Narrow gap IV-VI (e.g. Pb_1-xSn_xSe and PbTe) layers grown epitaxially on Si(111)-substrates by MBE exhibit high quality despite the large lattice and thermal expansion mismatch. A CaF₂ buffer layer is employed for compatibility. Due to easy glide of misfit dislocations in the IV-VI layers, thermal strains relax even at cryogenic temperatures and after many temperature cyclings. This is partly due to the NaCl-structure of the IV-VI materials and at variance to the zinkblende-type semiconductors. In addition, the high permittivities of the IV-VIs effectively shield the electric fields from charged defects. This makes the materials rather forgiving, higher quality devices are obtained from lower quality material, again at variance to Hg_1-xCd_xTe or InSb and related compounds. We describe ways to further improve device performance by lowering the dislocation densities in the lattice mismatched layers. This is achieved by temperature rampings, which drive out the threading dislocations from the active parts of the sensors. Presently, densities of 1 X 10⁶ cm^-2 in layers of a few micrometer thickness are obtained. These densities are sufficiently low in order not to dominate the leakage currents in real devices even at 77 K. Photovoltaic p-n or Schottky- barrier sensor arrays are delineated by using photolithography. At low temperatures, the ultimate sensitivities are presently limited by defects, mainly dislocations. At higher temperatures, the ultimate theoretical sensitivity was obtained with Schottky barrier devices, this despite the large mismatch and only 3 micrometer thickness of the layers. Due to the rather low temperatures used during the MBE and delineation (below 450 degrees Celsius), sensor arrays are obtained by postprocessing even on active Si-substrates.

Recent advances in metalorganic vapor phase epitaxy (MOVPE) of HgCdTe are reviewed that have impacted in situ growth of bandgap engineered IR detector devices. MOVPE can now be readily used to grow multilayer HgCdTe structures with complete flexibility in iodine donor and arsenic acceptor doping and tight control of alloy composition. 100% activation of both donor and acceptor dopants has been achieved and the mobilities and lifetimes of HgCdTe grown on lattice-matched CdZnTe are comparable to the best values achieved in HgCdTe by any epitaxial growth technique. The defects measured by etch pit density counts in multilayer structures with n-type and p- type regions are reported. Single-band IR detector device results are reported that have been grown in situ, for operation in the long wavelength (LW, 8 - 12 micrometer) and medium wavelength (MW, 3 - 5 micrometer) IR spectral bands. Their material characteristics and detector performances are reviewed and compared with theoretical modeling results.

We analyze numerically properties of small-size infrared photovoltaic devices based on complex two-dimensional Hg_{1- x}Cd_xTe heterostructures. An original iteration scheme was used to solve the system of nonlinear continuity equations and the Poisson equation. All quantities are expressed as functions of electric potential and Fermi quasi-levels. The results of calculations are presented as the maps showing spatial distribution of sensitivity and density of noise generation for 4 types of heterostructures. In addition, resulting parameters of the devices are summarized in the table. This approach may help to understand specific features of the heterostructural devices and optimize their performance. The simulations show viability of constructing devices with active region buried inside a wide gap material where existing potential barriers prevent adverse effects of both recombination of photogenerated carriers and thermal generation at surfaces, interfaces and contacts.

Mechanisms of incorporation of native defect and dopants in HgCdTe alloys are reviewed. Origin of the native defect related deep centers in limiting the minority carrier lifetime is explored. Primary and secondary mechanisms operative in the activation of n type and p type dopants in HgCdTe are discussed along with implications for fabrication of high performance detectors.

The quality of Hg_1-xCd_xTe epitaxial layers made of is strongly dependent on the crystalline properties of the substrate. The chemical compatibility and the small lattice mismatch between Hg_1-xCd_xTe and CdTe have been primary motivation for choosing CdTe as a substrate for Hg_1-xCd_xTe epitaxial layers. Nevertheless, the most important issue of epitaxial layers on the CdTe substrate is conditioned by lattice mismatch. In order to eliminate these problems we have replaced the CdTe substrate by Cd_0.95Zn_0.05Te which are lattice-matched to the Hg_1-xCd_xTe compound. In this work we conduct systematic experimental study of the two type of substrates: (111)B Cd_0.95Zn_0.05Te and (211)B Cd_0.95Zn_0.05Te. The (111)-oriented substrate remains attractive as the growth results in flat, featureless surfaces with excellent interfaces with lattice matched substrate. The (211)-oriented substrates combine the structural quality of (100) including the absence of twinning with the flat topography of (111)-oriented wafers. The Hg_1-xCd_xTe epilayers were grown from Te-rich solutions on (111)Cd_0.95Zn_0.05Te and (211)Cd_0.95Zn_0.05Te by a horizontal tipping liquid phase epitaxy technique. Characterization of the epilayers involved FTIR spectroscopy to determine both its thickness and composition. LPE film surface morphology was examined using microscope equipped with Nomarski phase contrast and atom force microscopy. The as- grown or annealed layers were measured by Hall effect at 300 and 77 K using Au or indium contacts.

Infrared sensor technology is critical to many commercial and military defense applications. Traditionally, cooled infrared material systems such as indium antimonide, platinum silicide, mercury cadmium telluride, and arsenic doped silicon (Si:As) have dominated infrared detection. Improvement in surveillance sensors and interceptor seekers requires large size, highly uniform, and multicolor IR focal plane arrays involving medium wave, long wave, and very long wave IR regions. Among the competing technologies are the quantum well infrared photodetectors based on lattice matched or strained III-V material systems. This paper discusses cooled IR technology with emphasis on QWIP and MCT. Details will be given concerning device physics, material growth, device fabrication, device performance, and cost effectiveness for LWIR, VLWIR, and multicolor focal plane array applications.

Because of the isotropic energy band structure of the (Gamma) electrons in GaAs/AlGaAs quantum well infrared photodetector (QWIP), normal incident radiation absorption is not possible so that the optical grating and its optimization become key requirements for the QWIPs. In this work we study the optical grating structure based on Huygen's principle and Kirchhoff's formula. The theory developed in this paper is used to design the optical grating for our GaAs/AlGaAs QWIPs with responding wavelength around 8.0 micrometer. The result for the photoconductive QWIPs of 1 X 128 focal plane array (FPA) working at 80 K is presented. The theoretical model has also been used to design the grating size for the 64 X 64 FPA which results in a good FPA device performance around 8.0 micrometer.

Generation-recombination noise in quantum well infrared photodetectors is considered theoretically. A numerical model and procedure for noise power calculation is proposed. It is shown that the transient response current due to extra charge generation from a QW is not described by a simple exponential model. Transient current is composed of fast and slow components. The autocorrelation coefficient is also non- exponential, having two components with different time scales and amplitudes. The noise power spectral density has a strong frequency dispersion at low frequencies, due to the modulation of the injection current from the emitter. Analytical model for non-equilibrium high-frequency noise is proposed.

A 15-period GaAs/Al_0.3Ga_0.7As superlattice infrared photodetector (SLIP) is presented. A 1500 Angstrom wide Al_0.22Ga_0.78As blocking barrier is introduced to reduce the dark current. The measured noise power spectral density of dark current at 77 K shows characteristics of i_n² equals 2qI_d rather than the generation-recombination noise. The spectral response is between 8 - 10 micrometer with (lambda) _p equals 9.3 micrometer. Background limited performance can be achieved up to 65 K. Assuming the noise of background photocurrent to be 2qI_pc, we estimate D*_BLIP equals 1.5 X 10¹⁰ cm(root)Hz/W. At 77 K, the detectivity is D* equals 3.5 X 10⁹cm(root)Hz/W with the bias voltage of 0.1 V. In comparison with conventional QWIPS, our SLIP has the advantage of higher performance at low bias voltage and lower power consumption.

Typical quantum well infrared photodetectors (QWIPs) exhibit rather narrow spectral bandwidth of 1 to 2 micrometer. For certain applications, such as spectroscopy, sensing of a broader range of infrared radiation is highly desirable. In this work, we report the design of five broadband (BB-) QWIPs sensitive over the 7 to 14 micrometer spectral range. Three n- type BB-QWIPs consisting of three, four, and five quantum wells of different thickness and/or composition in a unit cell, which are then repeated 20 times for the three and four quantum wells (QW) devices and 3 times for the five QWs device to create the BB-QWIP structures, are demonstrated. The three- well n-type In_xGa_1-xAs/Al_yGa_1-yAs BB-QWIP is designed to have a response peak at 10 micrometer, with a FWHM bandwidth that varies with the applied bias. A maximum bandwidth of (Delta) (lambda) /(lambda) _p equals 21% was obtained for this device at V_b equals -2 V. The four- well n-type In_xGa_1-xAs/GaAs BB-QWIP not only exhibits a very large responsivity of 2.31 A/W at 10.3 micrometer and V_b equals +4.5 V, but also achieves a broader bandwidth of (Delta) (lambda) /(lambda) _p equals 29% than the three-well device. The five-well n-type In_xGa_{1- x}As/GaAs BB-QWIP has achieved a FWHM bandwidth of (Delta) (lambda) /(lambda) _p equals 28% at V_b equals 1.75 V. In addition, two p-type In_xGa_1-xAs/GaAs BB-QWIPs with variable well thickness and composition, sensitive in the 7 - 14 micrometer spectral range, are also demonstrated. The variable composition p-type BB-QWIP has a very large FWHM bandwidth of (Delta) (lambda) /(lambda) _p equals 48% at V_b equals -1.5 V and T equals 40 K. The variable thickness p- type BB-QWIP was found to have an even broader FWHM bandwidth of (Delta) (lambda) /(lambda) _p equals 63% at V_b equals 1.1 V and T equals 40 K, with a corresponding peak responsivity of 25 mA/W at 10.2 micrometer. The results reveal that p-type BB- QWIPs have a broader and flatter spectral bandwidth but lower responsivity than that of n-type BB-QWIPs under similar operating conditions.

In order to tune the wavelength of lattice-matched QWIP detectors over the range from 4 - 20 microns, new designs are demonstrated for the first time which combine InGaAlAs and InGaAsP layers lattice-matched to InP and grown by gas-source molecular beam epitaxy. We demonstrate the first long- wavelength quantum well infrared photodetectors using the lattice-matched n-doped InGaAlAs/InP materials system. Samples with AlAs mole fractions of 0.0, 0.1, and 0.15 result in cutoff wavelengths of 8.5, 13.3, and 19.4 micrometer, respectively. A 45 degree facet coupled illumination responsivity of R equals 0.37 A/W and detectivity of D*_(lambda ) equals 1 X 10⁹ cm (root)Hz W^-1 at T equals 77 K, for a cutoff wavelength (lambda) _c equals 13.3 micrometer have been achieved. Based on the measured intersubband photoresponse wavelength, a null conduction band offset is expected for In_0.52Ga_0.21Al_0.27As/InP heterojunctions. We also report quantum well infrared photodetector structures of In_0.53Ga_0.47As/Al_0.48In_0.52As grown on InP substrate with photoresponse at 4 micrometer suitable for mid-wavelength infrared detectors. These detectors exhibit a constant peak responsivity of 30 mA/W independent of temperature in the range from T equals 77 K to T equals 200 K. Combining these two materials, we report the first multispectral detectors that combine lattice-matched quantum wells of InGaAs/InAlAs and InGaAs/InP. Utilizing two contacts, a voltage tunable detector with (lambda) _p equals 8 micrometer at a bias of V equals 5 V and (lambda) p equals 4 micrometer at V equals 10 V is demonstrated.

This paper presents the recent developments of large area focal plane 'pseudo' arrays for infrared (IR) imaging. The devices (called QWIP-LED) are based on the epitaxial integration of an n-type mid-IR (8 - 10 micrometer in the present study) GaAs/AlGaAs quantum well detector with light emitting diode. The originality of this work is to use n-type quantum wells for large detection responsivity. From these structures, very large area (approximately equals cm²) mesas are processed with V-grooves to couple the mid-IR light with the QW intersubband transitions. The increase of spontaneous emission by the mid-infrared induced photocurrent is detected with a CCD camera in the reflection configuration. As demonstrated earlier on p-type QWIP structures the mid-IR image of a blackbody object is up-converted to a near-IR transformed image with very small distortion.

In this letter, self-aligned dual implantation technique was successfully used to speed up the carrier transportation from sidewall quantum well (SQWL) to quantum wire (QWR) region in V-groove AlGaAs/GaAs QWR structure. Photoluminescence (PL) and time resolved photoluminescence (TRPL) show that the lateral confinement was enhanced after intermixing by intermixing the necking region. Lifetime was obviously enlonged after selective intermixing, which comes from the enhanced lateral carrier confinement. Strong hot exciton relaxation process in QWRs region is observed after selective intermixing.

The III-nitride materials are recognized as very promising candidates for the fabrication of optoelectronic devices for the visible and ultraviolet spectral ranges as well as for high-frequency electronic devices operating at high power levels and in caustic environments. At present, device quality materials preparation is making rapid progress and some devices have been successfully demonstrated and even commercialized. However, much of the basic materials, characterization data that is readily available in more conventional technologies is still lacking for the III-nitride materials. In this paper, a materials-theory-based device modeling methodology that is suitable for this unique situation of accelerated device exploration is discussed. EXamples of ultraviolet photodetectors in which the materials- theory-based device modeling technique has been used for device design and data analysis are presented.

We report the fabrication and characterization of Al_xGa_1-xN p-i-n photodiodes (0.05 less than or equal to X less than or equal to 0.30) grown on sapphire by low-pressure metalorganic chemical vapor deposition. The devices present a visible-rejection of about four orders of magnitude with a cutoff wavelength that shifts from 350 nm to 291 nm. They also exhibit a constant responsivity for five decades (30 mW/m² to 1 kW/m²) of optical power density. Using capacitance measurements, the values for the acceptor concentration in the p-Al_xGa_1-xN region and the unintentional donor concentration in the intrinsic region are found. Photocurrent decays are exponential for high load resistances, with a time constant that corresponds to the RC product of the system. For low load resistances the transient response becomes non- exponential, with a decay time longer than the RC constant.

In the recent years, the depletion of the stratospheric ozone layer has alerted the scientific community about the risks of a solar ultraviolet (UV) radiation overexposure. Biological research has confirmed the very important role of the UV-B (320 - 280 nm) and UV-A (400 - 320 nm) bands on the Earth biosystem. Al_xGa_1-xN semiconductor alloys, with a bandgap tunable between 3.4 eV and 6.2 eV, are the most suitable materials for the fabrication of solar UV detectors. In this paper we describe the fabrication and characteristics of AlGaN photoconductive and Schottky barrier photodetectors, with Al mole fractions up to 35%. Photoconductive detectors show very high gains, that decrease with increasing incident optical power. They present persistent photoconductivity effects, and a significant below-the-gap response. The physics of this behavior is discussed. On the other hand, AlGaN Schottky barrier photodetectors show a very fast response that is independent of the optical power, and their UV/visible rejection ratio is rather high. As the Al content increases, the evolution of the responsivity and cut-off wavelength is presented. Al_0.22Ga_0.78N Schottky barriers are very good candidates to monitor the UV-B band. The prospective applications of AlGaN photodiodes to determine the biological action of the solar UV radiation are also discussed.

Ultraviolet photodetectors have many military and commercial applications. However, for many of these applications, the photodetectors must be solar blind. This means that the photodetectors must have a cutoff wavelength of less than about 270 nm. Semiconductor based devices would then need energy gaps of over 4.6 eV. In the Al_xGa_1-xN system, the aluminum mole fraction, x, required is over 40%. As the energy gap is increased, doping becomes much more difficult, especially p-type doping. This report is a study of the electrical properties of Al_xGa_1-xN to enable better control of the doping. Magnesium doped p-type Al_xGa_{1- x}N has been studied using high-temperature Hall effect measurements. The acceptor ionization energy has been found to increase substantially with the aluminum content. Short-period superlattices consisting of alternating layers of GaN:Mg and AlGaN:Mg were also grown by low-pressure organometallic vapor phase epitaxy. The electrical properties of these superlattices were measured as a function of temperature and compared to conventional AlGaN:Mg layers. It is shown that the optical absorption edge can be shifted to shorter wavelengths while lowering the acceptor ionization energy by using short- period superlattice structures instead of bulk-like AlGaN:Mg. Silicon doped n-type films have also been studied.

High sensitivity visible-blind UV detectors were fabricated with organic semiconductors. The photo-sensitivity at 350 nm reaches 75 mA/Watt, corresponding to quantum efficiency of approximately 27% el/ph. The visible/UV suppression ratio is more than 10⁴ without optical filters. These UV detectors are of linear intensity dependence with fast response time. The simple fabrication process allows these UV detectors to be made in large size, in flexible forms or onto non-planar substrates with low cost. The fabrication process also allows these UV detectors to be integrated with electronic devices or optical devices.

The vulnerability of aircraft to missile attacks is briefly sketched. Aircraft and helicopter defensive electronic self- protection systems against missile attacks including passive and active radar and IR countermeasures are summarized. Radar and IR missile threat warnings are outlined. The development of UV missile threat warning is recounted. UV missile warning coverage is defined. UV missile plume photon emission is addressed. Solar UV emission and atmospheric transmission is reviewed. An AF atmospheric computer program called PLEXUS is applied to the UV spectral region to obtain the atmospheric transmission along paths extending from the upper atmosphere to the lower atmosphere. These atmospheric transmission numbers are used to evaluate the number of solar photons propagating through the upper atmosphere to lower tactical altitudes and into a UV missile-warning sensor viewing the sun directly. These numerical evaluations are accomplished in one- nanometer wide wavelength bins. The results are presented in a number of figures, which then serve to define the solar blind UV region. Detector spectral development objectives are then presented based on the results shown in the figures.

Low noise, high speed laser detectors sensitive to eye-safe laser wavelengths (near 1.5 micrometer) are of great importance in next-generation laser rangefinders, or laser radar applications. Improved signal-to-noise performance, and thus improved range or decreased laser power, can be achieved through the use of avalanche photodiodes (APD) in place of unity gain PIN detectors. The noise behavior of the APD is crucial in determining the overall performance of these systems. We will discuss intrinsic and extrinsic limits to performance for several different APD material systems sensitive to eye-safe lasers, including a detailed comparison of HgCdTe, InGaAs, and several other material systems.

Separate absorption and multiplication avalanche photodiode (SAM-APD) device structures, operating in the 1.1 - 1.6 micrometer spectral range, have been fabricated in the HgCdTe material system by molecular-beam epitaxy. These HgCdTe device structures, which offer an alternative technology to existing III-V APD detectors, were grown on CdZnTe(211)B substrates using CdTe, Te, and Hg sources with in situ In and As doping. The alloy composition of the HgCdTe layers was adjusted to achieve both efficient absorption of IR radiation in the 1.1 - 1.6 micrometer spectral range and low excess-noise avalanche multiplication. To achieve resonant enhancement of hole impact ionization from the split-off valence band, the Hg_1-xCd_xTe alloy composition in the gain region of the device, x equals 0.73, was chosen to achieve equality between the bandgap energy and spin-orbit splitting. The appropriate value of this alloy composition was determined from analysis of the 300 K bandgap and spin-orbit splitting energies of a set of calibration layers, using a combination of IR transmission and spectroscopic ellipsometry measurements. MBE-grown APD epitaxial wafers were processed into passivated mesa-type discrete device structures and diode mini-arrays using conventional HgCdTe process technology. Device spectral response, dark current density, and avalanche gain measurements were performed on discrete diodes and diode mini- arrays on the processed wafers. Avalanche gains in the range of 30 - 40 at reverse bias of 85 - 90 V and array-median dark current density below 2 X 10^-4 A/cm² at 40 V reverse bias have been demonstrated.

GaSb/AlSb/InAs is an attractive system for making low noise avalanche photodetectors (APD) due to possible resonant enhancement of hole impact ionization in Al_xGa_1-xSb and potential enhancement of electron impact ionization in GaSb/AlSb superlattices. We have employed molecular beam epitaxy (MBE) to fabricate device structures so that these effects could be further explored. The devices were grown on GaSb substrates and incorporated a p^-n⁺ one sided abrupt junction. The p^- multiplication region consisted of either bulk Al_0.04Ga_0.96Sb or 10 periods of alternating, 300 angstrom thick GaSb and AlSb layers. A short period, selectively doped InAs/AlSb superlattice was used as the n⁺ layer. Dark current suppression in these devices was found to be largely dependent on the InAs/AlSb superlattice configuration and the resulting band offset at the p^-n⁺ heterojunction. Notably, for devices with a 0.6 micrometer Al_0.04Ga_0.96Sb multiplication region and an optimized InAs/AlSb superlattice, an avalanche break down voltage of 13 V was observed. The dark current density for this device was 6 A/cm² at a multiplication factor of 10. Devices with GaSb/AlSb superlattice multiplication regions exhibited a higher breakdown voltage (18.5 V) and a lower dark current density (0.4 A/cm²) at comparable gain. Impact ionization rates in Al_0.04Ga_0.96Sb were studied by using 781 nm and 1645 nm laser light. The results were consistent with enhancement of hole impact ionization in Al_0.04Ga_0.96Sb.

In this paper a two-dimensional ensemble Monte Carlo particle method is used to simulate the metal-semiconductor-metal (MSM) photodetector response in the terahertz range of signal frequencies. We consider planar MSM photodetectors consisting of a GaAs absorbing layer with a system of Schottky contacts made 'back-to-back' on the above layer. The model takes into account the features of the carrier energy spectra, mechanisms of their scattering and a self-consistent electric field. The intrinsic transient response triggered by an ultra-short light pulse is calculated. The MSM frequency response is calculated using the Fourier transform of the obtained temporal dependences. It is shown that due to velocity overshoot effect exhibited by the photoelectrons, the MSM photodetector reveals rather high response to terahertz signals even if the contact spacing is relatively large. The frequency response of the MSM photodetectors utilizing the photoelectron velocity overshoot effect is compared with that of the MSM photodetectors with ultra-short carrier lifetime.

Results are presented for indium antimonide/indium aluminum antimonide (InSb/InAlSb) diodes grown by molecular beam epitaxy (MBE) for infrared detector applications. By lowering the substrate growth temperature during epitaxy it is possible to increase the dopant activation, both n and p-type. In addition, the Shockley-Read trap density is reduced by a factor X5 to approximately 2 X 10¹³ cm^-3 and the defect density in the MBE grown material falls to approximately 25 cm^-2. The application of these diodes with improved performance to 2D infrared detector arrays with enhanced detectivities operating at higher temperatures will be described. Conventional 2D arrays that operate at 80 K have also been fabricated. Typical noise equivalent temperature difference (NETD) is less than 10 mK for a 1.5 msec stare time.

In this paper, we review our research efforts on RCE high- speed high-efficiency p-i-n and Schottky photodiodes. Using a microwave compatible planar fabrication process, we have designed and fabricated GaAs based RCE photodiodes. For RCE Schottky photodiodes, we have achieved a peak quantum efficiency of 50% along with a 3-dB bandwidth of 100 GHz. The tunability of the detectors via a recess etch is also demonstrated. For p-i-n type photodiodes, we have fabricated and tested widely tunable devices with near 100% quantum efficiencies, along with a 3-dB bandwidth of 50 GHz. Both of these results correspond to the fastest RCE photodetectors published in scientific literature.

We demonstrate NIR (1.8 micrometer - 2.3 micrometer) resonant photo-detectors based on inter-band (Ecl- Ehhl) absorption in strain compensated, indium rich, InGaAs quantum wells (QW). Extremely low room temperature dark current densities are achieved by reduction of the active layer thickness combined with low defect density of the pseudomorphic strain compensated QWs. The weak absorption of the QW is enhanced by embedding the quantum well into a vertical resonant cavity. We present the experimental results for a demonstrator designed for a wavelength of 2 micrometer. The device, based on a single In_0.83Ga_0.17As quantum well and tensile strained barriers for strain compensation, exhibits a selectivity of 9 nm and 18% quantum efficiency. InP/InGaAs and Si/SiO₂ material systems are used for the bottom and top distributed Bragg reflectors (DBR) of the cavity, with 20 pairs and 2 pairs respectively. The semiconductor structure is grown by MOCVD. The top Si/SiO₂ DBR is deposited after fabrication of p-i-n planar photodiodes. Typical dark current densities are lower than 10^-7 A/cm² at -2 V bias. Conditions for extension of the operating wavelength up to 2.3 micrometer have been obtained experimentally using InAs/GaAs superlattice deposition to increase the thickness of the strained QW. A prospective tunable detector based on an actuable micro-machined air cavity and air/InP bottom DBR is proposed.

We designed, fabricated and characterized Al_xGa_{1- x}As/GaAs p-i-n resonant cavity enhanced (RCE) photodetectors with near-unity quantum efficiency. The peak wavelength is in the 780 - 830 nm region and post-process adjustable by recessing the top surface. Transit time limited bandwidth for these devices is in excess of 50 GHz. Possible applications of these detectors include conventional measurements of low light levels, quantum optical experiments that use pulsed sources and short-haul high speed communications.

The carrier lifetimes in In_xGa_1-xAs (InGaAs) and Hg_1-xCd_xTe (HgCdTe) ternary alloys for radiative and Auger recombination are calculated for temperature 300 K in the short wavelength range 1.5 less than (lambda) less than 3.7 micrometer. Due to photon recycling, an order of magnitude enhancements in the radiative lifetimes over those obtained from the standard van Roosbroeck and Shockley expression, has been assumed. This theoretical prediction has been confirmed by good agreement with experimental data for n-type In_0.53Ga_0.47As. The possible Auger recombination mechanisms (CHCC, CHLH and CHSH processes) in direct-gap semiconductors are investigated. In both n-type ternary alloys, the carrier lifetimes are similar, and competition between radiative and CHCC processes take place. In p-type materials the carrier lifetime are also comparable, however the most effective channels of Auger mechanisms are: CHSH process in InGaAs, and CHLH process in HgCdTe. Next, the performance of heterostructure p-on-n photovoltaic devices are considered. Theoretically predicted R_oA values are compared with experimental data reported by other authors. In_0.53Ga_0.47As photodiodes have shown the device performance within a factor of 10 of theoretical limit. However, the performance of InGaAs photodiodes decreases rapidly at intermediate wavelengths due to mismatch-induced defects. HgCdTe photodiodes maintain high performance close to ultimate limit over a wider range of wavelengths. In this context technology of HgCdTe is considerably advanced since the same lattice parameter of this alloy over wide composition range.

In this paper, we report on the growth and characterization of InAsSb alloys on GaAs and Si substrates for uncooled infrared photodetector applications. The fabrication and characterization of photodetectors from the grown layers are also reported. The photovoltaic and photoconductive devices were grown on (100) GaAs and Si substrates, respectively, using molecular beam epitaxy (MBE). The composition of InAs_1-xSb_x layers was 0.95 in both cases and cut-off wavelength of 7 - 8 micrometer has been obtained. At 300 K, the photovoltaic detectors on GaAs substrates resulted in a sharp cut-off wavelength of 7.5 micrometer with a peak responsivity as high as 0.32 V/W at 6.5 micrometer. For the photoconductive detectors on Si substrates, cut-off wavelength of 8 micrometer has been observed with a responsivity of 6.3 X 10^-2 V/W at 7 micrometer under an electric field of 420 V/m.

We report the first self-assembled InGaAs/InGaP quantum dot intersubband infrared photoconductive detectors (QDIPs) grown on semi-insulating GaAs substrate by low pressure metal organic chemical vapor deposition (MOCVD). The InGaAs quantum dots were constructed on an InGaP matrix as self assembling in Stranski-Krastanow growth mode in optimum growth conditions. The detector structure was prepared for single layer and multi-stacked quantum dots for active region. Normal incident photoconductive response was observed at a peak wavelength of 5.5 micrometer with a high responsivity of 130 mA/W, and a detectivity of 4.74 X 10⁷ cm Hz^1/2/W at 77 K for multi-stack QDIP. Low temperature photoresponse of the single quantum dot photodetector was characterized. Peak response was obtained between 16 K and 60 K. The detailed dark current noise measurements were carried on single and multistack quantum dot infrared detectors. High photoconductive gain as 7.6 X 10³ biased at 0.5 V results in increasing the intersubband carrier relaxation time as two order of magnitude compared quantum well infrared photodetectors.

We report the first demonstration of InAsSb/AlInSb double heterostructure detectors for room temperature operation. The structures were grown in a solid source molecular beam epitaxy reactor on semi-insulating GaAs substrate. The material was processed to 400 X 400 micrometer mesas using standard photolithography, etching, and metallization techniques. No optical immersion or surface passivation was used. The photovoltaic detectors showed a cutoff wavelength at 8 micrometer at 300 K. The devices showed a high quantum efficiency of 40% at 7 micrometer at room temperature. A responsivity of 300 mA/W was measured at 7 micrometer under a reverse bias of 0.25 V at 300 K resulting in a Johnson noise limited detectivity of 2 X 10⁸ cmHz^1/2/W.

The current-voltage characteristics of a superconductor-normal metal tunnel junction (SIN) is very sensitive to the temperature of the normal metal. Therefore SIN junction can be used as a thermometer which can be conveniently integrated into more complicated devices, for example bolometers. We estimate the effect of different types of noise on the sensitivity of such a thermometer. Shot noise of the tunnel junction, amplifier noise and the noise related to the fluctuations of the heat flow through the junction are considered. The performance of the bolometer with SIN junction as a temperature sensor is also discussed.

Real-time monitoring by multiwavelength phase modulated ellipsometry (PME) of the growth of plasma deposited microcrystalline Silicon ((mu) c-Si) is presented. Several growth models for process-monitoring are reviewed, and in particular the inhomogeneity in the (mu) c-Si layer is treated by allowing graded-index profile in the bulk. By also using the Bruggeman effective medium theory to describe the optical properties of (mu) c-Si, the monitoring of the crystallinity in the upper and lower part of the layer, together with the thickness is demonstrated. The inversion algorithms is very fast, with calculation times within 5 seconds using a standard Pentium computer. This opens up for precise control of surface roughness, bulk thickness, and crystallization of both the top and bottom interfaces of the layer during the elaboration of devices such as solar cells and thin film transistors.

6H-SiC single crystals are expected to be suitable substrates for thin film growth of the wide bandgap semiconductor (GaN, because it has a small lattice mismatch with GaN. Moreover, SiC single crystals are also expected for high-power and high- temperature electric applications because of its wide band gap, high breakdown voltage, high thermal conductivity and high temperature stability. Single crystals with large size used for electronic devices can be grown on seed crystals only by the modified Lely method based on sublimation deposition. But, single crystals have serious defects so called micropipes. These micropipes penetrate almost along the [001] direction. The internal strain around micropipes was investigated using the polarizing optical microscope for the purpose of clarifying the formation mechanisms and decreasing the amount of micropipes. A special interference figure was found around a micropipe under the crossed polars on the polarizing microscope. In this work, the special interference figure around micropipes due to internal stress was explained, and the magnitude and distribution of the stress was measured by means of photoelasticity and the mapping of Raman spectra.

We report the effect of annealing temperature on the near bandgap transmittance, absorption coefficient, as well as the evolution of shallow-level defects of arsenic-ion-implanted GaAs (referred as GaAs:As⁺) by using Fourier transform infrared spectroscopy. By either fitting the absorption curve with A(hv-E_g)^1/2 or extrapolating the ((alpha) hv)² curve to the abscissa, the blue shift of bandgap energy of RTA-annealed GaAs:As⁺ samples was found to increase from 1.35 eV (T_a equals 300 degrees Celsius) to 1.41 eV (T_a equals 800 degrees Celsius). The slightly perturbed absorption spectra at near bandgap region interpret that there are still a large amount of near-bandedge defects continuously distributed in the RTA-annealed GaAs:As⁺ samples. The diminishing of shallow-level defects with at higher annealing temperatures was also observed via the derivative absorption spectra.

In this paper we present the observation of the interband transition in the GaAs (100) surface Si-delta-doping potential. Samples with different surface doping concentrations (N_s equals undoped, 3.0 X 10¹², 6.3 X 10¹³, 2.4 X 10¹⁴ and 3.6 X 10⁴ cm^-2) have been studied at room temperature in the MBE high vacuum chamber using modulated photo-reflection (PR) spectroscopy technique. The MBE chamber guarantees that all the sample surfaces are free of oxidation or uncontrollable contamination. The optical transition of at GaAs bandedge around 1.41 eV is strong and is almost independent of N_s. A relatively weak feature above 1.42 eV has been observed which is clearly enhanced and blue-shifted following the increase of N_s. The experimental results have been analyzed and well explained based on the self-consistent Schrodinger-Poisson equations. The theoretical analysis indicates that it is not proper to attribute the PR spectral peak of 1.42 eV simply to be certain subband-related optical transition. The observed spectral peak of 1.42 eV is more likely to be related to the high-index confined-levels in the half-V-shape conduction band at the sample surface.

The surface passivation is essential for the fabrication of high performance HgCdTe photodiodes, especially for photodiodes with small junction area. The fabrication of the HgCdTe photodiodes passivated with a wide band gap epitaxial layer has been described. The planar double-layer heterojunction (DLHJ) structures used in fabrication Hg_{1- x}Cd_xTe photodiodes were grown on CdZnTe substrates by liquid phase epitaxy (LPE). The P⁺-n long wavelength infrared radiation (LWIR) photodiodes were fabricated by arsenic diffusion into n-type HgCdTe DLHJ structures. To improve the photodiode performance a thickness of n-type base layer was limited. The photodiodes performances were determined from measurements of the current-voltage and spectral response characteristics. The generation- recombination current was found to be dominant current around zero bias voltage at 77 K. The diodes without antireflection coating had a typical quantum efficiency of 60 percent. The performance of both type of p-n LWIR HgCdTe photodiodes (with and without the wide band gap epitaxial layer) have been compared.

Thin (approximately equals 1.5 micrometer) CdS films were prepared on glass by laser ablation using fluences of 2 - 5 Jcm^-2. We demonstrate that such an increase of the laser fluence turns the orientation of the c-axis of the films from perpendicular to parallel with respect to the substrate surface. The influence of this orientation variation on the optical properties of the films is studied by photocurrent, transmission and z-scan measurements. All experiments were carried out at 300 K using monochromatic light or the cw emission of argon and He-Ne lasers at 514.5 and 632.8 nm, respectively. The transmission threshold and the photocurrent maxima are shifted to shorter wavelengths and the transmission edge becomes steeper with increasing the laser fluence. The nonlinear absorption and refraction indices were evaluated for 514.5 nm and 632.8 nm by z-scan technique. It occurred that at 514.5 nm the photo-thermal heating due to effective absorption dominates and, therefore, refractive nonlinearities are not provable. At 632.8 nm, however, the samples are transmissive and refractive nonlinearities are clearly observed. Higher nonlinear coefficients of absorption and refraction were found for samples with parallel c-axis. As far as we are aware, this work represents the first study of the influence of the crystal direction on the photocurrent and z-scan features of oriented thin CdS films.

We present a new near-infrared photodetectors fabricated based on Hg₃In₂Te₆ semiconductor compound. This ternary compound is a direct-gap n-type semiconductor with the band gap of 0.74 eV and carrier concentration about 10¹³ cm^-3 at room temperature. Surface-barrier structures a transparent conducting metal oxide electrode-interfacial chemical grown oxide-semiconductor substrate with an active area from 3 to 50 mm² have been fabricated by chemical oxidation of Hg₃In₂Te₆ surface for the potential barrier's formation. The composition of oxide layer (40% In₂O₃, 50% TeO₂, and 10% HgO) was determined using XPS analysis. Tin-doped indium oxide (ITO) film (as transparent conducting electrode) was deposited over this layer by magnetron RF sputtering technique. The devices are very sensitive to light with the wavelength from 0.4 to 1.7 micrometer. A self-calibrated photodetectors, which permit 100% external quantum efficiency (within error not exceeding 2%) at wavelengths of 1.3 and 1.5 micrometer, have been developed. The photodetectors fabricated on thin Hg₃In₂Te₆ substrates have a low producing price and can be fabricated with a large photosensitive area. Photodetectors with an active area of 3 mm² exhibit the rise and fall times from 2 to 4 ns under 1.3 micrometer pulse irradiation. Both basic material aspects and devices fabrication technique is detailed discussed.

The effective mass model of the electronic structure and intersubband absorption in p-doped twinning superlattices is devised. As the bulk states basis, heavy hole (HH), light hole (LH) and split-off (SO) band Bloch functions, properly rotated according to the crystal orientation, are used. Furthermore, delta potential is introduced at the interface between two layers to model the corresponding microscopic potential. Peculiar electronic structure offers coupling between otherwise forbidden states in composite superlattices. This coupling arises from the change of both the off-diagonal terms of the velocity operator and the Luttinger parameters across two interfaces belonging to the superlattice period. The peak of the absorption coefficient for x polarized light (z is the growth direction) arises mainly from transitions between LH1 and HH2 miniband. This peak is located in the midwavelength infrared window for all three investigated semiconductors, GaAs, Ge, and Si. It is shown that the dipole matrix elements are responsible for absorption. It is important to note that the magnitude of the absorption coefficient agrees with the results of the pseudopotential theory. The results indicate the usefulness of this structure, which has recently been realized, in the state-of-art quantum well infrared photodetectors.

The Metal-Semiconductor-Metal (MSM) photodetector has attracted a great deal of interest because of its potential for high speed operation and low fabrication cost. In this paper, the parameters affecting the capacitance, sensitivity and bandwidth of the interdigitated MSM photodiode are examined. Models are presented for the detector capacitance, gain and transmission line effects. The proposed circuit models have been verified by experimental results.

We report the growth and characterization of Schottky based metal-semiconductor-metal ultraviolet photodetectors fabricated on lateral epitaxially overgrown GaN films. The lateral epitaxial overgrowth of GaN was carried out on basal plane sapphire substrates by low pressure metalorganic chemical vapor deposition and exhibited lateral growth rates more than 5 times as high as vertical growth rates. The spectral responsivity, the dependence on bias voltage, on incident optical power, and the time response of these photodetectors have been characterized. Two detector orientations were investigated: one with the interdigitated finger pattern parallel and the other perpendicular to the underlying SiO_x mask stripes.

Epitaxial growth techniques have made possible the fabrication of heteroepitaxial GaN films, which are of great interest for fabricating optical, high power and high frequency devices. A major problem for many device applications, however, is that these materials contain high densities of dislocations, between 10⁸ and 10¹⁰ cm^-2, which limit device performance. Recently, it has been found that reduced dislocation densities can be achieved using a lateral epitaxial overgrowth technique. Our optical and microstructural studios of un-coalesced and coalesced GaN layers indicate that most of the structural defects are confined only to the patterning apertures and that high quality material is present in the lateral epitaxial overgrown regions. PL measurements indicated that Si impurities have been incorporated in the epitaxial layers. Cathodoluminescence imaging shows the spatial distribution of recombination centers across the homoepitaxial layers.

This paper discusses the collaborative optical and electrical characterization of the first photovoltaic (PV) III-nitride based detectors grown and fabricated by the Air Force Research Laboratory (AFRL). These 2.6 micrometer thick, n-type GaN Schottky detector structures doped with Si were grown by molecular beam epitaxy (MBE) on (0001)-oriented sapphire substrates, and incorporated palladium (Pd) as the Schottky metal contact. Working Schottky-barrier detector sizes ranged from 50 micrometer to 1600 micrometer in diameter. Flood- illuminated spectral responsivities of these Schottky detectors were as high as 0.12 A/W (for a 1600 micrometer diameter device biased at -1.5 V) at a peak wavelength of 273 nm. The typical measured frequency response of these detectors was flat from dc to the chopper limit of 700 Hz, and the 1/e response time of a 1600 micrometer diameter Schottky- barrier GaN detector was found to be as low as 50 microsecond(s) at zero bias. Noise characterization of these detectors was also performed, and noise equivalent powers (NEPs) of sample GaN Schottky-barrier detectors are reported.

Since the first discovery, semiconductor infrared lasers and detectors have found many various applications in military, communications, medical, and industry sections. In this paper, the current status of semiconductor infrared lasers and detectors will be reviewed. Advantages and disadvantages of different methods and techniques is discussed later. Some basic physical limitations of current technology are studied and the direction to overcome these problems will be suggested.