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Gadolinium oxide (Gd2O3) was grown at low temperatures (250 °C) on Si(001) using molecular beam epitaxy. The crystal structure was investigated with X-ray diffraction experiments, where the crystal structure changes from cubic to monoclinic depending on the oxygen supply. We propose that Gibbs-Thomson effect is responsible for the change in crystal structure due to the initial nucleation of nanometer-sized islands and its variation in diameter depending on the growth conditions. The dielectric properties of the monoclinic structure were investigated using Capacitance-Voltage measurements, where platinum was used as contact material. Dielectric constants larger than 20 were obtained, which is higher than for the cubic structure. The monoclinic Gd2O3 layers suffer from flat-band voltage instabilities, which is most likely due to defects located at the interface or near-interface region of the Gd2O3 and silicon. Forming gas annealing results in an improvement of the dielectric behaviour.

The influence of growth conditions on the layer orientation, domain structure and crystal structure of gadolinium oxide (Gd2O3) on silicon (001) has been investigated. Gd2O3 was grown at low (250°C) and high (850°C) temperatures with different oxygen partial pressure as well as a temperature ramp up during growth. At low temperature, the cubic bixbyite type of crystal structure with space group Ia{\bar 3} was grown at low oxygen partial pressure. The layers consist of two domains oriented orthogonal to each other. The epitaxial relationships for the two domains were found to be Gd2O3(110)[001]||Si(001)[110] and Gd2O3(110)[001]||Si(001)[{\bar 1}10], respectively. Applying additional oxygen during growth results in a change in crystal and domain structures of the grown layer into the monoclinic Sm2O3-type of structure with space group C2/m with (20\bar 1) orientation and mainly two orthogonal domains with the epitaxial relationship Gd2O3(20\bar 1)[010]||Si(100)〈110〉 and a smooth surface morphology. Some smaller areas have two intermediate azimuthal orientations between these variants, which results in a six-domain structure. The change in crystal structure can be understood based on the Gibbs–Thomson effect caused by the initial nucleation of nanometre-sized islands and its variation in diameter with a change in growth conditions. The crystal structure remains stable even against a temperature ramp up during growth. The layers grown at high temperature exhibit a nanowire-like surface morphology, where the nanowires have a cubic crystal structure and are aligned orthogonal to each other along the 〈110〉 in-plane directions. An increase in oxygen supply results in a reduced length and increased number of nanowires due to lower adatom mobility. The results clearly indicate that both kinetic and thermodynamic factors have a strong impact on the crystal structure, epitaxial relationship and morphology of the grown layers.

The structural and morphological properties of gadolinium oxide (Gd

In this paper we investigate the initial stages of epitaxial growth of Ge on Si(1 1 1) and the impact of growth temperature on strain evolution in situ by reflection high energy electron diffraction (RHEED) for temperatures between 200 °C and 400 °C. The change in surface morphology from a flat wetting layer to subsequent islanding that is characteristic for Stranski–Krastanov growth is monitored by spot intensity analysis. The corresponding critical layer thickness is determined to 3.1

The structural and dielectric properties of gadolinium oxide (Gd 2O 3) grown on Si(001) depending on the epitaxial growth conditions were investigated. Gd 2O 3 layers were grown at temperatures between 250 ° C and 400 ° C with an oxygen partial pressure between 2 × 10 − 7 mbar and 5 × 10 − 7 mbar. The crystal structure of the Gd 2O 3 turns out to be monoclinic with rotational domains as revealed by x-ray diffraction measurements and transmission electron microscopy (TEM) investigations. The dielectric properties can be tuned with growth temperature, forming gas annealing, and an increase in oxygen partial pressure. Furthermore, the dielectric constant was found to increase with the layer thickness. This can be interpreted in terms of the presence of a two layer stack consisting an interfacial quasi-amorphous and monoclinic Gd 2O 3 on top, as confirmed by TEM. The value of around 33 was extracted for the dielectric constant of monoclinic Gd 2O 3, which is much higher than for cubic Gd 2O 3. The best Gd 2O 3 layers grown at 400 ° C and p O 2 = 5 × 10 − 7 mbar exhibit also a characteristic leakage current value J ( V fb − 1 V ) for a CET value of around 2 nm in the range of a few nA/cm 2, which enable the applicability in electronic devices.

The influence of growth conditions on the layer orientation, domain structure and crystal structure of gadolinium oxide (Gd

The step-flow growth condition of Si on Si (111) near the (7×7)–“1×1” surface phase transition temperature TC is analysed within the framework of Burton–Cabrera–Frank theory. In particular, coexistence of both surface phases well below TC and their specific influence on the step-flow growth behaviour is considered. We presume that under dynamical condition of growth, the surface initially covered by only the (7×7) phase separates into domains surrounded by “1×1” areas. On such a surface, the overall supersaturation should be reduced drastically compared to a surface with only (7×7), resulting in much larger critical terrace width for nucleation.

The step-flow growth condition of Si on Si(111) near the (7×7)-“1×1” surface phase transition temperature T C are analyzed within the framework of Burton-Cabrera-Frank theory. In particular, coexistence of both surface phases well below T C and their specific influence on the step-flow growth behavior are considered. We presume that under dynamical condition of growth, the surface initially covered by only the (7×7) phase separates into domains surrounded by “1×1” areas. On such a surface, the overall supersaturation should be reduced drastically compared to a surface with only (7×7), resulting in much larger critical terrace width for nucleation.

The impact of nitrogen doping on the growth electronic band structure and electrical properties of epitaxial Gd2O3 thin films on Si (111) has been investigated. Epitaxial layers of Gd2O3:N were grown on p-type Si (111) substrates by solid source molecular beam epitaxy technique using molecular N2O as the nitridation agent. Substitutional nitrogen incorporation into the dielectric layer was confirmed by secondary ion mass spectroscopy and X-ray photoelectron spectroscopy measurements. Significant reduction of the leakage current density and disappearance of capacitance-voltage hysteresis in the Gd2O3:N layers indicate that nitrogen doping in Gd2O3 successfully eliminates the adverse effects of the oxygen vacancy induced defects in the oxide layer. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A very promising way to realize advanced future devices is using single-crystalline, closely lattice matched oxides, which will be grown on the substrate of choice. The dielectric properties of such oxides are sensitive to small variations in structure and symmetry. It is known that thin layers of crystalline binary rare earth oxides can exhibit significant larger dielectric constants compared to bulk materials. For example, thin crystalline Gd2O3 films epitaxially grown on silicon exhibit dielectric constants up to 20 although the known bulk value is only around 13. The reason for that “enhancement effect” is not fully understood yet. Here, we will report about different investigations on strain-induced effects on dielectric properties. As model systems, we chose Gd2O3 and Nd2O3 having very similar bulk dielectric constants and band gaps. The crystalline structures are also identical. On the other hand, the lattice spacing in Nd2O3 is larger while that of Gd2O3 smaller than the lattice spacing in silicon; i.e. one layer should be under compressive strain and the other under tensile strain. First, we report on the dependence of the dielectric constant on layer thickness for epitaxial Gd2O3 on Si(111). The K-value strongly decreases with increasing layer thickness and reaches the bulk value at around 8 nm. Controlling the oxide composition in ternary (Gd1-xNdx)2O3 thin films enables us to tune the lattice mismatch to silicon, and thus the strain-induced variation in the dielectric constants of the layer from 13 (close to the bulk value) up to 20. Finally, we will show that solely tetragonal distortion of the cubic lattice is not sufficient to explain the huge lattice-mismatch induced enhancement in K-values. Thus, we will explain these effects by more severe strain induced structural phase deformations. Further, dielectric properties of epitaxial oxide thin films grown on Si have been found to improve significantly by incorporation of suitable dopants. We observe substantial reduction of the leakage current density in nitrogen-doped Gd2O3 layers. To achieve optimum electrical properties from such doped oxides it is important to understand the correlation between doping and the electronic structure of the material. X-ray photoelectron spectroscopy investigations revealed band gap narrowing in epitaxial Gd2O3 due to nitrogen doping, which leads to reduction in the valence band offset to Si. The observed reduction of the leakage current densities in the these layers with increasing nitrogen content suggests that nitrogen doping can be an effective route to eliminate the adverse effects of the oxygen vacancy induced defects in the oxide layers.

Development of surface morphology in Si molecular beam epitaxy on mesa-structured Si(111) at temperatures around the (7 × 7)-“1 × 1” surface phase transition was studied by atomic force microscopy. Significant changes in surface morphology were found for a small increase in temperature near the surface phase transition, accompanied by a strong increase in step-free area dimension. This behavior is reported for the first time for epitaxial silicon growth, but supports earlier in situ studies of the thermal decay of 2-dimensional islands and voids on Si(111) close to the surface phase transition using low energy electron microscopy by Hibino et al . (Phys. Rev. B 63, 245402, 2001). The observed changes in surface morphology clearly demonstrate the interplay of crystal growth and the surface phase transformation. In particular, the simultaneous appearance of two surface phases under certain conditions and their specific influence on the growth behavior are discussed with regard to this matter.

The (Nd1– x Gd x )2O3 thin films were grown on Si(001) in 2005 using molecular beam epitaxy followed by an in vacuo metallization of Pt contacts. Pt/(Nd1– x Gd x )2O3/Si(001) metal-oxide-semiconductor structures have been characterized several times over the period of 2005–2015 in order to investigate the stability of their physical and electrical properties. Core-level X-ray photoelectron spectroscopy (XPS) spectra remeasured during 2015 depict that the layer still retains its original properties that were measured in 2005. Although there has been some degradation of electrical properties such as capacitance and leakage current over several years, interestingly the midgap density of interface traps’ ( $D_{{\mathrm{it}}})$ value has been found to be decreased from $1.1 \times 10^{12}$ to $3.5 \times 10^{11}$ eV $^{-1}$ cm $^{-2} $ over the years. The formation of silicate-like interface inferred from XPS measurements can be attributed to such an improvement over this period. Time-dependent dielectric breakdown measurements under constant current stress and constant voltage stress were also carried out to investigate the reliability of the structure.

Creating rotation twins periodically in a defined distance within Si layers could lead to the formation of miscellaneous Si crystal structures. This could be realized by several growth and annealing cycles on heavily B-covered Si(1 1 1) exhibiting (√3×√3)R30° surface superstructure. However, surface defects due to imperfections of the B-induced surface structure give rise to an inhomogeneous Si nucleation, which limits the structure size. Therefore, surface structure formation induced by both adsorption and surface segregation of B on Si(1 1 1) and its influence on the Si molecular beam epitaxial growth mode has been investigated using ultraviolet photoelectron spectroscopy and accompanying reflection high-energy electron diffraction. Based on these studies, conditions have been established to prevent surface defects. Furthermore, annealing samples with 0.6 monolayers (ML) B buried below several ML Si at 1080 K results in a renewal of the B-induced Si surface structure without any defects. This indicates a dominance of B surface segregation over bulk diffusion, which becomes significant only above 1100 K.

In this work, an unconventional approach for epitaxial growth of Si on single-crystalline rare-earth oxide is presented using molecular beam epitaxy under ultra-high vacuum. Surface and bulk crystalline structures as well as chemical content were examined. Silicon-on-insulator layers were fabricated by encapsulated solid phase epitaxy on Si(111) substrate. The gadolinium oxide capping layer was removed by wet-chemical etching. The remaining silicon layer is single crystalline without any impurities and exhibits 7 × 7 reconstructed surface after annealing in very low silicon flux in the growth chamber. The thermal stability of the fabricated silicon-on-insulator structure was studied by step-wise heating under ultra-high vacuum conditions. The fabricated ultra-thin (10–15 nm) silicon-on-oxide layers remain structurally and chemically stable up to 900 °C.

We investigate molecular beam epitaxial overgrowth of Si template layers produced by different approaches on single-crystalline oxide grown on Si(111). Three approaches based on modified solid-phase epitaxy were found to be suitable for the subsequent Si epitaxial overgrowth. The crystalline quality and interface properties of single-crystalline silicon on single-crystalline oxide grown on Si(111) make the obtained structures suitable for silicon-on-insulator applications. First measurements of electrical properties of p-type samples indicate good electrical properties of the top Si layer. Supplemental investigations demonstrate that Si layers with thickness in the range of 10 nm remain stable during thermal annealing up to 900 °C in an ultra-high vacuum.

The influence of boron on Si molecular beam epitaxy was investigated as function of coverage and temperature by reflection high-energy electron diffraction (RHEED). The development of the boron-covered Si surface was studied additionally by ultraviolet photo electron spectroscopy (UPS) as function of boron coverage (c B ) and annealing condition. We found a direct correlation between the appearance of surface states in UPS and the transient growth behaviour observed in RHEED. For c B > 0.4 monolayer (ML) regular RHEED oscillations occur with a period typically for two bilayers (BLs), whereas for lower c B a transient behaviour with irregular intensity oscillations was observed in the initial growth stages. The appearance of this transient behaviour is discussed in terms of an initial surface defect-induced nucleation of BL-high Si islands and the formation of two BLs-high Si islands on top of the van der Waals-like perfect boron-covered surface, respectively. The occurrence of surface defects for c B < 0.6 ML is clearly established in UPS measurements, where an intensity peak slightly below the Fermi level (—0.4 eV) was visible. We suggest that this surface defect level is related to Si adatom dangling bonds acting also as preferential nucleation centres in epitaxy. Furthermore, surface defects results in a significant Fermi level pinning.

In this work, we investigated the epitaxial growth of Gd2O3 thin films on germanium layers grown by surfactant-mediated epitaxy on silicon (0 0 1) substrates. The influence of the lattice mismatch between Ge and Gd2O3 as well as the impact of the lower surface energy of Ge compared to Si on the growth process have been studied resulting in conditions for epitaxy of smooth Gd2O3 films without any interfacial layer on Ge. We determined the epitaxial relationship and the crystalline structure of these films using transmission electron microscopy and X-ray diffraction. The Gd2O3 layers grow in two orthogonal (0 1 1)-oriented domains of the cubic phase. They are relaxed and show structural perfection similar to that of Gd2O3 films grown on Si(0 0 1). No interfacial layer is observed between the Gd2O3 and Ge making this material combination particularly suitable for an application in high-mobility channel MOSFETs with equivalent oxide thickness below 1 nm.

In this work we show that by efficiently exploiting the growth kinetics during molecular beam epitaxy (MBE) one could create Si nanostructures of different dimensions. Examples are Si quantum dots (QD) or quantum wells (QW), which are buried into an epitaxial rare-earth oxide, e.g. Gd2O3. Electrical measurements carried out on Pt/Gd2O3/Si MOS capacitors comprised with Si-QD demonstrate that such well embedded Si-QD with average size of 5nm and density of 2x1012cm-2 exhibit very good charge storage capacity with suitable retention (~105s) and endurance (~105 write/erase cycles) characteristics. The Pt/Gd2O3/Si (metal-oxide-semiconductor (MOS)) basic memory cells with embedded Si-QD display large programming window (~1.5-2V) and fast writing speed and hence could be a potential candidate for future non-volatile memory application. The optical absorption of such Si-QD embedded into epitaxial Gd2O3 was found to exhibit a spectral threshold maximum up to 2.9+±0.1eV depending on their sizes, inferring a significant influence of quantum confinement on the QD/oxide interface band diagram. Ultra-thin single-crystalline Si-QW with epitaxial insulator (Gd2O3) as the barrier layers were grown by a novel approach based on cooperative vapor phase MBE on Si wafer with sharp interfaces between well and barriers. The current-voltage characteristics obtained for such structure exhibits negative differential resistance at lower temperature, making them a good candidate for resonant tunneling devices.

The influence of boron as a function of coverage on molecular beam epitaxial Si growth on Si(1 1 1) surface was systematically studied by reflection high-energy electron diffraction. At boron coverage above 0.3 monolayer regular oscillations occur with a period typically for two-bilayers, whereas at lower boron coverage a transient behaviour with irregular intensity oscillations was observed in the initial growth stages. This behaviour can be attributed to a superposition of a bilayer and a two-bilayer dominated growth mode. The appearance of these two growth modes is discussed in terms of an initial surface defect-induced nucleation of bilayer-high Si islands and the formation of two-bilayers-high Si islands on top of the van der Waals like boron-covered surface, respectively. We suggest that these surface defects are of intrinsic nature, and their number only depends on the amount of boron at the surface. The oscillations become regular during further Si deposition with a bilayer period, indicating a diminishing influence of the layer/substrate interface on the growth processes.

Two new methods for fabrication of silicon-on-isolator (SOI) structures are studied. The first one is based on the formation of a template single crystalline Si-layer and combines encapsulated solid-vapor-phase epitaxy of silicon on rare-earth-metal-oxide layer, developed for fabrication of oxide/silicon/oxide heterostructures, subsequent chemical etching of the second oxide layer, followed by vapor-phase epitaxial growth of silicon on the template-silicon layer. In the second method, crystalline silicon islands serve as template for further growth of crystalline Si layer. Structural investigations show no interface and no noticeable differences in structure quality between these two silicon sub-layers grown on each other. Silicon-substrate/oxide/silicon heterostructure exhibits transition of the substrate crystalline structure with A/B/A twinning relationship. Initial stage of deposition of the template silicon is crucial for its structural quality. [DOI: 10.1380/ejssnt.2009.405]

Using an unconventional approach, single crystalline Si-nanoclusters (Si-NCs) with uniform size and higher density were embedded into epitaxial rare earth oxide with two-dimensional spatial arrangements at a defined distance from the substrate using solid source molecular beam epitaxy (MBE) technique. The incorporated Si-NCs with average size of 5nm and density of 2x10^1^2cm^-^2 exhibit charge storage capacity with promising retention (~10^7s) and endurance (10^5 write/erase cycles) characteristics. The Pt/Gd"2O"3 (Si-NC)/Si (MOS) basic memory cells with embedded Si-nanoclusters display large programming window (~1.5-2V) and fast writing speed. With such properties demonstrated, we believe that the Si-NCs embedded in epitaxial Gd"2O"3 could be potential candidate for high density nonvolatile memory devices in the future.

The ability to integrate crystalline metal oxide dielectric barrier layers into silicon structures can open the way for a variety of novel applications which enhances the functionality and flexibility ranging from high-K replacements in future MOS devices to oxide/silicon/oxide heterostructures for nanoelectronic application in quantum-effect devices. We present results for crystalline gadolinium oxides on silicon in the cubic bixbyite structure grown by solid source molecular beam epitaxy. Additional oxygen supply during growth improves the dielectric properties significantly. Experimental results for Gd2O3-based MOS capacitors grown under optimized conditions show that these layers are excellent candidates for application as very thin high-K materials replacing SiO2 in future MOS devices. Epitaxial growth of lanthanide oxides on silicon without any interfacial layer has the advantage of enabling defined interfaces engineering. We will show that the electrical properties of epitaxial Gd2O3 thin films on Si substrates can further be improved significantly by an atomic control of interfacial structures. Finally, we will present a new approach for nanostructure formation which is based on solid-phase epitaxy of the Si quantum-well combined with simultaneous vapor-phase epitaxy of the insulator on top of the quantum-well. Ultra-thin single-crystalline Si buried in a single-crystalline insulator matrix with sharp interfaces was obtained by this approach on Si(111). In addition, structures consisting of a single-crystalline oxide layer with embedded Si nanoclusters for memory applications will also be demonstrated. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The ability to integrate low-dimensional crystalline silicon into crystalline insulators with high dielectric constant (high-k) can open the way for a variety of novel applications ranging from high-k replacement in future nonvolatile memory devices to insulator/Si/insulator structures for nanoelectronic applications. We will present an approach for nanostructure fabrication by incorporation of crystalline silicon into epitaxial oxide that is based on a solid-phase epitaxy of Si. In dependence on the preparation conditions we obtained nanostructures containing an either ultra-thin single-crystalline Si quantum-well buried in single-crystalline oxide matrix with sharp interfaces or Si-nanocrystals (ncs) embedded into single-crystalline oxide layer. As an example, we demonstrate the growth of Si buried in Gd2O3 and the incorporation of epitaxial Si clusters into single-crystalline Gd2O3 on silicon as well as silicon carbide substrates using molecular beam epitaxy. The leakage current of the obtained nanostructures exhibited negative differential resistance at lower temperatures. For structures containing Si-ncs a large hysteresis in capacitance-voltage measurements due to charging and discharging of the Si-ncs was obtained.

The Si/dielectric interface properties influence the device performance significantly. Often the interface is not stable and changes during and after the growth. The formation of SiOx and/or metal silicate interfacial layers can occur when these materials are deposited upon silicon or during subsequent annealing processes. The occurrence of an interfacial layer of SiOx or another low permittivity material will limit the highest possible gate stack capacitance, or equivalently, the lowest achievable equivalent oxide thickness (EOT) value. A good interface requires either that the oxide is amorphous, or that it is epitaxial and lattice-matched to the underlying silicon. Amorphous dielectrics are expected to be able to adjust the local bonding to minimize the number of Si dangling bonds at the interface. The alternative is to use an epitaxial oxide. This involves more effort, but it has the advantage of enabling defined interfaces engineering. Molecular beam epitaxy (MBE), known for its superior capability in atomic level engineering and interface control, has been used in the epitaxial growth of various high-K materials. Epitaxial growth on a clean surface requires matching in symmetry as well as in atomic spacing. Lanthanide oxides (LnO’s) form the most interesting group of insulators for epitaxial growth on silicon [1]. The LnO’s can have different oxygen compositions LnOx, with x ranging from 1 to 2 due to the multiple oxidation states (+2, +3, and +4) of the rare-earth metals. This leads to oxides with different stoichiometries (LnO, Ln2O3, and LnO2). All known Ln(II) oxides, like EuO, are not insulating. Different oxygen content can lead to different structural phases including two cubic phases, such as the calcium fluoride (CaF2) structure for the Ln(IV) only, and the manganese oxide (Mn2O3) or bixbyite structure for Ln(III). The bixbyite structure is based on the calcium fluorite structure, where 1/4 of the oxygen atoms have been removed from specific lattice sites. Some LnO’s also crystallize in the hexagonal lanthanum oxide structure, which is suitable for epitaxy only on Si(111). Also, monoclinic phases are known for various lanthanide oxides. For application in a Si-based device fabrication process, all lanthanide oxides exhibiting more than one valence state are not the best choice as epitaxial high-K materials because of the coexistence of phases with different oxygen content leading to different band alignments. In addition, stable mixed valence-state structures can occur for some LnO’s. For example, the mixed valence-state Pr6O11 is the most stable phase for praseodymium oxide. All lanthanide oxides displaying only one valence state are easier to handle due to the absence of transitions between phases with different oxygen content. Based on that argument, we will focus our discussion mainly on lanthanide (III) oxides (occurring as Ln2O3). Among them, gadolinium oxide (Gd2O3) has the lowest lattice mismatch to silicon. On Si(001) oriented surfaces, crystalline Ln2O3 grows as (110)-oriented domains, with two orthogonal inplane orientations (found experimentally for a large variety of binary metal oxides on Si(001)). In such a case, nearly 1:1 matching should occur along one direction. In the other direction, there would be roughly a 3:2 matching relation. For the calcium fluorite structure (LnO2), however, the 3:2 relation leads to unsaturated oxygen bonds that would create additional interface charges. Therefore, Ln2O3(110)//Si(001) heterostructures (with the metal oxide having the bixbyite structure) are favorable due to the lower oxygen content in that structure.

We will present results for crystalline gadolinium oxides on silicon in the cubic bixbyite structure grown by solid source molecular beam epitaxy. Additional oxygen supply during growth improves the dielectric properties significantly. Experimental results for Gd"2O"3-based MOS capacitors grown under optimized conditions show that these layers are excellent candidates for application as very thin high-k materials replacing SiO"2 in future MOS devices. We also will present a new approach for nanostructure formation which is based on solid-phase epitaxy of the Si quantum-well combined with simultaneous vapor-phase epitaxy of the insulator on top of the quantum-well. Ultra-thin single-crystalline Si buried in a single-crystalline insulator matrix with sharp interfaces was obtained by this approach on Si(111). Finally, the incorporation of crystalline Si islands into single-crystalline oxide layers will be demonstrated.

The impact of interface layer composition on electrical properties of epitaxial Gd"2O"3 thin films on Si(001) substrates have been investigated. The electrical properties of epitaxial Gd"2O"3 thin films were improved significantly by controlled modification of interface layer composition. The minimum capacitance equivalent thickness estimated for Pt/Gd"2O"3/Si MOS structures was as low as 0.76 nm with leakage current density of 15 mA/cm^2 at (V"g-V"F"B)=1 V. The corresponding density of interface states was found to be 2.3*10^1^2cm^-^2eV^-^1. We also find that a change in the interface layer composition significantly alters band alignment of Gd"2O"3 layer with respect to Si substrates.

We studied the growth and electrical properties of single crystalline mixed (Nd1 − xGdx)2O3 (NGO) thin films and compared the results with those of the binary Gd2O3 and Nd2O3 thin films, respectively. Epitaxial ternary NGO thin films were grown on Si(100) substrates using modified solid state molecular beam epitaxy. The films were characterized physically using various techniques. The capacitance equivalent oxide thickness of a 4.5 nm NGO thin film extracted from capacitance–voltage (C–V) characteristics was 0.9 nm, which is lower than all values reported earlier for other crystalline oxides. The leakage current density and the density of interface traps were 0.3 mA/cm2 at |Vg − VFB| = 1 V and 1.4 × 1012/cm2, respectively. These excellent electrical properties of NGO thin films demonstrate that such ternary oxides could be one of the promising candidates for gate dielectrics in the upcoming generations of complementary metal oxide semiconductor (CMOS) devices.

Single-crystalline double-barrier Gd 2 O 3 /Si/Gd 2 O 3 nanostructures on Si(1 1 1) were prepared using molecular beam epitaxy. Ultra-thin single-crystalline Si buried in a single-crystalline insulator matrix with sharp interfaces was obtained by a novel approach based on an epitaxial encapsulated solid-phase epitaxy. The I – V characteristic of the obtained nanostructures exhibited resonant tunneling at low temperatures.

We report about the formation of twinning-superlattice regions in Si epitaxial layers grown by multi-step molecular beam epitaxy on Si ( 1 1 1 ) ( 3 × 3 ) R 30 ∘ -B surfaces in which boron acts as a subsurfactant. Twinning-superlattice regions were formed by periodical arrangement of 180° rotation twins along the [1 1 1] direction separated by a few nanometers. The multi-step procedure consists of repeating several growth, boron deposition and annealing cycles on boron-predeposited undoped Si substrates. It is shown that the amount of subsurface boron and the growth mode influence the formation of twin boundaries. Only the nucleation of Si on the Si ( 1 1 1 ) ( 3 × 3 ) R 30 ∘ -surface covered by at least 1/3 ML subsurface boron results in the formation of 180° rotation twins. The presented technology should be suitable to prepare Si polytypes.

We have investigated the growth and electrical properties of crystalline Gd2O3 grown on 6H-SiC(0001) substrates by molecular beam epitaxy. Initially, Gd2O3 islands with hexagonal structure were formed. Further growth resulted in the formation of flat layers in a mixture of [111]-oriented cubic bixbyite and monoclinic structure. The fabricated capacitors with 14 nm Gd2O3 exhibited suitable dielectric properties at room temperature; such as a dielectric constant of e = 22 , a leakage current of 10−8 A/cm2@1 V and breakdown fields >4.3 MV/cm.

Epitaxial gadolinium oxide (Gd(2)O(3)) high-k dielectrics are investigated with respect to their CMOS compatibility in metal oxide semiconductor (MOS) capacitors and field effect transistors (MOSFE ...

We report about the formation of twinning-superlattice regions in Si epitaxial layers grown by molecular beam epitaxy on Si(1 1 1)( 3 × 3 )R30°-B surfaces. Twinning-superlattice regions were formed by periodical arrangement of 180° rotation twin boundaries along [1 1 1]-direction and are only separated by a few nanometers. The preparation method consists of repeating several growth, boron-deposition and annealing cycles on boron-predeposited undoped Si substrates. It is shown that the amount of subsurface boron and the growth mode influence the formation of twin boundaries. Only the nucleation of Si on the Si(1 1 1)( 3 × 3 )R30°-surface covered by at least 1 3 ML boron results in the formation of 180° rotation twins. The size of superlattice regions is restricted by surface morphology. However, the presented technology should also be suitable to prepare a new type of semiconductor heterostructure based on Si polytypes.

The different aspects of molecular beam epitaxy (MBE) for producing two-dimensional (Quantum well), one-dimensional (Quantum wire and rod), and zero-dimensional (Quantum dot) structures based on SiC for functional applications are discussed. Development and implementation of a suitable MBE growth procedure for fabrication of heteropolytypic layer sequences are demonstrated in context with thermodynamic considerations. Furthermore, the growth of onedimensional structures based on cubic wires and nanorod arrays, also grown on Si(111), is shown. Moreover, the perspectives of quantum dot structures and a novel way to form 3C-SiC-dot structures within α-SiC has been discussed.

Activation energies and atomic geometries associated with Si adsorption and desorption processes on the Si face of SiC (0001) have been obtained from in situ reflection high-energy electron diffraction measurements and ab initio pseudopotential calculations. During the initial stage of Si deposition, the SiC (0001) surface develops a series of superstructures, depending on temperature and Si coverage. A kinetic diagram of the structural transformations during Si exposure or annealing is outlined and the adsorption energy Eais determined. The latter is found to be strongly affected by the surface structure and coverage, changing from 4.5 eV to 1.6 eV as the geometry of the Si adlayer changes from [Formula: see text] to (3×3). These observations are interpreted with the help of ab initio pseudopotential calculations. It is concluded that [Formula: see text] measured on [Formula: see text] corresponds to adsorption at adatom vacancy while Ea3=1.6 eV measured on (3×3) corresponds to adsorption on top of a capping adatom. Finally, the development of surface structures from the [Formula: see text] surface to silicon-rich and carbon-rich extremes is discussed.

We show results for molecular beam epitaxial growth of praseodymium oxide on Si. On Si(1 0 0) oriented surfaces, crystalline Pr 2 O 3 grows as (1 1 0)-domains, with two orthogonal in-plane orientations. Epitaxial overgrowth with Si could not been realized so far. We obtain perfect epitaxial growth of hexagonal Pr 2 O 3 on Si(1 1 1). These layers can also be overgrown epitaxially with Si leading to novel tunnel structures. Crystalline Pr 2 O 3 on Si(0 0 1) is a promising candidate for highly scaled gate insulators, displaying sufficiently high- K value of around 30, ultra-low leakage current density, good reliability, and high electrical breakdown voltage. The Pr 2 O 3 /Si(0 0 1) interface exhibits the symmetric band alignment, desired for applying such material in both n- and p-type devices. The valence band as well as the conduction band offset to Si is above 1 eV. The electron masses can be assumed to be very heavy in the oxide. This effect together with the suitable band offsets leads to the unusually low leakage currents found experimentally. Finally, the integration of crystalline Pr 2 O 3 high- K gate dielectrics into a conventional CMOS process will be demonstrated.

In recent years, new types of semiconductor heterostructures consisting of only one material in different crystal structures, such as wurtzite/zinc-blende heterostructures (heteropolytypic structures) are under discussion. Such heterostructures maintain a completely defect-free, lattice matched, and coherent interface and effects due to different chemical constituents can be avoided. In this field, silicon carbide (SiC) is the most promising candidate because SiC crystallizes in more than two different stable structures. The preparation of heteropolytypic structures by only a change of the crystal structure during the growth is a great challenge and is realized only under well-defined conditions of molecular beam epitaxy (MBE). In this paper an overview is given of the results and conclusions of recent material research on the MBE growth, characterization and properties of SiC heteropolytypic structures and related materials.

Two types of nanostructures in SiC matrices are studied. Heterocrystalline structures are based on different polytypes, e.g. 3C-SiC and 4H-SiC. Multi-quantum well structures with 3C wells embedded in 4H barriers show photoluminescence (PL) that can be interpreted in terms of a type-II heterostructure character and a built-in electric field due to the pyroelectricity of 4H. Germanium is incorporated in 4H-SiC matrices in form of nanometer-sized crystals. Their size, crystal structure and orientation depend on the preparation conditions. A strong size-dependent luminescence from quantum states is predicted.

X-ray photoelectron spectroscopy (XPS) was performed to study the formation process of Pr2O3/Si(001) interfaces and films during epitaxial growth and postgrowth annealing. A significant shift in the Pr and O core-level binding energy was found accompanied by an analogous shift in the Pr2O3 valence band maximum. This shift depends on the oxide layer thickness and interface structure, as indicated by ab initio pseudopotential calculations. It is caused by interface dipoles in the Si/Pr2O3 interface region due to the accumulation of oxygen. Besides providing a insight into the growth mechanism and interface properties of high-K dielectrics on Si, our results also demonstrate the usefulness of in vacuo XPS for investigating epitaxial growth processes.

Multi-quantum well structures with 3C–SiC wells between α-SiC barriers were grown in a two-step procedure by solid-source molecular beam epitaxy. First, one-dimensional wire-like 3C–SiC was nucleated selectively on terraces of the well-prepared off-axis α-SiC(0 0 0 1) substrates at low temperature (T K ) . Next, 3C–SiC lamellae were incorporated into the hexagonal layer material via simultaneous step-flow growth mode of both the 3C–SiC nuclei and the hexagonal substrate material at higher T and Si-rich conditions. In comparison to homopolytypic SiC layers, photoluminescence investigations revealed additional signals, which can be explained by optical transitions within the thin cubic well layers.

Prototypical native defects, in particular monovacancies, are studied using ab initio density functional theory and the local spin-density approximation. Several properties such as the energetics, geometry, electronic structure, and spin states are discussed regarding their dependence on the chemical nature, the preparation conditions, and the polytype of the SiC crystal. Consequences of the defects are derived for the doping behaviour, electrical properties, and photoluminescence spectra.

The growth of SiC layers on hexagonal (or α-) SiC(0001) has been performed by solid-source MBE between 1300 and 1600 K. The α-SiC layers have been grown homoepitaxial via step-flow on off-axis substrates, whereas pseudomorphic cubic (or 3C-) SiC layers were obtained on α-SiC via nucleation and subsequent step-flow. Under more equilibrium-like conditions, 3C-layers nearly free of twin-boundaries were obtained. The SiC layers were of high quality and without unintentional doping, as revealed by photoluminescence investigations. The controlled growth of SiC heteropolytypic structures consisting of hexagonal and cubic polytypes, such as 4H/3C/4H-SiC(0001) and 6H/3C/6H SiC(0001), has also been demonstrated. Such structures were obtained by changing the growth conditions from lower temperatures (1550 K) and Si-rich Si/C ratio (3C-SiC) to higher temperatures (1600 K) and more C-rich Si/C ratio. On off-axis substrates, such heterostructures were also obtained by first nucleating selectively wire-like 3C-SiC nuclei on the terraces of well-prepared α-SiC(0001) substrates at low T (

The molecular beam epitaxial growth of Si on SiC(0 0 0 1), exhibiting the Stranski–Krastanov mode, was investigated by reflection high-energy electron diffraction (RHEED). In the initial stage some irregular RHEED oscillations were observed corresponding to both the occurrence of surface superstructures and monolayer coverages. After exceeding a critical coverage, Si island formation sets in. Two different kinds of islands were found with (1 1 1)/(0 0 0 1) and (1 1 0)/(0 0 0 1) epitaxial relationship. Under near equilibrium conditions, the critical coverage was found to be 1.4 monolayers and corresponds to the occurrence of a 3×3 superstructure remaining also as wetting layer after the island formation suggesting that the basic units (tetramers) forming the 3×3 superstructures act already as nucleation sites for the islands. Island formation at high deposition rates (R) and low temperatures (T) was found to be kinetically delayed, which can be described as a function of R and the diffusivity D by the relationship tc ∝ R/D . Si islands relatively uniform in size of some nanometers and a density of 1011/cm2 were obtained under these conditions. At lower R the critical thickness is only a function of T, indicating that the incorporation time of adatoms is the relevant time scale for surface diffusion. Ordered arrays of small dots were grown by decorating the steps on vicinal surfaces with Si islands.