Experimental and analytical investigation of incomplete ionization in p-Si/n-CdS heterojunctions at cryogenic temperatures

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Original abstract

The interplay between temperature-dependent band alignment and incomplete dopant ionization in p-Si/n-CdS heterojunctions was comprehensively investigated through a synergy of experimental characterization and advanced analytical modeling. Polycrystalline CdS thin films with a nominal thickness of approximately 3 μm were synthesized, exhibiting a well-defined direct optical bandgap of 2.42 eV, structured grain sizes ranging from 90 to 110 nm, a high surface coverage of 95÷97%, and a near-stoichiometric chemical composition. These optimized structural attributes provided high-quality interfaces essential for efficient interfacial charge transport. Current–voltage (I-V) characteristics revealed thermionic carrier injection mechanisms at low forward bias, demonstrating a strong dependency of macroscopic transport properties on the CdS layer thickness, substrate doping profiles, and post-growth ultrasonic processing. Under controlled 0.1 lux illumination, ultrasonic treatment significantly passivated interface trap states, enhancing the reverse photocurrent to 3.8∙10 6 A and yielding a substantial photocurrent gain spanning 3–35 times, complemented by an effective ambipolar mobility of 2.88 cm 2 V −1 s −1 . Comprehensive temperature-dependent analysis evaluated using advanced bandgap models showed that the dynamic Si—CdS energy bandgap difference narrows from 1.3889 eV at cryogenic temperatures (20 K) to 1.2109 eV at elevated thermal regimes (800 K), signaling a progressive reduction in heterojunction band offsets and carrier injection barriers. To quantify charge storage dynamics, a comprehensive analytical capacitance model explicitly incorporating incomplete impurity ionization was established and validated via capacitance–voltage (C-V) measurements. Elevating the nominal substrate dopant concentration from 1∙10 16 –1∙10 17 cm −3 expanded the built-in potential from 0.55 to 0.73 V and scaled the junction capacitance more than threefold. Conversely, severe dopant freeze-out governed by deep-level activation energies at 20 K suppressed the junction capacitance by approximately 45–50%, whereas thermal deviations from conventional full-ionization approximations remained strictly below 5% approaching room temperature. These insights isolate incomplete ionization as a dominant mechanism governing electrostatic charge redistribution in wide-temperature heterostructures, offering a rigorous quantitative framework for engineering high-performance low-temperature photodetectors, space-borne photovoltaics, and cryo-electronics.

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