Session J63: Optical Probes and Imaging of Defects

Multi-modal microscopy and spectroscopy of wide band gap semiconductors

The heart of materials science is the link between materials structure and properties. Traditionally, the small-scale structure of materials is often assessed using microscopy techniques, which may provide access to topography, crystallography, composition etc. The physical properties of material are more commonly assessed at a macroscopic level, often using spectroscopy techniques. Increasingly, however, advanced microscopes allow spectroscopy or other physical property measurements with nanoscale resolution.
One established technique that allows direct correlation of microscopy and spectroscopy is cathodoluminescence (CL) in the scanning electron microscope (SEM), in which the recombination of electron-hole pairs excited by the electron beam leads to emission of light that is collected and analysed spectroscopically. CL can be collected alongside structural/compositional data recorded in the SEM. This approach is often simply used for the collection of panchromatic CL images, mapping the total intensity of light emitted at each pixel. However, sophisticated CL systems allow not only the collection of time-integrated spectra, but also time-resolved measurements on the tens of picosecond timescale and data collection across a range of temperatures.
I will discuss the application of temperature- and time-dependent CL to a range of problems in wide-bandgap semiconductors, particularly defects and quantum wells in gallium nitride, addressing both the standard hexagonal form of this material and metastable cubic GaN. However, SEM only provides a limited subset of the structural information a materials scientist desires. Thus, I will introduce methods that allow different microscopy techniques to be brought to bear on exactly the same nanoscale structure, revealing more about its characteristics than any individual technique could achieve alone. In particular, I will discuss methods for combining SEM-CL with transmission electron microscopy, and scanning probe microscopy.   

Far-infrared spectroscopy of shallow thermal donors and dilute impurities in high-purity silicon

Silicon has uses as test-mass material for future gravitational-wave detectors and in high-resolution infrared spectroscopy in astronomy. The transparency of Si in the infrared region is critical to the effectiveness of these new devices. The temperature-dependent infrared transmission has been measured for high-purity silicon samples with impurity concentrations of ~10¹⁵ per cc. Measurements were made in a frequency range from 10–2000 cm^-1 and temperatures from 10–300K. At 10 K, silicon is transparent in the far infrared (10–600 cm^-1) apart from narrow absorption lines caused by residual impurities. These absorption lines are mostly due to hydrogen-like transitions in donor and acceptor atoms as well as interstitial oxygen and will have a significant effect on the optimal operation of the Si device. At higher temperatures the electrons are ionized by the thermal energy in the crystal and become free elections, causing a Drude-like response in the far-infrared. Using the results of our transmission spectra, we can determine the type of impurities in the sample and their concentration.
  

Multidimensional Coherent Spectroscopy of Erbium Doped GaAs Quantum Wells

Rare-earth ions are appealing for a broad array of applications. When embedded in crystalline matrices, they combine well protected spin and optically active transitions which are useful to store and process quantum information. When doped in semiconductors, they have been intensely investigated as a new class of electrically active infrared-emitting materials. To better understand the interaction of rare-earth doping, we investigate the effect of dilute erbium doping in GaAs quantum wells. When doped into GaAs, the Er³⁺ ion substitutes Ga³⁺ forming ErAs, which is known to have “coupled” magnetic and electronic properties. As a magnetic material, there is a strong exchange coupling between the relatively large local moments of the 4f and valence and conduction electrons near the Fermi level. Using multidimensional coherent spectroscopy, we attempt to elucidate excited-state structure and interactions of the Er³⁺ doping in GaAs at different doping concentrations. This spectroscopy can characterize excited state dynamics and interactions.   

Deep UV Photoluminescence and Chemical composition Analysis of Residual Impurities in Hexagonal boron nitride

Single crystal hexagonal boron nitride (h-BN) is an ultrawide (~6.0 eV) semiconductor under development for electronic, optoelectronic and nanophotonic devices. A key issue in these applications is the effect of impurities, especially oxygen, on its properties. Here the properties and compositions of h-BN and oxidized h-BN were compared. The deep UV photoluminescence spectra from h-BN samples were oxidized at 900 ^oC in dry and ambient air, had strong phonon-assisted band edge emissions along with a sharp atomic-like emission line at 4.09 eV, and its phonon replicas at 3.89 and 3.69 eV. The sharp emission line and phonon replicas were not observed in unoxidized sample. Comparative chemical composition analysis of the residual impurities was carried out using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) in order to identify the impurities related to the sharp transition. Our results of atomic like features could have potential applications in the solid-state single photon source for quantum information technologies.
  

Independent determination of carrier densities and analysis of different recombination channels in GaAs PL and EL through Raman scattering of LO phonon – plasmon coupled mode

Determining the carrier (electron) density n is critical in many material and device characterization. An ABC model is widely used to describe the carrier recombination, where the contributions of non-radiative, radiative, and Auger recombination are partitioned, respectively, into the three terms in R = An + Bn² + Cn³. In general, not only the partition is questionable, e.g., nonradiative contribution could be non-linear in n, but also a number of approximations are needed in order to determine n. It has been demonstrated recently that by combining photoluminescence (PL) and Raman imaging one can simultaneously obtain the spatial distribution of electron and hole density in a semiconductor material or device by first determining the electron density through Raman scattering of LO phonon – plasmon coupled (LOPP) mode [1]. This approach is extended to analyze carrier recombination processes in both PL and electro-luminescence (EL) with varying laser excitation and current injection level. The results indicate that nonradiative recombination typically exhibits non-linear dependence on n, as suggested by a recent analysis of EL in an InGaN LED [2].
[1] C. Hu et al., Light: Sci. & Appl. 7, 23 (2018).
[2] T. Gfroerer, J. Appl. Phys. 125, 204502 (2019).   

Understanding the Fluorescence Mechanism of Carbon Dots through Single-Molecule Excited-State Imaging

We employed scanning tunneling microscopy (STM) and single molecule absorption STM (SMA-STM) to study the fluorescence mechanism in carbon dots (Cdots). The red- and blue-fluorescing Cdot samples were previously purified and characterized in bulk with standard spectroscopic techniques. Carbon dots were deposited on a thin hybrid PtAu film over a sapphire substrate via solvent-based aerosol deposition. We first performed current imaging tunneling spectroscopy (CITS) to resolve the spatial electronic structure of the adsorbed Cdots, estimating the quantity of defects, whether they were active or inactive as optical absorbers, and measuring energy levels of dopants. Then we applied SMA-STM to spatially resolve electronically excited defects, and used scanning tunneling spectroscopy (STS) in situ to look at the density of states at these locations during laser excitation and otherwise. We observed significant spatial variations in the band structure and detected increased defect quantity as fluorescence red-shifts. We further identified the energy levels involved in fluorescence and ultimately discuss the most plausible fluorescence mechanism, which does not depend on the size of the Cdot.   

Charge sensing in CMOS devices using reflectometry

Spin qubits in silicon are great candidates for scalable quantum information processors due to their long coherence time and compatibility with industrial CMOS fabrication lines.[1] Usual spin-readout techniques imply spin-to-charge conversion and charge readout by transport measurement. This requires multiple leads and limits the scalability of the system. A solution is to do charge sensing using RF-reflectometry measurement[2] for which only one lead is necessary to control and read the qubit.[3] The critical part is to obtain an impedance matching, at low temperature, between the resonant circuit and the RF-line. Finding this matching condition can be challenging considering the temperature dependence of each of the tank circuit component and the sample-to-sample capacitance variability. A solution is to use a varactor[4] to adjust in situ the circuit impedance. In this talk, we report on optimization of reflectometry setups at low temperature and discuss the use of tunable capacitors for in situ tuning targeting high sensitivity RF-measurement for spin qubit readout.

[1] J. Yoneda, et al., Nat. Nano. 13, 102-106(2018)
[2] B. J. Villis, et al., Appl. Phys. Lett. 104, 233503(2014)
[3] P. Pakkiam, et al., Phys. Rev., 8, 041032(2018)
[4] N. Ares, et al., Phys. Rev., 5, 034011(2016)
  

Infrared Transmission of Ge:Mn Thick Films Prepared by Ion Implantation and Post-Annealing

Aproximately 3 micrometer thick Ge:Mn films were prepared by low temperature ion-implantation followed by post-annealing. XRD, SIMS, magnetometry and infrared transmission (100-6500 cm^-1) were used to characterize the films. The average Mn concentration in the amorphous implanted films is less than 0.3%. Annealing causes approximately 40-70% of Mn atoms to migrate to Mn-rich clusters or form Mn₅Ge₃ while the remainder enters the Ge lattice and creates free holes. If annealed at high enough temperature the amorphous films become polycrystalline. Solid phase epitaxy does not occur. A Drude peak and structure in the mid-infrared absorption observed by infrared transmission provide evidence for free holes. The data suggest that the maximum solubility of Mn in Ge is less than 0.08 %.   

Photon Statistics as an Analytical Tool in Solid-State Defect Systems

Photon correlation spectroscopy is a versatile and widely used analysis technique with a broad history of applications ranging from molecular spectroscopy to exploration of fundamental quantum mechanical concepts. More recently, this method has been used to study quantum emission from defect complexes in solid-state systems for a variety of quantum technological applications. The predominant use of photon statistics in solid-state systems is as an indicator of single-photon emission, a desirable property for applications such as quantum key distribution and quantum repeaters. However, a more thorough analysis of photon statistics offers the potential to reveal characteristics of the system such as electronic structure, multi-level optical dynamics, and responses to external fields. We briefly discuss the history and development of photon correlation spectroscopy as a measurement tool and outline methods for proper acquisition and analysis of photon statistics. In addition, we use this analysis framework to study quantum emitters in hexagonal boron nitride and discuss how these methods can allow the determination of their electronic structure and optical dynamics.   

Spatiotemporal Microwave Imaging of Photo-carrier Dynamics in Monolayer Semiconductors

The spatiotemporal evolution of photo-generated charge carriers in two-dimensional transition-metal dichalcogenides (TMDs) plays a crucial role for their optoelectronic applications. A comprehensive understanding of such dynamics, however, remains a challenging task. Here, we report the simultaneous spatial and temporal photoconductivity imaging in WS₂ monolayers by laser-illuminated microwave impedance microscopy. The diffusion length and carrier lifetime were directly measured. Time-resolved experiments indicate that the critical process for photo-carriers is the escape of holes from defect-induced trap states, which prolong the apparent lifetime of mobile electrons in the conduction band. This work provides fundamental knowledge on the defect-mediated spatiotemporal dynamics of charge carriers in 2D TMDs, paving the way for their applications in novel optoelectronic devices.
  

Photodegradation of Si-doped GaAs Nanowire

Researching optical effects in nanowires (NWs) may requires high pump intensity that under ambient conditions can degrade NWs due to thermal oxidation. In this work we investigated the photodegradation of a single Si-doped GaAs NW by laser heating in air. To understand the changes occurred on the NW we carried out scanning electron microscopy (SEM), and energy dispersive X-ray (EDS), micro-Raman (μ-RS), micro-photoluminescence (μ-PL) spectroscopies in laser damaged regions as well as in non-affected ones. From Stokes and anti-Stokes peaks we estimated that the oxidation process starts at 661 K, resulting in two new modes at 200 and 259 cm^-1. SEM and EDS showed a significant loss of As in the oxidized regions, but no erosion of the NW. μ-PL showed the near-band-edge emission of GaAs along the NW, as well as a new emission band at 755 nm corresponding to polycrystalline β-Ga₂O₃ formation. Our results also indicate that neither amorphous As, nor crystalline As, were deposited at the surface of the NW. Combining different experimental techniques, this study showed the formation of polycrystalline β-Ga₂O₃ by oxidation of the NW surface and the limits to perform spectroscopic investigations on individual GaAs NWs under ambient conditions.   

Defect induced photoluminescence in SnO2 nanostructures: Evaluation and Utilization for optical sensing and waveguide application

Despite having an excellent reputation in resistive sensing and profound optical properties, SnO₂ is less explored for optical sensing and waveguide application. In the current project SnO₂ quantum dots [QDs~ 2.4 nm] and 1D NSs [d~500nm], are utilized for photoluminescence (PL) based NH₃ sensing and waveguide application respectively. PL spectra collected from SnO₂ QDs using 325 nm excitation show the rise of peaks at 2.77 and 2.96 eV in the presence of NH₃. Sensor response (R) is calculated in different concentrations of NH_3, (10-500 ppm) using the formula R= (I_gas-I₀)/I₀ [I_gas and I₀, PL intensity in the presence and absence of NH₃]. Defects on the surface of QDs, supported by pre-edge SnM₅ resonance peak in XAS spectrum collected from QDs, form energy band just above the valance band and causes diminished PL peak at 2.77 and 2.96 eV. During the interaction, NH₃ provides electron to SnO₂ and enhance 2.77 and 2.96 eV related transition. Room temperature NH₃ detection with excellent selectivity and high recovery rate ensures the technical importance of this work. Further waveguide application using SnO₂ 1D NSs is realized by exciting single 1-D NSs with 325 nm LASER. Optical images and PL spectra were captured to confirm the defect induced PL is guided through 1D NSs.