1Department of Electrical and Computer Engineering, University of Illinois Urbana Champaign, Urbana IL 61801. 2Frederick Seitz Materials Research ...
SM2G.4.pdf
CLEO:2015 © OSA 2015
Diffusion Characterization Using Electron Beam Induced Current and Time-Resolved Photoluminescence of InAs/InAsSb Type-II Superlattices D. Zuo1, R. Liu1, J. Mabon2, Z.-Y. He3, S. Liu3, Y.-H. Zhang3, E. A. Kadlec4, B. Olson4, E. A. Shaner4, and D. Wasserman1
1 Department of Electrical and Computer Engineering, University of Illinois Urbana Champaign, Urbana IL 61801 Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 3 Center for Photonics Innovation and School of Electrical, Computer, and Energy Engineering, Arizona State University, Tempe, Arizona 85287 4 Sandia National Laboratories, Albuquerque, New Mexico 87185, USA 2
Abstract: We present the characterization of minority carrier diffusion length and surface recombination velocity, as well as vertical diffusivity and mobility by performing an electron beam induced current measurement in addition to an optical lifetime measurement. 2015 Optical Society of America OCIS codes: (250.0040) Detectors; (040.3060) Infrared; (290.1990) Diffusion
1. Introduction Novel photonic devices based on type-II superlattices (T2SLs) have seen a great deal of interest recently, in particular as a future replacement for II-VI alloys such as HgCdTe in infrared (IR) detection [1]. Theoretical performance of low effective band gap T2SLs, typically InAs/Ga(In)Sb and more recently InAs/InAsSb on GaSb substrates, has been shown to be superior to HgCdTe thanks to a variety of factors, including suppression of Auger recombination. The additional flexibility in altering layer thicknesses to tune the band gap also gives T2SLs a competitive edge. However, T2SLs suffer from defects and growth imperfections that limit their performance [2]. Electron beam induced current (EBIC) is a powerful technique that can be applied to already-fabricated devices in order to determine a sample’s surface recombination velocity as well as its minority carrier diffusion length in this vertical direction [3], with the only requirements being an electrically contacted sample that possesses a carrier collecting junction, i.e. a built-in electric field, and physical access to the junction cross-section via cleaving. Carriers are generated in the sample by kinetic scattering of a high-energy electron beam in a scanning electron microscope (SEM). Current is measured and correlated with the beam position to form a signal, which can be analyzed to determine the diffusion length as well as the surface recombination velocity by varying the energy of the beam. Combined with measuring the carrier recombination lifetime using time-resolved photoluminescence (TRPL) on the same T2SL sample, this enables determination of the minority carrier vertical diffusion coefficient and, via Einstein’s relation, the vertical hole mobility [4]. 2. Experiment and Results An nBn detector sample was grown via molecular beam epitaxy using InAs/InAsSb superlattices for the contacts and absorption layer, and a wide-bandgap InAs/AlAsSb superlattice for the electron-blocking barrier layer between the absorption region and the top n-contact. Devices were fabricated via a standard photolithography and chemical
Fig. 1. (a) Layer structure of the fabricated device used for measurements. (b) Spectral response data (grey points) with a smoothed average (red line), along with photoluminescence (blue line).
SM2G.4.pdf
CLEO:2015 © OSA 2015
wet etch into square mesas with top and bottom contacts using Ti/Pt/Au. The layer structure and device geometry are shown in Figure 1(a). The effective band gap of the detector, which was designed for approximately 5 µm, was verified via uncalibrated FTIR responsivity measurements and spectral photoluminescence (Figure 1(b)). EBIC measurements were carried out by cleaving the fabricated samples so as to bisect the etched mesas and mounting at a 90° angle with thermally conductive silver paste on a brass stage. Gold wire bonds were used to provide electrical connections to the device. Images were taken using a helium cooled cryostat stage in a JEOL 7000F SEM, with a Stanford SR570 preamplifier to measure the sample current. A series of scans were taken at 5.6 K with the beam energy of the SEM varied from 5 to 25 keV. Figure 2(a) shows the experimental data. To extract the minority carrier hole diffusion length, modeling of the recovered signal was carried out using standard EBIC theory [5], supplemented with Monte Carlo simulations to model the distributions of carriers generated by the electron beams at the specified energies [6]. In order to account for the electron hole pairs generated directly in the wide-bandgap barrier layer, we also incorporated a distribution of the energy absorbed in the barrier layers based on beam position. Figure 2(b) shows theoretical modeling results for the same beam energies as the experimental data. There was an excellent agreement between the measured and theoretical data and a hole diffusion length of 750 nm, and a surface recombination velocity of 2.78 × 104 cm/s were extracted. Figure 2(c) shows TRPL data obtained from the same sample, along with an exponential fit used to extract minority carrier lifetime. From the TRPL data we obtained a hole lifetime of 202 ns. From the EBIC model we obtained a hole diffusion length of 750 nm. From these data, we further calculate a vertical hole diffusivity of 2.78 × 10-2 cm2/s and a vertical hole mobility of 57.4 cm2/Vs.
Fig. 2. (a) EBIC data obtained at 5.6 K for various beam energies from the nBn detector. (b) Theoretical model used to extract carrier diffusion length. (c) Time-resolved photoluminescence data taken at 16 K for various optical pumping powers. A single-exponential fit (red dashed line) used to extract carrier lifetime is shown.
3. Conclusions We have demonstrated the use of EBIC measurements as a complementary measurement tool to obtain minority carrier diffusion lengths in an nBn infrared photodetector. By combining the results with TRPL data to measure minority carrier lifetime, we were able to estimate the vertical minority carrier diffusivity and mobility, key parameters for overall device performance. This characterization suite allows for a comprehensive study of carrier recombination in T2SLs and other novel materials. 4. Acknowledgements The authors gratefully acknowledge support from the Army Research Office Multi-Disciplinary Research Initiative, grant no. W911NF-10-1-0524. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. 5. References [1] D. L. Smith and C. Mailhiot, J. Appl. Phys., 62, 2545, 1987. [2] C. H. Grein, J. Garland, and M. E. Flatté, J. Electron. Mater., 38, 1800–1804, 2009. [3] J.-M. Bonard and J.-D. Ganière, J. Appl. Phys., 79, 6987, 1996. [4] S. L. Chuang, Physics of Photonic Devices, 2nd ed. New Jersey: Wiley, 2009. [5] C. Donolato, J. Appl. Phys., 76, 959, 1994. [6] D. Drouin, A. R. Couture, D. Joly, X. Tastet, V. Aimez, and R. Gauvin, Scanning, 29, 92–101, 2007.
