HgCdTe has become the detector material of choice for many development and production electro-optical systems whose applications cover the IR spectrum from 2 to 16 micrometers , with operating temperatures ranging from 300 K to 40 K and background flux levels from 1018 to 1012 photons/cm2-sec. At the base of this success is the ongoing development and perfection of the HgCdTe material from which these detectors are fabricated. This paper examines the expressions that describe leakage currents, signal current, and capacitance for HgCdTe p+-on-n diodes to identify the critical material properties and their influence on the performance of the resulting detectors. In addition to lifetime, doping density, mobility, and composition, the compositional grading within the absorbing layer must be managed to achieve the desired performance. Equally important are parameters such as size and uniformity which must be emphasized to meet IR FPA cost targets. Caution must also be taken not to attempt to reduce IR FPA cost at the expense of material quality, because doing so would most likely have the opposite effect.
HgCdTe MBE technology offers many advantages for the growth of multi-layer heterojunction structures for high performance IRFPAs. This paper reports data on major advances towards the fabrication of advanced detector structures, which have been made in MBE technology at Hughes Research Laboratories during the last couple of years. Currently device quality materials with desired structural and electrical characteristics are grown with the alloy compositions required for short-wavelength infrared (SWIR, 1 - 3 micron) to very long- wavelength infrared (VLWIR, 14 - 18 micron) detector applications. In-situ In (n-type) and As (p-type) doping developed at HRL have facilitated the growth of advanced multi-layer heterojunction devices. Thus, high performance IR focal plane arrays (128 X 128) with state-of-the-art performance have been fabricated with MBE-grown double-layer heterojunction structures for MWIR and LWIR detector applications. In addition, the growth of n-p-p-n multi-layer heterojunction structures has been developed and two-color detectors have been demonstrated. Recently, significant preliminary results on the heteroepitaxy growth of HgCdTe double-layer heterojunction structures on silicon have been achieved.
CdTe can be grown directly on silicon substrates by Molecular Beam Epitaxy. On Si(001), CdTe grows in the (111)B orientation. The homo-orientation on Si(001), Si(111) and Si(211) can be obtained if a buffer ZnTe epilayer is grown prior to CdTe growth. A systematic study of the growth of CdTe(111)B on Si(001) surface with different atomic step structures, defined by the miscut tilt angle (theta) and the tilt direction (phi) , has been carried out. Double domain and twin formation is very sensitive to tilt parameters. When growth conditions are optimized, single domain twin free layers are obtained with suitable tilt values. The best films which exhibit double crystal X-ray rocking curve FWHM of 60 arcsec have for tilt parameters (theta) equals 1 degree(s) and (phi) equals 30 degree(s). The heterointerface formation has been studied by photoelectron spectroscopy with synchrotron radiation. It was found that in the very first step of the growth, up to one monolayer of Te is absorbed on Si(001).
An investigation of native defect formation in n-HgCdTe was performed under conditions that reduce the possibility of mercury atom loss to define the contribution from complex defects to electrically active state formation. The short-term heat treatments were produced in vacuum by passing the current directly through a sample. The temperature measurement was carried out by registering the Planck radiation from the sample surface.
A theoretical program useful for predicting limiting performance of HgCdTe, InAs/InGaSb Type II superlattices and QWIP IR detectors is reviewed. Its principal ingredients include the calculation of bulk and superlattice band structures, absolute interband and intersubband optical absorption coefficients, radiative recombination neglecting photon recycling, non- radiative AM-1 and AM-7 Auger processes with or without the participation of shallow impurity levels, and the detectivity. The superlattice is viewed as a periodic lattice whose relevant band structure is described by K(DOT)p perturbation theory. Valence band strain splitting greater than the band gap is particularly important in suppressing AM-7 transitions. HgCdTe IR detectors are clearly superior to Type II superlattices in the MWIR. The band gap in Type II superlattices is due to intra-layer carrier confinement and is determined by layer widths. The resulting greater band gap uniformity and theoretically higher operating temperatures is promising for LWIR and VLWIR applications.
Acceptor doped, non-strained aluminum-free Quantum Well Intersubband Photodetectors lattice matched to GaAs with Ga0.79In0.21As0.59P0.41 wells and Ga0.62In0.38As0.22P0.78 barriers have been demonstrated on semi-insulating GaAs substrates. These devices which operate at normal incidence demonstrate a unique spectral response which extends from approximately 2 micrometers up to 10 micrometers . To explain such a broad spectral shape, a detailed theoretical analysis based on the 8 X 8 Kane Hamiltonian was necessary to probe all aspect of optical absorption. The results of this analysis revealed that spectral shape results from the influence of the Spin Split-off band on the band structure and the optical matrix.
Night vision technology is present in most military systems used today. The sensors range from night vision goggles to high performance infrared ()targetacquisition and tracking systems. These sensors have provided a distinct battlefield advantage and will continue to be important in future confrontations and peace-keeping missions. With the importance placed upon night vision, lower cost and wider availability of night vision technology is an important consideration. Sensors can be employed in greater numbers as the cost is reduced and as the sensor package becomes lighter and consumes lower power. Sensor packaging, conformal to the system configuration, helps the sensor readily adapt to new system configurations. Infrared sensors that do not require cryogenic cooling meet many of these characteristics. Recently, the excellent imaging performance of uncooled IR focal plane arrays have captured the attention of many system users and initiated the emergence of new applications. This paper reviews these application areas, explores new possibilities, and assesses the technology underway to further expand the realm of uncooled IR imaging. Technology advances, both in the sensor technology and the interface with the imaging system, will expand uncooled JR technology into additional military and commercial systems.
SBRC has developed a high-quality 320 X 240 room-temperature infrared FPA that operates in the 8 - 14 micrometers spectral band. The FPA is based upon the silicon microbolometer technology that has been licensed from Honeywell. This monolithic uncooled FPA utilizes a novel BiCMOS readout circuit that provides high sensitivity and excellent output uniformity. The 320 X 240 FPA operates at frame rates up to 60 Hz with a single output. The microbolometers were fabricated monolithically on the silicon readout circuits at SBRC using VOx as the bolometer material. An advanced microbridge structure design was used that achieves an optical fill-factor greater than 65% in the 48 micrometers X 48 micrometers pixels. The structure also provides excellent thermal isolation for high responsivity and sensitivity. Initial measurements indicate the FPAs are operating with an NETD sensitivity of about 100 mK for an f/1 aperture. This FPA is ultimately expected to operate at sensitivities of less than 20 mK. The FPA also demonstrate peak-to-peak output nonuniformities of less than 100 mV. The FPAs have been mounted in permanently-sealed vacuum packages with single- stage thermoelectric temperature stabilizers. These vacuum packages have been integrated into a camera system that has produced high-quality infrared imagery.
Pyroelectric detectors based on PbTiO3 (PTO) and Pb1-xCaxTiO3 (PCTx) were fabricated and evaluated on thin silicon micro electrical-mechanical structures (MEMS) utilizing a new sol-gel technique. The sol-gel approach allows arbitrary Pb:Ca ratio, and is fully compatible with existing CMOS processes. The novel precursor chemistry developed in our laboratory is not moisture sensitive, and has a shelf-life of many months. Layers of PTO precursor were spin-coated onto 4 micrometers thick silicon membrane structures. Heat treatment at 700 degree(s)C yielded crack-free films approximately 0.3 micrometers thick. Contacts to the top of the pyroelectric film and to the silicon membrane were made by thermal evaporation of aluminum. The reduced thermal capacitance and improved thermal isolation offered by the MEMS membrane structure have significantly improved the performance of our device over typical bulk single crystal or ceramic devices. Pyroelectric responsivity of 2.5 volt/watt was measured at (lambda) equals 10.6 micrometers for a first generation simple membrane structure without poling or electrical bias. SPICE analysis of a distributed thermal circuit model indicates the potential for two orders of magnitude improvement in device sensitivity for optimized membrane structures currently under construction. These results also indicate that a fully integrated high-performance room temperature imaging array for use in the far IR can be realized at low cost.
In this paper, we report on the growth of InSb on (100) Si and (111)B GaAs substrates and the growth of InAsSb alloys for longer wavelength applications. The fabrication and characterization of photodetectors based on these materials are also reported. Both photoconductive and photovoltaic devices are investigated. The photodiodes are InSb p-i-n structures and InSb/InAs1-xSbx/InSb double heterostructures grown on (100) and (111)B semi-insulating GaAs and Si substrates by low pressure metalorganic chemical vapor deposition and solid source molecular beam epitaxy. The material parameters for device structures have been optimized through theoretical calculations based on fundamental mechanisms. InSb p-i-n photodiodes with peak responsivities approximately 103 V/W were grown on Si and (111) GaAs substrates. An InAsSb photovoltaic detector with a composition of x equals 0.85 showed photoresponse up to 13 micrometers at 300 K with a peak responsivity of 9.13 X 10-2 V/W at 8 micrometers . The RoA product of InAsSb detectors has been theoretically and experimentally analyzed.
This paper presents an overview of semiconductor ultraviolet (UV) detectors that are currently available and associated technologies that are undergoing further development. At the beginning, the classification of UV detectors and general requirements imposed on these detectors are presented. Further consideration are restricted to modern semiconductor UV detectors, so the current state-of-the-art of different types of semiconductor UV detectors is presented. Hitherto, the semiconductor UV detectors have been mainly fabricated using Si. Industries such as the aerospace, automotive, petroleum, and others have continuously provided the impetus pushing the development of fringe technologies which are tolerant of increasingly high temperatures and hostile environments. As a result, the main effort are currently directed to a new generation of UV detectors fabricated from wide-band-gap semiconductors between them the most promising are diamond and AlGaN. The latest progress in development of AlGaN UV detectors is finally described in detail.
In SPIE Proceeding 2397 we demonstrated that there is a large payoff still to be gained by further improvements in the performance of solar blind UV detectors for astronomical purposes. We suggested that a particularly promising future technology is one based on the ability of investigators to produce high-quality films made of wide bandgap III-IV semiconductors. Here we report on significant progress we have made over the past year to fabricate and test single-pixel devices. The next step will be to measure and improve detective efficiency, measure the solar blindness over a larger dynamic range, and begin developing multiple-pixel designs.
Metalorganic chemical vapor deposition was used to deposit AlxGa1-xN active layers with varying aluminum compositions on basal plane sapphire substrate. AlxGa1-xN (x < 0.5) ultraviolet photodetectors have been fabricated and characterized with cut-off wavelengths as short as 260 nm. Carrier lifetimes on the order of 10 milliseconds were estimated from frequency dependent measurements of the responsivity.
The semiconductors gallium arsenide and polycrystalline diamond can both be obtained in a high resistivity form suitable for the fabrication of photosensors and detector arrays for the ultraviolet region of the electromagnetic spectrum. These materials have different energy bandgaps and chemical properties which make them complementary partners in providing photodetectors for coverage of the spectral range from near 100 nm in the vacuum ultraviolet to the near infrared at 870 nm. The readily available forms of these two semiconductors are different. GaAs is available in the form of single crystal wafers with uniform properties while polycrystalline diamond in a tight packing of crystallites with varying orientations providing only an average uniformity on the micron scale determined by the size of the crystallites. The GaAs MSM detectors were studied for use in monolithic GaAs-based charge-coupled device scanners for the ultraviolet spectroscopy. The polycrystalline diamond MSM devices are being investigated for hybrid scanners on silicon for the vacuum ultraviolet.
We have fabricated high speed Si metal semiconductor metal photodetector using 19F+ ion implantation in low doped Si. Bandwidths in excess of 6 GHz have been obtained representing more than an order of magnitude improvement over unimplanted counterparts. Measurements using short optical pulses show that the increase in bandwidth is primarily due to shorter carrier lifetime in implanted devices. In the absence of implantation, the response under short optical pulse excitation has a long decay with a time constant of approximately 0.35 ns. An optical fiber transmission experiment using a GaAs ((lambda) approximately 0.85 micrometers ) laser source and the implanted Si photodetector was carried out. Error-free transmission (BER < 10-11) with good receiver sensitivity was obtained at 2 Gb/s. These results demonstrate implanted Si can be used as a detector for short wavelength fiber optic communication systems for speeds up to a few Gb/s. Monolithic integration of this detector technology with conventional Si processing offer the potential for low cost receiver designs.
A feature of metal-semiconductor-metal (MSM) photodetectors is their low capacitance. In this paper, a new design formula is presented for the capacitance per unit area of an MSM photodetector. A comprehensive circuit model is also described which incorporates the optoelectronic conversion within a larger model which in turn accommodates the transmission line effects of the interdigitated structure. These models can be used to optimize the design of a receiver employing an MSM photodetector.
Planar metal-semiconductor-metal (MSM) devices fabricated on gallium arsenide (GaAs) are promising candidates for use as photodetectors in coherent optical communications and millimeter-wave phased-array applications. Their primary features are broad bandwidth, large responsivity, high power-handling capability, and compatibility with monolithic optoelectronic integrated circuits. We have characterized the performance of an interdigitated GaAs MSM photodetector grown by molecular beam epitaxy at 350 degree(s)C using a fast sampling technique in the time domain. A key factor for undoped GaAs material grown at this temperature is the optimal combination of both low dark current and high photocurrent. Experimental measurements are made of the temporal response of the MSM detector to optical impulses generated by a mode-locked titanium-sapphire (Ti:Al2O3) laser. Speed and responsivity are characterized over a range of optical powers and DC bias voltages. Results demonstrate that this device can switch up to 69% of the applied DC bias voltage under high optical pulsed power. Results also indicate responsivities exceeding 80 mV/pJ and bandwidths approaching 20 GHz. This high-efficiency, broad-bandwidth photodetector may find critical applications in the optical production of millimeter-wave signals by frequency conversion (mixing) and harmonic generation.
We will present high quality In0.53Ga0.47As which has been grown on semi- insulating (100) InP:Fe substrates by rare earth doped (Yb, Gd, and Er) liquid phase epitaxy using a graphite boat. The new earth ions, which are highly reactive, are thought to better impurities like O, C, and Si by reacting with these impurities and precipitating out in the melt, but not incorporating into the epitaxial layer to any significant amount.
We have succeeded in fabricating sensitive superconducting YBaCuO membrane bolometers by two original techniques on silicon and silicon-on-implanted-oxide (SIMOX) substrates. The first process is based on a silicon dry reactive ion etching (RIE) and the second on the selective SiO2 layer etching of a SIMOX substrate. On a 10 micrometers wide, 150 micrometers long 'RIE-type' suspended bridge, the best sensitivity and optical NEP are 1580 V/W and 2.10-12 W/(root)Hz respectively at 85 K, with a thermal response time of 750 microsecond(s) . These responsivity and NEP values, measured on non-optimized structures, are among the best reported for liquid nitrogen-cooled thermal detectors. In addition, very short response times in the order of 7 microsecond(s) are obtained on `SIMOX type' air-bridges.
A vertical resonant cavity detector for thermal imaging in the 8 - 9 micrometers wavelength range has been demonstrated, using a p-SiGe/Si quantum well structure on a silicon-on-insulator substrate with a 2 micrometers thick buried oxide layer. The photoresponse spectrum shows peaks at wavelengths corresponding to standing waves in the cavity, confirming resonant detection. The measured responsivity at the main cavity resonance near 8.7 micrometers , with 2 V bias, is 10 mA/W. This is several times larger than the responsivity typically observed, at the same wavelength and bias, for comparable non-resonant detectors grown on attenuating p+-Si substrates. The resonant device uses the Si/SiO2 interface as the buried mirror. The reflectance of this interface is particularly high between 7 - 9 micrometers , due of the dispersion of the refractive index near the oxide phonon absorption band.
We demonstrate the heterodyne detection of two CO2 laser signals offset in frequency up to 82.16 GHz using a multiple quantum well intersubband infrared photodetector. The high frequency is reached by down conversion using the detector itself as a microwave or millimeter-wave mixer.
Steady state computer analysis in the dark followed by time domain analysis in presence of optical radiation has been carried out to study the effect of ionization rates of charge carriers on the quantum efficiency and avalanche noise factors of p+nn+ and p+pnn+ structures of InP and GaAs avalanche photodetectors (APD). The simulation results show that the quantum efficiency of a GaAs APD is higher (80% at (lambda) equals 0.79 micrometers ) than that of an InP APD (64% at (lambda) equals 0.89 micrometers ) for both the structures. The results also suggest that the noise performance characteristics of both InP and GaAs APD's are sensitive functions of ionization rate data but the quantum efficiency is almost independent of ionization rates.