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Schottky barrier on n‐type GaN grown by hydride vapor phase epitaxy

A Schottky barrier on unintentionally doped n‐type GaN grown by hydride vapor phase epitaxy was obtained and characterized. Using vacuum evaporated gold as the Schottky barrier contact and aluminum for the ohmic contact, good quality diodes were obtained. The forward current ideality factor was n∼1.03 and the reverse bias leak current below 1×10−10 A at a reverse bias of −10 V. The barrier height φBn was determined to be 0.844 and 0.94 eV by current‐voltage and capacitance measurements, respectively.

Analysis of deep levels in n‐type GaN by transient capacitance methods

Transient capacitance methods were used to analyze traps occurring in unintentionally doped n‐type GaN grown by hydride vapor‐phase epitaxy. Studies by deep‐level transient spectroscopy (DLTS) and isothermal capacitance transient spectroscopy indicated the presence of three majority‐carrier traps occurring at discrete energies below the conduction band with activation energies (eV) ΔE1=0.264±0.01, ΔE2=0.580±0.017, and ΔE3=0.665±0.017. The single‐crystal films of GaN were grown on GaN formed by metal‐organic chemical‐vapor deposition and on sputter‐deposited ZnO; a similar deep‐level structure was found in both types of samples. Pulse‐width modulation tests using DLTS to determine the capture rates of the traps showed that the capture process is nonexponential, perhaps due to the high trap concentration. The origins of the deep levels are discussed in light of secondary‐ion‐mass‐spectroscopy analysis and group theory results in the literature.Transient capacitance methods were used to analyze traps occurring in unintentionally doped n‐type GaN grown by hydride vapor‐phase epitaxy. Studies by deep‐level transient spectroscopy (DLTS) and isothermal capacitance transient spectroscopy indicated the presence of three majority‐carrier traps occurring at discrete energies below the conduction band with activation energies (eV) ΔE1=0.264±0.01, ΔE2=0.580±0.017, and ΔE3=0.665±0.017. The single‐crystal films of GaN were grown on GaN formed by metal‐organic chemical‐vapor deposition and on sputter‐deposited ZnO; a similar deep‐level structure was found in both types of samples. Pulse‐width modulation tests using DLTS to determine the capture rates of the traps showed that the capture process is nonexponential, perhaps due to the high trap concentration. The origins of the deep levels are discussed in light of secondary‐ion‐mass‐spectroscopy analysis and group theory results in the literature.

Hydride vapor phase epitaxial growth of a high quality GaN film using a ZnO buffer layer

In hydride vapor phase epitaxial (HVPE) growth of GaN, the sputtered ZnO layer has been found to be one of the best buffer layers because of the fact that physical properties of ZnO are nearly analogous with those of GaN. With a ZnO buffer layer, the reproducibility of growing GaN single crystal by HVPE has been greatly improved. The GaN films grown by this method show excellent crystalline, electrical, and optical properties. In particular, the Hall mobility of 1920 cm2 V−1 s−1 at 120 K is the highest value that has ever been reported by HVPE.

Defect-reduced green GaInN/GaN light-emitting diode on nanopatterned sapphire

Green GaInN/GaN quantum well light-emitting diode (LED) wafers were grown on nanopatterned c-plane sapphire substrate by metal-organic vapor phase epitaxy. Without roughening the chip surface, such LEDs show triple the light output over structures on planar sapphire. By quantitative analysis the enhancement was attributed to both, enhanced generation efficiency and extraction. The spectral interference and emission patterns reveal a 58% enhanced light extraction while photoluminescence reveals a doubling of the internal quantum efficiency. The latter was attributed to a 44% lower threading dislocation density as observed in transmission electron microscopy. The partial light output power measured from the sapphire side of the unencapsulated nanopatterned substrate LED die reaches 5.2 mW at 525 nm at 100 mA compared to 1.8 mW in the reference LED.

Relaxation process of the thermal strain in the GaN/α-Al2O3 heterostructure and determination of the intrinsic lattice constants of GaN free from the strain

The relaxation process of the thermal strain in a GaN film due to the thermal expansion coefficient difference in the GaN(0001)/α-Al2O3(0001) heterostructure is studied by varying the film thickness of GaN in a wide range from 1 to 1200 µm. The lattice constant c has a large value of 5.191 A at a film thickness less than a few microns, while it decreases to about 150 µm, and becomes constant above 150 µm, indicating that the strain is almost completely relaxed. The intrinsic lattice constants of wurtzite GaN free from the strain, a0 and c0, are determined to be 3.1892±0.0009 and 5.1850±0.0005 A, respectively.

Relaxation Mechanism of Thermal Stresses in the Heterostructure of GaN Grown on Sapphire by Vapor Phase Epitaxy

Thermal strains and stresses due to the thermal expansion coefficient difference in GaN(0001)/α-Al2O3(0001) layered structures are studied by varying the film thickness of GaN from 0.6 to 1200 µm. The strain in GaN is greater in films of less than a few microns thickness. It is decreased in films of thickness from several to about a hundred microns, and is almost completely relaxed in those thicker than 100 µm. The stresses and strains in the heterostructure are calculated using a model in which relaxation due to cracking in the sapphire is considered. Three relaxation mechanisms of the thermal strain are found for different film thicknesses as follows: (a) only lattice deformation ( 20 µm).

Shallow donors in GaN—The binding energy and the electron effective mass

Abstract Fourier transform infrared absorption spectroscopy has been performed on a GaN epitaxial film grown by the hydride vapor phase epitaxy on sapphire substrate. We observe a transition at 215 cm −1 with a half width of 2 cm −1 , which we attribute to an electronic transition on the basis of temperature dependent measurements. We assign it to the 1s-2p transition of the residual shallow donors. Using effective mass theory neglecting anisotropies in the effective mass and dielectric constant the binding energy of the shallow donors is calculated to be (35.5 ± 0.5) meV. The electron effective mass is (0.236 ± 0.005) m o . Based on this knowledge, the energy separation between the donor-acceptor and the band-acceptor transitions as seen in photoluminescence at 45 K can be used to evaluate the donor concentration in the epitaxial films.

Deep levels in the upper band-gap region of lightly Mg-doped GaN

Deep levels in undoped and weakly Mg‐doped n‐type GaN films fabricated by metalorganic chemical vapor deposition were examined with deep level transient spectroscopy. Deep levels measured at 0.26 and 0.62 eV below the conduction band were found in relatively low concentrations of ∼2×1013 cm−3 in undoped GaN. Addition of small quantities of the Mg acceptor species by means of bis‐cyclopentadienyl magnesium (Cp2Mg) during growth corresponded to a significant increase in the concentration of the level at 0.62 eV. The concentration of the shallower level, found to be independent of the Cp2Mg addition, remained unchanged. These deep levels may detrimentally affect optical and electrical properties when fabricating p‐type GaN.

Determination of the Conduction Band Electron Effective Mass in Hexagonal GaN

The electron effective mass in hexagonal GaN films grown by metal organic vapor phase epitaxy on sapphire substrates is determined by cyclotron resonance experiments. Its value is m p* = 0.22±0.005 m o. Taking polaron effects into account the band edge mass is m b* = 0.20±0.005 m o. From the resonance linewidth a mobility of 3500 cm2/Vs at 6 K is obtained.

The growth of thick GaN film on sapphire substrate by using ZnO buffer layer

Abstract Sputtered ZnO layers have been used as buffer layers in the growth of GaN by hydride VPE. With these buffers we have not only improved the reproducibility of the growth of GaN but also achieved the preparation of single crystalline GaN films alone by etching buffer layers away. In this paper we have studied the effects of the ZnO buffer layer on GaN films.