Molecular Beam Epitaxy System Research Articles

Epitaxial Growth of GaN Films on Chemical-Vapor-Deposited 2D MoS2 Layers by Plasma-Assisted Molecular Beam Epitaxy.

The van der Waals epitaxy of wafer-scale GaN on 2D MoS2 and the integration of GaN/MoS2 heterostructures were investigated in this report. GaN films have been successfully grown on 2D MoS2 layers using three different Ga fluxes via a plasma-assisted molecular beam epitaxy (PA-MBE) system. The substrate for the growth was a few-layer 2D MoS2 deposited on sapphire using chemical vapor deposition (CVD). Three different Ga fluxes were provided by the gallium source of the K-cell at temperatures of 825, 875, and 925 °C, respectively. After the growth, RHEED, HR-XRD, and TEM were conducted to study the crystal structure of GaN films. The surface morphology was obtained using FE-SEM and AFM. Chemical composition was confirmed by XPS and EDS. Raman and PL spectra were carried out to investigate the optical properties of GaN films. According to the characterizations of GaN films, the van der Waals epitaxial growth mechanism of GaN films changed from 3D to 2D with the increase in Ga flux, provided by higher temperatures of the K-cell. GaN films grown at 750 °C for 3 h with a K-cell temperature of 925 °C demonstrated the greatest crystal quality, chemical composition, and optical properties. The heterostructure of 3D GaN on 2D MoS2 was integrated successfully using the low-temperature PA-MBE technique, which could be applied to novel electronics and optoelectronics.

E-Band InAs Quantum Dot Micro-Disk Laser with Metamorphic InGaAs Layers Grown on GaAs/Si (001) Substrate.

The direct growth of III-V quantum dot (QD) lasers on silicon substrate has been rapidly developing over the past decade and has been recognized as a promising method for achieving on-chip light sources in photonic integrated circuits (PICs). Up to date, O- and C/L-bands InAs QD lasers on Si have been extensively investigated, but as an extended telecommunication wavelength, the E-band QD lasers directly grown on Si substrates are not available yet. Here, we demonstrate the first E-band (1365 nm) InAs QD micro-disk lasers epitaxially grown on Si (001) substrates by using a III-V/IV hybrid dual-chamber molecular beam epitaxy (MBE) system. The micro-disk laser device on Si was characterized with an optical threshold power of 0.424 mW and quality factor (Q) of 1727.2 at 200 K. The results presented here indicate a path to on-chip silicon photonic telecom-transmitters.

Observation of an abrupt 3D-2D morphological transition in thin Al layers grown by MBE on InGaAs surface

Among superconductor/semiconductor hybrid structures, in situ aluminum (Al) grown on InGaAs/InAs is widely pursued for the experimental realization of Majorana Zero Mode quasiparticles. This is due to the high carrier mobility, low effective mass, and large Landé g-factor of InAs, coupled with the relatively high value of the in-plane critical magnetic field in thin Al films. However, growing a thin, continuous Al layer using the molecular beam epitaxy (MBE) is challenging due to aluminum's high surface mobility and tendency for 3D nucleation on semiconductor surfaces. A study of epitaxial Al thin film growth on In0.75Ga0.25As with MBE is presented, focusing on the effects of the Al growth rate and substrate temperature on the nucleation of Al layers. We find that for low deposition rates, 0.1 and 0.5 Å/s, the growth continues in 3D mode during the deposition of the nominal 100 Å of Al, resulting in isolated Al islands. However, for growth rates of 1.5 Å/s and above, the 3D growth mode quickly transitions into island coalescence, leading to a uniform 2D Al layer. Moreover, this transition is very abrupt, happening over an Al flux increase of less than 1%. We discuss the growth mechanisms explaining these observations. The results give new insights into the kinetics of Al deposition and show that with sufficiently high Al flux, a 2D growth on substrates at close to room temperature can be achieved already within the first few Al monolayers. This eliminates the need for complex cryogenic substrate cooling and paves the way for the development of high-quality superconductor-semiconductor interfaces in standard MBE systems.

The Stability Prediction and Epitaxial Growth of Boron Nitride Nanodots on Different Substrates.

Boron nitride (BN) is a wide-bandgap material for various applications in modern nanotechnologies. In the technology of material science, computational calculations are prerequisites for experimental works, enabling precise property prediction and guidance. First-principles methods such as density functional theory (DFT) are capable of capturing the accurate physical properties of materials. However, they are limited to very small nanoparticle sizes (<2 nm in diameter) due to their computational costs. In this study, we present, for the first time, an important computational approach to DFT calculations for BN materials deposited on different substrates. In particular, we predict the total energy and cohesive energy of a variety of face-centered cubic (FCC) and hexagonal close-packed (HCP) boron nitrides on different substrates (Ni, MoS2, and Al2O3). Hexagonal boron nitride (h-BN) is the most stable phase according to our DFT calculation of cohesive energy. Moreover, an experimental validation equipped with a molecular beam epitaxy system for the epitaxial growth of h-BN nanodots on Ni and MoS2 substrates is proposed to confirm the results of the DFT calculations in this report.

A complete comprehension of InGaAs capping layer deposition on InAs quantum dots by comparison of simulations, luminescence and X-ray diffraction

The mechanism of incorporation of InGaAs capping layer on InAs Quantum dots (QDs) are unclear in most of the literature studies. Various studies suggest vertical strain relaxation to be the cause of the longer wavelength emission of the dots. On the other hand, some suggest preferential In segregation on the periphery of QDs to be the reason for the enhancement of the dot size. In this study, the strain profile, electronic bands, and eigenstates of InAs QDs, with various InGaAs capping layer thickness, have been simulated using nextnano3 software. Further, the structures are grown by a molecular beam epitaxy (MBE) system. Comparison of ground-state emission energy obtained from PL and simulations, proves that strain relaxation alone is not sufficient to explain the shift in the emission wavelength, and indicate presence of a growth mechanism which alters the size of the QDs. From the in-plane and out-plane High-Resolution X-Ray Diffraction (HR-XRD) results, we have explained the complete dynamics involved in the InGaAs capping deposition, which was unclear in the literature up-to-date. Advantages of incorporation of InGaAs capping on the dots, like decrease in strain and defects, preservation of In, and increased activation energy are also observed in the study.

Stencil Lithography‐Assisted Fabrication of Triangular Grid Pb Architecture

AbstractManipulating Majorana zero modes (MZMs) holds significant promise in propelling the field of topological quantum computation. While scanning tunneling microscopy (STM) exhibits remarkable capabilities in characterizing MZMs, additional geometrical constraints are imperative for achieving precise and controllable manipulation of these zero modes. To this end, it is of great help to integrate a micro mask within a molecular beam epitaxy (MBE) system to grow specific patterns containing MZMs. Here, a scenario for manipulating the MZMs in the vortex cores based on a triangular grid architecture is proposed. Additionally, a stencil lithography setup that is compatible with a combined MBE and STM system is introduced and experimentally demonstrated. A fine‐tuning of the Pb patterns, between a hexagonal antidot lattice to a triangular grid array, is achieved by adjusting the spatial gap between the stencil mask and the underlying substrate. This study provides new possibilities for the fabrication and investigation of Majorana devices.

Dependence of WAL effect and enhancement of spin-orbit coupling effect in Bi1.2Sb0.8Te0.4Se2.6 low-temperature magnetic transport

The strong spin-orbit coupling and time-reversal symmetry in topological insulators (TIs) produce a new quantum state known as the topological surface state (TSS). The study of low-temperature magnetic transport in the TSS provides opportunities to explore exotic physical phenomena such as the spin Hall effect and the topological phase transition. An important goal in obtaining intrinsic TI systems is to eliminate internal defects and thereby reduce the contributions of metallic states in transport measurements. In this work, we are committed to overcoming this difficulty by elemental alloying. This paper reports the successful preparation of high-quality and stoichiometrically stable Bi1.2Sb0.8Te0.4Se2.6 (BSTS) thin films by innovatively combining the hybrid target structure with the laser molecular beam epitaxy (LMBE) system and low-temperature magnetic transport tests conducted on BSTS thin films of different thicknesses. Due to stronger spin-orbit coupling, we observed a weak anti-localization (WAL) effect, and the strength of the effect was affected by the film thickness and temperature in the low-temperature magnetic transport study. The dependence of the WAL effect provides a way to regulate the TSS.

Improving crystal quality of β-phase MgGaO thin films by using low-temperature homo-buffer layer

Ultra-wide bandgap (UWBG) semiconductors are promising for many applications, such as power electronics and deep-ultraviolet photonics. In this research, UWBG β-phase magnesium gallium oxide (MgGaO) thin films with a bandgap of 5.1 eV were grown using low-temperature homo-buffer layers in a plasma-assisted molecular beam epitaxy system. The role of the growth temperature and thickness of low-temperature buffer layer on the quality of the active layer was studied using x-ray diffraction and transmission electron microscopy and by analyzing the properties of metal–semiconductor–metal photodetector devices based on these films. It is found that lower buffer growth temperature at 300 °C leads to higher crystal quality of active layer. For the same low buffer growth temperature, different crystal quality in the active layer is attained with different buffer layer thickness. A buffer layer thickness at 40 nm has the best active layer quality with the highest photo current under 265 nm illumination and long decay time as a result of reduced recombination of photo-generated carriers through fewer defects in the active layer.

Competitive actions of MnSi in the epitaxial growth of Mn5Si3 thin films on Si(111)

Some magnetically ordered phases of the ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ crystal are proving to be prototypes for the study of the new fundamental spin physics related to the spontaneous breaking of the time-reversal symmetry despite a zero net magnetization. Here, we report on a route to grow epitaxial ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ thin films on Si(111). To this end, we use Mn and Si codeposition in a molecular beam epitaxy system and carefully tune the deposition rates, the growth temperature, and the annealing temperature. We assessed the silicide phase-formation and morphology using reflection high-energy electron diffraction, x-ray diffraction, high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy. Layers containing only ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ could be stabilized under very restrictive conditions, by tuning the Mn/Si flux ratio to match the compound stoichiometry and adjusting the substrate temperature during growth to 443 K. HRTEM imaging revealed the existence of an interfacial amorphous layer of few nanometers thickness. Annealing the heterostructure up to 573 K led to the formation of MnSi at the vicinity of the ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$/Si(111) interface, which significantly reduced the nucleation barrier of ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$. High-quality crystalline ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ thin films were then formed with the following epitaxial relationships: ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$(0001)$[01\overline{1}0]//\mathrm{MnSi}(111)[\overline{2}11]//\mathrm{Si}(111)[1\overline{1}0]$. Our experiments showed that the formation of MnSi is enhanced at a growth temperature above 473 K or for a longer annealing step, while the crystalline quality of the ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ overlayer is correspondingly degraded leading to textured thin films. The growth pathways and structural properties of the manganese silicides can be rationalized in terms of reactions maximizing the free-energy lowering rate. Moreover, we found that the magnetic and the magnetotransport properties can be used as an efficient tool to track both ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ crystallinity and proportion in the deposited layers.

InAs/GaAs quantum-dot lasers grown on on-axis Si (001) without dislocation filter layers.

InAs/GaAs quantum dot (QD) laser monolithically grown on silicon is one of the potential approaches to realizing silicon-based light sources. However, the mismatch between GaAs and Si generates a high density of threading dislocations (TDs) and antiphase boundaries (APBs), which trap carriers and adversely affect device performance. In this paper, we present a simple method to reduce the threading dislocation density (TDD) merely through GaAs buffer, eliminating the intricate dislocation filter layers (DFLs) as well as any intermediate buffer layers whose compositions are different from the target GaAs. An APB-free epitaxial 2.5 µm GaAs film was grown on exact Si (001) by metalorganic chemical vapor deposition (MOCVD) with a TDD of 9.4 × 106 cm-2. InAs/GaAs QDs with a density of 5.2 × 1010 cm-2 were grown on this GaAs/Si (001) virtual substrate by molecular beam epitaxy (MBE) system. The fabricated QD laser has achieved a single facet room temperature continuous-wave output power of 138 mW with a threshold current density of 397 A/cm2 and a lasing wavelength of 1306 nm. In this work, we propose a simplified method to fabricate high-power QD lasers, which is expected to promote the application of photonic integrated circuits.

Tailoring anomalous Hall effect by spin–orbit coupling in epitaxial Au/Fe4N bilayers

Anomalous Hall effect (AHE) is one of the most fascinating topics in condensed matter physics related to spin–orbit coupling (SOC). In this paper, we report on the AHE of high-quality epitaxial Au/Fe4N bilayer films, which were grown by a plasma-assisted molecular beam epitaxy system. A scaling involving multiple competing scattering mechanisms and a shunting model were adopted to analyze the AHE in detail. Compared with Fe4N single layers and Cu/Fe4N bilayers, the AHE of Au/Fe4N bilayers is dramatically modified by strong SOC of the Au layer. Analysis has shown that aside from extra scatterings from Au atoms that diffused from an Au layer to a Fe4N layer, both spin Hall effect of Au and magnetic proximity effect near the Au/Fe4N interface contribute to the modification of the AHE. Variation of coercivity with the change of current, which could be attributed to spin–orbit torque, implies that the measured AHE is a combination of the AHE of Fe4N and strong SOC of Au.

Probing the correlation between morphology and optical anisotropy in ZnTPP films grown at different temperatures

Films of tetraphenylporphyrins (TPP) have been recently investigated for their ability in protecting graphite electrode surfaces from degradation. The effectiveness of the protection depends on both the molecule–substrate and molecule–molecule interactions, which determine the type of film growth and its morphology. For instance, meso-tetraphenyl porphyrin-Zn(II) (ZnTPP) arranges differently depending on the substrate, conditions used for deposition and post-growth treatments. Since many parameters influence film morphology and strategies to reach an efficient protective coverage, here we investigate the role of the substrate temperature in determining morphological variations in thick ZnTPP films. ZnTPP molecules were sublimated by an organic molecular beam epitaxy (OMBE) system on a highly oriented pyrolytic graphite (HOPG) substrate, controlled in temperature by a variable temperature cryostat. The film morphology was characterized by atomic force microscopy (AFM), whereas the type of growth by reflectance anisotropy spectroscopy (RAS), which is highly sensitive to the orientation and type of arrangement of molecules on the substrate; changes in the film valence electronic structure were monitored in situ by UV photoemission spectroscopy (UPS). The comparison between AFM and RAS investigations clearly correlates a ZnTPP film morphology to the characteristic optical signal, both influenced by temperature conditions and diffusivity of molecules on the substrate.

Hydrogen-Assisted Molecular Beam Epitaxy of SiGeSn

Since its first synthesis in 2003 [1], the ternary alloy semiconductor SiGeSn has seen an increasing interest due to its unique properties in the field of Group-IV semiconductors [2–6]. SiGeSn not only allows the decoupling of its bandgap and lattice-constant by what it is predestined for the epitaxy of strain-reduced heterostructures on the Si platform. Furthermore, SiGeSn becomes a direct bandgap semiconductor at high Sn concentrations. Considering these properties, SiGeSn is an attractive material system for optoelectronic applications. An exemplary heterojunction optoelectronic device is the single confinement heterostructure laser diode, the still missing key component for the monolithic integration of the optical on-chip communication on Si.However, one of the greatest challenges faced by many Group-IV researchers is the epitaxy of SiGeSn bulk alloys with high crystal quality. The temperature induced segregation of Sn results in interstitial Sn atoms and subsequently in acceptor-like defect states, as it was already reported for GeSn [7].In this work, we show the molecular beam epitaxy (MBE) of SiGeSn structures under Hydrogen ambient. First of all, we determined the concentration of the acceptor-like defect states of undoped SiGeSn NA,defect on the basis of capacitance-voltage (C-V) measurements of SiGeSn pin diodes. Furthermore, we investigated Hydrogen-assisted epitaxy as an approach for the reduction of the acceptor-like defect states.All epitaxy experiments were performed in a 6” MBE system, where Si, Ge and Sn are used as matrix materials and B and Sb as dopants respectively. In particular, we investigated the influence of molecular (H2) as well as atomic (H) Hydrogen on the epitaxy process and therefore the quality of the grown structures. For this purpose, the MBE system was upgraded with a Hydrogen atom beam source for the generation of an atomic Hydrogen flux.Since the Sn segregation is affected mostly by the substrate temperature, this critical growth parameter is measured in-situ using an infrared pyrometer, which allows the observation of the dynamic processes on the sample surface [8] and the precise control of the substrate temperature at TSub = 200 °C during the SiGeSn epitaxy.All SiGeSn structures are based on Ge virtual substrates (Ge-VS) on Si(001) [9], followed by a 400 nm thick additional Ge buffer layer. In order to fulfil lattice matching of SiGeSn on Ge, a constant ratio of Si and Sn of Si/Sn = 3.67 was kept for all structures. The Sn concentration was varied in the range of 5 % < cSn < 10 %. For material characterization, 300 nm thick undoped SiGeSn layers were grown on this substrate layer stack (see Fig. 1a). A detailed characterization of the crystallinity and the surface roughness was performed using X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. The comparison of AFM micrographs of SiGeSn layers, grown with and without Hydrogen assistance (see Fig. 1b) show a clear reduction of the surface roughness due to Hydrogen assisted epitaxy.In order to determine NA,defect, C-V measurements on SiGeSn pin diodes (see Fig. 1c) were performed. The diode layer structure, also based on the previously described substrate layer stack, starts with a 400 nm thick p-doped SiGeSn bottom layer with an acceptor concentration of NA = 5x1019 cm-3, followed by an 300 nm thick undoped SiGeSn layer and a closing 200 nm thick n-doped SiGeSn top layer with a donor concentration of ND = 5x1019 cm-3. Subsequently, pin diodes were fabricated using a single mesa process consisting of four basic steps: Mesa structuring using inductive coupled plasma reactive ion etching (ICP-RIE) with HBr, mesa passivation with plasma enhanced chemical vapor deposition (PECVD) grown SiO2, oxide window opening using RIE with CHF3 and metallization using sputtered Al. An exemplary device can be seen in the scanning electron microscopy (SEM) image in Fig. 1d.The SiGeSn pin diodes were electrically characterised by means of C-V measurements using a Keithley 4200 semiconductor characterisation system. For further evaluation, the C-V characteristics of the pin diodes show linear behaviour when plotted on a C-2-V-scaled diagram (Fig. 1e). Under the assumption of a strongly asymmetric pn junction, (ND >> NA,defect), the slope m of the C-2-V characteristics is proportional to NA,defect. In this manner, we determined NA,defect for several SiGeSn pin diodes in dependence of the alloy composition. The results, as shown in Fig. 1f, show an acceptor-like defect concentration between 1017 cm-3 < NA,defect < 1018 cm-3, which is comparable with results reported by other groups for GeSn [7].In our presentation, we will discuss Hydrogen-assisted epitaxy as a promising approach for improving the SiGeSn crystal quality. Figure 1

(Invited) Enhanced Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes Using Photonic Crystal Structures

High efficiency ultraviolet (UV) light-emitting diodes (LEDs) are in high demand for a wide range of applications [1-3]. Due to its wide bandgap, covering from deep UV to near visible, AlGaN semiconductor has been intensively studied for high efficiency UV LEDs. However, the performance of AlGaN UV LEDs is still limited, especially in the deep UV wavelength region, due to large dislocation density and extremely inefficient p-type doping in AlGaN film. Therefore, AlGaN UV LEDs have been suffering from low internal quantum efficiency (IQE), poor light extraction efficiency (LEE), and low external quantum efficiency (EQE). Alternatively, AlInN alloy is relatively unexplored for light-emitters even though it holds great potential application in UV and visible light-emitting devices. Recent studies have reported that InAlN has a large optical gain for deep UV LEDs. In this context, we have successfully developed AlInN nanowire LEDs with strong emission in the UV wavelength regime. The AlInN nanowires achieved a high IQE of >50% and significantly strong Transverse Magnetic (TM) polarized emission at 299nm.The AlInN nanowire LED structures were spontaneously grown on n-Si (111) substrates under nitrogen rich conditions by a Veeco GEN II molecular beam epitaxy (MBE) system. The AlInN nanowire was grown on GaN nanowire template. The device active region consists of ~ 100 nm n-Al0.78In0.22N, 40 nm undoped Al0.75In0.25N well and 100 nm p-Al0.78In0.22N layers. The GaN nanowire segments were grown at ~ 800 °C, nitrogen flow rate of 0.5-1.0 sccm, and forward plasma power of ~ 400 W. However, the growth temperature of the active region was at ~ 670 ̶ 700 °C to increase the In incorporation in AlInN segments and the nitrogen flow rate was kept at 2.5 sccm. The MBE grown AlInN under nitrogen rich condition offers an effective approach to eliminate the composition inhomogeneity in the AlInN. The nanowires are vertically aligned to the Si substrate with quite uniform dimension, as illustrated in Figure 1(a). Figure 1(b) presents the structure of a single AlInN nanowire LED. The photoluminescence (PL) spectra of AlInN nanowires were measured using a 266 nm diode-pumped solid-state laser as the excitation source. The peak emissions vary from 280nm to 365nm by varying the Al/In composition in the AlInN layers. The peak emission is shifted to shorter wavelength when the substrate temperature gets increase which is attributed to the increased In adatom desorption at higher growth temperature, resulted in the reduced In composition in the AlInN segment. The AlInN/GaN nanowire exhibits relatively high IQE which is estimated of ~ 52% at room temperature [3, 4]. Illustrated in Figure 1(c), the UV nanowire LED holds strong emission spectra at ~299 nm with a negligible shift in the peak wavelength, attributing to the high crystalline quality and reduced quantum-confined Stark effect in the AlInN nanowires. The LED has a low turn-on voltage of ~5V and low reverse leakage current less, i.e., ~0.5 µA at -8.6V. Moreover, as shown in Figure 1(d), we have demonstrated that the LEE of the AlInN nanowire LEDs could reach ~ 63% using hexagonal photonic crystal nanowire structures which is significantly higher compared to that of the random nanowire arrays [5]. This study paves the way for the development of high-power ultraviolet light-emitters using AlInN nanowire arrays.

Desorption characteristics of selenium and tellurium thin films

The temperature-dependent desorption behavior of selenium and tellurium is investigated using a heated quartz crystal microbalance. Prior to heating the quartz crystal microbalance, selenium and tellurium films with varying thickness were deposited using thermal effusion cells in a molecular beam epitaxy system for subsequent determination of temperature-dependent mass loss of the deposited films. The desorption rate for tellurium was found to exhibit one sharp peak around 190 °C, indicating the loss of the entire film irrespective of film thickness within a temperature window of 20 °C, which was completely evaporated at 200 °C. Similar experiments for selenium revealed that the thermal desorption took place via a two-stage process with a smaller portion of the material desorbing within an even narrower temperature window of 5 °C at a much lower peak temperature of 65 °C, while most selenium desorbed within a temperature range of 10 °C around 90 °C. This two-stage behavior indicated the presence of at least two chemically distinct selenium species or binding states. The direct and quantitative determination of the chalcogen desorption process provides important insights into the kinetics of chalcogenide-based film growth and is in addition of applied benefit to the research community in the area of Se/Te capping and decapping of air sensitive materials as it provides temperature ranges and rates at which full desorption is achieved. Our work furthermore points toward the need for a more detailed understanding of the chemical composition state of atomic and molecular beams supplied from thermal evaporation sources during growth.

Demonstration of room-temperature continuous-wave operation of InGaAs/AlGaAs quantum well lasers directly grown on on-axis silicon (001)

Room-temperature continuous-wave operation of InGaAs/AlGaAs quantum well lasers directly grown on on-axis silicon (001) has been demonstrated. A 420 nm thick GaAs epilayer completely free of antiphase domains was initially grown on the silicon substrate in a metal-organic chemical vapor deposition system and the other epilayers, including four sets of five-period strained-layer superlattices and the laser-structural layers, were successively grown in a molecular beam epitaxy system. The lasers were prepared as broad-stripe Fabry–Pérot ones with a stripe width of 21.5 μm and a cavity length of 1 mm. Typically, the threshold current and the corresponding threshold current density are 186.4 mA and 867 A/cm2, respectively. The lasing wavelength is around 980 nm, and the slope efficiency is 0.097 W/A with a single-facet output power of 22.5 mW at an injection current of 400 mA. This advancement makes the silicon-based monolithic optoelectronic integration relevant to quantum well lasers more promising with an enhanced feasibility.

Solid Phase Epitaxy of Single Phase Two-Dimensional Layered InSe Grown by MBE.

Single-phase two-dimensional (2D) indium monoselenide (γ-InSe) film is successfully grown via solid phase epitaxy in the molecular beam epitaxy (MBE) system. Having high electron mobility and high photoresponsivity, ultrathin 2D γ-InSe semiconductors are attractive for future field-effect transistor and optoelectronic devices. However, growing single-phase γ-InSe film is a challenge due to the polymorphic nature of indium selenide (γ-InSe, α-In2Se3, β-In2Se3, γ-In2Se3, etc.). In this work, the 2D α-In2Se3 film was first grown on a sapphire substrate by MBE. Then, the high In/Se ratio sources were deposited on the α-In2Se3 surface, and an γ-InSe crystal emerged via solid-phase epitaxy. After 50 min of deposition, the initially 2D α-In2Se3 phase was also transformed into a 2D γ-InSe crystal. The phase transition from 2D α-In2Se3 to γ-InSe was confirmed by Raman, XRD, and TEM analysis. The structural ordering of 2D γ-InSe film was characterized by synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS).

Temperature and doping-dependent interplay between the direct and indirect optical response in buffer-mediated epitaxial germanium

The structural and optical properties of buffer mediated epitaxial germanium (Ge) layer were investigated and compared with bulk n-type and p-type Ge substrates. An interconnected dual-chamber molecular beam epitaxy (MBE) system was used to grow a 280 nm thin Ge epilayer on (100)GaAs substrate with an intermediate AlAs buffer layer. The lattice-matched, abrupt Ge/AlAs heterointerface was analyzed using cross-sectional transmission electron microscopic analysis, and no elemental interdiffusion was detected via secondary ion mass spectrometry. A strong direct gap transition, compared to the indirect gap transition, and a series of phonon-assisted transitions was observed by photoluminescence (PL) spectroscopy. In addition, the intensity of the direct gap recombination decreases with decreasing PL measurement temperatures, which was ascribed to the reduced density of Γ-valley electrons available for recombination at lower temperature. Furthermore, the intensity ratio between the direct and indirect optical transition drastically decreases with decreasing temperature in both n-type epitaxial and p-type bulk Ge. An empirical relation in both direct and indirect peak position with temperature was established. The observed strong luminescence in 280 nm thick epitaxial Ge at room temperature is vital for Ge-based photonic devices. In addition, the quality of the epitaxial Ge layer grown via MBE is on par with bulk Ge substrates.

Photodetector Arrays Based on MBE‐Grown GaSe/Graphene Heterostructure

AbstractA selective area growth of hexagonal GaSe on prepatterned graphene substrate and fabrication of a photodetector array is reported. The GaSe film is grown on the graphene substrates using a molecular beam epitaxy system and characterized for structural and morphological properties using X‐ray diffraction, Raman, and atomic force microscopy techniques. Preliminary observations indicate the growth of hexagonal GaSe layers along (001) direction on graphene film under certain optimum growth conditions. Subsequently, GaSe‐based Schottky diode photodetectors are fabricated in a single‐element and 16 × 16 crossbar array geometries. Current–voltage characteristics of the diodes show a rectifying behavior with a turn‐on voltage and a low leakage current of ≈2 V and ≈20 nA, respectively. Photoresponse characteristics of the diode are investigated by measuring wavelength‐dependent spectral response and temporal photocurrent response under illumination of different lights, including 532 nm laser and white light, with varied incident intensities. The spectral responsivity of the detector is found to be as high as 98 A W–1 at a wavelength of 500 nm with a turning‐on profile corresponding to the band edge of GaSe. The photo mapping capability of the detector array is also demonstrated in this report.

Experimental observation of pseudogap in a modulation-doped Mott insulator: Sn/Si(111)-()R30°

Unusual quantum phenomena usually emerge upon doping Mott insulators. Using a molecular beam epitaxy system integrated with cryogenic scanning tunneling microscope, we investigate the electronic structure of a modulation-doped Mott insulator Sn/Si(111)-()R30°. In underdoped regions, we observe a universal pseudogap opening around the Fermi level, which changes little with the applied magnetic field and the occurrence of Sn vacancies. The pseudogap gets smeared out at elevated temperatures and alters in size with the spatial confinement of the Mott insulating phase. Our findings, along with the previously observed superconductivity at a higher doping level, are highly reminiscent of the electronic phase diagram in the doped copper oxide compounds.