Novel oxides

We explore a variety of different semiconducting metal oxides. Most of them are sesquioxides with an M₂O₃          (M = metal) structural formula and occur as polymorphs, i.e. existing in several different crystal structures.

In₂O₃

Indium oxide is a technologically important material known for high conductivity when doped by Sn while maintaining transparency in the visible spectral range. Its typical applications are therefore found in transparent electronics, plasmonics or as front contacts for optical devices such as solar cells or thin film optical displays. [Feneberg et al., Phys. Rev. B 93, 045203 (2016).]

Here we explore In₂O₃ in its stable cubic form Bixbyite and the metastable corundum structure (α-In₂O₃).

Bixbyite In₂O₃

The stable form of indium oxide is cubic (bixbyite) and has 40 atoms in its unit cell. This leads to interesting properties, like 16 infrared active optical phonons, a variety of valence bands, and complications in first-principles calculations due to the extreme computational cost. We investigated single crystals and epitaxial thin films with different free-electron concentrations to study man-body effects on the band structure, the conduction band nonparabolicity, and effective electron masses. 

• Feneberg et al., Phys. Rev. B 93, 045203 (2016).

Furthermore, the dielectric function up to a photon energy of 40 eV yielded the possibility to benchmark first-principles calculations and shed light onto the effect of lattice screening.

• Schleife et al., New J. Phys. 20 053016 (2018).

 

Bixbyite In_xGa(_1-x)₂O₃

Alloys of In₂O₃ have been explored as well:

• Feldl et al., Appl. Phys. Lett. 119, 042101 (2021).
• Papadogianni et al., Phys. Rev. Materials 6, 033604 (2022).

 

α-In₂O₃

With α-In₂O₃ bandgap engineering in the α-In₂O₃–α-Ga₂O₃–α-Al₂O₃ system opens the possibility of tuning the bandgap from 3.8 to 8.8 eV.  We invetigate the Raman and IR-active phonon modes along with the absorption behavior at the bandgap.

•  Cuscó et al., Appl. Phys. Lett. 121, 062106 (2022).

 

 

 

Ga₂O₃ and their alloys

Gallium oxide is a wide band-gap semiconductor suitable for application in high-power electronic devices due to its high break-down voltage. It can crystallize in severall different polymorphs, which have similar basic properties such as a band-gap size of around 5 eV. Monoclinic β-Ga₂O₃ represents the thermodynamically stable phase. We explore the other less researched phases and their alloys in terms of their optical properties.

[Kracht et al., Phys. Rev. Applied 10, 024047 (2018).]

 

α-Ga₂O₃

Corundum-like Ga₂O₃ is a metastable polytype of gallia. Intriguingly, it has the same crystal structure than sapphire (α-Al₂O₃), is thus alloyable, and it can be donor doped. We are interested in the fundamental optical properties, its comparison to sapphire, the anisotropy, and the effective electron mass.

• Segura et al., Phys. Rev. Materials 1, 024604 (2017).
• Kracht et al., Phys. Rev. Applied 10, 024047 (2018).
•  Feneberg et al., Phys. Rev. Materials 2, 044601 (2018).

 

 

 

 

 

 

 

α-(Al_xGa_1-x)₂O₃

As Corundum-like Ga₂O₃ and sapphire (α-Al₂O₃) share the same crystall structure it is alloyable with Al. We are interested in the fundamental optical properties of the alloy. Specifically the anisotropy of the dielctric funtion.

• Kluth et al., Jpn. J. Appl. Phys. 62 051001 (2023).

 

 

 

α-(Ti_xGa_1-x)₂O₃

Similar to AlGa₂O₃ alloing with Ti offers the possibility of badgap energinering over a wide range. We explore the shift of the absorption     onset of only ~20 nm thick films on sapphire subtrate with incresing Ti-content.

• Kluth et al., Appl. Phys. Lett. 122, 092101 (2023).

 

 

 

γ-Ga₂O₃ and κ-Ga₂O₃

Besides the most explored β- and α-phase we are also interested in the fundamental optical proerties of the less explored cubic γ-Phase and the orthorhombic k-Phase, whereby specifically the anisotropy arising from the three different lattice parameters is important to understand.

• Ratcliff, et al., Adv. Mater. 34, 2204217 (2022).

•  Kluth et al., Appl. Phys. Lett. 127, 172106  (2025). 

 

 

 

 

Rutile-Ge_xSn_1-xO₂

The wide bandgap semiconductor rutile SnO₂ when heavily n-doped is well known to be a transparent conducting oxide (TCO) and can be used, e.g., in solar cells, displays, or smart windows. Rutile GeO₂, on the other hand, is a semiconductor of recent interest, as it is theoretically proven to allow an ambipolar doping and a high carrier mobility. We investigate rutile Ge_xSn_1-xO₂ thin films on TiO₂. An evaluation of the dielectric function yields the characteristic transition energies at the absorption onset, which lead to a bowing parameter of the bandgap.

• Kluth et al., Appl. Phys. Lett. 125, 122102 (2024).

 

ZnGa₂⁢O₄

The cubic spinel structure of ZnGa₂O₄ exhibits intrinsic n-type conductivity, caused by Ga_Zn antisites. We explore bulk single-crystals by spectroscopic ellipsometry and Raman spectroscopy. By analyzing the contribution of free carriers in the IR- region, experimental values of the electron effective mass are determined. Investigations in the visible to UV region allow the identification of the direct band gap of the inverse spinel structure and refinement of the direct band gap energy   of the normal spinel.

• Wüthrich et al., Phys. Rev. Materials 9, 064602 (2025).