593MeV Au irradiation of InP, GaP, GaAs and AlAs

NIM B Beam Interactions with Materials & Atoms Nuclear Instruments and Methods in Physics Research B 242 (2006) 363–366 www.elsevier.com/locate/nimb 593 MeV Au irradiation of InP, GaP, GaAs and AlAs W. Wesch a a,* , A. Kamarou a, E. Wendler a, S. Klaumünzer b Friedrich-Schiller-Universität Jena, Institut für Festkörperphysik, Max-Wien-Platz 1, D-07743 Jena, Germany b Hahn-Meitner-Institut Berlin, Glienicker Straße 100, D-14109 Berlin, Germany Available online 23 September 2005 Abstract The damage formation due to 593 MeV Au30+ ion irradiation at room temperature in InP, GaP, GaAs and AlAs was investigated as a function of the ion fluence using RBS/C spectrometry. In InP an amorphous layer is formed at a nuclear energy deposition being by two orders of magnitude smaller than that required to render the material amorphous by elastic collisions. The observed behaviour is explained in terms of the thermal spike model in which it is assumed that above a critical value of the electronic energy deposition melting around the ion trajectories followed by fast cooling and re-solidification causes the formation of amorphous tracks. In GaP, GaAs and AlAs only a slight increase of the damage concentration with the ion fluence is registered, amorphisation is not reached up to ion fluences of 3 · 1014 cm
2. The damaging in these materials can be attributed to nuclear energy deposition, because in accordance with data published in literature the critical electronic energy deposition for track formation is not reached with 593 MeV Au30+ ions.
2005 Elsevier B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Fk; 81.05.Ea Keywords: III–V compound semiconductors; Swift heavy ions; Defect formation; Electronic energy deposition 1. Introduction Whereas the effect of the nuclear energy deposition on ion irradiation-induced damage formation in most of the technologically relevant semiconductors is well studied, the investigation of the role of inelastic collision processes has received relatively scant attention in these materials. However, with the demand for complex and buried structures high irradiation energies are of growing importance for future device technologies. With rising ion energy the electronic energy loss of the ions increases and at very high energies (energy range of several 100 MeV to GeV) it exceeds the nuclear one by two orders of magnitude. Therefore the influence of the electronic energy deposition on materials modification needs to be studied systematically. It is well known that high electronic energy deposition during swift heavy ion (SHI) irradiation in insulators, some * Corresponding author. Tel.: +49 3641 947330; fax: +49 3641 947302. E-mail address: [email protected] (W. Wesch). 0168-583X/$ - see front matter
2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.095 inter-metallic compounds and metals leads to the formation of amorphous tracks around the ion trajectories (see references in [1]). Studies performed in the early 1990s on SHI irradiation of Si, Ge and GaAs with electronic energy depositions between 1 and 28 keV/nm showed a modification of the electrical properties of the materials due to the formation of point defects (e.g. [2,3] and references in [1]). However, the results do not yield clear evidence of ionisation-induced defect production in Si, Ge and GaAs by mono-atomic ion beams for an electronic energy deposition below 28 keV/nm [1]. The formation of discontinuous and continuous amorphous tracks in InP due to 250 MeV Xe irradiation (electronic energy deposition 19 keV/nm) was shown for the first time by experiments performed in our group [4] and confirmed by data published by Szenes et al. [5]. They also reported on track formation in InAs, InSb and GaSb due to SHI with electronic energy depositions between 27 and 30 keV/nm [5]. Evidence of discontinuous track formation in Ge single crystals was recently found by Komarov et al. for electronic energy depositions of about 40 keV/nm [6]. Track formation in solids is 364 W. Wesch et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 363–366 commonly described on the basis of the thermal spike model (e.g. [7] and references therein) assuming melting of the material along the ion trajectory up to depths for which the electronic energy loss exceeds a certain critical value, followed by fast cooling and re-solidification. How far such a model can also hold for semiconductors cannot clearly be concluded from the experimental data available up to now. The present paper reports results of a comparative study of damage formation by 593 MeV Au irradiation of InP, GaP, GaAs and AlAs. 2. Experimental (1 0 0) oriented InP, GaP and GaAs single crystals and 470 nm thick AlAs layers epitaxially grown on (1 0 0) GaAs and covered with a 20 nm thick GaAs cap layer were irradiated at the Hahn-Meitner-Institut Berlin with 593 MeV Au30+ ions at room temperature. The values of the nuclear and electronic energy deposition per ion and unit length, en and ee, respectively, calculated by the TRIM97 code [8] for the various materials are given in Table 1. The ion fluence NI was varied between 1011 and 3 · 1014 cm
2, and the ion flux was kept constant around 2.5 · 1010 cm
2 s
1. The irradiated samples were subsequently analysed by means of Rutherford backscattering spectrometry in channelling geometry (RBS/C) with 1.4 MeV He ions at a backscattering angle of 168. As a measure of the relative defect concentration, from the RBS/C spectra the difference in minimum yield, Dvmin, was calculated at a depth z = 300 nm below the sample surface. Dvmin is given by vir dam
Y vir Dvmin ¼ ðY dam al Þ=Y ra (Y al , Y al – aligned yield of al backscattered ions of the irradiated respectively virgin sample, Yra – backscattered yield for random incidence of the ions). 3. Results and discussion Fig. 1 shows the RBS/C spectra measured on InP after irradiation with various Au ion fluences NI. The backscattered yield continuously increases with the ion fluence both in the region of backscattering on In and P and reaches the random level at NI = 1 · 1013 cm
2 indicating the formation of an amorphous layer. The damage profiles calculated from the RBS/C spectra (not shown) exhibit constant defect concentrations over the whole depth monitored except for a thin surface layer being only slightly damaged. The behaviour is similar to that observed for 390 MeV Xe Fig. 1. RBS/C spectra of InP irradiated with various fluences of 593 MeV Au30+ ions at room temperature. irradiation [9] and is attributed to the high and almost constant electronic energy deposition. In order to understand the significant damage evolution it is helpful to introduce the number of displacements per lattice atom, ndpa, which is a measure of the nuclear energy deposition, according to ndpa ¼ ðN I N displ Þ=N 0 (N displ is the number of displacements per ion and unit length calculated by TRIM97 and N0 is the target atomic density). For 593 MeV Au ions N displ =2 nm
1 corresponding to ndpa = 2 · 10
3 dpa for NI = 1 · 1013 cm
2 where an amorphous layer is formed. This value is by about two orders of magnitude smaller than the corresponding value to render InP amorphous by elastic collision processes (ndpa = 0.2 dpa for 300 and 600 keV Se [10] and ndpa = 1 dpa for 100 keV B irradiation [11]). Consequently, the nuclear energy deposition is not responsible for the amorphisation of InP, but the high electronic energy deposition. As an example for the other materials investigated, Fig. 2 shows the RBS/C spectra of the system GaAs/ AlAs/GaAs. The backscattered yield between channel numbers 425 and 320 corresponds to backscattering at the As sub-lattice of the AlAs layer, backscattering on the Ga–As sub-lattices of the GaAs substrate is monitored Table 1 Energies ee and en deposited into electronic and nuclear processes, respectively, and number of displacements per ion and unit length, N displ , for InP, GaP, GaAs and AlAs irradiated with 593 MeV Au30+ ions calculated with the TRIM97 Monte Carlo code [8] ee [keV/nm] en [keV/nm] N displ [nm
1] InP GaP GaAs AlAs 28.64 0.088 2.05 28.95 0.082 1.03 32.87 0.107 2.75 25.44 0.076 0.68 Fig. 2. RBS/C spectra of GaAs/AlAs(470 nm)/GaAs(20 nm) irradiated with 593 MeV Au30+ ions for various ion fluences. W. Wesch et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 363–366 at channel numbers below 320. The backscattering on GaAs is superimposed by that on the Al atoms of the AlAs layer at channel numbers below 280. The existence of the thin GaAs cap layer represents itself in an increased backscattered yield at the surface edge of the random spectrum. It is clearly obvious that the backscattered yield in the region of the AlAs layer increases only weakly with the ion fluence indicating that ionisation stimulated processes do hardly cause damage. In the region of backscattering on GaAs, the backscattered yield increases slightly stronger with the ion fluence than in AlAs, but also in GaAs up to an ion fluence NI = 2 · 1014 cm
2 amorphisation does not occur. The results obtained for the various materials are summarised in Fig. 3. In InP, as already illustrated in the RBS/C spectra in Fig. 1, the relative defect concentration increases with the ion fluence until a maximum value due to the formation of an amorphous layer at NI = 1 · 1013 cm
2 is reached. Contrary, the other ma- terials exhibit only a weak increase of the relative defect concentration with increasing ion fluence, for NI < 3 · 1014 cm
2 the relative defect concentration is around or below 10%. Furthermore, the efficiency of damage formation decreases according to the sequence GaP–GaAs–AlAs which is similar to the behaviour observed for damage formation by elastic collision processes in these materials [12]. To obtain a relative defect concentration of 10% in GaAs, using e.g. 600 keV Se ions, a nuclear energy deposition of ndpa = 0.15 dpa is required [13]. The ion fluence NI = 3 · 1014 cm
2 of 593 MeV Au30+ ions corresponds to a nuclear energy deposition of ndpa  0.18 dpa which is in the same order of magnitude as that necessary to produce about 10% damage in GaAs. Consequently, the low damage concentration produced in GaAs with 593 MeV Au30+ ions is due to elastic collisions. The same can be assumed for GaP and AlAs. A discussion of the results in the framework of the thermal spike model [7] leads to the conclusion that the amorphisation of InP is a consequence of melting and 365 re-solidification due to the high electronic energy deposition. In case of 593 MeV Au30+ irradiation the value ee = 28.6 keV/nm exceeds the recently reported threshold value of ee  13 keV/nm necessary to melt InP with 250 MeV Xe17+ significantly [1,4]. In the other materials the threshold value is obviously not yet reached with the 593 MeV Au30+ ions, and the small damage concentrations observed are due to nuclear interaction. In accordance with that, from irradiations of GaAs with 20–40 MeV C60 cluster ions which caused continuous amorphous tracks a threshold value for track formation in GaAs of eth e ¼ 36 keV=nm was deduced [14] which is not reached with our irradiation conditions. 4. Summary In InP irradiation with 593 MeV Au30+ ions leads to the formation of an amorphous layer at a nuclear energy deposition being by about two orders of magnitude smaller than that required to render the material amorphous by elastic collision processes. The damaging is therefore attributed to the high electronic energy deposition and can be explained in the framework of the thermal spike model. Above a threshold value of the electronic energy deposition which is exceeded for these irradiation parameters, melting around the ion trajectories occurs followed by fast re-solidification leaving behind amorphous tracks which agglomerate to a continuous amorphous layer with increasing ion fluence. In GaAs, GaP and AlAs only weak damaging is observed which is obviously a consequence of the small amount of nuclear energy deposition. Probably, for these materials the threshold value of the electronic energy deposition to melt the material is not yet reached with 593 MeV Au30+ ions which is in agreement with results published by other groups. Acknowledgement This work was supported by the Bundesministerium für Bildung und Forschung (BMBF) under Contract No. 05KK1SJA/2. References Fig. 3. Differences in minimum yield, Dvmin, for the various materials irradiated with 593 MeV Au30+ ions, as a function of the ion fluence. [1] W. Wesch, A. Kamarou, E. Wendler, Nucl. Instr. and Meth. B 225 (2004) 111. [2] M. Levalois, J.P. Girard, G. Allais, A. Hairie, M.N. Metzner, E. Paumier, Nucl. Instr. and Meth. B 63 (1992) 25. [3] M. Levalois, P. Marie, Nucl. Instr. and Meth. B 156 (1999) 64. [4] O. Herre, W. Wesch, E. Wendler, P.I. Gaiduk, F.F. Komarov, S. Klaumünzer, P. Meier, Phys. Rev. B 58 (1998) 4832; W. Wesch, O. Herre, P.I. Gaiduk, E. Wendler, S. Klaumünzer, Nucl. Instr. and Meth. B 146 (1998) 341. [5] G. Szenes, Z.E. Horvath, B. Pecz, F. Paszti, L. Toth, Phys. Rev. B 65 (2002) 452061. [6] F.F. Komarov, P.I. Gaiduk, L.A. Vlasukova, A.J. Dydik, V.N. Yuvchenko, Vacuum 70 (2003) 75. [7] M. Toulemonde, C. Dufour, E. Paumier, Phys. Rev. B 46 (1992) 14362. 366 W. Wesch et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 363–366 [8] J.P. Biersack, J.F. Ziegler, The Stopping and Ranges of Ions in Matter, Vol. 1, Pergamon Press, Oxford, 1985. [9] A. Kamarou, W. Wesch, E. Wendler, S. Klaumünzer, Nucl. Instr. and Meth. B 225 (2004) 129. [10] W. Wesch, E. Wendler, T. Bachmann, O. Herre, Nucl. Instr. and Meth. B 96 (1995) 290. [11] E. Wendler, T. Opfermann, P.I. Gaiduk, J. Appl. Phys. 82 (1997) 5965. [12] W. Wesch, Nucl. Instr. and Meth. B 68 (1992) 342. [13] E. Wendler, Mater. Sci. Forum 248–249 (1997) 261. [14] A. Colder, B. Canut, M. Levalois, P. Marie, X. Poitier, S.M.M. Ramos, J. Appl. Phys. 91 (2002) 5853.