Steve Rozeveld

Structures and properties of disordered boron carbide coatings generated by magnetron sputtering

Disordered boron carbide coatings with their high hardness, high lubricity, and low surface friction have become the coatings of choice to enhance the wear performance of many existing products. These coatings have been successfully commercialized using a magnetron sputtering process. In this paper, the effects of one of the critical process parameters, bias voltage, on the chemistry, microstructure, and the properties of the coatings are discussed. In combination with microstructure examination, special emphasis was made on nanoscopic level chemical analyses in order to explain the effects of this process parameter. The substrate bias was found to have strong effects on the hardness and the stress of the coating, but it has little influence on the frictional characteristics of the coating. The results of the examination and the analyses of the coating using FTIR, XPS, TEM, PEELS, and SIMS revealed that the morphology of the coating changed from a columnar structure to a continuous solid structure as the substrate bias voltage increased from 0 to 200 V. Oxide species were found in between the columns, while the columns mainly consisted of boron carbide with a boron to carbon atomic ratio of about 4. The atomic ratio of boron to carbon appeared to be independent of the substrate bias.

Quantitative Contrast Evaluation of an Industry‐Style Rhodium Nanocatalyst with Single Atom Sensitivity

Aberration‐corrected electron microscopy opens new ways for material characterization. In catalyst research it will enable the observation of single atom arrangements, such as the location of promoter atoms on catalyst particles. However, quantitative procedures must be developed to account for dynamic contrast changes resulting from beam‐sample interactions and incoherent instrument aberrations. We demonstrate that at low acceleration voltage (80 kV), for which knock‐on damage is suppressed, the residual intensity fluctuations can be attributed to the presence of phonons resulting in 3D low frequency atom displacements. For rhodium [110] oriented particles it was found that the catalysts are platelets with an aspect ratio of about 0.2 and a surface roughness of ±1 atom. Observation of single surface atoms requires minimization of phonon‐induced motion.

Examination of lifetime-limiting failure mechanisms in CIGSS-based PV minimodules under environmental stress

In our study, Shell Solar Industries (SSI) minimodules were subjected to dry heat (85°C), damp heat (85°C/100% RH), and anaerobic/aerobic 85°C water baths. After 168 hrs exposure to moisture-containing environments, the SSI power generation decreased by over 50% of that of the original state. Analytical characterization performed before and after the exposure identified degradation of the Al:ZnO and Mo layers as likely device failure routes. To elucidate the observed degradation mechanism, individual Al:ZnO and Mo films were sputtered onto borosilicate glass and exposed to both 85°C/100% RH and a room temperature water bath. After 24 hrs the resistivity and optical transmission of the Al:ZnO films increased significantly following both exposure methods. XPS surface analysis of the films revealed changes in the O to Zn bonding ratio suggesting film hydration may have occurred. In addition, after 48 hours by both exposure methods the Mo films corroded, and the film resistivities increased. Our results show Al:ZnO layer degradation limits the lifetime of CIGSS based PV devices, whereas Mo degradation is considered a non-lifetime-limiting failure.

Electron Tomography of Nanostructured Materials – Towards a Quantitative 3D Analysis with Nanometer Resolution

Electron tomography has developed into a powerful technique to image the 3D structure of complex materials with nanometer resolution. Both, TEM and HAADF-STEM tomography exhibit tremendous possibilities to visualize nanostructured materials for a wide range of applications. Electron tomography is not only a qualitative tool to visualize nano¬structures, but recently electron tomographic results are also exploited to obtain quantitative measurements in 3D. We evaluated the reconstruction and segmentation process for a heterogeneous catalyst and, in particular, tried to assess the reliability and accuracy of the quantification process. Furthermore, a quantitative analysis of electron tomographic results was compared to macroscopic measurements.

Further investigation of the lifetime-limiting failure mechanisms of CIGSS-based minimodules under environmental stress

The lifetime-limiting failure mechanisms of CuInGaSSe (CIGSS) solar devices made by Shell Solar Industries (SSI) were investigated. In our study, SSI minimodules were exposed to dry-heat 85°C, damp-heat 85°C/85% RH and aerobic and anaerobic room temperature and 85°C water baths. After 200 hours exposure to moisture-containing environments, the average device performance decreased by more than 30% that of the initial state. The observed degradation was primarily due to losses in short circuit current density (Jsc) and fill factor (FF). The as-received device layers were relatively dense and free of voids. Interestingly, unreacted Cu-Ga particulates were found at the Mo-CIGSS interface. Corresponding with these particulates, CIS-rich defects were found within the bulk CIGSS material. Post-environmental weathering, Kirkendall-like voids were found in the Al:ZnO and CdS layers of these devices. Additionally, ToF-SIMS analysis revealed the Cu-Ga/CIS defects were enriched in Na and O. Our results indicate that in addition to moisture-induced failure of the window layers, the unreacted Cu-Ga particulates and the corresponding CIS-defects may facilitate moisture and/or oxygen-induced failure of these devices.

Optimization of electrical and optical properties in multilayer TCO thin film structures

Multilayer oxide thin film stacks of aluminum doped zinc oxide (AZO) and tin doped indium oxide (ITO) have been sequentially deposited on soda lime glass substrates by RF sputtering of AZO and ITO ceramic targets at a substrate temperature of 150 °C. The ratio of the AZO thickness to the ITO thickness is varied while keeping the total thickness of the stack constant. The electrical and optical properties of the multilayer stacks have been investigated as a function of this ratio and the number of interfaces. The experimental results are compared and their impact on device performance is demonstrated by simulations with validated AMPS-1D models. XRD and microscopy measurements have been carried out to understand the microstructure of the multilayer system and to establish its correlation with the opto- electric properties. The results have been evaluated for use of these multilayer TCO stacks as potential window layers for the photovoltaic solar cell applications.

Interfaces of Zinc Phosphide Magnesium Schottky Diodes

We report on the microanalysis of interfacial diffusion in zinc phosphide (Zn₃P₂) Schottky diodes. The Zn₃P₂ layers were grown by close-space sublimation on Ag-coated borosilicate glass substrates. The Schottky diodes were formed by depositing Mg layers directly on the Zn₃P₂ and thermally annealing the stack. Microanalysis revealed interdiffusion of Mg and Zn at the Zn₃P₂/Mg interface, as well as significant diffusion of Zn into the Ag back electrode layer. Such interfacial diffusion is expected to affect the electronic properties of the Zn₃P₂ layers and, therefore, the diode properties.

Development of a high-pressure CdS sputtering process for improved efficiency in CIGS-based photovoltaic devices

A new sputtered process for cadmium sulfide (CdS) has been developed that boosted efficiency of a PV device baseline (with single-stage, coevaporated CIGS) by ~ 2.7% (absolute) to an average of ~10%. The process is highly scalable to a roll-to-roll environment, with the maximum efficiency effect occurring at 0.1 mbar (~75 mTorr) sputtering pressure. The efficiency improvement is thought to be due to two main factors: composition and defects. The CdS film deposited at high pressure (HP) contains more oxygen (primarily as CdO) than one deposited at typical pressures, with oxygen content higher towards the CdS-CIGS interface. The HP process produces an interface with less CdS-CIGS intermixing, which results in a junction with ~ 5x fewer defects as measured by admittance spectroscopy. The performance improvement due to HP CdS occurs even with a very thin CdS layer (<;15 nm), thus greatly reducing the total amount of cadmium contained in the cells.

Aqueous-Base-Developable Benzocyclobutene (BCB)-Based Material Curable in Air

A positive-tone and aqueous-base-developable benzocyclobutene (BCB)-based dielectric material curable in air is described in this paper. The prepolymer is made from divinylsiloxane bisbenzocyclobutene (DVS-bisBCB) and BCB-acrylic acid. The formulation contains antioxidants that allow the prepolymer to cure in air and a diazonaphthaquinone to make it photosensitive. Patterned films have high resolution, and via openings are scum-free without a descum operation. Whether cured in nitrogen or in air, the prepolymer produces a film with optical, electrical, thermal, and mechanical properties desirable for many microelectronic applications, such as packaging applications and as a planarization or insulation layer in display applications

Control of CIGS roughness by initial selenization temperature

When formed by coevaporation, copper indium gallium diselenide (CIGS) films are typically more smooth than those formed by a two-step, precursor-selenization process. While some amount of roughness is desirable for minimizing reflection, extreme roughness or very sharp features can create a challenge in sputtering uniform, thin, transparent conductive oxide layers on top of the cell. NuvoSun, Inc. has determined that the surface roughness of the CIGS layer can be controlled by the initial temperature at which the CIGS precursor (PC) film is first exposed to a selenium flux during selenization. The resulting CIGS films are similar in roughness to coevaporated CIGS films, and the TCO layers on these smoother devices have fewer cracks and defects.