Gas Source Molecular Beam Epitaxy Research Articles

Photoconductive terahertz detectors with 105 dB peak dynamic range made of rhodium doped InGaAs

Rhodium (Rh)-doped In0.53Ga0.47As grown by gas-source molecular beam epitaxy is investigated as a terahertz (THz) detector antenna for optical excitation at 1550 nm. The 4d transition metal rhodium acts as a deep level and ultrafast trapping center. At a doping concentration around 8 × 1019 cm−3, InGaAs:Rh exhibits ideal properties for application as a THz antenna: an ultrashort carrier lifetime below 200 fs in combination with a mobility of 1010 cm2/Vs. The THz detectors fabricated from this sample show a record peak dynamic range of 105 dB and a bandwidth of up to 6.5 THz.

Impact of size distributions of Ge islands as etching masks for anisotropic etching on formation of anti-reflection structures

The impact of the size distributions of Ge islands on the structural and optical characteristics of anti-reflection structures was investigated. The variation of island size distribution was achieved through the variation of growth temperature in gas-source molecular beam epitaxy at 700 °C–800 °C. Ge islands were utilized as etching masks to form the anti-reflection structures. By using lower growth temperature of 700 °C and subsequent etching, larger texture without many hollows was formed in contrast to that using 800 °C. Broader size distribution of islands formed at 700 °C was found to lead to larger texture size. Smaller texture is formed by smaller islands and short etching, whereas larger texture is formed by erosion of smaller texture during long etching. A potential short-circuit current density of 36.75 mA cm−2 was obtained for the sample by the islands grown at 700 °C with reduced etching margins, comparable to that of conventional pyramid textures.

Mid-infrared type-I InAs/In0.83Al0.17As quantum wells grown on GaP and InP by gas source molecular beam epitaxy

Type-I InAs quantum wells have been grown on metamorphic In0.83Al0.17As buffers on (0 0 1)-oriented GaP and InP substrates by gas source molecular beam epitaxy. The structural and optical properties as well as strain conditions were characterized and investigated to compare the material quality of samples grown on GaP and InP substrates, respectively. Photoluminescence up to 2.8 μm has been observed for the quantum wells at room temperature. The quantum wells on GaP and InP show the similar photoluminescence intensity, X-ray diffraction peaks and Raman shift performances. These results indicate the potential to demonstrate type-I mid-infrared light sources on Si substrate using GaP as a template.

Formation of light-trapping structure using Ge islands grown by gas-source molecular beam epitaxy as etching masks

A high-performance light-trapping structure for Si was fabricated with an etching margin of only ∼1 µm using Ge islands grown by gas-source molecular beam epitaxy as etching masks. KOH solution containing isopropyl alcohol and HF + H2O2 + CH3COOH mixed solution were used as etchants. The reflectance of the structure was shown to be comparable to that of a conventional pyramid texture, which requires a larger etching margin of ∼10 µm. In addition, a potential short-circuit current density (p-Jsc) of 42.3 mA/cm2 was obtained for the sample after the deposition of indium tin oxide, which confirms that the light-trapping structure is applicable to crystalline Si solar cells with thinner wafers.

Transport properties of doped AlP for the development of conductive AlP/GaP distributed Bragg reflectors and their integration into light-emitting diodes

The transport properties of n- and p-doped AlP layers grown by gas-source molecular beam epitaxy are investigated. n- and p-types of conductivities are achieved using Si and Be with peak room-temperature mobilities of 59.6 cm2/Vs and 65.0 cm2/Vs for electrons and holes, respectively. Si-doping results are then used for the design of n-doped AlP/GaP distributed Bragg reflectors (DBRs) with an ohmic resistance of about 7.5 ± 0.1 Ω. The DBRs are integrated as bottom mirrors in GaP-based light-emitting diodes (LEDs) containing InGaP/GaP quantum dots. The functionality of the LED structure and the influence of the DBRs on the InGaP/GaP electroluminescence spectra are demonstrated.

MoS2 synthesis by gas source MBE for transition metal dichalcogenides integration on large scale substrates

We present in this paper the use of Gas Source Molecular Beam Epitaxy for the large-scale growth of transition metal dichalcogenides. Fiber-textured MoS2 co-deposited thin films (down to 1 MLs) are grown on commercially 200 mm wafer size templates where MX2 crystalline layers are achieved at temperatures ranging from RT to 550 °C. Raman Spectroscopy and photoluminescence measurements along with X-Ray Photoelectron Spectroscopy show that a low growth rate is essential for complete Mo sulfurization during MoS2 co-deposition. Finally, cross-section Transmission Electron Microscopy investigations are discussed to highlight the influence of SiO2 and Al2O3 used surfaces on MoS2 deposition.

Tuning the morphology of self-assisted GaP nanowires

Patterned arrays of self-assisted GaP nanowires (NWs) were grown on a Si substrate by gas source molecular beam epitaxy using various V/III flux ratios from 1–6, and various pitches from 360–1000 nm. As the V/III flux ratio was increased from 1–6, the NWs showed a change in morphology from outward tapering to straight, and eventually to inward tapering. The morphologies of the self-assisted GaP NWs are well described by a simple kinetic equation for the NW radius versus the position along the NW axis. The most important growth parameter that governs the NW morphology is the V/III flux ratio. Sharpened NWs with a stable radius equal to only 12 nm at a V/III flux of 6 were achieved, demonstrating their suitability for the insertion of quantum dots.

Effects of growth conditions on optical quality and surface morphology of InGaAsBi**Project supported by the National Natural Science Foundation of China (Grant No. 61434006) and the National Basic Research Program of China (Grant No. 2014CB643902).

The effects of Bi flux and pressure of AsH3 on Bi incorporation, surface morphology and optical properties of InGaAsBi grown by gas source molecular beam epitaxy are studied. It is found that using relatively low pressure of AsH3 and high Bi flux can strengthen the effect on the incorporation of Bi and increase its content linearly with Bi flux until it nearly reaches a saturation value. The result from Rutherford backscattering spectroscopy (RBS) confirms that the Bi incorporation can increase up to 1.13%. By adjusting Bi and As flux, we could improve the surface morphology of InGaAsBi sample. Room temperature photoluminescence shows strong and broad light emission at energy levels much smaller than the InGaAs bandgap.

Growth temperature optimization of GaAs-based In0.83Ga0.17As on InxAl1−xAs buffers

Improved quality of gas source molecular beam epitaxy grown In0.83Ga0.17As layer on GaAs substrate was achieved by adopting a two-step InxAl1−xAs metamorphic buffer at different temperatures. With a high-temperature In0.83Al0.17As template following a low-temperature composition continuously graded InxAl1−xAs (x = 0.05–0.86) buffer, better structural, optical and electrical properties of succeeding In0.83Ga0.17As were confirmed by atomic force microscopy, photoluminescence and Hall-effect measurements. Cross-sectional transmission electron microscopy revealed significant effect of the two-step temperature grown InAlAs buffer layers on the inhibition of threading dislocations due to the deposition of high density nuclei on GaAs substrate at the low growth temperature. The limited reduction for the dark current of GaAs-based In0.83Ga0.17As photodetectors on the two-step temperature grown InxAl1−xAs buffer layers was ascribed to the contribution of impurities caused by the low growth temperature of InAlAs buffers.

Self-catalyzed core-shell GaAs/GaNAs nanowires grown on patterned Si (111) by gas-source molecular beam epitaxy

We report structural studies on the epitaxial growth of GaAs/GaNAs core-shell nanowires (NWs) on patterned Si (111) substrates by self-catalyzed selective area growth using Gas-Source Molecular Beam Epitaxy. Epitaxial growth conditions were obtained using a combination of dry and time-sensitive wet etching of the SiO2 growth mask and native SiO2 layer, respectively. We found that higher growth temperatures resulted in a higher yield for the epitaxial growth of patterned self-catalyzed GaAs NWs on Si with an optimal temperature of 690 °C. The GaNAs shell growth at 500 °C was found to be conformal and maintained an epitaxial and dislocation-free interface with both the Si substrate and the GaAs nanowire. The micro-photoluminescence (μ-PL) measurement at 6 K revealed two bands peaking at 1.45 and 1.17 eV, which could be emission from the GaAs core and GaNAs shell. Transmission electron microscopy showed the zincblende crystal structure of GaAs and GaAs/GaNAs core-shell NWs with minimal twinning near the base of the GaAs nanowires and at the tips of the GaAs/GaNAs core/shell nanowires. This study illustrates the feasibility of the epitaxial growth of patterned GaAs with dilute nitride shells on Si substrates, which would have potential for Si-friendly intermediate band solar cells and telecom emitters.

AlN grown on Si(1 1 1) by ammonia-molecular beam epitaxy in the 900–1200 °C temperature range

We present a comprehensive study of AlN growth on Si(111) substrate by gas source molecular beam epitaxy with ammonia as nitrogen precursor in the high temperature range. We first demonstrate that the observation of the silicon 7×7 surface reconstruction by reflection high energy electron diffraction can be misleading as this technique is not sensitive to low density surface defects like SiC crystallites. A careful in situ cleaning procedure with annealing cycles at 1100°C allows getting rid of any surface defects, as shown by atomic force microscopy imaging. Then, we explore the effect of the growth temperature on the surface morphology and structural properties of 100nm thick AlN epilayers. At 1200°C, the growth proceeds with the step flow mode regime, which induces spiral-growth around screw-type dislocations and therefore surface roughening. On the other hand, a smooth surface morphology can be achieved by setting the temperature at 1100°C, which corresponds to the growth mode transition from two-dimensional nucleation to step flow. A further decrease of the growth temperature to 900°C leads to surface defects ascribed to polarity inversion domains. Similar defects are observed for growths performed at 1100°C when the NH3 flow is reduced below 100sccm. This points out the sensitivity of AlN to the surface stoichiometry.

Behaviors of beryllium compensation doping in InGaAsP grown by gas source molecular beam epitaxy

We report structural properties as well as electrical and optical behaviors of beryllium (Be)-doped InGaAsP lattice-matched to InP grown by gas source molecular beam epitaxy. P type layers present a high degree of compensation on the order of 1018 cm−3, and for Be densities below 9.5×1017 cm−3, they are found to be n type. Enhanced incorporation of oxygen during Be doping is observed by secondary ion mass spectroscopy. Be in forms of interstitial donors or donor-like Be-O complexes for cell temperatures below 800°C is proposed to account for such anomalous compensation behaviors. A constant photoluminescence energy of 0.98 eV without any Moss-Burstein shift for Be doping levels up to 1018 cm−3 along with increased emission intensity due to passivation effect of Be is also observed. An increasing number of minority carriers tend to relax via Be defect state-related Shockley-Read-Hall recombination with the increase of Be doping density.

Highly Ordered GeSi Quantum Dots Fabrication on Patterned Si Substrates with Gas Source Molecular Beam Epitaxy

Highly Ordered GeSi Quantum Dots Fabrication on Patterned Si Substrates with Gas Source Molecular Beam Epitaxy

GaP/GaNP Heterojunctions for Efficient Solar-Driven Water Oxidation.

The growth and characterization of an n-GaP/i-GaNP/p+ -GaP thin film heterojunction synthesized using a gas-source molecular beam epitaxy (MBE) method, and its application for efficient solar-driven water oxidation is reported. The TiO2 /Ni passivated n-GaP/i-GaNP/p+ -GaP thin film heterojunction provides much higher photoanodic performance in 1 m KOH solution than the TiO2 /Ni-coated n-GaP substrate, leading to much lower onset potential and much higher photocurrent. There is a significant photoanodic potential shift of 764 mV at a photocurrent of 0.34 mA cm-2 , leading to an onset potential of ≈0.4 V versus reversible hydrogen electrode (RHE) at 0.34 mA cm-2 for the heterojunction. The photocurrent at the water oxidation potential (1.23 V vs RHE) is 1.46 and 7.26 mA cm-2 for the coated n-GaP and n-GaP/i-GaNP/p+ -GaP photoanodes, respectively. The passivated heterojunction offers a maximum applied bias photon-to-current efficiency (ABPE) of 1.9% while the ABPE of the coated n-GaP sample is almost zero. Furthermore, the coated n-GaP/i-GaNP/p+ -GaP heterojunction photoanode provides a broad absorption spectrum up to ≈620 nm with incident photon-to-current efficiencies (IPCEs) of over 40% from ≈400 to ≈560 nm. The high low-bias performance and broad absorption of the wide-bandgap GaP/GaNP heterojunctions render them as a promising photoanode material for tandem photoelectrochemical (PEC) cells to carry out overall solar water splitting.

InGaAsBi materials grown by gas source molecular beam epitaxy

In this work, the In1−yGayAs1−xBix epilayers were optimized and grown successfully by V90 gas source molecular beam epitaxy (GSMBE). The effect of using an In0.53Ga0.47As buffer layer on the crystalline quality of In1−yGayAs1−xBix epilayers were investigated. With the buffer layer preparation, the surface RMS roughness value of 0.251nm, a maximum electron mobility of 5700cm2/Vs and a bulk carrier density of 2.9×1016cm3 are achieved for the In1−yGayAs1−xBix epilayer with the Bi content up to 3.1%.

Low-temperature growth of graphene on iron substrate by molecular beam epitaxy

Graphene has attracted a great deal of interest due to its fascinating properties and a wide variety of potential applications. Several methods have been used to achieve high-quality graphene films on different substrates. However, there have been only a few studies on graphene growth on iron (Fe) and the growth mechanism remains unclear. This paper systematically investigates temperature-dependent growth of graphene on Fe substrate by gas-source molecular beam epitaxy. Two-dimensional (2D), large-area graphene samples were grown on Fe thin films, and characterized by Raman, X-ray photoelectron spectroscopy, X-ray diffraction, optical microscopy, transmission electron microscopy and atomic force microscopy. It is found that graphene flakes can be grown on Fe at a growth temperature as low as 400°C and the optimized large-area graphene growth temperature is relatively low between 500°C and 550°C. The graphene growth on Fe that undergoes the formation and decomposition of iron carbide is discussed.

Growth of GaP and AlGaP on GaP(1 1 1)B using gas-source molecular-beam-epitaxy

We present an initial study of the influence of the growth parameters on the surface morphology and on the interface quality of homoepitaxial GaP(111) and heteroepitaxial GaP/AlGaP(111) grown on GaP(111)B substrates using Gas-Source Molecular Beam Epitaxy (GSMBE). Three different surface reconstructions are identified in the RHEED patterns during the growth runs. The Root Mean Square (RMS) surface roughness measured post-growth by AFM ranges from 3 to 10nm over 10×10µm2 areas, for a film thickness of 100–600nm. The results of 2θ-ω XRD scans on (111) and (311) planes reveal a stacking disorder in the AlGaP layer and further XRD phi-scan measurements on GaP (311) show strong peaks with 3-fold rotational symmetry and additional of 3-fold weak peaks indicating only a negligible fraction of the twinned crystal orientation in the substrate. TEM images of these samples show a smooth interface between the AlGaP layer and GaP substrate, and reveal the presence of a high density of extended defects such as stacking faults, twinning and dislocations lines in AlGaP layer whereas the GaP layer appears as pure Zinc-Blende. Further TEM analysis reveals composition and local strain variations for GaP/AlGaP samples associated with an undulated surface.

Growth and electrical properties of high-quality InGaAsBi thin films using gas source molecular beam epitaxy

The effects of Bi flux and In/Ga ratio on Bi incorporation and electrical properties of InGaAsBi grown by gas source molecular beam epitaxy were systematically studied. It is found that use of a low In/Ga ratio has an enhancement effect on the incorporation of Bi and its content increases linearly with Bi flux until reach a saturation. Incorporation of Bi induces p-type dopant that compensates the background electron concentration but does not degrade the electron mobility for the Bi content up to 6.2%. Up to 7.5% of Bi incorporation has been confirmed by Rutherford backscattering spectroscopy (RBS) and a maximum electron mobility of 5600 cm2·V−1·s−1 at room temperature was achieved in InGaAsBi with xBi = 6.2%, which is the highest value reported in InGaAsBi with xBi > 5%.

Abnormal variation of the growth rate under high NH3 injected regime in the growth of GaN by NH3-source MBE

Unusual growth-rate variation during GaN formation using gas-source MBE has been discussed with respect to the chemical reactions occurring in the transition layer. A series of samples were prepared to confirm the assumption by verifying the growth regime and the impacts on the crystal quality of the GaN film. We found that the growth rate can be varied along with the amount of NH3 supply even under NH3-rich condition with a fixed Ga flux. Two growth conditions were investigated for their impact on the transition layer. One was the atomic force microscopy result, which revealed that the adatom migration length is closely related to the transition layer formation. The other one is the photoluminescent spectra, which revealed that the luminescence property of GaN is strongly related to the transition layer.

Thermal annealing of lattice-matched InGaAs/InAlAs Quantum-Cascade Lasers

We describe the evolution of optical power, threshold current, and emission wavelength of a lattice-matched InGaAs/InAlAs Quantum-Cascade Laser (QCL) emitting at 13μm grown by gas-source molecular-beam epitaxy under thermal annealing. Pieces from the same 2-in wafer were annealed at 600°C, 650°C, or 700°C for 1h; one control piece remained unannealed. No change in threshold current and emission wavelength was observed. The slope efficiency and maximum emission power increase for the 600°C anneal, but higher annealing temperatures resulted in degraded performance. This result stands in contrast with the observation that strain-compensated structures cannot withstand annealing temperature of 600°C. Useful information for post-growth processing steps and the role of interface roughness in QCL performance are obtained.