Structural And Optical Properties Of ZnO/ZnMgO Multi-quantum Wells Grown By PLD

ZnO is a semiconductor with a direct wide band gap of 3.37 eV at room temperature (RT),nearly to that of GaN.Thus,ZnO is an ideal candidate for applications in shortwave optoelectronic devices,such as ultraviolet(UV) light-emitting devices and laser diodes.ZnO has mainly two advantages over GaN,i.e.,(1) low deposition temperature, low cost and innoxiousness,(2) large exciton binding energy of 60 meV in comparison to that of 26 meV in GaN.On the other hand,there are abundant nanostructures in ZnO, including nanowires,nanotubes,nanobelts,nanoloops,nanodots,and so on.Besides,it is well known that ZnO quantum-size-scale materials exhibit many novel characteristics different from bulk ZnO.In the paper,ZnO films and ZnO/ZnMgO quantum wells were grown by pulsed laser deposition(PLD).By optimizing the growth parameter(including substrate temperature, oxygen partial pressure,film thickness,and annealing treatment),the smooth ZnO films were achieved with a roughness no more than 1 nm.The growth parameter was then utilized to fabricate ZnO/ZnMgO quantum wells.The optical properties were analyzed by photoluminescence(PL) spectra.1.ZnO films were grown on Si(111) substrates by PLD.We investigate the dependence of the ZnO films on growth temperatures,oxygen pressures,and laser parameters.2.Effects of substrate temperatures on the structure,optical and electrical properties were investigated.The lowest resistivity of 5.6×10^-4Ωcm was achieved at the growth temperature of 350℃.The mechanism underlying the variation in the resistivity was revealed.3.A series of ZnO/ZnMgO multiple quantum wells(MQWs),with various well width and barrier width and barrier composition,were grown on Si(111) substrates controllably under the optimized growth conditions.TEM analysis reveals that the MQWs exhibit periodic structure with sharp interface.4.Photoluminescence analysis suggests that the MQWs show excitonic near-bandedge emission at room temperature.The well layer emission shows an obvious blueshift with a reduction in the well layer thickness.The excitons in the quantum wells have larger exciton binding energy than bulk ZnO.Besides,the exciton binding energy increases as the well layer thickness decreases.5.We find that the excitons are localized in the ZnO/ZnMgO single and multiple quantum wells at low temperatures by photoluminescence(PL) spectra.With an increase in temperature,the localized excitons are detrapped from the potential minima and transferred into free excitons.The depth of the potential minima is dependent of the well width and barrier width.