Localized Characterization of GaAs/AlGaAs Quantum Well Devices
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Localized Characterization of GaAs/AlGaAs Quantum Well Devices

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two-dimensional mapping, at pixel resolution, of semiconductor devices in terms of efficiency, electric field distribution, quantum well effects

two-dimensional mapping, at pixel resolution, of semiconductor devices in terms of efficiency, electric field distribution, quantum well effects

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  • Emission testing judges the nature of the sample only in two ways — either good or bad—without any other explanation
  • QE depends on the front surface reflectance of detector, optical absorption coefficient of the material (dependent on incident wavelength λ, t emperature T , the collection efficiencies of GaAs at the air/GaAs interface, and the length of the depletion region
  • QE depends on the front surface reflectance of detector, optical absorption coefficient of the material (dependent on incident wavelength λ, t emperature T , the collection efficiencies of GaAs at the air/GaAs interface, and the length of the depletion region
  • Enlarge diagram Define WW and NW
  • Slope as V changes
  • There is a general current increase with voltage in all areas
  • P33 average ~ 52% ± 3% P254 average ~ 79% ± 2% Draw ideal rectangle
  • ( dependent on t emperature T , the collection efficiencies of GaAs at the air/GaAs interface, the length of the depletion region)

Transcript

  • 1. LOCALIZED CHARACTERIZATION OF GaAs/AlGaAs QUANTUM WELL DEVICES Imee Rose Tagaca 25 March 2008 Adviser: Dr. Arnel Salvador Condensed Matter Physics Laboratory National Institute of Physics University of the Philippines-Diliman
  • 2. Introduction. TYPICAL LIFE CYCLE OF A p-i-n LAYER AT CMPL MBE growth Primary Characterization Device Fabrication Secondary Characterization or GOOD sample! Scenario 1 Scenario 2 BAD sample! strong emission weak emission no emission
  • 3.  
  • 4. Objectives of the Thesis
    • To find other means of evaluating the quality of a sample (good vs. bad) apart from using optical emission and therefore to explain the performance of the diodes
    • To know what is happening locally in the device while it is in operation (in terms of efficiency, electric field distribution, quantum well effects)
  • 5. Outline
    • Theory
      • Photocurrent
      • What makes a good device? QE*FF*ECE
      • Electric field effects in CQW
    • Methodology
      • SQW and CQW wafers
      • Device Fabrication
      • Characterization
    • Results
      • SQW diodes: spectra, I-V, images
      • CQW diodes: spectra, images
    • Abstract
  • 6. THEORY
      • Photocurrent
      • What makes a good device? QE*FF*ECE
      • Electric field effects in CQW
  • 7. P-I-N diode photocurrent P N I conduction band valence band - - +
  • 8. Theory. What makes a good device?
    • Quantum Efficiency η QE (photodetector)
    • - is the number of carriers (electron-hole pairs) that are generated in the depletion region per incident photon. [13, 17, 18]
    • Fill Factor FF (solar cell)
    • - describes the congruence of the I-V measurements to the ideal “square” I-V curve. [6]
    • Energy Conversion Efficiency η ECE (solar cell)
    • - is the percentage of power converted from absorbed light to electrical energy. [6]
  • 9. Theory. What makes a good device? QE*FF*ECE
    • Quantum Efficiency η QE (photodetector)
    • Fill Factor FF (solar cell)
    • Energy Conversion Efficiency η ECE (solar cell)
    Definitions I ph = photocurrent I rev = reverse current I 0 = dark current Λ = illumination wavelength P inc = incident illumination power P max = maximum diode output power I m = current at P max V m = voltage at P max I sc = short-circuit current V oc = open-circuit voltage h, c, e  universal constants
  • 10. Theory. Electric field effects in Coupled Quantum Well Unbiased V bias = 0 Resonance V bias = -V res beyond Resonance V bias > І - V res І NW = narrow well WW = wide well P N I conduction band valence band N WW NW
  • 11. METHODOLOGY
      • SQW and CQW recipe
      • Device Fabrication
      • Characterization
        • Photocurrent Spectroscopy
        • Optical-Beam Induced Current Imaging
  • 12. Methodology. SQW and CQW recipe P33 and P254 with 90Ǻ QW P295 with 110Ǻ/25Ǻ/50Ǻ CQW
  • 13. Methodology. Device Fabrication (old design). Apply photoresist (PR) After UV lithography and PR development
  • 14. Methodology. Device Fabrication (old design). After mesa etching and PR removal Apply PR After UV lithography and PR development Deposit metal contact After PR removal and metal lift-off
  • 15. Methodology. Device Fabrication (old design). SEM Mounting: needle probes
  • 16. Methodology. Device Fabrication (new design). Apply polyimide
  • 17. Methodology. Device Fabrication (new design). After UV lithography and polyimide development Apply PR After UV lithography and PR development Deposit metal contact After PR removal And metal lift off Deposit bottom metal contact
  • 18. Methodology. Device Fabrication (new design). Mounting: clip SEM Easier -to mount the sample -to align with optics -to integrate with the OBIC set-up
  • 19. Methodology. Characterization Set-ups chopper control Photocurrent Spectroscopy Optical-Beam Induced Current Imaging circuit circuit
  • 20. RESULTS
      • SQW diodes
        • spectra, I-V
        • images: reverse bias, forward bias, gradient maps, derived quantities
      • CQW diodes
        • spectra
        • images: reverse bias, ratio maps
  • 21. Results. Photocurrent (dotted) and Photoluminescence (solid) Spectra P33 (good SQW) Injection current ~ 200 µA P254 (bad SQW) Injection current ~ 5000µA Both samples behave as 90A SQW
  • 22. Results. Current-Voltage Curves P33 (good SQW) P254 (bad SQW) P254 has better I-V properties because of better metal contacts
  • 23. Results. Current maps for different reverse bias values Increasing reverse bias
  • 24. Results. Current maps for different reverse bias values For the same voltage, P33 produced higher OBIC P33 (good SQW) P254 (bad SQW)
  • 25. R. How to construct the gradient maps x x x x x Get SLOPE at each point (x, y) V = -1.0V V = -2.0V V = -3.0V V = -4.0V V = -5.0V
  • 26. Results. Positive gradient maps (reverse bias) P33 (good SQW) P254 (bad SQW) All OBIC values increase with bias  no negative gradient map
  • 27. Results. Positive gradient maps (forward bias) P33 (good SQW) P254 (bad SQW) All OBIC values increase with bias  no negative gradient map
  • 28. Results. Fill factor P33 (good) P254 (bad) FF average ~ 52% ± 3% FF average ~ 79% ± 2% P254 has better metal contacts
  • 29. Results. Energy conversion efficiency P33 (min = 0, max ~ 2.3%) ± 0.3% P254 (min = 0, max ~ 6.9% ) ± 0.7% P254 is more efficient in converting optical to electrical energy
  • 30. Results. quantum efficiency P33 (min~76% max~100%) ± 9% P254 (min~17%, max~42%) ± 3% P33 is more efficient in producing e-h carriers
  • 31. Results. Summary of SQW data
    • P33 (good) vs. P254 (bad)
      • P254 has better metal contacts than P33  P254 has higher FF and η ECE
      • P33 has better growth conditions or compositionally close to the design wafer  P33 has higher η QE
    • For both P33 and P254
      • Nonuniform η ECE suggests nonuniform diode output power; nonuniform η QE suggests nonuniform optical absorption coefficient of the material
      • In determining the emission, η QE is more influential than FF and η ECE . The performance of P33 can further be improved by optimizing the device fabrication (to raise FF and η ECE ). However, P254 has already reached its maximum potential as a device, limited by its low η QE
  • 32. Results. Photocurrent Spectra 1 2 3 1 2 3 Band diagram [27] 1-unbiased 2-at resonance 3-after resonance
  • 33. Results Theoretical calculation V res ~ -1.4V, coincides with experimental V bias ~1.4V where λ (803 and 850) transitions have closest responsivity values
  • 34. Results λ = 850nm λ = 803nm 0V -0.8V -1.4V -2.0V -3.0V Ratio (850/803)
  • 35. Results. Summary of CQW data
    • PC spectra proves that electric field effect is more pronounced in CQW, as expected.
    • Tuning the laser into the two associated intrawell transition λ s, the 2D current maps are acquired. T he consistent ratio values across the device at any bias indicate that the electric field applied is the uniform.
    • The ratio images follow the same trend exhibited by PC spectra wherein there is a relative intensity change at the two direct well transition λ s. The voltage at the peak ratio coincides with the calculated resonance voltage.
  • 36. Conclusion
    • Room-temperature spatially-resolved characterization for different bias voltages was performed on GaAs-based diodes. From this, we derived the two-dimensional (2D) topographies of external quantum efficiency, fill factor and energy conversion efficiency that give insight on the homogeneity of the characteristics at microscopic resolution for three operating functions: light-emitting, photovoltaic and photoconductive. Thus, we were able to compare the optoelectric properties of good and defective diodes. The samples used were GaAs/Al x Ga 1-x As single quantum well ( SQW ) p-i-n and coupled quantum well ( CQW ) p-i-n grown by molecular beam epitaxy and fabricated into 300 μm circular devices.
  • 37. Conclusion
    • In coupled quantum wells, the variation of the applied bias shifted the position of the QW energy levels with respect to each other, and hence, altered the relative carrier population densities in each QW. This was manifested as relative change in the spectral absorption of the 2 QW wavelength peaks. Ratio imaging revealed that the behavior is consistent across the device, indicative of the uniformity of electric field. This also translated to microscopic mapping of the interwell tunneling effect on the direct transitions, which is greatest at resonance.
  • 38. Acknowledgment
    • Raymund Sarmiento, Dr. Vernon Julius Cemine, and Dr. Carlo Mar Blanca for the use of the OBIC set-up and helpful discussions
    • Dr. Alipio Garcia for the growth of CQW and Jennifer Constantino for the deposition of metal contacts