Chemical Beam Epitaxy Research Articles

Impact of the substrate orientation on the N incorporation in GaAsN: Theoretical and experimental investigations

The growth orientation dependence of N incorporation on traditional (100) and high-index GaAs (311)A/B planes has been investigated by the first principle calculation and experimental measurement. Due to the electronegativity and atomic radius differences between As and N atoms, the GaN bond is much shorter than the corresponding GaAs bond as N occupies As atom site. The optimization to energetically preferred construction leads to the inward moving of N atom, while the displacement extent of three surface models exhibits considerable differences. More importantly, the theoretical result reveals a lower formation energy of N doping for N@(311)B GaAs surface and a larger one for N@(311)A GaAs surface as compared with the traditional (100) GaAs plane. The first principle investigations thus suggest the enhanced N incorporation in (311)B GaAsN, which is in good accordance with the secondary ion mass spectrometry (SIMS) measurement results of GaAsN layers by chemical beam epitaxy (CBE).

Thickness modulation and strain relaxation in strain-compensated InGaP/InGaP multiple-quantum-well structure grown by metalorganic molecular beam epitaxy on GaAs (100) substrate

We have investigated the structural features of a strain-compensated InGaP/InGaP multiple-quantum-well (MQW) structure on GaAs (100) substrate with a band-gap energy of around 1.7eV for solar cell applications. In transmission electron microscopy images, noticeable thickness modulation was observed in the barrier layers for a sample grown at the substrate temperature of 530°C. Meanwhile, the X-ray diffraction patterns indicated that strain relaxation predominantly occurred in the well layers. Decreasing the substrate temperature from 530 to 510°C was effective in suppressing both the thickness modulation and strain relaxation. Additionally, increasing the growth rate of the well layer further suppressed the thickness modulation. In room-temperature photoluminescence (PL) emission spectra, the sample grown at 510°C showed approximately 50 times higher PL peak intensity than the one grown at 530°C.

Observation of the quantum Hall effect in δ-doped SrTiO3.

The quantum Hall effect is a macroscopic quantum phenomenon in a two-dimensional electron system. The two-dimensional electron system in SrTiO3 has sparked a great deal of interest, mainly because of the strong electron correlation effects expected from the 3d orbitals. Here we report the observation of the quantum Hall effect in a dilute La-doped SrTiO3-two-dimensional electron system, fabricated by metal organic molecular-beam epitaxy. The quantized Hall plateaus are found to be solely stemming from the low Landau levels with even integer-filling factors, ν=4 and 6 without any contribution from odd ν's. For ν=4, the corresponding plateau disappears on decreasing the carrier density. Such peculiar behaviours are proposed to be due to the crossing between the Landau levels originating from the two subbands composed of d orbitals with different effective masses. Our findings pave a way to explore unprecedented quantum phenomena in d-electron systems.

First principles kinetic Monte Carlo study on the growth patterns of WSe2 monolayer

The control of domain morphology and defect level of synthesized transition metal dichalcogenides (TMDs) is of crucial importance for their device applications. However, current TMDs synthesis by chemical vapor deposition and molecular beam epitaxy is in an early stage of development, where much of the understanding of the process-property relationships is highly empirical. In this work, we use a kinetic Monte Carlo coupled with first principles calculations to study one specific case of the deposition of monolayer WSe2 on graphene, which can be expanded to the entire TMD family. Monolayer WSe2 domains are investigated as a function of incident flux, temperature and precursor ratio. The quality of the grown WSe2 domains is analyzed by the stoichiometry and defect density. A phase diagram of domain morphology is developed in the space of flux and the precursor stoichiometry, in which the triangular compact, fractal and dendritic domains are identified. The phase diagram has inspired a new synthesis strategy for large TMD domains with improved quality.

Nucleation and growth mechanism of self-catalyzed InAs nanowires on silicon

We report on the nucleation and growth mechanism of self-catalyzed InAs nanowires (NWs) grown on Si (111) substrates by chemical beam epitaxy. Careful choices of the growth parameters lead to In-rich conditions such that the InAs NWs nucleate from an In droplet and grow by the vapor–liquid–solid mechanism while sustaining an In droplet at the tip. As the growth progresses, new NWs continue to nucleate on the Si (111) surface causing a spread in the NW size distribution. The observed behavior in NW nucleation and growth is described within a suitable existing theoretical model allowing us to extract relevant growth parameters. We argue that these results provide useful guidelines to rationally control the growth of self-catalyzed InAs NWs for various applications.

Indium Antimonide Nanowires: Synthesis and Properties.

This article summarizes some of the critical features of pure indium antimonide nanowires (InSb NWs) growth and their potential applications in the industry. In the first section, historical studies on the growth of InSb NWs have been presented, while in the second part, a comprehensive overview of the various synthesis techniques is demonstrated briefly. The major emphasis of current review is vapor phase deposition of NWs by manifold techniques. In addition, author review various protocols and methodologies employed to generate NWs from diverse material systems via self-organized fabrication procedures comprising chemical vapor deposition, annealing in reactive atmosphere, evaporation of InSb, molecular/ chemical beam epitaxy, solution-based techniques, and top-down fabrication method. The benefits and ill effects of the gold and self-catalyzed materials for the growth of NWs are explained at length. Afterward, in the next part, four thermodynamic characteristics of NW growth criterion concerning the expansion of NWs, growth velocity, Gibbs–Thomson effect, and growth model were expounded and discussed concisely. Recent progress in device fabrications is explained in the third part, in which the electrical and optical properties of InSb NWs were reviewed by considering the effects of conductivity which are diameter dependent and the applications of NWs in the fabrications of field-effect transistors, quantum devices, thermoelectrics, and detectors.

Effect of Dot-Height Truncation on the Device Performance of Multilayer InAs/GaAs Quantum Dot Solar Cells

The effect of dot-height truncation on the device performance of multilayer InAs/GaAs quantum dot solar cells is investigated. The different structures were grown by chemical beam epitaxy, and an indium-flush process is used to control the dot height. A series of ten-layer samples with dots truncated at a height of 5 and 2.5 nm, respectively, are studied. Luminescence, atomic force microscopy, and high-resolution scanning transmission electron microscopy results indicate that the quantum dot properties are preserved up to the tenth layer for both structures. Under $\text{1}$ -sun illumination, the truncation of the dot height to 2.5 nm increased the short-circuit current density by $\text{0.7}$ mA/cm2 and the open-circuit voltage by $\text{31}$ mV. From the external quantum efficiency curves, limited to wavelengths $ > \text{500}$ nm, a $\text{1.46}$ -mA/cm2 current density enhancement is found over a GaAs reference cell. At least $\text{45}\%$ of this enhancement has been attributed uniquely to the presence of quantum dots in the structure.

In-situheight engineering of InGaAs / GaAs quantum dots by chemical beam epitaxy

This PDF file contains the editorial “Special Section Guest Editorial: Optics, Spectroscopy, and Nanophotonics of Quantum Dots” for JNP Vol. 10 Issue 03

In-situheight engineering of InGaAs / GaAs quantum dots by chemical beam epitaxy

This work reports on a chemical beam epitaxy growth study of InGaAs/GaAs quantum dots (QDs) engineered using an in-situ indium-flush technique. The emission energy of these structures has been selectively tuned over 225 meV by varying the dot height from 7 to 2 nm. A blueshift of the photoluminescence (PL) emission peak and a decrease of the intersublevel spacing energy are observed when the dot height is reduced. Numerical investigations of the influence of dot structural parameters on their electronic structure have been carried out by solving the single-particle one-band effective mass Schrodinger equation in cylindrical coordinates, for lens-shaped QDs. The correlation between numerical calculations and PL results is used to better describe the influence of the In-flush technique on both the dot height and the dot composition.

Optical and Electronic Properties of Two-Dimensional Layered Materials

Modern semiconductor devices have revolutionized wide-ranging technologies such as electronics, lighting, solar energy, and communication [1]. The semiconductor industry employs Si to fabricate electronic circuits, and GaAs, GaN, and other III–V materials for optoelectronics [2], with typical substrates consisting of wafers manufactured at high temperature. Precisely controlled thin films can be deposited on the substrate to achieve additional functionality, for example by chemical vapor deposition (CVD) or molecular beam epitaxy [3].

Growth of free-standing wurtzite AlGaN by MBE using a highly efficient RF plasma source

Ultraviolet light emitting diodes (UV LEDs) are now being developed for various potential applications including water purification, surface decontamination, optical sensing, and solid-state lighting. The basis for this development is the successful production of AlxGa1−xN UV LEDs grown by either metal-organic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE). Initial studies used mainly sapphire as the substrate, but this result in a high density of defects in the epitaxial films and now bulk GaN or AlN substrates are being used to reduce this to acceptable values. However, the lattice parameters of GaN and AlN are significantly different, so any AlGaN alloy grown on either substrate will still be strained. If, however, AlGaN substrates were available, this problem could be avoided and an overall lattice match achieved. At present, the existing bulk GaN and AlN substrates are produced by MOVPE and physical vapor transport, but thick free-standing films of AlGaN are difficult to produce by either method. The authors have used plasma-assisted MBE to grow free-standing AlxGa1−xN up to 100 μm in thickness using both an HD25 source from Oxford Applied Research and a novel high efficiency source from Riber to provide active nitrogen. Films were grown on 2- and 3-in. diameter sapphire and GaAs (111)B substrates with growth rates ranging from 0.2 to 3 μm/h and with AlN contents of 0% and ∼20%. Secondary ion mass spectrometer studies show uniform incorporation of Al, Ga, and N throughout the films, and strong room temperature photoluminescence is observed in all cases. For films grown on GaAs, the authors obtained free-standing AlGaN substrates for subsequent growth by MOVPE or MBE by removing the GaAs using a standard chemical etchant. The use of high growth rates makes this a potentially viable commercial process since AlxGa1−xN free-standing films can be grown in a single day and potentially this method could be extended to a multiwafer system with a suitable plasma source.

Electrical and structural properties of AlGaNAs alloys grown by chemical beam epitaxy

We correlate the structural properties of aluminium‐based dilute nitrides to their electrical properties. The effect of annealing on the carrier density and the mobility is measured on chemical beam epitaxy grown Al0.05Ga0.95N0.005As0.995 alloys. Both parameters increase with the annealing temperature due to the disappearance of N–C and N–H–VGa complexes. After annealing, a mobility of 60 cm2 V−1s−1 for a hole concentration of 1 × 1019 cm−3 are measured, which is similar to the values reported on GaAs with the same carrier concentration.

Reactive sputtering of III-N materials for applications in electronic devices

ABSTRACTGallium Nitride (GaN) and other III-N semiconductors are rapidly gaining importance in high power and high frequency electronic applications. III-N material based devices are fabricated on heterostructures that are usually grown by high vacuum techniques such as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). However, in many applications, it is necessary to regrow thin cap layers of III-N materials during device fabrication. One such application is regrowth of ohmic contacts to III-N devices. Heavily doped n+ GaN, or InGaN grown by MBE or MOCVD is used to obtain low resistance non-alloyed ohmic contacts to GaN based devices. However, from a commercial point of view, this becomes difficult because of the high cost and lack of availability of ultra high vacuum (∼1x10-10 Torr) techniques in most clean room facilities. Reactive sputtering provides a cheaper and more ubiquitous alternative for the growth of thin cap layers on parent MOCVD III-N heterostructures during device fabrication. In this work, we explore the possibility of using reactive sputtering as a method to grow III-N materials as ohmic contacts to GaN based devices.

Inhomogeneous nitrogen incorporation effects on the transport properties of GaAsN grown by CBE

In this paper, the Chemical Beam Epitaxy approach to GaAsN material growth has been investigated. Photoluminescence and Hall effect measurements were performed to clarify the influence of defects on the transport properties, those of holes in particular. The PL intensity of near-band emission (1.35eV-peak) for GaAs0.996N0.004 at low temperature (<100K) can be well fitted by considering the effects of N-related localized states, whilst the PL intensity was followed an Arrhenius dependence in the temperature region higher than 150K. It was concluded that the formation of N-related localized states, arising from the alloy disorder (N clusters), still cannot be avoided in GaAsN deposited by CBE. To further study its effects on the transport properties, we have also studied the contribution of various alloy scattering mechanisms to the temperature dependence of Hall mobility. The alloy scattering coefficient KAlloy-1 was calculated from the fitting of an alloy scattering mechanism. In addition, a pseudo-alloy scattering coefficient KAlloy-2 was theoretical deduced by assuming the nitrogen atoms were homogeneously incorporated along the growth direction. The ratio of those two values was studied: the departure from 1 (KAlloy-2−1/KAlloy-1−1>1) indicated that the N atoms were alloyed in a disordered arrangement. As a result, the corresponding alloy scattering mechanism was enhanced, contributing to the hole mobility decrease. However, compared to p-type GaAsN grown by MBE or MOCVD, our CBE-grown GaAsN showed a higher value of hole mobility.

Electrical characterization of Cu Schottky contacts to n-type GaAsN grown on (311)A/B GaAs substrates

Abstract The contact behavior of Cu on n-type GaAsN, grown on (100) and (311)A/B GaAs substrates by chemical beam epitaxy, has been investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements. Both N incorporation and growth orientation are found to influence the electrical properties of Cu/GaAsN Schottky diodes. The increasing N composition leads to an increase in the ideality factor and Schottky barrier height (C–V), whereas a decrease in the carrier concentration in samples with all adopted growth orientations. Compared with (100) and (311)B samples, the Schottky diodes constructed on (311)A GaAsN epilayer exhibit better performance with ideality factor close to 1 and higher barrier height. The interfacial chemical reaction between metal layer and surface atoms on the growth surface is believed to account for the observations.

Catalyst-free growth of InAs nanowires on Si (111) by CBE

We investigate a growth mechanism which allows for the fabrication of catalyst-free InAs nanowires on Si (111) substrates by chemical beam epitaxy. Our growth protocol consists of successive low-temperature (LT) nucleation and high-temperature growth steps. This method produces non-tapered InAs nanowires with controllable length and diameter. We show that InAs nanowires evolve from the islands formed during the LT nucleation step and grow truly catalyst-free, without any indium droplets at the tip. The impact of different growth parameters on the nanowire morphology is presented. In particular, good control over nanowire aspect ratio is demonstrated. A better understanding of the growth process is obtained through the development of a theoretical model combining the diffusion-induced growth scenario with some specific features of the catalyst-free growth mechanism, along with the analysis of the V/III flow ratio influencing material incorporation. As a result, we perform a full mapping of the nanowire morphology versus growth parameters which provides useful general guidelines on the self-induced formation of III–V nanowires on silicon.

The effects of the barrier and δ-doping on the electron tunneling in a nonmagnetic heterostructure

In this paper, we investigated the spin-dependent electron transport in a nonmagnetic nanostructure under the influence of the [Formula: see text]-doping which can be experimentally doped in the nanostructure with metal-organic chemical vapor deposition or molecular beam epitaxy. The numerical results indicate that the large spin polarization nearly 100% can be achieved in such a structure without the external magnetic field due to spin–orbit interactions and it can be controlled not only by the barrier, but also by the [Formula: see text]-doping. These interesting finds are very valuable for designing the [Formula: see text]-doping-tunable spintronic device based on the nonmagnetic nanostructure.

Double acceptor in p-type GaAsN grown by chemical beam epitaxy

The properties of the acceptor states in GaAsN grown by chemical beam epitaxy (CBE) are studied by analyzing their charges based on the Poole–Frenkel model. Deep level transient spectroscopy (DLTS) shows two acceptor levels at 0.11 and 0.19eV above the valence band maximum. The emission rates of carriers from these states are enhanced with increasing the electric field during the DLTS measurement, which indicates that the energies required for the emission are decreased. By analyzing this field-enhanced emission process, the polarizabilities of the levels at 0.11 and 0.19eV are found to be −1 (±0.1) and −2 (±0.1), respectively. In addition, these states have almost the same concentration. Therefore, we conclude that they originate from the same defect, acting as a double acceptor in GaAsN film grown by CBE.

Fabrication and characterization of multiband solar cells based on highly mismatched alloys

Multiband solar cells are one type of third generation photovoltaic devices in which an increase of the power conversion efficiency is achieved through the absorption of low energy photons while preserving a large band gap that determines the open circuit voltage. The ability to absorb photons from different parts of the solar spectrum originates from the presence of an intermediate energy band located within the band gap of the material. This intermediate band, acting as a stepping stone allows the absorption of low energy photons to transfer electrons from the valence band to the conduction band by a sequential two photons absorption process. It has been demonstrated that highly mismatched alloys offer a potential to be used as a model material system for practical realization of multiband solar cells. Dilute nitride GaAs1-xNx highly mismatched alloy with low mole fraction of N is a prototypical multiband semiconductor with a well-defined intermediate band. Currently, we are using chemical beam epitaxy to synthesize dilute nitride highly mismatched alloys. The materials are characterized by a variety of structural and optical methods to optimize their properties for multiband photovoltaic devices.

GaAs nanowires grown by Ga-assisted chemical beam epitaxy: Substrate preparation and growth kinetics

Growth kinetics of GaAs nanowires (NWs) on Si(111) substrates by Ga-assisted chemical beam epitaxy is studied as a function of growth conditions such as substrate temperature (Ts), V/III flux ratio and catalyst dimension. The preparation method for Si(111) substrates is optimized in order to obtain a thin surface oxide with a thickness around 0.5nm, allowing both the decomposition of metalorganic precursors and GaAs nucleation at oxide pinholes. The use of thinner oxides enables the growth of a GaAs layer whereas the utilization of thicker oxides could even inhibit GaAs nucleation. The successful self-formation of Ga droplets over this slightly oxidized Si surface has been observed by scanning electron microscopy (SEM), whose initial size is demonstrated to affect both the NW growth rate and the resultant NW aspect ratio. The formation of these droplets is crucial to enable the catalytic growth of NWs whose morphology is thoroughly analyzed by SEM, showing a self-organized array of vertically aligned match shaped GaAs NWs with a hexagonal footprint. In addition, the crystalline structure of NWs is monitored in-situ by reflection high energy diffraction, showing pure zincblende phase along the whole NW stem.In terms of better NW aspect ratio, higher crystalline quality and faster growth rates, the best NW growth conditions are found at Ts=580°C, using an effective flux ratio V/III≈0.8. Moreover, NW growth kinetics is demonstrated to be improved when using a pre-deposited Ga coverage of 7.5 monolayers, stabilized for 90s prior to the NW growth.