The Hall Carrier Mobility of AgBiTe 2 -Ag 2 Te Composite
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The Hall Carrier Mobility of AgBiTe2-Ag2Te Composite T. Sakakibara1,2, Y. Takigawa3, and K. Kurosawa4 1 Second Development Department, Aisin Seiki Co., Ltd., Kariya, 448-8650, Japan 2 Department of Materials Science and Energy Engineering, University of Miyazaki, Miyazaki, 889-2192, Japan 3 Department of Electronic Engineering, Osaka Electro-Communication University, Neyagawa, 572-8530, Japan 4 Department of Electrical and Electronic Engineering, University of Miyazaki, Miyazaki, 889-2192, Japan ABSTRACT We prepared a series of (AgBiTe2)1-x(Ag2Te)x (0≤x≤1) composite materials by melt and cool down [1]. The Hall coefficient and the electrical conductivity were measured by the standard van der Pauw technique over the temperature range from 93K to 283K from which the Hall carrier mobility was calculated. Ag2Te had the highest mobility while the mobility of AgBiTe2 was the lowest of all samples at 283K. However the mobility of the (AgBiTe2)0.125(Ag2Te)0.875 composite material was higher than the motility of Ag2Te below 243K. It seems that a small second phase dispersed in the matrix phase is effective against the increased mobility. INTRODUCTION Laser diodes are a key device to realize higher speed and larger capacity optical communication systems. Their performance largely depends on temperature control. In order to precisely control the temperature, one of the most promising methods is thermoelectric devices. However, the figure of merit for thermoelectric materials have not been remarkably improved for over forty years [2].The thermoelectric figure of merit, Z, is characterized by Z=α2σ/κ where α is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity. Good thermoelectric materials have large α and σ valuse and a small κ value. Chasmar and Stratton [3] and Rittener and Neumark [4] have calculated exact Z values using generalized Fermi-Dirac functions, and derived the following equation (1): Z∝
m *3 / 2 µ
(1)
κ ph G8.26.1
where m* is the effective mass, µ is the mobility, and κph is the lattice thermal conductivity. It suggests that the ideal thermoelectric material should satisfy the somewhat conflicting requirements of high effective mass, high mobility and low lattice thermal conductivity. Generally, the effective mass has a tendency to decrease, if the mobility increases. Goldsmid [5] discussed about the properties of high µ/κph materials and obtained the following equation (2):
µ ρA ≈ const. κ ph v
(2)
where ρ is the density, A is the average atomic-weight, and v is the sound velocity. Generally, if A increases, ρ would increase and v would decrease, suggesting that µ/κph might be improved. With such a theory, heavy metal alloys such as Bi2Te3 and PbTe show high Z values. On the other hand, thermoelectric materials have their own optimum operating temperature. With regard to the optimum temperature, Mahan proposed the “10kBT rule” [6] such that a good thermoelectric material has an energy gap of 10kBTop, where Top is the operating temperature. This equation denotes
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