SPIE Proceedings v. 9068 entitled: Technology of Thin Film & Application of Thin FilmA heterojunction with good rectifying properties in a wide temperature range from 20 K to 300 K was fabricated simply by depositing an as-grown La0.9Hf0.1MnO3 (LHMO) film on a commercial 0.7 wt% Nb-doped SrTiO3 single crystal substrate using pulsed laser deposition technique. The current-voltage behavior of the LHMO/STON is measured under applied magnetic fields varying between 0 and 5 T. The heterojunction shows a remarkable magnetoresistance which depends on both the temperature and bias voltages. The sign of the magnetoresistance as function of temperature at either forward or reverse bias voltage is extensively studied by the filling of electrons in the eg and t2g band. The good rectifying behaviors, the magnetic tunable properties and the maximum magnetoresistance obtained at room temperature make this simple heterojunction promising for practical applications. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personalLink_to_subscribed_fulltex

Chu, JH

Gao, J

Huang, J

Jiang, Y

Ren, B

Tang, K

Wang, CR

Wang, L

Wang, LJ

Wu, ZP

Zhang, WZ

HKU Scholars Hub

PROCEEDINGS OF SPIESPIEDigitalLibrary.org/conference-proceedings-of-spieRectifying properties and colossalmagnetoresistance inLa0.9Hf0.1MnO3 /Nb-0.7 wt%-dopedSrTiO3 heterojunctionLin Wang, Zhenping Wu, Yucheng Jiang, Bing Ren, JianHuang, et al.Lin Wang, Zhenping Wu, Yucheng Jiang, Bing Ren, Jian Huang, Ke Tang,Wenzhu Zhang, Ju Gao, Linjun Wang, "Rectifying properties and colossalmagnetoresistance in La0.9Hf0.1MnO3 /Nb-0.7 wt%-doped SrTiO3heterojunction," Proc. SPIE 9068, Eighth International Conference on ThinFilm Physics and Applications, 90681O (16 December 2013); doi:10.1117/12.2054009Event: Eighth International Conference on Thin Film Physics and Applications(TFPA13), 2013, Shanghai, ChinaDownloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 23 Jun 2019  Terms of Use: https://www.spiedigitallibrary.org/terms-of-use  Rectifying properties and colossal magnetoresistance in La0.9Hf0.1MnO3 /Nb-0.7 wt%-doped SrTiO3 heterojunction  Lin Wang1,*, Zhenping Wu2, Yucheng Jiang3, Bing Ren1, Jian Huang1, Ke Tang1, Wenzhu Zhang1, Ju Gao 3, and Linjun Wang1  1 School of Materials Science and Engineering, Shanghai University,  333 NanChen Road, BaoShan District, Shanghai, 200444, China 2 School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China 3 Department of Physics, The University of Hong Kong, Hong Kong, P. R. China    ABSTRACT A heterojunction with good rectifying properties in a wide temperature range from 20 K to 300 K was fabricated simply by depositing an as-grown La0.9Hf0.1MnO3 (LHMO) film on a commercial 0.7 wt% Nb-doped SrTiO3 single crystal substrate using pulsed laser deposition technique. The current-voltage behavior of the LHMO/STON is measured under applied magnetic fields varying between 0 and 5 T. The heterojunction shows a remarkable magnetoresistance which depends on both the temperature and bias voltages. The sign of the magnetoresistance as function of temperature at either forward or reverse bias voltage is extensively studied by the filling of electrons in the eg and t2g band. The good rectifying behaviors, the magnetic tunable properties and the maximum magnetoresistance obtained at room temperature make this simple heterojunction promising for practical applications.    Keywords: magnetoresistance, manganite, heterojunction Eighth International Conference on Thin Film Physics and Applications, edited by Junhao Chu, Chunrui Wang, Proc. of SPIE Vol. 9068, 90681O  © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2054009Proc. of SPIE Vol. 9068  90681O-1Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 23 Jun 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-use  I. INTRODUCTION  Perovskite manganites have been extensively studied for their huge magnetoresistance (MR) and many other interesting properties.1 Recently much attention has been paid to the manganite-based heterojunctions for their good rectifying properties similar to traditional semiconductor junctions. In addition to these, since a typical feature of the manganites is the strong dependence of their properties on magnetic filed, a spontaneous expectation is that the properties of these manganite-based heterojunctions can be modulated by the magnetic field, which is of special interest from the viewpoint of application. In fact, much effort has been devoted to this field. Although good results of negative MR have been reported in some magnetic tunnel junctions (MTJ),2,3 an extremely large positive MR (~180%) was found in La0.32Pr0.35Ca0.33MnO3 based p-n junction.4 Furthermore, a crossover from negative to positive MR of La0.7Ce0.3MnO3-SrTiO3-Nb heterojunction was observed by Sheng et al.5 It is understandable for certain manganite based artificial structures to be with a negative MR property since it may originate from the negative MR of the manganites. However it is very interesting and unbelievable that a structure consisting of a negative colossal MR material and a nonmagnetic material (such as Nb-doped SrTiO3) can show a large positive MR property. Besides, the maximum MR is usually observed at very low temperature or in the vicinity of transition temperature. This is inapplicable in semiconductor devices which must be operated at higher temperature even near room temperature. Thus it is highly desirable to find new materials and simple structures with a large MR at high temperature. Perovskite La0.9Hf0.1MnO3 (LHMO) is a tetravalent cation-doped manganite in which the La3+ ions of the parent compound LaMnO3 are substituted by Hf4+ ions. Our previous reports indicate that the LHMO film is in a mixed valence state of Mn2+ and Mn3+, implying an electron-doped conduction mechanism, and it show similar MR and electroresistance (ER) effect as hole-doped manganites.6,7 However, studies on this electron-doped manganite based heterojunction have seldom been reported so far.  In this piece of work, we successfully constructed a bilayer heterojunction by combining a manganite LHMO with a 0.7 wt % Nd-doped SrTiO3 (STON). The heterojunction shows excellent rectifying properties and a remarkable MR in a wide temperature range from 20 to 300 K. It is found that the filling of electrons in the interface of the heterojunction, which is highly affected by the temperature, plays a crucial role in the sign of MR.   II. EXPERIMENTAL  The epitaxial LHMO film was grown on a 0.7 wt% Nb-doped SrTiO3 single crystal substrate (LHMO/STON) by using pulsed laser deposition technique. A disk-shaped ceramic target of the stoichiometric La0.9Hf0.1MnO3 was synthesized by the standard solid reaction technique using high-purity powdered La2O3, HfO2 and MnO.8 The film deposition took place in pure oxygen with a pressure of ~0.5 mbar. The energy of the laser beam was ~400 mJ with a wavelength of 248 nm and a pulse frequency of 5 Hz. The substrate temperature was kept at 700 ℃ as measured by a k-type thermocouple inserted into the heater block. The thickness of the films (~280 nm) was measured by surface profile measuring system (DEKTAK3). After deposition, the chamber was vented to 1 bar with high purity oxygen and the sample was cooled down to room temperature. As confirmed by X-ray diffraction (SIEMENS D5000 with Cu-Kα radiation), they are single phase and epitaxially grown with the (001) axis normal to the films plane. The current-voltage behavior of LHMO/STON heterojunction was measured for temperature ranging from 20 to 300 K, and the measuring current was kept below 1 mA to minimize the heating effect.  III. RESULTS AND DISCUSSIONS  The I-V curves of LHMO/STON, measured at selected temperature (60 K,120 K, 180 K, 240 K and 300 K) under the fields of H = 0 and 5 T, are presented in Fig. 1. A schematic illustration of this heterojunction is also shown in the left top of Fig. 1. The positive bias is defined as the current flows from the LHMO film to the STON substrate. A magnetic filed was applied perpendicularly to the interface of the heterojunction. It is notable that the I-V curves exhibit an excellent rectifying property, whatever with or without magnetic fields, in a wide temperature range from 60 to 300 K which mimics the conventional bipolar junction. In the I-V curves, two particular voltages called diffusion voltage (Vd) and breakdown voltage (Vb) respectively are defined at which the current ⎟A⎜=1 mA, as marked in the right bottom of Fig. 1 by arrows. In order to analyze the junction more straightforward in the later part of this paper, the density of state (DOS) of Proc. of SPIE Vol. 9068  90681O-2Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 23 Jun 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-use  LHMO/STON heterojunction is plotted in Fig. 2. The gray areas mark the states being occupied by electrons. In this sketch, region Ⅰ denotes the LHMO homogenous region far away from the LHMO-STON interface, region Ⅱ the LHMO depletion layer closer to the interface, region Ⅲ the STON depletion layer closer to the interface, and region Ⅳ the STON homogenous region far away from the interface. Eg denotes the band gap between the t2g spin-down (t2g↓) band and eg1 spin-up (eg1↑) band. EF is near the bottom of t2g↓ band for region Ⅰ due to the low doping of tetravalent Hf ions in LHMO and slightly above the bottom of the Ti 3d conduction band for region Ⅳ. Noting that the EF of STON is generally higher than that of LHMO,9 when they are attached together, some electrons in STON will diffuse to LHMO to line up the Fermi lever, as shown in Fig. 2(a).    The Vd as function of temperature for LHMO/STON heterojunction measured under the fields of H = 0 and 5 T are shown in Fig. 3. Whatever with or without magnetic fields, Vd decreases monotonously as the temperature increasing. Here the decreasing Vd means a weakening built-in electric filed in the junction region or a lowering energy barrier at the LHMO-STON interface. When temperature increased, the depletion region becomes thinner due to thermionic emission of carriers, similar to the case of conventional bipolar junction, which accounts for the smaller Vd at high temperature. The temperature dependence of the MR ratio (I = 1 mA) is shown in the inset of Fig. 3. The most remarkable observation in our sample is that the positive MR increases with increasing temperature and the maximum (about 50%) are obtained at 300 K which is of much favorable to practical applications. As the schematic DOS shown in Fig. 2, both LHMO and STON should be regarded as degenerated semiconductor with corresponding band gap. When a forward bias is applied to the junction, the number of electrons filled in the t2g↓ band in the region Ⅱ decreases, as shown in Fig. 2(b). The positive MR shown in our LHMO/STON heterojunction can also be explained by the electron filling in the t2g↓ band in the region Ⅱ.10 As for the maximum MR existing at 300 K, which is different from ref. 9, is understandable when considering the difference of band diagram between LHMO and La0.9Sr0.1MnO3. For the LHMO/STON junction, with the increased temperature, the electron filling of the t2g↓ band in the region Ⅱ increases, as well as the positive MR in the inset of Fig. 3. However even when the temperature reach 300 K which is our maximum testing temperature (room temperature), the filling of electrons in region Ⅱ still could not reach the band edge of eg2↑ and the minority channel of t2g↓ still dominates the transport, as shown in Fig. 2(c). When a magnetic field is applied to the heterojunction, the spin polarization of eg1↑ band and t2g↓ band are both increased, thus less and less carriers can tunneling between them due to the scattering between carriers with antiparallel spins, which means larger and larger resistance, and positive MR are caused in the heterojunction. The Vb as function of temperature for LHMO/STON heterojunction measured under the fields of H = 0 and 5 T are presented in Fig. 4. The temperature dependence of the MR ratio (I = –1 mA) is presented in the inset of Fig. 4. It is notable that the MR undergo a crossover from positive to negative, and to positive again when the temperature rising from 60 K to 300 K. When a reverse bias is applied to the junction, the number of electrons filled in the t2g↓ band in the region Ⅱ increases, as shown in Fig. 2(d). As we mentioned before, at low temperature (60 K), there are electrons filling of the t2g↓ band in the region Ⅱ which caused the positive MR in the junction. When the temperature increases (100 K – 200 K), the electrons could start to fill the eg2↑ band in region Ⅱ and the majority channel dominates the transport, which causes the negative MR in the junction, as shown in Fig. 2(e). When we keep increasing the temperature (220 K – 300 K), as presented in Fig. 2(f), the t2g↓ band is almost full filled while the eg2↑ band is half filled. At this condition, the minority channel dominates the transport again, as well as the positive MR.  Based on our experiment results and explanations, it is obvious that the filling of electrons in the eg and t2g band, as well as the sign of MR, are highly affected by the temperature. Furthermore, it is possible to make the temperature at which the maximum MR happens near to the room temperature by precise picking the manganites and STON with suitable doping lever.   IV. CONCLUSION A LHMO/STON heterojunction, which shows excellent rectifying properties in a wide temperature range from 20 to 300 K, has been fabricated successfully. The sign of the MR as function of temperature at either forward or reverse bias voltage is extensively studied by the filling of electrons in the eg and t2g band. Our results reveal a possibility in controlling the temperature at which the maximum MR happens near the room temperature by precise Proc. of SPIE Vol. 9068  90681O-3Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 23 Jun 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-use  picking the manganites and STON with suitable doping lever, which is of special importance in practical applications. V. ACKNOWLEDGEMENTS This work is supported by a grant of the Research Grant Council of Hong Kong (HKU 701813), the National Natural Science Foundation of China (11074162, 61176072) and Special Research Foundation for Training and Selecting Outstanding Young Teachers of Universities in Shanghai (ZZSD12015, ZZSD12036).         REFERENCE  [1] Haghiri-Gosnet, A. M., and Renard, J.P.,“CMR manganites: physics, thin films and devices” J. of Phys. D: Appl. Phys. 36, R127- R150 (2003).  [2] Lu, Y., Li, X. W., Gong, G. Q., Xiao, G., Gupta, A., Lecoeur, P., Sun, J. Z., Wang, Y. Y., and Dravid, V. P., “Large magnetotunneling effect at low magnetic fields in micrometer-scale epitaxial La0.67Sr0.33MnO3 tunnel junctions” Phys. Rev. B 54, R8357- R8360 (1996). [3] Markna, J. H., Vachhani, P. S., Parmar, R. N., Kuberkar, D. G., Misra, P., Singh, B. N., kukreja, L. M., Rana, D. S., and Malik, S. K.,“Spin-dependent sum rules for X-ray absorption spectra” Europhys. Lett. 79, 17005-17010 (2007).  [4] Sun, J. R., Xiong, C. M., Zhao, T. Y., Zhang, S. Y., Chen, Y. F., and Shen, B. G., “Effects of magnetic field on the manganite-based bilayer junction ” Appl. Phys. Lett. 84, 1528-1530 (2004). [5] Sheng, Z. G., Song, W. H., Sun, Y. P., Sun, J. R., and Shen, B. G., “Crossover from negative to positive magnetoresistance in La0.7Ce0.3MnO3-SrTiO3-Nb heterojunctions” Appl. Phys. Lett. 87, 032501-032503 (2005). [6] Wang, L., and Gao, J.,“Strain effect on the transport properties in low-doped La0.9Hf0.1MnO3epitaxial films” J. Appl. Phys. 103, 07F702, (2008). [7] Gao, J., and Wang, L., “Current-induced electroresistance in tetravalent cation-doped La0.9Hf0.1MnO3 epitaxial thin films”Mater. Sci. Eng. B 144, 97-99 (2007). [8] Mandal P., and Das, S., “Transport properties of Ce-doped RMnO3 (R=La,Pr, and Nd) manganites”, Phy. Rev. B 56, 15073-15080 (1997). [9] Xiong, C. M., Zhao, Y. G., Xie, B. T., Lang, P. L., and Jin, K. J., “Unusual colossal positive magnetoresistance of the n-n heterojunction composed of La0.33Ca0.67MnO3 and Nb-doped SrTiO3”Appl. Phys. Lett. 88, 193507-193510 (2006). [10] Jin, K. J., Lu, H. B., Zhou, Q. L., Zhao, K., Cheng, B. L., Chen, Z. H., Zhou, Y. L., and Yang, G. Z., “Positive colossal magnetoresistance from interface effet in p-n junction of La0.9Sr0.1MnO3 and SrNb0.01Ti0.99O3”,  Phys. Rev. B. 71, 184428-184434 (2005).      Fig. 1. I-V curves of LHMO/STON heterojunction measured at selected temperature (60 K, 120 K, 180 K, 240 K and 300 K) under the fields of H=0 (solid symbol) and 5T (hollow symbol). The left top shows a schematic illustration of the present heterojunction.  Fig. 2. The schematic DOS of the LHMO/STON heterojunction at zero bias voltage (a), forward bias voltage (b-c) and reverse bias voltage (d-f), respectively. The gray areas mark the states being occupied by electrons.  Fig. 3. Diffusion voltage (Vd) as function of temperature for LHMO/STON heterojunction under the fields of H=0 and 5 T. The inset shows temperature dependence of MR (I = 1 mA).  Fig. 4. Breakdown voltage (Vb) as function of temperature for LHMO/STON heterojunction under the fields of H=0 and 5 T. The inset shows temperature dependence of MR (I = –1 mA).    Proc. of SPIE Vol. 9068  90681O-4Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 23 Jun 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-useI,VLaOOHfOiMuONb, .7a,t%-SrTiO3(1)) 0>.\ AI NO,LS lee II 0'1111'1     -1.0-0.50.00.51.0   60K-1.0-0.50.00.51.0  Current (mA)120K  180K-4 -2 0 2-1.0-0.50.00.51.0240K-2 0 2  300KVoltage (V)VbVd Fig. 1.     Fig. 2.  Proc. of SPIE Vol. 9068  90681O-5Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 23 Jun 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-use  0 50 100 150 200 250 3000.40.60.81.01.21.41.61.82.050 100 150 200 250 3001020304050  Diffision Voltage (Vd)Temperature (K) H=0 H=5MR(%)Temperature (K)MR(%)=(RH-R0)/R0*100I=1mA Fig. 3.   0 50 100 150 200 250 300-2.6-2.8-3.0-3.2-3.4-3.6-3.8-4.0-4.2-4.450 100 150 200 250 3000510152025MR(%)Temperature (T)  Breakdown Voltage (Vb)Temperature (K) H=0 H=5MR=(RH-R0)/R0*100%    I=-1mA Fig. 4. Proc. of SPIE Vol. 9068  90681O-6Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 23 Jun 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Rectifying properties and colossal magnetoresistance in La0.9Hf0.1MnO3 /Nb-0.7 wt%-doped SrTiO3 heterojunction

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Rectifying properties and colossal magnetoresistance in La0.9Hf0.1MnO3 /Nb-0.7 wt%-doped SrTiO3 heterojunction

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