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Abstract
In this paper, we demonstrate a bias-selective short/mid wave dual-band infrared detector based on GaSb bulk materials and InAs/GaSb type-II superlattices with 50% cutoff wavelengths of 1.55 μm and 4.62 μm, respectively. At 77 K, the short wave channel exhibited a peak quantum efficiency of 35.86% at 1.43μm and a dark current density of 8.41 × 10−5A/cm2 under the forward 5.0 V bias, thereby providing a Johnson-noise-limited detectivity of 7.63 × 1011cm · Hz1/2/W. The mid-wave channel showed a quantum efficiency of 10.45% at 4.0μm and dark current density of 4.17 × 10−3A/cm2 under -1.35 V bias, resulting in a detectivity of4.05 × 1010cm · Hz1/2/W. The cross-talk was very low in the short wave channel, but existed in the mid wave channel originated from the contribution of the residual built-in electric field in the short wave channel. Furthermore, the schematic band alignment of N-I-P-P-I-N back-to-back structure was also discussed for further optimization.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to the advantages of recognition capacity, strong resolution and anti-interference, dual-band and three-band infrared detector has always been a hot spot in the infrared detection and imaging [1–3]. Short/mid wave (SW/MW) dual-band infrared detectors can be applied in the field of missile warning, remote sensing, meteorology, airborne infrared and reconnaissance system. Some special systems require simultaneously to realize the high-contrast SWIR active image and the MWIR passive image, the integrated mid/short wave dual-band infrared detectors are optimal to reduce the volume and the system cost [4,5]. HgCdTe, quantum well and type-II superlattices (SLs) materials are usually used to fabricate the monolithic dual-band detectors. HgCdTe can tailor the cutoff wavelength from SWIR to LWIR, but suffering from the toxicity, high cost and complicated fabrication [6,7]. Quantum well has low quantum efficiency and large dark current [8]. Type-II InAs/GaSb SLs and InAs/GaSb/AlSb SLs were greatly concerned due to its tunable bandgap, large electron effective mass and high quantum efficiency [9–13]. SWIR detectors mainly include Ge and InGaAs. These materials are not suitable to fabricate monolithic dual-band IR detectors due to the large lattice mismatch with the dual-band detectors substrates, which resulted in the complex growth process thus the low repeatability and high cost [14,15] .InAs/GaSb/AlSb SLs and InAs/InAs1-xSbx/AlAs1-xSbx SLs can also be used as short wave infrared absorption materials,and have high performance, but the epitaxy growth is complicated [16,17]. Fortunately, GaSb material, as a optimal substrates for Sb-based mid wave IR detectors, is suitable to SWIR detector due to its bandgap of 0.75eV. Furthermore, the growth of GaSb has a wide growth temperature range without the composition adjustment [18,19].
In this paper, we combined GaSb bulk material and InAs/GaSb superlattices to fabricate a bias-selective short/mid wave dual-band IR detectors. The back-to-back nip-pin diodes were grown on n-GaSb (001) substrates by molecule beam epitaxy, in which the SWIR and MWIR channels can be selected by altering the polarity of the applied bias. The crystallization and the morphology of the dual-band IR materials were studied. The monolithic dual-band IR detectors with SiO2 passivation and the anodic sulfurization were fabricated. The spectra responsivity under different bias, the dark current, detectivity and band alignment were investigated.
2. Experimental methods
2.1. Material growth and detectors fabrication
The dual-band infrared materials were grown on an n-type GaSb (001) substrates using Veeco Epi Gen II molecular beam epitaxy system which equipped Sb2 and As2 valved cracker sources and Ga/In SUMO cells. The front-illuminated detectors are designed to n-i-p-p-i-n structure composed by two back-to-back p-i-n junctions as shown in Fig. 1(a), which makes each channel independent of each other without additional middle contact. The SWIR channel is GaSb bulk material. The MWIR channel is InAs/GaSb (8ML/8ML) superlattices (SLs) with InSb interface. A 0.8µm thick Te doped n-type ($n \approx 1.54 \times {10^{18}}c{m^{ - 3}}$) GaSb buffer layer was grown on the GaSb substrates to flatten the surface and facilitate subsequent layer growth. The MWIR channel was firstly grown to avoid the absorption of SWIR signals under front-side illumination conditions. The MWIR channel consists of an 0.5µm Si doped N-type ($n \approx 2.8 \times {10^{18}}c{m^{ - 3}}$) SLs contact layer, an 1.5µm undoped SLs absorption layer, and a 0.5µm Be doped p-type ($p \approx 2.15 \times {10^{18}}c{m^{ - 3}}$) SLs contact layer. The SWIR channel was composed of a 0.5µm Be doped p-type ($p \approx 2.15 \times {10^{18}}c{m^{ - 3}}$) GaSb contact layer, a 1.5µm undoped GaSb absorption layer, and a 0.5µm Te doped n-type ($n \approx 1.54 \times {10^{18}}c{m^{ - 3}}$) GaSb contact layer. In this structure the thick and heavy doping contact layer were adopted to reduce optical crosstalk and dark current.
The front incidence detectors with 500 µm diameter were fabricated by mesa etching using inductively coupled plasma (ICP). The etch depth was 5.012µm matching well with the design value 5µm and the dip angle of mesa was 80° in the cross-section SEM photo as shown in Fig. 1(b). It is not a smooth etching side especially at the bottom caused by the ICP etching damage. The hanging bonds on the side wall of the mesa were filled by the anodic sulfurization in the sulfide liquid of 24g $N{a_2}S \cdot 9{H_2}O$ and 500ml glycol under the ultrasonic stirring [20]. Then the SiO2 films were deposited for physical passivation by PECVD. The contact electrodes of Ti (50nm) / Pt (50nm) / Au (300nm) were prepared by the electron beam evaporation. Then the device was wire bonded within a chip fixed in a Dewar cooled by liquid nitrogen. We define the forward bias when the top contact electrode connecting the positive while the bottom contact electrode negative. Otherwise, it is defined as reverse bias.
2.2 Characterization of the materials and detectors
The microstructure of the GaSb bulk and InAs/GaSb superlattice was studied by the high-resolution X-ray diffraction (HRXRD). The surface morphology was investigated by atom force microscope (AFM). Normalized spectral response was performed at 77 K using a Bruker Vertex70 Fourier transform infrared (FTIR) spectrometer calibrated using a standard DTGS detector. The current–voltage characterization was measured using Agilent B1500 in the bias range from -2.0V to +6.0V at 77K.
3. Results and discussion
3.1 Microstructure and surface morphology of detectors material
The high-resolution X-ray diffraction (HRXRD) pattern of the materials is shown in Fig. 2(a). The periodic thickness of InAs/GaSb SLs can be calculated to $48.8769{{\AA}}$ from the distance of satellite peak of SLs, which is matched with the theoretical value $48.6148{{\AA}}$. The full width at half maxim (FWHM) of the zero-order satellite peak of SLs is 17.57 arcsec, which is better to the best level 36 arcsec [21]. The zero-order satellite peak of SLs and GaSb substrate peak are 109 arcsec apart, which shows the lattice mismatch is only -0.0905%. The atomic steps can be clearly seen in the AFM photo shown in Fig. 2(b). The RMS roughness is only $1.82{{\AA}}$ over $10{\mathrm{\mu}} \textrm{m} \times 10{\mathrm{\mu}} \textrm{m}$ area.
3.2 Photo-electronic performance of the detectors
The normalized spectral response under different bias before and after the calibration is shown in Figs. 3(a) and 3(b). Under the forward bias the detectors response obviously at SWIR range but no response at MWIR range because the internal electric field only locates at SWIR region and SWIR channel dominates the photo-generated current. Under the reverse bias MWIR channel dominates. The 50% cutoff wavelength of SWIR and MWIR channel is about 1.55µm and 4.62 µm respectively. SWIR responsivity increases with the forward bias. MWIR responsivity also enhances with bias and saturated at -1.35V. In MWIR channel there is an obvious peak at 1.6 µm shown in Fig. 3(b), which increases firstly from 0 to -1.0V then decreases gradually from -1.0V to -1.35V. It could be originated from the built-in electric field appeared at the interface between n-type GaSb buffer and p-type SL. The 1.0V reverse bias corresponds to the built-in potential difference.
Figure 4. shows the dark current density (Jd) and the differential resistance area product (RA) under the bias from -2.0V to +6.0V at 77K. The current-voltage characteristic indicates that the generated current in the depletion zone of both channels increases with the bias. For the SWIR channel the Jd and RA are $8.41 \times {10^{ - 5}}\textrm{A}/c{m^2}$ and $1.39 \times {10^4}\Omega \cdot \textrm{c}{\textrm{m}^2}$ at +5.0V. For the MWIR channel, they are $4.17 \times {10^{ - 3}}A/c{m^2}$ and $3.5 \times {10^1}\Omega \cdot \textrm{c}{\textrm{m}^2}$ at -1.35V bias. The zero-bias resistance area product R0A is $9.17 \times {10^6}\Omega \cdot \textrm{c}{\textrm{m}^2}$, corresponding to the dark current density $2.89 \times {10^{ - 7}}A/c{m^2}$. The RA value changes little with the bias from reverse to forward which indicates that the bias-related tunneling current plays a leading role in the dark current.
Fig. 4. Dark current density and differential resistance-area product vs. applied bias of detector at 77 K
Figures 5(a) and 5(b) show the spectral responsivity and quantum efficiency of the detector at 77K using a 500°C blackbody source without any anti-reflection coating. The SWIR channel exhibits a peak responsivity (Ri) of 0.415 A/W at 1.44µm under 5.0 V bias, while the MWIR channel shows 0.435 A/W at 2.70µm and 0.337 A/W at 4.0µm under -1.35V bias. Accordingly the SWIR channel exhibits a saturated QE of 35.86% at 1.43 µm, and the MWIR channel exhibits a peak QE 25.47% at 1.93 µm and 10.45% at 4.0 µm. In order to quantify the optical cross-talk, a selectivity parameter (S) was defined as follows: [22]
Fig. 5. Blackbody spectral responsivity (a) and quantum efficiency (b) of the dual-band detectors at 77K
The Johnson-noise-limited detectivity (D*) for both channels at 77 K can be calculated through the responsivity and RA product as shown in Fig. 6. The maximum D* for the SWIR channel is $7.63 \times {10^{11}}cm \cdot H{z^{1/2}}/W$ at 1.43µm under 5.0V. For MWIR channel the peak D* is $4.05 \times {10^{10}}cm \cdot H{z^{1/2}}/W$ at 2.6µm, and exceeds $3.07 \times {10^{10}}cm \cdot H{z^{1/2}}/W$ at 4.0µm under -1.35V.
3.3 Band alignment of detectors
In order to investigate the response mechanism of the dual-band bias-selective detectors, the schematic band alignment at zero bias, forward bias and reverse bias was simulated by next nanomat software as shown in Fig. 7. It is notable that the dependence of dual-band response on the bias is not from the band misalignment between SWIR and MWIR channel, but from the competition of the built-in electric field (Ebi) in the two back-to-back p-i-n junctions under reverse bias. The photo current comes from the separation and collection of photo-generated carriers in the intrinsic zone by Ebi. At zero bias in Fig. 6(a), both SWIR and MWIR channels have their respective Ebi which forms the barrier to each other and results in no response. At forward bias in Fig. 6(b), the Ebi in SWIR Channel enhances while MWIR channel is close to the flat band, so the photo-generated carriers in SWIR can be collected smoothly. With the forward bias increasing, the built-in potential difference (Vbi) is proportionally increasing thus the photo current and the responsivity improve continuously. At reverse bias in Fig. 6(c), the opposite scenario occurs that Ebi in MWIR channel enhance. It is also found the tiny barrier in the SWIR channel can affect the minor carrier. In addition the Vbi increases slowly with the bias, which is the reason that the responsivity saturates at -1.35V shown in Fig. 5(a).
Fig. 7. (a) schematic band alignment of the device at zero bias, conduction band alignment of the device at (b) forward bias and (c) reverse bias.
4. Conclusion
In this paper, a short-/mid-wave dual-band detector based on GaSb bulk material and InAs/GaSb superlattices with N-I-P-P-I-N structure was successfully fabricated and characterized. The 50% cutoff wavelengths are 1.55 µm and 4.62 µm for the SWIR channel and MWIR channel, respectively. The corresponding responsivity of the SWIR and MWIR channels is 0.415 A/W at 1.44 µm and 0.337 A/W at 4.0 µm. And the QEs of the SWIR and MWIR channels are 35.86% at 1.43 µm under 5.0V and 10.45% at 4.0 µm under -1.35V, respectively. At 77K, the D* for SWIR channel is $7.63 \times {10^{11}}cm \cdot H{z^{1/2}}/W$ at 1.43 µm under 5.0V, while it is $3.07 \times {10^{10}}cm \cdot H{z^{1/2}}/W$ at 4.0µm under -1.35V for the MWIR channel. The performance of the dual-band detectors is far from the expectations, some works such as reducing the bias, balancing the optical absorption in both channel and inserting the proper barrier layer should to be further optimized.
Funding
Doctoral Program Foundation of Institutions of Higher Education of China (20105303120002); Key Technologies Research and Development Program (2018YFA0209101); National Natural Science Foundation of China (11474248, 61534008, 61774130).
Disclosures
The authors declare no conflicts of interest.
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