Introduction: The 3–5 mm mid-wavelength infrared region (MWIR) is a suitable spectral domain for high-performance infrared (IR) imaging technology for medical applications (diagnosis assistance), industrial applications (process control) and military applications (night vision). This is mainly due to the low atmospheric absorption in this wavelength range and the important thermal contrast of the blackbody radiation near room temperature (RT). Commercially available infrared detectors operating in the MWIR are bulk InSb and HgCdTe (MCT) photon detectors, and intersubband quantum well infrared photodetectors (QWIPs) based on a GaAs=AlGaAs system. However, cooling is necessary at temperatures around that of liquid nitrogen to obtain high detectivity performance and high-speed detection [1]. The use of cryogenic coolers is not appropriate because of their short lifetime and the added power consumption, weight and cost. Uncooled thermal detectors using microbolometers have shown impressive progress during the past years. Nevertheless, these systems are mainly limited to specific static applications in industrial areas because they are inherently slow and convenient for IR imaging essentially in the 8–12 mm long wavelength infrared region (LWIR). Consequently, there is a need in alternative materials systems to fabricate uncooled photon detectors for high-speed infrared imaging. Type-II InAs=GaInSb [2, 3] and InAs=GaSb [4–6] superlattices (SLs) have been recently proposed as a new materials family for hightemperature operation in the 3–5 mm MWIR and 8–12 mm LWIR transparency bands of the atmosphere. These type-II SLs are attractive quantum systems because of their flexibility in designing the interband transitions over a wide range of wavelengths (from 3 to 30 mm) and their expected small dark current at high temperature, resulting from a reduced Auger recombination rate. Besides, uncooled type-II InAs=GaSb SL detectors have been demonstrated from photoconductor and photovoltaic devices operating in the LWIR region [5, 6]. Experiment: In this Letter we report results on p–i–n infrared detector based on the InAs=GaSb SL system, operating at RT in photovoltaic conditions in the MWIR domain. The detector structure was grown at 390 C on zinc-doped GaSb substrates (p 2 1018 cm3), in a RIBER Compact 21E solid-source molecular beam epitaxy (MBE) system equipped with arsenic and antimony valved cracker cells providing As2 and Sb2 species. It consists of a 120 nm-thick Be-doped (p 1 1018 cm3) GaSb buffer layer, an undoped InAs=GaSb SL active zone, and a 120 nm-thick Te-doped (n 1 1018 cm3) GaSb cap layer. The SL active region period is made of ten InAs monolayers (MLs) and ten GaSb MLs. The quality of the SL is improved by a strain compensation procedure thanks to the insertion of one InSb ML at the InAs on GaSb interface in each SL period, leading to a SL period structure InAs=InSb=GaSb (10=1= 10 MLs). The SL is perfectly lattice-matched to the GaSb substrate. Further details about growth conditions can be found in [7]. The unintentionally doped SL is composed of 150 periods, corresponding to a total active thickness of 1 mm. The relevant carrier concentration in the SL region was obtained on a 2 mm-thick SL structure specially grown on semi-insulating GaAs substrate. By using standard in-plane Hall and conductivity measurements, the SL revealed at 300K an intrinsically n-type doping with a concentration of 6 1016 cm3 and in-plane mobility value of 1.2 103 cm2=V s. This low mobility can be correlated to the presence of dislocations of this GaSb-based structure on GaAs substrate.
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A p–i–n superlattice photovoltaic detector is presented, operating uncooled in the 3–5 mm mid-wavelength infrared region. The active zone of the detector device, grown by molecular beam epitaxy on p-type GaSb substrate, is made of 150 periods of strain compensated InAs=InSb=GaSb (10=1=10 monolayers) superlattice (SL). The SL detector exhibited at 293K a cutoff wavelength of 5.9 mm, an absorption coefficient of 5 103 cm1 and a responsivity of 0.7 mA=W at 3.5 mm.
integrated PL intensity, a.u.
J.B. Rodriguez, P. Christol, A. Ouvrard, F. Chevrier, P. Grech and A. Joullie´
A Nicolet-870 Nexus Fourier transform infrared (FTIR) spectrometer was used to obtain the optical characteristics of the SL. The 150 period SL exhibited an optical absorption coefficient of 5 103 cm1 at 3.5 mm, measured under normal incidence. This value is excellent as it is comparable to those of narrow gap photodetector bulk materials in the MWIR spectral range [1]. Fig. 1 shows the photoluminescence (PL) spectra recorded at 80 and 293K. The PL peak positions due to C1-HH1 miniband transitions are centred on 210 meV and practically independent of the temperature, which is a signature of the misaligned type-II systems. The full width at half maximum (FWHM) of the PL peak at 80K is 22 meV. The temperature-dependent PL behaviour on the integrated PL intensity and on the FWHM of the PL peak is shown in the inset of Fig. 1. The behaviour is quite similar to that recently reported in high-quality type-II InAs=GaSb SL operating in the same spectral range [8]. The integrated PL intensity regularly decreases with temperature and displays a characteristic temperature T1 ¼ 82K.
photoluminescence intensity, a.u.
Uncooled InAs=GaSb superlattice photovoltaic detector operating in the mid-wavelength infrared range
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Fig. 1 Photoluminescence spectra of InAs=InSb=GaSb (10=1=10 MLs) superlattice recorded at 80 and 293K Inset: variations with temperature of integrated PL intensity and of PL peak full width at half maximum. Note that PL signals are distorted by water absorption lines
Cleaved 1 1 mm chips with indium stripe contacts taken frontside and backside were prepared to perform photovoltaic response. No antireflection coating or surface passivation was employed. The device shows poor rectifying characteristics with zero-bias resistance area product (R0A) values around 3 102 O cm2 at 293K and 0.5 O cm2 at 80K. A high R0A product is wanted to reduce the thermal noise and enhance the detectivity of the device. Noticeable improvement in R0A value can be achieved on the SL by using surface passivation [9]. The samples were illuminated through the front-side at normal incidence. An antimony-based optically pumped vertical cavity surface emitting laser with external cavity (OP-VECSEL), operating in continuous wave at 2.3 mm [10], was used to determine the absolute current responsivity. The VECSEL beam was focused on a 30 mm diameter spot and the power was monitored with a pyroelectric powermeter (MellesGriot). The photocurrent was measured through a standard preamplifier having a 24 000 (V=A) gain factor. This measure was repeated for different incident beam power values, and the slope of the current against incident power gave us the responsivity. Similar measurements on InSb cooled calibrated photodetector were performed to validate the experimental procedure.
Fig. 2 Absolute spectral responsivity of p–i–n photodetectors based on InAs=GaSb SL, recorded under no bias at 90, 190 and 293K
Fig. 2 shows the absolute spectral responsivity against IR wavelength and energy. The spectra were obtained, under no reverse bias, for three
ELECTRONICS LETTERS 17th March 2005 Vol. 41 No. 6
typical temperatures: 90, 190 and 293K (uncooled). Whatever the temperature, the photodetector exhibits well-defined cutoff energy at 210 meV (5.9 mm) while the photoresponse signal value increases by two decades between the RT and the low temperature (90K) situation. Fig. 3 presents the responsivity and the external quantum efficiency of the device at l ¼ 3.5 mm for temperatures varying from 90 to 293K. At l ¼ 3.5 mm, the measured response is 80 mA=W at 90K and equal to 0.7 mA=W at 293K.
Fig. 3 Current responsivity and external quantum efficiency of p–i–n device against temperature measured under no bias at l ¼ 3.5 mm (350 meV)
Conclusions: Uncooled operation of strain compensated InAs=GaSb p–i–n SL photovoltaic detector with cutoff wavelength of 5.9 mm was demonstrated. Investigations on device technology are in progress to improve these promising results on such non-optimised photodiodes. # IEE 2005 Electronics Letters online no: 20058045 doi: 10.1049/el:20058045
References 1
Rogalski, A.: ‘Photodetectors: status and trends’, Progr. Quantum Electron., 2003, 27, pp. 59–210 2 Johnson, J.L., Samoska, L.A., Gossard, A.C., Merz, J.L., Jack, M.D., Chapman, G.R., Baumgratz, B.A., Kosai, K., and Johnson, S.M.: ‘Electrical and optical properties of infrared photodiodes using the InAs=Ga1-xInxSb superlattice in heterojunctions with GaSb’, J. Appl. Phys., 1996, 80, pp. 1116–1127 3 Bu¨rkle, L., and Fuchs, F.: ‘InAs=(GaIn)Sb superlattice: a promising material system for infrared detections’, in ‘Handbook of infrared detection technologies’, M. Henini and M. Razeghi (Eds) (Elsevier, UK, 2002), Chap. 5, pp. 159–189 4 Razeghi, M., and Mosheni, H.: ‘GaSb=InAs superlattices for infrared FPAs’ in ‘Handbook of infrared detection technologies’, M. Henini and M. Razeghi (Eds) (Elsevier, UK, 2002), Chap. 6, pp. 191–232 5 Mosheni, H., Wojkowski, J., Razeghi, M., Brown, G., and Mitchel, W.: ‘Uncooled InAs=GaSb type-II infrared detectors grown on GaAs substrates for the 8-12 mm atmospheric windows’, IEEE J. Quantum Electron., 1999, 35, pp. 1041–1044 6 Rodriguez, J.B., Christol, P., Cerutti, L., Chevrier, F., and Joullie´, A.: ‘MBE growth and characterization of type-II InAs=GaSb superlattices for mid-infrared detection’, J. Crystal Growth, 2004 (to be published) 7 Mosheni, H., and Razeghi, M.: ‘Long-wavelength type-II photodiodes operating at room temperature’, IEEE Photonics Technol. Lett., 2001, 13, pp. 517–519 8 Wei, Y., Bae, J., Gin, A., Hood, A., Razeghi, M., Brown, G.J., and Tidrow, M.: ‘High quality type II InAs=GaSb superlattices with cutoff wavelength 3.7 mm using interface engineering’, J. Appl. Phys., 2003, 94, pp. 4720–4722 9 Gin, A., Wei, Y., Hood, A., Bajowala, A., Yazdanpanah, V., Razeghi, M., and Tidrow, M.: ‘Ammonium sulfide passivation of type-II InAs=GaSb superlattice photodiode’, Appl. Phys. Lett., 2004, 84, pp. 2037–2039 10 Cerutti, L., Garnache, A., Ouvrard, A., and Genty, F.: ‘High temperature continuous wave operation of Sb-based vertical external cavity surface emitting laser near 2.3 mm’, J. Cryst. Growth, 2004, 268, pp. 128–134
29 November 2004
J.B. Rodriguez, P. Christol, A. Ouvrard, F. Chevrier, P. Grech and A. Joullie´ (CEM2—Universite Montpellier II, UMR CNRS 5507, 34095 Montpellier, France) E-mail:
[email protected]
ELECTRONICS LETTERS 17th March 2005
Vol. 41 No. 6
