Ga Beam Equivalent Pressure Research Articles

Terahertz conductivity of the highly mismatched amorphous alloy, GaNBi

We report terahertz optical conductivity measurements of the highly mismatched alloy, GaNBi. We find that in these amorphous GaNBi epilayers grown using plasma assisted molecular beam epitaxy, the optical conductivity is enhanced in the samples grown at higher gallium beam equivalent pressure (BEP). The optical conductivity spectra in these pseudo-amorphous epilayers follow a Drude–Smith behaviour due to charge confinement effects. The direct current conductivity in the epilayers grown at the highest Ga BEP (3.1 × 10−7 Torr) show an increase of three orders of magnitude compared to the one grown at the lowest Ga BEP (2.0 × 10−7 Torr). Our measurements suggests a percolative transition from an insulating nature in the GaNBi epilayers grown at low Ga BEP to a highly conducting phase in the epilayers grown at high Ga BEP.

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.

An ab initio-based approach to the stability of GaN(0 0 0 1) surfaces under Ga-rich conditions

Structural stability of GaN(0 0 0 1) under Ga-rich conditions is systematically investigated by using our ab initio-based approach. The surface phase diagram for GaN(0 0 0 1) including (2×2) and pseudo-(1×1) is obtained as functions of temperature and Ga beam equivalent pressure by comparing chemical potentials of Ga atom in the gas phase with that on the surface. The calculated results reveal that the pseudo-(1×1) appearing below 684–973 K changes its structure to the (2×2) with Ga adatom at higher temperatures beyond 767–1078 K via the newly found (1×1) with two adlayers of Ga. These results are consistent with the stable temperature range of both the pseudo-(1×1) and (2×2) with Ga adatom obtained experimentally. Furthermore, it should be noted that the structure with another coverage of Ga adatoms between the (1×1) and (2×2)-Ga does not appear as a stable structure of GaN(0 0 0 1). Furthermore, ghost island formation observed by scanning tunneling microscopy is discussed on the basis of the phase diagram.

An ab initio-based approach to phase diagram calculations for GaN(0 0 0 1) surfaces

Surface phase diagrams of GaN(0 0 0 1)-(2 × 2) and pseudo-(1 × 1) surfaces are systematically investigated by using our ab initio-based approach. The phase diagrams are obtained as functions of temperature T and Ga beam equivalent pressure p Ga by comparing chemical potentials of Ga atom in the vapor phase with that on the surface. The calculated results imply that the (2 × 2) surface is stable in the temperature range of 700–1000 K at 10 −8 Torr and 900–1400 K at 10 −2 Torr. This is consistent with experimental stable temperature range for the (2 × 2). On the other hand, the pseudo-(1 × 1) phase is stable in the temperature range less than 700 K at 10 −8 Torr and less than 1000 K at 10 −2 Torr. Furthermore, the stable region of the pseudo-(1 × 1) phase almost coincides with that of the (2 × 2) with excess Ga adatom. This suggests that Ga adsorption or desorption during GaN MBE growth can easily change the pseudo-(1 × 1) to the (2 × 2) with Ga adatom and vice versa.

Surface morphology of homoepitaxial β-Ga 2O 3 thin films grown by molecular beam epitaxy

β-Ga 2O 3 thin films were grown on (100)-oriented β-Ga 2O 3 single crystal substrates by plasma-assisted molecular beam epitaxy. At the growth temperature over 800 °C and the Ga beam equivalent pressure of 1.1 × 10 − 7 Torr, the grown surfaces exhibited clear step and terrace structures and the root-mean-square roughness of 0.5 nm in atomic force microscopy. The successful step-flow growth is indebted to the strong cleavableness of (100) planes of β-Ga 2O 3. The off-axis direction, determined by the initial substrate polishing stage, significantly influences on the surface morphology, and it should be along [001] or [00-1] directions in order for smooth surfaces.

M -plane GaN layers grown by rf-plasma assisted molecular beam epitaxy with varying Ga∕N flux ratios on m-plane 4H-SiC substrates

A series of m-plane GaN layers with the Ga beam-equivalent pressure (BEP) as the only varied parameter was grown by rf-plasma assisted molecular beam epitaxy on m-plane 4H-SiC substrates using AlN buffer layers. The smoothest growth surfaces and most complete film coalescence were found for the highest Ga BEP corresponding to the Ga droplet accumulation regime. However, better structural quality as assessed by x-ray rocking curves was observed for growth at a lower Ga BEP value below the droplet limit. The variation of rocking curve widths for planes inclined with respect to the epilayer c axis followed a different trend with Ga BEP than those of reflections parallel to the c axis. The GaN layers were found to exhibit a large residual compressive strain along the a axis.

GaN-nanowhiskers: MBE-growth conditions and optical properties

Morphology and optical properties of GaN nanowhiskers grown by molecular beam epitaxy (MBE) have been studied in correlation with growth parameters. It was shown that the growth parameters can be tuned such that uniform, well separated and high-quality nanowhiskers are obtained. Using an optimized ramp of Ga beam equivalent pressure (BEP) during the growth, the tapering or coalescence of nanowhiskers can be suppressed. By increasing the growth temperature the density of nanowhiskers is reduced, but the crystalline quality is improved as can be concluded from cathodoluminescence (CL) results.

Microstructure of (Ga,Mn)As∕GaAs digital ferromagnetic heterostructures

We report on the microstructure of (Ga,Mn)As digital ferromagnetic heterostructures grown on GaAs (001) substrates by low-temperature molecular-beam epitaxy. The Mn concentration and the As4∕Ga beam equivalent pressure (BEP) ratio are varied in the samples containing periods of Mn sheets separated by thin GaAs spacer layers. Transmission electron microscopy studies reveal that decreasing the Mn doping concentration and reducing the BEP ratio lead to smaller composition fluctuations of Mn and more homogeneous (Ga,Mn)As layers with abrupt interfaces. Planar defects are found as the dominant defect in these heterostructures and their density is related to the magnitude of the composition fluctuation. These defects show a noticeable anisotropy in the morphologic distribution parallel to the orthogonal [110] and [11¯0] direction. Along the [11¯0] direction, they are stacking faults, which are preferentially formed in V-shaped pairs and nucleate at the interfaces between (Ga,Mn)As and GaAs layers. Along the [110] direction, the planar defects are isolated thin twin lamellae. The character of the planar defects and their configuration are analyzed in detail.

Interfaces in AlGaSb∕GaSb multiquantum well structures

The Al0.4Ga0.6Sb∕GaSb∕Al0.4Ga0.6Sb quantum well (QW) structures were grown by a molecular beam epitaxy on the GaSb substrates at different growth temperatures (Tg) and with different Sb4∕Ga beam equivalent pressure ratios (BEPR) and were studied using high-resolution x-ray diffraction, photoluminescence (PL), and transmission electron microscopy. The x-ray diffraction analysis showed that the AlGaSb∕GaSb interfaces have smooth Al composition profiles with graded region thicknesses in the range of 0.83nm–2.17nm, depending upon the growth conditions; the normal interfaces are generally sharper than the inverse ones. The low-temperature PL spectra from QWs show relatively broad peaks and energies of the peak emission that can be related to the growth parameters. Good agreement was obtained between the PL peak energies and the transition energies calculated using a QWs model that takes into account the one-dimensionally graded confining potential for describing the experimentally determined Al composition profiles. The interface broadening can be due to Al and Ga segregation, Al diffusion, or Al and Ga intermixing. The broadening increases with the growth conditions that enhance the surface mobility of group-III species, such as high Tg and low V∕III BEPRs. The relatively large full widths at half maxima of the PL features were attributed to the Al composition fluctuations in the barriers, instead of the well thickness ones.

Highly enhanced Curie temperature in low-temperature annealed [Ga,Mn]As epilayers

We report Curie temperatures up to 150 K in annealed Ga1−xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) suggest that the higher Curie temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material, suggesting that the free surface of Ga1−xMnxAs epilayers may be important in determining their physical properties.

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).

Growth kinetics, impurity incorporation, defect generation, and interface quality of molecular-beam epitaxy grown AlGaAs/GaAs quantum wells: Role of group III and group V fluxes

A systematic knowledge of the influence of molecular-beam epitaxy growth parameters on the properties of AlGaAs/GaAs quantum wells grown under realistic growth conditions is important in order to obtain optimal performance of modern electronic and optoelectronic devices. Photoluminescence (PL) was used to investigate the influence of the Ga-controlled growth rate in the range below standard growth rates of 1 μm/h down to 0.1 μm/h, and of the As:Ga beam equivalent pressure ratio in the range of 10 to 60, on the growth kinetics, the interface quality, and the impurity incorporation, at a substrate temperature Ts =620 °C. As compared to in situ reflection high-energy electron diffraction (RHEED) measurements, where no sample rotation is possible, PL has the advantage that realistic growth conditions can be used. A careful line shape analysis, together with infrared and time-resolved PL measurements gives information on the interface roughness, the impurity incorporation, and the deep trap concentration. Non-negligible desorption of Ga during growth is found for the range of conditions under study. The desorption is found to increase upon a decrease of As:Ga ratio. The interface roughness as well as the impurity and trap incorporation are found to decrease with decreasing growth rate, an optimum interface quality being obtained below 0.5 μm/h. At this optimal growth rate, increasing the As4:Ga ratio leads to a decrease of shallow impurity concentration and thus to a narrower line width, but to a simultaneous increase of defect generation. Optimal growth conditions are found at a beam equivalent pressure ratio of 15. The observed desorption kinetics and interface properties can be explained in accordance with existing theoretical simulations. Finally, growth interruption was found to lead to optimal formation of flat growth islands when the overall growth rate is lowered.