transistor

Design of a-Si:H/GaAs heterojunction bipolartransistors with improved DC and AC characteristics

F.G. Della Corte and F. Pezzimenti

Abstract: Results from a detailed simulation study are presented to help evaluate the pros and consof a wide-gap hydrogenated amorphous silicon emitter (a-Si:H) introduced to improve the injectionefficiency at the emitter–base junction of a GaAs bipolar transistor. For a set of deviceswithstanding the same maximum emitter–collector voltage, it is shown that the current gain,besides a net dependence on the defect state concentration at the emitter–base interface, is alsostrongly influenced by the base thickness and doping. The base thickness, however, has a weakimpact on the cut-off frequency, which in turn shows a clear dependence on the emitter electronmobility. The study predicts that the current thin film silicon technology would allow thefabrication of a transistor performing a DC current gain close to 3000 and a cut-off frequency inexcess of 15GHz. Owing to the simplicity of fabrication, such a device could represent an effectiveway of adding a bipolar stage to a GaAs MESFET IC without resorting to AlGaAs/GaAsheterostructures.

1 Introduction

n-p-n AlGaAs/GaAs heterojunction bipolar transistors(HBT) are widely used in analogue and digital appliancesfor wireless communication, optoelectronis and high speedcomputing. In fact the insertion of a wide-gap AlGaAsemitter permits fabrication of devices with a very highcurrent gain, which is in turn traded for larger bandwidths[1]. At present, the fabrication of these devices is based oneither molecular beam epitaxy (MBE) or metal organicchemical vapour deposition (MOCVD), two outstandingepitaxial technologies which, however, require expensiveand hazardous equipments.As an alternative to the use of AlGaAs/GaAs hetero-

junctions in particular applications, recently the fabricationof GaAs-based bipolar devices taking advantage ofwide-gap amorphous semiconductor regions has beenconsidered [2, 3]. In fact, with an energy gap exceeding1.7 eV, a-Si:H promises to allow the fabrication of high-gainHBT, while requiring a technology which is far simplerthan those based on classical heteroepitaxy. Such anapproach, in fact, has been already proposed and carefullydiscussed in the case of silicon HBTs performing anamorphous silicon (a-Si:H) or amorphous silicon carbide(a-SiC:H) emitter [4–7].As a contribution to the study of possible applications of

a-Si:H as a wide-gap semiconductor, in this work the DC,high frequency and switching characteristics of a GaAs-based HBT are investigated by means of numericalsimulations. The device, in particular, performs an a-Si:Hemitter, while the base and collector regions are made of

GaAs. A finite element simulator of semiconductor devices,which takes into account the distributed density of statestypical of amorphous materials, has been utilised through-out the work. For careful tuning of the electronic propertiesof the amorphous layer, the results of a previous study,which involved the fabrication and characterisation of adhoc devices [3], have been considered.

2 Characteristics of an a-Si:H/GaAs heterojunction

a-Si:H is known to be a highly defective semiconductor.Distortions in the co-ordination angles between Si atoms, inparticular, determine the onset of additional electronicstates close to the valence and conduction band limit, orUrbach tail states. Also, the lack of Si atoms in the crystallattice produces unsaturated, or dangling, bonds which inturn introduce additional allowed states in the middle of theenergy gap.The preferred deposition technique for thin film a-Si:H is

plasma-enhanced chemical vapour deposition (PECVD) ina SiH4 ambient, at temperatures of 200–280 1C. In fact, thistechnique ensures a high degree of hydrogenation, which inturn allows a tight control over the number of gap statesand the width of the energy gap [8]. In fact, depending onthe deposition technique and parameters, a-Si:H can showan energy gap Eg tunable in the range 1.6–1.9 eV. A typicaldistribution of states for a-Si:H is shown, for example, inFig. 1.Owing to the low deposition temperature allowed by

PECVD, the technology of a-Si:H is compatible with thatof many other semiconductors, and in particular with thatof GaAs. The potential advantages of the association ofthese two semiconductors come from the band offsetsestablished at the heterojunction. The two materials have infact almost the same electron affinity, with wa-Si:H¼ 3.93 eV[9] and wGaAs¼ 4.07 eV. Therefore, the energy gap differenceDEg is in practice completely located at the valence bandside, with a valence band offset DEV of up to 0.36 eV. Forthe emitter–base junction of a n-p-n HBT this could intheory result in an improvement of the current gain by a

The authors are with DIMET, Mediterranea University of Reggio Calabria,Loc. Feo di Vito, I-89100 Reggio Calabria, Italy

F.G. Della Corte is also with the Institute for Microelectronics andMicrosystems, CNR, Via Pietro Castellino 111, I-80131 Naples, Italy

r IEE, 2003

IEE Proceedings online no. 20030577

doi:10.1049/ip-cds:20030577

Paper first received 4th February and in revised form 24th April 2003

350 IEE Proc.-Circuits Devices Syst., Vol. 150, No. 4, August 2003

factor exp(DEV/kT)E106 [1], at room temperature. Inaddition, due to the lack of a crystal lattice in a-Si:H, nostrain exists at the interface whatever the energy gapdifference, a clear advantage in comparison with AlGaAs/GaAs heterostructures.Unfortunately, experimental works have shown that the

density of states at the amorphous/crystalline interface has anegative impact on the base current of a HBT [10].Moreover, the electron and hole mobilities in a-Si:H arevery low and the resulting poor conductivity can becompensated only in part by higher doping because thedoping efficiency is always low in a-Si:H [8].In an effort to gain more insight into such kinds of

heterostructures, and to identify the critical aspectsdetermining their electrical behaviour, a-Si:H/GaAs hetero-junctions have been studied recently [3]. In particular thecharacteristics of an experimental heterojunction p-i-n diodewere compared to those calculated by means of the finiteelement device simulator ATLAS [11], showing that limitingthe number of defects located at the interface below acritical level would dramatically improve the minoritycarrier injection ratio at the heterojunction. Such a level,moreover, is close to that allowed by the present technology.Moving from those considerations, in this paper, thepredicted DC, AC and switching characteristics of a GaAsbipolar transistor with a wide-gap a-Si:H emitter arepresented and discussed for the sake of drawing somedesign criteria.

3 HBT structure and simulation settings

A schematic representation of one half of the a-Si:H/GaAsHBT cell considered in this work is reported in Fig. 2. Thedevice is in particular designed for low-power GaAstechnology (VDDr5V).The fundamental geometrical and electrical characteris-

tics are summarised in Table 1 for the base and collectorregions, and in Table 2 for the a-Si:H emitter region.

The parameters NG,C and NG,V are, respectively, theintegrals over the whole energy spectrum of the twoGaussians representing the defects associated to thedangling bonds throughout the gap within the firstnanometre from the interface. Values five times lower wereconsidered for the same parameters in the remaining 4-nm-thick part of the emitter region. This thickness was chosento guarantee that the emitter region would not becompletely depleted at the bias points of interest. Detailsof the defect distribution model in the a-Si:H film arecontained in [3, 12].The a-Si:H(n) emitter stripe is 0.5mm wide, with an

emitter-to-base contact distance of 0.25mm. To compensatefor the low carrier mobility, the emitter region was assigneda doping level of 6� 1018 cm�3, which is close to thetechnological limit for a-Si:H.The base doping was changed between 5� 1017 and

2� 1018 cm�3, with the base thickness varying accordinglybetween 50 and 14nm to guarantee a base punch-throughvoltage of 5V.

dens

ity o

f sta

tes

vale

nce

band

tail states

midgap states

cond

uctio

n ba

nd

energy

Fig. 1 Typical density of states (DoS) for a PECVD grown,highly doped a-SiH film

sub-collector (n+)

collector (n)

base (p)

a-Si:H emitter (n+)

base contact

emitter contact

0.25 µm

0.4

µm

50 µm

Fig. 2 Schematic representation of one half of the simulateda-Si:H/GaAs HBT

Table 1: Simulation parameters in the GaAs regions

GaAs base GaAscollector

GaAssubcollector

Thickness (nm) 14–50 400 2000

Doping (cm�3) 5� 1017–2� 1018

(p-type)

1016 (n-type) 5�1018

(n-type)

tn, tp (ns) 5, 5 8, 8 1, 1

mn, mp (cm2 V�1 s�1) 1200, 60 8000, 400 1200, 60

Table 2: Simulation parameters in the a-Si:H emitter region

Thickness (nm) Doping (cm�3) NG,C (cm�3) NG,V (cm�3) mn (cm2 V�1 s�1) mp (cm2 V�1 s�1) Eg (eV) wa-Si:H (eV)

5 6�1018 (n-type) 3�1018 3�1018 0.1 0.01 1.8 4.1

Other parameters were set as in [3]

IEE Proc.-Circuits Devices Syst., Vol. 150, No. 4, August 2003 351

4 DC analysis

The devices analysed in our simulations are listed in Table 3with the relevant technological characteristics. For thecalculation of the device terminal currents we assigned adevice width of 50mm and therefore the emitter area was25mm2. In addition the considered collector effective areawas 75mm2.

The parameters WE, WB and NB are, respectively, theemitter thickness, the base thickness and the base doping. Inorder to compare devices designed for the same applica-tions, the base width was scaled in each HBT in a way suchthat they performed the same maximum base punch-through voltage, i.e. VCE,max¼ 5V. Therefore, for each basethickness, the base doping was incremented until weobserved the onset of an anomalous collector current atthe fixed VCE,max.To highlight the effect of the defect state concentration

on the device characteristics, the DC current gain forVCB¼ 0 was calculated as a function of the density of statesat the amorphous–crystalline interface. The simulationresults are shown in Fig. 3. From this analysis the effectof the interface quality on the current gain is clear. Inparticular, the existence of a type of threshold effect isobserved, with the IC/IB ratio rapidly increasing in alldevices for integral defect concentrations below 1019 cm�3.This should be interpreted as a direct consequence of theimproved injection efficiency at the heterojunction, as wasalready observed in [3]. It should be noted in this connectionthat modern PECVD techniques allow the deposition ofa-Si:H films showing integral defect concentrations wellbelow 1019 cm�3 in the bulk of the material; less has beenassessed, however, on the amorphous–crystalline interfaceregions.

The current gain is shown for the three devices in Fig. 4as a function of the emitter–base bias. In this case the defectstate concentration was set to 3� 1018 cm�3 at the interface,a value comparable with the emitter doping level, which isknown to be the major source of defects in highly extrinsicamorphous materials. With a maximum DC current gainclose to 2800, device B25 shows the best trade-off betweenthe collector efficiency and the detrimental base seriesresistance. It is worthwhile noting that the maximumcurrent gain is observed at an emitter–base bias of 1.3V,corresponding to a collector current ICE10

�3 A. Beyondthis bias point, a voltage drop develops across the resistiveamorphous region, determining the gradual reduction of theactual VBE fraction falling on the emitter–base junction. Atlower biases, a larger carrier recombination takes place inthe wider depleted portion of the base, determining a drasticdecrease of the current gain.

Device B25 shows the best DC characteristics. In fact,while device B50 suffers from an excessive recombination inthe wider depleted portion of the base, device B14 ispenalised by a poorer emitter junction injection efficiency asan effect of the higher base doping.In Fig. 5 the Gummel plots are shown for device B25

calculated at two different interface defect concentrations.The two devices perform comparable current gains.

total gap state concentration NG /1019, cm−3

curr

ent g

ain,

�D

C

0.4 0.8 1.2 1.6 2.00

500

1000

1500

2000

2500

3000

B50B25B14

Fig. 3 DC current gain as a function of the total gap stateconcentration at the amorphous–crystalline interface

B50

B14B25

0

500

1000

1500

2000

2500

3000

0.4 0.8 1.2 1.6 2.0 2.4

VCB = 0

base voltage VBE , V

curr

ent g

ain,

�D

C

Fig. 4 DC current gain as a function of the emitter–base bias forthe three simulated devices

Table 3: Parameters characterising the various simulateddevices

Device

B50 B25 B14

WE (nm) 5 5 5

WB (nm) 50 25 14

NB (cm�3) 5�1017 1018 2� 1018

base voltage VBE , V 0.4 0.8 1.2 1.6 2.0

−12

−10

−8

−6

−4

−2

0

−14

curr

ent I

C, I

B, 1

0X A

NG = 2 × 1018 cm−3

NG = 7 × 1018 cm−3

Fig. 5 Gummel plot for device B25 calculated at two differentinterface defect concentrationsBase and collector are held at same potential


However, at the low biases the base current is dominated bythe recombination taking place in the base regionof the electrons injected from the emitter; as a betterinterface allows a more efficient injection, a higherrecombination follows with respect to the more defectedheterojunction. This effect is clearly seen in Fig. 5, wherefor VBEo0.9V a higher base current is observed forNG,C¼NG,V¼ 2� 1018 cm�3.By raising the base bias, the interface recombination is

increased with respect to the base recombination. This effectis obviously more prominent in worse interfaces, resulting inan enhancement of the base current and a lowering of thedevice current gain.

5 AC and switching analysis

Small signal analysis of the devices B50, B25 and B14 wasperformed to assess their high-frequency behaviour. Pro-ceeding from a DC solution at an assigned bias, thesimulator applies a frequency-swept AC generator to thebase contact, with the device used as a common emitteramplification stage. We focused our attention on the smallsignal current gain bAC, the transconductance and thescattering parameter S21.The short circuit AC current gain bAC was calculated

with the devices biased in the active region, namely withVC¼ 3V, VE¼ 0 and VB ramping from 1.0 to 2.0V. Thequite large emitter–base polarisation is necessary to sustaina base current IB parallelly ramping from 10

�9 to 10�4A.As an example, the simulation results for VB¼ 1.6V areshown in Fig. 6 for NG,C¼NG,V¼ 3� 1018 cm�3. The threeplots are in fact coincident; for all of the devices the lowfrequency bAC is close to 50dB, while the cut-off frequencyft is approximately 8GHz. This substantial equivalence is anunusual result for bipolar transistors if one considers theprogressive base thinning from 50 to 14nm, and demon-strates that the base transit time gives a minimal contribu-tion to the overall emitter–collector transit time. In fact, thefactor that actually limits the high-frequency performancesof these devices is the emitter junction transit time, which inturn is kept high by the large resistance. We calculated, infact, that the emitter differential resistance re is in excess of60O, while common values in standard GaAs HBTs aregenerally in the range of a few ohms. This result furtherenforces the theory that extremely thin emitters arefundamental in amorphous/crystalline heterojunction bipo-lar devices.

The parameter S21, representing the voltage gain formatched load, is higher than 12dB for all the devices at lowfrequencies. The gain drops to unity at 9GHz for deviceB14, 8GHz for device B25 and 7GHz for device B50; thesefrequencies are therefore the respective maximum oscillationfrequencies. In the same frequency interval the devicetransconductance was also calculated as shown in Fig. 7. Itdrops from 430mS/mm, at low frequency, to 240, 195 and165mS/mm, respectively, for devices B14, B25 and B50, atabout 10GHz. Once again the maximum oscillationfrequency of 9GHz calculated for device B14 slightly differsfrom that of the 50nm thick base device B50.

The device high-frequency characteristics are obviouslyaffected by the quality of the amorphous layer. Table 4summarises the calculated cut-off frequencies ft for deviceB25 for various electron mobilities and for two defect stateconcentrations, namely 1018 and 3� 1018 cm�3. The simula-tions demonstrate that the cut-off frequency is remarkablysensitive to the electron carrier mobility, while we observedno dependence on the hole mobility, which was thereforefixed at 0.1 cm2V�1 s�1. It should be noted, in addition, thatthe mobility values considered in this analysis are consistentwith those reported for good quality a-Si:H films orstandard mc-Si:H (microcrystalline) films. A minor impacton the AC performances is conversely shown by the defectconcentration.

To evaluate the switching behaviour of the device,transient time simulations were performed while forcingdevice B25 to pass through the off-on-off states. The stepsignal shown on top of Fig. 8 was applied to the basecontact, while the collector current through a 50O loadresistance or the collector potential, also shown in Fig. 8,were monitored. The characteristic switching times are listedin Table 5, where ton, toff and ts are, respectively, the switch-on, switch-off and storage time, while tpLH (tpHL) is the

B50

B25

B14

frequency, Hz

−10

0

10

20

30

40

50

smal

l sig

nal c

urre

nt g

ain,

�A

C, d

B

60

10111010109108107

Fig. 6 AC current gain of devices B50, B25 and B14 at a base biasVBE¼ 1.6 V

0 2 4 6 8 10 frequency, GHz

0

100

200

300

400

500

tran

scon

duct

ance

gm

, mS

/mm

B50

B25

B14

Fig. 7 Calculated frequency dependence of the transconductancegm for the three described devices

Table 4: Calculated maximum cut-off frequencies

Device B25 Mobility (cm2 V�1 s�1)

mn¼0.1,mp¼0.01

mn¼ 0.2,mp¼ 0.01

mn¼0.5,mp¼0.01

NG,C¼NG,V¼ 1018 cm�3 ft¼ 8GHz ft¼16GHz ft¼ 30GHz

NG,C¼NG,V¼ 3� 1018 cm�3 ft¼ 7.5GHz ft¼15.2GHz ft¼ 29GHz


propagation delay measured between the instants where theinput and the output signals are at 50% of their respectiveevolution, with VC going from low to high (high to low).These values demonstrate that in spite of the presence of theamorphous layer, the charge accumulation and extinctionprocesses are controlled by the carrier lifetime in the GaAsbase. It should be noted that a similar conclusion hasalready been reported, drawn from experimental measure-ments carried on a-Si:H/GaAs p-i-n devices [13].

6 Conclusions

The impact of some geometrical and technological para-meters on the DC, AC and switching characteristics of a-Si:H/GaAs n-p-n HBTs has been analysed throughnumerical simulations, duly taking into account the gapstates in the amorphous semiconductor emitter. Theanalysis has demonstrated that the device DC character-istics are strongly dependent on the amorphous/crystalline

interface quality, which has a severe influence on thejunction injection efficiency. The current gain in particularrapidly increases for defect concentrations below 1019 cm�3.The calculated AC parameters have revealed a minimal

dependence on the base thickness, and therefore on the basetransit time. In fact, the factor that actually limits the high-frequency performances of these devices is the emitterjunction transit time, which in turn is kept high by the largeemitter resistance. The cut-off frequency is, however,strongly dependent on the amorphous layer electronicproperties, and in particular on the carrier mobility. A cut-off frequency in the range 15 to 30GHz is predicted in adevice with optimised base width and doping, if the emitterelectron mobility is assumed to be in the range 0.2 to0.5 cm2/s. An average propagation delay of 125ps has beenfinally calculated.

7 Acknowledgments

Simulations were performed at the Institute for Microelec-tronics and Microsystems of CNR, Naples, Italy.

8 References

1 Kroemer, H.: ‘Heterostructure bipolar transistors and integratedcircuits’, Proc IEEE, 1982, 70, pp. 13–25

2 Mimura, H., and Hatanaka, Y.: ‘Electrical properties of p-typeamorphous silicon-n-type crystalline gallium arsenide heterojunctions’,Jpn. J. Appl. Phys., 1985, 24, pp. 355–357

3 Della Corte, F.G., Rubino, A., and Cocorullo, G.: ‘Simulation studyand realisation of an a-Si:H emitter on GaAs’, Solid-State Electron.,1998, 42, pp. 1819–1825

4 Orpella, A., Bardes, D., Alcubilla, R., Marsal, L.F., and Pallares, J.:‘In situ-doped amorphous Si0.8C0.2 emitter bipolar transistors’, IEEEElectron Device Lett., 1999, 20, pp. 592–594

5 Li, P., Li, Y.Q., and Salama, C.A.T.: ‘A heterojunction bipolartransistor with a thin a-Si emitter’, IEEE Trans. Electron Devices,1994, 41, pp. 932–935

6 Sasaki, K., Rahman, M.M., and Furukawa, S.: ‘An amorphousSiC:H emitter heterojunction bipolar transistor’, IEEE Electron DeviceLett., 1985, 6, pp. 311–312

7 Symons, J., Ghannam,M., Neugroschel, A., Nijs, J., andMertens, R.:‘Silicon heterojunction bipolar transistors with amorphous andmicrocrystalline emitters’, Solid-State Electron., 1987, 30, pp. 1143–1145

8 Street, R.A.: ‘Hydrogenated amorphous silicon’ (Cambridge Uni-versity Press, Cambridge, 1991)

9 Matsuura, H., Okuno, T., Okushi, H., and Tanaka, K.: ‘Electricalproperties of n-amorphous/p-crystalline silicon hetero-junctions’, J.Appl. Phys., 1984, 55, pp. 1012–1019

10 Bonnaud, O., Sahnoune, M., Solhi, A., and Lhermite, H.: ‘Modelingthe base current of an a-Si:H/c-Si heterojunction bipolar transistor’,Solid-State Electron., 1992, 35, pp. 483–488

11 Silvaco Int.: ‘Atlas device simulation software user’s manualV.5.0.0.0.R’ (Silvaco Int., Patrick Henry Drive, Building 1, SantaClara, CA, USA, 1999)

12 Della Corte, F.G.: ‘Simulation Study of the DC and AC character-istics of an a-Si:H(n)/GaAs(p)/GaAs(n) heterojunction bipolartransistor’, Solid-State Electron., 2000, 44, pp. 2265–2271

13 Della Corte, F.G., Polichetti, T., Rubino, A., and Cocorullo, G.:‘Study of a-Si:H emitters for efficient carrier injection in GaAs bipolardevices’, J. Non-Cryst. Solids, 2000, 266–269, pp. 1049–1053

base

vol

tage

, V

0 0.5 1.0 1.5 2.0 2.5time, ns

colle

ctor

vol

tage

, V

−1

0

1

2

3

4

5

60

0.5

1.0

1.5

2.0

2.5

Fig. 8 Transient time simulation of the switching response ofdevice B25Signal on top is applied to the base while a 50O load is present on thecollector

Table 5: Characteristic switching times for device B25

ton toff ts tpHL tpLH

90ps 310ps 140ps 70ps 180ps


Study of electroluminescence in porous silicon forsensing applications

A. Tsargorodskaya, A.V. Nabok and A.K. Ray

Abstract: Electroluminescence (EL) of porous silicon (PS) samples was studied at anodicpolarisation using neutral electrolyte contact. SEM and AFMmorphology study of PS revealed itscomplex structure comprising features with micrometre and tens of nanometre dimensions. Theobserved features of EL, including its degradation during anodisation, were discussed in terms of aphenomenological model of a chemically modified silicon surface. The adsorption of polyelec-trolytes polyallylamine (PAA) and polysterylsulphonate (PSS) demonstrated sufficient improve-ment of EL stability. The effect of adsorption of protein bovine serum albumin (BSA) on ELquenching was studied.

1 Introduction

The large surface area of porous silicon (PS) has stimulatedrecent interest in its applications for sensor development.Due to a very high concentration of adsorption sites on thePS surface, the sensitivity of PS transducers to molecularadsorption can be increased by two orders of magnitudeover that of two-dimensional transducers. This approachhas become popular in recent years, and several experi-mental attempts have been made to study molecularadsorption and to perform different chemical and bio-chemical reactions in the PS matrix [1–6].Several physical principles were exploited in PS transdu-

cers, namely, optical Fabry–Perot interferometry [7],ellipsometry [8], photoluminescence (PL) [9] and electro-luminescence (EL) [10]. The latter seems to be the mostattractive because it combines the high sensitivity ofluminescence measurements with the much easier andcost-effective method of its excitation as compared to PL.Moreover, EL can be performed by simple measurementsof emitted light intensity at a fixed wavelength. Yet the useof an electrolyte contact in EL measurements allows easyoperation with chemical and bio-reactions in the aqueousphase. The main phenomenon to be exploited in PStransducers is the quenching of the luminescence due tomolecular adsorption and some products of chemicalreactions [3, 9, 11].However, the main drawback of both PL and EL

methods lies in the poor stability of the light emission,which decays rapidly due to a number of external factors,such as temperature, electric field, strong illumination andelectrochemical reactions [12–14]. Another important issueof PS is the reproducibility of its main characteristics andthe luminescence in particular. At present, despite a largenumber of publications on this topic, the formation of PSwith desirable properties is still a subject of intense scientific

pursuit. The mechanism of PL in PS is not finallyestablished. There exist five or six different physical modelsexplaining some particular features of PL and EL, but theyare not universal since there are a number of experimentalfacts contradicting those models [12].Some experimental work towards immunosensing has

been done recently using the PL in PS [3,9,11]. In particular,it was shown that the absorption of immune components inPS and the following immune reactions affect the intensityof PL. Much less work has been done so far on theapplication of EL for chemical and bio-sensing.The present study targets the improvement of the

reproducibility and stability of EL in PS, further develop-ment of the physical model of EL and the exploitation ofEL for chemical and bio-sensing. The latter narrowed thechoice of the electrolyte for this work. Instead of usingredox electrolytes and cathode luminescence, which canprovide high light emitting efficiency and stability [12], weperformed anodic EL in neutral electrolytes, such as trizmabuffer, compatible with proteins.

2 Experimental

2.1 Preparation of porous siliconPorous silicon (PS) layers were prepared by a standardmethod of anodisation of n-type /100S Si wafers ofresistivity in the range of (4.5–10) � 10�2 O �m in electrolytecontaining hydrofluoric acid [15].The experimental set-up used for the PS formation is

sketched in Fig. 1. Silicon samples were sealed against thePTFE electrochemical cell through a rubber O-ring, so thatthe front side of the silicon wafer was in contact with theelectrolyte. In the case of n-type silicon, holes necessary forpore formation were generated by illuminating the frontside of the sample with a 300 W solar simulator lamp. Inorder to provide uniform electrical potential and thushomogeneous current density across the sample, the backsides of the silicon wafers were coated with silver conductivepaint. The electrical contact between the Si wafer and apower supply was provided via a metal ring. The counterelectrode was a platinum spiral immersed in the electrolyte.The electrolyte was prepared by adding 49% HF to the(1:1) ethanol/water mixture up to a desired concentration ofabout 21%. The anodisation of the Si surfaces was carried

The authors are with the Nanotechnology Research Laboratories, SheffieldHallam University, School of Engineering, City Campus Pond Street, SheffieldS1 1WB, UK

r IEE, 2003

IEE Proceedings online no. 20030640

doi:10.1049/ip-cds:20030640

Paper first received 28th February and in revised form 1st May 2003


out at positive DC bias applied to the Si electrode with acurrent density of about 400Am�2. The etching timeusually lasted no longer than 15min. Following anodisa-tion, the samples were rinsed with Millipore water for10min and then in a stop-solution (acetone or ethanol) for5–7min, and finally dried with nitrogen. The resulting PSsamples had a circular shape with an area of about2.5 � 10�4m2.

2.2 Experimental methodsThe morphology of PS samples was studied by scanningelectron microscopy (SEM) and atomic force microscopy(AFM). The Philips XL40 ASEM/EDS instrument wasused, with typical accelerating voltages of 10–20kV. TheAFM study was carried out using the Nanoscope IVinstrument equipped with Si3N4 tips of 4nm radius. TheAFM pictures of the PS samples were obtained in a tappingmode and at a slow scan rate of about 0.1–0.2 s�1. Thelatter was chosen because of the high surface roughness ofPS samples previously observed by SEM at a larger scale.The electroluminescence measurements were performed

in a specially designed cell of 2 � 10�7m3 in volume having atransparent window and two (inlet and outlet) tubes toallow the injection of different solutions into the cell. Ametal syringe needle, serving as one of the tubes, providedelectrical contact to the electrolyte. PS samples were sealedagainst the cell through the rubber O-ring. Typically, anaqueous solution of 35mM/l trizma-base/HCl buffer (pH7.35) obtained from Sigmas was used as an electrolyte. AllEL experiments were carried out at room temperature atvariable (3–15V) positive (anodic) bias applied to the Sielectrode. EL spectra were registered in a wavelength rangeof 300–1000nm using the Spex 1681 monochromator and aCCD detector.The EL kinetics experiments were performed in situ at a

fixed wavelength of 633nm using the Anritsu ML 9001Aoptical power meter. The effect of adsorption of bovineserum albumin (BSA) from Sigmas, as a common protein,was studied by measuring the EL kinetics in response toconsecutive injections into the cell of pure trizma/HClbuffer and the buffer containing 1mg/ml of BSA. ELkinetics measurements were also deployed to study theeffect of adsorption of polyelectrolytes, namely polyallyla-mine hydrochloride (PAA) and polysterylsulphonate so-dium salt (PSS), both purchased from Aldrich. 2mg/mlsolutions of both PAA and PSS in millipore water wereused.

3 Results and discussion

3.1 Structural characterisation of PS samplesPS samples with different pore sizes and shapes wereobtained depending on the type of silicon and theparameters of electrochemical etching. Figures 2 and 3depict typical SEM and AFM images of porous siliconlayers. Typically, PS samples have a complex morphologycombining pores in micrometre and tens of nanometredimensions. The image in Fig. 2a shows a fractal structurewith islands separated by flat cross-shaped sections. Thelatter contain smaller pores visible as dark crosses andhaving the same orientation as the large ones (see alsoFig. 2a). The formation of pores of this shape is presumablycaused by selective electrochemical etching of the /100S Si

power supply

PTFE container

plastic window

light source

metal ring

silicon substrate

Pt electrode

electrolyte

Fig. 1 Set-up for the formation of porous silicon

Fig. 2 SEM images of porous silicona, b Top views of different fragments of PS surfacec Cross-section view of cleaved PS/Si sample


surface in the [001] and [010] directions. The image zoomedin on the island top shows nanoporous sponge structure(Fig. 2b), which was confirmed independently with AFM(see Fig. 3). The pores size in the range of 0.05–0.5mm wasevaluated from those images. The cross-section SEM image,presented in Fig. 2c, allows the evaluation of the poredepth, and thus the thickness of the PS layer. Mean valuesof the pore size and depth for different samples aresummarised in the Table 1. The porosity of the PS layerswas estimated to be 78% using a gravimetric method[12, 16].

3.2 General features of EL and the model of themodified surface of PSA typical set of anodic EL spectra measured in trizma/HClbuffer at 7V applied to a Si electrode is presented in Fig. 4a.Changes in both the intensity and peak position of ELduring the course of anodisation are shown in Fig. 4b. Atthe very beginning (up to 300 s) of anodisation the ELintensity is found to slightly rise along with a small blue shiftof the EL band, but the intensity decreases subsequentlyaccompanied by the red spectral shift. The transformationsin the EL peak depending on the bias voltage are shown inFig. 5. Here the luminescence intensity increases with thevoltage increased from 7 to 12V, and this is accompaniedby a very small blue shift of the EL band of about 5nm.Observed features of EL spectra are different from the

results reported earlier in [17, 18]. In those works EL spectrameasured at different voltages enveloped in the PL

spectrum were explained reasonably well in terms of thePS model having a wide size distribution of nanocrystallites(nanowires) and thus the band gap varied from 1.3–2.0 eV[17]. In that case, a larger applied voltage is expected to tunethe electron transfer to smaller nanocrystals causing a blueshift of the EL band.

12

34

µmx 1.000 µm/divz 150.000 nm/div

0°

view angle

light angle

Digital Instruments NanoScopeScan size 5.000 µmScan rate 0.2001 Hz

Image Data HeightNumber of samples 512

Data scale 150.0 nm

Fig. 3 AFM image of nanoporous section of PS

Table 1: Average pore size and depth for PS samples(SEM/AFM data)

Morphology features Pore size, mm Pore depth, mm

Islands B10 (Fig. 2a)

Big crosses betweenislands

6–25 (Fig. 2a)

Small crosses 1–2 (Fig. 2a) 7–17 (Fig. 2c)

Sponge structure ontop of the islands

0.5–0.05 (Fig. 2b,Fig. 3)

400 500 600 700 800 900 10000

200

400

600

800

1000

1140 s

780 s

510 s

330 s

270 s

180 s

EL

inte

nsity

, a.u

.

wavelength, nma

b

0 500 1000 1500 2000

0

200

400

600

800

1000

1200

0

1.8

1.7

1.6

1.5

EL

peak

pos

ition

, eV

EL

inte

nsity

, a.u

.

time, s

Fig. 4 EL Properties of PSa EL spectra of PS as a function of anodisation time shown nearrespective curvesb Time dependencies of the intensity and peak position of EL


In our experiments the observed energy blue shift of theEL band of about 10meV is much smaller than thatexpected from the above model. Additionally only the

crystallites with size of 3–5nm would result in a sufficientincrease in the band gap [19]. According to our morphologystudy, PS contains nanocrystals of about an order ofmagnitude larger, which is supposed to produce a negligibleeffect on the band gap. The luminescence in the visiblerange was observed earlier in mesoporous PS sampleshaving crystallites of submicron dimensions [12]. Thismeans that the presence of silicon crystals of a fewnanometers in size is not essential for the luminescence inthe visible range.A model illustrated by Fig. 6 is proposed for the

explanation of observed features of electroluminescence.First, it is assumed that the size of PS crystallites is not smallenough to provide quantum confinement conditions andthus to increase the band gap. Secondly, the surface ofporous silicon contains a large number of siloxane (S–H–O)groups which form a thin siloxane surface layer and can berepresented by characteristic HOMO and LUMO bands[20]. In fact, it is difficult to distinguish between siloxanelevels and the surface states of other origin, so that the

400 500 600 700 800 900 1000

0

10

20

30

4012 V

9 V

7 V

EL

inte

nsity

, a.u

.

wavelength, nm

Fig. 5 EL spectra of PS as a function of anodic bias voltage

h2 < h1

h+

h1

h+

H+/H (−2.5 eV)

O2−/O (−7 eV)

O2−/O

a

b c

Si siloxane

LUMO

HOMO

1.1 eV

1.5 − 1.7 eV

electrolyte

CB (−4eV)

VB

e−

O2−/Oe−

Fig. 6 Energy band diagrams of PS/electrolyte contacta At zero bias (positions of energy levels are shown in respect to the vacuum level)b At anodic bias applicationc After anodic oxidation of PS surface


surface of PS can be represented by a large concentration ofsurface states covering a wide energy range. Anotherconsequence of having a large density of surface states isthat the Fermi level is most likely pinned on the surface, andany external charge (due to external bias) will be screenedby recharging the surface states. In general, a partialscreening by the semiconductor space charge, and thus asmall band bending, may take place (see Figs. 6b and 6c).However, this would not change dramatically the proposedenergy band model.When no external bias is applied, the two nearest redox

energy levels in the electrolyte O2�/O and H+/H are lyingbelow the valence band and above the conduction bandedges, respectively (see Fig. 6a) [21]. The application of arelatively small external bias, which is just enough to matchthe O2�/O level with the valence band edge, will causeanodic oxidation of Si with holes supplied through thevalence band. However, there is no luminescence in thiscase. To observe luminescence a large positive bias (of 5–12V in our experiments) is required in order to match theO2�/O level with the LUMO siloxane band on the Sisurface (Fig. 6b). In such conditions, the injection ofelectrons from the O2�/O level to the HOMO level takesplace followed by an allowed electron transition betweenHOMO and LUMO siloxane bands and photon emission,e.g. luminescence. The energy of the emitted photondepends on the applied voltage but is limited by therelatively narrow HOMO band.The above process is, however, accompanied by anodic

oxidation of silicon. The voltage drop on that oxide layercauses lowering of the O2�/O level and therefore a decreasein the emitted photon energy (see Fig. 6c). This explains thered spectral shift of the EL band in our experiments. Alsothe presence of an oxide layer reduces the current throughthe contact and thus the injection of carriers andluminescence intensity.

3.3 Effect of molecular adsorption onelectroluminescenceThe effect of adsorption of BSA molecules on EL spectra isshown in Fig. 7. The consecutive EL spectra were measuredin pure trizma/HCl buffer and the solution of BSA intrizma/HCl. The reduction in EL intensity occurred at eachexposure to BSA on the background of the gradual decayof EL possibly due to anodic oxidation of PS. It is believedthat BSA molecules interact with the siloxane surface of PSand partially block the electron transfer to PS and thereforecause the reduction in EL.

The idea of stabilisation of the surface of PS by itschemical modification [22] was further developed here byusing adsorption of polyelectrolytes. Both types of poly-electrolytes, e.g. polycationic (PAA) and polyanionic (PSS),are supposed to form strong Coulomb bonding with the PSsurface containing both OH� and H+ groups as respectivebinding centres. The absorption of both PAA and PSS hasresulted in extended EL lifetime, as seen from the results ofEL kinetics measurements at a fixed wavelength of 633nmin Fig. 8. For this particular sample, the decay time of ELincreases from 120 s for untreated PS to 540 s for PS havingadsorbed polyelectrolyte layers. It should be noted thatunlike PAA, the adsorption of PSS causes substantialreduction in EL intensity (see Fig. 8). The most likelyreason for such behaviour is the passivation of the PSsurface with polyelectrolytes. The adsorption of a negativelycharged PSS polymer chain causes repelling of O2� ionsfrom the surface and thus the reduction in electron injectionand luminescence.

The presence of PAA on the surface of PS improves thestability of the EL response to adsorption of BSA, whichcan be seen from Fig. 9 representing the EL kinetics as aresponse on consecutive exposures to pure trizma/HClbuffer and 1mg/ml solution of BSA in trizma/HCl. Thedrops in EL signal are due to changes of the solution andalso correspond to the empty cell. BSAmolecules, which arenegatively charged at pH 7.35, interact well with the

400 500 600 700 800 900 1000

0

100

200

300

400

500

4

3

1

2

EL

inte

nsity

, a.u

.

wavelength, nm

Fig. 7 Series of consecutive EL spectra measured in pure 35mM/ltrizma/HCl buffer (1, 3) and 1 mg/ml solution of BSA in trizma/HCl (2, 4)

0 60 120 180 240 300 360 420 480 540

0

0.4

0.8

1.2

1.6

(iii)

(ii)

(i)

EL

inte

nsity

, a.u

. (×

10−1

0 )

time, s

Fig. 8 EL kinetics measured in different solutions(i) 35mM/l trizma/HCl(ii) 2mg/ml aqueous solution of PAA(iii) 2mg/ml aqueous solution of PSS

0 120 240 360 480 600 720

0

0.5

1.0

1.5

2.0

2.5

3.0

trizmatrizmatrizma

BSABSAPAA

trizma

time, s

EL

inte

nsity

, a.u

. (×

10−1

0 )

Fig. 9 EL kinetics: the effect of consecutive exposures to solutionsof PAA, trizma/HCl and BSA


positively charged PAA layer. It should be also noted that adramatic increase in EL intensity during exposure to PAAsolution is simply caused by the presence of Cl� counterions having a higher redox energy level than that for O2�/O.A nearly similar effect on EL was observed when NaClsolution was used as an electrolyte. In contrast to resultsshown in Fig. 9, adsorption of BSA on PSS should nothappen since both components are negatively charged. Thatis why the EL kinetics in Fig. 10 do not show a noticeabledifference between the response to trizma/HCl buffer andBSA.

4 Conclusions

The samples of porous silicon produced for electrolumines-cence study of molecular adsorption were investigated withSEM and AFMmethods, and the morphology revealed thepresence of pores having a wide range of dimensions frommicrometres down to tens of nanometres.EL measurements were performed at the anodic

polarisation of PS in neutral aqueous electrolytes, such astrizma/HCl or NaCl, which can be useful for bio-sensing.The observed features of EL cannot be explained in termsof the most common model of Si nanocrystallites havingband gap enlargement due to quantum confinement [12].Instead, the model of the Si surface, chemically modifiedwith siloxane groups [20] resulting in a very highconcentration of surface states, was successfully applied inour case.EL was found to be unstable and rapidly decaying due to

anodic oxidation of PS in aqueous electrolytes. Theproblem of EL stabilisation has to be addressed beforeproceeding with further chemical sensing research. It wasshown here, that the adsorption of polyelectrolytes on a PSsurface substantially improves the stability of EL andmakesthis method more suitable for chemical and bio-sensing.It was shown that adsorption of BSA molecules, as an

example of common proteins, caused reversible quenchingof EL. Since such adsorption is not selective and otherbiomolecules may cause a similar effect, further work on PSbiosensors will be focused on the investigation of highlyspecific bioreactions, such as antibody–antigen binding,using EL as an analytical tool. Such work towards thedevelopment of PS immune EL sensors is currentlyunderway.

5 Acknowledgments

The authors are grateful to Material Research Institute,Sheffield Hallam University for the use of their experi-mental facilities (SEM and AFM). Thanks are due to Dr.M. Simmonds (MRI, SHU) for his help with AFMmeasurements and J. Hill (The University of Sheffield,Physics Department) for EL spectral measurements.

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19 Yoffe, A.D.: ‘Low-dimensional systems: Quantum size effect andelectronic properties of semiconductor microcrystallites (zero-dimen-sional systems) and some quasi-two-dimensional systems’, Adv. Phys.,1993, 42, pp. 173–266

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0 120 240 360 480 600 720 840 960

0

0.25

0.50

0.75

1.00

1.25

1.50

BSA BSA trizmatrizma

BSA

PSS

time, s

EL

inte

nsity

, a.u

. (×

10−1

0 )

trizma

trizma

Fig. 10 EL kinetics: the effect of consecutive exposures tosolutions of PSS, trizma/HCl and BSA


transistor

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