Amorphous semiconductors, such as hydrogenated amorphous silicon (a-Si:H) and chalcogenide glasses, exhibit unique defect structures that differ fundamentally from those in crystalline materials. Unlike crystalline semiconductors, where defects arise primarily from deviations in periodic lattice arrangements, amorphous semiconductors possess intrinsic disorder that leads to distinct defect types and behaviors. Key defects in amorphous semiconductors include dangling bonds, valence alternation pairs (VAPs), and light-induced metastable defects, as exemplified by the Staebler-Wronski effect. These defects significantly influence electronic, optical, and stability properties, making their study critical for applications in photovoltaics, memory devices, and thin-film electronics.

In amorphous silicon, dangling bonds are among the most prevalent defects. These occur when a silicon atom lacks the required number of bonds to neighboring atoms due to the disordered network. In a perfect tetrahedral arrangement, each silicon atom forms four bonds, but in amorphous silicon, some atoms may form only three bonds, leaving one unsatisfied valence electron. These dangling bonds introduce electronic states within the bandgap, acting as recombination centers that degrade carrier mobility and lifetime. Hydrogen passivation is commonly employed to mitigate these defects, as hydrogen atoms bond with the dangling orbitals, reducing their electronic activity. The density of dangling bonds in a-Si:H typically ranges from 10^15 to 10^17 cm^-3, depending on deposition conditions and hydrogen content.

Chalcogenide glasses, such as those based on arsenic sulfide (As2S3) or selenium (Se), exhibit a different class of defects known as valence alternation pairs (VAPs). These defects arise from the flexibility of chalcogen atoms to adopt multiple coordination states. For instance, a sulfur atom in As2S3 may exist in a two-fold coordinated state (normal bonding) or transition into a one-fold or three-fold coordinated state under strain or illumination. The formation of positively charged three-fold coordinated chalcogen atoms (C3+) and negatively charged one-fold coordinated atoms (C1-) constitutes a VAP. These defects create localized states within the bandgap, influencing optical absorption and electronic transport. VAPs are also responsible for reversible structural changes exploited in phase-change memory devices.

The Staebler-Wronski effect is a metastable defect phenomenon observed in hydrogenated amorphous silicon under prolonged light exposure. It manifests as a reversible increase in dangling bond density, leading to a degradation of solar cell efficiency. The exact mechanism remains debated, but prevailing models suggest that light-induced breaking of weak Si-Si bonds or hydrogen diffusion plays a role. The defect density can increase by an order of magnitude under illumination, from 10^16 cm^-3 to 10^17 cm^-3, but annealing at temperatures around 150-200°C restores the initial state. This effect has profound implications for the long-term stability of a-Si:H-based photovoltaic devices.

A key distinction between defects in amorphous and crystalline semiconductors lies in their formation and electronic impact. In crystalline materials, defects such as vacancies, interstitials, and dislocations are well-defined and often predictable in their behavior due to the periodic lattice. In contrast, amorphous semiconductors lack long-range order, leading to a broader distribution of defect energies and configurations. The absence of translational symmetry means that defects in amorphous materials are not isolated perturbations but part of a continuous distribution of localized states. This results in band tails rather than discrete mid-gap states, complicating defect characterization and passivation strategies.

Thermodynamic stability also differs between amorphous and crystalline defects. In crystals, defects often follow well-defined formation energies, whereas amorphous networks exhibit a wider range of metastable configurations. For example, VAPs in chalcogenides can switch between coordination states with relatively low energy barriers, enabling applications in resistive switching devices. Similarly, the Staebler-Wronski effect highlights the metastable nature of light-induced defects in a-Si:H, where defect creation and annihilation depend on external stimuli rather than equilibrium thermodynamics.

Defects in amorphous semiconductors also exhibit unique interactions with dopants. In crystalline silicon, dopants such as phosphorus or boron substitute lattice sites predictably, altering carrier concentrations. In amorphous silicon, dopant incorporation is less uniform due to the disordered network, and dangling bonds can compensate dopant-induced carriers. This self-compensation effect limits doping efficiency and necessitates alternative approaches, such as defect passivation or modified deposition techniques.

The role of defects in electronic transport is another differentiating factor. In crystalline semiconductors, carrier mobility is primarily limited by phonon scattering or ionized impurities, with defects playing a secondary role at low concentrations. In amorphous semiconductors, defects dominate transport by trapping and releasing charge carriers, leading to dispersive conduction mechanisms. The mobility-lifetime product, a critical parameter for photovoltaic applications, is heavily influenced by defect density and energy distribution.

Optical properties of amorphous semiconductors are similarly affected by defects. Urbach tails, representing exponential absorption edges, arise from the disorder-induced distribution of electronic states. Defects contribute to sub-bandgap absorption, reducing the efficiency of optoelectronic devices. In chalcogenides, defect-related optical changes are exploited for holographic recording and optical switching, where light-induced defect generation alters refractive indices.

Efforts to mitigate defects in amorphous semiconductors have led to advanced passivation and alloying strategies. For a-Si:H, hydrogen dilution during deposition reduces dangling bond densities by promoting more ordered microstructure. In chalcogenides, compositional engineering with elements like germanium or tellurium can suppress VAP formation. Despite these advances, achieving defect-free amorphous materials remains challenging due to their inherent disorder.

Understanding defects in amorphous semiconductors is essential for optimizing their performance in applications ranging from solar cells to non-volatile memory. While crystalline semiconductors benefit from decades of defect engineering, amorphous materials require tailored approaches that account for their unique structural and electronic characteristics. Future research may focus on novel defect characterization techniques, advanced computational modeling, and innovative passivation methods to further unlock the potential of these disordered yet versatile materials.