Beam Equivalent Pressure Research Articles

Ab initio-based approach to elemental growth process on the InAs wetting layer grown on GaAs substrate

The surface reconstructions and migration behavior on InAs(001) and InAs(111)A wetting layer on GaAs substrate are systematically investigated by our ab initio-based approach and Monte Carlo simulations. The ab initio-based approach incorporates the chemical potentials of In atom and As4 molecule in the gas phase as functions of temperature T and beam equivalent pressure pAs4. The calculated surface phase diagrams imply that the respective stable surface structures of InAs(001) and InAs(111)A are the (2×4)α2 and the In-vacancy surfaces at conventional growth conditions at T∼700–750K and pAs4∼10−7–10−6Torr. On these surfaces, adsorption–desorption boundary for In atom appears below the growth temperatures. Therefore, In adatoms tend to desorb from the InAs(001) and the InAs(111)A surfaces at equilibrium. Kinetic behavior of In adatoms obtained by the Monte Carlo simulations reveal that life time τ and diffusion length L of In adatoms are small such as τ=1.59×10−3s and L=618nm on the InAs(001) and τ=4.35×10−14s and L=0.204nm on the InAs(111)A. On the basis of these results, the growth process on the InAs(111)A is discussed in terms of self-surfactant effect where simultaneous As adsorption makes In atom adsorb on the surface.

Plasma-Assisted Molecular Beam Epitaxy of InXGa1-XN Films on C-plane Sapphire Substrates

We report plasma-assisted molecular beam epitaxy of <TEX>$In_XGa_{1-X}N$</TEX> films on c-plane sapphire substrates. Prior to the growth of <TEX>$In_XGa_{1-X}N$</TEX> films, GaN film was grown on the nitride c-plane sapphire substrate by two-dimensional (2D) growth mode. For the growth of GaN, Ga flux of <TEX>$3.7{\times}10^{-8}$</TEX> torr as a beam equivalent pressure (BEP) and a plasma power of 150 W with a nitrogen flow rate of 0.76 sccm were fixed. The growth of 2D GaN growth was confirmed by <TEX>$in-situ$</TEX> reflection high-energy electron diffraction (RHEED) by observing a streaky RHEED pattern with a strong specular spot. InN films showed lower growth rates even with the same growth conditions (same growth temperature, same plasma condition, and same BEP value of III element) than those of GaN films. It was observed that the growth rate of GaN is 1.7 times higher than that of InN, which is probably caused by the higher vapor pressure of In. For the growth of <TEX>$In_xGa_{1-x}N$</TEX> films with different In compositions, total III-element flux (Ga plus In BEPs) was set to <TEX>$3.7{\times}10^{-8}$</TEX> torr, which was the BEP value for the 2D growth of GaN. The In compositions of the <TEX>$In_xGa_{1-x}N$</TEX> films were determined to be 28, 41, 45, and 53% based on the peak position of (0002) reflection in x-ray <TEX>${\theta}-2{\theta}$</TEX> measurements. The growth of <TEX>$In_xGa_{1-x}N$</TEX> films did not show a streaky RHEED pattern but showed spotty patterns with weak streaky lines. This means that the net sticking coefficients of In and Ga, considered based on the growth rates of GaN and InN, are not the only factor governing the growth mode; another factor such as migration velocity should be considered. The sample with an In composition of 41% showed the lowest full width at half maximum value of 0.20 degree from the x-ray (0002) omega rocking curve measurements and the lowest root mean square roughness value of 0.71 nm.

Effects of molecular beam epitaxy growth conditions on composition and optical properties of InxGa1−xBiyAs1−y

We describe the growth conditions of InxGa1−xBiyAs1−y (lattice-mismatched and matched) on InP substrates by molecular beam epitaxy and the resulting properties. Due to their anomalously narrow bandgaps and the presence of bismuth, these materials are promising for optoelectronics and thermoelectrics. Low growth temperature and moderate As/Bi beam equivalent pressure ratios are beneficial for Bi incorporation, in good qualitative agreement with GaBiyAs1−y on GaAs. Up to 6.75% bismuth is incorporated. High resolution x-ray diffraction and reciprocal space mapping show that InxGa1−xBiyAs1−y samples exhibit good crystalline quality and zero relaxation. The band gap is reduced in agreement with theoretical predictions. Lattice-matched samples have been produced with lattice mismatch ≤0.21%.

Ab initio-based approach to reconstruction, adsorption and incorporation on GaN surfaces

Reconstruction, adsorption and incorporation on various GaN surfaces are systematically investigated using an ab initio-based approach that predicts the surface phase diagram as functions of temperature and beam-equivalent pressure (BEP). The calculated results for GaN surface reconstructions with polar (0 0 0 1), nonpolar (1 1 −2 0), semipolar (1 −1 0 1) and semipolar (1 1 −2 2) orientations imply that reconstructions on GaN surfaces with Ga adlayers generally appear on the polar and the semipolar surfaces, while the stable ideal surface without Ga adsorption is found on the nonpolar GaN(1 1 −2 0) surface because it satisfies the electron counting rule. The hydrogen adsorption on GaN(0 0 0 1) and GaN(1 1 −2 0) realizes several surface structures forming N–H and Ga–NH2 bonds on their surfaces that depend on temperature and Ga BEP during metal-organic vapor-phase epitaxy (MOVPE). In contrast, the stable structures due to hydrogen adsorption on the semipolar GaN(1 −1 0 1) and GaN(1 1 −2 2) surfaces are not varied over the wide range of temperature and Ga BEP. This implies that the hydrogen adsorbed stable structures are expected to emerge on the semipolar surfaces during MOVPE regardless of the growth conditions. Furthermore, we clarify that Mg incorporation on GaN(1 −1 0 1) surfaces is enhanced by hydrogen adsorption consistent with experimental findings.

Growth of CuGaSe2 Layers on Closely Lattice-Matched GaAs Substrates by Migration-Enhanced Epitaxy

CuGaSe2 single-crystal films are grown on the As-stabilized (2×4) surface of (001) GaAs by migration-enhanced epitaxy (MEE), where Cu+Ga and Se are alternately deposited. The growth process is monitored by refraction high-energy electron diffraction (RHEED) in the [110] azimuth. Under the Cu-enriched growth condition, a deformed 4-fold pattern is observed in both Cu+Ga and Se deposition periods. The deformed 4-fold pattern is found to be related to the segregation of Cu2Se on the CuGaSe2 surface as confirmed by the results of X-ray diffraction (XRD) measurement. By reducing the beam equivalent pressure of Cu (Cu-BEP), clear 4-fold patterns appear in both Cu+Ga and Se deposition periods instead of deformed 4-fold patterns. Further reduction of Cu-BEP results in clear 4- and 2-fold patterns for Cu+Ga and Se deposition periods. Under these growth conditions, Cu2Se-segregation-free CGS growth is achieved. Thus, the CuGaSe2 single-crystal layers without Cu2Se-segregation are successfully grown on GaAs(001) substrates by optimizing the Cu-BEP.

Non-stoichiometric GaAsBi/GaAs (100) molecular beam epitaxy growth

The growth of III–V bismuthides is complicated by the low incorporation efficiency of Bi in GaAs, leading either to the formation of metallic Bi droplets or low layer composition fractions. Typically growth is performed between 280 and 350 °C and at near stoichiometric Ga:As fluxes in order to encourage Bi incorporation. However most work reported to date also utilises As 2 as the As overpressure constituent. It is found in this work that growth with As 4 allows high Bi composition films with the standard 1:20 Ga:As 4 beam equivalent pressure ratio (BEPR) utilised for higher temperature buffer layer growth. The Bi fraction versus Bi:As 4 BEPR is found to be initially linear, until a maximum value is obtained for a given temperature after which the continued oversupply of Bi results in the formation of droplets.

InGaAs Quantum Dots Grown by Molecular Beam Epitaxy for Light Emission on Si Substrates

The aim of this study is to achieve homogeneous, high density and dislocation free InGaAs quantum dots grown by molecular beam epitaxy for light emission on silicon substrates. This work is part of a project which aims at overcoming the severe limitation suffered by silicon regarding its optoelectronic applications, especially efficient light emission device. For this study, one of the key points is to overcome the expected type II InGaAs/Si interface by inserting the InGaAs quantum dots inside a thin silicon quantum well in SiO2 fabricated on a SOI substrate. Confinement effects of the Si/SiO2 quantum well are expected to heighten the indirect silicon bandgap and then give rise to a type I interface with the InGaAs quantum dots. Band structure and optical properties are modeled within the tight binding approximation: direct energy bandgap is demonstrated in SiO2/Si/InAs/Si/SiO2 heterostructures for very thin Si layers and absorption coefficient is calculated. Thinned SOI substrates are successfully prepared using successive etching process resulting in a 2 nm-thick Si layer on top of silica. Another key point to get light emission from InGaAs quantum dots is to avoid any dislocations or defects in the quantum dots. We investigate the quantum dot size distribution, density and structural quality at different V/III beam equivalent pressure ratios, different growth temperatures and as a function of the amount of deposited material. This study was performed for InGaAs quantum dots grown on Si(001) substrates. The capping of InGaAs quantum dots by a silicon epilayer is performed in order to get efficient photoluminescence emission from quantum dots. Scanning transmission electronic microscopy images are used to study the structural quality of the quantum dots. Dislocation free In50Ga50As QDs are successfully obtained on a (001) silicon substrate. The analysis of QDs capped with silicon by Rutherford Backscattering Spectrometry in a channeling geometry is also presented.

Formation Mechanisms of Islands on Cu-Alloyed GaN Grown by Plasma Assisted Molecular Beam Epitaxy

Cu-alloyed GaN epilayers were prepared by plasma assisted molecular beam epitaxy in a Ga-rich environment with Cu-to-Ga beam equivalent pressure ratios of 1.2 to 4.8%. Islands enriched with Cu are found on all the GaN epitaxial layers. The islands are composed of a Cu9Ga4 intermetallic phase and GaN with the orientation relationship: [111]Cu9Ga4//[1 10]GaN and (10 )Cu9Ga4//(0001)GaN. The formation mechanisms of the islands are discussed in detail. Wavelength dispersive X-ray spectroscopy analyses indicate that the 1.2 and 4.8% samples contain 0.10 ± 0.02 atom % Cu and 0.04 ± 0.03 atom % Cu, respectively. Both samples show ferromagnetic behavior with a magnetization of in a range of (2.3–5.1) × 103 A/m at saturation. A strategy to suppress the island formation and increase Cu incorporation is accordingly proposed and verified.

VI/II ratio-dependent growth and photoluminescence of cubic CdSe epilayers by molecular beam epitaxy

CdSe epilayers were grown on GaAs(0 0 1) substrates by molecular beam epitaxy (MBE) in a wide range of VI/II beam-equivalent-pressure (BEP) ratio from 1.1 to 8.5. X-ray diffraction (XRD) θ–2 θ scan reveals all the CdSe samples possess cubic zinc blende structure with (0 0 1) orientation. The film grown at a low VI/II ratio of 1.1 has rough surface with a three-dimensional (3D) growth mode, while two-dimensional (2D) growth can be established under Se-rich conditions at higher Se/Cd BEP ratios over 4.4. The growth rate increases monotonically with the VI/II ratio and becomes saturated when the ratio is raised to 8.5. The sample grown at a VI/II ratio of 8.5 shows the narrowest full-width at half-maximum (FWHM) of X-ray rocking curve (XRC) for (0 0 4) reflection. No near-band-edge (NBE) emission of CdSe is observed for the sample prepared at a low ratio of 1.1. The NBE emission from cubic CdSe at around 1.67 eV appears and its intensity is markedly enhanced by elevating the VI/II ratio. It is suggested that CdSe epilayer with good structural and optical properties can be prepared under a large VI/II BEP ratio.

Improving the photoluminescence properties of self-assembled InAs surface quantum dots by incorporation of antimony

This study investigates the effects of surfactant and segregation from InAs surface quantum dots (SQDs) by incorporating antimony (Sb) into the QD layers. The Sb surfactant effect extends planar growth and suppresses dot formation. Incorporating Sb can reduce the density of SQDs by more than two orders of magnitude. Photoluminescence (PL) reveals enhancement in the optical properties of InAs SQDs as the Sb beam equivalent pressure (BEP) increases. This improvement is caused by the segregation of Sb on the surface of SQDs, which reduces non-radiative recombination and suppresses carrier loss. The dark line at the SQDs surface in the transmission electron microscopic image suggests that the incorporated Sb probably segregates close to the surface of the SQDs. These results indicate a marked Sb segregation effect that can be exploited to improve the surface-sensitive properties of SQDs for biological sensing.

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 &amp;gt;50 to &amp;lt;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.

Non-self-sustained glow discharge with electrostatic confinement of electrons sustained by a fast neutral molecule beam

Experimental study of plasma produced at the nitrogen pressure 0.2–1 Pa in the chamber volume V ≈ 0.12 m3 as a result of injection into the chamber of a broad nitrogen molecule beam with 1–4 keV energy and 0.1–1 A equivalent current is carried out, and the study results are presented. Dependences of the plasma density distribution on the beam equivalent current Ib, energy Eb, and gas pressure p indicate a crucial role of fast molecules in gas ionization, and the probe characteristics reveal two groups of plasma electrons with the temperatures Te ∼ 0.4 eV and Te ∼ 16 eV. Immersion in plasma of an electrode isolated from the chamber and application to the electrode of a positive voltage U result in non-self-sustained discharge. When U changes from ∼0.5 to ∼1.5 V, the discharge current I rapidly rises to a certain value I*, and after that the rate of rise dI/dU drops by an order of magnitude. At U ∼ 10 V, the current I rises to I0 ≈ 1.5I*, and dI/dU once again drops by an order of magnitude. Current I0 specifies the number of electrons produced inside the chamber per second, and it grows up with Eb, Ib, and p. At U > 20 V, due to gas ionization by fast electrons emitted by the chamber and accelerated up to the energy ∼eU in the sheath between the plasma and the chamber walls, the current I rises again. When U grows up to ∼50 V, production of fast electrons with energies exceeding the ionization threshold begins inside the sheath, and the ionization intensity rises dramatically. At U > 150 V, contribution of fast electrons to gas ionization already exceeds the contribution of fast molecules, and the plasma density and its distribution homogeneity inside the chamber both grow up substantially. However, even in this case, the discharge is non-self-sustained, and only at U > 300 V it does not expire when the beam source is switched off.

MBE growth optimization of topological insulator Bi 2Te 3 films

We investigated the growth of the topological insulator Bi 2Te 3 on Si(1 1 1) substrates by means of molecular-beam epitaxy (MBE). The substrate temperature as well as the Bi and Te beam-equivalent pressure (BEP) was varied in a large range. The structure and morphology of the layers were studied using X-ray diffraction (XRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM). The layer-by-layer growth mode with quintuple layer (QL) as an unit is accomplished on large plateaus if the MBE growth takes place in a Te overpressure. At carefully optimized MBE growth parameters, we obtained atomically smooth, single-crystal Bi 2Te 3 with large area single QL covering about 75% of the layer surface. Angular-resolved photoelectron spectroscopy reveals a linear energy dispersion of charge carriers at the surface, evidencing topologically insulating properties of the Bi 2Te 3 epilayers.

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.

Study on temperature calibration and surface phase transition of GaAs crystal substrate in MBE growth by RHEED real-time monitoring

Using RHEED as a real-time monitoring tool, the MBE temperature measurement system was calibrated according to the relationship between GaAs (100) surface reconstruction phase and the substrate temperature, As4 beam equivalent pressure of the substrate. This approach can also be applied to other MBE systems. It provides an experimental basis of the growth of high-quality epitaxial thin films for studying of the surface roughness of InGaAs, the phase transformation process and the surface morphology.

Carbon as an acceptor in cubic GaN/3C–SiC

We report on carbon doping of cubic GaN by CBr4 using plasma-assisted molecular beam epitaxy. Cubic GaN:C samples were doped at different CBr4 beam equivalent pressures between 2×10−9 and 6×10−6mbar. The incorporated carbon concentration of up to 1×1020cm−3 was achieved in c-GaN:C as measured by secondary ion mass spectroscopy. The net donor/acceptor concentration was obtained by evaluation of capacitance-voltage data. Capacitance-voltage measurements on nominally undoped cubic GaN showed n-type conductivity. With an increase in CBr4 flux the conductivity type changed to p-type and for highest CBr4 flux an acceptor surplus of 1×1019cm−3 was obtained. The electrical properties of the c-GaN:C layers were investigated by current–voltage measurements and a decrease in the serial conductance by two orders of magnitude was demonstrated in c-GaN:C. A blue shift of the 2K donor–acceptor pair luminescence with an increase in carbon concentration was obtained.

Growth of InAs quantum dots and dashes on silicon substrates: Formation and characterization

Self assembled InAs quantum dots (QDs) are grown by solid source molecular beam epitaxy on (100) and (111) Si substrates using Stranski–Krastanov (SK) growth mode. Reflection high energy diffraction (RHEED) streak patterns confirm that the combination of the atomic hydrogen followed by thermal desorption is an efficient cleaning method. The evolution of size, density and shape of the QDs are ex-situ characterized by atomic-force microscopy (AFM). Different growth parameters such as InAs coverage, growth temperature, In-growth rate and V/III ratio are examined on differently prepared silicon surfaces including ex- and in-situ cleaning procedures. The ex-situ surface preparation consists of a pre-removal of the surface oxide by buffered HF (NH4/HF) treatment followed by an in-situ thermal atomic hydrogen (AH) assisted surface cleaning process at 500°C and a final thermal oxide desorption treatment in the growth chamber within a temperature range of 700–900°C. Additional improvement for the cleaning and growth is achieved by exposing the Si surface with Ga at low fluxes. The Ga treatment at a temperature of 560°C for 2min results in a strong reduction of the lateral size of InAs QDs from 50–70 down to 25–30nm and a significant enhancement in the homogeneity of the dot size and distribution. The InAs QDs density is strongly increased from 108 to 1011cm−2 at V/III ratio in the range of 15–35 (beam equivalent pressure values). InAs QD formations are not observed at temperatures as high as 500°C. Moreover, InAs quantum dashes are observed on [111] orientation and at higher In-growth rate of 0.3ML/s.