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Homojunction device architectures are explored through numerical simulations to extend the response of 4H-SiC APDs broadly throughout the UV spectrum. We report on device designs and experimental validation of approaches to improve both surface and bulk carrier collection, compatible with high gain Geiger mode operation.

Photodetectors in the ultraviolet spectral range are of great interest for applications such as fluorescence-free Raman spectroscopy and non-line-of-sight optical communications. These applications require single-photon sensitivity, resulting in the use of intensified CCDs or photomultiplier tubes (PMTs) that are bulky, fragile, operate at high voltages, and/or require active cooling. Silicon carbide avalanche photodiodes (SiC APD) show promise as a compact, rugged replacement, as they exhibit low noise, durability and high gain. In this paper we report on detailed studies of p+-p-i-n SiC APDs operating in both Geiger and linear mode. The single photon detection efficiency (SPDE) of these devices was measured as a function of excess bias using a pulsed 280 nm light emitting diode source focused through a 20 µm pinhole to a spot size 20% with dark count rate (DCR) of < 700 Hz in both gated and continuous operation. Comparison with linear mode operation shows that avalanche multiplication gain in these devices exceeds 5x106 under these conditions. Examination of linear mode gain vs. applied bias dependence suggests that a sluggish dependence corresponds to a poorer SPDE, which is likely associated with parasitic resistance in these devices. This resistance is consistent with the observed inverse dependence of calculated linear mode gain with increasing optical flux. A peak SPDE of 37% was measured at an excess bias of 3.9 V with a DCR of 7.3 kHz.

Infrared (IR) technology plays a critical role in various military and civilian applications including target acquisition, surveillance, night vision, and target tracking. IR sensors and systems operating from the short-wave infrared (SWIR) to long-wave infrared (LWIR) spectra are being developed for defense and commercial system applications. Performance of these IR systems is substantially limited by signal loss due to reflection off the IR substrates and optical components. Optical coatings with high antireflection (AR) characteristics can overcome this limitation and thus enhance the performance of IR systems. We are developing and advancing high-performance antireflection (AR) coatings for a wide range of spectral bands on various substrates for a variety of defense and commercial applications. The AR coatings enhance the transmission of light through optical components and devices by significantly minimizing reflection losses, providing substantial improvements over conventional thin-film AR coating technologies. The optical properties of ARcoated optical components and sensor substrates have been measured and fine-tuned to achieve high levels of performance. In this paper, we review our latest work on robust nanostructure-based AR coatings, including recent efforts in the development of the nanostructured AR coatings on silicon and CdZnTe substrates as well as on ZnSe lenses.

We report on improvement in the deep ultraviolet (DUV) photoresponse of SiC based detectors through the development of n- AlxGa1-xN / i-p SiC heterojunction photodiodes. Fabricated photodiodes have high external quantum efficiency (EQE), greater than 60%, over a wide spectral range from 215-255 nm that is ~10x enhancement in performance over comparable homogenous SiC photodiodes at the shortest wavelength. This is attributed to photogeneration of carriers within the SiC depletion region by DUV illumination of the diode through the n- AlxGa1-xN “window”, as compared to a typical homogenous SiC n-i-p structure where the carriers are photogenerated in the n-type neutral region, resulting in more efficient collection of holes through drift

Silicon carbide Schottky diodes with thick i-regions are reported. Compared with previously reported p-i-n photodiodes, a shift of the absorption peak from 270 nm to 350 nm was observed. The responsivity curves of the Schottky diode are modeled and compared with the experimental data.Silicon carbide Schottky diodes with thick i-regions are reported. Compared with previously reported p-i-n photodiodes, a shift of the absorption peak from 270 nm to 350 nm was observed. The responsivity curves of the Schottky diode are modeled and compared with the experimental data.

Heterogeneous aluminium gallium nitride (Alx Ga1-xN)/aluminium nitride (AlN)/silicon carbide (SiC) based n-i-p photodetectors have been demonstrated to effectively tailor the spectral response of SiC within the solar-blind regime. The differences in polarization at the hetero-interfaces resulting in negative polarization induced charge at the Alx Ga1-xN/AlN interface and positive polarization induced charge at the AlN/SiC interface has been exploited to create a large barrier to carrier transport across the interface. This barrier impedes the collection of photo-excited holes in the Alx Ga1-xN layers and enables the selective collection of electrons photo-excited to the Γ and L conduction band valleys of SiC while blocking the collection of electrons in the M valley. In this work, the influence of device design, including the AlN layer thickness and Alx Ga1-xN composition, on the spectral response is discussed. Thin AlN barrier layers are easily overcome by electrons generated in all valleys of 4H-SiC with increasing bias voltage while thicker barrier layers successfully minimize the collection of electrons in the M valley of SiC and therefore suppress the long-wavelength response >260 nm. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report enhanced performance of InGaN solar cells grown by metalorganic chemical vapor deposition through optimization of the annealing of the epitaxial wafer before device fabrication. We varied the annealing environment gas mixtures as well as temperatures to obtain the optimized annealing condition. It was found that the major improvement of the nitride solar cell efficiency after annealing is in the increase of the Voc. In addition, annealing at the reasonably moderate temperature of 550°C in O2 environment results in the highest-efficiency InGaN solar cell devices compared with devices annealed at different temperatures and in different gas environments.

High sensitivity deep ultraviolet (DUV) photodetectors operating at wavelengths shorter than 280 nm are useful for various applications, including chemical and biological identification, optical wireless communications, and UV sensing systems (1). While semiconductor avalanche photodiodes (APDs) can be more compact, lower cost and more rugged than the commonly used photomultiplier tubes (PMTs), commercially available devices such as silicon (Si) single photon counting APDs have poor DUV single photon detection efficiency. In contrast, silicon carbide (SiC) APDs are ideal for high-sensitivity detection applications, as they can possess very low dark currents, small k factor, and high gain (2). However, the responsivity of these devices diminishes at wavelengths shorter than 260 nm due to increasing absorption and carrier generation in the top doped layer of this device, the short diffusion length of minority carriers in this region, and the presence of a high density of surface states.

UV light emitting diodes (UVLEDs) emitting at wavelengths shorter than 365 nm remain in the research and development stage, with external quantum efficiencies and device operating lifetimes well below that of commercially available blue LEDs. These limitations are partially attributable to the large density of dislocations in these devices that arise due to heteroepitaxial growth of high AlN mole fraction III-Nitride semiconductors and act as non-radiatve recombination centers that reduce efficiency and increase debilitating heating. An approach to mitigating this problem is to incorporate nanometer scale compositional inhomogeneities within the AlGaN (NCI-AlGaN) active region of a UVLED that can enhance the internal quantum efficiency by concentrating carriers within regions of narrow bandgap surrounded by a wider band gap matrix. In this paper we report on the growth and characterization of NCI-AlGaN alloys deposited by plasma-assisted molecular beam epitaxy. Growth under N-limited and nearly stoichiometric growth conditions promote the spontaneous formation of these NCI regions. This is attributed to lower adatom mobility of group III and N species on the AlGaN layer surface under these conditions as well as the formation of beneficial surface faceting. Optical characterization by both temperature dependent, time-resolved and time-integrated photoluminescence demonstrates the capability for NCI regions to suppress non-radiative recombination in these active regions despite the presence of a large density of defects. However, reduction of the defect density, both dislocations and impurities, improves the performance of these active regions. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we report on the characterization of n - Al 0.51 Ga 0.49 N active regions and the fabrication of ultraviolet LEDs that contain self-assembled, nanometer-scale compositional inhomogeneities ( NCI - AlGaN ) with emission at ~290 nm. These active regions exhibit reduced integrated photoluminescence intensity and PL lifetime relative to 320 nm NCI - AlGaN active regions that have significantly lower AlN mole fraction, despite having more than an order of magnitude fewer threading dislocations, as measured by transmission electron microscopy. This behavior is attributed to nonradiative recombination associated with the presence of a larger density of point defects in the higher Al content samples. The point defects are ameliorated somewhat by the lower density of NCI AlGaN regions in the higher Al content samples, which leads to a larger concentration of carriers in the NCI and concomitant reduced radiative lifetime that may account for the high observed peak IQE (~ 25%). Prototype flip chip double heterostructure-NCI- ultraviolet light emitting diodes operating at 292 nm have been fabricated that employ a 50% NCI - AlGaN active region.

We report the structural, electrical, and optical properties of deep UV light emitting diodes (LEDs) grown by gas source molecular beam epitaxy with ammonia on sapphire and AlN/sapphire template substrates. AlN/sapphire substrates were grown by stress controlled hydride vapor phase epitaxy (HVPE). LEDs based on n- and p-type AlN/AlxGa1−xN (0.05 ≤ x ≤ 0.08) superlattices are demonstrated operating to wavelengths as short as 250 nm. We report a significant enhancement in the cathodoluminescence intensities (by factor of ∼100) and photoluminescence lifetimes in the AlxGa1−xN/AlyGa1−yN superlattices consisting of well material grown in the three dimensional mode. We interpret these observations in terms of formation of quantum well/quantum dot active regions.

Interest in developing ultraviolet emitters using the III-Nitride family of semiconductors has sparked considerable effort in fabricating AlGaN alloys that exhibit enhanced luminescence based on strong carrier localization, similar to their InGaN brethren. In this paper, we report on the growth of such alloys by plasma-assisted molecular beam epitaxy (PA-MBE) without the use of indium. This enhancement is attributed to the presence of nanoscale compositional inhomogeneities (NCIs) in these materials. The emission wavelength in these materials has been tuned between 275 nm and 340 nm by varying growth conditions. The effects of dislocations on double heterostructures (DHs) that employ an NCI AlGaN active region has been investigated, with an internal quantum efficiency as high as 32% obtained for the lowest dislocation density samples (3×1010 cm−2). Prototype DH-ultraviolet light emitting diodes (DH-UVLEDs) emitting at 324 nm were fabricated employing an NCI AlGaN alloy as the active region.

This work demonstrates two novel 4H-SiC-based photodiode structures that enhance the response from ~200 nm to 260 nm by increasing the absorption of DUV photons within the high-electric-field depletion region and more efficiently collecting photo-generated carriers through drift as opposed to diffusion, despite the presence of surface recombination. In particular, the two devices discussed in this work have replaced the heavily doped, top-illuminated, n + -layer of conventional p-n - -n + diodes by a semi-transparent metal contact to create a p-n - -metal based device and by an n-type, wider bandgap AlGaN layer to create a heterojunction 4H-SiC/AlGaN p-n - -n + based device.

Temperature-dependent carrier transport is investigated using ultrafast spectroscopy in a p-GaN/i-InGaN/n-GaN solar cell with heavily-doped layers to compensate for polarization charges at the hetero-interface. We observe a flip in the transport direction at 110 K.

UVC light output of 66 mW at 300 mA CW has been achieved from LEDs on AlN substrates with extensive photon extraction. Proper vessel design allows for efficient irradiation of a water sample for purification.

We report here enhanced solar energy harvesting using a hybrid solar cell with silicon solar cells (visible-infrared light) on bottom and an InGaN solar cell (UV light) on top. The InGaN solar cell with 30 QW periods has peak external quantum efficiency (EQE) of 40 % at 380 nm, an open circuit voltage (V oc ) of 2.0 V, a short circuit current (I sc ) of 0.8 mA/cm2, and fill factor of 55%. We have demonstrated that the application of an InGaN “active window” to a silicon solar cell counterbalances the encapsulation power loss typically suffered during production of a solar panel

The development of low cost and compact biological agent identification and detection systems, which can be employed in place-and-forget applications or on unmanned vehicles, is constrained by the photodetector currently available. The commonly used photomultiplier tube has significant disadvantages that include high cost, fragility, high voltage operation and poor quantum efficiency in the deep ultraviolet (240-260nm) necessary for methods such as fluorescence-free Raman spectroscopy. A III-Nitride/ SiC separate absorption and multiplication avalanche photodiode (SAM-APD) offers a novel approach for fabricating high gain photodetectors with tunable absorption over a wide spectrum from the visible to deep ultraviolet. However, unlike conventional heterojunction SAM APDs, the performance of these devices are affected by the presence of defects and polarization induced charge at the heterointerface arising from the lattice mismatch and difference in spontaneous polarization between the GaN absorption and the SiC multiplication regions. In this paper we report on the role of defect density and interface charge on the performance of GaN/SiC SAM APDs through simulations of the electric field profile within this device structure and experimental results on fabricated APDs. These devices exhibit a low dark current below 0.1 nA before avalanche breakdown and high avalanche gain in excess of 1000 with active areas 25x larger than that of state of the art GaN APDs. A responsivity of 4 A/W was measured at 365 nm when biased near avalanche breakdown.

A III-Nitride/silicon carbide (SiC) separate absorption and multiplication avalanche photodiode (SAM-APD) offers a novel approach for fabricating high gain photodetectors with tunable absorption over a wide spectrum from the visible to deep ultraviolet. However, unlike conventional heterojunction SAM-APDs, the formation of a polarization-induced charge at the heterointerface arising from spontaneous and piezoelectric polarization can dramatically affect the performance of this detector. In this report, we discuss the role of this interface charge on the performance of gallium nitride (GaN)/SiC SAM-APDs through simulations of the electric field profile within this device structure and experimental results on fabricated APDs. These devices exhibit a low dark current below 0.1 nA before avalanche breakdown and high avalanche gain in excess of 1000 with active areas 25 times larger than that of state-of-the-art GaN APDs. A responsivity of 4 A/W was measured at 365 nm when biased near avalanche breakdown.

III-Nitride/SIC separate absorption and multiplication avalanche photodiodes (SAM APDs) provide a new approach for realizing high sensitivity, high gain and low dark current detectors with a response that is tunable over a wide spectral range. However, the heterojunction interface plays a significant role in the performance of these detectors due to presence of 1) interface charge arising from the difference in polarization between III-Nitride and SiC as well as 2) defects at the interface resulting from lattice mismatch. In this paper we discuss the growth and fabrication of GaN/SiC SAM APDs and analyze the behavior in the context of these two factors.

Time-resolved photoluminescence and transmission electron microscopy results suggest that the density of point defects may have a more significant role than threading dislocations in the performance of UVLED AlGaN active regions emitting at shorter wavelengths.

The feasibility of attaining improved explosives detection and identification using complementary InN-based time-domain terahertz spectroscopy (TDTS) and laser-induced breakdown spectroscopy (LIBS) techniques employing 1550-nm femtosecond (fs) pulse technology has been investigated. The use of very low-energy, ultra-short, near-infrared laser pulses for both TDTS and LIBS has been demonstrated. Novel approaches exploiting polarization fields in wurtzite nitride semiconductors such as InN and GaN led to an enhancement of TMz generation by more than a factor of 3 relative to more conventional sources fabricated from these materials. These results could lead to improved TMz sources at fs fiber laser wavelengths of interest, with further advances in materials quality. Threshold LIBS pulse energies used in this investigation are the lowest reported to date, and gated detection is not required, potentially leading to an overall simpler and inexpensive system more amenable to field use. Finally, the use of low pulse energies in these studies currently available from more compact mode-locked fiber laser systems indicates potential for incorporation in field-deployable explosives-detection platforms.

Time-resolved photoluminescence studies of nitride semiconductors and ultraviolet light emitters comprised of these materials are performed as a function of pump intensity as a means of understanding and evaluating device performance. Comparison of time-resolved photoluminescence (TRPL) on UV LED wafers prior to fabrication with subsequent device testing indicate that the best performance is attained from active regions that exhibit both reduced nonradiative recombination due to saturation of traps associated with point and extended defects and concomitant lowering of radiative lifetime with increasing carrier density. Similar behavior is observed in optically pumped UV lasers. Temperature and intensity dependent TRPL measurements on a new material, AlGaN containing nanoscale compositional inhomogeneities (NCI), show that it inherently combines inhibition of nonradiative recombination with reduction of radiative lifetime, providing a potentially higher efficiency UV emitter active region.

An HVPE-grown n-InGaN/p-GaN single heterojunction LED with p-side down and emission at ~ 480nm has been demonstrated. Benefits of the p-down geometry for such an LED associated with polarity are discussed.

Optical characterization of nitride semiconductors and device testing of ultraviolet emitters and detectors comprised of these materials are employed in addressing the challenges faced in developing semiconductor-based, compact, low-cost, low-power-consumption biodetection systems. Comparison of time-resolved photoluminescence (TRPL) on UV LED wafers prior to fabrication with subsequent device testing indicate that the best performance is attained from active regions that exhibit both reduced nonradiative recombination due to saturation of traps associated with point and extended defects and concomitant lowering of radiative lifetime with increasing carrier density. Temperature and intensity dependent TRPL measurements on a new material, AlGaN containing nanoscale compositional inhomogeneities (NCI), show that it inherently combines inhibition of nonradiative recombination with reduction of radiative lifetime, providing a potentially higher efficiency UV emitter active region. In addition, testing of GaN avalanche photodiodes (APDs) on low defect density bulk GaN substrates indicates that for the first time GaN APDs with diameters as large as 50 microns exhibit reproducible gain greater than 1000. These results show promise for replacement of photomultipliers in biodetection systems.

AlGaN samples show intense room temperature photoluminescence from localized states that is significantly red-shifted, 200-400 meV, from band edge. Double-heterostructure light-emitting diodes were fabricated using this unique layer, with electroluminescence peak at 325 nm.

We have used subpicosecond time-resolved photoluminescence (TRPL) downconversion techniques to study the interplay of carrier localization and radiative and nonradiative processes in the active regions of light emitting III-nitride semiconductor ultraviolet optical sources, with the goal of identifying potential approaches that will lead to higher radiative efficiency. Comparison of TRPL in (In)AlGaN multiple quantum well active regions indicate that for addition of only 0.01 In content the PL decay time in an InAlGaN MQW is more than double that in an AlGaN MQW designed to emit at the same wavelength (360 nm), thus indicating the importance of indium for improvement of material quality, most likely through the suppression of point defects. This result is further underscored by TRPL data on 320 nm InAlGaN MQW active regions, which exhibit longer PL lifetimes than expected for growth on GaN templates with dislocation densities in the mid-10 8 cm -2 range. While the PL lifetimes in these InAlGaN MQWs improve for growth on lower dislocation density HVPE bulk GaN substrates, a similar phenomenon is not observed for deposition on nearly dislocation-free bulk AlN substrates, suggesting that defect generation in the MQWs associated with lattice mismatch or AlN surface preparation may play an important role. The pump intensity dependence of the time zero signal and the TRPL decays in the MQWs implies that internal electric field-induced recombination through the barriers and interface states plays an important role in the radiative efficiency of quantum well active regions for c-axis oriented materials and devices. The effect of these internal electric fields can be mitigated through the use of nonpolar MQWs. The combination of more intense time-integrated PL spectra and shorter PL lifetimes with decreasing well width in GaN/AlGaN MQWs grown on a-plane LEO GaN for low pump intensity suggests that the radiative lifetime becomes shorter due to the accompanying increase in exciton binding energy and oscillator strength at smaller well width in these high quality samples. Finally, it is demonstrated that compositional fluctuations in AlGaN active regions grown by plasma-assisted MBE can be employed to create spatial localization that enhances the luminescence efficiency and PL lifetime (300-400 ps) despite high defect density (>10 10 cm -2 ) by inhibiting movement of carriers to nonradiative sites. Significant enhancement of this phenomenon has been obtained in a DH LED structure grown on a lower defect density (mid-10 9 cm -2 ) AlGaN template, with PL lifetime increased by nearly a factor of two, corresponding to a defect density in the mid-10 7 cm -2 range, and only a 3.3 times drop in PL intensity when the temperature is raised from 12 K to room temperature, suggesting up to ~ 30% internal quantum efficiency.

High sensitivity deep ultraviolet (DUV) photodetectors sensitive at wavelengths shorter than 280 nm are useful for various applications, including chemical and biological identification, optical wireless communications, and UV sensing systems. State-of-the-art SiC APDs, employing a recessed top window, exhibit a high peak quantum efficiency (QE) of 60% at 268 nm, but the response of these devices diminishes at shorter wavelength due to increasing photo-generation of carriers in the top doped layer of this device, the short diffusion length of minority carriers in this doped region and the presence of a high density of surface states [1]. In this paper we explore two approaches for increasing the DUV photoresponse of 4H-SiC photodiodes by increasing the collection of DUV- generated carriers near the surface through drift within the high E-field region. As a result, the photo-generated carriers are more efficiently collected and the impact of surface state recombination is reduced. These approaches include replacing the illuminated top doped SiC layer of the diode with (1) a transparent metal contact or (2) a doped wider band gap III-Nitride semiconductor structure that is transparent in the spectral range of interest. Figure 1 shows the measured unity gain spectral response of a p-n--metal 4H-SiC diode reverse biased at 30V and consisting of a 2 µm-thick p +-SiC layer doped 2x1018 cm-3, a 350 nm-thick n- -SiC layer lightly doped 5x1015 cm-3, and employing a thin 5 nm transparent Ni metal top contact (broken red line). The response for a typical p-n--n+-SiC diode consisting of a 2 µm thick p +-SiC layer doped 2x1018 cm-3, a 480 nm thick n- -SiC layer lightly doped 5x1015 cm-3, and a 100 nm thick n +-SiC layer doped 4.5x1018 cm-3 is provided for comparison (solid-black line). A nearly flat QE between 40-45% is observed for the p-n--metal detector between 200- 270 nm that exceeds the performance of the conventional p-n--n+ diode for wavelengths shorter than 235 nm despite absorption losses within the metal contact across the spectral range of ~40-50%. This enhancement is attributed to improved collection of photogenerated carriers through drift within the high E-field region despite the presence of surface states. This conclusion is supported by modeling the expected photocurrent in the p-n- -metal (red open circles) and conventional p-n--n+ (black open squares) diodes. An alternative approach that can reduce absorption losses in the spectral range of interest is to employ a transparent III-Nitride semiconductor n-type doped layer. Figure 2 shows the measured spectral response of a photodiode having a 2 µm-thick p +-SiC layer doped 2x1018 cm-3, a 350 nm-thick n- -SiC layer lightly doped 5x1015 cm-3, a 10 nm-thick AlN layer, and a 100 nm-thick Al0.85Ga0.15N layer. The devices exhibit a peak unity gain QE of 55-58% between 265-250 nm that diminishes at shorter wavelength due to absorption in the top AlGaN layer. Increasing reverse bias above 40V results in an increase in response peaking around ~ 250 nm that is associated with inhomogeneous gain at wavelengths shorter than 255 nm (Figure 2- top). This is attributed to increased absorption associated with valence to higher conduction band transitions in SiC that ensures that these photons are absorbed near the interface between SiC and AlN, where photo-generated holes will have maximum transit distance through the depletion region as well as the larger impact ionization rate for holes over electrons for SiC. Calculations of the expected gain vs. wavelength within a p-i-n SiC diode illuminated from the n- region for varying magnitudes of E-field in the depletion region show similar trends consistent with this interpretation (Figure 2 inset). [1] Liu H., Mcintosh, D., Bai, X., Pan, H, Liu, M., Campbell J. C., and Cha H, IEEE Photonic Technol. Lett. 20, 1551 (2008).

Heterogeneous n-III-nitride/i-p silicon carbide (SiC) photodetectors have been demonstrated that enable the tailoring of the spectral response in the solar blind region below 280 nm. The negative polarization induced charge at the aluminum gallium nitride (AlxGa1−xN)/aluminum nitride (AlN) interface in conjunction with the positive polarization charge at the AlN/SiC interface creates a large barrier to carrier transport across the interface that results in the selective collection of electrons photoexcited to the Γ and L valleys of SiC while blocking the transport of electrons generated in the M valley. In addition, the AlxGa1−xN alloys act as transparent windows that enhance the collection of carriers generated by high energy photons in the fully depleted SiC absorption regions. These two factors combine to create a peak external quantum efficiency of 76% at 242 nm, along with a strong suppression of the long-wavelength response from 260 nm to 380 nm.

p-side-down, single heterostructure n-InGaN/p-GaN light emitting diodes grown by molecular beam epitaxy exhibited stable peak emission wavelengths as long as 550 nm for current densities in excess of 100 A/cm2, and minimal efficiency droop up to 150 A/cm2 without the use of an electron blocking layer. This behavior is consistent with the formation of a two-dimensional hole gas in the n-InGaN layer and a higher barrier to electron overflow in the conduction band due to the negative polarization charge at the n-InGaN/p-GaN interface.

A p-type interface charge control layer (PICCL) is introduced that enables optimization of the net positive polarization induced interface charge at the GaN/SiC interface in GaN/SiC separate absorption and multiplication avalanche photodiodes (SAM-APDs) by varying its thickness. The response from SAM-APDs with PICCL thickness less than 10 nm has an anomalous shape at all bias attributed to the collection of carriers generated directly in the SiC layer. Devices with a 15 nm thick PICCL exhibit GaN related response at high bias that is indicative of punch-through of the electric field into the GaN absorption region due to optimization of the net interface charge.

Near ultraviolet-sensitive separate absorption and multiplication avalanche photodiodes with GaN/SiC epitaxial layers grown on SiC substrate were fabricated. Dark current < 1 pA at 90% breakdown voltage, maximum multiplication gain of ∼105, and responsivity exceeding 4.2 A/W at 365 nm were achieved.

The incorporation of nanometer scale compositional inhomogeneities in AlGaN has a great promise for improving the performance of UV light emitting diodes based on these materials by suppressing nonradiative recombination associated with dislocations. In this article, the authors report on the growth and characterization of AlGaN containing nanometer scale compositional inhomogeneity (NCI-AlGaN) alloys deposited by plasma-assisted molecular beam epitaxy. Growth under N-limited and nearly stoichiometric growth is observed to enhance the photoluminescence intensity in AlGaN alloys by promoting the spontaneous formation of these NCI regions. This is attributed to the lower adatom mobility of group III and N species on the AlGaN layer surface under these conditions as well as the formation of beneficial surface faceting.

Lifetime measurements on single-chip, packaged 285 nm light-emitting diodes (LEDs) performed under constant current injection at 20 and 75 mA, were compared to the performance of unbiased LEDs baked at the equivalent operating junction temperatures. The thermally stressed devices showed a lesser degradation than those electrically stressed, indicating that elevated temperature alone does not cause degradation. Despite a decay to less than half of the initial power under current injection, time-resolved photoluminescence of the active region exhibits little change, while capacitance-voltage measurements imply that the reduced efficiency and power decay originate from the generation of point defects near the p-side of the p-n junction.