Lassana Ouattara

A cross-sectional scanning tunneling microscopy study of a quantum dot infrared photodetector structure

We report on cross-sectional scanning tunneling microscopy studies of a quantum dot infrared photodetector structure consisting of multiple InGaAs quantum wells containing InAs quantum dots and separated by GaAs. We have investigated the composition and size distribution of the InAs quantum dots in this structure. Using cross-sectional scanning tunneling microscopy images displaying atomic resolution, we reveal that the InAs quantum dots are strongly intermixed with the InGaAs quantum well layer. We estimate the size distribution of the dots and find this to be nonuniform. Further, defects related to dopants introduced in the structure are identified and their concentration is estimated.

Nanowire growth and dopants studied by cross-sectional scanning tunnelling microscopy

Using a crystalline embedding scheme it has recently become possible to study free-standing III–V nanowires with cross-sectional scanning tunnelling microscopy (XSTM). In the present paper we discuss how this novel method can be used for direct atomically resolved imaging of the interior of nanowires. We will focus in particular on two areas where this method provides unique possibilities. First we discuss the growth of the nanowire at the substrate as studied by XSTM and determine the facets of the nanowire growth on the surface and at the onset of free-standing nanowire growth. Second, we demonstrate how individual defects can be studied inside the wires, indicating a unique way for investigating dopant structures and concentrations in nanowires. We identify a carbon defect/dopant in the nanowire positioned on arsenic sites and establish quantitative limits on the defect density in the nanowires.

The Equivalence Principle Comes to School--Falling Objects and Other Middle School Investigations.

Comparing two objects falling together is a small-scale version of Galileo’s classical experiment, demonstrating the equivalence between gravitational and inertial mass. We present here investigations by a group of ten-year-olds, who used iPads to record the drops. The movie recordings were essential in the follow-up discussions, enabling the students to compare the different situations and to discern situations where air resistance was essential and where it could be neglected. By considering a number of familiar situations and simple investigations that can be performed, e.g., on a playground, students may come closer to an appreciation of the deep significance of the non-influence of mass on motion under gravity.

Stacked InAs quantum dots in InP studied by cross-sectional scanning tunnelling microscopy

We report on cross-sectional scanning tunnelling microscopy (XSTM) measurements on vertically stacked InAs quantum dots in InP barriers. We have investigated two-, five- and tenfold stacked quantum dot structures,grown by low-pressure metal-organic vapour phase epitaxy. The XSTM images reveal that the quantum dots are generally vertically well aligned, and have a truncated pyramidal shape in agreement with similar studies of InAs dots in GaAs. STM images displaying atomic resolution indicate that the dots have a pure InAs stoichiometry, with intermixing only occurring in the top and bottom dot rows. Further, we have investigated various anomalies (considered as defects) as observed in the quantum dot stacks. The origins of these anomalies are discussed and compared to theoretical predictions available so far. (Less)

Correlation lengths in stacked InAs quantum dot systems studied by cross-sectional scanning tunnelling microscopy

We have studied the influence of the InP spacer layer thickness on stacked InAs/InP quantum dots, using cross-sectional scanning tunnelling microscopy. We show that for a spacer layer thickness of up to 30 nm, the quantum dots are spatially correlated but for a separating distance of 50 nm the vertical ordering of the dots is lost. These values are the same as previously found for quantum dots in the InAs/GaAs system despite the large difference in lattice mismatch between the InAs/GaAs ( 7%) and InAs/InP ( 3%) systems. We show that the apparent similarities can be understood by a combination of intermixing in the dots and differences in dot size. Finally, we demonstrate that the size of the quantum dots is affected by their vertical correlation.

Direct observation of atomic scale surface relaxation in ortho twin structures in GaAs by XSTM

We have studied the (110) GaAs surface of a structure containing ortho twins by cross-sectional scanning tunnelling microscopy and we have compared the experimental results with ab initio density functional theory calculations and STM simulations. Both experimentally and theoretically we find that the surface of different twin crystallites are significantly displaced with respect to each other, parallel to the twin boundary. This result is explained by a surface relaxation of the atoms in the (110) GaAs surface and the difference between the atomic configuration of the ortho twins.

Mn diffusion in Ga1-xMnxAs/GaAs superlattices

Ga1−xMnxAs∕GaAs superlattices with Mn concentrations of 1% and 5% in the Ga1−xMnxAs layers and a GaAs spacer thickness of 4 and 60 GaAs monolayers have been studied by cross-sectional scanning tunneling microscopy. By achieving atomic resolution of the superlattices, we observe individual Mn atoms in the Ga1−xMnxAs layers and in the GaAs spacer. We find that about 20% of the total amount of Mn diffuses from the GaMnAs layers into the GaAs spacer layers. Our results can be related to previous measurements of the magnetic properties of short period Ga1−xMnxAs∕GaAs superlattices.

Spontaneous InAs quantum dot nucleation at strained InP/GaInAs interfaces

We present a cross-sectional scanning-tunneling microscopy investigation of twofold stacked InAs quantum dots in InP, between layers of GaInAs. The dots are vertically aligned, and images with atomic resolution show that the dots consist of pure InAs. Despite the intended twofold stacking of dots, three dots were often found in the stacks. The third dot formed immediately on top of the final InP layer, at the InP/GaInAs interface. Atomically resolved images of these spontaneously formed dots indicate that they also consist of pure InAs. The effect is discussed in terms of phase segregation of GaInAs and arsenic–phosphorus exchange reactions.

Motion on an Inclined Plane and the Nature of Science.

Friction is an important phenomenon in everyday life. All children are familiar with playground slides, which may thus be a good starting point for investigating friction. Motion on an inclined plane is a standard physics example. This paper presents an investigation of friction by a group of 11-year olds. How did they plan their investigations? What aspects of friction could they discern? What understanding of the nature of science was revealed—and developed—during their investigation and subsequent discussion with the teacher?