J. D. Cohen

Structural, defect, and device behavior of hydrogenated amorphous Si near and above the onset of microcrystallinity

High-hydrogen-diluted films of hydrogenated amorphous Si (a-Si:H) 0.5 μm in thickness and optimized for solar cell efficiency and stability, are found to be partially microcrystalline (μc) if deposited directly on stainless steel (SS) substrates but are fully amorphous if a thin n layer of a-Si:H or μc-Si:H is first deposited on the SS. In these latter cases, partial microcrystallinity develops as the films are grown thicker (1.5–2.5 μm) and this is accompanied by sharp drops in solar cell open circuit voltage. For the fully amorphous films, x-ray diffraction (XRD) shows improved medium-range order compared to undiluted films and this correlates with better light stability. Capacitance profiling shows a decrease in deep defect density as growth proceeds further from the substrate, consistent with the XRD evidence of improved order for thicker films.

Transient photocapacitance and photocurrent studies of undoped hydrogenated amorphous silicon

We have applied transient photocapacitance and transient junction photocurrent measurements to the study of undoped hydrogenated amorphous silicon (a‐Si:H) films, and find that the electronic optical transition from the dominant deep defect is very similar in energy to the D−→D0+e optical transition identified in n‐type doped a:Si:H films. In addition, we have observed a competing hole thermal transition, and we have obtained estimates of its thermal emission rate and of the thermal gap. We have used the difference between photocapacitance and photocurrent in the valence‐band tail region to determine the quantity (μτ)hNT at different temperatures. Finally, we have observed that light‐induced metastable defects have a hole capture cross section significantly larger than that of the intrinsic defects.

Saturated defect densities of hydrogenated amorphous silicon grown by hot-wire chemical vapor deposition at rates up to 150 Å/s

Hydrogenated amorphous-silicon (a-Si:H) is grown by hot-wire chemical vapor deposition (HWCVD) at deposition rates (Rd) exceeding 140 A/s (∼0.8 μm/min). These high rates are achieved by using multiple filaments and deposition conditions different than those used to produce our standard 20 A/s material. With proper deposition parameter optimization, an AM1.5 photo-to-dark-conductivity ratio of 105 is maintained at an Rd up to 130 A/s, beyond which it decreases. In addition, the first saturated defect densities of high Rd a-Si:H films are presented. These saturated defected densities are similar to those of the best HWCVD films deposited at 5–8 A/s, and are invariant with Rd up to 130 A/s.

Junction capacitance studies of deep defects in undoped hydrogenated amorphous silicon

Abstract Recent results obtained by applying two methods, drive-level capacitance profiling and transient photocapacitance spectroscopy, to the study of deep defects in undoped hydrogenated amorphous silicon (a-Si:H) are reviewed. From the first method, we illustrate how it is possible to obtain the density, energy distribution, electronic occupation and spatial distribution of light-induced metastable deep defects for a set of hydrogenated amorphous silicon samples with well-characterized impurity levels. These results strongly suggest an extrinsic carbon-related component to the degree of instability in some films. We also illustrate how a detailed comparison between transient photocapacitance and photocurrent spectra provides specific information about gap state dynamics and carrier-transport processes in a-Si:H films and, in particular, metastable changes in those properties.

Evidence for hole traps at the amorphous silicon/amorphous silicon–germanium heterostructure interface

Voltage-pulse stimulated capacitance transient measurements on a series a-Si:H/a-Si, Ge:H devices disclose a large density of hole traps at or very close to the heterojunction interface. The transient signal magnitude is independent of temperature or applied bias, ruling out charge polarization effects or a defect creation process caused by the filling pulse. While the areal density of such hole traps is considerable (within a factor of 2 of 1011 cm−2 for all samples) these traps do not appear to behave as recombination centers. Also, the treatment of the a-Si:H/a-Si,Ge:H interface during growth can significantly alter the concentration of these traps.

High quality a-Si:H films grown at high deposition rates

Intrinsic a-Si:H samples were grown with and without hydrogen (H{sub 2}) dilution of silane at different growth rates. They find that the dilution leads to a considerable reduction in the defect density, in particular at high growth rates. The defect density is particularly low for samples grown using H{sub 2} dilution conditions at growth rates as high as 10 {angstrom}/sec. Using transient photocapacitance measurements they find evidence for a small concentration of microcrystallites embedded in the amorphous films. An increase in the microcrystalline fraction correlates with a decrease in the defect density.

Correlation of optical and thermal emission processes for bound‐to‐free transitions from mobility gap states in doped hydrogenerated amorphous silicon

We have employed photocapacitance techniques to n‐type doped a‐Si: H Schottky diode samples and have deduced the density of states, g(E), in the upper half of the mobility gap versus the optical detrapping energy. This photocapacitance g(E) is found to be in good agreement with the density of states deduced by deep‐level transient spectroscopy measurements performed on the same samples. Small differences in the energy scale are observed in some samples which can be interpreted in terms of lattice relaxation effects. We have also applied sub‐band gap optical excitation in conjunction with the observation of capacitance transient measurements following voltage pulse excitation. This has allowed us to observe the direct competition between the thermal detrapping of electrons from gap states with optical detrapping from the same states. These measurements have allowed us to observe the thermal enhancement of the optical detrapping processes in a‐Si : H. They have also allowed us to explicitly measure for the ...

Capacitance Studies of Light-Induced Effects in Undoped Hydrogenated Amorphous Silicon

The effects of light saturation on the properties of undoped a-Si:H films were studied by a new capacitance profiling technique which can be used to directly determine changes in the dangling bond density of states near midgap. Coplanar conductivity and capacitance vs. temperature measurements save the changes in activation energies for electrical conductivity. These studies indicate that, while substantial increases in the dangling bond densities are observed for most samples, the detailed behavior of the light induced changes in these films are inconsistent with the creation of such defects by breaking weak valence band tail states.

Study of the Density of Bulk Gap States in Undoped a-Si:H by Depletion Width Modulation Electron Spin Resonance

The density of gap states in the bulk of intrinsic a-SiH has been probed using depletion width modulation electron spin resonance spectroscopy (DWM ESR) as well as complementary capacitance techniques. Our sample consists of a 2.85 micron thick layer of undoped a-Si:H sandwiched between a large area Pd Schottky barrier contact and a p + c-Si substrate, allowing charge-counting capacitance and spin-counting ESR measurements to be performed on the same sample. DWM ESR measures the spin change that occurs when electrons are removed from gap states by increasing reverse bias on a Schottky barrier. The phase of the signal indicates whether unpaired spins are created or destroyed with increasing bias, thus revealing the defect charge state at the Fermi level The DWM ESR signal from our sample indicates that removing electrons from gap states leads to a reduction of the spin signal. This is qualitatively consistent with a band of Do states near E F o . However, for all metastable states and at all temperatures studied, the magnitude of the DWM ESR signal account for only about 14% of the responding charge as determined by junction capacitance. This would occur only if the majority of the filled bulk gap states were doubly occupied with a small correlation energy U eff . In that case the majority of transitions caused by depletion width modulation would be D − /D + which would produce no spin change. Model calculations show that the small observed D o /D + DWM ESR signal would indeed result from the occupation statistics for a near zero value of U eff at the moderate temperatures of the experiment. Comparison with our data sets the overall limits If −20meV eff

The Effect of Oxygen Contamination on the Electronic Properties of Hot-Wire CVD Amorphous Silicon Germanium Alloys

A series of four a Si,Ge:H alloy samples with Ge fractions near 30 at.% were deposited by hot-wire CVD (HWCVD) using a Ta filament maintained at 1800oC. During film growth, the level of oxygen contamination was varied from less than 10 19 cm −3 to roughly 5 × 10 20 cm −3 using a controlled air-leak. The electronic properties of these films were then characterized using transient photocapacitance (TPC) and transient photocurrent (TPI) spectroscopy, as well as the drive-level capacitance profiling (DLCP) techniques. We observed an unexpected systematic improvement of the electronic properties of these HWCVD a Si,Ge:H with increasing oxygen impurity level, which was reflected by a decrease in the deduced Urbach energies. Comparing these with films co-deposited on stainless-steel versus p+ c-Si substrates, we found significantly better electronic properties in the latter case. Comparisons of the TPC and TPI spectra indicated a very high level of hole collection, consistent with these narrow bandtail distributions.