UV-ozone Cleaning Research Articles

A systematic study of the regrown interface impurities in unintentionally doped Ga-polar c-plane GaN and methods to reduce the same

Vertical gallium nitride (GaN) devices are strong candidates for next generation power electronics. Such vertical devices almost always require epitaxial regrowth of GaN. However, impurities present at the regrowth interfaces result in device degradation such as higher leakage current and lower breakdown voltage limiting its performance. While significant interface impurity concentration has been previously detected in the doped samples as well as in the etched-then-regrown samples, the origin of such interface impurity is still not well-understood. In this work, we perform a detailed characterization of the regrowth interface impurities of unintentionally doped (UID) Ga-polar GaN on c-plane sapphire substrate under various growth interruptions. This process (without any doping and without etch-then-regrowth) allows us to probe the relatively pristine regrown interface and thus better understand the origin of regrowth interface impurity. In the metal organic chemical vapor deposition (MOCVD) grown samples, we uncover a significant presence of silicon (Si) impurity concentration at the regrowth interface of an UID GaN for exposure both in air and in glovebox, with ∼88% higher impurity concentration for air exposure. This confirms an inevitable incorporation of Si impurities in the regrowth process originating from air, residual impurities in MOCVD environment and a further adsorption of impurities from the MOCVD chamber. We then demonstrate an ultraviolet (UV) ozone-based cleaning technique to reduce the concentration of Si impurities at the regrown interface. Using an iterative UV-ozone cleaning and wet etching treatment, we realize up to ∼61% reduction in the peak Si impurity concentrations therefore paving the way towards high performance vertical GaN devices.

Surface cleaning and sample carrier for complementary high-resolution imaging techniques

Nowadays, high-resolution imaging techniques are extensively applied in a complementary way to gain insights into complex phenomena. For a truly complementary analytical approach, a common sample carrier is required that is suitable for the different preparation methods necessary for each analytical technique. This sample carrier should be capable of accommodating diverse analytes and maintaining their pristine composition and arrangement during deposition and preparation. In this work, a new type of sample carrier consisting of a silicon wafer with a hydrophilic polymer coating was developed. The robustness of the polymer coating toward solvents was strengthened by cross-linking and stoving. Furthermore, a new method of UV-ozone cleaning was developed that enhances the adhesion of the polymer coating to the wafer and ensures reproducible surface-properties of the resulting sample carrier. The hydrophilicity of the sample carrier was recovered applying the new method of UV-ozone cleaning, while avoiding UV-induced damages to the polymer. Noncontact 3D optical profilometry and contact angle measurements were used to monitor the hydrophilicity of the coating. The hydrophilicity of the polymer coating ensures its spongelike behavior so that upon the deposition of an analyte suspension, the solvent and solutes are separated from the analyte by absorption into the polymer. This feature is essential to limit the coffee-ring effect and preserve the native identity of an analyte upon deposition. The suitability of the sample carrier for various sample types was tested using nanoparticles from suspension, bacterial cells, and tissue sections. To assess the homogeneity of the analyte distribution and preservation of sample integrity, optical and scanning electron microscopy, helium ion microscopy, laser ablation inductively coupled plasma mass spectrometry, and time-of-flight secondary ion mass spectrometry were used. This demonstrates the broad applicability of the newly developed sample carrier and its value for complementary imaging.

Electrochemical and Thermal Etching of Indium Tin Oxide by Solid-State Hybrid Organic–Inorganic Perovskites

We show that tin-doped indium oxide (ITO) can be thermally etched by the Bronsted acid salts methylammonium iodide (MAI), methylammonium lead triiodide (MAPbI3), and formamidinium lead triiodide (FAPbI3) in solid-state films and devices at common processing temperatures. More importantly, a series of reactions within an ITO/hybrid perovskite/Au device can be electrochemically induced near room temperature by applied cathodic voltages as low as −1.2 V. Cyclic voltammetry in this range leads to eventual In3+ leaching into the perovskite layer in the form of InI3, unambiguously identified by a binding energy signature of 445.9–446.3 eV measured by X-ray photoelectron spectroscopy. Furthermore, the etching is exacerbated by defects generated by O2-plasma treatment of the ITO compared to UV-ozone cleaning, lowering the reaction potential and the electrochemical stability window of an ITO/MAPbI3/Au device. Low-temperature, electrochemical reactivity at this interface has implications on operational stability, f...

Simple and versatile UV-ozone oxide for silicon solar cell applications

Semiconductor surface clean is sometimes perceived as costly but long recognized as pivotal in determining the final semiconductor device performance and yield. In this contribution, we investigated the effectiveness of crystalline silicon surface cleaning by a simple UV-ozone process in comparison to the industry standard RCA clean for silicon photovoltaic applications. We present a unique method of processing the silicon surface effectively by UV-ozone cleaning. Despite being simple, UV-ozone cleaning results in a superior surface passivation quality that is comparable to high-quality RCA clean. When used as a stack dielectric—UV-ozone oxide overlaid by aluminum oxide—the thickness of UV-ozone oxide plays an important role in determining the passivation quality. Of all treatment times, 15 min of UV-ozone treatment results in an outstanding passivation quality, achieving the effective carrier lifetime of 3 ms and saturation current density of 5 fA/cm2. In addition, we present a simple and effective technique to extract values of electron/hole capture cross-section for the purpose of analyzing the interface passivation quality from already measured surface recombination parameters of saturation current density, interfacial trap density and total fixed charge, instead of measuring on the separately prepared metal-insulated-semiconductor (MIS) samples by the techniques: frequency-dependent parallel conductance or deep-level transient spectroscopy.

Self-assembled vertically aligned Au nanorod arrays for surface-enhanced Raman scattering (SERS) detection of Cannabinol

Self-assembled multi-layered vertically aligned gold nanorod (AuNR) arrays have been fabricated by a simple preparation process that requires a balance between the particle concentration and the ionic strength of the solvent. An experimentally determined critical AuNR concentration of 2.0nM and 50mM NaCl produces well-ordered vertically aligned hexagonally close-packed AuNR arrays. We demonstrate surface treatment via UV Ozone cleaning of such samples to allow introduction of analyte molecules (benzenethiol and cannabinol) for effective surface enhanced Raman scattering detection. This is the first demonstration of the SERS analysis of cannabinol. This approach demonstrates a cost-effective, high-yield and simple fabrication route to SERS sensors with application in the screening for the cannabinoids.

Interfacial nucleation behavior of inkjet-printed 6,13 bis(tri-isopropylsilylethynyl) pentacene on dielectric surfaces

The performance of organic thin film transistors (OTFTs) is heavily dependent on the interface property between the organic semiconductor and the dielectric substrate. Device fabrication with bottom-gate architecture by depositing the semiconductors with a solution method is highly recommended for cost-effectiveness. Surface modification of the dielectric layer is employed as an effective approach to control film growth. Here, we perform surface modification via a self-assembled monolayer of silanes, a spin-coated polymer layer or UV-ozone cleaning, to prepare surfaces with different surface polarities and morphologies. The semiconductor is inkjet-printed on the surface-treated substrates as single-line films with overlapping drop assignment. Surface morphologies of the dielectric before film deposition and film morphologies of the inkjet-printed semiconductor are characterized with polarized microscopy and AFM. Electrical properties of the films are studied through organic thin-film transistors with bottom-gate/bottom-contact structure. With reduced surface polarity and nanoscale aggregation of silane molecules on the substrates, semiconductor nucleates from the interior interface between the ink solution and the substrate, which contributes to film growth with higher crystal coverage and better film quality at the interface. Surface treatment with hydrophobic silanes is a promising approach to fabrication of high performance OTFTs with nonpolar conjugated molecules via solution methods.

UV ozone treatment for improving contact resistance on graphene

Optimized UV ozone cleaning of graphene layers on SiO2/Si substrates is shown to improve contact resistance of e-beam evaporated Ti/Au contacts by three orders of magnitude (3 × 10−6 Ω-cm2) compared to untreated surfaces (4 × 10−3 Ω-cm2). Subsequent annealing at 300 °C lowers the minimum value achieved to 7 × 10−7 Ω-cm2. Ozone exposure beyond an optimum time (6 min in these experiments) led to a sharp increase in sheet resistance of the graphene, producing degraded contact resistance. The UV ozone treatment is a simple and effective method for producing high quality contacts to graphene.

UV–Ozone Cleaning of Supported Poly(vinylpyrrolidone)-Stabilized Palladium Nanocubes: Effect of Stabilizer Removal on Morphology and Catalytic Behavior

Poly(vinylpyrrolidone) (PVP)-stabilized Pd nanocubes were synthesized, deposited on a carbon-based support, and subsequently treated with UV-ozone (UVO) in order to eliminate the traces of PVP still present on the surface. Cubes, being a thermodynamically unfavorable shape, are very prone to restructuring to minimize the interfacial free energy and thus allow the assessment of their morphological stability during UVO cleaning. The process of PVP removal was monitored by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and in situ attenuated total reflection infrared spectroscopy (ATR-IR). High-resolution scanning electron microscopy (SEM) imaging was used to evaluate the morphology of the nanocubes. The effect of PVP removal was also studied in the hydrogenation of acetylene, showing a 4-fold increase of activity. This method can be applied to nanoparticles of other common shapes, which expose different crystal planes, in order to study the structure sensitivity of chemical reactions.

Investigation of pre-structured GaAs surfaces for subsequent site-selective InAs quantum dot growth

In this study, we investigated pre-structured (100) GaAs sample surfaces with respect to subsequent site-selective quantum dot growth. Defects occurring in the GaAs buffer layer grown after pre-structuring are attributed to insufficient cleaning of the samples prior to regrowth. Successive cleaning steps were analyzed and optimized. A UV-ozone cleaning is performed at the end of sample preparation in order to get rid of remaining organic contamination.

Growth and annealing of InAs quantum dots on pre-structured GaAs substrates

In this study, we investigated the effect of in situ annealing on InAs quantum dots site-selectively grown on pre-structured GaAs substrates. A morphological transition is observed with original double dots merging into one single dot during annealing. This is accompanied by a reduction of quantum dots originally nucleating between defined sites. The photoluminescence intensity of annealed site-selective quantum dots is compared to annealed self-assembled dots with linewidths of single dot emission of about 170 and 81μeV, respectively. UV-ozone cleaning is used to optimize the sample cleaning prior to quantum dot growth.

Anode Interfacial Tuning via Electron‐Blocking/Hole‐Transport Layers and Indium Tin Oxide Surface Treatment in Bulk‐Heterojunction Organic Photovoltaic Cells

AbstractThe effects of anode/active layer interface modification in bulk‐heterojunction organic photovoltaic (OPV) cells is investigated using poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and/or a hole‐transporting/electron‐blocking blend of 4,4′‐bis[(p‐trichlorosilylpropylphenyl)‐phenylamino]biphenyl (TPDSi2) and poly[9,9‐dioctylfluorene‐co‐N‐[4‐(3‐methylpropyl)]‐diphenylamine] (TFB) as interfacial layers (IFLs). Current–voltage data in the dark and AM1.5G light show that the TPDSi2:TFB IFL yields MDMO‐PPV:PCBM OPVs with substantially increased open‐circuit voltage (Voc), power conversion efficiency, and thermal stability versus devices having no IFL or PEDOT:PSS. Using PEDOT:PSS and TPDSi2:TFB together in the same cell greatly reduces dark current and produces the highest Voc (0.91 V) by combining the electron‐blocking effects of both layers. ITO anode pre‐treatment was investigated by X‐ray photoelectron spectroscopy to understand why oxygen plasma, UV ozone, and solvent cleaning markedly affect cell response in combination with each IFL. O2 plasma and UV ozone treatment most effectively clean the ITO surface and are found most effective in preparing the surface for PEDOT:PSS deposition; UV ozone produces optimum solar cells with the TPDSi2:TFB IFL. Solvent cleaning leaves significant residual carbon contamination on the ITO and is best followed by O2 plasma or UV ozone treatment.

Surface Chemistry of InP(100) After Wet and Electrochemical Etching: Is Digital Etching Possible?

The objective of this investigation was to develop a procedure for cleaning InP, to study the electrochemistry of InP(100) surfaces, and to look at the possibility of electrochemical digital etching of InP. Wet etches included pretreatment with and HF solutions, which left trace amounts of C and O. UV ozone cleaning of InP gave similar results but left the surface with a phosphate layer. Ion bombardment and annealing of InP resulted in a clean, well-ordered surface with a reconstruction. However, voltammetry studies of this surface, in various pH solutions, suggested the presence of some dispersed clusters of metallic In. Voltammetry in acid electrolytes, 10 mM HCl and 10 mM , evidenced oxidation of the metallic In clusters at to form a “stable” surface with a low-energy electron diffraction pattern. Reduction in the hydrogen evolution region, at pH 9, resulted in the dissolution of some P, probably as a phosphine species. This was supported by the appearance of small amounts of metallic In on the surface. Those results suggested that digital etching of InP may be possible. However, a more extensive investigation of P reduction showed it was not surface-limited, making it unsuitable for electrochemical digital etching.

Preparation of Ge (100) Substrates for High-Quality Epitaxial Growth of Group IV Materials

ABSTRACTWe compare various wet chemical treatments, in preparing high-quality Ge (100) surfaces suitable for molecular beam epitaxy (MBE). Various surface treatments are explored such as UV-ozone treatment followed by exposure to chemical solutions such as de-ionized (DI) water, hydrofluoric acid (HF), or hydrochloric acid (HCl). Chemical treatments to remove the oxide are performed in a nitrogen environment to prevent further formation of surface oxide prior to surface analysis. Following chemical treatments, in situ reflection high-energy electron diffraction (RHEED) analysis is performed to observe the surface evolution as a function of temperature. In a separate chamber, we analyze each sample, before and after chemical treatment by x-ray photoelectron spectroscopy (XPS) to directly determine the oxide desorption following each chemical treatment. Our results of this comparative study, the effectiveness of each chemical treatment, and the stability of the passivated surface suggest that UV ozone cleaning, followed by 10% HCl is the best choice for removing most of the oxide. Furthermore, we present evidence of high quality epitaxial growth of SnxGe1−x on wafers prepared by our method.

Kelvin probe investigations of metal work functions and correlation to device performance of organic light-emitting devices

Using the vibrating capacitor Kelvin probe technique, we have determined the contact potential difference (CPD) between a reference electrode and various metals acting as charge carrier injecting contacts in organic light-emitting devices (OLEDs). These investigations show that the work function of anode materials for OLEDs such as Pt, Au, and indium tin oxide depends strongly on the surface treatment and can be increased by more than 1 eV via oxygen plasma or UV-ozone cleaning. The device performance of multilayer OLEDs consisting of these anodes, copper-phthalocyanine (CuPc), N, N′-di(naphthalene-1-yl)- N, N′-diphenyl-benzidine (NPB), tris-(8-hydroxyquinolinato)aluminum (Alq 3), and a low-work-function metal cathode is correlated with the results of the CPD measurements. However, our investigations indicate that, apart from the measured work function, other factors such as the surface roughness and the binding energy of oxygen to the metal surface can significantly influence the injection properties and the long-term stability of the devices.

Low Temperature Silicon Surface Cleaning by HF Etching/Ultraviolet Ozone Cleaning (HF/UVOC) Method (I) –Optimization of the HF Treatment–

Several variations of fluoric acid (HF) treatments of silicon substrates were examined for their adaptability as a pretreatment method for a silicon epitaxy process. Treatments with and without distilled, deionized (DI) water rinse, of different HF concentrations, and of different methods of HF supply were tested and their residual carbonic impurity contents were measured using RHEED. As a result, HF treatments by themselves were found to be insufficient in passivating the surface dangling bonds irrespective of the method of HF supply: dipping into the solution or exposure to the vapor. The optimum procedure of HF treatment thus proposed is a succession of (a) HF dipping, (b) DI-water rinsing, (c) nitrogen-gas blowing, and (d) UV-ozone cleaning.

Uv-Ozone Cleaning of GaAs (100) Surfaces for Device Applications

ABSTRACTGaAs (100) substrates were subjected to UV-ozone, wet chemical, and thermal treatments prior to deposition of Ni contact structures. The use of UV-ozone showed marked improvement in the electrical characteristics compared to the wet chemical and thermal treatments alone. The major effect of UV-ozone is the nondestructive removal of carbonous contaminants. Results of C-V and I-V barrier height measurements on Ni contact structures fabricated on the treated surfaces are presented.

UV ozone cleaning of silicon substrates in silicon molecular beam epitaxy

This letter reports UV ozone cleaning of Si substrates for obtaining defect-free molecular beam epitaxial films by low-temperature in vacuum preheating. By using UV ozone cleaning, the high temperatures above 1200 °C required for removing surface carbon in the conventional method can be significantly lowered to below 1000 °C, since the UV ozone cleaning functions to remove carbon.

UV–Ozone Cleaning of GaAs for MBE

A simple method of removing carbon contamination from GaAs substrates using exposure to ozone and ultraviolet light is described.