cdscdte-heterojunction-analysis

# CdS/CdTe Heterojunction Analysis ## When to Use - Analyzing field distribution in CdS side of CdS/CdTe junctions - Modeling field quenching effects in copper-doped CdS - Understanding photoconductivity changes under bias - Predicting band alignment changes at heterojunction interface - Analyzing Schottky barrier behavior in CdS ## Prerequisites - Trap density in CdS (typically ~10^17 cm^-3) - Copper doping concentration (if applicable) - Bias conditions - Optical excitation level ## Procedure ### 1. Calculate Linear Field Distribution For first approximation in CdS: - Field increases linearly from bulk value to junction interface - Slope relates directly to trap density - Similar to Schottky barrier behavior Field profile: ``` F(x) = F_bulk + (F_max - F_bulk) * (x/d) ``` Where: - F_bulk: Field in bulk CdS - F_max: Maximum field at junction interface - d: CdS layer thickness ### 2. Determine Bias-Dependent Field Values | Bias Condition | Typical Field at Junction | |----------------|---------------------------| | Forward bias | ~20 kV/cm | | Near open circuit | Rapidly increasing | | Reverse bias | Can reach tunneling values | ### 3. Identify Field Quenching Onset **Critical threshold**: F ≥ 23 kV/cm At this field: - Barrier lowering δE = 2kT achieved - Holes released from slow copper recombination centers - Field quenching process initiates ### 4. Model Field Quenching Cascade When field reaches threshold: 1. **Initiation** (at 23 kV/cm): - Frenkel-Poole excitation releases trapped holes - Holes freed from Coulomb-attractive copper centers 2. **Cascade effect**: - Released holes increase valence band hole density - Holes captured by fast recombination centers - Enhanced recombination with conduction electrons 3. **Progressive quenching** (above 50 kV/cm): - Electron density markedly reduced - Photoconductivity quenched ### 5. Analyze Band Alignment Changes Field quenching consequences: - Distance between Fermi level and conduction band increases - Conduction band at interface moves up with changing bias - Band discontinuity develops (apparent electron affinity change) - Measured work function change: ~0.25 eV at Au contact ### 6. Predict Junction Behavior **Forward bias to open circuit**: - Conduction bands connected initially - Gradual separation develops - Field quenching begins affecting carrier density **Reverse bias**: - Significant band separation - Field quenching pronounced - Electron density substantially reduced at interface ## Output - Field distribution profile F(x) - Field quenching threshold status - Modified electron density at interface - Band alignment changes ## Constraints - Requires Coulomb-attractive hole centers for field quenching - Linear approximation does not account for field quenching at high fields - Density of copper centers must be sufficient for observable effect ## Key Parameters | Parameter | Typical Value | |-----------|---------------| | Trap density | 10^17 cm^-3 | | Field quenching onset | 23 kV/cm | | Marked quenching | >50 kV/cm | | Work function change | 0.25 eV | | Photoconductive electron density | ~10^18 cm^-3 (under optical excitation) |