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Thin films of GaNBi alloys with up to 12.5 at.% Bi were grown on sapphire using low-temperature molecular beam epitaxy. The low growth temperature and incorporation of Bi resulted in a morphology of nanocrystallites embedded in an amorphous matrix. The composition and optical absorption shift were found to depend strongly on the III:V ratio controlled by the Ga flux during growth. Increasing the incorporation of Bi resulted in an increase in conductivity of almost five orders of magnitude to 144 Ω-cm−1. Holes were determined to be the majority charge carriers indicating that the conductivity most likely results from a GaNBi-related phase. Soft x-ray emission and x-ray absorption spectroscopies were used to probe the modification of the nitrogen partial density of states due to Bi. The valence band edge was found to shift abruptly to the midgap position of GaN, whereas the conduction band edge shifted more gradually.
We have studied the low-temperature growth of GaNAs layers on sapphire substrates by plasma-assisted molecular beam epitaxy. We have succeeded in achieving GaN1-xAsx alloys over a large composition range by growing the films at temperature much below the normal GaN growth temperatures with increasing the As2 flux as well as Ga:N flux ratio. We found that the alloys with high As content x>0.1 are amorphous. Optical absorption measurements reveal a continuous gradual decrease of band gap from ˜3.4 eV to ˜1.4 eV with increasing As content. The energy gap reaches its minimum of ˜1.4 eV at the x˜0.6-0.7. For amorphous GaAsN alloys with x<0.3 the composition dependence of the band gap follows the prediction of the band anticrossing model developed for dilute alloys. This suggests that the amorphous GaN1-xAsx alloys have short-range ordering that resembles random crystalline GaN1-xAsx alloys. Such amorphous GaN1-xAsx alloys with tunable electronic structure may be useful as photoanodes in photo-electrochemical cells for hydrogen production.
High-energy particle irradiation has been used to control the free electron concentration and electron mobility in InN by introducing native point defects that act as donors. A direct comparison between theoretical calculations and the experimental electron mobility suggests that scattering by triply-charged donor defects limits the mobility in irradiated samples across the entire range of electron concentrations studied. Thermal annealing of irradiated films in the temperature range 425°C to 475°C results in large increases in the electron mobility that approach the values predicted for singly-ionized donor defect scattering. It is suggested that the radiation-induced donor defects are stable, singly-charged nitrogen vacancies, and triply-charged, relaxed indium vacancy complexes that are removed by the annealing.
Growth of non-polar III-nitrides has been an important subject recently due to its potential improvement on the efficiency of III-nitride-based opto-electronic devices. Despite study of non-polar GaN and GaN-based heterostructures, there are few reports on epitaxial growth of non-polar InN, which is also an important component of the III-nitride system. In this study, we report heteroepitaxial growth of non-polar InN on r-plane sapphire substrates using plasma-assisted molecular beam epitaxy. It is found that when a GaN buffer is used, the following InN film appears to be non-polar (1120) a-plane which follows the a-plane GaN buffer. The room temperature Hall mobility of undoped a-plane InN is around 250 cm2/Vs with a carrier concentration around 6×1018 cm-3. Meanwhile, if InN film is directly deposited on r-plane sapphire without any buffer, the InN layer is found to consist of a predominant zincblende (cubic) structure along with a fraction of the wurtzite (hexagonal) phase with increasing content with proceeding growth.
In this study, InN films with thickness up to 7.5 micron were prepared by molecular beam epitaxy (MBE) on (0001) sapphire and quasi-bulk GaN templates. Previously it has been challenging to grow InN film much beyond 1 micron because the growing surface tended to become rough. Techniques to overcome this limit have been developed. Various buffer techniques were used and compared to optimize the epitaxial growth. It was found that with increasing film thickness, Hall mobility will monotonically increase, while carrier concentration decreases. Hall mobility beyond 2100 cm2/Vs with carrier concentration close to 3×1017 cm−3 was obtained at room temperature. Compared with the lowest carrier concentration ∼2×1018 cm−3 obtained on thin InN films grown at the same condition, the conclusion is that impurities from the growth environment are not responsible for the high background doping of InN. Instead, some structural defects or substrate/buffer impurities may be the major source of the unintentional doping, which can be reduced by growing thicker films.
Some results on Mg and Be doping of InN will be reported as well. To date, all Mg and Be doping attempts have resulted in n-type material.
We propose a new approach for modeling of impurity diffusion at semiconductor heterointerfaces. The approach is based on the notion of a common energy reference for highly localized defects. It is shown that in the kick-out process, the segregation of group II acceptors is controlled by the valence band offsets among different constituent layers of the heterostructure. Extensive numerical modeling of the diffusion provides an explanation for the experimentally observed strong segregation of Zn and Be acceptors in the lattice matched InP/InGaAs, InP/InGaAsP and GaAs/AlGaAs heterostructures.
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