242 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 4, FEBRUARY 15, 2011 Noise Characterization of Midwave Infrared InAs/GaSb Superlattice pin Photodiode Katarzyna Jaworowicz, I. Ribet-Mohamed, C. Cervera, J. B. Rodriguez, and P. Christol Abstract—We report on noise characterization of a midwave infrared (MWIR) InAs/GaSb superlattice (SL) single detector. The SL structure was made of eight InAs monolayers (MLs) and eight GaSb MLs, with a total thickness of 2 m (440 SL periods). This structure exhibits a cut-off wavelength of 4.8 m at 77 K. Ex- tracted from current–voltage characteristics, zero-bias resistance area product above cm at 80 K was measured. Noise measurements were also performed under dark conditions. The measurements reveal the absence of noise above 30 Hz. Moreover, the detector under test remains Schottky noise-limited up to a bias voltage of 200 mV typically, which confirms the quality of the MWIR SL pin photodiode. Index Terms—Dark current, InAs/GaSb superlattice, infrared photodiodes, noise measurement. I. INTRODUCTION T HE 3–5 m mid-wavelength infrared range (MWIR) is an interesting spectral domain for high performance infrared (IR) imaging technology for medical applications (diagnosis as- sistance), industrial applications (process control) and military applications (night vision). Type-II InAs/GaSb superlattice (SL) photodiode has recently obtained tremendous results and can be now considered as an interesting alternative technology to the well-established InSb and HgCdTe MWIR photodetectors [1]. However, to enhance the device performances, a better knowl- edge of electrical properties [2], carrier-lifetime [3], gain [4] and, more importantly, noise measurements of SL photodiode [5]–[7] are still necessary. Indeed, noise measurements allow one to calculate the real detectivity, whereas most of the de- tectivity values reported in the literature use a theoretical ex- pression for the noise contribution which assumes a Johnson noise-limited detector. The analysis of the physical origin of the noise (Johnson noise, Schottky noise or noise) also allows one to estimate the potentiality of InAs/GaSb SL detector tech- nology. Paradoxically, only few papers deal with the study of noise in SL detectors: [6] and [7] focus on longwave infrared (LWIR) focal plane arrays (FPA) and single elements, respec- tively, while [5] reports measurements on MWIR SL detectors (but only at nearly zero bias voltage). Manuscript received September 15, 2010; revised November 08, 2010; ac- cepted November 13, 2010. Date of publication November 18, 2010; date of current version January 28, 2011. This work was supported in part by the French DGA. K. Jaworowicz and I. Ribet-Mohamed are with the ONERA, 91761 Palaiseau, France (e-mail: Katarzyna.Jaworowicz@onera.fr). C. Cervera, J. B. Rodriguez, and P. Christol are with the Institut d’Electron- ique du Sud (IES), Université Montpellier 2, UMR CNRS 5214, 34095 Mont- pellier, France (e-mail: christol@ies.univ-montp2.fr). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2010.2093877 In this letter we report on dark current and noise measure- ments performed on p-i-n MWIR InAs/GaSb SL single detector. II. EXPERIMENTS The detector structure was grown by molecular beam epitaxy (MBE) on p-doped GaSb substrate. It consists of a 200 nm-thick Be-doped ( cm ) GaSb buffer layer, an undoped InAs/GaSb SL active region surrounded by 10 periods of n and p-doped SL with similar period composition, and a 20 nm-thick Te-doped ( cm ) InAs cap layer. The unintention- ally doped SL comprises 440 periods, corresponding to a total thickness of 2 m; each period being made of 8 InAs mono- layers (MLs) and 8 GaSb MLs. This structure exhibits photolu- minescence emission in the MWIR, with a PL peak position at 4.8 m at 80 K [8]. Further details on MBE growth and struc- tural characterizations have been published elsewhere [9]. Circular mesa diodes were realized using standard pho- tolithography techniques with a mask set containing diodes with several diameters ranging from 40 to 465 m. AuGeNi and AuZn contact layers were deposited respectively on top of the mesa (n-type InAs cap layer) and on the back of the GaSb p-type substrate. A citric acid based solution was used for mesa etching and immediately after, a photoresist AZ-1518 was spun onto the sample and polymerized in order to protect the surface from ambient air [10]. Finally, the samples were indium-bonded and packaged in submounts. To perform dark current and dark noise measurements, the SL detectors were placed on the cold finger of a continuous flow SMC 7845.1 cryostat operated with helium or nitrogen. A BT 400 controller was used to reach the desired temperature with a resolution of 0.01 K. For dark current measurements, a Keithley 6430 subfem- toamperemeter was used to both apply the bias voltage and read the current delivered by the single detector. For dark current noise measurements, we used a Keithley 428 transimpedance amplifier and a spectrum analyzer (ONO SOKKI). In both cases, the experimental setup was controlled by a computer equipped with a GPIB (General Purpose Interface Bus) inter- face and dedicated Python software. III. RESULTS Fig. 1 plots several dark current density curves as a function of the bias voltage, for different operating temperatures (from 20 K up to 250 K). The sample under test is a cm area pixel. A dark current density of about 1.1 A cm at mV is measured at 80 K. At 20 K, the dark current is so low that we reach the limitations of our experimental setup (a dark current density of A cm corresponds to a dark current of 40 fA). From dark current density curves, the 1041-1135/$26.00 © 2010 IEEE