Objectives  HgCdTe short-wave infrared focal plane array(IRFPA) plays a significant role in infrared astronomical observations. Photon transfer curves (PTC) is an important method to characterize the detector performance. Measuring the gain of the detector by the PTC method is a prerequisite for characterizing detector's other performance metrics. In previous studies, the measured gain was typically the average gain of all pixels in the focal plane array. For infrared detectors, the gain across the array may be non-uniform. This research aims to test the gain distribution across different regions of a short-wave IRFPA using the PTC method and to improve gain uniformity through chip process optimization.

Methods  Mercury-cadmium-telluride (HgCdTe) material is used to develop short-wave IRFPA. The detector chip is based on an ion-implanted n-on-p structure. It features a resolution of 640×512 pixels with a pixel pitch of 15 micrometers. The PTC method is used to characterize the detector's gain. The test setup is illustrated in Fig.2. Two methods are employed for PTC testing: 1) maintaining a constant blackbody temperature while varying the exposure time, and 2) fixing the integration time while adjusting the blackbody temperature. The dark current and readout noise are measured after obtaining the gain.

Results and Discussions  The test results indicate that the gain is unevenly distributed across the 640×512 pixel array. The 640×512 array is divided into 10×8 groups, each consisting of a 64×64 pixel region.The gain for each group is calculated, as shown in Fig.5. By determining the standard deviation of the gain from this data, the non-uniformity of the gain is found to be 20.2%. To further verify the non-uniformity, each pixel's gain is tested by PTC method of fixing the integration time and adjusting the blackbody temperature. The heatmap of each pixel gain (Fig.7) also clearly shows that the gain distribution is uneven. The non-uniformity may be linked to the chip fabrication process. To investigate this, process parameters were adjusted, and after implementing improvements, the gain uniformity significantly improved, as shown in Fig.8, with the non-uniformity reduced to 0.3%. Additionally, the dark current and readout noise were measured under dark conditions. Figure 9 shows the variation of the detector's output signal with integration time under dark conditions. The slope of the line in the figure divided by the gain gives the detector's dark current, which is 2.2 e⁻/s. Figure 10 presents a histogram of the detector's noise, with the maximum value corresponding to the noise level of 67 e⁻.

Conclusion  The chip's fabrication process, particularly the stress caused by the polishing and thinning process, can propagate into the detector, affecting the noise of the detector's photosensitive elements and, consequently, the uniformity of the gain. By optimizing the process parameters and reducing polishing damage, noise can be decreased, thereby enhancing the uniformity of the gain. With the improved process, the gain non-uniformity of the IRFPA is reduced from 20.2% to 0.3%. Based on the gain obtained from the PTC, the detector's dark current is measured to be 2.2 e⁻/s, and the detector's noise is 67 e⁻. In this study, it was also observed that in the region where photon noise dominates, the linearity of the PTC deviates. The reasons for this deviation may be multifaceted, including material non-uniformity, non-effective pixels, non-uniformity of illumination during testing, and noise in the test system. In future technical improvements, it is necessary to enhance the performance of the detector, particularly by reducing detector noise.