Dr. Shabbir A. Bashar's Ph.D. Thesis

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

The term "ohmic" refers in principle to a (metal-semiconductor) contact which is non-injecting and has a linear I-V characteristic in both directions. In practice the contact is usually acceptable if it can supply the required current density with a voltage drop that is very small compared to the drop across the active region of the device even though it's behaviour may not be strictly linear.

Historically, metal-semiconductor contacts were predominantly used as rectifying contacts until suitable methods of fabricating p-n junctions became available. Then, these contacts began to assume a less significant role as ohmic contacts for transporting current into and out of p-n junctions. With greater understanding and technological advancements, there was a renaissance of the rectifying metal-semiconductor or Schottky contact in the 1960's. At the same time, the need for higher speed devices with their smaller and more complex geometries acted as the driving force behind the search for high performance ohmic contacts.

2.3.1 Theory of Ohmic Contacts

One of the most comprehensive papers that deals with the theory of ohmic contacts to III-V compound semiconductors is that by Rideout [25]. Nearly all practical metal-semiconductor contacts initially result in the formation of depletion layer Schottky barriers as shown in Figure 2.3. As such, they are essentially rectifying to begin with.

Figure 2.3: Schematic band energy diagram of a metal/n-semiconductor contact showing the three major current transport mechanisms: thermionic emission (TE), thermionic-field emission (TFE) and field-emission (FE).

The conduction properties of these contacts are determined by the actual transport mechanisms, most of which were discussed in section 2.2:

TE - thermionic emission of carrier, giving rise to rectifying behaviour

TFE - thermionic field emission or tunneling of hot carriers through the top of the barrier

FE - field emission or carrier tunneling through the entire barrier (the preferred mode in ohmic contacts)

In addition to these, recombination in the depletion region and the lowering of the barrier due to image force also affect, albeit to a much lesser extent, the behaviour of the ohmic contact. Taking the expressions for the first three dominant mechanisms, the current is determined by [26]:

exp(fb/kT)	for TE	(eqn. 2.14)
exp(fb/Eoocoth{Eo/kT})	for TFE	(eqn. 2.15)
exp(fb/Eoo)	for FE	(eqn. 2.16)

where Eoo is the tunneling parameter proportional to ÖND (the doping concentration).

Thus, for kT/Eoo >> 1, TE dominates and the contact is rectifying; for kT/Eoo << 1, FE dominates and the contact is ohmic while a mixed mode prevails for the condition kT/Eoo » 1. Note that both TE and TFE are temperature dependent while FE is not.

It is seen that there are several possible ways of achieving a good ohmic contact:

The most common method is to have a layer of very highly doped semiconductor (typically ND = 5e18cm-3 for n-type GaAs) immediately adjacent to the metal giving rise to a very narrow depletion/barrier width ( » 100Å). Increased conduction is then dominated by quantum mechanical tunneling.
Another approach is to have a negligible potential barrier, fb, to start with. In practice, this is harder to achieve for III-V covalent compounds such as GaAs where fb is essentially determined by interface states rather than the difference between the work function of the metal and the electron affinity of the semiconductor as theory predicts.
A third approach is to deliberately increase interface states aimed at reducing contact resistance by causing space-charge recombination to dominate. But, in practice, this has adverse effects on the device stability.

2.3.2 Practical Ohmic contacts

In most practical ohmic contacts the metal layers usually contain a suitable dopant species - donor or acceptor atoms. A heat treatment is used to drive the dopant into the semiconductor to form a n++ or p++ layer thus creating a tunneling metal-semiconductor junction required for enhanced ohmic behaviour. The quality of an ohmic contact is ultimately assessed by determining its specific contact resistance, rc, as discussed later in section 4.1.

Other desirable properties of ohmic contacts include good adhesion to the semiconductor, smooth surface morphology (particularly where near micron device geometry is concerned), ability to bond gold wires to connect the device to external circuitry and finally contact reliability. With these in mind, the practical ohmic contact system often consists of a "wetting agent" to promote adhesion, followed by the dopant species and finally a thick layer of Au for bonding purposes. Where indiffusion of the top Au poses potential reliability hazards, often a diffusion barrier is inserted between the gold and the dopant layer.

The most commonly used n-type contacts to GaAs is the Ni/AuGe/Ni/Au system. Thus many studies have been carried out to determine its alloying behaviour [27], effect of varying Ge (the dopant species) content [28] as well as investigation of the semiconductor surface cleanliness prior to metalisation [29]. However, this system has several ill effects such as "balling up" during annealing as a result of liquid phase reactions, non-uniform contact resistivity and vertical and lateral spiking. An alternative which addresses many of these issues is the Pd/Ge/Au system [30,31].

The usual p-type contact for GaAs is the Au/Zn/Au system [32] where Zn atoms act as acceptors. In devices such as HBTs base is very thin (>1000Å); hence junction shorting due to Au spiking is often more of a concern than the resistivity, especially for high temperature or high current applications. In this case, Pt/Ti/Au is used as an alternative [33] although there are no dopant species. It is argued that since the base doping is already very high (NA > 5e19cm-3) the need for extra acceptor atoms is diminished.

Pd/Ge based systems are known to have been used as both n and p-type contacts to InGaAs layers [34]. As in p-GaAs, Zn atoms also act as acceptors in p-InGaAs [35].

© 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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