First Principles Study of Metal/Bi 2 Te 3 Interfaces: Implications to Improve Contact Resistance
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1166-N05-01
First principles study of metal/Bi2Te3 interfaces: implications to reduce contact resistance Ka Xionga, Weichao Wanga, Husam N. Alshareefa,c, Rahul P. Guptaa, John B. Whited, Bruce E. Gnadea and Kyeongjae Choa,b,* a
Materials Science & Engineering Dept, The University of Texas at Dallas, Richardson, TX 75080, USA b Physics Dept, The University of Texas at Dallas, Richardson, TX 75080, USA c Materials Science & Engineering Dept, King Abdullha University of Science and Technology, Thuwal, Saudi Arabia d Marlow Industries, 10451 Vista Park Road, Dallas, TX 75238, USA
*[email protected] ABSTRACT We investigate the band offsets and stability for Ni/Bi2Te3 and Co/Bi2Te3 interfaces by first principles calculations. It is found that the surface termination strongly affects the band offsets. Ni and Co are found to form Ohmic contacts to Bi2Te3. The interface formation energies for Co/Bi2Te3 interfaces are much lower than those of Ni/Bi2Te3 interfaces. Our calculations are consistent with the experimental data.
INTRODUCTION The solid-state thermoelectric (TE) cooler has been of great interest in many applications because of its advantages of reliability, low-noise operation, miniaturization and high power density1-3. Commercial TE cooling devices use doped Bi2Te3 ((Bi,Sb)2Te3 for p-type and Bi2(Te,Se)3 for n-type) as the TE elements due to its high figure of merit (ZT ~1)4 at room temperature. To allow TE coolers to reach the next level of performance in terms of efficiency and power density, the size of the device needs to be scaled down. Consequently, the electrical resistance between the contact metal and the TE material plays an important role in the device performance5, as the material “figure of merit” (Z) is degraded by the contact resistance and the relationship between Z and ZD (device “figure of merit”) can be shown in the following equation L ZD = (1), Z L + 2rcσ
where L is the device length, rc is the contact resistance and σ is the bulk conductivity. The resulted maximum coefficient of performance (COP) of the device can be expressed as6 T 1 + Z DT − H Q TC TC COP = C = × (2), W TH − TC 1 + Z DT + 1 where QC is the heat pumping capacity, W is the input power. TC and TH represent the temperature of the cold side and the hot side, respectively. T is the average temperature. Therefore, from a device point of view, although a high Z of the material has been achieved, the COP can still be low due to the degradation of Z by the contact resistance etc. So merely seeking how to improve Z is not adequate. For current bulk TE devices, electroless Ni has been used as the contact metal with a contact resistance (rc) of ~ 5x10-6 Ω cm2. The contact resistance needs to be at least 10-100 times lower in order to maintain the required device scaling7. To see why the contact resistance is critical, we can do a simple quantitative analysis on how the contact resistance influences the device dimensions and hence the COP. The electrical conductivity (σ) of Bi2Te3 at room temperature is of order 1×103 (Ω•c
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