Investigating Solar Energy Materials

This article discusses the research carried out by Daniel Meysing, a Ph.D. candidate in the Chemical and Biological Engineering Department at the Colorado School of Mines in Golden, Colorado. His research group study the chemistry and energetics of radio frequency magnetron sputtering of the II-VI compounds utilized in thin-film solar cells. The Hiden EQP 500 was used for this work.

Solar Energy Material Study

Sputter deposition of II-VI compounds is usually considered a “black box,” where the input parameters (sputtering power, ambient composition, and pressure) are controlled, and the film properties (optical transmission, conductivity, and composition) are determined without in-depth knowledge of the process. The aim of the research work is to develop a greater knowledge of certain complex mechanisms involved in sputtering, which impact the optoelectronic properties of thin films.

Up to six sputtering guns can be accommodated in sputter-up configuration in the sputtering chamber, or the Sputter-Plasma Diagnostic (SPD) tool (Figure 1). The Hiden EQP 500 analyzer is installed on a rotating flange, and can be positioned normal to or off-axis to the sputtering target, and moved close to or further away from the target, using the z-axis motor. To use over different pressure regimes, the differentially pumped EQP can be fitted with orifices of different diameters. For this work, a 100 µm aperture is used to perform the analysis at ~10 mTorr sputtering pressure.

Schematic representation of the Sputter-Plasma Diagnostic tool.

Figure 1. Schematic representation of the Sputter-Plasma Diagnostic tool.

In the initial work, focus was placed on plasma-generated ion energy distributions (IED) during radio frequency magnetron sputtering of ZnS in a pure argon (Ar) under ambient conditions. Parameters, like pressure, power, and flight distance of S, Ar, and Zn ions, were studied. When compared to Zn and S, Ar ions have different IEDs, which exhibit a low-energy shoulder, which is a result of reflection at the target and Penning ionization reactions.

However, S and Zn IEDs possess one dominant peak, along with a relatively small high-energy shoulder. The energy of the Zn and S peaks in all instances are ~1.5 eV greater than that of the Ar peak. Varying the displacement between the EQP orifice and the target enabled the examination of the effect of flight distance (Figure 2). As predicted when the flight distance was increased, the arrival energy of ions was reduced due to increased collisions. Similarly an increase in pressure resulted in decreased ion energy.

Zinc sulfide ion energy distribution scans and Ar profiles as a function of pressure. (A) 36Ar profiles as a function of pressure; (B) 66Zn profiles as a function of pressure; (C) 32S, 36Ar, and 66Zn IED peak positions as a function of flight distance; (D) 32S, 36Ar, and 66Zn IED peak positions as a function of pressure.

Zinc sulfide ion energy distribution scans and Ar profiles as a function of pressure. (A) 36Ar profiles as a function of pressure; (B) 66Zn profiles as a function of pressure; (C) 32S, 36Ar, and 66Zn IED peak positions as a function of flight distance; (D) 32S, 36Ar, and 66Zn IED peak positions as a function of pressure.

Zinc sulfide ion energy distribution scans and Ar profiles as a function of pressure. (A) 36Ar profiles as a function of pressure; (B) 66Zn profiles as a function of pressure; (C) 32S, 36Ar, and 66Zn IED peak positions as a function of flight distance; (D) 32S, 36Ar, and 66Zn IED peak positions as a function of pressure.

Zinc sulfide ion energy distribution scans and Ar profiles as a function of pressure. (A) 36Ar profiles as a function of pressure; (B) 66Zn profiles as a function of pressure; (C) 32S, 36Ar, and 66Zn IED peak positions as a function of flight distance; (D) 32S, 36Ar, and 66Zn IED peak positions as a function of pressure.

Figure 2. Zinc sulfide ion energy distribution scans and Ar profiles as a function of pressure. (A) 36Ar profiles as a function of pressure; (B) 66Zn profiles as a function of pressure; (C) 32S, 36Ar, and 66Zn IED peak positions as a function of flight distance; (D) 32S, 36Ar, and 66Zn IED peak positions as a function of pressure.

Future Research

Future research will focus on studying oxygen incorporation at the time of reactive sputtering of CdS and ZnS in an ambient O2/Ar. Oxygen incorporation occurs through the substitution and/or reaction with sulfur. At the 2014 Rocky Mountain American Vacuum Society Symposium held in Denver, Colorado, the research groups poster titled “Energy-resolved quadrupole mass spectrometry in IIB-VIA sputtering investigations” secured third position.

This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.

For more information on this source, please visit Hiden Analytical.

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