The 'isothermal vapor phase epitaxy (iso-VPE) of mercury cadmium telluride on finite cadmium telluride substrate- systems,' consists in the complete transformation of starting CdTe films, on foreign-inert-substrates grown (by MOCVD or MBE), into compositionally controlled and uniform HgCdTe/foreign-substrate structures. The HgTe vapor phase growth and the HgTe/CdTe solid-state interdiffusion are the physical phenomena controlling the iso-VPE process evolution. Working at 530 degrees Celsius, under experimental conditions characterized by HgTe/CdTe interdiffusion rates higher than the HgTe growth rates, the possibility to prepare high quality Hg1-xCdxTe/Sapphire structures has been demonstrated and reported last year. Recently the same process has been successfully experimented on CdTe/Silicon substrates too. For the first time the HgCdTe/Silicon iso-VPE structures characteristics are reported in comparison with the equivalent HgCdTe/Sapphire films. Because of the good solid-state chemical compatibility between Hg1-xCdxTe alloys and silicon, well defined metallurgic interfaces between active layers and silicon substrates have been obtained. Rocking curves with FWHM in the 110 - 130 arcsec range have been measured on 15 micrometer thick HgCdTe/silicon iso-VPE structures. The films as-grown electrical characteristics are p-type, according to a mercury vacancy dominated defectivity.
A comprehensive study of the properties of Hg1-xZnxTe (MZTx) solid solutions is presented which is mainly focussed around x equals 0.15 corresponding to the detection at 10 micrometer. A comparison is done with Hg1-xCdxTe (MCTx). The experimental evidence of the Hg-Te bond strengthening, with ZnTe alloying, is discussed through the atomic bond length distribution, the atomic vibrations and macroscopic properties such as the infrared reflectivity, the low temperature specific heat, the linear thermal expansion coefficient and the hardness. The x dependence of some transitions in the Brillouin zone are compared with those found in MCTx. The conductivity type can be controlled through the concentration of mercury vacancies. The results on p-n conversion are presented using different techniques. This conversion is especially studied as a function of composition. The p type character of ZnTe dominates for x greater than 0.17. Experimental angle resolved photoemission studies (ARXPS) show that simple chemical treatments leave a stoichiometric compound up to the surface which is little damaged. Anodic oxides have been grown on MZT, their composition is deduced from spectroellipsometry. The interface density of states as measured on MIS structures is low but can be reliably weakened with anodic sulfidization.
Significant progress has been made in the technology for MBE growth of HgCdTe infrared focal-plane arrays on Si substrates since the initial demonstration of MBE HgCdTe-on- Si heteroepitaxy in 1989. In 1995, the first all-MBE-grown detector arrays on Si were produced through direct MBE growth of (112)B-oriented II-VI films on Si without III-V initiation layers, culminating in detector performance comparable to LPE-grown detectors on bulk CdZnTe substrates. This achievement was enabled by the development of two key contributing technologies: CdTe on Si buffer layer growth and HgCdTe p-on-n double-layer heterojunction growth using p-type chemical doping with As. The MBE process for deposition of high crystalline quality CdTe buffer layers has been developed so that x-ray rocking curve FWHM less than 75 arc-sec and near-surface etch pit densities (EPD) of 2 multiplied by 106 cm-2 are routinely achievable for 9-micrometer-thick CdTe buffer layers. The dependence of CdTe EPD on ZnTe initiation layer thickness, insertion of CdTe/CdZnTe strained layer superlattices, and thermal cycling to cryogenic temperatures has been investigated and is reviewed. HgCdTe baselayers deposited by MBE on these CdTe/Si composite substrates exhibit x-ray FWHM as low as 72 arc-sec and EPD of 3 - 20 multiplied by 106 cm-2. To demonstrate the potential for MBE growth of large-area HgCdTe FPAs on Si, detectors with 78 K cutoff wavelength of 7.8 micrometer have been fabricated in this HgCdTe/Si epitaxial material with array-average R0A product of 1.64 multiplied by 104 (Omega) -cm2 (0 FOV).
Progress in the development of narrow gap IV-VI-on-Si technology for infrared sensor arrays is reviewed. Epitaxial Pb1-xSnxSe layers, about 4 micrometer thick, are grown by molecular beam epitaxy onto 3 inch Si(111) substrates, and employing an intermediate CaF2 layer of only 2 nm thickness for compatibility reasons. Material quality is improved by proper growth conditions and annealing. Threading dislocation densities as low as 106 cm-2 are obtained in samples with 3 by 3 cm2 size after proper anneal. It seems that glissile threading dislocations sweep out across the edge of the samples, and, in addition, such dislocations are able to react with sessile ones and transform them to glissile. Infrared photodiodes with much higher resistance area products can be obtained which approach the theoretical limit in a certain temperature range with such improved material quality. If the Pb/Pb1-xSnxSe infrared Schottky-barrier sensors are described with a model which allows fluctuations of the barrier height, the saturation of the resistance-area products at low temperatures as well as ideality factors very much greater than 1 are explained, too.
Semiconducting YBaCuO thin films are a candidate for infrared bolometers operating at room temperature without the need for temperature control. Semiconducting YBaCuO thin films were fabricated by rf magnetron sputtering onto silicon substrates at room temperature. Room temperature deposition provides compatibility with CMOS technology to allow for the fabrication of low cost infrared imaging arrays with focal plane signal processing. The temperature coefficient of resistance (TCR) of the thin films was measured to be as high as 4% K-1 over a wide temperature range near 300 K. In this respect, amorphous YBaCuO thin films provided a higher TCR than epitaxial, tetragonal YBa2Cu3O6+x films. The noise voltage at 30 Hz was measured to be typically less than 1 (mu) V/Hz1/2. If the thin films are integrated into a typical air-gap thermal isolation structure, the projected responsivity Rv would be as high as 3.8 multiplied by 105 V/W with 1 (mu) A of current bias providing an estimated detectivity D* of 1.6 multiplied by 109 cm Hz1/2/W at a frame frequency of 30 Hz.
Mixed Pt/IR silicide Schottky barrier detectors are studied for operation in the 3 - 5 micrometer wavelength range. Thin films (2 - 10 nm) consisting of mixed Pt and IR layer sequences were deposited onto p-Si(100) substrates by e-beam evaporation in high vacuum (10-6 mbar). The deposited metal films were annealed in a lamp heater at 500 degrees Celsius for 30 min to obtain complete silicide formation. The resulting films were characterized by depth- sensitive methods (RBS and crater edge AES) as well as by current-voltage and photocurrent measurements. We found an enrichment of Pt at the silicide-silicon interface of all the films, caused by the enhanced silicidation reactivity of Pt-Si compared to that of IR-Si. A Schottky barrier height typical for PtSi was generally observed even for mixed layers. Only samples processed by IR deposition and silicidation prior to the Pt evaporation, showed an adjustable shift of the Schottky barrier height covering the whole range between the height of IRSi and that of PtSi (150 - 230 meV). The infrared responsivity of these films exceeded that of pure PtSi films.
This paper presents a comparison of theoretically predicted spatial NETD and mean temporal NETD as a function of the composition uniformity of Hg1-xCdxTe substrate used in the fabrication of IR detector arrays. The prediction of the spatial NETD is based on the residual spatial noise left behind in the FPA after implementing a linear two point nonuniformity compensation algorithm. The effect of using an optical filter on the spatial NETD is also included to show that the specifications on the composition uniformity of the Hg1-xCdxTe substrate can be partially relaxed by using an optical filter whose cut-off wavelength is less than the cutoff wavelength of the photodiodes of mean response. The composition uniformity leading to temporal noise rather than the spatial noise limited performances are suggested to be indicating the required composition uniformity of Hg1-xCdxTe substrate for focal plane arrays (FPA). The results are presented for both MWIR and LWIR HgCdTe hybrid FPAs.
Infrared detectors based on GaInSb/InAs strained layer superlattices promise focal plane arrays spanning the infrared. Response uniformity and operating temperature are projected to be superior to those of long-wavelength HgCdTe devices, without the low quantum efficiencies of multi- quantum well devices. Operation of such superlattice-based mid- and long-wave infrared detectors has been demonstrated, with zero bias impedances approaching the HgCdTe trend line and high quantum efficiencies. Two color, back-to-back diodes have also been demonstrated for the first time, in a p-n-p triple layer heterojunction. Various passivation approaches have been compared.
The composite infrared spctrometer (CIRS) is an important instrument for the upcoming Cassini mission for sensing infrared (IR) radiation from the Saturanian planetary system. We have delivered a linear, ten element, mercury cadmium telluride (HgCdTe) photoconductive detector array for use on focal plane 3 (FP3), which is responsible for detecting radiation from the 9.1 micrometer to 16.6 micrometer wavelength range. Reliable HgCdTe detectors require robust passivation, a low-stress zinc sulfide (ZnS) anti-reflection (AR) coating with good adhesion, and a proper optical cavity design to smooth out the resonance in the detector spectral response. During the development of CIRS flight array, we have demonstrated the potential of using an in-situ interfacial layer, such as SiNx, between ZnS and the anodic oxide. Such an interfacial layer drastically improves the adhesion between the ZnS and oxide, without degrading the minority carrier lifetime. We have also demonstrated the feasibility of applying a SiNx 'rain coat' layer over the ZnS to prevent moisture and other chemicals from attacking the AR coating, thus improving the long term reliability. This also enables device operation in a hazardous environment. The alumina/epoxy/HgCdTe/oxide/ZnS structure is a complicated multi-cavity optical system. We have developed an extensive device simulation, which enables us to make the optimal choice of individual cavity thickness for minimizing the resonance and maximizing the quantum efficiency. We have also used 0.05 micrometer alumina powder loaded epoxy to minimize the reflections at the epoxy/HgCdTe interface, thus minimizing the resonance.
We report, for the first time, experimental FIR detector results based on p-GaAs homojunction interfacial workfunction internal photoemission (HIWIP) structures. The MBE grown samples consist of a multilayer (p+- p--p+-p--...) structure. The detector shows high responsivity over a wide wavelength range with a bias tunable cutoff wavelength ((lambda) c). Changing the emitter layer (p+) doping concentration (Ne) will result in different (lambda) cs. For a detector with Ne equals 3 multiplied by 1018 cm-3, an effective quantum efficiency of 9.2% (at 26.3 micrometer) with (lambda) c equals 100 micrometer is obtained. Various experimental results are discussed.
P-type Hg1-xCdxTe epilayers either Hg-vacancy or As doped were etched in CH4/H2 electron cyclotron resonance plasmas and subsequently examined by Hall-effect and thermoelectric measurements with respect to changes of their electrical properties owing to the plasma exposure. The plasma was found to cause conversion from p- to n-type in a subsurface region extending up to 5 micrometer into the etched MCT epilayers. The observed type conversion was rather independent of sample temperature (60 to 100 degrees Celsius) and bias (0 to minus 50 V) and insensitive to the microwave power (100 to 250 W) and the exposure time used for the process. Annealing of the samples in sealed quartz ampules under Hg vapor at 150 to 200 degrees Celsius re- established p-type conduction in the type-converted layers. The type conversion and its reversibility are assumed to be correlated with the in- and outdiffusion of atomic hydrogen which presumably neutralizes the acceptors by the formation of complexes which are stable at temperatures T less than or equal to 100 degrees Celsius.
Generation and detection of high power short optical pulses are of interest for applications such as high speed switching and optically controlled microwave generation. Such systems based entirely on semiconductor technology are highly desirable. In our experiments significant enhancement in the response of metal-semiconductor-metal photoconductive switches fabricated on Fe doped semi-insulating InP with heavy ion N+3 implantation has been observed. The response tail of the devices was effectively eliminated, resulting in FWHM pulse widths reduction from 200 ps (when unimplanted) to less than 40 ps for 4.5 micrometer gap device. No appreciable decrease in the breakdown field was observed. The dependence of spectral sensitivity on implantation dose was also studied. By proper optimization of the detector circuit, and use of high power semiconductors lasers with saturable absorber, generation of microwave signals in excess of 25 GHz and several volts could be achieved.
By defining a grating coupling efficiency for quantum well infrared photodetectors (QWIPs) and including all possible high order diffracted waves, the authors calculate the spectral response of grating for QWIPs on the basis of modal expansion method (MEM). Numerical simulations show that the two dimensional grating can cover very wide spectral range, including 3 - 5 micrometer and 8 - 12 micrometer. The influence of the depth of grating and cavity width on the spectral response has been shown. A method of improving the response bandwidth of grating coupled QWIPs has been realized. A wideband bound-quasibound type QWIP has been fabricated. The measured detectivity (80 K) at the wavelength of 6.6 micrometer, 7.8 micrometer, 9.0 micrometer and 9.5 micrometer are 2.6 multiplied by 109, 2.1 multiplied by 109,4.8 multiplied by 109 and 4.4 multiplied by 109 cmHz1/2/W, respectively. The experiment has a good agreement with the theoretical results.
There has been significant progress during the past several years in photovoltaic (PV) HgCdTe technology for advanced long wavelength remote sensing applications. Useful cutoff wavelengths have been extended to beyond 17.0 micrometer. Junction quality has been improved to the point that D* greater than 3 multiplied by 1011 cm-(root)Hz/W can be achieved in arrays at temperatures of 60 - 65 K. The atmospheric infrared sounder (AIRS) instrument, scheduled for launch in the year 2000 as part of the NASA EOS Program, uses over 4000 PV HgCdTe detector elements organized into ten linear multiplexed arrays, with cutoff wavelengths extending as far as 15.0 micrometer at 60 K. The AIRS instrument also uses two long linear arrays of photoconductive (PC) HgCdTe detectors for the 13.7 - 15.4 micrometer band. These PC detector arrays have cutoff wavelengths of 16.0 - 17.0 micrometer and achieve D* values of 3 - 5 multiplied by 1011 cm-(root)Hz/W at 60 K. PV HgCdTe offers many advantages over PC HgCdTe for advanced remote sensing instruments: negligible 1/f noise, much higher impedance so that cold preamps or multiplexers are possible, configurational versatility with backside- illuminated two-dimensional arrays of closely spaced elements, 10X - 100X better linearity, dc coupling for measuring the total incident photon flux, and a (root)2 higher BLIP D* limit. In this paper we compare the relative merits of PV and PC HgCdTe for advanced remote sensing instruments, and we review recent data for both PV and PC HgCdTe arrays with cutoff wavelengths as long as 17.5 micrometer.
In the frame of technological preparation for scientific Earth observation, the European Space Agency (ESA) has undertaken a study program for a hyperspectral imager to be implemented on a low Earth orbit type satellite: HRIS (high resolution imaging spectrometer). The instrument is a pushbroom operating near 800 km altitude sun-synchronous orbit with 40 m sub-satellite spatial sampling, 30 km swath variable by plus or minus 30 degrees and a spectral coverage from 450 to 2350 nm at 10 nm average spectral sampling. One of the major challenges is therefore to develop, implement and operate a SWIR 140 multiplied by 768 pixels array tailored for the mission requirements, with main emphasis on low noise and high linearity over a large dynamic range. A SWIR focal plane breadboard has been studied and developed to be operated in the 1 to 2.35 micrometer range with the intention of demonstrating both performances and buttability. The hybrid technology is based on photovoltaic, HgCdTe buttable diode arrays coupled to full custom CMOS silicon multiplexing circuitry. Due to system constraint and as a result of trade-of at performances level, the focal plane is operated at 150 K. A similar SWIR focal plane (140 multiplied by 1024 pixels) is also intended to be the baseline in the frame of the ESA PRISM instrument (processes research by an imaging space mission).
Seven Aladdin sensor chip assemblies (SCAs) have been produced and nine more are in production. In this paper we cover the design, description, and measured performance of the Aladdin SCA. At over 7.5 square cm Aladdin is the largest single chip infrared array in use today. It is a hybrid assembly made up from an InSb detector coupled, via indium bumps, to a silicon readout. An array of this size is only possible because the InSb detector material is thinned to less than 10 microns which allows it to accommodate the InSb/silicon thermal mismatch. The Aladdin development program is a success and the resulting devices met most of the original design goals. Experience with Aladdin I led to improvements in the readout multiplexer design and, with these changes, we expect to meet the remaining goals. The new readouts have completed processing but testing has just started. The ALADDIN program is a joint collaboration between the National Optical Astronomy Observatories (NOAO) and the U.S. Naval Observatory (USNO) with Santa Barbara Research Center (SBRC).
Hughes has designed a large-area staring Si:As impurity band conduction (IBC) focal plane array specifically for high- background longwave infrared (LWIR) astronomy applications. We derived the design parameters by surveying leading astronomers for their requirements. This paper describes summary results of these requirements and how they were implemented in the design. We discuss preliminary detector and readout data that confirm satisfactory operation. We define current status and plans for fabrication and test of detector/readout hybrids.