The Investigation of Non-polar ZnO Growth on LaAlO3 Single Crystal Substrate

• Main Authors: Ho, Yen-Teng, 何焱騰
• Other Authors: Chang, ,Li
• Format: Others
• Language: zh-TW
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/77541368439954994012

博士 === 國立交通大學 === 材料科學與工程學系 === 99 === Abstract In this thesis, hetero-epitaxial growth of non-polar ZnO on LaAlO3 substrate by pulsed laser deposition method (PLD) has been investigated. LaAlO3 (LAO) single crystal has a pseudocubic structure at room temperature with lattice parameter of 3.791 Å, and it has been widely used to grow various dielectric oxide and high-temperature superconductor oxide films in the past. In this study, a-plane ZnO growth on (001)LaAlO3 has been demonstrated. In addition, the concept of substrate engineering has been applied to design (114) and (112) planes of LaAlO3 as substrates for growth of nonpolar ZnO, which can have a miscut angle of 19.47° and 35.26°, respectively, from (001) LAO. Thereafter, PLD was used to realize the growth of nonpolar ZnO. Furthermore, the results support the feasibility of the design from the concept of substrate engineering. The thesis is mainly composed of three topics: In the first topic, the study of non-polar ZnO growth on (001)LaAlO3 is presented. In the range of growth temperature from 300 to 850°C, pure a-plane ZnO can be successfully grown due to small lattice mismatch (3~5%), while growth at 150°C results in formation of c-plane ZnO co-existed with a-plane ZnO. As analyzed by x-ray diffraction (XRD) θ-2θ and ψ-scan, reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM), the microstructure of the a-plane ZnO films actually consists of L-shaped dual domains due to the cubic symmetry of atomic configuration of LAO substrate surface. Cross-sectional transmission electron microscopy (TEM) with selected-area diffraction (SAD) reveals that the a-plane ZnO film consists of two types of growth domains that are perpendicular to one another. The in- plane growth relationships of a-plane ZnO with LAO (001) substrate are: [0001]ZnO // [1 0]LAO and [1 00]ZnO // [1 0]LAO. Besides, a near band edge emission (NBE) of 3.30eV was measured by room temperature photoluminescence (PL) on this sample. As no evidence of green line emission was observed, the a-plane ZnO grown on LaAlO3 (001) has good optical characteristics, even though there are the dual- domain structure with boundaries. The second topic starts with the issue how to eliminate the dual domain structure of nonpolar ZnO on cubic symmetric substrate surface. It can be shown, from the concept of substrate engineering by enlarging the lattice distance of substrate along one direction, that (114) plane of LaAlO3 with a miscut angle of 19.47° from (001) is a proper one for nonpolar ZnO growth. The lattice mismatch of (114) LaAlO3 with a-plane ZnO is as small as 3 % in the direction //CZnO and 1.02% ⊥CZnO. After growth on this substrate by PLD, the ZnO thin film as examined by AFM as well as by RHEED exhibits a stripe-like morphology and two-fold symmetry, suggesting that it has a single domain structure on (114)LaAlO3. Examination of the RHEED pattern along [0001]ZnO azimuth, it is found that the c-axis lies on the substrate but the a-axis is tilted with an angle to the substrate normal. The epitaxial relationships of ZnO with substrate can be determined by XRD ω/2θ scan as (001)LAO//(11 0)ZnO and (112)LAO//(10 0)ZnO. In fact, cross-sectional TEM and SAD reveal that the growth plane of ZnO on (114) LaAlO3 is (13 0) which is an unconventional nonpolar plane observed for the first time. The room temperature PL spectrum of the ZnO film exhibits NBE at 3.29eV with FWHM of 87.5meV, suggesting that the (13 0)ZnO has good optical characteristics. In the last topic, the growth of m-plane ZnO on LaAlO3 substrate has been explored. Based on atomic arrangement of the (112) LAO which has a miscut angle of 35.26° from (001), it is shown that (112) LAO has better fit with m-plane ZnO by the same concept of substrate engineering as presented for (114) LAO with (13 0)ZnO. The lattice mismatch of (112) LAO with m-plane ZnO is small as 2.9 % in the direction //CZnO and 0.9%⊥CZnO. Structural characterization by AFM, RHEED and XRD 2θ and ψ-scan shows that the growth of m-plane ZnO on (112)LaAlO3 by PLD is in epitaxy with good crystallinity. The room temperature PL data also illustrate NBE at 3.28 eV. Therefore, it is suggested that (112) LaAlO3 can be a promising substrate for (1 00) ZnO growth.