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Interfacial reactions of palladium thin films on Ge(111) and Ge(001)
Thin Solid Films, 162 (1988) 295-303 PREPARATION AND CHARACTERIZATION
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INTERFACIAL REACTIONS OF PALLADIUM THIN FILMS ON Ge(111) AND Ge(001) Y. F. HSIEH AND L. J. CHEN Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu ( Taiwan ) (Received November 16, 1987; accepted April 5, 1988)
Planview and cross-sectional transmission electron microscopy (XTEM), Auger electron spectroscopy and X-ray diffraction (XRD) have been applied to study the interfacial reactions of palladium thin films on Ge(111) and Ge(001) with particular emphasis on the epitaxial growth of Pd2Ge on Ge(111). Polycrystalline palladium and Pd2Ge grains as well as epitaxial Pd2Ge regions were observed in as-deposited (111) samples. Epitaxial Pd2Ge was found to cover about 60~ of the germanium surface area in samples annealed at 160°C. The orientation relationships between epitaxial Pd2Ge and the germanium substrate were determined to be Pd2Ge[0001]llGe[lll] and Pd2Ge(1010)[IGe(~20). The Pd2Ge-Ge interface was found to be not very smooth. In samples annealed at 250-500 °C, only polycrystalline PdGe was found. Randomly oriented polycrystalline palladium and Pd2Ge were found in asdeposited (001) samples. In samples annealed at 160 °C, Pd2Ge and PdGe grains were found to coexist. Only polycrystalline PdGe grains were observed in samples annealed at 250-500 °C. XTEM observations and XRD data revealed the presence of an epitaxial Pd2Ge layer, about 20-30nm in thickness, locally between the palladium metal layer and germanium substrate in as-deposited (111) samples, irrespective of palladium metal layer thickness (30-220 nm) and deposition rate (0.1-0.7 nm s-1). The results for Pd2Ge and PdGe formation on germanium are compared with those for Pd2Si and PdSi formation on silicon.
1. INTRODUCTION Interest in field effect transistors built on semiconductors other than silicon stems from an increasing need for high speed, low power digital and analogue circuits.1 Germanium is one of the three most important semiconductors. For device applications, it is of particular interest because it has high mobility for both electrons and holes, making a high speed complementary field effect transistor a possibility2. Compared with the extensive studies of silicide formation, germanide formation received little attention 3-13. However, understanding of the metal-germanium 0040-6090/88/$3.50
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interactions is important not only for the development of radiation detector systems and integrated circuits based on germanium but also for novel applications. A prominent example is metal contacts for strained layer (Ge,Si)/Si heterostructures which show promise for use in optical communications, integrated optics on silicon, and other detector applications. Metal germanide compounds are in particular interesting in relation to the preparation of metallic contacts on GaAs 1'*-19. Previous studies have found that the germanide formation characteristics such as first phase nucleation, sequence of phase formation, growth kinetics and moving species are similar to those of the corresponding silicides 7'2°. It is therefore expected that the investigation of germanides will benefit greatly from past experience gained in studying silicides. The study of metal-semiconductor interaction using an elemental semiconductor other than silicon is also of fundamental interest. Among metal-germanium and metal-silicon systems, Pd/Ge and Pd/Si bear close similarity. For the Pd/Si system, Pd2Si (hexagonal, C22; a = 0.6493 nm and c = 0.3427 nm) and PdSi (orthorhombic, B31; a = 0.6133nm, b = 0.5599 nm and c = 0.3381 nm) were found to form at 100 °C and 700 °C respectively 3'4'21. Similarly, Pd2Ge (a = 0.6712nm and c = 0.3408nm) and PdGe (a = 0.6259nm, b = 0.5782nm and c = 0.3481 nm), which are structurally isomorphous to Pd2Si and PdSi respectively, were also observed to form sequentially in the Pd/Ge system 7'8'12'21. The Pd/Si system has received much attention in the past since (1) Pd2Si is formed at about 100°C, an anomalously low temperature for silicide formation, (2) Pd2Si is stable up to about 700 °C, an unusually high temperature for the first phase of a near noble metal/silicon system, and (3) Pd2Si can be grown epitaxially on Si(1 l 1) 3'4. A previous investigation of the growth kinetics and phase formation for the Pd/Ge system was performed by Rutherford backscattering spectrometry and the X-ray diffraction method 8. In view of the intimate relationships between microstructures and electrical characteristics of the contacts, a more thorough study on the microstructural aspects of the interfacial reactions is therefore highly desirable. In this paper, we report the results of a transmission electron microscopy (TEM) and X-ray diffraction (XRD) study of the interfacial reactions of palladium thin films on germanium with particular emphasis on the epitaxial growth of Pd2Ge on germanium. We note that the epitaxial growth of cobalt, nickel and platinum germanides on germanium has been reported previously 22-24. The epitaxial growth of silicides on silicon has been extensively investigated in recent years 25--31. 2.
EXPERIMENTAL PROCEDURES
Germanium wafers, (111) oriented and 2in in diameter, were cleaned in trichloroethylene and acetone, consecutively, followed by a deionized water rinse in an ultrasonic cleaner. After etching in a solution of HF, HNOa and CH3COOH (3: 5: 3) for 30 s and rinsing in deionized water, the samples were etched in a dilute H F solution immediately before being loaded into an electron gun evaporation chamber. The dilute H F dipping served to remove the surface oxide. Palladium thin films, 30 nm in thickness unless otherwise specified, were then electron gun deposited onto the germanium substrate at room temperature. The vacuum was maintained to
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be better than 1 x 10-6 Torr during depositions. The deposition rate was about 0.1 n m s - 1 . In order to study the possible dependence of epitaxial germanide formation on the thickness or deposition rate, palladium metal layers with thicknesses of 150 and 220nm and a deposition rate of 0.7nms -1 were also investigated. Heat treatments were carried out in a three-zone diffusion furnace in N 2 ambient. High purity N2 gas was first passed through a titanium getter tube maintained at 800 °C to reduce the 0 2 content. Annealings were conducted at 160-500 °C. The annealing time was 1 h at each temperature. The samples were then cut ultrasonically into discs, 3 mm in diameter. Planview specimens for TEM examinations were prepared chemically by perforating a small hole at the centre of the discs in a solution of H F and H N 0 3 (1:1). Cross-sectional samples were prepared using a procedure outlined by Sheng and Chang 32. T E M investigations were performed with a JEOL-200CX electron microscope operating at 200 kV. In order to ensure that the phases observed in as-deposited samples by TEM were not an artifact of T E M sample preparation, phase identification was also performed by XRD analysis using a Rigaku X-ray diffractometer. Auger electron spectroscopy (AES) combined with ion sputtering was applied to obtain depth-composition data. The surface composition was determined by AES with a base pressure down tO 8 x 10- lo Torr using 3 keV electrons with a 1 ~tA primary beam current. The beam size was about 1 ~tm. Composition depth profiles were obtained by Ar ÷ ion sputtering, 4 keV in Ar + ion energy and 2 m m x 2 mm in rastered area, at a sputtering rate of about 3 nm m i n - 1 under a pressure of about 2 x 10- 7 Torr. 3.
RESULTS AND DISCUSSION
3.1 Pd/Ge(lll) system Polycrystalline palladium and Pd2Ge grains as well as epitaxial Pd2Ge regions were observed in as-deposited samples. Epitaxial Pd2Ge was found to cover about 25% of the germanium surface area. Cross-sectional TEM (XTEM) observations revealed the presence of an epitaxial Pd2Ge layer interposed between the palladium layer and G e ( l l l ) substrates locally. XTEM micrographs of the as-deposited samples are shown in Fig. 1. The presence of Pd2Ge in the as-deposited samples was also confirmed by XRD analysis, where the X-ray spectrum is shown in Fig. 2. In samples annealed at 160 °C, the areal fraction of epitaxial Pd2Ge was found to be 60% with the rest being polycrystalline. The orientation relationships between
Fig. 1. XTEM micrographs of an as-deposited sample: (a) bright field micrograph, (b) dark field micrograph correspondingto the Pd~G¢(O001)diffractionspot.
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epitaxial Pd2Ge and Ge(lll) were analysed to be Pd2Ge[0001 ] IIGe[lll] and Pd2Ge(1010) IIGe(~20). An overlapping Pd2Ge[0001] IIGe[111] diffraction pattern and indexed pattern are shown in Fig. 3. XTEM observations revealed that the Pd2Ge-Ge interface is not very smooth. The thickness of the germanide layer was measured to range from 30 to 70 nm. In samples annealed at 250-500 °C, only polycrystalline PdGe was found. The average grain sizes were measured to be 100 nm, 160 nm and 250 nm respectively in samples annealed at 250 °C, 300 °C and 500 °C. An example is shown in Fig. 4. The results for interfacial reactions in (111) samples are summarized in Table I. 1K
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Fig. 2. XRD pattern of an as-deposited sample.
3.2 Pd/Ge(O01) system Randomly oriented polycrystalline palladium and Pd2Ge grains were found in as-deposited samples. In samples annealed at 160 °C, Pd2Ge and PdGe grains were found to coexist. An example is shown in Fig. 5. Only polycrystalline PdGe grains were observed in samples annealed at 250-500 °C. Phases formed and grain sizes in as-deposited and annealed Pd/Ge(001) samples are listed in Table II. Moffatt3a pointed out that there are six Pd-Ge compounds: PdsGe, PdaGe, Pd25Ge9, PdsGe2, Pd2Ge and PdGe in bulk systems. However, only two compounds, Pd2Ge and PdGe, were observed for thin film reactions in previous and present studies 7'a,a4,aS. It was reported that the phase formation sequence of the Pd-Ge system was one which violated the Walser-Bene rule 7. In a previous Rutherford backscattering and glancing angle XRD study, no Pd2Ge formation was found in as-deposited samples 8. Since Pd2Ge was detected in the XRD of as-deposited samples in the present investigation, the discrepancy cannot be attributed to the difference in sensitivity in phase detection of different instruments. A comparison of the deposition conditions of the present and earlier studies revealed that (1) chemical cleaning procedures prior to deposition were essentially the same, (2) vacua during depositions were similar (7× 10-7-1 x 10-6Torr compared with (2-6)x 10-TTorr), (3) vastly different
INTERFACIAL REACTIONS OF Pd FILMS
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Fig. 3. (a) Overlapping diffraction pattern for Pd2Ge[0001] IIGe[111] of a (111) sample annealed at 160 °C for 1 h; (b) indexed pattern of (a); (c) dark field corresponding to the (1010) diffraction spot of epitaxial Pd2Ge.
Fig. 4. (a) Bright field micrograph of an XTEM sample annealed at 500 °C for 1 h;(b) corresponding dark field micrograph of(a); (c) bright field micrograph of a (111) planview sample.
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Fig. 5. X T E M micrographs of a (001) sample annealed at 160 aC for 1 h: (a) bright field micrograph: (b)_ corresponding dark field micrograph of polycrystailine Pd2Ge. TABLE I PHASES FORMED AND SIZES OF GRAINS AND/OR EPITAXIAL REGIONS IN AS-DEPOSITED AND ANNEALED PALLADIUMTHIN FILMSON Ge(1 ] 1) SUBSTRATES
Temperature (°C)
As-deposited 160 250 300 500
Phases formed and sizes of polycrystalltne grains and/or epitaxial regions (length and width) (/am) Pd (0.01) and Pd2Ge (0.03), epitaxial Pd2Ge (0.5 x 0.16) Pd2Ge (0.1), epitaxial Pd2Ge (1.3 x 0.6) PdGe (0.1) PdGe (0.16) PdGe (0.25)
T A B L E II PHASES FORMEDAND SIZES OF GRAINS IN AS-DEPOSITEDAND ANNEALEDPALLADIUMTHIN FILMS ON Ge(001) SUBSTRATES
Temperature
Phasesformed and grain sizes (ttm)
(°C)
As-deposited 160 250 300 500
Pd (0.01), PdzGe (0.03) Pd2Ge (0.05), P d G e (0.08) PdGe(0.1) PdGe(0.14) PdGe (0.16)
palladium film thicknesses formed (30nm compared with 100-250nm), and (4) considerably different deposition rates were used (0.1nms -I compared with 0.4-0.8 nm s-1). In order to investigate the possible dependence of germanide formation on film thickness and deposition rate, samples with palladium films 150-220nm in thickness, deposited at a rate of 0.7nms -1, were also prepared. XTEM observations and XRD data revealed the presence of an epitaxial Pd2Ge layer interposed between the palladium layer and Ge(111) substrates locally in all deposited samples as described in an earlier section. The thicknesses of the epitaxial Pd2Ge layers were measured to be about 22nm, 28am and 30am in samples deposited with palladium films respectively 30 nm, 150 nm and 220 nm thick at rates of 0.1 and 0.7nms -1. As a consequence, the formation of a Pd2Ge layer in asdeposited (111) and (001) samples cannot be attributed to thickness and deposition effects. Abbati et al., utilizing synchrotron radiation photoemission, found that the
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composition of the reaction products of the Pd-Ge system reached a nearly constant value even at room temperature, indicating that something very similar to a welldefined compound is formed on deposition ar. Since it was previously established that in germanide formation the first nucleated compound is Pd2Ge, it may be speculated that the reaction product is probably Pd2Ge at room temperature 7. It is well known that chemical impurities, particularly those near the metal overlayer and semiconductor interfaces, can strongly influence the interfacial reactions. Specifically, a previous study of interfacial reactions in the Pt/Ge system found that a laterally uniform compound was formed at 250 °C in samples with germanium substrates appropriately cleaned before loading into the evaporation chamber, whereas no reaction was observed after annealings at 300-350°C in samples with inadequate substrate cleaning.9 Although the sample cleaning procedures prior to deposition were essentially the same in the present study and in an earlier study by Ottaviani et al., the fact that interfacial reactions occurred at a lower temperature in the present study is probably a reflection of cleaner Pd/Ge interfaces than those studied by Ottaviani et al. a In an effort to find the cleanliness of the Pd-Ge interfaces, Auger composition depth profiles were obtained. In all instances, minimal amounts of oxygen, carbon, nitrogen and fluorine were found to be present at Pd-Ge interfaces in as-deposited samples within the detection limit of the instrument. However, in the absence of the chemical analysis data for the samples studied by Ottaviani et al. the exact cause of the discrepancy remains elusive. In (111) samples, the orientation relationships between PdeGe and Ge(lll) were found to be identical to those between Pd2Si and Si(111). A straightforward analysis shows that the lattice mismatches along the three equivalent Ge(2]0) directions are approximately 3.5% at room temperature, which is close to those of the Pd2Si/Si system. Pd2Ge grown at low temperature on Ge(111) was previously found to be preferentially oriented with respect to the underlying substrate by Rutherford backscattering channelling analysisS. However, the orientation relationships between epitaxial Pd2Ge and Ge(111) were not determined in that study. As described in Section 1, Pd2Ge and PdGe bear many similarities to Pd2Si and PdSi respectively. Similar behaviours in Pd2Ge and Pd2Si formation, such as low temperature growth and epitaxy on corresponding semiconductor substrates, are therefore not unexpected. In this context, the vastxlifference between the formation temperatures of PdSi and PdGe, which are about 700 °C and 200 °C respectively, is indeed puzzling and deserves further inquiry. 4.
SUMMARY AND CONCLUSIONS
Planview and cross-sectional TEM, AES and XRD have been applied to study the interfacial reactions of palladium thin films on Ge(lll) and Ge(001) with particular emphasis on the epitaxial growth of Pd2Ge on Ge(111). 4.1 P d / G e ( l l l )
system
Polycrystalline palladium and PdeGe grains as well as epitaxial Pd2Ge regions were observed in as-deposited samples. Epitaxial Pd2Ge was found to cover about
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25% of the germanium surface area. In samples annealed at 160 °C, the areal fraction of epitaxial PdzGe was found to be 60% with the rest being polycrystalline. The orientation relationships between epitaxial PdzGe and the substrate were determined to be Pd2Ge[0001]llGe[111] and Pd2Ge(1010)tlGe(220). The Pd2Ge-Ge interface was found to be not very smooth. In samples annealed at 250-500 °C, only polycrystalline PdGe was found.
4.2 Pd/Ge(O01) system Randomly oriented polycrystalline palladium and Pd2Ge were found in asdeposited samples. In samples annealed at 160 °C, Pd2Ge and PdGe grains were found to coexist. Only polycrystalline PdGe grains were observed in samples annealed at 250-500 °C. XTEM observations and XRD data revealed the presence of an interposing epitaxial Pd2Ge layer between the palladium layer and Ge(lll) substrates in samples with various thicknesses (30-220 nm) and deposition rates (0.1-0.7 nm s- 1). The fact that interfacial reactions were found to occur at a lower temperature in the present study is probably a reflection of cleaner Pd-Ge interfaces than those studied by Ottaviani et al. The results for Pd2Ge and PdGe formation on germanium are compared with PdzSi and PdSi formation on silicon. ACKNOWLEDGMENT
The research was supported in part by the Republic of China National Science Council.
REFERENCES I S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 2nd edn., 1981. 2 J.J. Rosenberg, IEEE Trans. Electron. Devices, 30 (1983) 1602. 3 K . N . Tu and J. W. Mayer, in J. M. Poate, K. N. Tu and J. W. Mayer (eds.), Thin Films-Interdiffusion and Reactions, Wiley, New York, 1978, p.359. 4 M.A. Nicolet and S. S. Lau, in N. G. Einspruch and G. B. Larrabee (eds.), Materials and Process Characterization, Academic Press, New York, 1983, p. 329. 5 G.V. Samsonov and V. N. Bondarev, Germanides, Primary Source, New York, 1970. 6 A. Thanailaikis and D. C. Northrop, Solid State Electron., 16 (1973) 1383. 7 M . W . Wittmer, M . A . NicoletandJ. W. Mayer, ThinSolidFilms, 42(1977)51. 8 G. Ottaviani, C. Canali, G. Ferrari, G. Majni, M. Prudenziati and S. S. Lau, Thin Solid Films, 47 (1977) 187. 9 M.G. Grimaldi, L. Wielunski, M. A. Nicolet and K. N. Tu, Thin Solid Films, 81 (1981) 207. I0 G. Le Lay, Surf Sci., 132 (1984) 169. 11 T. Miller, Phys. Rev., 30 (1984) 570. 12 E.D. Marshall, C. S. Pai, D. M. Scott and S. S. Lau, M R S Symposia Proceedings, Vol. 47, Materials Research Society, Pittsburgh, PA, 1985, p. 161. 13 M. del Giudice, R. A. Butera, J. J. Joyce, M. W. Buckman and J. H. Weaver, M R S Symposia Proceedings, Vol. 54, Materials Research Society, Pittsburgh, PA, 1986, p. 91. 14 A.K. Sinha, T. E. Smith and H. J. Levinstein, IEEE Trans. Electron. Devices, 22 (1975) 218. 15 H. R. Grinolds and G. Y. Robinson, Appl. Phys. Lett., 34 (1979) 575. 16 H.R. Grinolds and G. Y. Robinson, Solid-State Electron., 23 (1980) 973.
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17 E.D. Marshall, C. S. Wu, D. M. Scott and S. S. Lau, M R S Symposia Proceedings, Vol. 25, Elsevier, Amsterdam, 1984, p. 63. 18 E.D. Marshall, W. X. Chen, C. S. Wu and S. S. Lau, Appl. Phys. Lett.. 47 (1985) 298. 19 E . D . Marshall, B. Zhang, L. C. Wang, P. F. Jiao, W. X. Chen, T. Sawada, S. S. Lau, K. L. Kavanagh and T. F. Kuench, J. Appl. Phys., 62 (1987) 942. 20 J. C. Bean, in J. C. Bean (ed.), Proc. 1st lnt. Symp. on Silicon Molecular Beam Epitaxy, Electrochemical Society, Pennington, NJ, 1985, p. 337. 21 Joint Committee on Powder Diffraction Standards, A S T M Powder Diffraction File, 1983 (International Center for Diffraction Data, Swarthmore, PA). 22 Y.F. Hsieh, L. J. Chen, E. D. Marshall and S. S. Lau, Appl. Phys. Left., 51 (1987) 1588. 23 Y.F. HsiehandL. J. Chen, J. Appl. Phys.,63(1988) l177. 24 Y.F. Hsieh, L. J. Chen, E. D. Marshall and S. S. Lau, Thin Solid Films, 162 (1988) 287. 25 S. Saitoh, H. Ishiwara and S. Furukawa, Appl. Phys. Lett., 37 (1980) 203. 26 R. T. Tung•J. M. P•ate•J. C. Bean•J. M. Gibs•nandD. C.Jac•bs•n• ThinS••idFi•ms•93(•982) 77. 27 L.J. Chen, J.W. MayerandK. N. Tu, ThinSolidFilms, 93(1982)91. 28 L.J. Chen, H. C. Cheng and W. T. Lin, MRS Symposia Proceedings, Vol. 54, Materials Research Society, Pittsburgh, PA, 1986, p. 245. 29 J.J. Chu, L. J. Chen and K. N. Tu, J. Appl. Phys., 62 (1987) 461. 30 C.S. Chang, C. W. Nieh, J. J. Chu and L. J. Chen, Thin Solid Films, 161 (1988) 263. 31 J.J. Chu, L.J. ChenandK. N. Tu, J. Appl. Phys.,63(1988) l163. 32 T.T. Sheng and C. C. Chang, IEEE Trans. Electron Devices, 23 (1976) 531. 33 W.G. Moffatt, The Handbook of Binary Phase Diagrams, Genium, Schenectady, NY, 1984. 34 E.D. Marshall and S. S. Lau, M R S Symposia Proceedings, Vol. 47, Materials Research Society, Pittsburgh, PA, 1985, p. 1. 35 G. Ottaviani, G. Majni and C. Canali, Appl. Phys., 18 (1979) 285. 36 I. Abbati, G. Rossi, L. Braicovich, I. Lindau and W. E. Spicer, Appl. Surf. Sci., 9 (198 I) 243.
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INTERFACIAL REACTIONS OF PALLADIUM THIN FILMS ON Ge(111) AND Ge(001) Y. F. HSIEH AND L. J. CHEN Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu ( Taiwan ) (Received November 16, 1987; accepted April 5, 1988)
Planview and cross-sectional transmission electron microscopy (XTEM), Auger electron spectroscopy and X-ray diffraction (XRD) have been applied to study the interfacial reactions of palladium thin films on Ge(111) and Ge(001) with particular emphasis on the epitaxial growth of Pd2Ge on Ge(111). Polycrystalline palladium and Pd2Ge grains as well as epitaxial Pd2Ge regions were observed in as-deposited (111) samples. Epitaxial Pd2Ge was found to cover about 60~ of the germanium surface area in samples annealed at 160°C. The orientation relationships between epitaxial Pd2Ge and the germanium substrate were determined to be Pd2Ge[0001]llGe[lll] and Pd2Ge(1010)[IGe(~20). The Pd2Ge-Ge interface was found to be not very smooth. In samples annealed at 250-500 °C, only polycrystalline PdGe was found. Randomly oriented polycrystalline palladium and Pd2Ge were found in asdeposited (001) samples. In samples annealed at 160 °C, Pd2Ge and PdGe grains were found to coexist. Only polycrystalline PdGe grains were observed in samples annealed at 250-500 °C. XTEM observations and XRD data revealed the presence of an epitaxial Pd2Ge layer, about 20-30nm in thickness, locally between the palladium metal layer and germanium substrate in as-deposited (111) samples, irrespective of palladium metal layer thickness (30-220 nm) and deposition rate (0.1-0.7 nm s-1). The results for Pd2Ge and PdGe formation on germanium are compared with those for Pd2Si and PdSi formation on silicon.
1. INTRODUCTION Interest in field effect transistors built on semiconductors other than silicon stems from an increasing need for high speed, low power digital and analogue circuits.1 Germanium is one of the three most important semiconductors. For device applications, it is of particular interest because it has high mobility for both electrons and holes, making a high speed complementary field effect transistor a possibility2. Compared with the extensive studies of silicide formation, germanide formation received little attention 3-13. However, understanding of the metal-germanium 0040-6090/88/$3.50
© ElsevierSequoia/Printedin The Netherlands
296
v . F . HSIEH, L. J. CHEN
interactions is important not only for the development of radiation detector systems and integrated circuits based on germanium but also for novel applications. A prominent example is metal contacts for strained layer (Ge,Si)/Si heterostructures which show promise for use in optical communications, integrated optics on silicon, and other detector applications. Metal germanide compounds are in particular interesting in relation to the preparation of metallic contacts on GaAs 1'*-19. Previous studies have found that the germanide formation characteristics such as first phase nucleation, sequence of phase formation, growth kinetics and moving species are similar to those of the corresponding silicides 7'2°. It is therefore expected that the investigation of germanides will benefit greatly from past experience gained in studying silicides. The study of metal-semiconductor interaction using an elemental semiconductor other than silicon is also of fundamental interest. Among metal-germanium and metal-silicon systems, Pd/Ge and Pd/Si bear close similarity. For the Pd/Si system, Pd2Si (hexagonal, C22; a = 0.6493 nm and c = 0.3427 nm) and PdSi (orthorhombic, B31; a = 0.6133nm, b = 0.5599 nm and c = 0.3381 nm) were found to form at 100 °C and 700 °C respectively 3'4'21. Similarly, Pd2Ge (a = 0.6712nm and c = 0.3408nm) and PdGe (a = 0.6259nm, b = 0.5782nm and c = 0.3481 nm), which are structurally isomorphous to Pd2Si and PdSi respectively, were also observed to form sequentially in the Pd/Ge system 7'8'12'21. The Pd/Si system has received much attention in the past since (1) Pd2Si is formed at about 100°C, an anomalously low temperature for silicide formation, (2) Pd2Si is stable up to about 700 °C, an unusually high temperature for the first phase of a near noble metal/silicon system, and (3) Pd2Si can be grown epitaxially on Si(1 l 1) 3'4. A previous investigation of the growth kinetics and phase formation for the Pd/Ge system was performed by Rutherford backscattering spectrometry and the X-ray diffraction method 8. In view of the intimate relationships between microstructures and electrical characteristics of the contacts, a more thorough study on the microstructural aspects of the interfacial reactions is therefore highly desirable. In this paper, we report the results of a transmission electron microscopy (TEM) and X-ray diffraction (XRD) study of the interfacial reactions of palladium thin films on germanium with particular emphasis on the epitaxial growth of Pd2Ge on germanium. We note that the epitaxial growth of cobalt, nickel and platinum germanides on germanium has been reported previously 22-24. The epitaxial growth of silicides on silicon has been extensively investigated in recent years 25--31. 2.
EXPERIMENTAL PROCEDURES
Germanium wafers, (111) oriented and 2in in diameter, were cleaned in trichloroethylene and acetone, consecutively, followed by a deionized water rinse in an ultrasonic cleaner. After etching in a solution of HF, HNOa and CH3COOH (3: 5: 3) for 30 s and rinsing in deionized water, the samples were etched in a dilute H F solution immediately before being loaded into an electron gun evaporation chamber. The dilute H F dipping served to remove the surface oxide. Palladium thin films, 30 nm in thickness unless otherwise specified, were then electron gun deposited onto the germanium substrate at room temperature. The vacuum was maintained to
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be better than 1 x 10-6 Torr during depositions. The deposition rate was about 0.1 n m s - 1 . In order to study the possible dependence of epitaxial germanide formation on the thickness or deposition rate, palladium metal layers with thicknesses of 150 and 220nm and a deposition rate of 0.7nms -1 were also investigated. Heat treatments were carried out in a three-zone diffusion furnace in N 2 ambient. High purity N2 gas was first passed through a titanium getter tube maintained at 800 °C to reduce the 0 2 content. Annealings were conducted at 160-500 °C. The annealing time was 1 h at each temperature. The samples were then cut ultrasonically into discs, 3 mm in diameter. Planview specimens for TEM examinations were prepared chemically by perforating a small hole at the centre of the discs in a solution of H F and H N 0 3 (1:1). Cross-sectional samples were prepared using a procedure outlined by Sheng and Chang 32. T E M investigations were performed with a JEOL-200CX electron microscope operating at 200 kV. In order to ensure that the phases observed in as-deposited samples by TEM were not an artifact of T E M sample preparation, phase identification was also performed by XRD analysis using a Rigaku X-ray diffractometer. Auger electron spectroscopy (AES) combined with ion sputtering was applied to obtain depth-composition data. The surface composition was determined by AES with a base pressure down tO 8 x 10- lo Torr using 3 keV electrons with a 1 ~tA primary beam current. The beam size was about 1 ~tm. Composition depth profiles were obtained by Ar ÷ ion sputtering, 4 keV in Ar + ion energy and 2 m m x 2 mm in rastered area, at a sputtering rate of about 3 nm m i n - 1 under a pressure of about 2 x 10- 7 Torr. 3.
RESULTS AND DISCUSSION
3.1 Pd/Ge(lll) system Polycrystalline palladium and Pd2Ge grains as well as epitaxial Pd2Ge regions were observed in as-deposited samples. Epitaxial Pd2Ge was found to cover about 25% of the germanium surface area. Cross-sectional TEM (XTEM) observations revealed the presence of an epitaxial Pd2Ge layer interposed between the palladium layer and G e ( l l l ) substrates locally. XTEM micrographs of the as-deposited samples are shown in Fig. 1. The presence of Pd2Ge in the as-deposited samples was also confirmed by XRD analysis, where the X-ray spectrum is shown in Fig. 2. In samples annealed at 160 °C, the areal fraction of epitaxial Pd2Ge was found to be 60% with the rest being polycrystalline. The orientation relationships between
Fig. 1. XTEM micrographs of an as-deposited sample: (a) bright field micrograph, (b) dark field micrograph correspondingto the Pd~G¢(O001)diffractionspot.
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epitaxial Pd2Ge and Ge(lll) were analysed to be Pd2Ge[0001 ] IIGe[lll] and Pd2Ge(1010) IIGe(~20). An overlapping Pd2Ge[0001] IIGe[111] diffraction pattern and indexed pattern are shown in Fig. 3. XTEM observations revealed that the Pd2Ge-Ge interface is not very smooth. The thickness of the germanide layer was measured to range from 30 to 70 nm. In samples annealed at 250-500 °C, only polycrystalline PdGe was found. The average grain sizes were measured to be 100 nm, 160 nm and 250 nm respectively in samples annealed at 250 °C, 300 °C and 500 °C. An example is shown in Fig. 4. The results for interfacial reactions in (111) samples are summarized in Table I. 1K
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Fig. 2. XRD pattern of an as-deposited sample.
3.2 Pd/Ge(O01) system Randomly oriented polycrystalline palladium and Pd2Ge grains were found in as-deposited samples. In samples annealed at 160 °C, Pd2Ge and PdGe grains were found to coexist. An example is shown in Fig. 5. Only polycrystalline PdGe grains were observed in samples annealed at 250-500 °C. Phases formed and grain sizes in as-deposited and annealed Pd/Ge(001) samples are listed in Table II. Moffatt3a pointed out that there are six Pd-Ge compounds: PdsGe, PdaGe, Pd25Ge9, PdsGe2, Pd2Ge and PdGe in bulk systems. However, only two compounds, Pd2Ge and PdGe, were observed for thin film reactions in previous and present studies 7'a,a4,aS. It was reported that the phase formation sequence of the Pd-Ge system was one which violated the Walser-Bene rule 7. In a previous Rutherford backscattering and glancing angle XRD study, no Pd2Ge formation was found in as-deposited samples 8. Since Pd2Ge was detected in the XRD of as-deposited samples in the present investigation, the discrepancy cannot be attributed to the difference in sensitivity in phase detection of different instruments. A comparison of the deposition conditions of the present and earlier studies revealed that (1) chemical cleaning procedures prior to deposition were essentially the same, (2) vacua during depositions were similar (7× 10-7-1 x 10-6Torr compared with (2-6)x 10-TTorr), (3) vastly different
INTERFACIAL REACTIONS OF Pd FILMS
299
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Fig. 3. (a) Overlapping diffraction pattern for Pd2Ge[0001] IIGe[111] of a (111) sample annealed at 160 °C for 1 h; (b) indexed pattern of (a); (c) dark field corresponding to the (1010) diffraction spot of epitaxial Pd2Ge.
Fig. 4. (a) Bright field micrograph of an XTEM sample annealed at 500 °C for 1 h;(b) corresponding dark field micrograph of(a); (c) bright field micrograph of a (111) planview sample.
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Y . F . HSIEH, L. J. CHEN
Fig. 5. X T E M micrographs of a (001) sample annealed at 160 aC for 1 h: (a) bright field micrograph: (b)_ corresponding dark field micrograph of polycrystailine Pd2Ge. TABLE I PHASES FORMED AND SIZES OF GRAINS AND/OR EPITAXIAL REGIONS IN AS-DEPOSITED AND ANNEALED PALLADIUMTHIN FILMSON Ge(1 ] 1) SUBSTRATES
Temperature (°C)
As-deposited 160 250 300 500
Phases formed and sizes of polycrystalltne grains and/or epitaxial regions (length and width) (/am) Pd (0.01) and Pd2Ge (0.03), epitaxial Pd2Ge (0.5 x 0.16) Pd2Ge (0.1), epitaxial Pd2Ge (1.3 x 0.6) PdGe (0.1) PdGe (0.16) PdGe (0.25)
T A B L E II PHASES FORMEDAND SIZES OF GRAINS IN AS-DEPOSITEDAND ANNEALEDPALLADIUMTHIN FILMS ON Ge(001) SUBSTRATES
Temperature
Phasesformed and grain sizes (ttm)
(°C)
As-deposited 160 250 300 500
Pd (0.01), PdzGe (0.03) Pd2Ge (0.05), P d G e (0.08) PdGe(0.1) PdGe(0.14) PdGe (0.16)
palladium film thicknesses formed (30nm compared with 100-250nm), and (4) considerably different deposition rates were used (0.1nms -I compared with 0.4-0.8 nm s-1). In order to investigate the possible dependence of germanide formation on film thickness and deposition rate, samples with palladium films 150-220nm in thickness, deposited at a rate of 0.7nms -1, were also prepared. XTEM observations and XRD data revealed the presence of an epitaxial Pd2Ge layer interposed between the palladium layer and Ge(111) substrates locally in all deposited samples as described in an earlier section. The thicknesses of the epitaxial Pd2Ge layers were measured to be about 22nm, 28am and 30am in samples deposited with palladium films respectively 30 nm, 150 nm and 220 nm thick at rates of 0.1 and 0.7nms -1. As a consequence, the formation of a Pd2Ge layer in asdeposited (111) and (001) samples cannot be attributed to thickness and deposition effects. Abbati et al., utilizing synchrotron radiation photoemission, found that the
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Pd
FILMS
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composition of the reaction products of the Pd-Ge system reached a nearly constant value even at room temperature, indicating that something very similar to a welldefined compound is formed on deposition ar. Since it was previously established that in germanide formation the first nucleated compound is Pd2Ge, it may be speculated that the reaction product is probably Pd2Ge at room temperature 7. It is well known that chemical impurities, particularly those near the metal overlayer and semiconductor interfaces, can strongly influence the interfacial reactions. Specifically, a previous study of interfacial reactions in the Pt/Ge system found that a laterally uniform compound was formed at 250 °C in samples with germanium substrates appropriately cleaned before loading into the evaporation chamber, whereas no reaction was observed after annealings at 300-350°C in samples with inadequate substrate cleaning.9 Although the sample cleaning procedures prior to deposition were essentially the same in the present study and in an earlier study by Ottaviani et al., the fact that interfacial reactions occurred at a lower temperature in the present study is probably a reflection of cleaner Pd/Ge interfaces than those studied by Ottaviani et al. a In an effort to find the cleanliness of the Pd-Ge interfaces, Auger composition depth profiles were obtained. In all instances, minimal amounts of oxygen, carbon, nitrogen and fluorine were found to be present at Pd-Ge interfaces in as-deposited samples within the detection limit of the instrument. However, in the absence of the chemical analysis data for the samples studied by Ottaviani et al. the exact cause of the discrepancy remains elusive. In (111) samples, the orientation relationships between PdeGe and Ge(lll) were found to be identical to those between Pd2Si and Si(111). A straightforward analysis shows that the lattice mismatches along the three equivalent Ge(2]0) directions are approximately 3.5% at room temperature, which is close to those of the Pd2Si/Si system. Pd2Ge grown at low temperature on Ge(111) was previously found to be preferentially oriented with respect to the underlying substrate by Rutherford backscattering channelling analysisS. However, the orientation relationships between epitaxial Pd2Ge and Ge(111) were not determined in that study. As described in Section 1, Pd2Ge and PdGe bear many similarities to Pd2Si and PdSi respectively. Similar behaviours in Pd2Ge and Pd2Si formation, such as low temperature growth and epitaxy on corresponding semiconductor substrates, are therefore not unexpected. In this context, the vastxlifference between the formation temperatures of PdSi and PdGe, which are about 700 °C and 200 °C respectively, is indeed puzzling and deserves further inquiry. 4.
SUMMARY AND CONCLUSIONS
Planview and cross-sectional TEM, AES and XRD have been applied to study the interfacial reactions of palladium thin films on Ge(lll) and Ge(001) with particular emphasis on the epitaxial growth of Pd2Ge on Ge(111). 4.1 P d / G e ( l l l )
system
Polycrystalline palladium and PdeGe grains as well as epitaxial Pd2Ge regions were observed in as-deposited samples. Epitaxial Pd2Ge was found to cover about
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25% of the germanium surface area. In samples annealed at 160 °C, the areal fraction of epitaxial PdzGe was found to be 60% with the rest being polycrystalline. The orientation relationships between epitaxial PdzGe and the substrate were determined to be Pd2Ge[0001]llGe[111] and Pd2Ge(1010)tlGe(220). The Pd2Ge-Ge interface was found to be not very smooth. In samples annealed at 250-500 °C, only polycrystalline PdGe was found.
4.2 Pd/Ge(O01) system Randomly oriented polycrystalline palladium and Pd2Ge were found in asdeposited samples. In samples annealed at 160 °C, Pd2Ge and PdGe grains were found to coexist. Only polycrystalline PdGe grains were observed in samples annealed at 250-500 °C. XTEM observations and XRD data revealed the presence of an interposing epitaxial Pd2Ge layer between the palladium layer and Ge(lll) substrates in samples with various thicknesses (30-220 nm) and deposition rates (0.1-0.7 nm s- 1). The fact that interfacial reactions were found to occur at a lower temperature in the present study is probably a reflection of cleaner Pd-Ge interfaces than those studied by Ottaviani et al. The results for Pd2Ge and PdGe formation on germanium are compared with PdzSi and PdSi formation on silicon. ACKNOWLEDGMENT
The research was supported in part by the Republic of China National Science Council.
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