MBE-grown GaAs Research Articles

Comparative REM and AFM investigations of the surface recovery of MBE-grown GaAs(0 0 1)-layers during annealing

The surface morphology of homoepitaxial, MBE-grown GaAs(001)-layers was investigated after annealing at temperatures in the range from 550°C to 620°C by ex situ reflection electron microscopy (REM) and atomic force microscopy (AFM). Both methods were found complementary in combining rapid aquisition of images at different places on the samples with high-resolution, undistorted imaging, respectively. On not misoriented substrates, both REM and AFM revealed a similar decrease of the densities of monolayer steps, islands and holes with increasing temperature. However, the detection probability of small monolayer islands and holes with sizes below 5 nm in AFM, and below 10 nm in REM, seems to be limited by a thin oxide layer. In REM, the lateral resolution in imaging direction is further reduced to about 100 nm by the strongly foreshortening perspective. On rough surfaces, the resolution in AFM is strongly reduced, such that step bunches on misoriented substrates could not be resolved into single steps with distance of below 20 nm. This, however, was clearly possible in REM, when the bunches were oriented about parallel to the viewing direction. On the other hand, virtually undistorted imaging in AFM allowed to detect anisotropic shapes of islands and holes, which developed during annealing.

ELectrical Characterization of Beryllium Doped Low Temperature MBE Grown GaAs

ABSTRACTThe electrical properties of low temperature MBE grown GaAs (LT-GaAs) in the n-i-n configuration have been studied. The mechanism of current rise in beryllium doped LT-GaAs is found to fit Frenkel-Poole type emission with a barrier height of 0.26 eV. However, this model does not fit undoped LT-GaAs. The breakdown field is considerably higher (up to 5.2* 105 V/cm) for beryllium doped films than undoped films, and depends on both growth temperature and beryllium concentration. Beryllium doping is also found to increase the resistivity of the preannealed films to values > 109 ω-cm.

Determination of the atomic geometry of the GaAs(001)2×4 surface by dynamical RHEED intensity analysis: The β2(2×4) model

Using dynamical reflection high-energy electron diffraction (RHEED) computations, experimental RHEED rocking curves from the MBE-grown As-rich GaAs(001)2×4 surface have been analysed to provide estimates of the surface structural parameters. Ab initio total-energy computations suggest that surfaces grown in such As-rich conditions are likely to exist in the so-called β2(2×4) phase. Therefore, it is assumed that the most appropriate GaAs(001)2×4 surface model with which to analyse the experimental data is that of the β2(2×4) phase. This phase has two As dimers in the surface layer and a third As dimer in the third layer. The experimental rocking curves analysed were measured in the primary beam azimuth, permitting estimates of those β2(2×4) phase surface structural parameters in the plane perpendicular to the primary beam azimuth. The best experiment–theory fit was obtained using a standard non-linear optimisation scheme (the Marquardt algorithm). We find that by using the β2(2×4) surface model we are able to get a better fit to the experimental data than in earlier work in which the data were analysed using the β(2×4) surface model. This supports the assumption that the β2(2×4) surface model is the most appropriate to describe the surface in the As-rich MBE growth conditions used to produce the sample surface upon which the RHEED experiments were performed. Our best-fit β2(2×4) surface structure is predominantly in agreement with that obtained from a surface X-ray diffraction study and from ab initio total-energy calculations. As in these other studies, our best-fit structure shows a large in-plane relaxation of the second layer threefold coordinated Ga atoms and an outward perpendicular displacement of the first layer As dimers. The good agreement in most details between all of these independent structure determinations suggests that we now have a largely consistent picture of the structure of the GaAs(001) β2(2×4) surface.

In situ RHEED, AFM, and REM investigations of the surface recovery of MBE-grown GaAs(0 0 1)-layers during growth interruptions

The evolution of the surface morphology of homoepitaxial, MBE-grown GaAs(0 0 1)-layers was studied during growth interruptions by in situ RHEED (reflection high-energy electron diffraction) as well as by ex situ AFM (atomic force microscopy) and REM (reflection electron microscopy). The recovery of the RHEED intensity during annealing was correlated with a significant decrease of the densities of steps, islands and holes, seen in AFM and REM images, obtained from quenched samples. While islands seem to be dissolved by Ostwald ripening, the densities of steps and holes were found to approach saturation values. The appearance of anisotropic shapes of islands and holes, and characteristic changes during annealing were attributed to different mechanisms governing surface kinetics. It is concluded that, while diffusion of monomers dominate the growth periods, formation and diffusion of GaAs molecules is responsible for the changes of anisotropy during annealing under As pressure.

Site control of self-organized InAs dots on GaAs substrates by in situ electron-beam lithography and molecular-beam epitaxy

We studied a site-control method for self-organized InAs quantum dots on GaAs substrates by a combination of in situ electron-beam (EB) lithography and molecular-beam epitaxy (MBE) using an ultrahigh-vacuum multichamber system. Small and shallow holes were patterned on a MBE-grown GaAs (001) surface by in situ EB writing and Cl2-gas etching. When more than a 1.4 monolayer of InAs was applied to the patterned surface, In(Ga)As dots were preferentially self-organized in the holes while dot formation around the holes was sufficiently suppressed. When we further increased the amount of InAs, the dots enlarged remarkably, presumably due to a stress-relaxation effect.

In-Situ Contactless Characterization of Microscopic and Macroscopic Properties of Si-doped MBE-Grown (2×4) GaAs Surfaces

Correlation between macroscopic electronic properties of the molecular beam epitaxy (MBE)-grown Si-doped GaAs (001) (2×4) surfaces with microscopic atomic structures was in situ investigated using ultrahigh vacuum scanning tunneling microscope (UHV STM), UHV contactless capacitance-voltage (C-V), UHV photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) techniques. Surface defects including kinks, steps, holes, islands and missing As atoms in As dimer rows were observed at the Si-doped (2×4) GaAs surfaces. Contactless C-V results directly showed the surface Fermi level pinning. The observed macroscopic C-V and PL behavior cannot be explained by the previous kink-acceptor model, assuming that each kink forms a discrete acceptor level, but it indicates Fermi level pinning by U-shaped continuous surface states.

In-Situ Regrowthi of GaAs Through Controlled Phase Transformations and Reactions of Thin Films on GaAs

ABSTRACTIn-situ depositions and reactions are utilized in the study of phase formation from solid phase reactions. We report on the formation of epitaxial GaAs and the formation of NiAs or Ni2Ga3 by the exposure of Ni3GaAs to As4 or Ga fluxes. In-situ annealing of Ni on MBE-grown GaAs leads to Ni3GaAs, and subsequent reaction with As4 or Ga drives regrowth of GaAs. The structures were analyzed by RBS, XRD, TEM, and in-situ electrical measurements.

Yb Luminescence Centres in MBE-Grown and Ion-Implanted GaAs

Yb Luminescence Centres in MBE-Grown and Ion-Implanted GaAs

The role of acceptor density in [formula omitted] based quantum well HEMTs

Numerical results based on a self-consistent solution of Poisson's and Schrődinger's equations for GaAs AlGaAs quantum well HEMTs are presented. It is shown that varying the acceptor density in a quantum well significantly affects the threshold voltage, the channel charge density, and hence the drain current. Particularly, near threshold and the subthreshold region, for a given gate bias a change of acceptor density from ∼ 10 13 to ∼ 10 16 cm −3 decreases the drain current by two orders of magnitude. For typical acceptor densities existing in the MBE-grown GaAs channel, a shift in the threshold voltage of 120 mV was obtained. The role of acceptor density is enhanced at increasing well width.

ZnSe ionicity control layer inserted in GaAs/CaF 2(111) interface

For heteroepitaxial growth of covalent crystals on ionic crystals which have very different ionicities, we propose a new approach of introducing a material which has intermediate ionicity to the heterointerface for the purpose of accommodation of the large ionicity difference. By introduction of a thin ZnSe layer to the interface of GaAs/CaF 2(111) as a buffer layer, we successfully improved the crystallinity of the MBE-grown GaAs layer, which was examined by the double crystal X-ray diffraction method. The ZnSe layer was grown by a 2-step growth method in which a few nm ZnSe was deposited at low temperature on an epitaxial CaF 2 film grown on Si(111) substrate and it was annealed in situ before overgrowth of GaAs. The growth conditions of the ZnSe layer such as deposition temperature, thickness, and annealing temperature were studied to obtain good crystallinity of the GaAs overlayer.

Improved MBE-grown GaAs using a novel, high-capacity Ga effusion cell

A novel PBN crucible has allowed the design of a new high-capacity Ga source incorporating a unique heat shielding cap. The extra heat shielding at the front of the source allows hot lip operation with considerably lower power than for previous designs, thus completely eliminating Ga droplets without an inordinately high heat load. Compared with another high-capacity, low-flux transient cell (an EPI 125DF), the new source operated with significantly less power and produced lower defect densities. Specifically, defect densities of 22–27 defects/cm 2 for ∼ 1 μ m thick GaAs layers have been obtained with the new source, compared with 88–92 defects/cm 2 from the 125DF. In addition, an 18.7 μm thick GaAs layer grown with the new cell had only 277 defects/cm 2 . The new source also produced GaAs with low background doping (< 2 × 10 13 /cm 3 ), low total trap density (< 2.0 × 10 12 /cm 3 ), and good uniformity over a 3 in diameter wafer (center-to-edge decrease in GaAs thickness of ∼ 2.2%). It also exhibited low flux transients (< 2%) and low-long term drift.

4.8 mW singlemode oxide confined top-surface emitting vertical-cavity laser diodes

The authors have optimised MBE-grown GaAs VCSELs emitting at a wavelength of 840 nm for maximum singlemode output power. Devices of 3.5 µm diameter show a record high singlemode CW output power of 4.8 mW and a butt-coupling efficiency into singlemode fibre of > 80%.

Kinetics on MBE-grown GaAs Surface during Annealing as Revealed by in-situ SEM Observation.

The behavior of monolayer-deep holes on the surface of (001) GaAs during post-growth annealing in molecular beam epitaxy was observed by in-situ scanning electron microscopy. Most small holes disappear immediately after growth. However, it was found that some are left and combine to form big holes. They extend in the [110] direction and coalesce with each other, while at the same time, shrink in the [110] direction. Finally, they shrink in both directions and disappear. It takes about 10min for all the holes to disappear, which is much longer than the growth interruption period usually employed to smooth heterointerfaces. The anisotropic behavior of big holes is discussed in relation to the reported growth anisotropy.

Tailoring of low-temperature MBE-grown GaAs for ultrafast photonic devices

In this paper, we review recent work on ultrafast nonlinear optical effects in low-temperature MBE-grown GaAs, and describe a rate-equation formalism that can be used to model the dynamic behaviour of the nonlinear absorption and refractive index. We show that under suitable conditions, this material exhibits the behaviour required for an ultrafast all-optical switch, and demonstrate how the material characteristics can be tailored for such applications by suitable control of growth and annealing conditions.

Stopping powers of GaAs for 0.3–2.5 MeV 1H and 4He ions

Ion backscattering and foil transmission methods have been used to determine the energy loss of 0.3–2.5 MeV 1H and 4He ions in crystalline bulk GaAs and thin foil GaAs grown by molecular beam epitaxy (MBE). The self-supporting GaAs sample foil was produced by floating the MBE-grown GaAs film from an AlAs GaAs backing by a lift-off process. The stopping powers, corresponding to energy loss of the ions in a nonchanneling direction in the crystal were extracted from the measurements. Ion backscattering and channeling were employed to study the effect of the crystal structure of the thin foil sample on the stopping powers deduced. Deviations from semi-empirical SRIM calculations (version 96.04) were observed in the case of 4He ions for which the stopping powers fall clearly below the calculated values. An average deviation of about 5% is found for the energies studied, from below the stopping power maximum at 0.8 to 2.2 MeV. The results obtained by the two independent experimental methods have been compared and discussed.

A NEW METHOD FOR STUDYING SEMICONDUCTING SURFACES IN AIR BY SCANNING TUNNELING MICROSCOPY

We used a new method for studying GaAs surfaces in air with a home-made scanning tunneling microscope. The oxide layer was chemically removed, and the sample was prevented from re-oxidation by a thin layer of mineral oil (NOJOL). MBE-grown GaAs layers and In As quantum dots were successfully imaged.

Observation of compensating Ga vacancies in highly Si-doped GaAs.

Positron annihilation experiments have been performed to study the type and concentration of compensating defects in highly Si-doped GaAs grown by molecular-beam epitaxy (MBE). The results show the presence of both Ga vacancies and negative ion defects, each of which act as acceptors in $n$-type GaAs. The concentrations of both types of defects increase strongly for Si concentrations exceeding 5 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. At [Si] \ensuremath{\ge}5 \ifmmode\times\else\texttimes\fi{} ${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, the concentrations of Ga vacancies and negative ions are comparable, and their sum represents a substantial fraction of the total concentration of Si itself. The results provide direct evidence that Ga vacancies play an important role in the electrical deactivation of highly Si-doped MBE-grown GaAs.

Deep level defects in GaAs on Si substrates grown by atomic hydrogen-assisted molecular beam epitaxy

The electrical activity of defects in GaAs p+n diodes grown on Si and GaAs substrates by both conventional molecular beam epitaxy (MBE) and atomic hydrogen-assisted MBE (H-MBE) were characterized by deep level transient spectroscopy. The well-known electron traps typical of MBE-grown GaAs were detected without the presence of any new levels in the upper half of the band gap. The trap densities and diode reverse saturation currents are significantly reduced in the homoepitaxial GaAs grown by H-MBE compared to that grown by MBE. The trap densities for the heteroepitaxial GaAs-on-Si grown by H-MBE have values higher than those of homoepitaxial GaAs grown by H-MBE at 330 °C, which are possibly affected by the residual dislocation density and stress. The reduction of trap density is attributed to in situ passivation of these defects by atomic H during the growth.

Surface geometry of MBE-grown GaAs(001) surface phases

The MBE-grown GaAs(001) surface exhibits various surface phases depending on the surface temperature and As/Ga flux ratio during the growth. Using scanning tunneling microscopy, reflection high energy electron diffraction and theoretical simulations, we have carried out a systematic study of various phases from As-rich c(4×4), 2×4 to Ga-rich 4×2 and 4×6, utilizing migration-enhanced epitaxy. Based on our thorough investigation, wc were able to propose a simple and unified structural model for the evolution of surface phases. The As-rich 2×4 phase consists of two As dimers on the (op layer and another As dimer on the third layer (Chadi's two-dimer model), while there is still a small window where a slightly As-poor phase may exist consisting of two As dimers on the lop layer and second layer relaxation (Northrup-Froyen model). As for the Ga-rich 4×2, we determined that the Beigelsen's two-Ga-dimer model fits best to our experimental and theoretical results, which consists of two Ga dimers on the top layer and an additional Ga dimer on the third layer, a mirror image of Chadi's two-dimer model for the As-rich 2×4 phase. Our direct observations reveal that there arc two distinct 4×6 phases.

Surface quality and atomic structure of MBE-grown GaAs(100) prepared by the desorption of a protective arsenic layer

Scanning tunneling microscopy (STM) was used to study clean GaAs(100) surfaces prepared by thermal desorption of a protective Arsenic layer. The GaAs samples were grown by MBE using an As 2 cracker cell. After transfer through atmosphere and insertion into the UHV chamber, clean, well ordered surfaces with different reconstructions were prepared by thermal annealing. The atomic structure and the morphology of the surfaces were found to depend sensitively on the annealing procedure. For several differently reconstructed surfaces STM images with atomic resolution were obtained similar to those recently published for in-situ-MBE investigated surfaces. These results demonstrate that the As-decapping technique is, in fact, a versatile tool for preparing well-defined GaAs(100) surfaces.