Effects of a ZnTe buffer layer on structural quality and morphology of CdTe epilayer grown on (001)GaAs by molecular beam epitaxy

The structural properties, crystalline quality and surface morphology of CdTe thin films without and with a ZnTe buffer layer grown on (001)GaAs by molecular beam epitaxy (MBE) have been studied. CdTe thin film directly prepared on GaAs substrate displays (111) orientation with an island growth mode, whereas the CdTe epilayers with a ZnTe buffer are (001)-oriented single-crystalline film with a two-dimensional (2D) growth mode. The morphology and surface root-mean-square (RMS) roughness of CdTe epilayers are also dramatically improved by using a ZnTe buffer. Furthermore, it is suggested that the high-temperature (HT) ZnTe buffer grown at 360 °C is more efficient for enhancing CdTe structural quality than the low-temperature (LT) one at 320 °C. The CdTe epilayer on the HT-ZnTe buffer shows a narrower full-width at half-maximum (FWHM) of double-crystal X-ray rocking curve (DCXRC) for (004) reflection and a smaller RMS roughness.     

The effect of growth parameters on CrN thin films grown by molecular beam epitaxy

In this paper, we report on the controlling of the effect of growth parameters such as substrate temperature and the ratio of Cr and N atoms on phase formation, surface morphology and crystallization of CrN(001) thin films grown by plasma-assisted molecular beam epitaxy on the MgO(001) substrate. The reflection high energy electron diffraction, atomic force microscopy, X-ray diffraction and scanning tunneling microscopy are used to characterize the thin films grown under various conditions. High-quality CrN(001) thin films are achieved at a substrate temperature 430 °C with a low Cr deposition rate.     

MBE growth and characterization of buried silicon oxide films on Si(100)

Silicon molecular beam epitaxy (Si-MBE) has been used to produce silicon oxide (SiO_x) films by evaporating Si on a heated Si(100) substrate in an ultra high vacuum system with an O₂ pressure of 10⁻⁶ to 10⁻⁴ mbar. Then the SiO_x films were overgrown with pure Si. The influence of the substrate temperature, the O₂ pressure and the Si deposition rate on the oxygen content in the SiO_x films and on the crystalline quality of the Si top-layer was investigated by Rutherford backscattering spectrometry and ion channeling. Epitaxial growth of the Si top-layer was observed up to a maximum concentration of ≈20 at.% oxygen content in the SiO_x film. Cross-sectional transmission electron microscopy shows that the structure of the SiO_x film changes duringa subsequent annealing procedure. Electron energy loss spectrometry proves that amorphous SiO₂ is formed and the development of holes indicates that the density of the as-grown SiO_x film is much lower than that of SiO₂. The specific for the as-grown SiO_x films was determined by I–V measurements.     

Surface and structural disorder in MBE and sputtering deposited Cu thin films revealed by X-ray measurements

Copper thin films have been deposited on Si substrates by molecular beam epitaxy (MBE) at different deposition rates varying from 1 up to 22 Å/s. X-ray reflectivity and θ-2θ measurements have shown that the surface roughness correlation length, the structural disorder and the grain dimensions are strongly affected by the deposition rate. Comparing these results with those obtained for sputtered deposited thin films with a low deposition rate (2.5 Å/s), a clear similarity between the MBE samples deposited with the highest deposition rate and the sputtering Cu films is observed. This result has been interpreted considering the different energies of the particles that approach the substrate in the two deposition techniques.     

Investigation of the growth of In2O3 on Y-stabilized ZrO2(100) by oxygen plasma assisted molecular beam epitaxy

Thin films of In₂O₃ have been grown on Y-stabilised ZrO₂(100) substrates by oxygen plasma assisted molecular beam epitaxy over a range of substrate temperatures between 650 °C and 900 °C. Growth at 650 °C leads to continuous but granular films and complete extinction of substrate core level structure in X-ray photoelectron spectroscopy. However with increasing substrate temperature the films break up into a series of discrete micrometer sized islands. Both the continuous and the island films have excellent epitaxial relationship with the substrate as gauged by X-ray diffraction and selected area electron diffraction and lattice imaging in high resolution transmission electron microscopy.     

Effects of low temperature ZnO and MgO buffer thicknesses on properties of ZnO films grown on (0001) Al2O3 substrates by plasma-assisted molecular beam epitaxy

We investigated the structural properties of Zn-polar ZnO films with low temperature (LT) ZnO and MgO buffer layers grown by plasma-assisted molecular beam epitaxy on (0001) c-Al₂O₃ substrates using X-ray diffraction and transmission electron microscopy (TEM). The effects of MgO buffer layer thickness and LT ZnO buffer layer thickness were also examined. The optimum thicknesses for better crystal quality were 8 and 40 nm. One-pair and two-pair LT ZnO/MgO buffer layers were employed, and the changes in the structural properties of the high-temperature (HT) ZnO films using such buffer layers were studied. Contrary to the general tendency of c-ZnO films, the HT ZnO films on the LT ZnO/MgO buffer layers showed higher full width at half maximum (FWHM) values for X-ray rocking curves (XRCs) with (0002) reflection than those with (101?1) reflection. Compared with the one-pair LT ZnO/MgO buffer layers, the FWHM values of (0002) XRCs markedly decreased, whereas those of (101?1) XRCs slightly increased due to the insertion of one more pair of LT ZnO/MgO buffer layers into the previous film with one-pair LT ZnO/MgO buffer layers. The cross-sectional TEM observations with the two-beam condition confirmed that the screw dislocation was the dominant threading dislocation type—a finding that agreed well with the XRC results. On the basis of the plan-view TEM observations, the densities of the total threading dislocations for the HT ZnO films with the one- and two-pair LT ZnO/MgO buffer layers were determined as 2.3 × 10⁹ cm^− 2 and 1.6 × 10⁹ cm^− 2, respectively. The results imply that the crystal quality of Zn-polar ZnO films can be improved by two-pair LT ZnO/MgO buffer layers, and types of threading dislocations can be modified by adjusting the buffer system.     

Properties and applications of MBE grown AlGaN

AlGaN epitaxial films have been grown on sapphire by plasma-induced molecular beam epitaxy (MBE) over the entire composition range from GaN to AlN. Structural and optical properties of the alloys have been investigated by X-ray diffraction (XRD), transmission electron and atomic force microscopy, Raman scattering, ellipsometry, optical transmission, and subgap absorption spectroscopy. Electron spin resonance has been used to study the dependence of intrinsic paramagnetic defects on Al mole fraction. N- and p-type doping with Si and Mg, respectively is found to become increasingly difficult with increasing Al content because of a continuous shift of the donor and acceptor levels deeper into the bandgap. Apart from the use of AlGaN as cladding layers in light emitting diodes, applications in MODFET transistors, solar blind photodetectors, surface acoustic wave devices and Bragg reflectors appear interesting and will be discussed briefly.     

Structural and optical properties of GaN films grown on GaAs substrates by molecular beam epitaxy

GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. X-ray diffraction and transmission electron microscopy indicate that this method promotes prismatic growth of c-oriented α-GaN. Photoluminescence studies show that the emission from cubic β-GaN inclusions dominates the spectrum.     

MBE生长GaAs薄膜表面形貌的RHEED图样研究

本文通过在GaAs(100)单晶衬底上MBE生长GaAs过程中形成的RHEED衍射图样,对GaAs薄膜的表面形貌进行研究。分析GaAs表面粗糙和生长时不发生RHEED强度振荡的原因。讨论在生长GaAs时出现In(Ga)As/GaAs(100)体系的RHEED衍射图样这种异常现象的原因。     

Structural characterization of molecular beam epitaxy grown ZnSe-based layers on GaAs substrates for blue–green laser diodes

ZnSe films and fully developed p-on-n laser structures, including CdZnSe-active and ZnSSe-guiding layers were grown by molecular beam epitaxy (MBE) on lattice matched p-GaAs, p-AlGaAs and p-GaInP buffer layers. The structural characteristics of these layers were studied by combined cross-section and planar view transmission electron microscopy (TEM). The defect density of the ZnS_xSe_1−x epilayers was shown to be very low, <10⁵ cm⁻². However, on their interfaces with the GaAs substrate, a high density of small dislocation loops and clusters of the order of 3×10¹⁰ cm⁻² was observed. In situ TEM experiments revealed that dislocations and stacking faults (SFs) were generated under the electron beam influence. From them, the perfect dislocations were confined at the ZnSe/GaAs interface, while the SFs propagated to the ZnSe overgrowth or the GaAs substrate, having one of their partial dislocations at the interface. The generation of dislocations under the electron beam was not related to radiation damage but to thermal strain, which was developed by the heating effect due to differential thermal dilatation. Defects around the active zone of fully developed p-on-n laser structures were also studied. The nature of such defects was defined by Burgers vector determination experiments. The critical role of growth variations, such as compositional changes resulting in strain, in the MBE process of II–VI materials was demonstrated. The destructive role of the defected guiding layers in the laser structure was shown.     

The structure of GaN layers grown on SiC and sapphire by molecular beam epitaxy

A comparative study of the defects at the interfaces and inside the layers was carried out in GaN/AlN epitaxial layers on SiC and sapphire. Whereas surface cleaning of the sapphire substrates is rather standardised now, the SiC substrates cleaning is still to optimise conditions, as the high densities of defects inside the epitaxial layers cannot be explained solely by the 3.54% lattice mismatch. The investigated specimens were grown by molecular beam epitaxy (MBE), either assisted by electron cyclotron resonance or an NH₃ gas source system to provide atomic nitrogen. Assuming that MBE is a growth technique more or less close to equilibrium, the observed defects are interpreted and a growth mechanism, for GaN layers on the stepped (0001) SiC and sapphire surfaces, is proposed.     

Effect of nitridation on crystallinity of GaN grown on GaAs by MBE

GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. Films are evaluated by X-ray diffraction and the dependence of crystalline quality on the nitridation temperature is studied. It is demonstrated that nitridation has to be performed at low-temperature to achieve c-oriented α-GaN. Higher nitridation temperature promotes formation of mis-oriented domains and β-GaN inclusions     

The influence of reactor pressure on qualities of GaN layers grown by hydride vapor phase epitaxy

Influence of reactor pressure on the quality of GaN layers grown by hydride vapor phase epitaxy (HVPE) has been studied. With the reactor pressure decreasing from 7 to 5 × 10⁴ Pa, improvements in structural, optical, and electrical properties of the GaN films have been observed.An investigation of the surface morphology of the GaN films reveals that the improvements arise from the change of the growth mode from an island-like mode at high pressures to a step-flow one at low pressures. These results clearly indicate that the reactor pressure, similar to the growth temperature, is one of the important parameters to control the qualities of HVPE-GaN epilayers.     

Morphology of strained and relaxed SiGe layers grown on high-index Si substrates

We have investigated the surface morphology of strained and relaxed SiGe layers grown on Si substrates with surface normals rotated off of the [001] axis towards [111] by 0, 13, and 25°. Atomic force microscopy has revealed surface corrugations in thin layers prior to plastic relaxation on each of the surfaces due to the initial deposition of the strained films. Thicker partially relaxed layers have previously been shown to contain networks of misfit dislocations which create patterns that are unique to each substrate orientation. We find on these relaxed layers that the surface corrugations are well aligned with the dislocation networks forming a modified crosshatch pattern on the off-axis substrates. More strikingly, though, we find that these corrugations are comprised of smaller nanostructured features which are also unique to each surface. Topographs of the unrelaxed surfaces show no such organization indicating a correspondence between the misfit dislocations and the surface corrugations.     

MBE方法在GaAs(001)衬底上外延生长GaSb膜

利用分子束外延方法(MBE)在GaAs(001)衬底上外延生长了GaSb薄膜,利用高分辨透射电子显微镜(HRTEM)、原子力显微镜(AFM)、Hall效应(HallEffect)和低温光荧光谱(LTPL)等手段对薄膜的晶体质量、电学性能和光学质量进行了研究。发现直接生长的GaSb膜表面平整,空穴迁移率较高。引入GaSb/AlSb超晶格可有效阻断进入GaSb外延层的穿通位错,对应的PL谱强度增强,材料的光学质量变好。     

EXAFS studies of plasma-enhanced MBE grown Group III-Nitrides

We have been studying the structural properties of Group III-Nitrides on Si substrates. We have shown from X-ray data that alloys of (AlGa)N and (InGa)N can be grown by plasma enhanced molecular beam epitaxy on oxidised (100) Si substrates with good control of the composition, over the entire range from InN to AlN. The composition of the alloys deduced from electron probe microanalysis data agree well with those from X-ray measurements assuming Vegard's law is valid for both alloy systems. SIMS studies show that the film composition is uniform in depth. EXAFS studies show no evidence for spinodal decomposition over the entire composition range for the (InGa)N alloys and indicate a monotonic variation in lattice parameter with increasing In mole fraction.     

Investigation of MBE grown GaAs/AlGaAs/InGaAs heterostructures

This paper reports on the influence of the In mole fraction variation (0.1≤x≤0.25) of MBE grown pseudomorphic GaAs/Al_yGa_1−yAs/In_xGa_1−xAs heterostructures on the material quality and the performance of the fabricated devices. For x=0.1–0.15, the carrier mobility in the samples was 4500 cm² V⁻¹ s (at 300 K) and 37 000 cm²V⁻¹ s (at 77 K) and decreased significantly at low temperatures as x was increasing up to 0.25. Transistors with gate length of 0.8 m and In_0.1Ga_0.9As channels exhibited transconductances of 200–220 mSm mm⁻¹ and output conductances of 0.15–0.20 mSm mm⁻¹, while gate-source breakdown voltages were 27–28 V. Delay times of the designed and fabricated ICs frequency dividers by 2 were 130–140 ps.     

Initial stages of growth of GaN over (0001) Al2O3 substrate using MBE: a crystallographic analysis of the defects

An AlN buffer layer grown on (0001) sapphire substrate by molecular beam epitaxy has been studied. It is found to be made of small grains having a common [0001] axis parallel to that of the substrate. Some grains are rotated around this axis and the angle rotation can reach 20° leading to a new epitaxial relationship (0001)_sap//(0001)_AlN and [11

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0]_sap//[21

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0]_AlN. A model for the atomic structure of one of these grain boundaries is proposed using high resolution electron microscopy and extensive image simulation.     

Nanoindention study of indium nitride thin films grown using RF plasma-assisted molecular beam epitaxy

In this study, we used an RF plasma-assisted molecular beam epitaxy (RF-MBE) system to grow single-crystalline indium nitride (InN) films onto aluminum nitride (AlN) buffer layers on Si (111) substrates. We then used nanoindentation techniques and reflection high-energy electron diffraction (RHEED) to study the influence of the c-axis-oriented InN films on the mechanical performance. From morphological observations, we compared the stiffness and resistance against contact-induced damage of the InN films in the presented shrinkage of the area. InN films prepared at growth temperatures of 440, 470, and 500 °C had nanohardnesses (H) of 3.6 ± 0.2, 4.5 ± 0.25, and 9.1 ± 0.8 GPa, respectively, and Young’s moduli (E) of 97.4 ± 1.2, 147.7 ± 1.8, and 176.0 ± 2.3 GPa, respectively.     

Growth and characterization of group-III impurity-doped semiconducting BaSi2 films grown by molecular beam epitaxy

Ga- or In-doped BaSi₂ films were grown on Si(111) by molecular beam epitaxy (MBE). The Ga-doped BaSi₂ showed n-type conductivity. The electron concentration and resistivity of the Ga-doped BaSi₂ depended on the Ga temperature; however, the electron concentration and resistivity could not be controlled properly. In contrast, the In-doped BaSi₂ showed p-type conductivity and its hole concentration was controlled in the range between 10¹⁶ and 10¹⁷ cm^− 3 at RT.