ZnAl2O4/α-Al2O3复合衬底的制备和GaN薄膜的生长

利用脉冲激光淀积法(PLD)在α-Al₂O₃衬底上淀积了ZnO薄膜, 通过ZnO和α-Al₂O₃固相反应制备了具有ZnAl₂O₄覆盖层的α-Al₂O₃衬底. X射线衍射谱(XRD)表明ZnAl₂O₄薄膜随反应时间由单一(111)取向转变为多晶取向, 然后变为不完全的(111)择优取向. 同时扫描电子显微镜(SEM)照片表明ZnAl₂O₄表面形貌随反应时间由均匀的岛状结构先变为棒状结构, 然后再变为突起的线状结构. 另外, XRD谱显示低温制备的ZnAl₂O₄在高温下不稳定. 在ZnAl₂O₄覆盖的α-Al₂O₃衬底上利用光加热低压MOCVD法直接生长了GaN薄膜. XRD测量表明随ZnAl₂O₄厚度增加GaN由c轴单晶变为多晶, 单晶GaN的摇摆曲线半高宽为0.4°. 结果表明薄ZnAl₂O₄覆盖层的岛状结构有利于GaN生长初期的成核, 从而提高了GaN的晶体质量.     

Synthesis of ZnAl2O4/α-Al2O3 complex substrates and growth of GaN films

With the solid phase reaction between pulsed-laser-deposited (PLD) ZnOfilm and α-Al2O3 substrate, ZnAl2O4/α-Al2O3 complex substrates were synthesized. X-ray diffraction (XRD) spectra show that as the reaction proceeds, ZnAl2O4 changes from the initial (111)-oriented single crystal to poly-crystal, and then to inadequate (111) orientation. Corresponding scanning electron microscope (SEM)images indicate that the surface morphology of ZnAl2O4 transforms from uniform islands to stick structures, and then to bulgy-line structures. In addition, XRDspectra present that ZnAl2O4 prepared at low temperature is unstable at the environment of higher temperature. On the as-obtained ZnAl2O4/α-Al2O3 substrates, GaN films were grown without any nitride buffer using light-radiation heating low-pressure MOCVD (LRH-LP-MOCVD). XRD spectra indicate that GaN film on this kind of complex substrate changes from c-axis single crystal to poly-crystal as ZnAl2O4 layer is thickened. For the single crystal GaN, its full width at half maximum (FWHM) of X-ray rocking curve is 0.4°. Results indicate that islands on thin ZnAl2O4 layer can promote nucleation at initial stage of GaN growth, which leadsto the (0001)-oriented GaN film.     

Pb(Zr0.53Ti0.47)O3/AlxGa1-xN/GaN异质结构的电容-电压特征

通过在调制掺杂Al0.22Ga0.78N/GaN异质结构上淀积Pb(Zr0.53Ti0.47)O3(PZT)铁电薄膜,我们发展了一种基于AlxGa1-xN/GaN异质结构的金属-铁电-半导体(MFS)结构.在高频电容-电压(C-V)特性测量中,发现Al0.22Ga0.78N/GaN异质界面二维电子气(2DEG)的浓度在-10V偏压下从1.56×1013cm-2下降为5.6×1012cm-2.由于PZT薄膜的铁电极化,在-10V偏压下可以观察到宽度为0.2V的铁电C-V窗口,表明在没有铁电极化方向反转的条件下,PZT/AlxGa1-xN/GaN MFS结构也能出现存储特性.因为2DEG带来的各种优点,可以认为AlxGa1-xN/GaN异质结构在以存储器为目标的MFS场效应晶体管中有重要应用.     

MOCVD生长InxGa1-Xn薄膜的表征

本文利用MOCVD方法在(0001)取向的蓝宝石衬底上实现了不同生工艺条件下的InxGa-xN薄膜的制备,并通过XRD、SEM、AFM等测量分析方法系统研究了生长工艺参数对InxGa1-xN薄膜的组分和性质的影响.InxGa1-xN薄膜的制备包括蓝宝石衬底表面上GaN缓冲层的生长以及缓冲层上InxGa1-xN薄膜的沉积两个过程.通过对所制备InxGa1-xN薄膜的XRD、SEM、AFM分析发现,调节生长温度和TMGa的流量可以有效控制InxGa1-xN薄膜中In的组分,并且随着生长温度的升高,InxGa1-xN薄膜的表面缺陷减少.     

在MOCVD系统中用预淀积In纳米点低温下合成生长InN

利用低压金属有机化学气相淀积(LP-MOCVD)系统,在(0001)蓝宝石衬底上采用预淀积In纳米点技术低温合成制备了立方相的InN薄膜.首先以TMIn作源在蓝宝石衬底表面预淀积了一层金属In纳米点,然后在一定条件下合成生长InN薄膜.X射线衍射谱(XRD)和X射线光电子发射谱(XPS)显示适当的预淀积In不仅能够促进InN的生长,同时还能够抑制金属In在InN薄膜中的聚集.原子力显微镜(AFM)观察表明,金属In纳米点不仅增强了成核密度,而且促进了InN岛的兼并.自由能计算表明预淀积的In优先和NH3分解得到的NH与N基反应生成InN.我们认为这种优先生成的InN为接下来InN的生长提供了成核位,从而促进了InN的生长.     

Bottom-up approaches to microLEDs emitting red,green and blue light based on GaN nanowires and relaxed InGaN platelets

Miniaturization of light-emitting diodes (LEDs) with sizes down to a few micrometers has become a hot topic in both academia and industry due to their attractive applications on self-emissive displays for high-definition televisions, augmented/mixed realities and head-up displays, and also on optogenetics, high-speed light communication, etc. The conventional top-down technology uses dry etching to define the LED size, leading to damage to the LED side walls. Since sizes of microLEDs approach the carrier diffusion length, the damaged side walls play an important role, reducing microLED performance significantly from that of large area LEDs. In this paper, we review our efforts on realization of microLEDs by direct bottom-up growth, based on selective area metal-organic vapor phase epitaxy. The individual LEDs based on either GaN nanowires or InGaN platelets are smaller than 1 μ in our approach. Such nano-LEDs can be used as building blocks in arrays to assemble microLEDs with different sizes, avoiding the side wall damage by dry etching encountered for the top-down approach. The technology of InGaN platelets is especially interesting since InGaN quantum wells emitting red, green and blue light can be grown on such platelets with a low-level of strain by changing the indium content in the InGaN platelets. This technology is therefore very attractive for highly efficient microLEDs of three primary colors for displays.