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Absorbtion

Absorbtion. FTIR: Fourier transform infrared spectroscopy ATR: attenuated total reflection MIR: multiple internal reflection (see JAP 94 (2003) 2707) RAIRS: reflection absorbtion infrared spectroscopy. Luminescence Spectroscopy. Electrons are moved into excited states using ….

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Absorbtion

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  1. Absorbtion • FTIR: Fourier transform infrared spectroscopy • ATR: attenuated total reflection • MIR: multiple internal reflection (see JAP 94 (2003) 2707) • RAIRS: reflection absorbtion infrared spectroscopy

  2. Luminescence Spectroscopy • Electrons are moved into excited states using … Photons  Photoluminescence (PL) Electron  Cathodoluminescence (CL) beam(SEM) Injected  Electroluminescence (EL) charge (p-n junction diodes) (electric current) PL EC hn > Eg EV

  3. Luminescence Spectroscopy • Electron transitions result in emission of characteristic light Phonon relaxation EC EV EC hn = EC – EV = Eg EV

  4. Luminescence Spectroscopy • Can measure wavelength of light to determine intrinsic (e.g., bandgap) and extrinsic (e.g., impurities, defects) material properties CB VB

  5. Types of Transitions • CB-to-VB (e-h) Phonon relaxation CB hn > Eg hn = Eg VB

  6. Types of Transitions • e-h can form a bound pair called an exciton due to Coulomb attraction • Similar to hydrogen atom with ionization energy: • Ex = (m*e4/2h2e2) n-2 n = 1, 2, … CB n =  n = 2 n = 1 hn = Eg - Ex VB

  7. Types of Transitions • Excitons Ex = (m*e4/2h2e2) n-2 n = 1, 2, … m* = reduced mass = (1/me + 1/mh)-1 Ex ~ few meV  Excitons only observed at low temperature

  8. Types of Transitions • Bound Excitons • Excitons may become bound to impurities • DoX : exciton bound to neutral donor • AoX : exciton bound to neutral acceptor From Pankove, Fig. 1-14, p. 16

  9. Types of Transitions • Shallow Transitions • e-D+ : e- may transition from CB to ionized donor (donor becomes neutral) • h-A- : e- may transition from ionized acceptor to VB (acceptor becomes neutral) • Ei ~ 10 meV (donors) • ~ 30 - 40 meV (acceptors) From Pankove, Fig. 6-24, p. 132

  10. Types of Transitions • Deep Transitions • Do-h : e- may transition from neutral donor to VB (donor becomes ionized) • e-Ao : e- may transition from CB to acceptor (acceptor becomes ionized) From Pankove, Fig. 6-25, p. 133

  11. Types of Transitions • Do-Ao Transitions • Transitions between neutral donors and neutral acceptors • Coulomb attraction between donors and acceptors • hn = Eg – Ed – Ea + e2/4per • r = donor-acceptor separation From Pankove, Fig. 6-38, p. 143

  12. Types of Transitions • Do-Ao Transitions: • r varies by discrete increments (lattice sites) From Yu & Cardona, Fig. 7.6, p. 346

  13. Quantum Wells hn = Eg + Ene + Enh- Ex Ene,h = (ħ2 / 2me,h*) (np/Lz)2 Eg Lz

  14. Quantum Wells Interface roughness M.A. Herman, D. Bimberg and J. Christen, “Heterointerfaces in Quantum Wells and Epitaxial Growth Processes: Evaluation by Luminescence Techniques”, J. Appl. Phys. 70, R1 (1991)

  15. Quantum Wells Interface roughness M.A. Herman, D. Bimberg and J. Christen, “Heterointerfaces in Quantum Wells and Epitaxial Growth Processes: Evaluation by Luminescence Techniques”, J. Appl. Phys. 70, R1 (1991)

  16. Luminescence Spectroscopy • Advantages: • Non-destructive • Sensitive: < 1012 cm-3 impurity detection • Monolayer detection capability in QWs • Disadvantages: • Peak assignment difficult • Difficult to quantify amount of impurity due to competing non-radiative recombination

  17. Luminescence Spectroscopy

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