Deep-level transient spectroscopy (DLTS) is a critical tool for investigating defects and traps in semiconductors, particularly in III-V and II-VI materials. These compound semiconductors exhibit unique defect behaviors due to their complex crystal structures and bonding characteristics. The technique’s high sensitivity to deep-level traps makes it indispensable for studying intrinsic defects like antisites and vacancies, as well as extrinsic dopant-related traps that influence carrier recombination and transport.

In III-V semiconductors such as GaAs and InP, intrinsic defects often dominate the trap spectra. GaAs, for instance, exhibits prominent arsenic antisites (As_Ga) and gallium vacancies (V_Ga), which introduce deep levels within the bandgap. DLTS studies reveal these defects as majority carrier traps with distinct emission signatures. As_Ga typically produces a peak near 0.5 eV below the conduction band, while V_Ga introduces multiple levels between 0.3 eV and 0.7 eV above the valence band. InP, on the other hand, shows phosphorus vacancies (V_P) and indium antisites (In_P) as key intrinsic defects. The Fermi-level pinning effect in III-V materials complicates defect analysis, as surface states and bulk defects interact to stabilize the Fermi level near mid-gap, masking some trap signatures.

Extrinsic dopant-related traps in III-V materials arise from intentional doping or unintentional impurities. Silicon, a common n-type dopant in GaAs, can form complexes with vacancies, creating Si_Ga-V_As defects detectable via DLTS. These complexes introduce levels at 0.1–0.3 eV below the conduction band, affecting carrier lifetimes. In p-type GaAs, carbon doping may lead to C_As-related traps, while transition metals like iron produce deep acceptors near Ev + 0.5 eV. The high defect densities in III-V materials, often exceeding 1e15 cm^-3, necessitate careful DLTS measurements to deconvolute overlapping peaks.

II-VI semiconductors, including CdTe and ZnO, present different challenges due to their ionic bonding and propensity for stoichiometric deviations. CdTe exhibits cadmium vacancies (V_Cd) and tellurium antisites (Te_Cd) as primary intrinsic defects. DLTS spectra of V_Cd show a prominent hole trap at Ev + 0.4 eV, while Te_Cd introduces deeper levels near Ev + 0.7 eV. The high mobility of Cd vacancies at room temperature complicates measurements, requiring low-temperature DLTS to capture their true concentrations. ZnO, with its wide bandgap, displays zinc vacancies (V_Zn) and oxygen antisites (O_Zn) as dominant intrinsic defects. V_Zn creates acceptor levels at Ev + 0.3 eV and Ev + 0.5 eV, while O_Zn acts as a deep donor near Ec - 1.0 eV.

Extrinsic defects in II-VI materials often involve group III or VII dopants. In CdTe, chlorine substituting for tellurium (Cl_Te) forms a shallow donor, but complexes like Cl_Te-V_Cd produce deep levels detectable by DLTS. ZnO doped with aluminum (Al_Zn) shows similar behavior, where Al_Zn-V_O complexes introduce traps near Ec - 0.5 eV. The high native defect densities in II-VI materials, sometimes exceeding 1e16 cm^-3, require DLTS to operate at high resolution to distinguish between closely spaced energy levels.

Material-specific challenges influence DLTS measurements in these systems. Fermi-level pinning in III-V materials can obscure defect signatures by saturating trap states, necessitating variable-temperature and filling-pulse techniques to isolate contributions. In II-VI materials, the high ionicity leads to defect metastability, where charge states switch under electric fields or illumination, complicating DLTS interpretation. Additionally, the high defect densities in both material classes demand careful calibration of the DLTS system to avoid signal saturation and ensure accurate trap concentration calculations.

The following table summarizes key defects and their DLTS signatures in III-V and II-VI semiconductors:

| Material | Defect Type | DLTS Peak (eV) | Trap Character |
|----------|------------|----------------|----------------|
| GaAs | As_Ga | Ec - 0.5 | Electron trap |
| GaAs | V_Ga | Ev + 0.3–0.7 | Hole trap |
| InP | In_P | Ec - 0.4 | Electron trap |
| InP | V_P | Ev + 0.2 | Hole trap |
| CdTe | V_Cd | Ev + 0.4 | Hole trap |
| CdTe | Te_Cd | Ev + 0.7 | Hole trap |
| ZnO | V_Zn | Ev + 0.3–0.5 | Hole trap |
| ZnO | O_Zn | Ec - 1.0 | Electron trap |

DLTS analysis must account for these material-specific behaviors to yield accurate defect profiles. In III-V materials, the technique helps identify traps that limit minority carrier lifetimes, while in II-VI systems, it reveals defects influencing conductivity and stability. Future advancements in DLTS instrumentation, such as high-speed digitizers and improved signal processing, will enhance its capability to resolve complex defect landscapes in these semiconductors.

The role of DLTS extends beyond defect identification; it provides quantitative data on trap concentrations and capture cross-sections, essential for material optimization. For example, in GaAs, reducing As_Ga concentrations below 1e14 cm^-3 improves device performance, while in CdTe, controlling V_Cd densities is critical for photovoltaic applications. The technique’s ability to probe both intrinsic and extrinsic defects makes it invaluable for advancing III-V and II-VI semiconductor technologies, despite the challenges posed by Fermi-level pinning and high defect densities.

In conclusion, DLTS remains a cornerstone technique for defect characterization in compound semiconductors. Its application to III-V and II-VI materials has unveiled critical insights into defect energetics and dynamics, guiding efforts to improve material quality and device performance. Continued refinement of DLTS methodologies will further solidify its role in semiconductor research and development.