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Heteroepitaxy of germanium thin films on silicon by ion sputtering
298
Journal of Crystal Growth 24/25 (1974) 298—301 © North-HollandPublishing Co.
HETEROEPITAXY OF GERMANIUM THIN FILMS ON SILICON BY ION SPUTTERING L. N. ALEKSANDROV, R. N. LOVYAGIN, 0. P. PCHELYAKOV and S. I. STENIN Institute of Semiconductor Physics of the Academy of Sciences, Novosibirsk, U.S.S.R. The successive stages of germanium film growth on the vicinal face of silicon near (111) have been studied in the range ofeffective thickness df from 10 A to 25000 A. It was shown that the growth of the trapezoidal islands of germanium began from the steps on the silicon substrate. After coalescence of big islands, from df > 1000 A, the vicinal germanium surface formed, which is parallel to the initial silicon surface. The 3 films with df = 25000 A had hole conductivity; the carrier concentration was from 1016 to l0’~cm mobility was from 600 to 1000 cm2/V sec at 300 °Kand from 2000 to 6000 cm2IV sec at 77 °K.
1. Introduction
of unsaturated hydrocarbons, C
The epitaxial system germanium—silicon differs in 1). In heteroessence the systemon of silicon silicon—silicon epitaxy from of germanium by sublimation and electron beam evaporation, triangular and hexagonal islands of germanium were observed in the early stages of growth with densities of about 4 x 106 cm2. The cause of the appearence of islands, their dependence on microrelief of the substrate, anisotropy of the growth and the successive stages of the film formation were not studied, In this work we describe the growth of germanium films on a vicinal silicon face near (111) by ion sputtering, using an independent discharge in argon. The physical conditions for epitaxy and the experimental method were described in refs. 2 and 3. 2. Experiment Rectangular silicon plates 18 x 5 x 0.4 mm3, n-type, doped with phosphorous and having a resistivity of 15 ohm cm were oriented to within ±3°of (ill). The substrates, after mechanical and chemical polishing, were degreased in boiling toluol and placed in the vacuum chamber between tantalum contacts, which were preheated in UHV at 1400 °Cfor 1 hr. After reaching a pressure less than 10-v torr, the chamber was heated to 200 °Cand gettered by silicon sputtering. The substrate was heated by an alternating current of about 10 A for 10—20 mm at about 1300 °C. Massspectrometer analysis shows that the partial pressures
2H2 and C2H4, which are responsible for the formation of f3-SiC on the silicon surface, were closewe toused 10—10 4). For sputtering a torr rotating germanium target which was p-type with resistivity of 5 ohm cm. Before sputtering it was placed with a magnet in a position screened from the substrate and bombarded by an electron beam at +600 V potential and with current densities from 2 to 3 mA/cm2 for 10 mm. Then the target was cleaned with argon ions at a potential of —600 V and current densities from 0.7 to 1 mA/cm2 for 5 mm. Before rotating the target into position, the substrate was heated in high vacuum for 1 mm at 1300 °C;the temperature was measured with an optical pyrometer. The cleaned target was placed above the substrate and at a distance of 5—6 mm, a potential of —600 V was applied to the target and gas was admitted abruptly; the pressure was between iO~ to 5 x i0-~ torr. Thus, the time of exposure of the clean silicon surface in argon before sputterring was minimal. The direction of the electric field between target and substrate was such that damage of the substrate by argon ions was impossible. Cleaning of the substrate surface by ions was not carried out. Condensation of germanium was carried out at substrate temperatures (t~) from 480 to 590 °C.The film growth rate, determined from the film thickness (measured with a binocular microscope MIS—I I at df = 0.4, 0.8,1.2 and 2.0 ~tmand the time ofdeposition, was about 7 A/sec. The film thickness df varied from 10 A to 25000 A. Small thicknesses were estimated from the
VI -4
299
HETEROEPITAXY OF GERMANIUM THIN FILMS ON SILICON BY ION SPUTTERING
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Fig. I. Structure of silicon surface before epitaxy: (a) replica, (b) RHEED pattern in the direction <110>.
calculated growth rate and the deposition time. In order to analyze the surface relief we used carbon replicas shadowed with gold at 5° to the surface of the cooled substrate. Structures of the substrate surface and the film were examined by RHEED2). 3. Results and discussion 3.1.
(b) Fig. 2. Surface of germanium films at early stages of epitaxy at T~ = 590 °C:(a) 4 = 10 A; (b) 4 = 40 A.
In contrast to the results of ref. 2, such surfaces did not contain pinning points for steps (i.e. etch pits or carbon particles). Fig. lb is a RHEED pattern of the
SUBSTRATE SURFACE BEFORE EPITAXY
On the substrate surface, prepared as described above, systems of micro- or macrosteps were formed which depended on the direction of the initial misorientation from (111). The relief of the surface before epitaxial deposition is shown on fig. la. The steps lie along <110> directions and the distance between the steps varied from 500 to 900 A; their height was approximately 30 A.
substrate in the <110> azimuth. 3.2. DEVELOPMENT OF THE RELIEF OF THE FILM SURFACE The film surface at df = 10 A and T 5 = 590 °Cis shown on fig. 2a. Many kinks are seen on the silicon steps. They are trapezoidal and triangular in form and grow into trapezoidal islands. These islands were observed at t/f = 40 A (fig. 2b). The density of the big
VI —4
300
L. N. ALEKSANDROV, R. N. LOVYAGIN, 0. P. PCHELYAKOV AND S. I. STENIN
islandswas5xlO8 cm2andtheirmeandimensionwas about 2500 . Comparison of the step forms on the initial substrate surface and in the initial stage of epitaxy (fig. 1 a and fig. 2a) shows that during the deposition of germanium, movement of the steps with the formation of macrokinks takes place. Later on, parts of these semi-islands grow into islands. Thus, in the initial stage of growth, adsorbed particles of germanium atoms and their complexes are added to the silicon surface and, as a result, parts of the germanium film are formed which are coherent with the substrate On reaching some critical c/f, according to our data not more than 10 A, the movement of the germanium steps is accelerated in some positions and thus leads to the formation of peninsulas and then to the formation of growth islands. The surfaces of germanium islands and silicon terraces appear to be parallel. It is known that the coefficient of diffusion of Ge in Si at T < 600 °Cis less than 10— 17 cm2/sec 5). Calculation shows that the thickness of the interdiffused germanium—silicon region was less than 3 A. In fig. 3a, b, surface relief of films with df = 400 A and df = 25000 A is shown; these films were obtained at T~ = 590 °C.Coalescence of the islands and formation of macrosteps was observed where they contacted. The vicinal germanium surface, parallel to the initial silicon surface, formed on the substrate when the thickness was more than 2000 A. At df = 25000 A only one system of macrosteps was observed. The macrosteps were parallel to the initial steps on the silicon surface, but their height was about 200 A and the distance between them was about 7000 A (fig. 3b). The appearance of the steps on the film is determined by the initial disorientation of the silicon surface from (111) and by the anisotropy of growth rates of the islands in various directions. The anisotropy is proved by the parallelism of the edges of the silicon terraces and islands, by their trapezoidal form (fig. 2b) and by the primary direction of the terraces of germanium at the coalescence stage (fig. 3a). It seems that the distance between macrosteps on the film surface (0.7 jtm) is determined by the distance between the active (big) growth islands (0.5 rim). Steps of two signs are seen between the film microsteps, which are from 20 to 40 A in height and which border the growth strip in the center of the terrace. (In this experiment microsteps of 6—10 A height could not
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Fig. 3. (a) Coalescense of germdnium islands and the formaoilgrowtlt ~,4~3±.51~3::C~9o C, (b) germanium
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be detected.) This can occur if the distance between the macrosteps is greater than the diffusion distance of adsorbed particles. As a result, parts of the particles will penetrate into the macrosteps and other parts will grow over the growth centers in the middle of the terrace. In this case, the increase in film thickness is due
VI -4
HETEROEPITAXY OF GERMANIUM THIN FILMS ON SILICON BY ION SPUTTERING
301
to the movement of microsteps and to layer growth on the terraces (fig. 4). Similar combined growth mechanisms were observed by us earlier in work on homoepitaxy of silicon2). However, in this case2) layer
~si
Fig. 4. The movement of steps during growth of germanium films when 4 > 1000 A, H is a microstep, h is a growth strip.
growth was the provided by the movement of monoatomic steps from dihedral angle of microsteps. The presence of the complexes in the movement of macro- and microsteps on the silicon surface was observed. More favorable conditions for the formation of complexes existed in the growth of germanium films (at higher supersaturation). Moreover, according to published data6 7), germanium vapor which reaches the substrate may serve as a source of complexes. For instance, in ion sputtering of silicon6) (the ion energy was 10 keV) the concentration of complexes with two or more atoms reaches 10 % of the flux of sputtered particles. Azimuth investigation of surface by RHEED shows that the films with df ? 40 A are without microtwins (fig. 5). Twin orientations of the growth islands were absent. The films with df = 25000 A were p-type, with
Fig. 5. RHEED pattern in the ‘110 azimuth of the film with d 1 = 25000 A, T, = 590 °C.
3 in the temperature hole cm rangeconcentration from 77 to 3001017 °Kand mobility 600—1000 cm2/V sec at 300 °Kand 2000—6000 cm2/V sec at 77 °K. References 1) A. G. Cullis and G. R. Booker, J. Crystal Growth 9 (1971) 107. 2) 0. P. Pchelyakov, R. N. Lovyagin, E. A. Krivorotov, A. I. Toropov, L. and S. I. Stenin, Phys. Status 3) L. N. Aleksandrov and R. N. Lovyagin, Thin Solid Films 20 (1974) 1. 4)
and R. N. Summergrad, AppI. Phys. Letters 11
5) G. L. McVay and A. R. DuCharme, J. AppI. Phys. 44 (1973) 1409. 6) C. Feldman and F. G. Satkiewicz, Thin Solid Films 12 (1972) 7) R. E. Honig, J. Chem. Phys. 21 (1953) 573.
VI-4
Journal of Crystal Growth 24/25 (1974) 298—301 © North-HollandPublishing Co.
HETEROEPITAXY OF GERMANIUM THIN FILMS ON SILICON BY ION SPUTTERING L. N. ALEKSANDROV, R. N. LOVYAGIN, 0. P. PCHELYAKOV and S. I. STENIN Institute of Semiconductor Physics of the Academy of Sciences, Novosibirsk, U.S.S.R. The successive stages of germanium film growth on the vicinal face of silicon near (111) have been studied in the range ofeffective thickness df from 10 A to 25000 A. It was shown that the growth of the trapezoidal islands of germanium began from the steps on the silicon substrate. After coalescence of big islands, from df > 1000 A, the vicinal germanium surface formed, which is parallel to the initial silicon surface. The 3 films with df = 25000 A had hole conductivity; the carrier concentration was from 1016 to l0’~cm mobility was from 600 to 1000 cm2/V sec at 300 °Kand from 2000 to 6000 cm2IV sec at 77 °K.
1. Introduction
of unsaturated hydrocarbons, C
The epitaxial system germanium—silicon differs in 1). In heteroessence the systemon of silicon silicon—silicon epitaxy from of germanium by sublimation and electron beam evaporation, triangular and hexagonal islands of germanium were observed in the early stages of growth with densities of about 4 x 106 cm2. The cause of the appearence of islands, their dependence on microrelief of the substrate, anisotropy of the growth and the successive stages of the film formation were not studied, In this work we describe the growth of germanium films on a vicinal silicon face near (111) by ion sputtering, using an independent discharge in argon. The physical conditions for epitaxy and the experimental method were described in refs. 2 and 3. 2. Experiment Rectangular silicon plates 18 x 5 x 0.4 mm3, n-type, doped with phosphorous and having a resistivity of 15 ohm cm were oriented to within ±3°of (ill). The substrates, after mechanical and chemical polishing, were degreased in boiling toluol and placed in the vacuum chamber between tantalum contacts, which were preheated in UHV at 1400 °Cfor 1 hr. After reaching a pressure less than 10-v torr, the chamber was heated to 200 °Cand gettered by silicon sputtering. The substrate was heated by an alternating current of about 10 A for 10—20 mm at about 1300 °C. Massspectrometer analysis shows that the partial pressures
2H2 and C2H4, which are responsible for the formation of f3-SiC on the silicon surface, were closewe toused 10—10 4). For sputtering a torr rotating germanium target which was p-type with resistivity of 5 ohm cm. Before sputtering it was placed with a magnet in a position screened from the substrate and bombarded by an electron beam at +600 V potential and with current densities from 2 to 3 mA/cm2 for 10 mm. Then the target was cleaned with argon ions at a potential of —600 V and current densities from 0.7 to 1 mA/cm2 for 5 mm. Before rotating the target into position, the substrate was heated in high vacuum for 1 mm at 1300 °C;the temperature was measured with an optical pyrometer. The cleaned target was placed above the substrate and at a distance of 5—6 mm, a potential of —600 V was applied to the target and gas was admitted abruptly; the pressure was between iO~ to 5 x i0-~ torr. Thus, the time of exposure of the clean silicon surface in argon before sputterring was minimal. The direction of the electric field between target and substrate was such that damage of the substrate by argon ions was impossible. Cleaning of the substrate surface by ions was not carried out. Condensation of germanium was carried out at substrate temperatures (t~) from 480 to 590 °C.The film growth rate, determined from the film thickness (measured with a binocular microscope MIS—I I at df = 0.4, 0.8,1.2 and 2.0 ~tmand the time ofdeposition, was about 7 A/sec. The film thickness df varied from 10 A to 25000 A. Small thicknesses were estimated from the
VI -4
299
HETEROEPITAXY OF GERMANIUM THIN FILMS ON SILICON BY ION SPUTTERING
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Fig. I. Structure of silicon surface before epitaxy: (a) replica, (b) RHEED pattern in the direction <110>.
calculated growth rate and the deposition time. In order to analyze the surface relief we used carbon replicas shadowed with gold at 5° to the surface of the cooled substrate. Structures of the substrate surface and the film were examined by RHEED2). 3. Results and discussion 3.1.
(b) Fig. 2. Surface of germanium films at early stages of epitaxy at T~ = 590 °C:(a) 4 = 10 A; (b) 4 = 40 A.
In contrast to the results of ref. 2, such surfaces did not contain pinning points for steps (i.e. etch pits or carbon particles). Fig. lb is a RHEED pattern of the
SUBSTRATE SURFACE BEFORE EPITAXY
On the substrate surface, prepared as described above, systems of micro- or macrosteps were formed which depended on the direction of the initial misorientation from (111). The relief of the surface before epitaxial deposition is shown on fig. la. The steps lie along <110> directions and the distance between the steps varied from 500 to 900 A; their height was approximately 30 A.
substrate in the <110> azimuth. 3.2. DEVELOPMENT OF THE RELIEF OF THE FILM SURFACE The film surface at df = 10 A and T 5 = 590 °Cis shown on fig. 2a. Many kinks are seen on the silicon steps. They are trapezoidal and triangular in form and grow into trapezoidal islands. These islands were observed at t/f = 40 A (fig. 2b). The density of the big
VI —4
300
L. N. ALEKSANDROV, R. N. LOVYAGIN, 0. P. PCHELYAKOV AND S. I. STENIN
islandswas5xlO8 cm2andtheirmeandimensionwas about 2500 . Comparison of the step forms on the initial substrate surface and in the initial stage of epitaxy (fig. 1 a and fig. 2a) shows that during the deposition of germanium, movement of the steps with the formation of macrokinks takes place. Later on, parts of these semi-islands grow into islands. Thus, in the initial stage of growth, adsorbed particles of germanium atoms and their complexes are added to the silicon surface and, as a result, parts of the germanium film are formed which are coherent with the substrate On reaching some critical c/f, according to our data not more than 10 A, the movement of the germanium steps is accelerated in some positions and thus leads to the formation of peninsulas and then to the formation of growth islands. The surfaces of germanium islands and silicon terraces appear to be parallel. It is known that the coefficient of diffusion of Ge in Si at T < 600 °Cis less than 10— 17 cm2/sec 5). Calculation shows that the thickness of the interdiffused germanium—silicon region was less than 3 A. In fig. 3a, b, surface relief of films with df = 400 A and df = 25000 A is shown; these films were obtained at T~ = 590 °C.Coalescence of the islands and formation of macrosteps was observed where they contacted. The vicinal germanium surface, parallel to the initial silicon surface, formed on the substrate when the thickness was more than 2000 A. At df = 25000 A only one system of macrosteps was observed. The macrosteps were parallel to the initial steps on the silicon surface, but their height was about 200 A and the distance between them was about 7000 A (fig. 3b). The appearance of the steps on the film is determined by the initial disorientation of the silicon surface from (111) and by the anisotropy of growth rates of the islands in various directions. The anisotropy is proved by the parallelism of the edges of the silicon terraces and islands, by their trapezoidal form (fig. 2b) and by the primary direction of the terraces of germanium at the coalescence stage (fig. 3a). It seems that the distance between macrosteps on the film surface (0.7 jtm) is determined by the distance between the active (big) growth islands (0.5 rim). Steps of two signs are seen between the film microsteps, which are from 20 to 40 A in height and which border the growth strip in the center of the terrace. (In this experiment microsteps of 6—10 A height could not
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Fig. 3. (a) Coalescense of germdnium islands and the formaoilgrowtlt ~,4~3±.51~3::C~9o C, (b) germanium
~°
be detected.) This can occur if the distance between the macrosteps is greater than the diffusion distance of adsorbed particles. As a result, parts of the particles will penetrate into the macrosteps and other parts will grow over the growth centers in the middle of the terrace. In this case, the increase in film thickness is due
VI -4
HETEROEPITAXY OF GERMANIUM THIN FILMS ON SILICON BY ION SPUTTERING
301
to the movement of microsteps and to layer growth on the terraces (fig. 4). Similar combined growth mechanisms were observed by us earlier in work on homoepitaxy of silicon2). However, in this case2) layer
~si
Fig. 4. The movement of steps during growth of germanium films when 4 > 1000 A, H is a microstep, h is a growth strip.
growth was the provided by the movement of monoatomic steps from dihedral angle of microsteps. The presence of the complexes in the movement of macro- and microsteps on the silicon surface was observed. More favorable conditions for the formation of complexes existed in the growth of germanium films (at higher supersaturation). Moreover, according to published data6 7), germanium vapor which reaches the substrate may serve as a source of complexes. For instance, in ion sputtering of silicon6) (the ion energy was 10 keV) the concentration of complexes with two or more atoms reaches 10 % of the flux of sputtered particles. Azimuth investigation of surface by RHEED shows that the films with df ? 40 A are without microtwins (fig. 5). Twin orientations of the growth islands were absent. The films with df = 25000 A were p-type, with
Fig. 5. RHEED pattern in the ‘110 azimuth of the film with d 1 = 25000 A, T, = 590 °C.
3 in the temperature hole cm rangeconcentration from 77 to 3001017 °Kand mobility 600—1000 cm2/V sec at 300 °Kand 2000—6000 cm2/V sec at 77 °K. References 1) A. G. Cullis and G. R. Booker, J. Crystal Growth 9 (1971) 107. 2) 0. P. Pchelyakov, R. N. Lovyagin, E. A. Krivorotov, A. I. Toropov, L. and S. I. Stenin, Phys. Status 3) L. N. Aleksandrov and R. N. Lovyagin, Thin Solid Films 20 (1974) 1. 4)
and R. N. Summergrad, AppI. Phys. Letters 11
5) G. L. McVay and A. R. DuCharme, J. AppI. Phys. 44 (1973) 1409. 6) C. Feldman and F. G. Satkiewicz, Thin Solid Films 12 (1972) 7) R. E. Honig, J. Chem. Phys. 21 (1953) 573.
VI-4