H. von Känel

High hole mobility in Si0.17Ge0.83 channel metal–oxide–semiconductor field-effect transistors grown by plasma-enhanced chemical vapor deposition

We report on effective hole mobility in SiGe-based metal–oxide–semiconductor (MOS) field-effect transistors grown by low-energy plasma-enhanced chemical vapor deposition. The heterostructure layer stack consists of a strained Si0.17Ge0.83 alloy channel on a thick compositionally-graded Si0.52Ge0.48 buffer. Structural assessment was done by high resolution x-ray diffraction. Maximum effective hole mobilities of 760 and 4400 cm2/Vs have been measured at 300 and 77 K, respectively. These values exceed the hole mobility in a conventional Si p-MOS device by a factor of 4 and reach the mobility data of conventional Si n-MOS transistors.

SILICON EPITAXY BY LOW-ENERGY PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION

A new technique for semiconductor epitaxy at low substrate temperatures is presented, called low-energy dc plasma enhanced chemical vapor deposition. The method has been applied to Si homoepitaxy at substrate temperatures between 400 and 600 °C and growth rates between 0.1 and 1 nm/s, using silane as the reactive gas. The quality of the Si films has been examined by reflection high-energy electron diffraction, scanning tunneling microscopy, cross-section transmission electron microscopy, and high-resolution x-ray diffraction. Two effects have been identified to lead to the formation of stacking faults after an initial layer of defect-free growth: (1) substrate bombardment by ions with energies in excess of 15 eV, and (2) hydrogen adsorption limiting the surface mobility of Si atoms and silane radicals. Both result in the accumulation of surface roughness, facilitating the nucleation of stacking faults when the roughness reaches a critical level. Defect introduction can be eliminated effectively by biasing...

A plasma process for ultrafast deposition of SiGe graded buffer layers

Low energy plasma enhanced chemical vapor deposition (LEPECVD) has been applied to the synthesis of Si-modulation doped field effect transistor structures, comprising a SiGe relaxed buffer layer and a modulation doped strained Si channel. A growth rate of at least 5 nm/s for the relaxed SiGe buffer layer is well above that obtainable by any other technique. Due to the low ion energies involved in LEPECVD, ion damage is absent, despite a huge plasma density. The structural quality of the LEPECVD grown SiGe buffer layers is comparable to that of state-of-the-art material. The electronic properties of the material were evaluated by growing modulation doped Si quantum wells on the buffer layers. We obtain a low temperature (2 K) Hall mobility of μH=2.5×104 cm2/Vs for the electrons in the Si channel at an electron sheet density of ns=8.6×1011 cm−2.

Epitaxy of fluorite-structure silicides : metastable cubic FeSi2 on Si(111)

Epitaxial FeSi2 of cubic symmetry has been grown for the first time on Si(111) by solid phase epitaxy (SPE) and molecular beam epitaxy (MBE). Structural investigation by reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM) suggest the phase to have the same CaF2 structure as CoSi2 and NiSi2. Ultraviolet photoelectron spectroscopy (UPS) and resistivity measurements show that the phase is metallic and undergoes an irreversible metal/semiconductor transition to the orthorhombic β-FeSi2 phase at a temperature depending on the thickness of the film. Hall effect measurements have revealed holes to be the dominant carrier species. High resolution TEM studies, together with the observation of an anomalous Hall effect as well as theoretical considerations, indicate that this cubic FeSi2 phase is magnetic, in contrast to CoSi2 and NiSi2.

Alternatives to thick MBE-grown relaxed SiGe buffers

Abstract We have investigated several growth concepts for strain relieved SiGe buffers as basis for high frequency transistors. Modulation doped quantum wells (MODQWs) were realized by molecular beam epitaxy (MBE) on top of thick graded buffers prepared by MBE, ultra-high vacuum chemical vapor deposition (UHVCVD) and low-energy plasma-enhanced CVD (LEPECVD). Additionally, thin buffers including a specific layer grown at low temperature (LT) were realized entirely by MBE. The overgrown thick CVD samples show comparable transport properties and thermal stabilities to those on thick graded MBE buffers. Mobilities of up to 90 000 cm 2 /V s have been measured at 30 K. Thin LT-MBE structures show slightly worse properties but are superior to conventional constant composition buffers.

Scattering mechanisms in high-mobility strained Ge channels

We report on the low-temperature mobility in remotely doped p-type strained Ge layers on relaxed Si0.3Ge0.7 virtual substrates, grown by low-energy plasma-enhanced chemical vapor deposition. A maximum mobility of 120 000 cm2 V−1 s−1 has been reached at 2 K, at a carrier sheet density of 8.5×1011 cm−2. Analysis of the mobility and Dingle ratio τ/τq as a function of sheet density suggests that remote impurity scattering is the limiting factor at low sheet densities, but that interface impurities become more important as the sheet density increases.

Universal shapes of self-organized semiconductor quantum dots: Striking similarities between InAs∕GaAs(001) and Ge∕Si(001)

The model systems for self-organized quantum dots formed from elemental and compound semiconductors, namely Ge grown on Si(001) and InAs on GaAs(001), are comparatively studied by scanning tunneling microscopy. It is shown that in both material combinations only two well-defined families of faceted and defect-free nanocrystals exist (and coexist). These three-dimensional islands, pyramids and domes, show common morphological characteristics, independently of the specific material system. A universal behavior is further demonstrated in the capping-passivation process that turns the nanocrystals in true quantum dots.

Ultralow dark current Ge/Si(100) photodiodes with low thermal budget

Vertical incidence photodiodes were fabricated from Ge grown epitaxially on Si(100) by low-energy plasma-enhanced chemical vapor deposition. Consideration of the energy band profiles of n-i-p and p-i-n heterostructures, and optimization of growth processes and thermal budget, allowed the performance of Ge photodectors integrated on Si(100) substrates to be optimized. Record low dark current density of Js=4.1×10−5 A/cm2 and external quantum efficiency at 1550 nm of η=32% were measured.

Shape preservation of Ge/Si(001) islands during Si capping

Coherently strained Ge/Si(001) islands were overgrown with Si at temperatures ranging from 300 to 550 °C. The induced shape changes were investigated at different stages of the capping process by scanning tunneling microscopy and high-resolution transmission electron microscopy. Islands were found to strongly flatten and intermix at temperatures above ∼450 °C. By contrast, a good shape preservation as well as the recovery of a flat Si surface above the buried islands can be achieved by low temperature capping at 300 °C followed by Si growth at 550 °C.

Monolayer resolution by means of x-ray interference in semiconductor heterostructures

We show that the interference of x‐ray wave fields in semiconductor heterostructures can be used to detect ultrathin layers having 1 monolayer thickness. A detailed theoretical and experimental investigation on Si/Six Ge1−x heterostructures is presented. The interference effect is studied experimentally by using a high‐resolution double‐crystal x‐ray diffractometer. The diffraction patterns are recorded in symmetrical as well as asymmetrical Bragg geometries and are analyzed by using the dynamical x‐ray diffraction theory for distorted crystals.