The Density-of-State Distribution in Undoped a-Si:H and a-SiGe:H Determined by Heterojunctions with c-Si
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THE DENSITY-OF-STATE DISTRIBUTION IN UNDOPED a-Si:H AND a-SiGe:H DETERMINED BY HETEROJUNCTIONS WITH c-Si HIDEHARU MATSUURA AND KAZUNOBU TANAKA Electrotechnical Laboratory, 1-1-4 Umezono,
Tsukuba,
Ibaraki 305,
Japan
ABSTRACT A novel technique has been proposed for determining the density-of-state (DOS) distribution in the mobility gap of highly resistive amorphous semiconductors, using amorphous/crystalline heterojunction structures. This technique has been tested and applied on undoped a-Si:H and a-SiGe:H films, covering the optical gap (E 0 ) range of 1.30 to 1.76 eV. For undoped a-Si:H with E -l 76 eV, the peak of the midgap DOS distribution has bee Ylocatedst 0.85 eN below the conduction band edge, E , with a value of 5.6xi0 cm eV . For undoped a-SiGe:H (E =1.55 eV) the same has been obtained 0.71 eV below --f le 0 E with a magnitude of 7.9xi0 cm eV . Those midgap states have been found to be correlated with singly-occupied dangling bonds.
INTRODUCTION The electronic properties of hydrogenated amorphous silicon (a-Si:H) films are critically linked with the density and distribution of localized states in the mobility gap of a-Si:H. In order to enhance the performance of a-Si:H based devices, a low density-of-state (DOS) distribution, g(E), in the mobility gap, is essential. Measurement of g(E) and an understanding of the nature of the gap states are, therefore, very important. Many techniques have been developed to determine g(E). These include both optical and electrical methods (1,2), while they have some limitations in their application. In the present work, we have focused on determining g(E) below the Fermi level (E ) in undoped a-Si:H films and also in undoped hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) films. These films play an important role in enhancing the performance of amorphous solar cells. A few groups have sought to infer the energy location of the dangling bonds in a-SiGe:H (3-6). Prior to this, the density of midgap states for undoped a-Si:H was obtained from the capacitance-voltage (C-V) characteristics of undoped (i.e., n-type) a-Si:H/p-type crystalline silicon (p c-Si) heterojunction structures under 100 kHz (7). This method will be called a steady-state heterojunctionmonitored capacitance (HMC) method in the following sections. The steady-state HMC method has been applied to undoped a-SiGe:H with the optical gap (E ) between 1.30 and 1.70 eV. Moreover, it has been demonstrated that the g(E) below EF in amorphous films (1.30 eV < E0 1.76 eV) can be determined from the experimental results of transient HMC measurements.
THEORY OF HETEROJUNCTION-MONITORED CAPACITANCE METHOD Steady-state regime The depletion region formed by an undoped a-Si:H (or a-SiGe:H)/p c-Si heterojunction is considered. When a reverse bias voltage (VR) is applied, it produces space-charge layers both in amorphous and crystalline semiconductors. Under the assumption that this p c-Si has only shallow acceptors, the space charge in the p c-Si is formed by negatively-charged acceptors. However, the amorphou
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