Stoichiometry, band alignment, and electronic structure of Eu2O3 thin films studied by direct and inverse photoemission: A reevaluation of the electronic band structure

Abstract

The electronic structure of Eu sesquioxide (Eu2O3) presents a significant challenge to the electronic structure theory due to the presence of correlated Eu semicore 4f electrons. The bandgap values do not agree between computational methods, and even experimentally, there are discrepancies between reports. Eu2O3 was grown epitaxially in a thin film form on n-type GaN (0001) by molecular beam epitaxy. The film was analyzed using UV and x-ray photoemission spectroscopies as well as inverse photoelectron spectroscopy in order to characterize both occupied and unoccupied states. Signatures of Eu2+ are detected after annealing in UHV or after exposure to air, which can be removed by subsequent O2 annealing. The sample reduction is shown to strongly affect the electronic structure. The bandgap of 4.3 eV, electron affinity of 2.2 eV, and band alignment to the substrate with a valence band offset of 0.2 eV for a stoichiometric Eu2O3 film were extracted from the measurements of the occupied and unoccupied electronic states. The electronic structure is interpreted in view of recent theoretical models, and the energy band alignment across the Eu2O3/GaN interface is discussed.

Evidence for the Eu 4f Character of Conduction-Band Edge at the Eu2O3 Surface Studied by Scanning Tunneling Spectroscopy

“…The experiments were performed in two separate ultra-high vacuum (UHV) systems. The Eu 2 O 3 film was grown on GaN(0001)/p-Si(111) substrates at 800 o C by MBE; the growth, sample and UHV system details are described in [1,4].The Eu 2 O 3 film revealed a streaky reflection high-energy electron diffraction (RHEED) pattern after formation, as shown in Figure 1. The rotational symmetry of the pattern was 6-fold, the same as the underlying GaN susbstrate pattern.…”

“…Before STS measurements, the sample was heated in O 2 at a pressure in the 10 -6 mbar range at around 600 o C for 30 min. This is required to restore the stoichiometry of the Eu 2 O 3 surface, which is shown to reduce significantly upon exposure to air [4]. The sample heating was performed by passing a direct current through the substrate.…”

“…The Eu atoms in the sesquioxide (Eu 2 O 3 ) have the highest possible oxidation state (+3), and therefore, during synthesis the stoichiometry of this form can be precisely achieved under oxygen-rich conditions [1]. Having a dielectric constant of about 14 [2] and a measured band gap of ~4.3-4.4 eV [3,4], Eu 2 O 3 is considered to be attractive as a high-K gate dielectric [5]. Also, epitaxial monoclinic B-type Eu 2 O 3 can act as a precursor in a topotactic transformation reaction for growing epitaxial rock salt EuO [6], which is an appealing ferromagnetic semiconductor for spintronics [7,8].…”

“…In a recent study combining normal and inverse photoemission, the details of the band structure, namely the band gap, ionization energy, and electron affinity, of stoichiometric Eu 2 O 3 thin films grown on n-type GaN(0001) have been elucidated [4]. In particular, it has been determined that the directly measured bandgap of Eu 2 O 3 is 4.3 eV, which agrees well with the values reported on the basis of optical measurements [7].…”

“…It is assumed in Ref. [4] that this narrowing is caused by weak unoccupied 4f states, strongly hybridized with 5d6s states, near the Eu 2 O 3 CB edge, while the bottom of CB in the Ln 2 O 3 series is typically determined solely by unoccupied Ln 5d6s states. However, a more detailed band structure near the CB edge of Eu 2 O 3 has not been determined experimentally in the earlier studies because it is challenging to probe the empty levels in general.…”