- Email: [email protected]
Ion beam analysis of thin doped ZnO layers
Nuclear Instruments and Methods in Physics Research B 219–220 (2004) 708–712 www.elsevier.com/locate/nimb
Ion beam analysis of thin doped ZnO layers Leszek S. Wielunski a
a,*
, D.H. Hill a, J. Quinn a, R.A. Bartynski a, P. Wu b, Y. Lu b
Department of Physics and Astronomy and Laboratory for Surface Modifications, Rutgers, The State University of New Jersey, 136 Frelinghuysen Rd., Piscataway, NJ 08854, USA b Department of Electrical and Computer Engineering, Rutgers, The State University of New Jersey, 94 Brett Rd, Piscataway, NJ 08854, USA
Abstract Thin films of ZnO and its ternary alloys Mgx Zn1x O have broad applications in transducers, resonators and filters. Similarly, transition metal- [TM-] doped ZnO is a promising candidate diluted magnetic semiconductor material for spintronic and spin-photonic applications. ZnO is a wide band gap semiconductor with direct energy band gap of 3.32 eV which can be increased upon Mg doping. Rutherford backscattering spectroscopy (RBS) with 2 MeV He ions has been used to monitor the thermal diffusion doping process of ZnO films grown on R-sapphire and silicon substrates. Concentration profiles of Mg and TM doping materials are measured as a function of processing conditions. This is a difficult case of RBS analysis as the signal of the TM dopants overlap significantly with that of Zn. Details of RBS spectra collection and analysis will be discussed. Results are compared with XPS measurements of dopant concentration and chemical state in the surface and near surface regions. 2004 Elsevier B.V. All rights reserved. PACS: 85.75.)d; 85.75.Hh; 85.70.Kh; 68.35.Fx; 81.15.Gh Keywords: Rutherford backscattering spectroscopy; Diffusion doping; ZnO; Mgx Zn1x O; Mnx Zn1x O; Nix Zn1x O; Vx Zn1x O; Spintronics
1. Introduction Essentially all semiconductor devices currently in use rely on electronic charge control. However, the magnetic moment of electron, the spin, can be used to communicate and store information in new devices such as spin transistors and spin memory, which are often grouped under the term spin* Corresponding author. Tel.: +1-732-445-0320; fax: +1-732445-4991. E-mail address: [email protected] (L.S. Wielunski).
tronics. There are many essential requirements for achieving practical spintronic devices. These include efficient electrical injection of spin-polarized carriers, the capability to transport the carriers within a semiconductor and conducting oxide, the ability to detect spin-polarized carriers, and external control of carrier transport. Substantial progress in these areas is reported by authors using spin aligners and time resolved photoluminescence [1] and electroluminescence polarization [2]. About 90% injection efficiency is reported into a nonmagnetic semiconductor device by measuring the polarization state of the light emitted from GaAs/
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.147
L.S. Wielunski et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 708–712
AlGaAs light-emitting diodes [3]. However, these results are obtained mostly at very low temperatures which limits their direct applications. Growing interest in the fundamental properties and potential applications of spin-transport electronic devices is stimulated by theoretical predictions of magnetic semiconductors with practical ordering temperatures. Application of the Zener model of ferromagnetism [4] predicts high Curie temperatures that exceeded room temperature for GaN and ZnO containing 5% of Mn [5]. Theoretical studies show that ferromagnetism in substitutionally-doped Mn–ZnO is induced by hole doping, while similar doping of ZnO with V, Cr, Fe, Co or Ni produces ferromagnetism without any additional carrier doping [6]. Many theoretical and experimental works related to these materials have been published recently [7,8] and progress in this field can be clearly seen in a new review paper [8] and the references therein. The vast majority of this work has used ZnO films grown by pulsed laser deposition. In this work we study ZnO based materials, grown either by MOCVD or an ion sputtering process, that have potential applications in room temperature spintronics. We use 2 MeV He RBS to study Mg, V, Mn, Fe and Ni doping processes in thin layers of ZnO on sapphire (R-Al2 O3 ) and silicon.
2. Experimental Two sets of ZnO films were measured. One set is comprised of thin films of ZnO (300–600 nm thick) deposited on a single crystal R-sapphire substrate using an MOCVD technique described in detail by Gorla and coworkers [9]. For the second set of samples, ZnO films were sputter deposited using either of two ZnO targets, one containing 15 mol% Mg and the other 2 wt% Ni. All of the MOCVD-grown films and one sputtered film (fabricated using the Ni-containing target) were then coated with a thermally evaporated thin layer (20–50 nm) of doping metal (V, Mn, Fe and/or Ni). These structures were exposed to annealing at 400, 600 and 700 C in order to promote diffusion of deposited metal into the ZnO film. In some cases, the remaining undiffused metal was removed
709
using ion etching. After preparation, the samples were placed in the Rutgers Tandem RBS analysis chamber and 2 MeV He RBS spectra were collected with a standard data collection system. The final well-diffused sample spectrum can be simulated and relative concentrations of sample constituencies well estimated. In the case of Mg doping, in order to separate the Mg and Zn signals, higher Heþþ beam energy (2.6 MeV) was used. The same sets of films were studied using X-ray photoelectron spectroscopy (XPS) using an upgraded Kratos XSAM 800 system. Samples were analyzed after a short cycle of Ar ion sputtering to remove residual contaminants and the concentration and chemical state of the species in the near surface region of the films was determined. XPS sputter depth profiling was then used to investigate the composition deeper into the film and compared to the RBS results. 3. Results and discussion Fig. 1 shows examples of 2 MeV He RBS spectra obtained from V, Mn, Fe and Ni deposited on thin ZnO layer on top of a sapphire (R-Al2 O3 ) substrate. For the V-coated sample, the metal is as-grown, while the other samples were annealed to 600 C for 60 min. On R-Al2 O3 , the ZnO film is highly epitaxial with large grains, and grows with the c-axis in plane. The RBS signal of the deposited metal overlaps significantly with the Zn signal from the ZnO film, which complicates the analysis. The experimental results are compared with simulated RBS spectra [10]. Details of the structures used in simulation are described in the figure caption. For the as-deposited V film, a thin sharp peak of the deposited metal RBS signal is observed on top of the broad plateau associated with the Zn of the film. The width of this peak is consistent with the thickness of deposited metal and indicates no significant diffusion prior to annealing. 1 In the 1 The simulations have an error of 4% in metal concentration. They are less accurate for oxygen owing to lower RBS sensitivity. In particular, oxygen concentration in the V overlayer (which is expected owing to exposure to the ambient) is difficult to estimate owing to the small thickness of the layer and the large background under the O-signal.
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(d)
Zn1-xNixO
ZnO
300
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R-Al2O2
Zn
2000 O 1000 0 50
100
150 200 Channel
250
300
Fig. 1. 2 MeV He RBS spectra from MOCVD-grown ZnO thin film on R-Al2 O3 with different thin doping metal layer on the top: (a) V, (b) Mn, (c) Fe and (d) Ni. The RBS peak of the doping metal overlaps with a broad Zn signal in all shown cases. The arrows indicate positions of the surface edge for each element. RBS simulations (shown as continuous line) are: (a) 220 · 1015 atoms of V0:55 O0:45 on 2000 · 1015 atoms of Zn0:5 O0:5 on Al2 O3 ; (b) 320 · 1015 atoms of Mn0:3 Zn0:2 O0:5 on 3000 · 1015 atoms of Zn0:5 O0:5 on Al2 O3 ; (c) 190 · 1015 atoms of Fe0:25 Zn0:25 O0:5 on 1900 · 1015 atoms of Zn0:5 O0:5 on Al2 O3 and (d) 200 · 1015 atoms of Ni0:46 Zn0:04 O0:5 on 100 · 1015 atoms of Ni0:2 Zn0:3 O0:5 on 100 · 1015 atoms of Ni0:1 Zn0:4 O0:5 on 2170 · 1015 atoms of Zn0:5 O0:5 on Al2 O3 .
annealed structures, the TM peak is broader and the analysis shows significant alloying with Zn indicating that diffusion has occurred. However, it is clear that, even after annealing, the TM concentration is strongly peaked within the first 20–30 nm of the film. Fig. 2(a) and (b) shows XPS spectra from the Mn- and Fe-doped films. The presence of the satellite feature in the solid curves (noted by arrows) indicates that there is a significant concentration of TM ions in the 2+ oxidation state, which is expected for substitutional doping. Fig. 2(c) shows the results of XPS sputter depth profiling of these films. Both the Zn:TM ratios and the thicknesses of the TM-doped regions agree well with those found by RBS. The profiles are rather uniform, except for Co which exhibits a significant slope throughout the probed region. Magnetic measurements show that the Fe- and Mn-doped films exhibit evidence for ferromagnetic behavior, with the Fe system having a Curie temperature near room temperature. Although these results do not prove substitutional doping in the films, they show a very promising trend. Fig. 3(a) shows the RBS spectrum from a 10 nm Mn overlayer on a sputter-deposited ZnO film on a Si substrate after annealing at 700 C for 12 h. Under these conditions, ZnO growth with the caxis normal to the substrate is preferred. In sharp contrast to the results obtained for ZnO grown on R-sapphire, there is only small Mn peak on the top of the broad Zn plateau. This peak corresponds, according to our RBS simulation, to about 14% (atomic) metal fraction of Mn and 14% of Ni in the first 4.5 · 1017 atoms/cm2 and 6% of Mn and 6% of Ni in the next 4.5 · 1017 atoms/cm2 . The remaining part of the ZnO film (5.45 · 1018 atoms/ cm2 ) contains about 4% (atomic) metal fraction of Ni in Zn. It must be pointed out that due to signal overlap in this case this fraction is not known precisely. Similar difficulties are observed with Fe, Mn or V doping concentration measurements (not shown). These results indicate, however, that TM diffusion is much more facile in ZnO films grown on Si as compared to R-Al2 O3 . This may be caused by the different crystallographic orientation of the ZnO film, or it may be a consequence of the larger density of grain boundaries in the sputtered films
L.S. Wielunski et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 708–712
Mn -doped ZnO
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Zn1-yNiyO
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545 Mg 0 50
100 Ni Mn Fe Co
Per Cent of Metal
80 60 40 20
(c) 0 0
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50
Fig. 2. (a) Mn 2p and (b) Fe 2p XPS spectra from MOCVDgrown ZnO films diffusion-doped with transition metals Mn and Fe; (c) XPS sputter depth profiles for these and similarly prepared Ni- and Co-doped films.
grown on Si substrates. Further studies to distinguish between these possibilities are in progress.
100
150 200 Channel
250
300
Fig. 3. 2 MeV He RBS spectra of (a) Mn diffusion-doped ZnO film grown from Ni-containing ZnO sputter target on a Si substrate; (b) ZnO film grown from Mg-containing sputter target (no TM) grown on an Al2 O3 substrate. The simulated spectra (shown as continuous line) are: (a) 450 · 1015 atoms of Mn0:07 Ni0:07 Zn0:36 O0:5 on 450 · 1015 atoms of Mn0:03 Ni0:03 Zn0:44 O0:5 on 5450 · 1015 atoms of Ni0:02 Zn0:48 O0:5 on 400 · 1015 atoms of SiO2 on Si and (b) 5670 · 1015 atoms of Mg0:081 Zn0:419 O0:5 on Al2 O3 . In order to separate Mg RBS signal from the Zn signal the same sample as in (b) was remeasured with higher energy 2.6 MeV He and the RBS spectrum is shown in panel (c). The simulated 2.6 MeV RBS spectrum (solid line) used the same sample structure as in 2 MeV case.
Turning now to the case of Mg doping, due to its substantial mass difference compared to Zn, the Mg RBS signal appears at lower energies in the spectrum. Fig. 3(b) shows a 2 MeV He RBS spectrum from a Mg-doped ZnO film on an Rsapphire substrate after annealing at 400 C. In
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L.S. Wielunski et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 708–712
this case, the Mg RBS signal from the sample surface is almost at the same energy as the Zn signal from ZnO/Al2 O3 interface and the Mg signal can be clearly seen in the region between channel 115 and 150 without interference from the strong Zn peak and O step. This permits one to measure a Mg concentration in the first 2.8 · 1018 atoms/cm2 of the film with high precision. The Mg concentration is constant in this region and it is 16% (atomic) metal fraction of Mg in Zn according to our RBS simulation. We point out, however, that in the spectral region spanned by the Mg signal there are background counts from the tail from Zn peak and the above result can be, to some extent, overestimate of Mg concentration. A separate RBS measurement has been performed for this sample with higher He energy: 2.6 MeV. Higher energy allows one to separate Mg signal from the low energy edge of the Zn peak and to observe the level of background below the Zn peak. Fig. 3(c) shows 2.6 MeV He RBS spectrum and the simulation for this sample. The level of the background below Zn peak is very low and it is not affecting substantially Mg concentration determination. In this case it is very clear that there is no Mg peak near the sample surface.
4. Conclusions We have used RBS analysis to characterize Mgdoped and TM-doped ZnO thin films that were grown on either R-Al2 O3 or Si substrates. In all cases, the analysis is complicated by overlap of the dopant signal with that of Zn. We find that TM diffusion is much more facile for ZnO films sputter deposited on Si as compared to the higher quality CVD-grown ZnO on R-sapphire. This may either be the result of the different orientation of the ZnO films on these two substrates, or the larger density of grain boundaries in ZnO grown on Si. For the
Mg-doped films, of interest for band gap engineering of ZnO, the Mg concentration and depth profile can be accurately determined by employing an appropriate choice of film thickness and/or beam energies that enables separation of the Mg signal from that of Zn in the film.
Acknowledgements This work has been funded by NJCHE: New Jersey Commission on Higher Education HighTech Workforce Excellence Grant #02-801020-08 and the National Science Foundation under grant ECS-0224166. L.W. would like to express his thanks to Theodore Madey for his scientific support and help with manuscript.
References [1] M. Oestreich, J. H€ ubner, D. H€agele, P.J. Klar, W. Heimbrodt, W.W. R€ uhle, D.E. Ashenford, B. Lunn, Appl. Phys. Lett. 74 (1999) 1251. [2] Y. Ohno, D.K. Young, B. Beschoten, F. Matsukura, H. Ohno, D.D. Awschalom, Nature 402 (1999) 790. [3] R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, L.W. Molenkamp, Nature 402 (1999) 787. [4] C. Zener, Phys. Rev. 81 (1951) 440; Phys. Rev. 83 (1951) 299. [5] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [6] K. Sato, H. Katayama-Yoshioda, Physica E 10 (2001) 251. [7] H. Ohno, Science 281 (1998) 951. [8] S.J. Pearton, C.R. Abernathy, M.E. Overberg, G.T. Thaler, D.P. Norton, N. Theodoropoulou, A.F. Hebard, Y.D. Park, F. Ren, J. Kim, L.A. Boatner, J. Appl. Phys. 93 (2003) 1. [9] C.R. Gorla, N.W. Emanetoglu, S. Liang, W.E. Mayo, Y. Lu, M. Wraback, H. Shen, J. Appl. Phys. 85 (1999) 2595. [10] SIMNRA version 4.40, Dr Matej Mayer, Forschungszentrum J€ ulich Institut f€ ur Plasmaphysik, D-52425, Germany.
Ion beam analysis of thin doped ZnO layers Leszek S. Wielunski a
a,*
, D.H. Hill a, J. Quinn a, R.A. Bartynski a, P. Wu b, Y. Lu b
Department of Physics and Astronomy and Laboratory for Surface Modifications, Rutgers, The State University of New Jersey, 136 Frelinghuysen Rd., Piscataway, NJ 08854, USA b Department of Electrical and Computer Engineering, Rutgers, The State University of New Jersey, 94 Brett Rd, Piscataway, NJ 08854, USA
Abstract Thin films of ZnO and its ternary alloys Mgx Zn1x O have broad applications in transducers, resonators and filters. Similarly, transition metal- [TM-] doped ZnO is a promising candidate diluted magnetic semiconductor material for spintronic and spin-photonic applications. ZnO is a wide band gap semiconductor with direct energy band gap of 3.32 eV which can be increased upon Mg doping. Rutherford backscattering spectroscopy (RBS) with 2 MeV He ions has been used to monitor the thermal diffusion doping process of ZnO films grown on R-sapphire and silicon substrates. Concentration profiles of Mg and TM doping materials are measured as a function of processing conditions. This is a difficult case of RBS analysis as the signal of the TM dopants overlap significantly with that of Zn. Details of RBS spectra collection and analysis will be discussed. Results are compared with XPS measurements of dopant concentration and chemical state in the surface and near surface regions. 2004 Elsevier B.V. All rights reserved. PACS: 85.75.)d; 85.75.Hh; 85.70.Kh; 68.35.Fx; 81.15.Gh Keywords: Rutherford backscattering spectroscopy; Diffusion doping; ZnO; Mgx Zn1x O; Mnx Zn1x O; Nix Zn1x O; Vx Zn1x O; Spintronics
1. Introduction Essentially all semiconductor devices currently in use rely on electronic charge control. However, the magnetic moment of electron, the spin, can be used to communicate and store information in new devices such as spin transistors and spin memory, which are often grouped under the term spin* Corresponding author. Tel.: +1-732-445-0320; fax: +1-732445-4991. E-mail address: [email protected] (L.S. Wielunski).
tronics. There are many essential requirements for achieving practical spintronic devices. These include efficient electrical injection of spin-polarized carriers, the capability to transport the carriers within a semiconductor and conducting oxide, the ability to detect spin-polarized carriers, and external control of carrier transport. Substantial progress in these areas is reported by authors using spin aligners and time resolved photoluminescence [1] and electroluminescence polarization [2]. About 90% injection efficiency is reported into a nonmagnetic semiconductor device by measuring the polarization state of the light emitted from GaAs/
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.147
L.S. Wielunski et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 708–712
AlGaAs light-emitting diodes [3]. However, these results are obtained mostly at very low temperatures which limits their direct applications. Growing interest in the fundamental properties and potential applications of spin-transport electronic devices is stimulated by theoretical predictions of magnetic semiconductors with practical ordering temperatures. Application of the Zener model of ferromagnetism [4] predicts high Curie temperatures that exceeded room temperature for GaN and ZnO containing 5% of Mn [5]. Theoretical studies show that ferromagnetism in substitutionally-doped Mn–ZnO is induced by hole doping, while similar doping of ZnO with V, Cr, Fe, Co or Ni produces ferromagnetism without any additional carrier doping [6]. Many theoretical and experimental works related to these materials have been published recently [7,8] and progress in this field can be clearly seen in a new review paper [8] and the references therein. The vast majority of this work has used ZnO films grown by pulsed laser deposition. In this work we study ZnO based materials, grown either by MOCVD or an ion sputtering process, that have potential applications in room temperature spintronics. We use 2 MeV He RBS to study Mg, V, Mn, Fe and Ni doping processes in thin layers of ZnO on sapphire (R-Al2 O3 ) and silicon.
2. Experimental Two sets of ZnO films were measured. One set is comprised of thin films of ZnO (300–600 nm thick) deposited on a single crystal R-sapphire substrate using an MOCVD technique described in detail by Gorla and coworkers [9]. For the second set of samples, ZnO films were sputter deposited using either of two ZnO targets, one containing 15 mol% Mg and the other 2 wt% Ni. All of the MOCVD-grown films and one sputtered film (fabricated using the Ni-containing target) were then coated with a thermally evaporated thin layer (20–50 nm) of doping metal (V, Mn, Fe and/or Ni). These structures were exposed to annealing at 400, 600 and 700 C in order to promote diffusion of deposited metal into the ZnO film. In some cases, the remaining undiffused metal was removed
709
using ion etching. After preparation, the samples were placed in the Rutgers Tandem RBS analysis chamber and 2 MeV He RBS spectra were collected with a standard data collection system. The final well-diffused sample spectrum can be simulated and relative concentrations of sample constituencies well estimated. In the case of Mg doping, in order to separate the Mg and Zn signals, higher Heþþ beam energy (2.6 MeV) was used. The same sets of films were studied using X-ray photoelectron spectroscopy (XPS) using an upgraded Kratos XSAM 800 system. Samples were analyzed after a short cycle of Ar ion sputtering to remove residual contaminants and the concentration and chemical state of the species in the near surface region of the films was determined. XPS sputter depth profiling was then used to investigate the composition deeper into the film and compared to the RBS results. 3. Results and discussion Fig. 1 shows examples of 2 MeV He RBS spectra obtained from V, Mn, Fe and Ni deposited on thin ZnO layer on top of a sapphire (R-Al2 O3 ) substrate. For the V-coated sample, the metal is as-grown, while the other samples were annealed to 600 C for 60 min. On R-Al2 O3 , the ZnO film is highly epitaxial with large grains, and grows with the c-axis in plane. The RBS signal of the deposited metal overlaps significantly with the Zn signal from the ZnO film, which complicates the analysis. The experimental results are compared with simulated RBS spectra [10]. Details of the structures used in simulation are described in the figure caption. For the as-deposited V film, a thin sharp peak of the deposited metal RBS signal is observed on top of the broad plateau associated with the Zn of the film. The width of this peak is consistent with the thickness of deposited metal and indicates no significant diffusion prior to annealing. 1 In the 1 The simulations have an error of 4% in metal concentration. They are less accurate for oxygen owing to lower RBS sensitivity. In particular, oxygen concentration in the V overlayer (which is expected owing to exposure to the ambient) is difficult to estimate owing to the small thickness of the layer and the large background under the O-signal.
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L.S. Wielunski et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 708–712 2000 V
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O 500 0 50
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O 1000
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ZnO
250 Fe
300 (c)
R-Al2O2 Counts
4000 Zn O 2000
0 50
100
150
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Counts
3000
(d)
Zn1-xNixO
ZnO
300
Ni
R-Al2O2
Zn
2000 O 1000 0 50
100
150 200 Channel
250
300
Fig. 1. 2 MeV He RBS spectra from MOCVD-grown ZnO thin film on R-Al2 O3 with different thin doping metal layer on the top: (a) V, (b) Mn, (c) Fe and (d) Ni. The RBS peak of the doping metal overlaps with a broad Zn signal in all shown cases. The arrows indicate positions of the surface edge for each element. RBS simulations (shown as continuous line) are: (a) 220 · 1015 atoms of V0:55 O0:45 on 2000 · 1015 atoms of Zn0:5 O0:5 on Al2 O3 ; (b) 320 · 1015 atoms of Mn0:3 Zn0:2 O0:5 on 3000 · 1015 atoms of Zn0:5 O0:5 on Al2 O3 ; (c) 190 · 1015 atoms of Fe0:25 Zn0:25 O0:5 on 1900 · 1015 atoms of Zn0:5 O0:5 on Al2 O3 and (d) 200 · 1015 atoms of Ni0:46 Zn0:04 O0:5 on 100 · 1015 atoms of Ni0:2 Zn0:3 O0:5 on 100 · 1015 atoms of Ni0:1 Zn0:4 O0:5 on 2170 · 1015 atoms of Zn0:5 O0:5 on Al2 O3 .
annealed structures, the TM peak is broader and the analysis shows significant alloying with Zn indicating that diffusion has occurred. However, it is clear that, even after annealing, the TM concentration is strongly peaked within the first 20–30 nm of the film. Fig. 2(a) and (b) shows XPS spectra from the Mn- and Fe-doped films. The presence of the satellite feature in the solid curves (noted by arrows) indicates that there is a significant concentration of TM ions in the 2+ oxidation state, which is expected for substitutional doping. Fig. 2(c) shows the results of XPS sputter depth profiling of these films. Both the Zn:TM ratios and the thicknesses of the TM-doped regions agree well with those found by RBS. The profiles are rather uniform, except for Co which exhibits a significant slope throughout the probed region. Magnetic measurements show that the Fe- and Mn-doped films exhibit evidence for ferromagnetic behavior, with the Fe system having a Curie temperature near room temperature. Although these results do not prove substitutional doping in the films, they show a very promising trend. Fig. 3(a) shows the RBS spectrum from a 10 nm Mn overlayer on a sputter-deposited ZnO film on a Si substrate after annealing at 700 C for 12 h. Under these conditions, ZnO growth with the caxis normal to the substrate is preferred. In sharp contrast to the results obtained for ZnO grown on R-sapphire, there is only small Mn peak on the top of the broad Zn plateau. This peak corresponds, according to our RBS simulation, to about 14% (atomic) metal fraction of Mn and 14% of Ni in the first 4.5 · 1017 atoms/cm2 and 6% of Mn and 6% of Ni in the next 4.5 · 1017 atoms/cm2 . The remaining part of the ZnO film (5.45 · 1018 atoms/ cm2 ) contains about 4% (atomic) metal fraction of Ni in Zn. It must be pointed out that due to signal overlap in this case this fraction is not known precisely. Similar difficulties are observed with Fe, Mn or V doping concentration measurements (not shown). These results indicate, however, that TM diffusion is much more facile in ZnO films grown on Si as compared to R-Al2 O3 . This may be caused by the different crystallographic orientation of the ZnO film, or it may be a consequence of the larger density of grain boundaries in the sputtered films
L.S. Wielunski et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 708–712
Mn -doped ZnO
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545 Mg 0 50
100 Ni Mn Fe Co
Per Cent of Metal
80 60 40 20
(c) 0 0
10 20 30 40 Approximate Sputter Depth (nm)
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Fig. 2. (a) Mn 2p and (b) Fe 2p XPS spectra from MOCVDgrown ZnO films diffusion-doped with transition metals Mn and Fe; (c) XPS sputter depth profiles for these and similarly prepared Ni- and Co-doped films.
grown on Si substrates. Further studies to distinguish between these possibilities are in progress.
100
150 200 Channel
250
300
Fig. 3. 2 MeV He RBS spectra of (a) Mn diffusion-doped ZnO film grown from Ni-containing ZnO sputter target on a Si substrate; (b) ZnO film grown from Mg-containing sputter target (no TM) grown on an Al2 O3 substrate. The simulated spectra (shown as continuous line) are: (a) 450 · 1015 atoms of Mn0:07 Ni0:07 Zn0:36 O0:5 on 450 · 1015 atoms of Mn0:03 Ni0:03 Zn0:44 O0:5 on 5450 · 1015 atoms of Ni0:02 Zn0:48 O0:5 on 400 · 1015 atoms of SiO2 on Si and (b) 5670 · 1015 atoms of Mg0:081 Zn0:419 O0:5 on Al2 O3 . In order to separate Mg RBS signal from the Zn signal the same sample as in (b) was remeasured with higher energy 2.6 MeV He and the RBS spectrum is shown in panel (c). The simulated 2.6 MeV RBS spectrum (solid line) used the same sample structure as in 2 MeV case.
Turning now to the case of Mg doping, due to its substantial mass difference compared to Zn, the Mg RBS signal appears at lower energies in the spectrum. Fig. 3(b) shows a 2 MeV He RBS spectrum from a Mg-doped ZnO film on an Rsapphire substrate after annealing at 400 C. In
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this case, the Mg RBS signal from the sample surface is almost at the same energy as the Zn signal from ZnO/Al2 O3 interface and the Mg signal can be clearly seen in the region between channel 115 and 150 without interference from the strong Zn peak and O step. This permits one to measure a Mg concentration in the first 2.8 · 1018 atoms/cm2 of the film with high precision. The Mg concentration is constant in this region and it is 16% (atomic) metal fraction of Mg in Zn according to our RBS simulation. We point out, however, that in the spectral region spanned by the Mg signal there are background counts from the tail from Zn peak and the above result can be, to some extent, overestimate of Mg concentration. A separate RBS measurement has been performed for this sample with higher He energy: 2.6 MeV. Higher energy allows one to separate Mg signal from the low energy edge of the Zn peak and to observe the level of background below the Zn peak. Fig. 3(c) shows 2.6 MeV He RBS spectrum and the simulation for this sample. The level of the background below Zn peak is very low and it is not affecting substantially Mg concentration determination. In this case it is very clear that there is no Mg peak near the sample surface.
4. Conclusions We have used RBS analysis to characterize Mgdoped and TM-doped ZnO thin films that were grown on either R-Al2 O3 or Si substrates. In all cases, the analysis is complicated by overlap of the dopant signal with that of Zn. We find that TM diffusion is much more facile for ZnO films sputter deposited on Si as compared to the higher quality CVD-grown ZnO on R-sapphire. This may either be the result of the different orientation of the ZnO films on these two substrates, or the larger density of grain boundaries in ZnO grown on Si. For the
Mg-doped films, of interest for band gap engineering of ZnO, the Mg concentration and depth profile can be accurately determined by employing an appropriate choice of film thickness and/or beam energies that enables separation of the Mg signal from that of Zn in the film.
Acknowledgements This work has been funded by NJCHE: New Jersey Commission on Higher Education HighTech Workforce Excellence Grant #02-801020-08 and the National Science Foundation under grant ECS-0224166. L.W. would like to express his thanks to Theodore Madey for his scientific support and help with manuscript.
References [1] M. Oestreich, J. H€ ubner, D. H€agele, P.J. Klar, W. Heimbrodt, W.W. R€ uhle, D.E. Ashenford, B. Lunn, Appl. Phys. Lett. 74 (1999) 1251. [2] Y. Ohno, D.K. Young, B. Beschoten, F. Matsukura, H. Ohno, D.D. Awschalom, Nature 402 (1999) 790. [3] R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, L.W. Molenkamp, Nature 402 (1999) 787. [4] C. Zener, Phys. Rev. 81 (1951) 440; Phys. Rev. 83 (1951) 299. [5] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [6] K. Sato, H. Katayama-Yoshioda, Physica E 10 (2001) 251. [7] H. Ohno, Science 281 (1998) 951. [8] S.J. Pearton, C.R. Abernathy, M.E. Overberg, G.T. Thaler, D.P. Norton, N. Theodoropoulou, A.F. Hebard, Y.D. Park, F. Ren, J. Kim, L.A. Boatner, J. Appl. Phys. 93 (2003) 1. [9] C.R. Gorla, N.W. Emanetoglu, S. Liang, W.E. Mayo, Y. Lu, M. Wraback, H. Shen, J. Appl. Phys. 85 (1999) 2595. [10] SIMNRA version 4.40, Dr Matej Mayer, Forschungszentrum J€ ulich Institut f€ ur Plasmaphysik, D-52425, Germany.