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Nonlinear voltage-current characteristics of MnTe films with island growth
Vacuum xxx (2017) 1e7
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Nonlinear voltage-current characteristics of MnTe films with island growth Liang Yang, Zhenhua Wang*, Da Li, Zhidong Zhang Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2016 Received in revised form 17 January 2017 Accepted 18 January 2017 Available online xxx
The structure and transport properties of Mn0.98Cr0.02Te and MnTe films prepared by pulsed laser el temdeposition were investigated. The metal-semiconductor transition was observed below the Ne perature of MnTe. A diode-like behavior near the transition temperature was shown in the voltagecurrent characteristics for these films grown with incompletely connected islands. The rough surface was proven to be necessary for the nonlinear voltage-current behavior. The Schottky barrier between the Ag electrode and the rough surface of the film contributed to the nonlinear voltage-current characteristic. The inhomogeneity of Cr along the direction of currents could not induce the diode-like behavior. The nonlinear current-voltage relationship resulted in the shift of metal-semiconductor transition temperature. © 2017 Published by Elsevier Ltd.
Keywords: Nonlinear current-voltage characteristic Diode-like behavior Island growth MnTe film
1. Introduction el MnTe is a kind of antiferromagnetic semiconductor with Ne temperature (TN) of 307 K [1]. MnTe is the sole kind of manganese chalcogenides which shows semiconductor properties with a direct band-gap of about 1.27 eV [2]. The stable structure of MnTe is hexagonal NiAs-type [2], and the lattice constants are a ¼ 4.148 Å and c ¼ 6.711 Å [3]. The structure, magnetic and transport properties of MnTe [4,5] and Mn1-xCrxTe [6,7] films were investigated. In the previous work [4e7], some of the films represent different characteristics from the bulk ones, such as ferromagnetism at room temperature and the anomaly of resistivity below TN [4,6,7]. In the MnTe films with preferred crystallographic plane of (110), the lattice strain is inhomogeneous, but the total effect is compressive strain perpendicular to the surface of the film, which results to a plump down around 150 K-250 K in the resistance-temperature curve for thick films [5]. The resistivity and susceptibility in these films were increased below 100 K due to a magneto-elastic coupling [8], which was obvious in the film with a preferred orientation of (110) [2]. Due to the sp-d exchange interaction between spins of band electrons and localized magnetic moments of Mn, the MnTe film exhibited a transition in resistivity from semiconducting to metallic behavior. The transition temperature can be
* Corresponding author. E-mail address: [email protected] (Z. Wang).
changed by doping Cr or Cd, because this interaction was destroyed by adding Cr or Cd [6,9]. Nonlinear current-voltage characteristics and diode behavior are usually observed in the p-n junction or the metalsemiconductor junction, due to the barrier at the interface [10]. The p-n junction could be formed in one kind of material by heterogeneous doping. For instance, diode-like behavior was found in Si nanowires [11]. For the interface of a metal and a semiconductor, the Schottky barrier height not only depends on the work function of the contacted metal and semiconductor, but also, can be influenced by surface conditions [10]. For example, Zn vacancies at the ZnO/Au interface raise the Schottky barrier height [12]. The Au/ZnO Schottky barrier was affected by the surface treatment [13]. It was also pointed out that oxygen plasma treatment changed the ohmic connection between Au and ZnO to a Schottky connection [14]. The nonlinear V-I relationship also exists in the resistive switch phenomenon, which has the hysteretic I-V characteristics [15]. The resistive switch phenomenon is usually attributed to a filament conduction path such as an oxygen vacancy filament [16], trapping of charge carriers, or a Mott transition induced by carriers doped at the interface [15]. In this work, MnTe and Mn1-xCrxTe films with island growth were prepared by pulsed laser deposition (PLD). The transport properties were studied from 350 K to 10 K, and a transition from metal to semiconductor was found in the range of el temperature of the MnTe. Diode-like 220 Ke300 K, below the Ne behavior and nonlinear voltage-current (V-I) characteristics were revealed near the transition temperature. It was understood that
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the nonlinear characteristic was induced by the Schottky barrier between the rough surface of the films and the Ag electrodes. 2. Material and methods Mn0.98Cr0.02Te and MnTe targets were prepared by a solidreaction of Mn (99.9%), Cr (99.9%) and Te (99.9%) elements at 725 C for 72 h in Ar. The prepared films were affected by many deposition parameters, such as the substrate temperature, the pulse frequency, the energy of each pulse, the material of the substrate, and the gas pressure in the deposition chamber. Here we focused on the roughness of the films, and we found that the roughness of the film was increased when increasing the deposition temperature. So the deposition temperature was set at 700 C which is the highest temperature of our PLD equipment. Mn0.98Cr0.02Te and MnTe films deposited at 700 C were prepared by PLD (laser wave-length 248 nm, LPX 300, Lambda Physik). At first the Si substrate with amorphous SiO2 surface of 1 mm thickness was heated to 700 C. Then the film was deposited using a frequency of 2 Hz. Finally the film was cooled down to room temperature at 7 C/min. A MnTe film deposited at room temperature was also prepared with frequency of 2 Hz, then it experienced the same heating and cooling processes as described above. The crystal structure of these films was examined by X-ray diffraction (XRD) in a D/max-gA diffractometer with Cu Ka radiation. The surface morphology of the films was examined by scanning electron microscopy (SEM) in an INSPECT F50 instrument, operating at 15 kV and 30 kV, respectively. The transmission electron microscopy (TEM) and the high-resolution transmission electron microscopy (HRTEM) images were obtained with a F20 instrument. The transport properties of the films were measured using a four probe method by a physical property measurement system (PPMS). The drive current was applied and the resistance and voltage were measured. The regions of the sample under the electrodes of Vþ and V- have different distances from the centre of the plasma plume. 3. Results and discussion The XRD results shown in Fig. 1 illustrate that both the Mn0.98Cr0.02Te and MnTe films deposited at 550 mJ and 700 C have the MnTe structure of NiAs-type with weak preferred orientation of (110). The diffraction peaks of Mn0.98Cr0.02Te film are shifted to higher angles, compared with those of MnTe film, implying that some Mn might be substituted by Cr, which can be seen clearly in the inset of Fig. 1. The lattice parameter, a, of Mn0.98Cr0.02Te film was calculated to be 4.131 Å. No impurity was observed from the XRD patterns. The transport property of the films in this paper was measured by the four-probe method, and a sketch of the samples is shown in Fig. 2(a). The size of the samples was about 8 3 mm2, and the radius of electrodes was about 1 mm. The distance between the Vþ and V- electrodes was 3e5 mm. The SEM images of Mn0.98Cr0.02Te film near the Vþ and V- electrodes are shown, respectively in Fig. 2(b) and (c). The islands were not completely connected, which reveals the feature of island growth and the gaps between islands are shown in the SEM images. Many cavities on the surface of grains near the Vþ electrode are observed and the density of cavities is low near the V- electrode. The islands, gaps and cavities are labeled in Fig. 2(b) and (c). The size of the islands is about 500 nm. The thickness of the film near both electrodes was estimated by side view SEM images to be about 270 nm. The substrate temperature was set at 700 C. The expanded plasma with high speed reaches the hot substrate. There the particles gather and form crystal nuclei. When the wettability of the deposited material to the substrate was weak, the particles tended to combine with
each other and form three-dimensional island-like nuclei, instead of a two-dimensional layer. Continuous sputtering added more particles to the surface and made the nuclei grow to form grains. The grains gradually grow and contact each other and then a rough film is formed. In addition there can be a layer followed by island growth mechanism during film growth. The TEM image in Fig. 3 shows that the island grains grew from the substrate to the surface, and the average size of islands is about 400 nm width and 250 nm height, consistent with SEM images. It is clearly seen from the inset of Fig. 3 that the grain is well crystallized. The calculated lattice parameters are a ¼ 4.234 Å and c ¼ 6.700 Å as marked in the HRTEM image. The different lattice parameters obtained in the HRTEM of the grain and the XRD are due to the different measurement modes. The XRD gives statistical data with many particles and shows some broad peaks, so it is difficult to get the exact spacing of a crystal. However, the HRTEM shows the spacing of only individual pairs of particles. The temperature-dependent resistance (R-T) curve of the Mn0.98Cr0.02Te film deposited at 550 mJ and 700 C was measured from 350 K to 10 K as shown in Fig. 4(a). The transport characteristic was measured by the four point probe method, with Ag paste applied as electrodes. The conducting behavior is similar to the MnTe thin films reported before [4,6,7]. The Mn0.98Cr0.02Te film shows a semiconductor characteristic in the range of 350 K-250 K and 70 K-10 K. The increase of the resistance below 60 K is caused by a magneto-elastic coupling [8]. In the range of 250 K-70 K, the resistivity increases as the temperature increases, with metallic character. The transition from metal to semiconductor was found at el about 250 K in the Mn0.98Cr0.02Te film, which is lower than the Ne temperature (TN) of MnTe. This phenomenon has been reported in the MnTe films deposited by molecular beam epitaxy (MBE) [4]. el temperature of bulk MnTe is about 307 K, the Although the Ne temperature can be reduced due to lack of close contact between crystal grains [17]. Above 250 K, the resistance increases as the temperature decreases, showing semiconductor behavior. Below 250 K, the degree of magnetic disorder influences the scattering of carriers in the antiferromagnetic film. The magnon scattering is el temperature, signifying the maximum value of strongest at the Ne resistance [8]. Then the disorder of magnetic moments is reduced
Fig. 1. XRD patterns of Mn0.98Cr0.02Te and MnTe film deposited at 700 C. The pink vertical lines represent the diffraction angle and diffraction intensity from PDF card of MnTe with NiAs-type structure. The diffraction peaks of the sample are marked by the corresponding Miller indexes.
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Fig. 2. (a) The schematic diagram of the sample in four probe measurement. Surface SEM images of the area near Vþ electrode (b) and V- electrode (c) of the Mn0.98Cr0.02Te film. The insets show the cross-section image of corresponding sample. The boundary of islands and gaps between islands are marked by yellow. The cavities on the surface are circled. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Sectional TEM image of Mn0.98Cr0.02Te film. The inset is HRTEM image of the area pointed by the arrow. The edge of islands is indicated by yellow lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
el temperature. In the as the temperature is decreased from the Ne range of 250 K-70 K, the effect of reduced magnon scattering is stronger than the effect of decreased carrier density, so the resistance will decrease with decreasing temperature. In the range of 70 K-10 K, because the density of thermally activated carriers decreases with decreasing temperature, the resistance increases. As shown in Fig. 4(b), obvious diode-like voltage-current (V-I) curves were obtained in the range of 180 K-340 K. The diode-like behavior is obvious near the transition temperature, and it becomes weak at temperatures far from it. From the above data, it is known that the diode-like behavior appears in the Mn0.02Cr0.98Te films, in which the islands are not completely connected. The structural or componential inhomogeneity between the two ends of the sample may be the origin of the diode-like behavior. In addition unequal Schottky barriers between the electrode and the film at Vþ and V- also should be considered as a possible reason. If the Schottky barriers at Vþ and V- were equal, the V-I curve would be centrosymmetric [14]. It is well known that the barrier is controlled by the work function and interface state [9]. Thus the rough surface will enhance the inequality in the Schottky barriers. To understand further, we investigated these possibilities by comparing experimental results as below. At first the influence of the rough film surface was considered. In order to exclude the possibility of the unequal Cr percentages at two ends of the sample, the MnTe film was prepared. The properties
Fig. 4. (a) Temperature dependence of resistance with temperature from 350 K to 10 K for the Mn0.98Cr0.02Te film on Si/SiO2 substrate at 700 C. (b) V-I characteristics of the Mn0.98Cr0.02Te film in the range of 180 K-340 K.
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Fig. 5. (a) R-T curve of MnTe film deposited at 700 C, with a laser of 500mJ. (b) V-I curves of the film at 220 K, 280 K and 320 K. (c) V-I curves of the film at 220 K and 240 K with applying positive current. (d) V-I curves of the film at 220 K and 240 K with applying negative current. (e) SEM image of the film. The inset shows the cross-section image of the sample.
of the MnTe film deposited at 700 C are displayed in Fig. 5. Ag served as electrodes in the electrical measurements. The R-T curve shown in Fig. 5(a) of the MnTe film illustrates similar conducting behavior to that of the Mn0.02Cr0.98Te films. The transition from semiconductor to metal was found at about 220 K, which is lower than the TN for MnTe. The diode-like behavior was achieved in the V-I curve above 220 K, as shown in Fig. 5(b). It is worth noting that
the transition temperature is shifted from about 220 K to 240 K by raising the measuring current from 0.01 mA to 0.1 mA, due to the nonlinear V-I relationship. Fig. 5(c, d) illustrate the V-I curves of the film at 220 K and 240 K when applying positive and negative currents respectively. When the current is 0.01 mA, the absolute value of the corresponding voltage at 220 K is larger than that at 240 K but when the current is 0.1 mA, the absolute value of the voltage at
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Fig. 6. (a) R-T curve of MnTe film deposited at room temperature with 500 mJ and annealed at 700 C. (b) V-I curves of the film at different temperatures. (c) SEM image of the film. The inset shows the cross-section image of the sample.
Fig. 7. (a) R-T curve of MnTe film deposited at 700 C, 550 mJ (Ag electrodes). (b) V-I curves of the film at different temperatures (Ag electrodes). (c) and (d) SEM images of the film at Vþ and V- (Ag electrodes). (e) R-T curve measured with In electrodes. (f) V-I curves measured with In electrodes.(g) and (h) SEM images of the film at V- and Vþ (In electrodes).
220 K is smaller than that at 240 K. So the point of maximum value of resistance is shifted from 220 K to 240 K when changing the current for measurements from 0.01 mA to 0.1 mA. Furthermore, the resistance obtained with measurement for a current of 0.1 mA is bigger than that measured with a current of 0.01 mA in the range of 350 K-220 K. The unequal resistance at 0.01 mA and 0.1 mA in the range of 350 K-220 K is also attributed to the nonlinear V-I relationship. The shape of the V-I curves is distinctly different from the V-I curve of a single p-n junction or metal-semiconductor contact [10,11]. This suggests that the existence of unequal Schottky barriers at the two ends of the film is the basic reason for the results. The SEM images illustrated in Fig. 5(e) show that the film is rough with islands incompletely connected. At this point, the roughness of the surface is a reason for the diode-like V-I curves. In order to further certify that the nonlinear V-I relationship is due to the rough surface formed by incompletely connected islands, the MnTe film with continuous island growth was synthesized by depositing the film at room temperature and then annealing at 700 C. When the deposition temperature was set at room
temperature, the plasma reached the cold substrate and formed a continuous and flat amorphous film. After annealing, the amorphous film crystallized to form a flat crystalline film. R-T properties of MnTe films with a flat surface are similar to those of the films with a rough surface. As shown in Fig. 6(a), the transition from metal to semiconductor is found at 290 K, which is near the TN of MnTe, indicating the maximum of magnon drag [8]. The V-I curves shown in Fig. 6(b) give linear features at each temperature. Here Ag was also used as electrodes during the measurement process. The SEM image in Fig. 6(c) displays that the film was still in island growth with some cracks, and the thickness was about 420 nm. However, the islands attached each other without cavities. The comparison of rough and flat MnTe films indicates that the rough surface of MnTe is necessary for the diode-like behavior. The origin of the large Schottky barrier between the rough MnTe film and electrodes was confirmed. We investigated the influence of different electrodes for MnTe films deposited at 700 C. Fig. 7(a), (b), (e) and (f) illustrate R-T and V-I curves measured with Ag and In electrodes, respectively. The R-T curves of the two samples are
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Fig. 8. (a) R-T curve of Mn0.95Cr0.05Te/MnTe junction deposited at room temperature (In electrodes). (b) V-I curves of the film at different temperatures (In electrodes). (c) SEM image of the film at the MnTe part. The insets show the enlarged image and the cross-section image of this part. (d) SEM image of Mn0.95Cr0.05Te film part. The inset shows the crosssection image of this part.
similar to that for MnTe films with a flat surface prepared at room temperature. The difference is that a nonlinear V-I characteristic occurs in the sample with Ag electrodes, while a linear V-I characteristic exists in the sample with In electrodes. Moreover, a hysteresis loop is observed in the V-I curves measured with Ag electrodes by applying negative current. Such a characteristic suggests the existence of a complicated interface, affected by oxidation and defects, between the film and the electrodes [15]. Ag electrodes are bonded to the film by silver paste, so the granular Ag with possible external oxidization in the paste makes a higher barrier between the electrodes and the film. The rough surface will also enhance the Schottky effect. Such a state will enhance the interface barrier, leading to more obvious nonlinear V-I relationship. The barrier will lower for the flat film surface with good electrode-film contact. Because of this, the V-I curve becomes linear. Considering the different work function and different contact condition of In and Ag, we could deduce that the larger Schottky barrier in the interface of rough MnTe film and Ag electrode contributed to the nonlinear V-I relationship. However, the film surfaces at Vþ and V- electrodes are almost same as shown in Fig. 7(c, d, g and h). That is why the samples did not give asymmetric diode-like behavior. Moreover, on account of the weak roughness of this sample compared with the film shown in Fig. 5(e), the result indicates that the roughness of the film is essential to the diode-like characteristic. A Mn0.95Cr0.05Te/MnTe junction was prepared to investigate the influence of componential inhomogeneity on the V-I relationship.
To amplify the possibly influence of component inhomogeneity, a Mn0.95Cr0.05Te film was prepared to replace the Mn0.98Cr0.02Te film, which may create more unbalanced composition distribution. A flat sample with the structure of Mn0.95Cr0.05Te/MnTe junction was prepared by depositing at room temperature and then annealing at 700 C. The Vþ and Iþ electrodes were set on the MnTe part, while the V- and I- electrodes were set on the Mn0.95Cr0.05Te part. Here all of the electrodes are In, in order to exclude the contribution of the Schottky barrier between MnTe and the electrodes to the nonlinear V-I curve. The RT curve as shown in Fig. 8(a) is similar to that for the MnTe film. As shown in Fig. 8(b), the V-I curve has approximately linear relationship; illustrating that 5% substitution of Mn by Cr does not induce apparent diode-like behavior. Both of the two parts are flat with grains connected completely, as shown in Fig. 8(c) and (d), which exclude the disturbance of the rough surface. This result testifies that the diode-like behavior is not attributed to the Cr inhomogeneity in the sample. 4. Conclusions The Mn0.98Cr0.02Te and MnTe films were prepared by PLD, showing the island growth when deposited at 700 C. With increasing the measure temperature, the resistance of all samples has semiconductor character in the temperature range of about 10 Ke70 K and has metallic character at higher temperature. There is a metal-semiconductor transition around the range of 220 K300 K, and the samples show semiconductor character above the
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transition temperature. The transition temperature of all samples el temperature of MnTe. The was found to be below the Ne nonlinear V-I characteristics and diode-like behavior are present in the films deposited at 700 C, of which the islands are incompletely connected and which show a rough surface. The high Schottky barrier between the incompletely connected grains and Ag electrodes contributes to the nonlinear V-I characteristic, and the diode-like V-I behavior was ascribed to the inhomogeneous Schottky barrier. The inhomogeneity of Cr does not induce diodelike behavior. The shift of metal-semiconductor transition temperature when enhancing the measuring current was attributed to the nonlinear V-I relationship. Acknowledgment This work was supported by the National Natural Science Foundation of China with Grant No. 51522104 and 51331006. References [1] K. Ozawa, S. Anzai, Y. Hamaguchi, Effect of pressure on the magnetic transition point of manganese telluride, Phys. Lett. 20 (2) (1966) 132e133. [2] W. Szuszkiewicz, E. Dynowska, B. Witkowska, B. Hennion, Spin-wave measurements on hexagonal MnTe of NiAs-type structure by inelastic neutron scattering, Phys. Rev. B 73 (10) (2006). [3] C. Reig, V. Munoz, C. Gomez, C. Ferrer, A. Segura, Growth and characterisation of MnTe crystals, J. Cryst. Growth 223 (3) (2001) 349e356. [4] W. Kim, I.J. Park, H.J. Kim, W. Lee, S.J. Kim, C.S. Kim, Room-temperature ferromagnetic property in MnTe semiconductor thin film grown by molecular beam epitaxy, IEEE Trans. Magn. 45 (6) (2009) 2424e2427.
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[5] L. Yang, Z.H. Wang, Z.D. Zhang, Electrical properties of NiAs-type MnTe films with preferred crystallographic plane of (110), J. Appl. Phys. 119 (4) (2016). [6] Z.H. Wang, D.Y. Geng, W.J. Gong, J. Li, Y.B. Li, Z.D. Zhang, Effect of adding Cr on magnetic properties and metallic behavior in MnTe film, Thin Solid Films 522 (2012) 175e179. [7] Z.H. Wang, D.Y. Geng, J. Li, Y.B. Li, Z.D. Zhang, Magnetic and Transport Properties of Mn0.98Cr0.02Te epitaxial films grown on Al2O3 substrates, J. Mater. Sci. Technol. 30 (2) (2014) 103e106. [8] J.B.C. Efrem D'Sa, P.A. Bhobe, K.R. Priolkar, A. Das, S.K. Paranjpe, R.B. Prabhu, P.R. Sarode, Low-temperature neutron diffraction study of MnTe, J. Magn, Magn. Mater 285 (1e2) (2005) 267e271. [9] S. Chandra, L.K. Malhotra, S. Dhara, A.C. Rastogi, Low-temperature dynamic susceptibility of thin Cd1-xMnxTe films, Phys. Rev. B 54 (1996) 13694. [10] D. Newmen, Semiconductor Physics and Devices: Basic Principles, McGrawHill Companies, New York, 2003. [11] K. He, J.-H. Cho, Y. Jung, S. Tom Picraux, J. Cumings, Silicon nanowires: electron holography studies of doped pen junctions and biased Schottky barriers, Nanotechnology 24 (11) (2013) 115703. [12] J.D. Hwang, Y.L. Lin, C.Y. Kung, Enhancement of the Schottky barrier height of Au/ZnO nanocrystal by zinc vacancies using a hydrothermal seed layer, Nanotechnology 24 (11) (2013) 115709. [13] B.J. Coppa, C.C. Fulton, S.M. Kiesel, R.F. Davis, C. Pandarinath, J.E. Burnette, R.J. Nemanich, D.J. Smith, Structural, microstructural, and electrical properties of gold films and Schottky contacts on remote plasma-cleaned, n-type ZnO {0001} surfaces, J. Appl. Phys. 97 (10) (2005) 103517. [14] H.L. Mosbacker, Y.M. Strzhemechny, B.D. White, P.E. Smith, D.C. Look, D.C. Reynolds, C.W. Litton, L.J. Brillson, Role of near-surface states in ohmicSchottky conversion of Au contacts to ZnO, Appl. Phys. Lett. 87 (1) (2005) 012102. [15] Akihito Sawa, Resistive switching in transition metal oxides, Mater. Today 11 (6) (2008) 28, 1717171. [16] Debanjan Jana, Subhranu Samanta, Siddheswar Maikap, Hsin-Ming Cheng, Evolution of complementary resistive switching characteristics using IrOx/ GdOx/Al2O3/TiN structure, Appl. Phys. Lett. 108 (1) (2016) 011605. el temperature on the [17] A.T. Raghavender, Nguyen Hoa Hong, Dependence of Ne particle size of MnFe2O4, J. Magn. Magn. Mater 32 (16) (2011) 2145.
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Nonlinear voltage-current characteristics of MnTe films with island growth Liang Yang, Zhenhua Wang*, Da Li, Zhidong Zhang Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2016 Received in revised form 17 January 2017 Accepted 18 January 2017 Available online xxx
The structure and transport properties of Mn0.98Cr0.02Te and MnTe films prepared by pulsed laser el temdeposition were investigated. The metal-semiconductor transition was observed below the Ne perature of MnTe. A diode-like behavior near the transition temperature was shown in the voltagecurrent characteristics for these films grown with incompletely connected islands. The rough surface was proven to be necessary for the nonlinear voltage-current behavior. The Schottky barrier between the Ag electrode and the rough surface of the film contributed to the nonlinear voltage-current characteristic. The inhomogeneity of Cr along the direction of currents could not induce the diode-like behavior. The nonlinear current-voltage relationship resulted in the shift of metal-semiconductor transition temperature. © 2017 Published by Elsevier Ltd.
Keywords: Nonlinear current-voltage characteristic Diode-like behavior Island growth MnTe film
1. Introduction el MnTe is a kind of antiferromagnetic semiconductor with Ne temperature (TN) of 307 K [1]. MnTe is the sole kind of manganese chalcogenides which shows semiconductor properties with a direct band-gap of about 1.27 eV [2]. The stable structure of MnTe is hexagonal NiAs-type [2], and the lattice constants are a ¼ 4.148 Å and c ¼ 6.711 Å [3]. The structure, magnetic and transport properties of MnTe [4,5] and Mn1-xCrxTe [6,7] films were investigated. In the previous work [4e7], some of the films represent different characteristics from the bulk ones, such as ferromagnetism at room temperature and the anomaly of resistivity below TN [4,6,7]. In the MnTe films with preferred crystallographic plane of (110), the lattice strain is inhomogeneous, but the total effect is compressive strain perpendicular to the surface of the film, which results to a plump down around 150 K-250 K in the resistance-temperature curve for thick films [5]. The resistivity and susceptibility in these films were increased below 100 K due to a magneto-elastic coupling [8], which was obvious in the film with a preferred orientation of (110) [2]. Due to the sp-d exchange interaction between spins of band electrons and localized magnetic moments of Mn, the MnTe film exhibited a transition in resistivity from semiconducting to metallic behavior. The transition temperature can be
* Corresponding author. E-mail address: [email protected] (Z. Wang).
changed by doping Cr or Cd, because this interaction was destroyed by adding Cr or Cd [6,9]. Nonlinear current-voltage characteristics and diode behavior are usually observed in the p-n junction or the metalsemiconductor junction, due to the barrier at the interface [10]. The p-n junction could be formed in one kind of material by heterogeneous doping. For instance, diode-like behavior was found in Si nanowires [11]. For the interface of a metal and a semiconductor, the Schottky barrier height not only depends on the work function of the contacted metal and semiconductor, but also, can be influenced by surface conditions [10]. For example, Zn vacancies at the ZnO/Au interface raise the Schottky barrier height [12]. The Au/ZnO Schottky barrier was affected by the surface treatment [13]. It was also pointed out that oxygen plasma treatment changed the ohmic connection between Au and ZnO to a Schottky connection [14]. The nonlinear V-I relationship also exists in the resistive switch phenomenon, which has the hysteretic I-V characteristics [15]. The resistive switch phenomenon is usually attributed to a filament conduction path such as an oxygen vacancy filament [16], trapping of charge carriers, or a Mott transition induced by carriers doped at the interface [15]. In this work, MnTe and Mn1-xCrxTe films with island growth were prepared by pulsed laser deposition (PLD). The transport properties were studied from 350 K to 10 K, and a transition from metal to semiconductor was found in the range of el temperature of the MnTe. Diode-like 220 Ke300 K, below the Ne behavior and nonlinear voltage-current (V-I) characteristics were revealed near the transition temperature. It was understood that
http://dx.doi.org/10.1016/j.vacuum.2017.01.015 0042-207X/© 2017 Published by Elsevier Ltd.
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the nonlinear characteristic was induced by the Schottky barrier between the rough surface of the films and the Ag electrodes. 2. Material and methods Mn0.98Cr0.02Te and MnTe targets were prepared by a solidreaction of Mn (99.9%), Cr (99.9%) and Te (99.9%) elements at 725 C for 72 h in Ar. The prepared films were affected by many deposition parameters, such as the substrate temperature, the pulse frequency, the energy of each pulse, the material of the substrate, and the gas pressure in the deposition chamber. Here we focused on the roughness of the films, and we found that the roughness of the film was increased when increasing the deposition temperature. So the deposition temperature was set at 700 C which is the highest temperature of our PLD equipment. Mn0.98Cr0.02Te and MnTe films deposited at 700 C were prepared by PLD (laser wave-length 248 nm, LPX 300, Lambda Physik). At first the Si substrate with amorphous SiO2 surface of 1 mm thickness was heated to 700 C. Then the film was deposited using a frequency of 2 Hz. Finally the film was cooled down to room temperature at 7 C/min. A MnTe film deposited at room temperature was also prepared with frequency of 2 Hz, then it experienced the same heating and cooling processes as described above. The crystal structure of these films was examined by X-ray diffraction (XRD) in a D/max-gA diffractometer with Cu Ka radiation. The surface morphology of the films was examined by scanning electron microscopy (SEM) in an INSPECT F50 instrument, operating at 15 kV and 30 kV, respectively. The transmission electron microscopy (TEM) and the high-resolution transmission electron microscopy (HRTEM) images were obtained with a F20 instrument. The transport properties of the films were measured using a four probe method by a physical property measurement system (PPMS). The drive current was applied and the resistance and voltage were measured. The regions of the sample under the electrodes of Vþ and V- have different distances from the centre of the plasma plume. 3. Results and discussion The XRD results shown in Fig. 1 illustrate that both the Mn0.98Cr0.02Te and MnTe films deposited at 550 mJ and 700 C have the MnTe structure of NiAs-type with weak preferred orientation of (110). The diffraction peaks of Mn0.98Cr0.02Te film are shifted to higher angles, compared with those of MnTe film, implying that some Mn might be substituted by Cr, which can be seen clearly in the inset of Fig. 1. The lattice parameter, a, of Mn0.98Cr0.02Te film was calculated to be 4.131 Å. No impurity was observed from the XRD patterns. The transport property of the films in this paper was measured by the four-probe method, and a sketch of the samples is shown in Fig. 2(a). The size of the samples was about 8 3 mm2, and the radius of electrodes was about 1 mm. The distance between the Vþ and V- electrodes was 3e5 mm. The SEM images of Mn0.98Cr0.02Te film near the Vþ and V- electrodes are shown, respectively in Fig. 2(b) and (c). The islands were not completely connected, which reveals the feature of island growth and the gaps between islands are shown in the SEM images. Many cavities on the surface of grains near the Vþ electrode are observed and the density of cavities is low near the V- electrode. The islands, gaps and cavities are labeled in Fig. 2(b) and (c). The size of the islands is about 500 nm. The thickness of the film near both electrodes was estimated by side view SEM images to be about 270 nm. The substrate temperature was set at 700 C. The expanded plasma with high speed reaches the hot substrate. There the particles gather and form crystal nuclei. When the wettability of the deposited material to the substrate was weak, the particles tended to combine with
each other and form three-dimensional island-like nuclei, instead of a two-dimensional layer. Continuous sputtering added more particles to the surface and made the nuclei grow to form grains. The grains gradually grow and contact each other and then a rough film is formed. In addition there can be a layer followed by island growth mechanism during film growth. The TEM image in Fig. 3 shows that the island grains grew from the substrate to the surface, and the average size of islands is about 400 nm width and 250 nm height, consistent with SEM images. It is clearly seen from the inset of Fig. 3 that the grain is well crystallized. The calculated lattice parameters are a ¼ 4.234 Å and c ¼ 6.700 Å as marked in the HRTEM image. The different lattice parameters obtained in the HRTEM of the grain and the XRD are due to the different measurement modes. The XRD gives statistical data with many particles and shows some broad peaks, so it is difficult to get the exact spacing of a crystal. However, the HRTEM shows the spacing of only individual pairs of particles. The temperature-dependent resistance (R-T) curve of the Mn0.98Cr0.02Te film deposited at 550 mJ and 700 C was measured from 350 K to 10 K as shown in Fig. 4(a). The transport characteristic was measured by the four point probe method, with Ag paste applied as electrodes. The conducting behavior is similar to the MnTe thin films reported before [4,6,7]. The Mn0.98Cr0.02Te film shows a semiconductor characteristic in the range of 350 K-250 K and 70 K-10 K. The increase of the resistance below 60 K is caused by a magneto-elastic coupling [8]. In the range of 250 K-70 K, the resistivity increases as the temperature increases, with metallic character. The transition from metal to semiconductor was found at el about 250 K in the Mn0.98Cr0.02Te film, which is lower than the Ne temperature (TN) of MnTe. This phenomenon has been reported in the MnTe films deposited by molecular beam epitaxy (MBE) [4]. el temperature of bulk MnTe is about 307 K, the Although the Ne temperature can be reduced due to lack of close contact between crystal grains [17]. Above 250 K, the resistance increases as the temperature decreases, showing semiconductor behavior. Below 250 K, the degree of magnetic disorder influences the scattering of carriers in the antiferromagnetic film. The magnon scattering is el temperature, signifying the maximum value of strongest at the Ne resistance [8]. Then the disorder of magnetic moments is reduced
Fig. 1. XRD patterns of Mn0.98Cr0.02Te and MnTe film deposited at 700 C. The pink vertical lines represent the diffraction angle and diffraction intensity from PDF card of MnTe with NiAs-type structure. The diffraction peaks of the sample are marked by the corresponding Miller indexes.
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Fig. 2. (a) The schematic diagram of the sample in four probe measurement. Surface SEM images of the area near Vþ electrode (b) and V- electrode (c) of the Mn0.98Cr0.02Te film. The insets show the cross-section image of corresponding sample. The boundary of islands and gaps between islands are marked by yellow. The cavities on the surface are circled. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Sectional TEM image of Mn0.98Cr0.02Te film. The inset is HRTEM image of the area pointed by the arrow. The edge of islands is indicated by yellow lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
el temperature. In the as the temperature is decreased from the Ne range of 250 K-70 K, the effect of reduced magnon scattering is stronger than the effect of decreased carrier density, so the resistance will decrease with decreasing temperature. In the range of 70 K-10 K, because the density of thermally activated carriers decreases with decreasing temperature, the resistance increases. As shown in Fig. 4(b), obvious diode-like voltage-current (V-I) curves were obtained in the range of 180 K-340 K. The diode-like behavior is obvious near the transition temperature, and it becomes weak at temperatures far from it. From the above data, it is known that the diode-like behavior appears in the Mn0.02Cr0.98Te films, in which the islands are not completely connected. The structural or componential inhomogeneity between the two ends of the sample may be the origin of the diode-like behavior. In addition unequal Schottky barriers between the electrode and the film at Vþ and V- also should be considered as a possible reason. If the Schottky barriers at Vþ and V- were equal, the V-I curve would be centrosymmetric [14]. It is well known that the barrier is controlled by the work function and interface state [9]. Thus the rough surface will enhance the inequality in the Schottky barriers. To understand further, we investigated these possibilities by comparing experimental results as below. At first the influence of the rough film surface was considered. In order to exclude the possibility of the unequal Cr percentages at two ends of the sample, the MnTe film was prepared. The properties
Fig. 4. (a) Temperature dependence of resistance with temperature from 350 K to 10 K for the Mn0.98Cr0.02Te film on Si/SiO2 substrate at 700 C. (b) V-I characteristics of the Mn0.98Cr0.02Te film in the range of 180 K-340 K.
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Fig. 5. (a) R-T curve of MnTe film deposited at 700 C, with a laser of 500mJ. (b) V-I curves of the film at 220 K, 280 K and 320 K. (c) V-I curves of the film at 220 K and 240 K with applying positive current. (d) V-I curves of the film at 220 K and 240 K with applying negative current. (e) SEM image of the film. The inset shows the cross-section image of the sample.
of the MnTe film deposited at 700 C are displayed in Fig. 5. Ag served as electrodes in the electrical measurements. The R-T curve shown in Fig. 5(a) of the MnTe film illustrates similar conducting behavior to that of the Mn0.02Cr0.98Te films. The transition from semiconductor to metal was found at about 220 K, which is lower than the TN for MnTe. The diode-like behavior was achieved in the V-I curve above 220 K, as shown in Fig. 5(b). It is worth noting that
the transition temperature is shifted from about 220 K to 240 K by raising the measuring current from 0.01 mA to 0.1 mA, due to the nonlinear V-I relationship. Fig. 5(c, d) illustrate the V-I curves of the film at 220 K and 240 K when applying positive and negative currents respectively. When the current is 0.01 mA, the absolute value of the corresponding voltage at 220 K is larger than that at 240 K but when the current is 0.1 mA, the absolute value of the voltage at
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Fig. 6. (a) R-T curve of MnTe film deposited at room temperature with 500 mJ and annealed at 700 C. (b) V-I curves of the film at different temperatures. (c) SEM image of the film. The inset shows the cross-section image of the sample.
Fig. 7. (a) R-T curve of MnTe film deposited at 700 C, 550 mJ (Ag electrodes). (b) V-I curves of the film at different temperatures (Ag electrodes). (c) and (d) SEM images of the film at Vþ and V- (Ag electrodes). (e) R-T curve measured with In electrodes. (f) V-I curves measured with In electrodes.(g) and (h) SEM images of the film at V- and Vþ (In electrodes).
220 K is smaller than that at 240 K. So the point of maximum value of resistance is shifted from 220 K to 240 K when changing the current for measurements from 0.01 mA to 0.1 mA. Furthermore, the resistance obtained with measurement for a current of 0.1 mA is bigger than that measured with a current of 0.01 mA in the range of 350 K-220 K. The unequal resistance at 0.01 mA and 0.1 mA in the range of 350 K-220 K is also attributed to the nonlinear V-I relationship. The shape of the V-I curves is distinctly different from the V-I curve of a single p-n junction or metal-semiconductor contact [10,11]. This suggests that the existence of unequal Schottky barriers at the two ends of the film is the basic reason for the results. The SEM images illustrated in Fig. 5(e) show that the film is rough with islands incompletely connected. At this point, the roughness of the surface is a reason for the diode-like V-I curves. In order to further certify that the nonlinear V-I relationship is due to the rough surface formed by incompletely connected islands, the MnTe film with continuous island growth was synthesized by depositing the film at room temperature and then annealing at 700 C. When the deposition temperature was set at room
temperature, the plasma reached the cold substrate and formed a continuous and flat amorphous film. After annealing, the amorphous film crystallized to form a flat crystalline film. R-T properties of MnTe films with a flat surface are similar to those of the films with a rough surface. As shown in Fig. 6(a), the transition from metal to semiconductor is found at 290 K, which is near the TN of MnTe, indicating the maximum of magnon drag [8]. The V-I curves shown in Fig. 6(b) give linear features at each temperature. Here Ag was also used as electrodes during the measurement process. The SEM image in Fig. 6(c) displays that the film was still in island growth with some cracks, and the thickness was about 420 nm. However, the islands attached each other without cavities. The comparison of rough and flat MnTe films indicates that the rough surface of MnTe is necessary for the diode-like behavior. The origin of the large Schottky barrier between the rough MnTe film and electrodes was confirmed. We investigated the influence of different electrodes for MnTe films deposited at 700 C. Fig. 7(a), (b), (e) and (f) illustrate R-T and V-I curves measured with Ag and In electrodes, respectively. The R-T curves of the two samples are
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Fig. 8. (a) R-T curve of Mn0.95Cr0.05Te/MnTe junction deposited at room temperature (In electrodes). (b) V-I curves of the film at different temperatures (In electrodes). (c) SEM image of the film at the MnTe part. The insets show the enlarged image and the cross-section image of this part. (d) SEM image of Mn0.95Cr0.05Te film part. The inset shows the crosssection image of this part.
similar to that for MnTe films with a flat surface prepared at room temperature. The difference is that a nonlinear V-I characteristic occurs in the sample with Ag electrodes, while a linear V-I characteristic exists in the sample with In electrodes. Moreover, a hysteresis loop is observed in the V-I curves measured with Ag electrodes by applying negative current. Such a characteristic suggests the existence of a complicated interface, affected by oxidation and defects, between the film and the electrodes [15]. Ag electrodes are bonded to the film by silver paste, so the granular Ag with possible external oxidization in the paste makes a higher barrier between the electrodes and the film. The rough surface will also enhance the Schottky effect. Such a state will enhance the interface barrier, leading to more obvious nonlinear V-I relationship. The barrier will lower for the flat film surface with good electrode-film contact. Because of this, the V-I curve becomes linear. Considering the different work function and different contact condition of In and Ag, we could deduce that the larger Schottky barrier in the interface of rough MnTe film and Ag electrode contributed to the nonlinear V-I relationship. However, the film surfaces at Vþ and V- electrodes are almost same as shown in Fig. 7(c, d, g and h). That is why the samples did not give asymmetric diode-like behavior. Moreover, on account of the weak roughness of this sample compared with the film shown in Fig. 5(e), the result indicates that the roughness of the film is essential to the diode-like characteristic. A Mn0.95Cr0.05Te/MnTe junction was prepared to investigate the influence of componential inhomogeneity on the V-I relationship.
To amplify the possibly influence of component inhomogeneity, a Mn0.95Cr0.05Te film was prepared to replace the Mn0.98Cr0.02Te film, which may create more unbalanced composition distribution. A flat sample with the structure of Mn0.95Cr0.05Te/MnTe junction was prepared by depositing at room temperature and then annealing at 700 C. The Vþ and Iþ electrodes were set on the MnTe part, while the V- and I- electrodes were set on the Mn0.95Cr0.05Te part. Here all of the electrodes are In, in order to exclude the contribution of the Schottky barrier between MnTe and the electrodes to the nonlinear V-I curve. The RT curve as shown in Fig. 8(a) is similar to that for the MnTe film. As shown in Fig. 8(b), the V-I curve has approximately linear relationship; illustrating that 5% substitution of Mn by Cr does not induce apparent diode-like behavior. Both of the two parts are flat with grains connected completely, as shown in Fig. 8(c) and (d), which exclude the disturbance of the rough surface. This result testifies that the diode-like behavior is not attributed to the Cr inhomogeneity in the sample. 4. Conclusions The Mn0.98Cr0.02Te and MnTe films were prepared by PLD, showing the island growth when deposited at 700 C. With increasing the measure temperature, the resistance of all samples has semiconductor character in the temperature range of about 10 Ke70 K and has metallic character at higher temperature. There is a metal-semiconductor transition around the range of 220 K300 K, and the samples show semiconductor character above the
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transition temperature. The transition temperature of all samples el temperature of MnTe. The was found to be below the Ne nonlinear V-I characteristics and diode-like behavior are present in the films deposited at 700 C, of which the islands are incompletely connected and which show a rough surface. The high Schottky barrier between the incompletely connected grains and Ag electrodes contributes to the nonlinear V-I characteristic, and the diode-like V-I behavior was ascribed to the inhomogeneous Schottky barrier. The inhomogeneity of Cr does not induce diodelike behavior. The shift of metal-semiconductor transition temperature when enhancing the measuring current was attributed to the nonlinear V-I relationship. Acknowledgment This work was supported by the National Natural Science Foundation of China with Grant No. 51522104 and 51331006. References [1] K. Ozawa, S. Anzai, Y. Hamaguchi, Effect of pressure on the magnetic transition point of manganese telluride, Phys. Lett. 20 (2) (1966) 132e133. [2] W. Szuszkiewicz, E. Dynowska, B. Witkowska, B. Hennion, Spin-wave measurements on hexagonal MnTe of NiAs-type structure by inelastic neutron scattering, Phys. Rev. B 73 (10) (2006). [3] C. Reig, V. Munoz, C. Gomez, C. Ferrer, A. Segura, Growth and characterisation of MnTe crystals, J. Cryst. Growth 223 (3) (2001) 349e356. [4] W. Kim, I.J. Park, H.J. Kim, W. Lee, S.J. Kim, C.S. Kim, Room-temperature ferromagnetic property in MnTe semiconductor thin film grown by molecular beam epitaxy, IEEE Trans. Magn. 45 (6) (2009) 2424e2427.
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