Charles M. Fortmann

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.

Temperature dependence of H radical etching in the deposition of microcrystalline silicon alloy thin films by HG-sensitized photo-CVD

Abstract The etching of a-Si:H by H radicals in Hg-sensitized photo-CVD is studied as a function of deposition temperature, etch temperature and film hydrogen content. For films deposited at the same temperature, etch rates increased as etch temperature decreased from 230°C to 100°C and were temperature independent for temperatures between 100°C and 40°C. Etch rate increased with decreasing deposition temperature, but did not change when hydrogen content was varied by changing pressure and dilution. Under conditions producing high etch rates, microcrystalline films of Si:B:H and SiGe:H were prepared at temperatures as low as 100°C.

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.

Design considerations for low band gap a-Sige:H alloy solar cells

Abstract The best low band gap (E g = 1.3 eV) a-Sige:alloy grown to date still have relatively poor electronic transport compared to that of unalloyed amorphous silicon. The poor electron transport is found to be at least partially due to poor electron mobility. Solar cell design is used to mitigate the performance loss. Thin a-Sige:H solar cells with Q.E. over 40% at 800nm and over 10% at 900nm have been prepared.

The relationship between hydrogen content, weak bond density and Staebler-Wronski defects in amorphous silicon

Abstract P-i-n solar cells were degraded by current injection at high temperature until steady state was achieved and quenched to room temperature for SW defect determination. The H content in the i-layer of the devices was systematically varied through reactor pressure. The effect of the H content on the steady state SW density is found to be associated with the weak bond density rather than with an alteration in the recombination rate.

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.

Hydrogen content and the goal of stable efficient amorphous-silicon-based solar cells

Abstract Solar cell and film analyses indicate that electron mobility in amorphous hydrogenated silicon-germanium decreases with increasing hydrogen C H and germanium C Ge contents. The hole mobility-lifetime product μτ is less dependent on germanium content than the electron μτ product. Thin (less than 1000 A) graded band gap alloy solar cells were prepared by photochemical vapor deposition with greater than 5% efficiency (at air mass 1.5) and 40% quantum efficiency at 800 nm. Unalloyed a-Si:H with C H values of 7% and 11% having similar annealed state dangling bond densities was prepared by photochemical vapor deposition. Under light exposure or high temperature current injection, high C H materials were markedly less stable.

Picosecond carrier dynamics in a-Si/sub 0.5/Ge/sub 0.5/:H measured with a free-electron laser

A picosecond time-resolved pump and probe experiment has been performed on a thin a-Si/sub 0.5/Ge/sub 0.5/:H film using the short optical pulses generated by the superconducting accelerator-pumped free-electron laser (FEL) at Stanford University. The FEL was tuned at 1.5 mu m. The probe beam was at the fundamental wavelength, and the pump beam was obtained after frequency doubling. The authors found that the transmission decreases due to photogeneration of carriers and then recovers on a 6 ps time scale. This recovery is the signature of ultrafast carrier recombination. It is faster than in a-Si:H for similar conditions. A possible origin of the difference is discussed. >

’’F‐etched a‐SI films’’

A model which suggests that a‐Si films deposited from (SiF4+H2) mixtures are subjected to strong ionic etching during growth is proposed. It is shown that many of the properties of these a‐Si films, such as high conductivity in doped layers and growth, H incorporation, and bandgap data, can be explained by this model. High conductivities are achieved without any detectable F in the film.

Critical Assessment of Sub-Bandgap Primary Photocurrent in a-Si:H Based Solar Cells.

The sub-bandgap primary photocurrent and the solar cell performance of a-Si:H p-i-n devices have been studied before and after light induced degradation. The results indicate significant discrepancy between the two methods when used to estimate the degree of degradation and the defect density in the i-layers. A preliminary explanation is proposed.