Quantitative analysis of carrier concentration and electric field in non-uniform quantum well infrared photodetectors

Semiconductor non-uniform quantum wells (NUQWs) improve long-wavelength infrared detection by resolving the trade-off between dark current and responsivity found in conventional uniform structures. By grading doping and barrier thicknesses, these designs enhance absorption in highly doped areas while strengthening electric fields in low-doped regions to trigger carrier multiplication. However, the actual carrier concentration profile within quantum wells (QWs) has long lacked effective experimental verification. In this work, we employed calibrated scanning spreading resistance microscopy (SSRM) to realize nanoscale quantitative analysis of the carrier distribution within GaAs/AlGaAs quantum well infrared photodetector (QWIP) structures. The non-uniform design successfully led to reduction of dark current by nearly 4 times compared to that of the uniform counterparts, thus elevating the background limited infrared photodetection (BLIP) temperature by 4 K. On the other hand, the responsivity failed to show the expected enhancement. Simulations revealed that the deviation of electron densities in the low-doped QWs induces a weak local electric field of approximately 1.7 kV cm^-1, which is significantly below the threshold field to trigger impact ionization. These findings guide the design of an optimized asymmetric gradient-doping structure capable of generating a strong electric field of up to 13.8 kV cm^-1 in the collection region, thereby achieving high gain while maintaining low dark current.