Electronic structures of wurtzite ZnO, BeO, MgO and p-type doping in Zn1-xYxO (Y = Mg, Be)

Using the density function theory within the generalized gradient approximation, the band structures of wurtzite ZnO, BeO and MgO have been calculated. The effective-mass parameters are fitted using the calculated eigenvalues. The Dresselhaus spin-orbit effect appears in the k[1 00] direction, and is zero in the high symmetry direction k[00 1]. The orderings of valence band split by the crystal-field and spin-orbit coupling in wurtzite ZnO, BeO and MgO are identified by analyzing the wave function characters calculated by projecting the wave functions onto p-state in the spherical harmonics. For wurtzite ZnO, the ordering of valence band is Still Gamma(7) > Gamma(9) > Gamma(7) due to the negative spin-orbit coupling splitting energy and the positive crystal-field splitting energy. Thus, the Thomas' conclusion is confirmed. For wurtzite BeO and MgO, although their orderings of valence bands are Gamma(7) > Gamma(9) > Gamma(7) too, the origins of their orderings are different from that of wurtzite ZnO. Zn1-x,YxO (Y = Mg, Be) doped with N and P atoms have been studied using first-principles method. The calculated results show that N atom doped in Zn1-x BexO has more shallow acceptor energy level with increasing the concentration of Be atom. (C) 2008 Elsevier B.V. All rights reserved.

Enhanced electroluminescence intensity of InGaN/GaN multi-quantum-wells based on Mg-doped GaN annealed in O-2

InGaN/GaN multi-quantum-well blue (461 +/- 4 nm) light emitting diodes with higher electroluminescence intensity are obtained by postgrowth thermal annealing at 720 C in O-2-ambient. Based on our first-principle total-energy calculations, we conclude that besides dissociating the Mg-H complex by forming H2O, annealing in O-2 has another positive effect on the activation of acceptor Mg in GaN. Mg can be further activated by the formation of an impurity band above the valence band maximum of host GaN from the passivated Mg-Ga-O-N complex. Our calculated ionization energy for acceptor Mg in the passivated system is about 30 meV shallower than that in pure GaN, in good agreement with previous experimental measurement. Our model can explain that the enhanced electroluminescence intensity of InGaN/GaN MQWs based on Mg-doped p-type GaN is due to a decrease in the ionization energy of Mg acceptor with the presence of oxygen. (C) 2008 American Institute of Physics.

Elasticity, band structure, and piezoelectricity of BexZn1-xO alloys

Lattice constants, elasticity, band structure and piezoelectricity of hexagonal wideband gap BexZn1-xO ternary alloys are calculatedusing firstprinciples methods. The alloys' lattice constants obey Vegard's law well. As Be concentration increases, the bulk modulus and Young's modulus of the alloys increase, whereas the piezoelectricity decreases. We predict that BexZn1-xO/GaN/substrate (x = 0.022) multilayer structure can be suitable for high-frequency surface acoustic wave device applications. Our calculated results are in good agreement with experimental data and other theoretical calculations. (c) 2008 Elsevier B.V. All rights reserved.

Effect on nitrogen acceptor as Mg is alloyed into ZnO

Our Raman measurement indicates that the intensity of the peaks (510 and 645 cm(-1)) related to nitrogen concentration is enhanced in MgZnO compared with that in ZnO. Using first-principles band structure methods, we calculated the formation energy and transition energy level for nitrogen acceptor in ZnO and random MgxZn1-xO (with x=0.25) alloy. Our calculations show that the incorporation of nitrogen can be enhanced as Mg is alloyed into ZnO, which agrees with our experiments. The acceptor energy level deeper in the alloy ascribes to the downward shift of the valence-band maximum edge in the presence of magnesium. (c) 2008 American Institute of Physics.

Elasticity, band-gap bowing, and polarization of AlxGa1-xN alloys

Elastic constants, the bulk modulus, Young's modulus, band-gap bowing coefficients, spontaneous and piezoelectric polarizations, and piezoelectric coefficients of hexagonal AlxGa1-xN ternary alloys are calculated using first-principles methods. The fully relaxed structures and the structures subjected to homogeneous biaxial and uniaxial tension are investigated. We show that the biaxial tension in the plane perpendicular to the c axis and the uniaxial tension along the c axis all reduce the bulk modulus, whereas they reduce and enhance Young's modulus, respectively. We find that the biaxial and uniaxial tension can enhance the bowing coefficients. We also find that the biaxial tension can enhance the total polarization, while the uniaxial tension will suppress the total polarization. (C) 2008 American Institute of Physics.

Epitaxial Growth of AlInGaN Quaternary Alloys by RF-MBE

AlInGaN quaternary alloys were successfully grown on sapphire substrate by radio-frequency plasma-excited molecular beam epitaxy (RF-MBE). AlInGaN quaternary alloys with different compositions were acquired by changing the Al cell's temperature. The streaky RHEED patterns were observed during AlInGaN quaternary alloys growth. Scanning Electron Microscope (SEM), Rutherford back-scattering spectrometry (RBS), X-Ray diffraction (XRD) and Cathodoluminescence (CL) were used to characterize the structural and optical properties of the AlInGaN alloys. The experimental results show that the AlInGaN quaternary alloys grow on the GaN buffer in the layer-by-layer growth mode. When the Al cell's temperature is 920 degrees C, the Al/In ratio in the AlInGaN quaternary alloys is about 4.7, and the AlInGaN can acquire better crystal and optical quality. The X-ray and CL full-width at half-maximum (FWHM) of the AlInGaN are 5arcmin and 25nm, respectively.

First principle study of Mg, Si and Mn co-doped GaN

Calculations of electronic structures and optical properties of Mg (or Si) and Mn co-doped GaN were carried out by means of first-principle plane-wave pesudopotential (PWP) based on density functional theory - The spin polarized impurity bands of deep energy levels were found for both systems. They are half metallic and suitable for spin injectors. Compared with GaN Mn, GaN Mn-Mg exhibits a significant increase in T-C 1 while the 1.3 eV absorption peak in GaN Mn disappears due to addition of Mg. In addition, a strong absorption peak due to T-4(1) (F) -> T-4(2) (F) transition of Mn4+ were observed near 1.1 eV. Nevertheless, GaN Mn-Si failed to show increase of T-C, and the absorption peak was not observed at the low energy side.

InGaN/GaN p-i-n Photodiodes Fabricated with Mg-Doped p-InGaN Layer

Mg-doped p-InGaN layers with In composition of about 10% are grown by metalorganic chemical vapor deposition (MOCVD). The effect of the annealing temperature on the p-type behavior of Mg-doped InGaN is studied. It is found that the hole concentration in p-InGaN increases with a rising annealing temperature in the range of 600 850 C, while the hole mobility remains nearly unchanged until the annealing temperature increases up to 750 C, after which it decreases. On the basis of conductive p-type InGaN growth, the p-In0.1Ga0.9N/i-In0.1Ga0.9N/n-GaN junction structure is grown and fabricated into photodiodes. The spectral responsivity of the InGaN/GaN p-i-n photodiodes shows that the peak responsivity at zero bias is in the wavelength range 350-400 nm.

Phase-separation suppression in GaN-rich side of GaNP alloys grown by metal-organic chemical vapor deposition

The GaN-rich side of GaNP ternary alloys has been successfully synthesized by light-radiation heating and low-pressure metal-organic chemical vapor deposition. X-ray diffraction (XRD) rocking curves show that the ( 0002) peak of GaNP shifts to a smaller angle with increasing P content. From the GaNP photoluminescence (PL) spectra, the red shifts from the band-edge emission of GaN are determined to be 73, 78 and 100 meV, respectively, in the GaNP alloys with the P contents of 1.5%, 5.5% and 7.5%. No PL peak or XRD peak related to GaP is observed, indicating that phase separation induced by the short-range distribution of GaP-rich regions in the GaNP layer has been effectively suppressed. The phase-separation suppression in the GaNP layer is associated with the high growth rate and the quick cooling rate under the given growth conditions, which can efficiently restrain the accumulation of P atoms in the GaNP layer.

Influence of Mg content on molecular-beam-epitaxy-grown ZnMgS ultraviolet photodetectors

Zn1-xMgxS-based Schottky barrier ultraviolet (UV) photodetectors were fabricated using the molecular-beam-epitaxy (MBE) technique. The influence of Mg content on MBE-grown Zn1-xMgxS-based UV photodetectors has been investigated in details with a variety of experimental techniques, including photoresponse (PR), capacitance-voltage, deep level transient Fourier spectroscopy (DLTFS) and photoluminescence (PL). The room-temperature PR results show that the abrupt long-wavelength cutoffs covering 325, 305 295. and 270 nm with Mg contents of 16%, 44%, 57%, and 75% in the Zn1-xMgxS active layers, respectively, were achieved. But the responsivity and the external quantum efficiency exhibited a slight decrease with the Mg content increasing. In good agreement with the PR results, both of the integrated intensity of the PL spectra obtained from Zn1-xMgxS thin films with different Mg compositions (x = 31% and 52%, respectively) and the DLTFS spectra obtained from Zn1-xMgxS-based (x = 5% and 45%, respectively) UV photodetector samples clearly revealed a significant concentration increase of the non-radiative deep traps with increasing Mg containing in the ZnMgS active layers. Our experimental results also indicate that the MBE-grown ZnMgS-based photodetectors can offer the promising characteristics for the detection of short-wavelength UV radiation. (C) 2004 Elsevier B.V. All rights reserved.

Synthesis of composite AIN powder mixed with sintering additives by the Lanxide method

Composite AlN powder, mixed with the sintering additive Y2O3, was synthesized by the direct nitridation of molten Al-Mg-Y alloys. The character of products was determined by means of electron microscopy, X-ray diffraction, granularmetric analysis and chemical composition analysis etc. The results show that the nitridation rate of the raw alloys is higher, and the nitridation products axe porous enough to be easily crushed. Composite AlN powder, obtained by the Lanxide method, has excellent characters such as high purity, especially low oxygen content, and narrow well-distributed grain size and so on.

Optical properties of phase-separated GaN1-xPx alloys grown by light-radiation heating metal-organic chemical vapour deposition

Based on the results of the temperature-dependent photoluminescence (PL) measurements, the broad PL emission in the phase-separated GaNP alloys with P compositions of 0.03, 0.07, and 0.15 has investigated. The broad PL peaks at 2.18, 2.12 and 1.83 eV are assigned to be an emission from the optical transitions from several trap levels, possibly the iso-electronic trap levels related to nitrogen. With the increasing P composition (from 0.03 to 0.15), these iso-electronic trap levels are shown to become resonant with the conduction band of the alloy and thus optically inactive, leading to the apparent red shift (80-160meV) of the PL peak energy and the trend of the red shift is strengthened. No PL emission peak is observed from the GaN-rich GaNP region, suggesting that the photogenerated carriers in the GaN-rich GaNP region may recombine with each other via non-radiation transitions.

Effects of the wave function localization in AlInGaN quaternary alloys

Using the first-principles band-structure method and the special quasirandom structures approach, the authors have investigated the band structure of random AlxInyGa1-x-yN quaternary alloys. They show that the wave functions of the band edge states are more localized on the InN sites. Consequently, the photoluminescence transition intensity in the alloy is higher than that in GaN. The valence band maximum state of the quaternary alloy is also higher than GaN with the same band gap, indicating that the alloy can be doped more easily as p-type. (c) 2007 American Institute of Physics.

Enhanced performance of p-GaN by Mg delta doping

The electrical and structural properties of Mg delta-doped GaN epilayers grown by MOCVD were investigated. Compared to uniform Mg-doping GaN layers, it has been shown that the delta-doping (delta-doping) process could suppress the dislocation density and enhance the p-type performance. The influence of pre-purge step on the structural properties of GaN was also investigated. The hole concentration of p-GaN decreases when using a pre-purge step. These results can be explained convincingly using a simple model of impurity incorporation under Ga-free growth condition. (C) 2007 Elsevier B.V. All rights reserved.

The growth of high-Al-content InAlGaN quaternary alloys by RF-MBE

High-Al-content InxAlyGa1-x-yN (x = 1-10%, y = 34-45%) quaternary alloys were grown on sapphire by radio-frequency plasma-excited molecular beam epitaxy. Rutherford back-scattering spectrometry, high resolution x-ray diffraction and cathodoluminescence were used to characterize the InAlGaN alloys. The experimental results show that InAlGaN with an appropriate Al/In ratio (near 4.7, which is a lattice-match to the GaN under-layer) has better crystal and optical quality than the InAlGaN alloys whose Al/In ratios are far from 4.7. Some cracks and V-defects occur in high-Al/In-ratio InAlGaN alloys. In the CL image, the cracks and V-defect regions are the emission-enhanced regions.