Defect evolution and accompanied change of electrical properties during the GaN growth by metalorganic chemical vapor deposition

The defect evolution and its correlation with electrical properties of GaN films grown by metalorganic chemical vapor deposition are investigated. It is found that the dislocation density decreases gradually during the growth process, and the dislocation reduction rate in the island coalescence process is especially rapid. The changes in electron mobility of GaN with the increase of growth time are mainly dependent on the dislocations acting as scattering centers. Furthermore, the variation of carrier concentration in GaN may be related with the point defects and their clusters. The quality of GaN could be improved by suitably increasing the film thickness. (C) 2009 Elsevier B.V. All rights reserved.

Structural, morphological, and magnetic characteristics of Cu-implanted nonpolar GaN films

Diluted magnetic nonpolar GaN:Cu films have been fabricated by implanting Cu ions into unintentionally doped nonpolar a-plane(1 1 (2) over bar 0) GaN films and a subsequent thermal annealing process. The structural, morphological and magnetic characteristics of the samples have been investigated by means of high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and superconducting quantum interference device (SQUID). The sample shows a clear ferromagnetism behavior at room temperature. It is significantly shown that with a Cu concentration as low as 0.75% the sample exhibits a saturation magnetization about 0.65 mu(B)/Cu atom. Moreover, the possible origin of the ferromagnetism for the sample was also discussed briefly. (C) 2009 Elsevier B. V. All rights reserved.

Determination of the tilt and twist angles of curved GaN layers by high-resolution x-ray diffraction

The full-width at half-maximum (FWHM) of an x-ray rocking curve (XRC) has been used as a parameter to determine the tilt and twist angles of GaN layers. Nevertheless, when the thickness of GaN epilayer reaches several microns, the peak broadening due to curvature becomes non-negligible. In this paper, using the (0 0 l), l = 2, 4, 6, XRC to minimize the effects of wafer curvature was studied systematically. Also the method to determine the tilt angle of a curved GaN layer was proposed while the Williamson-Hall plot was unsuitable. It was found that the (0 0 6) XRC-FWHM had a significant advantage for high-quality GaN layers with the radius curvature of r less than 3.5 m. Furthermore, an extrapolating method of gaining a reliable tilt angle has also been proposed, with which the calculated error can be improved by 10% for r < 2 m crystals compared with the (0 0 6) XRC-FWHM. In skew geometry, we have demonstrated that the twist angles deriving from the (2 0 4) XRC-FWHM are in accord with those from the grazing incidence in-plane diffraction (IP-GID) method for significantly curved samples.

The investigation on strain relaxation and double peaks in photoluminescence of InGaN/GaN MQW layers

Two emission peaks were observed in the low temperature photoluminescence (LTPL) spectra of an InGaN/GaN multiple quantum well (MQW) structure before and after nanopillar fabrication. After nanopillar fabrication it is found that among the two peaks the longer wavelength peak exhibits a clear blue shift and has a much stronger enhancement in LTPL intensity than the shorter one. Combined with x-ray diffraction and spatially resolved cathodoluminescence analyses, the difference induced by nanopillar fabrication is ascribed to different strain relaxation states in the lower and upper quantum well layers. It is found that the lower QW layers of the as-grown MQW which causes the longer wavelength PL peak are more strained, while the upper ones are almost fully strain-relaxed. Therefore, the nanopillar fabrication induces much less strain relaxation in the upper part of the MQW than in the lower one.

Analysis on the high luminous flux white light from GaN-based laser diode

We fabricated a phosphor-conversion white light using an InGaN laser diode that emits 445 nm and phosphor that emits in the yellow region when excited by the blue laser light. At 500 mA injection current the luminous flux and the luminous efficacy were 113 lm and 44 lm/W, respectively. The relationship of the luminous flux and the luminous efficacy of the white light with an injection current were discussed. Based on the evaluation method for luminous efficacy of light sources established by the Commission International de I'Eclairage (CIE) and the phosphor used in this experiment, a theoretical analysis of the experiment results and the maximum luminous efficacy of this white light fabrication method were also presented.

Influence of penetrating V-pits on leakage current of GaN based p-i-n UV detector

The leakage mechanism of GaN-based p-i-n (p-AlGaN/i-GaN/n-GaN) UV detector has been investigated. With the same dislocation density, devices made from material with higher density of V-pits on surface produce larger leakage current. SEM images show that some V-pits penetrate into i-GaN layer, sometimes even the n-GaN layer. If p-ohmic contact metal (Ni/Au) deposits in the V-pits, Schottky contact would be formed at the interface of metal and i-GaN, or form ohmic contact at the interface of metal and n-GaN. The existence of parallel Schottky junction and ohmic contact resistance enhances the leakage current greatly.

The impact of implantation dose on the characteristics of diluted-magnetic nonpolar GaN:Cu films

Diluted-magnetic nonpolar GaN:Cu films have been fabricated by implanting Cu ions into p-type nonpolar a-plane (1120) GaN films with a subsequent thermal annealing process. The impact of the implantation dose on the structural. morphological and magnetic characteristics of the samples have been investigated by means of high-resolution X-ray diffraction (HRXRD). atomic force microscopy (AFM), and superconducting quantum interference device (SQUID). The XRD and AFM analyses show that the structural and morphological characteristics of samples deteriorated with the increase of implantation dose. According to the SQUID analysis. obvious room-temperature ferromagnetic properties of samples were detected. Moreover, the saturation magnetization per Cu atom decreased as the implantation dose increased. (C) 2009 Elsevier B.V. All rights reserved.

Light extraction of GaN LEDs with 2-D photonic crystal structure

Ultraviolet photo-lithography is employed to introduce two-dimensional (2D) photonic crystal (PC) structure on the top surface of GaN-based light emitting diode (LED). PC patterns are transferred to 460-nm-thick transparent indium tin oxide (ITO) electrode by inductively coupled plasma (ICP) etching. Light intensity of PC-LED can be enhanced by 38% comparing with the one without PC structure. Rigorous coupled wave analysis method is performed to calculate the light transmission spectrum of PC slab. Simulation results indicate that total internal reflect angle which modulated by PC structure has been increased by 7 degrees, which means that the light extraction efficiency is enhanced outstandingly.

A wide-narrow well design for understanding the efficiency droop in InGaN/GaN light-emitting diodes

The electroluminescence efficiency at room temperature and low temperature (15 K) in a wide-narrow-well InGaN/GaN light-emitting diode with a narrow last well (1.5 nm) and a narrow next-to-last barrier (5 nm) is investigated to study the efficiency droop phenomenon. A reduced droop in the wide wells and a reduced droop at low temperatures reveals that inferior hole transportation ability induced Auger recombination is the root for the droop at high excitation levels.

Binding Energy and Spin-Orbit Splitting of a Hydrogenic Donor Impurity in AlGaN/GaN Triangle-Shaped Potential Quantum Well

In the framework of effective-mass envelope function theory, including the effect of Rashba spin-orbit coupling, the binding energy E-b and spin-orbit split energy Gamma of the ground state of a hydrogenic donor impurity in AlGaN/GaN triangle-shaped potential heterointerface are calculated. We find that with the electric field of the heterojunction increasing, (1) the effective width of quantum well (W) over bar decreases and (2) the binding energy increases monotonously, and in the mean time, (3) the spin-orbit split energy Gamma decreases drastically. (4) The maximum of Gamma is 1.22 meV when the electric field of heterointerface is 1 MV/cm.

A new p-n structure ultraviolet photodetector with p(-)-GaN active region

A new ultraviolet photodetector of employing p menus type GaN (p(-)-GaN) as the active layer is proposed. It is easy to obtain the p(-)-GaN layer with low carrier concentration. As a result, the depletion region can be increased and the quantum efficiency can be improved. The influence of some structure parameters on the performance of the new device is investigated. Through the simulation calculation, it is found that the quantum efficiency increases with the decrease of the barrier height between the metal electrode and the p(-)-GaN layer, and it is also found that the quantum efficiency can be improved by reducing the thickness of the p(-)-GaN layer. To fabricate the new photodetector with high performance, we should employ thin p(-)-GaN layer as the active layer and reduce the Schottky barrier height.

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.

Polarization effects simulation of AlGaN/GaN heterojunction by using a symbolistic delta-doping layer

Polarization effects in AlGaN/GaN heterojunction are simulated based on a traditional semiconductor device simulator. A delta doping layer is purposely inserted at the interface of the heterojunction in the simulation, so the ionized donors or acceptors can represent polarization-induced positive or negative fixed charges. The free electron distribution of single AlGaN/GaN heterostructures with Ga-face and N-face growth is compared, and the results of the simulation show that carrier confinement takes place only in the former structure. The dependence of sheet density of free electrons at the interface of Ga-face growth AlGaN/GaN on Al composition and the thickness of AlGaN is also investigated. The consistency of simulation results with the experiments and calculations reported by other researchers shows that this method can be effectively used to deal with the polarization effects in the simulation of GaN-based heterojunction devices. (C) 2004 Elsevier Ltd. All rights reserved.

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.

Dislocation scattering in a two-dimensional electron gas of an AlxGa1-xN/GaN heterostructure

The theoretical electron mobility limited by dislocation scattering of a two-dimensional electron gas confined near the interface of an AlxGa1-xN/GaN heterostructure is calculated. The accurate wave functions and electron distributions of the three lowest subbands for a typical structure are obtained by solving the Schrodinger and Poisson equations self-consistently. Based on the model of treating dislocation as a charged line, a simple scattering potential, a square-well potential, is utilized. The estimated mobility suggests that such a choice can simplify the calculation without introducing significant deviation from experimental data. It is also found that the dislocation scattering dominates both the low- and moderate-temperature mobilities and accounts for the nearly flattening-out behavior with increasing temperature. To clarify the role of dislocation scattering all standard scattering mechanisms are included in the calculation.