Helmut Jorke

The n-channel SiGe/Si modulation-doped field-effect transistor

At the heterointerface of Si1-xGex/Si the existence of two-dimensional carrier gas has recently been demonstrated. The electrons are confined inside the large-gap material Si. We report the first fabrication of n-channel modulation-doped SiGe/Si hetero field-effect transistors by use of molecular-beam epitaxial growth. Though neither layer sequence nor parasitic resistances were optimized, these first transistors exhibit an extrinsic transconductance of 40 mS/mm for a gate length of 1.6 µm. This value is higher than that of conventional Si MESFETs of comparable carrier concentration. Technological processing steps and device evaluation are described.

Low temperature kinetics of Si(100) MBE growth

Abstract In temperature ramped Si(100) molecular beam epitaxy films the crystalline /disorder transition seen at very low temperatures shifts to lower temperatures when the deposition rate is lowered. At deposition rates less than 0.025 nm s−1 no crystalline/disorder transition could be detected down to 120 °C. Cross-section transmission electron microscopy reveals the appearance of cone-like defects prior to the crystalline/disorder transition which are accompanied by twins. The results are described by a kinetic model which predicts the existence of a finite epitaxy width ΔE in the very low temperature growth regime.

Assessment of band offsets in Si/SiGe strained layer superlattices by vertical transport measurements

We report on current-voltage characteristics obtained from vertical carrier transport measurements at Si/SiGe superlattices. The period length of the superlattice was varied from 40 A up to 1000 A. The n-type layers were grown on (100) Si n+ substrates by molecular beam epitaxy. Within this range of period lengths both strained layer superlattices and incommensurate superlattices were achieved. An n+ top layer ensured low ohmic contacts. Strain symmetrization within the superlattice was achieved by initially growing an incommensurate SiGe buffer layer. Evaluation of room temperature current-voltage characteristics at small electric fields resulted in resistances which significantly depend on whether or not the superlattice was strained. Comparison of these experimental results with model calculations based on the Smoluchowski equation enables an estimation of the barrier height which can be approximately identified with the band offset of this particular system. As a result, strain in the superlattice considerably influences the conduction band offset.

Electron transport in bipolar transistors with biaxially strained base layers

In silicon npn bipolar junction transistors grown on (100) oriented substrate, at base doping levels in excess of 10/sup 20/ boron atoms/cm/sup 3/, strain induced splitting of the normally sixfold degenerated conduction band minimum becomes important and needs to be considered in modeling of injection currents. The biaxial tensile strain, originating in the smaller covalent radius of boron compared to silicon, induces a lowering of two valleys with heavy effective mass in vertical direction whereas the remaining four valleys are raised in energy. Using a coupled set of equations for the electron gas systems in the twofold and fourfold degenerated valleys, emitter and collector current formulas are derived. In the relevant case of strong f-type intervalley scattering rates compared to Auger recombination rates (which holds at least up to about 10/sup 21/ cm/sup -3/) collector currents are described by (V/sub BC/=0 V) j/sub C/=-e(D/sub n4/n/sub 4,0/+D/sub n2/n/sub 2,0/)/w(e/sup V(BE/V(th)/)-1) provided that the electron diffusion length is large compared to the base width w. D/sub n4/ D/sub n2/, and n/sub 4,0/, n/sub 2,0/ are diffusion constants and equilibrium minority carrier concentrations in the two electron gas systems, respectively. In Si/SiGe heterojunction bipolar transistors the conduction band situation in the base is similar to that in extremely heavily boron doped (homojunction) base layers as presence of Ge also causes the conduction band minimum to split (splitting is, however, of opposite sign). Thus, the transport model discussed here applies also to that kind of device.

Two-dimensional electron gas properties of symmetrically strained Si/Si1−xGex quantum well structures

Abstract n-type modulation-doped Si/Si 0.5 Ge 0.5 strained layer multiple quantum well structures have been grown on (100)Si substrates by molecular beam epitaxy. The concept of strain symmetrization has been applied by inserting a strain-relaxed SiGe buffer layer. The samples have been investigated by Hall measurements supplemented by Shubnikov-de Haas and Rutherfored backscattering analysis. We present mobility results on the two-dimensional electron gas in the silicon wells as a function of electron concentration, quantum well width, and spacer thickness. The influence of the sample parameters on the electron mobility is discussed. With an electron sheet concentration of 2.4×10 12 cm −2 per well, a well width of 10 nm, and a spacer thickness of 10 nm a Hall mobility at room temperature of 1280 cm 2 (V s) -1 has been achieved.

58.3: Recent Progress in Interference‐filter‐based Stereoscopic 3D LCD

A novel stereo 3D LCD for passive interference filter glasses is presented. A demonstrator based on a standard 120Hz LCD was set up. Stereoscopic image separation was realized in a time- sequential mode using a LED-based scanning backlight with two complementary spectra. A stereo brightness of 10 cd/m2 and a channel separation of 30:1 were achieved.

Tunnelling currents in very narrow p+–n+ junctions

Abstract Epitaxially grown p + –i–n + junctions were processed to diodes by mesa etching and contacted by Cr/Au metallization. Rim currents, which often plague mesa devices were reduced by a special design with the contact area separated from the mesa rim. The dependence on the current voltage characteristics on contact area, on temperature and on external load was investigated. Four different regimes were identified where Zener tunnelling, Esaki tunnelling, tunnelling through midgap states and diffusion of minority carriers dominate the current, respectively. Esaki tunnelling leads to negative differential resistance which causes oscillations depending on the external load. The results are interpreted by considering the nominal intrinsic width, the extension of the depletion layer and the abruptness of the doping transition.

Very low temperature growth and doping of silicon MBE layers

Abstract (100) Silicon molecular beam epitaxy films with etch pit densities below 10 3 cm -2 and X min values of 3.3%–3.9% were grown at very low temperatures (250–350 °C). δ-Doping at a monolayer Sb deposition shows a dopant activation of 0.45-0.81 with no detectable broadening at T s =200 °C growth temperature.

Assessment of transport parameters for the design of high speed Si/SiGe HBTs with compositionally graded base

In heterojunction bipolar transistors current gain cut-off frequencies f 1 are limited by time constants of electron transport across the base and the collector space charge zone, This point of view favours the use of small widths of the base. However, such a choice would increase the base resistance which is detrimental to the maximum frequency of oscillation f max , A way out of this trade-off is to accelerate minority carriers by using a graded or step base structure. Besides their potential to keep transit times low such sophisticated HBT structures may show, in addition, an active behaviour above the cut-off frequencies due to the transit time effect which is commonly ignored in bipolar transistors, A key parameter for that is the minority diffusion constant of electrons in the SiGe base. Our experimental study on this parameter in the present work indicates a value that is above that of minority carrier transport in pure silicon.

Growth and characterization of Si/Sb superlattices

SummaryArtificial Sim/Sb1 superlattices (m=15, 30, 60) were grown by molecular beam epitaxy at very low temperatures (200°C). Rutherford backscattering spectroscopy revealed the superlattices to be single crystalline with χmin values of less than 0.04. X-ray diffraction clearly showed the superlattice structure. However, residual surface segregation still activated at the low temperatures employed leads to a spread out of Sb atoms into the subsequently grown Si interspacing layers to an extent of about 2 nm.