Quantum Electronics Letters - IEEE Xplore

IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 47, NO. 12, DECEMBER 2011

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Quantum Electronics Letters Long Wavelength Infrared InAs/GaSb Superlattice Photodetectors with InSb-Like and Mixed Interfaces Yanhua Zhang, Wenquan Ma, Yulian Cao, Jianliang Huang, Yang Wei, Kai Cui, and Jun Shao

Abstract— We report on long wavelength infrared photodetectors using InAs/GaSb superlattices (SLs) with InSb-like and mixed interfaces (IFs). X-ray diffraction (XRD) measurements indicate that the SLs with mixed IFs have a narrower linewidth. The full-width at half-maximum of the -1st XRD satellite peak is 24 arcsec for the sample with InSb-like IFs and is only 17 arcsec for the sample with mixed IFs. However, in terms of infrared photodetection, InSb-like IFs are superior to the mixed ones. Stronger photoluminescence and photoresponse signals are observed for the sample with InSb-like IFs. Index Terms— InAs/GaSb, interfaces, long wavelength infrared photodetector, type II superlattice.

I. I NTRODUCTION

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HERE has been considerable interest in developing infrared photodetectors using type II InAs/GaSb shortperiod superlattices (SLs) ever since some advantages of the material for infrared photodetection were proposed in 1987 [1]. Type II InAs/GaSb SL structure is an alternative and a very promising material system for infrared detection of about 3 to 30 μm. For the detection of long wavelength infrared (LWIR) and very long wavelength infrared range, compared to HgCdTe, InAs/GaSb SL detector is expected to have a comparable performance due to the suppressed Auger recombination rate [2]–[4] while may demonstrate some advantages in terms of the material uniformity and cost [5]. In recent years, high-performance LWIR photodetectors using InAs/GaSb SLs have been reported [6]–[8]. However, one fundamental issue related to the interface (IF) selection seems still not very clear. That is, what kind of IF combination is beneficial to an enhanced performance of the SL photodetector? There are two types of natural IFs for InAs/GaSb SLs: the InSb-like IF for InAs-on-GaSb and the GaAs-like one for GaSb-onInAs [9]. For molecular beam epitaxy (MBE), by designing

Manuscript received July 12, 2011; revised September 7, 2011; accepted September 11, 2011. Date of current version November 1, 2011. This work was supported in part by the National Basic Research Program of China 973 Program, under Grant 2010CB327602 and Natural Science Foundation under Grant 61176014, Grant 61176075, and Grant 61021003. Y. H. Zhang, W. Q. Ma (corresponding author), Y. L. Cao, J. L. Huang, Y. Wei, and K. Cui are with the Laboratory of Nano-Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China (e-mail: [email protected]). J. Shao is with the National Laboratory for Infrared Physics, Shanghai Institute for Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2011.2168947

the shutter sequences, four types of IFs can be obtained [10]. For convenience, let’s call the InAs-on-GaSb IF the bottom one and the GaSb-on-InAs the top one. The majority of the literature seem to support that the SLs with the InSb-like IFs both at the bottom and at the top have better structural and optical or electrical properties than the ones with other three types of IFs [10]–[13]. However, there are also some reports supporting that the mixed IFs, i.e., the GaAs-like one at the bottom and the InSb-like one at the top, don’t degrade the structural and optical properties of the SLs [14]. We think that, in order to determine what kind of IF combination is favorable to photodetector application, the SL materials with different IF combinations must first have a very high and comparable structural quality. Hence, the contributions due to some other structural imperfections like dislocations can be removed and thus, such a comparison makes sense. In this paper, we report on LWIR InAs/GaSb SL photodetecors with the InSb-like and the mixed IFs. The as-grown SL materials demonstrate a very high structural quality reflected by a very small strain and a very narrow linewidth of x-ray diffraction (XRD) satellite peaks. The full width at half maximum (FWHM) of the -1st satellite peak is 24 arcsec for the sample with the InSb-like IFs and is only 17 arcsec for the one with the mixed IFs and the average strain of the SL materials of the two samples is in the order of magnitude of 10−4 . However, in terms of optical property, the SLs with the InSb-like IFs are superior to the ones with the mixed IFs. Stronger infrared photoluminescence (PL) and photoresponse signals are observed for the sample with the InSb-like IFs than the sample with the mixed IFs. The device structure is a p-i-n type along the growth direction. The samples were grown on semi-insulating GaSb (001) substrates by MBE using Sb2 and As2 . For the SL growth, the growth rate employed is 0.38 monolayer (ML)/s for InAs and is 0.5 ML/s for GaSb and the V/III beam-equivalent pressure ratio used is about 4 for both the InAs and the GaSb layers. After the growth of a 0.5 μm thick GaSb buffer layer doped to 2 × 1018 cm−3 using Be at 500 °C, the substrate was cooled down to 380 °C for the growth of the other layers. The p region is composed of the mentioned 0.5 μm thick GaSb buffer and 100 periods of InAs (8ML)/GaSb (8ML) SLs with the GaSb layer doped to 2 × 1018 cm−3 using Be. The active layer is made up of 300 periods of InAs (13ML)/GaSb (8.5ML) SLs. The n layer consists of 100 periods of InAs (8ML)/GaSb (8ML) SLs with the InAs layer doped to 2 × 1018 cm−3 with Si followed by a 200 Å thick InAs doped

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again to 2 × 1018 cm−3 using Si. The SLs in the p and the n layers are expected to have a 50% cutoff wavelength of about 5 μm. Namely, mid wavelength InAs/GaSb SLs are employed as the p and the n layers. In order to investigate the IF effect, we grew two samples between which the only difference lies in IF combinations. Fig. 1(a) and (b) illustrate the employed MBE shutter sequences of the two samples. One sample has the InSb-like bottom and top IFs, which we call sample A; the other has mixed IFs: the GaAs-like IF at the bottom and the InSb-like IF at the top, which we call sample B. For the two samples, the top IF is the same and is the InSb-like. In terms of the bottom IF, for sample B, the GaAs-like IF is generated by employing As soak time of 2 s after the GaSb deposition [15] while, for sample A, the InSb-like IF is formed by depositing 0.5 s of InSb and then, by introducing 1 s of Sb soak followed by 0.5 s of the growth stop in which all the shutters are closed. For the top IF, the difference between the two samples with respect to the stop time, the Sb soak time, and InSb deposition time results from minimizing the average strain of the SLs. For the two samples, since the bottom IF is different, the top InSb-like IF also should be different. In fact, Fig. 1(a) and (b) show our optimized shutter sequences for the two types of IF combinations. It can be seen that the handling of the shutter sequences is a very subtle process and for some procedures, the employed time between the shutter open and close is only 0.5 s, which is very close to the limit of the shutter open/close process. The structural characterization was performed using a Bede D1 high-resolution x-ray diffractometer with an open detector and the step angle of the ω scan used is 0.001°. Fig. 2(a) shows the XRD curves of the ω−2θ scan around the GaSb (004) reflection for the as-grown samples A (top curve) and B (bottom curve). It can be seen that the XRD curves reveal two series of very clear satellite peaks. One series, which is due to the LWIR SLs, is denoted as “L,” and the other series due to the SLs in the p and the n layers is denoted as “M.” From the spacing between the satellite peaks of the “L” series, the thickness of one period of the

Fig. 2. (a) XRD curves of the ω-2θ scan around the GaSb (004) reflection for samples with the InSb-like (top) and the mixed IFs (bottom). (b) and (c) shows the corresponding close-ups of the two samples around the 0-th and substrate peaks.

SLs is calculated to be 65.1 Å for sample A and 65.9 Å for sample B. Both the values are very close to our design. Fig. 2(b) and (c) show the close-ups around the range of the 0-th and the substrate peaks for samples A and B, respectively. In Fig. 2(b) and (c), L0 is the 0-th peak of the “L” series and M0 is the 0-th peak of the “M” series. It can be seen that L0 is located at the small angle side of the substrate peak for sample A while is at the high angle side for sample B. This implies that the SLs are compressively strained for sample A while are tensilely strained for sample B. The calculated strain is 3.1 × 10−4 for sample A and is −2.3 × 10−4 for sample B. Since the substrate and the 0-th peaks are very close to each other for the two samples, this makes the FWHM of the 0-th peak hard to determine. Therefore, the -1st peak is selected to measure the FWHM and the value is 24 arcsec for sample A and is only 17 arcsec for sample B. As can be seen, sample B, which has the mixed IFs, has a narrower XRD linewidth and a smaller strain value than sample A, which has InSb-like IFs. The revealed very narrow FWHM demonstrates that both the samples have a very high structural quality, which may be a prerequisite for the comparison of the optical quality of the two samples. For the PL measurement, since the top n-type SLs may block the PL excitation light, instead of using the p-i-n samples, we grew another two samples corresponding to samples A and B, and the corresponding structure comprises the 100 periods of the LWIR SLs deposited on a 0.5 μm thick GaSb buffer layer. The growth conditions of the two samples is exactly the same as that of the corresponding p-i-n type structure. The infrared PL was measured by a Fourier transform infrared (FTIR) spectrometer-based modulated PL technique with a spectral resolution of 12 cm−1 [16], [17]. The two structures were mounted on the same sample holder and were measured under exactly the same condition including the excitation power. Fig. 3(a) depicts the PL spectra of the two samples measured at 77 K. In spite of the fact that the period of the SLs is only 100, clear PL signal is observed. The PL peak position is about 9.4 μm for the sample with the InSblike IFs and is about 9.7 μm for the sample with the mixed IFs. The PL FWHM is about the same and is 26 meV for the two samples. Here the key point is that the PL intensity of the

ZHANG et al.: LONG WAVELENGTH INFRARED InAs/GaSb SUPERLATTICE PHOTODETECTORS

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sample with the InSb-like IFs is about 1.3 times as strong as that with the mixed IFs. This implies that the SLs with the InSb-like IFs have a better optical quality than the SLs with the mixed ones. To investigate the effect of the IF combinations on photoresponse, the two as-grown p-i-n samples were processed into circular mesa structures without surface passivation by photolithography and wet chemical etching. The diameter of the top ring contact is 340 μm and Ti/Au is used as the p and the n type Ohmic contact metal. The two samples were again mounted on the same sample holder and the spectral response (photocurrent in our case) was measured under exactly the same condition using a Bruker Vertex 70 FTIR spectrometer. The spectral responsivity was obtained by calibrating the photocurrent response by measuring the blackbody response with the blackbody source temperature set at 800 K. Fig. 4(a) depicts the measured responsivity spectra of samples A and B at zero bias at 77 K. The 50% cutoff wavelength is in LWIR range and is 9.6 μm for sample A and is 10.0 μm for sample B. The 50% cutoff wavelength nearly coincides with the PL peak position for the two samples. The responsivity maximum is at 7.7 μm for the two samples, but the responsivity value at this position is 3.2 A/W for sample A and is 2.2 A/W for sample B. Namely, the photoresponse at 7.7 μm is about 1.5 times as strong for sample A as for sample B. The responsivity R can be expressed as R = eη/ hν, where e is the electron charge, and η is the external quantum efficiency; h is the Planck’s constant and ν is the photon frequency. Therefore, for the two samples, at 7.7 μm, the quantum efficiency of the sample with the InSb-like IFs is about 1.5 times as high as that of the sample with the mixed IFs. The concrete value of η is 51.6% for sample A and is 35% for sample B and the corresponding η at the 50% cutoff wavelength is 20.8% at 9.6 μm for sample A and is 13.5% at 10.0 μm for sample B. The decrease of the photoresponse signals beyond 7.7 μm towards the high energy side is caused by the absorption of the incident IR light by the SLs in the p and the n layers, which have a 50% cutoff wavelength of about 5 μm. Here we need to point out that, in order to remove the inhomogeneous effect of the grown material, quarter 2-inch wafers are used

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for the growth of the two samples and the device dies are selected from the same area of the wafer for the comparison of the photoresponse measurements mentioned above. Both the samples are of very high structural quality and sample B even exhibits a narrower XRD linewidth and a smaller strain. Therefore, to account for the stronger PL and photoresponse observed for sample A, we rule out the possibility that sample B has more dislocations and has a rougher IFs than sample A since this point conflicts with the XRD results. Based on the early work [10], we propose that As-on-Ga antisite defects may be responsible for the degraded PL and photoresponse signals for sample B. These antisite defects are formed on the GaSb surface during the crossover of the GaAs-like interface [10] and they are very thin and thus, XRD probably can’t “see” them. However, they may cause nonradiative recombination resulting in degradation of PL and photoresonse signals. On the other hand, the difference between the two samples is reflected in the dark current [see the inset of Fig. 4(b)]. Moreover, more clear difference is actually revealed in the dependence of the dynamic resistance on the applied bias voltage as shown in Fig. 4(b). The dynamic resistance maximum is not at zero bias for the two samples, but the difference of the dynamic resistance at zero bias between the two samples is negligible. R0 A, the resistancearea product at zero bias, is also nearly the same and is 1.56 cm2 for sample A and is 1.43 cm2 for sample B. The value of the R0 A here is still small compared to the state-of-the-art results, for example, Ref. [6]. We think that the relatively small R0 A is due to the fact that there are no any surface passivation applied to the device structure of the two samples. Compared to sample A, a degradation of the dynamic resistance is observed for sample B as can be seen in Fig. 4(b). If the antisite defect explanation is correct, since the antisite defects are donors [10], based on Ref. [18], we propose that these donor-like antisite defects may attract the current and cause the dominant shunt current crowding, which consequently leads to an early onset of the breakdown mechanism. The result is degradation of the dynamic resistance with respect to the bias voltage as shown in Fig. 4(b).

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II. C ONCLUSION We have investigated LWIR photodetectors with different IFs. Both the samples with the InSb-like and the mixed IFs have a very high structural quality. The SLs with the mixed IFs has a narrower XRD linewidth and a smaller strain. However, PL and photoresponse measurements indicate that the InSblike IFs are superior to the mixed IFs. Our results indicate that the SLs with the InSb-like IF combination are preferential in terms of photodetector application. R EFERENCES [1] D. L. Smith and C. Mailhiot, “Proposal for strained type II superlattice infrared detectors,” J. Appl. Phys., vol. 62, no. 6, pp. 2545–2548, Sep. 1987. [2] E. R. Youngdale, J. R. Meyer, C. A. Hoffman, F. J. Bartoli, C. H. Grein, P. M. Yang, H. Ehrenreich, R. H. Miles, and D. H. Chow, “Auger lifetime enhancement in InAs–Ga1−x Inx Sb superlattices,” Appl. Phys. Lett., vol. 64, no. 23, pp. 3160–3162, Jun. 1994. [3] C. H. Grein, H. Cruz, M. E. Flatté, and H. Ehrenreich, “Theoretical performance of InAs/Inx Ga1−x Sb superlattice-based midwave infrared lasers,” Appl. Phys. Lett., vol. 65, no. 20, pp. 2530–2532, Nov. 1994. [4] H. Mohseni, V. I. Litvinov, and M. Razeghi, “Interface-induced suppression of the Auger recombination in type-II InAs/GaSb superlattices,” Phys. Rev. B, vol. 58, no. 23, pp. 15378–15380, Dec. 1998. [5] A. Rogalski, “Material considerations for third generation infrared photon detectors,” Infrar. Phys. Technol., vol. 50, nos. 2–3, pp. 240– 252, 2007. [6] S. Bogdanov, B.-M. Nguyen, A. M. Hoang, and M. Razeghi, “Surface leakage current reduction in long wavelength infrared type-II InAs/GaSb superlattice photodiodes,” Appl. Phys. Lett., vol. 98, no. 18, pp. 1835011–183501-3, May 2011. [7] D. Z. Y. Ting, C. J. Hill, A. Soibel, S. A. Keo, J. M. Mumolo, J. Nguyen, and S. D. Gunapala, “A high-performance long wavelength superlattice complementary barrier infrared detector,” Appl. Phys. Lett., vol. 95, no. 2, pp. 023508-1–023508-3, Jul. 2009. [8] N. Gautam, H. S. Kim, M. N. Kutty, E. Plis, L. R. Dawson, and S. Krishna, “Performance improvement of longwave infrared photodetector based on type-II InAs/GaSb superlattices using unipolar current blocking layers,” Appl. Phys. Lett., vol. 96, no. 23, pp. 231107-1– 231107-3, Jun. 2010. [9] J. B. Rodriguez, P. Christol, L. Cerutti, F. Chevrier, and A. Joullie, “MBE growth and characterization of type-II InAs/GaSb superlattices for mid-infrared detection,” J. Cryst. Growth, vol. 274, nos. 1–2, pp. 6– 13, 2005. [10] G. Tuttle, H. Kroemer, and J. H. English, “Effects of interface layer sequencing on the transport properties of InAs/AlSb quantum wells: Evidence for antisite donors at the InAs/AlSb interface,” J. Appl. Phys., vol. 67, no. 6, pp. 3032–3037, Mar. 1990. [11] N. Herres, F. Fuchs, J. Schmitz, K. Pavlov, J. Wagner, J. Ralston, P. Koidl, C. Gadaleta, and G. Scamarcio, “Effect of interfacial bonding on the structural and vibrational properties of InAs/GaSb superlattices,” Phys. Rev. B, vol. 53, no. 23, pp. 15688–15705, Jun. 1996. [12] G. R. Booker, P. C. Klipstein, M. Lakrimi, S. Lyapin, N. J. Mason, I. J. Murgatroyd, R. J. Nicholas, T. Y. Seong, D. M. Symons, and P. J. Walker, “Growth of InAs/GaSb source strained layer superlattices. II,” J. Cryst. Growth, vol. 146, nos. 1–4, pp. 495–502, Jan. 1995. [13] A. Y. Lew, S. L. Zuo, E. T. Yu, and R. H. Miles, “Anisotropy and growth-sequence dependence of atomic-scale interface structure in InAs/Ga1−x Inx Sb superlattices,” Appl. Phys. Lett., vol. 70, no. 1, pp. 75–77, Jan. 1997. [14] R. Kaspi, J. Steinshnider, M. Weimer, C. Moeller, and A. Ongstad, “Assoak control of the InAs-on-GaSb interface,” J. Cryst. Growth, vol. 225, nos. 2–4, pp. 544–549, May 2001. [15] J. R. Waterman, B. V. Shanabrook, R. J. Wagner, M. J. Yang, J. L. Davis, and J. P. Omaggio, “The effect of interface bond type on the structural and optical properties of GaSb/InAs superlattices,” Semicond. Sci. Technol., vol. 8, no. 1S, pp. S106–S111, 1993. [16] J. Shao, W. Lu, X. Lü, F. Y. Yue, Z. F. Li, S. L. Guo, and J. H. Chu, “Modulated photoluminescence spectroscopy with a step-scan Fourier transform infrared spectrometer,” Rev. Sci. Intrum., vol. 77, no. 6, pp. 063104-1–063104-6, Jun. 2006.

[17] J. Shao, L. Chen, W. Lu, X. Lü, L. Zhu, S. L. Guo, L. He, and J. H. Chu, “Backside-illuminated infrared photoluminescence and photoreflectance: Probe of vertical nonuniformity of HgCdTe on GaAs,” Appl. Phys. Lett., vol. 96, no. 12, pp. 121915-1–121915-3, Mar. 2010. [18] V. Gopal, E. Plis, J. B. Rodriguez, C. E. Jones, L. Faraone, and S. Krishna, “Modeling of electrical characteristics of midwave type II InAs/GaSb strain layer superlattice diodes,” J. Appl. Phys., vol. 104, no. 12, pp. 124506-1–124506-6, Dec. 2008.

Yanhua Zhang received the M.Sc. degree from the Beijing Institute of Technology, Beijing, China, in 2007. He is currently pursuing the Ph.D. degree with the Institute of Semiconductors, Chinese Academy of Sciences, Beijing. His current research interests include molecular beam epitaxy growth and characterization and device fabrication of quantum dot and type II InAs/GaSb superlattice infrared photodetectors.

Wenquan Ma received the B.Sc. degree in physics from Lanzhou University, Lanzhou, China, and the Ph.D. degree from Humboldt University, Berlin, Germany, in 2001. His thesis work was carried out at the Paul Drude Institute for Solid State Electronics, Berlin. He was a Post-Doctoral Fellow with the Physics Department, University of Arkansas, Fayetteville. He joined the Institute of Semiconductors, Chinese Academy of Sciences (CAS), Beijing, China, in October 2004. His current research interests include quantum dot and type II InAs/GaSb superlattice infrared photodetectors and self-organized synthesis of spatially ordered quantum dot arrays. Dr. Ma was a recipient of the Hundred Talents Program of the CAS in 2005.

Yulian Cao received the Ph.D. degree from the Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China, in 2006. Her thesis work was on the fabrication and characterization of quantum dot and quantum well lasers. Her current research interests include the fabrication of quantum dot and type II InAs/GaSb superlattice infrared photodetectors.

Jianliang Huang received the B.Sc. degree from the Beijing University of Posts and Telecommunications, Beijing, China, in 2007. He is currently pursuing the Ph.D. degree with the Institute of Semiconductors, Chinese Academy of Sciences, Beijing. His current research interests include molecular beam epitaxy growth and characterization and device fabrication of quantum dot and type II InAs/GaSb superlattice infrared photodetectors.

Yang Wei received the B.Sc. degree from Peking University, Beijing, China, in 2007. He is currently pursuing the Ph.D. degree with the Institute of Semiconductors, Chinese Academy of Sciences, Beijing. His current research interests include molecular beam epitaxy growth and characterization and device fabrication of quantum dot and type II InAs/GaSb superlattice infrared photodetectors.

ZHANG et al.: LONG WAVELENGTH INFRARED InAs/GaSb SUPERLATTICE PHOTODETECTORS

Kai Cui received the B.Sc. degree from Lanzhou University, Lanzhou, China, in 2008. He is currently pursuing the Ph.D. degree with the Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China. His current research interests include molecular beam epitaxy growth and fabrication of memory devices using type II GaAs/GaSb quantum dots.

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Jun Shao received the B.Sc. degree from Nanjing University, Nanjing, China, in 1986, and the Ph.D. degree from the University of Stuttgart, Stuttgart, Germany, in 2002. He is currently a Professor with the Shanghai Institute for Technical Physics, Chinese Academy of Sciences, Beijing, China. His current research interests include optical and magnetooptical studies of narrow bandgap semiconductor materials using Fourier transform infrared spectrometer-based photoreflectance and photoluminescence techniques.