Growth of thick AlN epilayers with droplet-free and atomically smooth surface by plasma-assisted molecular beam epitaxy using laser reflectometry monitoring

Low-temperature (<750 °C) growth of thick AlN epilayers on c-sapphire by plasma-assisted molecular-beam epitaxy under the Al-rich conditions (F_Al/F_N?<1.4) is reported here. Short periodic Al-flux interruptions controlled precisely by laser reflectometry ensure continuous growth of droplet-free and atomically smooth AlN films (rms<2 ML over 4 μm²) with a growth rate governed by the activated nitrogen flux. Lateral spreading of small accumulated Al clusters with their subsequent incorporation into the AlN layer during the Al-flux interruptions is supposed to be facilitated by activated nitrogen radicals. Strong influence of the remaining Al droplets on the subsequent growth of AlGaN/AlN superlattices is also demonstrated.     

A complementary geometric model for the growth of GaN nanocolumns prepared by plasma-assisted molecular beam epitaxy

In this article, we propose a new complementary geometrical growth mechanism, which may partially explain some of the apparent anomalies in our understanding of the growth of GaN nanocolumns by plasma-assisted molecular beam epitaxy (PA-MBE). This geometrical addition to any complete model for nanocolumn growth is based on the fact that most samples are grown using substrate rotation and it predicts an enhanced growth rate in the plane normal to the surface, i.e. vertically compared with the lateral growth rate of the columns. It also suggests a mechanism for the enhanced diffusion of gallium on the sidewalls of the columns even under strongly nitrogen-rich conditions. Finally, geometrical considerations also predict the growth of non-(0 0 0 1) oriented samples from the same mechanism. Some experimental evidence supporting this complementary geometrical model is presented.     

Quantum dot nanostructures and molecular beam epitaxy

In order to fulfil the requirements of the information society there is a growing demand for nanoelectronic devices with new or largely improved performances; these devices are based on low-dimensional carrier systems, and in particular on zero-dimensional ones, that have peculiar properties as compared to the three- and two-dimensional counterparts.

In this paper we review and discuss the basic features of the Molecular Beam Epitaxy growth of quantum dots that are very interesting archetypes of zero-dimensional nanostructures; quantum dots can be obtained by the three-dimensional growth of self-assembled nanoislands that takes place during the preparation of structures based on highly lattice-mismatched materials. Aspects of the morphological, electronic and optical properties of quantum dots will be reviewed and it will be shown how the energy of confined levels for carriers is determined by design and growth parameters of nanostructures and how quantum dot emission wavelengths can be tuned in the windows of optoelectronic and photonic interest, such as that at 0.98, 1.31 and 1.55 μm. An overview of quantum dot devices will be given, with particular attention paid to the quantum dot laser, unarguably the most important application of quantum dots so far.     

GaSb薄膜生长的RHEED研究

采用分子束外延技术,在GaAs衬底上生长GaSb薄膜时,利用反射式高能电子衍射仪(RHEED)对衬底表面清洁状况、外延层厚度等进行在线监控.通过RHEED讨论低温缓冲层对GaSb薄膜表面结构和生长机制的作用,可以估算衬底温度,并能计算出薄膜的生长速率.实验测量GaSb的生长周期为1.96s,每秒沉积0.51单分子层.低温缓冲层提高了在GaAs衬底上外延GaSb薄膜的生长质量.     

Growth of InN nanorods prepared by plasma-assisted molecular beam epitaxy with varying Cr thicknesses

This study investigates how the thickness of Cr deposited on the Si substrate after the nitridation process influences the AIN buffer layer and the InN nanorods. Atomic force microscopy results reveal that different thicknesses of Cr form varying sizes of CrN nanoislands. The results of scanning electron microscopy and X-ray diffraction show that a Cr deposition thickness of 10 nm results in CrN nanoislands after the nitridation process, improving the quality and density of InN nanorods. A Cr layer that was too thick led to polycrystalline InN growth. The results of transmission electron microscopy indicate a baseball bat-like InN nanorod growth mechanism.     

Influence of Be on N composition in Be-doped InGaAsN grown by RF plasma-assisted molecular beam epitaxy

Undoped and Be-doped InGaAsN layers were grown on GaAs substrates under the same growth conditions by radio frequency plasma-assisted molecular beam epitaxy. Increased tensile strain (Δa/a=3×10⁻³) was observed for Be-doped InGaAsN layers, compared to undoped InGaAsN layers. The strain is shown to originate from the increase in N composition related to Be incorporation, rather than solely from Be atoms substituting Ga atom sites (Be_Ga). A possible reason is the high Be–N bond strength, which inhibits the loss of N from the growth surface during epitaxial growth, thereby increasing the N composition in the Be-doped InGaAsN layer.     

Effects of real-time monitored growth interrupt on crystalline quality of AlN epilayer grown on sapphire by molecular beam epitaxy

Crack-free aluminum nitride (AlN) epilayers were grown on sapphire using growth-interrupt technique by radio-frequency assisted molecular beam epitaxy. In-situ reflectance spectroscopy was introduced for real-time monitoring of the growth of AlN epilayers. X-ray diffraction and atomic force microscopy measurements reveal that the threading dislocation density decreases considerably by using the growth-interrupt technique. Raman spectroscopy is used to characterize the residual stress of AlN epilayers. The optical transmittance and absorption spectra of AlN epilayers show a high transmittance and a sharp absorption edge.     

Molecular beam epitaxy (MBE) growth and structural properties of GaN and AlN on 3C-SiC(0 0 1) substrates

AlN and GaN was deposited by molecular beam epitaxy (MBE) on 3C-SiC(0 0 1) substrates on low-temperature (LT) GaN and AlN buffer layers. It is shown that not only GaN but also epitaxial AlN can be stabilized in the metastable zincblende phase. The zincblende AlN is only obtained on a zincblende LT-GaN buffer layer; on the other hand, AlN crystallizes in the wurtzite phase if it is grown directly on a 3C-SiC(0 0 1) substrate or on a LT-AlN buffer layer. The structural properties of the layers and in particular the orientation relationship of the wurtzite AlN on the 3C-SiC(0 0 1) were analyzed by conventional and high-resolution transmission electron microscopy.     

Effects of phosphorous beam equivalent pressure on GaInAsP/GaAs grown by solid source molecular beam epitaxy with a valve phosphorous cracker cell

We report the growth and characterization of GaInAsP films on GaAs substrates by solid source molecular beam epitaxy (SSMBE) using a valve phosphorous cracker cell at varied white phosphorous beam equivalent pressure (BEP). It is found that the GaInAsP/GaAs can be easily grown with the solid sources, and the incorporated phosphorous composition as a function of the beam equivalent pressure ratio, R=f_P/(f_P+f_As), can be well described by a parabolic relationship. With the increase of the incorporated phosphorous composition, the GaP-, InP-, InAs- and GaAs-like phonon modes shift towards opposite directions and their emission intensities also change. The first three modes shift to larger wave numbers while the last one shifts to smaller wave number. The lattice mismatch, Δa/a, of the materials grown with varied phosphorous BEP follows a linear relationship. Photoluminescence (PL) measurements reveal that as the phosphorous BEP ratio increases, the peak position or energy band gap of the material shifts towards higher energy; the full-width at half-maximum (FWHM) becomes narrower, and the luminescence intensity becomes higher. In addition, the materials also show smooth surfaces that do not change significantly with phosphorous beam equivalent pressure.     

Low temperature molecular beam epitaxy growth of cubic GaCrN

Cubic GaCrN layers are grown on MgO(0 0 1) substrates at 350–700 °C by plasma-assisted molecular beam epitaxy. Substrate temperature dependences of their structural and magnetic properties were systematically studied. It is found that the solubility limit of Cr atoms in cubic GaCrN is dramatically improved by the low temperature (350 °C) growth, though crystalline quality becomes poorer. It is also observed that the magnetic ordering increases with Cr content in the low Cr content region, but after showing highest ordering it decreases in the high Cr content region. The Cr content range showing ferromagnetic behavior increases with lowering substrate temperature. However, the magnetization vs. magnetic field curve shows “emaciated” hysteresis for the low temperature grown samples.     

Epitaxial growth of magnetic semiconductor EuO on silicon by molecular beam epitaxy

Functional oxides demonstrate a wide range of magnetic, optical and transport properties. Their integration with silicon promises significant advances in electronics. An important key in enabling brand‐new oxide technologies is the utilization of silicon/oxide epitaxy, thus making quality of the interface a critical issue. The progress depends on our ability to avoid formation of impurity phases at the interface and to tackle structural mismatch of the oxide and Si. We design a novel chemical protection of Si (001) surface on the submonolayer scale based on the surface metal silicide with the (1×5) reconstruction. This new technique is applied to the long‐standing problem of integration of a ferromagnetic semiconductor with Si. Direct epitaxial growth of EuO on Si without any buffer layer, so far inaccessible, is achieved by molecular beam epitaxy. The nucleation step, comprising first 10 monolayers of EuO, is followed by a distillation‐controlled growth. An alternative to standard capping procedures for EuO, based on controlled formation of an amorphous Eu₂O₃ layer, is devised. Crystal perfection of the films is established ex situ by x‐ray diffraction and Rutherford backscattering. Magnetic properties of the EuO films match those of the bulk.     

Growth of epitaxial ZnO films on Si (1 1 1) substrates with Cr compound buffer layer by plasma-assisted molecular beam epitaxy

Single crystalline ZnO film was grown on (1 1 1) Si substrate through employing an oxidized CrN buffer layer by plasma-assisted molecular beam epitaxy. Single crystalline characteristics were confirmed from in-situ reflection high energy electron diffraction, X-ray pole figure measurement, and transmission electron diffraction pattern, consistently. Epitaxial relationship between ZnO film and Si substrate is determined to be (0 0 0 1)_ZnO‖(1 1 1)_Si and [1 1 2¯ 0]_ZnO‖[0 1 1]_Si. Full-width at half-maximums (FWHMs) of (0 0 0 2) and (1 0 1¯ 1) X-ray rocking curves (XRCs) were 1.379° and 3.634°, respectively, which were significantly smaller than the FWHMs (4.532° and 32.8°, respectively) of the ZnO film grown directly on Si (1 1 1) substrate without any buffer. Total dislocation density in the top region of film was estimated to be ∼5×10⁹ cm⁻². Most of dislocations have a screw type component, which is different from the general cases of ZnO films with the major threading dislocations with an edge component.     

Selective growth and other applications of hydrogen-assisted molecular beam epitaxy

The usefulness of atomic hydrogen in molecular beam epitaxy has been demonstrated, centering around selective growth. Atomic hydrogen is effective for low-temperature cleaning of substrates, surfactant effects such as restrain of island growth and suppression of the surface migration of the adatoms and selective growth on masked or V-grooved substrates. These effects are dependent on substrate temperatures. The selective growth of GaAs has been successfully demonstrated at the conventional growth temperature and growth rate with the aid of atomic hydrogen. The main mechanism of the selective growth is the re-evaporation of Ga and As from mask materials such as SiN_x or SiO₂. Selective growth has also been observed on low-index crystal facets. On (111)A and (110) facets, no GaAs was deposited in the presence of atomic hydrogen, the flux of which is approximately the same as that of Ga. GaAs quantum wire structures have been fabricated on the substrates with V-shaped grooves. The efficient capture and confinement of carriers into wire regions have been observed by photolumenescence.     

Surface flattening by annealing after molecular beam epitaxy growth revealed by in-situ secondary electron microscopy

The consecutive evolution of the configurations of islands, steps and terraces on a GaAs(001) surface during annealing after growth was studied by in-situ observation using a secondary electron microscope. Three unexpected processes were revealed: some large two-dimensional islands grow, although most small ones shrink, step distances becomes more uniform rather than step bunching occurring, and multilayer high islands shrink and disappear. The latter two processes play an important role in surface flattening. The growth of islands is explained by thermodynamic equilibrium.     

Low-pressure HVPE growth of crack-free thick AlN on a trench-patterned AlN template

A thick AlN layer was grown on a trench-patterned AlN/sapphire template by low-presssure hydride vapor phase epitaxy (LP-HVPE). Compared with the AlN layer grown on a flat AlN/sapphire template, the AlN layer grown on the trench-patterned AlN/sapphire template had a crack-free and smooth surface. The typical full-widths at half-maximum (FWHMs) of X-ray rocking curves (XRC) for the (0 0 0 2), (1 0 1¯ 2), and (1 0 1¯ 0) diffractions of the AlN layer on the trench-patterned AlN/sapphire template were 132, 489, and 594 arcsec, respectively. In addition, atomic steps were observed on the AlN layer on the trench-patterned AlN/sapphire template, and the root-mean-square (RMS) roughness of the AlN layer was determined to be 0.602 nm by atomic force microscopy (AFM).     

In-situ second-harmonic generation study of the molecular beam epitaxy growth of GaAs

We report on the first observation of surface second-harmonic generation (SHG) during the molecular beam epitaxy (MBE) growth of GaAs homoepitaxy on a GaAs(100) substrate. We used a specific optical arrangement which enabled us to observe the surface SHG and the reflection high-energy electron diffraction (RHEED) pattern simultaneously. We found that the surface SH intensity increased according to a change from an As-stabilized to a Ga-stabilized surface due to an interruption of the As flux while keeping the Ga flux constant. We show that the surface SH intensity has its maximum value when there is about 1 monolayer (ML) of excess Ga on the surface.     

Effect of arsenic species on the kinetics of GaAs nanowires growth by molecular beam epitaxy

GaAs nanowires (NWs) are grown on GaAs (1 1 1) B substrates in a molecular beam epitaxy system, by Au-assisted vapor–liquid–solid growth. We compare the characteristics of NWs elaborated with As₂ or As₄ molecules. In a wide range of growth temperatures, As₄ leads to growth rates twice faster than As₂. The shape of the NWs also depends on the arsenic species: with As₄, regular rods can be obtained, while pencil-like shape results from growth with As₂. From the analysis of the incoming fluxes, which contributes to the NWs formation, we conclude that the diffusion length of Ga adatoms along the NW sidewalls is smaller under As₂ flux as compared to that under As₄ flux. It follows that As₂ flux is favourable to the formation of radial heterostructures, whereas As₄ flux is preferable to maintain pure axial growth.     

In situ film thickness and temperature control of molecular beam epitaxy growth by pyrometric interferometry

In this paper we present an overview as well as latest results of applying pyrometric interferometry (PI) to in situ film thickness and temperature measurements in molecular beam epitaxy facilities. We treat two PI configurations (PI and reflection supported PI) and discuss their different advantages and shortcomings. Physical basics, experimental set-ups as well as evaluation procedures are outlined. Special emphasis is given on long term experience at III-V MBE and the impact of PI on yield, reproducibility, number of required calibration runs and device performance.