| 1. |
- Benter, Sandra
(författare)
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Adventures of III-V Semiconductor Surfaces
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Tailoring the surface composition and morphology of materials to enable new electronic devices is important for a wide range of applications such as quantum computing or spintronics. A fundamental understanding of the changes induced in the surface during different process steps can help to establish new synthesis routes as well as devices. This thesis focuses on the manipulation of III-V semiconductor compounds, in particularthe surfaces of InAs and GaAs crystals.By implementing lithographically defined metal islands on the InAs surface, we push the boundaries of substrate annealing temperatures inhibiting the formation of In droplets locally. The employed Pd layer acts as a sink for freely diffusing In atoms above the congruent melting temperature. Here, As atoms go into gas phase at a higher rate compared to In due to the difference in vapor pressure. This lateral control over the concentration of In on the surface was investigated via scanning electron, atomic force as well as X-ray photoemission electron microscopy (XPEEM), and opens new pathways for epitaxy and the synthesis of heterostructures. Furthermore, theoretical studies have shown that the implementation of Bi atoms into the lattice of III-V compound semiconductors can facilitate band gap reduction and increased spin-orbit coupling desirable for fabricating of topological insulators. Particularly, the interaction of group III elements with Bi has attracted great interest. However, manufacturing these diluted Bismides is not trivial, since most approaches like molecular beam epitaxy, synthesis from the melt or metal organic vapor deposition suffer from limited and inhomogeneous Bi incorporation into the crystal.By following the approach of depositing Bi atoms onto a III-V sample and subsequent annealing, this thesis aims to synthesize and characterize heterostructures displaying III-V bulk properties and a surface made of III-As-Bi compounds. Different sample preparation routes were explored focusing on GaAs and InAs substrates with zinc blende (ZB) and wurtzite (WZ) crystal structure. The latter is only achievable in low-dimensional materials and will be employed in the form of InAs nanosheets. Part of this study focusses on the investigation of Bi-induced structural and chemical changes in the surface of the III-V compounds by utilizing surface sensitive techniques such as scanning tunneling microscopy, X-ray photoemission spectroscopy, low energy electron diffraction and XPEEM. Our results show that the mechanism of Bi incorporation is highly dependent on the underlying crystal structure, as well as process parameters such as time and substrate temperature. Additionally, first band structure measurements of InAs WZ crystal nanosheets collected via averaging angleresolved photoemission spectroscopy (ARPES) are presented. In contrast to other ZB crystal facets, a 2D electron gas (2DEG) is already detected after removing the native oxide and diminished after Bi deposition. We attribute the origin of the 2DEG to unique step and edge morphologies found on the WZ nanosheets. The thesis concludes with an ARPES study on InAs(111)B substrates presenting new electronic states inside the band gap based on the interaction of Bi and As atoms.
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| 2. |
- Benter, Sandra, et al.
(författare)
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Geometric control of diffusing elements on InAs semiconductor surfaces via metal contacts
- 2023
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Ingår i: Nature Communications. - 2041-1723. ; 14:1
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Tidskriftsartikel (refereegranskat)abstract
- Local geometric control of basic synthesis parameters, such as elemental composition, is important for bottom-up synthesis and top-down device definition on-chip but remains a significant challenge. Here, we propose to use lithographically defined metal stacks for regulating the surface concentrations of freely diffusing synthesis elements on compound semiconductors. This is demonstrated by geometric control of Indium droplet formation on Indium Arsenide surfaces, an important consequence of incongruent evaporation. Lithographic defined Aluminium/Palladium metal patterns induce well-defined droplet-free zones during annealing up to 600 °C, while the metal patterns retain their lateral geometry. Compositional and structural analysis is performed, as well as theoretical modelling. The Pd acts as a sink for free In atoms, lowering their surface concentration locally and inhibiting droplet formation. Al acts as a diffusion barrier altering Pd’s efficiency. The behaviour depends only on a few basic assumptions and should be applicable to lithography-epitaxial manufacturing processes of compound semiconductors in general.
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| 3. |
- Benter, Sandra, et al.
(författare)
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Tuneable 2D surface Bismuth incorporation on InAs nanosheets
- 2023
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Ingår i: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 15:21, s. 9551-9559
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Tidskriftsartikel (refereegranskat)abstract
- The chemical bonding at the interface between compound semiconductors and metals is central in determining electronic and optical properties. In this study, new opportunities for controlling this are presented for nanostructures. We investigate Bi adsorption on 2D wurtzite InAs (1120) nanosheets and find that temperature-controlled Bi incorporation in either anionic- or cationic-like bonding is possible in the easily accesible range between room temperature and 400 degrees C. This separation could not be achieved for ordinary zinc blende InAs(110) surfaces. As the crystal structures of the two surfaces have identical nearest neighbour configurations, this indicates that overall geometric differences can significantly alter the adsorption and incorporation. Ab initio theoretical modelling confirms observed adsorption results, but indicate that both the formation energies as well as kinetic barriers contributes to the observed temperature dependent behaviour. Further, we find that the Bi adsorption rate can differ by at least 2.5 times between the two InAs surfaces while being negligible for standard Si substrates under similar deposition conditions. This, in combination with the observed interface control, provides an excellent opportunity for tuneable Bi integration on 2D InAs nanostructures on standard Si substrates.
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| 4. |
- Liu, Yi, et al.
(författare)
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A 2D Bismuth-Induced Honeycomb Surface Structure on GaAs(111)
- 2023
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 17:5, s. 5047-5058
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional (2D) topological insulators have fascinating physical properties which are promising for applications within spintronics. In order to realize spintronic devices working at room temperature, materials with a large nontrivial gap are needed. Bismuthene, a 2D layer of Bi atoms in a honeycomb structure, has recently attracted strong attention because of its record-large nontrivial gap, which is due to the strong spin-orbit coupling of Bi and the unusually strong interaction of the Bi atoms with the surface atoms of the substrate underneath. It would be a significant step forward to be able to form 2D materials with properties such as bismuthene on semiconductors such as GaAs, which has a band gap size relevant for electronics and a direct band gap for optical applications. Here, we present the successful formation of a 2D Bi honeycomb structure on GaAs, which fulfills these conditions. Bi atoms have been incorporated into a clean GaAs(111) surface, with As termination, based on Bi deposition under optimized growth conditions. Low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/S) demonstrates a well-ordered large-scale honeycomb structure, consisting of Bi atoms in a √3 × √3 30° reconstruction on GaAs(111). X-ray photoelectron spectroscopy shows that the Bi atoms of the honeycomb structure only bond to the underlying As atoms. This is supported by calculations based on density functional theory that confirm the honeycomb structure with a large Bi-As binding energy and predict Bi-induced electronic bands within the GaAs band gap that open up a gap of nontrivial topological nature. STS results support the existence of Bi-induced states within the GaAs band gap. The GaAs:Bi honeycomb layer found here has a similar structure as previously published bismuthene on SiC or on Ag, though with a significantly larger lattice constant and only weak Bi-Bi bonding. It can therefore be considered as an extreme case of bismuthene, which is fundamentally interesting. Furthermore, it has the same exciting electronic properties, opening a large nontrivial gap, which is the requirement for room-temperature spintronic applications, and it is directly integrated in GaAs, a direct band gap semiconductor with a large range of (opto)electronic devices.
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| 5. |
- Marçal, Lucas A.B., et al.
(författare)
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Inducing ferroelastic domains in single-crystal CsPbBr3 perovskite nanowires using atomic force microscopy
- 2021
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Ingår i: Physical Review Materials. - 2475-9953. ; 5:6
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Tidskriftsartikel (refereegranskat)abstract
- Ferroelectric and ferroelastic domains have been predicted to enhance metal halide perovskite (MHP) solar cell performance. While the formation of such domains can be modified by temperature, pressure, or strain, established methods lack spatial control at the level of single domains. Here, we induce the formation of ferroelastic domains in CsPbBr3 nanowires at room temperature using an atomic force microscope (AFM) tip and visualize the domains using nanofocused x-ray diffraction with a 60 nm beam. Regions scanned with a low AFM tip force show orthorhombic 004 reflections along the nanowire axis, while regions exposed to higher forces exhibit 220 reflections. The applied stress locally changes the crystal structure, leading to lattice tilts that define ferroelastic domains, which spread spatially and terminate at {112}-type domain walls. The ability to induce individual ferroelastic domains within MHPs using AFM gives new possibilities for device design and fundamental experimental studies.
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| 6. |
- Yong, Zhihua, et al.
(författare)
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Tuning oxygen vacancies and resistive switching properties in ultra-thin HfO2 RRAM via TiN bottom electrode and interface engineering
- 2021
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Ingår i: Applied Surface Science. - : Elsevier BV. - 1873-5584 .- 0169-4332. ; 551
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Tidskriftsartikel (refereegranskat)abstract
- Resistive random access memory (RRAM) technologies based on non-volatile resistive filament redox switching oxides have the potential of drastically improving the performance of future mass-storage solutions. However, the physico-chemical properties of the TiN bottom metal electrode (BME) can significantly alter the resistive switching (RS) behavior of the oxygen-vacancy RRAM devices, yet the correlation between RS and the physico-chemical properties of TiN and HfOx/TiN interface remains unclear. Here, we establish this particular correlation via detailed material and electrical characterization for the purpose of achieving further performance enhancement of the stack integration. Two types of RRAM stacks were fabricated where the TiN BME was fabricated by physical vapor deposition (PVD) and atomic layer deposition (ALD), respectively. The HfOx layer in HfOx/PVD-TiN is more oxygen deficient than that of the HfOx/ALD-TiN because of more defective PVD-TiN and probably because pristine ALD-TiN has a thicker TiO2 overlayer. Higher concentration of oxygen vacancies induces a larger magnitude of band bending at the HfOx/PVD-TiN interface and leads to the formation of a higher Schottky barrier. Pulsed endurance measurements of up to 106 switches, with 10 μA ± 1.0 V pulses, demonstrate the potential of the studied ultra-thin-HfOx/TiN device stack for dense, large scale, and low-power memory integration.
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