- Email: [email protected]
In situ concentration monitoring in a vertical OMVPE reactor by fiber-optics-based Fourier transform infrared spectroscopy
,ounM., orCRVSTAL OROWTH ELSEVIER
Journal of Crystal Growth 169 ( 1996) 343-339
In situ concentration monitoring in a vertical OMVPE reactor by fiber-optics-based Fourier transform infrared spectroscopy S. Salim a,1,C.A. Wang b, R.D. Driver ‘, K.F. Jensen a,* a Department of Chemical h Lincoln
Engineering,
Lahorutoty,
Massachusetts
Massachusetts
’ Gulileo
Institute
Institute
Electra-Optics,
Received 30 October
oj’Trchno1og.v.
of Technology,
Srurbridge,
Cambridge.
Lexington,
Massachusetts
1995; accepted 29 January
Massachusetts
Massachusetts
02139. USA
02 17.3, USA
01.566, USA
I996
Abstract We describe fiber-optics-based Fourier transform infrared (FOB-FTIR) spectroscopy for in situ monitoring of input partial pressures of organometallic precursors in a vertical rotating-disk organometallic vapor phase epitaxy reactor. Detection limits as low as 0.05 Torr for trimethylgallium and 0.006 Torr for tritertiarybutylaluminum (TTBAl) are achieved using a 1 s scan time, which are comparable to established ultrasonic measurements. In addition, the FOB-FHR approach has the ability to detect parasitic Lewis acid-base interactions between organometallic precursors, as demonstrated for in situ measurements of TTBAl mixed with arsine, trimethylantimony or triethylantimony. Such observations are shown to provide insight into unexpected results in epitaxial growth.
1. Introduction Lack of process monitoring and feedback control of input partial pressures of organometallic precursors in organometallic vapor phase epitaxy (OMVPE) leads to difficulties in controlling the reproducibility of epitaxial thickness and composition. Precursors that are stored in liquid or solid form in stainless steel containers (“bubblers”) are introduced to the reactor by regulating a flow of carrier gas through the bubbler. The exact amount of reactant supplied to the reactor is not measured, but is estimated assuming the carrier gas is completely saturated with the
_ Corresponding author. ’ Present address: Praxair, Inc.. 175 East Park Drive. P.O. Box 44, Tonawanda, New York 14151-0044, USA. 0022.0248/96/$15.00 PII SOO22-0248(96)00
Copyright 0 1996 Published 19X-J
source vapor. Interactions between the reagents and the interior walls of the gas-handling system lead to variations in source delivery. This problem is particularly acute for solid precursors, such as trimethylindium (TMIn), and for precursors that have a low vapor pressure. Surface area changes during solid precursor evaporation often result in uncontrolled fluctuations in the delivery rate [I]. Low vapor pressure precursors are often maintained at a temperature higher than ambient, and consequently source condensation on colder sections of the gas handling system results in concentration transients. Current sensor technology is based upon speed of sound measurements in a binary mixture of the organometallic precursor and its carrier gas [l], and is successfully employed in monitoring TMIn delivery [2]. However, the system is limited to measurements in the gas delivery system and has been
by Elsevier Science B.V. All rights reserved
444
S. Salim et al./ Joumul of Crystal Growth 169 (1996) 443-449
restricted to line pressures above N 400 Torr. Moreover, being a physical measurement, it cannot detect the presence of different chemical species. Fourier transform infrared (FTIR) spectroscopy is suitable for OMVPE since it directly probes chemical bonds and is low in energy, thus avoiding photolysis of organometallic precursors which can accompany ultraviolet absorption spectroscopy [3]. Recently, we demonstrated applications of fiber-opticsbased (FOB) FTIR spectroscopy for remote in situ monitoring organometallic partial pressures in OMVPE gas-handling systems 141. The concentration measurement was based upon the absorbance of C-H vibrational stretching in the 2800 to 3000 cm- ’ spectral region. The HgCdTe detector and chalcogenide fiber technology used in those experiments limited measurements to conditions typical for an OMVPE gas delivery system. In this work, we present an improved FOB-FIIR approach based on fluoride fibers and an InSb detector for in situ concentration monitoring in OMVPE reactors.
2. Experimental system and procedure The FOB-FTIR system, supplied by Galileo Electro-Optics Corporation, is shown schematically in Fig. 1. The spectrometer is a KVB Analect Diamond-20 with a 4 cm-’ resolution and controlled by a Dell 486-50 MHz computer. The IR emission from the interferometer is coupled into a seven-fluoridefiber-bundle system through an optical fiber launcher.
Dell 486 50 MHz
The optical fibers guide the infrared radiation to the sensing region where the light is collimated into a 12 mm beam with a prealigned optical collimator module. After passing through the sampling volume, the collimated beam is refocused into the return fiber with a second prealigned optical module. Angular adjustment on both collimators allows optical throughput of the system to be optimized. The transmitted radiation is measured by a cooled InSb detector. Monitoring experiments were carried out in a vertical rotating-disk OMVPE reactor, which consists of a cylindrical quartz tube with a susceptor disk positioned perpendicular to the axis of the tube (Fig. 1) [5]. IR measurements were made perpendicular to the tube axis in the region midway between the porous plug flow distributor and the susceptor. The total H, carrier gas flow rate was 10 slm and the reactor pressure was either 150 or 760 Torr. Most measurements were performed with the reactor at room temperature. Typical organometallic flow rates were in the range of 1 to 5 seem, which represents conditions typically used in deposition of epitaxial layers. Numerous group 111 and V precursors were investigated including trimethylgallium (TMGal, triethylgallium (TEGa), trimethylindium (TMIn), tritertiarybutylaluminum (TTBAI), trimethylantimony (TMSb), triethylantimony (TESb), tertiarybutylphosphine (TBP), and tris-dimethylaminophosphine (DMAP). Flow visualization experiments [6], combined with fluid flow simulations and tracer gas measurements [7], indicated that no recirculating
Multi-channel InSb Detector
uxhaust
Fig. 1. Schematic diagram of the OMVPE reactor and FOB-FTIR
system.
S. Salim et al. / Journal
of Crystal
flows were present and that the concentrations of organometallic species would be uniform across the reactor. Under these flow conditions, no deposition occurred on the wall during the growth process, except on regions near the susceptor. Thus, it is possible to implement in situ monitoring with FOBFTIR through the quartz reactor wall. In order to compare the FOB-FTIR technique to conventional ultrasound measurements (Epison) [8], a 12.7 cm gas cell was placed in the vent line of the OMVPE system (see Fig. 1). This cell allowed the same organometallic precursor carrier gas mixture to pass through the two monitoring stations.
Growth
44s
169 (19961443-449
I
0.003 I
ii_ 3400
_
d
.I
3300
3200
3100
Wavenumber
3000
~.
2900
2800
(cm-‘)
Fig. 2. Infrared spectrum of 1% ASH, in Hz at 760 Torr. The overtone peaks spread between 2950 and 3200 cm-’ [IO].
3. Detection limits of the FOB-FTIR system In an earlier version of the FOB-FIIR system, the detection limit was determined by the HgCdTe detector and the use of a single fluoride fiber [3]. In the present setup, an order of magnitude improvement in the detection limit in the C-H stretching spectral region (2800 to 3000 cm- ’ > was achieved by using a more efficient InSb detector and a seven fluoridefiber bundle. The InSb setup results in a noise level of 0.001 absorbance units with a 1 s scan time, which is significantly shorter than the 45 s scan time required to reach a similar noise level with the previous system [3]. Based upon the 0.001 noise level and a IO cm path length, the detection limit for various organometallic sources varies from 0.05 Torr for TMGa to 0.006 Torr for TTBAl and DMAP, as shown in Table 1. Lower detection limits are obtained for those sources with a greater number of C-H bonds. The detection limits are appropriate for monitoring gas phase concentrations in a gas delivery system since they are much smaller than the typical input concentration levels (> 1 Torr). The short scan time also makes the technique useful for
Table I Detection
limits for selected organometallic
Scan time (s)
I
20
Noise level (ax)
I x to-’ I x lo-”
precursors
using
monitoring transients when the organometallic source is turned-on or shut-off. Further reduction in the noise level to 0.0001 absorbance units can be achieved by increasing the scan time to 20 s. This significant improvement in signal-to-noise level permits direct in situ probing of steady state chemical species in an OMVPE reactor, in which the organometallic concentration is typically two to three orders of magnitude lower than that in the delivery line. For example, in the present OMVPE reactor, typical group III organometallic concentrations used in growth are 100 ppm. For triethylgallium (TEGa), this concentration is significantly larger than minimum detection limits, which are 3 ppm at atmospheric pressure and 15 ppm at 150 Torr. It is also feasible to probe ASH, species in the reactor by monitoring overtone vibrational modes around 3 100 cm-’ [9, IO]. Fig. 2 shows a spectrum of 1% ASH, at atmospheric pressure, which is a typical level of ASH, used for epitaxial growth of AlGaAs 151. The same approach could also be implemented for other hydride sources, specifically NH, and PH,.
I and 20 s scan times
Detection limit at 25°C (Torr) TMGa
TEGa
TMln
TTBAI
TMSb
TESb
TBP
DMAP
0.05 0.005
0.02 0.002
0.05 0.005
0.006 O.OOQ6
0.06 0.006
0.02 0.002
0.008 0.0008
0.006 0.0006
446
S. Salim et al. /Journal
of Crystul Growth 169 (19%) 443-449
4. In situ monitoring of organometallic source concentrations in an OMVPE reactor The performance of the FOB-FTIR system for in situ steady-state measurement of precursor delivery was evaluated as a function of various H, bubbler flow rates and pressures to establish conditions at which the carrier gas was completely saturated with the organometallic reagent. In the first study, the delivery of TESb, which is a liquid at room temperature, was investigated for Hz bubbler flow rates of 0 to 500 seem and bubbler pressures of 200, 400 and 800 Torr. The bubbler was maintained at 25°C and the reactor pressure was 150 Torr. Fig. 3a shows the IR absorbance as a function of the calculated gas delivery rate, assuming saturation conditions inside the bubbler. The data fall on a straight line with a least-squares correlation coefficient equal to 1.OOO, indicating that the H2 was saturated with the TESb
0.01
(4
g 0.008 5
5: 0.006 $
*
vapor even at the highest flow rate and lowest pressure. Similar experiments were carried out for TMln, a solid source with well-known delivery transients [2]. The bubbler pressure was maintained at 800 Ton-, the reactor pressure was 760 Torr, and H, gas flow rate was varied from 0 to 500 seem. The bubbler temperature was maintained at 25°C and the TMIn bubbler was installed in a reverse-flow configuration such that the carrier gas is introduced to the bubbler through the designated outlet tube. In this manner, evaporation of TMIn is maximized. For comparison, the concentration of TMIn was also measured using the ultrasonic technique. Fig. 3b shows the IR absorbance for TMIn as a function of Hz flow rate to the bubbler, as well as TMIn flow rate based upon the ultrasonic measurement. There is excellent agreement between the FOB-FTIR and the ultrasonic measurements over the whole range of Hz flow rates. However, both measurements show increasing deviations from the predicted TMIn flow rate (solid line) with increasing H, flow rates. This discrepancy between measurements and calculated flow rates suggests that the H, carrier gas becomes increasingly under saturated with TMIn at high flow rates.
0.004 0.002 0
:,/:I::
0
2
4
6
6
10
TESb Flow Rate (seem)
iz Q LL
0.4
0.6
;
0.2
00’
100
200
300
400
500
600
Hydrogen Flow Rate (seem)
Fig. 3. (a) TESb absorbance (at 2958 cm-‘) as a function of calculated TESb flow assuming saturation conditions inside the bubbler. Reactor pressure: 150 Torr, TESb bubbler pressure 200 Torr (0 ), 400 Torr CO), and 800 Torr ( X ). (b) TMIn absorbance (at 2928 cm- ’ ) ( X ) and TMIn flow rates from ultrasonic measurements (0) as a function of Hz flow rate. The solid line represents TMln flow rates calculated on the basis of complete saturation.
5. Monitoring interactions between group III and V sources In addition to organometallic source delivery variations, potential interactions between group III and V organometallic sources upon mixing can reduce precursor concentrations and adversely affect growth performance. The ability of the FOB-FTIR technique to provide insight into such Lewis acid-base interactions in OMVPE growth of III-V materials is illustrated in this study by monitoring the pairwise interactions between TTBAI and ASH,, TTBAl and TMSb, and TTBAI and TESb. Monitoring experiments were conducted under flow ( 10 slm H?) and pressure conditions (150 Torr) similar to those used for film growth [ 1 1,121. The measurements were made with the susceptor at room temperature. The absorption spectra for TTBAl (dashed line, TTBAl = 0.7 seem) and a TTBAl-ASH, mixture
m (a)
2.0
2.0
1.5
1.5
m
zx :
3
1.0
m 2
0.5
1.0
i
0
0.5
. ;*L';
;I., *'11,
0
3100
3000
2900
Wavenumber
2800
(cm-l)
2700
3100
3000
2900
2800
Wavenumber
2700
(cm’)
Fig. 3. Comparison between infrared spectra of TTBAI (- - -1 and a mixture of TTBAI and ASH, (-_) for (a) V/III = IO and (h) V/III
= 100.
(solid line) are shown in Fig. 4a and 4b for V/III = 10 and V/III = 100, respectively. The TTBAI spectrum is characterized by absorption at 2847 cm’. For V/III = 10, no apparent interaction is observed since the spectra of TTBAl the TTBAl-ASH, mixture are identical. In contrast, for V/III = 100. the mixture spectrum shows an increase in absorbance at 2847 cm- ‘, which is likely caused by wall condensation of a low vapor pressure adduct between TTBAl and ASH,. The change in absorbance became apparent only after the mixture had been delivered to the reactor over 5 min. Earlier spectra did not show evidence of interactions. When the TTBAI-ASH, mixture was switched out of the reactor, IR absorp tion in the region 2800 to 3000 cm-’ was still detected. Upon heating the reactor wall to 5O”C, IR absorption was not observed, indicating that the wall deposit had evaporated. Thus, FOB-FTIR monitoring allowed both identification of a potential problem and its solution. The heating strategy is appropriate for weak adducts and reactor conditions with relatively high gas velocities. However, heating stronger adducts that would require increased temperatures could eventually result in the elimination of hydrocarbons and formation of low volatility solids. The spectra obtained for mixing of TTBAI and TMSb are shown in Fig. 5. The lower spectrum is for TTBAI. the middle for TMSb, the upper solid line for the TTBAI-TMSb mixture, and the upper dashed line for addition of the TTBAI and TMSb spectra. Upon mixing, adducts were produced as
evidenced by the emergence of a broad shoulder band around 2830 cm-‘. The degree of interaction decreased as the concentration of the organometallic reagents was reduced. For example, in Fig. 5b, by reducing the group V species to 2 seem while maintaining the same group III flow, less interaction is observed. When the TTBAI-TMSb mixture was switched out of the reactor, no absorption was observed, which indicates that this combination did not lead to any adduct condensation. The formation of gas-phase adducts should not affect the overall chemistry of the growth process since the adduct bond is relatively weak and breaks as the adduct reaches the high temperature regime of the reactor [13]. Adduct condensation, on the other hand. can be problematic for epitaxial growth since lower growth rates and higher impurity incorporation may result. Such a condensation phenomenon was observed for a mixture of TTBAI and TESb. The mixture spectrum shown in Fig. 6a (upper spectra, solid line) has a reduced absorbance signal compared to the addition spectrum (dashed line). indicating a reduction in the organometallic supply to the reactor, which results from Lewis acid-base adduct condensation. In situ monitoring experiments during turn-on and turnoff of TTBAI-TESb mixture into the reactor were performed. The spectrum taken immediately
TTBAI Tl&b
TM Sb
TTBAI
3000
2900
Wavenumber
2800
(cm-l)
3000
2900
Wavenumber
2800
(cm-l)
Fig. 5. Infrared aprctra of TTBAI. TMSb. and TTBAI-TMSh mixture of (a) V/III = 5.7, and (h) V/III = 2.8. Each graph shows the spectra of TTBAI (bottom). TMSh (middle), and TTBAI-TMSh mixture (top). The dashed line represents the simple addition of TTBAI and TMSh spectral intensities.
448
S. Salim et al. /Journal
ofCrystal Growth 169 (1996) 443-449
after the flow was switched in at 0.25 min shows mostly excess TESb (Fig. 6b), which is characterized by a triplet at 2956, 2917 and 2884 cm-‘, and the TTBAl-TESb adduct band at 2864 cm- ‘. The absorbance in the adduct band increased with time, indicating possible adduct evaporation. Further evidence of adduct evaporation is observed during turn-off experiments. Both TTBAl and TESb flows were switched out of the reactor at 6 min and the reactor was continuously purged with H,. The spectrum collected at 6.5 min shows an increase in absorption at 2864 cm-‘, reflecting the gas-phase Lewis acid-base adduct between TTBAl and TESb. The adduct was observed during the span of 9 min after shut-off. No evidence of alkyl substituent exchange was found in the mixed group III-V systems explored in this study. Adduct condensation and evaporation is consistent with observations made for growth of AlSb using TTBAl and TESb in the same reactor [ 121. Adduct condensation was observed as a powdery white deposit on the reactor inlet after growth. This condensate is likely also to be responsible for an Al memory effect. After growth of an AlSb layer, a GaSb cap layer was grown to prevent oxidation of
Growth ,.
is+terminated
,~~_
._~.
I~~
:.
I
0 --/-’
0
~L-~
10
20 Sputter
30
40
50
Time
Fig. 7. Auger depth profile analysis of multilayer AlSb-GaSb structure grown using a combination of TEGa, TTBAI and TESb.
the AlSb. However, Auger depth profiling of the epitaxial structure (Fig. 7) indicated the presence of Al in the GaSb cap layer and an Al layer on the surface. The source of aluminum, in this case, is the adduct which evaporates from the reactor inlet. The low decomposition temperature of TTBAl ( - 250°C) [14] and the relatively high substrate temperature drive the post-aluminum-deposition.
6. Summary
w
Off (6 min)
-KBAl
3000
2900
Wavenumber
2800
3000
(cm-l)
Wavenumber
2900
2800 (cm-l)
Fig. 6. (a) In situ FOB-FTIR spectra of TTBAI, TESb and TTBAI-TESb mixture of V/III = 2.0. The solid lines are the spectra of TI’BAI (bottom), TESb (middle), and TTBAI-TESb mixture (top). The dashed line represents the simple addition of TTBAl and TESb spectra. (b) Transient spectra during turn-on and turn-off flow of the TTBAl-TESb mixture (V/III = 2.0) to the reactor
The preceding examples establish FOB-FTIR spectroscopy as a useful technique for monitoring partial pressures of organometallic precursors in gas-handling systems, as well as in the OMVPE reactor. For on-line monitoring in the gas delivery system, detection limits of 0.05 to 0.006 Torr can be achieved for a one second sampling time with a 10 cm path length. This is significantly lower than the vapor pressure of organometallic sources used for OMVPE growth ( > 1 Torr). Improved detection limits are achieved by increasing the scan time, thereby allowing direct monitoring of organometallic and hydride species in an OMVPE reactor. The chemical selectivity of the FTIR technique gives an accurate measurement of the organometallic sources and allows detection of delivery problems caused by the
S. Salim et ul./Joumd
of Cystal
presence of foreign chemical species, chemical interactions and fluctuations in vapor pick-up from a bubbler.
Grobvth 169 flYY6I
449
443-449
[3] G.A. Hebner. K.P. Killeen and R.M. Biefield, J. Crystal Growth 98 (1989) 293. [4] S. Salim. K.F. Jensen and R.D. Driver, J. Crystal Growth 145 (1994) 28. [5] CA. Wang. S. Patnaik, J.W. Caunt Crystal Growth 93 (1988) 228.
Acknowledgements The MIT Lincoln Laboratory research portion was supported by the Department of the Air Force. The MIT Department of Chemical Engineering contribution was supported by the National Science Foundation (DMR 9023162). The authors thank A.C. Jones of Epichem for the TTBAl.
[IO] R. Robertson and J.J. Fox, Proc. R. Sot. A 120 (1928) 161. [I I] CA. Wang, S. Salim, K.F. Jensen and A.C. Jones, J. Electron. Mater. t 1996). in press.
[13] G. Wilkinson, Comprehensive Organometallic Vol. I (Pergamon, London, 1982).
[I] J.P.
J. Crystal
J.
[6] C.A. Wang, S.H. Groves, S.C. Palmateer, D.W. Weyburne and R.A. Brown, J. Crystal Growth 77 (1986) 136. [7] S. Patnaik, R.A. Brown and CA. Wang. J. Crystal Growth 96 (1989) 153. [8] Thomas Swan and Co. Ltd., Cambridge, UK. [9] G. Herzberg. Infrared and Raman Spectra of Polyatomic Molecules (Van Nostrand, New York. 1945). and references therein.
1121 C.A. Wang, M.C. Finn, S. Salim, K.F. Jensen Jones. Appl. Phys. Lett. 67 (1995) 1384.
References Stagg, Chemtronics 3 (1988) 44. [2] J.P. Stagg, J. Christer, E.J. Thrush and J. Crawley. Growth 120 (1992) 98.
and R.A. Brown,
1141 S. Salim. PhD Thesis, Massachusetts 1995.
and A.C. Chemistry,
Institute of Technology,
Journal of Crystal Growth 169 ( 1996) 343-339
In situ concentration monitoring in a vertical OMVPE reactor by fiber-optics-based Fourier transform infrared spectroscopy S. Salim a,1,C.A. Wang b, R.D. Driver ‘, K.F. Jensen a,* a Department of Chemical h Lincoln
Engineering,
Lahorutoty,
Massachusetts
Massachusetts
’ Gulileo
Institute
Institute
Electra-Optics,
Received 30 October
oj’Trchno1og.v.
of Technology,
Srurbridge,
Cambridge.
Lexington,
Massachusetts
1995; accepted 29 January
Massachusetts
Massachusetts
02139. USA
02 17.3, USA
01.566, USA
I996
Abstract We describe fiber-optics-based Fourier transform infrared (FOB-FTIR) spectroscopy for in situ monitoring of input partial pressures of organometallic precursors in a vertical rotating-disk organometallic vapor phase epitaxy reactor. Detection limits as low as 0.05 Torr for trimethylgallium and 0.006 Torr for tritertiarybutylaluminum (TTBAl) are achieved using a 1 s scan time, which are comparable to established ultrasonic measurements. In addition, the FOB-FHR approach has the ability to detect parasitic Lewis acid-base interactions between organometallic precursors, as demonstrated for in situ measurements of TTBAl mixed with arsine, trimethylantimony or triethylantimony. Such observations are shown to provide insight into unexpected results in epitaxial growth.
1. Introduction Lack of process monitoring and feedback control of input partial pressures of organometallic precursors in organometallic vapor phase epitaxy (OMVPE) leads to difficulties in controlling the reproducibility of epitaxial thickness and composition. Precursors that are stored in liquid or solid form in stainless steel containers (“bubblers”) are introduced to the reactor by regulating a flow of carrier gas through the bubbler. The exact amount of reactant supplied to the reactor is not measured, but is estimated assuming the carrier gas is completely saturated with the
_ Corresponding author. ’ Present address: Praxair, Inc.. 175 East Park Drive. P.O. Box 44, Tonawanda, New York 14151-0044, USA. 0022.0248/96/$15.00 PII SOO22-0248(96)00
Copyright 0 1996 Published 19X-J
source vapor. Interactions between the reagents and the interior walls of the gas-handling system lead to variations in source delivery. This problem is particularly acute for solid precursors, such as trimethylindium (TMIn), and for precursors that have a low vapor pressure. Surface area changes during solid precursor evaporation often result in uncontrolled fluctuations in the delivery rate [I]. Low vapor pressure precursors are often maintained at a temperature higher than ambient, and consequently source condensation on colder sections of the gas handling system results in concentration transients. Current sensor technology is based upon speed of sound measurements in a binary mixture of the organometallic precursor and its carrier gas [l], and is successfully employed in monitoring TMIn delivery [2]. However, the system is limited to measurements in the gas delivery system and has been
by Elsevier Science B.V. All rights reserved
444
S. Salim et al./ Joumul of Crystal Growth 169 (1996) 443-449
restricted to line pressures above N 400 Torr. Moreover, being a physical measurement, it cannot detect the presence of different chemical species. Fourier transform infrared (FTIR) spectroscopy is suitable for OMVPE since it directly probes chemical bonds and is low in energy, thus avoiding photolysis of organometallic precursors which can accompany ultraviolet absorption spectroscopy [3]. Recently, we demonstrated applications of fiber-opticsbased (FOB) FTIR spectroscopy for remote in situ monitoring organometallic partial pressures in OMVPE gas-handling systems 141. The concentration measurement was based upon the absorbance of C-H vibrational stretching in the 2800 to 3000 cm- ’ spectral region. The HgCdTe detector and chalcogenide fiber technology used in those experiments limited measurements to conditions typical for an OMVPE gas delivery system. In this work, we present an improved FOB-FIIR approach based on fluoride fibers and an InSb detector for in situ concentration monitoring in OMVPE reactors.
2. Experimental system and procedure The FOB-FTIR system, supplied by Galileo Electro-Optics Corporation, is shown schematically in Fig. 1. The spectrometer is a KVB Analect Diamond-20 with a 4 cm-’ resolution and controlled by a Dell 486-50 MHz computer. The IR emission from the interferometer is coupled into a seven-fluoridefiber-bundle system through an optical fiber launcher.
Dell 486 50 MHz
The optical fibers guide the infrared radiation to the sensing region where the light is collimated into a 12 mm beam with a prealigned optical collimator module. After passing through the sampling volume, the collimated beam is refocused into the return fiber with a second prealigned optical module. Angular adjustment on both collimators allows optical throughput of the system to be optimized. The transmitted radiation is measured by a cooled InSb detector. Monitoring experiments were carried out in a vertical rotating-disk OMVPE reactor, which consists of a cylindrical quartz tube with a susceptor disk positioned perpendicular to the axis of the tube (Fig. 1) [5]. IR measurements were made perpendicular to the tube axis in the region midway between the porous plug flow distributor and the susceptor. The total H, carrier gas flow rate was 10 slm and the reactor pressure was either 150 or 760 Torr. Most measurements were performed with the reactor at room temperature. Typical organometallic flow rates were in the range of 1 to 5 seem, which represents conditions typically used in deposition of epitaxial layers. Numerous group 111 and V precursors were investigated including trimethylgallium (TMGal, triethylgallium (TEGa), trimethylindium (TMIn), tritertiarybutylaluminum (TTBAI), trimethylantimony (TMSb), triethylantimony (TESb), tertiarybutylphosphine (TBP), and tris-dimethylaminophosphine (DMAP). Flow visualization experiments [6], combined with fluid flow simulations and tracer gas measurements [7], indicated that no recirculating
Multi-channel InSb Detector
uxhaust
Fig. 1. Schematic diagram of the OMVPE reactor and FOB-FTIR
system.
S. Salim et al. / Journal
of Crystal
flows were present and that the concentrations of organometallic species would be uniform across the reactor. Under these flow conditions, no deposition occurred on the wall during the growth process, except on regions near the susceptor. Thus, it is possible to implement in situ monitoring with FOBFTIR through the quartz reactor wall. In order to compare the FOB-FTIR technique to conventional ultrasound measurements (Epison) [8], a 12.7 cm gas cell was placed in the vent line of the OMVPE system (see Fig. 1). This cell allowed the same organometallic precursor carrier gas mixture to pass through the two monitoring stations.
Growth
44s
169 (19961443-449
I
0.003 I
ii_ 3400
_
d
.I
3300
3200
3100
Wavenumber
3000
~.
2900
2800
(cm-‘)
Fig. 2. Infrared spectrum of 1% ASH, in Hz at 760 Torr. The overtone peaks spread between 2950 and 3200 cm-’ [IO].
3. Detection limits of the FOB-FTIR system In an earlier version of the FOB-FIIR system, the detection limit was determined by the HgCdTe detector and the use of a single fluoride fiber [3]. In the present setup, an order of magnitude improvement in the detection limit in the C-H stretching spectral region (2800 to 3000 cm- ’ > was achieved by using a more efficient InSb detector and a seven fluoridefiber bundle. The InSb setup results in a noise level of 0.001 absorbance units with a 1 s scan time, which is significantly shorter than the 45 s scan time required to reach a similar noise level with the previous system [3]. Based upon the 0.001 noise level and a IO cm path length, the detection limit for various organometallic sources varies from 0.05 Torr for TMGa to 0.006 Torr for TTBAl and DMAP, as shown in Table 1. Lower detection limits are obtained for those sources with a greater number of C-H bonds. The detection limits are appropriate for monitoring gas phase concentrations in a gas delivery system since they are much smaller than the typical input concentration levels (> 1 Torr). The short scan time also makes the technique useful for
Table I Detection
limits for selected organometallic
Scan time (s)
I
20
Noise level (ax)
I x to-’ I x lo-”
precursors
using
monitoring transients when the organometallic source is turned-on or shut-off. Further reduction in the noise level to 0.0001 absorbance units can be achieved by increasing the scan time to 20 s. This significant improvement in signal-to-noise level permits direct in situ probing of steady state chemical species in an OMVPE reactor, in which the organometallic concentration is typically two to three orders of magnitude lower than that in the delivery line. For example, in the present OMVPE reactor, typical group III organometallic concentrations used in growth are 100 ppm. For triethylgallium (TEGa), this concentration is significantly larger than minimum detection limits, which are 3 ppm at atmospheric pressure and 15 ppm at 150 Torr. It is also feasible to probe ASH, species in the reactor by monitoring overtone vibrational modes around 3 100 cm-’ [9, IO]. Fig. 2 shows a spectrum of 1% ASH, at atmospheric pressure, which is a typical level of ASH, used for epitaxial growth of AlGaAs 151. The same approach could also be implemented for other hydride sources, specifically NH, and PH,.
I and 20 s scan times
Detection limit at 25°C (Torr) TMGa
TEGa
TMln
TTBAI
TMSb
TESb
TBP
DMAP
0.05 0.005
0.02 0.002
0.05 0.005
0.006 O.OOQ6
0.06 0.006
0.02 0.002
0.008 0.0008
0.006 0.0006
446
S. Salim et al. /Journal
of Crystul Growth 169 (19%) 443-449
4. In situ monitoring of organometallic source concentrations in an OMVPE reactor The performance of the FOB-FTIR system for in situ steady-state measurement of precursor delivery was evaluated as a function of various H, bubbler flow rates and pressures to establish conditions at which the carrier gas was completely saturated with the organometallic reagent. In the first study, the delivery of TESb, which is a liquid at room temperature, was investigated for Hz bubbler flow rates of 0 to 500 seem and bubbler pressures of 200, 400 and 800 Torr. The bubbler was maintained at 25°C and the reactor pressure was 150 Torr. Fig. 3a shows the IR absorbance as a function of the calculated gas delivery rate, assuming saturation conditions inside the bubbler. The data fall on a straight line with a least-squares correlation coefficient equal to 1.OOO, indicating that the H2 was saturated with the TESb
0.01
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vapor even at the highest flow rate and lowest pressure. Similar experiments were carried out for TMln, a solid source with well-known delivery transients [2]. The bubbler pressure was maintained at 800 Ton-, the reactor pressure was 760 Torr, and H, gas flow rate was varied from 0 to 500 seem. The bubbler temperature was maintained at 25°C and the TMIn bubbler was installed in a reverse-flow configuration such that the carrier gas is introduced to the bubbler through the designated outlet tube. In this manner, evaporation of TMIn is maximized. For comparison, the concentration of TMIn was also measured using the ultrasonic technique. Fig. 3b shows the IR absorbance for TMIn as a function of Hz flow rate to the bubbler, as well as TMIn flow rate based upon the ultrasonic measurement. There is excellent agreement between the FOB-FTIR and the ultrasonic measurements over the whole range of Hz flow rates. However, both measurements show increasing deviations from the predicted TMIn flow rate (solid line) with increasing H, flow rates. This discrepancy between measurements and calculated flow rates suggests that the H, carrier gas becomes increasingly under saturated with TMIn at high flow rates.
0.004 0.002 0
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0
2
4
6
6
10
TESb Flow Rate (seem)
iz Q LL
0.4
0.6
;
0.2
00’
100
200
300
400
500
600
Hydrogen Flow Rate (seem)
Fig. 3. (a) TESb absorbance (at 2958 cm-‘) as a function of calculated TESb flow assuming saturation conditions inside the bubbler. Reactor pressure: 150 Torr, TESb bubbler pressure 200 Torr (0 ), 400 Torr CO), and 800 Torr ( X ). (b) TMIn absorbance (at 2928 cm- ’ ) ( X ) and TMIn flow rates from ultrasonic measurements (0) as a function of Hz flow rate. The solid line represents TMln flow rates calculated on the basis of complete saturation.
5. Monitoring interactions between group III and V sources In addition to organometallic source delivery variations, potential interactions between group III and V organometallic sources upon mixing can reduce precursor concentrations and adversely affect growth performance. The ability of the FOB-FTIR technique to provide insight into such Lewis acid-base interactions in OMVPE growth of III-V materials is illustrated in this study by monitoring the pairwise interactions between TTBAI and ASH,, TTBAl and TMSb, and TTBAI and TESb. Monitoring experiments were conducted under flow ( 10 slm H?) and pressure conditions (150 Torr) similar to those used for film growth [ 1 1,121. The measurements were made with the susceptor at room temperature. The absorption spectra for TTBAl (dashed line, TTBAl = 0.7 seem) and a TTBAl-ASH, mixture
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2.0
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. ;*L';
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0
3100
3000
2900
Wavenumber
2800
(cm-l)
2700
3100
3000
2900
2800
Wavenumber
2700
(cm’)
Fig. 3. Comparison between infrared spectra of TTBAI (- - -1 and a mixture of TTBAI and ASH, (-_) for (a) V/III = IO and (h) V/III
= 100.
(solid line) are shown in Fig. 4a and 4b for V/III = 10 and V/III = 100, respectively. The TTBAI spectrum is characterized by absorption at 2847 cm’. For V/III = 10, no apparent interaction is observed since the spectra of TTBAl the TTBAl-ASH, mixture are identical. In contrast, for V/III = 100. the mixture spectrum shows an increase in absorbance at 2847 cm- ‘, which is likely caused by wall condensation of a low vapor pressure adduct between TTBAl and ASH,. The change in absorbance became apparent only after the mixture had been delivered to the reactor over 5 min. Earlier spectra did not show evidence of interactions. When the TTBAI-ASH, mixture was switched out of the reactor, IR absorp tion in the region 2800 to 3000 cm-’ was still detected. Upon heating the reactor wall to 5O”C, IR absorption was not observed, indicating that the wall deposit had evaporated. Thus, FOB-FTIR monitoring allowed both identification of a potential problem and its solution. The heating strategy is appropriate for weak adducts and reactor conditions with relatively high gas velocities. However, heating stronger adducts that would require increased temperatures could eventually result in the elimination of hydrocarbons and formation of low volatility solids. The spectra obtained for mixing of TTBAI and TMSb are shown in Fig. 5. The lower spectrum is for TTBAI. the middle for TMSb, the upper solid line for the TTBAI-TMSb mixture, and the upper dashed line for addition of the TTBAI and TMSb spectra. Upon mixing, adducts were produced as
evidenced by the emergence of a broad shoulder band around 2830 cm-‘. The degree of interaction decreased as the concentration of the organometallic reagents was reduced. For example, in Fig. 5b, by reducing the group V species to 2 seem while maintaining the same group III flow, less interaction is observed. When the TTBAI-TMSb mixture was switched out of the reactor, no absorption was observed, which indicates that this combination did not lead to any adduct condensation. The formation of gas-phase adducts should not affect the overall chemistry of the growth process since the adduct bond is relatively weak and breaks as the adduct reaches the high temperature regime of the reactor [13]. Adduct condensation, on the other hand. can be problematic for epitaxial growth since lower growth rates and higher impurity incorporation may result. Such a condensation phenomenon was observed for a mixture of TTBAI and TESb. The mixture spectrum shown in Fig. 6a (upper spectra, solid line) has a reduced absorbance signal compared to the addition spectrum (dashed line). indicating a reduction in the organometallic supply to the reactor, which results from Lewis acid-base adduct condensation. In situ monitoring experiments during turn-on and turnoff of TTBAI-TESb mixture into the reactor were performed. The spectrum taken immediately
TTBAI Tl&b
TM Sb
TTBAI
3000
2900
Wavenumber
2800
(cm-l)
3000
2900
Wavenumber
2800
(cm-l)
Fig. 5. Infrared aprctra of TTBAI. TMSb. and TTBAI-TMSh mixture of (a) V/III = 5.7, and (h) V/III = 2.8. Each graph shows the spectra of TTBAI (bottom). TMSh (middle), and TTBAI-TMSh mixture (top). The dashed line represents the simple addition of TTBAI and TMSh spectral intensities.
448
S. Salim et al. /Journal
ofCrystal Growth 169 (1996) 443-449
after the flow was switched in at 0.25 min shows mostly excess TESb (Fig. 6b), which is characterized by a triplet at 2956, 2917 and 2884 cm-‘, and the TTBAl-TESb adduct band at 2864 cm- ‘. The absorbance in the adduct band increased with time, indicating possible adduct evaporation. Further evidence of adduct evaporation is observed during turn-off experiments. Both TTBAl and TESb flows were switched out of the reactor at 6 min and the reactor was continuously purged with H,. The spectrum collected at 6.5 min shows an increase in absorption at 2864 cm-‘, reflecting the gas-phase Lewis acid-base adduct between TTBAl and TESb. The adduct was observed during the span of 9 min after shut-off. No evidence of alkyl substituent exchange was found in the mixed group III-V systems explored in this study. Adduct condensation and evaporation is consistent with observations made for growth of AlSb using TTBAl and TESb in the same reactor [ 121. Adduct condensation was observed as a powdery white deposit on the reactor inlet after growth. This condensate is likely also to be responsible for an Al memory effect. After growth of an AlSb layer, a GaSb cap layer was grown to prevent oxidation of
Growth ,.
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._~.
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40
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Fig. 7. Auger depth profile analysis of multilayer AlSb-GaSb structure grown using a combination of TEGa, TTBAI and TESb.
the AlSb. However, Auger depth profiling of the epitaxial structure (Fig. 7) indicated the presence of Al in the GaSb cap layer and an Al layer on the surface. The source of aluminum, in this case, is the adduct which evaporates from the reactor inlet. The low decomposition temperature of TTBAl ( - 250°C) [14] and the relatively high substrate temperature drive the post-aluminum-deposition.
6. Summary
w
Off (6 min)
-KBAl
3000
2900
Wavenumber
2800
3000
(cm-l)
Wavenumber
2900
2800 (cm-l)
Fig. 6. (a) In situ FOB-FTIR spectra of TTBAI, TESb and TTBAI-TESb mixture of V/III = 2.0. The solid lines are the spectra of TI’BAI (bottom), TESb (middle), and TTBAI-TESb mixture (top). The dashed line represents the simple addition of TTBAl and TESb spectra. (b) Transient spectra during turn-on and turn-off flow of the TTBAl-TESb mixture (V/III = 2.0) to the reactor
The preceding examples establish FOB-FTIR spectroscopy as a useful technique for monitoring partial pressures of organometallic precursors in gas-handling systems, as well as in the OMVPE reactor. For on-line monitoring in the gas delivery system, detection limits of 0.05 to 0.006 Torr can be achieved for a one second sampling time with a 10 cm path length. This is significantly lower than the vapor pressure of organometallic sources used for OMVPE growth ( > 1 Torr). Improved detection limits are achieved by increasing the scan time, thereby allowing direct monitoring of organometallic and hydride species in an OMVPE reactor. The chemical selectivity of the FTIR technique gives an accurate measurement of the organometallic sources and allows detection of delivery problems caused by the
S. Salim et ul./Joumd
of Cystal
presence of foreign chemical species, chemical interactions and fluctuations in vapor pick-up from a bubbler.
Grobvth 169 flYY6I
449
443-449
[3] G.A. Hebner. K.P. Killeen and R.M. Biefield, J. Crystal Growth 98 (1989) 293. [4] S. Salim. K.F. Jensen and R.D. Driver, J. Crystal Growth 145 (1994) 28. [5] CA. Wang. S. Patnaik, J.W. Caunt Crystal Growth 93 (1988) 228.
Acknowledgements The MIT Lincoln Laboratory research portion was supported by the Department of the Air Force. The MIT Department of Chemical Engineering contribution was supported by the National Science Foundation (DMR 9023162). The authors thank A.C. Jones of Epichem for the TTBAl.
[IO] R. Robertson and J.J. Fox, Proc. R. Sot. A 120 (1928) 161. [I I] CA. Wang, S. Salim, K.F. Jensen and A.C. Jones, J. Electron. Mater. t 1996). in press.
[13] G. Wilkinson, Comprehensive Organometallic Vol. I (Pergamon, London, 1982).
[I] J.P.
J. Crystal
J.
[6] C.A. Wang, S.H. Groves, S.C. Palmateer, D.W. Weyburne and R.A. Brown, J. Crystal Growth 77 (1986) 136. [7] S. Patnaik, R.A. Brown and CA. Wang. J. Crystal Growth 96 (1989) 153. [8] Thomas Swan and Co. Ltd., Cambridge, UK. [9] G. Herzberg. Infrared and Raman Spectra of Polyatomic Molecules (Van Nostrand, New York. 1945). and references therein.
1121 C.A. Wang, M.C. Finn, S. Salim, K.F. Jensen Jones. Appl. Phys. Lett. 67 (1995) 1384.
References Stagg, Chemtronics 3 (1988) 44. [2] J.P. Stagg, J. Christer, E.J. Thrush and J. Crawley. Growth 120 (1992) 98.
and R.A. Brown,
1141 S. Salim. PhD Thesis, Massachusetts 1995.
and A.C. Chemistry,
Institute of Technology,