Surface morphology during multilayer epitaxial growth of Ge(001)

PbS Quantum Dots Ink with Months-Long Shelf-Lifetime Enabling Scalable and Efficient Short-Wavelength Infrared Photodetectors

The phase-transfer ligand exchange of PbS quantum dots (QDs) has substantially simplified device fabrication giving hope for future industrial exploitation. However, this technique when applied to QDs of large size (>4 nm) gives rise to inks with poor colloidal stability, thus hindering the development of QDs photodetectors in short-wavelength infrared range. Here, it is demonstrated that methylammonium lead iodide ligands can provide sufficient passivation of PbS QDs of size up to 6.7 nm, enabling inks with a minimum of ten-week shelf-life time, as proven by optical absorption and solution-small angle X-ray scattering. Furthermore, the maximum linear electron mobility of 4.7 × 10-2 cm2 V-1 s-1 is measured in field-effect transistors fabricated with fresh inks, while transistors fabricated with the same solution after ten-week storage retain 74% of the average starting electron mobility, demonstrating the outstanding quality both of the fresh and aged inks. Finally, photodetectors fabricated via blade-coating exhibit 76% external quantum efficiency at 1300 nm and 1.8 × 1012 Jones specific detectivity, values comparable with devices fabricated using ink with lower stability and wasteful methods such as spin-coating.

Long-Wavelength Lead Sulfide Quantum Dots Sensing up to 2600 nm for Short-Wavelength Infrared Photodetectors

Lead sulfide nanoparticles (PbS NPs) are used in the short-wavelength infrared photodetectors because of their excellent photosensitivity, band gap tunability, and solution processability. It has been a challenge to synthesize high-quality PbS NPs with an absorption peak beyond 2000 nm. In this work, using PbS seed crystals with an absorption peak at 1960 nm, we report a successful synthesis of very large monodispersed PbS NPs having a diameter up to 16 nm by multiple injections. The resulting NPs have an absorption peak over 2500 nm with a small full width at half-maximum of 24 meV. To demonstrate the applications of such large quantum dots (QDs), broadband heterojunction photodetectors are fabricated with the large PbS QDs of an absorption peak at 2100 nm. The resulting devices have an external quantum efficiency (EQE) of 25% (over 50% internal quantum efficiency) at 2100 nm corresponding to a responsivity of 0.385 A/W and an EQE of ∼60% in the visible range.

Defect-free high Sn-content GeSn on insulator grown by rapid melting growth

GeSn is an attractive semiconductor material for Si-based photonics. However, large lattice mismatch between GeSn and Si and the low solubility of Sn in Ge limit its development. In order to obtain high Sn-content GeSn on Si, it is normally grown at low temperature, which would lead to inevitable dislocations. Here, we reported a single-crystal defect-free graded GeSn on insulator (GSOI) stripes laterally grown by rapid melting growth (RMG). The Sn-content reaches to 14.2% at the end of the GSOI stripe. Transmission electron microscopy observation shows the GSOI stripe without stacking fault and dislocations. P-channel pseudo metal-oxide-semiconductor field effect transistors (MOSFETs) and metal-semiconductor-metal (MSM) Schottky junction photodetectors were fabricated on these GSOIs. Good transistor performance with a low field peak hole mobility of 402 cm²/Vs is obtained, which indicates a high-quality of this GSOI structure. Strong near-infrared and short-wave infrared optical absorption of the MSM photodetectors at 1550 nm and 2000 nm were observed. Owing to high Sn-content and defect-free, responsivity of 236 mA/W@-1.5 V is achieved at 1550 nm wavelength. In addition, responsivity reaches 154 mA/W@-1.5 V at 2000 nm with the optical absorption layer only 200 nm-thick, which is the highest value reported for GeSn junction photodetectors until now.

Beneficial defects: exploiting the intrinsic polishing-induced wafer roughness for the catalyst-free growth of Ge in-plane nanowires

We outline a metal-free fabrication route of in-plane Ge nanowires on Ge(001) substrates. By positively exploiting the polishing-induced defects of standard-quality commercial Ge(001) wafers, micrometer-length wires are grown by physical vapor deposition in ultra-high-vacuum environment. The shape of the wires can be tailored by the epitaxial strain induced by subsequent Si deposition, determining a progressive transformation of the wires in SiGe faceted quantum dots. This shape transition is described by finite element simulations of continuous elasticity and gives hints on the equilibrium shape of nanocrystals in the presence of tensile epitaxial strain.

PACS

81.07.Gf; 68.35.bg; 68.35.bj; 62.23.Eg.

Reverse epitaxy of Ge: ordered and faceted surface patterns

Normal incidence ion irradiation at elevated temperatures, when amorphization is prevented, induces novel nanoscale patterns of crystalline structures on elemental semiconductors by a reverse epitaxial growth mechanism: on Ge surfaces irradiation at temperatures above the recrystallization temperature of 250 °C leads to self-organized patterns of inverse pyramids. Checkerboard patterns with fourfold symmetry evolve on the Ge (100) surface, whereas on the Ge (111) surface, isotropic patterns with a sixfold symmetry emerge. After high-fluence irradiations, these patterns exhibit well-developed facets. A deterministic nonlinear continuum equation accounting for the effective surface currents due to an Ehrlich-Schwoebel barrier for diffusing vacancies reproduces remarkably well our experimental observations.

Spectral phonon scattering from sub-10 nm surface roughness wavelengths in metal-assisted chemically etched Si nanowires

Frequency dependence in phonon surface scattering is a debated topic in fundamental phonon physics. Recent experiments and theory suggest such a phenomenon, but an independent agreement between the two remains elusive. We report low-temperature dependence of thermal conductivity in silicon nanowires fabricated using a two-step, metal-assisted chemical etch. By reducing etch rates down to 0.5 nm/s from the typical >100 nm/s, we report controllable roughening of nanowire surfaces and selectively focus on moderate roughness scales rather than the extreme scales investigated previously. This critically enables direct comparison with perturbation-based spectral scattering theory. Using experimentally characterized surface roughness, we show that a multiple scattering theory provides excellent agreement and explanation of the observed low-temperature dependence of rough surface nanowires. The theory does not employ any fitting parameters. A 5-10 nm roughness correlation length is typical in metal-assisted chemical etching and resonantly scatters dominant phonons in silicon, leading to the observed ~T(1.6-2.4) behavior. Our work provides fundamental and quantitative insight into spectral phonon scattering from rough surfaces. This advances applications of nanowires in thermoelectric energy conversion.

Nanopatterning of Si(001) for bottom-up fabrication of nanostructures

The epitaxial growth of Si on Si(001) under conditions at which the (2 × n) superstructure is forming has been investigated by scanning tunneling microscopy and Monte Carlo simulations. Our experiments reveal a periodic change of the surface morphology with the surface coverage of Si. A regular (2 × n) stripe pattern is observed at coverages of 0.7-0.9 monolayers that periodically alternates with less dense surface structures at lower Si surface coverages. The MC simulations show that the growth of Si is affected by step-edge barriers, which favors the formation of a rather uniform two-dimensional framework-like configuration. Subsequent deposition of Ge onto the (2 × n) stripe pattern yields a dense array of small Ge nanostructures.

High purity chemical etching and thermal passivation process for Ge(001) as nanostructure template

An advanced two-step cleaning process of the Ge(001) surface for nanoscience requirements is presented. First, wet-chemical etching with a variant of the Piranha solution (H(2)SO(4), H(2)O(2), H(2)O) is used to remove contaminants as well as the native oxide layer. Second, passivation of the surface is achieved by a rapid thermal oxidation step, leading to a homogeneous protective oxide layer. The thickness of the oxide layer is tuned to be thick enough to protect the surface, yet thin enough to be completely removed by thermal treatment in ultra-high vacuum. The application of this recipe results in an outstandingly clean and atomically flat surface, with carbon contamination at the detection limit of x-ray photoelectron spectroscopy. Scanning tunneling microscopy and electron diffraction reveal a long range ordered surface with typical terrace diameters of ~100 nm, suitable for the growth of atomic-scale nanostructures.

Preparation of the Ge(001) surface towards fabrication of atomic-scale germanium devices

We demonstrate the preparation of a clean Ge(001) surface with minimal roughness (RMS ~0.6 Å), low defect densities (~0.2% ML) and wide mono-atomic terraces (~80-100 nm). We use an ex situ wet chemical process combined with an in situ anneal treatment followed by a homoepitaxial buffer layer grown by molecular beam epitaxy and a subsequent final thermal anneal. Using scanning tunneling microscopy, we investigate the effect on the surface morphology of using different chemical reagents, concentrations as well as substrate temperature during growth. Such a high quality Ge(001) surface enables the formation of defect-free H-terminated Ge surfaces for subsequent patterning of atomic-scale devices by scanning tunneling lithography. We have achieved atomic-scale dangling bond wire structures 1.6 nm wide and 40 nm long as well as large, micron-size patterns with clear contrast of lithography in STM images.

Scaling and Coarsening in Epitaxial Growth

The results of recent theoretical and simulational studies of submonolayer and multilayer homoepitaxial growth are discussed. In the submonolayer regime, the results of kinetic Monte Carlo simulations are presented and shown to provide a quantitative explanation for the variation of the submonolayer island density, critical island size, island-size distribution and morphology as a function of temperature and deposition rate found in recent experiments. In multilayer growth, a realistic model for homoepitaxial growth on fcc and bcc lattices which takes into account the correct crystal structure is reviewed. The effects of instabilities which lead to mound formation and coarsening are discussed and a unified picture of the effects of attractive and repulsive interactions at ascending and descending steps on surface morphology and island nucleation is presented. An accurate prediction of the observed mound angle for Fe/Fe(100) deposition is obtained analytically and by kinetic Monte Carlo simulations. The general dependence of the mound angle, and mound coarsening behavior on temperature, deposition rate, and strength of the step barrier in bcc(100) and fcc(100) growth is also presented and compared with recent experiments.

Extensive studies on linear growth processes with spatiotemporally correlated noise in arbitrary substrate dimensions

An extensive analytical and numerical study on a class of growth processes with spatiotemporally correlated noise in arbitrary dimension is undertaken. In addition to the conventional investigation on the interface morphology and interfacial widths, we pay special attention to exploring the characteristics of the slope-slope correlation function S(r,t) and the [Q]-th degree residual local interfacial width w[Q](l,t), whose importance has been somewhat overlooked in the literature. Based on the above analysis, we give a plausible theoretical explanation about the various experimental observations of kinetically and thermodynamically unstable surface growth. Furthermore, through explicit examples, we show that the statistical methods of calculating the exponents (including the dynamic exponent z, the global roughness exponent α, and the local roughness exponent α(loc)), based on the scaling of S(r,t) and w[Q](l,t), are very reliable and rarely influenced by the finite time and/or finite-size effects. Another important issue we focus on in this paper is related to numerical calculation. For the specific class of growth processes discussed in this paper, we develop a very efficient and accurate algorithm for numerical calculation of the dynamics of interface configuration, the structure factor, the various correlation functions, the interfacial width and its variants in arbitrary dimensions, even with very large system size and very late time. The proposed systematical algorithm can be easily generalized to other linear processes and some special nonlinear processes.

Scale decomposition of molecular beam epitaxy

In this work, a study of epitaxial growth was carried out by means of the wavelets formalism. We showed the existence of a dynamic scaling form in a wavelet discriminated linear molecular beam epitaxy (MBE) equation where diffusion and noise are the dominant effects. We determined simple and exact scaling functions involving the scale of the wavelets when the system size is set to infinity. Exponents were determined for both correlated and uncorrelated noise. The wavelet methodology was applied to a computer model simulating linear epitaxial growth; the results showed very good agreement with analytical formulation. We also considered epitaxial growth with the additional Ehrlich-Schwoebel effect. We characterized the coarsening of mounds formed on the surface during the nonlinear phase using the wavelet power spectrum. The latter has an advantage over other methods, in the sense that one can track the coarsening in both frequency (or scale) space and real space simultaneously. Wavelets analysis also provides a quantitative tool for the characterization of the mounded surfaces through its concise scale discrimination. We showed that the averaged wavelet power spectrum (also called scalegram) over all the positions on the surface profile identified the existence of a dominant scale a(*), which increases with time following a power law relation of the form a(*)∼t(n), where [Formula: see text].

Renormalization of stochastic lattice models: epitaxial surfaces

We present the application of a method [C. A. Haselwandter and D. D. Vvedensky, Phys. Rev. E 76, 041115 (2007)] for deriving stochastic partial differential equations from atomistic processes to the morphological evolution of epitaxial surfaces driven by the deposition of new material. Although formally identical to the one-dimensional (1D) systems considered previously, our methodology presents substantial additional technical issues when applied to two-dimensional (2D) surfaces. Once these are addressed, subsequent coarse-graining is accomplished as before by calculating renormalization-group (RG) trajectories from initial conditions determined by the regularized atomistic models. Our applications are to the Edwards-Wilkinson (EW) model [S. F. Edwards and D. R. Wilkinson, Proc. R. Soc. London, Ser. A 381, 17 (1982)], the Wolf-Villain (WV) model [D. E. Wolf and J. Villain, Europhys. Lett. 13, 389 (1990)], and a model with concurrent random deposition and surface diffusion. With our rules for the EW model no appreciable crossover is obtained for either 1D or 2D substrates. For the 1D WV model, discussed previously, our analysis reproduces the crossover sequence known from kinetic Monte Carlo (KMC) simulations, but for the 2D WV model, we find a transition from smooth to unstable growth under repeated coarse-graining. Concurrent surface diffusion does not change this behavior, but can lead to extended transient regimes with kinetic roughening. This provides an explanation of recent experiments on Ge(001) with the intriguing conclusion that the same relaxation mechanism responsible for ordered structures during the early stages of growth also produces an instability at longer times that leads to epitaxial breakdown. The RG trajectories calculated for concurrent random deposition and surface diffusion reproduce the crossover sequences observed with KMC simulations for all values of the model parameters, and asymptotically always approach the fixed point corresponding to the equation proposed by Villain [J. Phys. I 1, 19 (1991)] and by Lai and Das Sarma [Phys. Rev. Lett. 66, 2899 (1991)]. We conclude with a discussion of the application of our methodology to other growth settings.

Epitaxial growth with pulsed deposition: submonolayer scaling and Villain instability

It has been observed experimentally that under certain conditions, pulsed laser deposition (PLD) produces smoother surfaces than ordinary molecular beam epitaxy (MBE). So far, the mechanism leading to the improved quality of surfaces in PLD is not yet fully understood. In the present work, we investigate the physical properties of a simple model for PLD, in which the transient mobility of adatoms and diffusion along edges is neglected. Analyzing the crossover from MBE to PLD, the scaling properties of the time-dependent nucleation density as well as the influence of Ehrlich-Schwoebel barriers, we find that there is indeed a range of parameters, where the surface quality in PLD is better than in MBE. However, since the improvement is weak and occurs only in a small range of parameters we conclude that deposition in pulses alone cannot explain the experimentally observed smoothness of PLD-grown surfaces.

Interfacial coarsening dynamics in epitaxial growth with slope selection

We investigate interfacial dynamics of molecular-beam epitaxy (MBE) growth in the presence of instabilities inducing formation of pyramids. We introduce a kinetic scaling theory which provides an analytic understanding of the coarsening dynamics laws observed in numerous experiments and simulations of the MBE. We address MBE growth on crystalline surfaces with different symmetries in order to explain experimentally observed differences between the growth on (111) and (001) surfaces and understand the coarsening exponents measured on these surfaces. We supplement our kinetic scaling theory by numerical simulations which document that the edges of the pyramids, forming a network across the growing interface, are essential for qualitative understanding of the coarsening dynamics of molecular-beam epitaxy.

Reentrant mound formation in GaAs(001) homoepitaxy observed by ex situ atomic force microscopy

A study of the surface morphology of homoepitaxial GaAs(001) by means of ex situ atomic force microscopy in air reveals the reentrance of mounding behavior at low growth temperatures. A transition from statistical roughening to organized mound formation is observed as the growth temperature is reduced. We show by means of growth simulations that the observed morphology is compatible with anisotropic adatom diffusion in the presence of an Ehrlich-Schwoebel barrier. The mechanism leading to this kind of adatom kinetics at low temperatures is interpreted in terms of surfactant acting arsenic condensing on the surface.

Fast coarsening in unstable epitaxy with desorption

Homoepitaxial growth is unstable towards the formation of pyramidal mounds when interlayer transport is reduced due to activation barriers to hopping at step edges. Simulations of a lattice model and a continuum equation show that a small amount of desorption dramatically speeds up the coarsening of the mound array, leading to coarsening exponents between 1/3 and 1/2. The underlying mechanism is the faster growth of larger mounds due to their lower evaporation rate.

Numerical test of the damping time of layer-by-layer growth on stochastic models

We perform Monte Carlo simulations on stochastic models such as the Wolf-Villain (WV) model and the Family model in a modified version to measure the mean separation l between islands in a submonolayer regime and the damping time t* of layer-by-layer growth oscillations in one dimension. The stochastic models are modified, allowing for diffusion within interval r upon deposition. It is found numerically that the mean separation and the damping time depend on the diffusion interval r, leading to the fact that the damping time is related to the mean separation as t* approximately l(4/3) for the WV model and t* approximately l(2) for the Family model. The numerical results are in excellent agreement with recent theoretical predictions.

Atomistic Processes in the Early Stages of Thin-Film Growth

Growth of thin films from atoms deposited from the gas phase is intrinsically a nonequilibrium phenomenon governed by a competition between kinetics and thermodynamics. Precise control of the growth and thus of the properties of deposited films becomes possible only after an understanding of this competition is achieved. Here, the atomic nature of the most important kinetic mechanisms of film growth is explored. These mechanisms include adatom diffusion on terraces, along steps, and around island corners; nucleation and dynamics of the stable nucleus; atom attachment to and detachment from terraces and islands; and interlayer mass transport. Ways to manipulate the growth kinetics in order to select a desired growth mode are briefly addressed.