DOE Solar Energy Technologies Program Peer Review. Denver, Colorado April 17-19, 2007

Transcription

1 DOE Solar Energy Technologies Program Peer Review Evaluation of Nanocrystalline Silicon Thin Film by Near-Field Scanning Optical Microscopy AAT Magnus Wagener and George Rozgonyi North Carolina State University Denver, Colorado April 17-19, 2007

2 OUTLINE Introduction Principle of near-field microscopy Experimental setup Example: Nanocrystalline silicon thin film Future work Near-field microscope development Approach using well controlled defect structures

3 Relevance/Objective Optimization of Silicon Crystal Growth and Wafer Processing for High Efficiency and High Mechanical Yield by developing methodologies for the evaluation of performance limiting defects. Develop near-field scanning optical microscopy for the characterization of nano-crystalline thin film PV systems.

4 Principle of near-field microscopy Resolution limit Far-field optical microscope Airy s disks Resolved Rayleigh Criterior Unresolved Rayleigh criterion: Δr = 0.6 λ n sin θ λ = 500nm, Δr ~ 320nm

5 Principle of near-field microscopy Near-field: definition Source Low spatial frequency High spatial frequency 10nm Distance is a low-pass filter

6 Principle of near-field microscopy Near-field optical microscope Far-field Z Illumination ~ λ Near-field Subwavelength aperture x detector Synge, Phylos. Mag. 6, 356 (1928)

7 Principle of near-field microscopy Near-field scanning optical microscope Z (NSOM) glass fiber probe Far-field Al-coating ~ λ Near-field Illumination detector x piezo

8 Experimental Sheer-Force Feedback System sample Tuning fork probe Amplitude (V) 7x10-3 6x10-3 5x10-3 4x10-3 3x10-3 2x10-3 1x10-3 piezo 2 mm Frequency (khz)

9 Experimental NSOM probe Scattered light detector I n-si sc-region p-type Si Imaging Modes Surface morphology Photocurrent map Scattered light map

10 Accomplishments Nano-Crystalline Silicon Thin Film Hot-Wire CVD 5.5 µm cm -3 n-type HWCVD cm -3 p-type CZ wafer Nano-Crystalline thin film samples were prepared by California Institute of Technology

11 Accomplishments SILICON THIN FILM (Hot-Wire CVD) Planar SEM surface Growth direction 1 µm Planar SEM Cross-section TEM Growth is predominantly polycrystalline, resulting in rough surface texture. Hydrogen passivation of grain-boundaries is vital: (IQE, shunt ) require low growth and device processing temperatures (dopant activation ) Images provided by C. E. Richardson, California Institute of Technology

12 Accomplishments Near-Field Imaging of Crystalline Silicon Thin Film Topography Photocurrent Reflectance 500 nm Max height = 320 nm Current contrast 5% Reflection contrast 50% Near-Field excitation not maintained between grains Wagener, ECS Transactions 3 (2006) p.365

13 Accomplishments NSOM OF BEVELED DEVICE The bevel structure dramatically improves the photocurrent contrast, and also complements the near-surface sensitivity of NSOM, making depth dependent imaging possible. 0.5 Wagener, ECS Transactions 3 (2006) p.365

14 Accomplishments near-field far-field 2.20 Near-field Far-field 2.15 NPC (na) Scan direction 500 nm Position (μm) NSOM probe Near-field imaging (~10 nm) sensitive to near-surface region (less than λ = 632 nm). ~10 nm Near-Field 400 nm

15 Accomplishments 500 nm 250 nm Photocurrent contrast 5% EBIC 100 µm

16 Schedule of Major Events/Milestones Constructed a Near-Field Scanning Optical Microscope (yr. 1). 1 µm Profiled the collection efficiency within dislocation clusters in cast silicon. Extended NSOM analysis to nano-crystalline silicon thin film grown by hot-wire CVD (yr. 2). 250 nm

17 Future Directions Future Plans (NSOM) Incorporate additional modes, i.e. - Carrier Lifetime Mapping - Nano-Raman Spectroscopy - Photoluminescence

18 Summary of Activities A Near-field Scanning Optical Microscope (NSOM) has been developed for the evaluation of performance limiting defects that require a high degree of spatial resolution. NSOM provides a means of evaluating grain-to-grain variations in the collection efficiency, as well as the extent by which grain-boundaries limit device performance in nano-crystalline materials.

19 Budget History Project Task(s) Total Value $99,500 $80,000 $83, $65, $71,000 Grand Total $399,000 5 year total