- Email: [email protected]
Characterization and device applications of p-type ZnO films prepared by thermal oxidation of sputter-deposited zinc oxynitride
Journal of Alloys and Compounds 695 (2017) 124e132
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Characterization and device applications of p-type ZnO films prepared by thermal oxidation of sputter-deposited zinc oxynitride A.E. Rakhshani* Physics Department, Faculty of Science, Kuwait University, PO Box 5969, Safat, 13060, Kuwait
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2016 Received in revised form 24 September 2016 Accepted 19 October 2016 Available online 19 October 2016
Formation of nitrogen-doped p-type ZnO (p-ZnO) and related homojunction devices has been reported extensively using a variety of techniques with the exception of thermal oxidation of ZnON that is a simple and less explored method. We converted sputter-deposited ZnON films to p-ZnO at relatively low oxidation temperatures (400e500 C) as compared with other reports. Chemical bonding states of nitrogen in p-ZnO films were examined by XPS analysis and the mechanism of p-type doping was discussed. Films were characterized by XRD, SEM, photocurrent spectroscopy, and photoluminescence (PL) measurement. Films were composed from nano-size crystallites with (100) preferential orientation. A shallow acceptor level with a 70e90 meV binding energy and a 1.72-eV deep level were measured and identified. PL thermal quenching of the films had activation energy of 34 meV. Schottky barrier (SB) and homojunction diodes, with two different types of n-ZnO, were fabricated on films and their parameters were compared with the reported data. The SB and both types of homojunction diodes had high rectification factors (104-105) and good device parameters. The conductance of homojunction diode based on sputter-deposited n-ZnO showed thermal activation energies of 0.08e0.12 eV, 0.16e0.20 eV, and 0.40 e0.50 eV. The homojunction diode based on n-ZnO nano-rods exhibited strong electroluminescence in the visible region from which two defect-related transition energies of 2.21 eV and 2.66 eV were obtained. In conclusion, good quality p-ZnO films and related devices can be prepared by thermal oxidation of ZnON films. © 2016 Elsevier B.V. All rights reserved.
Keywords: p-type ZnO Sputtering Thermal oxidation Homojunction diode Schottky diode Electroluminescence Photoluminescence
1. Introduction Zinc oxide (ZnO) is a transparent II-VI compound semiconductor with the wide band gap of 3.37 eV (300 K) and many interesting characteristics suitable for a wide range of electronic and optoelectronic applications. The fundamental properties of the intrinsically n-type ZnO (n-ZnO) [1] and p-type ZnO (p-ZnO) [2] are reviewed. Its high exciton binding energy (60 meV) makes the material a good candidate for the fabrication of ultraviolet (UV) and visible (defect-related emission) light-emitting diodes (LED) operating at room temperature [3,4]. Almost all of these devices have a heterojunction structure, where the counterpart of the easily prepared n-ZnO is another semiconductor (or polymer) with the ptype conductivity. For the fabrication of homojunction diodes, the preparation of stable p-ZnO is required. In principal, doping with the group V and group I elements in the periodic table should yield
* Tel.: þ965 99774732; fax: þ965 24819374. E-mail addresses: [email protected], [email protected]. http://dx.doi.org/10.1016/j.jallcom.2016.10.187 0925-8388/© 2016 Elsevier B.V. All rights reserved.
p-ZnO. In practice, this is a challenging task due to the self compensation effect introduced by the native donor-type defects including zinc interstitial (Zni) and oxygen vacancy (VO). Despite the existing difficulties in producing low resistivity and stable pZnO, homojunction light-emitting [2,5,6] and random laser [7] diodes have been realized using different p-type impurities including Li [5], Na [6], and N [7] for the preparation of p-ZnO films. Atomic nitrogen (N) is an effective p-type dopant in ZnO, likely due to its size matching with that for oxygen. Atomic nitrogen substituting oxygen, (N)O, acts as an acceptor whereas (N2)O behaves as a double donor. Nitrogen-doped p-ZnO films have been prepared using a variety of techniques [2]. Among these, thermal oxidation of sputter-deposited zinc nitride [8e12] and zinc oxynitride (ZnON) [13,14] films and sputtering of ZnO in nitrogen/argon plasma [15,16] are the simple routes. The concentration and mobility of holes in these films are reported to vary in the wide ranges of ~1015e1018 cm3 and ~0.1e100 cm2/V-s, respectively [2,8e16]. Also, the acceptor binding energy of nitrogen in doped films is measured by photoluminescence (PL) technique as 126 meV [17], 135 meV
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
[18], and 180 meV [19]. These wide variations in the electronic properties of nitrogen-doped films signify that still there is much to learn about nitrogen doping of ZnO, the films optoelectronic properties and their device applications. The aim of this study was to further develop the properties and device application of p-ZnO films by the simple route of oxidation of ZnON films. In this work we prepared sputter deposited ZnON precursor films that were converted to p-ZnO by thermal oxidation at a lower temperature as compared to 500e800 C used in similar works [13,14]. We report the doping mechanism, structural properties, photoelectrical characteristics and PL properties of these p-ZnO films. The results are discussed and compared with the published data on oxidized sputter-deposited ZnON and Zn3N2 films. The sputter-deposited Zn3N2 films generally contain considerable amount of oxygen and indeed can be regarded as ZnON. This is mainly due to the presence of residual oxygen in sputtering chambers and the high reactivity of oxygen with Zn [14]. We also discuss the preparation and characterization of Schottky barrier (SB) and homojunction diodes fabricated on p-ZnO films. These devices, as discussed in section 3, showed better characteristics than the reported devices prepared by oxidation of ZnON and Zn3N2 films. 2. Experimental Thin films of ZnON (250e1200 nm) were rf sputter-deposited (13.6 MHz, Torr International Inc magnetron sputtering model CRC-300) on glass and on stainless steel (SS) substrates. The substrates were cleaned ultrasonically in ethanol for 10 min. We used a 99.99% pure ZnO (or Zn) target with a diameter of 51 mm. The sputtering gas consisted of a mixture of N2, Ar and the residual O2 in the sputtering chamber. During deposition, the working pressure in the chamber was kept at 2.6 Pa and the flow rates for Ar and N2 (both, 99.99% pure) were maintained at 8 and 32 cm3/min, respectively (80% nitrogen in the gas mixture) using two independent mass flow controllers. The chamber was initially evacuated to 5.3 103 Pa and then the mixed gas was introduced. The rf power was kept at 40 W, unless otherwise stated. The substrate holder rotating at 3.0 rev/min was placed 7 cm far and parallel to the target. The temperature of substrates (not heated intentionally) reached to 30e50 C during the deposition. The target was sputter cleaned for 10 min before the opening of shutter to begin the deposition process. The films thickness and the rate of deposition (0.12 nm/s) were controlled by a quartz-crystal thickness monitor. For the conversion of the precursor ZnON films to p-ZnO, thermal oxidation was performed in a rapid thermal processor (Annealsys model AS-Micro) at 400e500 C for 5e120 min in dry air or in oxygen (atmospheric pressure). The converted p-ZnO films were characterized by scanning electron microscopy, SEM, (Joel, JSM-6300), X-ray photoelectron spectroscopy, XPS, (VG Scientific 200) and X-ray diffraction, XRD, (Siemens D500) using the CuKa line (0.15406 nm). The films' lateral photocurrent was measured at a suitable potential difference across two parallel conducting strips (1e2 mm separation) deposited on the film surface. The photocurrent normalized by the incident photon flux was measured at different incident wavelengths. The measurements were performed using a setup consisting of a grating monochromator (Sciencetech 9050), lock-in amplifier (Stanford Research SR 530), current amplifier (Keithley 428), mechanical chopper and light source (tungsten-halogen). PL measurements were performed in the temperature range of 80e300 K using the 355-nm line of a Nd:YAG laser, an optical cryostat (Oxford DN 1704) and a UV-VIS spectrometer (Ocean Optics HR2000þES). Heterojunction and SB diodes were fabricated (see the text) and characterized by the means of current-voltage (IV) and capacitance-voltage (CV) measurements using a source-measure unit (Keithley 236) and a CV
125
analyzer (Kiethley 590), respectively. 3. Results and discussion 3.1. Conversion of ZnON to p-ZnO films Fig. 1(a) shows the depth profiles of zinc, oxygen, nitrogen, and carbon (contaminant) in an as-grown film. Fig. 1(b) depicts the same plots after the film was oxidized at 400 C in air for 5 min. The Zn/(ZnþO) and N/(ZnþO) atomic ratios in the as-grown film were 0.60 and 0.045, respectively at a selected depth of 60 nm. In the oxidized film and at the same depth, these ratios changed to 0.54 and 0.004, respectively. This indicates the escape of nitrogen and to a much lesser extent zinc from the film. This is in agreement with the other reports that the thermal oxidation induces the out diffusion of nitrogen from the (N2)O and (N)O defects to form nitrogen bubles in the film [8,20]. Fig. 1(b) also shows that nitrogen is not present within 50 nm below the surface of the oxidized film, as a result of its out diffusion. The XPS N 1s plots for the as-grown and oxidized films are shown in Fig. 2. The plot of the as-grown film is composed from three peaks with the binding energies of 395.8 eV, 397.5 eV, and 403.5 eV. The latter two peaks also appear in the spectrum of the oxidized film, but with a slight shift, at 397.9 eV and 403.9 eV. The 395.8 eV peak is attributed to the NeZn bond in e.g. Zn3N2 [21]. The peak appears at 397.5 eV is attributed to the NeZn bond when N occupies the oxygen vacancy (VO) site to form the (N)O defect [14,22]. The peak at 403.5 eV is assigned to the (N2)O defect where N2 occupies the VO site [14,22]. The (N)O defect is believed to be a shallow acceptor in ZnO with the ionization energy of 170e200 meV, whereas (N2)O behaves as a double donor [1]. Apparently, the balance between the densities of the (N)O acceptors and the donor-type defects like (N2)O,VO and Zni (interstitial zinc) determines the type of conductivity in the film. It is evident from Fig. 2 that the oxidation process reduces considerably the Zn3N2 content of the film, lowers the concentration of the (N2)O double donors and increases the concentration of the (N)O acceptors. This leads to the conversion of the as-grown film to p-ZnO. The conversion of conductivity from n-type to p-type was verified by both the hot-probe technique and the Hall-effect measurement. The concentration of free holes in p-ZnO films prepared by oxidation at 400 C was in the range of 1017 e 1018 cm3. The holes concentration was reduced to 1015 e 1016 cm3 when the oxidation temperature was raised to 500 C. This was found to be consistent with the literature reports [9,10]. 3.2. Structural characterization Fig. 3(a) and (b) show the surface morphology of oxidized ZON films grown on glass and on SS substrates, respectively. The grain structure of the film on SS substrate is more pronounced, as expected, due to the polycrystalline nature of the substrate. The asgrown films had a poor crystalline structure. Their XRD patterns showed weak and broad diffraction lines as shown in Fig. 3(c), plot (i), for a 1200-nm film. As noticeable, the (002) diffraction line is weaker than the (100) line. This is in contrast to the un-doped ZnO films which normally show strong (002) preferential orientation when are deposited on amorphous or polycrystalline substrates. The incorporation of nitrogen in the film is associated with the deformation of ZnO lattice and, subsequently, the shift of the diffraction lines and the suppression of c-axis preferential orientation. Plot (ii) in Fig. 3(c) shows the XRD pattern of the same film which was oxidized at 500 C in air for 60 min. The oxidation process leads to the escape of nitrogen molecules from the lattice and hence improves the film crystallinity that is also associated with the increase of the crystallites size. The (100) preferential
126
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
Fig. 1. XPS-determined depth profile of elements in (a) as-grown and (b) oxidized films.
that for the oxidized reference film (n-ZnO). This can be attributed to the effect of nitrogen atoms occupying the oxygen sites in p-ZnO, knowing that the length of the ZneN bond is somewhat smaller than the length of the ZneO bond [23].
3.3. Photoelectrical characterization
Fig. 2. The XPS N 1s spectra for (a) the as-grown and (b) the annealed film at a depth of 100 nm below the surface.
orientation is also retained in the oxidized film. This is apparently a distinguished characteristic of nitrogen-doped ZnO films that is also reported by others [8e10]. For comparison, Fig. 3(d) shows the XRD pattern of an as-deposited ZnO film that exhibits a strong (002) orientation. This film was sputter deposited on glass under the same conditions as for ZnON films, but with the exception that nitrogen in the sputtering gas was replaced by oxygen. The Bragg equation (2dsinq ¼ l, where l ¼ 0.15406 nm and d is the spacing of the diffraction planes) was used to evaluate the lattice constants c and a of the films from their XRD diffraction angles, q. The Sherrer equation (L ¼ 0.94l/Bcosq, where B is the full width at half maximum of the line at the diffraction angle 2q) was used to evaluate the size of crystallites, L, in the film. The lattice constants of the as-grown and oxidized ZnO reference film were also determined from their XRD patterns. The unit cell volumes of the asdeposited and oxidized films were determined from their lattice constants and the results are listed in Table 1. The tabulated data shows the effect of oxidation heat treatment on the growth of crystallites for both samples. The oxidation heat treatment reduces the unit cell volume of the reference ZnO sample by 2.5% which is likely due to the effect of strain relaxation. The unit cell volume of the ZnON film is reduced by 6.0% after the oxidation process. This is almost twice of that for the reference sample. The escape of nitrogen molecules from the interstitial sites in ZnON, in addition to the strain relaxation, is accountable for this change. The unit cell volume in the oxidized ZnON film (p-ZnO) is even 2% smaller than
Fig. 4(a) depicts the dependence of photocurrent normalized to the incident photon flux, R, on the incident photon energy, E, for a p-ZnO and a nitrogen-free n-ZnO reference film. The p-ZnO film was formed by oxidation of a 250-nm ZON film at 400 C for 60 min. The reference film was sputter deposited exactly as for the ZnON precursor film except for the nitrogen in the sputtering gas that was replaced by oxygen. For the reference film the spectral response of R is entirely in the UV range as expected. As E approaches towards the band gap energy the optical absorption coefficient and, hence, the R value are both increased. With the increase of absorption coefficient, the photo-excited electron-hole pairs (EHP) are generated closer to the film surface. This enhances the recombination rate of EHPs due to the effect of surface states and, consequently, R starts to decrease as E exceeds ~3.4 eV. In contrast to the n-ZnO film, R shows a pronounced sub-band gap tail in the p-ZnO film. This implies that the photoconductivity response of the p-ZnO film in the visible range is due to the presence of nitrogen-related band gap defects. In the vicinity of a transition energy where the optical absorption coefficient, a, is high, R varies in proportion to a. Therefore, the direct transition energies, Ed, and the indirect transition energies, Ei, can be evaluated from the plots of (RE)2 and (RE)1/2 against E, respectively. Fig. 4(b) and its inset illustrate these plots which were reconstructed from the spectral response of R for the p-ZnO film in Fig. 4(a). The horizontal intercepts of the line fits to these plots yield Ed ¼ 3.18 eV and Ei ¼ 1.72 eV. A similar procedure gives only one direct transition energy Ed ¼ 3.25 eV for the n-ZnO film. The direct transition energy in the p-ZnO film is 70 meV smaller than the band gap energy of n-ZnO. This may imply that Ed ¼ 3.18 eV corresponds to a transition to the conduction band (CB) of p-ZnO from a shallow acceptor level located 70 meV above the valence band (VB) edge. This is in the vicinity of the 87-meV (0 K) activation energy of an acceptor level measured from the temperature dependence of the film PL, as discussed below. The indirect transition energy of Ei ¼ 1.72 eV in p-ZnO is assigned to the activation energy of a deep defect level. Since this transition was not detected in the n-ZnO film, it is attributed to the effect of nitrogen incorporation in ZnO. A PL peak energy of 1.70 eV is reported in nitrogen doped ZnO films and is assigned to the oxygen vacancy defects which are induced due to the incorporation of nitrogen [24].
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
127
Fig. 3. SEM surface views of the oxidized films grown on (a) glass and (b) stainless steel substrate. (c) XRD plots for (i) as-grown and (ii) oxidized film grown on glass. (d) XRD plot of a nitrogen-free n-ZnO film sputter-deposited on glass. In addition to the ZnO (002) diffraction line, sample holder (Pt/Rh) lines are also shown.
Table 1 The volume of unit cell, V, and the size of crystallites, L, in ZnON and in ZnO reference films before and after the oxidation process. Sample
ZnON ZnON (oxidized) ZnO ZnO (oxidized)
V
L
(nm)3
nm
0.0369 0.0348 0.0364 0.0355
8 21 15 30
3.4. Photoluminescence characterization The near band edge PL spectrum of a p-ZnO film measured at different temperatures (82e293 K) and the temperature dependence of the PL peak energy, E(T), and intensity, I, are shown in Fig. 5(a). For comparison, similar plots for the nitrogen-free n-ZnO reference film are also depicted in Fig. 5 (b). The p-ZnO film with a thickness of 103 nm was prepared by oxidation of ZnON at 400 C for 60 min. The resolved excitonic features of PL could not be observed even at 82 K. Nevertheless, the PL peak energy was in the range of 3.320e3.330 eV for both films at 82 K. This is very close to the 3.327 eV peak energy of the resolved free-electron to acceptor (FA) transition of nitrogen-doped ZnO at 83 K [25]. The variation of the PL peak energy with temperature can be well described by the Varshni equation E(T) ¼ E(0) e AT2/(TþB) [26] where E(0) denotes the transition energy at 0 K, A is a constant related to the excitationaverage phonon interaction and B is related to the Debye temperature [27]. The best fit of the Varshni equation to the peak energy data points in Fig. 5(a) is shown by a solid line corresponding to the fitting parameters A ¼ 0.738 meV/K and B ¼ 280 K which yields E (0) ¼ 3.350 eV. Taking E(0) ¼ 3.350 as the free-electron to acceptor transition at 0 K and taking the ZnO band gap at 0 K as
Fig. 4. The normalized photocurrent, R, against the incident photon energy, E, for (a) p-ZnO and n-ZnO films. (b) The variation of (R.E)2 and (R.E)1/2 (inset) with E for the pZnO film.
128
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
Eg(0) ¼ 3.437 eV [25,27,28], the acceptor binding energy was obtained from EA ¼ Eg(0) eE(0) þ kT/2 [25,27], as EA ¼ 87 meV (kT/2 has a negligible value at low temperatures). The result obtained (EA ¼ 87 meV) is favourably smaller than 140 meV [27] and 110 meV [25] reported for the nitrogen-doped ZnO films. The same analysis applied to the experimental results of the n-ZnO reference film, Fig. 5(b), yields A ¼ 0.738 meV/K, B ¼ 330 K, E(0) ¼ 3.324 eV, and EA ¼ 113 meV. The measured acceptor binding energy agrees well with the reported value of 113 meV for the undoped ZnO films [25]. The nature of this acceptor level in the undoped films is not clear. Fig. 5 illustrates the variation of PL intensity with temperature for the p-ZnO and n-ZnO films. The observed temperature quenching behaviour can be well described by equation I(T) ¼ I(0)/[1 þ Cexp (b/kT)] [29], where I(0) is the emission intensity at 0 K, C is a parameter and b is the activation energy in the thermal quenching process. For both films, the good fit of this equation to the data points (solid lines) yields b ¼ 34 meV which is within the range of 27e47 meV reported in the literature [27].
reduced the barrier height at the gold-film interface and formed a quasi ohmic contact. Gold is known to make ohmic contact to pZnO [2,16] since it acts as a p-type impurity [30]. The formation of ohmic contact at Au/p-ZnO interface resulted in a diode-type IV plot (Fig. 6) in which the forward current corresponds to the positive polarity of ohmic contact (Au) as it is expected for SB diodes formed on p-type semiconductors. The IV plot in Fig. 6 shows a high rectification factor (ratio of forward to reverse current) of r ¼ 1.1 104 (±1 V). The general form of the IV equation for SB diodes dominated by the thermionic emission of majority carriers over an effective barrier height F is I ¼ I0 [exp (qVj/nkT) e 1] where I0 ¼ AA*T2exp (F/kT) [31]. Here, q is the electronic charge, n is the
3.5. Schottky-barrier diodes To the best of our knowledge no study has been reported in the literature on SB devices fabricated on nitrogen-doped p-ZnO films. The IV plot of a diode we prepared on a p-ZnO film (thickness ~800 nm) is shown in Fig. 6. The film was formed by the oxidation of a ZnON film sputter-deposited on SS foil. The oxidation was performed at 500 C in air for 60 min. After the oxidation process, several 30-nm thin gold contacts (diameter, 2 mm) were thermally evaporated on the p-ZnO film. The IV plots of these devices initially showed symmetrical and nonlinear characteristics for both bias polarities (characteristic of two back-to-back diodes), implying that both gold and SS made blocking contacts to p-ZnO. A brief heat treatment of the completed device at 400 C (5e10 min) effectively
Fig. 6. Current-voltage characteristic of a Schottky diode based on a p-ZnO film deposited on flexible stainless steel substrate.
Fig. 5. Temperature dependence of the PL spectrum, its peak energy and intensity for (a) the p-ZnO film and (b) the undoped n-ZnO film.
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
129
ideality factor, k is Boltzmann's constant, T is the absolute temperature, Vj is the voltage drop across the junction (Vj ¼ V - IRs), and Rs is the diode series resistance; A (¼ 0.03 cm2) is the device area, and A* is the effective Richardson constant (A* ¼ 72 Acm2K2 using the hole effective mass m*p ¼ 0.6 m [28]). From the fit of the IV plot in Fig. 6 to the SB IV equation, the diode parameters were determined as n ¼ 2.0, I0 ¼ 0.6 nA, and Rs ¼ 330 U. From the I0 value, the barrier height at the SS-film interface was obtained as F ¼ 0.865 eV. The device capacitance measured at a test frequency of 100 kHz was C ¼ 260 pF and independent from the applied reverse biases, implying that the width of the depletion layer is comparable with the film thickness. From the measured capacitance, the depletion width was evaluated from C ¼ ε0εrA/W as W ¼ 760 nm, where ε0 is the free space permittivity, εr ¼ 7.5 was taken as the relative permittivity of ZnO, and A ¼ 0.03 cm2 was the device active area. The depletion width is related to the junction built-in potential, V0, and the effective density of acceptors, Na, through W ¼ [(2ε0εr V0)/(q Na)]1/2. Taking W ¼ 760 nm, one obtains Na ¼ 1.44 1015V0, where Na is in cm3 units. For an ideal SB formed on a p-type material, the position of the Fermi level from the valence band edge is Ef e Ev ¼ F e V0 and thus the density of holes can be expressed as p0 ¼ NVexp [(F e qV0)/kT]. Taking Na (¼ 1.44 1015 V0) ~ p0 and the effective density of states in the valence band NV ¼ 1.16 1019 cm3 (for m*p/m ¼ 0.6), one obtains V0 ¼ 8.06 103 exp [(F e qV0)/kT]. Substituting for F ¼ 0.865 eV, this equation was solved numerically to yield V0 ¼ 0.62 V, from which Na (~p0) ¼ 0.9 1015 cm3 was obtained. The result is in consistence with the p0 values determined by the Hall-effect measurement. 3.6. Homojunction diodes Homojunction diodes were fabricated from the sputter deposited p-ZnO and n-ZnO films with a configuration shown schematically in the inset of Fig. 7(a). The n-ZnO film with a thickness of 600e1000 nm was sputter deposited on a pre-cleaned SS substrate at room temperature. A ZnON film (thickness, 100 nm) was deposited on top of the n-ZnO film under the same conditions as mentioned previously. The stack layers were annealed in air at 400 C for the conversion of the top layer to p-ZnO. Finally, circular gold contacts (30 nm, 2 mm diameter) were thermally evaporated on the p-ZnO film. This was followed by a second annealing at 400 C for the formation of ohmic contact at the Au/p-ZnO interface. The forward current of the device corresponded to the positive polarity of gold. This verified the conversion of the top layer to pZnO and the formation of a junction at the interface of the two films. The junction at the SS/n-ZnO interface is apparently ohmic, otherwise the IV plot could not show a high rectification characteristic as observed. The device exhibited an excellent rectification factor of r ¼ 5 104 (±2.5 V), much greater than ~6 (±15 V) [6], ~10 (±7.5 V) [16], and 50 (±20 V) [7] reported in the literature. Snigurenko et al. could raise the diode rectification factor from 100 (±2 V) to 4 104 only by inserting an ultrathin Al2O3 layer between the p-ZnO and n-ZnO films to produce a p-i-n diode [32]. From the fit of the diode equation (empirically the same as that for SB), to the data points in Fig. 7(a), n ¼ 9.2 and I0 ¼ 4 nA were deduced. Apparently, the diode behaviour is far from being ideal (n ¼ 1). Despite this, the CV plot of the device shown in Fig. 7(b) follows the junction CV relationship expressed by C2 ¼ 2(Vo eV)/qε0εrA2N, where N ¼ NaNd/(Na þ Nd) [31]. Na and Nd are the uncompensated densities of the acceptors and donors in the p-ZnO and n-ZnO films, respectively. The horizontal intercept and the slope of the straight line fit to the data points in Fig. 7(b) measures V0 ¼ 0.50 V and N ¼ 4.2 1015 cm3. For the preparation of above device, the ZnON film was converted to p-ZnO by air annealing at 400 C which yields
Fig. 7. (a) The IV and (b) CV plots of a homojunction diode fabricated on flexible SS substrate. Both n-ZnO and p-ZnO films were prepared by sputtering. The device structure and bias polarity for the forward current are shown in the inset of Fig. 7(a).
Na ¼ 1.5 1017 cm3 [33]. Therefore from the measured value of N, the density of the uncompensated donors in the n-ZnO film was obtained as Nd ¼ 4.3 1015 cm3. It should be noted that the lower acceptor density of Na ¼ 0.9 1015 cm3 determined from the characteristics of the SB diode is due to the higher oxidation temperature of 500 C used for the conversion of ZnON to p-ZnO. This is consistent with several reports, including [9,10], that the oxidation of nitrogen-doped ZnO at 400e450 C yields a hole density in the range of 1017 cm3, one to two orders of magnitude greater than that in samples oxidized at 500 C. These heterojunction diodes showed weak electroluminescence (EL) under a forward bias. Only the emission integrated over all wavelengths in the UV-Vis range could be detected by a photomultiplier tube. The IV characteristics of a homojunction diode at some selected temperatures are shown in Fig. 8(a). The device rectification factor was reduced as the temperature was lowered. The variation of the device current with temperature at the constant biases of þ1.0 V and 1.0 V is also shown in Fig. 8. The variation of current with temperature follows the Arrhenious equation I ¼ I*exp (DE/kT) where DE is a weak bias-dependent activation energy, and I* is a constant. In the high temperature region, DE ¼ 0.4e0.5 eV was obtained at both biases for several devices. In the low temperature region, DE ¼ 0.08e0.12 eV was measured under the bias of þ1.0 V and DE ¼ 0.16e0.20 eV under the bias of 1.0 V. The interpretation of the measured activation energies is not straight forward. They may represent the grain boundary barrier heights as well as the binding energies of the impurity and defect levels. Nevertheless, it
130
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
(ZnO2 2 ) solution by successive dipping of the SS substrate into the zincate solution and the boiling water as described elsewhere [4]. Using a lower zincate concentration of 0.02 M, well-aligned n-ZnO nanorods, instead of a continuous film, could be grown. The film thickness and the dimensions of nanorods could be increased by increasing the number of dipping cycles. The structure of a typical device is schematically shown in the inset of Fig. 9(a). A 100-nm nZnO film was first deposited on SS to serve as a seed layer for the growth of the second n-ZnO nanorods layer (thickness ~1000 nm). On top of the nanorods layer, a 250-nm ZnON film was sputter deposited followed by its oxidation in pure oxygen at 400 C for 2 h and then gold contacts were deposited. The IV and CV characteristics of a typical device and its device parameters are shown in Fig. 9(a) and (b). This homojunction diode with device parameters superior to those reported in the literature [6,7,12,15,16,32], had a high current rectification factor (7.4 104 at ±1.5 V), a reasonably good ideality factor (1.7), a low reverse biased saturation current (1.0 nA), and a good junction built-in potential (0.75 V). The density of uncompensated donors in the solution-grown ZnO nanorods was obtained from the device CV characteristic as 7.5 1014 cm3. Due to a better crystalline structure of the solution-grown n-ZnO nanrods, these homojunction diodes exhibited better device parameters (current rectification, ideality factor, reverse biased saturation current, junction built-in potential) and much stronger EL as compared with the diodes formed from sputter-deposited n-ZnO films. To the best of our knowledge, homojunction diodes which are reported in the literature and are based on p-ZnO films formed by oxidation of Zn3N2 or ZnON have not exhibited EL property. Fig. 9(c) illustrates the images of a forward biased device in the light and dark backgrounds which were captured by an ordinary digital camera. The defect-related visible light emission through the exposed area of the gold contact is evident from Fig. 9(c). The device was carrying a forward current of 100 mA. Light emission could not be detected at reverse biases, as anticipated. Fig. 9(d) illustrates the variation of integrated EL intensity with the forward current. The inset of this figure shows a strong light emitted through a hole on the gold contact at a forward current of 80 mA. The spectrum of emission depicted in Fig. 9(e) covers the visible and a narrow part of the UV range (375e400 nm). This spectrum is composed from two broad peaks centered at ~466 nm (2.66 eV) and ~560 nm (2.21 eV), corresponding to the radiative transition of electrons through two band gap defect levels. The dominant role of these two defect levels prevented the band-to-band UV emission in the wavelength range of 365e380 nm. 4. Summary and conclusions
Fig. 8. (a) The IV plots of a homojunction diode at different temperatures. The variation of device current with temperature at a bias of (b) þ1.0 V, and (c) 1.0 V are also shown.
was noted that DE ¼ 0.4e0.5 eV is in the close vicinity of the hole binding energy of 0.46 eV in ZnO [34] and the 0.40-eV binding energy of acceptor-type NO cluster defects [35]. The measured value of DE ¼ 0.16e0.20 eV is also in good agreement with the 0.17e0.20 eV binding energy of the NO acceptor level [2,19]. Homojunction diodes were also prepared by replacing the sputter-deposited n-ZnO layer with the solution-grown n-ZnO film and n-ZnO nanorods. The p-ZnO film was prepared by sputtering as described previously. The n-ZnO film was grown from a 0.1 zincate
Thermal oxidation of sputter-deposited ZnON films was utilized to prepare p-ZnO films. The mechanism of nitrogen doping was described based on the results of XPS measurement. The p-ZnO films were formed from 21-nm large crystallites that had a preferential orientation of (100) parallel to the substrate as a result of nitrogen doping. The unit cell volume of the crystallites in p-ZnO was 2% smaller than that in undoped n-ZnO films due to the incorporation of nitrogen atoms in oxygen vacancy sites. Photocurrent and PL characterization of p-ZnO films revealed the presence of a shallow acceptor level with the binding energy of 70e90 meV and a nitrogen-related deep level located 1.72 eV above the valence band edge. The PL thermal quenching in these films had activation energy of 34 meV that is within the range of the reported values. SB diodes and light-emitting homojunction diodes with excellent device parameters were prepared on p-ZnO films. The SB formed at the SS/p-ZnO interface had a barrier height of 0.865 eV, built-in potential of 0.62 V, and ideality factor of 2.0. The diode had a good rectification factor of 1.1 104 (±1 V). Temperature
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
131
Fig. 9. (a) IV and (b) CV characteristics of a homojunction diode prepared from sputter-deposited p-ZnO and solution-grown n-ZnO; (c) visible light emission through a gold contact; (d) integrated EL intensity against the diode forward current; (e) the spectrum of light emitted through the gold contact.
dependence of the diode conductance revealed several defectrelated activation energies. Homojunction diodes fabricated from p-ZnO and sputter-deposited n-ZnO films showed a rectification factor of 5.0 104 (±2.5 V), ideality factor of 9.2, and built-in potential of 0.50 V. Homojunction diodes prepared from p-ZnO films and solution-grown n-ZnO nanorods showed better characteristics, including a rectification factor of 7.4 104 (±1.5 V), ideality factor of 1.7 and built-in potential of 0.75 V. Furthermore, these diodes exhibited strong EL in the visible region. As compared with the previous reports on p-ZnO films obtained by ZnON oxidation, films and devices prepared in this study have much improved characteristics. Acknowledgments The support of the Research Sector of Kuwait University under the research project SP03/14 is thankfully acknowledged. We also acknowledge the valuable technical support received from the general facility of the Faculty of Science (Projects GS 02/08 and GS03/01) and the general facility of the College of Engineering (Projects GE01/07 and GE01/08). References [1] A. Janotti, C.G. Van de Walle, Rep. Prog. Phys. 72 (2009) 126501.
[2] J.C. Fan, K.M. Sreekanth, Z. Xie, S.L. Chang, K.V. Rao, Prog. Mater. Sci. 58 (2013) 874. [3] Y.S. Choi, J.W. Kang, D.K. Hwang, S.J. Park, IEEE Trans. Electron Devices 57 (2010) 26. [4] A.E. Rakhshani, Appl. Surf. Sci. 311 (2014) 614. [5] W. Ko, S. Lee, N. Myoung, J. Hong, J. Mater. Chem. C 4 (2016) 142. [6] S.S. Lin, J.G. Lu, Z.Z. Ye, H.P. He, X.Q. Gu, L.X. Chen, J.Y. Huang, B.H. Zhao, Solid State Commun. 148 (2008) 25. [7] J. Huang, S. Chu, J. Kong, L. Zhang, C.M. Schwarz, G. Wang, L. Chernyak, Z. Chen, J. Liu, Adv. Opt. Mater 1 (2013) 179. [8] C.W. Zou, R.Q. Chen, W. Gao, Solid State Commun. 149 (2009) 2085. [9] C. Wang, Z. Ji, K. Liu, Y. Xiang, Z. Ye, J. Cryst. Growth 259 (2003) 279. [10] N.H. Erdogan, K. Kara, H. Ozdamar, R. Esen, H. Kavak, Appl. Surf. Sci. 271 (2013) 70. [11] Y.-P. Jin, B. Zhang, J.-Z. Wang, L.-Q. Shi, Chi. Phys. Lett. 33 (2016) 058101. [12] V. Kambilafka, A. Kostopoulos, M. Androulidaki, K. Tsagaraki, M. Modreanu, E. Aperathitis, Thin Solid Films 520 (2011) 1202. [13] I.V. Rogozin, A.N. Georgobiani, M.B. Kotlyyarevsky, V.I. Demin, L.S. Lepnev, Inorg. Mater 49 (2013) 568. [14] J.P. Zhang, L.D. Zhang, L.Q. Zhu, Y. Zhang, M. Liu, X.J. Wang, J. Appl Phys. 102 (2007) 114903. [15] S. Dhara, P.K. Giri, Thin Solid Films 520 (2012) 5000. [16] Z.W. Wang, Y. Yue, Y. Cao, Superlattice. Microst. 65 (2014) 7. [17] X.H. Wang, B. Yao, D.Z. Shen, Z.Z. Zhang, B.H. Li, Z.P. Wei, Y.M. Lu, D.X. Zhao, J.Y. Zhang, X.W. Fan, L.X. Guan, C.X. Cong, Solid State Commun. 141 (2007) 600. [18] S. Yamauchi, Y. Goto, T. Hariu, J. Cryst. Growth 260 (2004) 1. [19] J. Lu, Q. Liang, Y. Zhang, Z. Ye, S. Fujita, J. Phys. D. Appl. Phys. 40 (2007) 3177. [20] P. Fons, H. Tampo, A.V. Kolobov, M. Ohkubo, S. Niki, J. Tominaga, R. Carboni, F. Boscherini, S. Friedrich, Phys. Rev. Lett. 96 (2006) 045504. [21] F. Watanabe, H. Shirai, Y. Fujii, T. Hanajiri, Thin solid Films 520 (2011) 3729. [22] L. Li, C.X. Shan, B.H. Li, B. Yao, J.Y. Zhang, D.X. Zhao, Z.Z. Zhang, D.Z. Shen,
132
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
X.W. Fan, Y.M. Lu, J. Phys. D. Appl. Phys. 41 (2008) 245402. [23] C.H. Park, S.B. Zhang, S.H. Wei, S.B. Zhang, S.H. Wei, Phys. Rev. B 66 (2002) 073202. [24] F. Stavale, L. Pascua, N. Nilius, H.J. Freund, J. Phys. Chem. C 118 (2014) 13693. [25] Z.W. Wang, Y. Yue, Y. Cao, Vacuum 101 (2014) 313. [26] Y.P. Varshni, Physica 34 (1967) 149. [27] H.B. Ye, J.F. Kong, W.Z. Shen, J.L. Zhao, X.M. Li, J. Phys. D. Appl. Phys. 40 (2007) 5588. [28] K. Hummer, Phys. Stat. Sol. B 56 (1973) 249. [29] W.Z. Shen, S.C. Shen, J. Appl. Phys. 80 (1996) 5941.
[30] Y. Yan, M.M. Al-Jassim, S.H. Wei, Appl. Phys. Lett. 89 (2006) 181912. [31] D.A. Neamen, Semiconductor Physics and Devices: Basic Principles, McGraw Hill, New York, 2003. [32] D. Snigurenko, K. Kopalko, T.A. Krajewski, R. Jakiela, E. Guziewicz, Semicond. Sci. Technol. 30 (2015) 015001. [33] A.E. Rakhshani, A. Bumajdad, J. Kokaj, S. Thomas, Kuwait J. Sci. 41 (2014) 107. [34] S.T. Teklemichael, M.D. McCluskey, Nanotechnol. 22 (2011) 475703. [35] J. Furthmuller, F. Hachenberg, A. Schleife, D. Rogers, F. Hosseini Teherani, F. Bechstedt, Appl. Phys. Lett. 100 (2012) 022107.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Characterization and device applications of p-type ZnO films prepared by thermal oxidation of sputter-deposited zinc oxynitride A.E. Rakhshani* Physics Department, Faculty of Science, Kuwait University, PO Box 5969, Safat, 13060, Kuwait
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2016 Received in revised form 24 September 2016 Accepted 19 October 2016 Available online 19 October 2016
Formation of nitrogen-doped p-type ZnO (p-ZnO) and related homojunction devices has been reported extensively using a variety of techniques with the exception of thermal oxidation of ZnON that is a simple and less explored method. We converted sputter-deposited ZnON films to p-ZnO at relatively low oxidation temperatures (400e500 C) as compared with other reports. Chemical bonding states of nitrogen in p-ZnO films were examined by XPS analysis and the mechanism of p-type doping was discussed. Films were characterized by XRD, SEM, photocurrent spectroscopy, and photoluminescence (PL) measurement. Films were composed from nano-size crystallites with (100) preferential orientation. A shallow acceptor level with a 70e90 meV binding energy and a 1.72-eV deep level were measured and identified. PL thermal quenching of the films had activation energy of 34 meV. Schottky barrier (SB) and homojunction diodes, with two different types of n-ZnO, were fabricated on films and their parameters were compared with the reported data. The SB and both types of homojunction diodes had high rectification factors (104-105) and good device parameters. The conductance of homojunction diode based on sputter-deposited n-ZnO showed thermal activation energies of 0.08e0.12 eV, 0.16e0.20 eV, and 0.40 e0.50 eV. The homojunction diode based on n-ZnO nano-rods exhibited strong electroluminescence in the visible region from which two defect-related transition energies of 2.21 eV and 2.66 eV were obtained. In conclusion, good quality p-ZnO films and related devices can be prepared by thermal oxidation of ZnON films. © 2016 Elsevier B.V. All rights reserved.
Keywords: p-type ZnO Sputtering Thermal oxidation Homojunction diode Schottky diode Electroluminescence Photoluminescence
1. Introduction Zinc oxide (ZnO) is a transparent II-VI compound semiconductor with the wide band gap of 3.37 eV (300 K) and many interesting characteristics suitable for a wide range of electronic and optoelectronic applications. The fundamental properties of the intrinsically n-type ZnO (n-ZnO) [1] and p-type ZnO (p-ZnO) [2] are reviewed. Its high exciton binding energy (60 meV) makes the material a good candidate for the fabrication of ultraviolet (UV) and visible (defect-related emission) light-emitting diodes (LED) operating at room temperature [3,4]. Almost all of these devices have a heterojunction structure, where the counterpart of the easily prepared n-ZnO is another semiconductor (or polymer) with the ptype conductivity. For the fabrication of homojunction diodes, the preparation of stable p-ZnO is required. In principal, doping with the group V and group I elements in the periodic table should yield
* Tel.: þ965 99774732; fax: þ965 24819374. E-mail addresses: [email protected], [email protected]. http://dx.doi.org/10.1016/j.jallcom.2016.10.187 0925-8388/© 2016 Elsevier B.V. All rights reserved.
p-ZnO. In practice, this is a challenging task due to the self compensation effect introduced by the native donor-type defects including zinc interstitial (Zni) and oxygen vacancy (VO). Despite the existing difficulties in producing low resistivity and stable pZnO, homojunction light-emitting [2,5,6] and random laser [7] diodes have been realized using different p-type impurities including Li [5], Na [6], and N [7] for the preparation of p-ZnO films. Atomic nitrogen (N) is an effective p-type dopant in ZnO, likely due to its size matching with that for oxygen. Atomic nitrogen substituting oxygen, (N)O, acts as an acceptor whereas (N2)O behaves as a double donor. Nitrogen-doped p-ZnO films have been prepared using a variety of techniques [2]. Among these, thermal oxidation of sputter-deposited zinc nitride [8e12] and zinc oxynitride (ZnON) [13,14] films and sputtering of ZnO in nitrogen/argon plasma [15,16] are the simple routes. The concentration and mobility of holes in these films are reported to vary in the wide ranges of ~1015e1018 cm3 and ~0.1e100 cm2/V-s, respectively [2,8e16]. Also, the acceptor binding energy of nitrogen in doped films is measured by photoluminescence (PL) technique as 126 meV [17], 135 meV
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
[18], and 180 meV [19]. These wide variations in the electronic properties of nitrogen-doped films signify that still there is much to learn about nitrogen doping of ZnO, the films optoelectronic properties and their device applications. The aim of this study was to further develop the properties and device application of p-ZnO films by the simple route of oxidation of ZnON films. In this work we prepared sputter deposited ZnON precursor films that were converted to p-ZnO by thermal oxidation at a lower temperature as compared to 500e800 C used in similar works [13,14]. We report the doping mechanism, structural properties, photoelectrical characteristics and PL properties of these p-ZnO films. The results are discussed and compared with the published data on oxidized sputter-deposited ZnON and Zn3N2 films. The sputter-deposited Zn3N2 films generally contain considerable amount of oxygen and indeed can be regarded as ZnON. This is mainly due to the presence of residual oxygen in sputtering chambers and the high reactivity of oxygen with Zn [14]. We also discuss the preparation and characterization of Schottky barrier (SB) and homojunction diodes fabricated on p-ZnO films. These devices, as discussed in section 3, showed better characteristics than the reported devices prepared by oxidation of ZnON and Zn3N2 films. 2. Experimental Thin films of ZnON (250e1200 nm) were rf sputter-deposited (13.6 MHz, Torr International Inc magnetron sputtering model CRC-300) on glass and on stainless steel (SS) substrates. The substrates were cleaned ultrasonically in ethanol for 10 min. We used a 99.99% pure ZnO (or Zn) target with a diameter of 51 mm. The sputtering gas consisted of a mixture of N2, Ar and the residual O2 in the sputtering chamber. During deposition, the working pressure in the chamber was kept at 2.6 Pa and the flow rates for Ar and N2 (both, 99.99% pure) were maintained at 8 and 32 cm3/min, respectively (80% nitrogen in the gas mixture) using two independent mass flow controllers. The chamber was initially evacuated to 5.3 103 Pa and then the mixed gas was introduced. The rf power was kept at 40 W, unless otherwise stated. The substrate holder rotating at 3.0 rev/min was placed 7 cm far and parallel to the target. The temperature of substrates (not heated intentionally) reached to 30e50 C during the deposition. The target was sputter cleaned for 10 min before the opening of shutter to begin the deposition process. The films thickness and the rate of deposition (0.12 nm/s) were controlled by a quartz-crystal thickness monitor. For the conversion of the precursor ZnON films to p-ZnO, thermal oxidation was performed in a rapid thermal processor (Annealsys model AS-Micro) at 400e500 C for 5e120 min in dry air or in oxygen (atmospheric pressure). The converted p-ZnO films were characterized by scanning electron microscopy, SEM, (Joel, JSM-6300), X-ray photoelectron spectroscopy, XPS, (VG Scientific 200) and X-ray diffraction, XRD, (Siemens D500) using the CuKa line (0.15406 nm). The films' lateral photocurrent was measured at a suitable potential difference across two parallel conducting strips (1e2 mm separation) deposited on the film surface. The photocurrent normalized by the incident photon flux was measured at different incident wavelengths. The measurements were performed using a setup consisting of a grating monochromator (Sciencetech 9050), lock-in amplifier (Stanford Research SR 530), current amplifier (Keithley 428), mechanical chopper and light source (tungsten-halogen). PL measurements were performed in the temperature range of 80e300 K using the 355-nm line of a Nd:YAG laser, an optical cryostat (Oxford DN 1704) and a UV-VIS spectrometer (Ocean Optics HR2000þES). Heterojunction and SB diodes were fabricated (see the text) and characterized by the means of current-voltage (IV) and capacitance-voltage (CV) measurements using a source-measure unit (Keithley 236) and a CV
125
analyzer (Kiethley 590), respectively. 3. Results and discussion 3.1. Conversion of ZnON to p-ZnO films Fig. 1(a) shows the depth profiles of zinc, oxygen, nitrogen, and carbon (contaminant) in an as-grown film. Fig. 1(b) depicts the same plots after the film was oxidized at 400 C in air for 5 min. The Zn/(ZnþO) and N/(ZnþO) atomic ratios in the as-grown film were 0.60 and 0.045, respectively at a selected depth of 60 nm. In the oxidized film and at the same depth, these ratios changed to 0.54 and 0.004, respectively. This indicates the escape of nitrogen and to a much lesser extent zinc from the film. This is in agreement with the other reports that the thermal oxidation induces the out diffusion of nitrogen from the (N2)O and (N)O defects to form nitrogen bubles in the film [8,20]. Fig. 1(b) also shows that nitrogen is not present within 50 nm below the surface of the oxidized film, as a result of its out diffusion. The XPS N 1s plots for the as-grown and oxidized films are shown in Fig. 2. The plot of the as-grown film is composed from three peaks with the binding energies of 395.8 eV, 397.5 eV, and 403.5 eV. The latter two peaks also appear in the spectrum of the oxidized film, but with a slight shift, at 397.9 eV and 403.9 eV. The 395.8 eV peak is attributed to the NeZn bond in e.g. Zn3N2 [21]. The peak appears at 397.5 eV is attributed to the NeZn bond when N occupies the oxygen vacancy (VO) site to form the (N)O defect [14,22]. The peak at 403.5 eV is assigned to the (N2)O defect where N2 occupies the VO site [14,22]. The (N)O defect is believed to be a shallow acceptor in ZnO with the ionization energy of 170e200 meV, whereas (N2)O behaves as a double donor [1]. Apparently, the balance between the densities of the (N)O acceptors and the donor-type defects like (N2)O,VO and Zni (interstitial zinc) determines the type of conductivity in the film. It is evident from Fig. 2 that the oxidation process reduces considerably the Zn3N2 content of the film, lowers the concentration of the (N2)O double donors and increases the concentration of the (N)O acceptors. This leads to the conversion of the as-grown film to p-ZnO. The conversion of conductivity from n-type to p-type was verified by both the hot-probe technique and the Hall-effect measurement. The concentration of free holes in p-ZnO films prepared by oxidation at 400 C was in the range of 1017 e 1018 cm3. The holes concentration was reduced to 1015 e 1016 cm3 when the oxidation temperature was raised to 500 C. This was found to be consistent with the literature reports [9,10]. 3.2. Structural characterization Fig. 3(a) and (b) show the surface morphology of oxidized ZON films grown on glass and on SS substrates, respectively. The grain structure of the film on SS substrate is more pronounced, as expected, due to the polycrystalline nature of the substrate. The asgrown films had a poor crystalline structure. Their XRD patterns showed weak and broad diffraction lines as shown in Fig. 3(c), plot (i), for a 1200-nm film. As noticeable, the (002) diffraction line is weaker than the (100) line. This is in contrast to the un-doped ZnO films which normally show strong (002) preferential orientation when are deposited on amorphous or polycrystalline substrates. The incorporation of nitrogen in the film is associated with the deformation of ZnO lattice and, subsequently, the shift of the diffraction lines and the suppression of c-axis preferential orientation. Plot (ii) in Fig. 3(c) shows the XRD pattern of the same film which was oxidized at 500 C in air for 60 min. The oxidation process leads to the escape of nitrogen molecules from the lattice and hence improves the film crystallinity that is also associated with the increase of the crystallites size. The (100) preferential
126
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
Fig. 1. XPS-determined depth profile of elements in (a) as-grown and (b) oxidized films.
that for the oxidized reference film (n-ZnO). This can be attributed to the effect of nitrogen atoms occupying the oxygen sites in p-ZnO, knowing that the length of the ZneN bond is somewhat smaller than the length of the ZneO bond [23].
3.3. Photoelectrical characterization
Fig. 2. The XPS N 1s spectra for (a) the as-grown and (b) the annealed film at a depth of 100 nm below the surface.
orientation is also retained in the oxidized film. This is apparently a distinguished characteristic of nitrogen-doped ZnO films that is also reported by others [8e10]. For comparison, Fig. 3(d) shows the XRD pattern of an as-deposited ZnO film that exhibits a strong (002) orientation. This film was sputter deposited on glass under the same conditions as for ZnON films, but with the exception that nitrogen in the sputtering gas was replaced by oxygen. The Bragg equation (2dsinq ¼ l, where l ¼ 0.15406 nm and d is the spacing of the diffraction planes) was used to evaluate the lattice constants c and a of the films from their XRD diffraction angles, q. The Sherrer equation (L ¼ 0.94l/Bcosq, where B is the full width at half maximum of the line at the diffraction angle 2q) was used to evaluate the size of crystallites, L, in the film. The lattice constants of the as-grown and oxidized ZnO reference film were also determined from their XRD patterns. The unit cell volumes of the asdeposited and oxidized films were determined from their lattice constants and the results are listed in Table 1. The tabulated data shows the effect of oxidation heat treatment on the growth of crystallites for both samples. The oxidation heat treatment reduces the unit cell volume of the reference ZnO sample by 2.5% which is likely due to the effect of strain relaxation. The unit cell volume of the ZnON film is reduced by 6.0% after the oxidation process. This is almost twice of that for the reference sample. The escape of nitrogen molecules from the interstitial sites in ZnON, in addition to the strain relaxation, is accountable for this change. The unit cell volume in the oxidized ZnON film (p-ZnO) is even 2% smaller than
Fig. 4(a) depicts the dependence of photocurrent normalized to the incident photon flux, R, on the incident photon energy, E, for a p-ZnO and a nitrogen-free n-ZnO reference film. The p-ZnO film was formed by oxidation of a 250-nm ZON film at 400 C for 60 min. The reference film was sputter deposited exactly as for the ZnON precursor film except for the nitrogen in the sputtering gas that was replaced by oxygen. For the reference film the spectral response of R is entirely in the UV range as expected. As E approaches towards the band gap energy the optical absorption coefficient and, hence, the R value are both increased. With the increase of absorption coefficient, the photo-excited electron-hole pairs (EHP) are generated closer to the film surface. This enhances the recombination rate of EHPs due to the effect of surface states and, consequently, R starts to decrease as E exceeds ~3.4 eV. In contrast to the n-ZnO film, R shows a pronounced sub-band gap tail in the p-ZnO film. This implies that the photoconductivity response of the p-ZnO film in the visible range is due to the presence of nitrogen-related band gap defects. In the vicinity of a transition energy where the optical absorption coefficient, a, is high, R varies in proportion to a. Therefore, the direct transition energies, Ed, and the indirect transition energies, Ei, can be evaluated from the plots of (RE)2 and (RE)1/2 against E, respectively. Fig. 4(b) and its inset illustrate these plots which were reconstructed from the spectral response of R for the p-ZnO film in Fig. 4(a). The horizontal intercepts of the line fits to these plots yield Ed ¼ 3.18 eV and Ei ¼ 1.72 eV. A similar procedure gives only one direct transition energy Ed ¼ 3.25 eV for the n-ZnO film. The direct transition energy in the p-ZnO film is 70 meV smaller than the band gap energy of n-ZnO. This may imply that Ed ¼ 3.18 eV corresponds to a transition to the conduction band (CB) of p-ZnO from a shallow acceptor level located 70 meV above the valence band (VB) edge. This is in the vicinity of the 87-meV (0 K) activation energy of an acceptor level measured from the temperature dependence of the film PL, as discussed below. The indirect transition energy of Ei ¼ 1.72 eV in p-ZnO is assigned to the activation energy of a deep defect level. Since this transition was not detected in the n-ZnO film, it is attributed to the effect of nitrogen incorporation in ZnO. A PL peak energy of 1.70 eV is reported in nitrogen doped ZnO films and is assigned to the oxygen vacancy defects which are induced due to the incorporation of nitrogen [24].
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
127
Fig. 3. SEM surface views of the oxidized films grown on (a) glass and (b) stainless steel substrate. (c) XRD plots for (i) as-grown and (ii) oxidized film grown on glass. (d) XRD plot of a nitrogen-free n-ZnO film sputter-deposited on glass. In addition to the ZnO (002) diffraction line, sample holder (Pt/Rh) lines are also shown.
Table 1 The volume of unit cell, V, and the size of crystallites, L, in ZnON and in ZnO reference films before and after the oxidation process. Sample
ZnON ZnON (oxidized) ZnO ZnO (oxidized)
V
L
(nm)3
nm
0.0369 0.0348 0.0364 0.0355
8 21 15 30
3.4. Photoluminescence characterization The near band edge PL spectrum of a p-ZnO film measured at different temperatures (82e293 K) and the temperature dependence of the PL peak energy, E(T), and intensity, I, are shown in Fig. 5(a). For comparison, similar plots for the nitrogen-free n-ZnO reference film are also depicted in Fig. 5 (b). The p-ZnO film with a thickness of 103 nm was prepared by oxidation of ZnON at 400 C for 60 min. The resolved excitonic features of PL could not be observed even at 82 K. Nevertheless, the PL peak energy was in the range of 3.320e3.330 eV for both films at 82 K. This is very close to the 3.327 eV peak energy of the resolved free-electron to acceptor (FA) transition of nitrogen-doped ZnO at 83 K [25]. The variation of the PL peak energy with temperature can be well described by the Varshni equation E(T) ¼ E(0) e AT2/(TþB) [26] where E(0) denotes the transition energy at 0 K, A is a constant related to the excitationaverage phonon interaction and B is related to the Debye temperature [27]. The best fit of the Varshni equation to the peak energy data points in Fig. 5(a) is shown by a solid line corresponding to the fitting parameters A ¼ 0.738 meV/K and B ¼ 280 K which yields E (0) ¼ 3.350 eV. Taking E(0) ¼ 3.350 as the free-electron to acceptor transition at 0 K and taking the ZnO band gap at 0 K as
Fig. 4. The normalized photocurrent, R, against the incident photon energy, E, for (a) p-ZnO and n-ZnO films. (b) The variation of (R.E)2 and (R.E)1/2 (inset) with E for the pZnO film.
128
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
Eg(0) ¼ 3.437 eV [25,27,28], the acceptor binding energy was obtained from EA ¼ Eg(0) eE(0) þ kT/2 [25,27], as EA ¼ 87 meV (kT/2 has a negligible value at low temperatures). The result obtained (EA ¼ 87 meV) is favourably smaller than 140 meV [27] and 110 meV [25] reported for the nitrogen-doped ZnO films. The same analysis applied to the experimental results of the n-ZnO reference film, Fig. 5(b), yields A ¼ 0.738 meV/K, B ¼ 330 K, E(0) ¼ 3.324 eV, and EA ¼ 113 meV. The measured acceptor binding energy agrees well with the reported value of 113 meV for the undoped ZnO films [25]. The nature of this acceptor level in the undoped films is not clear. Fig. 5 illustrates the variation of PL intensity with temperature for the p-ZnO and n-ZnO films. The observed temperature quenching behaviour can be well described by equation I(T) ¼ I(0)/[1 þ Cexp (b/kT)] [29], where I(0) is the emission intensity at 0 K, C is a parameter and b is the activation energy in the thermal quenching process. For both films, the good fit of this equation to the data points (solid lines) yields b ¼ 34 meV which is within the range of 27e47 meV reported in the literature [27].
reduced the barrier height at the gold-film interface and formed a quasi ohmic contact. Gold is known to make ohmic contact to pZnO [2,16] since it acts as a p-type impurity [30]. The formation of ohmic contact at Au/p-ZnO interface resulted in a diode-type IV plot (Fig. 6) in which the forward current corresponds to the positive polarity of ohmic contact (Au) as it is expected for SB diodes formed on p-type semiconductors. The IV plot in Fig. 6 shows a high rectification factor (ratio of forward to reverse current) of r ¼ 1.1 104 (±1 V). The general form of the IV equation for SB diodes dominated by the thermionic emission of majority carriers over an effective barrier height F is I ¼ I0 [exp (qVj/nkT) e 1] where I0 ¼ AA*T2exp (F/kT) [31]. Here, q is the electronic charge, n is the
3.5. Schottky-barrier diodes To the best of our knowledge no study has been reported in the literature on SB devices fabricated on nitrogen-doped p-ZnO films. The IV plot of a diode we prepared on a p-ZnO film (thickness ~800 nm) is shown in Fig. 6. The film was formed by the oxidation of a ZnON film sputter-deposited on SS foil. The oxidation was performed at 500 C in air for 60 min. After the oxidation process, several 30-nm thin gold contacts (diameter, 2 mm) were thermally evaporated on the p-ZnO film. The IV plots of these devices initially showed symmetrical and nonlinear characteristics for both bias polarities (characteristic of two back-to-back diodes), implying that both gold and SS made blocking contacts to p-ZnO. A brief heat treatment of the completed device at 400 C (5e10 min) effectively
Fig. 6. Current-voltage characteristic of a Schottky diode based on a p-ZnO film deposited on flexible stainless steel substrate.
Fig. 5. Temperature dependence of the PL spectrum, its peak energy and intensity for (a) the p-ZnO film and (b) the undoped n-ZnO film.
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
129
ideality factor, k is Boltzmann's constant, T is the absolute temperature, Vj is the voltage drop across the junction (Vj ¼ V - IRs), and Rs is the diode series resistance; A (¼ 0.03 cm2) is the device area, and A* is the effective Richardson constant (A* ¼ 72 Acm2K2 using the hole effective mass m*p ¼ 0.6 m [28]). From the fit of the IV plot in Fig. 6 to the SB IV equation, the diode parameters were determined as n ¼ 2.0, I0 ¼ 0.6 nA, and Rs ¼ 330 U. From the I0 value, the barrier height at the SS-film interface was obtained as F ¼ 0.865 eV. The device capacitance measured at a test frequency of 100 kHz was C ¼ 260 pF and independent from the applied reverse biases, implying that the width of the depletion layer is comparable with the film thickness. From the measured capacitance, the depletion width was evaluated from C ¼ ε0εrA/W as W ¼ 760 nm, where ε0 is the free space permittivity, εr ¼ 7.5 was taken as the relative permittivity of ZnO, and A ¼ 0.03 cm2 was the device active area. The depletion width is related to the junction built-in potential, V0, and the effective density of acceptors, Na, through W ¼ [(2ε0εr V0)/(q Na)]1/2. Taking W ¼ 760 nm, one obtains Na ¼ 1.44 1015V0, where Na is in cm3 units. For an ideal SB formed on a p-type material, the position of the Fermi level from the valence band edge is Ef e Ev ¼ F e V0 and thus the density of holes can be expressed as p0 ¼ NVexp [(F e qV0)/kT]. Taking Na (¼ 1.44 1015 V0) ~ p0 and the effective density of states in the valence band NV ¼ 1.16 1019 cm3 (for m*p/m ¼ 0.6), one obtains V0 ¼ 8.06 103 exp [(F e qV0)/kT]. Substituting for F ¼ 0.865 eV, this equation was solved numerically to yield V0 ¼ 0.62 V, from which Na (~p0) ¼ 0.9 1015 cm3 was obtained. The result is in consistence with the p0 values determined by the Hall-effect measurement. 3.6. Homojunction diodes Homojunction diodes were fabricated from the sputter deposited p-ZnO and n-ZnO films with a configuration shown schematically in the inset of Fig. 7(a). The n-ZnO film with a thickness of 600e1000 nm was sputter deposited on a pre-cleaned SS substrate at room temperature. A ZnON film (thickness, 100 nm) was deposited on top of the n-ZnO film under the same conditions as mentioned previously. The stack layers were annealed in air at 400 C for the conversion of the top layer to p-ZnO. Finally, circular gold contacts (30 nm, 2 mm diameter) were thermally evaporated on the p-ZnO film. This was followed by a second annealing at 400 C for the formation of ohmic contact at the Au/p-ZnO interface. The forward current of the device corresponded to the positive polarity of gold. This verified the conversion of the top layer to pZnO and the formation of a junction at the interface of the two films. The junction at the SS/n-ZnO interface is apparently ohmic, otherwise the IV plot could not show a high rectification characteristic as observed. The device exhibited an excellent rectification factor of r ¼ 5 104 (±2.5 V), much greater than ~6 (±15 V) [6], ~10 (±7.5 V) [16], and 50 (±20 V) [7] reported in the literature. Snigurenko et al. could raise the diode rectification factor from 100 (±2 V) to 4 104 only by inserting an ultrathin Al2O3 layer between the p-ZnO and n-ZnO films to produce a p-i-n diode [32]. From the fit of the diode equation (empirically the same as that for SB), to the data points in Fig. 7(a), n ¼ 9.2 and I0 ¼ 4 nA were deduced. Apparently, the diode behaviour is far from being ideal (n ¼ 1). Despite this, the CV plot of the device shown in Fig. 7(b) follows the junction CV relationship expressed by C2 ¼ 2(Vo eV)/qε0εrA2N, where N ¼ NaNd/(Na þ Nd) [31]. Na and Nd are the uncompensated densities of the acceptors and donors in the p-ZnO and n-ZnO films, respectively. The horizontal intercept and the slope of the straight line fit to the data points in Fig. 7(b) measures V0 ¼ 0.50 V and N ¼ 4.2 1015 cm3. For the preparation of above device, the ZnON film was converted to p-ZnO by air annealing at 400 C which yields
Fig. 7. (a) The IV and (b) CV plots of a homojunction diode fabricated on flexible SS substrate. Both n-ZnO and p-ZnO films were prepared by sputtering. The device structure and bias polarity for the forward current are shown in the inset of Fig. 7(a).
Na ¼ 1.5 1017 cm3 [33]. Therefore from the measured value of N, the density of the uncompensated donors in the n-ZnO film was obtained as Nd ¼ 4.3 1015 cm3. It should be noted that the lower acceptor density of Na ¼ 0.9 1015 cm3 determined from the characteristics of the SB diode is due to the higher oxidation temperature of 500 C used for the conversion of ZnON to p-ZnO. This is consistent with several reports, including [9,10], that the oxidation of nitrogen-doped ZnO at 400e450 C yields a hole density in the range of 1017 cm3, one to two orders of magnitude greater than that in samples oxidized at 500 C. These heterojunction diodes showed weak electroluminescence (EL) under a forward bias. Only the emission integrated over all wavelengths in the UV-Vis range could be detected by a photomultiplier tube. The IV characteristics of a homojunction diode at some selected temperatures are shown in Fig. 8(a). The device rectification factor was reduced as the temperature was lowered. The variation of the device current with temperature at the constant biases of þ1.0 V and 1.0 V is also shown in Fig. 8. The variation of current with temperature follows the Arrhenious equation I ¼ I*exp (DE/kT) where DE is a weak bias-dependent activation energy, and I* is a constant. In the high temperature region, DE ¼ 0.4e0.5 eV was obtained at both biases for several devices. In the low temperature region, DE ¼ 0.08e0.12 eV was measured under the bias of þ1.0 V and DE ¼ 0.16e0.20 eV under the bias of 1.0 V. The interpretation of the measured activation energies is not straight forward. They may represent the grain boundary barrier heights as well as the binding energies of the impurity and defect levels. Nevertheless, it
130
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
(ZnO2 2 ) solution by successive dipping of the SS substrate into the zincate solution and the boiling water as described elsewhere [4]. Using a lower zincate concentration of 0.02 M, well-aligned n-ZnO nanorods, instead of a continuous film, could be grown. The film thickness and the dimensions of nanorods could be increased by increasing the number of dipping cycles. The structure of a typical device is schematically shown in the inset of Fig. 9(a). A 100-nm nZnO film was first deposited on SS to serve as a seed layer for the growth of the second n-ZnO nanorods layer (thickness ~1000 nm). On top of the nanorods layer, a 250-nm ZnON film was sputter deposited followed by its oxidation in pure oxygen at 400 C for 2 h and then gold contacts were deposited. The IV and CV characteristics of a typical device and its device parameters are shown in Fig. 9(a) and (b). This homojunction diode with device parameters superior to those reported in the literature [6,7,12,15,16,32], had a high current rectification factor (7.4 104 at ±1.5 V), a reasonably good ideality factor (1.7), a low reverse biased saturation current (1.0 nA), and a good junction built-in potential (0.75 V). The density of uncompensated donors in the solution-grown ZnO nanorods was obtained from the device CV characteristic as 7.5 1014 cm3. Due to a better crystalline structure of the solution-grown n-ZnO nanrods, these homojunction diodes exhibited better device parameters (current rectification, ideality factor, reverse biased saturation current, junction built-in potential) and much stronger EL as compared with the diodes formed from sputter-deposited n-ZnO films. To the best of our knowledge, homojunction diodes which are reported in the literature and are based on p-ZnO films formed by oxidation of Zn3N2 or ZnON have not exhibited EL property. Fig. 9(c) illustrates the images of a forward biased device in the light and dark backgrounds which were captured by an ordinary digital camera. The defect-related visible light emission through the exposed area of the gold contact is evident from Fig. 9(c). The device was carrying a forward current of 100 mA. Light emission could not be detected at reverse biases, as anticipated. Fig. 9(d) illustrates the variation of integrated EL intensity with the forward current. The inset of this figure shows a strong light emitted through a hole on the gold contact at a forward current of 80 mA. The spectrum of emission depicted in Fig. 9(e) covers the visible and a narrow part of the UV range (375e400 nm). This spectrum is composed from two broad peaks centered at ~466 nm (2.66 eV) and ~560 nm (2.21 eV), corresponding to the radiative transition of electrons through two band gap defect levels. The dominant role of these two defect levels prevented the band-to-band UV emission in the wavelength range of 365e380 nm. 4. Summary and conclusions
Fig. 8. (a) The IV plots of a homojunction diode at different temperatures. The variation of device current with temperature at a bias of (b) þ1.0 V, and (c) 1.0 V are also shown.
was noted that DE ¼ 0.4e0.5 eV is in the close vicinity of the hole binding energy of 0.46 eV in ZnO [34] and the 0.40-eV binding energy of acceptor-type NO cluster defects [35]. The measured value of DE ¼ 0.16e0.20 eV is also in good agreement with the 0.17e0.20 eV binding energy of the NO acceptor level [2,19]. Homojunction diodes were also prepared by replacing the sputter-deposited n-ZnO layer with the solution-grown n-ZnO film and n-ZnO nanorods. The p-ZnO film was prepared by sputtering as described previously. The n-ZnO film was grown from a 0.1 zincate
Thermal oxidation of sputter-deposited ZnON films was utilized to prepare p-ZnO films. The mechanism of nitrogen doping was described based on the results of XPS measurement. The p-ZnO films were formed from 21-nm large crystallites that had a preferential orientation of (100) parallel to the substrate as a result of nitrogen doping. The unit cell volume of the crystallites in p-ZnO was 2% smaller than that in undoped n-ZnO films due to the incorporation of nitrogen atoms in oxygen vacancy sites. Photocurrent and PL characterization of p-ZnO films revealed the presence of a shallow acceptor level with the binding energy of 70e90 meV and a nitrogen-related deep level located 1.72 eV above the valence band edge. The PL thermal quenching in these films had activation energy of 34 meV that is within the range of the reported values. SB diodes and light-emitting homojunction diodes with excellent device parameters were prepared on p-ZnO films. The SB formed at the SS/p-ZnO interface had a barrier height of 0.865 eV, built-in potential of 0.62 V, and ideality factor of 2.0. The diode had a good rectification factor of 1.1 104 (±1 V). Temperature
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
131
Fig. 9. (a) IV and (b) CV characteristics of a homojunction diode prepared from sputter-deposited p-ZnO and solution-grown n-ZnO; (c) visible light emission through a gold contact; (d) integrated EL intensity against the diode forward current; (e) the spectrum of light emitted through the gold contact.
dependence of the diode conductance revealed several defectrelated activation energies. Homojunction diodes fabricated from p-ZnO and sputter-deposited n-ZnO films showed a rectification factor of 5.0 104 (±2.5 V), ideality factor of 9.2, and built-in potential of 0.50 V. Homojunction diodes prepared from p-ZnO films and solution-grown n-ZnO nanorods showed better characteristics, including a rectification factor of 7.4 104 (±1.5 V), ideality factor of 1.7 and built-in potential of 0.75 V. Furthermore, these diodes exhibited strong EL in the visible region. As compared with the previous reports on p-ZnO films obtained by ZnON oxidation, films and devices prepared in this study have much improved characteristics. Acknowledgments The support of the Research Sector of Kuwait University under the research project SP03/14 is thankfully acknowledged. We also acknowledge the valuable technical support received from the general facility of the Faculty of Science (Projects GS 02/08 and GS03/01) and the general facility of the College of Engineering (Projects GE01/07 and GE01/08). References [1] A. Janotti, C.G. Van de Walle, Rep. Prog. Phys. 72 (2009) 126501.
[2] J.C. Fan, K.M. Sreekanth, Z. Xie, S.L. Chang, K.V. Rao, Prog. Mater. Sci. 58 (2013) 874. [3] Y.S. Choi, J.W. Kang, D.K. Hwang, S.J. Park, IEEE Trans. Electron Devices 57 (2010) 26. [4] A.E. Rakhshani, Appl. Surf. Sci. 311 (2014) 614. [5] W. Ko, S. Lee, N. Myoung, J. Hong, J. Mater. Chem. C 4 (2016) 142. [6] S.S. Lin, J.G. Lu, Z.Z. Ye, H.P. He, X.Q. Gu, L.X. Chen, J.Y. Huang, B.H. Zhao, Solid State Commun. 148 (2008) 25. [7] J. Huang, S. Chu, J. Kong, L. Zhang, C.M. Schwarz, G. Wang, L. Chernyak, Z. Chen, J. Liu, Adv. Opt. Mater 1 (2013) 179. [8] C.W. Zou, R.Q. Chen, W. Gao, Solid State Commun. 149 (2009) 2085. [9] C. Wang, Z. Ji, K. Liu, Y. Xiang, Z. Ye, J. Cryst. Growth 259 (2003) 279. [10] N.H. Erdogan, K. Kara, H. Ozdamar, R. Esen, H. Kavak, Appl. Surf. Sci. 271 (2013) 70. [11] Y.-P. Jin, B. Zhang, J.-Z. Wang, L.-Q. Shi, Chi. Phys. Lett. 33 (2016) 058101. [12] V. Kambilafka, A. Kostopoulos, M. Androulidaki, K. Tsagaraki, M. Modreanu, E. Aperathitis, Thin Solid Films 520 (2011) 1202. [13] I.V. Rogozin, A.N. Georgobiani, M.B. Kotlyyarevsky, V.I. Demin, L.S. Lepnev, Inorg. Mater 49 (2013) 568. [14] J.P. Zhang, L.D. Zhang, L.Q. Zhu, Y. Zhang, M. Liu, X.J. Wang, J. Appl Phys. 102 (2007) 114903. [15] S. Dhara, P.K. Giri, Thin Solid Films 520 (2012) 5000. [16] Z.W. Wang, Y. Yue, Y. Cao, Superlattice. Microst. 65 (2014) 7. [17] X.H. Wang, B. Yao, D.Z. Shen, Z.Z. Zhang, B.H. Li, Z.P. Wei, Y.M. Lu, D.X. Zhao, J.Y. Zhang, X.W. Fan, L.X. Guan, C.X. Cong, Solid State Commun. 141 (2007) 600. [18] S. Yamauchi, Y. Goto, T. Hariu, J. Cryst. Growth 260 (2004) 1. [19] J. Lu, Q. Liang, Y. Zhang, Z. Ye, S. Fujita, J. Phys. D. Appl. Phys. 40 (2007) 3177. [20] P. Fons, H. Tampo, A.V. Kolobov, M. Ohkubo, S. Niki, J. Tominaga, R. Carboni, F. Boscherini, S. Friedrich, Phys. Rev. Lett. 96 (2006) 045504. [21] F. Watanabe, H. Shirai, Y. Fujii, T. Hanajiri, Thin solid Films 520 (2011) 3729. [22] L. Li, C.X. Shan, B.H. Li, B. Yao, J.Y. Zhang, D.X. Zhao, Z.Z. Zhang, D.Z. Shen,
132
A.E. Rakhshani / Journal of Alloys and Compounds 695 (2017) 124e132
X.W. Fan, Y.M. Lu, J. Phys. D. Appl. Phys. 41 (2008) 245402. [23] C.H. Park, S.B. Zhang, S.H. Wei, S.B. Zhang, S.H. Wei, Phys. Rev. B 66 (2002) 073202. [24] F. Stavale, L. Pascua, N. Nilius, H.J. Freund, J. Phys. Chem. C 118 (2014) 13693. [25] Z.W. Wang, Y. Yue, Y. Cao, Vacuum 101 (2014) 313. [26] Y.P. Varshni, Physica 34 (1967) 149. [27] H.B. Ye, J.F. Kong, W.Z. Shen, J.L. Zhao, X.M. Li, J. Phys. D. Appl. Phys. 40 (2007) 5588. [28] K. Hummer, Phys. Stat. Sol. B 56 (1973) 249. [29] W.Z. Shen, S.C. Shen, J. Appl. Phys. 80 (1996) 5941.
[30] Y. Yan, M.M. Al-Jassim, S.H. Wei, Appl. Phys. Lett. 89 (2006) 181912. [31] D.A. Neamen, Semiconductor Physics and Devices: Basic Principles, McGraw Hill, New York, 2003. [32] D. Snigurenko, K. Kopalko, T.A. Krajewski, R. Jakiela, E. Guziewicz, Semicond. Sci. Technol. 30 (2015) 015001. [33] A.E. Rakhshani, A. Bumajdad, J. Kokaj, S. Thomas, Kuwait J. Sci. 41 (2014) 107. [34] S.T. Teklemichael, M.D. McCluskey, Nanotechnol. 22 (2011) 475703. [35] J. Furthmuller, F. Hachenberg, A. Schleife, D. Rogers, F. Hosseini Teherani, F. Bechstedt, Appl. Phys. Lett. 100 (2012) 022107.