Radio-frequency Plasma-assisted Molecular Beam Epitaxy Research Articles

Solar-Blind Photoresistors Based on $\hbox{Mg}_{0.48} \hbox{Zn}_{0.52}\hbox{O}$ Thin Films Grown on $r\hbox{-Al}_{2} \hbox{O}_{3}$ Substrates by Radio-Frequency Plasma-Assisted Molecular Beam Epitaxy

Solar-blind photoresistors based on Mg0.48Zn0.52O thin films were fabricated on r-plane sapphire substrates by radio-frequency plasma-assisted molecular beam epitaxy. High-quality a-plane-orientation single-phase wurtzite Mg0.48Zn0.52O thin films are demonstrated by X-ray diffraction and absorption spectra. The MgZnO photoresistors exhibit a large dark/photoresistance ratio up to 1.7 × 104 with the light intensity of 0.61 mW/cm2 at 260 nm. The spectral response shows a sharp response peak only in the solar-blind region with maximum responsivity of 1.5 × 10-3 Ω-1· W-1.

Growth and characterization of Cr-doped semi-insulating GaN templates prepared by radio-frequency plasma-assisted molecular beam epitaxy

Cr-doped semi-insulating GaN templates were grown using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). The samples were characterized by high resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The Cr incorporation was strongly dependent on the growth condition, which was primarily influenced by Cr cell temperature. The activation energy of dark conductivity was about 0.48eV which corresponds to the depth of the dominant electron traps pinning the Fermi level. The experimental results indicated that Cr doping did not affect the crystalline orientation of the GaN film but introduced more threading dislocations, and step-graded GaN/AlxGa1−xN superlattices (SLs) played an important role in hindering the penetration of the threading dislocations.

Observation of large Zeeman splitting in GaGdN/AlGaN ferromagnetic semiconductor double quantum well superlattices

Symmetric GaGdN/AlGaN (Gd concentration: 2%) and GaN/AlGaN double quantum well superlattices (DQW-SLs) were grown by radio-frequency plasma-assisted molecular-beam epitaxy on GaN (0001) templates. Atomic steps were observed on all the sample surfaces by atomic force microscope. X-ray diffraction θ/2θ scan curves exhibited well-defined satellite structures. Room temperature ferromagnetism was confirmed for the GaGdN/AlGaN DQW-SL samples by using alternating gradient magnetometer. Strong photoluminescence was observed from both GaGdN and GaN QWs at higher energy side of GaN excitonic peak. Magneto-photoluminescence spectra for GaGdN/AlGaN DQW-SL samples showed a large magnetic field dependence of the excitonic energy by applying a magnetic field up to 7T. The observed strong redshift of excitonic PL indicated an enhancement of Zeeman splitting of the free carrier energy levels in magnetic GaGdN/AlGaN DQW-SL. Enhanced g-factor was estimated to be about 60 for GaGdN/AlGaN DQW-SL sample with QW thickness of 1nm.

Growth of InGaN nanowires on a (111)Si substrate by RF‐MBE

AbstractWe have grown InGaN nanowires (NWs) on a (111)Si substrate directly by radio‐frequency plasma‐assisted molecular beam epitaxy (RF‐MBE), and investigated their In compositional distribution by energy dispersive x‐ray spectroscopy (EDX). The In composition line profiles of InGaN NWs measured by EDX are different at each position in the InGaN NWs, and In is most easily incorporated into the top of NWs. The In composition at the top of the NWs depends on the growth temperature and the In flux ratio. However, the In compositions at other positions is not dependent on these factors. Thus, these results might be due to the limited diffusion of In atoms from the top of the NWs towards the NWs/Si interface. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Mg Doping on the Photoluminescence of GaN:Mg Films by Radio-Frequency Plasma-Assisted Molecular Beam Epitaxy

We investigate undoped GaN and Mg-doped GaN grown by rf plasma-assisted molecular beam epitaxy (MBE) with different Mg concentrations by photoluminescence (PL) at low temperature, Hall-effect and XRD measurements. In the PL spectra of lightly Mg-doped GaN films, a low intensity near band edge (NBE) emission and strong donor-acceptor pair (DAP) emission with its phonon replicas are observed. As the Mg concentration is increased, the DAP and NBE bands become weaker and a red shift of these bands is observed in the PL spectra. Yellow luminescence (YL) is observed in heavily Mg-doped GaN. The x-ray diffraction is employed to study the structure of the films. Hall measurement shows that there is a maximum value (3.9 × 1018 cm−3) of hole concentration with increasing Mg source temperature for compensation effect. PL spectra of undoped GaN are also studied under N-rich and Ga-rich growth conditions. Yellow luminescences of undoped Ga-rich GaN and heavily Mg-doped GaN are compared, indicating the different origins of the YL bands.

Effect of mask material on selective growth of GaN by RF-MBE

The selective growth mechanism of GaN on a (0 0 0 1) GaN template using radio-frequency plasma-assisted molecular beam epitaxy is investigated. The effect of the mask material on selective growth is investigated using SiO 2 and Ti masks. Selective growth of GaN with excellent selectivity and a smooth surface is achieved by optimizing the growth temperature. The threshold temperatures for selective growth are 930 and 940 °C for SiO 2 and Ti masks, respectively. A high growth temperature is necessary to suppress the formation of polycrystals on the mask and to obtain selective growth. The different threshold temperatures are thought to be due to the different affinities between Ga adatoms and the two mask materials. Selective growth is also performed using wide masks. Re-evaporation of adatoms is found to be the principal process in the selective growth obtained using the SiO 2 mask, whereas adatom migration is the principal process with the Ti mask. Surface diffusion of adatoms on the Ti mask results in the formation of a denuded zone near the mask edge and protuberances on both sides of the selective growth.

Mg doped InN and confirmation of free holes in InN

We report a systematic investigation on Mg doped InN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy. Electrolyte capacitance voltage (ECV) combined with thermopower measurements find p-type conduction over an Mg concentration range. For InN:Mg in this p-type “window” the Seebeck coefficients dramatically change their signs from negative to positive when the thickness of undoped InN interlayer decreases to zero. This notable sign change of Seebeck coefficient explains the previous inconsistency between ECV and thermopower results and confirms the existence of mobile holes in the InN:Mg. Taking into account the undoped InN interlayer, the hole density and mobility are extracted.

In situ Investigation of Growth Mechanism during Molecular Beam Epitaxy of In-Polar InN

A real-time investigation of In-polar InN growth by radio-frequency plasma-assisted molecular beam epitaxy has been conducted using laser reflection and reflected high-energy electron diffraction (RHEED). Laser reflection intensity is very sensitive to surface changes in the growth front. Clear reflection intensity changes have been observed for In-stabilized surface, In adlayer, and In droplet formation. Simultaneously, the envelope of reflection intensity variation shows interference oscillation due to the increasing InN thickness. Laser reflection intensity increases during In deposition and decreases during the following N irradiation, in contrast to RHEED intensity. In situ monitoring using both techniques has demonstrated complementary information on the growth front. On the basis of the observation from both techniques, the growth mechanisms and roles of the In adlayer and droplets in the InN growth are discussed.

RF-MBE growth of Si doped cubic GaN and hexagonal phase incorporated c-AlGaN films on MgO(0 0 1) substrates

Silicon doped cubic GaN (c-GaN) films were grown on MgO (001) substrates by radio-frequency-plasma-assisted molecular beam epitaxy (RF-MBE). And the incorporation of hexagonal phase into cubic AlGaN (c-AlGaN) films was examined. The conduction type of the Si doped c-GaN and c-AlGaN films was n-type. The maximum electron concentration was 2.8×1020cm−3 for c-GaN and 1.3×1020cm−3 for c-AlGaN (Al content of 7–10% and hexagonal phase of about 30%). The cubic phase purity of the films was maintained near the value of undoped films of the same Al content. The maximum electron mobility was 27cm2/Vs for c-GaN and electron mobility decreased as the cubic phase purity of the film decreases. The main cause of electron scattering is the stacking faults associated with the hexagonal phase incorporation.

Effects of Surface Recombination on Exciton Dynamics in GaN Nanorods

We have investigated the optical properties of GaN nanorods with different diameters by micro-photoluminescence (PL) and time-resolved PL measurements grown on (111) Si substrates by using radio-frequency plasma-assisted molecular-beam epitaxy. In order to confirm the formation of the GaN nanorods and their physical dimension, we employed high resolution scanning electron microscopy. As the diameters of the nanorods decreased, the donor bound exciton (D 0 X) emission, which is mainly observed at low temperature for samples with larger diameters, rapidly quenched compared with the free exciton (FX). The activation energy of the FX for GaN nanorods with different diameters was also evaluated. The decay times are a few tens of ps for both the D 0 X and the FX transitions. The overall decay times decrease linearly with decreasing average diameter of the nanorods. Time-resolved PL studies show that surface recombination is dominant in determining the excitonic dynamics.

Cubic ZnxMg1−xO thin films grown by plasma-assisted molecular-beam epitaxy for optoelectronic applications

Heteroepitaxial ZnxMg1−xO thin films were grown on lattice-matched MgO (100) substrates using radiofrequency plasma-assisted molecular-beam epitaxy. High-quality epilayers with zinc concentrations ranging from x = 0 (MgO) to x = 0.65 were grown and characterized optically, structurally, and electrically. The ZnxMg1−xO films were found to maintain the rocksalt cubic (B1) crystal structure for concentrations z < 0.65, with a linear dependence of lattice constant on Zn concentration. X-ray diffraction (XRD) also revealed the emergence of phase segregation into wurtzite (B4) phase for the highest concentration film. The band gap energy of the films was successfully varied from 4.9 to 6.2 eV (253–200 nm), showing a linear relationship with Zn concentration. The strictly cubic films exhibit roughness on the order of 10 Å and resistivities of approximately 106 Ω·cm.

New MBE growth method for high quality InN and related alloys using in situ monitoring technology

AbstractA new radio‐frequency plasma‐assisted molecular beam epitaxy (RF‐MBE) method, named droplet elimination by radical beam irradiation (DERI), is utilized for the growth of InN. This method is composed of two series of growth processes: (1) InN growth under an In‐rich condition and (2) consecutive nitrogen radical beam irradiation. This method is simple, enabling the reproducible growth of a high‐quality thick InN film with a flat surface. This method is also effective for monitoring a nitrogen radical beam that is responsible for InN growth. This new growth method is also applied to InGaN growth. The results indicate a preferable Ga capturing process on an InGaN growing surface, particularly when the samples are grown under a metal‐rich condition.

InNSb single crystal films prepared on GaAs (001) substrates by molecular beam epitaxy

InNSb alloy films are prepared on GaAs (001) substrates by the N2 radio frequency plasma-assisted molecular beam epitaxy ( RF-MBE). The N composition and the micro-structure of the samples are characterized by atom force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. The measurement results reveal that the films have smooth surfaces and good crystalline quality, the N composition can reach 0.84%(from XRD) and most of the N atoms in the samples are at the sites of Sb atoms. The transport properties of the samples are also characterized, and the results demonstrate that our samples have lower carrier concentrations and higher mobilities. Owing to the introduction of N, a condside rable reduction of room-temperature magnetoresistance is observed.

TEM characterization of M ‐plane InN grown on (100) LiAlO2 substrate by RF‐MBE

AbstractIn this study, we characterized the microstructure of M ‐plane InN using transmission electron microscopy (TEM). The M ‐plane InN was grown on a (100) LiAlO2 substrate using radio‐frequency plasma‐assisted molecular beam epitaxy (RF‐MBE). We determined the epitaxial relationships of the M ‐plane InN and LiAlO2 using selected area electron diffraction (SAED). The epitaxial relationships were exactly the same as the M ‐plane GaN grown on the same substrate. Also, the M ‐plane InN was grown on the (100) LiAlO2 despite the high lattice mismatches between M ‐plane InN and LiAlO2. We observed a high density of stacking faults parallel to (0001)InN. The density was estimated about 1.8×106 cm–1, which was one order of magnitude higher than that of non‐polar GaN. The correlation between threading dislocations and stacking faults is investigated. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of M ‐plane (10‐10) InN on LiAlO2 (100) substrate

AbstractIn this study, we report the growth and characterization of M ‐plane InN films on LiAlO2 (100) substrates by radio‐frequency plasma assisted molecular beam epitaxy (RF‐MBE). InN films were grown at various temperatures and under various V/III ratios on the substrates. Pure M ‐plane InN films were successfully grown at a high temperature of 450 °C and under a slightly In‐rich condition, while the incorporation of C ‐plane phase was observed in M ‐plane InN films grown at low temperatures of less than 400 °C or under a N‐rich condition. These indicate that controls of growth temperature and V/III ratio are important for the growth of pure M ‐plane InN films. The in‐plane epitaxial relationships of M ‐plane InN on LiAlO2 (100) were [0001]InN // [010]LiAlO2 and [1‐210]InN // [001]LiAlO2. A surface electron accumulation layer on the obtained M ‐plane InN film is also discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of aligned CrN nanoclusters in Cr-delta-doped GaN

Cr-delta-doped GaN layers were grown by radio-frequency plasma-assisted molecular-beamepitaxy on GaN template substrates. Cr flux was supplied without nitrogen flow duringCr-delta-doping. Cr incorporation into a narrow thin layer region was confirmed with thedepth profile measured by secondary ion mass spectrometry. Structural propertiesand Cr atom alignments were studied with transmission electron microscopy. Itwas found that Cr-delta-doped GaN layers were coherently grown with Cr orCrGa nanoclusters in the delta-doped region for low temperature growth (350,500 °C). It was also found that aligned CrN nanoclusters (approximately 5 nm vertical thickness)with NaCl-type structure were formed in the delta-doped region for the growth at700 °C.

Growth of Semipolar InN on r-plane (10-12) Sapphire by RF-MBE

In this study, we report the growth of semipolar InN films on r-plane (10-12) substrates by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). Semipolar InN films were successfully obtained by controlling the substrate nitridation temperature between 400°C and 600°C. We characterized the epitaxial relationship between r-plane sapphire and the obtained semipolar films using X-ray diffraction (XRD). It is found that c-axis of semipolar InN was declined by 21°from the direction perpendicular to r-plane sapphire surface along a-axis of InN.

Influence of wet etching on the morphologies of Si patterned substrates and ZnO epilayers

A ZnO film with two-dimensional periodic structure was grown on Si substrate by radio-frequency plasma-assisted molecular beam epitaxy. The influence of wet-chemical etching on Si (100) and Si (111) substrates patterned with dot arrays was investigated for achieving a ZnO film with good periodic structure. X-ray diffraction and scanning electron microscopy mesurements demonstrate better crystalline quality and surface morphology of ZnO film grown on Si (111) than that on Si (100). The results suggest that the growth method is feasible for the fabrication of ZnO film with good periodic structure.

Abnormal photoluminescence properties of GaN nanorods grown on Si(111) bymolecular-beam epitaxy

We have studied the photoluminescence properties of GaN nanorods grown onSi(111) substrates by radio-frequency plasma-assisted molecular-beam epitaxy. Thehexagonal shaped nanorods with lateral average diameters from 30 to 150 nmare obtained by controlling the Ga flux with a fixed amount of nitrogen. As thediameters decrease, the main emission lines assigned as donor bound excitonsare blueshifted, causing a spectral overlap of this emission line with that of thefree exciton at 10 K due to the quantum size effect in the GaN nanorods. Thetemperature-dependent photoluminescence spectra show an abnormal behaviourwith an ‘S-like’ shape for higher diameter nanorods. The activation energy ofthe free exciton for GaN nanorods with different diameters was also evaluated.

Growth of single-crystalline ZnO film with two-dimensional periodic structure on Si(1 1 1) substrate by molecular beam epitaxy

Single-crystalline ZnO film with two-dimensional periodic structure was grown on Si(1 1 1) substrate by radio-frequency plasma-assisted molecular beam epitaxy. The influence of substrate orientations and growth temperatures was explored for achieving a high-quality ZnO film. It was found that ZnO epilayers grown on Si(1 1 1) substrate have better crystalline quality than the one on Si(1 0 0), and a higher growth temperature was further confirmed favorable for the attainment of a periodic morphology. The results suggest that our growth method is feasible for the fabrication of ZnO film with periodic structure, which is promising for applications in new photonic devices.