The electrical properties of low pressure chemical vapor deposition Ga doped ZnO thin films depending on chemical bonding configuration

Applied Surface Science 297 (2014) 125–129

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The electrical properties of low pressure chemical vapor deposition Ga doped ZnO thin films depending on chemical bonding configuration Hanearl Jung a , Doyoung Kim b , Hyungjun Kim a,∗ a b

School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea School of Electrical and Electronic Engineering, Ulsan College, 57 Daehak-ro, Nam-gu, Ulsan 680-749, Republic of Korea

a r t i c l e

i n f o

Article history: Received 12 July 2013 Received in revised form 6 December 2013 Accepted 16 January 2014 Available online 25 January 2014 Keywords: Ga doped ZnO Electrical properties Chemical bonding configuration

a b s t r a c t The electrical and chemical properties of low pressure chemical vapor deposition (LP-CVD) Ga doped ZnO (ZnO:Ga) films were systematically investigated using Hall measurement and X-ray photoemission spectroscopy (XPS). Diethylzinc (DEZ) and O2 gas were used as precursor and reactant gas, respectively, and trimethyl gallium (TMGa) was used as a Ga doping source. Initially, the electrical properties of undoped LP-CVD ZnO films depending on the partial pressure of DEZ and O2 ratio were investigated using Xray diffraction (XRD) by changing partial pressure of DEZ from 40 to 140 mTorr and that of O2 from 40 to 80 mTorr. The resistivity was reduced by Ga doping from 7.24 × 10−3  cm for undoped ZnO to 2.05 × 10−3  cm for Ga doped ZnO at the TMG pressure of 8 mTorr. The change of electric properties of Ga doped ZnO with varying the amount of Ga dopants was systematically discussed based on the structural crystallinity and chemical bonding configuration, analyzed by XRD and XPS, respectively. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Undoped ZnO thin films usually show n-type conductivity due to intrinsic defects such as oxygen vacancies and zinc interstitials. To improve the electrical conductivity of ZnO, doping with group III elements such as Al [1], In [2], and Ga [3] has been intensively investigated. Especially, Ga is predicted to be suitable for the ntype dopant for ZnO compared to other dopants such as Al (Al O: ˚ [5] due to the close match in covalent ˚ [4] and B (B O: 1.55 A) 1.82 A) ˚ and Zn O (1.97 A), ˚ which bonding length between Ga O (1.92 A) causes little lattice deformation even at high Ga concentration [6,7]. There have been reports on ZnO:Ga thin film deposition using various deposition techniques including sputtering [3], pulsed laser deposition (PLD) [8], molecular beam epitaxy (MBE) [9], sol–gel method [10], spray pyrolysis [11] and chemical vapor deposition (CVD) [12,13]. As for Ga doping precursor for CVD, triethyl gallium (TEG) [13–15], triisopropyl gallium (TIPGa) [16], trimethyl gallium (TMGa) [17] and mixtures [18] were reported. Especially, TEG and TMGa have been familiar precursors for GaAs deposition [19–21]. Although there have been some reports on plasma-enhanced [17,22] or thermal [23] CVD of Ga doped ZnO (ZnO:Ga) using TMGa,

∗ Corresponding author. Tel.: +82 2 2123 5773; fax: +82 2 313 2879. E-mail address: [email protected] (H. Kim). 0169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2014.01.096

reports on a systematical analysis of chemical bonding configurations depending on the amount of Ga dopant in ZnO:Ga and the analysis of relation between electrical properties and chemical bonding configuration of atoms are rare. In this work, we investigated the electrical properties of low pressure chemical vapor deposition (LP-CVD) Ga doped ZnO using DEZ as a Zn source and TMGa as a Ga source, respectively. The film properties of LP-CVD ZnO:Ga including structural crystallinity, electrical properties depending on partial pressures of DEZ and TMGa as key growth parameters were characterized by various analysis techniques. Especially, the electrical properties and bonding characteristics of ZnO:Ga related to the doping concentration of Ga were analyzed using Hall measurement and X-ray photoemission spectroscopy (XPS).

2. Experiment and data analysis A homemade LP-CVD system was used to deposit ZnO:Ga thin films on glass substrates. The main body is a quartz tube surrounded by heating elements and the sample-holder is located inside of the tube. N2 and O2 flows were controlled by mass flow controllers and the pressure inside of the tube was measured by a pirani gauge. The 5 sccm of N2 was used as the carrier gas for DEZ and TMGa, and each partial pressure was controlled using needle valves. The bubbler temperature containing DEZ was cooled down to 10 ◦ C to maintain proper vapor pressure, while the bubbler temperature

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Fig. 2. Resistivity, electron carrier concentration, and mobility of ZnO:Ga films as a function of the PTMGa at PDEZ = 90 mTorr and PO2 = 60 mTorr on glass substrate, Tsubstrate = 400 ◦ C.

Fig. 1. Resistivity, electron carrier concentration, and mobility of ZnO films (a) as a function of PDEZ at PO2 = 60 mTorr and (b) as a function of PO2 at PDEZ = 90 mTorr on glass substrate, Tsubstrate = 400 ◦ C.

for TMGa was maintained at room temperature. The growth temperature was constant at 400 ◦ C and the growth time was 10 min, resulting in a thickness of about 250–300 nm depending on other growth parameters such as precursor/reactant ratio. ZnO:Ga thin films were deposited with varying DEZ partial pressure (PDEZ , difference between based pressure and exposure pressure of DEZ with carrier gas) from 40 to 140 mTorr, the O2 partial pressure (PO2 , difference between based pressure and exposure pressure of O2 gas) from 40 to 80 mTorr, and the TMGa partial pressure (PTMGa , difference between based pressure and exposure pressure of TMGa with carrier gas) from 0 to 20 mTorr. The thickness of ZnO films was measured by Ellipsometry. Glass substrates were cleaned by dipping in acetone, isopropyl alcohol, and deionized water for 5 min using an ultrasonic bath, respectively, and then dried with nitrogen gas. The crystalline structure of ZnO:Ga films was investigated by X-ray diffraction (XRD) for a 2 range 10–80◦ . The resistivity, electron carrier concentration and Hall mobility of films were measured at room temperature by Hall measurement system. The chemical bonding configuration of films was performed by XPS. 3. Results and discussions Initially, the electrical properties of undoped ZnO thin films with changing PDEZ and PO2 were studied by Hall measurement system. Fig. 1 shows the resistivity, electron carrier concentration and Hall mobility of undoped CVD ZnO thin films with (a) varying DEZ partial pressure at constant PO2 of 60 mTorr and (b) varying O2 partial pressure at constant PDEZ of 90 mTorr, respectively. Fig. 1(a) shows that the electron carrier concentration of undoped ZnO films increases initially with increasing PDEZ and almost is saturated at around

5.44 × 1019 cm−3 at PDEZ of 90 mTorr. The Hall mobility increases with PDEZ and reaches a maximum value of 14.7 cm2 /V s at PDEZ of at 90 mTorr, then decreases with further increase in PDEZ . For the PO2 case shown in Fig. 1(b), the resistivity slightly decreases down to 7.24 × 10−3  cm until PO2 of 60 mTorr, and then sharply increases at PO2 > 60 mTorr. The electron carrier concentration continuously decreases with increasing PO2 . Thus, the lowest resistivity of LPCVD undoped ZnO (about 7.24 × 10−3  cm) is obtained at PDEZ of 90 mTorr and PO2 of 60 mTorr. Then, Ga doping in ZnO by using TMGa as a doping source was investigated. Fig. 2 shows the resistivity, electron carrier concentration and Hall mobility of LP-CVD ZnO:Ga thin films as a function of PTMGa . We set PDEZ and PO2 to 90 and 60 mTorr, at which the lowest resistivity was achieved for undoped ZnO thin film. With increasing PTMGa , the electron carrier concentration of ZnO:Ga thin films increases more than an order of magnitude, reaching maximum value of 5.32 × 1020 cm−3 at PTMGa of 8 mTorr. But further increase of PTMGa results in the decrease of the electron carrier concentration. From previous report, after certain doping level, Ga atoms tend to occupy interstitial sites and act as neutral defects, and even substitute oxygen site as acceptors [16]. In addition, when ZnO is highly doped by Ga atoms, the acceptor-like complex defects such as (GaZn – zinc vacancy VZn ) and (GaZn – oxygen interstitial Oi ) are easily formed. In the case of Ga-doped ZnSe, complexes such as (GaZn –VSe ) and (GaZn –VZn ) have been found and known as compensating native defects [24]. Also, Roberts et al. [25] suggest that Ga2 3+ Oi 2− is formed by doubly charged oxygen with two Ga3+ , serving as double–electron traps and thus reducing the carrier density. A similar trend was reported for ZnO:Ga deposited by various methods including CVD with other Ga precursor [16], DC sputtering [26], solution-processed [27], sol–gel [28]. Also, other dopants such as B [29] and In [30] have shown similar tendency because, after a certain doping level, dopants tend to occupy substitute oxygen site as an acceptor [16]. And the mobility increases until 8 mTorr of the PTMGa and is reached the maximum value of 18.3 cm2 /V s. However, with further increase in PTMGa , the mobility conversely begins to decrease. Fig. 3 shows XRD patterns of undoped ZnO thin film and ZnO:Ga films deposited at various PTMGa from 0 to 20 mTorr. XRD data show that undoped ZnO film and the ZnO:Ga films deposited at PTMGa ≤ 8 mTorr show a dominant (0 0 2) orientation, of which peak becomes sharper with increasing PTMGa . Then, at PTMGa > 8 mTorr, other diffraction peaks, such as (2 1 1), (1 0 1) and (1 0 0), begin to appear and (0 0 2) peak begins broaden. Thus, the amount of Ga atoms incorporated in ZnO film clearly influences the structural crystallinity of LP-CVD ZnO films. Also, the mean grain sizes of the films can be calculated using Debye-Scherrer formula:

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Fig. 3. XRD patterns of ZnO:Ga films as a function of PTMGa on glass substrate at PDEZ = 90 mTorr and PO2 = 60 mTorr, Tsubstrate = 400 ◦ C.

D = (0.9)/(B cos ) where D is the diameter of the crystallites forming the film,  is the wavelength of X-ray, B is full width at half-maximum (FWHM) in radians and  is the Bragg angle. From this formula, mean grain size increases from 148.5 A˚ (PTMGa of 0 mTorr) to 207.8 A˚ (PTMGa of 8 mTorr). However, at PTMGa > 8 mTorr, mean grain size decreases to 63.9 A˚ (PTMGa of 20 mTorr). The Hall mobility is expressed as 1 1 1 1 = + + H i g phonon where H is Hall mobility, i , g and phonon are mobilities corresponding to impurity scattering, grain boundary scattering and phonon scattering, respectively. phonon is affected by the temperature so it remains constant. The impurity scattering, i , should be decreased with increasing PTMGa since the amount of Ga atoms incorporated in ZnO film is increased. Hence, the tendency of the Hall mobility values to increase from 5.8 cm2 /V s to 17.3 cm2 /V s with increasing PTMGa indicates that the dominant factor of mobility at PTMGa ≤ 8 mTorr is grain boundary scattering g . From Fig. 3, the dominant orientation (0 0 2) is not changed and (0 0 2) peak becomes prominent as increasing at PTMGa ≤ 8 mTorr. This should be related to the reduced grain boundary by the increased grain size, which is beneficial for increasing g and H . But, as increasing PTMGa above 8 mTorr, many orientations of ZnO:Ga appear on surface and grain boundaries might be increased accompanied with the reduction of grain size. Therefore, the g and i would work consistently to decrease H from 17.3 cm2 /V s to 14.7 cm2 /V s. Structural crystallinity of ZnO:Ga and its relation of electrical properties with varying the amount of Ga dopant was previously reported [13], which shows similar tendency to our results. Especially, in the case of non-degenerate and polycrystalline semiconductors, the grain boundary scattering is a significant factor to reduce electron mobility [31]. This tendency is also observed for ZnO thin films with other dopants such as Al [32], B [29], In [33], F [34] and even a novel metal Ru [35]. Also, other deposition techniques of ZnO:Ga such as sol–gel [28], solution process [27], molecular beam epitaxy [36], atomic layer deposition [37], and DC magnetron sputtering [26] show same tendency in mobility with Ga doping. Accordingly, the resistivity of ZnO:Ga thin films decreases with increasing PTMGa and minimum value 2.05 × 10−3  cm is achieved at PTMGa = 8 mTorr and it conversely begins to increase due to decrease in mobility and electron carrier concentration.

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Fig. 4 shows the XPS spectra for Ga 2p3/2 , O 1s, Zn 2p and C 1s of ZnO:Ga thin films with varying PTMGa . Circular-dot-lines are experimental data and bold-solid-lines are fitted data. In Fig. 4(a), Ga 2p3/2 XPS peaks are deconvoluted into two Gaussian peaks. The peak at 1117.8 eV, dash-line, is assigned to the GaZn O bonds [8], where GaZn means Ga substitution for Zn in the lattice site. The other peak at 1118.7 eV, solid-line, may be attributed to the formation of Ga O clusters such as GaOx suboxides and oxides due to the intragrain congregation and grain-boundary segregation [38–40]. Also, O 1s curves could be deconvoluted by 3 curves as shown in Fig. 4(b). The low binding energy component of O 1s (O1) has a peak center at about 530.5 eV, the medium binding energy component centered at about 531.2 eV (O2) and the high binding energy component located at about 531.8 eV (O3) [8]. (O1) is attributed to O2− ions on wurtzite structure of hexagonal Zn2+ ion array, surrounded by Zn or the substitution of Ga atoms and the medium binding energy component (O2) is associated with changes of the concentration of O2− ions in the oxygen deficient regions, oxygen vacancy. The high binding energy component (O3) is usually attributed to adsorbed O2 [7,8]. And XPS data of Zn 2p and C 1s are shown in Fig. 4(c) and (d) but there is no change of Zn 2p and almost zero C 1s peak regardless of the amount Ga doping by TMGa. The atomic concentrations of Ga and O calculated from XPS data are listed in Fig. 4(e). It is clear that the GaOx peak area increases and the GaZn peak area decreases as PTMGa increases. Meanwhile, the (O1) peak shows a little change in intensity with negligible fluctuation in the PTMGa < 10 mTorr. Therefore, we infer that Ga atoms begin to form Ga oxides when Ga concentration reaches a critical concentration. And decrease in adsorbed O2 intensity (O3) with increasing GaOx intensity could be attributed to the consumption of oxygen molecules to form Ga2 O or Ga2 O3 in the films. These Ga oxides might degrade the crystallinity of ZnO:Ga thin films and it can also be seen in the XRD data. These XRD and XPS results can explain the observed electrical properties of LP-CVD ZnO:Ga thin films. The initial increase of electron carrier concentration with Ga doping is attributed to the substitution of Ga3+ ions at the Zn2+ sites creating an electron carrier density, which is supported by the small changed percent (O1) peak and increased percent of (O2) peak and the large percent of GaZn peak in PTMGa < 8 mTorr in Fig. 4(c). In PTMGa > 8 mTorr, the carrier concentration decreases but the percentage of (O2) peak is increased. So we suggest that the oxygen vacancies increase with decreased amount of substituted Ga atoms at PTMGa > 8 mTorr. But overall carrier concentration is decreased because of appeared GaOx which does not help to increase carrier concentration or appeared GaZn –VZn , GaZn –Oi and Ga2 3+ Oi 2− which even decrease carrier concentration. Meanwhile, the decrease of mobility at Ga atoms abundance condition (PTMGa > 8 mTorr) could be explained by changing the grain structure of ZnO:Ga films. As more Ga oxide is formed inside ZnO matrix at this high Ga concentration regime, the grain structure of ZnO becomes complicated with significant increase in grain boundary area, resulting in decrease in Hall mobility. The atomic ratios of ZnO:Ga depending on the PTMGa has been calculated from XPS data; see Fig. 5. It can be found that the ratio of Zn slightly decreases from 52.6% to 51.2% at PTMGa < 8 mTorr and it sharply decreases to 45.1% at PTMGa = 10 mTorr. Also, the atomic ratio of Ga slightly increases from 0% to 1.7% at PTMGa < 8 mTorr and it sharply increases to 7.7% at PTMGa = 10 mTorr. The slight decrease of Zn and increase of Ga atomic ratio at PTMGa < 8 mTorr indicate that more Ga atoms have been doped into the ZnO lattice by substituting Zn ions or occupying the position of Zn vacancies and this can be connected to the decrease of (O1) area in Fig. 4(e). However, sharply changes of the atomic ratios of Ga and Zn between 8 mTorr and 10 mTorr of PTMGa in Fig. 5 and increased the GaO peak area in

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Fig. 4. XPS data of LP-CVD ZnO:Ga on glass for (a) Ga 2p3/2 , (b) O 1s, (c) Zn 2p and (d) C 1s with the PTMGa . (e) The atomic concentrations calculated from XPS for ZnO:Ga films.

films are (0 0 2) and (2 1 1) when PDEZ and PTMGa are low, and additional peaks such as (1 0 1), (1 0 1) and (1 0 0) for ZnO:Ga are appeared when more TMGa gas is supplied. And it is found that Ga atoms can function as effective donors producing high electron carrier concentration up to 5.32 × 1020 cm−3 with low resistivity of about 2.05 × 10−3  cm at TMGa partial pressures of 8 mTorr. And small amount of Ga dopants (PTMGa ≤ 8 mTorr) can enhance the crystallinity of ZnO:Ga films which can increase electrical properties but more Ga dopants degrade the crystallinity of films. Also, we found that excessive doped Ga can act as acceptors by forming complex Ga compound to decrease carrier concentrations. And the change of electrical properties of ZnO:Ga films with various amounts of Ga dopant is systematically analyzed using chemical bonding configurations of Ga and O atoms from XPS, crystal structures from XRD, and Hall measurement. Fig. 5. Calculated atomic ratios of Zn, Ga and O from XPS spectra of LP-CVD ZnO:Ga films as a function of the PTMGa .

Fig. 4(e) indicate that Ga oxides are formed at PTMGa > 10 mTorr and they degrade the crystallinities and electrical properties of ZnO:Ga films. 4. Conclusion We investigated the electrical, structural crystallinity, and chemical properties of LP-CVD ZnO films and the effects of Ga doping using TMGa as a doping gas. We found that the structural crystallinity in ZnO:Ga is influenced by the amount of DEZ and TMGa. The preferential orientations of grain structure of ZnO

Acknowledgement This work was supported by the Industrial Strategic Technology Development Program (10041041, Development of non-vacuum and non-lithography based 5 ␮m width Cu interconnect technology for TFT backplane) funded by the Ministry of Knowledge Economy (MKE, Korea). References [1] J. Yoo, J. Lee, S. Kim, K. Yoon, I.J. Park, S.K. Dhungel, B. Karunagaran, D. Mangalaraj, J. Yi, High transmittance and low resistive ZnO:Al films for thin film solar cells, Thin Solid Films 480/481 (2005) 213–217.

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