Jia Haiqiang

Enhanced Photoluminescence of InGaN/GaN Green Light-Emitting Diodes Grown on Patterned Sapphire Substrate

Green InGaN/GaN based light-emitting diodes (LEDs) are fabricated both on planar and wet-etched patterned sapphire substrates by metalorganic vapour phase epitaxy (MOVPE). Their photoluminescence (PL) properties of the two samples are studied. The results indicate that the PL integral intensity of the green LED on the patterned substrate is nearly two times of that on the planar one within the whole measured temperature range. The enhanced PL intensity in the green LED on the patterned substrate is shown completely contributed from the extraction efficiency, but not from the internal quantum efficiency. The conclusion is supported by temperature-dependent PL analysis on the two samples, and the mechanisms are discussed.

The structural and optical properties of GaSb/InGaAs type-II quantum dots grown on InP (100) substrate

We have investigated the structural and optical properties of type-II GaSb/InGaAs quantum dots [QDs] grown on InP (100) substrate by molecular beam epitaxy. Rectangular-shaped GaSb QDs were well developed and no nanodash-like structures which could be easily found in the InAs/InP QD system were formed. Low-temperature photoluminescence spectra show there are two peaks centered at 0.75eV and 0.76ev. The low-energy peak blueshifted with increasing excitation power is identified as the indirect transition from the InGaAs conduction band to the GaSb hole level (type-II), and the high-energy peak is identified as the direct transition (type-I) of GaSb QDs. This material system shows a promising application on quantum-dot infrared detectors and quantum-dot field-effect transistor.

Stress Control in GaN Grown on 6H-SiC by Metalorganic Chemical Vapor Deposition

The strain in GaN epitaxial layers grown on 6H-SiC substrates with an AlN buffer by metalorganic chemical vapor deposition is investigated. It is found that the insertion of a graded AlGaN layer between the GaN layer and the AlN buffer can change the signs of strain. A compressive strain in an overgrown thick (2 μm) GaN layer is obtained. High-resolution x-ray diffraction, Raman spectroscopy and photoluminescence measurements are used to determine the strain state in the GaN layers. The mechanism of stress control by inserting graded AlGaN in subsequent GaN layers is discussed briefly.

Characteristics of GaN grown on 6H-SiC with different AlN buffers

Characteristics of GaN grown on 6H-SiC (0001) substrates using different thicknesses of AlN buffers are studied. It is found that the surface morphology and crystal quality of GaN film closely depends on the strain state of the AlN buffer. For a thicker AlN buffer, there are cracks on GaN surface, which make the GaN films unsuitable for applications. While for a thinner AlN buffer, more dislocations are produced in the GaN film, which deteriorates the performance of GaN. Possible generation mechanisms of cracks and more dislocations are investigated and a ~100 nm AlN buffer is suggested to be a better choice for high quality GaN on SiC.

Growth of semi-insulating GaN by using two-step AlN buffer layer

Semi-insulating GaN is grown by using a two-step AlN buffer layer by metalorganic chemical vapour deposition. The sheet resistance of as-grown semi-insulating GaN is dramatically increased to 1013 Ω/sq by using two-step AlN buffer instead of the traditional low-temperature GaN buffer. The high sheet resistance of as-grown GaN over 1013 Ω/sq is due to inserting an insulating buffer layer (two-step AlN buffer) between the high-temperature GaN layer and a sapphire substrate which blocks diffusion of oxygen and overcomes the weakness of generating high density carrier near interface of GaN and sapphire when a low-temperature GaN buffer is used. The result suggests that the high conductive feature of unintentionally doped GaN is mainly contributed from the highly conductive channel near interface between GaN and the sapphire substrate, which is indirectly manifested by room-temperature photoluminescence excited by an incident laser beam radiating on growth surface and on the substrate. The functions of the two-step AlN buffer layer in reducing screw dislocation and improving crystal quality of GaN are also discussed.

Growth of Highly Conductive n-Type Al0.7Ga0.3N Film by Using AlN Buffer with Periodical Variation of V/III Ratio

High quality and highly conductive n-type Al0.7Ga0.3N films are obtained by using AlN multi-step layers (MSL) with periodical variation of V/III ratios by low-pressure metalorganic chemical vapour deposition (LP-MOCVD). The full-width at half-maximum (FWHM) of (0002) and (1015) rocking curves of the Si-doped Al0.7Ga0.3N layer are 519 and 625 arcsec, respectively. Room temperature (RT) Hall measurement shows a free electron concentration of 2.9 × 1019 cm−3, and mobility of 17.8 cm2V−1s−1, corresponding to a resistivity of 0.0121 ω cm. High conductivity of the Si-doped AlGaN film with such high Al mole fraction is mainly contributed by a remarkable reduction of threading dislocations (TDs) in AlGaN layer. The TD reducing mechanism in AlN MSL growth with periodical variation of V/III ratio is discussed in detail.

Elimination of Crystallographic Wing Tilt of Canti-Bridged Epitaxial Laterally Overgrown GaN Films by Optimizing Growth Procedure

Canti-bridged epitaxial lateral overgrowth (CBELO) of GaN is performed by metalorganic chemical vapour deposition (MOCVD) on maskless V-grooved sapphire substrates prepared by wet chemical etching with different mesa widths. The wing tilt usually observed in ELO is not found in the CBELO GaN with wide mesa widths, while it can be detected obviously in the GaN with narrow mesa widths. The wing tilt of CBELO GaN grown on a grooved sapphire substrate with narrow mesa can be controlled by adjusting the thickness of the nucleation layer. The dependence of the wing tilt on the nucleation layer thickness is studied. Cross-sectional scanning electron microscopy is used to characterize the geometry of the wing regions, and double crystal x-ray diffraction is used to analyse the structural characteristics and to measure the magnitude of the crystalline wing tilt. It is found that the crystalline wing tilt can be eliminated completely by first growth of a thin nucleation GaN layer then the CBELO GaN. Possible reason and the origin of the wing tilt in CBELO GaN films are also discussed.

Growth and Characteristics of Epitaxial AlxGa1−xN by MOCVD

AlxGa1-xN epilayers with a wide Al composition range (0.2 ≤ x ≤ 0.68) were grown on AlN/sapphire templates by low-pressure metalorganic chemical vapour deposition (LP-MOCVD). X-ray diffraction results reveal that both the (0002) and (105) full widths at half-maximum (FWHM) of the AlxGa1−xN epilayer decrease with increasing Al composition due to the smaller lattice mismatch to the AlN template. However, the surface morphology becomes rougher with increasing Al composition due to the weak migration ability of Al atoms. Low temperature photoluminescence (PL) spectra show pronounced near band edge (NBE) emission and the NBE FWHM becomes broader with increasing Al composition mainly caused by alloy disorder. Meanwhile, possible causes of the low energy peaks in the PL spectra are discussed.

Reversible Carriers Tunnelling in Asymmetric Coupled InGaN/GaN Quantum Wells

Temperature-dependent photoluminescence (PL) and time resolved photoluminescence (TRPL) are performed to study the PL characteristics and carrier transfer mechanism in asymmetric coupled InGaN/GaN multiple quantum wells (AS-QWs). Our results reveal that abnormal carrier tunnelling from the wide quantum well (WQW) to the narrow quantum well (NQW) is observed at temperature higher than about 100K, while a normal carrier tunnelling from the NQW to the WQW is observed at temperature lower than 100K. The reversible carrier tunnelling between the two QWs makes it possible to explore new types of temperature sensitive emission devices. It is shown that PL internal quantum efficiency (IQE) of the NQW is enhanced to about 46% due to the assistant of the abnormal carrier tunnelling.

Three-Step Growth Optimization of AlN Epilayers by MOCVD

A three-step growth process is developed for depositing high-quality aluminium-nitride (AlN) epilayers on (001) sapphire by low pressure metalorganic chemical vapour deposition (LP-MOCVD). We adopt a low temperature (LT) AlN nucleation layer (NL), and two high temperature (HT) AlN layers with different V/III ratios. Our results reveal that the optimal NL temperature is 840–880°C, and there exists a proper growth switching from low to high V/III ratio for further reducing threading dislocations (TDs). The screw-type TD density of the optimized AlN film is just 7.86 × 106cm−2, about three orders lower than its edge-type one of 2 × 109cm−2 estimated by high-resolution x-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (TEM).