Plasma-assisted Molecular Beam Epitaxy Research Articles

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.

Investigation of Si incorporation in (010) β-Ga2O3 films grown by plasma-assisted MBE using diluted disilane as Si source and suboxide Ga2O precursor

Traditionally, elemental Ga and Si have been used to supply Ga and Si, respectively, in molecular beam epitaxy (MBE) to grow Si-doped β-Ga2O3. In this work, we investigated the feasibility of enhancing the β-Ga2O3 growth rate by using a Ga-suboxide precursor in a plasma-assisted MBE. Additionally, Si doping of β-Ga2O3 using diluted disilane and Ga-suboxide as the Si and Ga precursors, respectively, was studied. The growth rate and film quality under different suboxide fluxes were inspected. We found that Si concentration has an inverse relationship with Ga2O flux due to atom competition. A room-temperature mobility of 115 cm2/V s was measured for an electron concentration of 1.2 × 1017 cm−3 on the sample grown using a Ga2O beam equivalent pressure of 1.1 × 10−7 Torr and a disilane flow rate of 0.006 sccm. Temperature-dependent Hall characterization was performed on this sample, revealing compensating acceptor and neutral impurity densities of 2.70 × 1015 and 8.23 × 1017 cm−3, respectively.

Dislocation density as a factor compensating the polarization doping effect in graded p-AlGaN contact layers

One of the approaches to improve p-doping properties in (Al)GaN-based materials is using a method called polarization doping. In this study, the graded p-AlGaN contact layers were deposited using plasma-assisted molecular beam epitaxy with different III/N ratios. To study the influence of Ga flux on the structural and electrical properties of the grown graded p-AlGaN structures, the samples were investigated using reflection high-energy electron diffraction, atomic force microscopy, secondary ion mass spectrometry, X-ray diffraction, and Hall effect measurements. The electronic structure of the samples was investigated by X-ray absorption near edge spectroscopy, while the polarity of the layers was analyzed by means of transmission electron microscopy. Our study reveals that growing with a higher Ga flux increases Mg incorporation and changes its distribution in the graded layer. However, it was shown that the main factor drastically affecting the magnitude and the type of conductivity in the graded p-AlGaN structures is the dislocation density. The highest hole concentration of ∼3.0×1018 cm−3 was observed for sample grown with a low Ga flux.

Structural and surface characterizations of 2D β-In2Se3/3D β-Ga2O3 heterostructures grown on c-Sapphire substrates by molecular beam epitaxy

Integrating two-dimensional (2D) layered materials with wide bandgap β-Ga2O3 has unveiled impressive opportunities for exploring novel physics and device concepts. This study presents the epitaxial growth of 2D β-In2Se3/3D β-Ga2O3 heterostructures on c-Sapphire substrates by plasma-assisted molecular beam epitaxy. Firstly, we employed a temperature-dependent two-step growth process to deposit Ga2O3 and obtained a phase-pure (2¯01)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(\\overline{2 }01)$$\\end{document} β-Ga2O3 film on c-Sapphire. Interestingly, the in-situ reflective high-energy electron diffraction (RHEED) patterns observed from this heterostructure revealed the in-plane ‘b’ lattice constant of β-Ga2O3 ~ 3.038Å. In the next stage, for the first time, 2D In2Se3 layers were epitaxially realized on 3D β-Ga2O3 under varying substrate temperatures (Tsub) and Se/In flux ratios (RVI/III). The deposited layers exhibited (00l) oriented β-In2Se3 on (2¯01)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(\\overline{2 }01)$$\\end{document} β-Ga2O3/c-Sapphire with the epitaxial relationship of [112¯0]\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$[11\\overline{2 }0]$$\\end{document} β-In2Se3 || [010] β-Ga2O3 and [101¯0]\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$[10\\overline{1 }0]$$\\end{document} β-In2Se3 || [102] β-Ga2O3 as observed from the RHEED patterns. Also, the in-plane ‘a’ lattice constant of β-In2Se3 was determined to be ~ 4.027Å. The single-phase β-In2Se3 layers with improved structural and surface quality were achieved at a Tsub ~ 280 °C and RVI/III ~ 18. The microstructural and detailed elemental analysis further confirmed the epitaxy of 2D layered β-In2Se3 on 3D β-Ga2O3, a consequence of the quasi-van der Waals epitaxy. Furthermore, the β-Ga2O3 with an optical bandgap (Eg) of ~ 5.04 eV (deep ultraviolet) when integrated with 2D β-In2Se3, Eg ~ 1.43eV (near infra-red) can reveal potential applications in the optoelectronic field.

Molecular Beam Epitaxy of β-(InxGa1-x)2O3 on β-Ga2O3 (010): Compositional Control, Layer Quality, Anisotropic Strain Relaxation, and Prospects for Two-Dimensional Electron Gas Confinement.

In this work, we investigate the growth of monoclinic β-(InxGa1-x)2O3 alloys on top of (010) β-Ga2O3 substrates via plasma-assisted molecular beam epitaxy. In particular, using different in situ (reflection high-energy electron diffraction) and ex situ (atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, and transmission electron microscopy) characterization techniques, we discuss (i) the growth parameters that allow for In incorporation and (ii) the obtainable structural quality of the deposited layers as a function of the alloy composition. In particular, we give experimental evidence of the possibility of coherently growing (010) β-(InxGa1-x)2O3 layers on β-Ga2O3 with good structural quality for x up to ≈ 0.1. Moreover, we show that the monoclinic structure of the underlying (010) β-Ga2O3 substrate can be preserved in the β-(InxGa1-x)2O3 layers for wider concentrations of In (x ≤ 0.19). Nonetheless, the formation of a large amount of structural defects, like unexpected () oriented twin domains and partial segregation of In is suggested for x > 0.1. Strain relaxes anisotropically, maintaining an elastically strained unit cell along the a* direction vs plastic relaxation along the c* direction. This study provides important guidelines for the low-end side tunability of the energy bandgap of β-Ga2O3-based alloys and provides an estimate of its potential in increasing the confined carrier concentration of two-dimensional electron gases in β-(InxGa1-x)2O3/(AlyGa1-y)2O3 heterostructures.

Record peak current density of over 1500 kA/cm2 in highly scaled AlN/GaN double-barrier resonant tunneling diodes on free-standing GaN substrates

This work demonstrates high-performance AlN/GaN double-barrier resonant tunneling diodes (RTDs) with high peak current density grown by plasma-assisted molecular beam epitaxy on c-plane free-standing GaN substrates, featuring stable and repeatable negative differential resistance (NDR) characteristics at room temperature. By scaling down the barrier thickness of AlN barrier and the lateral mesa size of collector, the record peak current density of 1551 kA/cm2 is achieved along with a peak-to-valley current ratio (PVCR) of 1.24, which is attributed to the reduced resonant tunneling time under thinner AlN barrier and the suppressed external incoherent valley current by reducing the dislocation number contained in the RTD device with the smaller size of collector. By statistically analyzing the NDR performance of RTD devices with different thicknesses of AlN barrier, the average peak current density increases from 145.7 to 1215.1 kA/cm2, while the average PVCR decreases from 1.45 to 1.1, correspondingly, accompanying with a decreased peak voltage from 6.89 to 5.49 V, with downscaling the AlN barrier thickness from 1.5 to 1.25 nm. The peak current density obtained in this work is the highest value among all the reported nitride-based RTDs up until now while maintaining high PVCR value simultaneously, which illustrates that ultra-scaled RTD based on vertical quantum-well structure and lateral collector size is a valuable approach for the development of nitride-based RTDs with excellent NDR characteristics and reveals their great potential applications in high-frequency oscillation sources and high-speed switch circuits.

Optical characterizations of GaN/MoS2 van der Waals heterojunctions with different band alignments

The integration of gallium nitride (GaN) with 2D molybdenum disulfide (MoS2) to form GaN/MoS2 semiconductor heterojunctions will have high potential applications in novel electronics and optoelectronics. In this study, GaN thin films were grown on pulse-laser-deposited MoS2 layer by plasma-assisted MBE at different substrate temperatures (500 °C, 600 °C and 700 °C, respectively). The energy-band alignments of GaN/MoS2 semiconductor heterostructures were analyzed by X-ray photoelectron spectroscopy. The epitaxial growth conditions of GaN films influenced the band alignment of GaN/MoS2 heterojunction. Type-I heterostructure, a straddling relation between narrow-bandgap MoS2 and wide-bandgap GaN, was observed at the optimized growth temperature of 600 °C. At the same time, photoluminescence (PL) and photoreflectance spectroscopies were employed to analyze the optical properties of MoS2 and GaN/MoS2 heterostructures. The PL and exciton energy transition of 2D MoS2 layer can be enhanced by the capping layer GaN, especially for type-I band alignment structure.

Demonstration of controllable Si doping in N-polar AlN using plasma-assisted molecular beam epitaxy

In this study, 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. Through optimization of growth conditions, we obtained high-quality N-polar AlN films at 950 °C. However, our studies revealed that Si incorporation dramatically decreases at such high growth temperature. To enable higher Si incorporation, a hybrid low-temperature and high-temperature growth condition was developed by using Ga as a surfactant at low-temperature growth. By lowering the growth temperature of AlN to 750 °C, we were able to incorporate Si with concentrations as high as 2×1020 cm−3 and demonstrated an electron concentration as high as 1.25×1019 cm−3 at room temperature. The secondary ion mass spectrometry analysis revealed that, <0.2% Ga is incorporated in the AlN films grown with Ga as a surfactant at low temperature.

Silicon implantation and annealing in β-Ga2O3: Role of ambient, temperature, and time

Optimizing thermal anneals of Si-implanted β-Ga2O3 is critical for low resistance contacts and selective area doping. We report the impact of annealing ambient, temperature, and time on the activation of room temperature ion-implanted Si in β-Ga2O3 at concentrations from 5 × 1018 to 1 × 1020 cm−3, demonstrating full activation (>80% activation, mobilities >70 cm2/V s) with contact resistances below 0.29 Ω mm. Homoepitaxial β-Ga2O3 films, grown by plasma-assisted molecular beam epitaxy on Fe-doped (010) substrates, were implanted at multiple energies to yield 100 nm box profiles of 5 × 1018, 5 × 1019, and 1 × 1020 cm−3. Anneals were performed in an ultra-high vacuum-compatible quartz furnace at 1 bar with well-controlled gas compositions. To maintain β-Ga2O3 stability, pO2 must be greater than 10−9 bar. Anneals up to pO2 = 1 bar achieve full activation at 5 × 1018 cm−3, while 5 × 1019 cm−3 must be annealed with pO2 ≤ 10−4 bar, and 1 × 1020 cm−3 requires pO2 < 10−6 bar. Water vapor prevents activation and must be maintained below 10−8 bar. Activation is achieved for anneal temperatures as low as 850 °C with mobility increasing with anneal temperatures up to 1050 °C, though Si diffusion has been reported above 950 °C. At 950 °C, activation is maximized between 5 and 20 min with longer times resulting in decreased carrier activation (over-annealing). This over-annealing is significant for concentrations above 5 × 1019 cm−3 and occurs rapidly at 1 × 1020 cm−3. Rutherford backscattering spectrometry (channeling) suggests that damage recovery is seeded from remnant aligned β-Ga2O3 that remains after implantation; this conclusion is also supported by scanning transmission electron microscopy showing retention of the β-phase with inclusions that resemble the γ-phase.

Electrical properties and structural optimization of GaN/InGaN/GaN tunnel junctions grown by molecular beam epitaxy

The InGaN films and GaN/InGaN/GaN tunnel junctions (TJs) were grown on GaN templates with plasma-assisted molecular beam epitaxy. As the In content increases, the quality of InGaN films grown on GaN templates decreases and the surface roughness of the samples increases. V-pits and trench defects were not found in the AFM images. p++-GaN/InGaN/n++-GaN TJs were investigated for various In content, InGaN thicknesses and doping concentration in the InGaN insert layer. The InGaN insert layer can promote good interband tunneling in GaN/InGaN/GaN TJ and significantly reduce operating voltage when doping is sufficiently high. The current density increases with increasing In content for the 3 nm InGaN insert layer, which is achieved by reducing the depletion zone width and the height of the potential barrier. At a forward current density of 500 A/cm2, the measured voltage was 4.31 V and the differential resistance was measured to be 3.75 × 10−3 Ω·cm2 for the device with a 3 nm p++-In0.35Ga0.65N insert layer. When the thickness of the In0.35Ga0.65N layer is closer to the “balanced” thickness, the TJ current density is higher. If the thickness is too high or too low, the width of the depletion zone will increase and the current density will decrease. The undoped InGaN layer has a better performance than n-type doping in the TJ. Polarization-engineered tunnel junctions can enhance the functionality and performance of electronic and optoelectronic devices.

Demonstration of 82% relaxed In0.18Ga0.82N on porous GaN pseudo-substrate by plasma-assisted molecular beam epitaxy

In this study, the impact of InGaN film thickness and different compositionally graded structures on InGaN relaxation grown on tiled GaN-on-porous-GaN pseudo substrates (PSs) were studied. In addition, the impact of the degree of porosification on the In incorporation and relaxation of InGaN were examined. 82% relaxed 1μm thick In0.18Ga0.82N, which is equivalent to a fully relaxed In-composition of 15%, on porous GaN PS was obtained. Additionally, multi-quantum wells (MQWs) grown on the MBE InGaN-on-porous GaN base layers by MOCVD showed ∼85 nm redshift in comparison with MQWs grown on planar GaN. The developed InGaN-on-porous-GaN PSs can provide an alternative route to grow MQW with a high In content which is essential for high-efficiency nitride-based red LEDs.

(Invited) Gallium Nitride for Vertical Power Devices: Improving Morphology and Unintentional Impurities for Better Devices

While GaN has been researched for many years as a material for light emitting diodes, lasers, high electron mobility transistors, and other devices, further improvements to material purity are still needed to realize the full potential of this material for vertical device applications. To achieve high quality, high purity GaN for power devices, our research employs ammonia-assisted molecular beam epitaxy (NH3-MBE) and plasma-assisted molecular beam epitaxy (PA-MBE). Past studies of fast growth rates (more than 1 μm/hr) and ultra-low impurity incorporation revealed that both PAMBE and NH3-MBE can achieve low net doping levels, and NH3-MBE as low as 1x1015 cm-3.1 However, This work also indicated that impurity incorporation increased with increasing growth rate.Recent research efforts have focused on further improvements to morphology and unintentional dopants in NH3-MBE-grown homoepitaxial layers with fast growth rates. By optimizing growth conditions for faster growth rates and using indium as a surfactant, devices with drift layers up to 10 μm thick were grown. These layers exhibited smooth surface morphology as seen in AFM scans with RMS surface roughness as low as 0.21 nm for a 2x2 μm2 scan area.2 Adding the In surfactant also led to reduced incorporation of unintentional Si impurities in UID films as shown by SIMS depth profiles for GaN films grown with and without In. The extremely low impurity levels were further confirmed by CV measurements which indicated that the net doping density in the UID GaN layers decreased from 2x1016 cm-3 to 5x1015 cm-3 with the addition of the In surfactant.2 Based on these UID levels and the theoretical critical electric field of GaN (3 MV/cm), drift layers up to 29.5 μm and 7.4 μm can be depleted with a triangular electric field profile for net donor concentrations of 5x1015 cm-3 and 2x1016 cm-3, respectively. Devices with the same net concentrations with drift regions 3 μm thick can hold voltages of 854 V and 717 V, respectively.Based on the previously described studies of low-impurity GaN at different growth rates, we developed vertical p-n GaN power devices grown by NH3 MBE.3 These p-n diodes contained a 0.25 µm n+ region, 4 µm n- region with 3x1015 cm-3 unintentional carriers, 0.4 µm p+ region and 10nm p++ capping layer to improve the ohmic anode contact. The circular diodes were fabricated with Pd/Pt anode contacts and Ti/Au cathode contacts. Device diameters were between 80 and 100 µm. The devices featured a 15 µm field plate made from Ti/Au contacts on top of an Al2O3/Si3N4 dielectric stack.The diodes exhibited outstanding forward J-V behavior with a low specific on-resistance (Ron,sp) of 0.28 mΩ-cm2 and a minimum ideality factor of 1.36, which are among the best achieved among similar homoepitaxial GaN p-n diodes.3-5 The best diodes also exhibited a breakdown voltage of > 1 kV (equipment limit 1 kV). The reverse-voltage properties indicate that punch-through was achieved with a peak electric field (EC) >2.6 MV/cm, as confirmed by Silvaco simulations (Fig. 7). The combination of >1 kV breakdown voltage and on-resistance of 0.28 mΩ-cm2 represents one of the best performances of the p-n GaN diodes, achieved with a significantly thinner drift layer compared to comparable MOCVD GaN p-n diodes with drift layers more than 8 µm. Work is ongoing to develop NH3-MBE p-n diodes with indium surfactant-assisted drift layer growth which shows promise to further improve the surface morphology, reduce leakage, and enhance breakdown performance of the vertical devices.

Epitaxial lattice-matched AlScN/GaN distributed Bragg reflectors

We demonstrate epitaxial lattice-matched Al0.89Sc0.11N/GaN 10 and 20 period distributed Bragg reflectors (DBRs) grown on c-plane bulk n-type GaN substrates by plasma-assisted molecular beam epitaxy. Resulting from a rapid increase in in-plane lattice coefficient as scandium is incorporated into AlScN, we measure a lattice-matched condition to c-plane GaN for a Sc content of just 11%, resulting in a large refractive index mismatch Δn greater than 0.3 corresponding to an index contrast of Δn/nGaN = 0.12 with GaN. The DBRs demonstrated here are designed for a peak reflectivity at a vacuum wavelength of 400 nm, reaching a reflectivity of 0.98 for 20 periods. It is highlighted that AlScN/GaN multilayers require fewer periods for a desired reflectivity than other lattice-matched Bragg reflectors such as those based on AlInN/GaN multilayers.