Experimental evidence of Ga-vacancy induced room temperature ferromagnetic behavior in GaN films

We have grown Ga deficient GaN epitaxial films on (0001) sapphire substrate by plasma-assisted molecular beam epitaxy and report the experimental evidence of room temperature ferromagnetic behavior. The observed yellow emission peak in room temperature photoluminescence spectra and the peak positioning at 300 cm(-1) in Raman spectra confirms the existence of Ga vacancies. The x-ray photoelectron spectroscopic measurements further confirmed the formation of Ga vacancies; since the N/Ga is found to be >1. The ferromagnetism is believed to originate from the polarization of the unpaired 2p electrons of N surrounding the Ga vacancy. (C) 2011 American Institute of Physics. [doi:10.1063/1.3654151]

Estimation of background carrier concentration in fully depleted GaN films

Buffer leakage is an important parasitic loss mechanism in AlGaN/GaN high electron mobility transistors (HEMTs) and hence various methods are employed to grow semi-insulating buffer layers. Quantification of carrier concentration in such buffers using conventional capacitance based profiling techniques is challenging due to their fully depleted nature even at zero bias voltages. We provide a simple and effective model to extract carrier concentrations in fully depleted GaN films using capacitance-voltage (C-V) measurements. Extensive mercury probe C-V profiling has been performed on GaN films of differing thicknesses and doping levels in order to validate this model. Carrier concentrations as extracted from both the conventional C-V technique for partially depleted films having the same doping concentration, and Hall measurements show excellent agreement with those predicted by the proposed model thus establishing the utility of this technique. This model can be readily extended to estimate background carrier concentrations from the depletion region capacitances of HEMT structures and fully depleted films of any class of semiconductor materials.

Enhanced UV detection by non-polar epitaxial GaN films

Nonpolar a-GaN (11-20) epilayers were grown on r-plane (1-102) sapphire substrates using plasma assisted molecular beam epitaxy. High resolution x-ray diffractometer confirmed the orientation of the grown film. Effect of the Ga/N ratio on the morphology and strain of a-GaN epilayers was compared and the best condition was obtained for the nitrogen flow of 1 sccm. Atomic force microscopy was used to analyze the surface morphology while the strain in the film was quantitatively measured using Raman spectroscopy and qualitatively analyzed by reciprocal space mapping technique. UV photo response of a-GaN film was measured after fabricating a metal-semiconductor-metal structure over the film with gold metal. The external quantum efficiency of the photodetectors fabricated in the (0002) polar and (11-20) nonpolar growth directions were compared in terms of responsivity and nonpolar GaN showed the best sensitivity at the cost of comparatively slow response time. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Microstructure and Optical Properties of Nonpolar m-Plane GaN Films Grown on m-Plane Sapphire by Hydride Vapor Phase Epitaxy

Thick nonpolar (10 (1) over bar0) GaN layers were grown on m-plane sapphire substrates by hydride vapor phase epitaxy (HVPE) using magnetron sputtered ZnO buffers, while semipolar (10 (1) over bar(3) over bar) GaN layers were obtained by the conventional two-step growth method using the same substrate. The in-plane anisotropic structural characteristics and stress distribution of the epilayers were revealed by high. resolution X-ray diffraction and polarized Raman scattering measurements. Atomic force microscopy (AFM) images revealed that the striated surface morphologies correlated with the basal plane stacking faults for both (10 (1) over bar0) and (10 (1) over bar(3) over bar) GaN films. The m-plane GaN surface showed many triangular-shaped pits aligning uniformly with the tips pointing to the c-axis after etching in boiled KOH, whereas the oblique hillocks appeared on the semipolar epilayers. In addition, the dominant emission at 3.42eV in m-plane GaN films displayed a red shift with respect to that in semipolar epilayers, maybe owing to the different strain states present in the two epitaxial layers. [DOI: 10.1143/JJAP.47.3346]

Effect of indium-doped interlayer on the strain relief in GaN films grown on Si(111)

Crack-free GaN films have been achieved by inserting an Indoped low-temperature (LT) AlGaN interlayer grown on silicon by metalorganic chemical vapor deposition. The relationship between lattice constants c and a obtained by X-ray diffraction analysis shows that indium doping interlayer can reduce the stress in GaN layers. The stress in GaN decreases with increasing trimethylindium (TMIn) during interlayer growth. Moreover, for a smaller TMIn flow, the stress in GaN decreases dramatically when In acts as a surfactant to improve the crystallinity of the AlGaN interlayer, and for a larger TMIn flow, the stress will increase again. The decreased stress leads to smoother surfaces and fewer cracks for GaN layers by using an In-doped interlayer than by using an undoped interlayer. In doping has been found to enhance the lateral growth and reduce the growth rate of the c face. It can explain the strain relief and cracks reduction in GaN films. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Characterization of Thick GaN Films Directly Grown on Wet-Etching Patterned Sapphire by HVPE

Thick GaN films of high quality are directly grown on wet-etching patterned sapphire in a vertical hydride vapour phase epitaxy reactor. The optical and structural properties of GaN films are studied using scanning electronic microscopy and cathodoluminescence. Test results show that initial growth of hydride vapour phase epitaxy GaN occurs not only on the mesas but also on the two asymmetric sidewalls of the V-shaped grooves without selectivity. After the two-step coalescence near the interface, the GaN films near the surface keep on growing along the direction perpendicular to the long sidewall. Based on Raman results, GaN of the coalescence region in the grooves has the maximum residual stress and poor crystalline quality over the whole GaN film, and the coalescence process can release the stress. Therefore, stress-free thick GaN films are prepared with smooth and crack-free surfaces by this particular growth mode on wet-etching patterned sapphire substrates.

The field emission properties of nonpolar a-plane n-type GaN films grown on nano-patterned sapphire substrates

Si-doped nonpolar a-plane GaN films were grown on nanopatterned sapphire substrates by a low-pressure metal organic chemical vapor deposition (MOCVD) system. The structure, morphology and field emission properties of the sample were studied by means of high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and field emission measurement. The XRD analysis shows that the sample is a nonpolar a-plane (11 (2) over bar0) GaN film. The field emission measurement shows that the nonpolar GaN films exhibit excellent field emission properties with a threshold emission field of as low as 10 V/mu m at a current density of 0.63 mu A/cm(2), and a high field emission current density of 74 mA/cm(2) at an applied field of 24 V/mu m. Moreover, the Fowler-Nordheirn plot of the sample fits a near linear relation. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Role of edge dislocation and Si impurity in linking the blue luminescence and yellow luminescence in n-type GaN films

A close relationship is found between the blue and yellow luminescence bands in n-type GaN films, which are grown without intentional acceptor doping. The intensity ratio of blue luminescence to yellow luminescence (I-BL/I-YL) decreases with the increase in edge dislocation densities as demonstrated by the (102) full width at half maximum of x-ray diffraction. In addition, the I-BL/I-YL ratio decreases with the increase in Si doping. It is suggested that the edge dislocation and Si impurity play important roles in linking the blue and yellow luminescence.

The structure, morphology and Raman scattering study on Mn-implanted nonpolar a-plane GaN films

Dilute magnetic nonpolar GaN films with a Curie temperature above room temperature have been fabricated by implanting Mn ions into unintentionally doped nonpolar a-plane (1 1 (2) over bar 0) GaN films and a subsequent rapid thermal annealing (RTA) process. The impact of the implantation and RTA on the structure and morphology of the nonpolar GaN films is studied in this paper. The scanning electron microscopy analysis shows that the RTA process can effectively recover the implantation-indUced damage to the surface morphology of the sample. The X-ray diffraction and micro-Raman scattering spectroscopy analyses show that the RTA process can just partially recover the implantation-induced crystal deterioration. Therefore, the quality of the Mn-implanted nonpolar GaN films should be improved further for the application in spintronic devices. (C) 2009 Elsevier B.V. All rights reserved.

The structural, morphological and magnetic characteristics of Mn-implanted nonpolar a-plane GaN films

Dilute magnetic nonpolar GaN films have been fabricated by implanting Mn into unintentionally doped nonpolar a-plane GaN films at room temperature, and a subsequent rapid thermal annealing. The X-ray diffraction analysis shows that after rapid thermal annealing the peak of the GaN X-ray diffraction curve shifts to a lower angle, indicating a slight expansion of the GaN crystal lattice. Atomic force microscopy analysis shows that the annealing process does not change the morphology of the sample greatly. Magnetic property analysis indicates that the as-annealed sample shows obvious ferromagnetic properties. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Influence of implantation energy on the characteristics of Mn-implanted nonpolar a-plane GaN films

Nonpolar GaN Mn films have been fabricated by implanting Mn-ion into nonpolar a-plane (MO) GaN films at room temperature. The influence of implantation energy on the Structural, morphological and magnetic characteristics of samples have been investigated by means of stopping and range of ions in matter (SRIM) Simulation software, high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and superconducting quantum interference device (SQUID). According to the SQUID analysis, obvious room temperature ferromagnetic properties of samples were detected. Moreover, the implantation energy has little impact on the ferromagnetic properties of samples. The XRD and AFM analyses show that the structural and morphological characteristics of samples were severely deteriorated with the increase of implantation energy. (C) 2008 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.

Growth of crack-free GaN films on Si(111) substrate by using Al-rich AlN buffer layer

GaN epilayers were grown on Si(111) substrate by metalorganic chemical vapor deposition. By using the Al-rich AlN buffer which contains Al beyond stoichiometry, crack-free GaN epilayers with 1 mum thickness were obtained. Through x-ray diffraction (XRD) and secondary ion mass spectroscopy analyses, it was found that a lot of Al atoms have diffused into the under part of the GaN epilayer from the Al-rich AlN buffer, which results in the formation of an AlxGa1-xN layer at least with 300 nm thickness in the 1 mum thick GaN epilayer. The Al fraction x was estimated by XRD to be about 2.5%. X-ray photoelectron spectroscopy depth analysis was also applied to investigate the stoichiometry in the Al-rich buffer before GaN growth. It is suggested that the underlayer AlxGa1-xN originated from Al diffusion probably provides a compressive stress to the upper part of the GaN epilayer, which counterbalances a part of tensile stress in the GaN epilayer during cooling down and consequently reduces the cracks of the film effectively. The method using the Al diffusion effect to form a thick AlGaN layer is really feasible to achieve the crack-free GaN films and obtain a high crystal quality simultaneously. (C) 2004 American Institute of Physics.