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Molecular effect on surface topography of GaN bombarded with PF4 ions
Vacuum 86 (2012) 1638e1641
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Molecular effect on surface topography of GaN bombarded with PF4 ions A.I. Titov a, *, P.A. Karaseov a, V.S. Belyakov a, K.V. Karabeshkin a, A.V. Arkhipov a, S.O. Kucheyev b, A.Yu. Azarov c a
State Polytechnic University, Polytechnicheskaya 29, 195251 St.Petersburg, Russian Federation Lawrence Livermore National Laboratory, Livermore, CA 94551, USA c University of Oslo, PO Box 1048 Blindern, NO-0316 Oslo, Norway b
a b s t r a c t Keywords: Ion implantation GaN Molecular ions Surface topography Collision cascade density Swelling Sputtering
We study surface topography and thickness of GaN layers implanted at room temperature with 1.3 keV/amu F, P, and PF4 cluster ions. Results show that the density of collision cascades has a dramatic effect on the surface roughness and the thickness of implanted layers. Surface roughness increases with increasing cascade density. For very dense cascades produced by PF4 ions, the evolution of layer thickness is dominated by ion-induced sputtering. In contrast, for the case of P ions producing less dense cascades, ioninduced swelling is observed. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ion implantation is an attractive processing tool for the fabrication of GaN-based (opto)electronic devices (see, for example, reviews [1,2]). A successful application of ion implantation, however, requires understanding of the formation and mitigation of ion-beam-produced damage. In addition to defects in the crystalline lattice, radiation damage could manifest as changes of surface topography. For GaN, ion-implantation-induced amorphization is accompanied by material decomposition with the formation of nanoscale N2 bubbles and concomitant changes in the material density and surface topography [2e14]. These include swelling and an increase in surface roughness. High-dose irradiation at elevated temperatures is also known to cause efficient surface erosion [15,16]. In certain irradiation regimes, bombardment under conditions with a larger average density of collision cascades results in an increase in the efficiency of radiation damage build-up (see, for example, reviews [17,18]). Such cascade density effects can be conveniently studied by irradiation with cluster ions. In this case, a dependence of radiation damage processes on the cascade density is often referred to as the molecular effect. Collision cascades created by components of the cluster ion overlap in the nearsurface region, creating combined individual collision cascades with larger volumetric densities of atomic displacements than in
* Corresponding author. Department of Physical Electronics, St. Petersburg State Polytechnic University, Polytechnicheskaya 29, 195251 St. Petersburg, Russian Federation. Tel.: þ7 812 552 7516; fax: þ7 812 552 9516. E-mail address: [email protected] (A.I. Titov). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.12.014
the case of irradiation with atomic ions of the same velocity. For example, irradiation of GaN with atomic and molecular Bi1,2 ions [19] and PF4 ions [20] results in a significant increase in the level of stable radiation damage in the near-surface region with increasing density of collision cascades. This effect has been attributed to the formation of non-linear energy spikes in GaN, as discussed in detail in Ref. [20]. The formation of the nanoporous structure and changes in surface topography of ion-bombarded GaN are related to nearsurface radiation damage [2e14]. Hence, the density of collision cascades should influence these phenomena. In the present work, we will refer to the 5 atoms in PF4 as a cluster and we study the molecular effect in the evolution of surface topography of GaN irradiated with F, P, and small cluster (PF4) ions. 2. Experimental Wurtzite (0001) GaN epilayers, grown by metal-organic vapourphase epitaxy at the Ioffe Institute (St. Petersburg, Russian Federation) on c-plane sapphire substrates, were implanted at room temperature with F, P, and PF4 ions with an energy of 1.3 keV/amu over a wide dose range. Implantation was carried out at 7 off the [0001] direction in order to minimize channelling. As discussed in detail in [21], cascade density effects can be conveniently studied by cluster ion irradiation when the following parameters are kept constant: ion energy normalized to the atomic mass unit (amu), ion dose normalized to the number of displacements per atom (DPA), and ion beam flux normalized to DPA$s1. The irradiation conditions for PFn ions used in this study comply with these requirements. Beam flux was adjusted to yield 3.6 103 DPA$s1 in all three cases.
A.I. Titov et al. / Vacuum 86 (2012) 1638e1641
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Values of DPA were calculated with the TRIM code (version SRIM-2003.26) [22] with an effective threshold energy for atomic displacements of 25 eV for both Ga and N sub-lattices. Quoted DPA values are concentrations of ion-beam-generated lattice vacancies (gv) at the maximum of the nuclear energy loss profile normalized to the atomic concentration of GaN (8.85 1022 atoms cm3). In calculations of the vacancy concentrations generated by molecular ions, we used a linear approximation: gv (PF4) ¼ gv(P) þ 4 gv(F). Surface topography was studied in the tapping mode of atomic force microscopy (AFM) with a Nano-DST instrument manufactured by Pacific Nanotechnology. We also measured step heights between implanted and unimplanted regions of the surface. Such regions were made by masking the GaN surface by a cleaved piece of a silicon wafer that was in mechanical contact with the sample surface during irradiation. Surface topography was measured over 1 1 mm2 areas located far from the border of the implanted regions to avoid possible (but unlikely due to relatively low ion fluences used) effects of mask sputtering [23].
w0.2 nm, with a density of threading dislocations of w108 cm2. Nanoparticles form on the surface as a result of ion bombardment. More importantly, Fig. 1 shows that irradiation-induced changes in surface topography strongly depend on the ion cluster size. Larger clusters lead to the formation of larger nanoparticles. Hence, the density of collision cascades has a dramatic effect on surface roughness. This effect is better illustrated in Fig. 2(a), showing the dose dependence of RMS roughness. It reveals that surface roughness increases with increasing either dose or ion mass. In particular, surface roughness of PF4-irradiated samples is over an order of magnitude larger than that of F-bombarded surfaces for the same dose expressed in DPA units. Fig. 2(a) also reveals a threshold dependence of the evolution of roughness, with threshold doses corresponding to w15 DPA. Such a threshold behaviour is consistent with previous reports [2,3,7].
3. Results
Fig. 3(a) and (b) show three-dimensional AFM images of the border between implanted and virgin areas of samples irradiated with P and PF4 ions, respectively. Both an irradiation-induced increase in surface roughness and the formation of large steps between implanted and virgin areas are clearly seen in Fig. 3. Irradiation with P ions causes swelling of the implanted layer (Fig. 3(a)), which is consistent with a number of previous reports on ion-beam-induced porosity and swelling of GaN [2e14]. In contrast, Fig. 3(b) reveals that room-temperature irradiation with PF4 ions causes a reduction in layer thickness. A similar reduction in the
3.1. Surface roughness Fig. 1 shows AFM images of GaN surfaces bombarded with F, P, and PF4 ions to doses resulting in 30 DPA. The following ion doses (in 10 16 ions/cm2) correspond to 30 DPA: 3.2, 1.5, and 0.45 for F, P, and PF4 ions, respectively. It is seen from Fig. 1 that the surface of the virgin sample is relatively smooth with ledges that are <0.6 nm in height and with an arithmetic root mean square (RMS) roughness of
3.2. Thickness change of irradiated layers
Fig. 1. AFM images showing topography of the virgin GaN surface and surfaces after room-temperature irradiation with 1.3 keV/amu F, P, and PF4 ions to doses corresponding to 30 DPA. Full height scales are 4 nm in top two images and 40 nm in the two lower ones.
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A.I. Titov et al. / Vacuum 86 (2012) 1638e1641
a
b
c
Fig. 2. Ion dose dependences of (a) the RMS surface roughness (for 1 1 mm2 scans) of GaN surfaces irradiated at room temperature with different ions (as indicated in the legend) with an energy of 1.3 keV/amu and (b), (c) the step height between implanted and masked regions of the sample surface irradiated with (b) P and (c) PF4 ions.
thickness of implanted layers has recently been observed for a large dose (3 1016 cm2) room-temperature implantation with 500 keV Au atomic ions [7]. Fig. 2(b) reveals a threshold dose dependence of swelling in the case of P ion irradiation. A threshold dose of w5 DPA is, however, much lower than a threshold dose of w15 DPA for the development of surface roughness shown in Fig. 2(a). For PF4 ion irradiation, Fig. 2(c) reveals a monotonic dose dependence of the step height resulting from an ion-induced reduction of the implanted layer thickness.
displacements, the total number of displacements, and the displacement generation rate are the same for all three ions (F, P, and PF4). The only difference between these three irradiation cases is the density of collision cascades. We have previously found [20] that profiles of stable disorder differ significantly for these three cases. We have also successfully correlated [20] the damage build-up behaviour with calculated densities of collision cascades. As an example, Fig. 4(a) shows damage profiles (extracted from ion channeling spectra reported in [20]) for ion doses corresponding to 10 DPA. Surface defect peaks in profiles shown in Fig. 4(a) originate from surface amorphous layers whose thickness increases with increasing ion dose. Thicknesses of such surface amorphous layers in Fig. 4(a) are 6.7 and 12.1 nm for P and PF4 ion, respectively. Fig. 4(b) shows that, in the w10-nm-thick near-surface region, the cascade density strongly depends on the cluster size and exceeds the critical value of w0.8 at.%, above which non-linear effects in damage accumulation are expected for GaN [20]. A comparison of Fig. 2(a) and 4(b) reveals that irradiation under conditions with denser cascades results in larger surface roughness. We attribute this effect to a consequence of a more efficient surface amorphization for cluster ion irradiation. Indeed, the development of porosity in surface amorphous layers appears to be responsible for the evolution of surface topography [2e14]. Although heavilydamaged but still non-amorphous regions of GaN can develop pores, such pores have diameters up to a few nanometers [13]. More pronounced porosity develops only after lattice amorphization and is caused by ion-beam-induced decomposition of the amorphous phase, resulting in the formation of N2-filled spherical bubbles with diameters up to tens of nanometers, depending on irradiation conditions [2e14]. An increase in surface roughness with increasing cascade density can be explained by more efficient damage build-up leading to surface amorphization for conditions with denser cascades.
Φ
a
4. Discussion As stated above, irradiation conditions used in this work have been chosen so that depth profiles of ballistically generated
b
Fig. 3. AFM images showing the border between implanted and masked areas of GaN surfaces. Irradiation was done at room temperature with an energy of 1.3 keV/amu with (a) P ions and (b) PF4 ions to doses corresponding to (a) 40 DPA and (b) 30 DPA.
Fig. 4. (a) Depth profiles of relative disorder after implantation by different ions (as indicated in the legend) to doses resulting in 10 DPA. (b) Depth profiles of the average density of collision cascades in GaN bombarded by the atomic and cluster ions listed in the legend in (a) with an energy of 1.3 keV/amu.
A.I. Titov et al. / Vacuum 86 (2012) 1638e1641
Results from Fig. 2(c) and 3 also show that competing processes of sputtering have a dominating contribution for the case of PF4 ion bombardment. The sputtering coefficient increases with an increase in surface roughness due to a better overlap of collision cascades with the surface for an increase in the angle between the directions of ion impact and the sample normal [24]. Nonlinearity of collision cascades for cluster ions further increases the sputtering coefficient (see, for example [17,18,24,25]). Based on data from Fig. 2(c), we can estimate the sputtering coefficient S for PF4 ions as w40 or w70 depending on whether opposing swelling effects are ignored or taken into account, respectively. In such first-order estimates, we assume that the swelling contributions are identical for cases of P and PF4 ion bombardment. These large sputtering coefficients are comparable to a S value of w60 reported for 500 keV Au ion bombardment of GaN at room temperature to a dose of 3 1016 cm2 [7]. 5. Conclusions The results presented here show that the density of collision cascades has a dramatic effect on thickness of implanted layers and surface topography. For irradiation conditions with relatively low average densities of collision cascades in the near-surface region, swelling effects dominate. For cluster ions with a large near-surface density of collision cascades, sputtering effects dominate. Surface roughness increases with increasing ion mass/cluster size. For all ion used, we have found a threshold dependence of the evolution of roughness, with threshold doses corresponding to w15 DPA. Despite clear evidence of the molecular effect on surface topography and layer thickness of ion-irradiated GaN revealed here, more work is needed to better understand atomic-scale mechanisms of these effects. Acknowledgements Authors are grateful to Wsevolod Lundin from the Ioffe Institute (St. Petersburg, Russia) for the GaN samples used in these studies. Work in St. Petersburg was supported by RFBR (grant 10-08-91751).
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Work at LLNL was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344. References [1] Pearton SJ, Zolper JC, Shul RJ, Ren F. J Appl Phys 1999;86:1. [2] Kucheyev SO, Williams JS, Pearton SJ. Mater Sci Eng R 2001;33:51. [3] Kucheyev SO, Williams JS, Jagadish C, Craig VSJ, Li G. Appl Phys Lett 2000;77: 1455. [4] Kucheyev SO, Williams JS, Zou J, Jagadish C, Li G. Appl Phys Lett 2000;77:3577. [5] Molnar B, Qadri SB, Schiestel S, Stroud RM, Carosella CA. Mat Res Soc Symp Proc 2001;639. G11.53.1. [6] Jiang W, Zhang Y, Weber WJ, Lian J, Ewing RC. J Appl Phys Lett 2006;89: 021903. [7] Gao Y, Lan C, Xue J, Yan S, Wang Y, Xu F, et al. Nucl Instrum Methods Phys Res B 2010;268:3207. [8] Kucheyev SO, Williams JS, Zou J, Jagadish C, Li G. Nucl Instrum Methods Phys Res B 2001;175:214. [9] Katsikini M, Boscherini F, Paloura EC. Nucl Instrum Methods Phys Res B 2010; 268:241. [10] Kucheyev SO, Bradby JE, Li CP, Ruffell S, van Buuren T, Felter TE. Appl Phys Lett 2007;91:261905. [11] Zhang CH, Song Y, Sun YM, Chen H, Yang YT, Zhou LH, et al. Nucl Instrum Methods Phys Res B 2007;256:199. [12] Pagowska K, Ratajczak R, Stonert A, Turos A, Nowicki L, Sathish N, et al. Acta Phys Polonica A 2011;120:153. [13] Ishimaru M, Zhang Y, Weber WJ. J Appl Phys 2009;106:053513. [14] Ishimaru M, Zhang Y, Wang X, Chu Wei-Kan, Weber WJ. J Appl Phys 2011; 109:043512. [15] Kucheyev SO, Williams JS, Zou J, Jagadish C, Li G. Appl Phys Lett 2001;78:1373. [16] Usov I, Parikh N, Kudriavtsev Y, Asomoza R, Reitmeier Z, Davis R. J Appl Phys 2003;93:5140. [17] Thompson DA. Radiat Eff 1981;56:105. [18] Davies JA. In: Williams JS, Poate JM, editors. Ion implantation and beam processing. Sydney: Academic Press; 1984. [19] Kucheyev SO, Williams JS, Titov AI, Li G, Jagadish C. Appl Phys Lett 2001;78: 2694. [20] Kucheyev SO, Azarov AY, Titov AI, Karaseov PA, Kuchumova TM. J Phys D Appl Phys 2009;42:085309. [21] Titov AI, Azarov A Yu, Nikulina LM, Kucheyev SO. Nucl Instrum Methods Phys Res B 2007;256:207. [22] Ziegler JF, Biersack JP, Littmark U. The stopping and range of ions in solids, vol. 1. Oxford: Pergamon; 1985. p 109. [23] Macko S, Frost F, Ziberi B, Forster DF, Michely T. Nanotechnology 2010;21: 085301. [24] Andresen HH, Bay HL. In: Behrish R, editor. Sputtering by particle bombardment I. Springer; 1981. [25] Bouneau S, Brunelle A, Della-Negra S, Depauw J, Jacquet D, Le Beyec Y, et al. Phys. Rev B 2002;65:144106.
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Molecular effect on surface topography of GaN bombarded with PF4 ions A.I. Titov a, *, P.A. Karaseov a, V.S. Belyakov a, K.V. Karabeshkin a, A.V. Arkhipov a, S.O. Kucheyev b, A.Yu. Azarov c a
State Polytechnic University, Polytechnicheskaya 29, 195251 St.Petersburg, Russian Federation Lawrence Livermore National Laboratory, Livermore, CA 94551, USA c University of Oslo, PO Box 1048 Blindern, NO-0316 Oslo, Norway b
a b s t r a c t Keywords: Ion implantation GaN Molecular ions Surface topography Collision cascade density Swelling Sputtering
We study surface topography and thickness of GaN layers implanted at room temperature with 1.3 keV/amu F, P, and PF4 cluster ions. Results show that the density of collision cascades has a dramatic effect on the surface roughness and the thickness of implanted layers. Surface roughness increases with increasing cascade density. For very dense cascades produced by PF4 ions, the evolution of layer thickness is dominated by ion-induced sputtering. In contrast, for the case of P ions producing less dense cascades, ioninduced swelling is observed. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ion implantation is an attractive processing tool for the fabrication of GaN-based (opto)electronic devices (see, for example, reviews [1,2]). A successful application of ion implantation, however, requires understanding of the formation and mitigation of ion-beam-produced damage. In addition to defects in the crystalline lattice, radiation damage could manifest as changes of surface topography. For GaN, ion-implantation-induced amorphization is accompanied by material decomposition with the formation of nanoscale N2 bubbles and concomitant changes in the material density and surface topography [2e14]. These include swelling and an increase in surface roughness. High-dose irradiation at elevated temperatures is also known to cause efficient surface erosion [15,16]. In certain irradiation regimes, bombardment under conditions with a larger average density of collision cascades results in an increase in the efficiency of radiation damage build-up (see, for example, reviews [17,18]). Such cascade density effects can be conveniently studied by irradiation with cluster ions. In this case, a dependence of radiation damage processes on the cascade density is often referred to as the molecular effect. Collision cascades created by components of the cluster ion overlap in the nearsurface region, creating combined individual collision cascades with larger volumetric densities of atomic displacements than in
* Corresponding author. Department of Physical Electronics, St. Petersburg State Polytechnic University, Polytechnicheskaya 29, 195251 St. Petersburg, Russian Federation. Tel.: þ7 812 552 7516; fax: þ7 812 552 9516. E-mail address: [email protected] (A.I. Titov). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.12.014
the case of irradiation with atomic ions of the same velocity. For example, irradiation of GaN with atomic and molecular Bi1,2 ions [19] and PF4 ions [20] results in a significant increase in the level of stable radiation damage in the near-surface region with increasing density of collision cascades. This effect has been attributed to the formation of non-linear energy spikes in GaN, as discussed in detail in Ref. [20]. The formation of the nanoporous structure and changes in surface topography of ion-bombarded GaN are related to nearsurface radiation damage [2e14]. Hence, the density of collision cascades should influence these phenomena. In the present work, we will refer to the 5 atoms in PF4 as a cluster and we study the molecular effect in the evolution of surface topography of GaN irradiated with F, P, and small cluster (PF4) ions. 2. Experimental Wurtzite (0001) GaN epilayers, grown by metal-organic vapourphase epitaxy at the Ioffe Institute (St. Petersburg, Russian Federation) on c-plane sapphire substrates, were implanted at room temperature with F, P, and PF4 ions with an energy of 1.3 keV/amu over a wide dose range. Implantation was carried out at 7 off the [0001] direction in order to minimize channelling. As discussed in detail in [21], cascade density effects can be conveniently studied by cluster ion irradiation when the following parameters are kept constant: ion energy normalized to the atomic mass unit (amu), ion dose normalized to the number of displacements per atom (DPA), and ion beam flux normalized to DPA$s1. The irradiation conditions for PFn ions used in this study comply with these requirements. Beam flux was adjusted to yield 3.6 103 DPA$s1 in all three cases.
A.I. Titov et al. / Vacuum 86 (2012) 1638e1641
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Values of DPA were calculated with the TRIM code (version SRIM-2003.26) [22] with an effective threshold energy for atomic displacements of 25 eV for both Ga and N sub-lattices. Quoted DPA values are concentrations of ion-beam-generated lattice vacancies (gv) at the maximum of the nuclear energy loss profile normalized to the atomic concentration of GaN (8.85 1022 atoms cm3). In calculations of the vacancy concentrations generated by molecular ions, we used a linear approximation: gv (PF4) ¼ gv(P) þ 4 gv(F). Surface topography was studied in the tapping mode of atomic force microscopy (AFM) with a Nano-DST instrument manufactured by Pacific Nanotechnology. We also measured step heights between implanted and unimplanted regions of the surface. Such regions were made by masking the GaN surface by a cleaved piece of a silicon wafer that was in mechanical contact with the sample surface during irradiation. Surface topography was measured over 1 1 mm2 areas located far from the border of the implanted regions to avoid possible (but unlikely due to relatively low ion fluences used) effects of mask sputtering [23].
w0.2 nm, with a density of threading dislocations of w108 cm2. Nanoparticles form on the surface as a result of ion bombardment. More importantly, Fig. 1 shows that irradiation-induced changes in surface topography strongly depend on the ion cluster size. Larger clusters lead to the formation of larger nanoparticles. Hence, the density of collision cascades has a dramatic effect on surface roughness. This effect is better illustrated in Fig. 2(a), showing the dose dependence of RMS roughness. It reveals that surface roughness increases with increasing either dose or ion mass. In particular, surface roughness of PF4-irradiated samples is over an order of magnitude larger than that of F-bombarded surfaces for the same dose expressed in DPA units. Fig. 2(a) also reveals a threshold dependence of the evolution of roughness, with threshold doses corresponding to w15 DPA. Such a threshold behaviour is consistent with previous reports [2,3,7].
3. Results
Fig. 3(a) and (b) show three-dimensional AFM images of the border between implanted and virgin areas of samples irradiated with P and PF4 ions, respectively. Both an irradiation-induced increase in surface roughness and the formation of large steps between implanted and virgin areas are clearly seen in Fig. 3. Irradiation with P ions causes swelling of the implanted layer (Fig. 3(a)), which is consistent with a number of previous reports on ion-beam-induced porosity and swelling of GaN [2e14]. In contrast, Fig. 3(b) reveals that room-temperature irradiation with PF4 ions causes a reduction in layer thickness. A similar reduction in the
3.1. Surface roughness Fig. 1 shows AFM images of GaN surfaces bombarded with F, P, and PF4 ions to doses resulting in 30 DPA. The following ion doses (in 10 16 ions/cm2) correspond to 30 DPA: 3.2, 1.5, and 0.45 for F, P, and PF4 ions, respectively. It is seen from Fig. 1 that the surface of the virgin sample is relatively smooth with ledges that are <0.6 nm in height and with an arithmetic root mean square (RMS) roughness of
3.2. Thickness change of irradiated layers
Fig. 1. AFM images showing topography of the virgin GaN surface and surfaces after room-temperature irradiation with 1.3 keV/amu F, P, and PF4 ions to doses corresponding to 30 DPA. Full height scales are 4 nm in top two images and 40 nm in the two lower ones.
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A.I. Titov et al. / Vacuum 86 (2012) 1638e1641
a
b
c
Fig. 2. Ion dose dependences of (a) the RMS surface roughness (for 1 1 mm2 scans) of GaN surfaces irradiated at room temperature with different ions (as indicated in the legend) with an energy of 1.3 keV/amu and (b), (c) the step height between implanted and masked regions of the sample surface irradiated with (b) P and (c) PF4 ions.
thickness of implanted layers has recently been observed for a large dose (3 1016 cm2) room-temperature implantation with 500 keV Au atomic ions [7]. Fig. 2(b) reveals a threshold dose dependence of swelling in the case of P ion irradiation. A threshold dose of w5 DPA is, however, much lower than a threshold dose of w15 DPA for the development of surface roughness shown in Fig. 2(a). For PF4 ion irradiation, Fig. 2(c) reveals a monotonic dose dependence of the step height resulting from an ion-induced reduction of the implanted layer thickness.
displacements, the total number of displacements, and the displacement generation rate are the same for all three ions (F, P, and PF4). The only difference between these three irradiation cases is the density of collision cascades. We have previously found [20] that profiles of stable disorder differ significantly for these three cases. We have also successfully correlated [20] the damage build-up behaviour with calculated densities of collision cascades. As an example, Fig. 4(a) shows damage profiles (extracted from ion channeling spectra reported in [20]) for ion doses corresponding to 10 DPA. Surface defect peaks in profiles shown in Fig. 4(a) originate from surface amorphous layers whose thickness increases with increasing ion dose. Thicknesses of such surface amorphous layers in Fig. 4(a) are 6.7 and 12.1 nm for P and PF4 ion, respectively. Fig. 4(b) shows that, in the w10-nm-thick near-surface region, the cascade density strongly depends on the cluster size and exceeds the critical value of w0.8 at.%, above which non-linear effects in damage accumulation are expected for GaN [20]. A comparison of Fig. 2(a) and 4(b) reveals that irradiation under conditions with denser cascades results in larger surface roughness. We attribute this effect to a consequence of a more efficient surface amorphization for cluster ion irradiation. Indeed, the development of porosity in surface amorphous layers appears to be responsible for the evolution of surface topography [2e14]. Although heavilydamaged but still non-amorphous regions of GaN can develop pores, such pores have diameters up to a few nanometers [13]. More pronounced porosity develops only after lattice amorphization and is caused by ion-beam-induced decomposition of the amorphous phase, resulting in the formation of N2-filled spherical bubbles with diameters up to tens of nanometers, depending on irradiation conditions [2e14]. An increase in surface roughness with increasing cascade density can be explained by more efficient damage build-up leading to surface amorphization for conditions with denser cascades.
Φ
a
4. Discussion As stated above, irradiation conditions used in this work have been chosen so that depth profiles of ballistically generated
b
Fig. 3. AFM images showing the border between implanted and masked areas of GaN surfaces. Irradiation was done at room temperature with an energy of 1.3 keV/amu with (a) P ions and (b) PF4 ions to doses corresponding to (a) 40 DPA and (b) 30 DPA.
Fig. 4. (a) Depth profiles of relative disorder after implantation by different ions (as indicated in the legend) to doses resulting in 10 DPA. (b) Depth profiles of the average density of collision cascades in GaN bombarded by the atomic and cluster ions listed in the legend in (a) with an energy of 1.3 keV/amu.
A.I. Titov et al. / Vacuum 86 (2012) 1638e1641
Results from Fig. 2(c) and 3 also show that competing processes of sputtering have a dominating contribution for the case of PF4 ion bombardment. The sputtering coefficient increases with an increase in surface roughness due to a better overlap of collision cascades with the surface for an increase in the angle between the directions of ion impact and the sample normal [24]. Nonlinearity of collision cascades for cluster ions further increases the sputtering coefficient (see, for example [17,18,24,25]). Based on data from Fig. 2(c), we can estimate the sputtering coefficient S for PF4 ions as w40 or w70 depending on whether opposing swelling effects are ignored or taken into account, respectively. In such first-order estimates, we assume that the swelling contributions are identical for cases of P and PF4 ion bombardment. These large sputtering coefficients are comparable to a S value of w60 reported for 500 keV Au ion bombardment of GaN at room temperature to a dose of 3 1016 cm2 [7]. 5. Conclusions The results presented here show that the density of collision cascades has a dramatic effect on thickness of implanted layers and surface topography. For irradiation conditions with relatively low average densities of collision cascades in the near-surface region, swelling effects dominate. For cluster ions with a large near-surface density of collision cascades, sputtering effects dominate. Surface roughness increases with increasing ion mass/cluster size. For all ion used, we have found a threshold dependence of the evolution of roughness, with threshold doses corresponding to w15 DPA. Despite clear evidence of the molecular effect on surface topography and layer thickness of ion-irradiated GaN revealed here, more work is needed to better understand atomic-scale mechanisms of these effects. Acknowledgements Authors are grateful to Wsevolod Lundin from the Ioffe Institute (St. Petersburg, Russia) for the GaN samples used in these studies. Work in St. Petersburg was supported by RFBR (grant 10-08-91751).
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Work at LLNL was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344. References [1] Pearton SJ, Zolper JC, Shul RJ, Ren F. J Appl Phys 1999;86:1. [2] Kucheyev SO, Williams JS, Pearton SJ. Mater Sci Eng R 2001;33:51. [3] Kucheyev SO, Williams JS, Jagadish C, Craig VSJ, Li G. Appl Phys Lett 2000;77: 1455. [4] Kucheyev SO, Williams JS, Zou J, Jagadish C, Li G. Appl Phys Lett 2000;77:3577. [5] Molnar B, Qadri SB, Schiestel S, Stroud RM, Carosella CA. Mat Res Soc Symp Proc 2001;639. G11.53.1. [6] Jiang W, Zhang Y, Weber WJ, Lian J, Ewing RC. J Appl Phys Lett 2006;89: 021903. [7] Gao Y, Lan C, Xue J, Yan S, Wang Y, Xu F, et al. Nucl Instrum Methods Phys Res B 2010;268:3207. [8] Kucheyev SO, Williams JS, Zou J, Jagadish C, Li G. Nucl Instrum Methods Phys Res B 2001;175:214. [9] Katsikini M, Boscherini F, Paloura EC. Nucl Instrum Methods Phys Res B 2010; 268:241. [10] Kucheyev SO, Bradby JE, Li CP, Ruffell S, van Buuren T, Felter TE. Appl Phys Lett 2007;91:261905. [11] Zhang CH, Song Y, Sun YM, Chen H, Yang YT, Zhou LH, et al. Nucl Instrum Methods Phys Res B 2007;256:199. [12] Pagowska K, Ratajczak R, Stonert A, Turos A, Nowicki L, Sathish N, et al. Acta Phys Polonica A 2011;120:153. [13] Ishimaru M, Zhang Y, Weber WJ. J Appl Phys 2009;106:053513. [14] Ishimaru M, Zhang Y, Wang X, Chu Wei-Kan, Weber WJ. J Appl Phys 2011; 109:043512. [15] Kucheyev SO, Williams JS, Zou J, Jagadish C, Li G. Appl Phys Lett 2001;78:1373. [16] Usov I, Parikh N, Kudriavtsev Y, Asomoza R, Reitmeier Z, Davis R. J Appl Phys 2003;93:5140. [17] Thompson DA. Radiat Eff 1981;56:105. [18] Davies JA. In: Williams JS, Poate JM, editors. Ion implantation and beam processing. Sydney: Academic Press; 1984. [19] Kucheyev SO, Williams JS, Titov AI, Li G, Jagadish C. Appl Phys Lett 2001;78: 2694. [20] Kucheyev SO, Azarov AY, Titov AI, Karaseov PA, Kuchumova TM. J Phys D Appl Phys 2009;42:085309. [21] Titov AI, Azarov A Yu, Nikulina LM, Kucheyev SO. Nucl Instrum Methods Phys Res B 2007;256:207. [22] Ziegler JF, Biersack JP, Littmark U. The stopping and range of ions in solids, vol. 1. Oxford: Pergamon; 1985. p 109. [23] Macko S, Frost F, Ziberi B, Forster DF, Michely T. Nanotechnology 2010;21: 085301. [24] Andresen HH, Bay HL. In: Behrish R, editor. Sputtering by particle bombardment I. Springer; 1981. [25] Bouneau S, Brunelle A, Della-Negra S, Depauw J, Jacquet D, Le Beyec Y, et al. Phys. Rev B 2002;65:144106.