N. Carlsson

Study of the two‐dimensional–three‐dimensional growth mode transition in metalorganic vapor phase epitaxy of GaInP/InP quantum‐sized structures

Ga0.5In0.5P/InP quantum‐sized structures, grown by metalorganic vapor phase epitaxy, have been optically characterized by photoluminescence, cathodoluminescence, and photoluminescence excitation spectroscopy. Additional structural information has been obtained by atomic force microscopy. We find that the two‐dimensional layer‐by‐layer growth mode is limited to the growth of 1‐ML‐thick and, in part, 2‐ML‐thick quantum wells. The transition towards three‐dimensional Stranski–Krastanow island growth occurs before the second monolayer of InP is completed. To further study the dynamics of the island formation, growth interruptions were introduced between the InP deposition and the subsequent growth of the upper GaInP barrier. The two types of coherent islands show a quantum confinement in vertical direction, corresponding to about 2‐ and 3‐ML‐thick and about 9‐ and 10‐ML‐thick InP strained quantum wells.

Growth of self-assembled InAs and InAsxP1-x dots on InP by metalorganic vapour phase epitaxy

Abstract The formation of self-assembled InAs and InAs x P 1− x dots on InP has been studied, in particular with deposition conditions under which mainly coherent dots are developed. The samples were grown by metalorganic vapour phase epitaxy. Morphological investigations were performed by atomic force microscopy, with the instrument working in the contact mode as well as in the noncontact mode. Surface densities and height distributions were extracted, as a function of growth conditions. In addition, photoluminescence was used for investigations of the optical properties of capped InAs dots, formed under equivalent conditions. Comparisons between the two characterization techniques show a qualitative agreement with respect to the density of dots as well as their size homogeneity. It is also indicated that dots of binary InAs can be formed at deposition temperatures not higher than about 500°C. Elevated deposition temperatures in this process result in an unintentional alloying mechanism due to exchange reactions at the interface, leading to the formation of ternary InAs x P 1− x dots, which can be seen as a simultaneous increase in the energy of the light emission and the average dot size, indicating the widening of the energy gap in the quantum dots, which counteracts the decreased energy quantization in the larger dots formed at higher temperatures.

Alignment of InP Stranski–Krastanow dots by growth on patterned GaAs/GaInP surfaces

Stranski–Krastanow islands of InP nucleate in straight rows when deposited by metalorganic vapor phase epitaxy on linearly patterned GaInP/GaAs surfaces. The patterns were produced by overgrowth of lithographically defined W stripes 30° off from [110] on a GaAs(001) surface. Depending on the geometry of the grown GaAs/GaInP mesa stripes the islands were found to align either on top of the ridges, at the sidewall near to the mesa edge or at the bottom of the trenches. The highest density of almost equidistant coherent islands observed in some of the rows is in the order of 10 islands/μm, corresponding to a surface area density of 1010 InP islands/cm2. The maximum density of randomly distributed islands in the unpatterned area otherwise is only 2×109 islands/cm2. The results show a successful combination of overgrowth of conducting metal stripes and lateral geometrical positioning of dots in one in situ growth step.

In situ growth of nano-structures by metal-organic vapour phase epitaxy

Abstract Using spontaneous self-organization effects is an efficient way to produce nano-structures, as for instance quantum wires and quantum dots. This article is focused on the strain-induced self-organization, or “self-assembling” effect, producing quantum dots. Particularly the following aspects will be addressed: (i) the phenomenology of the 2D–3d morphology transition, (ii) the effects of materials choices and growth conditions on density, size and homogeneity of dots, and (iii) manipulations to get laterally aligned and vertically stacked dot structures.

Transmission electron microscopy investigation of the morphology of InP Stranski–Krastanow islands grown by metalorganic chemical vapor deposition

We have used transmission electron microscopy to determine the morphology of InP Stranski–Krastanow islands in GaInP, grown by metalorganic chemical vapor deposition at 580 °C. We investigated both capped and uncapped islands. It was found that the fully developed islands have the principal shape of truncated pyramids with a hexagonal base both before and after overgrowth. The planes defining the islands are of {001}, {110}, and {111} types. The base dimensions are 40–50 nm and 55–65 nm in the [110] and [110] directions, respectively, and the height is 12–18 nm.

Deep level transient spectroscopy of InP quantum dots

We report on the application of deep level transient spectroscopy to the study of electron emission from quantum dots. The results are presented for coherently grown InP dots embedded in Ga0.5In0.5P. We determine an emission activation energy of 220 meV for the one electron ground state of the dots. With increased average electron occupation in the dots we observe a systematic shift of the DLTS peak towards lower temperatures. This we interpret as being due to Coulomb charging of the dots. We extract an average Coulomb charging energy of 8–12 meV per added electron in the dot in agreement with our estimated value of 9 meV.

Case study of an InAs quantum dot memory: Optical storing and deletion of charge

We have studied self-assembled InAs quantum dots embedded in an InP matrix using photocapacitance and photocurrent spectroscopy. These dots are potentially promising for memories due to the large confinement energy for holes. In this work we have realized simple quantum dot memory by placing the dots in the space–charge region of a Schottky junction. Our measurements reveal that a maximum of about one hole can be stored per dot. We also find that illumination for an extended period deletes the stored charge. We show that these limitations do not reflect the intrinsic properties of the dots, but rather the sample structure in combination with deep traps present in the sample.

Observation of strain effects in semiconductor dots depending on cap layer thickness

We have investigated the photoluminescence emission energy of InP dots as a function of cap layer thickness. We find a strong blue‐shift with increasing cap layer thickness. The strain tensor in the dot as well as in the surrounding matrix has been modeled using finite element methods and the band gap has been calculated using deformation potential theory. We find good agreement between calculation and experiment. For uncapped dots we find that the emission energy is lower than for biaxially strained InP, and is indeed close to unstrained InP.

ELECTRICAL CHARACTERIZATION OF INP/GAINP QUANTUM DOTS BY SPACE CHARGE SPECTROSCOPY

An investigation of coherently grown InP quantum dots embedded in Ga0.5In0.5P by conventional space charge spectroscopy methods is reported. Deep level transient spectroscopy (DLTS) is used to obtain quantitative information on the electron emission from the dots. The applied field is found to significantly enhance the electron emission rates as seen by shifts in the peaks towards lower temperatures with increased field. Taking the field induced barrier lowering into account, the emission energy for the one electron ground state of the dot is determined as 240±10 meV. The correlation between the measured signal and the observed electron accumulation in capacitance–voltage measurements is clearly demonstrated. Further, studies of the electron emission when the average electron population in the dots was varied show that the emission energies are modified by the coulomb charging energy. Admittance measurements as a function of temperature, bias and frequency were also performed, and the results are qualitat...

Ga0.25In0.75As/InP quantum wells with extremely high and anisotropic two‐dimensional electron gas mobilities

Measurements of modulation‐doped Ga0.25In0.75As/InP quantum wells show, in the 〈−110〉 direction, a record electron mobility of 520 000 cm2/V s at 300 mK. A mobility difference of 15% between the 〈110〉 direction and the 〈−110〉 direction is observed. This anisotropy is tentatively attributed to an ordering effect. The mobilities at room temperature and at 77 K were 16 100 and 170 000 cm2/V s, respectively. By separating out the ionized impurity scattering from other scattering processes in the quantum well, we conclude that at low electron concentrations ionized impurity scattering is limiting the mobility, while alloy scattering has a strong influence on the mobility at high electron concentrations. From this result we determine the first experimental value of the alloy‐scattering potential as ΔV=0.3 eV.