Si Doping Concentration Research Articles

Study on the effects of Si-doping in molecular beam heteroepitaxial β-Ga2O3 films

β-Ga2O3, an emerging wide bandgap semiconductor material, holds significant potential for various applications. However, challenges persist in improving the crystal quality and achieving controllable doping of β-Ga2O3. In particular, the relationship between these factors and the mechanisms behind them are not fully understood. Molecular beam epitaxy (MBE) is viewed as one of the most sophisticated techniques for growing high-quality crystalline films. It also provides a platform for studying the effects of doping and defects in heteroepitaxial β-Ga2O3. In our study, we tackled the issue of Si source passivation during the MBE growth of Si-doped β-Ga2O3. We did this by using an electron beam vaporize module, a departure from the traditional Si effusion cell. Our research extensively explores the correlation between Si doping concentration and film properties. These properties include microstructure, morphology, defects, carrier conductivity, and mobility. The results from these investigations are mutually supportive and indicate that a high density of defects in heteroepitaxial β-Ga2O3 is the primary reason for the challenges in controllable doping and conductivity. These insights are valuable for the ongoing development and enhancement of β-Ga2O3-based device techniques.

Fractal dimension of heights facilitates mesoscopic mechanical properties in ternary hard film surfaces

Hardness of thin films is a noteworthy property in the electronic and mechanical industry and is generally observed to be dependent on the degree of roughening facilitated from surface heights' surface spatial heterogeneity at the mesoscopic observation scale. Nonetheless, owing to enhanced scale fluctuations and higher-order central moments, conventional parameters provide limitations and errors in capturing the spatial heterogeneity of surfaces. Herein, we have utilized scale-independent fractal parameters to analyze the spatial heterogeneity of surface heights in Ti1−xSixN ternary hard films deposited with varying Si doping concentrations using sputtering technique. The fractal dimension, lacunarity coefficient, Moran index, surface entropy, Otsu's separability, and fractal succolarity were computed to provide an overarching understanding of the surface heights' spatial heterogeneity. Principal component analysis was employed on the data sets to identify the parameter(s) accounting for the maximum variance and accordingly, the structure–property relation between spatial heterogeneity of surface and hardness is analyzed and discussed in the context of the fractal dimension of surface heights. The results indicate the possibility of mesoscopic surface engineering and, consequently, tuning of hardness and modulus of elasticity in Ti1−xSixN hard films by mere changing of surface spatial heterogeneity facilitated by the fractal dimension of surface heights.

Electrochemical lift-off of GaN films for GaN-on-GaN technology

Lifting off the native GaN substrate is an essential step in the fabrication of high-performance devices. In this study, we report a method to separate GaN thin films from GaN substrate through electrochemical (EC) lateral etching. By employing tetramethylammonium hydroxide to treat the dry-etched sidewalls, we addressed the issue of non-uniformity at the EC etching front. Meantime, we investigated the effect of Si doping concentration on the roughness of the lift-off GaN films. It is found that as increasing the doping concentration, the roughness decreases together with a reduced applied bias. Finally, we achieved an epitaxial-level smooth surface with a small roughness of only 0.3 nm.

Synthesis and comprehensive synchrotron-based structural analysis of Si-doped nanodiamond composite films deposited on cemented carbide

Si-doped nanodiamond composite (NDC) films were deposited on unheated WC − Co substrates by coaxial arc plasma deposition for advanced cutting tools applications. The nanoindentation test revealed changes in films hardness and Young's modulus attributed to various Si doping concentrations (0, 1, 5, and 20 at.%) and the catalytic effects of Co diffused from the substrate surface into the films. To reveal the physical origin behind these changes, the films' structure was examined by synchrotron-based Auger electron spectroscopy (AES), soft X-ray photoemission spectroscopy (XPS), and near edge X-ray absorption fine structure spectroscopy (NEXAFS). The AES and XPS analysis showed a consistent correlation between nanoindentation measurements and the estimated C sp3 fraction in the films. The NEXAFS spectra revealed intense C − C σ* resonance, consistent with the measured film hardness. The 1 at.% Si-doped film, deposited with an undoped NDC buffer layer to mitigate the Co catalytic effects, exhibited the highest hardness of 60 GPa, the largest C sp3 fraction, and the most intense CC σ* peak. Si doping resulted in the formation of C − Si sp3 bonds at the expense of C sp2 bonds, increasing the fraction of C sp3 bonds and enhancing film hardness. The synchrotron-based analysis effectively revealed the electronic states of NDC hard coatings, particularly after the removal of surface contaminants through mechanical polishing.

The surface tension of Ga2O3 melt measured by a drop-weight method in an optical floating-zone furnace

The surface tension of Ga2O3 melt is successfully measured using a drop-weight method in an optical floating-zone furnace that we have developed. The method is verified to be feasible by measuring the surface tension value of TiO2 melt and then comparing it with values in previous reports determined by other methods. We find that the surface tension of Si-doped Ga2O3 melt increases with the decrease in the Si doping concentration and reaches 527.9 mN m−1 for pure Ga2O3 melt. The surface tension of the unintentionally doped Ga2O3 melt is also measured to be 519.3 mN m−1 in the presence of some common contaminants appearing in Czochralski and edge-defined film-fed growth methods, including Ir, Al, and Si.

Band Gap Energy of Ion‐Doped Multiferroic NaFeO2 Nanoparticles

The band gap energy Eg of orthorhombic β‐ and hexagonal α‐NaFeO2‐bulk and nanoparticles is investigated for the first time based on a microscopic model combined with Green's function theory. Eg increases with decreasing nanoparticles size and is shape‐dependent. The competition between Coulomb and electron–phonon interactions is demonstrated. Moreover, Eg decreases with increasing Ge and Si doping concentration or increases by Mn, Cr, and Ni ion doping due to different radii of the doping and the host Fe ions, leading to varying strains. The substitution for Na ion by K or Li(Cu) ions can cause both enhancing or reducing of Eg. Thus, the band gap energy can be regulated by nanoparticles size, ion doping, electron–phonon, and Coulomb interactions.

Prediction of Optimal Thickness of InAs/InGaAs Quantum Well

In this work, some properties of the InAs/InGaAs quantum well (QW) were calculated, such as the wave functions and the charge density of the 2D free electron gas (2DEG) by solving the Poisson- Schroedinger equation. The thinner capping layer gives charge densities forming inside the QW that are higher than the thicker values. The optimal thickness of the capping layer can be 10 nm due to the most stable charge density and fully symmetrical wave functions. Our result indicates that higher charge densities can be found with higher Si-delta doping concentrations. However, the distance of the Si-delta doping also affects the charge population. The charge density linearly decreases with a higher Si-delta doping spacer; the thickness was chosen as 7nm. We performed the growth with different concentrations of Si with optimal thicknesses and compared them with the calculated values. There is good agreement between the simulations and experiments with the lower Si-doping concentrations.

Continuous Si doping in (010) and (001) β-Ga2O3 films by plasma-assisted molecular beam epitaxy

We report the continuous Si doping in β-Ga2O3 epitaxial films grown by plasma-assisted molecular beam epitaxy through the use of a valved effusion cell for the Si source. Secondary ion mass spectroscopy results exhibit that the Si doping profiles in β-Ga2O3 are flat and have sharp turn-on/off depth profiles. The Si doping concentration was able to be controlled by either varying the cell temperatures or changing the aperture of the valve of the Si effusion cell. High crystal quality and smooth surface morphologies were confirmed on Si-doped β-Ga2O3 epitaxial films grown on (010) and (001) substrates. The electronic properties of Si-doped (001) β-Ga2O3 epitaxial film showed an electron mobility of 67 cm2/Vs at the Hall concentration of 3 × 1018 cm−3.

Invesigation of the electronic structure and Optoelectronic properties of Si-doped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> using GGA+U method based on first-principle

In this work, the formation energy, band structure, state density, differential charge density and optoelectronic properties of undoped and Si doped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> are calculated using GGA+U method based on density functional theory. The results show that the Si-substituted tetrahedron Ga(1) is more easily synthesized in experiments, and the obtained <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> band gap and Ga 3d state peak are in good agreement with the experimental results, and the effective doping is more likely to be obtained under oxygen-poor conditions. After Si doping, the total energy band moves to the low-energy end, and Fermi level enters the conduction band, showing n-type conductive characterastic. Si 3s orbital electrons occupy the bottom of the conduction band, the degree of electronic coocupy is strengthened, and the conductivity is improved. The dielectric function ε2(ω) results show that with the increase of Si doping concentration, the ability to stimulate conductive electrons first increases and then decreases, which is in good agreement with the quantitative analysis results of conductivity. The optical band gap increases and the absorption band edge rises slowly with the increase of Si doping concentration. The results of absorption spectra show that Si-doped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> has strong deep ultraviolet photoelectric detection ability. The calculated results provide a theoretical reference for the further experimental investigation and the optimization innovation of Si-doped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> and relative device design.

Investigation of electronic structure and optoelectronic properties of Si-doped <i>β</i>-Ga<sub>2</sub>O<sub>3 </sub>using GGA+<i>U</i> method based on first-principle

In this work, the formation energy, band structure, state density, differential charge density and optoelectronic properties of undoped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> and Si doped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> are calculated by using GGA+<i>U</i> method based on density functional theory. The results show that the Si-substituted tetrahedron Ga(1) is more easily synthesized experimentally, and the obtained <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> band gap and Ga-3d state peak are in good agreement with the experimental results, and the effective doping is more likely to be obtained under oxygen-poor conditions. After Si doping, the total energy band moves toward the low-energy end, and Fermi level enters the conduction band, showing n-type conductive characteristic. The Si-3s orbital electrons occupy the bottom of the conduction band, the degree of electronic occupancy is strengthened, and the conductivity is improved. The results from dielectric function <i>ε</i><sub>2</sub>(<i>ω</i>) show that with the increase of Si doping concentration, the ability to stimulate conductive electrons first increases and then decreases, which is in good agreement with the quantitative analysis results of conductivity. The optical band gap increases and the absorption band edge rises slowly with the increase of Si doping concentration. The results of absorption spectra show that Si-doped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> has the ability to realize the strong deep ultraviolet photoelectric detection. The calculated results provide a theoretical reference for further implementing the experimental investigation and the optimization innovation of Si-doped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> and relative device design.

Acceleration of the yellow band luminescence in GaN layers via Si and Ge doping

A huge acceleration of the yellow band defect luminescence (YB) with increasing Si and Ge doping concentration in GaN layers has been observed and studied. The donor doping concentrations in the n-type GaN varied from 5 × 10 16 cm −3 to 1.5 × 10 19 cm −3 for undoped, Si-doped, and Ge-doped samples. Consequently, the fastest component of the photoluminescence (PL) decay time curve accelerated from 0.9 ms to 40.3 ns, and the mean decay time from 20 ms to 260 ns with increasing doping concentration. We have proposed an explanation based on a theoretical model of donor-acceptor pair transition (DAP) and electron-acceptor (e-A 0 ) recombination at higher dopant concentrations, which is supported by several measurement techniques as room-temperature radioluminescence (RL), PL measurements or thermally stimulated luminescence (TSL). Last but not least, we have shown a change in the morphology of samples with the increasing dopant concentration, especially 3D columns formation with the high level of Si-doping and its influence on the light extraction from the GaN layer. The paper shows an uncommon way of dealing with slow unwanted YB, which detriment the GaN luminescence properties. • Si and Ge doping of GaN may cause a huge acceleration of yellow band luminescence. • Steady-state photoluminescence intensity remains almost unchanged. • Donor-acceptor and electron-acceptor pair transitions explain the luminescence processes. • Highly Si-doped GaN growths in 3D columns, whereas Ge-doped remains smooth. • The light extraction is influenced by the morphology change.

Theoretical prediction of output performance of 63NiO-Si heterojunction betavoltaic cell

For the 63NiO-Si heterojunction betavoltaic nuclear battery, the energy deposition of the energy conversion material itself was simulated by Monte Carlo simulation, and the structure of the 63NiO-Si heterojunction was optimized based on the theoretical calculation results. When the thickness of 63NiO is 4 μm and the doping concentration of Si is 1 × 1015 cm−3, the short-circuit current density, open-circuit voltage, fill factor, and maximum output power density of the nuclear battery are 1.22 μA · cm−2, 3.17 V, 0.95, 3.67 μW · cm−2. In addition, the output performance of 63Ni/NiO-Si heterojunction betavoltaic nuclear cell was calculated in this study. Under the condition that the activity of the radioactive source and the thickness of NiO(63NiO) are the same in the two structures, the proposed structure (63NiO-Si) has greatly improved the output performance of the nuclear battery by reducing the energy lost from radioactive source self-absorption.

Sub-bandgap refractive indexes and optical properties of Si-doped β-Ga2O3 semiconductor thin films

In this article, we present a theoretical study on the sub-bandgap refractive indexes and optical properties of Si-doped β-Ga2O3 thin films based on newly developed models. The measured sub-bandgap refractive indexes of β-Ga2O3 thin film are explained well with the new model, leading to the determination of an explicit analytical dispersion of refractive indexes for photon energy below an effective optical bandgap energy of 4.952 eV for the β-Ga2O3 thin film. Then, the oscillatory structures in long wavelength regions in experimental transmission spectra of Si-doped β-Ga2O3 thin films with different Si doping concentrations are quantitively interpreted utilizing the determined sub-bandgap refractive index dispersion. Meanwhile, effective optical bandgap values of Si-doped β-Ga2O3 thin films are further determined and are found to decrease with increasing the Si doping concentration as expectedly. In addition, the sub-bandgap absorption coefficients of Si-doped β-Ga2O3 thin film are calculated under the frame of the Franz–Keldysh mechanism due to the electric field effect of ionized Si impurities. The theoretical absorption coefficients agree with the available experimental data. These key parameters obtained in the present study may enrich the present understanding of the sub-bandgap refractive indexes and optical properties of impurity-doped β-Ga2O3 thin films.

Ultra-low resistance n+GaN contacts for GaN HEMT applications using MOCVD selective area epitaxy in N2 carrier gas

Low resistance n+GaN contact materials were experimentally studied for GaN HEMT applications by selective area epitaxy regrowth on a patterned SiC substrate. Epitaxy was performed by metal organic chemical vapor deposition using 100% H2 or 100% N2 as the carrier gas. Thin film characterization demonstrated that n+GaN grown in N2 carrier gas has a superior morphology with improved crystalline quality to that grown in H2 carrier gas. The results also indicated that the surface morphology of n+GaN grown in N2 carrier gas is less sensitive to mask pattern density and micro-loading effects with Si doping concentrations up to 1 × 1020/cm3. Secondary ion mass spectrometry analysis shows that C and O impurity levels in n+GaN are one order of magnitude lower with N2 carrier gas than with H2. The electrical measurement of transmission line model structures shows an n+GaN sheet resistance of 15 Ω/sq and an Ohmic metal to n+GaN contact resistance of 0.02 Ω-mm for structures grown in N2 carrier gas. These values represent 7.1× and 2.5× improvements compared to H2 carrier gas.

Structural, electrical, and optical properties of Si-doped ZnO thin films prepared via supercycled atomic layer deposition

Si-doped zinc oxide (ZnO) thin films were fabricated via atomic layer deposition (ALD) using diethylzinc, Tris(dimethylamino)silane, and H2O as Zn, Si, and O sources, respectively. The ALD cycle of the (Zn-O) step and the (Zn-Si-O) step were separated and the Si concentration was controlled via changing the (Zn-Si-O) pulse cycle compared to the (Zn-O) step. This Si source pulse sequence (Zn-Si-O) is different from the conventional (O-Si) pulse sequence and was specially selected in this study to maximize the properties of the thin film. All of the films showed the hexagonal wurtzite structure of ZnO with the (100) plane producing the main diffraction peak when ALD was performed at 200 ℃. As the Si-doping concentration increased, the crystallinity of the films deteriorated and the Scherrer size was reduced from 29.69 to 28.10 nm, there were improvements in the electrical characteristics, the carrier concentration increased up to 8.10 × 1020 cm−3, and the resistivity decreased to 5.15 × 10-4 Ω·cm, which is lower than the lowest previously reported result for Si-ZnO thin films fabricated via ALD (9.29 × 10-4 Ω·cm). Moreover, the carrier mobility increased at first (Si-ZnO 1:29 supercycles) and then decreased with increasing Si concentration. The average transmittance of all of the films was above 80% and the optical band gap increased from 3.29 to 3.47 eV. The increased carrier concentration according to the Burstein-Moss effect was linearly related to the band gap shift while up-shifting the EF (Fermi level) toward the conduction band. We expect that our approach in this study will provide an experimental basis to optimize ZnO thin-film characteristics via modification of Si pulsing sequence.

Experimental study of threshold voltage shift for Si:HfO2 based ferroelectric field effect transistor

For a given three different Si doping concentrations at room and high temperatures, the threshold voltage shift (ΔV th) on silicon-doped hafnium-oxide-based ferroelectric field effect transistor (FeFET) is experimentally investigated. It turned out that charge trapping in the gate stack of FeFET (versus polarization switching in the gate stack of FeFET) adversely affects ΔV th. Charge trapping causes the positive ΔV th, while polarization switching causes the negative ΔV th. The dominance of polarization switching is predominantly determined by the total remnant polarization (2P r), which can be controlled by adjusting Si doping concentration in the hafnium-oxide layer. As the Si doping concentration increases from 2.5% to 3.6%, and 5.0%, 2P r decreases 19.8 μC cm−2 to 15.25 μC cm−2, and 12.5 μC cm−2, which leads to ΔVth of −0.8 V, −0.09 V, and +0.1 V, respectively, at room temperature. At high temperature, the effect of polarization switching is degraded due to the decreased P r, while the effect of charge trapping is very independent of temperature. For those three different Si doping concentrations (i.e. 2.5%, 3.6%, and 5.0%), at the high temperature, ΔV th of FeFET is −0.675 V, −0.075 V, and +0.15 V, respectively. This experimental work should provide an insight for designing FeFET for memory and logic applications.

Doping concentration dependent piezoelectric behavior of Si:HfO2 thin-films

Piezoelectric thin films are of current interest in science and industry for highly integrated nano-electro-mechanical-systems and sensor devices. In this study, the dependence of the piezoelectric properties on the doping concentration in Si:HfO2 thin films and their crystallographic origin are investigated. The Si:HfO2 films with a thickness of 20 nm and various Si doping concentrations in the range of 2.7–5.6 cat.% were examined. The relationship between the piezoelectric displacement and remanent polarization is studied during wake-up from the antiferroelectric-like pristine state until the cycled ferroelectric state, which reveals an application-dependent optimal doping concentration. Furthermore, the piezoelectric and ferroelectric properties, as well as the relative permittivity, were measured over wake-up, thus giving a glimpse at the underlying mechanism of the transition from a pristine antiferroelectric-like behavior to a ferroelectric/piezoelectric one, revealing a pre-existing polar phase that is reorienting during wake-up. The studied samples show a strong displacement and polarization dependence on the doping concentration. Hence, the stoichiometry is an excellent parameter for the application-specific adjustment of complementary metal–oxide–semiconductor compatible piezoelectric thin films.

InGaN Resonant-Cavity Light-Emitting Diodes with Porous and Dielectric Reflectors

InGaN based resonant-cavity light-emitting diode (RC-LED) structures with an embedded porous-GaN/n-GaN distributed Bragg reflector (DBR) and a top dielectric Ta2O5/SiO2 DBR were demonstrated. GaN:Si epitaxial layers with high Si-doping concentration (n+-GaN:Si) in the 20-period n+-GaN/n-GaN stacked structure were transformed into a porous-GaN/n-GaN DBR structure through the doping-selective electrochemical wet etching process. The central wavelength and reflectivity were measured to be 434.3 nm and 98.5% for the porous DBR and to be 421.3 nm and 98.1% for the dielectric DBR. The effective 1λ cavity length at 432nm in the InGaN resonant-cavity consisted of a 30 nm-thick Ta2O5 spacer and a 148 nm-thick InGaN active layer that was analyzed from the angle-resolved photoluminescence (PL) spectra. In the optical pumping PL spectra, non-linear emission intensity and linewidths reducing effect, from 6.5 nm to 0.7 nm, were observed by varying the laser pumping power. Directional emission pattern and narrow linewidth were observed in the InGaN active layer with bottom porous DBR, top dielectric DBR, and the optimum spacer layer to match the short cavity structure.

Leakage of holes induced by Si doping in the AlGaN first barrier layer in GaN/AlGaN multiple-quantum-well ultraviolet light-emitting diodes

The electroluminescence characteristics of GaN/AlGaN multiple-quantum-well ultraviolet light-emitting diodes (LEDs) with different concentrations of silicon impurities in the first AlGaN barrier layer near the N-type GaN region is investigated numerically. It is found that the LED's electroluminescence spectrum blueshifts and its peak intensity increases first and then decreases, as the Si-doping concentration increases. This is because that the effective potential height and width of the first barrier layer is reduced due to the screening effect by the ionization of silicon impurities. As a result, more electrons can be injected into the active region, enhancing the luminescence efficiency. However, when the impurity concentration is too high, the leakage of holes may become severe, leading to a reduction of the luminescence intensity for the most heavily doped sample at the injection current of 20 mA.

Si(111) electrode/electrolyte interfacial studied by in-situ second harmonic generation

Si(111) electrode has been widely used in electrochemical and photoelectrochemical studies. The potential dependent measurements of the second harmonic generation (SHG) were performed to study Si(111) electrode interface. At different azimuthal angles of the Si(111) and under different polarization combinations, the curve of the intensity of SHG with extern potential has a different form of line or parabola. Quantitative analysis showed that these differences in the potential-dependence can be explained by the isotropic and anisotropic contribution of the Si(111) electrode. The change in the isotropic and anisotropic contribution of the Si(111) electrode may be attributed to the increase in the doping concentration of Si(111) electrodes.