Influence of the grain boundary barrier height on the electrical properties of Gallium doped ZnO thin films

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Applied Surface Science

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Influence of the grain boundary barrier height on the electrical properties of

Gallium doped ZnO thin films

Chang-Feng Yu

_{, Sy-Hann Chen}

_{, Shih-Jye Sun}

_{, Hsiung Chou}

b_{Department of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan}

c_{Department of Physics and Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan}

a r t i c l e i n f o

Pulsed laser deposition Gallium doped zinc oxide Quantum size effect Barrier height

a b s t r a c t

The pulsed laser deposition (PLD) technique is used to deposit Gallium doped zinc oxide (GZO) thin films on glass substrates at 250 with different Gallium (Ga) doping concentration of 0, 1.0, 3.0 and 5.0%. The influence of Ga doping concentration on structure, chemical atomic compositions, electrical and optical properties was investigated by XRD, XPS, Hall measurement and UV spectrophotometer, respectively. The relationship between electrical properties and Ga doping concentration was clarified by analyzing the chemical element compositions and the chemical states on the GZO films. It is found that the carrier concentrations and oxygen vacancies in the GZO films increase with increasing Ga doping concentration. The lowest resistivity (3.63× 10−4_{ cm) and barrier height of grain boundaries (14 mV) were obtained}

with 3% Ga doping. In particular, we suppose the band gap of 5% Ga doping sample larger than that of 3% Ga doping sample is due to the quantum size effect from the amorphous structure rather than Moss–Burstein shift.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Transparent conducting oxides (TCO) with optical transmit-tance exceeding 80% in the visible region (550 nm) and resistivity less than 10−4 cm have been widely used in a variety of appli-cations. Most of the previous research on TCOs has been focused on indium-doped tin oxide (ITO)[1–2]. ITO thin films are often applied on opto-electrical devices due to their excellent conductiv-ity and transparency. However, the ITO films are expensive and not suitable for plasma deposition processes. Thus, the development of alternative TCO materials is necessary to resolve this serious prob-lem. Zinc oxide (ZnO) films have been investigated in recent years as TCOs because of their good electrical and optical properties in combination with their large band gap of 3.3 eV[3], abundance in nature, and lack of toxicity[4]. The electrical behavior of ZnO thin films could be significantly improved by replacing Zn atoms with higher valence elements, such as In, Al and Ga[5–10]. From many reports which have reported about the doping effect of impuri-ties on ZnO, they were widely demonstrated that Al and Ga were the best dopants for transparent conductive ZnO films. The Gal-lium doped zinc oxide (GZO) films have more advantages compared with alumina doped zinc oxide (AZO) films. The radius of Ga3+ (0.062 nm) is closer to that of Zn2+_{(0.074 nm) than that of Al}3+ (0.053 nm)[11–13]. Gallium is a good dopant because it is less

reac-∗ Corresponding author.

E-mail address:cfyu@mail.ncyu.edu.tw(C.-F. Yu).

tive with oxygen[14]and has less moisture resistance[10]than other dopants. For those reason, many researchers gradually focus on the study of GZO thin films.

Various techniques have been used to deposit ZnO-based films on different substrates, including metal-organic chemical vapor deposition (MOCVD) [15], molecular beam epitaxy (MBE) [16], pulsed laser deposition (PLD)[17], and spray pyrolysis deposition (SPD)[18]. Compared to other deposition methods, PLD is charac-terized by several advantages, such as low deposited temperature, good adhesion on substrates, and the easy deposition of alloys and compounds of materials with different vapor pressures [17,19]. GZO thin films which were deposited by PLD have shown better resistivity near ∼10−4_{ cm[20–22]. In this paper, high quality} GZO transparent conductive films were prepared by 355 nm PLD with various Ga doping concentrations. The dependence of the structural, electrical and optical properties of the GZO films on Ga doping concentration was investigated. The X-ray photoelectron spectroscopy (XPS) of oxygen vacancies have been used to probe the carrier concentration. The barrier height of grain boundaries at GZO thin films, with different percentages of Ga doping concen-tration, could be calculated from the temperature dependence on electrical mobility.

2. Experiment

In this article a reliable method was used to deposit the thin films of undoped and GZO thin films on glass substrates by PLD. The 1.0 in. diameter× 0.125 inch thick targets of the ablation PLD, with 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.

Fig. 1. XRD patterns of GZO thin films as a function of the Ga doping concentration.

0%, 1.0%, 3.0% and 5.0% Ga doping concentration, were fabricated by the combustion synthesis reaction technique. The pure (99.99%) ZnO and (99.999%) Ga2O3 powders were mixed with polyvinyl alcohol binder and water. The mixture was stirred, crushed into powder, dye palletized, and sintered at 1300◦C for 5 h. Undoped and various doped GZO thin films were evaporated in a PLD vacuum chamber with a base pressure of 1× 10−6_{Torr. Oxygen (99.99%)} gas was introduced in the chamber as the reactive gas and the working pressure was kept at 10 mTorr during deposition. The glass substrates were cut into standard sizes of 2.5 cm× 6 cm. All substrates were ultrasonically cleaned in acetone and dried in an oven until it was loaded into the deposition chamber. A stainless steel substrate holder was capable of being heated up to 400◦C. The deposition temperature was maintained at 250◦C. An Nd:YAG pulsed laser with a visible wavelength of 355 nm, pulse duration of about 7 ns, and 40.0 mJ/cm2_{energy density was focused on the} target to deposit the GZO thin films. The target to substrate dis-tance was kept at 4.5 cm. The crystalline structural analysis of GZO films was carried out by using X-ray diffraction (XRD). A UV-VIS spectrophotometer (Agilent 8453) was used to measure the opti-cal transmission spectra for wavelengths from 300 to 800 nm. The chemical composition and bonding were investigated by X-ray photoelectron spectroscopy (XPS). The electrical properties were measured in the four-point probe Van der Pauw configuration.

3. Results and discussions

Fig. 1shows the XRD spectra of the as-deposited GZO films. Peaks corresponding to crystallographic planes (0 0 2) and (1 0 3) are presented. It can be found fromFig. 1that various Ga-doping concentrations have changed the growth behaviors of GZO crys-talline, which now have preferred growth orientations along the (0 0 2) and (1 0 3) directions instead of (1 0 0), (0 0 2) and (1 0 1) directions. With the increase in Ga-doping concentration, the inten-sity of (0 0 2) and (1 0 3) reflection peaks also increased. When the Ga-doping concentration increased to 5.0%, the intensity of (0 0 2) and (1 0 3) reflection peaks decreased again. This implied that the Ga-doping concentration has a great influence on the crystalline GZO films. The greatest crystal quality is obtained for the Ga con-centration of 3% GZO thin films. XRD indicates that 3.0% doped GZO thin film is superior to the other GZO thin films with various Ga doped concentrations. This indicates that the crystal quality of the GZO thin film would be enhanced as the Ga doping concentration

Ga doping concentration (%)

Grain size (nm)

Fig. 2. Grain size along (0 0 2) plane plotted against Ga doping concentrations.

exceeds a certain limit (∼3.0%). It is well known that the conduc-tivity of GZO thin films increases as the crystal quality is enhanced. The highest peak appearing at the 2 of ∼34◦is due to ZnO (0 0 2) reflection. The average grain size d of GZO thin films was calcu-lated from the full-width at half-maximum (FWHM) of the XRD peaks using Scherrer’s formula[23]:

d = k ˇ cos 

where k is a constant of 0.94, ˇ is the FWHM,  is the wavelength of the X-ray (Cu K␣) and 2 is the location of the diffraction peak. The grain sizes were determined to be 3.287, 3.554, 3.271 and 3.739 nm from the FWHM of (0 0 2) peaks, for the films deposited with 0%, 1.0%, 3.0% and 5.0% dopant, respectively. The average grain size as a function of Ga doping is shown inFig. 2. In particular, the 5.0% Ga doping sample has the largest grain size in comparison with other doping samples. This larger grain size doped with 5% Ga has the largest mobility as shown inFig. 6. The grain sizes of GZO thin films change little with various Ga doping concentrations.

The effects of Gallium doping concentration on the optical prop-erties of GZO thin film growth and were studied. The optical transmission spectra for the wavelength range between 300 and 800 nm of the deposited GZO thin films with various Ga doping concentrations are shown inFig. 3. The average transmission spec-tra of GZO thin films all exceeded 80% when the GZO thin films were deposited with various Ga doping concentrations. InFig. 3, it is found that the transmission edges around 355–380 nm shifted to shorter wavelength as the Ga-doping concentration increases. The widening of the band gaps with the increase of the Ga doping con-centration might be due to the increase of the carrier density with the Moss–Burstein shift[24]except 5.0% doping. The Moss-Burstein effect was present in ZnO-based thin films and it was observed

300 400 500 600 700 800 0 20 40 60 80 100 0% 1% 3% 5% Wavelength(nm) Transmission(%)

Fig. 3. Transmission spectra for the GZO films deposited with various Ga doping

X Data

X Data

Binding Energy (eV)

1010 1020 1030 1040 1050 1060

Intensity(a. u.)

Zn 2p

0%

1%

3%

5%

Fig. 4. High resolution Zn 2p3/2XPS spectra of GZO thin films deposited with various

Ga doping concentrations.

that the optical band gap increased with increasing doping con-centration[25,26].Fig. 3exhibits an extra shoulder at short wave length region only for the 5.0% Ga doping sample (i.e. the band gap increase) in comparison with other doping samples. It implies that some amorphous structures present at 5% Ga doping sample. The optical band gap increase is possible due to the quantum size effect for the amorphous structure. The higher Ga doping maybe induce amorphous structures, in which vast oxygen vacancies could be produced in GZO thin films. The results of XPS (Fig. 5) show the oxygen vacancies increase with Ga doping.

Figs. 4 and 5show high-resolution XPS spectra of Zn 2p3/2(Fig. 4) and O 1s (Fig. 5) obtained from GZO films that grown with 0%, 1.0%, 3.0% and 5.0%, respectively. The change of chemical state in the GZO films grown with different doping concentrations was stud-ied.Fig. 4shows the core line of Zn 2p3/2with high symmetry. The binding energy of Zn 2p3/2obtained from the GZO films is observed at 1022.40± 0.10 eV[27]regardless of various doping concentra-tions. No metallic Zn peak with binding energy of 1021.50 eV[28] is observed. This result confirms that the largest majority of Zn atoms for all GZO films remain in the same formula valence states of Zn 2p3/2 within an oxygen deficient ZnO1−xmatrix. The bind-ing energy of the O1 s peak was calibrated by takbind-ing the C 1s peak (285 eV) as reference. Three peaks of the O 1s core level spectra can be consistently fitted by three nearly Gaussian functions as shown inFig. 5. The lower binding energy component (i.e. the main peak) OIlocated at 530.15± 0.15 eV is corresponded to the O–Zn bond [29]. The higher binding energy component OII(i.e. the shoulder peak) (531.25± 0.15 eV) is associated with O2−_{ions in the oxygen} deficient regions within the matrix of ZnO1−x[30](also known as VO-like bonding). The highest energy peak OIII(532.40± 0.15 eV) implies the presence of hydrated oxides species on the film surface [27]. It shows that the intensity of the OIIhigher binding energy component for GZO thin film deposited with 3.0% doping

concen-524 526 528 530 532 534

Binding Energy (eV)

Intensity( a. u. )

Fig. 5. The O 1s core level spectra with curve-fitting results obtained from GZO thin

films deposited with various Ga doping concentrations.

tration is higher than that of GZO thin film deposited with other doping concentrations (0% and 1.0%). On the other hand,Fig. 5 reveals that the relative intensity of the OIIpeak increases with increasing Ga doping concentration. It means that the concentra-tion of the oxygen vacancies is increased. Since oxygen vacancy can offer two electrons by doped with Gallium[31], the increase of Ga doping concentration could lead to an increase of carrier concentra-tion and oxygen vacancies. This result consists with the following electrical properties (Fig. 6). It shows that the increase of carrier concentration leds to the resistivity decreasing with increasing Ga doping concentration except 5% doping. This indicates that the GZO film grown with 3.0% doping concentration may has higher oxygen vacancy rather than that grown with 0% or 1.0% doping concen-tration, suggesting high conductivity (i.e. low resistivity). Thus, the change in the intensity of higher binding energy component OIImay be related to the variation of the concentration of stoichiometric oxygen in ZnO. The oxygen deficiency is increased with increas-ing carrier concentration. It is known that more oxygen-deficient film commonly exhibits a lower resistivity[31,32]. However, the OIIintensity of 5.0% Ga doping sample is higher than that of 3%

Gallium doping concentration (%)

0 1 3 5 3.0x1020 6.0x1020 9.0x1020 1.2x1021 1.5x1021 1.8x1021 2.1x1021 n Carrier concentration(cm -3 ) 0 1 3 5 2 4 6 8 10 12 μ Mobility(cm 2 Vs) 0 1 3 5 10-4 2.1x10-3 4.1x10-3 6.1x10-3 ρ Resistivity( Ω -cm)

ture would form and embed in GZO grains (as shown inFig. 3). It could increase the barrier height between the GZO grain bound-aries and reduce the conductivity of GZO poly-crystal thin film with 5.0% doping concentration.The carrier concentrations, carrier mobilities, and electric resistivities of the GZO thin films with dif-ferent Ga doping concentrations are plotted as functions of the Gallium concentration inFig. 6. When the Ga doping concentration increases, more carriers were released as more Gallium could be doped in the host lattice of ZnO crystal. The resistivity of the GZO thin film decreases as the Gallium concentration increases from 0% to 3.0%, but it increases as the Gallium concentration increases to 5.0%. Here it should be noted that the resistivity of 3.0% dop-ing sample is lower than that of 5.0% dopdop-ing sample. This result suggests that we can reduce the resistivity of the GZO thin film by adding a proper amount of Ga doping concentrations. The opti-mum Ga doping concentration is about 3.0% in the concentration range that was investigated in this study. According to the Hall measurement result inFig. 6, the electron concentration of 3.0% doping sample (1.89× 1021_cm−3_{) is higher than that of the other} various doping samples although the electron mobility of 3.0% dop-ing sample (9.12 cm2_{/V s) is lower than that of 5.0% doping sample} (9.88 cm2_{/V s). The best GZO thin film obtained with 3.0% doping} concentration has the lowest resistivity of 3.63× 10−4_{ cm. The} lower resistivity of the film deposited with 3.0% Ga-doping concen-tration is in good agreement with many other reports. They showed the optimum Ga-doping concentration of the best obtained electri-cal properties was in the range of 2.5–3%[33–35].Fig. 1shows that the 3.0% doping sample was deposited and formed the most com-pact structure and better crystalline. Gallium has the opportunity to replace Zn at regular places of the ZnO lattice and produces more oxygen vacancies, which caused the carrier concentration of the GZO films to rise as shown inFig. 6. The conductivity of the ZnO films should rise along with the increase of Ga doping concentra-tion. When the doping concentration is higher than the optimum value (3.0%), the small amount of amorphous structure would form in GZO thin films as shown inFig. 3. Amorphous structure could increase the barrier height between the GZO grain boundaries. The conductivity of GZO poly-crystal thin film with 5.0% doping con-centration would be reduced with higher barrier height. Below, the variation of barrier height of the GZO grain boundary with differ-ent Ga doping concdiffer-entrations will be extracted. It will verify that the barrier height of GZO grain boundary could be enhanced by amorphous structure.

The relative importance of the scattering mechanism is depen-dent on film quality and the carrier concentration. For intrinsic material with low carrier concentrations (<5× 1019_cm−3_{), the} observed mobility is limited by scattering at grain bound-aries. Conversely, in doped zinc oxide at high electron density (>5× 1020_cm−3_{) ionized impurity scattering dominates [36].} Impurity doping will increase the impurity density, but it also diminishes the activation energy for grain boundary scattering by raising the Fermi level. Temperature dependent Hall mea-surements were used to identify the mechanisms of the electron transport in the GZO films. Seto[37]derived a model for mobil-ity in polycrystalline semiconductor assuming that the electrical transport properties of the film are governed by carrier trapping at the grain boundary. The current flow between grains occurs by thermionic emission. This model was modified slightly to incorpo-rate Fermi–Dirac statistics, which is appropriate for degeneincorpo-rately doped TCO films[38]. The temperature dependence of the electron mobility gin the polycrystalline ZnO films can be described by

g= B T exp





Fig. 7. Relation between ln(gT) and 1/T for GZO thin films deposited with various

Ga doping concentrations. Ga doping concentration(%) 0 1 3 5 Barrier height(V) 0.012 0.014 0.016 0.018 0.020 0.022 Barrier height 0 2.0x10-3 4.0x10-3 6.0x10-3 8.0x10-3 10-2 1.2x10-2 Resistivity Resistivity( Ω cm)

Fig. 8. Variations in the barrier height and resistivity for GZO thin films deposited

with various Ga doping concentrations.

where q is the magnitude of the charge on an electron, a repre-sents the barrier height of the electrons at the grain boundaries and B is a constant related to the grain size. The barrier height acan be obtained from the slope of the ln(gT) versus 1/T line. A typical plot of ln(gT) versus 1/T of GZO film deposited with various Ga doping concentrations is shown inFig. 7. The plots of ln(gT) versus 1/T of the films deposited with various Ga doping concentrations were linear indicating that the ionized impurity scattering of the charge carriers is the dominant mechanism in these GZO films. The barrier height awas calculated from the slope of the above plots. The bar-rier height decreased from 0.048 V to 0.030 V with the increase in the doping concentration from 0% to 3.0% and increased to 0.056 V for the 5.0% doping concentration as shown inFig. 8. The decrease of the barrier height up to the 3.0% doping concentration is due to the increase in the doping concentration and the decrease in scattering of the charge carriers at the grain boundaries. The resistivity of GZO thin films would be reduced with the decrease of the barrier height. However, the increase of barrier height of the 5.0% doping sample is higher than that of 3.0% doping sample. It could be attributed to the forming of the amorphous structure in GZO thin films. It could cause the increase of scattering effect of the charge carriers at the grain boundaries and the increase of resistivity of poly-crystal GZO thin film.

4. Conclusions

In conclusion, GZO thin films were deposited on glass substrates at 250◦C using a pulsed laser deposition with the various Ga dop-ing concentrations. The effect of Ga dopdop-ing concentration on the GZO thin film structure, crystallinity, optical and electrical proper-ties was analyzed in this paper. It was found that the increase of Ga

tration and led to a low conductivity. The analysis of XPS indicated that the GZO films were oxygen deficient. The carrier concentra-tion and oxygen vacancies in the GZO thin films increased with an increase of Ga doping. Among the GZO thin films, the 3.0% Ga doped thin film exhibited the lowest resistivity as low as 3.63× 10−4_{ cm.} The temperature dependence of Hall mobility showed that the ion-ized impurity scattering dominates in these GZO thin films. The barrier heights of grain boundaries have calculated by using a plot of ln(gT) against (1/T). The lower barrier height would cause the lower resistivity in the GZO thin films.

Acknowledgement

This work was supported by the National Science Council in Taiwan through Grant No. NSC 99-2112-M-415-002-.

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