A Félvezető Nanoszerkezetek csoport kutatásait félvezetőkben illetve félvezető nanoszerkezetekben előforduló ponthibák kísérleti és elméleti jellemzésére fókuszáljuk. Új technikákat fejlesztünk illetve implementálunk, hogy adalékokat és hibákat jutassunk be félvezető (nano)szerkezetekbe, és vizsgáljuk azok tulajdonságait kísérleti és elméleti spektroszkópiával.
A félvezető nanoszerkezet előállításához olvasztásos, ion- és neutron-besugárzásos, kolloid vegyi és hasonló technikákat kombinálunk. Jelenleg a szilícium-karbid van a kutatás fókuszában, hogy egyedülálló in vivo biológiai képalkotó anyagot hozzunk létre. Aktívan dolgozunk a számítógépes anyagtudomány területén, és nagy tapasztalatunk van a sűrűségfunkcionál-elméleten (DFT) alapuló módszerek alkalmazásában szilárdtestekben és félvezetőkben. A kutatócsoport vezetője már 2002-ben is használta a legmodernebb hibrid sűrűségfunkcionálon alapuló módszereket a tömbi és nanoszerkezetű félvezetőkben előforduló ponthibák vizsgálatában. 2005-től az ún. GW-módszert is alkalmaztuk. Modern időfüggő DFT módszert használtunk nanoklaszterek abszorpciós spektrumának kiszámításához 2008 évtől. Ezeket az elméleteket alkalmaztuk új biológiai jelzőrendszerek, szilárdtestbeli kvantumbitek és spintronika, és napelem-cellák tervezésében.
A kutatócsoport honlapja
Design and fabrication of semiconductor nanostructures for bioimaging, quantum computing and 3rd generation photovoltaics. — The research team is active in three main different fields: develop new type of i) biomarkers, ii) quantum bits for quantum computation, and iii) 3rd generation solar cells. Next, we summarize our important achievements.
Development of biomarker systems. — Biologists urgently need biomarker systems which trace, e.g., cancer cells in the blood stream or provide fluorescent signals depending on the activity of neurons in brain. Such systems have been developed so far, but most of them are either unstable or toxic, thus they are not suitable for therapy. Our Semiconductor Nanostructures Research Group is, however, seeking such solutions that can be applied in vivo. Molecular-sized colloidal SiC nanoparticles are very promising candidates to realize bioinert non-perturbative fluorescent nanoparticles for in vivo bioimaging. These SiC nanoparticles are prepared by wet chemical etching of large SiC particles. However, the mechanism behind the etching process was far from being understood. We developed a no-photon exciton generation chemistry (NPEGEC) theory based on the experiments on SiC polytypes as a model semiconductor (see Fig. 1). Our theory applies to materials with a finite band gap. Furthermore, we could demonstrate the control over the size of SiC nanoparticles that we produce from the cubic layers of bulk cubic silicon carbide that contains hexagonal inclusions (Scientific Reports 7, 10599).
Figure 1. The mechanism “no-photon exciton generation chemistry” (NPEGEC) for stain etching of semiconductors. (A) The blue region depicts a semiconductor with a larger band gap that is resistive against etching while the yellow region represents a suitable material. A redox couple with redox potential higher (more negative) than the conduction band minimum (CBM) energy can inject electrons into the conduction band (I). The oxidized molecule itself, or the molecule formed after further transformation in the solution (II) can inject holes into the valence band (VB) with a maximum energy of VBM (III). The generated excitons can recombine with photon emission with energy hn or can lead to material dissolution. (B) In a material with spatially varying band structure selective etching is possible. The exciton Bohr radius limits the radius (R) of the final nanoparticle. (C) Patterned band structure in a macroscopic material can serve as a template for various nanostructures including patterned nanowires, anisotropic or uniform particles.
Nitrogen vacancy center (NV). — Significant results have been achieved in the research of solid-state quantum bits, which are the building blocks of a future implementation of the quantum computer. Diamond is a known host of solid state qubits and single photon emitters. NV center stands out among these qubits in terms of robustness of optical spin readout and initialization. The optical readout of electron spin is based on the intersystem crossing (ISC) between the optically active triplet states and the dark singlet states. However, the intricate details about the ISC processes were not fully understood. By combining the theory of dynamic Jahn-Teller system and first principles calculations, we could identify the ISC routes and their rates for the transition between the excited state triplet and a nearby singlet state (see Fig. 2) (Phys. Rev. B 96, 081115(R)).
Figure 2. NV center in diamond. (a) Schematic diagram of the structure of the negatively charged defect with the optimized carbon-nitrogen bond length. The symmetry axis of the defect in the diamond lattice is shown. (b) The calculated defect levels in the gap are depicted in the ground state where the curved arrow symbolizes the DSCF procedure for creating the triplet excited state. The e states are double degenerate. VB and CB correspond to valence and conduction bands, respectively. (c) The corresponding ground state and excited states are shown as well as the optical electron spin polarization cycle. The spin-orbit splitting λz is depicted that separates the sublevels in the triplet 3E excited state. The corresponding intersystem crossing rates between the 3E substates (˜A1,2, ˜E1,2 double group representations) and the singlet 1A1 are labeled by Gs. The tilde labels the vibronic nature of these states. The intersystem crossing (t± and tz) from the 1E to the triplet ground state is shown for the sake of completeness and closes the spin polarization cycle.
The diamond NV center can be used as a nanoscale sensor when engineered close to the diamond surface. However, the surface termination of diamond can affect the charge state and photo-stability of NV center that may compromise the sensitivity of NV center. We predict from first principles calculations that nitrogen-terminated (111) diamond would be ideal to maximize the sensitivity of near-surface NV centers (see Fig. 3). Furthermore, the array of I=1 nuclear spins of 14N isotopes on the surface can used to realize a quantum simulator of special spin systems (Nano Lett., 2017, 17 (4), pp 2294–2298).
Figure 3. The (111) surface of diamond terminated with nitrogen atoms. Nitrogen vacancy centers below the terminated surface enjoy a near-bulk physical environment, e.g. long spin coherence time, which makes them useful for quantum bit and nanometrological applications.
Divacancy defect in SiC. — Another prominent solid state qubit candidate is the so-called divacancy defect in SiC which has a high-electron-spin ground state. Divacancy qubit can be formed in cubic and hexagonal polytypes, however, the key magneto-optical parameters and rates were not known for these qubits. In collaboration with the Awschalom group at Chicago University, we characterized thoroughly these qubits (see Fig. 4). We found that an efficient spin-to-photon interface can be realized by these divacancy qubits at cryogenic temperature and resonant optical excitation. Furthermore, we identified a room temperature qubit in hexagonal SiC as Si-vacancy at the so-called cubic site in hexagonal SiC by means of first principles calculations. This Si-vacancy qubit has a great potential in thermometry and magnetometry applications at the nanoscale (Phys. Rev. X 7, 021046).
Figure 4. Dynamical model of the 3C-SiC divacancy. Left: An artistic view about the optical spin polarization of divacancy spins. Right: Diagram of the levels and major rates in the five-level rate-equation model. The transition rates and ground-state spin polarization are inferred from the combination of experimental data, group theory considerations and input from first principles calculations.
Furthermore, we studied nanosystems that are promising in biomarker and solar cell applications. The silicon nanoparticles (Si NPs) are very promising in various emerging technologies and for fundamental quantum studies of semiconductor nanocrystals. Heavily boron and phosphorus codoped fluorescent Si NPs can be fabricated with diameters of a few nanometers. However, very little is understood about the structure and origin of the fluorescence of these NPs. We performed a systematic time-dependent density functional study of hundreds of codoped Si NPs representing millions of configurations. We identified the most stable dopant configurations and a correlation between these configurations and their optical gaps. We find that particular dopant configurations result in emission in the second biological window, which makes these nanoparticles viable for deep-tissue bioimaging applications. We also found that the radiative lifetime of Si NPs is intrinsically long, thus the electron-hole pairs generated by illumination can principally be separated. This concludes that heavily doped Si NPs can be applied as an absorbant for Si based solar cells ( J. Phys. Chem. C 2017, 121, 27741−27750).
Design and fabrication of semiconductor nanostructures for bioimaging, quantum computing and 3rd generation photovoltaics
The research team is active in three main different fields, developing new type of i) biomarkers, ii) quantum bits for quantum computation, and iii) 3rd generation solar cells.
Diamond is a known host of solid state qubits and single-photon emitters. Our group seeks for novel potential qubit candidates in diamond, and carries out in-depth characterization by means of ab initio atomistic simulations. The properties of oxygen impurity in diamond is largely unexplored, however, they may form defects that can be useful for quantum computation. Oxygen can be introduced by implantation or during chemical vapor deposition. In the latter case, hydrogen can also enter diamond, thus complex formation of oxygen, vacancy and hydrogen could be expected. We examined theoretically several such complexes, some of which have already been observed, others which could potentially form (see Figure 1). Using hybrid density functional theory for the treatment of highly correlated orbitals, many measurable quantities are calculated. We highlight a result from this study: we showed that the neutral oxygen-vacancy complex isovalent with the negatively charged nitrogen-vacancy qubit (NV center) in diamond is not a good candidate for qubit because of the rapid non-radiative decay rate of its excited state.
Figure 1. Potential energy surface of the ground and excited states of OV(0). The different configurations of the defect are depicted in (a-e). (f) depicts the one-electron orbitals participating in this excitation process. An electron is promoted from the 1a' level to 3a'. (g) shows the ground state geometry in a different orientation. If the defect is excited at the 1.72 eV resonance, the system non-radiatively relaxes back through an intersystem crossing to its ground state without an emission of a photon. While the ground state of OV(0) is very similar to that of the well-known NV(-) system, the excitation process above is very different from that of NV(-) according to our results.
We implemented and applied theories to study the coupled electron-phonon systems that can be very important in understanding the photoexcitation spectrum of qubits and nanostructures. We showed that the phonon sideband of the photoluminescence spectrum for the silicon-vacancy qubit in diamond can only be interpreted by inclusion of the so-called Herzberg-Teller effect. This result was published in Physical Review B. We used methods that go beyond the Born-Oppenheimer approximation and involve many-body perturbation theory in the electron-phonon interaction. We showed that photoemission spectrum of diamondoids can only be understood by strongly coupled electron-phonon interaction (see Figure 2). In conjunction to defect qubits in diamond, we analyzed the optically detected magnetic resonance spectrum of the NV center in 13C-enriched diamond. We developed an effective spin-Hamiltonian model for this system that described well the observed spectra.
Figure 2. Photoemission spectrum (PES) of adamantane: theory and experiment. Our theoretical method includes the electron-electron correlation with the so-called GW calculation. In the ionization process, we also include electron-phonon interaction in order to properly describe the dynamic Jahn-Teller or polaronic nature of an ionized adamantane molecule. This effect is depicted on the left, if an electron is removed from the system, the atoms start vibrating. We could reproduce the ionization threshold at 9 eV, as well as the overall lineshape is fully ab initio - no empirical factors from experiments are used. The right panel depicts the structure of the adamantane molecule for which the PES was calculated.
Significant results have been achieved in the research of solid-state quantum bits, which are the building blocks of a future implementation of the quantum computer. A prominent candidate is the so-called divacancy defect in silicon carbide which has a high-spin ground state. This electron spin may interact with the nearby nuclear spins in the lattice that can naturally occur in SiC. We developed a detailed theory on the optical dynamic spin polarization of the nuclear spins driven by the coherent control of the electron spins of the point defect. Our simulations unraveled that certain nuclear spins can be optically spin-polarized at a given direction depending on the magnitude of a small external magnetic field, thus a bidirectional spin-polarization can be achieved without the need of radiofrequency excitation of the nuclear spins. The proof-of-principle measurement was carried out for proximate nuclear spins by Awschalom group at Chicago University for which theory predicted 25% spin inversion probability at a certain magnitude of the external magnetic field (see Figure 3). These results suggest the incorporation of optical dynamic spin polarization techniques into future quantum information processing and quantum sensing protocols. We contributed to the characterization of nitrogen-vacancy defect in hexagonal SiC, that might be a near-infrared (NIR) counterpart of the famous nitrogen-vacancy center in diamond that operates rather in the visible region.
Figure 3. Experimental and theoretical 29Si nuclear spin polarization and ODMR spectrum of the mS = 0 to mS = +1 spin transition of PL6 qubits in 4H-SiC at the GSLAC region. (a) The measured (points) and calculated (thick line) magnetic field dependence of the nuclear spin polarization of a 29Si nucleus at the SiIIb site. DNP is highly efficient up until the LAC-c, at which point it exhibits a sharp drop and reversal. Measurements are carried out at room temperature. (b) The experimental low-microwave-power ODMR spectrum. The measurements are carried out on an ensemble of PL6 divacancy-related qubits in 4H-SiC at room temperature. f0 = f0(B) describes the zero-field-splitting and Zeeman shift of the mS = +1 spin state. (c) Theoretical simulation of the ODMR spectrum which takes into account the DNP of the 29Si nucleus at the SiIIb site and the microwave transition strength in the mS = |0,+1> manifold. The green ellipsoids on (b) and (c) highlight the signs of the nuclear spin polarization reversal.
Biologists urgently need biomarker systems which trace, e.g., cancer cells in the blood stream or provide fluorescent signals depending on the activity of neurons in brain. Such systems have been developed so far, but most of them are either unstable or toxic, thus they are not suitable for therapy. Our Momentum Semiconductor Nanostructures Research Group is, however, seeking such solutions that can be applied in vivo. Molecular-sized colloid SiC nanoparticles (NP) are very promising candidates to realize bioinert non-perturbative fluorescent nanoparticles for in vivo bioimaging. Fluorescent water-soluble silicon carbide (SiC) nanocrystals have been previously identified as complex molecular systems of silicon, carbon, oxygen, and hydrogen held together by covalent bonds that made the identification of their luminescence centers unambiguous. Understanding the fluorescence of this complex system with various surface terminations in solution is still a scientific challenge. We showed that the combination of advanced time-resolved spectroscopy and ab initio simulations, aided by surface engineering, is able to identify the luminescence centers of such complex systems. We identified two emission centers of this complex system: surface groups involving carbon–oxygen bonds and a defect consisting of silicon–oxygen bonds that becomes the dominant pathway for radiative decay after total reduction of the surface (see Figure 4). The identification of these luminescent centers reconciles previous experimental results on the surface- and pH-dependent emission of SiC nanocrystals and helps design optimized fluorophores and nanosensors for in vivo bioimaging.
Figure 4. Surface- and environment-dependent luminescence of SiC NPs. The combination of advanced time-resolved spectroscopy and ab initio simulations, aided by surface engineering, is able to identify the luminescence centers of complex systems. Using such a method, SiOx-defect-related color centers (pink regions on NPs) at the surface of SiC NPs have been identified. From the experimental data, it is possible to build a framework for the surface-related luminescence which can describe the connection between luminescence and surface chemistry.