| 1. |
- Geijselaers, Irene, et al.
(författare)
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Atomically sharp, crystal phase defined GaAs quantum dots
- 2021
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 119:26
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Crystal phase defined heterostructures, or polytype heterostructures, are atomically sharp with no intermixing, which makes them ideal contenders for a wide number of applications. Although polytype quantum dots have shown promising results as single photon sources, a high degree of control on the dimensions and number of polytype quantum dots is necessary before any application can be developed.In this work we show results from optical characterization of highly controlled wz-zb GaAs quantum dots with sharp photoluminescence signal and a strong indication of 0D density of states. One band effective mass calculations show good agreement with the measured data. Radially confined nanowires with a single wz-zb GaAs interface also show sharp photoluminescence signal and a 0D density of states. This indicates the existence of quantum dot like states in the triangular wells formed at the wz-zb GaAs interface. These results show the potential of polytype quantum dots for physics and optics applications.
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| 2. |
- Geijselaers, Irene
(författare)
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Optical studies of crystal phase heterostructures
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- III-V semiconductors are commonly used for a variety of optical applications, such as LED based lights and solar sells. Most III-V semiconductors, such as GaAs and InP, exhibit the zinc-blende (zb) crystal structure, but in the form of nanowires it is also possible to create them in the wurtzite (wz) crystal structure. This allows for the creation of novel heterostructures consisting of the same compound, but different crystal structures, so called crystal phase orpolytype heterostructures. Unlike material heterostructures, these polytype heterostructures have atomically sharp interfaces with minimal strain. This could make them ideal candidates for a number of applications, such as single photon sources, and as an environment to study interesting physical phenomena such as electron crystals and quantum dots (Q-dots).In this work I have used photoluminescence (PL) spectroscopy and photoluminescence excitation (PLE) spectroscopy to investigate a number of InP and GaAs polytype heterostructures. PL and PLE are non-invasive optical techniques that use absorption and emission of photons to gain information about a number of semiconductor properties, such as bandgap, band structure and the energy of impurity levels. The PL and PLE measurements of single, nanowires are combined with electron microscopy. This allows for the correlation of the morphology and crystal structure quality with the optical properties of the polytype heterostructures. Finally, the measured optical properties are correlated to numerical calculations of electronic structure.This thesis consists of three papers. In paper I it is shown that the Fermi-level pinning at the semiconductor-air surface influences the perceived valence band offset between the wz and zb crystal structure in GaAs nanowires. In paper II the existence of a two dimensional electron gas at the wz-zb interface in modulation doped InP nanowires is demonstrated. Finally, paper III explores zb-GaAs Q-dots in narrow wz-GaAs nanowires. The existence of multiple Q-dot states is confirmed trough PLE, whose energies correlate with numerical calculations of the Q-dot energy levels. The results in this thesis pave the way towards the use of polytype heterostructures for the study of physical phenomena, such as electron crystals.
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| 3. |
- Geijselaers, Irene, et al.
(författare)
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Radial band bending at wurtzite–zinc-blende–GaAs interfaces
- 2018
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Ingår i: Nano Futures. - : IOP Publishing. - 2399-1984. ; 2:3
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Tidskriftsartikel (refereegranskat)abstract
- The band offset between wurtzite (wz) and zinc-blende (zb) GaAs is an important fundamental parameter in polytype heterostructure engineering. Since the interface has a type-II band alignment, it is reasonably straightforward to measure the band offset using photoluminescence (PL) e.g. on nanowires containing heterostructures between wz and zb GaAs. It has, however, been found that the transition energy in such heterostructures depends on the diameter of the nanowires which introduces uncertainties in the determined value of the band offset. In order to extract a more accurate value and to further elucidate the diameter-dependent behavior of the transition energy we have performed PL studies on a large set of GaAs nanowires. Those nanowires have different diameters and contain one wz–GaAs segment embedded in otherwise zb–GaAs. We have also studied the effect of a passivating capping layer of AlAs on the determined band offset. We find that our data is well explained by a diameter-dependent radial band bending in the nanowires. Combining modeling of the band bending with the experimental data we extract a band offset of about 125 meV and a p-type doping concentration of 1016 cm−3. Our results will improve the accuracy of future modeling of the electronic properties of wz–zb GaAs heterostructures and other engineered polytypic materials.
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| 4. |
- Geijselaers, Irene, et al.
(författare)
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Two-dimensional electron gas at wurtzite–zinc-blende InP interfaces induced by modulation doping
- 2020
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 116
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Tidskriftsartikel (refereegranskat)abstract
- The quality, such as long-range correlation and mobility, of a two-dimensional electron gas (2DEG) is limited by, among other factors, interface roughness, which is inherent to the use of compositional heterostructures. Polytypic heterostructures have atomically sharp interfaces and minimal strain, decreasing the interface roughness, which may increase the mobility and long-range correlation of 2DEGs. In this work, we show the formation of a 2DEG at the wurtzite–zinc blende interface in partially n-type-doped InP nanowires using power-dependent photoluminescence. We additionally determined the wurtzite–zinc blende InP valence band offset to be 35 meV
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| 5. |
- Jafari Jam, Reza, et al.
(författare)
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Template-assisted vapour-liquid-solid growth of InP nanowires on (001) InP and Si substrates
- 2020
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Ingår i: Nanoscale. - Cambridge : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 12:2, s. 888-894
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Tidskriftsartikel (refereegranskat)abstract
- We report on the synthesis of vertical InP nanowire arrays on (001) InP and Si substrates using template-assisted vapour-liquid-solid growth. A thick silicon oxide layer was first deposited on the substrates. The samples were then patterned by electron beam lithography and deep dry etching through the oxide layer down to the substrate surface. Gold seed particles were subsequently deposited in the holes of the pattern by the use of pulse electrodeposition. The subsequent growth of nanowires by the vapour-liquid-solid method was guided towards the [001] direction by the patterned oxide template, and displayed a high growth yield with respect to the array of holes in the template. In order to confirm the versatility and robustness of the process, we have also demonstrated guided growth of InP nanowire p-n junctions and InP/InAs/InP nanowire heterostructures on (001) InP substrates. Our results show a promising route to monolithically integrate III-V nanowire heterostructure devices with commercially viable (001) silicon platforms.
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| 6. |
- Karimi, Mohammad, 1988-, et al.
(författare)
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Intersubband Quantum Disc-in-Nanowire Photodetectors with Normal-Incidence Response in the Long-Wavelength Infrared
- 2018
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Ingår i: Nano letters (Print). - Washington : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 18:1, s. 365-372
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Tidskriftsartikel (refereegranskat)abstract
- Semiconductor nanowires have great potential for realizing broadband photodetectors monolithically integrated with silicon. However, the spectral range of such detectors has so far been limited to selected regions in the ultraviolet, visible, and near-infrared regions. Here, we report on the first intersubband nanowire heterostructure array photodetectors exhibiting a spectrally resolved photoresponse from the visible to long-wavelength infrared. In particular, the infrared response from 3 to 20 μm is enabled by intersubband transitions in low-bandgap InAsP quantum discs synthesized axially within InP nanowires. The intriguing optical characteristics, including unexpected sensitivity to normal incident radiation, are explained by excitation of the longitudinal component of optical modes in the photonic crystal formed by the nanostructured portion of the detectors. Our results provide a generalizable insight into how broadband nanowire photodetectors may be designed and how engineered nanowire heterostructures open up new, fascinating opportunities for optoelectronics.
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| 7. |
- Metaferia, Wondwosen, et al.
(författare)
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N-type doping and morphology of GaAs nanowires in Aerotaxy
- 2018
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 29:28
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Tidskriftsartikel (refereegranskat)abstract
- Controlled doping in semiconductor nanowires modifies their electrical and optical properties, which are important for high efficiency optoelectronic devices. We have grown n-type (Sn) doped GaAs nanowires in Aerotaxy, a new continuous gas phase mass production technique. The morphology of Sn doped nanowires is found to be a strong function of dopant, tetraethyltin to trimethylgallium flow ratio, Au-Ga-Sn alloying, and nanowire growth temperatures. High temperature and high flow ratios result in low morphological quality nanowires and in parasitic growth on the wire base and surface. Alloying and growth temperatures of 400 °C and 530 °C, respectively, resulted in good morphological quality nanowires for a flow ratio of TESn to TMGa up to 2.25 ×10-3. The wires are pure zinc-blende for all investigated growth conditions, whereas nanowires grown by metal-organic vapor phase epitaxy with the same growth conditions are usually mainly Wurtzite. The growth rate of the doped wires is found to be dependent more on the TESn flow fraction than on alloying and nanowire growth temperatures. Our photoluminescence measurements, supported by four-point probe resistivity measurements, reveal that the carrier concentration in the doped wires varies only slightly (1-3) ×1019 cm-3 with TESn flow fraction and both alloying and growth temperatures, indicating that good morphological quality wires with high carrier density can be grown with low TESn flow. Carrier concentrations lower than 1019 cm-3 can be grown by further reducing the flow fraction of TESn, which may give better morphology wires.
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| 8. |
- Sivakumar, Sudhakar, et al.
(författare)
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Doping of GaAsP nanowires grown by aerotaxy
- 2019
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Konferensbidrag (refereegranskat)abstract
- Nanowire (NW) Photovoltaics are a promising route for low-cost multi-bandgap tandem cells, which is taxing in terms of processing and lattice matching in traditional planar geometries.[1] III-V nanowires are more suitable for monolithic integration as they can assimilate the lattice mismatch to a larger extent.[2] GaAsP is a material that offers a wide tunability of the band gap from near infrared (Eg = 1.42 eV) to visible regions (Eg = 2.3 eV) and is one of the best-suited materials for growth on silicon.[3]Controlled synthesis of GaAs(1-x)P(x) NWs with a bandgap ranging from 1.42 to 1.90 eV (at 300K) through the scalable Aerotaxy [4] technique has already been reported.[5]The present work concerns doping of GaAsP NWs by controlling precursor (DEZn) flows during growth by Aerotaxy. Here we present structural, optical and electrical studies of doped GaAs(1-x)P(x) NWs by high-resolution TEM, Photoluminescence, 4-point probe and Hall measurements.
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