Abstract This work evaluates the passivation efficacy of thermal atomic layer deposited (ALD) Al 2 O 3 dielectric layer on self-catalyzed GaAs 1- x Sb x nanowires (NWs) grown using molecular beam epitaxy. A detailed assessment of surface chemical composition and optical properties of Al 2 O 3 passivated NWs with and without prior sulfur treatment were studied and compared to as-grown samples using x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and low-temperature photoluminescence (PL) spectroscopy. The XPS measurements reveal that prior sulfur treatment followed by Al 2 O 3 ALD deposition abates III–V native oxides from the NW surface. However, the degradation in 4K-PL intensity by an order of magnitude observed for NWs with Al 2 O 3 shell layer compared to the as-grown NWs, irrespective of prior sulfur treatment, suggests the formation of defect states at the NW/dielectric interface contributing to non-radiative recombination centers. This is corroborated by the Raman spectral broadening of LO and TO Raman modes, increased background scattering, and redshift observed for Al 2 O 3 deposited NWs relative to the as-grown. Thus, our work seems to indicate the unsuitability of ALD deposited Al 2 O 3 as a passivation layer for GaAsSb NWs.
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Molecular beam epitaxy of PtSe 2 using a co-deposition approach
Abstract The structural properties of co-deposited ultrathin PtSe 2 films grown at low temperatures by molecular beam epitaxy on c-plane Al 2 O 3 are studied. By simultaneously supplying a Se flux from a Knudsen cell and Pt atoms from an electron-beam evaporator, crystalline (001)-oriented PtSe 2 films were formed between 200 °C and 300 °C. The long separation between substrate and electron beam evaporator of about 60 cm ensured minimal thermal load. At optimum deposition temperatures, a ten times or even higher supply rate of Se compared to Pt ensured that the pronounced volatility of the Se was compensated and the PtSe 2 phase was formed and stabilized at the growth front. Postgrowth anneals under a Se flux was found to dramatically improve the crystalline quality of the films. Even before the postgrowth anneal in Se, the crystallinity of PtSe 2 films grown with the co-deposition method was superior to films realized by thermal assisted conversion. Postgrowth annealed films showed Raman modes with narrower peaks and more than twice the intensity. Transmission electron microscopy investigations revealed that the deposited material transitioned to a two-dimensional layered structure only after the postgrowth anneal. PtSe 2 growth was found to start as single layer islands that preferentially nucleated at atomic steps of the substrate and progressed in a layer-by-layer like fashion. A close to ideal wetting behavior resulted in coalesced PtSe 2 films after depositing about 1.5 PtSe 2 layers. Detailed Raman investigation of the observed PtSe 2 layer breathing modes of films grown under optimized co-deposition conditions revealed an interlayer coupling force constant of 5.0–5.6 × 10 19 N m −3 .
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- NSF-PAR ID:
- 10412531
- Date Published:
- Journal Name:
- 2D Materials
- Volume:
- 9
- Issue:
- 2
- ISSN:
- 2053-1583
- Page Range / eLocation ID:
- 025029
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Large area highly crystalline MoS2and WS2thin films were successfully grown on different substrates using radio-frequency magnetron sputtering technique. Structural, morphological and thermoelectric transport properties of MoS2,and WS2thin films have been investigated systematically to fabricate high-efficient thermal energy harvesting devices. X-ray diffraction data revealed that crystallites of MoS2and WS2films are highly oriented in 002 plane with uniform grain size distribution confirmed through atomic force microscopy study. Surface roughness increases with substrate temperature and it plays a big role in electron and phonon scattering. Interestingly, MoS2films also display low thermal conductivity at room temperature and strongly favors achievement of higher thermoelectric figure of merit value of up to 1.98. Raman spectroscopy data shows two distinct MoS2vibrational modes at 380 cm−1for E12gand 410 cm−1for A1g. Thermoelectric transport studies further demonstrated that MoS2films show p-type thermoelectric characteristics, while WS2is an n-type material. We demonstrated high efficient pn-junction thermoelectric generator device for waste heat recovery and cooling applications.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1088/2053-1583/ac606f
