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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.

We determined the optical and electrical characteristics of GaN-based, ultraviolet micro light emitting diodes (microLEDs). Such microLEDs are essential to next-generation high-resolution micro-displays. Square-shaped microLEDs of different sizes (side lengths: 5-50 μm) were designed. The peak emission wavelength of these devices shifted

Gallium-nitride and related materials have become suitable semiconductor platforms for next-generation power electronic devices. This materials system leverages unique properties of wider energy bandgap, higher carrier mobility, and ultrashort carrier recombination lifetime to achieve much improved bipolar switching characteristics when compared to Si- or SiC-based counterparts. In recent years, vertical GaN switches such as PIN rectifiers, junction barrier Schottky (JBS) diodes, and junction field-effect transistors (JFETs) have been actively sought in hopes to achieve low-loss device performance at kilo-volt (kV) operation range. Over the years, significant progresses in low-defect-density substrate preparation and high-quality epitaxial growth using metalorganic chemical vapor deposition (MOCVD) reactors, along with robust fabrication processing techniques helped achieve many impressive GaN-based power device demonstrations. As vertical GaN devices operate at the kV ranges, proper engineering of the electric field is a key to exploiting the ultimate device performance. To this end, several device-level design strategies such as field plates, beveled sidewalls, and junction termination extensions have been studied. In the context of realizing a planar device topology in GaN materials systems, ion implantation into GaN materials systems offer new opportunities to provide precise control of inhomogeneous carrier distribution for the creation of versatile doping profiles and geometries such as selectively doped regions or highly resistive regions at arbitrary locations in GaN devices. In this presentation, we will discuss a study of the floating guard rings (FGRs) designs that utilize a nitrogen-ion implantation isolation technique. The designs can effectively mitigate the electric field crowding effect and enable robust avalanching capability in vertical GaN PIN rectifiers. Using 1.2-kV GaN PIN rectifiers as design examples, a set of numerical simulation study was performed using a TCAD-Sentaurus software package. The results showed that a closely packed FGRs with a gap of adjacent FGRs in the range of 1-3 microns, a width of the FGRs in the range of 4 to 6 microns, along with a variable-width FGR design toward the outer rings can effectively suppress the peak electric field distribution at edge of the main junction and the surface of junction termination extensions. For example, consider a GaN PIN rectifier structure with a drift layer that consists of 6 microns of unintentionally doped (UID) GaN (n= 5×1015 cm-3) and a 4-micron-thick lightly doped GaN (n = 2×1016 cm-3). The FGR structures can be formed by creating highly resistive GaN regions in the epitaxially grown p-GaN layer using an ion implantation technique. In this design, five of the FGRs have 1-micron-wide undoped GaN and a 6-micron-wide p-GaN region in the p-GaN layer. The outermost FGR (the 6th FGR) is designed to have a 2-micron-wide undoped GaN and 5-micron-wide p-GaN extension. The resulting field distribution showed an effective suppression of the field crowding effect at the edge of the main junction (Figure 1(a)) and a smooth electric potential drop along the lateral direction can be achieved (Figure 1(b)). As shown in Figure 2, the electric field distribution at the vertical junction was plotted for two FGR designs with and without a metal plate on top of the p-GaN region. The electric field distribution of the main junction was also plotted for comparison. It is clearly shown that the peak electric field at the vertical junction is effectively reduced with the metal-plated FGRs by >10% when compared to a non-plated FGRs. Reduced peak electric field in typically high-electric-field regions can safeguard the device from experiencing premature breakdown for robust switching operations. We found that 6 FGRs with a total lateral extension of FGR designs were implemented in GaN PIN rectifiers for experimental validation. The fabrication processing of vertical GaN PIN rectifiers was grown on n-type free-standing or bulk GaN substrates using a MOCVD reactor. The cathode was formed by depositing Ti/Al-based ohmic contact on the backside of the wafer, and the anode electrode was formed on the topside p-type GaN layer. Nitrogen-ion implantation was employed to create FGR structures and served as a mean of inter-device isolation. Detailed design strategies of the FGRs, device processing and device characterizations will be discussed in the conference. Figure 1

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.

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 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.

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 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.

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 work, we investigate the thermal response of GaN PIN diodes grown on a sapphire substrate and compare the results to GaN PIN diodes grown on a free standing GaN substrate (FS-GaN). Until now, thermal characterization techniques have been developed to assess the temperature distribution across lateral devices. Raman thermometry has shown to accurately measure the temperature rise across the depth of the GaN layer. Implementing this technique to assess the temperature distribution across the depth of a vertical GaN device is more challenging since a volumetric depth average is measured. The use of TiO2 nanoparticles is shown to overcome this issue and reduce the uncertainty in the peak temperature by probing a surface temperature on top of the device. For the sapphire substrate, an additional temperature rise of about 15 K was seen on the surface of the PIN diode as compared to the average in the bulk. While the steady state thermal measurements show an accurate estimation of the device’s peak temperature, the PIN diodes are normally operated under pulsed conditions and the thermal response of these devices under periodic joule heating must be assessed. A recently developed transient thermoreflectance imaging technique (TTI) is used in this study to monitor transient temperature rise and decay of top metal contact. Under the same biasing conditions, the FS-GaN PIN diode is found to result in less than half the temperature rise obtained by the sapphire substrate diode. Extracting time constants, a longer rise and decay is also observed in the sapphire substrate diode.

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

An AlGaN deep ultraviolet laser diode design exploiting AlN substrates is presented, featuring an inverse-tapered p-waveguide layer. The 2-D optoelectronic simulation predicts lasing at 290 nm. Spatial balancing of the lasing mode to minimize optical loss in the p-Ohmic metallization is achieved through the use of a narrow bandgap yet transparent n-waveguide layer. Several electron blocking layer (EBL) designs are investigated and compared with a conventionally tapered EBL design. Through judicious volumetric redistribution of fixed negative polarization charge, inverse tapering may be exploited to achieve nearly flat valence band profiles free from barriers to hole injection into the active region, in contrast to conventional designs. Furthermore, proper selection of quantum well barrier and spacer compositions are demonstrated to reduce electron leakage from the active region. Numerical simulations demonstrate that the inverse tapered strategy is a viable solution for efficient hole injection in deep ultraviolet laser diodes operating at shorter wavelengths .

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)

The successful development of III-V nitride materials systems has led to active research of advanced electronic devices in the recent decades. III-nitride (III-N)-based devices have a bandgap engineering flexibility, large energy gap, direct band structure, and high carrier mobility when compared to Si- and SiC -based devices, providing new performance improvement opportunities for high-speed and energy-efficient electronics. Today, commercial GaN HFETs are available for microwave power amplifiers and power electronic circuits. On the other hand, the development of bipolar transistors and the related switch technologies are essential to fully exploit the potential of III-N materials systems. For instance, insulated-gate-bipolar transistors would require integrated bipolar transistor and unipolar transistor structures. The development of III-N bipolar switches in either two-terminal or three-terminal forms have therefore been actively sought. To this end, power switches that conduct the electric current in the vertical form are beneficial to achieve the ultimate Baliga’s Figure of Merit (BFOM) for switching. For GaN bipolar power switches development, low-defect-density native substrates, high-quality epitaxial growth technology, and proper device design and fabrication have become major focus areas of research. Early development of heterojunction bipolar transistors (HBTs) using GaN/InGaN heterojunctions at Georgia Tech showed promising device performance. These devices exhibited high-current gain of >100, high current drive density of >100 kA/cm2, and high d.c. power handling capability of > 3 MW/cm2. Small-signal microwave amplification with a cut-off frequency (f T) of 8GHz was reported. In this presentation, we will review the development status of III-N bipolar switches and use GaN rectifiers as a case study to discuss the device performance pertinent to the choices of GaN substrates prepared by different methods. The device structures were grown and fabricated at Georgia Tech and the device performance correlation with respect to the substrates were evaluated. High-power operation was demonstrated for homojunction GaN PIN rectifiers grown on a 2-inch bulk GaN substrate using a metalorganic chemical vapor deposition (MOCVD) reactor. The GaN substrate was prepared using a near-equilibrium ammonothermal (NEAT) method. The device fabrication employed an ion-implantation isolation, field plate, and electroplated copper electrodes. The fabricated devices have dimensions up to 1.1 mm in diameter and showed uniform V on of 3.6±0.1 V and an ideality factor of 2.08. A blocking voltage (BV) of >1.2 kV and the on-state resistance (R ON A) of 0.4 mΩ·cm2 were measured. For PIN rectifiers with a dimension of 1.1 mm in diameter, we reported a BV of 1,200 V and an on-state current drive of greater than 11 A (1 kA/cm2). These results demonstrated the feasibility of using bipolar carrier transport mechanisms and PN junctions in III-N devices for power amplifications and switching circuits.

There is an error in Fig. 4 of [1] in the TE 01 optical mode profile. The error is due to the author forgetting to rescale the x-axis when plotting the optical mode profile on the right y-axis. This oversight occurred because the optical mesh is different from the electrical mesh, and the correct figure is shown below. We wish to emphasize that this was merely a plotting error, and hence all of the results and conclusions presented in the manuscript remain unchanged. We sincerely regret the error.