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Abstract: A dual AlN buffer layer structure, including an isolated layer and a nucleation layer, is proposed to improve the growth of AlN films on sapphire substrate by metal organic chemical vapor deposition. This method is aimed to weaken the negative nitridation effect and improve lateral growth condition in the initial growth stage. It is found that suitably increasing the thickness of the nucleation layer is in favor of a better structural quality of the AlN film. An examination of surface morphology by atomic force microscopy suggests that the thicker the dual AlN buffer layer, the rougher the surface, and a higher quality of AlN epilayer is resulted. [Copyright &y& Elsevier]

Abstract: We have investigated the optical properties of thick InGaN film grown on GaN by cathodeluminescence (CL) spectroscopy. It is found that there is obvious In composition variation in both growth and lateral direction of InGaN film. The depth distribution of In composition is closely related to the strain relaxation process of InGaN film. Accompanied with the relaxation of compressive strain, the In composition of InGaN layer increases and the CL peak energy shifts towards red. Moreover, a rather apparent In composition fluctuation is found in the relaxed upper part of InGaN layer as confirmed by CL imaging. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: We have investigated the optical properties of AlGaN grown on sapphire. It is found that two main luminescence peaks occur in the cathodoluminescence (CL) spectra of AlGaN films, and their energy separation increases with the increase of Al source flux during the growth. Spatially resolved CL investigations have shown that the line splitting is a result of variation of AlN mole fraction within the layer. The Al composition varies in both lateral and vertical direction. It is suggested that the difference in the surface mobility of Al and Ga atoms, especially, its strong influence on the initial island coalescence process and the formation of island-like regions on the uneven film surface, is responsible for the Al composition inhomogeneity. [Copyright &y& Elsevier]

Abstract: The growth parameters which can modify In incorporation and affect electroluminescence (EL) properties of blue–violet InGaN/GaN multiple quantum wells (MQWs) during metalorganic chemical vapor deposition (MOCVD) are investigated. It is found that a suitable increase of trimethylindium (TMIn) flux during the growth of InGaN well can increase both EL intensity and EL peak wavelength. However, when the growth temperature of well decreases from 810 to 800°C, the EL intensity decreases although the EL peak wavelength increases. X-ray diffraction results demonstrate that the interface roughness plays an important role in determining the EL intensity of InGaN/GaN MQWs. It is suggested to grow blue–violet MQWs with high structural quality by suitably increasing the TMIn flux and at a relatively high growth temperature. [Copyright &y& Elsevier]

Three series of InGaN samples with different growth pressures are grown in a vertical metal organic chemical vapor deposition (MOCVD) system and the indium incorporation efficiency during InGaN layer growth is investigated. It is found that the indium content in InGaN layer decreases when the NH 3 flow rate increases at a higher growth pressure and it increases with the NH 3 flow rate at a lower growth pressure, This may be attributed to the higher dissociation rate of NH 3 into N 2 and H 2 at a higher growth pressure, leading to a higher H 2 concentration in reactor during InGaN growth. Therefore, changing growth conditions to suppress the dissociation of NH 3 into N 2 and H 2 can increase the indium incorporation efficiency during InGaN film growth. [ABSTRACT FROM AUTHOR]

A series of InGaN/GaN multi-quantum well (MQW) LEDs are grown by using metalorganic chemical vapor deposition (MOCVD) with similar structure but at different temperature, their structural and optical characteristics are investigated in detail. It is unexpectedly found that the output power of LEDs increases markedly when the growth temperature of InGaN QWs decreases from 820 °C to as low as 770 °C. This is attributed to the variation of localization states with the decrease of growth temperature. In the lower temperature (LT) grown InGaN/GaN MQWs, the formation of large and high indium content In-rich regions accompanied with defects is better suppressed, while the formation of shallower and more homogeneous localization states, behaving as effective luminescence centers, are more favored. Therefore, enhanced emission intensity is observed in the LT-grown InGaN/GaN MQW LEDs. [ABSTRACT FROM AUTHOR]

Abstract: The Al composition of metalorganic chemical vapor deposition (MOCVD)-grown AlGaN alloy layers is found to be greatly influenced by the parasitic reaction between ammonia (NH3) and trimethylaluminum (TMAl). The growth process of AlN is carefully investigated by monitoring the in situ optical reflection. The abnormal dependencies of growth rate on growth temperature, reactor pressure, and flux of NH3 are observed and can be well explained by the effect of parasitic reaction. The increase of growth rate with increasing flux of TMAl is found to depend on the growth temperature and reactor pressure due to the presence of parasitic effect. A relatively low growth temperature and a reduced reactor pressure are suggested for the effective decrease of parasitic reaction during the MOCVD growth of AlN and probably lead to a more effective incorporation of Al into the AlGaN layers. [Copyright &y& Elsevier]

Abstract The effect of carbon impurities on electrical resistivity of Mg doped GaN grown by metal organic chemical vapor deposition (MOCVD) was investigated. It was noted that when the growth rate increases or the growth pressure decreases, the resistivity of p-GaN increases and the quality of p-type conductivity of epitaxial GaN deteriorates. The intensity of yellow luminescence (YL) in photoluminescence (PL) spectra was strengthened at the same time. Due to the compensation effect of carbon impurities on magnesium, it makes a significant influence on the electrical resistivity in p-GaN. Carbon impurities may form deep donors, resulting in yellow luminescence (YL) in photoluminescence spectra and compensate Mg acceptors to increase the resistivity of p-GaN. Thus, the low resistivity p-type GaN films can be obtained only when we appropriately reduce growth rate and improve reaction chamber pressure to reduce the residual carbon impurity concentration. [ABSTRACT FROM AUTHOR]

A large field emission current density of 2.55 A/cm 2 at 20.9 V and a low turn-on voltage of 7.28 V is obtained from the Si-doped 50 nm-thick AlN film, synthesized by metalorganic chemical vapor deposition on the n-type SiC substrates, which is the best result reported for AlN film. Accompanying with atomic force surface micro-images, it is found that this current is achieved owing to a blunting process under a high voltage of 95 V, which can lead to a decrease of the root mean square roughness from 4.23 to 1.03 nm. [ABSTRACT FROM AUTHOR]

The electrical resistivity and compensation mechanism in heavily Mg doped p-GaN grown by metal organic chemical vapor deposition (MOCVD) are investigated. It is noted that when the Mg doping concentration is too high, the hole concentration measured by Hall test of p-GaN may decrease compared to the p-GaN samples with lower Mg doping concentration. This phenomenon suggests a possible self-compensation of Mg impurity in p-GaN. It is found that Mg concentration tested by secondary ion mass spectroscopy (SIMS) does not increase linearly when the Cp 2 Mg/TMGa flow rate ratio increases, and H concentration also varies with increasing Cp 2 Mg flow rate. The change of hole concentration may be attributed to the decrease of active acceptor (Mg Ga) concentration related to the self-compensation effect and the reduction of Mg incorporation rate. Actually, the integral intensity of blue luminescence (BL) caused by donor-acceptor pair (DAP) recombination in room temperature photoluminescence (PL) spectra is strengthened with increasing Cp 2 Mg flow rate at first, and then the BL intensity decreases when the Cp 2 Mg flow rate is over 100 sccm. The change of BL intensity is consistent with the conclusion that the acceptor concentration may decrease at a high Mg doping level. • Hole concentration may decrease when Cp 2 Mg flow rate increases too high in p-GaN.. • The concentration of H traces hole density because H is influenced by the Fermi level and hydrogen solubility in GaN.It found that H concentration may decrease when Cp2Mg flow exceeds 100sccm. [ABSTRACT FROM AUTHOR]