Akifumi Kasamatsu

Development of Complex Relative Permittivity Measurement System Based on Free-Space in 220–330-GHz Range

For measuring complex relative permittivity of more than 300 GHz in the THz region, time-domain spectroscopy (TDS) is usually used. On the other hand, a free-space method using a vector network analyzer (VNA) is used below 300 GHz. However, these methods have not been compared and continuity of complex relative permittivity measurement around 300 GHz has not been evaluated. We developed a system for measuring complex relative permittivity that can be operated at 220-330 GHz. This system is based on the free-space method. We compared the complex relative permittivity using a VNA and by TDS to evaluate our system and the continuity of complex relative permittivity measurement in the frequency domain. We first compared relative permittivity and dielectric loss between both methods. We then evaluated the measurement uncertainty in consideration of the thickness of materials under test (MUT), time span of the time-domain gating, linearity, stability, aperture alignment, and measurement repeatability. The dispersion in MUT thickness measurement was found to be the dominant source of uncertainty in measuring complex relative permittivity measurement. The maximum difference in relative permittivity and dielectric loss between both methods was less than 0.22 and 0.17 when MUT E was measured. For the measurement result with expanded uncertainty, the relative permittivity of VNA was 3.92±0.44 and that of TDS was 3.70±0.16, the dielectric loss of VNA was 0.30±0.52 and that of TDS was 0.13±0.02, respectively. The measured complex relative permittivity by using the VNA and by TDS were observed within the expanded uncertainty. We verified the availability of the measured complex relative permittivity by using our measurement system and the continuity of complex relative permittivity measurement at 300-GHz band.

20.1 A 300GHz 40nm CMOS transmitter with 32-QAM 17.5Gb/s/ch capability over 6 channels

The vast unallocated frequency band lying above 275GHz offers enormous potential for ultrahigh-speed wireless communication. An overall bandwidth that could be allocated for multi-channel communication can easily be several times the 60GHz unlicensed bandwidth of 9GHz. We present a 300GHz transmitter (TX) in 40nm CMOS, capable of 32-quadrature amplitude modulation (QAM) 17.5Gb/s/ch signal transmission. It can cover the frequency range from 275 to 305GHz with 6 channels as shown at the top of Fig. 20.1.1. Figure 20.1.1 also lists possible THz TX architectures, based on recently reported above-200GHz TXs. The choice of architecture depends very much on the transistor unity-power-gain frequency fmax. If the fmax is sufficiently higher than the carrier frequency, the ordinary power amplifier (PA)-last architecture (Fig. 20.1.1, top row of the table) is possible and preferable [1-3], although the presence of a PA is, of course, not a requirement [4,5]. If, on the other hand, the fmax is comparable to or lower than the carrier frequency as in our case, a PA-less architecture must be adopted. A typical such architecture is the frequency multiplier-last architecture (Fig. 20.1.1, middle row of the table). For example, a 260GHz quadrupler-last on-off keying (OOK) TX [6] and a 434GHz tripler-last amplitude-shift keying (ASK) TX [7] were reported. A drawback of this architecture is the inefficient bandwidth utilization due to signal bandwidth spreading. Another drawback is that the use of multibit digital modulation is very difficult, if not impossible. An exception to this is the combination of quadrature phase-shift keying (QPSK) and frequency tripling. When a QPSK-modulated intermediate frequency (IF) signal undergoes frequency tripling, the resulting signal constellation remains that of QPSK with some symbol permutation. Such a tripler-last 240GHz QPSK TX was reported [8]. However, a 16-QAM constellation, for example, would suffer severe distortion by frequency tripling. If the 300GHz band is to be seriously considered for a platform for ultrahigh-speed wireless communication, QAM-capability will be a requisite.

Strain-Relaxed Si1-xGex and Strained Si Grown by Sputter Epitaxy

Strained Si on our previously proposed strain-relief relaxed thin quadruple-Si1-xGex-layer buffer was formed by sputter epitaxy, the buffer relaxation mechanism and controllability of which were basically the same as those of gas-source molecular beam epitaxy (GS-MBE); the strained-Si crystallinity obtained by sputter epitaxy was largely comparable to that obtained by GS-MBE. By using sputter epitaxy, a flatter strained Si surface that exhibits almost no cross-hatch undulation morphology is obtained. The strain rate of the topmost 60-nm-thick strained Si layer grown on the quadruple-Si1-xGex-layer buffer with a total thickness of 240 nm and a top Ge content of 0.35 was 0.84% in the lateral direction. The results suggest that our environmentally light-load sputter epitaxy method can be applied to the fabrication of high-density Si/Si1-xGex strained devices.

Strain Distribution Analysis of Sputter-Formed Strained Si by Tip-Enhanced Raman Spectroscopy

Simultaneous nanometer-scale measurements of the strain and surface undulation distributions of strained Si (s-Si) layers on strain-relief quadruple-Si1-xGex-layer buffers, using a combined atomic force microscopy (AFM) and tip-enhanced Raman spectroscopy (TERS) system, clarify that an s-Si sample formed by our previously proposed sputter epitaxy method has a smoother and more uniformly strained surface than an s-Si sample formed by gas-source molecular beam epitaxy. The TERS analyses suggest that the compositional fluctuation of the underlying Si1-xGex buffer layer is largely related to the weak s-Si strain fluctuation of the sputtered sample.

Effects of boron dopants of Si (001) substrates on formation of Ge layers by sputter epitaxy method

The formation of Ge layers on boron-doped Si (001) substrates by our sputter epitaxy method has been investigated. The surface morphology of Ge layers grown on Si substrates depends on the substrate resistance, and flat Ge layers are obtained on Si substrates with 0.015 Ω cm resistivity. Highly boron-doped Si substrates cause a transition in the dislocation structure from complex dislocations with 60° dislocation glide planes to 90° pure-edge dislocations, resulting in the formation of flat Ge layers. Furthermore, we have found that the surface morphology of the Ge layers improves with increasing Ge layer thickness. Ge atoms migrating on the deposited Ge layers tend to position themselves at the reactive sites, where the reactivity is related to the number of bonding contacts between the Ge atom and the surface. This modifies the surface morphology, resulting in a flatter surface. Boron dopants together with the sputter epitaxy method effectively suppress the growth of Ge islands and result in the formation of flat Ge layers.

Investigation of Sn surface segregation during GeSn epitaxial growth by Auger electron spectroscopy and energy dispersive x-ray spectroscopy

The mechanism of Sn surface segregation during the epitaxial growth of GeSn on Si (001) substrates was investigated by Auger electron spectroscopy and energy dispersive X-ray spectroscopy. Sn surface segregation depends on the growth temperature and Sn content of GeSn layers. During Sn surface segregation, Sn-rich nanoparticles form and move on the surface during the deposition, which results in a rough surface owing to facet formation. The Sn-rich nanoparticles moving on the surface during the deposition absorb Sn from the periphery and yield a lower Sn content, not on the surface but within the layer, because the Sn surface segregation and the GeSn deposition occur simultaneously. Sn surface segregation can occur at a lower temperature during the deposition compared with that during postannealing. This suggests that the Sn surface segregation during the deposition is strongly promoted by the migration of deposited Ge and Sn adatoms on the surface originating from the thermal effect of substrate temperature, which also suggests that limiting the migration of deposited Ge and Sn adatoms can reduce the Sn surface segregation and improve the crystallinity of GeSn layers.

Low-Profile Terahertz Radar Based on Broadband Leaky-Wave Beam Steering

We demonstrate short-range terahertz radar based on a leaky-wave antenna with a beam steering capability. As a proof of concept, we develop a microstrip-based periodic leaky-wave antenna driven by a vector network analyzer. By sweeping the frequency from 235 to 325 GHz, beam steering from -23° to +15° across the broadside can be achieved with a nearly constant beam width of 4°. Small target detection is demonstrated by locating a metal cylinder with a diameter of 12 mm placed 46-86 mm in front of the antenna with a mean error of 2.4 mm. The use of a leaky-wave antenna can pave the way for developing a low-loss, low-profile, and wide-aperture terahertz radar. Importantly, it can be integrated with a solid-state source and a detector. The proposed approach is particularly promising for use with emerging small devices such as drones or wearable devices, where millimeter-wave radar is not suitable in terms of the resolution and system footprint.

A 300 GHz CMOS Transmitter With 32-QAM 17.5 Gb/s/ch Capability Over Six Channels

A 300 GHz transmitter (TX) fabricated using a 40 nm CMOS process is presented. It achieves 17.5 Gb/s/ch 32-quadrature amplitude modulation (QAM) transmission over six 5 GHz-wide channels covering the frequency range from 275 to 305 GHz. With the unity-power-gain frequency fmax of the NMOS transistor being below 300 GHz, the TX adopts a power amplifier-less QAM-capable architecture employing a highly linear subharmonic mixer called a cubic mixer. It is based on and as compact as a tripler and enables the massive power combining necessary above fmax without undue layout complication. The frequency-dependent characteristics of the cubic mixer are studied, and it is shown that even higher data rates of up to 30 Gb/s are possible at certain frequencies, where the channel signal-to-noise ratio is high. The design and the operation of the power-splitting and power-combining circuits are also described in detail. The measurements reported herein were all made “wired” via a WR3.4 waveguide.

Si/Ge Hole-Tunneling Double-Barrier Resonant Tunneling Diodes Formed on Sputtered Flat Ge Layers

We have demonstrated Si/Ge hole-tunneling double-barrier resonant tunneling diodes (RTDs) formed on flat Ge layers with a relaxation rate of 89% by our proposed method; in this method, the flat Ge layers can be directly formed on highly B-doped Si(001) substrates using our proposed sputter epitaxy method.

Demonstration of 20-Gbps wireless data transmission at 300 GHz for KIOSK instant data downloading applications with InP MMICs

We present 20-Gbps wireless ASK data transmission at 300 GHz with an all-electronic transmitter and receiver for KIOSK instant data downloading applications. The transmitter and receiver MMICs are based on 70-nm indium-phosphide-based high electron mobility transistor technologies of which the cut-off frequency (fmax) is approximately 700 GHz. For an experiment, the transmitter and receiver were packaged in a split-block waveguide and dedicated metallic housing, respectively. With 30-dBi and 25-dBi horn antennas for the transmitter and receiver, error free data transmission (bit error rate <; 1 × 10-9) was achieved up to 80-cm link distance.