Abstract
Increasing the operating temperature for infrared detectors is critical to reduce the size, weight and power of infrared (IR) systems. Such systems are essential to implement a compact and low-cost production of IR systems. For the Kunming Institute of Physics (KIP), HgCdTe standard p-on-n technology with indium doping and arsenic ion implantation technology has been optimized for many years and mid-wavelength IR (MWIR) detectors with excellent electro-optical performance were realized. This paper reports the latest results of the MWIR focal plane array (FPA) detector with a high operating temperature (HOT). Performances of the 1024×768@10 μm pitch MW detector working above 150 K were presented. The detector presenting a cut-off wavelength above 4.97 μm at 150 K has been developed. The noise-equivalent temperature difference (NETD), dark current and operability at different operating temperatures were attained. Additionally, the IR image taken with the MWIR HgCdTe-based FPA and processed at an operating temperature of 150 K was presented and retained an operability of 99.4%.Introduction
The infrared detector has been developed to the 3rd Gen sensor system using staring detector arrays operated at higher operating temperatures，with the aim of reducing size，weight and power consumption on the premise of ensuring the high performance ［1-4］. With the increase of operating temperature，the reduced cooling demand of cooled infrared detector can reduce the size and power of cryocooler，economize the application cost of infrared system，and improve the environmental adaptability，reliability，and lifetime of the detector ［5-9］. It has greatly optimized the IR-imaging systems which benefit the longer mission time，the easier thermal management，the reduced complexity of the system level and so on ［10-12］.
While HgCdTe offers several significant advantages，including close lattice matching for flexible control of the bandgap，readily available doping techniques，high electron mobility，high optical absorption coefficient and low carrier generation rate，making it the preferred material for demanding applications with high performance ［6，13-14］. There are mainly three kinds of device structure： Hg vacancy n-on-p structure，Au-doped n-on-n structure and extrinsic doped p-on-n structure［14，15］. In the early stage of HOT MWIR HgCdTe products of Sofradir and AIM，the operating temperature based on Hg vacancy n-on-p structure can reach 120 K by optimizing the material quality and the fabrication process ［8，15］. The HOT MWIR HgCdTe devices based on Au-doped n-on-p structure，as the transition products for AIM，can achieve an operating temperature of 140 K limited by the Au-diffusion stability in the HgCdTe ［10，15］. The extrinsic doped p-on-n devices using arsenic ion implantation or in-situ doping have more controllable and stable impurity concentrations，which have been adopted by RVS，TIS，AIM，BAE and others to achieve a temperature around 150 K or even higher ［6，8，11，15-21］.
In this paper，we report the study advance on the HgCdTe p-on-n MWIR FPAs. The MWIR HgCdTe FPAs showed a high operating temperature up to 150 K.
1 HgCdTe HOT MWIR FPAs
Consistent with the previous work about a 15 μm pitch 640×512 MWIR HgCdTe FPA，the detector with 1024×768 pixels and a 10 μm pixel pitch was fabricated using arsenic-ion implanted p-on-n planar junction device technology ［22］. MWIR HgCdTe epitaxial layers were grown on CdZnTe （111） B substrate by liquid phase epitaxy. N-type doping was obtained by indium incorporation during growth. The n-type doping concentration was about 4×1014 cm-3，the surface microdefect （Ф≥5 μm） density was less than 10 cm-2，and the dislocation etch pit density was less than 5×104 cm-2. The epitaxial HgCdTe films have the characteristics of low dislocation density and high crystal quality，which is the basic premise of low dark current and low noise detector ［23］. P-type doping was obtained by arsenic implantation and activation via a high-temperature annealing step under the condition of Hg saturation. Finally，the Hg vacancies in the materials were eliminated by low temperature Hg saturation annealing. It is noteworthy that the optimizing surface passivation and the interdiffusion annealing process are the key elements to suppress the surface-related current and noise ［24］. In order to optimize the surface state of HgCdTe films，removing the dangling bonds and optimizing the annealing condition are the main method. The HgCdTe detector chip and ROIC were hybridized by a flip-chip bonding technology. Then the CdZnTe substrate was removed completely and the anti-reflection film was prepared.
To support the SWaP applications，the Dewar was designed to be compact and low in heat leakage，which was integrated to a low-power miniaturized linear Stirling cryocooler （C351）. Using this feature，the optical axis is only 69.15 mm. Figure 1 shows the schematic diagram of the MWIR HgCdTe HOT integrated Dewar cooled assembly （IDCA）.
| Parameter | Typical performance |
|---|
| Format & pitch | 1024×768，10 μm |
| FPA operating temperature | 150 K |
| Spectral band | 3.7 - 4.97 μm |
| Pixel capacity | 4 Me-@IWR/6 Me-@ITR |
| Integration mode | ITR & IWR |
| Power consumption （FPAs） | 173 mW |
| NETD | ≤20 mK at 50% well fill capacity，F/2 |
| Operability | 99.4% |
| F# | 2 or 4 |
| Cooler | C351 |
| Weight （IDCA） | 250 g |
| Power consumption （IDCA） | 2.5 W （at 20 ℃） |
| Cool down time （IDCA） | 2.5 min （at 20 ℃） |
| Optical axis | 69.15 mm |
Table 1. Typical performances of the HgCdTe MWIR HOT IDCA
Figure 1.The photograph of the MWIR HgCdTe HOT IDCA
The performance of the FPAs is determined by several key properties with an operating temperature of 150 K. The NETD is normally defined by the ratio of the temporal noise to the responsivity. The FPAs offered an NETD of ≤20 mK at 50% well fill of its 6 Me- capacitor. The dark current was tested in a Dewar equipped with a blind cooling screen. The test results show a lower dark current of 5.2×10-7A/cm2 which is about a third of “Rule 07” ［27］. This FPA has a total of 4470 defective pixels （0.6%） that were defined as defective according to criteria. The overall IDCA power consumption including all the electronics is 2.5 W at room temperature，and the cool down time is 2.5 minutes. The typical performances of the HgCdTe HOT MWIR FPA are summarized in Table 1.
2 Performance characterization
This section will present the performances of the HgCdTe HOT MWIR FPAs. If not otherwise noted，all measurements in this section are from HgCdTe 1024×768，10 μm pitch detector arrays.
The current voltage （IV） plot，presented in Fig. 2，illustrates the excellent characteristics of these p-on-n photodiodes. The measurement was carried out at 150 K in front of a 293 K black body （black curve） and a 308 K black body （red curve）. The IV curves show that the reverse breakdown voltage is higher than 1 V，demonstrating the high quality of MWIR LPE device fabrication procedure. The photodiode shows a spectral signature with a cutoff wavelength equal to 4.97 μm at 150 K （Fig. 3）.
Figure 2.Current-voltage characteristics for the MWIR HgCdTe p-on-n photodiode，293 K black body，150 K operating temperature
Figure 3.The relative spectral response at 150 K operating temperature，λc= 4.97 μm
Figure 4 presents the dark-current characteristics of the p-on-n HgCdTe diodes. The dark current of the MWIR HgCdTe FPAs at different operating temperatures was tested in a Dewar equipped with a blind cooling screen，and compared with the predicted values of “Rule 07” and “Law 19” proposed by Tennant and Lee et al.，with the same composition ［27-28］. The dark current of the FPAs was nearly equal to the reported value of Sofradir and AIM in the range of 110-200 K. The dark current was 5.2×10-7 A/cm2 at an operating temperature of 150 K，which was lower than the predicted value by “Rule 07”，owing to high crystal quality HgCdTe films with low dislocation and microdefect density and low surface-related current and noise by optimizing surface passivation and interdiffusion annealing process ［13，14，24］. The dark current at the higher temperature showed a quicker increase compared with “Rule 07” that might be due to the increase of diffusion current and surface leakage current ［7，24-26］. The dark current at the lower temperature was inaccurate，possibly due to the reduced injection efficiency and input gate leakage in the direct injection circuit ［10］.
Figure 4.The dark current dependence on the reciprocal temperature of p-on-n MWIR FPAs versus the predicted values by “Rule 07” and “Law 19”
Meanwhile，the NETD has been measured under the half well fill condition at 308 K black body temperature for different operating temperatures in the range from 80 K to 200 K. The integration time has been adjusted for each operating temperature due to the reason that the wavelength became shorter with rising the operating temperature. Figure 5 shows the results as a function of temperature. With the operating temperature increasing，the impact on the dark current was obvious，which resulted in an NETD value slightly above 18 mK and a significantly reduced integration time at 160 K. The NETD remained constant at ~17 mK up to an operating temperature of 140 K，and increased rapidly with the operating temperature exceeding 160 K. The NETD was 17.1 mK at 150 K，and the NETD histogram of the FPAs is shown in Fig. 6.
Figure 5.The temperature dependence of the NETD for the FPAs，measured with an aperture of F/2 in front of a black body at a temperature of 20 ℃
Figure 6.The NETD histogram of FPAs at 150 K
The operability of the FPAs，plotted in Fig. 7 as a function of the operating temperature，is better than 99.4% up to 150 K. Figure 8 shows the blind pixel distribution of FPAs at 150 K that the black dots are blind pixels.
Figure 7.The temperature dependence of the operability for FPAs
Figure 8.The blind pixel distribution of 1 024×768 MWIR FPAs (above) and the central 512×384 area (below) at 150 K
The image at 150 K is presented in Fig. 9. It was recorded on a warm day and the image was corrected with the dead element concealment.
Figure 9.The IR-image at an operating temperature of 150 K from MWIR HgCdTe HOT FPAs
3 Conclusions
In conclusion，the MWIR HgCdTe HOT IDCA with 1024×768 pixels and a 10 μm pixel pitch exhibits high electro-optical performance and has an operating temperature of about 150 K. The dark current is nearly equivalent to the predicted value by “Rule 07”. The FPA presents a cutoff wavelength above 4.97 µm，NETD values of ~17.1 mK and operability of ~99.4% at 150 K. The optimized size，weight and power characteristics target an optical axis of 69.15 mm，a weight of 250 g and a typical power consumption of 2.5 W，which is an easy and fast integration into modern IR systems to support SWaP applications. Our next work will focus on the optimization of the device design and fabrication process to realize a higher operating temperature.
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