Multiatomic step formation on GaAs(001) vicinal surfaces during thermal treatment

We observed the surface morphology of vicinal GaAs(001) after thermal treatment in AsH₃/H₂ atmosphere by atomic force microscopy (AFM). Clear multiatomic steps were formed under the high temperature thermal treatment. Next, we investigated the mechanism of step bunching during thermal treatment by two experiments from the view point of Ga atom evaporation. One is the selective thermal treatment using a partially masked GaAs wafer, and the evaporation amount of Ga atoms was estimated by AFM. The other is the investigation of photoluminescence (PL) peak energy shifts for AlGaAs/GaAs single quantum wells with a thermal treatment process at the top of the GaAs quantum well layer, compared to those without thermal treatment. These results indicate that the evaporation hardly occurs during the thermal treatment process. Therefore, step bunching phenomena on GaAs(001) vicinal surfaces during thermal treatment are probably caused by migration of the atoms detached from upside steps and their re-incorporation to downside steps.     

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.     

Transmission electron microscopy observation of GaAs/Al0.3Ga0.7As T-shaped quantum well structure fabricated by glancing angle molecular beam epitaxy on GaAs(100) reverse-mesa etched substrates

GaAs/Al_0.3Ga_0.7As multi-layer structures were grown on GaAs (100) reverse-mesa etched substrates by glancing angle molecular beam epitaxy (GA-MBE). A(111)B facet was formed as a side-facet. Surface migration of Ga and Al atoms from the (100) flat region to the (111)B side-facet region has been investigated to fabricate T-shaped GaAs/AlGaAs quantum wells (QWs) under the condition that Ga and Al atoms impinge only an the (100) flat region and do not impinge on the (111)B side-facet. Observation of T-shaped GaAs/AlGaAs quantum wires (QWRs) by cross-sectional transmission electron microscopy (TEM) revealed that there is no migration of Al atoms from the (100) to the (111)B facet region at a substrate temperature (T_s) as high as 630°C, under a V/III ratio of 28 (in pressure ratio). On the other hand, very thin GaAs epitaxial layers grown on the (111)B side-facet region owing to the Ga migration were observed for substrate temperatures of 600 and 630°C. It was found that the mass flow of Ga atoms from the (100) region to the (111)B side-facet region increases, with the thermal activation energy of 2.0 eV, as the substrate temperature increases from 570 to 630°C. The GA-MBE growth on a reverse-mesa etched GaAs substrate at a low temperature 570°C or lower is desirable to fabricate a nm-scale GaAs/AlGaAs QWR structure with nm-scale precision.     

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。     

Fabrication and characterization of GaAs quantum well buried in AlGaAs/GaAs heterostructure nanowires

We developed a growth method for forming a GaAs quantum well contained in an AlGaAs/GaAs heterostructure nanowire using selective-area metal organic vapor phase epitaxy. To find the optimum growth condition of AlGaAs nanowires, we changed the growth temperature between 800 and 850 °C and found that best uniformity of the shape and the size was obtained near 800 °C but lateral growth of AlGaAs became larger, which resulted in a wide GaAs quantum well grown on the top (1 1 1)B facet of the AlGaAs nanowire. To form the GaAs quantum well with a reduced lateral size atop the AlGaAs nanowire, a GaAs core nanowire about 100 nm in diameter was grown before the AlGaAs growth, which reduced the lateral size of AlGaAs to roughly half compared with that without the GaAs core. Photoluminescence measurement at 4.2 K indicated spectral peaks of the GaAs quantum wells about 60 meV higher than the acceptor-related recombination emission peak of GaAs near 1.5 eV. The photoluminescence peak energy showed a blue shift of about 15 meV, from 1.546 to 1.560 eV, as the growth time of the GaAs quantum well was decreased from 8 to 3 s. Transmission electron microscopy and energy dispersive X-ray analysis of an AlGaAs/GaAs heterostructure nanowire indicated a GaAs quantum well with a thickness of 5−20 nm buried along the 〈1 1 1〉 direction between the AlGaAs shells, showing a successful fabrication of the GaAs quantum well.     

Nature of dislocations promoting growth in liquid phase epitaxy of gallium arsenide

Dislocations promoting growth in the course of liquid phase epitaxy (LPE) of GaAs layers on GaAs substrates are analysed by X-ray topography. The Burgers vectors are determined by comparing double-crystal back-reflection images with calculated misorientations taking into account surface relaxation. Any dislocation which generates a spiral of elementary steps is found to have a Burgers vector component parallel to the macroscopic growth direction. The nature of these growth promoting dislocations may be between pure screw and pure edge type. Defects which might be responsible for the generation of the observed concentric growth step patterns are below the detection limit of current X-ray topography.     

Structural analysis of AlGaAs quantum wires on vicinal (110)GaAs by transmission electron microscopy and energy dispersive X-ray spectroscopy

Giant step structures consisting of coherently aligned multi-atomic steps were naturally formed during the molecular beam epitaxy growth of Al_0.5Ga_0.5As/GaAs superlattices (SLs) on vicinal (110)GaAs surfaces misoriented 6° toward (111)A. The growth of AlAs/Al_xGa_1−xAs/AlAs quantum wells (QWs) on the giant step structures realized Al_x0Ga_1−x0As (x₀<x) quantum wires (QWRs). We studied the giant step structures and the QWRs by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). TEM observations revealed that the QWRs were formed at the step edges. The cross sections of the QWRs were as small as 10 nm×20 nm and the lateral distances between them were about 0.15 μm. We clarified the roles of the SLs to form the coherent giant step structures. From EDX analysis, it was estimated that the AlAs composition in the Al_0.5Ga_0.5As layers varied from 0.5 (terrace) to 0.41 (step edge). In the AlAs/Al_xGa_1−xAs/AlAs QWs, the AlAs compositional modulation and the confinement by the AlAs barriers led to the embedded Al_x0Ga_1−x0As regions. These results supported the existence of the Al_x0Ga_1−x0As QWRs on the giant step structures.     

Surface diffusion kinetics of GaAs and AlAs metalorganic vapor-phase epitaxy

We have investigated the surface kinetics during metalorganic vapor-phase epitaxy (MOVPE), using high-vacuum scanning tunneling microscopy (STM) observation of two-dimensional (2D) nuclei and denuded zones. Using Monte Carlo simulations based on the solid-on-solid model, from 2D nucleus densities we estimated the surface diffusion coefficients of GaAs and AlAs to be 2 × 10⁻⁶ and 1.5 × 10⁻⁷ cm²/s at 530°C, and the energy barriers for migration to be 0.62 and 0.8 eV, respectively. The 2D nucleus size in the [110] direction was about two times larger than that in the [ 10] direction. The size anisotropy is caused primarily by a difference in the lateral sticking probability (P_s) between steps along the [ 10] direction (A steps) and steps along the [110] direction (B steps). The P_s ratio was estimated to be more than 3:1. Denuded zone widths on upper terraces were 2 ± 0.5 times wider than those on lower terraces. This showed that P_s at descending steps was 10 to 3 × 10² times larger than P_s at ascending steps.     

Extremely flat interfaces in GaAs/AlGaAs quantum wells with high Al content(0.7) grown on GaAs (411)A substrates by molecular beam epitaxy

Effectively atomically flat interfaces over a macroscopic area (200 μm diameter) have been achieved in GaAs/Al_0.7Ga_0.3As quantum wells (QWs) with well widths of 3.6-12 nm grown on (411)A GaAs substrates by molecular beam epitaxy (MBE) for the first time. A single and very narrow photoluminescence peak (FWHM, full width at half maximum, is 6.1 meV) was observed at 717.4 nm for the QW with a well width of 3.6 nm at 4.2 K. The linewidth is comparable to that of growth-interrupted QWs grown on (100)-oriented GaAs substrates by MBE. A 1.5 μm thick Al_0.7Ga_0.3As layer with good surface morphology also could be grown on (411)A GaAs substrates in the entire growth temperature region of 580-700°C, while rough surfaces were observed in Al_0.7Ga_0.3As layers simultaneously grown on (100) GaAs substrates at 640-700°C. These results indicate that the surface of GaAs and Al_0.7Ga_0.3As grown on the (411)A GaAs substrates are extremely flat and stable on the (411)A plane.     

Mechanism of condensation of heteroepitaxial A3B5 layers pulse of moderate power

The mechanism of condensation of heteroepitaxial layers in laser deposition was studied on GaAs films on NaCl. Films were deposited in a superhigh vacuum using laser pulses of moderate power. Pulsed deposition proved to be more suitable for nucleation and growth than continuous deposition. However in this case it is important to compare the number of atoms deposited per pulse and the number of preferential adsorption centres in the substrate temperature of 310 °C. The density of dislocations in the films was about 10⁹ cm⁻². The main reasons for the dislocations are the excess of low volatile component and thermal stresses. Using a laser with quantum energy higher than the energy gap of the deposited semiconductor GaAs, which provides congruent evaporation, and making an additional deposition of As, we could reduce dislocations to 10⁷ cm⁻². A mechanism is proposed to explain the reduction of temperature in single crystal growth under laser deposition: high energy ion component of laser plasma gives rise to orienting defects on the substrate surface, and these defects determine epitaxial growth.     

In原子在GaAs(001)表面的成核与扩散研究

近年来,半导体量子点特别是InAs量子点的基本物理性质和潜在应用得到了广泛研究。许多研究者利用InAs量子点结构的改变以调制其光电特性。本文采用液滴外延法在GaAs(001)表面沉积了不同沉积量的In(3 ML、4 ML、5 ML),以研究In的成核机制和表面扩散。实验发现,随着In沉积量的增加,液滴尺寸(包括直径、高度)明显增大。不仅如此,在相同的衬底温度下,沉积量越大,液滴密度越大。利用经典成核理论,计算了GaAs(001)表面In液滴形成的临界厚度为0.57 ML,计算的结果与已报道的实验一致。从In原子在表面的迁移和扩散,以及衬底中Ga和液滴中的In之间的原子互混原理解释了In液滴形成和形貌演化的机理。实验中得到的In液滴临界厚度以及In液滴在GaAs(001)上成核机理,可以为制备InAs量子点提供实验指导。     

The growth of GaAs and InAs dots on etched mesas: The effect of substrate temperature on mesa profile and surface morphology on dot distribution

The molecular beam epitaxy (MBE) growth of GaAs and InAs quantum dots on etched mesas has been studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The [0 1 1]-oriented mesas are etched into (1 0 0) GaAs substrates, exposing (5 3 3)B sidewall facets. At a substrate temperature of 610 °C a top (1 0 0) plane is seen to evolve on a ridge mesa structure. Alternatively, if the overgrowth is carried out at 630 °C no such facet is seen, and the top ridge remains unchanged during GaAs growth. By controlling the mesa shape, either ordered lines of dots can be grown or the dot density can be varied from <5×10⁸ cm⁻² to >1×10¹¹ cm⁻² on the same substrate in pre-defined regions. The dot distribution observed on the mesa sidewalls and top is discussed in terms of net migration of adatoms from different facets, underlying step density, step height and surface curvature of the mesa top.     

Microampere laser threshold at 80°C with InGaAs/GaAs/InGaP buried heterostructre strained quantum well lasers

An extremely low CW threshold current of 670 μA and a high slope efficiency of 0.14 W/A at a high junction temperature of 80°C were obtained with a 200 μm long Al-free InGaAs/GaAs/InGaP buried heterostructure (BH) quantum well laser grown by three-step metal organic vapor phase epitaxy (MOVPE). The maximum energy conversion efficiency of a 500 μm long laser was as high as 50% at a output power level of 1 mW. Regrowth conditions of InGaP layers were found to be crucial for planarizing the grown surface to realize the high performances.     

Critical thickness of the 2-dimensional to 3-dimensional transition in GaSb/GaAs(001) quantum dot growth

The phase transformation from planar to quantum dot growth is driven by strain energy reduction at the cost of surface energy. By calculating and comparing the strain energies of monolayer thick GaSb and InAs films on GaAs(001), a critical thickness for the 2-dimensional to 3-dimensional phase transformation of about 1.2 ML was derived for the GaSb/GaAs quantum dot system. This value is in agreement with the direct observation of the effectively deposited amount of material using cross-sectional scanning tunneling microscopy. Deviating experimental literature values can be traced back to the neglect of the Sb-for-As exchange process.     

Anisotropic lateral growth in GaAs MOCVD layers on (001) substrates

For metalorganic chemical vapor deposition, a fast lateral growth rate is observed for the first time on (001) GaAs having round mesas. The lateral growth rate is greater than the vertical growth rate by a factor of 3–5. The lateral growth rates have anisotropy with respect to the crystallographic directions on the (001) surfaces. The fastest growth direction is the [110] and the slowest one is the [ 10]. The [110] and [ 10] growth rates were found to be strongly dependent on growth conditions, though the vertical one is independent. The [110] growth rate decreases with decreasing As pressure, while the [ 10] remains constant. As growth temperature increases, both the [110] and the [ 10] growth rates decrease. A simple model for the lateral growth mechanism is proposed from the consideration of atomic arrangements and the number of dangling bonds at [110] and [ 10] step sites. According to the model, the lateral growth rate is proportional to the number of bonds available for binding Ga atoms at step sites. The model can explain well the anisotropy in the lateral growth rate and its dependence on the growth conditions.     

Precise structure control of GaAs/InGaP hetero-interfaces using metal organic vapor phase epitaxy and its abruptness analyzed by STEM

Fabrication of abrupt InGaP on GaAs (InGaP/GaAs) and GaAs on InGaP (GaAs/InGaP) hetero-interfaces has been difficult using metal organic vapor phase epitaxy (MOVPE) due to the exchange of P and As during the fabrication steps. Indium (In) surface segregation during InGaP growth also degrades the abruptness. Here, the MOVPE gas-switching sequence to fabricate atomically abrupt hetero-interfaces was optimized and the effects of this optimization on the hetero-interface abruptness were quantitatively evaluated using the Z-contrast method with scanning transmission electron microscopy (STEM). Results revealed that (a) in the fabrication of InGaP/GaAs hetero-interface, the GaAs top layer should be stabilized using As-source gas supply, and the excess As layer on GaAs should be terminated using an additional supply of Ga species, and (b) in the fabrication of GaAs/InGaP interface, the InGaP layer should be grown using the flow modulation method to suppress In surface segregation. In conclusion, the abruptness of hetero-interfaces of InGaP/GaAs and GaAs/InGaP was improved by using these optimized gas-switching sequences.     

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.     

Magnitude and polarity of strain-induced fields in pseudomorphic In0.22Ga0.8As quantum well structures on (112) GaAs substrates

When pseudomorphic (In,Ga)As/(Al,Ga)As heterostructures are grown on certain surfaces of the general crystallographic planes of GaAs denoted by (11l), the strain in the structures induces an electric field because of the piezoelectric nature of the III-V semiconductors. We report the experimental determination of the direction and magnitude of the strain-induced electric field by photoluminescence spectroscopy on one of these substrate surfaces: the one for which l = 2. We confirm that for the two surfaces of the (112) GaAs substrate, the induced field is directed out of the substrate for the A surface and toward the substrate for the B surface.     

Structure characterization of MHEMT heterostructure elements with In<Subscript>0.4</Subscript>Ga<Subscript>0.6</Subscript>As quantum well grown by molecular beam epitaxy on GaAs substrate using reciprocal space mapping

The crystallographic parameters of elements of a metamorphic high-electron-mobility transistor (MHEMT) heterostructure with In_0.4Ga_0.6As quantum well are determined using reciprocal space mapping. The heterostructure has been grown by molecular-beam epitaxy (MBE) on the vicinal surface of a GaAs substrate with a deviation angle of 2° from the (001) plane. The structure consists of a metamorphic step-graded buffer (composed of six layers, including an inverse step), a high-temperature buffer of constant composition, and active high-electron-mobility transistor (HEMT) layers. The InAs content in the metamorphic buffer layers varies from 0.1 to 0.48. Reciprocal space mapping has been performed for the 004 and 224 reflections (the latter in glancing exit geometry). Based on map processing, the lateral and vertical lattice parameters of In_xGa_1–xAs ternary solid solutions of variable composition have been determined. The degree of layer lattice relaxation and the compressive stress are found within the linear elasticity theory. The high-temperature buffer layer of constant composition (on which active MHEMT layers are directly formed) is shown to have the highest (close to 100%) degree of relaxation in comparison with all other heterostructure layers and a minimum compressive stress.     

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.