Sören Jeppesen

Single quantum dots emit single photons at a time : Antibunching experiments

We have studied the photoluminescence correlation from a single InAs/GaAs self-assembled Stranski–Krastanow quantum dot under continuous, as well as under pulsed excitation. Under weak continuous excitation, where the single dot luminescence is due primarily to single exciton recombinations, antibunching is observed in the single dot emission correlation. Under weak pulsed excitation, the number of photons emitted by the quantum dot per pulse is close to one. We present data obtained under both conditions and are able to show that devices based on single quantum dots can be used to generate single photons.

Surface diffusion effects on growth of nanowires by chemical beam epitaxy

Surface processes play a large role in the growth of semiconductor nanowires by chemical beam epitaxy. In particular, for III-V nanowires the surface diffusion of group-III species is important to understand in order to control the nanowire growth. In this paper, we have grown InAs-based nanowires positioned by electron beam lithography and have investigated the dependence of the diffusion of In species on temperature, group-III and -V source pressure and group-V source combinations by measuring nanowire growth rate for different nanowire spacings. We present a model which relates the nanowire growth rate to the migration length of In species. The model is fitted to the experimental data for different growth conditions, using the migration length as fitting parameter. The results show that the migration length increases with decreasing temperature and increasing group-V/group-III source pressure ratio. This will most often lead to an increase in growth rate, but deviations will occur due to incomplete decomposition and changes in sticking coefficient for group-III species. The results also show that the introduction of phosphorous precursor for growth of InAs1−xPx nanowires decreases the migration length of the In species followed by a decrease in nanowire growth rate. (Less)

Transients in the Formation of Nanowire Heterostructures

We present results on the effect of seed particle reconfiguration on the growth of short InAs and InP nanowire segments. The reconfiguration originates in two different steady state alloy compositions of the Au/In seed particle during growth of InAs and InP. From compositional analysis of the seed particle, the In content in the seed particle is determined to be 34 and 44% during InAs and InP growth, respectively. When switching between growing InAs and InP, transient effects dominate during the time period of seed particle reconfiguration. We developed a model that quantitatively explains the effect and with the added understanding we are now able to grow short period (<10 nm) nanowire superlattices.

Optical detection of growth oscillations in high vacuum metalorganic vapor phase epitaxy

We report the first measurements of growth oscillations in high vacuum metalorganic vapor phase epitaxy (MOVPE). The reflection difference response of the surface is used for real‐time monitoring of the layer‐by‐layer growth of GaAs from triethylgallium (TEG) and arsine. The frequency of the optically detected growth oscillations is found to be proportional to the flux of TEG and to the growth rate. We expect our results to extend the more limited ranges of applicability offered by reflection high‐energy electron diffraction to allow the study of growth oscillations also in other MOVPE‐related growth techniques.

Reflectance-difference study of surface chemistry in MOVPE growth

With the advent of the reflectance-difference (RD) technique the electronic configurations of III–V surfaces can be probed also under the non-ultra-high vacuum (UHV) conditions prevailing during metalorganic vapour phase epitaxy (MOVPE). In this paper the surface chemistry involved in the MOVPE growth of GaAs from triethylgallium and arsine is studied in a growth system where the pressure can be changed from high-vacuum (10-3 Torr) to low-pressure (1 Torr) conditions. Studies of the GaAs surface during its exposure to triethylgallium and to arsine are presented. Growth oscillations detected in real time are used to characterize growth and to investigate three-dimensional island formation during Ga saturation. Finally, growth oscillations are used to study the kinetics of the regeneration of ideal, As-terminated, surface conditions.

Reflectance-difference detection of growth oscillations

We have observed damped oscillations measured by the reflectance-difference (RD) technique during growth of GaAs by vacuum chemical epitaxy (VCE) using triethylgallium (TEG) and arsine as precursors. The variation in growth rate with incident TEG flux (at 550°C) was found to be linear for growth rates between 0.1 and 0.4 monolayers/s. Our small, compact RD set-up measures, at nearly normal incidence, the difference between the reflectance of light polarized parallel to the [110] direction and parallel to the [110] direction. This gives information on the lower-symmetry surface since the bulk contribution nominally disappears due to the subtraction. Clearly resolved growth oscillations, having an amplitude of up to one fourth of the initial transient, can be obtained during one single time base scan. After arsine stabilization the RD can follow, in real time, the formation of up to 25 single monolayers, each one corresponding to one period of the growth oscillations. Our results prove that RD can be used during continuous growth providing excellent control of the growing GaAs surface.

Reflectance-difference probing of surface kinetics of (001) GaAs during vacuum chemical epitaxy

Abstract A reflectance-difference (RD) study of the kinetics of various surface processes involved in the growth of GaAs from triethylgallium and arsine is presented. During triethylgallium exposure of an As-stabilized (001) GaAs surface, an initial linear RD response is observed, similar to what has previously been reported for trimethylgallium. We show that in the case of triethylgallium an over-saturation of the surface occurs, which results in complex transients in the RD response and, for a critical dose, in a loss of the surface coherency as determined by the disappearance of RD growth oscillations. It is found that the arsine reactivity on the over-saturated surface is much higher that that on the Ga-stabilized surface. The arsine-induced RD transient is compared with the kinetics of the re-establishment of a perfect As-stabilized surface, using the amplitude of RD-detected growth oscillations as a probe of the status of the surface.

A comparison of RHEED reconstruction phases on (100) InAs, GaAs and InP

Abstract We have begun to compile a list of the surface reconstructions for (001) InAs, GaAs and InP. The symmetries that we have observed include 1 × 1, 2 × 1, 2 × 4, 4 × 6, 3 × 4, 4 × 3, 2 × 3, 3 × 3, 1 × 3, 3 × 1, 4 × 2, 4 × 1 and 8 × 2. These reconstructions were observed during various growth and non-growth conditions in order to make up phase diagrams that depend on the group V and III fluxes and the temperature. Some of the phase boundaries can serve as temperature reference points in our epitaxy machine. A comparison of the different semiconductor surfaces is made and the growth properties of these surfaces are discussed.

Assembling strained InAs islands by chemical beam epitaxy

Abstract We report on coherently strained InAs quantum-dots grown by chemical beam epitaxy on GaAs. The morphological phase transition of the InAs layer from two-dimensional to three-dimensional was characterized with reflection high-energy electron diffraction. The transition is found to be quasi-equilibrium in the slow deposition regime studied, to be approximately linear in InAs thickness, and to be suppressed both by higher temperature and As pressure. Patterned substrates were used to assemble the dots in specific locations. Conditions were found to align dots in chains of several-μm length, to place small numbers of dots in holes, and to grow dots only within patterns but not on adjoining flat surfaces. When capped with GaAs, the islands are optically active.

Control and understanding of kink formation in InAs–InP heterostructure nanowires

Nanowire heterostructures are of special interest for band structure engineering due to an expanded range of defect-free material combinations. However, the higher degree of freedom in nanowire heterostructure growth comes at the expense of challenges related to nanowire-seed particle interactions, such as undesired composition, grading and kink formation. To better understand the mechanisms of kink formation in nanowires, we here present a detailed study of the dependence of heterostructure nanowire morphology on indium pressure, nanowire diameter, and nanowire density. We investigate InAs-InP-InAs heterostructure nanowires grown with chemical beam epitaxy, which is a material system that allows for very abrupt heterointerfaces. Our observations indicate that the critical parameter for kink formation is the availability of indium, and that the resulting morphology is also highly dependent on the length of the InP segment. It is shown that kinking is associated with the formation of an inclined facet at the interface between InP and InAs, which destabilizes the growth and leads to a change in growth direction. By careful tuning of the growth parameters, it is possible to entirely suppress the formation of this inclined facet and thereby kinking at the heterointerface. Our results also indicate the possibility of producing controllably kinked nanowires with a high yield.