The influence of growth temperature and input V/III ratio on the initial nucleation and material properties of InN on GaN by MOCVD

The effects of growth temperature and V/III ratio on the InN initial nucleation of islands on the GaN (0 0 0 1) surface were investigated. It is found that InN nuclei density increases with decreasing growth temperature between 375 and 525 degrees C. At lower growth temperatures, InN thin films take the form of small and closely packed islands with diameters of less than 100 nm, whereas at elevated temperatures the InN islands can grow larger and well separated, approaching an equilibrium hexagonal shape due to enhanced surface diffusion of adatoms. At a given growth temperature of 500 degrees C, a controllable density and size of separated InN islands can be achieved by adjusting the V/III ratio. The larger islands lead to fewer defects when they are coalesced. Comparatively, the electrical properties of the films grown under higher V/III ratio are improved.

An Anomalous Gain Mechanism in GaN Schottky Barrier Ultraviolet Photodetectors

The gain mechanism in GaN Schottky barrier ultraviolet photodetectors is investigated by focused light beam. When the incident light illuminates the central region of the Schottky contact electrode, the responsivity changes very little with the increase of reverse bias voltage. However, when the incident light illuminates the edge region of the electrode, the responsivity increases remarkably with the increase of reverse bias voltage, and the corresponding quantum efficiency could be even higher than 100%. It is proposed that the surface states near the edge of the electrode may lead to a reduction of effective Schottky barrier height and an enhancement of electron injection, resulting in the anomalous gain.

Confinement factor and absorption loss of AlInGaN based laser diodes emitting from ultraviolet to green

Confinement factor and absorption loss of AlInGaN based multiquantum well laser diodes (LDs) were investigated by numerical simulation based on a two-dimensional waveguide model. The simulation results indicate that an increased ridge height of the waveguide structure can enhance the lateral optical confinement and reduce the threshold current. For 405 nm violet LDs, the effects of p-AlGaN cladding layer composition and thickness on confinement factor and absorption loss were analyzed. The experimental results are in good agreement with the simulation analysis. Compared to violet LD, the confinement factors of 450 nm blue LD and 530 nm green LD were much lower. Using InGaN as waveguide layers that has higher refractive index than GaN will effectively enhance the optical confinement for blue and green LDs. The LDs based on nonpolar substrate allow for thick well layers and will increase the confinement factor several times. Furthermore, the confinement factor is less sensitive to alloys composition of waveguide and cladding layers, being an advantage especially important for ultraviolet and green LDs.

Al composition variations in AlGaN films grown on low-temperature GaN buffer layer by metalorganic chemical vapor deposition

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. (C) 2008 Elsevier B.V. All rights reserved.

Stable multiplication gain in GaN p-i-n avalanche photodiodes with large device area

Visible-blind p-i-n avalanche photodiodes (APDs) were fabricated with high-quality GaN epilayers deposited on c-plane sapphire substrates by metal-organic chemical vapour deposition. Due to low dislocation density and a sophisticated device fabrication process, the dark current was as small as similar to 0.05 nA under reverse bias up to 20V for devices with a large diameter of 200 mu m, which was among the largest device area for GaN-based p-i-n APDs yet reported. When the reverse bias exceeded 38V the dark current increased sharply, exhibiting a bulk avalanche field-dominated stable breakdown without microplasma formation or sidewall breakdown. With ultraviolet illumination (360 nm) an avalanche multiplication gain of 57 was achieved.

Electron spin relaxation by nuclei and holes in single InAs quantum dots

Electron spin relaxation of charged excitons X+ and X2+ are investigated by time-resolved and polarization-resolved photoluminescence spectroscopy. For X+ configuration, the electron spin relaxation shows a typical decay curve induced by hyperfine interaction with nuclei, whereas for X2+ state the electron spin relaxation is affected not only by nuclei but also by electron-hole exchange interaction, leading to a power-law time dependence.

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.

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.

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.

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.

The effect of single AlGaN interlayer on the structural properties of GaN epilayers grown on Si (111) substrates

High-quality and nearly crack-free GaN epitaxial layer was obtained by inserting a single AlGaN interlayer between GaN epilayer and high-temperature AlN buffer layer on Si (111) substrate by metalorganic chemical vapor deposition. This paper investigates the effect of AlGaN interlayer on the structural proper-ties of the resulting GaN epilayer. It confirms from the optical microscopy and Raman scattering spectroscopy that the AlGaN interlayer has a remarkable effect on introducing relative compressive strain to the top GaN layer and preventing the formation of cracks. X-ray diffraction and transmission electron microscopy analysis reveal that a significant reduction in both screw and edge threading dislocations is achieved in GaN epilayer by the insertion of AlGaN interlayer. The process of threading dislocation reduction in both AlGaN interlayer and GaN epilayer is demonstrated.

Photoluminescence characteristics of GaAsSbN/GaAs epilayers lattice-matched to GaAs substrates

The photoluminescence (PL) characteristics of GaAsSbN/GaAs epilayers grown by molecular beam epitaxy (MBE) are carefully investigated. The results show that antimony (Sb) incorporation into GaNAs material has less influence on the N-induced localization states. For the same N concentration, GaAsSbN material can reach an emission wavelength near 1.3 mum more easily than GaInNAs material. The rapid thermal annealing (RTA) experiment shows that the annealing induced rearrangement of atoms and related blueshift in GaAsSbN epilayers are smaller than those in GaNAs and GaInNAs epilayers. The GaAsSbN material can keep a longer emission wavelength near 1.3 mum-emission even after the annealing treatment. Raman spectroscopy analysis gives further insight into the structure stability of GaAsSbN material after annealing. (C) 2004 Elsevier Ltd. All rights reserved.

Surface morphology of AlN buffer layer and its effect on GaN growth by metalorganic chemical vapor deposition

The in situ optical reflectivity measurements are employed to monitor the GaN epilayer growth process above low-temperature AlN buffer layer on c-plane sapphire substrate by metalorganic chemical vapor deposition. It is found that the lateral growth of GaN islands and their coalescence is promoted in the initial growth stage if the AlN buffer layer is treated with a long annealing time and has an optimal thickness: As confirmed by atomic force microscopy observations, the quality of GaN epilayers is closely dependent on the surface morphology of AlN buffer layer, especially the grain size and nuclei density after the annealing treatment. (C) 2004 American Institute of Physics.

Structural and optical properties of GaAsSb/GaAs heterostructure quantum wells

The structural and optical properties of MBE-grown GaAsSb/GaAs multiple quantum wells (MQWs) as well as strain-compensated GaAsSb/GaAs/GaAsP MQWs are investigated. The results of double crystal X-ray diffraction and reciprocal space mapping show that when strain-compensated layers are introduced, the interface quality of QW structure is remarkably improved, and the MQW structure containing GaAsSb layers with a high Sb composition can be coherently grown. Due to the influence of inserted GaAsP layers on the energy band and carrier distribution of QWs, the optical properties of GaAsSb/GaAs/GaAsP MQWs display a lot of features mainly characteristic of type-I QWs despite the type-II GaAsSb/GaAs interfaces exist in the structure. (C) 2004 Elsevier B.V. All rights reserved.

Investigations on V-defects in quaternary AlInGaN epilayers

The characteristics of V-defects in quaternary AlInGaN epilayers and their correlation with fluctuations of the In distribution are investigated. The geometric size of the V-defects is found to depend on the In composition of the alloy. The V-defects are nucleated within the AlInGaN layer and associated with threading dislocations. Line scan cathodoluminescence (CL) shows a redshift of the emission peak and an increase of the half width of the CL spectra as the electron beam approaches the apex of the V-defect. The total redshift decreases with decreasing In mole fraction in the alloy samples. Although the strain reduction may partially contribute to the CL redshift, indium segregation is suggested to be responsible for the V-defect formation and has a main influence on the respective optical properties. (C) 2004 American Institute Of Physics.