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The efficiency of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on uniformity of hole distribution between the QWs. Typically, transport between the QWs takes place via carrier capture into and thermionic emission out of the QWs. In InGaN/GaN QWs, the thermionic hole transport is hindered by the high quantum confinement and polarization barriers. To overcome this drawback, hole injection through semipolar QWs located at sidewalls of V-defects had been proposed. However, in the case of the V-defect injection, strong lateral emission variations take place. In this work, we explore the nature of these variations and the impact of the V-defects on the emission spectra and carrier dynamics. The study was performed by mapping electroluminescence (EL) and photoluminescence (PL) with a scanning near-field optical microscope in LEDs that contain a deeper well that can only be populated by holes through the V-defects. Applying different excitation schemes (electrical injection and optical excitation in the far- and near-field), we have shown that the EL intensity variations are caused by the lateral nonuniformity of the hole injection. We have also found that, in biased structures, the PL intensity and decay time in the V-defect regions are only moderately lower that in the V-defect-free regions thus showing no evidence of an efficient Shockley-–Read–Hall recombination. In the V-defect regions, the emission spectra experience a red shift and increased broadening, which suggests an increase of the In content and well width in the polar QWs close to the V-defects. [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]

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]

We investigate the bow of free standing (0001) oriented hydride vapor phase epitaxy grown GaN substrates and demonstrate that their curvature is consistent with a compressive to tensile stress gradient (bottom to top) present in the substrates. The origin of the stress gradient and the curvature is attributed to the correlated inclination of edge threading dislocation (TD) lines away from the [0001] direction. A model is proposed and a relation is derived for bulk GaN substrate curvature dependence on the inclination angle and the density of TDs. The model is used to analyze the curvature for commercially available GaN substrates as determined by high resolution x-ray diffraction. The results show a close correlation between the experimentally determined parameters and those predicted from theoretical model.

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]

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]

We describe the development of N-polar GaN-based high electron mobility transistors grown by N-2 plasma-assisted molecular beam epitaxy on C-face SiC substrates. High mobility AlGaN/GaN modulation-doped two-dimensional electron gas channels were grown, and transistors with excellent dc and small-signal performance were fabricated on these wafers. Large-signal dispersion was observed, and the trap states responsible for this were identified, and layer designs to remove the dispersive effects of these traps were demonstrated. Finally, an AlGaN-cap layer was used to reduce gate leakage in these devices, and a low-dispersion high breakdown voltage device was achieved. This detailed study of dispersion and leakage in N-polar GaN-based transistors establishes a technological base for further development of field effect devices based on N-polar III-nitrides. (C) 2007 American Institute of Physics.

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]

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]

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]

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 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]

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]

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]

The strain dependence of the free-exciton resonance energies in AlN epilayers and the values are analyzed by using an appropriated Hamiltonian assuming equibiaxial stress for the wurtzite crystal structure in order to obtain valence band parameters. The strain dependence of the valence band ordering, optical transition probability and free-exciton binding energy are analyzed.

The electrical and structural characterization of Mg-doped Al-rich p-type [Al.sub.0.69][Ga.sub.0.31]N alloys on SiC substrate grown by metal organic chemical vapor deposition is presented. Transmission electron microscopy has shown the generation of vertical spike-like defects with increasing Mg doping and the increase in resistivity with the increase in Mg doping.

The adsorption and subsequent desorption of Ga on (0001) GaN using simultaneous line-of-sight quadrapole mass spectroscopy (QMS) and reflection high-energy electron diffraction (RHEED) is investigated. It is concluded that the experimental results show the complementary nature of the information provided by the RHEED oscillations and the Ga-adlayer coverage measured by QMS.

The results of tensile strained AlN/GaN, AlGaN/GaN and compressive strained InGaN/GaN superlattices (SLs) grown by Ammonia MBE (NH3-MBE) are presented. A combination of atom probe tomography and high-resolution X-ray diffraction confirms that periodic heterostructures of high crystallographic quality are achieved. Strain induced misfit dislocations (MDs), however, are revealed by cathodoluminescence (CL) of the strained AlN/GaN, AlGaN/GaN and InGaN/GaN structures. MDs in the active region of a device are a severe problem as they act as non-radiative charge recombination centers, affecting the reliability and efficiency of the device. Strain compensated SL structures are subsequently developed, composed of alternating layers of tensile strained AlGaN and compressively strained InGaN. CL reveals the absence of MDs in such structures, demonstrating that strain compensation offers a viable route towards MD free active regions in III-Nitride SL based devices [ABSTRACT FROM AUTHOR]

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]

InAlN dipole diodes were developed and fabricated on both (0001) Ga-Face and (000...) N-face oriented GaN on sapphire templates by molecular beam epitaxy. The orientation and direction of the InAlN polarization dipole are functions of the substrate orientation and composition, respectively. Special consideration was taken to minimize growth differences and impurity uptake during growth on these orientations of opposite polarity. Comparison of devices on similarly grown structures with In compositions in excess of 50% reveals that dipole diodes shows poorer forward bias performance and exhibited an increase in reverse bias leakage, regardless of orientation. Similarly, (0001) Ga-face oriented InAlN at a lowered 40% In composition had poor device characteristics, namely, the absence of expected exponential turn on in forward bias. By contrast, at In compositions close to 40%, (000...) N-face oriented InAlN devices had excellent performance, with over five orders of magnitude of rectification and extracted barrier heights of 0.53- 0.62 eV; these values are in close agreement with simulation. Extracted ideality factors ranging from 1.08 to 1.38 on these devices are further evidence of their optimal performance. Further discussion focuses on the growth and orientation conditions that may lead to this discrepancy yet emphasizes that with proper design and growth strategy, the rectifying dipole diodes can be achieved with InAlN nitride dipole layers. [ABSTRACT FROM AUTHOR]

The design of isotype InGaN/GaN heterobarrier diode structures grown by ammonia molecular beam epitaxy is presented. On the (0001) Ga-polar plane, a structure consisting of a surface n+ GaN contact layer, followed by a thin InGaN layer, followed by a thick unintentionally doped (UID) GaN layer, and atop a buried n+ GaN contact layer induces a large conduction band barrier via a depleted UID GaN layer. Suppression of reverse and subthreshold current in such isotype barrier devices under applied bias depends on the quality of this composite layer polarization. Sample series were grown under fixed InGaN growth conditions that varied either the UID GaN NH3 flow rate or the UID GaN thickness, and under fixed UID GaN growth conditions that varied InGaN growth conditions. Decreases in subthreshold current and reverse bias current were measured for thicker UID GaN layers and increasing InGaN growth rates. Temperature-dependent analysis indicated that although extracted barrier heights were lower than those predicted by 1D Schrödinger Poisson simulations (0.9 eV-1.4 eV for In compositions from 10% to 15%), optimized growth conditions increased the extracted barrier height from ~11% to nearly 85% of the simulated values. Potential subthreshold mechanisms are discussed, along with those growth factors which might affect their prevalence. [ABSTRACT FROM AUTHOR]

Unipolar-light emitting diode like structures were grown by NH3 molecular beam epitaxy on c plane (0001) GaN on sapphire templates. Studies were performed to experimentally examine the effect of random alloy fluctuations on electron transport through quantum well active regions. These unipolar structures served as a test vehicle to test our 2D model of the effect of compositional fluctuations on polarization-induced barriers. Variables that were systematically studied included varying quantum well number from 0 to 5, well thickness of 1.5 nm, 3 nm, and 4.5 nm, and well compositions of In0.14Ga0.86N and In0.19Ga0.81N. Diode-like current voltage behavior was clearly observed due to the polarization-induced conduction band barrier in the quantum well region. Increasing quantum well width and number were shown to have a significant impact on increasing the turn-on voltage of each device. Temperature dependent IV measurements clearly revealed the dominant effect of thermionic behavior for temperatures from room temperature and above. Atom probe tomography was used to directly analyze parameters of the alloy fluctuations in the quantum wells including amplitude and length scale of compositional variation. A drift diffusion Schrödinger Poisson method accounting for two dimensional indium fluctuations (both in the growth direction and within the wells) was used to correctly model the turn-on voltages of the devices as compared to traditional 1D simulation models. [ABSTRACT FROM AUTHOR]

In hot-electron semiconductor devices, carrier transport extends over a wide range of conduction states, which often includes multiple satellite valleys. Electrical measurements can hardly give access to the transport processes over such a wide range without resorting to models and simulations. An alternative experimental approach however exists which is based on low-energy electron spectroscopy and provides, in a number of cases, very direct and selective information on hotelectron transport mechanisms. Recent results obtained in GaN crystals and devices by electron emission spectroscopy are discussed. Using near-band-gap photoemission, the energy position of the first satellite valley in wurtzite GaN is directly determined. By electro-emission spectroscopy, we show that the measurement of the electron spectrum emitted from a GaN p-n junction and InGaN/GaN light-emitting diodes (LEDs) under electrical injection of carriers provides a direct observation of transport processes in these devices. In particular, at high injected current density, high-energy features appear in the electro-emission spectrum of the LEDs showing that Auger electrons are being generated in the active region. These measurements allow us identifying the microscopic mechanism responsible for droop which represents a major hurdle for widespread adoption of solid-state lighting. [ABSTRACT FROM AUTHOR]

Atom probe tomography was used to achieve three-dimensional characterization of in situ Al2O3/GaN structures grown by metal organic chemical vapor deposition (MOCVD). Al2O3 dielectrics grown at three different temperatures of 700, 900, and 1000 °C were analyzed and compared. A low temperature GaN cap layer grown atop Al2O3 enabled a high success rate in the atom probe experiments. The Al2O3/GaN interfaces were found to be intermixed with Ga, N, and O over the distance of a few nm. Impurity measurements data showed that the 1000 °C sample contains higher amounts of C (4×1019/cm3) and lower amounts of H (7×1019/cm3), whereas the 700 °C sample exhibits lower C impurities (<1017/cm3) and higher H incorporation (2.2×1020/cm3). On comparing with Al2O3 grown by atomic layer deposition (ALD), it was found that the MOCVD Al2O3/GaN interface is comparatively abrupt. Scanning transmission electron microscopy data showed that the 900 °C and 1000 °C MOCVD films exhibit polycrystalline nature, while the ALD films were found to be amorphous.VC [ABSTRACT FROM AUTHOR]

The influence of alloy clustering on fluctuations in the ground state energy of the two-dimensional electron gas (2DEG) in AlGaN/GaN and InAlN/GaN heterostructures is studied. We show that because of these fluctuations, alloy clustering degrades the mobility even when the 2DEG wavefunction does not penetrate the alloy barrier unlike alloy disorder scattering. A comparison between the results obtained for AlGaN/GaN and InAlN/GaN heterostructures shows that alloy clustering limits the 2DEG mobility to a greater degree in InAlN/GaN heterostructures. Our study also reveals that the inclusion of an AlN interlayer increases the limiting mobility from alloy clustering. Moreover, Atom probe tomography is used to demonstrate the random nature of the fluctuations in the alloy composition. [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]

The dependence of electron mobility on growth conditions and threading dislocation density (TDD) was studied for n--GaN layers grown by ammonia-based molecular beam epitaxy. Electron mobility was found to strongly depend on TDD, growth temperature, and Si-doping concentration. Temperature-dependent Hall data were fit to established transport and charge-balance equations. Dislocation scattering was analyzed over a wide range of TDDs (∼2×106 cm-2 to ∼2×1010 cm-2) on GaN films grown under similar conditions. A correlation between TDD and fitted acceptor states was observed, corresponding to an acceptor state for almost every c lattice translation along each threading dislocation. Optimized GaN growth on free-standing GaN templates with a low TDD (∼2×106 cm-2) resulted in electron mobilities of 1265 cm2/Vs at 296K and 3327 cm2/Vs at 113 K. [ABSTRACT FROM AUTHOR]

Semipolar oriented laser diodes (LDs) have fundamental advantages over c-plane oriented LDs, however, the thickness and composition of InGaN waveguiding layers and AlGaN cladding are limited by the onset of stress relaxation via threading dislocation glide on the c-plane slip system. Limited area epitaxy minimizes misfit dislocation (MD) formation by preventing pre-existing TDs from entering a patterned mesa. We examine the effects of mesa height and facet evolution for LAE grown strained layers. Significant MD formation was suppressed by at least a factor of four for Al0.1Ga0.9N/GaN superlattices, enabling AlGaN-clad structures similar to those used in c-plane LDs. We then demonstrate AlGaN-clad blue (456 nm) LDs with threshold current density (Jth) of 4.5 kA/cm2 and GaN-clad true green (523 nm) LDs with Jth of 12 kA/cm2. TEM measurements of the green LD confirm reduction of MD densities with reduction of MD run length. LAE is effective for improving the optical confinement factor of semipolar LDs and offers a glimpse of the design space that may soon be available using low TD density bulk GaN substrates. [ABSTRACT FROM AUTHOR]

Quantitative measurements of interface state density and energy distribution profiles within Al2O3/GaN interfaces were obtained by constant capacitance deep level transient spectroscopy and deep level optical spectroscopy (CC-DLTS/DLOS). The new application of CC-DLOS to interface state measurement is described, which allows interrogation of very deep interface states. A series of Al2O3/GaN metal-insulator-semiconductor (MIS) devices prepared as a function of Al2O3 thickness via atomic layer deposition, on NH3-MBE-grown n-type Ga-polar GaN layers enabled a systematic study. The overall shape and magnitude of the interface trap distribution, Dit, were determined to be nearly identical, independent of Al2O3 thickness. The Al2O3/GaN Dit spectra had an overall U-shape with Dit ∼1012 cm-2 eV-1 near the conduction band edge, ∼1011 cm-2 eV-1 mid-gap, and ∼1014 cm-2 eV-1 near the valence band edge. However, the interface states near the GaN conduction band showed a slight inverse dependence on Al2O3 thickness, suggestive of annealing effect during deposition. The high near valence band state concentrations are consistent with expectations from residual carbon impurities at the GaN surface. A method for discriminating between bulk and interface states in the CC-DLTS signal is demonstrated, using the results on MIS capacitors in combination with spectroscopy results on a Schottky diode structure. [ABSTRACT FROM AUTHOR]

It has been long recognized that the capacitance-voltage (C-V) profiling technique only yields an apparent carrier concentration n⁁, which, in general, differs from the majority carrier concentration. However, it is still possible to extract the conduction band offset ΔEc and interface charge σi from n⁁, provided the doping profile and the exact hetero-interface position are known. We will show that in the case of σi∼1012cm-2, which is typical for the polar III-nitrides, an interface misplacement as small as 1 nm leads to a large error in the extracted ΔEc, making the technique challenging to use, even with the help of a computer-assisted reconstruction. We used one-sided p+-n junctions to experimentally investigate σi and ΔEc for Ga-face n-N GaN/AlGaN heterostructures through C-V profiling. The extracted σi matched the polarization charge reported by others. [ABSTRACT FROM AUTHOR]

The integration of III-nitride and non-III-nitride materials for tandem solar cell applications can improve the efficiency of the photovoltaic device due to the added power contributed by the III-nitride top cell to that of high-efficiency multi-junction non-III-nitride solar cells if the device components are properly designed and optimized. The proposed tandem solar cell is comprised of a III-nitride top cell bonded to a non-III-nitride, series-constrained, multi-junction subcell. The top cell is electrically isolated, but optically coupled to the underlying subcell. The use of a III-nitride top cell is potentially beneficial when the top junction of a stand-alone non-III-nitride subcell generates more photocurrent than the limiting current of the non-III-nitride subcell. Light producing this excess current can either be redirected to the III-nitride top cell through high energy photon absorption, redirected to the lower junctions through layer thickness optimization, or a combination of both, resulting in improved total efficiency. When the non-III-nitride cell's top junction is the limiting junction, the minimum power conversion efficiency that the III-nitride top cell must contribute should compensate for the spectrum filtered from the multi-junction subcell for this design to be useful. As the III-nitride absorption edge wavelength, λN, increases, the performance of the multi-junction subcell decreases due to spectral filtering. In the most common spectra of interest (AM1.5 G, AM1.5 D, and AM0), the technology to grow InGaN cells with λN < 520 nm is found to be sufficient for III-nitride top cell applications. The external quantum efficiency performance, however, of state-of-the-art InGaN solar cells still needs to be improved. The effects of surface/interface reflections are also presented. The management of these reflection issues determines the feasibility of the integrated III-nitride/non-III-nitride design to improve overall cell efficiency. [ABSTRACT FROM AUTHOR]

We have performed comprehensive studies of the optical, thermoelectric, and electrical properties of Mg doped InN with varying Mg doping levels and sample thicknesses. Room temperature photoluminescence spectra show a Mg acceptor related emission and the thermopower provides clear evidence for the presence of mobile holes. Although the effects of the hole transport are clearly observed in the temperature dependent electrical properties, the sign of the apparent Hall coefficient remains negative in all samples. We show that the standard model of two electrically well connected layers (n-type surface electron accumulation and p-type bulk) does not properly describe Hall effect in p-type InN. [ABSTRACT FROM AUTHOR]

This article presents a theoretical analysis of dislocation behavior and stress relaxation in semipolar III-nitride heteroepitaxy, e.g., for AlxGa1-xN and InyGa1-yN layers grown on {hh2hm}- or {h0hm}-type semipolar planes of GaN substrates. We demonstrate that the shear stresses on the unique inclined basal (0001) plane do not vanish for such growth geometries. This leads to the onset of relaxation processes in semipolar III-nitride heterostructures via dislocation glide in the basal slip systems <1120>(0001) and to the formation of misfit dislocations (MDs) with Burgers vectors of (a/3)<1120>-type at the semipolar heterointerface. Next we calculate the Matthews-Blakeslee critical thickness for MD formation in semipolar III-nitride layers together with the MD equilibrium spacings for complete misfit relaxation. The component of the MD Burgers vector normal to the film/substrate interface will cause a crystal lattice tilt in the epilayer with respect to the GaN substrate. The calculated magnitudes of the tilt angles are 0.62° and 0.67° for AlxGa1-xN and InyGa1-yN alloys with compositions of x = 0.20 and y = 0.07, respectively, grown in the (1122) semipolar orientation. The modeling results are discussed in light of recent experimental observations [A. Tyagi et al., Appl Phys. Lett. 95, 251905 (2009); E. Young et al., Appl. Phys. Express 3, 011004 (2010); and F. Wu et al., J. Appl. Phys. 109, 033505 (2011)] of MDs and crystal lattice tilt in semipolar III-nitride heteroepitaxial layers. [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]

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]

InxAl1-xN layers, with 0.09≤x≤0.23, were grown on GaN on both the In-polar and N-polar orientations by metal organic chemical vapor deposition. The impact of growth conditions, including temperature and the group-III flow rates, on the surface morphology and indium mole fraction was investigated. In-polar layers had a smooth surface morphology characterized by mounds which decreased in size with increasing supersaturation during growth. Smooth N-polar InAlN was achieved through the use of vicinal sapphire substrates with misorientation angles of 3°–5° toward <1010>GaN, and a trend of an increase in step bunching with decreasing supersaturation was observed for N-polar InAlN layers. The indium incorporation increased with decreasing growth temperature and increasing growth rate for both In-polar and N-polar layers. The indium incorporation was similar for both orientations on samples which were coloaded in our reactor. [ABSTRACT FROM AUTHOR]

As-grown and oxygen plasma treated n-type tin dioxide (SnO2) (101) thin films were investigated regarding the properties of Au contacts, surface structure, and band bending. The plasma treatment was performed in a conventional oxygen plasma cleaning system with maximum oxygen ion energies of 400 eV. Whereas the as-deposited SnO2 film formed non-Schottky contacts with Au, the oxygen plasma treated films formed Schottky contacts with Au. Capacitance-voltage and differential Hall measurements indicated the introduction of bulk electron traps up to several 100 nm below the surface due to the oxygen plasma treatment. Angle resolved x-ray photoelectron spectroscopy (AR-XPS) revealed a surface accumulation layer on the as-grown film that was absent after the plasma treatment. These measurements further revealed chemical differences between the as-deposited and the plasma treated SnO2 surfaces. All SnO2 films had Sn2+–O bonds. The AR-XPS Sn2+–O signal intensity increased after plasma treatment, indicating that the oxygen plasma damaged the SnO2 surface. Additionally, an O2 adlayer was formed due to the oxygen plasma treatment. The damaged layer due to oxygen ion bombardment, oxygen adsorption layer, and possibly bulk traps depleted the surface carriers which enabled the formation of a Schottky contact with Au. [ABSTRACT FROM AUTHOR]

Two-dimensional optical waveguide mode simulations have been employed to investigate the optimized device structures for ridge-waveguide (Al, In, Ga) N-based green (520 nm) laser diodes (LDs). The effects of thicknesses, alloy compositions, and doping densities of each epitaxially grown layers as well as ridge geometries on optical confinement factors (Γ) and waveguide absorption (α) were comprehensively surveyed. InyGa1-yN (y=0.07–0.1) guiding layers (GLs) with thickness more than 50 nm were effective for realizing high Γ and low α. To minimize the absorption by the anode metal, p-cladding layer (p-CL) was required to be more than 500 nm. At the same time, low index insulator such as SiO2 was preferable for the narrow ridge, where the thickness at the sidewall had to be more than 60 nm. We also found that InGaN barriers layers between the quantum wells (QWs) were superior to GaN barriers to increase Γ and reduce α. Moreover, a thicker last barrier between the topmost QW and the electron blocking layer was also effective to reduce α. Regarding the effect of Mg doping concentration on the absorption, the reduction in Mg in the p-CL and the p-GL was significant to reduce α. Generally, it was confirmed the design for typical 405 nm LDs can be applied for 520 nm LD with the inclusion of InGaN GLs and barriers for the QWs. [ABSTRACT FROM AUTHOR]

The polarity of GaN grown by plasma-assisted molecular beam epitaxy was inverted from N-face to Ga-face by simultaneously exposing the surface to Mg and activated N fluxes during a growth interruption at a reduced substrate temperature. Growth studies suggested that a MgxNy compound was responsible for inverting the crystal. The change in polarity was verified in situ by reflection high energy electron diffraction via GaN surface reconstructions, and ex situ by convergent beam electron diffraction and KOH etch studies. The surface of the inverted material showed smooth step flow features. Ga-face high electron mobility transistors with good dc and small signal performance were fabricated on the inverted epilayers. A drain-source current of 0.84 A/mm was measured at a gate-source voltage of +1 V. Current-gain cutoff and maximum oscillation frequencies of 22 and 53 GHz, respectively, were measured in these devices. The device performance is similar to that of Ga-face transistors with comparable dimensions. [ABSTRACT FROM AUTHOR]