The Army Research Laboratory (ARL) conducts a broad-based optoelectronics R and D program that addresses a number of Army applications. This program covers the full range of activities from basic materials development to component development and integration into higher levels of optoelectronic functionality. This paper addresses technology areas of interest to ARL including IR detection and imaging, IR sources, ladar, multifunction optoelectronic integration, diffractive optics, optoelectronic interconnects/processing, waveguide integrated optics, wide bandgap optoelectronics, and nonlinear optics. These areas represent a cross-section of the work conducted in the Sensors and Electron Deices Directorate of ARL. Space does not allow comprehensive discussion of the R and D program each of these technology thrust ares, but references are provided in each case so that the interested reader can pursue each of these topics further.

As the IR technology continues to advance, there is a growing demand for multispectral detectors for advanced IR systems with better target discrimination and identification. Both HgCdTe detectors and quantum well GaAs/AlGaAs photodetectors offer wavelength flexibility from medium wavelength to very long wavelength and multicolor capability in these regions. The main challenges facing all multicolor capability in these regions. The main challenges facing all multicolor devices are more complicated device structures, thicker and multilayer material growth, and more difficult device fabrication, especially when the array size gets larger an pixel size gets smaller. In the paper recent progress in development of two-color HgCdTe photodiodes and quantum well IR photodetectors is presented.

There has been significant interest in high quality growth of III-V IR materials on Si substrates for monolithic integration of the detector array with the read-out circuit. Growing the detector material on Si substrate also eliminates the need for yield lowering substrate thinning process in hybrid integration. While the large lattice mismatch between Si and these materials seems to be an important obstacle for growing device quality materials on Si, encouraging results have been achieved recently. A review of these results and successful operation of InSb p- i-n detectors on Si substrates are presented in this paper. The detector layers were grown by molecular beam epitaxy on GaAs coated Si substrates. Unintentionally doped InSb layers grown on semi-insulating GaAs substrates under similar growth conditions yielded a 77K Hall mobility of 9 by 10⁴ cm²/V-sec. Peak voltage responsivity of the detectors on Si substrates was higher than 10⁴ V/W at 77K with tunneling and shunt leakage limited zero-bias differential resistance. While there have been only few studies on the growth of III-V IR detectors on Si substrates, the recent result are encouraging for decreasing the cost and increasing the yield of IR detector systems.

Mercury Cadmium Telluride (HgCdTe) material growth, detector array fabrication and read out integrated circuit (ROIC) design and fabrication technologies have continued to advance and have led to the demonstration of high resolution, low noise and large format reliable hybrid IR Focal Plane Arrays (IRFPAs). MBE HgCdTe-based p-on-n planar heterostructure device technology has matured to a point that high performance IRFPAs are being fabricated routinely for applications in the 1-16micrometers spectral region. Control and flexibility have proven to be distinct advantages of MBE. Rapid advances in the commercial submicron Si-CMOS process continue to allow increasing functionality on ROICs. Hybrid focal pane arrays, formed by cold welding of indium columns deposited on the detector and the ROIC, are being fabricated to suit a broad range of military, civilian and scientific applications. High performance HgCdTe/CdZnTe 256 by 256, 640 by 480 and 1024 by 1024 focal plane arrays operating over a broad range of wavelengths, temperatures, and background radiation flux, have been produced. To mitigate issues associated with the thermal expansion coefficient mismatch between Si ROIC and CdZnTe substrate, growth of HgCdTe on alternate substrates, such as Si and sapphire, has been developed for large, 1024 by 1024 and 2048 by 2048 HgCdTe FPAs operating in the Short Wavelength IR (SWIR) 0.9-2.5 micrometers and mid wavelength IR 2.5-5.5 micrometers spectral bands. Simultaneous two-color IR imaging has been proven feasible, suing MBE in situ grow multilayer structures. Hybrid visible silicon imager, where detectors are processed on silicon and hybridized to the same ROIC fabricated originally for HgCdTe devices, is emerging as a competitive technology for imagin in the 0.3-1.05 micrometers spectral region. This paper provides an overview of the status of HgCdTe materials, detectors and FPA technologies at Rockwell Science Center.

Two different effects of annealing, 1) on the arsenic activation in the in-situ doped mercury cadmium telluride (HgCdTe) layers grown on silicon substrates by molecular beam epitaxy (MBE) and 2) on the CdTe passivant-HgCdTe interface leading to significant changes in the characteristics of metal-insulator-HgCdTe (MIS) and planar photovoltaic (PV) detectors are discussed here. On the arsenic activation, highly compensated n-type properties to 100 percent activation of arsenic up to a total arsenic concentration of 1-2 X 10¹⁸ cm^-3 and a decrease in activation thereafter are observed for annealing temperatures in the range of 235 to 450 degrees C. A range of annealing effects varying from unidentified structural defects acting as donors, probably due to donor arsenic tetramers or donor tetramers or donor tetramer clusters at 235 degrees C, to dissociation of bonds of neutral arsenic tetramer clusters to enable arsenic to occupy Te sites and behave as acceptors at 450 degrees C, are invoked to explain the arsenic activation mechanisms. The activation annealing, on the other hand, was found to have detrimental effect on the passivant-HgCdTe interface, possibility due to mercury diffusion during post-implant annealing. Capacitance-Voltage of MIS devices and current-voltage characteristics of planar diodes show tunneling limited performance with in-situ grown CdTe after annealing and show dramatic improvement in the performance characteristics when the in-situ grown CdTe is chemically removed and fresh CdTe passivation layer grown by MBE after arsenic activation annealing. Test structures containing mini arrays of square diodes with variable areas from 5.76 X 10^-6 cm² to 2.5 X 10^-3 cm² and MIS devices are used to establish the aforementioned effect. Under optimized conditions, state-of- the-art performance of the diodes in the mid-wavelength IR region with dynamic impedance on the order of 10⁷ Ohm- cm² is demonstrated.

We describe a mid-IR photovoltaic detector using InAsSb as active material, grown by MBE on a GaSb substrate. The purpose of this study is to show that quantum detectors can offer an alternative to thermal detectors for high temperature operation. With a 9 percent Sb content, InAsSb is lattice matched to GaSb and thus provides an excellent material quality, with Shokley-Read lifetimes of the order of 200 ns as measured by photoconductive gain measurements as well as time resolved photoconductivity experiments. The band gap of InAsSb corresponds to a wavelengths as well as time resolved photoconductivity experiments. The band gap of InAsSb corresponds to a wavelength of 5 microns at room temperature. This makes InAsSb an ideal candidate for rom temperature detection in the 3-5 microns atmospheric window. Photovoltaic structures are characterized by current voltage characteristics as a function of temperature. Using the absorption value obtained on the test samples, a detectivity of 7 by 10⁹ Jones can be obtained at a temperature of 250 K, which can easily be reached with Peltier cooling. This leads to a NETD lower than 80 mK.

IR sensing has been a key enabling technology in military systems providing advantages in night vision, surveillance, and ever more accurate targeting. Passive hyperspectral imagin, the ability to gather and process IR spectral information from each pixel of an IR image, can ultimately provide 2D composition maps of a scene under study. FInding applications such as atmospheric, and geophysical remote sensing, camouflaged target recognition, and defence against chemical weapons.

An uncooled IR imaging system that is based on thermomechanical sensing of IR radiation in conjunction with a visible optical readout has been developed. The system contains a focal plane array (FPA) consisting of bimaterial cantilever beams made of silicon nitride (SiN_x) and gold (Au) in each pixel. Absorption on incident IR radiation in the 8-14 micrometers wavelength range by SiN_x in each cantilever beam raises its temperature, resulting in proportional deflection due to mismatch in thermal expansion of the two cantilever materials. The FPA design involved maximizing the thermal resistance between the pixel and its surroundings, maximizing the thermomechanical response within the constraints of the pixel size, optimizing the pixel time response, and maximizing the IR absorption using thin film optics. Microfabrication of stress-balanced bimaterial cantilevers was achieved by varying the silicon concentration along the thickness of the SiN_x films in order to balance the residual tensile stress in the Au film and the Cr adhesion layer between Au and SiN_x. The optical readout utilized Fourier diffractive optics to simultaneously detect deflections of all cantilevers using a single light source. The results suggest that objects at temperatures as low as 30 degrees C can be imaged with the best noise-equivalent temperature difference (NETD) in the range of 2-5 K. It is estimated that further improvements that are currently being pursed can improved NETD below 5 mK.

Recent advances in micro-electro-mechanical systems (MEMS) have led to the development of uncooled IR detectors operate as micromechanical thermal detectors or micromechanical quantum detectors. We report on a new method for photon detection using electronic stresses in semiconductor microstructures. Photo-induced stress in semiconductor microstructures, is caused by changes in the charge carrier density in the conduction band and photon detection results from the measurement of the photon-induced bending of semiconductor microstructures. Small changes in position of microstructures are routinely measured in atomic force microscopy where atomic imaging of surfaces relies on the measurement of small changes in the bending of microcantilevers. Changes in the conduction band charge carrier density can result either from direct photo- generation of free charge carriers or from photoelectrons emitted from thin metal film surface in contact with a semiconductor microstructure which forms a Schottky barrier. In our studies we investigated three systems: (i) Si microstructures, (ii) InSb microstructures and (iii) Si microstructures coated with a thin excess electron-hole- pairs while for InSb photo-induced stress causes the crystal lattice to expand. We will present our results and discuss our findings.

This paper presents analysis of theoretical performance and experimental data of small size non-cooled long-wavelength multiple junction devices based on complex 2D Hg_1-xCd_xTe heterostructures. An original iteration scheme was used to solve the system of non-linear continuity equations and Poisson equation and to determine the performance of devices. All quantities are expressed as the functions of electric potential and Fermi quasi-levels. The results of calculations are presented as the maps showing spatial distribution of absorption coefficient and photoelectrical gain for two types of photodiode configurations: n⁺-on-p and p-on-n. This approach helps to understand specific features of the devices and optimize their performance. The simulations show that suitable choice of diode configuration limits reverse sign photovoltages in the contacts between p and n regions of the device. The multiple junction devise based on backside illuminated p-n photodiode were fabricated using LPE technique. The photovoltaic devices were prepared by connection in series of small area detectors. Another possibility of device technology is a stack of individual very thin photovoltaic detectors. It appears that the total thickness of the structures should be close to optimum thickness equal to 1.23/(alpha) and the thickness of lightly doped region of each element should be lower than the diffusion length. The theoretical predictions of detector performance have been compared with the experimental data.

The investigation of the InAs/Ga_1-xIn_xSb strained layer superlattice (SLS) has been largely motivated by the promise of overcoming limitations of current mature high-performance IR detectors, such as those using HgCdTe and extrinsic silicon. It also offers fundamentally superior performance over other newly emerging III-V bandgap- engineered materials such as QWIPs. The inherent properties of the InAs/GaInSb SLS have identified it as an attractive alternative for niche VLWIR applications requiring high performance under low backgrounds at operating temperatures > 40K. If this material system proves to meet the stringent demands of VLWIR applications, it will most certainly play a significant role as an alternative materials for photovoltaic focal pane arrays operating in the LWIR and MWIR regimes as well. This paper is an overview of SLS technology development, and focuses on critical development needs as seen from the perspective of the IR detector industry.

We report a set of high-quality InAs/InGaSb type-II photodetectors grown on GaSb substrates with cutoff wavelengths form 11 to 21 micrometers . The SL structural parameters were very repeatable between samples as evidenced by the consistency of the SL periods and the long wavelength photoresponse cut-off. The measured photoresponse spectra were in excellent agreement with the calculated absorption spectrum. Very low background carrier concentrations were achieved in this samples set. Based on the study, the optimum growth temperature for type-II photodetectors is between 390 to 410 C with a post growth annealing at 495 to 510 C. Thickness non-uniformity of type-II photodiodes was less than 1 percent across 2-inch wafers. We have also demonstrated photodetectors with good performance from 10 to 18 micrometers , directly grown on compliant InGaAs/GaAs substrates.

We report on the growth and characterization of type-II IR detectors with a InAs/GaSb superlattice active layer in the 15-19 micrometers wavelength range. The material was grown by molecular beam epitaxy on semi-insulating GaAs substrates. The material was processed into photoconductive detectors using standard photolithography, dry etching, and metalization. The 50 percent cut-off wavelength of the detectors is about 15.5 micrometers with a responsivity of 90mA/W at 80K. The 90 percent-10 percent cut-off energy width of the responsivity is only 17meV which is an indication of the uniformity of the superlattices. These are the best reported values for type-II superlattices grown on GaAs substrates.

Growth and characterization of type-II detectors for mid-IR wavelength range is presented. The device has a p-i-n structure is designed to operate in the non-equilibrium mode with low tunneling current. The active layer is a short period InAs/GaSb superlattice. Wider bandgap p-type AlSb and n-type InAs layers are used to facilitate the extraction of both electronics and holes from the active layer for the first time. The performance of these devices were compared to the performance of devices grown at the same condition, but without the AlSb barrier layers. The processed devices with the AlSb barrier show a peak responsivity of about 1.2A/W with Johnson noise limited detectivity of 1.1 X 10¹¹ cm X Hz^1/2/W at 8 micrometers at 80 K at zero bias. The details of the modeling, growth, and characterizations will be presented.

We analyze waveguide InP photodetectors for millimeter wave applications. We start with the PIN waveguide photodetector pointing out key problems like optical coupling, microwave access and maximum available power. To benefit form an internal gain we introduce the waveguide InP heterojunction phototransistor showing its ability to operate up to 60 GHz.

High-speed photodetectors operating at 1.3 and 1.55 micrometers are important for long distance fiber optic based telecommunication applications. We fabricated GaAs based photodetectors operating at 1.3 micrometers that depend on internal photoemission as the absorption mechanism. Detectors using internal photoemission have usually very low quantum efficiency. We increased the quantum efficiency using resonant cavity enhancement effect. Resonant cavity enhancement effect also introduced wavelength selectivity which is very important for wavelength division multiplexing based communication systems. The top-illuminated Schottky photodiodes were fabricated by a microwave-compatible monolithic microfabrication process. The top metal layer serves as the top mirror of the Fabry-Perot cavity. Bottom mirror is composed of 15 pair AlAs/GaAs distributed Bragg reflector. We have used transfer matrix method to simulate the optical properties of the photodiodes. Our room temperature quantum efficiency measurement and simulation of our photodiodes at zero bias show that, we have achieved 9 fold enhancement in the quantum efficiency, with respect to a similar photodetector without a cavity. We also investigated the effect of reverse bias on quantum efficiency. Our devices are RC time constant limited with a predicted 3-dB bandwidth of 70 GHz.

We investigate the microwave characteristics of traveling- wave photodetector using the finite-difference time-domain method. This method enables us to consider the full configuration of the device to be studied with no assumption. Therefore we obtain accurate simulation result, though the geometry of ridge-type coplanar waveguide photodetector is complicated. Physical phenomena such as attenuation and dispersion are shown in time domain. We offer two design parameters, the width of PIN region and the thickness of i-layer, and analyze TWPDs property in frequency domain. Microwave loss, characteristic impedance, and RF refractive index are shown and we find out that there exists a tradeoff condition for design.

Bandwidth of traveling-wave photodetectors is limited not only by the optical absorption coefficient and the velocity mismatches, but also by the drift time of photo-generated carriers in the i-layer. The bandwidth limitation effect of the optical absorption coefficient and the velocity mismatches have already been considered and analyzed by presenting the velocity-mismatch impulse response. In this paper, we present a modified velocity-mismatch impulse response considering the effect of the transit time. The modified impulse response is modeled by the optical waveguide analysis and the segmentation of i-layer. The frequency impulse response is also obtained by the FFT of the modified impulse response. It is found that bandwidth limitation effect of the transit time is more serious than the effect of velocity mismatch, especially at high frequency.

The impact ionization rates and coefficients are computed for Al_xGa_1-xSb avalanche photodiodes with alloy compositions near the resonance between the energy gap and the spin-orbit splitting. Realistic psuedopotential band structures and wavefunctions including spin-orbit corrections are employed to evaluate the electron and hole initiated impact ionization rates. We address seemingly contradictory experimental data regarding the presence of an enhancement in the hole to electron impact ionization coefficient ratio near the resonance between the energy gap and the spin-orbit splitting. The hole to electron impact ionization coefficient ratio shows no enhancement at high electric fields. However, an enhancement due to the resonance in the band structure is predicted for weak fields.

This paper presents the recent developments of device models for quantum dot IR photodetectors (QDIPs) and for imagers based on the integration of these photodetectors with light emitting diodes (LEDs). We derive analytical formulas for the dark current and the responsivity in QDIPs based on different QD structures and the QDIP-LED contrast transfer characteristic as functions of the structural parameters and the bias voltage. It is shown that the characteristics of QDIPs are strongly affected by the effect of electron accumulation in QDs close to the emitter contact. The main effect limiting QDIP-LED imager resolution is associated with the processes of photon reabsorption and reemission in the device LED part.

We have studied the dependence of the well doping density in n-type GaInAs/InP quantum well IR photodetectors (QWIPs) grown by low-pressure metalorganic chemical vapor deposition. Three identical GaInAs/InP QWIP structures were grown with well sheet carrier densities of 1 by 10¹¹ cm^-2, 3 by 10¹¹ cm^-2, and 10 by 10¹¹ cm^-2; all three samples had very sharp spectral response at (lambda) equals 9.0 micrometers . We find that there is a large sensitivity of responsivity, dark current, noise current, and detectivity with the well doping density. Measurements revealed that the lowest-doped samples had an extremely low responsivity relative to the doping concentration while the highest-doped sample had an excessively high dark current relative to doping. The middle-doped sample yielded the optimal results. This QWIP had a responsivity of 33.2 A/W and operated with a detectivity of 3.5 by 10¹⁰ cmHz^1/2W^-1 at a bias of 0.75 V and temperature of 80 K. This responsivity is the highest value reported for any QWIP in the (lambda) equals 8-9 micrometers range. Analysis is also presented explaining the dependence of the measured QWIP parameters to well doping density.

Multi-quantum well structures of Ga_xIn_1-xAs_yP_1-y were grown by metalorganic chemical vapor deposition for the fabrication of quantum well IR photodetectors. The thickness and composition of the wells was determined by high-resolution x-ray diffraction and photoluminescence experiments. The intersubband absorption spectrum of the Ga_0.47In_0.53As/InP, Ga_0.38In_0.62As_0.80P_0.20 (1.55 micrometers )/InP, and Ga_0.27In_0.73As_0.57P_0.43 (1.3 micrometers )/InP quantum wells are found to have cutoff wavelengths of 9.3 micrometers , 10.7 micrometers , and 14.2 micrometers respectively. These wavelengths are consistent with a conduction band offset to bandgap ratio of approximately 0.32. Facet coupled illumination responsivity and detectivity are reported for each composition.

Al_xGa_1-xN material system, whose bandgap lies in the 3.42-6.2 eV range, is extremely interesting for visible and solar blind UV photodetector applications. This paper describes the device performances of Al_xGa_1-xN UV Schottky barrier photodetectors for visible-blind applications grown on c-oriented sapphire, with a detailed balance with the basic materials properties. Conventional low temperature grown AlN or GaN were used in all applications. High quality Schottky barrier photodiodes made of Epitaxial Lateral Overgrown (ELOG) GaN are also presented. All Schottky barrier devices show a fast time response, a high UV-visible rejection factor, and high absolute values of above bandgap responsivities. A new application of AlGaN UV Schottky barrier photodetectors to monitor the biological action of the solar UV radiation, as well as the device performance of high quality GaN and AlGaN Metal Semiconductor Metal with cutoff wavelengths as short as 310 nm, are described in detail.

The detection of light in the UV portion of the electromagnetic spectrum is critical to a number of applications. Until very recently, the primary means of light detection in the UV was with either silicon photodiodes or photomultiplier tubes, both of which have serious drawbacks. With the advent of optoelectronic devices fabricated in the ternary alloy of AlGaN, the possibility exists to produce high-performance solid-state photodetector arrays sensitive to the visible-blind and solar-blind regions of the spectrum. In this paper, we discuss recent advances in the area of UV photodetectors fabricated on GaN and AlGaN. Various device structures are presented, and their peculiar characteristics discussed in terms of responsivity, dark current, gain, temporal response, and frequency response. Models describing the current transport mechanisms and the quantum efficiencies of these photodiodes are discussed. Special emphasis is given to novel device structures that improve on the temporal, spectral, and electrical characteristics of AlGaN-based photodiodes. Specifically, results for a transparent recessed-window p-i- n device, and a semi-transparent electrode device structure are described. Finally, the results of a separate absorption, charge, and multiplication avalanche photodetector are presented. This device structure resulted in a stable gain of > 10 at a reverse bias of approximately 40 V.

There is currently a strong interest in developing solid- state, UV photodetectors for a variety of applications. Some of these are early missile threat warning, covet space to space communications, flame monitoring, UV radiation monitoring and chemical/biological reagent detection. The III-Nitride material system is an excellent candidate for such applications due to its wide, reagent detection. The III-Nitride material system is an excellent candidate for such applications due to its wide, direct bandgaps and robust material nature. However, despite many inherent material advantages, the III-Nitride material system typically suffers from a large number of extended defects which degrade material quality and device performance. One technique aimed at reducing defect densities in these materials is lateral epitaxial overgrowth (LEO). In this work, we present a preliminary comparison between AlGaN UV, solar-blind p-i-n photodiodes fabricated form LEO GaN and non-LEO GaN. Improvements in both responsivity and rejection ratio are observed, however, further device improvements are necessary. For these, we focus on the optimization of the p- i-n structure and a reduction in contact resistivity to p- GaN and p-AlGaN layers. By improving the structure of the device, GaN p-i-n photodiodes were fabricated and demonstrate 86 percent internal quantum efficiency at 362 nm and a peak to visible rejection ratio of 10⁵. Contact treatments have reduced the contact resistivity to p-GaN and p-AlGaN by over one order of magnitude form our previous results.

GaN homojunction and AlGaN/GaN heterojunction UV photodiodes were successfully fabricated and tested. The p⁺/n mesa devices were grown on a n-type 6H-SiC substrate. Photoresponse was observed in these deices from 206 nm to the cutoff wavelength of GaN. Peak responsivity values of 111 mA/W and 123 mA/W were observed at 360 nm for unpackaged homojunction and heterojunction devices, respectively. In packaged device, the peak responsivity increased to 124 and 147 mA/W for the homojunction and heterojunction devices, respectively. High breakdown voltages in excess of 100 V for the homojunction and 70 V for the heterojunction devices were obtained with dark current densities of 3 by 10^-11 A/cm² and 1 by 10^-10 A/cm² A/cm² at -1V bias at room temperature, respectively. These result show that homojunction and heterojunction visible-blind detectors can be fabricated in the AlGaN/GaN material system on SiC substrates.

Among the current flame detectors, UV selective photosensor has the highest performance thanks to its high ability to follow the turn down of burners. However, its utilization is limited to industrial-use because of its lifetime of 1-2 years, operation temperature below 120 degrees C, and its high system cost including power supply of 300V and cooling. Accordingly, solid sate flame detector is strongly desired. The required elements for the UV flame detection is as follows: a) photo current given by low intensity light should be greater than dark current; b) high rejection to the background is indispensable. AlGaN was chosen through material selection and feasibility study had been conducted. For a), using pn-GaN grown on single buffer layer, the leakage current comes from both mesa sidewall leakage and tunneling current through PN junction. Both leakages were approximately 2 orders larger to detect 1nW/cm², but thought to be within outreach using multi-buffer crystal growth technique and sensor design. For b), AlGaN photocurrent fabricated on the multibuffer layer has shown responsivity difference of 3 orders at the bandage of 4.4 eV, which satisfied the high rejection in UV region of 300- 400nm.

High performance UV detectors have been fabricated using group III-nitride materials grown by molecular beam epitaxy. GaN PIN detectors exhibit near quantum efficiency limited responsivity, sharp spectral cutoff, and high shunt resistance of several hundred mega-ohms for 0.5 mm² active area devices. Comparison of PIN and Schottky devices is presented. The capabilities of group III-nitride based UV detectors is discussed in relation to suitability in UV sensing applications such as high temperature flame sensing, UV-B solar radiation monitoring, and high intensity UV dosimetry.

We report on the improved quantum efficiency of both GaN homojunction and Al_xGa_1-xN/GaN heterojunction photodiodes using a recessed window device structure. A very high quantum efficiency of 77 percent at 357 nm and also a much improved quantum efficiency at the solar blind wavelengths were achieved. A spatial non-uniformity problem on the large area devices was observed with 2D raster scan photocurrent measurements. The spatial non-uniformity is attributed to an electric field crowding effect that is primarily caused by the high resistivity of the p-GaN layer with the aid of Medici simulations.

In this paper, we report on the growth by molecular beam epitaxy (MBE), the fabrication and the characterization of GaN diodes on HVPE n⁺-GaN/sapphire and ELO-HVPE n⁺-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^-8 A/cm² for p-n junctions grown on ELO-HVPE n⁺-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⁺-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⁺-GaN/sapphire substrate is attributed to the absorption and collection in the thick n⁺ GaN substrate.

Lateral epitaxial overgrowth (LEO) has recently become the method of choice to reduce the density of dislocations in heteroepitaxial GaN thin films, and is thus expected to lead to enhanced performance devices. We present here the LEO growth and characterization of GaN films by low pressure metalorganic chemical vapor deposition. Various substrates were used, including basal plane sapphire and oriented Si substrates. The steps in the LEO growth technology will be briefly reviewed. The characterization results will be discussed in detail. The structural, electrical and optical properties of the films were assessed through scanning, atomic and transmission electron microscopy, x-ray diffraction, capacitance-voltage, deep level transient spectroscopy, photoluminescence, and scanning cathodoluminenscence measurements. Single-step and double- step LEO GaN was achieved on sapphire. Similarly high quality LEO grown GaN films were obtained on sapphire and silicon substrates. Clear and dramatic reduction in the density of defects are observed in LEO grown materials using the various characterization techniques mentioned previously.

In this paper, recent developments in the electrical characterization and doping of Al_xGa_1-xN will be reviewed. The properties important for the development of solar-blind UV photodetectors will be stressed. For many of the military and commercial applications of UV photodetectors, the photodetectors must be solar-blind with cutoff wavelengths of less than about 280 nm. This means that for devices is that for devices based on the Al_xGa_1-xN system, the aluminum mole fraction for the active region is nearly 40 percent. One of the implications for devices is that as the energy gap is increased, doping becomes much more difficult. Therefore, one of the main thrusts of this paper will be the p-type and n-type doping of Al_xGa_1-xN. In addition to the study of the doping of bulk-like Al_xGa_1-xN, the use of Al_xGa_1-xN based superlattices to reduce the dopant ionization energy will be presented. Because GaN is likely to be used for contact layers in solar-blind devices and as an active layer in visible-blind devices, the electrical properties of this better studied binary material will be reported. The role of electrically active defects and unintentional dopants will also be discussed.

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 10⁹ A/cm². Doping concentration and barrier height were determined to be 9 X 10¹⁶ cm^-3 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^-9 W/Hz^1/2.

Femtosecond nonlinear optical techniques have been employed in the study of carrier dynamics and transport in UV detector materials. Visible femtosecond pulses derived from the signal beam of a 250 kHz regenerative amplifier-pumped optical parametric amplifier were frequency doubled to obtain pulses tunable from 250 nm to 375 nm. Time-resolved reflectivity experiments indicate that the room-temperature carrier lifetime in GaN grown by double lateral epitaxial overgrowth is about 3 times longer than that of GaN grown on sapphire without benefit of this technique. The electron velocity-field characteristics and saturation velocity in GaN have been obtained form time-resolved studies of electroabsorption in a GaN p-i-n diode. The peak steady- state velocity of 1.9 X 10⁷ cm/s in this device occurs at 225 kV/cm. Time-resolved transmission measurements have been used to monitor ultrafast carrier relaxation phenomena in a thin AlGaN layer with bandgap in the solar blind region of the spectrum. Excitation intensity and wavelength dependent studies of the photoinduced bleaching decays suggest that they are primarily governed by trapping in a high density of sub-bandgap defect levels.

A low temperature plasma enhanced chemical reactionary synthesis was applied to grow of InN thin polycrystalline layers. Insensitive Ti-wire evaporation was carried out during all the time of deposition process to reduce oxygen contamination inside of reactor as well as in growing films. A wide variety of deposition parameters like plasma power, deposition time and pressure, substrate's temperature and reactionary gas flow rate were intentionally changed and studied to obtain the high quality textured InN layers up to 2000 nm of thickness. AFM and SEM investigations of natural surface morphology and fractured edge of InN layers on ceramics indicates clearly a surface texture with a pyramidal form of grains. Optical absorption and reflectance spectra of InN textured layers at room temperature in visible and near IR regions were taken to determine direct band gap energy, electron plasma resonances energy, damping constant as well as optical effective mass of electrons. Some TO and LO phonon features of InN textured layers in NIR and Raman spectra are observed and discussed.

An analysis of spike-like noise for the thermal detector prepared with Pb(Zr,Ti)O₃-Pb(Sb_0.5Nb_0.5) (PZT- PSN) pyroelectric ceramics as sensing element was conducted by measuring its oven noise as a function of the JFET characteristics, gate resistance, low-temperature heat treatment, chemical composition and grain size of pyroelectric ceramic. Pyroelectric wafers were prepared by mixed oxide technique, and thermal sensor fabricated with a PZT-PSN ceramic wafer, JFET, chip-type gate resistor and TO- 5 package with AR coated Si-window. White noises depended on the characteristics of JFET, gate resistance and chemical composition of puyroelectric materials. Output spikes originated in JFET were removed by the pyroelectric sensing wafer with high capacitance. Pyroelectric element generated the temperature-induced transient noise during cooling, which were remarkably reduced in their amplitude and frequency by heat-treating at low temperature and by decreasing the grain size of pyroelectric ceramic. The spike-like transient noise is caused by the twinning and domain switching to reduce the thermally induced elastic energy within the sensing element, originated in the different thermal expansion between pyroelectric ceramic and alumina substrate.

A novel tipping boat for liquid phase epitaxial growth of mercury cadmium telluride from Te-rich solutions has been proposed. By optimizing the growth parameters and the construction of graphite boat it was possible to obtain in situ Hg_1-xCd_xTe heterostructures. The successful fabrication of long wavelength Hg_1-yCd_yTe/Hg_1-xCd_xTe heterostructures on semi-insulating CdZnTe substrates is presented. The heterostructures consist of a thin 1-2 micrometers layer on n-type 10-15 micrometers thick HgCdTe epilayer. The characterization of double-layer heterostructures was carried out using different methods. Variations in the layer thickness were determined by microscopic examination of cleaved samples. Average composition was determined from an IR absorption measurement on the central area of the layers. Chemical analysis and the Cd, Hg and Te profile compositions at different depths of the epitaxial layers were performed using secondary ion mass spectrometry. Transport properties were measured in temperature range of 77-300 K using the Van der Pauw arrangement. The paper also reports an experimental innovation of the in situ preparation of the mercury cadmium telluride heterostructures using the tipping method from Te- rich solutions performed in a one-zone reactor.

The metal-semiconductor-metal (MSM) photodetector has the desirable attributes of large bandwidth and ease of fabrication. The lateral structure of the MSM detector allows easy incorporation into optoelectronic integrated circuits. In this paper, a new, simplified, broad-band model of the MSM detector is presented. Practical MSM detectors often exhibit an undesirable low frequency gain that is bias-dependent. It is shown in this paper that the trapping process involved in producing this gain can be modeled, in part, by including a passive equivalent circuit within the circuit model of the detector. The components of the equivalent circuit are related to the trap lifetime and the probability of a hole becoming trapped. The nonlinear effect resulting from the saturation of the electron velocity is modeled as a nonlinear current source whose magnitude is a function of bias voltage. The distributed nature of the interdigitated structure is modeled by a single set of coupled transmission lines. The trapping model and associated current sources are incorporated at the ends of the transmission lines to produce the overall model for the compete detector. Tests have shown that the proposed model provides good agreement with previously published experimental results.