Defects In Wide Bandgap Semiconductors Research Articles

(Invited) Tackling Interface Challenges for Shallow Diamond Color Centers

Spin defects in wide-bandgap semiconductors, such as color centers in diamond, can be highly sensitive to local (nanoscale) changes in magnetic field, temperature, and strain. These solid-state quantum sensors have certain advantages over their atomic counterparts owing to their room-temperature operation without the need for vacuum components and the relative ease of photonic- and RF-component integration. However, near-surface quantum defects exhibit spin decoherence and most of the light emitted is trapped within the bulk crystal due total internal reflection at the interface. In this presentation, I will summarize our recent work towards better understanding and addressing these interface challenges, including the modeling and experimental characterization of radiative emission of near-surface emitters, design of nanophotonic components to improve light extraction, and surface analysis and chemical termination techniques aimed at improving spin coherence of emitters.

Sensing spin wave excitations by spin defects in few-layer-thick hexagonal boron nitride.

Optically active spin defects in wide bandgap semiconductors serve as a local sensor of multiple degrees of freedom in a variety of "hard" and "soft" condensed matter systems. Taking advantage of the recent progress on quantum sensing using van der Waals (vdW) quantum materials, here we report direct measurements of spin waves excited in magnetic insulator Y3Fe5O12 (YIG) by boron vacancy [Formula: see text] spin defects contained in few-layer-thick hexagonal boron nitride nanoflakes. We show that the ferromagnetic resonance and parametric spin excitations can be effectively detected by [Formula: see text] spin defects under various experimental conditions through optically detected magnetic resonance measurements. The off-resonant dipole interaction between YIG magnons and [Formula: see text] spin defects is mediated by multi-magnon scattering processes, which may find relevant applications in a range of emerging quantum sensing, computing, and metrology technologies. Our results also highlight the opportunities offered by quantum spin defects in layered two-dimensional vdW materials for investigating local spin dynamic behaviors in magnetic solid-state matters.

Nanoscale Infrared Spectroscopic Characterization of Extended Defects in 4H-Silicon Carbide.

Extended defects in wide-bandgap semiconductors have been widely investigated using techniques providing either spectroscopic or microscopic information. Nano-Fourier transform infrared spectroscopy (nano-FTIR) is a nondestructive characterization method combining FTIR with nanoscale spatial resolution (∼20 nm) and topographic information. Here, we demonstrate the capability of nano-FTIR for the characterization of extended defects in semiconductors by investigating an in-grown stacking fault (IGSF) present in a 4H-SiC epitaxial layer. We observe a local spectral shift of the mid-infrared near-field response, consistent with the identification of the defect stacking order as 3C-SiC (cubic) from comparative simulations based on the finite dipole model (FDM). This 3C-SiC IGSF contrasts with the more typical 8H-SiC IGSFs reported previously and is exemplary in showing that nanoscale spectroscopy with nano-FTIR can provide new insights into the properties of extended defects, the understanding of which is crucial for mitigating deleterious effects of such defects in alternative semiconductor materials and devices.

Experimental evidence for spin–triplet states in Titanium implanted AlN film: An electron spin resonance study

Spin carrying defects in wide-bandgap semiconductors are promising candidates for the development of quantum information and quantum sensing technology. Here we report experimental evidence for the formation of paramagnetic defects in AlN films, subjected to high-energy hydrogen and titanium ion implantation. X-ray diffraction and Raman spectroscopy show that ion species and implantation process could alter the amount of local strain in AlN films. Electron spin resonance spectroscopy reveals resonance peaks with g-value higher than the one of a free electron (2.0023) for both H-implanted and Ti-implanted AlN films. The origin of these peaks is identified and attributed to different types of point defects. For the Ti-implanted film, in addition to the central peak, a half-field resonance peak has been detected and assigned to spin–triplet (S=1). These results provide new insight into the electronic spin state of point defects in AlN, which opens up interesting perspectives for applications to quantum devices.

Влияние комбинированного ионного и электронного облучения на полосу люминесценции 2 eV в гексагональном нитриде бора

Point defects in wide-bandgap semiconductors, in particular in hexagonal boron nitride, are promising candidates for single-photon emitters, used in quantum informatics. We investigated cathodoluminescence of ion beam induced defects in hexagonal boron nitride, as well as the effect of prolonged electron irradiation on the intensity of the luminescence. It has been shown that the intensity of both band-to-band emission and defect related emission decreased after ion irradiation, and during subsequent electron irradiation the intensity of 2 eV luminescence band increased, whereas the intensity of other bands remained unchanged.

Identification of the defect dominating high temperature reverse leakage current in vertical GaN power diodes through deep level transient spectroscopy

Deep level defects in wide bandgap semiconductors, whose response times are in the range of power converter switching times, can have a significant effect on converter efficiency. We use deep level transient spectroscopy (DLTS) to evaluate such defect levels in the n-drift layer of vertical gallium nitride (v-GaN) power diodes with VBD ∼ 1500 V. DLTS reveals three energy levels that are at ∼0.6 eV (highest density), ∼0.27 eV (lowest density), and ∼45 meV (a dopant level) from the conduction band. Dopant extraction from capacitance–voltage measurement tests (C–V) at multiple temperatures enables trap density evaluation, and the ∼0.6 eV trap has a density of 1.2 × 1015 cm−3. The 0.6 eV energy level and its density are similar to a defect that is known to cause current collapse in GaN based surface conducting devices (like high electron mobility transistors). Analysis of reverse bias currents over temperature in the v-GaN diodes indicates a predominant role of the same defect in determining reverse leakage current at high temperatures, reducing switching efficiency.

Challenges of Overcoming Defects in Wide Bandgap Semiconductor Power Electronics

The role of crystal defects in wide bandgap semiconductors and dielectrics under extreme environments (high temperature, high electric and magnetic fields, intense radiation, and mechanical stresses) found in power electronics is reviewed. Understanding defects requires real-time in situ material characterization during material synthesis and when the material is subjected to extreme environmental stress. Wide bandgap semiconductor devices are reviewed from the point of view of the role of defects and their impact on performance. It is shown that the reduction of defects represents a fundamental breakthrough that will enable wide bandgap (WBG) semiconductors to reach full potential. The main emphasis of the present review is to understand defect dynamics in WBG semiconductor bulk and at interfaces during the material synthesis and when subjected to extreme environments. High-brightness X-rays from synchrotron sources and advanced electron microscopy techniques are used for atomic-level material probing to understand and optimize the genesis and movement of crystal defects during material synthesis and extreme environmental stress. Strongly linked multi-scale modeling provides a deeper understanding of defect formation and defect dynamics in extreme environments.

Stability and molecular pathways to the formation of spin defects in silicon carbide

Spin defects in wide-bandgap semiconductors provide a promising platform to create qubits for quantum technologies. Their synthesis, however, presents considerable challenges, and the mechanisms responsible for their generation or annihilation are poorly understood. Here, we elucidate spin defect formation processes in a binary crystal for a key qubit candidate—the divacancy complex (VV) in silicon carbide (SiC). Using atomistic models, enhanced sampling simulations, and density functional theory calculations, we find that VV formation is a thermally activated process that competes with the conversion of silicon (VSi) to carbon monovacancies (VC), and that VV reorientation can occur without dissociation. We also find that increasing the concentration of VSi relative to VC favors the formation of divacancies. Moreover, we identify pathways to create spin defects consisting of antisite-double vacancy complexes and determine their electronic properties. The detailed view of the mechanisms that underpin the formation and dynamics of spin defects presented here may facilitate the realization of qubits in an industrially relevant material.

(Invited) Nanoscale Investigation of Extended Defects in Wide Bandgap Semiconductors By Exploiting Phonon-Resonant Near-Field Interaction

The use of wide-bandgap semiconductor materials like SiC and GaN can dramatically improve the performance of electronic devices at high powers and temperatures. However, the propensity of extended defects in these materials does challenge their implementation in commercial electronic and optical applications. Spectroscopic and microscopic tools for identifying and characterizing these defects typically offer either spectroscopic or microscopic information exclusively and/or may be destructive. Here, we show how extended defects within 4H-SiC manifest in the nanoscale infrared phonon response probed by scattering-type scanning near-field optical microscopy (s-SNOM), a nondestructive method capable of simultaneously collecting topographic and spectroscopic information with frequency-independent nanoscale spatial resolution (≈20 nm).We correlate the s-SNOM response of various defects in 4H-SiC with UV-photoluminescence, secondary electron and electron channeling contrast imaging, and transmission electron microscopy. We identify evidence of step-bunching, recombination-induced stacking faults, and threading screw dislocations, and also demonstrate the interaction of surface phonon polaritons with extended defects. Phonon-enhanced infrared nanospectroscopy and spatial mapping via s-SNOM thus offer significant insights into extended defects within emerging semiconductor materials and devices. It thus serves as an important diagnostic tool to help advance material growth efforts for electronic, photonic, phononic, and quantum optical applications.[1] B. Hauer, C. E. Marvinney, et al. Advanced Functional Materials 30, 1907357 (2020).

Probing Charge Dynamics in Diamond with an Individual Color Center.

Control over the charge states of color centers in solids is necessary to fully utilize them in quantum technologies. However, the microscopic charge dynamics of deep defects in wide-band-gap semiconductors are complex, and much remains unknown. We utilize a single-shot charge-state readout of an individual nitrogen-vacancy (NV) center to probe the charge dynamics of the surrounding defects in diamond. We show that the NV center charge state can be converted through the capture of holes produced by optical illumination of defects many micrometers away. With this method, we study the optical charge conversion of silicon-vacancy (SiV) centers and provide evidence that the dark state of the SiV center under optical illumination is SiV2-. These measurements illustrate that charge carrier generation, transport, and capture are important considerations in the design and implementation of quantum devices with color centers and provide a novel way to probe and control charge dynamics in diamond.

X-ray topography of crystallographic defects in wide-bandgap semiconductors using a high-resolution digital camera

A high-resolution CMOS camera equipped with a scintillator and relay lenses was used to record X-ray topographic images of wide-bandgap semiconductor wafers, including 4H-SiC, GaN, AlN, and β-Ga2O3. The images were compared with those recorded with nuclear emulsion plates (NEPs) and a CCD camera at the same sample site. Fine structures of dislocation contrasts, which allowed for accurate identification of the dislocation types, could be observed using the CMOS camera. The results suggest that the CMOS camera can provide high-quality images that are comparable with the NEPs; hence, the reported approach is a promising one for real-time dislocation observation in power devices under operation.

Identification and thermal stability of point defects in neutron-irradiated hexagonal boron nitride (h-BN)

Point defects in wide-bandgap semiconductors, such as hexagonal boron nitride (h-BN), have driven enormous amounts of research due to their interesting optical properties such as quantum emission at room temperature. Defect engineering by particle irradiation is a very interesting strategy for producing quantum emitters in a controlled way. The identification of most point defects is done by electron paramagnetic resonance and related advanced techniques. Following our recent works, we present new data about point defects produced by neutron irradiation at room temperature and investigate their thermal stability in crystalline, flake and ultrafine h-BN powder. Our experimental data indicate the formation of the nitrogen antisite defect next to a nitrogen vacancy in its neutral charge state, (NBVN)0, with spin S= 1/2 in all types of h-BN showing strong axial hyperfine interaction (A || ≈ 95 MHz and A ⊥ ≈ 5 MHz). It is thermally stable up to ~600 ºC, which is similar to the disappearance of the negative charged boron vacancy, . In addition, after neutron irradiation of h-BN powder samples, a spin S= 1/2 point defect is observed presenting an axial superhyperfine interaction with two equivalent nitrogen nuclei (14N, I = 1, 99.68%), which we tentatively assign to the CNVB defect. It is thermally stable up to ~850 ºC. Another, yet-unidentified S = 1/2 center acts as a charge compensating defect at specific annealing temperatures. Our experimental results are compared and discussed with theoretical models available in the literature.

Defects in SiC for Quantum Computing

AbstractMany novel materials are being actively considered for quantum information science and for realizing high-performance qubit operation at room temperature. It is known that deep defects in wide-band gap semiconductors can have spin states and long coherence times suitable for qubit operation. We theoretically investigate from ab-initio density functional theory (DFT) that the defect states in the hexagonal silicon carbide (4H-SiC) are potential qubit materials. The DFT supercell calculations were performed with the local-orbital and pseudopotential methods including hybrid exchange-correlation functionals. Di-vacancies in SiC supercells yielded defect levels in the gap consisting of closely spaced doublet just above the valence band edge, and higher levels in the band gap. The divacancy with a spin state of 1 is charge neutral. The divacancy is characterized by C-dangling bonds and a Si-dangling bonds. Jahn-teller distortions and formation energies as a function of the Fermi level and single photon interactions with these defect levels will be discussed. In contrast, the anti-site defects where C, Si are interchanged have high formation energies of 5.4 eV and have just a single shallow defect level close to the valence band edge, with no spin. We will compare results including the defect levels from both the electronic structure approaches.

Magnetic-field-dependent quantum emission in hexagonal boron nitride at room temperature

Optically addressable spins associated with defects in wide-bandgap semiconductors are versatile platforms for quantum information processing and nanoscale sensing, where spin-dependent inter-system crossing transitions facilitate optical spin initialization and readout. Recently, the van der Waals material hexagonal boron nitride (h-BN) has emerged as a robust host for quantum emitters, promising efficient photon extraction and atom-scale engineering, but observations of spin-related effects have remained thus far elusive. Here, we report room-temperature observations of strongly anisotropic photoluminescence patterns as a function of applied magnetic field for select quantum emitters in h-BN. Field-dependent variations in the steady-state photoluminescence and photon emission statistics are consistent with an electronic model featuring a spin-dependent inter-system crossing between triplet and singlet manifolds, indicating that optically-addressable spin defects are present in h-BN.

Control of Spin Defects in Wide-Bandgap Semiconductors for Quantum Technologies

Deep-level defects are usually considered undesirable in semiconductors as they typically interfere with the performance of present-day electronic and optoelectronic devices. However, the electronic spin states of certain atomic-scale defects have recently been shown to be promising quantum bits for quantum information processing as well as exquisite nanoscale sensors due to their local environmental sensitivity. In this review, we will discuss recent advances in quantum control protocols of several of these spin defects, the negatively charged nitrogen-vacancy (NV $^{-}$ ) center in diamond and a variety of forms of the neutral divacancy $({\text{VV}}^{0})$ complex in silicon carbide (SiC). These defects exhibit a spin-triplet ground state that can be controlled through a variety of techniques, several of which allow for room temperature operation. Microwave control has enabled sophisticated decoupling schemes to extend coherence times as well as nanoscale sensing of temperature along with magnetic and electric fields. On the other hand, photonic control of these spin states has provided initial steps toward integration into quantum networks, including entanglement, quantum state teleportation, and all-optical control. Electrical and mechanical control also suggest pathways to develop quantum transducers and quantum hybrid systems. The versatility of the control mechanisms demonstrated should facilitate the development of quantum technologies based on these spin defects.

Ultra Large Scale Manufacturing Challenges of Silicon Carbide and Gallium Nitride Based Power Devices and Systems

The potential of wide band gap (WBG) semiconductors for manufacturing ultra-high performance power devices and systems is well known [1]. Historically, silicon carbide research is as old as is the discovery of transistors [2]. In recent years, significant progress has been made in reducing the bulk crystal defects. As a result, for niche applications some SiC and GaN products are currently in the market. However, silicon dominates the power devices and systems market. In an earlier publication [3] we pointed out that bulk crystal defects and process induced defects are still the major road blocks in creating a SiC and GaN semiconductor based power devices and power systems market. This is due to the fact that best values of performance, reliability and yield of semiconductor products can be obtained only when the microstructure is homogenous and minimum defect density is observed [4]. It is a well-known fact in semiconductor manufacturing that other than feature size reduction, the prime reason for the success of silicon manufacturing for low power and power devices has been the reduction of defect density. Currently the defect density of SiC and GaN based products is several orders of magnitude higher than the defect density of Si based semiconductor products. The general notion in the WBG semiconductor based power devices and power system community has been that older generation of silicon manufacturing is good enough to manufacturer WBG based power devices [5]. The prime reason for the success of silicon manufacturing is that the process control of semiconductor manufacturing has evolved from classical statistical process control (SPC) to advanced process control (APC). In other words, batch processing has been replaced by single wafer processing [6]. The key objective of this paper is to demonstrate that without making major process control changes in SiC and GaN processing, these materials will have only niche markets in power semiconductor devices and systems manufacturing. REFERENCES R. Singh, K. Shenai, G.F. Alapatt and S.M. Evon ,"Semiconductor Manufacturing for Clean Energy Economy", Invited Paper, Proc. IEEE Energy Tech 2013 Technology Frontiers in Sustainable Power and Energy, May 21-23, 2013, Case Western Reserve University, published by IEEE, 9781467344449/13 , 2013G. K. Teal, “Single crystals of germanium and silicon—Basic to the transistor and integrated circuit”, IEEE Trans. on Electron Dev. vol. 23, no.7, pp. 621 – 639, 1976K. Shenai, A. Christou, M. Dudley, B. Ragothamachar, and R. Singh,” Crystal Defects in Wide Bandgap Semiconductors”, ECS Transactions, vol. 61 pp. 283-293, 2014R. Singh, V. Parihar, K. F. Poole and K. Rajkanan, “Semiconductor Manufacturing in the 21st Century”, Semiconductor Fabtech, 9th Edition, pp. 223-232, 1999R, Singh and R. Thakur, “Chip Making’s Singular Future”, IEEE Spectrum, February 1, 2005, http://spectrum.ieee.org/semiconductors/materials/chip-makings-singular-futureR. Ivester, “Accelerating Innovation: PowerAmerica Is Up and Running”, Office of Energy Efficiency and Renewable Energy, February 3, 2015, http://energy.gov/eere/amo/articles/accelerating-innovation-poweramerica-and-running-0 (accessed: April 15, 2015)

Design of defect spins in piezoelectric aluminum nitride for solid-state hybrid quantum technologies

Spin defects in wide-band gap semiconductors are promising systems for the realization of quantum bits, or qubits, in solid-state environments. To date, defect qubits have only been realized in materials with strong covalent bonds. Here, we introduce a strain-driven scheme to rationally design defect spins in functional ionic crystals, which may operate as potential qubits. In particular, using a combination of state-of-the-art ab-initio calculations based on hybrid density functional and many-body perturbation theory, we predicted that the negatively charged nitrogen vacancy center in piezoelectric aluminum nitride exhibits spin-triplet ground states under realistic uni- and bi-axial strain conditions; such states may be harnessed for the realization of qubits. The strain-driven strategy adopted here can be readily extended to a wide range of point defects in other wide-band gap semiconductors, paving the way to controlling the spin properties of defects in ionic systems for potential spintronic technologies.

Polarization Spectroscopy of Defect-Based Single Photon Sources in ZnO.

Point defects in wide bandgap semiconductors are promising candidates for future applications that necessitate quantum light sources. Recently, defect-based single photon sources have been observed in ZnO that are very bright and remain photoactive from 4.5 K to room temperature. Despite several investigations, the structure and electronic states of these emitters remain unknown. In this work, we establish a procedure to distinguish a Z dipole from an XY dipole when studying quantum emitters that are randomly oriented. Our cryogenic and room temperature polarization measurements collectively establish that these unidentified ZnO quantum emitters have a Z dipole. We show that the associated absorption and emission dipoles are parallel within experimental uncertainty for all 32 individuals studied. Additionally, we apply group theory and find that, assuming the defect symmetry belongs to a point group relevant to the ZnO wurtzite lattice, the ground and excited states are orbital singlets. These results are a significant step in identifying the structure and electronic states of defect-based single photon sources in ZnO.

(Invited) Crystal Defects in Wide Bandgap Semiconductors

The state-of-the-art power switching devices made from SiC and GaN semiconductors contain a high density of crystal defects [1]. Most of these defects are present in starting wafers and some are generated during device processing. There is little conclusive evidence so far on the exact role crystal defects paly on device performance, manufacturing yield, and more importantly, long-term field-reliability especially when devices are operating under extreme environments. This paper will review the current state-of-the-art of material defects in SIC and GaN power semiconductors, and the potential role these defects may have on high-density power switching electronics. A review of silicon technology development and manufacturing evolution will be made to draw a parallel between the state of silicon and wide bandgap power electronics.[1] K. Shenai et al, “Current status and emerging trends in wide bandgap (WBG) semiconductor power devices,” ECS J. Solid State Sci. and Tech. 2(8), N3055-N3063, Jul 2013

(Invited) Crystal Defects in Wide Bandgap Semiconductors

The state-of-the-art power switching devices made from SiC and GaN semiconductors contain a high density of crystal defects. Most of these defects are present in starting wafers and some are generated during device processing. There is little conclusive evidence so far on the exact role that the crystal defects paly on device performance, manufacturing yield, and more importantly, long-term field-reliability especially when devices are operating under extreme stressful environments. This paper provides a review of the current state-of-the-art of SiC and GaN power semiconductor material technology, and the potential impact crystal defects may have on high-density power switching electronics. A review of silicon technology development and manufacturing evolution is made to draw a parallel between silicon and wide bandgap (WBG) semiconductor power electronics.