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To explore the origin of low conversion efficiency of GaAsN solar cells, the effects from a key deep level (E1) at about 0.3–0.4 eV below the conduction band and interface states between the GaAsN emitter and GaAs front surface field layer were investigated by numerical simulation. Our results show that a high deep level concentration in GaAsN layers and a large interface recombination velocity are necessary to fit the experimental data. Interface recombination plays an important role in reducing the quantum efficiency in the short-wavelength region. Its effect can be weakened by applying a thin emitter and a high doping level in the GaAs surface field layer. The abundant existence of E1 is responsible for the high contribution of Shockley–Read–Hall recombination from the space-charged region. Its density should be reduced to

We compared the N incorporation and optical emission in GaAsN epilayers grown on (3 1 1)A/B and (1 0 0) GaAs substrates using a chemical beam expitaxy system. Over the growth-temperature range 420 –460 °C, N composition was enhanced 2–3 times for the epitaxial growth following [3 1 1]B orientation, but reduced in the [3 1 1]A direction. Both (3 1 1) A and B substrates are effective to weaken the photoluminescence emission from the deep levels as compared with the (1 0 0) plane. The deep-level emission can be further suppressed for all substrates by increasing the growth temperature and/or performing postgrowth annealing. However, in contrast to the continuous increase in total emission intensities of (3 1 1)B sample, a decreasing tendency was recorded for (3 1 1)A with the rise in growth temperature. The optimum growth temperature and annealing conditions for better crystal quality were found to depend on the growth orientation and surface polarity. These results present a potential approach to improving the N incorporation efficiency in Ga(In)AsN materials through adopting high-index substrates such as (3 1 1)B.

Magneto-transport in an AlGaN/GaN heterostructure has been studied at 4.2K and it was found that the magneto-resistance is negative at low fields. The negative components disappeared by inserting a thin AlN film between the AlGaN barrier layer and the GaN layer. The negative component is attributed to the presence of alloy disorder at the hetero-interface. It increases linearly followed by saturation, which is characteristic to the percolation transport in random potential.

Low-field Hall and Shubnikov–de Haas (SdH) measurements were performed on two-dimensional electron gas of Al0.24Ga0.76N/GaN and Al0.24Ga0.76N/AlN/GaN heterostructures at a low temperature. A dramatic improvement in electron mobility is observed in Al0.24Ga0.76N/AlN/GaN heterostructures with 1 and 2 nm thick AlN interlayers. A further increase in the AlN thickness degrades the transport behaviour. Transport and quantum scattering times were derived from the corresponding Hall mobility and the oscillatory part of the SdH signal. The ratio of the transport scattering time and the quantum scattering time increases from 3.87 in the heterostructure without the AlN interlayer to 14.34 with the 1 nm AlN interlayer and to 12.45 with the 2 nm AlN interlayer. A modified scattering calculation due to charged dislocation with a finite length suggests that dislocation related long-range scattering event tends to dominate both transport and quantum scattering times when a thin AlN interlayer is introduced.

Magnetotransport measurements were performed on a series of AlxGa1−xN/GaN heterostructures with different Al compositions (x = 0.15, 0.20 and 0.30) at 4.2 K. Adopting a fast Fourier transform method, we analysed the Shubnikov–de Hass oscillations due to the two-dimensional electron gas to derive the quantum scattering time (τq). It was found that the quantum scattering time in the ground subband decreases with increasing Al composition: 0.194 ps (x = 0.15), 0.174 ps (x = 0.20) and 0.123 ps (x = 0.30), respectively. To discern the predominant scattering process, the scattering time limited by interface roughness, the residual impurity and the alloy disorder were investigated numerically by including inter-subband scattering. We found that enhanced interface roughness scattering dominates both the transport and quantum scattering time in the ground subband.