SiGe Quantum Dots Research Articles

Quantitative Model of Heterogeneous Nucleation and Growth of SiGe Quantum Dot Molecules

Using a multiscale approach combining continuum model with first-principles calculation, we develop a quantitative theoretical model for heterogeneous nucleation and the growth of a quantum dot molecule-a few islands "strain bonded" by a pit in heteroepitaxy of thin films, in contrast to homogeneous nucleation and growth of isolated strain islands on the surface. We show that the critical size and energy barrier for island nucleation next to a pit is substantially reduced with the increasing pit size, but the reduction approaches an upper bound of ~85% and ~72% for the size and barrier, respectively. Our model also predicts a self-limiting effect on island growth, resulting from an intriguing interplay between island-pit attraction and island-island repulsion, that drives the island size to increase linearly with the pit size, which explains a long-standing puzzle of experimental observation.

Excitation intensity dependence of photoluminescence spectra of SiGe quantum dots grown on prepatterned Si substrates: Evidence for biexcitonic transition

The pumping intensity ($I$) dependence of the photoluminescence (PL) spectra of perfectly laterally two-dimensionally ordered SiGe quantum dots on Si(001) substrates was studied. The PL results from recombinations of holes localized in the SiGe quantum dots and electrons localized due to the strain field in the surrounding Si matrix. The analysis of the spectra revealed several distinct bands, attributed to phonon-assisted recombination and no-phonon recombination of the excitonic ground state and of the excited excitonic states, which all exhibit a linear $I$ dependence of the PL intensity. At approximately $I\ensuremath{\ge}3\phantom{\rule{0.28em}{0ex}}\text{W}\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{\ensuremath{-}2}$, additional bands with a nearly quadratic $I$ dependence appear in the PL spectra, resulting from biexcitonic transitions. These emerging PL contributions shift the composite no-phonon PL band of the SiGe quantum dots to higher energies. The experimentally obtained energies of the no-phonon transitions are in good agreement with the exciton and biexciton energies calculated using the envelope function approximation and the configuration interaction method.

Electronic structure of double Ge quantum dots in Si

Theoretical investigations of the electronic structure of elastically stressed double Ge quantum dots in Si performed in the six-band kp approximation with the Bir-Pikus Hamiltonian and with the configuration interaction method are reviewed. The existence of the antibonding ground state of holes has been revealed. It has been found that, when quantum dots approach each other, the exchange energy of two-particle states has a minimum at the point of the intersection of bonding and antibonding levels; the singlet and triplet states at this point are degenerate. For the lowest spin singlet, it has been revealed that Coulomb correlations in the motion of two holes are manifested in the localization of the two-particle wavefunction at opposite quantum dots when the distance between the dots increases. It has been shown that the degree of entanglement of the singlet quantum states reaches 50% in the case of the manifestation of such spatial correlations.

Increased conductance of individual self-assembled GeSi quantum dots by inter-dot coupling studied by conductive atomic force microscopy.

The conductive properties of individual self-assembled GeSi quantum dots (QDs) are investigated by conductive atomic force microscopy on single-layer (SL) and bi-layer (BL) GeSi QDs with different dot densities at room temperature. By comparing their average currents, it is found that the BL and high-density QDs are more conductive than the SL and low-density QDs with similar sizes, respectively, indicating the existence of both vertical and lateral couplings between GeSi QDs at room temperature. On the other hand, the average current of the BL QDs increases much faster with the bias voltage than that of the SL QDs does. Our results suggest that the QDs’ conductive properties can be greatly regulated by the coupling effects and bias voltages, which are valuable for potential applications.

Formation of coupled three-dimensional GeSi quantum dot crystals

Coupled three-dimensional GeSi quantum dot crystals (QDCs) are realized by multilayer growth of quantum dots (QDs) on patterned SOI (001) substrates. Photoluminescence spectra of these QDCs show non-phonon (NP) recombination and its transverse-optical (TO) phonon replica of excitons in QDs. With increasing excitation power, peak energies of both the NP and TO peaks remain nearly constant and the width of the TO peak decreases. These anomalous features of the PL peaks are attributed to miniband formation due to strong coupling of the holes and the emergence of quasioptical phonon modes due to periodic scatters in ordered GeSi QDs.

Ge out diffusion effect on SiGe nanoring formation

Based on atomic force microscopy, high-angle annular dark-field scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, and Raman spectroscopy, Ge outdiffusion effects on SiGe quantum dots to form nanorings are studied using the ultrahigh vacuum chemical vapor deposition. The epitaxial Si layer grown on quantum dots with SiH4 and H2 precursors cannot cover the whole quantum dots, and the central area is passivated by the hydrogen. After annealing longer than 1 h at 500 °C in vacuum, the Ge atoms at the central area of quantum dots diffuse outward to form nanorings. However, the epitaxial Si layer grown with SiH4 and He precursors can cap all over the quantum dots, and the Si cap layer retards the Ge outdiffusion at the central area of quantum dots, resulting in 4 h annealing time. Moreover, for the uncapped quantum dots, the annealing in H2 also prevents the Ge outdiffusion at the central area and no nanorings are observed up to 5 h annealing at 500 °C.

Ordering of Epitaxial Quantum Dots on Nanomembranes

Semiconductor nanomembranes (NMs) provide fascinating opportunities to create unique structures and electronic devices owing to their mechanical flexibility. A fascinating question is whether the growth mediated by such flexibility can lead to the formation of ordered epitaxial surface nanostructures. By using computational modeling, we investigate the energetics of ordering of SiGe quantum dots (QDs) on both Si(001) and Si(111) NMs. We calculate the interaction energies for quantum dots grown on one side and on both sides of the NM and assembled in a square lattice for the Si(001) surface and in a hexagonal lattice for the Si(111) surface. Our calculations show that for QDs grown on the Si(001) NM the interaction energy possesses a minimum at a well-defined spacing only when the QDs are positioned on both sides in a square array and aligned along the [110] direction. The predicted QD ordering, spacing, and other features are in excellent agreement with recent experimental results. For QDs grown on the Si(111) NM, our calculations predict that ordered QDs can be achieved for both one-side and both-side growth, albeit with different QD spacings. The present work suggests that semiconductor NMs are a fascinating template for the self-assembled growth of ordered QDs.

Self-assembled SiGe quantum dots embedded in Ge matrix by Si ion implantation and subsequent annealing

We have fabricated SiGe quantum dots (QDs) by means of a two-step Si ion implantation followed by thermal rapid thermal annealing (RTA) method. SiGe QDs with the 4–6 nm diameter are formed uniformly in the near-surface region of Ge substrate. The RTA processes are performed at 800 and 900 °C for 15 s, respectively. Both experimental and theoretical analysis indicates that the higher temperature (900 °C) RTA can enhance the growth of SiGe QDs. Two photoluminescence peaks are observed near 572 and 581 nm at room temperature. The mechanism of the luminescence from SiGe QDs is discussed.

The microstructure of vertically coupled quantum dots ensembles by EXAFS spectroscopy

The microstructure of samples containing Ge/Si, GaN/AlN, and InAs/AlAs (Ge, GaN, and InAs quantum dots (QDs) in the Si, AlN, and AlAs matrices, respectively) multilayer heterostructures has been investigated by EXAFS spectroscopy. The effect of the effective thicknesses of Si (or AlN) barrier layers, number of Ge (or GaN) layers in a sandwich, and annealing temperature on the size and shape of Ge (or GaN) clusters, interfacial diffusion in these systems, and formation of three-dimensional ordered ensembles of QDs has been established.

Local interaction imaging by SiGe quantum dot probe

Local interaction imaging of cleaved InAs surface (110) using a high aspect ratio (HAR) SiGe quantum dot (QD) probe was demonstrated by frequency-modulation mode noncontact atomic force microscopy under atmospheric pressure. The local contrast image with pronounced brightness is mainly attributed to no contact potential difference between the HAR SiGe-QD probe apex and the sample surface, and low frequency-change of the HAR SiGe-QD probe.

Controlling thermal conductance through quantum dot roughening at interfaces

We examine the fundamental phonon mechanisms affecting the interfacial thermal conductance across a single layer of quantum dots (QDs) on a planar substrate. We synthesize a series of Ge${}_{x}$Si${}_{1\ensuremath{-}x}$ QDs by heteroepitaxial self-assembly on Si surfaces and modify the growth conditions to provide QD layers with different root-mean-square (rms) roughness levels in order to quantify the effects of roughness on thermal transport. We measure the thermal boundary conductance (${h}_{\mathrm{K}}$) with time-domain thermoreflectance. The trends in thermal boundary conductance show that the effect of the QDs on ${h}_{\mathrm{K}}$ are more apparent at elevated temperatures, while at low temperatures, the QD patterning does not drastically affect ${h}_{\mathrm{K}}$. The functional dependence of ${h}_{\mathrm{K}}$ with rms surface roughness reveals a trend that suggests that both vibrational mismatch and changes in the localized phonon transport near the interface contribute to the reduction in ${h}_{\mathrm{K}}$. We find that QD structures with rms roughnesses greater than 4 nm decrease ${h}_{\mathrm{K}}$ at Si interfaces by a factor of 1.6. We develop an analytical model for phonon transport at rough interfaces based on a diffusive scattering assumption and phonon attenuation that describes the measured trends in ${h}_{\mathrm{K}}$. This indicates that the observed reduction in thermal conductivity in SiGe quantum dot superlattices is primarily due to the increased physical roughness at the interfaces, which creates additional phonon resistive processes beyond the interfacial vibrational mismatch.

Inhomogeneous low temperature epitaxial breakdown during Si overgrowth of GeSi quantum dots

The low temperature epitaxial breakdown of inhomogeneously strained Si capping layers is investigated. By growing Si films on coherently strained GeSi quantum dot surfaces, we differentiate the effects of surface roughness, strain, and growth orientation on the mechanism of epitaxial breakdown. Using atomic force microscopy and high resolution cross-sectional transmission electron microscopy, we find that whereas local lattice strain of up to 2% has a negligible effect, growth on higher-index facets such as {113} significantly reduces the local breakdown thickness. Nanoscale growth mound formation is observed above all facet orientations. Because diffusion lengths depend directly on the surface orientation, we relate the variation in epitaxial thickness to the low temperature stability of specific growth facets and the average size of kinetically limited growth mounds.

Thermal conductivity of Si–Ge quantum dot superlattices

Quantum dot superlattices (QDSLs) have been proposed forthermoelectric applications as a means of increasing thermal conductivity,σ, and reducing the lattice thermal conductivity,κl, to increase the dimensionless thermoelectric figure of merit,ZT. To fully exploit the thermoelectric potential of Si–Ge quantum dot superlattices(QDSLs), we performed a thorough study of the structural interplay of QDSLs withκl using Green–Kubo theory and molecular dynamics. It was found that the resultingκl has less dependence on the arrangement of the dots than to dot size and spacing.In fact, regardless of arrangement or concentration, QDSLs show a minimumκl at a dot diameter of 1.4–1.6 nm and can reach values as low as 0.8–1.0 W mK−1, increasingZT by orders of magnitude over bulk Si and Ge. The drastic reduction of thermal conductivityin such a crystalline system is shown to be the result of both the stress caused by the dotsas well as the quality of the Si–Ge interface.

Ordered GeSi nanorings grown on patterned Si (001) substrates

An easy approach to fabricate ordered pattern using nanosphere lithography and reactive iron etching technology was demonstrated. Long-range ordered GeSi nanorings with 430 nm period were grown on patterned Si (001) substrates by molecular beam epitaxy. The size and shape of rings were closely associated with the size of capped GeSi quantum dots and the Si capping processes. Statistical analysis on the lateral size distribution shows that the high growth temperature and the long-term annealing can improve the uniformity of nanorings.PACS code1·PACS code2·moreMathematics Subject Classification (2000) MSC code1·MSC code2·more

Influencing factors on the size uniformity of self-assembled SiGe quantum rings grown bymolecular beam epitaxy

The size uniformity of self-assembled SiGe quantum rings, which are formed by cappingSiGe quantum dots with a thin Si layer, is found to be greatly influenced by the growthtemperature and the areal density of SiGe quantum dots. Higher growth temperaturebenefits the size uniformity of quantum dots, but results in low Ge concentration as well asasymmetric Ge distribution in the dots, which induces the subsequently formedquantum rings to be asymmetric in shape or even broken somewhere in the ridge ofrings. Low growth temperature degrades the size uniformity of quantum dots, andthus that of quantum rings. A high areal density results in the expansion andcoalescence of neighboring quantum dots to form a chain, rather than quantum rings.Uniform quantum rings with a size dispersion of 4.6% and an areal density of7.8×108 cm − 2 are obtained at the optimized growth temperature of640°C.

Bias-dependent conductive characteristics of individual GeSi quantum dots studied byconductive atomic force microscopy

The bias-dependent electrical characteristics of individual self-assembled GeSi quantumdots (QDs) are investigated by conductive atomic force microscopy. The results reveal thatthe conductive characteristics of QDs are strongly influenced by the applied bias. At low (−0.5 to − 2.0 V) andhigh (−2.5 to − 4.0 V) biases, the current distributions of individual GeSi QDs exhibit ring-like and disc-likecharacteristics respectively. The current of the QD’s central part increases more quicklythan that of the other parts as the bias magnitude increases. Histograms of the magnitudeof the current on a number of QDs exhibit the same single-peak feature at low biases, anddouble- or three-peak features at high biases, where additional peaks appear at large-currentlocations. On the other hand, histograms of the magnitude of the current on the wettinglayers exhibit the same single-peak feature for all biases. This indicates the conductivemechanism is significantly different for QDs and wetting layers. While the small-currentpeak of QDs can be attributed to the Fowler–Nordheim tunneling model at low biasesand the Schottky emission model at high biases respectively, the large-currentpeak(s) may be attributed to the discrete energy levels of QDs. The results suggestthe conductive mechanisms of GeSi QDs can be regulated by the applied bias.

Amorphous SiOx nanowires catalyzed by metallic Ge for optoelectronic applications

Amorphous SiOx nanowires, with diameters of ∼20nm and lengths of tens of μm, were grown from self-organized GeSi quantum dots or GeSi alloy epilayers on Si substrates. The morphologies and yield of these amorphous nanowires depend strongly upon the synthesis temperature. Comparative experiments indicate that the present SiOx nanowires are induced by metallic Ge as catalysts via the solid liquid solid growth mechanism. Two broad peaks centered at 410nm and 570nm were observed in photoluminescence spectrum, indicating that such SiOx nanowires have the potential applications in white light-emitting diodes, full-colour display, full-colour indicator and light sources.

气源MBE外延自组装GeSi量子点的光致荧光

Self-assembled GeSi quantum dots(QDs) were grown by gas source molecular beam epitaxy(MBE).Morphology and optical properties of the QDs were studied by atomic force microscopy(AFM) and photoluminescence(PL) spectra.QDs structure grown by gas source MBE at lower temperature showed a higher QDs coverage,lower defect and impurity density.Below 200 K,carriers are trapped in QDs as excitons with bonding energy of about 17 meV.The transport process changes as increasing the temperature to 200 K.By fitting the temperature dependence curves of PL integrated intensity,the activation energy of 129 meV representing the energy difference between the wetting layer and QDs was obtained.

Recent progress in infrared detector technologies

In the paper, fundamental and technological issues associated with the development and exploitation of the most advanced infrared detector technologies are discussed. In this class of detectors both photon and thermal detectors are considered. Special attention is directed to HgCdTe ternary alloys on silicon, type-II superlattices, uncooled thermal bolometers, and novel uncooled micromechanical cantilever detectors. Despite serious competition from alternative technologies and slower progress than expected, HgCdTe is unlikely to be seriously challenged for high-performance applications, applications requiring multispectral capability and fast response. However, the nonuniformity is a serious problem in the case of LWIR and VLWIR HgCdTe detectors. In this context, it is predicted that type-II superlattice system seems to be an alternative to HgCdTe in long wavelength spectral region. In well established uncooled imaging, microbolometer arrays are clearly the most used technology. Present state-of-the-art microbolometers are based on polycrystalline or amorphous materials, typically vanadium oxide (VO x ) or amorphous silicon (α-Si), with only modest temperature sensitivity and noise properties. Basic efforts today are mainly focused on pixel reduction and performance enhancement. Attractive alternatives consist of low-resistance α-SiGe monocrystalline SiGe quantum wells or quantum dots. In spite of successful commercialization of uncooled microbolometers, the infrared community is still searching for a platform for thermal imagers that combine affordability, convenience of operation, and excellent performance. Recent advances in MEMS systems have lead to the development of uncooled IR detectors operating as micromechanical thermal detectors. Between them the most important are biomaterial microcantilevers.

Formation of Nanopits in Si Capping Layers on SiGe Quantum Dots.

In-situ annealing at a high temperature of 640°C was performed for a low temperature grown Si capping layer, which was grown at 300°C on SiGe self-assembled quantum dots with a thickness of 50 nm. Square nanopits, with a depth of about 8 nm and boundaries along 〈110〉, are formed in the Si capping layer after annealing. Cross-sectional transmission electron microscopy observation shows that each nanopit is located right over one dot with one to one correspondence. The detailed migration of Si atoms for the nanopit formation is revealed by in-situ annealing at a low temperature of 540°C. The final well-defined profiles of the nanopits indicate that both strain energy and surface energy play roles during the nanopit formation, and the nanopits are stable at 640°C. A subsequent growth of Ge on the nanopit-patterned surface results in the formation of SiGe quantum dot molecules around the nanopits.