Highly resistive gallium nitride (GaN) is an essential material for power optoelectronic applications. While carbon doping is widely used to achieve semi-insulating properties in GaN, the persistent photoconductivity (PPC) arising from deep-level defect traps remains a major obstacle for high-speed power switching. This study demonstrates a novel approach: leveraging the ultrahigh photoresponsivity of GaN:C (up to 2.1 A & centerdot;cm/W & centerdot;kV, surpassing alternatives such as GaN:Fe) and employing defect-selective optical control to effectively quench the PPC. By synchronizing a short infrared (1064 nm) quenching pulse with UV (385 nm) excitation in an epitaxially grown GaN:C layer on a heavily doped n-type GaN substrate, we achieve a dramatic reduction in photocurrent fall time by approximately 293 & times; (from 470 to 1.6 mu s), increasing modulation bandwidth from 745 Hz to nearly 218 kHz. This advancement not only establishes a new pathway for controlling PPC in GaN:C but also enables the practical integration of GaN:C in fast power switching devices. Enhanced modulation bandwidth, along with GaN:C excellent photoresponsivity, makes it a promising candidate for optically controlled high-voltage, high-power electronic systems, such as photoconductive semiconductor switches (PCSSs) used in pulsed-power drivers, high-power microwave (HPM) sources, and high-voltage gate drivers for wide bandgap (WBG) power electronics.

Semi-insulating manganese-doped gallium nitride (GaN:Mn) layers epitaxially grown on unintentionally doped GaN substrates were used as photoconductors in optically addressable light valves (OALVs) to withstand higher operational laser fluences compared to current state-of-the-art OALVs where bismuth silicon oxide (BSO; Bi12SiO20) layers are used as photoconductors. GaN:Mn promises to be an exciting material for optoelectronic operations due to its large laser fluence handling capability and photoresponsivity near the band edge. The laser damage thresholds for the semi-insulating epitaxial GaN:Mn layer and the n-type substrate layer were measured to be 2.4 and 4.2 J/cm(2), respectively. These are 6-10 times higher than that of BSO (0.4 J/cm(2)). These measurements were performed by exposing similar to 200 sites on the samples to increasing fluence levels from a Gaussian pulsed Nd:YAG laser system (1064 nm) operating at a 5 Hz repetition rate with a 3 ns pulse width. Photoresponsivity of the GaN:Mn material was investigated at discrete wavelengths of 447, 405, and 380 nm. The peak photoresponsivity was observed under an illumination wavelength of 380 nm and is attributed to stronger absorption. The OALV was fabricated by attaching a 110-mu m-thick GaN:Mn layer grown on a 280-mu m-thick n-GaN layer to a 3-mm-thick BK7 optical window. A twisted nematic E7 liquid crystal was introduced to the 5 mu m gap between the two components. Transmission levels of >90% were achieved for the fabricated OALVs for a peak voltage of 40 V, constrained by transmission "bleed-through".

Whispering gallery mode resonator sensors are nondisruptive optical sensors that can detect and monitor perturbations in a gaseous environment. Through its resonant properties of peak wavelength, amplitude, and quality factor (Q factor), changes in concentration can be quantified within seconds and monitored over days with great stability. In addition, the small footprint, low cost, and high sensitivity are ideal properties for a disposable sensor that can be utilized in extreme environments. The large Q factor of the resonant cavity enables long interaction lengths and amplifies the effect of small changes in the background refractive index, which is detectable in picometer shifts of the resonance wavelength. However, this measurement is susceptible to changes in other environmental factors such as temperature, pressure, and humidity, which manifest on the picometer wavelength scale, reinforcing the need to decouple the variables. In this work, we compare the spectral response of different diameter resonators to carbon dioxide, nitrogen, and its mixtures, observing the spectral shifting and broadening of the cavity resonance near 1550 nm. In addition, the effect of environmental temperature on spectral shifting due to the thermo-optic effect is characterized and quantified. Lastly, the gas concentrations are changed in real time to showcase the tracking and recovery capabilities of the resonator sensor.

AbstractOptically addressable light valves (OALVs) are specialized optical components utilized for spatial beam shaping in various laser‐based applications, including optics damage mitigation, and enhanced functionality in diode‐based additive manufacturing requiring high intensities. Current state‐of‐the‐art OALVs employ photoconductors such as Bismuth Silicon Oxide (BSO) or Bismuth Germanium Oxide (BGO), which suffer from limited laser‐induced damage thresholds (LiDT) and inadequate thermal conductivities, thus restricting their use in high peak and average power applications. Aluminum nitride (AlN), an emerging ultra‐wide band gap (UWBG) III–V semiconductor, offers promising optoelectronic properties and superior thermal conductivity (>300 Wm−1K−1 at 298° K, compared to BSO's 3.29 Wm−1K−1). In this study, the first AlN‐based OALVs are designed, fabricated, and experimentally demonstrated using commercially available single‐crystal AlN substrates. These AlN‐based OALVs have shown clear superiority over BSO and BGO‐based devices. Design considerations for OALVs incorporating UWBG photoconductors are discussed, and the photoresponsivity from defect‐mediated sub‐bandgap absorption in AlN crystals is verified as sufficient for OALVs operating under high light fluences. The optimum driving voltage for the AlN‐based OALV is determined to be ≈ 45 Vpp at 100 Hz, achieving a transmittance of 91.3%, an extinction ratio (ER) of more than 100, and a 51:1 image contrast.

The evolution of pathogens has increased the demand for a sensing and detection platform, capable of qualifying constituents in real time. Whispering Gallery Mode Resonators provide an ideal biochemical sensing platform due to their low cost, high sensitivity, and low impact on the analyte. These resonators have high quality factors and possess the ability to detect minute changes in the local environment, as the light traveling on the surface of the resonator, when at resonance interacts with the surrounding medium for interaction lengths on the order of ~10-100cm’s . These changes in physical properties are captured through shifts of the resonance wavelength, resonance dip intensity, and/or quality factor. In this work, we provide our design of a 3-D printed microfluidic cell that is compatible with our taper and sphere coupling scheme developed from our previous work. Initially, the baseline performance of the resonator fluidic system was established by measuring the resonance wavelength shift due to refractive index change from water to phosphate buffered saline (PBS). Next, we showcase our biofunctionalization procedure and measure the accumulation of pathogens, such as E. Coli and Influenza A, on the resonator’s surface. The presence of these biological analytes results in small changes in the resonator’s diameter and refractive index, which manifests in real time as a red shift of the resonance wavelength on the picometer scale. Finally, we develop the foundation for a silicon integrated circuit chip resonator system, resulting in a further reduction of our system’s footprint.

Substrate temperature, RF power, and ICP power were investigated for their effects on GaN micropillar sidewall roughness and etch characteristics. Elevated substrate temperature was shown to improve the sidewall etch morphology at low RF powers (reduced physical bombardment) and low ICP powers (lower plasma densities). Increased lateral etching is observed with both increased ICP power and substrate temperature, which both act to increase the chemical component of the etch. Etch conditions with a high chemical driving force resulted in faceting along the a-plane on the sidewalls. This faceting produced extremely smooth surfaces with root-mean-square roughness (Rq) as low as 0.20 nm which is comparable to typical epitaxy-ready surfaces and smaller than the a-plane lattice spacing of 0.3186 nm. The smooth surfaces produced in this study enable possibilities for laser facets or for new device structures that require high quality surfaces for GaN regrowth.

This work demonstrates a novel optoelectronic device with the potential for use as a high-frequency, high-power RF source or amplifier. The device is a gallium–arsenide coplanar waveguide with a small gap in the signal trace for optical illumination. A confined charge cloud is generated by illumination through an aperture in an opaque mask over this gap. An electric field above the threshold for negative differential mobility (NDM) enables pulse compression, which prevents the charge cloud from spreading temporally during the drift process. Due to the NDM phenomenon, the output electrical pulse is temporally compressed compared to the input optical pulse. This phenomenon is demonstrated using three different experiments with varied laser pulsewidth (28–700 ps) and device geometry (50- and 100- $\mu \text{m}$ -length gaps). A 66% reduction in the full-width at half-maximum of the electrical pulse relative to the input optical pulse was demonstrated. This novel coupled optoelectronic device opens avenues for high-frequency, high-power, compact devices that could enable next-generation satellite communication systems with faster data rates and longer ranges.

The realization of vertical GaN devices requires deep plasma etching and is contingent on high mask selectivity. In this work, we show that SiO2 can be an effective mask material for deep etching GaN with GaN:SiO2 selectivities greater than 40—higher than the conventionally reported 15 for metal hard masks such as nickel. Ultrahigh SiO2 selectivities were achieved by introducing Al and AlCl into the Cl2-Ar inductively coupled plasma, which reacts with the SiO2 mask surface to form an etch-resistant aluminum silicate surface layer. This mechanism provides a low-contamination pathway to etch deep GaN microdevices.

The choice of carrier wafer was found to significantly influence etch rates, selectivity, and morphology in GaN micropillar etching in a Cl2-Ar high-density inductively coupled plasma. 7 × 7 mm2 GaN on sapphire chips with a plasma-enhanced chemical vapor deposition SiO2 hard mask was etched on top of 4-in. fused silica, silicon carbide, silicon, sapphire, aluminum nitride, and high purity aluminum carriers. Silicon and silicon carbide carriers reduced GaN:SiO2 selectivity because incidental SiClx and CClx etch products from the carriers attack the SiO2 mask. Aluminum nitride and high-purity aluminum carriers yielded the highest GaN:SiO2 selectivities due to the deposition of Al-based etched by-products, while the highest GaN etch rate was achieved using the sapphire carrier since it was the most inert carrier and did not sink any Cl2. Results indicate that SiO2 and Al may be used as passivation materials during GaN etching, as vertical profiles were achieved when SiO2 or Al is redeposited from the fused silica and aluminum carriers, respectively. Floor pitting, trenching, sidewall roughness, and faceting were all influenced by carrier wafer type and will be discussed.

Shrinking the volumetric footprint of gas sensors is desirable as it allows for nonintrusive, nonperturbing gas mixture analysis and access to tight enclosures. Micro-resonators are a perfect candidate for these sensors as their size parameter (~micron) is minimal, and the typical surface propagating whispering gallery modes can interact with an analyte without disrupting the environment. The large, quality factor (Q) of these resonant cavity modes enables long interaction lengths on the order of 100s of centimeters between the optical field and analyte. Thus, the presence of a gas different than the nominal environment will result in a shift of the resonant properties, including the resonant wavelength, amplitude, and quality factor, that can be detected in real-time. To illustrate this effect, we utilized a spherical micro resonator on the end of a piece of optical fiber, formed using standard ball lens fabrication, and excited the resonant modes using a tapered optical fiber connected to tunable Infrared laser. The resonator was fixed in contact with the tapered region of fiber, and the assembly was placed inside an in-house, optically coupled, vacuum-tight vessel for gas testing. We compared the spectral response of air, pure CO2, and pure N2 gas, observing spectral shifting and broadening of the cavity resonances. In addition, the effect of vessel temperature on resonance peak position due to the thermo-optic effect was investigated and quantified. Lastly, a feedback arm was added to the setup to reduce signal noise and automated data analysis was implemented to improve data clarity.

This paper discusses ongoing research at Lawrence Livermore National Laboratory (LLNL) that investigates the effectiveness of a whispering gallery mode micro-resonator as a biosensor. Whispering Gallery mode resonators have properties such as ultrahigh quality factors (Q factors up to 10(11)), very high power density, and small mode volume that make them suitable for sensing applications1. In this work, silica microspheres (spheres on the order of 250 mu m) are used as resonators. These resonators are coupled to a tapered optical fiber connected to an infrared laser. Using critical coupling techniques, resonant wavelengths (wavelengths of zero power transmission) are produced. The resonant wavelengths of the coupled system are dependent upon properties of the microsphere such as diameter and index of refraction. Conjugation of biological organisms to the sphere causes a small change in these properties and thus creates a shift in resonant wavelengths (free spectral range) which can be characterized and used as a sensor. This paper will discuss microsphere and taper fabrication, the tested functionalization process, and the effect conjugation has on the microsphere Q factors. Future work includes real time analysis of biological organism conjugation and bringing the sensor down to the chip sized scale.

This paper discusses ongoing research at Lawrence Livermore National Laboratory (LLNL) that investigates the effectiveness of spherical micro-resonators, coupled to a symmetrically tapered optical fiber, as a gas sensor. We will discuss silica-based microspheres and optimized tapered fiber coupling systems to detect greenhouse gases, i.e. CO2 in this context. The coupling setup is designed to be portable and amenable to different controlled environments, from constrained and controlled geometries to open and flexible enclosures. 3D-printed spherical resonator and tapered-fiber holders were made to satisfy different requirements. We produced microspheres for absorption spectroscopy of targeted gas and fabricated tapers by HF etching, using an HF-resistant fixture for safer handling and reduced waste. Detection within loose enclosures was performed as a preliminary study, where we observed spectral shift and broadening in the cavity resonances induced by the gaseous environments. Optically coupled vacuum-tight vessels have been designed and built to understand environmental effects.

In this article, we focus on the physical modeling of the nonlinear operation of intrinsic photoconductive semiconductor switches (PCSS) based on 4H-SiC using coupled electrical and optical simulations to provide performance bounds of the switch as a function of material and geometry parameters, as well as applied bias. We also conduct a full design-space exploration to identify the optimal operating and design conditions to maximize the compound metric fopPout, where fop is the maximum operating frequency, and Pout is the maximum output power the switch can provide. We quantify that a 10-μm long and 5-μm thick 4H-SiC PCSS can deliver output power density greater than 2W/mm at 150 GHz when triggered by a 0.325-μm laser with intensity of 3 kW/cm2. The output power density can be significantly enhanced by increasing the optical generation rate as well as by using thicker SiC to improve its absorption characteristics. A brief discussion of signal distortion and electrostatic screening effects at high optical bias is included. Finally, we present an analytic model of charge cloud propagation and the frequency of operation based on the physics, material parameters, and geometry of the PCSS. The model accurately captures fop of 4H-SiC PCSS over a broad range of laser spot size, device length, and electrical bias applied at the contacts.

B $$_{12}$$ P $$_{2}$$ , a wide bandgap semiconductor, has been previously reported to self-heal from high-energy electron bombardment as determined from transmission electron microscopy. However, the effect of irradiation on the electronic transport properties of B $$_{12}$$ P $$_{2}$$ has not been studied. We report electrical transport measurements on B $$_{12}$$ P $$_{2}$$ after $$\alpha $$ irradiation. The temperature-dependent Hall effect was measured on p-type B $$_{12}$$ P $$_{2}$$ after successive $$\alpha $$ irradiation from $$^{210}$$ Po over a fluence range of $$1\times \,10^{11}{-}1\times 10^{13}\,\alpha /\hbox {cm}^{2}$$ to test for self-healing of B $$_{12}$$ P $$_{2}$$ to $$\alpha $$ bombardment. Prior to irradiation, B $$_{12}$$ P $$_{2}$$ displayed electrical transport characteristics of both hopping and impurity/defect band conduction due to background impurities and growth-induced defects. Fluences of $$5\times 10^{12}\,\alpha /\hbox {cm}^{2}$$ or greater caused a decrease in the hole mobility and an increase in defect band conduction, suggesting defect accumulation in B $$_{12}$$ P $$_{2}$$ .

Most electrical characterization of radiation damage to semiconductors is conducted on full devices or on low‐doped material. However, evaluating the radiation hardness is challenging in less mature semiconductor systems where low‐doped material is unavailable and full devices are difficult to realize. Herein, temperature‐dependent Hall effect measurements are used to demonstrate α particle‐induced radiation effects in p‐type 4H‐SiC with a room temperature hole concentration of . The 4H‐SiC is irradiated by α particles from a 210Po source over a fluence of –. Modeling the hole concentration as a function of temperature shows that α radiation causes hole compensation through the introduction of hole traps. The radiation also induces a reduction in hole mobility due to an increase in defect‐related scattering centers. At low temperatures and increasingly higher fluences, the conduction mechanism changes from band conduction to another mechanism.

This paper presents the design of a SiC-based photoconductive semiconductor switch (PCSS) in which a picosecond laser pulse generates excess free electrons and holes that are rapidly separated by applying a lateral electric field across the switch. Due to the high velocity and low recombination rate of carriers in high-quality, semi-insulating SiC, the PCSS can potentially operate at terahertz frequency at 10's of watts of output power, thereby improving the frequency-power-size trade-off of high-electron mobility transistors and traveling wave tubes. The key contributions of this paper are as follows. First, we quantify the impact of material properties, doping, traps, laser spot size, and electric field on the transient response of the switch. Second, we develop a new compact model that can describe the performance of the PCSS over broad operating conditions. Excellent agreement of the model against numerical data is demonstrated. Finally, we identify an upper bound on the frequency of operation of the switch and obtain "frequency versus length" and "frequency versus laser spot size" scaling under low optical generation conditions.

For quantum computing to become fault tolerant, the underlying quantum bits must be effectively isolated from the noisy environment. It is well known that including an electromagnetic bandgap around the qubit operating frequency improves coherence for superconducting circuits. However, investigations of bandgaps to other environmental coupling mechanisms remain largely unexplored. Here we present a method to enhance the coherence of superconducting circuits by introducing a phononic bandgap around the device operating frequency. The phononic bandgaps block resonant decay of defect states within the gapped frequency range, removing the electromagnetic coupling to phonons at the gap frequencies. We construct a multi-scale model that derives the decrease in the density of states due to the bandgap and the resulting increase in defect state T1 times. We demonstrate that emission rates from in-plane defect states can be suppressed by up to two orders of magnitude. We combine these simulations with theory for resonators operated in the continuouswave regime and show that improvements in quality factors are expected by up to the enhancement in defect T1 times. Furthermore, we use full master equation simulation to demonstrate the suppression of qubit energy relaxation even when interacting with 200 defects states. We conclude with an exploration of device implementation including tradeoffs between fabrication complexity and qubit performance.

For quantum computing to become fault tolerant, the underlying quantum bits must be effectively isolated from the noisy environment. It is well known that including an electromagnetic bandgap around the qubit operating frequency improves coherence for superconducting circuits. However, investigations of bandgaps to other environmental coupling mechanisms remain largely unexplored. Here we present a method to enhance the coherence of superconducting circuits by introducing a phononic bandgap around the device operating frequency. The phononic bandgaps block resonant decay of defect states within the gapped frequency range, removing the electromagnetic coupling to phonons at the gap frequencies. We construct a multi-scale model that derives the decrease in the density of states due to the bandgap and the resulting increase in defect state $T_1$ times. We demonstrate that emission rates from in-plane defect states can be suppressed by up to two orders of magnitude. We combine these simulations with theory for resonators operated in the continuous-wave regime and show that improvements in quality factors are expected by up to the enhancement in defect $T_1$ times. Furthermore, we use full master equation simulation to demonstrate the suppression of qubit energy relaxation even when interacting with 200 defects states. We conclude with an exploration of device implementation including tradeoffs between fabrication complexity and qubit performance.

Gallium nitride (GaN) has excellent material properties for power switching applications. The availability of low-defect density bulk GaN substrates has generated increased interest in vertically-structured devices [1]-[2]. By defining the electric field vertically, devices can sustain higher voltages within a smaller device footprint when compared to lateral topologies [2]. Despite record performance in vertical bulk-based devices, vertical heteroepitaxial devices remain attractive as a low-cost alternative [3]-[7]. A key design parameter is the drift layer thickness which, to first order, determines the maximum achievable breakdown voltage. For GaN-on-Si, drift layers have been limited to ~ 3 μm due to film quality issues from lattice and thermal mismatches [4], [6]. For growth on sapphire, films can be achieved. To date, drift layers ≤ 7 μm -thick and hard breakdown voltages have been reported for p-i-n GaN-on-sapphire [3], [5]. This may be due to difficulties with high-quality deep mesa etching. Recently, we reported exceptionally smooth -deep GaN etching which enables the use of thicker drift layers and simplified processing [8]. Here, vertical p-i-n GaN-on-sapphire diodes with a 10 μm -thick drift layer are demonstrated with 1.1 kV soft breakdown (defined at 0.1 A/cm 2 ) and irreversible hard breakdown . These results are, to our knowledge, the highest breakdown voltage reported for GaN diodes on foreign substrates and were achieved without edge termination techniques due to our high-quality mesa isolation etch.