Molecular Beam Epitaxy Technique Research Articles

Effective Mg Incorporation in CdMgO Alloy on Quartz Substrate Grown by Plasma-Assisted MBE

The development of CdMgO ternary alloy with a single cubic phase is challenging but meaningful work for technological advancement. In this work, we have grown a series of Cd1-xMgxO ternary random alloys with various Mg concentrations (x = 0, 30, 32, 45, and 55%) on quartz substrate by plasma-assisted molecular beam epitaxy (PA-MBE) technique. The structural investigations of alloys were performed using the X-ray diffraction (XRD) technique. The decreases in average crystallite size and lattice parameters were observed with an increase in Mg content in the alloys. XRD analysis confirms a single cubic phase is obtained for alloy compositions. The elemental and morphological studies were carried out using energy dispersive x-ray (EDX) spectroscopy and atomic force microscope (AFM) technique, respectively. The optical investigation was carried out using UV-Vis spectroscopy. The optical bandgaps were estimated using the Tauc relation and it was varied from 2.34 eV to 3.47 eV by varying the Mg content from zero to 55% in the alloys. The Urbach energy increases from 112 meV to 350 meV which suggests a more disordered localized state with an increase in Mg incorporation in the alloys.

Observation of Gapped Topological Surface States and Isolated Surface Resonances in PdTe2 Ultrathin Films.

The superconductor PdTe2 is known to host bulk Dirac bands and topological surface states. The coexistence of superconductivity and topological surface states makes PdTe2 a promising platform for exploring topological superconductivity and Majorana bound states. In this work, we report the spectroscopic characterization of ultrathin PdTe2 films with thickness down to three monolayers (ML). In the 3 ML PdTe2 film, we observed spin-polarized surface resonance states, which are isolated from the bulk bands due to the quantum size effects. In addition, the hybridization of surface states on opposite faces leads to a thickness-dependent gap in the topological surface Dirac bands. Our photoemission results show clearly that the size of the hybridization gap increases as the film thickness is reduced. The observation of isolated surface resonances and gapped topological surface states sheds light on the applications of PdTe2 quantum films in spintronics and topological quantum computation.

Low-Frequency Noise Characteristics of (Al, Ga)As and Ga(As, Bi) Quantum Well Structures for NIR Laser Diodes.

Fabry-Perot laser diodes based on (Al, Ga)As and Ga(As, Bi) with single or multiple parabolic or rectangular-shaped quantum wells (QWs) emitting at the 780-1100 nm spectral range were fabricated and investigated for optimization of the laser QW design and composition of QWs. The laser structures were grown using the molecular beam epitaxy (MBE) technique on the n-type GaAs(100) substrate. The photolithography process was performed to fabricate edge-emitting laser bars of 5 μm by 500 μm in size. The temperature-dependent power-current measurements showed that the characteristic threshold current of the fabricated LDs was in the 60-120 mA range. Light and current characteristics were almost linear up to (1.2-2.0) Ith. Low-frequency 10 Hz-20 kHz electrical and optical noise characteristics were measured in the temperature range from 70 K to 290 K and showed that the low-frequency optical and electrical noise spectra are comprised of 1/f and Lorentzian-type components. The positive cross-correlation between optical and electrical fluctuations was observed.

A Method to Probe the Interfaces in La2−xSrxCuO4-LaSrAlO4-La2−xSrxCuO4 Trilayer Junctions

C-axis trilayer cuprate Josephson junctions are essential for basic science and digital circuit applications of high-temperature superconductors. We present a method for probing the interface perfection in La2−xSrxCuO4 (LSCO)-LaSrAlO4 (LSAO)-La2−xSrxCuO4 trilayer junctions. A series of LSCO-LSAO superlattices with atomically smooth surfaces and sharp interfaces were grown by the atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) technique. We have systematically varied the thickness of LSCO and LSAO layers with monolayer precision. By studying the mutual inductance and electrical transport in these superlattices, we detect the non-superconducting (“dead”) layers at the interfaces and quantify their thicknesses. Our results indicate that two optimally doped LSCO monolayers just above and below the one monolayer LSAO barrier are no longer superconducting, rendering the actual barrier thickness of five monolayers. Next, we have shown that introducing a protective highly-overdoped LSCO layer reduces the thickness of dead layers by one or two monolayers.

Fabrication of GaN-based MSM droplet triboelectric nanogenerator by the conjunction of photovoltaic and triboelectric effect

The present work focuses on the fabrication of Gallium Nitride based triboelectric nanogenerators for energy harvesting from moving NaCl droplets of various concentrations between 0.1 M and 1.0 M. Fabricated Droplet-Based (DB)-TENG is found to generate a voltage of ∼18.3 V, current of 0.9 µA and power of 64 µW at a load resistance of 150 MΩ by drop casting the NaCl droplet over GaN thin film. The direct growth of GaN on Aluminium Nitride over different substrate including Si(n + +), Si (100), Sapphire, and Si (111) is achieved using the Laser Molecular Beam Epitaxy technique, for the improved device performance, a substrate-dependent comparison analysis is also conducted. The effect of ultraviolet (UV) light of wavelength 325 nm on the DB-TENG has also been studied to enhance the induced signal and the maximum output voltage of ∼ 42 V, current of 8.8 µA and maximum output power of 824 µW were measured with 150 MΩ load resistance. The results indicate an enhancement in the generated voltage by 3 times and the current by one order upon UV illumination. This work is used for energy harvesting in the form of mechanical energy and photon energy by coupling the photovoltaic and triboelectric effects.

Investigating defects in InGaN based optoelectronics: from material and device perspective

III-nitride optoelectronics have revolutionized solid-state lighting technology. However, non-radiative defects play a major bottleneck in determining the performance of InGaN-based optoelectronics devices. It becomes especially challenging when high indium is required to be incorporated to obtain emission at higher wavelength (>500 nm). In this research article, we are going to discuss our investigation on the origin of defects in InGaN-based optoelectronics devices from the material and device perspective and characterize them through various techniques. This article broadly consists of two parts. In the first part, we investigate defects in InGaN based optoelectronics from a material point of view. Here, we discuss the challenges in the growth of InGaN planar (2-dimensional) and nanowires (1-dimensional) with high indium (≥20%) incorporation using the plasma-assisted molecular beam epitaxy (PA-MBE) technique. Photoluminescence spectroscopy (PL) has been performed to characterize these grown samples to assess their optical quality. Atomic force microscopy (AFM) has been employed to characterize the surface morphology of grown InGaN layers. High-resolution transmission electron microscopy (HRTEM) and scanning electron microcopy (SEM) are also used to characterize InGaN planar and nanowire samples grown under various process conditions. In the second part, we investigate the role of defects on InGaN optoelectronics from a device point of view. Here, we discuss the fabrication of InGaN multi-quantum well-based light emitting diodes (LEDs). Temperature-dependent current versus voltage measurements are carried out to investigate the role of defects on carrier dynamics under forward and reverse bias conditions. Frequency-dependent capacitance versus voltage (CV) and conductance versus voltage (GV) techniques are employed extensively to characterize defects in fabricated InGaN LEDs.

Quantum transport and mobility spectrum of topological carriers in (001) SnTe/PbTe heterojunctions

Measurements of magnetotransport in SnTe/PbTe heterojunctions grown by the molecular beam epitaxy technique on (001) undoped CdTe substrates were performed. At low magnetic fields, quantum corrections to conductivity were observed that may be attributed to the presence of topological states at the junction interface. For a sample with a 5-nm-thick SnTe layer, the data analysis suggests that midgap states are actually gapped. However, the phase coherence effects in 10 and 20 nm SnTe/PbTe samples are fully explained assuming the existence of gapless Dirac cones. Magnetotransport at higher magnetic fields is described in the framework of mobility spectrum analysis (MSA). We demonstrate that the electron- and holelike peaks observed simultaneously for all SnTe/PbTe heterojunctions may originate from the concave and convex parts of the energy isosurface for topological states---and not from the existence of quasiparticles both carrying negative and positive charges. This interpretation is supported by numerical calculations of conductivity tensor components for gapless (100) Dirac cones, performed within a classical model and based on the solutions of the Boltzmann transport equation. Our approach shows the feasibility of MSA in application to magnetotransport measurements on topological matter.

Improving structure design of active region of InAs quantum dots by using InAs/GaAs digital alloy superlattice

A 1.3-μm InAs quantum dot laser has been successfully fabricated on a GaAs(100) substrate by molecular beam epitaxy (MBE) technique through using InAs/GaAs digital alloy superlattices instead of the conventional InGaAs layer. The samples grown by conventional growth method and the digital alloy superlattice growth method are characterized by atomic force microscope (AFM) and photoluminescence (PL) spectroscopy. It is found that 8-period sample possesses a low quantum dot density and poor luminescence performance. With the increase of the number of growth periods, the quantum dot density of the sample increases and the luminous performance improves. This indicates that the quality of the grown sample improves with the increase of InAs/GaAs period of the InGaAs layer. When the total InAs/GaAs period is 32, the quantum dot density of the sample is high and the luminescence performance is good. After the experimental measurement, the sample DAL-0 fabricated by conventional growth method and the sample DAL-32 (32-periods InAs/GaAs digital alloy superlattices) are utilized to fabricate quantum dot laser by standard process. The performances of two types of quantum dot lasers obtained with different growth methods are characterized. It is found that the InAs quantum dot lasers fabricated by the sample grown by digital alloy superlattice method have good performances. Under continuous wave operation mode, the threshold current is 24 mA corresponding to a threshold current density of 75 A/cm<sup>2</sup>. The highest operation-temperature reaches 120 ℃. In addition, InAs quantum dot laser using digital alloy superlattice has good temperature stability. Its characteristic temperature is 55.4 K. Compared with the traditional laser, the InAs quantum dot laser grown by InAs/GaAs digital alloy superlattice has good performance in terms of threshold current density, output power and temperature stability, which indicates that high-quality laser can be obtained by this growth method. Using the InAs/GaAs digital alloy superlattice growth method, the InGaAs composition can be changed without changing the temperature of the source oven. Thus InAs quantum dot lasers with different luminescence wavelengths can be obtained through this growth method. The InAs/GaAs digital alloy superlattice structure can be used to realize different averaging of In content in the growth structure. The method provides a new idea for designing and growing the active region of quantum dot laser.

Pressure-dependent bandgap study of MBE grown {CdO/MgO} short period SLs using diamond anvil cell

Semiconductor superlattices (SLs) have found widespread applications in electronic industries. In this work, a short-period SL structure composed of CdO and MgO layers was grown using a plasma-assisted molecular beam epitaxy technique. The optical property of the SLs was investigated by absorption measurement at room temperature. The ambient-pressure direct bandgap was found to be 2.76 eV. The pressure dependence of fundamental bandgap has been studied using a diamond anvil cell technique. It has been found that the band-to-band transition shifts toward higher energy with an applied pressure. The bandgap of SLs was varied from 2.76 to 2.87 eV with applied pressure varied from 0 to 5.9 GPa. The pressure coefficient for the direct bandgap of SLs was found to be 26 meV/GPa. The obtained experimental result was supported by theoretical results obtained using density functional theory calculations. The volume deformation potential was estimated using the empirical rule. We believe that our findings may provide valuable insight for a better understanding of {CdO/MgO} SLs toward their future applications in optoelectronics.

Observation of interfacial strain relaxation and electron beam damage thresholds in Al0.3In0.7N/GaN heterostructures by transmission electron microscopy

The structural quality of indium-rich Al0.3In0.7N grown by metal modulated epitaxy (MME), previously demonstrating x-ray diffraction (XRD) figures of merit ∼11 times better than the previous literature is investigated to explain the origin of such a large quality improvement. Four-dimensional scanning transmission electron microscope was used to map the lattice parameter near the AlInN/GaN heterojunction and indicate a 5.4% lattice constant change, suggesting 75% relaxation within ∼2 nm from the interface. Cross-sectional TEM Moiré fringes are observed at the AlInN/GaN heterointerface, indicating that there are misfit dislocations between AlInN and GaN which, while rare, have been observed for other highly mismatched In-rich III-Nitrides. The TEM measurements show regions of contrast indicating larger scale variations in strain, but defect contrast associated with dislocations and/or intrinsic basal stacking faults was minimal, indicating a good quality AlInN film and confirming prior XRD results. Significant electron beam induced damage can occur and depended strongly on operational conditions. The damage threshold current density was estimated using time-dependent TEM to be ∼5.7 A/cm2, significantly lower than from prior studies of InGaN. Damage also strongly depends on the thickness of the TEM foil examined and occurred at thicknesses greater than found for InGaN. The present study suggests that the MME technique is an excellent candidate for growing high-quality indium-rich AlInN films as compared to the traditional molecular beam epitaxy or metal organic chemical vapor deposition techniques.

Epitaxial Growth of One‐Monolayer Pb1−xBix Alloy Films

Motivated by recent theoretical reports of intrinsic topological superconductivity in Pb3Bi alloy systems where large Rashba gap and type II van Hove singularity coexist, one‐monolayer Pb 1− x Bi x alloy films have been successfully grown by molecular beam epitaxy technique and systematically investigated by scanning tunneling microscopy and spectroscopy in this work. There coexist two different stable phases in the Pb 1− x Bi x alloyed films: 3 × 3 semiconducting phase with a bandgap as large as 0.6 eV, and 3√3 × 3√3 metallic phase. Furthermore, the formation of the two different Pb 1− x Bi x monolayer films can be tuned by changing the Bi content and annealing processes. These findings pave the way for further exploration of superconductivity and topological properties of the Pb x Bi 1− x systems.

Growth of uniform Mg-doped p-AlGaN nanowires using plasma-assisted molecular beam epitaxy technique for UV-A emitters

In this manuscript, we have shown the growth and extensive structural and optical characteristic of the uniformly Mg-doped Al0.23Ga0.77N (UV-A region, λ ∼ 323 nm) nanowire. The Kelvin probe force microscopy was employed to determine the profile of holes in p-type AlGaN nanowires by measuring the work function changes induced by Mg incorporation. The influence of surface band bending on doping concentration has thoroughly been discussed. Our experiment confirms the homogeneous incorporation of Mg throughout the nanowire without any top surface Mg segregation. In this work, we have also demonstrated a comprehensive analysis of acceptor states induced thermal quenching behaviour in the optical transition of Mg-doped AlGaN nanowire. We propose a phenomenological model, based on the rate equation which confirms that achieving higher ‘hole’ (p-doping) concentration in AlGaN nanowire (>1018 cm−3) is more conducive than the planar counterpart if the growth of NWs is carried out at optimized process conditions. This rate equation-based model has also demonstrated the influence of sidewall surface passivation in those AlGaN nanowires.

Optical properties of LMBE grown c-axis oriented GaN thin films using Surface Plasmon Resonance technique

Present work reports the growth and optimisation of thin films of Gallium nitride (GaN) with preferred c-axis (0002) orientation on Si (111) substrate using the Laser Molecular Beam Epitaxy technique. For the development of optically efficient GaN thin film, the nitrogen gas flow during film deposition has been optimised and film properties are measured using the highly sensitive Surface Plasmon Resonance (SPR) technique in OTTO configuration. Structural, morphological and optical properties of the GaN thin films were studied using XRD, AFM, PL and FTIR spectroscopy. Varying the deposition gas flow from 1.3 sccm to 4 sccm led to a decrease in refractive index values from 2.77 to 2.21. By using the SPR technique, the refractive index (n) dispersion of GaN thin films was also analysed with the incident radiated wavelength.

Ohmic-Schottky conversion of ZnO/metal contact modulated by a plasma surface treatment method

Herein, we present a systematical investigation with regard to the Ar/N2/O2 plasma surface treatments on the ZnO thin films prepared by a radio frequency plasma-assisted molecular beam epitaxy (RF MBE) technique. The as-deposited ZnO thin films are exposed to the Ar, N2 and O2 plasma atmospheres under different RF powers. The conductivity and the surface morphology of the post-treated ZnO thin films are strongly depending on the treatment species and the RF powers. Higher RF power results in lower resistance and reduces the grain boundaries of the surface. Indium and gold metals are designed as the Ohmic and Schottky contact electrodes, respectively. The ZnO/Au contacts based on both pre-treated films and Ar/N2 plasma-treated films show obvious Ohmic contact behaviors. However, In/ZnO/Au the devices fabricated on the ZnO thin film which exposes to the O2 plasma show distinct rectified behavior, indicating the conversion of ZnO/Au contact from Ohmic type to Schottky type. The O2 plasma treatment may play two key roles within the contact conversion: 1, removing the surface contaminants such as carbon, absorbed oxygen and hydroxide; 2, repairing the defects (Vo), thus reducing the near surface donor density and defect assisted hopping transportation.

A vertical CsPbBr3/ZnO heterojunction for photo-sensing lights from UV to green band.

In this work, we have reported a vertical CsPbBr3/ZnO heterojunction photodetector for photo-sensing lights from UV to visible band. The ZnO thin film is deposited on the c-sapphire substrate through a molecular beam epitaxy (MBE) technique, and then the CsPbBr3 thin film is synthesized on the as-prepared ZnO film layer by using a solution processing method. The as-prepared CsPbBr3/ZnO heterostructure presents type-II energy band structure induced by the energy band offset effect, which can promote the separation and extraction efficiencies of the photo-generated electron-hole pairs. Compared with the CsPbBr3 based metal-semiconductor-metal (MSM) structure photodetector, the heterojunction photodetector presents higher responsivity and detectivity of 630 µA/W and 7 × 109 Jones. While compared with the ZnO based MSM structure photodetector, the heterojunction device reveals much faster response speeds of 61 µs (rise time) and 1.4 ms (decay time). These findings demonstrate that the CsPbBr3/ZnO heterojunction photodetector is promising for constructing next generation perovskite based optoelectronic devices.

Interface superconductivity in FeSe thin films on SrTiO grown by the PLD technique

In this study, we fabricate 5–30 nm thick films of FeSe/STO using pulsed laser deposition (PLD). The grown films exhibit superconductivity with an onset that is much higher than that of bulk FeSe under ambient pressure. The observed values are exceptionally high in terms of the strain vs. relationship of the same material established so far. Furthermore, increases as the film thickness decreases, except for films thinner than 10 nm. This thickness dependence of is in good agreement with the results reported for films grown by molecular beam epitaxy (MBE) that exhibit interface superconductivity. These results indicate the realization of interface superconductivity in PLD-grown FeSe/STO. Furthermore, our PLD technique requires no post-annealing to realize interface superconductivity, which is different from MBE techniques. Because the PLD technique has the advantage that various interfaces can be fabricated easily by simply altering the target materials, our results open novel routes to study interface superconductivity toward higher by systematic control of the interface.

High-operating temperature far-infrared Si:Ga blocked-impurity-band detectors

Silicon-based blocked impurity band (BIB) detectors have become the preferred candidate for the astronomical observation field because of their excellent ability for far-infrared detection, easy integration with the readout circuit, and potential for large-scale preparation. We fabricate Si:Ga BIB far-infrared detectors by a molecular beam epitaxy technique with an impressive blackbody specific detectivity of 4.21 × 1011 cm Hz1/2 W−1 at 10 K and nearly uniform broadband response between 2.5 and 20 μm. A response mechanism with variable temperature is described minutely by the varying temperature optoelectronic characterization and theoretical calculation as well as energy band diagram. The substantial results indicate that the responsivity of the detector can steadily maintain up to 26 K for far-infrared. This paper not only increases the accessibility of BIB detectors' fabrication tools but also provides an approach of high-operating temperature far-infrared detectors for astronomy explorations.

Electronic Properties of Fully Strained La1-x Sr x MnO3 Thin Films Grown by Molecular Beam Epitaxy (0.15 ≤ x ≤ 0.45).

The structural, electronic, and magnetic properties of Sr-hole-doped epitaxial La1–xSrxMnO3 (0.15 ≤ x ≤ 0.45) thin films deposited using the molecular beam epitaxy technique on 4° vicinal STO (001) substrates are probed by the combination of X-ray diffraction and various synchrotron-based spectroscopy techniques. The structural characterizations evidence a significant shift in the LSMO (002) peak to the higher diffraction angles owing to the increase in Sr doping concentrations in thin films. The nature of the LSMO Mn mixed-valence state was estimated from X-ray photoemission spectroscopy together with the relative changes in the Mn L2,3 edges observed in X-ray absorption spectroscopy (XAS), both strongly affected by doping. CTM4XAS simulations at the XAS Mn L2,3 edges reveal the combination of epitaxial strain, and different MnO6 crystal field splitting give rise to a peak at ∼641 eV. The observed changes in the occupancy of the eg and the t2g orbitals as well as their binding energy positions toward the Fermi level with hole doping are discussed. The room-temperature magnetic properties were probed at the end by circular dichroism.

Theory of MBE Growth of Nanowires on Adsorbing Substrates: The Role of the Shadowing Effect on the Diffusion Transport.

A new model for nanowire growth by molecular beam epitaxy is proposed which extends the earlier approaches treating an isolated nanowire to the case of ensembles of nanowires. I consider an adsorbing substrate on which the arriving growth species (group III adatoms for III-V nanowires) may diffuse to the nanowire base and subsequently to the top without desorption. Analytical solution for the nanowire length evolution at a constant radius shows that the shadowing of the substrate surface is efficient and affects the growth kinetics from the very beginning of growth in dense enough ensembles of nanowires. The model fits quite well the kinetic data on different Au-catalyzed and self-catalyzed III-V nanowires. This approach should work equally well for vapor-liquid-solid and catalyst-free nanowires grown by molecular beam epitaxy and related deposition techniques on unpatterned or masked substrates.

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.