Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H9: Optoelectronic Properties of Quantum Dots 
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Sponsoring Units: DCMP Chair: Michael Scheibner, University of California, Merced Room: A105 
Tuesday, March 16, 2010 8:00AM  8:12AM 
H9.00001: 2DEG depthdependent leakage currents in GaAs/AlGaAs heterostructures Bernard Matis, Dong Ho Wu A quantumdot device needs to be operated at low enough temperatures so that the charging energy of the device exceeds the thermal fluctuation energy. As the charging energy is known to be inversely proportional to the device size, one might be able to elevate the operating temperature of a quantumdot device by reducing the device size. In an attempt to measure the operating temperature as a function of device size, we fabricated quantumdot devices in different sizes ranging from 100 nm to 1 $\mu $m. These devices were fabricated on GaAs/AlGaAs heterostructure substrates with 2DEG depth of 40 nm to 160 nm. We used a shallower 2DEG for a quantumdot device with a smaller lateral dimension. For the shallower 2DEG, we observed larger leakage currents between the top layer and the 2DEG layer. To prevent these leakage currents that disrupt the quantum device, we had to limit the miniaturization of our device to a certain size. In this talk we will discuss this limitation and the functional dependence between the 2DEG depth and the leakagecurrent strength. [Preview Abstract] 
Tuesday, March 16, 2010 8:12AM  8:24AM 
H9.00002: ZeroBias Anomaly in Quantum Point Contacts Yuan Ren, WingWa Yu, Seyed Hadi Ebrahimnejad, Sergey Frolov, Joshua Folk, Werner Wegscheider Quantum point contacts (QPCs) are narrow constrictions between large reservoirs of twodimensional electron gas, with conductance quantized in units of $G = 2e^2/h$ at zero magnetic field. Despite decades of investigation, some conductance features of QPCs remain mysterious, one of which is known as the zerobias anomaly (ZBA)  a conductance peak centered at $V_{\mathrm{sd}}=0$ when the sourcedrain voltage $V_{\mathrm{sd}}$ is swept. Most previous work has focused on ZBAs around the 0.7 structure ($2e^2/h\ga G\ga e^2/h$). Here, we report measurements of the ZBA over a wide range of conductance, from the lowconductance limit ($G\sim10^{4} e^2/ h$) to well above the first plateau ($G\sim46 e^2/h$). The qualitatively different dependences on inplane magnetic field that are observed for ZBAs in different conductance regimes provide insight into the various physical mechanisms responsible for this feature. [Preview Abstract] 
Tuesday, March 16, 2010 8:24AM  8:36AM 
H9.00003: Heisenberg Backaction in Quantum Point Contact Qubit Detectors Carolyn Young, Aashish Clerk Quantum point contacts (QPCs) are widely used for the readout of quantum electronic systems. Surprisingly, it has not been fully appreciated that charge noise fluctuations in the QPC are the source of the fundamental Heisenberg backaction associated with QPC measurement. In this talk, we derive a rigorous quantum limit on the magnitude of the QPC charge noise [1]. Without specifying the precise geometry of the detector or its coupling to the device being measured, we obtain a lower bound that depends exclusively on intrinsic QPC properties that are directly measurable in experiment. A random phase approximation treatment of charge screening is also presented which includes a description of spatial variations in the QPC charge and density of states. Finally, we discuss the implications of our results for QPCinduced dephasing in twoelectron spin qubits [2]. \\[4pt] [1] C.E. Young and A.A. Clerk, arXiv:0910.4942v1. \\[0pt] [2] J.R. Petta et. al., Science {\bf 309}, 2180 (2005). [Preview Abstract] 
Tuesday, March 16, 2010 8:36AM  8:48AM 
H9.00004: Tunable Conductance Anomaly in Asymmetrically Gated Quantum Point Contacts Phillip Wu, Peng Li, Albert Chang We have observed conductance anomalies in asymmetrically gated quantum point contacts formed in AlGaAs/GaAs two dimensional electron gas. The point contacts can be set in a regime with conductance anomaly that can be tuned continuously from $0.3*2e^2/h$ to $0.7*2e^2/h$, as well as in a regime without any anomalous features below $2e^2/h$. Differential conductance versus source drain bias ($dI/dV_{sd}$) in either regimes show strikingly contrasting behavior. [Preview Abstract] 
Tuesday, March 16, 2010 8:48AM  9:00AM 
H9.00005: Analysis of electron tunneling events with the hidden Markov model Matthew House, Hong Wen Jiang The charge fluctuations of a quantum dot in a semiconductor heterostructure can be observed using a charge sensor which observes electrons transiting on and off of the quantum dot in real time. As electrons tunnel onto and off of the quantum dot, the conductance of the charge sensor shows stochastic transitions between two levels in a pattern that is commonly referred to as a random telegraph signal. Information about the quantized states of electrons on the quantum dot can be inferred from measurements of the charge sensor conductance. We have applied the hidden Markov model (HMM) formalism to the problem of extracting information about quantum dot states and transition rates from charge sensor data. HMM theory provides a mathematical approach for inferring the details of a stochastic process from indirect observations. We discuss how this applies to the analysis of charge sensor data in quantum dot experiments. We apply HMMs to simulated and experimental data, and demonstrate its usefulness in extracting the electron transition rates. Data analysis by HMM is much more robust against noise than previous approaches, and has the potential to infer whether tunneling events correspond to the ground state or to excited states of the quantum dot, information which is not directly observable by the charge sensor. [Preview Abstract] 
Tuesday, March 16, 2010 9:00AM  9:12AM 
H9.00006: Coherent excitonic resonances of natural quantum dots studied with optical 2D Fourier transform spectroscopy Mark Siemens, Galan Moody, Alan Bristow, Xingcan Dai, Denis Karaiskaj, Steven Cundiff, Allan Bracker, Daniel Gammon Electronic structure and dynamics can be captured by optical 2DFouriertransform spectroscopy (2DFTS), which tracks the phase of the nonlinear signal during two time delays of a multipulse excitation sequence. We use optical 2DFTS [1] to study the coherent response of an ensemble of interfacial ``natural'' GaAs quantum dots (QD) [2], found within the monolayer fluctuations of a quantum well (QW). Low temperature ($\sim $6K) spectra show excitonic resonances from both the QD and the QW. We simultaneously extract homogenous and inhomogeneous linewidths of both QW and QD states, indicating slow dephasing and size distribution in the QDs. Additionally, variation of the population time delay and lattice temperature reveals a coupling from the QW states to the lower energy QD mediated by incoherent phonon interactions. \newline [1] S. T. Cundiff, T. Zhang, A. D. Bristow, D. Karaiskaj, X. Dai, Acc. Chem. Res. 42, 1423 (2009). \newline [2] D. Gammon, E.S. Snow, B.V. Shanabrook, D.S. Katzer, and D. Park, PRL 76, 3005 (1996). [Preview Abstract] 
Tuesday, March 16, 2010 9:12AM  9:24AM 
H9.00007: Quantum confined Stark effect in direct excitonic states in single InAs/GaAs quantum molecules Swati Ramanathan, Mauricio Garrido, Kushal C. Wijesundara, Eric Stinaff, Allan Bracker, Dan Gammon We present a study of the quantum confined Stark effect (QCSE) for direct excitonic states in individual vertically coupled selfassembled 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 shift is found to decrease and eventually change sign. We present data on this effect, and attribute this shift to a change in the permanent dipole moment of the dots. In addition, we study this effect as a function of charge state of the quantum dot, to see how the additional charge affects the dipole moment, and therefore the slope of the charged excitonic state. [Preview Abstract] 
Tuesday, March 16, 2010 9:24AM  9:36AM 
H9.00008: InAs Lateral Quantum Dot Molecules With Controllable Configurations Michael Yakes, Allan Bracker, Cory Cress, Joe Tischler, Danny Kim, Alex Greilich, Dan Gammon Well controlled, verticallystacked quantum dot molecules (QDMs) are now routinely grown for optical investigations. For device applications, laterally coupled dots offer compatibility with existing gate technologies and advantages in scalability. One promising technique for creating laterally coupled QDMs is to use gallium droplet epitaxy to form homoepitaxial mounds which serve as a template for further dot growth. In this presentation, we will describe new growth techniques that can be used to control the configuration of lateral QDMs. In addition, by capping first layer QDMs and growing additional straincoupled dots, we demonstrate flexible and uniform three dimensional QDM configurations. We use AFM and cross sectional STM to study the morphology of asgrown and capped QDMs. These results demonstrate that these QDMs are excellent candidates for investigations of electron tunneling using photoluminescence (PL) spectroscopy. We will present ensemble PL spectra and discuss techniques for extending this method to create QDMs that can be measured using single molecule PL spectroscopy. [Preview Abstract] 
Tuesday, March 16, 2010 9:36AM  9:48AM 
H9.00009: Direct and indirect exciton lifetimes in InAs/GaAs coupled quantum dots Kushal C. Wijesundara, Mauricio Garrido, Swati Ramanathan, Eric Stinaff, Allan Bracker, Dan Gammon 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. Timeresolved photoluminescence was used to study the temporal dynamics of excitons in InAs/GaAs CQDs where the carrier distribution was controlled with an applied electric field along the molecular axis. We observed an overall increase in the exciton lifetimes with applied electric field. Indirect excitons demonstrated lifetime increase of more than a factor of 3 relative to the lifetimes of spatially direct excitons. This is attributed to the reduction in wave function overlap between electron and hole with applied electric field. The dependence of these lifetimes on barrier thickness along with timeresolved polarization measurements gives additional insight into the dynamics of these molecular states. [Preview Abstract] 
Tuesday, March 16, 2010 9:48AM  10:00AM 
H9.00010: Carrier dynamics in InGaAs/GaAs quantum dots excited by degenerate pumpprobe technique K.N. Chauhan, D.M. Riffe We have studied ultrafast carrier dynamics in a single layer of selfassembled In$_{0.4}$Ga$_{0.6}$As/GaAs quantum dots (QDs) using femtosecond degenerate pumpprobe differential reflectivity. The measurements were done with an 800 nm, 28 fs Tisapphire oscillator. The growth process of QDs consists of two steps, low temperature growth and high temperature annealing. Specifically, the InGaAs QD structures are fabricated on ntype GaAs(001) using molecular beam epitaxy (MBE). The In$_{0.4}$Ga$_{0.6}$As layer is deposited at 390400 $^{\circ}$C followed by QDs self assembly at 450540 $^{\circ}$C. Finally, these QDs are caped with a 10 nm or 100 nm layer of GaAs. Measured width and height of these QDs are typically 33 nm and 6 nm respectively. Dots annealed at higher temperature have larger base area (width and length) and reduced height as compared to those annealed at lower temperature. We have developed a rate equation model to describe the carrier dynamics and fit the reflectivity data. Dynamics depends on the size of the quantum dots: larger QDs have faster dynamics as compared to smaller dots. Additionally, dynamics are slower at higher excitation levels. [Preview Abstract] 
Tuesday, March 16, 2010 10:00AM  10:12AM 
H9.00011: PhononInduced Dephasing of Excitons in Semiconductor Quantum Dots: Multiple Exciton Generation, Fission, and Luminescence Angeline Madrid, HyeonDeuk Kim, Bradley Habenicht, Oleg Prezhdo Phononinduced dephasing processes that govern optical line widths, multiple exciton (ME) generation (MEG), and ME fission (MEF) in semiconductor quantum dots (QDs) are investigated by ab initio molecular dynamics simulation. Using Si QDs as an example, we propose that MEF occurs by phononinduced dephasing and, for the first time, estimate its time scale to be 100 fs. In contrast, luminescence and MEG dephasing times are all sub10 fs. Generally, dephasing is faster for higherenergy and higherorder excitons and increased temperatures. MEF is slow because it is facilitated only by lowfrequency acoustic modes. Luminescence and MEG couple to both acoustic and optical modes of the QD, as well as ligand vibrations. The detailed atomistic simulation of the dephasing processes advances understanding of exciton dynamics in QDs and other nanoscale materials. [Preview Abstract] 

H9.00012: ABSTRACT WITHDRAWN 
Tuesday, March 16, 2010 10:24AM  10:36AM 
H9.00013: Disorderinduced linear magnetoresistance in composite films of selfassembled MnAs nanoparticles in GaAs Don Heiman, Hannah Johnson, Steve Bennett, Radhika Barua, Laura Lewis The discovery of novel effects arising from the interplay of charge carriers with structural disorder is expected to contribute to future device applications. Strong electrical disorder in semiconductors provided by compositional inhomogenities has led to anomalously large magnetoresistance (MR) which increases linearly with increasing magnetic field and does not appear to saturate even at high fields.[1] In our study, we find that composite films containing selfassembled MnAs nanoparticles embedded in GaAs exhibit nonsaturating positive MR which is linear in magnetic fields up to H=14 T. A MR of 900{\%} is observed at 25 K. The magnitude of the linear MR is found to be proportional to the carrier mobility over the measured temperature range, T=25 to 300 K. Furthermore, the crossover field, where the MR dependence on field crosses over from quadratic to linear behavior is found to be inversely proportional to carrier mobility. These proportionalities are obeyed over 4 orders of magnitude of carrier concentration. This remarkable relationship linking the MR to the average macroscopic carrier mobility is explained by a model of random disorder in the nanoscale mobility. Work supported by NSF grant DMR097007.\\[4pt] [1] R. Xu, et al., Nature 390, 57 (1997). [Preview Abstract] 
Tuesday, March 16, 2010 10:36AM  10:48AM 
H9.00014: Carrier thermal escape in families of InAs/InP selfassembled quantum dots Guillaume Gelinas, Ali Lanacer, Richard Leonelli, Remo A. Masut, Philip J. Poole The temperature evolution of the photoluminescence spectra of singlelayer InAs/InP (001) selfassembled quantum dots samples was measured from 10 K to 300 K. To understand the thermal quenching of their multimodal emission, we developed a coupled rateequation model that includes the effect of carrier thermal escape from a quantum dot to the wetting layer and to the InP matrix, recapture and/or nonradiative recombination. Our model reproduces the temperature dependence with a single set of parameters. We find that the main escape mechanism of the carriers in the quantum dots is through thermal emission to the wetting layer. The activation energy for this process is found to be close to onehalf the energy difference between wetting layer and a quantum dot family, as measured by photoluminescence excitation experiments. This indicates that electrons and holes exit the InAs quantum dots as correlated pairs. [Preview Abstract] 
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