8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
I-V characteristics of La0.7Sr0.3MnO3/ SiO2 /Si MOS structure
S. Chattopadhyay, P. Dey, T. K. Nath*
Dept. of Physics and Meteorology
Indian Institute Technology Kharagpur, 721302
*Corresponding Author: tnath@phy.iitkgp.ernet.in
Abstract
An experimental study of p-silicon (Si) La0.7Sr0.3MnO3 (LSMO) junction in which the LSMO and silicon are separated by a thin interfacial silicon dioxide (SiO2) layer has been made. A generalized approach is taken towards the interfacial study of both the LSMO as a metal and the Si as the semiconductor. The SiO2 film is grown on Si by annealing it at 500ºC in oxygen atmosphere. The LSMO film of about 64 nm thick has been grown on SiO2 at 800ºC substrate temperature in 10-1 mbar oxygen pressure by Pulsed Laser Deposition technique. The XRD diffraction pattern showed polycrystalline nature of LSMO. The interpretation of the current-voltage characteristic for this model has been discussed. The ideality factor obtained is greater than 2 which may be responsible for thermo-ionic emission. The reverse saturation current obtained from forward bias I-V characteristics is about 1 μA/cm2.
Introduction
Photo-carriers injection induces the resistance or magnetic or electrical changes to open up a new way of exploring the mechanism in the strongly correlated electronic systems [1-4]. Perovskite-based p–n junctions are sensitive to light illumination and thus revealing their potential for photonic device applications [5-8]. Conventional semiconductor p–n junction theory, in which majority carriers and the built-in field dominate the formation of photo voltage, is applied to explain the photovoltaic effect in perovskite-based p-n junctions. Recently, much works have been carried out with the perovskite based oxide semiconductors and semimetals heterojunctions with different semiconductors [9-11]. Although it is still an open question to model the interface between different perovskite-type oxides. Previous studies reveal that the transport process in manganite heterostructures can be approximated by p-n junctions or Schottky type junctions [12-13]. In some report
detailed I-V characteristics showed that the LSMO/NSTO junctions are well described by Schottky type barrier [14-15]. The most of the studies focused on the transport process based on current-voltage characteristics. Generally the current-voltage characteristic is governed by the tunneling of conduction electrons between the junctions. However, the presence of edge leakage makes the transport mechanism complicated. As a result there is difficulty in the extraction of actual barrier height.
In this present work we are introducing the MOS like structure with half-metallic LSMO and semiconducting p-Si with thin insulating SiO2 interfacial layer. Since the carrier concentration is generally large in LSMO and the barrier width is correspondingly thin in transition-metal oxide junctions, the present analysis should be quite generally relevant for understanding oxide junction characteristics. A detailed analysis of the interface transport properties at room temperature is presented. We show that the MOS junction of such non epitaxial films does not show good rectifying behavior. The X-ray diffraction and the current - voltage (I-V) studies have been discussed.
Experimental Details
The LSMO powder was synthesized through chemical pyrophoric reaction process where we have employed stoichiometric mixtures of high purity La2O3 (99.99 %), SrCO3 (99.9+ %) and Mn(CH3COO)2 (99.0 %). After final grinding and pelletization of LSMO powders, the pelletized sample was first heated at 800°C for 12 h, then at 1000°C for 12 h and at 1200°C for another 12 h, with intermediate grinding. Final sintering of the target was carried out at 1200°C for 24 h.
La0.7Sr0.3MnO3 film on p-Si was prepared by Pulsed Laser Deposition process using 248 nm KrF excimer laser with 10 Hz rep. rate. The (100) p-Si substrate was at first cleaned by DI water and Acetone in ultrasonic vibrator for 20 min. Then it was put into 1:1 H2SO4 and H2O2 mixture. After removing the substrate from this solution it was finally cleaned by HF. This well cleaned Si substrate was used for deposition of thin film. The film was deposited on Si using following steps.
1. The well cleaned substrate was first heated at 500ºC at the 10-1 mbar O2 pressure for 20 min. The thin oxide layer was formed over Si in this process.
2. A portion of the substrate was musked using another Si. The LSMO film was deposited over another portion at 800C at 10-1 mbar O2 atmosphere. The laser pulse was applied on the LSMO target for 200 second at frequency 10 Hz. The laser power was 300 mJ.
3. The oxide layer of the other part was cleaned by HF and Ohmic contact was made using indium metal.
Results and Discussion
The X-ray diffraction data (pattern) using Cu-Kα of the LSMO films deposited on p-Si substrates has been shown in Fig.1. The multipeaks of LSMO shows the non-epitaxial nature of the LSMO film. Figure 2 shows the current density-voltage (J-V) characteristics of a typical LSMO/SiO2/Si structured heterojunction measured at room temperature. It can be seen that the junction exhibits a diode-like behavior. The ideality factor η is greater than 2 at room temperature. The observed large ideality factors exclude several possible forward current transport mechanisms across the junction barrier, namely thermionic emission, minority carrier injection, and recombination degeneration. The voltage dependence of the junction current can be expressed as [16] 122exp()expexpBqeVJATkTkTδφχη−=−
. (1)
where ()34t
2 Amqhkπ= .χ (eV) is the mean barrier height presented by the film,δ(Å) is the oxide thickness k is Boltzmann’s constant. Bφ is the barrier height of the junction. The effective mass for LSMO is taken as 4 me [17]. The parameter me is the free electron rest mass. Reverse saturation current is measured from the extrapolation of the ln J - V curve to the y-axis at voltage = 0 (Fig. 3). The reverse saturation current is found to be ~ 0.88 μA/cm2. The effective barrier height is estimated to be ~ 1.29 eV.
Conclusions
The LSMO/SiO2/Si structure is prepared by the pulsed laser deposition technique where the oxide layer was formed by annealing at 500ºC temperature in O2 atmosphere. The J-V characteristics are well defined by general metal oxide semiconductor junctions. The junctions’ exhibit MOS diode-like behavior but it does not act like rectifying type. An attempt is made to determine the dominant current transport mechanism in the junction. The effective barrier height of the junction is found to be ~ 1.29 eV.
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Figures
304050050100150
6020304050607080050000100000150000200000 (104) LSMO(024) LIntensity
SMO
(125) LSMO
2θ (degree)
(400) Si
Fig1: XRD pattern of LSMO film on Si substrate. Inset shows the enlarged view of XRD scan with the variation of 2θ in the range from 30º to 60º.
-15-10-50510152025
-6-4-202430 Voltage (Volt)Current density (μA/cm2)
Fig 2: J - V characteristics of LSMO/SiO2/ p-Si MOS junction
0.00.51.01.52.0-3-2-101234 ln(J)Voltage (
V)
Fig 3: ln (J) - V plot to determine reversesaturation curent
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