Characterization of hydrogenated amorphous silicon thin films by the spectral shape of their photoluminescence

Thin Sohd Fdms, 124 (1985) 185-189 PREPARATION AND CHARACTERIZATION

185

CHARACTERIZATION OF HYDROGENATED AMORPHOUS SILICON THIN FILMS BY THE SPECTRAL SHAPE OF THEIR PHOTOLUMINESCENCE* H. NANTO AND T. MINAMI

Department of Eleetrwal and Electronw Engmeermg, Kanazawa Instttute of Technology, 7-10ogtgaoka, Nonoteh-machl, P 0 Kanazawa South. lshtkawa 921 (Japan) Y. MISHIMA AND M. HIROSE

Department of Electrtcal Engmeermg, Htroshtma Umverstty, Htgashthtroshtma, Htroshtma 724 (Japan) (Received August 13, 1984, accepted October 11, 1984)

A detailed study of the shape of photoluminescence (PL) spectra was carried out for thin films of hydrogenated amorphous silicon (a-Si: H). PL spectra in films with different physical properties could be analysed using two gaussian emission bands located at 1.20 and 1.35eV. The linewidth and the intensity of the PL spectrum showed a strong dependence on the quality of the a-Si: H films. We found a correlation between the PL linewidth and the film quality. The large PL linewidth observed for poor quality films is attributed to disorder broadening. The fatigue effect (changes in the PL intensity) and the Staebler-Wronski effect (changes in the conductivity) produced in films as a result of exposure to light were also investigated on the basis of the above result. Analysis of the shapes of the PL spectra revealed that the disorder in a-Si: H films was enhanced by exposure to strong light.

]. INTRODUCTION

Hydrogenated amorphous silicon (a-Si:H) prepared by the glow discharge decomposition of silane, which has potential applications in optoelectronic devices such as solar cells, generally exhibits photoluminescence (PL). In recent years PL has been extensively studied in a-Si: H films since the efficiency and spectral shape of PL depend on the sample preparation conditions and provide useful information on recombination and trapping at localized states. Street e t al. t have shown experimentally that the intensity of the PL peak which occurs at photon energies in the range 1.2-1.5 eV decreases with increasing electron spin resonance (ESR) spin density. This result suggests that PL provides a useful method of characterizing a-Si:H films. However, there are few reports regarding the relation between the shape of the PL spectra and the film quality for a-Si: H thin films. A detailed study of the causal relation between the shape of the PL spectra and the film quality which was carried out for a-Si: H films of varying quality defined by physical parameters such as the density of gap states and the dark conductivity is * Paper presented at the Sixth International Conference on Thin Fdms, Stockholm, Sweden, August 13-17, 1984 0040-6090/85/$3 30

© Elsevier Sequoia/Printed in The Netherlands

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reported in this paper. A correlation was found between the peak intensity and peak halfwidth of the PL spectrum and the film quality. The reversible degradat!on phenomena observed for the PL (fatigue effect) and the conductivity (StaeblerWronski effect), which have recently been studied by many workers, are also investigated on the basis of the above result. 2.

EXPERIMENTAL PROCEDURE

Undoped a-Si: H thin films were prepared by the glow discharge decomposition of 11~o silane diluted with hydrogen gas at a substrate temperature of 573 K. The deposition conditions were as follows: r.f. power, 5 W (13.56 MHz); total gas pressure, 0.15 Torr; gas flow rate, 12-15 standard cm 3 mm-1. Two types of sample were prepared on various substrates by applying d.c. magnetic fields of zero (type 1 film) and 0.8 kG (type 2 film) perpendicular to the substrate surface (poor quality films can be prepared by applying a d.c. magnetic field2'3), n + crystalline silicon (c-Si), stainless steel (SUS) and fused quartz glass were used as substrates. The films were simultaneously deposited onto these substrates in a reactor. The PL measurements were carried out at 80 K using excitation by the 514.5 nm line of an argon ion laser. The PL was dispersed by a monochromator and detected at a cooled S-1 type of photomultiplier. All PL spectra were corrected for the diffraction efficiency of the grating and the optical response of the photomultiplier. 3.

RESULTS AND DISCUSSION

The density N(E,) of gap states near the midgap determined by the capacitance-voltage method 2-4 and the dark conductivity at room temperature were about 1016 cm-3 eV-1 and 2 × 10 - 9 ~ - 1 cm-1 respectively for type 1 films and about 1018 c m - 3 eV- 1 and 4 x 10-11 f~- 1 c m - 1 respectively for type 2 films (Table I). It should be noted that the density of gap states of type 2 films is two orders of magnitude greater than that of type 1 films. This suggests that the quality of type 2 films is poorer than that of type 1 films 3. Figure 1 shows typical PL spectra for type 1 (S-l) and type 2 (S-2) films deposited onto c-Si substrates. It can be seen that the shape of the PL spectrum of the S-1 film differs from that of the S-2 film and that the

TABLE I PHYSICAL

PROPERTIES

OF

THE

FILMS

AND

THE

PEAK

HALFWIDTH

AND

INTENSITY

RATIO

OF

P H O T O L U M I N E S C E N C E SPECTRA

Film

N(E,) (cm-3 eV t)

S-1

~

S-2

~ 10 is

1016

Dark conductivity (f~- t c m - I)

State

2x

10 -9

4 x 10 -11

H W (eV)

Total HW (eV)

A- B A+B

Peak A (1 20 eV)

Peak B (1 35 eV)

As deposited Fatigued

0.167 0.167

0231 0 252

0398 0 419

0108 0 237

As deposited Fatigued

0135 0 135

0306 0 325

0441 0 459

0041 0 055

CHARACTERIZATION OF

a-Si:H

THIN FILMS

187

5-1

5-1 z

~10

5-2

z

i °'

z

¢.

z

10

10

t2 14 16 PHOTON ENERGY(eV)

12 14 16 PHOTON ENERGY(eV)

Fig 1. Typ]cal PL spectra of S-1 and S-2 films

~-10

,-,O! L,~

10

(a)

12 PHOTON

I/, 16 ENERGY (eV)

10

12 14 16 PHOTON ENERGY ( eV )

(b)

Fig 2 Normahzed PL spectra for (a) S-2 and (b) S-I films which were analysed using two gausslan emission bands.

PL intensity in the S-2 film is lower than that in the S-1 film. To investigate the variation in the PL spectral shape in detail, the PL spectrum was analysed using gaussian emission bands. The PL spectra in both the S-1 and the S-2 films could be analysed using two gaussian emission bands located at 1.20 eV (peak A) and 1.35 eV (peak B) as shown in Fig. 2. The peak halfwidth (HW) and the peak intensity ratio ( A - B)/(A + B) of each PL spectrum for the S-1 and S-2 films are given in Table I. The following features should be noted. (1) The PL mtensity in the S-2 film, which was of poor quality, was lower than that in the S-1 film. This result is compatible with the relation between the ESR spin density and the PL intensity reported by Street et al. 1 (2) The ratio (A - B)/(A + B) for the S-2 film was less than that for the S- 1 film. (3) The total HW (the sum of the HWs of peaks A and B) for the S-2 film was larger than that for the S-1 film. These features allow us to conclude that the HW and the intensity of PL spectra strongly depend on the quality of a-Si:H films and that the HW of the PL spectrum is related to the film quality. The same features in the PL spectrum were observed in films deposited onto SUS substrates. The PL linewidth for amorphous semiconductors is generally determined by thermal broadening and disorder broadenlng. The linewidth of the PL spectrum measured at 80 K is dominated by disorder broadening since the PL linewidth of a-Si:H films is independent of the temperature up to 100 K s. We believe that the large PL linewidth for S-2 films can be attributed

188

H. NANTO et al.

to disorder broadening as a result of an enhancement in the electron-phonon interaction and an increase in the energy distribution of gap states because of the increasing &sorder introduced into the film. Evidence for the increase in the disorder is provided by the large number of gap states in the S-2 film (Table I). The fatigue and Staebler-Wronski effects produced in a-Si: H films as a result of exl~osure to strong light were also investigated on the basis of the above result. The fatigue effect was observed under exposure to light not only at 80 K but also at room temperature. Figure 3, curves b and c, shows typical PL spectra for S-1 films after exposure to an 8 0 0 m W argon laser beam at room temperature and 8 0 K respectively. The PL spectra were measured at 80 K under the same exotatlon conditions (50 mW). It should be noted that the degradation in the PL intensity after fatigue at 80 K is greater than that at room temperature. The same fatigue effect was also observed m the S-2 film. It was confirmed that the PL mtensity can be recovered to the initial state by anneahng at 423 K, in vacuum. Figure 4 shows the normahzed PL spectra in as-deposited and fatigued films. It can be seen that the PL intensity in the region with higher photon energy is substantially decreased by exposure to light. The H W and peak intensity ratio of the fatigued PL spectra for S-1 and S-2 films are also given in Table I. The following features can be seen from Table I and Fig. 3. (1) The H W of the PL spectrum after fatigue is larger than that before fatigue. (2) The decrease m the PL intensity and the increase in the H W of peak B are much larger than those of peak A. (3) The decrease m the PL intensity after fatigue at 80 K is larger than that after fatigue at RT. The Staebler-Wronski effect was observed under the same exposure conditions as the fatigue effect on the PL. It was confirmed that the conductivity was decreased on exposure to hght at both 80 K and room temperature, and that it could be

a

l.;"/I

(a)

t,"

I"-II~ I

0

10 -zQ5

Ns " ~

W

0 10

12 14 1.6 PHOTON ENERGY(eV)

!

t

tb) ~ a I ~ 10 12 1/-, 16 2: PHOTON ENERGY (eV)

F~g 3. P L spectra m S-1 obtained at 80 K by e x o t a t l o n using a 50 m W argon laser curve a, initial s p e c t r u m , curve b, spectrum o b t a i n e d after e x p o s u r e to an 800 m W a r g o n laser b e a m for 30 m m at r o o m t e m p e r a t u r e , curve c, spectrum o b t a i n e d after exposure to an 800 m W a r g o n laser b e a m for 30 m m at 80 K Fig 4 N o r m a h z e d P L s p e c t r a m ( a ) a n a s - d e p o s l t e d a n d ( b ) a f a t l g u e d S - I e x p o s u r e to an 800 m W argon laser b e a m for 60 m m

film The fattgue was caused by

CHARACTERIZATIONOF a-Si:H THIN FILMS

189

restored to the initial state by annealing at 423 K in vacuum. The first striking result is the increase In the HW of the PL spectrum after fatigue. By analogy with the dependence of the H W of the PL spectrum on the film quality, we can suggest that the disorder m the a-SI:H films is enhanced by exposure to light. The disorder broadening effect for the linewidth of peak B was much larger than that of peak A. Peak A (1.20 eV) and peak B (1.35 eV) may be caused by the radiative transition between the tailed states in the band gap, since about 1.7 eV of the optical band gap energy of these films is used 3. The states responsible for the transition in peak A are more localized than those responsible for the transition in peak B because the photon energy of peak A is less than that of peak B. Thus the observation that the effect of disorder broadening for peak B is much larger than that of peak A is reasonable. The second major feature is the fact that the degree of degradation in the PL intensity after fatigue at 8 0 K is larger than that after fatigue at room temperature. We believe that the recovery process 6'7, which is caused by an annealing effect at the film surface owing to heating by the argon laser beam, competes with the fatigue process during the exposure to light at room temperature. 4. CONCLUSION We showed that the PL spectrum, particularly the linewidth and the intensity of the PL peak, depends on the quality of a-Si: H films. We found a correlation between the PL linewidth and the film quality. The large linewidth of the PL spectrum for a poor quality film was explained by disorder broadening. From an investigation of the dependence of the PL spectral shape on fatigue, we found that the disorder in a-Si:H films was enhanced by exposure to light. We can conclude that shape analysis of PL spectra is an effective method of evaluating the quality of a-Si:H thin films. ACKNOWLEDGMENTS The authors wish to acknowledge S. Nakamura, T. Nakazawa, T. Nakamori and H. Taniguchi for their technical assistance with the experiments. REFERENCES 1 R A, Street,J.C KmghtsandD K. Blegelson, Phys Rev B, 18(1978) 1880. 2 T Yamamoto, Y Mishtma, M. Htroseand Y. Osaka, Jpn J Appl Phys., 20 (1981) 185 3 Y MIshlma, M Hlrose, I Suemune,M Yamamshl and Y Osaka, J Phys. (Parts), Colloq C4, 42 (1981)447 4 M. Htrose,T Suzukiand G. H Doehler,Appl Phys Lett, 34 (1979)234 5 C TsangandR A. Street, Phys Rev. B, 19(1979)3028 6 J I Pankoveand J. E Berkeyhelser,Appl. Phys. Lett., 37 (1980)705. 7 M. Tomozane,F Hasegawa,M. Kawabeand Y Nanmchi,Jpn. J. Appl. Phys, 21 (1982)L497