Modeling growth rate of HfO2 thin films grown by metal–organic molecular beam epitaxy
Introduction
The rapid progress of complementary metal-oxide-semiconductor (CMOS) integrated circuit technology since the late 1980s resulted in the reduction of the gate oxide thickness. With the reduction of gate dielectric thickness to a few nanometers, higher dielectric constant material than the conventional SiO2 (i.e. high-k material) is needed to overcome the problem of an exponential increase in the leakage current level due to direct tunneling. Many researches for high-k dielectric materials such as ZrO2, Ta2O5, Al2O3, HfO2, TiO2, silicates (ZrSixOy and HfSixOy), STO, and BST have been performed [1], [2], [3], [4], [5]. Among these candidates, HfO2 is one of the most highlighted high-k gate insulators because of its high dielectric constant (25–30), wide band-gap energy (5.68 eV), high breakdown field (15–20 MV/cm2), and good thermal stability on Si substrate. The growth method of high-k materials is also an important factor to determine the property of gate dielectric layer. In our experiments, we searched appropriate experimental conditions and examined the characteristics of HfO2 films using metal–organic molecular beam epitaxy (MOMBE) system. MOMBE is one of the powerful techniques obtaining abrupt interface and controlled thickness of films, mainly due to source evaporation at a controlled rate under ultra high vacuum condition [6]. In this paper, the relationships between the growth rate of HfO2 films grown by MOMBE and process variables of MOMBE systems are investigated. The electrical characteristics of HfO2 films are investigated by high frequency (HF) capacitance–voltage (C–V) and current–voltage (I–V) measurements. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) were also performed to analyze the HfO2 films grown by MOMBE systems.
Section snippets
Experiment
HfO2 thin film was grown on a p-type Si (100) substrate, of which the native oxide was chemically eliminated by (50:1) H2O/HF solution prior to growth by MOMBE. Hafnium-tetra-butoxide [Hf(O·t-C4H9)4] was chosen as the MO precursor because it has a appropriate vapor pressure and relatively low decomposition temperature. High-purity (99.999%) oxygen gas was used as the oxidant. Hf-t butoxide was introduced into the main chamber using Ar as a carrier gas through a bubbling cylinder. The bubbler
Result and discussion
The metal-oxide-semiconductor (MOS) capacitor structure Au/HfO2/p-type Si was fabricated to measure its electrical characteristics. The high frequency (1 MHz) C–V curve and I–V curve of HfO2 films grown by MOMBE are shown in Fig. 2. From measured capacitance and physical thickness, the dielectric constant and the equivalent oxide thickness can be obtained. Actually, the total capacitance must be calculated as series connection of HfO2 layer capacitance and SiO2 layer capacitance because an
Conclusion
The characteristics of the HfO2 dielectric layer on the p-type Si substrate by MOMBE process were investigated. HfO2 films grown by MOMBE had a high dielectric constant (k=20–22). It was identified that there existed the fixed oxide charge (∼8×1011 cm−2) and interface state density (∼1×1012 eV−1cm−2) in the HfO2 layer. Leakage current density of HfO2 film grown by MOMBE is about 10−9–10−7 A/cm2 in −2 to 2 V gate voltage. It is observed that the growth rate of HfO2 film was affected by the substrate
Acknowledgements
This research was supported by the MIC (Ministry of Information and Communication), Korea, under the ITRC (Information Technology Research Center) support program supervised by the IITA (Institute of Information Technology Assessment)
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Impact of the crystal structure of HfO2 on the transport properties of model HfO2 /Si/ HfO2 silicon-on-insulator field-effect transistors: A combined DFT-scattering theory approach
2009, Physical Review B - Condensed Matter and Materials Physics