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Porous silicon

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1995 Semicond. Sci. Technol. 10 1187

(http://iopscience.iop.org/0268-1242/10/9/001)

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Semicond. Sci. Technoi. 10 (1995) 1187-1207. Printed in the UK

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TOPICAL REVIEW

Porous silicon

B Hamilton Department of M60 IQD, UK

Physics, UMIST, PO Box 88, Sackville Street, Manchester

Received 5 January 1995, accepted for publication 17 March 1995

Abstract. This paper attempts to review the field of research into light emission from porous silicon. The driving force behind such research is the tantalizing goal of adding optoelectronic functions to the already impressive array of electronic functions provided by silicon-based devices. A silicon technology with included light emission would move even closer to complete dominance of the electronics market. After several years of research effort. the fundamental mechanisms of light emission are still not completely resolved. This is not surprising: porous silicon has many attributes of a new and complex material, and its study requires a truly interdisciplinary effort involving electrochemistry, surface science, structural and chemical microscopy on the atomic scale and detailed optical spectroscopy. This paper tries to connect these various threads; inevitably what emerges will only serve as a rather selective 'snapshot' of a still developing and often perplexing field.

1. Introduction

The dominance of silicon in the electronics industry is almost complete, at least in terms of volume: the worldwide market for silicon-based devices and systems depending on them is huge. The comparatively small but important markets which silicon does not^ fulfil are those of ultra high-speed devices and optoelectronics, in particular optical communications. In fact virtually all optoelectronic functions requiring high-speed modulation rely on compound semiconductor devices; fibre-optic-based optical communications systems rest firmly on InP-based lasers and modulators, whereas GaAs-based devices supply the near-infrared and visible emission required for short- range communication and disc r e d w r i t e functions.

It is a curious fact that although silicon is the material which essentially fed the information technology revolution, much of the highly successful international research effort into semiconductor physics during the past 15 years has been devoted to II-V semiconductors. The search for novel physical phenomena based on reduced dimensionality-superlattice, quantum well and latterly quantum wire and dot structures-has been a major driving force for condensed matter physics. Improved device functionality has emerged both in the fields of high-speed transport and optics, which have strengthened the III-V industry in these areas.

A picture emerges of a silicon industry dealing with a rather mature technology, able to fulfil many of the growing demands of an information-dependent culture. Materials-based research which will underpin a future silicon indushy currently centres around silicon-germanium heterojunction devices, novel configurations for reduced power consumption in portable systems, cheap thin film

0268-1242/95/091187+21519.50 6 1995 IOP Publishing Ltd

devices and nanoscale fabrication. Of course the latter topic holds out the possibility of novel functionality which exploits the quantum regime of electron behaviour, and so connects with some of the work reviewed here. Porous silicon burst into this arena several years ago, offering at least a possibility that silicon technology might eventually yield light-emitting devices. The fact is that under optical excitation. porous silicon does produce light with high efficiency, and furthermore with an emission spectrum which can be 'tuned' from the near-infrared to the green by varying porosity. Further processing by rapid oxidation extends the emission into the bludviolet region of the spectrum.

Clearly then, one driving force behind research into light emission from porous silicon is the hope that having finally understood the basic mechanisms, it might be possible to make an electrically excited LED or laser which could, ultimately, be integrated into a complex chip. This notion should not be seen as one of simply enlarging the functionality of silicon; discoveries of new functionality traditionally end up finding applications in semiconductor technology. In their turn, both the rapidly oxidizing silicon surface and the semiconductor laser respectively were dismissed either as a nuisance which would prevent development or as lacking in applications! It would be wrong therefore to rule out, say, optical interconnect applications for porous silicon, provided that it could be developed into a stable electroluminescent system, compatible with integrated circuit processing.

In order to even contemplate real applications for porous silicon devices we must understand the basic radiative processes and must have a clear view of how to optimize the porous skeleton and how to control and perhaps to take advantage of its enormous surface area.

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B Hamilton

Finally we must learn how to electrically excite the luminescence. In the final analysis it may prove impossible to achieve these goals, or some other less complex form of optically functional silicon may emerge. Many issues are currently being pursued, including modification of porous silicon and new ways to process material and contacts, and these will be touched upon below. However, the central issue remains the origin of the light, and its relationship to the atomic-scale structure and the associated electronic structure of the porous layer. The main aim of this review, then, is to try to draw together the threads of evidence which are guiding workers in the field towards understanding the physics of the material. The story to date, although largely qualitative, is complicated and has generated lively debate. This being so, it is politic to simply state at the outset the four most commonly held views on the origin of the visible luminescence:

(i) The visible and near-infrared light is the result of of quantum confinement shifts of the silicon energy gap due to particle localization in nanometre scale structures (wire or dot) which make up the porous skeleton.

(ii) The luminescence originates from surface molecular species which coat the porous skeleton, and which result from the electrochemical processing.

(iii) The light originates from radiative decay at surfacdinterface states, the character of which are partly determined by nanocrystaltine particles within the porous layer.

(iv) Hydrogenated amorphous silicon is a product of the invasive electrochemistry and is responsible for the emission.

In attempting to revicw the field, it is necessary to subdivide the information; first, in section 2, an overview of the main optical phenomena is presented. Section 3 thcn deals, in a.simple way, with the electrochemical process involved in pore formation, leading on to a brief review of pore morphology and microstructure. In section 4 some of the issues involved with the surface of porous silicon are discussed in order to provide a firmer basis for the review of the debate surrounding luminescence mechanisms in section 5. Section 6 deals with some issues concerning oxidized porous silicon which shed light on some fundamental aspects of the material. Finally section I outlines some attempts to make simple device stmctures and also the considerable problems involved.

2. An overview of the optical phenomena associated with porous silicon

Highly porous Si, processed using electrochemical etching methods, exhibits strong photoluminescence; efficiencies of several per cent have been routinely reported. Spectra are broad, but peak wavelengths can be 'tuned' over a wide range in the near-infrared and visible, by varying the porosity. These facts, which were first noted by Canham in 1990 [l] , remain the key points underpinning a wide-ranging and interdisciplinary research effort. It is interesting to note though that the observation that ultra small silicon crystallites, passivated by hydrogen, and with

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Wavevector

Figure 1. The energy band structure of crystalline silicon. The indirect energy gap leads to slow band to band radiative decay transitions which require the participation of momentum-conserving phonons.

emission wavelengths which depend on size, pre-dates the first report of porous silicon luminescence [2].

A natural starting point for a review of porous silicon is a comparison with the optical emission properties associated with crystalline silicon. The energy band structure of any semiconductor dictates many of the observed luminescence properties. Silicon has an indirect enera gap, shown in figure 1. A well known consequence of this that the radiative efficiency of Si is low at room temperature. The indirect gap dictates that electron-hole recombination across the gap requires the involvement of momentum-conserving phonons; the matrix element for the transition is thus small. Note that this is not a fundamental limit to the radiative efficiency, it simply results in a long radiative lifetime; the calculated radiative lifetime of moderately doped silicon at room temperature is in the millisecond regime. With such a slow radiative decay process, injected carriers inevitably recombine through non- radiative shunt paths, and the net recombination lifetime (though very sample dependent) is orders of magnitude shorter than the radiative lifetime. However, if all competing shunt paths like deep electron states or surfaces did not exist, then silicon would be a perfect emitter at close to 1 pm. Unfortunately this remains a hypothetical case, though data exist which demonstrate clearly that effective removal of the surface shunt path by hydride passivation results in long minority carrier decay times and large increases in radiative efficiency [3].

At low temperatures, Si becomes more optically active. This is principally because certain optical decay channels become thermally stabilized. For example, the free exciton population under injection conditions grows, and more importantly shallow impurities or defects can stably bind excitons. The initial capture event for an exciton into an impurity state may be fast; if the exciton can remain trapped for a sufficient time, i.e. is not thermally ionized from the

Porous silicon

1090 1110 113 1150

Figure 2. An example of a low-temperature photoluminescence spectrum of high-quality p-type bulk clystalline silicon. The sharp structure is due to the decay of excitons which are stably bound to the boron impurities a t low temperatures

2

Photon energy h"el

impurity potential, decay may occur with a matrix element determined partially by the impurity. Radiative decay times for such transitions vary considerably, but even though non-radiative 'branching' usually occurs for such impurity- localized excitons it is a relatively simple matter to measure the associated luminescence spectra. Such spectroscopy is an active field of semiconductor physics, and the reader is referred to a comprehensive review by Davies for more detail [4].

The luminescence spectra associated with bulk crystalline Si are typically highly structured and well defined, whereas porous Si luminescence is strikingly different. Figure 2 shows the photoluminescence spectrum from a lightly boron-doped sample [4], this is a high-quality version of the sort of wafer which might be used as a starting point for processing into porous material. Whilst the emission from the p-type wafer shows the characteristic sharp features due to the decay of excitons trapped at boron acceptors, that from porous material is both broad and significantly shifted above the three dimensional gap; this is clear from figure 3. which shows some typical examples of spectra measured on freshly prepared porous material. Although the porous Si emission is blue shifted, it is clear from figure 1 that the emission energies lie far below the lowest direct gap at the r point in the Brouillon zone. In fact it is clear that none of the light emission observed from porous Si, even the blue emission discussed later, is associated with the three-dimensional direct gap.

The spectral emission from porous silicon is not confined to a single band. The band shown in figure 3 [5 ] is often called the visible or slow band. and is the one first reported. This band can actually be shifted systematically between the near-infrared and the yellowlgreen region of the visible spectrum. Other radiating systems exist and are crucial to the emerging story of porous Si, but we shall use the properties of the visible band to obtain an overview of the luminescence properties. Table 1 gives a brief resume of the various bands observed to date with comments on their origin.

1 g1.1

0.9 o i 0.2 o. i 0.6 o:8 1 (1-Porosity)

I

Figure 3. (a) An illustration of the way in which the 'visible band' vanes with porosity. All samples in this data set came from the same p+ substrate and were processed in the same electrochemical cell. (b) The variation of peak energy with porosity for the same band. Although porosity is only an indirect assessment of size, it is clear that the blue shill becomes faster a t the highest porosities.

Although all conductivity types of Si wafer have now been demonstrated to yield the visible band, most work has been canied out on pt material. This is largely to do with the fact that the anodic electrochemical dissolution, which requires a large supply of holes, is most readily and precisely established in p-type material (see section 3).

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Table 1. Luminescence bands obselved for porous silicon.

Energy range (ev) Key properties 1.2-2.2 ('visible band')

Sensitive to porosity in most reported cases: sensitive to surface passivation, especially but not exclusively hydride passivation; strong temperature dependence of decay time; convolution of at least two components; dominant band in freshly anodized material. Usually weak compared with visible band;

sensitive to surface oxidation condition.

strong in high temperature oxidized material fast decay time,

0.8-1.3 ('IR band') sensitive to porosity;

2.5-2.8 Weak dependence on porosity ('blue band')

The visible band displays a quite remarkable variation of peak wavelength with porosity. Figure 3 shows this for small pieces of the same p substrate, processed in the same electrochemical cell to different degrees of porosity and measured under identical excitation conditions [SI, It is common practice to measure the visible band in freshly etched material. This behaviour has been reproduced by many groups, and though there are small variations the key features are now established. The onset of luminescence requires a threshold porosity of around 45%; below this value only the luminescence attributes of crystalline, non- porous silicon are observed. The visible band moves smoothly from its threshold peak energy of 1.3 eV to 2.0 eV at a porosity of 90%. At and beyond this porosity, the film is mechanically fragile and porosities are difficult to reproduce and to measure. The sensitivity of wavelength to porosity increases dramatically as the porosity increases. Since in a general sense the characteristic size of the porous skeleton reduces with increasing porosity, this wavelength sensitivity was one of first attributes to underpin the search for quantum confinement effects.

It was rapidly discovered [7] that the visible band is not completely stable following anodization; both wavelength shifts and efficiency changes occur when porous Si layers are stored in ambient conditions. In general blue shifting of the emission occurs, with a peak shift of 0.5 eV being recorded for a three year storage period, though all effects seem to saturate after around one year. Interestingly the quantum efficiency often changes little with ambient storage and may increase [7]. Another early discovery was the optical fatigue of the visible band which is especially pronounced for short-wavelength, high-power Laser excitation [SI. These instability problems relate to surface chemical and electronic structure, issues of vital importance for the understanding and control of porous Si.

In addition to the issues of stability, the visible band turns out to have rather complex spectral and temporal properties. The emission contained within the spectral envelope consists of more than one emission band [91. By using time-gated detection it has been demonstrated that both fast and slow components are typically present with distinctive spectral shapes. Figure 4 [9] demonstrates this, and shows that the fast component peaks at significantly higher photon energies. The data of figure 4 were obtained

1190

I

I r

Photon energy [ e V I

Figure 4. The 'visible' emission band is not a single system. This figure shows that time domain measurements reveal that at least two spectral bands are present.

from a p- layer of around 80% porosity which had a fully stabilized native oxide (i.e. is fully aged). More recently, fast high-energy components have been associated with specific types of oxidation of porbus Si. However, for porous silicon which has received no additional surface treatment, aside from ambient aging, the slow component is by far the most important and accounts for almost all of the measured quantum efficiency.

The detailed temporal behaviour of the slow component of the visible band varies according to which spectral bundle of the rather broad band is measured, and also on the measurement temperature. Also when measured over several decades, the decay profile is never completely exponential. There is broad agreement about the general form of the temperature variation of the decay time for this band. At low temperatures ( 4 0 K) the decay can be very long, of the order of milliseconds; as the temperature is increased the lifetime quenches to the microsecond regime. There are some differences in detail from sample to sample, which makes it difficult to fit the data to detailed kinetic models; however, figure 5 demonstrates the trend which has been established by many workers.

Porous silicon

10 1 10 100

Temperature (K1

Figure 5. The temperature dependence of the decay time of the main component of the visible band measured at 1.8 eV. The data are taken from t'Hoofl G W et a/ 1992 Appl. Phys. Lett 61 2344.

0.16-

c 0 , 0 8 i I

-0 0Lj-I 0 50 100 150 ZOO 250 3 0

Temperature IK I

Figure 6. The spectral peak of the visible band moves in a very uneven way with temperature. The detailed movement is sample dependent. The effect is illustrated here for p+ material of around 85% porosity. This trend is typical of a disordered system, but can be very marked for porous silicon.

This shortening of the total lifetime at high temperatures is always accompanied by a quenching of the luminescence efficiency. This shows in a rather unambiguous way that non-radiative channels are opening up to the excited carrier populations as the temperature is raised. This is rather a familiar picture in semiconductors and is often associated with disorder, or more specifically particle localization within the potential energy minima resulting from disorder. It suggests that at low temperatures excitons (for example) created by the optical pumping rapidly localize into sites which have good radiative efficiency. that is to say the local non-radiative channel is slow compared with the measured lifetime of typically 1 ms. As the temperature is raised. the thermalization time out of these 'radiative sites' becomes shorter and the exciton is free to explore larger volumes of the porous skeleton, and to find much more efficient non-radiative paths.

The notion that disorder plays a role in the luminescence mechanisms of porous Si is compelling given the enormous complexity of the porous skeleton, and indeed other simple observations support this view. One such observation is the spectral shift of the visible band as the temperature of the sample is raised and an example i s shown in figure 6. This occurs because at the lowest temperatures particles bind efficiently into the deepest potential fluctuations and the luminescence signal

1.0 20 3.0 4.0 5.0 6.0 70

i o 4

Excitation Energy (eV)

Energy (eV)

Figure 7. The photoluminescence excitation spectrum of a porous silicon layer. There is some similarity to that of bulk silicon especially in the higher energy regions towards the direct gap

is weighted in favour of the lower energies characteristic of the deep fluctuations. At higher temperatures shallower potential fluctuations become statistically more significant, shifting the mean of the spectral distribution to higher energies. This is exactly what is observed in disordered alloy quantum wells [IO], but the effect is much more dramatic in porous silicon [ I l l ,

Other spectral features may reveal the presence of disorder phenomena, such as the Stokes shift between emission and absorption bands, and the relationship between the Stokes shift and luminescence linewidth. For any situation in which luminescence is dependent on the details of localizing potentials, the absorption process itself is more representative of the band structure of the solid; it is not unevenly weighted by the defect phenomena. A practical difficulty in obtaining absorption data from porous silicon is that one of the spectral regions of interest is well above the three-dimensional gap of the underlying substrate. The strong substrate absorption inevitably masks the processes in the porous layer. For this reason many measurements rely on photoluminescence excitation spectroscopy (PLE) which is well suited to the measurement of thin surface layers. One of the first reported PLE measurements is shown in figure 7 [12].

The first impression of such data is that it is rather reminiscent of the absorption of bulk silicon, with strong absorption above 3.4 eV corresponding to the three-

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B Hamilton

dimensional direct gap. Once again, though, we run into the complexity of the material in the interpretation of the data. 'we do not know in detail the macroscopic optical constants of the porous silicon layer, though the refractive index has been shown to decrease with increasing porosity. Clearly some caution must be exercised in assigning an optical thickness w . The role of internal light scattering is likely to complicate the estimate of optical thickness. Calculation of the dielectric functions of the altered layer is difficult because of the complexity of the layer, though some attempts have been made [13]. Even in the limit of an optically very thin surface layer, for which the PLE method truly measures the absorption processes, the experiment will average the whole ensemble of size distributions; this will inevitably lead to a smearing of the data. Such smearing would be particularly enhanced if size effects were present in the optical density of states functions.

In order to get a better feeling for the material structure which gives rise to these very distinctive optical properties of porous silicon, we now turn to a review of the fabrication process: this includes some insight into the crucial role that high-resolution microscopy has played in the interpretation of the material properties.

3. Porous Silicon formation and microstructure

For many years porous silicon formation has been used as one of the mahy processing techniques for device isolation. The FIPOS process, (full isolation by porous oxidized silicon) makes use of hydrofluoric acid (HF) as an electrolyte in an anodic electrochemical reaction; HF, it seems, is the only known electrolyte which can anodically dissolve silicon in an efficient manner. The basic electrochemical phenomena involved in the FIPOS process and optically active porous silicon are essentially the same, except that the latter usually has significantly higher porosity and in the limit of such high porosity additional electrochemical reactions may occur. The electrochemistry of nanostructured silicon is still a developing field. and only the simplest of views can be presented here.

In principle the production of a porous silicon layer is not demanding; a carefully constructed electrochemical cell along the lines of that illustrated in figure 8 [14] is all that is required. The cell and the electrolyte system must be formed from high-purity material, and good control of the operating characteristics is required. Nevertheless. processing of centimetre-size samples with good macroscopic uniformity is not difficult. Ironically, the ease of fabrication is in stark contrast to the complex range of characterization methods which have been employed in an attempt to understand the porous material.

In the cell, the SifHF interface forms an elec- trode/electrolyte barrier system. The potential harriers and electric field distributions across even the equilibrium sys- tem are rather involved, depending on the doping character- istics of the semiconductor and the chemical composition of the electrolyte. However, the gross feature of the barrier is its 'double layer' attribute. There exists finite regions of space over which the interfacial electric fields are spread and the potential barrier evolves; these are the Hehnoltz

1192

Ammeter - I Cathode- h -Anode

-Magnetic stirrer

\. , , . . . . . . , U ca e: 0

*

- U Silicon - ca

Potential Distribution

.s ................................ U Figure 8. A simple schematic diagram of a basic electrochemical cell used for anodization. The potential distribution across the electrolytelsilicon system is shown below.

layer in the electrolyte, and the depletion layer in the semi- conductor. A schematic diagram [ZO] of the barrier system is shown in figure 8.

The dissolution of silicon occurs only under anodic conditions, and the primary process leading to massive removal of Si atoms is considered to be the formation of silicon fluoride molecules, SiF,. Various routes are possible in principle 115-171. Perhaps the simplest example proposed are for Si dissolution involves the divalent state [I81

Si + 2HF + ne' + , 3 5 2 + 2H' + (2 - n)e-.

Here it is assumed that holes take part, i.e. holes are freely available in the silicon to feed the reaction. This requirement is easily fulfilled by p' material, but low or even n-type conductivity is not a fundamental barrier to the process because optical excitation can always be used to generate an excited hole population. The SiFz formed in the above reaction may then be removed by other chemical reactions [18, 191. It has emerged, however [19], that the number of electrons consumed in the initial electrochemical reaction, that is the number n in the above equation, is

Figure 9. The electrochemical regimes available for silicon processing as a function of the I-V characteristic of the electrochemical cell. Region A: pore formation, region B: transition, region C: electropolishing.

greater than 2. It seems therefore that both the divalent and tetravalent Si dissolution occur simultaneously

Si + 4HF + (4 - n)e+ + SiF4 + 4H+ +ne-.

Again, several routes are possible for the removal of the SiF4.

What is achieved in practice depends on the precise anodizing conditions, for example the anodic potential, and .ranges from a layer of uniform porosity to complete removal or electropolishing. The current-voltage relationship of the sample-cell system reveals the various regimes of electrochemistry. This is shown in figure 9 [ZO]. In the region of low applied potential (A) the current is generally exponential with voltage (the Tafel region), with a slope of typically 60 mV per decade; this value is clearly an indication of the physics of the potential banier and the

in this region, the silicon removal being driven primarily by the above reactions. At significantly higher potentials, the electropolishing regime is entered, resulting in complete removal of the porous layer, Electropolishing results from the formation of an anodic oxide which is dissolved by the HF, any irregularities in the silicon topography being removed due to the divergence of th e electric field lines at regions of dielectric with a consequent enhancement at any Si features [221. One proposal for the electrochemistry of this oxidation process [23] is the following reaction

Si + 40H- +ne+ -+ Si(OH)4 + (4 - n)e-

..... ...... ..... associated carrier transport [20, 211. Pore formation occurs ~i~~~~ 10, The simpleS. model for pore formation, based essentially on impedance to current flow, leads to columnar pores (a). A more complex model based a diffusion-controlled mechanism of pore formation leads to the sort of multiply interconnected or spongy porous layers ofien obselved in TEM measurements (b),

the processing of porous silicon sensitive to the HF concentration in the electrolyte, low HF concentrations and hence low oxide removal rates favouring electropolishing; these trends have been established. experimentally [20].

Whilst the simplified electrochemistry discussed so far can explain Si removal, it does not account for the spatial selectivity which results in pore formation. In fact Pore morphologY does depend on conductivity type and several models have been proposed to explain this crucial feature of the processing, most of them resting

Si02 + 6HF + H2SiF6 + 2H20. on built-in inhomogeneities in the original Si wafer as the trigger for pore formation. The wafer conductivity

At low potentials, in the Tafel region, the oxide formation and electrochemical details then dictate the detailed pore rate is too low to compete with Si removal and porous evolution. silicon results. At high potentials oxide formation is One of the earliest attempts to explain pore formation enhanced and surpasses the oxide dissolution rate, resulting is due to Beale and co-workers [24, 251, and is based on in electropolishing. It is to be expected that this interplay the barrier properties coupled with the spatial variation between oxide formation and removal rates should make of impedance to current flow. The essence of the

1193

4. Si02 + 2Hz0.

me oxide formation rate is in competition with its dissolution rate governed by

B Hamilton

model is that small inhomogeneities on the wafer surface cause enhanced current flow and locally rapid removal of Si. The original depression is enlarged, leading to pore formation. The nature of the inhomogeneity is not specified; it could be some macroscopic perturbation of surface morphology, or even a defect at the atomistic level. In its simplest form this model led to the expectation that silicon between the pores will ultimately become depleted, simply because the dimensions of the remaining silicon 'columns' is insufficient to support the space charge width. The impedance offered to the current path into the silicon column is then held to grow rapidly and current flows preferentially down the electrolyte and into the wafer at the bottom of the pore, as illustrated in figure 10(a) [271. This provides a possible mechanism for producing columnar structures.

This way of describing the pore evolution does seem to go some way towards explaining the gross morphology of the porous skeleton in p+ silicon. It is suggested that the heavily doped wafer leads to a narrow space charge layer in the semiconductor, tunnelling phenomena are enhanced Gust as in Schottky baniers to degenerate semiconductors) and the impedance to current at the base of a pore is significantly lowered. However, the model does not explain the pore morphology observed in p- material; this typically consists of massively interconnected network which is uniformly distributed across the film.

Smith et al [26] have shown that a more complex pore morphology may be explained if pore evolution is limited, at least partially by the rate of diffusive transport of the hole to the reaction point at the electrolytic interface. The diffusion-limited case arises because the impedance offered by the barrier system is much higher than in the pf case, tunnelling being much weaker in the wide, low-field barrier system of the lightly doped semiconductor. This analysis still accounts for a faster than average reaction rate at a pore tip, and hence elongation of the pore. It also makes the interconnected network a more reasonable expectation, as shown in figure IO@). A pore E, initiated on the sidewall of an existing pore A, will be in better communication with the diffusing hole flux in that region of silicon between pores A and B until the tip of pore E approaches the tip of B within two hole diffusion lengths. This is rather a complex, at simplest. two-dimensional diffusion problem, but the significant sidebranching is a fundamental feature of the observed morphology.

Other models of pore formation have been discussed. The possible effect of quantum confinement in residual silicon structures has been proposed [27] as way of enhancing carrier depletion effects and hence limiting pore growth in the limit of very small structures. Alternative electrochemical schemes have also been proposed for the Si removal process [19, 281, which draw closer analogies with the formation of porous aluminium. The theoretical simulation of pore structure for a variety of possible electrochemical conditions is given by Parkhutik etal 1291.

No complete understanding of pore morphology exists, and it is likely that improvements in our understanding will come about through the application of high-resolution microscopy. Transmission electron microscopy has already

1194

proved essential in probing the porous structure. Porous silicon layers are inevitably fragile and this makes their evaluation more difficult, and necessitates the development of some novel approaches to specimen preparation. Measured pore sizes can vary from -100 nm (macroporous) down to c 2 nm (mesoporous). As a very approximate guide to published data it appears that lightly doped p- type silicon produces a fine network of pores whereas heavily doped p-type material produces more of a columnar structure [24, 25, 30, 311. For lightly doped n-type silicon, the pores in general take up a more crystallographic form with typical dimensions of several tens of nanometres propagating in the (100) direction. This attribute has even played a role in VLSI device isolation by trench formation. Un l i e the case of p-type silicon, as n-type doping level increases, the pore dimension increases and hence the interpore spacing decreases.

These general comments on pore morphology must be taken as a rough guide only. The detail form of the layer depends on the precise anodization conditions used, and very high resolution imaging can often real more complex geometry, leading to a fractal view of the altered layer. For example it has been known since the early work of Beale et al [32] that the columnar pore arrangement in pt silicon is heavily branched. It is also possible to produce mesoporous n+ silicon with -5 nm pore dimensions (33). The key question which high- resolution electron microscopy has attempted to address concerns the detailed relationship between porosity and luminescence. This has been reviewed by Cullis [34], who highlighted the need to avoid ion beam milling or other invasive specimen preparation methods for the preparation of electron transparent samples of porous material, which is easily amorphized and chemically modified. Figure 11 is taken from that review; it demonstrates well the key issues concerning the microstructure of p-type porous silicon in the transition from relatively low-porosity weakly luminescent material to high-porosity strongly luminescent material. The pictures represent bright field (001) projections. For the weakly emitting material, the Si skeleton comprises mainly rod-like structures with a range of diameters, the smallest being around 5 nm. The corresponding electron diffraction patterns indicate completely crystalline material. Figure 1 l(b) illustrates material of higher porosity than (a) which gave stronger luminescence. The microstructure is now finer with silicon structures down to 3 nm clearly visible. Arcing of the electron diffraction spots indicates misalignment of the Si columns. The electron diffraction pattern now shows more severe misalignment of the Si skeleton, but still indicates crystalline material. As porosity grows, these trends are continued. These TEh4 data, then, point to a correlation between a reduced characteristic size distribution of the silicon porous skeleton and the switching on of strong luminescence. In particular, column or particle sizes of around 3 nm or smaller are present in highly luminescent material.

Ultra high-resolution microscopy will continue to play a key role in porous silicon research and it can be anticipated that advances in microscopy will add more vital information

Porous silicon

regarding the relationship between microstructure and luminescence. Whatever new insight is gained from microscopy, though, it remains true that increasing the porosity of the material inevitably increases the surface area. Surface chemical interactions and the influence of the surface on electronic-properties area key areas of investigation.

4. Surface effects on porous silicon

The 'internal' surface area of porous silicon is very large; several hundred square metres per cubic centimetre of porous material is typical. It is reasonable therefore to expect that the surface itself might play a direct role in some of the observed luminescence behaviour, or that the surface would exert important effects on the 'bulk' behaviour of the material. A good deal of effort has been expended on investigating these issues, which still remain at the heart of the debate on the origin of the light emission. One of the earliest [2] and most graphic attributes of surface chemistry is the role of hydrogen coverage. After anodization in the HF-based electrolyte, a surface rich in Si-H bonds can be routinely observed using infrared local mode absorption spectroscopy. Bonds involving one (Si-H), two (Si-H2) and three (Si-H3) hydrogen atoms are normally present and both stretching and scissor vibrational modes can be seen [35-381. Figure 12(n) [ 5 ] shows a typical absorption spectrum measured for a freshly prepared porous layer (curve (a)). Compared to that of, say, an unprocessed Si wafer, the H bond-related absorption is dramatically increased. Other Vibrational features can be seen which are common to both porous layers and bare wafers; these correspond to S i S i stretching modes and to (probably bulk) Si-0-Si, interstitial 0 asymmetric stretching modes. These and other 0-related modes assume much more significance for oxidized material. The figure shows the evolution of the local mode structure as the sample is annealed in vacuum (curve (b)) and also in nitrogen at 300 "C for 5 (curve (c)) and 10 min (curve (d)). The vacuum anneal completely removes the H-related features, whilst the nitrogen anneals promote 0-related modes, probably due to weak 0 contamination.

It was noted above that the process of atmospheric aging has an indeterminate effect on the luminescence, and may cause it to increase. Such aging causes a broadening of the H-related absorption modes and also a growth of 0-related modes. However, by far the most dramatic phenomenon associated with H coverage is observed following desorption on a large scale during vacuum anneal. This causes a complete quenching of the luminescence. Figure 12(b) illustrates the luminescence spectrum for a freshly prepared sample. After vacuum anneal no luminescence can be seen, though some weak recovery is observed if the vacuum anneal is followed by a nitrogen anneal. This recovery is significant, even though no H-related absorption can be measured. By immersing the annealed sample in HF for a few seconds, both the luminescence and surface H bonds measured by absorption are dramatically restored. Very small shifts in peak wavelength are seen due to this cycle, and these are

1195

Figure 11. High-resolution data obtained from TEM measurements of porous p-type silicon. The trend in characteristic sizes of the remaining silicon skeleton as a function of porosity is clear. The associated optical characteristics are described in the text.

B Hamilton

(4 si.0-si related m o d s

J- defamation

I

IIIP 1 1

I I I I I

500' lMXl 1500 2m 2500 cni' Wavenumbers

Energy (eV) Figure 12. (a) The infrared absorption spectrum of prepared 4550% porosity silicon is rich in Si-H bond-related transitions curve (a), vacuum anneal for 2 min at 400 'C removes these modes completely. Further annealing in nitrogen at 300 'C for 5 (curve c) or 10 min serves only to weakly promote 0-related bonds. (b). The luminescence spectrum of the same sample: as-prepared (full curve), after the vacuum plus the 10 minute nitrogen anneals (dotted line) and finally after immersion in HF (broken curve)

not surprising since one expects small changes in the silicon skeleton to Lake place; we don't have precisely the same sample at the end of the sequence. There is no doubt. however. that surface hydrogen coverage plays a key role in the luminescence behaviour of freshly prepared porous Si, and that removal and replacement of the H coverage leads to reversible quenching and restoration of the luminescence. The electronic role of H is not fully understood, but removal of H has been shown to increase the Si dangling

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bond density measured by electron spin resonance [39]. Since the dangling bond is known to be a powerful non- radiative recombination centre [40], a straightforward role of H as a passivating centre is suggested. This notion is much connected with the debate surrounding the radiative mechanisms which operate in porous silicoa

There is no doubt then that the surface plays an important role and that H bonding is necessw to sustain the luminescence. Furthermore the vibrational assignments suggest that simple Si-H, bonds account for some, possibly most, of the surface hydrogen. It is unrealistic, however, to expect that this bonding arrangement accounts for all of the surface chemistry and some considerable effort has been devoted to probing for other surface constituents which might bear on electronic processes. There are several important candidates for surface bonding, based simply on the processing environment of the wafer; to date most reported work has been aimed at probing the involvement of oxygen, fluorine, or organic radicals of varying complexity.

Although surface Si-F bonds play an important role in the dynamics of pore evolution, it seems that they are not stable on the free surface after processing. Probably they are replaced via a hydrolysis reaction, by Si-OH bonds which themselves can dissociate into Si-H or Si-0-Si bonds by reaction with the atmosphere [41].

The role of oxygen is important. Simple exposure to air causes surface oxidation of all silicon, and the effect is enhanced for porous silicon. After all, this fact led directly to the development of the FIPOS process mentioned above. The luminescence aging of porous silicon is connected the incorporation of 0 into the surface bonding arrangement. The detailed form of 0-modified surface bonding has been analysed by Kat0 et al [36]. Low-temperature ( d o 0 "C, for times of less than 50 min) oxidation was used; this might be regarded as a sort of accelerated aging process. Using IR local mode absorption, the Si-H transitions are seen to broaden and shift somewhat. These spectral changes were attributed to the incorporation of 0 into the Si back bond(s) associated with the S-H, atomic arrangement. For example if 0 is incorporated into one of the three back bonds of S-H, the Si-H stretching mode transition was calculated to shift from 2090 cm-l to 2127 cm-', and by considering all possible sites for 0 incorporation into back bonds the general changes which occur in 'lightly' oxidized material are accounted for. This picture of 0 incorporation leaves all surfaces terminated with an Si-H bond; only the back bonds are broken. The spectral deconvolution leading which led to this picture rests on the assumption that the Si-H stretching mode transition is located at 2090 cm-', but it should be noted that some debate exists regarding thc precise vibrational nature of this transition.

Aging, and non-aggressive oxidation also cause increased absorption in all peaks that relate to Si-0 vibrational modes. The effect of further increasing the 0 content of porous Si causes yet more changes, and is currently being used as a modification process to stabilize the material. Oxidation is also potentially useful in helping to evaluate the luminescence mechanisms and the role of the dangling bond in quenching luminescence; these issues are discussed further in section 7.

Porous silicon

5.1. The quantum confmement mechanism

The idea that the surviving silicon skeleton contains within it structures small enough to exhibit quantum confinement effects such as opening up of the bandgap was the first proposal for a mechanism for porous silicon luminescence. This suggestion represents a simple explanation based on a well established attribute of the material, i.e. the existence of nanoparticles in the layer. Figure 13 is an example of the way in which the quantum confinement mechanism is often viewed. As we have seen, nanometre sizes for crystalline Si particles are amply proven from TEM data and are also confirmed by an analysis of the optic phonon Raman lineshape [45]. Energy shifts due to confinement on a scale comparable to the particle size are a universal feature of quantum mechanics, but proof that such a mechanism is correct must rest on a direct observation of luminescence from the nanoparticles and supporting evidence on the interconnection between the geometly of the nanoparticle and the emission wavelength.

Such direct evidence does not exist for porous silicon. However, a direct observation of red luminescence from oxidized isolated Si nanoparticles with characteristic sizes of below 5 nm has been reported [46, 471. Such observations demonstrate that isolated particles can luminesce, but the detailed chemical arrangement of the oxidized nanoparticles complicates the interpretation of the luminescence process

It remains the case that the blue shift of the visible luminescence with increasing porosity in p+ material is one of the key observations linking the light output to nanoparticle size. However, porosity in itself is a quantitative measurement of the fractional mass removed, but is not a quantitative measure of nanopaaicle size. So. for example one can imagine crudely that a film of a particular porosity might consist of large Si particles and extremely large voids, or of very smal1.nanopartick.s and moderate size voids. In p+ material, in which the morphology is known to exhibit size reduction of the nanoparticles as porosity increases, the increased sensitivity of the luminescence blue shift to porosity is precisely what would be expected from quantum size effects provided that the light originated from recombination within the Si nanoparticles.

The slow decay rate of the visible band is really what one might expect from an indirect semiconductor; and if the interior of the residual silicon nanoparticles were perfectly crystalhe and therefore presented a shunt-free environment with no non-radiative recombination centres and with completely passivated surfaces, they might offer the perfect environment for light emission. However, the non-exponential decay indicates that such an idealized notion is unlikely. In fact this behaviour is not unlike that observed in amorphous silicon, in which carrier trapping in the tail states plays a dominant role.

The photoluminescence attributes are rather different for n-type material. In general, for such material there is no systematic variation of blue shift of the visible band with porosity. For example, marginally porous layers, e.g. less than 40%, have been shown to exhibit strong visible luminescence with the same general character as that seen

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Although IR absorption has emerged as a powerful tool in the surface analysis of porous silicon, other techniques, principally x-ray photoelectron spectroscopy ( X P S ) and SIMS, have been used. SIMS analysis has confirmed the presence of hydrogen and Ruorine as the major surface species of freshly anodized material, whilst oxygen, carbon and nitrogen were detected at lower concentrations [42]. The SIMS data also c o n h that F is not stable but reduces with atmospheric exposure, presumably due to the hydrolysis reaction mentioned above, and indeed an increase in surface hydrogen to be expected from this process is also observed using SIMS. The other important changes revealed by SIMS measurements are a build-up of carbon and oxygen with prolonged exposure to the atmosphere.

The XPS technique has revealed fluorine, carbon and oxygen on porous silicon surfaces [43], in broad agreement with the SIhlS data. Evidence in support of a fluorine- admixed Si02 surface phase has also been claimed, based on XPS analysis [44]. This possibility, of surface layers with rather complex chemistry, for example an Si-0-F- H system, though more difficult to analyse experimentally than simple Si-H bonding arrangements, is the basis for one of the models suggested for the luminescence, to which we now turn.

5. The light emission process

Having reviewed the basic features of the material it is possible. by looking in a little more detail at particular pieces of experimental evidence, to try to glean what is cumently understood about the basic light emission mechanism. This of course is the pivotal question surrounding porous silicon. Until it is answered progress towards any technological goals will be limited.

It must be readily acknowledged that the complexity of the material provides fertile ground for the proposal of differing models for the light emission process. Porous silicon is a richly interconnected system of small particles and intricate surface topology. This fact alone leads naturally on to the expectation of electronic disorder with its associated defect and interface states; furthermore the large surface area, generated in a chemically varied environment adds the possibility of partial surface coverage with complex molecular films. All of these attributes have formed the basis for hypothesis regarding the light emission, and at the time of writing all of these generic schemes receive support, often zealous. In keeping with the spirit of this lively debate, this section is presented by analysing some of the data which either support or undermine current models. In the current literature, by far the largest attention has been paid to investigations which have been designed around the hypothesis that quantum confinement plays a key role; by far the largest number of reports discuss this issue. Accordingly, this aspect is given more emphasis here, though the correctness of the hypothesis is not proven.

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Figure 14. The optical transmission spectrum of free-standing porous silicon films.

to the PLE measurements reported above, there have been successful attempts to measure absorption in free- standing films, free of any complications associated with the substrate. The absorption measurements often appear to be rather more sensitive to the low-energy tail, near and even below the three-dimensional gap of silicon. Several such measurements seem to indicate that the threshold for absorption in porous silicon is higher in energy than in bulk silicon and that the threshold moves to higher energies with increasing porosity. Figure 14 demonstrates this for films originally processed from both p and p+ substrates [27]. The up-shift in energy was more marked for the p+ material which was found to have smaller nanoparticle sizes than the p material. These observations are consistent with the opening up of the gap due to confinement and would strongly support i t if it were to be confirmed that silicon nanoparticles and not some other phase of material were dominating the absorption spectrum.

Looked at in more detail, the absorption edge of porous silicon does not wholly support a simple quantum confinement model. Photothermal deflection spectroscopy has shown [49] that the absorption strength increases roughly exponentially above the luminescence peak. Whilst it might be argued that a size distribution of nanoparticles might partly explain such data, the same experiments show that absorption occurs significantly below the gap of three-

from P-tyPe material [481. This is an imPortant Point. and dimensional silicon. The existence of significant Urhach is further highlighted by the fact that the pore morphology tails in the density of states is of course a qualitative is much more macroscopic in nature with tYPicallY large measure of departure from crystallinity and is reminiscent widely spaced and crystallographically oriented pores. This of the behaviour of amorphous silicon. Whatever the picture is at variance with the quantum confinement model. origin of the density of states low-energy tails, its existence However, the fact that the macropores have much smaller implies strongly some significant degree of electronic structure on the sidewalls is a further complication which localization. means that we cannot rule out a nanoparticle explanation Falling back on the evidence relating to the for the luminescence. luminescence spectrum, the modification of the emission

A key test of low dimensionality in any electronic characteristics by post-anodization processing has been system is a measure of the density of states functions for used to variously support or oppose confinement models, electrons and holes, and it was noted above that, in the The blue shift of the luminescence peak as a result of case of porous silicon, optical ahsorption is in principle oxidation followed by HF dipping was first suggested as a fundamentally better measurement of these (or more supporting evidence, since the consumption (by oxidation) precisely of the joint optical density of states). In addition and subsequent removal of silicon is expected to reduce the

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Figure 13. Schematic diagrams illustrating the some of the structures envisaged for the optically active material, according to the quantum confinement hypothesis. ( a ) The transition from quantum wire through oxidized nanoparticles to porous glass [7]. (b) The aligned nanocrystalline or wire structures consistent with EPR data. (From Harvey J F e t a/ 1993 NATO AS/ Series voI244, p 179.) (c) An electronic view of how an exciton localized in a nanoparticle might suffer three possible fates for radiative decay giving rise to three luminescence bands. (From Koch F 1993 Mat. Res. Soc. Symp. Proc. 298 319.)

Porous silicon

- c 3

;i .. 10000

E 8000 P 5 6000

5 4000

- 2000

.. - - h

c

c -

850 800 750 700 650 Wavelength l nml

Figure 15. An example of one effect of immersing porous silicon in ethanol: (1) is the ‘as-prepared spectrum’, (2) is for 1 min of immersion, (3) for 3 min, (4) for 10 min and (5) for 60 min. The luminescence was measured in situ

overall size of all silicon components in the porous skeleton. To counterbalance this, reports of red shifts with hydrogen loss [SO] would not be expected to affect the particle size, though very small effects due to strain might be expected to produce small wavelength shifts. The influence of low- energy ‘processing’, essentially immersion in a variety of organic fluids, has been shown to have large and nearly reversible effects on the emission spectrum. Effects due to acetic acid, propanol and ethanol have been reported [SI, 521. Figure IS [SI] shows the rather dramatic effect of immersion in ethanol for times of up to 1 h. The detailed chemical interaction with the porous skeleton has not been analysed for these organic treatments, but it seems reasonable to assume that they involve surface or near- surface effects, and such effects are expected to impinge only weakly on optical transitions with energies determined mainly by size quantization.

The debate on quantum confinement has been underpinned by attempts to calculate the electronic structure of silicon nanoparticles, and hence to predict optical properties, in particular the transition energies and matrix elements. Effective mass theory (EMT), which has been so successful in predicting the properties of epitaxially grown low-dimensional smctures, has been applied to small silicon structures typical of those know to exist in porous silicon. One such calculation [53] was based on the notion that for cubic structures with sides greater than IO atoms long, bounded by (100) planes, EMT represents a plausible approximation for the description of wavefunctions. Simple envelope functions and confinement energy shifts result. The infinite barrier approximation at the cube boundary leads to an optical matrix element which is an oscillatory function of the cube size. This is difficult to test in a real system because the ensemble of sizes present in a given film inevitably smears the effect. However, the overall trend for radiative lifetime variation with confinement energy shift for the optical transition shift, which of course relates to cube size, is predicted by EMT to vary rapidly: approximately as the inverse cube of confinement shift. Some workers have noted that the measured lifetime of the visible band can vary with

1 + +

0.5 1.5 2.5 3.5 4.5 Olnml

Figure 16. The optical gap predicted by LCAO theory for nanometre-size silicon crystallites, as a function of size.

porosity and therefore with peak photon energy [54], and there is rough agreement with the EMT prediction and the experimental data.

Of course, the very small sizes of some crystallites observed in porous siiicon must eventually limit the applicability of EMT, and point to the need for first principles calculations. One such calculation [5S] has been performed for wire structures, spanning wire thicknesses which vary from the thii, molecular, limit of polysilane to structures which are essentially bulk-like. The calculation was performed for wires with axes in the [OOI] direction, bounded by (110) surfaces which were assumed to be fully terminated with hydrogen atoms. A supercell approach was used with the basic unit cell of the wire repeating in space in order to retain three-dimensional periodicity. Such a calculation is far removed from the effective mass approach, using a first-principles pseudopotential for the Si ions and a bare Coulomb potential for the H ions; the exchange-correlation energy and potential were included using a local density approximation. It is interesting to compare the results of such a calculation with EMT. For the confinement up-shift, agreement was good for wire diameters of greater than 23 A; above this value the EMT prediction appears to be an overestimate. The calculation also yielded a radiative lifetime of around 380 ps for a wire with 72 atoms in the unit cell, i.e. in broad agreement with experiment based on the notion that small crystallites yield the photon output of the visible band. The authors noted, however. that such a long radiative lifetime implies that the high quantum efficiency of this band is largely a consequence of the small non-radiative competition rather than the lifting of the momentum selection rule.

A recent calculation using the linear combination of atomic orbitals (LCAO) technique has been used 1561 to calculate the optical properties of wires and crystallites (cylindrical and spherical shapes). This form calculation should yield information on both conduction and valence band properties of the structures. A key result, the calculated optical gap as a function of diameter, is shown in figure 16. The crystallites show the greatest sensitivity to size; this is the intuitive result based on the fact that confinement is in three dimensions. The authors also illustrate the Coulomb electron-hole interaction energy

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which makes only a small difference to the total calculated energy gap. An interesting comparison is made with the experimentally measured photoluminescence energy measured not from porous silicon but from hydrogen- passivated silicon crystallites produced by nucleation from the gas phase [57].

The result for the wires is a little more complex, showing anisotropy between different wire directions. The authors note that the visible band, which is tunable between 1.4 and 2.2 V, would be consistent with characteristic structure sizes of between 2.5 and 4.5 nm. The exponent relationship between gap and diameter, in the visible band energy window. was found to follow D-',39 rather than D-2 predicted by EMT. However, the calculation predicted an inverse square law at larger D values where EMT is valid.

The LCAO calculation also dealt with recombination and optical absorption. It was concluded that the strong confinement in silicon (43 A) induces band mixing and dipole allowed transitions. The optical matrix elements, though, remain small and the radiative decay rates as function of transition energy show strong scatter. Partly this results from the oscillatory behaviour induced by the dependence of the matrix element on the overlap in k space of the electron and hole wavefunctions; this was a point which emerged also from the EMT formalism. For the case of the crystallites, the LCAO calculation also demonstrated that the radiative rate was also sensitive to the symmetry representation of the Td point group which varies greatly as the size of the crystallite is varied. This effect was shown to be more sensitive at lower temperatures.

The optical absorption coefficient based on the above calculation, for a crystallite of 3.86 nm diameter is shown in figure 17(a) [%I. This shows that the major absorption strength is in the ultraviolet, with an absorption 'edge' near to 3.5 eV, i.e. close to the direct edge of bulk silicon. The spectral shape is also very structured and bears a superficial resemblance to what might be expected from a molecular system. When viewed on a more sensitive scale. figure 17(b), the calculated absorption coefficient for this crystallite does show that the transition is allowed down to the calculated gap energy, but with small oscillator strength. The absorption coefficient shows a quadratic dependence on photon energy above threshold, unlike that of bulk silicon which shows a linear dependence. The blue shift of absorption edge with porosity (assuming an attendant reduction of crystallite size) reported above is then generally predicted by the LCAO calculation, and the predicted non linear shape has also been recorded experimentally [58].

These three illustrations serve only to review the trend in calculations of small structures, and are by no means exhaustive. They underline the point that in general there is no fundamental disagreement between theory and the quantum confinement model for the main emission band observed from porous silicon. They also highlight the fact that the regime of solid at the heart of the debate is tantalizingly poised between one which is comfortably crystalline and populated with electrons in Bloch states, and one which is better described by a molecular framework This notion is very much the theme of the surface film and defect models reviewed next.

1200

E l e v 1

Figure 17. LCAO prediction for the optical absorption coefficient of a 3.86 nm crystallite.

5.2. Molecular films, interfaces and defects

Since it is clear that surface hydrogen coverage is an important criterion for light emission, at least in unprocessed porous silicon, several groups have explored the possibility that the hydrogen does not simply play a passivating role (i.e. dangling bond saturation), but is somehow involved directly in the radiative process. Two main candidates have emerged; surface hydride species and a class of compounds known generically as siloxenes. Although other variations have been suggested, these two examples are illustrative of the key ideas.

The idea that surface hydride species of the form S a I are directly involved in the luminescence process stems largely from the fact that particle size distribution, and in particular size reduction attempts, are not universally consistent with the quantum confinement model. Luminescence from only moderately porous p+ silicon (20%) has been reported (591 which did not show a blue shift with repeated HF dipping, and with increased porosity. On the other hand, this material did show all of the well known attributes of surface hydride coverage, i.e. luminescence could be quenched and restored by hydride removal and replacement. A further report of luminescence peaking at 1.7 eV measured for n-type samples sample of less than 10% porosity bas been made [60]. Particle sizes of around 200 nm were found in this material and it was claimed that side pores did.not exist. Such a size distribution is not appropriate for

Porous silicon

and clearly show that .the optical gap shrinkage of the amorphous silicon matches well to the red shift of the porous silicon sample.

A somewhat more complex model for the involvement surface molecular species in the form of siloxene related compounds has been suggested 1641. Siloxene in its simplest form has the chemical composition SiaO& and can be prepared from C a s h via the reaction

3CaSiz + 6HCI + 3Hz0 = Si6O& + 3CaC12 + 3H2.

The existence of such compounds has been known for some time, and their fluorescence in the green region of the visible spectrum is also well known in the chemical literature [64]. The initial suggestion was that siloxene or closely related compounds are a by-product of the electrochemical processing of silicon, which is rich in Si. H and 0 atoms. The tuning of the luminescence was suggested tentatively to result from chemical variations to the basic structure, for example by substituting other ligands for the H-terminated Si bonds in the sixfold Si ring of the isolated molecule. Many of the features of porous silicon luminescence were also seen in the fluorescence of siloxene: tunability of wavelength, electroluminescence during anodic oxidation (see section 7), luminescence fatigue and non-exponential decay.

The tunability of the chemical structure of the siloxenes and its link to emission wavelength were the main question marks which militated against the siloxene explanation soon after it was suggested. In part this shortcoming was due to a lack of understanding of the physical chemistry of these materials. More recently [65] it has been demonstrated how crystalline films on silicon substrates can in principle be produced by evaporation of calcium followed by reaction with HCI, i.e. a potential planar technology. Perhaps more importantly for the present debate, recent quantum chemical simulations of siloxene [66] crystals have led to a better understanding of the stability of the system and the way in which modification by oxygen incorporation can change the electronic properties.

The idealized crystalline siloxene structure is shown in figure 19(n) [65], and consists of a silicon plane, terminated by OH and H radicals on opposite side of the plane. Calculations suggest that this form is metastable; insertion of 0 into the bonds of the Si plane gains 1 eV per Si- @Si bond. Therefore annealing the structure is likely to transform it into that shown in figure 19(b) [651. The stoichiometry remains the same, but now that all the 0 atoms have been incorporated into what was the Si plane notice that isolated Si6 rings begin to appear.

The optical properties of the metastable and annealed structures are quite different. The metastable structure fluoresces near 2.6 eV, and has a relatively sharp absorption edge at only a slightly higher energy, i.e. a fairly small Stokes shift, The quantum chemical calculations predict that the Si plane present in the metastable form is a direct-gap semiconductor with a gap of 2.7 eV at the point, broadly consistent with the experimental data. The annealed structure fluoresces in the red, near 2 eV, and shows a much broader absorption edge and a very much larger Stokes shift. This is much less like the properties

1201

1.75 "80 7 1.70t v =

T

1,501 1 ohm-cm porous Si 1.45 T a 6 : H (Yamasakl et al.)

1.40

100 200 300 400 500 600 Temperature ["Cl

Figure 18. A comparison of the measured shift of the luminescence spectra of amorphous and porous silicon as a result of annealing. In both cases the loss of hydrogen from the system is implicated.

producing quantum size effects. The same report also detailed measurements of p-type material which was subject to cyclic (atmospheric) oxidation and HF dipping. This is without doubt an obvious way to thin the microstructure, and pore enlargement with an associated reduction in average particle size is to be expected. However, what was in fact observed was a cyclic shift of peak wavelength from 720 nm to 680 nm, the shorter wavelength reappearing after the HF dip.

A futther observation [60] relating to size distribution is that high-temperature (up to 1200 "C) annealing of porous silicon under UHV conditions causes a collapse of the microstructure, to .the point where the material resembles a collection roughly spherical particles having a dimension of a few hundred nanometres. The luminescence is also quenched. When such material is dipped in HF, luminescence is restored, even though the particle size distribution is unchanged.

These inconsistencies' with the quantum confinement model appear to leave the presence of the surface hydride species as the only completely consistent factor in determining whether or not luminescence is present. The plausibility of this idea gains support from work on hydrogenated amorphous silicon [61], deposited from the vapour phase, with high H content. Luminescence from such material is in the range 1.3 to 2.08 eV, and blue shifts with increasing H content. This was explained in terms of polysilane complexes: (SiHz)" or hydride complexes. Wavelength variation may be a feature of the luminescence of both entities because the 'gap' of the polysilanes depends on the chain length, and the SiH, species, according to tight binding calculations, produce bonding states deep in the silicon at energies which depend on the H content of the molecule [62].

On a more practical note, an interesting comparison has been made [60] between the red shift of the luminescence of porous silicon induced by annealing, (in an argon atmosphere), compared with the red shift induced in the luminescence of hydrogenated amorphous silicon by similar processing [63]. The data are shown in figure 18 [60],

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expected of a direct gap semiconductor, and is similar to the measured properties of porous silicon. Another similarity between annealed siloxene and porous silicon lies in the involvement of the triplet exciton in the low- temperature luminescence, probed by optically detected magnetic resonance [66]. These experiments also support the idea that the exciton is strongly localized, on a scale compatible with the size of the sixfold Si ring expected to be present in annealed siloxene.

It has been proposed that the sixfold silicon ring is the basic luminescence 'centre' for both annealed siloxene and porous silicon [65]. The spectral properties of properties of both are similar and the EPR measurements have identified the Si dangling bond as the key non-radiative shunt for both. It has been argued that coalescing pores will produce fragmentation of monolayer silicon that could lead to the formation of the ring structure. Furthermore it was noted by the authors of [65] that such a process might be a simple explanation for the fact that strong luminescence has been reported from porous amorphous silicon [671, which appears to relax crystallinity as an absolute prerequisite for the luminescence.

The siloxene model, like the quantum confinement model, has many appealing features. However, at the present time it seems not to explain in a simple way the smooth shift of wavelength with porosity which is probably the key result. It must he remembered that this band can be reliably tuned down to less than 1.4 eV. A more complete statement seems to be required about the way in which the sixfold silicon ring, or perhaps some perturbation to it, might allow such gross tunability. In contrast, wavelength tunability with size is a natural feature of quantum confinement.

6. Oxidized porous silicon

Oxidation of porous silicon has received much attention recently mainly because it produces stable material with additional emission at short wavelengths, often referred to as the blue or fast band. Not only is this luminescence relatively immune to thermal degradation but it exhibits decay times in the nanosecond regime, a potentially useful attribute for devices. It must be stressed, though, that the visible or slow band remains (with some spectral modification) in oxidized material, and rapid oxidation for typically 30 s at 900 "C can give stable material which emits strongly at these longer wavelengths. Higher-temperature processing than this tends to remove the visible band [5, 681, leaving only the fast band, and of course a much more fully oxidized material structure.

Figure 20 shows representative spectra of oxidized porous silicon. It has been noted by several groups that the intensity of the visible band drops rapidly with oxidation for oxidizing temperatures up to 600 "C, and then rises with processing temperature until the melting point of silicon is reached. Figure 20(a) [69] shows that the visible band remains, in a broadened blue shifted form, but also that a higher energy band emerges which extends into the blue. Of course the latter band requires pumping with an appropriately short wavelength source. Figure 20(b) shows

-,-

Figure 19. (a) The idealized structure of the Si planes in as prepared siloxene: the planes are terminated by H or OH radicals on opposite sides. (b) Siloxene after the ordered insertion of 0. isolated SiB rings now appear, and these may be the luminescent centres responsible for emission from modified siloxenes.

1202

Energy (eV)

Figure 20. Photoluminescence spectra typical of rapidly oxidized porous silicon. (a) The broadening and blue shifting of the visible band and the appearance of the high energy band (from 1691). (b) Besides the visible band (a) measured at 10 K, a lower energy band, the infrared band, is produced. There is strong competition between the infrared and visible bands which depends on temperature; spectra (b), (c) and (d) were measured at 10, 70 and 300 K respectively.

another key result; the oxidation also produces a low energy band known as the infrared band. This band which is below the energy of the bulk gap competes with the visible band, but at low temperawes becomes as efficient as, or more efficient than, the visible emission.

Electron paramagnetic resonance has been applied to most forms of porous silicon. By far the most important defect to be observed is the dangling bond or Pbo centre. This centre has been known for some time to be present at the S i S i 0 2 interface [70]; it is a [ill] axially symmetric system with the unbonded orbital directed along one of the four equivalent [ 11 I] directions. The density of this centre increases when the visible luminescence is quenched by annealing and it accordingly correlates with hydrogen loss from the surface [711. The centre has also been detected via its influence on the intensity of the visible band as the magnetic field is swept through resonance [72]. The general conclusion is that the dangling bond is a key non-radiative shunt path for the visible band.

The optically detected magnetic resonance (ODMR) experiment shows a large effect for the infrared band; the intensity of the hand increases by around 15% at resonance. This compares with values of typically 0.01% variation for the visible band. The data, as shown in figure 21 [73], indicate something slightly more complex than a simple P ~ o

Porous silicon

1.22 1.24 1.26 Magnetic f ield IT1

Figure 21. The strong ODMR signal measured for the low-energy or IR band [73]. These data prove that the PbO centre (together with another centre) is directly involved in the luminescence process responsible for the IR band

centre. The lowest-field peak with an isotropic g value of 2.013 does not belong to the dangling bond, but may belong to a localized hole. The two higher peaks are signatures of the Pbo; their g values are 811 = 2.0017 and gl = 2.0085, where the parallel direction of !he magnetic field is along the [ 11 11 direction. This anisotropy is exactly that found in the EPR spectrum of the dangling bond. The strong effect in ODMR may imply that the dangling bond is directly involved in the infrared band.

The origin of the blue band is of course of great interest. One obvious question to he asked is that since it is much faster than the visible band [74] could it be the true signature of a direct energy gap in a quantum confined system? This idea has been supported by several groups who point to the similarities which exist between the blue band and emission from direct-gap semiconductors. Unfortunately the blue band is most easily seen in highly porous (oxidized) material, and so its wavelength dependence on porosity cannot be easily explored; i.e. even this rather uncertain size control has not been explored for particles which remain in the system. Besides which, the rather aggressive oxidations used modify the skeleton of the porous layer. Other workers have supported the view that the blue band originates from SO?. For example, it has been reported [69] that the luminescence does not shift spectrally with increasing oxidation time (up to 50 min). It has also been noted that oxidized planar silicon wafers give similar luminescence [75]. In both these references, the suggestion that defects in the Si02 glass might be responsible for the emission was made.

7. Electroluminescence and devices

The goal of research into porous silicon is without doubt the realization of an efficient LED. The wavelength range of

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Figure 22. An indium tin oxidelporous silicon heterojunction LED, showing the device structure and electrical characteristics.

any potential device is actually a secondary consideration since any wavelength band has applications. It has become painfully apparent that to fabricate such a device is hugely difficult, and quantum efficiencies of 10-6 are typical; even then large drive voltages, in excess of tens of volts for example are often required.

A variety of structures with solid state contacts have been tried, ranging from simple indium tin oxide, to attempts at p n junction technology. Figures 22 and 23 show two devices which are representative of the sort of structures appearing in the literature. Figure 22 demonstrates a simple heterojunction technology using conducting (and transparent) indium tin oxide [761. Rectification is observed for such contacts with light emission occurring only for one polarity of voltage. This particular diode structure produced a rather narrow emission band, about 20 nm wide, centred on 580 nm, significantly narrower than a typical room-temperature photoluminescence spectrum. Although the reasons for this are not clear, it may he that the electroluminescence excitation excites only a subset of the porous layer.

A device based on a porous p-n technology is shown in figure 23 [77]. This device is based on an n-type wafer with a surface-implanted pf layer. Anodization was carried out using illumination so that the n-type material as well as the implanted p layer became porous. The vertical porosity profile depends very much on the doping profile, but the active region was thought to be a nanoporous region which straddled the metallurgical p n junction. The quantum efficiency of the device was measured to be lO-4. An interesting attribute of this structure is that the emission hand seen in electroluminescence can he tuned by the wavelength used to excite the layer during the anodization process; the shorter the excitation wavelength, the shorter the emission wavelength. Although the reasons for this are not completely established, the authors point out that in order to supply holes in porous

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Figure 23. A porous Si p n junction device fabricated by anodization under illumination of a boron implanted n-type wafer. (a ) Device structure and I-V characteristics. (b ) The electroluminescence output for devices anodized with different wavelengths of light.

n-type material, and hence for anodization to proceed, the exciting photon must be strongly absorbed within the small silicon particles characteristic of nanoporous material. Following the qualitative expectations of quantum confinement, very small particles would require short wavelengths for absorption by the opened up energy gaps. Whatever the detail, it was demonstrated that electroluminescence ranging from the infrared to blue could be produced by varying the anodization excitation from infrared to ultraviolet. The electroluminescence data are also shown in figure 23.

A variety of device structures have been tried, but the key result, the external quantum efficiency, remains low. It seems an obvious conclusion that the contact technology to date fails because it does not provide volume excitation of the porous film. What is required is a contact technology which transports energy into the whole of the film and then facilitates local minority carrier injection. Such a scheme would also demand a continuous current path to the wafer. Of course optical excitation fulfils all of the required conditions without the need for current continuity; it is the perfect excitation source. It has become increasingly clear

Wavelength (nm)

Figure 24. (a) The transient (integrated) light emission from porous silicon during anodization. (b) The spectral detail of the light emitted during the transient period.

that some innovation in solid state contact philosophy is required in order to achieve better success.

Liquid contacts represent a state of suitability which is intermediate between photon beams and solid state contacts, and the study of such systems has yielded some important results for our understanding of luminescence processes in porous silicon. This work has utilized electrolytic solutions in order to transport charge carriers to the interior of the porous film. In 'fact the anodic dissolution process used to create the porous layer is accompanied by some rich luminescence detail 178, 791. The key observations for the light emitted during the anodic oxidation process are shown in figure 24. Firstly there is a time delay before any emission is observed and the authors ascribe this to the fact that the porosity is simply too low in the early stages of anodization for any light to be seen. After this delay, the luminescence builds up to some peak value and then decays to zero. The quenching is always associated with a sharp rise in the required anodic potential (to sustain a constant anodic current). Although it is estimated from the total anodic charge transferred (Qo in figure 24) that the film porosity is only around 50% at the point of quenching, the authors conclude that at this point the electrical connectivity between the small crystallites in the film, which are assumed to be the source of light, is broken. The electrochemical activity then switches to oxidation of the base of the pores; a process which is associated with the increased anodic potential. Actually, light emission is

Porous silicon

also observed in this high anodic potential regime, but it is of short wavelength and is thought to be associated with processes occurring in the oxide layers 1801.

The spectral distribution of the light transiently emitted during the initial stages of anodization, though, shows remarkable similarity to the visible or slow band, and it would be surprising if its origin were not the same as the optically pumped emission. This is shown in figure 24, which also shows that as anodization proceeds the luminescence peak blue shifts. This shift has been analysed by the authors in terms of a tunnelling-limited escape mechanism for excited carriers which leads to non- radiative recombination. A model was described for a tunnelling process occurring through small regions which connect optically active crystallites to non-radiative bulk silicon sinks. The yodel demonstrates that because these regions, which may be even smaller than the crystallites, are particularly sensitive to shape (and hence bandgap) changes, the predicted variation of tunnelling flux with time would produce the observed blue shift. They also commented that the oxidation-induced thinning of nanopdcles mentioned above cannot account for the blue shift: it would be too small. These results then provide some additional insight into the non-radiative processes, and are supported by the qualitative observation that the radiative efficiency of the fluorescent species, perhaps the nanoparticles, seems to increase as the porosity and hence the barriers to tunnelling transport increases.

Strictly speaking, many would wish to label these observations of light emission during anodization as chemiluminescence, rather than electroluminescence, and there do remain some problems in understanding the electron injection process in the overall ,scheme of the luminescence. The hole of course is provided directly by the anodic reaction. One possibility for the electron supply channel may lie in the existence of intermediate species being formed during the oxidation process of the silicon wafer; such intermediate species may have energy levels high enough to allow electron injection [SI]. The key point here is that the structure is undergoing chemical transformation during the light emission process. However. luminescence using liquid contacts has also been achieved under cathodic conditions [82], and cathodic injection does not modify the chemistry of the material. The principle of liquid contact electroluminescence is therefore firmly established.

8. Conclusions

Research into the physics of light emission from porous silicon is motivated by a very practical desire to extend the functionality of silicon. The fact that enthusiasm remains strong for research into this complex material is a testimony to this huge technological prize and the challenge of bringing together the impressively wide ranging interdisciplinary tools needed to deal with structures of almost fractal complexity. It is the sheer scale of the complexity which has limited the interpretation of data and the success of attempts to make devices.

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B Hamilton

As regards the basic luminescence mechanism. it would be wrong to give the impression that views do not remain divided; quantum structures, surface hydride species, amorphous silicon and more complex molecular arrangements like siloxene derivatives all have their protagonists. It may be that a combination of some or even all these are present in porous silicon, but it is more satisfying to think that there is a single mechanism operating. Such satisfaction is rooted in the traditional approach of physics, which finds elegance in obtaining the most complete solution possible for model systems. This luxury, though, is not available to, say, biologists who deal with systems in which complexity is the intrinsic dominant feature.

On balance at the present level of knowledge, the quantum confinement model has most support. It is easy to understand why this should be the case: size confinement and the associated energy shifts are absolutely fundamental features of quantum mechanics. So, whatever additional mechanisms might be invoked in porous silicon, quantum size effects should occur if very small singlecrystal entities are formed. The quantum particle or wire hypothesis for the optical activity, then, is viewed as a development of a straightforward and basic rule of physics. Of course, in practice this intrinsic effect may be overwhelmed by the optical activity of other chemical species, or there may be reasons why quantum sized crystallites simply do not fluoresce with adequate efficiency to explain the obsewed light emission. Such objections, though, are details, especially since the alternative hypotheses are on the whole more complex than quantum confinement involving exotic molecular species for which detailed knowledge of the electronic structure may be missing.

The current debate therefore tends support the view that whilst the jury is still out on all of the possible mechanisms, the quantum confinement model remains the simplest explanation: and until better or more innovative experimental techniques prove otherwise, simplicity holds sway.

Looking back over the past several years of research into porous silicon, however, it seems that this effort is very much part of a paradigm shift in condensed matter physics, and particularly in materials research. The movement towards understanding materials systems of great complexity is now evidenr One can think of numerous examples, including semicofiductor superlattices, high-T, superconducting materials, hierarchical biological structures characteristic of living matter, and many more. Probably this research will in the end prove to be valuable even i f devices do not result, because at the very least it has led to one of the most successful periods interdisciplinary work and experimental development for many years.

Acknowledgments

The author wishes acknowledge the financial support of the EPSRC and from the ESPRIT programme for maintaining his involvement in porous silicon research. Special thanks are due to his collaborators Ursel Bangert, Phil Dawson, Spyros Gardelis and Robert Pettifer for their unstinting efforts and critical approach.

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