Zhenyang Zhong

Site-controlled and size-homogeneous Ge islands on prepatterned Si (001) substrates

We report on a combination of lithography and self-assembly techniques which results in long-range two-dimensionally ordered Ge islands. Island lattices with perpendicular but also with obliquely oriented unit vectors were realized. Quantitative analysis of the island topographies demonstrates that the size dispersion of these islands is smaller than that found on flat substrates. Furthermore, island formation on the patterned substrates is observed for a smaller amount of Ge deposition. However, with further Ge deposition an increasing amount is incorporated into the sidewalls.

Two-dimensional periodic positioning of self-assembled Ge islands on prepatterned Si (001) substrates

Two-dimensional (2D) periodic arrays of Ge islands were realized on prepatterned Si (001) substrates by solid-source molecular-beam epitaxy. Atomic-force microscopy images demonstrate that the Ge islands are formed in the 2D laterally ordered pits of patterned substrates. The 2D periodicity of the substrate pattern is replicated throughout a stack of Ge island layers by strain-driven vertical ordering. Photoluminescence spectra of the ordered Ge islands show well-resolved peaks of the no-phonon signal and the transverse-optical phonon replica. These peaks are observed at nearly the same energy as those of random Ge islands deposited under the same conditions on unpatterned Si substrates.

Positioning of self-assembled Ge islands on stripe-patterned Si(001) substrates

Self-assembled Ge islands were grown on stripe-patterned Si(001) substrates by solid source molecular beam epitaxy. The surface morphology obtained by atomic force microscopy and cross-sectional transmission electron microscopy images shows that the Ge islands are preferentially grown at the sidewalls of pure Si stripes along the [−110] direction at 650 °C or along the trenches, whereas most of the Ge islands are formed on the top terrace when the patterned stripes are covered by a strained GeSi buffer layer. Reducing the growth temperature to 600 °C results in a nucleation of Ge islands both on the top terrace and at the sidewall of pure Si stripes. A qualitative analysis, based on the growth kinetics, demonstrates that the step structure of the stripes, the external strain field, and the local critical wetting layer thickness for the islands formation contribute to the preferential positioning of Ge islands on the stripes.

Ge island formation on stripe-patterned Si(001) substrates

Self-assembled Ge islands were grown by solid-source molecular-beam epitaxy on the submicron stripe-patterned Si(001) substrates at 650 °C. Atomic-force microscopy shows that the Ge islands grow preferentially at the sidewall of the Si stripes, oriented along the [−110] direction. The migration of the Ge adatoms from the top terrace down to the sidewall accounts for the island formation at the sidewall of the stripes. However, most of the Ge islands are formed on the top terraces when the patterned stripes are covered by a strained GeSi multilayer buffer prior to Ge island growth. Apparently, the strained buffer layer acts as a stressor and contributes to the preferential growth of islands on the top terrace.Self-assembled Ge islands were grown by solid-source molecular-beam epitaxy on the submicron stripe-patterned Si(001) substrates at 650 °C. Atomic-force microscopy shows that the Ge islands grow preferentially at the sidewall of the Si stripes, oriented along the [−110] direction. The migration of the Ge adatoms from the top terrace down to the sidewall accounts for the island formation at the sidewall of the stripes. However, most of the Ge islands are formed on the top terraces when the patterned stripes are covered by a strained GeSi multilayer buffer prior to Ge island growth. Apparently, the strained buffer layer acts as a stressor and contributes to the preferential growth of islands on the top terrace.

The fabrication and application of patterned Si(001) substrates with ordered pits via nanosphere lithography

A new scalable approach has been developed for fabricating large-scale pit patterns with controllable periodicity on Si(001) substrates. The fabrication processes start with self-assembling a monolayer of polystyrene (PS) spheres on hydrogenated Si(001) substrates. A novel net-like mask in combination of the Au pattern thermally evaporated in between the PS spheres and the Au-catalyzed SiO(2) around them is naturally formed. After selective etching of Si by KOH solution, two-dimensionally ordered pits with a periodicity equal to the diameter of the PS spheres in the range from micrometers to less than 100 nm can be obtained. The shape of the pits can be modulated by controlling the chemical etching time. Such pit-patterned Si substrates facilitate the formation of ordered Si-based nanostructures, such as ordered self-assembled GeSi quantum dots, by deposition of Ge using molecular beam epitaxy.

Effect of overgrowth temperature on shape, strain, and composition of buried Ge islands deduced from x-ray diffraction

We have investigated a series of samples containing SiGe islands capped at different growth temperatures. A layer of islands formed by deposition of 5 ML of pure Ge was capped with Si, deposited at temperatures of 460, 540, and 630 °C, respectively. The Ge composition profile and the shape of the buried islands are deduced from x-ray diffraction data. While for capping at high substrate temperatures a significant dilution of the Ge content and a flattening of the islands occur, capping at low temperatures maintains a high aspect ratio and a high Ge content of the islands. The maximum in-plane strain in the island remains as high as 0.005 for capping at low temperatures.

Large-Area Au-Nanoparticle-Functionalized Si Nanorod Arrays for Spatially Uniform Surface-Enhanced Raman Spectroscopy

In this study, large-area hexagonal-packed Si nanorod (SiNR) arrays in conjunction with Au nanoparticles (AuNPs) were fabricated for surface-enhanced Raman spectroscopy (SERS). We have achieved ultrasensitive molecular detection with high reproducibility and spatial uniformity. A finite-difference time-domain simulation suggests that a wide range of three-dimensional electric fields are generated along the surfaces of the SiNR array. With the tuning of the gap and diameter of the SiNRs, the produced long decay length (>130 nm) of the enhanced electric field makes the SERS substrate a zero-gap system for ultrasensitive detection of large biomolecules. In the detection of R6G molecules, our SERS system achieved an enhancement factor of >107 with a relative standard deviation as small as 3.9-7.2% over 30 points across the substrate. More significantly, the SERS substrate yielded ultrasensitive Raman signals on long amyloid-β fibrils at the single-fibril level, which provides promising potentials for ultrasensitive detection of amyloid aggregates that are related to Alzheimers disease. Our study demonstrates that the SiNRs functionalized with AuNPs may serve as excellent SERS substrates in chemical and biomedical detection.

Temperature dependence of ordered GeSi island growth on patterned Si (001) substrates

Statistical information on GeSi islands grown on two-dimensionally pit-patterned Si substrates at different temperatures is presented. Three growth regimes on patterned substrates are identified: (i) kinetically limited growth at low growth temperatures, (ii) ordered island growth in an intermediate temperature range, and (iii) stochastic island growth within pits at high temperatures. A qualitative model based on growth kinetics is proposed to explain these phenomena. It can serve as a guidance to realize optimum growth conditions for ordered islands on patterned substrates.

Large-area ordered P-type Si nanowire arrays as photocathode for highly efficient photoelectrochemical hydrogen generation.

In this Article, we report the successful fabrication of large-area ordered Si nanowire arrays (NWAs) by a cost-effective and scalable wet-etching process in combination with nanospheres lithography technique. The periodical Si NWAs are further investigated as photocathode for water splitting, with excellent hydrogen evolution performances with a maximum photocurrent density of 27 mA cm(-2) achieved, which is ∼2.5 times that of planar Si and random Si nanowires electrode. The greatly improved PEC performance can be attributed to the patterned and ordered NWs structure as a result of enhancement of the light harvesting as well as charge transportation and collection efficiency.

Influence of GaN domain size on the electron mobility of two-dimensional electron gases in AlGaN/GaN heterostructures determined by x-ray reflectivity and diffraction

X-ray reflectivity and diffraction measurements were performed on Ga-face AlGaN/GaN heterostructures to determine the influence of interface roughness scattering and GaN domain boundaries scattering on the electron mobility of polarization induced two-dimensional electron gases. From simulations of the specular reflectivity, the root-mean-square roughness of the AlGaN/GaN interfaces was obtained. In reciprocal space maps, laterally elongated streaks passing through the Bragg peaks have been observed, which are attributed to column-like domains in the GaN buffer layers. The relationship between electron mobility measured by Hall effect and the interface roughness on one hand, and the column domain size on the other hand, demonstrates that the interface roughness scattering is not limiting the electron mobility, whereas the transport properties of the two-dimensional electron gas degrade with decreasing size of columnar domains in the GaN layer.