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Recent developments with group III nitrides suggest AlxGa1-xN based LEDs can be new alternative commercially viable deep ultra-violet light sources. Due to a significant difference in the lattice parameters of GaN and AlN, AlxGa1-xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE) using a novel RF plasma source. Thick wurtz-ite AlxGa1-xN films were grown by PA-MBE on 2-inch GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1-xN samples. Growth rates of AlxGa1-xN up to 3 µm/h have been demonstrated. Our novel high efficiency RF plasma source allowed us to achieve free-standing bulk AlxGa1-xN layers in a single day's growth, which makes our MBE bulk growth technique commercially viable. [ABSTRACT FROM AUTHOR]

We have studied time-integrated and time-resolved photoluminescence (PL) spectra and the transport properties of cubic GaN layers doped with different amounts of either magnesium (p-type) or silicon (n-type). In Mg doped material a broad blue peak was observed whose origin in cubic material has not been well established. Our temperature dependent and time resolved PL results as well as the observation of strong self-compensation in Mg-doped cubic GaN have allowed us to present a model for the origin of this emission in cubic material. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Transmission Electron Microscopy of Mg doped GaN1-xAsx samples, grown by MBE at low temperatures, show substantial structural changes for samples that are semi-insulating and those with high or low conductivity. The conductive samples show p-type conductivity as evidence from the positive thermopower values. All the Mg doped samples show phase segregation: cubic GaAs and GaN grains (a mixture of cubic and some hexagonal) phases within an amorphous matrix. The best conductive samples show cubic GaAs grains with high density of stacking faults embedded into an amorphous matrix. The samples that are less conductive have lower ratio of the amorphous to the crystalline phase of the samples and much lower density of stacking faults. Higher Mg concentration is expected in the amorphous parts of the samples The semi-insulating samples that have either low Mg concentration or low As show grains of GaAs and GaN attached to each other with no evidence of the amorphous phase between them. There are no SFs in these grains. It is possible that the presence of the GaN between the GaAs grains lead to semi-insulating material properties since p-type doping of GaN is more difficult. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We have discovered materials with a wide tunability of the band gap that opens the possibility to achieve a desirable material for multi-junction solar cells. Two different alloy systems have been studied. GaN1-xAsx alloys over the entire composition range were grown by plasma-assisted MBE at low temperature on crystalline and amorphous (glass) substrates. For growth on sapphire the alloys are amorphous in the composition range of 0.171-xAsx alloys ranges from 3.4-0.7 eV, covering a large part of the solar spectrum, therefore this material is potentially useful for solar energy conversion. The substitution of N by Bi in GaN forming GaN1-x Bix is extremely difficult since Bi and N have even larger differences in size and electronegativity. TEM studies of GaN1-xBix with x up to 0.2 show that the alloy is not entirely amorphous. Small crystallites embedded in an amorphous matrix are observed and Bi segregation was confirmed by Electron Energy Loss studies. A strong reduction in band gap is observed in samples with increasing Bi, suggesting the formation of a random alloy but low energy absorption tail may be related to the presence of the crystallites observed by TEM (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]