Narrow Results

Tunneling currents and surface leakage currents are both contributors to the overall dark current which limits many semiconductor devices. Surface leakage current is generally controlled by applying a post-epitaxial passivation layer; however, surface passivation is often expensive and ineffective. Band-to-band and trap assisted tunneling currents cannot be controlled through surface passivants, thus an alternative means of control is necessary. Unipolar barriers, when appropriately applied to standard electronic device structures, can reduce the effects of both surface leakage and tunneling currents more easily and cost effectively than other methods, including surface passivation. Unipolar barriers are applied to the p-type region of a conventional, MBE grown, InAs based pn junction structures resulting in a reduction of surface leakage current. Placing the unipolar barrier in the n-type region of the device, has the added benefit of reducing trap assisted tunneling current as well as surface leakage currents. Conventional, InAspn junctions are shown to exhibit surface leakage current while unipolar barrier photodiodes show no detectable surface currents.

GaInSb and AlGaInSb compositionally graded buffer layers grown on GaSb by MBE were used to develop unrelaxed InAs_1-xSb_x epitaxial alloys with strain-free native lattice constants up to 2.1% larger than that of GaSb. The in-plane lattice constant of the strained top buffer layer was grown to be equal to the native, unstrained lattice constant of InAs_1-xSb_x with given x. The InAs_0.56Sb_0.44 layers demonstrated a photoluminescence (PL) peak at 9.4 μm at T = 150 K. The minority carrier lifetime measured at 77 K for InAs_0.8Sb_0.2 was 250 ns.

Gallium Oxide has attracted a great deal of attention for power electronics due to its large bandgap (∼4.8 eV) as well as availability of cost effective, large area, high quality substrates. In this article, we will discuss advancements in homoepitaxial and heteroepitaxial growth of β-(Al,Ga)₂O₃ and their heterostructures via different growth techniques with the emphasis on metal organic chemical vapor deposition, molecular beam epitaxy, and halide vapor phase epitaxy. We will also review various device structures demonstrated for high power applications.

The application of thermal annealing at various annealing temperatures (473–1073 K) has been shown to significantly modify surface morphology of platinum (Pt) metal contacts on AlGaN/GaN/AlN heterostructure grown on silicon by plasma-assisted molecular beam epitaxy (PA-MBE). Structural analysis of the AlGaN/GaN samples used for the Pt Schottky contacts fabrication were performed by using high resolution X-ray diffraction (HR-XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Pt metal contacts were then deposited on the samples followed by current–voltage (I–V) characterization. Thermally-treated samples showed significant decrease in current compared with untreated samples. From the I–V measurements, the Schottky barrier height (SBH) and ideality factor (n) were calculated. We found that the lowest value of SBH obtained was 0.526 eV at 873 K annealing temperature. Unfortunately, there are no values for the SBH and ideality factor at 1073 K annealing temperature. The SEM analysis has shown some island formation at high annealing temperature due to the difference of surface energies between thin metal films and AlGaN that causes dewetting. We suggest that the reason for the barrier height reduction is due to the metal island formation on the samples.

In this paper, the growth and characterization of epitaxial Al_0.29Ga_0.71N grown on Si(111) by RF-plasma assisted molecular beam epitaxy (MBE) are described. The Al mole fraction was derived from the HR-XRD symmetric rocking curve (RC) ω/2θ scans of (0002) plane as x = 0.29. PL spectrum of sample has shown sharp and intense band edge emission of GaN without the existence of yellow emission band, showing that it is comparable in crystal quality of the sample when compared with previous reports. From the Raman measurement of as-grown Al_0.29Ga_0.71N layer on GaN/AlN/Si sample. We found that the dominant E₂ (high) phonon mode of GaN appears at 572.7 cm^-1. The E₂ (high) mode of AlN appears at 656.7 cm^-1 and deviates from the standard value of 655 cm^-1 for unstrained AlN. Finally, AlGaN Schottky photodiode have been fabricated and analyzed by mean of electrical characterization, using current–voltage (I–V) measurement to evaluate the performance of this device.

An experimental investigation was conducted to explore the effect of inserting a single AlGaN interlayer between AlN epilayer and GaN/AlN heterostructures on Si (111) grown by molecular beam epitaxy (MBE). It is confirmed from the scanning electron microscopy (SEM) that the AlGaN interlayer has a remarkable effect on reducing the tensile stress and dislocation density in AlN top layer. Capacitance–voltage (C–V) measurements were conducted to study the electrical properties of AlN/GaN heterostructures. While deriving the findings through the calculation it is suggested that the AlGaN interlayer can significantly reduce the value of effective oxide charge density and total effective number of charges per unit area which are $1.37\times 10^{-6}{\mathrm{\text{C/cm}}}^2$ and $8.55\times 10^{12}{\mathrm{\text{cm}}}^{-2}$, respectively.

Ag thin films deposited on Ru (0001) surface by molecular beam epitaxy, at temperatures of 20°C and 450°C, have been investigated using reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. For both growth temperatures, the in situ RHEED patterns of the Ag films exhibited an in-plane six-fold symmetry, indicating that the Ag deposit is in epitaxy with the Ru buffer surface. At RT, the RHEED technique indicated a three-dimensional growth (3D), while a layer-by-layer growth (2D) takes place at HT. The AFM images showed a granular structure of the surface of the deposited Ag layers with a large variation of the roughness with the growth temperature. XRD analysis gave indication of a strongly textured thin film along the growth direction. The lattice mismatch between the Ag and Ru is at the origin of a stress at the interface and defects structure in the film.

Chlorine is one of the most used species to produce n-type ZnSe epilayers. In this paper, we present Secondary Ion Mass Spectrometry (SIMS) profiles of a series of Cl-doped ZnSe samples, which were grown by Molecular Beam Epitaxy (MBE) technique on GaAs substrates. These profiles have been used to examine the limitation of SIMS analysis of narrow Cl-delta layers. In order to convert SIMS raw data to quantified data, the depth profile from a Cl-implanted standard sample has been used to estimate the "useful ion yield" of chlorine and thus the instrumental sensitivity for chlorine in a ZnSe matrix. The "useful ion yield" and detection limit of chlorine in the ZnSe host matrix were calculated to be 4.7 × 10^-7 and 5 × 10¹⁷ atoms/cm³, respectively.

We have investigated the effect of Bi on the heteroepitaxial growth of Co on Cu by reflection high-energy electron diffraction (RHEED) measurements. It was found that Bi enhanced the layer-by-layer growth of Co on the Cu(111) surfaces at 100°C. The dependence of the growth on Bi layer thickness suggested that there existed a suitable amount of Bi surfactant layer that enhanced smoother layered growth. On the contrary, for the case of Co growth on Cu(100), Bi depressed the layer-by-layer growth of Co on Cu(100). The surface segregation effect of Bi was also studied by Auger electron spectroscopy (AES).

This article reports the use of plasma-assisted molecular beam epitaxy (MBE) to grow AlN on Si(111) substrate at 850°C under UHV conditions for 15, 30, and 45 min. The films were characterized by high-resolution X-ray diffraction (HR-XRD) and micro-Raman spectroscopy. XRD measurement revealed that the AlN was epitaxially grown on Si(111). Micro-Raman result showed that all the allowed Raman modes of AlN and Si were clearly visible. Fourier transform infrared (FTIR) spectroscopy has been used to investigate the A₁ (LO) and E₁ (TO) modes with frequencies (890–899) cm^-1 and (668–688) cm^-1, respectively. The results are in good agreement with reported phonon frequencies of AlN grown on Si(111).

Besides SiC, a group III-V nitrides are also suitable large-band gap semiconductor materials for high-temperature gas sensor devices. In this paper, we present the study of the H₂ sensitive device fabricated based on n-type GaN wafer. The GaN thin film with AlN buffer layer was grown on sapphire by RF plasma-assisted molecular beam epitaxy (RF-MBE). A few monolayers of AlN were deposited using high flux Al before the growth of the AlN buffer layer. This step is speculated to be able to form a better relaxed layer for the subsequent growth of the AlN buffer layer. Pt contacts with thickness of about 150 nm were then deposited on the GaN/AlN/Al₂O₃ using the sputtering system. Gas detection was carried out at room temperature. Prior to the current–voltage (I–V) measurements, the samples were annealed at temperatures ranging from 200°C to 600°C in N₂ ambience. A significant change of current in the Pt/ GaN gas sensor was observed for the 600°C annealed sample when exposed to 0.5% H₂ in N₂ gas.

Nitrogen plasma-assisted molecular beam epitaxy (PAMBE) deposited GaN thin films on (111) n-type silicon substrate with different thickness AlN buffer layers are investigated and distinguished by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and Raman scattering. The thickness of AlN buffer layer ranged from 200 nm to 300 nm. Besides that, the electrical characteristics of the GaN thin film for ultraviolet detecting utilizations are studied by calculating the photo current/dark current ratio on a metal-semiconductor-metal (MSM) photodiode with and without the illumination of Hg-lamp source. The devices have been tested over room temperature (RT). The photocurrent analysis, together with the study of Schottky barrier height (SBH) development, ascertain that the principal mechanism of photo transport is thermionic emission. The photocurrent value is rigorously dependent on Schottky barrier height. The GaN/AlN(200 nm)/n-Si MSM photodiode produces the highest photo/dark current ratio for the lowest strain that consists of the GaN film grown on the AlN (200 nm) buffer layer.

Photoluminescence (PL) measurements performed in a series of InAs self-assembled quantum dot (QD) structures grown by molecular beam epitaxy (MBE) on GaAs (100) substrates using a two-stage "nucleation-augmented" growth method show that an InAs QD "nucleation" layer grown at a fast growth rate, followed by a slowly grown InAs "augmented" layer, dramatically increases the dot density and improves the PL intensity. Besides red-shift in peak wavelength, the PL intensity was found to increase as the growth rate of the InAs augmented layer was reduced. The increased PL intensity was due to a higher dot density arising from the nucleation layer and an improved optical quality caused by the growth interruption in the augmented layer.

Experimental data are presented on variations of the in-plane lattice constant of Ge and Si films in the course of the MBE film growth on the silicon (100) surface. The in-plane lattice constant of the silicon film is shown to alter as the film grows; the changes reflect the process of relaxation of elastic strains that result from the misfit of the germanium and silicon lattice constants. Due to the presence of germanium islands, a considerably thicker silicon film is required to provide the strain relaxation. The dependence of distortion penetration depth to the silicon film on the effective germanium film thickness is obtained. TEM studies indicate the vertical ordering of the germanium island layers when the thickness of the Si layer in between Ge layers is not sufficient to provide the full strain relaxation.

The Quantum Dots/Graphene (QDs/GR) composites have attracted numerous interests caused by its unique physical and chemical properties in past few decades. The shortages of the single QD and graphene materials could be remedied by the synergistic effects from QDs/GR composite materials; meanwhile, some unique phenomena and superior physical properties were also produced. The QDs/GR composites processed better photocatalytic activities, higher photon capture abilities and excellent optical responsibilities. Therefore, they were widely applied in various techniques. Here, we reviewed and discussed recent research progresses about the QDs/GR composites and focused on their industrial preparation and commercial applications. Among these synthetic methods, ion beam sputtering deposition (IBSD) and molecular beam epitaxy (MBE) were discussed in detail because they could be directly applied in commercial industry for preparing size-tunable quantum dots. In another part, the applications of the QDs/GR composites were also discussed, the advanced physical and chemical properties promoted these composites to have numerous potential for being applied in photodetectors, lithium ion batteries, solar cells, supercapacitors and other devices. The appropriate synthetic method for QDs/GR materials is highly dependent on the requirements of its applications. We firmly believe that the direct synthesis technique of ideal QDs/GR composite for specific applications is a challenge and research emphasis for scientist and engineers in future.

Quantum dots (QDs) play an important role in fabricating electronic, photonic, and optoelectronic devices with enhanced functions and performance. There are two typical epitaxial methods for growing semiconductor QDs, one is stress-driven self-assembly, i.e., Stranski–Krastanov (S–K), and the other is droplet epitaxy (DE). The former was developed faster in earlier years and usually referred to as conventional method while the latter has attracted increasing interest in recent years driven by increasing requirement for high quality QDs towards quantum computing applications. Physically, the nucleation of QDs in both methods are self-assembly surface reactions, which have limited site-control for nucleation on flattened substrate. Pre-patterning of substrate was developed to regularly locate the nucleation sites but may also cause undesired contaminations. Laser-interference-assisted growth has emerged in the past few years, which provides a non-contact in-situ patterning for the nucleation of QDs arrays. This overview provides a glimpse of recent advances in epitaxial growth of QDs towards their advanced photonic and optoelectronic applications.

Gallium Oxide has attracted a great deal of attention for power electronics due to its large bandgap (~ 4.8 eV) as well as availability of cost effective, large area, high quality substrates. In this article, we will discuss advancements in homoepitaxial and heteroepitaxial growth of β-(Al,Ga)₂O₃ and their heterostructures via different growth techniques with the emphasis on metal organic chemical vapor deposition, molecular beam epitaxy, and halide vapor phase epitaxy. We will also review various device structures demonstrated for high power applications.

GaInSb and AlGaInSb compositionally graded buffer layers grown on GaSb by MBE were used to develop unrelaxed InAs_1-xSb_x epitaxial alloys with strain-free native lattice constants up to 2.1% larger than that of GaSb. The in-plane lattice constant of the strained top buffer layer was grown to be equal to the native, unstrained lattice constant of InAs_1-xSb_x with given x. The InAs_0.56Sb_0.44 layers demonstrated a photoluminescence (PL) peak at 9.4 μm at T = 150 K. The minority carrier lifetime measured at 77 K for InAs_0.8Sb_0.2 was 250 ns.