Final Remarks

Future civilian and military infrared and terahertz detector applications face many critical challenges. The main efforts are directed to decreasing the size, weight, and power consumption (SWaP) of infrared imaging systems by increasing the operating temperature of detector arrays. Nowadays, HgCdTe is the most widely used variable bandgap semiconductor for IR photodetectors, including uncooled operation, and stands as a for alternative technologies. However, in spite of sixty years of development history of the HgCdTe ternary alloy system, its ultimate HOT performance limit has not been achieved.

In Kinch’s monograph [1] it is clearly shown that “the ultimate cost reduction for an IR system will only be achieved by the room-temperature- operation of depletion-current-limited arrays with pixel densities that are fully consistent with background- and diffraction-limited performance due to the system optics. This mandates the use of an IR material with a long S-R lifetime. Currently, the only material that meets this requirement is HgCdTe.” Kinch predicted that large-area ultra-small pixel diffraction- limited and background-limited photon-detecting MW and LW HgCdTe FPAs, operating at room temperature, would be available within the next five years [2].

At present, the Rule 07 metric (specified in 2007 [3]) is not a proper approach for prediction of the HgCdTe detector and system performance, nor as a reference benchmark for alternative technologies (often used especially for type-II superlattices and colloidal quantum dots). In Section 3.3, it was shown, that, for sufficiently long SRH carrier lifetime in HgCdTe ternary alloy, what is experimentally supported by Teledyne at a doping level below 5xlO^l3cmthe internal P-i-N HgCdTe photodiode current is suppressed and the detector performance is limited by the background radiation. HgCdTe photodiodes, operating in the longer-wavelength infrared range (above 3-pm), guarantee achieving more than one order of magnitude higher detectivity (above 10¹⁰ Jones) in comparison with the value predicted by Rule 07. This new benchmark [4], Law 19, corresponds exactly to the detector background-limited performance curve for room temperature. In this context, our paper evaluates a new class of emerging 2D material technologies for HOT infrared photodetectors.

At present, the performance of single room-temperature 2D material photodetectors, operating in infrared and terahertz spectral ranges, is comparable to that of standard commercial photodetectors (InGaAs and HgCdTe). Black phosphorus-arsenic alloys and noble TMD photodetectors have emerged as candidates for detection of long-wavelength infrared radiation with higher detectivities in comparison with commercial HgCdTe photodiodes. However, the instability of the black phosphorus surface, due to chemical degradation under ambient conditions, remains a major impediment to its prospective applications. More promising are stable noble TMD photodetectors like PdSe₂/MoS₂ heterojunctions, with record detectivity in the LWIR range at room temperature. However, their practical application lies in perfect material synthesis and processing. Due to mature HgCdTe technology, it is rather difficult for 2D material to compete with HgCdTe photodiodes. To achieve it, 2D material photodetectors have a long way to go.

The future applications imaging infrared systems require:

• • Higher pixel sensitivity,
• • Further increases in pixel count to above 10^s pixels (mosaicking may be used) with pixel size decreasing to about 5-pm for both cooled and uncooled LWIR applications,
• • Cost reduction in imaging array systems through the use of the integration of detectors and signal processing functions (with much more on-chip signal processing) and less cooling sensor technology,
• • Improvement in the functionality of imaging arrays through the development of multispectral sensors.

Small-pitch IR FPAs will require the development of larger effective ROIC well capacities per unit area, possibly faster optics than//l, and improved hybridization technologies compared with those dominating currently in IR arrays fabrication. Leveraging deeply scaled CMOS process technology enables designers to miniaturize pixel pitch and/or increase on-chip processing capability, depending on application-specific needs. Array sizes will continue to increase but perhaps at a rate that falls below the Moore’s Law curve. An increase in array size is already technically feasible. However, the market forces that have demanded larger arrays are not as strong now that the megapixel barrier has been broken.

The first 2D material-based imaging sensors operating in the visible and short wavelength infrared region have been demonstrated [5]. Dual band vertical GaSe/GaSb heterostructure linear array (16x1) was fabricated by the MBE growth technique in 2017 [6]. Also 388x288 20-pm pixel gra- phene-QD-based CMOS-integrated sensor has since been demonstrated [7]. Graphene is particularly attractive due to its compatibility with silicon and its monolithic integration with CMOS-integrated circuits. However, development of photodetectors toward high performance, high integration, and a large focal plane is particular critical for practical applications of 2D photodetectors in industry. In a way, graphene has been a victim of its own success, getting people overly excited about it and attracting unrealistic expectations. High-quality 2D materials are the promising next- generation alternative, although it will take several decades of research, development, and, most importantly, billions of dollars of investment at the national and international level to become the ultimate benchmark for standard electronic materials and devices. From the other side, the evolutionary path of Si technology, driven by Moore’s Law of Scaling, seems to be narrowing and fast approaching an end, simply due to the fundamental limitations of Si at the atomic scale [8]. In fact, it is unlikely that 2D technology will supplant Si, but instead it may coexist with Si technology [9]. This, too, will require significant research and resource investment in large area growth of 2D materials at temperatures compatible with silicon- based technology.

Next, we follow Ref. [10] to compare different detector technologies existing on the global market, with emerging 2D photodetectors. Table 1 provides a snapshot of the current state of development of LWIR detectors fabricated from different material systems. Note that TRL means technology readiness level. The highest potential TRL (ideal maturity) achieves a value of 10. The highest actual level of maturity (TRL = 9) is credited to HgCdTe photodiodes and microbolometers, with QWIPs offering a slightly lower maturity, with TRL = 8. The type-II A^IHB^V superlattice structures have great potential for LWIR spectral range applications, with performance comparable to HgCdTe for the same cutoff wavelength. Considerable progress toward mature superlattice and barrier detector technologies, including their commercialization, has been observed in the

(Continued)

Note: TRL - technology readiness level.

past decade, with 2D material detector technology being at a very early stage of development (TRL = 1-2).

Development of a scalable 2D material detector arrays concept, with appropriate fabrication tools, can be a noteworthy indicator that the 2D materials can enable the development of new detector systems. The combination of scalability, the prospects for integration with Si-platforms, and the potential for implementing flexible devices can make 2D material attractive for future generations of detection systems. However, at the present stage of detector technology, it will be rather difficult to improve their position in a mature global market detector family in the near future. From an economical point of view and a future technology perspective, an important aspect concerns industry fabrication of detector arrays with high operability, spatial uniformity, temporal stability, scalability, produc- ibility, and affordability. All these aspects are at a stage of infancy in terms of manufacturability.

References

• 1. M.A. Kinch, State-of-the-Art Infrared Detector Technology, SPIE Press, Bellingham, 2014.
• 2. M.A. Kinch, “An infrared journey”, Proceedings of SPIE 9451, 94512B (2015).
• 3. W.E. Tennant, D. Lee, M. Zandian, E. Piquette, and M. Carmody, “MBE HgCdTe technology: A very general solution to IR detection, described by ‘Rule 07’, a very convenient heuristic”, Journal of Electronic Materials 37, 1406-1410 (2008).
• 4. D. Lee, P. Dreiske, J. Ellsworth, R. Cottier, A. Chen, S. Tallarico, A. Yulius, M. Carmody, E. Piquette, M. Zandian, and S. Douglas, “Law 19 - The ultimate photodiode performance metric”, Extended Abstracts. The 2019 U.S. Workshop on the Physics and Chemistry of II-VI Materials, pp. 13-15,2019.
• 5. M. Long, P. Wang, H. Fang, and W. Hu, “Progress, challenges, and opportunities for 2D material based photodetectors”, Advanced Functional Materials 29, 1803807 (2018).
• 6. P. Wang, S. Liu, W. Luo, H. Fang, F. Gong, N. Guo, Z.-G. Chen, J. Zou, Y. Huang, X. Zhou, J. Wang, X. Chen, W. Lu, F. Xiu, and W. Hu, “Arrayed van der Waals broadband detectors for dual-band detection”, Advanced Materials 29, 1604439 (2017).
• 7. S. Goossens, G. Navickaite, C. Monasterio, S. Gupta, J.J. Piqueras, R. Perez, G. Burwell, I. Nikitskiy, T. Lasanta, T. Galan, E. Puma, A. Centeno, A. Pesquera, A. Zurutuza, G. Konstantatos, and F. Koppens, “Broadband image sensor array based on graphene-CMOS integration”, Nature Photonics 11, 366-371 (2017).

• 8. International Roadmap for Devices and Systems™. 2018 Update. More Moore, https://irds.ieee.org/images/files/pcif/2018/ 2018IRDS_MM. pdf
• 9. N. Briggs, S. Subramanian, Z. Lin, X. Li, X. Zhang, K. Zhang, K. Xia, D. Geohegan, R. Wallace, L.-Q. Chenl, M. Terrones, A. Ebrahimi, S. Das, J. Redwing, C. Hinkle, K. Momeni, A. van Duin, V. Crespi, S. Kar, and J.A. Robinson, “A roadmap for electronic grade 2D materials”, 2D Materials 6, 022001 (2019).
• 10. Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays, Committee on Developments in Detector Technologies; National Research Council, 2010, http://www.nap.edu/catalog/12896.html