Amorphous Silicon Characterization

Method 1: Tauc Plot

Procedure

1. Plot (hν × α(hν))^1/2 on y-axis
2. Plot photon energy hν on x-axis
3. Identify linear region of the plot
4. Extrapolate linear region to x-axis
5. 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

1. Obtain absorption coefficient vs photon energy data
2. Identify photon energy where α = 3 × 10^3 cm^-1
3. 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

Material Quality Indicators

• Smaller band tails → better electronic quality
• EC increases in a-SiGe alloys (degraded transport)
• EV dominates hole transport limitation3b:["$","$L43",null,{"content":"$44","frontMatter":{"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."}}]