High Resistivity Float Zone Silicon Research Articles

All-silicon, low-cross-talk terahertz waveguide crossing based on effective medium.

All-silicon effective-medium-clad waveguides are a promising candidate for an integrated terahertz platform with high efficiency and broad bandwidth. Waveguide crossings are essential circuit components, allowing for wave routing over shorter paths to increase circuit density. However, the simple intersection of two orthogonal effective-medium-clad waveguides results in terahertz wave scattering, leading to relatively high cross talk. In this work, a low-loss, 40% fractional bandwidth crossing utilizing Maxwell-Garnet effective-medium theory and wavefront planarization techniques is proposed. This monolithic structure is fabricated on a single high-resistivity float-zone silicon wafer using a deep reactive ion etching process with a modest 4.4 mm diameter (4.03λ0) structure footprint. Experimentally verified results show low insertion loss, less than 1 dB, and average cross talk level of -39dB for both E11x and E11y operating modes, over 220-330 GHz with a 40% fractional bandwidth. This waveguide crossing can be foreseen as a useful routing component for terahertz all-silicon integrated circuits. The proposed techniques are applicable to other dielectric waveguide platforms at infrared and optical frequencies.

Design and Application of Optical System Based on Terahertz Time-of-Flight Imaging

Using the penetration and transientness of Terahertz pulses and the photoconductive delayed detection method, the tomographic imaging of the interior of the object based on the time of flight can be realized. By comparing the transmittance of several crystals and polymers in the Terahertz band, Polymethylpentene (TPX) was selected as the material of Terahertz lens, and High Resistivity Float Zone Silicon (HRFZ-Si) as the material of beam splitter; By comparing the focal depth of normal incidence and oblique incidence optics system, the normal incidence optical system was used to realize time-of-flight imaging; Using the idea of optical-mechanical integration, the corresponding optical camera is designed according to the normal incidence optical system, and the peak-to-valley map and thickness map of the Lithium cobaltate coating of aluminum foil were obtained by time-of-flight imaging, and the defects such as complete shedding of the coating and uneven thickness were observed. The results show that the normal incidence optical system can realize Terahertz time-of-flight imaging and is applicable in the industrial nondestructive testing field.

Design and Application of Optical System Based on Terahertz Time-of-Flight Imaging

Using the penetration and transientness of Terahertz pulses and the photoconductive delayed detection method, the tomographic imaging of the interior of the object based on the time of flight can be realized. By comparing the transmittance of several crystals and polymers in the Terahertz band, Polymethylpentene (TPX) was selected as the material of Terahertz lens, and High Resistivity Float Zone Silicon (HRFZ-Si) as the material of beam splitter

All-optical high-speed modulation of THz transmission through silicon core optical fibers.

High speed optical modulation of THz radiation is of interest for information processing and communications applications. In this paper infrared femtosecond pulses are used to generate free carriers that reduce the THz transmission of silicon based waveguides over a broad spectral range. Up to 96% modulation is observed from 0.5 to 7 THz in an optical fiber with a 210 µm diameter gold-doped silicon core. The observed carrier recombination time of 2.0 ± 0.2 ns makes this material suitable for high speed all-optical signal processing. These results show both enhanced modulation depth and reduced carrier lifetime when compared to the performance of a high resistivity float zone silicon rectangular guide with comparable cross sectional area.

Broadband infrared and THz transmitting silicon core optical fiber

Silicon waveguide structures are a viable alternative for the transmission of signals over a wide range of frequencies, and new fabrication methods are key to increased applications. In this work, THz transparency of silicon-core, silica clad fibers, refined using a traveling solvent method, is demonstrated. The ≈ 200 µm core of these fibers is shown to have good transmission from 4.8–9 µm and 1–7 THz. Fibers were drawn on a conventional optical fiber tower using the scalable molten core technique and CO2 laser annealed, resulting in large-grain crystalline cores with broadband transmission. The spectral properties are comparable to those of rectangular guides of similar cross-sectional area cut from high resistivity float zone silicon wafers.

Characteristics of Effective-Medium-Clad Dielectric Waveguides

Effective-medium-clad dielectric waveguides are purely built into a single high-resistivity float-zone silicon wafer with their claddings defined by deep subwavelength perforations. The waveguides are substrate-free while supporting both $E_{11}^x$ and $E_{11}^y$ modes with low loss and low dispersion. This article extends the investigations of the waveguides by analyzing the dispersion, cross-polarization, and crosstalk together with the characteristics of bends and crossings over an operation frequency range of 220–330 GHz (WR-3 band). Taking the $E_{11}^x$ mode as an example, the experimental results show an average measured attenuation coefficient of 0.075 dB/cm and a group velocity dispersion ranging from around $\pm$ 10 ps/THz/mm across the whole band. A crosstalk level below $-$ 10 dB is measured for parallel waveguides with a separation of 0.52 $\lambda _{0}$ at 300 GHz. The realized waveguides show a bending loss ranging from 0.500 to 0.025 dB per bend and a crosstalk at crossing below $-$ 15 dB from 220 to 330 GHz. Due to the different dispersion characteristics, the $E_{11}^y$ mode has similar performances but with its operation frequency range reduced to 260–330 GHz. Limited by the measurement setup, a cross-coupling between the $E_{11}^x$ and $E_{11}^y$ modes is measured to be below $-$ 20 dB over the whole band. This in-depth investigation of effective-medium-clad waveguides will form a basis for terahertz-integrated platforms.

Interference elimination based on the inversion method for continuous-wave terahertz reflection imaging.

We propose a novel approach based on the inversion method to eliminate interference in the continuous-wave (CW) terahertz (THz) reflection imaging. Through the study on the imaging window of the CW-THz reflection imaging with the interference mechanism, inverse processing is introduced to realize the interference elimination. Based on the theoretical calculation, high resistivity float-zone silicon (HRFZ-Si) with high refractive index is selected as the imaging window to improve the dynamic range of the THz image. The interference elimination method is verified experimentally by a CW-THz reflection imaging system based on a THz quantum cascade laser (QCL) lasing at 4.3THz. The reflectivities of liquid samples of water and ethanol are restored by the interference elimination method, which corresponds well with the theoretical calculation. Moreover, the interference elimination method is performed on THz images of fresh biological tissues. The image contrast of tissue can be greatly enhanced with the accurate reflective information.

Investigation of nitrogen enriched silicon for particle detectors

This article explores the viability of nitrogen enriched silicon for particle physics application. For that purpose silicon diodes and strip sensors were produced using high resistivity float zone silicon, diffusion oxygenated float zone silicon, nitrogen enriched float zone silicon and magnetic Czochralski silicon. The article features comparative studies using secondary ion mass spectrometry, electrical characterization, edge transient current technique, source and thermally stimulated current spectroscopy measurements on sensors that were irradiated up to a fluence of 1015 neq/cm2. Irradiations were performed with 23 MeV protons at the facilities in Karlsruhe (KIT), with 24 GeV/c protons at CERN (PS-IRRAD) and neutrons at the research reactor in Ljubljana. Secondary ion mass spectrometry measurements give evidence for nitrogen loss after processing, which makes gaining from nitrogen enrichment difficult.

Terahertz waveguiding in glass-clad silicon wafers

The waveguiding properties of high-resistivity float zone silicon slab waveguides are characterized over the spectral range from 0.5 to 7.5 THz. Waveguide modes and dispersion are observed for lengths of 1.2 cm and silicon thicknesses from 40 to 300 µm. The influence of core thickness and cladding glass attenuation is characterized, and modeled transmitted pulse shapes compare well to the measured signals. Fused silica cladding allows propagation in the 40 µm thick wafer, demonstrating the feasibility of developing flexible semiconductor core fibers for THz transmission.

On-chip single-mode high-Q terahertz whispering gallery mode resonator

The chip-scale terahertz (THz) devices and systems are helpful to enable actual applications in THz communication and sensing. Here, we demonstrate an on-chip THz whispering gallery mode resonator (WGMR) delivering a high-quality (Q) factor TE single mode, fabricated on a high-resistivity float zone silicon wafer by complementary metal–oxide–semiconductor compatible technology. For a TE mode in a WGMR, it is the first time to achieve a Q factor of 1210 at 0.4925 THz with 28 dB extinction ratio, to the best of our knowledge. With this excellent WGMR, we have demonstrated a stable and highly sensitive sensor for particle distance estimation. Our investigation provides an effective approach to fabricate high-performance integrated THz-WGMRs for chip-scale systems.

Ultra-high Q terahertz whispering-gallery modes in a silicon resonator

We report on the first experimental demonstration of terahertz (THz) whispering-gallery modes (WGMs) with an ultra-high quality factor of 1.5 × 104 at 0.62 THz. The WGMs are observed in a high resistivity float zone silicon spherical resonator coupled to a sub-wavelength silica waveguide. A detailed analysis of the coherent continuous wave THz spectroscopy measurements combined with a numerical model based on Mie-Debye-Aden-Kerker theory allows us to unambiguously identify the observed higher order radial THz WGMs.

2D tomographic terahertz imaging using a single pixel detector.

A 2D tomographic terahertz imaging set-up using a single pixel imaging approach is realized, where a liquid helium cooled bolometer is utilized as a bucket detector and a mercury-arc lamp serves as a broadband terahertz source. The different patterns of the terahertz radiation, which are necessary for the single pixel imaging approach, are realized by spatially addressed photodoping of a high resistivity float zone silicon window, employing a near-infrared laser diode, which is spatially modulated by a digital micromirror device. The two investigated sample objects have cylindrical and cuboid shapes and consist of polypropylene. Both sample shapes cause strong influences of refraction, reflection and diffraction, which distort the measured projections and thus have to be considered in the tomographic reconstruction. In order to consider these effects, a model is developed which combines refraction and diffraction effects by a hybrid approach using ray tracing and scalar diffraction theory yielding finally projections of the sample objects. These simulated projections are compared to the measured projections and show a good agreement between the experimental results and the developed model. In accordance with this result, an optimization problem is formulated, which offers an approach for tomographic reconstruction using the developed model.

First results on DEPFET Active Pixel Sensors fabricated in a CMOS foundry—a promising approach for new detector development and scientific instrumentation

DEPFET Active Pixel Sensors (APS) have been introduced as focal plane detectors for X-ray astronomy already in 1996. Fabricated on high resistivity, fully depleted silicon and back-illuminated they can provide high quantum efficiency and low noise operation even at very high read rates. In 2009 a new type of DEPFET APS, the DSSC (DEPFET Sensor with Signal Compression) was developed, which is dedicated to high-speed X-ray imaging at the European X-ray free electron laser facility (EuXFEL) in Hamburg. In order to resolve the enormous contrasts occurring in Free Electron Laser (FEL) experiments, this new DSSC-DEPFET sensor has the capability of nonlinear amplification, that is, high gain for low intensities in order to obtain single-photon detection capability, and reduced gain for high intensities to achieve high dynamic range for several thousand photons per pixel and frame. We call this property “signal compression”.Starting in 2015, we have been fabricating DEPFET sensors in an industrial scale CMOS foundry maintaining the outstanding proven DEPFET properties and adding new capabilities due to the industrial-scale CMOS process. We will highlight these additional features and describe the progress achieved so far. In a first attempt on double-sided polished 725 μm thick 200 mm high resistivity float zone silicon wafers all relevant device related properties have been measured, such as leakage current, depletion voltage, transistor characteristics, noise and energy resolution for X-rays and the nonlinear response. The smaller feature size provided by the new technology allows for an advanced design and significant improvements in device performance. A brief summary of the present status will be given as well as an outlook on next steps and future perspectives.

Processing and characterization of edgeless radiation detectors for large area detection

The edgeless or active edge silicon pixel detectors have been gaining a lot of interest due to improved silicon processing capabilities. At VTT, we have recently triggered a multi-project wafer process of edgeless silicon detectors. Totally 80 pieces of 150mm wafers were processed to provide a given number of detector variations. Fabricated detector thicknesses were 100, 200, 300 and 500µm. The polarities of the fabricated detectors on the given thicknesses were n-in-n, p-in-n, n-in-p and p-in-p. On the n-in-n and n-in-p wafers the pixel isolation was made either with a common p-stop grid or with a shallow p-spray doping. The wafer materials were high resistivity Float Zone and Magnetic Czochralski silicon with crystal orientation of <100>. In this paper, the electric properties on various types of detectors are presented. The results from spectroscopic measurement show a good energy resolution of the edge pixels, indicating an excellent charge collection near the edge pixels of the edgeless detector.

Improved Vertical p-Type Radio Frequency Metal-Base Transistors

This letter reports radio frequency (RF) transistors using a high-resistivity p-type float zone silicon wafer. A high-density uniform organic semiconducting single layer is deposited using thermal evaporation technique, and RF transistors with a base layer of about 60 nm are fabricated. We report on a transistor exhibiting a cutoff frequency (fT) of 630 kHz. In addition to the cutoff frequency, other important figures of merit for the RF transistor are also presented, including the common-emitter current gain and on/off ratio.

Degradation of high-resistivity float zone and magnetic Czochralski n-type silicon detectors subjected to 2-MeV electron irradiation

Particle-tracking detectors made on high-resistivity (HR) float zone (FZ) silicon are widely used in high-energy physics experiments. It is known that the incorporation of oxygen in the FZ Si can lead to some improvement in the radiation hardness of the material. In this contribution we investigate the effects of 2 MeV electron irradiation, up to a fluence of 5×10 16 e/cm 2, on the electrical and carrier lifetime properties of p-on-n silicon diodes fabricated on different substrate materials, including HR standard and oxygenated FZ, as well as HR magnetic Czochralski silicon, with a higher intrinsic oxygen contents. A progressive degradation of the characteristics is observed for all devices, pointing to a generation of bulk damage. Interestingly, a significant increase of the effective carrier concentration is observed after the highest fluences for all materials. Under the limited experimental conditions studied, no significant changes are observed for diode characteristics subjected to a thermal annealing treatment at 80 °C. This degradation in the electrical properties should be taken into account for the use of such HR Si materials under high-energy electron environments.

Monolithic multichannel secondary electron detector for distributed axis electron beam lithography and inspection

The attractiveness of electron beam systems would be greatly enhanced if the throughput could be improved. One approach, described previously by the authors employs a uniform axial magnetic field to focus thousands of electron beams simultaneously [D. S. Pickard et al., J. Vac. Sci. Technol. B 21, 2709 (2003); T. R. Groves and R. A. Kendall, ibid., 16, 3168 (1998)]. The beamlets never combine to form a common crossover, thereby avoiding the throughput limitations due to space charge blurring. With this approach, one challenge was to fashion a detection scheme that maintains a tight beamlet packing density (250μm pitch) while minimizing cross-talk between adjacent secondary electron signals, either by crossing trajectories or within the detector. A pin-diode-based detector was investigated as a potential component of the multielement detection scheme for the authors’ system. The detector features a two-dimensional array of elements on high resistivity float-zone silicon. The detector attributes that were attractive to their application include modest internal amplification (&amp;gt;5000 at 25kV), fast response time (measured at &amp;lt;10ns), ability to be made compact and with dense packed electrodes (&amp;lt;250μm), low electrode capacitance (&amp;lt;1pF), and ability for (complementary metal-oxide semiconductor) circuitry to be integrated directly onto the detector array so that low noise amplification of each signal can be performed. This detector requires a retarding field for the primary beam, which accelerates the secondary electrons to energies sufficient to excite a large number of internal secondaries.

Characterization of irradiated detectors fabricated on p-type silicon substrates for super-LHC

An upgrade of the large hadron collider (LHC), the Super-LHC (SLHC), towards higher luminosities is currently being discussed as an extension of the LHC physics program. The goal of the SLHC is to operate at a luminosity of 10 35 cm −2 s −1 (10 times larger than that of the LHC one). Thus, the operation of the SLHC implies a need to upgrade the detectors of the LHC experiments. The current tracking system of ATLAS will not cope with that luminosity. New solutions must be investigated to improve the radiation tolerance of the semiconductor detector. p-Type bulk sensors are being considered for the ATLAS tracking system for the SLHC. Microstrip detectors fabricated by CNM-IMB on p-type high-resistivity float zone silicon have been irradiated with neutrons at the TRIGA Mark II reactor in Ljubljana up to a fluence of 10 16 cm −2 (as expected in the innermost region of the ATLAS upgrade) and have been characterized at IFIC Laboratory. The collected charge, after irradiation, has been measured by infrared laser illumination. The leakage current of those sensors is also reported.

Ultimate limits for the radiation hardness of silicon strip detectors for sLHC

The new SuperLHC upgrade will impose severe restrictions on the radiation hardness of silicon detectors since a maximum fluence of 10 16 particles / cm 2 is foreseen in the innermost region. Microstrip detectors have been fabricated in p-type high resistivity float zone silicon at CNM facilities, been irradiated at the TRIGA reactor in Ljubljana to a fluence of 10 16 neutrons / cm 2 and characterized at IFIC laboratory. The total collected charge before and after irradiation in the detectors has been measured by Sr 90 beta source and by infrared laser illumination. The results show that even after this extreme radiation fluence, p-type substrate detectors collect 3500 electrons when biased at 800 V, which is enough charge to induce a measurable signal with standard readout electronics. P-type strip detectors could be suitable for the middle and even inner regions of sLHC.

A blue-violet enhanced BDJ photodetector and its applications in the probe chip measurements of the LEDs for solid-state lighting

A blue-violet enhanced buried double p–n junction (BDJ) photodetector with its characteristics is reported in this manuscript. It was fabricated on high resistivity float-zone silicon substrate by using ion implantation and exhibits low leakage currents. It can measure both the light intensity as a photometric detector and the wavelength of quasi-monochrome light as a wavelength sensor simultaneously. It was optimized to operate at wavelength from 380 nm to 780 nm in which the light emitting diodes (LEDs) for solid-state lighting is most interested. The resolution for the wavelength measurement of the photodetector is better than 1 nm. The probe chip tests of violet, blue, green and red LED chips have been demonstrated using the fabricated detector. This simple, fast and reliable method provides an alternative to low cost and fast examination technique for LED chips in vast production.