Materials Science and Engineering B72 (2000) 197–199

Gap states and stability of rapidly deposited hydrogenatedamorphous silicon films

S.H. Lin *, Y.C. Chan, D.P. Webb, Y.W. LamDepartment of Electronic Engineering, City Uni6ersity of Hong Kong, 83 Tat Chee A6enue, Kowloon, Hong Kong

Abstract

The sub-band gap absorption spectrum a(h6) in a-Si:H films prepared at high deposition rates was measured by the constantphoto-current method (CPM) for photon energy ranging from 0.8 to 1.7 eV in a thermally-annealed state and light-soaked state.The Simmons–Taylor theory and occupation statistics of correlated defects are used to model the distribution of band tail andgap states. It is found that the density of gap states increases after light soaking, however, there is no evident change found inthe density and distribution of band tail states. Measurements of the light-induced changes find that the photoconductivitydecreases by less than one order of magnitude after long time of light illumination for the high rate deposited a-Si:H. Thisdemonstrates that the high rate deposited samples have relatively high stability compared with conventionally deposited a-Si:H.© 2000 Elsevier Science S.A. All rights reserved.

Keywords: PECVD; a-Si:H; Sub-band gap absorption; Density of states; CPM; SWE

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1. Introduction

The density of states (DOS) and the stability are veryfundamental properties of hydrogenated amorphous sil-icon (a-Si:H) thin films with respect to its applications.Susceptibility to light induced degradation (the Stae-bler–Wronsky effect) is known to be an intrinsic prop-erty limiting application of a-Si:H films [1,2]. In thiseffect, dark conductivity and photoconductivity de-crease, and correspondingly the sub-band gap absorp-tion increases [3], when a-Si:H films are exposed tostrong light illumination. This effect has been proved tobe associated with the increase of density of states inthe band gap. Self-consistent analyses on the steadystate photoconductivity and sub-band gap absorptionindicate that both neutral (D0) and charged defectstates (negatively charged D− and positively chargedD+ states) exist in undoped a-Si:H films in annealedstates, and both are created during light soaking [4,5].

In this paper, we report the results of the light-in-duced degradation of photoconductivity and increase ofgap state density measured by CPM in our high ratedeposited a-Si:H films. This is the first report of SW

effect and the density of sub-band gap states in thishigh rate deposited a-Si:H.

2. Experimental details

The undoped a-Si:H films used in this study weredeposited onto Corning 7059 glass substrates at a highrate (\10 A, s−1) by means of a PECVD techniqueunder optimum conditions [6,7]. Pure silane (SiH4) wasused for the deposition, with preparation conditions asfollows: a SiH4 flow rate of 35 sccm, a substrate tem-perature of 330°C, a total reaction pressure of 0.45Torr, and rf discharge power densities of 500, 600 and800 mW cm−2.

Optical transmission spectroscopy (OTS) was carriedout on a Perkin–Elmer model 567 spectrometer over awavelength range from 600 to 1100 nm. The opticalabsorption coefficient a(h6) and film thickness werecalculated from the transmission spectrum using nu-merical analysis taking into account surface roughness[8]. The optical gaps were derived from the Tauc rela-tion [9]. The CPM experiments were carried out on thesamples in coplanar configuration. A schematic dia-gram of the CPM measurement apparatus is shown inFig. 1. A standard feedback loop was used to keep thephotocurrent constant. The annealed state (state A) was* Corresponding author.

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S.H. Lin et al. / Materials Science and Engineering B72 (2000) 197–199198

Fig. 1. Schematic of the apparatus for CPM measurement.

Fig. 2. Absorption spectra determined by optical transmission (OTS-open symbol) and sub-band gap absorption spectra (CPM-filledsymbol) of high rate rf discharge power density of 500, 600 and 800mW cm−2. All samples are in the annealed state.

obtained by heating the samples at temperature of200°C for 10 h in high vacuum and dark environment.The light soaked states (state B) were obtained byilluminating the sample under a light intensity of 100mW cm−2 light from a 250 W Tungsten light sourcewith the IR part filtered by a CuSO2 solution.

3. Results and discussion

The principal properties of the films used are shownin Table 1. The deposition rate increases from 10.5 to18.1 A, s−1 with increasing power density, however, theoptical gap simultaneously decreases from 1.69 to 1.66eV and the ratio of photoconductivity to dark conduc-tivity decreases from 1.0×104 to 5×103. These resultsindicate quality degradation introduced by increasing rfdischarge power density.

The CPM method is based on the assumption of onedominating recombination path and one majority typeof carrier during monochromatic illumination. As theillumination wavelength is scanned, the photocurrentthrough the sample is kept constant by varying illumi-nation intensity. For homogeneous excitation [10,11],the relative absorption coefficient can be determinedfrom [12] aCPM(h6)8 (1/Nphh6), where Nph(h6) is thenumber of photons required to maintain photocurrentat photon energy h6. The absolute sub-band gap ab-sorption may then be obtained by normalizing theresults in the vicinity of about 1.7 eV to the opticalabsorption [10,11].

A significant increase in a(1.2 eV) from 0.3 to 7.0cm−1 upon increasing rf power density from 500 to 800mW cm−2 can be found in Fig. 2 for annealed films.Using the relation [5] ND0=a(1.2 eV)×3×1016 cm−3,the neutral defect states ND0 are found to be 9.0×1015

cm−3 for the 500 mW cm−2, 7.5×1016 cm−3 for the600 mW cm−2 and 2.1×1017 cm−3 for the 800 mWcm−2 films. Although these values are potentially anunderestimate of ND0 by up to 15% [5], it is clear thatthe sub-band gap density increases significantly withincreasing rf discharge power density.

The photoconductivity in the best film in Fig. 2 (500mW cm−2) decreases by a factor of 2 after 10 h light

Fig. 3. The light-induced degradation in photoconductivity sph of thea-Si:H film deposited at rf discharge power density of 500 mW cm−2.For comparison, the data obtained on conventionally depositeddevice-quality a-Si:H film and ‘layer by layer’ deposited high-qualitya-Si:H film are also shown [13].

Table 1A summary of experimental results of undoped a-Si:H films

a-Si:H films 500 mW cm−2 600 mW cm−2 800 mW cm−2

18.1Deposition 10.5 12.7rate (A, s−1)

Film thickness 760.8817.7 1144.8(nm)

1.69Eopt (eV) 1.67 1.661.0×104sph/sdark 6×103 5×103

S.H. Lin et al. / Materials Science and Engineering B72 (2000) 197–199 199

illumination (see Fig. 3). It is obvious that the lightstability of our high rate deposited a-Si:H film is betterthan that of conventionally deposited device-qualitya-Si:H film [13] and is comparable to that of a high-quality ‘layer by layer’ deposited sample [13] after 10 hof light soaking.

For the study of light induced degradation of thesample shown in Fig. 3, the sample was again annealedto state A. Then, measurements of sub-band gap ab-sorption were performed after light soaking the samplefor 300 and 600 min. The results are shown in Fig. 4.For comparison, the result obtained in the annealedstate (state A) is plotted again in Fig. 4. It can be seenthat the sub-band gap absorption at photon energy of1.2 eV increases from 0.3 to 2.2 cm−1 on light soaking.This is due to the increase of the density of sub-bandgap states, which causes the light-induced degradationin photoconductivity shown in Fig. 3. In Fig. 4, thesolid lines are the best fits to the experimental datausing a band gap states model based on Simmons–Tay-lor statistics [4,5]. The energy distribution of gap statesincluding negatively charged D− states, neutral D0

states and positively charged D+ states is illustrated inthe inset. The model assume extended states withparabolic distributions, exponential conduction and va-lence band-tail states and a Gaussian D0 distributioncentered at 0.82 eV, with half-width of 0.12 eV andcorrelation energy of 0.3 eV. The parameters werechosen to give a good eye fit to all the data simulta-neously. The total density of state D−, D0 and D+

obtained by the fitting, are found to be 8.2×1015,7.8×1015 and 8.2×1015 cm−3 for the annealed state;2.3×1016, 2.5×1016 and 2.3×1016 cm−3 for the 300min light soaked state and 6.4×1016, 6.8×1016 and

6.4×1016 cm−3 for the 600 min light soaked state. Inthe annealed state, the characteristic energy of theconduction and valence band tail, which are adjustableparameters in the fitting, are found to be 30 and 50meV by the best fit, and are about the same for the twolight soaked states. The results indicate that the densityof sub-band gap states, including both of neutral andcharged defect, are greatly increased upon light soak-ing. However, the band tail states show no evidentchange after light soaking.

4. Conclusions

Measurements of light induced changes in photocon-ductivity and sub-band gap absorption from CPM ofa-Si:H films deposited at high-rates are reported here. Afitting process taking into account Simmons–Tayloroccupation statistics has been used with the absorptioncoefficient spectra to determine the sub-band gap statedensities. Satisfactory agreement between the fittingresults and the experimental data were obtained. Fromthe fitting results, the slope of the conduction band andvalence band tail exhibits no evident change betweenthe annealed and light soaked states. However, thedensities of sub-band gap state are significantly raisedby light soaking and increase with light soaking time upto 600 min. However, measurement of light inducedchanges in photoconductivity on our high rate de-posited film shows improved stability compared to con-ventional low rate deposited a-Si:H samples and iscomparable to that of device-quality a-Si:H prepared atlow rate by a ‘layer by layer’ technique.

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Fig. 4. Sub-band gap absorption coefficient of the undoped a-Si:Hfilm deposited at rf discharge power density of 500 mW cm−2,measured in the annealed state, and after 300 and 600 min lightsoaking. The solid lines are best fits to the experimental data..