M J Kearney

The effect of alloy scattering on the mobility of holes in a quantum well

The effect of alloy scattering on the mobility of holes in a quantum well is considered from a theoretical perspective. Issues relating to the definition and value of the alloy potential are discussed, and results are presented showing how the alloy scattering component of the overall mobility varies with temperature, carrier density and alloy concentration. Screening is allowed for and found to be important at low temperatures. By considering other scattering mechanisms such as interface impurities, surface roughness and acoustic phonons, we argue that alloy scattering is not a significant limiting factor as regards device applications, contrary to recent claims.

On the importance of considering the incident spectrum when measuring the outdoor performance of amorphous silicon photovoltaic devices

Conventional measurement practice for the outdoor performance evaluation of solar cells does not make use of the complete spectrum, relying instead on the total irradiance as measured, say, with a pyranometer. In this paper it is shown that this can result in significant errors for solar cells having wide band gaps, in particular, for amorphous silicon solar cells. Two effects are investigated. The first relates to quantifying the typical errors associated with instantaneous measurements; what one might term the calibration of devices. The second relates to quantifying the impact of neglecting variations in the spectrum on the estimation of the annual energy production. It is observed that the fraction of the spectrum falling in the spectrally useful range for amorphous silicon can vary by as much as +10% to −15% with respect to standard test conditions at the test site used in this study, which translates directly into performance variations of similar magnitude. The relationship between changes due to spectral variations as opposed to variations in device temperature is also investigated. The results show that there is a strong case for investigating spectral effects more thoroughly, and explicitly including the measurement of the spectral distribution in all outdoor performance testing.

The influence of the measurement environment on the accuracy of the extraction of the physical parameters of solar cells

This article concerns the influence of measurement conditions on the extraction of a solar cells equivalent circuit parameters. Influences previously not investigated are considered, thus helping to define suitable measurement strategies. The influences of measurement environments are investigated, as is the influence of the fitting algorithm chosen. It is shown that the number of measurement points for the current-voltage characteristic can have an important effect on the accuracy of the parameters extracted. The stability of the system is of minor importance, as long as the variations in the measurement conditions are monitored. We also show that the Marquardt-Levenberg algorithm using a least squares error criterion and a hybrid algorithm employing an area criterion outperform other choices of fitting algorithm.

Scattering mechanisms affecting hole transport in remote‐doped Si/SiGe heterostructures

Boron modulation‐doped Si/SiGe heterojunctions have been grown by molecular beam epitaxy. The two‐dimensional hole gas formed along the heterojunction, just inside the alloy, has a sheet density in the range 2–5×1011 cm−2 and a typical mobility at 5 K of 2000 cm2 V−1 s−1. An explanation for the magnitude of the mobility is sought by considering likely scattering mechanisms, namely those due to remote impurities, interface roughness, alloy disorder, and interface impurities. A self‐consistent model is used to determine the sheet density in terms of structural and energy parameters and dopant concentrations in the heterostructure. It is shown that the presence of negatively charged impurities at the heterojunction provides the basis for a consistent interpretation of the experimental results.

On the low-temperature mobility of holes in gated oxide Si/SiGe heterostructures

A detailed comparison is made between theory and experiment for the low-temperature mobility of holes in gated oxide, coherently strained Si/SiGe heterostructures. We conclude that the mobility is mainly limited by interface impurities, conventional surface roughness and strain fluctuations; by contrast, we argue that alloy scattering is comparatively weak. Comments regarding possible mobility degradation due to oxide formation are also made.

Performance of amorphous silicon double junction photovoltaic systems in different climatic zones

To date the majority of investigations into the performance of amorphous silicon photovoltaic systems have been limited to single sites, and therefore the conclusions from such studies are unlikely to be as generic as they might at first appear. This paper compares data collected from different systems across the world in Brazil, Hong Kong, Spain, Switzerland, and the United Kingdom. All systems have been operating for a number of years, and are employing double junction amorphous silicon devices of a similar age manufactured by RWE Solar. The data are analysed for performance variations reflecting the different climatic zones, and the variations are explained on the basis of operating temperature, incident irradiation and seasonal spectral shift.

Analysis of hole mobility and strain in a Si/Si0.5Ge0.5/Si metal oxide semiconductor field effect transistor

The hole mobility has been measured in a metal-oxide semiconductor field-effect transistor featuring a 6 nm Si0.5Ge0.5 conduction channel. Physical characterization and analysis of the electrical properties confirm that the channel is fully strained. At a total sheet carrier concentration, ps, of 1012 cm−2, the 300 K mobility of 220 cm2 V−1 s−1 is double that of a Si control device (and at 4 K has a peak value of 1800 cm2 V−1 s−1). This improvement is largely maintained up to ps ~ 1013 cm−2, despite the onset of conduction in a parallel Si channel at the oxide interface. Comparing the measurements with calculations suggests that the mobility is affected more strongly by interface roughness than by alloy scattering.

Experimental investigation of spectral effects on amorphous silicon solar cells in outdoor operation

The effect of varying spectrum on PV output is often underestimated in the case of amorphous silicon photovoltaic devices. This paper gives an indication of the order of magnitude of the seasonal variation of the useful irradiance in a maritime climate and also shows that this will involve a direct change in efficiency. This can be expected to be in the range of 15 percent around the annual average, thus explaining the seasonal performance. The spectral effects are investigated by distinguishing between a primary and a secondary effect. The primary effect is dependent on the availability of useful spectral irradiance while the secondary effect depends also on the spectral composition of the light in the useful range. It is shown that the secondary effect is especially significant for double junction devices.

Direct evidence for a piezoelectriclike effect in coherently strained SiGe/Si heterostructures

A hybrid acoustic spectroscopy technique has been used to demonstrate the (reversible) conversion of high frequency electric fields into longitudinal acoustic waves within a modulation‐doped pseudomorphic Si/Si0.88Ge0.12/Si heterostructure. This provides compelling evidence for the existence of a piezoelectriclike coupling within such structures.

The transport-time to state-lifetime ratio in semiconductor quantum-well alloys: a multiple scattering analysis

A theoretical study is made of how the transport-time to state-lifetime ratio ( t / s ) varies as a function of sheet carrier density in semiconductor quantum-well alloys at low temperature. Various sources of ionized impurity scattering are considered, in addition to alloy scattering, using a self-consistent multiple-scattering theory proposed originally by Gold and Gotze. To begin, we present the theory of Gold and Gotze in its most transparent form. We then examine electron transport in In1-x Gax As quantum wells and hole transport in Si1-x Gex quantum wells. The interplay between alloy scattering and ionized impurity scattering leads to interesting behaviour, and to a significant reduction in the value of t / s as compared to systems where alloy scattering is not a factor. Multiple-scattering effects become progressively more important as the sheet carrier density decreases, and indications are given as to when lowest-order transport theory ceases to be adequate.