摘要
ZnO是一种重要的宽带隙半导体材料,与GaN的带隙宽度基本上相同,是制备紫外半导体发光器件和半导体激光器的理想材料。与GaN相比,ZnO不仅具有生长温度低、无毒、原料成本相对低廉等优点,还具有高达60 meV的激子束缚能(后者仅为24 meV),因而获得受激发射的阈值要低得多。另一方面,ZnO还具有丰富的纳米结构,包括纳米线、纳米管、纳米带、纳米环、纳米点等等。当材料尺寸降低至量子尺度时,将出现许多不同于体材料的性能。
本文采用脉冲激光沉积(PLD:pulsed laser deposition)方法进行ZnO薄膜与ZnO/ZnMgO量子阱的生长。为了探索量子阱的生长条件,我们首先研究了生长温度、氧压、缓冲层及退火处理等对制备的ZnO薄膜的形貌特征和晶体质量的影响,获得了取得平整薄膜的最佳生长参数。在此基础上,实现了ZnO/ZnMgO量子阱的可控生长,并且深入分析了其光致发光性能。现简要介绍如下:
1、利用PLD方法探索了ZnO薄膜的生长工艺。本文通过改变生长温度、氧压、激光器等参数,分析了衬底温度、薄膜厚度、缓冲层、退火处理等对薄膜的形成和形貌的影响。
2、探索了衬底温度的改变对ZnMgO:Al合金薄膜的结构与电学性能的影响,当生长温度为350℃获得了最低电阻率为5.6×10~(-4)Ωcm的薄膜,分析了电阻率发生改变的原因,为今后制备调制掺杂超晶格结构打下了良好的基础。
3、在最佳生长条件下,通过改变阱层厚度、势垒层厚度及组分等结构参数在Si(111)衬底上可控地制备出一系列ZnO/ZnMgO多量子阱,透射电镜(TEM)结果表明这些量子阱具有良好的周期性结构。
4、通过对光致发光(photoluminescence,PL)结果分析表明,量子阱室温下的带边发光具有激子特征。随着势阱层的厚度减小,势阱层PL发射峰发生蓝移,这是量子约束(尺寸)效应。我们还从实验结果中发现,量子阱具有高于体材料的激子束缚能,并且激子束缚能随着势阱层厚度的减小而增加。
5、通过对ZnO/ZnMgO单量子阱和多量子阱的PL谱随温度的变化情况发现,量子阱中的激子在低温下是“局域化”的,随着温度的上升,激子逐渐地“去局域化”转化为自由激子。本文还发现,“势谷”的深度受势阱层的厚度和势垒层的组分影响。
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~(-4)Ωcm 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.
引文
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