ZnO Growth on Si with Low-Temperature CdO and ZnO Buffer Layers by Molecular-Beam Epitaxy

Journal of Electronic Materials, Apr 2006 by Xiu, F X, Yang, Z, Zhao, D T, Liu, J L, Et al

Low-temperature (LT) buffer-layer techniques were employed to improve the crystalline quality of ZnO films grown by molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra show that CdO, as a hetero-buffer layer with a rock-salt structure, does not improve the quality of ZnO film grown on top. However, by using ZnO as a homo-buffer layer, the crystalline quality can be greatly enhanced, as indicated by PL, atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman scattering. Moreover, the buffer layer grown at 450°C is found to be the best template to further improve the quality of top ZnO film. The mechanisms behind this result are the strong interactions between point defects and threading dislocations in the ZnO buffer layer.

Key words: ZnO, buffer layer, atomic force microscopy (AFM), photoluminescence (PL), x-ray diffraction (XRD), Raman scattering

INTRODUCTION

With a large exciton binding energy of 60 meV, ZnO has been considered as a promising material for next-generation optoelectronic devices, such as blue-light emitting and short-wavelength laser diodes with low thresholds in the ultraviolet region.1 While many researchers are focusing on fabricating p-type ZnO films, much attention has been paid to the quality of undoped ZnO films, which serve as the basis for the development of reliable and reproducible p-type films. Some native defects, such as zinc interstitials (Zn^sub i^) and oxygen vacancies (V^sub 0^), are known to provide electrons to compensate holes from acceptors,2 thus making p-type ZnO even harder to fabricate. Therefore, eliminating these native defects and dislocations becomes a rather important issue for the development of ZnO-based optoelectronic devices.

For potential integration with Si-based circuits and devices, the growth of high-quality ZnO film on Si is of great importance. The main obstacles to overcome, however, are large lattice mismatch and avoiding an amorphous SiO^sub x^ layer generated in the interface, because Si is easily oxidized in oxygen environments.'5 To solve these problems, use of ZnS,4 Zn metal layer,5 CdF^sub 2^,6 GaN,7 and nitridation of Si surface8 have been studied. But little progress has been made so far. Currently, low-temperature (LT) buffer-layer techniques are being paid much attention, and remarkable improvement in the crystalline quality of ZnO have been reported with various growth techniques.9-12 However, optimization of buffer layers grown by MBE has not been performed, and temperature effects of buffer layers on ZnO crystalline quality have not yet been studied.

In this paper, LT-CdO-buffer- and LT-ZnO-bufferassisted ZnO films were grown and characterized by low-temperature-photoluminescence (LT-PL), atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman scattering. Temperature-dependent growth conditions for buffer layers were reported, and growth mechanisms were analyzed.

EXPERIMENTAL PROCEDURES

ZnO films were grown on Si (100) with LT CdO and ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted MBE. Elemental zinc (5N), cadmium (6N), and oxygen gas (5N) were used as molecular beam sources. Zinc and cadmium were evaporated by LT effusion cells. Oxygen plasma was generated by an ECR plasma source. Si (100) substrates are n-type wafers with resistivity of 20-30 ohm cm. All of these substrates were cleaned by the Piranha-HF method and dried with nitrogen gas.

During growth, several steps were followed. In step I, Si substrates were thermally cleaned at 650°C for 10 min. In step II, CdO and ZnO buffer layers were deposited on Si at different growth temperatures of 350, 450, and 550°C. In step III, ZnO films were grown on top of buffer layers at 550°C. For all these samples, growth conditions for top ZnO films remained the same while growth temperatures of the buffer layers were different, as shown in Table I. In addition, a ZnO film without a buffer layer was grown directly on a Si substrate for comparison with the samples mentioned above.

PL spectra were measured by the excitation from a 325-nm He-Cd laser at 8 K. A Nanoscope III AFM was employed to study surface morphologies. A Bruker Advanced D8 x-ray diffractometer was utilized to investigate ZnO crystalline quality. A θ-2θ scan was performed to determine growth orientations. A Renishaw micro-Raman spectrometer 2000 with visible (488 nm) excitation lasers was used to measure Raman scattering spectra at room temperature.

RESULTS AND DISCUSSION

We have attempted to use CdO as a hetero-buffer layer to improve ZnO crystalline quality. LT-PL measurements were carried out at 8 K to study optical properties of ZnO/CdO/Si films, as shown in Fig. 1. Strong near-band-edge UV emissions associated with neutral-donor-bound excitons (D^sup o^X) are observed at 3.355 eV, which are attributed to native defects (V^sub o^ and Zn^sub i^)2 and/or H incorporation.13 For samples with CdO buffers (samples a-c), however, broad peaks between 2.80 and 3.20 eV are found besides D^sup o^X emissions. These peaks are mainly associated with nonradiative recombination centers in the forbidden gap of ZnO as a result of high-density dislocations in these films. In addition, some Cd atoms might diffuse into top ZnO layers during growth; therefore a thin layer of CdZnO could be developed in the interface between ZnO and CdO, contributing to these peaks. Moreover, when compared with those from sample (d), the full-width at half-maximum (FWHM) values remain the same after employing CdO buffer layers, indicating that CdO layers with a temperature region of 350-550°C and a thickness of 0.5 µm may not be ideal conditions for improving ZnO crystalline quality.