Jörg Schulze

Scaling the vertical tunnel FET with tunnel bandgap modulation and gate workfunction engineering

In this paper, we look into the scaling issues of a vertical tunnel field-effect transistor (FET). The device, a gated p-i-n diode based on silicon, showed gate-controlled band-to-band tunneling from the heavily doped source to the intrinsic channel. An exponentially increasing input characteristics, perfect saturation in the output characteristics, and off-currents of the order of 1 fA//spl mu/m for sub-100-nm channel lengths were observed. Further, with a /spl delta/p/sup +/ SiGe layer at the p-source end, improvements in the device performance in terms of on-current, threshold voltage and subthreshold swing were shown, albeit trading off the off-currents which increase with Ge content x. We show here that the tunnel FET performance is nearly independent of channel length scaling L and with /spl delta/p/sup +/ SiGe layer, scaling t/sub ox/ is not critical to tunnel FET scaling. Further, with gate workfunction engineering, the tunnel FET can be tuned to achieve a high on-current as well as very low off-currents. Due to the perfect saturation in the output characteristics, the device looks good for sub-100-nm low-power analog devices.

Performance Enhancement of Vertical Tunnel Field-Effect Transistor with SiGe in the δp+ Layer

The metal–oxide–semiconductor (MOS)-based vertical tunnel field effect transistor (FET) on silicon has been proposed earlier and which showed gate-controlled band-to-band tunneling from the valence band in the heavily doped δp+ layer at source to the conduction band in the inversion channel. In this work, using 2D computer simulation, we further investigate the device performance enhancement with SiGe in the δp+ layer. On-current as well as threshold voltage are seen to improve considerably and meet the roadmap technology requirements. We also show that unlike the conventional MOSFET, the subthreshold swing of the vertical tunnel FET is not limited to the theoretical value of 60 mV/dec at room temperature.

Grazing incidence small angle x-ray scattering from free-standing nanostructures

We develop the theory for grazing incidence small-angle x-ray scattering (GISAXS) from nanometer-sized naked islands on a flat substrate in the framework of the distorted-wave Born approximation (DWBA). The scattered wave amplitude is composed of four terms, including all combinations of scattering from the islands and reflection from the substrate. We apply this theory to x-ray measurements on Ge islands grown on Si(111), and show that we can determine the full triangular symmetry of these islands. The results also show that the DWBA must be used for smooth substrates near the angle of total external reflection. We finally discuss the advantages of GISAXS as compared to transmission small angle x-ray scattering for determining the symmetry of nanostructures.

Germanium-tin p-i-n photodetectors integrated on silicon grown by molecular beam epitaxy

GeSn heterojunction p-i-n diodes with a Sn content of 0.5% are grown with a special low temperature molecular beam epitaxy. The Sn incorporation in Ge is facilitated by a very low temperature growth step in order to suppress Sn surface segregation. Diodes with sharp doping transitions are realized as double mesa structures with a diameter from 1.5 up to 80 μm. An optical responsivity of these GeSn diodes of 0.1 A/W at a wavelength of λ=1.55 μm is measured. In comparison with a pure Ge detector the optical responsivity is increased by factor of 3 as a result of Sn caused band gap reduction.

A simulation approach to optimize the electrical parameters of a vertical tunnel FET

Using two-dimensional device simulations, the electrical parameters of gated tunnel field-effect transistor (FET) are optimized with a SiGe delta doped layer in the source region. In order to prove the validity of the simulation models we compare simulation results with the experimentally realized tunnel FET on silicon and show that it gives a good match. It is shown that the incorporation of pseudomorphic strained-Si/sub 1-x/Ge/sub x/ layers leads to a significant performance increase. Furthermore, it becomes evident that the improvements are not a direct consequence of bandgap lowering but rather an indirect consequence of tunnel barrier width lowering. This leads to an asymmetry in the n- and the p-channel performance.

GeSn p-i-n detectors integrated on Si with up to 4% Sn

GeSn heterojunction photodetectors on Si substrates were grown with Sn concentration up to 4%, fabricated for vertical light incidence, and characterized. The complete layer structure was grown by means of ultra low temperature (100 °C) molecular beam epitaxy. The Sn content shifts the responsivity into the infrared, about 310 nm for the 4% Sn sample. An increase of the optical responsivity for wavelengths higher than 1550 nm can be observed with increasing Sn content. At 1600 nm, the optical responsivity is increased by more than a factor of 10 for the GeSn diode with 4% Sn in comparison to the Ge reference diode.

Room-Temperature Electroluminescence From GeSn Light-Emitting Pin Diodes on Si

In this letter, a GeSn light-emitting pin diode integrated on Si via a Ge buffer is demonstrated and it is compared with a light-emitting pin diode made from pure, unstrained Ge on Si. The diode layer structures are grown with a special low-temperature molecular beam epitaxy process. The pseudomorphic GeSn layers (1.1% Sn content) on the Ge buffer are compressively strained. Both light-emitting pin diodes clearly show direct bandgap electroluminescence emission at room temperature. The electroluminescence peak of the GeSn light-emitting pin diode is shifted by 20 meV into the infrared region compared to the electroluminescence peak of the unstrained Ge light-emitting pin diode. The shift is due to the lower bandgap of GeSn and the influence of strain.

GeSn Heterojunction LEDs on Si Substrates

GeSn on Si light-emitting diodes (LEDs) is investigated for different Sn concentrations up to 4.2% and they are compared with an LED made from pure Ge on Si. The LEDs are realized from in-situ doped pin junctions in GeSn on Ge virtual substrates. The device structures are grown with a special ultra-low temperature molecular beam epitaxy process. All LEDs clearly show direct bandgap electroluminescence emission at room temperature. The light intensity of the compressively strained GeSn LEDs increases with higher Sn concentration. The in-plane strain of the LEDs is determined with reciprocal space mapping. The bandgap energies of the emitting GeSn layer are calculated from the emission spectra.

GeSn-on-Si normal incidence photodetectors with bandwidths more than 40 GHz.

GeSn (Sn content up to 4.2%) photodiodes with vertical pin structures were grown on thin Ge virtual substrates on Si by a low temperature (160 °C) molecular beam epitaxy. Vertical detectors were fabricated by a double mesa process with mesa radii between 5 µm and 80 µm. The nominal intrinsic absorber contains carrier densities from below 1 · 10(16) cm(-3) to 1 · 10(17) cm(-3) for Ge reference detectors and GeSn detectors with 4.2% Sn, respectively. The photodetectors were investigated with electrical and optoelectrical methods from direct current up to high frequencies (40 GHz). For a laser wavelength of 1550 nm an increasing of the optical responsivities (84 mA/W -218 mA/W) for vertical incidence detectors with thin (300 nm) absorbers as function of the Sn content were found. Most important from an application perspective all detectors had bandwidth above 40 GHz at enough reverse voltage which increased from zero to -5 V within the given Sn range. Increasing carrier densities (up to 1 · 10(17) cm(-3)) with Sn contents caused the depletion of the nominal intrinsic absorber at increasing reverse voltages.

Direct bandgap narrowing in Ge LED’s on Si substrates

In this paper we investigate the influence of n-type doping in Ge light emitting diodes on Si substrates on the room temperature emission spectrum. The layer structures are grown with a special low temperature molecular beam epitaxy process resulting in a slight tensile strain of 0.13%. The Ge LEDs show a dominant direct bandgap emission with shrinking bandgap at the Γ point in dependence of n-type doping level. The emission shift (38 meV at 10²⁰cm⁻³) is mainly assigned to bandgap narrowing at high doping. The electroluminescence intensity increases with doping concentrations up to 3x10¹⁹cm⁻³ and decreases sharply at higher doping levels. The integrated direct gap emission intensity increases superlinear with electrical current density. Power exponents vary from about 2 at low doping densities up to 3.6 at 10²⁰cm⁻³ doping density.