- Email: [email protected]
Incorporation of phosphorus during gas phase epitaxy
Journal of Crystal Growth 120 (1992) 150—154 North-Holland
~
CRYSTAL GROWTH
Incorporation of phosphorus during gas phase epitaxy K.A. Jones, J.R. Flemish Electronics Technology and Dei’ices Laboratory, lort Moninouth, New Jersey 07703-5601, USA
H. Shen Ceo-Centers, inc. Hopatcong, New Jersey 07849, USA
and
V.S. Ban Epitaxx, Inc., Princeton, New Jersey 08540, USA
The PH
3 desorption rate can he reduced and the decomposition rate increased, thereby increasing the P incorporation efficiency by replacing TMIn with mCI created by the pyrolysis of DEIn. mCI generated by cracking DEInCI and uncracked PH~ could be used for CBE growth of lnP provided that H on the P113 can be used to remove CI from the mCI. Evidence is presented that this is possible. Evidence is also provided that P is more readily incorporated during OMVPE growth using TBP than P11 because PH2 is a primary pyrolysis product, and it is less likely to desorh and more likely to decompose than P113.
1. Introduction As is suggested by the large V/Ill ratio required to grow InP by OMVPE and the requirement that PH3 be cracked for the growth of InP by CBE, P is not readily incorporated from PH3 using these growth techniques. This implies that the desorption rate constant, kd, of PH3 is much larger than both the adsorption, k%, and decomposition, kt, rate constants. (The rate constants are illustrated in fig. 1.) In this paper we examine ways in which k. and/or kt can be increased relative to k~1. One approach to increasing k0 is to modify the growing semiconductor surface so that PH3 is more strongly adsorbed. This is possibly what occurs to some extent during the hydride growth of InP when PH3 adsorbs onto InCI which itself is adsorbed onto the lnP surface. One potential disadvantage with regard to OMVPE and CBE growth is that InCl is a low vapor pressure mate0022-0248/92/$05.00 © 1992
—
rial. However, this can be overcome easily by cracking diethylindium chloride, DEInC1, in a hot wall OMVPE reactor or CBE cracker. We examinc this approach by studying the growth of lnP in a hot wall reactor. The question is then raised whether all of the PH3 decomposes to P2 and P4 and the InCl reacts with the H2 carrier gas to form In and HCI, or whether PH3 reacts directly with the InCI. In the latter case cracked DEInCI could also be used in conjunction with uncracked PH3 for CBE growth. We examine whether PH3 participates directly in the hydride growth process by altering the total flow rates, growth temperature, and amount of HCI injected into the growth zone of a hydride reactor in which InGaAsP is grown and interpreting the changes in the composition. Another attractive feature of using InCl as a replacement for TMIn is that selective area epitaxy is more easily achieved [1—31.In fact, the improved selectivity could he more readily
Elsevier Science Publishers B.V. All rights reserved
K.A. Jones et a!.
/ Incorporation of phosphorus during gas phase epitaxy
15 1
7
Fig. 1. Schematic illustration of the adsorption rate, ka, desorption rate, k
4, and the decomposition rate, k~,constants.
achieved because the InCl molecule is strongly adsorbed on the InP surface while at the same time it is not strongly adsorbed on e.g. the Si3N4 surface. Turning to ways in which k~can be increased, we examine molecules in which at least one relatively strong P—H bond is replaced by a weaker bond. A leading candidate is tertiarybutyl phosphine (TBP), (CH3)3—C—PH2, which has been demonstrated to grow high quality InP films with a V/Ill ratio ten times smaller then that used with PH3 [41.The reduced V/Ill ratio could be attributed to a larger k~,which in turn could be attributed to the weaker P—C bond, 63 kcal/mol, when compared to the P—H bond, 98 kcal/mol
[51. 2. Procedure Films were grown by injecting DEInCI into the source of a hydride reactor with the In boat removed. The parameters were similar to those used by us to grow InGaAsP. This includes a
total flow rate of 4670 SCCM (28 cm/s linear velocity), DEInCI flow rate of 50 SCCM, PH3 flow rate of 25 SCCM, source zone temperature of 838°C,mixing zone temperature of 820°C,and deposition zone temperature of 700°C[6]. To study whether PH3 can react directly with mCI, the DEInCI was replaced by In and Ga boats with HC1 flowing over them. Flow rates were adjusted to grow In0725Ga0275As058P042, which is approximately lattice matched to InP and emits near 1.30 ~m. First, all of the flow rates were cut in half, and the compositions were determined using a combination of double crystal X-ray diffraction and photoreflectance measurements [7]. Measurements could not be made accurately on samples grown at lower flow rates because the surfaces contained numerous morphological defects. Material emitting at 1.30 j.~m, In073Ga027As0595P0403, was grown at 700 and 670°C.The temperature range was limited by the fact that it was difficult to get deposition above 700°Cand the surface was covered with morphological defects for films grown below 670°.The effect on the composition of introducing up to 5
152
K.A. Jones et al.
/ incorporation
of phosphorus during gas phase epitaxy
in the cell through the open gate valves by the
septum
movable rod. In situ FTIR measurements can be
valve
transfer rod
-
gas inlet ~
stainless steel tube
made using a heated gas cell, but its maximum
operating temperature is 300°C. A mass spectrometer is being purchased for in situ measurements at the higher temperatures.
furnace
H
sample holder
We have grown InP films using DEIn and PH3, but have not yet optimized the process. The fact that epitaxial films can be grown suggests that It is likely DEInCI thatdoes onedecompose of the products in theis source InCI, as zone. the
va I e gas outlet~ to pumps
3. Results and discussion
In—C bond strength at 47 kcal/mol [81is substan-~—-
gate valves
-.~——gascell sample holder seat Fig. 2. Schematic drawing of the system used to study the decomposion of TBP.
SCCM of HCI into the growth zone was recorded, and the change in the PH3/AsH3 ratio necessary to restablish lattice matching after 3 SCCM of HC1 was also determined along with the new lattice matched composition. These results are compared with theoretical predictions using the computation methods described in ref. [61. Decomposition studies of TBP were carried out in the batch reactor displayed in fig. 2. A mechanical and turbo pump are attached to it, and pressures in the few mTorr range can be attained easily. Reactant gases are controllably fed into it with mass flow controllers, and reactant liquids can be injected through a septum with a microliter syringe. This enables one to inject reactants that are liquid at room temperature without using a carrier gas. Product gases flow into the attached, evacuated gas cell when the gate valves are opened, and they are analyzed ex-situ with an FTIR spectrometer. Adsorbed species on a GaAs or InP crystal exposed to different ambients can also be analyzed via transmission FTIR when the exposed crystal is placed
tially lower than those of other bonds. Moreover, the ethyls can be removed by /3-hydrogen elimination reactions. Our primary concern at the moment is C contamination, which Yoshida [91 found to be a problem for GaAs in an analogous case. However, Buchan et al. [101 recently found no appreciable C contamination in their GaAs grown in a hot wall reactor using DEGaC1 and AsH3. When the flow rates are cut in halve during growth of the InGaAsP in the hydride reactor, the P fraction decreases as one would expect because more of the PH3 has had time to decompose at the lower flow rates. However, as shown in fig. 2a, the change is small. For growth at 700°C, the GaC1 : InCl and PH3 : AsH3 ratios were 0.038 and 4.00, whereas they were 0.054 and 4.00 at 670°C. The corresponding theoretical values at 700°Cwere 0.060 and 8.56, and 0.073 and 6.90 and 670°C.These values were computed using the assumption that both of the hydrides had decomposed to form elemental dimers and tetramers. The theoretical PH3 AsH3 ratios are larger, possibly because P is more readily incorporated than is predicted -due to the presence of PH3. As seen if figs. 3a and 3b, introducing small amounts of extra HCI downstream can have a considerable effect. Extra HCI reduces the deposition of all species because it is a reaction product, but it reduces the species with the larger incorporation coefficient at a slower rate. Be-
K.A. Jones et al.
/ Incorporation of phosphorus during gas phase epitaxy
153
cause the relative stabilities are GaP> GaAs> InAs> InP [11], the Ga fraction, x, is increased,
We note that one could grow lattice matched superlattices of the form In1~Ga~Asi~P5/
and this is seen to be the case for both theory and experiment. However, theory predicts that the P (fraction y) incorporation should increase for this range of composition, but it decreases. This could possibly be due to a reduced requirement of H from the PH3 at the higher HCI concentration. Also shown in fig. 3 is the change in the composition as the PH3 : AsH3 ratio is reduced to achieve lattice matching after 3 SCCM of extra HCI has been added. Both theory and experiment show that the % Ga is virtually unaffected. (Note that the data points for varying the extra HCI and then the PH3 : AsH3 do not quite coincide because they were obtained on a different sequence of runs.) At 700°Cthe experimental value of the PH3 : AsH3 ratio changes from 4.00 to 1.61 to obtain lattice matching, and at 670°Cthe corresponding experimental ratios are 4.00 and 1.40. Less PH3 is required at the lower temperature, possibly because less has decomposed and is therefore more readily incorporated.
Inl~GauAsiLPc over a range limited by the amount of extra HCI that can be added simply by simultaneously altering the extra HC1 flow rate and the PH3 : AsH3 ratio. For the lattice matched compositions in fig. 3, x changed from 0.26 to 0.34 and y changed from 0.41 to 0.25. Results from the literature also suggest that all of the PH3 does not decompose outside of the deposition chamber in a hot wall reactor, and that it can participate directly in the growth. Some years ago Ban [12] established that not all of the PH3 decomposes in a hot wall reactor. Others have determined that it reacts more efficiently with mCI than elemental P and H2 do [13], and the growth rate of hydride-VPE grown InP increases with the PH3 concentration [14]. Additionally, Goodridge and Hasdell [15] determined that adding PH3 to Cl-VPE grown InP increased its growth rate. Because the rate determining step in the growth of the Ill—V compounds by either Cl-VPE or hydride-VPE is be-
700°C
0.50
_______________________________________
“0
~
0 20
‘.s.’
015
.‘.
0•10a
•
0.30
I
0.35
.
~
.5.
010b
.
0.40
‘S.’.’
0.15
0.45
0.25
x •
flows initially set for lattice matching
O extra HCI added
o — -- - -
I
V
0.20
• and
I
.
045
full
0.25
670°C
0.50
.
extra HCI with AsH3 PH3 flows adjusted calculated change in comp as HCI is added calc. change in comp. as AsH3 and PH3 adjusted
.
•
0.30
I
0.35
0.40
0.45
x •
flows initially set for lattice matching 0 extra HCI added 0 extra HCI with AsH3 PH3 flows adjusted — calculated change in comp as HCI is added --- calc. change in comp. as Ask3 and PH3 adjusted
Fig. 3. (a) The theoretical effect of varying the extra HCI flow ( ) and PH3/AsH3 ratio (—--) on the composition of tnGaAsP grown at 700°C. the lattice matched compositions (s) and (•), and the experimental effects of varying the extra HCI concentration (~)and the PH3:AsH3 ratios (0). (b) The theoretical effect of varying the extra HCI flow ( ) and PH3/A5H3 ratio ( —) on the composition of InGaAsP grown at 670°C.The lattice matched composition (.), and the experimental effects of varying the extra HCI concentration (~O)and the PH3:AsH3 ratios (o).
154
K.A. Jones et al.
/ incorporation
lieved to be the reaction between the adsorbed MCI and H2 [16], it is reasonable to assume that PH3 enhances the growth rate by reacting with MC1 directly to form HCI. Our initial results support those of Li et al. [5] that a primary reaction in the decomposition of TBP is cleavage of the P—C bond to form (CH3)3C + PH2, and that PH2 is adsorbed on the substrate surface possibly because the reaction is surface catalyzed. Because the PH2 radical has an unsatisfied bond, it is likely that it will be more strongly bonded to the surface with a subsequent reduction in kd. We cite the somewhat analogous and much more frequently studied case of NH3 adsorbing and decomposing on Pt [17]. The authors found that the adsorption energy of NH2, Ed(NH2) = 46.5 kcal/mol, whereas Ed(NH3) 12 kcal/mol. In addition, the activation energy for the surface decomposition of NH2 9 kcal/mol, whereas for NH3 it was 16 kcal/mol. This implies that k1(NH2)> k1(NH3). The authors also show that NH is more strongly adsorbed than NH2 and that it decomposes more rapidly. Thus if PH3 adsorbed on a semiconductor surface follows a similar reaction, it will quite readily decompose to P once the first H is removed. It has been shown that AsH3 dissociatively adsorbs on GaAs to form AsH2(a) + H(a), but readily desorbs associatively as AsH3 [18]. Perhaps the difference between this and TBP is that neighboring H(a) are not readily available in the case of TBP. If, as suggested, the critical step for increased P incorporation in OMVPE is the creation of the PH2 radical, one could abruptly alter the P composition by turning an ArF excimer laser on and off, as Sam and Yardley [19] have shown that as much as 10% of the PH3 can be photolyzed to form PH 2 =
of phosphorus during gas phase epitaxy
sensitivity of the morphology to the growth ternperature will have to be dealt with. Evidence is also presented that H from the PH3 reacts directly with InCl, so that DEInCI can be used along with PH3 to grow InP films by CBE. Evidence is also presented that the V/Ill ratio for the growth of lnP is greatly reduced when TBP is substituted for PH3 because PH2 is formed by bond scission, and it is less likely to desorb from the growing surface. This suggests that the P composition in, e.g., GaAsP or lnGaAsP could be abruptly increased using an excimer laser to form PH2 from PH3.
References [1] V.S.
Ban, G.C. Erickson, S. Mason and G.H. Olsen, J. Electrochem. Soc. 137 (1991) 2904. [2] T.F. Kuech, M.A. Tischler and R. Potemski, AppI. Phys. Letters 54 (19989) 910. [31MS. Goorsky, T.F. Kuech and R.M. Potemski, J. Elcctrochem. Soc. 138 (1991) 1817. 14] F.G. Kellcrt, J.S. Whelan and K.T. Chan, J. Electron. Mater. 18 (1989) 355. [5] S.H. Li. C.A. Larsen. N.J. Buchan and G.B. Stringfellow, J. Electron. Mater. 18 (1989) 457. [6] J.R. Flemish, K.A. Jones, A. Tripathi, VS. Ban and C.H. Park, J. Electrochem. Soc. 138 (1991) 1427. [7] J.R. Flemish, H. Shen, K.A. Jones, M. Dutta and V.S. Ban. J. AppI. Phys. 70 (1991) 2152. [81 M.G. Jacko and S.J.W. Price. Can. J. Chem. 42 (1964) 1198. [9] M. Yoshida, J. Crystal Growth 88 (1988) 16. 110] NI. Buchan. T.F. Kuech, M.A. Tischler, G. Scilla, F. Cardone and R. Potemski, J. Electrochcm. Soc. 138 (1991) 2789. Ill] H. Seki, H. Eguchi and H. Kobayashi, J. Crystal Growth 24/25 91974) 225. [12] V.S. Ban. J. Electrochem. Soc. 118 (1971) 1473. [13] P.J. Born and D.S. Robertson. J. Mater. Sci. II (1976) 395.
[141R.F. 115]
4. Conclusions The P incorporation efficiency during the OMVPE growth of InP can be increased if the TMIn is replaced by mCi, which can be formed by the thermal decomposition of DEInCI. However, problems with C contamination and the
[16] [17] [18] [19]
Karlicek. Jr., D. Mitcham, J.C. Ginocchio and B. Ilammarlund, J. Electrochem. Soc. 134 (1987) 470. l.H. Goodridge and N.B. Hasdell, in: Proc. 11th intern. Symp. on GaAs and Related Compounds, Biarritz, 1984, Inst. Phys. Conf. Ser. 74, Ed. B. de Cremoux (Inst. Phys., Bristol, 1985) p. 205. D.W. Shaw. J. Crystal Growth 31(1975)130. J.J. Vajo, W. Tsai and W.H. Weinberg. J. Phys. Chem. 89 (1985) 3243. J S Foord, private communication. C.L. Sam and J.T. Yardley, J. Chen,. Phys. 69 (1978) 4621.
~
CRYSTAL GROWTH
Incorporation of phosphorus during gas phase epitaxy K.A. Jones, J.R. Flemish Electronics Technology and Dei’ices Laboratory, lort Moninouth, New Jersey 07703-5601, USA
H. Shen Ceo-Centers, inc. Hopatcong, New Jersey 07849, USA
and
V.S. Ban Epitaxx, Inc., Princeton, New Jersey 08540, USA
The PH
3 desorption rate can he reduced and the decomposition rate increased, thereby increasing the P incorporation efficiency by replacing TMIn with mCI created by the pyrolysis of DEIn. mCI generated by cracking DEInCI and uncracked PH~ could be used for CBE growth of lnP provided that H on the P113 can be used to remove CI from the mCI. Evidence is presented that this is possible. Evidence is also provided that P is more readily incorporated during OMVPE growth using TBP than P11 because PH2 is a primary pyrolysis product, and it is less likely to desorh and more likely to decompose than P113.
1. Introduction As is suggested by the large V/Ill ratio required to grow InP by OMVPE and the requirement that PH3 be cracked for the growth of InP by CBE, P is not readily incorporated from PH3 using these growth techniques. This implies that the desorption rate constant, kd, of PH3 is much larger than both the adsorption, k%, and decomposition, kt, rate constants. (The rate constants are illustrated in fig. 1.) In this paper we examine ways in which k. and/or kt can be increased relative to k~1. One approach to increasing k0 is to modify the growing semiconductor surface so that PH3 is more strongly adsorbed. This is possibly what occurs to some extent during the hydride growth of InP when PH3 adsorbs onto InCI which itself is adsorbed onto the lnP surface. One potential disadvantage with regard to OMVPE and CBE growth is that InCl is a low vapor pressure mate0022-0248/92/$05.00 © 1992
—
rial. However, this can be overcome easily by cracking diethylindium chloride, DEInC1, in a hot wall OMVPE reactor or CBE cracker. We examinc this approach by studying the growth of lnP in a hot wall reactor. The question is then raised whether all of the PH3 decomposes to P2 and P4 and the InCl reacts with the H2 carrier gas to form In and HCI, or whether PH3 reacts directly with the InCI. In the latter case cracked DEInCI could also be used in conjunction with uncracked PH3 for CBE growth. We examine whether PH3 participates directly in the hydride growth process by altering the total flow rates, growth temperature, and amount of HCI injected into the growth zone of a hydride reactor in which InGaAsP is grown and interpreting the changes in the composition. Another attractive feature of using InCl as a replacement for TMIn is that selective area epitaxy is more easily achieved [1—31.In fact, the improved selectivity could he more readily
Elsevier Science Publishers B.V. All rights reserved
K.A. Jones et a!.
/ Incorporation of phosphorus during gas phase epitaxy
15 1
7
Fig. 1. Schematic illustration of the adsorption rate, ka, desorption rate, k
4, and the decomposition rate, k~,constants.
achieved because the InCl molecule is strongly adsorbed on the InP surface while at the same time it is not strongly adsorbed on e.g. the Si3N4 surface. Turning to ways in which k~can be increased, we examine molecules in which at least one relatively strong P—H bond is replaced by a weaker bond. A leading candidate is tertiarybutyl phosphine (TBP), (CH3)3—C—PH2, which has been demonstrated to grow high quality InP films with a V/Ill ratio ten times smaller then that used with PH3 [41.The reduced V/Ill ratio could be attributed to a larger k~,which in turn could be attributed to the weaker P—C bond, 63 kcal/mol, when compared to the P—H bond, 98 kcal/mol
[51. 2. Procedure Films were grown by injecting DEInCI into the source of a hydride reactor with the In boat removed. The parameters were similar to those used by us to grow InGaAsP. This includes a
total flow rate of 4670 SCCM (28 cm/s linear velocity), DEInCI flow rate of 50 SCCM, PH3 flow rate of 25 SCCM, source zone temperature of 838°C,mixing zone temperature of 820°C,and deposition zone temperature of 700°C[6]. To study whether PH3 can react directly with mCI, the DEInCI was replaced by In and Ga boats with HC1 flowing over them. Flow rates were adjusted to grow In0725Ga0275As058P042, which is approximately lattice matched to InP and emits near 1.30 ~m. First, all of the flow rates were cut in half, and the compositions were determined using a combination of double crystal X-ray diffraction and photoreflectance measurements [7]. Measurements could not be made accurately on samples grown at lower flow rates because the surfaces contained numerous morphological defects. Material emitting at 1.30 j.~m, In073Ga027As0595P0403, was grown at 700 and 670°C.The temperature range was limited by the fact that it was difficult to get deposition above 700°Cand the surface was covered with morphological defects for films grown below 670°.The effect on the composition of introducing up to 5
152
K.A. Jones et al.
/ incorporation
of phosphorus during gas phase epitaxy
in the cell through the open gate valves by the
septum
movable rod. In situ FTIR measurements can be
valve
transfer rod
-
gas inlet ~
stainless steel tube
made using a heated gas cell, but its maximum
operating temperature is 300°C. A mass spectrometer is being purchased for in situ measurements at the higher temperatures.
furnace
H
sample holder
We have grown InP films using DEIn and PH3, but have not yet optimized the process. The fact that epitaxial films can be grown suggests that It is likely DEInCI thatdoes onedecompose of the products in theis source InCI, as zone. the
va I e gas outlet~ to pumps
3. Results and discussion
In—C bond strength at 47 kcal/mol [81is substan-~—-
gate valves
-.~——gascell sample holder seat Fig. 2. Schematic drawing of the system used to study the decomposion of TBP.
SCCM of HCI into the growth zone was recorded, and the change in the PH3/AsH3 ratio necessary to restablish lattice matching after 3 SCCM of HC1 was also determined along with the new lattice matched composition. These results are compared with theoretical predictions using the computation methods described in ref. [61. Decomposition studies of TBP were carried out in the batch reactor displayed in fig. 2. A mechanical and turbo pump are attached to it, and pressures in the few mTorr range can be attained easily. Reactant gases are controllably fed into it with mass flow controllers, and reactant liquids can be injected through a septum with a microliter syringe. This enables one to inject reactants that are liquid at room temperature without using a carrier gas. Product gases flow into the attached, evacuated gas cell when the gate valves are opened, and they are analyzed ex-situ with an FTIR spectrometer. Adsorbed species on a GaAs or InP crystal exposed to different ambients can also be analyzed via transmission FTIR when the exposed crystal is placed
tially lower than those of other bonds. Moreover, the ethyls can be removed by /3-hydrogen elimination reactions. Our primary concern at the moment is C contamination, which Yoshida [91 found to be a problem for GaAs in an analogous case. However, Buchan et al. [101 recently found no appreciable C contamination in their GaAs grown in a hot wall reactor using DEGaC1 and AsH3. When the flow rates are cut in halve during growth of the InGaAsP in the hydride reactor, the P fraction decreases as one would expect because more of the PH3 has had time to decompose at the lower flow rates. However, as shown in fig. 2a, the change is small. For growth at 700°C, the GaC1 : InCl and PH3 : AsH3 ratios were 0.038 and 4.00, whereas they were 0.054 and 4.00 at 670°C. The corresponding theoretical values at 700°Cwere 0.060 and 8.56, and 0.073 and 6.90 and 670°C.These values were computed using the assumption that both of the hydrides had decomposed to form elemental dimers and tetramers. The theoretical PH3 AsH3 ratios are larger, possibly because P is more readily incorporated than is predicted -due to the presence of PH3. As seen if figs. 3a and 3b, introducing small amounts of extra HCI downstream can have a considerable effect. Extra HCI reduces the deposition of all species because it is a reaction product, but it reduces the species with the larger incorporation coefficient at a slower rate. Be-
K.A. Jones et al.
/ Incorporation of phosphorus during gas phase epitaxy
153
cause the relative stabilities are GaP> GaAs> InAs> InP [11], the Ga fraction, x, is increased,
We note that one could grow lattice matched superlattices of the form In1~Ga~Asi~P5/
and this is seen to be the case for both theory and experiment. However, theory predicts that the P (fraction y) incorporation should increase for this range of composition, but it decreases. This could possibly be due to a reduced requirement of H from the PH3 at the higher HCI concentration. Also shown in fig. 3 is the change in the composition as the PH3 : AsH3 ratio is reduced to achieve lattice matching after 3 SCCM of extra HCI has been added. Both theory and experiment show that the % Ga is virtually unaffected. (Note that the data points for varying the extra HCI and then the PH3 : AsH3 do not quite coincide because they were obtained on a different sequence of runs.) At 700°Cthe experimental value of the PH3 : AsH3 ratio changes from 4.00 to 1.61 to obtain lattice matching, and at 670°Cthe corresponding experimental ratios are 4.00 and 1.40. Less PH3 is required at the lower temperature, possibly because less has decomposed and is therefore more readily incorporated.
Inl~GauAsiLPc over a range limited by the amount of extra HCI that can be added simply by simultaneously altering the extra HC1 flow rate and the PH3 : AsH3 ratio. For the lattice matched compositions in fig. 3, x changed from 0.26 to 0.34 and y changed from 0.41 to 0.25. Results from the literature also suggest that all of the PH3 does not decompose outside of the deposition chamber in a hot wall reactor, and that it can participate directly in the growth. Some years ago Ban [12] established that not all of the PH3 decomposes in a hot wall reactor. Others have determined that it reacts more efficiently with mCI than elemental P and H2 do [13], and the growth rate of hydride-VPE grown InP increases with the PH3 concentration [14]. Additionally, Goodridge and Hasdell [15] determined that adding PH3 to Cl-VPE grown InP increased its growth rate. Because the rate determining step in the growth of the Ill—V compounds by either Cl-VPE or hydride-VPE is be-
700°C
0.50
_______________________________________
“0
~
0 20
‘.s.’
015
.‘.
0•10a
•
0.30
I
0.35
.
~
.5.
010b
.
0.40
‘S.’.’
0.15
0.45
0.25
x •
flows initially set for lattice matching
O extra HCI added
o — -- - -
I
V
0.20
• and
I
.
045
full
0.25
670°C
0.50
.
extra HCI with AsH3 PH3 flows adjusted calculated change in comp as HCI is added calc. change in comp. as AsH3 and PH3 adjusted
.
•
0.30
I
0.35
0.40
0.45
x •
flows initially set for lattice matching 0 extra HCI added 0 extra HCI with AsH3 PH3 flows adjusted — calculated change in comp as HCI is added --- calc. change in comp. as Ask3 and PH3 adjusted
Fig. 3. (a) The theoretical effect of varying the extra HCI flow ( ) and PH3/AsH3 ratio (—--) on the composition of tnGaAsP grown at 700°C. the lattice matched compositions (s) and (•), and the experimental effects of varying the extra HCI concentration (~)and the PH3:AsH3 ratios (0). (b) The theoretical effect of varying the extra HCI flow ( ) and PH3/A5H3 ratio ( —) on the composition of InGaAsP grown at 670°C.The lattice matched composition (.), and the experimental effects of varying the extra HCI concentration (~O)and the PH3:AsH3 ratios (o).
154
K.A. Jones et al.
/ incorporation
lieved to be the reaction between the adsorbed MCI and H2 [16], it is reasonable to assume that PH3 enhances the growth rate by reacting with MC1 directly to form HCI. Our initial results support those of Li et al. [5] that a primary reaction in the decomposition of TBP is cleavage of the P—C bond to form (CH3)3C + PH2, and that PH2 is adsorbed on the substrate surface possibly because the reaction is surface catalyzed. Because the PH2 radical has an unsatisfied bond, it is likely that it will be more strongly bonded to the surface with a subsequent reduction in kd. We cite the somewhat analogous and much more frequently studied case of NH3 adsorbing and decomposing on Pt [17]. The authors found that the adsorption energy of NH2, Ed(NH2) = 46.5 kcal/mol, whereas Ed(NH3) 12 kcal/mol. In addition, the activation energy for the surface decomposition of NH2 9 kcal/mol, whereas for NH3 it was 16 kcal/mol. This implies that k1(NH2)> k1(NH3). The authors also show that NH is more strongly adsorbed than NH2 and that it decomposes more rapidly. Thus if PH3 adsorbed on a semiconductor surface follows a similar reaction, it will quite readily decompose to P once the first H is removed. It has been shown that AsH3 dissociatively adsorbs on GaAs to form AsH2(a) + H(a), but readily desorbs associatively as AsH3 [18]. Perhaps the difference between this and TBP is that neighboring H(a) are not readily available in the case of TBP. If, as suggested, the critical step for increased P incorporation in OMVPE is the creation of the PH2 radical, one could abruptly alter the P composition by turning an ArF excimer laser on and off, as Sam and Yardley [19] have shown that as much as 10% of the PH3 can be photolyzed to form PH 2 =
of phosphorus during gas phase epitaxy
sensitivity of the morphology to the growth ternperature will have to be dealt with. Evidence is also presented that H from the PH3 reacts directly with InCl, so that DEInCI can be used along with PH3 to grow InP films by CBE. Evidence is also presented that the V/Ill ratio for the growth of lnP is greatly reduced when TBP is substituted for PH3 because PH2 is formed by bond scission, and it is less likely to desorb from the growing surface. This suggests that the P composition in, e.g., GaAsP or lnGaAsP could be abruptly increased using an excimer laser to form PH2 from PH3.
References [1] V.S.
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4. Conclusions The P incorporation efficiency during the OMVPE growth of InP can be increased if the TMIn is replaced by mCi, which can be formed by the thermal decomposition of DEInCI. However, problems with C contamination and the
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