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AZO TCOs for CIGS Solar Cell Applications

Transparent Conducting Oxide (TCO) Al doped ZnO (AZO) is becoming popular as a transparent conducting oxide (TCO) replacement for ITO. We have discussed the economic and transparency advantages of AZO previously [4 ]. Applications of this TCO include the front transparent window and contact on thin film amorphous silicon and CuInGaSe (CIGS) solar cells. AZO is also used in gas sensing and surface acoustic wave (SAW) devices and UV emitters. The favored deposition process is DC reactive magnetron sputtering of Al:doped ZnO targets, and Al2O3/ZnO targets are used with RF sputtering. In the recent report reviewed here, a target 99.9% pure Zn alloyed with 2% (m/m) Al was used [5].

The researchers deposited films using O2 gas flow and target voltage settings that occur within the hysteresis transition region, O2 flows 20-45 sccm and voltage drop 325 – 600 V. To overcome the instability of such operation, plasma emission was used to control the gas flow by monitoring the metal emission line. A set of transparency and resistivity values was determined at different voltage settings and nearly constant power. The table lists some examples of transparency (transmittance average from 450 – 500 nm) and resistance. Close examination shows large variations in resistance occur with small changes in power and voltage, suggesting a strong relationship with voltage. Sample A25 has a sheet resistance of 7 ohm/sq for a thickness of 260 nm, yet the P and V values are not substantially different compared to the other tests, making this result exceptional. Perhaps another sensitive parameter or two must have been encountered to achieve this low R. The resistance of the other high transmittance samples ranges from 300 to 900 ohm/sq for thicknesses 500 – 600 nm.

Spectroscopic ellipsometry (SE) modeling to determine the absorption coefficients of these samples suggested a positive correlation relating smaller absorption with lower resistance. The other conclusion that can be drawn is that oxygen and voltage deposition parameters for producing low sheet resistances with AZO are very critical; and from experience, we can conclude that this sensitivity is greater than that for ITO deposition.

Obtaining resistivity and transmission >80% with AZO (and ZnO-based materials in general) that is as low as that obtained with ITO is proving to be difficult, independent of the deposition technique used. A recent paper demonstrates the advantages of the sputtering technique variation, high power pulsed magnetron sputtering (HPPMS), to achieve 37 ohm/sq sheet resistance with 150 nm thick AZO films on 200° C substrates. The best obtained using reactive DC magnetron sputtering (RDCMS) was 50 ohm/sq [6]. ITO films of thickness ~500 nm deposited on 300° C substrates by both HPPMS and RDCMS gave about the same resistance, <3 ohm/sq,="" and="" ~85%="" transmission.="" on="" unheated="" substrates,="" the="" resistivity="" is="" 10="" x="" higher.="" process="" control="" based="" on="" plasma="" emission="" was="" able="" to="" stabilize="" the="" discharge="" in="" the="" transition="" mode,="" and="" this="" seems="" to="" be="" the="" process="" needed="" to="" achieve="" the="" minimum="" resistivity="" for="" azo="" and="" similar="" compositions.="" voltage="" and="" o2="" flow="" rate="" have="" large="" influences="" film="" roughness="" (scatter)="" and="" resistivity,="" and="" the="" reader="" is="" encouraged="" to="" consult="" the="" paper="" for="">

Materion Microelectronics & Services is developing AZO target compositions with varied Al doping concentrations to produce films with lower sheet resistance, and parallel efforts are being directed toward deposition process optimization.

References 1. CMN: V18 Issue 1 (Jan 2008). 2. A. Nemeth, Cs Major, M. Fried, Z. Labadi, and I. Barsony, ¡§Spectroscopic ellipsometry of transparent conductive ZnO layers for CIGS solar cell applications¡¨, Thin Solid Films, 516, 7016 (2008). 3. V. Sittinger, F. Ruske, W. Werner, C. Jacobs, B. Szyszka, and D. J. Christie, ¡§High power pulsed magnetron sputtering of transparent conducting oxides¡¨, Thin solid Films 516, 5847 (2008).