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Preparation and characterization of transparent conducting ZnS:Al films
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Solid State Sciences 11 (2009) 224e232 www.elsevier.com/locate/ssscie
Preparation and characterization of transparent conducting ZnS:Al films P. Prathap a, N. Revathi a, Y.P.V. Subbaiah b, K.T. Ramakrishna Reddy a,*, R.W. Miles c a
Thin Film Laboratory, Department of Physics, Sri Venkateswara University, Chandragiri Road, Tirupati e 517 502, Andhra Pradesh, India b Department of Physics, Yogi Vemana University, Kadapa e 516 003, India c School of Engineering and Technology, Northumbria University, Newcastle NEI 8ST, UK Received 7 February 2008; received in revised form 21 March 2008; accepted 23 April 2008 Available online 1 May 2008
Abstract Al-doped ZnS films were deposited using close-spaced evaporation of the powders synthesized by chemical precipitation method. The films were prepared for different Al concentrations in the range 0e10 at.% on glass substrates kept at 300 C. The effect of Al-doping on ZnS composition, microstructure and optoelectronic properties of as-grown ZnS layers was determined using appropriate techniques. The films were polycrystalline and showed (111) preferred orientation for all the doping concentrations in spite of an additional phase of Al2S3 observed at higher dopant levels. The surface morphological studies indicated that the Al incorporation had a considerable effect on the surface roughness of the films. The optical measurements indicated that the optical energy band gap decreased slightly with the increase of dopant concentration without affecting the optical transmittance characteristics significantly. The electrical analysis indicated that the resistivity of the layers changed significantly with the doping concentration in the layers. The change of photoluminescence behaviour of the as-grown ZnS:Al films with dopant concentration was also studied. Ó 2008 Published by Elsevier Masson SAS. PACS: 68.55.J; 81.15.E; 73.61.G; 78.66.F Keywords: ZnS films; Close-spaced evaporation; Structure; Optical properties
1. Introduction ZnS is a potential candidate in optoelectronic field despite its high resistive nature (107 U cm). Its applications can be further extended if the layers were low resistive, which leads to a decrease of series resistance of the fabricated device. For instance, it can be used as window layer in the fabrication of thin film solar cells due to its higher optical band gap of w3.6e3.8 eV, which led to the increase in the blue response of the cell [1]. Most of the successful reports on solar cells employ highly toxic CdS layers as a window/buffer. The basic requirements for the window layers are low electrical resistivity and high optical transmittance. The resistivity can be decreased either by the creation of excess metal atoms or by doping the * Corresponding author. Tel.: þ91 877 2249666x272; fax: þ91 877 2249611. E-mail address: [email protected] (K.T. Ramakrishna Reddy). 1293-2558/$ - see front matter Ó 2008 Published by Elsevier Masson SAS. doi:10.1016/j.solidstatesciences.2008.04.020
material with suitable dopant. The decrease of resistivity by the creation of excess metal atoms may not be durable when the layers are exposed to different atmospheres. Therefore, an effective way to improve the electrical conductivity of ZnS films is to dope the layers by suitable dopants preferably from III group metals like Al, Ga and In without affecting their optical transmittance characteristics. This can produce shallow donor levels leading to an improvement in the conductivity of the layers. In this direction, very less efforts have been made to produce low resistive ZnS films. Olsen et al. prepared conducting Al-doped ZnS films by evaporation and they studied the variation of optical and electrical properties with dopant concentration [2]. But they did not clearly mention the dopant percentage in the layers. Lee et al. prepared In-doped ZnS films and studied their electrical properties as a function of dopant concentration [3]. Qi et al. [4] studied the effect of copper and aluminum doping on the luminescence properties of ZnS films. Karar [5] studied the effect of Cu/Al and codoping of Cl1 and
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(NO3)1 on the luminescence properties of ZnS crystals. Apart from the microcrystalline films, nanocrystalline films can offer distinct advantages like effective optical absorption, lower recombination losses and pave a way for the fabrication of low dimensional device structures. [6]. In view of the potentiality and versatility of ZnS compared to other wide band gap semiconductors, we have adopted an inexpensive chemical method to prepare Al-doped ZnS powders. This method is a promising way to produce low resistive films over a large area due to the uniform distribution of dopant atoms in ZnS matrix rather than forming concentration gradient with a lower lattice distortion. ZnS:Al powders thus synthesized were used to grow the layers by close-spaced evaporation followed by the characterization of the films using different techniques. The results obtained in the evaluation of the physical behaviour of the as-grown ZnS:Al films are reported and discussed. 2. Experimental Al-doped ZnS films were prepared on glass substrates by close-spaced evaporation (CSE) technique with different dopant concentrations that vary in the range 0e10 at.%. ZnS:Al powders required for evaporation were synthesized in the laboratory by chemical precipitation method. In the preparation process, we did not use chloride-based precursors in order to avoid the formation of ZnCl2 complexes in the precipitate, which degrade the crystalline quality and electrical characteristics of the grown films. The films were grown under the optimized deposition conditions achieved in our earlier study to deposit undoped ZnS layers [7]. The substrate temperature was maintained at 300 C with the substrate fixed at a distance of w5 cm from the source. The rate of deposition and film ˚ /s and 300 nm, respectively. The thickness were kept as 30 A experimental arrangement of CSE technique was reported elsewhere [8]. ZnS:Al powder in the form of a pellet was placed in a molybdenum boat and covered with quartz wool and a perforated molybdenum sheet to avoid bumping of the material during evaporation. The system was then pumped
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down to a base pressure of 5 106 mbar. The substrates were sputter cleaned in glow discharge prior to evaporation in order to remove any impurities present on the substrates. The vacuum system was again pumped down to a pressure of 5 106 mbar. The rate of deposition and thickness of the experimental films were monitored using the quartz crystal thickness monitor placed just below the substrate holder. The deposition process and film thickness were controlled by a shutter placed between the source and substrate. The structural and morphological studies were carried out using Siefert X-ray diffractometer and Vecco atomic force microscope, respectively. The elemental composition of the layers was studied using VG Microtech ESCA2000 X-ray photoelectron spectrometer. The spectral transmittance of the films was recorded as a function of wavelength that varied in the range 300e1500 nm using Hitachi UVeVISeNIR spectrophotometer. The photoluminescence properties were studied using YVON fluorescence spectrophotometer. The electrical resistivity of the layers was measured using the two-probe method. Silver was used as electrodes for measuring the resistivity of the layers. The temperature dependence of electrical conductivity of the ZnS films was studied in the range 150e450 K. 3. Results and discussion All the Al-doped ZnS powders synthesized using chemical precipitation method showed the (111) plane as the dominant orientation corresponding to the cubic structure of ZnS. The other peaks observed at 47.61 and 56.37 were attributed to the (220) and (311) orientations of cubic ZnS. The evaluated crystallite size and lattice constant of the powder were found ˚ , respectively. Fig. 1(a) and (b) to be w4.8 nm and 4.774 A shows the typical X-ray diffractogram and EDAX spectrum of ZnS powder with an ‘Al’ concentration of 6 at.%, respectively. The elemental composition was found to be Zn ¼ 44.75 at.%, S ¼ 49.12 at.% and Al ¼ 6.13 at.%. The powder samples prepared for different Al-dopant concentrations were pelletized and evaporated on to glass substrates in order to prepare Al-doped ZnS films at predetermined conditions.
Fig. 1. (a) XRD and (b) EDAX spectra of the ZnS:Al powder.
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The visual observation of as-grown layers was pinhole free, uniform and pale yellow in colour. The as-deposited films were smooth and transparent. They had a distinct advantage that the adhesion of Al-doped film to the substrate was better when compared to the undoped films as well as those deposited by the direct evaporation of ZnS powder as was tested by the Scotch tape method. The thickness of the layers was approximately 0.3 mm. 3.1. Compositional analysis X-ray photoelectron spectroscopy (XPS) measurements were performed to know the dopant incorporation, chemical composition and state of the elements present in the layers. The XPS scans were recorded before as well as after the Arþ bombardment and no changes were observed in the binding energies and peak shapes due to the Arþ bombardment. Fig. 2 shows the typical XPS spectrum of ZnS layers grown at a dopant concentration of 6 at.%, recorded in the range 0e1200 eV. The XPS spectrum exhibited the binding energies corresponding to Zn 2p3/2, Zn 2p1/2 and S 2p3/2 at 1022 eV, 1046 eV and 162.5 eV, respectively. The observed binding energies are in good agreement with the reported data [9]. It has been observed that in the case of Al 2p3/2 centered at 74.02 eV, which originates from Al3þ rather than metallic Al (72.8 eV), the Al atoms dissolved in ZnS matrix substituted Zn sites successfully. No other compounds related to sulfates or organic compounds were observed. In order to verify the homogeneity of Al content in the layers, one of the optimized films prepared for an Al concentration of 6 at.% had been etched using Ar and the XPS signal was analyzed for successive etching times, which showed no significant change in the XPS signal strength as a function of etching time. This confirms the uniformity of dopant concentration across the layer thickness. There was a small peak in the XPS spectrum at 284.96 eV, which is an indication of carbon inclusion in the films as the evaporant was synthesized from the organic precursors. The binding energies of the constituent elements shifted slightly towards lower
Fig. 2. XPS spectrum of ZnS films deposited for an Al-doping concentration of 6 at.%.
energies and peak shapes of core levels remained unchanged after doping. This may be due to the formation of lattice distortion caused by Al-doping. 3.2. Structural analysis The crystallographic studies made using X-ray diffraction analysis clearly indicated the effect of doping concentration on the crystalline quality of the grown layers. The X-ray diffraction spectra of ZnS:Al films for different Al-doping concentrations in the range 0e10 at.% are shown in Fig. 3. The comparison of XRD patterns of as-grown undoped and Al-doped films did not show the essential difference in the growth orientations and other phases of films in the investigated doping concentration range 0e6 at.%. The (111), (220) and (311) reflections observed in the XRD spectra of the films grown in the entire Al-doping concentration correspond to the cubic structure of ZnS. El Hichou et al. [10] reported cubic and a combination of cubic and hexagonal phases, respectively, for Al- and Sn-doped ZnS films grown by spray pyrolysis technique. In the present study, the as-grown films showed purely cubic structure for all the dopant concentrations. Moreover, it is interesting to note that the crystallinity of the films was not seriously affected by the incorporation of Al in ZnS lattice where the intensity of the (111) peak increased with the increase of dopant concentration to 6 at.% while other orientations such as (220) and (311) could not be developed significantly. Above this critical value of dopant percentage, a small indication of Al2S3 phase could be observed that was confirmed by the origin of other reflections like (116), (300) and (119) that correspond to Al2S3 phase along with the poor crystallinity. This indicated the phase segregation at higher dopant concentration. The initial increase of crystallinity with doping concentration might be due to the incorporation of Al atoms in the Zn vacancies present in the samples. There were reports in the literature that the crystallinity of the host material could not be disturbed noticeably by the incorporation of foreign dopant atoms upto certain-doping level [11]. Similar increase of crystallinity with dopant concentration was also reported by Machado et al. [12] for In-doped ZnO films prepared by electrodeposition. This analysis shows that the presence of aluminum helps to improve the crystallinity of ZnS films. When a material is doped with a suitable dopant, generally at lower dopant concentrations, new X-ray diffraction peaks may not appear, instead a gradual shift in the angular positions of the XRD peaks, corresponding to the host material are observed. This shift in the diffraction angle is expected because of the changes that occur in the lattice parameters of the host lattice on the incorporation of dopant atoms. The position of (111) peak gradually shifted to higher diffraction angles with the increase of Al-doping content in the layers. However, the (111) peak of Al-doped ZnS film shifted back to lower diffraction angle at higher doping levels, above 6 at.%. This suggests that Al-doping might have caused some lattice disorder in ZnS films although the cubic structure was maintained. The variation of lattice constant ‘a’ as a function of aluminum concentration is
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Fig. 3. XRD spectra of Al-doped ZnS films deposited for different dopant levels.
˚ for shown in Fig. 4. The lattice constant was found to be 5.416 A ˚ for an Alundoped ZnS films, which decreased to 5.383 A ˚) doping concentration of 6 at.%. As the radius of Al3þ (0.54 A ˚ ) the substitution of Al3þ in is lower than that of Zn2þ (0.74 A Zn2þ lattice sites could decrease the lattice parameter. However, the lattice constant increased with further increase of doping level above 6 at.%, which could be due to the decrease of number of substitutional Al3þ ions. The Al atoms may be out diffused from Zn2þ sites at higher dopant concentrations (>6 at.%), resulting in the interstitial incorporation of Al3þ in the ZnS crystal lattice. This analysis shows that the solubility limit of Al in ZnS might be as high as 6 at.%.
The change of strain in the as-grown films as a function of Al-doping concentration in ZnS layers, as shown in Fig. 5, was evaluated using the relation [13] 3¼
a a0 100 a0
ð1Þ
where ‘a’ is the lattice constant of ZnS films and a0 is the unstrained bulk lattice parameter. The undoped films showed tensile strain that changed to compressive strain with the increase of Al-doping density upto 6 at.%. In general, dopant atoms can occupy substitutional or interstitial positions in the
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nucleation and coalescence phases of film growth. Goyal et al. [15] reported similar observations for indium doped zinc oxide films and explained the decrease of grain size at higher In-doping levels in terms of interstitial inclusion of dopant atoms in ZnO lattice, which results in poor crystallinity of the films. In polycrystalline films, the texture coefficient gives a measure of the preferred orientation compared to a randomly oriented sample. The value of Ci with dopant concentration was evaluated from the relation [4,13] Texture coefficient; Ci ¼
Fig. 4. Change of lattice constant with Al-doping density.
crystal lattice. The increase of compressive strain with the increase of doping levels might be due to the substitutional incorporation of dopant atoms whereas the increase of lattice constant at higher Al-doping levels (>6 at.%) might be due to the interstitial occupation of Al atoms. The lattice strain present in the as-grown films might be caused by several factors such as stoichiometric deviations, difference in the thermal expansion coefficients of the grown film and the substrate, etc. However, we thought that strain induced by doping might be dominant in the present case as all other deposition parameters were kept constant. The mean crystallite size, D, of the layers was calculated using Scherrer’s equation [14]. The crystallite size initially increased from 42 nm to 56 nm in the dopant concentration range, 0e6 at.% and decreased to 31 nm with further increase of the dopant concentration to 10 at.%. This might be due to the limited solubility of the dopant atoms in the host lattice. This implies that Al had a considerable effect on the
Fig. 5. Variation of lattice strain with Al-doping concentration.
ðIi =Ioi Þ PN ð1=NÞ i¼1 ðIi =Ioi Þ
where Ii is the intensity of a generic peak in the spectrum, Ioi is the intensity of a generic peak for a completely random sample (JCPDS) and N is the number of reflections considered in the analysis. The variation of crystallite size and texture coefficient (Ci) with Al-doping concentration is shown in Fig. 6. The value of Ci is peaked at the dopant concentration of 6 at.%, which shows better crystallinity of the grown samples. 3.3. Morphological analysis Fig. 7 shows the AFM images of the films grown for three typical Al-doping densities of 0 at.%, 6 at.% and 10 at.%. The photographs indicated that the films were smooth and continuous without any pinholes/cracks. The surface morphology of the Al-doped samples is found to be similar to that of undoped one. The surface of the layer is covered with equal sized grains that are uniformly distributed. The grains on the film surface were more compact and have dense structure. It was also found that there were variations in the grain size with the dopant concentration. The grain size increases with the increase of Al concentration followed upto 6 at.% by a decrease at higher dopant concentration although the films were deposited using the same deposition conditions. The grain size of the films had a maximum at an Al-doping
Fig. 6. The variation of crystallite size and texture coefficient with Al-dopant level.
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electron beam evaporation. The grain size evaluated from AFM measurements closely matched with the value calculated from the XRD data. The evaluated average surface roughness for the films was nearly constant, w1.02 nm upto Al concentration of 6 at.% and its value increased to 1.57 nm in spite of the decrease of grain size beyond this doping value. 3.4. Optical studies Fig. 8 shows the optical transmittance spectrum of ZnS:Al films with Al-doping percentage. Doping of ZnS films with ‘Al’ shows significant changes in the transmittance characteristics in the visible region. The average transmittance of pure ZnS films in the visible region is about 85% whereas ZnS films doped with Al concentration of 6 at.% showed a transmittance of 91% that decreased to 65% with further increase of aluminum doping level to 10 at.%. The increase of optical transmittance could be due to the better crystallinity and grain size whereas the decrease of transmittance at higher Al-doping concentrations (>6 at.%) could be attributed to the lower grain size, formation of additional phases and higher defect density that resulted in poor crystallinity of the layers [17]. Moreover, the sharpness of the absorption edge increased with doping concentration up to 6 at.% and decreased thereafter. This shows that the structural disorder seems to be increased at higher Al-doping concentrations (>6 at.%), which is also supported by the structural analysis of the layers in the present study. The optical band gap was determined from the plots of (ahn)2 vs photon energy, hn by extrapolating the linear region of the plot onto the energy axis. All the films grown for different Al concentrations showed the direct band gap nature and the evaluated energy band gap with dopant concentration is shown in Table 1. It varied from 3.79 eV to 3.72 eV when the doping concentration varied from 0 at.% to 6 at.%.
Fig. 7. AFM pictures of Al-doped ZnS films grown with different Al-doping densities.
concentration of 6 at.% and decreased beyond this dopant level. The variation of grain size with doping concentration implies that there might be some important effect of Al on the nucleation and coalescence of islands during the process of film growth. Megahid et al. [16] observed a similar behaviour of grain size variation in In-doped CdS films deposited by
Fig. 8. Optical transmittance vs wavelength spectra of ZnS:Al films.
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Table 1 The variation of energy band gap and activation energy with Al-doping concentration Al-doping concentration (at.%)
Energy band gap (eV)
Activation energy (meV) 150e310 K
310e450 K
0 2 4 6 8 10
3.79 3.79 3.75 3.72 3.65 3.54
21 19 15 11 14 19
240 209 157 113 138 159
According to Pintle et al. [18], the doping of thin films is effective when the maximum variation of band gap is lower than 2%. In the present study also, the variation in band gap with Al-doping is <2% and hence it could be inferred that the effective doping of Al occurred in ZnS upto 6 at.%. Generally it is observed that the energy band gap decreases linearly by doping materials, which is due to the formation of shallow levels or impurity levels in the band gap region. Hankare et al. [19] reported a similar variation of band gap for Pb-doped CdSe films with the dopant concentration. In this study, the effect of doping on the energy band gap is marginal upto the dopant concentration of 6 at.%. This marginal variation of band gap could be explained on the basis of the fact that the structural defects such as voids and dangling bonds, which caused the generation of undesirable localized states in the band gap could be decreased with the introduction of Al into the ZnS lattice and hence the decrease of structural disorder that reduces band tailing into the band gap region [20]. Therefore, the linear decrease of band gap with the dopant concentration could be compensated by improvement in the crystalline quality of the layers which was also supported by the XRD analysis. 3.5. Photoluminescence studies Fig. 9 shows the room temperature photoluminescence spectra of Al-doped ZnS layers when excited with the radiation of wavelength 325 nm. The spectra clearly indicated the effect of Al incorporation on the photoluminescence characteristics of ZnS films. The PL spectra of Al-doped layers are much different from that of undoped ZnS layers and the luminescence intensity increased with the increase of Al concentration although the change in the peak position is marginal. The de-convolution of pure ZnS films gives two peaks at 391 nm and 436 nm. The luminescence peak located around 391 nm is associated with the zinc vacancies [21] while the other emission at 436 nm is associated with zinc interstitials [22]. With the incorporation of Al into ZnS structure, an additional peak could also be observed at 470 nm and its intensity increased with the increase of dopant concentration upto 6 at.%. The observed luminescence at 470 nm could be ascribed to the self-activated luminescence that resulted from the donoreacceptor pair (DAP) transition [23]. The doped Al impurities can form both the isolated Al donors as well as compensated acceptors, the density of which depends on the level of the dopant. The donor
Fig. 9. Photoluminescence spectra of ZnS:Al films.
states could be produced by isolated Al donors whereas the acceptor states could be generated in the form of Zn vacancyeAl complex, (VZnZnAl). Therefore, the luminescence and electron transport properties are largely controlled by the compensation of Al donors. The density of these states increased with the increase of Al-doping level, which increases the probability of recombination of DAP. This led to an enhanced luminescence intensity at higher doping densities (w6 at.%). The decrease of luminescence intensity at larger doping levels (>6 at.%) could be due to the formation of Al donor complex that induces the structural disorder caused by the increase of non-radiative recombination. Similar decrease of luminescence intensity at higher doping level was reported by Oh et al. for Al-doped ZnSe films deposited by molecular beam epitaxy [24]. A broad luminescence spectrum with low intensity was observed for the films deposited at a dopant concentration of 10 at.%. This analysis shows that the compensation of Al donors is dominant at higher doping levels. 3.6. Electrical studies The ZnS films in their non-stoichiometric form are known to have n-type conductivity in which the carriers are generated from interstitial zinc or sulfur vacancies. The electrical resistivity of ZnS films as a function of Al-dopant concentration is shown in Fig. 10. It could be observed that the resistivity of the grown films strongly depends on the Al-dopant level in ZnS films. It decreased from 2.7 105 U cm to 24 U cm with the increase of Al concentration from 0 at.% to 6 at.% and again increased to 6.6 103 U cm with further increase of Al concentration in ZnS layers upto 10 at.%. The present trend of resistivity can be explained in terms of solubility limit of dopant atoms in the host lattice. For Al-doping concentration lower than 6 at.%, every Al atom that substitutes Zn atom in ZnS lattice can contribute one extra free carrier to the conduction band as the valence difference between
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Fig. 11. ln s vs T1 plots for ZnS:Al films. Fig. 10. Change of electrical resistivity of ZnS films with Al-dopant concentration.
Al3þ and Zn2þ is 1. As the doping level is increased, more Aldopant atoms occupy Zn lattice sites that results in more charge carriers. However, after certain level of doping, the dopant atoms cannot occupy the zinc sites due to their limited solubility. Since the radius of Al atom is less than the radius of Zn atom, excess Al atoms can occupy the interstitial position in ZnS lattice [25]. Alternatively, it can be precipitated at the grain boundaries and form Al2S3 phase as observed from the XRD analysis. This indicated that the maximum solubility of Al atoms in ZnS lattice could reach approximately 6 at.%. Hence, the increase of resistivity at higher doping levels might be due to impurity scattering or grain boundary potential effects. Alam and Cameron [26] also reported similar results in Al-doped ZnO films deposited by solegel process. The formation (AlZnVZn) acceptor complexes that induce the self-compensation of the shallow donor impurity levels may be also responsible for the increase of resistivity as reported by Prokesch et al. [27], which are identified in Al-doped ZnSe films. The probability of formation of (AlZnVZn) complexes increases with the increase of dopant concentration [28]. Therefore, the increase of resistivity was observed at higher dopant concentrations in the present films. The temperature dependent electrical conductivity was also studied in the range 150e450 K for all the samples and the activation energies were evaluated. Fig. 11 shows the Arrhenius plots of ZnS:Al films deposited in the dopant concentration range 0e10 at.%. It can be observed from these plots that the variation of ln s with temperature showed two slopes, where the change occurred around the annealing temperature of 310 K. The evaluated activation energies in the two regions are shown in Table 1. It could be observed that the activation energy decreased with doping concentration upto 6 at.%. The decrease of activation energy could be due to the decrease of energy band gap and effective doping of ZnS. Further, the increase of crystallinity could also contribute to the observed decrease of activation energy. The activation energy was found
to increase again with further increase of dopant concentration that could be due to the formation of additional phases, which caused the increase of the grain boundary potential and poor crystallinity of the grown layers. 4. Conclusions Al-doped ZnS films were successfully prepared by closespaced evaporation of the powders synthesized by chemical precipitation method at different Al-doping concentrations. The chemical composition of the layers analyzed by using XPS measurements indicated that the dopant was distributed uniformly across the layer thickness. The layers showed predominantly (111) orientation corresponding to cubic structure of ZnS. However, an additional Al2S3 phase was observed at higher Al-dopant concentrations. The surface morphological analysis revealed that the dopant atoms played some role in the modification of surface roughness and the layers prepared at the dopant concentration of 6 at.% showed an average roughness of 1.02 nm. These layers showed a higher optical transmittance of 91% with an energy band gap of 3.73 eV. The as-grown layers showed closely separated photoluminescence peaks that consisted of Zn and S vacancies and their corresponding interstitials and also the Al-dopant. The luminescence intensity also decreased at higher doping levels, >6 at.%. The layers prepared at the dopant concentration of 6 at.% showed a lower electrical resistivity of 24 U cm and the resistivity was found to decrease beyond this dopant concentration. These layers showed two different regions in the Arrhenius plots and the layers prepared at an optimized dopant level showed lower activation energies of 11 meV and 113 meV in the low and high temperature regions, respectively. References [1] T. Miyawaki, M. Ichimura, Mater. Lett. 61 (2007) 4683. [2] L.C. Olsen, R.C. Bohara, D.L. Barton, Appl. Phys. Lett. 34 (1979) 528.
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Solid State Sciences 11 (2009) 224e232 www.elsevier.com/locate/ssscie
Preparation and characterization of transparent conducting ZnS:Al films P. Prathap a, N. Revathi a, Y.P.V. Subbaiah b, K.T. Ramakrishna Reddy a,*, R.W. Miles c a
Thin Film Laboratory, Department of Physics, Sri Venkateswara University, Chandragiri Road, Tirupati e 517 502, Andhra Pradesh, India b Department of Physics, Yogi Vemana University, Kadapa e 516 003, India c School of Engineering and Technology, Northumbria University, Newcastle NEI 8ST, UK Received 7 February 2008; received in revised form 21 March 2008; accepted 23 April 2008 Available online 1 May 2008
Abstract Al-doped ZnS films were deposited using close-spaced evaporation of the powders synthesized by chemical precipitation method. The films were prepared for different Al concentrations in the range 0e10 at.% on glass substrates kept at 300 C. The effect of Al-doping on ZnS composition, microstructure and optoelectronic properties of as-grown ZnS layers was determined using appropriate techniques. The films were polycrystalline and showed (111) preferred orientation for all the doping concentrations in spite of an additional phase of Al2S3 observed at higher dopant levels. The surface morphological studies indicated that the Al incorporation had a considerable effect on the surface roughness of the films. The optical measurements indicated that the optical energy band gap decreased slightly with the increase of dopant concentration without affecting the optical transmittance characteristics significantly. The electrical analysis indicated that the resistivity of the layers changed significantly with the doping concentration in the layers. The change of photoluminescence behaviour of the as-grown ZnS:Al films with dopant concentration was also studied. Ó 2008 Published by Elsevier Masson SAS. PACS: 68.55.J; 81.15.E; 73.61.G; 78.66.F Keywords: ZnS films; Close-spaced evaporation; Structure; Optical properties
1. Introduction ZnS is a potential candidate in optoelectronic field despite its high resistive nature (107 U cm). Its applications can be further extended if the layers were low resistive, which leads to a decrease of series resistance of the fabricated device. For instance, it can be used as window layer in the fabrication of thin film solar cells due to its higher optical band gap of w3.6e3.8 eV, which led to the increase in the blue response of the cell [1]. Most of the successful reports on solar cells employ highly toxic CdS layers as a window/buffer. The basic requirements for the window layers are low electrical resistivity and high optical transmittance. The resistivity can be decreased either by the creation of excess metal atoms or by doping the * Corresponding author. Tel.: þ91 877 2249666x272; fax: þ91 877 2249611. E-mail address: [email protected] (K.T. Ramakrishna Reddy). 1293-2558/$ - see front matter Ó 2008 Published by Elsevier Masson SAS. doi:10.1016/j.solidstatesciences.2008.04.020
material with suitable dopant. The decrease of resistivity by the creation of excess metal atoms may not be durable when the layers are exposed to different atmospheres. Therefore, an effective way to improve the electrical conductivity of ZnS films is to dope the layers by suitable dopants preferably from III group metals like Al, Ga and In without affecting their optical transmittance characteristics. This can produce shallow donor levels leading to an improvement in the conductivity of the layers. In this direction, very less efforts have been made to produce low resistive ZnS films. Olsen et al. prepared conducting Al-doped ZnS films by evaporation and they studied the variation of optical and electrical properties with dopant concentration [2]. But they did not clearly mention the dopant percentage in the layers. Lee et al. prepared In-doped ZnS films and studied their electrical properties as a function of dopant concentration [3]. Qi et al. [4] studied the effect of copper and aluminum doping on the luminescence properties of ZnS films. Karar [5] studied the effect of Cu/Al and codoping of Cl1 and
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(NO3)1 on the luminescence properties of ZnS crystals. Apart from the microcrystalline films, nanocrystalline films can offer distinct advantages like effective optical absorption, lower recombination losses and pave a way for the fabrication of low dimensional device structures. [6]. In view of the potentiality and versatility of ZnS compared to other wide band gap semiconductors, we have adopted an inexpensive chemical method to prepare Al-doped ZnS powders. This method is a promising way to produce low resistive films over a large area due to the uniform distribution of dopant atoms in ZnS matrix rather than forming concentration gradient with a lower lattice distortion. ZnS:Al powders thus synthesized were used to grow the layers by close-spaced evaporation followed by the characterization of the films using different techniques. The results obtained in the evaluation of the physical behaviour of the as-grown ZnS:Al films are reported and discussed. 2. Experimental Al-doped ZnS films were prepared on glass substrates by close-spaced evaporation (CSE) technique with different dopant concentrations that vary in the range 0e10 at.%. ZnS:Al powders required for evaporation were synthesized in the laboratory by chemical precipitation method. In the preparation process, we did not use chloride-based precursors in order to avoid the formation of ZnCl2 complexes in the precipitate, which degrade the crystalline quality and electrical characteristics of the grown films. The films were grown under the optimized deposition conditions achieved in our earlier study to deposit undoped ZnS layers [7]. The substrate temperature was maintained at 300 C with the substrate fixed at a distance of w5 cm from the source. The rate of deposition and film ˚ /s and 300 nm, respectively. The thickness were kept as 30 A experimental arrangement of CSE technique was reported elsewhere [8]. ZnS:Al powder in the form of a pellet was placed in a molybdenum boat and covered with quartz wool and a perforated molybdenum sheet to avoid bumping of the material during evaporation. The system was then pumped
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down to a base pressure of 5 106 mbar. The substrates were sputter cleaned in glow discharge prior to evaporation in order to remove any impurities present on the substrates. The vacuum system was again pumped down to a pressure of 5 106 mbar. The rate of deposition and thickness of the experimental films were monitored using the quartz crystal thickness monitor placed just below the substrate holder. The deposition process and film thickness were controlled by a shutter placed between the source and substrate. The structural and morphological studies were carried out using Siefert X-ray diffractometer and Vecco atomic force microscope, respectively. The elemental composition of the layers was studied using VG Microtech ESCA2000 X-ray photoelectron spectrometer. The spectral transmittance of the films was recorded as a function of wavelength that varied in the range 300e1500 nm using Hitachi UVeVISeNIR spectrophotometer. The photoluminescence properties were studied using YVON fluorescence spectrophotometer. The electrical resistivity of the layers was measured using the two-probe method. Silver was used as electrodes for measuring the resistivity of the layers. The temperature dependence of electrical conductivity of the ZnS films was studied in the range 150e450 K. 3. Results and discussion All the Al-doped ZnS powders synthesized using chemical precipitation method showed the (111) plane as the dominant orientation corresponding to the cubic structure of ZnS. The other peaks observed at 47.61 and 56.37 were attributed to the (220) and (311) orientations of cubic ZnS. The evaluated crystallite size and lattice constant of the powder were found ˚ , respectively. Fig. 1(a) and (b) to be w4.8 nm and 4.774 A shows the typical X-ray diffractogram and EDAX spectrum of ZnS powder with an ‘Al’ concentration of 6 at.%, respectively. The elemental composition was found to be Zn ¼ 44.75 at.%, S ¼ 49.12 at.% and Al ¼ 6.13 at.%. The powder samples prepared for different Al-dopant concentrations were pelletized and evaporated on to glass substrates in order to prepare Al-doped ZnS films at predetermined conditions.
Fig. 1. (a) XRD and (b) EDAX spectra of the ZnS:Al powder.
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The visual observation of as-grown layers was pinhole free, uniform and pale yellow in colour. The as-deposited films were smooth and transparent. They had a distinct advantage that the adhesion of Al-doped film to the substrate was better when compared to the undoped films as well as those deposited by the direct evaporation of ZnS powder as was tested by the Scotch tape method. The thickness of the layers was approximately 0.3 mm. 3.1. Compositional analysis X-ray photoelectron spectroscopy (XPS) measurements were performed to know the dopant incorporation, chemical composition and state of the elements present in the layers. The XPS scans were recorded before as well as after the Arþ bombardment and no changes were observed in the binding energies and peak shapes due to the Arþ bombardment. Fig. 2 shows the typical XPS spectrum of ZnS layers grown at a dopant concentration of 6 at.%, recorded in the range 0e1200 eV. The XPS spectrum exhibited the binding energies corresponding to Zn 2p3/2, Zn 2p1/2 and S 2p3/2 at 1022 eV, 1046 eV and 162.5 eV, respectively. The observed binding energies are in good agreement with the reported data [9]. It has been observed that in the case of Al 2p3/2 centered at 74.02 eV, which originates from Al3þ rather than metallic Al (72.8 eV), the Al atoms dissolved in ZnS matrix substituted Zn sites successfully. No other compounds related to sulfates or organic compounds were observed. In order to verify the homogeneity of Al content in the layers, one of the optimized films prepared for an Al concentration of 6 at.% had been etched using Ar and the XPS signal was analyzed for successive etching times, which showed no significant change in the XPS signal strength as a function of etching time. This confirms the uniformity of dopant concentration across the layer thickness. There was a small peak in the XPS spectrum at 284.96 eV, which is an indication of carbon inclusion in the films as the evaporant was synthesized from the organic precursors. The binding energies of the constituent elements shifted slightly towards lower
Fig. 2. XPS spectrum of ZnS films deposited for an Al-doping concentration of 6 at.%.
energies and peak shapes of core levels remained unchanged after doping. This may be due to the formation of lattice distortion caused by Al-doping. 3.2. Structural analysis The crystallographic studies made using X-ray diffraction analysis clearly indicated the effect of doping concentration on the crystalline quality of the grown layers. The X-ray diffraction spectra of ZnS:Al films for different Al-doping concentrations in the range 0e10 at.% are shown in Fig. 3. The comparison of XRD patterns of as-grown undoped and Al-doped films did not show the essential difference in the growth orientations and other phases of films in the investigated doping concentration range 0e6 at.%. The (111), (220) and (311) reflections observed in the XRD spectra of the films grown in the entire Al-doping concentration correspond to the cubic structure of ZnS. El Hichou et al. [10] reported cubic and a combination of cubic and hexagonal phases, respectively, for Al- and Sn-doped ZnS films grown by spray pyrolysis technique. In the present study, the as-grown films showed purely cubic structure for all the dopant concentrations. Moreover, it is interesting to note that the crystallinity of the films was not seriously affected by the incorporation of Al in ZnS lattice where the intensity of the (111) peak increased with the increase of dopant concentration to 6 at.% while other orientations such as (220) and (311) could not be developed significantly. Above this critical value of dopant percentage, a small indication of Al2S3 phase could be observed that was confirmed by the origin of other reflections like (116), (300) and (119) that correspond to Al2S3 phase along with the poor crystallinity. This indicated the phase segregation at higher dopant concentration. The initial increase of crystallinity with doping concentration might be due to the incorporation of Al atoms in the Zn vacancies present in the samples. There were reports in the literature that the crystallinity of the host material could not be disturbed noticeably by the incorporation of foreign dopant atoms upto certain-doping level [11]. Similar increase of crystallinity with dopant concentration was also reported by Machado et al. [12] for In-doped ZnO films prepared by electrodeposition. This analysis shows that the presence of aluminum helps to improve the crystallinity of ZnS films. When a material is doped with a suitable dopant, generally at lower dopant concentrations, new X-ray diffraction peaks may not appear, instead a gradual shift in the angular positions of the XRD peaks, corresponding to the host material are observed. This shift in the diffraction angle is expected because of the changes that occur in the lattice parameters of the host lattice on the incorporation of dopant atoms. The position of (111) peak gradually shifted to higher diffraction angles with the increase of Al-doping content in the layers. However, the (111) peak of Al-doped ZnS film shifted back to lower diffraction angle at higher doping levels, above 6 at.%. This suggests that Al-doping might have caused some lattice disorder in ZnS films although the cubic structure was maintained. The variation of lattice constant ‘a’ as a function of aluminum concentration is
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Fig. 3. XRD spectra of Al-doped ZnS films deposited for different dopant levels.
˚ for shown in Fig. 4. The lattice constant was found to be 5.416 A ˚ for an Alundoped ZnS films, which decreased to 5.383 A ˚) doping concentration of 6 at.%. As the radius of Al3þ (0.54 A ˚ ) the substitution of Al3þ in is lower than that of Zn2þ (0.74 A Zn2þ lattice sites could decrease the lattice parameter. However, the lattice constant increased with further increase of doping level above 6 at.%, which could be due to the decrease of number of substitutional Al3þ ions. The Al atoms may be out diffused from Zn2þ sites at higher dopant concentrations (>6 at.%), resulting in the interstitial incorporation of Al3þ in the ZnS crystal lattice. This analysis shows that the solubility limit of Al in ZnS might be as high as 6 at.%.
The change of strain in the as-grown films as a function of Al-doping concentration in ZnS layers, as shown in Fig. 5, was evaluated using the relation [13] 3¼
a a0 100 a0
ð1Þ
where ‘a’ is the lattice constant of ZnS films and a0 is the unstrained bulk lattice parameter. The undoped films showed tensile strain that changed to compressive strain with the increase of Al-doping density upto 6 at.%. In general, dopant atoms can occupy substitutional or interstitial positions in the
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nucleation and coalescence phases of film growth. Goyal et al. [15] reported similar observations for indium doped zinc oxide films and explained the decrease of grain size at higher In-doping levels in terms of interstitial inclusion of dopant atoms in ZnO lattice, which results in poor crystallinity of the films. In polycrystalline films, the texture coefficient gives a measure of the preferred orientation compared to a randomly oriented sample. The value of Ci with dopant concentration was evaluated from the relation [4,13] Texture coefficient; Ci ¼
Fig. 4. Change of lattice constant with Al-doping density.
crystal lattice. The increase of compressive strain with the increase of doping levels might be due to the substitutional incorporation of dopant atoms whereas the increase of lattice constant at higher Al-doping levels (>6 at.%) might be due to the interstitial occupation of Al atoms. The lattice strain present in the as-grown films might be caused by several factors such as stoichiometric deviations, difference in the thermal expansion coefficients of the grown film and the substrate, etc. However, we thought that strain induced by doping might be dominant in the present case as all other deposition parameters were kept constant. The mean crystallite size, D, of the layers was calculated using Scherrer’s equation [14]. The crystallite size initially increased from 42 nm to 56 nm in the dopant concentration range, 0e6 at.% and decreased to 31 nm with further increase of the dopant concentration to 10 at.%. This might be due to the limited solubility of the dopant atoms in the host lattice. This implies that Al had a considerable effect on the
Fig. 5. Variation of lattice strain with Al-doping concentration.
ðIi =Ioi Þ PN ð1=NÞ i¼1 ðIi =Ioi Þ
where Ii is the intensity of a generic peak in the spectrum, Ioi is the intensity of a generic peak for a completely random sample (JCPDS) and N is the number of reflections considered in the analysis. The variation of crystallite size and texture coefficient (Ci) with Al-doping concentration is shown in Fig. 6. The value of Ci is peaked at the dopant concentration of 6 at.%, which shows better crystallinity of the grown samples. 3.3. Morphological analysis Fig. 7 shows the AFM images of the films grown for three typical Al-doping densities of 0 at.%, 6 at.% and 10 at.%. The photographs indicated that the films were smooth and continuous without any pinholes/cracks. The surface morphology of the Al-doped samples is found to be similar to that of undoped one. The surface of the layer is covered with equal sized grains that are uniformly distributed. The grains on the film surface were more compact and have dense structure. It was also found that there were variations in the grain size with the dopant concentration. The grain size increases with the increase of Al concentration followed upto 6 at.% by a decrease at higher dopant concentration although the films were deposited using the same deposition conditions. The grain size of the films had a maximum at an Al-doping
Fig. 6. The variation of crystallite size and texture coefficient with Al-dopant level.
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electron beam evaporation. The grain size evaluated from AFM measurements closely matched with the value calculated from the XRD data. The evaluated average surface roughness for the films was nearly constant, w1.02 nm upto Al concentration of 6 at.% and its value increased to 1.57 nm in spite of the decrease of grain size beyond this doping value. 3.4. Optical studies Fig. 8 shows the optical transmittance spectrum of ZnS:Al films with Al-doping percentage. Doping of ZnS films with ‘Al’ shows significant changes in the transmittance characteristics in the visible region. The average transmittance of pure ZnS films in the visible region is about 85% whereas ZnS films doped with Al concentration of 6 at.% showed a transmittance of 91% that decreased to 65% with further increase of aluminum doping level to 10 at.%. The increase of optical transmittance could be due to the better crystallinity and grain size whereas the decrease of transmittance at higher Al-doping concentrations (>6 at.%) could be attributed to the lower grain size, formation of additional phases and higher defect density that resulted in poor crystallinity of the layers [17]. Moreover, the sharpness of the absorption edge increased with doping concentration up to 6 at.% and decreased thereafter. This shows that the structural disorder seems to be increased at higher Al-doping concentrations (>6 at.%), which is also supported by the structural analysis of the layers in the present study. The optical band gap was determined from the plots of (ahn)2 vs photon energy, hn by extrapolating the linear region of the plot onto the energy axis. All the films grown for different Al concentrations showed the direct band gap nature and the evaluated energy band gap with dopant concentration is shown in Table 1. It varied from 3.79 eV to 3.72 eV when the doping concentration varied from 0 at.% to 6 at.%.
Fig. 7. AFM pictures of Al-doped ZnS films grown with different Al-doping densities.
concentration of 6 at.% and decreased beyond this dopant level. The variation of grain size with doping concentration implies that there might be some important effect of Al on the nucleation and coalescence of islands during the process of film growth. Megahid et al. [16] observed a similar behaviour of grain size variation in In-doped CdS films deposited by
Fig. 8. Optical transmittance vs wavelength spectra of ZnS:Al films.
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Table 1 The variation of energy band gap and activation energy with Al-doping concentration Al-doping concentration (at.%)
Energy band gap (eV)
Activation energy (meV) 150e310 K
310e450 K
0 2 4 6 8 10
3.79 3.79 3.75 3.72 3.65 3.54
21 19 15 11 14 19
240 209 157 113 138 159
According to Pintle et al. [18], the doping of thin films is effective when the maximum variation of band gap is lower than 2%. In the present study also, the variation in band gap with Al-doping is <2% and hence it could be inferred that the effective doping of Al occurred in ZnS upto 6 at.%. Generally it is observed that the energy band gap decreases linearly by doping materials, which is due to the formation of shallow levels or impurity levels in the band gap region. Hankare et al. [19] reported a similar variation of band gap for Pb-doped CdSe films with the dopant concentration. In this study, the effect of doping on the energy band gap is marginal upto the dopant concentration of 6 at.%. This marginal variation of band gap could be explained on the basis of the fact that the structural defects such as voids and dangling bonds, which caused the generation of undesirable localized states in the band gap could be decreased with the introduction of Al into the ZnS lattice and hence the decrease of structural disorder that reduces band tailing into the band gap region [20]. Therefore, the linear decrease of band gap with the dopant concentration could be compensated by improvement in the crystalline quality of the layers which was also supported by the XRD analysis. 3.5. Photoluminescence studies Fig. 9 shows the room temperature photoluminescence spectra of Al-doped ZnS layers when excited with the radiation of wavelength 325 nm. The spectra clearly indicated the effect of Al incorporation on the photoluminescence characteristics of ZnS films. The PL spectra of Al-doped layers are much different from that of undoped ZnS layers and the luminescence intensity increased with the increase of Al concentration although the change in the peak position is marginal. The de-convolution of pure ZnS films gives two peaks at 391 nm and 436 nm. The luminescence peak located around 391 nm is associated with the zinc vacancies [21] while the other emission at 436 nm is associated with zinc interstitials [22]. With the incorporation of Al into ZnS structure, an additional peak could also be observed at 470 nm and its intensity increased with the increase of dopant concentration upto 6 at.%. The observed luminescence at 470 nm could be ascribed to the self-activated luminescence that resulted from the donoreacceptor pair (DAP) transition [23]. The doped Al impurities can form both the isolated Al donors as well as compensated acceptors, the density of which depends on the level of the dopant. The donor
Fig. 9. Photoluminescence spectra of ZnS:Al films.
states could be produced by isolated Al donors whereas the acceptor states could be generated in the form of Zn vacancyeAl complex, (VZnZnAl). Therefore, the luminescence and electron transport properties are largely controlled by the compensation of Al donors. The density of these states increased with the increase of Al-doping level, which increases the probability of recombination of DAP. This led to an enhanced luminescence intensity at higher doping densities (w6 at.%). The decrease of luminescence intensity at larger doping levels (>6 at.%) could be due to the formation of Al donor complex that induces the structural disorder caused by the increase of non-radiative recombination. Similar decrease of luminescence intensity at higher doping level was reported by Oh et al. for Al-doped ZnSe films deposited by molecular beam epitaxy [24]. A broad luminescence spectrum with low intensity was observed for the films deposited at a dopant concentration of 10 at.%. This analysis shows that the compensation of Al donors is dominant at higher doping levels. 3.6. Electrical studies The ZnS films in their non-stoichiometric form are known to have n-type conductivity in which the carriers are generated from interstitial zinc or sulfur vacancies. The electrical resistivity of ZnS films as a function of Al-dopant concentration is shown in Fig. 10. It could be observed that the resistivity of the grown films strongly depends on the Al-dopant level in ZnS films. It decreased from 2.7 105 U cm to 24 U cm with the increase of Al concentration from 0 at.% to 6 at.% and again increased to 6.6 103 U cm with further increase of Al concentration in ZnS layers upto 10 at.%. The present trend of resistivity can be explained in terms of solubility limit of dopant atoms in the host lattice. For Al-doping concentration lower than 6 at.%, every Al atom that substitutes Zn atom in ZnS lattice can contribute one extra free carrier to the conduction band as the valence difference between
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Fig. 11. ln s vs T1 plots for ZnS:Al films. Fig. 10. Change of electrical resistivity of ZnS films with Al-dopant concentration.
Al3þ and Zn2þ is 1. As the doping level is increased, more Aldopant atoms occupy Zn lattice sites that results in more charge carriers. However, after certain level of doping, the dopant atoms cannot occupy the zinc sites due to their limited solubility. Since the radius of Al atom is less than the radius of Zn atom, excess Al atoms can occupy the interstitial position in ZnS lattice [25]. Alternatively, it can be precipitated at the grain boundaries and form Al2S3 phase as observed from the XRD analysis. This indicated that the maximum solubility of Al atoms in ZnS lattice could reach approximately 6 at.%. Hence, the increase of resistivity at higher doping levels might be due to impurity scattering or grain boundary potential effects. Alam and Cameron [26] also reported similar results in Al-doped ZnO films deposited by solegel process. The formation (AlZnVZn) acceptor complexes that induce the self-compensation of the shallow donor impurity levels may be also responsible for the increase of resistivity as reported by Prokesch et al. [27], which are identified in Al-doped ZnSe films. The probability of formation of (AlZnVZn) complexes increases with the increase of dopant concentration [28]. Therefore, the increase of resistivity was observed at higher dopant concentrations in the present films. The temperature dependent electrical conductivity was also studied in the range 150e450 K for all the samples and the activation energies were evaluated. Fig. 11 shows the Arrhenius plots of ZnS:Al films deposited in the dopant concentration range 0e10 at.%. It can be observed from these plots that the variation of ln s with temperature showed two slopes, where the change occurred around the annealing temperature of 310 K. The evaluated activation energies in the two regions are shown in Table 1. It could be observed that the activation energy decreased with doping concentration upto 6 at.%. The decrease of activation energy could be due to the decrease of energy band gap and effective doping of ZnS. Further, the increase of crystallinity could also contribute to the observed decrease of activation energy. The activation energy was found
to increase again with further increase of dopant concentration that could be due to the formation of additional phases, which caused the increase of the grain boundary potential and poor crystallinity of the grown layers. 4. Conclusions Al-doped ZnS films were successfully prepared by closespaced evaporation of the powders synthesized by chemical precipitation method at different Al-doping concentrations. The chemical composition of the layers analyzed by using XPS measurements indicated that the dopant was distributed uniformly across the layer thickness. The layers showed predominantly (111) orientation corresponding to cubic structure of ZnS. However, an additional Al2S3 phase was observed at higher Al-dopant concentrations. The surface morphological analysis revealed that the dopant atoms played some role in the modification of surface roughness and the layers prepared at the dopant concentration of 6 at.% showed an average roughness of 1.02 nm. These layers showed a higher optical transmittance of 91% with an energy band gap of 3.73 eV. The as-grown layers showed closely separated photoluminescence peaks that consisted of Zn and S vacancies and their corresponding interstitials and also the Al-dopant. The luminescence intensity also decreased at higher doping levels, >6 at.%. The layers prepared at the dopant concentration of 6 at.% showed a lower electrical resistivity of 24 U cm and the resistivity was found to decrease beyond this dopant concentration. These layers showed two different regions in the Arrhenius plots and the layers prepared at an optimized dopant level showed lower activation energies of 11 meV and 113 meV in the low and high temperature regions, respectively. References [1] T. Miyawaki, M. Ichimura, Mater. Lett. 61 (2007) 4683. [2] L.C. Olsen, R.C. Bohara, D.L. Barton, Appl. Phys. Lett. 34 (1979) 528.
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