Surface passivation is a critical technique in semiconductor technology aimed at reducing defect states at surfaces and interfaces, thereby improving electronic performance. Unpassivated surfaces often exhibit dangling bonds and trap states that degrade carrier lifetimes, increase recombination, and impair device efficiency. Effective passivation methods include chemical passivation, dielectric capping, and field-effect passivation, each tailored to specific material systems and applications such as solar cells and metal-oxide-semiconductor field-effect transistors (MOSFETs).

Chemical passivation involves terminating dangling bonds with atoms or molecules that neutralize electronic defects. Hydrogenation is one of the most widely used methods, particularly for silicon surfaces. Atomic hydrogen diffuses into the silicon lattice, saturating dangling bonds at the surface and within grain boundaries. This process reduces interface trap densities from above 1e12 cm⁻² eV⁻¹ to below 1e10 cm⁻² eV⁻¹, significantly enhancing minority carrier lifetimes. Hydrogenation is routinely employed in silicon solar cells, where surface recombination velocities can be reduced from over 1e6 cm/s to below 10 cm/s after treatment.

Sulfur passivation is another chemical method, particularly effective for III-V semiconductors like GaAs and InP. Thiol-based solutions or (NH₄)₂S treatments form stable sulfur bonds with surface atoms, reducing mid-gap states. For GaAs, sulfur passivation can lower surface recombination velocities by an order of magnitude, improving photoluminescence (PL) intensity by up to 100 times. However, sulfur layers are often unstable in ambient conditions, necessitating additional dielectric capping for long-term stability.

Dielectric capping involves depositing thin insulating layers that both chemically passivate defects and provide a physical barrier against contamination. Silicon dioxide (SiO₂) is the most common passivation layer for silicon, grown thermally or via plasma-enhanced chemical vapor deposition (PECVD). High-quality SiO₂ reduces interface trap densities to below 1e10 cm⁻² eV⁻¹, crucial for MOSFET gate oxides. However, SiO₂ may introduce fixed charges that affect flat-band voltage, requiring precise thickness control.

Aluminum oxide (Al₂O₃) has emerged as a superior alternative, especially for crystalline silicon solar cells. Deposited via atomic layer deposition (ALD), Al₂O₃ provides excellent chemical passivation due to its high fixed negative charge density (~1e13 cm⁻²), which induces field-effect passivation by repelling minority carriers from the surface. This dual mechanism enables surface recombination velocities below 5 cm/s, making Al₂O₃ a key enabler of high-efficiency passivated emitter and rear contact (PERC) solar cells.

Field-effect passivation leverages electrostatic screening to minimize carrier recombination without direct chemical bonding. It is achieved by introducing fixed charges in dielectric layers or via external bias. In silicon solar cells, Al₂O₃’s negative charges create an accumulation layer for holes, reducing electron recombination. For n-type silicon, positively charged layers like silicon nitride (SiNₓ) are used instead. Field-effect passivation is also critical in MOSFETs, where gate dielectrics must suppress interface traps while maintaining low leakage currents.

Applications in solar cells highlight the importance of passivation. In crystalline silicon photovoltaics, rear-side passivation with Al₂O₃ or SiO₂/SiNₓ stacks has pushed efficiencies above 24%. For perovskite solar cells, surface passivation is even more critical due to their high defect densities. Lewis base molecules like thiophene or pyridine can coordinate with undercoordinated lead atoms, reducing non-radiative recombination and improving open-circuit voltages. Dielectric layers like Al₂O₃ or TiO₂ are also explored, though their deposition must avoid damaging the perovskite lattice.

In MOSFETs, passivation directly impacts threshold voltage stability and channel mobility. High-k dielectrics like hafnium oxide (HfO₂) are used alongside interfacial SiO₂ layers to reduce defect densities while maintaining gate control. For GaN-based high-electron-mobility transistors (HEMTs), silicon nitride (SiNₓ) passivation suppresses surface traps that otherwise cause current collapse, enabling stable high-frequency operation.

Characterization techniques are essential for evaluating passivation quality. Photoluminescence (PL) spectroscopy measures radiative recombination efficiency, with higher PL intensity indicating lower non-radiative losses. For silicon, effective carrier lifetimes exceeding 1 ms signify excellent passivation. Deep-level transient spectroscopy (DLTS) quantifies trap densities and energy levels, distinguishing between bulk and interface defects. In MOSFETs, capacitance-voltage (C-V) profiling reveals interface trap densities, while conductance methods assess their time-dependent response.

Despite its successes, surface passivation faces challenges. Hydrogen passivation in silicon can degrade at elevated temperatures, requiring robust capping layers. For compound semiconductors like GaAs, achieving both low interface traps and minimal Fermi-level pinning remains difficult. In perovskites, passivation layers must not impede charge extraction, necessitating ultra-thin or selectively conductive coatings.

Future developments may explore hybrid passivation schemes combining chemical, dielectric, and field-effect mechanisms. Atomic-scale engineering using machine learning could optimize passivant selection and deposition parameters. Advances in in-situ characterization will further elucidate passivation dynamics, enabling real-time process control.

In summary, surface passivation is indispensable for optimizing semiconductor device performance. By mitigating defect states through chemical, dielectric, and field-effect strategies, it enhances efficiency in solar cells, reliability in MOSFETs, and functionality across emerging technologies. Continued innovation in passivation methods will be pivotal for next-generation electronic and optoelectronic applications.