ZnO Heteroepitaxy on Sapphire Using a Novel Buffer Layer of Titanium Oxide: Crystallographic Behavior

Abstract

A novel buffer layer consists of titanium oxide grown on a-sapphire by low-pressure chemical vapor deposition using titanum-tetra-iso-propoxide and oxygen gas was used for ZnO epitaxial growth at temperature as low as 340˚C by plasma-assisted epitaxy using radio-frequency oxygen-gas plasma. XRD and RHEED indicated (0001)Ti 2 O 3 layer in corundum crystal system was epitaxially grown on the substrate in an in-plane relationship of [1-100]Ti 2 O 3 // [0001]Al 2 O 3 by uniaxial phase-lock system. Growth behavior of ZnO layer was significantly dependent on the Ti 2 O 3 buffer-layer thickness, for example, dense columnar ZnO-grains were grown on the buffer layer thinner than 10 nm but the hexagonal pyramid-like grains were formed on the thin buffer layers below 2 nm. RHEED observations showed ZnO layer including the pyramid-like grains was epitaxially grown with single-domain on the thin buffer layer of 0.8 nm in the in-plane relationship of [1-100]ZnO//[1-100]Ti 2 O 3 //[0001]Al 2 O 3 , whereas the multi-domain was included in ZnO layer on the buffer layer above 10 nm.

ZnO Heteroepitaxy on Sapphire Using a Novel Buffer Layer of Titanium Oxide: Optoelectronic Behavior

“…In contrast, when the buffer layer was grown at 320˚C, the PL-property was more sensitive to the buffer layer thickness, but the thickness could be optimized at 0.8 nm for both of A-E x intensity and the intensity ratio. The layer grown on the optimized buffer layer grown at 320˚C showed smooth surface partially including hexagonal pyramid-like grains as shown in Figure 3(b), where the number of pyramidlike grain was most reduced in the ZnO layer on the optimized buffer layer [10]. It was found that the pyramid-like grains were originated from surface roughness and dislocations on a-sapphire and could be prevented by chemical etching of the substrate using a hot 3H 2 SO 4 + H 3 PO 4 solution.…”

“…The layer was epitaxially grown on the buffer layer with the epitaxial relationship of ZnO//[1-100] Ti 2 O 3 , whereas the layer was polycrystallized on a-sapphire. The hexagonal pyramid-like grains with the facets according to the epitaxial relationship as shown in the inset of (b) were observed in ZnO layer on the buffer layer, which indicated the two-dimensional growth was enhanced on the buffer layer as described elsewhere [10]. In the near band-edge PL-feature, strong and sharp exciton emissions could be observed from ZnO layer on the buffer layer, in contrast to board and weak emissions from poly-crystallized ZnO layer directly grown on a-sapphire.…”

“…Details of the PAE-apparatus and the growth process were described elsewhere [4,10]. ZnO growth rate was different by change of the growth mode on c-, a-and the buffer layer as shown elsewhere [10], but effective Zn/O supply ratio during the growth was kept at 1.0 by control of Znflux determined by dependence of the growth rate on the Zn-flux.…”

“…Details of the apparatus and the condition was shown elsewhere [10]. In the case of buffer layer growth for ZnO layer, the thickness was controlled by the growth period estimated from the growth rate.…”

“…Therefore, suitable two-dimensional ZnO growth at low temperatures with reducing impurity-donors and the intrinsic donor-defects is required for the optoelectronic device applications. For the purpose, we previously demonstrated drastic improvement of ZnO growth by Ti 2 O 3 buffer layer on a-sapphire [10], where two-dimensional epitaxial growth of ZnO layer was successfully achieved without the rotational domain.…”