Session H7: Dopants and Defects in Semiconductors: Spin Related Transport

Anomalous organic magnetoresistance from competing carrier-spin-dependent interactions with localized electronic and nuclear spins

Transport of carriers through disordered electronic energy landscapes occurs via hopping or tunneling through various sites, and can enhance the effects of carrier spin dynamics on the transport. When incoherent hopping preserves the spin orientation of carriers, the magnetic-field-dependent correlations between pairs of spins influence the charge conductivity of the material. Examples of these phenomena have been identified in hopping transport in organic semiconductors and colloidal quantum dots, as well as tunneling through oxide barriers in complex oxide devices, among other materials. The resulting room-temperature magnetic field effects on the conductivity or electroluminescence require external fields of only a few milliTesla. These magnetic field effects can be dramatically modified by changes in the local spin environment. Recent theoretical and experimental work has identified a regime for low-field magnetoresistance in organic semiconductors in which the spin-relaxing effects of localized nuclear spins and electronic spins interfere1. The regime is studied experimentally by the controlled addition of localized electronic spins, through the addition of a stable free radical (galvinoxyl) to a material (MEH-PPV) that exhibits substantial room-temperature magnetoresistance (~ 20%). The magnetoresistance is initially suppressed by the doping, as the localized electronic spin mixes one of the two spins whose correlation controls the transport. At intermediate doping, when one spin is fully decohered but the other is not, there is a regime where the magnetoresistance is insensitive to the doping level. For much greater doping concentrations the magnetoresistance is fully suppressed as both spins that control the charge conductivity of the material are mixed. The behavior is described within a theoretical model describing the effect of carrier spin dynamics on the current. Generalizations to amorphous and other disordered crystalline semiconductors will also be described. This work was supported by DOE and an ARO MURI and was done in collaboration with N. J. Harmon, K. Sahin-Tiras, Y. Wang and M. Wohlgenannt.   

Near-infrared induced charge dynamics of the nitrogen vacancy center in diamond

The nitrogen-vacancy (NV) center in diamond is a key functional element in emerging quantum technologies such as nodes in quantum information processing and nanoscale sensors for condensed matter physics and biology. Recent efforts to optimize the NV's functionality lead to the discovery of photoinduced charge-state switching between the negative (NV$^{\mathrm{-}})$ and neutral (NV$^{\mathrm{0}})$ states which holds great potential to enhance the fidelity of spin readout. While the charge state dynamics under visible illumination have been studied, the effect of infrared light remains unexplored. Here, we use a tunable, pulsed infrared source to illuminate NV centers under various spin and optical states. Precise time-domain control of visible, microwave, and infrared pulses together with single-shot charge readout allows for the direct probing of spin and charge dynamics induced by the infrared light. This new understanding is relevant for the development of advanced protocols to leverage the NV's complete spin, charge, and optical dynamics for quantum control and sensing applications.   

Formation and Annealing Behaviors of Qubit Centers in 4H-SiC from First Principles

Inspired by finding that the nitrogen-vacancy center in diamond is a qubit candidate, similar defects in silicon carbide have drawn considerable interest. However, the generation and annealing behaviors of these defects remain unclear. Using first-principles calculations, we describe the equilibrium concentrations and annealing mechanisms based on the diffusion of silicon vacancies. The formation energies and energy barriers along different migration paths, which are responsible for the formation rates, stability, and concentrations of these defects, are investigated. The effects on these processes of charge states, annealing temperature, and crystal orientation are also discussed. These theoretical results are expected to be useful in achieving controllable generation of these defects in experiments.   

Coherence studies on silicon vacancies in SiC generated via proton irradiation

Single spins in defects in wide bandgap semiconductors are the canonical platform for scalable quantum technologies in the solid state. The silicon vacancy (VSi) in silicon carbide has very recently been shown to exhibit similar spin coherence to the diamond nitrogen vacancy center but in a material that has a mature technological base for fabrication and is an order of magnitude cheaper. Additionally, SiC has several polytypes, allowing the engineering of the spin behavior of the silicon vacancy. In this work, we generate ensembles of VSi via proton irradiation of 4H-SiC. We then measure the spin lifetimes and coherence times of ensembles of defect spins via optically detected magnetic resonance and Hahn pulse techniques. We show that the spin coherence time is strongly dependent on distance from the proton damage layer, therefore setting important parameters for the fabrication of long-lifetime single defect devices.   

Magnetoresistance Phenomena in a Variety of Amorphous Semiconductors and Insulators

We report on near zero-field magnetoresistance (MR) phenomena in a variety of amorphous semiconductors and insulators. We utilize electrically detected magnetic resonance (EDMR) measurements at multiple fields and frequencies to complement MR measurements. EDMR, the electrically detected analog of electron paramagnetic resonance (EPR), provides both information about the chemical nature and energy levels of point defects involved. Semiconductors in this study include a-BC:H, a-C:H, diamond-like carbon (DLC), and a-Si:H. Insulators include a-SiN:H, a-SiOC:H, a-SiCN:H. In hydrogenated amorphous systems, near featureless EPR and EDMR spectra are often difficult to analyze. We utilize multiple field and frequency EDMR results including ultra-low field/frequency ($\nu \quad =$ 85 MHz, B $=$ 3 mT) EDMR measurements to provide insight into defect chemistry in these systems. We have also made EDMR and MR conditions over a wide range of metal/semiconductor heterojunction and metal/insulator/semiconductor biasing conditions. By comparing variable bias measurements with band diagrams, we gain an elementary understanding of defect energy levels. We believe our results will be of significant importance for understanding defect mediated spin-dependent transport in these systems.   

Exploration of Defects in 4H-SiC MOSFETs via Spin Dependent Charge Pumping

4H-SiC MOSFETs have great promise for use in high temperature and high voltage applications. Unfortunately, defects at the SiC/SiO$_{\mathrm{2}}$ interface reduce the performance of these devices. Previously, our group utilized electrically detected magnetic resonance (EDMR) detected via spin dependent recombination (SDR) to identify such SiC/SiO$_{\mathrm{2}}$ interface defects utilizing the bipolar amplification (BAE) biasing scheme; we observed SiC silicon vacancies, E-prime centers, and hydrogen complexed E-prime centers. All of these defects must have levels around the middle of the SiC band gap because they are effective recombination centers. We expanded our studies to include EDMR detection via spin dependent charge pumping (SDCP) at low field, X band, and K band, allowing EDMR exploration of nearly the entire 4H-SiC band gap. Perhaps the most important finding of the (nearly) full band gap measurements is the absence of the carbon dangling bond spectrum in the SDCP. Additionally, in nMOSFETs, we observe an SDCP EDMR spectrum dominated by a silicon vacancy, whereas in pMOSFETs, we also observe a strong, nearly isotropic single line spectrum with g $=$ 2.00244 and 2.00248 when the c-axis is nearly parallel and perpendicular to the magnetic field, respectively. The results suggest that silicon vacancy centers dominate nMOSFET interfaces whereas additional defects clearly play important roles in pMOSFETs.   

Double electron-electron resonance measurements of diamond to determine {\it T}$_2$ dependence on concentration of paramagnetic impurities

A nitrogen-vacancy (NV) center in diamond is a promising candidate for investigation of fundamental sciences and applications to a nanoscale magnetic field sensing device because of unique properties of a NV center in diamond including capability to detect optically detected magnetic resonance (ODMR) signals from a single NV and initialize its spin state. Fundamental studies and applications of NV centers relay on coherent control of the NV centers that is limited by decoherence time ({\it T}$_2$) and, as often observed, {\it T}$_2$ is limited by paramagnetic impurity contents in diamond crystals. In this work, we will investigate {\it T}$_2$ dependence on concentration of nitrogen impurities in type-Ib and type-IIa diamond crystals. For precise determination of the nitrogen concentration, we employ a home-built high-frequency electron spin resonance spectrometer which enables broadband double electron-electron resonance spectroscopy with high spectral resolution.~[1,2] \\$[1]$ F.~H.~Cho, V.~Stepanov and S.~Takahashi, Rev. Sci. Instrum. \textbf{85} , 075110 (2014). \\$[2]$ F.~H.~Cho, V.~Stepanov, C.~Abeywardana and S.~Takahashi, Methods Enzymol. 563 , \textbf{95} (2015).   

Electrical detection and imaging of individual phosphorus and silicon-dangling bonds states at the crystalline silicon to silicon dioxide interface

Nuclear spins of phosphorus [P] donor atoms in crystalline silicon are promising qubit candidates, but utilizing these systems for scalable quantum devices will require the ability to probe individual donors on atomic length scales and address these systems by application of well-controlled electric fields$^{1}$. In this talk we focus on identifying individual P donor and P$_{b}$ (dangling bond) states by measuring electric current through a crystalline silicon (100) - SiO$_{2}$ interface, observing charge flow through individual pairs of P donors and highly localized ({\AA}-range) silicon dangling bond states. The experiments were conducted with neutral P donor states using a low-temperature (T $=$ 4.3K) ultra-high vacuum scanning probe microscope with a quartz tuning fork sensor that allows simultaneous AFM and local current measurements in complete darkness. This so called conduction-atomic force microscopy experiment$^{2}$ yields images of the dangling bond states coupled to individual phosphorus donors. I-V responses on these isolated highly localized charge percolation paths further support the hypothesis that individual P-donor - P$_{b}$ states are being addressed, and that spin-states may be probed using spin-dependent charge-carrier recombination current between $^{31}$P and the interface defects. [1] B. E. Kane, Nature \textbf{393}, 133 (1998); [2] S. Kremmer et, al., Surf. Interface Anal. \textbf{33}, 168 (2002).   

ESR Experiments on a Single Donor Electron in Isotopically Enriched Silicon

In this talk we will discuss electron spin resonance experiments in single donor silicon qubit devices fabricated at Sandia National Labs. A self-aligned device structure consisting of a polysilicon gate SET located adjacent to the donor is used for donor electron spin readout. Using a cryogenic HEMT amplifier next to the silicon device, we demonstrate spin readout at 100 kHz bandwidth and Rabi oscillations with 0.96 visibility. Electron spin resonance measurements on these devices show a linewidth of 30 kHz and coherence times T2* $=$ 10 us and T2 $=$ 0.3 ms. We also discuss estimates of the fidelity of our donor electron spin qubit measurements using gate set tomography. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000.   

Nanoscale thermal imaging using a scanning spin probe.

We use a 30-nm diamond-nanocrystal-hosted nitrogen-vacancy (NV) center attached to the apex of a silicon tip as a local temperature sensor. First, we apply an electrical current to heat up the tip to a predefined operating temperature and rely on the NV to monitor the small thermal changes the tip experiences as it is brought into contact with surfaces of varying thermal conductivity. With the aid of a combined AFM/confocal setup, we image engineered microstructures with nanoscale resolution, and attain excellent agreement between the thermal conductivity and topographic maps [1]. Given the small mass of the NV-hosting diamond nanoparticle, our technique shows a fast time response of order hundred microseconds, limited by the heat dissipation time of the tip. In a second approach, we heat nanostructured gold deposited on glass substrate by injecting a direct current. By monitoring the frequency shift of NV spin transitions upon scanning the AFM tip we reconstruct nanometer-resolved temperature maps. Our technique promises multiple applications ranging from the investigation of phonon dynamics in nanostructures to the characterization of heterogeneous phase transitions in various solid-state systems. [1] A. Laraoui, et al., Nat. Commun, in press.   

Optimizing the Growth of (111) Diamond for Diamond Based Magnetometry

Magnetometers based on nitrogen vacancy (NV) ensembles have recently achieved sub-picotesla sensitivities [Phys. Rev. X 5, 041001(2015)], putting the technique on par with SQUID and MFM magnetometry.Typically these sensors use (100) oriented diamond with NV centers forming along all four (111) crystal orientations.This allows for vector magnetometry, but is a hindrance to the absolute sensitivity. Diamond grown on (111) oriented substrates through microwave plasma enhanced chemical vapor deposition(MP-CVD) provides a promising route in this context since such films can exhibit preferential orientation greater than 99\% [Appl. Phys. Lett.104, 102407(2014)]. An important challenge though is to achieve sufficiently high NV center densities required for enhancing the sensitivity of an NV ensemble magnetometer.We report systematic studies of the MP-CVD growth and characterization of (111) oriented diamond, where we vary growth temperature, methane concentration, and nitrogen doping. For each film we study the Nitrogen to NV ratio, the $NV^{-}$ to $NV^{0}$ ratio, and alignment percentage to minimize sources of decoherence and ensure preferential alignment. From these measurements we determine the optimal growth parameters for high sensitivity, NV center ensemble scalar magnetometry.   

First principles identification of divacancy configurations in 4H- and 6H-SiC

Based on the combination of ab initio simulations and group theory considerations it was proposed earlier that the high spin divacancy defect in silicon carbide (SiC) with similar electronic structure as the NV center in diamond could be utilized as a solid state quantum bit [1]. Recent demonstrations have shown coherent manipulation of divacancy and related defect spins in 4H- [2], 6H- and 3C-SiC [3]. In hexagonal SiC polytypes, point defects can exist in numerous different configurations. Associating potentially interesting photoluminescence (PL) centers with their microscopic configurations is of great importance as quantum information applications often require single defect control. In our study, we carry out large-scale first principles calculation to identify the aforementioned divacancy related point defects. By resolving accuracy issues of ab initio supercell techniques, we were able to obtain convergent PL energies, zero-field-splitting, and hyperfine parameters. Our results confirm pervious assignment of the divacancy related PL1-PL4 PL lines in 4H-SiC to their microscopic structure, provide the identification of QL1- QL6 PL lines in 6H-SiC, as well as propose defect configurations for the unknown PL5-PL6 centers in 4H-SiC that yields strong signal at room temperature. [1] A. Gali et al., Mater. Sci. Forum 645-648, 395 (2010). [2] W. F. Koehl et al., Nature 479, 84 (2011). [3] A. L. Falk et al., Nature Commun. 4, 1819 (2013).