P type transparent conducting CU AL o thin films prepared by PE MOCVD 2

Chapter Properties of Cu-Al-O films grown from acac precursors Wang Yue cannot provide accurate information as the core level peak It is still clear that in the CuLMM peak (calibrated), the main peak was at a binding energy around 337.5eV, which was from Cu+, and a small peak was at a lower binding energy around 335.5eV, which was from Cu2+ CuLMM Intensity (counts) O 800 C O 750 C + O Cu 700 C 2+ Cu 325 330 335 340 345 350 Binding Energy (eV) Figure 4-15 XPS Auger spectra of copper LMM peak of the 350°C annealed films, temperatures shown in the figure are growth temperatures With an increase of growth temperature, the intensity of the peak at lower binding energy increased too The modified Auger parameters estimated here for Cu+ and Cu2+ were 1848.7eV and 1851.8eV, respectively, which were consistent with the reference values for Cu+ and Cu2+ To decide the percentage of Cu+, the spectra of Cu2p3/2 were fitted by the software provided with the XPS instrument In the fitting process, the difference in Cu2p3/2 binding energies of Cu+ and Cu2+, the GL (Gaussian-Lorentzian) ratio and the FWHM (Full Width at Half Maximum) were fixed P-type transparent conducting Cu-Al-O thin films 75 Chapter Properties of Cu-Al-O films grown from acac precursors Wang Yue The peak fitting results are listed in Table 4-6 The Cu+ content was about 80% and the content of Cu2+ increased with the increase of growth temperature This provides further proof that the co-doping theory17 may work in the present films Table 4-6 The content of Cu+ and Cu2+ calculated from peak fitting results Sample Cu+(%) Cu2+(%) Grown at 700°C 88.7 11.3 Grown at 750°C 82.0 18.0 Grown at 800°C 78.8 21.2 As discussed in the previous section, Cu2+ can act as an n-type dopant when it replaces Cu+ With the increase of growth temperature, more Cu2+ could be regarded as more n-type doping that helped to form energetically favorable acceptor-donor-acceptor complexes, and resulted in a shallower acceptor level and consequential higher conductivity 4.3.3 The effect of oxygen flow rate on the properties of Cu-Al-O films In this section the change of properties with the oxygen flow rate will be discussed The growth conditions were: O2 flow rate at to 20sccm, Ar flow rate at 30sccm, working pressure at 150mTorr, plasma power at 50W and the substrate temperature at 700°C The film thickness was measured to be from 120 to 180nm, equivalent to a growth rate of 2-2.5nm/min XRD was employed with a fixed incidence angle of degree The XRD results are shown in Figure 4-16 As indicated in the previous section, the hump at 21.20° is from the quartz substrate There are mainly two peaks around 43.46° (2.083Å) and 50.57° P-type transparent conducting Cu-Al-O thin films 76 Chapter Properties of Cu-Al-O films grown from acac precursors Wang Yue (1.806Å) in all the films, except the film grown at the lowest oxygen flow rate that showed one more small peak at 53.60° (1.710Å) Here the XRD spectra were similar to that being analyzed before, thus the XRD peaks might be from β-CuAlO2 Intensity (a.u.) • β-CuAlO2 • • 20sccm 12sccm 8sccm 6sccm 4sccm 10 20 30 40 50 60 70 Scattering Angle 2θ (deg.) Figure 4-16 XRD spectra of as-deposited films grown at different oxygen flow rates Because of charging, the morphologies from SEM of films grown at 12sccm and 20sccm were not shown Figure 4-17 shows the SEM pictures of the annealed films grown at oxygen flows of 4−8sccm No big difference in the morphology was observed in the films grown at different oxygen flow rates except for the particle size The particle size decreased from about 45 to 15nm when the oxygen flow rate increased Figure 4-18 shows the transmittance of the as-deposited and annealed films in the range of 300-1100nm The inset shows the transmittance at 650nm The films were annealed in the RHF1400 Carbolite furnace at 350°C for minutes in air P-type transparent conducting Cu-Al-O thin films 77 Chapter Properties of Cu-Al-O films grown from acac precursors Wang Yue (a) (b) (c) Figure 4-17 Morphology of 350°C annealed films which were grown at different oxygen flow rates of (a) 4sccm, (b) 6sccm and (c) 8sccm P-type transparent conducting Cu-Al-O thin films 78 Chapter Properties of Cu-Al-O films grown from acac precursors Wang Yue 80 As-deposited D CB 60 50 E 40 30 20 10 (a) A 100 Transmittance at 650nm(%) Transmittance (%) 70 80 60 40 20 12 16 20 Oxygen Flow Rate(sccm) 300 500 700 900 1100 Wavelength(nm) 90 B D 60 100 45 30 15 300 E C Transmittance at 650nm (%) Transmittance (%) After annealing A 75 (b) 80 60 40 20 12 16 20 Oxygen Flow Rate(sccm) 500 700 900 1100 Wavelength(nm) Figure 4-18 Transmittances of (a) as-deposited and (b) 350°C annealed Cu-Al-O films grown at different oxygen flow rates, A: 4sccm, B: 6sccm, C: 8sccm, D: 12sccm and E: 20sccm Before annealing, with the increase of oxygen flow rate, there was no much difference in the transmittance for the films grown at different oxygen flow rates except the one at 4sccm, which had lowest transmittance in the visible range After annealing, the transmittance of every film was greatly improved to be above 70% in the near-infrared P-type transparent conducting Cu-Al-O thin films 79 Chapter Properties of Cu-Al-O films grown from acac precursors Wang Yue range Above about 500nm, the film at 6sccm showed the lowest transmittance, while other films showed similar transmittance One obvious change due to annealing was observed that one absorption peak near 600nm disappeared after annealing To explain this phenomenon, the absorbance of as-deposited films as a function of photon energy is shown in Figure 4-19 0.5 A Absorbance 0.4 C E 0.3 D B 0.2 0.1 1.5 1.8 2.1 Eg 2.4 2.7 3.0 hν (eV) Figure 4-19 Absorbances (plot against photon energy) of as-deposited films grown from acac precursors at different oxygen flow rates, A: 4sccm, B: 6sccm, C: 8sccm, D: 12sccm and E: 20sccm Eg is the absorption edge All spectra showed an absorption edge Eg around 2.2eV It cannot be determined if this is direct bandgap or indirect bandgap from this figure.26 From the plot (αhν)1/2 against hν, an indirect bandgap of the film grown at 4sccm was estimated to be 2.25eV, which was very close to this value The absorption peak below Eg can be explained by an excitonic effect.26 An exciton is a bound electron-hole pair, usually free to move together through the crystal It is formed because of strong Coulomb attraction between the electron and the hole All excitons are unstable with respect to P-type transparent conducting Cu-Al-O thin films 80 Chapter Properties of Cu-Al-O films grown from acac precursors Wang Yue the ultimate recombination process in which the electron drops into the hole.27 This is why the absorption peak disappeared after annealing Theoretically, the optical absorption edge is perfectly abrupt; however, there are several effects that make the model change The excitonic effect mentioned above is one reason The existence of an impurity bandgap and the emission of a phonon with absorption of a photon are also possible reasons of this band tail absorption.26 Optical Bandgap (eV) 4.3 4.1 before annealing 3.9 3.7 after annealing 3.5 12 16 20 Oxygen Flow Rate (sccm) Figure 4-20 Optical bandgap versus oxygen flow rate for as-deposited ( ) and 350°C annealed ( ) films grown from acac precursors The direct optical bandgaps of the films deduced from absorption spectra versus oxygen flow rate are plotted in Figure 4-20 For as-deposited films, the bandgap decreased first, and then increased A minimum point at 8sccm was observed After annealing, the bandgap still had similar trend with the lowest point at 8sccm It can be concluded that the relationship between the bandgap and the oxygen flow rate is not linear and the film grown at 8sccm has the smallest bandgap The change of the P-type transparent conducting Cu-Al-O thin films 81 Chapter Properties of Cu-Al-O films grown from acac precursors Wang Yue optical bandgap cannot be explained by the change of carrier concentration due to the immeasurability of the carrier concentration The conductivity of all the films was not very good and the Hall effect measurement was not applicable for these films Therefore, the two-probe method was employed to measure the resistance The films grown at oxygen flow rates of 4sccm and 20sccm showed very poor conductivity while their resistances were beyond the range of measurement The resistances of other films were in the range of to 25MΩ Roughly, the films with the lower resistivity had smaller bandgap After annealing, the resistance was increased by about 50% As discussed in last section, annealing resulted in a decrease of defects concentration (mainly interstitial oxygen) and a consequential increase of resistivity XPS spectra Cu2p of the annealed films (Figure 4-21) were investigated after sputtering-clean and all spectra were plotted after the calibration by using the C1s peak as 284.8eV The peak at a high binding energy (952.5eV) is Cu2p1/2 whose appearance is due to the spin doublet separation resulted from ionization There is a satellite peak between peak 2p3/2 and peak 2p1/2, which is called a shake-up line This shake-up line came out because some ions were left in an excited state a few electron volts above the ground state leading to an increase in the binding energy of the emitted photoelectron.25 The existence of this peak is an evidence of the existence of Cu2+ The Cu2p3/2 peaks of all films appeared around 932.6eV, showing the dominance of Cu+ in the films However, there is some difference in the shake-up peak at 943.8eV For sample A (oxygen flow rate was 4sccm), there was no such satellite peak at all P-type transparent conducting Cu-Al-O thin films 82 Chapter Properties of Cu-Al-O films grown from acac precursors Wang Yue This satellite peak was seen clearly for samples C (8sccm) and E (20sccm), meaning that more Cu2+ was formed at higher oxygen flow rates Intensity (counts) Cu2p3/2 Cu2p1/2 Shake-up peak 927 932 937 942 947 E C B A 952 957 Binding Energy (eV) Figure 4-21 XPS Cu2p spectra of 350°C annealed films grown at different oxygen flow rates, A: 4sccm, B: 6sccm, C: 8sccm and E: 20sccm, D: 12sccm is not included because of too low counts The binding energy of Cu2p3/2 for every film was not exactly the same after calibration This was attributed to a chemical shift, which was related to the chemical environment of the element A higher binding energy means a stronger bond to the nucleus or looser bonds with adjacent atoms.25 Therefore, it is known from the figure that when oxygen flow rate was at an intermediate value, the bonds of copper atom with the adjacent oxygen atoms were less strong It is noticed that the samples with less strong Cu-O bond had better conductivity This is possibly because the positive holes move mainly in the CuO2 layer whose structure is much more open than AlO6 layer Less strong Cu-O bond means longer bond length, which gives holes more space to move P-type transparent conducting Cu-Al-O thin films 83 Chapter Properties of Cu-Al-O films grown from acac precursors Wang Yue 4.4 Further Discussion on Film Properties 4.4.1 Structural properties The rhombohedral structure was normally found in delafossite compounds To lay stress on the layer structure, the hexagonal description was often used, which means that the structure diagram was drawn referring to the hexagonal axis28 (Figure 4-22) c axis Figure 4-22 Rhombohedral ABO2 in hexagonal description, the vertical direction is c axis (adapted from R N Attili, M Uhrmacher, K P Lieb, and L Ziegeler, Phys Rev B53, 600 (1996)) Figure 4-23 shows a rhombohedral lattice in which the primitive cell is defined by the rhombohedral axes a1, a2, a3; but a non-primitive hexagonal unit cell can be chosen by adopting the axes A1, A2 and C 29 The latter has lattice points at 000, 1 and 3 2 3 P-type transparent conducting Cu-Al-O thin films 84 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue 5.2 Experimental Cu-Al-O films were prepared in a 13.56MHz RF plasma enhanced CVD apparatus Mixed metal-organic precursors Cu(dpm)2 and Al(dpm)3 of a mole ratio 1:1 were sublimated at about 150°C The vapor was then carried by Ar gas into the reaction chamber The reactive gas, O2, was introduced into the chamber through another inlet Mass flow controllers were employed to maintain the flow of both the carrier and reactive gases By using a turbo molecular pump, a reactor base pressure was kept at 3×10-6Torr Commercial (Asahi Glass Co., Ltd.) quartz substrates of dimension 10mm×10mm were employed They were cleaned by successive ultrasonic cleaning in analytically pure ethanol, acetone and ethanol before being introduced into the reactor Prior to the deposition, the substrates were heated at 400°C in the reactor of the base pressure for 1hour for further thermal cleaning and degassing The deposition parameters employed were: substrate temperatures from 650°C to 830°C, O2 flow rates from 20 to 35sccm, carrier gas Ar flow rates from 20 to 30sccm, RF discharge power 200W, and working pressure 75mTorr The morphology was examined using a field-emission SEM (Philips XL30 FEGSEM) The thickness was measured by an Alpha-step 500 surface profiler or/and by cross-section SEM An XRD apparatus (Philips X’pert-MPD) and an atomicresolution TEM (Philips CM300 FEG-TEM) were employed to reveal the structure of the sample The type of conductivity was determined by employing the Hall-effect method (BIO-RAD HL5500PC) and the Seebeck technique The chemical composition in the film was estimated by using EDX (Philips XL30 FEG-SEM) The P-type transparent conducting Cu-Al-O thin films 95 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue transmittance and absorbance of the film were measured by employing the UV-visible spectrophotometer (Shimadzu UV-1601) The valence of copper and depth profile of the film were examined by XPS (VG ESCLALAB MKII) The depth profile was also investigated by secondary ion mass spectrometry (SIMS) (Cameca IMS 6f SIMS) 5.3 Results and Discussions 5.3.1 A typical sample The deposition parameters employed in this section were: the substrate temperature at 830°C, the oxygen flow at 30sccm, the Ar flow at 30sccm, the pressure at 75mTorr and the plasma power at 200W The annealing of the as-deposited samples was carried out in air in the Carbolite furnace RHF1400 at 350°C for minutes, 10 minutes and 15 minutes, respectively The film thickness was estimated by cross-section SEM and confirmed by using an Alpha-step 500 surface profiler (at steps generated by removing part of the asdeposited film without damaging the substrate) The thickness of the film was determined to be about 120nm, and the deposition rate was 2nm/min By using EDX, it was found that the ratio of Cu to Al was 1.2:1.0 for the as-deposited sample Such a ratio was slightly higher than the original ratio of Cu to Al in the mixed precursors, possibly due to the different vaporization rates of Cu(dpm)2 and Al(dpm)3 and the deposition rates of the relevant species To determine the structure and phases in the film, XRD was employed P-type transparent conducting Cu-Al-O thin films 96 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue The peaks in the spectra were very weak so the y-axis was plotted in logarithm scale to amplify the signals There were peaks and humps at 43.38° (2.087Å), 64.58° Intensity (a.u.) (1.444Å) (estimated) and 74.28° (1.277Å) (weak) in the XRD spectra (Figure 5-1) 36.55 61.17 B 43.38 64.58 74.28 A 30 40 50 60 70 Scattering Angle 2θ (deg.) 80 Figure 5-1 XRD spectra of the film prepared at 830°C from dpm precursors, A: as-deposited, B: annealed at 350°C for 10 minutes Compared with powder diffraction pattern files,4 the peak at 43.38° could be from βCuAlO2, hexagonal CuAlO2 or Cu precipitate After annealing at 350°C for 10 minutes, these peaks disappeared and some new and weak peaks emerged which could be attributed to rhombohedral CuAlO2 or Cu2O (refer to section 4.3.2) As mentioned in last chapter, metal copper was excluded and only β-CuAlO2 would decompose at 360°C so the observed peak for the as-deposited film might come from the β-CuAlO2 phase To get more information about the structure, a high resolution TEM was employed Two high-resolution images of the as-deposited film are shown in Figure 5-2 The P-type transparent conducting Cu-Al-O thin films 97 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue grain sizes in A and B were all about 12nm It is clearly seen that these crystallites were surrounded by amorphous regions (a) (b) Figure 5-2 High-resolution TEM images of the as-deposited film, images (a) and (b) are for two typical nanograins After careful measurements, the lattice spacings in Figure 5-2(a) and (b) were found to be 2.15±0.02Å and 2.46±0.02Å, respectively These values could correspond to rhombohedral CuAlO2 or cubic Cu2O, similar to the observation in last chapter As P-type transparent conducting Cu-Al-O thin films 98 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue mentioned before, the compound found in the as-deposited film was β-CuAlO2, which possibly decomposed to Cu2O under the bombardment of high-energy electrons or upon heating Rhombohedral CuAlO2 possibly existed in the film with preferred orientation and could not only be revealed by XRD (a) (b) Figure 5-3 SEM pictures showing the morphology of the copper aluminum oxide films: (a) asdeposited and (b) annealed at 350°C for 10 minutes Figure 5-3 shows the morphology of the as-deposited and annealed films Threedimensional fractal island growth was observed In this growth mode (Volmer-Weber growth)5 small clusters are nucleated directly on the substrate surface and then grow P-type transparent conducting Cu-Al-O thin films 99 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue into islands of the film material This growth mode takes place when the film atoms are more strongly bound to each other than to the substrate The film presented a loose structure with irregularly shaped particles whose sizes range from 20 to 40nm After annealing at 350°C for 10min, the film appeared more compact with larger particle size Seebeck voltages at the cold end of all the as-deposited and annealed samples were positive, indicating p-type conductivity The Hall effect measurement also led to the same conclusion based on the positive sign of the Hall coefficients The mobility and carrier concentration were also measured by the Hall effect method at room temperature Table 5-1 lists the room-temperature conductivity, Hall coefficient, mobility and carrier concentration of the as-deposited film and the annealed films Unfortunately, no reproducible Hall measurement data could be obtained for the asdeposited film Therefore, such data are not available It is seen from Table 5-1 that the conductivity became slightly higher after annealing However, the conductivity and carrier concentration of annealed films were reduced when the annealing time increased The mobility of the annealed film slightly increased with the annealing time Such a higher conductivity after annealing can be explained by the co-doping effect (the co-doping theory was described in section 4.3.1) When the film was placed in air for annealing, ambient oxygen oxidized some Cu+ to Cu2+ resulting in more n-type codopants and a further reduction of the electrostatic energy of CuAlO2 This would enhance the sample conductivity However, the concentration of interstitial oxygen, which is the source of p-type charge carriers, was reduced by the annealing, leading to a decrease of conductivity With these two competing effects, the conductivity P-type transparent conducting Cu-Al-O thin films 100 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue increased first and then decreased gradually with annealing time Referring to Appendix C (Hall effect), it is known that with the existence of conduction electrons in p-type semiconductors, experimental Hall coefficient and mobility became smaller while the experimental carrier concentration became larger Table 5-1 Conductivity, Hall coefficient, Hall mobility and carrier concentration of the asdeposited and 350°C annealed films (“⎯” means not measurable) Conductivity Hall (S·cm-1) coefficient Mobility Carrier (cm2·V-1·s-1) concentration (cm3·c-1) (×1019cm-3) As-deposited film 15.66 ⎯ ⎯ ⎯ Annealing for minutes 17.08 +0.055 0.936 11.67 Annealing for 10 minutes 16.79 +0.061 1.03 10.31 Annealing for 15 minutes 16.02 +0.068 1.096 9.19 In comparison with the work of Kawazoe et al.,6 it is noticed that the present films have conductivities about two-orders higher than that of CuAlO2 films prepared by the laser-ablation technique The conductivity of the present p-type film is so high that it is only two orders lower than that of the best n-type transparent films, and this is sufficient for some applications More importantly, the highly conductive films were produced by the PE-MOCVD technique, one of the most commonly used techniques in the wafer fabrication industry The transmittances of the as-deposited and annealed films are shown in Figure 5-4 The transmittance of the original sample was 18-56% in the range of 300-1100nm After being annealed in air, the film became slightly more transparent and the annealed samples had similar transmittances (averagely 16-58%) in the same range P-type transparent conducting Cu-Al-O thin films 101 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue Transmittance (%) 60 50 B C 40 D A 30 20 10 300 500 700 900 1100 Wavelength (nm) Figure 5-4 The optical transmission of the as-deposited and annealed films grown from dpm precursors A, B, C and D stand for the as-deposited film and the films annealed at 350°C for 5, 10 and 15 minutes, respectively 7.0 B DCA 5.0 4.0 15 (αhν) (x10 ) 6.0 3.0 2.0 1.0 0.0 2.0 2.5 3.0 3.5 4.0 Energy (hν) 4.5 5.0 Figure 5-5 A plot of (αhν)2 against hν for the determination of optical bandgap for the film grown from dpm precursors A, B, C and D stand for the as-deposited film and the films annealed at 350°C for 5, 10 and 15 minutes, respectively P-type transparent conducting Cu-Al-O thin films 102 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue The direct optical bandgaps of the films were deduced from the absorption measurement Figure 5-5 shows a plot of (αhν)2 against hν, where α and hν denote absorption coefficient and photon energy, respectively The optical bandgaps were estimated to be 3.75eV, 3.67eV, 3.64eV and 3.60eV for the as-deposited sample and the samples annealed at 350°C for 5, 10 and 15 minutes, respectively These gaps are all larger than the direct bandgap of the film reported by Kawazoe et al.6 (~3.5eV) Similar to the discussion in Chapter 4, the large bandgap is probably due to the quantum effect The Burstein shift may also be the reason for the large bandgap because the film has much higher carrier concentration than that (~1017cm-3) of Kawazoe et al.6 The annealing shrank the direct bandgap, accompanying with the decrease of the carrier concentration There are mainly two theories concerning the change of bandgap with carrier concentration: Burstein-Moss shift and bandgap narrowing, which will be described in detail in a later discussion part Briefly, Burstein-Moss shift means that the increase of carrier concentration widens the bandgap for the materials with large carrier concentration The bandgap narrowing theory suggests that the interaction between carriers and carrier-impurity scattering shrink the direct bandgap in a certain carrier concentration range.8, Here the decrease of bandgap with annealing is related to a decrease in carrier concentration, in consistency with the Burstein-Moss theory Activation energies of the positive carriers were determined from the temperature dependence of electrical conductivity The activation energy Ea, conductivity σ and temperature T are related by P-type transparent conducting Cu-Al-O thin films 103 Chapter Properties of Cu-Al-O films grown from dpm precursors σ = σ exp( Wang Yue − Ea ) …………… ………………(5-2) kT where σ0 is a constant, and k is Boltzmann’s constant The natural logarithms of the inverse of the resistances as a function of temperature, measured from RT (room temperature) to 60°C in air, are plotted in Figure 5-6 1000/T (1000/K) 3.3 3.2 3.1 1000/T (1000/K) 3.0 3.3 3.1 3.0 -9.48 ln(1/R) -9.45 -9.60 ln(1/R) -9.57 3.2 -9.63 -9.51 -9.54 -9.66 -9.69 -9.57 (a) a -9.72 300 310 320 330 (b) b -9.60 300 340 310 3.2 3.1 330 340 1000/T (1000/K) 1000/T (1000/K) 3.3 320 T (K) T (K) 3.0 3.3 3.2 3.1 3.0 2.9 -11.3 -11.08 -11.4 ln(1/R) ln(1/R) -11.12 -11.16 -11.5 -11.6 -11.20 (c)c -11.24 300 310 320 330 d (d) -11.7 340 T (K) 300 310 320 330 340 350 T (K) Figure 5-6 The natural logarithm of the inverse of resistance plotted as a function of temperature for (a) as-deposited film, and the films annealed at 350°C for (b) 5min, (c) 10min and (d) 15min The unit of resistance R is ohm A linear relationship between ln(1/R) and 1/T was clearly shown for all the figures It is evident that all the films had typical semiconductive characteristics From the curves, the activation energies were estimated to be 30meV, 32meV, 40meV and P-type transparent conducting Cu-Al-O thin films 104 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue 77meV for the as-deposited, 5min, 10min and 15min annealed samples, respectively With the increase of annealing time, the activation energy increased The activation energies of these samples were smaller than the values of 100-200meV reported by Kawazoe et al.6 as well as the value of 120meV of the film prepared from acac precursors in section 4.3.1 For a non-degenerate system, the activation energy contains two parts: carrier generation energy and energy barrier for carrier mobility For a degenerate system, carrier concentration is independent of temperature15 so the activation energy only accounts for carrier movement From the data of carrier concentrations and conductivities, the films reported elsewhere possibly belonged to the former system and the present films possibly belonged to the latter system Thus the activation energy reported in this section was only responsible for carrier transport, which was thermal emission over the barriers.10 In this section, a p-type transparent Cu-Al-O film fabricated by PE-MOCVD from Cu(dpm)2 and Al(dpm)3 precursors was reported for the first time The roomtemperature conductivity of the as-deposited film reached 15.66S·cm-1 After postannealing in air at 350°C, the p-type conductivity was further increased to a value as high as 17.08S·cm-1 The optical bandgap of the as-deposited film was 3.74eV The bandgap decreased to 3.60eV after annealing at 350oC for 15 Compared with other reported p-type TCOs, the conductivity difference between p-type and n-type TCOs was much narrowed To further ascertain the conduction mechanism and the optical properties of the copper aluminum oxide, various growth conditions and film properties are studied in the following sections P-type transparent conducting Cu-Al-O thin films 105 Chapter Properties of Cu-Al-O films grown from dpm precursors 5.3.2 Wang Yue Effect of growth temperature on the properties of Cu-Al-O films Comparing section 5.3.1 with section 4.3.1, the film prepared from dpm precursors had much better conductivity than the film from acac precursors The former one also showed better adhesion with the substrate Therefore it is more important to investigate the films prepared from dpm precursors As is well known, the substrate temperature is a significant factor governing electrical properties of semiconductor thin films In this part, properties of copper aluminum oxide films at different substrate temperatures will be presented The substrate temperature was varied from 650 to 800°C, while the oxygen flow was kept at 30sccm, the Ar flow was 20sccm, the pressure was 75mTorr and the plasma power was 200W Growth Rate (nm/min) 2.5 1.5 0.95 1.00 1.05 1.10 1000/Tsub (1/K) Figure 5-7 The growth rate plotted on a natural logarithm scale versus the inverse of growth temperature Tsub for Cu-Al-O films prepared from dpm precursors It was found that the adhesion of these films deposited from dpm precursors was much better than those from acac precursors The latter films were easily scratched off from P-type transparent conducting Cu-Al-O thin films 106 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue the substrate but the dpm films were well-bonded and difficult to remove No matter which precursors were used, the adhesion was always improved after annealing The thickness of the films grown at 650°C, 700°C, 750°C and 800°C were 75nm, 101nm, 135nm and 189nm, respectively Figure 5-7 shows the natural logarithm of the growth rate γgrow versus growth temperature With the increase of the growth temperature, the logarithm of growth rate increased linearly This agreed with the Arrhenius law (Eq.4-1) Compared with the films prepared from acac precursors (Figure 4-6), this result fitted better with the Arrhenius law Intensity (a.u.) • β-CuAlO2 • O • 800 C • O 750 C O 700 C O 650 C 10 20 30 40 50 60 70 80 Scattering Angle 2θ (deg.) Figure 5-8 XRD spectra of films prepared from dpm precursors at different temperatures The XRD result (Figure 5-8) was similar to that in the previous section It is also observed that the film grown at higher temperatures showed higher intensity of peaks, which demonstrated that a high growth temperature promoted the growth of the β- P-type transparent conducting Cu-Al-O thin films 107 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue CuAlO2 phase The film grown at 650°C did not show the characteristic peaks of βCuAlO2 Transmittance (%) 100 O 650 C 80 O 700 C 60 O 750 C 40 O 800 C 20 300 (a) 500 700 900 1100 Wavelength (nm) 100 Transmittance (%) O 650 C O 700 C O 800 C 80 60 O 750 C 40 20 300 (b) 500 700 900 1100 Wavelength (nm) Figure 5-9 Transmittances of Cu-Al-O films grown at different temperatures prepared from dpm precursors: (a) original data and (b) after normalization to the thickness of 100nm The films grown at different temperatures showed quite different transmittances (Figure 5-9(a)) The lower the growth temperature, the more transparent was the film P-type transparent conducting Cu-Al-O thin films 108 Chapter Properties of Cu-Al-O films grown from dpm precursors Wang Yue The transmittance of the film grown at 650°C even exceeded 90%, while the transmittance of the film grown at 800°C was below 40% (a) (b) (c) Figure 5-10 Morphology of the as-deposited films prepared at (a) 700°C, (b) 750°C and (c) 800°C The film at 650°C is not shown because of charging P-type transparent conducting Cu-Al-O thin films 109 ... possibility of the existence of metal copper and CuAl 2O4 , suggesting that the major phase was CuAlO2 (β-CuAlO2 and rhombohedral CuAlO2) and minor phases were Cu2 O and Al2 O3 However, β-CuAlO2 is a... P- type transparent conducting Cu- Al- O thin films 83 Chapter Properties of Cu- Al- O films grown from acac precursors Wang Yue 4.4 Further Discussion on Film Properties 4.4.1 Structural properties... chapter reported the first success of highly conductive p- type transparent copper aluminum oxide films prepared by PE- MOCVD A study of p- type transparent Cu- AlO films grown from acetylacetonate