Plasma-assisted Molecular Beam Epitaxy Growth Research Articles

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.

Влияние постростовой обработки плазмой азота на структурные и оптические свойства InGaN

The effect of cooling conditions in the plasma-assisted molecular-beam epitaxy growth on the structural and optical properties of InGaN nanostructures is studied. It is shown that cooling of the samples without nitrogen plasma contributes to the suppression of phase separation in InGaN nanostructures. The integrated intensity of photoluminescence from these nanostructures increased by a factor of 2.

Effect of Nitrogen Plasma Treatment on the Structural and Optical Properties of InGaN

The effect of cooling conditions in the plasma-assisted molecular-beam epitaxy growth on the structural and optical properties of InGaN nanostructures is studied. It is shown that cooling of the samples without nitrogen plasma contributes to the suppression of phase separation in InGaN nanostructures. The integrated intensity of photoluminescence from these nanostructures increased by a factor of 2. Keywords: InGaN, silicon, nanostructures, photoluminescence, structural properties, optical properties, molecular beam epitaxy, nitrogen plasma.

Evaluation of nanoindentation characteristics of cubic InN epilayer grown by Molecular Beam Epitaxy

The potential of the III-nitride semiconductor materials for modern optoelectronic applications as diodes, transistors, LEDs, and photovoltaics has prompted the mechanical characterization of small volumes as thin films. In this paper, the load-displacement curves of cubic indium nitride (c-InN) obtained during the nanoindentation with a Berkovich indenter were investigated. c-InN was obtained by plasma-assisted molecular beam epitaxy growth on c-GaN/MgO (100). The thickness of the c-GaN buffer layer used in all the films studied was 350 nm to eliminate the substrate's effect on the material studied properties. The c-InN thickness is around 180 nm. The reflection high energy electron diffraction and the X-ray diffraction results show that the c-GaN buffer and c-InN grown layers had a high cubic zincblende phase with more than 97%. The obtained value of the hardness is 12.5 ± 0.4 GPa, and the value for Young's modulus is 365.6 ± 7 GPa with a Poisson's ratio of 0.3.

Growth of zinc-blende GaN on muscovite mica by molecular beam epitaxy

The mechanisms of plasma-assisted molecular beam epitaxial growth of GaN on muscovite mica were investigated. Using a battery of techniques, including scanning and transmission electron microscopy, atomic force microscopy, cathodoluminescence, Raman spectroscopy and x-ray diffraction, it was possible to establish that, in spite of the lattice symmetry mismatch, GaN grows in epitaxial relationship with mica, with the [11–20] GaN direction parallel to [010] direction of mica. GaN layers could be easily detached from the substrate via the delamination of the upper layers of the mica itself, discarding the hypothesis of a van der Waals growth mode. Mixture of wurtzite (hexagonal) and zinc blende (ZB) (cubic) crystallographic phases was found in the GaN layers with ratios highly dependent on the growth conditions. Interestingly, almost pure ZB GaN epitaxial layers could be obtained at high growth temperature, suggesting the existence of a specific GaN nucleation mechanism on mica and opening a new way to the growth of the thermodynamically less stable ZB GaN phase.

N-polar GaN/AlN resonant tunneling diodes

N-polar GaN/AlN resonant tunneling diodes are realized on a single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy growth. The room-temperature current–voltage characteristics reveal a negative differential conductance (NDC) region with a peak tunneling current of 6.8 ± 0.8 kA/cm2 at a forward bias of ∼8 V. Under reverse bias, the polarization-induced threshold voltage is measured at ∼−4 V. These resonant and threshold voltages are well explained with the polarization field, which is opposite to that of the metal-polar counterpart, confirming the N-polarity of the resonant tunneling diodes (RTDs). When the device is biased in the NDC-region, electronic oscillations are generated in the external circuit, attesting to the robustness of the resonant tunneling phenomenon. In contrast to metal-polar RTDs, N-polar structures have the emitter on the top of the resonant tunneling cavity. As a consequence, this device architecture opens up the possibility of seamlessly interfacing—via resonant tunneling injection—a wide range of exotic materials with III-nitride semiconductors, providing a route towards unexplored device physics.

High nitrogen flux plasma-assisted molecular beam epitaxy growth of InxGa1-xN films

Growth of efficient III-N based light emitting devices by plasma assisted molecular beam epitaxy has been elusive, even though the technique has attractive advantages in comparison to metal organic chemical vapor deposition. Modern high-flux radio frequency plasma systems could remedy this issue by enabling growth of InxGa1-xN at higher temperatures than previously possible, likely improving the material quality. In this work, active nitrogen fluxes of up to 3.5 μm/h GaN-equivalent growth rate were employed to grow InxGa1-xN alloys. InxGa1-xN growth rates of 1.3 μm/h were demonstrated at growth temperatures of 550 °C and 600 °C with maximum film compositions of In0.25Ga0.75N and In0.21Ga0.79N, respectively. A composition of In0.05Ga0.95N was observed in a film grown at 700 °C with smooth step-terrace morphology.

P-Type Nonpolar a-ZnO:N Thin Films on r-Sapphire Substrates Grown by Molecular Beam Epitaxy

We have grown nonpolar nitrogen (N) doped a-plane zinc oxide (ZnO) films on r-plane sapphire substrates in order to eliminate the self-polarization component in the growth direction, which decreases the doping efficiency of N acceptors. Nonpolar a-ZnO:N films were grown by plasma-assisted molecular beam epitaxy (PA-MBE) using Zn metal and a plasma source of O2 and NO mixed gas. It was confirmed by reflection high-energy electron diffraction and x-ray diffraction that single phase a-plane ZnO:N films were grown on the r-plane sapphire substrates. After the PA-MBE growth, the post-annealing was performed in an oxygen atmosphere. Photoluminescence experiments showed donor–acceptor pair emissions increase with increasing the annealing temperature (≤ 700°C). AC magnetic field Hall effect measurements revealed that n-type conduction of the as-grown films clearly changed to the p-type at the annealing temperature of 650°C. The resistivity, hole concentration, and mobility were ρ = 3.4 Ω cm, p = 8.0 × 1017 cm−3, and μ = 2.3 cm2/Vs, respectively.

Efficient suboxide sources in oxide molecular beam epitaxy using mixed metal + oxide charges: The examples of SnO and Ga2O

Sources of suboxides, providing several advantages over metal sources for the molecular beam epitaxy (MBE) of oxides, are conventionally realized by decomposing the corresponding oxide charge at extreme temperatures. By quadrupole mass spectrometry of the direct flux from an effusion cell, we compare this conventional approach to the reaction of a mixed oxide + metal charge as a source for suboxides with the examples of SnO2 + Sn → 2 SnO and Ga2O3 + 4 Ga → 3 Ga2O. The high decomposition temperatures of the pure oxide charge were found to produce a high parasitic oxygen background. In contrast, the mixed charges reacted at significantly lower temperatures, providing high suboxide fluxes without additional parasitic oxygen. For the SnO source, we found a significant fraction of Sn2O2 in the flux from the mixed charge that was basically absent in the flux from the pure oxide charge. We demonstrate the plasma-assisted MBE growth of SnO2 using the mixed Sn + SnO2 charge to require less activated oxygen and a significantly lower source temperature than the corresponding growth from a pure Sn charge. Thus, the sublimation of mixed metal + oxide charges provides an efficient suboxide source for the growth of oxides by MBE. Thermodynamic calculations predict this advantage for further oxides as well, e.g., SiO2, GeO2, Al2O3, In2O3, La2O3, and Pr2O3.

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.

BBr3 as a boron source in plasma-assisted molecular beam epitaxy

Boron is a difficult material to use in a molecular beam epitaxy (MBE) reactor due to its high melting point as a pure compound. Consequently, there is interest in exploring alternative sources for B in MBE. In this paper, the authors detail the construction and operation of a novel BBr3 injection system for plasma-assisted MBE growth and show results for BGaN thin films grown using readily available low purity BBr3 as a proof of concept for the source. The BBr3 system enables the growth of coherent BGaN films with a concentration up to 3% B on the group III site and thicknesses up to 280 nm as determined by high resolution x-ray diffraction. Atom probe tomography and secondary ion mass spectroscopy results of a B0.03Ga0.97N film indicate a high level of Br impurity on the order of 1 × 1019 atoms/cm3 and atmospheric contamination consistent with a low purity source. BBr3 is successful as a B source for high crystal quality BGaN films; however, the Br incorporation from the source limits the applications for this material.

Spontaneous intercalation of Ga and In bilayers during plasma-assisted molecular beam epitaxy growth of GaN on graphene on SiC

The formation of a self-limited metallic bilayer is reported during the growth of GaN by plasma-assisted molecular beam epitaxy on graphene on (0001) SiC. Depending on growth conditions, this layer may consist of either Ga or In, which gets intercalated between graphene and the SiC surface. Diffusion of metal atoms is eased by steps at SiC surface and N plasma induced defects in the graphene layer. Energetically favorable wetting of the (0001) SiC surface by Ga or In is tentatively assigned to the breaking of covalent bonds between (0001) SiC surface and carbon buffer layer. As a consequence, graphene doping and local strain/doping fluctuations decrease. Furthermore, the presence of a metallic layer below GaN opens the way to the development of devices with a spontaneously formed metallic electrode on their back side.

Enhanced photoelectrochemical performance of InGaN-based nanowire photoanodes by optimizing the ionized dopant concentration

InGaN-based nanowires (NWs) have been extensively studied for photoelectrochemical (PEC) water splitting devices owing to their tunable bandgap and good chemical stability. Here, we further investigated the influence of Si doping on the PEC performance of InGaN-based NW photoanodes. The Si dopant concentration was controlled by tuning the Si effusion cell temperature (TSi) during plasma-assisted molecular beam epitaxy growth and further estimated by Mott-Schottky electrochemical measurements. The highest Si dopant concentration of 2.1 × 1018 cm−3 was achieved at TSi = 1120 °C, and the concentration decreased with further increases in TSi. The flat-band potential was calculated and used to estimate the conduction and valence band edge potentials of the Si-doped InGaN-based NWs. The band edge potentials were found to seamlessly straddle the redox potentials of water splitting. The linear scan voltammetry results were consistent with the estimated carrier concentration. The InGaN-based NWs doped with Si at TSi = 1120 °C exhibited almost 9 times higher current density than that of the undoped sample and a stoichiometric evolution of hydrogen and oxygen gases. Our systematic findings suggest that the PEC performance can be significantly improved by optimizing the Si doping level of InGaN-based NW photoanodes.

Influence of substrate temperature on incorporation of magnesium into Zn1-xMgxO layers growth by molecular beam epitaxy

Plasma-assisted molecular beam epitaxy growth of ZnO and Zn1-xMgxO epilayers on Al2O3 (11–20) substrates is described. The influence of the substrate temperature on the growth rate and magnesium concentration in Zn1-xMgxO epilayers was studied in detail. It was found that the composition of magnesium in Zn1-xMgxO epilayers increased with increasing substrate temperature. The composition variation in the alloys as a function of the growth temperature could be explained by the difference of vapor pressure of Mg and Zn at high growth temperature. Theoretical approaches to surface evaporation of metal atoms under different growth condition are difficult. Here, we describe an uncomplicated method determine quantities experimental to obtain. These results can be extended to other types of ternary or quaternary materials grown by MBE technique.

True-blue laser diodes with tunnel junctions grown monolithically by plasma-assisted molecular beam epitaxy

We demonstrate true-blue 450 nm tunnel junction (TJ) laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy (PAMBE). The absence of hydrogen during PAMBE growth allows us to achieve TJs with low resistance. We compare TJ LDs with LDs of standard construction with p-type metal contact. For both types of LD, the threshold current density is around 3 kA/cm2 and the slope efficiency is 0.5 W/A. We do not observe any significant changes in optical losses and differential gain in TJ LDs compared with standard LDs. The differential resistivity of the TJs for current densities higher than 2 kA/cm2 is below 10−4 Ω·cm2.

Effect of surface roughness, chemical composition, and native oxide crystallinity on the orientation of self-assembled GaN nanowires on Ti foils

We report on plasma-assisted molecular beam epitaxial growth of almost randomly oriented, uniformly tilted, and vertically aligned self-assembled GaN nanowires (NWs), respectively, on different types of polycrystalline Ti foils. The NW orientation with respect to the substrate normal, which is affected by an in situ treatment of the foil surface before NW growth, depends on the crystallinity of the native oxide. Direct growth on the as-received foils results in the formation of ensembles of nearly randomly oriented NWs due to the strong roughening of the surface induced by chemical reactions between the impinging elements and Ti. Surface nitridation preceding the NW growth is found to reduce this roughening by transformation of the uppermost layers into TiN and TiOxNy species. These compounds are more stable against chemical reactions and facilitate the growth of uniformly oriented GaN NW ensembles on the surface of the individual grains of the polycrystalline Ti foils. If an amorphous oxide layer is present at the foil surface, vertically oriented NWs are obtained all across the substrate because this layer blocks the transfering of the epitaxial information from the underlying grains. The control of NW orientation and the understanding behind the achievement of vertically oriented NWs obtained in this study represent an important step towards the realization of GaN NW-based bendable devices on polycrystalline metal foils.

Single-crystal N-polar GaN p-n diodes by plasma-assisted molecular beam epitaxy

N-polar GaN p-n diodes are realized on single-crystal N-polar GaN bulk wafers by plasma-assisted molecular beam epitaxy growth. The current-voltage characteristics show high-quality rectification and electroluminescence characteristics with a high on currents ∼10 kA/cm2, low off currents <10−5 A/cm2, on/off current ratio of >109, and interband photon emission. The measured electroluminescence spectrum is dominated by a strong near-band edge emission, while deep level luminescence is greatly suppressed. A very low dislocation density leads to a high reverse breakdown electric field of ∼2.2 MV/cm without fields plates—the highest reported for N-polar epitaxial structures. The low leakage current N-polar diodes open up several potential applications in polarization-engineered photonic and electronic devices.

Growth kinetics of O-polar BexMgyZn1-x-yO alloy: Role of Zn to Be and Mg flux ratio as a guide to growth at high temperature

We studied the effect of the substrate temperature, in the range from 450 °C to 500 °C, on the required Zn to (Be + Mg) flux ratio for plasma-assisted molecular beam epitaxy growth of O-polar BexMgyZn1-x-yO on (0001)-GaN/sapphire templates. Achievement of single-crystalline BexMgyZn1-x-yO with improved optical and structural qualities required relatively high substrate temperatures, which necessitated the Zn to (Be + Mg) flux ratio to be increased from 3.9 at 450 °C to 8.3 at 500 °C. This resulted in a reduction of Mg incorporation from 25% to 15% for a fixed Be content of ∼3%. With increasing Zn to (Be + Mg) ratio, 15 K photoluminescence energy for the dominant emission remained unchanged at around 3.75 eV and 3.55 eV for the samples grown at 475 °C and 500 °C, respectively. These findings readily suggest a kinetic limitation of Mg and Be incorporation into wurtzite BexMgyZn1-x-yO lattice, resulting in the formation of second phase due mainly to the enhanced surface mobility of Mg adatoms and, therefore, an increase in the probability of the formation of Mg-rich clusters. An increase in the in-plane lattice parameter, deduced from the Reflection High Energy Electron Diffraction, at the onset of the phase segregation suggests the formation of the wurtzite phase MgO rich alloy(s).

Growth of free-standing bulk wurtzite AlxGa1−xN layers by molecular beam epitaxy using a highly efficient RF plasma source

The recent development of group III nitrides allows researchers world-wide to consider AlGaN based light emitting diodes as a possible new alternative deep ultra–violet light source for surface decontamination and water purification. In this paper we will describe our recent results on plasma-assisted molecular beam epitaxy (PA-MBE) growth of free-standing wurtzite AlxGa1−xN bulk crystals using the latest model of Riber's highly efficient nitrogen RF plasma source. We have achieved AlGaN growth rates up to 3µm/h. Wurtzite AlxGa1−xN layers with thicknesses up to 100μm were successfully grown by PA-MBE on 2-inch and 3-inch GaAs (111)B substrates. After growth the GaAs was subsequently removed using a chemical etch to achieve free-standing AlxGa1−xN wafers. Free-standing bulk AlxGa1−xN wafers with thicknesses in the range 30–100μm may be used as substrates for further growth of AlxGa1−xN-based structures and devices. High Resolution Scanning Transmission Electron Microscopy (HR-STEM) and Convergent Beam Electron Diffraction (CBED) were employed for detailed structural analysis of AlGaN/GaAs (111)B interface and allowed us to determine the N-polarity of AlGaN layers grown on GaAs (111)B substrates. The novel, high efficiency RF plasma source allowed us to achieve free-standing AlxGa1−xN layers in a single day's growth, making this a commercially viable process.

Plasma-assisted molecular beam epitaxy growth diagram of InGaN on (0001)GaN for the optimized synthesis of InGaN compositional grades

A PAMBE (plasma-assisted molecular beam epitaxy) growth diagram of InGaN is established for the metal-face (0001) orientation at 575 °C. This growth diagram allows identifying different growth regimes during the epitaxy of InGaN, namely the N-rich, the Ga-rich, the intermediate In-rich, and the In-accumulation regimes. InGaN compositional grades are then grown by ramping the Ga and In fluxes in parallel to remain in either the intermediate In-rich or the N-rich regime. The two samples exhibit distinct structural characteristics, surface morphologies, and optical properties. The growth diagram proves to be a useful tool in controlling the metal fluxes and thereby the stoichiometry conditions on the surface during the growth of compositionally graded layers.