C-sapphire Substrates Research Articles

Strain-induced yellow to blue emission tailoring of axial InGaN/GaN quantum wells in GaN nanorods synthesized by nanoimprint lithography

GaN nanorods (NRds) with axial InGaN/GaN MQWs insertions are synthesized by an original cost-effective and large-scale nanoimprint-lithography process from an InGaN/GaN MQWs layer grown on c-sapphire substrates. By design, such NRds exhibit a single emission due to the c-axis MQWs. A systematic study of the emission of the NRds by time-resolved luminescence (TR-PL) and power dependence PL shows a diameter-controlled luminescence without significant degradation of the recombination rate thanks to the diameter-controlled strain tuning and QSCE. A blueshift up to 0.26 eV from 2.28 to 2.54 eV (543 nm to 488 nm) is observed for 3.2 nm thick InGaN/GaN QWs with an In composition of 19% when the NRds radius is reduced from 650 to 80 nm. The results are consistent with a 1-D based strain relaxation model. By combining state of the art knowledge of c-axis growth and the strong strain relieving capability of NRds, this process enables multiple and independent single-color emission from a single uniform InGaN/GaN MQWs layer in a single patterning step, then solving color mixing issue in InGaN based nanorods LED devices.

Nanocrystalline equiatomic CoCrFeNi alloy thin films: Are they single phase fcc?

The bulk quaternary equiatomic CoCrFeNi alloy is studied extensively in literature. Under experimental conditions, it shows a single-phase fcc structure and its physical and mechanical properties are similar to those of the quinary equiatomic CoCrFeMnNi alloy. Many studies in literature have focused on the mechanical properties of bulk nanocrystalline high entropy alloys or compositionally complex alloys, and their microstructure evolution upon annealing. The thin film processing route offers an excellent alternative to form nanocrystalline alloys. Due to the high nucleation rate and high density of defects in thin films synthesized by sputtering, the kinetics of microstructure evolution is often accelerated compared to those taking place in the bulk. Here, thin films are used to study the phase evolution in nanocrystalline CoCrFeNi deposited on Si/SiO2 and c-sapphire substrates by magnetron co-sputtering from elemental sources. The phases and microstructure of the films are discussed in comparison to the bulk alloy. The main conclusion is that second phases can form even at room temperature provided there are sufficient nucleation sites.

Strain-induced variation of bandgap in (111) In2O3 epitaxial films grown on c-sapphire substrates by a pulsed laser deposition technique

The structural and electronic properties of In2O3 epitaxial films grown on c-sapphire substrates by a pulsed laser deposition technique are studied as functions of various growth conditions. Our study shows that growth temperature, growth time, oxygen partial pressure, and post-growth cooling conditions play important roles in governing the magnitude and character of the overall change in unit cell volume (volume strain) of the lattice. It is found that biaxial strain, which is developed due to lattice and thermal expansion coefficient mismatches between the layer and the substrate, leads to overall compression of the lattice. Interestingly, a hydrostatic tensile strain is found to coexist with biaxial strain in samples, which, following growth, are cooled to room temperature at a rate much slower than the natural rate of cooling. It has been observed that the overall strain can be varied from compressive to tensile by changing the growth parameters. The study further shows a systematic variation of the bandgap (from 3.8 to 3.1 eV) with a change in the unit cell volume of these layers, demonstrating the strong influence of strain on the band structural properties of the material.

Enhanced phase transition and infrared photoresponse characteristics in VO2(M1) thin films synthesized by DC reactive sputtering on different substrates

VO2(M1) films synthesized by sputtering on quartz and c-sapphire substrates exhibited good semiconductor-to-metal transition (SMT) and IR photoresponse properties.

Laser synthesis of non-volatile memristor structures based on tantalum oxide thin films

The amorphous thin TaOx films (x ≤ 5) of different thicknesses with different levels of oxygen vacancy content, which are promising for use as an active region of non-volatile memristors, were fabricated by the pulsed laser deposition in a drop-free mode on the c-sapphire substrates. The films were obtained from tantalum metal targets at substrate temperatures of 25 °C and 350 °C in the oxygen pressure range from 0.5 to 50 mTorr. The relationship between the partial oxygen pressure and the optical, electrical, and chemical properties of the films was studied. It was found that the amorphous TaOx phase, which is a source of defects associated with oxygen deficiency, required for the creation of memristors, appears only at low oxygen pressures in the chamber (~ 5 × 10−4 Torr), this leads to low values resistivity in the films (~ 10−3 Ohm · cm). Memristor structures Ag(Pt)/TaOx/TaOy/Ta/c-Al2O3 (x > y) in cross-bar geometry have been created that do not require electroforming, due to the choice of design and deposition conditions. Memristor using two mechanisms of resistive switching based on oxygen vacancies and silver cations showed very reliable RS performance with memory window performance RON/ROFF ~ 103 and RESET operating voltage ~ 1 V.

Influence of precursor thin-film quality on the structural properties of large-area MoS2 films grown by sulfurization of MoO3 on c-sapphire

In recent years, molybdenum disulfide (MoS2) has been investigated due to its unique electronic, optical, and mechanical properties with a variety of applications. Sulfurization of pre-deposited MoO3 layers is one of the methods of the preparation of large-area MoS2 thin films. The MoO3 layers have been grown on c-sapphire substrates, using two different techniques (rf sputtering, pulsed laser deposition). The films were subsequently annealed in vapors of sulfur at high temperatures what converted them to MoS2 films. The quality of MoS2 is strongly influenced by the properties of the precursor MoO3 layers. The pre-deposited MoO3, as well as the sulfurized MoS2, have been characterized by several techniques including Raman, Rutherford backscattering spectroscopy, atomic force microscopy, scanning electron microscopy, and X-ray diffraction. Here we compare two types of MoS2 films prepared from different MoO3 layers to determine the most suitable MoO3 layer properties providing good quality MoS2 films for future applications.

Carrier recombination dynamics in green InGaN-LEDs with quantum-dot-like structures

Exciton localization phenomena are considered here to comprehend the high internal quantum efficiency in InGaN/GaN multiple-quantum-well structures having discrete quantum dots (QDs) prepared by metal–organic-chemical-vapor deposition method on c-sapphire substrates. Spectroscopic results from the variable-temperature steady-state-photoluminescence and time-resolved photoluminescence (TRPL) are investigated. While the exciton localization is enhanced by strong localized states within the InGaN/GaN QDs–the impact of free carrier recombination cannot be ignored. The observed non-exponential decay in TRPL measurements is explained using a model by meticulously including localized exciton, non-radiative and free carrier recombination rates. A new method is proposed to calculate the internal quantum efficiency, which is supplementary to the traditional approach based on temperature-dependent photoluminescence measurement.

High-temperature low-pressure chemical vapor deposition of β-Ga2O3

Recent advancements in β-Ga2O3 materials’ growth and device developments are briefly reviewed with the focus on low-pressure chemical vapor deposition (LPCVD) of β-Ga2O3. β-Ga2O3 films are grown on off-axis c-sapphire and (010) β-Ga2O3 substrates via high-temperature LPCVD (HT-LPCVD) with growth temperatures ranging between 950 and 1050 °C. The effects of HT-LPCVD growth conditions on material properties are comprehensively studied. With relatively higher growth temperatures, an increased O2 flow rate is required to maintain β-Ga2O3 crystalline quality with high electron mobility. The growth rate of the HT-LPCVD β-Ga2O3 film scales with the increase of growth temperature and O2 flow rate. The film growth rate is strongly related to the sapphire substrate off-axis angle, which determines the preferred nucleation sites from the step edges. The transport properties of samples grown on substrates with different off-axis angles are compared. The optimized growth temperature for obtaining films with high electron mobility varies with the off-axis angle. From this comprehensive study, high-quality β-Ga2O3 films grown on c-sapphire are achieved with room temperature mobilities of 126 cm2/V s (6° off-axis c-sapphire), 116 cm2/V s (8° off-axis c-sapphire), and 119 cm2/V s (10° off-axis c-sapphire) at carrier concentrations of mid-1016 cm−3. β-Ga2O3 LPCVD homoepitaxy optimized at elevated growth temperatures is performed on Fe-doped semi-insulating (010) Ga2O3 substrates. With controllable Si doping, record-high room temperature mobilities of 156 cm2/V s (150 cm2/V s) are achieved with doping concentrations of 3 × 1016 cm−3 (1.5 × 1017 cm−3) at a growth temperature of 1050 °C. Secondary ion mass spectroscopy analysis shows more than an order of magnitude reduction of C and H impurity incorporation with increased growth temperature. Results from this work demonstrate that HT-LPCVD growth is a promising method to produce high-quality β-Ga2O3 films on both off-axis c-sapphire and native Ga2O3 substrates with a fast growth rate and superior transport properties critical for high power device applications.

Ammonia assisted low temperature growth of In2O3 (111) epitaxial films on c-sapphire substrates by chemical vapor deposition technique

The authors report the growth of bixbyite In2O3 (111) epitaxial layers on c-plane sapphire substrates by a chemical vapor deposition route, in which growth takes place under the flow of oxygen and ammonia in a furnace. Indium metal is used as the source for indium. It has been found that In2O3 films with high epitaxial quality can be grown by optimizing the growth temperature and the flow rate of NH3. Ammonia plays a catalytic role in the growth process. At growth temperatures less than 550 °C, inclusion of a rhombohedral phase, which is known to be thermodynamically stable only at high pressure, has been detected in the film. X-ray photoelectron spectroscopy does not show the presence of nitrogen in these films. An x-ray diffraction study reveals a sharp increase of disorder in these films as the growth temperature increases beyond 550 °C. The bandgap of the material is also found to decrease with the increase of disorder.

Growth, structural and optical properties of coherent κ-(AlxGa1−x)2O3/κ-Ga2O3 quantum well superlattice heterostructures

High quality heteroepitaxial (001)-oriented κ-(AlxGa1−x)2O3/κ-Ga2O3 quantum well superlattice heterostructures were deposited by tin-assisted pulsed laser deposition on c-sapphire substrates. Sharp superlattice fringes up to the ninth order in XRD patterns for Al-contents up to about 50 at. % confirm excellent structural quality and smooth interfaces in the multilayers on par with reports on homoepitaxial superlattices in the monoclinic modification. By employing elliptically segmented targets, the Al-content in the barrier layers of the superlattices was systematically varied in a range of 0.1 ≤ x ≤ 0.5 in a controlled and quasi-continuous manner. An in-depth investigation employing XRD 2θ-ω scans and reciprocal space map measurements on superlattices with different periods as well as single quantum well samples suggests coherent growth of the superlattices for application-relevant quantum well widths. The critical thickness for coherent growth of κ-Ga2O3 on κ-(AlxGa1−x)2O3 was further estimated to be at least 50 nm and 3 nm for x = 0.2 and x = 0.3, respectively. We determined absorption energies in optical transmission spectra for superlattices with x = 0.3 well below the bandgap of the barrier layers that decrease with increasing quantum well width suggesting transitions between localized states in the quantum wells as their origin. These results render superlattices in the metastable orthorhombic phase of Ga2O3 as a promising active layer for quantum well infrared photodetector applications.

Low-Temperature Laser Synthesis of Thin Electrochromic WO3 Films

Amorphous dielectric WO3 films with a surface roughness from 4 to 5 nm have been grown by room-temperature pulsed laser deposition on quartz and c-sapphire substrates using metallic targets. We have examined the effect of the nature of the substrates and the oxygen pressure during the growth process on the transmission spectrum of the WO3 films in the range from 400 to 2000 nm. The parameters of the films have been shown to depend on the oxygen pressure during the growth process. Raising the oxygen pressure from 20 to 60 mTorr during the film growth process increases the transmission of the WO3 films from 40 to 75% in the visible and UV spectral regions and from 10 to 70% in the IR. With increasing oxygen pressure during film growth, the band gap of the WO3 films increases from 3.01 to 3.34 eV in the case of sapphire substrates and from 2.95 to 3.42 eV in the case of quartz substrates, being only slightly dependent on the nature of the substrate. Using a WO3 film grown at room temperature, we have fabricated the first thin-film liquid-electrolyte electrochromic cell. Its transmission in the spectral range from 300 to 900 nm drops by 30% at an applied voltage of 2.5 V, with a coloration time on the order of 2 min.

Solubility limit and material properties of a κ-(AlxGa1−x)2O3 thin film with a lateral cation gradient on (00.1)Al2O3 by tin-assisted PLD

A ternary, orthorhombic κ-(AlxGa1−x)2O3 thin film was synthesized by combinatorial pulsed laser deposition on a 2 in. in diameter c-sapphire substrate with a composition gradient. Structural, morphological, and optical properties were studied as a function of the alloy composition. The thin film crystallized in the orthorhombic polymorph for Al contents of 0.07 ≤ x ≤ 0.46, enabling bandgap engineering from 5.03 eV to 5.85 eV. The direct optical bandgap and the c-lattice constant, as well, show a linear dependence on the cation composition. XRD measurements, especially 2θ-ω- and ϕ-scans, revealed the growth of κ-(AlxGa1−x)2O3 in [001]-direction and in three rotational domains. The surface morphology was investigated by atomic force microscopy and reveals root mean square surface roughnesses below 1 nm. Furthermore, the dielectric function (DF) and the refractive index, determined by spectroscopic ellipsometry, were investigated in dependence on the Al content. Certain features of the DF show a blue shift with increasing Al concentration.

PLD of thin electrochromic WO3 films at room temperature

The amorphous dielectric WO3films have been obtained by the droplet-free pulsed laser deposition method on the quartz and c-sapphire substrates at room temperature and the oxygen pressure from 20 to 60 mTorr. The dependence of the parameters of the obtained films on the oxygen pressure during the growth has been established. The transmission of the WO3 films increases over the whole region under study from 200 to 1000 nm with increase of the oxygen pressure in the process of the film growth on the substrates of both types. The surface roughness of the films poorly depended on the oxygen pressure during the film deposition and was 4-5 nm. The thin-film electrochromic cell with a liquid electrolyte was produced on the basis of the obtained WO3 film. The cell transmission over the spectrum range from 300 to 900 nm is reduced by 30% at a voltage of 2.5 V during the staining time of the order of two minutes.

Laser formation of thin-film memristor structures based on vanadium dioxide

The thin films of VO2 and the metal-oxide-metal (MOM)-structures of Au/VO2/VO2-x/Au based on them, which are promising for the use in neuromorphic electronic devices, have been obtained by the method of pulsed laser drop-free deposition on the c-sapphire substrates at room temperature. Using the cyclic I-V characteristics, a memristive effect has been revealed in the vertical geometry of the Au/VO2/VO2-x/Au MOM-structures. The x value was varied in the course of their growth by changing the pressure of buffer oxygen from 0.1 to 40 mTorr in the vacuum chamber, which provided the needed conductivity in the depleted injection layer. The dependence of memristive properties on the thickness of the semiconductor layer and concentration of the oxygen vacancies has been established. The oxygen pressure in the PLD method has been determined, at which the volatile behavior of the memristor resistive switching starts to appear at an oxide region thickness of 10/30 nm.

Optimization of parameters for generating nitrogen plasma in plasma-assisted MOCVD growth of InGaN thin films

The deposition of InGaN thin films by plasma-assisted metalorganic chemical vapor deposition is achieved using nitrogen plasma as a nitrogen source. The generation of nitrogen plasma is optimized using optical emission spectroscopy, and the plasma is dominated by excited molecular nitrogen, which emits in the range 300–420 nm. The emission intensity of the plasma significantly depends on the flow rate of nitrogen gas and heater temperature and are optimally 70 SCCM and 650 °C, respectively. A further increase in these parameters results in a decrease in the intensity of the nitrogen plasma emission. An optimal flow rate and heater temperature are used to grow InGaN thin films on c-sapphire substrates. InGaN thin films grown with a TMIn vapor concentration (xv) of 0%, 50%, and 100% at a growth temperature of 650 °C are highly oriented to the (0002) plane in a hexagonal structure. The film grown with a vapor concentration of 50% has an indium concentration of 55% and no indication of phase separation. Increasing the growth temperature above 650 °C results in a decrease in the growth rate.

The plasmonic nanoparticles with controlled optical properties

The gold and silver plasmon nanoparticles have been synthesized on the c-sapphire and p-silica substrates by the pulsed laser deposition method. It has been demonstrated that the variation of the thickness of as-grown gold and silver films permits producing the plasmon nanoparticles with different size and density. It provides the retuning of the frequency of surface plasmon resonance in wide spectral region.

Pulsed laser deposition of the ion-conducting LiCoO2 films

Pulsed laser deposition on the n-type and p-type sapphire and silicon substrates produced the ion-conducting LiCoO2 films with a thickness of 10 to 100 nm. The films were synthesized from the compound LiCoO2:Li2O targets with the concentrations of Li2O 5% and 10% at the substrate temperature of 25 to 500 °C. The optical and electrical properties and surface morphology of the films obtained were investigated depending on the type of the substrate and the deposition temperature. The surface roughness of the LiCoO2 films weakly depended on the type of substrate and increased from 7 to 35 nm with a decrease in the substrate temperature from 500 to 25 °C, respectively. The transmission of the LiCoO2 films increased, on the average, by 30% over the entire spectral range under study from 200 to 1000 nm with a change in the substrate temperature from 25 to 500 °C. It has been established that the LiCoO2 films deposited on the silicon substrates exhibit the resistivity of the order of 10−7 Ω cm for the films prepared on high-conducting n- and p-type silicon, and of the order of 10-1 Ω cm for those prepared on high-resistance silicon of n-type. The LiCoO2 films deposited on the c-sapphire substrates display dielectric properties.

Epitaxial stabilization of single phase κ-(InxGa1−x)2O3 thin films up to x = 0.28 on c-sapphire and κ-Ga2O3(001) templates by tin-assisted VCCS-PLD

High-quality (InxGa1−x)2O3 thin films in the orthorhombic κ-phase were grown by pulsed-laser deposition (PLD) on c-sapphire substrates as well as PLD-grown κ-Ga2O3 thin film templates. We varied the In-content 0 ≤ x ≤ 0.38 of the layers using a single, elliptically segmented, and tin-doped (In0.4Ga0.6)2O3/Ga2O3 target, employing the vertical continuous composition spread (VCCS) PLD-technique. A stoichiometric transfer of In and Ga from the target to the thin films has been confirmed, suggesting that the formation of volatile Ga2O and In2O suboxides is not a limiting factor in the tin-assisted growth mode. For all x, the thin films crystallized predominantly in the κ-modification as demonstrated by XRD 2θ-ω scans. However, for x > 0.28, phase separation of the cubic bixbyite and the κ-phase occurred. The κ-Ga2O3 template increased the crystalline quality of the κ-(InxGa1−x)2O3 thin film layers remarkably. Epitaxial, but relaxed growth with three in-plane rotational domains has been found for all thin films by XRD ϕ-scans or reciprocal space map measurements. Smooth surface morphologies (Rq < 3 nm) for all phase pure thin films were evidenced by atomic force microscopy measurements, making them suitable for multilayer heterostructures. The composition-dependent in- and out-of plane lattice constants follow a linear behavior according to Vegard’s law. A linear relationship can also be confirmed for the optical bandgaps that demonstrate the feasibility of bandgap engineering in the energy range of 4.1–4.9 eV. The results suggest κ-(InxGa1−x)2O3 as a promising material for heterostructure device applications or photodetectors.

Influence of phase composition on optical properties of TiO2: Dependence of refractive index and band gap on formation of TiO2-II phase in thin films

Atomic layer deposition was applied to grow TiO2 thin films containing anatase, rutile and mixture of TiO2-II and rutile phases with the aim to investigate the dependence of optical parameters on the formation of metastable TiO2-II phase. The films were grown at a substrate temperature of 450 °C using TiCl4 and O3 as the precursors. The crystal structure obtained depended on the substrates used. The films deposited on amorphous SiO2 substrates contained only the anatase phase when the film thicknesses did not exceed 88 nm while in thicker films, anatase with inclusions of rutile was formed. The films deposited on α-Al2O3 substrates contained epitaxial rutile without traces of other crystalline phases on α-Al2O3(0 1 1̄ 2) (r-sapphire) and epitaxial TiO2-II with inclusions of rutile phase on α-Al2O3(0 0 0 1) (c-sapphire) substrates. The films that contained TiO2-II had refractive indices of 2.8 at a wavelength of 500 nm and 2.61–2.67 at 633 nm. These values were higher than those of anatase films (2.6 and 2.48–2.49, respectively) and rutile films (2.74 and 2.57–2.62, respectively). Optical band gaps of 3.28–3.36 eV obtained for the films containing TiO2-II were by 0.20 ± 0.08 eV wider than the band gaps of rutile films and by 0.06 ± 0.05 eV narrower than the band gaps of anatase films. A band gap value of 3.00 ± 0.08 eV was measured for a non-epitaxial film that was deposited on amorphous SiO2 and contained anatase with inclusions of rutile.

Thermally stimulated current spectroscopy of traps in CVD grown monolayer MoS2

We have investigated defects in large area monolayer MoS2 films using thermally stimulated current (TSC) spectroscopy. Films are grown on c-sapphire substrates using a microcavity based chemical vapor deposition technique. A theoretical framework to analyze TSC data for a two dimensional semiconductor has been developed. The study reveals the existence of two traps with average activation energies of 670 and 850 meV. The density of these traps shows an increase followed by saturation as the sample goes through repeated thermal cycles in vacuum. Interestingly, the density returns to its initial level when the sample is exposed to the ambient condition for a sufficiently long time, suggesting that these defects are passivated by certain adsorbate groups/molecules in the ambient condition. It has been found that annealing in a sulfur environment substantially reduces the concentration of these traps. This indicates that the traps must be related to sulfur deficiency related defects.