J. Phys. Chem. 1992,96, 10439-10443 (29) Grant, J. T.; Haas, T. W. Sur/. Sci. 1971, 24, 332. (30) Wik, B.; Mrozek, P.; Jablonski, A.; Joswik, A. Surf. InZe&ce Anal. 1986,8, 121. (31) Carslaw, H. S.;Jaeger, J. C. Conduction of Hear in Solids, 2nd ed.; Oxford University Rees: Oxford, 1986. (32) Houle, F. A.; Hinsberg, W. D. To be submitted for publication. (33) (a) Gillcspie, D. T. J. Comput. Phys. 1976, 22,403. Turner, J. S. J. Phys. Chem. 1977,81,2379. (b) Bunker, D. L.; Garrett, B.; Kleindeinst, T.; Long, 111, G. S.Combust. Flame 1974, 23, 373. (34) LarioLapis, E.; Raptis, Y. S.J . Appl. Phys. 1985, 57, 5123. (35) Hicks, J. M.; Urbach, L. E.; Plummer, E. W.;Dai, H. L. Phys. Rev. Lctr. 1988, 61, 2588. (36) Touloukian, Y. S.;Powell, R. W.; Ho, C. Y.; Klemens, P. G. ThermoDhvsical Promrties of Matter. IFI/Plenum: New York. 1970; Vol. 1. i3i) Touloukan, Y. 8.;Buyco, E: H. Thermophysical Properties of Matter, IFI/Plenum: New York, 1970; Vol. 4. (38) Ho, C. Y.; Powell, R. W.; Liley, P. E. J. Phys. Chem. Ref. Data 1974, 3 (Suppl. l), 1-588. (39) Wicks, C. E.; Block, F. E. Thermodynamic Properties of 65 Elements-their oxides. halides. carbides and nitrides. Bulletin 605; U S . Bureau of Mines: Washington, DC, 1963, p 103. (40) Palik, E. D., Ed. Handbook of Oprical Consranrs of Solids; Academic: Orlando, 1985. (41) Weaver, J. H.; K r a h , C.; Lynch, D. W.; Koch, E. E. Physics Data: &tical Romrties of Metals, 18-1; Fachinfonnationzentm Energie, -~ Physik, Mathematik GmbH: Karlsruhs, 1981. (42) Adamson, A. W . Physical Chemistry of Surfaces, 4th cd.;Wiley: New York, 1982; Chapters 16 and 17. (43) Houle, F. A.; Yeh, L. I. J . Phys. Chem. 1992, 96, 2691. (44) Lewis, K. E.; Golden, D. M.; Smith, G. P. J. Am. Chem. Soc. 1984, 106,3905. (45) Umbach, E.; Menzel, D. Surf. Sci. 1983, 135, 199. (46) Kohrt, C.; Gomer, R. Surf. Sci. 1971, 21.77. (47) Robinson, P. J.; Holbrook, K. A. Unimolecular Reactions; Wiley: London, 1972.
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Comparison of Chemical Schemes for SI Atomic Layer Epitaxy S.M. Gates IBM Research Division, T.J. Watson Research Center, Yorktown Heights, New York 10598 (Received: August 3, 1992)
Chemical schemes for Si atomic layer epitaxy (ALE) are discussed using a two-step sequence of reactions: (1) surface chlorination, using a chlorosilane molecule, and (2) reduction (C1removal), using atomic H or a silane molecule. The schemes are compared in terms of equilibrium thermodynamics, to select the most promising schemes. All of the proposed processes based on atomic H are spontaneous (thermodynamically downhill). Two reactions using SizHsare endothermic,but may be thermally driven at useful Si-growth temperatures, due to a large, positive entropy change for the reaction.
I. Introduction
Thin-filmgrowth using cycles of self-limiting adsorption steps, with resulting layer-by-layer growth, is known as atomic layer epitaxy (ALE).***Extension of ALE from binary materials to elementalsemiconductors such as Si or diamond is a research area of current interest. Concerning Si growth, motivationsto develop this method are fine control over deposited film thickness, insensitivity of growth rate to changes in promsing parameters, and the goal of depositingvery abrupt '&doped" layers. Uniform growth rate Over large areas is another useful feature of ALE. The possibilityOf laya-by-laya Of singlaelementmaterials, such as Si and diamond, is also a fascinating challenge which is stimulating research in the surface chemistry of these materials. The only previous example of Si-film growth by ALE used self-limiting adsorption of Si2H6at temperatures ( r ) below the H2desorption temperature, and rapid photothermal heating with an excimer laser pulse to desorb the surface hydr~gen.~ It is not clear what chemical schemes will be most useful for isothermal Si ALE at low temperature. First, we examine thermodynamicsof example reactions from the ALE literature on the growth of binary materials. These example ALE schemes use reactions that are 'downhill" ther-
modynamically (negative AG). Second, potential schemes for Si ALE using chlorosilanes and H/Cl exchange chemistry4" are compared with respect to equilibrium thermodynamics. Equilibrium is not reached in film-growthreactors, but the hypothetical equilibrium case serves as a qualitative guide in choosing facile reactions that are 'downhill" thermodynamically. Also,a closed reactor for investigating ALE chemistries with the reactants and f h allowed to reach equilibrium in each cycle has been recently discussed.6 Finally, Si reactions that are promising on the basis of thermodynamics are discussed with regard to other factors.
II. Thermodynamics of ALE Schemes A. Tables of T h e r m o d y ~ mData. i~ Tables I-IV summarize the equilibrium thermodynamics of the net reactions considered for ALE. The state labels a r t omitted from the reactions in all the tables, for simplicity. All reactants,and H2and HCl products, are gases. The remaining products are solids. Tabulations of standard thermodynamic function^^^ were used to calculate W and AGO, both at 300 K. Also, &So was calculated (not shown), and an estimate of AG at 775 K (est AG) was made assuming that AH is independent of T, using est AG = AHO - (775 K X S O ) .
0022-365419212096-10439$03.00/0 Q 1992 American Chemical Society
10440 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
Gates
TABLE I: Example ALE Reactions from Literature
---
reaction ZnCI, H B ZnS 2HC1 2Ta& i H 2 0 Ta2OJ lOHCl 2AIC13 + 3H2O A1203 + 6HCl GaCl AsH3 GaAs H 2 HCl
+ + +
+
+
+ +
AHO at 300 K, kcal/mol -24.9 -58 -75.6
-35.8
AGO at 300 K, kcal/mol -16.3 -59 -74.0 -30.0
est AG at 770 K, kcal/mol -2.7 -60 -7 1.6 -20.9
comment refs 1, 2, 10: footnote u refs 2, IO;footnote u ref 2; footnote u refs 12. 13
"Thermodynamic data from refs 7 and 8.
B. ALE of ZnS, Ta200S, A1203md CaAs. Thermodynamics of four example reactions from the literature for the ALE growth of binary materials have been analyzed, with the results summarized in Table I. Literature references given in the table describe the use of these in ALE studies. Three examples use a metal halide reaction with either H2S or H20.10 The products are HC1 gas and the metal sulfide or oxide. The last example is GaAs g r ~ w t h ~ and ~ - lis~ more complicated. The growth of ZnS from H2S and ZnCI2g a d 0 is one of the most successful ALE processesl*2and is used industrially to make electroluminescent displays. Addition of different metal ion dopants to ZnS is used to produce different colors of electroluminescence.'*2 All four example reactions in Table I are exothermic. The standard enthalpy changes (AH")at 300 K are all negative. These succasful ALE schemes use the formation of strong bonds in both the solid and gas products to drive the reaction. Also, for all these reactions AS is sufficientlysmall that AG remains negative below T=800K. The ZnS formation reaction is characterized by a substantial negative entropy change (AS), and the Gibbs free energy change (AG)is therefore sensitive to T. While AG is negative at low T, it becomes positive at sufficiently high T. Lower reaction temperatures should favor this reaction, provided the Tis sufficient to overcome the kinetic barriers to HCl desorption and lattice ordering. The Ta20S and A1203growth reactions are very similar in thermodynamics. Both are quite exothermic, and both have AS approximately equal to zero. At all useful oxide growth temperatures, AG is negative and large in magnitude for both reactions. Several schemes for GaAs ALE have been reported."-'4 The GaCl reaction was selected because tabulated data for GaCl are readily available, in contrast to GaR3where R = methyl or ethyl. A furnace at 1050 K was used to produce GaCl in one case,12and gas-phase pyrolysis is proposed to result in the use of GaCl(eth~1)~ in GaCl formation, with GaCl being the adsorbing Ga species.I3 The GaCl Gaks reaction is similar to the ZnS reaction in Table I, in that both reactions have negative W and negative AS. Compared to the ZnS reaction, AS is smaller in magnitude for the GaCl -.GaAs reaction. Hence, the T dependence of AG is smaller for GaAs growth using GaCl, compared to the ZnS reaction. C. SiQJ12 Readam for S i ALE Dichlorosilaneis widely used for the epitaxial growth of Si in chemical vapor deposition (CVD) reactors at total pressures in the range 1-760 Torr. It is cheap and readily available. Useful fundamental information is available regarding thermodynamic data,lS Si surface reaction^,^^^'^ and 9 ~ ~SiC12H2 We use SiC12H2as a typical ALE film g r 0 w t h ~ ~with chlorosilaneto illustrate the general pattern of two reaction steps for all of the Si ALE schemes considered here. This pattern is shown schematically in Figure 1. The chlorination step consists of exposing the clean Si surface to a chlorosilane (SiC12H2,for example) at T = 775 K (500 "C). The balanced stoichiometric reactions shown in Tables 11-IV consider all hydrogen atoms in the chlorosilane (marked H') desorbing from the surface as dihydrogen (H'2) in the chlorination step. On the clean Si( 100) surface, thermal desorption of monolayer (ML) of H as H2 requires =200 s at 775 K.20 The mult is a Si surface terminated with C1. This surface is stable (free of measurable C1 desorption) in vacuum for several hundred seconds at 775 K.'6,21 Ideally, 1 ML of C1 should be formed in the chlorination step to ensure that a self-limiting layer with a maximum amount of Si is adsorbed. If H remains coadsorbed with C1, HCl desorption will occur.
-
CHLORINATION
REDUCTION 6 HCI
t
6 H*
3 H;
Si
3 SiCl&
Si
Si
CI CI CI
F i e 1. Schematic representation of the formal scheme used in Tables 11-V and throughout the text, for Si atomic layer epitaxy. Each growth cycle consists of one chlorination and one reduction step.
TABLE II: SiCIzHz RWC~~OM'
AIP at
-- -
reaction 2SiCI2Hi + SiH4 3Si + 2H'2 + 4HCl 3SiCI2Hi + Si2H6 5Si + 3H'2 + 6HCI 4SiC12Hi + SiJHs 7Si + 4H'* + 8HC1 SiCI2Hi + 2H' Si + H'2 + 2HCl SiC12H2 Si + 2HCI SiCI2H2 SiC12 + H2
AGO at est AGat 300K, 300K, 775 K, kcal/mol kcal/mol kcal/mol +53.4 +33.1 +0.2
+73.2
+39.7
-14.6
-73.8 +23.3 +26.6
-74.6 +11.3 +12.5
+94.3 -73.4 +30.8 +35.4
"SiCI2H2thermodynamic data from ref 15.
Recently, our experiments have suggested SiC12H2and SiCIH3 do not adsorb at 775 K exactly as depicted, because some HCl desorption occurs during the chlorination step.21 The fully chlorinated molecules SiC14and Si2C&should behave exactly as depicted in Figure 1, both forming 1 ML of surface monochloride. A formal scheme is necessary, in order to write stoichiometric equations and compare the molecules we wish to consider. The reduction step consists of using a reducing agent to remove the C1 adlayer. Both silanes and atomic hydrogen (H) are compared here for use as this reducing agent. The stoichiometric coefficient of the reducing agent is selected to convert all C1 atoms in the chlorosilane to HCI in the reduction step. At 775 K,the rate of HC1 desorption is =20 times that of SiCI2desorption.16 The upper l i t of T for a stable C1 layer is
