(Ga, Gd, As) film growth on GaAs substrate by low-energy ion-beam deposit

(Ga, Gd, As) film was fabricated by the mass-analyzed dual ion-beam epitaxy system with the energy of 1000 eV at room temperature. There was no new peak found except GaAs substrate peaks (0 0 2) and (0 0 4) by X-ray diffraction. Rocking curves were measured for symmetric (0 0 4) reflections to further yield the lattice mismatch information by employing double-crystal X-ray diffraction. The element distributions vary so much due to the ion dose difference from AES depth profiles. The sample surface morphology indicates oxidizing layer roughness is also relative to the Gd ion dose, which leads to islandlike feature appearing on the high-dose sample. One sample shows ferromagnetic behavior at room temperature. (C) 2003 Elsevier B.V. All rights reserved.

Growth and properties of GaN on Si (111) substrates with AlGaN/AlN buffer layer by NH3-GSMBE

We describe the growth of GaN on Si (111) substrates with a AlGaN/AlN buffer layer by NH3-GSMBE. The influence of the AlN and AlGaN buffer layer thickness on the crack density of GaN has been investigated. It is found that the optimum thickness is 120 nm and 250 nm for AlN and AlGaN layers, respectively. The full width at half maximum of the GaN (0002) peak in the triple-crystal x-ray rocking curve measurement is about 15 arcmin.

Crack-free GaN/Si(111) epitaxial layers grown with InAlGaN alloy as compliant interlayer by metalorganic chemical vapor deposition

A new method is demonstrated to be effective in reducing mismatch-induced tensile stress and suppressing the formation of cracks by inserting InAlGaN interlayers during the growth of GaN upon Si (1 1 1) substrate. Compared with GaN film without quaternary interlayer, GaN layer grown on InAlGaN compliant layers shows a five times brighter integrated PL intensity and a (0 0 0 2) High-resolution X-ray diffraction (HRXRD) curve width of 18 arcmin. Its chi(min), derived from Rutherford backscattering spectrometry (RBS), is about 2.0%, which means that the crystalline quality of this layer is very good. Quaternary InAlGaN layers, which are used as buffer layers firstly, can play a compliant role to endure the large mismatch-induced stress and reduce cracks during the growth of GaN epitaxy. The mechanisms leading to crack density reduction are investigated and results show that the phase immiscibility and the weak In-N bond make interlayer to offer tenability in the lattice parameters and release the thermal stress. (c) 2005 Elsevier B.V. All rights reserved.

Effects of different modified underlayer surfaces on growth and optical properties of InGaN quantum dots

InGaN/GaN quantum dots were grown on the sapphire (0 0 0 1) substrate in a metalorganic chemical vapor deposition system. The morphologies of QDs deposited on different modified underlayer (GaN) surfaces, including naturally as grown, Ga-mediated, In-mediated, and air-passivated ones, were investigated by atomic force microscopy (AFM). Photo luminescence (PL) method is used to evaluate optical properties. It is shown that InGaN QDs can form directly on the natural GaN layer. However, both the size and distribution show obvious inhomogeneities. Such a heavy fluctuation in size leads to double peaks for QDs with short growth time, and broad peaks for QDs with long growth time in their low-temperature PL spectra. QDs grown on the Ga-mediated GaN underlayer tends to coalesce. Distinct transform takes place from 3D to 2D growth on the In-mediated ones, and thus the formation of QDs is prohibited. Those results clarify Ga and In's surfactant behavior. When the GaN underlayer is passivated in the air, and together with an additional low-temperature-grown seeding layer, however, the island growth mode is enhanced. Subsequently, grown InGaN QDs are characterized by a relatively high density and an improved Gaussian-like distribution in size. Short surface diffusion length at low growth temperature accounts for that result. It is concluded that reduced temperature favors QD's 3D growth and surface passivation can provide another promising way to obtain high-density QDs that especially suits MOCVD system. (c) 2004 Elsevier Ltd. All rights reserved.

Crack-free InAlGaN quaternary alloy films grown on Si(111) substrate by metalorganic chemical vapor deposition

Crack-free In0.08Al0.25Ga0.67N quaternary films, with and without thick (> 1.5 mum) high-temperature-GaN (HTGaN) interlayer, have been grown on Si(1 1 1) substrates by a low-pressure metalorganic chemical vapor deposition (MOCVD) system. Mole fractions of In and Al in quaternary alloy layers are determined by Energy dispersive spectroscopy (EDS) and Rutherford backscattering spectrometry (RBS), which are recorded as similar to8% and similar to25-27%, respectively. High-resolution X-ray diffraction (HRXRD) and room temperature photoluminescence (RT-PL) results evidence the film's single crystal structure and the existence of local In- and/or Al-rich regions. Compared with GaN film grwon on Si(1 1 1) substrate, no crack is observed in the quaternary ones. Two explanations are proposed. First, mismatch-induced strain is relaxed significantly due to gradual changes of In concentration. Second, the weak In-N bond is likely to break when the sample is cooled down to the room temperature, which is expected to favor the releasing of thermal stress. (C) 2004 Elsevier B.V. All rights reserved.

Spatial distribution of deep level defects in crack-free AlGaN grown on GaN with a high-temperature AlN interlayer

The deep level luminescence of crack-free Al0.25Ga0.75N layers grown on a GaN template with a high-temperature grown AlN interlayer has been studied using spatially resolved cathodoluminescence (CL) spectroscopy. The CL spectra of Al0.25Ga0.75N grown on a thin AlN interlayer present a deep level aquamarine luminescence (DLAL) band at about 2.6 eV and a deep level violet luminescence (DLVL) band at about 3.17 eV. Cross-section line scan CL measurements on a cleaved sample edge clearly reveal different distributions of DLAL-related and DLVL-related defects in AlGaN along the growth direction. The DLAL band of AlGaN is attributed to evolve from the yellow luminescence band of GaN, and therefore has an analogous origin of a radiative transition between a shallow donor and a deep acceptor. The DLVL band is correlated with defects distributed near the GaN/AlN/AlGaN interfaces. Additionally, the lateral distribution of the intensity of the DLAL band shows a domainlike feature which is accompanied by a lateral phase separation of Al composition. Such a distribution of deep level defects is probably caused by the strain field within the domains. (c) 2006 American Institute of Physics.

Crack control in GaN grown on silicon (111) using In doped low-temperature AlGaN interlayer by metalorganic chemical vapor deposition

The effects of In doped low-temperature (LT) AlGaN interlayer on the properties of GaN/Si(111) by MOCVD have been investigated. Using In doping LT-interlayer can decrease the stress sufficiently for avoiding crack formation in a thick (2.0 mu m) GaN layer. Significant improvement in the crystal and optical properties of GaN layer is also achieved. In doping is observed to reduce the stress in AlGaN interlayer measured by high-resolution X-ray diffraction (HRXRD). It can provide more compressive stress to counteract tensile stress and reduce crack density in subsequent GaN layer. Moreover, as a surfactant, indium is observed to cause an enhanced PL intensity and the narrowed linewidths of PL and XRD spectra for the LT-interlayer. Additionally, the crystal quality of GaN layer is found to be dependent on the growth parameters of underneath In-doped LT-AlGaN interlayer. The optimal parameters, such as TMIn flow rate, TMAl flow rates and thickness, are achieved to obtain nearly 2.0 mu m thick crack free GaN film with advanced optical and crystal properties. (c) 2005 Elsevier B.V. All rights reserved.

In0.25Ga0.75As growth on low-temperature thin buffer layers formed on GaAs (001) substrate

High-quality In0.25Ga0.75As films were grown on low-temperature (LT) ultra-thin GaAs buffer layers formed on GaAs (0 0 1) substrate by molecular beam epitaxy. The epilayers were studied by atomic force microscopy (AFM), photo luminescence (PL) and double crystal X-ray diffraction (DCXRD), All the measurements indicated that LT thin buffer layer technique is a simple but powerful growth technique for heteroepitaxy. (C) 2002 Elsevier Science B.V. All rights reserved.

Statistical investigation on morphology development of gallium nitride in initial growth stage

Morphology of Gallium Nitride (GaN) in initial growth stage was observed with atomic force microscopy (AFM) and scanning electron microscopy (SEM), It was found that the epilayer developed from islands to coalesced film. Statistics based on AFM observation was carried out to investigate the morphology characteristics. It was found that the evolution of height distribution could be used to describe morphology development. Statistics also clearly revealed variation of top-face growth rate among islands. Indium-doping effect on morphology development was also statistically studied. The roughening and smoothing behavior in morphology development was explained. (C) 2002 Elsevier Science B.V. All rights reserved.

Hydrogen behavior in GaN epilayers grown by NH3-MBE

Hydrogen behavior in unintentionally doped GaN epilayers on sapphire substrates grown by NH3-MBE is investigated. Firstly, we find by using nuclear reaction analysis (NRA) that with increasing hydrogen concentration the background electron concentration increases, which suggests that there exists a hydrogen-related donor in undoped GaN, Secondly, Fourier transform infrared (FTIR) absorption and X-ray photoelectron spectroscopy (XPS) reveal Further that hydrogen atom is bound to nitrogen atom in GaN with a local vibrational mode at about 3211 cm(-1) Hence, it is presumed that the hydrogen-related complex Ga. . .H-N is a hydrogen-related donor candidate partly responsible for high n-type background commonly observed in GaN films. Finally, Raman spectroscopy results of the epilayers show that ill addition to the expected compressive biaxial strain, in some cases GaN films suffer from serious tensile biaxial strain. This anomalous behavior has been well interpreted in terms of interstitial hydrogen lattice dilation. (C) 2001 Elsevier Science B.V. All rights reserved.

Observation of "ghost" islands and surfactant effect of surface gallium atoms during GaN growth by molecular beam epitaxy

We observe "ghost" islands formed on terraces during homoepitaxial nucleation of GaN. We attribute the ghost islands to intermediate nucleation states, which can be driven into "normal" islands by scanning tunneling microscopy. The formation of ghost islands is related to excess Ga atoms on the surface. The excess Ga also affect island number density: by increasing Ga coverage, the island density first decreases, reaching a minimum at about 1 monolayer (ML) Ga and then increases rapidly for coverages above 1 ML. This nonmonotonic behavior points to a surfactant effect of the Ga atoms.

Annealing behavior of InAs/GaAs quantum dot structures

We investigate the annealing behavior of InAs layers with different thicknesses in a GaAs matrix. The diffusion enhancement by strain, which is well established in strained quantum wells, occurs in InAs/GaAs quantum dots (QDs). A shift of the QD luminescence peak toward higher energies results from this enhanced diffusion. In the case of structures where a significant portion of the strain is relaxed by dislocations, the interdiffusion becomes negligible, and there is a propensity to generate additional dislocations. This results in a decrease of the QD luminescence intensity, and the QD peak energy is weakly affected.

High responsivity ultraviolet photodetector based on crack-free GaN on Si (111)

A Schottky-based metal-semiconductor-metal photodetector is fabricated on 1 mu m-thick, crack-free GaN on Si (I 11) substrate using an optimized AlxGal-xN/AlN complex buffer layer. It exhibits a high responsivity of 4600A/W at 366nm which may be due to both a crack-free sample and high internal gain. The relationship between responsivity and bias voltage is also investigated. The experiment results indicate that the responsivity increases with the bias voltage and shows a tendency to saturate. (c) 2007 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim.

Electroluminescence behavior of ZnO/Si heterojunctions: Energy band alignment and interfacial microstructure

n-ZnO/p-Si heterojunction light-emitting diodes (LEDs) show weak defect-related electroluminescence (EL). In order to analyze the origin of the weak EL, the energy band alignment and interfacial microstructure of ZnO/Si heterojunction are investigated by x-ray photoelectron spectroscopy. The valence band offset (VBO) is determined to be 3.15 +/- 0.15 eV and conduction band offset is -0.90 +/- 0.15 eV, showing a type-II band alignment. The higher VBO means a high potential barrier for holes injected from Si into ZnO, and hence, charge carrier recombination takes place mainly on the Si side rather than the ZnO layer. It is also found that a 2.1 nm thick SiOx interfacial layer is formed at the ZnO/Si interface. The unavoidable SiOx interfacial layer provides to a large number of nonradiative centers at the ZnO/Si interface and gives rise to poor crystallinity in the ZnO films. The weak EL from the n-ZnO/p-Si LEDs can be ascribed to the high ZnO/Si VBO and existence of the SiOx interfacial layer.

Hydrogen behavior in GaN epilayers grown by NH3-MBE

Hydrogen behavior in unintentionally doped GaN epilayers on sapphire substrates grown by NH3-MBE is investigated. Firstly, we find by using nuclear reaction analysis (NRA) that with increasing hydrogen concentration the background electron concentration increases, which suggests that there exists a hydrogen-related donor in undoped GaN, Secondly, Fourier transform infrared (FTIR) absorption and X-ray photoelectron spectroscopy (XPS) reveal Further that hydrogen atom is bound to nitrogen atom in GaN with a local vibrational mode at about 3211 cm(-1) Hence, it is presumed that the hydrogen-related complex Ga. . .H-N is a hydrogen-related donor candidate partly responsible for high n-type background commonly observed in GaN films. Finally, Raman spectroscopy results of the epilayers show that ill addition to the expected compressive biaxial strain, in some cases GaN films suffer from serious tensile biaxial strain. This anomalous behavior has been well interpreted in terms of interstitial hydrogen lattice dilation. (C) 2001 Elsevier Science B.V. All rights reserved.