Radio-frequency Plasma-assisted Molecular Beam Epitaxy Research Articles

Suppression of Mixing of Metastable Zincblende Phase in GaN Crystal Grown on ScAlMgO4 Substrates by Radio‐Frequency Plasma‐Assisted Molecular Beam Epitaxy

ScAlMgO4 (SAM) substrates have attracted significant attention as template substrates for fabricating bulk GaN crystals. Radio‐frequency plasma‐assisted molecular beam epitaxy (RF‐MBE) is preferred for the growth of the first GaN layer. However, the metastable zincblende (ZB) phase is mixed on the SAM substrate with a smaller terrace width, coupled with lower growth temperatures in MBE. Herein, the time evolution of GaN growth on SAM substrates is investigated to elucidate the mechanism underlying ZB mixing. ZB‐GaN is generated as stacking faults during the coalescence of the initially grown wurtzite‐ (WZ‐) GaN islands on adjacent SAM terraces, owing to the extraordinarily large step height close to three bilayers of WZ‐GaN. Therefore, high‐temperature growth on a SAM substrate with wider terraces suppresses the generation of the ZB phase in the initial growth stage. ZB‐GaN also decreases with an increase in the growth time. Based on these results, a 940 nm‐thick GaN layer is grown at 750 °C on the SAM substrate with a terrace width of 820 nm. A pure WZ‐GaN layer from the GaN/SAM interface with a flat surface with a root mean square value of below 1 nm and a dislocation density of 7.3 × 109 cm−2 is obtained.

Effect of two-step cubic GaN buffer layer on the radio-frequency plasma-assisted molecular beam epitaxy growth of cubic AlN films grown on MgO (001) substrates

Cubic AlN (c-AlN) films were grown on MgO (001) substrates through the use of a two-step cubic GaN (c-GaN) buffer layer using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) technique. The two-step c-GaN buffer layer was found to be crucial for the epitaxial growth of c-AlN, as it showed a specific epitaxial relationship between c-AlN and c-GaN along the MgO [100] zone axis. On the other hand, the c-AlN films grown with the one-step and nonGaN buffer layers showed columnar grain structure. The optical study found that the c-AlN films grown with the two-step c-GaN buffer layer had an indirect bandgap energy of 4.99 ± 0.02 eV, which is consistent with previous experimental data and theoretical calculations. The c-AlN films grown with the one-step and nonGaN buffer layers had indirect bandgap energies of 5.00 ± 0.02 eV and 5.18 ± 0.02 eV, respectively. These results highlight the importance of the two-step c-GaN buffer layer in enhancing the MBE growth of c-AlN on MgO (001) substrate.

Growth of single crystalline Si on graphene using RF-MBE: Orientation control with an AlN interface layer

Graphene integration with commonly used Si is very promising for high-performance devices in the future. The fabrication of Si on graphene has gained more attention to realize promising device design and to develop high-quality Si thin-film technology. Nevertheless, attempts to develop Si on graphene are very few, and films are often of poor-quality. Introducing an AlN atomic layers as an interface layer, this paper reports the growth of single crystalline Si on graphene using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). We focus on the initial growth process of Si on graphene and clarify the epitaxial relationship between Si and graphene. It is observed that the initial Si nuclei rotate randomly on the surface if deposited directly on the graphene, which implies that orientation control of the initial Si layers on the epitaxial graphene is very important to obtain high-quality layers. The proposed thin AlN intermediate layer to the graphene surface greatly enhances the orientation of initial Si nuclei and results in a single crystalline Si layer on the epitaxial graphene.

The influence of AlN buffer layer on the growth of self-assembled GaN nanocolumns on graphene

GaN nanocolumns were synthesized on single-layer graphene via radio-frequency plasma-assisted molecular beam epitaxy, using a thin migration-enhanced epitaxy (MEE) AlN buffer layer as nucleation sites. Due to the weak nucleation on graphene, instead of an AlN thin-film we observe two distinguished AlN formations which affect the subsequent GaN nanocolumn growth: (i) AlN islands and (ii) AlN nanostructures grown along line defects (grain boundaries or wrinkles) of graphene. Structure (i) leads to the formation of vertical GaN nanocolumns regardless of the number of AlN MEE cycles, whereas (ii) can result in random orientation of the nanocolumns depending on the AlN morphology. Additionally, there is a limited amount of direct GaN nucleation on graphene, which induces non-vertical GaN nanocolumn growth. The GaN nanocolumn samples were characterized by means of scanning electron microscopy, transmission electron microscopy, high-resolution X-ray diffraction, room temperature micro-photoluminescence, and micro-Raman measurements. Surprisingly, the graphene with AlN buffer layer formed using less MEE cycles, thus resulting in lower AlN coverage, has a lower level of nitrogen plasma damage. The AlN buffer layer with lowest AlN coverage also provides the best result with respect to high-quality and vertically-aligned GaN nanocolumns.

Molecular beam epitaxy of InAlN alloys in the whole compositional range

The fabrication of high-quality InxAl1−xN alloys over the whole composition is very challenging. Controlling the In/(In + Al) beam flux ratio and the growth temperature, this paper reports the fabrication of single crystalline InxAl1−xN alloys over the whole In composition by radio-frequency plasma-assisted molecular beam epitaxy. A comprehensive systematic study on the structural and vibrational properties of the InxAl1−xN alloys for the whole In composition has been carried out experimentally by Raman spectroscopy and theoretically by the forced vibrational method. The InxAl1−xN alloys show broad Raman peaks in the intermediate range of In composition. The appearance of Raman inactive B1 (High) mode has been confirmed by the experimental and theoretical results. The B1 (High) and A1 (LO) modes show one-mode behavior, whereas the E2 (High) mode shows the two-mode behavior in the whole In composition range. The observed Raman modes from the grown InxAl1−xN alloys well match with the calculated phonon modes of the InxAl1−xN alloys. These results provide an in-depth understanding of the growth of whole compositional InxAl1−xN alloys and the fabrication of optoelectronic devices using these promising materials.

Electronic engineering of transition metal Zn-doped InGaN nanorods arrays for photoelectrochemical water splitting

The fast charge carrier recombination and slow oxidation kinetics of ternary semiconductor InGaN as promising photocatalyst impede the PEC performance. Herein, we fabricate transition metal zinc (Zn) doped InGaN nanorods arrays by radio-frequency plasma-assisted molecular beam epitaxy. The doping obviously reduces indium atoms composition, the aggregation of In–In and induces the deep energy level. This greatly decreases the defects and improves the valence band potential of InGaN nanorods, which is beneficial for the rapid carrier separation efficiency with the decreased photogenerated carrier recombination rate and improved water oxidation kinetics. Significantly, Zn doped InGaN nanorods photoanode shows three times higher photocurrent density of 1.65 mA/cm2 at 1.23 V versus reversible hydrogen electrode (RHE) compared to undoped InGaN nanorods (0.58 mA/cm2). More importantly, after loading Au nanoparticles, the maximum applied bias photo-to-current efficiency of Zn-doped nanorods photoanode reaches 1.33%, which is superior to the recent results of reported Zn doped based photoanodes. This efficient doping strategy not only bridges the gaps of heteroatom doped InGaN nanorods based photoelectrodes, but also provides deep insights into controlling the electronic structure, and crystallinity of photoelectrodes for enhanced solar converting efficiency.

In Situ Synchrotron X-ray Diffraction Reciprocal Space Mapping Measurements in the RF-MBE Growth of GaInN on GaN and InN

In this work, in situ synchrotron X-ray diffraction reciprocal space mapping (RSM) measurements were carried out for the radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) growth of GaInN on GaN and InN layers, which were also grown by RF-MBE on commercialized GaN/c-sapphire templates. In situ XRD RSM measurements were performed using an MBE apparatus directly coupled to an X-ray diffractometer at the beamline of the synchrotron radiation facility SPring-8. It was observed in situ that both lattice relaxation and compositional pulling occurred during the initial growth stage, reducing the strain of GaInN on GaN and InN. Different initial growth behaviors of GaInN on GaN and InN were also observed from the results of the evolution of GaInN integrated peak intensities.

Fabrication of InN on epitaxial graphene using RF-MBE

This paper reports the fabrication of InN layers on the epitaxial graphene (EG) using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). Prior to the fabrication of InN, single crystalline EG with step and terrace structure was formed on 6H-SiC (0001) substrate in an Ar ambient by the Si sublimation method. Single crystalline epitaxial layers of InN with smooth surfaces are successfully fabricated on the EG using RF-MBE. InN layers with terrace and step structure are grown on the graphene surface up to 2MLs, and InN are grown in a layer by layer 2D growth mode. If the number of layers is increased above 3 MLs, the terrace and steps disappear, and the growth mode changes to 3D mode. The Raman spectroscopy analysis shows that the interfacial stress is reduced for the InN layer grown on the EG surface. The quality of the grown InN layer on the EG surface achieved at present is comparable to the InN film grown on sapphire. This work opens the possibility of growing high-quality InN layers on the EG surface in the near future.

Enhancement of InN Luminescence by Introduction of Graphene Interlayer.

Indium nitride (InN) luminescence is substantially enhanced by the introduction of a multilayer graphene interlayer, mitigating the lattice mismatch between the InN epilayer and the Gallium nitride (GaN) template on a sapphire substrate via weak van der Waals interaction between graphene and nitride layers. The InN epilayers are deposited by radio-frequency plasma-assisted molecular beam epitaxy (MBE), and are characterized by spatially-resolved photoluminescence spectroscopy using confocal microscopy. A small blue shift of the emission band from the band gap evidences a low density of equilibrium carriers, and a high quality of InN on multilayer graphene. A deposition temperature of ~375 °C is determined as optimal. The granularity, which is observed for the InN epilayers deposited on multilayer graphene, is shown to be eliminated, and the emission intensity is further enhanced by the introduction of an aluminum nitride (AlN) buffer layer between graphene and InN.

Si doping mechanism in Si doped GaAsN

The Si doping mechanism is systematically investigated in dilute nitride GaAsN grown by radio frequency plasma assisted molecular beam epitaxy (RF-MBE). We change growth temperature, Si impurity concentration ([Si]), and nitrogen composition ([N]). The relationship between Si activation ratio (α) and [N] are evaluated using X-ray diffraction (XRD) 2θ-ω and Hall effect measurement. As [N] in GaAsN increases, α drastically decreases, which is ascribed to mechanisms of inactive Si donors such as a cluster of Si and N at an As site ((Si-N)As). We find that the main factor of inactive Si donors in GaAsN depends on both [Si] and [N].

Vertical GaN nanocolumns grown on graphene intermediated with a thin AlN buffer layer

We report on the self-assembled growth of high-density and vertically-oriented n-doped GaN nanocolumns on graphene by radio-frequency plasma-assisted molecular beam epitaxy. Graphene was transferred to silica glass, which was used as the substrate carrier. Using a migration enhanced epitaxy grown AlN buffer layer for the nucleation is found to enable a high density of vertical GaN nanocolumns with c-axis growth orientation on graphene. Furthermore, micro-Raman spectroscopy indicates that the AlN buffer reduces damage on the graphene caused by impinging active N species generated by the radio-frequency plasma source during the initial growth stage and nucleation of GaN. In addition, the grown GaN nanocolumns on graphene are found to be virtually stress-free. Micro-photoluminescence measurements show near band-edge emission from wurtzite GaN, exhibiting higher GaN bandgap related photoluminescence intensity relative to a reference GaN bulk substrate and the absence of both yellow luminescence and excitonic defect emission. Transmission electron microscopy reveals the interface of GaN nanocolumns on graphene via a thin AlN buffer layer. Even though the first few monolayers of AlN on top of graphene are strained due to in-plane lattice mismatch between AlN and graphene, the grown GaN nanocolumns have a wurtzite crystal structure without observable defects. The results of this initial work pave the way towards realizing low-cost and high-performance electronic and optoelectronic devices based on III-N semiconductors grown on graphene.

Surface and bulk electronic structures of unintentionally and Mg-doped In0.7Ga0.3N epilayer by hard X-ray photoelectron spectroscopy

The surface and bulk electronic structures of In0.7Ga0.3N epilayers are investigated by angle-resolved hard X-ray photoelectron spectroscopy (HX-PES) combined with soft X-PES. The unintentionally and Mg-doped In0.7Ga0.3N (u-In0.7Ga0.3N and In0.7Ga0.3N:Mg, respectively) epilayers are grown by radio-frequency plasma-assisted molecular beam epitaxy. Here three samples with different Mg concentrations ([Mg] = 0, 7 × 1019, and 4 × 1020 cm−3) are chosen for comparison. It is found that a large downward energy band bending exists in all samples due to the formation of a surface electron accumulation (SEA) layer. For u-In0.7Ga0.3N epilayer, band bending as large as 0.8 ± 0.05 eV occurs from bulk to surface. Judged from the valence band spectral edge and numerical analysis of energy band with a surface quantum well, the valence band maximum (VBM) with respect to Fermi energy (EF) level in the bulk is determined to be 1.22 ± 0.05 eV. In contrast, for In0.7Ga0.3N:Mg epilayers, the band bending increases and the VBM only in the bulk tends to shift toward the EF level owing to the Mg acceptor doping. Hence, the energy band is considered to exhibit a downward bending structure due to the coexistence of the n+ SEA layer and Mg-doped p layer formed in the bulk. When [Mg] changes from 7 × 1019 to 4 × 1020 cm−3, the peak split occurs in HX-PES spectra under the bulk sensitive condition. This result indicates that the energy band forms an anomalous downward bending structure with a singular point due to the generation of a thin depleted region at the n+ p interface. For In0.7Ga0.3N:Mg epilayers, the VBM in the bulk is assumed to be slightly lower than EF level within 0.1 eV.

Surface and bulk electronic structures of heavily Mg-doped InN epilayer by hard X-ray photoelectron spectroscopy

To evaluate the polarity, energy band diagram, and oxygen (O) distribution of a heavily Mg-doped InN (InN:Mg+) epilayer with a Mg concentration of 1.0 ± 0.5 × 1020 cm−3, the core-level and valence band (VB) photoelectron spectra are investigated by angle-resolved soft and hard X-ray photoelectron spectroscopies. The InN:Mg+ epilayers are grown by radio-frequency plasma-assisted molecular beam epitaxy. In this doping level, the polarity inversion from In-polar to N-polar occurs with the increase in the Mg flow rate under the same growth conditions, and the VB spectrum clearly indicates the direction of polarity of InN:Mg+, which is N-polar. The energy band diagram is considered to exhibit a two-step downward bending structure due to the coexistence of the n+ surface electron accumulation layer and heavily Mg-doped p+ layer formed in the bulk. The O concentration rapidly increases until ∼4 nm with respect to the surface, which is deduced to be one of the reasons of the formation of the anomalous two-step energy band profile.

Linear and nonlinear behaviour of near-IR intersubband transitions of cubic GaN/AlN multi quantum well structures

The linear and nonlinear behaviour of intersubband transitions of cubic GaN/AlN multi quantum well (QW) structures in the IR spectral region is investigated. In this study photoluminescence, IR absorption as well as pump-probe measurements are done. Two cubic GaN/AlN multi quantum wells with Si content of NSi ~1019cm−3 in the cubic GaN quantum wells were grown on 3C-SiC (001) substrate by radio-frequency plasma-assisted molecular beam epitaxy. A broad IR absorption with a FWHM of 370meV was found with a maximum at 0.7eV, corresponding to the intersubband transition of the multi quantum wells. The nonlinear optical measurement reveals a clear change of transmission for a pump pulse with an angle of incidence of 65°. Furthermore, transmission electron microscopy measurements are used to determine the real layer thicknesses. These thickness values are exploited in the calculation with the Schrödinger-Poisson solver nextnano³. The simulated transition energies agree very well with the experimental data for the photoluminescence and the absorption measurement.

High Efficiency Photoelectrochemical Water Splitting Using Graphene/GaN Nanowire Photoelectrode

GaN based photoelectrode with a nanowire structure have been studied due to superior performance of water splitting ability. Self‐organized GaN nano‐rod on silicon has been demonstrated, and a direct growth via vapour–liquid–solid using metal particle as a catalyst has been reported. Nonowire structure has benefit with short minority carrier diffusion lengths and large surface-to-volume ratio. It allows minor carrier to easily reach to surface without recombination, so that water splitting ability could be increased. However, this structure has photocorrosion problem. For solving this problem, we covered graphene material on the tips of nanowires as a passivation layer. Graphene has been known to have excellent mechanical, thermal, optical, and electrical properties. For these reasons, we studied the photoelectrochemical water splitting properties of graphene coated GaN nanowire (graphene/GaN nanowire) photoelectrode. GaN nanowire sample without graphene was used for the reference. The GaN nanowire structures were grown on n-type Si(111) substrates using radio-frequency plasma-assisted molecular beam epitaxy. The graphene was grown by chemical vapor deposition method and exfoliated to surface of GaN nanowire. As a result, compared with non-coated GaN nanowire sample, the graphene/GaN nanowire phoeoelectrode showed higher photocurrent density. Also, we could know that it has much higher photocurrent stability. We will present the results and the reasons for higher efficiency and stability of this graphene coated GaN nanowire sample at this conference. Reference[1] Allon I. Hochbaum, and Peidong Yang, Chem. Rev., 110 (2010) 527–546[2] B. AlOtaibi, M. Harati, S. Fan, S. Zhao, H. P. T. Nguyer, M. G. Kibiria, and Z. Mi, Nanotechnology, 24 (2013) 175401

RHEED에 의한 GaN, InN 핵생성층의 열처리 효과 분석

GaN and InN epilayers with nucleation layer (LT-buffer) were grown on (0001) sapphire substrates by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). As-grown and annealed GaN and InN nucleation layers grown at various growth condition were observed by reflection high-energy electron diffraction (RHEED). When temperature of effusion cell for III source was very low, diffraction pattern with cubic symmetry was observed and zincblende nucleation layer was flattened easily by annealing. As cell temperature increased, LT-GaN and LT-InN showed typical diffraction pattern from wurtzite structure, and FWHM of (10-12) plane decreased remarkably which means much improved crystalline quality. Diffraction pattern was changed to be from streaky to spotty when plasma power was raised from 160 to 220 W because higher plasma power makes more nitrogen adatoms on the surface and suppressed surface mobility of III species. Therefore, though wurtzite nucleation layer was a little hard to be flattened compared to zincblende, higher cell temperature led to easier movement of III surface adatoms and resulted in better crystalline quality of GaN and InN epilayers.

Biosensing operations based on whispering-gallery-mode optical cavities in single 1.0-µm diameter hexagonal GaN microdisks grown by radio-frequency plasma-assisted molecular beam epitaxy

Biosensing operations based on a whispering-gallery-mode optical cavity in a single hexagonal GaN microdisk of approximately 1.0 µm diameter were demonstrated here. The sharp resonant peak in the photoluminescence spectrum obtained from the microdisk in aqueous sucrose solution redshifts with a change in sucrose concentration. The results indicate that an extremely small microdisk could be used as an optical transducer for sensing sugar, namely, as a biosensor. Furthermore, we investigate the relationship between the diameter of the microdisk and the sensitivity of the biosensor.

Photoluminescence excitation spectroscopy of excited states of an asymmetric cubic GaN/Al0.25Ga0.75N double quantum well grown by molecular beam epitaxy

Optical transitions involving higher energy levels of cubic AlGaN quantum wells are investigated by means of photoluminescence excitation spectroscopy. An asymmetric cubic GaN/AlxGa1−xN double quantum well (QW) structure with an Al content of x = 0.25 ± 0.03 was grown on a 3C-SiC(001) substrate exploiting radio-frequency plasma-assisted molecular beam epitaxy. The photoluminescence excitation data reveals two emission bands, which are assigned to the first electron and the third heavy hole (e1–hh3) and the second electron and the second heavy hole (e2–hh2) energy level of the wide QW. Besides in the narrow QW no higher energy levels can be observed. The experimental data is in good agreement with theoretical calculations using a Schrödinger–Poisson solver based on an effective mass model (nextnano3). The exciton binding energy was calculated considering the confinement of the QWs and also the energy dependency of the effective mass for excited energy levels.

Joint Raman spectroscopy and HRXRD investigation of cubic gallium nitride layers grown on 3C-SiC

Cubic gallium nitride (GaN) films are analyzed with highresolution X-ray diffraction (HRXRD) and Raman spectroscopy. Several cubic GaN layers were grown on 3C-SiC (001) substrate by radio-frequency plasma-assisted molecular beam epitaxy. The layer thickness of the cubic GaN was varied between 75 and 505 nm. The HRXRD analysis reveals a reduction of the full-width at half-maximum (FWHM) of omega scans for growing layer thicknesses, which is caused by a partial compensation of defects. The Raman characterization confirms well-formed c-GaN layers. A more detailed examination of the longitudinal optical mode hints at a correlation of the FWHM of the Raman mode with the dislocation density, which shows the possibility to determine dislocation densities by Ramanspectroscopy on a micrometer scale, which is not possible by HRXRD. Furthermore, this Raman analysis shows that normalized Raman spectra present an alternative way to determine layer thicknesses of thin GaN films.

(Invited) High Performance Deep Ultraviolet Nanowire Light Emitting Diodes and Lasers

Deep ultraviolet light sources are important for a broad range of applications including water purification, disinfection, and medical diagnostics. The currently reported AlGaN planar light emitting diodes (LEDs) and lasers generally exhibit poor performance, due to the presence of large densities of defects and dislocations and the inefficient p-type doping. In this context, we have investigated the molecular beam epitaxial growth and characterization of nearly defect-free AlGaN nanowire heterostructures grown directly on Si substrate. We have demonstrated AlGaN nanowire LEDs that can operate in the UV-B (280-320nm) and UV-C (<280 nm) bands, which exhibit excellent current current-voltage characteristics. We have further demonstrated electrically injected AlGaN nanowire lasers on Si that can exhibit relatively low threshold current densities (~ 100 A/cm2). In this work, catalyst-free AlGaN nanowires were grown directly on Si substrate by radio-frequency plasma-assisted molecular beam epitaxy. Due to the diffusion controlled growth process, such self-organized AlGaN nanowires exhibit unique core-shell structures, with the presence of Al-rich AlGaN shell. As a consequence, the nonradiative surface recombination can be greatly suppressed. By varying the Al/Ga flux ratio during the growth process, the emission wavelengths can be controllably varied from ~ 210 nm to >340 nm. The nanowire heterostructures exhibit relatively high internal quantum efficiency (~ 80%) at room-temperature in the UV-C band. We have further identified that Mg-dopant incorporation can be significantly enhanced in AlGaN nanowires, compared to their planar counterparts, due to the reduced formation energy for Al(Ga)-substitutional Mg-doping. The resulting AlN nanowire LEDs can exhibit excellent current-voltage characteristics, with a turn-on voltage of 6 V. We have also demonstrated that self-organized AlGaN nanowire arrays can function as a high Q optical cavity, due to the Anderson localization of light. The AlGaN nanowire lasers consist of n-AlGaN cladding layer, i-AlGaN active region, and p-AlGaN cladding layer. The devices were fabricated using standard photolithography and metallization techniques. Under continuous wave biasing conditions, the devices can exhibit very low threshold current density. For devices operating in the UV-AII (~ 320-340 nm) band, the threshold current density is ~ 12 A/cm2 at 6 K. The threshold current density increases slightly to ~ 200 A/cm2 for operation wavelengths in the UV-C band. The measured threshold current densities are nearly two to three orders of magnitude lower, compared to conventional AlGaN quantum well lasers under either optical pumping or electrical injection.