Electrical characterizations of asymmetric InAs/GaSb superlattice MWIR photodiodes R. Taalat a , J.B. Rodriguez a , C. Cervera a , I. Ribet-Mohamed b , P. Christol a,⇑ a Institut d’Electronique du Sud, UMR-CNRS 5214, Université Montpellier 2, 34095 Montpellier, France b ONERA, Chemin de la Hunière, 91761 Palaiseau, France article info Article history: Available online 29 December 2012 Keywords: InAs/GaSb superlattice Photodiode Dark current abstract InAs/GaSb superlattice pin photodiodes, having an asymmetric period design, exhibited cut-off wave- length in the midwave infrared domain (MWIR) at 5 lm at 77 K. Electrical characterizations including dark-current and capacitance–voltage measurements were performed on single detectors in the temper- ature range (77–300 K). The SL photodiode measurements revealed carrier concentrations of about 6  10 14 cm 3 at 77 K, dark-current densities J =4  10 8 A/cm 2 at 77 K for V bias = 50 mV and the mea- sured R 0 A product is higher than 1.5  10 6 X cm 2 at 77 K. Comparison to classical pin diodes with sym- metric period design show that the differential resistance area product is improved by more than one order of magnitude. This result obtained demonstrates the strong influence of the period on the electrical properties of SL MWIR photodiodes. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction During the last decade, photodetector based on type-II InAs/ GaSb superlattice (SL) material has emerged as an attractive sys- tem for specific military and civilian imagery applications in the 3–5 lm mid-wavelength infrared (MWIR) spectral domain. Indeed, since the first works of Yang and Bennet in 1994 on MWIR pin pho- todiode [1], impressive progresses were obtained on SL detectors thanks the growth control of the binary InAs/GaSb SL quantum structure by molecular beam epitaxy (MBE) [2–4] and the progress on the processing of antimonide-based (Sb-based) SL devices [5–7], resulting in the demonstration of high-performance MWIR focal plane arrays (FPA) [8–11]. Recently, a class of unipolar SL detectors named nBn [12], M-structure [13] or C-BIRD [14] permitted the improvement of the detector signal-to-noise ration by eliminating the generation- recombination dark-current (J GR ) usually limiting the low temper- ature noise of standard pin photodiodes. In the MWIR domain, these advanced heterostructure designs allowed to ameliorate the performances of SL photodiodes at high temperatures [11] but improvement of SL quantum structure and SL material properties are still necessary in order to compete with the well- established MWIR detector technologies based on InSb and HgCdTe materials. Another way to enhance performances is to focus studies on SL active zone of conventional photodiode configuration based on pin junction. The InAs/GaSb SL period composition governs the mate- rial properties of the active zone material such as the carrier effec- tive masses which determine the effective density of states and then the intrinsic carrier concentration (n i ). It is well-known that the diminution of n i reduces the J GR (proportional to n i ) at low tem- perature and the J diff contribution (proportional to n 2 i ) at high tem- perature. Most of the SL periods reported for the MWIR domain are composed of a symmetric period design, i.e. the same thickness of InAs and GaSb layers [6,8–10,12,15], and the choice of the SL per- iod is made to obtain a given cut-off wavelength, never to improve the detector signal-to-noise ratio. Thanks the properties of the type-II broken gap InAs/GaSb SL quantum structure, it is possible to address a given cut-off wavelength by using an asymmetric per- iod design i.e. when, for example, the InAs layer is thicker than the GaSb one [16]. In this paper, we report on electrical characterizations of MWIR pin SL photodiodes with active zone made of asymmetric period composed of 7 InAs monolayers (MLs)/4 GaSb MLs (7/4 SL struc- ture), showing cut-off wavelength at 5 lm (77 K). The results show the strong influence of the SL period design on the electrical prop- erties of SL MWIR photodiodes. 2. SL device fabrication and experimental measurements The InAs/GaSb SL structure for the MWIR domain was grown on p-type GaSb substrate by Molecular Beam Epitaxy (MBE). The structure, described in the inset of Fig. 1, consists of a 200 nm Be-doped (p+ type doping 1  10 18 cm 3 ) GaSb buffer layer, a non intentionally doped (nid) InAs/GaSb SL active zone, and a 1350-4495/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.infrared.2012.12.006 ⇑ Corresponding author. Tel.: +33 467 524 368; fax: +33 467 544 842. E-mail address: christol@ies.univ-montp2.fr (P. Christol). Infrared Physics & Technology 59 (2013) 32–35 Contents lists available at SciVerse ScienceDirect Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared