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β-Ga2O3 is a promising ultra-wide bandgap semiconductor with melt-grown substrates that are scalable, particularly in the (001) orientation. In this study, we report on the heated-phosphoric acid etching of (001) β-Ga2O3. A wagon wheel pattern with spokes aligned to a range of specific crystallographic directions was fabricated on (001) β-Ga2O3. At 160 °C, the (001) etch rate was 1.47 μm/h, which is comparable to etch rates obtained via dry etch techniques. The etched (001) surface had a morphology that was smoother than an inductively coupled plasma etched surface. All spokes possessed relatively smooth sidewalls. Spokes oriented along the [100] direction exhibited minimal SiO2 mask undercut rates at lower etch temperatures and symmetric trapezoidal profiles with near vertical (010)-like sidewalls, which are ideal for device structures with a trench geometry. Spokes oriented along the [010] direction exhibited significant SiO2 mask undercut rates and asymmetric trapezoidal profiles with different sidewall angles. These spokes also possessed reduced sidewall angles, which is favorable for field management at device corners and edges. The etch process was used to realize a dense array of [100]-oriented trenches with a height of 1.5 μm, a base width of 2.0 μm, and a mask width of 0.8 μm. The work highlights the potential for ion damage-free, standalone heated-H3PO4 etching as a viable alternative to the dry etching of (001) β-Ga2O3 for high-performance device applications. [ABSTRACT FROM AUTHOR]

β-Ga2O3 is a promising ultra-wide bandgap semiconductor with melt-grown substrates that are scalable, particularly in the (001) orientation. In this study, we report on the heated-phosphoric acid etching of (001) β-Ga2O3. A wagon wheel pattern with spokes aligned to a range of specific crystallographic directions was fabricated on (001) β-Ga2O3. At 160 °C, the (001) etch rate was 1.47 μm/h, which is comparable to etch rates obtained via dry etch techniques. The etched (001) surface had a morphology that was smoother than an inductively coupled plasma etched surface. All spokes possessed relatively smooth sidewalls. Spokes oriented along the [100] direction exhibited minimal SiO2 mask undercut rates at lower etch temperatures and symmetric trapezoidal profiles with near vertical (010)-like sidewalls, which are ideal for device structures with a trench geometry. Spokes oriented along the [010] direction exhibited significant SiO2 mask undercut rates and asymmetric trapezoidal profiles with different sidewall angles. These spokes also possessed reduced sidewall angles, which is favorable for field management at device corners and edges. The etch process was used to realize a dense array of [100]-oriented trenches with a height of 1.5 μm, a base width of 2.0 μm, and a mask width of 0.8 μm. The work highlights the potential for ion damage-free, standalone heated-H3PO4 etching as a viable alternative to the dry etching of (001) β-Ga2O3 for high-performance device applications. [ABSTRACT FROM AUTHOR]

The development of high performance wide-bandgap AlGaN channel transistors with high current densities and reduced Ohmic losses necessitates extremely highly doped, high Al content AlGaN epilayers for regrown source/drain contact regions. In this work, we demonstrate the achievement of semi-metallic conductivity in silicon (Si) doped N-polar Al0.6Ga0.4N grown on C-face 4H-SiC substrates by molecular beam epitaxy. Under optimized conditions, the AlGaN epilayer shows smooth surface morphology and a narrow photoluminescence spectral linewidth, without the presence of any secondary peaks. A favorable growth window is identified wherein the free electron concentration reaches as high as ∼1.8 × 1020 cm−3 as obtained from Hall measurements, with a high mobility of 34 cm2/V·s, leading to a room temperature resistivity of only 1 mΩ·cm. Temperature-dependent Hall measurements show that the electron concentration, mobility, and sheet resistance do not depend on temperature, clearly indicating dopant Mott transition to a semi-metallic state, wherein the activation energy (Ea) falls to 0 meV at this high value of Si doping for the AlGaN films. This achievement of semi-metallic conductivity in Si doped N-polar high Al content AlGaN is instrumental for advancing ultrawide bandgap electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

Influence of the barrier layer on the electrical properties of V/Al-based Ohmic contact is investigated by comparing the surface morphology and alloying results of V/Al/Ni/Au (with barrier Ni) and V/Al/Cr/Au (with barrier Cr) contacts on n-AlGaN (Al molar fraction>60%) after 850 °C annealing. Due to the clustering of Ni during annealing, a thin Al layer (AlOx) and a number of Al-Au alloy clusters are formed at the contact interface of n-AlGaN, which increases the contact resistance of the V/Al/Ni/Au Ohmic contacts. In contrast, the annealed V/Al/Cr/Au electrodes cover the surface uniformly, which not only contributes to the increased contact area, but also suppresses the formation of high resistance products. Thanks to the Cr barrier layer, the specific contact resistivity of V/Al-based Ohmic contact is reduced by 26%. [ABSTRACT FROM AUTHOR]

This study reports on the formation of self-organized silicon (Si) nanostructures by 75 eV helium (He) plasma irradiation at normal incidence without the presence of impurities. In contrast to the featureless surface after normal incidence argon (Ar) ion beam irradiation without the co-deposition of impurities, the Si surface exhibits the development of faceted nanostructures under 75 eV He plasma irradiation. The faceted structures are interspersed with valleys that extend in two orthogonal directions, imparting a mountain-like morphology to the surface. Our investigation verifies that the He bubbles align themselves along the direction perpendicular to the surface underneath these valleys. Furthermore, the presence of He bubbles induces distortion in the surface layer and leads to the formation of an amorphous Si layer. The underlying mechanism driving this surface evolution could be attributed to the instability induced by the presence of He bubbles. [ABSTRACT FROM AUTHOR]

We propose a strain relaxed template (SRT), which consists of an InGaN decomposition layer (DL) and GaN protecting layers grown at three different temperatures as decomposition stop layers (DSLs), to enhance the indium incorporation in quantum wells. The high-temperature growth of the DSL decomposed the InGaN DL and created voids inside to release the strain of the as-grown templates. Although the surface morphology slightly degraded with the DL-DSL SRT, the emission wavelength over the 4-in. wafer was uniform with a standard deviation of 3.4 nm. In addition, the chip containing DL-DSL SRT exhibited an average redshift of 15 nm in peak wavelength compared to the chip without DL-DSL SRT, and the full widths at half-maximum of all samples were below 55 nm. Finally, we achieved an InGaN red LED chip using the DL-DSL SRT structure, exhibiting a red emission of 634 nm at 10 A/cm2 with an external quantum efficiency of 1.3%. The high-efficiency and uniform emission wavelength across the epi-wafer demonstrate the great potential of inserting a DL-DSL SRT to mass-produce high-performance, long-wavelength InGaN LEDs. [ABSTRACT FROM AUTHOR]

Ferroelectric materials have attracted the interest of current innovation in terms of high nonvolatile storage, light weight, smart, portable, and better functioned memories. In this work, we present all MXene (Mo2TiC2Tx/FE-Ti3C2Tx/Mo2TiC2Tx) flexible memory device with substrate-free growth in ambient conditions. The surface morphology, elemental and structural analysis confirmed synthesis of the MXene sheets. X-ray diffraction, Raman, and FTIR spectra showed the presence of TiO2 inside FE-Ti3C2Tx MXene that contributed toward the ferroelectric behavior of MXene as is evident by its hysteretic polarization curves. The extraordinary device performance with 103 on/off ratio, significant reproducibility, visible stable behavior up to 200 cycles, and a data retention of 104 s demonstrate MXene's promise to be employed in ferroelectric random access memory devices. [ABSTRACT FROM AUTHOR]

In this work, the effect of in situ SiNx grown with different carrier gas on the structural and electrical properties of the SiNx/AlGaN/GaN MIS-HEMTs is studied. It was found that the growth rate of SiNx grown with N2 as carrier gas (N2-SiNx) is more sensitive to different growth conditions, while the growth rate of SiNx grown with H2 as carrier gas (H2-SiNx) is very stable due to the inhibiting effects of H2 carrier gas on the SiH4–NH3 forward reactions. More importantly, a continuous and smooth SiNx growth at the initial stage can be realized with H2 carrier gas due to its faster surface migration, leading to a decent surface morphology and sharp interface of H2-SiNx. As a result, the SiNx passivated device with H2 as carrier gas shows improved performance compared to that with N2 as carrier gas, featuring ultra-low interface-state density of 2.8 × 1010 cm−2 eV−1, improved on- and off-state current, reduced threshold voltage shift, and mitigated current collapse, especially after long-term electrical stress. These results not only elaborate on the growth mechanisms of in situ SiNx with different carrier gases but also highlight the advances of H2 as carrier gas for in situ SiNx growth, providing an effective strategy to tailor the passivation schemes for GaN-based devices. [ABSTRACT FROM AUTHOR]

We report a thickness-dependent flexoresistance effect caused by strain gradient in SrTiO3 (STO) epitaxial thin films. The strain gradient in STO was induced by using an atomic force microscope diamond-coated tip. The depolarization field caused by the strain gradient changes the electrical state of the film, resulting in a decrease in the resistance of the films by 4–6 orders of magnitude. By testing the current–voltage (I–V) curves of STO films with different thicknesses, it is found that the flexoresistance behavior of STO films is obvious thickness dependent: the thicker the film, the higher the force threshold to trigger the film to enter a highly conductive state. The comparison of the surface morphology before and after the I–V test with loading shows that only when the thickness of the STO film is below 8 nm, the film can enter the conductive state from the insulating state without damaging the surface. During the electrical state switching process, the strain gradient caused by tip loading play an important role in the modification of the film barrier. [ABSTRACT FROM AUTHOR]

A series of La0.7K0.3−xSrxMnO3 (0.00 ≤ x ≤ 0.3) films were fabricated on LaAlO3 substrates using a sol-gel spin-coating technique. The relationship between electrical transport properties and a microstructure was explained via the Jahn–Teller effect, a phenomenological percolation model, and a double-exchange mechanism. The results revealed the strong impact of K and Sr co-occupying A-sites on the formation of Mn3+/Mn4+ pairs, crystal structure, conductivity, surface morphology, and in-plane and out-of-plane strains. The as-fabricated La0.7K0.25Sr0.05MnO3 films showed a peak temperature coefficient of resistivity of 16.63% K−1 at 298.3 K. [ABSTRACT FROM AUTHOR]

Due to the excellent properties, C4F7N/CO2 mixture is the most concerned eco-friendly SF6 alternative gas. For a better understanding of the surface discharge characteristics, the surface discharge pattern of C4F7N/CO2 mixture under negative impulse voltages is investigated in this work. The morphology of the surface discharge is obtained by an optical method and a dust figure method. The structure of the surface discharge is established, which from the outside to the inside consists of electron cloud, streamer, streamer stem, and leader. The propagation pattern of the surface discharge in C4F7N/CO2 mixture under negative impulse follows the stepwise expansion pattern. The structure of the surface discharge in C4F7N/CO2 mixtures with different ratios and SF6 has no obvious difference, and the propagation pattern is also the same. The results of this work can help to better understand the surface discharge phenomenon and the insulation characteristics of C4F7N/CO2 mixture. [ABSTRACT FROM AUTHOR]

The magnetic ground state of LaMnO3 (LMO) thin film is still a controversial issue, even though various mechanisms, such as cation/anion non-stoichiometry, epitaxial strain, interfacial charge reconstruction, and orbital ordering, have been proposed. Here, exchange bias (EB) was introduced into a high-quality epitaxial LMO thin film via relatively low oxygen growth pressure. The EB in LMO was modulated by +2 V gating via ionic liquid method with increased EB field (HEB), coercivity (HC), blocking temperature (TB), and reduced ferromagnetic (FM) magnetization. However, the −2 V gating has a much weaker tunability. By investigating the change of structure, surface morphology, and Mn oxidation state in LMO thin films, the modulation of magnetic properties is attributed to the creation/annihilation of oxygen vacancy in an LMO thin film. The suppressed FM phase in LMO can be ascribed to reduced Mn valence, structure disorders, and structure transition. However, the enriched antiferromagnetic phase results from the transition of the pseudocubic structure to the distorted orthorhombic structure. This work not only highlights the importance of functional defects in perovskite oxides but also sheds light on the potential of electric-field modulation of magnetism in spintronic devices. [ABSTRACT FROM AUTHOR]

Triboelectric nanogenerators (TENGs) capable of efficiently harvesting various mechanical energies from the environment are an effective way to solve the energy supply problem of wearable electronic devices. A series of flexible and lightweight TENGs with different surface morphologies of "dots"–"lines"–"honeycombs" hierarchical micro-structures are developed using low-cost and easily accessible background paper as a substrate. Polydimethylsiloxane films with dots–lines–honeycombs hierarchical micro-structures are prepared by the spin coating to obtain larger contact areas and, thus, enhance the triboelectric effect. Under the same test conditions, the honeycombs TENG (H-TENG) with complex surface morphology has the largest output performance with an open-circuit voltage of 55 V and a short-circuit current of 2.7 μA, which is 2.1 and 2.5 times that of the flat TENG, respectively. By combining with nylon gloves, the H-TENG can harvest the mechanical energy during hand movement and realize sensing of hand movement patterns through the difference of electrical signals. In addition, this device can power the liquid crystal display screen through a rectifier circuit and capacitor. This simple and efficient TENG device shows great potential in the wearable field. [ABSTRACT FROM AUTHOR]

Low-voltage, solution-processed oxide thin-film transistors (TFTs) have shown great potential in next-generation low-power, printable electronics. However, it is now still quite challenging to obtain low-voltage oxide TFTs with both high mobility and stability, especially for solution-processed ones. In this work, La-doped InZnO (IZO:La) channel for high performance and stable TFTs is developed using a simple solution process. The effects of La composition in IZO:La on the film and TFT performance are systematically investigated. It is confirmed that the incorporation of appropriate La could control the carrier concentration, improve the surface morphology, and passivate the oxygen-related defects, leading to a reduced trap density both at dielectric/channel interface and within the channel layer. As a result, the optimized TFTs with 1% La dopants exhibit the best overall performance, including a low operating voltage of 1 V, a high mobility of 14.5 cm2/V s, a low subthreshold swing of 109 mV/dec, a turn-on voltage close to 0 V, and negligible changes of performance under both positive and negative bias stresses. This work might support the development of all-solution-processed oxide TFT backplanes for battery-powered active-matrix displays. [ABSTRACT FROM AUTHOR]

By combining the exclusive characteristics of a two-dimensional MoS2 layered material with a conventional key material, aluminum nitride (AlN), a flexible supercapacitor electrode can be realized. We present a bendable electrode that is straightforwardly grown on stainless-steel foil via a binder-free sputtering route. The inherent merits of good conductive pathways among MoS2 nanolayers and enriched pseudocapacitive and dielectric activity from AlN nanoflowers enable synergism of the intermixed porous structure. This unique surface morphology facilitates sulfur and nitrogen edges to make insertion/de-insertion of Li-ions more feasible to store electrochemical energy. The MoS2-AlN@SS hybrid working electrode achieves a gravimetric capacitance of 372.35 F/g at a 5 mV/s scan rate with a wide potential window of 2 V in a 1 M Li2SO4 electrolytic aqueous solution. The composite thin film of better adhesion with the current collector exhibits a remarkably high specific power of 28.05 W h/kg at a specific power of 0.26 kW/kg, simultaneously an advanced cycling lifespan of 93% over 5000 charge–discharge cycles. The capacity of the hybrid electrode is almost unperturbed under bending from 0° to 175°, while only ∼5% degradation in capacitance was noticed at a flexing angle of 175°. These distinctive features of this electrode material elucidate the practical applicability and recommend it as a promising candidate in wearable bendable supercapacitors. [ABSTRACT FROM AUTHOR]

We report on the absence of strain relaxation mechanism in Al0.6Ga0.4N epilayers grown on (0001) AlN substrates for thickness as large as 3.5 μm, three-orders of magnitude beyond the Matthews–Blakeslee critical thickness for the formation of misfit dislocations (MDs). A steady-state compressive stress of 3–4 GPa was observed throughout the AlGaN growth leading to a large lattice bow (a radius of curvature of 0.5 m−1) for the thickest sample. Despite the large lattice mismatch-induced strain energy, the epilayers exhibited a smooth and crack-free surface morphology. These results point to the presence of a large barrier for nucleation of MDs in Al-rich AlGaN epilayers. Compositionally graded AlGaN layers were investigated as potential strain relief layers by the intentional introduction of MDs. While the graded layers abetted MD formation, the inadequate length of these MDs correlated with insignificant strain relaxation. This study emphasizes the importance of developing strain management strategies for the implementation of the single-crystal AlN substrate platform for III-nitride deep-UV optoelectronics and power electronics. [ABSTRACT FROM AUTHOR]

Laser terahertz (THz) emission spectroscopy has broken the diffraction limit of THz frequencies and offers multifaceted spectroscopic and imaging capabilities for understanding the light–matter interaction in various quantum and energy materials. However, this advanced technique has not yet been applied in the recently extensively studied spintronic THz emission process, in which the material surface morphology may play an important role. Here, we conduct THz emission microscopy on 5.4-nm thick Pt/CoFeB/W heterostructures and obtain twice enhanced THz by tightly focusing the pumping laser, delicately choosing the radiation location on nanofilms and coating gold nanorods. Through THz emission mapping, the material surface morphology and its modification have a strong correlation with THz emission performance from spintronic emitters. Our proposed femtosecond fiber laser driven spintronic THz emission microscopy can provide exciting possibilities for studying surface morphology sensitive THz emission materials and microdomain ultrafast dynamics for low-dimensional small samples via further coupling optical microscopy. [ABSTRACT FROM AUTHOR]

AC-poling of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) single crystals with a thickness of 0.06–0.16 mm was studied in this paper. Compared with DC-poled samples, enhancements in piezoelectric and dielectric properties can be obtained when the thickness is above 0.1 mm. However, inconsistency in poling effects was found in the crystals with thickness below 0.1 mm. To elucidate why such scaling effect arises, surface roughness was measured by an atomic force microscopy to correlate surface morphology and poling effects. It was found that non-uniform surface roughness led to inconsistent and decreased properties. Furthermore, temperature-dependent dielectric permittivity spectra were measured to explore how crystal thickness affects the thermal stability of ferroelectric phases. It is noted that complex changes in crystallographic symmetries emanate by decreasing thickness. Such phenomena can be attributed to more influential effects of surface morphology when thickness is reduced. We hope this work suggests a clue for solving the scaling effects of AC-poling on relaxor-PbTiO3 single crystals. [ABSTRACT FROM AUTHOR]

Thin films of Bi1.05La0.05FeO3 (BLFO) have been deposited on a CaRuO3 buffering layer on SrTiO3 using pulsed laser deposition. Then, a Pt thin layer was in situ sputtered on the surface of BLFO. The crystallization characteristic and surface morphology of the BLFO layer were checked. Spin transporting behaviors of the Pt/BLFO structure were investigated at room temperature. Interestingly, a magnetoresistance effect was observed at the interface of the Pt/BLFO bilayer. The spin mixing conductance at the interface between BLFO and Pt was estimated after a corrections of Pt thickness. This implies that the manipulation of spin–orbit coupling is possible at the interface of a single-phase multiferroic (e.g., BLFO) and a heavy metal (e.g., Pt). [ABSTRACT FROM AUTHOR]

C4F7N/CO2 is a promising mixture to replace SF6 in an electrical insulation field for its environment friendly property. Understanding surface discharge behaviors of the C4F7N/CO2 mixture is essential for designing an effective insulation system. In this work, surface discharge propagation characteristics in the C4F7N/CO2 mixture under positive impulse voltages are investigated. The surface discharges are detected electrically and observed by optical imaging. The residual surface charge distribution is acquired by an electrostatic probe method and a dust figure method after surface discharge. Comparative studies in CO2 and SF6 are further conducted. Results exhibit some significant differences between CO2 and C4F7N/CO2 mixtures in the propagation pattern and morphology of surface discharges. The surface discharge in the C4F7N/CO2 mixture develops in a stepwise expansion pattern, which is a result of alternative formation of streamers and leaders. A leader channel and a streamer development region are clearly distinguished in the optical image. The dust figure shows that no charge is deposited on the leader channel area after discharge, proving that the leader channel is electrically neutral and of high conductivity. The residual surface charge distribution in the C4F7N/CO2 mixture is obtained and resembles pine branches. A little difference is found in the surface discharge propagation characteristics between the C4F7N/CO2 mixture and SF6. [ABSTRACT FROM AUTHOR]

Based on theoretical predictions, it has been demonstrated that it is possible to purposefully change the elemental composition and position of the adsorbed particles on the GaN surface, thereby controlling the surface energy and morphology of GaN. Comparison of experimental data obtained by reflected high-energy electron diffraction and the calculated concentration of ammonia fragments on the GaN surface, and surface energy showed that the movement of adsorbed ammonia fragments into strongly bound states is an effective mechanism to control the GaN morphology. The minimum value of equivalent NH3 beam pressure at different temperatures to prevent the conversion of the two-dimensional (2D) GaN layer to three-dimensional (3D) islands has been established. It was shown that the boundary between the 2D and 3D states on the surface is defined by the elemental composition of adsorbed particles on the surface and the temperature dependence of the surface energy of the facets of islands. [ABSTRACT FROM AUTHOR]

Using the semibulk approach, p-InxGa1−xN semibulk (p-SB) templates were grown with an indium content ranging from 2.4% to 15.2% via metalorganic chemical vapor deposition. When compared to optimized bulk p-GaN, the hole concentration in p-SB with an In content of ∼15.2% increased by two orders of magnitude from 5.22 × 1017 to 5.28 × 1019 cm−3. The resistivity and mobility of the templates decreased gradually from 3.13 Ω · cm and 3.82 cm2/V s for p-GaN to 0.24 Ω · cm and 0.48 cm2/V s for p-SB with an In content of 15.2%. Temperature dependent Hall measurements were conducted to estimate the activation energy of the p-SB template. The p-SB with the In content of ∼15.2% is estimated to have an activation energy of 29 meV. These heavily doped p-SB templates have comparable material qualities to that of GaN. The atomic force microscopy height retraces of p-SB films show device quality surface morphology, with root mean square roughness ranging from 2.53 to 4.84 nm. The current results can impact the performances of several nitride-based devices, such as laser diodes, LEDs, solar cells, and photodetectors. [ABSTRACT FROM AUTHOR]

Here, we present a binder less sputtering approach for controllable growth of copper nitride (Cu3N) nanoflakes incorporated into 2D layered molybdenum disulfide (MoS2) nanoworms directly grown on flexible stainless steel (SS) substrate. The formation of the intermixed nanostructure is revealed by surface morphology. Moreover, the porous structure and good conductivity, and the presence of sulfur and N2 edges facilitate the synergistic effect favor more pathways for insertion and desertion of electrolyte ions (Na+). The optimized composite electrode achieves an outstanding specific capacitance (215.47 F/g at 0.5 A/g) along with remarkable elongated cycle life (∼90% retention over 2000 cycles at 9.5 A/g). Additionally, the electrode (of dimensions 3 × 1 cm2) shows high energy density (∼30 Wh/kg at a power density of 138 W/kg), extended potential window (1 V), fair mechanical stability, and pliability (retains ∼91% specific capacitance at 175° bending angle). The contemporary method provides a cathode material for practically applicable supercapacitors with superior flexibility and desirable electrochemical properties. [ABSTRACT FROM AUTHOR]

Shape memory polymers (SMPs) with tunable wettability and reversible adhesion through adjusting the surface morphology have great prospects in functional devices. However, current functional devices based on SMPs are mainly concentrated on the manipulation of diverse liquid droplets in the air; it is still a challenge to achieve the reversible tuning of underwater oil wettability and adhesion on the microstructured SMP surface. Herein, we fabricate a kind of SMP micropillar array through one-step femtosecond laser irradiation, which could achieve the reversible tuning of underwater oil wettability and adhesion between the tilted and upright states. It is worth noting that the deformed micropillar arrays show underwater oleophobicity and have a larger adhesion force to the oil droplet than that of upright micropillar arrays, which is mainly related to the variation of the contact area between the oil droplet and the micropillar array. Finally, we demonstrate the lossless oil droplet transfer underwater by using the micropillar array surface with different states. We believe the present work will bring some insights for researchers in the field of dynamically responsive tunable underwater oil droplet manipulation. [ABSTRACT FROM AUTHOR]

We report on the growth of β-Ga2O3 thin films using trimethylgallium (TMGa) as a source for gallium and pure O2 for oxidation. The growth rate of the films was found to linearly increase with the increase in the molar flow rate of TMGa and reach as high as ∼6 μm/h at a flow rate of 580 μmol/min. High purity, lightly Si-doped homoepitaxial β-Ga2O3 films with a good surface morphology, a record low temperature electron mobility exceeding 23 000 cm2/V s at 32 K, and an acceptor concentration of 2 × 1013 cm−3 were realized, showing an excellent purity film. Films with room temperature (RT) electron mobilities ranging from 71 cm2/V s to 138 cm2/V s with the corresponding free carrier densities between ∼1.1 × 1019 cm−3 and ∼1.5 × 1016 were demonstrated. For layers with the doping concentration in the range of high-1017 and low-1018 cm−3, the RT electron mobility values were consistently more than 100 cm2/V s, suggesting that TMGa is suitable to grow channel layers for lateral devices, such as field effect transistors. The results demonstrate excellent purity of the films produced and confirm the suitability of the TMGa precursor for the growth of device quality β-Ga2O3 films at a fast growth rate, meeting the demands for commercializing Ga2O3-based high voltage power devices by metalorganic chemical vapor deposition. [ABSTRACT FROM AUTHOR]

In this Letter, in situ Mg doping in β-Ga2O3 was demonstrated via metalorganic chemical vapor deposition (MOCVD) epitaxy. The electrical insulating property of the Mg acceptors in β-Ga2O3 was found to be intrinsically activated in the as-grown Mg-doped β-Ga2O3 thin films. Growth conditions for MOCVD β-Ga2O3 were further explored and optimized at a lower growth temperature regime, leading to a better confinement of the Mg-doping profile. Detailed analysis of Mg diffusion characteristics revealed a diffusion barrier energy Ebarrier ∼ 0.9 eV for Mg in MOCVD β-Ga2O3, which is likely related to an interstitial-assisted process. Surface morphologies and electron transport were characterized on samples grown with different growth temperatures and Mg doping levels. The MOCVD growth method demonstrated its feasibility to grow semi-insulating Mg-doped β-Ga2O3 epilayers with controllable Mg incorporation while maintaining good material quality and smooth surface morphology. From capacitance-voltage charge profiling, it is verified that the Mg-doped buffer layer grown at the substrate-epilayer interface could effectively compensate the charge accumulation at the interface. The in situ acceptor doping of Mg in MOCVD β-Ga2O3 will provide versatility for designing β-Ga2O3 power devices. [ABSTRACT FROM AUTHOR]

Time-resolved photoluminescence measurements are reported for InAsSbBi alloys grown by molecular beam epitaxy with Bi mole fractions ranging from 0 to 0.8%, yielding minority carrier lifetimes on the order of hundreds of nanoseconds. The minority carrier lifetimes extracted from the time-resolved photoluminescence measurements are comparable to those of lattice-matched InAsSb grown at the same respective temperatures. Nomarski imaging shows that smooth, droplet-free surface morphologies are obtained in 1 μm thick InAsSbBi epilayers grown at temperatures between 360 and 380 °C. The alloy composition-dependent bandgap energies for the InAsSbBi samples are determined from temperature-dependent steady-state photoluminescence measurements and compared with the tetragonal distortion measured by x-ray diffraction to determine the Sb and Bi mole fractions of each sample. The minority carrier lifetime and the achievable extension of the InAsSb(Bi) cut-off wavelength are analyzed as functions of alloy composition and compared with the performance of InAsSb layers with similar growth parameters. [ABSTRACT FROM AUTHOR]

In this work, sulfur addition has been employed on the boron-doped diamond growth process, and a significant regulation of the boron doping and the growth behavior has been realized by microwave plasma chemical vapor deposition. It is interesting to find that the sulfur incorporation will lead to an accordant evolution on the boron doping efficiency, hole mobility and concentration, crystal quality, surface morphology, and growth rate. In the presence of sulfur with appropriate dosage, for a boron-to-carbon ratio of only 2.5 ppm in gas phase during growth, a very high doping concentration of 1.2 × 1019 at/cm3 has been achieved, indicative of a very efficient boron doping. Besides, the hole mobility of the sample is 853 cm2/V s at 300 K, which is better than the state of the art for p-type doping in diamond. The regulation mechanism of the sulfur addition will be discussed from the point of view of sulfur-induced plasma change and possible B–S complex formation. This study may provide an effective way for high-quality p-type conductive diamond layer growth and further for the potential diamond-based opto-electronic device applications. [ABSTRACT FROM AUTHOR]

A high-sensitivity 4H–SiC temperature sensor and an alpha detector have been fabricated using additively printed metal contacts. The surface morphology and electrical conductivity of the printed electrodes were established prior to Schottky diode development. 4H–SiC Schottky diodes with direct-write printed silver contacts on the 5 μm-thick epilayer on 4H–SiC were characterized electrically in terms of the forward and reverse current–voltage and high-frequency capacitance–voltage characteristics. The turn-on voltage of the Schottky diodes, as established from the forward current–voltage characteristics measured up to a temperature of 400 °C, showed a linear temperature dependence. Schottky diodes with direct-write printed Ag electrodes were able to measure alpha particles emitted from Americium-241. The high temperature and radiation response of the Schottky diodes show their suitability for multi-modal sensor fusion on the 4H–SiC platform for harsh environment applications. [ABSTRACT FROM AUTHOR]

An air-coupled piezoelectric ultrasonic transducer (PUT) was fabricated to assess the effects of a nano-scale embossed Al diaphragm on the acoustic emission characteristics. A 310-μm-thick multilayer piezoelectric film and a 300-μm-thick Al plate were used for the PUT. It was found that the flexural modulus of the Al plate was decreased slightly from 3.73 GPa to 3.41 GPa by the effect of the embossed surface morphology, having an array of Al bumps with dimensions of 800 nm × 800 nm and a height of 320 nm. The decreased flexural modulus had a strong effect on the decrement of the acoustic resonant frequency and on the enhancement of the sound power radiation for the PUT. In comparison to a sound pressure level (SPL) of 117.8 dB at 30 cm away from a flat PUT at 41.4 kHz, the embossed PUT could result in a significantly enhanced SPL of 121.7 dB at the secondary resonant frequency of 40.5 kHz for the implementation of a smaller transducer. This result can motivate an efficient design parameter for better performance of a PUT through modifications of the nm-scale embossed surface. [ABSTRACT FROM AUTHOR]

Antimony selenosulfide, Sb2(SxSe1−x)3, has been considered as a promising light harvesting material for low-cost, non-toxic, and stable solar cell applications. However, current preparation methods of Sb2(SxSe1−x)3 suffer from low-quality films, which hampers the performance improvement in Sb2(SxSe1−x)3-based solar cells. Herein, we develop a pulsed laser deposition technique to fabricate antimony selenosulfide films with flat and compact surface morphology and high crystallinity. The composition of the as-obtained films can be conveniently tuned via varying molar ratios of Sb2S3 and Se in targets. At optimized conditions, we fabricate planar heterojunction solar cells and then obtain a significantly improved power conversion efficiency of 7.05%. Our research offers a facile and robust preparation method for Sb2(SxSe1−x)3 films with enhanced photovoltaic properties. [ABSTRACT FROM AUTHOR]

Hexagonal LuFeO3 exhibiting ferroelectricity and weak ferromagnetism is grown on metal-polar GaN by molecular-beam epitaxy. The oxide films exhibit smooth surface morphologies and are found to be single crystalline with an epitaxial relationship related by a 30° in-plane rotation relative to the GaN crystal structure. The LuFeO3 layers grown on GaN exhibit room-temperature ferroelectricity and low-temperature magnetic ordering. This epitaxial integration creates a heterostructure platform to explore and exploit the coupling of the ferroelectricity and magnetism of oxides with the strong spontaneous and piezoelectric polarization and the unique electronic and photonic properties of nitride semiconductors. [ABSTRACT FROM AUTHOR]

AlGaN films were grown on face-to-face annealed sputter-deposited AlN/sapphire (FFA Sp-AlN) templates via metalorganic vapor phase epitaxy (MOVPE), and the growth behavior of the AlGaN films was investigated. The sapphire substrates with small off-cut exhibited poor surface flatness of AlGaN grown on the FFA Sp-AlN templates owing to the formation of large hillock structures. To understand the origin of these hillock structures, the crystallinity and surface morphology of conventional fully MOVPE-grown AlN/sapphire (MOVPE-AlN) templates and the FFA Sp-AlN template were comprehensively studied. The screw- and mixed-type threading dislocation density of the FFA Sp-AlN template was estimated to be approximately 1.8 × 106 cm−2, which was two orders of magnitude lower than that of the MOVPE-AlN template. Consequently, the uniquely observed growth of the hillock structures in the FFA Sp-AlN templates was attributed to their low density of screw- and mixed-type threading dislocations. The large surface off-cut sapphire substrates suppressed the hillock structures on the FFA Sp-AlN templates. The improvement in surface flatness resulted in better optical properties of multiple quantum wells grown on the AlGaN layer. These results demonstrate a promising method for achieving highly efficient and cost effective AlGaN based deep ultraviolet light-emitting diodes. [ABSTRACT FROM AUTHOR]

Boron rich BAlN alloys have been grown on 2-inch sapphire substrates by Metal-Organic Vapor Phase Epitaxy. The surface morphology of BAlN alloys exhibits a transition stage from a completely two-dimensional to a three-dimensional granular surface with an increased trimethylaluminum/group III (TMAl/III) ratio. Only a shift in the position of the 002 plane reflection peak to higher diffraction angles in the 2 θ − ω scan along with a decrease in intensity was observed, specifying formation of layered BAlN alloys up to a TMAl/III ratio of 14. AlN phase separation was observed while increasing the TMAl/III ratio to 25, supporting SEM observations. Secondary-ion mass spectrometry measurements confirmed the presence of up to 17% Al in layered BAlN alloy systems. A cross sectional transmission electron microscopy (TEM) study confirmed the layered nature of single phase BAlN alloys. It also revealed the presence of wurtzite Al rich BAlN phases in a matrix of layered hexagonal B rich BAlN. Band to band transition around 5.86 eV has been observed, which shifted slightly to lower energy with increasing Al incorporation. The bowing parameter (C) in boron rich BAlN alloy systems was evaluated to be around 0.65 ± 0.05 eV. Encouraging results were obtained on boron rich BAlN alloy formation, motivating further exploration of growth conditions and study of BAlN fundamental properties for applications in deep UV optoelectronics. [ABSTRACT FROM AUTHOR]

Currently, chemical vapor deposition (CVD) has attracted widespread attention for preparing high efficiency and large-scale perovskite solar cells (PSCs). The surface morphology and crystallinity of precursor lead iodide (PbI2) films play a critical role in the subsequent CVD process. Here, we use a self-formed PbI2-dimethyl sulfoxide adduct in precursor PbI2 for fabricating a high quality perovskite film. The results show that a larger grain size and excessive PbI2 in the perovskite film effectively suppress carrier recombination, which is helpful to improve the device stability and efficiency. It is noted that the device maintained an excellent stability with almost the same efficiency after 1000 h (in a dry N2 glovebox, 25 °C). Finally, the PSC by using the CVD method exhibits a high power conversion efficiency of 17.9% with a bandgap of 1.60 eV, an open voltage (VOC) of 1140 mV, a short-circuit current density (JSC) of 20.1 mA/cm2, and a fill factor of 78%. This method provides guidance for preparing large-area solar cells or applications in tandem solar cells. [ABSTRACT FROM AUTHOR]

A facile strategy was developed to simultaneously improve the performance and stability of perovskite solar cells (PSCs). It involves the dissolution of an ultranarrow bandgap material, (2,2′-((2Z,2′Z)-(((4,4,9,9-tetrakis (4-hexylphenyl)-4,9-dihydro-s-indaceno [1,2-b:5,6-b′] dithiophene-2,7-diyl) bis (4-((2-ethylhexyl) oxy) thiophene-5,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)) dimalononitrile (IEICO-4F)), in chlorobenzene (antisolvent), the use of the solution in the treatment of perovskite films during spin-coating, and the fabrication of solar cells using the treated perovskite films. IEICO-4F formed a charge transfer bridge at the perovskite/Spiro-OMeTAD interface and improved the charge extraction and transport. Furthermore, the addition of IEICO-4F facilitated the crystallization, improved the surface morphology, and enhanced the passivation of trap sites of perovskite films. Meanwhile, a reliable power conversion efficiency exceeding 20% for CH3NH3PbI3-based cells and 15.72% for CsPbBrI2-based all-inorganic PSCs was realized. These values surpass those of the control devices (i.e., 18.66% and 13.30%, respectively). [ABSTRACT FROM AUTHOR]

In this paper, high-index Bi2Se3(2045) films are achieved on a Bi-modified Si(211) substrate with corrugated surface morphologies composed of striplike 19.5° tilted (001) facets by molecular beam epitaxy. The chemical bonds between Bi2Se3(001) faces and Bi-absorbed nanoscale faceted (111) terraces on Si(211) surfaces provide solid lattice constraints which favor the coherent epitaxy of Bi2Se3 films on the Bi-modified Si(211) substrate as compared to the H-passivated Si(211) substrate. This makes Bi2Se3(2045) grown on Bi-modified Si(211) more suitable for device applications. Temperature-dependent I-V measurements of the Bi2Se3(2045)/Si(211) heterojunction are performed which yield a Schottky barrier height of ФB = 0.22 eV at the heterointerface. [ABSTRACT FROM AUTHOR]

Controlling the surface morphology at the subwavelength scale is one of the cornerstones of modern nanophotonics. Femtosecond laser-induced surface structuring is a viable technique for the large-scale formation of nano- and microscale structures. A typical example is the formation of one-dimensional laser-induced periodic surface structures (LIPSSs), which can lead to strong modifications of optical and wetting properties of the material surface. Creating two-dimensional (2D) patterned structures, however, is a more challenging and rewarding task. Here, we demonstrate a single step method for fabricating various subwavelength structures on the cobalt (Co) surface using different laser fluences (0.12–0.24 J/cm2) and time delay (0–30 ps) between double pulses. More importantly, we can control the geometry and organization of the formed structures demonstrating spherical, triangular, rhombic, and high spatial frequency LIPSSs using two temporally delayed orthogonally polarized femtosecond laser beams. We show that the laser fluence and delay time between the two beams are the controlling parameters for realizing the different surface morphologies. We provide a numerically supported, phenomenological model to explain the formed 2D structures. Our model employs elements from both the scattered surface-wave interference and the self-organization theories that are commonly used to explain uniform surface structures. [ABSTRACT FROM AUTHOR]

Surfaces with tunable topological features enable important applications, such as optical devices, precision metrology, adhesion, and wetting. In this study, we demonstrate a facile method to fabricate and control the surface morphologies by combining thin film wrinkling and thermal expansion. This approach utilizes self-assembled surface wrinkling induced by shape recovery of shape memory polymers (SMPs) and localized thermal expansion caused by Joule heating. Recovering the prestrain in the SMP substrate induces global wrinkling of the thin film on the substrate. Joule heating in the SMP by a heating wire embedded in the substrate induces thermal expansion of the substrate in a localized area, which leads to the disappearance of the wrinkling pattern. This effect is reversed when heating is stopped, leading to reversible and repeatable tuning of the surface morphology in a controllable localized surface region. With metal coating, the SMP surface can be switched from specular to diffuse reflectance in response to external Joule heating. Finally, we demonstrate a smart micromirror device with its diffuse reflectance tunable between 13.5% and 81.9% in the visible light region. This approach provides a method to modulate surface diffusivity by controlling its surface morphologies, with potential applications in optical display and optical microelectromechanical systems devices. [ABSTRACT FROM AUTHOR]

We demonstrated the fabrication of a rare-earth-free ferromagnetic L10-type Fe–Ni alloy (L10-FeNi) by pulsed laser deposition (PLD). We deposited Fe and Ni on Cu(001) by alternating monoatomic deposition via automatically stabilized laser ablation. We examined the structural properties, magnetic properties, and surface morphology of the alloy specimens as the growth temperature (Ts) was varied. We adequately confirmed the construction of the most prominent L10-FeNi phase at 300 °C, which is significantly higher than previously reported growth temperatures, indicating that PLD followed by thermal treatment promoted two-dimensional growth of the adsorbent. The formation process of L10-FeNi was investigated from the standpoint of surface thermodynamics, and the results suggest that the surface free energy of PLD and its highly instantaneous deposition process by PLD played key roles. Our findings are expected to lead to advanced methods for the fabrication of L10-FeNi. [ABSTRACT FROM AUTHOR]

Liquid marbles can be characterized using elastic solid models consisting of a liquid surrounded by a soft solid membrane. The elastic properties of liquid marbles determine the amount of compression under a given external force. This is an important property as the elasticity of liquid marbles determines their morphology under a given stress. We show that the stress-strain relationship of liquid marbles can be described by σ * Bo = 0.6 [ 1 / (1 − ε h r o ) 2 − 1 ] , where Bo is the Bond number, σ* is the normalised stress, and ε h r 0 is the strain measured with respect to the equivalent radius of the liquid marble. This stress-strain relationship could pave the way for the development of microfluidic devices with robust liquid marbles. [ABSTRACT FROM AUTHOR]

In this letter, we demonstrate a crack and strain free AlN epilayer with a thickness of 10.6 μm grown on a pyramidal patterned sapphire substrate by metalorganic chemical vapor deposition. The full width at half maximum of the X-ray rocking curve was 165/185 arcsec for (002)/(102) planes, respectively. The total threading dislocation density was less than 3 × 108 cm−2. The dislocation evolution and the coalescence process were probed by transmission electron microscopy and scanning electron microscopy. A dual coalescence of the AlN epilayer was observed, which can effectively relax strain during the heteroepitaxy process. Owing to the approximately entire strain relaxation demonstrated by reciprocal space mapping and Raman shift, the surface morphology was crack-free and atomically smooth with a root-mean-square roughness of 0.14 nm. Temperature dependent Raman spectra showed the Raman linewidth of 4.3 cm−1 at 300 K which was comparable to that of bulk AlN; it also demonstrated good crystalline quality of the AlN epilayer. [ABSTRACT FROM AUTHOR]

The mechanisms governing "compositional pulling" during the growth of AlxGa1−xN wells are investigated. Gallium-rich AlxGa1−xN wells grown on high dislocation density AlN/sapphire templates exhibit asymmetric and diffuse composition profiles, while those grown on low dislocation density native AlN substrates do not. Furthermore, strain in all AlxGa1−xN wells is found to be pseudomorphic, ruling it out as the dominating driving force. Rather, the high threading dislocation density of the AlN template is considered to play the defining role. We propose that a transient surface morphology is introduced during dislocation mediated spiral growth, which, in conjunction with process supersaturation, determines the Ga incorporation. These findings provide insights into compositional pulling in high Ga content AlxGa1−xN grown on AlN and provide a route to grow thicker wells with very abrupt interfaces on native AlN substrates. [ABSTRACT FROM AUTHOR]

A study on the effect of fine particulate matter on the surface morphologies of conductors subjected to high positive dc voltages has been undertaken. A global distribution pattern of particulate matter was observed in the shape of parallel chains consisting of particle agglomerations. Based on a coupling model of electric and force fields, the dynamic progress of particles near conductor surfaces was investigated and the effects of forces exerted on the particles were analyzed. The experimental observations agreed well with calculated results. It was found that the interactions between particles played a critical role in the formation of agglomerations and parallel chains. The surface roughness and the corona discharge intensity of the conductors increased significantly as a result of the variation of surface morphologies. [ABSTRACT FROM AUTHOR]

The molecular orientation and the electronic structure at molecular donor/acceptor interfaces play an important role in the performance of organic optoelectronic devices. Here, we show that graphene substrates can be used as templates for tuning the molecular orientation of pentacene (PEN), selectively driving the formation of either face-on or edge-on arrangements by controlling the temperature of the substrate during deposition. The electronic structure and morphology of the two resulting C60/PEN heterointerfaces were elucidated using ultraviolet photoelectron spectroscopy and atomic force microscopy, respectively. While the C60/PEN (edge-on) interface exhibited a vacuum level alignment, the C60/PEN (face-on) interface exhibited a vacuum level shift of 0.2 eV, which was attributed to the formation of an interface dipole that resulted from polarization at the C60/PEN boundary. [ABSTRACT FROM AUTHOR]

We present a study of the surface morphology of SrRuO3 thin films grown on TiO2 terminated (001) SrTiO3 substrates using an off-axis RF magnetron sputtering deposition technique. We investigated the step bunching formation and the evolution of the films by varying deposition parameters. The thin films were characterized using atomic force microscopy methods, allowing us to study the various growth regimes of SrRuO3 as a function of the growth parameters. We observe a strong influence of both the miscut angle and growth temperature on the evolution of the SrRuO3 surface morphology. In addition, a thickness dependence is present. Remarkably, the formation of a smooth, regular, and uniform “fish-skin” structure at the step-bunch transition is observed. The fish-skin morphology results from the merging of 2D flat islands predicted by previous models. The direct observation of surface evolution allows us to better understand the different growth regimes of SrRuO3 thin films. [ABSTRACT FROM AUTHOR]

We present a study of the surface morphology of SrRuO3 thin films grown on TiO2 terminated (001) SrTiO3 substrates using an off-axis RF magnetron sputtering deposition technique. We investigated the step bunching formation and the evolution of the films by varying deposition parameters. The thin films were characterized using atomic force microscopy methods, allowing us to study the various growth regimes of SrRuO3 as a function of the growth parameters. We observe a strong influence of both the miscut angle and growth temperature on the evolution of the SrRuO3 surface morphology. In addition, a thickness dependence is present. Remarkably, the formation of a smooth, regular, and uniform “fish-skin” structure at the step-bunch transition is observed. The fish-skin morphology results from the merging of 2D flat islands predicted by previous models. The direct observation of surface evolution allows us to better understand the different growth regimes of SrRuO3 thin films. [ABSTRACT FROM AUTHOR]

In this paper, N-polar GaN films with different V/III ratios were grown on vicinal C-face SiC substrates by metalorganic chemical vapor deposition. During the growth of N-polar GaN film, the V/III ratio was controlled by adjusting the molar flow rate of ammonia while keeping the trimethylgallium flow rate unchanged. The influence of the V/III ratio on the surface morphology of N-polar GaN film has been studied. We find that the surface root mean square roughness of N-polar GaN film over an area of 20 × 20 μm2 can be reduced from 8.13 to 2.78 nm by optimization of the V/III ratio. Then, using the same growth conditions, N-polar InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) were grown on the rough and the smooth N-polar GaN templates, respectively. Compared with the LED grown on the rough N-polar GaN template, dramatically improved interface sharpness and luminescence uniformity of the InGaN/GaN MQWs are achieved for the LED grown on the smooth N-polar GaN template. [ABSTRACT FROM AUTHOR]

The insertion of an InGaN underlayer (UL) is known to strongly improve the performance of InGaN/GaN quantum well (QW) based blue light emitting diodes (LEDs). However, the actual physical mechanism responsible for it is still unclear. We thus conduct a systematic study and investigate different hypotheses. To this aim, InGaN/GaN single (S) QWs are grown on sapphire and GaN free-standing substrates with or without InGaN UL. This allows us to conclude that (i) improvement of LED performance is due to a higher internal quantum efficiency of the InGaN/GaN SQW and (ii) reduction of structural defects is not at play. Furthermore, we show that neither the surface morphology nor the strain of the top GaN layer before the growth of the QW is affected by the InGaN UL. Finally, we find that the beneficial effect of the InGaN UL is still present after 100 nm of GaN. This result combined with band structure modelling rules out the hypothesis of higher QW oscillator strength induced by a reduction of the internal electric field due to band bending. In conclusion, we demonstrate that the increase in InGaN/GaN QW efficiency is the consequence of a reduction of non-radiative recombination centers in the QW itself, independent of the dislocation density. [ABSTRACT FROM AUTHOR]

In this paper, we study the adsorption/desorption kinetics of gallium on (1-100) m-GaN during molecular-beam epitaxy on bulk substrates. We demonstrate the stabilization of a laterally contracted biatomic layer of Ga containing up to ≈2.5 times the (0001) surface atom density, on top of the GaN(1-100) surface. By assessing the surface morphology of m-GaN layers grown under different Ga/N flux ratios, we show that the presence of the Ga bilayer allows the growth of atomically smooth m-GaN without accumulation of Ga droplets and in conditions (substrate temperature and Ga/N ratio) that are compatible with the growth of c-GaN. [ABSTRACT FROM AUTHOR]