Photoconductivity in compensated a-Si:H and the effect of bias light on the drift mobility

Journal of Non-Crystalline Solids 164-166 (1993) 607-610 North-Holland

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Photoconductivity in compensated a-Si:H and the effect of bias light on the drift mobility P. Tzanetakis ~, H. Fritzsche b, M.Q. Tran b, M. AndroulidakP and E. Rynes b ~Foundation for Research and Technology-Hellas, P.O. Box 1527, Heraklion 71110, Crete, Greece. b James Franck Institute, University of Chicago, Chicago, IL 60637 USA We measured the photoconductivity ~p of compensated hydrogenated amorphous silicon (a-Si:H) between 4.2 and 400K. The PH3/SiH4 = B2H6/SiH4 doping ratios ranged from 2.5 × 10 -6 to 10 -2. Potential fluctuations strongly degrade (rp. Subgap absorption and defect concentration are unreliable indicators for material quality. The drift mobility obtained from the initial decay of ~p was found to increase at surprisingly small bias light intensities. 1. I N T R O D U C T I O N

3. R E S U L T S

Potential fluctuations have been invoked to explain a number of observations such as the difference in thermopower and conductivity activation energies [1] and the persistent photoconductivity [2] in compensated a-Si:H but our understanding of their effect on the transport properties is still incomplete. Howard et al. [3] observed a rapid drop of the time-of-flight drift mobilities of electrons and holes with increasing compensation in a-Si:H and attributed this to the confinement of the carriers in potential wells of the fluctuations, Such confinement should affect extended state conduction more severely than charge carrier hopping in localized states. To explore whether this can be used as a distinguishing leature of the two conduction processes, we studied the effect of compensation on the photoconductivity between 4K and 350K. 2. E X P E R I M E N T A L

DETAILS

About l # m thick compensated a-Si:H samples were prepared at 500K by RF plasma deposition. Equal concentrations of PH3 and B2H6 were added to the Sill4 plasma gas, ranging from 2.5 × 10 -6 to 10 -2. The samples were contacted with NiCr electrodes. The steady state photoconductivity ap was measured with a tungsten halogen lamp filtered for 800 > A > 540 nm at an intensity of 7 m W c m -2. The decay time was measured with a transient recorder and light pulses from red light emitting diodes,

Sample properties are listed in Table 1. The optical gap Eopt and coefficient B were obtained from a Tauc plot of the absorption coefficient (ahu) 1/2 = B(hu - Eopt). The Urbach slope E0 and the defect concentration ND were determined by photothermal deflection spectroscopy and the hydrogen concentration from the Si-H stretching mode at 2000 cm -1. The 2100 cm -1 component was negligible. The dark conductivity of all annealed samples was between 10 -1° and 10 -11 o h m -1 cm -1 at 300K, hence the Fermi level EF is close to midgap. The annealed samples were exposed at 300K for 30 min to red light producing a photocarrier generation rate of G 6 × 102°cm-3s -1 to diminish degradation during measurements of crp. The ratio D = A(Tp/op due to this exposure is listed in Table 1. Fig. 1 shows an Arrhenius plot of the normalized photoconductivity measured with G = (5 + 1) × 1019 c m - 3 s -1. In order to include the low t e m p e r a t u r e region we plot the same data against log T in Fig. 2. Between 4.2 and 10K ap is essentialy constant. Compensation has the largest effect on c~p in the intermediate temperature range between about 70K and 250K. The relatively high photoconductivity of the least doped samples 2.5 × 10 -6, 1 and 2 × 10 -5, undergoes thermal quenching (TQ) above 150K and closely approaches the c~p values of the 10 -4 and 2.5 × 10 -4 samples above 300K. The most hearily doped 5 x 10 -4 and 10 -2 samples have significantly reduced ~p/eG values in the whole temperature range.

0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.

P. Tzanetakis et al. I Photoconductivity in compensated a-Si.'H

608

Table 1 Characteristics of compensated samples. C-Doping

Eopt eV

B (cmeV) -1/2

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CH at %

ND 1016 cm -3

D

2.5 x 10 .6 1 x 10 -5 2 x 10 -5 1 x 10 -4 2.5 x 10 -4 5 × 10 -4 10 .2

1.82 1.81 1.81 1.81 1.81 1.78 1.62

800 750 750 770 770 740 650

59 50 57 59 60 69 100

7.4 8.2 7.8 8.6

2.8 1.6 1.0 0.6 1.5 6 2

0.43 0.44 0.31 0.35 0.20 0.05 0.0

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Fig. 2 T e m p e r a t u r e dependence of normalized photoconductivity of a-Si:H containing different C-doping ratios.

In Fig. 3 we show o'p/eG in the region of thermal quenching for the 10 -5 doped sample together with the exponent 7 of the relation ap --, G "r. In order to demonstrate that this power law is not valid at all T or a wide range of G, we show in Fig. 3 measurements for different G values, Using the method of Hoheisel and Fuhs [4] we determined the drift mobility PD from the ini-

tial decay of the steady state photoconductivity. Fig. 4 shows the temperature dependence o f / t O for the 2 × 10 -~ C-doped sample. In uncompensated samples the magnitude of #D suggsested a transition from hole conduction above T Q to electron conduction below T Q [5]. The kink in the T-dependence of PD between 200K and 150K resembles such a transition. The change in the

P. Tzanetakis et al. I Photoconductivity in compensated a-Si:H

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magnitude of #D is however less pronounced in the present samples presumably because the difference in electron and hole mobilities decreases as the C-doping concentration is increased [3]. If the potential fluctuations diminish #D, then one should expect that screening by a large concentration of photocarriers might increase #D to its normal value. For this purpose we measured YD as a function of the generation rate G of bias light (e=electron charge). The value of #D WaS determined as before from the initial decay of the incremental #D produced by the LED. The results f°r the 2 × 10-5 sample are sh°wn in Fig. 5. The mobility increases by about a factor 10 at the largest bias intensities. The results were unchanged within the intensity range of the LED, 0.15 < e G < 2.3C/cmas, and when the duty cycle of the bias light was reduced by a factor 10 to avoid heating.

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610

P. Tzanetakis et al. / Photoconductivity in compensated a-Si:H

4. D I S C U S S I O N Our samples agree well with those of Howard el al. [3]. A decrease in the optical gap and an increase in the Urbach slope begin at a doping of 5 × 10 -4 and become strong at 10 -2. The concentrations of hydrogen and of dangling bond defects remain constant and the light-induced degradation D vanishes at high doping levels, Except for the two most heavily doped samples which we shall discuss separately, most of the characteristics listed in Table 1 do not change much with increasing C-doping; the photoconductivity, however, drops by several orders of magnitude in the intermediate temperature range 70K < T < 150K. The sensitized part of % that disappears above 150K by the process of TQ has vanished completely at a doping of 10 -4. At T>300K, where T Q has removed this sensitized part, ~p is nearly independent of doping up to 2.5 × 10 -4. If the majority photocarriers are electrons in the sensitized region below TQ but holes about TQ, then the rapid decrease of the sensitized crp may be caused by the sharp decrease of the electron mobility with doping. Howard et al. [3] found that the electron and hole mobilities become the same at dopings of 3 × 10 -4, which agree well with the doping of 10 -4 at which electrons and holes become indistinguishable in

duction. The gradual transition from hopping to band conduction with increasing T is noticeable in the three least doped samples between 40K and 100K. At high doping levels crp is decreased by nearly the same factor which suggests that hopping dominates at all T. If all recombination processes originate from extended states and photoconduction occurs solely in these states by one kind of carrier then there exists a close relation between 7 and the slope dlnap/dT, that is, 7 = 1 at the temperature where the slope is zero, and 7 reaches a maximum where the slope has its largest negative value. 6 Fig. 3 shows that at least one of the above conditions is not fulfilled by the 10 -5 doped sample. They are fulfilled at smaller dopings. We suspect that this is due to an increasing hopping contribution to crp as the band mobility is decreased by potential fluctuations. We attempted to reduce the potential fluctuations by increasi.ng the concentration of charge carriers available for screening with bias light. We observe in Fig. 5 an order of magnitude increase in drift mobilities. A more detailed analysis of the data is required, however, before we can associate this effect with the expected reduction of potential fluctuations.

Crp(T).

ACKNOWLEDGEMENTS

At doping levels exceeding 5 × 10 -4 one observes a lowering of Eopt and a broadening of the tails. The internal electric fields will be severe, the concept of a mobility edge ceases to have meaning, and photoconduction will occur by hopping. Not only the photoconductivity but also the drift ranges of majority and minority carriers will be strongly reduced by potential fluctuations, while N o remains unchanged as shown in Table 1. ND is therefore an unreliable indicator of material quality.

We wish to thank M. Choi and M. Harvey for their competent assistance. The work was supported by NSF DMR 9108109 and by the Materials Laboratory of the University of Chicago funded by the NSF.

At very low temperatures crp is due to down hopping of photocarriers through localized tail states and hence is unaffected by the decrease in electron or hole mobilities. At very high dopings the drop of Crp/eG by nearly a factor of 10 may be partly due to the broadening of the tails and stronger localization. Band conduction is much more strongly reduced by potential fluctuation than hopping con-

3 J.A. Howard and R.A. Street, Phys. Rev. B44 (1991) 7935. 4 M. Hoheisel and W. Fuhs, Philos. Mag. B57 (1988) 411. 5 H. Fritzsche, B.-G. Yoon, D.-Z. Chi and M.Q. Tran, J. Noncryst. Solids 141 (1992) 123. 6 J.Z. Liu and S. Wagner, Phys. Rev. B39 (1989) 11156.

REFERENCES 1 H. Overhof and W. Beyer, Philos. Mag. 43 (1981) 433. 2 A.J. Hamed, Phys. Rev. B44 (1991) 5585.