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The recent progress in III-nitride tunnel junction (TJ) contacts for optoelectronic devices is summarized in this review paper. Due to the rapid developments of various III-nitride based optoelectronic devices for different emerging applications, TJs are of interest for these devices to achieve more efficient device performance. This review paper first briefly describes the working principles of TJs and the advantages of employing TJs in optoelectronic devices. The significant advancements in III-nitride TJs realized by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are addressed, as well as their TJ device performances. Lastly, the critical challenges for employing III-nitride TJs in wide bandgap ultraviolet (UV) light emitters are discussed for future investigations. [ABSTRACT FROM AUTHOR]

Femtosecond pump-probe experiments in a transmission geometry were performed on Sn-doped n-type β-Ga2O3. With the pump and probe wavelengths below the bandgap, the differential transmission signal was determined by the free electron dynamics. Differential transmission decay times and their spectral dependence were used to evaluate electron-phonon scattering for polar optical (PO) and intervalley phonons. The obtained average electron-PO phonon scattering time is 4.5 ± 0.4 fs, while the electron scattering to and from the side valley is 80 ± 5 fs. The energy between the absolute and second lowest conduction band minima is estimated to be 2.6 ± 0.1 eV. [ABSTRACT FROM AUTHOR]

Ultra-wide bandgap (UWBG) semiconductors are attracting increasing research interest for power device applications. While promising results have been reported for various materials, it remains unclear which material and technology will succeed. Many figure of merits (FOMs) were derived for power device applications to guide material choices, including the widely used Baliga's FOM (BFOM) to describe the resistive loss of power devices, and Baliga's high-frequency FOM (BHFFOM) to further consider the switching loss. However, key underlying assumptions for those widely cited FOMs, including the assumption of shallow hydrogenic dopants, tend to fail for UWBG semiconductors. In this work, we revisit several important FOMs in describing vertical power electronics to properly account for both incomplete ionization and background compensation effects. We suggest that it is necessary to include the dopant ionization term (for example n/Nd in an n-drift layer) for both BFOM and BHFFOM to fully capture the potential of the UWBG semiconductors for power device applications. Incomplete dopant ionization in materials like diamond and AlN substantially lowers their FOMs for power switching, leading to high conductive and switching losses. Due to the availability of shallow donors, low background impurity compensation, and bulk substrates, β-Ga2O3 promises the best performance among the investigated materials. The modified FOMs offer a valuable guidance in material choices for power device applications. [ABSTRACT FROM AUTHOR]

Hole self-localization into polaron states is a phenomenon characteristic of many wide bandgap compounds including transparent semiconducting oxide β-Ga2O3. While this effect has been recognized for some time, its dynamics remained elusive, and self-localization has often been considered instantaneous. In this work, using the two-color ultrafast pump–probe technique, we have measured the hole self-localization time in β-Ga2O3. At room temperature, this time has been found to be 0.5 ps increasing to 1.1 ps at 10 K. In terms of the configuration coordinate diagram, at room temperature, tunneling and thermal activation contribute to the process with similar weights. The hole self-trapping coefficient is estimated to be 6.4 × 10−8 cm3/s. [ABSTRACT FROM AUTHOR]

The surface roughness on different length scales of In2O3(001) films is detrimental to potential semiconductor applications of this material. One type of surface roughness consists of square shaped islands with edge lengths on the micron scale. This islanding was observed in In2O3(001) films grown on Y-stabilized ZrO2(001), YSZ(001), by plasma-assisted molecular beam epitaxy under oxygen-rich and under In-rich growth conditions. Our reflection high-energy electron diffraction observations during the initial phase of the growth showed that the islanding is caused by a nucleation of the In2O3 with incomplete wetting of the substrate and not by a strain-induced breakup of the film. By systematic variation in the growth parameters Indium-to-oxygen beam flux ratio (VI/III ratio) and substrate temperature we found that the wetting of YSZ(001) by the In2O3(001) film is thermodynamically not preferred and that kinetic limitations, such as low substrate temperatures or (to a lesser extent) high VI/III ratios, were able to force nucleation of continuous films. A growth diagram was constructed that shows the parameter regions with forced or inhibited wetting. The nucleation with inhibited wetting can be applied to the formation of square shaped In2O3(001) islands whose size increases and areal density decreases with decreasing VI/III ratio. To realize continuous, smooth In2O3(001) films under In-rich conditions, a thin nucleation layer was grown at lower substrate temperature to completely wet the substrate. The bulk of the film was grown on top of this nucleation layer at higher temperature to ensure a high crystalline quality. [ABSTRACT FROM AUTHOR]

The active regions of ultraviolet light emitting diodes (UVLEDs) for UVB and ultra-violet band C wavelengths are composed of AlGaN alloy quantum barriers (QBs) and quantum wells (QWs). The use of alloy QBs and QWs facilitates the formation of percolative paths for carrier injection but also decreases carrier confinement within the QWs. We applied the recently developed Localization Landscape (LL) theory for a full 3D simulation of the LEDs. LL theory describes the effective quantum potential of the quantum states for electrons and holes in a random disordered system with a high computational speed. The results show that the potential fluctuations in the n-AlGaN buffer layer, QWs, and QBs provide percolative paths for carrier injection into the top (p-side) QW. Several properties due to compositional disorder are observed: (1) The peak internal quantum efficiency is larger when disorder is present, due to carrier localization, than for a simulation without fluctuations. (2) The droop is larger mainly due to poor hole injection and weaker blocking ability of the electron blocking layer caused by the fluctuating potentials. (3) Carriers are less confined in the QW and extend into the QBs due to the alloy potential fluctuations. The wave function extension into the QBs enhances TM emission as shown from a k·p simulation of wave-functions admixture, which should then lead to poor light extraction. [ABSTRACT FROM AUTHOR]

Recently, we have reported on the use of the titanium-zirconium-molybdenum alloy (TZM) as an autoclave material for acidic ammonothermal growth of bulk GaN using chloride based mineralizers. Here, we discuss the trends of growths with temperatures ranging from 650 to 900 °C at pressures around 100 MPa in a TZM autoclave. The resulting crystal quality as measured by x-ray diffraction appears to be dependent upon growth thickness. Secondary ion mass spectrometry studies of impurities such as O, H, Cl, and Fe show a tendency to decrease with increasing growth thickness, with concentrations ranging from 10 18 to 10 20 cm −3 for O and H, and 5 × 10 16 to 5 × 10 18 cm −3 for Fe. Cl did not have a standard to quantify the concentration. Ti increased in concentration with increasing growth rate with a range of 4 × 10 14 –1 × 10 17 cm −3 . C had steady concentrations between 3 × 10 16 and 5 × 10 17 cm −3 and the Mo concentration was measured in the low 10 16 cm −3 or less. Zr did not have a standard and thus could not be quantified but was near the detection limit in all cases. The instability of the ammonia solvent at high temperatures requires very high pressures for ammonothermal growth above 800 °C but short-term growth has been demonstrated up to 900 °C. [ABSTRACT FROM AUTHOR]

β-Ga2O3 is of great interest for power electronic devices with efficiency beyond current generation Si, 4H-SiC, and GaN devices due to its large breakdown electric field of ∼8 MV/cm. However, taking advantage of this large field strength in power diodes requires device engineering to reduce leakage current that arises at high electric fields. In this work, we elucidate the current transport mechanisms of metal/TiO2/β-Ga2O3 diodes, showing that thermionic emission is an excellent descriptor of current in forward and reverse bias. It is shown that tunneling current is greatly suppressed, and consequently, that the diodes with the TiO2 interlayer can block orders of magnitude more current than Schottky barrier diodes with the same barrier heights. Finally, a 1200 V diode structure is designed based on the derived transport models, and calculated on- and off-state current characteristics closely align with those of state-of-the-art 4H-SiC commercial devices, indicating that this diode structure is ready to enable the realization of β-Ga2O3 power diodes. [ABSTRACT FROM AUTHOR]

A comprehensive study of Sn doping in In2O3 during plasma-assisted molecular beam epitaxy (PA-MBE) is given, covering growth aspects and application-relevant aspects such as structural and transport properties. Single crystalline, (001) oriented indium oxide (In2O3) thin films were grown on Y-stabilized ZrO2(001) and systematically doped with 1018 cm−3 to 6 × 1021 cm−3 tin (Sn) by PA-MBE. The Sn incorporation was proportional to the Sn flux up to a Sn concentration of ≈1020 cm−3 indicating well-controllable doping in this regime. Toward higher Sn concentrations the Sn incorporation was increasingly impeded, which could be somewhat mitigated by increasing the oxygen-to-indium flux ratio. The surface faceting of undoped In2O3(001) during growth under oxygen rich conditions was prevented by doping to Sn concentrations [ABSTRACT FROM AUTHOR]

Time-resolved photoluminescence measurements have been performed on Mg-doped GaN for Mg concentrations in the low- to mid-1019 cm−3. As-grown and annealed (600–675 °C) samples were studied. In the as-grown samples, the nonradiative carrier lifetime was found to be about 200 ps and nearly independent of the Mg concentration. Upon annealing, the carrier lifetimes shorten to ∼150 ps but, again, show little dependence on the annealing temperature. The analysis of possible Shockley–Read–Hall recombination centers and their behavior during doping and annealing suggests that the main nonradiative recombination center is the Mg–nitrogen vacancy complex. The weak dependence of the PL decay times on temperature indicates that carrier capture into this center has a very low potential barrier, and the nonradiative recombination dominates even at low temperatures. [ABSTRACT FROM AUTHOR]

Abstract: AlN bulk single crystals were grown by spontaneous nucleation on the side wall of a hot pressed BN crucible, using both aluminum metal and AlN powder as source materials in an open system. Optimization of reactor pressure, temperature profile, and crucible design yielded AlN crystals with an a-plane platelet morphology. The crystals had low (112¯0) ω rocking curve full-width at half-maximum values of 17–60arcsecs indicative of their high crystalline quality. Further, the addition of 1% indium by weight to the source material was found to significantly influence growth. When Al metal was used as a source material, the addition of indium to the source material resulted in an increase in the average crystal size by roughly an order of magnitude. When AlN powder was used as a source material, the indium addition resulted in a decrease in the total number of nucleation sites. The results show that the addition of indium is a promising method for improving the physical vapor transport growth of AlN. [Copyright &y& Elsevier]

The InAlN alloy system has bandgap energies that span the entire visible range and is a promising candidate for bandgap engineering. To facilitate bandgap engineering, the bandgap bowing for the entire composition range must be well characterized. In rich InxAl1−xN (x > 0.60), films were grown via plasma assisted molecular beam epitaxy on freestanding GaN substrates. The InxAl1−xN film compositions were determined using high resolution X-ray diffraction. On-axis 0002 ω-2θ scans were used to determine the c spacing of the films, reciprocal space maps of the 1 ¯ 015 peaks were used to determine that the films were completely relaxed, and the film compositions were calculated from that information assuming Vegard's law. The bandgaps were independently measured using absorbance spectroscopy and fitting to the linear region of a Tauc plot (dependence of α2 hν on hν) as determined by interpolating the second derivative of the data and selecting a region of low curvature. Compared with the literature, the data were fit to a composition-independent bandgap bowing model, and a bowing parameter of b = 4.0 ± 0.2 eV was calculated, which is consistent with previous results. [ABSTRACT FROM AUTHOR]

β-Ga2O3 is a promising ultra-wide bandgap semiconductor with melt-grown substrates that are scalable, particularly in the (001) orientation. In this study, we report on the heated-phosphoric acid etching of (001) β-Ga2O3. A wagon wheel pattern with spokes aligned to a range of specific crystallographic directions was fabricated on (001) β-Ga2O3. At 160 °C, the (001) etch rate was 1.47 μm/h, which is comparable to etch rates obtained via dry etch techniques. The etched (001) surface had a morphology that was smoother than an inductively coupled plasma etched surface. All spokes possessed relatively smooth sidewalls. Spokes oriented along the [100] direction exhibited minimal SiO2 mask undercut rates at lower etch temperatures and symmetric trapezoidal profiles with near vertical (010)-like sidewalls, which are ideal for device structures with a trench geometry. Spokes oriented along the [010] direction exhibited significant SiO2 mask undercut rates and asymmetric trapezoidal profiles with different sidewall angles. These spokes also possessed reduced sidewall angles, which is favorable for field management at device corners and edges. The etch process was used to realize a dense array of [100]-oriented trenches with a height of 1.5 μm, a base width of 2.0 μm, and a mask width of 0.8 μm. The work highlights the potential for ion damage-free, standalone heated-H3PO4 etching as a viable alternative to the dry etching of (001) β-Ga2O3 for high-performance device applications. [ABSTRACT FROM AUTHOR]

Deep level defects were characterized in Ge-doped (010) β-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy (PAMBE) using deep level optical spectroscopy (DLOS) and deep level transient (thermal) spectroscopy (DLTS) applied to Ni/β-Ga2O3:Ge (010) Schottky diodes that displayed Schottky barrier heights of 1.50 eV. DLOS revealed states at EC − 2.00 eV, EC − 3.25 eV, and EC − 4.37 eV with concentrations on the order of 1016 cm−3, and a lower concentration level at EC − 1.27 eV. In contrast to these states within the middle and lower parts of the bandgap probed by DLOS, DLTS measurements revealed much lower concentrations of states within the upper bandgap region at EC − 0.1 – 0.2 eV and EC − 0.98 eV. There was no evidence of the commonly observed trap state at ∼EC − 0.82 eV that has been reported to dominate the DLTS spectrum in substrate materials synthesized by melt-based growth methods such as edge defined film fed growth (EFG) and Czochralski methods [Zhang et al., Appl. Phys. Lett. 108, 052105 (2016) and Irmscher et al., J. Appl. Phys. 110, 063720 (2011)]. This strong sensitivity of defect incorporation on crystal growth method and conditions is unsurprising, which for PAMBE-grown β-Ga2O3:Ge manifests as a relatively “clean” upper part of the bandgap. However, the states at ∼EC − 0.98 eV, EC − 2.00 eV, and EC − 4.37 eV are reminiscent of similar findings from these earlier results on EFG-grown materials, suggesting that possible common sources might also be present irrespective of growth method. [ABSTRACT FROM AUTHOR]

Using x-ray photoemission spectroscopy (XPS) and current-voltage (I-V) measurements of Hg contacts we show that the surface electron accumulation layer of In2O3 can be removed by an oxygen plasma treatment. For the untreated sample, XPS measured a downward band bending toward the surface and a conduction band peak, and the I-V curve was linear which indicated the presence of a surface accumulation layer. After the treatment an upward bending, the absence of the conduction band peak, and a nonlinear I-V curve indicated the absence of the surface accumulation layer. The sheet resistance of the surface accumulation layer of >45 kΩ was deduced from the increase of the total sheet resistance upon the treatment. The removal of the surface electron accumulation layer opens up the possibility to use Schottky contacts for electrical characterization and device applications of semiconducting In2O3. [ABSTRACT FROM AUTHOR]

Temperature dependent Hall measurements were applied to demonstrate, semiconducting n-type high-quality In2O3(001) and (111) thin films with nondegenerate unintentionally doped (UID) electron concentrations on the order of 1017 cm-3 at room temperature, which is below those of bulk single crystals. The freeze-out of the electron concentration in all samples suggests a shallow UID donor with ionization energy in the 17 meV range. Postgrowth annealing in oxygen allowed to further decrease the UID electron concentration with simultaneous mobility increase, suggesting oxygen vacancies to be the shallow UID donors. Hall mobilities above 200 cm2/V s at room temperature and 1000 cm2/V s at low temperatures exceed those of bulk single crystals and demonstrate high material quality and purity. [ABSTRACT FROM AUTHOR]

The article discusses light-emitting diodes (LEDs), and the impact they are having on energy-efficient lighting. LEDs can help reduce greenhouse gases and energy consumption in white-light sources due to their energy-efficiency. The developments in LEDs have been made possible due to research into semiconductor materials, and the article discusses these developments in detail.

Laser assisted atom probe tomography is used to characterize the alloy distribution in BGaN. The effect of the evaporation conditions applied on the atom probe specimens on the mass spectrum and the quantification of the III site atoms is first evaluated. The evolution of the Ga++/Ga+ charge state ratio is used to monitor the strength of the applied field. Experiments revealed that applying high electric fields on the specimen results in the loss of gallium atoms, leading to the overestimation of boron concentration. Moreover, spatial analysis of the surface field revealed a significant loss of atoms at the center of the specimen where high fields are applied. A good agreement between X-ray diffraction and atom probe tomography concentration measurements is obtained when low fields are applied on the tip. A random distribution of boron in the BGaN layer grown by molecular beam epitaxy is obtained by performing accurate and site specific statistical distribution analysis. [ABSTRACT FROM AUTHOR]

The V-defect is a naturally occurring inverted hexagonal pyramid structure that has been studied in GaN and InGaN growth since the 1990s. Strategic use of V-defects in pre-quantum well superlattices or equivalent preparation layers has enabled record breaking efficiencies for green, yellow, and red InGaN light emitting diodes (LEDs) utilizing lateral injection of holes through the semi-polar sidewalls of the V-defects. In this article, we use advanced characterization techniques such as scattering contrast transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy, x-ray fluorescence maps, and atom probe tomography to study the active region compositions, V-defect formation, and V-defect structure in green and red LEDs grown on (0001) patterned sapphire and (111) Si substrates. We identify two distinct types of V-defects. The "large" V-defects are those that form in the pre-well superlattice and promote hole injection, usually nucleating on mixed (Burgers vector b = ± a ± c) character threading dislocations. In addition, "small" V-defects often form in the multi-quantum well region and are believed to be deleterious to high-efficiency LEDs by providing non-radiative pathways. The small V-defects are often associated with basal plane stacking faults or stacking fault boxes. Furthermore, we show through scattering contrast transmission electron microscopy that during V-defect filling, the threading dislocation, which runs up the center of the V-defect, will "bend" onto one of the six { 10 1 ¯ 1 } semi-polar planes. This result is essential to understanding non-radiative recombination in V-defect engineered LEDs. [ABSTRACT FROM AUTHOR]

The efficiency of high-power operation of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on the realization of uniform hole distribution between the QWs. In long wavelength InGaN/GaN QW LEDs, the thermionic interwell hole transport is hindered by high GaN barriers. However, in polar LED structures, these barriers may be circumvented by the lateral hole injection via semipolar 10 1 ¯ 1 QWs that form on the facets of V-defects. The efficiency of such carrier transfer depends on the transport time since transport in the semipolar QWs is competed by recombination. In this work, we study the carrier transfer from the semipolar to polar QWs by time-resolved photoluminescence in long wavelength (green to red) LEDs. We find that the carrier transfer through the semipolar QWs is fast, a few tens of picoseconds with the estimated room temperature ambipolar diffusion coefficient of ∼5.5 cm2/s. With diffusion much faster than recombination, the hole transport from the p-side of the structure to the polar QWs should proceed without a substantial loss, contributing to the high efficiency of long wavelength GaN LEDs. [ABSTRACT FROM AUTHOR]

The large polarization barriers between the quantum wells and quantum barriers in long-wavelength GaN-based light-emitting diodes (LEDs) inhibit their performance by requiring excess driving voltages to reach standard operating current densities. Lateral injection of carriers directly into quantum wells is required to circumvent this issue. V-defects are naturally occurring inverted hexagonal defects with semipolar 10 1 ¯ 1 -plane sidewalls generated on surface depressions from threading dislocations. LEDs engineered to intentionally generate V-defects below the active region of the LED can achieve lateral carrier injection through the V-defect sidewalls and have already been able to demonstrate world record wall-plug efficiencies for LEDs in the green-red wavelengths. V-defects can be enlarged during kinetically limited growth where the growth rate of the c-plane GaN is faster than that of their sidewalls, leaving them unfilled. We report on the metal organic chemical vapor deposition growth conditions required to fill in V-defects with p-GaN during epitaxial growth of the LED post the active region. Circular transmission length measurements of Pd/Au contacts processed on p-GaN surfaces with various amounts of unfilled V-defects showed no significant difference in their sheet resistance and specific contact resistance. J–V measurements of LEDs grown with varying unfilled V-defect densities showed no significant difference in the forward bias regime. However, in the reverse bias regime, catastrophic breakdown occurred at markedly lower voltages for samples with larger unfilled V-defect densities. This suggests that unfilled V-defects may act as hotspots for device failure, and planarizing LED surfaces may help prevent early degradation of LED devices. [ABSTRACT FROM AUTHOR]

In this work, we report on the growth of (001) β-(AlxGa1−x)2O3 films in molecular beam epitaxy via metal oxide-catalyzed epitaxy. Films with Al contents up to 15% were grown and the Al content was measured with atom probe tomography. A relationship between the Al content and the out-of-plane lattice parameter was derived for both (001) and (100) orientations. Transmission electron microscopy showed no evidence of extended defects in (001) β-(AlxGa1−x)2O3, and reciprocal space maps confirmed that β-(AlxGa1−x)2O3 films were coherently strained to (001) β-Ga2O3. Sn was also demonstrated to act as a surfactant for (001) β-(AlxGa1−x)2O3 growth, allowing for high-quality, uniform films with smooth morphologies. [ABSTRACT FROM AUTHOR]

In this paper, we describe the influence of the intrinsic indium fluctuation in the InGaN quantum wells on the carrier transport, efficiency droop, and emission spectrum in GaN-based light emitting diodes (LEDs). Both real and randomly generated indium fluctuations were used in 3D simulations and compared to quantum wells with a uniform indium distribution. We found that without further hypothesis the simulations of electrical and optical properties in LEDs such as carrier transport, radiative and Auger recombination, and efficiency droop are greatly improved by considering natural nanoscale indium fluctuations. [ABSTRACT FROM AUTHOR]

We report on the material challenges of the growth of highly conductive n-AlGaN in germicidal ultraviolet light emitting diodes (GUV LEDs), with the degradation of the surface morphology of thick highly doped n-AlGaN due to the Si anti-surfactant effect. Threading dislocation inclination, increasing relaxation, and eventual cracking were observed with epitaxial n-AlGaN films thicker than 400 nm, along with an increasing Ga composition with the same metalorganic flows. With the optimization of the n-AlGaN conductivity in previous works, thin n-AlGaN films with high conductivity along with a smoothing superlattice were incorporated in GUV LED devices, resulting in LEDs with 285 nm electroluminescence, a low forward voltage of 4.2 V with a peak external quantum efficiency (EQE) of 10.6% and a peak wall-plug efficiency of 8.6% below 1 A/cm2, and an EQE of 5.5% at 20 A/cm2. [ABSTRACT FROM AUTHOR]

Hole injection through V-defect sidewalls into all quantum wells (QWs) of long wavelength GaN light emitting diodes had previously been proposed as means to increase efficiency of these devices. In this work, we directly tested the viability of this injection mechanism by electroluminescence and time-resolved photoluminescence measurements on a device in which QW furthest away from the p-side of the structure was deeper, thus serving as an optical detector for presence of injected electron–hole pairs. Emission from the detector well confirmed that, indeed, the holes were injected into this QW, which could only take place through the 10 1 ¯ 1 V-defect sidewalls. Unlike direct interwell transport by thermionic emission, this transport mechanism allows populating all QWs of a multiple QW structure despite the high potential barriers in the long wavelength InGaN/GaN QWs. [ABSTRACT FROM AUTHOR]

We demonstrate Schottky barrier engineering using PtOx/thin Pt Schottky contacts combined with edge termination using a high permittivity dielectric (ZrO2) field-plate for high-voltage vertical β-Ga2O3 diodes. A systematic study of baseline bare Pt/β-Ga2O3, PtOx/thin Pt/β-Ga2O3, and PtOx/β-Ga2O3 Schottky diode characteristics was performed, which revealed that the PtOx/thin Pt/β-Ga2O3 contact can combine the advantages of both PtOx and Pt, allowing better reverse blocking performance than plain metal Pt/β-Ga2O3 Schottky diodes and lower turn-on voltage than plain oxidized metal PtOx/β-Ga2O3 ones. Moreover, the thin Pt interlayer in the PtOx/thin Pt/β-Ga2O3 anode contact configuration, deposited by e-beam deposition, also provides plasma-free interface at the Schottky junction as opposed to the direct sputter deposited PtOx contacts of the PtOx/β-Ga2O3 diodes. We further implemented a high permittivity dielectric (ZrO2) field plate in PtOx/thin Pt/β-Ga2O3 diodes that assisted in edge-field management and enabled a breakdown voltage to ∼2.34 kV. These results indicate that the PtOx/thin Pt/β-Ga2O3 Schottky contact, combined with a high permittivity field-plate, will be promising to enable Schottky barrier engineering for high-performance and efficient vertical β-Ga2O3 power switches. [ABSTRACT FROM AUTHOR]

In this work, we report the growth of Mg-doped (010) β-Ga2O3 via plasma-assisted molecular beam epitaxy. Mg concentrations from 2 × 1016 to 8 × 1020 cm−3 with sharp doping profiles were realized. The Mg doping incorporation in β-Ga2O3 showed little dependence on the growth temperature and Ga:O flux ratio. Annealing at temperatures from 925 to 1050 °C resulted in significant diffusion, thus limiting the application of Mg-doped β-Ga2O3 to lower temperature growth techniques and processing. Mg accumulation near the sample surface after diffusion gives insight into the rich point–defect interaction that may play an important role in Mg diffusion. [ABSTRACT FROM AUTHOR]

The polarity of GaN grown by plasma-assisted molecular beam epitaxy was inverted from N-face to Ga-face by inserting a composite AlN/aluminum oxide (AlOx) interlayer structure at the inversion interface. The change in polarity was verified in situ by reflection high energy electron diffraction via intensity transients and postgrowth surface reconstructions, and ex situ by convergent beam electron diffraction and etch studies in an aqueous potassium hydroxide solution. The inverted materials showed smooth surfaces and good electrical properties. AlGaN/GaN high electron mobility transistors fabricated on the inverted epilayers showed good dc and high frequency performance. A current-gain cutoff frequency (fT) of 21 GHz and maximum oscillation frequency (fmax) of 61 GHz were measured in devices with a gate length of 0.7 μm. These data compare favorably to those of Ga-face AlGaN/GaN devices with a similar structure grown on Si-face SiC substrates. [ABSTRACT FROM AUTHOR]

We investigate the unexpected high-energy electroluminescence (EL) peaks observed in long-wavelength InGaN light-emitting diodes (LEDs) with ground state emission peaks between ∼495 and 685 nm by studying the EL spectra of LEDs with varying quantum well (QW) thicknesses and indium compositions. In addition to the ground state emission, two high-energy emission peaks were observed in the LEDs with thick QWs and high indium compositions. The less energetic high-energy emission peak (2.4–2.6 eV) is attributed to the optical transitions involving excited states. Factors influencing the excited state transitions, such as the QW thickness and indium compositions, were also examined by simulations to better understand the occurrence of these transitions. The more energetic high-energy emission peak (2.8–3.1 eV) originates from V-defect sidewalls and was verified through micro-photoluminescence measurements. Identification of the high-energy emission peaks is essential as it enables targeted epitaxial or growth optimizations to minimize or eliminate these undesirable emission peaks. This work demonstrates the importance of using thin QWs to suppress the unwanted high-energy emissions due to excited state transitions and V-defect sidewalls for long-wavelength InGaN LEDs. [ABSTRACT FROM AUTHOR]

Atomic layer etching (ALE) was performed on (Al, In, Ga)N thin films using a cyclic process of alternating Cl2 gas absorption and Ar+ ion bombardment in an inductively coupled plasma etcher system. The etch damage was characterized by comparing photoluminescence of blue single quantum well light-emitting diodes before and after the etch as well as bulk resistivities of etched p-doped layers. It was found that etched surfaces were smooth and highly conformal, retaining the step-terrace features of the as-grown surface, thus realizing ALE. Longer exposures to the dry etching increased the bulk resistivity of etched surfaces layers slightly, with a damaged depth of ∼55 nm. With further optimization and damage recovery, ALE is a promising candidate for controlled etching with atomic accuracy. It was found that Al0.1Ga0.9N acts as an etch barrier for the ALE etch, making it a suitable etch to reveal buried V-defects in III-nitride light emitting diodes. [ABSTRACT FROM AUTHOR]

We report a systematic investigation on the anisotropic etching behavior of β-Ga2O3. A wagon wheel pattern was designed and fabricated on (010)-oriented β-Ga2O3 substrates. The wet etching in hot phosphoric acid was found to be effective in reducing the sidewall roughness caused by plasma dry etching. The angular dependence of the sidewall etch rate and inclination angles after wet etching was evaluated. The fins aligned along the [001] direction showed nearly vertical sidewalls after wet etching and a fast sidewall etch rate, making it feasible for the fabrication of ultrascaled vertical channel devices. The fins aligned in the angular range between the [203] and [201] directions showed slanted sidewalls with high tilt angles, which could be useful in reducing the peak electric field near the corners of high power devices. The observed anisotropic etching in this work offers a low-cost, low damage, and controllable fabrication method for ultrascaled and advanced device structures. [ABSTRACT FROM AUTHOR]

We report on the measurement of the lateral distribution of the junction current of an electrical biased p-n GaN diode by electron emission microscopy using a low-energy electron microscope. The vacuum level at the surface of the diode was lowered by deposition of cesium to achieve negative electron affinity, allowing overflow electrons at the surface of the biased diodes to be emitted and their spatial distribution imaged. The results were compared to the literature, and a good match with analytical solutions by Joyce and Wemple [J. Appl. Phys. 41, 3818 (1970)] was obtained. [ABSTRACT FROM AUTHOR]

We report on the controllable nitrogen doping of β-Ga2O3 as a deep acceptor dopant using ammonia diluted in nitrogen (NH3/N2) as a source of active nitrogen in the metal organic chemical vapor deposition epitaxy. The effects of the NH3/N2 flow rate and substrate temperature on the incorporation efficiency, reproducibility, and controllability of N doping into Ga2O3 were studied using secondary ion mass spectrometry measurements. With the increase in the NH3/N2 molar flow rate from ∼2 × 10−8 to ∼2 × 10−6 mol/min, the N impurities incorporated into the β-Ga2O3 increased linearly from ∼1 × 1018 to ∼2 × 1020 cm−3. At low substrate temperatures (<800 °C), hydrogen was incorporated into the film accompanying nitrogen with comparable concentrations. Despite this, the current–voltage measurements showed that the N and H co-doped films were resistive with a measured resistance of >70 MΩ for a film grown with [N] ≈ [H] of ∼8 × 1018 cm−3. X-ray on-axis (020) and off-axis (111) rocking curve ω-scans and atomic force microscopy measurements show no influence of NH3/N2 dopant on the structural and surface quality of the films. However, the presence of H promoted the growth of (110) and ( 1 ¯ 10) facets elongated along the [001] direction. At high growth temperatures (≥950 °C), the H concentration in the films was reduced by nearly ∼10×, but with a slight increase in the concentration of N. The results show that controllable and repeatable nitrogen doping into β-Ga2O3 can be achieved using ammonia to obtain deep acceptor doping or compensation needed for device engineering in β-Ga2O3-based power electronic devices. [ABSTRACT FROM AUTHOR]

Electron emission spectroscopy was performed on metalorganic chemical vapor deposition grown p-n−-n+ junctions with p-thicknesses ranging from 50 to 300 nm, doped with [Mg] = 3.5 × 1019 cm−3. By measuring the decreasing emitted electron intensity from a cesiated p-GaN surface with increasing p-thickness, we were able to extract the minority carrier diffusion length of electron in p-type GaN, Le = 26 ± 3 nm. The measured value is in good agreement with literature reported values. The extrapolated electron current at the n− region–p-GaN interface is in reasonable agreement with the simulated electron current at the interface. [ABSTRACT FROM AUTHOR]

Scanning tunneling electroluminescence (STL) microscopy is performed on a 3 nm‐thick InGaN/GaN quantum well (QW) with [In] = 0.23 such that the main light emission occurs in the green. The technique is used to map the radiative recombination properties at a scale of a few nanometers and correlate the local electroluminescence map with the surface topography simultaneously imaged by scanning tunneling microscopy. While the expected green emission is observed all over the sample, measurements performed on a 500 nm × 500 nm area around a 150 nm‐large and 2.5 nm‐deep hexagonal defect reveal intense emission peaks at higher energies close to the defect edges, features which are not visible in the macrophotoluminescence spectrum of the sample. Via a fitting of the local tunneling electroluminescence spectra, quantitative information on the fluctuations of the intensity, peak energy, width, and phonon replica intensity of the different spectral contributions is obtained, which provides information on carrier localization in the QW. This procedure also indicates that the carrier diffusion length on the probed area of the QW is shorter than 50 nm. [ABSTRACT FROM AUTHOR]

Until recently, the electrical efficiency of green nitride light-emitting diodes (LEDs) was considerably lower than that of blue LEDs. This is particularly surprising as one would expect a reduced forward voltage with increasing emission wavelength. In this paper, we theoretically investigated the impact of the number of quantum wells on the forward voltage of III-nitride LEDs with x = 0.15 (blue) and x = 0.24 (green) InxGa1–xN QWs. The simulated dependence of current density (J) on applied diode bias (V) shows a significant increase of 1.9 V in the forward voltage between one and five quantum well (QW) c-plane green LED structures. Artificially turning off the polarization fields in the simulation does not entirely suppress this effect. Due to the large band offsets in the green LED multiple QW stack, simulations indicate a sequential band filling of the QW sequence. This mechanism should not be limited to c-plane LEDs and could also be present in nonpolar or semipolar devices. [ABSTRACT FROM AUTHOR]

Time- and spectrally-resolved scanning near-field optical microscopy was applied to study spatial variations of photoluminescence (PL) spectra and carrier dynamics in polar InGaN/GaN single quantum wells (QWs) emitting from 410 nm to 570 nm. The main attention was devoted to variations of PL properties and carrier dynamics around V-defects. The PL intensity, peak wavelength, and linewidth, as well as the radiative and nonradiative recombination times, were found to be different in V-defect-rich and defect-free regions. The radiative lifetime close to the defects was longer up to several times, which is attributed to an increased electron and hole wave function separation in the QW plane. PL decay times, measured using excitation and collection through the near-field probe, were one to two orders of magnitude shorter than PL decay times measured in the far field. This shows that the near-field PL decay and the integrated PL intensity are primarily determined by the carrier out-diffusion from under the probe. Only in the immediate vicinity of the V-defects, the near-field PL decays due to the nonradiative recombination at dislocations. The area of such enhanced nonradiative recombination is limited to just a few percent of the total QW area. This shows that recombination via dislocations and V-defects does not play a decisive role in the overall nonradiative recombination and internal quantum efficiency of polar InGaN/GaN QWs. [ABSTRACT FROM AUTHOR]

Ga2O3 is emerging as an important electronic material. Alloying with Al2O3 is a viable method to achieve carrier confinement, to increase the bandgap, or to modify the lattice parameters. However, the two materials have very different ground-state crystal structures (monoclinic β-gallia for Ga2O3 and corundum for Al2O3). Here, we use hybrid density functional theory calculations to assess the alloy stabilities and electronic properties of the alloys. We find that the monoclinic phase is the preferred structure for up to 71% Al incorporation, in close agreement with experimental phase diagrams, and that the ordered monoclinic AlGaO3 alloy is exceptionally stable. We also discuss bandgap bowing, lattice constants, and band offsets that can guide future synthesis and device design efforts. [ABSTRACT FROM AUTHOR]

Increased attention has been paid to the thermal management of β-Ga2O3 devices as a result of the large thermal resistance that can present itself in part due to its low intrinsic thermal conductivity. A number of die-level thermal management approaches exist that could be viable for thermal management. However, they have not been assessed for β-Ga2O3 devices exclusively. Here, we explore the limits of various die level thermal management schemes on a β-Ga2O3 metal–semiconductor field-effect transistor using numerical simulations. The effects of the various cooling approaches on the device channel temperature were comprehensively investigated, along with guidance for material selection to enable the most effective thermal solutions. Among various cooling strategies, double side cooling combined with a heat spreader used in the active region of the device can suppress the device thermal resistance to as low as 11 mm °C/W, achieving a maximum dissipated power density as high as 16 W/mm for a junction temperature limit of 200 °C. A multi-finger transistor thermal model was also developed to assess the potential of β-Ga2O3 devices for higher output power applications. Overall, this numerical study shows that it is possible to achieve high power β-Ga2O3 device operation with appropriate die-level thermal management solutions. [ABSTRACT FROM AUTHOR]

By studying low radiative efficiency blue III-nitride light emitting diodes (LEDs), we find that the ABC model of recombination commonly used for understanding efficiency behavior in LEDs is insufficient and that additional effects should be taken into account. We propose a modification to the standard recombination model by incorporating a bimolecular nonradiative term. The modified model is shown to be in much better agreement with the radiative efficiency data and to be more consistent than the conventional model with very short carrier lifetimes measured by time-resolved photoluminescence in similar, low radiative efficiency material. We present experimental evidence that a hot carrier-generating process is occurring within these devices, in the form of measurements of forward photocurrent under forward bias. The forward photocurrent, due to hot carrier generation in the active region, is present despite the lack of any "efficiency droop"—the usual signature of band-to-band Auger recombination in high-quality III-nitride LEDs. Hot carrier generation in the absence of band-to-band Auger recombination implies that some other source of hot carriers exists within these low radiative efficiency devices, such as trap-assisted Auger recombination. [ABSTRACT FROM AUTHOR]

Mg-doped indium oxide (In2O3) thin films were grown by plasma-assisted molecular beam epitaxy with Mg-concentrations ranging from 1017 to 6×1020 cm-3. In this concentration range Mg was incorporated into In2O3 without discernable impediment nor formation of secondary phases. Despite the role of Mg as acceptor, the films were n-type conductive in the as-grown state or after annealing in vacuum. For Mg-concentrations well in excess of the unintentional donor concentration annealing in oxygen resulted in semi-insulating films without detectable p-type conductivity. These results strongly suggest oxygen vacancies to act as shallow donors in In2O3 that can overcompensate the Mg acceptors. [ABSTRACT FROM AUTHOR]

Due to the disparity between observed gallium nitride (GaN) growth under conditions for which literature reports normal solubility, GaN solubility in supercritical NH 3 -Na containing solutions was re-evaluated. Isothermal gravimetric experiments on polycrystalline GaN were performed in the temperature range ( T =415–650 °C) for which retrograde growth of GaN routinely occurs ( P ≈ 200 MPa, molar NH 3 :Na fill ratio =20:1). Two previously-unreported error contributions to the gravimetric determination of GaN solubility were identified: Ga-alloying of exposed Ni-containing components, and the presence of a dense, Ga-absorbing Na-rich, second phase under these conditions. Due to the inability to measure Ga-alloying of the exposed autoclave wall for each experiment, considerable scatter was introduced in the refined GaN solubility curve. No clear dependence of GaN solubility on temperature was resolvable, while most solubility values were determined to be within a band of 0.03–0.10 mol% GaN, normalized by fill NH 3 . [ABSTRACT FROM AUTHOR]

This paper discusses promising materials for use as internal, non-load bearing components as well as molybdenum-based alloys for autoclave structural components for an ammonothermal autoclave. An autoclave was constructed from the commercial titanium–zirconium–molybdenum (TZM) alloy and was found to be chemically inert and mechanically stable under acidic ammonothermal conditions. Preliminary seeded growth of GaN was demonstrated with negligible incorporation of transition metals (including molybdenum) into the grown material (<10 17 cm −3 ). Molybdenum and TZM were exposed to a basic ammonothermal environment, leading to slight degradation through formation of molybdenum nitride powders on their surface at elevated temperatures (T>560 °C). The possibility of a ‘universal’, inexpensive, liner-free ammonothermal autoclave capable of exposure to basic and acidic chemistry is demonstrated. [ABSTRACT FROM AUTHOR]

The electrical performances of III-nitride blue micro-light-emitting diodes (µLEDs) with different tunnel junction (TJ) epitaxial architectures grown by metalorganic chemical vapor deposition are investigated. A new TJ structure that employs AlGaN is introduced. The current density–voltage characteristic is improved by incorporating AlGaN layer above the n-side of the TJ layer, and the effects of the AlGaN/GaN superlattices is examined. Based upon the data from band diagram simulation, net positive polarization charge is formed at the AlGaN/GaN interface, which results in a reduction in tunneling distance and increase in tunneling probability. Moreover, similar electrical improvement is observed in various device dimensions and is independent of operating current density, suggesting that AlGaN/GaN biaxial tensile strain or current spreading is not the main contribution for the improvement. Finally, the effects on the efficiency performances are determined. While the maximum external quantum efficiency of the TJ devices remains identical, the wall-plug efficiency of µLEDs is enhanced significantly by the proposed AlGaN-enhanced TJ design. This work reveals the possibility of energy efficient TJ contact with high transparency in the visible wavelength range. [ABSTRACT FROM AUTHOR]

We have studied multicarrier contributions to the conductivity in low concentration (with Hall concentration below 1 × 10 18 cm − 3 ) nondoped bulk n-InN samples using Shubnikov-de Haas (SdH) in the tilted magnetic field as well as variable field resistivity tensor measurements. In some samples, the Shubnikov-de Haas effect also revealed, besides 3D electron gas, high mobility 2D electron contributions (with transport and quantum mobilities reaching the values of 5060 cm 2 /V s and 1800 cm 2 /V s, respectively, for one of our samples), which could hardly be assigned to the surface electrons as the latter are commonly believed to have mobility too low to be detected at magnetic fields not exceeding 12 T in our SdH measurements. The values of the effective masses derived from the temperature dependences of the SdH oscillation amplitudes scale with the concentration of 2D channels and are typical for low concentration InN, thus confirming that these contributions are presumably located on the InN side of some interfaces. This is one of the first experimental evaluations of the effective mass of nonsurface-related 2D electron gas in InN grown on GaN. [ABSTRACT FROM AUTHOR]

The correlative use of X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and atom probe tomography (APT) allows the structure of substitutionally alloyed half-Heusler compound CoTi1-xFexSb to be characterized at the nanometer scale. For x = 0.2, 0.3, and 0.5, XRD patterns indicate an epitaxial cube-on-cube growth with no detectable secondary phases and are suggestive of high crystalline quality for all three of the films. This is supported using atomic resolution STEM images, which revealed the absence of secondary phases or polycrystalline regions, thus confirming the half-Heusler structure of the CoTi1-xFexSb films. For all three samples, the APT reconstructions were optimized to resolve the atomic planes in the [001] growth direction allowing the exploitation of reliable datasets. The presence of Fe-rich phases in the samples with Fe atoms sitting on Ti crystallographic sites is revealed. A strong phase separation is observed for x = 0.2 and x = 0.3 and is almost no longer observed for x = 0.5 in good agreement with the magnetic properties of the films. [ABSTRACT FROM AUTHOR]

Deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) were used to characterize defect states throughout the bandgap of unintentionally-doped InxAl1−xN grown by metal organic chemical vapor deposition for x = 0.18 (nominally lattice-matched) and x = 0.15 compositions. DLTS revealed broad peaks with energy levels of EC − 0.23 eV and 0.38 eV for In0.18Al0.82N and In0.15Al0.85N, respectively, tracking the difference in their conduction band minima [S. Schulz et al., Appl. Phys. Express 6, 121001 (2013)]. Capture kinetics studies revealed logarithmic filling behavior, which with the broad peaks, implies that an extended defect source is likely, consistent with threading dislocation densities (TDD) of ∼1 × 109 cm−2 measured for both structures. However, the trap concentration did not track the detailed TDD variation but instead followed the background oxygen content, which varied between 1.2 × 1018 cm−3 and 1.8 × 1018 cm−3 for the samples. Taken together with the logarithmic capture kinetics, this implies that dislocation-oxygen complexes could be the source for this trap. In spite of the high oxygen content in the samples, this state did not reveal DX-like behavior, supporting the assertion of an oxygen-dislocation complex as its likely source. DLOS also revealed additional states at EC − 1.63 eV, 2.09 eV, and 3.59 eV for In0.18Al0.82N and analogous states at EC − 1.70 eV, 2.70 eV, and 3.90 eV within In0.15Al0.85N. Lighted capacitance-voltage measurements indicated that the near mid-gap (EC − 2.09 eV and 2.70 eV) and near valence band (EC − 3.59 eV and 3.90 eV) states are their primary sources for carrier compensation. [ABSTRACT FROM AUTHOR]

The measurement of the energy distribution of vacuum emitted electrons from InGaN/GaN lightemitting diodes (LEDs) has proven essential in understanding the efficiency loss mechanism known as droop. We report on the measurement and identification of a new low-energy feature in addition to the previously measured three peaks present in the electron emission spectrum from a forward biased LED. Photoemission measurements show that the two low-energy peaks correspond to photoemitted electrons from each of the p-contact metals, palladium and gold. We confirm that the mid and high-energy peaks are due to electrons which have transited the p-type region of the device and have been emitted from the semiconductor surface from the bulk ᴦ-valley or a high-energy side valley. [ABSTRACT FROM AUTHOR]

We report ultra-high responsivity of epitaxial (Sn x Ga 1 − x) 2 O 3 (TGO) Schottky UV-C photodetectors and experimentally identified the source of gain as deep-level defects, supported by first principles calculations. Epitaxial TGO films were grown by plasma-assisted molecular beam epitaxy on (−201) oriented n-type β- Ga 2 O 3 substrates. Fabricated vertical Schottky devices exhibited peak responsivities as high as 3.5 × 10 4 A/W at −5 V applied bias under 250 nm illumination with sharp cutoff shorter than 280 nm and fast rise/fall time in milliseconds order. Hyperspectral imaging cathodoluminescence (CL) spectra were examined to find the mid-bandgap defects, the source of this high gain. Irrespective of different tin mole fractions, the TGO epilayer exhibited extra CL peaks at the green band (∼2.20 eV) not seen in β- Ga 2 O 3 along with enhancement of the blue emission-band (∼2.64 eV) and suppression of the UV emission-band. Based on hybrid functional calculations of the optical emission expected for defects involving Sn in β- Ga 2 O 3 , VGa–Sn complexes are proposed as potential defect origins of the observed green and blue emission-bands. Such complexes behave as acceptors that can efficiently trap photogenerated holes and are predicted to be predominantly responsible for the ultra-high photoconductive gain in the Sn-alloyed Ga 2 O 3 devices by means of thermionic emission and electron tunneling. Regenerating the VGa–Sn defect complexes by optimizing the growth techniques, we have demonstrated a planar Schottky UV-C photodetector of the highest peak responsivity. [ABSTRACT FROM AUTHOR]

AlGaN germicidal ultraviolet (GUV) light emitting diodes (LEDs) are one of the most promising disinfection technologies in fighting the COVID-19 pandemic; however, GUV LEDs are still lacking in efficiency due to low p-type doping efficiency in p-AlGaN. The most successful approach for producing conductive p-type AlGaN is the implementation of a polarization-enhanced short period Al $_{\mathbf {x}}$ Ga $_{\mathbf {1-}\mathbf {x}}$ N/Al $_{\mathbf {y}}$ Ga $_{\mathbf {1-}\mathbf {y}}$ N superlattice (SL) structure, which enhances hole injection and reduces device operating voltage. In this report, we investigated different aspects of the superlattice including the Al $_{\mathbf {x}}$ Ga $_{\mathbf {1-}\mathbf {x}}$ N and Al $_{\mathbf {y}}$ Ga $_{\mathbf {1-}\mathbf {y}}$ N alloy constituent compositions, ${x}$ and ${y}$ , period thickness, total thickness, and Mg dopant concentration in terms of LED performance as well as electrical, optical, and morphological characteristics. The polarization-enhanced p-type doping in the AlGaN superlattice was also investigated computationally, giving excellent agreement with experimental results. Highly efficient UVC LEDs (279 nm) with EQE of 2% at 5 A/cm2 were demonstrated. A maximum output power of 5.5 mW (56 mW/mm2) was achieved at 100 mA. [ABSTRACT FROM AUTHOR]