MBE Technique Research Articles

Raman scattering spectroscopy of MBE grown thin film topological insulator Bi2-xSbxTe3-ySey.

BSTS epitaxial thin film topological insulators were grown using the MBE technique on two different types of substrates i.e., Si (111) and SiC/graphene with Bi0.7Sb1.6Te1.8Se0.9 and Bi0.9Sb1.5Te1.8Se1.1, respectively. The crystallographic properties of BSTS films were investigated via X-ray diffraction, which showed the strongest reflections from the (0 0 l) facets corresponding to the rhombohedral phase. Superior epitaxial growth, homogeneous thickness, smooth surfaces, and larger unit cell parameters were observed for the films grown on the Si substrate. Polarization dependent Raman spectroscopy showed a weak appearance of the Ag mode in cross--polarized geometry. In contrast, a strong Eg mode was observed in both parallel and cross-polarized geometries which correspond to the rhombohedral crystal symmetry of BSTS films. A redshift of Ag and Eg modes was observed in the Raman spectra of BSTS films grown on the Si substrate, compared to those on SiC/graphene, which was directly associated with the unit cell parameter and composition of the films. Raman spectra showed four fundamental modes with asymmetric line shape, and deconvolution of the peaks resulted in additional modes in both the BSTS thin films. The sum of relative ratios of linewidths of fundamental modes (Ag and Eg) of BSTS films grown on Si substrate was lower, indicating a more ordered structure with lower contribution of defects as compared to BSTS film grown on SiC/graphene substrate.

Towards Efficient Electrically-Driven Deep UVC Lasing: Challenges and Opportunities

The major issues confronting the performance of deep-UV (DUV) laser diodes (LDs) are reviewed along with the different approaches aimed at performance improvement. The impact of threading dislocations on the laser threshold current, limitations on heavy n- and p-doping in Al-rich AlGaN alloys, unavoidable electron leakage into the p-layers of (0001) LD structures, implementation of tunnel junctions, and non-uniform hole injection into multiple quantum wells in the active region are discussed. Special attention is paid to the current status of n- and p-type doping and threading dislocation density reduction, both being the factors largely determining the performance of DUV-LDs. It is shown that most of the above problems originate from intrinsic properties of the wide-bandgap AlGaN semiconductors, which emphasizes their fundamental role in the limitation of deep-UV LD performance. Among various remedies, novel promising technological and design approaches, such as high-temperature face-to-face annealing and distributed polarization doping, are discussed. Whenever possible, we provided a comparison between the growth capabilities of MOVPE and MBE techniques to fabricate DUV-LD structures.

Structural insight using anomalous XRD into Mn2CoAl Heusler alloy films grown by magnetron sputtering, IBAS, and MBE techniques

Inverse Heusler alloy Mn2CoAl thin films, known as a spin-gapless semiconductor (SGS), grown by three different methods—ultra-high vacuum magnetron sputtering, Ar-ion beam assisted sputtering, and molecular beam epitaxy—are investigated by comparing their electric transport properties, microstructures and atomic-level structures. Of the samples, the Mn2CoAl thin film grown by molecular beam epitaxy consists of Mn- and Co-rich phases, the structures of which are determined to be the L21B-type and disordered L21-type, respectively, according to anomalous X-ray diffraction analysis. None of them forms the XA-type structure expected for SGS Heusler alloy, although they all exhibit SGS characteristics. To validate the SGS characteristics, it is necessary to extract not only the magnetic and electric transport properties but also information about microstructures and atomic-scale structures of the films including defects such as atomic swap.

Electroluminescence study of InGaN/GaN QW based p-i-n and inverted p-i-n junction based short-wavelength LED device using laser MBE technique

In the present work, the Laser Molecular Beam Epitaxy (Laser MBE) technique has been used for the fabrication of InGaN/GaN quantum well LEDs. A comparative study was performed for analysing the performance of LEDs with two different device structures i.e conventional (p-GaN/QW/i-GaN/n-GaN/Substrate) and inverted (n-GaN/QW/i-GaN/p-GaN/Substrate). The fabricated QW based Inverted p-i-n junction and conventional (p-i-n) LEDs exhibited non-linear rectifying current-voltage characteristics with a forward threshold voltage of about 0.7 V and 0.5 V, respectively. Room temperature electroluminescence was observed with increasing injection current in both conventional and inverted junction LED, where LEDs with 5 QW periods showed carrier localization effect and higher external quantum efficiency (EQE) at fixed injection current. The emission in fabricated QW LEDs was found to exhibit a spectral blue shift with increasing injection current. Convention LEDs (topmost layer: p-type GaN) exhibited higher luminous efficiency (i.e., 2.83 lm/W) than the Inverted LEDs (top most layer: n-type GaN) luminous efficiency (i.e., 1.56 lm/W). The efficiency droop observed is 14% in the conventional and 8% in the inverted LED device, respectively. This study focuses on the demonstration of room temperature electroluminescence in InGaN/GaN based LEDs grown using Laser MBE technique. The successful utilization of Laser MBE technique for growth of InGaN/GaN quantum wells can be a revolutionary step towards the establishment of a reliable, cost-effective, and ammonia-free process for fabrication of LED.

Double Schottky metal–semiconductor–metal based GaN photodetectors with improved response using laser MBE technique

Double Schottky metal–semiconductor–metal based GaN photodetectors with improved response using laser MBE technique

The study of the AlN/Si(111) epitaxial structures grown by PA MBE via coalescence overgrowth of AlN nanocolumns

The AlN/Si(111) epitaxial structures were synthesized by coalescence overgrowth of AlN nanocolumns using PA MBE technique. Such epitaxial structures can be used as a buffer layer for obtaining high quality AlN and GaN layers. Structural, electrical and chemical properties of these samples were studied. For the first time it was demonstrated that the etching of the obtained type of AlN/Si(111) structures in KOH can become a promising method for obtaining high quality free-standing AlN and GaN.

Mg3N2 nanocrystallites formation during the GaN:Mg layers growth by the NH3-MBE technique

The work is devoted to the study of p-GaN: Mg epitaxial layers grown by the ammonia MBE technique. We find that the conductivity of GaN layers doped with Mg does not change with a postgrowth heat treatment. Formation of Mg3N2 nanocrystallites on GaN surface during epitaxial growth of the GaN layer with a high magnesium doping level was detected by the RHEED technique for the first time. It was shown that the Mg3N2 nanocrystallites formation competes with the acceptor states formation process. It has been proposed that the growth temperature can be applied as an additional “tuning” mechanism which affects the Mg incorporation into the growing GaN:Mg layers.

Mn5Ge3 ultra-thin films on GaAs (111)B substrates: Influence of initial growth conditions

We report the interaction of Mn 5 Ge 3 ultra-thin films with GaAs (111)B substrates under distinct initial conditions by monitoring in-situ the in-plane crystal structure and surface chemistry using reflection high energy electron diffraction and X-ray photoelectron spectroscopy. The order of Mn and Ge adatoms arriving on the GaAs surface determines the interface elemental stoichiometry , crystalline quality, and films thermal stability. When the growth starts by opening the Mn and Ge cells simultaneously is sufficient to inhibit the strong reactions with the GaAs (111) surface. First-principles calculations show that the Mn 5 Ge 3 epitaxial growth on the GaAs (111)B is feasible, with a lattice mismatch of 2.3% which is in agreement with the value of at least 2.6% estimated within the accuracy of our RHEED measurements. Results have implications in the design of MBE growth strategies to achieve Mn 5 Ge 3 /GaAs semiconductor heterostructure . • Mn–Ge deposit on GaAs. • MBE technique for deposit. • First principles calculations.

Use of quantum effects as potential qualifying metrics for "quantum grade silicon".

Across solid state quantum information, materials deficiencies limit performance through enhanced relaxation, charge defect motion or isotopic spin noise. While classical measurements of device performance provide cursory guidance, specific qualifying metrics and measurements applicable to quantum devices are needed. For quantum applications, new materials metrics, e.g., enrichment, are needed, while existing, classical metrics like mobility might be relaxed compared to conventional electronics. In this work, we examine locally grown silicon superior in enrichment, but inferior in chemical purity compared to commercial-silicon, as part of an effort to underpin the materials standards needed for quantum grade silicon and establish a standard approach for intercomparison of these materials. We use a custom, mass-selected ion beam deposition technique, which has produced isotopic enrichment levels up to 99.99998 % 28Si, to isotopically enrich 28Si, but with chemical purity > 99.97% due the MBE techniques used. From this epitaxial silicon, we fabricate top-gated Hall bar devices simultaneously on the 28Si and on the adjacent natural abundance Si substrate for intercomparison. Using standard-methods, we measure maximum mobilities of at an electron density of cm-2 and at an electron density of cm-2 at K for devices fabricated on 28Si and natSi, respectively. For magnetic fields T, both devices demonstrate well developed Shubnikov-de Haas (SdH) oscillations in the longitudinal magnetoresistance. This provides transport characteristics of isotopically enriched 28Si, and will serve as a benchmark for classical transport of 28Si at its current state, and low temperature, epitaxially grown Si for quantum devices more generally.

In-situ and post deposition analysis of laser MBE deposited GaN films at varying nitrogen gas flow

Highly crystalline c-axis oriented GaN thin films have been realized on c-plane sapphire substrate using Laser MBE technique. The deposition was carried out under the influence of nitrogen gas plasma at varying nitrogen gas flow to study the modifications in the structural, surface morphological and electrical properties of the as grown films. The GaN films were in-situ analysed using Reflection High Energy Electron Diffraction (RHEED) technique and X-ray diffraction technique and AFM imaging were utilized for the structural and morphological analysis of prepared thin films. The grown films were utilized for the realization of p-n junction structures upon integration with p-type Mg:GaN. IV characteristics revealed rectifying diode like behaviour for the GaN p-n junction structures grown at lower gas pressures, which is highly promising for the development of LEDs using Laser MBE technique.

Surface evolution of native silicon oxide layer and its effects on the growth of self-assisted VLS GaAs nanowires

This research focuses on the impact of native SiO2 layer evolution, occurring on the surfaces of Si(111) substrates on the morphological and structural properties of self-assisted GaAs nanowires. GaAs nanowires growth were grown on Si(111) substrates, already covered with native SiO2 developing in different states with identical growth parameters including growth temperature, growth time, and Ga and As flux, using self-assisted VLS process by MBE technique. Results from nanowire samples and substrates were compared to understand the correlation between the changes in appearance of native SiO2 layer and the changes in the growth pattern of the nanowires.

Influence of substrate temperature on incorporation of magnesium into Zn1-xMgxO layers growth by molecular beam epitaxy

Plasma-assisted molecular beam epitaxy growth of ZnO and Zn1-xMgxO epilayers on Al2O3 (11–20) substrates is described. The influence of the substrate temperature on the growth rate and magnesium concentration in Zn1-xMgxO epilayers was studied in detail. It was found that the composition of magnesium in Zn1-xMgxO epilayers increased with increasing substrate temperature. The composition variation in the alloys as a function of the growth temperature could be explained by the difference of vapor pressure of Mg and Zn at high growth temperature. Theoretical approaches to surface evaporation of metal atoms under different growth condition are difficult. Here, we describe an uncomplicated method determine quantities experimental to obtain. These results can be extended to other types of ternary or quaternary materials grown by MBE technique.

Developing of dual junction GaInP/GaAs solar cell devices: effects of different metal contacts

DJ GaInP/GaAs SC structure was designed by using analytical solar cell model. The electrical parameters (Jsc, Voc, FF and η) were calculated by determining optimum conditions for improving the performance of the SCs. Considering the optimization conditions in design of SC, lattice and current matched DJ GaInP/GaAs SC structure was grown using MBE technique. Alloy composition and lattice constants of each layers in the structure were estimated from measured XRD data. To evaluate of effects on conversion efficiency of different metal contact materials, SC devices were fabricated by photolithographic technique. Two types of front-side electrodes, which included Au and Au/Ti metals, were separately fabricated on devices and denoted as S1 and S2, respectively. Performance of the S1 and S2 were determined using I–V measurements under the AM1.5 illuminations. S2 possesses 4.16% enhancement in conversion efficiency compared to that of S1. The better performance of the S2 can be attributed to having higher Isc and Voc due to higher conductivity of titanium as well as good adhesion on GaAs. In addition, Al2O3/TiO2 anti-reflective coating effect on performance of the S1 and S2 was also investigated. Sputtered anti-reflective layer increased the efficiencies from 14.65 to 15.72% and 15.26 to 16.90% for S1 and S2, respectively.

Phase separation suppression in InxGa1−xN on a Si substrate using an indium modulation technique

A high quality, single phase InGaN film is fabricated on a GaN/Si (111) substrate by optimizing the pulse patterned In supply with a plasma-assisted MBE technique. Compositional phase separation in InGaN is considerably suppressed. The optical and structural properties of the single phase InGaN epitaxial film are consistently confirmed by atomic force microscopy, X-ray diffraction and photoluminescence measurements. We propose a growth mechanism for single phase InGaN in terms of optimal incorporation and surface migration of In atoms.

Migration mechanisms and diffusion barriers of carbon and native point defects in GaN

Carbon related defects are readily incorporated in $\mathrm{GaN}$ due to its abundance during growth both with MBE and MOCVD techniques. Employing first-principles calculations, we compute the migration barriers of carbon interstitials and we discuss possible relevant mechanisms of diffusion in the wurtzite $\mathrm{GaN}$ crystal. In addition, we calculate the migration barriers for the diffusion of the native defects of the crystal, i.e., gallium and nitrogen interstitials and vacancies. The minimum energy path and the migration barriers of these defects are obtained using the nudged elastic band method with the climbing image modification. In addition, the dimer method is used to independently determine the results. The results yield a quantitative description of carbon diffusion in $\mathrm{GaN}$ allowing for the determination of the most preferable migration paths.

The impact of RF-plasma power in carrier relaxation dynamics of unintentional doped GaN epitaxial layers grown by MBE

In this work, unintentionally doped GaN samples were prepared on GaN template by radio frequency (RF)-plasma MBE technique using two different RF-plasma powers. Photoluminescence (PL), steady state photoconductivity (PC) and ultrafast optical pump-probe spectroscopy measurements have been carried out to characterize the samples. The effect of RF-plasma power towards unintentional doping and giving rise to yellow luminescence (YL) is discussed. Our PC measurements show relatively faster decay for sample grown with higher RF-plasma power. In addition, the ultrafast optical pump-probe spectroscopy results show the presence of various defect levels with different relaxation times. A faster ultrafast relaxation time from the conduction band to the closest defect level and conduction band to the next defect level was observed for the sample grown with higher plasma power. A comparatively low defect density and faster carrier relaxation observed in higher RF-plasma power grown samples is caused by lower impurities and gallium vacancies. The results imply that RF-plasma power is very important parameter for the growth of epitaxial GaN films and undesirable impurities and gallium vacancies might get incorporated in the epitaxial GaN films.

Crystalline Tetrahedral Phases Al1-xBxPSi3 and Al1-xBxAsT3 (T = Si, Ge) Via Reactions of Al(BH4)3 and M(TH3)3 (M = P, As)

Crystalline (III-V)1-y(IV2)y hybrid semiconductors Al1-xBxPSi3 and Al1-xBxAsSi3 have been grown on Si(001) substrates via reactions of Al(BH4)3 and M(SiH3)3 (M = P, As). These films are characterized for structure, composition, and phase morphology The materials form by interlinking III-V-IV3 tetrahedral units that construct a cubic lattice with average diamond-like symmetry containing isolated III-V pairs. This synthetic approach was extended to alloy systems in which the III-V pairs are imbedded in a Ge-matrix using As(GeH3)3. These materials are grown on Ge/Si(001) platforms and a comparative study of two growth methods is presented. One describes a low pressure CVD route via reactions of Al(BH4)3 and As(GeH3)3, and the other is a gas-source MBE technique combining Al atoms from an effusion cell with the germyl-arsene precursor. Both sets of films are studied using aberration-corrected STEM. Globally the results show that Al(BH4)3 acts as a carbon-free source of Al compatible with CVD protocols.

First-principles study of migration mechanisms and diffusion of carbon in GaN

Carbon related defects are readily incorporated in GaN due to its abundance during growth both with MBE and CVD techniques. Employing first-principles calculations we compute the migration barrier of neutral carbon interstitials in the wurtzite GaN crystal. The Minimum Energy Path (MEP) and the migration barriers of these defects are obtained using the Nudged Elastic Band (NEB) method with the climbing image modification (CI-NEB). In addition, the Dimer method is used to verify the results. The results yield a quantitative description of carbon diffusion in the crystal allowing for the determination of the most probable migration paths.

Crystalline Tetrahedral Phases Al1-xBxPSi3 and Al1-xBxAsT3 (T = Si, Ge) Via Reactions of Al(BH4)3 and M(TH3)3 (M = P, As)

Hybrid (III-V)-IV alloys with the general formula (III-V)x(IV)5-2x represent a new class of semiconductors that are designed to enhance the optoelectronic capabilities of current silicon and germanium-based technologies for applications in Si-based photovoltaics. Recent studies have produced mono-crystalline, phase pure alloys with compositions (AlP)xSi5-2x (x = 0-1.0) These materials are grown as thin layers directly on Si(001) substrates via reactions of molecular P(SiH3)3 and Al atomic beams using gas-source MBE techniques. The reactions generate AlPSi3 tetrahedral units that combine to form a diamond-like structure in which Al and P are imbedded as isolated bonded pairs within a host tetrahedral Si lattice. The presence of discrete Al-P dimers prevents phase segregation and AlP precipitation. The intact incorporation of the AlPSi3 building blocks fixes the material stoichiometry yielding bulk crystals with reproducible AlPSi3 compositions and Si rich analogues depending on the reaction temperature. However, difficulties associated with precise control of the Al flux in the MBE experiments can lead to minor variations of the atomic distribution at the nanoscale whose impact on material properties has yet to be determined. This issue can be potentially resolved by replacing the solid aluminum source by a chemically suitable molecular precursor that combines readily with P(SiH3)3 to produce the target materials using chemical vapor deposition (CVD). Development of CVD routes is highly desirable since they are more practical and cost effective than MBE for the large-scale, high-throughput fabrication of technologically relevant materials. This study describes the use of Al(BH4)3, a carbon-free inorganic hydride of Al, as a molecular source to synthesize (AlP)xSi5-2x systems via reactions with P(SiH3)3. These reactions yield crystalline alloys with novel Al1-xBxPSi3(x = 0.04-0.06) compositions which are grown lattice-matched on Si(001) substrates. The new materials have been characterized for structure, composition, phase purity and optical response by spectroscopic ellipsometry, high-resolution XRD, XTEM, EELS and EDS, which indicate the formation of single-phase mono-crystalline layers with tetrahedral structures based on the AlPSi3 parent phase. Raman scattering of the Al1-xBxPSi3films supports the presence of substitutional B in place of Al and provides strong evidence that the boron is bonded to P in the form of isolated pairs, as expected on the basis of the AlPSi3 prototype. The substitution of small-size B atoms is facilitated by the stabilizing effect of the parent lattice, and it is highly desirable for promoting full lattice matching with Si as required for Si-based solar cell designs. The substitution of B also increases the bond-length disorder leading to a significantly enhanced absorption relative to Si and AlPSi3 at E < 3.3 eV which may be beneficial for PV applications. Analogous reactions of As(SiH3)3 with Al(BH4)3 produce Al1-xBxAsSi3 crystals in which the B incorporation is limited to doping concentrations at 1020 atoms/cm3. In both cases the classical Al(BH4)3 acts as an efficient delivery source of elemental Al to create crystalline group (III-V)-IV hybrid materials comprising light, earth-abundant elements with possible applications not only in the fields of Si-based photovoltaic technologies but also as light-element refractory solids.

Growth and characterization of Ga doped ZnSnAs2 thin films on InP substrates

AbstractWe have grown Ga doped ZnSnAs2 thin films on InP substrates using all‐solid source MBE technique. Using the optimum substrate temperature of 340 ºC and Zn:Sn:As4 beam equivalent pressure ratio (BEPR) of 24:1:52, samples were prepared with different Ga concentration achieved by changing the Ga flux. Compositional analysis was performed using electron microprobe microanalysis (EMPA). We found that Ga concentration on the cation sites is 20.9% and 8.7%. XRD patterns of Ga doped samples showed only assignable to ZnSnAs2:Ga and InP substrate. The lattice constant decreases linearly from 0.5862 to 0.5829 nm with decreasing Ga concentration. The observed concentration dependence indicates that the lattice constant of the thin films can be controlled by Ga doping concentration. Hall coefficient and resistivity of Ga 8.7% doped sample can be well explained by the impurity band model. We were able to resolve the experimentally obtained carrier concentration pexp and mobility μexp into valence band carrier concentration pv with mobility μv and acceptor band carrier concentration pa with mobility μa. The computed apparent values papp and μapp are in good agreement with pexp and μexp, respectively. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)