Ambipolar Diffusion Coefficients in a-SiC:H Alloys in Steady-State and Transient Grating Measurements
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AMBIPOLAR DIFFUSION COEFFICIENTS IN a-SiC:H ALLOYS IN STEADYSTATE AND TRANSIENT GRATING MEASUREMENTS H. WEINERT*, M. PETRAUSKAS$, J. KOLENDA$, A. GALECKAS$, F. WANG+, and R.
SCHWARZ` (*)Humboldt-Universitlit zu Berlin, Berlin, Germany ($)Vilnius University, Vilnius, Lithuania (+)Technical University of Munich, Garching, Germany Abstract
Recently we have determined surprisingly large values of the ambipolar diffusion coefficient D of 3 - 9 cm2 /s in amorphous silicon-based alloys and a-Si:H/a-SiC:H multilayer structures from transient grating decays in the psec time domain. The steady-state photocarrier grating method, however, resulted in much lower D values (-10-4 cm2 /s) in the same samples. Since high carrier densities of typically 1019 cm-3 are reached in the psec domain, the Einstein relation may no longer be valid. The large diffusivity of non-equilibrium carriers decreases, however, rapidly in time due to energy relaxation and carrier recombination until a stable trap occupation under steady-state condition is reached.
1. Introduction It is generally found that the ambipolar diffusion coefficient in hydrogenated amorphous silicon (a-Si:H) equals about 10-4 cm2/s in steady-state measurements [1-3]. This is inferred from the ambipolar diffusion length measurement done with the steady-state photocarrier grating method SSPG [4]. In a recent paper Devlen and Schiff [5] have discussed the puzzling difference of the ambipolar diffusion coefficient D obtained in transient grating experiments (TG) of the order of 10-2 cm2 /s [6] compared to the much lower steady-state values. The authors suggest an explanation based on the saturation of bandtail states which then allows large diffusion coefficients. We have added a new piece of information to the confusion with the publication of results ofD values well above 1 cm2 /s under very high carrier generation rates in psec transient grating experiments [7]. Altogether it becomes evident that D is strongly dependent on the carrier density which changes from 1013 up to 1019 cm-3 in the above mentioned experiments and on the time scale considered. We will also discuss the unrealistically large carrier mobility which would follow from Einstein's relation. 2. Experimental details We investigated bulk films of hydrogenated amorphous silicon (a-Si:H), alloy films of a-SiC:H, and a:Si:H/a-SiC:H multilayer structures that were deposited on Corning glass substrates by decomposition of silane and methane gas mixtures in a conventional RF glow discharge system. The total film thickness was 1.2 grm for all films. The thickness of the a-Si:H sublayer in the multilayers was varied between 3 and 1000 A while keeping the barrier-to-well thickness ratio constant at 1:1.3. The energy gap of wells and barriers was 1.7 and 2.02 eV, respectively. To measure the ambipolar diffusion coefficient D at low carrier densities we first determined the ambipolar diffusion length LD with the SSPG method using HeNe laser Mat. Res. Soc. Symp. Proc. Vol. 297.
11993 Materials Research Society
498
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