Study on interface electrical properties and carrier transport behavior of long-wave p-on-n HgCdTe

Objective The p-on-n double-layer heterojunction mercury cadmium telluride (HgCdTe) infrared detector exhibits low dark current and high R0A, demonstrating unique advantages in long-wave infrared detection applications. While numerous analytical studies on p-on-n double-layer heterojunction devices have been conducted internationally, reports on long-wave focal plane arrays (FPAs) based on this structure remain limited in domestic research. The electrical properties of the HgCdTe/passivation layer interface and the carrier transport behavior in HgCdTe significantly influence the performance of p-on-n HgCdTe infrared detectors. Currently, there is a scarcity of evaluation methods for the electrical characteristics of p-on-n HgCdTe detectors, coupled with insufficient in-depth investigation into the interfacial electrical behavior between HgCdTe and passivation layers, as well as the carrier transport mechanisms in HgCdTe bulk materials. The passivation and annealing techniques employed during the fabrication of p-on-n double-layer hetero-mesa junctions play a critical role in reducing device dark current and suppressing defective pixels, warranting prioritized research efforts.Methods This study systematically investigates the electrical characteristics of the interface between passivation layers and HgCdTe by fabricating metal-insulator-semiconductor (MIS) structures under different passivation conditions and characterizing them through dynamic capacitance-voltage (C-V) measurements (Fig.1). Additionally, to address the limitations of conventional HgCdTe Hall devices - including significant measurement errors and inadequate characterization of carrier transport properties due to multi-carrier effects-a novel Hall bar device was developed (Fig.2). This customized design enables measurement of magnetic-field-dependent carrier mobility spectra under different annealing conditions, facilitating calculation of carrier concentration, mobility, and conductivity contributions for annealing optimization. Finally, focal plane array (FPA) performance was evaluated for devices fabricated with optimized passivation and annealing processes.Results and Discussions Ellipsometer, Atomic Force Microscope(AFM) and Scanning electron microscopy (SEM) results demonstrate that after passivation optimization, the number of micro-voids between the passivation layer and HgCdTe is significantly reduced, grain boundary defects are diminished, and substantial improvements are observed in the compactness, uniformity, and mesa coverage of the passivation layer (Figs.3-5). C-V measurements reveal that optimized passivation conditions effectively improve the electrical state of the HgCdTe/passivation interface, reducing the fixed interface charge density from 2.8×1011 cm−2 to 9.4×1010 cm−2 and the slow interface state density from 4.0×1010 cm−2 to 3×1010 cm−2 (Fig.6). I-V characterization indicates that passivation optimization significantly enhances the reverse-bias characteristics of the device, leading to reduced dark current and improved diode performance (Fig.7). Multi-carrier mobility spectrum analysis shows that annealing optimization markedly improves electron mobility, increasing from the order of 104 cm2/V·s to 105 cm2/V·s, which effectively suppresses carrier recombination and enhances the performance of long-wave infrared detectors (Fig.8). Focal plane array (FPA) testing results confirm that the long-wave infrared FPA exhibits uniform response and good device performance at 77 K. The device dark current reaches 1.8 pA, with a corresponding dark current density of 2.9×10−7 A/cm2, which is generally consistent with the empirical Rule 22 prediction (Fig.11).Conclusions Through the optimization of passivation conditions, the mesa coverage capability and compactness of the passivation layer are significantly improved, while the number of voids in the passivation layer is markedly reduced. The C-V characteristics of MIS devices demonstrate that the optimized passivation process effectively improves the electrical properties of the HgCdTe/passivation interface, reducing the fixed interface charge density from 2.8×1011 cm−2 to 9.4×1010 cm−2 and the slow interface state density from 4.0×1010 cm−2 to 3×1010 cm−2. Furthermore, the optimized passivation layer effectively suppresses surface leakage current and improves the reverse-bias characteristics of the devices. In addition, a novel Hall bar-structured device was designed in this work to characterize and analyze the magnetic-field-dependent carrier mobility spectra under different annealing conditions. Through annealing optimization, high-quality electron transport performance with mobility up to 105 cm2/V·s was achieved. Finally, by combining the optimized passivation and annealing processes, a p-on-n long-wave double-layer hetero-mesa junction device was successfully fabricated. Under 77 K operation, the device exhibits a dark current of 1.8 pA and a dark current density of 2.9×10−7A/cm2, which aligns well with the theoretical prediction of Rule 22. This achievement establishes a solid foundation for the development of high-performance HgCdTe infrared detectors.