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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.

The voltage dependent spectral response in the 200–400 nm range was measured in n+-n--p SiC avalanche photodiodes with ultrathin n+ layers varying in the thickness from 60 to 120 nm. The unity gain responsivity in the far ultraviolet (200–250 nm) spectral region increases significantly with bias, leading to a 2–3 times enhancement at 200 nm for devices with 120 nm and 60 nm n+ layers, respectively. At the onset of gain, new spectral response peaks appear at 226 nm and 240 nm for the devices with the 60 nm and 120 nm thick n+ layers, respectively, both significantly blue shifted from the zero bias peak at 265 nm. Modeling indicates that the enhanced far ultraviolet unity gain responsivity results from the improved collection of carriers photogenerated near the illuminated surface of the device as the depletion of the n-region extends toward the surface, thereby mitigating surface recombination. The emergence of the peaks at higher bias and their larger blue shift with decreasing n+ layer thickness can be explained by the longer transit distance across the multiplication region for the holes with improved collection photogenerated near the illuminated surface, which leads to a larger enhancement in a multiplication gain with increasing bias for 200 nm photoexcitation over that for 380 nm. Devices employing the thicker 120 nm n+ layer achieve a multiplication gain greater than 5 × 106 at 12 pW 240 nm illumination, and ∼ 12 nA/cm2 dark current at gain of 1000 suitable for single photon counting.

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

III-Nitride/SiC heterostructure devices are a promising approach for improving the performance of SiC devices such as bipolar transistors and avalanche photodiodes. However, the performance of these devices should critically depend on the properties of the hetero-interface that will likely lie within an active region of the device and impact carrier transport. Importantly, intermixing at the hetero-interface can effect doping profiles in these structures as constituent atoms of each semiconductor act as a dopant in the other. In this paper we explore the impact of in situ substrate preparation and migration enhanced epitaxy (MEE) on the nucleation and impurity concentration of thin AlN films grown by plasma-assisted molecular beam epitaxy on 4H-SiC. The surface morphology of the samples were examined by atomic force microscopy and the composition of the films were studies by secondary ion mass spectroscopy (SIMS) and depth profiling x-ray photoemission spectroscopy (XPS). The MEE approach promotes the nucleation of AlN growth on SiC in a 2D mode while suppressing the migration of Al into SiC. Active N that leaks around the closed shutter during in-situ preparation prevents the nucleation of AlN in a 2D growth mode at lower substrate temperatures that is attributed to GaN island formation at the hetero-interface.

The reduction of undesirable gas phase reaction between trimethylaluminum and NH3 was achieved by spatially separating the precursors using N2 purge line during AlN growth by metal organic chemical vapor deposition (MOCVD). Under this condition, it was shown that the growth pressure has a strong impact on the surface morphology independent of pre-reaction. For 0.8-μm-thick AlN grown on (0001) sapphire substrates, increasing pressure from 200 to 500 Torr drastically increased a root-mean-squared surface (r.m.s.) roughness from 0.48 to 33 nm. This morphological change was previously attributed to the pre-reaction. Less pre-reaction also allowed us to investigate the pressure dependence of impurity (carbon and oxygen) incorporation in AlN as the growth rate was no longer affected by the pressure. Unlike GaN, the carbon level almost doubled with increasing pressure from 200 to 500 Torr. By optimizing the surface morphology (r.m.s. roughness from 33 to 0.62 nm) at 500 Torr, the carbon concentration in AlN decreased from 5 × 1018 to 7 × 1017 cm−3. Although there was no improvement in the structural quality, this uniquely designed MOCVD could further improve the material quality of AlN by reducing the impurity level.

The strain relaxation process of undoped and Si-doped AlxGa1−xN with various compositions and thicknesses on semipolar (2 0 2 ¯ 1) bulk GaN substrate was investigated via the cathodoluminescence (CL) and high-resolution x-ray diffraction (XRD) analyses. CL analyses showed that the strain relaxation occurred through three different mechanisms. At first, the relaxation was initiated by the basal-plane (BP) misfit dislocations (MDs) along [1 1 2 ¯ 0] followed by the crack formation along [1 0 1 ¯ 4] . As the Al composition and/or thickness of AlGaN layer further increased, the crack density saturated and non-basal plane (NBP) MDs almost parallel to [1 0 1 ¯ 4] started emerging due to the prismatic slip. Observed crack spacing was 40–60 μm at which the energy of exposing new surfaces from the cracks is higher than Peierls potential associated with the prismatic slip. By introducing Si (2.0–6.0 × 1018 cm−3) into the film, the formation of both BP and NBP MDs could be suppressed within our growth conditions. However, Si doping was not effective in suppressing the crack formation but there was no significant change of its density either.

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.

Strain-compensated InGaN/AlGaN structures can enable the growth of thick layers of InGaN epitaxial films far beyond the critical thickness for InGaN grown pseudomorphically to GaN. In this paper, we demonstrate the epitaxial growth of high-quality strain-compensated a-plane In0.12Ga0.88N/Al0.19Ga0.81N superlattices up to 5 times thicker than the critical thickness on free-standing a-plane GaN substrates by plasma-assisted molecular beam epitaxy (PA-MBE). The superlattices consist of 50 to 200 periods of 10 nm thick In0.12Ga0.88N and 6 nm thick Al0.19Ga0.81N layers. The structures are characterized using a double crystal X-ray diffractometer, asymmetric reciprocal space mapping, and atomic force microscopy. We use X-ray diffraction to determine the strain, composition, degree of relaxation, and superlattice period of our samples. The structural characteristics of periodic structures containing from 50 to 200 periods are compared to single layer, uncompensated In0.12Ga0.88N films. A 100 period structure exhibited only 15% relaxation compared to 69% relaxation for the bulk In0.12Ga0.88N film grown with the same total InGaN thickness but without strain-compensating layers. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

III-V materials, which exhibit high absorption coefficients and charge carrier mobility, are ideal templates for solar energy conversion applications. This work describes the photoelectrochemistry research in several IIIV/electrolyte junctions as an enabler for device design for solar chemical reactions. By designing lattice-matched epitaxial growth of InGaP and GaP on GaAs and Si, respectively, extended depletion region electrodes achieve photovoltages which provide an additional boost to the underlying substrate photovoltage. The InGaP/GaAs and GaP/Si electrodes drive hydrogen evolution currents under aqueous conditions. By using nanowires of InN and InP under carefully controlled growth conditions, current and capacitance measurements are obtained to reveal the nature of the nanowire-electrolyte interface and how light is translated into photocurrent for InP and a photovoltage in InN. The materials system is expanded into the III-V nitride semiconductors, in which it is shown that varying the morphology of GaN on silicon yields insights to how the interface and light conversion is modulated as a basis for future designs. Current extensions of this work address growth and tuning of the III-V nitride electrodes with doping and polarization engineering for efficient coupling to solar-driven chemical reactions, and rapid-throughput methods for III-V nanomaterials synthesis in this materials space.

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.

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 formation of polarization-induced charge at the hetero-interface arising from spontaneous and piezoelectric polarization can dramatically affect the performance of this detector. In this paper we report on the role of this interface charge on the performance of GaN/SiC SAM APDs.

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)

The optical and structural properties of AlGaN active regions containing nanoscale compositional inhomogeneities (NCI) grown on low dislocation density bulk AlN substrates are reported. These substrates are found to improve the internal quantum efficiency and structural quality of NCI-AlGaN active regions for high Al content alloys, as well as the interfaces of the NCI with the surrounding wider bandgap matrix, as manifested in the absence of any significant long decay component of the low temperature radiative lifetime, which is well characterized by a single exponential photoluminescence decay with a 330 ps time constant. However, room temperature results indicate that non-radiative recombination associated with the high point defect density becomes a limiting factor in these films even at low dislocation densities for larger AlN mole fractions.

Temperature dependent time-resolved photoluminescence is used to study the development of active regions for optoelectronic devices employing AlGaN nanostructures for deep-UV emission. The changing importance of dislocation versus point defects and their relationship to different forms of carrier localization are discussed. The results presented suggest that AlGaN nanostructure development for deep-UV emitters require both point defect/impurity suppression for improved efficiency and lower dislocation density for improved interface quality. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Semipolar (202¯1) InGaN/AlGaN multiple quantum wells (MQWs) with a low In content (

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 present material and device characterization of 280 nm semiconductor ultraviolet light emitting diodes. These devices exhibit low series resistance, wavelength stability with increasing current, and have a half-life in excess of 570hrs, depending upon the injection current. Time-resolved photoluminescence studies of these materials prior to fabrication have been correlated with the device performance. We also discuss the potential for use in water purification.

The use of optical gating by frequency downconversion to study UV optoelectronic devices provides information on carrier recombination dynamics with sub-picosecond resolution. Measurement of active LEDs undergoing current injection and measurement of stimulated emission from UV laser cavities is demonstrated.

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.

AlGaN epilayers grown by plasma-assisted molecular beam epitaxy and exhibiting high internal quantum efficiency (up to 30%) are incorporated into double-heterostructure devices grown on base layers of varying defect density. Growth of these AlGaN active layers, having increased emission from localization of carriers in regions of nanoscale compositional inhomogeneities, is found to benefit from base layers of reduced defect density, including thick AlGaN templates grown by hydride vapor phase epitaxy. Nonlinear radiative processes are observed at high optical excitation for layers grown on lower defect base layers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Subpicosecond time-resolved photoluminescence (TRPL) has been used to compare the room temperature carrier dynamics in Al0.1Ga0.9N/Al0.3Ga0.7N multiple quantum well (MQW) structures simultaneously deposited on a high quality free standing HVPE GaN substrate (dislocation density ∼1 × 107cm–2) and 1 µm MOCVD GaN template on sapphire. The PL lifetime of ∼500 ps in the MQW on GaN substrate is about 5 times longer than that for the MQW on GaN template, with a concomitant increase in CW PL intensity. This behavior is attributed primarily to an increase in nonradiative lifetime associated with a 100 times reduction in dislocation density in the GaN substrate. The observation that the PL lifetime in the MQW falls short of the ∼900 ps dominant decay time in the GaN substrate may be indicative of generation of additional defects and dislocations due to substrate surface preparation, strain relaxation, and nonoptimal growth temperature associated with the difference in heating of the thin GaN template on sapphire and the thick GaN substrate. An extended PL rise time of greater than 20 ps for the MQW emission when above barrier pumping is employed implies that both wells and barriers are of high quality. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An interferometric technique employing femtosecond pump-probe reflectivity measurements has been used to study the direct generation, propagation, and detection of coherent longitudinal acoustic (LA) phonon pulses in bulk GaN and AlGaN thin films at frequencies in the 100 to 200 GHz range. Photoreflectance and resonant Raman studies suggest that the dominant mechanism in the generation of the LA phonon pulse is photoexcited carrier transport-induced screening of the near-surface electric field associated with residual strain in the incompletely relaxed GaN or AlGaN epilayers. This field screening component in the strain pulse generation can be as much as two orders of magnitude larger than the deformation potential component, in contrast to theoretical calculations for the multiple quantum well (MQW) case, for which the two components are of comparable magnitude. The technique has been used to directly measure the sound velocity in GaN and AlGaN of various Al contents. The results are in good agreement with those calculated from elastic moduli.

The indium gallium nitride (InGaN) ternary alloy system offers an ideal semiconductor basis as a light absorber for driving chemical reactions such as the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). It is a direct bandgap semiconductor whose energy can be tuned for optimal solar absorption and overcoming kinetic overpotential losses, it can be grown epitaxially as a single crystal for high material quality, and it exhibits relatively high corrosion resistance in aqueous solutions for a III-V material. In this work, we report on the design, synthesis, and photoelectrochemical studies of InGaN in an anode configuration with a goal of using this material as a light absorber basis for driving solar fuels reactions such as the OER and the HER. Studies are shown reporting on how reactive vapor synthesis of GaN and InN with different structuring affects the light conversion properties and band diagram electrostatics in an electrochemical junction with ferrocene and cobaltocene redox pairs. This structure is extended to thin films of GaN and InGaN grown by molecular beam epitaxy for high efficiency light conversion, with the resulting electrochemical junction also made with ferrocene and cobaltocene. The effects of ternary alloying, doping, and the two different c-plane polarities (III-polar and V-polar) on the junction are investigated using the same electrochemical junction to evaluate the light conversion properties in relation to the electrostatics; specifically, tuning the bandgap energy for optimal light absorption, determining the c-plane polarity configuration for the electric fields to have the correct band alignment, and incorporating silicon doping to create a conductive substrate. The combination of these engineering techniques extends the depletion region of the active layer for high efficiency light absorption as similarly demonstrated for epitaxial phosphide films [1], and is utilized toward the goal of high performance solar energy conversion. [1] V. Parameshwaran, X. Xu, and B. Clemens, Journal of the Electrochemical Society, vol. 163, no. 8, pg. H714-H721 (2016).

We report on the fabrication of periodically poled AlN structures using inductive coupled plasma (ICP) etching and plasma-assisted molecular beam epitaxy (PAMBE). Periodically poled AlN structures are fabricated by first depositing Al polar material on c-plane sapphire substrates and then inverting the polarity through Mg overdoping. Subsequently, a grating with periods between 250 and 750 nm is defined by e-beam lithography and etched into the film by inductive coupled plasma etching to expose stripes of Al-polar material. ICP etching was optimized to produce vertical sidewalls and smooth Al-polar regions. Finally, the patterned substrate is regrown by PAMBE to realize a periodically poled AlN structure. The sharpness of the interface between the Al and N polar regions is found to be dependent upon ICP etch conditions and III-V flux during regrowth. Under III-rich conditions, the alignment of the grating with crystallographic orientation has an important role in the final surface morphology. While lower III-V flux ratios prevent faceting, regrowth of Al-polar material is inhibited near the sidewalls of trenches with high aspect ratios. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

We report on the fabrication and evaluation of flip-chipped double heterostructure UVLEDs operating at 320 nm that employ a bulk (80nm) AlGaN active region containing nanoscale compositional inhomogeneities (NCI), deposited by plasma-assisted molecular beam epitaxy. The devices were deposited on AlGaN templates grown by hydride vapor phase epitaxy and contain electron and hole injection layers grown by metalorganic chemical vapor deposition. A packaged, 300 μm x 300 μm device with 3% internal quantum efficiency has a peak output power of 0.56 mW at 90 mA DC current that corresponds to an external quantum efficiency of 0.15%. Nearly constant external quantum efficiency is observed for DC current density up to 100 A/cm2, suggesting that the NCI regions effectively suppress the deleterious effects of polarization fields at low injection currents that should inhibit radiative recombination in double heterostructure devices. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report structural and optical properties of AlxGa1–xN (0.4 < x < 0.5)/AlyGa1–yN (0.6 < y ≤ 1) quantum structures grown by gas source molecular beam epitaxy with ammonia on (0001) sapphire substrates. The structures are designed for light emission at ∼ 280 nm. The AlxGa1–xN (0.4 < x < 0.5) well material was grown under two dimensional (2D), three dimensional (3D), and (2D+3D) conditions by the varying group-III/ammonia ratio. The formation of nanoscale islands, or quantum dots (QDs), in the wells grown in 3D and (2D+3D) modes was observed using transmission electron microscopy. Optical properties are investigated using room temperature cathodoluminescence and time-resolved photoluminescence. Systematic studies allow us to obtain well growth conditions to produce ∼ 60 fold intensity enhancement over purely two-dimensional structures. Under these conditions, corresponding to deposition ∼ 10 monolayers of well material, we obtain emission at ∼ 280 nm with narrowest line width and longest photoluminescence decay time. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlGaN-based quantum structures were grown using gas-source molecular beam epitaxy with ammonia. Quantum structures are formed in the wells of Al0.4Ga0.6N/Al0.55Ga0.45N superlattices, as confirmed by transmission electron microscopy. Optical properties are investigated using cathodoluminescence and time-resolved photoluminescence. We obtain ~60 fold intensity enhancement over two-dimensional growth. For conditions corresponding to deposition of ~10 monolayers of well material, we obtain narrow emission at 280 nm and long ~320 ps photoluminescence decay time.

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.

High sensitivity ultraviolet (UV) avalanche photodiodes (APDs) are useful for various applications, including chemical and biological identification, optical wireless communications, and UV sensing systems. State-of-the-art SiC APDs exhibit a high peak quantum efficiency (QE) of 60% at 268 nm, but the response of these devices diminishes at longer wavelengths due to weak absorption and at shorter wavelengths 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]. Recently, we have demonstrated that heterostructure III-Nitride/SiC diodes have promise for improving the efficiency of SiC based detectors throughout the UV spectrum. However, these devices require growth techniques for initiating two-dimensional growth on lattice mismatch substrates as well as suppressing impurity migration at the hetero-interface. p- SiC In this paper we explore the impact of in situ substrate preparation and migration enhanced epitaxy (MEE) on the nucleation and impurity concentration of thin AlN films grown by plasma-assisted molecular beam epitaxy on 4H-SiC. A series of 30nm AlN films were deposited on i-p epitaxial SiC structures grown on 4 degree miscut 4H-SiC. The in-situ preparation techniques explored include annealing at 740 °C while depositing and desorbing Ga with and without N plasma active and the shutter closed. The AlN films were grown at temperatures of 600 °C and 740 °C using a standard approach without growth interruption under N limited conditions or employ an MEE, or interrupted growth approach. The surface morphology of the samples were examined by atomic force microscopy and the composition of the films were studies by depth profiling x-ray photoemission spectroscopy (XPS). Figure 1 compares the morphology and the composition profile of the AlN film grown by MEE (sample A) and the standard process (sample B) with the N plasma off during the Ga preparation. Sample A exhibits an atomically smooth, 2D surface, and a nearly stoichiometric Al to N composition in the AlN layer that drops off at the heterointerface. In contrast, sample B has a higher roughness on a 2x2 mm scale with an absence of discernable steps on the substrate. XPS results indicate a slightly larger Al to N composition in the film (no visible droplets) and a detectable presence of Al in the SiC layer indicating migration. These results suggest that MEE approach can both promote the nucleation of AlN growth on SiC in a 2D mode as well as suppress the migration of Al into SiC. Figure 2 demonstrates the impact of an active N plasma source during the Ga preparation process despite closing the source shutter on the morphology and interface composition of the AlN films. Sample C was prepared with the N plasma on and was grown at 600 °C. The film exhibits a rough surface morphology despite the use of MEE, consistent with the observations of Okumura et al. [4]. Sample D, grown under similar conditions at a higher substrate temperature of 740 °C exhibits a 2D morphology characterized by micro-steps and possible step-bunching. Sample E was grown at 600 °C similar to sample D, but with the N plasma off during Ga deposition, exhibits an atomically smooth surface morphology consistent with that of the substrate. XPS studies of sample D indicate the presence of GaN as well as metallic Ga at the hetero-interface in contrast to that of sample E. This suggests that active N that leaks around the closed shutter leads to GaN island formation that prevents the nucleation of AlN in a 2D growth mode at lower substrate temperatures. The potential partial nitridation of the SiC surface during this process does not impede the nucleation of AlN in a 2D growth mode based upon studies of films grown on substrates annealed at 740 °C with the N plasma active but the shutter closed (data not shown). Figure 1

Deep ultraviolet (DUV) avalanche photodiodes (APD) sensitive at wavelengths shorter than 260 nm have numerous applications, including chemical and biological identification and water quality monitoring. Wide bandgap materials such as silicon carbide (SiC) and the III-nitrides (AlInGaN) are well suited for UV detector applications. SiC devices have high avalanche gain, low dark current and low excess noise owing to a low k-factor (k=0.01) due to the much higher impact ionization coefficient for holes relative to electrons; however, these devices have poor response at wavelengths shorter than 260 nm related to carrier trapping by surface states. Conversely, the III-nitrides have a direct bandgap which can be engineered for efficient absorption in the DUV regime, but suffer from high leakage currents, especially for lattice mismatched growth, and difficulty in p-type doping in high AlN mole fraction alloys. To overcome these challenges, this work targets separate absorption and multiplication avalanche photodiodes (SAM-APDs) utilizing an aluminum gallium nitride (AlxGa1-xN) absorption layer and a SiC multiplication region.

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

Photoelectrochemical cells are devices that can convert solar radiation to hydrogen gas through a water decomposition process. In this process, energy is converted from incident photons to the bonds of the generated H2 molecules. The solar radiation absorption, electron-hole pair splitting, and photoelectrolysis half reactions all occur in the vicinity of the electrode-electrolyte interface. As a result, engineering the electrode material and its interaction with the electrolyte is important in investigating and improving the energy conversion process in these devices. III-V nitride materials are promising candidates for photoelectrochemical energy applications. We demonstrate solar-to-hydrogen conversion in these cells using p-type GaN and n-type InGaN as a photocathode and photoanode material, respectively. Additionally, we demonstrate heteroepitaxial MOCVD growth of GaP on Si, enabling future work in developing GaPN as a photocathode material.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

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.

We present ultraviolet pump-terahertz probe and photoconductive decay measurements on nanoscale compositionally inhomogeneous AlGaN. Results compared to photoluminescence and time-resolved photoluminescence measurements indicate luminescence from the material is limited by hole trapping.

Despite the recent progress in the development of III-Nitride semiconductor based ultraviolet light emitting diodes (UVLEDs), commercially available devices emitting at wavelengths shorter than 370 nm still suffer from poor wall plug efficiencies on the order of ~ 1%. One factor limiting the performance of these devices is the presence of a high density of dislocations that arise from the typical heteroepitaxial growth on c-plane sapphire substrates that result in non-radiative recombination. Previously we have reported on the growth of AlGaN active regions containing self-assembled nanometer scale compositional inhomogeneities (NCI-AlGaN) that demonstrate enhanced luminescence efficiency despite the presence of a large dislocation density [1]. This phenomenon is attributed to (1) the high density of NCI regions that improves the probability of carriers recombining radiatively within them rather than non-radiatively at a structural defect and (2) the subsequent concentration of carriers within the narrower band gap NCI regions that suppresses non-radiative recombination and enhances radiative efficiency due to the reduced radiative lifetime at high carrier density [2,3]. Nevertheless, time-resolved photoluminescence (TRPL) studies show that the reduction of dislocation density in these materials further improves the active region performance [4], a phenomenon that may be due to improved transport from the wider band gap matrix to the NCI and concomitant higher carrier concentration therein. Bulk AlN has significant advantages over conventional c-plane sapphire as a substrate for III-Nitride based UVLEDs including a low dislocation density (etch pit density

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.