Ga-stabilized Surface Research Articles

Analytical–Monte Carlo model of the growth of In nanostructures during droplet epitaxy on the triangle-patterned GaAs substrates

Kinetic Monte Carlo simulations combined with the analytical nucleation theory are used to study droplet epitaxy of indium on the GaAs substrates patterned with triangular-shaped nanoholes. The simulation results demonstrate that the growth on the Ga-stabilized surface provides much better selectivity than the formation of nanostructures on the As-stabilized surface because of the stronger bonding with As atoms of the substrate. We estimate technological parameters which allow obtaining required characteristics of nanostructures with precise positioning and complete filling of holes. The distance between nanostructures is considered to be controlled by the growth temperature at an appropriate interhole distance. At the same time, dimensions of nanostructures are outlined by the geometrical parameters of holes and can be adjusted by the deposition thickness.

Behavior of Ga atoms deposited on GaAs (111)B and (111)A surfaces

We investigate the behavior of Ga atoms deposited on GaAs (111)B and (111)A surfaces by the intensity measurement of the specular spot of the reflection high-energy electron diffraction. Characteristics of these two surfaces are very different. For (111)B surface, we reveal that the deposited Ga atoms fill the Ga vacancy of 19×19 reconstruction until the surface is fully covered by one mono-layer of Ga. Excess atoms deposited on the fully covered surface form droplets and act as a Ga source during the recovery process. The surface recovers initial condition by supplying As. Alternate formation of As-stabilized and Ga-stabilized surfaces controlled by growth sequence enables the migration-enhanced epitaxy on the GaAs (111)B surface. For (111)A surface, in contrast, the stable surface is 2×2 reconstruction even under As irradiation. Deposition of Ga only forms droplets on the surface. The MEE mode growth is not suitable on this surface since As-stabilized surface is hard to form.

Reconstruction dependence of the etching and passivation of the GaAs(001) surface

The microscopic nature of the selective interaction of iodine with an As- and Ga-stabilized GaAs(001) surface has been investigated by the photoelectron emission and ab initio calculations. The adsorption of iodine on the Ga-stabilized (4 × 2)/c(8 × 2) surface leads to the formation of the prevailing chemical bond with gallium atoms; to a significant redistribution of the electron density between the surface Ga and As atoms; and, as a result, to a decrease in their binding energy. Iodine on the As-stabilized (2 × 4)/c(2 × 8) surface forms a bond predominantly with surface arsenic atoms. Such a selective interaction of iodine with the reconstructed surfaces gives rise to the etching of the Ga-stabilized surface and the passivation of the As-stabilized surface; this explains the layer-by-layer (“digital”) etching of GaAs(001) controlled by the reconstruction transitions on this surface.

New Ga-enriched reconstructions on the GaAs(001) surface

To prepare structure-ordered GaAs(001) surfaces at low temperatures, GaAs(001) surfaces coated with native oxides were exposed in an atomic hydrogen flow in the temperature range 280–450 °C. The new Ga-enriched GaAs(001) surfaces with the (4 × 4) and (2 × 4)/c(2 × 8) reconstructions were prepared and studied by the methods of X-ray photoelectron spectroscopy, low-energy electron diffraction, and high-resolution characteristic electron energy loss spectroscopy. For the GaAs(001)-(2 × 4) surface, the structure of the Ga-stabilized surface has been proposed and ab initio computed within the (2 × 4) Ga-trimer unit cell model.

Formation of ultrathin SiNx∕Si interface control double layer on (001) and (111) GaAs surfaces for ex situ deposition of high-k dielectrics

In order to realize pinning-free high-k dielectric metal-insulator-semiconductor (MIS) gate stack on (001) and (111)B oriented GaAs surfaces using the Si interface control layer (Si ICL) concept, formation of a SiNx∕Si ICL double layer was investigated as a chemically stable structure on (001) and (111)B surfaces which allows ex situ deposition of HfO2 high-k dielectric films without losing the benefit of Si ICL. First, Si ICLs grown by molecular beam epitaxy (MBE) on (001) and (111)B GaAs surfaces with various initial surface reconstructions were investigated in detail by reflection high energy electron diffraction and x-ray photoelectron spectroscopy (XPS) investigations at each step of the interface formation. Large shifts of the surface Fermi level position toward unpinning were observed after Si ICL growth on appropriately formed Ga-stabilized surfaces. It was found that Si layers grow epitaxially with Si–Ga bonds at the Si∕GaAs interface and Si–As termination on top, suggesting surfactant roles played by As atoms. Then, an ultrathin SiNx buffer film was formed on the Si ICL by its in situ partial nitridation in the MBE chamber. An XPS analysis of the resultant SiNx∕Si ICL double layer formed on (001) and (111)B surface indicated that the structure is chemically stable against air exposure on both surfaces in the sense that it prevents the host GaAs surface from subcutaneous oxidation, although SiNx film itself partially turns into SiOxNy. Finally, high-k MIS capacitors were formed by ex situ deposition of HfO2 on the SiNx∕Si ICL/GaAs structure after transferring the sample through air. The capacitance-voltage (C-V) analysis indicated that the MIS interface is completely pinning-free with a minimum interface state density in the range of low 1011cm−2eV−1.

Ga-rich GaAs(0 0 1) surfaces observed by STM during high-temperature annealing in MBE

Ga-rich GaAs (0 0 1) surfaces are successfully observed by scanning tunneling microscopy (STM) during high-temperature annealing in molecular beam epitaxy. With a substrate temperature of 550°C, reflection high-energy diffraction patterns and reflectance anisotropy spectra confirm a (4×2) Ga-stabilized surface. STM images clearly show alteration of the surface reconstructions while scanning. It is postulated that detaching and attaching of Ga adatoms may be the cause of this surface dynamics. For these conditions it is determined that ζ(4×4), ζ2(4×4) and ζ(4×6) reconstructions co-exist on the surface. The ζ2(4×4) reconstruction contains a Ga tetramer cluster.

Electrical activation of Te and Se in GaAs at extremely heavy doping up to 5×10 20 cm −3 prepared by intermittent injection of TEG/AsH 3 in ultra-high vacuum

Doping characteristics of group VI elements (Te and Se) on (1 0 0)-oriented GaAs are investigated at an extremely heavy doping level up to 5×10 20 cm −3 on the basis of the surface stoichiometry control to improve the incorporation of impurities. By changing the gas injection sequences, the surface stoichiometry before the introduction of impurity precursors is controlled. The impurity concentration is measured by secondary ion mass spectroscopy (SIMS) analysis and the activation ratio is determined in conjunction with the results of Hall effect measurements. It is shown that the incorporation of Te and Se is extremely enhanced when DETe and DESe are exposed on the Ga-stabilized surface. From the electrical measurements and SIMS results, the segregation of defects due to doped impurity atoms is strongly suggested. The defect formation mechanism of heavily impurity doped GaAs is discussed in view of the formation of impurity–defect complex and the control of site-occupation of doped impurities.

Growth of InAs nanocrystals on GaAs(1 0 0) by droplet epitaxy

Self-assembled InAs nanocrystals were grown by droplet epitaxy on a GaAs(1 0 0) substrate at a relatively low growth temperature of 160°C. The growth mode of InAs varies as a function of the surface stabilizing conditions of the GaAs(1 0 0) substrate. Three-dimensional InAs nanocrystals with a height of 30–40 nm and a base size of 350 nm were grown on the Ga-stabilized surface. However, a two-dimensional layer of InAs with a root-mean-square roughness of 1.4–3.5 Å was formed on the As-stabilized surface. The island density was higher on the vicinal surface than that on the singular surface. The islands were preferentially aligned along the 〈1 0 0〉 direction, i.e., parallel to the step edges of the substrate. The base edges of islands on the vicinal surface were also aligned along the 〈1 0 0〉 direction. These results suggest that the conditions of substrate surface, as well as the strain energy due to the lattice misfit between InAs and GaAs play important roles in the formation of self-assembled InAs nanocrystals by droplet epitaxy.

Dynamics of Ga Clusters on GaAs(001) Surface

AbstractDynamics of Ga clusters and GaAs epitaxial growth on a GaAs (001) surface were successfully observed by a system in which scanning tunneling microscopy (STM) and molecular beam epitaxy (MBE) were incorporated into one unit rather than in separate chambers. With the substrate temperature of 528°C, reflection high-energy diffraction (RHEED) patterns showed a (4×6) Ga-stabilized surface reconstruction and dynamics of steps and islands were clearly observed. The detaching and attaching of small Ga clusters might cause the change of steps and islands. It seems that the small Ga clusters migrated with the diameter of about 0.8 to 1.2 nm and around the steps and islands. These clusters could be observed only when they were detached from or attached to the steps and islands. Moreover, even under the substrate temperature of 440 oC and the As4 partial pressure of 2×10−6 torr, STM images were clearly observed. After 0.1 ML Ga was additionally supplied to the sample by migration enhanced epitaxy mode, step flow growth occurred, resulting in an additional GaAs layer grown on the B-step side. Moreover, the c(4×4) As-stabilized surface reconstruction was moderate. It seems that there is an equilibrium additional layer of As amorphous adatoms on the c(4x4) surface reconstructions.

Effects of substrate preheating for the growth of [formula omitted] by MOCVD

A process for the growth of ZnCdTe (100) GaAs heteroepitaxial films using metalorganic chemical vapor deposition (MOCVD) has been developed. It is found that substrate baking pretreatment deeply affects the characteristics of Zn x Cd 1 − x Te( x < 0.09) on (100)GaAs substrate (such as ZnCdTe film orientations, the Zn composition in the ZnCdTe compound alloys and the quality of ZnCdTe epilayer). We compared measurements on the same set of samples by photoreflectance (PR) and photoluminescence (PL). It was found that baking temperature (or baking time) may affect the relative contribution of each transition, resulting in a shift of the transition energy. We speculate that (100)GaAs substrates may undergo decomposition at a high baking temperature, leading to the GaAs surface change from As-stabilized surface to Ga-stabilized surface, resulting in better film PL quality and different film orientation. At higher baking temperature (or baking time), the ZnCdTe epilayer tends towards a (1 1 1) orientation and the film quality is obviously improved. Up to a temperature of around 640°C, the quality begins to decline sharply due to the destruction surface.

Structural transformation of As-stabilized surfaces caused by Ga-deposition detected by time-resolved surface photo-absorption

Evolution of surface structures during conversion from various As-stabilized surfaces to Ga-stabilized ones is observed by surface photo-absorption. Desorption of As dimers parallel to [110] is observed during the structural change of c(4 × 4) and (2 × 4)γ. The evolution of Ga- and As-related peaks is isolated in surface dielectric anisotropy spectra, which are determined using p- and s-polarized surface photo-absorption spectra during the conversion of (2 × 4)β to a Ga-stabilized (4 × 2) surface.

Structural transformation of As-stabilized surfaces caused by Ga-deposition detected by time-resolved surface photo-absorption

Evolution of surface structures during conversion from various As-stabilized surfaces to Ga-stabilized ones is observed by surface photo-absorption. Desorption of As dimers parallel to [110] is observed during the structural change of c(4 × 4) and (2 × 4) γ. The evolution of Ga- and As-related peaks is isolated in surface dielectric anisotropy spectra, which are determined using p- and s-polarized surface photo-absorption spectra during the conversion of (2 × 4) β to a Ga-stabilized (4 × 2) surface.

Surface reconstruction of sulfur-terminated GaAs(001) observed during annealing process by scanning tunneling microscopy

The surface reconstruction of in-situ prepared sulfur-terminated (S-terminated) and sulfur-protected (S-protected) GaAs(001) is studied by scanning tunneling microscopy (STM) at high temperatures of up to 260°C. We demonstrate a new reordering process from the (4 × 6) Ga-stabilized surface to the (2 × 6) S-stabilized surface and a novel method of air protection using a S passivation layer. Moreover, the in-situ observation of the annealing process of this S-protection layer is performed by high-temperature STM, avoiding the adsorption from environments and verifying that the (2 × 6) structure still becomes dominant on the S-terminated GaAs(001) surface without the effects of As atoms.

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.

Growth of Cubic GAN on (001) GAAS

ABSTRACTWe present a study of the growth of cubic GaN films on (001) GaAs by molecular beam epitaxy. Our investigations focus on the nucleation stage as well as on the subsequent growth of GaN. The phenomenon of epitaxial growth at this extreme mismatch (20%) is demonstrated to arise from a coincidence lattice between GaAs and GaN. The presence of a high-density of stacking faults in the GaN layer is explained within this understanding as being a natural consequence of the coalescence of perfectly relaxed nuclei. We furthermore analyze the growth kinetics of GaN via the surface reconstruction transitions observed upon an impinging Ga flux, from which we obtain both the desorption rate of Ga as well as the diffusion coefficient of Ga adatoms on the Ga-stabilized GaN surface. The diffusivity of Ga is found to be very low at the growth temperatures commonly used during molecular beam epitaxy, which provides an explanation for the microscopic surface roughness observed on our samples.

Pulsed trimethylgallium scattering from As-stabilized and Ga-stabilized surfaces

We investigated trimethylgallium (TMG) scattering from differently prepared Ga-stabilized GaAs(100) surfaces in comparison with that from an As-stabilized surface using a pulsed TMG beam. The obtained sticking coefficients of TMG as functions of TMG exposure indicated that, although clean Ga-stabilized surfaces are reactive to TMG, these surfaces are inactivated by TMG exposure of about 1.5 monolayers, independent of the stoichiometry of the prepared surface. The time-of-flight (TOF) spectrum of TMG scattered from the inactivated surfaces had only the component corresponding to scattering without surface residence, while the component with surface residence was dominant for scattering from an As-stabilized surface. This feature (without surface residence) is the same as that in the scattering from the mask surface of selective-area growth. The sticking coefficients and TOF spectra for Ga-stabilized surfaces were essentially the same for the various Ga coverages of the prepared surfaces. This suggested that the adsorbed methyl group rather than the surface Ga plays an important role in the inactivation of a Ga-stabilized surface.

First real time observation of reconstruction transition associated with Ga droplet formation and annihilation during molecular beam epitaxy of GaAs

The growing surface of GaAs(001) by molecular beam epitaxy under excess Ga flux and a continuous supply of As 4 has been studied in-situ by scanning electron microscope and microprobe reflection high energy diffraction. Dynamical behavior of reconstruction transition associated with formation and annihilation of Ga droplets is observed in real time. It is shown that the transition between As- and Ga-stabilized surfaces occurs not uniformly but by the expansion of domains, the size of which is as large as on the order of a few hundreds μm. Ga droplets are formed inside the domains which show the Ga-stabilized reconstruction. There are two modes for the annihilation of Ga droplets, depending on the droplet density. The mechanisms for both the formation and the annihilation of domains and Ga droplets are discussed.

Arsenic Coverages and Surface Structures of As-Stabilized GaAs (001) Surfaces during Metalorganic Chemical Vapor Deposition Observed by Reflectance Difference

We study temporal evolutions of surface anisotropy change detected optically at wavelengths of 633 nm and 488 nm during the formation of Ga-stabilized surfaces to characterize As-stabilized (001) GaAs surfaces grown by metalorganic chemical vapor deposition (MOCVD). Using the evolution of Ga dimer concentration, we find that effective As coverages, which are defined as the amount of As on the surface that can be incorporated into the crystal, are close to 1 for the c(4×4) surface and As-richer surfaces, although these surfaces have actual As coverages of more than 1. We also find that three kinds of well-defined As-stabilized surfaces with different As coverages, c(4×4)+α, c(4×4), and (2×4), can exist in MOCVD environments depending on the AsH3 partial pressures, with c(4×4)+α being a c(4×4)-like surface with additional As. A fast As desorption with a time constant of <0.5 s at 600°C and at 570°C accompanies the conversion from c(4×4)+α to c(4×4). Another As desorption with a time constant of 2 s at 600°C and 4 s at 570°C accompanies the conversion from c(4×4) to the least-As-rich (2×4) surface with θAs=0.6-0.7.

Cluster model study on GaAs epitaxial crystal growth by arsenic molecular beam: II. Mechanism involving a GaAs 2 intermediate cluster

The mechanisms of the GaAs epitaxial crystal growth by Ga atomic and As 2 molecular beams on Ga-stabilized and As-stabilized GaAs(100) surfaces are studied with the use of the cluster model and the Hartree-Fock method. We propose the mechanism involving the formation of the GaAs 2 cluster as an intermediate. When the As 2 beam is irradiated on the surface, the GaAs 2 cluster is generated from As 2 and a free Ga atom without energy barrier. This GaAs 2 is adsorbed on the As-stabilized surface and gives a new Ga-layer on the surface and the As 2 remains to sb activation energy. On the other hand, the adsorption of this GaAs 2 on the Ga-stabilized surface gives a new As layer and the remaining Ga atom migrates on the surface to make a new Ga layer. The mechanism involving the dissociative adsorption of As 2 studied previously and the presently proposed mechanism involving the GaAs 2 intermediate cluster are compared and it is concluded that the present mechanism is more favorable than the previous one. We summarize the overall reaction mechanism of the GaAs epitaxial crystal growth by the Ga atomic and As 2 molecular beams.

New selective molecular-beam epitaxial growth method for direct formation of GaAs quantum dots

Numerous GaAs epitaxial microcrystals having an average base size of 700 Å×700 Å surrounded mainly by (111) and (110) facets were fabricated on a sulfer-terminated (S-terminated) GaAs (001) substrate by sequentially supplying Ga and As molecular beams. The S-terminated GaAs (001) surface was produced by exposing the surface to a sulfur vapor in the molecular-beam epitaxy system immediately after obtaining a Ga-stabilized surface. The growth of GaAs microcrystals on the S-terminated substrate is caused by a vapor–liquid–solid mechanism. The process consists of forming Ga droplets on the inert surface and reacting the droplets with As to produce GaAs microcrystals. This method termed droplet epitaxy is thought to be a promising growth method for fabricating the GaAs quantum dots.