Nanoscale Potential Fluctuation in Non-Stoichiometric Hafnium Suboxides

We study the structure of nonstoichiometric HfO_x films with variable composition using methods of X-ray photoelectron spectroscopy and spectroscopic ellipsometry. HfO_x, to a first approximation, is a mixture of HfO₂ and Hf metal with a small amount (~10–15%) of hafnium sub-oxide HfO_y (y<2).

In modern silicon devices SiO₂ is superseded by high-κ dielectrics, such as HfO₂, ZrO₂, Ta₂O₅ etc. Hafnia (HfO₂) permittivity depends on the modification, varies in the range of 12–40. HfO₂ is the promising material for CMOS devices, DRAM capacitors, and memory insulator in SONOS-type flash memory cells [High Permittivity Gate Dielectric Materials, S. Kar (Ed.), Springer Series in Advanced Microelectronics 43 (2013)]. Of great interest is the use of nonstoichiometric hafnium suboxides HfO_x (x<2). Variation of HfO_x chemical composition (stoichiometry) leads to changes in its electronic structure, which opens up the possibility of controlling the physical (optical and electrical) properties. Comparing with tetrahedral compounds of silicon, coordination numbers of Hf and O atoms in HfO₂ are high. Thus, is not clear which model describes the structure of nonstoichiometric HfO_x. Purpose of the present work is to study the atomic and electronic structure of variable composition HfO_x.

HfO_x films were produced using Hf target ion beam sputtering deposition in oxygen. The composition (x-parameter) of HfO_x was defined by partial oxygen pressure. For our experiments we grew three sets of HfO_x samples at the partial oxygen pressures of 4.4×10^-4Pa, 1.0×10^-3Pa, and 3.6×10^-3Pa. In these conditions, we produced two sets of the samples of nonstoichiometric (x<2) films and one set of almost stoichiometric composition (x≈2), respectively.

The core-level Hf4f_7/2-Hf4f_5/2 and valence band spectra of HfO_x films were obtained using XPS machine with monochromatic Al Kα radiation. The dispersive refractive index and absorption coefficient of HfO_x films were determined by means of spectroscopic ellipsometry.

The higher-level electronic-structure calculations were performed using the density functional theory with the QUANTUM ESPRESSO software package.

XPS spectra of Hf4f_7/2-Hf4f_5/2 level in HfO_x variable composition are shown in Fig.1. For HfO_x, grown at high pressures of oxygen, a peak is observed at an energy corresponding to the stoichiometric HfO₂. Decreasing the oxygen pressure during HfO_x synthesis accompanied by the appearance of XPS peaks corresponding to metal Hf. With decreasing of oxygen pressure, the intensity of the peaks corresponding to metal hafnium increases. The decomposition of the spectra indicates that in addition to HfO₂ and Hf the films have a nonstoichiometric phase of HfO_y. Corresponding to this phase peaks are located approximately in the middle between the Hf- and HfO₂-related peaks (Fig.1). So, according to XPS HfO_x is a mixture of stoichiometric HfO₂, metal Hf, and nonstoichiometric phase of HfO_y.

Experimental valence band spectra of HfO₂ is in good agreement with simulated spectra for m-HfO₂ as shown in Fig.2. A proof of the existence of nonstoichiometric hafnia in our films can be obtained from the comparison of the experimental valence-band XPS with the corresponding one calculated from the first principles for m-HfO₂ with neutral oxygen vacancy and polyvacancy (Fig.2). The calculation satisfactorily describes the experiment, with the fact that the XPS spectra are compared for HfO₂ in amorphous and monoclinic phases. The presence of the neutral oxygen vacancy in m-HfO₂leads to the defect levels at 3.2eV.

Fig.3 shows the spectra of absorption coefficient α for HfO_x. Bandgap of amorphous HfO₂ is E_g=5.6eV [V.V. Afanas’ev et al., J. Appl. Phys. 102, 081301 (2007)]. HfO_x absorption coefficient increases monotonically with increasing of photon energy E in the range of 1.1-4.5eV. At E>4.5eV a sharp increase of α is observed. In the range of 1.1-4.5eV absorption is low due Hf metallic clusters. Absorption at E>4.5eV is caused by nonstoichiometric HfO_y presence.

According to XPS and optical absorption experiment data HfO_x consists of metal Hf and nonstoichiometric HfO_y. HfO_y can be placed between HfO₂ and Hf, inside HfO₂, inside Hf. Fig.4(a) shows planar model of HfO_x in terms of intermediate structure model (IM). According to this model HfO_x consists of three phases: HfO₂, HfO_y и Hf. Fig.4(b) shows energy diagram of HfO_x. Electron affinity оf HfO₂ is 2.0eV [W. Zheng et al., J. Phys. Chem. A 109, 11521 (2005)]. According to IM space fluctuations of chemical composition cause space fluctuations of bandgap in HfO_x (Fig.4(b)). Bandgap HfO_x varies in the range of 0-5.6eV. The bottom of the conduction band E_c against the energy of an electron in a vacuum E₀ varies in the range of 2.0-4.0eV, which leads to E_c fluctuation scope of 2.0eV. The valence band ceiling E_v against E₀ varies in the range of 3.9-7.7eV.