Preparation and characterization of hydrogenated amorphous germanium and hydrogenated amorphous germanium carbide thin films

Abstract

<p>Hydrogenated amorphous germanium (a-Ge:H) and germanium carbide (a-Ge[subscript] 1-xC[subscript] x:H) films were prepared by rf sputtering of a polycrystalline Ge target in a vacuum ~4 x 10[superscript]-7 Torr at various rf power 50 ≤ P ≤ 600 W (0.27-3.3 W/cm[superscript]2), target-substrate distance 1 ≤ d ≤ 7[superscript]'', varying partial pressures of Ar, H[subscript]2, and C[subscript]3H[subscript]8, and flow rates f. The vibrational and opto-electronic properties such as infrared (IR) absorption, optical gap, electron-spin-resonance (ESR) signals, and conductivities vary with deposition conditions. The photoconductivity [sigma][subscript] ph, in particular, was carefully monitored as a function of the deposition conditions to optimize it;The concentration of Ge-H bonds and the optical gap (E[subscript] g), generally decrease as P is increased. E[subscript] g of the a-Ge[subscript] 1-xC[subscript] x:H films range from 0.85-2.3 eV. The ESR results range from 2 x 10[superscript]17 to 2 x 10[superscript]19 spins/cm[superscript]3. Results of annealing showed the enhanced segregation effect of Ge-C bonds ≥300°C. The evolution of bonded hydrogen with temperature is studied;Deposition rates (R[subscript] d) of a-Ge:H films are estimated and compared. The thermalization curve for a Ge target is constructed. R[subscript] d was found to decrease exponentially with increasing d, to decrease with increasing partial pressures of H[subscript]2 and C[subscript]3H[subscript]8 and increasing flow rates. R[subscript] d is maximal at some P[subscript] Ar and is relatively insensitive to the substrate temperature T[subscript] s, and rises linearly with the rf power;Hydrogen incorporation markedly increased [sigma][subscript] ph. The dark conductivity [sigma][subscript] d and [sigma][subscript] ph increase with increasing d, up to an optimal value at d ~ 6[superscript]'', increase at f < 1 sccm, and increase with P and T[subscript] s. Incorporation of significant amounts of carbon sharply reduces [sigma][subscript] ph. However, a small amount has little effect on [sigma][subscript] ph. Although all increases in [sigma][subscript] ph are significant relative to those of previously reported rf sputtered a-Ge:H, the results in this work are still much lower than those of glow discharged films, which are lower than those of rf magnetron sputtered films grown in high vacuum (<5 x 10[superscript]-9 Torr). The parallel behavior of [sigma][subscript] d and [sigma][subscript] ph, the relatively long lifetimes of the carriers [tau] ~ 0.8 ms but rather low mobility [mu] ~ 5 x 10[superscript]7 cm[superscript]2/V-sec and low subgap absorption suggest that the transport is dominated by defects near the mobility edge. The failure in identifying the effect of electron bombardment of the growing film by using biased screen suggests that the high values of [sigma][subscript] ph reported from rf magnetron sputtering are probably partially due to the high background vacuum. ftn*DOE Report IS-T-1377. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.</p>