Abstract
High quality heteroepitaxial (001)-oriented κ-(AlxGa1−x)2O3/κ-Ga2O3 quantum well superlattice heterostructures were deposited by tin-assisted pulsed laser deposition on c-sapphire substrates. Sharp superlattice fringes up to the ninth order in XRD patterns for Al-contents up to about 50 at. % confirm excellent structural quality and smooth interfaces in the multilayers on par with reports on homoepitaxial superlattices in the monoclinic modification. By employing elliptically segmented targets, the Al-content in the barrier layers of the superlattices was systematically varied in a range of 0.1 ≤ x ≤ 0.5 in a controlled and quasi-continuous manner. An in-depth investigation employing XRD 2θ-ω scans and reciprocal space map measurements on superlattices with different periods as well as single quantum well samples suggests coherent growth of the superlattices for application-relevant quantum well widths. The critical thickness for coherent growth of κ-Ga2O3 on κ-(AlxGa1−x)2O3 was further estimated to be at least 50 nm and 3 nm for x = 0.2 and x = 0.3, respectively. We determined absorption energies in optical transmission spectra for superlattices with x = 0.3 well below the bandgap of the barrier layers that decrease with increasing quantum well width suggesting transitions between localized states in the quantum wells as their origin. These results render superlattices in the metastable orthorhombic phase of Ga2O3 as a promising active layer for quantum well infrared photodetector applications.
Highlights
In recent years, research interest in the wide bandgap semiconductor Ga2O3 has been growing tremendously
We investigated the structural properties of our multilayers in depth by x-ray diffraction (XRD) and will show the coherent growth of the SLs in κ-modification in a broad composition range up to Al-contents in the barrier layers of about
Peaks appear on the low angle side of the SL0 reflections as well for larger tw, both the double layer thickness (DLT) vs the QW pulse number and the c-lattice constants of the SL0 reflections vs the inverse DLT still show a linear dependence where all extracted parameters agree well with those of the tB series, see Fig. S5 in the supplementary material. We further investigated this by reciprocal space map (RSM) measurements around the asymmetric SL0 (139) reflections, see Fig. 5(b) for selected samples with xAl = 0.3
Summary
Research interest in the wide bandgap semiconductor Ga2O3 has been growing tremendously. The fact that it is the only known polymorph that is predicted to exhibit a spontaneous and large electrical polarization of 23 μC/cm along its c-direction attracted interest in the scientific community This value is three times higher than that of AlN and surpasses the one of GaN even by one order of magnitude.. Polarization discontinuities at heterointerfaces, e.g., to the κ-(AlxGa1−x)2O3 or κ-(InxGa1−x)2O3 alloy system or other materials, are utilized, which result in polarization charges These charges can be compensated by free electrons that are localized as two-dimensional electron gas (2DEG) at the interface. The Al- and In-alloy systems of κ-Ga2O3 would be even more promising for such applications, since scitation.org/journal/apm the larger polarization differences can be expected to localize 2DEGs with even higher sheet charge carrier densities increasing the effectivity of the devices. The larger bandgap of Ga2O3 of about 4.9 eV for the κ-phase compared to that of GaN1 (Eg ≈ 3.4 eV)
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