| 531. |
|
|
| 532. |
- Khranovskyy, Volodymyr, et al.
(författare)
-
Crystal phase engineered quantum wells in ZnO nanowires
- 2013
-
Ingår i: Nanotechnology. - : Institute of Physics: Hybrid Open Access. - 0957-4484 .- 1361-6528. ; 24:21
-
Tidskriftsartikel (refereegranskat)abstract
- We report the fabrication of quantum wells in ZnO nanowires (NWs) by a crystal phase engineering approach. Basal plane stacking faults (BSFs) in the wurtzite structure can be considered as a minimal segment of zinc blende. Due to the existing band offsets at the wurtzite (WZ)/zinc blende (ZB) material interface, incorporation of a high density of BSFs into ZnO NWs results in type II band alignment. Thus, the BSF structure acts as a quantum well for electrons and a potential barrier for holes in the valence band. We have studied the photoluminescence properties of ZnO NWs containing high concentrations of BSFs in comparison to high-quality ZnO NWs of pure wurtzite structure. It is revealed that BSFs form quantum wells in WZ ZnO nanowires, providing an additional luminescence peak at 3.329 eV at 4 K. The luminescence mechanism is explained as an indirect exciton transition due to the recombination of electrons in the QW conduction band with holes localized near the BSF. The binding energy of electrons is found to be around 100 meV, while the excitons are localized with the binding energy of holes of ~5 meV, due to the coupling of BSFs, which form QW-like structures.
|
|
| 533. |
- Khranovskyy, Volodymyr, et al.
(författare)
-
Effect of oxygen exposure on the electrical conductivity and gas sensitivity of nanostructured ZnO films
- 2009
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 517:6, s. 2073-2078
-
Tidskriftsartikel (refereegranskat)abstract
- Nanostructured ZnO films (Undoped and Ga, Co, Mn doped) were exposed to oxygen (1-80 vol.%) at temperature range of 300-500 degrees C in order to reveal the ambience-temperature effect oil the electrical conductivity. The dominant effect of ambient influence via oxygen absorption was observed: the intensity of conductivity decrease was found to be proportional with temperature and tends to saturate with time. It is demonstrated that oxygen absorption occurs accordingly to diffusion law and the quantifying of oxygen diffusion was realized for different samples. It is revealed that the type of dopant affects the diffusion in ZnO and the tendency to increase the diffusion intensity with dopant content has been observed. After oxygen saturation the reversible effect of oxygen adsorption became dominant and contributed to the films conductivity. Oxygen exposure undoped ZnO films revealed high sensitivity for oxygen content change in the ambience therefore they have been preceded further for gas sensor design and the detailed investigation of films sensing properties has been carried out.
|
|
| 534. |
- Khranovskyy, Volodymyr, et al.
(författare)
-
Heteroepitaxial ZnO nano hexagons on p-type SiC
- 2010
-
Ingår i: JOURNAL OF CRYSTAL GROWTH. - : Elsevier BV. - 0022-0248. ; 312:2, s. 327-332
-
Tidskriftsartikel (refereegranskat)abstract
- ZnO single crystal nanohexagons have been grown heteroepitaxially on p-type Si-face 4H-SiC substrates with 8 degrees miscut from to [0 0 0 1] by catalyst-free atmospheric pressure metalorganic chemical vapor deposition and characterized by x-ray diffraction, scanning and transmission electron microscopy as well as energy disperse x-ray and cathodoluminescence analyses. The as-grown ZnO nanohexagons have a pillar shape terminated by a and c plane facets, and are aligned along the growth direction with the epitaxial relation [0 0 0 1](ZnO) parallel to[0 0 0 1](4H-SiC) and [1 0 (1) over bar 0](ZnO) parallel to[1 0 (1) over bar 0](4H-SiC). The ZnO nanohexagons demonstrate intense UV emission (lambda(NBE)=376 nm) and negligible defect-related luminescence.
|
|
| 535. |
- Khranovskyy, Volodymyr, et al.
(författare)
-
Photoluminescence study of basal plane stacking faults in ZnO nanowires
- 2014
-
Ingår i: Physica. B, Condensed matter. - : Elsevier. - 0921-4526 .- 1873-2135. ; 439, s. 50-53
-
Tidskriftsartikel (refereegranskat)abstract
- We have investigated the photoluminescence (PL) of ZnO nanowires (NWs) containing a high density (similar to 1 x 10(6) cm(-1)) of basal plane stacking faults (BSFs). It was observed that the BSFs result in a specific PL peak at similar to 3.329 eV along with a donor bound excitonic emission (D degrees X) peak at 5 K. The observed BSF-related emission is of excitonic type and possesses longer PL lifetime than D degrees X (similar to 360 ps vs. similar to 70 ps). Via comparison of the microstructural and the PL properties of the ZnO NWs, it is shown that the observed BSF-related emission is due to the formation of crystal phase quantum wells (QWs). This is explained by the fact that BSF in wurtzite (WZ) ZnO is the thinnest segment of zinc blende (ZB) phase ZnO inserted in the WZ matrix, resulting in band alignment of type II due to the conduction and valence band offsets of ZB with respect to WZ ZnO. The mechanism of the BSF related PL is suggested to be an indirect exciton transitions clue to the recombination of electrons confined in the ZB QWs to holes in the WZ barriers localized near the BSFs.
|
|
| 536. |
- Khranovskyy, Volodymyr, et al.
(författare)
-
Selective homoepitaxial growth and luminescent properties of ZnO nanopillars
- 2011
-
Ingår i: NANOTECHNOLOGY. - : Institute of Physics; 1999. - 0957-4484 .- 1361-6528. ; 22:18, s. 185603-
-
Tidskriftsartikel (refereegranskat)abstract
- High spatial density ZnO nanopillars (NPs) have been fabricated on catalyst-and pattern-free Si wafers using atmospheric pressure metal organic chemical vapor deposition (APMOCVD) at a moderate temperature (500 degrees C). The nanopillar diameter is similar to 35 nm and the length is similar to 150 nm, with a density of similar to 2 x 10(9) cm(-2). The growth evolution of the nanopillars, providing the (0001)(NP) parallel to (0001)(ZnO) (grain) parallel to (100)(Si) (surface) epitaxial relationship, is extensively studied by scanning and high resolution transmission microscopy. The approach to obtaining the ZnO 1D structures is explained in terms of selective homoepitaxial growth via the crystallographic anisotropy of the seeding layer. The advanced PL properties of ZnO NPs, e. g. indications of free excitonic and absence of defect emission, are related to their single crystalline nature within one pillar and most probably better stoichiometry and less contamination. The observed efficient monochromatic UV emission from the ZnO NPs at room temperature points toward their potential application as building blocks for nanoscale optoelectronic devices.
|
|
| 537. |
- Khranovskyy, Volodymyr, et al.
(författare)
-
Surface morphology effects on the light-controlled wettability of ZnO nanostructures
- 2012
-
Ingår i: Applied Surface Science. - : Elsevier. - 0169-4332 .- 1873-5584. ; 258:20, s. 8146-8152
-
Tidskriftsartikel (refereegranskat)abstract
- ZnO nanostructures of diverse morphology with shapes of corrals and cabbages as well as open and filled hexagons and sheaves prepared by APMOCVD technique, are investigated with water contact angle (CA) analysis. The as-grown ZnO nanostructures exhibit pure hydrophobic behavior, which is enhanced with the increase of the nanostructures surface area. The most hydrophobic structures (CA = 124 degrees) were found to be the complex nanosheaf, containing both the macro-and nanoscale features. It is concluded that the nanoscale roughness contributes significantly to the hydrophobicity increase. The character of wettability was possible to switch from hydrophobic-to-superhydrophilic state upon ultra violet irradiation. Both the rate and amplitude of the contact angle depend on the characteristic size of nanostructure. The observed effect is explained due to the semiconductor properties of zinc oxide enhanced by increased surface chemistry effect in nanostructures.
|
|
| 538. |
- Khromov, Sergey, et al.
(författare)
-
Atom probe tomography study of Mg doped GaN layers
- 2014
-
Ingår i: Nanotechnology. - : Institute of Physics Publishing (IOPP). - 0957-4484 .- 1361-6528. ; 25:27, s. 275701-
-
Tidskriftsartikel (refereegranskat)abstract
- Atom probe tomography studies on highly Mg-doped GaN(0001) layers with concentrations 5×1019 cm-3 and 1×1020 cm-3 were performed. Mg cluster formation was observed only in the higher doped sample whereas in the lower doped sample the Mg distribution was homogeneous. CL measurements showed that the emission normally attributed to stacking faults was only present in the lower doped layers ([Mg] = 1.5×1019 and 5×1019 cm-3), but absent in the higher-doped layer, where Mg clusters were detected. Mg clusters are proposed to produce a screening effect thereby destroying the exciton binding on the SFs thus rendering them optically inactive.
|
|
| 539. |
- Khromov, Sergey, et al.
(författare)
-
Correlation between Si doping and stacking fault related luminescence in homoepitaxial m-plane GaN
- 2013
-
Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 103, s. 192101-
-
Tidskriftsartikel (refereegranskat)abstract
- Si-doped GaN layers grown by metal organic vapor phase epitaxy on m-plane GaN substrates were investigated by low-temperature cathodoluminescence (CL). We have observed stacking fault (SF) related emission in the range of 3.29–3.42 eV for samples with moderate doping, while for the layers with high concentration of dopants, no CL lines related to SFs have been noted. Perturbation of the SF potential profile by neighboring impurity atoms can explain localization of excitons at SFs, while this effect would vanish at high doping levels due to screening.
|
|
| 540. |
- Khromov, Sergey, 1986-
(författare)
-
Doping effects on the structural and optical properties of GaN
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Today there is a strong drive towards higher efficiency light emitters and devices for power electronics based on GaN and its ternary compounds. Device performance can be improved in several ways on the material level. Development of bulk GaN to substitute sapphire and SiC as substrate materials can allow lower defect density epitaxial GaN layers to be grown. Using nonpolar homoepitaxial layers alleviates the problem of polarization fields present in polar GaN epilayers. This thesis advances the field by attacking outstanding problems related to doping and its influence on structural and optical properties of GaN. Optical and structural investigations were performed on bulk GaN grown by halide vapor phase epitaxy (HVPE) and on polar and nonpolar epitaxial GaN grown by metal organic chemical vapor deposition (MOCVD), doped with different impurities: Mg, Si, O or C. Optical characterization was done using photoluminescence (PL), time-resolved photoluminescence (TRPL), and cathodoluminescence (CL) in-situ scanning electron microscope, whereas structural properties were studied by means of transmission electron microscopy (TEM) and atom probe tomography (APT).A correlation between Mg doping levels and stacking fault (SF) concentration in highly Mg-doped c-plane homoepitaxial GaN layers is found. Increasing Mg concentrations, from 2×1018 cm-3 to 5×1019 cm-3, coincides with increasing density of small, 3-10 nm-sized, SFs. Emission lines ascribed to SFs are observed in CL in all the studied samples. The observed SF-related luminescence can be explained by a model where Mg atoms interacting with the nearby SF changes the confinement for holes and leads to a pronounced defectrelated luminescence. Non-polar m-plane homoepitaxial GaN layers with Mg concentration of 2×1018 cm-3 and 3×1019 cm-3 exhibits high density of basal SFs as well as a number of prismatic SFs. Instead of normally observed in nonpolar GaN SF-related broad lines several sharp lines are detected in the 3.36-3.42 eV region. Their relation to donor-acceptor pair recombination (DAP) was dismissed by calculating the DAP energies and fitting with the measured spectra. The sharp lines are tentatively explained by some impurities bound to point defects or SFs. The origin of two Mg related acceptor bound exciton (ABE) peaks in the emission spectra is also proposed: narrower ABE1 peak at 3.466 eV is identified as coming from a substitutional Mg atom. Broader emission at 3.454 eV is deemed to be coming from a Mg acceptor atom perturbed by a nearby SF. Additionally, Mg cluster formation in the highest doped sample ([Mg] = 1×1020 cm-3) was revealed by APT.Simultaneous doping by Si and O was studied for HVPE grown bulk GaN. Doping with O concentration from 1017 cm-3 leads to a decrease in the Si concentration to less than 1016 cm-3. Si incorporation is believed to be suppressed by the competing Ga-vacancy-O incorporation process. Bandgap narrowing by 6 meV due to high doping was observed. Donor bound exciton (DBE) lifetime was obtained from TPRL experimental data and it is found to decrease with increasing doping. In non-polar m-plane homoepitaxial GaN Si doping influences the SF-related luminescence. At moderate Si concentrations excitons are bound to the impurity atoms or impurity-SF complex. Proximity of impurity atoms changes the potential for SF creating localization for charge carriers resulting in SF-related emission. At dopant concentrations higher than the Mott limit screening destroys the carrier interaction and, thus, the exciton localization at impurity-SF complex.Finally, C-doped HVPE grown bulk GaN layers were studied by TEM, CL, and TRPL. Enhanced yellow line (YL) luminescence was observed with increasing C doping. Stability of YL in a wide temperature range (5-300 K) confirms that YL is due to a deep defect, likely CN-ON complex. Low-temperature CL mapping reveals a pit-like structure with different luminescence properties in different areas. DBE emission dominates in CL spectra within the pits while in pit-free areas, in contrast, two ABE lines typical for Mg-doped GaN are observed.
|
|
