MOMBE growth characteristics of antimonide compounds

This paper describes the MOMBE (metalorganic molecular beam epitaxy) growth characteristics of antimonide compounds using TMIn (trimethylindium), TEGa (triethylgallium) and TIBAl (triisobutylaluminium) as group III sources, and As₄, Sb₄, TEAs (triethylarsine) and TESb (triethylstibine) as group V sources. Large differences in the growth characteristics of GaAs and GaSb MOMBE are observed. These are explained, using a theoretical consideration of the growth mechanism, by the difference in the effective surface coverage of excess As and Sb atoms during the growth. The use of TEAs and TESb instead of As₄ and Sb₄ alters the growth rate variation of both GaAs and GaSb with substrate temperature (T_sub), which results from the interaction of alkyl Ga species with the alkyl radicals coming from the thermally cracked TEAs and TESb. The alkyl exchange reaction process is observed in the growth of AlGaSb using TIBAl and TEGa, where the incorporation rate of Al is suppressed by the coexistence of TEGa on the growth surface, in the low T_sub region. This is caused by the formation of an ethyl-Al bond which is stronger than the isobutyl-Al bond. The composition and the growth rate variations of InGaSb with T_sub are similar to those of InGaAs, which are closely related to the MOMBE growth process and are quite different from those of MBE (molecular beam epitaxy) and MOVPE (metalorganic vapor phase epitaxy) growth. In the MOMBE growth of InAsSb and GaAsSb using TEAs and TESb, the composition variation with T_sub is weaker than that of MBE. This is a superior point of MOMBE growth for the composition control. The electrical and optical properties of MOMBE grown films as well as the quantum well structures are also described.     

Surface diffusion lenght during MOMBE and CBE growth measured by μ-RHEED

The growth rates of layers grown on a mesa-etched (001) GaAs surface were measured by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED) from the period of the RHEED intensity oscillation in real time. The diffusion lenght of the surface adatoms of column III elements was determined from the gradient of the variation of the growth rates in the cases of MBE, MOMBE using trimethylgallium (TMGa) and CBE using TMGa or triethylgallium (TEGa) and arsine (AsH₃). The obtained values of the diffusion lengths were of the order of a micrometer in every case of the source-material combination. In the case of metalorganic materials as Ga source, it was found that the diffusion length was larger than that of Ga atom from metal Ga source. Since the substrate temperature of the present experiment is high enough to decompose TMGa and TEGa on the surface, Ga adatoms are considered to be responsible to the surface diffusion. Therefore, it is considered that the derivatives of the metalorganic molecules such as methyl radicals affect the diffusion of Ga adatoms.     

Characteristics of GaSb growth using various gallium and antimony precursors

We report the epitaxial growth of GaSb using trimethylgallium (TMGa) or triethylgallium (TEGa) with trimethylantimony (TMSb), triethylantimony (TESb), or trisdimethylaminoantimony (TDMASb) in a low-pressure vertical rotating-disk reactor. Growth is kinetically limited for TMGa in the temperature range 560 to 640°C, and is mass-transport limited for TEGa in the range 525 to 640°C. A minimum V/III ratio is necessary to obtain stoichiometric GaSb, and is dependent on the pyrolysis temperature of Ga and Sb precursors. Featureless morphology is achieved for layers grown with TMGa or TEGa and TMSb, while surface defects are observed for layers grown with TEGa and TESb or TDMASb. These observations are consistent with Fourier transform infrared measurements, which indicate interactions between TEGa and TESb or TDMASb. All nominally undoped layers are p-type, with overall superior properties being obtained for layers grown with TEGa and TMSb. However, growth conditions that yield layers with the best electrical properties do not necessarily correspond to the same conditions for highest optical quality.     

Simultaneous reflection high-energy electron diffraction oscillations and mass spectroscopy investigations during molecular beam epitaxy growth of (001) GaAs - smooth surfaces or stoichiometric films?

Film composition and surface morphology of molecular beam epitaxy (MBE)-grown GaAs(001) surfaces were investigated in situ as a function of flux ratio J_Ga/J_As4. The flux of As₄ molecules desorbing from the sample surface was measured with a quadrupole mass spectrometer (QMS) simultaneously with the observation of reflection high energy electron diffraction (RHEED) intensity oscillations. The incorporation ratio, given by the number of incorporated Ga atoms per As atom, was calculated independent of both the QMS data and the decreasing growth rate for growth conditions with As deficiency, as obtained from the RHEED oscillation frequency. Stoichiometric growth was found up to a flux ratio J_Ga/J_As4 ≈ 0.9. At flux ratios of 1.1 to 1.2, a minimum of the damping of the RHEED oscillation amplitude indicates a very smooth growth front profile, but Ga excess appears to be incorporated mainly in As sites without disturbing the crystal lattice. This assumption was confirmed by photoluminescence (PL) spectroscopy of films grown at a flux ratio J_Ga/J_As4 of 1.2. An additional PL peak was observed, which indicates the incorporation of Ga atoms on As sites.     

Heavily carbon-doped p-type (In)GaAs grown by gas-source molecular beam epitaxy using diiodomethane

Heavily carbon-doped p-type In_xGa_1−xAs (0≤x<0.49) was successfully grown by gas-source molecular beam epitaxy using diiodomethane (CH₂I₂), triethylindium (TEIn), triethylgallium (TEGa) and AsH₃. Hole concentrations as high as 2.1×10²⁰ cm⁻³ were achieved in GaAs at an electrical activation efficiency of 100%. For In_xGa_1−xAs, both the hole and the atomic carbon concentrations gradually decreased as the InAs mole fraction, x, increased from 0.41 to 0.49. Hole concentrations of 5.1×10¹⁸ and 1.5×10¹⁹ cm⁻³ for x = 0.49 and x = 0.41, respectively, were obtained by a preliminary experiment. After post-growth annealing (500°C, 5 min under As₄ pressure), the hole concentration increased to 6.2×10¹⁸ cm⁻³ for x = 0.49, probably due to the activation of hydrogen-passivated carbon accepters.     

A novel technique to reduce the concentration of carbon in LEC gallium arsenide

An in-situ method has been developed to reduce the concentration of carbon in the liquid encapsulated growth of gallium arsenide. Sublimated As₂O₃ was bubbled through a GaAs melt to remove carbon via the production of CO₂. When a melt of known high starting carbon concentration ( 30 × 10¹⁴ atoms/cm³) was bubbled with As₂O₃, the subsequent crystal contained 4 × 10 ¹⁴ carbon atoms/cm³. This low carbon crystal was subsequently carbon doped and regrown, demonstrating the ability to return As₂O₃ treated GaAs to desired carbon concentrations and electrical properties.     

Heavily carbon-doped p-type InGaAs by MOMBE

Carbon-doped In_xGa_1−xAs layers (x=0−0.96) were grown by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMG), solid arsenic (As₄) and solid indium (In) as sources of Ga, As and In, respectively. The carrier concentration is strongly affected by growth temperature and indium beam flux. Heavy p-type doping is obtained for smaller In compositions. The hole concentration decreases with the indium composition from 0 to 0.8, and then the conductivity type changes from p to n at x=0.8. Hole concentrations of 1.0×10¹⁹ and 1.2×10¹⁸ cm^-3 are obtained for x=0.3 and 0.54, respectively. These values are significantly higher than those reported on carbon-doped In_xGa_1−xAs by MBE. Preliminary results on carbon-doped GaAs/In_xGa_1−xAs strained layer superlattices are also discussed.     

Heavily carbon-doped GaAs grown on various oriented GaAs substrates by MOMBE

Heavily carbon-doped GaAs epitaxial layers have been grown simultaneously on (100), (111)A, (111)B, (411)A, (411)B and (711)A semi-insulating (SI) GaAs substrates by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMG) and elemental As (As₄). The hole concentration and surface flatness strongly depend on the substrate orientation. The highest carbon incorporation was observed for the layers grown on a (411)A substrate with a hole concentration of 1.0 × 10²¹ cm^{− 3} and a lattice mismatch of Δd/d = −0.48%. Atomic force microscope (AFM) images reveal that the epilayers grown on (411)A substrates exhibit extremely flat surfaces, although these layers contain the highest carbon concentration.     

Surface reconstructions and growth mode transitions of AlAs(100)

The surface reconstructions of AlAs(100) layers grown by molecular beam epitaxy (MBE) on GaAs(100) were mapped as a function of substrate temperature and arsenic flux. Three main reconstructions were observed - a c(4×4) at lower temperatures and higher arsenic fluxes, a (2×4) at middle temperatures, and a (3×2) at higher temperatures and lower arsenic fluxes. Growth of AlAs on AlAs(100) is layer-by-layer for the high temperature and low temperature reconstructions. In the mid-temperature region, AlAs grows rough on (2×4) reconstructed AlAs(100) as indicated by rapidly damped reflection high-energy electron diffraction (RHEED) intensity oscillations and the appearance of three-dimensional (3D) features. The addition of fractional layers of Ga enhances the smooth growth of AlAs. A metastable (5×2) reconstruction was observed when a fraction of a layer of Ga was present on the surface. The results indicate that Ga segregates during the growth of AlAs on GaAs(100) at temperatures at least as low as 500°C, and that annealing at temperatures above 700°C removes most of the Ga from the surface.     

Influence of Be on N composition in Be-doped InGaAsN grown by RF plasma-assisted molecular beam epitaxy

Undoped and Be-doped InGaAsN layers were grown on GaAs substrates under the same growth conditions by radio frequency plasma-assisted molecular beam epitaxy. Increased tensile strain (Δa/a=3×10⁻³) was observed for Be-doped InGaAsN layers, compared to undoped InGaAsN layers. The strain is shown to originate from the increase in N composition related to Be incorporation, rather than solely from Be atoms substituting Ga atom sites (Be_Ga). A possible reason is the high Be–N bond strength, which inhibits the loss of N from the growth surface during epitaxial growth, thereby increasing the N composition in the Be-doped InGaAsN layer.     

Atomic-scale controlled incorporation of ultrahigh-density Si doping sheets in GaAs

Delta-doped GaAs:Si with doping densities up to 4×10¹⁴ cm⁻² has been grown by molecular beam epitaxy (MBE) at a substrate temperature of 590°C. To promote an ordered incorporation and thus avoid clustering of Si atoms, vicinal GaAs(001) surfaces 2° misoriented towards (111)Ga were used and Si was supplied in pulses. As evidenced by real-time reflection high-energy electron diffraction (RHEED) measurements an ordered incorporation of Si atoms on Ga sites along the step edges takes place. Although the ordered (3×2) structure degrades at high coverages, unusual high sheet carrier concentrations are obtained by pulsed delta-doping for doping concentrations >10¹³ cm⁻², as revealed by Hall measurements. The surface conditions during GaAs overgrowth have a strong influence on the free electron concentration, too. Raman scattering by local vibration modes and secondary ion mass spectrometry (SIMS) measurements are used to show that this is related to segregation effects as well as to a modification of the site occupancy.     

Confinement of δ Be at the one monolayer level in GaAs

We investigate growth dependences to the planar confinement of Be near the 1 monolayer, ML, level when δ doped in GaAs. We examine concentration dependences on the rates of segregation and diffusion, as well as the effect of As₂, As₂ and AsH₃ overpressure on Be confinement during growth. Evidence from both chemical profiling and surface electron diffraction point to a Be surface dimerization process that drives dopant segregation.     

Top-seeded solution growth of LiB3O5

We report on the top-seeded solution growth of LiB₃O₅ from an excess B₂O₃ solution. Parameters investigated include the Li₂O/B₂O₃ ratio, rotation rate, pulling rate, cooling rate, and seed direction. With careful control of the above parameters, we have grown clear crystals of 25 × 30 × 20 mm in size. Selected nonlinear optical properties of these crystals are reported. Observations concerning the occurrence of unstable growth (inclusions, hopper growth) are discussed, and methods to eliminate the unstable growth are suggested.     

Optical third-harmonic generation from some high-index glasses

Optical third-harmonic generation from some high-index glasses was investigated. The highest χ⁽³⁾ was obtained from As₂S₃ glass 2.2 × 10⁻¹² esu/ This value was 100 times higher than that of pure SiO₂ glass and comparable with that of the polymer with monomer-doping, which are known as organic materials with quite high χ⁽³⁾. From the relationship between χ⁽³⁾ and composition, sulfide glasses were found to have higher χ⁽³⁾ than oxide or semiconductor-doped oxide glasses with similar refractive indices.     

The growth and physics of ultra-high-mobility two-dimensional hole gas on (311)A GaAs surface

We report on the molecular beam epitaxy growth of modulation-doped GaAs-(Ga,Al)As heterostructures on the (311)A GaAs surface using silicon as the acceptor. Two-dimensional hole gases (2DHGs) with low-temperature hole mobility exceeding 1.2×10⁶ cm² V⁻¹ s⁻¹ with carrier concentrations as low as 0.8×10¹¹ cm⁻² have been obtained. This hole mobility is the highest ever observed at such low densities by any growth technique. We also report the first observation of persistent photoconductivity in a 2DHG. An analysis of the number density and temperature dependence of the mobility leads us to conclude that the mobility is limited by phonon scattering above 4 K and interface scattering at lower temperatures.     

Reflection high-energy electron diffraction intensity oscillations during growth of (Al,Ga)As on GaAs(111)A

We have made a reflection high-energy diffraction (RHEED) intensity oscillation study of the growth of (Al,Ga)As on GaAs and (Al,Ga)As (111)A surfaces. The RHEED intensity oscillations during growth of (Al,Ga)As are dependent on both growth temperature and As₄:Ga flux ratio. In addition, the oscillation period decreases as the Al fraction in the (Al,Ga)As is increased. Changes in the oscillation period during growth of the first few monolayers of both GaAs on (Al,Ga)As and (Al,Ga)As on GaAs may indicate the intermixing of Al and Ga near the heterointerface.     

As2S3 and AgI thin layers as ion sensitive membranes

The possibilities of using thin layers of As₂S₃ and AgI as ion sensitive membranes for ion selective field-effect transistors (ISFETs) are investigated. The thin films have been prepared by vacuum deposition on static and rotating substrates. The As₂S₃ layers were additionally doped with silver. The influence of the type of substrates and preparation conditions on electrochemical properties of the layers was studied. Electrochemical measurements revealed a reasonable sensitivity of chalcogenide and halide layers to silver and iodide ions, respectively. The near Nernstian behavior of sensitivity of As₂S₃ layers to Ag⁺ ions and of AgI layers to I⁻ ions is observed. The results obtained are promising for the development of ISFETs.     

A glass-forming system with compound-forming tendency: As4S6---P4S10

Glasses in the quasi-binary system (As₄S₆)_x(P₄S₁₀)_1−x x = 0.1, …, 1.0, are produced and studied by thermal analysis, X-ray, and Raman spectroscopy. The phase diagram of the system and the critical cooling rate for glass formation both have their maximum at x = 0.5, corresponding to the compound As₂P₂S₈; the X-ray structure of recrystallized samples can be described as a sum of the As₂P₂S₈ (x = 0.5)- and the P₄S₁₀-structure (As₄S₆ not visible); Raman spectra of the glasses are again sums of As₂P₂S₈- and As₄S₆/P₄S₁₀-spectra. All these observations support the assumption that a stable building block corresponding to the 1:1 compound As₂P₂S₈ and surplus As₄S₆ (or P₄₁₀) are the essential elements of the structure in the glasses.     

Growth mechanisms in excimer laser photolytic deposition of gallium nitride at 500°C

The mechanisms of controlling laser-induced chemical vapour deposition of GaN at substrate temperatures between 350 and 650°C have been investigated. Ultraviolet (193 nm) photolytic decomposition of trimethylgallium (TMGa) and ammonia (NH₃) precursors was examined in this range. Laser-induced fluorescence studies support the view that the dissociated intermediate fragments GaCH₃ and NH are the reacting species in GaN film formation, irrespective of substrate temperature. It was found that two crystal phases coexist in films grown at substrate temperatures below 500°C, wurtzite crystal structure with (0002) orientation forms at substrate temperatures above 500°C. The growth rate increases with both NH₃/TMGa ratio, and TMGa flow rate, while the temperature dependence shows a thermal activation energy of 0.2 eV which is smaller by a factor of five than that of films prepared by conventional thermal CVD. The large NH₃/TMGa ratios needed to achieve stoichiometry are interpreted in terms of the two-photon dissociation cross section of NH₃.     

Growth by self-flux method and post-annealing effect of Bi2Sr2Ca1Cu2Ox single crystals

Post-annealing effects on superconducting characteristics have been studied in Bi₂Sr₂Ca₁Cu₂O_x single crystals grown by a conventional flux method. Also, favorable growth conditions and the effect of the pre-sintering process on the starting materials for flux growth have been examined. The best superconducting behavior is obtained in post-annealed crystals grown from pre-sintered powder materials. The critical current density J_c estimated from magnetization hysteresis in annealed crystals grown with pre-sintered materials is roughly 8×10⁵ A/cm² (Hc) and 5×10⁴ A/cm² (Hc) at 4.2 K at zero field.