Daniel Vanhart

Growth morphology and electrical/optical properties of Al-doped ZnO thin films grown by atomic layer deposition

The authors report electrical and optical characterization of zinc oxide (ZnO) and Al-doped zinc oxide (AZO) films grown by atomic layer deposition (ALD). A detailed analysis of ZnO growth morphology is presented with the help of atomic force microscopy imaging, roughness analysis, and x-ray photoelectron spectroscopy surface chemistry information. Initially the film grew as islands, which coalesced to complete the substrate coverage at 50 ALD cycles. The AZO films to be used as transparent conducting oxides for solar cell applications were grown on single crystalline Si (100) and float-glass substrates at temperatures from 150–325 °C. The amount of aluminum doping was varied from 2 to 8 %. The AZO film with 5% Al exhibited the highest conductivity in the film, which increased as the growth temperature increased. Hall effect measurements of an AZO film of thickness 575 nm doped at 5% on silicon and glass substrates showed a sheet resistance (Rs) of 100 Ω/□, which improved further to 25 Ω/□ after annealing...

Electrical and Optical Degradation Studies on AZO Thin Films Under Cyclic Bending Conditions

Aluminum-doped zinc oxide (AZO) thin film is a promising candidate for low-cost transparent conductive oxide applications. AZO thin films have good electrical and optical properties; however, their reliability must be carefully considered in manufacturing and usage of flexible devices such as flexible solar cells. Cyclic bending leads to both electrical and optical degradation of AZO thin films. Therefore, this paper was designed to investigate the effects of the bending frequency, bending diameter, number of bending cycles, and film thickness on the electrical and optical degradation of AZO films, which were deposited on polyimide (PI) substrates using a radio frequency-magnetron sputtering technique. The cycling was conducted on identical specimens under controlled study parameters. The design of experiments technique was used to determine the significant factors on the percentage change in electrical resistance (PCER) and the degradation of average transmittance (DAT), considering a wavelength range from 660 to 950 nm. The result shows that the PCER is very sensitive to number of bending cycles, particularly in the first 300 cycles. After 1000 cycles, the mean PCER of 425- and 600-nm AZO films are about 300% and 600%, respectively. The DAT is sensitive to number of bending cycles, bending diameter, and bending frequency. The mean DAT is about 2.8% after 1000 bending cycles.

Moisture-Induced Surface Corrosion in AZO Thin Films Formed by Atomic Layer Deposition

Aluminum-doped zinc oxide (AZO) thin film is a viable alternative to tin-doped indium oxide, the dominant transparent conducting oxide used in solar cells. The durability of the AZO thin films grown by atomic layer deposition technique, which is known to form layers with atomic layer precision, is studied. The AZO films were subjected to the harsh environmental conditions of varying temperatures and humidity, and their changes in surface morphology and conductivity are investigated. Four different combinations of temperature (100°C and 20°C) and relative humidity (100% and 20%) were used. It was found that the films exposed to the high-moisture and temperature conditions resulted in surface corrosion and lowered conductivity. However, SEM cross-sectional images showed that the bulk of the film was unaffected. The corroded surface had contaminants deposited from the measurement chamber as observed from XPS elemental analysis. Detailed phase analysis showed the presence of zinc hydroxide and zinc carbonate inside the corroded regions.

Analysis And Characterization Of Electrically Conductive Adhesives

Electrically conductive adhesives (ECAs) are considered an alternative to solder interconnection in microelectronic circuit packaging. In an effort to understand the performance of this class of materials, several chemical analytical techniques have been employed to characterize polymer chemistry, filler morphology and surface chemistry. These techniques include fourier transform mass spectroscopy pyrolysis (FTMS), optical microscopy, confocal laser scanning microscopy (CLSM), scanning electron microscopy with energy dispersive x‐ray analysis (SEM/EDX), auger electron spectroscopy (AES) and photo‐electron spectroscopy (XPS). Samples studied were conductive adhesive layers and deposits, plated metal surfaces, cross‐sections of bonded joints and fractured bonds. Electrical contact resistance data were also taken on bonded joints. Three commercial adhesives were studied. It was found that thermal stability varied among the three adhesives with one adhesive showing degradation at as low as 200 °C. Differences were also noted in silver flake size, morphology and apparent contact area. Excellent contact resistance stability is achieved with two formulations on a palladium‐nickel alloy surface. In contrast, a hard gold surface yielded unstable contact resistance. Fractured bond surfaces were studied for both stable and unstable joints to understand mechanisms for contact resistance degradation. The increase in contact resistance is most likely caused by oxygen ingress along an interfacial bond where the adherend surface is smooth. The matrix polymer oxidized at the interface when exposed to temperatures above the glass transition temperature. Tin surfaces cause increasing contact resistance and nickel surfaces give an initial contact resistance that is more than two orders of magnitude higher than noble metal surfaces. Oxide formation in both cases is the likely cause for the unstable and high resistances. Various analytical techniques have been used to characterize and differentiate electrically conductive adhesives. Differences among three adhesives are noted and correlated to contact resistance performance.

Annealing of FeS 2 nano-crystal thin film

With a band gap of 0.95 eV and high absorption coefficient (~105 cm-1), FeS2 is ideal for use as a p-type heterojunction partner in a solar cell. Although pyrite is abundant in nature, getting the right phase for thin films is difficult due to the various phases of iron sulfides. We propose an ink based process for attaining the pyrite phase. Our experimental process involves use of low cost non-toxic chemicals for synthesis. The process involves reacting iron (II) chloride and sulphur complexes made with solvents like oleylamine, octadecylamine and diphenyl ether. The reaction provides perfect nano crystals dispersed in a carbon based solution which is later subjected to centrifugation to separate the crystals. After multiple cleaning cycles, the crystals were dispersed in chloroform for uniform suspension. SEM image of the film formed by drop casting revealed that the nanocrystals were perfect cubes. EDX analysis showed the iron to sulfur atomic percentage ratio 1:1. The nano-crystal films were annealed at temperatures from 400°C to 600°C. The organic residues were removed after annealing but the film annealed at 600°C converted into FeS and consisted of amorphous region. A detail analysis of the films using SEM/EDX/XRD/FTIR/Raman is presented in this article.

Development of zinc phosphide as a p-type absorber

In the present contribution, we report a simple and repeatable process for the synthesis of zinc phosphide on two different substrates, zinc foil and zinc evaporated on molybdenum-coated glass. Zinc phosphide has been an important candidate for optoelectronic applications and has also been explored in the lithium ion batteries. Zinc phosphide is synthesized from earth-abundant constituents, zinc and phosphorous. Trioctylphosphine (TOP) is used as a source of phosphorous which reacts with zinc and results in the growth of zinc phosphide. Zinc phosphide has been successfully synthesized in both continuous thin film and nanowires form around ~ 350°C. The synthesized zinc phosphide phase was characterized using SEM, EDS, XRD and XPS. Possible growth mechanism is discussed.