Growth and structural characterization of epitaxial semipolar ZnO thin films on LaAlO3

• Main Authors: Tian, Jr-Sheng, 田志盛
• Other Authors: Chang, Li
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/86271093108420976094

博士 === 國立交通大學 === 材料科學與工程學系所 === 102 === 　　Wurtzite ZnO, a direct wide bandgap semiconductor, is promising for optoelectronic applications. However, the polarization along [0001]ZnO usually degrades luminescent properties. To reduce the polarization effect, the growth of ZnO along semipolar directions is desirable. 　　In this study, growth of semipolar (11-2-2) ZnO thin films has been sucessfully demonstrated on (112) LaAlO3-buffered LSAT substrate by pulsed laser deposition. We investigated the structural characteristics and the growth mechanism of (11-2-2) ZnO by using high resolution X-ray diffraction, transmission electron microscopy, atomic force microscopy, and photoluminescence (PL). The main results are listed below. 1. High quality LAO buffer is ahchieved on (112) LSAT ((LaAlO3)0.29(Sr2AlTaO6)0.35) substrates by PLD. The LAO buffer layer has a twin-free structure, implying the phase transformation from cube to rhombohedron at 813 K during cooling does not occur, and is cation nonstoichiometric (La/Al atomic ratio = 1.13). 2. The epitaxial relationship of (11-2-2) ZnO on (112) LAO/LSAT is [1-100]ZnO // [1-10]LAO/LSAT, and the polar axis [000-1]ZnO points to the surface. 3. The ZnO films show good crystalline quality, such as narrrow X-ray diffraction rocking curves (near 0.1°), and large lateral coherent length (over 1 micron). 4. [1-100]ZnO-oriented stripes and pits are the main characteristics of (11-2-2) ZnO surface morphology. The stripes are constituted of two facets, and the pits are corresponding to the termination positions of threading dislocations at the surface. 5. In low temperature (10 K) PL spectra, the main peak with a FWHM value below 10 meV is neutral-donor-bound exciton recombination. For room temperature PL spectra, the near-band-edge emissions mainly consist of free exciton recombination, free electron to neutral acceptor transition, and their longitudinal-optical-phonon-assisted transitions. 6. The misfit strain of (11-2-2) ZnO on (112) LAO buffer along [11-23]ZnO is relaxed by the formation of misfit dislocations (MDs) with b = 1/6[11-23]ZnO and b = 1/6[11-20]ZnO. The latter causes slight tilt of (11-2-2)ZnO plane toward (110)LAO from (112)LAO plane. Along [1-100]ZnO, MDs with b = 1/3[1-210]ZnO are responsible for the misfit strain relaxation. 7. The basal stacking faults in (11-2-2) ZnO films are rarely found (the density is below 10^4 cm^-1), while planar defects bound by {10-10}ZnO and extending along [000-1]ZnO are observed and resemble inversion domain boundary. Only perfect dislocations with b = a、c and a + c are shown, and the density near the surface of 1.6 micron ZnO films is 1 ×10^9 cm^-2. 8. Growth of nonpolar (10-10) and semipolar (11-2-2) ZnO on (112) LAO substrates can be obtained by annealing the substrate surface in vacuum and oxygen atmosphere prior to ZnO deposition, respectively. It is shown that (10-10) ZnO is grown on atomic flat (112)LAO plane, whereas (11-2-2) ZnO is grown on faceted (112) LAO surface with (001)LAO and (110)LAO facets in atomic scale. 9. By using the growth conditions of (11-2-2) and (10-10) ZnO, (11-2-1) and (13-40) ZnO can be achieved on (114) LAO substrates, respectively. In addition, it is found that (11-2-2) and (11-2-1) have the same orientation relationships with LAO ( (000-1)ZnO // (110)LAO and [1-100]ZnO // [1-10]LAO ). 　　We further discuss the origin of MDs and the mechanism for the reduction of dislocation density with increasing film thickness. The atomic configurations of (11-2-2) ZnO/LAO and (10-10) ZnO/LAO interfaces are also modeled to account for their epitaxial relationships.