We present an original model for the nucleation of charge domain instabilities in semiconductor materials exhibiting negative differential drift velocity, and identify diffusion as the microscopic process which inhibits domain formation. Multiple contributing factors associated with conventional Gunn diode designs are identified which conspire to degrade negative differential drift velocity in GaN to a far greater extent than in GaAs or InP, necessitating a new paradigm for GaN‐based Gunn oscillators. A simple triode structure very similar to that of a high‐electron‐mobility transistor is proposed, and Gunn oscillation is demonstrated through stochastic solution of the Boltzmann transport equation.

Future applications for emerging AlN semiconductor electronics and optoelectronics are facilitated by emerging doping technologies enabled by low temperature, non-equilibrium epitaxy. Defect and impurity compensation can be reduced by controlling the surface chemistry with reducing compensating vacancy concentrations being a key driver for lower temperature growth. Contrary to common understanding, low temperature, metal-rich vacuum processes are shown to have higher diffusion lengths than high temperature nitrogen-rich methods. This feature can be utilized to inhibit silicon-DX center formation without compromises in crystal quality. First principles calculations identify the valence split-off band as the dominant hole band contributing to impurity band formation (as opposed to the heavy and light hole bands in other nitrides). This anomalous band structure causes an impurity band to form at dopant concentrations similar to GaN even though AlN has a deeper isolated acceptor energy and results in hole mobilities that are substantially higher than possible in GaN. AlN hole concentrations of ∼4.4 × 1018 cm−3 and 0.045 Ω cm resistivity and electron concentrations of ∼6 × 1018 cm−3 and ∼0.02 Ω cm resistivity are shown and offer substantial promise for future generations of AlN bipolar electronic and optical devices.

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.

The objective of this article is to demonstrate a general approach to the problem of designing a novel laser diode. This involves first understanding prior work in this and related fields (literature survey), and then acquiring a thorough understanding of the relevant physics. This in turn enables the development and/or selection of appropriate models and tools for that problem followed by partitioning the problem into smaller solvable chunks. This involves identifying the key contributing factors affecting poor laser performance (sources of optical loss, poor injection efficiency, high thermal resistance, etc.), quantifying and ranking their relative importance, and finally finding solutions to these problems. The final step involves putting all these pieces back together to arrive at the final laser design. This step is challenging due to the strong coupling between electrical, optical, and thermal physics in a laser diode. Any design changes must be considered through all three lenses. Herein, this approach to laser design will be demonstrated in the context of a novel III–N‐based ultraviolet vertical‐cavity surface‐emitting laser diode (UV‐VCSEL).

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.

Nitride-based vertical-cavity surface-emitting lasers (VCSELs) at wavelengths below 400 nm have drawn tremendous interest as they are potentially utilized in various applications as highly-efficient and temperature-insensitive optical sources. One example is for a compact, energy efficient, portable ytterbium-ion-based atomic clock that requires a high-stability CW optical pump at near ultraviolet (UV) wavelengths. In this work, we have used metalorganic chemical vapor deposition (MOCVD) to grow optically pumped UV VCSEL epitaxial structures and UV resonant-cavity light-emitting diodes (RCLEDs) to demonstrate several design concepts targeting the ~360-370nm UVA spectral range. The UV VCSELs are designed using a proprietary III-N device simulation tool developed at Georgia Institute of Technology. Previously, we have demonstrated a variety of optically pumped III-N VCSELs lasing at ~375 nm that showed the feasibility of building the near-UV coherent light source on GaN materials platform. The test structures consisted of a five-pair InGaN/Al0.15Ga0.85N multiple-quantum-well (MQW) active region embedded in Al0.1Ga0.9N spacers with the total optical cavity thickness of 3λ. The lowest threshold incident power density for optically pumped VCSELs was estimated to be ~270 kW/cm2 under pulsed conditions at the room temperature. In further work, we implemented a hybrid distributed Bragg reflectors (DBR) combination to demonstrate vertical resonant cavity light emitting diodes (RCLEDs) targeting at 370-nm peak emission wavelengths. The hybrid mirrors use a dielectric DBR with twelve pairs of ¼-wavelength HfO2/SiO2 layers as the topside mirror, and a semiconductor-based mirror composed of five periods of ¾λ air-gap/Al0.05Ga0.95N DBR at the bottom side of the vertical cavity. The RCLED structure comprises three distinct regions: (1) bottom DBR mirror of five-pair 3λ/4-air-gap/ Al0.045Ga0.95fN, (2) a 6λ-cavity LED active region of n-Al0.08Ga0.92N (n~8x1018cm-3) spacer/InGaN-AlGaN MQW/p-AlGaN electron blocking/p-Al0.08Ga0.92N (p~3x1017cm-3) spacer/graded p +-AlGaN, and (3) a 12-pair λ/4 HfO2/SiO2 dielectric DBR as the top-side mirror. The device fabrication employed a nitrogen ion-implantation isolation to form a current confinement aperture. The mesa-type devices have the anode and the cathode electrodes on the topside of the wafer. The air-gap DBRs were formed using optical lithography and subsequent inductively coupled plasma dry etching to create a deep trench, followed by laterally removing sacrificial layers using a conductivity-selective electrochemical etching process. Once formed, this airgap DBR provides a high reflectivity of ~99.9% covering the spectral range from 360 to 400 nm while the dielectric DBR has a reflectivity of >99% covering the range from 350 to 405 nm. The fabricated RCLEDs can be operated at a current density >50 kA/cm2 in the pulsed current mode with a peak emission wavelength of ~375 nm at the room temperature. Details of MOCVD growth, device fabrication, and characterization will be presented.

III-N VCSELs undergo severe self-heating which limits the output optical power. This makes thermal management a critical design consideration. The three most common VCSEL structures (hybrid VCSELs, flip-chip VCSELs and ELOG VCSELs) have been studied using advanced self-consistent electro-opto-thermal numerical simulations. The key geometric and material parameters affecting the thermal resistance of these devices have been identified. Our simulations suggest that some of the proposed solutions and design modifications can increase the maximum optical output power by as much 100%. This manuscript also describes the correct method of using numerical simulation in device design—to predict trends and isolate the key factors affecting device performance.

We report Monte Carlo simulation results of 300nm InP heterojunction bipolar transistors driven to exhibit distortion. IM3 distortion is typically explained by collector velocity modulation. Full-band ensemble Monte Carlo simulations implicate intervalley transfer as an additional source of distortion under conditions of high current, low voltage, and high doping. Simulations reveal that intervalley transfer promotes the formation of traveling accumulation domains which result in high frequency distortion more significant than that caused by velocity modulation. Special care must be taken when designing high current HBT power amplifiers in order to mitigate this effect.

Theoretical studies of heat generation and diffusion in Si devices generally assume that hot electrons in Si lose their energy mainly to optical phonons. Here, we briefly review the history of this assumption, and using full-band Monte Carlo simulationswith electron-phonon scattering rates calculated using the rigid-ion approximation and both empirical pseudopotentials and Harris potentialswe show that, instead, electrons lose as much as 2/3 of their energy to acoustic phonons. The scattering rates that we have calculated have been used to study hot-electron effects, such as impact ionization and injection into SiO2, and are in rough agreement with those obtained using density functional theory. Moreover, direct subpicosecond pump-probe experimental results, some of them dating back to 1994, are consistent with the predictions of our model. We conclude that the study of heat generation and dissipation in nanometer-scale Si devices may require a substantial revision of the assumptions that have been considered common wisdom so far.

Velocity overshoot in heterojunction bipolar transistors has long been recognized to significantly reduce collector signal delay below that predicted under the assumption of a constant saturated electron velocity [1], [2]. This phenomenon has proven critical for high frequency operation of InP DHBTs which has resulted in cutoff frequencies above 1 THz [3]. In order to better understand the operation of these devices, we further explore the microscopic electron dynamics through simulation. We report modulated collector signal delay results from static and dynamic full band ensemble Monte Carlo simulation of a 300nm InGaAs/InP DHBT with a 300 nm collector.

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.

Optically pumped VCSELs with a 1λ thick optical cavity lasing at 375 nm have been demonstrated using a pulsed 248 nm KrF excimer laser source. To realize a high-reflectivity mirror on the bottom of the cavity, five-period airgap/ Al0.05Ga0.95N DBRs with a large refractive index contrast have been employed while the top mirror was formed by dielectric DBRs consisting of twelve pairs HfO2/SiO2. The lowest threshold incident power density measured at room temperature was estimated to be ~270 kW/cm2. The achieved optically pumped VCSEL demonstrates the possibility that the airgap/AlxGa1-xN DBRs can be used as a mirror for injection laser 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 lambda = 375 nm using a novel hybrid-mirror 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/cm(2) with a peak emission wavelength at lambda = 375 nm and a full-width-half-maximum (FWHM) of 9.3 nm at room temperature.

InP double-heterojunction bipolar transistors (DHBTs) have demonstrated power gain cutoff frequencies (f max ) above 1THz under low collector voltage due to electron velocity overshoot in the InP drift collector [1] [2]. Under higher collector voltage, however, a quick onset of Γ-L scattering limits the average electron velocity to the saturation velocity (Fig. 1(a)-(c)), leading to a Johnson's figure-of-merit (JFOM) second to GaN HEMTs and a limited transistor power bandwidth for InP DHBTs [3]. Here we propose a velocity-engineered device structure called the segmented-collector DHBT (SC-DHBT) that incorporates p-type scattering layers within the drift collector to reduce the electron kinetic energy and force a greater electron distribution into the low effective mass Γ- valley for extended velocity overshoot (Fig. 3(a)). Transport simulations show the collector transit time τc is reduced from 1.23ps in the reference design to 0.90ps in a double scatterer design at V cb =5V, J c = 1mA/um 2 . The proposed SC-DHBT design is suited for large power bandwidth power amplifiers.

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.

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.

The current-voltage characteristics and metastability in GaN p++/n++ homojunction tunnel diodes and n++/p++/i/n tunnel-contacted diodes grown via metal modulated epitaxy have been investigated. The room temperature negative differential resistance (NDR) beginning at ∼1.35 V is reported for GaN homojunction devices grown on sapphire. The NDR vanishes, and the conductivity increases as multiple I-V sweeps are performed, thus suggesting that charge trapping states with long trap lifetimes exist at defect sites, and these traps play a crucial role in the tunneling mechanism. Additionally, the use of extremely high n-type (ND ∼ 4.6 × 1020 cm−3) and p-type (NA ∼ 7.7× 1020 cm−3) doping results in a near linear characteristic with minimal rectification at current densities less than 200 A/cm2 and soft rectification above this current density. Forward-bias tunneling and NDR are still present at 77 K. The highest silicon-doped n++/p++/i/n tunnel-contacted pin diode demonstrates a turn-on voltage of 3.12 V, only 0.14 V higher than that of the pin control diode, and an improved specific on-resistance of 3.24 × 10−4 Ω cm2, which is 13% lower than that of the control pin diode.

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 cm(2)/(V . 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.

Indium-free AlGaN-based distributed Bragg reflectors (DBRs) in the UV spectrum are known to have very low reflectivities due both to the low refractive index contrast as well as limitations imposed by the critical thickness of AlGaN alloys (tensile strain of similar to 2.41% for AlN on GaN). Near-bandedge excitonic resonances influence the real part of AlGaN's dielectric function, which sharply increases its refractive index as the photon energy approaches the bandgap. Furthermore, heavy doping (Si: 10(20) cm(-3)) can modify the plasma frequency of AlGaN, leading to a reduction in its refractive index. Hence, judiciously choosing the high index material to exploit excitonic resonances and using heavy doping to reduce the refractive index of the low index material can enhance the index contrast and enable growth of epitaxial DBRs with higher reflectivities. We have demonstrated this technique both experimentally and by simulations for wavelengths ranging from 240 to 370 nm. Typically, over 50 epitaxial pairs are needed to achieve a mirror whose reflectivity exceeds 99%, but this can be shrunk down to 20-30 epitaxial pairs by depositing silver/aluminum underneath the epitaxial DBR stack. Silver and aluminum exhibit >90% reflectivity at the AlGaN/metal interface between wavelengths ranging from >360 to 180-670 nm, respectively. A thinner DBR stack also reduces the thermal resistance, which would allow the VCSEL to achieve higher peak output powers, and simultaneously reduce overall tensile strain.