Thin films of Cu2ZnSnS4 (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been deposited by spray pyrolysis technique onto soda‐lime glass substrates held at a substrate temperature (Ts) of 643 K. The effect of copper salt and thiourea concentrations on the formation of Cu2ZnSnS4 thin films is investigated. CZTS films formed under optimized conditions are found to be polycrystalline in nature with kesterite structure. The lattice parameters are found to be a = 0.543 nm and c = 1.086 nm. The optical band gap of these films is found to be 1.43 eV. It is found to increase with decrease in copper salt concentration in the solution.

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The Dirac surface states of topological insulators offer a unique possibility for creating spin polarized charge currents due to the spin-momentum locking. Here we demonstrate that the control over the bulk and surface contribution is crucial to maximize the charge-to-spin conversion efficiency. We observe an enhancement of the spin signal due to surface-dominated spin polarization while freezing-out the bulk conductivity in semiconducting Bi 1.5 Sb 0.5 Te 1.7 Se 1.3 below 100 K. Detailed measurements up to room temperature exhibit a strong reduction of the magnetoresistance signal between 2 − 100 K, which we attribute to the thermal excitation of bulk carriers and to the electron-phonon coupling in the surface states. The presence and dominance of this effect up to room temperature is promising for spintronic science and technology.

The effect of substrate temperature and post-deposition annealing on the growth and properties of Cu 2 ZnSnSe 4 thin films, a potential candidate for a solar cell absorber layer, is investigated. The substrate temperature (T s ) is chosen to be in the range 523-673 K and the annealing temperature (T pa ) is kept at 723 K. Powder x-ray diffraction (XRD) patterns of as-deposited films revealed that the films deposited at T s = 523 K and 573 K contain Cu 2−x Se as a secondary phase. Single phase, polycrystalline Cu 2 ZnSnSe 4 films are obtained at T s = 623 K and films deposited at T s = 673 K have ZnSe as a secondary phase along with Cu 2 ZnSnSe 4 . Direct band gap of as-deposited CZTSe films is found to lie between 1.40 eV and 1.65 eV depending on T s . XRD patterns of post-deposition annealed films revealed that the films deposited at T s = 523-623 K are single phase CZTSe and films deposited at T s = 673 K still contain ZnSe secondary phase. CZTSe films are found to exhibit kesterite structure with the lattice parameters a = 0.568 nm and c = 1.136 nm. Optical absorption studies of post-deposition annealed films show that there is a slight increase in the band gap on annealing, due to decrease in the Cu content. Electrical resistivity of the films is found to lie in the range 0.02-2.6 cm depending on T s .

Phase change alloys are used for non-volatile random access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase. However, this contrast has never been directly related to the electronic band structure. Here, we employ photoelectron spectroscopy to map the relevant bands for metastable, epitaxial GeSbTe films. The constant energy surfaces of the valence band close to the Fermi level are hexagonal tubes with little dispersion perpendicular to the (111) surface. The electron density responsible for transport belongs to the tails of this bulk valence band, which is broadened by disorder, i.e., the Fermi level is 100 meV above the valence band maximum. This result is consistent with transport data of such films in terms of charge carrier density and scattering time. In addition, we find a state in the bulk band gap with linear dispersion, which might be of topological origin. arXiv:1708.08787v2 [cond-mat.mtrl-sci] 17 Jan 2018 AUTHOR CONTRIBUTIONS M.M. provided the idea of the experiment. J.K., M.L. and C.P. carried out all (S)ARPES experiments under the supervision of E