K. Werner

Technology for high‐performance n‐channel SiGe modulation‐doped field‐effect transistors

A fabrication process for n‐channel SiGe modulation‐doped field‐effect transistors has been developed in which the strain in the heterostructure is completely preserved by taking into account an appropriate temperature range for the formation of ohmic contacts to source drain by implantation. It yields a low specific contact resistance of 7.1×10−5 Ω cm2 to be compared to 5.7×10−2 Ω cm2 for alloyed Au/Sb contacts. With the same thermal upper limit the application of different gate oxides has been studied. All oxides show negligible gate leakage enabling a superior gate voltage swing compared to devices with a Schottky gate. Application of thermal oxide grown at 650 °C yields a superior field‐effector transistor performance compared to devices with sputter and plasma‐enhanced chemical‐vapor deposition deposited oxides due to its lower and negative charge level. Gate recessing of 0.5 μm Schottky gates yields a transconductance increase up to 310 mS/mm.

Gas source MBE growth of SiSiGe device materials

Abstract This contribution reviews the status quo for the growth of Si SiGe device materials on Si(001) substrates by Si SiGe gas source MBE (GSMBE) technology. The technology is compared to related growth techniques like ultra-high vacuum chemical vapour deposition (UHV-CVD) and low pressure CVD (LP-CVD) in terms of equipment, process conditions and precursors used. Materials aspects like n- and p-type dopant behaviour, Ge incorporation and interface qualities are compared to those of the competing technologies. Realised devices based on GSMBE material such as heterostructure bipolar transistors (HBTs), (electro-)luminescent quantum well structures and modulation doped heterostructures for two-dimensional electron and hole gases are reviewed. Particular emphasis is given to devices that exploit selective area epitaxy (SAE). The limits of SAE in GSMBE are discussed. Devices employing SAE into small structures for the manufacture of e.g. semiconducting quantum point contacts are given. Summarising, we point to the particular strongholds of GSMBE within the device and materials research field; a field that demands versatility and flexibility but not necessarily high throughput. An outlook to the envisaged future role of GSMBE within research and production is given.

Reactive ion etching mechanism study on Si/GexSi1−x

Abstract We investigate the reactive ion etching characteristics of epitaxial Ge x Si 1− x ( x ≤ 0.25) alloys in SF 6 O 2 He and SiCl 4 / Cl 2 / He plasmas. A high resolution patterning process for Ge 0.25 Si 0.75 using SF 6 / O 2 / He as a dry etch gas mixture has been developed. A model is proposed to explain the anomalously high etch rates of Ge x Si 1− x in both gas mixtures as well as the different elemental enrichments reported in literature from in situ XPS studies on plasma exposed reaction layers.

Roughening of SiGe layers grown with gas-source MBE: dependence on Ge concentration and growth temperature

Abstract We have investigated the roughening of SiGe layers grown from disilane and germane on Si(001) using gas-source molecular beam epitaxy (GSMBE). Si 1 − x Ge x layers were grown in the temperature range of 500–650°C while the Ge concentration x was varied between 0.06–0.28. Photoluminescence spectroscopy (PL) was used to calibrate the Ge concentration in SiGe layers. During growth the SiGe surfaces were inspected with reflection high-energy electron diffraction (RHEED) and after deposition with scanning electron microscopy (SEM). At low temperatures and Ge concentrations we found smooth (2D) layers. However, for high temperatures and Ge concentrations we obtained rough {118} and {113} faceted layers (3D). Comparison with solid-source MBE (SSMBE) results (Bean et al. [J. Vac. Sci. Technol. A 2 (1984) 436]) reveal that 3D growth in GSMBE is observed for lower Ge concentrations and temperatures. This difference can be explained by the much lower growth rates used in our experiments. At higher growth rates the surface atom diffusion length is reduced and the development of the thermodynamically favourable 3D phase is suppressed.

High‐resolution reactive ion etching and damage effects in the Si/GexSi1−x system

The reactive ion etching characteristics of epitaxial GexSi1−x(x≤0.25) alloys in SF6/O2/He and SiCl4/Cl2/He plasmas have been studied. A high‐resolution patterning process for Ge0.25Si0.75 using SiCl4/Cl2/He as a dry etch gas mixture has been developed. The etch rate and etch profile of Ge0.25Si0.75 are found to be strongly influenced by the substrate temperature and the oxygen content in the SF6/O2/He gas mixture. Conductance measurements of dry etched narrow wires in a Si/Ge0.2Si0.8 two‐dimensional hole gas at 4.2 K reveal less sidewall damage for gas mixture SiCl4/Cl2 compared with SF6/O2/He, SiCl4/Cl2/He, and SF6/O2. Scattering in the conducting wires is studied using weak localization theory.

Nanofabrication of Si point contacts using gas-source MBE: towards single-crystalline devices

We describe a nanofabrication process for three-dimensional Si point contacts with a large single-crystalline constriction region. The process is based on solid-source MBE to grow electrodes of the point contacts, on gas-source MBE (GSMBE) to grow the constriction region and on e-beam lithography and wet etching to produce nanoholes in the insulator. From TEM and SEM analysis we find that the GSMBE-grown faceted crystal, which forms the constriction region, is larger than the nanohole as a result of epitaxial lateral overgrowth. For the process conditions used the largest crystal is four times larger than the hole, so that the ideal point contact structure is approached. Room temperature I-V measurements on fabricated devices are linear and behave according to the Maxwell resistance, as expected for diffusive transport.

Plasma oxidation of Si and SiGe

Abstract This paper reports the latest progress in the oxidation of Si and SiGe by plasma anodisation at a temperature below 150°C. The interface state density for the SiGe sample is substantially reduced and the surface inversion is observed. The interfacial stability is tested under the biased temperature stress.

Chemical vs. physical factors in dry etching induced damaged in the Si/Ge x Si 1−x system

Abstract Dry etching induced damage in n-type strained Si Si 1-x Ge x heterostructures has been studied by conductance and low field magnetoconductance measurements on quantum wires at cryogenic temperatures. Wires were etched in SF 6 O 2 or Cl2 plasma at different ion energies. The chemical constitution of the sidewalls was measured by XPS, showing Ge enrichment after fluorine and Si enrichment after chlorine plasma processing. The overall XPS and electrical data are analysed in terms of dry etching induced damage from direct ion impact and a component originating from surface charges and traps due to oxidation of the sidewall reaction layer.

Fabrication of submicron SiSiGe double barrier resonant tunneling structures

We have developed a process for the fabrication of submicron Si/SiGe double barrier resonant tunneling structures. All patterning is done by electron beam lithography. Mesas with diameters down to 480 nm are etched by reactive ion etching with an anisotropy of 7:1. The etch is followed by planarization and etch-back, contact hole etching and metallization. The low temperature I-V characteristics exhibited tunneling through 2D quantum well states and through defect states. Furthermore, we observed random telegraph noise in the current and a switching effect dependent on bias history. These effects are probably also caused by defect states.

Silicon point contacts: Nanofabrication, molecular beam epitaxial growth, and transport measurements

We describe a fabrication process for three‐dimensional Si point contacts. The process is based on molecular beam epitaxy to grow electrodes of the devices and on electron‐beam lithography and wet etching to produce holes in the insulator. From scanning electron microscopy and transmission electron microscopy analysis we find that the lower electrode has a perfect crystalline structure, while the upper electrode has a composite structure consisting of a single‐crystalline column grown out of the hole in the insulator and a poly‐crystalline environment. The smallest holes realized have a diameter of 40 nm. I–V curves of point contacts with a boron concentration of about 5×1018 cm−3 and with diameters in the range 200–2000 nm are linear at 300 K and indicate behavior according to the Maxwell resistance, as expected for diffusive transport. Below 40 K variable‐range hopping is the transport mechanism. In this temperature range quadratic I–V curves develop, which can be understood from electric field activate...