Average Dot Size Research Articles

Raman scattering of Ge dot superlattices

Self-organised Ge dot superlattices grown by molecular beam epitaxy of Ge and Si layers utilizing Stranski-Krastanov growth mode were investigated by Raman spectroscopy. An average size of Ge quantum dots was obtained from transmission electron microscopy measurements. The strain and interdiffusion of Ge and Si atoms in Ge quantum dots were estimated from the analysis of frequency positions of optical phonons observed in the Raman spectra. Raman scattering by folded longitudinal acoustic phonons in the Ge dot superlattices was observed and explained using of elastic continuum theory.

An optical study of self-assembled InxGa1−xAs/GaAs quantum dots embedded in a two-dimensional electron gas

We studied the low temperature (77 K) photomodulated reflection and transmission as well as the photoluminescence at 2.2 K of a self-assembled InxGa1−xAs quantum dot layer grown on a (100) GaAs substrate in the vicinity of a two-dimensional electron gas. The dot layer was grown without rotation of the substrate in order to achieve a gradual variation of the In concentration along the wafer diameter. This resulted in an increase in the density of quantum dots along the In concentration gradient, which is reflected in a characteristic dependence of the relative intensities of the spectral lines. A consistent assignment of the optical structure observed in all spectra leads to an estimate of the average value of the Fermi energy in the conduction band of the wetting layer (EF≃13.4 meV). The variation of this Fermi energy along the composition gradient can be obtained from the spectra, and an estimate of the gradient of the density of quantum dots along this direction can be made. A careful comparison of the variation of the critical energy of the different lines suggests that the average quantum dot size depends on the In molar fraction of the alloy, which is seen to vary more or less linearly across the wafer diameter.

Growth of vertically self-organized InGaAs quantum dots with narrow inhomogeneous broadening

We have fabricated vertically self-organized multiple sets of In0.6Ga0.4As quantum dots (QDs) on GaAs (001) that combines the concepts of variable amount deposition and shape stabilization and size equalization of QDs. The inhomogeneous broadening of optical emission from these dots reached a record low value of 18.4 meV at a wavelength of ∼1185 nm (4 K). The seed layer and the second dot layer have essentially the same dot density of ∼250 μm−2 due to the high degree of dot vertical alignment. The deposition amount for the second dot layer was selected to be 9 monolayers, which resulted in dots with convergent lateral size (∼62 nm) and stabilized facets, close to {011}. The third layer, with the same amount of InGaAs as the second layer, had a dot density of ∼350 μm−2, an average lateral dot size of ∼71 nm, an average dot height of ∼11 nm, and shallower side facets close to {023}.

Growth and luminescence properties of self-organized ZnSe quantum dots

Self-organized ZnSe quantum dots (QDs) were grown on (100) ZnS/GaAs surfaces to study the relation of the size dispersion and luminescence. The exact dot sizes were obtained by measurements of atomic force microscope with its tip calibration and transmission electron microscope. The average dot size was 2.0 nm high and 11 nm in its diameter and the density was 1×1010 cm−2. Transition energies of ZnSe QDs were calculated using these measured dot sizes. These calculated peaks were in reasonable agreement with measured photoluminescence (PL) peaks. It was also revealed that the broadening of the PL spectra from ZnSe QDs were consistently explained by the dot size distribution.

Optical investigation of CdS quantum dots in Langmuir-Blodgett films

Structures with CdS quantum dots produced by the Langmuir–Blodgett (LB) technique were investigated by Raman, IR, and UV spectroscopies. The confinement effect of longitudinal optical (LO) phonons in CdS quantum dots was investigated by Raman spectroscopy. Surface vibrational modes of CdS quantum dots were observed in IR spectra. It was shown experimentally that the frequency of the surface vibrational modes depends on the properties of the surrounding media. An average size of CdS quantum dots of about 3–6.4 nm was obtained from the analysis of UV measurements.

Molecular-beam epitaxy of InAs on anodized GaAs substrates

InAs dots were grown on anodized (0 0 1) GaAs substrates by molecular-beam epitaxy. It was found that InAs dots were selectively grown at the bottom of beehive-like etch-pits formed by anodization of GaAs substrates in an NH 4OH solution. The average size of InAs dots and its coefficient of variation on anodized substrates was larger than that on unpatterned substrates at the total amount of deposited In atoms less than about 4.0 ML, and vice versa at the deposited In amount of 10 ML. In the case of 10.0-ML InAs growth, one or two dots were grown at a bottom of beehive-like etch-pits on the anodized substrates, and the average size of InAs dots and its coefficient of variation on the anodized substrate were about two-third times smaller than those on the unpatterned substrate.

In-plane polarized intraband absorption in InAs/GaAs self-assembled quantum dots

In-plane polarized photoinduced intraband absorption is reported in InAs/GaAs self-assembled quantum dots in the 90--600-meV energy range. This in-plane absorption mainly originates from bound-to-continuum transitions in the conduction and valence bands and lies in the 4--8-\ensuremath{\mu}m spectral range. The continuum is constituted either by the dot quasibound states hybridized with the wetting layer subbands or by the delocalized states of the barriers. A weak in-plane polarized bound-to-bound hole transition is also observed at 11 \ensuremath{\mu}m. Its in-plane absorption cross section is estimated to be around $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{\mathrm{cm}}^{2}.$ The energy position of the p-polarized and in-plane polarized intraband transitions is analyzed as a function of the average quantum dot size. An experimental energy diagram of the quantum dots is presented, as deduced from the interband and intraband measurements. The existence, directions, and lengths of the intraband transition dipoles are compared to the ones deduced from the numerical resolution of the three-dimensional effective mass Schr\odinger equation taking into account the flat lens-shape geometry of the dots.

Atomic force microscopy studies of ZnSe self-organized dots fabricated on ZnS/GaP

ZnSe self-organized dot structures on ZnS thin films were fabricated by the molecular beam epitaxy technique. In situ reflection high-energy electron diffraction studies reveal that growth interruption is required for the formation of the dot structure. Atomic force microscopy (AFM) images of the dots taken within the same day of growth reveal that the dot density increases with increasing ZnSe coverage. A density of 18 μm−2 was achieved with a coverage of 8.0 ZnSe monolayers. AFM images taken at later times (up to six months later) show ripening effects. The average dot size measured at various times after growth is consistent with the prediction of the Ostwald ripening model with a growth time constant of 4±1 days for the structure with a coverage of 8.0 ZnSe monolayers. The dot size and density in the fully ripened state are essentially independent of the initial ZnSe coverage.

Carbon-induced germanium dots: Kinetically-limited islanding process prevents coherent vertical alignment

Pregrowth of a small amount of C on a Si(001) substrate induces small Ge quantum dots. We present a structure where an initial layer of these C-induced Ge dots is followed by five layers of four monolayers Ge, each layer separated by a 2 nm thick Si spacer. Although the critical thickness for planar growth of a single Ge layer is not exceeded, a vertically aligned stack of Ge islands is formed. If we substitute the pure Ge layers by a fivefold stack of C-induced Ge dots, no vertical island correlation is observed. The phenomenon is explained by the strongly kinetically limited process of C-induced Ge island formation, itself. Photoluminescence experiments on stacks of 0.16 monolayers C/2.2 monolayers Ge dots, where we systematically varied the Si spacer thickness and the number of dot layers, suggest that the average dot size increases if the Si spacer is kept thinner than 10 nm.

Growth of self-assembled InAs and InAs xP 1− x dots on InP by metalorganic vapour phase epitaxy

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.

Light emission from individual InAs/GaAs self-assembled quantum dots excited by tunneling current injection

Emission from individual InAs quantum dots by tunneling current injection using a scanning tunneling microscope (STM) is reported. By scanning the STM tip above the self-assembled InAs quantum dot structures, a spatially resolved scanning tunneling luminescence (STL) image was measured, which contained some peaks originating from InAs quantum dots. The width of these peaks are of the same size as the average dot size, suggesting that the resolution of this method is much higher than 20 nm.

Intermixing induced changes in the radiative emission from III–V quantum dots

We have examined the effect of thermally induced interdiffusion on the luminescence emission from red and infrared emitting self-assembled III–V quantum dots. Three different combinations of dot/barrier materials have been investigated: InAlAs/AlGaAs, InGaAs/AlGaAs and InGaAs/GaAs. In all cases, thermal intermixing was found to result in significant blueshifts of the photoluminescence (PL) emission. In addition, narrowing of the linewidth of the inhomogeneously broadened PL peak was observed. Both effects were found to be strongly dependent on the material system and average dot size. InAlAs/AlGaAs quantum dots exhibited the greatest linewidth reduction after intermixing, indicating this to be a promising method of achieving narrower luminescence lines for devices such as red-emitting zero-dimensional lasers.

Highly uniform InGaAs/GaAs quantum dots (∼15 nm) by metalorganic chemical vapor deposition

We report the direct deposition of strained InGaAs-dot structures with a diameter of about 15 nm on GaAs surfaces by metalorganic chemical vapor deposition growth. High resolution scanning electron micrographs show highly uniform quantum-sized dots formed by the Stranski–Krastanow growth mode. The sharp photoluminescence emission band of buried dot structures indicates efficient carrier capture and a homogeneous heterointerface. The average dot size and area dot density can be controlled accurately by growth temperature, and InGaAs deposition thickness, respectively.

Synthesis, Structural Characterization, and Optical Spectroscopy of Close Packed CdSE Nanocrystallites

ABSTRACTWe describe a method to synthesize optical quality thin and thick films of close packed CdSe nanocrystallites (quantum dots). The average dot size is tunable from ∼12-150 Å in diameter with ∼3.5% rms size distribution. The identity of each particle and the high monodispersity of the sample are maintained in the films as revealed by small-angle x-ray scattering, transmission electron microscopy, and optical spectroscopy. We use small-angle x-ray diffraction patterns to obtain form factors for the individual dots and to generate radial distribution functions for the densely packed films. We observe the random close packing of dots in the solid state using transmission electron microscopy. Comparing optical spectra of particles close packed in a film with those of nanocrystallites dispersed in alkanes reveals similar optical resonances in absorption while the emission lineshape is modified and its position is red shifted. Differences in film emission are consistent with electronic energy transfer between close packed dots within the inhomogeneous distribution of the sample.

Effect of size nonuniformity on the absorption spectrum of a semiconductor quantum dot system

The interband optical absorption of a nonuniform semiconductor quantum dot system is calculated. The effect of dot size variation on the resolvability of the absorption peaks is estimated. The dots are assumed to be cubic, with a size distribution described by a Gaussian function. It is shown that the total absorption spectrum of such a dot system depends strongly on the dot size distribution described by the parameter ξ, the ratio of the standard deviation of the dot size to the average dot size of the system. The range of ξ values for which the absorption peaks are resolvable is given.