Beam Equivalent Pressure Ratio Research Articles

Microstructure Characterization of Cu-Alloyed GaN Grown by Plasma Assisted Molecular Beam Epitaxy

Cu-alloyed GaN epilayers were prepared by plasma assisted molecular beam epitaxy with Cu-to-Ga beam equivalent pressure ratio of 1.2 to 4.8 %. Islands enriched with Cu were found on the GaN epitaxial layers grown in a Ga-rich environment. The islands are composed of a Cu9Ga4 intermetallic phase and GaN between which an orientation relationship was identified as [111]Cu9Ga4//[1 10]GaN and (10)Cu9Ga4//(0001)GaN. X-ray spectroscopy analyses indicated that the 1.2 % and 4.8 % samples contains 0.10 0.02 wt.%Cu and 0.04 0.03 wt.%Cu, respectively.

Growth and thermal conductivity analysis of polycrystalline GaAs on chemical vapor deposition diamond for use in thermal management of high-power semiconductor lasers

The authors demonstrate the growth of polycrystalline GaAs thin films on polycrystalline chemical vapor deposition (CVD) diamond by low-temperature molecular beam epitaxy. The low-temperature GaAs (LT-GaAs) layer is easily polished compared to the CVD diamond, and this process results in a reduction of rms surface roughness from >50 to <5 nm. This makes the LT-GaAs on diamond layer an ideal wafer-bonding interface for high-power semiconductor devices. The samples were grown at 0.2 μm/h with a substrate temperature of 250 °C and a 1:8 III/V beam equivalent pressure ratio. The samples were analyzed by x-ray powder diffraction, atomic force microscopy for surface roughness, and in situ reflective high-energy electron diffraction during molecular beam epitaxy growth. The authors also measure the thermal conductivity of the GaAs layer on CVD diamond using pump-probe time domain thermoreflectance.

Dependence of MBE-grown ZnSnAs2:Mn epitaxial film properties on Mn doping level

We have prepared Mn-doped ZnSnAs2 thin films with varying Mn doping content (2.1%, 2.7%, and 5.0%) using molecular beam epitaxy, by changing the Mn-to-Sn beam equivalent pressure ratio during growth. All the samples were grown on InP(001) substrates using the optimum substrate temperature Ts=300°C previously reported. As a reference, an un-doped ZnSnAs2 epitaxial film was also prepared. In-situ reflection high-energy electron diffraction observations revealed the transformation from streaky to spotty suggesting increasing surface roughening with increasing Mn-content. From high-resolution X-ray diffraction studies, the lattice constant was found to increase with increasing Mn-doping level. The 5% Mn-doped ZnSnAs2 epitaxial film exhibited a Curie temperature of ∼314 K, as revealed from measurement of the zero-field cooled temperature dependence of the remanent magnetization using a magnetic property measurement system superconducting quantum interference device magnetometer. A hysteretic M-H curve was also obtained even at 300 K. From the M-H curve measured at 5K, the magnetic moment was computed to be 1.1μB per Mn atom.

Effect of Growth Mode on Eu-Incorporation and Luminescence of Eu-Doped GaN Epitaxial Film Grown by Plasma-Assisted Molecular Beam Epitaxy

The growth mode of europium (Eu)-doped GaN epitaxial films grown on a GaN template by rf plasma-assisted molecular beam epitaxy (PAMBE) was investigated with different III/V ratios under a constant Eu beam equivalent pressure ratio [PEu/(PEu+PGa)]. The reflection high-energy electron diffraction (RHEED) patterns and atomic force microscopy (AFM) images revealed the transition of the growth mode from three-dimensional (3D) to step-flow/two-dimensional (2D) by increasing the III/V ratio. When the films were grown in the 3D growth mode, Eu concentrations estimated by Rutherford backscattering spectrometry/channeling (RBS/channeling) were almost constant, although the III/V ratios varied. However, when the growth mode was transferred from 3D to step-flow/2D, precipitates on the surface abruptly increased while the Eu concentration abruptly decreased, indicating the abrupt degradation of Eu-incorporation in the film. Luminescence sites of Eu3+ were sensitive to the III/V ratio, and Eu atoms have different luminescence sites in both growth modes. Furthermore, luminescence efficiency abruptly increased when the growth mode was transferred from 3D to step-flow/2D.

Optical and structural characterization of GaN thin films at different N to Ga flux ratios

GaN films were grown on Si(111) substrates under various beam equivalent pressure (BEP) ratios by plasma-assisted molecular beam epitaxy. The optical properties for the grown samples were studied over a wide spectral range from 200 to 3300 nm using the reflectance spectrum only. It was found that increasing the N/Ga BEP ratio from 17.9 to 46.1 increases the refractive index (n) from 2.05 to 2.38 at wavelength 630 nm (for example), while the optical energy gap (Eg) were found to be in the range between 3.325 to 3.35 eV with no specific trend. The structural properties for the grown films were studied through two types of rocking curve measurements; normal rocking curve (ω-scan) and triple axis rocking curve (ω/2θ-scan). It was found that with decreasing the N/Ga ratio from 46.1 to 17.9 the full width at half maximum decreases from 0.62° to 0.58° for ω-scan and from 0.022° to 0.021° for ω/2θ-scan. Thus, our results showed a clear correlation between the optical-structural parameters and the BEP ratios of N and Ga.

Optimization of VI/II pressure ratio in ZnTe growth on GaAs(0 0 1) by molecular beam epitaxy

ZnTe epilayers were grown on GaAs(0 0 1) substrates by molecular beam epitaxy (MBE) at different VI/II beam equivalent pressure (BEP) ratios ( R VI/II) in a wide range of 0.96–11 with constant Zn flux. Based on in situ reflection high-energy electron diffraction (RHEED) observation, two-dimensional (2D) growth mode can be formed by increasing the R VI/II to 2.8. The Te/Zn pressure ratios lower than 4.0 correspond to Zn-rich growth state, while the ratios over 6.4 correspond to Te-rich one. The Zn sticking coefficient at various VI/II ratios are derived by the growth rate measurement. The ZnTe epilayer grown at a R VI/II of 6.4 displays the narrowest full-width at half-maximum (FWHM) of double-crystal X-ray rocking curve (DCXRC) for (0 0 4) reflection. Atomic force microscopy (AFM) characterization shows that the grain size enlarges drastically with the R VI/II. The surface root-mean-square (RMS) roughness decreases firstly, attains a minimum of 1.14 nm at a R VI/II of 4.0 and then increases at higher ratios. It is suggested that the most suitable R VI/II be controlled between 4.0 and 6.4 in order to grow high-quality ZnTe epitaxial thin films.

Initial growth stage evaluation of high quality ZnSe films deposited on glass substrate

Initial growth stage evaluation of high quality ZnSe films deposited on a glass substrate was investigated. The self-limiting monolayer epitaxial process was used to pregrowth the buffer layer for a zinc selenide (ZnSe) film deposited. After alternating depositions for several cycles, the growth mode was changed to the molecular beam deposition mode under growth conditions. Films deposited at substrate temperatures of 250–350 °C and Se/Zn beam equivalent pressure ratios of 0.77–1.87 were investigated. The crystal structure and the preferred orientation of as-grown ZnSe films were examined using x-ray diffraction patterns. The optical properties of the ZnSe films were revealed by photoluminescence spectra. The characteristics of the ZnSe films with and without a buffer layer were compared and discussed in detail. Finally, the results demonstrate how the quality of ZnSe film can be improved on glass substrates for application to various devices.

Photoluminescence Studies of Mg-Doped AlxGa1-xAs Epitaxial Layers Grown by Molecular Beam Epitaxy

Magnesium (Mg)-doped AlxGa1-xAs epitaxial layers were grown by molecular beam epitaxy (MBE) at various substrate temperatures, Al mole fractions, and As/Ga beam equivalent pressure (BEP) ratios. The optical properties were investigated by photoluminescence (PL) spectroscopy. The PL emission peak intensity was increased and the emission peak was slightly blue shifted from 700.7 to 697.3 nm when the substrate temperature was increased. The full width at half maximum (FWHM) was decreased from 29.6 to 19 meV when the substrate temperature was increased. The other growth parameters, such as the As/Ga BEP ratio and the Al mole fraction, also affected the optical properties of the Mg-doped AlxGa1-xAs epilayers. The anomalous behavior of the blue- and red-shift of the PL peak energy was observed also to be temperature dependent.

Investigation on the properties of molecular beam deposited ZnSe films

Molecular beam deposition was used to deposit high quality zinc selenide (ZnSe) thin film on glass substrate at various substrate temperatures and Se/Zn beam equivalent pressure ratios. The best growth condition was identified by analyzing characteristics such as the full width at half-maximum and peak intensity of the (111) preferred orientation in X-ray diffraction patterns. The dependence of lattice constant, growth rate, film composition and optical property on a variety of growth parameters was discussed in detail to identify how the quality of ZnSe film can be improved.

MBE growth of Mn-doped ZnSnAs 2 thin films

ZnSnAs 2 thin film doped with ∼4% Mn was grown on (0 0 1) InP substrates by all elemental solid source molecular beam epitaxy using the previously determined optimum substrate temperature of 300 °C and Zn:Sn:As 4 beam equivalent pressure ratio of 24:1:52. In this work, we have employed slower growth rate by halving the beam equivalent pressures of Zn, Sn and As 4 we have used in our earlier reports, while maintaining the same beam equivalent pressure ratio. From the high-resolution X-ray diffraction (HRXRD) data, the computed lattice constant a is 5.867 Å, which is in very good agreement with the reported lattice constant value of the bulk ZnSnAs 2 chalcopyrite, suggesting that lowering the growth rate leads to a transition to a more chalcopyrite structure. The average composition ratio of Zn:Sn:As:Mn according to electron probe micro-analysis (EPMA) studies is 1:0.826:2.42:0.076. Hysteretic M– H curves were obtained at 5, 100, 300 and 320 K using a superconducting quantum interference device (SQUID) magnetometer. The Curie temperature was estimated to be ∼330 K from the zero-field cooled temperature dependence of magnetization. The possible origin of this above-room-temperature ferromagnetism is discussed.

Optimum growth of ZnSe film by molecular beam deposition

Zinc selenide (ZnSe) thin film has been grown originally on Coning 7059 glass substrate by molecular beam deposition (MBD). Optimum growth condition has been determined rapidly by comparing and analyzing the relative characteristics of X-ray diffraction spectrum. In this paper, films deposited at different substrate temperatures as a function of Zn/Se beam equivalent pressure (BEP) ratios are investigated. As-grown ZnSe films are polycrystalline and have a cubic (zinc-blende type) structure by the X-ray diffraction technique. The composition of ZnSe film was determined by Energy dispersive spectroscopic (EDS) analysis. Optical properties of ZnSe film were characterized by photoluminescence spectra. Structural parameters such as lattice constant, grain size, strain, dislocation density calculated are correlated with various growth conditions. The dependence of film properties on various substrate temperatures and Se/Zn BEP ratios has been discussed in detail. Finally, high quality of ZnSe film has been successfully obtained and ready for device application.

Low-Temperature Growth of InAs on Glass and Plastic Film Substrates by Molecular-Beam Deposition

InAs layers were deposited on glass substrates by molecular-beam deposition at substrate temperatures of 180–240 °C at As/In beam equivalent pressure (BEP) ratios of 3–30. X-ray diffraction (XRD) patterns indicated that the InAs layers are polycrystalline and are more preferentially textured in the (111) plane when deposited at higher temperatures and lower As/In BEP ratios. Room-temperature Hall effect measurement showed that the films deposited at an As/In BEP ratio of 30 exhibit n-type conduction with electron concentrations of approximately 3 ×1018 cm-3, being almost independent of substrate temperature. With decreasing As/In BEP ratio from 30 to 3 at a fixed substrate temperature of 240 °C, the electron concentration decreased to 1.5 ×1018 cm-3 and the electron mobility increased to 610 cm2/(V·s). Preliminary results of the deposition on plastic substrates showed an electron mobility of 460 cm2/(V·s) when the layer was deposited at 280 °C at an As/In BEP ratio of 30.

Molecular beam epitaxy of CdSe epilayers and quantum wells on ZnTe substrate

We have grown zinc-blende cadmium selenide (CdSe) epilayers on ZnTe-(001) substrate by molecular beam epitaxy (MBE). By controlling the substrate temperature and beam-equivalent pressure (BEP) ratio, of Se to Cd, we determined the most suitable growth condition based on reflection high-energy electron diffraction (RHEED) pattern. At a substrate temperature of 280°C and a BEP ratio of 3.6, the RHEED pattern showed a V-like feature, indicating a rough surface with facets. As the substrate temperature was increased to 360°C at the same BEP ratio, a V-like RHEED pattern moved to a clear streaky pattern. Moreover when the BEP ratio was increased to 4.8 at 360°C of substrate temperature, a clear (2×1) reconstruction of the CdSe layer was observed. A CdSe/CdMgSe single quantum well structure was also grown on ZnTe-(001) substrate by MBE. The RHEED pattern showed a clear (2×1) surface reconstruction during the growth. By photoluminescence measurement, a good optical property of the structure was obtained.

Growth Mechanism of GaAs Microdisk Structures by Area-Selective Epitaxy Using Migration-Enhanced Epitaxy

We have carried out area-selective epitaxy of GaAs microdisk structures on a SiO2-masked GaAs substrate by migration-enhanced epitaxy. We have successfully grown “damage-free” disks at 590 °C. It is found that, for the structures grown on (111)B substrates, the facets surrounding the disks are controlled by the V/III beam equivalent pressure ratio. This characteristic is caused by the reduced growth rate in the [111]B direction under high V/III ratios. Using an optimized growth condition, uniform hexagonal GaAs disks surrounded by six vertical {110} facets and a flat (111)B top are grown. The microdisks grown on (001) substrates, however, show no flat-top structures, regardless of the V/III ratio, probably due to the fact that the growth rate in the (001) direction is not sensitive to the V/III ratio. In the latter case, square-pyramidal structures surrounded by four inclined {011} facets are obtained.

Thallium incorporation during TlInAs growth by low-temperature MBE

Molecular-beam epitaxial growth of Tl x In 1− x As has been performed on InAs substrates at substrate temperatures between 150 and 250 °C. Optical microscopy revealed that Tl droplets decrease in size and in density with decreasing growth temperature, and vanish at lower temperatures than 175 °C. Secondary ion mass spectrometry (SIMS) revealed that Tl concentration shows almost flat depth profiles in the samples grown at 150–175 °C, but shows concave profiles in the samples grown at 200–225 °C. Electron probe micro analysis (EPMA) as well as SIMS showed that the Tl mole fraction x in the samples grown at 150–175 °C increases up to 2.7% almost in proportion with Tl flux. X-ray diffraction and cross-sectional transmission electron microscopy studies showed that monocrystalline growth is prevented if the beam equivalent pressure ratio of Tl/In is beyond 4%.

MBE growth of MgGeAs2:Mn on GaAs substrate

AbstractManganese doped II–IV–V2 chalcopyrites are promising candidates for room temperature ferromagnetic semiconductors. In this category, we report a new material MgGeAs2:Mn for MBE growth on GaAs substrates. Firstly, we investigated the growth of MgGeAs2 on GaAs (001) and (111)B. Stoichiometric growth conditions were established with assistance of XPS measurements. On GaAs (001), RHEED, XRD and TEM revealed phase separation with the formation of Mg3As2 and columnar grains with composition modulation. On GaAs(111)B, we obtained single crystalline films of 35 nm with a smooth surface at an optimized growth temperature of 560 °C and a beam equivalent pressure ratio Mg:Ge:As = 1:3.1:800. The lattice constant of the chalcopyrite phase is nearly matched to GaAs for both (001) and (111)B growth, in contrast to the theoretical prediction of a 6.1% mismatch. With flux variations of ±10%, Hall measurements at room temperature showed n‐type conduction (n = 4 × 1017 ∼ 3 × 1018 cm–3) in germanium rich samples and p‐type conduction (p = 8 × 1018 ∼ 2 × 1019 cm–3) in magnesium rich samples.Two ways of manganese incorporation were tried on (111)B grown MgGeAs2: (1) in‐situ solid state reaction by annealing a manganese layer deposited on top of MgGeAs2 and (2) co‐evaporation of manganese during the host material growth. Using method (2), manganese was incorporated by replacing 20% of magnesium without structural change. RHEED and XRD did not reveal the existence of additional phases. However, the preliminary magnetization and transport measurement didn't reveal a ferromagnetic signal in these samples. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimization of GaAs (110) quantum well material growth technology by reflection high energy electron diffraction

Recently, there is an increasing interest in the molecular beam epitaxy growth of various high quality heterostructures on the nonpolar GaAs(110) surface for the unique properties which arise from this unconventional orientation. Considering that the Ga and As atoms are coplanar in GaAs(110) surface, the range of bestgrowth temperature is small. It is difficult to find the best growth condition by observing the change of reflection high energy electron diffraction (RHEED) pattern because this kind of (1×1) RHEED pattern is insensitive to growth temperature and V/III beam equivalent pressure ratio. In the process of the GaAs(110) quantum well growth, we observed the single and double period variation of oscillation of RHEED intensity. This implies that there are two growth modes (monolayer-by-monolayer and bilayer-by-bilayer) of GaAs quantum wells growing on the GaAs (110) substrate under different growth conditions. The measurements of transmission electron microscopy and photoluminescence at room temperature showed that the quantum wells have very bad optical property under the bilayer_by_bilayer growth mode, while the quantum wells grown under the monolayer-by-monolayer growthmode have much better optical property with rough interfaces. By means of RHEEDoscillations, high quality quantum wells have been grown on GaAs (110) substrate under optimized growth conditions.

Structural and optical characterization of GaSb layers on Si (001) substrates

We investigated the growth of GaSb layers on Si (001) substrates by molecular beam epitaxy (MBE). Epilayers were grown as a function of Sb4/Ga beam equivalent pressure ratio (BEPR). They were then characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), photoluminescence (PL), micro Raman scattering analysis. We confirmed that the optimum condition to grow relative high quality GaSb layer in this study was Sb4/Ga=20. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Accurate Determination of Acceptor Densities and Acceptor Levels in Undoped InGaSb from Temperature Dependence of Hole Concentration

Without any assumptions regarding residual impurity species and intrinsic defects in an undoped semiconductor, it is experimentally demonstrated that the densities and energy levels of impurities and defects can be precisely determined by a graphical peak analysis method based on Hall-effect measurements, referred to as free-carrier-concentration spectroscopy (FCCS). By FCCS, the number of acceptor species in p-type undoped In0.2Ga0.8Sb epilayers is determined, and the densities and energy levels of these acceptor species are accurately estimated. Two acceptor species, whose acceptor levels are EV+25 meV and EV+86 meV, are detected, where EV is the valence band maximum. The density of the EV+25 meV acceptor increases with Sb4/(In+Ga) flux beam equivalent pressure (BEP) ratio, whereas the density of the EV+86 meV acceptor decreases with increasing BEP ratio. These observations are not consistent with the conventional assumption that these acceptor species are VSb+ and VSb2+ in GaSb-based semiconductors, where VSb is the Sb vacancy.

Molecular beam epitaxial growth window for high-quality (Ga,In)(N,As) quantum wells for long wavelength emission

We grow high-quality (Ga,In)(N,As) quantum wells containing 36% In and 4.5% N by molecular beam epitaxy, with a low As pressure and low substrate temperature growth concept. A V/III beam equivalent pressure ratio of 5 and a substrate temperature of 375°C lead to highly regular ten-period multiple quantum well structures having abrupt interfaces and smooth surfaces. By varying the quantum well width from 4to8nm, we observe 1.34–1.6μm emission of narrow linewidth (⩽50meV) at room temperature after annealing. The large conduction band offset of 410meV estimated from calculations is beneficial for a material system considered for high temperature laser operation.