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• Abstract

SECTION I

## INTRODUCTION

Multicolor detection, along with high operation temperature and large format arrays, are the characteristics of the third generation of infrared (IR) imaging systems. Data collection in separate IR bands is highly beneficial for military and civil applications involving identification of temperature differences and determination of the thermal characteristics of an object. Detectors based on interband [Mercury–Cadmium–Telluride (MCT)] and intersubband [quantum well infrared detectors (QWIPs)] transitions have been the dominant technologies for such applications [1], [2].

Difficulties in the epitaxial growth of MCT and low electron effective mass $(0.009\ {\rm m}_{o})$ resulting in large dark current due to tunneling especially at longer wavelengths [3] affect the development of multispectral cameras based on MCT. With respect to MCT detectors, QWIPs have a number of advantages, including the use of standard manufacturing techniques based on a mature GaAs growth and processing technologies, highly uniform and well-controlled MBE growth on larger area GaAs wafers (6 in.), high yield and, thus, low cost, and more thermal stability. However, they have larger dark currents and lower quantum efficiencies compared with the interband devices. On the contrary, the basic material properties of InAs/GaSb type-II strained layer superlattices (SLS) provide a prospective benefit in the realization of dual-color imagers. The strain in InAs/GaSb type-II SLS system facilitates suppression of interband tunneling [4] and Auger recombination [5] processes. Moreover, the larger effective mass in SLS leads to a reduction of tunneling currents compared with MCT detectors of the same bandgap. By optimizing the oscillator strength in this material system, a large quantum efficiency and responsivity can be obtained.

Despite the prominent advantages of SLS technology, only a few reports of dual-band detectors based on InAs/GaSb SLS have been published so far. The dual-color SLS cameras [MWIR (4 $\mu\hbox{m}$)/MWIR (5 $\mu\hbox{m}$) and MWIR (7.7 $\mu\hbox{m}$)/LWIR (10 $\mu\hbox{m}$)] possess a vertical detector design based on two back-to-back InAs/GaSb SLS photodiodes separated by a common ground contact layer [6], [7]. These detectors are based on conventional p-i-n design which is characterized by high diffusion and Shockley–Read–Hall (SRH) generation-recombination dark currents. Moreover, they require multiple contacts per pixel resulting in a complicated processing scheme and expensive specific read-out circuits. The MWIR/LWIR detectors based on nBn design [8], [9] are expected to reduce the dark current level due to elimination of depletion region in detector heterostructure. In addition, multicolor detectors with nBn design do not require multiple contacts per pixel, which reduces the cost and complexity associated with the FPA fabrication process.

Typical dual-band SLS detector with nBn design is composed by two narrow band-gap absorbers (n) the different IR regions separated by a 100-nm-thick wide-band-gap material layer with a large barrier for electrons and no barrier for holes (B). As a result, the majority carrier current between the two electrodes is blocked by the large energy offset, while there is no barrier for photogenerated minority carriers. However, photocarriers in this case are the low mobility holes. It would be desirable to have the inverted structure, (i.e., pBp detector) in which the photocarriers are the higher mobility electrons.

In this paper, we report a dual-band InAs/GaSb SLS detector with a pBp architecture. The dual-band pBp SLS detector has shown the shot noise limited detectivity at 77 K of $5 \times 10^{11}$ Jones (at $\lambda = 5\ \mu\hbox{m}$ and $V_{b} = +0.1\ \hbox{V}$) and $2.6 \times 10^{10}$ Jones (at $\lambda = 9\ \mu\hbox{m}$ and $V_{b} = -0.4\ \hbox{V}$) for MWIR and LWIR absorbers, respectively. The maximum values of quantum efficiency were estimated to 41% (MWIR absorber) and 25% (LWIR absorber) at $V_{b} = +0.4\ \hbox{V}$ and $V_{b} = -0.7\ \hbox{V}$. The performance of the pBp detector is superior to that of previously reported dual-band (MWIR/LWIR) QWIP detectors and is comparable with that of dual-band MCT detectors in the MWIR band.

SECTION II

## EXPERIMENTAL DETAILS

The devices studied in this work were grown on the Te-doped epiready (100) GaSb substrate by a solid source MBE VG-80 system. The active region of pBp dual-band detector structure was formed by LWIR (14 ML InAs/7 ML GaSb:Be SLS, p-type) and MWIR (10 ML InAs/4 ML GaSb:Be SLS, p-type) absorbing regions with thicknesses of 2 $\mu\hbox{m}$ separated by the 100-nm barrier (16 ML InAs/4 ML AlSb:Be SLS, $p = 1 \times 10^{16}\ \hbox{cm}^{-3}$). The doping level of MWIR and LWIR absorbing regions was estimated as $p = 1 \times 10^{16}\ \hbox{cm}^{-3}$ based on GaAs equivalent.

Semiempirical pseudopotential method [10] integrated with a SENTAURUS TCAD simulation platform [11] was used for calculation of band lineup of LWIR and MWIR absorbing regions with the barrier. The energy band diagram of active region of pBp detector showing conduction band (CB) and valence band (VB) energies under zero, forward $(V_{b} = +0.1\ \hbox{V})$, and reverse biases $(V_{b} = -0.1\ \hbox{V})$ at 77 K is presented in Fig. 1. Special care was taken to minimize the conduction band offset for the unhindered movements of the photocarrier electrons between contacts. The designed conduction band offset was equal 9 meV.

Fig. 1. Energy band diagram of pBp detector calculated at 77 K under zero (top), reverse (middle), and forward (bottom) biases.

Device fabrication was initiated with a standard optical photolithograpy to define $410\ \mu\hbox{m} \times 410\ \mu\hbox{m}$ square mesa devices with apertures ranging from 25 to 300 $\mu\hbox{m}$. Etching was performed using inductively coupled plasma (ICP) reactor with $\hbox{BCl}_{3}$ gas. Resulting etch depth was 4.7 $\mu\hbox{m}$, which corresponds to the middle of the bottom contact layer of the detector. Next, ohmic contacts were evaporated on the bottom and top contact layers using Ti (500 $\hbox{\rm{\AA}}$)/Pt (500 $\hbox{\rm{\AA}}$)/Au (3000 $\hbox{\rm{\AA}}$) in both cases. Finally, devices were passivated by SU-8 2002 photoresist [12] after a short (40 s) dip in a phosphoric acid-based solution $(\hbox{H}_{3}\hbox{PO}_{4}: \hbox{H}_{2} \hbox{O}_{2}: \hbox{H}_{2}\hbox{O} = 1: 2: 20)$ that is intended to remove native oxide film formed on the etched mesa sidewalls. The resulting thickness of SU-8 passivation film was 1.5 $\mu\hbox{m}$. The cross section of fabricated pBp dual-band detector is shown in Fig. 2.

Fig. 2. Cross section of fabricated pBp dual-band detector.
SECTION III

## RESULTS AND DISCUSSION

The normalized spectral response of a pBp dual-band detector under different polarities of applied bias was measured at 77 K with a Fourier transform IR spectrometer (FTIR), as shown in Fig. 3. Under forward bias, which is defined as positive voltage applied to the top contact, the photogenerated carriers are collected from the MWIR absorber. When the device is under reverse bias, which is defined as negative voltage applied to the top contact, the photogenerated carriers from the LWIR absorber are collected, while those from the MWIR absorber are blocked by the barrier. Thus, a two-color response is obtained under two different bias polarities. The zero-response cutoff wavelengths $(\lambda_{100\%})$ of MWIR and LWIR absorbers were equal to 7.8 $\mu\hbox{m}$ and 12.0 $\mu\hbox{m}$, respectively (77 K).

Fig. 3. Normalized spectral response of dual-band pBp detector measured at 77 K. MWIR and LWIR responses are observed under forward and reverse bias polarity, respectively.

Current–voltage ($IV$) characteristics were measured for temperatures ranging from 30 to 300 K. Representative dark current density versus applied bias curve measured at 77 K for a $400\ \mu\hbox{m} \times 400\ \mu\hbox{m}$ device is shown in Fig. 4. Fig. 5 shows the dark current density as a function of detector temperature measured at +0.1 V and −0.1 V of applied bias. At 77 K, the dark current density was equal to $6 \times 10^{-5}\ \hbox{A/cm}^{2}$ at +0.1 V (MWIR absorber) and $5 \times 10^{-4}\ \hbox{A/cm}^{2}$ at −0.1 V (LWIR absorber).

Fig. 4. Dark current density as a function of applied bias measured at 77 K.
Fig. 5. Dark current density as a function of detector temperature measured for +0.1 V (MWIR absorber) and −0.1 V (LWIR absorber) of applied bias.

At high temperatures, the extracted activation energy of MWIR absorber (0.128 eV) is found to be very close to the nominal value of the optical band gap (0.159 eV). This behavior indicates that the current is dominated by a diffusion mechanism. The saturation observed at lower temperatures is probably due to the trap-assisted tunneling [13]. For the LWIR absorber, the value of activation energy at higher temperatures (0.078 eV) was proportional to the 2/3 Eg (0.103 eV), indicating that the LWIR absorber is limited by the SRH generation process.

The responsivity of dual-band pBp detector with an aperture of 100 $\mu \hbox{m}$ was measured under a $2\pi$ field of view (FOV) with a calibrated blackbody source at 800 K. Fig. 6 shows the measured responsivity for MWIR and LWIR absorbers.

Fig. 6. Responsivity of pBp dual-band detector measured as function of applied bias.

One of the concerns in multiband detectors is the spectral crosstalk. Ideally, the MWIR absorber should only respond for wavelengths below 7.8 $\mu\hbox{m}$, and the LWIR absorber should respond to wavelengths between 7.8 $\mu\hbox{m}$ and 12 $\mu\hbox{m}$. However, the InAs/GaSb superlattice detectors are broadband detectors and are susceptible to spectral crosstalk; in other words, the LWIR absorber will detect all the radiation below 12 $\mu\hbox{m}$. We measured black body broadband responsivity and QE for the presented detector structure. In order to define the responsivity and QE at particular wavelengths, additional measurements need to be performed with set of narrow band filters. These measurements are currently being undertaken.

The shot noise limited $D^{\ast}$ was evaluated at 5 $\mu\hbox{m}$ and 9 $\mu \hbox{m}$ for MWIR and LWIR absorbers, respectively, using the equation TeX Source $$D^{\ast} = {R \over \sqrt{{4k_{B}T \over R_{d}A_{d}} + 2qJ}}\eqno{\hbox{(1)}}$$ where $R$ is responsivity, $k_{B}$ is the Boltzman constant, $T$ is temperature, $R_{d}$ is dynamic resistance, and $A_{d}$ is diode area, $q$ is the electronic charge, and $J$ is the dark current density.

Values of specific detectivity at different temperatures are shown in Fig. 7. At 77 K, the peak $D^{\ast}$ has reached $5 \times 10^{11}$ Jones $(V_{b} = +0.1\ \hbox{V}, \lambda = 5\ \mu\hbox{m})$ and $2.6 \times 10^{10}$ Jones $(V_{b} = -0.4\ \hbox{V}, \lambda = 9\ \mu \hbox{m})$. The corresponding values of responsivity and quantum efficiency were 1.6 A/W and 39% (MWIR) and 1.3 A/W and 17% (LWIR).

Fig. 7. Specific detectivity of pBp dual-band detector measured as function of applied bias.

It should be noted that the detector is operated under small values of applied bias compared with nBn dual-band SLS detector [9], thus illustrating the advantages of the pBp design. Performance comparison of dual-band single element detectors based on InAs/GaSb SLS with pBp design, QWIP, and MCT is shown in Table 1. It should be noted that all the parameters in Table 1 are stated at $\lambda_{50\%\ {\rm cutoff}}$. The dual-band InAs/GaSb SLS detector with pBp design showed superior performance to the QWIP detectors both in MWIR and LWIR bands and comparable performance with MCT detectors in the MWIR band. Better performance of a dual-band HgCdTe detector in the LWIR band is attributed to the considerably thicker (10 $\mu\hbox{m}$) absorbing layer of MCT detector.

TABLE 1 Performance comparison of dual-band single element detectors based on InAs/GaSb SLS with pBp architecture, QWIP, and MCT
SECTION IV

## CONCLUSION

In conclusion, we have designed and demonstrated dual-band response from InAs/GaSb SLS detectors with a pBp architecture. Diffusion-limited behavior of dark current at higher temperatures was observed for MWIR absorber. At 77 K, the peak $D^{\ast}$ has reached $5 \times 10^{11}$ Jones $(V_{b} = +0.1\ \hbox{V}, \lambda = 5\ \mu\hbox{m})$ and $2.6 \times 10^{10}$ Jones $(V_{b} = -0.4\ \hbox{V}, \lambda = 9\ \mu \hbox{m})$. The corresponding values of responsivity and quantum efficiency were 1.6 A/W and 39% (MWIR) and 1.3 A/W and 17% (LWIR).

### ACKNOWLEDGMENT

The authors would like to thank the user service center at the University of New Mexico for sample fabrication and testing.

## Footnotes

This work was supported by the Navy SBIR Phase I Contract N68936-10-C-0080. Corresponding author: E. A. Plis (e-mail: elena.plis@gmail.com).

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