In order to determine an energetic distribution of localized states, a new method is proposed. The method is based on the analysis of the phase shift between a sinusoidally modulated excitation light and its inducing photocurrent. The theoretical relation between the phase shift and its relevant localized states is derived assuming a trap-limited band conduction of unipolar photocarriers. The energetic profile of the localized states can be calculated from a modulation frequency dependence of the phase shift. The method has been applied to CdS crystal and the validity has been confirmed. It is suggested that the method is useful for amorphous semiconductors in which the localized states are distributed quasi-continuously in the band gap.
Electrotechni cal_ Laboratory L-1,-4 Umezono, Sakura-mura, N'ihari-gun, Ibaraki 305 * 0n leave from Sharp Corporation, 26L3-L lchinomoto-cho, Tenri-c]-ty, Nara 632 We have measured C-V characteristics and temperature dependence of J-V characteristics of undoped a-Si:H heterojunctions formed on p-type c-Si substrates with different resistivities. It has been found out that an abrupt heterojunction model is valid for a-Si:H/p c-Si heterojunctions, and the electron affinity of a-Si:H has been estimated as 3.93 t 0.07 eV from C-V characteristics. The forward cument of all the junctions studied.shows voltage and temperature dependence expressed as exp(-M ^/kT) x exp(Av), where ot". and A are constants independent of vg]{age and templratu?5. in. reverse current is-p*roportional to exp(-AE^-/kT) x (V^_V)'r', where M is constant independent of voltage and temperature and Vo-is the diffusion voltage.
We have investigated the current-voltage characteristics of metal/undoped a-Si: H/n
+ (or p
+) c-Si structures using Mg, Au, Pt and Al. It has been indicated that the energy band of undoped a-Si: H shows a downward bending against Mg, being analogous to n
+
a-Si: H/undoped a-Si: H contact, while the band shows an upward bending against Au, Pt and Al. Mg/undoped a-Si: H contact is found to allow injection of electrons into the conduction band of a-Si: H, providing us a good ohmic contact property. This contact does not exhibit the thermal-degradation at least up to 100°C.
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