Imaging Arrays

It is expected that the successful implementation of a new class of CQD IR technology may match the broad impact of cheap CMOS cameras that are widely used today [24]. SWIR cameras, built on CQD thin-film photodiodes, and fabricated monolithically on silicon ROICs, have been launched [25,26]. Figure 8.12 shows two PbS CQD cameras fabricated by IMEC and SWIR Vision Systems. The IMEC’s prototype imager has a resolution of 758 X 512 pixels and a 5-pm pixel pitch. The CQD photodiodes on the silicon substrate achieve an external quantum efficiency greater than 60% at 940 nm wavelength, and above 20% at 1450 nm, allowing uncooled operation, with a dark current comparable to that in commercial InGaAs photodetectors. The Acuros cameras, with a format up to 1920 X 1080 (2.1 megapixels, 15-pm pixel pitch), deliver 0.4 to 1.7 pm broadband, visible- to-SWIR, high-resolution images, at far superior cost points compared to InGaAs SWIR cameras (Fig. 8.12). In Table 8.3, the performance specifications of the Acuros camera are collated.

Figure 8.13 compares images taken with a visible and SWIR CQD camera during heavy rain over an ocean coast. Fligher resolution images taken by the SWIR CQD camera were associated with reduced raindrop scattering [25].

New PbS CQD sensors integrated in camera modules with standard or SWIR lenses fabricated by (a) IMEC and (b) SWIR Vision Systems

FIGURE 8.12 New PbS CQD sensors integrated in camera modules with standard or SWIR lenses fabricated by (a) IMEC and (b) SWIR Vision Systems.

TABLE 8.3 Performance Comparison of SWIR Cameras

Acuros™ CQD™

InGaAs

Maturity

New

High

Spectrum

400-1700 nm

900-1700 nm

Quantum efficiency

15% average

65-80%

Resolution

  • 640x512
  • 1280x1024
  • 1920x1080
  • 640x512
  • 1280x1024

Pixel pitch

15 pm -* ~2 pm

15 pm -» ~12 pm

Noise-equivalent irradiance

6x 109 photons/cm2/s

~109 photons/cm2s

Operability

>99%

>99%

Cost

Low materials $ Low processing $

Low materials $ Low processing $

Images obtained from the CMOS visible camera

FIGURE 8.13 Images obtained from the CMOS visible camera (left) and SWIR Acuros camera (right), during heavy rain over a Pacific Coast inlet, obscuring the 3-km distant shoreline. SWIR image enhances long-range visibility due to reduced raindrop scattering (after Ref. [25]).

At present, CQD cameras are used in newer applications that require high-definition, low-cost imaging on smaller pixels without extreme sensitivity [24-26]. It can be predicted that increasing the dot size, while maintaining a good mono-dispersion, carrier transport, and quantum efficiency, will improve/maintain low noise levels. Due to continuous development of deposition and synthesis techniques, much higher performances will be achieved in the future.

Present Status of CQD Photodiodes

In this section, we compare the performance of ideal P-i-N HgCdTe photodiodes with the present status of CQD photodiodes operated at room temperature. The theoretical estimation of HgCdTe photodiode performance, together with the experimental data, are described in detail in Section 3.3.

Figure 3.12(b) compares the temperature dependence of detectivity for different material systems with a cutoff wavelength of ~5 pm, namely commercially available HgCdTe photodiodes and HgTe CQD photodiodes. The experimental data collected are also included. The estimated detectivity values for CQD photodiodes are located below those for HgCdTe photodiodes. As is shown, at temperatures above 200 K, the theoretically predicted detectivity for HgCdTe photodiodes is limited by background. The semiempirical rule Rule 07, widely popular in the IR community as a reference for other technologies, is found not to fulfill primary expectations. Rule 07 coincides well with a theoretically predicted curve for an Auger-suppressed p-on-n HgCdTe photodiode, with a doping concentration in the active region equal to 1015 cnr3. As indicated in Section 3.3, at the present stage of HgCdTe technology, the doping concentration is almost two orders of magnitude lower (mid-1013 cm'3).

All experimental data listed in Figs. 3.16 and 8.14 indicate a sub-BLIP photodetector performance by CQD photodetectors. Both figures also clearly show that the detectivity values of CQD photodetectors are inferior in comparison with those of HgCdTe photodiodes. Moreover, the theoretical predictions indicate a possible further performance improvement of HgCdTe devices after decreasing the i-doping level in P-i-N photodiodes. For doping levels of 5 X 1013 cm'3, the photodiode performance can be limited by background radiation in a spectral band above 3 pm. It is shown that, in this spectral region, the D' is limited, not by the detector itself, but by background photon noise at a level above 1010 Jones in the LWIR range (more than one order of magnitude above Rule 07).

In the past decade, considerable progress has been made in the fabrication of SWIR and MWIR CQD photodetectors, together with their integration into thermal imaging cameras. In spite of this, the performance of CQD photodetectors is inferior in comparison with HgCdTe photodiodes. It seems that the PbS CQD photodetectors, characterized by multispec- tral sensitivity and detectivity comparable with InGaAs devices (which are currently the most common in commercial applications), have been located at the best position in the IR-material family at the present time.

Between different material systems used in the fabrication of HOT LWIR photodetectors, only HgCdTe ternary alloy can fulfill the required expectations: low doping concentration (1013 cm'3) and high SRH carrier

Room-temperature spectral detectivity curves of the commercially available photodetectors [PV Si and InGaAs, PC PbS and PbSe, HgCdTe photodiodes

FIGURE 8.14 Room-temperature spectral detectivity curves of the commercially available photodetectors [PV Si and InGaAs, PC PbS and PbSe, HgCdTe photodiodes (solid lines, Ref. [27]). The experimental data for different types of CQD photodetectors are marked by dot points [3,9,28,29]. Spectral detectivity of new and emerging T2SL IB QCIPs (dashed lines) are also included [30]. PC: photoconductor; PV: photodiode.

lifetime (above 1 ms). In this context, it will be rather difficult to rival HgCdTe photodiodes with CQD photodetectors. The above estimates do, however, provide further encouragement for achieving low-cost and high-performance MWIR and LWIR HgCdTe focal plane arrays operated under HOT conditions.

 
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