We investigate the electronic structures and stability for Ni/Bi2Te3, NiTe/Bi2Te3, Co/Bi2Te3 and CoTe2/Bi2Te3 interfaces by first-principles calculations. It is found that the surface termination strongly affects the band alignment. Ni and Co are found to form Ohmic contacts to Bi2Te3. The interface formation energy for Co/Bi2Te3 interfaces is much lower than that of Ni/Bi2Te3 interfaces. Furthermore, we found that NiTe on Bi2Te3 is more stable than Ni, while the formation energies for Co and CoTe2 on Bi2Te3 are comparable.

A detailed study of the impact of surface preparation and postdeposition annealing on contact resistivity for sputtered Ni and Co contacts to thin-film Bi 2 Te 3 is presented. The specific contact resistivity is obtained using the transfer length method. It is observed that in situ sputter cleaning using Ar bombardment before metal deposition gives a surface free of oxides and other contaminants. This surface treatment reduces the contact resistivity by more than 10 times for both Ni and Co contacts. Postdeposition annealing at 100°C on samples that were sputter-cleaned further reduces the contact resistivity to Ͻ10 Thermoelectric ͑TE͒ coolers have been extensively used in the optoelectronic, automotive, space, and semiconductor industries where low device operational temperature is key to device performance in terms of speed and reliability.1-3 However, despite these advantages the use of TE devices has been limited for high watt density applications. 4 To allow TE coolers to reach the next level in terms of performance and power density, the thickness of the device needs to be scaled down.5 Contact resistance becomes a serious limitation to the efficiency of TE material based solid-state coolers with thermoelement leg lengths Ͻ100 m, as can be seen in Fig. 1, where the ratio of device figure-of-merit ͑Z d ͒ and material figureof-merit ͑Z m ͒ is plotted vs the thermoelement leg length ͑L͒ as a function of contact resistance. 6 The relationship between Z d and Z m is given by Eq. 1where L is the thermoelement leg length, r c is the contact resistance, and is the bulk conductivity.As can been seen in Fig. 1a, for a device leg length of 100 m, Z d /Z m drops from 0.9 to 0.5 as the contact resistivity increases from 5 ϫ 10 −7 to 5 ϫ 10 −6 ⍀ cm 2 . The resulting drop in the device dimensionless figure-of-merit Z d T impacts the coefficient of performance ͑COP͒ of the device as shown in Fig. 1b, which can be expressed aswhere T c and T h represent the temperature of the cold side and the hot side, respectively. Therefore, from a device point of view, although a high Z material can be achieved, the device COP can still be low due to the degradation of Z due to the contact resistivity. For thin TE materials, the losses become even more extreme and low electrical contact resistivity of Ͻ10 −7 ⍀ cm 2 is needed to minimize the impact of contact resistance on COP. High contact resistance at the electrode/TE material interface remains a challenge that is limiting widespread adoption of TE technology in many applications.For industry standard TE devices, electroless plated Ni is used as a diffusion barrier to Cu and solder components such as Sn. However, electroless Ni gives a relatively high contact resistivity ͑Ͼ5 ϫ 10 −6 ⍀ cm 2 ͒. 9 Bi 2 Te 3 is a small bandgap semiconductor with a bandgap of 0.16 eV. Therefore it is theoretically possible to obtain a very low contact resistance, Ͻ10 −7 ⍀ cm 2 , for an ideal metalsemiconductor contact; however, real metal-semiconductor interfaces are far more complex.10 Extrinsic factors ...

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