Defects in GaN films grown on Si(111) substrates by metal-organic chemical vapour deposition

We report the transmission-electron microscopy study of the defects in wurtzitic GaN films grown on Si(111) substrates with AIN buffer layers by the metal-organic chemical vapour deposition method. The In0.1Ga0.9N/GaN multiple quantum well (MQW) reduced the dislocation density by obstructing the mixed and screw dislocations passing through the MQW. No evident reduction of the edge dislocations density by the MQW was observed. It was found that dislocations with screw component can be located at the boundaries of sub-grains slightly in-plane misoriented.

Microstructure of GaN films grown on Si(111) substrates by metalorganic chemical vapor deposition

We report the transmission electron microscopy (TEM) study of the microstructure of wurtzitic GaN films grown on Si(I I I) substrates with AlN buffer layers by metalorganic chemical vapor deposition (MOCVD) method. An amorphous layer was formed at the interface between Si and AlN when thick GaN film was grown. We propose the amorphous layer was induced by the large stress at the interface when thick GaN was grown. The In0.1Ga0.9N/GaN multiple quantum well (MQW) reduced the dislocation density by obstructing the mixed and screw dislocations from passing through the MQW. But no evident reduction of the edge dislocations by the MQW was observed. It was found that dislocations located at the boundaries of grains slightly in-plane misoriented have screw component. Inversion domain is also observed. (C) 2003 Elsevier B.V. All rights reserved.

Stress and its effect on optical properties of GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire

The stress states in unintentionally doped GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire, and their effects on optical properties of GaN films were investigated by means of room-temperature confocal micro-Raman scattering and photoluminescence techniques. Relatively large tensile stress exists in GaN epilayers grown on Si and 6H-SiC while a small compressive stress appears in the film grown on sapphire. The latter indicates effective strain relaxation in the GaN buffer layer inserted in the GaN/sapphire sample, while the 50-nm-thick AlN buffer adopted in the GaN/Si sample remains highly strained. The analysis shows that the thermal mismatch between the epilayers and the substrates plays a major role in determining the residual strain in the films. Finally, a linear coefficient of 21.1+/-3.2 meV/GPa characterizing the relationship between the luminescent bandgap and the biaxial stress of the GaN films is obtained. (C) 2003 American Institute of Physics.

Time-resolved photoluminescence studies of AlInGaN alloys

We study the two samples of AIInGaN, i.e., 1-mum GaN grown at 1030degreesC on the buffer and followed by a 0.6-mum-thick epilayer of AIInGaN under the low pressure of 76 Torr and the AIInGaN layer deposited directly on the buffer layer without the high-temperature GaN layer, by temperature-dependent photoluminescence (PL) spectroscopy and picosecond time-resolved photoluminescence (TRPL) spectroscopy. The TRPL signals of both the samples were fitted well as a stretched exponential decay at all temperatures, indicating significant disorder in the material. We attribute the disorder to nanoscale quantum dots or discs of high indium concentration. Temperature dependence of dispersive exponent beta shows that the stretched exponential decay of the two samples comes from different mechanisms. The different depths of the localization potential account for the difference, which is illustrated by the results of temperature dependence of radiative recombination lifetime and PL peak energy.

Effects of reactor pressure on GaN nucleation layers and subsequent GaN epilayers grown on sapphire substrate

The influence of reactor pressure on GaN nucleation layer (NL) and the quality of subsequent GaN on sapphire is studied. The layers were grown by low-pressure metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire substrates and investigated by in situ laser reflectometry, atomic force microscope, scanning electron microscope, X-ray diffraction and photoluminescence. With the increase of reactor pressure prior to high-temperature GaN growth, the size of GaN nuclei formed after annealing decreases, the spacing between nucleation sites increases and the coalescence of GaN nuclei is deferred. The optical and crystalline qualities of GaN epilayer were improved when NLs were deposited at high pressure. The elongated lateral overgrowth of GaN islands is responsible for the quality improvement. (C) 2003 Elsevier Science B.V. All rights reserved.

High-resolution X-ray diffraction analysis of the Bragg peak integrated intensity in highly mismatched III-N epilayers

A new method of measuring the thickness of GaN epilayers on sapphire (0 0 0 1) substrates by using double crystal X-ray diffraction was proposed. The ratio of the integrated intensity between the GaN epilayer and the sapphire substrate showed a linear relationship with the GaN epilayer thickness up to 2.12 mum. It is practical and convenient to measure the GaN epilayer thickness using this ratio, and can mostly eliminate the effect of the reabsorption, the extinction and other scattering factors of the GaN epilayers. (C) 2003 Elsevier Science B.V. All rights reserved.

Numerical simulation of nc-Si : H/c-Si heterojunction solar cells

AMPS simulator, which was developed by Pennsylvania State University, has been used to simulate photovoltaic performances of nc-Si:H/c-Si solar cells. It is shown that interface states are essential factors prominently influencing open circuit voltages (V-OC) and fill factors (FF) of these structured solar cells. Short circuit current density (J(SC)) or spectral response seems more sensitive to the thickness of intrinsic a-Si:H buffer layers inserted into n(+)-nc-Si:H layer and p-c-Si substrates. Impacts of bandgap offset on solar cell performances have also been analyzed. As DeltaE(C) increases, degradation of VOC and FF owing to interface states are dramatically recovered. This implies that the interface state cannot merely be regarded as carrier recombination centres, and impacts of interfacial layer on devices need further investigation. Theoretical maximum efficiency of up to 31.17% (AM1.5,100mW/cm(2), 0.40-1.1mum) has been obtained with BSF structure, idealized light-trapping effect(R-F=0, R-B=1) and no interface states.

The growth morphologies of GaN layer on Si(111) substrate

The growth morphologies of metalorganic chemical vapor deposition (MOCVD) grown GaN layer on Si(111) substrate were studied using atomic force microscopy and transmission electron microscopy. It was found that the growth process of GaN/Si(111) consisted of two cycles of island growth and coalescence. These two cycles process differs markedly from that of one cycle process reported. The stress of evolving GaN layers on Si(111) was characterized by measuring the lattice constant c of GaN using X-ray diffraction (XRD) technique. It was proposed that the large tensile stress within the film during growth initiated this second island growth cycle, and the interaction between the GaN islands with high orientational fluctuation on the buffer layer induced this large tensile growth stress when coalescence occurred. (C) 2002 Elsevier Science B.V. All rights reserved.

Structural characteristic of cubic GaN nucleation layers on GaAs(001) substrates by MOCVD

The structural characteristic of cubic GaN (C-GaN) nucleation layers on GaAs(0 0 1) substrates by metalorganic chemical vapor deposition was in detail investigated first by X-ray diffraction (XRD) measurements, using a Huber five-circle diffractometer and an intense synchrotron X-ray source. The XRD results indicate that the C-GaN nucleation layers are highly crystallized. Phi scans and pole figures of the (1 1 1) reflections give a convincing proof that the GaN nucleation layers show exactly cubic symmetrical structure. The GaN(1 1 1) reflections at 54.74degrees in chi are a measurable component, however (002) components parallel to the substrate surface are not detected. Possible explanations are suggested. The pole figures of {1 0 (1) over bar 0} reflections from H-GaN inclusions show that the parasitic H-GaN originates from the C-GaN nucleation layers. The coherence lengths along the close-packed [1 1 1] directions estimated from the (1 1 1) peaks are nanometer order of magnitude. (C) 2002 Elsevier Science B.V. All rights reserved.

A novel line-order of InAs quantum dots on GaAs

A novel line-order of InAs quantum dots (QDs) along the [1, 1, 0] direction on GaAs substrate has been prepared by self-organized growth. After 2.5 monolayer InAs deposition, QDs in the first layer of multi-layer samples started to gather in a line. Owing to the action of strong stress between layers, almost all the dots of the fourth layer gathered in lines. The dots lining up tightly are actually one-dimensional superlattice of QDs, of which the density of electronic states is different from that of isolated QDs or quantum wires. The photoluminescence spectra of our multi-layer QD sample exhibited a feature of very broad band so that it is suitable for the active medium of super luminescent diode. The reason of dots lining up is attributed to the hill-and-valley structure of the buffer, anisotropy and different diffusion rates in the different directions on the buffer and strong stress between QD layers. (C) 2002 Published by Elsevier Science B. V.

A new method to fabricate InGaN quantum dots by metalorganic chemical vapor deposition

A new method to form nanoscale InGaN quantum dots using MOCVD is reported, This method is much different from a method. which uses surfactant or the Stranski-Krastannow growth mode. The dots were formed by increasing the energy barrier for adatoms, which are hopping by surface passivation, and by decreasing the growth temperature. Thus, the new method can be called as a passivation-low-temperature method. Regular high-temperature GaN films were grown first and were passivated. A low-temperature thin layer of GaN dot was then deposited on the surface that acted as the adjusting layer. At last the high-density InGaN dots could be fabricated on the adjusting layer. Atomic force microscopy measurement revealed that InGaN dots were small enough to expect zero-dimensional quantum effects: The islands were typically 80 nm wide and 5 nm high. Their density was about 6 x 10(10) cm(-2). Strong photoluminescence emission from the dots is observed at room temperature, which is much stronger than that of the homogeneous InGaN film with the same growth time. Furthermore, the PL emission of the GaN adjusting layer shows 21 meV blueshift compared with the band edge emission of the GaN due to quantum confine effect. (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.

A geometrical model of GaN morphology in initial growth stage

Based on morphology observed by atomic force microscopy, a geometrical model was proposed in order to explain the statistical results obtained from morphology observation on GaN in initial growth stage. Four parameters were introduced to describe the morphology characteristics in this model. Least-square fitting of height distribution was performed. The height distribution derived from the model agreed well with that obtained from experimental records. It was also found that the model should be further advanced to understand the growth of GaN in initial growth stage. (C) 2002 Elsevier Science BY. All rights reserved.

Indium doping effect on GaN in the initial growth stage

Three minutes' growth was carried out to investigate the indium-doping effect on initially grown GaN. Indium-doped and undoped samples were grown by low-pressure metalorganic vapor phase epitaxy. Atomic force microscope observation revealed that In-doping modified the morphology of the nuclei. Indium-doping also enhanced wetting between the buffer and nuclei layers, which was also supported by optical transmission. Photoluminescence suggested that indium-doping obviously enhanced band-edge related emission even in the nucleation stage. X-ray diffraction performed on samples grown for 20 minutes indicated improvement of the crystalline quality through indium-doping. The mechanism of the indium-doping effect was discussed.

High-quality metamorphic HEMT grown on GaAs substrates by MBE

Metamorphic high electron mobility transistor (M-HEMT) structures have been grown on GaAs substrates by molecular beam epitaxy (MBE). Linearly graded and the step-graded InGaAs and InAlAs buffet layers hal e been compared, and TEM, PL and low-temperature Hall have been used to analyze the properties of the buffer layers and the M-HEMT structure. For a single-delta-doped M-HEMT structure with an In0.53Ga0.47As channel layer and a 0.8 mum step-graded InAlAs buffer layer, room-temperature mobility of 9000 cm(2)/V s and a sheet electron density as high as 3.6 x 10(12)/cm(2) are obtained. These results are nearly equivalent to those obtained for the same structure grown on an InP substrate. A basic M-HEMT device with 1 mum gate was fabricated, and g(m) is larger than 400 mS/mm. (C) 2001 Elsevier Science B.V. All rights reserved.