Epitaxial growth of Ge_(1-x)Sn_x films with x up to 0.14 grown on Ge(00l) at low temperature

We characterize the structures of Ge1-xSnx films with x up to 0.14 grown on Ge(00l) by molecular-beam epitaxy at low temperature. The results show that Ge1-xSnx films are fully strained even at high Sn composition. The in-plane lattice parameters remain exactly the same as that of the substrate. Depth sensitivity analysis of the lattice parameters indicates that the strains of the epitaxial films are all in homogeneity. The films are fully strained. Poisson ratios, the force constants for the bonds between Ge and Sn are estimated and discussed in the present paper. Raman results show Ge–Ge, Ge–Sn,Sn–Sn vibrational modes. The Sn–Sn bond aggregation may respond to the high quality of our films. The fully strained epitaxy films with high content of Sn may be useful in designing the high quality GeSn films.     

Two crucial factors influencing quality of GaAs on Ge substrate

High-quality GaAs films with fine surfaces and GaAs/Ge interfaces on Ge have been achieved via molecular beam epitaxy. The influence of low temperature annealing and low temperature epitaxy on the quality of the film when GaAs is grown on a （100） 6 ° offcut towards [111] Ge substrate are investigated by analyzing and comparing the GaAs films that are fabricated via three different processes. A low temperature annealing process after high temperature annealing and a low temperature epitaxy process after the initial GaAs growth play a vital role in improving the quality of GaAs film on a Ge substrate.     

Strained and strain-relaxed epitaxial Ge1-xSnx alloys on Si（100） substrates

Epitaxial Ge1-xSnx alloys are grown separately on a Ge-buffer/Si(100) substrate and directly on a Si(100) substrate by molecular beam epitaxy (MBE) at low temperature. In the case of the Ge buffer/Si(100) substrate, a high crystalline quality strained Ge0.97Sn0.03 alloy is grown, with a χmin value of 6.7% measured by channeling and random Rutherford backscattering spectrometry (RBS), and a surface root-mean-square (RMS) roughness of 1.568 nm obtained by atomic force microscopy (AFM). In the case of the Si(100) substrate, strain-relaxed Ge0.97Sn0.03 alloys are epitaxially grown at 150°C-300°C, with the degree of strain relaxation being more than 96%. The X-ray diffraction (XRD) and AFM measurements demonstrate that the alloys each have a good crystalline quality and a relatively flat surface. The predominant defects accommodating the large misfit are Lomer edge dislocations at the interface, which are parallel to the interface plane and should not degrade electrical properties and device performance.     

Effect of substrate temperature on the morphological,structural,and optical properties of RF sputtered Ge_(1-x)Sn_x films on Si substrate

In this study, Ge_(1-x)Sn_x alloy films are co-sputtered on Si(100) substrates using RF magnetron sputtering at different substrate temperatures. Scanning electron micrographs, atomic force microscopy(AFM), Raman spectroscopy, and x-ray photoemission spectroscopy(XPS) are conducted to investigate the effect of substrate temperature on the structural and optical properties of grown Ge Sn alloy films. AFM results show that RMS surface roughness of the films increases from 1.02 to 2.30 nm when raising the substrate temperature. This increase could be due to Sn surface segregation that occurs when raising the substrate temperature. Raman spectra exhibits the lowest FWHM value and highest phonon intensity for a film sputtered at 140?C. The spectra show that decreasing the deposition temperature to 140?C improves the crystalline quality of the alloy films and increases nanocrystalline phase formation. The results of Raman spectra and XPS confirm Ge–Sn bond formation. The optoelectronic characteristics of fabricated metal-semiconductor-metal photodetectors on sputtered samples at room temperature(RT) and 140?C are studied in the dark and under illumination. The sample sputtered at 140?C performs better than the RT sputtered sample.     

Characterization of Thick GaN Films Directly Grown on Wet-Etching Patterned Sapphire by HVPE

Thick GaN films of high quality are directly grown on wet-etching patterned sapphire in a vertical hydride vapour phase epitaxy reactor. The optical and structural properties of GaN films are studied using scanning electronic microscopy and cathodoluminescence. Test results show that initial growth of hydride vapour phase epitaxy GaN occurs not only on the mesas but also on the two asymmetric sidewalls of the V-shaped grooves without selectivity. After the two-step coalescence near the interface, the GaN films near the surface keep on growing along the direction perpendicular to the long sidewall. Based on Raman results, GaN of the coalescence region in the grooves has the maximum residual stress and poor crystalline quality over the whole GaN film, and the coalescence process can release the stress. Therefore, stress-free thick GaN films are prepared with smooth and crack-free surfaces by this particular growth mode on wet-etching patterned sapphire substrates.     

Superconducting Properties of MgB2 with Different Lattice Parameters

We investigate the elecronic structures and superconductivities of new superconductor MgB2 with different lattice parameters by means of multiple-Scattered-Xa calculation.It is shown that the lattice parameters have great effect on the superconductivity of MgB2.The results of this paper show that the density of states at the Fermi level decreases with the decreasing lattice parameters.The effects are very more sensitive substantially to the change of the lattice parameter a as compared with that of the lattice paramter c.Correspondingly.the superconducting transition temperature TC decreases correspondingly mainly with deceasing the lattice parameter a .Our results are in good agreement with the experiments.     

GaN layers with different polarities prepared by radio frequency molecular beam epitaxy and characterized by Raman scattering

GaN layers with different polarities have been prepared by radio-frequency molecular beam epitaxy (RF-MBE) and characterized by Raman scattering. Polarity control are realized by controlling Al/N flux ratio during high temperature AlN buffer growth. The Raman results illustrate that the N-polarity GaN films have frequency shifts at $A_{1}$(LO) mode because of their high carrier density; the forbidden $A_{1}$(TO) mode occurs for mixed-polarity GaN films due to the destroyed translation symmetry by inversion domain boundaries (IDBS); Raman spectra for Ga-polarity GaN films show that they have neither frequency shifts mode nor forbidden mode. These results indicate that Ga-polarity GaN films have a better quality, and they are in good agreement with the results obtained from the room temperature Hall mobility. The best values of Ga-polarity GaN films are 1042 cm$^{2}$/Vs with a carrier density of 1.0$\times $10$^{17}$~cm$^{ - 3}$.     

Growth of Mn5Ge3 ultrathin film on Ge（111）

The growth of Mn5Ge3 ultrathin films with different thicknesses, prepared by solid phase epitaxy, is studied. The results of scanning tunnelling microscopy and low energy electron diffraction studies show that the film can be formed and it is terminated with a （√3 × √3） R30° surface reconstruction when the thickness of Mn exceeds 3 monolayers. The magnetic properties show that the Curie temperature is about 300 K and the T^2-dependent behaviour is observed to remain up to 220 K.     

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.     

A k·p analytical model for valence band of biaxial strained Ge on（001） Si1-xGex

In this paper,the dispersion relationship is derived by using the k·p method with the help of the perturbation theory,and we obtain the analytical expression in connection with the deformation potential.The calculation of the valence band of the biaxial strained Ge/（001）Si1-xGex is then performed.The results show that the first valence band edge moves up as Ge fraction x decreases,while the second valence band edge moves down.The band structures in the strained Ge/（001）Si 0.4 Ge 0.6 exhibit significant changes with x decreasing in the relaxed Ge along the [0,0,k] and the [k,0,0] directions.Furthermore,we employ a pseudo-potential total energy package（CASTEP） approach to calculate the band structure with the Ge fraction ranging from x = 0.6 to 1.Our analytical results of the splitting energy accord with the CASTEP-extracted results.The quantitative results obtained in this work can provide some theoretical references to the understanding of the strained Ge materials and the conduction channel design related to stress and orientation in the strained Ge pMOSFET.     

Effects of Substrate Temperature on Helium Content and Microstructure of Nanocrystalline Titanium Films

Helium-charged nanocrystalline titanium films have been deposited by HeAr magnetron co-sputtering. The effects of substrate temperature on the helium content and microstructure of the nanocrystalline titanium films have been studied. The results indicate that helium atoms with a high concentration are evenly incorporated in the deposited titanium films. When the substrate temperature increases from 60℃ to 350℃ while the other deposition＇parameters are fixed, the helium content decreases gradually from 38.6 at.% to 9.2at.%, which proves that nanocrystalline Ti films have a great helium storage capacity. The 20 angle of the Bragg peak of （002） crystal planes of the He-charged Ti film shifts to a lower angle and that of （100） crystal plane is unchanged as compared with that of the pure Ti film, which indicates that the lattice parameter c increases and a keeps at the primitive value. The grain refining and helium damage result in the diffraction peak broadening.     

Molecular beam epitaxy growth of GaAs on an offcut Ge substrate

Molecular beam epitaxy growth of GaAs on an offcut Ge (100) substrate has been systemically investigated. A high quality GaAs/Ge interface and GaAs film on Ge have been achieved. High temperature annealing before GaAs deposition is found to be indispensable to avoid anti-phase domains. The quality of the GaAs film is found to strongly depend on the GaAs/Ge interface and the beginning of GaAs deposition. The reason why both high temperature annealing and GaAs growth temperature can affect epitaxial GaAs film quality is discussed. High quality In_0.17Ga_0.83As/GaAs strained quantum wells have also been achieved on a Ge substrate. Samples show flat surface morphology and narrow photoluminescence line width compared with the same structure sample grown on a GaAs substrate. These results indicate a large application potential for III--V compound semiconductor optoelectronic devices on Ge substrates.     

A k·p analytical model for valence band of biaxial strained Ge on（001） Si_1-xGe_x

In this paper,the dispersion relationship is derived by using the k·p method with the help of the perturbation theory,and we obtain the analytical expression in connection with the deformation potential.The calculation of the valence band of the biaxial strained Ge/(001)Si1-xGex is then performed.The results show that the first valence band edge moves up as Ge fraction x decreases,while the second valence band edge moves down.The band structures in the strained Ge/(001)Si 0.4 Ge 0.6 exhibit significant changes with x decreasing in the relaxed Ge along the [0,0,k] and the [k,0,0] directions.Furthermore,we employ a pseudo-potential total energy package(CASTEP) approach to calculate the band structure with the Ge fraction ranging from x = 0.6 to 1.Our analytical results of the splitting energy accord with the CASTEP-extracted results.The quantitative results obtained in this work can provide some theoretical references to the understanding of the strained Ge materials and the conduction channel design related to stress and orientation in the strained Ge pMOSFET.     

Structure,Electrical,and Optical Properties of Nb-doped BaTiO3 Thin Films Grown by Laser Molecular Beam Epitaxy

Structure,electrical,and optical properties of Nb-doped BaTiO3 (Nb:BTO) thin films on MgO substrates grownby laser molecular beam epitaxy with increasing Nb content were investigated.The Nb:BTO thin films with high crystallinity are epitaxially grown on MgO substrates.With more Nb-doped content,the impurity phases are found in Nb:BTO thin films.Hall measurement at room temperature confirms that the charge carriers of the Nb:BTO thin films are n-type.When the Nb-doped content increases,the carrier concentration and carrier mobility increase.Meanwhile the optical transmittance decreases with the increase of the Nb-doping,and the width of the forbidden band in each group is not affected by the presence of Nb in the samples.Raman spectra show that the structural phase transition may occur with the increase of the Nb-doping content,in the meantime more defects and impurities exist in the Nb:BTO thin films.     

Oxygen Pressure Dependence of Properties of Epitaxial LaAlO3 Films Grown on Si （100）

Heteroepitaxial LaAlO3 films were grown on a SrTiO3/Si (100) substrate by laser molecular beam epitaxy under different oxygen pressures, and their properties such as crystallinity and electrical characteristics were experimentally investigated using the various measurement methods. The results show that most properties depend mainly on the deposition oxygen pressure. The crystallinity and the C-V and I-V characteristics can be greatly improved with the increasing oxygen deposition pressure. Moreover, after annealed at 1050~C in N2 ambient, the C-V and I-V characteristics of LAO films deposited at the lower oxygen pressure are also improved due to the decrease of oxygen vacancies in LAO films.     

Effects of high temperature rapid thermal annealing on Ge films grown on Si（001） substrate

Tensile strain,crystal quality,and surface morphology of 500 nm thick Ge films were improved after rapid thermal annealing at 900 C for a short period(20 s).The films were grown on Si(001)substrates by ultra-high vacuum chemical vapor deposition.These improvements are attributed to relaxation and defect annihilation in the Ge films.However,after prolonged(20 s)rapid thermal annealing,tensile strain and crystal quality degenerated.This phenomenon results from intensive Si–Ge mixing at high temperature.     

Microstructure and strain analysis of GaN epitaxial films using in-plane grazing incidence x-ray diffraction

This paper investigates the major structural parameters,such as crystal quality and strain state of (001)-oriented GaN thin films grown on sapphire substrates by metalorganic chemical vapour deposition,using an in-plane grazing incidence x-ray diffraction technique.The results are analysed and compared with a complementary out-of-plane xray diffraction technique.The twist of the GaN mosaic structure is determined through the direct grazing incidence measurement of (100) reflection which agrees well with the result obtained by extrapolation method.The method for directly determining the in-plane lattice parameters of the GaN layers is also presented.Combined with the biaxial strain model,it derives the lattice parameters corresponding to fully relaxed GaN films.The GaN epilayers show an increasing residual compressive stress with increasing layer thickness when the two dimensional growth stage is established,reaching to a maximum level of-0.89 GPa.     

Microstructure and strain films using in-plane grazing analysis of GaN epitaxial incidence x-ray diffraction

This paper investigates the major structural parameters, such as crystal quality and strain state of （001）-oriented GaN thin films grown on sapphire substrates by metalorganic chemical vapour deposition, using an in-plane grazing incidence x-ray diffraction technique. The results are analysed and compared with a complementary out-of-plane x- ray diffraction technique. The twist of the GaN mosaic structure is determined through the direct grazing incidence t of （100） reflection which agrees well with the result obtained by extrapolation method. The method for directly determining the in-plane lattice parameters of the GaN layers is also presented. Combined with the biaxial strain model, it derives the lattice parameters corresponding to fully relaxed GaN films. The GaN epilayers show an increasing residual compressive stress with increasing layer thickness when the two dimensional growth stage is established, reaching to a maximum level of-0.89 GPa.     

Improved crystal quality of GaN film with the in-plane lattice-matched In_(0.17)Al_(0.83)N interlayer grown on sapphire substrate using pulsed metal organic chemical vapor deposition

We report on an improvement in the crystal quality of GaN film with an In0.17Al0.83N interlayer grown by pulsed metal–organic chemical vapor deposition, which is in-plane lattice-matched to GaN films. The indium composition of about 17% and the reductions of both screw and edge threading dislocations(TDs) in GaN film with the InAlN interlayer are estimated by high resolution X-ray diffraction. Transmission electron microscopy(TEM) measurements are employed to understand the mechanism of reduction in TD density. Raman and photoluminescence measurements indicate that the InAlN interlayer can improve the crystal quality of GaN film, and verify that there is no additional residual stress induced into the GaN film with InAlN interlayer. Atomic force microscopy measurement shows that the InAlN interlayer brings in a smooth surface morphology of GaN film. All the results show that the insertion of the InAlN interlayer is a convenient method to achieve excellent crystal quality in GaN epitaxy.     

Improved crystal quality of GaN film with the in-plane lattice-matched Ino.17Alo.s3N interlayer grown on sapphire substrate using pulsed metal-organic chemical vapor deposition

We report on an improvement in the crystal quality of GaN film with an Ino.17Alo.83N interlayer grown by pulsed metal-organic chemical vapor deposition, which is in-plane lattice-matched to GaN films. The indium composition of about 17% and the reductions of both screw and edge threading dislocations （TDs） in GaN film with the InA1N interlayer are estimated by high resolution X-ray diffraction. Transmission electron microscopy （TEM） measurements are employed to understand the mechanism of reduction in TD density. Raman and photoluminescence measurements indicate that the InA1N interlayer can improve the crystal quality of GaN film, and verify that there is no additional residual stress induced into the GaN film with InA1N interlayer. Atomic force microscopy measurement shows that the InA1N interlayer brings in a smooth surface morphology of GaN film. All the results show that the insertion of the InA1N interlayer is a convenient method to achieve excellent crystal quality in GaN epitaxy.