Proceedings Volume 4650

Photodetector Materials and Devices VII

Gail J. Brown, Manijeh Razeghi
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Proceedings Volume 4650

Photodetector Materials and Devices VII

Gail J. Brown, Manijeh Razeghi
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 21 May 2002
Contents: 7 Sessions, 30 Papers, 0 Presentations
Conference: Symposium on Integrated Optoelectronic Devices 2002
Volume Number: 4650

Table of Contents

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Table of Contents

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  • Photodetectors for High-Speed Applications
  • Avalanche Photodiodes
  • UV Photodetectors
  • Section
  • UV Photodetectors
  • Infrared Photodetectors and Focal Plane Arrays
  • Quantum Well and Superlattice Detectors
  • Section
  • Poster Session
  • Infrared Photodetectors and Focal Plane Arrays
Photodetectors for High-Speed Applications
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Optoelectronic components for optical information processing applications
The advances in the design and fabrication of optoelectronic components such as microlaser arrays, photodetectors and free-space optical interconnection elements have driven the creation of ever more ambitious demonstrator systems. In this paper, the progress made to date on two separate demonstrator project, which have been under investigation at Heriot-Watt University, will be reviewed. A description of some of the enabling technologies used in the creation of these systems will be provided and the potential for scaling the architectures described up to sizes where the computational advantages of the optics-in-computing paradigm become highly attractive will be outlined.
Optic fiber clock distribution in high-speed video memory
Peter B. Kosel, Amir Amin Hanjani
An electronic system based on a novel high-speed massive video memory array using an optical fiber clock distribution network has been investigated for the generation of random patterns for testing high-resolution color video monitors with screen sizes in the realm of 4096 by 4096 pixels. For frame rates in the range of 30 to 100 per second with 256 (28) to 4096 (212) intensity levels for each primary color the speed requirement amounts to 1.21x1010 to 6.04x1010 bits per second. The massive memory makes use of high-speed MSM photodetectors, optical receiver amplifiers and gallium arsenide charged coupled devices which are integrated on GaAs chips. These chips are assembled into 16 planes of multi-chip modules with 32 GaAs chips per plane. Only GaAs CCDs have been found to provide the short access times required to achieve the above data rates that exceed the capabilities of current silicon-based DRAMs. For proper operation clock skew must be eliminated, therefore, a 2-phase laser driven optical fiber distribution network has been considered. In addition, the photodetectors and amplifiers driving the CCDs must have speeds that do not compromise the access times of the CCD registers. To meet all requirements the design was implemented with optical fiber v-groove coupling to the MSM monolithic detectors and high-speed preamplifiers that are fabricated with the same technology as used for the fabrication of the CCDs.
GaAs PIN photodetectors for 10 Gbit/s data communication
Lars Dillner, Roger Loow, Elsy Odling, et al.
For high speed data communication, Zarlink has developed GaAs PIN detectors with three different diameters of the apertures: 40 micrometers , 55 micrometers and 70 micrometers . To minimize the capacitance we have chosen to use a mesa structure on a semi-insulating substrate. At íV2 Volt bias the capacitances are 110 fF, 160 fF and 230 fF for the 40 micrometers , 55 micrometers and 70 micrometers detectors respectively. For all diameters the series resistance is about 5 (Omega) and the responsivities are 0.6 A/W. At-V10 Volt bias, the dark current is less than 100 pA. Link experiments show open eye diagrams at 10 Gbit/s for a 70 micrometers unamplified PIN detector, both at room temperature and at 90 degree(s)C.
Novel germanium photodetectors fabricated with a diffused junction
Charles B. Morrison, Rengarajan Sudharsanan, Moran Haddad, et al.
Germanium (Ge) photodetectors are fabricated by growing epitaxial III-V compounds on Ge substrates and by in-situ formation of the PN junction by MOVPE. After material growth, Ge photodetectors are mesa-etched using conventional optoelectronic device processing techniques. By varying the Ge substrate resistivity and the device area, Ge photodetector properties such as reverse leakage current, capacitance, and shunt resistance have been engineered. Such devices have demonstrated leakage currents below 50(mu) A/cm2 at -0.1 V bias. For optoelectronic applications that require high temperature operation, high shunt resistance detectors exhibit leakage currents below (mu) A/cm2 at 80 degree(s)C. Low capacitance devices have measured as little as 275 pF at 0V bias for a 1 mm diameter detector. High shunt resistance devices are a low cost alternative to conventional InGaAs photodiodes in applications such as laser monitor diodes.
Avalanche Photodiodes
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Wafer-bonded InGaAs/silicon avalanche photodiodes
Alexandre Pauchard, Phil Mages, Yimin Kang, et al.
Wafer-bonded avalanche photodiodes (APDs) combining InGaAs for the absorption layer and silicon for the multiplication layer have been fabricated. The reported APDs have a very low room-temperature dark current density of only 0.7 mA/cm2 at a gain of 10. The dark current level is as low as that of conventional InGaAs/InP APDs. High avalanche gains in excess of 100 are presented. The photodiode responsivity at a wavelength of 1.31 micrometers is 0.64 A/W, achieved without the use of an anti-reflection coating. The RC-limited bandwidth is 1.45 GHz and the gain-bandwidth product is 290 GHz. The excess noise factor F is much lower than that of conventional InP-based APDs, with values of 2.2 at a gain of 10 and 2.3 at a gain of 20. This corresponds to an effective ionization rate ratio keff as low as 0.02. The expected receiver sensitivity for 2.5 Gb/s operation at (lambda) = 1.31 um using our InGaAs/silicon APD is -41 dBm at an optimal gain of M = 80.
Frequency response of multilayer avalanche photodiodes: structural effects
The frequency response of SAM-APD devices is calculated from the response of each layer using matrix algebra. Most of the results apply to devices with absorption region of InGaAs and avalanche region of InP and they assume uniform carrier ionization coefficients and velocities. The effect of the width of each layer, carrier ionization ratio and velocities on the multilayer structure frequency response has been investigated. A change of the absorption region width changes the 3-dB bandwidth at low avalanche gains whereas a change in the avalanche region width only affects the frequency response at high avalanche gains. When the ionization ratio decreases an increase of the 3-dB bandwidth is observed at high avalanche gains. The frequency response seems to be very sensitive to the carrier velocities mainly the hole velocity. In order to include the strong dependence of the ionization coefficients on the electrical field, the avalanche region was modeled piecewise uniform by breaking it into three layers. The frequency response of this structure is seen to be similar to the one obtained when uniform ionization coefficients are considered assuming they are assigned the mean value of the corresponding ionization coefficients in the three layers.
Characterization of Geiger mode avalanche photodiodes for fluorescence decay measurements
John Carlton Jackson, Don Phelan, Alan P. Morrison, et al.
Geiger mode avalanche photodiodes (APD) can be biased above the breakdown voltage to allow detection of single photons. Because of the increase in quantum efficiency, magnetic field immunity, robustness, longer operating lifetime and reduction in costs, solid-state detectors capable of operating at non-cryogenic temperatures and providing single photon detection capabilities provide attractive alternatives to the photomultiplier tube (PMT). Shallow junction Geiger mode APD detectors provide the ability to manufacture photon detectors and detector arrays with CMOS compatible processing steps and allows the use of novel Silicon-on-Insulator(SoI) technology to provide future integrated sensing solutions. Previous work on Geiger mode APD detectors has focused on increasing the active area of the detector to make it more PMT like, easing the integration of discrete reaction, detection and signal processing into laboratory experimental systems. This discrete model for single photon detection works well for laboratory sized test and measurement equipment, however the move towards microfluidics and systems on a chip requires integrated sensing solutions. As we move towards providing integrated functionality of increasingly nanoscopic sized emissions, small area detectors and detector arrays that can be easily integrated into marketable systems, with sensitive small area single photon counting detectors will be needed. This paper will demonstrate the 2-dimensional and 3-dimensional simulation of optical coupling that occurs in Geiger mode APDs. Fabricated Geiger mode APD detectors optimized for fluorescence decay measurements were characterized and preliminary results show excellent results for their integration into fluorescence decay measurement systems.
UV Photodetectors
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Structural, optical, and electrical characterization of ZnO/Zn0.8Mg0.2O quantum wells for UV applications
Agisilaos A. Iliadis, Soumya Krishnamoorthy, Wei Yang, et al.
The structural, optical, and electrical characteristics of An0.8Mg0.2O/ZnO/Zn0.8Mg0.2O quantum well heterostructures, are reported. The structures consist of a ZnO quantum well, with thickness of 6 nm, 8 nm, or 50 nm, placed between two Zn0.8Mg0.2O barriers, with a thickness of 7nm. The epitaxial layers are grown by pulsed laser deposition (PLD) on c-cut sapphire substrates. Resonant tunneling action in the 6 nm and 8 nm width single quantum well has been observed. Negative differential resistance (NDR) peaks were evident at room temperature and at 200 K in this system for the first time. X-ray diffraction spectra showed high crystal quality, and pulsed photoluminescence measurements showed high quality hetero- interfaces with a FWHM of 5.6 meV at 77K. The photoluminescence (PL) transitions in the quantum wells occurred at wavelengths of 345.55 nm and 348.22nm for the 6 nm and 8 nm well width respectively. The current-voltage characteristics of the structures showed the negative differential resistance peaks at RT and 200 K, making this type of wide band-gap semiconductor material system a very promising system for applications both in electron transport and in UV detector devices.
High-speed visible-blind GaN-based ITO-Schottky photodiodes
Necmi Biyikli, Ibrahim Kimukin, Tolga Kartaloglu, et al.
In this paper we present our efforts on the design, fabrication and characterization of high-speed, visible-blind, GaN-based ultra-violet (UV) photodiodes using indium-tin-oxide (ITO) Schottky contacts. ITO is known as a transparent conducting material for the visible and near infrared part of the electromagnetic spectrum. We have investigated the optical properties of thin ITO films in the ultraviolet spectrum. The transmission and reflection measurements showed that thin ITO films had better transparencies than thin Au films for wavelengths greater than 280 nm. Using a microwave compatible fabrication process, we have fabricated Au and ITO based Schottky photodiodes on n-/n+ GaN epitaxial layers. We have made current-voltage (I-V), spectral quantum efficiency, and high-speed characterization of the fabricated devices. I-V characterization showed us that the Au-Schottky samples had better electrical characteristics mainly due to the larger Schottky barrier. However, due to the better optical transparency, ITO-Schottky devices exhibited higher quantum efficiencies compared to Au-Schottky devices. ITO-Schottky photodiodes with ~80 nm thick ITO films resulted in a maximum quantum efficiency of 47%, whereas Au-Schottky photodiode samples with ~10 nm thick Au films displayed a maximum efficiency of 27% in the visible-blind spectrum. UV/visible rejection ratios over three orders of magnitude were obtained for both samples. High-frequency characterization of the devices was performed via pulse-response measurements at 360 nm. ITO-Schottky photodiodes showed excellent high-speed characteristics with rise times as small as 12 psec and RC-time constant limited pulse-widths of 60 psec.
Section
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Future of AlxGa1-xN materials and device technology for ultraviolet photodetectors
Thanks to advances in the quality of wide bandgap AlxGa1-xN semiconductors, these materials have emerged as the most promising approach for the realization of photon detectors operating in the near ultraviolet from 200 to 365 nm. This has in turn spurred the need for such devices in an increasing number of applications ranging from water purification to early missile threat warning systems. Nevertheless, the control of the material quality and doping, and the device technology remain tremendous challenges in the quest for the realization of high performance photodetectors. Design of the photodetector structure is one of the key issues in obtaining high performance devices; especially the thickness of the intrinsic region for p-i-n photodiodes is a crucial value and needs to be optimized. We compare the performance of the p-i-n photodiodes with different widths for the depletion region, which shows a trade-off between speed and responsivity of the devices. Furthermore, another challenge at present is the realization of low resistivity wide bandgap p-type AlxGa1-xN semiconductors. We present here recent advances and propose future research efforts in the enhancement of the AlxGa1-xN p-type conductivity through the use of polarization fields in AlxGa1-xN/GaN superlattice structures.
UV Photodetectors
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Advances in wide-bandgap semiconductor-based photocathode devices for low-light-level applications
The basic requirement for an imaging low-light level system (one capable of single photon counting) is that the device has low dark current. Photocathode based devices have the advantage over solid state devices in this regard as the dark current is inherently low. A further requirement for UV detectors is the necessity to suppress the sensitivity in the red, and wide-band gap semi-conductors fill this role well. For nitride based semi-conductors, there is still the issue of making p-type material and making alloys with Al or In to move the red cutoff to the blue (Al) or red (In). Regardless of the material (e.g. another choice is diamond) coupling the resulting photocathode to a device such as a micro-channel plate (MCP) is necessary to produce imaging. Based on advances we have made both in the production of p- type GaN photocathodes, diamond photocathodes, and read-outs of Si MCPs, we are on the verge of making high quality UV imaging systems for astronomy and other low-light level applications. In this paper we will review the progress that has been made over the past few years and provide an update with recent results.
Femtosecond studies of fundamental materials issues in III-nitride ultraviolet photodetectors
We have used femtosecond time-resolved optical techniques to study fundamental materials issues in III-nitride semiconductors relevant to the realization of high quality ultraviolet photodetectors. Intensity dependent pump-probe reflectivity and transmission measurements have been employed in the investigation of carrier dynamics in AlGaN alloys with Al content ranging from ~0.15 to 0.4. For the Al0.15Ga0.85N sample, the intensity dependence of the (Delta) R decay suggests that at high intensity the shallow traps are saturated and ultrafast nonradiative recombination dominates the carrier dynamics. For the Al0.25Ga.75N and Al0.4Ga0.6N samples (Delta) R decays faster with decreasing intensity and changes sign. Moreover, the decays are faster for a given in tensity in the higher Al content sample. This behavior suggests that in these cases the dynamics are governed by trapping at localized states that become deeper and more numerous as the Al content increases. Within this context the sign change in (Delta) R in A;0.4Ga0.6N may be indicative of the onset of photoinduced absorption associated with the excitation of carriers from the localized states to the bands, which has also been observed in time-resolved transmission measurements. This localization may be associated with alloy fluctuations that broaden the absorption edge of the material and degrade the long-wavelength performance of photodetectors. In addition, time-resolved electroabsorption measurements on an AlGaN/GaN heterojunction p-i-n photodiode have been used to study the transient electron velocity overshoot for transport in the c-direction in wurzite GaN. The velocity overshoot is observed at fields well below the field at which the calculated peak steady-state velocity occurs, and it increases with electric field up to ~320 kV/cm, at which field a peak velocity of 7.25x107 cm/s is attained within the first 200 fs after photoexcitation. These results are consistent with theoretical Monte Carlo calculations incorporating a GaN full-zone band structure, which show that because of band nonparabolicity in the (Gamma) valley the majority of electrons do not attain sufficient energy to effect intervalley transfer until they are subjected to higher fields (>325kV/cm). This behavior may have important implications for avalanche photodiodes, for which electrons are promoted to higher lying bands for participating in the avalanche process.
Solar-blind AlGaN-based UV photodetectors grown on Si (111) substrates
Jose L. Pau, Elias Munoz Merino, Miguel A. Sanchez-Garcia, et al.
In this work, efficient solar-blind metal-semiconductor-metal (MSM) photodiodes grown on Si (111) by molecular beam epitaxy are reported. Growth details are described, including the use of different kinds of buffer layers. AlGaN samples using an AlGaN/GaN superlattice (SL) as a buffer showed the presence of cracks, while AlGaN samples on an AlN buffer were crack-free. The additional strain introduced by the SL and the increase of the lattice mismatch between Si and AlGaN when the Al content increases, are responsible for the cracking. MSM photodiodes were fabricated and characterized using such layers. UV detectors obtained on the sample with cracks presented a dark current above 100 pA at 5 V, while in the crack-free photodiodes the dark current was below 10 pA at 30 V. The ultraviolet/visible contrast was also reduced in order of magnitude due to the presence of cracks. Peak responsivity values of 14 mA/W at 5 V and of 16 mA/W at 10 V were obtained for the photodetectors with cracks and for the crack-free photodetectors, respectively. The spectral noise density was 1 x 10-24 A2/Hz at 5 V for the detectors with cracks, showing at low frequencies a 1/f-type behavior. For the crack-free photodetectors, the spectral noise density value was below the system detection limits (1 x 10-26 A2/Hz) at 10 V. A detectivity value of 5.2 x 1010 cmxHz1/2xW-1 at 10 V was estimated for the crack-free photodiodes.
Infrared Photodetectors and Focal Plane Arrays
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Comparison of performance limits of infrared detector materials
The fundamental parameters of infrared (IR) detection are discussed to compare a wide range of materials. In comparative studies both photon and thermal detectors are considered. More attention is paid to photon detectors such as : HgCdTe photodiodes, InSb photodiodes, quantum well IR photoconductors (QWIPs) and doped silicon photoconductors. Different competitive technologies in long wavelength IR(LWIR) and very LWIR(VLWIR) spectral ranges with emphasis on the material properties, device structure, and their impact on FPA performance are considered. The potential performance of materials as infrared detectors is examined utilizing the (alpha) /G ratio, where (alpha) is the absorption coefficient and G is the thermal generation. It results that LWIR QWIP cannot compete with HgCdTe photodiode as the single device, especially at higher temperature operation (>70K), due to fundamental limitations associated with intersubband transitions. However, the advantage of HgCdTe is less distinct in temperature range below 50K due to problems involved in an HgCdTe material (p- type doping, Shockley-Read recombination, trap-assisted tunneling, surface and interface instabilities). Even though QWIP is a photoconductor, several its properties such as high impedance, fast response time, long integration time, and low power consumption, well comply requirements of fabrication of large FPAs. Due to the high material quality at low temperature, QWIP has potential advantages over HgCdTe for VLWIR FPA applications in terms of the array size, uniformity, yield and cost of the systems. State of the art of QWIP and HgCdTe FPAs provides similar performance figure of merit, because they are predominantly limited by the readout circuits. Decision of the best technology implementation is therefore driven by the specific needs of a system.
Recent development in infrared FPAs with multispectral 128exp2 IRCMOS
J. P. Baylet, Jean-Paul Zanatta, Damien Chance, et al.
In this paper we present a status of the activity of the LETI infrared laboratory in the field of HgCdTe infrared multispectral detectors. The multilayer doped structures needed to achieve two color pixels are grown by molecular beam epitaxy (MBE) (211)HgCdTe on lattice matched CdZnTe substrates. The device structure is n+ppn and is spatially coherent. The long wavelength layer is a planar like n+/p diode and is made by ion implantation while the shorter wavelength p-n diode is made in-situ during the MBE growth using Indium impurity doping. The last junction is isolated by mesa etch. The detectors are interconnected by indium bumps to a CMOS readout circuit. One or two indium bumps per pixel are used to address sequentially or simultaneously the two wavelengths, the detector pitch being 50micrometers or 60micrometers respectively. Elementary detectors exhibit performances in each band which are very close to those obtained in single color detectors with our standard technology. The Si-CMOS read-out circuits are specially designed to optimize the best performance of the IRCMOS focal plane arrays (FPA) in both wavelengths. The electro-optical performances of a two color IRCMOS FPA with a complexity of 128x128 pixels (pitch of 50micrometers ) operating sequentially within the (3-5micrometers ) middle wavelength infrared range (MWIR) at 77K will be presented.
Amorphous silicon technology improvement at CEA-LETI
Eric Mottin, Astrid Bain, Pierre Castelein, et al.
The emergence of uncooled infrared detectors has opened new opportunities for IR imaging both for military and civil applications. Infrared imaging sensors that operate without cryogenic cooling have the potential to provide the military or civilian users with infrared vision capabilities packaged in a camera of extremely small size, weight and power. Uncooled infrared sensor technology has advanced rapidly in the past few years. Higher performance sensors, electronics integration at the sensor, and new concepts for signal processing are generating advanced infrared focal plane arrays. This would significantly reduce the cost and accelerate the implementation of sensors for applications such as surveillance or predictive maintenance. We present the uncooled infrared detector operation principle and the development at CEA/LETI from the 256 x 64 with a pitch of 50 micrometers to the 320 x 240 with a pitch of 35 micrometers . LETI has been involved in Amorphous Silicon uncooled microbolometer development since 1992. This silicon IR detection is now well mastered and matured so that industrial transfer of LETI technology was performed in 2000 towards Sofradir. Industrial production of 320 x 240 microbolometer array with 45micrometers pitch is then started. After a description of the technology and the methodology for reliability enhancement, we present the readout circuit architectures designs and its evolution from the 256 x 64 array to the different version of 320 x 240 arrays. Electro-optical results obtained from these IRCMOS are presented. NEDT close to 30 mK is now obtained with our standard microbolometer amorphous silicon technology.
QWIP-LED pixelless imaging
Hui Chun Liu, Emmanuel Dupont, M. Byloos, et al.
This paper discusses issues related to the imaging performance of pixel-less quantum well infrared photodetectors integrated with light emitting diodes. Our latest imaging results are shown. Analytical expressions are derived for evaluating noise equivalent temperature difference. Areas that need improvement are pointed out.
Quantum Well and Superlattice Detectors
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Whither P-type GaAs/AlGaAs QWIP?
P-type GaAs/AlGaAs quantum well infrared photodetectors (QWIP) represent a complementary technology to the well developed and already commercialized n-type GaAs/AlGaAs QWIP technology. Since n-QWIPs require grating couplings for normal incidence absorption, p-type GaAs/AlGaAs QWIPs have emerged as a viable alternative in some applications. In this paper, progress in optimizing the performance of p-type GaAs/AlGaAs QWIPs through modeling, growth, and characterization is described. Our approach begins with the theoretical design of p-QWIPs based on calculations of optical absorption. Next, samples are grown by MBE according to the theoretical designs and their characteristics measured. p-type QWIPs were optimized with respect to the well and barrier widths, alloy concentration, and dopant concentration; resonant cavity devices were also fabricated and the temperature dependent photoresponse was measured. Based on the progress to date, it is now possible to make some comparisons between the n- and p-type approaches. Further avenues for improvement of p-QWIP photoresponse are being explored by exploiting the rich physics of this coupled multi-band system.
Pulsed-laser-induced quantum well intermixing on the InGaAs/InGaAsP MQW waveguide photodetector
Na Li, Teik Kooi Ong, Yuen Chuen Chan, et al.
In this paper, we report our research work on the application of the pulsed-laser-induced disordering (P-LID) technique in InGaAs/InGaAsP MQW waveguide photodetector. A Q-switched Nd-YAG laser with wavelength of 1.064 micrometers was used to irradiate on the InGaAs/InGaAsP quantum well materials, annealing process at 625 degree(s)C for 120s was followed. A maximum bandgap shift of up to 112meV has been observed. The variety of photocurrent curves indicated that the cut-off wavelength of the photodetector becomes shorter with increasing of intermixing strength.
Backside illuminated In(Ga)As/InAsSbP DH photodiodes for methane sensing at 3.3 um
Boris A. Matveev, Nonna V. Zotova, Sergey A. Karandashev, et al.
We describe Gd doped backside illuminated In(Ga)As double and single heterostructure photodiodes with InAsSbP cladding layers grown onto heavily doped n+-InAs transparent substrate of the episide-down bonding design. The advantages of the construction include improvement of material quality due to rare earth gettering effect and the possibility of coupling with fibers or immersion lenses through the contact free surface. The report presents U-I, spectral response and sensitivity of narrow band (3.1-3.4 micrometers ) photodiodes at 20divided by180 degree(s)C with RoA product as high as 2 (Omega) cm2 at room temperature and serial resistance as low as 0.1 (Omega) .
Type-II InGaAs/GaAsSb superlattice for photodetection in the near infrared
Gail J. Brown, Joseph E. Van Nostrand, S. M. Hedge, et al.
The optical properties of an (formula available in paper) type-II superlattice lattice matched to InP(001) was characterized by photo luminescence and near infrared photoresponse. The samples were designed for optical emission near 1.8micrometers and were grown by molecular beam epitaxy. At 4K, a strong type-II luminescence at 1.8micrometers (689meV0 with a full width at half maximum (FWHM) of 18 meV was observed. Similarly, the onset of the band edge photoresponse occurred at 1.8micrometers (693 meV) at 10K. We believe this to be the first observation of both luminescence and photoresponse from the InGaAs/GaAsSb/InP materials system grown by any technique.
InAs/InGaSb Type-II strained layer superlattice IR detectors
Vaidya Nathan, K. Alex Anselm, C.H. Thompson Lin, et al.
InAs/InGaSb type2 strained layer superlattice (SLS) combines the advantages of III-V materials technology with the strong, broad-band absorption, and wavelength tunability of HgCdTe. In fact, the significantly reduced tunneling and Auger recombination rates in SLS compared to those in HgCdTe should enable SLS detectors to outperform HgCdTe. We report the results of our investigation of InAs/InGaSb type2 strained layer superlattices (SLS)for LWIR photovoltaic detector development. We modeled the band structure, and absorption spectrum of SLS's, and achieved good agreement with experimental data. We systematically investigated the SLS growth conditions, resulting in good uniformity, and the elimination of several defects. We designed, developed and evaluated 16x16 array of 13 micron cutoff photovoltaic detectors. Photodiodes with cutoff wavelengths of 13 and 18microns were demonstrated, which are the longest wavelengths demonstrated for this material system. Quantum efficiencies commensurate with the superlattice thickness were demonstrated and verified at AFRL. The electrical properties show excessive leakage current, most likely due to trap-assisted tunneling.
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Origin of improved luminescence efficiency after annealing of Ga(In)NAs materials grown by molecular beam epitaxy
Wei Li, Markus Pessa, Tommy Ahlgren, et al.
Positron-annihilation measurements and nuclear reaction analysis (utilizing the 14N(d, p)15N and 14N(d, a)12C reactions) in conjunction with Rutherford backscattering spectrometry in the channeling geometry were used to study the defects in as-grown Ga(In)NAs materials grown by molecular beam epitaxy (MBE) using a radio-frequency (rf) plasma nitrogen source. Our data unambiguously show the existence of vacancy-type defects, which we attribute to Ga vacancies, and nitrogen interstitials in the as-grown Nitride-Arsenide epilayers. These point defects, we believe, are responsible for the low luminescence efficiency of as-grown Ga(In)NAs materials and the enhanced diffusion process during annealing.
Noninvasive electrical characterization of semiconductor and interface
Samir Kasouit, Bernard Drevillon, Joao Conde, et al.
The Spectroscopic Ellipsometry and the Time Resolved Microwave Conductivity (TRMC) are efficient tools for in-situ non invasive characterizations during the growth of semiconductors and interfaces. From ellipsometry, one estimates the optical absorption, structural composition of the material in the bulk and near the interface. The TRMC measures the transient microwave reflectivity induced by carriers photogenerated by a pulsed laser. From TRMC, one may estimate the mobility of the carriers in a thin film or in bulk materials, the carrier lifetime in the bulk or near the surface. Particularly, we characterize microcrystalline silicon : electron and hole mobility, electron mobility inside the grain, trapping.
Two-photon excited emission probing of thin film CdS formed by various techniques
Bruno Ullrich, Raoul Schroeder
We demonstrate that two-photon spectroscopy is a powerful tool to study the intrinsic optical, interfacial and electronic properties of thin film semiconductors. The emission properties of CdS films formed by close-spaced vapor transport (CSVT) on calcium fluoride, by spray pyrolysis on pyrex, and by pulsed-laser deposition (PLD) on glass were investigated. The films were excited with ultrashort (200 fs) laser pulses at 1.54 eV at room temperature. Though the impinging photon energy is far below the bandgap of CdS (2.45 eV), the excitation caused green bandgap emission due to two-photon absorption. Notably, the emission revealed the intrinsic properties of the films independent of preparation method, doping and substrate. The influence of the substrate/CdS interface on the emission was probed by comparing the spectra measured at face and rear. The PLD films revealed a clear dependence on the experimental geometry by shifting the back emission 40 meV towards lower energies with respect to the front emission. In contrast to this, the emission of films formed by spray pyrolysis and CSVT did not show geometry dependence. Additionally, we show that two-photon spectroscopy is capable of probing bandgap shrinkage. The knowledge of the latter is very useful for the design of laser cavities.
Quantitative secondary ion mass spectrometry (SIMS) of III-V materials
Patrick Van Lierde, Chunsheng Tian, Bruce Rothman, et al.
Secondary ion mass spectrometry (SIMS) provides direct methods to characterize the chemical composition of III-V materials at major, minor and trace level concentrations as a function of layer depth. SIMS employs keV primary ions to sputter the surface and sensitive mass spectrometry techniques to mass analyze and detect sputtered secondary ions which are characteristic of the sample composition. In-depth compositional analysis of these materials by SIMS relies on a number of its unique features including: (1) keV primary ion sputtering yielding nanometer depth resolutions, (2) the use of MCs+ detection techniques for quantifying major and minor constituents, and (3) ion implant standards for quantifying trace constituents like dopants and impurities. Nanometer depth resolution in SIMS sputtering provides accurate detection of diffusion of dopants, impurities and major constituents. MCs+ refers to the detection of molecular ions of an element (M) and the Cs+ primary beam. MCs+ minimizes SIMS matrix effects in analysis for major and minor constituents, thus providing good quantification. This paper presents a SIMS study of AlxGa(1-x)As structures with three different x values. MCs+ (M=Al or Ga) data are presented for the accurate determination of major and minor components. Rutherford backscattering spectrometry (RBS) and x-ray diffraction (XRD) data were cross-correlated with the MCs+ results. Three specimens with different x values were ion implanted with H, C, O, Mg, Si, Zn and Se to study quantification of trace levels. SIMS data acquired on a double focusing instrument (CAMECA IMS-4f) and a quadrupole instrument (PHI ADEPT 1010) are also compared. Lastly, we discuss our efforts to improve the analysis precision for p- and n-type dopants in AlGaAs which currently is +/- 3% (1 sigma).
Poster Session
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Assessment of colossal magnetoresistive manganite thin films for infrared detector applications
Paul P. Donohue, Rex Watton, Michael A. Todd, et al.
Recently there has been great interest in doped manganite thin films exhibiting colossal magnetoresistance (CMR), in particular for magnetic sensing applications. These perovskite oxides exhibit a metal-insulator transition at the Curie temperature which is accompanied by a large change of resistance as the material loses its ferromagnetic properties. Since this resistance change occurs over a relatively narrow temperature range it is accompanied by very large temperature coefficients of resistance in the region of the phase transition, making these materials ideal candidates for infrared detectors utilizing resistance bolometers. This paper reports measured physical and electrical properties, the latter including 1/f noise, of doped manganite thin film CMR material deposited by pulsed- laser deposition. The potential performance of CMR based resistance bolometer devices is reported.
Improved velocity mismatch in traveling-wave coplanar waveguide photodetector with thick ground-metal-line
Sang-Jun Yoon, Seong-Hae Ok, Young-Seol Yoon, et al.
Bandwidth of a traveling-wave photodetector (TWPD) is limited by the optical absorption coefficient, velocity and impedance mismatches, and the drift time of photo-generated carriers in the intrinsic region. In these parameters, velocity and impedance mismatches have much influence on the bandwidth of TWPD. In this paper, we focus on mismatches, and propose a novel design to enhance the bandwidth. In the new structure, the thickness of a ground electrode increases as much as the ridge thickness. It forces the structure to have characteristics similar to a coplanar waveguide. We simulate this structure by finite different time domain Method in three dimensions and look-over frequency dependent parameters by Fourier transform for the detailed analysis of microwave characteristics such as characteristic impedance, microwave effective index, and microwave attenuation of TWPD. As a result, we obtain 50 (Omega) impedance matching and 89.7 % velocity matching using our novel structure.
Surface leakage current in HgCdTe photodiodes
This study describes fabrication of heterojunction HgCdTe photodiodes passivated with a wide band-gap CdTe epitaxial layer. The current-voltage characteristics of these photodiodes with and without passivation have been investigated. It is shown that for reverse bias the measured I-V characteristics can be explained by a surface tunneling current and surface generation current. The breakdown voltage is observed to decrease monotonically with increasing temperature, a trend that is directly opposite to what would be expected from a pure tunneling mechanism. Additional information on surface limitations is obtained from analyzing the R0A product as a function of temperature. The performance of both type of p-n VLWIR HgCdTe photodiodes (with and without the passivating layer) have been compared.
Infrared Photodetectors and Focal Plane Arrays
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Type-II InAs/GaSb superlattices and detectors with lambda c >18m
Manijeh Razeghi, Yajun Wei, Aaron Gin, et al.
The authors report the most recent advances in type II InAs/GaSb superlattice materials and photovoltaic detectors. Lattice mismatch between the substrate and the superlattice has been routinely achieved below 0.1%, and less than 0.0043% as the record. The FWHM of the zeroth order peak from x-ray diffraction has been decreased below 50 arcsec and a record of less than 44arcsec has been achieved. High performance detectors with 50% cutoff beyond 18 micrometers up to 26 micrometers have been successfully demonstrated. The detectors with a 50% cut-off wavelength of 18.8 micrometers showed a peak current responsivity of 4 A/W at 80K, and a peak detectivity of 4.510 cm x Hz1/2/W was achieved at 80K at a reverse bias of 110mV under 300K 2(pi) FOV background. Some detectors showed a projected 0% cutoff wavelength up to 28~30 micrometers . The peak responsivity of 3Amp/Watt and detectivity of 4.2510 cm x Hz1/2/W was achieved under -40mV reverse bias at 34K for these detectors.