- Email: [email protected]
The potential of ion beam techniques for the development of indium nitride
ARTICLE IN PRESS
Journal of Crystal Growth 269 (2004) 50–58
The potential of ion beam techniques for the development of indium nitride Heiko Timmersa,*, Santosh K. Shresthaa, Aidan P. Byrneb,c a
School of Physical, Environmental and Mathematical Sciences, University of New South Wales at the Australian Defence Force Academy, Canberra ACT 2600, Australia b Department of Nuclear Physics of the Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia c Department of Physics, Australian National University, Canberra ACT 0200, Australia
Abstract Several types of indium nitride films presently available have been studied with ion beam techniques including Elastic Recoil Detection (ERD) analysis and radioisotope implantation combined with Perturbed Angular Correlation spectroscopy. Severe beam-induced effusion of nitrogen during ERD analysis can successfully be modelled and accurate compositional information is obtained. All types of films analysed are nitrogen-rich. Perturbed Angular Correlation spectroscopy has been demonstrated to be able to detect indium clustering in indium nitride. Indium nitride is extremely sensitive to irradiation with heavy ions both at keV- and MeV-energies. For keV-energies heavy ion fluences of the order of 1014 cm2 can be expected to result in dissociation and significant nitrogen loss. r 2004 Elsevier B.V. All rights reserved. PACS: 23.20.En; 61.72.Vv; 61.80.Jh; 68.55.Nq; 81.05.Ea; 81.40.Wx; 82.80.Ej; 82.80.Yc Keywords: A1. Characterization; A1. Composition; A1. Ion beam analysis; A1. Irradiation; B1. Indium nitride; B2. Semiconducting III–V materials
1. Introduction Beams of swift ions are extensively used in the development and processing of semiconductor materials to achieve modifications such as, for example, n- or p-type doping, the creation of damage-layers to trap metal contaminants, elec*Corresponding author. Tel.: +61-2-6268-8768; fax: +61-26268-8786. E-mail address: [email protected] (H. Timmers).
trical isolation, the controlled alteration of emission wavelengths, or structuring on the nanometre-scale [1]. For indium nitride [2] few results about its response to energetic ions are yet available despite of the importance ion beam technology may have for future processing of devices based on this material. Furthermore, resistance to high-energy ion damage is an important requirement for the extraterrestrial deployment of possible photovoltaic cells employing indium nitride. Importantly, such resistance, or
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.05.033
ARTICLE IN PRESS H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
at least an understanding of any damage processes, is a necessary prerequisite for the successful characterisation of indium nitride with ion beam techniques [3]. In view of the rudimentary knowledge of several fundamental materials properties for indium nitride including the value of the electronic band gap [4–7], the role of oxygen incorporation, or the importance of indium clusters [8], and persistent challenges to grow stoichiometric and crystalline material, analytical ion beam techniques appear well suited to aid further development. Quantitative compositional depth-profiles may be obtained from elastic ion scattering, the crystallinity and amorphisation of the material can be studied with ion channelling, while the local lattice environment and defects may be investigated through the ion implantation of radioactive probe nuclei [3]. Considering irradiation with 100 keV Si ions, Williams [1] has compared indium nitride to other semiconductors and expects that the amorphisation of crystalline indium nitride should occur at an ion fluence of B3 1014 ions/cm2, which is only slightly larger than that for silicon and contrasts sharply with the value for gallium nitride. The latter is more than two orders of magnitude larger because defect mobility results in dynamic annealing [1]. Bombarding indium nitride with F ions of similar energies (40–300 keV) and comparable fluences (8 1013–2 1015 cm2), Pearton et al.
51
[9] observed an increase in sheet resistance of up to two orders of magnitude. An increase in resistivity was also found by Emtsev et al. [10] following the irradiation of indium nitride with 1 1015– 2 1016 cm2 150 keV protons and has been attributed to nitrogen vacancies. Wu et al. [11] only measured a minute increase in resistivity following the exposure of indium nitride to 2 MeV protons with fluences in the range of 0.5 1014– 2.2 1014 cm2 and observed no significant change in photoluminescence. They do not comment, however, on the possibility of nitrogen release. While the results by Emtsev et al. [10] may indicate the loss of some nitrogen during exposure, Kosiba et al. [12] unambiguously found that much slower 1 keV Ar ions severely deplete nitrogen in indium nitride over much of the range of the Ar ions, well before the remaining indium atoms are sputtered. Given that the dissociation energy of indium nitride is very low, nitrogen loss during ion beam exposure may indeed be expected to be significant. This is also suggested by recent results [13] for ( thick indium 100 keV electrons. For a 100 A 2 nitride film only 170 kC/cm E1024 electrons/cm2 are sufficient to remove all the nitrogen. Table 1 gives an overview of the stopping forces in indium nitride for the ion species and energies discussed [14]. The table also includes the stopping forces for the ion beams used in the present work, in which indium nitride thin films, grown with a variety of
Table 1 Atomic numbers Z; nuclear and electronic stopping forces dE=dx; and ion ranges x for the ion beams discussed in this paper. [14]. Ion energy and species
Atomic number Z
Nuclear stopping force dE=dxjnuc: (keV/mm)
Electronic stopping force dE=dxjel: (keV/mm)
Ion range x (mm)
150 keV protons 2 MeV protons
1 1
0.15 0.02
101.1 29.4
2 MeV He
2
0.27
254
7.2
21.3 75.1 165.1 215.3 70.4 117.8 142.7
0.003 0.04 0.21 0.13 0.05 0.05 0.07
1 keV Ar 16 keV O 100 keV F 100 keV Si 62.5 keV Cu 109 keV In 160 keV In
18 8 9 14 29 49 49
206.4 113.6 98.3 234.2 877.4 1802 1848
200 MeV Au
79
186.2
18380
1.4 43.4
21.4
ARTICLE IN PRESS 52
H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
techniques, have been irradiated with keV- and MeV-heavy ions to obtain compositional and structural information. This has demonstrated the potential ion beam analysis holds for the further development of indium nitride materials. Importantly, it has also highlighted the extreme sensitivity of indium nitride to beam-induced nitrogen depletion, which may jeopardise the possibility of any future ion beam processing.
2. Experimental details Four different types of indium nitride films have been studied. They included films grown with molecular beam epitaxy (MBE) on an aluminium nitride buffer layer [15], two MBE-growth films without buffer layer [16], a film grown by remote plasma-enhanced chemical vapour deposition (RPE-CVD) [17], and a series of films grown with radio-frequency (RF) reactive ion sputtering of a metallic indium target in a nitrogen atmosphere [18]. Film thicknesses were in the range 300–1500 nm. In order to measure film composition and obtain elemental depth-profiles Elastic Recoil Detection (ERD) analysis was performed using a 200 MeV 197Au ion beam delivered by the 14UD Pelletron accelerator at the Australian National University [19,20]. The electrical beam current incident on the films was typically 0.05 nA, corresponding to a particle current of 3 108 s1. Total fluences of projectile ions were of the order of f ¼ 123 1012 cm2. In order to allow the detection of recoil ions, the sample was tilted relative to the beam, so that the acute angle between the sample surface normal and the beam axis was 67.5 , while the ERD-detector [21,22] was located at a recoil angle of 45 relative to the beam direction. As part of the development of a negative ion implanter for radioisotope implantation to facilitate materials studies [23], an MBE-grown indium nitride film [15] was exposed over an area of 40 mm2 to heavy ions at keV-energies. The film structure was 1500 nm InN/220 nm GaN/10 nm AlN/ sapphire, however, the ions penetrated only the first 80 nm of the indium nitride. The purpose
of this experiment was the introduction of the radioisotope 111In into the film. It was implanted as 125 keV 111In16O molecular ions with the mass number A ¼ 127: Only a minute implanted activity of 0.1 nCi was achieved though. In order to realise maximum throughput for the radioisotope, the mass window of the implanter was set rather wide, so that ions with neighbouring mass numbers, i.e. A ¼ 126 or 128 may have not been fully suppressed. Indeed, the electrical beam current incident on the sample was measured as (4.270.4) nA, which could have not arisen from the few 111In16O ions. The parasitic stable beam 63 consisted most likely of either 63Cu Cu65Cu 2 or molecular ions originating from the copper sputter-cathode of the implanter ion source. On impact the 125 keV molecular Cu 2 ions dissociate into two elemental Cu ions which penetrate the indium nitride with an energy of 62.5 keV each. From the duration of the implantation and a careful measurement of the implanted area, which is clearly visible on the film sample, a total implanted fluence of (4.670.5) 1015 cm2 has been inferred. After removal from the implantation chamber, both the implanted area and also an unexposed part of the film were analysed with ERD in the same way as described above. The same radioisotope implanter [23] was used to introduce activities of 0.5 mCi of 111In, corresponding to a total fluence of 5 1011 cm2, into the first 80 nm of two indium nitride films grown by RF-sputtering [18]. In this case, the mass window of the implanter was restricted to A ¼ 127 and no parasitic stable beam was measured. Despite of the low incident fluence, the brown indium nitride films were discoloured in the beam spot area. Perturbed Angular Correlation (PAC) spectroscopy [3] was then performed on both of the samples. The radioisotope 111In dominantly decays via electron capture to 111Cd. The quadrupole moment of the intermediate state of a subsequent g–g cascade in 111Cd then interacts with the electric field gradient at the lattice location of the 111Cd nucleus. This causes a perturbation of the anisotropic emission pattern of the second g-ray with respect to the direction of the first one. The resulting time-dependence of this emission anisotropy RðtÞ is measured in PAC
ARTICLE IN PRESS H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
53
spectroscopy and can reveal information about the immediate lattice environment of the 111In(111Cd) probe.
3. The stoichiometry of indium nitride films The determination of the compositional stoichiometry of indium nitride is routinely attempted using Rutherford Backscattering Spectrometry (RBS) with typically 2 MeV 4He projectile ion beams. Results for films on standard substrates, such as sapphire, are generally poor, since the intense signals from the indium and the aluminium in the substrate obscure the weak nitrogen signal. Some success is possible when the films are grown on light substrates such as carbon, see for example Ref. [18]. An unambiguous separation of the nitrogen signal, however, can be achieved with ERD [3,20] analysis using energetic heavy ion projectiles. The experimental data collected during the ERD measurements in this work have been analysed as discussed previously [24,25]. As expected from results for silicon nitride [26] and gallium nitride [20] films, the nitrogen in the indium nitride films was depleted during analysis by the incident 200 MeV 197Au beam. Fig. 1 shows that the depletion rate is not constant and considerably larger for indium nitride than for gallium nitride. For comparison results for a MOCVD-grown GaAs(N) film, which contains a small fraction of nitrogen, are also shown as an inset in Fig. 1. In this case no nitrogen depletion occurs under identical experimental conditions. For indium nitride the depletion is immediate, while for the gallium nitride film analysed a threshold fluence of about 5 1011 cm2 is observed below which no depletion takes place. The nitrogen depletion process in indium nitride can successfully be described in analogy to models developed for the beam-induced hydrogen depletion of hydrogenated amorphous carbon films [27]. The passage of a swift ion breaks electronic bonds and thus activates nitrogen atoms, which then form strongly bound nitrogen molecules N2. Following formation, the N2 molecules easily diffuse out of the film. Fits based on this bulk
Fig. 1. The nitrogen-to-metal ratio Z measured with ERD analysis as a function of incident fluence f of 200 MeV 197Au ions for an indium nitride film (open circles), a gallium nitride film (filled diamonds) and a GaAs(N) film (triangles, inset). The solid curves in the main figure are fits with the bulk molecular recombination model. The dashed curve is a similar fit, which however also considers a delayed onset of the nitrogen depletion in gallium nitride. The extrapolation of the original Z is indicated on the y-axis.
molecular recombination model are also shown in Fig. 1 and are in excellent agreement with both the data for indium nitride and that for gallium nitride. It was found that the beam-induced nitrogen depletion was similar for all four types of indium nitride films. While the films grown by RFsputtering essentially completely lose their nitrogen, for the other types of films some of the nitrogen is retained. In the case of the MBE-films grown with aluminium buffer layer, nitrogen gas is trapped at the substrate interface underneath the film and de-laminates the film producing blisters. Due to the good agreement between model and data, the original nitrogen content could be established for all samples analysed. Since the number of indium atoms in the films remained unchanged during ERD analysis and any depletion of oxygen and carbon was linear with fluence and could easily be taken into account in the interpretation of the data, the stoichiometry of all films studied with ERD has been determined. While some of them included detectable amounts of hydrogen, this has been neglected for the purpose of this discussion.
ARTICLE IN PRESS H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58 MBE Cornell 1.35
Nitrogen/Indium
1.25
MBE Ioffe
RF-sputtered RPECVD
54
1.15
N/In
1.05 0.95
1.89 eV
0.85 15 at-% 10 at-% 5 at-%
Oxygen
5 at-%
Carbon 2 2.5 Band gap (eV)
Fig. 2. Stoichiometries of the different types of indium nitride films studied in this work as they have been measured with ERD analysis. Details are given in the text.
The results of the ERD analysis are displayed in Fig. 2. It is significant that all four types of indium nitride films are nitrogen-rich. The MBE films have nitrogen-to-indium ratios Z in the range 1.02–1.07. The nitrogen excess for the film from RPE-CVD with a measured ratio of Z ¼ 1:10 is only slightly larger, while all the films grown with RF-sputtering are considerably more nitrogenrich with nitrogen-to-indium ratios in the range 1.13–1.31. Interestingly, the nitrogen content of the films from RF-sputtering appears to be correlated to the band gap derived from optical transmission spectroscopy [6] and can consistently be extrapolated to the nominal indium nitride band gap of 1.89 eV [4]. The same set of films stands out due to a high oxygen content which is always of the order of 10 at% and more. In contrast, the films from MBE-growth and notably from RPE-CVD contain only 1 at% or less oxygen. The MBE-films from Cornell University impress with oxygen contents below 0.01 at%. This oxygen is almost entirely due to surface oxidation or surface contamination. Similarly, these MBE-grown films do not show any significant incorporation of carbon, while this is an issue for the other three types of films with carbon contents in the range of 1–3 at%. In the case of the MBE-films grown at the Ioffe Institute the carbon is not uniformly distributed. While the films have a carbon content of about
Fig. 3. Energy spectrum for carbon recoil ions from an indium nitride film grown by MBE on sapphire [16]. The thick solid curve is an averaged representation of the measured spectrum shown as a thin curve. The correlation between the energy spectrum and the film structure is indicated. The intense recoil yield in the energy range 16–18 MeV demonstrates the existence of an 80 nm wide interface region.
1 at%, which is five times that of the sapphire substrate, a wide (B80 nm) interface exists with an increased carbon content of about 3 at%. The nonuniform carbon profile for one of these films is shown in Fig. 3. The analysis of the ERD measurements has demonstrated that the detailed understanding of the severe modifications of the indium nitride samples by the incident heavy ion beam was essential for a correct interpretation of the results. With this understanding important information has been obtained, notably, accurate stoichiometries showing nitrogen-excess, the level of oxygen and carbon incorporation, and elemental depthprofiles.
4. Nitrogen depletion of indium nitride by heavy ions at keV-energies While exposure to relatively low fluences (1012 cm2) of energetic 200 MeV 197Au ions has shown that indium nitride is considerably more sensitive to ion beam damage under these circumstances than comparable semiconductors such as for example gallium nitride, it is not clear to what extent these findings may be extrapolated to the
ARTICLE IN PRESS H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
experimental conditions for the ion beam implantation of heavy ions, which typically involves much lower energies of the order of 100 keV. The exposure of MBE-grown material to a fluence of B5 1015 cm2 62.5 keV Cu ions as described above can, however, give some guidance. Fig. 4 shows the results of the ERD measurements on this sample for regions inside and outside the area exposed to the Cu ions, respectively. While the energy spectrum of the indium recoil ions is unchanged in Fig. 4(a), the high-energy edge of the spectrum for nitrogen recoil ions from inside the exposed area of the film is shifted by 1.2 MeV to lower energies, when compared to that for the unexposed film. This implies that the bombardment by the Cu ions has completely removed the nitrogen from the near-surface region of the film. Using tabulated stopping powers [14] and a simulation routine [28] it has been verified that the depleted region corresponds to a depth interval of 80 nm, which is consistent with the penetration of 62.5 keV Cu ions. Interestingly, while the original film has negligible oxygen content, the oxygen spectrum for the irradiated area in Fig. 4(c) indicates oxidation over (a)
(b)
(c)
(d)
Fig. 4. ERD energy spectra for recoil ions from an MBEgrown indium nitride film. Dashed curves correspond to an asgrown region of the film, whereas solid curves represent the results for a region, which had been exposed to 62.5 keV Cu ions. While the indium spectrum in (a) is unchanged, the difference between the two nitrogen spectra in (b) indicates that the copper ions have completely removed the nitrogen in an 80 nm thick layer at the surface. (c) storage in air has resulted in the oxidation of this metallic indium layer in the irradiated region. (d) the carbon signals from both regions are similar.
55
the depth interval depleted of nitrogen, which presumably occurred, when the film was removed from the implantation chamber and stored in air. Fig. 4(d) shows that the carbon signal is unchanged. Since the stopping forces for other heavy ions typically used in ion implantation at keV-energies are rather similar to those for Cu ions, see Table 1, it may be concluded that for implantation fluences of 1015 cm2 indium nitride can be completely depleted of nitrogen, while significant nitrogenloss may already be expected for fluences of the order of 1014 cm2. Notably, this is of the order of the fluence for which amorphisation is observed for silicon and for which Williams [1] expects amorphisation to take place in indium nitride. The possibility of such an implantation-induced modification of indium nitride without the dissociation of indium and nitrogen is thus questionable.
5. Evidence for metallic indium clusters in indium nitride? Fig. 5 shows the time-dependence of the g-ray emission anisotropy measured with PAC spectroscopy for the radioisotope probe 111In(111Cd) in indium nitride films grown with RF-sputtering. In the figure the result is compared with that for a metallic indium sample. It is apparent that the data for indium nitride reflect those for metallic indium, with a pronounced dip at a time close to 400 ns, albeit with much smaller amplitude. By fitting the results for indium nitride a quadrupole interaction frequency of nQ ¼ ð1571Þ MHz has been extracted which agrees with that expected for polycrystalline indium metal. Annealing at 200 C did not alter the time-dependency of the anisotropy. An equivalent result has already been obtained by Lorenz and Vianden [29] for indium nitride from MBE-growth, who have also observed and interpreted changes of the anisotropy time-dependency for annealing at temperatures above 200 C. Both studies suggest the presence of metallic indium clusters in indium nitride. For the data in Fig. 5 the fit suggests that up to 16% of the 111 In(111Cd) probes are located in a metallic
ARTICLE IN PRESS H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
56
(a)
could imply that some nitrogen was removed during the implantation of the probe nuclei, even though in this case the incident fluence may have been as low as 5 1011 cm2. Such implantationinduced nitrogen-loss could explain the observation of the quadrupole interaction frequency for polycrystalline indium metal in the indium nitride films, since the nitrogen would preferentially be removed along the track of the ion and in the vicinity of its final location.
(b)
6. Conclusions
Fig. 5. The time-dependence RðtÞ of the g-ray emission anisotropy measured with Perturbed Angular Correlation (PAC) spectroscopy for (a) indium metal and (b) indium nitride using a 111In(111Cd) probe. The dips in the spectra near 400 ns are characteristic of polycrystalline indium metal. The solid curve in (b) is a fit to the data. Details are given in the text.
indium environment. This is a significant result, since the incorporation of indium clusters or droplets is associated with the apparent low electronic band gap of some types of indium nitride films [8]. The latter is highly contested and presently unexplained [4–7]. A thorough study of the implantation process needed to introduce the radioisotope probes before PAC spectroscopy is, however, necessary. While Lorenz and Vianden [29] do not comment on any compositional changes of the indium nitride during the implantation of the 160 keV 111In ions in their study, they quote an implantation fluence of 2 1013 cm2 which is only an order of magnitude lower than that for which significant nitrogen loss may be expected based on the experimental results discussed in Section 4. The discolouring of the implantation beam spot area on the two films from RF-sputtering studied in the present work
Despite the severe nitrogen depletion induced by the projectile beam, the strength of Elastic Recoil Detection (ERD) analysis to generate an individual depth-profile for each chemical element in indium nitride can be exploited to obtain accurate compositional information for this material. The depletion process has been understood as being dominated by the effusion of nitrogen molecules and can successfully be modelled. The analysis presented here has shown that the indium nitride films currently available from MBE, RPE-CVD and RF-sputtering are nitrogen-rich, while the oxygen and carbon content of those films has been determined unambiguously. Importantly though, the analysis has also demonstrated that indium nitride is dramatically more sensitive to ion beam exposure than other comparable materials. While the potential of ion beam techniques for the development of this material is considerable, in the context of both, materials-characterisation and materials-engineering, this extreme sensitivity has to be carefully considered. The present controversy about the correct value of the electronic band gap for indium nitride may be aided by studies with the radioisotope probe 111 In(111Cd) and Perturbed Angular Correlation spectroscopy, which has been demonstrated here to be able to detect indium clustering in indium nitride in agreement with a previous study. Such clusters or droplets of indium metal are suggested to possibly be responsible for the apparent observation of band gaps well below the nominal value of 1.89 eV. Similar to ERD analysis though, any dissociation of the indium nitride during the
ARTICLE IN PRESS H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
radioisotope implantation and subsequent nitrogen loss has to be either identified or excluded, so that reliable results can be obtained. At present it is not clear, if there is any significant nitrogen loss at the relatively low-fluences required to introduce the radioisotope probes. In contrast, the small number of results yet available for heavy ion implantation at energies in the range of 10– 200 keV clearly indicate that significant nitrogen loss can be expected at fluences of the order of 1014 cm2, if not before that. This is significant, because it is anticipated that such ion fluences would for example be required to amorphise crystalline indium nitride in any future ion beam processing. It is therefore questionable, if standard ion implantation techniques, as they are extensively used in the manufacture of other semiconductor materials, might be applicable in the case of indium nitride.
Acknowledgements The provision of samples of indium nitride films suitable for ion beam analysis by Bill Schaff, Cornell University, by Sergei Ivanov, Ioffe Institute, and by K. Scott Butcher and Marie Wintrebert-Fouquet, Macquarie University, is gratefully acknowledged. The authors are also grateful to Qiang (Michael) Gao, Australian National University, for making available a GaAs(N) film and would like to thank Reiner Vianden for guidance with the analysis of the PAC results.
References [1] J.S. Williams, Mater. Sci. Eng. A 253 (1998) 8. [2] A.G. Bhuiyan, A. Hashimoto, A. Yamamoto, J. Appl. Phys. 94 (5) (2003) 2779. [3] G. Schatz, A. Weidinger, Nuclear Condensed Matter Physics: Nuclear Methods and Applications, 2nd Edition, Wiley, New York, 1996, 290pp., ISBN 0471954799 and references therein. [4] T.L. Tansley, C.P. Foley, J. Appl. Phys. 60 (1986) 2092. [5] J. Wu, W. Walukiewicz, K.M. Yu, J.W. III Arger, E.E. Haller, H. Lu, W.J. Schaff, Y. Saito, Y. Nanishi, Appl. Phys. Lett. 80 (2002) 3967.
57
[6] K.S.A. Butcher, M. Wintrebert-Fouquet, P.P.-T. Chen, H. Timmers, S.K. Shrestha, Mater. Sci. Semicond. Process. 6 (2003) 351. [7] B.R. Nag, Phys. Stat. Sol. (b) 237 (2) R1-R2 (2003) and references therein; see also several contributions to the Proceedings of the International Indium Nitride Workshop, 16–20 November 2003, Fremantle, Australia, J. Crystal Growth (2004). [8] S.V. Ivanov, Proceedings of the International Indium Nitride Workshop, 16–20 November 2003, Fremantle, Australia, J. Crystal Growth (2004). [9] S.J. Pearton, C.B. Vartuli, R.J. Shul, J.C. Zolper, Mater. Sci. Eng. B 31 (1995) 309. [10] V.V. Emtsev, V.Yu. Davydov, E.E. Haller, A.A. Klochikhin, V.V. Kozlovskii, G.A. Oganesyan, D.S. Poloskin, N.M. Shmidt, V.A. Vekshin, A.S. Usikov, Physica B 308–310 (2001) 58. [11] J. Wu, W. Walukiewicz, K.M. Yu, W. Shan, J.W. Ager III, E.E. Haller, Hai Lu, W.J. Schaff, W.K. Metzger, S. Kurtz, J. Appl. Phys. 94 (10) (2003) 6477. [12] R. Kosiba, G. Ecke, V. Cimalla, L. SpieX, S. Krischok, J.A. Schaefer, O. Ambacher, W.J. Schaff, Nucl. Instrum. Methods B (2004), in press. [13] K.A. Mkhoyan, J. Silcox, Appl. Phys. Lett. 82 (6) (2003) 859. [14] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Matter, Pergamon Press, New York, 1985; Stopping power compilation SRIM-2003, http:// www.srim.org/. [15] H. Lu, W.J. Schaff, J. Hwang, H. Wu, G. Koley, L.F. Eastman, Appl. Phys. Lett. 79 (2001) 1489. [16] V. Tu Davydov, A.A. Klochikhin, V.V. Emtsev, D.A. Kurdyukov, S.V. Ivanov, V.A. Vekshin, F. Bechstedt, J. Furthmuller, J. Aderhold, J. Graul, A.V. Mudryi, H. Harima, A. Hashimoto, A. Yamamoto, E.E. Haller, Phys. Status Solidi B 234 (2002) 787. [17] M. Wintrebert-Fouquet, K.S.A. Butcher, P.P.-T. Chen, Proceedings of the International Indium Nitride Workshop, 16–20 November 2003, Fremantle, Australia, J. Crystal Growth (2004). [18] S. Kumar, Li Mo, T.L. Motlan, Tansley, Jpn. J. Appl. Phys. 35 (1996) 2261. [19] H. Timmers, G.R. Palmer, T.R. Ophel, D.J. O’Connor, R.G. Elliman, Nucl. Instrum. Methods B 136–138 (1998) 611. [20] H. Timmers, T.D.M. Weijers, R.G. Elliman, Nucl. Instrum. Methods B 190 (1–4) (2002) 393. [21] H. Timmers, T.R. Ophel, R.G. Elliman, Nucl. Instrum. Methods B 161–163 (2000) 19. [22] H. Timmers, T.R. Ophel, R.G. Elliman, Nucl. Instrum. Methods A 447 (2000) 536. [23] S.K. Shrestha, H. Timmers, A.P. Byrne, D.H. Chaplin, A.V.J. Edge, W.D. Hutchison, Proceedings of the 13th Australian Conference on Nuclear Techniques of Analysis, Lucas Heights, Australia, 2003, pp. 197–200, ISBN 0 9577217 9 X. [24] S.K. Shrestha, H. Timmers, K.S.A. Scott Butcher, M. Wintrebert-Fouquet, Curr. Appl. Phys. 4 (2004) 237.
ARTICLE IN PRESS 58
H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
[25] S.K. Shrestha, K.S.A. Butcher, M. Wintrebert-Fouquet, H. Timmers, Nucl. Instrum. Methods 219–220 (2004) 686. [26] H. Timmers, T.D.M. Weijers, R.G. Elliman, J. Uribasterra, H.J. Whitlow, E.-L. Sarwe, Nucl. Instrum. Methods B 190 (2002) 428. [27] M.E. Adel, O. Amir, R. Kalish, L.C. Feldman, J. Appl. Phys. 66 (1989) 3248.
[28] M. Mayer, SIMNRA User’s Guide, Report IPP 9/113, Max-Planck-Institut fur . Plasmaphysik, Garching, Germany, 1997; M. Mayer, AIP Conference Proceedings 475 (1999) 541. [29] K. Lorenz, R. Vianden, Phys. Status Solidi (c) 0 (1) (2002) 413.
Journal of Crystal Growth 269 (2004) 50–58
The potential of ion beam techniques for the development of indium nitride Heiko Timmersa,*, Santosh K. Shresthaa, Aidan P. Byrneb,c a
School of Physical, Environmental and Mathematical Sciences, University of New South Wales at the Australian Defence Force Academy, Canberra ACT 2600, Australia b Department of Nuclear Physics of the Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia c Department of Physics, Australian National University, Canberra ACT 0200, Australia
Abstract Several types of indium nitride films presently available have been studied with ion beam techniques including Elastic Recoil Detection (ERD) analysis and radioisotope implantation combined with Perturbed Angular Correlation spectroscopy. Severe beam-induced effusion of nitrogen during ERD analysis can successfully be modelled and accurate compositional information is obtained. All types of films analysed are nitrogen-rich. Perturbed Angular Correlation spectroscopy has been demonstrated to be able to detect indium clustering in indium nitride. Indium nitride is extremely sensitive to irradiation with heavy ions both at keV- and MeV-energies. For keV-energies heavy ion fluences of the order of 1014 cm2 can be expected to result in dissociation and significant nitrogen loss. r 2004 Elsevier B.V. All rights reserved. PACS: 23.20.En; 61.72.Vv; 61.80.Jh; 68.55.Nq; 81.05.Ea; 81.40.Wx; 82.80.Ej; 82.80.Yc Keywords: A1. Characterization; A1. Composition; A1. Ion beam analysis; A1. Irradiation; B1. Indium nitride; B2. Semiconducting III–V materials
1. Introduction Beams of swift ions are extensively used in the development and processing of semiconductor materials to achieve modifications such as, for example, n- or p-type doping, the creation of damage-layers to trap metal contaminants, elec*Corresponding author. Tel.: +61-2-6268-8768; fax: +61-26268-8786. E-mail address: [email protected] (H. Timmers).
trical isolation, the controlled alteration of emission wavelengths, or structuring on the nanometre-scale [1]. For indium nitride [2] few results about its response to energetic ions are yet available despite of the importance ion beam technology may have for future processing of devices based on this material. Furthermore, resistance to high-energy ion damage is an important requirement for the extraterrestrial deployment of possible photovoltaic cells employing indium nitride. Importantly, such resistance, or
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.05.033
ARTICLE IN PRESS H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
at least an understanding of any damage processes, is a necessary prerequisite for the successful characterisation of indium nitride with ion beam techniques [3]. In view of the rudimentary knowledge of several fundamental materials properties for indium nitride including the value of the electronic band gap [4–7], the role of oxygen incorporation, or the importance of indium clusters [8], and persistent challenges to grow stoichiometric and crystalline material, analytical ion beam techniques appear well suited to aid further development. Quantitative compositional depth-profiles may be obtained from elastic ion scattering, the crystallinity and amorphisation of the material can be studied with ion channelling, while the local lattice environment and defects may be investigated through the ion implantation of radioactive probe nuclei [3]. Considering irradiation with 100 keV Si ions, Williams [1] has compared indium nitride to other semiconductors and expects that the amorphisation of crystalline indium nitride should occur at an ion fluence of B3 1014 ions/cm2, which is only slightly larger than that for silicon and contrasts sharply with the value for gallium nitride. The latter is more than two orders of magnitude larger because defect mobility results in dynamic annealing [1]. Bombarding indium nitride with F ions of similar energies (40–300 keV) and comparable fluences (8 1013–2 1015 cm2), Pearton et al.
51
[9] observed an increase in sheet resistance of up to two orders of magnitude. An increase in resistivity was also found by Emtsev et al. [10] following the irradiation of indium nitride with 1 1015– 2 1016 cm2 150 keV protons and has been attributed to nitrogen vacancies. Wu et al. [11] only measured a minute increase in resistivity following the exposure of indium nitride to 2 MeV protons with fluences in the range of 0.5 1014– 2.2 1014 cm2 and observed no significant change in photoluminescence. They do not comment, however, on the possibility of nitrogen release. While the results by Emtsev et al. [10] may indicate the loss of some nitrogen during exposure, Kosiba et al. [12] unambiguously found that much slower 1 keV Ar ions severely deplete nitrogen in indium nitride over much of the range of the Ar ions, well before the remaining indium atoms are sputtered. Given that the dissociation energy of indium nitride is very low, nitrogen loss during ion beam exposure may indeed be expected to be significant. This is also suggested by recent results [13] for ( thick indium 100 keV electrons. For a 100 A 2 nitride film only 170 kC/cm E1024 electrons/cm2 are sufficient to remove all the nitrogen. Table 1 gives an overview of the stopping forces in indium nitride for the ion species and energies discussed [14]. The table also includes the stopping forces for the ion beams used in the present work, in which indium nitride thin films, grown with a variety of
Table 1 Atomic numbers Z; nuclear and electronic stopping forces dE=dx; and ion ranges x for the ion beams discussed in this paper. [14]. Ion energy and species
Atomic number Z
Nuclear stopping force dE=dxjnuc: (keV/mm)
Electronic stopping force dE=dxjel: (keV/mm)
Ion range x (mm)
150 keV protons 2 MeV protons
1 1
0.15 0.02
101.1 29.4
2 MeV He
2
0.27
254
7.2
21.3 75.1 165.1 215.3 70.4 117.8 142.7
0.003 0.04 0.21 0.13 0.05 0.05 0.07
1 keV Ar 16 keV O 100 keV F 100 keV Si 62.5 keV Cu 109 keV In 160 keV In
18 8 9 14 29 49 49
206.4 113.6 98.3 234.2 877.4 1802 1848
200 MeV Au
79
186.2
18380
1.4 43.4
21.4
ARTICLE IN PRESS 52
H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
techniques, have been irradiated with keV- and MeV-heavy ions to obtain compositional and structural information. This has demonstrated the potential ion beam analysis holds for the further development of indium nitride materials. Importantly, it has also highlighted the extreme sensitivity of indium nitride to beam-induced nitrogen depletion, which may jeopardise the possibility of any future ion beam processing.
2. Experimental details Four different types of indium nitride films have been studied. They included films grown with molecular beam epitaxy (MBE) on an aluminium nitride buffer layer [15], two MBE-growth films without buffer layer [16], a film grown by remote plasma-enhanced chemical vapour deposition (RPE-CVD) [17], and a series of films grown with radio-frequency (RF) reactive ion sputtering of a metallic indium target in a nitrogen atmosphere [18]. Film thicknesses were in the range 300–1500 nm. In order to measure film composition and obtain elemental depth-profiles Elastic Recoil Detection (ERD) analysis was performed using a 200 MeV 197Au ion beam delivered by the 14UD Pelletron accelerator at the Australian National University [19,20]. The electrical beam current incident on the films was typically 0.05 nA, corresponding to a particle current of 3 108 s1. Total fluences of projectile ions were of the order of f ¼ 123 1012 cm2. In order to allow the detection of recoil ions, the sample was tilted relative to the beam, so that the acute angle between the sample surface normal and the beam axis was 67.5 , while the ERD-detector [21,22] was located at a recoil angle of 45 relative to the beam direction. As part of the development of a negative ion implanter for radioisotope implantation to facilitate materials studies [23], an MBE-grown indium nitride film [15] was exposed over an area of 40 mm2 to heavy ions at keV-energies. The film structure was 1500 nm InN/220 nm GaN/10 nm AlN/ sapphire, however, the ions penetrated only the first 80 nm of the indium nitride. The purpose
of this experiment was the introduction of the radioisotope 111In into the film. It was implanted as 125 keV 111In16O molecular ions with the mass number A ¼ 127: Only a minute implanted activity of 0.1 nCi was achieved though. In order to realise maximum throughput for the radioisotope, the mass window of the implanter was set rather wide, so that ions with neighbouring mass numbers, i.e. A ¼ 126 or 128 may have not been fully suppressed. Indeed, the electrical beam current incident on the sample was measured as (4.270.4) nA, which could have not arisen from the few 111In16O ions. The parasitic stable beam 63 consisted most likely of either 63Cu Cu65Cu 2 or molecular ions originating from the copper sputter-cathode of the implanter ion source. On impact the 125 keV molecular Cu 2 ions dissociate into two elemental Cu ions which penetrate the indium nitride with an energy of 62.5 keV each. From the duration of the implantation and a careful measurement of the implanted area, which is clearly visible on the film sample, a total implanted fluence of (4.670.5) 1015 cm2 has been inferred. After removal from the implantation chamber, both the implanted area and also an unexposed part of the film were analysed with ERD in the same way as described above. The same radioisotope implanter [23] was used to introduce activities of 0.5 mCi of 111In, corresponding to a total fluence of 5 1011 cm2, into the first 80 nm of two indium nitride films grown by RF-sputtering [18]. In this case, the mass window of the implanter was restricted to A ¼ 127 and no parasitic stable beam was measured. Despite of the low incident fluence, the brown indium nitride films were discoloured in the beam spot area. Perturbed Angular Correlation (PAC) spectroscopy [3] was then performed on both of the samples. The radioisotope 111In dominantly decays via electron capture to 111Cd. The quadrupole moment of the intermediate state of a subsequent g–g cascade in 111Cd then interacts with the electric field gradient at the lattice location of the 111Cd nucleus. This causes a perturbation of the anisotropic emission pattern of the second g-ray with respect to the direction of the first one. The resulting time-dependence of this emission anisotropy RðtÞ is measured in PAC
ARTICLE IN PRESS H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
53
spectroscopy and can reveal information about the immediate lattice environment of the 111In(111Cd) probe.
3. The stoichiometry of indium nitride films The determination of the compositional stoichiometry of indium nitride is routinely attempted using Rutherford Backscattering Spectrometry (RBS) with typically 2 MeV 4He projectile ion beams. Results for films on standard substrates, such as sapphire, are generally poor, since the intense signals from the indium and the aluminium in the substrate obscure the weak nitrogen signal. Some success is possible when the films are grown on light substrates such as carbon, see for example Ref. [18]. An unambiguous separation of the nitrogen signal, however, can be achieved with ERD [3,20] analysis using energetic heavy ion projectiles. The experimental data collected during the ERD measurements in this work have been analysed as discussed previously [24,25]. As expected from results for silicon nitride [26] and gallium nitride [20] films, the nitrogen in the indium nitride films was depleted during analysis by the incident 200 MeV 197Au beam. Fig. 1 shows that the depletion rate is not constant and considerably larger for indium nitride than for gallium nitride. For comparison results for a MOCVD-grown GaAs(N) film, which contains a small fraction of nitrogen, are also shown as an inset in Fig. 1. In this case no nitrogen depletion occurs under identical experimental conditions. For indium nitride the depletion is immediate, while for the gallium nitride film analysed a threshold fluence of about 5 1011 cm2 is observed below which no depletion takes place. The nitrogen depletion process in indium nitride can successfully be described in analogy to models developed for the beam-induced hydrogen depletion of hydrogenated amorphous carbon films [27]. The passage of a swift ion breaks electronic bonds and thus activates nitrogen atoms, which then form strongly bound nitrogen molecules N2. Following formation, the N2 molecules easily diffuse out of the film. Fits based on this bulk
Fig. 1. The nitrogen-to-metal ratio Z measured with ERD analysis as a function of incident fluence f of 200 MeV 197Au ions for an indium nitride film (open circles), a gallium nitride film (filled diamonds) and a GaAs(N) film (triangles, inset). The solid curves in the main figure are fits with the bulk molecular recombination model. The dashed curve is a similar fit, which however also considers a delayed onset of the nitrogen depletion in gallium nitride. The extrapolation of the original Z is indicated on the y-axis.
molecular recombination model are also shown in Fig. 1 and are in excellent agreement with both the data for indium nitride and that for gallium nitride. It was found that the beam-induced nitrogen depletion was similar for all four types of indium nitride films. While the films grown by RFsputtering essentially completely lose their nitrogen, for the other types of films some of the nitrogen is retained. In the case of the MBE-films grown with aluminium buffer layer, nitrogen gas is trapped at the substrate interface underneath the film and de-laminates the film producing blisters. Due to the good agreement between model and data, the original nitrogen content could be established for all samples analysed. Since the number of indium atoms in the films remained unchanged during ERD analysis and any depletion of oxygen and carbon was linear with fluence and could easily be taken into account in the interpretation of the data, the stoichiometry of all films studied with ERD has been determined. While some of them included detectable amounts of hydrogen, this has been neglected for the purpose of this discussion.
ARTICLE IN PRESS H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58 MBE Cornell 1.35
Nitrogen/Indium
1.25
MBE Ioffe
RF-sputtered RPECVD
54
1.15
N/In
1.05 0.95
1.89 eV
0.85 15 at-% 10 at-% 5 at-%
Oxygen
5 at-%
Carbon 2 2.5 Band gap (eV)
Fig. 2. Stoichiometries of the different types of indium nitride films studied in this work as they have been measured with ERD analysis. Details are given in the text.
The results of the ERD analysis are displayed in Fig. 2. It is significant that all four types of indium nitride films are nitrogen-rich. The MBE films have nitrogen-to-indium ratios Z in the range 1.02–1.07. The nitrogen excess for the film from RPE-CVD with a measured ratio of Z ¼ 1:10 is only slightly larger, while all the films grown with RF-sputtering are considerably more nitrogenrich with nitrogen-to-indium ratios in the range 1.13–1.31. Interestingly, the nitrogen content of the films from RF-sputtering appears to be correlated to the band gap derived from optical transmission spectroscopy [6] and can consistently be extrapolated to the nominal indium nitride band gap of 1.89 eV [4]. The same set of films stands out due to a high oxygen content which is always of the order of 10 at% and more. In contrast, the films from MBE-growth and notably from RPE-CVD contain only 1 at% or less oxygen. The MBE-films from Cornell University impress with oxygen contents below 0.01 at%. This oxygen is almost entirely due to surface oxidation or surface contamination. Similarly, these MBE-grown films do not show any significant incorporation of carbon, while this is an issue for the other three types of films with carbon contents in the range of 1–3 at%. In the case of the MBE-films grown at the Ioffe Institute the carbon is not uniformly distributed. While the films have a carbon content of about
Fig. 3. Energy spectrum for carbon recoil ions from an indium nitride film grown by MBE on sapphire [16]. The thick solid curve is an averaged representation of the measured spectrum shown as a thin curve. The correlation between the energy spectrum and the film structure is indicated. The intense recoil yield in the energy range 16–18 MeV demonstrates the existence of an 80 nm wide interface region.
1 at%, which is five times that of the sapphire substrate, a wide (B80 nm) interface exists with an increased carbon content of about 3 at%. The nonuniform carbon profile for one of these films is shown in Fig. 3. The analysis of the ERD measurements has demonstrated that the detailed understanding of the severe modifications of the indium nitride samples by the incident heavy ion beam was essential for a correct interpretation of the results. With this understanding important information has been obtained, notably, accurate stoichiometries showing nitrogen-excess, the level of oxygen and carbon incorporation, and elemental depthprofiles.
4. Nitrogen depletion of indium nitride by heavy ions at keV-energies While exposure to relatively low fluences (1012 cm2) of energetic 200 MeV 197Au ions has shown that indium nitride is considerably more sensitive to ion beam damage under these circumstances than comparable semiconductors such as for example gallium nitride, it is not clear to what extent these findings may be extrapolated to the
ARTICLE IN PRESS H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
experimental conditions for the ion beam implantation of heavy ions, which typically involves much lower energies of the order of 100 keV. The exposure of MBE-grown material to a fluence of B5 1015 cm2 62.5 keV Cu ions as described above can, however, give some guidance. Fig. 4 shows the results of the ERD measurements on this sample for regions inside and outside the area exposed to the Cu ions, respectively. While the energy spectrum of the indium recoil ions is unchanged in Fig. 4(a), the high-energy edge of the spectrum for nitrogen recoil ions from inside the exposed area of the film is shifted by 1.2 MeV to lower energies, when compared to that for the unexposed film. This implies that the bombardment by the Cu ions has completely removed the nitrogen from the near-surface region of the film. Using tabulated stopping powers [14] and a simulation routine [28] it has been verified that the depleted region corresponds to a depth interval of 80 nm, which is consistent with the penetration of 62.5 keV Cu ions. Interestingly, while the original film has negligible oxygen content, the oxygen spectrum for the irradiated area in Fig. 4(c) indicates oxidation over (a)
(b)
(c)
(d)
Fig. 4. ERD energy spectra for recoil ions from an MBEgrown indium nitride film. Dashed curves correspond to an asgrown region of the film, whereas solid curves represent the results for a region, which had been exposed to 62.5 keV Cu ions. While the indium spectrum in (a) is unchanged, the difference between the two nitrogen spectra in (b) indicates that the copper ions have completely removed the nitrogen in an 80 nm thick layer at the surface. (c) storage in air has resulted in the oxidation of this metallic indium layer in the irradiated region. (d) the carbon signals from both regions are similar.
55
the depth interval depleted of nitrogen, which presumably occurred, when the film was removed from the implantation chamber and stored in air. Fig. 4(d) shows that the carbon signal is unchanged. Since the stopping forces for other heavy ions typically used in ion implantation at keV-energies are rather similar to those for Cu ions, see Table 1, it may be concluded that for implantation fluences of 1015 cm2 indium nitride can be completely depleted of nitrogen, while significant nitrogenloss may already be expected for fluences of the order of 1014 cm2. Notably, this is of the order of the fluence for which amorphisation is observed for silicon and for which Williams [1] expects amorphisation to take place in indium nitride. The possibility of such an implantation-induced modification of indium nitride without the dissociation of indium and nitrogen is thus questionable.
5. Evidence for metallic indium clusters in indium nitride? Fig. 5 shows the time-dependence of the g-ray emission anisotropy measured with PAC spectroscopy for the radioisotope probe 111In(111Cd) in indium nitride films grown with RF-sputtering. In the figure the result is compared with that for a metallic indium sample. It is apparent that the data for indium nitride reflect those for metallic indium, with a pronounced dip at a time close to 400 ns, albeit with much smaller amplitude. By fitting the results for indium nitride a quadrupole interaction frequency of nQ ¼ ð1571Þ MHz has been extracted which agrees with that expected for polycrystalline indium metal. Annealing at 200 C did not alter the time-dependency of the anisotropy. An equivalent result has already been obtained by Lorenz and Vianden [29] for indium nitride from MBE-growth, who have also observed and interpreted changes of the anisotropy time-dependency for annealing at temperatures above 200 C. Both studies suggest the presence of metallic indium clusters in indium nitride. For the data in Fig. 5 the fit suggests that up to 16% of the 111 In(111Cd) probes are located in a metallic
ARTICLE IN PRESS H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
56
(a)
could imply that some nitrogen was removed during the implantation of the probe nuclei, even though in this case the incident fluence may have been as low as 5 1011 cm2. Such implantationinduced nitrogen-loss could explain the observation of the quadrupole interaction frequency for polycrystalline indium metal in the indium nitride films, since the nitrogen would preferentially be removed along the track of the ion and in the vicinity of its final location.
(b)
6. Conclusions
Fig. 5. The time-dependence RðtÞ of the g-ray emission anisotropy measured with Perturbed Angular Correlation (PAC) spectroscopy for (a) indium metal and (b) indium nitride using a 111In(111Cd) probe. The dips in the spectra near 400 ns are characteristic of polycrystalline indium metal. The solid curve in (b) is a fit to the data. Details are given in the text.
indium environment. This is a significant result, since the incorporation of indium clusters or droplets is associated with the apparent low electronic band gap of some types of indium nitride films [8]. The latter is highly contested and presently unexplained [4–7]. A thorough study of the implantation process needed to introduce the radioisotope probes before PAC spectroscopy is, however, necessary. While Lorenz and Vianden [29] do not comment on any compositional changes of the indium nitride during the implantation of the 160 keV 111In ions in their study, they quote an implantation fluence of 2 1013 cm2 which is only an order of magnitude lower than that for which significant nitrogen loss may be expected based on the experimental results discussed in Section 4. The discolouring of the implantation beam spot area on the two films from RF-sputtering studied in the present work
Despite the severe nitrogen depletion induced by the projectile beam, the strength of Elastic Recoil Detection (ERD) analysis to generate an individual depth-profile for each chemical element in indium nitride can be exploited to obtain accurate compositional information for this material. The depletion process has been understood as being dominated by the effusion of nitrogen molecules and can successfully be modelled. The analysis presented here has shown that the indium nitride films currently available from MBE, RPE-CVD and RF-sputtering are nitrogen-rich, while the oxygen and carbon content of those films has been determined unambiguously. Importantly though, the analysis has also demonstrated that indium nitride is dramatically more sensitive to ion beam exposure than other comparable materials. While the potential of ion beam techniques for the development of this material is considerable, in the context of both, materials-characterisation and materials-engineering, this extreme sensitivity has to be carefully considered. The present controversy about the correct value of the electronic band gap for indium nitride may be aided by studies with the radioisotope probe 111 In(111Cd) and Perturbed Angular Correlation spectroscopy, which has been demonstrated here to be able to detect indium clustering in indium nitride in agreement with a previous study. Such clusters or droplets of indium metal are suggested to possibly be responsible for the apparent observation of band gaps well below the nominal value of 1.89 eV. Similar to ERD analysis though, any dissociation of the indium nitride during the
ARTICLE IN PRESS H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
radioisotope implantation and subsequent nitrogen loss has to be either identified or excluded, so that reliable results can be obtained. At present it is not clear, if there is any significant nitrogen loss at the relatively low-fluences required to introduce the radioisotope probes. In contrast, the small number of results yet available for heavy ion implantation at energies in the range of 10– 200 keV clearly indicate that significant nitrogen loss can be expected at fluences of the order of 1014 cm2, if not before that. This is significant, because it is anticipated that such ion fluences would for example be required to amorphise crystalline indium nitride in any future ion beam processing. It is therefore questionable, if standard ion implantation techniques, as they are extensively used in the manufacture of other semiconductor materials, might be applicable in the case of indium nitride.
Acknowledgements The provision of samples of indium nitride films suitable for ion beam analysis by Bill Schaff, Cornell University, by Sergei Ivanov, Ioffe Institute, and by K. Scott Butcher and Marie Wintrebert-Fouquet, Macquarie University, is gratefully acknowledged. The authors are also grateful to Qiang (Michael) Gao, Australian National University, for making available a GaAs(N) film and would like to thank Reiner Vianden for guidance with the analysis of the PAC results.
References [1] J.S. Williams, Mater. Sci. Eng. A 253 (1998) 8. [2] A.G. Bhuiyan, A. Hashimoto, A. Yamamoto, J. Appl. Phys. 94 (5) (2003) 2779. [3] G. Schatz, A. Weidinger, Nuclear Condensed Matter Physics: Nuclear Methods and Applications, 2nd Edition, Wiley, New York, 1996, 290pp., ISBN 0471954799 and references therein. [4] T.L. Tansley, C.P. Foley, J. Appl. Phys. 60 (1986) 2092. [5] J. Wu, W. Walukiewicz, K.M. Yu, J.W. III Arger, E.E. Haller, H. Lu, W.J. Schaff, Y. Saito, Y. Nanishi, Appl. Phys. Lett. 80 (2002) 3967.
57
[6] K.S.A. Butcher, M. Wintrebert-Fouquet, P.P.-T. Chen, H. Timmers, S.K. Shrestha, Mater. Sci. Semicond. Process. 6 (2003) 351. [7] B.R. Nag, Phys. Stat. Sol. (b) 237 (2) R1-R2 (2003) and references therein; see also several contributions to the Proceedings of the International Indium Nitride Workshop, 16–20 November 2003, Fremantle, Australia, J. Crystal Growth (2004). [8] S.V. Ivanov, Proceedings of the International Indium Nitride Workshop, 16–20 November 2003, Fremantle, Australia, J. Crystal Growth (2004). [9] S.J. Pearton, C.B. Vartuli, R.J. Shul, J.C. Zolper, Mater. Sci. Eng. B 31 (1995) 309. [10] V.V. Emtsev, V.Yu. Davydov, E.E. Haller, A.A. Klochikhin, V.V. Kozlovskii, G.A. Oganesyan, D.S. Poloskin, N.M. Shmidt, V.A. Vekshin, A.S. Usikov, Physica B 308–310 (2001) 58. [11] J. Wu, W. Walukiewicz, K.M. Yu, W. Shan, J.W. Ager III, E.E. Haller, Hai Lu, W.J. Schaff, W.K. Metzger, S. Kurtz, J. Appl. Phys. 94 (10) (2003) 6477. [12] R. Kosiba, G. Ecke, V. Cimalla, L. SpieX, S. Krischok, J.A. Schaefer, O. Ambacher, W.J. Schaff, Nucl. Instrum. Methods B (2004), in press. [13] K.A. Mkhoyan, J. Silcox, Appl. Phys. Lett. 82 (6) (2003) 859. [14] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Matter, Pergamon Press, New York, 1985; Stopping power compilation SRIM-2003, http:// www.srim.org/. [15] H. Lu, W.J. Schaff, J. Hwang, H. Wu, G. Koley, L.F. Eastman, Appl. Phys. Lett. 79 (2001) 1489. [16] V. Tu Davydov, A.A. Klochikhin, V.V. Emtsev, D.A. Kurdyukov, S.V. Ivanov, V.A. Vekshin, F. Bechstedt, J. Furthmuller, J. Aderhold, J. Graul, A.V. Mudryi, H. Harima, A. Hashimoto, A. Yamamoto, E.E. Haller, Phys. Status Solidi B 234 (2002) 787. [17] M. Wintrebert-Fouquet, K.S.A. Butcher, P.P.-T. Chen, Proceedings of the International Indium Nitride Workshop, 16–20 November 2003, Fremantle, Australia, J. Crystal Growth (2004). [18] S. Kumar, Li Mo, T.L. Motlan, Tansley, Jpn. J. Appl. Phys. 35 (1996) 2261. [19] H. Timmers, G.R. Palmer, T.R. Ophel, D.J. O’Connor, R.G. Elliman, Nucl. Instrum. Methods B 136–138 (1998) 611. [20] H. Timmers, T.D.M. Weijers, R.G. Elliman, Nucl. Instrum. Methods B 190 (1–4) (2002) 393. [21] H. Timmers, T.R. Ophel, R.G. Elliman, Nucl. Instrum. Methods B 161–163 (2000) 19. [22] H. Timmers, T.R. Ophel, R.G. Elliman, Nucl. Instrum. Methods A 447 (2000) 536. [23] S.K. Shrestha, H. Timmers, A.P. Byrne, D.H. Chaplin, A.V.J. Edge, W.D. Hutchison, Proceedings of the 13th Australian Conference on Nuclear Techniques of Analysis, Lucas Heights, Australia, 2003, pp. 197–200, ISBN 0 9577217 9 X. [24] S.K. Shrestha, H. Timmers, K.S.A. Scott Butcher, M. Wintrebert-Fouquet, Curr. Appl. Phys. 4 (2004) 237.
ARTICLE IN PRESS 58
H. Timmers et al. / Journal of Crystal Growth 269 (2004) 50–58
[25] S.K. Shrestha, K.S.A. Butcher, M. Wintrebert-Fouquet, H. Timmers, Nucl. Instrum. Methods 219–220 (2004) 686. [26] H. Timmers, T.D.M. Weijers, R.G. Elliman, J. Uribasterra, H.J. Whitlow, E.-L. Sarwe, Nucl. Instrum. Methods B 190 (2002) 428. [27] M.E. Adel, O. Amir, R. Kalish, L.C. Feldman, J. Appl. Phys. 66 (1989) 3248.
[28] M. Mayer, SIMNRA User’s Guide, Report IPP 9/113, Max-Planck-Institut fur . Plasmaphysik, Garching, Germany, 1997; M. Mayer, AIP Conference Proceedings 475 (1999) 541. [29] K. Lorenz, R. Vianden, Phys. Status Solidi (c) 0 (1) (2002) 413.