Optical properties and bandgap evolution of ALD HfSiOx films
© Yang et al.; licensee Springer. 2015
Received: 16 November 2014
Accepted: 30 December 2014
Published: 5 February 2015
The Erratum to this article has been published in Nanoscale Research Letters 2015 10:378
Hafnium silicate films with pure HfO2 and SiO2 samples as references were fabricated by atomic layer deposition (ALD) in this work. The optical properties of the films as a function of the film composition were measured by vacuum ultraviolet (VUV) ellipsometer in the energy range of 0.6 to 8.5 eV, and they were investigated systematically based on the Gaussian dispersion model. Experimental results show that optical constants and bandgap of the hafnium silicate films can be tuned by the film composition, and a nonlinear change behavior of bandgap with SiO2 fraction was observed. This phenomenon mainly originates from the intermixture of d-state electrons in HfO2 and Si-O antibonding states in SiO2.
With the downscaling of CMOS devices, high-k materials are required to replace SiO2 as gate dielectrics in order to decrease the direct tunneling leakage current and, at the same time, maintain the gate capacitance at a certain value [1-7]. Among the potential candidates, hafnium silicate was chosen as the first generation of high-k dielectrics for its high dielectric constant and excellent thermal stability [8,9]. Compared to other deposition methods used for hafnium silicate film fabrication, atomic layer deposition (ALD) has the advantages of precise film thickness and stoichiometry control, which are of great significance to optimize the material especially for the shrinking devices [10-15].
Since accurate determination of the optical properties is an essential prerequisite for device simulations and gives the opportunity to improve material preparation, we have applied vacuum ultraviolet (VUV) spectroscopic ellipsometry to investigate the optical characteristics of a set of hafnium silicate films in this work. It will also gain us an insight into the effect of film composition on the electrical performance and chemical states of hafnium silicate dielectric films.
The targeted HfSiOx thin films were deposited on lightly doped p-type Si (100) substrates by a BENEQ TFS-200 ALD system (BENEQ Oy, Espoo, Finland) at 200°C. The Si wafers were cleaned via RCA cleaning process at first, then prior to film growth, they were cleaned again in a diluted HF solution (50:1) to remove the native oxide and passivate the silicon dangling bonds followed by a deionized water rinsing and drying in N2. During deposition process, precursors for Hf, Si, and O were TEMAH, TDMAS, and O2 plasma respectively. TEMAH was kept at 80°C in a stainless bottle, and TDMAS was kept at room temperature. The O2 plasma was activated at the power of 100 W. Typical pulsing sequences during the ALD process are 1/3/2/2 s (TEMAH/Ar purge/O2 plasma/Ar purge) and 2/2/2/2 s (TDMAS/Ar purge/O2 plasma/Ar purge) for the growth of HfO2 and SiO2 films, respectively. For the HfSiOx films, SiO2 percentage was controlled by deposition cycle ration of HfO2: SiO2. Pure HfO2, SiO2, and five groups of HfSiOx samples with different atomic compositions were prepared.
For optical characterization, each sample was measured using a Woollam variable-angle vacuum ultraviolet spectroscopic ellipsometer (SE), and the data were analyzed with the software Complete EASE by J.A. Woollam. The measurements were taken at two angles of incidence, 67.5° and 75°, with a spectroscopic range of 0.6 to 8.5 eV. Then, to determine the optical properties of the target samples, such as layer thickness and optical constants, the model-based analysis were carried out. Complete EASE includes a wide range of built-in functions, such as Lorentz, Gaussian, Drude, Tauc-Lorentz, and Cody-Lorentz. These functions can be used to approach a wide variety of thin film, ranging from dielectrics and organics to semiconductors and metals. In this work, the Cauchy dispersion relation was adopted for the determination of the films thickness and the optical properties were analyzed with the Gaussian dispersion model.
Results and discussion
Thickness of samples by Cauchy dispersion relation
9.80 ± 0.015
12.00 ± 0.020
10.39 ± 0.021
9.36 ± 0.026
8.80 ± 0.026
11.36 ± 0.039
11.13 ± 0.049
In the equations above, the P stands for the Cauchy principal part of the integral. The Gaussian oscillator model employs four setting parameters: the amplitude A, the broadening parameter B r, the center energy E n, and the non-dispersive term ε 1 (∞). The two fitting parameters B r and E n are in units of energy while A and ε 1 (∞) are dimensionless. ε 1 (∞) represents the contribution of the optical transitions at higher energies and appears as an additional fitting parameter .
The evolution of extinction coefficient k is shown in Figure 2b. For all samples, k saturates to zero in the visible region, suggesting the realization of high quality HfSiOx films in terms of optical properties. An abrupt increase in the extinction coefficient for higher photon energy is due to the fundamental bandgap absorption in the films. Moreover, as can be seen in Figure 2b, there is also a mall band tail below the gap. This weak absorption is attributed to the Urbach tail which exists below the bandgap of amorphous materials and due to the disorder of the amorphous network [23,24]. Furthermore, with the increase of Si concentration, decrease in the magnitude of the exponential tail can be observed, and similar results were obtained by the Cody-Lorentz model and Tauc-Lorentz model . According to J. Price et al., this phenomenon is attributed to Si atoms filling the O2 vacancies/defects in the HfO2 films. By filling these vacancies, there is less disorder and therefore, less intraband absorption.
The optical properties of ALD hafnium silicate films, together with pure HfO2 and SiO2 films as references, were investigated systematically based on Gaussian dispersion model. According to the results, optical constants and bandgap of the hafnium silicate films can be tuned by the film composition, and a nonlinear change behavior of bandgap with SiO2 fraction was observed. This phenomenon mainly originates from the intermixture of d-state electrons in HfO2 and Si-O antibonding states in SiO2.
This work was supported by the NSFC (61076114, 61106108) and Shanghai Educational Develop Foundation (10CG04), SRFDP (20100071120027), and the S&T Committee of Shanghai (1052070420).
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