The structures and optical properties of two-step growth m-plane ZnO epifilm by pulse laser deposition
Cheng-An Chien1*, Hou-Ren Chen1, ChinChia Kuo1, Wen-Feng Hsieh1
1Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan
* presenting author:簡呈安, email:yuni0428@hotmail.com
ZnO epi-layers grow preferentially with c-plane normal will the quantum-confined Stark effect (QCSE). To improve the quantum efficiency, devices with ZnO of nonpolar orientations, such as a-plane and m-plane, have been proposed.
The extra-oriented domains exist in the direct growth m-ZnO epifilms on m-sapphires are attributed to the strain relaxation mechanism along the c-axis of m-ZnO. The extra domains solve the strain problem at the interface of the ZnO layer and sapphire. However, the broadband emission at 3.17 eV due to the surface-bound exciton (SX) emission, which is caused by the exciton being trapped to the domain boundaries.¹ We will use two-step growth method (low- and high-temperature ZnO) for eliminating extra domains to reduce SXs. The LT m-ZnO buffers are grown with thickness of ~4 to 160 nm at 175℃, and raise the growth temperature to improve crystal quality for growing main m-ZnO epilayer at 600℃. The LT grown m-ZnO buffers show pure m-plane oriented without extra domains when the film thickness is below 67 nm, detectable extra domains appear for the thickness over 67 nm. Therefore, the optimal thickness of LT-buffer should be below 67 nm. We defined the m-[1100], a-[1120], and c-[0001] axes of the wurtzite ZnO lattice as the three orthogonal axes x, y and z of the orthorhombic lattice, respectively. The results show that the LT m-ZnO buffer is under a tensile strain along x-axis, and compressive strain along c-axis. However, the small strain along the y-axis results from domain matching that leads to the lattice mismatch less than 0.08 %. They reveal opposite tendency of the x-axis (m-[1100]) with respect to the z-axis (c-[0001]). The strain increases with increasing thickness of LT-buffer from 4.7 to 67 nm and strain relaxation occurs when the buffer thickness is thicker than 67 nm. At the same time, the extra domains have been observed. Therefore. The LT-ZnO buffer is the optimal thickness one with thickness < 67 nm. Then, we investigated the epitaxial growth of m-ZnO on LT-buffer at 600℃ (HT m-ZnO films). The strain of, x- and z-axes of the HT m-ZnO films are still tensile and compressive. The HT m-ZnO epilayers show smaller strains than LT-buffer. The tendency of strains for the HT m-ZnO opposites to the LT-buffer that the strains of HT ZnO layers gradually decrease with increasing thickness of LT-buffer until 67 nm. There is few extra domains observed for HT growth m-ZnO on thinner LT-buffers having thickness from 1.7 to 47 nm. With thick enough LT-buffer from 47 to 67 nm the strain has significantly reduced but with not observable extra domains, the HT growth m-ZnO films on these LT-buffers have the better crystal quality. The PL spectra measured in 13 K shows three dominant peaks at 3.362, 3.32 eV, and 3.26 eV, which are attributed to D0X, BSFs, and DAP emission, respectively. However, the crystal quality has opposite tendency to BSF emission. The BSF/D0X intensity ratio increases with increasing thickness of LT m-ZnO buffer, implying BSF could help the strains relaxation of HT m-ZnO layers as increasing thickness of LT m-ZnO.

This work is partially supported by Ministry of Science and Technology of Taiwan under grants NSC 102-2112-M-009-016-MY3, and MOST 103-2221-E-009-106-MY3

1. C. C. Kuo, B. H. Lin, Song Yang, W. R. Liu, W. F. Hsieh and C.-H. Hsu, Appl. Phys. Lett. 101, 011901 (2012)


Keywords: Non-polar ZnO, Extra Domains, Basel Stacking Fault, pulse laser deposition