PHYSICAL REVIEW B VOLUME 27, NUMBER 12 15 JUNE 1983 Study of gap states in hydrogenated amorphous silicon by transient and steady-state photoconductivity measurements C. - Y. Huang* The Standard Oil Company (Ohio), Research Laboratories, 3092 Broadway Avenue, Cleveland, Ohio 44115 S. Guha and S. J. Hudgens Energy Conversion Devices, Inc. , 1675 O'. Maple Road, Troy, Michigan 48084 (Received 29 November 1982) The energy distribution of gap states in amorphous hydrogenated silicon has been investi- gated by transient photoconductivity (TPC) and steady-state photoconductivity (SPC) mea- surements. Both TPC and SPC measurements show that the shallow states decrease ex- ponentially with energy away from the conduction-band edge with a characteristic tempera- ture of 300 K whereas the deep states decrease with a characteristic temperature of about 1000 K. The transition energy is located around 0. 3 eV from the conduction-band edge. The density of states at the Fermi level was obtained from frequency dependence of the capacitance (C-co curve) of Schottky diodes. The derived density-of-states distribution as ob- tained from TPC, SPC, and C-co measurements agrees with results obtained from field- effect and C- V measurements. I. INTRODUCTION It is now well established that amorphous hydro- genated silicon (a-Si:H) films are characterized by a distribution of states in the mobility gap. These states which originate from the tailing of the band edges due to the lack of long-range order' and also from dangling bonds, defects, and impurities in the material have pronounced effect on the photovolta- ic properties of the films. Study of these gap states has therefore received a great deal of attention in re- cent years. The most widely used method for determining gap-state distribution is based on the measurement of field effect. The results show a density of around 10' — 10' cm eV ' near the midgap in good quality material; the density increases as one approaches the band edges. Estimates of density of states (DOS) have also been obtained from voltage dependence of capacitance (C-V) and temperature and frequency dependence of capacitance (C T co)-- of Schottky diodes and metal-oxide — semiconductor (MOS) structures. C- V results show a midgap den- sity of around 2)&10' cm eV '; the density in- creases as one approaches the conduction-band edge. C-T-co studies " which provide only DOS at the Fermi level g(EF) give a midgap density close to 5X10' cm eV '. g(E+) has been found to be higher in films where the Fermi level is closer to the conduction-band edge again indicating that DOS in- creases as one moves away from the midgap. Deep-level transient-capacitance spectroscopy (DLTS) has been used extensively for studying deep traps in crystals. ' This technique which relies on the measurement of the time dependence of capaci- tance of Schottky diodes or p-n junctions as traps empty out has recently been used to obtain DOS in a-Si:H. In striking contrast to the results discussed earlier, DI. TS studies' ' show a minimum in DOS at around 0. 4 eV from the conduction-band edge with the gap states rising as one approaches the midgap. The minimum value obtained from DLTS is also much lower — less than 10' cm eV '. It has been argued'" that the higher values of DOS ob- tained from field-effect, C-V, or C-T-co measure- ments, may be due to the influence of surface states. On the other hand, interpretation of DLTS data in amorphous semiconductors with a continuous distri- bution of gap states is quite complex and is limited in its application orily to samples which have been doped to increase their conductivity. Clearly there is a need for measurement of DOS by independent techniques to resolve this problem. The study of transient photoconductivity of amorphous semiconductors provides a useful tool for obtaining information about the states in the gap. When electron-hole pairs are created by pulsed optical excitation, the photocurrent is found to de- crease with time even when the carrier density remains a constant. This can be explained' by the