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Platinum Metals Rev., 1999, 43, (1), 2

Platinum Metals in Ohmic Contacts to III-V Semiconductors

  • D. G. Ivey
  • Department of Chemical and Materials Engineering, University of Alberta, Canada

Article Synopsis

Platinum metals, particularly platinum and palladium, are used extensively in low resistance, relatively stable and laterally uniform ohmic contacts made to semiconductor devices formed from Group III and V elements. Palladium is primarily utilised as one component of a multilayer metallisation structure, improving adhesion to the semiconductor, while platinum is commonly used as a diffusion barrier to minimise interdiffusion between metal and semiconductor components. This paper describes the roles of platinum and palladium in ohmic contacts, gives examples of their typical utilisation and outlines the design considerations associated with the formation of reliable ohmic contacts.

Compound semiconductors formed from elements in Groups III and V (III-V) of the Periodic Table, such as GaAs and InP and related ternaries and quaternaries, have become important materials for a number of microelectronic and optoelectronic applications. These include field effect transistors (FET), junction field effect transistors (JFET), high electron mobility transistors (HEMT) and heterojunction bipolar transistors (HBT), as well as photonic devices, such as long-wavelength laser diodes, light-emitting diodes (LED) and photodetectors and solar cells.

These devices are operated at high current densities and, as technology develops, are continually undergoing a reduction in size. In order to link the active regions of the semiconductor devices to the external circuit, contacts with low electrical resistance (ohmic contacts) are required. The small dimensions of these devices place severe processing constraints on the materials used in the construction of these ohmic contacts. The main requirements of ohmic contacts are that they should have low contact resistance, be thermally stable and have good adhesion and lateral uniformity. These are all directly affected by the microstructure of the contact.

Selected Ohmic Contacts to III-V Semiconductors

Contact Doping level, cm-3 Minimum contact resistance, rc, Ω cm2 Annealing temperature, °C Refs.
n -Type Pd-InP 1 × 1018 7 × 10-5 300 − 350 1,2
Pd/Ge/Au-InP 1 × 1017 2.5 × 10-6 300 − 375 3
Pd/Ge-InP 1 × 1017 6 × 10-6 400 − 450 3
Pd/Ge-GaAs 5 × 1018 4 × 10-7 400 − 415 4
p -Type Pd/Pt/Au/Pd-InGaP/GaAs 3 × 1019 < 10-6 415 − 440 5
Zn/Pd/Pt/Au-AIGaAs/GaAs 8.5 × 10-7 440 6
Ti/Pt/Au-InGaAs 5 × 1018 6 × 10-6 450 7
Ti/Pt-InGaAs 5 × 1018 4 × 10 -6 450 8
Ti/Pt-InGaAs 1.5 × 1018 3.4 × 10-8 450 9
Pd/Zn/Pd/Au-InP 2 × 1018 7 × 10-5 420 − 425 10

Platinum metals are used extensively in ohmic contacts to III-V semiconductors, as diffusion barriers or reactive components in multilayer metallisation schemes. Examples of typical contacts are given in the Table, together with their minimum contact resistances.

Formation of Ohmic Contacts

Metal-semiconductor contacts can be divided into two types, based on their current-voltage characteristics. Contacts which show rectifying characteristics are called Schottky barriers or rectifying contacts, and contacts which have linear current-voltage behaviour are referred to as ohmic contacts. In practice, a contact is ohmic if the voltage drop across it is negligible compared with the voltage drop across the device or specimen – the contact is thus not affecting the current/voltage (I/V) characteristics of the device. There are several methods for forming low resistance ohmic contacts, but manufacture of the most common commercial contacts always involves the formation of a very heavily doped (> 5 × 1018 cm-3) thin layer of semiconductor material on the semiconductor surface, immediately adjacent to the metal, see Figure 1. The depletion region produced at the interface between the metal and the semiconductor is then so thin that field emission or tunneling of charge carriers may take place, giving a contact with a very low resistance at zero bias.

Fig. 1

(a) Schematic of an ohmic contact structure to an n -type semiconductor;

(b) Simplified plot of doping concentration of semiconductor in (a);

(c) Simplified energy band diagram of metallisation/semiconductor interface region

Production of the highly doped surface can be achieved in two ways. One method involves growing a heavily doped epitaxial layer, for example, by chemical vapour deposition (CVD) on the semiconductor prior to metal deposition. The epitaxial layer is often a material with a lower bandgap, such as In0.53Ga0.47As (Eg = 0.75 eV) which is lattice matched to InP, or a graded InxGa1-xAs layer (graded to InAs at the surface) (11). These contacts should not, at least in theory, require subsequent annealing and are therefore referred to as non-alloyed contacts. In practice, most of these contacts are still annealed to attain optimum values of contact resistance.

The other method of producing the doped, thin layered surfaces is by using an external dopant. The dopant, for example, zinc for p -type and germanium for n -type semiconductors, is deposited as a thin layer (of a few to tens of nanometres in thickness) on the semiconductor by electron beam evaporation and is usually driven into the semiconductor by solid state diffusion, which requires heating the contact. These contacts are termed alloyed contacts and the dopant metal is usually part of a multilayer metallisation scheme, the other possible metal layers including adhesion layers, diffusion barriers and capping layers to prevent oxidation during annealing.

In designing contacts to III-V semiconductors a number of factors have to be considered (12), the most important probably being the contact resistance. Several other important factors have to be taken into account, as contacts should be stable over a wide temperature range, have good lateral uniformity and shallow diffusion depths. These aspects have become particularly important as device dimensions are continually being decreased in size. However, it is difficult to meet all the requirements in a given contact metallisation. In order to achieve low contact resistance, interdiffusion between the metallisation components and the semiconductor is needed and this can compromise contact stability and uniformity.

The electrical properties of ohmic contacts are defined by a specific contact resistance, rc, (13):

(i)

where V is the voltage and J is the current density. For a homogeneous contact of area A with a uniform current density, the contact resistance Rc is simply:

The measured resistance R will be approximately equal to Rc for most sample geometries when rc ≥ 10-2 Ω cm2. For lower values of rc, the spreading resistance of the semiconductor and the series resistance of the connecting wires and semiconductor substrate must be taken into account. Specific contact resistances of < 10 4 Ω cm2 are required for ohmic behaviour, although values in the 10-6 to 10-7 Ω cm2 range are most desirable.

Platinum Metals in Ohmic Contacts

Palladium and platinum are common components in ohmic contact metallisation schemes to III-V semiconductors. Many of the original III-V ohmic contacts were gold-based and although these contacts possess low resistance values, they are plagued by problems of stability and uniformity of interface structures. Platinum metals based contacts are generally less reactive and produce more uniform interfaces. Common features of the platinum metals include ease of deposition, good oxidation resistance, relatively low reaction temperatures with III-V compound semiconductors (which greatly enhances contact adhesion to the semi-conductor) and compound formation with both the Group III and Group V elements (12). Reactions which occur during annealing are quite similar and are summarised below:

  • Palladium and platinum react with compound III-Vs at low temperatures to form metal-rich ternary phases. Many of these are amorphous, particularly those with InP.

  • Annealing of the ternary phases results in their decomposition into binary phases and/or elemental species.

Portions of isothermal sections (at 600°C) of the phase diagrams, determined from bulk specimens for Pd-In-P, Pt-In-P, Pd-Ga-As and Pt-Ga-As systems are shown in Figures 2 and 3. Note, the similarities between the systems, particularly the three-phase region bounded by InP or GaAs, and the binary phases with the Group III and Group V elements. Annealing of thin palladium or platinum layers on the III-V semiconductor should then result in the formation of the phases in the three phase region.

Fig. 2

Isothermal sections at 600°C of ternary phase diagrams (14–16):

(a) Pd-In-P

(b) Pt-In-P

Fig. 3

Isothermal sections at 600°C of ternary phase diagrams (14–16):

(a) Pd-Ga-As

(b) Pt-Ga-As

Palladium in Ohmic Contacts

Palladium can be utilised on its own to form ohmic contacts to III-V semiconductors (for example, Pd/n -type InP with a contact resistance of 7 × 10-5 Ω 2 cm2 (1, 2)). However, palladium is generally one component in a multilayer metallisation, where its primary functions are to serve as an adhesion layer and to initiate reactions with the underlying semiconductor. The annealing behaviour of a layer of palladium (several tens of nanometres thick) on InP or GaAs plays an important role in how the contact functions.

In the Pd/InP system, see Figure 2(a), an amorphous ternary phase (Pd=3InP) forms during palladium deposition and grows on annealing (17, 18). At ≈ 225°C, crystalline islands of cubic Pd2InP (L12-type structure) begin to nucleate and grow at the amorphous layer/InP interface. After annealing at 225 to 275°C, Pd2InP grows into a continuous, epitaxial layer (40–50 nm thick). Two other ternary phases, Pd5InP and Pd2InP(II) (of composition and structure similar to Pd2InP) then form and also grow epitaxially. At 300 to 350°C, all the palladium is consumed and Pd2InP(II) is the only phase present on InP.

For the Pd/GaAs system, see Figure 3(a), a hexagonal ternary phase, Pd5(GaAs)2, forms and grows epitaxially on GaAs during palladium deposition (19, 20). Growth of this phase continues during subsequent annealing (< 250°C), with palladium being the dominant diffusing species. A second ternary phase, Pd4(GaAs), forms at higher annealing temperatures (up to ≈ 350°C) (19, 20). This phase also demonstrates an orientation relationship with GaAs. Growth of the ternary phases consumes the palladium by temperatures of ≥ 300 to 325°C.

Further annealing of palladium metallisations results in decomposition of the ternary phases and subsequent formation of binary compounds. In the case of GaAs: PdGa and PdAs2 have been reported to form in one study (19), while Pd2Ga and Pd2As were reported in another study (20). According to the Pd-Ga-As phase diagram, Figure 3(a), PdGa and PdAs2 are the stable phases in contact with GaAs.

Annealing samples of Pd/InP at temperatures ≥ 400°C results in decomposition of Pd2InP(II) to PdIn and PdP2 (17), which are the equilibrium phases in contact with InP (Fig. 2 (a)).

Specific Contacts

As mentioned above, palladium can be used by itself in ohmic contacts, however, its use in this way is rare. Instead, in practice, palladium is utilised as an integral component in many contacts, with its primary role being that of an adhesion layer to the III-V semiconductor. Ge/Pd One particular contact structure of interest is Ge/Pd, which was originally applied to n -type GaAs (21–24) (and later to InP, see below (3, 25)). Ohmic contact formation is based on a solid phase regrowth mechanism (21). According to this mechanism, films of two elements, M and M′ (palladium and germanium for this specific case) are deposited sequentially on a III-V semiconductor, AB (such as GaAs). At low annealing temperatures, the M metal film adjacent to the semiconductor reacts with AB to form a ternary phase MxAB:

The second film (M′ or Ge) is chosen so that it drives the decomposition of MxAB and forms a stable binary compound with M. M′ should also be a suitable dopant for AB.

AB regrows, doped with M′, epitaxially on the AB substrate. For the Ge/Pd-GaAs system annealing temperatures are in the range 300 to 500°C. In this contact, the onset of ohmic behaviour corresponds to the decomposition of Pd4GaAs, which is driven by inward diffusion of germanium, and results in a highly doped regrown semiconductor layer. Specific contact resistances of the order of 5 × 10-7 to 1 × 10-6 Ω cm2 have been achieved for both n -type and p -type GaAs, see the Table. Selecting the correct Ge:Pd ratio is crucial for fabricating uniform and stable contacts: the Ge:Pd atomic ratio must slightly exceed 1. This ensures that the only stable binary phase that forms is PdGe and this prevents the formation of other binary phases, such as Pd2Ge, PdAs2 and PdGa. In addition, any excess germanium is available for doping purposes.

For actual devices, comprising Pd/Ge contacts to n -type GaAs, a capping layer (for example, titanium/platinum) is needed to prevent the oxidation of germanium during subsequent device processing (4). Figure 4(a) shows an image of a platinum/titanium capped sample, while Figure 4(b) shows contact oxidation and deterioration for an uncapped sample. The Ti/Pt remains inert relative to the ohmic contact until annealing above ≈ 550°C, whereupon complex reactions result in rapid deterioration of the microstructure (26). Figure 4(c) shows the initial decomposition of PdGe at 550°C. Pd/Ge-InP Similar Pd/Ge contacts have since been fabricated to n -type InP (3, 25). Pd2InP forms initially followed by decomposition to PdGe and regrown InP. Contact resistances in the region of 10-6 Ω cm2 have been achieved, see the Table.

Fig. 4

TEM cross-section images of Pd/Ge contacts to n -type GaAs:

(a) with a Ti/Pt capping layer

(b) without a Ti/Pt capping layer (4)

(c) the contact in (a) has been annealed at 550°C and shows initial decomposition of PdGe (26)

Platinum in Ohmic Contacts

Reactions for the Pt/InP system are similar to those for the Pd/InP system. The main difference is that the reaction temperatures tend to be higher for platinum, which is not surprising as platinum has a higher melting point than palladium. Ternary amorphous phase formation begins at about 325°C, followed at ≈ 350°C by the formation of a crystalline ternary phase (Pt5InP) (27). Pt5InP does not form at the amorphous layer/semiconductor interface, but at the amorphous layer/metallisation interface, and as such exhibits no preferred orientation with the InP substrate. This is in contrast to Pd/InP where crystalline ternary phases nucleate, and then grow with a preferred orientation, at the semiconductor surface. A second ternary phase forms at 400°C which is followed by decomposition of both ternary phases to binary phases at temperatures higher than 450°C. The final phases are PtIn2 and PtP2, which agree with those expected from the bulk Pt-In-P phase diagram, see Figure 2 (b) (14, 28).

Pt/GaAs reactions, see Figure 3(b), are similar to those of Pt/InP, with PtAs2 and PtGa binary phases forming at temperatures ≈ 550°C (29–31).

Non-Alloyed Contacts The main application of platinum in ohmic contacts to III-V semiconductors (including InGaAs and InGaAsP) is in the so-called non-alloyed contacts. These usually consist of titanium and platinum layers, with or without a gold capping layer, deposited sequentially on the semiconductor (7, 8, 32). Gold, which can be deposited either before or after contact annealing, is needed to permit wire bonding and solder bonding of the device to a suitable submount. Deposition of gold prior to annealing permits deposition of all the metal layers in one sequence, without breaking the vacuum, although gold may reduce contact stability during subsequent annealing.

The titanium layer improves adhesion to the underlying semiconductor, while the platinum layer acts as a diffusion barrier for gold. Good contacts to a number of semiconductors have been obtained using this scheme, see the Table, giving contact resistance values depending very much on the initial doping levels in the semiconductors. The original doping levels in the semiconductors are generally quite high, so that many of these contacts are ohmic without being annealed or require very little annealing to attain ohmic behaviour, hence, the term non-alloyed contacts.

In reality, many of these contacts are annealed to optimise the electrical properties and therefore undergo fairly significant interfacial reactions, making the terminology “non-alloyed” something of a misnomer. An example of one of these contacts to p-type InP, and the inter-facial reactions involved, is discussed below.

Ti/Pt/Au is the standard p -side metallisation used for a number of InP-based laser and optical wave-guide devices. p -Type InGaAs is grown as a lattice matched capping layer (In0.53Ga0.47AS) on InP. Its low band gap (Eg ≈ 0.75 eV) relative to InP (Eg = 1.35 eV) and high dopant solubility (≈ 1010 cm-3 for zinc) permit the formation of low resistance contacts. Specific contact resistances as low as 3 × 10-8 Ω cm2 (for high doping levels) have been reported, see the Table.

A series of TEM cross-section micrographs, showing microstructure development during annealing are shown in Figure 5 (33). Reaction between titanium and InGaAs begins at ≈ 275°C and shows up as small dark contrast features at the Ti/InGaAs interface, Figure 5(b). These have been identified as metallic indium particles, which nucleate uniformly across the Ti/InGaAs interface and grow to ≈ 100 nm in size, see Figures 5(c) and 5(d). The indium particles are pyramidal in shape and grow along low energy {110} and {111} planes, Figures 5(d) and 6. Indium is a low melting point metal (157°C), which can dissolve up to ≈ 1.4 weight per cent gallium and up to ≈ 1 weight per cent zinc (the dopant in InGaAs). Both the gallium and zinc impurities lower the melting point of indium, which can affect the long term stability of devices. TiAs also forms at the Ti/InGaAs interface and thickens with increasing annealing temperature, see Figure 5(d). Depletion of indium and arsenic from the InGaAs layers results in the formation of gallium-rich InGaAs.

Fig. 5

TEM cross-section micrographs of Ti/Pt/Au contacts to p -type InGaAs (33):

(a) as deposited; (b) 275°C; (c) 300°C; (d) 350°C; (e) and (f) are schematics of (b) and (d), respectively

Fig. 6

TEM plan view images of indium particles at the Ti/InGaAs interface (33):

(a) 275°C and (b) 350°C

Platinum is an effective barrier, preventing inward diffusion of gold and outward diffusion of the Group III elements (indium and gallium), up to annealing temperatures of 450°C. At higher temperature significant interdiffusion between titanium and platinum occurs.

Some metallisations incorporate both palladium and platinum. One example was given previously (Pd/Ge/Ti/Pt to n -type GaAs, in Figure 4(a)), although the platinum was essentially inert and acted solely as a capping layer. However, in some instances, both palladium and platinum participate in metallisation/semiconductor reactions. This is shown in both Figures 7(a) and 7(b) for a multilayer p-contact Pd/Pt/Au/Pd, utilised for InGaP/GaAs heterojunction bipolar transistors (HBT) (5). The metal layers are deposited directly onto the emitter layer (InGaP) of the HBT, with palladium and platinum reacting with and consuming the InGaP during annealing to form an ohmic contact with the underlying base material (p-type GaAs). Lateral uniformity is particularly importam because of the thin nature of the base layer (< 100 nm). An outer layer of palladium protects the gold during subsequent device processing; dry etching sputters the gold but not the palladium. The doping level in the base layer is high enough (> 1019 cm-13) that additional dopant is not required to achieve low contact resistances. The initial reaction between palladium and platinum and InGaP results in the formation of a 5-component amorphous layer (Pd, Pt, In, Ga and P), which crystallises to (PtxPd1-x)5(InyGa1-y)P (0 ≤ x, y ≤ 1) at the Pt/amorphous layer interface. Annealing at temperatures > 415°C results in the complete consumption of InGaP and partial decomposition of the GaAs base layer, resulting in a relatively uniform contact consisting of (PtxPd1-x)5(InyGa1-y)P, PtAs2 and PdGa, see Figure 7 (b). In this structure, contact resistance values < 10-6 Ω cm2 are attainable.

Fig. 7

TEM cross-section images and related schematics of Pd/Pt/Au/Pd contact to an InGaP/GaAs heterojunction bipolar transistor (5):

(a) as deposited and (b) annealed at 440°C

Summary

Platinum and palladium are common components in ohmic contacts to III-V semiconductors. The metals react readily with the semiconductor at relatively low temperatures, forming ternary phases initially, followed by binary phase formation. Palladium is utilised primarily as an adhesion layer to the semiconductor, while the major role of platinum is as a diffusion barrier. In both cases, palladium and platinum act to improve contact stability and hence the reliability of the device to which they are attached.

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