I. INTRODUCTION

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ChooseTop of pageABSTRACTI. INTRODUCTION <<II. DESIGN, GROWTH, AND F...III. RESULTSIV. ConclusionREFERENCESThe detection of mid-wavelength infrared (MWIR) light is of growing interest for a variety of scientific and military applications. The recent launch of the James Webb Space Telescope has sparked an increased interest in MWIR space imaging due to its inclusion of multiple MWIR capable detectors.1,21. G. Bagnasco, M. Kolm, P. Ferruit, K. Honnen, J. Koehler, R. Lemke, M. Maschmann, M. Melf, G. Noyer, P. Rumler, J.-C. Salvignol, P. Strada, and M. T. Plate, in Cryogenic Optical Systems and Instruments XII (SPIE, 2007), pp. 174–187.2. G. H. Rieke, M. E. Ressler, J. E. Morrison, L. Bergeron, P. Bouchet, M. García-Marín, T. P. Greene, M. W. Regan, K. G. Sukhatme, and H. Walker, Publ. Astron. Soc. Pac. 127, 665 (2015). https://doi.org/10.1086/682257 MWIR detection under 5 μm is useful in gas sensing systems for environmental monitoring. In particular, the detection of water, methane, and ammonia is possible in the 3–3.5 μm range.33. D. Popa and F. Udrea, Sensors 19, 2076 (2019). https://doi.org/10.3390/s19092076 In military applications, MWIR detection is used in night vision systems.44. M. Razeghi and B.-M. Nguyen, Rep. Prog. Phys. 77, 082401 (2014). https://doi.org/10.1088/0034-4885/77/8/082401The implementation of avalanche photodiodes (APDs) for the above applications can provide an advantage over traditional photodetectors as the internal gain mechanism can lead to higher receiver sensitivities due to an improved system signal-to-noise ratio. The internal gain of APDs is achieved through the random process of impact ionization, creating a shot noise described byishot2=2qIphoto+IdarkΔfM2F(M),(1)where q is the elementary charge, M is the avalanche gain, Iphoto and Idark are the photo- and dark current, F(M) is the excess noise factor, and Δf is the measurement bandwidth. The excess noise factor in an APD can be represented using the simple local field model55. R. J. McIntyre, IEEE Trans. Electron Devices ED-13, 164 (1966). https://doi.org/10.1109/t-ed.1966.15651where k is defined as the ratio between β, the hole impact ionization coefficient, and α, the electron impact ionization coefficient. Using this model, an ideal material would have a k-factor equal to zero, in which the excess noise factor asymptotically approaches two with increasing gain.HgCdTe is a widely used materials system for MWIR detection and amplification with devices exhibiting a high gain, up to 5300,66. J. Rothman, G. Perrais, G. Destefanis, J. Baylet, P. Castelein, and J.-P. Chamonal, in Infrared Technology and Applications XXXIII (SPIE, 2007), pp. 475–484. and near unity excess noise.7,87. A. Singh, V. Srivastav, and R. Pal, Opt. Laser Technol. 43, 1358 (2011). https://doi.org/10.1016/j.optlastec.2011.03.0098. A. Kerlain, G. Bonnouvrier, L. Rubaldo, G. Decaens, Y. Reibel, P. Abraham, J. Rothman, L. Mollard, and E. De Borniol, J. Electron. Mater. 41, 2943 (2012). https://doi.org/10.1007/s11664-012-2087-5 However, in addition to environmental concerns, HgCdTe is a difficult materials system to work with specifically regarding the low defect density crystal growth and p-type doping. There have been several recently proposed solutions to these problems; however, their implementations are still pending.99. W. Lei, J. Antoszewski, and L. Faraone, Appl. Phys. Rev. 2, 041303 (2015). https://doi.org/10.1063/1.4936577 These growth difficulties have prompted the search for alternative materials systems for MWIR detection and amplification.A popular III–V alternative to HgCdTe for detection below ∼3.5 μm is InAs as it too has a low excess noise factor near unity.1010. W. Sun, Z. Lu, X. Zheng, J. C. Campbell, S. J. Maddox, H. P. Nair, and S. R. Bank, IEEE J. Quantum Electron. 49, 154 (2013). https://doi.org/10.1109/jqe.2012.2233462 Several device implementations exist;11–1311. A. R. J. Marshall, C. H. Tan, M. J. Steer, and J. P. R. David, IEEE Photonics Technol. Lett. 21, 866 (2009). https://doi.org/10.1109/lpt.2009.201962512. P. J. Ker, A. R. J. Marshall, A. B. Krysa, J. P. R. David, and C. H. Tan, IEEE J. Quantum Electron. 47, 1123 (2011). https://doi.org/10.1109/jqe.2011.215919413. J. Huang, C. Zhao, B. Nie, S. Xie, D. C. M. Kwan, X. Meng, Y. Zhang, D. L. Huffaker, W. Ma, X. Meng, Y. Zhang, Y. Zhang, D. L. Huffaker, W. Ma, W. Ma, and W. Ma, Photon. Res. 8, 755 (2020). https://doi.org/10.1364/prj.385177 however, when operating at 77 K, in order to reduce the impact of band-to-band tunneling on dark current, the gain peaks at ∼30. Operating at higher temperatures yields increased gains, up to ∼300; however, the dark current of these devices is poor due to band-to-band tunneling.10,14,1510. W. Sun, Z. Lu, X. Zheng, J. C. Campbell, S. J. Maddox, H. P. Nair, and S. R. Bank, IEEE J. Quantum Electron. 49, 154 (2013). https://doi.org/10.1109/jqe.2012.223346214. B. S. White, I. C. Sandall, X. Zhou, A. Krysa, K. McEwan, J. P. R. David, and C. H. Tan, J. Lightwave Technol. 34, 2639 (2016). https://doi.org/10.1109/jlt.2016.253127815. W. Sun, S. J. Maddox, S. R. Bank, and J. C. Campbell, in 72nd Device Research Conference (IEEE, 2014), pp. 47–48. With InAs, there is a trade-off between the high gain and low dark current density. Additionally, InAs structures incorporate thick intrinsic regions, up to 8-µm thick, to achieve gain while keeping the electric field low enough to suppress band-to-band tunneling. These thick intrinsic regions not only make it difficult to grow but also reduce the transit time bandwidth of the device.

Many of the aforementioned devices are relatively simple structures where absorption and multiplication occur in the same region. However, a separate absorption, charge, and multiplication (SACM) structure can be used to decouple the absorption and multiplication. This structure is useful as it alleviates band-to-band tunneling issues that plague narrow bandgap materials required to absorb MWIR light. In an SACM, the intermediate charge layer establishes an electric field profile that is high in the multiplier, promoting impact ionization, and low in the absorber, reducing the impact of band-to-band tunneling. With the proper materials system, these structures can be implemented to achieve high gain IR detection.

In recent years, digitally grown AlxIn1−xAsySb1−y (hereafter referred to as AlxInAsSb) lattice matched to GaSb1616. S. J. Maddox, S. D. March, and S. R. Bank, Cryst. Growth Des. 16, 3582 (2016). https://doi.org/10.1021/acs.cgd.5b01515 has been used to realize low-excess-noise, high-gain APDs17,1817. M. E. Woodson, M. Ren, S. J. Maddox, Y. Chen, S. R. Bank, and J. C. Campbell, Appl. Phys. Lett. 108, 081102 (2016). https://doi.org/10.1063/1.494237218. S. R. Bank, J. C. Campbell, S. J. Maddox, M. Ren, A. K. Rockwell, M. E. Woodson, and S. D. March, IEEE J. Sel. Top. Quantum Electron. 24, 1 (2018). https://doi.org/10.1109/jstqe.2017.2737880 with k-factors of 0.01–0.05 and gains above 100 for visible and near-infrared (NIR) detection. Additionally, in this wavelength range, AlxInAsSb layers have been successfully grown as a random alloy lattice matched to both InP and GaSb substrates for use in solar cells19,2019. S. Tomasulo, M. Gonzalez, M. P. Lumb, M. E. Twigg, I. Vurgaftman, J. R. Meyer, R. J. Walters, and M. K. Yakes, in 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC) (IEEE, 2019), pp. 3187–3190.20. J. Kret, J. Tournet, S. Parola, F. Martinez, D. Chemisana, R. Morin, M. de la Mata, N. Fernández-Delgado, A. A. Khan, S. I. Molina, Y. Rouillard, E. Tournié, and Y. Cuminal, Sol. Energy Mater. Sol. Cells 219, 110795 (2021). https://doi.org/10.1016/j.solmat.2020.110795 and avalanche photodiodes with a gain of 15 and k-factor of ∼0.02.2121. S. H. Kodati, S. Lee, B. Guo, A. H. Jones, M. Schwartz, M. Winslow, N. A. Pfiester, C. H. Grein, T. J. Ronningen, J. C. Campbell, and S. Krishna, Appl. Phys. Lett. 118, 091101 (2021). https://doi.org/10.1063/5.0039399 SACM APDs have been demonstrated in the materials system for IR detection and amplification with cutoff wavelengths of 1.7-µm,22,2322. M. Ren, S. J. Maddox, M. E. Woodson, Y. Chen, S. R. Bank, and J. C. Campbell, Appl. Phys. Lett. 108, 191108 (2016). https://doi.org/10.1063/1.494933523. Y. Lyu, X. Han, Y. Sun, Z. Jiang, C. Guo, W. Xiang, Y. Dong, J. Cui, Y. Yao, D. Jiang, G. Wang, Y. Xu, and Z. Niu, J. Cryst. Growth 482, 70 (2018). https://doi.org/10.1016/j.jcrysgro.2017.10.035 2.1-µm,2424. A. H. Jones, S. D. March, S. R. Bank, and J. C. Campbell, Nat. Photonics 14, 559 (2020). https://doi.org/10.1038/s41566-020-0637-6 and 2.9-µm.2525. A. H. Jones, S. D. March, A. A. Dadey, A. J. Muhowski, S. R. Bank, and J. C. Campbell, IEEE J. Quantum Electron. 58, 1 (2022). https://doi.org/10.1109/jqe.2022.3149532 Specifically, the ∼2.9-µm cutoff device in Ref. 2424. A. H. Jones, S. D. March, S. R. Bank, and J. C. Campbell, Nat. Photonics 14, 559 (2020). https://doi.org/10.1038/s41566-020-0637-6 has a unity-gain external quantum efficiency of 47% at 2 µm and can achieve gains up to 350.In this paper, we present an SACM APD design using digitally grown Al0.05InAsSb as the absorber for MWIR detection and digitally grown Al0.7InAsSb as the multiplier for low excess noise amplification (hereafter referred to as 3.5-µm cutoff SACM APD). This device achieves gains more than double that of the state-of-the-art InAs-based detectors and achieves gain-normalized dark current densities over two orders of magnitude lower than that of the previous MWIR Al0.15InAsSb-based detector.2525. A. H. Jones, S. D. March, A. A. Dadey, A. J. Muhowski, S. R. Bank, and J. C. Campbell, IEEE J. Quantum Electron. 58, 1 (2022). https://doi.org/10.1109/jqe.2022.3149532 This device also achieves a longer wavelength cutoff than the InAs and previous MWIR Al0.15InAsSb devices.

II. DESIGN, GROWTH, AND FABRICATION

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ChooseTop of pageABSTRACTI. INTRODUCTIONII. DESIGN, GROWTH, AND F... <<III. RESULTSIV. ConclusionREFERENCESEpitaxial layers for the 3.5-µm cutoff SACM APD, displayed in Fig. 1(a), were grown as a digital alloy via molecular beam epitaxy on an n-type GaSb substrate as discussed in a previous publication.1616. S. J. Maddox, S. D. March, and S. R. Bank, Cryst. Growth Des. 16, 3582 (2016). https://doi.org/10.1021/acs.cgd.5b01515 Figure 1(b) shows the simulated zero bias energy band diagram for the device with different regions of the device numbered. The simulation was performed using Ansys Lumerical CHARGE. Region 1 is the n-contact. Region 2 is the wide bandgap Al0.7InAsSb multiplication layer. Region 3 contains an Al0.7InAsSb p-type charge layer and an Al0.7-0.15InAsSb bandgap grading region. By decreasing the Al concentration in AlxInAsSb, the bandgap energy is reduced.1616. S. J. Maddox, S. D. March, and S. R. Bank, Cryst. Growth Des. 16, 3582 (2016). https://doi.org/10.1021/acs.cgd.5b01515 These layers in region 3 establish the electric field profile and provide a smooth transition between the multiplier and absorber. Region 4 is an Al0.15InAsSb layer that helps prevent high electric field buildup in the narrow bandgap regions of the grading layer (region 3), reducing dark current contributions from band-to-band tunneling. A previous study2525. A. H. Jones, S. D. March, A. A. Dadey, A. J. Muhowski, S. R. Bank, and J. C. Campbell, IEEE J. Quantum Electron. 58, 1 (2022). https://doi.org/10.1109/jqe.2022.3149532 confirms the utility of this layer through a comparison of two SACM APD designs with and without this region. Region 5 is the Al0.05InAsSb narrow bandgap absorber that has been lightly p-type doped to convert the doping polarity from n- to p-type, which in return helps prevent energy band sagging. Finally, region 6 acts as the p-contact as well as a diffusion barrier for electrons generated in the absorber. Figure 2 shows an x-ray diffraction pattern for the grown epitaxy shown in Fig. 1(a) with the GaSb substrate and AlInAsSb superlattice fringe peaks labeled.

Circular mesas were defined using standard lithography techniques. Mesas were formed by first partially reactive ion etching with an inductively coupled plasma (ICP) through the low Al containing absorber and charge layers. A Cl2:N2 (8 sccm/20 sccm) plasma was used with 300 W of ICP power. The remainder of the mesa was wet etched into the n-type Al0.3InAsSb contact layer using a C6H8O7:H3PO4:H2O2:H2O (10 g:6 ml:3 ml:60 ml) solution. This wet etch also acts to clean up the surface damage caused during the dry etch. After forming the mesas, Ti/Au (10 nm/100 nm) contacts were deposited on both the p- and n-contact layers using e-beam evaporation. Finally, the mesa sidewalls were passivated with SU-8 to reduce surface leakage current.

IV. Conclusion

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ChooseTop of pageABSTRACTI. INTRODUCTIONII. DESIGN, GROWTH, AND F...III. RESULTSIV. Conclusion <<REFERENCES

In this paper, we have demonstrated a high-gain, low-excess-noise, 3.5-µm cutoff SACM APD based on the digitally grown AlxInAsSb materials system. Under 2-µm illumination, maximum gains in excess of 850 are achievable, more than double that of InAs. The unity-gain external quantum efficiency attains a peaks of ∼54% (1.02 A/W) at ∼2.35 µm and maintains an external quantum efficiency of ∼24% (0.58 A/W) at 3 µm. Additionally, a low excess noise factor is achieved scaling with a k of ∼0.04. At a gain of ∼850, the gain-normalized dark current density is ∼0.05 mA/cm2 at 100 K. This device achieves gains more than double that of the state-of-the-art InAs detectors and achieves dark current densities over two orders of magnitude lower than that of the previously best MWIR Al0.15InAsSb-based detector. These results are promising for MWIR detection applications as the increased gains can lead to higher receiver sensitivities and offer an attractive III–V-based alternative to HgCdTe-based APDs.