**1. Introduction**

The fact that ohmic contacts provide an almost unimpeded transfer of majority carriers across an interface makes them an essential part of nanoelectronic device fabrication. The interface control processes of producing ohmic contacts in germanium-based technology, such as the local incorporation of dopant atoms at the metal-germanium interface and the insertion of an interlayer into the interface, result in contacts that have values of resistivity which are very sensitive to the interlayer thickness and the temperature of annealing used during the fabrication process. These aspects of the interface control processes will be examined in this chapter.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

We present a review of some of the novel interface control processes developed for the fabrication of NiGe/*n*-Ge and PdGe/*n*-Ge Schottky and ohmic contacts.

**2.2. NiGe contacts**

the Ge substrate.

*2.2.1. Cyclically stacked NiGe contacts*

cyclically stacked ones, were annealed in nitrogen (N<sup>2</sup>

One of the concerns regarding NiGe contacts on *n*-type Ge substrates is that other phases of the Ni/Ge system apart from NiGe are formed below 250°C. Another concern is the reaction of the deposited Ni film and the Ge substrate, which increases the interface roughness. Suppression of this interface reaction by the use of cyclic stacking, as explained in Section 2.1.3 of the previous chapter, is advantageous in obtaining a flat interface between NiGe and the Ge substrates. This was done in an investigation carried out by Motoki [12]. The wafers used in this study were *n*-type Ge (100) with a doping density of 4.0 × 10<sup>16</sup> cm−3. These substrates were treated with HF after which sets of Ni/Ge (0.5 nm/1.3 nm) layers were cyclically stacked eight times using RF magnetron sputtering. The thickness of the layers corresponded to an atomic ratio between Ni and Ge of 1 to 1, as in the phase NiGe. As explained in Section 2.1.3 of the previous chapter, the concept behind this process is to suppress the interface reaction, upon annealing, between the deposited Ni and the Ge substrate, hence reducing the number of interface electron energy states. The samples configuration is illustrated in **Figure 1**.

Interface Control Processes for Ni/Ge and Pd/Ge Schottky and Ohmic Contact Fabrication: Part Two

Two samples of cyclically stacked Ni/Ge were produced, one with 8 Ni/Ge cycles (referred to as sets in the figures) and the other with 16 cycles. In order to see if cyclic stacking produces improved results, two other samples were prepared with Ni films of thickness 3.0 and 5.5 nm respectively on Ge substrates without cyclic stacking, for comparison. The four types of samples, including the

from 200 to 500°C for 1 min. Four-terminal sheet resistance measurements were carried out on the samples as explained in Section 2.3 of the previous chapter. **Figure 2** shows experimental results of the sheet resistivity (*ρsh*) of the films as a function of the annealing temperature. We see a large decrease in sheet resistivity for the sample with a 3.0 nm-thick Ni film and no cyclic stacking within the temperature range from 200 to around 300°C. This is attributed to the formation of the NiGe phase. When the annealing temperature is over 350°C, the sheet resistivity shows a large increase owing to thermal instability. In the sample with a 5.5 nm-thick Ni layer and no cyclic stacking, the temperature range of the NiGe thermal phase stability is wider than that for the

**Figure 1.** Cyclically stacked samples to suppress the interface reaction, upon annealing, between the deposited Ni and

) gas at annealing temperatures that ranged

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