Second harmonic generation in a sputtered LiNbO3 film on MgO

Phase-matched optical second harmonic generation with a Nd:YAG laser and a GaAs laser was observed in an rf sputtering LiNbO₃ film guide deposited on a (111)MgO substrate. The efficiency of the SHG with a GaAs laser (TM₀ - TM₂) was about 10^-4 and the conversion efficiency with a Nd:YAG laser (TM₀ - TM₁) was about 10^-6.     

Nucleation and ripening of seeded InAs/GaAs quantum dots

We have investigated the nucleation and ripening of pairs of InAs/GaAs quantum dot layers separated by thin (2–20 nm) GaAs spacer layers. Reflection high energy electron diffraction (RHEED) measurements show that the 2D–3D transition in the second layer can occur for less than 1 monolayer deposition of InAs. Immediately after the islanding transition in the second layer chevrons were observed with included angles as low as 20° and this angle was seen to increase continuously to 45±2° as more material was deposited. Atomic force microscopy showed the dot density in both layers to be the same. It is proposed that surface morphology can radically alter processes that determine the nucleation and ripening of the 3D islands.     

Influence of initial growth parameters on the structural and optical properties of GaAs on (001) Si

The structural and optical properties of GaAs on (001) Si substrates were investigated by transmission electron microscopy (TEM) and low-temperature photoluminescence (PL). It was found that the success of the two-step growth technique is controlled by the quality (morphology and defect density) of the low-temperature grown AlGaAs nucleation layer. GaAs epilayers grown on low V/III ratio AlGaAs nucleation layers exhibit improved surface morphologies and structural properties. These results were confirmed by optical measurements where it was shown that the best PL response was obtained from GaAs epilayers in which the initial AlGaAs nucleation layers were deposited at a low V/III ratio.     

Arsenic-free GaAs substrate preparation and direct growth of GaAs/AlGaAs multiple quantum well without buffer layer

High-temperature treatment of GaAs substrate without As flux in a preparation chamber was investigated as a substrate surface cleaning method for molecular beam epitaxial (MBE) growth. Oxide gases such as CO and CO₂ were almost completely desorbed at a temperature above which Ga and As started to evaporate from the substrate. During the cleaning at a temperature as high as 575°C for 30 min, about 100 nm thick GaAs was evaporated from the substrate, but its surface maintained mirror-like smoothness and showed streak pattern with surface reconstruction pattern in the reflection high energy electron diffraction (RHEED) observation. Direct growth of GaAs/Al GaAs quantum well (QW) structures was tried on such surfaces without introducing any buffer layers. The QW structure showed photoluminescence with both intensity and full width at half maximum comparable with those for the QW grown on the substrate cleaned by the conventional method with introducing a GaAs buffer layer.     

Stability of GaAs oxide under metalorganic molecular beam epitaxy process

Well-defined oxide of GaAs can be used as a mask material for selective-area metalorganic molecular beam epitaxy (MOMBE) of GaAs. In this study, the reaction between triethylgallium (TEG) and the GaAs oxide layer was studied using a quadrupole mass spectrometer (QMS) and an atomic force microscope (AFM). Results of the QMS observation showed that TEG was reflected on the GaAs oxide surface until the start of desorption of the GaAs oxide, and the GaAs oxide layer was desorbed from the wafer after a large time delay from the start of TEG supply. AFM images showed that many holes appeared on the GaAs oxide surface during the desorption of the GaAs oxide. The effect of incident TEG upon the stability of the GaAs oxide mask is discussed.     

Growth of InGaAs/GaAs heterostructures with abrupt interfaces on the monolayer scale

Segregation processes entail severe deviations from the nominal composition profiles of heterostructures grown by molecular beam epitaxy for most semiconductor systems. It is, however, possible to compensate exactly these effects, as shown here for InGaAs/GaAs. The deposition of a one-monolayer-thick indium-rich prelayer of InGaAs (or of a sub-monolayer amount of InAs) prior to growth of In_xGa_1−xAs allows forming a perfectly abrupt In_xGa_1−xAs-on-GaAs interface. Thermal annealing can furthermore be performed at the GaAs-on-InGaAs inter face, so as to desorb surface indium atoms and suppress In incorporation in the GaAs overlayer. This powerful approach has been validated from a detailed study of the surface composition at various stages of the growth of InGaAs/GaAs quantum wells, as well as from high-resolution transmission electron microscopy and photoluminescence investigations.     

Ti-adsorption on Se-prereacted GaAs (110) surfaces studied by SXPS

This study is embedded in the broader context of reactive metal overlayers and passivation on GaAs. High resolution synchrotron-radiation photoemission experiments for Ti coverages on Se-reacted GaAs (110) surfaces show that, contrary to clean Ti-reacted GaAs (110) interface, there is no initial disruption (submonolayer reaction) of the surface involving both Ga and As atoms during early stage of interface formation. However, a delayed Ti involved reaction appears at a trigger coverage of about 1 ML involving only As atoms from the prereacted As Se interface configuration continued by Stranski-Krastanov growth mode. A second reacted phase starts to form two Ga Ti involved configurations replacing Ga Se bonds near a Ti coverage of 4 ML. The preferential chemical trapping of Ti by Se atoms is associated with a smaller interface thickness very likely <10 ML instead of 50 ML in the case of clean Ti/GaAs (110) interface caused by mobile As atoms.     

Reaction kinetics for the CBE growth of GaAs from triethylgallium; computer modelling studies incorporating recent surface spectroscopic data

A new model for the decomposition of triethylgallium on GaAs(100), with kinetic parameters derived from the results of surface science experiments, is presented. Deficiencies of early models are corrected by including surface coverage and site blocking effects, and lateral interactions between absorbed DEG species are included. The model successfully predicts variations in the rate of CBE growth of GaAs with substrate temperature, and addresses effects induced by variations in arsenic overpressure. This dependence of growth rate on the arsenic flux is modelled by computing the steady state concentrations of absorbed arsenic as a function of temperature and As₂ and TEG flux. Excess arsenic is shown to inhibit GaAs growth by blocking sites for TEG absorption.     

Surface morphology of carbon-doped GaAs grown by MOVPE

The development of the surface structures of carbon-doped epitaxial GaAs layers grown by metalorganic vapor phase epitaxy was investigated by atomic force microscopy (AFM). Carbon-doped GaAs layers were grown using trimethyl gallium and a mixture of AsH₃/TMAs. The AFM micrographs were quantitatively analyzed through the determination of the height-height and height-difference correlation functions, which yields both the short and long range surface structures. The incorporation of carbon leads to the progressive roughening of the GaAs surface as well as an increase in surface correlation length. The high concentration of surface-adsorbed methyl radicals are suggested to lead to the diminution of growth rate and change in surface structure.     

In situ reflectance difference spectroscopy of ZnSe-based semiconductor surfaces

We will give a brief report on applications of reflectance difference spectroscopy (RDS) for in situ growth monitoring during MOVPE. The experiments were made on GaAs(001) substrates using dimethylzinc-triethylamine, ditbutylselenide and as metalorganic precursors in a laminar flow reactor. We analyzed the influence of the in situ GaAs-substrate preparation on the optical response of the surface. The growth of ZnSe was investigated and compared to data obtained in an MBE process. Spectra at various stages of growth and time-dependent kinetic RDS records were measured during deposition and fitted with a three-layer growth model. Furthermore we utilized the surface sensitivity of the RDS technique to demonstrate surface kinetic processes during p-doping with tbutylamine.     

Growth process studies by reflectance anisotropy spectroscopy on MOVPE ZnSe

The growth of ZnSe on GaAs by metal organic vapour phase epitaxy (MOVPE) has been studied using reflectance anisotropy spectroscopy (RAS). The RA spectra of ZnSe are significantly different for growth on initially Se- or Zn-exposed GaAs surfaces. The Se-terminated GaAs (001) RA spectrum has Se-dimer-related features at 3.3 and 5.1 eV, and the large, high energy peak dominates during ZnSe growth on this surface. Transmission electron microscopy (TEM) analysis has been used to show that these large RA signals arise from anisotropic surface corrugation of the growing ZnSe epilayer. Under initially Zn-stabilised growth conditions, the ZnSe epilayer RA spectrum is largely featureless, showing only a weak peak at 4.7 eV and a dip at 5.1 eV. The corresponding surface anisotropy is greatly reduced in comparison with growth from the initially Se-terminated surface. These observations reveal RAS to the an important technique for ensuring the desired initial GaAs surface since the grown ZnSe surface morphology is critically dependent on the pre-growth substrate surface treatment. However, as the characteristic ZnSe RA spectra are relatively insensitive to changes in substrate temperature and VI–II ratio, RAS is of more limited use as an in-growth surface probe for MOVPE-grown ZnSe.     

Application of X-rays to the Study of the Surface Roughness

The paper presents problems of an application of grazing incidence X-ray reflectivity as a tool for investigations of the surface roughness. The theoretical calculations are based on Fresnel theory. The surface roughness reduces the reflected amplitude therefore a damping factor describing an influence of that is introduced into equations. The results of computer simulations have been used for estimation of the measured surface roughness of thin epitaxial Si film and GaAs single crystal.     

Substituted imidazolium phenylphosphonate salts for nonlinear optical applications

Three candidates for potential applications as new nonlinear optical materials were investigated using x-ray crystallography to probe the structure and powder second harmonic generation to elicidate the nonlinear response of the materials. Three structures are reported: 1-benzylimidazolium phenylphosphonate, 2-ethylimidazolium phenylphosphonate, and 2-methylimidazolium phenylphosphonate. Of these three salts two, 2-ethylimidazolium phenylphosphonate and 2-methylimidazolium phenylphosphonate, are noncentrosymmetric and therefore display a nonlinear optical response.     

Thermodynamic analysis of GaAs growth by molecular beam epitaxy at the surface structure transition from 3 × 1 to 4 × 2

The molecular beam epitaxy (MBE) growth of GaAs layers on a single crystal is studied in relation with the stability domain of the GaAs compound in the Ga-As binary system. The growth parameters, i.e. The Ga and As impinging atomic flows, are compared to the necessary flows as calculated by thermodynamics. In order to take into account in the real growth situation, which is not strictly at equilibrium, the flow balance at the surface of the crystal between the impinging flows and the growth and evaporated flows is written for quasi-equilibrium growth conditions, including condensation and evaporation coefficients that split the “so-called” sticking coefficient in parts related to the condensation or the evaporation phenomenon for each gaseous species. A comparison between the quasi-equilibrium simulation of the growth and the experiment is made with the assumption that the surface structure transition from gallium-stabilized to arsenic-stabilized surface corresponds to the growth of a GaAs crystal at its solidus boundary rich in gallium. The surface structure transitions are observed by reflection high energy electron diffraction (RHEED) and the impinging atomic flows are carefully calibrated and also controlled by RHEED oscillations as observed after gallium or arsenic excess as deposited on the surface. The results show that the growth is effectively performed close to equilibrium conditions as evidenced by the values of the condensations and evaporation coefficients. The evaporation coefficient of gallium is 0.4, showing that this component is the supersaturated one at the surface, and this value agrees with theoretical predictions for an evaporation process (during growth) controlled by the surface diffusion process of monoatomic species between steps.     

Real-time scanning microprobe reflection high-energy electron diffraction observations of the cleaning process of GaAs substrates

GaAs surface cleaning using atomic hydrogen (H·) prior to molecular beam epitaxy has been compared to the conventional thermal treatment of GaAs surfaces. The surface morphology was observed in real time using in situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED). GaAs surfaces have been found to be uniformly cleaned at temperatures of about 400°C using the H· treatment. On the other hand, a local initiation of the desorption of the oxide layer has been observed during the conventional thermal treatment at about 580°C.     

Determination of the Thickness of Chemically Removed Thin Layers on GaAs VPE Structures

Thinning of epitaxial GaAs layers was studied during the surface etching, with a special attention to submicron epitaxial structures, like MESFET or varactor-type structures. Each chemical treatment influences the crystal surface during the device preparation processes, though the possible thinning of the active layer is small. Therefore a method allowing determination of thicknesses as small as at about 20 nm of the layer removed by chemical etching from GaAs VPE structures was applied. Using special multilayered structures and a continuous electrochemical carrier concentration depth profiling, the influence of the layer thickness inhomogeneity and of some measurement errors can be minimized. Some frequently used etchants and the influence of different – so called – non-etching processes were compared in different combinations. It was shown that besides the direct etching a change of the surface conditions occurs, which influences the etch rate in the succeeding etching procedure.     

Effects of surface reconstruction on CdTe/GaAs(001) interface structure

CdTe/GaAs(001) heterostructures were fabricated by molecular beam epitaxy on chemically etched and thermally deoxidized GaAs(001) substrates, as well as GaAs(001) (3×1) buffer layers grown in situ by molecular beam epitaxy. Different growth protocols were also explored, leading to Te-induced (6×1) or (2×1) surface reconstructions during the early growth stage. High-resolution cross-sectional transmission electron microscopy was used to examine the final interface structure resulting from the different substrate preparations, and surface reconstructions. The (2×1) surface reconstruction led to pure (001) growth, while the (6×1) reconstruction led to an interface which included small (111)-oriented inclusions. In addition, deposition on etched and deoxidized GaAs(001) wafers led to preferential CdTe growth within etch pits and resulted in a macroscopically rough interface region.     

引入Si掺杂层调控InGaAs/GaAs表面量子点的光学特性

在InGaAs/GaAs表面量子点(SQDs)的GaAs势垒层中引入Si掺杂层,以研究Si掺杂对InGaAs/GaAs SQDs光学特性的影响。荧光发光谱(PL)测量结果显示,InGaAs/GaAs SQDs的发光强烈依赖于Si掺杂浓度。随着掺杂浓度的增加, SQDs的PL峰值位置先红移后蓝移; PL峰值能量与激光激发强度的立方根依赖关系由线性向非线性转变;通过组态交互作用方法发现SQDs的PL峰位蓝移减弱;时间分辨荧光光谱显示了从非线性衰减到线性衰减的转变。以上结果说明Si掺杂能够填充InGaAs SQDs的表面态,并且改变表面费米能级钉扎效应和SQDs的荧光辐射特性。本研究为深入理解与InGaAs SQDs的表面敏感特性关联的物理机制和载流子动力学过程,以及扩大InGaAs/GaAs SQDs传感器的应用提供了实验依据。     

Effect of atomic hydrogen in highly lattice-mismatched molecular beam epitaxy

Effects of atomic hydrogen on the growth of lattice-mismatched InAs/GaAs and GaAs/InP systems have been investigated. The irradiation of atomic H has resulted in a delay of the onset of formation of three-dimensional islands maintaining flat surface morphology and increase of the critical layer thickness (CLT) from 4 to 10 å in the case of the InAs/GaAs system. The effect of atomic H was shown to be dependent on the growth conditions such as the growth temperature and V/III flux ratio. The increase of CLT with atomic H irradiation may be explained by the uniform distribution of the total misfit stress in the plane of the surface as a result of enhanced two-dimensional growth by atomic H acting as a surfactant.     

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.