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To aid the development of AlN-based optoelectronics, it is essential to identify the defects that cause unwanted light absorption and to minimize their impact. Using hybrid functional calculations, we investigate the role of native defects and their complexes with oxygen, a common impurity in AlN. We find that Al vacancies are the source of the absorption peak at 3.4 eV observed in irradiated samples and of the luminescence signals at 2.78 eV. The absorption peak at ∼4.0 eV and higher, and luminescence signals around 3.2 and 3.6 eV observed in AlN samples with high oxygen concentrations are attributed to complexes of Al vacancies and oxygen impurities. We also propose a transition involving Al and N vacancies and oxygen impurities that may be a cause of the absorption band peaked at 2.9 eV.

We investigate the impact of incorporating realistic In profiles in simulations of c-plane InGaN/GaN light-emitting diodes. Simulations based on a drift-diffusion model typically overestimate the onset voltage, but have usually been based on the assumption of ideal quantum wells with a square In profile. We demonstrate that more realistic profiles lead to significant modifications of current-density-versus-voltage characteristics, and explain the effects based on changes in the band diagram and carrier overlap.

Using first-principles methods and 8-band k·p simulations, we study the electronic structure of an ultrathin quantum-well system consisting of a single layer of InN inserted in GaN matrix. Experimental photoluminescence and electroluminescence emission peaks for such structures have been reported in the wavelength region between 380 to 450 nm. In contrast, our calculations show an energy difference between the electron and hole states around 2.17 eV (573 nm). Possible origins of the experimental light emission are examined. We suggest that the experimental emission may be due to recombination of electrons (holes) in GaN with holes (electrons) in the quantum well.

We use theoretical modeling to investigate the effect of polarization fields and non-radiative Auger recombination on the efficiency-droop and green-gap problems of polar and nonpolar nitride light-emitting diodes. The dependence of radiative and nonradiative recombination rates on electron-hole wave-function overlap is analyzed. Device designs that minimize the polarization fields lead to higher efficiency, not because the internal quantum efficiency is improved at a given carrier density but because they can be operated at a lower carrier density for a given current density.

We investigate the influence of polarity on carrier transport in single-quantum-well and multiple-quantum-well (MQW) light-emitting diodes (LEDs) grown on the semipolar (202¯1) and (2021¯) orientations of free-standing GaN. For semipolar MQW LEDs with the opposite polarity to conventional Ga-polar c-plane LEDs, the polarization-related electric field in the QWs results in an additional energy barrier for carriers to escape the QWs. We show that semipolar (2021¯) MQW LEDs with the same polarity to Ga-polar c-plane LEDs have a more uniform carrier distribution and lower forward voltage than (202¯1) MQW LEDs.

We report indium incorporation properties on various nonpolar and semipolar free-standing GaN substrates. Electroluminescence characterization and x-ray diffraction (XRD) analysis indicate that the semipolar (202¯1¯) and (112¯2) planes have the highest indium incorporation rate among the studied planes. We also show that both indium composition and polarization-related electric fields impact the emission wavelength of the quantum wells (QWs). The different magnitudes and directions of the polarization-related electric fields for each orientation result in different potential profiles for the various semipolar and nonpolar QWs, leading to different emission wavelengths at a given indium composition.

Defects may cause compensation or act as recombination centers in Mg-doped GaN. Using hybrid functional calculations, we investigate the effects of nitrogen vacancies (VN) and Mg-vacancy complexes (MgGa-VN) on the electrical and optical properties of GaN. We find that MgGa-VN are compensating centers in p-type but electrically inactive in n-type GaN. They give rise to a broad emission at 1.8 eV, explaining the red luminescence that is frequently observed in Mg-doped GaN, regardless of the Fermi level. Nitrogen vacancies are also compensating centers in p-type GaN and likely contribute to the yellow luminescence that has been observed in Mg-doped GaN.

We report the effects of Mg doping in the barriers of semipolar (202¯1) multiple-quantum-well light-emitting diodes (LEDs) with long emission wavelengths (>500 nm). With moderate Mg doping concentrations (3 × 1018–5 × 1018 cm−3) in the barriers, the output power was enhanced compared to those with undoped barriers, which suggests that hole transport in the active region is a limiting factor for device performance. Improved hole injection due to Mg doping in the barriers is demonstrated by dichromatic LED experiments and band diagram simulations. With Mg-doped AlGaN barriers, double-quantum-well LEDs with orange to red emission (λ > 600 nm) were also demonstrated.

The optical polarization ratio of spontaneous emission was investigated by electroluminescence measurements for semipolar (202¯1¯) InGaN/GaN light-emitting diodes, covering the blue to green spectral range. Devices fabricated on semipolar (202¯1¯) substrates exhibit polarization ratios ranging from 0.46 at 418 nm to 0.67 at 519 nm. These polarization ratios are significantly higher than those reported on semipolar (202¯1) devices. The valence band energy separation is extracted from spectral measurements and is consistent with the increased polarization ratio and theoretical predictions. Quantum well interdiffusion induced valence band mixing is suggested as a possible explanation for the low experimental value of polarization ratio observed for the (202¯1) devices.

The effect of strain on the valence-band structure of (11math2) semipolar InGaN grown on GaN substrates is studied. A k⋅p analysis reveals that anisotropic strain in the c-plane and shear strain are crucial for deciding the ordering of the two topmost valence bands. The shear-strain deformation potential D6 is calculated for GaN and InN using density functional theory with the Heyd–Scuseria–Ernzerhof hybrid functional [ J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 124, 219906 (2006)] . Using our deformation potentials and assuming a pseudomorphically strained structure, no polarization switching is observed. We investigate the role of partial strain relaxation in the observed polarization switching.

Using first-principle electronic structure calculations, we show a metal- semiconductor transition of a metallic graphene nanoribbon with zigzag edges induced by substitutional doping of Nitrogen or Boron atoms at the edges. A field effect transistor consisting of a metal-semiconductor-metal junction can then be constructed by selective doping of the ribbon edges. The current-voltage characteristics of such a prototype device is determined by the first-principle quantum transport calculations., 4 pages, 3 figures. Accepted in APL

From ab initio calculations, a nanoelectronic switch is proposed based on telescoping double-walled carbon nanotubes (TDWCNTs), which can be conveniently turned on or off via the relative motion of tubes along the tube axis. It is found that the quantum conductance of switch periodically oscillates with the overlapping length of the two shells of the TDWCNT, which is mainly attributed to the variation of coupling effect between π electronic states of the two shells. The switching of TDWCNTs is determined by the distributions of delocalized frontier molecular orbitals of this system.