Nanoscale Potential Fluctuation in Non-Stoichiometric Hafnium Suboxides
Authors: Orlov Oleg M. , Krasnikov Gennady Ja. , Gritsenko Vladimir A. , Kruchinin Vladimir N. , Perevalov Timofey V. , Aliev Vladimir Sh. , Islamov Damir R. , Prosvirin Igor P. 
Published: February 2020

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In modern silicon devices SiO2 is superseded by high-κ dielectrics, such as HfO2, ZrO2, Ta2O5 etc.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 HfOx. 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 E
0 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 E
0 varies in the range of 3.9-7.7eV.

Figure 1