Thomas Unold

Deep defect structure and carrier dynamics in amorphous silicon and silicon-germanium alloys determined by transient photocapacitance methods

A detailed comparison between transient junction photocurrent and photocapacitance spectra can be used to examine separately the majority and minority carrier processes in amorphous semiconductors. Such methods are employed both on intrinsic samples of hydrogenated amorphous silicon (a-Si:H) and also amorphous silicon-germanium alloys (a-Si, Ge:H) with a Tauc gap near 1.3 eV. It is demonstrated how this method can be used not only to map out the deep defect distribution in such samples, but also to determine the effective μτ products for the minority carrier motion.

Enhancement of light-induced degradation in hydrogenated amorphous silicon due to carbon impurities

The effect of carbon impurities in a‐Si:H samples at low concentrations (1 at. % to less than 0.1 at. %) has been investigated using capacitance profiling methods on samples whose carbon content was intentionally modulated spatially during growth. We have found a strong correlation between the secondary‐ion mass spectroscopy determined carbon content and the susceptibility of these samples to light‐induced metastable defect creation. No correlation was found with respect to the variation in total hydrogen content of these samples.

Electronic mobility gap structure and deep defects in amorphous silicon‐germanium alloys

Amorphous hydrogenated silicon‐germanium alloys have been studied using a variety of junction‐capacitance techniques to establish the dependence of the mobility gap electronic structure and the density of deep defects on the germanium content. The Urbach tail slope is observed to be nearly constant over the whole alloy range. The energy position of the dominant deep defect band near midgap is deduced and evidence for a shallower unoccupied defect band undergoing a large lattice relaxation is also observed. The total density of deep defects is found to increase exponentially with increasing germanium content and the details of this increase are shown to be consistent with a weak bond to dangling bond conversion model.

Density of states and carrier dynamics in amorphous silicon germanium alloys and amorphous germanium

Several a-Si1−xGex:H alloys and a-Ge:H grown by PECVD have been investigated using transient and steady state junction capacitance techniques. From transient photocapacitance and transient photocurrent measurements we estimate (μτ)h of order 10−10cm2/V and deep defect densities of order 1016cm−3 for our best a-Si1−xGex:H sample. The a-Ge:H samples investigated exhibited a large defect density (Nd℞1018cm−3) but a low Urbach parameter (Eu=50meV). We found a large role of lattice relaxation for the dominant deep defect band in a-Si1−xGex:H.

Electronic mobility gap structure and the nature of deep defects in amorphous silicon-germanium alloys grown by photo-CVD

Abstract Amorphous hydrogenated silicon-germanium alloys grown by photo-CVD have been studied using a variety of steady-state and transient junction-capacitance techniques. The dependence of the electronic mobility gap structure, the density of deep defects, and the carrier trapping properties on the germanium content has been investigated systematically. The Urbach tail slope is found to be nearly constant over the whole alloy range. The dominant defect band is found to track the midgap energy position, and the density of deep defects increases exponentially with increasing germanium content. These results are fully consistent with weak bond breaking theories and suggest that the quality of amorphous silicon-germanium alloys and a-Ge:H is inherently inferior to pure a-Si:H materials.

The carbon impurity dependence of light induced metastable effects in hydrogenated amorphous silicon

Abstract The effect of carbon impurities on the creation of light induced metastable defects in undoped a-Si:H has been investigated using capacitance profiling techniques to characterize samples with spatially varying carbon content. It was found that there was a significant correlation between the increase in metastable states with light-soaking and the impurity level within different regions of each sample. We conclude that even trace levels of carbon impurities (at levels of 0.2–2.0at%) increase the susceptibility to light-induced metastable degradation.

Light bias CPM study of the density of states in n-type amorphous silicon

The authors measure subgap absorption on n-type amorphous silicon using the absolute constant photocurrent method. They find that for typical monochromator probe beam intensities the measurement is not significantly influenced by lifetime changes. When the measurement is performed under light bias, an apparent increase in the defect absorption coefficient is observed, but no change in the photoexcitation threshold or spectral shape of the absorption band is seen. The authors show that this increase is likely due to a bias-light amplification of spectrally dependent lifetime changes. The measurements suggest a larger electron capture cross section of positive valence band tail states compared to neutral dangling bonds.

Electronic properties of amorphous silicon and amorphous silicon‐germanium alloys

We review results obtained by employing two junction capacitance methods—drive‐level capacitance profiling and transient photocapacitance spectroscopy—to a‐Si:H and a‐Si,Ge:H alloy samples. We discuss how these measurements disclose the dominant deep defect bands and their carrier trapping dynamics in both a‐Si:H and the alloy samples. In particular these methods allow us to separate electron from hole processes and provide evidence for the existence of two distinct defect bands in the alloys. The deduced dependence of total defect density vs Ge fraction is found to agree with the predictions of a spontaneous defect formation model.

Effect of C Impurities in a-Si:H as Measured by Drive-Level Capacitance, Photo Current, and Electron Spin Resonance

The effect of C impurities in a-Si:H in levels of 0.4 to 2.6 at. % were studied over a wide range of metastable defect densities. Three complimentary experimental techniques [electron spin resonance (ESR), drive-level capacitance (DLC) and photo-current] were employed to track the materials defect density with light soaking and annealing, as well as Urbach energies, midgap absorption and mobility gaps energies as a function of the C content. Our results show C impurities have a definite effect on the initial and saturated defect densities, as well as the midgap absorption and Urbach energies at levels 1 at. % and above. The results indicate that C acts mainly as a center for increased disorder in the material which results in an increase in the bandtail widths, and consequently an increase in intrinsic defects. Comparison to the ESR and drive-level data show an excellent agreement between these two techniques in determining the bulk defect densities in a-Si:H.

The effects of impurities on the light-induced degradation of hydrogenated amorphous silicon

Abstract Amorphous silicon samples with an intentionally modulated impurity content at concentrations below 1 at.% were grown by the glow discharge deposition technique. The impurity profiles were determined by secondary-ion mass spectroscopy (SIMS) measurements and the spatial distributions of deep defects were determined by junction capacitance profiling measurements before and after light-induced degradation. For the case of intentionally added carbon, we found a significant enhancement in the concentration of light induced defects in regions with higher impurity levels such that a carbon level of 0.5 at.% leads to an additional (1–2) × 10 16 cm −3 increase in defects after light soaking.