Yee-Chia Yeo

In the first part of this two-part article, a ladder transmission line model (LTLM), featuring elimination of parasitic metal resistances and high accuracy for extracting specific contact resistivity ρc, is developed and verified by simulation. In Part II, the LTLM is applied experimentally to Ni contacts formed on the heavily Ga-doped Ge0.95Sn0.05 film. Two types of Ni/p⁺-Ge0.95Sn0.05 LTLM structures, single-cell and multicell LTLMs, are fabricated. Ultralow ρc values of 8.6 ± 2 x 10⁻¹⁰ Ω-cm² and 1.1 ± 0.3 x 10⁻⁹ Ω-cm² are extracted from the single-cell and multicell LTLMs, respectively. The good agreement of the extracted ρc from the single-cell and multicell structures validates the ρc extraction by the LTLM. ρc is also extracted by using refined transmission line model (RTLM). In contrast, the extracted ρc by the RTLM is more than two times larger due to the ignored parasitic metal resistance. The proposed LTLM is a useful tool for an accurate assessment of ρc in advanced contact technology.

A ladder transmission line model (LTLM) that features capability to eliminate parasitic metal resistance from contact metal and access electrodes, a simple fabrication process, and a sub-10⁻¹⁰ Ω· cm² resolution is demonstrated for accurate extraction of the specific contact resistivity ρc. The proposed LTLM is verified by technology computer-aided design (TCAD) simulation and a two-layer distributed-resistor network model. After eliminating the parasitic metal resistance, the current spreading and crowding effects are the main factors that contribute to the extraction error in LTLM. The effects of current spreading and crowding on the extraction of ρc in LTLM are further quantitatively analyzed and design guidelines to achieve a sub-10⁻¹⁰ Ω · cm² resolution of ρc extraction are provided. The details of theoretical modeling and simulation of LTLM are documented in Part I. The experimental demonstration of extracting sub-10⁻⁹ Ω · cm² ρc using the proposed LTLM is provided in Part II.

This paper reports the Au-free GaN power integration platform and a complete integration scheme from devices to functional sub-circuits, and to application-oriented GaN converter ICs. The design and experimental demonstration of all GaN DC-DC converter IC with high level of integration is presented. Through the developed GaN power integration platform, devices are monolithically integrated and functional sub-circuits are demonstrated, which have realized expected application-oriented functions and are feasible for high-level integration. The all GaN converter IC with monolithically integrated high-side gate driver, pulse width modulation (PWM) feedback controller and over-current protection circuits is proposed, numerically analyzed, experimentally demonstrated and characterized. It can realize stable 10 V output with constant output ripples below 4% from 15-30 V input line voltage. Stable output with constant ripples can be maintained according to designed feedback control when input and load conditions are abruptly changed. When subjected to over-current incident, the converter IC can be protected according to desired over-current threshold values within one duty cycle period. The developed all GaN power integration platform, together with functional sub-circuits and DC-DC converter IC, can be a practical verification of all GaN IC scheme oriented towards power conversion application, and a useful reference for all GaN IC designs.

High-performance GeSn multiple-quantum-well (MQW) photodiode is demonstrated on a 200 mm Ge-on-insulator (GeOI) photonics platform for the first time. Both GeSn MQW active layer stack and Ge layer (top Ge layer of GeOI after bonding) were grown using a single epitaxy step on a standard (001)-oriented Si substrate (donor wafer) using a reduced pressure chemical vapor deposition (RPCVD). Direct wafer bonding and layer transfer technique were then employed to transfer the GeSn MQW device layers and Ge layer to a 200 mm SiO₂-terminated Si handle substrate. The surface illuminated GeSn MQW photodiode realized on this platform exhibits an ultra-low leakage current density of 25 mA/cm² at room temperature and an enhanced photo sensitivity at 2 μm of 30 mA/W as compared to a GeSn MQW photodiode on Si at 2 μm. The underlying GeOI platform enables monolithic integration of a complete suite of photonics devices operating at 2 μm band, including GeOI strip waveguides, grating couplers, micro-ring modulators, Mach–Zehnder interferometer modulators, etc. In addition, Ge CMOS circuits can also be realized on this common platform using a “photonic-first and electronic-last” processing approach. In this work, as prototype demonstration, both Ge p- and n-channel fin field-effect transistors (FinFETs) were realized on GeOI simultaneously with decent static electrical characteristics. Subthreshold swings of 150 and 99 mV/decade at |V_D| = 0.1 V and drive currents of 91 and 10.3 μA/μm at |V_G-V_TH| = 1 V and |V_D| = 0.75 V were achieved for p- and n-FinFETs, respectively. This works illustrates the potential of integrating GeSn (as photo detection material) on GeOI platform for Ge-based optoelectronics integrated circuits (OEICs) targeting communication applications at 2 μm band.

A metal-resistance-aware transmission line model (MRA-TLM) is developed to eliminate the parasitic metal resistance from the extraction of specific contact resistivity ρc in the metal/semiconductor contact. The proposed MRA-TLM is verified by Synopsys technology computer-aided design (TCAD) simulation, and a superior extraction accuracy as compared to the conventional TLM-based method is achieved. The proposed MRA-TLM is experimentally demonstrated in the metal/p⁺-Ge0.95n0.05 (p⁺-GeSn) contacts. Ultralow ρc values of 7.5 x 10⁻¹⁰ and 2.1 x 10⁻⁹ Ω · cm² are extracted by MRA-TLM for Ni/p⁺-GeSn and Ti/p⁺-GeSn contacts, respectively, which are in good agreement with 9.5 x 10⁻¹⁰ and 2.6 x 10⁻⁹ Ω · cm² extracted numerically by the two-layer distributed-resistor-network model (TDM), qualifying the proposed MRA-TLM for the extraction of ultralow ρ c down to sub-10⁻⁹ Ω · cm² regime. In contrast, the ρc extracted by the conventional TLM-based method is more than three times higher due to the parasitic metal resistance.

We report high-speed photo detection at two-micron-wavelength achieved by a GeSn/Ge multiple-quantum-well (MQW) p-i-n photodiode, exhibiting a 3-dB bandwidth (f_3-dB) above 10 GHz for the first time. The epitaxy of device layer stacks was performed on a standard (001)-oriented 300 mm Si substrate by using reduced pressure chemical vapor deposition (RPCVD). The results showed promise for large-scale manufacturing. To our knowledge, this is also the first photodiodes-on-Si with direct radio-frequency (RF) measurement to quantitatively confirm high-speed functionality with tens of GHz f_3-dB at 2 µm, which is considered as a promising candidate for the next data communication window. This work illustrates the potential for using GeSn to extend the utility of Si photonics in 2 µm band integrated optical transceivers for communication applications.

GeSn multiple-quantum-well rib waveguide on Si substrate was fabricated with vertical p-i-n photodiode structure and was characterized at two-micron-wavelength. Enhanced external photo response was achieved as compared to surface-illuminated counterparts, thanks to the waveguide scheme.

GeSn multiple-quantum-well microdisks are demonstrated on an 8-inch insulating platform, which was formed using direct wafer bonding technique. Whispering-gallery modes are identified through FDTD simulation, showing potential of such ultra-compact cavities for Group-IV lasing.