53 research outputs found
Sharp MIR plasmonic modes in gratings made of heavily doped pulsed laser-melted Ge1-xSnx
No full textPlasmonic structures made out of highly doped group-IV semiconductor materials are of large interest for the realization of fully integrated mid-infrared (MIR) devices. Utilizing highly doped Ge1-xSnx alloys grown on Si substrates is one promising route to enable device operation at near-infrared (NIR) wavelengths. Due to the lower effective mass of electrons in Sn compared to Ge, the incorporation of Sn can potentially lower the plasma wavelength of Ge1-xSnx alloys compared to that of pure Ge. However, defects introduced by the large lattice mismatch to Si substrates as well as the introduction of alloy scattering limit device applications in practice. Here, we investigate pulsed laser melting as one strategy to increase material quality in highly doped Ge1-xSnx alloys. We show that a pulsed laser melting treatment of our Ge1-xSnx films not only serves to lower the material’s plasma frequency but also leads to an increase in active dopant concentration. We demonstrate the application of this material in plasmonic gratings with sharp optical extinction peaks at MIR wavelengths
Titanium and Nickel as alternative materials for mid Infrared plasmonic
No full textFinding suitable materials with low Drude losses for plasmonic applications at mid-infrared wavelengths is challenging. Highly doped Germanium and Germanium Tin alloys have been investigated to that end but are difficult to utilize at wavelengths closer to the near infrared region. In this work, we present results on the fabrication and optical characterization of Titanium and Nickel comb antennas. Extinction spectra were obtained via Fourier Transform Infrared (FTIR) Spectroscopy and verified via Finite Difference Time Domain (FDTD) simulation. The measured spectra show distinct extinction peaks in the range between 4 μm and 11 μm which can be correlated to a plasmonic mode forming at the antenna substrate interface showing that it is possible to excite localized plasmonic modes at the interface between such antennas and a Silicon substrate
Strained Ge Channels with High Hole Mobility Grown on Si Substrates by Molecular Beam Epitaxy
No full textStrained modulation-doped quantum wells (QW) offer a huge potential for semiconductor device applications due to their high mobility. The material Ge is particularly interesting here, exhibiting the highest bulk hole-mobility of all known semiconductors. However, the growth for Ge QW structures is quite complex and a special virtual substrate (VS) technique is needed. The VS is commonly grown with thicknesses of over 1 μm , making it difficult for integration with other devices on a single chip. In this paper, we report on the growth of a 15 nm thick strained Ge QW on top of a Si1−xGex VS and Si0.2Ge0.8 buffer layer, using Molecular Beam Epitaxy. The Si1−xGex VS is grown by deposition of 100 nm Ge with a subsequent high-temperature annealing step, followed by a 100 nm thick Si0.2Ge0.8 buffer layer. The resulting two-dimensional hole gas reaches a hole mobility of over 8⋅104cm2V−1s−1 with a corresponding sheet carrier density of 5.7⋅1011cm−2 at 8 K. The Ge QW is further analysed, and comparing it to a sample with a higher VS thickness, a possible limitation of the mobility due to background doping is being discussed. These results show that complementary metal-oxide-semiconductor (CMOS) compatible device integration of the Ge QW is possible, thin buffer layers suffice for the mobilities achieved and background doping limits the low-temperature mobility
High mobility Ge 2DHG based MODFETs for low-temperature applications
No full textModulation-doped field-effect transistors (MODFET) that are typically based on either a two-dimensional electron gas or a two-dimensional hole gas (2DHG) are highly suitable for low-temperature applications due to the high mobilities attained by the charge carriers in the device channel. The Ge 2DHG is particularly interesting, since Ge exhibits the highest hole mobility among all semiconductors. In this paper, we report on the temperature-dependent direct current-characteristics of normally-on MODFETs based on a high mobility Ge 2DHG. Here, we specifically investigate device characteristics down to cryogenic temperatures and analyze the impact of different MOD-doping on device performance. We find that the largest On-Off ratio of I-On/I-Off = 3.2 x 10(6) and lowest sub-threshold swing (SS) of SS = 64 mV dec(-1) at a temperature of T = 7.5 K are attained for a MOD-doping of N-A = 5 x 10(17) cm(-3), while the highest effective mobility mu(eff) = 864 cm(2) V(-1)s(-1) is obtained from a device with a MOD-doping of N-A = 1.1 x 10(18) cm(-3). Furthermore, while effective mobilities in the devices are strongly reduced compared to Hall mobilities measured in the unstructured Ge 2DHGs, both quantities can be seen to follow a similar trend. Our results motivate further investigations of Ge 2DHG MODFETs for applications in cryogenic low-noise amplifiers, besides their well-established potential for spintronic applications
A mid-infrared laser microscope for the time-resolved study of light-induced protein conformational changes
No full textWe have developed a confocal laser microscope operating in the mid-infrared range for the study of light-sensitive proteins, such as rhodopsins. The microscope features a co-aligned infrared and visible illumination path for the selective excitation and probing of proteins located in the IR focus only. An external-cavity tunable quantum cascade laser provides a wavelength tuning range (5.80-6.35 mu m or 1570-1724 cm(-1)) suitable for studying protein conformational changes as a function of time delay after visible light excitation with a pulsed LED. Using cryogen-free detectors, the relative changes in the infrared absorption of rhodopsin thin films around 10(-4) have been observed with a time resolution down to 30 ms. The measured full-width at half maximum of the Airy disk at lambda= 6.08 mu m in transmission mode with a confocal arrangement of apertures is 6.6 mu m or 1.1.. Dark-adapted sample replacement at the beginning of each photocycle is then enabled by exchanging the illuminated thin-film location with the microscope mapping stage synchronized to data acquisition and LED excitation and by averaging hundreds of time traces acquired in different nearby locations within a homogeneous film area. We demonstrate that this instrument provides crucial advantages for time-resolved IR studies of rhodopsin thin films with a slow photocycle. Time-resolved studies of inhomogeneous samples may also be possible with the presented instrument
Plasmonics-integrated Ge PIN-photodetectors
No full textRefractive index sensors can be designed for high sensitivities at small device foot-print. Using a complementary metal-oxide-semiconductor compatible process for fabrication paves the way to cheap devices enabling integrated biosensors. We present results on combining vertical Ge PIN photodetectors with metallic nanostructures such as Al nanohole arrays placed directly on top of the diode. The interaction of plasmonic resonances and photonic modes such as waveguide modes or optical modes in nanostructured photodetectors can potentially be exploited to design refractive index sensors with very high sensitivities. We discuss how the interplay of material properties and device geometry can be tailored for applications in integrated biosensing
Optical characterization of highly n-type doped Ge0.95Sn0.05 rod antennas on Si(001) substrates
No full textPlasmonic excitations in metal nanostructures can be used to control and manipulate optical energy in the visible and infrared spectrum and have been used to enable biosensing, to enhance absorption and quantum yields for photovoltaics and to enhance the energy efficiency of light-emitting devices. For light at mid-infrared (MIR) wavelengths, metals become less suitable for plasmonic applications as a result of the high Drude losses. At those wavelengths, highly doped semiconductors are potential alternatives. The doped group IV alloy Ge x Sn y has been shown to be particularly interesting. We present results on the fabrication and optical characterization of rod antennas fabricated from highly n-type doped Ge 0.95 Sn 0.05 (n-Ge x Sn y ) on Si(001) substrates. Extinction spectra were obtained via Fourier Transform Infrared (FTIR) Spectroscopy. To verify the measurement results the behavior of the n-Ge 0.95 Sn 0.05 rod antennas was simulated. The results show that the n-Ge 0.95 Sn 0.05 rod antennas absorb MIR radiation through plasmonic excitation. Two different peaks could be observed in the extinction spectra of the n-Ge 0.95 Sn 0.05 rod antennas and attributed to two locally separated plasmonic modes. One mode forms on the n-Ge 0.95 Sn 0.05 /Si interface of the n-Ge 0.95 Sn 0.05 rod antennas, the other one forms at the n-Ge 0.95 Sn 0.05 /air interface. We discuss possible applications of this type of n-Ge 0.95 Sn 0.05 rod antennas for MIR sensing
Optofluidic sensor system with Ge PIN photodetector for CMOS-compatible sensing
No full textVertical optofluidic biosensors based on refractive index sensing promise highest sensitivities at smallest area footprint. Nevertheless, when it comes to large-scale fabrication and application of such sensors, cheap and robust platforms for sample preparation and supply are needed—not to mention the expected ease of use in application. We present an optofluidic sensor system using a cyclic olefin copolymer microfluidic chip as carrier and feeding supply for a complementary metal–oxide–semiconductor compatibly fabricated Ge PIN photodetector. Whereas typically only passive components of a sensor are located within the microfluidic channel, here the active device is directly exposed to the fluid, enabling top-illumination. The capability for detecting different refractive indices was verified by different fluids with subsequent recording of the optical responsivity. All components excel in their capability to be transferred to large-scale fabrication and further integration of microfluidic and sensing systems. The photodetector itself is intended to serve as a platform for further sophisticated collinear sensing approaches
Electrical Characterization of SiGeSn/Ge/GeSn-pin-Heterodiodes at Low Temperatures
No full textThe combination of the ternary alloy Si x Ge l-x-y Sn y with Gel -y Sn y is very promising for electrooptical applications in the near infrared regime up to 2.5 μm wavelength. With the tunable bandgap at a non-varying lattice constant SixGe l-x-y Sny is predestined for the lattice matched growth on a Ge virtual substrate and the integration of pseudomorphic Gel-ySny layers with high Sn content (> 10 %). The main challenge of the growth of such alloys is to achieve a low density of defects. However, in the last few years there was a major progress in growing highly doped SixGe l-x-y Sny layers with good crystal quality. In this work we investigate the electrical characteristics of a SixGe l-x-y Sny/Ge/Ge l-y Sny -pin-heterodiode in a temperature range from 300 K to 8 K. This temperature depended measurement provides the opportunity for a more precise characterization of such diodes. A linear relation between reciprocal temperature and the ideality factor is found. With the extrapolation of this relation up to room temperature the ideality factor of the diode is calculated (n = 1.22). From the temperature dependent reverse current the activation energy is determined ( EA=0.178 eV). We discuss the possibility to utilize such diodes for near infrared electrooptical applications
Polarization-resolved surface-enhanced infrared spectra with nanosensors based on self-organized gold nanorods
No full textBiosensors are becoming ubiquitous in the study of biomolecules, as, by modifying shape size and environment of metallic nanostructures it is now possible to engineer the field so to monitor subtle transient changes in molecular conformation at the level of a single biolayer. In this paper, we present a first step towards a polarization-resolved study of light-induced conformational changes of transmembrane proteins. We exploit a platform of self-organized gold nanorods on SiO2 substrates to enhance the infrared reflection absorption spectroscopy and to perform difference spectroscopy (i.e., spectrum under visible light ON minus spectrum under visible light OFF) on a light-sensitive transmembrane protein with simultaneous visible light illumination from the backside of the chip. The broad size distribution of nanorods allows us to probe with high sensitivity the modifications of the vibrational peaks over the entire fingerprint region. We show that it is possible to identify dissimilarities in the difference spectra, which in turn implies that we are monitoring over a broadband spectrum not only the chemical bonds with the dipole moment aligned orthogonally to our substrate/nanorod surface but also those with different orientation
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