Solid Source Molecular Beam Epitaxy Research Articles

Hole mobility in Strained Si/Relaxed SiGe/Si(110) hetero structures studied by gated Hall measurements

Strained Si/relaxed SiGe/Si(110) heterostructures are gaining interest because the high hole mobility of them are suitable for realization of high-performance Si-wafer-based CMOS devices. Because the theoretical hole effective mass of the (110)-oriented tensile-strained Si is less than half of that of the unstrained Si. The strained Si layer thickness is one of the parameters that determine the device characteristics, because the strain and the crystalline quality of the strained Si layer depends on it. In this study, the samples with various strained Si layer thicknesses (20–136 nm) were grown on SiGe/Si(110) structures using the solid source molecular beam epitaxy (SSMBE) method. The electrical characteristics were evaluated by the gated Hall measurements in which the Hall measurements were performed with applying gate bias voltage. We confirmed that the mobility and the density of the carriers measured in the gated Hall measurements correspond to those of the holes driven by the gate bias voltage. The hole mobility is the highest for the sample with the 20 nm thick strained Si and decreases with increasing the strained Si layer thickness. This dependence of the hole mobility can be explained as results of the reduction of the lattice strain and the increase of the crystalline defects as the strained Si layer thickness increases.

High electron mobility InP grown by solid source molecular beam epitaxy

High-quality InP is grown by solid source molecular beam epitaxy using white phosphorus source in a three-zone valved cracker cell. The growth temperature, growth rate and substrate desorption process have been adjusted to optimize the structural, electrical and optical properties. The photon and carrier scattering dynamics are investigated by temperature-dependent photoluminescence and electron Hall mobility. The 5-μm-thick sample presents stronger photoluminescence, solo carrier recombination scheme and inferior impurity scattering comparing to the 1-μm-thick sample. The high electron mobility of 137000 cm2/V·s at 77 K has been achieved for the 5-μm-thick InP layer.

Electronic and optical properties of highly boron-doped epitaxial Ge/AlAs(001) heterostructures

The impact of elemental boron (B) doping on the structural, optical, and magnetotransport properties of epitaxial Ge/AlAs/GaAs(001) heterostructures, grown by solid-source molecular beam epitaxy, was comprehensively investigated. Cross-sectional transmission electron microscopy analysis revealed atomically abrupt Ge:B/AlAs and AlAs/GaAs heterointerfaces and a lack of observable long-range defect formation or B segregation in the epitaxial Ge:B layer. Spectral broadening observed in the measured temperature-dependent photoluminescence spectra suggested valence band mixing during recombination, implying a splitting of the valence band heavy- and light-hole degeneracy due to residual strain resulting from substitutional B incorporation in the Ge epilayer. Temperature-dependent magnetotransport analysis of the B-doped Ge thin films exhibited the tell-tale signature of antilocalization, indicating observable spin–orbit interaction in the Ge:B system. Moreover, the temperature- and magnetic field-dependent magnetotransport results indicate the presence of single-carrier, p-type conduction in the Ge:B film, further affirming the successful incorporation and activation of B at a high concentration (∼4 × 1019 cm−3) and elimination of parallel conduction via the large-bandgap AlAs buffer. Together, these results provide insights into the effects of heavy doping (via elemental solid-source doping) on Ge-based heterostructures and their feasibility in future electronic and photonic applications.

1.73 eV AlGaAs/InGaP heterojunction solar cell grown by MBE with 18.7% efficiency

AbstractWe report on AlGaAs‐based heterojunction solar cells grown by solid source molecular beam epitaxy (MBE). We investigate InGaP and AlGaAs material quality and we demonstrate significant efficiency improvements by combining the best of each alloy: a thick p‐AlGaAs base with tunable bandgap, and a thin 50 nm n‐InGaP emitter separated by a thin intrinsic AlGaAs layer. We report a certified solar cell conversion efficiency of 18.7% with a 2‐μm‐thick AlGaAs layer and a bandgap of 1.73 eV, suitable for high efficiency Si‐based tandem devices.

Gas Sensitivity of In0.3Ga0.7As Surface QDs Coupled to Multilayer Buried QDs

A detailed analysis of the electrical response of In0.3Ga0.7As surface quantum dots (SQDs) coupled to 5-layer buried quantum dots (BQDs) is carried out as a function of ethanol and acetone concentration while temperature-dependent photoluminescence (PL) spectra are also analyzed. The coupling structure is grown by solid source molecular beam epitaxy. Carrier transport from BQDs to SQDs is confirmed by the temperature-dependent PL spectra. The importance of the surface states for the sensing application is once more highlighted. The results show that not only the exposure to the target gas but also the illumination affect the electrical response of the coupling sample strongly. In the ethanol atmosphere and under the illumination, the sheet resistance of the coupling structure decays by 50% while it remains nearly constant for the reference structure with only the 5-layer BQDs but not the SQDs. The strong dependence of the electrical response on the gas concentration makes SQDs very suitable for the development of integrated micrometer-sized gas sensor devices.

High-Performance Growth of Terahertz Quantum Cascade Laser Structures by Solid Source MBE

High performance growth of terahertz quantum cascade lasers (THz QCLs) based on hybrid bound-to-continuum transition and resonant phonon extraction is presented by solid source molecular beam epitaxy. The corresponding accurate control of layer thickness and alloy composition during growth, with precise calibration of the growth rate of Ga and Al by reflection high energy electron diffractometry and high resolution XRD techniques are studied in detail. By utilizing surface plasmon ridge waveguide device structures, the THz QCLs lasing at 3.3 THz with maximum powers of 426 mW at 10 K and 213 mW at 80 K are realized in pulsed mode and the maximum lasing temperature achieves 110 K.

Вертикально-излучающие лазеры спектрального диапазона 1.55 мкм, изготовленные по технологии спекания гетероструктур, выращенных методом молекулярно-пучковой эпитаксии из твердотельных источников

The GaAs-InGaAsP heterointerfaces formation have been studied and optimized using a direct intermolecular wafer bonding (fusion)of an active region heterostructure on an InP substrate and distributed Bragg reflector heterostructures on GaAs substrates for the fabrication of hybrid heterostructures of long-wave vertical-cavity surface-emitting lasers (VCSEL). The heterostructures were grown by solid-source molecular beam epitaxy. It was shown that in the case of incomplete removal of oxide films during the preparation of the wafers before fusion and/or the presence of adsorbed water on the wafer surfaces, the fused interface contains a large number of amorphous inclusions, most likely related to the III-group oxides. Optimization of the formation regimes of a buried tunnel junction on the surface of the InP-based heterostructure made it possible to reduce the surface roughness down to 1 nm and to ensure the thickness of the GaAs-InGaAsP fused interface < 5 nm, with no dislocations or other extended defects in the region of the fused heterointerfaces. The 1.55 μm-range VCSELs fabricated from the hybrid heterostructures created by using the developed technology demonstrate efficient lasing under continuous wave pumping over a wide temperature range, which indicates the high optical quality of the fused heterointerfaces in the VCSEL heterostructure.

High performance waveguide uni-travelling carrier photodiode grown by solid source molecular beam epitaxy.

The first waveguide coupled phosphide-based UTC photodiodes grown by Solid Source Molecular Beam Epitaxy (SSMBE) are reported in this paper. Metal Organic Vapour Phase Epitaxy (MOVPE) and Gas Source MBE (GSMBE) have long been the predominant growth techniques for the production of high quality InGaAsP materials. The use of SSMBE overcomes the major issue associated with the unintentional diffusion of zinc in MOVPE and gives the benefit of the superior control provided by MBE growth techniques without the costs and the risks of handling toxic gases of GSMBE. The UTC epitaxial structure contains a 300 nm n-InP collection layer and a 300 nm n++-InGaAsP waveguide layer. UTC-PDs integrated with Coplanar Waveguides (CPW) exhibit 3 dB bandwidth greater than 65 GHz and output RF power of 1.1 dBm at 100 GHz. We also demonstrate accurate prediction of the absolute level of power radiated by our antenna integrated UTCs, between 200 GHz and 260 GHz, using 3d full-wave modelling and taking the UTC-to-antenna impedance match into account. Further, we present the first optical 3d full-wave modelling of waveguide UTCs, which provides a detailed insight into the coupling between a lensed optical fibre and the UTC chip.

Indium antimonide photovoltaic cells for near-field thermophotovoltaics

Indium antimonide photovoltaic cells are specifically designed and fabricated for use in a near-field thermophotovoltaic device demonstrator. The optimum conditions for growing the p-n junction stack of the cell by means of solid-source molecular beam epitaxy are investigated. Then processing of circular micron-sized mesa structures, including passivation of the side walls, is described. The resulting photovoltaic cells, cooled down to around 77 K in order to operate optimally, exhibit excellent performances in the dark and under far-field illumination by thermal sources in the [600–1000] °C temperature range. A short-circuit current beyond 10 μA, open-circuit voltage reaching almost 85 mV, fill factor of 0.64 and electrical power at the maximum power point larger than 0.5 μW are measured for the cell with the largest mesa diameter under the highest illumination. These results demonstrate that these photovoltaic cells will be suitable for measuring a near-field enhancement of the generated electrical power.

Passively mode-locked semiconductor quantum dot on silicon laser with 400 Hz RF line width.

Mode-locked InAs/InGaAs quantum dot lasers emitting optical frequency combs centered at 1310 nm are promising sources for high-speed and high-capacity communication applications. We report on the stable optical pulse train generation by a monolithic passively mode-locked edge-emitting two-section quantum dot laser based on a five-stack InAs/InGaAs dots-in-a-well structure directly grown on an on-axis (001) silicon substrate by solid-source molecular beam epitaxy. Optical pulses as short as 1.7 ps at a pulse repetition rate or inter-mode beat frequency of 9.4 GHz are obtained. A minimum pulse-to-pulse timing jitter of 9 fs, corresponding to a repetition rate line width of 400 Hz, is demonstrated. The generated optical frequency combs yield exceptional low amplitude jitter performance and comb widths exceed 5.5 nm at a -3 dB criteria, containing more than 100 comb carriers.

Effects of nitrogen incorporation and thermal annealing on the optical and spin properties of GaPN dilute nitride alloys

Here, we report on the structural, optical and magneto-optical properties of as-grown and thermal annealed GaPN thin films containing different N concentrations grown by Solid-Source Molecular Beam Epitaxy on GaP substrates. A wide range of characterization techniques were used to investigate the effect of N incorporation and thermal annealing effect of GaPN/GaP epilayers for a N content between 0.5% and 3%. Our results have shown that as we increase the N content the PL peak energy presents a red shift which is explained by a band gap reduction. However, important effects of exciton localization on optical properties were also evidenced from cw and time resolved PL (PLRT) and magneto-PL. Our results have demonstrated that as the concentration of N is increased, exciton localization effect is enhanced. A substantial improvement of the optical quality and spin properties of the epilayers has been observed after thermal annealing which are significantly more important for samples with higher nitrogen contents.

Poisson ratio and bulk lattice constant of (Sr0.25La0.75)CrO3 from strained epitaxial thin films

About 30 nm thick (001)-oriented (Sr0.25La0.75)CrO3 (SLCO) epitaxial thin films were grown by solid-source oxide molecular beam epitaxy on four different single-crystalline cubic or pseudocubic (001)-oriented oxide substrates, LaAlO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, SrTiO3, and DyScO3, which result in lattice mismatch ranging from –2% to +1.7%. All the films are of high-quality, flat, and strained by the substrates. By assessing the evolution of the out-of-plane lattice parameter as a function of the in-plane lattice parameter of the samples, we determine both the Poisson ratio (ν = 0.32) and the bulk lattice constant (ab = 3.876 Å) of SLCO. The Poisson ratio significantly differs from LaCrO3 (ν = 0.23), and the (SrxLa1−x)CrO3 solid solution appears to obey structural Vegard's law. Since SLCO is the only one p-type transparent conductive oxide of perovskite structure and has promising thermoelectric properties, integrating SLCO in heterostructures and devices is, therefore, of paramount importance, which confers on our results their strong interest. Besides, the method used here can be straightforwardly applied to other complex oxides.

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.

The investigation of wafer-bonded multi-junction solar cell grown by MBE

We report on the room-temperature wafer-bonded InGaP/GaAs//InGaAsP/InGaAs four-junction solar cell grown by all solid-source molecular beam epitaxy (MBE). The material growth and structure optimization of multi-junction solar cell were investigated. The effect of growth temperature on phosphide-related material of the solar cell was discussed. The lattice-matched InGaP/GaAs tandem cell on GaAs substrate and InGaAsP- or InGaAsP/InGaAs-tandem cell on InP substrate were grown separately by MBE. Room-temperature wafer bonding technique was used to integrate the subcells into multi-junction solar cells. The current limiting of multi-junction cells was analysed and the device structure of four-junction solar cell was optimized. The best wafer-bonded four-junction solar cell reached a conversion efficiency of 42% under concentration.

Structural properties of strained epitaxial La1+δCrO3 thin films

Epitaxially strained La1+δCrO3 (LCO) thin films, with δ ranging between −0.25 (Cr-rich) and 0.25 (La-rich), were grown on SrTiO3(001) substrates by solid-source molecular beam epitaxy. The effect of the cationic-stoichiometry deviation (δ) on the structural properties (surface structure, morphology, terminations, and unit-cell parameters) is reported. Whereas deviation from stoichiometry does not significantly affect the structural quality (all films keep a perovskite structure, are epitaxial, atomically flat, fully strained, and present mosaicity below 0.1°); increasing |δ| leads to an increase of the out-of-plane lattice parameter and of the unit-cell volume. These results are of importance for further studies on structure–property relationships in the view to build enhanced LCO-based devices.

Molecular beam epitaxy growth and temperature-dependent electrical characterization of carbon-doped GaAs on GaAs(1 1 1)B

Here we present a study of the solid source molecular beam epitaxy growth of carbon doped GaAs(1 1 1)B oriented layers in a wide doping concentration range from 1016 to 1020cm-3. The layers have been fabricated on GaAs(1 1 1)B substrates with a 1° miscut towards (2 1 1) and carbon was supplied by a heated graphite filament source. Atomic force microscopy revealed smooth surfaces (root mean square roughness 0.3 nm) up to the highest carbon concentration. The carbon concentration was determined by secondary ion mass spectroscopy. All samples exhibit p-type conductivity and temperature-dependent Hall- and resistivity measurements show carrier freeze out for doping concentration in the 1016 cm−3 and 1017 cm−3 range, whereas we observe degenerated behavior for carbon concentrations above ca. 1×1019 cm−3. The hole mobility decreases with increasing carbon concentration. The acceptor ionization energy for carbon was determined via low-temperature photoluminescence measurements to 26.3 meV. From the free hole density determined from Hall-measurements at 300 K and the overall carbon concentration, we estimate that approximately 100% of the carbon atoms are electrically active acceptors for carbon concentrations below 1×1018 cm−3. For concentrations above 1×1019 cm−3 compensation sets in and the doping efficiency is well below 100%. For comparison, samples in the standard (1 0 0)-orientation have been produced and carbon incorporation behavior during growth is very similar.

Impact of Rotation Rate on Bismuth Saturation in GaAsBi Grown by Molecular Beam Epitaxy

GaAs1−xBix has been grown by solid-source molecular beam epitaxy using varying substrate rotation rates. Changes in local bismuth saturation were studied by varying the Bi/Ga pressure ratio across the wafer. Films were grown on both GaAs and InGaAs buffer layers with varying indium content to change the strain conditions of the bismide layer and the out-of-plane growth rate. All samples demonstrated vertical composition modulations with a period of ∼ 4 nm that tracked with the rate of growth per substrate rotation cycle. The thermal stability of these composition modulations was shown to behave similarly to bulk GaAsBi. Bismide composition modulations are attributed to the low growth temperature and the varying Bi/Ga pressure ratio across the sample rather than the varying V/III ratio.

Structure Design and Analysis of 2 μm InGaAsSb/AlGaAsSb Muti-Quantum Well Laser Diode with Carrier Blocking Layer

A low threshold current density of 2 μm InGaAsSb/AlGaAsSb muti-quantum well (MQW) laser diode with carrier blocking layer (CBL) is demonstrated by simulation and fabrication. The carrier leakage is found to be theoretically suppressed for the devices with CBL. All the laser wafers are grown with a solid source Molecular Beam Epitaxy(MBE) System. Experimental results reveal the samples with CBL exhibits ultra-low threshold current densities of 142 A/cm2 and high slope efficiency of 0.158 W/A, which is better than 215 A/cm2 and 0.122 W/A achieved in the conventional InGaAsSb/AlGaAsSb LDs at room temperature. This improvement in device performance comes from meticulously designing the carrier blocking layers to increase carrier confinement and injection efficiency.

Growth optimization and characterization of regular arrays of GaAs/AlGaAs core/shell nanowires for tandem solar cells on silicon

With a band gap value of 1.7 eV, Al0.2Ga0.8As is one of the ideal III–V alloys for the development of nanowire-based Tandem Solar Cells on silicon. Nevertheless, growing self-catalysed AlGaAs nanowires on silicon by solid-source molecular beam epitaxy is a very difficult task due to the oxidation of Al adatoms by the SiO2 layer present on the surface. Here we propose a nanowire structure including a p.i.n radial junction inside an Al0.2Ga0.8As shell grown on a p-GaAs core. The crystalline structure of such self-catalysed nanowires grown on an epi-ready Si(111) substrate (with a thin native SiO2 layer) was investigated by transmission electronic microscopy and photoluminescence. I(V) measurements performed on single nanowires have shown a diode-like behaviour corresponding to the radial p.i.n junction inside the Al0.2Ga0.8As shell. Moreover, a current generation under the electron beam was evidenced over the entire radial junction along the nanowires by means of electron beam induced current (EBIC) microscopy. The same structure was reproduced on patterned substrates with a SiO2 mask, producing an ordered hexagonal array. High and uniform yields from 83% to 87% of vertical nanowires were obtained on 0.9 × 0.9 cm2 patterned areas. EBIC mapping performed on these nanowires confirmed the good electrical properties of the radial junction within the nanowires.

Anomalous arsenic diffusion at InGaAs/InP interface

The anomalous arsenic (As) diffusion at the In0.53Ga0.47As/InP interface grown by solid source molecular beam epitaxy has been reported in this work. Heterointerface samples were grown using group-V switches of P2 to As2 or to As4. High-resolution x-ray diffraction (XRD), photoluminescence (PL), 3D atom probe (3DAP) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) data have shown that, an up to hundred-nanometer-thick composition-graded InAsyP1-y layer formed in the InP buffer layer for the sample grown by P2 to As2. This observation demonstrates a rapid replacement of P by As atoms at the heterointerface and a deep As diffusion into the lower InP buffer. In contrary, a sharper heterointerface without the composition-graded layer was obtained in the sample grown using P2 to As4. These findings give insight into the distinct properties of different As species in the InGaAs/InP interface growth.