Radio-frequency Molecular Beam Epitaxy Research Articles

Substrate Terrace Width Dependence of Direct Growth of GaN on ScAlMgO4 by Radio‐Frequency Molecular Beam Epitaxy

ScAlMgO4 (SAM) substrates have attracted considerable attention as platforms for GaN growth in recent years because GaN can be grown directly on SAM without any buffer layer. Herein, the effect of the terrace width of SAM substrates on direct GaN growth is investigated using radio‐frequency molecular beam epitaxy (RF‐MBE). A flat and single wurtzite (WZ)‐phase GaN film is grown on a SAM substrate with a large terrace width. In contrast, a rough GaN film with a mixture of WZ and zinc‐blende (ZB) phases is obtained on SAM with a small terrace width. The SAM step height of 0.8 nm corresponds to three GaN molecular layers. Therefore, ZB stacking faults may be generated during the coalescence of islands located on adjacent terraces, although only the WZ‐GaN islands grow in the initial growth stage. This indicates that SAM substrates with a larger terrace width, that is, a smaller off‐cut angle, are preferable for obtaining a flat WZ‐GaN film.

Effect of Dry Oxidation and Thermal Annealing on AlN/GaN/AlN/Si (111) and Evaluation of its Electrical Characteristics

We proposed a technique for improving the platinum (Pt) Schottky contact dark current of the AlN/GaN/AlN/Si(111) substrate. The AlN/GaN/AlN/ heterostructure sample was successfully grown on a silicon substrate by radio frequency molecular beam epitaxy. The high quality of the interlayer heterostructure sample was verified by transmission electron microscopy (TEM). From the TEM image, a good quality single interface layer with spacing less than 1 nm was detected. The strong significant peaks obtained by X-ray diffraction measurement indicated that the sample has a high structural quality for each grown layer. Dry oxidation and thermal annealing were used in conjunction to effectively reduce the leakage current of the Schottky contact of the AlN/GaN/AlN/Si(111) substrate. Energy-dispersive X-ray analysis revealed the presence of the element oxygen. Dry oxidation enhanced the surface roughness and surface-active area of the samples. Al2O3 contributed to the low leakage current of the Pt Schottky contact of the AlN/GaN/AlN/Si(111) substrate. The Al2O3 layer acted as an insulator layer, and retarded the current flow of devices.

Behaviour of Raman B1 (high) mode and evaluation of crystalline quality in the InxGa1–xN alloys grown by RF-MBE

InxGa1–xN ternary alloys are very promising for a variety of applications. However, high-quality growth of InxGa1–xN alloys, particularly in the intermediate In composition range, is very difficult. This study reports on a systematic analysis of the Raman spectra from the InxGa1–xN alloys grown by radio-frequency molecular beam epitaxy (RF-MBE) for the whole In compositional range, particularly in the intermediate range of In composition. The B1 (high) mode, which is inherently Raman inactive is observed for the InxGa1–xN alloys grown by RF-MBE. The behaviour of Raman inactive B1 (high) mode is studied for the evaluation of InxGa1–xN quality, which is found to vary with the In composition and the temperature of growth. The crystallinity of the InxGa1–xN alloys can be assessed using B1 (high) mode’s relative signal intensity and full-width at half-maximum, which are well agreed with the reflection high energy electron diffraction and X-ray diffraction analyses. The optimum growth temperature for the InxGa1–xN alloys grown by RF-MBE in the intermediate range of In composition is also discussed.

Point defects in BaSi2 thin films for photovoltaic applications studied by positron annihilation spectroscopy

Barium di-silicide (BaSi2) is a very promising absorber material for high-efficiency thin-film solar cells, due to its suitable bandgap, high light absorption coefficient, and long minority-carrier lifetime. In this study, we compare the nanostructure, layer composition, and point defects of BaSi2 thin films deposited by Radio Frequency (RF) sputtering, Thermal Evaporation (TE), and Molecular Beam Epitaxy (MBE), using Doppler Broadening Positron Annihilation Spectroscopy (DB-PAS) depth profiling, Raman spectroscopy, and x-ray diffraction. Our DB-PAS study on thermally annealed RF-sputter deposited and on TE-deposited BaSi2 layers, in a comparison with high quality BaSi2 films produced by MBE, points to the presence of vacancy-oxygen complexes and Si or Ba mono-vacancies, respectively, in the (poly)crystalline BaSi2 films. The degree of near-surface oxidation increases, going from MBE and TE to the industrially applicable RF-sputtered deposition synthesis. The use of a-Si capping layers on the thermally annealed RF-sputtered BaSi2 films leads to a clear reduction in sub-surface oxidation and improves the quality of the BaSi2 films, as judged from DB-PAS.

Growth Mechanism of InN Nucleation Layers on Epitaxial Graphene Using Metal Organic Vapor Phase Epitaxy and Radio-Frequency Molecular Beam Epitaxy

This paper presents a growth mechanism and a comparative study of the InN nucleation layers grown on epitaxial graphene (EG) by metal organic vapor phase epitaxy (MOVPE) and radio-frequency molecular beam epitaxy (RF-MBE). Before growing InN nucleation layers, the EG surface was formed on 4° off 6H-SiC substrates (both Si- and C-faces) by thermal annealing in Ar ambient. The MOVPE grown InN nucleation layers show three-dimensional (3D) growth from the very beginning due to lack of nucleation centers and for the long migration length on the EG surface. The MOVPE InN on the EG formed on Si-face SiC is nucleated with the step edge, while it is nucleated randomly over the entire EG surface on C-face SiC. In contrast, the RF-MBE InN nucleation layers grown on EG formed on Si-face SiC show a two-dimensional growth mode up to 3 ML, and then a further increase of nucleation layer changes the growth mode to 3D mode. The formation of adsorption sites on the graphene surface by introduction of defects plays a signif...

Highly-efficient GaN-based light-emitting diode wafers on La 0.3 Sr 1.7 AlTaO6 substrates.

Highly-efficient GaN-based light-emitting diode (LED) wafers have been grown on La0.3Sr1.7AlTaO6 (LSAT) substrates by radio-frequency molecular beam epitaxy (RF-MBE) with optimized growth conditions. The structural properties, surface morphologies, and optoelectronic properties of as-prepared GaN-based LED wafers on LSAT substrates have been characterized in detail. The characterizations have revealed that the full-width at half-maximums (FWHMs) for X-ray rocking curves of GaN(0002) and GaN(10-12) are 190.1 and 210.2 arcsec, respectively, indicating that high crystalline quality GaN films have been obtained. The scanning electron microscopy and atomic force microscopy measurements have shown the very smooth p-GaN surface with the surface root-mean-square (RMS) roughness of 1.3 nm. The measurements of low-temperature and room-temperature photoluminescence help to calculate the internal quantum efficiency of 79.0%. The as-grown GaN-based LED wafers have been made into LED chips with the size of 300 × 300 μm2 by the standard process. The forward voltage, the light output power and the external quantum efficiency for LED chips are 19.6 W, 2.78 V, and 40.2%, respectively, at a current of 20 mA. These results reveal the high optoelectronic properties of GaN-based LEDs on LSAT substrates. This work brings up a broad future application of GaN-based devices.

Achieve high-quality InGaN/GaN multiple quantum wells on La0.3Sr1.7AlTaO6 substrates

High-quality InGaN/GaN multiple quantum wells (MQWs) have been epitaxially grown on La0.3Sr1.7AlTaO6 (LSAT) (111) substrates by radio-frequency molecular beam epitaxy. The as-grown InGaN/GaN MQWs are characterized by X-ray technique of high-resolution X-ray diffraction (HRXRD), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL) for structural and optical properties. The clear and sharp Pendellösung fringes from the typical HRXRD θ–2θ scan patterns reveal the abrupt InGaN/GaN interfaces and well designed MQWs periodicities, which are confirmed by the HRTEM measurement. A sharp and narrow PL peak of InGaN/GaN MQWs grown on LSAT substrates is observed at 445nm with a full width at half-maximum of 22nm conducted at room temperature, which shows the high optoelectronic properties. This work presents an alternative substrate for achieving high-quality InGaN/GaN MQWs for the potential application of optoelectronic devices.

Investigation on the structural properties of GaN films grown on La0.3Sr1.7AlTaO6 substrates

Gallium nitride (GaN) films with excellent structural, electrical and optical properties have been epitaxially grown on La0.3Sr1.7AlTaO6 (LSAT) (111) substrates by radio-frequency molecular beam epitaxy at low temperature. The GaN films grown at 500 °C exhibits high crystalline quality with the (0002) and (10−12) full width at half maximum of 0.056° and 0.071°. There is a maximum of 1.1-nm-thick interfacial layer existing between the as-grown GaN and LSAT (111) substrate, and the as-grown about 300-nm-thick GaN films are almost fully relaxed only with a 0.0094% in-plane tensile strain. Hall and photoluminescence (PL) measurements also reveal outstanding electrical and optical properties of the as-grown GaN films on LSAT. This achievement brings the prospect for achieving highly-efficient GaN-based optoelectronic devices on LSAT (111) substrates.

AlGaN/GaN HEMTs on Si by MBE with regrown contacts and fT = 153 GHz

AbstractAlGaN/GaN high‐electron‐mobility transistors (HEMT) have been grown by radio frequency molecular beam epi‐taxy (RF‐MBE) on 3″ Si substrates. A record low contact resistance Rc ˜ 0.11 Ω.mm has been achieved for GaN HEMTs on Si by using non alloyed ohmic contacts regrown by MBE. Owing to the low contact resistance a 75‐nm gate length unpassivated HEMT shows intrinsic current gain cut‐off frequency fT =153 GHz, a high saturation drain current density> 1.3 A/mm and a low RON of 1 Ω.mm, among the best reported for HEMTs on Si. With further scaling GaN HEMTs on Si can compete in the high‐performance RF arena with similar devices on SiC, while exploiting the many advantages of integration with Si. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN Nanowires Grown on a Graphite Substrate by Radio Frequency Molecular Beam Epitaxy

Self-assembled GaN and InGaN nanowires (NWs) were directly grown on a highly oriented pyrolytic graphite (HOPG) substrate by radio frequency molecular beam epitaxy (RF-MBE). The GaN NWs grown on HOPG had a shorter length and larger diameter than those grown conventionally on a Si(111) substrate based on the analysis of scanning electron microscopy (SEM) images. They were also found to have good crystalline qualities with few stacking faults, as determined by scanning transmission electron microscopy (STEM) analysis. Photoluminescence (PL) measurements at room temperature indicated that the luminescence intensity of the GaN NWs on HOPG is stronger than that of NWs grown on a Si(111) substrate; however, the InGaN NWs on both substrates showed similar tendencies with respect to the shift of the luminescence wavelength as a function of the growth temperature. While In incorporation decreased as the growth temperature increased, the uniformity of the InGaN NW arrays improved rapidly.

Quantum dots-templated growth of strain-relaxed GaN on a c-plane sapphire by radio-frequency molecular beam epitaxy

We investigated the quantum dots-templated growth of a (0001) GaN film on a c-plane sapphire substrate. The growth was carried out in a radio-frequency molecular beam epitaxy system. The enlargement and coalescence of grains on the GaN quantum dots template was observed in the atom force microscopy images, as well as the more ideal surface morphology of the GaN epitaxial film on the quantum dots template compared with the one on the AlN buffer. The Ga polarity was confirmed by the reflected high energy electron diffraction patterns and the Raman spectra. The significant strain relaxation in the quantum dots-templated GaN film was calculated based on the Raman spectra and the X-ray rocking curves. Meanwhile, the threading dislocation density in the quantum dots-templated film was estimated to be 7.1 × 107 cm−2, which was significantly suppressed compared with that of the AlN-buffered GaN film. The room-temperature Hall measurement showed an electron mobility of up to 1860 cm2/V · s in the two-dimensional electron gas at the interface of the Al0.25Ga0.75N/GaN heterojunction.

Study of Surface Morphology Control and Investigation of Hexagonal Indium Nitride Nanorods Grown on GaN/Sapphire Substrate

Heteroepitaxial growth of metal-catalyst-free indium nitride (InN) nanorods on GaN/sapphire substrates by radio-frequency metal-organic molecular beam epitaxy (RF-MOMBE) system was investigated. We found that different N/In flow ratios together with the growth temperatures greatly influenced the surface morphology of InN nanorods and their structural properties. The InN nanorods have been characterized in detail using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM). Optical property was evaluated by photoluminescence (PL) measurements. At lower growth temperatures, InN nanorods were successfully grown. A pronounced two-dimensional growth mode was observed at higher growth temperature of 500 degrees C, and these films showed preferred orientation along the c-axis. XRD patterns and SEM images reveal that InN nanorods has high quality wurtzite structure with FWHM approaching 900 arcsec, and they have uniform diameters of about 150 nm and length of about 800 nm. Meanwhile, no metallic droplet was observed at the end of the nanostructured InN, and this is strong evidence that the nanorods are grown via the self-catalyst process. The PL peak at 0.8 eV is attributed to the quantum confinement and Moss-Burstein effects. These observations provide some valuable insights into the physical-chemical process for manufacturing InN nanorods devices.

Effects of Mg doping on optical and electrical properties of GaNAs multiple quantum wells

AbstractWe have evaluated the effects of Mg doping on the optical and the electrical property of GaNAs/GaAs multiple quantum wells (MQWs) with high N composition (11∼17%) grown by radio‐frequency molecular beam epitaxy (RF‐MBE). The results of Van der Pauw measurements revealed strong n‐type conduction by Mg doping. The blue‐shifts and enhanced intensities in photoluminescence by Mg doping suggest the type‐II band structure of GaNAs/GaAs MQWs and carrier generation by interstitial Mg atoms (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Studies on the InGaGdN/GaN magnetic semiconductor heterostructures grown by plasma-assisted molecular-beam epitaxy

InGaGdN single-layer and InGaGdN/GaN superlattice (SL) structures were grown on the MOVPE-grown GaN (0001) template substrates by radio frequency molecular beam epitaxy. X-ray diffraction spectroscopy and X-ray absorption fine structure data showed no existence of a secondary phase such as GdN or InN and the Gd atom occupation at the group-III site. Magnetization versus magnetic field curves exhibited clear hysteresis and saturation at both 10 and 300K. The InGaGdN/GaN SL sample showed higher saturation magnetization per volume than the InGaGdN single-layer sample.

Rapid growth and characterization of InN nanocolumns on InGaN buffer layers at a low ratio of N/In

c-Axis-aligned InN nanocolumn arrays were vertically grown on 3 μm GaN epilayers with InGaN buffer layers by radio-frequency molecular beam epitaxy without any catalysts. X-ray diffraction, transmission electron microscopy, and field-emission scanning electron microscope were used to study the structural properties of the nanocolumns. It has been found that without InGaN buffers, InN films, rather than nanocolumns, were grown even at the same N/In ratio. In addition, high-quality InN nanocolumns can grow faster on InGaN buffers. The growth mechanism was discussed and the joint actions of the gas–solid and Volmer–Weber modes promote the nucleation and the growth of InN nanocolumns.

Nanoscale indium variation along InGaN nanopillars grown on (1 1 1) Si substrates

The nanoscale structural properties of InGaN nanopillars grown on (1 1 1) Si substrates, by radio frequency molecular beam epitaxy, were explored using electron microscopy techniques. Single phase, single crystalline initial nanostructures merged to form final nanopillars. Given that tuning of the band gap is controlled by the InN mole fraction, the compositional homogeneity along the c-axis of the nanopillars was examined and correlated with their luminescence properties. High growth temperature and desorption phenomena led to a negligible indium incorporation in the initial stages of growth. Here, among other possible explanations, indium accumulation at the tip area of nanopillars is interpreted by the arrangement of In clusters at steps of the top surface of merged nanopillars and the formation of a thin In-adlayer. The limited mobility of In atoms in the absence of appropriate migration paths restrained their diffusion along the c-axis, resulting in a 20% In-content at the tips of nanopillars, and hence high luminescence intensity at the true blue region. Narrow, non-coalesced nanopillars with fine reconstructed top surface exhibited a higher growth rate and were found In-free.

RED LIGHT EMITTING SCHOTTKY DIODES ON p-TYPE GaN/AlN/Si(111) SUBSTRATE

High quality GaN layers doped with Mg were grown on Si (111) substrates using high temperature AlN as buffer layer by radio-frequency molecular beam epitaxy. From the Hall measurements, fairly uniform high hole concentration as high as (4–5) × 1020 cm -3 throughout the GaN was achieved. The fabrication of the device is very simple. Nickel ohmic contacts and Schottky contacts using indium were fabricated on Mg -doped p-GaN films. The light emission has been obtained from these thin film electroluminescent devices. Thin film electroluminescent devices were operated under direct current bias. Schottky and ohmic contacts used as cathode and anode were employed in these investigations. Alternatively, two Schottky contacts could be probed as cathode and anode. Thin film electroluminescent devices were able to emit light. However, electrical and optical differences could be observed from the two different probing methods. The red light color could be observed when the potential between the electrodes was increased gradually under forward bias of 8 V at room temperature. Electrical properties of these thin film electroluminescent devices were characterized by current–voltage (I–V) system, the heights of barriers determined from the I–V measurements were found to be related to the electroluminescence.

Structural and magnetic properties of GaGdN/GaN superlattice structures

GaGdN/GaN superlattices (SLs) having various GaGdN and GaN layer thicknesses were grown on Al 2O 3 (0001) substrates by radio frequency molecular beam epitaxy and their structural and magnetic properties were investigated. By changing the thickness of GaGdN and GaN layer in the GaGdN/GaN SL, we find that the lattice constant in the c-direction of GaGdN is larger than that of GaN. All SLs samples exhibited ferromagnetic characteristics at 10 K and 300 K. Comparing with the GaGdN single layer sample, the magnetization of SL samples is larger at the same unit volume of GaGdN. Magnetization data show that the SL samples having relatively thinner GaGdN layer and thicker GaN layer have larger magnetic moment per Gd atom. This phenomenon can be explained by considering that the carriers (electrons) flow from the GaN layers and are accumulated in the GaGdN layers; thinner GaGdN layer has higher carrier density, suggesting the carrier-enhanced magnetization in SL structure.

Epitaxial Growth of AlInGaN Quaternary Alloys by RF-MBE

AlInGaN quaternary alloys were successfully grown on sapphire substrate by radio-frequency plasma-excited molecular beam epitaxy (RF-MBE). AlInGaN quaternary alloys with different compositions were acquired by changing the Al cell's temperature. The streaky RHEED patterns were observed during AlInGaN quaternary alloys growth. Scanning Electron Microscope (SEM), Rutherford back-scattering spectrometry (RBS), X-Ray diffraction (XRD) and Cathodoluminescence (CL) were used to characterize the structural and optical properties of the AlInGaN alloys. The experimental results show that the AlInGaN quaternary alloys grow on the GaN buffer in the layer-by-layer growth mode. When the Al cell's temperature is 920 degrees C, the Al/In ratio in the AlInGaN quaternary alloys is about 4.7, and the AlInGaN can acquire better crystal and optical quality. The X-ray and CL full-width at half-maximum (FWHM) of the AlInGaN are 5arcmin and 25nm, respectively.

InGaN nanorod arrays grown by molecular beam epitaxy: Growth mechanism structural and optical properties

Vertically c-axis-aligned InGaN nanorod arrays were synthesized on c-plane sapphire substrates by radio-frequency molecular beam epitaxy. In situ reflection high-energy electron diffraction was used to monitor the growth process. X-ray diffraction, transmission electron microscopy, field-emission scanning electron microscope, and photoluminescence were used to investigate the structural and optical properties of the nanorods. The growth mechanism was studied and a growth model was proposed based on the experimental data. A red shift of photoluminescence spectrum of InGaN nanorods with increasing growth time was found and attributed to the partial release of stress in the InGaN nanorods.