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Volume 2999 Photodetectors: Materials and Devices II

Gail J. Brown, Manijeh Razeghi April 1997

Conference Location: San Jose, CA, USA Conference Date: Wednesday 12 February 1997

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Infrared Photodetectors I

Select this Article		State of infrared photodetectors and materials Thomas N. Casselman Proc. SPIE 2999, 2 (1997); http://dx.doi.org/10.1117/12.271169 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract In the last 10 years, infrared technology has seen a dramatic increase in new detector materials and structures as well as new and more complex detector architectures. This paper reviews the newest materials, growth technologies, and detector architectures that have been developed recently.

Select this Article		Radiative lifetime in semiconductors: influence of photon recycling Christoph H. Grein, Henry Ehrenreich, and E. Runge Proc. SPIE 2999, 11 (1997); http://dx.doi.org/10.1117/12.271179 \| Cited 1 time Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract A microscopic theory of radiative recombination including the effects of photon reabsorption has ben developed which does not make the traditional assumptions of spherical constant energy surfaces, Maxwell-Boltzmann statistics for carriers, and constant momentum matrix elements. Numerical results illustrating the effects of photon recycling will be discussed for superlattice and bulk infrared detector materials. The calculations employ realistic K.p band structures and matrix elements. The lifetime enhancement can be greater than an order of magnitude.

Select this Article		Pixelless infrared-imaging device concept Hui C. Liu Proc. SPIE 2999, 19 (1997); http://dx.doi.org/10.1117/12.271185 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract We present a new concept for infrared imaging using a pixel- less up-conversion device together with a CCD. The concept is applicable to wavelengths longer than the CCD response range. A specific implementation using quantum well infrared photodetectors integrated with light emitting diodes is discussed in detail.

Select this Article		GaInAsSb and InAsSbP photodetectors for mid-infrared wavelengths Zane A. Shellenbarger, Michael G. Mauk, Mark Gottfried, Joseph D. Lesko, and Louis C. DiNetta Proc. SPIE 2999, 25 (1997); http://dx.doi.org/10.1117/12.271198 \| Cited 2 times Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Progress on mid-IR photodetectors fabricated by the liquid phase epitaxial growth of GaInAsSb and InAsSbP on GaSb substrates is reported. Both p/n junction and avalanche photodiode structures were fabricated. Preliminary results indicate that these devices can have higher detectivity with lower cooling requirements than commercially available detectors in the same wavelength range. Room temperature detectivity of 1.85 X 10⁸ cmHz^1/2/W was measured for GaInAsSb detectors, while room-temperature avalanche multiplication gain of 20 was measured on AlGaAsSb/GaInAsSb avalanche photodiodes. Infrared p/n junction detectors made from GaInAsSb and InAsSbP showed cut-off wavelengths of 2.3 micrometers and 4.4 micrometers respectively.

Select this Article		Isothermal vapor phase epitaxy as a versatile technology for infrared photodetectors Krzysztof Adamiec, Miroslaw Grudzien, Zenon Nowak, Jaroslaw Pawluczyk, Jozef Piotrowski, Jarek Antoszewski, John M. Dell, Charles A. Musca, and Lorenzo Faraone Proc. SPIE 2999, 34 (1997); http://dx.doi.org/10.1117/12.271208 \| Cited 3 times Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract We report here the use of isothermal vapor phase epitaxy to grow 3D Hg_1-xCd_xTe heterostructures for photoconductive, photovoltaic and photoelectromagnetic infrared detectors operated at near room temperatures. A reusable two-zone atmospheric pressure growth system has been developed.the system makes it possible not only to grow epilayers but also to perform in situ other processes such as high temperature annealing to control the compositional grading, the low temperature annealing for reduction of native acceptor concentration, and doping with foreign impurities. The required various composition profiles have been theoretically predicted and then implemented changing the temperature and mercury pressure during growth and subsequent thermal treatment. In addition, post-growth etching, substrate shaping, selective epitaxy, and negative epitaxy have been used to achieve 3D band gap profiles. The photoconductors were based on lightly p-type doped epilayers. Low diffusion length, weak absorption of radiation and a very low junction resistance makes it difficult to obtain useful performance of longwavelength photovoltaic devices operating at near room temperature. This was overcome with development of multiple heterojunction photovoltaic devices in which short elements were connected in series. To improve the performance of any type of heterostructure photodetector, monolithic optical immersion has been used. Detectivities as high as 1 X 10⁸ cmHz^1/2/W and 1 X 10⁹ cmHz^1/2/W were obtained at (lambda) equals micrometers and temperatures of 300 K and 220 K, respectively.

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Infrared Photodetectors II

Select this Article		Design issues relating to low-temperature dark current in quantum well infrared photodetectors Anjali Singh and David A. Cardimona Proc. SPIE 2999, 46 (1997); http://dx.doi.org/10.1117/12.271211 \| Cited 1 time Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Quantum well infrared photodetectors (QWIPs) have emerged as a viable contender for many remote sensing applications, even in the space environment where low background fluxes are involved. There are, however, several issues that still need to be resolved in order to achieve optimum performance at low operating temperatures. One important issue is the dark current, which is dominated by Fowler-Nordheim and trap-assisted tunneling in this regime. In order to decrease the dark current of QWIPs at low temperatures, we are investigating these mechanisms to better understand them. Our preliminary investigations have uncovered another possible problem; offsets in the I-V characteristics which could impair the compatibility of a QWIP array with a readout circuit. In this paper we discuss these design issues.

Select this Article		Sb-based infrared materials and photodetectors for the near-room-temperature applications Jedon D. Kim, Erick J. Michel, H. Mohseini, Joseph S. Wojkowski, J. J. Lee, and Manijeh Razeghi Proc. SPIE 2999, 55 (1997); http://dx.doi.org/10.1117/12.271212 \| Cited 1 time Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract We report on the growth of InSb, InAsSb, and InTlSb alloys for infrared photodetector applications. The fabrication and characterization of photodetectors based on these materials are also reported. Both photoconductive and photovoltaic devices are investigated. The materials and detector structures were grown on (100) and (111)B semi-insulating GaAs and GaAs coated Si substrates by low pressure metalorganic chemical vapor deposition and solid source molecular beam epitaxy. Photoconductive detectors fabricated from InAsSb and InTlSb have been operated in the temperature range from 77 K to 300 K. The material parameters for photovoltaic device structures have been optimized through theoretical calculations based on fundamental mechanisms. InSb p-i-n photodiodes with 77 K 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 R_oA product of InAsSb detectors has been theoretically and experimentally analyzed.

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Photodiodes for Optical Communication I

Select this Article		Improvement of avalanche photodetectors through integration of InGaAs and Si Aaron R. Hawkins, Weishu Wu, and John E. Bowers Proc. SPIE 2999, 68 (1997); http://dx.doi.org/10.1117/12.271213 \| Cited 1 time Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Near infrared avalanche photodetectors have been constructed by wafer fusing epitaxial layers of InGaAs to Si. This integration combines the light absorption properties of InGaAs with the avalanche multiplication properties of Si. We concentrate here on two of the advantages these detectors have: desirable gain sensitivity properties, and high gain- bandwidth-products which would make them ideal for optical communication applications. Measurements are shown that illustrate very gradual gain increases with increasing device voltages as well as gain-bandwidth-products of over 300 GHz.

Select this Article		Techniques for the design and simulation of interdigitated MSM photodetectors Laurence W. Cahill Proc. SPIE 2999, 78 (1997); http://dx.doi.org/10.1117/12.271170 \| Cited 1 time Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract The metal-semiconductor (MSM) photodetector attracts a great deal of interest as a result of its high bandwidth and low fabrication costs. In this paper a broad-band circuit model for the interdigitated MSM photodetector is presented. The circuit model can be used for both design and simulation purposes. The circuit model can also take into account nonlinear effects so that the practical behavior of the photodetector can be more faithfully represented.

Select this Article		Equivalent circuit modeling of metal-semiconductor-metal photodiodes with transparent conductor electrodes Sean L. Rommel, David N. Erby, Wei Gao, Paul R. Berger, George J. Zydzik, W. W. Rhodes, H. M. O'Bryan, Deborah L. Sivco, and Alfred Y. Cho Proc. SPIE 2999, 86 (1997); http://dx.doi.org/10.1117/12.271171 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Metal-semiconductor-metal (MSM) photodiodes with electrodes fabricated from the transparent conductor cadmium tin oxide (CTO) have been shown to double photoresponsivity. Their bandwidths, however, are significantly lower than those of MSMs fabricated with standard Ti/Au contacts. Though MSMs are generally believed to be limited by the transit time of electrons, it is possible the larger resistivity of CTO has become a significant factor, making the MSMs RC time constant limited instead. Previous models of MSMs only account for one of the two back-to-back Schottky diodes. A new model which takes into account both the forward and reverse biased junctions has been developed from the small signal model of a Schottky diode. This new model was fit to data obtained from S-parameter measurements, and incorporates both the transit time response and RC time constant response.

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Quantum Well Infrared Photodetectors I

Select this Article		Improved performance III-V quantum well IR photodetectors: review of current and potential focal plane technology Lewis T. Claiborne Proc. SPIE 2999, 94 (1997); http://dx.doi.org/10.1117/12.271178 \| Cited 2 times Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract The maturation of the III-V materials technology has provided an opportunity for the development of a producible and affordable class of IR detector arrays. Designs based on the GaAs compounds permit the realization of multiple quantum well IR photodetectors (QWIPs) which are useful for long wavelength focal plane arrays with sizes demonstrated up to 640 X 480. Similar designs using InP based materials can cover an even broader IR spectral region with lattice matched structures. QWIP demonstrations have been made for midwave detectors and very long wave detectors as well. New detector structures that improve optical performance and reduce bias current can lead to higher performance QWIPs which approach the performance of mercury cadmium telluride at moderate operating temperatures. These developments offer the possibility of practical, large, affordable IR focal plane arrays in the near future.

Select this Article		Optical coupling mechanisms in quantum well infrared photodetectors Sumith V. Bandara, Sarath D. Gunapala, John K. Liu, Winn Hong, and Jin S. Park Proc. SPIE 2999, 103 (1997); http://dx.doi.org/10.1117/12.271180 \| Cited 2 times Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Light coupling systems, such as gratings are required because quantum well infrared photodetectors do not respond to normal incident light due to the quantum mechanical selection rules associated with intersubband transitions. The resolution of the photolithography and accuracy of the etching become key issues in producing smaller grating features sizes especially in shorter wavelengths. An enhancement factor of three due to 2D periodic grating fabricated on a QWIP structure was observed. Variation of the enhancement factor with groove depth and feature size of the grating can be theoretically explained.

Select this Article		Simplified quantum well infrared photodetectors Meimei Z. Tidrow Proc. SPIE 2999, 109 (1997); http://dx.doi.org/10.1117/12.271181 \| Cited 1 time Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Abstract not available

Select this Article		Dry etching processes for fabrication of QWIPs and other detector structures Stephen J. Pearton Proc. SPIE 2999, 118 (1997); http://dx.doi.org/10.1117/12.271182 \| Cited 4 times Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract The fabrication of QWIPs and various grating structures for detectivity improvements requires the ability to selectively or non-selectively or non-selectively etch GaAs/AlGaAs, GaAs/InGaP, AlInAs/InGaAs and other heterostructure systems with high resolution. For selective etching of GaAs over ALGaAs, plasma chemistries involving BCl₃ or SiCl₄ with additions of SF₆ or CF₄ can provide selectivities over 600:1 when the ion energies and fluxes in the plasma are relatively low. The selectivity is severely reduced when the ion energy exceeds approximately 150 eV, because of sputter-enhanced removal of the AlF₃ which provides the etch-stop reaction. Fluorine-free, chlorine- based chemistries provide equi-rate etching of Al_xGa_1-xAs across the entire composition range as long as water vapor is excluded from the reactor chamber. This is readily achievable in load-locked systems, and BCl₃ is an attractive choice as the plasma chemistry because of its ability to get water vapor and to attack the native oxide on III-V materials. The CH₄/H₂ plasma chemistry is selective for InGaAs over AlInAs at low ion energies, and also provides smooth controlled etching of II-VI compounds such as HgCdTe and related materials. ECR plasma sources have a number of advantages over conventional RIE systems, including higher etch rates, but typically display lower etch selectivities because of the very high ion fluxes. We also describe new etch processes developed for the III-N materials such as GaN, AlN and InN. Their high bond energies relative to the more common compound semiconductors have generally led to reports of low etch rates and the need for very high dc self-biases, but we show it is possible to use new plasma chemistries such as ICl under ECR conditions which produce efficient etching of these materials.

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Quantum Well Infrared Photodetectors II

Select this Article		Effect of compressive strain on the performance of p-type quantum well infrared photodetectors Sheng S. Li and Jerome T. Chu Proc. SPIE 2999, 132 (1997); http://dx.doi.org/10.1117/12.271183 \| Cited 1 time Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract A detailed study of the performance of compressively strained p-type III-V quantum well infrared photodetectors (p-QWIPs) is presented in this work. Three device structures composed of InGaAs/GaAs, InGaAs/AlGaAs, and InGaAs/AlGaAs/GaAs for normal incidence absorption have been fabricated and analyzed, with the results being compared with similar reported unstrained p-QWIPs. In all three QWIP structures, the quantum well layers are under biaxial compressive strain ranging from -0.8 to 2.8 percent, while the barrier layers are lattice matched to the substrate. The detection peaks of the quantum well infrared photodetectors ranged from 7.4 micrometers to 10.4 micrometers . The detectors utilized the bound-to-continuum, bound-to-quasi- bound, and step bound-to-miniband intersubband transitions for infrared detection. The results showed that responsivities of up to 90 mA/W and detectivities from 10⁹ to over 10¹⁰ cm (root) Hz/W are achieved under moderate applied bias and at reasonable operating temperatures, demonstrating the viability of the strained layer p-doped quantum well infrared photodetectors for staring focal plane array applications.

Select this Article		Very long wavelength GaAs/GaInP quantum well infrared photodetectors Christopher L. Jelen, Steven Slivken, Gail J. Brown, and Manijeh Razeghi Proc. SPIE 2999, 144 (1997); http://dx.doi.org/10.1117/12.271184 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract We demonstrate long wavelength quantum well infrared photodetectors with GaAs quantum wells and GaInP barriers grown using gas-source molecular beam epitaxy. Wafers were grown with varying well widths. The optimum well width was 75 angstrom, which resulted in a detection peak at 13 micrometers and a cutoff wavelength of 15 micrometers . Dark current measurements of the samples with 15 micrometers cutoff wavelength show low dark current densities. The dark current characteristics have been investigated as a function of temperature and electron density in the well and compared to a model which takes into account thermionic emission and thermally assisted tunneling. The model is used to extract a saturation velocity of 1.5 X 10⁵ cm/s for electrons. The photoelectron lifetime before recapture has been deduced from this carrier velocity and photoconductive gain measurements. The lifetime is found to be approximately 5 ps. Preliminary focal plane array imaging is demonstrated.

Select this Article		Nonlinear photoresponse of quantum well infrared photodetectors at high-excitation power Maxim Ershov, Hui C. Liu, Margaret Buchanan, and Zbigniew R. Wasilewski Proc. SPIE 2999, 153 (1997); http://dx.doi.org/10.1117/12.271210 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Nonlinear photoresponse effects at high excitation power in quantum well infrared photodetectors (QWIPs) are studied both experimentally and theoretically. The photoconductivity nonlinearity is mainly caused by a redistribution of the electric potential at high power, which leads to a decrease of electric field in the bulk of the QWIP. As a result of the decreased field, the photoexcited electron escape probability and drift velocity decrease, resulting in a decrease of responsivity. These effects are strongly influenced by QWIP structural parameters and operating conditions. In QWIPs with a few QWs the IR power required to observe a decrease of responsivity is much lower than that needed to cause the saturation of the intersubband absorption. Key factors in designing a QWIP with a suppressed nonlinearity are discussed.

Select this Article		Can coupling with remote conduction bands cause a sufficient normal-incidence absorption in n-type direct-gap semiconductor quantum wells? Rui Q. Yang Proc. SPIE 2999, 161 (1997); http://dx.doi.org/10.1117/12.271186 \| Cited 4 times Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Surprisingly, several experiments have reported that normal- incidence light absorption due to inter-conduction-subband transitions in direct-gap semiconductor quantum wells is a s strong as in-plane-incidence absorption.In contrast to other models, a recent theoretical study claimed that a 14-band k.p model including multiband coupling terms due to remote- conduction bands is able to explain the experimental results. In the percent work, a concise formulation extends the model beyond 14 bands. Nevertheless, after re-deriving the optical transition matrix elements, this analysis clearly shows that the oscillator strength for the in-plane polarized optical intersubband transition due to the multiband coupling effects is much smaller than the oscillator strength for the normal-to-plane polarized optical intersubband transition. These results indicate that the multiband coupling effects due to remote-conduction bands cannot cause a sufficient in-plane polarized optical intersubband transition to produce the observed normal- incidence absorption in the desirable n-type III-V compound semiconductor quantum wells.

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Photodiodes for Optical Communication II

Select this Article		High-efficiency high-speed (60-GHz) InGaAsP multimode waveguide photodetectors Jean C. Renaud, Francois A. Deborgies, Y. Combemale, Robert R. Blondeau, Jean-Pascal Duchemin, Jean-Pierre Vilcot, Joseph Harari, and Didier J. Decoster Proc. SPIE 2999, 170 (1997); http://dx.doi.org/10.1117/12.271187 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Abstract not available

Select this Article		InP photodetectors for millimeter wave applications based on edge-coupled heterojunction phototransistors Jerome Van de Casteele, Vincent Magnin, Jean-Philippe Gouy, Jean-Pierre Vilcot, Joseph Harari, Sophie Maricot, and Didier J. Decoster Proc. SPIE 2999, 176 (1997); http://dx.doi.org/10.1117/12.271188 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Abstract not available

Select this Article		Wide-spectral response photodetectors based on microcrystalline hydrogenated silicon thin films Manuela Vieira, Alessandro Fantoni, Svetoslav Koynov, and Reinhard Schwarz Proc. SPIE 2999, 183 (1997); http://dx.doi.org/10.1117/12.271189 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Microcrystalline hydrogenated silicon based p-i-n photodiodes with an increased infrared sensitivity have been deposited by a new cyclic CVD method. The microcrystalline deice, when compared to the amorphous counterpart, presents an increased collection efficiency, dependent on bias voltage, in the red and near-infrared spectral region. The shape of the spectral response also depends on the details of the quality of the individual layers, like conductivity and thickness of the p-type contact layer. AS model for the electronic transport of (mu) c-Si:H p-i-n photodetectors is presented and supported by numerical simulation.

Select this Article		Noise analysis of the detection unit pixel a-Si:H Peter Balco, Jean-Marie Peransin, and Bernard P. Orsal Proc. SPIE 2999, 193 (1997); http://dx.doi.org/10.1117/12.271190 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract We present experimental and numerical simulation results on the noise levels generated in hydrogenated amorphous silicon PIN diodes. THe diodes have the tow structures forming a PIXEL arrangement.A parallel study is developed to compare the noise sources and to precise their location. Firstly we have led the analysis and the modelization of voltage - current curves separately for PIN and NIP diodes. The electrical model valid for both structures is designed. Secondly the noise for these structures is examined for the frequency range from 10Hz to 200kHz. Three different noise generators are extracted from analysis of the spectra. It is shown, that shot noise, 1/f noise and f^-1/2 noise sources are present in the dark forward current. The model to explain white noise levels in designed and its validity is verified by experimental data. The dominant typical noise generator for the hydrogenated amorphous semiconductor takes the form of f^-1/2 spectra, it is discussed as noise source which is formed by the composition of trap noise generators placed in the intrinsic layer and near the interface N⁺N^-.

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Near-Infrared Photodetectors

Select this Article		High-quantum-efficiency 2.2-um InGaAs MOCVD photodiodes Steven Wojtczuk, Peter C. Colter, and Murzy D. Jhabvala Proc. SPIE 2999, 204 (1997); http://dx.doi.org/10.1117/12.271191 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Photodiodes responding in the 0.8-2.3 micrometers wavelength range are of interest in a wide range of applications, from wind- shear detection systems which use eyesafe 2.1 micrometers lasers to differential absorption LIDAR aerosol measurements of CO₂. In this paper, we report on uncooled, broadband, 2.25 micrometers lattice-mismatched 0.55eV In_0.72Ga_0.28As photodiode arrays, in which the cutoff wavelength has been 'extended' from the 1.65 micrometers which is standard for 0.74eV In_0.53Ga_0.47As lattice-matched to InP wafers. In_xGa_1-xAs step-grading layers were used to transition from the InP wafer to the final In_0.72Ga_0.28As photodiode material during the metal organic chemical vapor deposition epitaxial growth. Linear 64 X 1 photodiode arrays were made with an independently-verified external quantum efficiency above 50 percent from 0.8 to 2.2 micrometers using MgF₂/ZnS dual layer antireflection coating. Average 300 degree K area-normalized dark current for these N/P diodes was 5 X 10^-5 A/cm² at 10mV reverse bias.

Select this Article		Monolithic InGaAs-on-silicon shortwave infrared detector arrays Abhay M. Joshi, Rene Brown, Eugene A. Fitzgerald, Xinde Wang, Steve M. Ting, and Mayank T. Bulsara Proc. SPIE 2999, 211 (1997); http://dx.doi.org/10.1117/12.271192 \| Cited 1 time Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract We have designed and developed 1, 16, 256, and 512 element linear monolithic InGaAs-on-silicon infrared detector array for the 1-3 micrometers SWIR band. A methodology of monolithically integrating InGaAs photodetectors and high density complex CMOS readout electronics all on a single silicon substrate has been developed. The innovation consists of an improved 'selective' epitaxial technique that significantly reduces the misfit dislocation density caused by the severe lattice mismatch between the In_xGa_1-xAs photodetector's absorption region and the silicon substrate. The individual pixel size of (40 X 40) and (80 X 80) micrometers ² exhibits room temperature RoA product of 40 to 45 (Omega) - cm². The InGaAs photodetectors are operated at a zero bias voltage to eliminate leakage current integration, reduce the 1/f noise, and maintain a uniform bias for light detection. The CMOS readout circuitry for each individual pixel consists of an integrating pre-amplifier, a CDS signal processor, and a voltage to current converter. The column scanner for the linear array is a D type shift register with a master clear signal reset once per frame. The master clock signal shifts the bit through the registers one column per clock cycle. When a pixel is selected, the output signal at the voltage to current converter is coupled to a transimpedance amplifier. This amplifier drives the line capacitance of the output bus line to achieve 25 MHz output speed. Power dissipation of less than 100 mW has been demonstrated for 10 MHz operation.

Select this Article		Monolithic active pixel InGaAs focal plane arrays for near-infrared imaging Jiten Sarathy, George A. Gasparian, Michael J. Lange, Marshall J. Cohen, Gregory H. Olsen, Dong-Su Kim, Pavel Studenkov, and Stephen R. Forrest Proc. SPIE 2999, 225 (1997); http://dx.doi.org/10.1117/12.271193 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract We present the preliminary results of our investigation into the demonstration of a 16 X 16 element 'active-pixel sensor' array for imaging in the 0.5-2.5 micrometers wavelength spectrum. Each pixel consists of an InGaAs photodiode integrated with InP JFET-based signal processing electronics. An InP-based reset switch in each pixel is addressable using peripheral row and column select switches. The pixel architecture is based on a source-follower circuit. Each pixel has only four devices - three JFETs and a photodetector. This architecture can potentially lead to pixel fill factors in excess of 60 percent. The InP-based JFETs have extremely low leakage currents and high ON/OFF switching ratios. Each photodiode is independently addressed by applying a voltage to the reset terminal. Once the reset transistor Qa is switched ON, the photocurrent drives the output terminal using the source-follower circuit. The Sorce Follower Array architecture facilitates monolithic integration of the InP JFETs and InGaAs photodetectors with up to 2.5micrometers -wavelength cut-off. One important aspect of the JFET design is the p-type InP epilayer under the channel region of the JFET. This grown in p-n junction isolates the channel from the substrate thereby reducing the leakage current of the JFET.

Select this Article		Photodetectors of slit and sandwich types based on CdS and CdS_1-xSe_x films obtained using MOCVD method from dithiocarbamates Ludmila V. Zavyalova, Sergey V. Svechnikov, and Vladimir G. Tchoni Proc. SPIE 2999, 235 (1997); http://dx.doi.org/10.1117/12.271194 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Here we report the results of working out an original, simple in control and not requiring expensive equipment MOCVD-method for depositing films of semiconductor compounds A₂B₆. Dithiocarbamates (DTC) are used as starting materials. The compounds are stable, easily synthesized, cheap and low toxic. Atoms of metal and sulfur in the DTC are strongly bonded. The DTC could be easily dissolved in various organic solvents. The experimental unit for film deposition comprises a spraying apparatus, a substrate heater, and a quartz cylinder for separation of a reaction zone from ambience. The process of film deposition is carried out in air conditions. Films of CdS, bright-yellow, transparent, having mirror smooth surface at thickness less than 2 mkm and rough surface at thickness 8-12 mkm, were deposited by spraying cadmium dithiocarbamate, that is DTC with radical C₂H₅, solution in pyridine on substrates heated to 240-280 degrees C. Deposition rate was 60-90 nm/min. Films obtained were of hexagonal modification, polycrystalline, textured, with low, at the level of centipercents content of oxygen and carbon. Slit type photodetectors based on CdS and CdS_1-xSe_x of 1.0 mkm thickness have dark conductivity (sigma) _d equals 10^-9 divided by 10^-8 Ohm^-1cm^-1 and photoconductivity (sigma) _ph equals 10^-2 divided by 10^-1 Ohm^-1cm^-1 at 200 lux. Industrially suitable technology for production of photopotentiometer on the base of these films was developed. Sandwich-type photodetectors In₂S₃ - CdS: Cu, Cl - In with 8-12 mkm thickness have the same value of photoconductivity and the light-to-dark ratio is 10⁶ divided by 10⁷. Based on sandwich-type photodetectors, a hybrid structure of pyroelectric-photodetector as a resonant-type coordinate-sensitive detector was developed.

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UV Photodetectors I

Select this Article		Advances in astronomical UV image sensors and associated technologies Charles L. Joseph Proc. SPIE 2999, 244 (1997); http://dx.doi.org/10.1117/12.271197 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Large-format ultraviolet image sensors have been and are actively being developed for a variety of space-borne astronomy missions. The detector is historically one of the most problematic parts of any astronomical spacecraft and it plays a critical role in the overall capability of the instrument. There are numerous detector systems with one being ideal for all applications. This paper focuses on the physical processes responsible for the inherent strengths and weaknesses of a few important UV image sensors as well as the recent technological progress that has been mae to improve performance of these devices.

Select this Article		Requirements and design considerations of UV and x-ray detectors for astronomical purposes Melville P. Ulmer Proc. SPIE 2999, 259 (1997); http://dx.doi.org/10.1117/12.271195 \| Cited 1 time Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Since our previous review, new developments have occurred which make it interesting to compare wide bandgap semiconductors with low temperature detectors. We compare the strengths and weaknesses of each of these devices. We show that there is a need for both types of devices. Furthermore, single or small-array versions of the low temperature devices have been shown to work as single photon-counting devices. This gives the low temperature devices a distinct advantage over wide bandgap detectors. The ability to operate at room temperature and to be able to use standard indium bump bonding techniques to allow for relatively routine fabrication of 500 X 500 pixel formats should, however, provide impetus to further development of wide bandgap semiconductor devices.

Select this Article		Intrinsic Al_xGa_1-xN photodetectors for the entire compositional range Danielle Walker, Xiaolong Zhang, Adam W. Saxler, Patrick Kung, Jianren Xu, and Manijeh Razeghi Proc. SPIE 2999, 267 (1997); http://dx.doi.org/10.1117/12.271200 \| Cited 2 times Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Al_xGa_1-x ultraviolet photoconductors with cut- off wavelengths from 365 nm to 200 nm have been fabricated and characterized. Various characteristics of the devices, such as photoresponse, voltage-dependent responsivity, frequency-dependent responsivity and noise spectral density, were measured and cross-referenced with optical, electrical and structural characteristics of the material to provide information about the mechanisms taking place during detection. The maximum detectivity reached 5.5 X 10⁸ cm X Hz^1/2/W at a modulating frequency of 14 Hz. The effective majority carrier lifetime in Al_xGa_1-xN materials, derived from frequency-dependent photoconductivity measurements, has been estimated to be from 6 to 35 msec. The frequency-dependent noise-spectrum shows that it is dominated by Johnson-noise at high frequencies for low Al-composition samples.

Select this Article		AlGaN ultraviolet detectors Manijeh Razeghi and Antoni Rogalski Proc. SPIE 2999, 275 (1997); http://dx.doi.org/10.1117/12.271196 \| Cited 2 times Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Hitherto, the semiconductor ultraviolet (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 efforts are currently directed to anew 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 described in detail.

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UV Photodetectors II

Select this Article		Growth models of GaN thin films based on crystal chemistry: hexagonal and cubic GaN on Si substrates Hitoshi Ohsato and Manijeh Razeghi Proc. SPIE 2999, 288 (1997); http://dx.doi.org/10.1117/12.271199 \| Cited 1 time Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract We have been presented crystallographic growth models of GaN thin films on the (alpha) -Al₂O₃ substrates based on the crystal chemistry: electronegativity, chemical bonds, Pouling's rules in the background of mineralogy. We have introduced an extended atomic distance mismatch in crystal growth models and reported epitaxial growth models and reported epitaxial growth model with edge-type dislocation and bridge-type model growing with some roots contacting with substrates. In this paper, we presented growth models of GaN on Si substrates for an example of crystallographic growth model of thin film and discussed the growth conditions of different hexagonal and cubic phases. Crystal chemistry should have been performed effectively on the same aspects of epitaxial growth.

Select this Article		Growth of III-nitrides for photodetector applications Ian T. Ferguson, Chuong A. Tran, Robert F. Karlicek, Jr., Zhe Chuan Feng, Richard A. Stall, Shaohua Liang, W. Cai, Yuxin Li, Y. Liu, and Y. Lu Proc. SPIE 2999, 298 (1997); http://dx.doi.org/10.1117/12.271209 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Interdigital metal-semiconductor-metal (MSM) and p-n UV photodetectors have been successfully grown and fabricated from GaN based materials. The MSM devices were produced using two types of GaN; high-resistive GaN and Mg doped GaN. For the high-resistive GaN detector, the lowest dark current is approximately 0.1 nA and the UV responsivity of the device was about 460 A/W at a DC bias of 30 V. The Mg doped GaN exhibited larger gains, 1150 A/W at 2.0 V, but at much higher dark currents, 400 nA. The high gain in this device is not well understood but has attributed to an 'avalanche' effect and is under further investigation. The feasibility of a photovoltage detector structure based on alloys of GaN has also been proven. A GaN/GaInN structure exhibited a cut- off at 2.9 eV with a responsivity of 0.28 A/W at zero bias for an active region of only 500 angstrom thick.

Select this Article		High-sensitivity photodetectors made with charge-transfer polymer blends Gang Yu, Jun Gao, Cuiying Yang, and Alan J. Heeger Proc. SPIE 2999, 306 (1997); http://dx.doi.org/10.1117/12.271201 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Blending organic semiconductors with different electron affinities results in an interpenetrating bi-continuous network of internal donor/acceptor (D/A) heterojunctions. These nano-scale D/A junctions show efficient charge separation and charge transfer. The interpenetrating bi- continuous networks of the donor and the acceptor phases also allow the separated carriers to be collected effectively at the anode and cathode contacts. Typical materials used for the donor phase are conjugated polymers (MEH-PPV). Typical materials used for the acceptor phase are conjugated polymer CN-PPV or fullerene molecules. These photosensitive materials are soluble to common organic solvents, and are processable at room temperature. Photodiodes and photovoltaic cells are fabricated with high quantum efficiencies. The carrier collection efficiency and energy conversion efficiency of MEH-PPV:C₆₀ photovoltaic cells are approximately 29 percent electrons/photon and approximately 3 percent under illumination of 20 mW/cm² at 430 nm, two orders of magnitude higher than that in devices with MEH-PPV alone. The photosensitivity and the quantum yields increase to 0.26 A/W and approximately 75 percent electrons/photon at reverse bias of -2V, even higher than those in UV-enhanced Si photodiodes at the same wavelength. Large size photodetectors and image sensors have been fabricated with these materials.

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High-Temperature Superconductors I

Select this Article		Highly sensitive and wideband optical detector in patterned YBa2Cu3O7-d thin film Ken'ichi Tanaka, Yuichi Harada, Martin Danerud, Matsuo Sekine, and Motoichi Ohtsu Proc. SPIE 2999, 316 (1997); http://dx.doi.org/10.1117/12.271202 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract We have demonstrated an optical detector based on patterned YBa₂Cu₃O_7-(delta ) thin films. The responses up to 18 GHz were observed using two wavelength lasers. The amplitude of the light was modulated up to 18 GHz by an optical heterodyne mixing system. The responsivity of the detectors is over 50 V/W for different wavelength lasers below 1 Hz and the responsivity is 4 mV/W at the 780 nm wavelength and 30 mV/W at 1.5 micrometers in the region from 3 GHz to 18 GHz. Our detector is one of the promising candidates for highly sensitive and wideband optical receiver.

Select this Article		Wavelength-selective photodetectors based on dye-superconductor assemblies Sara J. Eames, David C. Jurbergs, Steven M. Savoy, Jianai Zhao, and John T. McDevitt Proc. SPIE 2999, 326 (1997); http://dx.doi.org/10.1117/12.271203 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract A strategy for increasing the wavelength selectivity and responsivity of hybrid dye/superconductor optical sensors is described. Here, reflective 'mirror layers' deposited on the top surface of YBa₂Cu₃O_7-(delta ) thin film devices are used to enhance the optical performance characteristics of such hybrid sensors. Quantification of the wavelength-selectivity for such detector structures is detailed for both dye/high-T_c superconductor and dye/mirror-layer/high-T_c superconductor systems. Optical response studies of the structures suggest that the inclusion of the mirror layer serves to enhance the wavelength-selectivity of the detector. Consequently, only the on-resonance signals captured by the dye layer are effectively sensed by the superconductor element. Measurements of the spectral response properties of the mirror layer-modified hybrid detectors show that energy transfer between the dye and superconducting elements is not diminished by the presence of this reflective layer.

Select this Article		Case for superconductivity associated primarily with charge-reservoirs rather than cuprate planes John D. Dow Proc. SPIE 2999, 335 (1997); http://dx.doi.org/10.1117/12.271204 \| Cited 1 time Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Evidence that high-temperature superconductivity originates in the charge-reservoir layers rather than in the cuprate- planes of the host materials is discussed. Hence PrBa₂Cu₃O₇ superconducts when grown under conditions that minimize Ba-site Pr. The supercurrent in high- temperature superconductors is located primarily in the layers where the holes are and the magnetic moments are not. Gd_2-zCe_zCuO₄ fails to superconduct because Gd⁺³ has L equals 0 and does not have its Cooper-pair- breaking limited by crystal-field splitting. The observed charges-transferred in these materials are compatible with charge-reservoir superconductivity, and not with cuprate- plane superconductivity. The observed ionic charge of Ce in Nd_2-zCe_zCuO₄ and its homologues is consistent with p-type superconductivity, doping by the p- type defects pair, and a size effect on the superconductivity. The chemical trends in T_c and the amount of Ni or Zn required to destroy superconductivity by pair-breaking clearly indicate that the primary superconducting condensate is in the charge-reservoir layers.

Select this Article		Room-temperature operation of a YBaCuO microbolometer Zeynep Celik-Butler, Donald P. Butler, Agha Jahanzeb, John E. Gray, and C.M. Travers Proc. SPIE 2999, 348 (1997); http://dx.doi.org/10.1117/12.271205 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Linear arrays of 40 X 40 micrometers ² to 60 X 60 micrometers ² microbolometers utilizing a semiconducting YBaCuO thin film as the IR sensitive material were fabricated and tested. This material displays a high temperature coefficient of resistance at room temperature, which makes it very attractive as an uncooled IR detector. The compatibility of semiconducting YBaCuO with Si micromachining techniques and CMOS technology and its ease of fabrication into thin films is attractive to the fabrication of inexpensive, high performance infrared detectors and imaging arrays. The room temperature responsivity, R_v, of the detectors was measured using 100 Hz chopped broad band infrared radiation to be as high as 10⁴ V/W at 0.8 (mu) A current bias, providing a detectivity of 10⁷ cm Hz^1/2/W. The thermal conductance, G, of the isolation structure was measured to be a relatively high 10^-5 W/K which limited the responsivity of the detector. Since R_v varies as 1/G, the responsivity and hence detectivity may be increased by up to two orders of magnitude by improving the thermal isolation. Zero bias responsivity was also observed on the structures, which was interpreted as pyroelectric IR detection. This was confirmed with independent pyroelectric current measurements.

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High-Temperature Superconductors II

Select this Article		Terahertz radiation from high-Tc superconducting films excited by femtosecond optical pulses Masanori Hangyo Proc. SPIE 2999, 358 (1997); http://dx.doi.org/10.1117/12.271206 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Ultrashort electromagnetic pulses with approximately 0.5 ps width have been radiated into free space from current biased superconducting YBa₂Cu₃O₇ (YBCO) films by exciting femtosecond laser pulses. The Fourier spectrum of them extends up to approximately 3 THz. The characteristics of the radiation are studied in detail and the mechanism of the radiation is discussed in relation to the relaxation of photoexcited quasi particles By the improvement of the device structure and the collecting optics, the radiation power can be increased to approximately 0.5 (mu) W. A new type THz radiation from YBCO films under an external magnetic field or in the flux-trapped state is found and its preliminary result is described.

Select this Article		Spectral-component monitoring and control of pulsed laser deposition of YBCO films Rand R. Biggers, Paul T. Murray, David B. Mast, I. Maartense, T. L. Peterson, D. Dempsey, C. Varanasi, S. Murray, D. P. Lubbers, S. Laube, B. Lovett, Eric K. Moser, J. L. Brown, D. C. Liptak, and John D. Busbee Proc. SPIE 2999, 371 (1997); http://dx.doi.org/10.1117/12.271207 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract We examine the time-resolved spectral components emitted at approximately 327 nm and approximately 550 nm in YBCO plumes during pulsed laser deposition of thin films using a KrF excimer laser at (lambda) equals 248 nm. The studied emission signals last for approximately 20 microsecond(s) ec, and show variations when process parameters such as laser power, laser excitation voltage, beam focus, chamber pressure, substrate temperature, pulse repetition rate, and target rotation rate are changed. These signals are also dependent on other factors such as target wear and age of the laser gas mixture. Spectral-component monitoring is a supplementary method of real-time plume evaluation, and allows observation of changes both prior to deposition and during the actual deposition. Adjustments can be made to the process parameters to make the plume conform to criteria necessary for the growth of films with specific qualities. The use of these spectral components as real-time process- control state variables for more reproducible fabrication of high quality thin films will be assessed.

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Poster Session

Select this Article		MSM-photodetectors with corrugated metal-semiconductor-interface based on III-V semiconductors Nikolas L. Dmitruk, Olga Y. Borkovskaya, Olga I. Mayeva, Sergey V. Mamikin, and Oxana B. Yastrubchak Proc. SPIE 2999, 384 (1997); http://dx.doi.org/10.1117/12.271172 \| Cited 1 time Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract The results of investigation for two types of GaAs(InP) based MSM photodetectors are described: 1) on quasiperiodical surfaces as light radiation detectors with enhanced photosensitivity throughout the spectral sensitivity range of device; 2) on holographic gratings as detectors having selective sensitivity respect to wavelength, polarization and incidence angle of light. The electrophysical parameters of Au(Ag)-GaAs(InP) MSM- photodetectors - current transport mechanism, interface recombination velocity, minority carrier diffusion length etc. - the spectral, angular, and polarization characteristics of their photosensitivity have been determined. Surface plasmon polariton excitation is shown to be an additional mechanism of photosensitivity enhancement for MSM photodetectors with corrugated surface. The passivation of corrugated surfaces in aqueous solution of sulfides was used for further improvement of photodetector sensitivity. The optimum regimes of chemical processing of texturation and passivation have been determined.

Select this Article		Comparative study of surface-plasma enhanced-grating coupled detectors Kannan Krishnaswami, Robert J. Tremblay II, William T. Goodhue, and Aram S. Karakashian Proc. SPIE 2999, 391 (1997); http://dx.doi.org/10.1117/12.271173 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract Gold and silver Schottky diodes were fabricated on a structured surface of an n-type GaAs substrate. The structures surface, consisting of a square-wave grating with a period of 450 nm and a depth of 15 nm, was fabricated using the techniques of holography and wet chemical etching. Measurements of the reflected intensity and photocurrent were made at the HeNe wavelength of 633 nm over a range of incident angels. It was observed that the grating coupler enhanced the photodetection for p-polarized light through surface plasmon coupling at the resonant angle. A comparative study of similarly fabricated Au and Ag Schottky diodes were made, along with a comparison to flat, non- structured devices from the same substrate. Fabrication induced shift in the angular position of the resonant angle is also demonstrated.

Select this Article		Monolithically integrated resonant-cavity photodetectors for measurement and feedback control of spontaneous and stimulated emission James A. Lott Proc. SPIE 2999, 402 (1997); http://dx.doi.org/10.1117/12.271174 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract This paper presents a study of the physics and applications of resonant cavity pin photodetectors strategically positioned within the optical structure of vertical microcavity surface emitting devices. Conventional AlGaAs resonant cavity light-emitting diodes and vertical cavity surface emitting lasers (VCSELs) emitting near 780 nm are examined. However, the structures include resonant cavity detectors placed within the upper or lower distributed Bragg reflectors. The detectors consist of an undoped quantum well absorbing layer positioned at an antinode of the resonant standing wave.A classical model is developed to examine the detector's spectral responsivity. This embedded detector scheme provides an effective means for studying the spontaneous and stimulated emission components of microcavity light emitting devices. The photocurrent from the intracavity detector is proportional to the microcavity output power over a broad emission range. This is especially useful for the control of VCSEL output power by means of a feedback circuit.

Select this Article		High-performance guard ring p-i-n photodetectors for digital fiber optic communications Babatunde Odubanjo, Chen-Show Wang, and Wen I. Wang Proc. SPIE 2999, 414 (1997); http://dx.doi.org/10.1117/12.271175 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract A novel guard ring p-i-n photodiode is designed and fabricated to solve the slow-tail problem associated with typical p-i-n photodiodes. The device incorporates an additional diffused p⁺ region around the active layer to circumvent the problem of slower responses due to diffusion currents. The effectiveness of the guard ring in providing low Bit-Error-Rate is quantified. The Extinction Ratio of the device with a 20nsec pulse is in excess of 31 dB.

Select this Article		Time dependence of the magnetization in Ba2Sr2CaCu2Ox single crystal under zero field cooled condition Jin T. Wang, Wayne K. Dawson, Manford Chinkhota, C. L. Lin, M. Xu, and Ted Chen Proc. SPIE 2999, 424 (1997); http://dx.doi.org/10.1117/12.271176 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract We have measured the time-dependence of the magnetization in Ba₂Sr₂CaCu₂O_x single crystal under the zero field cooled conditions with various magnetic fields ranging from 100 Oe to 1500 Oe along C axis at different temperatures ranging from 15K to 35K. The magnetic moment decays logarithmically with time. The decay curves have been fitted with a nonlinear logarithmic function based on the Anderson-Kim thermally activated flux creep model. We have obtained the relation between the fitting parameters and the parameters provided by the Anderson-Kim model. It is believed that our experimental results agree with the predictions made by the theoretical model.

Select this Article		Normal-incidence infrared modulators based on InAs/GaSb/AlSb quantum wells grown by molecular beam epitaxy Jody Alperin, Qinghong Du, and Wen I. Wang Proc. SPIE 2999, 431 (1997); http://dx.doi.org/10.1117/12.271177 Online Publication Date: Aug 30, 2004 Full Text: \| Download PDF
	+ -	Show Abstract We report normal incidence infrared electroabsorption modulation utilizing the Stark effect to induce (Gamma) -L transitions in asymmetric AlSb/InAs/Al_0.4Ga_0.6Sb/GaSb/AlSb quantum wells on a undoped GaSb substrate grown by molecular beam epitaxy. The normal incidence measurements of the fabricated devices were performed under various electric fields at T equals 77K using a Fourier transform infrared spectrometer. The modulation absorption was found to be directly proportional to applied bias. The largest infrared absorption at 5 micrometers with an absorption coefficient of 3200 cm^-1 was obtained at 14 V reverse bias. Our results indicate the potential of this novel structure for application in normal incidence modulators.

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