Open Access
Add to BibSonomy
Abstract
GaSb-based infrared (IR) photodetectors are moving from a developmental phase into manufacturing, requiring among other things a shift to larger wafer diameters and volumes. We report on the multi-wafer molecular beam epitaxy (MBE) growth of mid-wave IR nBn photodetector structures on 5-inch GaSb and 6-inch GaAs substrates. The 5 × 5-inch and 4 × 6-inch multi-wafer configurations exhibited excellent cross-wafer uniformity of standard epiwafer characteristics, including morphology, and structural and optical properties. Large-area mesa diode characteristics from these epiwafers are comparable to those grown on smaller diameter substrates. The results represent an important technological path toward next-generation large-format IR detector array applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The GaSb-based 6.1 Å lattice constant family of materials and heterostructures provides rich bandgap engineering possibilities and have received considerable attention for their potential and demonstrated performance in infrared (IR) detection and imaging applications [1,2]. The type-II broken energy bandgap alignments of InAs/Ga(In)Sb-based strained-layer superlattices (SLS) can be tailored over a wide wavelength range (λ = 3–30 µm) [3] and advanced structure designs have demonstrated additional reduction of dark current, enhanced quantum efficiency and improved zero-bias dynamic resistance-area product (R0A) [4–7]. The unipolar barrier design, commonly noted as nBn or XBn [8–10], demonstrates great promise in high operating temperature (HOT) regimes (~150K). These barrier detectors may be implemented using a variety of architectures including the typical SLS alloy system above [8], bulk InAsSb and AlGaAsSb layers [9,10], or GaSb-free SLS with InAs/InAsSb where enhanced minority carrier lifetimes has been demonstrated [11].
The GaSb-photodetector technology must continue to develop and improve, stepping out of research laboratories working with 2- and 3-inch substrates and toward general III-V manufacturing processes based on large-format platforms in order to truly compete with HgCdTe in both performance and cost. As IR detector sizes continue to increase and wafer die yield requirements become ever more demanding, there is a strong push to explore the routine use of 5-inch and 6-inch diameter substrates that are prevalent in the wireless and photonics industries. Previously, we have demonstrated the growth of high quality 5-inch and 6-inch MWIR detector structures by MBE using a single-wafer configuration [12]. In this work, we report on the multi-wafer MBE growth of MWIR nBn detector structures using large-format 5 × 5-inch GaSb and 4 × 6-inch GaAs platforms. Material properties and large-area mesa diode characteristics from these epiwafers will be presented and results will be compared with those grown on smaller batch size MBE reactor platform.
2. Experimental setup
The generic bulk nBn detector structure design, with target cutoff wavelength (λc) of ~4 µm, consisted of a ~4 µm thick InAsSb absorber layer, a thin AlAsSb barrier, and a Si-doped InAsSb top contact layer [9]. As opposed to the more complicated SLS detector structure designs, the bulk nBn design is well suited to evaluate epitaxial growth capability because it requires few shutter actions, meaning surface morphology is more directly related to epilayer quality than to other mechanical growth issues such as shutter-related defects. The bulk, mixed group-V alloys are also very sensitive to substrate temperature uniformity and to group-V flux distribution uniformity, which are vital to a reliable growth process. This uniformity of growth conditions over a wide-area growth front is a critical challenge to manufacture Sb-detectors on large-diameter substrates, as nominally identical performance must be achieved across the entire wafer to enable fabrication of very large area next-generation FPAs. The bulk nBn structures described in this work were grown at IQE on an Oxford-VG V-150 solid source MBE tool using multi-5-inch and 6-inch platens. Group-III molecular beams (In, Ga, Al) were produced via conical effusion cells, and the group-V (As, Sb) flux was controlled by valved cracker sources. Details of the MBE growths of bulk nBn structures have been previously described [12,13]. Te-doped n-type GaSb(100) substrates used in this work were supplied by IQE-Wafer Technology and IQE-Galaxy with epi-ready surface finish.
To evaluate the quality of the detector epiwafers, and thus the suitability of the process on the large-format platform, various tests were performed to characterize surface morphology, structural properties, and optical quality across the whole wafer. Surface morphology is a critical first-pass evaluation of epiwafer quality and uniformity, as any abnormalities, roughness, or high-density defect patterns may result in fabrication problems and degraded device parameters. This is particularly important for the growth of metamorphic nBn structures where the large lattice mismatch between the GaAs substrate and the GaSb-based active detector layers can manifest into rough and highly-corrugated surface if the buffer layers are not fully optimized. Epiwafer surface morphology was evaluated using a Nomarski contrast optical microscope, a Veeco D500 Atomic Force Microscopy (AFM) tool, and an optical-based Tencor Surfscan 6220 defect mapping tool. Important epitaxial layer parameters and structural quality were measured using High Resolution X-ray Diffraction (HRXRD). Optical properties of the epitaxial photodetector material were assessed by low-temperature photoluminescence (LT-PL). Standard optical lithography and wet chemical etching processes were used to fabricate large-area mesa diodes, with test chips wire-bonded and packaged in chip carriers for dark current and spectral measurements. All diode testing were done at 150K with single-pass, front-side flood illumination without anti-reflection coating or passivation. Details of our LT-PL and large-area diode processing and testing setup have been reported elsewhere [14].
3. Experimental results
3.1 nBn MWIR detector structures grown on a 5 × 5-inch GaSb platform
To support the continual advancement of large-format FPA applications, it is important to evaluate the production readiness of large-diameter epiwafers. Here, we report on material and large-area device characterization of bulk InAsSb/AlAsSb nBn MWIR detector structures grown on GaSb substrates using a 5 × 5-inch production platform as depicted in Fig. 1(a).
Fig. 1 Surface morphology of an nBn MWIR detector structure grown on a 5 × 5-inch GaSb platform: (a) schematic of platen configuration with an outer ring of 5 × 5-inch wafers and a 3-inch witness wafer in the center, (b) full wafer Surfscan map showing low defect density (1.3−50 µm2) of 72 /cm2, (c) Nomarski contrast optical microscope images, and (d) AFM images showing smooth surface with low rms roughness of <3 Å (5 µm × 5 µm area scan).
All 5-inch epiwafers grown via this configuration exhibited excellent surface morphology, with low defect densities where the majority of defects observed were general random MBE defects (Fig. 1(b)). Nomarski images (Fig. 1(c)) revealed smooth, mirror like surface, and AFM images (Fig. 1(d)) indicated low rms roughness of 2.3 Å in the 5 µm × 5 µm area scan. These results compare favorably to similar structures grown on multi-4-inch configurations [13].
Long-range structural properties of the bulk nBn epiwafer layers were evaluated by HRXRD measurements. The InAsSb absorber (tensile strain) and the AlAsSb barrier (compressive strain) alloy compositions were intentionally skewed from perfect lattice-matching using opposite strain with respect to the GaSb substrate. These strain conditions provided clear, independent evaluation of each ternary alloy with minimal interference in the x-ray peaks. The cross-wafer uniformity was assessed by measuring five points from edge to edge. The two endpoints were taken very close to the edge of the wafer, typically within 5−10 mm (Fig. 2(a)). Figure 2(b) shows an overlay of the five x-ray spectra; the spectra have been aligned to the angular position of the substrate peak (sharpest peak in the center of the spectra), and the intensities were intentionally offset to enable clearer depiction of the trend across the wafer. The peak from the thick InAsSb absorber layer, just to the right of the substrate peak, has the same intensity as the substrate; the AlAsSb barrier peak to the left of the substrate is broader and less intense, as expected since the barrier layer was much thinner. The peak positions for each alloy vary by less than 150 ppm (<0.03 Sb mole fraction) across the wafer. The nearly-identical overlay of all the spectra demonstrates excellent across-wafer uniformity in alloy composition control and crystalline quality for the 5 × 5-inch GaSb growth platform.
Fig. 2 Multi-point HRXRD and PL measurements across an nBn MWIR detector structure grown on a 5 × 5-inch GaSb platform. (a) Schematic showing the orientation of the measurement points, (b) overlay of 5-point HRXRD spectra, the intensity of each scan has been intentionally offset to allow for a clearer depiction of the trend in the peaks across the wafer, and (c) 3-point 77K PL spectra across the same wafer.
Low-temperature photoluminescence (LT-PL) is another tool used to evaluate detector material quality and epiwafer uniformity. It is generally accepted by the Sb-based IR detector community that PL is a good method to screen epiwafers prior to FPA fabrication, due to the positive correlation of the PL parameters of wavelength and intensity with the detector parameters of cut-off wavelength and dark current [15]. Figure 2(c) shows the 3-point overlay of the 77K PL spectra measured across the same 5-inch bulk nBn wafer used in the HRXRD study. The PL peak was sharp with FWHM around 21 meV; the total variation in PL wavelength of <0.1 µm is comparable to similar structures grown on multi-4-inch configurations [13].
Beyond good epitaxial material quality, IR photodetector epiwafers must also demonstrate desirable device performance. Once the epiwafer material parameters were confirmed, wafer samples were processed and fabricated into large-area mesa diodes (500 × 500 µm2) for evaluation at typical operating temperatures of 140 and 150K. All diodes showed low dark current, remaining in the 10−6 A/cm2 range up to 150K at turn-on (Fig. 3(a)). Bias-dependent and spectral QE curves from the same set of diodes are shown in Figs. 3(b) and 3(c), respectively. At 150K, the fabricated devices exhibited cut-off wavelengths of 4.2 µm as targeted, and estimated quantum efficiencies of ~57% at −0.2 V turn-on based on a calibrated blackbody source passed through a narrow bandpass IR filter centered at 3.1 µm.
Fig. 3 Large-area mesa diode test data taken at 140K and 150K on a bulk nBn MWIR epiwafer grown using the 5 × 5-inch configuration: (a) J-V, (b) QE versus bias, and (c) spectral QE plots.
3.2 Metamorphic nBn MWIR detector structures grown on a 4 × 6-inch GaAs platform
An alternate route to realize large-format Sb-based IR photodector FPA is to explore the use of mature 6-inch GaAs substrate technology via metamorphophic growths. Various metamorphic buffers (M-buffers), including bulk-steps or graded ternary or quaternary architectures, have been used for large lattice mismatched growths, with a prime example being the successful development and commercialization of InP/InGaAs/InAlAs M-HEMTs on GaAs substrates [16,17]. In addition, graded ternary buffers have been employed on GaSb substrates to achieve InAsSb compositions beyond 6.1 Å and thus enable access to longer transition wavelengths [18]. The M-buffer must span the range of mismatched lattice constants and accommodate the inherent strain while minimizing the propagation of dislocations into the device layers. For commercial purposes, the growth process must be achieved on a production growth platform and the resulting epiwafers must have low roughness for device processing and reliability requirements. We have previously investigated the single-wafer MBE growth of GaSb-alloys and photodetectors on 6-inch GaAs substrates using various M-buffer architectures [19]. Using the optimized step-graded M-buffer approach, we extended our work to the growth of metamorphic bulk nBn on a 4 × 6-inch GaAs platform (Fig. 4(a)). The optimized M-buffer employed a single-step buffer growth of GaSb on GaAs. A ~0.5 µm thick tensile strained AlAsxSb1-x/ AlAsySb1-y strained layer superlattices (SLS) was embedded within the 4 µm thick GaSb bulk layer. This SLS has proven to be an effective dislocation filter and help improves surface roughness [19]. The same generic bulk nBn photodetector structure discussed in section 3.1 above was then grown on top of this GaSb-on-GaAs M-buffer.
Fig. 4 Surface morphology of a metamorphic nBn MWIR detector structure: (a) Schematic showing the 4 × 6-inch platen configuration, (b) full wafer Surfscan map showing low defect density (1.3−50 µm2) of 75 /cm2, (c) Nomarski contrast optical microscope image, and (c) AFM image showing smooth surface with low rms roughness of 11 Å (20 µm × 20 µm area scan).
Surfscan maps from all 6-inch M-nBn epiwafers grown using the 4 × 6-inch platen configuration exhibit uniform defect pattern (Fig. 4(b)). Nomarski image (Fig. 4(c)) revealed a slightly textured surface characteristics of single-step M-buffer growth, and AFM measurements indicate rms roughness of <7 Å and <12 Å for 5 µm × 5 µm and 20 µm × 20 µm area scans (Fig. 4(d)), respectively, which is excellent for large-mismatch growths. No degradation in surface morphology was observed when extending the metamorphic growth process from single- to multi- 6-inch wafer configuration [19].
Good structural properties was evidenced by sharp HRXRD peaks shown in Fig. 5(a). The 5-point measurements across the full 6-inch wafer overlay consistently, with good across-wafer uniformity in alloy composition control (<0.01 Sb mole fraction) and in the crystalline quality (XRD FWHM = 119-127 arcsec) for the 4 × 6-inch metamorphic growth platform. The corresponding 5-point overlay of the 77K PL spectra are shown in Fig. 5(b), revealing good uniformity in terms of PL wavelength and FWHM across the full 6-inch wafer. All PL peaks are sharp with FWHM around 17 meV and total wavelength variation of <0.1 µm, comparable to values measured on structures grown on lattice-matched GaSb substrates.
Fig. 5 Multi-point HRXRD and PL measurements across a M-nBn MWIR detector structure grown on the 4 × 6-inch GaAs platform; data points are color-coded in reference to Fig. 4a. (a) Overlay of 5-point HRXRD spectra, the intensity of each scan has been intentionally offset to allow for a clearer depiction of the trend in the peaks across the wafer and (c) 5-point PL spectra taken at 77K across the same wafer.
Large-area mesa diode mesa structures were fabricated from the 5 locations depicted in Fig. 4(a) to assess device quality and uniformity across the 6-inch M-nBn wafer. All diodes exhibit typical J-V, bias-dependent and spectral QE characteristics for this type of detector design (Fig. 6). All diodes showed low dark current in the 1 × 10−5 A/cm2 range at −0.2 V and 150K. Cut-off wavelengths are ~4.0 µm as targeted, with quantum efficiencies of ~60% at −0.2 V turn-on. The device characteristics of these metamorphic structures are comparable to the same structures grown on lattice-matched GaSb substrates (Fig. 3), with marginally higher QE and ~50% higher dark current. The strong photoresponsivity and tight distribution on diodes fabricated on all 5 points across the whole 6-inch GaAs wafer is indicative of excellent material quality and uniformity.
Fig. 6 Multi-point 150K large-area mesa diode characteristics measured across a M-nBn MWIR epiwafer grown GaAs substrate using the 4 × 6-inch platen configuration: (a) J-V, (b) QE versus bias, and (c) spectral QE spectra (5-point measurements based on Fig. 6a).
4. Conclusion
The commercialization of GaSb-based IR photodetectors will inevitably require high quality epitaxial material grown on larger diameter substrates at increased volumes. IQE has successfully transferred our proven GaSb epiwafer growth process to a multi-5-inch and multi-6-inch growth platform in order to support this production trend. Using a straight-forward bulk ~4 µm InAsSb nBn design as a control, multi-wafer 5 × 5-inch growths produced good device performance with high QE and low dark, and demonstrated excellent uniformity and epiwafer characteristics including surface morphology, HRXRD crystal structure, and 77K PL. For the first time, we extended the process to multi-wafer 4 × 6-inch growths of InAsSb M-nBn on GaAs substrates using a M-buffer architecture. Again, multi-point characterization, including AFM, Nomarski, HRXRD, and PL, confirmed excellent uniformity across the 6-inch diameter epiwafers. Large area mesa diode QE (~60%) and dark current (~1 × 10−5 A/cm2) values measured on the M-nBn epiwafers were very comparable to the measurements of the nBn structures grown on 5-inch GaSb substrate and on smaller MBE platforms. Thus the MBE growth process is preparing for increased volume manufacture of large diameter epiwafers for current and next-generation applications, including large-format FPA’s, for this important IR technology.
Acknowledgments
The authors would like to thank Brent Bartholomew, Cherrie Lynn Heisey, Thang Pham, Steve Silknitter, Kanwar Singh, Mark Strzelecki, and Jason Zuber for assistance in MBE growths; Ryan Flick, Dominic Monfre, Michael Renninger and Ying Wu for material characterization and diode processing and testing; Mike Tokar, David Muffley, Ronald Hutton and Kevin Schild for reactor maintenance and equipment support; and Mark Furlong, Becky Martinez and Patrick Flint for GaSb substrate support.
References and links
1. D. L. Smith and C. Mailhiot, “Proposal for strained type II superlattice infrared detectors,” J. Appl. Phys. 62(6), 2545–2548 (1987). [CrossRef]
2. D. H. Chow, R. H. Miles, J. R. Söderström, and T. C. McGill, “Growth and characterization of InAs/Ga1−xInxSb strained-layer superlattices,” Appl. Phys. Lett. 56(15), 1418–1420 (1990). [CrossRef]
3. Y. Wei, A. Gin, M. Razeghi, and G. J. Brown, “Type II InAs/GaSb superlattice photovoltaic detectors with cutoff wavelength approaching 32 µm,” Appl. Phys. Lett. 81(19), 3675–3677 (2002). [CrossRef]
4. F. Fuchs, U. Weimar, W. Pletschen, J. Schmitz, E. Ahlswede, M. Walther, J. Wagner, and P. Koidl, “High performance InAs/Ga1-xInxSb superlattice infrared photodiodes,” Appl. Phys. Lett. 71(22), 3251–3253 (1997). [CrossRef]
5. A. Hood, D. Hoffman, B. M. Nguyen, P. Y. Delaunay, E. Michel, and M. Razeghi, “High differential resistance type-II InAs/GaSb superlattice photodiodes for the long-wavelength infrared,” Appl. Phys. Lett. 89(9), 093506 (2006). [CrossRef]
6. E. H. Aifer, J. G. Tischler, J. H. Warner, I. Vurgaftman, W. W. Bewley, J. R. Meyer, J. C. Kim, L. J. Whitman, C. L. Canedy, and E. M. Jackson, “W-structured type-II superlattice long-wave infrared photodiodes with high quantum efficiency,” Appl. Phys. Lett. 89(5), 053519 (2006). [CrossRef]
7. D. Z.-Y. Ting, C. J. Hill, A. Soibel, S. A. Keo, J. M. Mumolo, J. Nguyen, and S. Gunapala, “A high-performance long wavelength superlattice complementary barrier infrared detector,” Appl. Phys. Lett. 95(2), 023508 (2009). [CrossRef]
8. G. Bishop, E. Plis, J. B. Rodriguez, Y. D. Sharma, H. S. Kim, L. R. Dawson, and S. Krishna, “nBn detectors based on InAs/GaSb type-II strain layer superlattice,” J. Vac. Sci. Technol. B 26(3), 1145–1148 (2008). [CrossRef]
9. S. Maimon and G. W. Wicks, “nBn detector, an infrared detector with reduced dark current and high operating temperature,” Appl. Phys. Lett. 89(15), 151109 (2006). [CrossRef]
10. P. Klipstein, O. Klin, S. Grossman, N. Snapi, B. Yaakobovitz, M. Brumer, I. Lukomsky, D. Aronov, M. Yassen, B. Yofis, A. Glozman, T. Fishman, E. Berkowitz, O. Magen, I. Shtrichman, and E. Weiss, “MWIR InAsSb XBn detectors for high operating temperatures,” Proc. SPIE 8012, 76602Y (2010). [CrossRef]
11. E. H. Steenbergen, B. C. Connelly, G. D. Metcalfe, H. Shen, M. Wraback, D. Lubyshev, Y. Qiu, J. M. Fastenau, A. W. K. Liu, S. Elhamri, O. O. Cellek, and Y.-H. Zhang, “Significantly improved minority carrier lifetime observed in long-wavelength infrared III-V type-II superlattice comprised of InAs/InAsSb,” Appl. Phys. Lett. 99(25), 251110 (2011). [CrossRef]
12. A. W. K. Liu, D. Lubyshev, Y. Qiu, J. M. Fastenau, M. J. Furlong, M. Tyberg, B. Martinez, A. Mowbray, and B. Smith, “MBE Growth of Sb-based Bulk nBn Infrared Photodetector Structures on 6-inch GaSb Substrates,” Proc. SPIE 9451, 94510T (2015). [CrossRef]
13. J. M. Fastenau, D. Lubyshev, Y. Qiu, A. W. K. Liu, E. J. Koerperick, J. T. Olesberg, and D. Norton Jr., “Sb-based IR photodetector epiwafers on 100 mm GaSb substrates manufactured by MBE,” Infrared Phys. Technol. 59, 158–162 (2013). [CrossRef]
14. D. Lubyshev, J. M. Fastenau, M. Kattner, P. Frey, A. W. K. Liu, and M. J. Furlong, “Large-format Multi-wafer Production of 5” GaSb-based Photodetectors by Molecular Beam Epitaxy,” Proc. SPIE 10177, 1017718 (2017). [CrossRef]
15. C. L. Canedy, E. H. Aifer, J. H. Warner, I. Vurgaftman, E. M. Jackson, J. G. Tischler, S. P. Powell, K. Olver, J. R. Meyer, and W. E. Tennant, “Controlling dark current in type-II superlattice photodiodes,” Infrared Phys. Technol. 52(6), 326–334 (2009). [CrossRef]
16. W. E. Hoke, P. S. Lyman, C. S. Whelan, J. J. Mosca, A. Torabi, K. L. Chang, and K. C. Hsieh, “Growth and characterization of metamorphic Inx(AlGa)1−xAs/InxGa1−xAs high electron mobility transistor material and devices with X=0.3–0.4,” J. Vac. Sci. Technol. B 18(3), 1638–1641 (2000). [CrossRef]
17. D. I. Lubyshev, J. M. Fastenau, X.-M. Fang, Y. Wu, C. Doss, A. Snyder, W. K. Liu, M. S. M. Lamb, S. Bals, and C. Song, “Comparison of As- and P-based Metamorphic buffers for High Performance InP Heterojunction Bipolar Transistor and High Electron Mobility Transistor Applications,” J. Vac. Sci. Technol. B 22(3), 1565–1569 (2004). [CrossRef]
18. W. L. Sarney, S. P. Svensson, H. Hier, G. Kipshidze, D. Donetsky, D. Wang, L. Shterengas, and G. Belenky, “Structural and luminescent properties of bulk InAsSb,” J. Vac. Sci. Technol. B 302(2), 02B105 (2012). [CrossRef]
19. J. M. Fastenau, D. Lubyshev, Y. Qiu, A. W. K. Liu, E. J. Koerperick, J. T. Olesberg, and D. Norton Jr., “MBE Growth of GaSb-based Photodetectors on 6 inch Diameter GaAs Substrates via Select Buffers,” J. Vac. Sci. Technol. B 31(3), 03C106 (2013). [CrossRef]
