ICMCTF2014 Session C5-1: Thin Films for Active Devices

Wednesday, April 30, 2014 1:30 PM in Sunset

Wednesday Afternoon

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1:30 PM C5-1-1 Recent Progress in Understanding Free-charge Carrier and Structural Properties of InN Thin Films
Vanya Darakchieva (Linköping University, IFM, Sweden)
A sustainable energy future is one of the major global challenges today pointing to the acute need for improved materials. InN(Indium Nitride)-based materials provide unique opportunities for device applications targeting both i) increasing the fraction of energy supply coming from sustainable energy sources and ii) improving the efficiency of energy conversions and end uses. The revision of the band gap energy of wurtzite InN from 1.9 eV to 0.6 eV has opened the way for the application of InGaN and InAlN alloy thin films in a new class of highly efficient solar cells and visible light emitting diodes for solid state lighting. Consequently, considerable research interest has been focused on InN, related alloys and low-dimensional structures. However, many of the fundamental properties of InN-based materials remain still controversial and the related metrology is challenging.

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.

2:10 PM C5-1-3 Carrier Transport in Undoped CdO Films Grown by Atmospheric-pressure Chemical Vapor Deposition
Tomoaki Terasako, Ken Ohmae, Sho Shirakata (Ehime University, Japan)
Recently, cadmium oxide (CdO) films have been attracted much attention because of their relatively high Hall mobility values. Yan et al. reported that the Hall mobility value of 609 cm2/Vs was achieved on the epitaxial CdO:Sn film prepared by pulse laser deposition (PLD) [1]. In our previous paper, we have reported the successful growth of undoped CdO films on c- and r-plane sapphire substrates (denoted hereafter by “c-Al2O3” and “r-Al2O3”, respectively) by the atmospheric-pressure CVD (AP-CVD) using Cd powder and H2O as source materials [2]. The highest Hall mobility of 178 cm2/Vs was obtained on the CdO/r-Al2O3 film with the carrier concentration n of 4.2×1019 cm-3. To clarify the carrier transport mechanism, we have investigated the temperature dependences of the electrical properties of the undoped CdO films in this paper.

The direct optical gap energies (Egdis) determined by extrapolating the linear portions of the -(αhν)2 curves were distributed in the range from 2.34 to 2.38 eV, which agree well with the reported Egdi values (~2.37 eV) for the films grown by PLD [3]. It was also found that the variation of Egdi 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.

2:30 PM C5-1-4 Large Polarization in Lead Free Ferroelectric Thin Films Fabricated by Pulsed Laser Deposition
Yesappa Kolekar (University of Pune, India); Anagh Bhaumik (Missouri State University, US); Chintalapalle C. V. Ramana (University of Texas at El Paso, US); Kartik Ghosh (Missouri State University, US)

We report ferroelectricity with a large remnant and saturation polarization close to a morphotropic phase boundary (MPB) lead (Pb) free (0.5)BaTi0.8Zr0.2O3-(0.5)Ba0.7Ca0.3TiO3, (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.

2:50 PM C5-1-5 Investigating in Via-Contact-Type Amorphous Indium-Gallium-Zinc-Oxide Thin Film Transistors Two-Stage Rise Capacitance-Voltage Characteristics Degradation in Different Environment
Jhe-Ciou Jhu, Ting-Chang Chang (National Sun Yat-Sen University, Taiwan); Geng-Wei Chang, Ya-Hsiang Tai (National Chiao Tung University, Taiwan)
Electrical characteristics and the effect of negative bias stress (VGS-VT=-30V) are investigated in via-contact-type a-InGaZnO thin film transistors in different environments. Current-voltage and capacitance-voltage measurements are utilized to investigate the impact of water vapor on device characteristics as well as degradation behaviors caused by a negative bias stress. We observed that threshold voltage (VT) shifts positively with negative gate bias applied. It is also found that the gate to channel capacitance (CGC) curves exhibit an anomalous two-stage rise with the first stage similar to that of initial. Negative bias stress brings about electron trapping in the etch stop layer in H2O vapor environment, and this is further verified through modulating the water vapor partial pressure. The quantity of electron trapping in the etch stop layer was calculated from ISE-TCAD simulation. Also, the result shows good agreement with CGC curve shifts phenomenon calculated by ISE-TCAD simulations.
3:30 PM C5-1-7 Device Applications of Energetically Deposited Metal Oxide and Carbonaceous Thin Films
Jim Partridge, Edwin Mayes, Billy Murdoch, Masturina Kracica (RMIT University, Australia); Salim Elzwawi, Martin Allen (University of Canterbury, New Zealand); Marcela Bilek (University of Sydney, Australia); Dougal McCulloch (RMIT University, Australia)

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 Zn1-xMgxO 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.

3:50 PM C5-1-8 Bipolar Memristive Properties of TiO2 Thin Film on Pt/p++Si
Suheyla Gullulu, Tevhit Karacali, Hasan Efeoglu (Ataturk University, Turkey)

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).

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