"Study of Indium Tin Oxide (ITO) for Novel Optoelectronic Devices"
Ph.D. thesis by Shabbir A Bashar




5.4 Ohmic Contacts to ITO

The application of Indium Tin Oxide in a solid state device, for example, requires a low resistance ohmic contact to the ITO which may itself be an ohmic contact or a Schottky contact. Ohmic contacts are defined as a metal-semiconductor contact with a linear or near linear current voltage characteristic; these have been discussed in greater detail in the following chapter. Direct gold wire bonding to thin ITO films giving a low resistance contact is not easily achieved without undue damage to either or both the underlying ITO and/or the device. Therefore, following the ITO deposition and appropriate pattern definition, the contact metal or the pad is usually thermally evaporated by means of liftoff lithography. Bonds can then be made to the ITO device with as much ease as for devices with conventional contacts.

Figure 5.19: Schematic scale diagram of the plan view of a high speed ITO/n-GaAs Schottky photo diode showing the importance of a good ohmic contact to the ITO active area (Diagram after Wang et al [71]). Any region outside the "isolation" rectangle was proton bombarded prior to device fabrication.

Where typical contact areas in micro-electronic devices are less than a few 100mm2, it is very important for the contact resistance to be less than a few tens of ohms corresponding to specific contact resistance, rc, of approximately 1e-5 Wcm2. A schematic scale diagram of the plan view of a high speed ITO/n-GaAs Schottky photo diode is shown in Figure 5.19; it shows the importance of a good ohmic contact in order to access the active region of the device which is formed by an ITO/n-GaAs Schottky contact.

Weijtens et al [154] have used TiW as barrier layers to obtain low resistive, ohmic contacts between ITO and Al or Al/Si. They reported a value of 1.5e-6 Wcm2 for the rc after annealing in 4N2:1H2 gas mixtures at 450C or 500C.


5.4.1 Metalisation Schemes Used

Initial tests with Au deposited on bare ITO produced electrically good contacts but lacked mechanical strength and were unable to sustain the stress during gold wire bonding. Hence the suitability of three convenient metalisation systems for metal/ITO ohmic contacts were investigated. These are shown in Table 5.6. Their choice was dictated by a need to keep the metal/ITO contacts as close in composition to conventional metal/n+ - or metal/p+ - semiconductor for overall processing compatibility and the ease of device fabrication.

Layer Set A Set B Set C
5 - - Au (1000)
4 Au (1000) - Ti (500)
3 Ni (100) - Ni (100)
2 AuGe (300) Al (1500) AuGe (300)
1 Ni (50 ) Ni (50 ) Ni (50 )

Table 5.6: Details the ohmic contact schemes applied to ITO

The Ni/AuGe/Ni/Au metalisation scheme is the most widely used scheme in our laboratories for making conventional n-type ohmic contacts to a wide ranging substrate materials and devices. Therefore, ITO contacts can be made conveniently by placing the sample into the evaporation chamber in tandem with a wide variety of other devices requiring an n-type ohmic contact. The Ni/Al system provides a cheap and easy way of making ohmic contacts to ITO layers. It is particularly useful when metals are deposited exclusively for making ohmic contacts to ITO devices as opposed to the usual case when a number of other devices are fabricated in tandem. Finally, the Ni/AuGe/Ni/Ti/Au metalisation scheme was used in the fabrication of n-type ohmic contacts to transparent gate HEMT devices. This was chosen because unlike Set A, this scheme retains a smooth surface morphology following rapid thermal annealing (RTA).


 5.4.2 Results and Conclusion

Each of the contacts were annealed using the following conditions: 5 hours at 240C followed by another 5 hours at 340C in H2/N2 ambient. The TLM method was once again used to appraise these contacts. Recall that Rsh is the sheet resistance of the mesa (ITO in this case) while Rsk is the modified sheet resistance directly under the metal pads deposited on the ITO. The results are shown in Table 5.7.

Metalisation Rsh (ITO) [W/] Rsk [W/] rc [x 1e-4 Wcm2]
Set A Unannealed 978.2 31.1 41.7 1.6 2.6 0.3
Annealed 50.6 1.7 0.98 0.06 0.28 0.04
Set B Unannealed 664.6 48.0 7.4 0.5 5.4 0.4
Annealed 24.1 2.6 1.4 0.1 0.8 0.1
Set C Unannealed 1273.2 68.6 93.3 12.1 3.4 0.4
Annealed 76.0 3.4 1.8 0.2 0.4 0.02

Table 5.7: Effect of annealing on the electrical properties of various ohmic contacts to ITO films (typically 2000 thick).

Comparing the electrical properties of the three metalisation systems investigated, it is seen that all the unannealed characteristics of the contacts are very similar - Rsk and rc in the region of 50W/ and 3e-4 Wcm2 respectively. This is in spite of the sheet resistances, Rsh, of the corresponding ITO material varying over a much greater range: 1273.2 W/ to 664.6 W/. The Rsk after annealing for all three metalisations reduces by an order and lies approximately in the range 1 to 2 W/ while rc drops to the range 3 to 8e-5 Wcm2.

Hence it may be concluded that while the various metalisation schemes used here ultimately make no significant difference to electrical properties of the metal/ITO contacts, they all are all equally suitable for the purpose of further work in this investigation since excellent ohmic contacts to ITO have been realised.

1998: Shabbir A. Bashar (in accordance with paragraph 8.2d, University of London Regulations for the Degrees of M.Phil. and Ph.D., October 1997). The Copyright of this thesis rests with the author, and no quotation from it or information derived from it may be published without the prior written consent of the author.
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