Comparison of nanostructure characteristics of ZnO grown on GaN and sapphire

Abstract

The immature developments of p-type ZnO and ZnO-related ternary or quaternary compound and the small lattice mismatch between ZnO and GaN stimulate interest in the hybrid growth of ZnO and GaN. In this research, we compared the nanostructures and optical properties of two ZnO thin-film samples grown under the same conditions but on different underlying materials ͑sapphire and GaN͒. With the high growth temperature of 450 °C, both samples show domain structures. However, in contrast to the sample of ZnO on sapphire, the sample of ZnO on GaN starts with smaller domains ͑a few tens of nanometers in width͒ of almost vertical boundaries at the interface and evolves into larger domains ͑around 100 nm in width͒. The 30°twist of the basal plane in ZnO with respect to its underlying sapphire does not occur in the sample of ZnO on GaN. Within a domain of ZnO on sapphire, misfit and threading dislocations can be periodically observed along the interface with the separation of about 8 nm. The transition of the lattice structure across the interface between ZnO and GaN was quite smooth, indicating the high heterojunction quality in this sample. The x-ray diffraction results showed that the crystalline quality of ZnO on GaN is slightly better than that of ZnO on sapphire. However, based on the thermal quenching behavior of integrated photoluminescence intensity, we estimated that the optical quality of the two samples is about the same.

ZnO Wurtzite Single Crystals Prepared by Nanorod-Assisted Epitaxial Lateral Overgrowth

“…As such, fabricated films have high dislocation densities . For high efficiency optical and electronic devices, a key requirement common to both ZnO and GaN is to grow epitaxial layers with low defect density, which depends strongly on the choice of substrates as well as on the crystal quality of intermediate or buffer layers. − Several approaches aimed at reducing the dislocation density in GaN layers have been successfully applied. The representative methods are epitaxial lateral overgrowth (ELO) and pendeoepitaxy (PE), which result in low defect density in selected areas. − However, these techniques are not effective for ZnO films, and the systematic studies on the development of novel methods are lacking.…”

Enhanced near-band-edge emission from a-plane ZnO thin films on SrTiO3 substrates

“…The c-ZnO thin film exhibits the sixfold symmetry. In details, the epitaxy relationship is described by ZnO [11][12][13][14][15][16][17][18][19][20]//STO [1][2][3][4][5][6][7][8][9][10] and ZnO [11][12][13][14][15][16][17][18][19][20]//STO [2-1-1]. The lattice mismatch between ZnO and STO in this epitaxy mode is only *1.9 %, much smaller than that between c-ZnO and sapphire, and the lattice match geometry is schematically shown in Fig.…”

“…However, defects are easily generated in epitaxial ZnO films on the sapphire due to the large lattice mismatch of 18 % [14], including misfit dislocations [15,16] and twin domains [14,17,18]. On the other hand, increasing attentions are paid to obtain in-plane polar epitaxial ZnO films, such as (10-10)-oriented (m-plane) and (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)-oriented (a-plane) ones [19]. It is known that ZnO has its intrinsic electrical polarity along the c-axis in the wurtzite structure, and thus, the c-ZnO thin films inevitably suffer the quantum-confined Stark effect (QCSE), which reduces the exciton binding energy and limits the optical performance.…”

“…Successful preparations of (0001)-oriented ZnO (c-ZnO) films on various substrates including sapphire have been well reported [12,13]. However, defects are easily generated in epitaxial ZnO films on the sapphire due to the large lattice mismatch of 18 % [14], including misfit dislocations [15,16] and twin domains [14,17,18]. On the other hand, increasing attentions are paid to obtain in-plane polar epitaxial ZnO films, such as (10-10)-oriented (m-plane) and (11-20)-oriented (a-plane) ones [19].…”

“…For the c-ZnO films, the epitaxy relationship is described by ZnO [10-10]//STO [1][2][3][4][5][6][7][8][9][10] and ZnO [11][12][13][14][15][16][17][18][19][20] …”