Phase pure β-(Al x Ga 1−x ) 2 O 3 thin films are grown on (001) oriented β-Ga 2 O 3 substrates via metalorganic chemical vapor deposition. By systematically tuning the precursor molar flow rates, the epitaxial growth of coherently strained β-(Al x Ga 1−x ) 2 O 3 films is demonstrated with up to 25% Al compositions as evaluated by high resolution x-ray diffraction. The asymmetrical reciprocal space mapping confirms the growth of coherent β-(Al x Ga 1−x ) 2 O 3 films (x < 25%) on (001) β-Ga 2 O 3 substrates. However, the alloy inhomogeneity with local segregation of Al along the ([Formula: see text]) plane is observed from atomic resolution STEM imaging, resulting in wavy and inhomogeneous interfaces in the β-(Al x Ga 1−x ) 2 O 3 /β-Ga 2 O 3 superlattice structure. Room temperature Raman spectra of β-(Al x Ga 1−x ) 2 O 3 films show similar characteristics peaks as the (001) β-Ga 2 O 3 substrate without obvious Raman shifts for films with different Al compositions. Atom probe tomography was used to investigate the atomic level structural chemistry with increasing Al content in the β-(Al x Ga 1−x ) 2 O 3 films. A monotonous increase in chemical heterogeneity is observed from the in-plane Al/Ga distributions, which was further confirmed via statistical frequency distribution analysis. Although the films exhibit alloy fluctuations, n-type doping demonstrates good electrical properties for films with various Al compositions. The determined valence and conduction band offsets at β-(Al x Ga 1−x ) 2 O 3 /β-Ga 2 O 3 heterojunctions using x-ray photoelectron spectroscopy reveal the formation of type-II (staggered) band alignment.
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Local Electronic Structure of Molecular Heterojunctions in a Single‐Layer 2D Covalent Organic Framework
The synthesis of a single‐layer covalent organic framework (COF) with spatially modulated internal potentials provides new opportunities for manipulating the electronic structure of molecularly defined materials. Here, the fabrication and electronic characterization of COF‐420: a single‐layer porphyrin‐based square‐lattice COF containing a periodic array of oriented, type II electronic heterojunctions is reported. In contrast to previous donor–acceptor COFs, COF‐420 is constructed from building blocks that yield identical cores upon reticulation, but that are bridged by electrically asymmetric linkers supporting oriented electronic dipoles. Scanning tunneling spectroscopy reveals staggered gap (type II) band alignment between adjacent molecular cores in COF‐420, in agreement with first‐principles calculations. Hirshfeld charge analysis indicates that dipole fields from oriented imine linkages within COF‐420 are the main cause of the staggered electronic structure in this square grid of atomically–precise heterojunctions.
more » « less- NSF-PAR ID:
- 10461003
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Materials
- Volume:
- 31
- Issue:
- 3
- ISSN:
- 0935-9648
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Epitaxial growth of κ-phase Ga 2 O 3 thin films is investigated on c-plane sapphire, GaN- and AlN-on-sapphire, and (100) oriented yttria stabilized zirconia (YSZ) substrates via metalorganic chemical vapor deposition. The structural and surface morphological properties are investigated by comprehensive material characterization. Phase pure κ-Ga 2 O 3 films are successfully grown on GaN-, AlN-on-sapphire, and YSZ substrates through a systematical tuning of growth parameters including the precursor molar flow rates, chamber pressure, and growth temperature, whereas the growth on c-sapphire substrates leads to a mixture of β- and κ-polymorphs of Ga 2 O 3 under the investigated growth conditions. The influence of the crystalline structure, surface morphology, and roughness of κ-Ga 2 O 3 films grown on different substrates are investigated as a function of precursor flow rate. High-resolution scanning transmission electron microscopy imaging of κ-Ga 2 O 3 films reveals abrupt interfaces between the epitaxial film and the sapphire, GaN, and YSZ substrates. The growth of single crystal orthorhombic κ-Ga 2 O 3 films is confirmed by analyzing the scanning transmission electron microscopy nanodiffraction pattern. The chemical composition, surface stoichiometry, and bandgap energies of κ-Ga 2 O 3 thin films grown on different substrates are studied by high-resolution x-ray photoelectron spectroscopy (XPS) measurements. The type-II (staggered) band alignments at three interfaces between κ-Ga 2 O 3 and c-sapphire, AlN, and YSZ substrates are determined by XPS, with an exception of κ-Ga 2 O 3 /GaN interface, which shows type-I (straddling) band alignment.more » « less
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