Beam Equivalent Pressure Ratio Research Articles

Molecular beam epitaxy growth and characterization of GePb alloys

Pb based group-IV alloys such as GePb have been gaining interest as a potential alternative for infrared detectors, quantum materials, and high-speed electronic devices. Challenges remain in their growth due to the extremely low solid solubility of Pb in the Ge–Pb system. This paper reports molecular beam epitaxy growth of GePb alloy thin films on Ge(100) substrates. Effusion cells of Ge and Pb are used to control the flux ratio independently. The optimal substrate temperature is found to be near the thermocouple temperature of 300 °C based on the characterization of the grown films using high-resolution x-ray diffraction. A large change in the Ge:Pb beam equivalent pressure ratio from 10:1 to 1:1 results in only a minimal increase of the Pb composition from 0.74% to 2.84% as estimated from Raman spectroscopy and Rutherford backscattering spectrometry. Scanning electron microscopy images show a large volume of Pb islands on the surface that form into either long trapezoidal rods or uniform droplets, with increasing Pb flux and growth time the density of Pb islands increased.

Nanosecond recombination lifetimes and spin relaxation times in (110) InGaAs/AlGaAs quantum wells at room temperature

Quantum wells in InGaAs/AlGaAs with (110) orientation are attractive as active layers in spin-controlled lasers with circularly polarized emission, while the spin relaxation time is expected to be larger than for (100)-oriented layers. However, the hitherto reported recombination lifetimes (40 ps) and spin relaxation times (440 ps) of (110) InGaAs/AlGaAs structures are insufficient. Here it is shown that higher growth temperatures and higher V/III beam equivalent pressure ratios than previously used in crystal growth by molecular beam epitaxy lead to recombination and spin relaxation times in the nanosecond range at RT, meeting the requirements for application in spin lasers.

Low-temperature grown MnAs/InAs/MnAs double heterostructure on GaAs (111)B by molecular beam epitaxy

We used MBE to grow a double heterostructure of MnAs/InAs/MnAs on GaAs(111)B at low temperature for vertical spin FET applications. To confirm the ideal development environment, we primarily prepared single InAs thick layer (∼1.2 μm) at low growth temperature (∼250 °C) with varied V/III ratio (As/In = 2, 10, 20) due to the challenge of growing InAs at much lower temperature than its usual growth temperature (∼480 °C). We measured their structural and electrical properties and found an optimum condition at V/III ratio of 10. Afterwards, we prepared the double heterostructure at low temperature (∼250 °C), again varying the As/In beam equivalent pressure ratio to find its influence on the overall quality of the structure. Using atomic force microscopy, we observed the surface roughness variation corresponding to V/III ratio variation of InAs. We confirmed the growth of three individual thick layers of MnAs and InAs by cross-sectional analysis using scanning electron microscopy and energy-dispersive X-ray spectroscopy. Using a superconducting quantum interference device magnetometer, we found in-plane easy magnetization and observed the effect of top and bottom MnAs layers on the hysteresis curve. We also found the existence of ferromagnetic behavior of MnAs layers at RT MH measurements. The MnAs/InAs/MnAs double heterostructure on GaAs(111)B, in our opinion, has potential as a structure for spin FETs.

Optimized molecular beam epitaxy process for lattice-matched narrow-bandgap (0.8 eV) GaInNAsSb solar junctions

High performance narrow-bandgap GaInNAsSb solar cells are instrumental for the development of lattice-matched GaAs-based solar cells with more than four junctions. To this end a comprehensive optimization process including the effects of growth temperature, As/III beam equivalent pressure ratio, and Sb flux on the performance of 0.8 eV GaInNAsSb solar cells grown by molecular beam epitaxy is reported. For this, sets of GaInNAsSb p-i-n solar cell structures with 5–6% nitrogen compositions were fabricated, while varying the key growth parameters. The quantum efficiency and current generation increased significantly when the narrow gap materials were grown at elevated growth temperatures, close to phase separation. A further improvement in the current generation was observed by employing lower As/III beam equivalent pressure ratios. The best GaInNAsSb cell exhibited about 94% peak external quantum efficiency and generated a short-circuit current of 17.7 mA/cm 2 with AM1.5D (1000 W/m 2 ) illumination at wavelengths above 900 nm without employing a back surface reflector. Our analysis indicates that the best cell is already close to being absorption limited. While the N composition should be kept as low as possible (i.e., ≲5%) to achieve high performance, increasing the Sb flux generally results in improved the material quality, i.e., leading to a slight improvement for the open-circuit voltages and fill factors. In addition, it was found that the phase separation observed at the growth temperature of 480 °C could effectively be inhibited by employing higher Sb fluxes. • Optimized MBE growth for 0.8 eV GaInNAsSb solar junctions with 5–6% N content. • High current obtained using high growth temperatures and lower As pressures. • High peak external quantum efficiency of 94% demonstrated. • Background doping level as low as 1.1 × 10 15 cm −3 for GaInNAsSb with 6% N. • Phase separation is suppressed, and power output improved by optimizing Sb flux.

Study of the type-I to type-II band alignment transition in InAs(Sb)/GaAs quantum dot nanostructures

InAs(Sb)/GaAs self-assembled quantum dots (QDs) were grown by molecular beam epitaxy though incorporating Sb into InAs with increased beam equivalent pressure ratio, x = Sb2/(Sb2+As2), of x = 0, 0.15, 0.25, and 0.5. From photoluminescence measurements, we observed a transition of the band alignment from type-I for the InAs QDs to type-II for InAsSb QDs. Thus, after adding Sb into the QDs these type II InAsSb QDs have different characteristics from the well-studied type-I InAs QDs regarding to their excitation intensity dependence, temperature dependence, and time dependence of the luminescence. Additionally, the Sb distribution in the QDs and the wetting layer was studied. These results indicate that we are able to fabricate InAsSb QDs with a type II band alignment, which may be good optical materials for the development of intermediate-band photovoltaic cells.

Growth of high material quality InAs/GaSb type-II superlattice for long-wavelength infrared range by molecular beam epitaxy

By optimizing the V/III beam-equivalent pressure ratio, a high-quality InAs/GaSb type-II superlattice material for the long-wavelength infrared (LWIR) range is achieved by molecular beam epitaxy (MBE). High-resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM), and Fourier transform infrared (FTIR) spectrometer are used to characterize the material growth quality. The results show that the full width at half maximum (FWHM) of the superlattice zero-order diffraction peak, the mismatching of the superlattice zero-order diffraction peak between the substrate diffraction peaks, and the surface roughness get the best results when the beam-equivalent pressure (BEP) ratio reaches the optimal value, which are 28 arcsec, 13 arcsec, and 1.63 Å, respectively. The intensity of the zero-order diffraction peak is strongest at the optimal value. The relative spectral response of the LWIR detector shows that it exhibits a 100% cut-off wavelength of 12.6 μm at 77 K. High-quality epitaxial materials have laid a good foundation for preparing high-performance LWIR detector.

Improved growth quality of epitaxial ZnTe thin films on Si (111) wafer with ZnSe buffer layer

To achieve high-quality and low-cost ZnTe epitaxial films, a low-temperature (LT, 200 °C) ZnSe buffer layer with a thickness of 5 nm was grown on Si (111) wafers prior to the epitaxy of ZnTe layers by molecular beam epitaxy. Reflection high-energy electron diffraction patterns reveal that ZnTe epilayers grown at 335 °C with a thickness of 100 nm on the LT-ZnSe buffer layer present a completely two-dimensional growth mode. Film surfaces present smooth and flat morphology with the lowest surface roughness of ∼2.2 nm at a Te/Zn beam equivalent pressure ratio of 1.6. X-ray diffraction θ-2θ scanning indicates that the out-of-plane structure of ZnTe films is highly preferred with the (111) growth orientation of ZnSe/Si. X-ray diffraction φ scanning further confirms that the in-plane structure of ZnTe epilayer is also a single-crystal orientation growth. Thus, high-quality ZnTe (111) single-crystal epilayer with the lowest full-width at half-maximum of ∼200 arc sec and the smallest dislocation density of ∼1.12 × 108 cm−2 was grown on Si (111) wafers. In addition, the ZnSe buffer layer could alleviate the residual stress effectively for the heteroepitaxy of Si wafers. The most balanced residual stress for ZnTe/ZnSe/Si with σx of ∼96.5 MPa and σy of ∼94.2 MPa was obtained, which is essential for achieving high-performance and good mechanical properties of ZnTe-based devices.

Density-controlled growth of vertical InP nanowires on Si(111) substrates

A procedure to achieve the density-controlled growth of gold-catalyzed InP nanowires (NWs) on (111) silicon substrates using the vapor-liquid-solid method by molecular beam epitaxy is reported. We develop an effective and mask-free method based on controlling the number and the size of the Au-In catalyst droplets in addition to the conditions for the NW nucleation. We show that the NW density can be tuned with values in the range of 18 µm−2 to <0.1 µm−2 by the suitable choice of the In/Au catalyst beam equivalent pressure (BEP) ratio, by the phosphorous BEP and the growth temperature. The same degree of control is transferred to InAs/InP quantum dot-nanowires, taking advantage of the ultra-low density to study by micro-photoluminescence the optical properties of a single quantum dot-nanowires emitting in the telecom band monolithically grown on silicon. Optical spectroscopy at cryogenic temperature successfully confirmed the relevance of our method to excite single InAs quantum dots on the as-grown sample, which opens the path for large-scale applications based on single quantum dot-nanowire devices integrated on silicon..

Growth Control of High-Performance InAs/GaSb Type-II Superlattices via Optimizing the In/Ga Beam-Equivalent Pressure Ratio**Supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB0402402 and 2016YFB0400601), the National Basic Research Program of China (Grant No. 2015CB351902), the National Science and Technology Major Project (2018ZX01005101-010), the National Natural Science Foundation of China (Grant Nos. 61835011 and U1431231), the

The performance of type-II superlattice (T2SL) long-wavelength infrared devices is limited by crystalline quality of T2SLs. We optimize the process of growing molecular beam epitaxy deposition T2SL epi-layers on GaSb (100) to improve the material properties. Samples with identical structure but diverse In/Ga beam-equivalent pressure (BEP) ratio are studied by various methods, including high-resolution x-ray diffraction, atomic force microscopy and high-resolution transmission electron microscopy. We find that appropriately increasing the In/Ga BEP ratio contributes to improving the quality of T2SLs, but too large In BEP will much more easily cause a local strain, which can lead to more InSb islands in the InSb interfaces. The InSb islands melt in the InSb interfaces caused by the change of chemical potential of In atoms may result in the “nail” defects covering the whole T2SLs, especially the interfaces of GaSb-on-InAs. When the In/Ga BEP ratio is about 1, the T2SL material possesses a lower full width at half maximum of +1 first-order satellite peak, much smoother surface and excellently larger area uniformity.

Influence of MBE growth parameters on GaSb/GaAs quantum dot morphology

Abstract In this paper, growth of self-assembled type-II GaSb/GaAs quantum dots by molecular beam epitaxy technique is presented. The effect of various growth parameters such as substrate temperature, Sb/Ga beam equivalent pressure ratio, growth rate and total mono-layer coverage on surface morphology is analyzed. The GaSb quantum dots exhibit high anisotropy along and directions under specific growth conditions. The anisotropy is discussed on the basis of incorporation of background Arsenic and anisotropic diffusion of Gallium adatoms on the GaSb surface. The low temperature photoluminescence measurements in GaAs capped samples show recombination in the type-II quantum dots and the wetting layer.

A Study of GaAs1–xSbx Axial Nanowires Grown on Monolayer Graphene by Ga-Assisted Molecular Beam Epitaxy for Flexible Near-Infrared Photodetectors

We report the successful growth of high-quality GaAs1–xSbx nanowires on monolayer graphene/SiO2/p-Si (111) using molecular beam epitaxy (MBE) for the application of a flexible near-infrared photodetector. A systematic and detailed study of NW growth parameters, namely, growth temperature, V/III beam equivalent pressure (BEP) ratio, and Ga shutter opening duration, has been carried out. Growth of vertical ⟨111⟩ oriented nanowires on graphene with 4 K photoluminescence emission in the range 1.24–1.38 eV has been achieved. The presence of a weak D mode in Raman spectra of NWs grown on graphene suggests that NW growth did not alter the intrinsic properties of the monolayer graphene. High-resolution transmission electron microscopy and a selective area diffraction pattern confirmed the zinc-blende crystal structure of the NWs. This study suggests that Sb as a surfactant plays a critical role in the surface engineering of the substrate, leading to the superior optical quality of NWs exhibiting a higher 4 K phot...

Molecular beam epitaxial growth of GaAsSb/GaAsSbN/GaAlAs core-multishell nanowires for near-infrared applications

We report on the bandgap engineering of the GaAsSb/GaAsSbN heterostructured nanowires (NWs) in the core–shell architecture using the unique properties of dilute nitride material system for near-infrared photodetection. A high density of vertical GaAsSb/GaAsSb(N)/GaAlAs core-multishell configured NWs with well faceted, smooth surface morphology has been grown on Si (111) substrates using Ga-assisted molecular beam epitaxy. A low Sb content GaAsSb core has been shown to enable the coherently strained growth of dilute nitride shell with higher Sb content in GaAsSbN shell NWs. A systematic study of N and V/III beam equivalent pressure ratios is carried out to achieve the large band-gap reduction, while successfully incorporating higher Sb content in the dilute nitride shells (GaAs1−xSbxN; x = 0.27). The incorporation of N acts to relieve strain and provide a smooth surface morphology as well as redshift the 4K photoluminescence (PL) peak energy by ∼160 meV in comparison to a non-nitride shell. The selected area diffraction pattern confirms zinc-blende structure in all the NWs and did not show any noticeable planar defects in dilute nitride NWs. We successfully, thus demonstrate GaAsSb/GaAsSbN/GaAlAs core–shell NWs by engineering the lattice strain of nitride shell with the non-nitride ternary core, for extending the 4K photoemission up to 1.43 μm.

MBE growth strategy and optimization of GaAsBi quantum well light emitting structure beyond 1.2 μm

GaAs1−xBix/AlGaAs quantum wells (QWs) with varying As/Ga beam equivalent pressure (BEP) ratios were grown by solid source molecular beam epitaxy at a relatively high temperature of 350–400 °C intended for light emitting applications with wavelengths beyond 1.2 μm. Both the Bi content and the photoluminescence (PL) intensity were found to be highly dependent on As2 flux, especially for the case of growing GaAsBi at a relatively high temperature. A graded index separate confinement GaAsBi/AlGaAs single QW with 5.8% Bi exhibited a strong PL emission at 1.22 μm. The growth strategy to incorporate considerable Bi into GaAs at a relatively high temperature through meticulous control of the As/Ga BEP ratio and compensation of Bi flux is demonstrated to be effective in guaranteeing a high Bi content as well as an optimal optical performance of GaAsBi QWs.

Influence of Molecular Beam Epitaxy (MBE) Parameters on Catalyst-Free Growth of InAs Nanowires on Silicon (111) Substrate

The effect of molecular beam epitaxy parameters on catalyst-free growth of InAs nanowires using oxide templates on Si (111) substrate has been studied. Two different approaches, i.e., thermal and plasma-enhanced chemical vapor deposition, were used to prepare the oxide templates. The length, diameter, density, and quality of the nanowires depended strongly on the substrate temperature, As/In beam equivalent pressure ratio, and growth rate, whereas the type, thickness, and uniformity of the oxide were found to affect only the density of the nanowires. Nanowires could be grown effectively in only a very narrow range of these growth parameters. The surface morphological, structural, and optical properties of the nanowires were assessed using various techniques including field-emission scanning electron microscopy, high-resolution x-ray diffraction analysis, transmission electron microscopy, and Raman spectroscopy.

Molecular beam epitaxy and characterization of AlGaN nanowire ultraviolet light emitting diodes on Al coated Si (0 0 1) substrate

We have demonstrated the epitaxial growth of AlGaN nanowires on Al coated Si (0 0 1) substrate. The as-grown nanowires feature diameters of >200 nm and relatively uniform height distribution. AlGaN nanowires with emission wavelengths from 340 nm to 288 nm have been successfully achieved by varying Al/Ga beam equivalent pressure ratio and growth temperature. Detailed structural characterization suggests that AlGaN nanowires grown on Al template are free of dislocations. We have further demonstrated functional AlGaN nanowire deep ultraviolet (DUV) light emitting diodes, which exhibit a turn-on voltage of 7 V and a single peak EL emission at 288 nm. The realization of high quality AlGaN nanostructures on reflective Al template provides a promising approach for achieving high efficiency DUV light emitters.

Investigation of MBE-Growth of Mid-Wave Infrared Hg1−xCdxSe

Undoped mid-wave infrared Hg1−xCdxSe epitaxial layers have been grown to a nominal thickness of 8–14 μm on GaSb (211)B substrates by molecular beam epitaxy (MBE) using constant beam equivalent pressure ratios. The effects of growth temperature from 70°C to 120°C on epilayer quality and its electronic parameters has been examined using x-ray diffraction (XRD) rocking curves, atomic force microscopy, Nomarski optical imaging, photoconductive decay measurements, and variable magnetic field Hall effect analysis. For samples grown at 70°C, the measured values of XRD rocking curve full width at half maximum (FWHM) (116 arcsec), root mean square (RMS) surface roughness (2.7 nm), electron mobility (6.6 × 104 cm2 V−1 s−1 at 130 K), minority carrier lifetime (∼ 2 μs at 130 K), and background n-type doping (∼ 3 × 1016 cm−3 at 130 K), indicate device-grade material quality that is significantly superior to that previously published in the open literature. All of these parameters were found to degrade monotonically with increasing growth temperature, although a reasonably wide growth window exists from 70°C to 90°C, within which good quality HgCdSe can be grown via MBE.

Bismides: 2D structures and quantum dots

The growth and characterization of ternary GaAsBi and quaternary GaInAsBi compound quantum wells (QWs) on GaAs substrates is presented in this study. The influence of technological parameters, such as different growth modes, substrate temperatures, beam equivalent pressure ratios and thermal treating on structural and luminescent properties of QWs is discussed. The complex structural investigations using x-ray diffraction, atomic force microscopy and high-resolution transmission electron microscopy revealed high crystal structure, smooth surfaces and abrupt interfaces of both GaAsBi and GaInAsBi QWs. The temperature dependent photoluminescence measurements demonstrated emission wavelengths up to 1.43 µm in room temperature PL spectra measured for GaAsBi/GaAs QWs containing 12% Bi, whereas GaInAsBi QWs with 4.2% of bismuth inserted between GaAs barriers has reached 1.25 µm. Moreover, the annealing at high temperatures of GaAsBi/AlAs QWs stimulated agglomeration of bismuth to quantum dots in the well layers, emitting at 1.5 µm. The achieved wavelengths are the longest ones declared for the GaAsBi and GaInAsBi QW structures grown on the GaAs substrate, therefore bismide-based QWs are the promising structures for applications in infrared devices.

GaAsBi/GaAs MQWs grown by MBE using a two-substrate-temperature technique

Eleven periods of GaAsBi/GaAs multiple quantum wells (MQWs) on (100) GaAs substrates were grown by molecular beam epitaxy (MBE) using the two-substrate-temperatures (TST) technique. In the TST technique, the substrate temperature was set at TGaAsBi = 350 °C for GaAsBi layers and TGaAs = 550 °C for GaAs layers for the growth of MQWs. The structural and optical characterization of GaAsBi/GaAs MQWs using TST technique was carried out for growth optimization by changing As4/Ga beam equivalent pressure (BEP) ratio. Atomic force microscope observation shows high As4/Ga BEP ratio supply can prevents roughening of the GaAsBi layer surface of MQWs. High resolution x-ray diffraction analysis results reveal Bi incorporation enhancement with increasing As4/Ga BEP ratio. The reciprocal space mapping shows highly strained MQWs layers can be grown without relaxation with respect to the GaAs substrate at optimized growth conditions. At room temperature 1.22 μm photoluminescence emission was obtained from GaAs0.962Bi0.038/GaAs MQWs grown using TST technique.

GaAsSb/InAs core-shell nanowires grown by molecular-beam epitaxy

In this paper, we report for the first time the growth of GaAsSb/InAs core-shell nanowires (NWs) on Si (111) substrates by molecular-beam epitaxy. We first grow high-quality GaAsSb NWs as cores using self-catalyzed technique, then cover these cores by InAs shells. For the growth of InAs shells, we find that the optimum beam equivalent pressure ratio of As/In is only ∼5.2, but the growth temperature window is larger (from 460 to 540 °C). Also, the thickness of InAs shells increases almost linearly with extending the growth time. Because of the high-quality of GaAsSb cores, the InAs shells are continuous, smooth, and without stacking faults or twins adopted from the cores. Temperature dependent I-V curves measured from the field-effect transistor devices based on these GaAsSb/InAs core-shell NWs indicate that GaAsSb/InAs NWs without any intentional doping are electrically conductive. The growth of GaAsSb/InAs core-shell NWs opens up new possibilities for both fundamental studies and future device applications based on such natural p-n junctions.

Foreign-catalyst-free growth of InAs/InSb axial heterostructure nanowires on Si (111) by molecular-beam epitaxy

Epitaxial high-quality InAs/InSb axial heterostructure nanowires are of great interest due to their distinct advantages in fundamental research as well as applications in semiconductor electronic and quantum devices. Currently, nearly all the growth of InAs/InSb axial heterostructure nanowires is assisted with foreign catalysts such as Au, and work on foreign-catalyst-free growth of InAs/InSb axial heterostructure nanowires is lacking. Here we report on the growth of InAs/InSb axial heterostructure nanowires on Si (111) substrates by molecular-beam epitaxy without using any foreign catalysts. The Sb/In beam equivalent pressure (BEP) ratio is found to have important influence on the heterostructure nanowire morphology, and InSb nanowires can be epitaxially grown on InAs nanowire stems with a hexagonal prism and nanosheet-like shapes when the Sb/In BEP ratio varies from 10 to 20. Transmission electron microscopy studies reveal that the InAs nanowire stems have a mixture of zincblende (ZB) and wurtzite (WZ) crystal structures, while InSb nanowire parts have a pure ZB crystal structure free of stacking faults. Composition analysis of axial heterostructure nanowires provides clear evidence that the InSb nanowires are epitaxially grown on InAs nanowires in an In self-assisted vapor–liquid–solid manner. This study paves a new route for growing narrow-gap semiconductor heterostructures with strong spin–orbit interaction for the study of topological states, and the growth manner presented here is expected to be used to grow other In-based axial heterostructure nanowires.