an example, many works report about the redistribution of carbon inside the silicide layer and its role on the electrical properties of the contacts (focusing on ...
Ohmic contacts to SiC Fabrizio Roccaforte, Francesco La Via and Vito Raineri CNR-IMM, sezione di Catania, Italy
1.
Introduction
Ohmic contacts play a very important role in the signal transfer to and from the semiconductor and the external circuitry. In particular, the resistance of the contact must be negligible with respect to that of the bulk device (the device on-resistance), since a voltage drop at the contact adds an undesired contribution to the dissipated power, thus decreasing the efficiency of the system.1 Because of its excellent properties such as a wide band gap, a high critical electric field, a high thermal conductivity and a high electron saturation velocity, silicon carbide (SiC) is one of the most prominent candidates for the next generation semiconductor devices.2,3 However, although most of the common processes of silicon technology, like ion implantation, oxidation, self aligned silicides formation, etc., have been applied to SiC, and a variety of high-power and high-frequency SiC devices (Schottky diodes, MESFETs, VJFETs, SITs, etc.) have been demonstrated in the last years,3,4,5,6 the best expression of the potentialities of the material remains strictly related to the improvement of some technological concerns. Among them, one of the key technology issues which may limit the performances of SiC electronic devices is that of ohmic contacts. The specific contact resistance ρc is the most important figure of merit of ohmic contacts, being an intrinsic interfacial property, which is independent of the contact geometry. It has been demonstrated that high-frequency and high-power SiC electronics devices, particularly those in which a high current density flows horizontally (MESFETs), need to have ohmic contacts with values of ρc in the range 10-5-10-6 Ωcm2.7,8 Now, it is known that in order to form a good ohmic contact to a semiconductor, a metal resulting into a low Schottky barrier with the semiconductor material must be selected. In these terms, the wide band gap of SiC, about 3 times higher than that of silicon, results for almost all the metals into Schottky barrier height values larger than 1-1.2 eV, this latter making particularly difficult to obtain low specific resistance ohmic contacts. For all these reasons, a great deal of effort has been spent in the last decades to study the physics of metal contacts on SiC and to achieve low resistivity reliable ohmic contacts. The interest of the scientific community towards the material and the device related issues resulted in a good number of review papers dealing with ohmic contacts.2,7,9,10,11,12 Metal/SiC contacts are generally non-ohmic after metal deposition, due to the high Schottky barriers at the interface, determining the rectifying properties. Hence, in order to overcome this problem, beyond using heavily doped material for creating tunnel ohmic contacts, the most common method to form an ohmic contact to SiC is the deposition of a metal layer followed by an annealing process. In fact, the post-deposition annealing may result into a reaction of the metal with the SiC (i.e. with the formation of silicides, carbides, or ternary phases), with a consequent reduction of the barrier height or the barrier thickness.
1
A large number of ohmic contacts materials have been investigated in the last decades, both in terms of structural characterization and of electrical performances. For ohmic contacts on the n-type SiC material, the most promising metal is Ni. Since the early works of Crofton et al.,13 it has been demonstrated that Ni films annealed at temperatures in the range 900-1000°C can form good ohmic contacts on n-type SiC with specific contact resistance values as low as ~1×10-6 Ωcm2.13 The physical problems related to the formation of nickel silicides ohmic contacts were widely investigated. As an example, many works report about the redistribution of carbon inside the silicide layer and its role on the electrical properties of the contacts (focusing on aspects like the Schottky to ohmic transition, the thermal stability, the ease of device bonding, etc.). Moreover, the control of the thermal budget required for ohmic contacts formation is an important factor to achieve a good process compatibility for practical SiC devices. For the p-type material, due to the larger values of the Schottky barrier heights, ohmic contact formation is even more difficult than in the n-type material. Much research activity has been focused on Al/Ti contacts, which resulted in specific contact resistance in the ~10-5 Ωcm2 range.14,15 In the numerous reported works, big attention has been given to the possible mechanisms of ohmic contact formation of Al/Ti alloys on SiC, which however remain still under discussion. In addition to the low resistance, the thermal stability of ohmic contacts is required for SiC devices like sensors operating at high temperatures and in harsh environments, for the aerospace applications, satellites, nuclear power instruments, etc. Recent works reported on long term stability of Ni- and Ti-based contacts16,17,18 up to 600°C in oxidizing atmosphere and up to 1000°C in inert ambient. This paper reviews the most significant work done in the last decade on ohmic contacts on silicon carbide. First, some basic concepts related to the Schottky barrier formation, the physics of ohmic contacts and the specific contact resistance measurements techniques are briefly reported. Thereafter, the major results on ohmic contacts to n-type and p-type SiC will be discussed, with particular attention to some scientific aspects concerning the physics of contact formation (i.e. in particular for the cases of annealed Ni and Al/Ti contacts, respectively). Examples of innovative applications on practical devices are also reported, focusing on new technology processes such as the simultaneous formation of ohmic contacts on n- and p-type SiC for vertical power MOS devices proposed by Kiritani et al.19 and the single metal contact technology developed by Tanimoto et al.20 for ohmic and rectifying contacts in MESFETs.
2.
Metal-Semiconductor Contacts
Metal-semiconductor contacts fall into two basic categories, the ohmic and the rectifying (or Schottky) contacts. An ohmic contact has a linear and symmetric current-voltage characteristic for positive and negative applied voltages and a negligible resistance compared with that of the bulk of the device.1 Conversely, a rectifying Schottky contact is characterized by the current flow for only one voltage sign.
2
Fig. 1: Energy band diagram for a metal-semiconductor (n-type) contact, in the case Φm>Φs before (a) and after (b) they are brought into contact, showing the formation of a rectifying contact with a Schottky barrier height ΦB.
In order to introduce the important physical parameters which affect the contact performance, the classical description of the Schottky barrier formation is briefly reported. Fig. 1a shows the energy band diagram of a metal and an n-type semiconductor before they are brought into contact. The metal work function Φm and the semiconductor work function Φs are the energies required to bring an electron from the Fermi level of the material to the vacuum. The energy difference between the vacuum level and the bottom of the semiconductor conduction band Ec is defined as semiconductor electron affinity χs. When the metal and the semiconductor are brought into contact, provided the semiconductor work function Φs is lower than the metal work function Φm, electrons will flow from the n-type semiconductor to the metal, leaving behind a positively charged donors region over the depletion width W. This charge flow will proceed until the thermodynamic equilibrium is reached and the two Fermi levels line up. In this way the energy level of the electrons in the semiconductor will be raised near the contact by an amount Vbi, as shown in Fig. 1b. The difference between the metal work function Φm and the semiconductor electron affinity χs is defined as the Schottky barrier height ΦB:
Φ B = (Φ m − χ s ) (1) The Schottky barrier height is the most important parameter in a metal-semiconductor contact and it determines the electrical behavior of both an ohmic or a Schottky contact. The Schottky barrier can be seen as the energy necessary for electrons in the metal to penetrate into the semiconductor. On the other hand, the build-in potential Vbi is the barrier for electrons on the semiconductor side. It is worth noting that the Schottky barrier height ΦB is almost independent of the semiconductor doping concentration ND. Actually, there exists a weak dependence of ΦB on ND through the image force lowering of the barrier ∆ΦB (∆ΦB ∝ ND1/4V1/4).21 On the other hand, the barrier width W depends on the doping level, being W ∝ ND-1/2 . In general, if Φm>Φs, a rectifying contact is formed. For lightly doped semiconductors (< 1×1017 cm-3) the main conduction mechanism is the thermoionic emission, i.e. the carriers having sufficient thermal energy to surmount the Schottky barrier can pass from a material to the other. In this case, the application of a voltage V across a metal-
3
semiconductor junction, according to the thermoionic emission theory,21 leads to a current density J through the contact given by:
J=A T e **
2
−
qΦ B kT
qV ⎞ ⎛ nkT ⎜ e − 1⎟ ⎟ ⎜ ⎠ ⎝
(2)
where A** is the effective Richardson constant, q is the electron charge, k is the Boltzmann constant, T the absolute temperature and n is the ideality factor. If the condition Φm1×1019 cm-3), kT/E00
