Epitaxial κ-(AlxGa1−x)2O3 thin films and heterostructures grown by tin-assisted VCCS-PLD

Abstract

The structural, surface, and optical properties of phase-pure κ-(AlxGa1−x)2O3 thin films on c-sapphire and STO(111):Nb substrates as well as on MgO(111) and κ-Ga2O3 templates are reported as a function of alloy composition for x < 0.4. The thin films were grown by tin-assisted pulsed laser deposition (PLD). For the variation of the Al-content, we utilized radially segmented PLD targets that enable the deposition of a thin film material library by discrete composition screening. Growth on κ-Ga2O3 (001) thin film templates enhanced the phase pure growth window remarkably up to x = 0.65. The crystallization of the κ-phase was verified by X-ray diffraction 2θ-ω-scans for all samples. Both in- and out-of-plane lattice constants in dependence on the Al-content follow a linear relationship according to Vegard’s law over the complete composition range. Atomic force microscope measurements confirm smooth surfaces (Rq ≈ 1.4 nm) for all investigated Al-contents. Furthermore, bandgap tuning from 4.9 eV to 5.8 eV is demonstrated and a linear increase in the bandgap with increasing Al-content was observed.

Band Offsets at κ-([Al,In]xGa1–x)2O3/MgO Interfaces

“…However, it should be noted that in principle every core level could be used for this purpose. Supplementary Figure S4 and are in agreement with recently determined values from transmission spectroscopy 24,25 . It should be mentioned that for higher In contents the band gap determined by this method might be overestimated, as the onset of the inelastic background and the core level peak start to partially overlap, making a determination of the constant background difficult.…”

“…Subniveau transition energies in the conduction band of QWs can therefore be tuned up to a value of 1.1 eV such that detectable wavelengths from the far IR up to the visible might be covered by QWIPs in the investigated composition range. Figure 5: Summary of the energy levels at the a) (InxGa1-x)2O3/MgO interface and b) at the (AlxGa1-x)2O3/MgO interface as determined from XPS (red) and transmission 23,24 (blue) measurements. The measured energy levels render the (AlxGa1-x)2O3/(InxGa1-x)2O3/(AlxGa1x)2O3 a promising heterostructure for quantum wells.…”

“…The elliptically-segmented target for the κ-(InxGa1-x)2O3 alloy layers was a (In0.4Ga0.6)2O3/Ga2O3 target with additional 1.5 at.% SnO2 in each segment and for the κ-(AlxGa1-x)2O3 alloy layers a (Al0.4Ga0.6)2O3/Ga2O3 or a (Al0.2Ga0.6)2O3/Ga2O3 target with additional 2 at.% SnO2 in each segment for both targets was employed, respectively. The exact target geometries can be found in References 24,25,45 . The presence of tin is necessary to facilitate the growth of the Ga2O3 based layers in the metastable orthorhombic modification for the PLD deposition method [18][19][20]23 .…”

“…Recently, one report even achieved conductive doping of this phase by Si or Sn incorporation, which is necessary for a variety of device applications 36 . However, in PLD and MBE growth, tin is necessary as catalyst to stabilize the orthorhombic modification at all, but is not incorporated in the thin film layer as active donor element [18][19][20][23][24][25]44 . Further, several reports already deal with the alloying of this phase with indium 23,25,38 or aluminum 24,40 in a broad composition range, including bandgap engineering and the evolution of lattice constants.…”

Structural and Elastic Properties of α‐(AlxGa1−x)2O3 Thin Films on (11.0) Al2O3 Substrates for the Entire Composition Range

“…[ 6 ] The second‐most stable structure is the orthorhombic κ ‐phase, for which ternary PLD thin films were already reported. [ 7–11 ] The third‐most stable polymorph is the rhombohedral α‐phase.…”