Ultrafast dynamics in ZnO/ZnMgO multiple quantum wells (MQW)

1. Ultrafast dynamics in ZnO/ZnMgO multiple quantum wells (MQW) Speaker: Ho-Jei Ton Advisor: Jia-Hon Lin

2. Outline • Introduction • Experimental sample and technique • Experimental results and discussion • Summary

3. Introduction • ZnO is a potential material in short-wavelength optoelectronics applications. • ZnO: • Large band gap (~3.3eV) • Large exciton binding energy (~60meV) result in a stable exciton distribution and efficient excitonic emission. • ZnO MQW exhibit even greater exciton stability compared to ZnO bulk, due to enhancement of the binding energy and the reduction of the exciton-phonon coupling.

4. Introduction • Upon high excitation, Yamamoto et al observed hot carrier cooling on the older of less than 1ps in ZnO thin film. • Sun et al reported the observation of an extremely fast thermalization time on the older of 200fsat low excitation and a prolonged carrier cooling process at high excitation in ZnO thin film.

5. Introduction • Using an optical Kerr gate technique, Takeda et al studied ultrafast TRPL in ZnO thin films. • Johnson et al reported the observation of a 400~1000fs fast transient in ZnO nanowires and nanoribbons. • TRPL experiments were performed at the near-band-gap region in ZnO/ZnMgO quantum wells to elucidate the basic properties.

6. Introduction • In this study, pump-probe technique is used at room temperature. • For a probe energy above band edge, longitudinal phonon (LO) scattering is probed with a subpicosecond cooling time. • For a probe energy near the band gap, the exciton-acoustic phonon scattering result in a relaxation time of a few picoseconds.

7. Optical and Acoustical Phonons • Acoustic mode: • Associated with the motion of the center of mass of the two atoms. • Optical mode: • Vibration induced by photon with different atoms have opposite charges.

8. Experimental sample and technique Experimental set up: • Ti-sapphire: duration (100fs) • repetition rate (1kHz) • Pump pulse fixed to 3.64eV (above band gap), • but is lower than the absorption energy of • ZnMgO • Excitation density: 1mJ cm-2/pulse

9. Photoluminescence experiment Light source: The fundamental pulses (800nm) from the Ti-sapphire regenerative amplifier (duration:100fs, repetition rate:1kHz). PL experiment set up: Excitation wavelength at 266nm Detector output slit : 0.25nm

10. PL spectra of ZnO/ZnMgO MQW Excitation at 266nm Emission peak at 3.45eV FWHM : 15.5nm PL spectra of ZnO/ZnMgO MQW • This PL band is blue shifted with respect to • bulk ZnO due to strong quantum confinement • and weak confined Stark effect. Stark effect: the virtual excitation of electron-hole pairs leads to a high-energy shift of exciton absorption line. • As temperature increasing, recombination • is dominant by non-radiative centre activation. The shoulder is caused by LO phonon scattering

11. PL spectra of ZnO/ZnMgO MQW PL spectra at room-temperature The PL spectrum is dominated by exciton recombination Makino T, Chia C H, Tuan N T, Sun H D, Segawa Y, Kawasaki M, Ohtomo A, Tamura K and Koinuma H 2000 Appl. Phys. Lett. 77 975

12. Absorption spectra of ZnO/ZnMgO MQW Absorption spectra of ZnO/ZnMgO The absorption spectra is similar to the ZnO bulk, because the ZnO layer (500nm) is much larger than the wells.

13. Experimental results and discussion • The process of carrier dynamics • electron-hole pairs are generated by 3.64eV excitation in the excited state and instantaneously convert partially into excitons. • these hot carriers and excitons rapidly relax down by mainly LO phonon scattering, resulting in a quasi-equilibrium states. • in the band gap region acoustic phonon scattering can result in a slow relaxation due to small phonon energy. • electrons or excitons eventually recombinationradiatively or non-radiatively.

14. Time evolution of transmission probed at 3.44eV • The fast decay in above band edge region is attributed to large LO phonon-assisted relaxation, which is similar to observations in ZnO film.

15. Time evolution of transmission probed at 3.3eV • In this below band gap region, the dominant relaxation mechanism should be exciton-acoustic phonon scattering, and it cause a smaller coupling strength.

16. Summary • In the above band gap region, strong LO-phonon scattering result in effective carrier relaxation with a cooling time of 700~850 fs. • In the near band gap region, a relatively long cooling time (6ps) is due to a weak carrier-acoustic or exciton-acoustic phonon coupling strength.