nBn based infrared detectors using type-II InAs/(In,Ga)Sb superlattices

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Normal incidence single pixel photodiodes were fabricated using standard lithography with apertures ranging from 25-300 µm in diameter. Processing was initiated with the formation of ohmic contacts on the n-type top contact layer followed by dry etching of the device to the top of the barrier (etch depth 100 nm) for the mesa definition. Then the wafer was patterned with the photoresist, and a deep dry etch (etch depth ~2 µm) to the middle of the bottom n-type contact layer was performed. Finally, an ohmic contact was evaporated on the bottom contact layer. Ti (500 Å) / Pt (500 Å) /Au (3000 Å) was used as n-contact metal for both top and bottom contacts. Devices were then wire bonded to a leadless chip carrier for further characterization. Spectral measurements were performed using a FTIR spectrometer and a Keithley 428 preamplifier. Figure 9 shows the normalized spectral response (obtained by dividing the photocurrent of the SLS detector with that obtained using a pyroelectric detector) for a 300 µm diameter device for two different temperatures at the different polarities of applied bias. For the nBn detector structure, forward bias is defined as a negative voltage applied to the top contact of the detector. As depicted in Figure 9, the nBn LWIR detector structure demonstrated two color response (λ c1 ~ 3.5 µm and λ c2 ~ 8.0 µm) under different polarity of applied bias. Schematic band diagrams for the nBn LWIR detector under forward (negative voltage on the top) and reverse (positive voltage on the top) biases are shown in Figure 9(b) and Figure 9(c), respectively. Under forward bias, the photo carriers are collected from the absorber. When the device is under reverse bias, the photo carriers from the heavily doped (n-type) contact layer are collected, while those from the absorber are blocked by the barrier. Heavily doped InAs in the top contact layer results in a larger optical bandgap due to the Moss-Burstein effect and is the source of the MWIR signal. Thus, the two color response come from the detector absorber (LWIR signal) and top contact layer (MWIR signal). 4. NBN INAS/GASB SLS DUAL COLOR (MWIR/LWIR) DETECTORS On the basis of the results obtained from the MWIR and LWIR nBn detectors, we designed a two color nBn structure 10 . The heterostructure schematic of such structure is presented in Figure 10 (a). The growth procedure started with the deposition of a 480 nm bottom contact layer formed by 8 ML InAs:GaTe (n=4 x 10 18 cm −3 )/8 ML GaSb SLS. Then a 1.8 µm thick LWIR absorber consisting of unintentionally doped 9 ML InAs/5 ML In 0.25 Ga 0.75 Sb SLS was grown followed by a 1.5 µm thick MWIR absorber composed of 8 ML InAs/8 ML GaSb SLS. The thicknesses of the LWIR and MWIR absorbers were designed to be approximately the same. A 100 nm Al0.2Ga0.8Sb barrier separated the two absorbers. The structure was capped with a ~ 0.1 µm top contact layer with the same composition and doping level as the bottom contact layer. SL (n) 97 nm MWIR Contact SL nid 1.5 µm MWIR Absorber Al 0.2 GaSb 100 nm Barrier SL nid 1.8 µm LWIR Absorber SL (n) 480 nm MWIR Contact GaSb:Te 2” Substrate Counts/s GaSb 10 4 SLMWIR SL LWIR 0 0 +1 LWIR 10 3 10 2 10 1 +1 MWIR -1 LWIR -2 LWIR -1 MWIR -2 MWIR +2 MWIR +2 LWIR AlGaSb Barrier 10 0 27 28 29 30 31 32 33 34 Ω/2θ ( ο ) (a) (b) Figure 10. (a) The heterostructure schematic and (b) (004) XRD scan of multispectral (LWIR/MWIR) nBn SLs detector Proc. of SPIE Vol. 6940 69400E-8
Symmetric (004) X-ray scan was performed on the multispectral nBn SLs sample and is presented in Figure 8 (b). The lattice mismatch of LWIR and MWIR SLs to the GaSb substrate is 0.13% (compressive strain) and –0.04% (tensile strain), respectively. The full width at half maximum (FWHM) of the 1 st satellite peak of the LWIR SL is equal to 25 arcsec (45 arcsec for the MWIR SLs). The periods of LWIR and MWIR SLs were measured from the fringe spacing of the SLs peaks and were found to be 42.2 Å and 48.2 Å for the LWIR and MWIR SLs, respectively. The detector material was processed to operate at normal incidence as single pixel photodiodes with apertures ranging from 25 to 300 µm. The processing procedure was similar to that described earlier for nBn LWIR single element detectors in Section 2. The relative spectral responses of the bicolor structure are shown in Fig. 11 (a). The data clearly show cutoff wavelengths λ c1 ~ 4.5 µm and λ c2 ~ 8 µm under different polarities of applied bias. When a forward bias is applied, the carriers from the LW absorber are collected, and when a reverse bias is applied, the carriers from the MW absorber are collected. It is to be noted that there will be barriers between unintentionally doped LWIR layer and the bottom n-contact that can impede the transport. An interesting feature in the detector’s responsivity vs. wavelength behavior was noticed while the spectral response of the multispectral nBn detector within the range of applied bias from 0 to + 0.1V was being investigated (Figure 11 (b)). The data clearly indicate that the response cutoff wavelength shifts from MWIR to LWIR when the applied bias voltage varies between 0 and +0.1 V. As the applied bias voltage increases from 0 to +0.1 V, the intensity of the MWIR response decreases whereas the intensity of LWIR response increases. Due to this bias dependent feature of the spectral response, a single detector can be operated at multiple biases sequentially, whereby the detector’s spectral response changes in accordance with the applied bias. Consequently, one of the advantages of nBn design is that it does not require multiple contacts per pixel, which reduces the cost and complexity associated with the fabrication process. Normalized spectral response (a.u.) 1.0 0.8 0.6 0.4 0.2 0.0 V b = + 0.5 V V b = -0.6 V 3 4 5 6 7 8 9 10 Wavelength (µm) 100 K 150 K Spectral response (a.u) 0.06 0.05 0.04 0.03 0.02 0.01 0.00 2 3 4 5 6 7 8 9 10 Wavelength(µm) 0 V + 0.01 V + 0.05 V + 0.09 V + 0.1 V Figure 11. (a) Normalized spectral response of the multispectral nBn detector at different temperatures and different polarity of applied bias (b) Spectral response of the multispectral nBn detector within a range of applied bias from 0 to +0.1V 5. SUMMARY AND CONCLUSION The development of nBn based detectors for MWIR and LWIR spectral regions using type-II InAs/(In,Ga)Sb SL has been presented. First, we demonstrated a 320 x 256 FPA with cutoff wavelength of 4.2 µm at 77K. The average value of the dark current density (1 x 10 -7 A/cm 2 at 77K (V b =0.7V) proved comparable to that reported for the state-ofthe-art MWIR SLs photodiodes utilizing some passivation schemes. At 77K, FPA reveals an average NEDT of 23.8 mK Proc. of SPIE Vol. 6940 69400E-9
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nBn based infrared detectors using type-II InAs/(In,Ga)Sb superlattices

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