Proceedings of the 9th IEEE International Conference on Nano/Micro Engineered and Molecular Systems April 13-16, 2014, Hawaii, USA
Epitaxial Growth Controlled Tailoring of MetalInsulator (MI) Transition Properties of Rare Earth Correlated Oxides A. Iqbal, S. A. Khan, N.U. Rahman, T. Faraz Abstract— Strongly correlated electron devices using Metal Insulator Transition (MIT) Oxides are prospective alternatives along the new generation of high speed devices based on novel mechanisms. Taking the advantages of correlated electrons which are capable of forming a variety of electronic phases, MIT Oxides and Phase Change Materials (PCM) are treated as the frontiers of emergent device research. With the prospect of downsizing devices to the nanoscale regime, benefits over conventional semiconductor devices are attained. Aided by recent advances in fabrication technology, considerable improvements have been achieved to tailor the Metal-Insulator (MI) transition properties of MIT Oxides. In this study, the tailoring of MI transition properties for a particular group of MIT Oxides, namely the transition metal perovskite oxides of RNiO3 family are studied on the epitaxial platform. Finally, antiferromagnetism characteristics and anonymous resistivity inherent within those oxides are studied.
Keywords— Metal Insulator Transition (MIT) Oxides, Correlated Electron Devices, Metal-Insulator (MI) Transition, Antiferromagnetism.
I
I. INTRODUCTION
N recent years, correlated oxides such as transition metal perovskite oxides of RNiO3 family (R: La, Pr, Nd, Sm, Eu) have been intensively studied. Except LaNiO3 (which is metallic), all other remaining nickelates of the RNiO3 family are semiconductors at low temperatures and metallic above a critical transition temperature. This temperature is the Insulator to Metal Transition Temperature (TMIT) which is characterized by a change in the sign of δρ/δT (where ρ is resistivity) from a negative semiconducting state to a positive metallic state. As of late, there have been major advances in the growth of thin-film oxides by a variety of techniques such as Molecular Beam Epitaxy (MBE), sputtering, Chemical Vapor Deposition (CVD) and Pulsed Laser Deposition (PLD). These technological advances have renewed interest on the experimental and theoretical studies of Metal Insulator Transition (MIT) Oxides, which was set in motion by Morin’s
A. Iqbal is a lecturer in the Department Electrical and Electronic Engineering, BRAC University, Bangladesh. (E-mail:
[email protected]) S.A. Khan is a researcher at Control & Applications Research Centre, BRAC University, Bangladesh. (E-mail:
[email protected]). N.U. Rahman has graduated from the Department Electrical and Electronic Engineering, Islamic University of Technology, Gazipur, Bangladesh. (E-mail:
[email protected]). T. Faraz is a Project Engineer of Control and Application Research Centre (CARC), BRAC University, Bangladesh. (E-mail:
[email protected])
978-1-4799-4726-3/14/$31.00 © 2014 IEEE
seminal paper [1] on phase transitions in binary transitionmetal oxides in 1959. Success in the preparation of high quality thin films of these oxides has opened numerous opportunities for the applications of these films in thermal and optical switches, thermo-chromatic coatings, bolometers, actuators, non-volatile memory etc. [2,3]. However, the temperature range of functionality of these films is still limited. To make the correlated electron devices more applicable and useful, it is important to improve the quality of these films and extend their temperature range of functionality. Previous studies have shown that variation in TMIT depends on the preparation conditions and substrate-film misfit and dislocation. Ramanathan et al. [4] have reported the epitaxy and strain effects on the Metal-Insulator (MI) transition of SmNiO3 thin films. Davinder et al. [5] have studied the effects of film thickness on the MI transition properties of NdNiO3. All these studies have investigated the possibility of tuning the MI transition of some well known rare earth nickelates. But a detailed, generalized study of epitaxial growth related controlling parameters for tailoring the transition properties of MIT Oxides is yet to be undertaken. In this paper, we present a detailed study of some prospective controlling parameters which have significant effects on the MI transition of all members in RNiO3 family. In particular, this paper focuses on the possible tuning of TMIT with an aim of making the aforementioned MIT oxides prospective for device application. Moreover, this paper also studies recently reported [6] antiferromagnetism (AF) and anonymous resistivity of rare earth nickelates to further assist the tailoring of MI transition characteristics. Procedure for Paper Submission II.TAILORING OF METAL-INSULATOR (MI) TRANSITION PROPERTIES A. Effect of Substrate on TMIT Limited works have been performed to uncover suitable substrates for the growth of MIT Oxides and study the corresponding effects on MI Transition behavior. Epitaxial growths of NdNiO3 on LaAlO3 (LAO), SiO2, MgO and SrTiO3 (STO) [7], SmNiO3 on Si, LaAlO3 (LAO) and SrLaAlO4 (SLAO) [4], PrNiO3 on LAO, STO and Sapphire [8] have been reported previously. The concept of lattice-mismatch epitaxial strain has been used to tailor the electronic properties of MIT oxides [4,5,7,9] because of its efficient uses in emerging semiconductor heterostructures. In strain layer epitaxy, after a critical thickness (hc) has been reached, most of the elastic strain energy in the deposited material encourages
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the formation of dislocation and only a residual strain exists there. But it has been found [10-13] that, even this small residual strain is sufficient to modify the electrical properties of the deposited thin film. The correlation between TMIT and substrate materials is still vague in a sense that different MIT oxides show dissimilar tailoring of TMIT on the basis of compressive or tensile strain in the epitaxial layer. For the epitaxial growth of NdNiO3, it is observed that as the state of the thin film layer changes from tensile strain to compressive strain on substrate, TMIT gradually increases [7]. On the other hand, SmNiO3 shows completely opposite substrate effects. Tailoring of TMIT for SmNiO3 increases in tensile strain whereas it decreases in compressive strain. Table I evidently summarizes this effect with experimental values. TABLE I EFFECTS OF SUBSTRATE ON TMIT
NdNiO3 (3.81)
SmNiO3 (3.80)
Substrate with Lattice Parameter in Å
Lattice Mismatch (%)
Type Strain
of
TMIT (K)
LAO (3.79) STO (3.90) MgO (4.21) LAO (3.79) SLAO (3.75) Si (5.43)
-0.527
Compressive
280
2.3
Tensile
200
9.5
Tensile
60
-0.263
Compressive
360
-1.33
Compressive
335
28.10
Tensile
455
C. EFFECT OF DEPOSITION TEMPERATURE ON TMIT AND RESISTIVITY MI transition is also affected by thin film deposition temperature and thus it can be another prospective controlling parameter for tailoring MI transition. It has been observed that the film deposited at lower temperature is completely metallic, without showing any significant MI transition. Again film deposited at a too high temperature shows metallic behavior. In practice, the RNiO3 shows an increase in TMIT and resistivity as the film deposition temperature increases. After reaching the maximum value, both of them decrease with deposition temperature. This effect has been reported for NdNiO3 film on LaNiO3 substrate. 600 550 500 Resistivity (micro-ohm-cm)
MIT Oxide with Lattice Parameter in Å
nm) on LAO substrate was investigated [4]. It reported the ratio of Ni3+ and Ni2+ as approximately 75% of relative concentration. But it fell down to 54% due to the increase of Ni2+ cations in a thicker film which has a greater ionic radius than Ni3+ cations. As a result oxygen internalization decreases. So oxygen non-stoichiometry increases with film thickness and vice versa.
B. Effect of Thickness of the Film on TMIT MIT Oxides have, in principle, advantages in device scaling because correlated oxides have essentially metallic electron densities (1022 to 1023 cm−3), even in their insulating phases [14]. This will undoubtedly help to ensure a sufficient number of carriers in nanoscale devices and hence avoid the limitation of density fluctuations, which are becoming increasingly important in conventional semiconductor devices. So the scaling of MIT Oxides and its effects on TMIT is crucial for its effective use in nanoscale devices [15]. Continuous change in phase transition behavior of MIT Oxides has been reported as the film is scaled [5]. The effect of strain at film-substrate interface becomes more significant with the scaling of the film. For NdNiO3 on LAO substrate and SmNiO3 on LAO and SLAO substrate, if the thickness of the film increases, TMIT shifts to a higher temperature. Similarly, TMIT shifts to a lower temperature when the thickness of the film decreases [4,7]. Not only TMIT but also resistivity of the MIT Oxides and oxygen non-stoichiometry are related with thickness of the film. The resistivity of PrNiO3 film on LAO substrate decreases with the thickness of the film [8] and the film becomes metallic. In order to relate oxygen non-stoichiometry with scaling for RNiO3, a sufficiently thin film of SmNiO3 (80
450 400 350 300 250 200 150 600
620
640
660 680 700 720 740 Deposition Temperature (K)
760
780
800
Figure 1. Resistivity Versus Deposition Temperature Curve for NdNiO3 Film on LaNiO3 Substrate
Possible explanation of deposition temperature effects on the resistivity can be drawn by the film’s X-ray diffraction pattern reported in [5]. Films deposited at temperature below 400ͼC are amorphous and they begin to crystalline when deposition temperature exceeds 400˚C. Hence films show low resistivity as well as poor MI transition with low TMIT. Films gradually become polycrystalline at deposition temperatures below 700˚C. Films deposited at 750˚C show the best c-axis textured XRD pattern with preferred orientation. As the deposition temperature increases further, the film quality degrades and NiO phase is observed in XRD pattern. This is due to the fact that at higher deposition temperature, the NdNiO3 phase gets disassociated into Nd2NiO4 and NiO phases. Zang et al. [16] observed the metal-to-metal transition in this phase.
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135
III. ANTIFERROMAGNETISM CHARACTERISTIC AND
MI Transition Temperature (K)
130
ANONYMOUS RESISTIVITY
125
120
115
110
105
100 700
710
720
730 740 750 760 770 Deposition Temperature (K)
780
790
800
Figure 2. MI Transition Temperature (TMIT) Versus Deposition Temperature Curve for NdNiO3 Film on LaNiO3 Substrate
D. EFFECTS OF DOPING ON TMIT AND UNIT CELL VOLUME Very few efforts have been reported to study the tailoring of TMIT when RNiO3 is doped with different materials or other rare earth materials. Continuous control of TMIT in wide temperature ranges have been demonstrated in solid solutions such as Sm1−xNdxNiO3 and La1−xEuxNiO3 bulk polycrystals [17,18]. Ca doped SmNiO3 (Sm1-xCaxNiO3) which is deposited on (001)-oriented LaAlO3 substrate by keeping the substrate temperature at 600°- 650°C [6], is a prospective candidate for the tuning of TMIT with change in doping(x). In Sm1-xCaxNiO3 films, metal insulator transition appears around 370K and it has been noticed that this temperature decreases to room temperature only by 1%-2% of Ca doping [6]. Again TMIT of this film gradually decreases with increase in Ca content and then completely vanishes for Ca content above 10%. Another rare earth material, Nd doped SmNiO3 (Sm1-xNdxNiO3) on NdGaO3 substrate also shows systematic decrease in TMIT with increase of Nd content. It has been reported that for Sm1xNdxNiO3, TMIT decreases from 378K for x=0 to 199K for x=1 [19]. Similar characteristics also have been observed for different doping component in RNiO3, e.g., Nd1-xCax/SrxNiO3, La1-xEuxNiO3 etc. So for every instance, it is seen that TMIT decreases with the increase of doping content. Change in doping content also has an effect on unit cell volume. A distinct change in unit cell volume is observed by varying doping content. In terms of Sm1-xCaxNiO3, a tiny reduction in unit cell volume is observed with increase in Ca content up to 3% [6]. It is then followed by an increase in unit cell volume with Ca content greater than 3%. This phenomenon is a consequence of the substitution of Sm3+ by Ca2+ where the ionic radius of Ca2+ is a little larger than Sm3+ which can distort NiO6 octahedron or Ni-O-Ni bond angle. Similar observations have been found for Sr or Ca doped NdNiO3 films [20].
While transitioning through the insulating phase, some rareearth nickelates (RNiO3) show antiferromagnetism (AF), a property where the electrons of materials do not align themselves with the same magnetic polarity. In this case RNiO3 materials illustrate a characteristic opposite to ferromagnetism. This phenomenon does not exist for a long period of time and generally decreases with the increase of TMIT. The temperature at which this characteristic occurs is referred as Neel temperature (TN). The AF transition for RNiO3 with R = Sm and Eu occurs much lower than TMIT whereas the AF transition coincides with TMIT for RNiO3 with R = Pr and Nd [6]. Reported values show SmNiO3 undergoing MI transition at 403K with TN at 225 K and PrNiO3 shows MI transition at 135K with TN also at 135K. In terms of doping of rare-earth nickelates with other materials (e.g. Ca/Co) or other rare earth materials (e.g. Sr/Eu/Nd), the appearance of AF and its characteristic depend on the choice of dopant material and the amount of doping. In case of Ca doped rare earth nickelates like Sm1-xCaxNiO3, AF transition depends on the value of x. The peaks in –dln(ρ)/dT shift towards the lower temperature with Ca doping. Here, a kink is found below TMIT at around 200K where Ca doping is actually less than 4% [6]. So it can be said that TN can be found within the x limit of 0≤x≤0.04. On contrary, only one peak in the –dln(ρ)/dT plot is observed when the doping is higher than 4%, indicating a possible coincidence of the MI and the AF transitions [6]. But the AF effect vanishes around 10% Ca doping. However it is difficult to determine TN from magnetic measurement of Sm1-xCaxNiO3 films, because the AF arrangement of the low spin Ni3+ ions delivers only a tiny contribution to the magnetic properties of the whole material.. In case of Co doping materials like SmNi1-xCoxO3, AF appears up to 20% of Co doping [21,22]. From the suppression doping percentage, it can be concluded that Ca doping is more effective in suppressing AF than Co doping. It has also been reported that 10% of Ca or Sr doping in NdNiO3 bulk polycrystals is enough to suppress the MI transition thoroughly [6]. So it can be articulated that Nd1xCax/SrxNiO3 will show AF transition which is similar to Sm1xCaxNiO3 with TN at around 200K and the effect will sustain more or less 10% of Ca/Sr doping. In case of other rare earth materials like Sm1-xNdxNiO3 and La1-xEuxNiO3 systematic MI transitions are observed. Sm1-xNdxNiO3 on NdGaO3 substrate shows MI transition from 378K to 199K for x=0 to x=1. According to the aforementioned discussion, there must be an AF effect on the insulating phase. So an anonymous resistivity peak at around 200K corresponding to the AF ordering in Ni sublattice is observed with different dopant materials. IV. CONCLUSION The tailoring of MI transition properties of MIT Oxides is crucial for its effective contribution towards post-silicon devices. Continuous attempts are being undertaken to utilize the potential of phase-change functions in transition metal
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oxides to implement sensors and nonvolatile memories. By controlling the epitaxial growth related parameters, it becomes plausible to scrupulously tailor the MI transition properties of rare earth nickelates. This work performs a detailed study on tailoring the MI transition properties of MIT Oxides by controlling some parameters e.g., choice of substrate, thickness and deposition temperature of the film and doping concentration. It also advocates the future prospect of selecting an optimum combination of epitaxial growth controlled parameters to devise MI transition pertinently for correlated electron devices.
[20] J.A. Alonso et al., “Influence of carrier injection on the metal–insulator transition in electron-and hole-doped R1-xAxNiO3 perovskites,” Phys. Rev.B., vol.52:13563-13569, 1995. [21] M.Castro et al., “Study of the phase transition in SmNiO3,” J. Phys.:Condens Matter, 11, 405, 1999. [22] J.Blasco et al., “Electronic and magnetic phase diagram of SmNi 1xCoxO3,” Phys. Rev.B., vol.59:14424, 1999.
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