QD Height Research Articles

Self-assembling mechanism of Si-QDs on thermally grown SiO2

The self-assembling formation of Si quantum dots (Si-QDs) on as-grown SiO2 layers was shown by controlling the early stages of low-pressure chemical vapor deposition of SiH4. The QD height and radius distributions assessed by atomic force microscopy and scanning electron microscopy images revealed that the Si-QDs become hemispherical due to them being rate-limited by aggregation, which reduces the surface energy at substrate temperatures above ∼580 °C. Moreover, at temperatures below ∼580 °C, semi-ellipsoidal shaped Si-QDs are formed because the precursor supply is a dominant factor.

Evaluation of different capping strategies in the InAs/GaAs QD system: Composition, size and QD density features

In this work, two different strategies to preserve InAs/GaAs QDs against decomposition during the capping process have been compared structurally and optically. They are based on: (i) the control of the growth parameters of the capping layer (CL), such as growth rate, and (ii) the nature of the CL, such as the use of GaAsSb strain-reducing layers (SRL). For this, we have statistically determined the average size, composition and areal density in sample populations of dozens of QDs as well as the wetting layer features. Faster growth rates and the presence of Sb in the CL results in an increase of both average QD height as well as the In content but the thorough comparison among all the samples allowed us to draw the following important inferences. i) QD volume is a better parameter than QD height to assess QD decomposition and PL redshift; ii) higher QD volumes are not always linked to higher In contents and iii) a reduction in the QD density is observed in both strategies related to the increase of the average volume. The results are discussed in terms of the mechanisms that might operate during the capping process, such as surface coverage, intermixing or segregation.

Effect of parameter variations on the static and dynamic behaviour of a self-assembled quantum-dot laser using circuit-level modelling

We report a new circuit model for a self-assembled quantum-dot (SAQD) laser made of InGaAs/GaAs structures. The model is based on the excited state and standard rate equations, improves the previously suggested circuit models and also provides and investigates the performance of this kind of laser. The carrier dynamic effects on static and dynamic characteristics of a SAQD laser are analysed. The phonon bottleneck problem is simulated. Quantum-dot lasers are shown to be quite sensitive to the crystal quality outside and inside quantum dots. The effects of QD coverage factor, inhomogeneous broadening, the physical source of which is the size fluctuation of quantum dots formed by self-assembly of atoms, and cavity length on the SAQD laser characteristics are analysed. The results of simulation show that an increase in the cavity length and in the QD coverage factor results in the growth of the output power. On the other hand, an increase in the coverage factor and a degradation of inhomogeneous broadening lead to an increase in the modulation bandwidth. The effect of the QD height (cylindrical shape) and stripe width of the laser cavity on QD laser modulation is also analysed.

Interband optical transitions of a strained InxGa1−xAs/GaAs quantum dot/wetting layer with various In mole fractions

In this paper, a numerical approach for calculating interband optical transitions of an InxGa1−xAs/GaAs conical shaped quantum dot is presented with different In mole fraction. The electronic structure was calculated by solving one-band effective-mass Schrödinger equation taking into account the strain effects. The simulations are performed by the employing finite element method in cylindrical co-ordinates. The wetting layer thickness and QD height effects on the S and P bands energies are also investigated to be a guidance of optimal structure design for optoelectronic devices. The obtained numerical results indicate a red shift in photoluminescence spectra with increasing wetting layer thickness which are in a good agreement with experimental data.

Proposal of high efficiency solar cells with closely stacked InAs/In0.48Ga0.52P quantum dot superlattices: Analysis of polarized absorption characteristics via intermediate–band

We present a theoretical study of the electronic structures and polarized absorption properties of quantum dot superlattices (QDSLs) using wide–gap matrix material, InAs/In0.48Ga0.52P QDSLs, for realizing intermediate–band solar cells (IBSCs) with two–step photon–absorption. The plane–wave expanded Burt–Foreman operator ordered 8–band k·p theory is used for this calculation, where strain effect and piezoelectric effect are taken into account. We find that the absorption spectra of the second transitions of two–step photon–absorption can be shifted to higher energy region by using In0.48Ga0.52P, which is lattice–matched material to GaAs substrate, as a matrix material instead of GaAs. We also find that the transverse magnetic polarized absorption spectra in InAs/In0.48Ga0.52P QDSL with a separate IB from the rest of the conduction minibands can be shifted to higher energy region by decreasing the QD height. As a result, the second transitions of two–step photon–absorption by the sunlight occur efficiently. These results indicate that InAs/In0.48Ga0.52P QDSLs are suitable material combination of IBSCs toward the realization of ultrahigh efficiency solar cells.

Growth of ZnO quantum dots on Si nano ripples

Substrates with nano-scale ripples are excellent templates for the deposition of semiconductor nanostructures. We have prepared quasi periodical nano-scale ripples on Si (100) substrates with spatial wavelength λ from ~ 70 to ~ 150 nm by ion beam sputtering. ZnO QDs with diameters from 17 to ~ 30 nm and heights from 2 to ~ 4 nm have been successfully deposited by reactive ion beam sputter deposition. The QD size and distribution were found to be dependent both on growth conditions and spatial wavelength of the nano-scale ripple. On substrates with λ ~ 150 nm, ZnO QDs were distributed evenly across the wafer, while on substrates with λ ~ 70 nm, ZnO QDs were preferentially located along the crest of the nano-scale ripples. As the QD height decreases from ~ 4 to 2 nm, room temperature photoluminescence UV emission energy blue shifts by 80 meV. Possible sources of the blue shift are presented.

Physics of novel site controlled InGaAs quantum dots on (1 1 1) oriented substrates

Recent work has shown that site-controlled dots (QD) grown on (1 1 1)B GaAs substrates, pre-patterned with tetrahedral pyramidal recesses (Baier et al., 2006) [1], (Pelucchi et al., 2007) [2], (Zhu et al., 2007) [3] are suitable for the generation of single and entangled photons (Young et al., 2009) [4]. We recently introduced InGaAs/GaAs site controlled QD structures which demonstrated record breaking spectral purity, and we showed that increasing the indium concentration of the active region allows easy tunability of the emission wavelength (Mereni et al., 2009) [5,6]. We present here the first theoretical analysis of the emission energies and optical properties of this system as a function of QD height and In concentration. We model the dots using an 8 band k.p theory chosen to provide the best convergence and performance for structures oriented specifically along the (1 1 1) crystallographic direction.

Direct comparison of recombination dynamics in cubic and hexagonal GaN/AlN quantum dots

We report on time-integrated and -resolved photoluminescence (PL) data on self-assembled GaN quantum dots (QD's) embedded in AlN, in both cubic [zinc-blende (ZB)] and hexagonal [wurtzite (Wz)] crystallographic phases. The comparison of the optical properties of ZB and Wz nitride QD's allows us to distinguish pure dimensionality effects from the influence of the large polarization-induced electric fields present in the Wz nanostructures. Specifically, the PL energy of the ZB QD's is always higher than the bulk cubic GaN band-gap energy, in contrast to the Wz QD's where a 7-MV/cm polarization field gives rise to below-gap PL emission for sufficiently large QD's. As a further consequence of the internal field, the low-temperature PL decay times of the Wz QD's are significantly longer than the ZB ones, and increase strongly with the QD height in contrast to the ZB ones, which exhibit only a small size dependence. For both types of QD's, the PL intensity is found to be weakly dependent on temperature, underscoring the strong zero-dimensional exciton localization in the GaN/AlN system. In spite of the strong localization, however, we show that the nonradiative channels cannot be neglected and have a significant contribution in the PL decay time for both ZB and Wz QD's.

Growth and optical characterizations of InAs quantum dots on InP substrate: towards a 1.55μm quantum dot laser

Abstract The surface morphology changes associated with the formation of InAs/InP(3 1 1)B quantum dots grown according to a proposed growth procedure (double cap) have been investigated using atomic force microscopy (AFM). We show that the deposit of an InP capping layer thinner than the highest dot, followed by the annealing under phosphorous overpressure, leads to the smoothing of the growth front. It induces a drastic reduction of the dot height and of its dispersion. Transmission electron microscopy and photoluminescence experiments show a clear correlation between the QD height and the deposited InP layer thickness. Using this modified capping growth process, a 1.55 μm emission wavelength with a narrower PL linewidth (50 meV) is achieved. Finally, we report ground state laser emission from QDs at 1.52 μm, which supports the DC process for the fabrication of QD devices emitting in the 1.5 μm range.

Origin of the inhomogenous broadening and alloy intermixing in InAs/GaAs self-assembled quantum dots

The photoluminescence (PL) spectra of a single-layer and a multilayer sample of self-assembled InAs/GaAs quantum dots (QD's) intermixed by thermal annealing at various temperatures, have been investigated. Intermixing is found to change both the optical transition energy and the intersublevel spacing of the QD energy levels. The linewidth (which is due to inhomogeneous broadening) of the PL emission peaks of both samples, are very similar, hence showing that the intermixing process is very homogeneous over 25 layers of QD's, whatever the annealing technique used. Moreover it is observed that the width of the PL peaks decreases for increasing interdiffusion down to about 5 meV for the ground-state transition of the multilayer sample. A reduction of the peak width is also observed for higher-energy states within the same ensemble of dots. The present paper shows that this effect can only be explained by some variation of the effective confining potential. Theoretical calculations have shown that the QD height, rather than the diameter, the volume, the composition or the strain, appears to be the key parameter that controls the sharpness of the PL linewidths in the investigated samples. Our model allows the identification of the main mechanisms involved in the inhomogeneous broadening of the optical transitions for the InAs/GaAs QD system.