Paramagnetic centers in hydrogenated microcrystalline silicon, c-Si:H have been studied using dark and light-induced electron-spin resonance ͑ESR͒. In dark ESR measurements only one center is observed. The g values obtained empirically from powder-pattern line-shape simulations are g ʈ ϭ2.0096 and g Ќ ϭ2.0031. We suggest that this center may be due to defects in the crystalline phase. During illumination at low temperatures, an additional ESR signal appears. This signal is best described by two powder patterns indicating the presence of two centers. One center is asymmetric (g ʈ ϭ1.999, g Ќ ϭ1.996), while the other is characterized by large, unresolved broadening such that unique g values cannot be obtained. The average g value for this center is 1.998. The light-induced signal, which we interpret as coming from carriers trapped in the band tails at the crystalline grain boundaries, remains for at least several minutes after the light is turned off. Although the time scales of the decay curves are very different for two samples prepared by different techniques, both decays can be fitted using the assumption of recombination due to distant pairs of electrons and holes trapped in localized band-tail states.
This paper presents the development of low dark current amorphous silicon (a-Si:H) based heterojunction photodiodes. A series of p-i-n and n-i-p structures have been deposited by plasma-enhanced chemical vapor deposition (PECVD). Junction properties and carrier transport are investigated in terms of dark and light current-voltage characteristics, time dependence of the dark current, and spectral photoresponse measurements. It is demonstrated that a thin (∼4 nm) undoped a-SiC:H buffer layer introduced between the p and i layers reduces the leakage current and improves the diode ideality factor. A dark current density of ∼10 pA/cm2 at reverse bias of 1 V was achieved for the n-i-p structure. Optimization of device design for further improvement of dark current and photoresponse is discussed.
The pulsed PECVD involves modulation of standard 13.56 MHz RF plasma in the kHz range. This allows an increase in the electron density during the 'ON' cycle, while in the 'OFF' cycle neutralizing the ions responsible for dust formation in the plasma. In this work, we report the development of state of the art nano-crystalline Si (nc-Si:H) materials using Pulsed PECVD technique with 220 crystallite orientation, grain size of ~200 Å, low O concentration and a minority carrier diffusion length, L d , of ~1.2 µm. The crucial effects of p/i interface and the incubation layer have been investigated and an efficiency of ~7.5% for single junction nc-Si:H p-in device has been achieved for the i-layer thickness of 1.4 µm, using non optimized textured substrates.
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