Artificially ordered Bi/Sb superlattice alloys: Fabrication and transport properties

Abstract

We have fabricated Bi/Sb superlattice alloys that are artificially ordered on the atomic scale using molecularbeam epitaxy. We observe that by changing the superlattice period thickness, the electronic structure can be ''tuned'' from a semimetal, through zero gap, to a narrow-gap semiconductor. These unique properties, which are distinct from those in random alloys, are believed to be a consequence of ordered atomic configurations.

Thermoelectric transport properties of n-doped and p-doped Bi0.91Sb0.09 alloy thin films

“…27,28 Differences between thin films grown on CdTe͑111͒ and bulk alloy may arise from the effects of strain. 27,28 In addition, in BiSb alloy thin films grown on CdTe͑111͒, the Sb concentration with the largest power factor has also shifted to lower Sb concentration ͑7%-9% Sb͒ and the power factor peaks at a significantly higher temperature ͑250 K͒ than previously reported for the bulk alloy ͑80 K͒. In Fig.…”

“…The 9% Sb concentration was chosen since that alloy shows the highest resistivity versus temperature, arising from a maximal effective thermal band gap of E g0 ϭ40 meV determined by the temperature dependent electrical resistivity. 27,28 For the undoped ͑intrinsic͒ sample the electron contribution to the conduction is dominant since the mobility of the electrons is an order of magnitude higher than that of the holes. The addition of the group VI Te donor to the semiconducting BiSb increases the Fermi level, effectively resulting in a one-carrier material.…”

“…The sample preparation is reported elsewhere. [22][23][24][25][26][27][28] The growth direction of the Bi 0.91 Sb 0.09 alloy film on CdTe͑111͒B is parallel to the trigonal axis. We first deposited a 3000 Å CdTe buffer layer on the CdTe substrate at 250°C, followed by deposition of the Bi 0.91 Sb 0.09 layer at a rate of 0.4 Å/s and at a growth temperature of 83°C.…”

“…6 In 1 m thick Bi 1Ϫx Sb x alloy thin films on CdTe͑111͒B, we have observed several differences relative to the bulk system. 27,28 The 3.5% and 5.1% Sb alloys ͑corresponding to semimetals in bulk͒ show semiconducting behavior. ͓As mentioned in the Introduction, the semiconducting behavior in bulk occurs at alloy compositions between 7% and 22% Sb with a maximum band gap ͑18-20 meV͒ at xϭ0.12-0.15.]…”

Thermoelectric figure of merit as a function of carrier propagation angle in semiconducting superlattices

“…1 Approaches involving the use of thin-film structures, 2 such as those based on quantum-confinement effects, 3 phonon-blocking/electrontransmitting superlattices, 4-6 and heterostructures, 7,8 are promising candidates for ZT enhancement. Superlattices in particular have made great strides, [9][10][11][12][13][14][15] with ZT ¼ 2.4 reported in p-type Bi 2 Te 3 /Sb 2 Te 3 superlattices at 300 K. 9 One difficulty in raising ZT is that the phonon and electron transport properties cannot be independently tuned since they arise from the same underlying crystal structure. Furthermore, it has generally been thought that the best directions in a superlattice are the cross-plane direction and the direction parallel to the planes and that the best of these two orientations is the one with the highest mobility.…”