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Electro- and opto-resistive switching behaviors of the Nb doped SrTiO3 films
Current Applied Physics 13 (2013) 768e774
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Electro- and opto-resistive switching behaviors of the Nb doped SrTiO3 films Ashvani Kumar, Joonghoe Dho* Department of Physics, Kyungpook National University, Daegu 702-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 April 2012 Received in revised form 24 October 2012 Accepted 3 December 2012 Available online 12 December 2012
Nb doped SrTiO3 (Nb:STO) films were deposited on (100) SrTiO3 substrates using a pulse laser deposition technique. The effects of deposition pressure on their structural, electrical and optical properties were investigated. Decrease in deposition pressure lead to decrease in grain size and average surface roughness. Various optical parameters such as refractive index, extinction coefficient, and band gap were calculated by applying the envelop or extrapolation methods using the transmittance data obtained from a UV/Vis spectrophotometer. A systematic decrease in resistivity and increment of negative charged carriers was observed with decreasing deposition pressure. Experimental results exhibited electro- and opto-resistive switching behaviors with a resistive transition from high resistance state to low resistance state on the application of current-pulse or UV light. Multi-level resistance states have also been demonstrated using a train of current pulses of different magnitudes or a simultaneous application of current and UV light. Such observed phenomenon makes Nb:STO a potential candidate to be used in future for the fabrication of multi-level memory devices and transparent thin films transistors. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nb:STO thin films Electrical properties Electro- and opto-resistive switching Multi-level memory
1. Introduction Strontium titanate SrTiO3 (STO) has been widely studied because of its high dielectric constant, excellent optical properties and a simple cubic structure with stable stoichiometry. It is known that the fundamental optical band gap of STO in the bulk form is 3.22 eV, which is attributed to the transitions from 2p oxygen states to 3d titanium states [1]. However, a slight doping with n-type or ptype impurities (e.g. Nb, In) or an induction of oxygen vacancies promotes STO to behave alternatively as a transparent conducting oxide or a superconductor [2e4]. Doped-STO emerged as a material of potential use because of its conductive nature that makes it promising for variety of applications [5e7]. While making highly conductive STO films for various electronic applications, it becomes very important to control a series of the conductivity by doping technique because the doping technique is more accurate than that of oxygen deficiency control. In recent years, Nb doped STO (Nb:STO), an n-type semiconductor, has attracted considerable interest as it exhibited huge hysteresis in currentevoltage characteristics required for high speed nonvolatile resistance random access memories (Re-RAM) applications [8e13]. In Nb:STO, the appropriate carrier concentration can be achieved by controlling the Nb doping level that provides an efficient way to control the * Corresponding author. E-mail address: [email protected] (J. Dho). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2012.12.001
switching characteristics and facilitates the formation of the Schottky junction with enough trapping density between the electrode and the semiconductor. The resistive switching phenomenon have been explained by various models such as formation of conducting filaments and the field-induced emission of trapped carriers and refilling of trap sites near the interface [13e 17]. Apart from this, transparent conducting oxides have been extensively studied to exploit their high transparency to visible light and good electrical properties for future industrial applications such as transparent electrodes, transparent thin film transistor (TFT), and transparent displays [18,19]. In light of the above, the objective of the present study was to deposit good quality Nb:STO films using pulse laser deposition technique and to investigate (a) the effect of varied oxygen partial pressure on their structural, electrical and optical properties and (b) the electro- and opto-resistive switching behaviors to meet the demand of multi-level memory devices. In the modern era of technology, dual responsive or multi responsive materials have also gained great interest because they allow multiple channels data storage and can increase integration densities compared with single-responsive materials. This dual responsive behavior permits the manipulation of various signals with alternating operation mode. Here, we made a successful attempt to explore the possibility of multi-channel data storage using Nb:STO films for the first time. Optical and electrical control signals were alternatively used as operation modes.
A. Kumar, J. Dho / Current Applied Physics 13 (2013) 768e774
2. Experimental details Nb doped SrTiO3 (Nb:STO) films were grown on (100) SrTiO3 single crystal substrates using the pulse laser deposition technique (PLD). A self-made 0.5 wt% Nb doped STO target was used to deposit these films. A Nd:YAG laser with a wavelength of 355 nm and a frequency of 5 Hz was used. Prior to deposition, the chamber was evacuated to a base pressure of 105 Torr and then backfilled with the mixture of 80% Ar þ 20% O2 gas to the desired pressure. The deposition pressure was varied during the growth of these films. The films deposited under the gas pressures of 1 mtorr and 0.75 mtorr will be hence forth known as sample S1 and S2, while the film deposited under vacuum (2 105 torr) as sample S3, respectively. The films were deposited at a substrate temperature of 550 C and thereafter, naturally cooled to room temperature. No post-annealing was performed after deposition. All the summarized parameters have been listed in Table 1. The crystalline properties of these films were studied using a Rigaku D/Max-2500 X-ray diffractometer of Cu Ka radiation in q/ 2q-scan and u-scan modes. Surface morphology was studied using a scanning probe microscope (NanoScope IIIa Multimode). Optical properties were measured using a commercial UV/VIS Spectrophotometer, Ocean optics usb2000. Electrical and opto-electrical measurements of these films were performed using a helium cryocooler and Keithley instruments (current source 2600 and nanovoltmeter 4284) in conjunction with a homemade UV exposure system. The used UV light had a central wavelength of 365 nm with a spectral full-width at a half maximum of w15 nm. The size of the illumination spot was a diameter of about 2 mm and the distance between the sample and the UV lamp was maintained around 5 mm, and the power density was kept at a value below w5 mW/cm2 in order to minimize the heating effect during UV illumination. The four probe technique was used to measure the resistance, while a typical two probe method was adopted to measure the voltage versus current (VeI) characteristics. The electric contacts over the samples were made by silver (Ag) target using sputtering system. 3. Results and discussion 3.1. Microstructural properties Nb:STO films were epitaxially grown on STO substrate evident by the XRD pattern as no distinguished peak was observed other than substrate peaks. The surface morphology of sample S1, S2 and S3 are shown in Fig. 1(a), (b) and (c), respectively. Sample S1 exhibited relatively large grains with blurred inter-grain boundaries while sample S3 showed very small grains with high contrast of dark and bright colors. Sample S2 showed the intermediate state of sample S1 and S3, respectively. The root mean square roughness values of sample S1, S2 and S3 were obtained from AFM micrographs and found to be about 1.09, 0.34 and 0.22 nm, respectively. A systematic decrease in the grain size was observed with decrease in deposition pressure that can be explained using the following expression of mean free path (l) [20]:
l ¼ 2:33 1020
T
(1)
P dm
769
where T (K) is the temperature, P (Torr) is the pressure and dm (cm) is the molecular diameter. The mean free path is the account of the collision probability, which is the dominant factor influencing the mean diameters of the oxide nanoparticles. At a relatively higher ambient pressure, frequent collision of ablated species with gas molecules cause energy loss of ablated particles while approaching to the substrate. Therefore, the insufficient surface mobility of the ablated particles inhibits the formations of clusters with clean boundaries as shown in Fig. 1(a). In case of low deposition pressure, a lower collision probability results in a high surface mobility of the ablated particles and thus the formation of small clusters with clean boundaries as can be seen in, Fig. 1(c). The lower number of collisions between ablated species and the gas molecules can bring the formation of oxygen deficient films that was in agreement with our results. 3.2. Optical properties Semiconductor oxide films demonstrate different optical properties than that of corresponding bulk materials [21]. In case of thin films, the optical properties depend on various factors such as crystalline fraction, grain size and their orientation, surface roughness, and defect density. In order to explore the effect of deposition pressure on the optical properties of Nb:STO films, the following studies were carried out. The spectral variations of transmission for the Nb:STO films, including the substrate, were measured over the wavelength range from 300 to 900 nm. Fig. 2(a) shows the transmittance spectra of STO substrate and samples S1, S2 and S3, respectively. Average transmittance of samples S1, S2, and S3 were estimated using the transmittance of the substrate as reference and found to be w91, 52, and 32%, respectively in the visible range of electromagnetic spectrum. A clear decrease in transmittance with decrease in deposition pressure was observed, which was more prominent toward higher wavelengths. The reason may be attributed to the fact that the decrease in O2 partial pressure lead to generation of oxygen vacancies and defect density, which results in more scattering and absorption. Optical constants were deducted using envelope method to explore the optical properties of these films in detail. In the region of low absorption, the incident light traverses the film several times and the interference fringes are subsequently produced. A necessary condition for a good fringe pattern is that the difference between the refractive index of the deposited film and the substrate should be as pronounced as possible. In the present case, the ‘sample S1’ did not exhibit any fringe pattern in transmittance spectra while sample S2 and S3 showed the fringes of significant amplitude. In previous case it was quite obvious as there was ideally no optical interference while in the latter the formation of fringes could be due to change in refractive index of the deposit films. The refractive index (n) was estimated from the transmission spectrum by using the following expressions [22]:
1=2 i1=2 h n ¼ N þ N2 n2o n21
(2)
Table 1 Various parameters of Nb doped SrTiO3 films deposited at varied pressures and environment. Sample name S1 S2 S3
Gas used (Ratio) Ar:O2 (4:1) Ar:O2 (4:1) Vacuum
Deposition pressure (mtorr) 1.0 0.75 0.02
Thickness (nm) e 298 25 280 20
Band gap (eV) 3.12 3.16 3.18
Carrier concentration (cm3) 12
(2.43 0.77) 10 (2.14 0.33) 1017 e
Mobility (cm2/V s) 8.17 0.73 5.95 0.85 e
770
A. Kumar, J. Dho / Current Applied Physics 13 (2013) 768e774
Fig. 2. (a) Transmittance spectra of samples S1, S2, S3 and STO substrate; (b) Extinction coefficient and (c) calculated optical band gap of samples S1, S2 and S3, respectively. Inset of Fig. 2(a) represents the process to select the value of Tmax and Tmin at same wavelength.
and
N ¼
Fig. 1. AFM micrographs of (a) sample S1, (b) sample S2 and (c) sample S3, respectively.
n2o þ n21 Tmax Tmin þ no n1 2 Tmax Tmin
(3)
where no (z1.0) and n1 (¼2.39) are the refractive indices of air and substrate, respectively. Tmax and Tmin are maximum and minimum
A. Kumar, J. Dho / Current Applied Physics 13 (2013) 768e774
transmittance values at the same wavelength, which were obtained from the envelope method as seen in inset of Fig. 2(a). The calculated values of the average refractive index for sample S2 and S3 were 2.39 and 2.40, respectively. The enhanced density of sample S3 packed by small grains could be the possible reason for the slight increment of refractive index. By knowing the refractive indices, the thickness (t) of the film can be estimated by using the following relation [22]:
t ¼
M l1 l2 2 nl1 $l2 nl2 $l1
(4)
where M is the number of oscillations between the two extrema and the value of M is equal to 1 between two consecutive maxima and minima. l1, nl1 and l2, nl2 are the corresponding wavelengths and indices of refraction. The thicknesses calculated for sample S2 and S3 were found to be 298 25 and 280 20 nm, respectively. Furthermore, the optical absorption coefficient was calculated from the following relation [23]:
T ¼
ð1 RÞ2 expðatÞ 1 R2 expð2atÞ
(5)
where R and T are the spectral reflectance and transmittance and t is the film thickness. For greater optical density (at > 1), the interference effects due to internal reflections as well as reflectance at normal incidence are negligible, and the previous equation can be approximated as
TzexpðatÞ
(6)
The optical absorption coefficient (a) is given by the approximate formula,
1 t
a ¼ lnðTÞ
(7)
where t and T are the film thickness and the measured transmittance of the film, respectively. The calculated value of ‘a’ was used to measure the extinction coefficient (k), which is the measure of total optical losses caused by both absorption and scattering. The following relation was used to measure ‘k’:
k ¼
al
(8)
4p
All the samples showed the decreasing trend of extinction coefficient with decrease in wavelength (Fig. 2(b)), manifested that the scattering and absorption was more prominent in the visible region. The value of ‘k’ was of the order of 102 throughout the wavelength range in case of sample S1 and decreased to 101 for sample S2 and S3 that clearly support the fact that decrease in transmission spectra with decrease in deposition pressure was due to the enhancement of absorption and scattering factors due to the increased number of oxygen vacancies and defect density. On the basis of inter-band absorption theory, the optical band gap of these films was calculated using the following relation [24]:
ahy ¼ A hy Eg
n
771
shown in Fig. 2(c). The calculated values of the band gap were found to be 3.12, 3.16, 3.18 and 3.22 eV for the samples S1, S2, S3 and STO substrate, respectively. The observed small rise in band gap from sample S1 to S3 could be understood on the basis of ‘Burstein-Moss effect’ explaining the filling of the conduction band with increasing carrier density in doped semiconductors. The filling of the conduction band may start with increase in doping concentrations because of the finite density of states, and thus the ‘band filling’ process can prevent the absorption transition from the top of the valance band to the bottom of the conduction band. As a result, the fundamental edge of the absorption transitions shifts from Ec Ev ¼ Eg, as in case of the undoped semiconductors, to Ef Ev > Eg, in doped n-type semiconductors [25,26]. 3.3. Electrical properties The temperature dependence of electrical resistance for all the samples is shown in Fig. 3. It was observed that decrease in oxygen partial pressure ðPO2 Þ, during the growth, leads to a drastic change in the electrical behavior and resistance of these samples as the sample S1 and S2 exhibited semiconducting behavior while sample S3 showed metallic behavior. The possible reasons could be as follows; (i) the critical amount of PO2 may result in grain growth of stoichiometric STO and a creation of niobium oxides such as Nb2O5, while relatively low PO2 generates the singly ionized oxygen vacancies and prevent the formation of Nb2O5, which result in an efficient substitution of niobium (Nbþ5) to titanium (Tiþ4) and an increase of electron carriers density in the system [27,28], (ii) difference in ‘Nb’ doping level with the PO2 may lead to different level of lattice distortion, which can also modify the electronic band structure or energy level and, in turn, affect the electrical resistance. Sample S2 revealed metal to semiconductor transition around 150 K during subsequent heating and cooling cycle that can be understand on the basis of carrier freeze-out phenomenon as in the oxygen deficient SrTiO3d film [29,30]. A drastic increase in resistivity with decreasing temperature was due to the low density of donors (i.e. oxygen vacancies in the present case) resulted in the separation of the donor level from the bottom of conduction band. Below w150 K, majority of free electrons start shrinking down to the lower donor level and get trapped. The concentration and mobility of the negative charged carriers, obtained using Hall’s measurement, for the sample S1 and S2 were found to be (2.43 0.77) 1012 cm3, 8.17 0.73 cm2/V s and (2.14 0.33) 1017 cm3, 5.95 0.85 cm2/V s, respectively. These measurements were not possible for sample S3 because of its highly conductive nature. In conclusion it is worth mentioning that a decrease in PO2 resulted in enhancement of conductivity with very high negative-charged carrier concentration. The samples S2 and S3 made at lower oxygen pressures have more oxygen defects, which induce an increase of carrier density and thus a decrease of resistance, than the sample S1 made at a higher oxygen pressure. Since a low resistance state induced by defects precludes an observation of a small resistance change with the application of current pulse or UV light, the sample S1 was mainly used for the following studies on electro- and opto-resistive switching behavior. 3.4. Electro- and opto-resistive switching behaviors
(9)
where A is the probability parameter for the transition, Eg is the band gap of the material, h and n are the incident photon energy and the transition coefficient, respectively. The reported value of n is 2 for the measurement of an indirect band gap and 1/2 for a direct band gap. Here, the direct band gap of the Nb:STO films was evaluated by extrapolating the straight line part of the curves, (ahy)1/2 ¼ 0, as
The voltage versus current (VeI) characteristics of sample S1 and S3 are shown in Fig. 4(a) and (b), respectively. The dc voltage was swept from 0 / þVmax / 0 / Vmax / 0. Sample S1 exhibited non-linear and hysteresis behaviors in the VeI curve, while sample S3 showed a linear ohmic behavior without any hysteresis (Fig. 4(b)). The rectifying and hysteresis behaviors observed in the sample S1 indicate Schottky contacts at the
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Fig. 3. Electrical resistance versus temperature curves of sample S1, S2 and S3, respectively.
Nb:STO/Ag interfaces which probably cause the resistance switching (RS) between the high and low resistance states. The inset of Fig. 4(a) shows the ln (I/V) versus V1/2 plot for the high resistive state of the IeV curve that could be fitted linearly with the linear regression value (R2) of 0.992. Such a linear relation in the high voltage region suggested that the conduction mechanism in the high resistance state can be explained by Pool-Frenkel emission, which is dominated by bulk effect resulting from the lowering of the coulomb potential barrier of a trap site due to increased electric field. Therefore, the conduction mechanism in the high resistive state was presumably governed by the formation of conducting paths due to the electron hopping between the trap states which are probably introduced by the Nb doping and oxygen vacancies. Further, the sample S1 was exposed to a train of the current pulses that displayed the switching of the system between two resistance states that correspond to the high and low values of current. Interestingly, the system switched reversibly between two well defined resistance states, where the high resistance state can be assimilated to a logic ‘1’ and the low-resistance to a logic ‘0’ as shown in Fig. 5(a). A series of current pulses of different amplitudes was applied to the sample S1, which displayed the corresponding changes of resistance as can be seen in Fig. 5(b). The observed behavior was characteristic of multi-level switching of resistance. The systematic decrease in resistance with increasing the magnitude of applied current could be possibly due to the reduced Schottky barrier height, formation of conduction path in oxide regime or the lowering of the coulomb potential barrier of a trap site. The change in resistance to different magnitude of applied
Fig. 4. VeI characteristics of samples (a) S1 and (b) S3, respectively. Inset of figure (a) shows the plot of ln (I/V) against square root of voltage.
current is plotted in Fig. 5(c), and it was fitted by an exponential function R ¼ 0.18 exp(I/0.22) þ 0.11, where R and I is the resistance and the current in MU and mA, respectively. The above relation suggested that the resistance was more responsive to the small amplitude of the pulse and was almost saturated at higher amplitude. In order to investigate the photo-induced effect, resistance versus time characteristics were acquired using an illumination of UV light. The change in resistance corresponding to the ‘ON’ and ‘OFF’ state of UV light was found to be almost 7.1% as shown in Fig. 6. The observed decrease in resistance under the illumination of UV light could be due to the boosting of electron concentration. At the onset of UV radiations, the charge carrier density varies with time as n ¼ bkIt, where ‘bkI’ and ‘t’ represent the true quantum yield and time, respectively. Under UV illumination, a major part of the excited carriers jumps to the conduction band and is responsible for the sudden decrease in resistance value. Presumably, a majority of excited electrons go back to the valence band through relaxation process but a fraction of excited carriers captured by the trapping centers stay at the conduction band quite a while. Consequently, the system reaches to an equilibrium state as represented by the
A. Kumar, J. Dho / Current Applied Physics 13 (2013) 768e774
773
Fig. 6. Electrical resistance versus time curve correspond to ON/OFF state of UV light. The UV light with the power density of w3.6 mW/cm2 was used.
trapping levels [31]. Encircled region in Fig. 6 exhibited the recovery of initial value of resistance after termination of UV light, which manifested the recombination process with an exponential time dependence. For the first time, an approach has been made to enhance the storage channels by both the control parameters, i.e. current and UV light. The main idea was to enhance the integration density of the device by external means without any change in its dimension. Sample S1 was exposed to a current pulse and its change in resistance was examined with and without UV exposure as shown in Fig. 7. It was observed that in the dark, referred to as Channel A, the resistance exhibited two states of almost 42% apart correspond to low and high current values that can be assigned as logic 1 and 0, respectively. Similarly, under the UV illumination, the resistance decreased to almost 15% of the dark value, which was referred as channel B. Channel B also showed two resistance states with a difference of w34%, which was corresponds to low and high values of the current pulse that can also be assimilated as logic 1 and 0, respectively. The lower states of resistance in channel A and B showed the difference of w3% that was sufficient to distinguish these states. Therefore, the reading and writing can become possible at two channels by using both the control parameters alternatively. This may arise as a potential application in the emerging era of optoelectronic technology.
Fig. 5. (a) Change in electrical resistance corresponds to a train of current pulse; (b) Relative change in electrical resistance with different magnitude of current pulse and (c) variation of electrical resistance with increasing magnitude of current with an exponential fit.
saturating nature of the photo-induced change of resistance. On the cessation of UV radiations, the resistance increased rapidly to the initial dark resistance level due to the recombination of excited carriers at the trap centers. The decay of the carrier density after the UV termination can be represented by n ¼ bkIsnet/s, where sn is the electron lifetime and s signifies the effect of trap centers on the dynamics of charge carriers and it depends on the density of
Fig. 7. Resistance versus time curve corresponds to a current pulse with and without UV exposure. The UV light with the power density of w5.5 mW/cm2 was used.
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4. Conclusion Structural, optical and electrical properties of Nb:STO films were found be dependent of deposition pressure. Decrease in deposition pressure leads to decrease in mean free path caused less number of collisions between ablated particles and gas species, which prevent gas phase nucleation and resulted in small grain size and enhanced packed density. Average transmittance in the visible region was found 91, 52 and 32% for sample S1, S2 and S3, respectively. The observed decrease in transmittance was due to the increased absorption and scattering as evidenced by increased extinction coefficient. Electrical resistivity was found to be decreased with decreasing oxygen partial pressure because of the generation of more carriers such as singly ionized vacancies and free electrons. Sample S1 and S2 demonstrated electro- and opto-resistive switching behaviors, while S3 did not exhibit any such behavior due to its conductive nature. Sample S1 exhibited almost 50.0 and 7.1% of difference in high and low resistance states on the application of current pulse and UV light, respectively. In addition, multi-level switching has been demonstrated successfully by the alternative use of current pulse and UV light. These application of Nb:STO can provide new dimensions to the device technologies. Acknowledgments This work was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2010-0007902). References [1] M. Capizzi, A. Frova, Phys. Rev. Lett. 25 (1970) 1298. [2] N. Shanthi, D.D. Sarma, Phys. Rev. B 57 (1998) 2153.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Electro- and opto-resistive switching behaviors of the Nb doped SrTiO3 films Ashvani Kumar, Joonghoe Dho* Department of Physics, Kyungpook National University, Daegu 702-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 April 2012 Received in revised form 24 October 2012 Accepted 3 December 2012 Available online 12 December 2012
Nb doped SrTiO3 (Nb:STO) films were deposited on (100) SrTiO3 substrates using a pulse laser deposition technique. The effects of deposition pressure on their structural, electrical and optical properties were investigated. Decrease in deposition pressure lead to decrease in grain size and average surface roughness. Various optical parameters such as refractive index, extinction coefficient, and band gap were calculated by applying the envelop or extrapolation methods using the transmittance data obtained from a UV/Vis spectrophotometer. A systematic decrease in resistivity and increment of negative charged carriers was observed with decreasing deposition pressure. Experimental results exhibited electro- and opto-resistive switching behaviors with a resistive transition from high resistance state to low resistance state on the application of current-pulse or UV light. Multi-level resistance states have also been demonstrated using a train of current pulses of different magnitudes or a simultaneous application of current and UV light. Such observed phenomenon makes Nb:STO a potential candidate to be used in future for the fabrication of multi-level memory devices and transparent thin films transistors. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nb:STO thin films Electrical properties Electro- and opto-resistive switching Multi-level memory
1. Introduction Strontium titanate SrTiO3 (STO) has been widely studied because of its high dielectric constant, excellent optical properties and a simple cubic structure with stable stoichiometry. It is known that the fundamental optical band gap of STO in the bulk form is 3.22 eV, which is attributed to the transitions from 2p oxygen states to 3d titanium states [1]. However, a slight doping with n-type or ptype impurities (e.g. Nb, In) or an induction of oxygen vacancies promotes STO to behave alternatively as a transparent conducting oxide or a superconductor [2e4]. Doped-STO emerged as a material of potential use because of its conductive nature that makes it promising for variety of applications [5e7]. While making highly conductive STO films for various electronic applications, it becomes very important to control a series of the conductivity by doping technique because the doping technique is more accurate than that of oxygen deficiency control. In recent years, Nb doped STO (Nb:STO), an n-type semiconductor, has attracted considerable interest as it exhibited huge hysteresis in currentevoltage characteristics required for high speed nonvolatile resistance random access memories (Re-RAM) applications [8e13]. In Nb:STO, the appropriate carrier concentration can be achieved by controlling the Nb doping level that provides an efficient way to control the * Corresponding author. E-mail address: [email protected] (J. Dho). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2012.12.001
switching characteristics and facilitates the formation of the Schottky junction with enough trapping density between the electrode and the semiconductor. The resistive switching phenomenon have been explained by various models such as formation of conducting filaments and the field-induced emission of trapped carriers and refilling of trap sites near the interface [13e 17]. Apart from this, transparent conducting oxides have been extensively studied to exploit their high transparency to visible light and good electrical properties for future industrial applications such as transparent electrodes, transparent thin film transistor (TFT), and transparent displays [18,19]. In light of the above, the objective of the present study was to deposit good quality Nb:STO films using pulse laser deposition technique and to investigate (a) the effect of varied oxygen partial pressure on their structural, electrical and optical properties and (b) the electro- and opto-resistive switching behaviors to meet the demand of multi-level memory devices. In the modern era of technology, dual responsive or multi responsive materials have also gained great interest because they allow multiple channels data storage and can increase integration densities compared with single-responsive materials. This dual responsive behavior permits the manipulation of various signals with alternating operation mode. Here, we made a successful attempt to explore the possibility of multi-channel data storage using Nb:STO films for the first time. Optical and electrical control signals were alternatively used as operation modes.
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2. Experimental details Nb doped SrTiO3 (Nb:STO) films were grown on (100) SrTiO3 single crystal substrates using the pulse laser deposition technique (PLD). A self-made 0.5 wt% Nb doped STO target was used to deposit these films. A Nd:YAG laser with a wavelength of 355 nm and a frequency of 5 Hz was used. Prior to deposition, the chamber was evacuated to a base pressure of 105 Torr and then backfilled with the mixture of 80% Ar þ 20% O2 gas to the desired pressure. The deposition pressure was varied during the growth of these films. The films deposited under the gas pressures of 1 mtorr and 0.75 mtorr will be hence forth known as sample S1 and S2, while the film deposited under vacuum (2 105 torr) as sample S3, respectively. The films were deposited at a substrate temperature of 550 C and thereafter, naturally cooled to room temperature. No post-annealing was performed after deposition. All the summarized parameters have been listed in Table 1. The crystalline properties of these films were studied using a Rigaku D/Max-2500 X-ray diffractometer of Cu Ka radiation in q/ 2q-scan and u-scan modes. Surface morphology was studied using a scanning probe microscope (NanoScope IIIa Multimode). Optical properties were measured using a commercial UV/VIS Spectrophotometer, Ocean optics usb2000. Electrical and opto-electrical measurements of these films were performed using a helium cryocooler and Keithley instruments (current source 2600 and nanovoltmeter 4284) in conjunction with a homemade UV exposure system. The used UV light had a central wavelength of 365 nm with a spectral full-width at a half maximum of w15 nm. The size of the illumination spot was a diameter of about 2 mm and the distance between the sample and the UV lamp was maintained around 5 mm, and the power density was kept at a value below w5 mW/cm2 in order to minimize the heating effect during UV illumination. The four probe technique was used to measure the resistance, while a typical two probe method was adopted to measure the voltage versus current (VeI) characteristics. The electric contacts over the samples were made by silver (Ag) target using sputtering system. 3. Results and discussion 3.1. Microstructural properties Nb:STO films were epitaxially grown on STO substrate evident by the XRD pattern as no distinguished peak was observed other than substrate peaks. The surface morphology of sample S1, S2 and S3 are shown in Fig. 1(a), (b) and (c), respectively. Sample S1 exhibited relatively large grains with blurred inter-grain boundaries while sample S3 showed very small grains with high contrast of dark and bright colors. Sample S2 showed the intermediate state of sample S1 and S3, respectively. The root mean square roughness values of sample S1, S2 and S3 were obtained from AFM micrographs and found to be about 1.09, 0.34 and 0.22 nm, respectively. A systematic decrease in the grain size was observed with decrease in deposition pressure that can be explained using the following expression of mean free path (l) [20]:
l ¼ 2:33 1020
T
(1)
P dm
769
where T (K) is the temperature, P (Torr) is the pressure and dm (cm) is the molecular diameter. The mean free path is the account of the collision probability, which is the dominant factor influencing the mean diameters of the oxide nanoparticles. At a relatively higher ambient pressure, frequent collision of ablated species with gas molecules cause energy loss of ablated particles while approaching to the substrate. Therefore, the insufficient surface mobility of the ablated particles inhibits the formations of clusters with clean boundaries as shown in Fig. 1(a). In case of low deposition pressure, a lower collision probability results in a high surface mobility of the ablated particles and thus the formation of small clusters with clean boundaries as can be seen in, Fig. 1(c). The lower number of collisions between ablated species and the gas molecules can bring the formation of oxygen deficient films that was in agreement with our results. 3.2. Optical properties Semiconductor oxide films demonstrate different optical properties than that of corresponding bulk materials [21]. In case of thin films, the optical properties depend on various factors such as crystalline fraction, grain size and their orientation, surface roughness, and defect density. In order to explore the effect of deposition pressure on the optical properties of Nb:STO films, the following studies were carried out. The spectral variations of transmission for the Nb:STO films, including the substrate, were measured over the wavelength range from 300 to 900 nm. Fig. 2(a) shows the transmittance spectra of STO substrate and samples S1, S2 and S3, respectively. Average transmittance of samples S1, S2, and S3 were estimated using the transmittance of the substrate as reference and found to be w91, 52, and 32%, respectively in the visible range of electromagnetic spectrum. A clear decrease in transmittance with decrease in deposition pressure was observed, which was more prominent toward higher wavelengths. The reason may be attributed to the fact that the decrease in O2 partial pressure lead to generation of oxygen vacancies and defect density, which results in more scattering and absorption. Optical constants were deducted using envelope method to explore the optical properties of these films in detail. In the region of low absorption, the incident light traverses the film several times and the interference fringes are subsequently produced. A necessary condition for a good fringe pattern is that the difference between the refractive index of the deposited film and the substrate should be as pronounced as possible. In the present case, the ‘sample S1’ did not exhibit any fringe pattern in transmittance spectra while sample S2 and S3 showed the fringes of significant amplitude. In previous case it was quite obvious as there was ideally no optical interference while in the latter the formation of fringes could be due to change in refractive index of the deposit films. The refractive index (n) was estimated from the transmission spectrum by using the following expressions [22]:
1=2 i1=2 h n ¼ N þ N2 n2o n21
(2)
Table 1 Various parameters of Nb doped SrTiO3 films deposited at varied pressures and environment. Sample name S1 S2 S3
Gas used (Ratio) Ar:O2 (4:1) Ar:O2 (4:1) Vacuum
Deposition pressure (mtorr) 1.0 0.75 0.02
Thickness (nm) e 298 25 280 20
Band gap (eV) 3.12 3.16 3.18
Carrier concentration (cm3) 12
(2.43 0.77) 10 (2.14 0.33) 1017 e
Mobility (cm2/V s) 8.17 0.73 5.95 0.85 e
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Fig. 2. (a) Transmittance spectra of samples S1, S2, S3 and STO substrate; (b) Extinction coefficient and (c) calculated optical band gap of samples S1, S2 and S3, respectively. Inset of Fig. 2(a) represents the process to select the value of Tmax and Tmin at same wavelength.
and
N ¼
Fig. 1. AFM micrographs of (a) sample S1, (b) sample S2 and (c) sample S3, respectively.
n2o þ n21 Tmax Tmin þ no n1 2 Tmax Tmin
(3)
where no (z1.0) and n1 (¼2.39) are the refractive indices of air and substrate, respectively. Tmax and Tmin are maximum and minimum
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transmittance values at the same wavelength, which were obtained from the envelope method as seen in inset of Fig. 2(a). The calculated values of the average refractive index for sample S2 and S3 were 2.39 and 2.40, respectively. The enhanced density of sample S3 packed by small grains could be the possible reason for the slight increment of refractive index. By knowing the refractive indices, the thickness (t) of the film can be estimated by using the following relation [22]:
t ¼
M l1 l2 2 nl1 $l2 nl2 $l1
(4)
where M is the number of oscillations between the two extrema and the value of M is equal to 1 between two consecutive maxima and minima. l1, nl1 and l2, nl2 are the corresponding wavelengths and indices of refraction. The thicknesses calculated for sample S2 and S3 were found to be 298 25 and 280 20 nm, respectively. Furthermore, the optical absorption coefficient was calculated from the following relation [23]:
T ¼
ð1 RÞ2 expðatÞ 1 R2 expð2atÞ
(5)
where R and T are the spectral reflectance and transmittance and t is the film thickness. For greater optical density (at > 1), the interference effects due to internal reflections as well as reflectance at normal incidence are negligible, and the previous equation can be approximated as
TzexpðatÞ
(6)
The optical absorption coefficient (a) is given by the approximate formula,
1 t
a ¼ lnðTÞ
(7)
where t and T are the film thickness and the measured transmittance of the film, respectively. The calculated value of ‘a’ was used to measure the extinction coefficient (k), which is the measure of total optical losses caused by both absorption and scattering. The following relation was used to measure ‘k’:
k ¼
al
(8)
4p
All the samples showed the decreasing trend of extinction coefficient with decrease in wavelength (Fig. 2(b)), manifested that the scattering and absorption was more prominent in the visible region. The value of ‘k’ was of the order of 102 throughout the wavelength range in case of sample S1 and decreased to 101 for sample S2 and S3 that clearly support the fact that decrease in transmission spectra with decrease in deposition pressure was due to the enhancement of absorption and scattering factors due to the increased number of oxygen vacancies and defect density. On the basis of inter-band absorption theory, the optical band gap of these films was calculated using the following relation [24]:
ahy ¼ A hy Eg
n
771
shown in Fig. 2(c). The calculated values of the band gap were found to be 3.12, 3.16, 3.18 and 3.22 eV for the samples S1, S2, S3 and STO substrate, respectively. The observed small rise in band gap from sample S1 to S3 could be understood on the basis of ‘Burstein-Moss effect’ explaining the filling of the conduction band with increasing carrier density in doped semiconductors. The filling of the conduction band may start with increase in doping concentrations because of the finite density of states, and thus the ‘band filling’ process can prevent the absorption transition from the top of the valance band to the bottom of the conduction band. As a result, the fundamental edge of the absorption transitions shifts from Ec Ev ¼ Eg, as in case of the undoped semiconductors, to Ef Ev > Eg, in doped n-type semiconductors [25,26]. 3.3. Electrical properties The temperature dependence of electrical resistance for all the samples is shown in Fig. 3. It was observed that decrease in oxygen partial pressure ðPO2 Þ, during the growth, leads to a drastic change in the electrical behavior and resistance of these samples as the sample S1 and S2 exhibited semiconducting behavior while sample S3 showed metallic behavior. The possible reasons could be as follows; (i) the critical amount of PO2 may result in grain growth of stoichiometric STO and a creation of niobium oxides such as Nb2O5, while relatively low PO2 generates the singly ionized oxygen vacancies and prevent the formation of Nb2O5, which result in an efficient substitution of niobium (Nbþ5) to titanium (Tiþ4) and an increase of electron carriers density in the system [27,28], (ii) difference in ‘Nb’ doping level with the PO2 may lead to different level of lattice distortion, which can also modify the electronic band structure or energy level and, in turn, affect the electrical resistance. Sample S2 revealed metal to semiconductor transition around 150 K during subsequent heating and cooling cycle that can be understand on the basis of carrier freeze-out phenomenon as in the oxygen deficient SrTiO3d film [29,30]. A drastic increase in resistivity with decreasing temperature was due to the low density of donors (i.e. oxygen vacancies in the present case) resulted in the separation of the donor level from the bottom of conduction band. Below w150 K, majority of free electrons start shrinking down to the lower donor level and get trapped. The concentration and mobility of the negative charged carriers, obtained using Hall’s measurement, for the sample S1 and S2 were found to be (2.43 0.77) 1012 cm3, 8.17 0.73 cm2/V s and (2.14 0.33) 1017 cm3, 5.95 0.85 cm2/V s, respectively. These measurements were not possible for sample S3 because of its highly conductive nature. In conclusion it is worth mentioning that a decrease in PO2 resulted in enhancement of conductivity with very high negative-charged carrier concentration. The samples S2 and S3 made at lower oxygen pressures have more oxygen defects, which induce an increase of carrier density and thus a decrease of resistance, than the sample S1 made at a higher oxygen pressure. Since a low resistance state induced by defects precludes an observation of a small resistance change with the application of current pulse or UV light, the sample S1 was mainly used for the following studies on electro- and opto-resistive switching behavior. 3.4. Electro- and opto-resistive switching behaviors
(9)
where A is the probability parameter for the transition, Eg is the band gap of the material, h and n are the incident photon energy and the transition coefficient, respectively. The reported value of n is 2 for the measurement of an indirect band gap and 1/2 for a direct band gap. Here, the direct band gap of the Nb:STO films was evaluated by extrapolating the straight line part of the curves, (ahy)1/2 ¼ 0, as
The voltage versus current (VeI) characteristics of sample S1 and S3 are shown in Fig. 4(a) and (b), respectively. The dc voltage was swept from 0 / þVmax / 0 / Vmax / 0. Sample S1 exhibited non-linear and hysteresis behaviors in the VeI curve, while sample S3 showed a linear ohmic behavior without any hysteresis (Fig. 4(b)). The rectifying and hysteresis behaviors observed in the sample S1 indicate Schottky contacts at the
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Fig. 3. Electrical resistance versus temperature curves of sample S1, S2 and S3, respectively.
Nb:STO/Ag interfaces which probably cause the resistance switching (RS) between the high and low resistance states. The inset of Fig. 4(a) shows the ln (I/V) versus V1/2 plot for the high resistive state of the IeV curve that could be fitted linearly with the linear regression value (R2) of 0.992. Such a linear relation in the high voltage region suggested that the conduction mechanism in the high resistance state can be explained by Pool-Frenkel emission, which is dominated by bulk effect resulting from the lowering of the coulomb potential barrier of a trap site due to increased electric field. Therefore, the conduction mechanism in the high resistive state was presumably governed by the formation of conducting paths due to the electron hopping between the trap states which are probably introduced by the Nb doping and oxygen vacancies. Further, the sample S1 was exposed to a train of the current pulses that displayed the switching of the system between two resistance states that correspond to the high and low values of current. Interestingly, the system switched reversibly between two well defined resistance states, where the high resistance state can be assimilated to a logic ‘1’ and the low-resistance to a logic ‘0’ as shown in Fig. 5(a). A series of current pulses of different amplitudes was applied to the sample S1, which displayed the corresponding changes of resistance as can be seen in Fig. 5(b). The observed behavior was characteristic of multi-level switching of resistance. The systematic decrease in resistance with increasing the magnitude of applied current could be possibly due to the reduced Schottky barrier height, formation of conduction path in oxide regime or the lowering of the coulomb potential barrier of a trap site. The change in resistance to different magnitude of applied
Fig. 4. VeI characteristics of samples (a) S1 and (b) S3, respectively. Inset of figure (a) shows the plot of ln (I/V) against square root of voltage.
current is plotted in Fig. 5(c), and it was fitted by an exponential function R ¼ 0.18 exp(I/0.22) þ 0.11, where R and I is the resistance and the current in MU and mA, respectively. The above relation suggested that the resistance was more responsive to the small amplitude of the pulse and was almost saturated at higher amplitude. In order to investigate the photo-induced effect, resistance versus time characteristics were acquired using an illumination of UV light. The change in resistance corresponding to the ‘ON’ and ‘OFF’ state of UV light was found to be almost 7.1% as shown in Fig. 6. The observed decrease in resistance under the illumination of UV light could be due to the boosting of electron concentration. At the onset of UV radiations, the charge carrier density varies with time as n ¼ bkIt, where ‘bkI’ and ‘t’ represent the true quantum yield and time, respectively. Under UV illumination, a major part of the excited carriers jumps to the conduction band and is responsible for the sudden decrease in resistance value. Presumably, a majority of excited electrons go back to the valence band through relaxation process but a fraction of excited carriers captured by the trapping centers stay at the conduction band quite a while. Consequently, the system reaches to an equilibrium state as represented by the
A. Kumar, J. Dho / Current Applied Physics 13 (2013) 768e774
773
Fig. 6. Electrical resistance versus time curve correspond to ON/OFF state of UV light. The UV light with the power density of w3.6 mW/cm2 was used.
trapping levels [31]. Encircled region in Fig. 6 exhibited the recovery of initial value of resistance after termination of UV light, which manifested the recombination process with an exponential time dependence. For the first time, an approach has been made to enhance the storage channels by both the control parameters, i.e. current and UV light. The main idea was to enhance the integration density of the device by external means without any change in its dimension. Sample S1 was exposed to a current pulse and its change in resistance was examined with and without UV exposure as shown in Fig. 7. It was observed that in the dark, referred to as Channel A, the resistance exhibited two states of almost 42% apart correspond to low and high current values that can be assigned as logic 1 and 0, respectively. Similarly, under the UV illumination, the resistance decreased to almost 15% of the dark value, which was referred as channel B. Channel B also showed two resistance states with a difference of w34%, which was corresponds to low and high values of the current pulse that can also be assimilated as logic 1 and 0, respectively. The lower states of resistance in channel A and B showed the difference of w3% that was sufficient to distinguish these states. Therefore, the reading and writing can become possible at two channels by using both the control parameters alternatively. This may arise as a potential application in the emerging era of optoelectronic technology.
Fig. 5. (a) Change in electrical resistance corresponds to a train of current pulse; (b) Relative change in electrical resistance with different magnitude of current pulse and (c) variation of electrical resistance with increasing magnitude of current with an exponential fit.
saturating nature of the photo-induced change of resistance. On the cessation of UV radiations, the resistance increased rapidly to the initial dark resistance level due to the recombination of excited carriers at the trap centers. The decay of the carrier density after the UV termination can be represented by n ¼ bkIsnet/s, where sn is the electron lifetime and s signifies the effect of trap centers on the dynamics of charge carriers and it depends on the density of
Fig. 7. Resistance versus time curve corresponds to a current pulse with and without UV exposure. The UV light with the power density of w5.5 mW/cm2 was used.
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4. Conclusion Structural, optical and electrical properties of Nb:STO films were found be dependent of deposition pressure. Decrease in deposition pressure leads to decrease in mean free path caused less number of collisions between ablated particles and gas species, which prevent gas phase nucleation and resulted in small grain size and enhanced packed density. Average transmittance in the visible region was found 91, 52 and 32% for sample S1, S2 and S3, respectively. The observed decrease in transmittance was due to the increased absorption and scattering as evidenced by increased extinction coefficient. Electrical resistivity was found to be decreased with decreasing oxygen partial pressure because of the generation of more carriers such as singly ionized vacancies and free electrons. Sample S1 and S2 demonstrated electro- and opto-resistive switching behaviors, while S3 did not exhibit any such behavior due to its conductive nature. Sample S1 exhibited almost 50.0 and 7.1% of difference in high and low resistance states on the application of current pulse and UV light, respectively. In addition, multi-level switching has been demonstrated successfully by the alternative use of current pulse and UV light. These application of Nb:STO can provide new dimensions to the device technologies. Acknowledgments This work was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2010-0007902). References [1] M. Capizzi, A. Frova, Phys. Rev. Lett. 25 (1970) 1298. [2] N. Shanthi, D.D. Sarma, Phys. Rev. B 57 (1998) 2153.
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