【作者】 张宏海；

【导师】 叶志镇； 潘新花；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2014， 博士

【摘要】 ZnO作为一种直接禁带半导体材料,具有3.37eV的禁带宽度以及60meV的激子束缚能,在短波长光电器件中具有非常大的应用潜力。ZnO基光电器件应用的一个关键问题是实现能带裁剪从而制备量子阱结构,即能带工程。ZnMgO合金薄膜禁带宽度可以大范围调节,且其晶格常数变化不大,因此是开展ZnO能带工程的首选材料之一。为了实现ZnMgO合金在光电器件和量子阱中的应用,首先要制备优质的ZnMgO单晶薄膜和ZnO/ZnMgO多量子阱结构。同时,异质结是构成量子阱和超晶格的基础,要实现高效的载流子注入和限域作用,异质结界面能带结构的测定和设计至关重要。鉴于上述情况,我们开展了高晶体质量、低缺陷密度的ZnMgO单晶薄膜生长研究；在此基础上,研究Mg组分和生长取向对ZnMgO/ZnO异质结界面能带偏移的影响；最后采用P-MBE法制备了ZnO/ZnMgO量子阱结构,通过光致发光谱研究其光学性能。本文的主要研究内容和结论为：1.采用射频等离子体辅助分子束外延(P-MBE)技术在c面蓝宝石衬底上生长了优质的ZnMgO单晶薄膜。通过引入MgO缓冲层技术,ZnMgO合金薄膜室温电子迁移率达到了66cm2V-1s-1,高分辨XRD摇摆曲线(002)面半峰宽仅47arcsec,螺型位错密度降低至4×106cm-2,显著提高了其晶体质量。2.采用光电子能谱法测定了Zn1-xMgxO/ZnO异质结的能带带阶。Zn1-xMgxO/ZnO异质结的界面能带排列为type-I型结构,其导带带阶和价带带阶比值AEc/ΔEv分别为1.5(x=0.1),1.8(x=0.15),2.0(x=0.2)。这说明Zn1-xMgxO/ZnO异质结中能带带阶的偏移量和比值与Mg组分密切相关。3.采用脉冲激光沉积法在c面和r面蓝宝石衬底上制备了极性和非极性取向的Na掺杂ZnMgO薄膜。Hall测试显示a面非极性取向的ZnMgO:Na薄膜呈现p型导电信号,空穴浓度为3.5×1016cm-3,而c面极性取向薄膜的导电类型为补偿型导电。为了探索p型导电机理,我们研究了不同生长取向ZnMgO/ZnO异质结的能带结构。极性和非极性异质结的价带偏移量分别为0.07eV和0.02eV,能带结构均为type-I型,极性ZnMgO/ZnO异质结中的自发极化效应是造成两种异质结能带带阶值差别的主要原因。我们认为,非极性ZnMgO相对于极性价带下移量更小,导致Nazn受主能级变浅,且导带上移量更大引起施主能级变深,这两者的综合作用是非极性ZnMgO:Na薄膜中p型导电的来源。4.采用P-MBE技术在蓝宝石衬底上制备了一系列5个周期的ZnO/ZnMgO多量子阱结构。高分辨XRD测试结果表明这些量子阱具有良好的周期性结构。通过PL测试分析,量子阱在低温下可以分辨出局域激子发光峰(LE),自由激子发光峰(FE)以及LE的多个声子伴线,同时LE的半峰宽均小于10meV,表明了这些量子阱具有非常好的质量。随着势阱层的厚度减小或者势垒组分增加,LE发射峰不断蓝移,量子约束效应增强。5.通过比较不同Mg组分的ZnO/Zn1-xMgxO多量子阱结构变温PL谱发现,高组分势垒层的多量子阱结构(MQWs)中局域激子发光峰位随温度升高呈现S型变化,而低组分中LE则随温度升高单调红移。S型变化是由于高组分势垒中激子的局域化效应引起的,随着温度升高,激子逐渐去局域化转为自由激子。通过分析变温PL变化规律,不同势垒组分量子阱中的激子束缚能分别为64meV (x=0.1)和76meV(x=0.2),证明了激子束缚能随势垒组分增大而增强。更多还原

【Abstract】 ZnO has been extensively studied for potential applications in optoelectronic devices owing to its unique properties, including high electron mobility, wide direct band gap (3.37eV at room temperature), and large exciton binding energy (60meV). One of the important capabilities involved in constructing optical and electrical confinement structures is band gap engineering. The ternary alloy semiconductor, Zn1-xMgxO, is considered as one of the best candidates for increasing the band gap energy of ZnO because of its large band gap and small lattice-mismatch with ZnO. In order to develop ZnO-based heterostructures and deep ultraviolet devices, the most important issues are to fabricate high quality Zn1-xMgxO films and ZnO/Zn1-xMgxO multiple quantum wells (MQWs).It is well known that heterojunctions consisting of the fundamental quantum wells and superlattice structures have been playing an important role in fabricating optoelectronic devices because of the carrier confinement effect and high carrier injection ratio properties. Furthermore, the valence band maximum (VBM) strongly influences the capability of p-type doping in Zn1_-MgxO films. Therefore, it is necessary to design the ideal valence and conduction band offsets of Zn1-xMgxO/ZnO heterojunctions.In this work, we focus on the growth and characterization of high quality ZnMgO thin films using intervening epitaxial MgO buffer layers; on the basis of this, we investigate the Mg composition and polarity orientation dependent band offsets of Zn1-xMgxO/ZnO heterojunctions, in an attempt to reveal the p-type doing mechanism. After that, ZnO/Zn1-xMgxO MQWs were fabricated and the relative optical properties were analyzed by photoluminesence (PL) spectra. The work includes:1. High quality epitaxial Zn1-xMgxO films were fabricated on c-plane sapphire substrates using MgO buffer layer by plasma-assisted molecular beam epitaxy. The quality of the Zn1-xMgxO epilayers is manifested by a Hall mobility of more than60cmV-1S-1at room temperature, and x-ray diffraction rocking curve full-width at half-maximum of47arcsec. A screw dislocation density of4x10cm-2is estimated by x-ray diffraction.2. The valence band offsets (AEv) of Zn1-xMgxO/ZnO heterojunctions grown by plasma-assisted molecular beam epitaxy were measured by photoelectron spectroscopy. From the directly obtained AEv values, the related conduction band offsets (ΔEC) were deduced. All the Zn1-xMgxO/ZnO heterojunctions exhibit a type-Ⅰ band alignment with the ΔEC/ΔEV estimated to be1.5,1.8,2.0for x=0.10,0.15and0.20, respectively. The band offsets of Zn1-xMgxO/ZnO heterojunctions depend on Mg composition.3. Polar and non-polar ZnMgO:Na films that were fabricated on c-plane and r-plane sapphire substrates using intervened ZnO layers by pulsed laser deposition. Hall-effect measurements indicate that the a-plane ZnMgO:Na film exhibits p-type conductivity with a carrier concentration of about3.5×1016cm-3, while the polar film shows a compensatory conductivity. Meanwhile, the dependence of the band alignment on the orientation of the ZnMgO/ZnO heterojunctions has been investigated using photoelectron spectroscopy. The heterojunctions form in the type-Ⅰ straddling alignment with valence band offsets of0.07(0.02) eV for the (non-)polar heterojunction. The difference in valence band offsets is primarily attributed to the spontaneous polarization effect. We propose that the smaller valence band offsets and larger conduction band offsets would reduce the NaZn acceptor level and enhance the relative intrinsic donor levels. Such effects consequently lead to p-type conductivity in non-polar ZnMgO:Na films.4. A series of5-period ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs) with different well layer thicknesses in the range of3-10nm have been fabricated on (0001) sapphire substrates by plasma-assisted molecular beam epitaxy (MBE) with combined MgO and low-temperature ZnO thin film as buffer layers. The good quality of ZnO/Zn0.9Mg0.1O MQWs is evidenced by the observation of readily resolved Pendellosung fringes and the small full-width at half-maximum (FWHM) value of exciton emission as low as8.3meV, as well as the observation of high order phonon replicas. The dominated photoluminescence (PL) peak in the MQWs shows a systematic blueshift with decreasing well width, which is consistent with a quantum confinement effect.5. In addition, we investigaed the temperature-dependent photoluminescence (PL) measurements of ZnO/Zn1-xMgxO MQWs with different barrier compositions. The PL band in the well layers is dominated by localized excitons (LEs), free excitons (FEs), and two longitudinal optical (LO) phonon replicas of the LE emission. The LE emission of higher Mg compositional barrier exhibits a significant blue shift of about70meV with respect to the lower one. The mechanisms of carrier dynamics and localization are investigated within the temperature range14-300K. As the temperature increases, luminescence from the excitons localized in the well layers shows an’S-shaped’shift in the high-barrier MQWs, whereas a monotonic red shift is observed in the low-barrier MQWs. The’S-shaped’shift behavior is associated with delocalization of the excitons in the potential minima induced by interface fluctuations or alloy disorder. Large exciton binding energies of64meV and76meV were deduced, demonstrating efficient quantum confinement in the ZnO/ZnMgO MQWs.更多还原