Dr. Martin Paul Geller
University of Duisburg-Essen
- Internal photoeffect from a single quantum emitter
Lochner, P. and Kerski, J. and Kurzmann, A. and Wieck, A.D. and Ludwig, A. and Geller, M. and Lorke, A.
Physical Review B 103 (2021)We demonstrate by time-resolved resonance fluorescence measurements on a single self-assembled quantum dot an internal photoeffect that emits electrons from the dot by an intraband excitation. We find a linear dependence of the optically generated emission rate on the excitation intensity and use a rate equation model to deduce the involved rates. The emission rate is tunable over several orders of magnitude by adjusting the excitation intensity. Our findings show that a process that is well known in single atom spectroscopy (i.e., photoionization) can also be observed in the solid state. The results also quantify an important, but mostly neglected, mechanism that may fundamentally limit the coherence times in solid-state quantum optical devices. © 2021 American Physical Society.
view abstract 10.1103/PhysRevB.103.075426
- Quantum Sensor for Nanoscale Defect Characterization
Kerski, J. and Lochner, P. and Ludwig, Ar. and Wieck, A.D. and Kurzmann, A. and Lorke, A. and Geller, M.
Physical Review Applied 15 (2021)The optical and electronic properties of semiconductors are strongly affected by structural and stoichiometric defects. The precise incorporation of dopants and the control of impurities are essentially what makes semiconductors useful materials for a broad range of devices. The standard defect and impurity characterization methods are sensitive only on a macroscopic scale, like the most widely used method of deep-level transient spectroscopy (DLTS). We perform time-resolved measurements of the resonance fluorescence of a single self-assembled (In,Ga)As quantum dot (QD) at low temperatures (4.2K). By pulsing the applied gate voltage, we are able to selectively occupy and unoccupy individual defects in the vicinity of the dot. We address the exciton transition of the QD with a tunable diode laser. Our time-resolved measurements exhibit a shift of the resonance energy of the optical transition. We attribute this to a change of the electric field in the dot's vicinity, caused by electrons tunneling from a reservoir to the defect sites. Furthermore, we are able to characterize the defects concerning their position and activation energy by modeling our experimental data. Our results thus demonstrate how a quantum dot can be used as a quantum sensor to characterize the position and activation energy of individual shallow defects on the nanoscale. © 2021 authors. Published by the American Physical Society.
view abstract 10.1103/PhysRevApplied.15.024029
- Defect spectroscopy on the dielectric material aluminum oxide
Oing, D. and Geller, M. and Stahl, L. and Kerski, J. and Lorke, A. and Wöhrl, N.
Scientific Reports 10 (2020)A method for defect characterization is presented that allows to measure the activation energy, capture cross-section, and defect density in dielectric materials. This is exemplarily performed on aluminum oxide thin films deposited on hydrogen-terminated diamond. During the measurement, samples were illuminated using a 405 nm laser, charging the defects while simultaneously measuring the surface conductivity of the diamond at different temperatures. By applying the standard boxcar evaluation known from deep-level transient spectroscopy, we found five different defect levels in Al 2O 3. One can be identified as substitutional silicon in aluminum oxide, while the others are most likely connected to either aluminum interstitials or carbon and nitrogen impurities. © 2020, The Author(s).
view abstract 10.1038/s41598-020-69240-3
- Real-Time Detection of Single Auger Recombination Events in a Self-Assembled Quantum Dot
Lochner, P. and Kurzmann, A. and Kerski, J. and Stegmann, P. and König, J. and Wieck, A.D. and Ludwig, Ar. and Lorke, A. and Geller, M.
Nano Letters 20 (2020)Auger recombination is a nonradiative process, where the recombination energy of an electron-hole pair is transferred to a third charge carrier. It is a common effect in colloidal quantum dots that quenches the radiative emission with an Auger recombination time below nanoseconds. In self-assembled QDs, the Auger recombination has been observed with a much longer recombination time on the order of microseconds. Here, we use two-color laser excitation on the exciton and trion transition in resonance fluorescence on a single self-assembled quantum dot to monitor in real-time single quantum events of the Auger process. Full counting statistics on the random telegraph signal give access to the cumulants and demonstrate the tunability of the Fano factor from a Poissonian to a sub-Poissonian distribution by Auger-mediated electron emission from the dot. Therefore, the Auger process can be used to tune optically the charge carrier occupation of the dot by the incident laser intensity, independently from the electron tunneling from the reservoir by the gate voltage. Our findings are not only highly relevant for the understanding of the Auger process but also demonstrate the perspective of the Auger effect for controlling precisely the charge state in a quantum system by optical means. © 2020 American Chemical Society.
view abstract 10.1021/acs.nanolett.9b04650
- Author Correction: Contrast of 83% in reflection measurements on a single quantum dot (Scientific Reports, (2019), 9, 1, (8817), 10.1038/s41598-019-45259-z)
Lochner, P. and Kurzmann, A. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lorke, A. and Geller, M.
Scientific Reports 9 (2019)In the original version of this Article, the authors Andreas D. Wieck and Arne Ludwig were incorrectly affiliated with ‘Solid State Physics Laboratory, ETH Zurich, 8093 Zurich, Switzerland’. The correct affiliation is listed below. Ruhr-Universität Bochum, Lehrstuhl für Angewandte Festkörperphysik, D-44780 Bochum, Germany This error has now been corrected in the PDF and HTML versions of the Article. © 2019, The Author(s).
view abstract 10.1038/s41598-019-54298-5
- Contrast of 83% in reflection measurements on a single quantum dot
Lochner, P. and Kurzmann, A. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lorke, A. and Geller, M.
Scientific Reports 9 (2019)We report on a high optical contrast between the photon emission from a single self-assembled quantum dot (QD) and the back-scattered excitation laser light. In an optimized semiconductor heterostructure with an epitaxially grown gate, an optically-matched layer structure and a distributed Bragg reflector, a record value of 83% is obtained; with tilted laser excitation even 885%. This enables measurements on a single dot without lock-in technique or suppression of the laser background by cross-polarization. These findings open up the possibility to perform simultaneously time-resolved and polarization-dependent resonant optical spectroscopy on a single quantum dot. © 2019, The Author(s).
view abstract 10.1038/s41598-019-45259-z
- Gas dependent hysteresis in MoS2 field effect transistors
Urban, F. and Giubileo, F. and Grillo, A. and Iemmo, L. and Luongo, G. and Passacantando, M. and Foller, T. and Madauß, L. and Pollmann, E. and Geller, M.P. and Oing, D. and Schleberger, M. and Di Bartolomeo, A.
2D Materials 6 (2019)We study the effect of electric stress, gas pressure and gas type on the hysteresis in the transfer characteristics of monolayer molybdenum disulfide (MoS2) field effect transistors. The presence of defects and point vacancies in the MoS2 crystal structure facilitates the adsorption of oxygen, nitrogen, hydrogen or methane, which strongly affect the transistor electrical characteristics. Although the gas adsorption does not modify the conduction type, we demonstrate a correlation between hysteresis width and adsorption energy onto the MoS2 surface. We show that hysteresis is controllable by pressure and/or gas type. Hysteresis features two well-separated current levels, especially when gases are stably adsorbed on the channel, which can be exploited in memory devices. © 2019 IOP Publishing Ltd.
view abstract 10.1088/2053-1583/ab4020
- Nonequilibrium carrier dynamics in self-assembled quantum dots
Applied Physics Reviews 6 (2019)Self-assembled quantum dots are still one of the best model systems for artificial atoms in a solid-state environment, where the electronic states can be accessed by electrical and optical means. This article focuses on nonequilibrium carrier dynamics in these quantum dots, using the ability of recent developments in electrical and optical spectroscopy techniques. All-electrical transconductance spectroscopy is introduced, where a two-dimensional electron gas serves as a fast and sensitive detector for the electron/hole dynamics and charge/spin state preparation and relaxation in an ensemble of dots. Latest results on single electron tunneling and nonequilibrium Auger recombination in a single quantum dot using a high-resolution optical experiment (the time-resolved resonance fluorescence) are summarized. This article concludes with a perspective view on a future combination of both techniques toward an electro-optical measurement toolbox to link the coherent control of quantum states by optical means with an electrical preparation of electron charge and spin states. © 2019 Author(s).
view abstract 10.1063/1.5091742
- Optical Detection of Single-Electron Tunneling into a Semiconductor Quantum Dot
Kurzmann, A. and Stegmann, P. and Kerski, J. and Schott, R. and Ludwig, Ar. and Wieck, A.D. and König, J. and Lorke, A. and Geller, M.
Physical Review Letters 122 (2019)The maximum information of a dynamic quantum system is given by real-time detection of every quantum event, where the ultimate challenge is a stable, sensitive detector with high bandwidth. All physical information can then be drawn from a statistical analysis of the time traces. We demonstrate here an optical detection scheme based on the time-resolved resonance fluorescence on a single quantum dot. Single-electron resolution with high signal-to-noise ratio (4σ confidence) and high bandwidth of 10 kHz make it possible to record the individual quantum events of the transport dynamics. Full counting statistics with factorial cumulants gives access to the nonequilibrium dynamics of spin relaxation of a singly charged dot (γ↑↓=3 ms-1), even in an equilibrium transport measurement. © 2019 American Physical Society.
view abstract 10.1103/PhysRevLett.122.247403
- Patterning of diamond with 10 nm resolution by electron-beam-induced etching
Dergianlis, V. and Geller, M. and Oing, D. and Wöhrl, N. and Lorke, A.
Nanotechnology 30 (2019)We report on mask-less, high resolution etching of diamond surfaces, featuring sizes down to 10 nm. We use a scanning electron microscope (SEM) together with water vapor, which was injected by a needle directly onto the sample surface. Using this versatile and low-damage technique, trenches with different depths were etched. Cross sections of each trench were obtained by focused ion beam milling and used to calculate the achieved aspect ratios. The developed technique opens up the possibility of mask- and resist-less patterning of diamond for nano-optical and electronic applications. © 2019 IOP Publishing Ltd.
view abstract 10.1088/1361-6528/ab25fe
- Photon Noise Suppression by a Built-in Feedback Loop
Al-Ashouri, A. and Kurzmann, A. and Merkel, B. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
Nano Letters 19 (2019)Visionary quantum photonic networks need transform-limited single photons on demand. Resonance fluorescence on a quantum dot provides the access to a solid-state single photon source, where the environment is unfortunately the source of spin and charge noise that leads to fluctuations of the emission frequency and destroys the needed indistinguishability. We demonstrate a built-in stabilization approach for the photon stream, which relies solely on charge carrier dynamics of a two-dimensional hole gas inside a micropillar structure. The hole gas is fed by hole tunneling from field-ionized excitons and influences the energetic position of the excitonic transition by changing the local electric field at the position of the quantum dot. The standard deviation of the photon noise is suppressed by nearly 50% (noise power reduction of 6 dB) and it works in the developed micropillar structure for frequencies up to 1 kHz. This built-in feedback loop represents an easy way for photon noise suppression in large arrays of single photon emitters and promises to reach higher bandwidth by device optimization. © 2018 American Chemical Society.
view abstract 10.1021/acs.nanolett.8b03486
- Tunable carrier density and high mobility of two-dimensional hole gases on diamond: The role of oxygen adsorption and surface roughness
Oing, D. and Geller, M. and Lorke, A. and Wöhrl, N.
Diamond and Related Materials 97 (2019)The transport properties of two-dimensional hole gases (2DHGs) on chemical-vapor-deposition (CVD)-grown diamond are investigated. A hydrogen plasma treatment and exposure to ambient atmosphere are used to establish and tailor the properties of the 2DHG. The transport parameters of the 2DHGs (namely carrier density and mobility) are characterized by temperature-dependent Hall measurements. The importance of the surface oxygen adsorption, determined by X-ray photoelectron spectroscopy (XPS), on the carrier density and mobility is shown. Hall measurements reveal that for oxygen concentrations below 2.2% (relative XPS signal) the carrier density is increasing from 1.4 ∙ 1010 cm−2 to 1.5 ∙ 1013 cm−2 with increasing oxygen adsorption. For oxygen concentrations above 2.2%, the charge carrier density decreases again. The carrier density remains constant over a temperature range between 4.2 K and 325 K. At room temperature, the mobility increases with decreasing carrier concentration. The opposite behavior is observed for 4.2 K. By decreasing the surface roughness to 8.2 nm, we were able to increase the mobility to above 250 cm2/V s at room temperature for a carrier density of 1.2 ∙ 1013 cm−2. This is among the highest values reported for 2DHGs on diamond. © 2019 Elsevier B.V.
view abstract 10.1016/j.diamond.2019.107450
- All-electrical measurement of the triplet-singlet spin relaxation time in self-assembled quantum dots
Eltrudis, K. and Al-Ashouri, A. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Geller, M. and Lorke, A.
Applied Physics Letters 111 (2017)We have measured the spin relaxation time of an excited two-electron spin-triplet state into its singlet ground state in self-assembled InAs/GaAs quantum dots. We use a time-resolved measurement scheme that combines transconductance spectroscopy with spin-to-charge conversion to address the |s↑,p↑〉 triplet state, where one electron is in the quantum dot s-shell and a second one in the p-shell. The evaluation of the state-selective tunneling times from the dots into a nearby two-dimensional electron gas allows us to determine the s- and p-shell occupation and extract the relaxation time from a rate equation model. A comparably long triplet-to-singlet spin relaxation time of 25 μs is found. © 2017 Author(s).
view abstract 10.1063/1.4985572
- Charge-driven feedback loop in the resonance fluorescence of a single quantum dot
Merkel, B. and Kurzmann, A. and Schulze, J.-H. and Strittmatter, A. and Geller, M. P. and Lorke, A.
Physical Review B - Condensed Matter and Materials Physics 95 (2017)We demonstrate a feedback loop that manifests itself in a strong hysteresis and bistability of the exciton resonance fluorescence signal. Field ionization of photogenerated quantum dot excitons leads to the formation of a charged interface layer that drags the emission line along over a frequency range of more than 30GHz. These measurements are well described by a rate equation model. With a time-resolved resonance fluorescence measurement we determined the buildup times for the hole gas in the orders of milliseconds. This internal charge-driven feedback loop could be used to reduce the spectral wandering in the emission spectra of single self-assembled quantum dots. © 2017 American Physical Society.
view abstract 10.1103/PhysRevB.95.115305
- Electron dynamics in transport and optical measurements of self-assembled quantum dots
Kurzmann, A. and Merkel, B. and Marquardt, B. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
Physica Status Solidi (B) Basic Research 254 (2017)The tunneling dynamics between self-assembled quantum dots (QDs) and a charge reservoir can be measured in an all-electrical or optical detection scheme. In all-electrical transconductance spectroscopy, a two-dimensional electron gas is used to probe the evolution of the many-particle states inside an ensemble of QDs from non-equilibrium to equilibrium. The optical detection scheme measures the tunneling dynamic into a single self-assembled dot. The work done and results obtained using these different measurement techniques are reviewed and compared within this article. We will show that transconductance spectroscopy is sensitive to a time-dependent density of states and enables preparation of non-equilibrium charge and spin states for future applications in quantum information processing. The optical resonance fluorescence measurements on the electron dynamics demonstrates the influence of the exciton states on the charge-carrier dynamics and enables a systematic study of the Auger recombination in self-assembled dots. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/pssb.201600625
- Giant magneto-photoelectric effect in suspended graphene
Sonntag, J. and Kurzmann, A. and Geller, M. P. and Queisser, F. and Lorke, A. and Schützhold, R.
New Journal of Physics 19 (2017)We study the optical response of a suspended, monolayer graphene field-effect transistor structure in magnetic fields of up to 9 T (quantum Hall regime). With an illumination power of only 3 μW, we measure a photocurrent of up to 400 nA (without an applied bias) corresponding to a photo-responsivity of 0.13 A W-1, which we believe to be one of the highest values ever measured in single-layer graphene. We discuss possible mechanisms for generating this strong photo-response (17 electron-hole pairs per 100 incident photons). Based on our experimental findings, we believe that the most likely scenario is a ballistic two-stage process including carrier multiplication via Auger-type inelastic Coulomb scattering at the graphene edge. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
view abstract 10.1088/1367-2630/aa739d
- Auger Recombination in Self-Assembled Quantum Dots: Quenching and Broadening of the Charged Exciton Transition
Kurzmann, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
Nano Letters 16 (2016)In quantum dots (QDs), the Auger recombination is a nonradiative process in which the electron-hole recombination energy is transferred to an additional carrier. It has been studied mostly in colloidal QDs, where the Auger recombination time is in the picosecond range and efficiently quenches the light emission. In self-assembled QDs, on the other hand, the influence of Auger recombination on the optical properties is in general neglected, assuming that it is masked by other processes such as spin and charge fluctuations. Here, we use time-resolved resonance fluorescence to analyze the Auger recombination and its influence on the optical properties of a single self-assembled QD. From excitation-power-dependent measurements, we find a long Auger recombination time of about 500 ns and a quenching of the trion transition by about 80%. Furthermore, we observe a broadening of the trion transition line width by up to a factor of 2. With a model based on rate equations, we are able to identify the interplay between tunneling and Auger rate as the underlying mechanism for the reduced intensity and the broadening of the line width. This demonstrates that self-assembled QDs can serve as an ideal model system to study how the charge recapture process, given by the band-structure surrounding the confined carriers, influences the Auger process. Our findings are not only relevant for improving the emission properties of colloidal QD-based emitters and dyes, which have recently entered the consumer market, but also of interest for more visionary applications, such as quantum information technologies, based on self-assembled quantum dots. © 2016 American Chemical Society.
view abstract 10.1021/acs.nanolett.6b01082
- Optical Blocking of Electron Tunneling into a Single Self-Assembled Quantum Dot
Kurzmann, A. and Merkel, B. and Labud, P.A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M. P.
Physical Review Letters 117 (2016)Time-resolved resonance fluorescence (RF) is used to analyze electron tunneling between a single self-assembled quantum dot (QD) and an electron reservoir. In equilibrium, the RF intensity reflects the average electron occupation of the QD and exhibits a gate voltage dependence that is given by the Fermi distribution in the reservoir. In the time-resolved signal, however, we find that the relaxation rate for electron tunneling is, surprisingly, independent of the occupation in the charge reservoir - in contrast to results from all-electrical transport measurements. Using a master equation approach, which includes both the electron tunneling and the optical excitation or recombination, we are able to explain the experimental data by optical blocking, which also reduces the electron tunneling rate when the QD is occupied by an exciton. © 2016 American Physical Society.
view abstract 10.1103/PhysRevLett.117.017401
- Photoelectron generation and capture in the resonance fluorescence of a quantum dot
Kurzmann, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
Applied Physics Letters 108 (2016)Time-resolved resonance fluorescence on a single self-assembled quantum dot (QD) is used to analyze the generation and capture of photoinduced free charge carriers. We directly observe the capture of electrons into the QD as an intensity reduction of the exciton transition. The exciton transition is quenched until the captured electron tunnels out of the dot again in the order of milliseconds. Our results demonstrate that even under resonant excitation, excited free electrons are generated and can negatively influence the optical properties of a QD. © 2016 Author(s).
view abstract 10.1063/1.4954944
- Electron-beam induced nano-etching of suspended graphene
Sommer, B. and Sonntag, J. and Ganczarczyk, A. and Braam, D. and Prinz, G. and Lorke, A. and Geller, M.
Scientific Reports 5 (2015)Besides its interesting physical properties, graphene as a two-dimensional lattice of carbon atoms promises to realize devices with exceptional electronic properties, where freely suspended graphene without contact to any substrate is the ultimate, truly two-dimensional system. The practical realization of nano-devices from suspended graphene, however, relies heavily on finding a structuring method which is minimally invasive. Here, we report on the first electron beam-induced nano-etching of suspended graphene and demonstrate high-resolution etching down to ~7 nm for line-cuts into the monolayer graphene. We investigate the structural quality of the etched graphene layer using two-dimensional (2D) Raman maps and demonstrate its high electronic quality in a nano-device: A 25 nm-wide suspended graphene nanoribbon (GNR) that shows a transport gap with a corresponding energy of ~60 meV. This is an important step towards fast and reliable patterning of suspended graphene for future ballistic transport, nano-electronic and nano-mechanical devices.
view abstract 10.1038/srep07781
- The effect of charged quantum dots on the mobility of a two-dimensional electron gas: How important is the Coulomb scattering?
Kurzmann, A. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
Journal of Applied Physics 117 (2015)We have investigated the influence of a layer of charged self-assembled quantum dots (QDs) on the mobility of a nearby two-dimensional electron gas (2DEG). Time-resolved transconductance spectroscopy was used to separate the two contributions of the change in mobility, which are: (i) The electrons in the QDs act as Coulomb scatterers for the electrons in the 2DEG. (ii) The screening ability and, hence, the mobility of the 2DEG decreases when the charge carrier density is reduced by the charged QDs, i.e., the mobility itself depends on the charge carrier concentration. Surprisingly, we find a negligible influence of the Coulomb scattering on the mobility for a 2DEG, separated by a 30nm tunneling barrier to the layer of QDs. This means that the mobility change is completely caused by depletion, i.e., reduction of the charge carrier density in the 2DEG, which indirectly influences the mobility. © 2015 AIP Publishing LLC.
view abstract 10.1063/1.4907217
- Tuning the tunneling probability between low-dimensional electron systems by momentum matching
Zhou, D. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Geller, M. and Lorke, A.
Applied Physics Letters 106 (2015)We demonstrate the possibility to tune the tunneling probability between an array of self- assembled quantum dots and a two-dimensional electron gas (2DEG) by changing the energy imbalance between the dot states and the 2DEG. Contrary to the expectation from Fowler-Nordheim tunneling, the tunneling rate decreases with increasing injection energy. This can be explained by an increasing momentum mismatch between the dot states and the Fermi-circle in the 2DEG. Our findings demonstrate momentum matching as a useful mechanism (in addition to energy conservation, density of states, and transmission probability) to electrically control the charge transfer between quantum dots and an electron reservoir. © 2015 AIP Publishing LLC.
view abstract 10.1063/1.4922738
- 3 ns single-shot read-out in a quantum dot-based memory structure
Nowozin, T. and Beckel, A. and Bimberg, D. and Lorke, A. and Geller, M.
Applied Physics Letters 104 (2014)Fast read-out of two to six charges per dot from the ground and first excited state in a quantum dot (QD)-based memory is demonstrated using a two-dimensional electron gas. Single-shot measurements on modulation-doped field-effect transistor structures with embedded InAs/GaAs QDs show read-out times as short as 3ns. At low temperature (T=4.2K) this read-out time is still limited by the parasitics of the setup and the device structure. Faster read-out times and a larger read-out signal are expected for an improved setup and device structure. © 2014 AIP Publishing LLC.
view abstract 10.1063/1.4864281
- Asymmetry of charge relaxation times in quantum dots: The influence of degeneracy
Beckel, A. and Kurzmann, A. and Geller, M. P. and Ludwig, Ar. and Wieck, A.D. and König, J. and Lorke, A.
EPL 106 (2014)Using time-resolved transconductance spectroscopy, we study the tunneling dynamics between a two-dimensional electron gas (2DEG) and self-assembled quantum dots (QDs), embedded in a field-effect transistor structure. We find that the tunneling of electrons from the 2DEG into the QDs is governed by a different time constant than the reverse process, i.e., tunneling from the QDs to the 2DEG. This asymmetry is a clear signature of Coulomb interaction and makes it possible to determine the degeneracy of the quantum-dot orbitals even when the individual states cannot be resolved energetically because of inhomogeneous broadening. Our experimental data can be qualitatively explained within a master-equation approach. © CopyrightEPLA, 2014.
view abstract 10.1209/0295-5075/106/47002
- Time-resolved transconductance spectroscopy on self-assembled quantum dots: Spectral evolution from single- into many-particle states
Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M. P.
Physical Review B - Condensed Matter and Materials Physics 89 (2014)Using transconductance spectroscopy we study the tunneling dynamics of electrons from a two-dimensional electron gas (2DEG) into excited and ground states of a layer of self-assembled InAs quantum dots (QDs). From an initially selected nonequilibrium condition, we observe the charging dynamics of the QD states and their spectral evolution for one- and two-electron configurations. Furthermore, we measure the electron emission from the QD states into the 2DEG for the corresponding evolution of the QD-hydrogen and QD-helium spectra. The comparison with theoretically predicted energies, as well as the evaluation of the dynamics in charging and emission, allows us to separate and identify ground and excited electron configurations in the spectral evolution and discuss in detail the observed maxima in the different spectra. © 2014 American Physical Society.
view abstract 10.1103/PhysRevB.89.155430
- Phase-locked indistinguishable photons with synthesized waveforms from a solid-state source
Matthiesen, C. and Geller, M. and Schulte, C.H.H. and Le Gall, C. and Hansom, J. and Li, Z. and Hugues, M. and Clarke, E. and Atatüre, M.
Nature Communications 4 (2013)Resonance fluorescence in the Heitler regime provides access to single photons with coherence well beyond the Fourier transform limit of the transition, and holds the promise to circumvent environment-induced dephasing common to all solid-state systems. Here we demonstrate that the coherently generated single photons from a single self-assembled InAs quantum dot display mutual coherence with the excitation laser on a timescale exceeding 3 s. Exploiting this degree of mutual coherence, we synthesize near-arbitrary coherent photon waveforms by shaping the excitation laser field. In contrast to post-emission filtering, our technique avoids both photon loss and degradation of the single-photon nature for all synthesized waveforms. By engineering pulsed waveforms of single photons, we further demonstrate that separate photons generated coherently by the same laser field are fundamentally indistinguishable, lending themselves to the creation of distant entanglement through quantum interference. © 2013 Macmillan Publishers Limited. All rights reserved.
view abstract 10.1038/ncomms2601
- Role of the ligand layer for photoluminescence spectral diffusion of CdSe/ZnS nanoparticles
Braam, D. and Mölleken, A. and Prinz, G.M. and Notthoff, C. and Geller, M. P. and Lorke, A.
Physical Review B - Condensed Matter and Materials Physics 88 (2013)The time-resolved photoluminescence (PL) characteristics of single CdSe/ZnS nanoparticles, embedded in a poly(methyl methacrylate) layer, is studied at room temperature. We observe a strong spectral jitter of up to 55 meV, which is correlated with a change in the observed linewidth. We evaluate this correlation effect using a simple model, based on the quantum confined Stark effect induced by a diffusing charge in the vicinity of the nanoparticle. This allows us to derive a mean distance between the center of the particle and the diffusing charge of approximately 5 nm on average, as well as a mean charge carrier displacement within the integration time. These results are reproducible, even for particles which exhibit strong blueing, with shifts of up to 150 meV. Both the statistics and its independence of core-shell alterations lead us to conclude that the charge causing the spectral jitter is situated in the ligands or on its surface. © 2013 American Physical Society.
view abstract 10.1103/PhysRevB.88.125302
- Spatial high resolution energy dispersive X-ray spectroscopy on thin lamellas
Notthoff, C. and Winterer, M. and Beckel, A. and Geller, M. and Heindl, J.
Ultramicroscopy 129 (2013)For conventional samples and measurement geometries the spatial resolution of energy dispersive X-ray spectroscopy is limited by a tear drop shaped emission volume to about 1 μm. This restriction can be substantially improved using thin samples and high acceleration voltage. In this contribution the spatial resolution of energy dispersive X-ray spectroscopy in a scanning electron microscope using thin lamella samples is investigated. At an acceleration voltage of 30kV, an edge resolution down to Δd edge = 40 ± 10 nm is observed performing linescans across an interface, using an 80nm thin sample prepared from a GaAs/AlAs-heterostructure. Furthermore, Monte-Carlo simulations of pure elements ranging from sodium to mercury are performed for different sample thicknesses. From the simulations we can derive a simple empirical formula to predict the spatial resolution as a function of sample thickness. © 2013 Elsevier B.V.
view abstract 10.1016/j.ultramic.2013.02.008
- Momentum matching in the tunneling between 2-dimensional and 0-dimensional electron systems
Beckel, A. and Zhou, D. and Marquardt, B. and Reuter, D. and Wieck, A.D. and Geller, M. and Lorke, A.
Applied Physics Letters 100 (2012)We investigate the tunneling rates from a 2-dimensional electron gas (2DEG) into the ground state of self-assembled InGaAs quantum dots. These rates are strongly affected by a magnetic field perpendicular to the tunneling direction. Surprisingly, we find an increase in the rates for fields up to 4 T before they decrease again. This can be explained by a mismatch between the characteristic momentum of the quantum dot ground state and the Fermi momentum k F of the 2DEG. Calculations of the tunneling probability can account for the experimental data and allow us to determine the dot geometry as well as k F. © 2012 American Institute of Physics.
view abstract 10.1063/1.4728114
- N-GaAs/InGaP/p-GaAs core-multishell nanowire diodes for efficient light-to-current conversion
Gutsche, C. and Lysov, A. and Braam, D. and Regolin, I. and Keller, G. and Li, Z.-A. and Geller, M. and Spasova, M. and Prost, W. and Tegude, F.-J.
Advanced Functional Materials 22 (2012)Heterostructure n-GaAs/InGaP/p-GaAs core-multishell nanowire diodes are synthesized by metal-organic vapor-phase epitaxy. This structure allows a reproducible, selective wet etching of the individual shells and therefore a simplified contacting of single nanowire p-i-n junctions. Nanowire diodes show leakage currents in a low pA range and at a high rectification ratio of 3500 (at ±1V). Pronounced electroluminescence at 1.4 eV is measured at room temperature and gives evidence of the device quality. Photocurrent generation is demonstrated at the complete area of the nanowire p-i-n junction by scanning photocurrent microscopy. A solar-conversion efficiency of 4.7%, an open-circuit voltage of 0.5 V and a fill factor of 52% are obtained under AM 1.5G conditions. These results will guide the development of nanowire-based photonic and photovoltaic devices. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/adfm.201101759
- Time-resolved detection of many-particle hole states in InAs/GaAs quantum dots using a two-dimensional hole gas up to 77 K
Nowozin, T. and Marent, A. and Bimberg, D. and Beckel, A. and Marquardt, B. and Lorke, A. and Geller, M.
Physica Status Solidi (C) Current Topics in Solid State Physics 9 (2012)We demonstrate the detection of many-particle hole states in self-organized InAs/GaAs quantum dots (QDs) using an adjacent two-dimensional hole gas (2DHG) as detector for temperatures up to 77 K. capacitance-voltage (C-V) measurements as well as time-resolved current measurements in the 2DHG resolve a structure of six distinct peaks which are related to the many-particle hole states in the QD ensemble. The time constants of the capture and emission processes for each individual many-particle hole state are extracted and the underlying emission processes are identified. An equivalent circuit model yields the gate voltage depedent lever-arm and the level-splittings of the many-particle hole states. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/pssc.201100218
- Transverse rectification in density-modulated two-dimensional electron gases
Ganczarczyk, A. and Rojek, S. and Quindeau, A. and Geller, M. P. and Hucht, A. and Notthoff, C. and König, J. and Lorke, A. and Reuter, D. and Wieck, A.D.
Physical Review B - Condensed Matter and Materials Physics 86 (2012)We demonstrate tunable transverse rectification in a density-modulated two-dimensional electron gas (2DEG). The density modulation is induced by two surface gates, running in parallel along a narrow stripe of 2DEG. A transverse voltage in the direction of the density modulation is observed, i.e., perpendicular to the applied source-drain voltage. The polarity of the transverse voltage is independent of the polarity of the source-drain voltage, demonstrating rectification in the device. We find that the transverse voltage U y depends quadratically on the applied source-drain voltage and nonmonotonically on the density modulation. The experimental results are discussed in the framework of a diffusion thermopower model. © 2012 American Physical Society.
view abstract 10.1103/PhysRevB.86.085309
- All-electrical transport spectroscopy of non-equilibrium many-particle states in self-assembled quantum dots
Marquardt, B. and Geller, M. and Baxevanis, B. and Pfannkuche, D. and Wieck, A.D. and Reuter, D. and Lorke, A.
AIP Conference Proceedings 1399 (2011)We demonstrate an all-electrical prepartion and detection of many-particle spin states in self-assembled QDs. The excited states of the first three 'QD elements' are measured, i. e. the degenerate s-, p- and d-shells of QD-Hydrogen (one electron), the many-particle triplet and singlet states of QD-Helium (two electrons) and the many-particle states of QD-Lithium (three electrons). Comparison with exact diagonalization calculations fully accounting for the effects of Coulomb interaction enables us to reveal unambiguously the different charge and spin configurations. © 2011 American Institute of Physics.
view abstract 10.1063/1.3666398
- Mobility and carrier density in nanoporous indium tin oxide films
Gondorf, A. and Geller, M. P. and Weißbon, J. and Lorke, A. and Inhester, M. and Prodi-Schwab, A. and Adam, D.
Physical Review B - Condensed Matter and Materials Physics 83 (2011)The charge-carrier concentration and mobility of various nanoporous indium tin oxide films have been studied by Hall measurements and optical spectroscopy. The carrier density obtained by Hall measurements is found to be significantly lower than that derived from optical spectroscopy. The results of these complementary measurement techniques thus support recent theoretical work on granular conducting layers by Kharitonov and Efetov, which predicts that the Hall carrier concentration n of such a system is reduced by a geometrical factor. As expected, the mobility inside the particles, derived from optical transmission spectra (22 - 35 cm2Vs), is much higher than the macroscopic Hall mobility (0.12-0.7cm2Vs). © 2011 American Physical Society.
view abstract 10.1103/PhysRevB.83.212201
- Optical properties of heavily doped GaAs nanowires and electroluminescent nanowire structures
Lysov, A. and Offer, M. and Gutsche, C. and Regolin, I. and Topaloglu, S. and Geller, M. P. and Prost, W. and Tegude, F.-J.
Nanotechnology 22 (2011)We present GaAs electroluminescent nanowire structures fabricated by metal organic vapor phase epitaxy. Electroluminescent structures were realized in both axial pn-junctions in single GaAs nanowires and free-standing nanowire arrays with a pn-junction formed between nanowires and substrate, respectively. The electroluminescence emission peak from single nanowire pn-junctions at 10 K was registered at an energy of around 1.32 eV and shifted to 1.4 eV with an increasing current. The line is attributed to the recombination in the compensated region present in the nanowire due to the memory effect of the vapor-liquid-solid growth mechanism. Arrayed nanowire electroluminescent structures with a pn-junction formed between nanowires and substrate demonstrated at 5 K a strong electroluminescence peak at 1.488 eV and two shoulder peaks at 1.455 and 1.519 eV. The main emission line was attributed to the recombination in the p-doped GaAs. The other two lines correspond to the tunneling-assisted photon emission and band-edge recombination in the abrupt junction, respectively. Electroluminescence spectra are compared with the micro-photoluminescence spectra taken along the single p-, n-and single nanowire pn-junctions to find the origin of the electroluminescence peaks, the distribution of doping species and the sharpness of the junctions. © 2011 IOP Publishing Ltd.
view abstract 10.1088/0957-4484/22/8/085702
- Role of oxygen on microstructure and thermoelectric properties of silicon nanocomposites
Schierning, G. and Theissmann, R. and Stein, N. and Petermann, N. and Becker, A. and Engenhorst, M. and Kessler, V. and Geller, M. and Beckel, A. and Wiggers, H. and Schmechel, R.
Journal of Applied Physics 110 (2011)Phosphorus-doped silicon nanopowder from a gas phase process was compacted by DC-current sintering in order to obtain thermoelectrically active, nanocrystalline bulk silicon. A density between 95 and 96 compared to the density of single crystalline silicon was achieved, while preserving the nanocrystalline character with an average crystallite size of best 25 nm. As a native surface oxidation of the nanopowder usually occurs during nanopowder handling, a focus of this work is on the role of oxygen on microstructure and transport properties of the nanocomposite. A characterization with transmission electron microscopy (TEM) showed that the original core/shell structure of the nanoparticles was not found within the sintered nanocomposites. Two different types of oxide precipitates could be identified by energy filtered imaging technique. For a detailed analysis, 3-dimensional tomography with reconstruction was done using a needle-shaped sample prepared by focused ion beam (FIB). The 3-dimensional distribution of silicon dioxide precipitates confirmed that the initial core/shell structure breaks down and precipitates are formed. It is further found that residual pores are exclusively located within oxide precipitates. Thermoelectric characterization was done on silicon nanocomposites sintered between 960 C and 1060 C with varying oxygen content between room temperature and 950 C. The higher sintering temperature led to a better electrical activation of the phosphorus dopant. The oxidic precipitates support densification and seem to be able to reduce the thermal conductivity therefore enhancing thermoelectric properties. A peak figure of merit, zT, of 0.5 at 950 C was measured for a sample sintered at 1060 C with a mean crystallite size of 46 nm. © 2011 American Institute of Physics.
view abstract 10.1063/1.3658021
- Spatially resolved photoelectric performance of axial GaAs nanowire pn-diodes
Lysov, A. and Vinaji, S. and Offer, M. and Gutsche, C. and Regolin, I. and Mertin, W. and Geller, M. and Prost, W. and Bacher, G. and Tegude, F.-J.
Nano Research 4 (2011)The spatially resolved photoelectric response of a single axial GaAs nanowire pn-diode has been investigated with scanning photocurrent and Kelvin probe force microscopy. Optical generation of carriers at the pn-junction has been shown to dominate the photoresponse. A photocurrent of 88 pA, an open circuit voltage of 0.56 V and a fill factor of 69% were obtained under AM 1.5 G conditions. The photocurrent followed the increasing photoexcitation with 0.24 A/W up to an illumination density of at least 90 W/cm2, which is important for potential applications in concentrator solar cells. © 2011 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
view abstract 10.1007/s12274-011-0155-4
- Synthesis and ink-jet printing of highly luminescing silicon nanoparticles for printable electronics
Gupta, A. and Khalil, A.S.G. and Offer, M. and Geller, M. P. and Winterer, M. and Lorke, A. and Wiggers, H.
Journal of Nanoscience and Nanotechnology 11 (2011)The formation of stable colloidal dispersions of silicon nanoparticles (Si-NPs) is essential for the manufacturing of silicon based electronic and optoelectronic devices using cost-effective printing technologies. However, the development of Si-NPs based printable electronics has so far been hampered by the lack of long-term stability, low production rate and poor optical properties of Si- NPs ink. In this paper, we synthesized Si-NPs in a gas phase microwave plasma reactor with very high production rate, which were later treated to form a stable colloidal dispersion. These particles can be readily dispersed in a variety of organic solvents and the dispersion is stable for months. The particles show excellent optical properties (quantum yields of about 15%) and long-term photoluminescence (PL) stability. The stable ink containing functionalized Si-NPs was successfully used to print structures on glass substrates by ink-jet printing. The homogeneity and uniformity of large-area printed film was investigated using photoluminescence (PL) mapping. Copyright © 2011 American Scientific Publishers.
view abstract 10.1166/jnn.2011.4184
- The influence of charged InAs quantum dots on the conductance of a two-dimensional electron gas: Mobility vs. carrier concentration
Marquardt, B. and Beckel, A. and Lorke, A. and Wieck, A.D. and Reuter, D. and Geller, M.
Applied Physics Letters 99 (2011)Using time-resolved transport spectroscopy, we investigate the influence of charge-tunable InAs quantum dots (QDs) on the conductance of a nearby two-dimensional electron gas (2DEG). Loading successively electrons into the self-assembled QDs decreases the carrier concentration and mobility in the 2DEG. We are able to quantify how these transport properties change for each additional charge in the s- or p-shell. It is found that mobility and carrier concentration contribute equally to the overall change in conductance. © 2011 American Institute of Physics.
view abstract 10.1063/1.3665070
- The QD-flash: A quantum dot-based memory device
Marent, A. and Nowozin, T. and Geller, M. and Bimberg, D.
Semiconductor Science and Technology 26 (2011)We demonstrate the large potential of III-V compound semiconductors for a novel type of Flash memory. The concept is based on self-organized III-V quantum dots (QDs). Here the advantages of the most important semiconductor memories, the dynamic random access memory and the Flash are merged. A non-volatile memory with fast access times (>10 ns) and good endurance (>1015 write/erase cycles) as an ultimate solution seems possible. A storage time of 1.6 s at 300 K in InAs/GaAs QDs with an additional Al0.9Ga 0.1As barrier was demonstrated and a retention time of 106 years is predicted by us for GaSb QDs in an AlAs matrix. In addition, a minimum write time of 6 ns is obtained for InAs/GaAs QDs. First prototypes with all-electrical data access prove the feasibility of the concept. The stored information is read-out by a two-dimensional hole gas underneath the QD layer. Time-resolved drain-current measurements demonstrate the memory operations. © 2011 IOP Publishing Ltd.
view abstract 10.1088/0268-1242/26/1/014026
- Time-resolved high-temperature detection with single charge resolution of holes tunneling into many-particle quantum dot states
Nowozin, T. and Marent, A. and Hönig, G. and Schliwa, A. and Bimberg, D. and Beckel, A. and Marquardt, B. and Lorke, A. and Geller, M. P.
Physical Review B - Condensed Matter and Materials Physics 84 (2011)We demonstrate detection of many-particle hole states in InAs/GaAs quantum dots with single charge resolution up to a temperature of 75 K. Capacitance-voltage measurements as well as time-resolved current measurements in an adjacent two-dimensional hole gas are used to determine the emission and capture time constants from 4 K up to 130 K. A transition from pure tunneling to thermally assisted tunneling is observed with increasing temperature. An equivalent circuit model gives access to the energy level splittings of the many-particle hole states and explains the broadening of the peaks at higher temperatures. © 2011 American Physical Society.
view abstract 10.1103/PhysRevB.84.075309
- Transport spectroscopy of non-equilibrium many-particle spin states in self-assembled quantum dots
Marquardt, B. and Geller, M. and Baxevanis, B. and Pfannkuche, D. and Wieck, A.D. and Reuter, D. and Lorke, A.
Nature Communications 2 (2011)Self-assembled quantum dots (QDs) are prominent candidates for solid-state quantum information processing. For these systems, great progress has been made in addressing spin states by optical means. In this study, we introduce an all-electrical measurement technique to prepare and detect non-equilibrium many-particle spin states in an ensemble of self-assembled QDs at liquid helium temperature. The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method. An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum. These experiments are a starting point for an all-electrical control of electron spin states in self-assembled QDs above liquid helium temperature. © 2011 Macmillan Publishers Limited. All rights reserved.
view abstract 10.1038/ncomms1205
- XeF2 gas-assisted focused-electron-beaminduced etching of GaAs with 30 nm resolution
Ganczarczyk, A. and Geller, M. P. and Lorke, A.
Nanotechnology 22 (2011)We demonstrate the gas-assisted focused-electron-beam (FEB)-induced etching of GaAs with a resolution of 30 nm at room temperature. We use a scanning electron microscope (SEM) in a dual beam focused ion beam together with xenon difluoride (XeF2) that can be injected by a needle directly onto the sample surface. We show that the FEB-induced etching with XeF2 as a precursor gas results in isotropic and smooth etching of GaAs, while the etch rate depends strongly on the beam current and the electron energy. The natural oxide of GaAs at the sample surface inhibits the etching process; hence, oxide removal in combination with chemical surface passivation is necessary as a strategy to enable this high-resolution etching alternative for GaAs. © 2011 IOP Publishing Ltd.
view abstract 10.1088/0957-4484/22/4/045301
- A 2D electron gas for studies on tunneling dynamics and charge storage in self-assembled quantum dots
Marquardt, B. and Moujib, H. and Lorke, A. and Reuter, D. and Wieck, A.D. and Geller, M.
Lecture Notes of the Institute for Computer Sciences, Social-Informatics and Telecommunications Engineering 36 LNICST (2010)The carrier tunneling dynamics of self-assembled InAs quantum dots (QD) is studied using time-resolved conductance measurements of a nearby two-dimensional electron gas (2DEG). The coupling strength (tunneling time) between the QDs and the 2DEG is adjusted by different thicknesses of the spacer layers. We demonstrate a strong influence of charged QDs on the conductance on the 2DEG, even for very weak coupling, where standard C-V spectroscopy is unsuitable to investigate the electronic structure of these QDs. © Institute for Computer Sciences, Social-Informatics and Telecommunications Engineering 2010.
view abstract 10.1007/978-3-642-11731-2_22
- A two-dimensional electron gas as a sensitive detector for time-resolved tunneling measurements on self-assembled quantum dots
Geller, M. and Marquardt, B. and Lorke, A. and Reuter, D. and Wieck, A.D.
Nanoscale Research Letters 5 (2010)A two-dimensional electron gas (2DEG) situated nearby a single layer of self-assembled quantum dots (QDs) in an inverted high electron mobility transistor (HEMT) structure is used as a detector for time-resolved tunneling measurements. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG which allows us to probe the tunneling dynamics between the 2DEG and the QDs time resolved. Measurements of hysteresis curves with different sweep times and real-time conductance measurements in combination with an boxcar-like evaluation method enables us to unambiguously identify the transients as tunneling events between the s- and p-electron QD states and the 2DEG and rule out defect-related transients. © 2010 The Author(s).
view abstract 10.1007/s11671-010-9569-2
- A two-dimensional electron gas as a sensitive detector to observe the charge carrier dynamics of self-assembled QDs
Marquardt, B. and Geller, M. and Lorke, A. and Reuter, D. and Wieck, A.D.
Physica E: Low-Dimensional Systems and Nanostructures 42 (2010)The carrier tunneling dynamics of self-assembled InAs quantum dots (QD) is studied using a time-resolved conductance measurement of a nearby two-dimensional electron gas (2DEG). The investigated heterostructures consist of a layer of QDs with different coupling strengths to a 2DEG, adjusted by different thicknesses of the spacer layers. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG, even for very weak coupling between the QD layer and the 2D system, where standard capacitance (C)voltage (V) spectroscopy is unsuitable to investigate the electronic structure of these QDs. © 2009 Elsevier B.V. All rights reserved.
view abstract 10.1016/j.physe.2010.02.010
- A voltage-tunable in-plane diode in a two-dimensional-electron system
Ganczarczyk, A. and Voßen, S. and Geller, M. and Lorke, A. and Reuter, D. and Wieck, A.D.
Physica E: Low-Dimensional Systems and Nanostructures 42 (2010)We present a voltage-tunable in-plane diode in a nanoscale, two-dimensional electron system, whose functionality is mainly determined by the sample geometry. The diode consists of a narrow semiconductor channel, confined by etched insulating trenches. An applied voltage along the channel modulates the effective width of the conducting channel, depending on the sign of the applied voltage. This behavior results in a diode-like IV -characteristic. The tunability of the device is achieved by two in-plane side gates, which are able to widely tune the I (V) -characteristic of this rectifier. In the normally-off regime, this tunable in-plane diode works as a half-wave rectifier with sharply defined turn-on voltage. The value of the turn-on voltage depends on the side-gate voltage. © 2009 Elsevier B.V. All rights reserved.
view abstract 10.1016/j.physe.2009.10.018
- Electroluminescence from silicon nanoparticles fabricated from the gas phase
Theis, J. and Geller, M. and Lorke, A. and Wiggers, H. and Wieck, A. and Meier, C.
Nanotechnology 21 (2010)Electroluminescence from as-prepared silicon nanoparticles, fabricated by gas phase synthesis, is demonstrated. The particles are embedded between an n-doped GaAs substrate and a semitransparent indium tin oxide top electrode. The total electroluminescence intensity of the Si nanoparticles is more than a factor of three higher than the corresponding signal from the epitaxial III-V semiconductor. This, together with the low threshold voltage for electroluminescence, shows the good optical properties of these untreated particles and the efficient electrical injection into the device. Impact ionization by electrons emitted from the top electrode is identified as the origin of the electrically driven light emission. © 2010 IOP Publishing Ltd.
view abstract 10.1088/0957-4484/21/45/455201
electron beam lithography
focused ion beam