Sub-nanometer Surface Roughness Research Articles

Ferroelectric Al1−xBxN–GaN heterostructures

This report demonstrates Al0.93B0.7N thin films grown epitaxially on n-type GaN (0002)/c-plane sapphire substrates by reactive magnetron sputtering at 300 °C. At 200 nm film thickness, the Al0.93B0.07N layers exhibit partially relaxed substrate-induced epitaxial strain, a 0.16° wide (0002) rocking curve, in-plane crystallographic registry, and sub-nanometer surface roughness. Electrically, the stack shows robust hysteresis over three frequency decades, a remanent polarization of ∼125 μC/cm2, a strongly frequency dependent coercive field, highly uniform dc leakage currents, and endurance >106 field cycles. This report validates possibilities for ferroelectric nitride integration into conventional III-nitride heterostructures with high crystalline fidelity, high electrical resistivity, and persistent hysteresis. Such materials are synthesizable at thermal budgets and temperatures compatible with back-end-of-the-line boundary conditions.

Femtosecond-laser-enabled simultaneous figuring and finishing of glass with a subnanometer optical surface.

We demonstrate simultaneous figuring and surface finishing of glass using a femtosecond laser. For the first time, to the best of our knowledge, we have achieved deterministic material removal with nanometer precision and maintained sub-nanometer surface roughness without inducing any mid-spatial-frequency errors to the initial surface. A dynamic pulse propagation model is established to predict the interaction process, including plasma generation and surface temperature. The interactive modeling and the experiments enable the selection of a set of laser parameters to achieve controllable optical figuring and finishing. This demonstration shows the potential for using femtosecond lasers for advanced freeform optic forming, finishing, and reduction of detrimental mid-spatial-frequency errors, and laser-ablation-based patterning used for fabrication of integrated photonics and lasers.

Efficient and slurryless ultrasonic vibration assisted electrochemical mechanical polishing for 4H–SiC wafers

This paper proposes a slurryless, highly efficient polishing method called ultrasonic vibration assisted electrochemical mechanical polishing (UAECMP) to realize 4H–SiC wafers with subnanometer surface roughness. UAECMP involves using ultrasonic vibration to simultaneously assist anodic oxidation of the SiC surface and mechanical removal of the generated oxide layer. The performance of UAECMP was evaluated by experiments and theoretical analyses. For a 4H–SiC (0001) surface, UAECMP achieved a material removal rate (MRR) of 14.54 μm/h, which was 4.5 times greater than that of ordinary electrochemical mechanical polishing (ECMP) and 290 times greater than that of mechanical polishing. Ultrasonic vibration increased the anodic oxidation rate by introducing local transient strain to the SiC surface and increasing the temperatures of the polishing area and electrolyte. The effect increased with the amplitude of the ultrasonic vibration. However, increasing the ultrasonic vibration amplitude also increased the surface roughness due to the large fluctuations of polishing marks caused by the grinding stone and SiC surface impact and the increasing residual oxide. Therefore, we propose a high-efficiency and -quality polishing process for SiC wafers that combines UAECMP and ECMP. The proposed polishing process may help simplify the existing manufacturing process for SiC wafers.

Fabrication of freeform optical components by fluidic shaping

Freeform optical components enable advanced manipulation of light that is not possible with traditional optical systems. However, their fabrication relies on machining processes that are complex, time-consuming, and require significant infrastructure. Here we present the ability to shape liquid volumes and solidify them into desired freeform components, enabling rapid prototyping of freeform components with high surface quality. The method is based on controlling the minimum energy state of the interface between a curable optical liquid and an immersion liquid, by dictating a geometrical boundary constraint. We provide an analytical solution for the resulting topography given a predefined boundary and demonstrate the fabrication of freeform components with sub-nanometer surface roughness within minutes. Such a fabrication capability, that allows for rapid prototyping of high-quality components, has the potential to answer an unmet need in the optical design industry—allowing researchers and engineers to rapidly test freeform design concepts. It can be further envisioned to be expanded to an industrial scale, allowing for mold-less fabrication of freeform optics.

Modeling and experimenting of disc hydrodynamic polishing process for ultra-smooth surfaces

The machining of ultra-smooth surfaces has always been a significant research topic in the fields of optics fabrication. In this paper, disc hydrodynamic polishing (DHDP) process with a novel polishing tool is proposed to produce ultra-smooth surface with sub-nanometer scale surface roughness (Ra < 1 nm) and undamaged sub-surface. In order to explore the principle of the ultra-smooth surface generation in the DHDP process, the material removal mechanism and evolution mechanism of micro-morphology of polished surface were firstly studied by preliminary experiments. Then, theoretical material removal and surface simulation modeling are established on the basis of computational fluid dynamics (CFD), fractal theory for rough surfaces, particle contact mechanics, and erosion mechanisms in DHDP. Finally, a series of polishing experiments was performed on fused quartz glass to verify the theoretical model, which could be applied to predict surface topography and guide the ultra-smooth surface polishing process. The results indicated that the peak-to-valley (PV) value and waviness of the workpiece were reduced in the nanometer level under the erosion action of abrasive particles. Moreover, the surface roughness Ra and PV values decreased with the increase of rotation speed and polishing time. The analysis of power spectral density (PSD) results illustrates that high rotation speed of tool and polishing time (less than 60 min) can reduce the high-spatial-frequency errors distinctly, and the longer polishing time (above 60 min) can also influence on the mid-spatial-frequency errors.

Achievement of sub-nanometer surface roughness of bearing steel via chemical mechanical polishing with the synergistic effect of heterocyclic compounds containing N and S

Achievement of sub-nanometer surface roughness of bearing steel via chemical mechanical polishing with the synergistic effect of heterocyclic compounds containing N and S

Study of surface roughness of lifted-off epitaxial lateral overgrown GaN layers for the n-DBR mirror of a III-nitride vertical-cavity surface emitting laser

In this work, we propose using the low defect density wing region arising from epitaxial lateral overgrowth (ELO) for the development of Group III-nitride flip-chip vertical-cavity surface emitting lasers (VCSELs). The ELO wing is intended to be incorporated within the VCSEL cavity, supporting the n-side distributed Bragg reflector (DBR) mirror, and must therefore be very smooth. We measure the surface morphology of the interface surface of the ELO material after separation from the growth substrate, finding that the interface roughness changes with the composition and thickness of the ELO mask. Sub-nanometer surface roughness suitable for the placement of the DBR mirror is achieved using mask layers terminating in Si3N4, or via 300 nm thick sputtered SiO2.

High-temperature etching of SiC in SF6/O2 inductively coupled plasma

In this work, we demonstrate an effective way of deep (30 µm depth), highly oriented (90° sidewall angle) structures formation with sub-nanometer surface roughness (Rms = 0.7 nm) in silicon carbide (SiC). These structures were obtained by dry etching in SF6/O2 inductively coupled plasma (ICP) at increased substrate holder temperatures. It was shown that change in the temperature of the substrate holder in the range from 100 to 300 °C leads to a sharp decrease in the root mean square roughness from 153 to 0.7 nm. Along with this, it has been established that the etching rate of SiC also depends on the temperature of the substrate holder and reaches its maximum (1.28 µm/min) at temperatures close to 150 °C. Further temperature increase to 300 °C does not lead to the etching rate rising. The comparison of the results of the thermally stimulated process and the etching with a water-cooled substrate holder (15 °C) is carried out. Plasma optical emission spectroscopy was carried out at different temperatures of the substrate holder.

Homogenous and ultra-shallow lithium niobate etching by focused ion beam

Focused ion beam (FIB) milling has been used for fast prototyping of lithium niobate (LiNbO3, LN) devices with feature size from sub-to hundreds of micrometers. However, a promising and challenging depth range of tens-of-nanometers or below is rarely attended. Moreover, the surface roughness, related closely with device performances, is particularly non-negligible for such an ultra-shallow etching. Here, the surface roughness evolution was studied on ultra-shallow FIB etched LN structures. It was found that the inhomogeneous etching of the metallic film, coated on LN surface to avoid charge accumulation, had a detrimental effect on the LN surface roughness control. By thinning the gold thickness to 7 nm, sub-nanometer surface roughness was reported for etching depth of several nanometers. This work paves the way towards a homogenous and ultra-shallow FIB milling of LN nano-structures.

Molecular beam epitaxial growth of scandium nitride on hexagonal SiC, GaN, and AlN

RF plasma assisted MBE growth of scandium nitride (ScN) thin films on Ga-polar GaN (0001)/SiC, Al-polar AlN (0001)/Al2O3, and Si-face 6H-SiC (0001) hexagonal substrates is found to lead to a face centered cubic (rock salt) crystal structure with (111) out-of-plane orientation instead of hexagonal orientation. Cubic (111) twinned patterns in ScN are observed by in situ electron diffraction during epitaxy, and the twin domains in ScN are detected by electron backscattered diffraction and further corroborated by X-ray diffraction. The epitaxial ScN films display very smooth, subnanometer surface roughness at a growth temperature of 750 °C. Temperature-dependent Hall-effect measurements indicate a constant high n-type carrier concentration of ∼1 × 1020/cm3 and an electron mobility of ∼20 cm2/V s.

Nano- to atomic-scale epitaxial aluminum films on Si substrate grown by molecular beam epitaxy

We demonstrate nano- to atomic-scale epitaxial aluminum film growth on Si(111) substrate by molecular beam epitaxy. Excellent quality of these aluminum films, including sub-nanometer surface roughness, narrow linewidth of X-ray diffraction peak, clear transmission electron diffraction, and high optical reflectivity in ultra-violet, have been obtained with a reproducible growth recipe. The atomic-scale metallic aluminum film is formed by the self-limiting oxidation on the 3-nm-thick sample in air and the metallic state is confirmed with X-ray photoemission spectroscopy. Our work paves the way to future integration of aluminum-based plasmonic and superconducting devices on Si platform.

Single crystal hybrid perovskite field-effect transistors

The fields of photovoltaics, photodetection and light emission have seen tremendous activity in recent years with the advent of hybrid organic-inorganic perovskites. Yet, there have been far fewer reports of perovskite-based field-effect transistors. The lateral and interfacial transport requirements of transistors make them particularly vulnerable to surface contamination and defects rife in polycrystalline films and bulk single crystals. Here, we demonstrate a spatially-confined inverse temperature crystallization strategy which synthesizes micrometre-thin single crystals of methylammonium lead halide perovskites MAPbX3 (X = Cl, Br, I) with sub-nanometer surface roughness and very low surface contamination. These benefit the integration of MAPbX3 crystals into ambipolar transistors and yield record, room-temperature field-effect mobility up to 4.7 and 1.5 cm2 V−1 s−1 in p and n channel devices respectively, with 104 to 105 on-off ratio and low turn-on voltages. This work paves the way for integrating hybrid perovskite crystals into printed, flexible and transparent electronics.

CdZnTe Substrate Surface Preparation Technology at ASELSAN, Inc. for Molecular Beam Epitaxy Growth of High Quality HgCdTe Epilayers

High quality CdZnTe substrates with 4% ZnTe mole fraction are used for epitaxial growth of HgCdTe infrared detector layers. Molecular Beam Epitaxy (MBE) growth of HgCdTe epilayers requires high quality surface layer with sub-nanometer surface roughness values as well. We report on the development of a CdZnTe substrate surface preparation process for MBE growth of high quality HgCdTe epilayers. Surface preparation processes were performed on both commercially available CdZnTe substrates and substrates obtained from in-house grown CdZnTe boules. We developed a multi-step substrate surface processing flow to achieve sub-nanometer surface roughness, low total thickness variation (TTV), and wax or slurry residue free CdZnTe substrate surfaces. Each process step was optimized with the aim of removing the subsurface damage caused by the previous process step; so that we reduce the amount of damaged layer needed to be etched prior to epitaxy. Our developed surface preparation process can be applicable to commercially available CdZnTe substrates with high surface roughness and high TTV. This process was also optimized for as-cut CdZnTe slices. We are capable of processing typically 25 mm × 25 mm CdZnTe substrates with achievable surface roughness values (Rrms) down to below 0.5 nm.

Combination of thermal scanning probe lithography and ion etching to fabricate 3D silicon nanopatterns with extremely smooth surface

An emerging and extremely enabling route towards rapid prototyping of complex topographies with nanoscale precision and with highly controlled surface position is thermal scanning probe lithography. This technique, coupled to the recent advances in dry etching, allows pattern transfer into the target material at high vertical amplification and could therefore provide access to novel devices. However, the dry etch processing used to transfer the resist pattern into the structural material results in pronounced surface roughening, ultimately diminishing the functional quality of the nanostructures. To access the area of applications such as nano-photonics, nanoelectronics, nano-mechanics and nano-fluidics to name a few, a better control of surface morphology down to the sub-nm scale is a key request. Here, we explore different ion-beam polishing techniques for the surface roughness reduction of thermal scanning probe lithography patterns in silicon. Direct ion beam etch was optimized to smoothen thermal scanning probe lithography features and further extended to a polymer mask technique. The critical step of introducing the polymer mask for the ion beam etching enables extremely smooth three-dimensional surface contours at a sub-nanometer surface roughness.

Reduction of the surface roughness of Ge-on-insulator layers up to sub-nanometer range by chemical mechanical polishing

We are investigating the thermoelectric characteristics of Ge and SiGe nanostructures for realizing high power generator efficiency. In this paper, we investigated the influence of the thinning process on the surface roughness of a direct wafer-bonded p-type Ge-on-insulator (GOI) layer in order to realize an ultra-thin GOI substrate with extremely low surface roughness for the fabrication of Ge and SiGe nanostructures. The wafer thinning process was performed by chemical mechanical polishing (CMP) and wet chemical etching (WCE). A very smooth GOI layer with sub-nanometer (0.3 nm) surface roughness, suitable for nanostructure fabrication, was achieved using CMP compared to WCE process.

Uncovering Two Competing Switching Mechanisms for Epitaxial and Ultrathin Strontium Titanate-Based Resistive Switching Bits.

Resistive switches based on anionic electronic conducting oxides are promising devices to replace transistor-based memories due to their excellent scalability and low power consumption. In this study, we create a model switching system by manufacturing resistive switches based on ultrathin 5 nm, epitaxial, and grain boundary-free strontium titanate thin films with subnanometer surface roughness. For our model devices, we unveil two competing nonvolatile resistive switching processes being of different polarities: one switching in clockwise and the other in counterclockwise direction. They can be activated selectively with respect to the effective switching voltage and time applied to the device. Combined analysis of both processes with electrical DC-methods and electrochemical impedance spectroscopy reveals that the first resistive switching process is filament-based and exhibits counterclockwise bipolar resistive switching. The R(OFF)/R(ON) resistance ratio of this process is extremely stable and can be tuned in the range 5-25 depending on the switching voltage and time. Excitingly, at high electric field strength a second bipolar resistive switching process was found. This process is clockwise and, therefore, reveals the opposite polarity switching direction when compared to the first one. Both processes do not obstruct each other, consequently, stable 1, 2, or even 3 crossover current-voltage (I-V) characteristics can be addressed for the memory bits. Equivalent circuit model analysis and fitting of impedance characteristics unequivocally show for the created grain boundary free switches that the oxide's defects and its carrier distribution close to the electrode interface contribute to the resistive switching mechanism. The addressability of two sets of resistive ON and OFF states in one device through electric field strength and switching time offers exciting new operation schemes for memory devices.

Corrective finishing of a micro-aspheric mold made of tungsten carbide to 50 nm accuracy.

The increasing demand on the optical performance of micro-aspheric glass lenses used for consumer, medical, as well as industrial applications places a high requirement on the surface quality of the molds used for replicating these lenses. However, it is difficult, almost impossible to generate a supersmooth surface with extremely high form accuracy by using the current cutting or grinding techniques. Therefore, this paper presents a corrective polishing process to finish micro-aspheric molds made of tungsten carbide by applying the vibration-assisted polishing method, aiming to obtain tens-of-nanometer form accuracy and subnanometer surface roughness. A polishing system which provides precise position, angle, and force control in 5 degrees of freedom (DOF)is employed for the experiments, and a tilting angle control method is introduced for the ease of the precisely controlling polishing force so as to keep the material removal rate stable. Then a dwell time algorithm is proposed by considering the scanning path of the polishing tool and the tilting angle of the workpiece. The experimental results show that after corrective polishing, the form accuracy of a micro-aspheric mold with high numerical aperture (NA) is successfully improved from 230nm peak-to-valley (PV) to under 50nm PV, while the surface roughness is reduced from 7.2nm root-mean-square (rms) to 0.5nm rms.

A-SixC1−x:H thin films with subnanometer surface roughness for biological applications

The characterization of a-SixC1−x:H thin films by plasma-enhanced chemical vapor deposition with high hydrogen dilution for biological applications is addressed. A root mean square roughness less than 1 nm was measured via atomic force microscopy for an area of 25 μm2. Structural analysis was done using Fourier transform infrared spectroscopy in the middle infrared region. It was found that under the deposition conditions, the formation of Si–C bonds is promoted. Electrical dark conductivity measurements were performed to evaluate the effect of high hydrogen dilution and to find the relation between carrier transport properties and the structural arrangement. Conductivities of the order of 10−7 to 10−9 S/cm at room temperature for methane–silane gas flow ratio from 0.35 to 0.85 were achieved, respectively. UV-visible spectra were used to obtain the optical band gap and the Tauc parameter. Optical band gap as wide as 3.55 eV was achieved in the regime of high carbon incorporation. Accordingly, deposition under low power density and high hydrogen dilution reduces the roughness, improves the structure of the network, and stabilizes the film properties as a greater percentage of carbon is incorporated. The biofunctionalization of a-SixC1−x:H surfaces with NH2-terminated self-assembled monolayers was obtained through silanization with 3-aminopropyltrimethoxysilane. This knowledge opens a window for the inclusion of these a-SixC1−x:H thin films in devices such as biosensors.

Molybdenum Oxides Deposited by Modulated Pulse Power Magnetron Sputtering: Stoichiometry as a Function of Process Parameters

Molybdenum oxide films were deposited using modulated pulse power magnetron sputtering (MPPMS) from a molybdenum target in a reactive environment where the flow rate of oxygen was varied from 0 sccm to 2.00 sccm. By varying the amount of reactive oxygen available during deposition, the composition of the films ranged from metallic Mo to fully stoichiometric MoO3, when the molybdenum target became poisoned, due to the formation of a dielectric surface oxide coating. Film compositions were verified using high energy resolution x-ray photoelectron spectroscopy. Target poisoning occurred at an oxygen flow rate of 1.25 sccm and reversed when the flow rate decreased to about 1.00 sccm. MoO3 films deposited via MPPMS had densities of 3.8 g cm−3, 81% of the density of crystalline α-MoO3 as determined by x-ray reflectivity (XRR). In addition, XRR and atomic force microscopy data showed sub-nanometer surface roughness values. From spectroscopic ellipsometry, the measured refractive index of the MoO3 films at 589 nm was 1.97 with extinction coefficient values <0.02 at wavelengths above the measured absorption edge of 506 nm (2.45 eV).

A New Route for Fabricating On‐Chip Chalcogenide Microcavity Resonator Arrays

High-yield production and on-chip integration of high-Q chalcogenide microresonators with various sizes and geometrical shapes (spherical, spheroidal, and ellipsoidal) and with sub-nanometer surface roughness is achieved. The high-throughput and low-cost production of chalcogenide spherical microresonators works via inducing a Plateau-Rayleigh instability within a multimaterial fiber in extended lengths.