Daniel Alquier

Evidence of electrical activity of extended defects in 3C-SiC grown on Si

In this paper, we demonstrate the high electrical activity of extended defects found in 3C–SiC heteroepitaxially grown layer on (100) silicon substrates. Cross-sectional scanning transmission electron microscopy analysis was performed to reveal the defects while scanning spreading resistance microscopy aimed to study their electrical behavior. Using this technique, complete layer resistance cartography was done. The electrical activity of the extended defects in 3C–SiC was clearly evidenced. Furthermore, the defect activity was estimated to be higher than that of heavily nitrogen doped (5×1018 cm−3) 3C–SiC layer.

Experimental observation and analytical model of the stress gradient inversion in 3C-SiC layers on silicon

A detailed study of the static bending of micro-cantilevers has been performed for structures created from thin 3C-SiC films grown on (100) and (111) oriented silicon substrates. The biaxial stress distribution in the direction of the film normal has been evaluated based on analysis of the deformation profiles of clamped-free 3C-SiC beams of various thicknesses. Surprisingly, the obtained results clearly indicate that for as-grown samples of both studied orientations, the absolute value of the intrinsic stress increases from the interface to the surface of the film. We propose a simple analytical model of a relaxation process that explains in a quantitative way this unexpected phenomenon of stress gradient inversion.

Low Stress Heteroepitaxial 3C-SiC Films Characterized by Microstructure Fabrication and Finite Elements Analysis

Chemical vapor deposition in the low pressure regime of a high quality 3C-SiC film on silicon (100)-oriented substrates was carried out using silane (SiH 4 ), propane (C 3 H 8 ), and hydrogen (H 2 ) as the silicon supply, carbon supply, and gas carrier, respectively. The resulting bow in the freestanding cantilever structures was evaluated by an optical profilometer, and the residual gradient stress (σ 1 ) in the films was calculated to be approximately between 15 and 20 MPa, which is significantly lower than the previously reported 3C-SiC on Si films. Finite element simulations of the stress field in the cantilever have been carried out to separate the uniform contribution (σ 0 ), related to the SiC/Si interface, from the gradient one (σ 1 ), related to the defects present in the SiC epilayer.

Fabrication of field-effect transistors and functional nanogenerators using hydrothermally grown ZnO nanowires

The present work demonstrates the production of single crystalline ZnO nanowires (NWs) using the low temperature hydrothermal process and their integration as the active channel material and piezoelectric elements in single NW field-effect transistors (FETs) and functional nanogenerators (NGs), respectively. Even though hydrothermally grown ZnO NWs show high levels of excess free carriers ≫1018 cm−3, we show that an optimized thermal annealing process at just 450 °C in atmospheric air sufficiently reduces this level to around ∼3.7 × 1017 cm−3. The excess free carrier suppression is verified by assessing the field-effect transport behaviour in a single NW FET. The single device is found to exhibit good performance metrics, including low off-state current (pA range), high on-state current (in the 10 s of μA range) and moderate effective mobility (∼10 cm2 V−1 s−1). The functional NGs are based on vertically grown ZnO NWs with ∼7 μm thick polydimethylsiloxane (PDMS) polymer matrix. We show that a NG incorporating annealed ZnO NWs can continuously generate higher output voltages and power compared to a reference device based on as-grown ZnO NWs. This included peak output voltage of ∼109 mV and an output power density of ∼16 μW cm−3. We envisage that this approach of thermal annealing may find practical applications in other areas of hydrothermal ZnO NW research, including high performance NW FETs and piezoelectric energy harvesters.

Fabrication of high performance field-effect transistors and practical Schottky contacts using hydrothermal ZnO nanowires

The production of large quantities of single crystalline semiconducting ZnO nanowires (NWs) at low cost can offer practical solutions to realizing several novel electronic/optoelectronic and sensor applications on an industrial scale. The present work demonstrates high-density single crystalline NWs synthesized by a multiple cycle hydrothermal process at ∼100 °C. The high carrier concentration in such ZnO NWs is greatly suppressed by a simple low cost thermal annealing step in ambient air at ∼450 °C. Single ZnO NW FETs incorporating these modified NWs are characterized, revealing strong metal work function-dependent charge transport, unobtainable with as-grown hydrothermal ZnO NWs. Single ZnO NW FETs with Al as source and drain (s/d) contacts show excellent performance metrics, including low off-state currents (fA range), high on/off ratio (10(5)-10(7)), steep subthreshold slope (<600 mV/dec) and excellent field-effect carrier mobility (5-11 cm(2)/V-s). Modified ZnO NWs with platinum s/d contacts demonstrate excellent Schottky transport characteristics, markedly different from a reference ZnO NW device with Al contacts. This included abrupt reverse bias current-voltage saturation characteristics and positive temperature coefficient (∼0.18 eV to 0.13 eV). This work is envisaged to benefit many areas of hydrothermal ZnO NW research, such as NW FETs, piezoelectric energy recovery, piezotronics and Schottky diodes.

Interaction between dislocations and He-implantation-induced voids in GaN epitaxial layers

In this letter, we demonstrate that helium high-dose implantation is able to produce voids in GaN and we describe the behavior of material dislocations under these conditions. Two main types of nanovoids are encountered after annealing: cylindrical and pyramidal nanovoids. During their thermal evolution, these vacancy-type defects are interacting with dislocations favoring their local annihilation. The experimental results demonstrate a short-range interaction between nanovoid layer and dislocations, thus having potential applications for the improvement of GaN epitaxial layers quality.

Delta-Doping of Epitaxial GaN Layers on Large Diameter Si(111) Substrates

We report on silicon n-type delta (δ)-doping of gallium nitride (GaN) epitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on silicon (111) substrates. In a series of group III–nitride epitaxial structures a ~1-µm-thick Si bulk-doped GaN layer is replaced by 100, 50, 10, 5, or 1 Si δ-doped planes. While Si bulk-doping of GaN aggrandizes the in-plane tensile stress and the wafer bow with respect to undoped structures, δ-doping is found to reduce both stress and wafer bow. Two-dimensional carrier sheet densities between 1012 and 1013 cm-2 per δ-doped plane and electron mobilities of 1429 cm2 V-1 s-1 are achieved.

Electrothermally driven high-frequency piezoresistive SiC cantilevers for dynamic atomic force microscopy

Cantilevers with resonance frequency ranging from 1 MHz to 100 MHz have been developed for dynamic atomic force microscopy. These sensors are fabricated from 3C-SiC epilayers grown on Si(100) substrates by low pressure chemical vapor deposition. They use an on-chip method both for driving and sensing the displacement of the cantilever. A first gold metallic loop deposited on top of the cantilever is used to drive its oscillation by electrothermal actuation. The sensing of this oscillation is performed by monitoring the resistance of a second Au loop. This metallic piezoresistive detection method has distinct advantages relative to more common semiconductor-based schemes. The optimization, design, fabrication, and characteristics of these cantilevers are discussed.

Zinc oxide sheet field-effect transistors

The present work investigates charge carrier transport in back-gated field-effect transistors based on ZnO sheets (BG ZS-FETs). The ZSs used in this work have been synthesized via the catalytic-assisted vapor-liquid-solid process inside a horizontal quartz tube furnace at around 950 °C. The BG ZS-FETs were constructed as bottom-gate top-contact structures using suspended and non-suspended ZS as the active channel material. Assessment of key device performance metrics revealed excellent n-channel behavior with low off-state current in the femtoamp range, high on-state current (∼2 μA/μm), high on-to-off current ratio (>107), a steep sub-threshold swing of around 190 mV/dec, and field-effect carrier mobility of around 60 cm2/Vs. Temperature dependent charge transport studies reveal excessive mobility degradation in the non-suspended device while the same parameter in the suspended case appeared fairly stable. The present work is envisaged to benefit ongoing research towards the development of high performanc...

Process Parameters Influence on Specific Contact Resistance (SCR) Value for TiAl Ohmic Contacts on GaN Grown on Sapphire

In this work, Ti/Al bilayer sputtered ohmic contacts on n-type Gallium Nitride films were studied as a function of process parameters such as Ti thickness, surface cleaning procedure or annealing temperature. Epilayers, with doping concentration of 5.8x1018 at.cm-3, were grown on sapphire using AlN buffer layer. Electrical characterizations were made using circular Transfer Length Method (cTLM) patterns with a four probes equipment. Specific Contact Resistance (SCR) was then extracted for all the process conditions. Our results show that surface treatment is not a critical step in the ohmic contact process while annealing temperature has a larger impact. Finally, SCR values of 1x10-5 Ω.cm2 can be reproducibly achieved, which is of high interest for future devices fabrication using this material.