Bernhard Kowalski

Assembling strained InAs islands on patterned GaAs substrates with chemical beam epitaxy

The assembly of strained InAs islands was manipulated through growth on patterned GaAs substrates with chemical beam epitaxy. Conditions were found to selectively place the islands in patterns features but not on surrounding unpatterned fields. Chains of islands having 33 nm minimum periods were formed in trenches, and single or few islands were grown in arrays of holes. When capped with GaAs, the islands behave as quantum dots and are optically active.

Stacking InAs islands and GaAs layers: Strongly modulated one‐dimensional electronic systems

With chemical beam epitaxy we stacked small InAs islands, separated by thin GaAs layers. Reflection electron diffraction during growth showed that after a seed‐layer growth, subsequent depositions require less InAs to form the islands. At 5 K the stacks have narrower luminescence peaks at lower energies than single island layers, and the stacks luminesce at room temperature. For 4‐nm‐high pyramidal islands with 20‐nm‐wide bases, we observed vertical periods down to 5.4 nm, small enough to couple quantum mechanically. The electronic structures possible for this class of objects should be sufficient for designing and observing room temperature quantum mechanical phenomena.

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.

Optically detected spin resonance of conduction band electrons in InGaAs/InP quantum wells

Optically detected spin resonance was used to measure the effective g-value of electrons at the conduction band minimum in type-I quantum wells. The experiments showed that the spin resonance is induced by electric dipole transitions, and hence is not limited by the short carrier lifetime that renders magnetic dipole transitions impossible. The spin splittings obtained are strongly anisotropic and dependent on quantum well thickness. A calculation without adjustable parameters, using a three-band Kane model, agrees with the experimental data. The bulk effective g-value of used in this calculation was measured on a thick sample.

Carrier-modulated, microwave-detected Shubnikov-de Haas oscillations in two-dimensional systems

It was recently shown that Shubnikov–de Haas (SdH) oscillations observed in conventional resistance measurements can be dramatically enhanced by light‐induced carrier modulation [S. E. Schacham, E. J. Haugland, and S. A. Alterovitz, Appl. Phys. Lett. 61, 551 (1992)]. Here we report on a similar observation in the case of contact‐free, microwave‐detected SdH oscillations. In the original version of this nondestructive technique [P. Omling, B. Meyer, and P. Emanuelsson, Appl. Phys. Lett. 58, 931 (1991)], magnetic‐field modulation was applied in order to enhance the sensitivity. If, instead, the carrier concentration is modulated by illumination, we show that a similar enhancement in the sensitivity of the signal is obtained. We demonstrate that very simple microwave equipment can be used for the measurements, and that the accessible magnetic‐field region can be extended, allowing for contact‐free transport investigations in the high magnetic‐field region.

Manipulating InAs island sizes with chemical beam epitaxy growth on GaAs patterns

We demonstrate the manipulation of InAs island size through growth on patterned GaAs substrates. Using chemical beam epitaxy, we deposited islands on patterns consisting of concentric circular trenches. One sample was left uncapped for atomic force microscopy, and another was capped with GaAs for micro-photoluminescence. Luminescence images taken at particular energies show that at different orientations along the circular arcs, the islands have different luminescence energies and thus different sizes. In correlating the island distributions found by atomic force microscopy images with the luminescence, we conclude that the InAs islands that form in high-density one-dimensional chains are smaller than those that are not in chains.

Level crossing of nanometer sized InAs islands in GaAs

We used polarization spectroscopy to detect level crossings in the fine structure of excitons in strained InAs islands grown on [001] GaAs. The crossings gave rise to quasi-resonant peaks, when monitoring the circularly polarized photoluminescence (PL) as a function of magnetic field. The peaks could also be detected as increases of the PL intensity. The resonant magnetic field was strongly dependent on detection energy within the PL emission peak. This energy selection is equivalent to monitoring a specific size or small interval within the broader size and shape distribution inherent to the growth process. The resonance was observed to shift to a higher magnetic field, when increasing the angle between field and sample growth direction. Basic arguments based on quantum confinement and the exciton fine structure can qualitatively account for the observations. Together with hole effective g-values the level crossing fields can be used to calculate the zero magnetic field splitting of the exciton fine structure.

Directional dependence of InAs island formation on patterned GaAs

Strained, coherent InAs islands were grown in the Stranski-Krastanov growth mode on patterned GaAs by chemical beam epitaxy. The present work represents initial steps in quantitatively understanding the placement and growth of islands in patterns. The trade-offs between regular pattern features and better island nucleation are demonstrated by a deposition on a pattern of concentric circles. InAs mass transport is also measured, where material is seen to move on the order of a micron to and from different local orientations. An important practical result is that the pattern definition for island alignment need not be finer than 0.25 μm, within the reach of present-day optical lithography.

Vertically-Stacked InAs Islands Between GaAs Barriers Grown by Chemical Beam Epitaxy

We report vertically-aligned InAs islands separated by GaAs barriers thin enough for electronic coupling. Thinner barriers reduced the InAs critical-thickness for island formation. Transmission electron microscopy revealed well-aligned islands with all detected islands in complete stacks. Atomic force microscopy showed the top islands of uncapped stacks are fully formed. The photoluminescence peak was sharper and shifted to lower energy compared to a single-layer growth. We attribute this shift to island-to-island electronic coupling and to the smaller compressive strain in the center of the composite structure.

The conduction band spin splitting in type-I strained and unstrained (GaIn)As/InP quantum wells

We present the first successful optically detected magnetic resonance experiments (ODMR) on strained and unstrained GaxIn1−xAs/InP type-I quantum wells. The resonances attributed to electrons are in general anisotropic and detailed results are given for the composition range 0.4 < x < 0.6. Extrapolating to the binary end points, i.e. InAs and GaAs close agreement with existing results is found. The anisotropy is explained within a simple model using subband calculations. It also explains why single-sided p-type modulation doping reduces the electron-hole wavefunction overlap, increases the radiative life-times and allows for optical detection of magnetic resonance. A detailed discussion why hole resonances can be ruled out is given.