Due to long Auger lifetime and other advantages, InAs/Gasb superlattice material system has become a candidate material for replacing HgCdTe (MCT).^[1,2] However, the detector fabricated by an InAs/Gasb superlattice material system has a relatively high dark-current. The high generation–recombination (GR) dark current is caused by a low Shockley–Rad–Hall (SRH) lifetime, which results from the original defects of the GaSb layer.^[3–5]

As a typical III–V ternary compound semiconductor material, the structure of InAsSb is stable. There is a stable chemical bond between indium (In) and arsenic (As) or antimony (Sb). Compared with MCT, the InAsSb has a high carrier mobility and relatively small dielectric constant and self-diffusion coefficient at room temperature. Therefore, the InAsSb has become an important candidate for fabricating midwavelength infrared (MWIR) detectors and optical gas sensors.

As figure 1(a) shows, the diffusion current at low temperature is several orders of magnitude lower than the generation–recombination current, while at high temperature it is several orders greater.^[6] Define T₀ as the “cross-over” temperature at which the diffusion and generation–recombination currents are equal then we will be able to use a function of the bandgap of the InAsSb absorbing layer to estimate the values of T₀.^[7] For an nBn detector, there exists a depletion region in the wide bandgap barrier, so the generation–recombination dark current is suppressed.^[7–9] Figure 1(a) shows a typical Arrhenius plot of the dark current in a pin detector. The slope of the lower line is about half that of the upper line. When multiplied by Boltzman’s constant, these two slopes essentially correspond to the activation energy for generation–recombination and diffusion limited behavior, respectively. The dotted line is an extension of the diffusion limited behavior to the case of low temperature. At temperatures below T₀, the nBn detector presents the following two important advantages.

Fig. 1.

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	Fig. 1. (a) Arrhenius plot of dark current in pin detector (solid line) and nBn detector (dotted line).[6] (b) Band profile of nBn detector with n-type barrier layer, at operating bias.

(i) The dark current density is lower than that of pin detector working at the same temperature.

(ii) The nBn detector can operate at a higher temperature than the pin detector under the same dark current.

They are indicated by solid and dotted arrows in Fig. 1, respectively. Figure 1(b) shows the band profile of an nBn detector with an n-type barrier layer. Further discussion of the principles of nBn detector can be found in other papers.^[10–12]

In this work, our samples were grown with Veeco Gen20 molecular beam epitaxy (MBE) system with group III cells and group V valved cracker cells in this experiment. The rate ratio of As to Sb was kept at 0.91:0.09 throughout the growth progress, and the growth rate of In was 0.5 ML/s, that was measured with reflection high energy electron diffraction (RHEED) prior to growth. We used flux monitor and GaSb (001) surface reconstruction to calibrate the V/III ratio and substrate temperature. For more imformation about growth progress and techniques readers may refer to other relevent literature.

As figure 2 shows, the nBn detcetor consists of a 0.5–μm-thick n-GaSb buffer layer, 1-μm-thick n⁺-InAs_0.91Sb_0.09 (5 × 10¹⁷ cm⁻³) bottom contact layer, 2.5-μm-thick n-InAs_0.91Sb_0.09 (1 × 10¹⁶ cm⁻³) absorption layer, 0.2-μm-thick n-doped AlAs_0.08Sb_0.92 barrier layer, and 0.25-μm-thick n⁺-InAsSb (5 × 10¹⁷ cm⁻³) top contact layer. The other sample, InAsSb pin detector, was grown for comparison. The pin structure was grown with GaSb buffer layer, 0.5-μm-thick p⁺-InAsSb (1 × 10¹⁸ cm⁻³), 2-μm-thick undoped InAsSb active layer, and 0.5-μm-thick n⁺-InAsSb (1.2 × 10¹⁸ cm⁻³) layer which is a top contact layer. The doping in nBn sample and that in pin sample were achieved by silicon (Si) and beryllium (Be) during growth, respectively. We used high resolution x-ray diffraction (HRXRD) and atomic force microscopy (AFM) to characterize the layer structure and material quality.

Fig. 2.

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	Fig. 2. Epitaxial structure of (a) nBn detector and (b) pin.

Figure 3 shows the HRXRD and AFM image of an nBn sample. The sample exhibits clear atomic step with a root mean square (RMS) surface roughness of 1.82 Å over an area of 10 μm × 10 μm. The HRXRD rocking curve shows that the splitting between InAsSb layer and GaSb layer is 64 arcsec and the full width at half maximum (FWHM) of InAsSb layers is ∼ 50 arcsec in many experiments. As shown by the surface morphology, AFM, and HRXRD data, the growth conditions of InAsSb layers are appropriate, which is the reason why we can achieve a thick enough InAsSb layer without dislocations or other growth defects introduced by lattice mismatch. The AlAsSb barrier layer can also be optimized, as InAsSb layer is optimized. The pin sample is also characterized by the AFM and HRXRD. The pin sample exhibits a clear atomic step of 1.99-Å RMS surface roughness over an area of 10 μm × 10 μm and a 40-arcsec splitting between InAsSb layers and GaSb substrate, but we do not show these data here. Figure 4 shows the splitting mapping over a whole 3″ wafer. Each colour in Fig. 4 corresponds to a splitting value and the splitting varies radially between the center and edge of the wafer, with a total splitting value of about 60 arcsec. We believe that these HRXRD results demonstrate a high uniformity for MBE growth. After characterization, we use a standard detector fabrication process, which includes 4 times lithography process and etching process by inductively coupled plasma (ICP).

Fig. 3.

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	Fig. 3. (a) AFM of an area of 10 μm × 10 μm for nBn sample; (b) HRXRD rocking curve for nBn sample.

Fig. 4.

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	Fig. 4. HRXRD mapping of nBn sample on 3″ wafer.

Figure 5 shows dark current density characteristics of nBn detector and pin detector at temperatures ranging from 150 K to 300 K. The dark current density of nBn detector is 8.95 × 10⁻⁶ A/cm² and 1.7 A/cm² under −0.2-V bias voltage at 150 K and 300 K, respectively. The constant slope in the figure indicates the absence of GR dark current. There is only slightly difference from the theory (T^s exp (−E/kT) (E is the activation energy), which results from other reason, such as surface leakage current.

Fig. 5.

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	Fig. 5. Arrhenius plot of dark current in pin detector (red dot) and nBn detector (blue dot) under −0.2 V at 300 K.

Figure 6 shows the curves of quantum efficiency (QE) versus λ of pin and nBn detectors at room temperature. We use a Fourier transform infrared spectrometer and a blackbody to measure the spectral response, and the quantum efficiency, respectively. As figure 6 shows, pin detector has a 100% cut-off wavelength of 4.8 μm and 50% cut-off wavelength of 4.4 μm. The QE of pin detector achieves its maximum of 41.4% at 4.1 μm. As figure 6 shows, the nBn detector has a 100% cut-off wavelength of 4.75 μm at 300 K. The QE of nBn detector reaches its peak of 30.1% at 4.05 μm under 0-V bias; when we apply a −0.2-V bias voltage to nBn detector, the QE maximum goes up to 63.4%. As bias increases, the QE values of nBn detectors increase.^[13] QE maximum value under −0.2 V reaches > 90% of that under −0.6 V in our nBn detector. The QE values of these detectors-based InAsSb are all higher than those of InAs/GaSb type-II supperlattice pin detectors. There is a little difference in cut-off wavelength between nBn detector and pin detector. It is believed that the cut-off wavelength difference results from little difference in ratio among V group elements caused by MBE growth and difference in doping.

Fig. 6.

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	Fig. 6. Curves of QE versus λ of pin and nBn detectors under different bias voltages at 300 K.

According to the measured spectral data, such as J and RA, we can calculate specific detectivity (D^*) values of pin and nBn detectors from the following equation:

Fig. 7.

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	Fig. 7. Curves of D* versus λ of pin and nBn detectors under different bias voltages at 300 K.

In summary, Arrhenius plot of J_dark–1/T shows that GR current is completely suppressed in InAsSb nBn detectors in a temperature range from 150 K to 300 K, which limits the performance of pin detector. The nBn detector achieves a QE of ∼ 63.4% under −0.2 V at 300 K, which is higher than the QE values of type-II InAs/Gasb superlattice MWIR detectors. There is still room for improvement, but these good experimental data show that the InAsSb nBn detector may be a candidate for MWIR detector.