AVS2016 Session EM-ThP: EMPD Poster Session

Work supported by the Semiconductor Research Corporation under Contract No. 2012-KJ-2359 and the National Science Foundation under Grant No. CBET-1066231.

H. Zheng, S. W. King, V. Ryan, Y. Nishi and J.L. Shohet, Appl. Phys. Lett. 104, 062904 (2014).

2 A. Pusel, U. Wetterauer, and P. Hess, Phys. Rev. 81, 645 (1998).

3 T.V. Rakhimova, A.T. Rakhimov, Yu A. Mankelevich, D.V. Lopaev, A.S. Kovalev, A.N. Vasil’eva, S.M. Zyryanov, J. Phys. D: Appl. Phys. 47, 025102 (2014).

4 J. Lee and D.B. Graves, J. Phys. D: Appl. Phys. 44, 325203 (2011)

Fabrication of high quality Indium rich- In_1-xGa_xN layers is still a challenge due to the immiscibility between the binaries InN and GaN. The lack of lattice-matched substrate is an additional challenge for the growth of In_1-xGa_xN layers. The lattice mismatch between the substrate template and the In_1-xGa_xN layer generate residual strains and threading dislocations at the interface that propagate into the In_1-xGa_xN layers, and consequently, degrade the quality of the In_1-xGa_xN epilayer. To mitigate these effects, different approaches such as the use of buffer layers (e.g. GaN, InN, AlN) between the In_1-xGa_xN layers are explored.

In this contribution, we present our findings on the effects of substrate treatments on the structural and optoelectronic properties of In_1-xGa_xN layers grown by Migration-Enhanced, Plasma-Assisted MOCVD (MEPA-MOCVD). Furthermore, the analysis consist the data for In_1-xGa_xN layers grown on different nitrided sapphire substrates as well as layers grown on sapphire substrates templates, containing InN, GaN or AlN interlayers.

The optoelectronic properties – e.g. free carrier concentration, the mobility of the carriers, high-frequency dielectric constant ε_∞, and layer thickness in the In_1-xGa_xN layers have been analyzed by simulating the reflectance spectra, obtained via Fourier Transform Infra-red (FTIR) spectroscopy, using a multilayer stack model and a dielectric function based on Lorentz-Drude model. AFM topography has been used to study the surface morphology of the layers. Raman spectroscopy has been utilized to analyze the local crystallinity (E₂(high) mode) of the In_1-xGa_xN layers as well as the composition via the shift of the A₁(LO) mode and its broadening with In_1-xGa_xN target composition.

The quaternary chalcogenide Cu₂ZnSnS₄ (CZTS) is composed of earth-abundant elements and has interesting optoelectronic properties that project it as an important candidate for thin-film photovoltaics. Among several synthesis methods, sulfurization of metallic stacked layers has been heavily used because it does not rely on toxic compounds and provides good control over the final stoichiometry of the sample [1-3]. We present an in-depth structural and compositional analysis of CZTS synthesized by this technique. A first study exhibits preferential segregation of hexagonal SnS₂ when increasing the sulfurization pressure in a closed annealing chamber. Variations in film morphology suggest that different reaction pathways take place as the pressure is raised. Formation of gaseous SnS is favored at lower pressure, while nucleation of solid SnS₂ preferentially occurs at higher pressure. In addition to this investigation, an individual grain study sheds light on the complexity of this material system. Elemental analysis shows significant grain-to-grain variations in composition despite dealing with an overall close-to-ideal stoichiometry. High resolution Raman spectroscopy indicates that this is accompanied by grain-to-grain structural variations as well. The intensity from the 337 cm^-1 Raman peak, generally assigned to the kesterite phase of CZTS, remains constant over a large area of the sample. On the other hand, signals from secondary phases at 376 cm^-1 (copper-tin-sulfide) and 351 cm^-1 (zinc-sulfide) show significant variation over the same area. These results demonstrate how a seemingly homogeneous CZTS thin film can actually have considerable structural and compositional variations that are often overlooked.

References

[1] Dhakal, T. P.; Peng, C.; Tobias, R. R.; Dasharathy, R.; Westgate, C. R. Characterization of a CZTS thin film solar cell grown by sputtering method. Sol. Energy2014, 100, 23–30.

[2] Hong, S.; Kim, C. Characteristics of Cu2ZnSnS4 Thin Films Fabricated by Sulfurization of Two Stacked Metallic Layers. Mol. Cryst. Liq. Cryst.2014, 602, 134–143.

[3] Cheng, A.-J.; Manno, M.; Khare, A.; Leighton, C.; Campbell, S. A.; Aydil, E. S. Imaging and phase identification of Cu2ZnSnS4 thin films using confocal Raman spectroscopy. J. Vac. Sci. & Technol. A: Vacuum, Surfaces, Films2011, 29.

ABSTRACT We have shown previously that doping a poly(4-vinyl pyridine)/pyridine gel with an ester group-containing polymer (e.g. poly(butyl methacrylate)) (polymer acids as additives) expands the wavelength range of the gel photoelectrical sensitivity from the uv into the infra-red. Here we characterize the temporal response of the gel resistivity and demonstrate its ability to operate as a bi-functional bolometer. At constant room temperature, the bolometer can function as a rapidly responding IR detector. It can also respond to temperature variation produced by hot air, but with a much longer time constant. Measurements are shown in the figure below.

1. b)

   Figure. a) Time dependence of the resistance changes of the polymer gel due to IR irradiation at 1 μm. The down pointing arrows indicate switch-on of the radiation source while the up pointing arrows indicate switch-off. b) Time dependence of the resistance changes of the polymer gel in response to temperature variation produced by hot air. The maximum temperature change was 4̊C (26̊C - 30̊C).

   The fractional change in resistance caused by IR irradiation at 1mm is ΔR/R₀ = 0.13; the fractional change caused by hot air induced temperature variation - ΔR/R₀ = 0.06/1°C. The relaxation rate of the IR response following switch-off is 260%/s. The temperature-induced relaxation curve could be fit to an exponential function with two time constants – t₁=0.148s and t₂=17.11s. Analysis of the relaxation of the photo-response was limited by the time resolution of the resistance measurements, i.e. 0.02s.

   Polymer/liquid pyridine interactions² are considered to be responsible for this interesting functionality of the polymer blend and they will be discussed.

   References:

1. E. Vaganova, E. Wachtel, A. Goldberg, S.Yitzchaik, J. Phys. Chem. C, 2012, 116, 2508.

2. B. Sedlacek, J. Spevachek et al. Macromolecules, 1984, 17, 825.

Two proton fluences were chosen (~2 × 10¹⁵cm-2 and ~2 × 10¹⁴cm-2).The proton-induced effects on HfOx RR AM cell include forming rate, modification to forming voltage, resistance of high resistance state (HRS) and shifts in set/reset voltage. After proton irradiation, no RRAM cells were formed and ended up in the low resistance state (LRS) and no changes were observed in the forming voltage of irradiated RRAM cells even when exposed to very high fluence(~2 × 10¹⁵cm-2).

An increase in the resistance of HRS was observed in proton-irradiated RRAM cells. RRAM cells irradiated with 60 keV protons have a higher increase in their HRS state than RRAM cells irradiated with 5 MeV protons. The shift in values of the set voltage can be seen on the I-V characteristic of the proton-irradiated RRAM cell. It is very likely that there is an annealing process occurs and it might be a result of defect reordering after proton irradiation.

The shift in set voltage after 5 MeV proton irradiation (fuence ~2 × 1015cm-2) is from 3.5 V to 7 V. The shift in set voltage after 60 keV proton irradiation (fluence ~2 × 10¹⁵cm-2) is from 3.5 to 11 V. Such shifts of set voltages may create problems in real device applications. These shifts a likely to be be attributed to atomic-structure changes in HfOx caused by proton irradiation.

This work was supported by the Semiconductor Research Corporation under Contract No. 2012-KJ-2359, by the National Science Foundation under Grant No. CBET-1066231.

Reference:

[1] H.-S. Philip Wong, H-Y Lee, S. Yu, Y. S. Chen, Y. Wu, P-S Chen, B. Lee, F. T. Chen, and M-J Tsai, “Metal–oxide RRAM,” Proceedings of the IEEE 100 1951 (2012).

SiGe_x is a potential semiconductor for next generation transistors because it could be incorporated into current, silicon-based semiconductor manufacturing processes and it would improve transistor performance due to the high carrier mobility of Ge. Despite these advantages, one major challenge is to reduce the number of Ge defects at the SiGe/dielectric interface because they degrade electrical performance of the transistor. While a relatively stable SiO₂ layer can be grown on Si with relatively few defects, the same is not true for Ge. One approach to forming a high quality interface is to remove Ge oxides by passivating the Ge atoms on the surface. SiGe and Ge metal oxide semiconductor capacitors (MOSCaps) were fabricated (10 nm Al₂O₃) and tested to evaluate the effect of sulfur-based chemical passivation on electrical performance. The (100) faces of three SiGe_x substrates – x = 0.25, 0.5, 0.75 – and one Ge substrate were cleaned and treated with one of two ammonium sulfide, (NH₄)₂S, wet chemistries: (NH₄)₂S/H₂O (1:100 v/v) or (NH₄)₂S/HCl/HF/H₂O (1:0.15:0.15:100 v/v). The surfaces of control MOSCaps were cleaned only. The contact area on each device was 12.6 µm².

Average capacitance in accumulation for SiGe_x MOSCaps (x = 0.25, 0.5) was 199 ± 5.4 and 205 ± 22 pF, with a DC bias sweep from -2 to +2 V at 1 MHz. The same capacitance was 42 ± 1.3 pF for SiGe_x (x = 0.75) and 457 ± 12 pF for the Ge MOSCaps. Both (NH₄)₂S treatments increased the accumulation capacitance by 6% and 34%, on average, for SiGe_x (x = 0.25, 0.5), 8% for SiGe_x (x = 0.75), and 8% for Ge MOSCaps. Similarly, V_FB shifted -2 V (SiGe_x (x = 0.25, 0.5)) and -.14 V (SiGe_x (x = 0.75) and Ge) with respect to the controls. While flatband shifting is at least due to reduction of oxide defects in the Al₂O₃ layer, the low magnitude of accumulation capacitance (according to oxide thickness calculations) suggests that there are other oxide layers present.

Half of the SiGe_x MOSCaps (x = 0.25, 0.75) were annealed in forming gas. V_FB shifted +3 V for SiGe_x MOSCaps (x = 0.25) with respect to non-annealed results, which is indicative of a reduction in negatively charged bulk oxide defects . SiGe_x (x = 0.75), and Ge MOSCaps possibly had less bulk oxide defects because their V_FB were within ± 0.5 V before and after annealing. Less bulk oxide defects in these MOSCaps suggest that nucleation and growth of the Al₂O₃ layer on these surfaces may differ from that of the SiGe_x (x = 0.25) surface. Among all three SiGe_x (x = 0.25) MOSCaps, the one treated with (NH₄)₂S and acid and annealed resulted in the flatband voltage closest to 0 V, as well as the lowest capacitance.

Solid-state direct-conversion neutron detectors, wherein a semiconductor detector heterostructure is made up of a neutron absorbing material, are capable in principle of very high neutron detection efficiencies. High-efficiency direct-conversion detectors have not yet been achieved in practice, however, because of challenges in finding suitable materials that simultaneously meet the necessary criteria, including high neutron absorption, high mobility–lifetime product, and low leakage current. Uranium-oxide-based semiconductors make up a promising class of neutron detection materials as uranium undergoes neutron-induced fission to yield very high energy primary reaction products, which can in turn create a large number of electron–hole pairs—two-to-four orders of magnitude higher than in the case of boron and lithium, the materials commonly studied for direct-conversion detectors. This additional charge can help to overcome limitations in charge transport properties, such as high leakage current and low charge carrier mobility, typically seen in candidate neutron-absorbing semiconductor materials. Of the uranium oxides, UO₂ has been studied the most, and literature reports show that its resistivity and charge carrier mobility vary widely with stoichiometry and microstructure. Very few studies on the electrical transport properties of U₃O₈ exist, with one reporting values of 10⁴ Ω cm for resistivity and 1 cm²/ V s for mobility (George & Karkhanavala, 1963). Like for UO₂ and other semiconductors, however, these properties would be expected to vary widely. To determine the range of the possible charge transport properties in U₃O₈, as well as how they vary with material composition and microstructure, a rigorous study is necessary. We report the results of charge transport measurements using a range of techniques, including four-point van der Pauw resistivity and DC Hall, on sintered U₃O₈ pellets of varying stoichiometry and grain size. Additionally, we report results from ultraviolet and x-ray photoelectron spectroscopy toward probing the electronic structure of the U₃O₈ surface toward the development of suitable electrical contacts for this material.

George, A. M., & Karkhanavala, M. D. (1963). Studies on the electrical properties of uranium oxides I. Electrical conductivity of alpha U3O8. J. Phys. Chem. Solids, 24, 1207–1212. doi:10.1103/PhysRevB.81.205324

In the present work, temperature-resistance effect is found in carbon black/Polydimethylsiloxane composites. The composites were made by mixing the carbon black, aluminium oxide power and polydimethylsiloxane (PDMS). Interdigitated copper electrodes were obtained using chemical etching method on the flexible polyimide substrate in order to determine the resistance change of the composites at different temperatures. The resistances of as-fabricated composites at different temperatures are measured using multimeter. From the results, we found out that the resistance increases dramatically with temperature increasing, and the fitting result shows that it’s an exponential increase. This composite has the potential to fabricate the flexible and high-precision temperature sensor.