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Absorbtion

Discover the principles and applications of luminescence spectroscopy, including techniques like FTIR and RAIRS, and how they are used to study electron transitions and excitons in materials. Learn about quantum wells, interface roughness, and types of transitions seen in luminescence spectroscopy.

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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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