InAs/(GaIn)Sb short-period superlattices for focal plane arrays

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Figure 5. (a) Responsivity and corresponding quantum efficiency at 77 K. (b) Large area test diode with optical window in the top contact metalization for electro-optical characterization. a combination of mechanical lapping, wet-chemical polishing and wet-chemical etching to a thickness of about 20 µm. The detector hybrid is anti-reflection coated and finally integrated into a stand-alone camera system. The I-V characteristics of the diodes were measured on test diodes of the same size and geometry (37 x 37 µm 2 ) as the FPA diodes. Figure 4b shows the total dark current vs. bias voltage of a diode with a cut-off wavelength (5%) of 5.4 µm. At 0 V bias, the total dark current is below 1 pA at 77 K. The diodes are generation-recombination limited and show background-limited performance (BLIP). Figure 4a illustrates the R0A product of a set of 15 randomly selected diodes. The value of the R0A product equals 4x10 5 Ωcm 2 (a) at 77 K and shows a very homogeneous behavior apart from diode #15 which is presumably damaged by a bulk defect. At 67 K R0A beyond (b) Figure 6. Histogram of the NETD (a) and mapping of dead pixels (b) in a 256 x 256 InAs/GaSb shortperiod superlattice FPA integrated on IDCA. 1x10 6 Ωcm 2 have been measured. To determine responsivity and quantum efficiency electrooptical measurements have been performed under front side illumination on large area test diodes with an optical window in the top contact metalization (see Figure 5b). Due to the absence of an optical reflector in this configuration, the infrared radiation passes the diode only once. Caused by the mirror metalization on top of each pixel, responsivity and quantum efficiency are roughly doubled in FPAs which are illuminated from the backside in the camera system. The spectrally resolved current responsivity of such a test diode is plotted in Figure 5a for single optical path. A current responsivity of 0.7 A/W was measured at a wavelength of 5 µm, resulting in a quantum efficiency of η = 17 % for single optical path. Thus, the quantum efficiency is estimated to be around η = 30 % in FPA detector pixels under backside illumination. After flip-chip bonding and removal of the GaSb substrate, the detector chips were mounted in an integrated detector cooler assembly with a 1 W linear Stirling cooler. An important figure of merit is the noise-equivalent temperature difference (NETD) of the camera system. The histogram over 256 x 256 detector pixels at an integration time of 6.5 ms and with F/2 optics is
Figure 7. Thermal images of the first fully integrated 256 x 256 Mid-IR superlattice camera with 9.4 mK NETD. plotted in Figure 6a. Excellent spatial uniformity is obtained over the array, resulting in a median NETD value of 9.4 mK at a detector temperature of 73 K. For an integration time of 1.25 ms a NETD value of 25 mK has been measured. The very good homogeneity of the NETD across the entire 256 x 256 array is illustrated in Figure 6b which shows a mapping of defective pixels. In total, pixel outages are well below 1% and appear statistically distributed, predominantly as single pixel faults without large clusters. The excellent image quality delivered from the InAs/GaSb SL mid-IR camera system is shown in Figure 7. The fine structure of the clothing proves the good spatial resolution and the low cross talk between neighboring detector elements in the FPA. With this camera demonstrator, it is obvious, that InAs/GaSb SLs are in the position to provide low NETD values at short integration times comparable with HgCdTe detectors. The excellent performance of the camera system demonstrates the high potential of InAs/GaSb SLs for the fabrication of future IR imaging systems. 3. InAs/GaInSb SHORT-PERIOD SUPERLATTICES FOR 8-10 µm The main technological challenge for the fabrication of large format FPAs with pixel sizes of 40 µm and less is the occurrence of surface leakage currents. Since surface leakage currents in low band gap devices are mostly due to tunneling of electrons, these currents increase exponentially when the band gap of the device shifts to longer wavelengths. Besides efficient suppression of surface leakage currents, a passivation layer suitable for production purposes must withstand various treatments occurring during the subsequent processing and integration up to the camera level. Therefore, ammonium-sulfide treatments, which have been investigated by several groups [7,8], seem difficult to be incorporated in a production process. Up to now the reproducibility and long-term stability achieved by the deposition of a dielectric passivation layer is not sufficient for photodiodes in the 8 – 12 µm range. Therefore, we investigated the epitaxial overgrowth of InAs/(GaIn)Sb short-period superlattice mesa devices with lattice matched AlxGa1-xAsySb1-y by MBE [9]. The approach is demonstrated using an InAs/(Ga0.75In0.25)Sb short period superlattice p-i-n photodiode with a cut-off wavelength around 10 µm at 77 K grown by MBE on undoped 2” (001)-GaSb substrate. MBE growth started with a 500 nm thick Al0.5Ga0.5As0.04Sb0.96 lattice matched buffer layer followed by a p-type contact layer comprised of 700 nm GaSb:Be (3x10 18 cm -3 ). The active region contains 190 periods of an 8.6 ML InAs / 5.0 ML (Ga0.75In0.25)Sb superlattice structure. The active region is terminated by a 30 nm GaSb cap layer. The lower 90 periods of the SL were p-doped with 1x10 17 cm -3 Be in the Ga0.75In0.25Sb layers. The next 40 SL-periods were not intentionally doped followed by 60 periods with an n-doping of 1x10 17 cm -3 Si in the InAs layers.
Page 1 and 2: Copyright 2005 Society of Photo-Opt
Page 3: Figure 3. SEM image of a 256 x 256
Page 7 and 8: Figure 9a shows a SEM image of mesa

InAs/(GaIn)Sb short-period superlattices for focal plane arrays

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