A critical investigation of a-Si:H photoconductivity generated by subgap absorption of light

316

Journal of Non-Crystalline Solids 114 (1989) 316-318 North-Holland

A CRITICAL INVESTIGATION OF a-Si:H PHOTOCONDUCTIVITY GENERATED BY SUBGAP ABSORPTION OF LIGHT S. LEE, Satyendra KUMAR, and C. R. WRONSKI Center for Electronic Materials and Processing, Electrical Engineering Department, Pennsylvania State University, University Park, PA 16802, U.S.A. N. MALEY Coordinated Science Laboratory, University of lllinois, Urbana, IL 61801, U.S.A. Dual beam photoconductivity (DBP) method has been used to characterize the sub-bandgap optical absorption of a-Si:H. Results are presented for different a-Si:H films which illustrate the effects of the electron quasi-Fermi level displacement due to changes in bias light intensity and temperature. These results demonstrate that DBP offers a sensitive and reliable method for studying the densities and energy distribution of intrinsic and light induced defects and their effect on recombination kinetics. 1. INTRODUCTION Photoconductivity, generated by photons with energies less than the optical bandgap is used to characterize the gap states in a-Si:H based materials and device structures. 1'2 The use of photoconductivity to measure the large changes in sub-bandgap optical absorption, c~(hu), requires that the effects of the changes in carrier generation rates, G Cr/~-38 -1, on the photoconductive carrier lifetime, r, and the response time, To, be taken into account 3. To overcome this, two methods are used; the constant photocurrent method (CPM) 1'2 and the dual beam photoconductivity (DBP). 4 In CPM this is done by maintaining the photocurrents over the entire range of photon energies at the low values obtained in the regions ( c ~ l c m -1) which have a correspondingly long response times. In DBP on the other hand, both ~" and ro are maintained at a constant value by a dc volume generated G, which is superimposed on the smaller ac monochromatic component,

g(ht,) cm-3s -1, used to probe cr(hv). Under these conditions the monochromatic component of the photoconductivity ap(hv) is linear with monochromatic light intensity, F(hv), and for uniformly absorbed light is directly proportional to ~(hv)

of the monochromatic light. Both T and ~ depend on the intrinsic photoconductivity of the material and G, but for G :>:> g are independent of ~(hv) so that the value and photon energy dependence can be obtained by normalizing volume absorbed a(hv) to values of cz(hv) determined directly from optical measurements. The DBP method not only improves the signal-to-noise ratio of the measured photocurrents because of the higher ro but using different values of G also allows the

Oc~(hu)

to be probed for different separations of the electron and hole quasi-Fermi levels, EI~ and Efp, respectively. As Elm and

Ely

move apart they allow more gap states to act as

recombination centers and their occupancy by electrons (in the case of n-type photoconductivity) is reflected in a(hv) and Oc~(hu). This can lead to large changes in

Oc~(hu) particularly in the cases where there is a nonlinear dependence of the photoconductivity on G. We have investigated the use of DBP as a reliable and powerful method of determining Oa(hv) using a wide variety of a-Si:H films. 2. EXPERIMENTAL DETAILS Intrinsic and lightly phosphorous doped a-Si:H films

where ~ is the photogeneration quantum efficiency and

were studied using the experimental technique described elsewhere,s The photoconductivities were measured using approximately l # m thick films on quartz substrates hav-

is a constant determined by ~'o and the chopping frequency

ing NiCr and Cr/n + coplanar electrodes. In the DBP mea-

a(hv) = q#rtcO~(hv)

(1)

0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

S. Lee et al./ Investigation of a-Si:H photoconductivity

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Spectral dependence of Oct for annealed a-Si:H film measured by DBP with different bias light (G)

Spectral dependence of Oct for a-Si:H film in annealed and light soaked state measured at two different G

surement the bias light generation rates, G, between 10]5-

to 2.5× 1019 cm-3s -I which corresponds to a displacement

1019cm-as -1 were obtained with red light (~ > 600rim).

of the quasi-Fermi levels from 0.17 to 0.32eV above the

The monochromatic photon fluxes used (he from 1.8 to

Fermi level• For the low level of G, the subgap data ob-

0.8eV) were between 1015 to 1012cm-2s -1, and were chop-

tained by DBP is in good agreement with the CPM data

ped at frequencies from ~10Hz to 1000Hz.

shown which was measured independently. The fall-off in

In these measurements care was taken to ensure that

subgap absorption at hu,,,leV indicates that aph(hv) is

there were no appreciable interface and surface related ef-

dominated by states ~0.SeV below the conduction band

fects and for annealed films there was no detectable light

edge. It is important to note here that for the change in C

induced degradation generated by the bias light. In or-

(and photoconductivity) of ~-,104, there is only a change

der to observe small changes in Oct(he) fringe averaging

of ~2 in 8ct(hu) for hv between ~1.4 and 0.geV which

was carried out using the Fourier transform methodology developed by Wiedman et el. 6

density of midgap states, Nr, estimated from the integration of subgap absorption data 7 varies from 4x1015 to

3. RESULTS AND DISCUSSIONS For G>>g(hv) the spectral dependence of aph(hu) was found to be independent of chopping frequency and of the monochromatic light intensity, F(hv). The value of 0c~(he) in the energy range h u < l . S e V were found to depend upon the quality of the film, its photoconductivity, the exponent "7 (where crph = ~ )

is consistent with the virtually constant value of v. The

and the value of G. In

Fig. 1 we show the results for Oct(he) obtained with DBP at room temperature for an intrinsic a-Si:H film in the annealed state which had a conductivity activation energy of 0.88eV and 7=0.94. In Fig. 1 G is changed from 6.0x10 z5

9x1015 cm-3eV -1 with increasing G values. It can also be pointed out that accurate fringe averaging is necessary to discern the small changes in 9ct(hv) that are observed in the subgap region. An example of the observed differences in 0ct(hu) obtained between the soaked and annealed state of the film shown in Fig. 1 are illustrated in Fig. 2. The differences in subgap 8ct(hv) of the soaked state are again relatively small when G is changed by ,,,104 but are different to those in the annealed state, as is the energy dependence of

S. Lee et aL /Investigation of a-Si:H photoconductivity

318

the higher densities of defects ( N r ~ l x l 0 '7 cm-3eV -1)

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present in even lightly doped a-Si:H films. The values of 0a(hu) at lower temperatures were then obtained by superimposing the values of 0~(hv)
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ment with the optical measurements on a-Si:H reported by Cody et al.s The independence of 0ol(hv) in the energy

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range 0.8 to 1.4eV clearly illustrates that even though we (D

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have very high densities of trapped electrons in the conduction band tail, their effect on the DBP measurements of deep gap states is negligible. It also indicates that 0 is

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independent of the measurement temperature. In conclusion the results presented here illustrate that DBP offers a sensitive and reliable method of studying subgap absorption. The ability to study the effects of both

Spectral dependence of 0c~ measured on a slightly P doped a-Si:H (EI=0.f5eV) film at three different temperatures

bias light and temperature offers a powerful tool for investigating the densities and energy distribution of deep lying gap states as well as their effect on recombination kinetics.

0c~(hv). At the lower value of (~ the difference in 6c~(hv)

ACKNOWLEDGMENTS

is very small and only the change in the energy dependence can be discerned. However with the higher (7, both

larex Thin Film Division for the a-Si:H films and helpful

differences in the energy dependence and magnitude can

discussions. We also thank M. Gunes and R. Dawson for

be seen reflecting the effect of moving E tn. Also higher values of N~ are obtained which increase from ,-,7×10 is

assistance in the photoconductivity and optical measure-

to 2.5x1016 with increasing G. The variation in measurement temperature offers an independent way of moving the quasi-Fermi levels and probing the defects in a-Si:H. Although the corresponding

Research Institute and in part by the Ben Franklin Technology Center of Central and Northern Pennsylvania.

changes in photoconductivity may lead to a temperature dependence of e: and ~" (Eqn. 1), crph(ht/) should still reflect

9(hu) and 0c~(hv). Thus unlike CPM where the change in T, To have to be taken into account, 2 DBP can be directly used to study 0c~(hz+). However, in both methods the normalization has to take into account the changes in c~ and optical band gap, Eg, with temperature. In Fig. 3 we show the DBP results for a lightly doped (EI=O.55eV) a-Si:H film obtained at 298,220, and 160K. The effects of changes in/~I~ with temperature on the occupation of the defect states at ~0.8eV below Ec are minimized so as not

We wish to thank M. Bennett and S. Wiedman of So-

ments. This work was supported by the Electrical Power

REFERENCES 1. M. Vanecek, J. Kocka, J. Stuchlik, Z. Kozisek, O. Stika, and A. Triska, Solar Energy Mater. 8 (1983) 411. 2. K. Pierz, H. Mell, and J. Terukov, J. Non-Cryst. Solids 77&78 (1985) 547. 3. A. Rose, Concepts in Photoconductivity and Allied Problems, (Interscience, New York, 1963). 4. C. R. Wronski, B. Abeles, T. Tiedje, and G. D. Cody, Solid State Commun. 44 (1982) 1423. 5. C. R. Wronski, B. Abeles, and G. D. Cody, Solar Cells 2 (1980) 245. 6. S. Wiedman, M. S. Bennett, and J. L. Newton, Mater. Res. Soc. Proc. 95 (1987) 145.

to affect the values of Oct(by) below ,,,1.4eV. In Fig. 3 the 298K values of 0~(hl/) were obtained by normaliza-

7. C. R. Wronski et al, in AlP Conf. Proc. 157 (1987) 70.

tion to a at 1.7eV where the values of ,,~lOcm -~ reflect

8. G. D. Cody, T. Tiedje, B. Abeles, B. Brooks, and Y. Goldstein, Phys. Rev. Lett. 47 (1981) 1480.