Plasma-assisted Molecular Beam Epitaxy Research Articles

Generation of GHz surface acoustic waves in (Sc,Al)N thin films grown on free-standing polycrystalline diamond wafers by plasma-assisted molecular beam epitaxy

Abstract Telecommunication of the next generation demands filters that can operate in the 10 GHz range with sufficient bandwidths. For surface-acoustic-wave (SAW) devices this prerequisite translates into high sound velocities and high piezoelectric couplings. Wurtzite AlN on diamond, which exploits the strong piezoelectricity of AlN with the very high SAW velocity of diamond, has been considered a promising platform. A significant boost (up to a factor of 4) of the piezoelectric response can be obtained by alloying AlN with Sc. Here, the main challenge lies in the synthesis of highly-oriented thin (Sc,Al)N films on diamond. In this work, we aim at establishing a platform for SAW devices using plasma-assisted molecular beam epitaxy for the deposition of Sc0.2Al0.8N on diamond. We investigate the structural properties related to SAW generation gearing towards applications at high frequencies. To this end, we prepare (Sc,Al)N thin films using plasma-assisted MBE on polished polycrystalline diamond wafers and demonstrate the efficient generation of SAW modes with frequencies up to 8 GHz. Systematic studies of the dependence of the SAW velocity and electromechanical coupling coefficient on the Sc0.2Al0.8N film thickness is presented for various SAW modes. Our result demonstrates the potential of this material combination for future application that requires large bandwidth in the ultra-high frequency range.

Monolayer-scale AlN/GaN digital alloys grown by plasma-assisted molecular beam epitaxy

The efficiency of usual AlGaN based deep ultraviolet light-emitting devices is still quite low. The difficulties are basically originated from the fundamental material properties of AlGaN. This work has adopted monolayer-scale (AlN)m/(GaN)n ordered digital alloys (DAs) as alternatives to AlGaN random alloys, m and n are the numbers of monolayers. X-ray diffraction scans have demonstrated clear satellite peaks, verifying good periodicity of AlN/GaN DAs grown by molecular beam epitaxy (MBE), and transmission electron microscopy results have revealed atomically sharp and smooth interfaces and quite precise m:n values agreeing well with designs. The electron densities of Si-doped (AlN)m/(GaN)n DAs with high equivalent Al compositions are significantly higher than those of conventional AlGaN:Si random alloys grown in the same MBE system. Si dopant ionization energies in DAs are only 2–5 meV, much lower than that for usual random alloys. The red shift of the light emission for DAs with thinner AlN barriers has suggested strong coupling between the GaN wells and thus formation of a miniband in a vertical direction. The results have demonstrated the potential of the (AlN)m/(GaN)n DAs as electronically functional alternatives for various device applications.

High-Quality Single Crystalline Sc0.37Al0.63N Thin Films Enabled by Precise Tuning of III/N Atomic Flux Ratio during Molecular Beam Epitaxy

We attained wurtzite ScxAl1−xN (0.16 ≤ x ≤ 0.37) thin films by varying the Sc and Al fluxes at a fixed active nitrogen flux during plasma-assisted molecular beam epitaxy. Atomic fluxes of Sc and Al sources via measured Sc percentage in as-grown ScxAl1−xN thin films were derived as the feedback for precise determination of the ScxAl1−xN growth diagram. We identified an optimal III/N atomic flux ratio of 0.78 for smooth Sc0.18Al0.82N thin films. Further increasing the III/N ratio led to phase separation under N-rich conditions, validated by the observation of high-Sc-content hillocks with energy-dispersive X-ray spectroscopy mapping. At the fixed III/N ratio of 0.78, we found that phase separation with high-Al-content hillocks occurs for x > 0.37, which is substantially lower than the thermodynamically dictated threshold Sc content of ~0.55 in wurtzite ScxAl1−xN. We postulate that these wurtzite-phase purity degradation scenarios are correlated with adatom diffusion and the competitive incorporation process of Sc and Al. Therefore, the ScxAl1−xN growth window is severely restricted by the adatom kinetics. We obtained single crystalline Sc0.37Al0.63N thin films with X-ray diffraction (002)/(102) ω rocking curve full-width at half-maximums of 2156 arcsec and 209 arcsec and surface roughness of 1.70 nm. Piezoelectric force microscopy probing of the Sc0.37Al0.63N epilayer validates unambiguous polarization flipping by 180°.

Fabrication and characterization of modulation-doped β-(AlxGa1−x)2O3/Ga2O3 tri-metal FET utilizing ultra-high vacuum deposition based on plasma-assisted molecular beam epitaxy

Fabrication and characterization of modulation-doped β-(AlxGa1−x)2O3/Ga2O3 tri-metal FET utilizing ultra-high vacuum deposition based on plasma-assisted molecular beam epitaxy

GaN thickness dependence of GaN/AlN superlattices on face-to-face-annealed sputter-deposited AlN templates

Recently, deep-ultraviolet (DUV) light-emitting devices have attracted attention for various applications. GaN/AlN superlattices have emerged as a promising alternative for achieving high-efficiency DUV emission. To fabricate superlattices with high crystal quality and abrupt interfaces, we have utilized face-to-face-annealed sputter-deposited AlN template substrates characterized by a flat surface and low dislocation density. Furthermore, radio-frequency plasma-assisted molecular beam epitaxy with in situ reflection high-energy electron diffraction monitoring was employed for the growth process. The growth of the superlattices follows a specific sequence. Step 1: AlN growth, Step 2: conversion of Al droplets to AlN, Step 3: GaN growth, and Step 4: evaporation of Ga droplets. This study explored the impact of GaN thickness on the GaN/AlN superlattice. The GaN thickness was linearly controlled by changing the duration of Step 3. This approach allowed for the growth of a flat GaN layer up to 1 monolayer (ML) and achieved superlattices with abrupt interfaces. Single-peak cathodoluminescence (CL) emission at 240–245 nm was observed from the superlattices, with the peak shift toward longer wavelengths as the GaN thickness increased. In contrast, quantum dot-like GaN islands were generated with a thickness of over 1 ML, induced by compressive strain. Superlattices with thicker GaN exhibited broad CL emission with multiple peaks. However, the AlN barrier layer reduced the surface roughness and maintained abrupt interfaces within the superlattices. Therefore, to obtain sharp single-peak UV emission from GaN/AlN superlattices, the growth sequence should be controlled to obtain flat GaN layers without dots.

Epitaxial Growth and Doping of N-Polar AlN by Plasma-Assisted Molecular Beam Epitaxy

In recent years, AlN has garnered significant attention in the research community due to its excellent electronic properties. AlN, which is an ultra-wide bandgap material with a direct band gap of 6.2 eV, has a high breakdown electric field (12 MV/cm) and high thermal conductivity. In addition, band engineering can be achieved by alloying AlN with Gallium (Ga), Indium (In), and Scandium (Sc), and two-dimensional electron gas (2DEG) is formed at the AlGaN/AlN interface due to strong piezoelectric and spontaneous polarization. Furthermore, AlN has high-temperature stability, which is suitable for application in harsh environments. Due to these unique properties, AlN has gained attention as a promising candidate for the next generation of high power electronic and optoelectronic applications.In this talk, we present the demonstration of controllable Si doping in N-polar AlN films grown on single crystal AlN substrates by plasma assisted molecular beam epitaxy (PAMBE). Through optimization of growth conditions, we obtained high quality N-polar AlN films at high temperatures. However, our studies revealed that Si incorporation dramatically decreases at such high growth temperature. To enable higher Si incorporation, a hybrid growth scheme was developed, using a combination of low-temperature and high-temperature growth condition and by using Ga as a surfactant at low growth temperature. By lowering the growth temperature of AlN to 750°C, we were able to incorporate Si with concentrations as high as 2x1020 cm-3 and demonstrated an electron concentration as high as 1.25x1019 cm-3 at room temperature.

Super-Nernstian potentiometric response of InN/InGaN quantum dots by fractional electron transfer

The potentiometric response of InN/InGaN quantum dots (QDs) on Si (111) is experimentally studied and modeled as a function of the In content and morphology of the InGaN layer below the QDs due to the changing N flux in stationary plasma-assisted molecular beam epitaxy. For isolated core–shell InGaN nanowires formed for N-rich growth, sub-Nernstian response with Cl− anions as the test analyte is observed. For compact columnar InGaN layers formed in a very narrow range of N flux at the N-rich to metal-rich growth transition, a maximum super-Nernstian response of 100 mV/decade is achieved, provided the In content is high. With reducing N flux and In content, low super-Nernstian response and finally sub-Nernstian response are re-established for compact planar GaN layers. The maximum super-Nernstian response and the transition to sub-Nernstian response are quantitatively modeled by the quantum partition of electrons inside and outside of the QDs and consequent fractional electron transfer in the artificial chemical reaction of the QDs with the anions.

Pinpointing lattice-matched conditions for wurtzite ScxAl1−xN/GaN heterostructures with x-ray reciprocal space analysis

Using comprehensive x-ray reciprocal space mapping, we establish the precise lattice-matching composition for wurtzite ScxAl1−xN layers on (0001) GaN to be x = 0.14 ± 0.01. 100 nm thick ScxAl1−xN films (x = 0.09–0.19) were grown in small composition increments on c-plane GaN templates by plasma-assisted molecular beam epitaxy. The alloy composition was estimated from the fit of the (0002) x-ray peak positions, assuming the c-lattice parameter of ScAlN films coherently strained on GaN increases linearly with Sc-content determined independently by Rutherford backscattering spectrometry [Dzuba et al., J. Appl. Phys. 132, 175701 (2022)]. Reciprocal space maps obtained from high-resolution x-ray diffraction measurements of the (101¯5) reflection reveal that ScxAl1−xN films with x = 0.14 ± 0.01 are coherently strained with the GaN substrate, while the other compositions show evidence of relaxation. The in-plane lattice-matching with GaN is further confirmed for a 300 nm thick Sc0.14Al0.86N layer. The full-width-at-half-maximum of the (0002) reflection rocking curve for this Sc0.14Al0.86N film is 106 arc sec and corresponds to the lowest value reported in the literature for wurtzite ScAlN films.

Electrical properties of ScN(111) layers grown on semi-insulating GaN(0001) by plasma-assisted molecular beam epitaxy

Electrical properties of ScN(111) layers grown on semi-insulating GaN(0001) by plasma-assisted molecular beam epitaxy

Patterning optimization for device realization of patterned GaAsSbN nanowire photodetectors

Vertically grown nanowires (NWs) are a research interest in optoelectronics and photovoltaic applications due to their high surface to volume ratio and good light trapping capabilities. This study presents the effects of process and design parameters on self-catalyzed GaAsSbN NWs grown by plasma-assisted molecular beam epitaxy on patterned silicon substrates using electron beam lithography. Vertical alignment of the patterned NWs examined via scanning electron microscopy show the sensitivity of patterned NW growth to the parameters of NW diameter, pitch, dose time, etching techniques and growth plan. Diameters range from 90 nm to 250 nm. Pitch lengths of 200 nm, 400 nm, 600 nm, 800 nm, 1000 nm, and 1200 nm were examined. Dry etching of the oxide layer of the silicon substrate and PMMA coating is performed using reactive ion etching (RIE) for 20 s and 120 s respectively. Comparisons of different HF etch durations performed pre and post PMMA removal are presented. Additionally, the report of an observed surfactant effect in dilute nitride GaAsSbN NWs in comparison to non-nitride GaAsSb is presented. Optimizations to patterning, RIE, and HF etching are presented to obtain higher vertical yield of patterned GaAsSbN NWs, achieving ∼80% of the expected NW µm2. Room temperature and 4 K photoluminescence results show the effect of nitride incorporation for further bandgap tuning, and patterned pitch on the optical characteristics of the NWs which gives insights to the compositional homogeneity for NWs grown at each pitch length.

MOLECULAR BEAM EPITAXY GROWTH OF HIGH-QUALITY InN NANORODS ON Si (111) SUBSTRATE

The growth of high-quality InN nanorods (NRs) on Si (111) by plasma-assisted molecular beam epitaxy (PAMBE) is reported. X-ray diffraction and Raman spectroscopy investigations indicates that the NRs are wurtzite, c-axis oriented and single crystalline. Low temperature photoluminescence emissions with peak energy of ~ 0.75eV was observed indicating the high quality of the nanostructures. This study unravels a novel strategy for the successful growth of high-quality InN NRs on Silicon which is highly promising for applications in next generation nanodevices.

Characterization of the p-NiO/n-GaN heterojunction and development of ultraviolet photodiode

Nickel oxide semiconductor is a good p-type material for fabricating a p-n-heterojunction with n-type GaN semiconductor. The p-NiO/n-GaN heterojunction, which produces a type II band bias, has significant prospects for creating photosensitive devices. Herein, it is shown that a selective ultraviolet photodetector can be created based on this heterojunction. p-NiO films were formed using magnetron sputtering onto n-doped GaN epitaxial layers grown by plasma assisted molecular beam epitaxy. The structural, optical, and electronic properties of the formed nickel oxide films were studied. A mechanism for reducing dislocation density and conductivity and increasing transparency due to film annealing is proposed. It has been established that the current–voltage characteristics of the p-NiO/n-GaN heterojunction are typical for diode structures and show selective sensitivity to ultraviolet radiation. The band model and sensitivity of the fabricated ultraviolet photodiode were evaluated. The photosensitivity at zero voltage was 3.11 mA/W, the quantum yield was 0.011, and the detection ability was 8.69 × 109 Jones. The work shows the potential of using the p-NiO/n-GaN heterojunction to create ultraviolet photodetectors.

Growth of AlGaN alloys with ∼ 90 % AlN mole fraction by plasma-assisted molecular beam epitaxy: Indication of phase-segregation effects

AlGaN alloys with very high AlN mole fraction (∼90 %) were grown by plasma-assisted molecular beam epitaxy (PA-MBE) and their alloy properties were investigated by high-resolution X-ray diffraction (HR-XRD) measurements. Growth was carried out employing a series of group III/V ratios and for samples grown under a high group-III regime, phase-segregation in the alloy was evident from a characteristic splitting of the AlGaN (0002) peak in the HR-XRD pattern. With decreasing excess group-III conditions the separation of peak positions continuously reduced, till a single peak was observed. However, for samples where such splitting was absent, spontaneous superlattice peaks were seen at lower incident angles of the XRD patterns indicating the presence of long-range atomic ordering (LRAO). Thus, for AlGaN alloys with extremely high Al content, effects of phase-segregation, or of LRAO were observed, depending on the kinetics of growth. These results on phase-segregation effects are expected to promote the development of high-efficiency deep ultraviolet emitters for skin-safe germicidal action by mitigating the detrimental effect of dislocations and related non-radiative recombination centers through carrier localization processes at potential minima.

Low-defect and stress-free AlN(0001) nanoprisms and microrods selectively grown on micro-patterned c-sapphire substrate by plasma-assisted molecular beam epitaxy

This paper describes different growth modes of AlN layers on micro-cone patterned c-sapphire substrates (μ-PSSs) using plasma-assisted molecular beam epitaxy. Ordered arrays of AlN nanoprisms and microrods were selectively grown on the tips of μ-PSS's microcones according to a bottom-up formation mechanism using sequential migration enhanced and metal-modulated epitaxy (MME) under metal-rich growth conditions at 820 °C. Transmission electron microscopy revealed structurally perfect AlN regions above the tips of the μ-PSSs, which initiate as inverted nanopyramids with {1011¯} side faces, evolving into hexagonal nanoprisms with orientations of {11¯00} and (0001) for side and top surfaces, respectively. The diameter and height of these ordered hexagonal nanoprisms, which have a 60% probability of nucleating, were about 1 μm. Long-term MME growth of these nanoprisms in both vertical and lateral directions led to the formation of AlN(0001) microrods with a maximum possible diameter of two micrometers and a height of up to 6 μm. Atomic force microscopy revealed a mixed step-flow and 2D nucleation growth mechanism for the flat tops of these AlN nanoprisms and microrods with an average surface roughness of 1–2 monolayers. Micro-Raman spectroscopy demonstrated narrow E2 (high) linewidths of 3.8 and 4.2 cm−1 for essentially stress-free AlN nanoprisms and microrods, respectively.

Evidence of two-dimensional lateral quantum confinement in self-formed core–shell InGaN nanowires on Si (111) emitting in the red

The proof of strong two-dimensional lateral quantum confinement in the In-rich core of red-light emitting self-formed core–shell InGaN nanowires is given. The nanowires are directly grown on Si (111) by plasma-assisted molecular beam epitaxy. After the initial InGaN nucleation, straight nanowires with quantum-size core radius determined by x-ray diffraction, transmission electron microscopy, and energy dispersive x-ray mappings develop. Detailed comparison of the photoluminescence from the core, the In contents of the core and shell, and the core radius with theoretical modeling reveals a parabolic confinement potential with large ground state quantum confinement energies of electrons and holes. Such strong lateral quantum confinement in a vertical quantum wire active region is ideal for the performance of optoelectronic devices, in particular of our reported red InGaN light emitting diode with high brightness and color stability.

Molecular beam epitaxy and characterization of ferroelectric quaternary alloy Sc0.2Al0.45Ga0.35N

In this study, we demonstrate ferroelectricity in high-quality monocrystalline quaternary alloy ScAlGaN. Sc0.2Al0.45Ga0.35N films are grown by plasma-assisted molecular beam epitaxy and exhibit a surface roughness of 0.5 nm, limited by the roughness of the underlying molybdenum template. Polarization-electric field and positive-up-negative-down measurements reveal unambiguous ferroelectric switching with a coercive field of ∼5.5 MV cm−1 at 10 kHz and high remanent polarization of ∼150 μC cm−2. Time-dependent measurements suggest that the polarization reversal behavior adheres to the Kolmogorov–Avrami–Ishibashi model and follows a scheme of domain nucleation and growth. Detailed piezoresponse force microscopy studies further elucidate the evolution of polarity reversal domains in wurtzite nitride ferroelectrics and support the notion that the growth of inversion domains occurs via an in-plane motion of the domain walls. The realization of functional ferroelectric quaternary alloys in the wurtzite nitride family extends beyond being a technical demonstration. The additional degree of bandgap, band alignment, lattice parameter, and piezoelectric constant tunability achievable through quaternary alloys unveils a vast dimension through which wurtzite nitride ferroelectrics can be optimally engineered for a broad variety of high-performance electronic, optoelectronic, and acoustic devices and systems.

Metal-modulated epitaxy of Mg-doped Al0.80In0.20N-based layer for application as the electron blocking layer in deep ultraviolet light-emitting diodes

This work reports the growth and characterization of p-AlInN layers doped with Mg by plasma-assisted molecular beam epitaxy (PAMBE). AlInN was grown with an Al molar fraction of 0.80 by metal-modulated epitaxy (MME) with a thickness of 180 nm on Si(111) substrates using AlN as buffer layers. Low substrate temperatures were used to enhance the incorporation of indium atoms into the alloy without clustering, as confirmed by X-ray diffraction (XRD). Cathodoluminescence measurements revealed ultraviolet (UV) range emissions. Meanwhile, Hall effect measurements indicated a maximum hole mobility of 146 cm2/(V∙s), corresponding to a free hole concentration of 1.23 × 1019 cm−3. The samples were analyzed by X-ray photoelectron spectroscopy (XPS) estimating the alloy composition and extracting the Fermi level by valence band analysis. Mg-doped AlInN layers were studied for use as the electron-blocking layer (EBL) in LED structures. We varied the Al composition in the EBL from 0.84 to 0.96 molar fraction to assess its theoretical effects on electroluminescence, carrier concentration, and electric field, using SILVACO Atlas. The results from this study highlight the importance and capability of producing high-quality Mg-doped p-AlInN layers through PAMBE. Our simulations suggest that an Al content of 0.86 is optimal for achieving desired outcomes in electroluminescence, carrier concentration, and electric field.

Observation of skyrmion-induced topological Hall effects in Mn3.5Au0.5N films

The Mn4-xAuxN films were grown on MgO(001) substrates by plasma assisted molecular beam epitaxy. The magnetic characteristics and spin transport properties are systematically studied. A sign reversal of anomalous Hall coefficient is observed at 160 K in Mn3.5Au0.5N. This phenomenon can be attributed to the competition between electron-like and hole-like anomalous Hall effects (AHEs). The mutual eliminating of electron-like and hole-like AHEs gives rise to zero resistivity of 160 K at high field. A butterfly-like curve with two humps is observed at 160 K, which can be ascribed to the skyrmion-induced topological Hall effects (THEs). It is thus confirmed that the transverse resistivity in Mn3.5Au0.5N comprises of three components: electron-like AHEs, hole-like AHEs, and skyrmion-induced THEs. An effective method was employed to analyze THEs in the material systems that contain both electrons and holes. The THEs in Mn3.5Au0.5N can be observed after electron-like AHEs are subtracted down from transverse resistivity.

Accommodating a hexagonal [formula omitted]- phase Mn2N film on a cubic MgO (001) substrate

The thin films of c-plane dominated hexagonal ζ- phase Mn2N were successfully grown on a cubic MgO (001) substrate directly using plasma-assisted molecular beam epitaxy. The surface was comprehensively studied through experimental and theoretical approaches. Reflection high energy electron diffraction revealed two pseudo-cubic domains along [100]MgO and [110]MgO, and one hexagonal domain, which is 30° apart from the [100]MgO direction. Scanning tunneling microscopy was used to image and resolve the high quality surface, revealing a distorted hexagonal surface structure. Furthermore, atomic resolution of the hexagonal domain with a 2 × 2 reconstructed surface is presented. For the c-plane surface, theoretical investigations were carried out using first-principle studies to determine the surface formation energy for various reconstructed surfaces. The theoretical study shows that the nitrogen terminated 2 × 2 structure with a manganese adatom on the surface is the most stable reconstruction that reproduces the experimental results. A corresponding simulated scanning tunneling microscopy model is also presented, providing strong support for the experimentally observed in-plane lattice structures. The manganese:nitrogen stoichiometry within the bulk and surface is in good agreement with the expected ratio of 2:1.

Epitaxial Growth of GaN Films on Chemical-Vapor-Deposited 2D MoS2 Layers by Plasma-Assisted Molecular Beam Epitaxy.

The van der Waals epitaxy of wafer-scale GaN on 2D MoS2 and the integration of GaN/MoS2 heterostructures were investigated in this report. GaN films have been successfully grown on 2D MoS2 layers using three different Ga fluxes via a plasma-assisted molecular beam epitaxy (PA-MBE) system. The substrate for the growth was a few-layer 2D MoS2 deposited on sapphire using chemical vapor deposition (CVD). Three different Ga fluxes were provided by the gallium source of the K-cell at temperatures of 825, 875, and 925 °C, respectively. After the growth, RHEED, HR-XRD, and TEM were conducted to study the crystal structure of GaN films. The surface morphology was obtained using FE-SEM and AFM. Chemical composition was confirmed by XPS and EDS. Raman and PL spectra were carried out to investigate the optical properties of GaN films. According to the characterizations of GaN films, the van der Waals epitaxial growth mechanism of GaN films changed from 3D to 2D with the increase in Ga flux, provided by higher temperatures of the K-cell. GaN films grown at 750 °C for 3 h with a K-cell temperature of 925 °C demonstrated the greatest crystal quality, chemical composition, and optical properties. The heterostructure of 3D GaN on 2D MoS2 was integrated successfully using the low-temperature PA-MBE technique, which could be applied to novel electronics and optoelectronics.