I have sputtered thin film of ZnO over glass plate. It appears to be less transparent than plain glass. I tried to measure the resistance of this thin film by using a simple multimeter. It is not showing any resistance values. I don't know why this is the case. But in the literatures they have reported for ZnO as some lesser values of resistance in the range of milli ohm.
Have you tried increasing the oxigen concentration during the sputtering process? Depending on the system you will need different concentrations to achive nonconductive ZnO films. I hope this helps.
ZnO thin films show a wide range of resistivity variation from ~10^‒4 to ~ 10^8Ωcm,
depending on the growth process condition. Carcia et al. showed that the resistivity of ZnO films is increased from ~10^‒2 to ~ 10^8 Ωcm by increasing oxygen partial pressure in sputtering method.
http://scitation.aip.org/content/aip/journal/apl/82/7/10.1063/1.1553997
The about two comment are good. Depending on the O2 partial pressure during deposition will play a crucial role in resistivity because of variation of oxygen vacancy in the thin film. Also the growth temperature has great influence. Room temperature growth of ZnO will produce film of giga-ohm order resistance whether deposition at 500C will produce Mega-ohm order resistance of the film.
Instead of using multimeter, I think it is better to use some other techniques like four probe measurement to find the resistance.
It all boils down to the thickness of your film in addition to other growth parameters. If its nm thick, then you can only measure sheet resistance using 4-point probe or Van de Pauw methods then you can determine your resistivity from the measured sheet resistance.
It is possible to get very low resistivities from thicker films of ZnO, however you should also understand that there is always a trade between transmittance and conductivity. Generally, more conducting films (lower resistivity) are less transmitting.
Dear Raja
Please note the following:
1) As the ZnO is not colorless, it seems that it is heavily doped. This explains the high conductivity.
2) How did you measure resistance? Across the very thin film? This may show very small resistance. If you measure resistance in the xy plane with some distance between the connections you will see higher resistance (use the 4-probe please).
Try to illuminate it. ZnO films may possess very interesting long-term photoconductance, e.g. see this ref and citations to this paper:
[1] S. Studenikin, N. Golego, and M. Cocivera, "Optical and electrical properties of undoped ZnO films grown by spray pyrolysis of zinc nitrate solution," Journal of Applied Physics, vol. 83, pp. 2104-2111, 1998.
[2] S. A. Studenikin, N. Golego, and M. Cocivera, "Density of band-gap traps in polycrystalline films from photoconductivity transients using an improved Laplace transform method," Journal of Applied Physics, vol. 84, pp. 5001-5004, 1998.
[3] S. Studenikin, N. Golego, and M. Cocivera, "Carrier mobility and density contributions to photoconductivity transients in polycrystalline ZnO films," Journal of Applied Physics, vol. 87, pp. 2413-2421, 2000.
I hope it helps. S
Additionally (of above advices) you can consider the thickness of your sample. May be it is very thicker.
Resistance of any transition metal oxide thin thin film depends on oxygen content in the film. Depending on the degree of deviation from the stoichiometric composition resistance of the material change.
Thanks for all your valuable replies.
My thin films of ZnO is yellowish in color, seems it is Zn rich and O deficient. I have done the sputtering only in Ar atm. I tried for annealing the sample at 500 C. but it too doesn't changed the color of the thin films.
I suggest to etch a meander path on the film. This has two benefits:
1. it increases the total resistance of the overall film structure;
2. makes it possible to measure the resistance in a 4 wire configuration (http://www.allaboutcircuits.com/vol_1/chpt_8/9.html).
The meander path can be etched by photolitography (better) or even using a simple dremel, if you have a steady hand.
Could you more explain your measurement technique? I am familiar with four-point probe which gives precise results in a huge range of material conductivity.
you should try an accurate method of resistance measurement like four probe or HMS Hall measurement technique
ZnO as thin film can be so resistant and reistivity can be found within kiloohm.cm - 10^-6 ohm cm.
you can read J Sol-Gel Sci Technol (2010) 55:335–342
DOI 10.1007/s10971-010-2258-x
"No resistance" on the multimeter meaning very low or very high? I suspect it's very high, cause putting multimeter probes on a very conductive ZnO thin film would measure a few hundreds of ohm up to kilo ohms. If you wanted to get a conducting ZnO film, this is not one. So it cannot be oxygen deficient (oxygen vacancies make the film more conductive and Zn enrichment makes it black). Annealing in air will not help making it conductive; you could bake it in vacuum or something else than oxygen for this. Maybe you should reduce Ar pressure in sputtering if you want a more conducting film.
sputtered ZnO films are normally resistive.
whether dc or rf (diode or magnetron sputtered films)
whether you use an oxide or a metal target.
1) To make conducting and semi conducting ZnO films you have to control the oxygen partial pressure in the sputtering gas and use (Ar+ O2) mixtures), or dope the ZnO films appropriately with Aluminum and make AZO films
2) the yellowish color is due to the oxygen non stoichiometry in thedeposited ZnO films.
3) ZnO films prepared at high pressure in 100% oxygen are colour-less and highly transparent.
4 Try to sputter at higher percentage of gas (say 25 to 45 mTorr).
K. Sreenivas
To measure the resistivity of ZnO films, you have to prepare a capactior structure in the (metal-insulator-metal) configuration.
Deposit your ZnO films on a metallised glass piece, On top of the SnO films try to depost top dot electrodes of a defined area, and then
using a three-terminal arrangement, apply some dc volts (10 to 15), and measure the leakage current. In this way you will be able measure the resistivity of the deposited ZnO films.
In case your films are highly resistive, you may need an electrometer capable of measuring currents in the nano to pico amps.
K. Sreenivas
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