Ga Beam Research Articles

First-principles studies of kinetics in epitaxial growth of III-V semiconductors

We demonstrate how first-principles calculations using density-functional theory (DFT) can be applied to gain insight into the molecular processes that rule the physics of materials processing. Specifically, we study the molecular beam epitaxy (MBE) of arsenic compound semiconductors. For homoepitaxy of GaAs on GaAs(001), a growth model is presented that builds on results of DFT calculations for molecular processes on the beta2-reconstructed GaAs(001) surface, including adsorption, desorption, surface diffusion and nucleation. Kinetic Monte Carlo simulations on the basis of the calculated energetics enable us to model MBE growth of GaAs from beams of Ga and As_2 in atomistic detail. The simulations show that island nucleation is controlled by the reaction of As_2 molecules with Ga adatoms on the surface. The analysis reveals that the scaling laws of standard nucleation theory for the island density as a function of growth temperature are not applicable to GaAs epitaxy. We also discuss heteroepitaxy of InAs on GaAs(001), and report first-principles DFT calculations for In diffusion on the strained GaAs substrate. In particular we address the effect of heteroepitaxial strain on the growth kinetics of coherently strained InAs islands. The strain field around an island is found to cause a slowing-down of material transport from the substrate towards the island and thus helps to achieve more homogeneous island sizes.

Measurement of the hyperfine structure of the 4d^2D_3/2, 5/2 levels and isotope shifts of the 4p^2P_3/2 → 4d^2D_3/2 and 4p^2P_3/2 → 4d^2D_5/2 transitions in gallium 69 and 71

The hyperfine structure of the 4d2D3/2,5/2 levels of 69,71Ga is determined. The 4p2P3/2→4d2D3/2 (294.50-nm) and 4p2P3/2→4d2D5/2 (294.45-nm) transitions are studied by laser-induced fluorescence in an atomic Ga beam. The hyperfine A constant measured for the 4d2D5/2 level is 77.3±0.9 MHz for 69Ga and 97.9±0.7 MHz for 71Ga (3σ errors). The A constant measured for the 4d2D3/2 level is -36.3±2.2 MHz for 69Ga and -46.2±3.8 MHz for 71Ga. These measurements correct sign errors in the previous determination of these constants. For 69Ga the hyperfine B constants measured for the 4d2D5/2 and the 4d2D3/2 levels are 5.3±4.1 MHz and 4.6±4.2 MHz, respectively. The isotope shift is determined to be 114±8 MHz for the 4p2P3/2→4d2D3/2 transition and 115±7 MHz for the 4p2P3/2→4d2D5/2 transition. The lines of 71Ga are shifted to the blue. This is in agreement with previous measurement.

Nucleation and growth mechanisms during MBE of III–V compounds

In this paper we discuss our recent results on the homoepitaxial growth of GaAs from beams of Ga and As 2( 4) and of InAs on GaAs from In and As 2( 4) beams. Experimental measurements are based on in-situ reflection high energy electron diffraction (RHEED) and scanning tunnelling microscopy (STM), while their analysis relies on kinetic Monte Carlo (KMC) simulations and the application of rate equations. We emphasise the comparative behaviour on all three low-index substrate orientations and the importance of surface reconstruction. Wherever possible we use an atomistic approach and in the homoepitaxial studies we are principally concerned with nucleation effects occurring at the sub-monolayer stage, including site-specific adatom incorporation and arsenic molecule dissociation pathways, especially the essential requirement of a mobile precursor state. Some results for vicinal plane growth are also presented. In the case of the InAs system, we have investigated strain relaxation processes using STM images, while specifically for growth on GaAs(001)-2×4 and c(4×4) surfaces we have studied the formation of so-called self assembled quantum dots (SADs) in relation to the classical Stranski–Krastanov mechanism and the associated strain effects. Although it is frequently stated that this accounts fully for dot formation, we have found many anomalies and believe the process, and indeed the final structures, to be much more complex than generally believed.

In situ studies of III–V semiconductor film growth by molecular beam epitaxy

In situ studies of the fundamental processes involved in film growth by molecular beam epitaxy (MBE) have reached the point where atomistic models can be developed and compared directly with theoretical predictions. In this paper, we discuss two material systems to illustrate aspects of growth chemistry, adatom incorporation and strain relaxation. We deal first with the homoepitaxial growth of GaAs(0 0 1) from beams of Ga, As 2 and As 4, starting from an historical perspective and then relating the very early work to present day scanning tunnelling microscopy (STM) studies, made in conjunction with kinetic Monte Carlo (KMC) simulations which take account of the actual surface structure. We then demonstrate the uniqueness of (0 0 1) growth in comparison with growth on other low-index orientations. The InAs–GaAs system provides the archetypal illustration of the effects of misfit and we indicate how in situ studies with reflection high-energy electron diffraction (RHEED) and STM have shown that again growth on (0 0 1) oriented substrates is completely different from that observed on (1 1 0) or (1 1 1) orientations in terms of growth mode and strain relaxation behaviour. The formation of quantum dots is not simply strain-related.

As4 incorporation kinetics in GaAs (001) molecular-beam epitaxy

A kinetic model of epitaxial growth on a Ga-stabilized GaAs (001) surface from As4 and Ga beams is proposed. Elementary surface processes are studied: adsorption-desorption of As4, bimolecular reaction of As4*, and incorporation of As 2 chem in lattice sites. The model correctly describes the experimental results for the growth rate at low and high As4 pressures. The role of As4 desorption from the surface in the epitaxial growth of GaAs crystals is analyzed.

Nanostructured Arrays Formed by Finely Focused Ion Beams

AbstractAmorphous, polycrystalline, and single crystal nanometer dimension particles can be formed in a variety of substrates by ion implantation and subsequent annealing. Such composite colloidal materials exhibit unique optical properties that could be useful in optical devices, switches, and waveguides. However colloids formed by blanket implantation are not uniform in size due to the nonuniform density of the implant, resulting in diminution of the size dependent optical properties. The object of the present work is to form more uniform size particles arranged in a 2-dimensional lattice by using a finely focused ion beam to implant identical ion doses only into nanometer size regions located at each point of a rectangular lattice. Initial work is being done with a 30 keV Ga beam implanted into Si. Results of particle formation as a function of implant conditions as analyzed by Rutherford backscattering, x-ray analysis, atomic force microscopy, and both scanning and transmission electron microscopy will be presented and discussed.

GaSb-Growth Study by Realtime Crystal-Growth Analysis System Using Synchrotron Radiation Photoelectron Spectroscopy

A system has been developed for realtime crystal-growth analysis by photoelectron spectroscopy, using vacuum ultraviolet (VUV) light from synchrotron radiation. This system consists of a VUV synchrotron radiation beamline and growth/analysis apparatus in which photoelectron spectroscopy is performed. A “non-scanning” photoelectron spectroscopy measurement method that uses only electron energy dispersion on a multi-channel detector achieves sub-second resolution. This system was used to monitor the GaSb-growth process by alternately supplying Ga and Sb molecular beams; the changes in the chemical state were analyzed with sub-second resolution. The Ga supply resulted in a Ga-full surface; Ga droplets formed on this surface, in which the Sb atoms remained lying on the Ga layer. The Sb supply did not change the ratio of Ga and Sb atoms at the surface from the initial value. These results show that, exposure to an Sb flux causes a crystal surface to once again become an Sb-rich surface although Ga droplets were previously formed during exposure to a Ga flux a GaSb surface was thus successfully grown.

Recombination velocity at oxide–GaAs interfaces fabricated by in situ molecular beam epitaxy

The recombination velocity at oxide–GaAs interfaces fabricated by in situ multiple-chamber molecular beam epitaxy has been investigated. Ga2O3, Al2O3, SiO2, and MgO films have been deposited on clean, atomically ordered n- and p-type (100) GaAs surfaces using molecular beams of Ga–, Al–, Si–, and Mg oxide, respectively. Based on the internal quantum efficiency measured for incident light power densities 1≤P0≤104 W/cm2, the interface recombination velocity S has been inferred using a self-consistent numerical heterostructure device model. While Al2O3–, SiO2–, and MgO–GaAs structures are characterized by an interface recombination velocity which is comparable to that of a bare GaAs surface (≂ 107 cm/s), S observed at Ga2O3–GaAs interfaces is as low as 4000–5000 cm/s. The excellent Ga2O3–GaAs interface recombination velocity is consistent with the previously reported low interface state density in the mid 1010 cm−2 eV−1 range.

Characterization of Y2SiO5:Ce, YAG:Tb AND YAG:Eu RGB Phosphor Triplet for Field Emission Display Application

ABSTRACTThe spectral response and outgassing characteristics of the three new, low-voltage phosphors combustion synthesized and electrophoretically deposited for application in field-emission flatpanel displays, are presented. The phosphors, forming a candidate Red-Blue-Green (RGB) triplet are YAG:Eu, YAG:Tb and Y2SiO5:Ce. These cathodoluminescent materials are tested with electron-beam excitation at currents up to 50 x03BC;A within the 200-2000V (eg. "low-voltage") and 3000-8000V (eg. "medium voltage") ranges. The spectral coordinates, as compared with industrially-manufactured P22 phosphors in low voltage operation, are reasonable; however, there is considerable difference in the green coordinates, and the red and green materials show significant satellite intensities. Phosphor outgassing, as a function of time, is measured by a residual gas analyzer at fixed 50 gA beam current in the low-voltage range. We find that after two hours of excitation, levels of outgassed CO, CO2 and H2 stabilize to low values.

Periodic supply epitaxy: a new approach for the selective area growth of GaAs by molecular beam epitaxy

Selective growth of GaAs on a GaAs substrate with a SiO 2 patterned mask has been carried out by molecular beam epitaxy at 630°C, using molecular beams of Ga and As 4 as sources and employing a periodic supply epitaxy technique for the first time. The GaAs crystalline nuclei deposited on the mask surface are decomposed and the atoms adsorbed on the mask surface are evaporated or allowed to migrate until they are captured by the single crystalline epitaxial layer. A wide depleted zone is then formed and the polycrystals of GaAs are located beyond this area. The surface migration is shown to influence drastically the thickness of the grown layer. The epitaxial layer obtained is flat and smooth even for the (001) substrates, since the lateral flux prevents the decomposition of GaAs on the free surface of the epitaxial layer. The growth technique is discussed in details, together with an analysis of the important parameters for the selective growth by periodic supply epitaxy.

Epitaxial growth of GaN on Si (100)/sapphire (0001) using RF plasma-assisted ionized source beam epitaxy

A new approach to epitaxial GaN growth utilizing radiofrequency plasma-assisted ionized source beam epitaxy (PAISBE) is described. To facilitate the reaction between Ga and atomic nitrogen, the Ga beam was partially ionized and accelerated. The reactive atomic nitrogen flux was obtained from an RF discharge nitrogen plasma with controllable power and frequency. A grid was placed at the exit end of the plasma tube to prevent Ninf2up+ from hitting the substrate. The growth parameters were chosen to systematically investigate the effects of Ga-beam ionization and grid bias. The crystal quality of the PAISBE-grown GaN was analysed by RHEED and x-ray diffraction. The FWHM of a 0.4-μm-thick GaN epilayer grown with ionized Ga beam and RF nitrogen plasma with 140W power was measured to be 25 min at (0002) diffraction peak. This and other preliminary data from the present study indicate that PAISBE is a promising technique for GaN growth.

Selective area growth of GaAs using a Ga beam with a step-function lateral intensity profile

Selective area growth of GaAs has been carried out in order to investigate the surface diffusion of Ga atoms using molecular beam epitaxy (MBE) with the aid of a Ga beam with a lateral step-function intensity profile. This step-function profile was obtained using a closely fitted GaAs shadow mask. When the mask edge was parallel to [01 1 ], a (311)A facet was typically observed near the edge of the Ga beam, while in the case of the mask edge parallel to [011], a (111)B facet was formed. MBE growth simulation based on the diffusion model was carried out in order to understand the mechanism of this selective area growth. The calculated results were in good agreement with the experimental results, and the diffusion lengths of Ga atoms were determined to be 0.10 μm along [011] direction on the (100) GaAs surface, 0.37 μm along [233] direction on the (311)A GaAs surface and 0.17 μm along [21 1 ] direction on the (111)B GaAs surface during MBE growth. These diffusion lengths seem to be smaller than those previously observed, which is probably due to a large V III ratio in the region of the substrate close to the mask edge.

Selective area growth of GaAs using a Ga beam with a step-function lateral intensity profile

Selective area growth of GaAs using a Ga beam with a step-function lateral intensity profile

Precise thickness measurement within a few monolayers by X-ray diffraction from InGaAs/GaAs strained-layer superlattices

We propose a technique to measure the thickness of a GaAs layer with a precision of a few monolayers (MLs) by high-resolution X-ray diffraction (HRXRD) from InGaAs/GaAs strained-layer superlattices (SLSs) on GaAs substrates. Using this technique to monitor the GaAs growth rate, we have successfully controlled the Ga beam flux within ± 1% in molecular beam epitaxy (MBE) growth for continuous 40 runs during four days by increasing the Ga cell temperature to compensate the decrease of the Ga beam flux caused by the consumption of the Ga source. Precise thickness measurements are also demonstrated in the growth on InP substrates by using InAlGaAs/InGaAs SLSs and InAlGaAs/InAlAs SLSs.

Precise thickness measurement within a few monolayers by X-ray diffraction from InGaAs/GaAs strained-layer superlattices

We propose a technique to measure the thickness of a GaAs layer with a precision of a few monolayers (MLs) by high-resolution X-ray diffraction (HRXRD) from InGaAs/GaAs strained-layer superlattices (SLSs) on GaAs substrates. Using this technique to monitor the GaAs growth rate, we have successfully controlled the Ga beam flux within ± 1% in molecular beam epitaxy (MBE) growth for continuous 40 runs during four days by increasing the Ga cell temperature to compensate the decrease of the Ga beam flux caused by the consumption of the Ga source. Precise thickness measurements are also demonstrated in the growth on InP substrates by using InAlGaAs/InGaAs SLSs and InAlGaAs/InAlAs SLSs.

Growth and characterization of GaN on sapphire (0001) using plasma-assisted ionized source beam epitaxy

A new approach to expitaxial GaN growth using the plasma-assisted ionized source beam epitaxy (PAISBE) is described. To facilitate the growth of GaN, an accelerated partially ionized Ga source beam was used along with an atomic nitrogen beam from an rf-discharge nitrogen plasma. To control the amount of N+2 ions leaving the discharge tube (thus participating in the GaN growth), a grid was installed at the exit end of the plasma tube. The growth parameters under control were the power and frequency of the rf discharge, the energy and fraction of ionization of the Ga beam, the Ga and N fluxes, and the substrate temperature. The crystal quality and surface morphology of the PAISBE-grown GaN films on sapphire (0001) were analyzed using reflection high-energy electron diffraction, x-ray diffraction, and scanning electron microscopy. The best films were obtained with a partially ionized Ga beam at an rf power of 140 W, which exhibited a full width at half maximum of 25 min at the (0002) diffraction peak for a 0.4-μm-thick film. The buffer layer was a GaN film grown at 400 °C. Results illustrating the effects of various control parameters and, in particular, those demonstrating the advantages of PAISBE are presented.

Growth of GaSe on As-passivated Si(111) substrates

We have studied the growth of GaSe on As-passivated Si(111) substrates by molecular beam epitaxy (MBE) for a wide range of substrate temperatures (375–500°C) and Se Ga beam equivalent pressure (BEP) ratios (∼ 30 to > 200). We see three distinct regions when our data are plotted as a phase diagram in Se Ga BEP and growth temperature. For high Se Ga BEP ratios and high substrate temperatures, continuous layered structure GaSe films are formed on the substrate. Reflection high energy electron diffraction (RHEED) indicates that these films are flat during growth, but after growth is stopped, the surface goes through a transformation and becomes rough on a microscopic scale. At low Se Ga BEP ratios and high substrate temperatures, small Ga droplets form on the substrate surface, and no GaSe forms on the substrate. GaSe films with Ga droplets on the surface are formed when either the substrate temperatures or the Se Ga BEP ratio is low. In this paper we will discuss possible mechanisms for growth (or the inhibition of growth) in the three regions of our phase diagram and compare our results with those already published for MBE growth of GaSe on Si(111)7 × 7 and GaAs(111).

In-situ observation of GaAs selective epitaxy on GaAs (111)B substrates

Selective area epitaxy has been carried out by μ-RHEED (reflection high energy electron diffraction)/SEM (scanning electron microscopy) MBE (molecular beam epitaxy) on (111)B patterned substrates masked by SiO2 and conditions for selective growth have been determined as a function of substrate temperature and Ga beam intensity. Below the critical Ga flux, a smooth epitaxial layer grows without any deposition of polycrystals on the masked area. However, in the high temperature region (over 660°C), the epilayer grown in the window is rather rough, due to the high re-evaporation rate of Ga atoms. In the high Ga flux region, polycrystalline deposition on the mask occurs and the coverage of the masked area by poly-GaAs nuclei decreases from the region close to the window to the center of the mask. It has been found that there is a strong lateral surface flow of Ga atoms from the window to the mask region.

Real-time control of molecular beam epitaxy by optical-based flux monitoring

We have developed a real-time molecular beam epitaxy control system based on Al and Ga atomic beam resonant absorption of hollow cathode lamp emission. By continuously monitoring the absorption of Al and Ga beams during growth, this system accurately determines instantaneous growth rates and then integrates these over time to determine layer thickness. This information is used in real time for effusion cell shutter (and hence layer thickness) control. The accuracy and flexibility of this system is demonstrated here by growing AlAs/GaAs distributed Bragg reflectors with consistent layer thicknesses even though effusion cell temperatures were intentionally varied. In each instance the system automatically detected and compensated for the different growth rates, resulting in DBRs with center wavelengths controlled to within 0.3% of the target wavelength.

Characterization of AlAs/GaAs ultrathin-layer superlattices grown by migration enhanced epitaxy at low temperatures

This paper describes the effect of As coverage during migration enhanced epitaxy (MEE) on the structural and optical quality of (AlAs) 10(GaAs) 10 ultrathin-layer superlattices (ULSs) grown at low temperatures. The As coverage during MEE was in-situ determined by analyzing the oscillation waveform of reflection high energy electron diffraction intensity. Experimental results on Raman scattering and photoluminescence spectroscopy have revealed that high quality ULSs can be grown under a slightly As-excess condition although the optical quality deteriorates with increasing the excess of As adsorption. When the ULSs are grown completely free from excess As adsorption, on the other hand, the width of the ULS structure tends to be high with a considerable fluctuation. The increased width has been attributed to the presence of column III droplets on the surface after the irradiation of Ga and Al beams.