Mid-wavelength infrared (MWIR) photodetectors play a crucial role in a wide range of applications, including gas detection, imaging, astronomy, science, and environmental monitoring^[1–4]. However, the operating temperature is limited by significantly increasing dark current as the operating temperature rises, which is associated with the nature of the narrow bandgap the materials used. The dark current, typically due to the thermal excitation process, will rise exponentially as the temperature increases.

Over the past decades, many material systems have been exploited to work on MWIR spectra. Bulk narrowband gap InSb-based MWIR photodetectors are currently commercially mature, representing a dark current density of 1×10−9A/cm2 under −50mV bias at 77 K^[5]. Another commercially matured material option is HgCdTe (MCT). Simulation models were established^[6,7] and experimental nBn type MCT devices have subsequently been realized with a saturation dark current about 0.54A/cm2 under −0.8V at 180 K^[8]. Moreover, narrow bandgap material InAs is also a candidate for MWIR detection. At room temperature, a dark current density of 2.5×10−2A/cm2 under −30mV and a peak responsivity of 1.47 A/W were obtained from InAs-based devices^[9]. A type II superlattice (T2SL) material system based on 6.1 Å (1 Å = 0.1 nm) lattice constant family (i.e. InAs/GaSb/AlSb) has also been utilized on the MWIR spectra. Interband cascade MWIR photodetectors based on InAs/GaSb superlattices have demonstrated a dark current of 3.6×10−7A/cm2 and 7.3×10−3A/cm2 under −5mV bias at 77 and 295 K, respectively, with a responsivity of 0.167 A/W at 4 µm^[10].

However, there are several drawbacks for the material system above that limit the optoelectronic performance of the devices. Less flexibility of tuning the bandgap for a wide spectral response is the main issue for the bulk materials InAs or InSb, while the performance of HgCdTe-based MWIR photodetector devices is restricted by the nonuniform growth of large array and high cost CdZnTe substrate^[11]. The T2SL InAs/GaSb system is a good candidate for MWIR detection, but the InAs/GaSb interfaces bring a strong scattering effect that shortens the photon-generated carriers’ lifetime and increases dark current, resulting in a limitation on its operating temperature^[12]. Thus, T2SL Ga-free InAs/InAsSb has been investigated since 2012 due to the potential long carrier lifetime of 412 ns^[13]. Several attempts on improving the growth quality of InAs/InAsSb have been made to enhance the structural and optical properties of the InAs/InAsSb absorption layer^[14]. As for the optoelectronic performance of InAs/InAsSb-based MWIR photodetectors, researchers at Northwestern University have optimized InAs/InAsSb-based MWIR photodetectors with an AlAsSb barrier in both planar and mesa configurations. These detectors demonstrated a dark current density of 0.44A/cm2 under −50mV at 300 K and 2.5×10−2A/cm2 at 150 K^[15,16]. Jiang et al. demonstrated an InAs/InAsSb-based photodetector with a cutoff wavelength of 3.5 µm, presenting a peak responsivity of 0.56 A/W under a reverse bias voltage of −130mV^[17]. Huang et al. proposed an MWIR photodetector using an InAs/InAsSb absorption layer and obtained a peak responsivity of 2.16 A/W at a temperature of 160 K^[18]. Also, high-speed MWIR photodetectors exploiting higher p-doped InAs/InAsSb absorbers have been reported by Huang et al. that provide a large 3-dB bandwidth^[19].

In this paper, we reported photodetectors with reduced dark current at higher temperatures using nBn type T2SL InAs/InAsSb MWIR photodetectors. This was achieved through the use of a lattice-matched AlAsSb layer as the electron barrier, which provides a large conduction band offset (CBO) between the InAs/InAsSb absorption layer and the barrier layer, employing an Sb soaking technique to improve the T2SL interfaces, which lowers the Shockley–Read–Hall (SRH)-related recombination current, and using a SiO2 layer for passivation to suppress the surface leakage current that is related to dangling bonds.

To target MWIR photodetection, we employed a strain-balanced structure of (3n,n)−InAs/InAs0.6Sb0.4 T2SL (n is the thickness of InAsSb, while 3n is the thickness of InAs)^[20]. The structure was grown on an n-GaSb substrate by molecular beam epitaxy (MBE). After the deposition of a 200 nm n-GaSb buffer layer, a 200 nm thick heavily n-doped InAs/InAsSb bottom contact layer was deposited, followed by a 1 µm thick unintentionally doped InAs/InAsSb T2SL layer. Then an undoped 10 nm-AlAsSb lattice matched to GaSb was grown, followed by a top contact layer of heavily n-doped InAs/InAsSb T2SL with narrower period thickness, which has wider bandgap energy acting as a window layer as well. The growth condition was set at a pre-optimized temperature of 400°C–430°C with a growth rate of 1 µm/h and a beam equivalent pressure (BEP) of 3×10−6mbar for Sb. An Sb soaking technique that we developed previously was applied to improve the interfaces of the T2SL^[21]. After the growth of InAs, both In and As shutters closed for 20 s to purge residual As; then 3 s later, Sb was deposited and followed by immediate growth of InAsSb. The mesa was dry-etched by an inductive coupled plasma (ICP) instrument, and then a 400 nm thick SiO2 passivation layer was deposited over the sidewall by plasma-enhanced chemical vapor deposition (PECVD).

Figure 1 shows the high-resolution X-ray diffraction (HRXRD) pattern, which indicates the crystalline quality of the epilayer materials. The narrow superlattice 0th peak with a full width at half-maximum (FWHM) of 62 arcsec indicates an extremely high crystalline quality of InAs/InAsSb superlattice and abrupt interfaces.

Furthermore, the small angular separation between the superlattice 0th peak and the GaSb substrate reflects the well strain-balanced InAs/InAsSb superlattice with respect to the GaSb substrate. As shown in Fig. 1, this angular separation value is found to be about 126 arcsec, which leads to a lattice mismatch of 0.10% according to Eq. (1)^[22]. This indicates the InAs/InAsSb absorption layer is closely lattice matched to GaSb, Δa/a=(sin(θsubstrate)/sin(θsubstrate+Δθ))−1,(1)where Δθ stands for the angular separation between the epilayer and the substrate.

The simulation result reveals the T2SL has a structure of 48Å−InAs/11Å−InAs0.62Sb0.38.

Figure 2(a) shows the simulation results of the 48Å−InAs/11Å−InAs0.62Sb0.38 T2SL band diagram using SILVACO TCAD 2022 software. It reveals a cutoff wavelength at about 5.9 µm at 300 K, which covers the important atmospheric window of 3–5 µm and a spectral range that gives strong absorption to most molecules.

The schematic configuration of the fabricated mesa type nBn InAs/InAsSb device is shown in Fig. 2(b). The scanning electron microscope (SEM) image of the passivated mesa sidewall and the microscope image of the device are shown in Figs. 2(c) and 2(d), respectively.

Temperature-dependent dark current characteristics and the differential resistance area product (RA) of a 200 µm diameter InAs/InAsSb nBn device are shown in Figs. 3(a) and 3(b). At 97 K, the device shows a dark current density of 6.28×10−6A/cm2 and an RA of 3.20×104Ω·cm2 under a bias of −600mV. At 297 K, under −600mV, the dark current density increased to 0.31A/cm2 and a reduced RA of 1.32Ω·cm2 was obtained under similar bias. The inset of Fig. 3(a) represents the dark current density under −600mV as a function of perimeter to area ratio (P/A) according to Ref. [23]. The dark current density shows an ultraweak dependence on the detector size, leading to an ultralow extracted surface current density. This is associated with the precisely controlled ICP etching process and the high quality of the densely deposited SiO2 passivation layer over the mesa sidewall. Figure 3(c) shows the Arrhenius plot of the photodetector under a reverse bias of −600mV. The extracted activation energy E_a of 239 meV can be concluded in the temperature range of 157 to 297 K. This is close to the energy bandgap of the designed InAs/InAsSb superlattice structure, which reveals that diffusion current dominates the whole dark current in the temperature range. At lower operating temperatures, e.g. from 157 to 97 K, E_a reduces to 53 meV, indicating a tunneling dark current component dominates in this temperature range. This suggests that the generation-recombination (G-R) current is eliminated due to the nBn structure.

Table 1 summarizes the dark current characteristics of InAs/InAsSb MWIR devices from several research works; our InAs/InAsSb nBn MWIR photodetectors demonstrate an ultralow dark current density compared to that of other groups.

Figure 4 shows the spectral photoresponse under different reverse biases at 237 K. A cutoff wavelength of 5.5 µm that is consistent with the bandgap simulation was obtained for this nBn InAs/InAsSb device. With the increasing reverse bias, the photoresponse increases simultaneously and nearly saturates at a reverse bias voltage of around −600mV. A peak responsivity of 0.39 A/W was obtained at 237 K.

Figure 5 shows the calculated detectivity of this nBn InAs/InAsSb photodetector device under reverse bias voltage of −600mV at an operating temperature of 97 and 237 K, respectively. At 97 K, the peak detectivity is about 1.2×1010cm·Hz1/2/W, while peak detectivity decreases to 2.1×109cm·Hz1/2/W as the temperature rises up to 237 K.

In summary, we demonstrated nBn T2SL InAs/InAsSb MWIR photodetectors containing an AlAsSb electron barrier with several attempts to suppress dark current density and increase operating temperature. An ultralow dark current density of 6.28×10−6A/cm2 and 0.31A/cm2 was obtained under reverse bias −600mV at 97 and 297 K, respectively. This dark current density level is lower than that of the reported InAs/InAsSb MWIR photodetectors, to the best of our knowledge. The device under study showed a cutoff wavelength of 5.5 µm. Also, peak responsivity of 0.39 A/W and peak detectivity of 2.1×109cm·Hz1/2/W were obtained of this nBn InAs/InAsSb device at a high operating temperature of up to 237 K.