Indium Tin Oxide Thin Films With Both Near-Infrared Transparency and Excellent Resistivity

Abstract

An indium tin oxide film containing by weight about 90% In.sub.2O.sub.3 and about 10% SnO.sub.2 is prepared using a low-energy deposition sputter process on a substrate. The indium tin oxide film thus obtained has a carrier concentration on the order of 10.sup.20/cm.sup.3 and a carrier mobility greater than 30 cm.sup.2/Vs. The low carrier concentration results in an increased transmission in the near infra-red region, while the high carrier mobility results in good conductive properties.

Claims

1. An indium tin oxide film comprising by weight about 90% In.sub.2O.sub.3 and about 10% SnO.sub.2, wherein the indium tin oxide film is about 350 nm in thickness.

2. The indium tin oxide film of claim 1, wherein In.sub.2O.sub.3 and SnO.sub.2 are 90% and 10% by weight respectively.

3. The indium tin oxide film of claim 2, wherein the indium tin oxide film is 350 nm in thickness.

4. The indium tin oxide film of claim 2, wherein In.sub.2O.sub.3 and SnO.sub.2 combined are present at 99.99% by weight.

5. The indium tin oxide film of claim 2, wherein a carrier concentration is on the order of 10.sup.20/cm.sup.3.

6. The indium tin oxide film of claim 2, wherein the indium tin oxide film provides a carrier mobility greater than 15 cm.sup.2/Vs.

7. The indium tin oxide film of claim 6, wherein the carrier mobility is greater than 30 cm.sup.2/Vs.

8. The indium tin oxide film of claim 2, wherein the indium tin oxide film provides a transmission greater than 75% from 700 nm to 1500 nm.

9. The indium tin oxide film of claim 2, wherein the indium tin oxide film provides a resistivity on the order of 10.sup.−4 Ω.Math.cm.

10. A method for a sputter deposition of an indium tin oxide film comprising: providing an indium tin oxide target comprising In.sub.2O.sub.3 and SnO.sub.2 in a ratio of about 90:10 by weight; providing argon as a sputter gas at a pressure of about 3.5 millitorr; and sputtering the indium tin oxide target on a substrate at a radio frequency power density of about 1.0 W/cm.sup.2 to obtain the indium tin oxide film.

11. The method of claim 10 wherein the sputtering is continued for a time sufficient to produce the indium tin oxide film having a thickness of about 350 nm.

12. The method of claim 10 wherein the sputter gas is at a pressure of 3.5 millitorr.

13. An indium tin oxide film comprising by weight about 90% In.sub.2O.sub.3 and about 10% SnO.sub.2, wherein the indium tin oxide film is about 350 nm in thickness, further wherein the indium tin oxide film is prepared by a method for a sputter deposition comprising the steps of: providing an indium tin oxide target comprising In.sub.2O.sub.3 and SnO.sub.2 in a ratio of about 90:10 by weight; providing argon as a sputter gas at a pressure of about 3.5 millitorr; and sputtering the indium tin oxide target on a substrate at a radio frequency power density of about 1.0 W/cm2 to obtain the indium tin oxide film.

14. The indium tin oxide film of claim 13, wherein the In.sub.2O.sub.3 and SnO.sub.2 are 90% and 10% by weight respectively, further wherein the indium tin oxide film provides a transmission greater than 75% from 700 nm to 1500 nm and a carrier concentration on the order of 10.sup.20/cm.sup.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 is a graph of transmission (T), reflection (R) and absorption (A) for Sample ITO#1, Sample ITO#2 and glass (as a reference).

[0024] FIG. 1A is a breakout graph of FIG. 1 showing transmission (T) for Sample ITO#1, Sample ITO#2 and glass (as a reference).

[0025] FIG. 1B is a breakout graph of FIG. 1 showing reflection (R) for Sample ITO#1, Sample ITO#2 and glass (as a reference).

[0026] FIG. 1C is a breakout graph of FIG. 1 showing absorption (A) for Sample ITO#1, Sample ITO#2 and glass (as a reference).

DETAILED DESCRIPTION OF THE INVENTION

[0027] In this invention, an indium tin oxide film is prepared. The indium tin oxide film contains about 90% In.sub.2O.sub.3 by weight and about 10% SnO.sub.2 by weight, and is about 350 nm in thickness. The indium tin oxide film has a bulk concentration on the order of 10.sup.20/cm.sup.3 and a carrier mobility greater than 15 cm.sup.2/Vs.

[0028] In a preferred embodiment, the indium tin oxide film is 99.99% pure, and contains by weight 90% In.sub.2O.sub.3 and 10% SnO.sub.2.

[0029] In another preferred embodiment, the indium tin oxide film is 350 nm in thickness.

[0030] In another preferred embodiment, the bulk concentration of the indium tin oxide film is −3.68×10.sup.20/cm.sup.3; the carrier mobility is 36.9 cm/Vs; the resistivity is 4.59×10.sup.−4 Ω.Math.cm; and the transmission in the 700 nm to 1500 nm portion of the near infra-red region is greater than 75%. The negative sign for the carrier bulk concentration indicates that the carrier material is an n-type material whereby electrons are the majority charge carrier.

[0031] In another preferred embodiment, the bulk concentration of the indium tin oxide film is −3.06×10.sup.20/cm.sup.3; the carrier mobility is 42.3 cm/Vs; the resistivity is 4.82×10.sup.−4 Ω.Math.cm; and the transmission in the 700 nm to 1500 nm portion of the near infra-red region is greater than 75%.

[0032] In this invention, a particular sputter process is used for the deposition of the ITO film having the enhanced properties—most notably, high near-IR transparency and low resistivity.

[0033] The various aspects of the sputter process are explained as follows:

1. An indium tin oxide sputter target is chosen that contains In.sub.2O.sub.3 and SnO.sub.2 in a 90:10 ratio by weight. The 90:10 ratio by weight represents a low percentage of SnO.sub.2 in deposition. The indium tin oxide film formed contains the In.sub.2O.sub.3 and SnO.sub.2 in 99.99% by total weight.
2. Argon alone is used as the sputter gas.
3. An example of the partial pressure of argon in the sputter chamber is 3.5 mTorr. The 3.5 mTorr pressure is somewhat higher than that commonly used for ITO depositions where <1 mTorr is typical.
4. An example of the radio frequency (RF) power used in the deposition is 47 W on a 75 mm diameter target. This RF power level correlates to a power density of 1.06 W/cm.sup.2, lower than the >4 W/cm.sup.2 typically used to create ITO films at high deposition rates.
5. The sputter deposition of the indium tin oxide target on a substrate is continued for a time sufficient to produce a film of a chosen thickness. The thickness is chosen based on the desired values of NIR transmission and resistivity. An example of the film thickness is 350 nm.

[0034] The optimum combination of power and pressure is determined by mapping out a response curve with respect to the characteristic properties, for example, resistivity and transmission, using Design of Experiment (DoE) techniques.

[0035] A high carrier concentration and a correspondingly high absorption in the NIR is a common problem faced in ITO. In its application as a TCO in photovoltaics, surmounting these problems can lead to an increase in short-circuit current and a high fill factor.

[0036] As discussed above, as with all transparent conducting films, a compromise must be made between conductivity and transparency. Although increasing the thickness and the concentration of charge carriers increases the material's conductivity, it decreases transparency. A thickness of about 350 nm represents a balance between conductivity and transparency. Thicker films may be expected to provide a higher conductivity counter-balanced by a lower transmission, as is well known in the field.

[0037] The thickness during the deposition may be monitored using well-established methods. For example, using a quartz crystal monitor and further calibrated using a Dektak® profilometer [Bruker Nano, Inc., 3400 E. Britannia Drive, Suite 150, Tucson, Ariz. 85706] to confirm that the correct thickness is reached.

[0038] Using conventional processing methods, the ITO films have a high carrier concentration on the order of 10.sup.21/cm.sup.3 and a low mobility of approximately 15 cm.sup.2/Vs. The ITO films produced according to the current invention have a carrier concentration an order of magnitude lower and a higher mobility.

Examples

[0039] Using the following parameters, film samples ITO #1 and ITO #2 were prepared.

1. Glass, such as a soda lime glass plate, was used as a substrate.
2. An indium tin oxide sputter target contained In.sub.2O.sub.3 and SnO.sub.2 in a 90:10 ratio by weight.
3. Argon as the sputter gas was set at 3.5 mTorr in the sputter chamber.
4. Radio frequency (RF) power at 47 W was used on a 75 mm diameter circular target. This RF power level correlates with a power density of 1.06 W/cm.sup.2.
3. The sputter deposition was continued until the film thickness reached 350 nm.

[0040] The properties of ITO #1 and ITO #2 are presented in Table 1. The variations between samples “ITO#1” and “ITO#2” are attributed to process-to-process variability.

TABLE-US-00001 TABLE 1 Hall measurement and sheet resistance ITO#1 ITO#2 Bulk concentration [cm.sup.−3] −3.68 × 10.sup.20 −3.06 × 10.sup.20 Mobility [cm/Vs] 36.9 42.3 Resistivity [Ωcm]  4.59 × 10.sup.−4  4.82 × 10.sup.−4 Sheet resistance [Ω/.square-solid.] 15.7 16.0

[0041] The Hall Effect measurements were used to ascertain the carrier concentration, mobility and resistivity.

[0042] A four point probe was used for the sheet resistance measurement.

[0043] A Dektak® profilometer was used for thickness measurement.

[0044] A spectrophotometer was used for transmission and reflectance measurements, with additional calculation of the band gap. The transmission (T), reflection (R) and absorption (A) for Sample ITO#1, Sample ITO#2 and glass (as a reference) are plotted in FIGS. 1 and 1A-1C. As shown in FIG. 1A, the transmission from 700-1500 nm is 75-83%.

[0045] The foregoing presents particular embodiments of a system embodying the principles of the invention. Those skilled in the art will be able to devise alternatives and variations which, even if not explicitly disclosed herein, embody those principles and are thus within the scope of the invention. Although particular embodiments of the present invention have been shown and described, they are not intended to limit what this patent covers. One skilled in the art will understand that various changes and modifications may be made without departing from the scope of the present invention as literally and equivalently covered by the following claims.