An investigation of MBE growth of (Zn, Mg)Se using RHEED oscillations

We have used reflection high-energy electron diffraction (RHEED) oscillations to measure the growth rates of Zn_1−x.Mg_xSe and ZnSe as functions of substrate temperature, II/VI flux ratio, and electron-beam filament current. Although the ZnSe and Zn_1−xMg_xSe growth rates vary with these growth parameters, the growth rate difference attributable to the addition of magnesium does not depend on them, if growth is done with a selenium excess. Thus, unlike the other common components of II/VI materials (Zn, Cd, S, Se and Te), the sticking coefficient of magnesium is independent of substrate temperature and II/VI flux ratio. Electron-beam-induced desorption of magnesium is also negligible. For growth with a selenium excess, the composition of Zn_1−xMg_xSe films determined by RHEED oscillations agrees well with X-ray fluorescence measurements. However, if growth is done without an excess of selenium, then a simple composition calculation using the Zn_1−xMg_xSe and ZnSe growth rates indicates a magnesium content which is lower than the true magnesium content. This occurs because of a decrease in the sticking coefficient of zinc induced by the presence of magnesium.     

Dynamic RHEED observations of the MBE growth of GaAs

Detailed measurements have been made of the specular beam intensity in RHEED patterns from static and growing GaAs surfaces. The basic parameters investigated were substrate temperature and electron beam azimuth. The results have provided further understanding of growth dynamics and surface disorder, respectively. There is a significant trend away from two-dimensional growth at the higher temperatures, which also correspond to more Ga-rich surface structures. Conversely, surface disorder is apparently greater during growth at the lower temperatures, where the structure is As-rich. The static As-stable 2×4 surface is, however, the most ordered and the most closely two-dimensional. It has also been shown that ordered, two-dimensional growth can be initiated from excess Ga adatom populations.     

RHEED from stepped surfaces and its relation to RHEED intensity oscillations observed during MBE

Multiple scattering theory is used to calculate the intensities of reflection high energy electron diffraction from periodic arrays of surface steps. The intensities are found to depend strongly on the direction of the incident beam azimuth. When the incident beam azimuth is parallel to the step edges, both the specular and diffracted beam intensities are diminished with respect to the intensities from a flat surface. When the incident beam azimuth is perpendicular to the edges, the intensities of all the beams are of the same order of magnitude as for a flat surface but some of the peak heights are oscillatory functions of the number of atoms in the topmost layer. These peak intensity oscillations are very similar to the intensity oscillations observed during molecular beam epitaxial film growth.     

The application of RHEED intensity effects to interrupted growth and interface formation during MBE growth of GaAs/(Al,Ga)As structures

There is a diffraction-induced phase effect in the RHEED intensity oscillation technique used in MBE, whereby intensity maxima only correspond to monolayer completion for very restricted conditions. In particular, the angle of incidence of the primary beam is extremely critical. The effect occurs because the total intensity at the measured position of the specular beam is always derived from at least two different diffraction processes, which do not have the same phase relation to monolayer formation. It can be accomodated either by a systematic series of measurements to establish an empirical relationship between incidence angle and phase, or by Fourier transform techniques. Unless full account is taken of this purely diffraction-induced effect, very misleading results can be obtained for the time constants of the recovery period following cessation of growth and this is illustrated for GaAs. The effect also has important implications for the growth-interrupt technique. In addition, it is shown that for heterojunction formation in the GaAs/(Al, Ga)As system, adatom (Ga and Al) migration lengths are of greater importance than the position in the monolayer at which the composition is changed, and that RHEED can provide only limited information on the interface structure.     

Origin of azimuthal effect of RHEED intensity oscillations observed during MBE

On the basis of calculated RHEED intensities from stepped surfaces, the or igin of the azimuthal dependence of RHEED intensity oscillations observed during Si MBE growth on Si(001) is discussed. By combining the azimuthal dependence of RHEED intensity variations from stepped surface with a model growth mechanism, we obtain the result that in a certain incident beam azimuth one period of the oscillations corresponds to a biatomic height and to a monatomic height in other azimuths. The result is quite consistent with a recently reported experiment.     

Combined study of RHEED spot profiles and intensity oscillations during MBE growth of Ge on Ge(111)

The problem to define particular points in the growth sequence and to obtain quantitative data on the step-terrace structure of the growing surface is addressed in an analysis of RHEED spot profiles and intensity oscillations recorded simultaneously during MBE growth of Ge on Ge(111).     

反射式高能电子衍射实时监控的分子束外延生长GaAs晶体衬底温度校准及表面相变的研究

以反射式高能电子衍射(RHEED)作为实时监测工具,根据GaAs(100)表面重构相与衬底温度、As₄等效束流压强之间的关系,对分子束外延(MBE)系统中衬底测温系统进行了校准,这种方法也适用于其他的MBE系统.为生长高质量的外延薄膜材料、研究InGaAs表面粗糙化及相变等过程提供了实验依据. 关键词： 分子束外延 反射式高能电子衍射 表面重构 温度校准     

In situ characterization of MBE grown GaAs and Al x Ga1−x As films using RHEED,SIMS, and AES techniques

A combination of the surface diagnostic techniques Auger electron spectroscopy (AES), reflection high energy electron diffraction (RHEED), and secondary ion mass spectroscopy (SIMS) was used in order to get more detailed information on basic processes which lead to the formation of high quality monocrystalline GaAs and Al _x Ga_1−x As films by molecular beam epitaxy (MBE) under ultra-high vacuum conditions. The formation and changes of reconstructed surface structures on (100) GaAs as a function of growth parameters were observedduring growth by RHEED. AES was used to determine the relative ratio of Ga/As on the surface for different reconstructed structures, to investigate the impurity contamination on substrate surfaces and grown films, and to study the surface segregation of Sn in MBE GaAs during doping. Finally, intentional and unintentional impurities incorporated during the growth of GaAs and Al _x Ga_1−x As by MBE were detected by the SIMS technique immediately after growth within the reaction chamber.     

RHEED observation of BaTiO3 thin films grown by MBE

Ferroelectric BaTiO₃ thin films with a thickness of 10 monolayers (ML) were epitaxially grown on SrTiO₃(0 0 1) substrates by very slow deposition using molecular beam epitaxy (MBE). The investigations were carried out by two growth methods: (i) codeposition and (ii) alternate deposition of the metal elements in an oxygen atmosphere. In situ observation of reflection high-energy electron diffraction confirmed that an epitaxial cube-on-cube structure was prepared. After the deposition, X-ray diffraction measurements were carried out. The 10-ML-thick BaTiO₃ films were highly c-axis oriented single crystals with good film quality.     

Polarity control in MBE growth of III-nitrides,and its device application

Until recently, molecular beam epitaxy (MBE) has been behind metalorganic chemical vapor deposition (MOCVD) as a growth technique for III-nitride thin films, due to the lack of nitrogen source powerful enough for the growth in vacuum and the understanding of growth mechanism. We have clarified that the quality of GaN epilayers on sapphire substrates grown by N2 plasma-assisted MBE can be much improved by realizing Ga-polarity growth mode, which enables us to fabricate HFETs using the MBE-grown AIGaN/GaN 2DEG structures. The Ga-polarity growth mode was found to be achieved by Al high temperature buffer process, In flux exposure etc., and directly confirmed by coaxial impact collision ion scattering spectroscopy (CAICISS) technique. The relation between the surface reconstruction structure of GaN epilayers and the lattice polarity of the epilayers is also shown.     

MBE growth of nitride-based photovoltaic intersubband detectors

In this work, we present the plasma-assisted molecular-beam epitaxial growth of quantum well infrared photodetector (QWIP) structures, including the Si-doped GaN/AlN short-period superlattice of the active region, conductive AlGaN claddings and integration of the final device. The growth of Si-doped GaN/AlN multiple quantum well (QW) structures is optimized by controlling substrate temperature, metal excess and growth interruptions. Structural characterization confirms a reduction of the interface roughness to the monolayer scale. P-polarized intersubband absorption peaks covering the 1.33–1.91 μm wavelength range are measured on samples with QW thickness varying from 1 to 2.5 nm. The absorption exhibits Lorentzian shape with a line width around 100 meV in QWs doped 5×10¹⁹ cm⁻³. To prevent partial depletion of the QWs owing to the internal electric field, we have developed highly-conductive Si-doped AlGaN cladding layers using In as a surfactant during growth. Complete ISB photodetectors with 40 periods of 1 nm-thick Si-doped GaN QWs with 2 nm-thick AlN barriers have been grown on conductive AlGaN claddings, the Al mole fraction of the cladding matching the average Al content of the active region. Temperature-dependent photovoltage measurements reveal a narrow (90 meV) detection peak at 1.39 μm.     

MBE growth of AlGaAs on patterned GaAs substrates

We have experimentally and theoretically investigated the molecular-beam epitaxy growth of AlGaAs layers on ridge structures of patterned GaAs(0 0 1) substrates. While the surface morphologies were imaged by scanning electron microscopy, the local Al concentrations were mapped by spatially resolved micro-photoluminescence spectroscopy. Both, the surface morphologies and the profiles of the Al concentration could be well modeled by calculations based on a rate-equation which accounts for the migration of Ga adatoms and for the different growth rates of GaAs and AlAs.     

Solid source MBE growth of quantum cascade lasers

High material quality is the basis of quantum cascade lasers (QCLs). Here we report the solid source molecular beam epitaxy (MBE) growth details of realizing high quality of InGaAs/InAlAs QCL structures. Accurate control of material compositions, layer thickness, doping profile, and interface smoothness can be realized by optimizing the growth conditions. Double crystal x-ray diffraction discloses that our grown QCL structures possess excellent periodicity and sharp interfaces. High quality laser wafers are grown in a single epitaxial run. Room temperature continuous-wave (cw) operation of QCLs is demonstrated.     

Nucleation effects during MBE growth of Sn-Doped GaAs

The morphology, structure and composition of Sn films on GaAs substrates have been investigated in relation to the predeposition technique used with Sn doping of MBE-grown GaAs films. It has been shown that 3-D aggregates of Sn are formed and a similar morphology has been identified in the Sn which accumulates at the surface of all Sn-doped GaAs films. The way in which this relates to donor incorporation is considered and a simple model of incorporation kinetics proposed which is shown to be consistent with observation.     

Sb-mediated growth of high-density InAs quantum dots and GaAsSb embedding growth by MBE

Self-assembled InAs quantum dots (QDs) with high-density were grown on GaAs(0 0 1) substrates by antimony (Sb)-mediated molecular beam epitaxy technique using GaAsSb/GaAs buffer layer and InAsSb wetting layer (WL). In this Sb-mediated growth, many two-dimensional (2D) small islands were formed on those WL surfaces. These 2D islands provide high step density and suppress surface migration. As the results, high-density InAs QDs were achieved, and photoluminescence (PL) intensity increased. Furthermore, by introducing GaAsSb capping layer (CL), higher PL intensity at room temperature was obtained as compared with that InGaAs CL.     

在Si-Ge晶体外延生长中的RHEED花样研究

在晶体衍射理论基础上,利用运动学理论解释了反射式高能电子衍射（reflection high energy electron diffraction, RHEED）在Si-Ge晶体外延生长过程中不同阶段出现的花样．尤其研究了晶体岛状生长之后出现的RHHED透射式衍射花样,并给出了相应的解释．解释了硅锗外延生长过程中的多晶环图案和孪晶衍射图案的含义,并给出各个生长阶段演化的工艺条件． 关键词： 硅锗外延生长 反射式高能电子衍射 表面重构 透射式衍射花样     

MBE growth of ZnSe nanowires on oxidized silicon substrate

We report the growth of single crystalline ZnSe nanowires on oxidized Si(100) substrates by molecular-beam epitaxy using Au nano-particles as the catalysts. It was found that average length decreased while the average diameter increased as we increased the temperature from 230 to 320 ^°C. It was also found that crystal quality of the ZnSe nanowires prepared at 320 ^°C was poorer than the ZnSe nanowires prepared at 230 ^°C and 280 ^°C.     

Ion-beam-assisted MBE growth of semi-spherical SiGe/Si nano-structures

Semi-spherical SiGe/Si nano-structures of a new type are presented. Epitaxial islands of 30–40 nm in base diameter and 11 nm in height and having a density of about 6×10¹⁰ cm^-2 were produced on (001) Si by molecular beam epitaxial growth of Si/Si_0.5Ge_0.5 layers with in situ implantation of 1-keV As⁺ ions. It was found by cross-section transmission electron microscopy that the islands have a complicated inner structure and consist of a micro-twin nucleus and semi-spherical nano-layers of various SiGe compositions. The nature of the surface patterning is interpreted by stress relaxation through implantation-induced defects. Received: 12 July 2001 / Accepted: 4 September 2001 / Published online: 2 October 2001     

A3B5 nanowhiskers: MBE growth and properties

The structural properties of GaAs nanowhiskers (NWs) grown by molecular beam epitaxy (MBE) are investigated. Under optimal growth conditions, the aspect ratio of MBE grown GaAs NWs is higher than 100. The maximum length of NWs is several times (up to 10) larger than the effective thickness of deposited GaAs. A kinetic model of the diffusion-induced NW rowth is used to predict the dependence of NW length on the technologically controlled MBE growth conditions. The obtained results demonstrate that the NW growth is controlled by the adatom diffusion towards their tip rather than by the conventional vapor-liquid-solid mechanism. The growth conditions influence on the NW morphology may be used for the controlled fabrication of NWs by MBE for different applications. Presented at the X-th Symposium on Suface Physics, Prague, Czech Republic, July 11–15, 2005.