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.
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 H<sub>2</sub> 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
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
We report the structural and optical properties of Al<sub>x</sub>Ga<sub>1-x</sub>N/Al<sub>y</sub>Ga<sub>1-y</sub>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<sub>y</sub>Ga<sub>1-y</sub>N, 0.3<y<0.45, wells (nominally 2-4 nm thick) and Al<sub>x</sub>Ga<sub>1-x</sub>N, 0.55<x<1, barriers (nominally 5 nm thick). All
the structures were completed with a 10 nm thick cap layer of AlN. We observed a significant enhancement in the
cathodoluminescence intensities and longer photoluminescence lifetimes for QW structures grown in the 3D mode, as
confirmed by spotty reflection high energy electron diffraction patterns. These effects are attributed to the formation of
AlGaN quantum dots in the well materials.
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<sup>10</sup>cm<sup>-2</sup>) 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.
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 (300-400 ps) despite high defect density (>10<sup>10</sup>cm<sup>-2</sup>) 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<sup>9</sup>cm<sup>-2</sup>) AlGaN template, with PL lifetime increased by nearly a factor of two, corresponding to a defect density in the mid-10<sup>7</sup> cm<sup>-2</sup> 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 <i>p-i-n</i> 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 Γ 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<sup>16</sup> cm<sup>-3</sup>. For n-type GaN layers doped with Si, concentration Nd-Na was controlled from 10<sup>16</sup> to 10<sup>19</sup> cm<sup>-3</sup>. P-type GaN layers were fabricated using Mg doping with concentration Na-Nd ranging from 4x10<sup>16</sup> to 3x10<sup>18</sup> cm<sup>-3</sup>, for various samples. Zn doping also resulted in p-type GaN formation with concnetration N<sub>D</sub>-N<sub>A</sub> in the 10<sup>17</sup> cm<sup>-3</sup> 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<sup>7</sup> cm<sup>-2</sup> 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<sup>8</sup>cm<sup>-2</sup> 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<sup>10</sup>cm<sup>-2</sup>) 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<sup>9</sup>cm<sup>-2</sup>) AlGaN template, with PL lifetime increased by nearly a factor of two, corresponding to a defect density in the mid-10<sup>7</sup> cm<sup>-2</sup> 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.
We have used femtosecond time-resolved reflectivity and luminescence downconversion techniques to study carrier relaxation, localization, and recombination in III-nitride semiconductors. Intensity dependent, frequency degenerate pump-probe reflectivity measurements employing near-bandgap excitation provide information about initial carrier localization, subsequent ultrafast heat generation due to nonradiative recombination or trapping in states deep in the bandgap, and photoinduced absorption associated with excitation of carriers from localized states to the bands. These phenomena and their experimental signatures are illustrated for Al<sub>0.25</sub>Ga<sub>0.75</sub>N and Al<sub>0.4</sub>Ga<sub>0.6</sub>N samples, in which the photoinduced change in reflectivity <i>ΔR </i>decays faster with decreasing intensity and changes sign, with faster decays for a given intensity in the higher Al content sample. This behavior suggests that in these cases the dynamics are governed by trapping at localized states associated with alloy fluctuations that become deeper and more numerous as the Al content increases. Within this context the sign change and subsequent temporal evolution of <i>ΔR </i>may be indicative of ultrafast heat generation and/or photoinduced absorption, depending upon A1 content. Nondegenerate pump-probe reflectivity experiments designed to separate the electronic contributions of the <i>ΔR </i>decays from the slower thermal components by using a sub-bandgap probe are used to measure carrier lifetime in GaN. Comparison with data obtained from frequency degenerate experiment sunder identical excitation conditions employing a near bandgap probe indicate that in the frequency degenerate case the decay times in <i>ΔR </i>are inflated due to the presence of an additional long-lived component with the same sign as the electronic contribution. The sign and power dependence of this slow decay suggest that it may be associated with screening of a surface electric field by carriers trapped in deep states. In addition, a new technique is presented that employs luminescence downconversion using an ultrashort gating pulse to enable the characterization of UV light emission from III-nitride semiconductors with subpicosecond temporal resolution. This technique also allows one to measure PL rise times and fast components of multiple decays in the subsequent time evolution of the PL intensity. Comparison of luminescence emission intensity and lifetime in GaN and AlGaN with ~0.1 Al content grown homoepitaxially on GaN templates with the same quantities measured in heteroepitaxial layers grown on sapphire indicate significant improvement in the homoepitaxial layers due to reduction in defect density. Fast (<15 ps) initial decays in the AlGaN are attributed to localization in shallow traps associated with alloy fluctuations, with subsequent recombination through gap states.
Vertical geometry Schottky barrier photodiodes have been fabricated on n-GaN films grown by molecular beam epitaxy (MBE). Vertical mesas were fabricated by RIE and Schottky barriers were achieved by depositing Ni/Pt/Au metal contacts. I-V measurements show near ideal diode behavior, with reverse saturation current density of 1 X 10<SUP>9</SUP> A/cm<SUP>2</SUP>. Doping concentration and barrier height were determined to be 9 X 10<SUP>16</SUP> cm<SUP>-3</SUP> and 1.0V respectively, using C-V measurements. The diodes were then evaluated as UV photodiodes. The responsivity was measured to be 0.18A/W, corresponding to a quantum efficiency of 70 percent. Spectral response showed a sharp transition at 365 nm, and more than five orders of magnitude visible light rejection. Low frequency noise measurements indicate that 1/f noise is the dominant source of noise. The detectivity was determined to be 1.3 X 10<SUP>-9</SUP> W/Hz<SUP>1/2</SUP>.
In this paper, we report on the growth by molecular beam epitaxy (MBE), the fabrication and the characterization of GaN diodes on HVPE n<SUP>+</SUP>-GaN/sapphire and ELO-HVPE n<SUP>+</SUP>-GaN/sapphire substrates. Specifically, such diodes were fabricated in the form of vertical schottky diodes or p-n junctions. In both cases we have seen a dramatic decrease in the leakage current in the reverse direction which is consistent with the reduction of threading dislocations in the active area of the device. The lowest leakage current measured at -5 V bias was approximately 10<SUP>-8</SUP> A/cm<SUP>2</SUP> for p-n junctions grown on ELO-HVPE n<SUP>+</SUP>-GaN/sapphire substrates. The spectral response of the vertical schottky diodes were evaluated and compared to similar devices grown wholely by MBE on sapphire substrates. The device grown on HVPE n<SUP>+</SUP>-GaN/sapphire substrate shows nearly ideal responsivity below 355 nm but also poorer visible light rejection than the fully grown MBE device. The observed exponential tail in the spectral response of the vertical schottky grown on the HVPE n<SUP>+</SUP>-GaN/sapphire substrate is attributed to the absorption and collection in the thick n<SUP>+</SUP> GaN substrate.