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GaAs layers
Journal of Crystal Growth 451 (2016) 120–125
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Hot-wall-epitaxy growth and electrical/optical characterization of epitaxial BaAl2S4/GaAs layers J.W. Jeong a, K.J. Hong a,n, T.S. Jeong b, C.J. Youn b a
Department of Physics, Chosun University, Gwangju 501-759, South Korea School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center (SPRC), Chonbuk National University, Jeonju 561-756, South Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 March 2016 Received in revised form 30 June 2016 Accepted 7 July 2016 Communicated by Roberto Fornari Available online 7 July 2016
The BaAl2S4 layers, which were identified to be a cubic structure in space group Pa3, were grown using the hot wall epitaxy method attached with the reservoir tail. Also, the coincidence lattice mismatch of the grown layers was shown to be 2.52% due to the influence of the compressive strain. The Hall effect measurement showed the different temperature-dependent decrease of mobility at a temperature above 100 K. One was T 1/2 at the temperature range of 100 o To 180 K and the other was T 3/2 at the temperature of T4180 K. The mobility decreased in proportion to T1 at a low temperature range of T o 100 K. Three PC peaks obtained from the photocurrent (PC) spectra were corresponding to band-to-band transitions, which were observed over the temperature range. These PC peaks were caused by the transition of electrons from the three valence band states in order of increasing energy to the conduction band states, respectively. By analyzing absorption and PC results, the band gap variation has been compared and matched well with Eg(T)¼ Eg(0) 7.556 10 4T2/(T þ523), where Eg(0) is estimated to be 4.0596, 4.1053, and 4.1094 eV. & 2016 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A3. Hot wall epitaxy B1. BaAl2S4 B2. Semiconducting ternary compounds
1. Introduction Ternary chalcogenide compounds of II-III2-VI4 type are attractive for use in optoelectronics and nonlinear optics [1–3]. Barium aluminum sulfide (BaAl2S4) of these materials is the photoconductive and luminescent materials with band gap energy of 3.98 eV at room temperature [4]. Its crystal structure is a cubic system in space group Pa3. Thus, it has another aspect when a rare earth element is wedged into its compound. The rare earth element of Eu is activated for blue light emitting barium thioaluminate (BaAl2S4:Eu) phosphor composition having luminescent emission centered at 470–490 nm according to Eu concentration [5–8]. And, the structure of BaAl2S4:Eu has two types forming a cubic lattice crystal structure and a face centered orthorhombic crystal lattice structure [9]. Therefore, the BaAl2S4 compound is a particularly attractive material due to the emission characteristics for optoelectronic devices operating in blue and ultraviolet wavelength regions as a performance of full color display. So, the bulk and the layer of BaAl2S4 had been grown through a chemical transport reaction and electron beam evaporation methods, respectively [8,10,11]. However, in order to achieve better device n
Corresponding author. E-mail address: [email protected] (K.J. Hong).
http://dx.doi.org/10.1016/j.jcrysgro.2016.07.006 0022-0248/& 2016 Elsevier B.V. All rights reserved.
performance, it is of primary importance to grow high quality crystals and investigate their fundamental properties. Its crystal growth is not easy owing to the high reactivity with moisture and high melting point (41250 °C). Moreover, this behavior leads to a stoichiometric deviation in the crystal during the growth or the additional thermal treatment. The stoichiometric deviation mainly occurs when the partial vapor pressure of S atoms is higher than those of the Ba and Al atoms during growth. Thus, these stoichiometric defects are responsible for the formation of native defects and self compensation in BaAl2S4. Furthermore, they can disturb the achievement of the performance device. Therefore, low-temperature growth method is requested at this experiment to suppress the stoichiometric deviation. In order to grow BaAl2S4 layers, the hot-wall-epitaxy (HWE) method, which is one of the low-temperature growth technologies, was introduced in this experiment. HWE method has been especially designed to grow epilayers under condition of near thermodynamic equilibrium [12]. In this research, the BaAl2S4 layers were first-time grown on the GaAs substrate by means of the HWE method. The structure and crystal quality on the BaAl2S4/GaAs layers were found from double crystal X-ray diffraction (DCXD). Thus, the electric/optical characterization was extracted through the Hall effect, optical absorption, and photocurrent (PC) experiments. Based on these results, the behaviors of electrical/optical characterization as
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functions of the temperature were discussed in this paper.
2. Experimental procedures Prior to the growth of the BaAl2S4 layer, the polycrystalline ingot was sintered. The starting materials were 6 N-purity shottypes of Ba, Al, and S (all from Aldrich). After these materials had been weighed in stoichiometric proportions then sealed in evacuated quartz tubes. Fig. 1 shows the horizontal furnace of the Bridgman method used to form polycrystalline BaAl2S4. As shown in Fig. 1, the sealed ampoule was placed in the synthesis furnace and was continually rotated at a rate of 1 revolution per minute. In order to avoid an explosion of the ampoule due to the S vapor pressure, we gradually increased the temperature of the ampoule to 500 °C and then maintained that temperature for 24 h. Thereafter, the temperature of the ampoule was increased gradually to 1150 °C, which temperature was then maintained for 48 h. After these processes, a polycrystalline BaAl2S4 ingot was extracted. Fig. 2 presents the HWE apparatus used for the growth of the BaAl2S4 layer. This HWE apparatus constructed by the electric furnaces of three stages was made of a quartz tube with kanthal wire. A substrate holder is located in the furnace between the first stage labeled as a substrate and the second stage labeled as a source. Here, one of the roles of the substrate holder serves as a lid to close the top of the source tube. And the reservoir stage is posited at the bottom of the vertical quartz tube and its purpose is to uniformly maintain the total partial pressure on each element in the quartz tube. Each part of the quartz tube is connected to the temperature controller, so as to independently control the temperature of each quartz tube. In order to grow the BaAl2S4 layer, we used the polycrystalline BaAl2S4 ingot as the HWE source and the semi-insulating GaAs (100) wafer as the substrate. Here, the adoption of the GaAs (100) wafers provides a prototype for understanding the detailed nature of that growth process. Thus, the (100) phase is the favored growth substrate, partly because of the ease with which it may be cleaned and partly because of the difficulty of achieving good HWE growth on other phases such as (111) [13]. The GaAs substrate was cleaned ultrasonically for 1 min in successive baths of trichloroethylene, acetone, methanol, and 2-propanol and etched for 1 min in a solution of H2SO4:H2O2:H2O (5:1:1). The substrate was degreased in organic solvents and rinsed with deionized water (18.2 MΩ. Here, ultra pure water hardly brings electricity, so its resistance is very high). After the substrate was dried off, the substrate was immediately loaded onto the substrate holder in the HWE apparatus and was annealed
Fig. 2. Schematic diagram of the HWE apparatus.
at 580 °C for 20 min to remove the residual oxide on the surface of the substrate. On the other hand, to find the optimum growth conditions, the grown BaAl2S4/GaAs layers were analyzed by DCXD measurements. The stoichiometric composition was measured using an energy dispersive X-ray spectrometer (EDS). Also, the electric property was achieved by Hall effect measurement of the van der Pauw method. In order to take PC measurements, we fabricated two Au electrodes with a coplanar geometry on the BaAl2S4/GaAs by using an e-beam evaporator, and we confirmed Ohmic contacts of the electrodes by using current-voltage measurements. The distance between electrodes was 1.5 mm. After the electrodes had been connected to a wire, the sample was mounted on the holder of a low-temperature cryostat. Then, a bias of 2 V was applied to the circuit. The measurement of the PC spectrum was done while the monochromatic light emitted from a xenon lamp (Output power: 500 W) was illuminated on the sample. At this time, the monochromatic light was normalized, and its intensity was 18 μW/cm2. Optical absorption was used to measure the band gap energy using an ultraviolet visible near-infrared (UV–vis-NIR) spectrophotometer (Hitachi, U-3501). These measurements were achieved while varying the temperature from 10 to 293 K.
3. Results and discussion 3.1. Growth and structural characteristic
Fig.1. Horizontal furnace of the Bridgman method.
Fig. 3 presents the XRD curves of polycrystalline BaAl2S4. These diffraction peaks were measured by means of the X-ray powder method. As shown in Fig. 3, this synthesized polycrystalline crystallized into a cubic structure [14]. By using the extrapolation method, the lattice constant of a0 was extracted to be 12.6435 Å. This value concurred well with that of 12.660 Å obtained by Peters et al [15]. Also, Eisenmann et al [16]. reported that its value is 12.650 Å. Table 1 displays the composition of each element between the synthesized polycrystalline and the grown layer obtained from the EDS measurement. As shown in Table 1, the weight percent of the Ba and Al components in sintered polycrystalline increased a little bit, while that of the S components decreased. It suggests that its cause is due to the high vapor
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Fig. 3. XRD curves of polycrystalline BaAl2S4.
Table 1 Composition of each element in the synthesized polycrystalline and the grown layers. Here, “Before” and “After” indicate before and after synthesis of polycrystalline BaAl2S4, respectively. Element Synthesized polycrystalline (Units: wt%)
Ba Al S
Before
After
HWE growth (Units: wt%) Layer
42.98 16.88 40.14
43.00 16.91 40.09
42.99 16.95 40.06
pressure of S elements. Then, the synthesized polycrystalline BaAl2S4 ingot was used as a source material of HWE as shown in Fig. 2. The heating temperature of the source material was fixed at 630 °C throughout the experimental repetition process. Also, the temperature of the reservoir was determined at the partial vapor pressure of the source maintained at Pmin, which is identified to the minimum value of the total pressure in the chamber, as the vapor pressure of the reservoir [17]. And the reservoir tube was filled by the sulfur powder. At this time, the vapor pressures of the source material and sulfur were 2/3Pmin and 1/3Pmin, respectively. When Pmin was about 10 1 Torr during the growth, the vapor pressure of sulfur was about 10 2 Torr and then the temperature of the reservoir was determined to be 110 °C. Under these conditions, the BaAl2S4/GaAs layer was grown by changing the substrate temperature from 400 to 440 °C. In order to extract out the optimum growth temperature, the DCXD measurement was conducted. The observation of DCXD curve implies that the grown layer is a high quality crystalline, because its observation at the low quality crystal is not easy. Fig. 4 displays the intensity and the full width at half maximum (FWHM) values of the DCXD rocking curve obtained as a function of the substrate temperature. As shown in Fig. 4, the layer grown at 420 °C has the highest DCXD intensity among several growth temperatures and its FWHM was a minimum value. This fact indicates that the most suitable substrate temperature is 420 °C. While the source temperature and the reservoir temperature were maintained at 630 and 110 °C, respectively, the most suitable substrate temperature for the growth turned out to be 420 °C. As shown in Table 1, the component ratios of the initial mole fraction were continuously maintained during the layer growth. It suggests that the vapor pressure of the reservoir is responsible for this situation because the stoichiometry during the layer growth was well adjusted by the proper temperature of the reservoir. Also, this result indicates that the BaAl2S4/GaAs layer consisting of each Ba, Al, and S element is well formed in chemical bonding. Under the optimum substrate temperature, the thickness and growth rate of the BaAl2S4/GaAs layer were obtained to be 2.4 μm and 1.21 Å/s, respectively.
Fig. 4. Intensity and FWHM values of the DCXD rocking curve as a function of the substrate temperature.
Fig. 5. XRD and DCXD rocking curves on the BaAl2S4/GaAs layers grown at the optimum substrate temperature of 420 °C.
Fig. 5 shows the XRD and DCXD rocking curves on the BaAl2S4/GaAs layers grown at the optimum substrate temperature of 420 °C. As shown in Fig. 5, two peaks were appeared in XRD rocking curve and other diffraction peaks except these peaks were not observed. Here, the low and dominant peaks correspond to the diffraction peaks of the BaAl2S4 (312) and the GaAs (400) plane, respectively. It means that the orientation of the BaAl2S4 layer grown on the GaAs (100) substrate is converted to the (312) plane. Such a phase conversion was observed in the CdTe epilayer grown on the GaAs (100). Faurie et al [18]. reported that the orientation of the CdTe epilayer was related to the pre-annealing process used to remove the residual oxide on the surface of the substrate. They concluded that the growth of the CdTe (100) or (111) planes on the GaAs (100) was possible by controlling the annealing temperature and annealing time of the substrate. As shown in the subfigure of Fig. 5, the DCXD spectrum displayed the diffraction curve on the BaAl2S4 (312) phase of 26.327° and its FWHM was a 238 arcsec.
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Fig. 6. A simple lattice model with CLS between the layer and substrate.
From these results, it indicates that the layer was epitaxially grown along the (312) direction on to the GaAs (100) substrate. Thus, the BaAl2S4/GaAs layer is cubic structure because the peak position of the (312) plane is equal to that of the (312) plane in Fig. 3. As Fig. 5 shows, two XRD peaks between layer and substrate are a very large separation meaning the presence of a lattice mismatch. Here, the inter-planar spacing (dhkl), which is the distance between crystallographic planes belonging to the family (hkl) on BaAl2S4 (312) and GaAs (400), were calculated to be 3.3825 and 1.4140 Å, respectively. In fact, the distance between two planes is so far that epitaxial growth can hardly be expected. However, this large lattice mismatch can be explained by a coincidence site lattice (CSL), which is proposed by Trampert et al [19]. Fig. 6 shows a simple lattice model with CLS between the layer and substrate. As shown in Fig. 6, the coincidence between the substrate and layer lattices would occur when mdsubstrate ¼ndlayer, defined by a pair of integers (m, n). So, the substrate and layer having lattice spacing close to an integer ratio m/n may form an epitaxial interface described by a coincidence lattice [20,21]. The coincidence lattice mismatch δ, which expressed the deviation from true coincidence, is given by
δ% =
mdsubstrate − ndlayer × 100, mdsubstrate
(1)
where m and n are integers meaning CSL between the substrate and layer, respectively. Then, a positive or negative value of δ gives rise to a tensile strain or compressive strain in the layer, respectively. Here, every third BaAl2S4 (312) plane corresponds to every seventh GaAs (400) plane. When a pair of integers (m, n) is (7, 3), the substrate and layer have a coincidence lattice. These seven and three coincident lattices reduce the lattice misfit to 2.52%, which is a reasonable value for the epitaxial growth. Thus, it suggests that the BaAl2S4/GaAs layer is influenced by the compressive strain due to negative value of the coincidence lattice mismatch during layer growth. Therefore, these results can give a significant influence on the electrical and optical properties in the BaAl2S4/GaAs layer. 3.2. Electrical/optical characteristics Fig. 7 shows the electron mobility on the BaAl2S4/GaAs layer as a function of the temperature T. As shown in Fig. 7, the mobility increased with increasing temperature up to 100 K and it decreased with increasing temperature at temperatures higher than 100 K. Its maximum value at the apex of 100 K was 9.95 102 cm2 /V s. Using Mattiessen's rule, the total mobility can be expressed as the sum of the scattering rates caused by the phonon and impurity. There are two prominent scattering mechanisms of phonon
Fig. 7. Electron mobility as a function of the temperature T.
scattering (lattice scattering) and impurity scattering [22]. Lattice scattering becomes dominant at high temperatures, its mobility decreases with increasing temperature. This lattice scattering is attributed to three phonons of a) deformation potential acoustic phonons, b) piezoelectric acoustic phonons, and c) polar optical phonons. Mobility dominated by acoustic phonon interaction is expected to be proportional to T 3/2, while the mobility due to piezoelectric acoustic phonons scattering only is expected to be proportional to T 1/2. However, impurity scattering by dislocations becomes less significant at higher temperatures and its mobility increases with increasing temperature. As shown in Fig. 7, the mobility decreased as a function of T 3/2 at the high temperature range (T 4180 K). This indicates that scattering at the high temperature range is mainly due to the acoustic phonon mode of lattice vibrations through a deformation potential [22]. Thus, at the intermediate temperature range (100 oTo 180 K), the mobility gradually decreased as a function of T 1/2 with increasing temperature. Such a behavior is attributed to the piezoelectric potential scattering [23]. Therefore, at the high temperatures over 100 K, the mobility was scattered by the piezoelectric and deformation acoustic phonons. While, at the low temperature range (T o100 K), the mobility increased as a function of T1 with increasing temperature. It associates that scattering can be analyzed by the dislocation scattering model [24]. It is estimated that the scattering due to the change of dislocation in layers is dominant up to 100 K. Optical band gap of the BaAl2S4/GaAs layer was extracted through PC and absorption measurements. Generally, for light with a variable photon energy (Ep) near the band gap energy (Eg), light with lower-energy photons (Ep oEg) is transmitted without being absorbed and light with higher-energy photons (Ep 4Eg) is absorbed. The strongly-absorbed excitation creates a high density of free electron-hole pairs (EHPs) near the surface. Ultimately, EHP generation is used to designate the valence-band electron as part of a covalent bond [25]. Therefore, when the covalent bond is broken, the electron is free to move in the crystal lattice of the layer. The required energy to break a covalent bond is Eg, If the photon energy is Ep oEg, it is not sufficient to break a covalent bond and free an electron for conduction. At this time, electrons do not occupy the forbidden states between the valence and the conduction bands, so a photon with Ep oEg is not absorbed and passes through the layer. Therefore, for a typical photosensitive layer, the lifetime for surface excitation for Ep 4Eg is much shorter
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extended to longer wavelengths and it will be observed as a rough spectrum. At this time, the defect states include vacancies, interstitial atoms, or dislocations, etc. As shown in Fig. 8, the PC curve located to the long wavelength direction is a smooth gradient. Consequently, it means that the grown layers are high quality crystal. On the other hand, the optical band gap of the BaAl2S4/GaAs layer can be extracted from the traditional method of the optical absorption coefficient measurement. When photons are incident on a material, the intensity, I, transited photon can be expressed by the Lambert-Beer-Bouguer law [29]:
Ι = Ι0 exp ( −αd),
(2)
where I0 and I are the intensities of photon entering and leaving the layer, respectively. Also, α and d are optical absorption coefficient and thickness of the layer, respectively. Under the absorption conditions, I is exponentially reduced while traveling through the layer and α is strongly depended to the incident photon energy (hν). And Eq. (2) is rewritten as:
α = − (1/d) 1n (I /I0 )
(3)
Therefore, from the optical absorption measurement, the relation between α and hν was extracted from the optical absorption spectra. The absorption coefficient as a function of photon energy in the allowed direct transition can be expressed as [30]: Fig. 8. Characteristic PC spectra of the BaAl2S4/GaAs layer measured with the temperature variation. Here, the dotted line represents the peak A of the PC peaks obtained at each the temperature.
than the lifetime for volume excitation for Ep o Eg. Consequently, the signal obtained from the PC measurement is related to the absorption of photons, leading to the generation of free charge particles in the conduction band and/or in the valence band. For the PC measurement, when the photon energy is Ep 4 Eg, the absorbed photons create electron and hole carriers. And then, if an external electric field is applied, the electrons and holes move in opposite directions. These carriers instantly flow out through the electrodes and produce PC signals. At this time, a maximum peak in the PC spectrum occurs when a transition is made from surface excitation to volume excitation with increasing wavelength [26]. The PC peak gives rise to one of the following mechanisms: 1) Band-to-band transitions, 2) Impurity-level-to-bandedge transitions, 3) Ionization of donors, and 4) Deep levels in band gap. Fig. 8 presents the PC spectra of the BaAl2S4/GaAs layer measured with the temperature variation. As shown in Fig. 8, three PC peaks appeared at specific temperatures from 10 to 293 K and were shifted toward the short-wavelength region with decreasing PC measurement temperature. Three peaks observed at 293 K were located to 311.5 (3.9803), 308.0 (4.0255), and 307.8 nm (4.0281 eV), respectively. Those observed at 10 K were 305.4 (4.0598), 302.0 (4.1055), and 301.7 nm (4.1096 eV). The three peaks were labeled as peaks A, B, and C to indicate the transitions from the three valence-band levels in order of increasing energy. Thus, the PC peaks are known to be due to the combined influences of the spin-orbit interaction and the noncubic crystalline field, and they are related to each direct band gap [27]. These PC peaks are attributed to the transition of electrons from the valence band to the conduction band. This suggests that the three PC peaks are caused by band-to-band transitions following mechanism 1) [26,28]. Moreover, the PC slope toward the longer wavelength region of the spectrum is related to the crystal quality. If there exists the optically active traps, then it takes along time to attain a steady-state value after vanishing the monochromatic light. As a result, PC response will be observed to a rough spectrum. Furthermore, the spectral PC response caused by the defect states is
(αhv)2 = A (hv − Eg ),
(4)
where, h is the Planck constant and A is a constant because it do not involve phonons due to the lattice vibrations in electron bandto-band transitions. According to Eqs. (4), (αhν)2 linearly depends on the photon energy. Fig. 9 displays the plots of (αhν)2 versus photon energy for different temperatures. As shown in Fig. 9, the optical band gap was obtained by extrapolating the linear portions of the respective curves to zero [31]. Generally, the magnitude of the band gap for any material varies with temperature and pressure. Therefore, the band gap as a function of temperature and pressure can be expressed as [28]
ΔEG = (∂EG/∂P )T ΔP + (∂EG/∂T )P ΔT
(5)
When A¼ (∂EG/∂T)P and B ¼(∂EG/∂P)T, the band gap at constant pressure is EG ¼ EG(0) þAT, where EG(0) is the band gap at 0 K. For most photoconductor materials, the value of A is a negative quantity. Therefore, the band gap as a function of temperature can be well described by following Varshni's empirical equation [32,33]:
Fig. 9. Plots of (αhν)2 versus photon energy for different temperatures.
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behavior was attributed to the dislocation scattering. At the higher temperature than 100 K, the scattering of mobility was decreased to two different reasons. One had a function of T 3/2 at the temperature of T4 180 K and the other one showed a function of T 1/2 at the temperature range of 100 oT o180 K. The former was mainly caused by the scattering of the acoustic phonon mode of lattice vibrations through a deformation potential and the latter was owing to the piezoelectric potential scattering. From the PC measurement, three PC peaks labeled as peaks A, B, and C were observed. The three PC-peaks were caused by band-to-band transitions due to the transition of electrons from the three valence band states in order of increasing energy, respectively, to the conduction band states, respectively. Thus, these PC-peak energies shifted toward the short wavelength region with decreasing temperature. Through the analysis of absorption and PC results, the band gap variation has been compared and matched well by using Eg(T)¼ Eg(0) 7.556 10 4T2/(T þ523), where Eg(0) is estimated to be 4.0596, 4.1053, and 4.1094 eV for peaks A, B and C, respectively. Fig. 10. Energy variation of the optical absorption and PC-peak energy as a function of temperature.
Eg (T ) = Eg (0) − aT 2/(b + T ),
(6)
where a and b are constants. Also, Eg(0) is the band gap energy at 0 K. Fig. 10 shows the energy variation as a function of the temperature of the BaAl2S4/GaAs layer, which was extracted from PC and absorption measurements. The PC energies of peak A corresponding to the band-to-band transition were well consistent with the absorption energies contrasted with the same temperature. In fact, the absorption experiment has been known to be inaccurate for obtaining the band gap energy because of the difficulty in defining the position of the absorption edge. This discrepancy comes from the fact that the band gap energy is obtained by fitting the experimental spectra to theoretical models. Contrary to this process, the band gap energy through the PC method can be instantly found by measuring the position of the PC peak. Consequently, the PC spectroscopy is a useful method for determining the band gap of the BaAl2S4/GaAs layer. As shown in Fig. 10, these energies increased with decreasing temperature and had a nonlinear relationship. When a, b, and Eg(0) are taken to be 7.556 10 4 eV/K, 523 K, and 4.0596 eV for peak A, respectively. The curves plotted by Eq. (6) closely fit the experimental values. Thus, by fitting Eq. (6), Eg(0) values for peaks B and C are extracted to be 4.1053 and 4.1094 eV, respectively.
4. Conclusions The epitaxial BaAl2S4 layers were first grown on GaAs substrate through the HWE method. At reservoir temperature of 110 °C maintaining vapor pressure of 1/3Pmin, the optimum growth temperature of the substrate and the source extracted out to be 420 and 630 °C, respectively. Under these optimum substrate temperature, the thickness and growth rate were obtained to be 2.4 μm and 1.21 Å/s, respectively. Thus, it was confirmed that the grown BaAl2S4 layer, which formed the cubic structure in space group Pa3, was epitaxially accumulated along the (312) direction onto the GaAs (100) substrate and its FWHM on DCXD was 238 arcsec. According to the CSL model, the coincidence lattice mismatch of layer was 2.52% with the integer pair of (7, 3) and the layer was influenced by the compressive strain. By the Hall effect measurement, the mobility behavior depending on temperature was found. During increasing temperature to 100 K, the mobility increased as a function of T1 and such a
Acknowledgments This study was supported by research funds from Chosun University, 2015.
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Hot-wall-epitaxy growth and electrical/optical characterization of epitaxial BaAl2S4/GaAs layers J.W. Jeong a, K.J. Hong a,n, T.S. Jeong b, C.J. Youn b a
Department of Physics, Chosun University, Gwangju 501-759, South Korea School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center (SPRC), Chonbuk National University, Jeonju 561-756, South Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 March 2016 Received in revised form 30 June 2016 Accepted 7 July 2016 Communicated by Roberto Fornari Available online 7 July 2016
The BaAl2S4 layers, which were identified to be a cubic structure in space group Pa3, were grown using the hot wall epitaxy method attached with the reservoir tail. Also, the coincidence lattice mismatch of the grown layers was shown to be 2.52% due to the influence of the compressive strain. The Hall effect measurement showed the different temperature-dependent decrease of mobility at a temperature above 100 K. One was T 1/2 at the temperature range of 100 o To 180 K and the other was T 3/2 at the temperature of T4180 K. The mobility decreased in proportion to T1 at a low temperature range of T o 100 K. Three PC peaks obtained from the photocurrent (PC) spectra were corresponding to band-to-band transitions, which were observed over the temperature range. These PC peaks were caused by the transition of electrons from the three valence band states in order of increasing energy to the conduction band states, respectively. By analyzing absorption and PC results, the band gap variation has been compared and matched well with Eg(T)¼ Eg(0) 7.556 10 4T2/(T þ523), where Eg(0) is estimated to be 4.0596, 4.1053, and 4.1094 eV. & 2016 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A3. Hot wall epitaxy B1. BaAl2S4 B2. Semiconducting ternary compounds
1. Introduction Ternary chalcogenide compounds of II-III2-VI4 type are attractive for use in optoelectronics and nonlinear optics [1–3]. Barium aluminum sulfide (BaAl2S4) of these materials is the photoconductive and luminescent materials with band gap energy of 3.98 eV at room temperature [4]. Its crystal structure is a cubic system in space group Pa3. Thus, it has another aspect when a rare earth element is wedged into its compound. The rare earth element of Eu is activated for blue light emitting barium thioaluminate (BaAl2S4:Eu) phosphor composition having luminescent emission centered at 470–490 nm according to Eu concentration [5–8]. And, the structure of BaAl2S4:Eu has two types forming a cubic lattice crystal structure and a face centered orthorhombic crystal lattice structure [9]. Therefore, the BaAl2S4 compound is a particularly attractive material due to the emission characteristics for optoelectronic devices operating in blue and ultraviolet wavelength regions as a performance of full color display. So, the bulk and the layer of BaAl2S4 had been grown through a chemical transport reaction and electron beam evaporation methods, respectively [8,10,11]. However, in order to achieve better device n
Corresponding author. E-mail address: [email protected] (K.J. Hong).
http://dx.doi.org/10.1016/j.jcrysgro.2016.07.006 0022-0248/& 2016 Elsevier B.V. All rights reserved.
performance, it is of primary importance to grow high quality crystals and investigate their fundamental properties. Its crystal growth is not easy owing to the high reactivity with moisture and high melting point (41250 °C). Moreover, this behavior leads to a stoichiometric deviation in the crystal during the growth or the additional thermal treatment. The stoichiometric deviation mainly occurs when the partial vapor pressure of S atoms is higher than those of the Ba and Al atoms during growth. Thus, these stoichiometric defects are responsible for the formation of native defects and self compensation in BaAl2S4. Furthermore, they can disturb the achievement of the performance device. Therefore, low-temperature growth method is requested at this experiment to suppress the stoichiometric deviation. In order to grow BaAl2S4 layers, the hot-wall-epitaxy (HWE) method, which is one of the low-temperature growth technologies, was introduced in this experiment. HWE method has been especially designed to grow epilayers under condition of near thermodynamic equilibrium [12]. In this research, the BaAl2S4 layers were first-time grown on the GaAs substrate by means of the HWE method. The structure and crystal quality on the BaAl2S4/GaAs layers were found from double crystal X-ray diffraction (DCXD). Thus, the electric/optical characterization was extracted through the Hall effect, optical absorption, and photocurrent (PC) experiments. Based on these results, the behaviors of electrical/optical characterization as
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functions of the temperature were discussed in this paper.
2. Experimental procedures Prior to the growth of the BaAl2S4 layer, the polycrystalline ingot was sintered. The starting materials were 6 N-purity shottypes of Ba, Al, and S (all from Aldrich). After these materials had been weighed in stoichiometric proportions then sealed in evacuated quartz tubes. Fig. 1 shows the horizontal furnace of the Bridgman method used to form polycrystalline BaAl2S4. As shown in Fig. 1, the sealed ampoule was placed in the synthesis furnace and was continually rotated at a rate of 1 revolution per minute. In order to avoid an explosion of the ampoule due to the S vapor pressure, we gradually increased the temperature of the ampoule to 500 °C and then maintained that temperature for 24 h. Thereafter, the temperature of the ampoule was increased gradually to 1150 °C, which temperature was then maintained for 48 h. After these processes, a polycrystalline BaAl2S4 ingot was extracted. Fig. 2 presents the HWE apparatus used for the growth of the BaAl2S4 layer. This HWE apparatus constructed by the electric furnaces of three stages was made of a quartz tube with kanthal wire. A substrate holder is located in the furnace between the first stage labeled as a substrate and the second stage labeled as a source. Here, one of the roles of the substrate holder serves as a lid to close the top of the source tube. And the reservoir stage is posited at the bottom of the vertical quartz tube and its purpose is to uniformly maintain the total partial pressure on each element in the quartz tube. Each part of the quartz tube is connected to the temperature controller, so as to independently control the temperature of each quartz tube. In order to grow the BaAl2S4 layer, we used the polycrystalline BaAl2S4 ingot as the HWE source and the semi-insulating GaAs (100) wafer as the substrate. Here, the adoption of the GaAs (100) wafers provides a prototype for understanding the detailed nature of that growth process. Thus, the (100) phase is the favored growth substrate, partly because of the ease with which it may be cleaned and partly because of the difficulty of achieving good HWE growth on other phases such as (111) [13]. The GaAs substrate was cleaned ultrasonically for 1 min in successive baths of trichloroethylene, acetone, methanol, and 2-propanol and etched for 1 min in a solution of H2SO4:H2O2:H2O (5:1:1). The substrate was degreased in organic solvents and rinsed with deionized water (18.2 MΩ. Here, ultra pure water hardly brings electricity, so its resistance is very high). After the substrate was dried off, the substrate was immediately loaded onto the substrate holder in the HWE apparatus and was annealed
Fig. 2. Schematic diagram of the HWE apparatus.
at 580 °C for 20 min to remove the residual oxide on the surface of the substrate. On the other hand, to find the optimum growth conditions, the grown BaAl2S4/GaAs layers were analyzed by DCXD measurements. The stoichiometric composition was measured using an energy dispersive X-ray spectrometer (EDS). Also, the electric property was achieved by Hall effect measurement of the van der Pauw method. In order to take PC measurements, we fabricated two Au electrodes with a coplanar geometry on the BaAl2S4/GaAs by using an e-beam evaporator, and we confirmed Ohmic contacts of the electrodes by using current-voltage measurements. The distance between electrodes was 1.5 mm. After the electrodes had been connected to a wire, the sample was mounted on the holder of a low-temperature cryostat. Then, a bias of 2 V was applied to the circuit. The measurement of the PC spectrum was done while the monochromatic light emitted from a xenon lamp (Output power: 500 W) was illuminated on the sample. At this time, the monochromatic light was normalized, and its intensity was 18 μW/cm2. Optical absorption was used to measure the band gap energy using an ultraviolet visible near-infrared (UV–vis-NIR) spectrophotometer (Hitachi, U-3501). These measurements were achieved while varying the temperature from 10 to 293 K.
3. Results and discussion 3.1. Growth and structural characteristic
Fig.1. Horizontal furnace of the Bridgman method.
Fig. 3 presents the XRD curves of polycrystalline BaAl2S4. These diffraction peaks were measured by means of the X-ray powder method. As shown in Fig. 3, this synthesized polycrystalline crystallized into a cubic structure [14]. By using the extrapolation method, the lattice constant of a0 was extracted to be 12.6435 Å. This value concurred well with that of 12.660 Å obtained by Peters et al [15]. Also, Eisenmann et al [16]. reported that its value is 12.650 Å. Table 1 displays the composition of each element between the synthesized polycrystalline and the grown layer obtained from the EDS measurement. As shown in Table 1, the weight percent of the Ba and Al components in sintered polycrystalline increased a little bit, while that of the S components decreased. It suggests that its cause is due to the high vapor
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Fig. 3. XRD curves of polycrystalline BaAl2S4.
Table 1 Composition of each element in the synthesized polycrystalline and the grown layers. Here, “Before” and “After” indicate before and after synthesis of polycrystalline BaAl2S4, respectively. Element Synthesized polycrystalline (Units: wt%)
Ba Al S
Before
After
HWE growth (Units: wt%) Layer
42.98 16.88 40.14
43.00 16.91 40.09
42.99 16.95 40.06
pressure of S elements. Then, the synthesized polycrystalline BaAl2S4 ingot was used as a source material of HWE as shown in Fig. 2. The heating temperature of the source material was fixed at 630 °C throughout the experimental repetition process. Also, the temperature of the reservoir was determined at the partial vapor pressure of the source maintained at Pmin, which is identified to the minimum value of the total pressure in the chamber, as the vapor pressure of the reservoir [17]. And the reservoir tube was filled by the sulfur powder. At this time, the vapor pressures of the source material and sulfur were 2/3Pmin and 1/3Pmin, respectively. When Pmin was about 10 1 Torr during the growth, the vapor pressure of sulfur was about 10 2 Torr and then the temperature of the reservoir was determined to be 110 °C. Under these conditions, the BaAl2S4/GaAs layer was grown by changing the substrate temperature from 400 to 440 °C. In order to extract out the optimum growth temperature, the DCXD measurement was conducted. The observation of DCXD curve implies that the grown layer is a high quality crystalline, because its observation at the low quality crystal is not easy. Fig. 4 displays the intensity and the full width at half maximum (FWHM) values of the DCXD rocking curve obtained as a function of the substrate temperature. As shown in Fig. 4, the layer grown at 420 °C has the highest DCXD intensity among several growth temperatures and its FWHM was a minimum value. This fact indicates that the most suitable substrate temperature is 420 °C. While the source temperature and the reservoir temperature were maintained at 630 and 110 °C, respectively, the most suitable substrate temperature for the growth turned out to be 420 °C. As shown in Table 1, the component ratios of the initial mole fraction were continuously maintained during the layer growth. It suggests that the vapor pressure of the reservoir is responsible for this situation because the stoichiometry during the layer growth was well adjusted by the proper temperature of the reservoir. Also, this result indicates that the BaAl2S4/GaAs layer consisting of each Ba, Al, and S element is well formed in chemical bonding. Under the optimum substrate temperature, the thickness and growth rate of the BaAl2S4/GaAs layer were obtained to be 2.4 μm and 1.21 Å/s, respectively.
Fig. 4. Intensity and FWHM values of the DCXD rocking curve as a function of the substrate temperature.
Fig. 5. XRD and DCXD rocking curves on the BaAl2S4/GaAs layers grown at the optimum substrate temperature of 420 °C.
Fig. 5 shows the XRD and DCXD rocking curves on the BaAl2S4/GaAs layers grown at the optimum substrate temperature of 420 °C. As shown in Fig. 5, two peaks were appeared in XRD rocking curve and other diffraction peaks except these peaks were not observed. Here, the low and dominant peaks correspond to the diffraction peaks of the BaAl2S4 (312) and the GaAs (400) plane, respectively. It means that the orientation of the BaAl2S4 layer grown on the GaAs (100) substrate is converted to the (312) plane. Such a phase conversion was observed in the CdTe epilayer grown on the GaAs (100). Faurie et al [18]. reported that the orientation of the CdTe epilayer was related to the pre-annealing process used to remove the residual oxide on the surface of the substrate. They concluded that the growth of the CdTe (100) or (111) planes on the GaAs (100) was possible by controlling the annealing temperature and annealing time of the substrate. As shown in the subfigure of Fig. 5, the DCXD spectrum displayed the diffraction curve on the BaAl2S4 (312) phase of 26.327° and its FWHM was a 238 arcsec.
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Fig. 6. A simple lattice model with CLS between the layer and substrate.
From these results, it indicates that the layer was epitaxially grown along the (312) direction on to the GaAs (100) substrate. Thus, the BaAl2S4/GaAs layer is cubic structure because the peak position of the (312) plane is equal to that of the (312) plane in Fig. 3. As Fig. 5 shows, two XRD peaks between layer and substrate are a very large separation meaning the presence of a lattice mismatch. Here, the inter-planar spacing (dhkl), which is the distance between crystallographic planes belonging to the family (hkl) on BaAl2S4 (312) and GaAs (400), were calculated to be 3.3825 and 1.4140 Å, respectively. In fact, the distance between two planes is so far that epitaxial growth can hardly be expected. However, this large lattice mismatch can be explained by a coincidence site lattice (CSL), which is proposed by Trampert et al [19]. Fig. 6 shows a simple lattice model with CLS between the layer and substrate. As shown in Fig. 6, the coincidence between the substrate and layer lattices would occur when mdsubstrate ¼ndlayer, defined by a pair of integers (m, n). So, the substrate and layer having lattice spacing close to an integer ratio m/n may form an epitaxial interface described by a coincidence lattice [20,21]. The coincidence lattice mismatch δ, which expressed the deviation from true coincidence, is given by
δ% =
mdsubstrate − ndlayer × 100, mdsubstrate
(1)
where m and n are integers meaning CSL between the substrate and layer, respectively. Then, a positive or negative value of δ gives rise to a tensile strain or compressive strain in the layer, respectively. Here, every third BaAl2S4 (312) plane corresponds to every seventh GaAs (400) plane. When a pair of integers (m, n) is (7, 3), the substrate and layer have a coincidence lattice. These seven and three coincident lattices reduce the lattice misfit to 2.52%, which is a reasonable value for the epitaxial growth. Thus, it suggests that the BaAl2S4/GaAs layer is influenced by the compressive strain due to negative value of the coincidence lattice mismatch during layer growth. Therefore, these results can give a significant influence on the electrical and optical properties in the BaAl2S4/GaAs layer. 3.2. Electrical/optical characteristics Fig. 7 shows the electron mobility on the BaAl2S4/GaAs layer as a function of the temperature T. As shown in Fig. 7, the mobility increased with increasing temperature up to 100 K and it decreased with increasing temperature at temperatures higher than 100 K. Its maximum value at the apex of 100 K was 9.95 102 cm2 /V s. Using Mattiessen's rule, the total mobility can be expressed as the sum of the scattering rates caused by the phonon and impurity. There are two prominent scattering mechanisms of phonon
Fig. 7. Electron mobility as a function of the temperature T.
scattering (lattice scattering) and impurity scattering [22]. Lattice scattering becomes dominant at high temperatures, its mobility decreases with increasing temperature. This lattice scattering is attributed to three phonons of a) deformation potential acoustic phonons, b) piezoelectric acoustic phonons, and c) polar optical phonons. Mobility dominated by acoustic phonon interaction is expected to be proportional to T 3/2, while the mobility due to piezoelectric acoustic phonons scattering only is expected to be proportional to T 1/2. However, impurity scattering by dislocations becomes less significant at higher temperatures and its mobility increases with increasing temperature. As shown in Fig. 7, the mobility decreased as a function of T 3/2 at the high temperature range (T 4180 K). This indicates that scattering at the high temperature range is mainly due to the acoustic phonon mode of lattice vibrations through a deformation potential [22]. Thus, at the intermediate temperature range (100 oTo 180 K), the mobility gradually decreased as a function of T 1/2 with increasing temperature. Such a behavior is attributed to the piezoelectric potential scattering [23]. Therefore, at the high temperatures over 100 K, the mobility was scattered by the piezoelectric and deformation acoustic phonons. While, at the low temperature range (T o100 K), the mobility increased as a function of T1 with increasing temperature. It associates that scattering can be analyzed by the dislocation scattering model [24]. It is estimated that the scattering due to the change of dislocation in layers is dominant up to 100 K. Optical band gap of the BaAl2S4/GaAs layer was extracted through PC and absorption measurements. Generally, for light with a variable photon energy (Ep) near the band gap energy (Eg), light with lower-energy photons (Ep oEg) is transmitted without being absorbed and light with higher-energy photons (Ep 4Eg) is absorbed. The strongly-absorbed excitation creates a high density of free electron-hole pairs (EHPs) near the surface. Ultimately, EHP generation is used to designate the valence-band electron as part of a covalent bond [25]. Therefore, when the covalent bond is broken, the electron is free to move in the crystal lattice of the layer. The required energy to break a covalent bond is Eg, If the photon energy is Ep oEg, it is not sufficient to break a covalent bond and free an electron for conduction. At this time, electrons do not occupy the forbidden states between the valence and the conduction bands, so a photon with Ep oEg is not absorbed and passes through the layer. Therefore, for a typical photosensitive layer, the lifetime for surface excitation for Ep 4Eg is much shorter
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extended to longer wavelengths and it will be observed as a rough spectrum. At this time, the defect states include vacancies, interstitial atoms, or dislocations, etc. As shown in Fig. 8, the PC curve located to the long wavelength direction is a smooth gradient. Consequently, it means that the grown layers are high quality crystal. On the other hand, the optical band gap of the BaAl2S4/GaAs layer can be extracted from the traditional method of the optical absorption coefficient measurement. When photons are incident on a material, the intensity, I, transited photon can be expressed by the Lambert-Beer-Bouguer law [29]:
Ι = Ι0 exp ( −αd),
(2)
where I0 and I are the intensities of photon entering and leaving the layer, respectively. Also, α and d are optical absorption coefficient and thickness of the layer, respectively. Under the absorption conditions, I is exponentially reduced while traveling through the layer and α is strongly depended to the incident photon energy (hν). And Eq. (2) is rewritten as:
α = − (1/d) 1n (I /I0 )
(3)
Therefore, from the optical absorption measurement, the relation between α and hν was extracted from the optical absorption spectra. The absorption coefficient as a function of photon energy in the allowed direct transition can be expressed as [30]: Fig. 8. Characteristic PC spectra of the BaAl2S4/GaAs layer measured with the temperature variation. Here, the dotted line represents the peak A of the PC peaks obtained at each the temperature.
than the lifetime for volume excitation for Ep o Eg. Consequently, the signal obtained from the PC measurement is related to the absorption of photons, leading to the generation of free charge particles in the conduction band and/or in the valence band. For the PC measurement, when the photon energy is Ep 4 Eg, the absorbed photons create electron and hole carriers. And then, if an external electric field is applied, the electrons and holes move in opposite directions. These carriers instantly flow out through the electrodes and produce PC signals. At this time, a maximum peak in the PC spectrum occurs when a transition is made from surface excitation to volume excitation with increasing wavelength [26]. The PC peak gives rise to one of the following mechanisms: 1) Band-to-band transitions, 2) Impurity-level-to-bandedge transitions, 3) Ionization of donors, and 4) Deep levels in band gap. Fig. 8 presents the PC spectra of the BaAl2S4/GaAs layer measured with the temperature variation. As shown in Fig. 8, three PC peaks appeared at specific temperatures from 10 to 293 K and were shifted toward the short-wavelength region with decreasing PC measurement temperature. Three peaks observed at 293 K were located to 311.5 (3.9803), 308.0 (4.0255), and 307.8 nm (4.0281 eV), respectively. Those observed at 10 K were 305.4 (4.0598), 302.0 (4.1055), and 301.7 nm (4.1096 eV). The three peaks were labeled as peaks A, B, and C to indicate the transitions from the three valence-band levels in order of increasing energy. Thus, the PC peaks are known to be due to the combined influences of the spin-orbit interaction and the noncubic crystalline field, and they are related to each direct band gap [27]. These PC peaks are attributed to the transition of electrons from the valence band to the conduction band. This suggests that the three PC peaks are caused by band-to-band transitions following mechanism 1) [26,28]. Moreover, the PC slope toward the longer wavelength region of the spectrum is related to the crystal quality. If there exists the optically active traps, then it takes along time to attain a steady-state value after vanishing the monochromatic light. As a result, PC response will be observed to a rough spectrum. Furthermore, the spectral PC response caused by the defect states is
(αhv)2 = A (hv − Eg ),
(4)
where, h is the Planck constant and A is a constant because it do not involve phonons due to the lattice vibrations in electron bandto-band transitions. According to Eqs. (4), (αhν)2 linearly depends on the photon energy. Fig. 9 displays the plots of (αhν)2 versus photon energy for different temperatures. As shown in Fig. 9, the optical band gap was obtained by extrapolating the linear portions of the respective curves to zero [31]. Generally, the magnitude of the band gap for any material varies with temperature and pressure. Therefore, the band gap as a function of temperature and pressure can be expressed as [28]
ΔEG = (∂EG/∂P )T ΔP + (∂EG/∂T )P ΔT
(5)
When A¼ (∂EG/∂T)P and B ¼(∂EG/∂P)T, the band gap at constant pressure is EG ¼ EG(0) þAT, where EG(0) is the band gap at 0 K. For most photoconductor materials, the value of A is a negative quantity. Therefore, the band gap as a function of temperature can be well described by following Varshni's empirical equation [32,33]:
Fig. 9. Plots of (αhν)2 versus photon energy for different temperatures.
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behavior was attributed to the dislocation scattering. At the higher temperature than 100 K, the scattering of mobility was decreased to two different reasons. One had a function of T 3/2 at the temperature of T4 180 K and the other one showed a function of T 1/2 at the temperature range of 100 oT o180 K. The former was mainly caused by the scattering of the acoustic phonon mode of lattice vibrations through a deformation potential and the latter was owing to the piezoelectric potential scattering. From the PC measurement, three PC peaks labeled as peaks A, B, and C were observed. The three PC-peaks were caused by band-to-band transitions due to the transition of electrons from the three valence band states in order of increasing energy, respectively, to the conduction band states, respectively. Thus, these PC-peak energies shifted toward the short wavelength region with decreasing temperature. Through the analysis of absorption and PC results, the band gap variation has been compared and matched well by using Eg(T)¼ Eg(0) 7.556 10 4T2/(T þ523), where Eg(0) is estimated to be 4.0596, 4.1053, and 4.1094 eV for peaks A, B and C, respectively. Fig. 10. Energy variation of the optical absorption and PC-peak energy as a function of temperature.
Eg (T ) = Eg (0) − aT 2/(b + T ),
(6)
where a and b are constants. Also, Eg(0) is the band gap energy at 0 K. Fig. 10 shows the energy variation as a function of the temperature of the BaAl2S4/GaAs layer, which was extracted from PC and absorption measurements. The PC energies of peak A corresponding to the band-to-band transition were well consistent with the absorption energies contrasted with the same temperature. In fact, the absorption experiment has been known to be inaccurate for obtaining the band gap energy because of the difficulty in defining the position of the absorption edge. This discrepancy comes from the fact that the band gap energy is obtained by fitting the experimental spectra to theoretical models. Contrary to this process, the band gap energy through the PC method can be instantly found by measuring the position of the PC peak. Consequently, the PC spectroscopy is a useful method for determining the band gap of the BaAl2S4/GaAs layer. As shown in Fig. 10, these energies increased with decreasing temperature and had a nonlinear relationship. When a, b, and Eg(0) are taken to be 7.556 10 4 eV/K, 523 K, and 4.0596 eV for peak A, respectively. The curves plotted by Eq. (6) closely fit the experimental values. Thus, by fitting Eq. (6), Eg(0) values for peaks B and C are extracted to be 4.1053 and 4.1094 eV, respectively.
4. Conclusions The epitaxial BaAl2S4 layers were first grown on GaAs substrate through the HWE method. At reservoir temperature of 110 °C maintaining vapor pressure of 1/3Pmin, the optimum growth temperature of the substrate and the source extracted out to be 420 and 630 °C, respectively. Under these optimum substrate temperature, the thickness and growth rate were obtained to be 2.4 μm and 1.21 Å/s, respectively. Thus, it was confirmed that the grown BaAl2S4 layer, which formed the cubic structure in space group Pa3, was epitaxially accumulated along the (312) direction onto the GaAs (100) substrate and its FWHM on DCXD was 238 arcsec. According to the CSL model, the coincidence lattice mismatch of layer was 2.52% with the integer pair of (7, 3) and the layer was influenced by the compressive strain. By the Hall effect measurement, the mobility behavior depending on temperature was found. During increasing temperature to 100 K, the mobility increased as a function of T1 and such a
Acknowledgments This study was supported by research funds from Chosun University, 2015.
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