Abrupt Interfaces Research Articles

Abrupt SiGe-to-Si interface: influence of chemical vapor deposition processes and characterization by different metrology techniques

Future electronic (gate-all-around transistors) and photonic devices (electro-absorption modulator and laser stack) will require SiGe/Si multilayer stacks with abrupt interfaces. Chemical vapor deposition (CVD) is a proven method to epitaxially grow such multilayer structures. However, the abruptness of the compositional change is limited by Ge segregation occurring during Si layer growth on a (Si)Ge surface. Here, we study the compositional abruptness at the interface between Si0.70Ge0.30 and Si layers epitaxially grown in a CVD reactor. The ultra-thin interface layer between SiGe and Si is characterized by transmission electron microscopy, secondary ion mass spectrometry and spectroscopic ellipsometry (SE). Our results show that a Si layer grown using a chlorinated chemistry produces the most abrupt interface. For Si deposition processes with non-chlorinated chemistry, lower growth temperature and a higher order hydride precursor result in a reduced transition thickness. Furthermore, we show that the interface thickness measured by the different metrology techniques are in good agreement, suggesting that SE can be used to evaluate the ultra-thin interface at the Si0.70Ge0.30-to-Si interface, resulting in considerably shorter development time in a production environment.

Producing Atomically Abrupt Axial Heterojunctions in Silicon-Germanium Nanowires by Thermal Oxidation.

Compositional abruptness of the interfaces is one of the important factors to determine the performance of Group IV semiconductor heterojunction (Si/Ge or Si/SiGe) nanowire devices. However, forming abrupt interfaces in the nanowires using the common vapor-liquid-solid (VLS) method is restricted because large solubility of Si and Ge in the Au eutectic liquid catalyst makes gradual composition change at the heterojunction after switching the gas phase components. According to the VLS growth mechanism, another possible approach to form an abrupt interface is making a change of the semiconductor concentration in the eutectic liquid before precipitation of the second phase. Here we show that the composition in AuSiGe eutectic liquid on SiGe nanowires of low Ge concentration (≤6%)can be altered by thermal oxidation at 700 °C.During the oxidation process, only Si is oxidized on the surface of the eutectic liquid, and the Ge/Si ratio in the eutectic liquid is increased. The subsequently precipitated SiGe step at the liquid/solid interface has a higher Ge concentration (∼20%), and a compositionally abrupt interface is produced in the nanowires. The growth mechanism of the heterojunction includes diffusion of Si and Ge atoms on nanowire surface into the AuSiGe eutectic liquid and step nucleation at the liquid/nanowire interface.

Fabrication of atomically abrupt interfaces of single-phase TiH2 and Al2O3

We report the fabrication of atomically abrupt interfaces of titanium dihydride (δ-TiH2) films and α-Al2O3(001) substrates. With the assistance from reactive hydrogen in plasma, single-phase δ-TiH2 epitaxial thin films were grown on α-Al2O3(001) substrates using the reactive magnetron sputtering technique. Scanning transmission electron microscopy measurements revealed an atomically abrupt interface at the δ-TiH2(111) film and Al2O3(001) substrate. These results indicate that the reactive magnetron sputtering has great potential to deposit various epitaxial thin films of hydrides restricted by the hydrogenation limit. The fabrication of high-quality hydride epitaxial thin films with atomically controlled interfaces paves the way for future hydride electronics.

Toward the Atomically Abrupt Interfaces of SiOx/Semiconductor Junctions

A novel simple method to produce a high-quality interface between semiconductor (e.g., Ge or III-V) and dielectric film (i.e., SiO2) is suggested. A nonoxygen element is deposited directly on an oxidized substrate at T ≥ 300 °C, leading to semiconductor reduction and dielectric film growth. As a result, the formed interface is atomically abrupt and has a low density of defect gap states. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Self-Driven Photodetector and Ambipolar Transistor in Atomically Thin GaTe-MoS2 p-n vdW Heterostructure.

Heterostructure engineering of atomically thin two-dimensional materials offers an exciting opportunity to fabricate atomically sharp interfaces for highly tunable electronic and optoelectronic devices. Here, we demonstrate abrupt interface between two completely dissimilar material systems, i.e, GaTe-MoS2 p-n heterojunction transistors, where the resulting device possesses unique electronic properties and self-driven photoelectric characteristics. Fabricated heterostructure transistors exhibit forward biased rectifying behavior where the transport is ambipolar with both electron and hole carriers contributing to the overall transport. Under illumination, photoexcited electron-hole pairs are readily separated by large built-in potential formed at the GaTe-MoS2 interface efficiently generating self-driven photocurrent within <10 ms. Overall results suggest that abrupt interfaces between vastly different material systems with different crystal symmetries still allow efficient charge transfer mechanisms at the interface and are attractive for photoswitch, photodetector, and photovoltaic applications because of large built-in potential at the interface.

Oxide Thin Film Heterostructures on Large Area, with Flexible Doping, Low Dislocation Density, and Abrupt Interfaces: Grown by Pulsed Laser Deposition

Advanced Pulsed Laser Deposition (PLD) processes allow the growth of oxide thin film heterostructures on large area substrates up to 4-inch diameter, with flexible and controlled doping, low dislocation density, and abrupt interfaces. These PLD processes are discussed and their capabilities demonstrated using selected results of structural, electrical, and optical characterization of superconducting (YBa2Cu3O7−δ), semiconducting (ZnO-based), and ferroelectric (BaTiO3-based) and dielectric (wide-gap oxide) thin films and multilayers. Regarding the homogeneity on large area of structure and electrical properties, flexibility of doping, and state-of-the-art electronic and optical performance, the comparably simple PLD processes are now advantageous or at least fully competitive to Metal Organic Chemical Vapor Deposition or Molecular Beam Epitaxy. In particular, the high flexibility connected with high film quality makes PLD a more and more widespread growth technique in oxide research.

An improved three-dimensional full-vectorial finite-difference imaginary-distance beam propagation method

A modified alternating direction implicit approach is proposed to discretize the three-dimensional full-vectorial beam propagation method (3D-FV-BPM) formulation along the longitudinal direction. The cross-coupling terms (CCTs) are neglected at the first sub-step, and then double used at the second substep. The order of two substeps is reversed for each transverse electric field component so that the CCTs are always expressed in an implicit form, thus the calculation is efficient and stable. Based on the multinomial inter-polation, a universal finite difference scheme with a high accuracy is developed to ap-proximate the 3D-FV-BPM formulation along the transverse directions, in which the dis-continuities of the normal components of the electric field across the abrupt dielectric in-terfaces are taken into account and can be applied to both uniform and non-uniform grids. The corresponding imaginary-distance procedure is first applied to a buried rectangular and a GaAs-based deeply-etched rib waveguide. The field patterns and the normalized propagation constants of the fundamental and the first order modes are presented and the hybrid nature of the full-vectorial guided-modes is demonstrated, which shows the validity and utility of the present approach. Then the modal characteristics of the deeply- and shallow-etched rib waveguides based on the InGaAsp/InGaAsP strained multiple quantum wells in InP substrate are investigated in detail. The results are necessary for modeling and the design of the planar lightwave circuits or photonic integrated circuits based on these waveguides.

Structure of ultrathin SiO2/Si(111) interfaces studied by photoelectron spectroscopy

Device-grade ultrathin (9–22 Å) films of silicon dioxide, prepared from crystalline silicon by remote-plasma oxidation, are studied by soft x-ray photoelectron spectroscopy (SXPS). The 2p core-level spectra for silicon show evidence of five distinct states of Si, attributable to the five oxidation states of silicon between Si0 (the Si substrate) and Si4+ (the thin SiO2 film). The relative binding energy shifts for peaks Si1+ through Si4+ (with respect to Si0) are in agreement with earlier work. The relatively weaker signals found for the three intermediate states (I1, I2, and I3) are attributed to silicon atoms at the abrupt interface between the thin SiO2 film and substrate. Estimates of the interface state density from these interface signals agree with the values reported earlier of ∼2 monolayers (ML). The position and intensity of the five peaks are measured as a function of post-growth annealing temperature, crystal orientation, and exposure to He/N2 plasma. We find that annealing produces more abrupt interfaces (by reducing the suboxide interface state density), but never more abrupt than ∼1.5 monolayers. We observe a 15%–20% drop in the interface thickness (in particular the “Si2+” peak intensity) with increasing annealing temperature. Somewhat different behavior is observed with small amounts of nitrogen in the SiO2 film where an apparent increase in interface state density is seen. A quantitative analysis is presented which explores the effects of these sample preparation parameters in terms of relative interface state density and modeling of the SXPS data.

Ultrahigh Electron Mobilities in Si1-xGex/Si/Si1-xGex Heterostructures with Abrupt Interfaces Formed by Solid-Phase Epitaxy

Si1-xGex/Si/Si1-xGex heterostructures with a flat channel were successfully fabricated by molecular-beam epitaxy combined with solid-phase epitaxy. The solid-phase epitaxial growth completely suppressed Ge segregation between the channel layer and the doped layer. Cross-sectional transmission-electron-microscope images revealed that misfit dislocations were limited to the bottom part of the buffer layer and to the substrate, and that the heterointerfaces were very smooth and flat. The electron mobility for a sample with a graded buffer layer was nearly one order of magnitude higher than without such a layer. Ultrahigh mobility of 8.0×105 cm2V-1s-1 was obtained at 15 K for a sample with a graded buffer layer (x=0.2).

Study of SI1−xGEx / SI / SI1−xGEx Heterostructures with abrupt interfaces for ultrahigh mobility FETS

AbstractWe grew Si1−x, Gex / Si / Si1−xGex heterostructures with abrupt interfaces for a high-speed device application. In order to eliminate the interface and the impurity scatterings, a two-stepsolid-phase epitaxy (SPE) was developed newly. This SPE can obtain an abrupt interface between the upper Si1−xGex. and the Si-channel layers as well as a sharp Sb-doping profile in the upper Si1−xGex layer. Single-step SPE growth of the Si1−xGex. layer including the Sb-doped layer caused solid state diffusion of Sb and made the doping-profile difficult to control. A two-step SPE, (that is, two independent SPE processes forming the Si1−xGex. and the Sb-doped layers), obtained a sharp Sb depth profile and high electrical activation in the Sb-doped Si1−xGex. layer. However, the number of modulation-doped carriers in the Si channel layer was small. Calculation of the carrier distribution between the channel and the doped layer suggests that the carrier density in the channel is low when the Sb-doping profile is extremely sharp; thus, precise control of the modulation-doped-carrier density by optimizing the thickness of the Sb-doped layer is crucial for operating modulation-doped field-effect transistors (MODFETs).

Photoluminescence of Si1-xGex/Si Quantum Wells with Abrupt Interfaces Formed by Segregant-Assisted Growth

Photoluminescence (PL) of Si1-x Gex /Si quantum wells with abrupt interfaces formed by segregant-assisted growth (SAG) is reported. It was found that SAG-grown samples show spectral red-shift compared to those grown by conventional solid-source molecular beam epitaxy due to the suppression of Ge surface segregation during growth. Crystal quality of SAG-grown samples is discussed based on the excitation variation of PL intensity.

Interface structure and optical properties of quantum wells and quantum boxes

We investigate two types of GaAs–GaAlAs interfaces in single quantum wells: (a) The abrupt interface produced by growth interruption during molecular-beam epitaxy; (b) the interdiffused interface produced by Ga+ ion implantation and annealing. The abrupt interfaces are shown to contain islands with sizes ≲1.5 μm. The ion implanted and interdiffused interfaces are characterized by an error function Al concentration profile. The Ga and Al interdiffusion is found to be markedly enhanced (up to ≊100 times at 900 °C) following the ion implantation and annealing. Quantum well boxes (QWB) are produced using the implantation enhanced interdiffusion (IEI) process. The new cathodoluminescence spectral lines observed in the analysis of QWB’s are attributed to three-dimensional carrier confinement in the potential wells induced by locally enhanced interdiffusion.

Formation of Abrupt Interfaces between Surface Silicon and Buried SiO2 Layers by Very High Dose Oxygen-Ion Implantation

Formation of Abrupt Interfaces between Surface Silicon and Buried SiO2 Layers by Very High Dose Oxygen-Ion Implantation