N-type 3C-SiC Research Articles

Piezojunction effect in heterojunctions under external bias for ultrasensitive strain sensing

This paper investigates for the first time the piezojunction effect in heterojunctions under external bias for ultrasensitive strain sensing. As a proof of concept, we used sensing devices made of 3C-SiC/Si heterostructure with vertically aligned electrodes. Applying the beam bending method to characterize the sensing effect, the bending strain was introduced along the typical orientation [100] or [110] on (100) Si plane. Experimental results show a linear relationship between the relative change in the forward current and the applied strain from 0 to 500 ppm, decreasing under the tensile strain while increasing under the compressive strain. At the forward bias of 8 V, the obtained gauge factors (GFs) are 199.7 for [100] orientation and 173.1 for [110] orientation, which significantly enhance about 630 % and 540 % compared to the highest GF of n-type 3C-SiC in the literature. Interestingly, the GFs of the n+-3C-SiC/p-Si heterostructure are positive in contrast to the negative GFs of n-3C-SiC thin films. The results were explained by the strain modulation on the band split and electron mass shift along the out-of-plane direction as well as by the change in the barrier height, depletion region width, and carrier concentrations under the forward bias. The ultrasensitive piezojunction effect in the 3C-SiC/Si heterojunction demonstrated in this study can pave the way toward developing ultrasensitive mechanical sensors.

Piezo-optoelectronic coupling in heterojunctions for self-powered and ultrasensitive pressure sensing

It is highly desirable to discover sensing mechanisms for developing self-powered and ultrasensitive pressure sensing. Here, we report that the piezo-optoelectronic coupling in a n+-3C-SiC/p-Si heterojunction not only enables energy harvesting but also features massive strain sensitivity. To demonstrate the proof of the concept, a pressure sensing device is developed with active and reference electrodes arranged on SiC along strain-sensitive [100] and strain-insensitive [110] orientations, respectively. Under the illumination of 637-nm laser at 2.0 mW, experimental results show the great response of the photogenerated voltage to the change in both static and dynamic pressures. Gauge factor is found to be −670.2 which is much larger than the highest gauge factor of n-type 3C-SiC. The underlying physics behind is the strain modulation on the effective mass and energy level of photogenerated charge carriers. This results in the change in carrier concentration detected by the electrodes along the strain-sensitive and strain-insensitive orientations. These findings could contribute significantly to the development of self-powered mechanical sensors using solid-state electronics.

Highly sensitive and robust 3C-SiC/Si pressure sensor with stress amplification structure

SiC based pressure sensors show tremendous promise for harsh environment applications thanks to their excellent mechanical, electrical, thermal, and chemical properties. This paper presents the design, fabrication, and characterisation of a highly sensitive and robust 3C-SiC/Si pressure sensor. The sensor utilises a stress amplification structure consisting of four Si pillars built up from the 3C-SiC/Si membrane, supporting a series of released n-type 3C-SiC sensing elements. When pressure is applied to the diaphragm, the pillars act to locally concentrate and amplify strain in the 3C-SiC sensing elements, resulting in over 7 times higher stresses/strains in these sensing elements compared to a traditional structure. Additionally, the front side of the sensor is fully covered by a 3C-SiC thin film, which provides a strong chemical protective capability, allowing the sensor to operate in harsh chemically corrosive environments. The robust device utilises the full Wheatstone bridge to negate the effects of temperature. Experimental results show that the fabricated sensor is highly stable, repeatable, has a high sensitivity of 0.276 mV/V/kPa and a maximum non-linearity of 2.2 % in the 0–100 kPa region. The results indicate that this smart-structure pressure sensor is promising for applications that require highly precise pressure sensing in aggressively corrosive environments.

Exploring n‐type SiC film with ultralow work function Cs coating surface for solar cell anode

In this study, the Cs adsorption behavior on n-type 3C-SiC (0001)-(2 × 1) and -(3 × 2) reconstructed surfaces are investigated via first-principles calculations. The potentiality of 3C-SiC as the anode material of solar cell based on photon-enhanced thermionic emission (PETE) is discussed. Various doping configurations are considered for the determination of an optimum n-type doping structure. Cs-covered (3 × 2) surface exhibits stronger stability compared with (2 × 1) surface. The charge transfer caused by Cs adsorption produces a [Cs+-SiC] dipole moment directing from Cs adatom to reconstructed surface, resulting in an effective declination of work function. Specifically, 0.5 ML Cs coverage surface provides an ultralow work function with 0.9 eV, demonstrating its potential application in PETE devices. Combining with a photoemission model, the maximum conversion efficiency can attain 30% utilizing SiC as an anode. Moreover, the Mulliken charge analysis provides the details of surface charge transfer and implies the inner mechanism of work function alteration. This study provides guiding significance for the synthesis of SiC material with low work function and an effective theoretical method for the design of high-performance anode materials in application of PETE devices. The adsorption mechanism of Cs on n-type doped 3C-SiC(0001)-(2 × 1) and -(3 × 2) reconstructed surfaces and its application as the anode material of solar cell are systematically investigated. For one Cs adatom adsorption, Cs-covered (3 × 2) surfaces present stronger stability compared with (2 × 1) surfaces. Cs-decorated (3 × 2) surface with 0.5 ML coverage exhibits an ultralow work function as low as 0.9 eV. Based on the photoemission model, the maximum conversion efficiency of PETE devices with SiC anode can reach 30%. Highlights Cs-decorated SiC surfaces with ultralow work function are suitable as PETE anode materials. Cs adsorption mechanism on 3C-SiC(0001)-(2 × 1) and -(3 × 2) surfaces are explored. The optimum n-type doped (2 × 1) and (3 × 2) surfaces are, respectively, determined. 0.5 ML Cs-adsorbed (3 × 2) surfaces show an ultralow work function of 0.9 eV. Cs adsorption induces a [Cs+-SiC] dipole moment on (2 × 1) and (3 × 2) surfaces.

Cubic silicon carbide/zinc oxide heterostructure fuel cells

Multifunctional semiconductor cubic silicon carbide (3C-SiC) is employed for fuel cell electrolyte, which has never been used before. n-type 3C-SiC can be individually employed as the electrolyte in fuel cells, but delivers insufficient open circuit voltage and minuscule current density due to its electronic dominant property. By introducing n-type ZnO to form an n–n 3C-SiC/ZnO heterostructure, significant enhancements in the ionic conductivity of 0.12 S/cm and fuel cell performance of 270 mW cm−2 are achieved at 550 °C. It is found that the energy band bending and build-in electric field of the heterostructure play the pivotal role in the ionic transport and suppressing the electronic conduction of 3C-SiC, leading to a markable material ionic property and fuel cell performance. These findings suggest that 3C-SiC can be tuned to ionic conducting electrolyte for fuel cell applications through the heterostructure approach and energy band alignment methodology.

ScAlN/3C-SiC/Si platform for monolithic integration of highly sensitive piezoelectric and piezoresistive devices

This paper reports on a platform for monolithic integration of piezoelectric and piezoresistive devices on a single chip using the ScAlN/3C-SiC/Si heterostructure. Surface acoustic wave devices with an electromechanical coupling of 3.2% and an out-of-band rejection as high as 18 dB are demonstrated using the excellent piezoelectric properties of ScAlN and low acoustic loss of 3C-SiC. Additionally, a large piezoresistive effect in the low-doped n-type 3C-SiC(100) thin film has been observed, which exceeds the previously reported values in any SiC thin films. The growth of the n-type 3C-SiC thin film was performed using the low pressure chemical vapor deposition technique at 1250 °C and the standard micro-electro-mechanical systems process is used for the fabrication of 3C-SiC piezoresistors. The piezoresistive effect was measured using the bending beam method in different crystallographic orientations. The maximum gauge factor is –47 for the longitudinal [100] orientation. Using the longitudinal and transverse gauge factors for different crystallographic orientations, the fundamental piezoresistive coefficients of the low-doped n-type 3C-SiC thin film are measured to be π11=(−14.5±1.3)×10−11 Pa−1, π12=(5.5±0.5)×10−11 Pa−1, and π44=(−1.7±0.7)×10−11 Pa−1.

High interfacial quality metal-oxide-semiconductor capacitor on (111) oriented 3C-SiC with Al2O3 interlayer and its internal charge analysis

Metal-oxide-semiconductor (MOS) capacitors with various gate dielectrics were fabricated on (111) oriented n-type 3C-SiC. Deposited SiO2 by sputtering without an interlayer (IL) and thermally grown SiO2 show deteriorated capacitance–voltage (C–V) characteristics and high interface trap density (Dit) over 1011–1012 cm−2 eV−1. By inserting an IL, C–V and leakage current characteristics are improved. In particular, an atomic layer deposited (ALD) Al2O3-IL is suitable for 3C-SiC, which successfully achieved low Dit in the order of 1010 cm−2 eV−1. MOS capacitor with the same gate dielectric on n-Si shows contradictory characteristics. Structural analysis shows it is considered the flat and uniform interface at Al2O3/3C-SiC leads good electrical characteristics. The 3C-SiC MOS capacitor with Al2O3-IL showed slightly negative flatband voltage and it is induced by negative interfacial dipole generated at the Al2O3/3C-SiC interface. This 3C-SiC MOS structure can be fabricated at low process temperature (<600 °C), which means the overall process for device application can be designed more flexibly.

Electron transport in bulk n-doped 3C-SiC by using a non-equilibrium quantum kinetic theory

In this paper we present a study on the charge transport in bulk n-type doped semiconductor 3C-SiC (in both, transient and steady state) using a non-equilibrium quantum kinetic theory derived from the method of nonequilibrium statistical operator (NSO), which furnishes a clear description of the irreversible phenomena that occur in the evolution of the analyzed system. We obtain theoretically the dependence on the applied electric field of the basic macrovariables: the “electron drift velocity” and the “non-equilibrium temperature”. The “peak points” in time evolution of this macrovariables are derived and analyzed.

Spin transport in n-type 3C–SiC observed in a lateral spin-pumping device

3C–SiC is a promising platform for semiconductor spintronics because it consists of light group-IV elements and has a zinc blende structure. To demonstrate spin transport in this attractive semiconductor, we conducted an experiment of spin-pump-induced spin transport through n-type 3C–SiC. A spin current is injected from Ni80Fe20 into the SiC channel under ferromagnetic resonance of the Ni80Fe20. The DC electromotive force caused by the inverse spin Hall effect in an adjacent metal detector attached to the SiC is detected as a manifestation of the spin current transport. This indicates 1.2-μm-long spin transport through the n-type 3C–SiC at room temperature. This achievement is the first step in the investigation of the physics of 3C–SiC for further spintronics study and its application.

Fabrication and Characterization of Ohmic Contacts to 3C-SiC Layers Grown on Silicon

This paper reports on the formation and characterization of Ohmic contacts to n-type and p-type type 3C-SiC layers grown on silicon substrates. In particular, Ohmic contact behavior was obtained either using Ni or Ti/Al/Ni layers annealed at 950°C. The values of the specific contact resistance ρc estimated by means of circular TLM (C-TLM) structures varied in the range ~ 10-3-10-5 Ωcm2, depending on the doping level of the 3C-SiC layer. A structural analysis performed by X-Ray Diffraction (XRD) allowed to identify the main phases formed upon annealing, i.e., Ni2Si and Al3Ni2. The morphology of the reacted contacts depended on that of the underlying substrate. The results can be useful for the development of a variety of devices on the cubic 3C-SiC polytype.

Deep Electronic Levels in n-Type and p-Type 3C-SiC

In recent times, 3C-SiC is gaining more and more interest in terms of applications for optoelectronics and quantum computing. Cubic SiC exhibits a number of luminescent defects in the near infrared originating from deep electronic levels. Temperature dependent photoluminescence measurements were conducted on n-type and p-type 3C-SiC in order to investigate the formation of dopant related point defects as well as intrinsic point defects and defect complexes. The results indicate a number of VSi, VC and VCCSi related defects which might be suitable candidates for future optoelectronic applications.

Fabrication of a Monolithic Implantable Neural Interface from Cubic Silicon Carbide.

One of the main issues with micron-sized intracortical neural interfaces (INIs) is their long-term reliability, with one major factor stemming from the material failure caused by the heterogeneous integration of multiple materials used to realize the implant. Single crystalline cubic silicon carbide (3C-SiC) is a semiconductor material that has been long recognized for its mechanical robustness and chemical inertness. It has the benefit of demonstrated biocompatibility, which makes it a promising candidate for chronically-stable, implantable INIs. Here, we report on the fabrication and initial electrochemical characterization of a nearly monolithic, Michigan-style 3C-SiC microelectrode array (MEA) probe. The probe consists of a single 5 mm-long shank with 16 electrode sites. An ~8 µm-thick p-type 3C-SiC epilayer was grown on a silicon-on-insulator (SOI) wafer, which was followed by a ~2 µm-thick epilayer of heavily n-type (n+) 3C-SiC in order to form conductive traces and the electrode sites. Diodes formed between the p and n+ layers provided substrate isolation between the channels. A thin layer of amorphous silicon carbide (a-SiC) was deposited via plasma-enhanced chemical vapor deposition (PECVD) to insulate the surface of the probe from the external environment. Forming the probes on a SOI wafer supported the ease of probe removal from the handle wafer by simple immersion in HF, thus aiding in the manufacturability of the probes. Free-standing probes and planar single-ended test microelectrodes were fabricated from the same 3C-SiC epiwafers. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on test microelectrodes with an area of 491 µm2 in phosphate buffered saline (PBS) solution. The measurements showed an impedance magnitude of 165 kΩ ± 14.7 kΩ (mean ± standard deviation) at 1 kHz, anodic charge storage capacity (CSC) of 15.4 ± 1.46 mC/cm2, and a cathodic CSC of 15.2 ± 1.03 mC/cm2. Current-voltage tests were conducted to characterize the p-n diode, n-p-n junction isolation, and leakage currents. The turn-on voltage was determined to be on the order of ~1.4 V and the leakage current was less than 8 μArms. This all-SiC neural probe realizes nearly monolithic integration of device components to provide a likely neurocompatible INI that should mitigate long-term reliability issues associated with chronic implantation.

Electrical properties of n-type 3C-SiC epilayers in situ doped with extremely high levels of phosphorus

Low temperature heteroepitaxy of cubic silicon carbide (3C-SiC) on silicon substrates is key to the low-cost and mass scale hetergeneous integration of 3C-SiC into the semiconductor market. Low temperature growth also opens up the opportunity to dope 3C-SiC in situ during the epitaxial growth with standard Si based n-type and p-type dopants. In situ doping offers many advantages over ion implantation, such as complex doping profiles, more abrupt interfaces and minimal crystal damage. In this study, 3C-SiC thin films have been doped with phosphorus to a range of concentrations during epitaxial growth on standard silicon (Si) substrates. Both the material and electrical properties of the films have been investigated. Hall effect measurements and secondary ion mass spectroscopy profiling confirm 100% electrically active n-type dopants up to 2 × 1020 cm−3. The process offers extreme control over the 3C-SiC electrical properties without relying on post-growth ion implantation and high temperature activation annealing, enabling the formation of more complex 3C-SiC based devices and low resistance contacts.

Strain Effect in Highly‐Doped n‐Type 3C‐SiC‐on‐Glass Substrate for Mechanical Sensors and Mobility Enhancement

This work reports the strain effect on the electrical properties of highly doped n-type single crystalline cubic silicon carbide (3C-SiC) transferred onto a 6-inch glass substrate employing an anodic bonding technique. The experimental data shows high gauge factors of -8.6 in longitudinal direction and 10.5 in transverse direction along the [100] orientation. The piezoresistive effect in the highly doped 3C-SiC film also exhibits an excellent linearity and consistent reproducibility after several bending cycles. The experimental result was in good agreement with the theoretical analysis based on the phenomenon of electron transfer between many valleys in the conduction band of n-type 3C-SiC. Our finding for the large gauge factor in n-type 3C- SiC coupled with the elimination of the current leak to the insulated substrate could pave the way for the development of single crystal SiC-on-glass based MEMS applications.

Fabrication of N-doped 3C-SiC nanobelts with selected [formula omitted] top surface and their enhanced transverse piezoresistance behaviours

The piezoresistive effect of SiC low-dimensional nanostructures, together with their exclusive benefits of superior mechanical characteristics and could be integrated into the flexible devices that can withdraw large strain, promise to make a wide range of sensing devices application possible in the severe working conditions. In this work, we reported the transverse piezoresistance behaviours of n-type 3C-SiC nanobelts with selected (11¯0) top surface and N dopants. The single-crystalline SiC nanobelts were achieved by the means of direct pyrolysis of polymeric precursors without catalysts, which had an average N doping level of ~ 6.02 at% and a typical width-to-thickness ratio of ~ 8. It was disclosed that the transverse piezoresistance coefficient π[11¯0] reached the maximum 10.29 × 10–11 Pa−1 at a fixed external stress of 67.03 nN. Correspondingly, the gauge factor (GF) was calculated to be of ~ 61.7, which is higher than all the reported n-type SiC, intrinsic SiC, as well as most of the p-type SiC counterparted. The findings shed light on the fact that present n-type SiC nanobelts have the potential of becoming an outstanding contestant for the development of robust pressure sensors having elevated sensitivities.

A large pseudo-Hall effect in n-type 3C-SiC(1 0 0) and its dependence on crystallographic orientation for stress sensing applications

The pseudo-Hall effect in n-type single crystal 3C-SiC(100) with low carrier concentration has been investigated. Low pressure chemical vapor deposition was used to grow the single crystal n-type 3C-SiC(100) and Hall devices were fabricated by photolithography and dry etch processes. A large pseudo-Hall effect was observed in the grown thin films which showed a strong dependence on the crystallographic orientation. N-type 3C-SiC(100) with low carrier concentration shows a completely different behavior of pseudo-Hall measurements as compared to the p-type 3C-SiC(100). Contrary to p-type, the effect is maximum along [100] crystallographic orientation and minimum along [110] orientation. Moreover, the observed pseudo-Hall effect is 50% larger than p-type with higher carrier concentration grown by the same process which makes n-type 3C-SiC(100) with low carrier concentration more suitable material for designing highly sensitive micro-mechanical sensors.

Photo-electrochemical water splitting in silicon based photocathodes enhanced by plasmonic/catalytic nanostructures

Photoelectrochemical water splitting is a promising method to produce H2 by making use of solar energy. In this paper we report on a photocathode made by p-type crystalline Si covered with an n-type 3C-SiC polycrystalline film, acting as protective layer and transparent emitter. The photoelectrodes exhibit a saturated photocurrent above 30mAcm−2. No decay is observed after 9h under constant current stress at 1kWm−2 with AM1.5G spectrum illumination. Improvement of the photocurrent value is achieved by covering the 3C-SiC emitter with Au or Pt nanoparticles. Under suitable metal nanoparticles deposition conditions, compared to the samples without nanoparticles, two major effects are observed: first the onset potential is considerably reduced, and second, higher saturated photocurrent is found, up to 38mAcm−2, i.e. with a 27% increase. Optical and micro-structural studies on the nanoparticles provide insights on the origin of the observed effects.

Piezo-Hall effect and fundamental piezo-Hall coefficients of single crystal n-type 3C-SiC(100) with low carrier concentration

This article reports the results on the piezo-Hall effect in single crystal n-type 3C-SiC(100) having a low carrier concentration. The effect of the crystallographic orientation on the piezo-Hall effect has been investigated by applying stress to the Hall devices fabricated in different crystallographic directions. Single crystal n-type 3C-SiC(100) and 3C-SiC(111) were grown by low pressure chemical vapor deposition at 1250 °C. Fundamental piezo-Hall coefficients were obtained using the piezo-Hall effect measurements as P11 = (–29 ± 1.3) × 10−11 Pa−1, P12 = (11.06 ± 0.5)× 10−11 Pa−1, and P44 = (–3.4 ± 0.7) × 10−11 Pa−1. It has been observed that the piezo-Hall coefficients of n-type 3C-SiC(100) show a completely different behavior as compared to that of p-type 3C-SiC.

Pseudo-Hall Effect in Single Crystal n-Type 3C-SiC(100) Thin Film

This article reports the first results on stress induced pseudo-Hall effect in single crystal n-type 3C-SiC(100) grown by LPCVD process. After the growth process, Hall devices were fabricated by standard photolithography and dry etching processes. The bending beam method was employed to study the stress induced changes in the electrical response of the fabricated Hall devices. It has been observed that when stress is applied to the 3C-SiC(100) Hall devices, the offset voltage of the Hall devices varies linearly with the applied compressive and tensile stresses which is called, the pseudo-Hall effect. The variation of the offset voltage of these Hall devices is also proportional to the applied input current. This variation of the offset voltage with the applied compressive and tensile stresses shows that single crystal n-type 3C-SiC(100) can be used for stress sensing applications.

Temperature behavior of the conduction electrons in the nitrogen-doped 3C SiC monocrystals as studied by electron spin resonance

The temperature behavior of the electron spin resonance (ESR) spectra of nitrogen donors in n-type bulk 3C SiC monocrystals with (ND − NA) ≈ 1017 cm−3 was studied at T = 10–50 K. The triplet lines due to the hyperfine (hf) interaction with 14N nuclei (I = 1, 99.6%) along with a single line with similar isotropic g values of 2.0050(3) were observed in the ESR spectrum of n-type 3C SiC monocrystals in the temperature interval from 10 to 35 K. The observed reduction of the hf splitting for the nitrogen donor residing cubic position (Nk) in the temperature interval from 15 to 35 K was attributed to the motion narrowing effect of the hf splitting. With further increase of the temperature up to 35 K, only one single line with a Lorentzian lineshape was observed in the ESR spectrum of n-type 3C SiC, which was previously assigned in the literature to the unknown deeper donor center. Based on the temperature behavior of the ESR linewidth, integral intensity, and g-value, we have attributed this signal to the conduction electrons (CEs). The temperature dependence of the CE ESR linewidth was described by an exponential law (Orbach process) with the value of the activation energy ΔE ≈ 40 meV close to the energy separation between 1 s(E) excited energy level and conduction band for nitrogen donors. The nitrogen donor pairs were found in the ESR spectrum of n-type 3C SiC. The electrical characteristics of 3C SiC sample were studied by using the contact-free microwave conductivity. The energy ionization of nitrogen donor Ei = 51.4 meV was obtained from the fitting of the experimental data with the theory.