Growth of thin zirconium and zirconium oxides films on the n-GaN(0 0 0 1) surface studied by XPS and LEED
Introduction
The insulating oxide layers, especially used in electronic devices and sensors, were based in the past mainly on SiO2. Due to increasing miniaturization transistor gates with silicon dioxide reached the limit of the thickness for which the tunnelling current leakage has unacceptably high values [1]. Such behaviour forced attempts to find new insulating materials. The promising ones are zirconium and zirconium dioxide [2], [3]. They exhibit a high thermal stability, low thermal conductivity, high durability, very low thermal neutron absorption and high resistance to corrosion. Moreover, zirconium dioxide is a high-κ gate dielectric. All these properties cause that both Zr and ZrO2 are materials very widely used in many applications, such as catalysis, medical, biomechanical and electronic devices, fuel cell technology and photonics [4], [5], [6], [7].
On the other hand, nowadays there is a huge interest in gallium nitride. It is characterized by high thermal stability, large breakdown electric field, good electron mobility and thermal conductivity. As a wide band-gap semiconductor, gallium nitride has attracted interest due to its wide applications in light emitters and detectors, high frequency field effect transistors or high power microwaves devices [8], [9]. One of the important requirements for these systems is a more reliable and thermally stable Schottky contact on n-type GaN. A good example is ZrN/Zr/GaN structure, where the Zr/GaN interface has proper thermal stability [10], [11]. That kind of nitride-based semiconductor devices typically consist of many layers of various materials, so it is important to understand processes involved in the interfaces formation, their chemical composition and structure as well as surface reconstructions in adsorbed layers.
Section snippets
Material and methods
The samples used were 10 μm thick n-GaN (Si-doped, 1018 cm−3) deposited on Al2O3 substrates (producer: Technologies and Devices International, An Oxford Instruments Company). Typical size of samples was about 4 × 8 mm2. Before inserting into the vacuum chamber, the substrates were degreased in alcohol, washed in distilled water and dried in air. Before the Zr deposition the GaN surface was annealed in cycles at the temperature about 800 °C to remove the surface oxide and carbon contaminations.
Thin
Results and discussion
The LEED optics and the XPS analyzer were used to investigate the bonding environment and to determine the main structural characteristics of the topmost layer of the surface. Fig. 1 presents the Ga 3d, N 1s, O 1s and Zr 3p XPS spectra observed for the clean and adsorbate-covered samples in the first experiment (in UHV).
For the virgin sample, although the substrate was cleaned and annealed, there is still a small amount of oxygen detected in the XPS spectrum. That is why O 1s spectrum is also
Conclusions
The experiments performed under various condition have shown that during Zr deposition onto the n-GaN(0 0 0 1) surface zirconium oxides and nitrides are produced in a different ratio. In the experiment performed in UHV, ZrN and ZrNxOy were the dominant compounds in adlayer, while in the experiment under oxygen pressure (10−7 Torr) – ZrOx (x < 2) and ZrO2. In both cases gallium was not involved in forming the adlayer. The LEED observations and the analysis of the substrate XPS signal decrease
Acknowledgments
The work was supported by Wroclaw Research Centre EIT+ within the project “The Application of Nanotechnology in Advanced Materials” – NanoMat (POIG.01.01.02-02-002/08) co-financed by the European Regional Development Fund (operational Programme Innovative Economy, 1.1.2) and by the University of Wrocław under the grant 1347/M/IFD/13.
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