Chapter 4. Surface defects created by kev Xe ion irradiation on Ge

81 Chapter 4 Surface defects created by kev Xe ion irradiation on Ge 4.1. Introduction As high energy ions penetrate into a solid, those ions can deposit kinetic energy in two processes: electronic excitation and ionization, and nuclear collision. Kinetic energy transfer between penetrating ions and target atoms takes place through collective processes of nuclear collisions in a displacement. The cascade evolution consists of three stages. The first stage lasts ~ 0.3 ps, i.e., approximately the period of a lattice vibration: in this stage, which is called collision cascade, the penetrating ions and displaced atoms slow down until they no longer displace more lattice atoms. In the second stage lasting from ~ 0.3 ps to ~ 3 ps, the collision cascade evolves into a thermal spike where atoms are set into violent motion; the deposited energy is converted into heat, and the high temperature in a thermal spike causes local melting in the crystal interior [1-5]. In the last stage after ~ 3 ps, the melt zone rapidly cools at the rate on the order of 10 14 o C/s and resolidifies, attaining ambient temperature after ~ 10 ps and leaving behind vacancy-rich core [6]; only in simple lattice structures like fcc or bcc metals the atoms can return to the original crystal structures during resolidification process, while in more complicated structures like ordered alloys, compound semiconductors or oxides a highly disordered nonequilibrium phase is frozen in [6].

82 In metals local meting causes volume expansion in a thermal spike, and therefore pressure builds up inside the melt zone. If this melt zone is formed close to the surface, the pressure buildup may not be contained, and bulk atoms can be pushed toward the surface by viscous flow [3-5] or microexplosion [4,5], resulting in surface craters [7-13]. However, molecular dynamics simulation of 5 kev Ge ion irradiation on Ge, which has a negative volume of melting, demonstrated depression of surface layers but no surface crater formation [4]; the density of solid Ge is 5.32 g/cm 3 at room temperature and the density of liquid Ge is 5.49 g/cm 3 at 937 o C, the melting temperature of Ge. Surface crater formation due to high energy heavy ion impact strongly depends on cascade energy and cascade energy density [7], and has been observed experimentally on Au(001) irradiated by 10-500 kev Bi ions [7], on Au(001) bombarded by 50-400 kev Xe ions [8], on Ge(001) irradiated by 20 kev Ga ions [9], and on Pt(111) irradiated by 5 kev Xe ions [10]. The yield of surface craters, which is the number of surface crater produced per incident ion, is typically on the order of 1 percent due to the fluctuations in energy deposition in individual collision cascades [7,8,10]. If the bulk nanocavities discussed in chapter 3 appear on the surface during ion irradiation, crater-like surface defects will be formed. Those bulk nanocavities can appear on the surface through migration of nanocavities toward the surface or exposure of nanocavities by ion etching removal of material above the nanocavities. Nanocavities can migrate in the crystal interior through surface diffusion, volume diffusion and vapor transport processes during thermal annealing [14,15].

83 In addition to being able to create surface craters, a displacement cascade can enhance the mobility of precipitates and nanocavities through the interaction of melt zones with precipitates and nanocavities [16-18]. Discrete He nanocavity motion was observed during 400 kev Ar ion irradiation on Au [16,17], where dense collision cascades form, whereas no migration of He nanocavities was observed during 200 kev Xe ion irradiation on Al, where dilute collision cascades form [17,18]. Donnelly et al. also observed disappearance of many He nanocavities in Au during 400 kev Ar ion irradiation and proposed that some nanocavities may appear on the surface while most nanocavities are likely to be disintegrated by displacement cascades [16,17]. In my experimental results, large surface defects, which resemble surface craters but will be called pits, are formed on the Ge surfaces irradiated by Xe ions with energies ranging from 650 ev to 20 kev at temperatures between 245 o C and 305 o C; the term pits used in this chapter implies surface defects more than one bilayer or monolayer deep and formed in a different manner than surface craters created by single ion displacement cascades. These pits appear only when the Ge starting surfaces are prepared by ion etching at T 400 o C and thus nanocavities are present in the bulk. Also the number of pits initially increases and then decreases with increasing ion fluence. These results suggest the pits are not surface craters and support the proposed formation mechanism of pits, which is sudden annihilation of bulk nanocavities on the surface through the interaction of a melt zone with a bulk nanocavity during kev Xe ion irradiation.

84 4.2. Results and discussion 4.2.1. Formation of pits Fig. 4.1 shows STM images of the Ge surfaces following 5 kev Xe ion irradiation on the Ge starting surfaces displayed in Fig. 2.3 for 18 seconds with the ion fluence of 5.6 10 14 ions/cm 2 ; this ion fluence corresponds to 1.2 bilayer removal for Ge(111) and 2.7 monolayers removal for Ge(001). Fig. 4.1(a) and (b) are Ge(111) surfaces irradiated at 275 o C and 305 o C respectively, and Fig. 4.1(c) and (d) are Ge(001) surfaces irradiated at 275 o C and 305 o C respectively. Large pits are formed on the Ge surfaces while no pits are observed on the Ge starting surfaces. Small area scans in Fig. 4.2 show the pits are surrounded by closely spaced steps. The size of the pits is larger at 305 o C than at 275 o C and on Ge(111) than on Ge(001); the average diameter of the pits in Fig. 4.1(a) is 24 nm and that in Fig. 4.1(b) is 40 nm, either of which is much larger than the diameter of the surface craters created by 20 kev Ga ions on Ge(001) at room temperature, ~ 6 nm [9]. Fig. 4.3 displays STM images of the Ge surfaces following 5 kev Xe ion irradiation for 3 minutes with the ion fluence of 5.6 10 15 ions/cm 2 ; the ion fluence of 5.6 10 15 ions/cm 2 corresponds to 12 bilayers removal for Ge(111) and 27 monolayers removal for Ge(001). Fig. 4.3(a) and (b) are Ge(111) surfaces etched at 275 o C and 305 o C respectively, and Fig. 4.3(c) is Ge(001) surface etched at 305 o C. Fig. 4.4 shows STM images of the Ge surfaces following 5 kev Xe ion irradiation for 30 minutes with the ion fluence of 5.6 10 16 ions/cm 2 for Fig. 4.4(a), (c) and (d), and 3.7 10 16 ions/cm 2 for Fig. 4.4(b); the ion fluence of 5.6 10 16 ions/cm 2 corresponds to 120 bilayers removal for

85 Ge(111) and 270 monolayers removal for Ge(001), and the ion fluence of 3.7 10 16 ions/cm 2 corresponds to 77 bilayers removal for Ge(111). Fig. 4.4(a) and (b) are Ge(111) surfaces etched at 275 o C and 305 o C respectively, and Fig. 4.4(c) and (d) are Ge(001) surfaces etched at 275 o C and 305 o C respectively. The number and size of the pits diminish both on Ge(111) and Ge(001) compared to Fig. 4.1. The number of pits is counted from STM images and plotted as a function of ion fluence for 650 ev, 5 kev and 20 kev Xe ion irradiation experiments in Fig. 4.5; for the experiments with Xe ion energy lower than 20 kev, Ge starting surfaces are prepared by 5 kev Xe ion etching at 520 o C for 30 minutes with the ion fluence of 5.6 10 16 ions/cm 2, and for the 20 kev Xe ion irradiation experiments Ge starting surfaces are prepared by 5 kev Xe ion etching at 520 o C for 53 minutes with the ion fluence of 5.6 10 16 ions/cm 2. The error bars are inserted assuming the statistics of the pits follows Poisson distribution where the error is the square root of the average. More pits are formed on Ge(001) than on Ge(111) both at 275 o C and 305 o C, and at 275 o C than at 305 o C both on Ge(001) and Ge(111). The pits completely disappear after 5 kev Xe ion irradiation with the ion fluence of 3.9 10 17 ions/cm 2 on Ge(111). If the pits are surface craters, the number of pits should increase and the size of pits should grow with increasing ion fluence as observed in porous structure on Ge [9,19]. Therefore Fig. 4.4 and 4.5 suggest that pits are not surface craters, which are created by microexplosion or viscous flow through single ion displacement cascades.

86 Fig. 4.6 displays STM images of the Ge surfaces following 650 ev Xe ion irradiation on the starting surfaces displayed in Fig. 2.3 at 245 o C for 12 minutes 40 seconds with the ion fluence of 1.3 10 15 ions/cm 2 ; this ion fluence corresponds to 1.2 bilayer removal for Ge(111) in Fig. 4.6(a) and 2.7 monolayers removal for Ge(001) in Fig. 4.6(b) [20], the same amount of material removed with the Ge surfaces shown in Fig. 4.1. Pits as large as those produced by 5 kev Xe ions are observed with lower density than Fig. 4.1. Surface crater formation strongly depends on cascade energy and cascade energy density [7], and thus a 650 ev Xe ion is improbable to create a 40 nm-diameter crater. Therefore, Fig. 4.6 also suggests that the pits are not surface craters. Fig. 4.7 shows annealing experiments of irradiated Ge(111) at 305 o C: Fig. 4.7(a) is the Ge(111) surface irradiated by 5 kev Xe ions at 305 o C for 5 seconds with the ion fluence of 1.6 10 14 ions/cm 2, corresponding to removal of 0.33 bilayer or 0.11 nm thickness, Fig. 4.7(b) is the Ge(111) surface subsequently annealed for 30 minutes at 305 o C, and Fig. 4.7(c) is the Ge(111) surface irradiated by 5 kev Xe ions at 305 o C for 1 minute with the ion fluence of 1.9 10 15 ions/cm 2, corresponding to removal of 3.9 bilayers or 1.3 nm thickness, Fig. 4.7(d) is the Ge(111) surface subsequently annealed for 30 minutes at 305 o C. Individual pits disappear with the annealing at 305 o C, while larger pits formed by coalescence of individual pits remain without much change in size after the annealing at 305 o C.

87 4.2.2. Formation mechanism of pits Diminishing number and size of pits with increasing ion fluence and formation of pits by 650 ev Xe ions suggest pits are not surface craters. 5 kev Xe ion irradiation at 275 o C on a Ge(111) buffer layer, which had been grown at 365 o C with the thickness of in an in situ MBE chamber, was performed to examine the interaction of 5 kev Xe ions with the starting surface prepared without ion etching at high temperature; this experiment revealed vacancy islands but no pits. Also annealing of the Ge(111) starting surface shown in Fig. 2.3(a) for 18 seconds at 305 o C was performed and neither pits nor vacancy islands were observed. The result that 650 ev Xe ions can create large pits suggests the formation of pits does not depend on ion energy as strongly as the formation of surface craters. The observation that the number of pits diminishes with the increasing ion fluence > 10 16 ions/cm 2 implies the formation of pits depends on processes that diminish with ion irradiation. The fact that pits do not appear with 5 kev Xe ion irradiation on an MBE grown Ge(111) buffer layer suggests the formation of pits is associated with the bulk nanocavities discussed in chapter 3. As discussed in chapter 3, since there is a denuded zone for bulk nanocavities with the depth of ~ 4 nm from the surface where no nanocavities are observed, the formation of pits is not due to exposing bulk nanocavities by ion etching removal of material above the nanocavities considering the formation of pits after 0.11 nm thick Ge surface layer removal by 5 kev Xe ions in 4.7(a). In addition, the result that annealing of Ge(111) starting surface at 305 o C does not reveal pits

88 suggests the formation of pits is not mainly due to thermal migration of bulk nanocavities toward the surface. Therefore I propose that the pits are produced due to migration of bulk nanocavities toward the surface through the interaction of subsurface displacement cascades with bulk nanocavities; this interaction is responsible for the enhanced mobility of bulk nanocavities and thus for the formation of pits on the surface. Lacking the atomic level understanding of the interaction of a displacement cascade with a nanocavity, I propose that a collision cascade initiated adjacent to a nanocavity will form a melt zone that will allow the nanocavity to deform into the molten region, and if this melt zone intersects the surface, the nanocavity can be pulled out toward the surface during recrystallization process. Once appearing on the surface, pits become larger due to thermal diffusion of surface vacancies or adatoms, hence larger pits are formed at 305 o C than at 275 o C in Fig. 4.1 and individual pits eventually disappear with the annealing at 305 o C in Fig. 4.7(b) and (d). This proposed formation mechanism of pits can elucidate the existence of a denuded zone for bulk nanocavities, starting surfaces without pits, and behaviors of pits shown in Fig. 4.1-4.7. First, the denuded zone for bulk nanocavities with the depth of ~ 4 nm from the surface can be formed by 5 kev Xe ion etching at 520 o C since the nanocavities remaining in the zone will be annihilated on the surface due to the proposed interaction of displacement cascades with nanocavities. The depth of the denuded zone is approximately equal to the predicted 5 kev Xe ion penetration depth in Ge with ion incidence angle of 50 o from surface normal. Second, once those nanocavities appear on

89 the surface at 520 o C, pits flatten rapidly through surface diffusion of vacancies or adatoms to eventually disappear and therefore no pits are observed on the Ge starting surfaces. Third, with increasing ion fluence the number of pits initially increases as more bulk nanocavities appear on the surface, and eventually decreases as the bulk nanocavities are consumed. The number of pits formed on Ge(111) at 305 o C by 5 kev Xe ions shown in Fig. 4.5 and the areal density of nanocavities formed in the Ge(111) sample at 500 o C are plotted in Fig. 4.8; the areal density of nanocavities is calculated by integrating the average nanocavity density shown in Fig. 3.3 with base of 10 nm-thick depth. The similarity between the two plots also suggests that the pits are formed by annihilation of nanocavities. In addition, since higher energy ions can produce deeper displacement cascades, the result that 20 kev Xe ions produce more pits than 5 kev Xe ions by a factor of 4 with the ion fluence of ~ 1 10 14 ions/cm 2 in Fig. 4.5 also suggests that the pits are formed by annihilation of nanocavities. The initial increase in the number of pits on Ge(001) is not linear with the ion fluence compared to the almost linearity on Ge(111) and more pits are observed on Ge(001) than on Ge(111) in Fig. 4.5; since the number of pits depends on the areal density of nanocavities, I believe there is a difference in the formation of bulk nanocavities in Ge(111) and Ge(001) probably due to the difference in the areal density of surface sites, which is proportional to sink strength of the surface, in the Eq. (1-21); Ge(001) with the surface site density of 6.25 10 14 atoms/cm 2 is a weaker sink for vacancies compared to Ge(111) with the surface site density of 1.45 10 15 atoms/cm 2. In

90 other words, only nanocavities large enough to absorb vacancies in competition with the surface can be nucleated and in the presence of stronger sink such as Ge(111) nanocavities larger than in Ge(001) can survive the competition; therefore, smaller number of larger pits results on Ge(111) than Ge(001). 4.2.3. Individual impact of 20 kev Xe ions on Ge Fig. 4.9 shows STM images of the Ge(111) surfaces following 20 kev Xe ion irradiation at 275 o C; the ion fluence is 7.5 10 12 ions/cm 2 corresponding to 0.016 bilayer removal for 4.9(a), 6.8 10 13 ions/cm 2 corresponding to 0.14 bilayer removal for 4.9(b), 2.0 10 14 ions/cm 2 corresponding to 0.41 bilayer removal for 4.9(c) and 9.0 10 14 ions/cm 2 corresponding to 1.9 bilayer removal for 4.9(d). Two types of prominent surface defects are observed: one is large pits formed by the annihilation of bulk nanocavities on the surface as discussed in section 4.2.2 and the other is small pits. The difference in size between large and small pits is a factor of 2 in Fig. 4.9(a), and as ion irradiation continues the size difference between those two types of pits becomes more prominent. The small pits are more than one bilayer deep in comparison to one bilayer deep vacancy islands, and thus small pits are not formed by sputtering removal of surface atoms. Just like large pits, the number of small pits initially increases and then decreases with increasing ion fluence as shown in Fig. 4.10, and the number of small pits created at 215 o C is larger than at 275 o C. The initial increase in the number of small pits is not linear in Fig. 4.10, which can also be seen in Fig. 4.9(a) and (b); though the ion fluence is

91 9 times larger in Fig. 4.9(b) than in Fig. 4.9(a), the number of small pits is not proportionally larger. Also, in Fig. 4.9 and 4.10, the number of small pits decreases dramatically with the ion fluence of ~ 1 10 14 ions/cm 2. I believe the small pits are annealed out much like the large pits in Fig. 4.7 with the resulting vacancies absorbed by the large pits or attached to the step edges or vacancy islands, and also recombine with the adatoms created along with the small pits during ion irradiation; hence the yield of the small pits in Fig. 4.10 is not constant. Fig. 4.11 displays STM images of the Ge(111) surfaces following 20 kev Xe ion irradiation with the ion fluence of 2.3 10 13 ions/cm 2 corresponding to 0.048 bilayer removal; the sample temperature during ion irradiation is 190 o C for 4.11(a), 215 o C for 4.11(b), and 245 o C for 4.11(c). Adatom islands are formed at 190 o C and 215 o C but not at 245 o C, and the number of small pits diminishes while the size of small pits grows as temperature increases from 215 o C to 245 o C. Small area scans reveal small pits and adatom islands more clearly as shown in Fig. 4.12 and the size of small pits grows as temperature increases from 190 o C to 215 o C. Also the density of adatom islands decreases while the size of adatom islands grows as temperature increases from 190 o C to 215 o C. Fig. 4.13 shows STM images of the Ge(111) surfaces following 20 kev Xe ion irradiation with the ion fluence of 6.8 10 13 ions/cm 2 corresponding to 0.14 bilayer removal; the sample temperature during ion irradiation is 190 o C for 4.13(a), 215 o C for 4.13(b), 245 o C for 4.13(c), and 305 o C for 4.13(d). Comparing Fig. 4.13(a) and (b) with

92 Fig. 4.11(a) and (b), larger but less adatom islands are formed at 190 o C and 215 o C, and the size of small pits does not vary much, while the number of small pits is smaller in Fig. 4.13(c) than Fig. 4.11(c) though the ion fluence is increased by a factor 3. At 305 o C almost no small pits are observed. In addition, large pits are observed in Fig. 4.13(c) and (d) but not in Fig. 4.11(c). I propose that small pits and adatoms are produced via surface damage creation process by single 20 kev Xe ion impact on Ge(111) since I did not obtain experimental evidence that the small pits are surface craters as observed on Ge(001) bombarded by 20 kev Ga ions in Ref. 9. Besides, since the small pits are more than one bilayer deep, the small pits are not vacancy islands. 20 kev Xe ion irradiation on Ge(111) with the ion fluence of 7.5 10 12 ions/cm 2 at room temperature revealed small pits, which are not surrounded by rims, whose size is ~ 1.5 nm compared to ~ 6 nm surface craters observed in Ref. 9; these small pits on the Ge(111) surface rather resembles small scale damage features in Ref. 9. In addition, the yield of small pits produced by 20 kev Xe ions on Ge(111) is ~ 0.5 %, which is a factor of 5 larger than the yield of surface craters in Ref. 9. Since 20 kev Xe ions can deposit the same amount of kinetic energy in a shallower depth than 20 kev Ga ions, if 20 kev Ga ions can create surface craters, 20 kev Xe ions can also create surface craters. However, the absence of surface craters on Ge(111) irradiated by 20 kev Xe ions and low yield of ~ 0.1 % for surface crater formation by 20 kev Ga ions on Ge(001) lead me to believe the possibility that during 20 kev Ga ion bombardment doubly or higher charged Ga ions or clustered Ga ions were formed in the ion beam with low but not negligible probability and those ions produced

93 surface craters. Threshold energy for Xe ions to create surface craters on Ge should be higher than 20 kev, although I do not yet know the exact threshold energy. Since the atomic mass of Ga is 70, 20 kev Ga + 2 ions would not have much different impact compared to 20 kev Xe + ions whose mass is 131. However, I believe 40 kev Ga 2+ 2 or higher charged clustered Ga ions accelerated by 20 kv would be able to create surface craters on Ge. For example, Merkle and Jäger observed that threshold energy for surface crater formation on Au decreases from 50 kev for Bi + ions to 12 kev for Bi + 2 ions due to increased deposited energy density for Bi + 2 ions [7]. The small pits created by surface damage creation can recombine with adatoms and disappear as temperature increases considering the absence of adatom islands on the Ge(111) surface and almost unchanged size of small pits in Fig. 4.11(c) and Fig. 4.13(c), and therefore less small pits are observed at 245 o C and 305 o C in Fig. 4.11 and 4.13. Also as more adatoms are produced by ion irradiation, small pits are more likely to be filled with adatoms produced through the formation of nearby small pits. Fig. 4.14 shows an STM image of the Ge(111) surface annealed for 90 seconds at 275 o C after producing the surface shown in Fig. 4.13(b). Though large pits are not observed in Fig. 4.13(b), annealing the surface at 275 o C reveals large pits. As discussed in section 4.2.2, since thermal annealing alone cannot cause bulk nanocavities to migrate toward and appear on the surface, those large pits should be present in Fig. 4.13(b) but are not clearly distinguished from small pits. In agreement with the proposed mechanism of large pits formation discussed in section 4.2.2, this appearance of large pits is due to

94 thermal diffusion of surface vacancies or adatoms while small pits disappear during annealing.

95 (a) (b) (c) (d) Fig. 4.1. STM images of Ge irradiated by 5 kev Xe ions for 18 seconds on the starting surfaces shown in Fig. 2.3 with the ion flux of 3.1 10 13 ions/cm 2 s, corresponding to 1.2 bilayer removal from the Ge(111) surface in (a) and (b), and 2.7 monolayers removal from the Ge(001) surface in (c) and (d). The scan size is 600 600 nm 2. Temperature of samples during ion irradiation is: (a) 275 o C (b) 305 o C (c) 275 o C (d) 305 o C.

Fig. 4.2. High resolution STM images of Ge irradiated by 5 kev Xe ions at 305 o C corresponding to (a) Fig. 4.1(b) and (b) Fig. 4.1(d). The scan size is (a) 300 300 nm 2 and (b) 150 150 nm 2. 96

97 (a) (b) (c) Fig. 4.3. STM images of Ge irradiated by 5 kev Xe ions for 3 minutes on the starting surfaces shown in Fig. 2.3 with the ion flux of 3.1 10 13 ions/cm 2 s, corresponding to 12 bilayers removal from the Ge(111) surface in (a) and (b), and 27 monolayers removal from the Ge(001) surface in (c). The scan size is 600 600 nm 2. Temperature of samples during ion irradiation is: (a) 275 o C (b) 305 o C (c) 305 o C.

98 (a) (b) (c) (d) Fig. 4.4. STM images of Ge irradiated by 5 kev Xe ions for 30 minutes (in (a), (b) and (d)) and 20 minutes (in (b)) on the starting surfaces shown in Fig. 2.3 with the ion flux of 3.1 10 13 ions/cm 2 s, corresponding to 120 bilayers removal from the Ge(111) surface in (a), 77 bilayers removal from the Ge(111) surface in (b), and 270 monolayers removal from the Ge(001) surface in (c) and (d). The scan size is 600 600 nm 2. Temperature of samples during ion irradiation is: (a) 275 o C (b) 305 o C (c) 275 o C (d) 305 o C.

99 ion fluence (cm -2 ) 1000 10 13 10 14 10 15 10 16 10 17 100 10 20 kev 5 kev 650 ev 1 0.01 0.1 1 10 100 thickness removed (nm) Fig. 4.5. The number of pits as a function of the ion fluence. Filled symbols are Ge samples irradiated by 5 kev Xe ions. Sample orientation and temperature are : (111), 305 o C; : (111), 275 o C; : (001), 305 o C; : (001), 275 o C. Open symbols are Ge(111) samples irradiated by 20 kev Xe ions. Sample temperature is : 275 o C, : 215 o C, : 245 o C and : 305 o C. and + are Ge(001) and Ge(111) samples irradiated by 650 ev Xe ions at 245 o C, respectively.

100 Fig. 4.6. STM images of Ge irradiated by 650 ev Xe ions at 245 o C for 12 minutes 40 seconds on the starting surfaces shown in Fig. 2.3 with the ion fluence of 1.3 10 15

ions/cm 2, corresponding to 1.2 bilayer removal from the Ge(111) surface in (a), and 2.7 monolayers removal from the Ge(001) surface in (b). The scan size is 600 600 nm 2. 101

102 (a) (b) (c) (d) Fig. 4.7. STM images of Ge(111) (a) irradiated by 5 kev Xe ions at 305 o C for 5 seconds with the ion flux of 3.1 10 13 ions/cm 2 s, corresponding to 0.33 bilayer removal from the surface (b) subsequently annealed for 30 minutes at 305 o C (c) irradiated by 5 kev Xe ions at 305 o C for 1 minute with the ion flux of 3.1 10 13 ions/cm 2 s, corresponding to 3.9 bilayers removal from the surface (d) subsequently annealed for 30 minutes at 305 o C. The scan size is 600 600 nm 2.

103 100 nanocavities formed in Ge(111) at 500 o C 10 pits formed on Ge(111) at 275 o C 1 0.1 1 10 100 1000 depth (nm) Fig. 4.8. The number of pits and nanocavities as a function of depth from the Ge surface. The number of pits is obtained from STM images as in Fig. 4.5 and the number of nanocavities is obtained from TEM images as in Fig. 3.4. The areal density of nanocavities is calculated by integrating the average nanocavity density shown in Fig. 3.4 with base of 10 nm-thick depth.

104 (a) (b) (c) (d) Fig. 4.9. STM images of Ge(111) irradiated by 20 kev Xe ions at 275 o C with the ion fluence of (a) 7.5 10 12 ions/cm 2, corresponding to 0.016 bilayer removal from the surface (b) 6.8 10 13 ions/cm 2, corresponding to 0.14 bilayer removal from the surface (c) 2.0 10 14 ions/cm 2, corresponding to 0.41 bilayer removal from the surface (d) 9.0 10 14 ions/cm 2, corresponding to 1.9 bilayer removal from the surface. The scan size is 400 400 nm 2.

105 thickness removed (nm) 10 4 0.01 0.1 1 215 o C 1000 100 275 o C 10 10 13 10 14 10 15 ion fluence (cm -2 ) Fig. 4.10. The number of small pits as a function of the ion fluence. The error bars are inserted assuming the statistics of small pits follows Poisson distribution.

106 (a) (b) (c) Fig. 4.11. STM images of Ge(111) irradiated by 20 kev Xe ions with the ion fluence of 2.3 10 13 ions/cm 2, corresponding to 0.048 bilayer removal from the surface at (a) 190 o C (b) 215 o C (c) 245 o C. The scan size is 400 400 nm 2.

107

108 Fig. 4.12. STM images of Ge(111) irradiated by 20 kev Xe ions with the ion fluence of 2.3 10 13 ions/cm 2, corresponding to 0.048 bilayer removal from the surface at (a) 190 o C (b) 215 o C. The scan size is 200 200 nm 2.

109 (a) (b) (c) (d) Fig. 4.13. STM images of Ge(111) irradiated by 20 kev Xe ions with the ion fluence of 6.8 10 13 ions/cm 2, corresponding to 0.14 bilayer removal from the surface at (a) 190 o C (b) 215 o C (c) 245 o C (d) 305 o C. The scan size is 400 400 nm 2.

110 Fig. 4.14. STM image of Ge(111) annealed for 90 seconds at 275 o C after obtaining the surface displayed in Fig. 4.13(b). Large pits which were not observed in Fig. 4.13(b) are observed. The scan size is 400 400 nm 2. 4.3. References

111 [1] D. N. Seidman, R. S. Averback, and R. Benedek, Phys. Stat. Sol. 144, 85 (1987). [2] T. Diaz de la Rubia, R. S. Averback, R. Benedek, and W. E. King, Phys. Rev. Lett. 59, 1930 (1987). [3] R. S. Averback and Mai Ghaly, J. Appl. Phys. 76, 3908 (1994); Mai Ghaly and R. S. Averback, Phys. Rev. Lett. 72, 364 (1994). [4] Mai Ghaly, R. S. Averback, and T. Diaz de la Rubia, Nucl. Instr. Meth. Phys. Res. B 102, 51 (1995). [5] R. S. Averback, M. Ghaly and H. Zhu, in Film Synthesis and Growth Using Energetic Beams. Mat. Res. Soc. Symp. Proc. 388 (Materials Research Society, Pittsburgh, 1995) pp. 337-348. [6] P. Ehrhart, W. Schilling, and H. Ullmaier in Encyclopedia of Applied Physics, edited by George L. Trigg (VCH Publishers, New York, 1996). [7] K. L. Merkle and W. Jäger, Philos. Mag. A 44, 741 (1981). [8] S. E. Donnelly and R. C. Birtcher, Phys. Rev. B 56, 13599 (1997); R. C. Birtcher and S. E. Donnelly, Mat. Chem. Phys. 54, 111 (1998). [9] P. Bellon, S. Jay Chey, Joseph E. Van Nostrand, Mai Ghaly, David G. Cahill, and R. S. Averback, Surf. Sci. 339, 135 (1995). [10] Christian Teichert, Michael Hohage, Thomas Michely, and George Comsa, Phys. Rev. Lett. 72, 1682 (1994). [11] G. Foti, G. Vitali, and J. A. Davies, Rad. Effects 32, 187 (1977). [12] I. H. Wilson, N. J. Zheng, U. Knipping, and I. S. T. Tsong, Phys. Rev. B 38, 8444 (1988).

112 [13] I. H. Wilson, Y. J. Chen, J. B. Xu, R. A. B. Devine, and C. Jeynes, Surf. Int. Anal. 24, 881 (1996). [14] P. J. Goodhew and S. K. Tyler, Proc. R. Soc. Lond. A 377, 151 (1981). [15] H. Yamaguchi, I. Hashimoto, H. Mitsuya, K. Nakamura, E. Yagi, and M. Iwaki, J. Nucl. Mater. 161, 164 (1989). [16] S. E. Donnelly, R. C. Birtcher, C. Templier, and V. Vishnyakov, Phys. Rev. B 52, 3970 (1995) [17] S. E. Donnelly, R. C. Birtcher, C. Templier, R. Valizadeh, and V. Vishnyakov, Mat. Res. Soc. Symp. Proc. 373, 243 (1995). [18] R. C. Birtcher, S. E. Donnelly, and C. Templier, Phys. Rev. B 50, 764 (1994). [19] I. H. Wilson, J. Appl. Phys. 53, 1698 (1982). [20] A. L. Southern, William R. Willis, and Mark T. Robinson, J. Appl. Phys. 34, 153 (1963); G. S. Anderson, J. Appl. Phys. 37, 2838 (1966). The sputtering yield of 650 ev Ar ion is 1.3 on Ge(111) and Ge(001), which is approximately the sputtering yield of 650 ev Xe ion on Ge(111) and Ge(001).