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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.

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.

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.

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)

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.

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.

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.

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.

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)

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.

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.

We have developed AlGaN films deposited by plasma assisted molecular beam epitaxy (PA-MBE) that can possess enhanced internal quantum efficiency (> 30%) due to the presence of nanometer scale compositional inhomogeneities (NCI-AlGaN) within a wider bandgap matrix that inhibit nonradiative recombination through the large defect densities (> 10 10 cm -2 ) in these materials. Time- and temperature-dependent studies of the UV photoluminescence from these NCI AlGaN films as a function of growth conditions have been performed with the goal of optimizing the emission efficiency. Measurements of radiative and nonradiative lifetimes in conjunction with modeling indicate that the NCI AlGaN inherently combines inhibition of nonradiative recombination with reduction of radiative lifetime, providing a potentially higher efficiency UV emitter active region.

We report the structural and optical properties of Al x Ga 1-x N/Al y Ga 1-y N quantum wells (QWs) structures grown by gas source molecular beam epitaxy with ammonia on sapphire (0001) substrates. QWs structures consist of five pairs of Al y Ga 1-y N, 0.3 x Ga 1-x N, 0.55

Ultraviolet photoluminescence from AlxGa1-xN epilayers exhibiting strong carrier localization is time-resolved with system response down to 300 fs using optical gating by frequency downconversion in a nonlinear crystal.

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.

Femtosecond time-resolved and continuous wave optical techniques have been used to study fundamental nanoscale materials issues in III-nitride semiconductors relevant to the realization of high quality ultraviolet light emitters and photodetectors. It is demonstrated that compositional fluctuations in AlGaN active regions grown by plasma-assisted MBE can be employed to create nanoscale spatial localization that enhances the luminescence efficiency and PL lifetime (300-4(X) 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. Femtosecond, time-resolved electroabsorption measurements of nanoscale high field transport in an AlGaN/GaN heterojunction p-i-n diode show an onset of velocity overshoot at an electric field of -105 kV/cm for transport in the c-direction of wurzite GaN. Theoretical Monte Carlo calculations employing a full band structure indicate that at fields below -300 kV/cm this velocity overshoot is associated primarily with band nonparabolicity in the valley related to a negative electron effective mass. In addition, these calculations show that similar behavior is not expected for transport in the basal plane until much higher fields are attained, with important implications for the design of high power, high frequency electronics and avalanche photodetectors.

In this paper we report on the fabrication and characterization of GaN, AlGaN, and AlN layers grown by hydride vapor phase epitaxy (HVPE). The layers were grown on 2-inch and 4-inch sapphire and 2-inch silicon carbide substrates. Thickness of the GaN layers was varied from 2 to 80 microns. Surface roughness, Rms, for the smoothest GaN layers was less than 0.5 nm, as measured by AFM using 10 μm x 10 μm scans. Background Nd-Na concentration for undoped GaN layers was less than 1x10 16 cm -3 . For n-type GaN layers doped with Si, concentration Nd-Na was controlled from 10 16 to 10 19 cm -3 . P-type GaN layers were fabricated using Mg doping with concentration Na-Nd ranging from 4x10 16 to 3x10 18 cm -3 , for various samples. Zn doping also resulted in p-type GaN formation with concnetration N D -N A in the 10 17 cm -3 range. UV transmission, photoluminescence, and crystal structure of AlGaN layers with AlN concentration up to 85 mole.% were studied. Dependence of optical band gap on AlGaN alloy composition was measured for the whole composition range. Thick (up to 75 microns) crack-free AlN layers were grown on SiC substrates. Etch pit density for such thick AlN layers was in the 10 7 cm -2 range.

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.

Femtosecond UV luminescence downconversion is used to study hot carrier relaxation in homoepitaxial GaN through measurement of PL rise times for varying carrier excitation energies and densities.

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.

AlGaN samples grown by plasma-assisted molecular-beam epitaxy on sapphire (0001) substrates, with 20%–50% Al content and without the use of indium, show intense room-temperature photoluminescence that is significantly redshifted, 200–400meV, from band edge. This intense emission is characterized by a long room-temperature lifetime (∼375ps) comparable to that seen in low defect density (∼108cm−2) GaN. Room-temperature monochromatic cathodoluminescence images at the redshifted peak reveal spatially nonuniform emission similar to that observed in In(Al)GaN alloys and attributed to compositional inhomogeneity. These observations suggest that spatial localization enhances the luminescence efficiency despite the high defect density (>1010cm−2) of the films by inhibiting movement of carriers to nonradiative sites.