ShaneLogic/CdS/CdTe 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) |