| name | Amorphous Silicon Characterization |
| description | Determine optical band gap and band tail characteristics in amorphous silicon and its alloys using Tauc plot, E04 method, and mobility edge analysis. Use when characterizing a-Si:H materials, analyzing Urbach tails, or determining electronic structure parameters for amorphous semiconductor devices. |
Amorphous Silicon Characterization
When to Use
- Measuring optical band gap of a-Si:H or a-Si alloys
- Analyzing band tail widths (Urbach tails)
- Determining mobility edges in amorphous semiconductors
- Characterizing material quality for device applications
- Comparing electrical vs optical band gaps
Prerequisites
- Optical absorption coefficient data α(hν)
- Photon energy data (hν)
- Understanding of band structure concepts
Key Concepts
Band Tails
Localized states extending into the band gap due to disorder.
Mobility Edges
Energy separating localized from delocalized states.
Optical vs Electrical Band Gap
- Optical (Tauc): From absorption measurements
- Electrical: From conductivity or internal photoemission
- Electrical gap is typically 50-100 meV larger
Method 1: Tauc Plot
Procedure
- Plot (hν × α(hν))^1/2 on y-axis
- Plot photon energy hν on x-axis
- Identify linear region of the plot
- Extrapolate linear region to x-axis
- X-intercept = Tauc optical band gap (ET)
Formula Basis
(αhν)^1/2 = B(hν - ET)
Where B incorporates transition matrix elements.
Notes
- Most widely used method
- Assumes parabolic band edges
- Proportionality constant B not usually studied separately
Method 2: E04 Method
Procedure
- Obtain absorption coefficient vs photon energy data
- Identify photon energy where α = 3 × 10^3 cm^-1
- This energy value = E04
Advantages
- Simpler than Tauc analysis
- No extrapolation required
- Useful for comparative studies
Band Tail Analysis
Valence Band Tail (EV)
- Described as 'Urbach' tail of spectrum
- Typical value for a-Si:H: EV = 50 meV
- Contributes to low hole mobility
Conduction Band Tail (EC)
- Smaller than valence band tail
- Best a-Si:H: EC ≈ 22 meV
- Increases markedly for a-SiGe alloys
Mobility Edge Definition
- Energy separating localized from delocalized electrons
- Referred to as conduction and valence band mobility edges (Mott 1987)
- Differs slightly from optical band gap
Procedure Summary
Complete Characterization
Step 1: Obtain Optical Data
- Measure absorption coefficient α(hν)
- Cover energy range around expected band gap
Step 2: Determine Optical Band Gap
- Apply Tauc plot method for ET
- Apply E04 method for comparison
Step 3: Analyze Band Tails
- Extract Urbach tail energy from low-α region
- Compare EV and EC values
Step 4: Relate to Electrical Properties
- Note that electrical gap > Tauc gap by 50-100 meV
- Use internal photoemission for electrical gap if needed
Typical Values for a-Si:H
| Parameter | Value | Notes |
|---|
| ET (Tauc) | 1.7-1.8 eV | Deposition dependent |
| E04 | ~1.6 eV | Lower than Tauc |
| EV | 50 meV | Urbach tail |
| EC | 22 meV | Best material |
| Electrical gap | 1.8-1.9 eV | 50-100 meV > ET |
Material Quality Indicators
- Smaller band tails → better electronic quality
- EC increases in a-SiGe alloys (degraded transport)
- EV dominates hole transport limitation
