Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session G41: Defects in NitridesFocus
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Sponsoring Units: DMP Chair: Suraj Cheema, University of California, Berkeley Room: Room 319 |
Tuesday, March 7, 2023 11:30AM - 12:06PM |
G41.00001: Optical Absorption of point defects in AlN crystals as measured by photo-induced electron paramagnetic resonance Invited Speaker: Mary Ellen Zvanut AlN is actively pursued as a substrate for high power electronics due to its large bandgap (~ 6 eV) and high critical field (15 MV/cm). The ultrawide bandgap also enables production of emitters and detectors that are operational in the ultraviolet. We have performed 10 GHz room temperature electron paramagnetic resonance (EPR) measurements in combination with illumination to study a prominent point defect in AlN. The photo-induced EPR technique offers an advantage over conventional absorption measurements because the defect itself is monitored, rather than the material as a whole. |
Tuesday, March 7, 2023 12:06PM - 12:18PM |
G41.00002: Towards tunable single-pixel LEDs with Eu-doped GaN active layers Hayley J Austin, Brandon J Mitchell, Jun Tatebayashi, Yasufumi Fujiwara, Volkmar G Dierolf Doping rare-earth ions in the active layers of GaN-based materials has been useful for achieving precise emission colors. In particular, Eu3+ can be excited electrically and emits red (~620nm) from its 5D0 state, as well as green, yellow and orange from its 5D1 state. By adjusting excitation conditions, the population ratio between the two states can be controlled resulting in smooth color tunability from red to yellow. Modeling work for the excitation dynamics of the defect-ion complex after creation or injection of electron hole pairs takes a local defect into account, where the carriers are captured, recombine, and transfer energy to the Eu3+. Multiple excitations of the same defect complex before the Eu3+ can relax causes the 5Dj states’ populations to redistribute. Furthermore, by considering the local defect as a blue emitter, our model can predict tunability parameters within a larger color space due to the mixing of the primary colors: red, green, and blue. The model assumes that the reshuffling process takes place within one complex and that energy is not transferred to others. To test the validity of this assumption, various samples co-doped with Eu3+ and other RE ions are studied. |
Tuesday, March 7, 2023 12:18PM - 12:30PM |
G41.00003: Optical Dipole Structure and Orientation of GaN Defect Single Photon Emitters Yifei Geng, Jialun Luo, Gregory D Fuchs, Farhan Rana Defect-based single-photon emitters play an important role in quantum information technologies. The study of quantum emitters in technologically mature wide bandgap semiconductors, such as the wide bandgap nitrides, is attractive for photonic integration. GaN has recently been shown to host very bright defect single photon emitters in the 600-700 nm wavelength range but very little is known thus far about their structure and origin [1,2]. We study these emitters in HVPE grown Wurtzite GaN epitaxial layers using the method of defocused imaging whereby the radiation pattern in the Fourier plane is recorded. This techniques enables the detection of the far-field emission pattern and the orientation of the optical dipole moment. Our experimental results show the defect emitters around 680 nm wavelength have dipoles oriented almost parallel to the crystal c-axis. Our results call into question the view that these defect emitters are related to crystal dislocations [2] and show that different emitters exhibit different characteristics. The measured optical dipole orientations suggest that these defect emitters could be substitute impurity atoms or complexes with dipole orientations along the Ga-N bonds and involve small lattice distortions. We will discuss the nature of GaN defect emitters in the light of our measurements. [1] arXiv:2206.12636 (2022), [2] Adv. Mat. 29, 1605092 (2017). |
Tuesday, March 7, 2023 12:30PM - 12:42PM |
G41.00004: First-principles studies of the role of cation disorder on electronic structure of ZnTiN2 Sijia Ke, Jeffrey B Neaton Cation disorder is often found in divalent ternary nitride semiconductors, with significant consequences for optoelectronic properties. For example, recently synthesized wurtzite-derived ZnTiN2 exhibits cation disorder on its Zn and Ti sites, and has a measured band gap of about 2 eV, 1.5 eV smaller than that predicted for its cation-ordered structure by density functional theory (DFT) calculations with hybrid functionals [1]. ZnTiN2 has great potential in photoelectric applications due to its band gap in the visible, good integration with high-performing semiconductors, and the potential for a stable self-passivating surface layer (e.g., ZnO, TiO2) under operating conditions. Using state-of-the-art first principles DFT calculations with optimally-tuned range-separated hybrid functionals, we demonstrate that cation disorder in ZnTiN2 creates locally charge-unbalanced nitrogen centered motifs that further introduce states near band edges, leading to band gap reduction. We study how the cation disorder affects local chemical environment, energetics, electronic structure, and optoelectronic properties in ZnTiN2 and discuss its properties in light of possible photocatalytic applications. |
Tuesday, March 7, 2023 12:42PM - 12:54PM |
G41.00005: Identification of an Oxygen Defect in Hexagonal Boron Nitride Song Li, Adam Gali Single-photon emitters in hexagonal boron nitride have been extensively studied recently as they are promising building blocks for quantum information processing. Although numerous defect-related single-photon sources have been found, except for the boron vacancy (VB), their identification is still elusive. Electron paramagnetic resonance (EPR) could be a powerful tool to identify the structure of point defects. A spin S = 1/2 point defect D3 was observed recently in experiment [1]. Based on the unique EPR line shape, we proposed VB + XN defect that containing VB with one nearest nitrogen atom replaced by an impurity atom “X”. In the VB + XN model, the impurity atom should have very low natural abundance of nuclear spin isotopes, for example, carbon (13C = 1.1%) and oxygen (17O = 0.037%). However, we found CNVB cannot keep C2v symmetry and could easily transform to VNCB. Formation energy calculations show that the VNCB is the dominant defect over CNVB [2]. Instead, the negatively charged ONVB well reproduces the hyperfine constant and EPR spectra by means of the many-body perturbation theory method on top of hybrid density functional calculations. To our surprise, it also produces a coherent emission around 2 eV with well agreement with previously recorded PL spectrum of some quantum emitters, according to our calculations [3]. |
Tuesday, March 7, 2023 12:54PM - 1:06PM |
G41.00006: Formation, structure, and properties of GaN nanowire polytype heterostructures Abby Liu, Hongling Lu, Rachel S Goldman Semiconductor polytype heterostructures, which consist of adjacent layers of lattice-matched materials differing only in their atomic stacking sequences, offer opportunities for exceptional electronic device performance. For example, interfaces between wurtzite (WZ) and zincblende (ZB) polytypes are expected to result in the formation of a polarization-doped 2-dimensional electron gas, with both high carrier concentration and high carrier mobility. In the case of GaN, we recently discovered a metal-mediated molecular-beam epitaxy (MBE) process to nucleate ZB GaN films and/or nanowire (NW) ensembles directly on silicon (Si) [1]. For both films and NW ensembles, reflection-high energy electron diffraction and x-ray diffraction reveal ZB-to-WZ transformations at thicknesses ~ 20 nm. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) reveals ZB NWs with axes oriented ~37° from the Si [001] surface normal. Interestingly, the NW axis orientation remains fixed as it transforms to the WZ polytype. Furthermore, for thicknesses in excess of 45 nm, alternating ZB and WZ polytypes are observed. To determine the electronic states and band-offsets at the ZB/WZ GaN NW interfaces, STEM electron energy loss spectroscopy (EELS) studies are in progress. |
Tuesday, March 7, 2023 1:06PM - 1:18PM |
G41.00007: Theoretical Interpretations of Sb Incorporation in GaN Yujie Liu, Ishtiaque Ahmed Navid, Zetian Mi, Emmanouil Kioupakis Dilute concentrations of Sb doping in GaN induce large band gap-bowing and tunable photoluminescence (PL) at room temperature from the UV to the green spectral regions. However, the incorporation of Sb into the host nitride material, both within the bulk and on the surface, remains unclear. Previous work found that Sb is preferentially incorporated as an anion (Sb3–) into the N sublattice. Here, we use first-principles calculations to explore the possibility that Sb may also be incorporated as a cation (Sb3+, Sb5+) into the metal sublattices. Our bulk defect thermodynamic analysis finds that SbGa is energetically preferable compared to SbN under both N-rich and Ga-rich conditions in n-type samples. We next explore surface defect thermodynamics and quantify the site-selectivity of Sb incorporation for different crystallographic orientations. Our calculated band structures suggest the simultaneous presence of Sb3- and Sb5+. Our calculated phonon spectra of Sb in different charge states successfully explain the origin of the experimentally observed additional Raman peaks of Sb-doped GaN. Our current results could be extended to other group-V dopants in GaN such as As, P, or Bi. |
Tuesday, March 7, 2023 1:18PM - 1:30PM |
G41.00008: Investigating the strain-tunable properties of ScAlN through first-principles calculations Mahlet Molla, Woncheol Lee, Ping Wang, Ding Wang, Zetian Mi, Emmanouil Kioupakis Though ScAlN exhibits ferroelectricity due to the incorporation of ScN and its layered hexagonal phase, the strain on the wurtzite basal plane due to alloying or epitaxy also contributes to the ability of this material to exhibit polarization switching without reaching its dielectric breakdown limit. Since the applied field that allows for polarization switching is smaller than the dielectric strength of ScAlN, understanding the strain-tunable properties of the alloy is critical to obtaining a method to lower the energy barrier for ferroelectric switching. In this work, we investigate how increasing the strain of ScAlN through the incorporation of indium or by epitaxial lattice mismatch affects its dielectric permittivity and polarization switching energy barrier. We utilized density functional perturbation theory (DFPT) phonon calculations at the Brillouin zone center to determine the electronic and ionic contribution to the dielectric constant and performed nudged elastic band (NEB) calculations to quantify the effects of In composition on the polarization switching barrier of the alloy system. This work on the strain-tunable properties of ScAlN can be generalized to other III-nitride alloys like ScAlGaN and provides insight into the polarization switching mechanism of these systems. |
Tuesday, March 7, 2023 1:30PM - 1:42PM |
G41.00009: Ion beam channeling studies of scandium incorporation into ScAlN Erdem Ozdemir, Joshua Cooper, Thai-Son Nguyen, Joseph Casamento, Debdeep Jena, Huili Grace Xing, Rachel S Goldman Recently, the incorporation of scandium (Sc) into wide bandgap semiconductors has generated great interest due to the possibility of transforming pyroelectric materials into promising piezoelectric and ferroelectrics. Indeed, the incorporation of Sc into AlN and GaN facilitates a piezoelectric response if the wurtzite (WZ) polytype is maintained. ScN typically crystallizes in the rocksalt (RS) polytype although metastable WZ polytypes have been reported. Here, we investigate the mechanisms for Sc incorporation into MBE-grown ScxAl1-xN layers using channeling Rutherford backscattering spectrometry (RBS/c). Using 1.5 MeV He2+, we measured [0001] channeling, random, and angular yield scans of ScxAl1-xN layers grown on AlN-on-Al2O3 templates. For the AlN-on-Al2O3 templates, the [0001] minimum yield is 0.6, and the in-plane angular yield scans exhibit similar half-depths, suggesting the presence of mixed RS-WZ polytypes within the Sc0.3Al0.7N layers. To determine the site occupancy of Sc, RBS/c studies along the [1100] and [11-20] directions are in progress. |
Tuesday, March 7, 2023 1:42PM - 1:54PM |
G41.00010: Transport properties of heavily Si doped high Al mole fraction AlxGa1−xN grown by MBE on single-crystal AlN substrates Chandrashekhar P Savant, Ryan Page, Thai-Son Nguyen, Kevin Lee, Vladimir Protasenko, Huili Grace Xing, Debdeep Jena Controlled n-type doping in ultrawide bandgap AlGaN alloys is critical for power electronics, deep UV photonics applications. While the bandgap can be increased by increasing Al, a sharp increase in resistivity at above ∼80% Al has been reported in Si doped AlGaN, though the exact mechanism remains unclear. In this study, Si-doped AlGaN films with a wide range of doping concentrations and Al% are grown by molecular beam epitaxy on low dislocation density, single crystal AlN substrates. Electronic transport of these films was studied to investigate the upper limits of both Al% and Si doping. Degenerate doping and high electron density were achieved in films with up to 85% Al. Above this, the dopant thermal activation energy sharply increased; electron thermal activation energies were measured in samples with Al% up to and including binary AlN, which exhibited activation energy of 200 meV. Several mechanisms to explain this sharp increase in dopant activation energy, including the stabilization of deep DX-centers; decreased permittivity from decreased polar optical phonon screening, are identified. Additionally, the self-compensation phenomenon at very high donor densities was investigated. By minimizing the impacts of foreign impurities and native defect complexes, carrier densities of 7.6 x 1019 cm−3 several times higher than maximum reported in similar MBE, MOCVD grown films are achieved. Furthermore, the degenerate doping is pushed to higher Al of up to 85%. This work demonstrates that AlGaN layers with >5.5eV bandgaps can be made highly conductive for electron transport in optoelectronic devices. |
Tuesday, March 7, 2023 1:54PM - 2:06PM |
G41.00011: Extending the room temperature spin coherence time of boron vacancies in hexagonal boron nitride Martin Schalk, Andreas V Stier, Roberto Rizzato, Chenjiang Qian, Stephan Mohr, Joachim P Leibold, Georgy V Astakhov, Ulrich Kentsch, Manfred Helm, Dominik Bucher, Jonathan J Finley Optically active spins in hexagonal boron nitride (hBN) attract increased attention recently due to room temperature operation and the possible co-integration of the spin defect sensors into ultra-thin semiconductor heterostructures. The VB- spin (S=1) can be addressed by means of optically detected magnetic resonance (ODMR). However, the optical readout contrast of the VB- spin state is relatively low and decays quickly, rendering the use for quantum sensing applications limited until more efficient microwave control and optimized defect creation with long spin phase coherence times are developed. Here, we demonstrate coherent control of the VB- spin defects and utilize a pulsed dynamical decoupling protocol CPMG to extend the T2 coherence time by up to two orders of magnitude. With the increased spin coherence time we address new applications such as the detection of AC magnetic fields and sequences for ambient quantum imaging. |
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