Speaker: Ho-Jei Ton Advisor: Jia-Hon Lin

1. Room-temperature luminescence of excitons in ZnO/MgZnO multiple quantum wells on lattice-matched substrates Speaker: Ho-Jei Ton Advisor: Jia-Hon Lin

2. Outline • Introduction • Experimental Setup • Result & Discussion • Summery

3. Introduction • ZnO • optoelectric applications in visible and ultraviolet (UV) regions. • large exciton binding energy (60meV) permits excitonic recombination. • A lower pumping threshold can be expected, if an exciton-related recombination rather than an electron-hole plasma recombination is used.

4. Introduction • In previous fabrication of ZnO/MgZnO MQW grew on sapphire showed the quantum confinement effect clearly at 4.2 K from Excitonic photoluminescence (PL). • However there were some drawbacks attributed to the rough interface due to lattice-mismatched substrate (18%). • The Mg concentration of x=0.2 (MgxZn1-xO) is close to the limit to obtain clear electronic structures. • In this work, SaAlMgO4 with (0001) orientation as a substrate, the lattice constant of which matches that of ZnO with 0.08%. LW=1.7nm&3.1nm LB=6.2nm (10 period)

5. Introduction PL of ZnO on SCAM Atomic force microscope image • ScAlMgO4 (SCAM) which matches the lattice constant of ZnO with 0.08%. (a)&(b): polished sapphire (0001) (c)&(d): polished ScAlMgO4(0001) (e)&(f): cleaved ScAlMgO4(0001) “A” and “B” are resonance energy positions of longitudinal A- and B-excitons, respectively.

6. Experimental Setup • Sample: [ZnO/Mg0.12Zn0.88O]10& [ZnO/Mg0.27Zn0.73O]10 • Laser molecular-beam epitaxy • well width (Lw): 6.9~46.5 Å • barrier width (LB): 50 Å • PL: • excited by (CW) He-Cd laser (325nm) • Monitored using a monochromator with a charge-coupled device • Absorption: • carry out using a xenon lamp

7. PL and absorption spectra in [ZnO(Lw)/Mg0.12ZnO0.88O]10 MQWs measured at 5K for (Lw=17.5A, 6.91A) • Both PL and absorption peaks shifted towards the higher energy side as Lw decrease. • This shift was due to the quantum confinement effect. • n=1 is the lowest exciton absorption, n≧2 means the excited states of the exciton or higher interband transition. • B+LO, B+2LO, B+3LO correspond to exciton-phonon complex transitions.

8. Well width dependence of the peak energies of PL and absorption • The exciton absorption energy shifted to a higher energy as the barrier height increased. • The PL peaks showed a stokes shift due to spatial fluctuation that is more sensitive effect for very thin well. • Since x=0.27 is above the solubility limit, the inhomogeneity of band gaps in barrier layers induces the depth fluctuation and enhancement of the exciton energy.

9. The lattice mismatching on sapphire • Both peak energies have the maximum at Lw of 15Å when sapphire substrates were used. • The critical thickness is cased by poor controllability of Lw due to the lattice mismatching.

10. Temperature dependence of PL and absorption spectra in [ZnO(23.5Å)/Mg0.27Zn0.73O]10 on SCAM • The temperature limit is 150K for sapphire, however SCAM improve the quality to reach the lowest temperature.

11. Summery • SCAM, such a mismatching condition enable to attain high controllability of layer thickness. • The quantum confinement effect for the exciton energy could be confirmed experimentally if the well width is equal to 1.5 unit cells or more. • The bright excitonic PL was confirmed at RT. • Such high quality MQWs opens up numerous possibilities for UV optoelectric devices