Structural characterization of ultra-thin (0 0 1) silicon films bonded onto (0 0 1) silicon wafers:: a transmission electron microscopy study

Ultra-thin (0 0 1) silicon films (thickness less than 25 nm) directly bonded onto (0 0 1) silicon wafers have been investigated by transmission electron microscopy. Twist interfacial dislocations accommodate the twist between the two crystals, whereas tilt interfacial dislocations accommodate the tilt resulting from the residual vicinality of the initial surfaces. In low-twist angle grain boundaries, twist interfacial dislocations are dissociated and no precipitates are detected. In high-twist angle grain boundaries, there is no dissociation and a high density of silicon oxide precipitates is observed at the interface. Tilt interfacial dislocations are pinned by these precipitates, they are more mobile than precipitates. Without precipitates, their lines are straighter than those with precipitates, and this is especially when the bonded wafers are annealed at a high temperature. When no precipitates are present, tilt interfacial dislocations are associated by pairs, and we demonstrate that each tilt interfacial dislocations introduce a diatomic interfacial step at the interface.     

A high Schottky barrier between Ni and S-passivated n-type Si(1 0 0) surface

By minimizing surface states with sulfur passivation, a record-high Schottky barrier is achieved with nickel on n-type Si(1 0 0) surface. Capacitance–voltage measurements yield a flat-band barrier height of 0.97 eV. Activation-energy and current–voltage measurements indicate ～0.2-eV lower barriers for the Ni/Si(1 0 0) junction. These results accompany a previously-reported record-high Schottky barrier of 1.1 eV between aluminum and S-passivated p-type Si(1 0 0) surface. The operation of these metal/Si(1 0 0) junctions changes from majority-carrier conduction, i.e., a Schottky junction, to minority-carrier conduction, i.e., a p–n junction, with the increase in barrier height from 0.97 eV to 1.1 eV. Temperature-dependent current–voltage measurements reveal that the Ni/S-passivated n-type Si(1 0 0) junction is stable up to 110 °C.     

Implant temperature dependence of transient-enhanced diffusion in silicon (1 0 0) implanted with low-energy arsenic ions

The diffusion of arsenic implanted into silicon at low ion energies (2.5 keV) has been studied with medium-energy ion scattering, secondary ion mass spectrometry and four-point probe measurements. The dopant redistribution together with the corresponding damage recovery and electrical activation produced by high-temperature (550–975°C) rapid thermal anneals has been investigated for a range of substrate temperatures (+25, +300 and −120°C) during implant. Initial results show an implant temperature dependence of the damage structure and arsenic lattice position prior to anneal. Solid-phase epitaxial regrowth was observed following 550°C, 10 s anneals for all implant temperatures and resulted in approximately 60% of the implanted arsenic moving to substitutional positions. Annealing at 875°C resulted in similar arsenic redistribution for all implant temperatures. Following annealing at 925°C, transient-enhanced diffusion was observed in all samples with more rapid diffusion in the +25°C samples than either the −120 or +300°C implants, which had similar dopant profiles. In the 975°C anneal range, similar rates of implant redistribution were observed for the +300 and +25°C implants, while diffusion in the −120°C sample was reduced. These observations are discussed qualitatively in terms of the nature and density of the complex defects existing in the as-implanted samples.     

Raman spectroscopic assessments of structural orientation and residual stress in wurtzitic AlN film deposited on (0 0 1) Si

In this paper, polarized Raman spectroscopy is applied to quantitatively assess crystallographic alteration and interfacial residual stress with a micron-scale resolution in highly 〈0 0 0 1〉 oriented (textured) polycrystalline wurtzitic AlN films grown on (0 0 1)-oriented Si substrates. Raman selection rules for the wurtzite structure of AlN were explicitly put forward and a set of Raman tensor elements determined from experimentally retrieved angular dependences of Raman band intensities upon in-plane rotation measurements. An appreciably high degree of homogeneity in the AlN film (i.e., with respect to both in-plane and out-of-plane Euler angles, retrieved according to the proposed spectroscopic algorithm) could be observed in spectral line scans randomly selected on the cross-section of the film/substrate system. These characterizations indicated negligible structural alterations, such as grain tilting and twisting during film growth. However, a non-uniform stress distribution in the AlN film along the film thickness direction was found, which remained stored during manufacturing of the AlN film. A quite remarkable magnitude of compressive residual stress (∼−1.5 GPa) could be measured at the film/substrate interface. Finally, a Raman (non-destructive) statistical characterization of the film system in terms of micromechanical homogeneity by spectral surface mapping is presented, which provides a prompt overall view of the film quality. The proposed procedure should generally be applicable in crystallographic and micromechanical quality control of electronic film devices exhibiting a Raman spectrum.     

Dissociation of sexithiophene on Al(1 1 1) surface

The growth of sexithiophene ultra-thin films on an atomically clean Al(1 1 1) surface has been investigated by angle-resolved ultraviolet photoemission spectroscopy. Although sexithiophene on Al(1 1 1) represents a weakly interacting system, sexithiophene molecules were found to dissociate even below room temperature, presumably via reactive sites. The mechanism is distinctly different to that claimed on the reverse system, i.e. aluminum evaporated on thiophene oligomers films, where isolated aluminum atoms were reported to covalently bond to thiophene rings and thus dramatically affect the molecule geometry and the conjugation. The effect of the insertion of an ultra-thin separator film of sexiphenyl, on the dissociation of sexithiophene was also examined.     

Praseodymium oxide growth on Si(1 0 0) by pulsed-laser deposition

We report on the pulsed-laser deposition of high-K praseodymium oxide films onto Si(1 0 0) surfaces by laser-ablating a sintered Pr₆O₁₁ target. Optical microscope, SEM, and AFM investigations reveal two kinds of Pr_xO_y-surface structures, which are identified as: (a) large-scaled particles, and (b) ordered structures (rods) of different size with different orientations. The size of the particles increases with laser wavelength. The size of the ordered surface structures strongly depends on the substrate temperature. For the first time, we show characteristic Pr-Raman signals which confirm the crystalline quality of the grown layer. They also indicate that the silicon layer at the Si–Pr_xO_y interface is under compressive stress.     

Spin dynamics in (1 1 0)-oriented quantum wells

Quantum structures of III–V semiconductors grown on (1 1 0)-oriented substrates are promising for spintronic applications because they allow us to engineer and control spin dynamics of electrons. We summarise the theoretical ideas, which are the basis for this claim and review experiments to investigate them.     

Low threading dislocation density Ge deposited on Si (1 0 0) using RPCVD

Epitaxial Ge layer growth of low threading dislocation density (TDD) and low surface roughness on Si (1 0 0) surface is investigated using a single wafer reduced pressure chemical vapor deposition (RPCVD) system. Thin seed Ge layer is deposited at 300 °C at first to form two-dimensional Ge surface followed by thick Ge growth at 550 °C. Root mean square of roughness (RMS) of ∼0.45 nm is achieved. As-deposited Ge layers show high TDD of e.g. ∼4 × 10⁸ cm⁻² for a 4.7 μm thick Ge layer thickness. The TDD is decreasing with increasing Ge thickness. By applying a postannealing process at 800 °C, the TDD is decreased by one order of magnitude. By introducing several cycle of annealing during the Ge growth interrupting the Ge deposition, TDD as low as ∼7 × 10⁵ cm⁻² is achieved for 4.7 μm Ge thick layer. Surface roughness of the Ge sample with the cyclic annealing process is in the same level as without annealing process (RMS of ∼0.44 nm). The Ge layers are tensile strained as a result of a higher thermal expansion coefficient of Ge compared to Si in the cooling process down to room temperature. Enhanced Si diffusion was observed for annealed Ge samples. Direct band-to-band luminescence of the Ge layer grown on Si is demonstrated.     

Temperature-controlled self-organized InP nanostructures grown on GaAs(1 0 0) substrate by MOCVD

The self-organized InP nanostructures grown on GaAs(0 0 1) substrates by metalorganic vapor deposition were examined in detail using atomic force microscopy. By properly selecting growth temperature, three kinds of nanostructure, islands, pits and ripples were formed. For growth temperature of 400–450 °C, the surface morphologies were governed by islands; but, for the growth temperature of 500 °C, the formation of surface ripples instead of islands was presumably due to the combination effect of temperature-controlled surface kinetics and strain effect. On the other hand, the observation of enhanced growth of pits upon a high-temperature annealing (at 685 °C for 90 s) indicated that the strained InP epitaxial film would be morphologically stabilized by taking the form of pits formation.     

Surface reconstructions on Sb-irradiated GaAs(0 0 1) formed by molecular beam epitaxy

Reconstructed surfaces on Sb-irradiated GaAs(0 0 1) formed by molecular beam epitaxy have been studied by in-situ scanning tunneling microscopy (STM). The reflection high-energy electron diffraction patterns showed (2×3) [or weak (4×3)] structure. The step density was about five times higher than that of GaAs(0 0 1)-c(4×4) surface. It was found that there were swinging dimer rows along to the [1 1¯ 0] direction, which seemed not to consist of a specified reconstruction. We proposed two (2×3)-structure models for these swinging dimers. By first-principles calculation, we found that the proposed models were stable and with energy difference was 0.17 eV, indicating the coexistence of the two structures. Moreover, we proposed three (4×3) reconstruction models based on these (2×3) models. The electron counting rule was applied for these models, indicating that there was an excessive amount of electrons. By two bias-alternative STM images, it was found that the many spots appear only in empty-state. These might be segregated Ga or Sb cluster and strongly relate to the excessive amount of electrons.     

(Ga,Mn)As on patterned GaAs(0 0 1) substrates: Growth and magnetotransport

A new type of (Ga,Mn)As microstructures with laterally confined electronic and magnetic properties has been realized by growing (Ga,Mn)As films on -oriented ridge structures with (1 1 3)A sidewalls and (0 0 1) top layers prepared on GaAs(0 0 1) substrates. The temperature- and field-dependent magnetotransport data of the overgrown structures are compared with those obtained from planar reference samples revealing the coexistence of electronic and magnetic properties specific for (0 0 1) and (1 1 3)A (Ga,Mn)As on a single sample.     

Properties of high sensitivity ZnO surface acoustic wave sensors on SiO2/(1 0 0) Si substrates

The properties of ZnO/SiO₂/Si surface acoustic wave (SAW) Love mode sensors were examined and optimized to achieve high mass sensitivity. SAW devices A and B, were designed and fabricated to operate at resonant frequencies around 0.7 and 1.5 GHz. The ZnO films grown by pulsed laser deposition on SiO₂/Si demonstrated c-axis growth and the fabricated devices showed guided shear horizontal surface acoustic wave (or Love mode) propagation. Acoustic phase velocity in the ZnO layer was measured in both devices A and B and theoretical and experimental evaluation of the mass sensitivity showed that the maximum sensitivity is obtained for devices with ZnO guiding layer thicknesses of 340 nm and 160 nm for devices A and B, respectively. The performance of the SAW sensors was validated by measuring the mass of a well-characterized polystyrene–polyacrylic acid diblock copolymer film. For the optimized sensors, maximum mass sensitivity values were as high as 4.309 μm²/pg for device A operating at 0.7477 GHz, and 8.643 μm²/pg for device B operating at 1.5860 GHz. The sensors demonstrated large frequency shifts per applied mass (0.1–4 MHz), excellent linearity, and extended range in the femto-gram region. The large frequency shifts indicated that these sensors have the potential to measure mass two to three orders of magnitude lower in the atto-gram range.     

Selective growth experiments on gallium arsenide (1 0 0) surfaces patterned using UV-nanoimprint lithography

We describe a nanoimprint lithography (NIL) process and subsequent solid-source molecular beam epitaxy (SSMBE) growth of III–V semiconductors on patterned substrates. In particular, growth of GaAs, GaInAs, and GaInP, and effects of growth temperature were studied using AFM, SEM, and XRD. It turns out that selective growth of GaAs on patterned substrates is relatively straightforward, but GaInAs and GaInP are more challenging. For the first time, GaInP has been selectively grown on UV-NIL-patterned substrates using SSMBE.     

Wetting layer formation in superlattices with Ge quantum dots on Si(1 0 0)

The influence of parameters of germanium deposition on wetting layer thickness was studied during the growth on the Si(1 0 0) surface. A non-monotone dependence of the thickness on growth temperature was discovered and accounted for by changing the mechanism of the layer-by-layer growth. The conclusion was supported by changing the mode of oscillations of the reflection high-energy electron diffraction (RHEED) specular beam. In addition, wetting layer thickness is strongly affected by the replication number and thickness of the silicon spacer due to accumulation of elastic strains throughout the structure.     

Highly ordered C60 films on epitaxial Fe/MgO(0 0 1) surfaces for organic spintronics

Hybrid interfaces between ferromagnetic surfaces and carbon-based molecules play an important role in organic spintronics. The fabrication of devices with well defined interfaces remains challenging, however, hampering microscopic understanding of their operation mechanisms. We have studied the crystallinity and molecular ordering of C₆₀ films on epitaxial Fe/MgO(0 0 1) surfaces, using X-ray diffraction and scanning tunneling microscopy (STM). Both techniques confirm that fcc molecular C₆₀ films with a (1 1 1)-texture can be fabricated on epitaxial bcc-Fe(0 0 1) surfaces at elevated growth temperatures (100–130 °C). STM measurements show that C₆₀ monolayers deposited at 130 °C are highly ordered, exhibiting quasi-hexagonal arrangements on the Fe(0 0 1) surface oriented along the [1 0 0] and [0 1 0] directions. The mismatch between the surface lattice of the monolayer and the bulk fcc C₆₀ lattice prevents epitaxial overgrowth of multilayers.     

Crystalline orientation dependence of electrical properties of Mn Germanide/Ge(1 1 1) and (0 0 1) Schottky contacts

We have investigated the crystalline orientation dependence of the electrical properties of Mn germanide/Ge(1 1 1) and (0 0 1) Schottky contacts. We prepared epitaxial and polycrystalline Mn₅Ge₃ layers on Ge(1 1 1) and (0 0 1) substrates, respectively. The Schottky barrier height (SBH) estimated from the current density-voltage characteristics for epitaxial Mn₅Ge₃/Ge(1 1 1) is as low as 0.30 eV, while the SBH of polycrystalline Mn₅Ge₃/Ge(0 0 1) is higher than 0.56 eV. On the other hand, the SBH estimated from capacitance-voltage characteristics are higher than 0.6 eV for both samples. The difference of these SBHs can be explained by the local carrier conduction through the small area with the low SBH regions in the epitaxial Mn₅Ge₃/Ge(1 1 1) contact. This result suggests the possibility that the lowering SBH takes place due to Fermi level depinning in epitaxial germanide/Ge(1 1 1) contacts.     

Effect of high-temperature annealing on lanthanum aluminate thin films grown by ALD on Si(1 0 0)

Amorphous lanthanum aluminate thin films were deposited by atomic layer deposition on Si(1 0 0) using La(ⁱPrCp)₃, Al(CH₃)₃ and O₃ species. The effects of post-deposition rapid thermal annealing on the physical and electrical properties of the films were investigated. High-temperature annealing at 900 °C in N₂ atmosphere leads to the formation of amorphous La-aluminosilicate due to Si diffusion from the substrate. The annealed oxide exhibits a uniform composition through the film thickness, a large band gap of 7.0 ± 0.1 eV, and relatively high dielectric constant (κ) of 18 ± 1.     

Epitaxial growth of LaAlO3 on Si(0 0 1) using interface engineering

In this work, the potentiality of molecular beam epitaxy techniques to prepare epitaxial lanthanum aluminate (LaAlO₃) films on Si(0 0 1) is explored. We first demonstrate that the direct growth of LaAlO₃ on Si(0 0 1) is impossible : amorphous layers are obtained at temperatures below 600 °C whereas crystalline layers can be grown at higher temperatures but interfacial reactions leading to silicate formation occur. An interface engineering strategy is then developed to avoid these reactions. SrO and SrTiO₃ have been studied as buffer for the subsequent growth of LaAlO₃. Only partial LaAlO₃ epitaxy is obtained on SrO whereas high quality layers are achieved on SrTiO₃. However both SrO and SrTiO₃ appear to be unstable with respect of Si at the growth temperature of LaAlO₃ (700 °C). This leads to the formation of relatively thick amorphous interfacial layers. Despite their instability at high temperature, these processes could be used for the fabrication of twins-free LaAlO₃ templates on Si, and for the fabrication of complex oxide/Si heterostructures for various applications.     

Morphological evolution and lateral ordering of uniform SiGe/Si(0 0 1) islands

By investigating the morphological evolution during epitaxial growth of Ge on Si(0 0 1) substrates, we find that highly uniform distributions of islands can be obtained. The islands are no longer domes but they consist of barns, which are bounded by steeper facets. A detailed morphological analysis indicates the presence of facets at their base, which are not stable for Ge but for Si. Finally, we show that long-range ordering of highly uniform SiGe barns can be obtained when the growth is performed on patterned Si(0 0 1) substrates.     

Photoelectron spectroscopy (XPS) and photoelectron diffraction (XPD) studies on the system hafnium silicide and hafnium oxide on Si(1 0 0)

Continuous down-scaling of silicon based transistors results in device lengths of less than 100 nm. This requires a reduction of the gate dielectric thickness to less than 15Å which is not possible for SiO₂ due to an increasing leakage current. One of the most promising candidates for a replacement material for the gate dielectric is HfO₂ [Wilk GD, Wallace RM, Anthony JM. J Appl Phys 2001; 89:5243].In this work we applied X-ray photoelectron spectroscopy (XPS) and photoelectron diffraction measurements in order to study the interface of hafnium oxide to Si(1 0 0). The high resolution measurements were performed with synchrotron radiation at beamlines 5 and 11 at DELTA (Dortmund). For the first time, photoelectron diffraction patterns for this system were recorded. The spectral resolution allowed to separate different spectral components.The preparation of hafnium oxide films on Si(1 0 0) was performed by evaporation of hafnium at a partial oxygen background pressure of . Three different spectral components were observed in the hafnium 4f photoemission signal by high resolution XPS. The photoelectron signals with binding energies shift of 3.1 and 1.2 eV with respect to signal of hafnium silicide were assigned to hafnium dioxide and hafnium silicate, respectively. The corresponding high-resolution diffraction patterns result from different local environments for each component. The experimental patterns are compared with simulations for a model structure of hafnium silicide.