amorphous-silicon-characterization

# 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 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 | 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