Session P35: Nanostructure Fabrication, Quantum Point Contacts, and Single Electron Transistors

Atomic-Scale Modeling of Shape Stability-Regimes and Stacking in InAs/GaAs Quantum Dot Nanostructures

From a thermodynamic point of view, quantum dot (QD) growth is governed by the balance between energy gain due to strain relief and energy cost due to formation of QD side facets and edges. Both contributions are accounted for by an interatomic potential of the Abell-Tersoff type that we developed recently. We relax realistic InAs/GaAs QD nanostructures using this interatomic potential and compare the resulting total energies. To investigate the experimentally observed shape sequence of `hut'-like QD's dominated by \{317\} facets and `dome'-like QD's dominated by \{101\} facets, we compare the energy of a homogenous InAs film and differently sized InAs QD's with either shapes. We identify three regimes: For coverages below about 1.9 monolayers InAs the film is most stable, followed by small `hut'-like QD's and larger `dome'-like QD's. This is in line with the experimentally deduced critical coverage for the 2D to 3D growth transition, and the shapes of small and larger QD's. We can also explain the growth correlation in QD stacks: Our calculated potential-energy surfaces of free-standing QD's in different lateral positions above overgrown QD's show an energy gain of about 20 meV per In atom for the experimentally observed vertical QD alignment.   

Raman Spectroscopy of InAs/GaAs Quantum Dots Patterned by Nano-indentation

Patterns of InAs/GaAs quantum dots (QDs) grown by the combination of nanoindentation technique and molecular beam epitaxy were studied. The resulting QDs tend to preferentially nucleate on indented areas rather than other regions. We studied the strain on the indentations, regions surrounding the indents, and non-indented areas. The QD LO mode for the patterned areas shifted by 8 cm-1 when compared to the non-patterned area. The biaxial strain in the indented areas producing this shift is four times larger than that in non-indented areas, explaining the QD preference within these areas. This larger strain suggests that QDs on the indentations can be formed by depositing a smaller InAs amount than that required to form QDs on non-indented areas, thus obtaining QDs only on the pattern.   

Self-Assembled Unstrained InGaAs Quantum Dashes

We describe a technique for MBE-based fabrication of unstrained quantum dashes with Al$_{x}$In$_{y}$Ga$_{1-x-y}$As alloys lattice-matched to InP substrates. Templates for lattice-matched quantum dash growth are obtained by combining molecular beam epitaxy with \textit{in situ} etching by arsenic bromide. A seed layer of self-assembled InAs quantum dashes is converted into nanotrench templates through overgrowth followed by strain-enhanced etching. We have explored limitations on the accessible range of alloy compositions imposed by the etch process and found that strain-induced etching is limited to compounds with low Al content. Nanotrench templates can be filled with lattice-matched alloys of varied compositions to define barriers and quantum wires that could lead to optoelectronic devices in a spectral range around 1.5 $\mu$m. Here we also present Atomic Force Microscopy and Photoluminescence data obtained from self assembled unstrained In$_{0.53}$Ga$_{0.47}$As Quantum Dashes.   

Microanalysis of quantum dots with type II band alignments

We will discuss the structural characterization of a system consisting of undoped self-assembled InSb quantum dots having a type II band alignment with the surrounding In$_{0.53}$Ga$_{0.47}$As matrix. This differs from systems using conventional type-I quantum dots that must be doped and that rely on intersubband transitions for infrared photoresponse. Type II dots grown in a superlattice structure combine the advantages of quantum dots (3-dimensional confinement) with the tunability and photovoltaic operation of the type II superlattice. We grew a high surface density of InSb quantum dots with a narrow distribution of sizes and shapes and free of dislocations within the body of the dots. The dots are relaxed due to an array of misfit dislocations confined at the basal dot/matrix interface. This makes burying the dots with InGaAs not feasible without generating dislocations due to the large dot/matrix lattice mismatch. We are experimenting with strain-compensating or graded strain overlayers to lower the lattice mismatch.   

Feasibility Study of Directed Self-Assembly of Semiconductor Quantum Dots

Strain mismatched semiconductors are used to form Self-Assembled Quantum Dots (SAQDs). An important step in developing SAQD technology is to control randomness and disorder in SAQD arrays. There is usually both spatial and size disorder. Here, it is proposed to use spatially varying heating as a method of to direct self-assembly and create more ordered SAQD arrays or to control placement of single dots or dot clusters. The feasibility of this approach is demonstrated using a 2D computational model of Ge dots grown in Si based on finite element analysis of surface diffusion and linear elasticity.   

Controlling the self-assembly of Ge quantum dots grown by pulsed laser deposition

Growth dynamics and morphology of self-assembled Ge quantum dots (QD) on Si(100)-(2x1) by nanosecond pulsed laser deposition are studied by in situ reflection high-energy electron diffraction (RHEED) and post deposition atomic force microscopy (AFM). The effects of the laser fluence and substrate temperature on the QD formation are investigated. The QD density increased dramatically (from 3$\times $10$^{7}$ cm$^{-2}$ to 6.3$\times $10$^{8}$ cm$^{-2})$, while the average lateral size decreased (from 362 nm to 107 nm) when the laser fluence was increased from 23 J/cm$^{2 }$to 70 J/cm$^{2}$. Their shape also changed from large huts, observed at 23 J/cm$^{2}$, to domes observed at the highest fluence. At 150$^{\circ}$ C, misaligned QDs formed resulting in diffused RHEED pattern. At 400$^{\circ}$ C and 500$^{\circ}$ C, transmission RHEED patterns were observed indicating the growth of oriented hut and dome QDs. Around 600$^{\circ}$ C, the QDs were formed on top of textured surfaces.   

Defect engineering in periodic gradient-index optical thin films

For thin film deposition with obliquely incident vapour flux, ballistic shadowing limits growth to nucleation sites, forming a porous columnar microstructure. Combined with advanced substrate rotation in a technique known as glancing angle deposition (GLAD), precisely controlled nanoscale architectures are formed. \textit{In situ} variation of the angle of incidence provides dynamic control of the resulting film porosity, allowing the design of continuously varying periodic refractive index profiles to produce thin film interference filters. Intentional nanostructural defects can be introduced, such as uniaxial and biaxial constant index layers or index profile discontinuities, creating defect modes in the filter optical stopbands. Structural and optical characterizations of these periodic structures were performed, with the goal of understanding the relationship between the spectral properties of the film and the engineered nanostructure, demonstrating the high degree of control obtainable over the resulting filter properties using the GLAD process.   

Strontium titanate transformation to highly conductive nanolayers

Developing fabrication methods for electronically active nanostructures is an important challenge of modern science and technology. Fabrication efforts for crystalline materials have been focused on state-of-the-art epitaxial growth techniques. These techniques are based on deposition of precisely controlled combinations of various materials on a heated substrate. We report a method that does not require deposition and transforms a nanoscale layer of a complex crystalline compound into a new material using low energy Ion Beam Preferential Etching (IBPE). We demonstrate this method by transforming a widely used insulator model system, SrTiO3, into a transparent conductor. Most significantly, the resistivity decreases with decreasing temperature as 2.5 power of T and eventually falls below that of room temperature copper. These transport measurements imply a crystal quality in the conduction channel comparable to that obtained with the highest quality growth techniques. The universality of low energy IBPE implies wide potential applicability to fabrication of other nanolayers. David W. Reagor, Vladimir.Y. Butko, Nature Materials, v.4, 593, August 2005.   

Dependency of quantum pumping on transmission mode and dot size

We have used e-beam lithography to fabricate sub-micron metal gates on a two dimensional electron gas with mean free path on the order of several micrometers. Negative biases were applied to the metal gates to confine electrons in a small area ($\sim \mu $m$^{2})$ forming a so-called quantum dot. Two quantum point contacts (QPCs) served the entrance and exit of electrons in the dot are located in line. Quantum charge pumping phenomena of the open dot in the absence of an external bias was observed using two independent ac voltages with the same frequency, 1$\sim $80MHz, but a phase difference between them. Similar pumping results were reported by Marcus et al. earlier. However, due to the differences in the geometrical arrangements, the behaviors are somehow different including that our pumping current is one order more magnitude bigger and does not increase linearly with frequency for the entire measuring range. Moreover, we found that the pumping current seems increase with decreasing transmission mode numbers of the two QPCs. When the mode number goes to zero and the open dot transforms to closed dot, the pumping current vanishes. The results and measurements of the dependences of quantum charge pumping on transmission mode and dot size will be presented and discussed.   

Local Density of States of a Quantum Point Contact Near Pinchoff

Over the last decade, there has been great interest in how electrons flow through a quantum point contact (QPC) as it is just opened up, before a fully transmitting 1D conduction channel is available. Remarkably, there does not seem to be a smooth transition from tunneling to ballistic transport. Instead, a shoulder appears in the conductance versus channel width, at a conductance of roughly 0.7 times that of an open spin-degenerate channel. Experiments have built a consensus that this so-called ``0.7 structure'' is related to electron spin and electron-electron interaction, but the detailed description remains controversial. To study this system, we have made devices where one of the two QPC gates is actually a tunnel barrier to a third lead, fabricated on a GaAs/AlGaAs heterostructure. With this third lead, it is possible to probe the density of states in the QPC channel from the side as the QPC opens from pinchoff through the first channel. We acknowledge support from the ONR Young Investigator Program, Award No. N00014-01-1-0569 and a Research Corporation Research Innovation Award, No. RI1260.   

Single electron transistors in GaN/AlGaN heterostructures.

We study transport properties of two single-electron transistors (SETs) in a GaN/AlGaN heterostructure. The first SET accidentally formed in a quantum point contact near pinchoff. Its small size produces large energy scales: a charging energy of 7.5 meV, and well-resolved excited states. The second, intentionally-fabricated SET is much larger. Hundreds of consecutive, uniformly-spaced Coulomb oscillations yield a charging energy of 0.85 meV. Excited states are not resolvable in Coulomb diamonds, and Coulomb blockade peak height remains constant with increasing temperature, indicating that transport is through multiple quantum levels even at the 450 mK base electron temperature of our measurements.   

Current Biased Real Time Charge Detection in a Single Electron Transistor

Measurements of charge fluctuations in an AlGaAs/GaAs single electron transistor (SET) are presented. The SET consists of a lateral quantum dot created by confining a two-dimensional electron gas using nanometer-size surface electrodes. The charge on the quantum dot is detected by changes in conductance of a nearby quantum point contact. We discuss noise and bandwidth characteristics of our charge detection method, which uses commercially available voltage amplifiers. Our real time charge detection capabilities are used to investigate charge dynamics on the SET in a magnetic field parallel to the two-dimensional electron gas. This work is supported by the U.S. Army Research Office under Contract No. W911NF-05-1-0062, by the National Science Foundation under Grant No. DMR-0353209, and in part by the NSEC Program of the National Science Foundation under Award No. PHY-0117795.   

Real Time Electron Hopping Phenomena in a Single-Electron Transistor

Utilizing a current-biased quantum-point-contact charge sensor, we observe electrons hopping on and off a AlGaAs/GaAs single-electron transistor (SET) in real time. An electron tunnels between the extended states in the leads and the lowest-energy state localized in the lateral quantum dot created by nanometer-size surface electrodes. We observe changes in the tunneling rates, caused by the spin splitting in a magnetic field B applied parallel to the 2DEG. We have also observed single-electron photo-ionization of the SET by application of microwave radiation. This work is supported by the ARO (W911NF-05-1-0062), the NSF (DMR-0353209) and in part by the NSEC Program of the NSF (PHY-0117795).   

Shot noise and strong feedback effects in nanoelectromechanical systems

Quantum nanoelectromechanical systems have attracted much attention recently, offering potential for applications as well as insight into fundamental physics. Using a quantum noise approach, we study theoretically a nanomechanical oscillator coupled to a superconducting single-electron transistor (SSET). Incoherent Cooper pair tunneling processes in the SSET can lead to a negative damping instability, where the oscillator's amplitude increases as it absorbs energy from the SSET \footnote{Clerk, Bennett, NJP {\bf 7}, 238 (2005).}. Here, we focus on the current noise of the SSET in the negative damping regime, in which the growing amplitude of the oscillator becomes large enough that the motion of the oscillator and the dynamics of the SSET depend strongly on each other. We describe the inherent non-linearity of this regime using effective, energy-dependent damping and temperature, and discuss characteristic timescales for dynamics in the system. The current noise is of particular interest because it can be directly observed, and current experiments are probing this regime \footnote{K. Schwab {\it et al.} (in preparation).}.   

Finite size effects in the decay of metastable states in one-dimensional resonant tunneling structures

We study the current switching in a double-barrier resonant tunneling structure in the regime where the current-voltage characteristic exhibits intrinsic bistability, so that in a certain range of bias two different steady states of current are possible. Near the upper boundary $V_{th}$ of the bistable region the upper current state is metastable, and because of the shot noise it eventually decays to the stable lower current state. We find the time of this switching process in strip-shaped devices, with the width small compared to the length. The mean switching time $\tau$ increases exponentially as the bias $V$ is tuned inside the bistable region from its boundary value $V_{th}$. The one-dimensional geometry of the problem enables us to obtain analytically exact expressions for the exponential factor and to calculate the prefactor of $\tau$ for an arbitrary length of the strip. Furthermore, we evaluate the mean time of switching in ring-shaped devices, with the widths small compared to their diameters.