hydrogen content, electrical properties and stability of glow discharge amorphous silicon

Solar Energy Materials 2 (1979) 93-106 © North-Holland Publishing Company

HYDROGEN CONTENT, ELECTRICAL PROPERTIES AND STABILITY OF GLOW DISCHARGE AMORPHOUS SILICON

D. I. JONES, R. A. GIBSON, P. G. Le COMBER and W. E. SPEAR Carnegie Laboratory of Physics, University of Dundee, Dundee DDI 4HN, Scotland

Received 3 July 1979

The hydrogen content, Cn, the room temperature photoconductivity and the dark conductivity and its activation energy have been investigated in a-Si specimens produced in three different preparation units by the glow discharge technique. Hydrogen evolution measurements were made on films deposited on glass and aluminium substrates and checked by nuclear resonance experiments. Cu values range between 3 and 18 a te , indicating a definite dependence on the preparation unit. Annealing experiments show that in all specimens the incorporated hydrogen is stable up to 400° C, irrespective of the initial Ca value. Photoconductivity, like Cn, decreases rapidly at temperatures above 400°C, showing that additional defect states are introduced. Changes in room temperature dark conductivity and activation energy take place above 500°C and indicate a change in transport mechanism. The results on as-deposited specimens provide no evidence for any direct connection between Cn and photoconductive or conductive properties. Photostability experiments showed that 2 h exposure to 20 mW cm -2 illumination had a negligible effect on the electrical properties of some of the specimens, suggesting that the large photostructural changes recently reported in the literature are not an inherent property of glow discharge a-Si.

I. Introduction

The possibility of substitutional doping [1, 2] of amorphous silicon (a-Si) films prepared by the glow discharge decomposition of silane has stimulated much interest in this material and its applications [3]. An essential requirement [2] for the sensitive doping of any amorphous semiconductor is that it should be produced with a low density of gap states g(8) and the results of extensive field effect measurements in our laboratory [4--6] have demonstrated that glow discharge a-Si is most suitable in this respect. The question then arises as to the reason for the much lower g(e) in this material compared with evaporated or sputtered a-Si. It was suggested by Brodsky [7] and others that the incorporation of hydrogen, present in the silane glow discharge, is of fundamental importance in determining the properties of the films. It is generally argued that hydrogen tends to saturate 'dangling bonds' on internal defects of the random network and thereby reduces the density of gap states. This suggestion was supported by the work of Paul et al. [8] who demonstrated that doping of sputtered a-Si is possible by adding hydrogen to the sputtering gas.

The presence of hydrogen in glow discharge films was first reported by Triska et al. [9] who observed hydrogen evolution from specimens heated in vacuum. Subsequent-

93

94 D. I. Jones et al. / Glow discharoe amorphous silicon

ly, the hydrogen content of these films was determined in several laboratories using techniques such as nuclear resonance (Brodsky et al. [,10]), the absorption in the Si-H vibrational spectrum (Brodsky et al. [-11], Knights et al. [-12]), as well as the thermal evolution technique [,10, 13, 14]. The experiments showed that the specimens investigated contained typically 15 a t~ H if deposited on a substrate held at 250°C, and 35 a t~ (or more) for a substrate at room temperature; it was also established that the hydrogen content depended to some extent on factors associated with the deposition process [10, 15-17].

On the bdsis of the above work it has generally been concluded [18, 19] that strong hydrogenation is an essential property of all a-Si deposited in a plasma and provides the 'key' for the low density of gap states which allows control of electronic properties by doping. On the other hand the view has been put forward in publications by the Dundee group [-5, 20] that this is by no means the only reason. An equally important factor, essential for viable electronic properties, is the method of preparation itself. This point is brought out in a recent paper [,21], where the product #z of carrier mobility and recombination life time obtained from photoconductivity measurements, is compared for a-Ge and a-Si specimens prepared by different techniques. As the recombination of the excess carriers proceeds through gap states, the value of z is largely determined by the overall density of states in the mobility gap. Comparison of ~z values for a-Ge produced by the glow discharge technique and by sputtering in a hydrogen atmosphere [-22] shows that the photosensitivity in the latter is several orders of magnitude smaller; the difference is less extreme in the case of a-Si. The results clearly suggest that the preparation technique has an important bearing on the electronic properties. In spite of the presence of hydrogen in the sputtering plasma and the appreciable amount incorporated into the specimens, it appears that the overall density of gap states in sputtered films remains substantially higher than in specimens prepared by glow discharge decomposition.

The presence of hydrogen in an a-semiconductor such as a-Si raises a number of interesting problems. ESR measurements carried out in several laboratories [-23-25] on glow discharge specimens deposited near 250°C show that the spin density is extremely low, about 1015 cm -3. When the specimen is heated, the spin density increases as some of the Si-H bonds are broken and ultimately reaches values between 5 x 1018 and 1019 cm -3 when all the hydrogen has been evolved. If one associates about 10 dangling bonds with each unpaired spin [,26], then with 5 x 1022 Si atoms in the structure, 0.1-0.2 a t~ H should be required to saturate all dangling bonds. It is therefore relevant to ask whether the large excess of hydrogen generally incorporated into the material is essential to achieve the desirable electronic properties or whether it merely reflects our present inability to control the plasma and deposition conditions to give a lower hydrogen content. Another important question concerns the effect of the hydrogen on the thermal stability of the electronic properties of the material, particularly as it has been suggested that hydrogen may be responsible for large photostructural changes observed under illumination [27].

In this paper we shall attempt to answer some of these questions by comparing the electrical and photoconductive properties of a considerable number of a-Si specimens prepared under different conditions and with hydrogen content ranging from 3 to 18

D. I. Jones et al. / Glow discharge amorphous silicon 95

at%. The thermal stability of the properties is investigated in systematic annealing experiments and results for the photostability of specimens prepared in this laboratory are reported.

2. Experimental details

2.1. Specimen preparation

The specimens investigated were prepared by the decomposition of silane in an rf glow discharge [28] using one of three different preparation units. In unit A the rf field is coupled inductively into the silane gas; this apparatus, described in ref. [28], was used in most of our earlier work for specimens of a few cm 2 in area. Unit B is essentially a scaled-up version of A in which the rf field can be applied either inductively or capacitively to produce films covering a total area of 75 cm 2. The third preparation unit, C, is capacitatively coupled and designed for uniform deposition over large areas of up to 200 cm 2. The specimens were deposited on 7059 glass and, in some cases, on aluminium substrates, both held at 250°C. Specimen thicknesses lay between 0.5 and 1.2 #m.

2.2. Measurement of hydrogen content

Three methods have been used by previous workers to determine the hydrogen concentration, Ca, of a-semico.nductor specimens. In the first the film is heated in vacuum to the crystallisation temperature (or above) and the quantity of evolved hydrogen is measured by one of several techniques [9, 10, 14--16]. The second method [10, 17] relies on the nuclear reaction 1SN + 1H ~ 12 C + 4He + y. The nitrogen ions are accelerated towards the a-Si target where they react with the hydrogen present, producing y-rays. CH is then determined by photon counting, having first calibrated the system by means of a crystalline sample implanted with a known hydrogen concentration [10] or by some other technique. The third approach consists of measuring the infrared absorption in the Si-H vibrational band and calibrating this quantity in terms of hydrogen content by one of the methods mentioned above [11, 15, 29].

In most of the following experiments we have used the evolution technique. A known area of the specimen, typically 1 to 2 cm 2, was held on a nickel holder and placed near one end of a long fused quartz (vitrosil) tube which was evacuated to a pressure of 5 x 10- 6 Torr, or lower. The horizontal tube passed through a moveable furnace which surrounded just over half the length of the tube. The furnace, initially positioned at the end of the tube furthest away from the specimen, was heated to (950 + 50)°C. It was then moved in steps towards the specimen to raise its temperature to around 800°C at a fairly constant rate of 100°C min-1. Slower rates of heating (~20°C min -1) produced no significant difference in the amount of hydrogen evolved or in the temperature at which this occurred. The quantity of hydrogen was measured on a mass spectrometer inserted between the quartz tube and the pumping system. For reasonably consistent results it was found necessary to calibrate the mass spectrometer at the end of each evolution experiment. This was done by introducing

96 D. I Jones et al. / Glow discharge amorphous silicon

hydrogen into the system at a known rate through a "standard" leak. The rate of flow of hydrogen through the leak had previously been calibrated and was checked periodically. The mass spectrometer signal was recorded both as a function of time and of specimen temperature. The total amount of hydrogen evolved was then determined from the integrated area under the mass spectrometer signal-time graph. The minimum hydrogen content that could be detected with this system is estimated to be about 0.5 at~o. The CH values quoted in the following were calculated as the ratio Nn/(Nn + Nsi) expressed as a percentage, where Nrt and Nsi refer to the number of hydrogen and silicon atoms in the sample, respectively.

2.3. Annealin 9 experiments

A series of experiments to investigate the thermal stability of the incorporated hydrogen involved annealing the specimens, measuring their electrical properties, and then determining the amount of hydrogen remaining in the sample after each annealing step. In these experiments a large area specimen was cut into smaller pieces. Each piece was then annealed at a constant temperature, TA, for 15 min in a vacuum better than 5 × 10 -6 Torr. Electrical measurements, to be described later, were completed on each piece before the remaining hydrogen was evolved and measured.

3. The hydrogen content of the a-Si fdms

3.1. Evolution measurements with different substrates

The temperature dependence of hydrogen evolution and the hydrogen content has been determined on over 40 specimens with the following general results. Detect- able hydrogen evolution began at T>400°C and was complete between 700 and 800 ° C. The peak of the hydrogen evolution occurred between 550 and 650°C, with the position of the maximum depending on the specimen. In general, these temperatures are somewhat higher than those reported by other authors. For example, Brodsky [7] observed maximum evolution at T=500°C for samples deposited at 250°C. Similarly, Tsai et al. 113] found hydrogen evolution extending over the temperature range from 250 to 750°C for a specimen deposited at 270°C.

The evolution experiments revealed interesting differences between specimens deposited in the same preparation run on 7059 glass and on aluminium substrates. In the latter the hydrogen evolution was complete at about 580°C and the total measured amount was approximately twice that found in the corresponding specimen deposited on glass. To investigate this discrepancy, Cn values for a number of specimens on glass substrates were checked by nuclear reaction measurements at the Max-Planck Institute, Heidelberg, and at Yale University. In fig. 1 the hydrogen concentration (CH)sR determined from these experiments is compared with the evolution results (Cn)Ev for the same samples. It can be seen that the points lie close to the line (Cn)Ev = 0.5(Cr0sR, suggesting that only about half the hydrogen incorporated into the film is evolved on heating. Further measurements by the nuclear reaction


6

j +

0 /. 8 12 16

(C H ),R%

Fig. 1. Hydrogen content of a-Si samples deposited on 7059 glass measured by the evolution technique, (CH)EV, plotted against the corresponding value (CH)NR determined by the nuclear reaction technique. The nuclear reaction measurements were carried out at Yale (filled circles) and at Heidelberg (open circles).

technique confirmed that no detectable hydrogen remained in the film heated to 800°C. It is therefore concluded that the undetected 50~ of the hydrogen must have diffused into the 0.75 mm thick glass substrate. In the following work the CH values for specimens deposited on glass substrates have been corrected for this effect.

The comparison in fig. I also implies that evolution results for specimens deposited on aluminium substrates are in substantial agreement with values from the nuclear reaction technique, and this has been confirmed in a number of independent measurements. We believe that this fact and also the observed rapid evolution of hydrogen at about 580°C are a consequence of the formation of an AI-Si eutectic known to occur at 577°C 1-30]. It is likely that the structural changes involved cause a rapid and complete evolution of the hydrogen originally incorporated.

3.2. Hydrogen content as a function of annealing temperature

The results in fig. 2 show the dependence of the hydrogen content of a number of a-Si specimens on the annealing temperature TA. As described in section 2.3, a piece of the specimen was annealed for 15 min and then the remaining hydrogen content (Cn)rA was determined. The fraction (CH)r^/(CH)RT is plotted in fig. 2 against TA for four specimens prepared on unit C and one on unit B (O). The hydrogen content (Cn)RT of the un-annealed specimens ranged from 5 to 11 a t~ (see section 3.3). The interesting result is that irrespective of the initial (CH)RT value the thermal stability of the hydrogen in all specimens extends to a temperature of 400 ° C, which may of course depend to some extent on the annealing period.

3.3. Hydrogen content and electrical properties

The results presented in fig. 3 and discussed in this section are based on experi-

98 D. 1. Jones et al. / Glow discharge amorphous silicon

1.0

CH)T& iCHIRT

0-8

0.6

0.4

0-2

' ' T i

I 0 ,, , ,

0 2O0 LO0

O o

, i

600 TA (°C)

Fig. 2. Hydrogen conten t (CH}TA ofa-Si specimens after annealing to the temperature T A, normalised to the un-annealed values {CH)~T, plotted as a function of TA. The initial un-annealed values Of CH were: e5 at%; © 8.6 at%; x 8.8 at%; +9.6 at%; Fql0.7 at%.

{eV)

0.80 ~ - ~ - - - -

C H Ranges: I

_ _ i0 5

. . . . . 11o' 0 ,4 8 t2 CH 16 %

Fig. 3. Mean conductivity activation energy ~ and room temperature photoconductivity a~ from over forty specimens prepared on the A, B and C units, plotted as a function of the mean hydrogen content of the specimens. The error bars denote standard errors in the mean and the horizontal lines at the top of the figure show the spread of Cn values for each deposition system.

D. L Jones et al. / Glow discharge amorphous silicon 99

mental data from over 40 a-Si specimens prepared by the three units described in section 2.1. Evolution measurements led to the ranges of Cr~ shown at the top of the figure and to the plotted mean values of CH for the three groups of specimens. In spite of the fact that parameters such as deposition temperature, flow rate, gas pressure and rf power were substantially the same in all depositions, there are systematic differences in the hydrogen content of specimens prepared by units A, B and C. These could be caused by the method of rf coupling, by geometrical factors or by other as yet unexplored variables. Specimens produced on the inductively coupled system A generally had the highest hydrogen content (an average of ~ 14 ate), whereas those from the capacitatively coupled large area unit C contained on average 6 a t ~ and, in some cases, as little as 3 a t~ of hydrogen. The highest values (~- 14 a te ) are similar to those reported by Fritzsche [31] and by Brodsky et al. [10] for films deposited at substrate temperatures of about 250°C.

What is the effect of the different hydrogen content on the electrical properties of the specimens? To investigate this point we measured for each specimen the temperature dependence of the conductivity, tr, and the room temperature photoconductivity, try. The experiments were made in a surface cell configuration with evaporated electrodes, a few millimetres wide and separated by a 250 #m gap; an applied potential of 50 V was used. The i(V) characteristics of the samples were linear, both in the dark and under illumination. The room temperature photoconductivity was determined at a photon flux of 7 x 10 ~4 photons cm-2 s- 2. Measurements were also made over a range of intensities to investigate the dependence of trw on intensity.

Fig. 3 shows trw and the conductivity activation energy e~ as a function of Cn. Each point represents the mean value and the standard error for the above quantities, obtained for each of the three groups of specimens. Perhaps the most striking feature of the results is the order of magnitude increase in trw in going from specimens C to B. At first sight it may be tempting to associate this sensitisation with the increasing CH, although the subsequent decrease in aw from specimens B to A would be difficult to explain. We believe that there is no direct connection between trw and CH, mainly on the basis that within each group of specimens there is no recognisable Cn dependence of the electrical properties. Thus suggests that the differences in the mean values of at~ for specimens from units A, B and C are largely introduced by differences in preparation conditions. The recent work of Anderson and Spear [32] on the photoconductivity in doped a-Si showed that a~ is critically dependent on the Fermi energy position and on the density of empty, positively charged recombination centres lying just above ef in the undoped specimen. Slight n-type doping moved ef through these states and produced a remarkable sensitisation of trp¢ (by a factor of 20 or more) as a result of the increase in recombination life time of excess electrons. We suggest that a mechanism of this kind determines the values of trp~ in fig. 3. This view is strongly supported by the corresponding changes in e~ values indicated in the figure. Thus the increase in photosensitivity between specimens C and B is associated with the movement of et towards e~ by about 0.08 eV. In this instance the change is not caused by doping but by the slightly different preparation conditions in units B and C. These determine the detailed state distribution in the region of the density of state minimum in the centre of the mobility gap. We therefore conclude that there exists no direct

100 D. 1. Jones et al. / Glow discharge amorphous silicon

correlation between the level of photoconductivity and the hydrogen content in undoped glow discharge a-Si specimens, at least within the range of CH investigated here.

4. Dependence of electronic properties on annealing temperature and on illumination

It has been shown in section 3.2 that annealing of a specimen at a temperature TA will decrease its hydrogen content in accordance with fig. 2. We shall now investigate the effect of systematic annealing in photoconductivity and conductivity of a number of specimens. The procedure has been described in section 2.3. After annealing a piece of the specimen at the required TA, aluminium electrodes were evaporated onto the sample for the measurements, which were carried out as described in section 3.3.

4.1. Photoconductivity

The results obtained for five specimens with different hydrogen contents are presented in fig. 4. (ap¢)TA denotes the-photoconductivity measured at room temperature after the specimen had been annealed at temperature TA and (a~)~T is the room temperature photoconductivity before annealing. The ratio of these quantities is plotted against TA in fig. 4. Within the accuracy of the measurements, (apc)rA remains

(dPC)r A ( z,~°C

1'0 ~ ~

\ 10 2

\\ 1INs ~

15 3 . . . . . . . . ',

i

i 10 ~ - t--

x

i ldSo 26o 40o

(c~i ~)

ld 15

. . . . 1~ 6

i 1(~17

( , _ _ _ 10 ~° £'\ l

6o0 8oo T~ (°C)

Fig. 4. Room temperature photoconductivity (O'pc)T A after annealing to the temperature TA, normalised to the un-annealed value (apc)RX, plotted as a function of T A. The initial values of (~rF)RT varied from 5 x 10 -7 (fl cm) -1 to 3 × 10 -5 (fl cm) -1. The samples are denoted by the same symbols as in fig. 2. The broken line shows the reciprocal of the spin density Ns as a function of annealing temperature (from ref. [13]).


substantially constant up to TA ~300°C, but annealing to 400°C reduces the photoconductivity by about a factor of two. Between 450 and 600°C (ar~)rA drops rapidly by several orders of magnitude.

It is interesting to note the close similarity of the results with those in fig. 2 which showed the corresponding CH ratio as a function of TA. The comparison suggests that the rapid drop in (at,) r A arises from the increased density of gap states as the hydrogen is removed by the annealing process. These defects drastically decrease the carrier life time T and hence the photoconductivity. The appearance of paramagnetic defect centres with increasing TA is clearly shown by measurements of the spin density Ns. In fig. 4 we include results obtained by Tsai et al. [13], plotted as Ns t vs. TA. The curve shows the same general trend as the photoconductivity data, although the decrease in Ns 1 begins at a lower temperature, reflecting the earlier evolution of hydrogen from Tsai et al.'s specimens [13-[.

Finally, in table 1, we compare aw for a number of un-annealed specimens with others annealed to have the same hydrogen content. For example, specimens C2 and C3, prepared in unit C, each had a total hydrogen content of 3.8 atg/o and aw ~- 10-6 (fl cm)-1; samples C4 and C5, originally with higher CH, were annealed at 500 and 470°C to reduce their hydrogen content to the same value. As a result, their photoconductivity dropped to 1.9x 10 -8 and 1.2x 10 -7 (t~ cm) -1, respectively. The examples stress the significance of the defect density, whether introduced by annealing or by the preparation method, in determining a sensitive property such as photoconductivity; they also demonstrate that CIj is of little relevance in this case.

4.2. Conductivity

The effects of annealing ".~t the electrical conductivity and the activation energy of a number of specimens prepa.,'ed on the B and C units have been investigated. In fig. 5 the temperature dependence of a is shown for a sample deposited on the C gear after annealing to the TA values given in the figure. Unlike the photoconductivity results, which led to the rapid decrease in a~ for TA ~" 400°C, a(T) above room temperature

Table 1

Comparison of photoconductivity in a number of un-annealed samples (as prepared) with specimens annealed at the given TA to have the same hydrogen content CH

CH O'pc Sample (at%) T A (f~ cm)- 1

C2 3.8 as prepared 1.2 x 10- 6 C3 3.8 as prepared 1.0 x 10- 6 C4 3.8 500°C 1.9 x 10 - s C5 3.8 47(fC 1.2 x 10 -7 C1 7.2 as prepared 1.3 x 10-6 B1 7.2 480°C 3.8 x 10 -T

102 D. 1. Jones et al. /' Glow discharge amorphous silicon

10 -~

l~cm)

io ~

! . . . . I ' ' ' ' I

- 4 - - - - ~ . . . . . . .

J

1 0 4 . . . . . . . . . . . . . . . . . . . . .

- i ? I . . . . , . . . . i , , J

2.5 3.0 3.5 4.0 10"3/T (K 4)

Fig. 5. Temperature dependence of the electrical c o n d u c t i v i t y cr for a s a m p l e depos i t ed o n the C gea r a f te r

annealing to the TA values given for each curve•

1.0 t;.. (eV)

0.8

0.6

0.4

0,2

4t

i _ [3

g I +

O

+

o 4 3~o ~ ' ' 400 500 600 700 TA (°C)

Fig. 6. Conductivity activation ene rgy s . n e a r r o o m temperature plotted as a func t ion of the annealing temperature TA for the same samples shown in figs. 2 a n d 4.

shows little change up to T A values exceeding 500 ° C. This can also be seen in fig. 6 in which the conductivity activation energy e~ in the same temperature range is plotted as a function of TA for the samples shown in fig. 4. e, is essentially constant up to TA = 500°C and then decreases abruptly to about 0.16 eV between 500 and 550°C. The insensitivity to annealing up to T A'-~ 500°C is somewhat surprising since, at this temperature, the results in fig. 2 show that only about a third of the original hydrogen remains in the specimen and both the photoconductivity data and the ESR results of Tsai et al. [13] clearly indicate an increase in the localised state density as soon as hydrogen is evolved. Evidently, photoconductivity is a very much more sensitive


tool for the detection of defects than the dark conductivity, as we also found in recent work on the effect of defects introduced by ion implantation [33].

It should, however, be stressed that the measurements in fig. 6 refer to the temperature range between 300 and 400 K. If the experiments are extended to lower T, as has been done with a few specimens, it is found that e, in this temperature range begins to decrease already at annealing temperatures closer to 400°C. In other words, the drop in eo in fig. 6 now moves towards the left. It is fairly certain that this characteristic change in eo marks a transition to a new conduction path in which transport is associated with hopping through the defect states produced by the hydrogen evolution. The higher T^, the larger will be the defect density and, as a consequence, the transition to hoppirrgtransport will take place at progressively higher T. This interpretation is in agreement with that for the photoconductivity results, but, as already pointed out, the latter are a more sensitive method for the detection of the defects.

4.3. Photo-stability of a-Si films

Large changes in the electronic properties of a-Si films after illumination have been reported by Staebler and Wronski [27] and by Tanielian et al. [34]. Staebler and Wronski illuminated their specimens with approximately 200 mW/cm 2 for four hours and afterwards noted the following changes: (i) the room temperature conductivity aRT in the dark had decreased by four orders of magnitude, (ii) e, had increased from 0.57 to 0.87 eV, and (iii) the intensity dependence ofa~ had changed from an essentially bimolecular mechanism with v=0.5, (defined by a ~ I v) to a mainly monomolecular one with v = 0.9. The original properties before illumination could be restored by heating the sample to 150°C. Tanielian et al. [34] used a much smaller light intensity (1 mW/cm 2) in their experiments, but nevertheless found a change of about a factor of 50 after illuminating for less than an hour.

As we had not observed effects of this kind in previous work it appeared of interest to carry out similar experiments with specimens prepared in this laboratory. Fig. 7 summarises the results for one of the specimens prepared in unit B. The full lines represent the room temperature dark conductivity trRr measured after various stages of treatment, denoted by (a) to (f). The numbers above these lines are the corresponding values of e, in eV. The initial measurements, (a), were made within a day of the deposition of the sample. The sample was then stored in air for 123 days and remeasured (stage (b)). After this period the value of aRT has decreased from 1.6 x 10-s (~ cm)- 1 by a factor of four and e, has increased from 0.69 to 0.80 eV. The sample was then raised in temperature to 150°C over a period of one hour, held at this temperature for about 15 min and then remeasured (stage (c)) on cooling. This had increased aRT to 1.0 X 10 -s eV and reduced e, to 0.71 eV, close to the initial values. Since these measurements were made in the gap cell configuration it is most likely that the changes were caused by absorbed gases on the specimen surface, particularly by water vapour.

The sample was then illuminated in stage (d) for two hours with about 20 mW/cm 2 of white light from a quartz-iodine lamp. During this illumination the photoconductivity decreased from 1.5 x 10 -4 (f~ cm)- x to 0.9 x 10 -4 (t~ cm)- 1. After illumination, the electrical properties of the specimen were again measured (stage (e)). In contrast

104 D. I. Jones et al. / Glow discharge amorphous silicon

(.O.cm) -1

10-6

I I r LIOHT

1 0 - ~ _ _ _

. . . . ~p£ . . . .

E~(eV) 069 0.71

10 .8 ~ 0.80 I

(a) (b) (c) (d)

io- 01 i I I o 1 2

t (hours)

0.76 0 .6g

(e) (f)

Fig. 7. Room temperature dark conductivity O'RT (full line) and photoconductivity ape (broken line) of an a-Si specimen measured after various treatments (denoted by (a) to (13 (see text). The numbers above the aRT results are the corresponding values of the conductivity activation energy e, in eV. The full line in step Id) shows the time dependence of the room temperature photoconductivity under illumination with

20 mW cm -2 of white light.

to the four orders of magnitude changes in aRT observed by Staebler and Wronski [27] and the 50-fold change reported by Tanielian et al. [34] at much lower intensities, the illumination only decreased the room temperature conductivity of this specimen to 0.8 x 10 -s (~) cm) -1, compared with the value of 1.0 x 10 -8 before illumination. Finally, in stage (f), annealing to 150°C after the strong illumination produced only slight changes in aRx or e,. The dashed lines in fig. 7 represent the room temperature values ofar~ at various stages. It is worth noting that a~ before and after illumination is almost identical, which suggests that the density of photostructural defects introduced into the specimen must be very small.

Similar measurements on other specimens prepared in this laboratory showed conductivity changes of up to a factor of four after illumination. It must therefore be concluded that the large photostructural changes observed by the above-mentioned authors are not an inherent property of glow discharge a-Si, but probably are dependent on the details of the preparation technique. The results in fig. 7 encourage us in the belief that these effects, unwanted in most applications, can be almost completely eliminated by suitable preparation conditions.

5. Conclusions

In the following we should like to summarise the main conclusions that can be drawn from the work described in the paper.

1. Amorphous Si specimens prepared in this laboratory by the rf glow discharge decomposition of silane contain between 3 and 18 at% of hydrogen. Different preparation units give distinct ranges of CH and it is concluded that the hydrogen content

D. 1. Jones et al. / Glow discharoe amorphous silicon 105

of the specimens depends on the detailed preparation conditions. On this basis it may be possible with further development of the technique to decrease Cn to 1 at% or below and thus approach more closely the expected minimum hydrogen content (section 1).

2. The thermal stability of the incorporated hydrogen extends to temperatures of 400°C for heating periods of 15 min. This applies to all specimens investigated irrespective of their initial hydrogen content.

3. In experiments on over 40 specimens we found no evidence for any direct connection between Cn and the photoconductive and conductive properties of the films. The differences that arise between the groups of specimens from the three preparation units are almost certainly caused by inherent differences in preparation conditions, unconnected with Ca.

4. Photoconductivity measurements are a sensitive method for detecting an increase in the defect density. The rapid drop in tr~ after annealing the specimens at temperatures above 400 ° C is caused by the increased density of gap states as the hydrogen is removed. However, the room temperature dark conductivity and its activation energy are affected only after annealing the specimen to temperatures above 500°C. It is concluded that the sudden decrease in activation energy marks a transition to a new conduction path in which transport is by hopping, probably through defect states produced by hydrogen evolution.

5. The specimens investigated here show little evidence for the large changes in electrical properties after strong illumination that have recently been reported in the literature. Photostability experiments suggest that such effects, undesirable in most applications, are not an inherent property of glow discharge a-Si and can largely be eliminated by suitable preparation conditions.

Acknowledgements

The authors would like to thank the Science Research Council for support of this project. They are greatly indebted to Stewart Kinmond, who prepared most of the specimens, and to S. Kalbitzer (Max-Planck Institut fur Kernphysik, Heidelberg), and W. A. Lanford (Yale University) for measuring the hydrogen content of some of the films by the nuclear reaction technique.

References

[1] W. E. Spear and P. G. Le Comber, Solid State Commun. 17 (1975) 1193. [2] W. E. Spear and P. G. Le Comber, Phil. Mag. 33 (1976) 935. I-3] See, for example, Proc. 7th Intern. Conf. on Amorphous and Liquid Semiconductors, Edinburgh

(1977) ed. W. E. Spear (CICL, Univ. of Edinburgh, 1977). I-4] W. E. Spear and P. G. Le Comber, J. Non-Crystalline Solids 8-10 (1972) 727. 1-5] A. Madan, P. G. Le Comber and W. E. Spear, J. Non-Crystalline Solids 20 (1976) 239. 1-6] A. Madan and P. G. Le Comber, p. 377 of ref. [3]. 1-7] M. H. Brodsky, Thin Solid Films 50 (1978) 57.

106

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hydrogen content, electrical properties and stability of glow discharge amorphous silicon

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