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

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Figure 8. Cross-section schematic of a InAs/GaInSb superlattice for the 8-12 µm spectral range passivated by MBE overgrowth with lattice matched Al0.5Ga0.5As0.04Sb0.96. A full wafer process using standard optical lithography with a mask set containing various test diodes of different size and geometry was used to process the wafer. A cross section scheme of a completely processed mesa detector is presented in Figure 8. The processing sequence starts with the mesa definition which was done wet-chemically using citric acid. In order to ensure homogeneous wet etching and overgrowth of both sidewalls of rectangular shaped mesas, the mesa mask was aligned at 45 degree to the natural cleavage planes of the crystal. In this way two slightly positive facets for both the (100) and (010) oriented mesa sidewalls were achieved. After mesa etching, great care was taken to remove the resist completely. The sample was then re-inserted into the MBE chamber and the residual surface oxide was thermally desorbed. Subsequently, epitaxial overgrowth of the mesa-structured sample with a 150 m thick Al0.5Ga0.5As0.04Sb0.96 passivation layer was performed. During oxide desorption and overgrowth group-V stabilization of the semiconductor surface was ensured by a sufficiently high Sb-flux. Figure 9. (a) SEM image of a 256x256 InAs/GaInSb superlattice FPA with 40 µm pitch for 8-12 µm directly after overgrowth with lattice-matched Al0.5Ga0.5As0.04Sb0.96. (b) Edge of such a FPA pixel at higher magnification. The smooth surface morphology without polycrystalline phases is evident.
Figure 9a shows a SEM image of mesas with 40 µm pitch in a 256x256 FPA arrangement after epitaxial overgrowth with the 150 nm thick Al0.5Ga0.5As0.04Sb0.96 layer. In Figure 9b the corner of such an FPA pixel is shown at higher magnification. The excellent surface morphology without polycrystalline phases is evident. In order to prevent oxidation of the Al-containing passivation layer, a 200 nm thick silicon nitride layer was deposited after removing the sample from the MBE system. Since the Fermi level at the p-n junction on the diode sidewall is defined by the Al0.5Ga0.5As0.04Sb0.96 layer, the electrical properties of the photodiode are not affected by this dielectric layer. The silicon nitride was used as an etch mask for wet chemical etching of the Al0.5Ga0.5As0.04Sb0.96 passivation and the GaSbcap layer. The wet chemical etchant chosen for this process step offers a sufficiently high selectivity to the Incontaining SL. Finally, ohmic n- and p-contact metalization layers were deposited. Various test diodes were wire-bonded after processing and characterized by dark I-V measurements. Since surface leakage currents occur at the mesa sidewalls, dark current measurements in dependence of the perimeter-to-area (P/A) ratio were carried out to separate the surface leakage currents from the bulk contributions. A convenient measure for this investigation is the R0A product, since it is inversely proportional to the square of the limiting Johnson noise contribution [10]. (R 0 A) -1 (Ω -1 cm -2 ) 10 3 10 2 10 1 10 0 10 -1 10 -2 EDGE LENGTH OF QUADRATIC MESA (µm) 800 700 600 500 400 300200 200 100 Dielectric passivation Passivation by overgrowth λ c = 10 µm (a) 77 K (b) 77 K 0.02 0.04 0.06 PERIMETER-TO-AREA (µm -1 ) 70 µm edge length P/A = 0.057 µm -1 10 3 10 2 10 1 10 0 10 -1 0 2 4 6 8 10 12 10-2 DIODE # Figure 10. (a) Inverse R0A product vs. P/A ratio of InAs/GaInSb superlattice photodiodes with 10 µm cut-off at 77 K. (b) Histogram of the (R0A) -1 product of the diodes from (a) with 70 µm edge length (P/A = 0.057 µm -1 ). Figure 10a shows the (R0A) -1 in dependence of the P/A ratio on a logarithmic scale at 77 K. Since the P/A ratio depends on the size and geometry of the mesa, the edge length of a quadratic mesa is depicted on the upper scale for convenience. The (R0A) -1 data of the sample passivated by overgrowth is compared with a dielectrically passivated sample with 10 µm cut-off wavelength. R0A values obtained with a dielectric passivation are low and the inverse R0A increases linearly with increasing P/A ratio. Obviously the employed dielectric layer is not well suited for the passivation of FPA pixels with 40 µm diameter and less. In contrast, the R0A product obtained with the passivation by AlGaAsSb overgrowth is more than three orders of magnitude larger compared to the dielectric passivation. Furthermore, the best diodes align along the dashed line corresponding to R0A ≈ 10 Ωcm 2 which is the known bulk value of this material. The independency of (R0A) -1 on the (P/A) ratio indicates suppression of surface leakage currents (R 0 A) -1 (Ω -1 cm -2 )
Page 1 and 2: Copyright 2005 Society of Photo-Opt
Page 3 and 4: Figure 3. SEM image of a 256 x 256
Page 5: Figure 7. Thermal images of the fir

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

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