ICMCTF2014 Session C5-1: Thin Films for Active Devices

In this talk, the doping mechanisms, electronic and structural properties of InN-based materials will be discussed in relation to their application in photovoltaics and optoelectronics. Details on the microstructure and impurity incorporation and dynamics in InN films with different surface orientations and dopings will be given. Special attention will be paid to the phenomenon of electron accumulation at the surfaces of InN-based materials, observed or expected to occur only in very few semiconductors. The challenges in understanding and assessment of doping mechanisms in InN-based materials will be described. The effect of Mg doping on the experimentally determined lattice parameters of InN will be discussed and combined with first principle calculations of the size and deformation potential effects in order to draw conclusions about the origin of strain in the films. Finally, the precise measurement of the free-charge carrier properties (effective mass, concentration and mobility) of undoped and Mg-doped InN films by the unique Optical Hall effect will be presented.

The direct optical gap energies (E_g^dis) determined by extrapolating the linear portions of the hν-(αhν)² curves were distributed in the range from 2.34 to 2.38 eV, which agree well with the reported E_g^di values (~2.37 eV) for the films grown by PLD [3]. It was also found that the variation of E_g^di as a function of carrier concentration n is well fitted by the calculated curve based on both the band gap widening caused by Burstein-Moss shift and the band gap narrowing caused by the electron-electron and electron-impurity interactions.

Hall effect measurements were performed at 90-330 K. For all the films, no remarkable changes were observed on the temperature (T) vs. carrier concentration n curves, indicating that these films were n-type degenerate semiconductors. According Bruneaux et al. [4], the barrier heights at grain boundaries can be determined from the slopes of the 1000/T vs. ln (μT) curves. The calculated barrier heights for the CdO films were smaller than the thermal energy at 300 K. This result indicates that the grain boundary scattering plays a minor role in the carrier transport compared with the intra-grain scattering. The gradients of the μ-T curves (denoted by ⊿μ/⊿T) can be divided into two regions: (1) Region I (n<7×10¹⁹ cm^-3); ⊿μ/⊿T<0 and μ increasing with increasing n, and (2) Region II (n>7×10¹⁹ cm^-3); μ independent of both T and n (⊿μ/⊿T=0). The dependence of ⊿μ/⊿T on n demonstrates the continuous transformation of the dominant intra-grain scattering mechanism from the phonon scattering mechanism to the ionized impurity scattering with increasing n.

<<References>>

[1] M. Yan et al. : Appl. Phys. Lett. 78 (2001) 2342.

[2] T. Terasako et al. : Surf. & Coat. Technol. 201 (2007) 8924.

[3] K. M. Yu et al. : J. Appl. Phys. 111 (2012) 123505.

[4] J. Bruneaux et al. : Thin Solid Films 197 (1991) 129.

We report ferroelectricity with a large remnant and saturation polarization close to a morphotropic phase boundary (MPB) lead (Pb) free (0.5)BaTi_0.8Zr_0.2O₃-(0.5)Ba_0.7Ca_0.3TiO₃, (BZT- 0.5 BCT), thin films. High quality thin films of Pb free BZT- 0.5BCT were grown on Pt/Ti/SiO₂/Si substrate using pulsed laser deposition (PLD). The structural and ferroelectric properties have been studied using X-ray diffraction (XRD), Raman spectroscopy, and polarization switching measurements. The Raman spectroscopic data in combined with the XRD data confirm the high quality BZT- 0.5 BCT thin films on Pt/Ti/SiO₂/Si substrate. Polarization versus electric field data shows a square hysteresis loop with a large remnant and saturation polarization and a small coercive field (~1.5 kV/cm) which is essential for practical device applications. We found that the thin films of BZT-0.5BCT close to MPB exhibits a large remnant polarization of 118 m C/cm² and a saturation polarization of 120 m C/cm², which are much higher than any previously reported values for any ferroelectric thin film. The polarization switching dynamics are well correlated with structural distortion and phonon vibration observed in XRD and Raman spectroscopy. The observed results may stimulate to develop new Pb free ferroelectric thin films for future non-volatile random access memory and many other high-tech applications.

We report on the properties and device applications of energetically deposited metal oxide and carbonaceous thin films. Using both filtered cathodic vacuum arc and high impulse power magnetron sputtering systems, unintentionally n-doped ZnO thin films have been deposited with sufficient quality to support Schottky diodes and field effect transistors . The films exhibit high transparency, moderate carrier concentrations, and Hall mobilities up to 30 cm²/V.s. Their properties can be further improved by post-deposition annealing. Bandgap tuning has been accomplished in energetically deposited Zn_1-xMg_xO films with varying Mg content and Schottky detectors sensitive to ultraviolet light in the UVB region have been formed on these films. Featuring graphitic carbon anodes, these detectors exhibit UVB-visible rejection ratios up to 10⁴. Detailed micro-structural, optical and electrical characterisation of the thin films will be presented and related to the characteristics of all devices.

Current voltage characteristics of TiO₂ at nano scale have unique property under certain conditions which defined as memristor in electronics. In general Metal/Metal Oxide/Metal structures are expected to be a candidate for memristor applications. The memristive behavior is modeled with filament formation, voltage driven oxygen vacancies and Poole Frenkel emission from the electronic states. The well known and the first memristor is based on TiO₂ film having nm thickness. TiO₂ thin films fabricated using reactive RF sputtering with 20ccm Ar and 0.46ccm O₂ having 6N purity. A sequential sputter is used for 10nm Pt or Ti film on p⁺⁺Si and then 10nm TiO₂ grown with 0.027A/sec rate. Top Pt metallization has done onto TiO₂ using lift off technique or Al using vacuum deposition and shadow mask. I-V measurements were carried out using Keithley 2400 with current compliance. Voltage scanning rate was changed from 0.5V/sec to 0.001V/sec. In general reverse current remained several order of magnitude smaller than forward case. Abrupt switching did not observed but a gradual switching between ON and OFF states were observed during continuous cycling. In some cases ON/OFF resistances of some the samples were increased to order of 10¹² Ohm during voltage cycling. By the time much lower resistance occurred but when cycling repeated later, the resistance goes up again. This observation indicates trapping and detrapping of electronic states may control the resistance. This proses to be activated by kT energy or carriers driven by built in electrical field at the interface. This situation require further temperature dependent measurement and time base analysis of I(t).