E. A. Stinaff

Ohio University · Department of Physics and Astronomy

Chemical vapor deposition (CVD) is the most common fabrication method for transition metal dichalcogenides (TMDs) where direct chemical vapor phase reaction between an oxide transition metal and chalcogen powder results in formation of high-quality crystals of TMDs. However, in this method the nucleation is often random with incomplete nucleation a...

Transition metal dichalcogenides such as MoS2, which can be produced in monolayer form, have attracted attention because of their interesting and potentially useful electrical and optical properties. These properties often depend sensitively on material properties such as defect density and crystallinity. Here we investigate the effects of post‐gro...

A chemical vapor deposition (CVD) growth model is presented for a technique resulting in naturally formed 2D transition metal dichalcogenide (TMD)-based metal-oxide-semiconductor structures. The process is based on a standard CVD reaction involving a chalcogen and transition metal oxide-based precursor. Here however, a thin metal oxide layer formed...

Coupled quantum dots (CQDs) that consist of two InAs QDs stacked along the growth direction and separated by a relatively thin tunnel barrier have been the focus of extensive research efforts. The expansion of available states enabled by the formation of delocalized molecular wavefunctions in these systems has led to significant enhancement of the...

The difficulty of processing two-dimensional (2D) transition metal dichalcogenide (TMD) materials into working devices with any scalability is one of the largest impediments to capitalizing on their industrial promise. Here, we describe a versatile, simple, and scalable technique to directly grow self-contacted thin-film materials over a range of T...

Scalable fabrication of two-dimensional materials-based devices with consistent characteristics remains a significant impediment in the field. Here, we report on as-grown monolayer MoS2 metal-semiconductor-metal photodetectors produced using a CVD process which results in self-contacted two-dimensional material-based devices. The photodetectors sho...

In 0.06 Ga 0.94 N/GaN superlattices (SLs) grown on sapphire (0001) by metalorganic chemical vapor deposition were studied before and after europium (Eu) ion implantation to understand the strain induced-effects in the SL structure. The implanted SLs were investigated as a function of the thermal annealing temperature up to 1000 °C in nitrogen ambie...

A chemical vapor deposition process resulting in self-contacted as-grown 2D materials-based devices is described by Eric Stinaff and co-workers. The center of the cover shows an as-grown MoS2 based photodiode device. Clockwise, the top four images show optical, second harmonic generation, Raman, and photoluminescense (PL) analysis of the monolayer...

Electronic and optical properties of InAs/GaAs nanostructures grown by the droplet epitaxy method are studied. Carrier states were determined by k·p theory including effects of strain and In gradient concentration for a model geometry. Wavefunctions are highly localized in the dots. Coulomb and exchange interactions are studied and we found the sys...

While new species and properties of two-dimensional (2D) materials are being reported with extraordinary regularity, a significant bottleneck in the field is the ability to controllably process material into working devices. We report a chemical vapor deposition process to selectively grow 2D material in a deterministic manner around lithographical...

While new species and properties of two-dimensional (2D) materials are being reported with extraordinary regularity, a significant bottleneck in the field is the ability to controllably process material into working devices. We report a chemical vapor deposition process to selectively grow 2D material in a deterministic manner around lithographical...

Carrier dynamics in monolayer MoS2 have been investigated using broadband femtosecond transient absorption spectroscopy (FTAS). A tunable pump pulse was used while a broadband probe pulse revealed ground and excited state carrier dynamics. Interestingly, for pump wavelengths both resonant and nonresonant with the A and B excitons, we observe a broa...

Interdot transitions in the emission spectra of a quantum dot molecule may be used as a sensitive nanoscale probe to measure electric fields. Here, we demonstrate this potential by monitoring the temporal behavior of photovoltaic band flattening in a Schottky diode structure using a two-color excitation scheme. First, a continuous wave laser is tun...

Carrier dynamics in monolayer MoS2 have been investigated using broadband femtosecond transient absorption spectroscopy (FTAS). A tunable pump pulse was used while a broadband probe pulse revealed ground and excited state carrier dynamics. Interestingly, for pump wavelengths both resonant and non-resonant with the A and B excitons, we observe a gro...

To support quantum technologies that require entangled photon pairs and/or heralded photons for operation, a photon pair source was developed that uses periodically poled lithium niobate (PPLN) waveguides that are coupled to optical fibers. Both Ti-indiffused and annealed proton-exchanged (APE) waveguide technologies were studied, and waveguide/fib...

Nanorod of in situ Yb-doped InGaN and undoped InGaN have been grown on (0001) sapphire substrates by plasma assisted molecular beam epitaxy (MBE). Selected regions on Yb-doped InGaN sample show single dominant near band edge emission (NBE) in green, yellow or orange color due to the variation of In content. Temperature dependent PL peak energy of I...

Using the physically separated electron and hole of an interdot exciton in a quantum dot molecule we have studied local electric fields with extremely high resolution. By monitoring the interdot exciton energy we have measured an electric field generated through non-resonant excitation in a Schottky device. A maximum optically generated field of ∼3...

We report the effects of tunnel coupling on the Quantum-Confined Stark Effect (QCSE) for excitons in InAs/GaAs coupled quantum dots (CQDs). As the barrier separating the dots is reduced, the zero-field dipole moment and the polarizability are both found to increase. This systematic variation as a function of barrier thickness is due to factors incl...

It has been shown that vertically stacked InAs quantum dots may form quantum dot molecules (QDMs) where the tunneling of the carriers results in molecular wavefunction formation. These states are potentially useful for the preparation and manipulation of entangled spins, necessary components for quantum information processing. It has also been prev...

We will present a theoretical study of the properties of self assembled InAs/GaAs quantum rings. These nanostructures are grown by metal droplet epitaxy and do not follow the traditional strain driven growth model. For certain growth conditions, two quantum dots are formed on the ring structure which then, in a sense, acts as a wetting ring. A `wet...

The use of metal droplet epitaxy may provide a novel method of growing laterally coupled nanostructures. We will present optical studies of InAs/GaAs nanostructures which result in twin quantum dots (QD) formed on a quantum ring (QR). Previous studies have investigated the coupling between vertically grown quantum dot pairs. Here we have used photo...

Bright sources of entangled photons are of great interest in the quantum information community, and the non-linear optical process of Spontaneous Parametric Downconversion (SPDC) is a well-known means to create entangled photons. Additionally, periodic polling has emerged as a viable choice for quasi-phase matching the downconverted photons renderi...

We study electrically tunable self-assembled InAs quantum dot molecules through photoluminescence (PL) and time-resolved PL measurements. For the model we assume quantum dots with cylindrical symmetry, for which the confinement potentials have been modeled as narrow quantum wells in the growth and in-plane directions matched to parabolic potentials...

Polarization sensitive spectroscopy of self assembled quantum dots (QDs) has been shown to yield important information about spins associated with the charge carriers in various excitonic states. As pairs of quantum dots are brought together, and the formation of molecular states via tunneling becomes relevant, the interactions that determine the p...

Tunable exciton relaxation rates are observed in individual vertically coupled semiconductor quantum dots (CQDs). An applied electric field is used to tune the energy difference between the spatially direct (SD) and indirect (SI) excitons in InAs CQDs. The intensity and lifetime of the SI exciton is found to vary as a result of wave-function distri...

Tuning the relative energy levels in coupled quantum dots with an applied electric field results in controllable spin interactions of bound carriers. These interactions may provide new directions in engineering these systems for optical and spintronic applications. Using polarization resolved photoluminescence experiments we observe spin dependent...

We observe modulations in radiative lifetimes and intensities of the spatially indirect exciton as the InAs/GaAs coupled quantum dot system is tuned between molecular and atomic like states. With standard time-resolved single photon counting techniques the measured lifetimes were found to vary between 0.3 and 2.0 ns which resulted in modulations of...

The polarization state of charged excitons in coupled InAs/GaAs dots can reveal useful information about the spin state of its charge carriers. In this study, we examine the complete polarization state through Stokes parameter measurements to relate the polarization parameters of the luminescence to the spin configurations of the various charged ex...

We measure circular polarization memory of neutral exciton states with polarization dependent photoluminescence spectra. As a consequence of anisotropic exchange interaction a low degree of circular polarization memory was observed in the spatially direct and indirect excitons where they anticross. With applied electric field as we tune the exciton...

Optoelectronic control of quantum dots is a thriving area of research with impact on fundamental physics and quantum information devices. Time-resolved photoluminescence experiments, carried out in charge tunable coupled quantum dots, have demonstrated non-monotonic behavior of neutral indirect exciton lifetimes over a wide range of applied electri...

Coupled quantum dots (CQDs) can provide a sensitive probe of the electric field within a device. With non-resonant excitation above the wetting layer (WL) energy, optical generation of an electric field within the CQD structure was observed. By alternating this non-resonant excitation the temporal response of the optically generated electric field...

Nonresonant optical excitation of a coupled quantum dot system was seen to generate a shift in the electric-field-dependent photoluminescence spectra. By monitoring the interdot recombination associated with an electron and hole in different dots we were able to precisely monitor the internal electric field generated. Power, wavelength, and applied...

We present a study of the quantum confined Stark effect (QCSE) for direct excitonic states in individual vertically coupled self-assembled InAs/GaAs quantum dots (QDs). The QCSE in coupled QDs is seen to be a function of barrier separation where for large barriers the shift is similar to that observed in single QDs. As the barrier is reduced the sh...

Coupled quantum dots (CQDs) have potential as components in next generation electronic devices as well as being excellent systems for investigations into quantum mechanical coupling. Understanding the lifetimes of various excitations is a key element for potential applications of these systems. Time-resolved photoluminescence was used to study the...

Spin manipulation in coupled quantum dots is of interest for quantum information applications. Control of the exchange interaction between electrons and holes via an applied electric field may provide a promising technique for such spin control. Polarization dependent photoluminescence (PL) spectra were used to investigate the spin dependent intera...

Excited states in single quantum dots (QDs) have been shown to be useful for spin state initialization and manipulation. For scalable quantum information processing it is necessary to have multiple spins interacting. Therefore, we present initial results from photoluminescence excitation studies of excited states in coupled quantum dots (CQDs). Due...

Recent photoluminescence excitation (PLE) experiments have revealed the unexpected resonant creation of a positive trion in a coupled InAs/GaAs quantum dot system. Positive trion creation is a two photon process requiring the second photon to have a different energy from the first due to the presence of the photogenerated hole. This leads us to con...

Identification and manipulation of the exchange interaction between different spin configurations may be useful for implementing quantum logic operations. Coupled quantum dots offer the possibility of controlling the exchange interaction by continuously tuning between direct and indirect excitonic configurations. The effect of the anisotropic part...

Optical pumping of electron spins and negative photoluminescence polarization are observed when interface quantum dots in a GaAs quantum well are excited nonresonantly by circularly polarized light. Both observations can be explained by the formation of long-lived dark excitons through hole spin relaxation in the GaAs quantum well prior to exciton...

We report on the ability to fabricate super low density InGaAs semiconductor ring-shaped nanocrystals on a GaAs (100) surface by molecular beam epitaxy. Specifically, we demonstrate densities down to 2.3 x 10(6) cm(-2) with only self-assembled methods based on droplet epitaxy. This is several orders of magnitude lower than conventional nanostructur...

We present the polarization dependent photoluminescence from coupled InAs quantum dots. We find polarization memory signatures consistent with those reported for single quantum dots. We also observe a continuous change in the polarization memory of the positively charged exciton as it changes charge configuration due to a hole tunnelling as a funct...

Polarized photoluminescence of the InAs/GaAs coupled quantum dot system was studied, and circular polarization memory signatures of the neutral exciton, the positive trion and the negative trion are reported. Our samples are Stranski-Krastanow dots, vertically separated by a GaAs barrier. We obtain results for circular polarization memory that are...

Spins in a quantum dot molecule are of interest for possible quantum information and spintronic applications. By studying in detail the polarization dependent photoluminescence in the region where the ground state energy levels are in resonance and therefore behaving molecular-like we can gain insight into the various relevant interactions. Vertica...

An understanding of the excited states in coupled quantum dots is a necessary step in the road towards a coherent control of this system. Photoluminescence excitation studies were performed on an InAs/GaAs coupled quantum dot system embedded in a Schottky diode structure. The ground states of the positive trion, negative trion and neutral exciton a...

We present photoluminescence studies of the molecular neutral biexciton-exciton spectra of individual vertically stacked InAs/GaAs quantum dot pairs. We tune either the hole or the electron levels of the two dots into tunneling resonances. The spectra are described well within a few-level, few-particle molecular model. Their properties can be modif...

Through optical spectroscopy of Quantum Dot Molecules we observe spin interactions and gfactors that depend on electric field. We describe how these effects could be used to control spin states and optically gate spin interactions.

Coupling between two closely spaced quantum dots is observed by means of photoluminescence spectroscopy. Hole coupling is realized by rational crystal growth and heterostructure design. We identify molecular resonances of different excitonic charge states, including the important case of a doubly charged quantum dot molecule.

The photoluminescence spectrum of an asymmetric pair of coupled InAs quantum dots in an applied electric field shows a rich pattern of level anticrossings, crossings and fine structure that can be understood as a superposition of charge and spin configurations. We present a theoretical model that provides a description of the energy positions and...

Most self-assembled quantum dot molecules are intrinsically asymmetric with inequivalent dots resulting from imperfect control of crystal growth. The authors have grown vertically aligned pairs of In As / Ga As quantum dots by molecular beam epitaxy, introducing intentional asymmetry that limits the influence of intrinsic growth fluctuations and al...

The coherent quantum coupling of carriers in vertically stacked asymmetric pairs of quantum dots in applied electric fields manifests itself in rich photoluminescence spectral patterns of level crossings and anticrossings. These patterns arise from configurations of charges and spins in optically excited coupled quantum dots. We present a theoretic...

Spin interactions between particles in quantum dots or quantum dot molecules appear as fine structure in the photoluminescence spectra. Using the understanding of exchange interactions that has been developed from single dot spectra, we analyze the spin signatures of coupled quantum dots separated by a wide barrier such that inter-dot interactions...

We present a magnetophotoluminescence study of individual vertically stacked InAs/GaAs quantum dot pairs separated by thin tunnel barriers. As an applied electric field tunes the relative energies of the two dots, we observe a strong resonant increase or decrease in the g factors of different spin states that have molecular wave functions distribut...

The interaction between spins in coupled quantum dots is revealed in distinct fine structure patterns in the measured optical spectra of InAs/GaAs double quantum dot molecules containing zero, one, or two excess holes. The fine structure is explained well in terms of a uniquely molecular interplay of spin exchange interactions, Pauli exclusion and...

Recent optical spectroscopy of excitonic molecules in coupled quantum dots (CQDs) tuned by electric field reveal a richer diversity in spectral line patterns than in their single quantum dot counterparts. We developed a theoretical model that allows us to classify energies and intensities of various PL transitions. In this approach the electric fie...

An asymmetric pair of coupled InAs quantum dots is tuned into resonance by applying an electric field so that a single hole forms a coherent molecular wave function. The optical spectrum shows a rich pattern of level anticrossings and crossings that can be understood as a superposition of charge and spin configurations of the two dots. Coulomb inte...

An asymmetric pair of coupled InAs quantum dots is tuned into resonance by applying an electric field so that a single hole forms a coherent molecular wave function. The optical spectrum shows a rich pattern of level anticrossings and crossings that can be understood as a superposition of charge and spin configurations of the two dots. Coulomb inte...

We report the observation of excited states of holes for acceptorlike excitons bound to isolated nitrogen impurities in GaAs:N under high hydrostatic pressures. Appearance of a large absorption resonance (5K) in optical transmission and photoluminescence excitation spectroscopies leads to the identification of the 2S excited hole state associated w...

With the ability to create coupled pairs of quantum dots, the next step towards the realization of semiconductor based quantum information processing devices can be taken. However, so far little knowledge has been gained on these artificial molecules. Our photoluminescence experiments on single InAs/GaAs quantum dot molecules provide the systematic...

We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge-tunable quantum dots. The spectrum exhibits a p-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange inter...

We compare binding energies for positive and negative trions in a series of narrow GaAs quantum wells and in ``natural'' quantum dots defined by quantum well thickness fluctuations. We assign photoluminescence features to oppositely charged trions through a combination of charging behavior, luminescence polarization, and spin fine structure. Negati...

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the net charge from positive to neutral to negative with a charge-tunable heterostructure. Negative photoluminescence polarization memory is enhanced by optical pumping of ground state electron spins, which we prove with the first measure...

Using polarization-sensitive photoluminescence and photoluminescence excitation spectroscopy, we study single InAs/GaAs self-assembled quantum dots. The dots were embedded in an n-type, Schottky diode structure allowing for control of the charge state. We present here the exciton, singly charged exciton (positive and negative trions), and the twice...

We report on the first observation of the excited states of the hole for acceptor-like excitons bound to isolated nitrogen impurities (N 2 x 10^18 cm-3) in GaAs under pressure. A large absorption-resonance in both transmission and photoluminescence excitation (PLE) spectroscopy leads to identification of the 2S3/2 excited-hole-state associated with...

We report a large and unexpected suppression of the free electron spin-relaxation in lightly doped n-GaAs bulk crystals. The spin-relaxation rate shows a weak mobility dependence and saturates at a level 30 times less than that predicted by the Dyakonov-Perel theory. The dynamics of the spin-orbit field differs substantially from the usual scheme:...

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the charge from positive to neutral to negative using a Schottky diode. We observe that photoluminescence polarization memory has the same sign as the net charge of the dot. Optical pumping of ground state electron spins enhances this eff...

Studies of polarization effects in individual semiconductor quantum dots will be discussed. Spin lifetimes are deduced from changes in the observed photoluminescence polarization as a function of transverse magnetic field (Hanle effect). Results from Hanle measurements performed on neutral, positively, and negatively charged excitons in GaAs/AlGaAs...

We present new insights into the problem of the isoelectronic nitrogen (N) impurity in gallium arsenide (GaAs). By performing photoluminescence (PL), photoluminescence excitation (PLE), transmission, and time decay measurements through a broad range of pressures, we were able to positively identify and track previously unseen states arising from th...

We report He-temperature experiments on narrow, -cleaved platelet cavities of N-doped GaAs loaded into diamond cells for intense photopumping. We find that strong direct-donor exciton PL predominates thru 20 kbar. At 22 kbar the electronic level of isolated N abruptly transforms from a conduction band resonance into the intensely emitting single-ex...

Time-decays have been collected at 1.8 - 6 K for photoluminescent states in GaAs:N versus pressure (0 - 100 kbar) in diamond cells. We summarize this beginning at 70 - 80 kbar, wherein the NX (A and B) exciton states of GaAs:N most clearly resemble their corresponding A and B states in GaP:N, with X_1-binding energies of ~ 32 meV. Here, single expo...

We report 1.8-6 K PL and PLE at pressures up to 100 kbar involving N impurities in high-purity n-type GaAs at concentrations (> ~ 10^19 cm -3) just short of host alloying. Samples were loaded into gasketed diamond cells containing a hydrostatic (>1 in 10^4) ^4 He-pressure-medium. From atmosphere we find Gamma _1-donor-exciton ( ~ 6-meV deep) PL pre...

Electrical charge is the fundamental physical quantity that makes modern electronic devices possible. However, recent advances in nanofabrication and materials physics have created the opportunity to use electron spin for next-generation devices. Spin is a quantum mechanical property with great potential to bring about entirely new technologies for...