Short communication
Epitaxial growth of zinc oxide thin films on silicon

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Abstract

Epitaxial zinc oxide thin films were grown on Si(1 1 1) using aluminum nitride and magnesium oxide/titanium nitride buffer layers. The resultant films were examined using transmission electron microscopy, X-ray diffraction, electrical conductivity, and photoluminescence spectroscopy. The following epitaxial relationships were observed in the ZnO/AlN/Si(1 1 1) heterostructure: ZnO[0 0 0 1] || AlN[0 0 0 1] || Si[1 1 1] along the growth direction, and ZnO[21¯1¯0] || AlN[21¯1¯0] || Si[011¯] along the in-plane direction. Domain-matching epitaxial growth of TiN on Si(1 1 1) substrate allows successful epitaxial growth of MgO and ZnO layers in a ZnO/MgO/TiN/Si(1 1 1) heterostructure. The epitaxial relationships observed for this heterostructure were ZnO[0 0 0 1] || MgO/TiN/Si[1 1 1] along the growth direction and ZnO[21¯1¯0] || MgO/TiN/Si[011¯] along in-plane direction. The resultant ZnO films demonstrate excellent electrical and optical properties. ZnO thin films exhibit extremely bright ultraviolet luminescence with relatively weak green-band emission.

Introduction

Zinc oxide is an attractive II–VI semiconductor that has great potential for using in ultraviolet- blue semiconductor lasers and light emitting diodes. It has a hexagonal wurtzite structure, with lattice parameters of a = 3.2498 Å and c = 5.2066 Å. ZnO thin films have been grown using several different deposition techniques, including pulsed laser deposition (PLD) [1], [2], RF-magnetron sputtering [3], spray pyrolysis [4], etc. Sapphire (α-Al2O3)(0 0 0 1) substrates have been widely used for growing zinc oxide thin films. The mechanism for ZnO/(α-Al2O3) epitaxial growth is characterized by domain-matching epitaxy, in which there is an in-plane epitaxial orientation between the film and the substrate that corresponds to a 30° or 90° rotation of the film with respect to the substrate (0 0 0 1) basal plane [5]. Rotation allows six planes of the zinc oxide film to match approximately seven planes of the sapphire substrate. Zinc oxide thin films on sapphire are used in light emitting diodes, in which emission from both sides is desired. On the other hand, epitaxial growth of zinc oxide on a silicon substrate is necessary to integrate these films with microelectronic devices.

In this work, we report epitaxial growth of zinc oxide thin films on silicon(1 1 1) substrates using aluminum nitride and magnesium oxide/titanium nitride buffer layers. Aluminum nitride has a hexagonal wurtzite structure similar to that of zinc oxide, with lattice parameters of a = 3.11 Å and c = 4.98 Å. The epitaxial growth of aluminum nitride thin films on Si(1 1 1) occurs by means of domain matching epitaxy [6], in which integral multiples of the major lattice planes of the film and the substrate match across the film–substrate interface. Zinc oxide thin films can be grown on aluminum nitride by means of lattice-matching epitaxy, since the lattice misfit between these two materials is ∼4%. On the other hand, titanium nitride has a cubic sodium chloride structure, with a lattice parameter of 4.24 Å. Titanium nitride has been grown on Si(1 0 0) by means of cube-on-cube domain-matching epitaxy, in which four lattice planes of titanium nitride match three lattice planes of silicon with less than 4% misfit [7]. Magnesium oxide has a sodium chloride structure, with a lattice parameter of 4.22 Å. Growth of titanium nitride thin films on magnesium oxide substrates and magnesium oxide thin films on titanium nitride substrates occurs by means of lattice-matching epitaxy, since titanium nitride and magnesium oxide possess nearly matched lattice parameters and crystal structures [8], [9]. ZnO/AlN/Si(1 1 1) and ZnO/MgO/TiN/Si(1 1 1) heterostructures were studied by transmission electron microscopy, X-ray diffraction, electrical conductivity, and photoluminescence spectroscopy.

Section snippets

Experimental procedures

ZnO/AlN/Si(1 1 1) and ZnO/MgO/TiN/Si(1 1 1) heterostructures were grown by pulsed laser deposition using a KrF excimer laser (λ = 248 nm, ts = 25 ns) in a high vacuum chamber. Multi-layered heterostructures were formed by sequential deposition using a multiple target holder. Silicon(1 1 1) substrates were cleaned in 10% hydrofluoric acid solution to remove any surface oxide. The resulting silicon substrate retained a hydrogen-terminated surface. Commercially obtained stoichiometric aluminum nitride,

ZnO/AlN/Si(1 1 1) heterostructure

The X-ray diffraction pattern (intensity versus 2θ) of the ZnO/AlN/Si(1 1 1) heterostructure is shown in Fig. 1. The diffraction pattern contains Si(1 1 1), AlN(0 0 0 2), AlN(0 0 0 4), ZnO(0 0 0 2), and ZnO(0 0 0 4) reflections. The X-ray diffraction results reveal that the ZnO and AlN films are crystalline with remarkable alignment of ZnO[0 0 0 1] || AlN[0 0 0 1] || Si[1 1 1] growth.

Transmission electron microscopy diffraction studies confirm the single crystalline epitaxial nature of zinc oxide in the ZnO/AlN/Si(1 1 1)

Concluding remarks

ZnO/AlN/Si(1 1 1) and ZnO/MgO/TiN/Si(1 1 1) heterostructures have been developed that allow the growth of single-crystal zinc oxide films on silicon substrates. These heterostructures were examined using X-ray diffraction and transmission electron microscopy. In the ZnO/AlN/Si(1 1 1) heterostructure, a domain-matching epitaxial relationship was observed between the aluminum nitride buffer layer and the Si(1 1 1) substrate. A lattice-matching epitaxial relationship was observed between the aluminum

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