High Quality AlN with a Thin Interlayer Grown on a Sapphire Substrate by Plasma-Assisted Molecular Beam Epitaxy

We report an AlN epi-layer grown on sapphire by plasma-assisted molecular beam epitaxy with a thin interlayer structure. The effects of growth mode on threading dislocations (TDs) and surface morphology are studied. Then an interlayer structure grown under a V/Ⅲ ratio of 1 is adopted to improve the AlN crystalline quality. By optimizing the thickness of the interlayer, the TD density and surface roughness can be reduced simultaneously.     

GaAs Based InAs/GaSb Superlattice Short Wavelength Infrared Detectors Grown by Molecular Beam Epitaxy

InAs/GaSb superlattice （SL） short wavelength infrared photoconduction detectors are grown by molecular beam epitaxy on GaAs（O01） semi-insulating substrates. An interfacal misfit mode AISb quantum dot layer and a thick GaSb layer are grown as buffer layers. The detectors containing a 200-period 2 ML/S ML InAs/GaSb SL active layer are fabricated with a pixel area of 800×800 μm^2 without using passivation or antirefleetion coatings. Corresponding to the 50% cutoff wavelengths of 2.05μm at 77K and 2.25 μm at 300 K, the peak detectivities of the detectors are 4 × 10^9 cm·Hz^1/2/W at 77K and 2 × 10^8 cm·Hz1/2/W at 300K, respectively.     

Optimization of Metamorphic InGaAs Quantum Wells on GaAs Grown by Molecular Beam Epitaxy

We investigate the molecular beam epitaxy growth of metamorphic InxGal-xAs materials （x up to 0.5） on GaAs substrates systematically. Optimization of structure design and growth parameters is aimed at obtaining smooth surface and high optical qualdty. The optimized structures have an average surface roughness of 0.9-1.8 nm. It is also proven by PL measurements that the optical properties of high indium content （55%） InGaAs quantum wells are improved apparently by defect reduction technique and by introducing Sb as a surfactant. These provide us new ways for growing device quality metamorphic structures on GaAs substrates with long-wavelength emissions.     

Interfaces in InAs/GaSb Superlattices Grown by Molecular Beam Epitaxy

Short period InAs（4ML）/GaSb（SML） superlattices （SLs） with InSb- and mixed-like （or Ga1-xInxAs1-ySby- like） interfaces （IFs） are grown by molecular-beam epitaxy （MBE） on （001） GaSh substrates at optimized growth temperature. Raman scattering reveals that two kinds of IFs can be formed by controlling shutter sequences. X-ray diffraction （XRD） and atomic force microscopy （AFM） demonstrate that SLs with mixed-like IFs are more sensitive to growth temperature than that with InSb-like IFs. The photoluminescence （PL） spectra of SLs with mixed-like IFs show a stronger intensity and narrower line width than with InSb-like IFs. It is concluded that InAs/GaSb SLs with mixed-like IFs have better crystalline and optical properties.     

Molecular Beam Epitaxy of GaSb on GaAs Substrates with AlSb Buffer Layers

We investigate the molecular beam epitaxy growth of GaSb films on GaAs substrates using AlSb buffer layers. Optimization of AlSb growth parameter is aimed at obtaining high GaSh crystal quality and smooth GaSh surface. The optimized growth temperature and thickness of AlSb layers are found to be 450℃ and 2.1 nm, respectively. A rms surface roughness of 0.67 nm over 10 × 10 μm^2 is achieved as a 0.5 μm GaSh film is grown under optimized conditions.     

Growth and Characteristics of Epitaxial AlxGa1-xN by MOCVD

Al_xGa_1-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 (10^-15) full widths at half-maximum (FWHM) of the Al_xGa_1-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.     

Properties of Strain Compensated Symmetrical Triangular Quantum Wells Composed of InGaAs/InAs Chirped Superlattice Grown Using Gas Source Molecular Beam Epitaxy

We investigate the properties of symmetrical triangular quantum wells composed of InGaAs/InAs chirped superlattice, which is grown by gas source molecular beam epitaxy via digital alloy method. In the quantum well structure tensile AlInGaAs are used as barriers to partially compensate for the significant compressive strain in the wells, the strain compensation effects are confirmed by x-ray measurement. The photoluminescence spectra of the sample are dominated by the excitonic recombination peak in the whole temperature range. The thermal quenching, peak energy shift and line-width broadening of the PL spectra are analysed in detail, the mechanisms are discussed.     

Effect of Ⅲ/Ⅴ Ratio of HT-AIN Buffer Layer on Polarity Selection and Electrical Quality of GaN Films Grown by Radio Frequency Molecular Beam Epitaxy

We investigate the effect of A/N ratio of the high temperature （HT） AIN buffer layer on polarity selection and electrical quality of GaN films grown by radio frequency molecular beam epitaxy. The results show that low Al/N ratio results in N-polarity GaN films and intermediate Al/N ratio leads to mixed-polarity GaN films with poor electrical quality. GaN films tend to grow with Ga polarity on Al-rich AIN buffer layers. GaN films with different polarities are confirmed by in-situ reflection high-energy electron diffraction during the growth process. Wet chemical etching, together with atomic force microscopy, also proves the polarity assignments. The optimum value for room-temperature Hall mobility of the Ga-polarity GaN film is 703cm^2/V.s, which is superior to the N-polarity and mixed-polarity GaN films.     

Structural and Photoluminescence Properties for Highly Strain-Compensated InGaAs/InAlAs Superlattice

The effects of strain compensation are investigated by using twenty periods of highly strain-compensated InGaAs/InAlAs superlattice. The lattice mismatches of individual layers are as high as about 1%, and the thicknesses are close to critical thicknesses. X-ray diffraction measurements show that lattice imperfectness is not serious but still present, though the structural parameters are within the range of theoretical design criteria for structural stability. Rough interfaces and composition fluctuations are the primary causes for lattice imperfectness. Photoluminescence measurements show the large thermally activated nonradiative recombination in the sample. In addition, the recombination process gradually evolves from excitonic recombination at lower temperatures to band-to-band recombination at higher temperatures, which should be considered in device applications.     

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 × 10^19 cm^-3, and mobility of 17.8cm^2V^-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.     

Highly Strained Si Films with Ultra-low Dislocation Density Grown on Virtual Substrates of Thin Thickness

By using compositionally graded SiGe films as virtual substrates, tensile strained Si films with the strain of 1.5% and the threading dislocation density less than 1.0 × 10^5 cm-2 are successfully grown in micron size windows by molecular beam epitaxy （MBE）. The thickness of the virtual substrates was only 33Onto. On the surface of the s-Si films no cross-hatched lines resulting from misfit dislocations could be observed. We attribute these results to the edge-induced strain relaxation of the epitaxial films in windows, and the patterned virtual substrates with compositionally graded SiGe films.     

CoPt alloy grown on the WSe2(0 0 0 1) van der Waals surface

The structural and magnetic properties of 3-nm-thick CoPt alloys grown on WSe₂(0 0 0 1) at various temperature are investigated. Deposition at room temperature leads to the formation of a chemically disordered fcc CoPt alloy with [1 1 1] orientation. Growth at elevated temperatures induces L1₀ chemical order starting at 470 K accompanied with an increase in grain size and a change in grain morphology. As a consequence of the [1 1 1] growth direction, the CoPt grains can adopt one of the three possible variants of the L1₀ phase with tetragonal c-axis tilted from the normal to the film plane direction at 54°. The average long-range order parameter is found to be 0.35(±0.05) and does not change with the increase in the deposition temperature from 570 to 730 K. This behavior might be related to Se segregation towards the growing facets and surface disorder effects promoted by a high surface-to-volume ratio. Magnetic studies reveal a superparamagnetic behavior for the films grown at 570 and 730 K in agreement with the film morphology and degree of chemical order. The measurements at 10 K reveal the orientation of the easy axis of the magnetization lying basically in the film plane.     

Effect of Reactor Pressure on Qualities of GaN Layers Grown by Hydride Vapour Phase Epitaxy

The influence of reactor pressure on GaN layers grown by hydride vapour phase epitaxy （HVPE） is investigated. By decreasing the reactor pressure from0. 7 to 0.5 mm, the GaN layer growth mode changes from the island-like one to the step flow. The improvements in structural and optical properties and surface morphology of GaN layers are observed in the step flow growth mode. The results clearly indicate that the reactor pressure, similarly to the growth temperature, is One of the important parameters to influence the qualities of GaN epilayers grown by HVPE, due to the change of growth mode.     

Influence of AlN Buffer Thickness on GaN Grown on Si(111) by Gas Source Molecular Beam Epitaxy with Ammonia

Hexagonal GaN is grown on a Si(111) substrate with AlN as a buffer layer by gas source molecular beam epitaxy (GSMBE) with ammonia. The thickness of AlN buffer is changed from 9 to 72nm. When the thickness of AlN buffer is 36nm, the surface morphology and crystal quality of GaN is optimal. The in-situ reflection high energy electron diffraction (RHEED) reveals that the transition to a two-dimensional growth mode of AlN is the key to the quality of GaN. However, the thickness of AlN buffer is not so critical to the residual in-plane tensile stress in GaN grown on Si(111) by GSMBE for AlN thickness between 9 to 72nm.     

Realization of Ultraviolet Electroluminescence from ZnO Homojunction Fabricated on Silicon Substrate with p-Type ZnO:N Layer Formed by Radical N2O Doping

ZnO homojunction light-emitting diodes are fabricated on Si（100） substrates by plasma assisted metal organic chemical vapour deposition. A p-type layer of nitrogen-doped ZnO film is formed using radical N2O as the acceptor precursor. The n-type ZnO layer is composed of un-doped ZnO film. The device exhibits desirable rectifying behaviour with a turn-on voltage of 3.3 V and a reverse breakdown voltage higher than 6 V. Distinct electrolumineseence emissions centred at 395nm and 490nm are detected from this device at forward current higher than 20mA at room temperature.     

Laser Molecular Beam Epitaxy of Multilayer Heterostructure SrNb0.05Ti0.95O3/La0.9Sr0.1MnO3 in 10000 Unit-Cell Layers

Ten thousands of unit-cell multilayer heterosturctures, [SrNb0.05 Ti0.95O3/La0.9 Sr0.1MnO3]3 （SNTO/LSMO）, have been epitaxial grown on SrTiO3 （001） substrates by laser molecular beam epitaxy. The monitor of insitu. reflection high-energy electron diffraction demonstrates that the heterosturctures are layer-by-layer epitaxial growth. Atomic force microscope observation indicates that the surface of the heterosturcture is atomically smooth. The measurements of cross-sectional low magnification and high-resolution transmission electron microscopy as well as the corresponding selected area electron diffraction reveal that the interfaces are of perfect orientation, and the epitaxial crystalline structure shows the orientation relation of SNTO（001）//LSMO（001）, and SNTO[100]//LSMO[100].     

Growth of highly disordered InGaP on (100) GaAs by molecular beam epitaxy with a GaP decomposition source

High-quality, lattice-matched InGaP on exact (100) GaAs was successfully grown by molecular beam epitaxy with a GaP decomposition source. The ordering parameter (η) of the InGaP is investigated as a function of the growth temperature. η is as low as 0.22 and almost insensitive to the growth temperature below 460 °C. It increases abruptly around 475 °C and has a maximum value of 0.35 at ≈490 °C. Double crystal X-ray diffraction and a low-temperature photoluminescence spectrum reveal that the present growth method is robust and provides better quality InGaP compared to other state-of-the-art growth technologies. Received: 20 November 2000 / Accepted: 27 January 2001 / Published online: 21 March 2001     

Growth and Annealing Study of Mg-Doped AlGaN and GaN/AlGaN Superlattices

Mg-doped AlGaN and GaN/AlGaN superlattices are grown by metalorganic chemical vapour deposition （MOCVD） Rapid thermal annealing （RTA） treatments are carried out on the samples. Hall and high resolution x-ray diffraction measurements are used to characterize the electrical and structural prosperities of the as-grown and annealed samples, respectively. The results of hall measurements show that after annealing, the Mg-doped AIGaN sample can not obtain the distinct hole concentration and can acquire a resistivity of 1.4 ×10^3 Ωcm. However, with the same annealing treatment, the GaN/AlGaN superlattice sample has a hole concentration of 1.7 × 10^17 cm-3 and a resistivity of 5.6Ωcm. The piezoelectric field in the GaN/AlGaN superlattices improves the activation efficiency of Mg acceptors, which leads to higher hole concentration and lower p-type resistivity.     

Local environment of nitrogen in GaNyAs1−y epilayers on GaAs (0 0 1) studied using X-ray absorption near edge spectroscopy

X-ray absorption near-edge spectroscopy (XANES) is used to study the N environment in bulk GaN and in GaN_yAs_1−y epilayers on GaAs (0 0 1), for y∼5%. Density-functional optimized structures were used to predict XANES via multiple-scattering theory. We obtain striking agreement for pure GaN. An alloy model with nitrogen pairs on Ga accurately predicts the threshold energy, the width of the XANES ‘white line’, and features above threshold, for the given X-ray polarization. The presence of large quantitities of N-pairs may point to a role for molecular N₂ in epitaxial growth kinetics.     

Evolutions of Crystal Structure, Stoichiometry and Electrochemical Behavior with Co Substitution in LiNi1-yCoyO2 Positive Electrodes

LiNi1-yCoyO2（0.1 ≤ y ≤ 0.4） positive electrode materials are synthesized by a chemical method with stoichiometric acetates of related cations. Their crystal structure, stoichiometry and electrochemical behaviors versus Co concentration are investigated by x-ray diffraction, synchrotron-based x-ray absorption fine structure and galvanostatic cycling ts. The results reveal that the non-stoiehiometric Ni^2＋, Li/Ni cation mixing and polarization are reduced as the amount of Co substitution increases, clearly indicating that the Co element is a medium for easily oxidizing Ni^2＋ to Ni^3＋ during the synthesis process.