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The effect of target nitridation on structural properties of InN grown by radio-frequency reactive sputtering
Thin Solid Films 422 (2002) 28–32
The effect of target nitridation on structural properties of InN grown by radio-frequency reactive sputtering Motlan1, E.M. Goldys*, T.L. Tansley Macquarie University, 2109 Sydney, Australia Received 26 November 2001; received in revised form 27 June 2002; accepted 25 August 2002
Abstract The structure and composition of indium nitride (InN) films grown by radio frequency reactive sputtering as a function of target nitridation have been investigated. X-ray diffraction shows that the films are primarily polycrystalline with preferred (0 0 2) orientation indicating the c-axis of the hexagonal InN structure perpendicular to the substrate. Scanning electron microscopy shows that films grown from non-nitrided targets are characterised by smaller grain size and rougher surfaces, with no observable structure. Films grown with pre-nitrided targets have a continuous columnar morphology with relatively even surfaces. X-ray photoelectron spectroscopy and Rutherford backscattering techniques were used to quantify the amounts of In, N and O contents. 䊚 2002 Elsevier Science B.V. All rights reserved. PACS: 61.66 Fn; 68.55 ya; 81.15.cd; 87.64. Fb Keywords: Sputtering; Indium nitride; Morphology; Composition
1. Introduction Indium nitride (InN) has the narrowest band gap of ;1.89 eV w1x among the isostructural III–V nitride compounds. Because of this energy and its direct band gap, it has a potential application for visible-light optoelectronic devices and for high-efficiency solar cell w2x, optical coating w3x, and the relatively wider band gap allows high temperature operation. The small effective mass of 12mo w4x and thus high mobility will provide high speed electronic devices. InN has been proven very difficult to prepare in stoichiometric form, and difficulties in its preparation are widely known. Firstly, InN has a low dissociation temperature, it is reported that this material may decompose at 600 8C in vacuum w5x. Secondly, the traditional thermodynamic equilibrium techniques used for the preparation of semiconductors are inappropriate for the *Corresponding author. E-mail address: [email protected] (E.M. Goldys). 1 Permanent address: Department of Physics, Faculty of Mathematics and Science, State University of Medan, Indonesia.
growth of InN because of extremely high vapour pressure at its melting point, and need to replaced by kinetically controlled methods. Various growth techniques such as chemical vapour deposition, and recently molecular beam epitaxy, ion plating, magnetron sputtering, and radio-frequency (r.f.) sputtering have been used. The best electrical parameters reported so far were obtained in films grown by r.f. reactive sputtering with a relatively low carrier concentration of the order of 1016 cmy3 w4x. It has been found that the crystalline quality of InN layers grown on sapphire was improved by substrate nitridation w6x. In this paper, we present the effect of target nitridation on the structural properties of InN grown by the reactive r.f. sputtering. Formation of thin films by r.f. reactive sputtering involves kinetic interaction between plasma and target. The intensity of the plasma and the number of ionised reactive gas atoms are highly dependent on pressure and flow rate of the reactive gas, in this case nitrogen. The present work shows the need for precise control of the pressure and the flow rate, both of which affect the structure and compositional stoichiometry of the resulting films.
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 7 7 2 - 1
Y. Motlan et al. / Thin Solid Films 422 (2002) 28–32
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2. Experiment The system used for the deposition consists of a vacuum chamber housing the target and substrates, a pumping system which consists of rotary and diffusion pumps, r.f. power supply, a matching unit, a mass flow control system and an r.f. power source. The target was prepared by heating an ingot of nominal 99.9999% In placed in a pyrex container while under vacuum (;10y7 Torr). The InN thin films were grown on crown glass substrate cleaned with an ammonium oxalate based paste then with isopropyl alcohol, followed by ultrasonic cleaning. The substrate temperature was 80 8C. The gas pressure was varied between 10 and 50 mTorr. During the deposition, the r.f. sputtering power was varied between 30 and 50 W. The structural and morphological properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectroscopy (RBS) measurements. Films are grown in the thickness range 0.3–1.2 mm, depending on deposition time, with a uniformity of "5% over an area of 1 cm2 typical thickness. 3. Results and discussion As grown films are primarily polycrystalline and XRD measurements reveal a preferred (0 0 2) orientation indicating the c-axis of the hexagonal InN structure perpendicular to the substrate. Weak reflections from (1 0 0), (1 0 3), and (0 0 4) planes were also observed. All these peaks are attributable to InN w7–10x. The crystallographic structure of indium nitride thin films changes as the target ages. The pure indium surface of new metal target is exposed to the plasma initially. This is converted after approximately 40 h exposure into a fully nitrided target with a polycrystalline InN surface. Fig. 1 shows the effects of the degree of target nitridation on as grown InN film structure. During the early phase (from fresh metallic target to approximately 40 h of nitridation-in-growth) four reflection planes parallel to the film surface are found, as shown in Fig. 1a. During the mature target phase (after 100 h nitridation), only two reflection planes, (0 0 2) and (0 0 4) remain, as shown in Fig. 1b indicating strong c-axis orientation. As the target become, fully nitrided the (1 0 0) plane reappears and persists for the rest of the target aging processes as shown in Fig. 1c and d. In the first stage, where the target is not yet nitrided, the resulting films are deficient in nitrogen. During the second stage, more nitrogen is available on the surface of the target and the resulting film is stoichiometric, well oriented with all crystallites having (0 0 2) and (0 0 4) planes parallel to the growth surface. At the third stage, where the target has been nitrided for several hundred hours, the target has gradually absorbed oxygen
Fig. 1. XRD intensities in r.f. sputtered InN films as a function of target nitridation. Target ages are (a) 17 h; (b) 37 h; (c) 89 h; (d) 345 h.
either from atmospheric contamination during sample transfers or from residual impurities in the vacuum system, gas supply etc. We later describe the measurement of oxygen content of the films and show that the reappearance of (1 0 0) planes coincides with presence of oxygen. The mechanism for the change of the crystalline orientation and the change in lattice constant are not fully understood, we suggest that the nitrogen concentration influences the crystal structure since the more the target is nitrided, the higher the nitrogen flux ejected onto the substrate to form indium nitride films w11x. As the nitrided fraction of the target increases, the indium-sputtering rate from the target, and the nitrogen gettering rate onto it decrease. This has the effect of increasing the VyIII ratio at the growth surface w12x. In common with most III–V semiconductor growth, a considerable excess of the group V precursor appears necessary for stoichiometry. Formation of the nitride during reactive sputtering is determined by the ratio of reactive species at the growth surface w9x, consequently, nitrogen incorporation into the growing films is promoted by a nitrided target. The surface morphology of the films was verified by using SEM. Fig. 2 shows a significant difference in the morphologies of the films grown from non-nitrided and
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Y. Motlan et al. / Thin Solid Films 422 (2002) 28–32
Fig. 3. A typical XPS spectrum of an InN thin film after sputter etch using argon ions for 10 min.
Fig. 2. Scanning electron micrographs of cleaved cross-sections of InN films grown from (a) a fresh In target and (b) a nitrided target. The bars in the photographs indicate 100 nm.
nitrided targets. Films grown from non-nitrided targets show smaller grain size and rougher surfaces, with no observable structure. Films grown with nitrided targets have a continuous columnar structure with relatively even surfaces. All films show some degree of porosity, also visible in Fig. 2. Typical XPS spectra of various elements present in the films are presented in Fig. 3. The sample examined in Fig. 3 was grown from a nitrided target and has been pre-etched for 10 min to remove contaminants deposited during exposure to the air. The typical configuration of InN elements before and after target nitridation in the subsurface of the films is shown in Table 1. Apart from a high amount of oxygen (27% of atomic concentration), the average ratio of In to N in our film is approximately 2.2. The composition of the films is not homogeneous through the bulk of the films. A typical elemental composition through the bulk of the films for both nitrided and non-nitrided samples is shown in Fig. 4. It can be seen that oxygen dominates at the surfaces of the films and decreases dramatically after 5 min of sputtered etching. Samples grown from a non-nitrided target show a dominant oxygen concentration and a high contamination by carbon. The samples grown from nitrided target
show less oxygen, no carbon and both nitrogen and indium elemental fractions are increased. These results are in a good agreement with XRD data which show better oriented lattice planes. Although indium content still exceeds the nitrogen content, the increase in nitrogen is very significant with a nitrided target. The properties of InN thin films in term of nonstoichiometry, the amount of oxygen present, and the fluctuating composition of the films through the bulk, results from difficulties faced in stabilising growth parameters during the deposition process. The most significant parameters are: nitrogen gas pressure, r.f. power, target current, and deposition rate. All these parameters are highly interrelated. Predicting the critical ranges of these parameters using nitrogen gas as both the bombarding and reactive species has not been possible. The only theoretical model available has concentrated on argon as the principal sputtering species with nitrogen acting principally as the reactive species but also contributing to sputtering w12x. Unfortunately, in the nitrogen-only sputtering system available in this work, independent control of sputtering and reaction rates is much less flexible than an argon–nitrogen system that has the extra parameter of ratio reactivey nonreactive species under control. It is worth noting, however, that systems with independent AryN2 control Table 1 Typical elemental composition of InN films grown by r.f. reactive sputtering before and after target nitridation using XPS measurement Sample
Non-nitrided, 噛3 Non-nitrided, 噛4 Nitrided, 噛1 Nitrided, 噛5
Atomic concentration % In
N
O
33 36 44 44
13 13 32 20
45 47 17 28
The uniformity of the film is shown in Fig. 4.
Y. Motlan et al. / Thin Solid Films 422 (2002) 28–32
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Fig. 5. Depth profile indicating variation of composition between two samples grown at a different growth rate. Open symbols (噛1) sample grown at a high growth rate, solid symbols (噛2) sample grown at a low growth rate. The identity of elements is indicated in figure.
Fig. 4. Depth profile illustrating the composition of InN through the bulk of the film (a) grown from a new In target; (b) grown from a nitrided target. Different symbols correspond to different elements, as indicated in figure.
have also not succeeded producing low-carrier-concentration, high mobility InN. We can use the theoretical studies of AryN2 system to give some information on the parameters of a N2 system. The nitrogen gas pressure will interact strongly with the other critical parameters, namely target sputtering rate, distance between Nmax (maximum indium atomic density in the plasma as a function of N2 discharge pressure) and target surface, and maximum indium neutral atom density Nmax w13x. Anderson et al. w14x found that in the range of interest small changes in the gas pressure can have a very significant effect on film bombardment by the sputtered atom. Blom et al. w15x found that the minimum partial pressure of N2 mixed argon nitrogen system needed to form a nitride is strongly dependent on the total flow. They point out that it is more relevant to control the actual mass flow of reactive gas than to rely on the partial pressure, since a small change in gas pressure alters the plasma discharge conditions, which can be roughly determined
from the intensity and colour of the plasma. Therefore in this experiment we relied more on the nitrogen gas pressure rather than the gas flow because the former was easier to control. We also found that the concentration of nitrogen in the indium nitride films is influenced by the deposition rate Fig. 5 shows that the nitrogen concentration is higher at higher growth rates. Both samples were grown from nitrided target, but sample 噛1 was grown with 20 mTorr nitrogen gas pressure and sample 噛2 grown with 50 mTorr nitrogen gas pressures. The growth rate of sample 噛1 and 噛2 are 0.66 and 0.36 nmymin, respectively. Such lower growth rate at higher pressure reflects the reduced mean free path in the plasma leading to an increase in the oxygen contamination w12x. These results indicate that low sputtering pressures are necessary if good quality InN film are to be obtained. The same results were obtained in growth by r.f. magnetron sputtering w9x. The composition of InN thin films was also investigated by RBS. In these measurements we used indium nitride films deposited on glassy carbon. The RBS analysis indicated the composition of the films was uniform throughout the depth of the film. The atomic concentrations were obtained by the profile fitting method, the general results indicating more nitrogen less oxygen than XPS. Table 2 shows typical relative amounts of In, N and O using RBS measurements. Table 2 Relative amounts of In, N, O using RBS measurement Sample
N2 pressure (mTorr)
Power (W)
In (%)
N (%)
O (%)
噛6 噛7 噛8
20 50 20
30 30 50
34.5 36.6 37.0
34.5 28.4 46.0
31.0 35.0 17.0
Y. Motlan et al. / Thin Solid Films 422 (2002) 28–32
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4. Conclusion The structural and morphological properties of r.f. sputtered InN have been examined by various approaches, with particular focus on evolution of films with nitridation of the target. The films are polycrystalline with (0 0 2) preferred orientation perpendicular to the film substrates. The Bragg reflections are highly dependent on the nitrogen content, since the films suffer from nitrogen depletion. A strong relationship is found between the stoichiometry and the film structure. The crystallite sizes are also influenced by the film stoichiometry; with films grown from nitrided target having larger crystallite size. XPS depth profiling measurements show that the elemental content of the films is not homogeneous through the bulk of the film while the RBS measurements indicate improved stoichiometry for growth from a nitrided target. References w1x T.L. Tansley, C.P. Foley, J. Appl. Phys. 59 (1986) 3241. w2x O. Takai, J. Ebisawa, Y. Hisamatsu, Proceedings of the Seventh International Conference on Vacuum Metallurgy, Tokyo, Japan, November 26–30, 1982, p. 137.
w3x K.L. Westra, R.P.W. Lawson, M.J. Brett, J. Vac. Sci. Tech. A 6 (1988) 1373. w4x C.P. Foley, T.L. Tansley, Phys. Rev. B 33 (1986) 1430. w5x H. Takeda, T. Hada, Toyama Kogyoukute Semon Gakko Kiyo 11 (1977) 73. w6x T. Tsuchiya, H. Yamano, O. Miki, A. Wakahara, A. Yoshida, Jpn. Appl. Phys. 38 (1999) 1884. w7x O. Takai, J. Ebisawa, Y. Hisamatsu, Proceedings of the Seventh International Conference on Vacuum Metallurgy, Tokyo, Japan, November 26–30, 1982, p. 129. w8x N. Asai, Y. Inoue, H. Sugimura, O. Takai, Thin Solid Films 332 (1998) 267. w9x Q. Guo, N. Shingai, Y. Mitsuishi, M. Nishio, H. Ogawa, Thin Solid Films 243–344 (1999) 524. w10x O. Takai, K. Ikuta, Y. Inoue, Thin Solid Films 318 (1998) 148. w11x L.I. Maissel, R. Glang, Handbook of Thin Film Technology, McGraw-Hill Book Company, New York, 1970. w12x S. Berg, H.O. Blom, T. Larsson, C. Neder, J. Vac. Sci. Technol. A 5 (1987) 202. w13x B.R. Natarajan, A.H. Eltouky, J.E. Greene, Thin Solid Films 69 (1980) 201. w14x D.A. Anderson, G. Moddel, M.A. Paesler, W. Paul, J. Vac. Sci. Technol. 16 (1979) 906. w15x H.O. Blom, S. Berg, T. Larson, Thin Solid Films 130 (1985) 307.
The effect of target nitridation on structural properties of InN grown by radio-frequency reactive sputtering Motlan1, E.M. Goldys*, T.L. Tansley Macquarie University, 2109 Sydney, Australia Received 26 November 2001; received in revised form 27 June 2002; accepted 25 August 2002
Abstract The structure and composition of indium nitride (InN) films grown by radio frequency reactive sputtering as a function of target nitridation have been investigated. X-ray diffraction shows that the films are primarily polycrystalline with preferred (0 0 2) orientation indicating the c-axis of the hexagonal InN structure perpendicular to the substrate. Scanning electron microscopy shows that films grown from non-nitrided targets are characterised by smaller grain size and rougher surfaces, with no observable structure. Films grown with pre-nitrided targets have a continuous columnar morphology with relatively even surfaces. X-ray photoelectron spectroscopy and Rutherford backscattering techniques were used to quantify the amounts of In, N and O contents. 䊚 2002 Elsevier Science B.V. All rights reserved. PACS: 61.66 Fn; 68.55 ya; 81.15.cd; 87.64. Fb Keywords: Sputtering; Indium nitride; Morphology; Composition
1. Introduction Indium nitride (InN) has the narrowest band gap of ;1.89 eV w1x among the isostructural III–V nitride compounds. Because of this energy and its direct band gap, it has a potential application for visible-light optoelectronic devices and for high-efficiency solar cell w2x, optical coating w3x, and the relatively wider band gap allows high temperature operation. The small effective mass of 12mo w4x and thus high mobility will provide high speed electronic devices. InN has been proven very difficult to prepare in stoichiometric form, and difficulties in its preparation are widely known. Firstly, InN has a low dissociation temperature, it is reported that this material may decompose at 600 8C in vacuum w5x. Secondly, the traditional thermodynamic equilibrium techniques used for the preparation of semiconductors are inappropriate for the *Corresponding author. E-mail address: [email protected] (E.M. Goldys). 1 Permanent address: Department of Physics, Faculty of Mathematics and Science, State University of Medan, Indonesia.
growth of InN because of extremely high vapour pressure at its melting point, and need to replaced by kinetically controlled methods. Various growth techniques such as chemical vapour deposition, and recently molecular beam epitaxy, ion plating, magnetron sputtering, and radio-frequency (r.f.) sputtering have been used. The best electrical parameters reported so far were obtained in films grown by r.f. reactive sputtering with a relatively low carrier concentration of the order of 1016 cmy3 w4x. It has been found that the crystalline quality of InN layers grown on sapphire was improved by substrate nitridation w6x. In this paper, we present the effect of target nitridation on the structural properties of InN grown by the reactive r.f. sputtering. Formation of thin films by r.f. reactive sputtering involves kinetic interaction between plasma and target. The intensity of the plasma and the number of ionised reactive gas atoms are highly dependent on pressure and flow rate of the reactive gas, in this case nitrogen. The present work shows the need for precise control of the pressure and the flow rate, both of which affect the structure and compositional stoichiometry of the resulting films.
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 7 7 2 - 1
Y. Motlan et al. / Thin Solid Films 422 (2002) 28–32
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2. Experiment The system used for the deposition consists of a vacuum chamber housing the target and substrates, a pumping system which consists of rotary and diffusion pumps, r.f. power supply, a matching unit, a mass flow control system and an r.f. power source. The target was prepared by heating an ingot of nominal 99.9999% In placed in a pyrex container while under vacuum (;10y7 Torr). The InN thin films were grown on crown glass substrate cleaned with an ammonium oxalate based paste then with isopropyl alcohol, followed by ultrasonic cleaning. The substrate temperature was 80 8C. The gas pressure was varied between 10 and 50 mTorr. During the deposition, the r.f. sputtering power was varied between 30 and 50 W. The structural and morphological properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectroscopy (RBS) measurements. Films are grown in the thickness range 0.3–1.2 mm, depending on deposition time, with a uniformity of "5% over an area of 1 cm2 typical thickness. 3. Results and discussion As grown films are primarily polycrystalline and XRD measurements reveal a preferred (0 0 2) orientation indicating the c-axis of the hexagonal InN structure perpendicular to the substrate. Weak reflections from (1 0 0), (1 0 3), and (0 0 4) planes were also observed. All these peaks are attributable to InN w7–10x. The crystallographic structure of indium nitride thin films changes as the target ages. The pure indium surface of new metal target is exposed to the plasma initially. This is converted after approximately 40 h exposure into a fully nitrided target with a polycrystalline InN surface. Fig. 1 shows the effects of the degree of target nitridation on as grown InN film structure. During the early phase (from fresh metallic target to approximately 40 h of nitridation-in-growth) four reflection planes parallel to the film surface are found, as shown in Fig. 1a. During the mature target phase (after 100 h nitridation), only two reflection planes, (0 0 2) and (0 0 4) remain, as shown in Fig. 1b indicating strong c-axis orientation. As the target become, fully nitrided the (1 0 0) plane reappears and persists for the rest of the target aging processes as shown in Fig. 1c and d. In the first stage, where the target is not yet nitrided, the resulting films are deficient in nitrogen. During the second stage, more nitrogen is available on the surface of the target and the resulting film is stoichiometric, well oriented with all crystallites having (0 0 2) and (0 0 4) planes parallel to the growth surface. At the third stage, where the target has been nitrided for several hundred hours, the target has gradually absorbed oxygen
Fig. 1. XRD intensities in r.f. sputtered InN films as a function of target nitridation. Target ages are (a) 17 h; (b) 37 h; (c) 89 h; (d) 345 h.
either from atmospheric contamination during sample transfers or from residual impurities in the vacuum system, gas supply etc. We later describe the measurement of oxygen content of the films and show that the reappearance of (1 0 0) planes coincides with presence of oxygen. The mechanism for the change of the crystalline orientation and the change in lattice constant are not fully understood, we suggest that the nitrogen concentration influences the crystal structure since the more the target is nitrided, the higher the nitrogen flux ejected onto the substrate to form indium nitride films w11x. As the nitrided fraction of the target increases, the indium-sputtering rate from the target, and the nitrogen gettering rate onto it decrease. This has the effect of increasing the VyIII ratio at the growth surface w12x. In common with most III–V semiconductor growth, a considerable excess of the group V precursor appears necessary for stoichiometry. Formation of the nitride during reactive sputtering is determined by the ratio of reactive species at the growth surface w9x, consequently, nitrogen incorporation into the growing films is promoted by a nitrided target. The surface morphology of the films was verified by using SEM. Fig. 2 shows a significant difference in the morphologies of the films grown from non-nitrided and
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Y. Motlan et al. / Thin Solid Films 422 (2002) 28–32
Fig. 3. A typical XPS spectrum of an InN thin film after sputter etch using argon ions for 10 min.
Fig. 2. Scanning electron micrographs of cleaved cross-sections of InN films grown from (a) a fresh In target and (b) a nitrided target. The bars in the photographs indicate 100 nm.
nitrided targets. Films grown from non-nitrided targets show smaller grain size and rougher surfaces, with no observable structure. Films grown with nitrided targets have a continuous columnar structure with relatively even surfaces. All films show some degree of porosity, also visible in Fig. 2. Typical XPS spectra of various elements present in the films are presented in Fig. 3. The sample examined in Fig. 3 was grown from a nitrided target and has been pre-etched for 10 min to remove contaminants deposited during exposure to the air. The typical configuration of InN elements before and after target nitridation in the subsurface of the films is shown in Table 1. Apart from a high amount of oxygen (27% of atomic concentration), the average ratio of In to N in our film is approximately 2.2. The composition of the films is not homogeneous through the bulk of the films. A typical elemental composition through the bulk of the films for both nitrided and non-nitrided samples is shown in Fig. 4. It can be seen that oxygen dominates at the surfaces of the films and decreases dramatically after 5 min of sputtered etching. Samples grown from a non-nitrided target show a dominant oxygen concentration and a high contamination by carbon. The samples grown from nitrided target
show less oxygen, no carbon and both nitrogen and indium elemental fractions are increased. These results are in a good agreement with XRD data which show better oriented lattice planes. Although indium content still exceeds the nitrogen content, the increase in nitrogen is very significant with a nitrided target. The properties of InN thin films in term of nonstoichiometry, the amount of oxygen present, and the fluctuating composition of the films through the bulk, results from difficulties faced in stabilising growth parameters during the deposition process. The most significant parameters are: nitrogen gas pressure, r.f. power, target current, and deposition rate. All these parameters are highly interrelated. Predicting the critical ranges of these parameters using nitrogen gas as both the bombarding and reactive species has not been possible. The only theoretical model available has concentrated on argon as the principal sputtering species with nitrogen acting principally as the reactive species but also contributing to sputtering w12x. Unfortunately, in the nitrogen-only sputtering system available in this work, independent control of sputtering and reaction rates is much less flexible than an argon–nitrogen system that has the extra parameter of ratio reactivey nonreactive species under control. It is worth noting, however, that systems with independent AryN2 control Table 1 Typical elemental composition of InN films grown by r.f. reactive sputtering before and after target nitridation using XPS measurement Sample
Non-nitrided, 噛3 Non-nitrided, 噛4 Nitrided, 噛1 Nitrided, 噛5
Atomic concentration % In
N
O
33 36 44 44
13 13 32 20
45 47 17 28
The uniformity of the film is shown in Fig. 4.
Y. Motlan et al. / Thin Solid Films 422 (2002) 28–32
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Fig. 5. Depth profile indicating variation of composition between two samples grown at a different growth rate. Open symbols (噛1) sample grown at a high growth rate, solid symbols (噛2) sample grown at a low growth rate. The identity of elements is indicated in figure.
Fig. 4. Depth profile illustrating the composition of InN through the bulk of the film (a) grown from a new In target; (b) grown from a nitrided target. Different symbols correspond to different elements, as indicated in figure.
have also not succeeded producing low-carrier-concentration, high mobility InN. We can use the theoretical studies of AryN2 system to give some information on the parameters of a N2 system. The nitrogen gas pressure will interact strongly with the other critical parameters, namely target sputtering rate, distance between Nmax (maximum indium atomic density in the plasma as a function of N2 discharge pressure) and target surface, and maximum indium neutral atom density Nmax w13x. Anderson et al. w14x found that in the range of interest small changes in the gas pressure can have a very significant effect on film bombardment by the sputtered atom. Blom et al. w15x found that the minimum partial pressure of N2 mixed argon nitrogen system needed to form a nitride is strongly dependent on the total flow. They point out that it is more relevant to control the actual mass flow of reactive gas than to rely on the partial pressure, since a small change in gas pressure alters the plasma discharge conditions, which can be roughly determined
from the intensity and colour of the plasma. Therefore in this experiment we relied more on the nitrogen gas pressure rather than the gas flow because the former was easier to control. We also found that the concentration of nitrogen in the indium nitride films is influenced by the deposition rate Fig. 5 shows that the nitrogen concentration is higher at higher growth rates. Both samples were grown from nitrided target, but sample 噛1 was grown with 20 mTorr nitrogen gas pressure and sample 噛2 grown with 50 mTorr nitrogen gas pressures. The growth rate of sample 噛1 and 噛2 are 0.66 and 0.36 nmymin, respectively. Such lower growth rate at higher pressure reflects the reduced mean free path in the plasma leading to an increase in the oxygen contamination w12x. These results indicate that low sputtering pressures are necessary if good quality InN film are to be obtained. The same results were obtained in growth by r.f. magnetron sputtering w9x. The composition of InN thin films was also investigated by RBS. In these measurements we used indium nitride films deposited on glassy carbon. The RBS analysis indicated the composition of the films was uniform throughout the depth of the film. The atomic concentrations were obtained by the profile fitting method, the general results indicating more nitrogen less oxygen than XPS. Table 2 shows typical relative amounts of In, N and O using RBS measurements. Table 2 Relative amounts of In, N, O using RBS measurement Sample
N2 pressure (mTorr)
Power (W)
In (%)
N (%)
O (%)
噛6 噛7 噛8
20 50 20
30 30 50
34.5 36.6 37.0
34.5 28.4 46.0
31.0 35.0 17.0
Y. Motlan et al. / Thin Solid Films 422 (2002) 28–32
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4. Conclusion The structural and morphological properties of r.f. sputtered InN have been examined by various approaches, with particular focus on evolution of films with nitridation of the target. The films are polycrystalline with (0 0 2) preferred orientation perpendicular to the film substrates. The Bragg reflections are highly dependent on the nitrogen content, since the films suffer from nitrogen depletion. A strong relationship is found between the stoichiometry and the film structure. The crystallite sizes are also influenced by the film stoichiometry; with films grown from nitrided target having larger crystallite size. XPS depth profiling measurements show that the elemental content of the films is not homogeneous through the bulk of the film while the RBS measurements indicate improved stoichiometry for growth from a nitrided target. References w1x T.L. Tansley, C.P. Foley, J. Appl. Phys. 59 (1986) 3241. w2x O. Takai, J. Ebisawa, Y. Hisamatsu, Proceedings of the Seventh International Conference on Vacuum Metallurgy, Tokyo, Japan, November 26–30, 1982, p. 137.
w3x K.L. Westra, R.P.W. Lawson, M.J. Brett, J. Vac. Sci. Tech. A 6 (1988) 1373. w4x C.P. Foley, T.L. Tansley, Phys. Rev. B 33 (1986) 1430. w5x H. Takeda, T. Hada, Toyama Kogyoukute Semon Gakko Kiyo 11 (1977) 73. w6x T. Tsuchiya, H. Yamano, O. Miki, A. Wakahara, A. Yoshida, Jpn. Appl. Phys. 38 (1999) 1884. w7x O. Takai, J. Ebisawa, Y. Hisamatsu, Proceedings of the Seventh International Conference on Vacuum Metallurgy, Tokyo, Japan, November 26–30, 1982, p. 129. w8x N. Asai, Y. Inoue, H. Sugimura, O. Takai, Thin Solid Films 332 (1998) 267. w9x Q. Guo, N. Shingai, Y. Mitsuishi, M. Nishio, H. Ogawa, Thin Solid Films 243–344 (1999) 524. w10x O. Takai, K. Ikuta, Y. Inoue, Thin Solid Films 318 (1998) 148. w11x L.I. Maissel, R. Glang, Handbook of Thin Film Technology, McGraw-Hill Book Company, New York, 1970. w12x S. Berg, H.O. Blom, T. Larsson, C. Neder, J. Vac. Sci. Technol. A 5 (1987) 202. w13x B.R. Natarajan, A.H. Eltouky, J.E. Greene, Thin Solid Films 69 (1980) 201. w14x D.A. Anderson, G. Moddel, M.A. Paesler, W. Paul, J. Vac. Sci. Technol. 16 (1979) 906. w15x H.O. Blom, S. Berg, T. Larson, Thin Solid Films 130 (1985) 307.