High-resistivity Si Substrates Research Articles

The Effect of the Barrier Layer on the Uniformity of the Transport Characteristics of AlGaN/GaN Heterostructures on HR-Si(111).

The high transport characteristics of AlGaN/GaN heterostructures are critical components for high-performance electronic and radio-frequency (RF) devices. We report the transport characteristics of AlGaN/GaN heterostructures grown on a high-resistivity (HR) Si(111) substrate, which are unevenly distributed in the central and edge regions of the wafer. The relationship between the composition, stress, and polarization effects was discussed, and the main factors affecting the concentration and mobility of two-dimensional electron gas (2DEG) were clarified. We further demonstrated that the mechanism of changes in polarization intensity and scattering originates from the uneven distribution of Al composition and stress in the AlGaN barrier layer during the growth process. Furthermore, our results provide an important guide on the significance of accomplishing 6 inch AlGaN/GaN HEMT with excellent properties for RF applications.

Heteroepitaxial growth of CaGe2 films on high-resistivity Si(111) substrates and its application for germanane synthesizing

Realizing a germanane (GeH) film on insulating substrates is strongly required to understand its intrinsic electric properties in discussing the potential applications for the next-generation field-effect transistor channels. This study investigates the heteroepitaxial growth of CaGe2—the precursor of the GeH—films on a high resistive substrate of FZ-Si(111) using a molecular beam epitaxy with the deposition temperature (Tdepo) as a variable. X-ray diffraction and Raman spectroscopy analyses indicated that a metastable polytype of 2H was dominated in the epitaxial CaGe2 films; the Tdepo must be less than 700 °C to prevent Si diffusion from the substrates. We also synthesize GeH films by immersing some CaGe2 films in HCl acid solutions at a low temperature of −40 °C. Although the obtained grain size of GeH was still as small as several hundred nanometers, we believe that the present study is the first step in obtaining the GeH continuous films directly on insulators necessary for electrical characterizations.

Ultralow-Supersaturation Al Pretreatment toward Low Dislocation Density and Low Radio Frequency Loss GaN/AlN Epi-Stacks on High-Resistivity Si Substrates

Ultralow-Supersaturation Al Pretreatment toward Low Dislocation Density and Low Radio Frequency Loss GaN/AlN Epi-Stacks on High-Resistivity Si Substrates

Stability, metallicity, and magnetism in niobium silicide nanofilms

Modern superconducting qubits based on two-dimensional (2D) transmons typically involve the growth of Nb thin films on high-resistivity Si substrates. Since imperfections at the Nb-Si heterointerface have been implicated as a source of two-level systems that limit quantum coherence times, detailed characterization and understanding of niobium silicide interfacial layers are critical to improving superconducting qubit technology. While bulk binary intermetallic niobium silicide phases are well understood, the thermodynamic phase stability and properties of ultrathin niobium silicides, such as those found at the Nb-Si heterointerface in 2D transmons, have not yet been explored. Here, we report finite-sized effects for ultrathin niobium silicide films using density functional theory calculations and predict nanoscale stabilization of ${\mathrm{Nb}}_{6}{\mathrm{Si}}_{5}$ over the bulk $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Nb}}_{5}{\mathrm{Si}}_{3}$ phase. This result is consistent with our experimental observations of a niobium silicide interfacial layer between a sputtered Nb thin film and the underlying Si substrate. Furthermore, our calculations show that ${\mathrm{Nb}}_{6}{\mathrm{Si}}_{5}$ nanofilms are nonmagnetic, making them superior to nanofilms of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Nb}}_{5}{\mathrm{Si}}_{3}$ that exhibit antiferromagnetic correlations detrimental to long coherence times in superconducting qubits. By providing atomic-scale insight into niobium silicide nanofilms, this paper can help guide ongoing efforts to optimize Nb-Si heterointerfaces for long coherence times in superconducting qubits.

Low-loss characteristics of coplanar waveguides fabricated by directly bonding metal foils to high-resistivity Si substrates

Coplanar waveguides (CPWs) are fabricated by directly bonding 17 μm Al foils to Si substrates with different resistivities and wet etching. Their RF characteristics are compared with those of CPWs fabricated on 0.8 μm thick evaporated Al layers. A lower insertion loss is observed for Si substrates with higher resistivity irrespective of the thickness of conductors because the substrate loss is decreased. For a 10000-Ω · cm Si substrate, Al-foil-based CPWs reveal a ∼0.7 dB cm−1 lower insertion loss in comparison with evaporated-Al-based CPWs at 1 GHz. These features agree with the calculation, which implies the superiority of direct bonding of metal foils for realizing low-loss passive components.

Low RF loss and low dislocation density of GaN grown on high-resistivity Si substrates

The conductive channel induced by aluminum (Al) diffusion into high-resistivity Si substrates is one of the main contributors to RF loss of GaN-on-Si RF devices. However, it has been unclear how and when the Al diffuses into the substrates. Here, we study the origins of the Al impurity in the substrates. It’s revealed that trimethylaluminum (TMAl) flow rate during the pretreatment is a considerable contributor to the Al diffusion. By restricting the TMAl preflow rate, high-quality GaN layers with RF loss of 0.3 dB mm−1 at 10 GHz and dislocation density of 1.6 × 109 cm−2 have been achieved on Si substrates.

Computational Analysis of a 200 GHz Phased Array Using Lens-Coupled Annular-Slot Antennas

We report the design, simulation, and analysis of a THz phased array, using lens-coupled annular-slot antennas (ASAs) for potential beyond 5G or 6G wireless communications. For a prototype demonstration, the ASA employed was designed on a high resistivity Si substrate with a radius of 106 μm, and a gap width of 6 um for operation at 200 GHz. In order to achieve higher antenna gain and efficiency, an extended hemispherical silicon lens was also used. To investigate the effect of the silicon lens on the ASA phased array, a 1 × 3 array and 1 × 5 array (the element distance is 0.55λ) were implemented with a silicon lens using different extension lengths. The simulation shows that for a 1 × 3 array, a ±17° scanning angle with an about −10 dB sidelobe level and 11.82 dB gain improvement (compared to the array without lens) can be achieved using a lens radius of 5000 μm and an extension length of 1000 μm. A larger scanning angle of ±31° can also be realized by a 1 × 5 array (using a shorter extension length of 250 μm). The approach of designing a 200 GHz lens-coupled phased array reported here is informative and valuable for the future development of wireless communication technologies.

Investigation of Temperature Sensing Capabilities of GaN/SiC and GaN/Sapphire Surface Acoustic Wave Devices

This paper proposes high sensitivity temperature sensors based on single port surface acoustic wave (SAW) devices with GHz resonance frequencies, developed on GaN/SiC and GaN/Sapphire, which permit wide range, accurate temperature determinations. In contrast with GaN/Si SAW based temperature sensors, SiC and Sapphire substrates enable the proper functionality of these devices up to 500°C (773 K), as the high resistivity Si substrate becomes conductive at temperatures exceeding 250°C (523 K) due to the relative low bandgap (and high intrinsic carrier concentrations). Low temperature measurements were carried out using a cryostat between -266°C (7 K) and room temperature (RT) while the high temperature measurements are made on a modified RF probe station. A polynomial fit was used below RT and a linear approximation was evidenced between RT and 500°C (773 K). The structures were simulated at different selected temperatures based on a method that couples Finite Element Method (FEM) and Coupling of Modes (COM). The measured temperature coefficient of frequency (TCF) is about 46 ppm/°C for GaN/SiC SAWs and reaches values of 96 ppm/°C for GaN/Sapphire SAW in the temperature range 25 – 500°C (298 K – 773 K).

Effects of substrate phonon absorption on the resonance behavior of metal–insulator–metal metamaterial terahertz absorbers

We have investigated the effects of substrate phonon absorption on the resonance behavior of metal–insulator–metal (MIM) double-layer metamaterial absorbers (MMAs) in the terahertz (THz) frequency range. A sharp resonant absorption dip is clearly observed for a metamaterial-on-ground-plane (MMOGP) structure fabricated on a semi-insulating (SI) GaAs substrate when THz radiation is incident from the surface metamaterials (MMs) side. However, when the THz is incident from the substrate side to the ground-plane-on-metamaterial (GPOMM) structures fabricated on a SI GaAs substrate, we find that the resonance dip is almost merged into the broad background of acoustic phonon absorption. The resonant absorption is recovered when the GaAs substrate is replaced with a high-resistivity Si substrate. These findings demonstrate that the choice of substrates is very important to suppress the absorption by acoustic phonons absorption in the THz range and achieve high-quality factor resonance.

GaAs-based microelectromechanical terahertz bolometers fabricated on high-resistivity Si substrates using wafer bonding technique

We have fabricated GaAs-based microelectromechanical systems' (MEMSs) terahertz bolometers on high-resistivity Si substrates by using a wafer-bonding technique. In contrast to polar GaAs, nonpolar Si has very small absorption in the terahertz (THz) frequency range. The wafer-bonded MEMS bolometers show a large responsivity even in the Reststrahlen band of GaAs, where the responsivity vanishes in the conventional MEMS bolometers fabricated on GaAs substrates. Furthermore, we have observed two peaks in the responsivity spectrum near the TO and LO phonon frequencies of GaAs, which originate from an interplay between strong reflection in the Reststrahlen band and strong absorption at the TO phonon frequency in the GaAs MEMS beam. The present result demonstrates that the wafer-bonded MEMS bolometers are a very good candidate for the room-temperature, fast, and sensitive broadband THz detection.

Investigation and reduction of RF loss induced by Al diffusion at the AlN/Si(111) interface in GaN-based HEMT buffer stacks

GaN-based high electron mobility transistors (HEMT) on Si (111) substrates have large potential for applications in the 5G telecommunication field. However, for this potential to be fully realized, all loss mechanisms need to be minimized. It is known that typical metal-organic chemical vapor deposition (MOCVD) processes used to grow the GaN epitaxial layers can cause considerable parasitic conductivity at the interface of the AlN nucleation layer to the high-resistivity Si substrate, leading to reduced gain and power added efficiency in amplifiers. Reducing this parasitic conductivity is hence of utmost importance to render GaN-on-Si a significant contributor to next-generation 5G power amplifier technology. In this work, we employ secondary ion mass spectroscopy, spreading resistance profiling and insertion loss measurements up to 28 GHz using coplanar waveguides fabricated on the epitaxial layer stacks to study the origin and characterize the parasitic conductivity. While a single heat-up process in an AIXTRON G5+ reactor chamber cleaned using Cl2 does not introduce any extra dopants in the Si substrate, the epitaxial growth of (Al,Ga)N-based HEMT buffer layer stacks leads to the diffusion of Al and, to a lower extent, Ga acceptors into the Si substrate. Optimization of the MOCVD process towards lower growth temperatures leads to a strong reduction of density of diffused acceptors. This reduction goes in line with a significant decrease of the insertion loss from 0.45 dB mm−1 to only 0.20 dB mm−1 at a frequency of 28 GHz.

Optimization of Oxygen Plasma Treatment on Ohmic Contact for AlGaN/GaN HEMTs on High-Resistivity Si Substrate

The oxygen plasma surface treatment prior to ohmic metal deposition was developed to reduce the ohmic contact resistance (RC) for AlGaN/GaN high electron mobility transistors (HEMTs) on a high-resistive Si substrate. The oxygen plasma, which was produced by an inductively coupled plasma (ICP) etching system, has been optimized by varying the combination of radio frequency (RF) and ICP power. By using the transmission line method (TLM) measurement, an ohmic contact resistance of 0.34 Ω∙mm and a specific contact resistivity (ρC) of 3.29 × 10–6 Ω∙cm2 was obtained with the optimized oxygen plasma conditions (ICP power of 250 W, RF power of 75 W, 0.8 Pa, O2 flow of 30 cm3/min, 5 min), which was about 74% lower than that of the reference sample. Atomic force microscopy (AFM), energy dispersive X-ray spectroscopy (EDX), and photoluminescence (PL) measurements revealed that a large nitrogen vacancy, which was induced near the surface by the oxygen plasma treatment, was the primary factor in the formation of low ohmic contact. Finally, this plasma treatment has been integrated into the HEMTs process, with a maximum drain saturation current of 0.77 A/mm obtained using gate bias at 2 V on AlGaN/GaN HEMTs. Oxygen plasma treatment is a simple and efficient approach, without the requirement of an additional mask or etch process, and shows promise to improve the Direct Current (DC) and RF performance for AlGaN/GaN HEMTs.

The Behavior of Gold Metallized AlN/Si- and AlN/Glass-Based SAW Structures as Temperature Sensors.

Thin AlN piezoelectric layers have been deposited on high resistivity Si and glass substrates by reactive RF magnetron sputtering, in order to manufacture one-port gigahertz operating surface acoustic wave (SAW)-type resonators to be used as temperature sensors. The growth morphology surface topography, crystallographic structure, and crystalline quality of the AlN layers have been analyzed. Advanced nanolithographic techniques have been used to manufacture structures having interdigitated transducers with fingers and finger interdigit spacing width in the range of 250-170 nm. High resonance frequency ensures the increase of the sensitivity, but also of its normalized value, the temperature coefficient of frequency (TCF). The resonance frequency shift versus temperature has been measured in the -267°C-+150°C temperature range, using a cryostat setup adapted for on wafer microwave measurements up to 50 GHz. The sensitivity and the TCF were determined in the 25 °C-150 °C temperature range.

Integration of multi-layer black phosphorus into photoconductive antennas for THz emission

We report the fabrication, characterization, and modeling of photoconductive antennas (PCAs) using 40 nm thin-film flakes of black phosphorus (BP) as the photoconductor and hexagonal boron nitride (hBN) as a capping layer to prevent oxidation of BP. Dipole antennas were fabricated on oxidized high-resistivity Si substrates, and BP and hBN flakes were picked up and transferred onto the antenna inside a nitrogen glovebox. The transfer matrix technique was used to optimize the thickness of BP and hBN for maximum absorption. BP flakes were aligned with the armchair axis along the anode–cathode gap of the antenna, with crystal orientation measured using reflection anisotropy. Photocurrent imaging under illumination with 100 fs pulses at 780 and 1560 nm showed a bias-dependent maximum photocurrent localized to the antenna gap with a peak photoconductivity of 1 (2) S/cm in the linear regime of bias for excitation at 780 (1560) nm. Photocurrent saturation in bias (pump fluence) occurred at approximately 1 V (0.25mJ/cm2). Device performance was modeled numerically by solving Maxwell’s equations and the drift–diffusion equation to obtain the photocurrent density in response to pulsed laser excitation, which was largely in qualitative agreement with the experimental observations. THz output computed from surface current density suggests that BP THz PCA performance is at least comparable to more traditional devices based on low-temperature-grown GaAs. These devices represent a step toward high-performance THz photoconductive antennas using BP.

Low-Frequency Noise Investigation of GaN/AlGaN Metal–Oxide–Semiconductor High-Electron-Mobility Field-Effect Transistor With Different Gate Length and Orientation

In this article, GaN/AlGaN metal–oxide–semiconductor high-electron-mobility field-effect transistors (MOSHEMTs) fabricated on high-resistivity Si (111) substrates have been evaluated using low-frequency (LF) noise measurement. The noise power spectral density (PSD) of devices with different lengths and channel orientations has been characterized in linear operation. No noticeable differences in the electrical and noise PSD characteristics have been observed between the GaN [ $1\overline {1}00$ ] and [ $11\overline {2}0$ ] channel orientations. While most devices are dominated by 1/ ${f}$ noise, originating from number fluctuations, for long devices ( ${L} \ge 1.1~\mu \text{m}$ ), additional generation–recombination (GR) noise has been observed, originating from traps in the GaN layer.

Al diffusion at AlN/Si interface and its suppression through substrate nitridation

One of the challenges for GaN-on-Si radio frequency (RF) device applications is the RF loss, which is mainly associated with a parasitic channel formed at the interface of AlN and high-resistivity Si substrates. However, the type of conductivity and formation mechanism of the parasitic channel remains controversial. Here, we report unambiguous evidence of Al diffusion at the AlN/Si interface and its effect on RF loss. Hall measurements reveal p-type conductivity at the interface. By combining with secondary ion mass spectroscopy measurements, the p-type conductivity is attributed to the Al diffusion from the AlN layers into the Si substrates, with Al being an acceptor in Si. Experimental data and simulations are in good agreement. We also demonstrate that substrate nitridation can indeed promote the formation of an amorphous silicon nitride layer, which plays a role in suppressing the Al diffusion and, thus, reducing the RF loss.

The influence of AlN nucleation layer on RF transmission loss of GaN buffer on high resistivity Si (111) substrate

A sufficiently low transmission loss in radio frequency (RF) is one of the critical requirements for GaN-on-Si RF devices to achieve high performance. We have systematically studied the mechanism and effect of the AlN nucleation layer on the RF loss of the GaN-on-Si device buffer stack. Our results show that the RF loss is strongly influenced by the growth parameters of the AlN nucleation layer during epitaxial process. It is observed that the AlN nucleation layer grown at a low thermal budget with a low density of deep surface pits can efficiently reduce the AlN/Si interface loss by suppressing the conductive channel at AlN/Si interface which is governed largely by the thermal diffusion of Al and Ga into the Si substrate. By optimizing the growth process of the AlN nucleation layer, the RF loss of the GaN-on-Si device buffer can be dramatically reduced by up to ∼40%.

Magnetotransport and superconductivity in InBi films grown on Si(111) by molecular beam epitaxy

Bismuth-containing compounds inherit the high spin-orbit coupling and bandgap bowing effects of the Bi atom. Here, we report the growth of InBi films using molecular beam epitaxy. By growing in a Bi-rich regime, we obtain coalesced and crystalline films with a sharp interface to the high-resistivity Si(111) substrate. Temperature-dependent transport and resistivity measurements exhibit a nonlinear Hall effect and parabolic magnetoresistance, suggesting two-carrier semimetallic behavior. In In-rich films, metallic temperature-dependent resistivity is observed. In Bi-rich films, we observed semiconductorlike temperature-dependent resistivity as well as superconductivity.

(Invited) Cluster-Preforming-Deposited Si-Rich W Silicide: A New Contact Material for Advanced CMOS

With recent shrinkage of Si CMOS, the contact resistance at source/drain (S/D) and the external series resistance are becoming determining factors of device performance. Low Schottky barrier height (SBH) at the metal/semiconductor interface is required to reduce the specific contact resistivity. It is well known, however, that for most metallic species, silicide/Si junctions always show a high electron SBH to n-Si owing to the Fermi-level pinning in the range between the mid gap and the valence band edge. This fact imposes a fundamental difficulty for obtaining low contact resistance and high performance in CMOS. Many researchers have already addressed this issue; yet, an unconventional contact material is strongly required. In this work, we demonstrate a novel contact material with low SBH for both n- and p-FET together with excellent diffusion barrier properties: the WSi n (n = 12) composed of W-atom-encapsulated Si n cage clusters.[1] The WSi n film was prepared by the newly developed Cluster-Preforming Deposition (CPD) method.[2] In a hot-wall thermal deposition system, hydrogenated W-atom-encapsulated WSi n H x clusters with well-controlled n values (n ≤ ~12) are preformed by reaction of WF6 and SiH4 in the gas phase. The WSi n H x clusters are then deposited onto a substrate, where they are thermally dehydrogenated and coalesce to the WSi n film with less hydrogen content. The residual fluorine concentration in the film was less than ~0.1 at. % because of the sufficient reduction reaction of WF6 with SiH4 in the gas phase. The formed films have an amorphous structure similar to the hydrogenated amorphous Si, but they are extremely stable against annealing up to 1100 °C.[3] The power formation is entirely suppressed under the CPD condition of very high flow ratio of SiH4/ WF6. We also confirmed that the CPD WSi n exhibits an excellent step coverage over a high-aspect-ratio hole array with 40-nm diameter and 2-μm depth. We evaluated the insertion effect of WSi n film of 5-10 nm thickness into Schottky diodes with high resistivity Si substrates: W/WSi n /n-Si (~2 Ω·cm) and W/WSi n /Ge/p-Si (~5 Ω·cm). The electron SBH in the inserted diodes decreases with n, and in particular, is most reduced to 0.32 eV when n is 12. This indicates that the WSi n film has a Fermi level close to the Si conduction band edge and sufficiently passivates the interfacial states at the Si surface because WSi n is composed of almost Si. This SBH reduction effect is kept even after annealing up to at 700 °C. On the other hand, for the W/WSi n /Ge/p-Si junction, the hole SBH once increases by the insertion but is reduced with annealing temperature, approaching the charge neutrality level close to the valence band edge of Ge, e.g. 0.51 eV with n = 12 after annealing at 600 °C. This property enables the WSi12 film to reduce both the electron and hole SBHs simultaneously in CMOS. Furthermore, the WSi n is a narrow gap semiconductor with a low offset to the band edge of Si or Ge. This band alignment leads to the negligible insertion resistance. To reduce the parasitic series resistance at S/D, it is effective to adopt a thinner barrier film for a low resistance electrode such as Cu. Thus, the barrier properties of the WSi n film against Cu diffusion were investigated using the two test structures: MOS capacitors with a 20-nm thermal oxide and p+-n diodes (junction depth of 100 nm) with and without the WSi n insertion (n = 12, thickness = 5 nm). The WSi n film exhibited excellent diffusion barrier properties for Cu contact; from the TDDB lifetime of the Cu MOS capacitors, the diffusion barrier height was estimated to be 1.33 eV under 5 MV/cm stress and the Cu on Si diodes were stable against annealing up to 600 °C. In conclusion, the insertion of the CPD WSi n (n = 12) film is demonstrated to reduce the electron and hole SBH to 0.32 eV at W/n-Si and to 0.51 eV at W/Ge/p-Si junctions, while significantly preventing the Cu diffusion with a TDDB barrier height of 1.33 eV under 5 MV/ cm stress. Consequently, this film is a promising contact material for S/D in CMOS. [1] IEDM, 22.5, (2017). [2] J. Chem. Phys., 144, 084703 (2016). [3] J. Appl. Phys., 121, 225308, (2017).

Low-Loss Coplanar Waveguides on GaN-on-Si Substrates

Low-loss coplanar waveguides (CPWs) on AlGaN/GaN HEMT heterostructures grown on high-resistivity Si substrates (GaN-on-Si) are demonstrated. Performance has been evaluated from 0.1 to 20 GHz, and loss as low as 0.27 dB/mm at 20 GHz is achieved. The performance of CPWs fabricated with ion implant- and mesa etching-based processes are compared, and finite element simulations are used to analyze the CPW loss mechanisms. In this frequency range, the dielectric loss associated with the GaN-on-Si substrate is below 0.065 dB/mm, compared to approximately 0.01 dB/mm for reference GaN-on-SiC CPW lines. The extra loss in the GaN-on-Si CPWs is due to a parasitic conductive layer with a sheet resistance of $2\times 10^{4} \,\,\Omega /\Box $ . The low loss obtained, combined with the economic and integration advantages of GaN-on-Si substrates, makes them promising for GaN monolithic microwave integrated circuits.