2015 MRS Spring Meeting

Apr 6, 2015 ‐ Apr 10, 2015


The 2015 MRS Spring Meeting and Exhibit was held in San Francisco, California. Technical symposia were held within the following topical clusters: Energy, Nanomaterials, Electronics and Photonics, Soft and Biomaterials, and General – Fabrication and Characterization. Access to all of the content below is free – simply login with your MRS username and password for complete access. Don’t have a username and password? Create a free account.


Sessions

Infrared-Light-Responsible Perovskite Solar Cells Consisting of Sn and Pb

Apr 9, 2015 2:00pm ‐ Apr 9, 2015 2:30pm

Identification: C8.02

The certified efficiency of perovskite solar cells is now 17.9% and is reaching that of inorganic solar cells such as 20.4% of poly Si solar cells. According to our simulation, 20.4 % efficiency is obtained (IPCE:0.8 and FF:0.7) by using photo-absorber with 900nm absorption spectrum edge, when 0.3 V was taken as the voltage loss from the ideal voltage. We focused on infrared-photoconversion, because the infrared-responsible solar cells should be useful for the bottom cells of the tandem cells and for enhancing the efficiency of single cells by harvesting light up to 900 nm after the band gap tuning. It is well-known that CH3NH3SnI3 Perov Sn) has absorption in the area of infrared. However, it was difficult to handle the Perov Sn in open air condition. We found that the mixture of Sn perovskite and Pb perovskite (CH3NH3Sn(0.5)Pb(0.5)I3, Perov Pb/Sn) has improved stability under air with maintaining the photoconversion in the infrared area. The IPCE of the 950nm was 0.2. However, it was found that the absolute photoconversion efficiency (IQE: Internal quantum efficiency) was 0.75 (75%). The low IPCE in the infrared area was explained by the loss of light uptake. Therefore, it was concluded that the photon confinement in the layer of Perov Pb/Sn is needed for enhancing the conversion. The charge recombination between electrons in porous titania and P3HT for Perov Pb and that in Perov Pb/Sn(1:1) was 600?sec and 30?sec respectively. In order to retard the charge recombination, porous titania was passivated by wide gap oxide thin layers such as Al2O3, ZrO2 and Y2O3. Among then, Y2O3 passivation gave the best results. Urbach tail measured by Photo Acoustic Spectroscopy was compared between Perov Pb/Sn and Perov Pb and it was found that the Urbach tail for the Perov Pb/Sn was lower than that of Perov Pb. The tail and band gaps of Perov Pb/Sn varied, depending on the substrates from which Perov Pb/Sn crystals grow, suggesting that Perov Pb/Sn has gradient structures affected by the surface of the substrates and Sn:Pb ratio changed from the bottom layer to the top layer. The relationship between the structure of Perov Sn/Pb and the substrate surface is reported in detail.

Speaker(s):

Effects of Electrical Stress and High-Energy Electron Irradiation on the InGaP/GaAs Heterojunction Phototransistor

Apr 9, 2015 2:00pm ‐ Apr 9, 2015 2:15pm

Identification: CC11.03

A heterojunction phototransistor (HPT) is very attractive compared with photodiodes because of its good compatibility with a heterojunction bipolar transistor (HBT), high photoresponse even at low bias voltage and immunity from avalanche noise [1]. Since InGaP has an advantage over AlGaAs in the material properties and fabrication process, the InGaP emitter has been actively employed to replace the AlGaAs in the AlGaAs/GaAs HPT. Although there have been many reports on the study of electrically-stressed InGaP/GaAs HBTs [2], there is a little or none on the electrically-stressed InGaP/GaAs HPTs. In this paper, we report the effects of electrical stress on the characteristics of InGaP/GaAs HPTs in detail. We gave an electrical stress, which was much smaller than that usually given to the HBTs, at room temperature and an elevated temperature to the InGaP/GaAs HPTs with and without the emitter-ledge passivation, which has demonstrated the improved performance of HBTs. The HPT epilayers were grown on S.I. GaAs (100) substrate by metal-organic chemical vapor deposition (MOCVD). Although the structure and design are similar to the HBTs, the emitter area is 160,800 ?m2, much larger than those of typical HBTs in order to increase the collector photocurrent. The electrical stress was given to the phototransistors by keeping a collector current of 60 mA (corresponding to a current density of 37 A/cm2) for 1 hour at room temperature. The electrical stress was found to be too small to affect the common-emitter current gain and photocurrent of both HPTs without and with the InGaP emitter-ledge passivation at room temperature. However, both current gain and photoresponse at 420 K decreased significantly in the electrically-stressed HPTs, but those of the HPT with the emitter-ledge passivation were still higher than those of the HPT without the emitter-ledge passivation. The effect of the electrical stress was more significant if it was given at 420 K even for the period reduced to 15 min. There was a significant decrease in the current gain and photoresponse even at room temperature in the HPT without the emitter-ledge passivation, while a little decrease was observed in the HPT with the emitter-ledge passivation. The effect of the electrical stress on the photoresponse was also much more significant than that on the current gain, and the emitter-ledge passivation was more effective in suppressing the degradation of the HPTs. Furthermore, for a potential application of the InGaP/GaAs HPTs in space, we will irradiate the HPTs with high-energy electrons at the Japan Atomic Energy Agency and characterize their degradation as a function of the energy and fluence. References [1] S. Chandrasekhar et al., IEEE Photon. Technol. Lett 5, 1316 (1993). [2] W. Liu et al., IEEE Electron Device Lett 14, 301 (1993).

Speaker(s):
  • Phuc Than, The University of Electro-Communications

Engineered Microstructure Hydroxyapatite Granules for Tailored Drug Release Rate

Apr 9, 2015 2:00pm ‐ Apr 9, 2015 2:15pm

Identification: GG7.03

Developed biomaterials and their concept of controlled drug delivery system are proven their excellence in this research. The biomaterial, prepared using hydroxyapatite (HAp), shows a hollow structure with the presence of connections between inner and outer surface to load drug carriers. The poly (lactic-co-glycolic acid) nanoparticles as a drug carrier contain dexamethasone, which is known to cause osteoinduction. Surface of the drug carriers are modified using polyethyleneimine and therefore is able to conjugate to the surface of HAp granules. The hollow HAp granules, containing the drug carriers on inner and outer surface, show the controlled drug release rate compared to the granules, containing the drug carriers only on the outer surface. The pores that are designed for insertion of drug carriers and also the preosteoblast. Consequently, they influenced on the cellular behavior; the first is that the cell proliferation and the second is that the early stage of osteogenic differentiation. The effects of controlled release rate is evident up through the two weeks after the cell seeding, results in increase of osteogenic differentiation. From the above results, the drug carriers loaded hollow HAp granules are shown to be patient-specific biomaterials and exhibit the potential for hard tissue regeneration.

Speaker(s):

Flexible Phosphorene Transistors: Materials, Devices, Amplifiers and Systems

Apr 9, 2015 2:00pm ‐ Apr 9, 2015 2:15pm

Identification: O8.02

Two dimensional atomic sheets, such as graphene and transition metal dichalcogenides (TMDs), have been widely studied as electronic materials for flexible nanoelectronics applications due to the high flexibility enabled by their natural 2D layered crystal structure. However, with the growing need for both high speed and low power consumption in realistic applications, TMDs with relatively low mobility and graphene with zero band gap are facing critical challenges to satisfy practical requirements. Recently, few-layer phosphorene, a new candidate in the portfolio of 2D crystals, has demonstrated high room temperature mobility and high on/off ratio, which is very attractive for advanced flexible nanoelectronics. In this work, we present the first phosphorene flexible field effect transistors (BP-FETs), fundamental circuits and an audio receiver. For BP-FETs based on exfoliated phosphorene flakes with thickness between 5nm to 15nm, clear ambipolar characteristics and negligible hysteresis were achieved, attributed to a dielectric capping layer, which significantly enhanced long-term air stability. Outstanding device performance were achieved at room temperature; hole mobility and current on/off ratio are 300 cm2/Vs and 105, respectively. With significantly enhanced ambipolar characteristics, electron mobility of 100 cm2/Vs was observed. In this work, high performance electronic circuit blocks, including digital inverters, frequency doublers, inverting and non-inverting amplifiers were realized for the first time on plastics. Furthermore, we demonstrate a phosphorene flexible radio receiver which effectively demodulates amplitude modulated audio signals. In conclusion, our results indicate that few layer black phosphorus is the most competitive 2D material for future high speed and low power flexible electronics beyond the low mobility of TMDs and zero bandgap of graphene.

Speaker(s):

Atomic Layer Deposition of Crystalline SrHfxTi1-xO3 Directly on Ge (001) for High-K Dielectric Applications

Apr 9, 2015 2:15pm ‐ Apr 9, 2015 2:30pm

Identification: AA10.03

We demonstrate the growth of crystalline strontium titanate, SrTiO3 (STO), and strontium hafnate, SrHfO3 (SHO), directly on Ge via atomic layer deposition (ALD). Both STO (a ~ 3.905 �) and SHO (a ~ 4.069 �) have good lattice match to the Ge (001) surface (a/?2 ~ 3.992 �), yielding a ~2.2% tensile and ~1.9% compressive strain in the epitaxial film, respectively. After thermal deoxidation, the Ge substrate is transferred in vacuo to the deposition chamber where a thin film of STO / SHO is deposited by ALD. Following a post-deposition anneal, the perovskite film becomes crystalline with epitaxial registry to the underlying Ge (001) substrate. The 2�1 reconstructed, clean Ge (001) surface is a necessary template to achieve crystalline films upon annealing. In situ x-ray photoelectron spectroscopy confirms stoichiometric films with no GeOx formation or carbon impurities. The STO and SHO films exhibit excellent crystallinity, as shown by x-ray diffraction and transmission electron microscopy. Capacitor structures using the crystalline STO dielectric show a high permittivity (k~90), but also high leakage current (~10 A/cm2 at +1 eV). The unfavorable conduction band offset (and high leakage current) of STO on Ge is circumvented by growing the Hf-based perovskite, SHO. The SHO films have favorable electronic properties, with satisfactory band offsets with Ge (> 2 eV), low leakage current (< 10-5 A/cm2 at an applied field of 1 MV/cm) at an equivalent oxide thickness of 1 nm, and a reasonable dielectric constant (k~15). The interface trap density (Dit) is estimated to be ~2-5 � 1012 cm-2 eV-1 under the current growth and anneal conditions. Some interfacial reaction is observed between SHO and Ge at temperatures above ~650 �C, which may contribute to the observed Dit value. In efforts to improve electrical performance of the crystalline perovskite dielectric, including leakage current, permittivity, and Dit, we have recently studied crystalline SrHfxTi1-xO3 (SHTO) grown directly on Ge by ALD. SHTO benefits from a reduced leakage current over STO and a higher permittivity than SHO. The SHTO films crystallize at a relatively lower temperature than SHO (600 �C vs. 650 �C), which limits the formation of hafnium germanide. In addition, the lattice constant of SrHfxTi1-xO3 (x~0.5) is estimated to be 4.014 �, yielding minimal (~0.6%) compressive strain in the epitaxial film. By minimizing the epitaxial strain and maintaining an abrupt interface, the SHTO films are expected to reduce Dit at the oxide-Ge interface. We will report our recent results on the growth, characterization, and electrical performance of epitaxial SHTO films on Ge for next-generation high-k dielectric applications, and compare them against STO and SHO films grown directly on Ge(001).

Speaker(s):

Unexpected Electronic Structure of Air-Stable Lead-Free Perovskite Variant Cs2SnI6 for Photovoltaic Application

Apr 9, 2015 2:30pm ‐ Apr 9, 2015 2:45pm

Identification: C8.03

Lead-based halide perovskites with the chemical formula of APb(II)X3 (A = Cs, CH3NH3, or CH2NH=CH2; X = I, Br or Cl) have attracted enormous interest for solar cell applications, primarily due to the high photoelectric conversion efficiency (PCE) up to 19.3%. However, the stability and toxicity are the major issues restricting the commercialization of these lead perovskites based PVs. ASn(II)X3 perovskites are lead-free, but they are extremely sensitive to the ambient air. To resolve the issues, an air-stable class of perovskite materials with the chemical formula of A2Sn(IV)X6, a defect variant of the ASn(II)X3, has been very recently introduced to PV applications. Particularly, Cs2SnI6 exhibits an ideal bandgap of 1.26 eV, high carrier mobility, and have demonstrated high PCE up to 7.8%. On the other hand, the electronic structure and the chemical bonding nature of Cs2SnI6 have not been understood properly; i.e., Sn in Cs2SnI6 is expected to be the +4 charge state upon assumption of Cs+ and I-, which would lead to expectation that the conduction band minimum is formed by Sn 5s similar to SnO2. Here, we performed hybrid density functional theory calculations with the HSE06 functionals for Cs2SnI6. We found that the band gap is formed mainly by anti-bonding and bonding states of I 5p � I 5p slightly hybridized with Sn 5s because the Cs2SnI6 structure is composed of isolated Cs ions and [SnI6] clusters. Unexpectedly, the Sn 5s state is very deep, ~7 eV deeper than the I 5p valence band (VB), and has little contribution to the VB; while, Sn 5s has more contribution to the conduction band. As a consequence, the charge state of Sn is not +4, but much closer to +2 like in CsSnI3 and SnO. Then, the apparent charge state of I is a bit smaller than -1 and to be of -4/6, which difference comes from the direct covalent bonds between the adjacent I atoms in the [SnI6] cluster. We also studied the defect physics and revealed the origin of ambipolar conduction in Cs2SnI6. These results break the conventional common sense about chemical bonding nature in ionic semiconductors and provide a guiding principle to design new pervoskite-based PV materials.

Speaker(s):

Memristive Properties of Metal/Manganite Devices: Correlation of Charge-Carrier Transport and Redox-State at the Interface

Apr 9, 2015 2:30pm ‐ Apr 9, 2015 2:45pm

Identification: TT13.04

Resistance random access memory (RRAM), which utilizes two or more resistive states of a material system for data storage, has attracted considerable attention as a future non-volatile memory concept. A large variety of binary oxides and complex transition metal oxides exhibit different resistance states at opposite polarities of electrical stimulation and could thereby be employed as RRAM. It has become widely accepted that resistive switching in oxides is in most cases connected with a voltage-driven oxygen vacancy movement and a resulting redox process. However, the current knowledge of the microscopic details of the redox-processes is very limited. Besides the n-conducting oxides which mostly exhibit filamentary resistive switching, there exists another class of bipolar resistive switching oxide systems, such as several manganites, for which it was demonstrated that the high and low resistive state currents scale with the electrode area implying that forming and switching take place beneath the whole electrode. We have shown that after electroforming of Ti/Pr0.48Ca0.52MnO3/SrRuO3 thin film devices, the chemical changes at the interface between the Ti electrode and the PCMO layer dominate the resistance of this multilayer stack. The reactive metal electrode forms an oxide layer at the Ti/PCMO interface prior to device operation, and applying a voltage to the stack was shown to increase the thickness of the TiO2 layer at the interface, and to deplete the underlying PCMO of oxygen further. Based on the knowledge of the electroforming mechanism, we present an encompassing view of the field-induced valence change during resistive switching at the Ti/PCMO interface. The chemical changes at the Ti/PCMO interface in four different resistive states are investigated by Hard X-ray Photoelectron Spectroscopy (HAXPES), and correlated to the transport-mechanism of charge-carriers across the multilayer stack for each state. A notable difference between the high and low resistive states can be explained through a convolution of several conduction mechanisms, which is in agreement with the observed chemical changes. [1] F. Borgatti, C. Park, A. Herpers, F. Offi, R. Egoavil, Y. Yamashita, A. Yang, M. Kobata K. Kobayashi, J. Verbeeck, G. Panaccione and R. Dittmann, Nanoscale 5, 3954 (2013) [2] A. Herpers, C. Lenser, C. Park, F. Offi, F. Borgatti, G. Panaccione, S. Menzel, R. Waser and R. Dittmann, Adv. Mat. 26, 2730 (2014)

Speaker(s):

Self-Powered n-MgxZn1-xO/p-Si Photodetector Improved by Alloying-Enhanced Piezo-Phototronic Effect

Apr 9, 2015 2:45pm ‐ Apr 9, 2015 3:00pm

Identification: P8.04

ZnO based nanowires and thin films are widely utilized in piezotronics, piezo-phototronics and nanogenerators. The performance of piezotronics and piezo-phototronic strongly depends on the magnitude of piezopotential. By alloying Mg into ZnO, MgxZn1-xO of wider band gaps has been regarded as a good candidate for solar-blind UV sensors. In this study, high-performance self-powered ZnO-based photodetectors are developed by enhancing piezopotential through alloying with Mg. A series of n-MgxZn1-xO thin films (from x = 0 to 0.2) with strong c-axis preferred orientation in the wurtzite phase are grown on p-type Si substrate by magnetron co-sputtering. The Mg content is determined by energy dispersive spectroscopy (EDS) analysis in conjunction with photoluminescence (PL) where the results confirm the increasing trend on band gap with increasing Mg. The piezotronics and piezo-phototronics properties of photodetectors made of selective preferentially orientated MgxZn1-xO thin films are studied by investigating the coupling of electrical and optical characteristics on strain, which reveals different piezotronic phenomena with different Mg contents. The MgxZn1-xO thin films are of wurtzite structure with highly preferred 0002 orientation and the photodetectors exhibit unique characteristics of self-power, ultrafast response, and superior stability over time. The performance of the self-powered photodetectors enhances with Mg content due to the increase of the piezoelectric coefficient by the alloying process, and thus an enhanced piezopotential. An enhancement of up to 100% in the output current and voltage can be achieved through the piezo-phototronic effect. More notably, the sensitivity of the Mg0.20Zn0.80O self-powered photodetector is more than 6-fold higher than that of the Mg0.05Zn0.95O photodetector as a result of alloying with more Mg. The higher sensitivity with the higher Mg content in the MgZnO compound is attributed to the increased piezoelectric coefficient. The results demonstrate that the piezoelectric coefficient of MgxZn1-xO increases with Mg content, resulting in an enhancement of the effective piezopotential, which is responsible for the higher sensitivity of self-powered photodetectors. The developed material is thus a promising candidate for ultrahigh sensitive photodetectors.

Speaker(s):
  • Yen-Yu Chen, National Cheng Kung University, Feng Chia University

Polarization Dynamics in Ferroelectric Relaxors Studied by Multidimensional Piezoresponse Force Microscopy

Apr 9, 2015 2:45pm ‐ Apr 9, 2015 3:00pm

Identification: TT13.05

The unique properties of the ferroelectric relaxors are thought to be due to compositional and charge disorder in these systems [1,2], which can manifest at atomic and mesoscopic length scales. However, little is known about polarization dynamics in the prototypical ferroelectric relaxor system (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) at the mesoscopic level. Here, we develop a multidimensional piezoresponse force microscopy (PFM) based technique [3] to study the voltage and time dependent response of the single crystal relaxor PMN-0.28PT. Measurements reveal that for small bias, there is little perturbation, but the relaxation increases for larger bias pulse amplitudes. Furthermore, mesoscale heterogeneity is evident in relaxation amplitudes, with two distinct amplitudes present. To gain insight into the disorder, the entire dataset was further analyzed by independent component analysis. These reveal the presence of a disorder component to the response, which is found to be inversely correlated to the piezoresponse at that area. The difference is postulated to arise from the mixture of rhombohedral and field-induced tetragonal phases present in the probed volume of the tip, which can vary spatially depending on local defects, chemical inhomogeneity and elastic clamping effects. These studies show the utility of PFM-based spectroscopies in combination with big-data style analyses in mapping the disorder that underpins functionality of the ferroelectric relaxors. This research was sponsored by the Division of Materials Sciences and Engineering, BES, DOE (RKV, SVK). A portion of this research was conducted at and partially supported by (SJ, MO) the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. NBG acknowledges funding from the US national Science Foundation through grant # DMR-1255379. References 1. A. A. Bokov, Z. �G. Ye, J. Mater. Sci. 41, 31 (2006). 2. F. Jiang, S. Kojima, C. Zhao, C. Feng, Appl. Phys. Lett 79, 3938 (2001). 3. S. V. Kalinin, B. J. Rodriguez, J. Budai, S. Jesse, A. Morozovska, A. A. Bokov, Z.-G. Ye, Phys. Rev. B. 81, 064107 (2010).

Speaker(s):

Electrowetting on Bio-Inspired Soft Liquid-Infused Film (EWOLF): Complete Reversibility and Controlled Droplet Oscillation Suppression for Fast Optical Imaging

Apr 9, 2015 3:15pm ‐ Apr 9, 2015 3:30pm

Identification: GG7.05

Electrowetting on dielectric (EWOD), owing to its ability to electrically manipulate tiny individual droplets without involving movable mechanical parts, has received much attention in the past two decades [1, 2]. Despite tremendous promise, the use of solid dielectric layer between the aqueous droplet and underlying electrode is associated with inevitable physical and chemical heterogeneities [3, 4], leading to limited functionalities. For example, owing to the large contact angle (CA) hysteresis, contact line pinning [5] as well as CA saturation [6] at high voltage, it remains challenging to achieve reversible electrowetting with a large degree of switchability in ambient conditions. Moreover, activating droplet in EWOD is vulnerable to pronounced oscillation in response to an abrupt external stimulus, resulting in elongated time for the droplet to reach its equilibrium state [7]. Here, we demonstrate a new paradigm of electrowetting on bio-inspired soft liquid-infused film (EWOLF) that allows for the enhanced reversibility and faster response time to reach the steady state simultaneously. The liquid-infused film is achieved by locking a liquid lubricant in a porous membrane through the delicate control of wetting properties of the liquid and solid phases. Taking advantage of the negligible contact line pinning at the liquid-liquid interface [8, 9], the droplet response in EWOLF can be electrically addressed with enhanced degree of switchability and reversibility compared to the conventional EWOD. Moreover, we show that the infiltration of liquid lubricant phase in the porous membrane also efficiently enhances the viscous energy dissipation, suppressing the droplet oscillation and leading to fast response without sacrificing the desired electrowetting reversibility. Meanwhile, we find that the enhanced damping effect associated with the EWOLF can be tailored by manipulating the viscosity and thickness of liquid lubricant. We also demonstrate the feasibility of developing adaptive liquid lens for fast focusing using the as-proposed EWOLF. References: [1] B. Berge, C. R. Acad. Sci. II 317, 157 (1993). [2] H. J. J. Verheijen and M. W. J. Prins, Langmuir 15, 6616 (1999). [3] G. Manukyan, J. M. Oh, D. van den Ende, R. G. H. Lammertink, and F. Mugele, Phys. Rev. Lett. 106, 014501 (2011). [4] G. McHale, C. V. Brown, M. I. Newton, G. G. Wells, and N. Sampara, Phys. Rev. Lett. 107, 186101 (2011). [5] X. M. Chen, R. Y. Ma, J. T. Li, C. L. Hao, W. Guo, B. L. Luk, S. C. Li, S. H. Yao, and Z. K. Wang, Phys. Rev. Lett. 109, 116101 (2012). [6] J. Liu, M. R. Wang, S. Chen, and M. O. Robbins, Phys. Rev. Lett. 108, 216101 (2012). [7] S. R. Annapragada, S. Dash, S. V. Garimella, and J. Y. Murthy, Langmuir 27, 8198 (2011). [8] A. Lafuma and D. Qu�r�, EPL 96, 56001 (2011). [9] T. S. Wong, S. H. Kang, S. K. Y. Tang, E. J. Smythe, B. D. Hatton, A. Grinthal, and J. Aizenberg, Nature 477, 443 (2011).

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