Search

We report high-performance homojunction GaN avalanche photodiodes (APDs) grown on a low-defect GaN substrate and fabricated with an ion-implantation isolation method. High-quality p-i-n GaN layers were grown using metalorganic chemical vapor deposition (MOCVD), and an effective device isolation method using optimized nitrogen ion implantation was developed to provide significant leakage current suppression on the etched mesa sidewalls. Fabricated GaN APDs showed an ultralow dark current density 106 at a reverse bias of 71.5 V under deep-ultraviolet (DUV) illumination at $\lambda =280$ nm. A further temperature-dependent study of the dark current indicated that a trap-assisted tunneling process predominates the leakage current component that also contributes to the carrier multiplication process near the avalanching breakdown.

Front-illuminated GaN-based p-i-n ultraviolet (UV) avalanche photodiodes (APDs) were grown by metalorganic chemical vapor deposition on a free-standing GaN substrate. X-ray diffraction measurements confirm the high crystal quality of the grown structure. Atomic force microscopy measurements show well-developed atomic step-flow morphologies with sub-nanometer root-mean-square surface roughness for scan sizes of 1 × 1 $${\mu m}^{2}$$ and 5 × 5 $${\mu m}^{2}$$ . Mesa etching is one of the major challenges in the fabrication of these devices. Inductively coupled plasma mesa etching creates damage which leads to an increase of the dark current, premature device breakdown, and noise. Front-illuminated GaN-based UV APDs were fabricated using ion-implantation isolation to reduce the sidewall leakage current and improve reliability. The devices show a very low reverse-bias low leakage current density of below 10−10 A/cm2 up to − 35 V. A maximum multiplication gain of 1 × 106 under λ = 355 nm illumination was demonstrated (device limited to a current density of 10 A/cm2). The spectral responsivity of an 82-μm-diameter APD shows a peak responsivity of 1.05 A/W under UV illumination at λ = 375 nm at the avalanche-break-down voltage.

Flexible electronics and mechanically bendable devices based on Group III-N semiconductor materials are emerging; however, there are several challenges in manufacturing, such as cost reduction, device stability and flexibility, and device-performance improvement. To overcome these limitations, it is necessary to replace the brittle and expensive semiconductor wafers with single-crystalline flexible templates for a new-bandgap semiconductor platform. The substrates in the new concept of semiconductor materials have a hybrid structure consisting of a single-crystalline III-N thin film on a flexible metal tape substrate which provides a convenient and scalable roll-to-roll deposition process. We present a detailed study of a unique and simple direct epitaxial growth technique for crystallinity transformation to deliver single-crystalline GaN thin film with highly oriented grains along both a-axis and c-axis directions on a flexible and polycrystalline copper tape. A 2-dimensional (2D) graphene having the same atomic configuration as the (0001) basal plane of wurtzite structure is employed as a seed layer which plays a key role in following the III-N epitaxy growth. The DC reactive magnetron sputtering method is then applied to deposit an AlN layer under optimized conditions to achieve preferred-orientation growth. Finally, single-crystalline GaN layers (∼1 μm) are epitaxially grown using metal organic chemical vapor deposition (MOCVD) on the biaxially-textured buffer layer. The flexible single-crystalline GaN film obtained using this method provides a new way for a wide-bandgap semiconductor platform pursuing flexible, high-performance, and versatile device technology.

The thermal effect of the growth temperature on interface morphology and stimulated emission in ultraviolet AlGaN/InGaN multiple quantum wells (MQWs) are experimentally investigated. During the MOCVD epitaxial growth of AlGaN/InGaN MQWs, the ramping rate from a lower temperature for InGaN quantum wells (QWs) to a higher one for AlGaN quantum barriers (QBs) is intentionally changed from 1.0°C/s to 4.0°C/s. Atomic force microscopy images show that the surface morphology of InGaN QWs, which is improved by a thermal effect when the growth temperature rises to the set value of the AlGaN QBs, varies with different temperature ramping rates. The results of stimulated emission indicate that the threshold pumping power density of MQWs is decreased with increasing temperature ramping rate from 1.0°C/s to 3.0°C/s and then slightly increased when the ramping rate is 4.0°C/s. This phenomenon is believed to result from the thermal degradation effect during the temperature ramp step. A long-time high-temperature annealing will reduce the density of indium-rich microstructures as well as the corresponding localized state density, which is assumed to contribute to the radiative recombination in the InGaN QWs. Given the great difference between optimal growth temperatures for AlGaN and InGaN layers, a higher ramping rate would be more appropriate for the growth of ultraviolet AlGaN/InGaN MQWs.

We report the development of high-performance GaN ultraviolet avalanche photodiode technology with uniform dark current characteristics. These devices exhibited high-avalanche-gain (>105) in linear-mode operation in 6x6 array form with pixel sizes up to 70x70 μm2.

Although both III-N laser diodes (LDs) and LEDs employ electron blocking layers (EBLs) to reduce electron leakage from the active region, LDs typically operate at far higher current densities than LEDs. Shortcomings of the common rectangular EBL are discussed. Two alternative EBL designs have been systematically studied using numerical simulation: the inverse-tapered EBL and the inverse-tapered step-graded EBL. It is shown that the efficacy of each of these EBL designs depends strongly on the operational current density, suggesting that the EBL design considerations for III-N LDs and LEDs are fundamentally different.

Front-illuminated p-i-n GaN-based ultraviolet (UV) avalanche photodiodes (APDs) were grown by metalorganic chemical vapor deposition (MOCVD) on 25 mm dia. bulk Ammono® n-GaN substrate having a low etch pit density (EPD) less than 5 × 104 [cm-2] and processed into 6×6 APD arrays. The devices employed N-ion implantation to achieve sidewall passivation. Evaluation of these 6×6 arrays will help to confirm the uniformity of the epitaxial materials and device processing. The maximum avalanche gain reached ~ 3×105 at the breakdown (current limited). The dark current density was 10-9 A/cm2 at reverse bias up to -20 V and the APDs exhibited a reverse breakdown voltage of 81 ± 1 V for all 36 devices without any leaky devices, confirming a high uniformity of the growth and fabrication processes.

A top-illuminated deep-ultraviolet Al0.6Ga0.4N p-i-n avalanche photodiode (APD) structure was designed and grown by metalorganic chemical vapor deposition on an AlN bulk substrate and on two different quality AlN/sapphire templates, and APDs were fabricated and tested. The APD devices with a circular diameter of 20 μm have demonstrated a distinctive reverse-bias breakdown behavior. The reverse breakdown voltage of the APDs is approximately −140 V, which corresponds to a breakdown electric field of 6–6.2 MV/cm for the Al0.6Ga0.4N material as estimated by Silvaco TCAD simulation. The APDs grown on the AlN bulk substrate show the lowest leakage current density of −8 A/cm2 (at low reverse bias) compared to that of the devices grown on the AlN templates. From the photocurrent measurement, a maximum gain (current limited) of 1.2 × 104 is calculated. The average temperature coefficients of the breakdown voltage are negative for APD devices fabricated from both the AlN bulk substrate and the AlN templates, but these data show that the coefficient is the least negative for the APD devices grown on the low-dislocation-density AlN bulk substrate.

We report high-performance GaN avalanche photodiodes using a novel ion implanted isolation technique. The devices showed low dark current, an equivalent noise power of < 5×10−16 WHz−0.5 and avalanche gain of 7×105 at λ = 280 nm.

Finally, we acknowledge the funding support from KAUST Baseline BAS/1/1664-01-01, KAUST Competitive Research Grant URF/1/3437-01-01, and the GCC Research Program REP/1/3189-01-01. Y.J.P, T.D, and R.D.D. at Georgia Tech acknowledge the funding support from NSF under Grant No. DMR-1410874 and the DARPA under Grant No. W911NF-15-1-0026, and also the support of the Georgia Research Alliance and the Steve W. Chaddick Endowed Chair in Electro-Optics.

Lateral hole injection into AlGaN-based ultraviolet (UV) vertical-cavity light-emitting lasers (VCSELs) has been studied via numerical simulation. For blue and violet vertical cavity light emitters, indium tin oxide (ITO) is most commonly used as a transparent current spreading layer to increase the overlap between the optical mode and the radial current profile. However, ITO has very high optical losses in the UV spectrum, so alternative schemes for lateral current spreading have been investigated for use in UV-VCSELs. A modulation doped short-period superlattice (MD-SPSL) has been proposed as a transparent lateral current spreading layer in UV-VCSELs. The narrow bandgap unintentionally doped (uid) material maintains a high mobility due to reduced impurity scattering and has a high free hole concentration due to modulation doping, thus forming highly conductive channels which aid lateral hole transport. This has been shown to partially mitigate current crowding around the current aperture. To account for imperfect modulation doping due to the magnesium memory effects and other factors, the effect of varying the hole mobility in the uid-narrow bandgap layer of the MD-SPSL from 13–300 cm2/( $\text {V}\cdot \text {s}$ ) on the threshold current and slope efficiency has also been studied. Employing an MD-SPSL results in a significant reduction in the threshold current and slope efficiency compared to ITO, and the extent of the improvement depends on the hole mobility in the uid-AlGaN layer.

Front-illuminated GaN-based separate absorption and multiplication (SAM) ultraviolet (UV) avalanche photodiodes (APDs) with various photon detection areas are demonstrated grown by metalorganic chemical vapor deposition on bulk GaN native substrates with low dislocation density. By adopting a front-illuminated UV-APD structure with a thin AlGaN window layer, no additional etching of the substrate for the reduction of strong UV absorption is required. The epitaxial layer structure of the p-i-p-i-n SAM UV-APDs consists of a Mg-doped ${p}$ -Al0.05Ga0.95N window layer to minimize UV absorption at the top surface region and a Mg-graded ${p}$ -GaN charge layer to serve as a field-termination layer. The onset point of the breakdown voltage ( $V_{\text {BR}}$ ) is around 73 V for all SAM-APDs with different mesa areas ranging from 1963 to 10 $000~\mu \text{m}^{2}$ , which is a lower $V_{\text {BR}}$ than the typical p-i-n UV-APDs with the similar thickness of undoped layer, where the photon absorption and multiplication processes take place simultaneously. Under UV-light illumination at $\lambda =340$ nm, the SAM-APDs exhibit high avalanche gains greater than $8.0\times 10^{5}$ at a reverse bias of $V_{R}> 72.5$ V

We have investigated the microstructure of B x Al 1-x N films grown by flow-modulated epitaxy at 1010 °C, with B/(B + Al) gas-flow ratios ranging from 0.06 to 0.18. The boron content obtained from X-ray diffraction (XRD) patterns ranges from x = 0.02 to 0.09. On the other hand, boron content deduced from the aluminum signal in the Rutherford backscattering spectra (RBS) ranges from x = 0.06 to 0.16, closely following the gas-flow ratios. Transmission electron microscopy indicates the sole presence of a wurtzite crystal structure in the BAlN films, and a tendency towards columnar growth for B/(B + Al) gas-flow ratios below 0.12. For higher ratios, the BAlN films exhibit a tendency towards twin formation and finer microstructure. Electron energy loss spectroscopy has been used to profile spatial variations in the composition of the films. The RBS data suggest that the incorporation of B is highly efficient for our growth method, while the XRD data indicate that the epitaxial growth may be limited by a solubility limit in the crystal phase at about 9%, for the range of B/(B + Al) gas-flow ratios that we have studied, which is significantly higher than previously thought.

We report the current progress of our development of near-ultraviolet (NUV) III-nitride vertical-cavity LED emitters and avalanche photodetectors grown by metalorganic chemical vapor deposition (MOCVD). The III-N emitters are designed to be UV vertical-cavity surface-emitting lasers operating at 369.5nm. We describe the development of the growth and processing of an air-gap/AlGaN distributed Bragg reflector (DBR) consisting of five-pairs of quarter-wavelength layers of Al0.12Ga0.88N and air-gap regions created by selective chemical etching. A 4-6λ cavity was employed in the laser structure. We also report on the electrical and optical emission characteristics of these microcavity emitters. The photodetectors are GaN- and AlGaN-based p-i-n avalanche photodiodes (APDs) designed for front-side illumination. We report on the electrical and optical detection characteristics of these photodetectors.

Free-standing gallium nitride (GaN) substrates are in high demand for power devices, laser diodes, and high-power light emitting diodes (LEDs). SixPoint Materials Inc. has begun producing 2&rdquo, GaN substrates through our proprietary Near Equilibrium AmmonoThermal (NEAT) growth technology. In a single 90 day growth, eleven c-plane GaN boules were grown from free-standing hydride vapor phase epitaxy (HVPE) GaN substrates. The boules had an average X-ray rocking curve full width at half maximum (FWHM) of 33 &plusmn, 4 in the 002 reflection and 44 &plusmn, 6 in the 201 reflection using 0.3 mm divergence slits. The boules had an average radius of curvature of 10.16 &plusmn, 3.63 m. The quality of the boules was highly correlated to the quality of the seeds. A PIN diode grown at Georgia Tech on a NEAT GaN substrate had an ideality factor of 2.08, a high breakdown voltage of 1430 V, and Baliga&rsquo, s Figure of Merit of &gt, 9.2 GW/cm2. These initial results demonstrate the suitability of using NEAT GaN substrates for high-quality MOCVD growth and fabrication of high-power vertical GaN switching devices.

Two different structures of AlGaN/InGaN ultraviolet (UV) multiple quantum wells (MQWs) were grown in a metalorganic chemical vapor deposition (MOCVD) system, and their performance under optically pumped stimulated emission were experimentally investigated. During the MOCVD epitaxial growth of the AlGaN/InGaN MQWs, the growth rate of the AlGaN quantum barriers (QBs) was intentionally reduced to improve the surface morphology. Atomic-force microscopy (AFM) images show that the AlGaN QBs have a smooth surface with clear step flow patterns. The surface morphology of InGaN QWs was improved by thermal annealing effect when the growth temperature rose to the one of the AlGaN QBs. With optical confinement layers on both the n- and p-sides, the threshold pumping power density of optical stimulated emission for AlGaN/InGaN MQWs was determined to be 168 kW/cm2. In order to reduce the negative effect of the interface between AlGaN QBs and InGaN QWs, another MQW structure with a larger quantum well thickness was designed and epitaxial grown. The optical investigation of sample B showed a threshold pumping power density of 124 kW/cm2, which is 26% lower than sample A.

We report III-N surface-emitting resonant-cavity light-emitting diodes (RCLEDs) at λ = 375 nm using a novel hybridmirror approach. The hybrid mirrors consist of 5 pairs of air-gap/AlGaN distributed Bragg reflector (DBR) at the bottom side of the vertical cavity and HfO2/SiO2 dielectric DBR (DDBR) on the top to facilitate the formation of a resonant cavity for nitride-based surface light emitting diodes. The air-gap/AlGaN DBR replaces the conventional thick stack of semiconductor DBR to achieve high reflectivity. Hybrid-mirror III-N RCLEDs with airgap/AlGaN DBR mirror were fabricated and the results showed that the III-N RCLEDs achieved high current density operation up to 40 kA/cm2 with a peak emission wavelength atλ = 375 nm and a full-width-half-maximum (FWHM) of 9.3 nm at room temperature.

Front-illuminated GaN p-i-p-i-n separate-absorption and multiplication avalanche photodiode (SAM-APD) epitaxial structures were grown by metalorganic chemical vapor deposition (MOCVD) on n-type bulk GaN substrates and fabricated into 4×4 arrays with a large detection area of 100×100 μm2. The SAM-APD array showed a uniform distribution of dark current density of JDark

We report high-responsivity GaN/InGaN heterojunction phototransistors (HPTs) grown on sapphire substrates. Under the ultraviolet (UV) photon illumination from the front side of the wafer, an HPT shows broad photoresponse spectrum with the short-wavelength cutoff wavelength well beyond $\lambda = 280$ nm, and the UV-to-visible-band rejection ratio is $> 8 \times 10^{3}$ . The responsivity ( $R_{\lambda }$ ) of HPT is greater than 8 A/W at $\lambda = 373$ nm, and is greater than 3 A/W at $\lambda = 280$ nm as the device is biased at $V_{\mathrm{ CE}}=10$ V. As the HPT is biased at the near breakdown voltage ( $V_{\mathrm{ CE}}> 35$ V), the responsivity performance was enhanced due to the carrier multiplication, resulting in $R_{\lambda }> 100$ A/W at $V_{\mathrm{ CE}}= 40$ V for $P_{\mathrm{ opt}}=1.73~\mu \text{W}$ /cm2 at $\lambda = 373$ nm. These results demonstrate that GaN/InGaN HPTs can achieve low light detection with a broadband photon response in the near-UV-to-deep-UV spectral ranges.

GaN p-i-n ultraviolet avalanche photodiodes (UV-APDs) were fabricated from epitaxial structures grown on low-dislocation-density free-standing GaN substrates to form $4\times 4$ UV-APD arrays with a device size of $75\times 75~\mu \text{m}^{{ {2}}}$ . The devices in the UV-APD array showed a uniform and reliable distribution of breakdown voltage ( $V_{\text {BR}}$ ) and leakage current density. The average $V_{\text {BR}}$ of the 16 devices in one of the UV-APD arrays was 96±0.6 V, and the average dark current density ( $J_{R\_{}{\text {Dark}}}$ ) and photocurrent density ( $J_{R\_{}{\text {Photo}}}$ ) were measured to be (6.5±1.8) $\times 10^{-7}$ and (5.7±1.1) $\times 10^{{\text {-6}}}$ A/cm $^{{{2}}}$ at the reverse bias voltage of $V_{R}=48$ V (50% of the average onset point of $V_{\text {BR}}$ ), respectively. The reliable device performance was confirmed by performing multiple reverse bias $I$ – $V$ scans for the selected devices in the UV-APD array. We also observed the significantly enhanced spectral responsivity from the 142 to 5485 mA/W due to the strong carrier impact ionization at high reverse bias.

We report an electrically conducting 40-pair silicon doped Al 0.12 Ga 0.88 N/GaN distributed Bragg reflector (DBR) grown by metalorganic chemical vapor deposition on a silicon doped n -type GaN template. Due to the relatively small lattice mismatch between AlGaN and GaN, strain managing layers are not required for crack-free n -DBR growth. The DBR demonstrates a peak reflectivity of 91.6% at 368 nm with stopband of 11 nm. In addition, the 40-pair n -DBR shows the vertical resistance of 5.5 Ω, which corresponds to bulk resistivity of 0.52 Ω cm, near the maximum measured current of 100 mA.

An atomic-scale phase transition in heterophase epitaxy (HPE) of GaN on a 900 nm-wide v-grooved Si(001) substrate is reported. Two different incorporation mechanisms of adatoms sequentially occur for the hexagonal (h-) to cubic (c-) phase transition: orientation- and phase-dependent incorporation (ODI and PDI). Epitaxy begins with ODI that results in preferential growth of h-GaN individually aligned to opposing Si(111) facets inside a v-groove but incurs a structural instability by crystallographic mismatch at the groove bottom. This instability is relieved by an abrupt transition to c-phase, initiating from single or multiple atomic sites uniquely arranged atop the mismatch along the groove. Epitaxy proceeds with PDI that allows μm-scale c-GaN extended from these sites while suppressing growth of h-GaN. An important condition for HPE and the stability of c-GaN in further growth is derived from equilibrium crystal shape.

Unlike laser diodes made of traditional III-V materials (III-AsP), III-Nitride laser diodes and LEDs suffer from reduced injection efficiency due to greater electron leakage [1]. The overflow of electrons out of the active region into the adjacent p-type quasi-neutral region (QNR) is primarily due to the asymmetry between majority carrier conductivities in p- and n-GaN [2], where the electron conductivity in n-GaN is more than 40 times greater than the hole conductivity in p-GaN [3], [4]. Furthermore, carrier leakage is exacerbated by interfacial spontaneous and piezoelectric polarization charges at III-N heterojunctions. To stem electron leakage, an electron blocking layer (EBL) is typically employed between the last quantum barrier (QB) and p-type quasi-neutral region (QNR), and this EBL is typically a thin layer of wide bandgap material.

We report a high-aluminum-containing ([Al] $\,\,{\sim }\,\,0.6$ ) AlGaN multiple-quantum well (MQW) double-heterojunction (DH) emitter employing an inverse-tapered-composition AlGaN:Mg p-type waveguide grown on a $c$ plane Al-polar AlN bulk substrate. Using numerical simulations, we have determined that the inverse-tapered p-type waveguide design is necessary for high [Al] containing p-n junction devices as any valence band discontinuity at the junction will limit the vertical hole transport and induce a larger voltage-drop across the structure. The fabricated ultraviolet MQW DH emitter can sustain a DC current of at least 500 mA and a pulsed current of at least 1.07 A, which corresponds to a current density of 10 and 18 kA/cm $^{2}$ at maximum measured voltage of 15 and 20 V with the measured series resistance of 15 and 11 $\Omega $ , respectively.

We report a homojunction gallium nitride (GaN) p-i-n rectifier fabricated on free-standing GaN substrates with the breakdown voltage $>800$ V and low specific on-resistance ( $R_{\mathrm{{\scriptscriptstyle ON}}}A$ ). At 298 K, $R_{\mathrm{{\scriptscriptstyle ON}}}A$ is 0.28 $\text{m}\Omega $ -cm $^{2}$ at the current density ( $J$ ) of 2.5 kA/cm $^{\vphantom {R^{R^{}}}2}$ and the corresponding Baliga’s figure of merit is $>2.5$ GW/cm $^{2}$ . At a given temperature, $R_{\mathrm{{\scriptscriptstyle ON}}}A$ values decrease with $J$ due to conductivity modulation in the drift region. The ambipolar lifetime ( $\tau _{a})$ is also determined by open-circuit voltage decay measurement. The value for $\tau _{a }$ is 9.6 ns at 298 K and it monotonically increases to 22 ns at 448 K. The reverse $I$ – $V$ measurement reveals the reverse leakage current mechanism is mainly attributed to a field-assisted ionization process from deep-level centers in the space-charge region. The analysis of $T$ – $I$ – $V$ curve yields the Poole-Frenkel coefficient ( $\sim 3.1\times 10^{-4} ~\text {eV} \cdot \text {V}^{-1/2}\cdot {\rm cm}^{-1/2})$ and a deep-level trap ( $\sim 0.7$ eV) at zero bias.

We studied temperature dependence of crystalline quality of AlN layers at 1050–1250 °C with a fine increment step of around 18 °C. The AlN layers were grown on c -plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD) and characterized by X-ray diffraction (XRD) ω-scans and atomic force microscopy (AFM). At 1050–1068 °C, the templates exhibited poor quality with surface pits and higher XRD (002) and (102) full-width at half-maximum (FWHM) because of insufficient Al atom mobility. At 1086 °C, the surface became smooth suggesting sufficient Al atom mobility. Above 1086 °C, the (102) FWHM and thus edge dislocation density increased with temperatures which may be attributed to the shorter growth mode transition from three-dimension (3D) to two-dimension (2D). Above 1212 °C, surface macro-steps were formed due to the longer diffusion length of Al atoms than the expected step terrace width. The edge dislocation density increased rapidly above 1212 °C, indicating this temperature may be a threshold above which the impact of the transition from 3D to 2D is more significant. The (002) FWHM and thus screw dislocation density were insensitive to the temperature change. This study suggests that high-quality AlN/sapphire templates may be potentially achieved at temperatures as low as 1086 °C which is accessible by most of the III-nitride MOCVD systems.

In this paper, we have investigated the high temperature characteristics of monolithically integrated LED and MOS-channel HEMT in GaN. The monolithically integrated LED/MOSC-HEMT was implemented by using a selective epi removal approach. The temperature dependence of optical and electrical characteristics of the integrated LED is found to be comparable to that of discrete GaN based LEDs. On-resistance of the MOSC-HEMT shows gradual increase with temperature (~1.6× increase from 25 °C to 225 °C) whereas LED LOP shows rapid decrease with temperature (~6× decrease from 25 °C to 225 °C). Light output of the integrated LED is modulated by the MOSC-HEMT gate bias up to 225 °C.

Current understanding of phonons treats them as plane waves/quasi-particles of atomic vibration that propagate and scatter. The problem is that conceptually, when any level of disorder is introduced, whether compositional or structural, the character of vibrational modes in solids changes, yet nearly all theoretical treatments continue to assume phonons are still waves. For example, the phonon contributions to alloy thermal conductivity (TC) rely on this assumption and are most often computed from the virtual crystal approximation (VCA). Good agreement is obtained in some cases, but there are many instances where it fails—both quantitatively and qualitatively. Here, we show that the conventional theory and understanding of phonons requires revision, because the critical assumption that all phonons/normal modes resemble plane waves with well-defined velocities is no longer valid when disorder is introduced. Here we show, surprisingly, that the character of phonons changes dramatically within the first few percent of impurity concentration, beyond which phonons more closely resemble the modes found in amorphous materials. We then utilize a different theory that can treat modes with any character and experimentally confirm its new insights. In solids, atoms continuously vibrate in collective motions with quantized amplitudes that can be described in terms of quasiparticles known as phonons—which are responsible for heat transfer and sound. Phonons are usually treated as waves that propagate and scatter, but this approach can sometimes fail when dealing with materials with disorder. A team of researchers from the Georgia Institute of Technology, led by Asegun Henry, show that by focussing on phonon correlation rather than scattering, it is possible to more accurately capture the changes in vibrational mode behavior as a function of disorder.

In this work, we report a polarized Raman study on the vibrational properties of the InAs/InAs1-xSbx SLs as well as selected InAs1-xSbx alloys, all grown on GaSb substrates by either MBE or MOCVD, from both growth surface and cleaved edge.

In this study, pioneering research was performed on GaN p-i-n diodes for the first ever assessment of surface temperature distribution by incorporating the use of infrared (IR) thermography, thermoreflectance thermal imaging, Raman thermometry, and thermal simulations. Each technique was advanced in order to obtain self-consistent results with higher accuracy. A two-temperature emissivity calibration procedure was utilized for IR measurements to acquire a temperature map of the p-contact metallization. Higher spatial resolution thermoreflectance thermal imaging was performed with a diverse range of illumination wavelengths including 470 nm and 530 nm. To confirm the results of thermoreflectance, TiO2 thermal nanoprobes were deposited on the device surface which enabled Raman thermometry to be performed on the p-contact metallization. Coherence of the techniques was then validated through thermal modeling. The results suggest that IR thermography, when using the two-temperature emissivity correction procedure, gives reasonable results at high power dissipating conditions. Thermoreflectance and nanopowder assisted Raman thermometry are viable options for GaN vertical device temperature assessment. However, results from Raman thermometry possess relatively large uncertainties and thermoreflectance measurements require multiple illumination wavelengths to ensure the validity of the measured temperatures that are derived from the thermoreflectance calibration coefficient.

Vertical-cavity surface-emitting lasers (VCSELs) are known to have advantages of lower threshold current operation, circular and low-divergence output beam, and lower temperature sensitivity compared to edge-emitting laser diodes. In conventional VCSELs, the formation of a current aperture plays a vital role in the device characteristics. Low laser thresholds and single-transverse-mode operation would not be possible without a well-defined current aperture to confine carriers to generate photons between the two distributed Bragg reflectors. Since the introduction of the controlled oxidation process for the AlxGa1-xAs material system by Dallesasse and Holonyak in 1989, most VCSELs have employed oxidation for current aperture formation as well as optical confinement and this technique has become one of the most commonly used fabrication techniques for traditional III-V compound semiconductor infrared VCSELs. However, for III-N emitters operating at wavelengths in the ultraviolet to green wavelength range, the formation of Al-based native oxide layers has not proven feasible. As a result, various current-confinement techniques have been studied such as, selective-area growth of buried AlN, oxidizing AlInN, and selective activation of acceptors. In this work, we report an ion-implantation process which is effective for carrier confinement and defines a current aperture for our III-N ultraviolet microcavity light-emitting diodes (MCLEDs). The devices have peak emission wavelength of ~371.4 nm with the spectral linewidth of 5.1 nm at the highest pulsed current injection level of 15 kA/cm2. Further discussion on the material growth, material characterization, implantation parameters, as well as numerical simulation for structural design will be presented in the conference.

The field of ultraviolet (UV) photonics at wavelengths λ

The III-N wide-bandgap alloys in the AlInGaN system have many important and unique electrical and optical properties which have been exploited to develop deep-ultraviolet (DUV) optical devices operating at wavelengths

The growth of high quality AlN epitaxial films relies on precise control of the initial growth stages. In this work, we examined the influence of the trimethylaluminum (TMAl) pretreatment of sapphire substrates on the structural properties, crystal quality and growth modes of heteroepitaxial AlN films on (0 0 0 1) sapphire substrates. Without the pretreatment, the AlN films nucleated on the smooth surface but exhibited mixed crystallographic Al- (N-) polarity, resulting in rough AlN film surfaces. With increasing the pretreatment time from 1 to 5 s, the N-polarity started to be impeded. However, small islands were formed on sapphire surface due to the decompostion of TMAl. As a result, small voids became noticeable at the nucleation layer (NL) because the growth started as quasi three-dimensional (3D) but transformed to 2D mode as the film grew thicker and got coalesced, leading to smoother and Al-polar films. On the other hand, longer pretreatment time of 40 s formed large 3D islands on sapphire, and thus initiated a 3D-growth mode of the AlN film, generating Al-polar AlN nanocolumns with different facets, which resulted into rougher film surfaces. The epitaxial growth modes and their correlation with the AlN film crystal quality under different TMAl pretreatments are also discussed. The KAUST authors would like to acknowledge the support of GCC Research Program REP/1/3189-01-01, Baseline BAS/1/1664-01-01, and Equipment BAS/1/1664-01-07. The work at QU was supported by GCC Research Program GCC-2017-007. The work at Georgia Institute of Technology was supported in part by DARPA under grant W911NF-15-1-0026 and NSF under grant DMR-1410874. RDD acknowledges the additional support of the Steve W Chaddick Endowed Chair in Electro-Optics and Georgia Research Alliance. Scopus

We report the successful epitaxial growth of biaxial GaN films using metal-organic vapor phase epitaxy on amorphous surfaces (native oxide covered Si wafer) through biaxial metal buffer layers of Mo and W. The biaxial metal films were grown using a dynamic oblique angle deposition technique. An epitaxial relationship of GaN(0002)//Mo(110) in the out-of-plane orientation and GaN[ 10 1 ¯ 0 ]//Mo[001] in the in-plane orientation was found by a detailed X-ray diffraction analysis. Similar epitaxial results have been found for GaN/W. These biaxial GaN films on biaxial Mo and W buffer layers were used as seeds to grow GaInN/GaN multiple quantum well structures (MQWs). A high intensity, narrow peak, and uniform luminance space distribution of photoluminescence signal was observed from these MQWs. This work opens up an alternative strategy for the growth of GaN-based films on amorphous substrates using biaxial metal buffer layers.

The advent of a wide-bandgap GaN p-n junction has enabled highly efficient blue light-emitting diodes (LEDs) in GaInN/GaN heteroepitaxy. The system also enables a much wider range of emission wavelengths. We summarize progress in epitaxial materials development of green, yellow, and orange direct-emitting LEDs that bypass the steps of external phosphor conversion to achieve higher stability, efficiency, and higher overall color rendering quality for wider adoption of one of the widest power savings resources so far identified.

Achieving high p-type conductivity is one of the most significant bottlenecks to the efficient operation of wurtzite III-N based optoelectronic and microelectronic devices. Through judicious volumetric redistribution of fixed negative polarization charge, compositionally graded layers may be exploited to achieve nearly flat valence band profiles free from electrostatic barriers to hole injection into the active region. This may potentially ameliorate problems associated with poor p-type conductivity and inefficient hole transport that complicates the design of DUV laser diodes and light emitting diodes. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)