Realization of optically active strained InAs island quantum boxes on GaAs(100) via molecular beam epitaxy and the role of island induced strain fields

We report here the realization of strained InAs three-dimensional islands on GaAs(100) with optical characteristics that reveal lateral quantum confinement (i.e. quantum box behavior). The importance of the cap layer growth conditions and methodology in achieving optically active InAs islands and the existence, range, and impact of island-induced strain fields on the cap layer growth are uncovered via marker layer experiments. Strong optical emission from the InAs islands is observed in correlation with the transmission electron microscope (TEM) observation of uniform coherent islands under optimized growth conditions. Photoluminescence excitation (PLE) spectroscopy reveals the presence of the energy transitions due to the three-dimensional electronic confinement in such InAs islands. The InAs islands buried under the GaAs were found to be quite stable upon annealing to 100°C higher than the growth temperature.     

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.     

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。     

Atomic force microscope observation of the initial stage of InAs growth on GaAs substrates

The initial stage of InAs growth on GaAs(001) substrates has been investigated by atomic force microscopy. Three-dimensional (3D) islands of a uniformly small size and high density were observed at the initial stage not only for low substrate temperature (T_s) but also for low V/III ratio. This is explained by a simple model based on the difference in the growth rate between strained and relaxed InAs surfaces. The 3D islands are found to agglomerate after the growth is interrupted under As pressure. We suppress the agglomeration and obtain an atomically flat InAs surface.     

Nucleation of islands in GaAs molecular beam epitaxy studied by in-situ scanning electron microscopy

We studied two-dimensional nucleation of islands in the molecular beam epitaxy (MBE) of GaAs by in-situ scanning electron microscopy and found that the islands do not appear until about 1/3 monolayer is deposited. In contrast to what is expected from the previous simple nucleation and growth model, we observed islands to form continuously.     

GaAs initial growth on InAs(001) vicinal surfaces observed by scanning tunneling microscopy

GaAs initial growth on InAs surfaces misoriented by 2° toward the [110] and [1 0] directions was investigated by scanning tunneling microscopy (STM). In the STM images of both InAs vicinal surfaces after GaAs deposition, white lines running in the [1 0] direction, corresponding to the grown GaAs surface, were observed. Almost all of the lines were attached only to steps running in the [110] direction (B-type steps) on both InAs surfaces; that is, the lines were seldom attached to steps running in the [1 0] direction (A-type steps). These results indicate that the B-type steps are more favorable for the sticking of deposited Ga atoms than the A-type steps during GaAs initial growth on InAs vicinal surfaces.     

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.     

Uniformity improvement of selectively-grown InGaAs micro-discs on Si

Dislocation-free InGaAs micro-disks, having a diameter of approximately 5 μm and a thickness of approximately 0.2 μm, have been grown on Si(111) substrate patterned with SiO₂, with excellent uniformity in shape. This shape uniformity was dependent on the initial growth of InAs on Si windows. For the growth of well-controlled lateral islands, it is mandatory to cover the openings completely with a single-domain InAs crystal prior to the subsequent growth of InGaAs, the Ga content of which is essential for lateral growth. Moreover, the shape uniformity of InAs islands strongly affected the uniformity of the InGaAs islands' shape. On the basis of these observations, the diameter of Si openings was reduced to 1 μm to make it easier to fill the openings with InAs without failure. A two-step growth of InAs was devised to achieve void-less nucleation and lateral growth at the same time. It was also found that the recess at the boundary between Si and the SiO₂ mask played a vital role to limit the lateral size of InAs islands: when an InAs island hit that boundary, the lateral growth was suppressed and a stable {?110} facet emerged, and the growth seemed to wait for all the {?110} sidewalls to emerge. This mechanism compensated variation in the progress of InAs growth due to scattered timing of InAs nucleation. Therefore, for the purpose of improving the shape uniformity of InGaAs micro-disks, transition from InAs to InGaAs growth should be attempted after all the sidewalls are covered with {?110} facets and before vertical growth starts.     

Directional solidification of InxGa1−xAs

We have investigated a constitutional supercooling and segregation phenomena in In_xGa_1−xAs crystals unidirectionally solidified in a vertical system. The constitutional supercooling generates characteristic fluctuations of composition along the growth direction and this can be explained by a free nucleation ahead from the growth interface. The macroscopic compositional profiles of the grown crystals suggest that a transport of solute is mainly dominated by the diffusion. Such a growth mode is partly attributed to the difference in density between InAs and GaAs.     

Oriented growth of whiskers of AIIIBV compounds by VLS-mechanism

The oriented growth of GaAs, GaP, InAs and GaInAs whiskers on the same (GaAs, GaP) or different (InAs/GaAs, GaInAs/GaAs) substrates was studied. A detailed morphological study of GaAs whiskers on polar A(III), B(111 ) and non-polar (001), (011) substrates was performed. The growth conditions for ordered (perpendicular to substrate) growth on the A(111) and B(111 ) faces were determined. There were found discrete spectra of whisker systems on all substrates with the preferential growth of “arsenic” B{111 } faces. The dependence of the growth rate on the whisker diameter is typical for the vapour-liquid-solid (VLS) mechanism and is used for the determination of kinetic coefficients for polar faces. There was observed a periodic instability in growth of InAs and GaInAs whiskers.     

Transmission electron microscopy study of an AlN nucleation layer for the growth of GaN on a 7-degree off-oriented (0 0 1) Si substrate by metalorganic vapor phase epitaxy

We have investigated the morphology of the high-temperature-grown AlN nucleation layer and its role in the early stage of GaN growth, by means of transmission electron microscopy. The nitride was selectively grown on a 7-degree off-oriented (0 0 1) patterned Si substrate by metalorganic vapor phase epitaxy. AlN was deposited on the inclined unmasked (1 1 1) facet in the form of islands. The size of the islands varied along the slope, which is attributable to the diffusion of the growth species in the vapor phase. The GaN nucleation occurred at the region where rounded AlN islands formed densely. The threading dislocations were observed to generate in the GaN nucleated region.     

Misfit Strain Relaxation by Dislocations in InAs Islands and Layers Epitaxially Grown on (001)GaAs Substrates by MOVPE

The misfit dislocation configurations in InAs islands as well as in more or less continuous layers grown on (001) oriented GaAs substrates were studied by weak-beam and high-resolution electron microscopy. The islands are confined by {101} and {111} facets where the aspect ratio (height/lateral extension) can be affected by the growth conditions. It is possible to grow well-defined islands as well as relatively continuous layers by MOVPE under As-stabilized conditions. At constant deposition parameters the growth is characterized by islands of different sizes (but with constant aspect ratio) in various strain states depending on their dislocation content. Coherently strained islands without any dislocation can be observed for heights up to 23 ML InAs, or otherwise, up to a maximal island extension of about 12 nm (for the particular aspect ratio ≈︂0.585). With further increase of island height and lateral extension, the introduction of dislocations becomes favourable. Independent of the island size, the layer thickness and the dislocation density, a residual elastic strain of about ε^r = —0.8% remains after relaxation. This means, about 88% of the total misfit strain of ε = —6.686 × 10^—2 were compensated by Lomer dislocations. These sessile Lomer dislocations lie in the island interior only, where single 60° dislocations were observed exclusively in their near-edge regions. With increasing island size and/or layer thickness some close-spaced 60° dislocations occur additionally within the interfacial region. The Lomer dislocations that are always located 4 monolayers (ML) above the InAs/GaAs interfacial plane result from the well-known fusion of two 60° slip dislocations. These 60° dislocations have been nucleated 7 … 8 ML above the interface at surface steps on the {111} facets confining the islands. Based on our experimental observations a new mechanism is proposed that explains the origin of these 60° dislocations. Their further fusion to sessile Lomer dislocations that compensate the misfit strain most efficiently occurs in the way as commonly accepted.     

Formation of Ge-diffusion-suppressed GaAs layers and InAs quantum dots on Ge/Si substrates

Crystal growth of GaAs layers and InAs quantum dots (QDs) on the GaAs layers was investigated on Ge/Si substrates using ultrahigh vacuum chemical vapor deposition. Ga-rich GaAs with anti-site Ga atoms grown at a low V/III ratio was found to suppress the diffusion of Ge into GaAs. S-K mode QD formation was observed on GaAs layers grown on Ge/Si substrates with Ga-rich GaAs initial layers, and improved photoluminescence from 1.3 μm-emitting InAs QDs was demonstrated.     

In-situ fabrication of three-dimensionally confined GaAs and InAs volumes via growth on non-planar patterned GaAs(001) substrates

Three-dimensionally confined GaAs/AlGaAs and InAs/GaAs structures on 100 oriented square mesas patterned onto GaAs(001) substrates are realized, in-situ, via size-reducing molecular beam epitaxy. Two stages of mesa top pinch-off involving {103} and subsequently {101} side facets are revealed. GaAs and InAs quantum boxes with lateral linear dimensions down to 40 nm and confined by AlGaAs and GaAs, respectively, are reported. For InAs, the strain relief in mesas is found to enhance the well known 2 ML thickness for three-dimensional island formation on unpatterned substrates to, remarkably, >5 ML for mesa size 75 nm. Cathodoluminescence emission from the InAs on the mesa top attests to its good optical quality.     

Stranski-Krastanov formation of InAs quantum dots monitored during growth by reflectance anisotropy spectroscopy and spectroscopic ellipsometry

In this study the successful application of reflectance anisotropy spectroscopy (RAS) and spectroscopic ellipsometry (SE) for the in-situ investigation of InAs quantum dot growth on GaAs (001) in Stranski-Krastanov growth mode is reported. Both techniques provide the precise determination of the growth mode transition from two-dimensional InAs layer to three-dimensional island growth. In order to optimize the growth conditions, spectral and time-resolved measurements were performed for different growth parameters (temperatures, growth rates and V/III ratios). For high temperature deposition large additional anisotropies, caused by clusters elongated in the [110] direction were found. Decreasing the deposition rate (0.5 to 0.125 ML/s) also results in the formation of large clusters, as decreases in reflectivity due to larger stray light losses prove. Finally, increasing the AsH₃ partial pressure leads to an earlier onset of island formation and an enhanced tendency for cluster formation.     

In situ monitoring of CdTe nucleation on GaAs (100) using spectroscopic ellipsometry

In situ spectroscopic ellipsometry was used to monitor the nucleation behavior of CdTe grown on vicinal GaAs (100) substrates by organometallic vapor phase epitaxy. CdTe was grown on GaAs (100) substrates of exact and 2° off towards 110 orientations. A spectroscopic ellipsometer was used to collect in situ data at 44 wavelengths from 4000–7000 Å. The Bruggeman's effective medium approximation was employed to determine the variation of the epilayer volume fraction with thickness, which was an indirect way of monitoring the expected island growth behavior. The Stranski-Krastonov (layer plus island) mode of growth was clearly observed for CdTe growth. The growth on the 2° off substrate was also “denser” than that on exact (100), which implied that coalescence of the islands occurred at lower thickness. This was expected since island nucleation is most favored along the ledges on the surface whose spacing decreases with increasing misorientation. A simple nucleation model, assuming cylinder-like islands, was able to fit the experimental data quite well, lending support to the island growth model.     

Structure and optical properties of self-assembled InAs/GaAs quantum dots with In0.15Ga0.85As underlying layer

A high density of 1.02×10¹¹ cm⁻² of InAs islands with In_0.15Ga_0.85As underlying layer has been achieved on GaAs (1 0 0) substrate by solid source molecular beam epitaxy. Atomic force microscopy and PL spectra show the size evolution of InAs islands. A 1.3 μm photoluminescence (PL) from InAs islands with In_0.15Ga_0.85As underlying layer and InGaAs strain-reduced layer has been obtained. Our results provide important information for optimizing the epitaxial structures of 1.3 μm wavelength quantum dots devices.     

Effect of growth interruptions on ultra-thin InAs/InP quantum wells grown by chemical beam epitaxy

We have studied the effect of growth interruptions on 2 monolayer thick InAs/InP strained quantum wells (QWs) grown by chemical beam epitaxy. The main feature is the formation of up to 8 monolayer thick InAs islands during As₂ annealing of the QW. Their formation is characterized by a two- to three-dimensional transition of the reflection high energy electron diffraction (RHEED) pattern and by multiple-line photoluminescence spectra. This interpretation of the data is confirmed by transmission electron microscopy (TEM).     

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.