Session D20: Focus Session: Growth and Properties of Novel Semiconductor and Related Nanostructures

Silicon Nanomembranes

Silicon nanomembranes (SiNMs) are extremely flexible, strain-engineered, defect-free, thin \underline {single-crystal} sheets, with thicknesses from several 100 nm to less than 10 nm. Their novelty is several-fold: they are flexible, they are readily transferable to other hosts and conform and bond easily, they are stackable, and they can take on a large range of shapes (tubes, spirals, ribbons, wires) by engineering the strain and patterning the geometry. One can thus think of SiNMs as having inexact and tunable dimensionality, from 3-D to 0-D (when growth of quantum dots is included). Many properties of bulk Si are modified by thinness, strain, shape, and size, including band structure and quantum properties, electronic transport, phonon distributions, and mechanical properties. Because they are so close, the two surfaces of the membrane can influence each other's behavior, and the surface also becomes a significant influence on overall SiNM properties. After a review of SiNM fabrication, strain engineering, and transfer, we overview some of the unexpected physical and electronic properties of SiNMs. These include surface transfer doping via surface structures or adsorbed layers, through-membrane elastic interactions to create periodic strain lattices, energy level splitting and shifting with strain and quantum size effects, and orientation-dependent mobility enhancement with strain. SiNMs provide the potential for new or enhanced application of Si in fast flexible electronics; quantum electronics, new nanophotonic, optoelectronic, and thermoelectric devices; and chemical and biological sensors. These applications will be briefly outlined.   

Controlled displacement of nanoscale structures using an electron wind force

Electromigration is widely used to drive mass transfer in the fabrication of nanogaps, and will be a crucial issue for the structural stability, reliability and performance of nanoscale electronic devices. Using a combination of scanning tunneling microscopy and scanning electron microscopy, we directly observe the biased motion of monatomic islands driven by the electron wind force on patterned single-crystal Ag(111) thin films. The island motion can be steered by changing the direction of the applied electric current. For monatomic adatom islands, the biased motion is opposite to the current direction and along the wind force direction, while vacancy islands move in the opposite direction. The measured dependence of the drift velocity on the island size, yields the product of the diffusion constant and the magnitude of the wind force, giving the diffusion constant $D$ = 1.56 $\times $ 10$^{6}$/$z*$ (nm$^{2}$/s) for an effective charge of $z*$ =360 [1]. The wind force acts even more strongly on C$_{60}$-decorated structures, as observed by directionally-oriented bending of step edges. The wind force needed to cause the observed structure distortions is $F$ = 0.13 meV/nm, about 3 times the corresponding wind force acting on the bare steps. [1] A. Bondarchuk, et al. PRL 99, 206801 (2007).   

Lateral alloy segregation in thin heteroepitaxial films

We have studied the segregation and alloy formation of thin heteroepitaxial films. We use an atomistic strain model that has a cubic geometry and includes nearest neighbor bonds, next nearest neighbor bonds, and bond bending terms. Our motivation is the well established fact that for many heteroepitaxial systems growth proceeds in the Stranski-Krastanov growth mode, where islands form after the formation of a wetting layer. Recent results indicate that intermixing and thus vertical variations of the alloy concentration are a crucial factor in controlling the formation and thickness of the wetting layer. Our results suggest that in addition to vertical segregation there is also lateral segregation. Thermodynamically, the system prefers to have one big feature of the epilayer material that is embedded in the substrate but is near the surface. In practise, there will be a typical separation distance of these features because of kinetic limitations. We postulate that this lateral segregation and the separation of these features is ultimately responsible for the lateral placement of islands on the surface.   

Two Species Diffusion Model of Self-Organized Evolution on Patterned GaAs(001) Surfaces*

We report on numerical simulations of the self-organized evolution on GaAs(001) surface, pre-patterned with square arrays of pillars, during homo-epitaxial growth. Our experiments showed that lithographically fabricated, flat-topped cylindrical pillars evolved into a universal, downward paraboloidal shape, for initial diameters of the pillar ranging from 0.7um to several microns. In modeling this behavior, we construct a two-species diffusion model to simulate the growth. We consider the diffusion of both the Ga atom and As$_{2}$ dimers deposited on terraces between the concentric loop steps which make up the sidewall of the pillar. The stoichiometry for incorporating the diffusing Ga atoms and As$_{2}$ dimers into solid GaAs at the step edges produces boundary conditions that couple the flux of both diffusising species. We compare the results of our numerical simulation to the observed self-organization of the topography. *supported by the Lab for Physical Sciences and by~NSF{\#} DMR-0705447.~~   

Directed Matrix Seeding of Nitride Semiconductor Nanocrystals

The controlled formation of semiconductor nanocomposites offers a unique opportunity to tailor functional materials with a variety of novel properties. A promising approach to nanocomposite synthesis is matrix-seeded growth, which involves ion-beam-amorphization of a semiconductor film, followed by nanoscale re-crystallization via annealing. In this work, we are studying the formation and evolution of N ion-implanted InAs and GaAs (InAs:N, GaAs:N). The InAs:N and GaAs:N nanocomposites are synthesized using 100keV ion-implantation with a dose of 5x10$^{17}$cm$^{-2}$, at 77K and 300C, respectively. In all cases, the as-implanted structures are primarily amorphous, and after appropriate rapid thermal annealing (RTA) sequences, zincblende (ZB) InN and GaN [1] nanocrystals are formed. We are also developing a novel approach to \textit{direct} the seeding of nanostructure arrays, using a combination of focused-ion-beam (FIB) implantation in combination with conventional ion implantation. To date, we have demonstrated the selective positioning of wurtzite (WZ) and ZB GaN nanocrystals using 75keV and 100keV N implantation, followed by FIB patterning and 800C RTA. The growth mechanisms and structural evolution of nitride crystallites will also be discussed. [1] X. Weng, et al, \textit{J. Appl. Phys}., \textbf{92} 4012 (2002)   

Nanofabrication of carbon materials

We demonstrate a process for fabrication of nanostructures on the surfaces of highly oriented pyrolytic graphite (HOPG) and glassy carbon (GC) samples. Using hole-mask colloidal lithography (HCL), nanosized etch masks with three different feature diameters were prepared by identical processes on each of the two surface types. Oxygen reactive ion etching (RIE) was then used to transfer the mask pattern onto the surfaces. The structures were characterized using atomic force- (AFM), scanning electron microscopy (SEM) and optical spectrophotometry. The identical preparation schemes applied to the two materials yield structures with remarkably different shape and sizes. For example the process that yields 361 nm high and 37 nm diameter structures on glassy carbon yields 120 nm high and 119 nm diameter structures on HOPG. In general, the diameters of the fabricated GC nano-features are always at least 80 nm smaller than those of the corresponding HOPG structures, and the GC structure heights are more than three times that of the HOPG structures. These differences are attributed to different (an)isotropic etching behavior of the two materials.   

Fabrication of Metallic Nanoporous Films by Selective Chemical Etching

The objective of this study is to synthesize and characterize different nanoporous structures by chemical etching. The experiments were conducted on three different materials: We treated 6 carat white gold (Au/Ag alloy, 1:3 ratio by weight) with 70{\%} HNO$_{3}$ to grow Au nanoporous, the 50/50 solder wire (Pb/Sn alloy) with 93{\%} H$_{2}$SO$_{4}$ to create Pb porous and Imitation Italian gold leaf (Cu/Zn alloy, 82/18 by wt. {\%}) with NaOH solution (5 gm NaOH per 100 ml distilled H$_{2}$O) for Cu porous. The free-standing porous films have been analyzed by scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), high quality x-ray mapping (XRM). We observed the composition of the porous materials at every stage of chemical dealloying and conducted tests with different process parameters to optimize the size of self-ordered porous structures. Our experiments resulted in sponge like Au nanoporous of 10-200 nm, Pb pores of 10-300 nm and Cu pores of 10-150 nm. The results showed a technically improved fabrication of different nanoporous materials with high surface area and well defined pore morphology.   

Formation of Periodic 2D Metallic Nanostructures by Template-Assisted Electrodeposition

Two-dimensional ordered metallic nanostructures on solid surface with specific patterns may have potential applications in photonics and optoelectronics. Yet it remains a challenge to produce regular nanostructures over a large area with low cost and with a simple method. Here we report a novel method to fabricate well-aligned copper nanowire array on silicon surface by template-assisted electrodeposition. The template is introduced onto silicon surface by nanoimprinting. With our previously reported unique electrodeposition system [1-2], we find that the array of straight copper wires with their width varying from 400 nm to 20 nm can be fabricated. The wire width can be tuned by the control parameters in electrodeposition. It is shown that this method is not limited to straight wires only. It can be used to form more complicated patterns. The physical properties of the metallic nanostructures are also discussed. [1] M. Zhang, S. Lenhert, M. Wang, L. Chi, et al., Adv. Mater. 16, 409 (2004) [2] M. Wang, S. Zhong, X. Yin, J. Zhu, et al., Phys. Rev. Lett., 86, 3827 (2001)   

Phase transition induced surface electronic states on Pb/Si(111) surface

It is known that the 1$\times $1 phase of a monolayer Pb on Si(111) surface at room temperature may undergo a phase transition into a $\surd $7x$\surd $3 phase at a low temperature below 250K. We use scanning tunneling spectroscopy to study electronic structures on both 1$\times $1 and $\surd $7x$\surd $3 phases. Our observation reveals that the electronic structures of Pb overlayer are significantly affected because of phase transition. In tunneling spectra there appears two distinct peaks on $\surd $7x$\surd $3 phase but they disappear on 1$\times $1 phase, indicating that the phase transition can induce the formation of the surface electronic states on $\surd $7x$\surd $3 phase. Moreover, the peak intensity is location-dependent and the relative strength at the low-energy peak can be reversed at the high-energy peak. These phenomena can be qualitatively explained by Kronig-Penney model.   

Fabrication of High-Aspect-Ratio Nanogaps

For nanoscale electrical characterization and device fabrication it is often desirable to fabricate planar metal electrodes with separations well below 100 nm running parallel over a macroscopic width. In this work we demonstrate a self-aligned process to accomplish this goal using a thin Cr film as a sacrificial etch layer. The resulting gaps can be as small as 10 nm and have aspect ratios exceeding 1000, with excellent interelectrode isolation. Two separate lithographic patterning steps are used to define first and second electrodes while the interelectrode separation is controlled by the oxidation of a Cr layer deposited upon the first electrode. Advantageously, only a $\mu $m-alignment of first and second electrodes is required and the described method effectively does not have limitations on the gap width while the length of the gap is controlled by the Cr layer thickness. In addition to fabrication of Ti/Au electrodes on Si substrates, our technique was also demonstrated to work for other electrode metals (Pt, Fe, etc.) even on such relatively reactive substrates as magnetite, F$_{3}$O$_{4}$, films, thus demonstrating the flexibility and utility of this method.   

Subtle role played by H in Si thin-film growth from radicals: key atomic-scale mechanisms revealed by DFT calculations

Breaking silane molecules and creating reactive radicals in the gas phase is an efficient strategy for growing Si films at high growth rates and/or moderate temperatures. In a seminal experimental paper [1], the possibility of obtaining crystalline growth down to T$\sim$200$^\circ$C, was clearly demonstrated under high dilution of radicals in H. Several interpretations, in some cases controversial, have been given for explaining this evidence. Here we shall show that a clear understanding can be reached by relying on DFT calculations. Starting by a fully hydrogenated Si(001)(1$\times$2) surface, typical of low-temperature growth, we first illustrate the role played by SiH$_3$ in removing adsorbed H, therefore creating empty sites for further SiH$_3$ adsorption [2]. The adsorbed sylil, however, is frozen in its initial, non-epitaxial configuration, so that crystalline growth cannot take place. We demonstrate that further incoming hydrogen can easily transform silyl into SiH$_2$ which, in turn, incorporates into epitaxial sites crossing a barrier of only $\sim$1 eV [3], compatible with Ref. [1] conditions. [1] C.C. Tsai et al., J. Non-Cryst. Solids 114, 151 (1989). [2] S. Cereda et al., Phys. Rev. B 75, 235311 (2007), Phys. Rev. Lett. (in press).   

Kinetic Monte Carlo Simulation of Plasma Deposition of Silicon Thin Films

We report results from kinetic Monte Carlo simulations of plasma deposition of silicon thin films under conditions that render the SiH$_{3}$ radical the dominant deposition precursor. The transition probabilities for the various kinetic events accounted for in the simulations are based on first-principles density functional theory (DFT) calculations of the corresponding optimal pathways on the H-terminated Si(001)-(2$\times $1) surface and on molecular-dynamics simulations on hydrogenated amorphous silicon film surfaces. The relevant surface transport and reaction processes include SiH$_{3}$ diffusion, SiH$_{3}$ chemisorption and insertion into Si-Si bonds, surface H abstraction reactions, surface hydride dissociation reactions, as well as SiH$_{4}$ and Si$_{2}$H$_{6}$ desorption into the gas phase. Surface etching is predominantly observed over the 373-640 K temperature range. The surface compositions obtained are in good agreement with experimental measurements on films deposited under similar growth conditions. At 500 K, surface SiH$_{2}$ formed by surface trihydride dissociation reactions is the dominant surface hydride species.   

Isotropic interfaces in a structurally anisotropic organic thin film

We investigate the interfacial boundary fluctuations of Acridine-9-Carboxylic Acid (ACA) deposited on Ag(111) using UHV STM. The ACA molecule is anisotropic in shape and intermolecular interactions, and has been shown to exhibit a disordered 2D gas phase on Ag(111) at low coverage\footnote{B. Xu et al., J. Phys. Chem. B 110, 1271 (2006)}. At higher coverage, the molecules arrange in domains of ordered chain-like structures which coexist with the disordered phase. We measure the real-time fluctuations at the phase boundaries, and show that these fluctuations are governed by molecular exchange between the two phases. Due to structural anisotropy, there are two types of domain boundaries with significantly different molecular interactions. Surprisingly, the fluctuation magnitudes, mobilities, and free energies are nearly equal for the two boundary types. A lattice-gas statistical model is presented which includes the influence of molecular conformations on substrate interactions, and reproduces the essential features observed experimentally: domain shapes, boundary fluctuations, and phase densities.