The influence of nitrogen on the optical and electrical properties of sputtered a-Si56Ge44:N:H films

Journal of Non-Crystalline Solids 135 (1991)204-210 North-Holland

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The influence of nitrogen on the optical and electrical properties of sputtered a-Si56Ge44" N" H films T. D r i i s e d a u 1 FB Physik der Unit,ersitiit Kaiserslautern, Postfach 3049, W-6750 Kaiserslautern, Germany

Received 14 March 1991 Revised manuscript received 19 June 1991

The influence of nitrogen concentration below 10 22 c m 3 on the properties of a - S i 5 6 G e 4 4 : N : H films prepared by dc-magnetron sputtering has been investigated. Optical properties in the strong and weak absorbing region are described in terms of the Tauc-gap and the single-oscillator expression after Wemple and DiDomenico, respectively. All characteristic energies show a dependence on nitrogen content and, excepting the dispersion energy, a dependence on annealing after deposition. Films containing about 10 21 N atoms c m - 3 have a dark conductivity of nearly 10 - 6 (1"~ cm) I which is one order above the value of nitrogen free films. This N content leads to an enhanced photoconductivity of ritzy"= 5x 10 - 7 cm2/V at maximum compared with rt/x~-= 4× 10 s cme/V without nitrogen.

1. Introduction T h e application of h y d r o g e n a t e d a m o r p h o u s s i l i c o n - g e r m a n i u m alloys (a-Si t_xGe x : H ) in multijunction solar cells causes persistent interest in preparation and the physical properties of this material [1]. Extensive studies on films deposited by plasma e n h a n c e d chemical v a p o u r deposition ( P C V D ) were p e r f o r m e d by Mackenzie et al. [2] and Stutzmann et al. [3]. F u r t h e r papers on this topic were reviewed in a recent article [1]. Several authors reported on the alloy preparation by reactive m a g n e t r o n sputtering (MSP) from a composite target [4,5] or dual m a g n e t r o n sources [6,7]. T h e properties of the sputtered films were reported to be similar to that of PCVD-films. Over the past decade, a great n u m b e r of papers concerning the electrical and optical properties of a-SiNx : H have b e e n published as cited in the literature [8,9]. M u c h less work was carried out on nitrogenated h y d r o g e n a t e d g e r m a n i u m (a-

1 On leave from Inst. Exp. Physik der TU 'Otto yon Guericke', PSF 124, O-3100 Magdeburg, Germany.

G e N x : H) [10-12]. T h e influence of nitrogen content on the electrical properties of these films has not yet been systematically studied. The first p a p e r dealing with nitrogen incorporation into sputtered a-Sil_xGe x : H was published by Banerjee et al. [13]. A detailed IR-spectrometric investigation on a-Si56Ge44 : N : H films has recently b e e n p e r f o r m e d [14]. T h e aim of the present work was to describe the d e p e n d e n c e of the electrical and optical properties of these films on nitrogen content and a post-deposition annealing procedure.

2. Experimental details T h e d c - m a g n e t r o n deposition apparatus is described in an earlier p a p e r [15]. F u r t h e r details of preparation and IR-spectroscopic m e a s u r e m e n t s of the a-Si56Ge44 : H : N films were subject of the recent work [14]. T h e target consisted of a 125 m m diameter single crystalline silicon wafer on which four chips of crystalline g e r m a n i u m 22 m m in diameter were mounted. Two deposition cycles were carried out with deposition time tD = 15

0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

T. Driisedau / hl]htence of N on sputtered a-Sis~Ge44: N: H fihns rain, hydrogen partial pressure p ( H : ) = 0.3 Pa, dc power PDC = 90 W, substrate temperature T s = 200 ° C, target to substrate distance dvs = 55 mm (sample set I) and t D = 25 rain, p(H 2) = 0.4 Pa, P,l,, = 110 W, T s = 180°C and d T s = 80 mm (sample set I1). The argon partial pressure was always kept at the value of p ( A r ) = 0.6 Pa. During each deposition cycle, six samples were deposited using nitrogen partial pressures of p(N z) = 0, 2, 4, 8, 16, 32 mPa. Optical transmission measurements were performed in the wavelength region A = 0 . 5 . . . 2 . 5 ~ m by means of a Specord U V / V I S and a Specord N1R double beam spectrometer (VEB Carl Zeiss, Jena). The method introduced by Manifacier et al. [16] using a least squares calculation in which two envelope functions are fitted to the interference minima and maxima in the region of weak absorption was employed. From these analytical functions, one is able to calculate initial values of the films refractive index, n(A). Sample thickness, d~, and integer interference order, m, were determined from a plot of 4n/A versus m~,, where the interference of greatest wavelength was arbitrarily chosen to m~, = 1. Then the resulting straight line has a slope of 1/d~. Its intercept of ordinate equals m J d ~ , with m~ being the 'error' in order with m = m,, + m~. Hereafter the initial n(A) values are corrected by the interference condition

4nd~ = mA,

(1)

The dispersion of refractive index obtained in this way was examined to obey the single-oscillator expression suggested by Wemple and DiDomenico [17,18]:

n2(hu)=EDE,i/[E2-(hu)

2] + 1 ,

(2)

where E D and E 0 are the dispersion energy and single-oscillator energy, respectively, by plotting 1 / ( n 2 - 1) vs. the square of photon energy (hu) z. Transmission measurements in the region of strong absorption (transmission T R = 0.002... 0.02) were used for the determination of the optical bandgap exploiting the Tauc procedure. The absorption coefficient was determined with regard to multiple reflections of the optical system air-film-glass [19].

205

Conductivity measurements were performed at room temperature in a coplanar arrangement with evaporated AI electrodes of 10 mm length and 1 mm interelectrode spacing with an electrical field of 100 V / c m . Photoconductivity was excited with filtered light from a tungsten lamp at two wavelengths A = 6(}0, 800 nm with an irradiance of 1 = 2 m W / c m 2. Post-deposition annealing was carried out at a temperature of TA = 493 K under vacuum of a pressure below 1 Pa for 1 h.

3. Results

As already reported for the preparation of a-SiN,. : H using the identical apparatus [9], the introduction of nitrogen into the magnetron discharge reduces the target voltage in the present case. Comparing the case of a nitrogen free atmosphere with that of maximum nitrogen pressure of p(N z) = 32 mPa, the voltage drops from 420 to 370 V and from 520 to 445 V for sample set I and II, respectively. Note that the distance between the magnetron and the rotatable grounded substrate holder which acts as front electrode for the target varies from one deposition cycle to another. Within the first deposition cycle, the deposition rate decreases monotonicly with nitrogen pressure from 0.64 to 0.61 n m / s according to a sample thickness of d~ = 570... 550 nm. Surprisingly, during the second cycle the opposite behaviour appears with rate increasing with nitrogen pressure from 0.37 to 0.40 n m / s ( d ~ = 560... 610 nm). Figure 1 shows a typical plot of 1 / ( n 2 - 1) vs. (hu) ~ for the samples of set lI. The dispersion of the refractive index is described quite well in terms of the single-oscillator expression given in eq. (2) with exception of photon energies hu > 1.6 eV where a systematic deviation downward from the straight line appears. In the same manner, the validity of the Tauc expression a(~h-uu= !/B(hu-E T) for the prepared films is demonstrated in fig. 2. The incorporation of nitrogen causes a widening of the optical gap, E x, and in addition at higher amounts it results in a decrease of the slope v ~ . The same influence of nitrogen was observed for a - S i : H [8,9], a - G e : H

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[10] and the a - S i l _ x G e x : H alloy [13]. In the further presentation, all film properties will be plotted versus the nitrogen partial pressure, p(N2). In the previous p a p e r [14], it was shown by I R spectrometry that, for the films of set II, the nitrogen content, c N, is proportional to p ( N 2) in the range of c N = 8:6 × 10 2o cm -3 at 2 mPa to

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Fig. 3. Dependence of the Tauc gap, ET, on nitrogen pressure, P(N2), for set 1 and II samples (circles and squares, respectively). Data were measured after deposition (open symbols) and after anneal (full symbols, see text). All lines are drawn as visual guide.

c N = 1 × 1022 cm -3 at 32 mPa. Figure 3 displays the dependence of the optical gap on nitrogen pressure and the influence of the post-deposition annealing. The values of E T taken after deposition for the two sample sets differ in the range of lowest and highest p(N 2) values. The annealing procedure causes a general shrinkage of the gap and the dependence of E T on nitrogen becomes very similar for both sample sets. In the recent paper on sputtered a-SiN x : H [9], the authors pointed out the fact that small amounts of incorporated nitrogen result in a slight increase of the Tauc slope, v~-. An identical behaviour appears in the present case, where the maxima in ~ are placed near a nitrogen pressure of p ( N 2) = 8 mPa with exception of set II samples (after deposition) where no maximum was observed. For a better understanding of the annealing effects, the following facts should be mentioned. From RBS and I R spectrometry, the concentration of hydrogen and s i l i c o n / g e r m a n i u m in nitrogen-free films are estimated to be c . = 10 22 c m - 3 and C s i + G e = 4 × 10 22 c m - 3 , respectively. Further, the film thickness was found to decrease

T. Driisedau / Influence of N on sputtered a-Sis~Ge 44 : N: H films

with annealing by nearly the identical value of Ad s = 30 nm for all films investigated. Simultaneously, the high wavelength refractive index, n~, obtained from the extrapolation to hu = 0 in the single-oscillator plot (see fig. 2) increases by about 0.1. The refractive index measured after deposition amounts to n~ = 3.5 for a-Si56Ge44:H, a value which is also representative for PCVD material [20] and it decreases monotonicly with increasing nitrogen content down to n~ = 3.1. Figure 4 shows the dependence of the singleoscillator energy, E 0, on nitrogen partial pressure and annealing. Incorporation of nitrogen raises the E o values of the films whereas annealing causes their decrease by about 0.2 eV in average. Because of the similarities in the dependence of E T and E o on p ( N 2) and annealing, one should expect the existence of a relation between both characteristic energies. Indeed a plot of singleoscillator energy versus optical Tauc gap reveals a linear dependence in the interval treated: Eo=

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(3)

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influence of annealing on the dispersion energy is evident within experimental uncertainty. Previous conductivity measurements on nitrogen-free a-Si56Ge44:H films showed changes of their electrical properties with ageing, i.e., a slight decrease of the dark conductivity and an increase of the photoconductivity by a factor of five at maximum over a period of about four months. An annealing procedure as described above was performed to accelerate this process and to obtain a steady state in the present case. From the hydrogen evolution experiments of Beyer [21], one should assume that, for the annealing temperatures used, only an insignificant part of the hydrogen escapes from the film. Dark and photoconductivities measured after this annealing are disphtyed for set II samples in fig. 6. Both dark and the photoconductivity first increase with the nitrogen partial pressure up to maximum values at about p(N 2) = 8 mPa which equals a nitrogen concentration of c N = 3 × 10 21 c m - 3 . The smaller photoconductivity due to excitation with 800 nm wavelength light is related to the decreased absorption compared with 600 nm light. The data of set I samples revealed a behaviour similar to fig. 6. However, these samples are in general of

208

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smaller photoconductivity and larger dark conductivity so that the ratio of both does not exceed % J ~ r D = 2 . The normalized photoconductivity r/t~r (see, for example, ref. [22]) due to data of fig. 6 is r//zr = 4 x 10 s c m 2 / V (no nitrogen) and r / # ' r = 5 × 10 - 7 c m 2 / V ( p ( N 2) = 4 mPa).

4. Discussion

We must keep in mind that the single oscillator energy though obtained from the dispersion of the refractive index in the absorption-free region is located near the maximum of the imaginary part of the dielectric function [18]. Hence the influence of nitrogen incorporation on E 0 and E T and the observed relation between both characteristic energies can be explained in the same manner as in the case of a-SiNx : H [8] (see also ref. [9] and refs. therein). Si-Si bonding states are removed from the top of the valence band and replaced by S i - N bonds located at lower energies, thus widening the gap. Simultaneously, the average energy for an electron transition from the valence into the conduction band, which is the interpretation of E o given by Davis

et al. [8], must increase. The proportionality factor between E 0 and E v estimated for crystalline materials to 1.5 [17] seems to be higher in the case of a - S i : H - b a s e d alloys and in the interval 2 - 3 [8,231. Quite similar observations with respect to the influence of annealing on porous rf-magnetron sputtered a - S i : H (i.e. shrinkage of optical gap and sample thickness, increase of the refractive index) have been made by other authors (ref. [23] and refs. cited therein). From these investigations, it must be concluded that the influence of annealing in the present case has to be interpreted by the diffusion of hydrogen and a more dense packing of the amorphous network. Hence, the top of the valence band is filled with bonding states and the characteristic energies E T and E 0 decrease. Surprisingly, the annealing procedure does not influence the dispersion energy. The dependence of E D on nitrogen content is similar to the linear relation of E D on oxygen content, x, assumed by Wemple for SiO x [18]. Let us now focus our view on the conductivity data. There are two competing effects of nitrogen on the Fermi level as already reported for aSiN,.:H (see refs. [9,24] and refs. cited therein) which should be responsible for the observed maximum of the conductivity in fig. 6: (i) the appearance of a small part of nitrogen (probably within NHx-groups) acting as donor states to raise EF; (ii) the widening of the mobility gap and related downward shift of the Fermi-level. Considering both effects, an estimation of the upper limit of doping efficiency for the data of fig. 6 is given as follows. Using Mott's equation (see, for example, ref. [9]) the distance of Fermi level to conduction band mobility edge at room temperature is E c - E ~ = 0 . 5 2 eV and E c - E v = 0.47 eV for c N = 0 and c N = 3 x 10 21 cm -3, respectively. Simultaneously, the optical gap widens by about 60 m e V (cf. fig. 3). Therefore, the Fermi level has to be shifted by doping not more than 110 meV towards E c. From spin densities of n ~ = 3 x 1018 cm -3 measured for the nitrogen free films, which are in approximate agreement with the data of Stutzmann et al. [3] and Beyer [21], a density of states around E v not

T. Driisedau / Influence of N on sputtered a-Si,s~Ge 44 : N: H films

exceeding N = 1019 cm 3 eV-~ is concluded. The maximum concentration of donor-like states is /7D = 1.1 X 1018 cm -3 which equals 3.7 x 10 -4 of the built-in nitrogen. However, the observed effect differs only slightly from the increase of dark conductivity by two orders of magnitude achieved for heavy phosphorus doping of sputtered aSi70Ge30:H [25]. Although P-doping is far more efficient in PCVD-films, an enhancement of photoconductivity by one order of magnitude requires the addition of about 10 4 ppm PH 3 [26] to the plasma. This leads to the conclusion that the enhancement of photoconductivity by N-incorporation is not only a simple consequence of the upward shift of E F and the accompanying transition of D ° into D - states. Further, nitrogen incorporation must improve structural properties of the films which are of importance for photoconductivity, i.e., reduction of tail or defect states. This may be due to a hydrogen concentration raised with nitrogen content of the films as reported for a-SiN, : H and a-Si56Ge44 : N: H [9,14]. It is worth noting that a very similar effect on the r//zr-products of PCVD-material was observed by MacKenzie and Paul [22] investigating fluorine incorporation into their films.

5. Conclusions

(i) The dispersion of the refractive index in the long wavelength (absorption-free) region is well described by the single-oscillator expression. (ii) Incorporation of nitrogen in sputtered aSi56Si44:N:H films results in an increase of the Tauc gap and the single oscillator energy whereas post-deposition annealing results in a lowering of both characteristic energies. (iii) Nitrogen incorporation leads to a reduction of the dispersion energy which is not affected by the annealing procedure. (iv) The observed increase of dark conductivity with nitrogen to concentrations of c N = 3 x 10 21 cm -3 is caused by N-related donor states where the doping efficiency is below 4 x 10 -4. (v) Nitrogen incorporation enhances the photoconductivity by more than one order of magnitude at maximum.

209

The author wishes to acknowledge Professor H. Fiedler who encouraged his work on amorphous semiconductors and Mrs U. Schmidt, Dr B. Schr6der, Dr St. J~iger and Mr V. Kirbs for valuable discussions and additional measurements, respectively. He thanks Mrs S. Heider and Mrs I. Wollscheid for typing the manuscript and the preparation of drawings, respectively.

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7". Driisedau / Influence o f N on sputtered a-Si56Ge 44 : N: H films

[22] K.D. MacKenzie and W. Paul, J. Non-Cryst. Solids 97&98 (1987) 1055. [23] St. J~iger, PhD thesis, TU Magdeburg (1990). [24] J. Robertson and M.J. Powell, Mater. Res. Soc. Symp. Proc. 49 (1985) 215.

[25] P.K. Banerjee, J.M.T. Pereira, G.A. Hanidu and S.S. Mitra, J. Non-Cryst. Solids 104 (1988) 190. [26] J. Wind, G. Kr6tz, V. Petrova-Koch, G. Miiller and P.P. Deinel, J. Non-Cryst. Solids 114 (1989) 531.