- Email: [email protected]
Mass spectrometric studies of phosphine pyrolysis and OMVPE growth of InP
148
Journal of Crystal Growth 85 (~987)148~-153 North-Holland, Amsterdam
MASS SPECTROMETRIC STUDIES OF PHOSPHINE PYROLYSIS AND OMVPE GROWTH OF InP C.A. LARSEN, N.!. BUCHAN and G.B. STRINGFELLOW Departments of Materials Science and Engineering and Electrical Engineering, University of Utah, Salt Lake City, Utah 84112, USA
The mechanism of PH
3 decomposition was studied by using D2 as a carrier gas and analyzing the reaction products with a mass spectrometer. The effects of InP and silica surfaces were investigated. The only gaseous product below 6000 C is H2. Since any gas phase H atoms would produce HD, the reaction occurs entirely on the surface. The slow step is the unimolecular removal of the first hydrogen atom, with an activation energy of 36.0 kcal/mole on InP surfaces. The reaction on lnP is first order for PH3 concentrations as high as 15%, so the surface is not saturated at those conditions. When trimethylindium (TMIn) is added to the gas mixture, the mechanism changes dramatically, probably proceeding via an unstable intermediate adduct of TMIn and PH3 which eliminates CH4 upon formation. This concerted reaction lowers the pyrolysis temperatures of both PH3 and TMIn.
1. Introduction Phosphine (PH3) is almost universally used as the phosphorus source in organometallic vapor phase epitaxy (OMVPE) of 111/V materials such as InP, GaInP and GaAsP [1—3].The OMVPE technique has advanced to the point where high purity 111/V compounds and alloys can be produced and complex semiconductor device structures, including superlattices, are grown with byperabrupt interfaces [4,5]. It is imperative that further research in the field begin to focus heavily on the fundamental aspects of the process such as fluid dynamics in reactors and phenomena at the growth surface. It is also vital to understand the mechanisms whereby precursors such as trimethylindium (TMIn), trimethylgallium (TMGa), arsine (AsH3) and PH3 decompose to give the final crystal layers. Such basic studies are necessary to refine and improve the existing technology, including development of such advanced techniques as photon assisted growth [6] and atomic layer epitaxy [7]. In this paper we report the results of our investigations of the thermal decomposition of PH3. We have employed a technique which to our knowledge is a new approach to the system, namely the use of deuterium (D2) as a carrier gas to label
the gaseous reaction products. After leaving the reaction zone the products are analyzed with a mass spectrometer. Except for the difference in mass, D2 is chemically similar to the H2 usually used as the carrier in OMVPE reactors, so the method can provide useful information on growth mechanisms. For instance, interactions between PH3 and the ambient may be separated from reaction steps involving only PH3 molecules. As will be seen, gas and surface steps may also be distinguished. 2. Background A few studies of PH3 pyrolysis have been reported. Hinshelwood and Topley [8] investigated the decomposition of pure PH3 in bulbs of silica or porcelain. A strong surface effect was found, with first order heterogeneous kinetics persisting up to 771°C. Devyatykh et al. [9] decomposed PH3 on glass and silicon surfaces. The activation energy was slightly higher on silicon (55.3 versus 44.2 kcal/mol). Larsen and Stringfellow [10] found that the reaction is homogeneous above 800 °C. Addition of a small amount of powdered silica had little effect on the decomposition rate, but powdered InP and GaP greatly enhanced the pyrolysis.
0022-0248/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
CA. Larsen et a!.
/ Mass spectrometric studies of phosphine pyrolysis
3. Experimental
~49
4. Results
The experiments were performed in a standard flow system. The reaction chamber was a 4 mm inside diameter silica tube placed in a furnace with a hot zone 41.5 cm long. The PH3 was electronic grade obtained from Matheson Gas Company. The D2 was C.P. grade from Linde Specialty Gases. The D2 was found to contain less than 1% HD and only a trace of H2. To study the effects of silica surfaces on the reaction the surface area inside the2 reactor was increased approxiby packing with silicatochips haymately 300 cm ing diameters between 0.42 and 0.76 mm. For the InP experiments the tube walls were coated directly by decomposing a mixture of TMIn and PH3, and adjusting the furnace position and ternperature so that the entire tube length was covered, TMIn was introduced into the system by passing the D2 through a TMIn sublimer held at 20°C (where the vapor pressure is 1.7 Torr) before mixing with the PH3 flow. The products were sampled by means of a calibrated leak and analyzed using a CVC 2000 time-of-flight mass spectrorneter. Reactions in the ionization region of the mass spectrometer, such as fragmenting of the parent species and formation of new species, were taken into account and incuded in background corrections.
400
500
600
The dependence of PH3 pyrolysis on carrier gas and surface type is shown in fig. 1. The percent pyrolysis is given as a function of temperature. The three curves on the right, designated as (a), are for experiments in an unpacked tube using D2, H2, and N2 as the carrier as indicated. From earlier work [10] we know that the reaction proceeds homogeneously in this case. Essentially no difference is observed for the three gases. This is different than which the situation group III organometallics, pyrolyze atfor lower temperatures in H 2 than in N2 [10,11]. It may be concluded that the slow step in PH3 pyrolysis does not involve any species other than the PH3 or partially decomposed groups such as PH2. The middle curve (b) is the result for the increased surface area. In agreement with ref. [8] an increase in conversion is apparent due to the packing. The lack of a surface effect with silica powder reported earlier [10] was due to the low actual surface area presented to the PH3, since the powder occupied only a small portion of the bottom of the tube. In the present work the chips uniformly filled the reactor and the entire surface was exposed to the gases. Finally, a thin coating of InP on the unpacked tube walls gives curve c. We have assumed that the
700
800
Temperature (°C) 2 silica tube with N Fig. 1. Percent PH3 decomposition versuspacking temperature: with D(a) 60 cm 2 InP coating with2 (D), D H2 2 (c) 60cm 2.
900
1000
((~),and D2
(0); (b) 300 cm
2 silica
150
CA. Larsen et a!.
__
RT
JL~
~
2 I
.ll~_1l
.....~
10
550°C
575°C
I 30X
ML
~
20
30
40
I
JJJL~ 0
10
20
~roba~tYT~f
n~
~2th~:~nto
H+D2sHD+D is only 8 kcal/mol [12] so the reaction would be rapid. Therefore all of the H atoms produced in
2
I
30X
l.
Mass spectrometric studies of phosphine pyrolysis
1
I
500°C
0
/
30
40
Fig. 2. Mass spectra of PH2 decomposition in D2 on InP at room temperature, 500, 550 and 5750 C.
surface area is the same as for the original tube, that is, the deposited InP film is smooth and non-porous. As is seen, even such a small surface area of InP has a very large catalytic effect on the reaction rate. The most striking results of this study are shown in fig. 2, which is a series of representative mass spectra of the products over a range of temperatures for the InP catalyzed reaction. The key feature is the rise in the H2 peak with increasing pyrolysis, while the HD peak is virtually unchanged. The small HD signal is in fact present in the room temperature spectrum and is from the trace impurity in the D2 source. So the only gaseous product of the reaction is H2, with no contribution from the D2. Similar results were obtained for the silica packed tube. Unfortunately the latter case is complicated by a small increase in HD from the reaction
the reaction are on the InP (or silica) surface and desorb molecularly. There is a small contribution to the mass spectra from PH2D. This may be interpreted as evidence that PH2 is a stable intermediate, with a lifetime which permits some interaction with D2 before decomposing further. This conclusion is stiii tentative. The kinetics of the reaction are demonstrated in fig. 3. The plot is of ln(p/p0) versus residence time, the latter being varied by changing the flow rate. The experiments were done at 550°C over an InP surface with a PH3 mole fraction of 15%. The straight line clearly shows that the reaction is first order in PH3. From fig. 1 the rate limiting step is known to be independent of other species. So the slow step is a unimolecular bond breaking. Furthermore, it can be concluded that the surface is not completely covered by PH3 under these conditions of temperature and pressure. This is so because heterogeneous reaction rates are propor-
3.0
2.0
-
H2+D2—s2HD, which we found to be purely homogeneous over silica. The exchange becomes important in the same temperature range at which the PH3 decomposes for the particular packing conditions we chose. At the lower temperatures at which pyrolysis occurs with the InP catalyst the H2/D2 exchange is negligible. The H2 can come only from recombination of H atoms adsorbed on the surface. Any atomic hydrogen formed in the gas phase has a high
1.0
0
5
10
15
TIME (sec) Fig. 3. Plot of —ln(p/p0) versus residence time for 15% PH3 in D2 over InP at 550°C.
C.A. Larsen et a!.
/
tional to the surface coverage 0 which is given by the Langmuir isotherm: 0 = bP ‘(1 + bP” 1~
~‘
where b is the equilibrium constant for adsorption and P is the pressure of the adsorbate over the surface. At high pressures or low temperatures 0 is independent of pressure and the surface is saturated. But if bP is small, then the coverage is a linear function of pressure as observed. The significance for crystal growth is that most OMVPE reactors are operated with PH3 mole fractions of one to two percent, for which case there is an abundance of sites for PH3 adsorption. When TMIn is added to the reaction mixture several new phenomena are observed. First, as shown in fig. 4, the decomposition temperature is lowered as the ratio of TMIn to PH3 increases, When a 47.2: 1 ratio is used there is almost no difference from the case when pure PH3 is decomposed over InP. A 2.1 : 1 ratio lowers the temperature for 50% pyrolysis by approximately 250°C, to a temperature lower than that at which TMIn alone decomposes [10,13]. The middle curve is for a 4.2: 1 ratio of PH3 to TMIn and shows that one fourth of the PH3 decomposes at the same low temperatures as for the 2.1 : 1 ratio. The rest of the PH3 requires higher temperatures. Thus it appears that as long as there is TMIn available there is a reaction pathway with an energy barrier even lower than for the InP surface catalyzed case. The 2.1 :1
curve (and to some extent the curve for the 4.2 : I ratio) is complicated by the inclusion of all P-contaming species which reach the mass spectrometer, including methylphosphines which decompose at higher temperatures than PH3. This is the reason for the plateau above 300 °C. The second important point, which is discussed in more detail in refs. [14,15], is that while the mass spectra for TMIn decomposition show large amounts of CH3D due to the reaction of free CH3 radicals with D2, TMIn—PH3 mixtures yield only CH4. Because of the low concentrations of PH3 used in this study any methyl radicals which are removed from the TMIn by homolytic fission will react with a D2 molecule before encountering a PH3 molecule and abstracting a hydrogen atom. So the reaction proceeds via an intermediate species involving both TMIn and PH3 molecules which favors simultaneous elimination of a methyl radical from TMIn and a hydrogen atom from PH3.
5. Discussion Based on our findings the following mechanism is proposed for PH3 pyrolysis on surfaces: PH PH d 1 a s PH3(ads) —p PH2(ads) + H(ads), (2) PH2 (ads) P(ads) + 2 H(ads), (3) —~
—~
2 H(ads) ~oo
~
PH3•TMIO-D2
//1/
40
~
5/036/98 2 05 /024/993
20
300
350
400 450 500 550 Temperature (°C)
—
H2(g),
(4)
P2(g).
(5)
has only a small energy barrier. Step (2), then, is the rate limiting process in the mechanism, followed rapidly by step (3). Steps (1) and (2) may in fact be combined as dissociate adsorption: PH3
250
—
Step (1) is a simple adsorption and probably
60
0 200
2 P(ads)
-°
7/
80
151
Mass spectrometric studies of phosphine pyrolysis
600
650
2
PH2(ads)
+
H(ads)
(1’)
700
Fig. 4. Effects of TMIn on PH3 pyrolysis in an unpacked InP coated tube, with a constant PH3 concentration of 15% in D 2 and increasing concentrations of TMIn. The PH3 : TMIn ratios for the data are: (v) ~ (0) 47.2:1; (a’) 4.2:1; (0) 2.1 :1. Included in the curves are contributions from methylphosphines.
.
.
.
.
which would have a significant activation energy. To date the only study of PH adsorption at elevated temperatures [16] has shown that PH adsorbed on silicon (100) begins to dissociate at 200°C and is completely decomposed at 400°C. Another argument for step (1’) is the rate increase
152
CA. Larsen eta!.
/ Mass spectrometric studies ofphosphinepyrolysis
by InP compared to silica. This is a common phenomenon for group V hydrides [17—19].An Arrhenius plot of our data gives an activation energy of 36.0 kcal/mol for the decomposition on InP. This is to be compared with 46.0 kcal/mol on glass [1] and a PH2—H bond strength of 90.3 kcal/mol. The rate enhancement is due to significant weakening of the PH2—H bond upon adsorption and would make adsorption and dissociation more likely in the same step. Steps (4) and (5) are simple recombinations of adsorbed atomic species. The deuterated phosphines may arise from the homogeneous reaction
products. Didchenko et al. [26] found that the adduct was unstable above 0°C. Larsen and Stringfellow [10] also found no evidence for its existence. Based on these observations we coneluded that methane elimination occurs simultaneously with formation of the adduct, or at least before the adduct can dissociate to the original molecules (which should occur within a few vibrational periods). So the probable form of the transition state is a four-center species:
CH (C H3)2 I
— —
— —
H PH2
PH2 + D2 PH2D + D • Similar unstable adducts leading to alkane if some of the PH groups desorb before further elimination are common for other Ga and In decomposition takes place. Another possibility is compounds [27]. After the elimination, the resultthe surface recombrnation ing molecule, (CH3)2In: PH2, is probably a stable PH2(ads) + D(ads) PH2D(g). species which migrates to the growth surface before further decomposition takes place. Similar The extent to which this reaction occurs depends studies of atmospheric pressure OMVPE growth on the amount of adsorbed D atoms. Adsorption of GaAs using trimethylgallium (TMGa) and AsH3 on 111/V surfaces has only recently been of interusing the D2-tracer technique indicate a similar est, but there is some evidence for chemisorption mechanism operated in this system [28]. For GaAs of D2 on InP [21,22]. On the other hand, we found growth, the reduction in pyrolysis temperature is no catalysis of H 2/D2 exchange on silica surfaces significant for both TMGa and AsH3 and the but still saw PH2D peaks, indicating that the reaction in a D2 ambient produces mainly CH4. reaction proceeds in the gas phase. Another route to PH2D is the reaction [23] 6. Conclusion PH3 + D PH2D + H, We have investigated the decomposition of one but this would require a source of D atoms. of the common OMYPE precursors, PH3, over a The results of TMIn—PH3 mixtures clearly inrange of conditions using D2 labelling to follow dicate a transition state not found for either comthe reaction mechanism. The rate does not depend pound alone. A possible form of this intermediate on other gases present. The reaction is of first is (CH3)31n: PH3, a gas phase Lewis acid—base order, so the surface is not saturated even at a adduct. Analogous adducts between other Ill/V PH3 concentration of 15% at 550°C. The slow species are well known in OMVPE systems [24,25]. step is the removal of the first H atom, with an The In—P bond in this adduct would weaken both activation energy of 36.0 kcal/mol on InP. This the In—C and P—H bonds by electronic induction and all subsequent reaction steps occur entirely on effects, accounting for the lower pyrolysis temperthe surface when InP or a large surface area of atures observed. However, this bond weakening silica is present. When TMIn is added the reaction would lead to CH3D and HD formation upon proceeds via an unstable adduct leading to decomposition if the adduct as written were the methane elimination as the first step in the mechafavored species. As noted, we do not observe these nism. -~
.
•
—*
-*
CA. Larsen et a!.
/
Mass spectrometric studies of phosphine pyrolysis
Acknowledgement This research was supported by research grant from NASA, Langley Research Center, Contract Number NAG4608.
References [1] L.D. Zhu, K.T. Chan and J.M. Ballantyne, App!. Phys. Letters 47 (1985) ~ [2] J.S. Yuan, M.T. Tsai, C.H. Chen, R.M. Cohen and G.B. Stringfellow, J. Appl. Phys. 60 (1986) 1346. [3] M. Takeyasu, T. Soga, S. Sakai and M. Umeno, Japan. J. Appl. Phys. Letters 25 (1986) L297. [4] R.M. Biefeld, J. Electron Mater. 14 (1986) 193. [5] B.I. Miller, E.F. Schubert, U. Koren, A. Ourmazd, A.H. Dayem and R.J. Capik, AppI. Phys. Letters 49 (1986) 1384. [6] P. Balk, H. Heinecke, N. Pütz, C. Plass and H. Lüth, J. Vacuum Sci. Technol. A4 (1986) 711. [7] C.H. Goodman and M.V. Pessa, J. AppI. Phys. 60 (1986) R65. [8] C.N. Hinshelwood and T. Topley, J. Chem. Soc. 125 (1924) 393. [9] G.G. Devyatykh, V.M. Kedyarkin and A.D. Zorin, Russ. J. Inorg. Chem. 14 (1969) 1055. [10] C.A. Larsen and G.B. Stringfellow, J. Crystal Growth 75 (1986) 247. [11] M. Yoshida, H. Watanabe and F. Uesugi, J. Electrochem. Soc. 132 (1985) 677. [12] SW. Benson, Thermochemical Kinetics, 2nd ed. (Wiley, New York, 1976) p. 191.
153
[13] N.J. Buchan, C.A. Larsen and GB. Stringfellow, unpublished data. [14] N.!. Buchan, C.A. Larsen and G.B. Stringfellow, App!. Phys. Letters, to be published. [15] GB. Stringfellow, N.!. Buchan and C.A. Larsen, in: Proc. Mater. Res. Soc. Meeting, Anaheim, CA, April 1987. [16] M.L. Yu, D.J. Vitkavage and B.S. Meyerson, J. App!. Phys. 59 (1986) 4023. [17] l.A. Frolov, E.M. Kitaev, B.L. Drul and E.B. Sokolov, Russ. J. Phys. Chem. 51(1977) 651. [18] J. Nishizawa and T. Kurabayashi, J. Electrochem. Soc. 130 (1983) 413. [19] M.J. Cherng, H.R. Jen, C.A. Larsen, G.B. Stringfellow, H. Lundt and P.C. Taylor, J. Crystal Growth 77 (1986) 408. [20] N.J. Friswell and B.G. Gowenlock, Advan. Free-Radical Chem. 2 (1967) 1. [21] F. Bartels, L. Surkamp, H.J. Clements and W. Mönch, J. Vacuum Sci. Technol. Bi (1985) 756. [22] D.J. Frankel, J. Anderson, G.L. Lapeyre and H.H. Farrell, J. Vacuum Sci. Technol. B3 (1985) 1093. [23] H.W. Melville and J.L. Bolland, Proc. Roy. Soc. (London) A160 (1937) 384. [24] R.H. Moss and J.S. Evans, J. Crystal Growth 55 (1981) 129. [25] F. Maury and G. Constant, J. Crystal Growth 62 (1983) 568. [26] R. Didchenko, J.D. Alix and R.H. Toeniskoetter, J. Inorg. Chem. 4 (1966) 35. [27] D.G. Tuck, in: Comprehensive Organometallic Chemistry, Eds. G. Wilkinson, F.G.A. Stone and E.W. Abel (Pergamon, New York, 1982) p. 711. [28] C.A. Larsen, N.J. Buchan and G.B. Stringfellow, paper presented at the Electronic Materials Conf., Santa Barbara, CA, June 1987.
Journal of Crystal Growth 85 (~987)148~-153 North-Holland, Amsterdam
MASS SPECTROMETRIC STUDIES OF PHOSPHINE PYROLYSIS AND OMVPE GROWTH OF InP C.A. LARSEN, N.!. BUCHAN and G.B. STRINGFELLOW Departments of Materials Science and Engineering and Electrical Engineering, University of Utah, Salt Lake City, Utah 84112, USA
The mechanism of PH
3 decomposition was studied by using D2 as a carrier gas and analyzing the reaction products with a mass spectrometer. The effects of InP and silica surfaces were investigated. The only gaseous product below 6000 C is H2. Since any gas phase H atoms would produce HD, the reaction occurs entirely on the surface. The slow step is the unimolecular removal of the first hydrogen atom, with an activation energy of 36.0 kcal/mole on InP surfaces. The reaction on lnP is first order for PH3 concentrations as high as 15%, so the surface is not saturated at those conditions. When trimethylindium (TMIn) is added to the gas mixture, the mechanism changes dramatically, probably proceeding via an unstable intermediate adduct of TMIn and PH3 which eliminates CH4 upon formation. This concerted reaction lowers the pyrolysis temperatures of both PH3 and TMIn.
1. Introduction Phosphine (PH3) is almost universally used as the phosphorus source in organometallic vapor phase epitaxy (OMVPE) of 111/V materials such as InP, GaInP and GaAsP [1—3].The OMVPE technique has advanced to the point where high purity 111/V compounds and alloys can be produced and complex semiconductor device structures, including superlattices, are grown with byperabrupt interfaces [4,5]. It is imperative that further research in the field begin to focus heavily on the fundamental aspects of the process such as fluid dynamics in reactors and phenomena at the growth surface. It is also vital to understand the mechanisms whereby precursors such as trimethylindium (TMIn), trimethylgallium (TMGa), arsine (AsH3) and PH3 decompose to give the final crystal layers. Such basic studies are necessary to refine and improve the existing technology, including development of such advanced techniques as photon assisted growth [6] and atomic layer epitaxy [7]. In this paper we report the results of our investigations of the thermal decomposition of PH3. We have employed a technique which to our knowledge is a new approach to the system, namely the use of deuterium (D2) as a carrier gas to label
the gaseous reaction products. After leaving the reaction zone the products are analyzed with a mass spectrometer. Except for the difference in mass, D2 is chemically similar to the H2 usually used as the carrier in OMVPE reactors, so the method can provide useful information on growth mechanisms. For instance, interactions between PH3 and the ambient may be separated from reaction steps involving only PH3 molecules. As will be seen, gas and surface steps may also be distinguished. 2. Background A few studies of PH3 pyrolysis have been reported. Hinshelwood and Topley [8] investigated the decomposition of pure PH3 in bulbs of silica or porcelain. A strong surface effect was found, with first order heterogeneous kinetics persisting up to 771°C. Devyatykh et al. [9] decomposed PH3 on glass and silicon surfaces. The activation energy was slightly higher on silicon (55.3 versus 44.2 kcal/mol). Larsen and Stringfellow [10] found that the reaction is homogeneous above 800 °C. Addition of a small amount of powdered silica had little effect on the decomposition rate, but powdered InP and GaP greatly enhanced the pyrolysis.
0022-0248/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
CA. Larsen et a!.
/ Mass spectrometric studies of phosphine pyrolysis
3. Experimental
~49
4. Results
The experiments were performed in a standard flow system. The reaction chamber was a 4 mm inside diameter silica tube placed in a furnace with a hot zone 41.5 cm long. The PH3 was electronic grade obtained from Matheson Gas Company. The D2 was C.P. grade from Linde Specialty Gases. The D2 was found to contain less than 1% HD and only a trace of H2. To study the effects of silica surfaces on the reaction the surface area inside the2 reactor was increased approxiby packing with silicatochips haymately 300 cm ing diameters between 0.42 and 0.76 mm. For the InP experiments the tube walls were coated directly by decomposing a mixture of TMIn and PH3, and adjusting the furnace position and ternperature so that the entire tube length was covered, TMIn was introduced into the system by passing the D2 through a TMIn sublimer held at 20°C (where the vapor pressure is 1.7 Torr) before mixing with the PH3 flow. The products were sampled by means of a calibrated leak and analyzed using a CVC 2000 time-of-flight mass spectrorneter. Reactions in the ionization region of the mass spectrometer, such as fragmenting of the parent species and formation of new species, were taken into account and incuded in background corrections.
400
500
600
The dependence of PH3 pyrolysis on carrier gas and surface type is shown in fig. 1. The percent pyrolysis is given as a function of temperature. The three curves on the right, designated as (a), are for experiments in an unpacked tube using D2, H2, and N2 as the carrier as indicated. From earlier work [10] we know that the reaction proceeds homogeneously in this case. Essentially no difference is observed for the three gases. This is different than which the situation group III organometallics, pyrolyze atfor lower temperatures in H 2 than in N2 [10,11]. It may be concluded that the slow step in PH3 pyrolysis does not involve any species other than the PH3 or partially decomposed groups such as PH2. The middle curve (b) is the result for the increased surface area. In agreement with ref. [8] an increase in conversion is apparent due to the packing. The lack of a surface effect with silica powder reported earlier [10] was due to the low actual surface area presented to the PH3, since the powder occupied only a small portion of the bottom of the tube. In the present work the chips uniformly filled the reactor and the entire surface was exposed to the gases. Finally, a thin coating of InP on the unpacked tube walls gives curve c. We have assumed that the
700
800
Temperature (°C) 2 silica tube with N Fig. 1. Percent PH3 decomposition versuspacking temperature: with D(a) 60 cm 2 InP coating with2 (D), D H2 2 (c) 60cm 2.
900
1000
((~),and D2
(0); (b) 300 cm
2 silica
150
CA. Larsen et a!.
__
RT
JL~
~
2 I
.ll~_1l
.....~
10
550°C
575°C
I 30X
ML
~
20
30
40
I
JJJL~ 0
10
20
~roba~tYT~f
n~
~2th~:~nto
H+D2sHD+D is only 8 kcal/mol [12] so the reaction would be rapid. Therefore all of the H atoms produced in
2
I
30X
l.
Mass spectrometric studies of phosphine pyrolysis
1
I
500°C
0
/
30
40
Fig. 2. Mass spectra of PH2 decomposition in D2 on InP at room temperature, 500, 550 and 5750 C.
surface area is the same as for the original tube, that is, the deposited InP film is smooth and non-porous. As is seen, even such a small surface area of InP has a very large catalytic effect on the reaction rate. The most striking results of this study are shown in fig. 2, which is a series of representative mass spectra of the products over a range of temperatures for the InP catalyzed reaction. The key feature is the rise in the H2 peak with increasing pyrolysis, while the HD peak is virtually unchanged. The small HD signal is in fact present in the room temperature spectrum and is from the trace impurity in the D2 source. So the only gaseous product of the reaction is H2, with no contribution from the D2. Similar results were obtained for the silica packed tube. Unfortunately the latter case is complicated by a small increase in HD from the reaction
the reaction are on the InP (or silica) surface and desorb molecularly. There is a small contribution to the mass spectra from PH2D. This may be interpreted as evidence that PH2 is a stable intermediate, with a lifetime which permits some interaction with D2 before decomposing further. This conclusion is stiii tentative. The kinetics of the reaction are demonstrated in fig. 3. The plot is of ln(p/p0) versus residence time, the latter being varied by changing the flow rate. The experiments were done at 550°C over an InP surface with a PH3 mole fraction of 15%. The straight line clearly shows that the reaction is first order in PH3. From fig. 1 the rate limiting step is known to be independent of other species. So the slow step is a unimolecular bond breaking. Furthermore, it can be concluded that the surface is not completely covered by PH3 under these conditions of temperature and pressure. This is so because heterogeneous reaction rates are propor-
3.0
2.0
-
H2+D2—s2HD, which we found to be purely homogeneous over silica. The exchange becomes important in the same temperature range at which the PH3 decomposes for the particular packing conditions we chose. At the lower temperatures at which pyrolysis occurs with the InP catalyst the H2/D2 exchange is negligible. The H2 can come only from recombination of H atoms adsorbed on the surface. Any atomic hydrogen formed in the gas phase has a high
1.0
0
5
10
15
TIME (sec) Fig. 3. Plot of —ln(p/p0) versus residence time for 15% PH3 in D2 over InP at 550°C.
C.A. Larsen et a!.
/
tional to the surface coverage 0 which is given by the Langmuir isotherm: 0 = bP ‘(1 + bP” 1~
~‘
where b is the equilibrium constant for adsorption and P is the pressure of the adsorbate over the surface. At high pressures or low temperatures 0 is independent of pressure and the surface is saturated. But if bP is small, then the coverage is a linear function of pressure as observed. The significance for crystal growth is that most OMVPE reactors are operated with PH3 mole fractions of one to two percent, for which case there is an abundance of sites for PH3 adsorption. When TMIn is added to the reaction mixture several new phenomena are observed. First, as shown in fig. 4, the decomposition temperature is lowered as the ratio of TMIn to PH3 increases, When a 47.2: 1 ratio is used there is almost no difference from the case when pure PH3 is decomposed over InP. A 2.1 : 1 ratio lowers the temperature for 50% pyrolysis by approximately 250°C, to a temperature lower than that at which TMIn alone decomposes [10,13]. The middle curve is for a 4.2: 1 ratio of PH3 to TMIn and shows that one fourth of the PH3 decomposes at the same low temperatures as for the 2.1 : 1 ratio. The rest of the PH3 requires higher temperatures. Thus it appears that as long as there is TMIn available there is a reaction pathway with an energy barrier even lower than for the InP surface catalyzed case. The 2.1 :1
curve (and to some extent the curve for the 4.2 : I ratio) is complicated by the inclusion of all P-contaming species which reach the mass spectrometer, including methylphosphines which decompose at higher temperatures than PH3. This is the reason for the plateau above 300 °C. The second important point, which is discussed in more detail in refs. [14,15], is that while the mass spectra for TMIn decomposition show large amounts of CH3D due to the reaction of free CH3 radicals with D2, TMIn—PH3 mixtures yield only CH4. Because of the low concentrations of PH3 used in this study any methyl radicals which are removed from the TMIn by homolytic fission will react with a D2 molecule before encountering a PH3 molecule and abstracting a hydrogen atom. So the reaction proceeds via an intermediate species involving both TMIn and PH3 molecules which favors simultaneous elimination of a methyl radical from TMIn and a hydrogen atom from PH3.
5. Discussion Based on our findings the following mechanism is proposed for PH3 pyrolysis on surfaces: PH PH d 1 a s PH3(ads) —p PH2(ads) + H(ads), (2) PH2 (ads) P(ads) + 2 H(ads), (3) —~
—~
2 H(ads) ~oo
~
PH3•TMIO-D2
//1/
40
~
5/036/98 2 05 /024/993
20
300
350
400 450 500 550 Temperature (°C)
—
H2(g),
(4)
P2(g).
(5)
has only a small energy barrier. Step (2), then, is the rate limiting process in the mechanism, followed rapidly by step (3). Steps (1) and (2) may in fact be combined as dissociate adsorption: PH3
250
—
Step (1) is a simple adsorption and probably
60
0 200
2 P(ads)
-°
7/
80
151
Mass spectrometric studies of phosphine pyrolysis
600
650
2
PH2(ads)
+
H(ads)
(1’)
700
Fig. 4. Effects of TMIn on PH3 pyrolysis in an unpacked InP coated tube, with a constant PH3 concentration of 15% in D 2 and increasing concentrations of TMIn. The PH3 : TMIn ratios for the data are: (v) ~ (0) 47.2:1; (a’) 4.2:1; (0) 2.1 :1. Included in the curves are contributions from methylphosphines.
.
.
.
.
which would have a significant activation energy. To date the only study of PH adsorption at elevated temperatures [16] has shown that PH adsorbed on silicon (100) begins to dissociate at 200°C and is completely decomposed at 400°C. Another argument for step (1’) is the rate increase
152
CA. Larsen eta!.
/ Mass spectrometric studies ofphosphinepyrolysis
by InP compared to silica. This is a common phenomenon for group V hydrides [17—19].An Arrhenius plot of our data gives an activation energy of 36.0 kcal/mol for the decomposition on InP. This is to be compared with 46.0 kcal/mol on glass [1] and a PH2—H bond strength of 90.3 kcal/mol. The rate enhancement is due to significant weakening of the PH2—H bond upon adsorption and would make adsorption and dissociation more likely in the same step. Steps (4) and (5) are simple recombinations of adsorbed atomic species. The deuterated phosphines may arise from the homogeneous reaction
products. Didchenko et al. [26] found that the adduct was unstable above 0°C. Larsen and Stringfellow [10] also found no evidence for its existence. Based on these observations we coneluded that methane elimination occurs simultaneously with formation of the adduct, or at least before the adduct can dissociate to the original molecules (which should occur within a few vibrational periods). So the probable form of the transition state is a four-center species:
CH (C H3)2 I
— —
— —
H PH2
PH2 + D2 PH2D + D • Similar unstable adducts leading to alkane if some of the PH groups desorb before further elimination are common for other Ga and In decomposition takes place. Another possibility is compounds [27]. After the elimination, the resultthe surface recombrnation ing molecule, (CH3)2In: PH2, is probably a stable PH2(ads) + D(ads) PH2D(g). species which migrates to the growth surface before further decomposition takes place. Similar The extent to which this reaction occurs depends studies of atmospheric pressure OMVPE growth on the amount of adsorbed D atoms. Adsorption of GaAs using trimethylgallium (TMGa) and AsH3 on 111/V surfaces has only recently been of interusing the D2-tracer technique indicate a similar est, but there is some evidence for chemisorption mechanism operated in this system [28]. For GaAs of D2 on InP [21,22]. On the other hand, we found growth, the reduction in pyrolysis temperature is no catalysis of H 2/D2 exchange on silica surfaces significant for both TMGa and AsH3 and the but still saw PH2D peaks, indicating that the reaction in a D2 ambient produces mainly CH4. reaction proceeds in the gas phase. Another route to PH2D is the reaction [23] 6. Conclusion PH3 + D PH2D + H, We have investigated the decomposition of one but this would require a source of D atoms. of the common OMYPE precursors, PH3, over a The results of TMIn—PH3 mixtures clearly inrange of conditions using D2 labelling to follow dicate a transition state not found for either comthe reaction mechanism. The rate does not depend pound alone. A possible form of this intermediate on other gases present. The reaction is of first is (CH3)31n: PH3, a gas phase Lewis acid—base order, so the surface is not saturated even at a adduct. Analogous adducts between other Ill/V PH3 concentration of 15% at 550°C. The slow species are well known in OMVPE systems [24,25]. step is the removal of the first H atom, with an The In—P bond in this adduct would weaken both activation energy of 36.0 kcal/mol on InP. This the In—C and P—H bonds by electronic induction and all subsequent reaction steps occur entirely on effects, accounting for the lower pyrolysis temperthe surface when InP or a large surface area of atures observed. However, this bond weakening silica is present. When TMIn is added the reaction would lead to CH3D and HD formation upon proceeds via an unstable adduct leading to decomposition if the adduct as written were the methane elimination as the first step in the mechafavored species. As noted, we do not observe these nism. -~
.
•
—*
-*
CA. Larsen et a!.
/
Mass spectrometric studies of phosphine pyrolysis
Acknowledgement This research was supported by research grant from NASA, Langley Research Center, Contract Number NAG4608.
References [1] L.D. Zhu, K.T. Chan and J.M. Ballantyne, App!. Phys. Letters 47 (1985) ~ [2] J.S. Yuan, M.T. Tsai, C.H. Chen, R.M. Cohen and G.B. Stringfellow, J. Appl. Phys. 60 (1986) 1346. [3] M. Takeyasu, T. Soga, S. Sakai and M. Umeno, Japan. J. Appl. Phys. Letters 25 (1986) L297. [4] R.M. Biefeld, J. Electron Mater. 14 (1986) 193. [5] B.I. Miller, E.F. Schubert, U. Koren, A. Ourmazd, A.H. Dayem and R.J. Capik, AppI. Phys. Letters 49 (1986) 1384. [6] P. Balk, H. Heinecke, N. Pütz, C. Plass and H. Lüth, J. Vacuum Sci. Technol. A4 (1986) 711. [7] C.H. Goodman and M.V. Pessa, J. AppI. Phys. 60 (1986) R65. [8] C.N. Hinshelwood and T. Topley, J. Chem. Soc. 125 (1924) 393. [9] G.G. Devyatykh, V.M. Kedyarkin and A.D. Zorin, Russ. J. Inorg. Chem. 14 (1969) 1055. [10] C.A. Larsen and G.B. Stringfellow, J. Crystal Growth 75 (1986) 247. [11] M. Yoshida, H. Watanabe and F. Uesugi, J. Electrochem. Soc. 132 (1985) 677. [12] SW. Benson, Thermochemical Kinetics, 2nd ed. (Wiley, New York, 1976) p. 191.
153
[13] N.J. Buchan, C.A. Larsen and GB. Stringfellow, unpublished data. [14] N.!. Buchan, C.A. Larsen and G.B. Stringfellow, App!. Phys. Letters, to be published. [15] GB. Stringfellow, N.!. Buchan and C.A. Larsen, in: Proc. Mater. Res. Soc. Meeting, Anaheim, CA, April 1987. [16] M.L. Yu, D.J. Vitkavage and B.S. Meyerson, J. App!. Phys. 59 (1986) 4023. [17] l.A. Frolov, E.M. Kitaev, B.L. Drul and E.B. Sokolov, Russ. J. Phys. Chem. 51(1977) 651. [18] J. Nishizawa and T. Kurabayashi, J. Electrochem. Soc. 130 (1983) 413. [19] M.J. Cherng, H.R. Jen, C.A. Larsen, G.B. Stringfellow, H. Lundt and P.C. Taylor, J. Crystal Growth 77 (1986) 408. [20] N.J. Friswell and B.G. Gowenlock, Advan. Free-Radical Chem. 2 (1967) 1. [21] F. Bartels, L. Surkamp, H.J. Clements and W. Mönch, J. Vacuum Sci. Technol. Bi (1985) 756. [22] D.J. Frankel, J. Anderson, G.L. Lapeyre and H.H. Farrell, J. Vacuum Sci. Technol. B3 (1985) 1093. [23] H.W. Melville and J.L. Bolland, Proc. Roy. Soc. (London) A160 (1937) 384. [24] R.H. Moss and J.S. Evans, J. Crystal Growth 55 (1981) 129. [25] F. Maury and G. Constant, J. Crystal Growth 62 (1983) 568. [26] R. Didchenko, J.D. Alix and R.H. Toeniskoetter, J. Inorg. Chem. 4 (1966) 35. [27] D.G. Tuck, in: Comprehensive Organometallic Chemistry, Eds. G. Wilkinson, F.G.A. Stone and E.W. Abel (Pergamon, New York, 1982) p. 711. [28] C.A. Larsen, N.J. Buchan and G.B. Stringfellow, paper presented at the Electronic Materials Conf., Santa Barbara, CA, June 1987.