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A gas handling system for MOMBE growth
282
Journal of Crystal Growth 105 (1990) 282—288 North-Holland
A GAS HANDLING SYSTEM FOR MOMBE GROWTh S.D. HERSEE and J.M. BALLINGALL General Electric Company, Electronics Laboratory (EP3-12O~l,Syracuse, New York 1322!. USA
This paper describes the operating principles and performance of a gas handling system, which was designed specifically for the metalorganic molecular beam epitaxy (MOMBE) growth of Ill—V compound semiconductors. The system employs three-parameter control (carrier flow, reagent vapor pressure and bubbler pressure) to deliver a stable and reproducible flow of each metalorganic reagent. This approach permits a very wide range of reagent flux to be conveniently accessed. This flux can be exactly predicted, using a modification to the simple theoretical expression that has been used previously. Data for the repeated growth of GaAs FET structures show that the growth rate is stable and has an excellent day-to-day reproducibility. A novel compensated vent/run gas switching system is used to eliminate pressure transients that can otherwise occur when switching the reagent gases in and Out of the MOMBE growth chamber. This system has been used for the growth of GaAs and A1GaAs and we show that the growth rate stabilizes within one monolayer during a change of 111—V composition. This allows us to grow heterojunctions with monolayer abruptness as determined by RHEED oscillations and photoluminescence studies of narrow quantum well samples.
I. Introduction MOMBE combines the accurately metered and long-lived MO gas sources of metalorgamc vapor phase epitaxy (MOVPE) with the vacuum environment of molecular beam epitaxy (MBE) and has the potential for a higher uniformity, reproducibility and throughput than both of its predecessors. In MOVPE, the use of gaseous (normally metalorgamc or hydride) sources and mass flow controllers has been very successful in providing a stable and reproducible flow of group III and group V reagents to the reaction vessel. The flow of gaseous reagents is stable and does not suffer from the beam collimation and attendent uniformity problems that arise in MBE type Knudsen cells, when the elemental charge depletes. However in MOVPE, once the gaseous reagent flux has been injected into the reaction chamber, then hydrodynamic problems such as gas-phase depletion, convection and turbulence, will frequently degrade the uniformity and reproducibility of the growing layer and limit the scale-up potential of this technology. In MBE, these hydrodynamic problems are eliminated by virtue of the molecular flow environment of the growth chamber. 0022-0248/90/$03.50 © 1990
—
In this paper we describe the operation of a gas handling system designed specifically for the MOMBE growth of Ill—V compounds.
2. Design principles 2.1. Control of reagent flux To maintain a molecular flow regime during growth the mean free path of the injected gas molecules should be greater than the distance between the injector and the substrate. In a typical growth chamber this distance is 10 to 20 cm, so a working pressure of 10 Torr or less will allow an adequate mean free path. With currently available pumping speeds this translates into a maximum total gas flow into the chamber of typically 50 SCCM. Given this limitation the MOVPE type “carrier-gas” method of extracting MO reagent from a bubbler, initially appears unsuitable for MOMBE, and it is tempting to eliminate the carrier gas and extract reagent simply by pumping on the bubbler. However, we have concluded [11]that the use of a small carrier flow offers advantages over the alternative “carrier free” approaches.
Elsevier Science Publishers B.V. (North-Holland)
‘~
S.D. Hersee, J.M. Ballingall / Gas handling system for MOMBE growth
In the pioneering MOMBE work of Veuhoff et a!. [2], a precision leak valve was placed between the reagent bubbler and the MOMBE chamber. Drift in the conductance of the manual leak valve (for example, due to partial clogging) presents a problem with this approach and the reagent flux requires frequent calibration. In the work of Tsang [3] mass flow controllers were inserted after the reagent bubblers to accurately control and momtor the reagent flow. In later work of Tsang et al. [4], hydrogen carner gas was mtroduced but the mass flow controllers remained downstream from the bubbler. While not a problem with most reagents, from MOVPE experience we have found that the heating action of the mass flow controller can partially decompose some of the less table metalorganic sources. This can lead to contamination of the reagent and can eventually block the gas line, If we use a small carrier flow the mass flow controller can be placed upstream from the bubbier, and potential problems of clogging or decomposition in the gas lines avoided. In addition to this the presence of a carrier flow allows a wide choice of reagent flux to be conveniently selected, as described below, When we use a carrier gas, the flow of MO reagent (Fr) is a function of the carrier flow (F~),
283
the reagent saturated vapor pressure (Pr) and the pressure in the bubbler (Pb), as described in eq. (1), which was first shown by Schaus et al. [5]: F = F / i~ ,~ 1 —
r
Cl
1k b/
In a real system the bubbler pressure is measured downstream from the bubbler and we have shown [1] that the reagent flow is exactly described by eq. (2): —
Here, ~d is the downstream bubbler pressure, measured by a manometer placed downstream from the bubbler and separated from the bubbler by a short length of tubing of conductance C~(see fig. 1). As described above, high carrier flows (Ft) cannot be tolerated in MOMBE so we must use another approach to extract a reasonable flow of reagent from the bubbler. The reagent vapor pressure (‘~r) increases rapidly with temperature and this can be used to increase the molar flow of the MO reagent. However, this approach is limited as too high a temperature will cause the reagent to decompose. Also, the bubbler temperature can not be changed quickly as it requires a stabilization time of typically several hours.
Downstream Pressure, ~d (~. - ~..
—FJ—~I J
Carrier Gas Flow, Fc
..‘~
.“
,,..-
-
Automatic Pressure Control
j
~ Pressure Control Valve
Conductance, CS
Reagent Vapor Pressure, ~r
Reagent Flux, Fr = Pd.Pr+(!Q~±~!t) Cs Fig. 1. Three-parameter control of reagent flow.
etwork
S.D. Hersee, f.M. Ballingall
284
/
Gas handling system for MOMBE growth
In this work we emphasize downstream bubbler pressure, (ed) as the main parameter in the control of reagent flux. We find, that if the carrier flow and reagent temperature are fixed, then a wide range of reagent flux can be controllably accessed simply by changing the bubbler pressure. In the case of reagents with a low vapor pressure (e.g. TEAl, 0.14 Torr at 25°C) the use of a low bubbler pressure allows a high but accurately controlled reagent flux to be extracted without resort to excessive bubbler temperatures or carrier flows,
We have designed a state of the art cornpensated vent/run gas switching system to eliminate unwanted flow transients associated with gas switching. The vent/run concept [6] is that the MO flows are first stabilized in a vent or waste line and then switched into the run line (the growth chamber) when required. Starting and stopping growth is then achieved by switching this stabilized flow in and out of the growth chamber rather than opening and closing the MO source container. The superiority of vent/run switching by comparison with the open/close approach is described in detail in ref. [7].
and run lines are not balanced. These transients must be avoided as they translate directly into variations in growth rate and composition. Several schemes have been devised to maintain the vent and run lines at constant and equal pressure and these generally employ a manometer and an automatic pressure controller (APC) in a feedback loop to maintain a given pressure value. Such systems provide equalized vent and run line pressures up to the moment of switching. However, when a flow is switched from the vent to the run line the pressure in the run line will rise and there will inevitably be a small time delay before this is detected and corrected by the APC system. This small rise in reactor line pressure will change ~d and will therefore affect the flow of reagent (f.), as shown by eq. (2). Our novel MOMBE manifold design eliminates this transient by using a compensation flow in parallel with each reagent’ ‘flux (fig. 2). The cornpensation flow is set to be equal to the flow in the reagent line and is vent/run switched in opposition to the reagent flow, the source flow (carrier + reagent), 1~,and the carrier-gas compensationflow, P~omp, are equal flows which can be switched into either the vent line or the reactor line. The valves V1, V2, V3 and V4 are ganged such that
It has also been shown [7] that even when vent/run switching is employed it can also suffer from flux transients if the pressures in the vent
when the source flow is in the vent line the cornpensation flow is in the reactor line and vice versa. These four valves are operated simultaneously so
2.2. Compensated vent/run gas switching
~L~mber
Low Pressure Vent Line Fig. 2. Compensated vent/run gas switching.
Vent (no growth) Run (growth) 0
=
valve open; C
=
valve closed.
V1
V2
V3
V4
0 C
C 0
0 C
C 0
S.D. Hersee, f.M. Ballingall
/ Gas handling system for MOMBE growth
that when the source flow is added to the reactor line the equivalent compensation flow is subtracted from the reactor line. The total flow (and more importantly the pressure) in both the reactor and the vent lines therefore remains constant during switching, thus eliminating all unwanted flow transients. Using this gas switching system we have grown GaAs/A1GaAs heterojunctions with monolayer abrupt interfaces, as described below,
285
The vent line is maintained at low pressure (0.1 Ton) by pumping through a filtered mechanical pump. A ballast flow of dry nitrogen is added at the pump inlet to maintain a viscous flow and avoid oil backstreaming into the vent manifold [8]. The vent/run air-operated valves are connected directly to the group III gas injector at the source flange. Each group III reagent enters the injector through a separate port and then passes through a short high-conductance mixing zone before entering the growth chamber. The remainder of the system is a standard MBE system (Vacuum Generators, UK; V8OH) equipped with a liquid nitrogen trapped 3600 1/s diffusion pump on the main deposition chamber. After baking the background pressure in the main growth chamber is typically 8 x lO~ Ton. Small carrier flows of hydrogen (1 to 10 SCCM per bubbler) are used to deliver TEGa (triethylgalhum), TEAl (triethylaluminum) and TiBA1 (triisobutylaluminum) reagents to the MOMBE growth chamber. The MOMBE layers were grown at a substrate temperature of 550°Cwith an arsenic flux (from a conventional solid arsenic Knudsen cell) of between 6 X i05 and 3.5 X iO~ mbar. The orien-
3. Experimental details These two principles, three-parameter reagent flux control and compensated vent/run switching, are combined in our gas handling system, which is shown schematically in fig. 3. For clarity only one reagent cell is shown, however the full system has the capacity for six rea~it bubblers. For each reagent cell we use hydrogen carrier gas flow, MO bubbler temperature and bubbler pressure as fully controllable variables. The metalorganic reagents are contained in commercial stainless steel bubblers (type * 65270, CVD Inc.) with vertical inlet and outlet ports.
GE Aerospace
Control
EJ.i
I
Reagent
Mass Flow
componsw
___________
Low Pressure Vent Line
~BE
Filter Nitrogen Balls
P>1.5 x 102m bar MechanIcal
Fig. 3. A complete reagent cell.
286
S.D. Hersee, f.M. Ballingall
/
Gas handling system for MOMBE growth
~ ~—AIAs—--—~.-~--AIGaAs—-~ Fig. 4. RHEED oscillations during (a) growth of GaAs and (b) growth of AlAs and AlGaAs.
tation of the undoped semi-insulating substrates was either (100) or 2°-off (100) and they were prepared as for conventional MBE growth. 4. Results Under the arsenic rich growth conditions used in this MOMBE work at 550°Cthe binary Ill—V compound growth rate is controlled by the flux of group III reagent [9].The group III reagent flux is therefore proportional to the deposition rate of the Ill—V compound and the latter can be conveniently calculated from reflection high energy electron diffraction (RHEED) oscillation measurements [9]. Figs. 4a and 4b show typical RHEED oscillation traces for the MOMBE growth of GaAs, AlAs and AlGaAs from TEGa, TEA! and solid arsenic.
lThl 20
Using the three-parameter control approach a wide range of reagent flux is obtained for TEGa, TEAl and TiBA1. Controllable and reproducible growth rates up to 3 p~mh1 have been achieved for GaAs grown from triethylgallium, up to 0.7 jsrn h~ for AlAs grown from triethylaluminum and up to 2 js m h ‘ for AlAs grown from tnsobutylaluminum. The variation of growth rate, as determined from RHEED oscillation measurements, with downstream pressure is shown for GaAs and AlAs in figs. 5 and 6. We find that the best fit to the experimental data is obtained by choosing a finite (rather than infinite) conductance value (Ce) as indicated by eq. (2). Fig. 7 shows the variation of AlAs growth rate (grown from triisobutylaluminum) with carrier gas flow for two pressure settings. Again the growth rates were determined from RHEED oscillation measurements.
e.G 40
60
80
100
120
Downstream Pressure (Torn
Fig. 5. Variation of GaAs growth rate with downstream pressure.
till 2.0
4.0
6.0
8.0
10.0
12.0
Downstream Pressure (Torr)
Fig. 6. Variation of AlAs growth rate with downstream pressure.
S.D. Hersee, f.M. Ballingall 2.0
1.3
Gas handlingsystem for MOMBE growth
287
1019
‘
1.7
~
/
~
0E
~
N
-~
Undoped Buffer
Substrate
1017
-
1:: : I
0.0
2.0
4.0
6.0
8.0
10.0
1015
12.0
1010
1
Carrier Flow, Fc (sCcm)
2
Distance From Surface (~tm)
Fig. 7. Variation of AlAs growth rate with carrier flow for two settings of downstream pressure.
Fig. 8. Doping concentration profiles of four FET growths.
In each case the flows are well controlled and repeatable and are accurately described by eq. (2). Any given reagent flow could be obtained with an excellent day to day reproducibility. Fig. 8 show the doping concentration profiles, as measured by the Polaron (electrochemical capacitance—voltage) profiler, for multiple growths of a simple FET N~/N/undoped buffer structure, grown four times over a period of three days. All
of these profiles are the same within the experimental accuracy of the measurement, which is 5% for doping concentration and thickness. The compensated vent/run system was designed to provide a rapid gas switching for the growth of abrupt heterojunctions and doping spikes. Heterojunction abruptness was measured by analysis of the RHEED oscillation period as a function of time, as used by Tsang et a!. [9]. This
718 0W4 G~A~13.4
0W3 GM~25.4
0W2 G~A~5O
-
818 OWl
—
O
I
.
GoAs75
GoA~O.31rn,
~
741
a
820
772
~J
680
i~l
780 Wavelength (nm)
852
880
Fig. 9. (a) The multi-quantum well (MQW) structure (inset). (b) Photoluminescence spectrum of the MQW structure.
288
S.D. Hersee, f.M. Ballingall
/ Gas handling system for MOMBE growth
shows that for each group III reagent the growth rate stabilized within one monolayer, see fig. 4. If vent/run switching was performed without the compensation flow, then a longer stabilization period of typically 5 monolayers was observed, During this period the growth rate typically fell to 95% of its initial value and then recovered. However, the degree of instability was different for each reagent cell and further work is needed to determine whether this was due to differences in the time constants of each control loop (mass flow control and pressure control) or to differences in the physical properties of each reagent. Photoluminescence (PL) measurements were performed at 4 K on multiquantum well samples of the type shown inset in fig. 9, we observe a doublet for AlGaAs (705 and 710 nm), peaks due to the four quantum wells (718, 741, 772 and 789 nm), the GaAs excitonic peaks (818 and 820 nm), and the GaAs carbon acceptor at 831 nm. At higher detector gain we can also see a phonon replica of the acceptor peak at 852 nm. The quanturn well thickness as calculated from the PL peak energy, is consistently within one monolayer of the expected value, which confirms the RHEED result that the heterojunctions are indeed monosayer aurupt. In this sample the full width at half maximum of the PL peak is 7.9 meV for the 14 A quantum well and 5 meV for the 47 A well.
5. Conclusions It has been shown that this design of gas handling system conveniently provides a wide range of reagent flux with excellent stability and reproducibility. This approach allows the mass flow controller to be placed upstream from the bubbler
thereby avoiding potential problems of clogging. In addition it has proved very useful in obtaining reasonable flux rates (and therefore reasonable growth rates) from reagents with a very low vapor pressure, such as triethylalurninum. The novel compensated vent/run gas switching system allows the growth of heterojunctions with monolayer abruptness. These properties are essential if MOMBE is to seriously challenge MOVPE and MBE as a source of quality epitaxy for advanced Ill—V devices and circuits.
Acknowledgements The authors wish to thank Glenn Tessmer for the photoluminescence measurements and Baker Drake for expert technical assistance. The support of Alan W. Swanson and Walter J. Butler is also greatly appreciated.
References [1] S.D. Hersee and J.M. Ballingall. J. Vacuum Sci. Technol. A8 (1990) 800. [2] E. Veuhoff, W. Pletschen. P. Balk and H. LUih. J. Crystal Growth 55 (1981) 30. [3] W.T. Tsang, J. Electron. Mater. 15 (1986) 235. [4] W.T. Tsang, E.F. Schubert. T.H. Chiu. J. Cunningham. E.G. Burkhardt. J.A. Ditzenburger and F. Agyekum, Appl. Phys. Letters 51(1987) 761. [5] CF. Schaus, W.J. Schaff and JR. Shealy, J. Crystal Growth 77 (1986) 360. [6] S.D. Hersee, M. Baldy and P. Assenat. J. Physique 43 [7] [8] [9]
(1982) C5193. J.S. Roberts, N.J. Mason and M. Robinson, J. Crystal Growth 68 (1984) 422. T.A Heppell. Vacuum 37 (1987) 593. W.T. Tsang, T.H. Chiu. J. Cunningham and A. Robertson. AppI. Phys. Letters 50 (1987) 1376.
Journal of Crystal Growth 105 (1990) 282—288 North-Holland
A GAS HANDLING SYSTEM FOR MOMBE GROWTh S.D. HERSEE and J.M. BALLINGALL General Electric Company, Electronics Laboratory (EP3-12O~l,Syracuse, New York 1322!. USA
This paper describes the operating principles and performance of a gas handling system, which was designed specifically for the metalorganic molecular beam epitaxy (MOMBE) growth of Ill—V compound semiconductors. The system employs three-parameter control (carrier flow, reagent vapor pressure and bubbler pressure) to deliver a stable and reproducible flow of each metalorganic reagent. This approach permits a very wide range of reagent flux to be conveniently accessed. This flux can be exactly predicted, using a modification to the simple theoretical expression that has been used previously. Data for the repeated growth of GaAs FET structures show that the growth rate is stable and has an excellent day-to-day reproducibility. A novel compensated vent/run gas switching system is used to eliminate pressure transients that can otherwise occur when switching the reagent gases in and Out of the MOMBE growth chamber. This system has been used for the growth of GaAs and A1GaAs and we show that the growth rate stabilizes within one monolayer during a change of 111—V composition. This allows us to grow heterojunctions with monolayer abruptness as determined by RHEED oscillations and photoluminescence studies of narrow quantum well samples.
I. Introduction MOMBE combines the accurately metered and long-lived MO gas sources of metalorgamc vapor phase epitaxy (MOVPE) with the vacuum environment of molecular beam epitaxy (MBE) and has the potential for a higher uniformity, reproducibility and throughput than both of its predecessors. In MOVPE, the use of gaseous (normally metalorgamc or hydride) sources and mass flow controllers has been very successful in providing a stable and reproducible flow of group III and group V reagents to the reaction vessel. The flow of gaseous reagents is stable and does not suffer from the beam collimation and attendent uniformity problems that arise in MBE type Knudsen cells, when the elemental charge depletes. However in MOVPE, once the gaseous reagent flux has been injected into the reaction chamber, then hydrodynamic problems such as gas-phase depletion, convection and turbulence, will frequently degrade the uniformity and reproducibility of the growing layer and limit the scale-up potential of this technology. In MBE, these hydrodynamic problems are eliminated by virtue of the molecular flow environment of the growth chamber. 0022-0248/90/$03.50 © 1990
—
In this paper we describe the operation of a gas handling system designed specifically for the MOMBE growth of Ill—V compounds.
2. Design principles 2.1. Control of reagent flux To maintain a molecular flow regime during growth the mean free path of the injected gas molecules should be greater than the distance between the injector and the substrate. In a typical growth chamber this distance is 10 to 20 cm, so a working pressure of 10 Torr or less will allow an adequate mean free path. With currently available pumping speeds this translates into a maximum total gas flow into the chamber of typically 50 SCCM. Given this limitation the MOVPE type “carrier-gas” method of extracting MO reagent from a bubbler, initially appears unsuitable for MOMBE, and it is tempting to eliminate the carrier gas and extract reagent simply by pumping on the bubbler. However, we have concluded [11]that the use of a small carrier flow offers advantages over the alternative “carrier free” approaches.
Elsevier Science Publishers B.V. (North-Holland)
‘~
S.D. Hersee, J.M. Ballingall / Gas handling system for MOMBE growth
In the pioneering MOMBE work of Veuhoff et a!. [2], a precision leak valve was placed between the reagent bubbler and the MOMBE chamber. Drift in the conductance of the manual leak valve (for example, due to partial clogging) presents a problem with this approach and the reagent flux requires frequent calibration. In the work of Tsang [3] mass flow controllers were inserted after the reagent bubblers to accurately control and momtor the reagent flow. In later work of Tsang et al. [4], hydrogen carner gas was mtroduced but the mass flow controllers remained downstream from the bubbler. While not a problem with most reagents, from MOVPE experience we have found that the heating action of the mass flow controller can partially decompose some of the less table metalorganic sources. This can lead to contamination of the reagent and can eventually block the gas line, If we use a small carrier flow the mass flow controller can be placed upstream from the bubbier, and potential problems of clogging or decomposition in the gas lines avoided. In addition to this the presence of a carrier flow allows a wide choice of reagent flux to be conveniently selected, as described below, When we use a carrier gas, the flow of MO reagent (Fr) is a function of the carrier flow (F~),
283
the reagent saturated vapor pressure (Pr) and the pressure in the bubbler (Pb), as described in eq. (1), which was first shown by Schaus et al. [5]: F = F / i~ ,~ 1 —
r
Cl
1k b/
In a real system the bubbler pressure is measured downstream from the bubbler and we have shown [1] that the reagent flow is exactly described by eq. (2): —
Here, ~d is the downstream bubbler pressure, measured by a manometer placed downstream from the bubbler and separated from the bubbler by a short length of tubing of conductance C~(see fig. 1). As described above, high carrier flows (Ft) cannot be tolerated in MOMBE so we must use another approach to extract a reasonable flow of reagent from the bubbler. The reagent vapor pressure (‘~r) increases rapidly with temperature and this can be used to increase the molar flow of the MO reagent. However, this approach is limited as too high a temperature will cause the reagent to decompose. Also, the bubbler temperature can not be changed quickly as it requires a stabilization time of typically several hours.
Downstream Pressure, ~d (~. - ~..
—FJ—~I J
Carrier Gas Flow, Fc
..‘~
.“
,,..-
-
Automatic Pressure Control
j
~ Pressure Control Valve
Conductance, CS
Reagent Vapor Pressure, ~r
Reagent Flux, Fr = Pd.Pr+(!Q~±~!t) Cs Fig. 1. Three-parameter control of reagent flow.
etwork
S.D. Hersee, f.M. Ballingall
284
/
Gas handling system for MOMBE growth
In this work we emphasize downstream bubbler pressure, (ed) as the main parameter in the control of reagent flux. We find, that if the carrier flow and reagent temperature are fixed, then a wide range of reagent flux can be controllably accessed simply by changing the bubbler pressure. In the case of reagents with a low vapor pressure (e.g. TEAl, 0.14 Torr at 25°C) the use of a low bubbler pressure allows a high but accurately controlled reagent flux to be extracted without resort to excessive bubbler temperatures or carrier flows,
We have designed a state of the art cornpensated vent/run gas switching system to eliminate unwanted flow transients associated with gas switching. The vent/run concept [6] is that the MO flows are first stabilized in a vent or waste line and then switched into the run line (the growth chamber) when required. Starting and stopping growth is then achieved by switching this stabilized flow in and out of the growth chamber rather than opening and closing the MO source container. The superiority of vent/run switching by comparison with the open/close approach is described in detail in ref. [7].
and run lines are not balanced. These transients must be avoided as they translate directly into variations in growth rate and composition. Several schemes have been devised to maintain the vent and run lines at constant and equal pressure and these generally employ a manometer and an automatic pressure controller (APC) in a feedback loop to maintain a given pressure value. Such systems provide equalized vent and run line pressures up to the moment of switching. However, when a flow is switched from the vent to the run line the pressure in the run line will rise and there will inevitably be a small time delay before this is detected and corrected by the APC system. This small rise in reactor line pressure will change ~d and will therefore affect the flow of reagent (f.), as shown by eq. (2). Our novel MOMBE manifold design eliminates this transient by using a compensation flow in parallel with each reagent’ ‘flux (fig. 2). The cornpensation flow is set to be equal to the flow in the reagent line and is vent/run switched in opposition to the reagent flow, the source flow (carrier + reagent), 1~,and the carrier-gas compensationflow, P~omp, are equal flows which can be switched into either the vent line or the reactor line. The valves V1, V2, V3 and V4 are ganged such that
It has also been shown [7] that even when vent/run switching is employed it can also suffer from flux transients if the pressures in the vent
when the source flow is in the vent line the cornpensation flow is in the reactor line and vice versa. These four valves are operated simultaneously so
2.2. Compensated vent/run gas switching
~L~mber
Low Pressure Vent Line Fig. 2. Compensated vent/run gas switching.
Vent (no growth) Run (growth) 0
=
valve open; C
=
valve closed.
V1
V2
V3
V4
0 C
C 0
0 C
C 0
S.D. Hersee, f.M. Ballingall
/ Gas handling system for MOMBE growth
that when the source flow is added to the reactor line the equivalent compensation flow is subtracted from the reactor line. The total flow (and more importantly the pressure) in both the reactor and the vent lines therefore remains constant during switching, thus eliminating all unwanted flow transients. Using this gas switching system we have grown GaAs/A1GaAs heterojunctions with monolayer abrupt interfaces, as described below,
285
The vent line is maintained at low pressure (0.1 Ton) by pumping through a filtered mechanical pump. A ballast flow of dry nitrogen is added at the pump inlet to maintain a viscous flow and avoid oil backstreaming into the vent manifold [8]. The vent/run air-operated valves are connected directly to the group III gas injector at the source flange. Each group III reagent enters the injector through a separate port and then passes through a short high-conductance mixing zone before entering the growth chamber. The remainder of the system is a standard MBE system (Vacuum Generators, UK; V8OH) equipped with a liquid nitrogen trapped 3600 1/s diffusion pump on the main deposition chamber. After baking the background pressure in the main growth chamber is typically 8 x lO~ Ton. Small carrier flows of hydrogen (1 to 10 SCCM per bubbler) are used to deliver TEGa (triethylgalhum), TEAl (triethylaluminum) and TiBA1 (triisobutylaluminum) reagents to the MOMBE growth chamber. The MOMBE layers were grown at a substrate temperature of 550°Cwith an arsenic flux (from a conventional solid arsenic Knudsen cell) of between 6 X i05 and 3.5 X iO~ mbar. The orien-
3. Experimental details These two principles, three-parameter reagent flux control and compensated vent/run switching, are combined in our gas handling system, which is shown schematically in fig. 3. For clarity only one reagent cell is shown, however the full system has the capacity for six rea~it bubblers. For each reagent cell we use hydrogen carrier gas flow, MO bubbler temperature and bubbler pressure as fully controllable variables. The metalorganic reagents are contained in commercial stainless steel bubblers (type * 65270, CVD Inc.) with vertical inlet and outlet ports.
GE Aerospace
Control
EJ.i
I
Reagent
Mass Flow
componsw
___________
Low Pressure Vent Line
~BE
Filter Nitrogen Balls
P>1.5 x 102m bar MechanIcal
Fig. 3. A complete reagent cell.
286
S.D. Hersee, f.M. Ballingall
/
Gas handling system for MOMBE growth
~ ~—AIAs—--—~.-~--AIGaAs—-~ Fig. 4. RHEED oscillations during (a) growth of GaAs and (b) growth of AlAs and AlGaAs.
tation of the undoped semi-insulating substrates was either (100) or 2°-off (100) and they were prepared as for conventional MBE growth. 4. Results Under the arsenic rich growth conditions used in this MOMBE work at 550°Cthe binary Ill—V compound growth rate is controlled by the flux of group III reagent [9].The group III reagent flux is therefore proportional to the deposition rate of the Ill—V compound and the latter can be conveniently calculated from reflection high energy electron diffraction (RHEED) oscillation measurements [9]. Figs. 4a and 4b show typical RHEED oscillation traces for the MOMBE growth of GaAs, AlAs and AlGaAs from TEGa, TEA! and solid arsenic.
lThl 20
Using the three-parameter control approach a wide range of reagent flux is obtained for TEGa, TEAl and TiBA1. Controllable and reproducible growth rates up to 3 p~mh1 have been achieved for GaAs grown from triethylgallium, up to 0.7 jsrn h~ for AlAs grown from triethylaluminum and up to 2 js m h ‘ for AlAs grown from tnsobutylaluminum. The variation of growth rate, as determined from RHEED oscillation measurements, with downstream pressure is shown for GaAs and AlAs in figs. 5 and 6. We find that the best fit to the experimental data is obtained by choosing a finite (rather than infinite) conductance value (Ce) as indicated by eq. (2). Fig. 7 shows the variation of AlAs growth rate (grown from triisobutylaluminum) with carrier gas flow for two pressure settings. Again the growth rates were determined from RHEED oscillation measurements.
e.G 40
60
80
100
120
Downstream Pressure (Torn
Fig. 5. Variation of GaAs growth rate with downstream pressure.
till 2.0
4.0
6.0
8.0
10.0
12.0
Downstream Pressure (Torr)
Fig. 6. Variation of AlAs growth rate with downstream pressure.
S.D. Hersee, f.M. Ballingall 2.0
1.3
Gas handlingsystem for MOMBE growth
287
1019
‘
1.7
~
/
~
0E
~
N
-~
Undoped Buffer
Substrate
1017
-
1:: : I
0.0
2.0
4.0
6.0
8.0
10.0
1015
12.0
1010
1
Carrier Flow, Fc (sCcm)
2
Distance From Surface (~tm)
Fig. 7. Variation of AlAs growth rate with carrier flow for two settings of downstream pressure.
Fig. 8. Doping concentration profiles of four FET growths.
In each case the flows are well controlled and repeatable and are accurately described by eq. (2). Any given reagent flow could be obtained with an excellent day to day reproducibility. Fig. 8 show the doping concentration profiles, as measured by the Polaron (electrochemical capacitance—voltage) profiler, for multiple growths of a simple FET N~/N/undoped buffer structure, grown four times over a period of three days. All
of these profiles are the same within the experimental accuracy of the measurement, which is 5% for doping concentration and thickness. The compensated vent/run system was designed to provide a rapid gas switching for the growth of abrupt heterojunctions and doping spikes. Heterojunction abruptness was measured by analysis of the RHEED oscillation period as a function of time, as used by Tsang et a!. [9]. This
718 0W4 G~A~13.4
0W3 GM~25.4
0W2 G~A~5O
-
818 OWl
—
O
I
.
GoAs75
GoA~O.31rn,
~
741
a
820
772
~J
680
i~l
780 Wavelength (nm)
852
880
Fig. 9. (a) The multi-quantum well (MQW) structure (inset). (b) Photoluminescence spectrum of the MQW structure.
288
S.D. Hersee, f.M. Ballingall
/ Gas handling system for MOMBE growth
shows that for each group III reagent the growth rate stabilized within one monolayer, see fig. 4. If vent/run switching was performed without the compensation flow, then a longer stabilization period of typically 5 monolayers was observed, During this period the growth rate typically fell to 95% of its initial value and then recovered. However, the degree of instability was different for each reagent cell and further work is needed to determine whether this was due to differences in the time constants of each control loop (mass flow control and pressure control) or to differences in the physical properties of each reagent. Photoluminescence (PL) measurements were performed at 4 K on multiquantum well samples of the type shown inset in fig. 9, we observe a doublet for AlGaAs (705 and 710 nm), peaks due to the four quantum wells (718, 741, 772 and 789 nm), the GaAs excitonic peaks (818 and 820 nm), and the GaAs carbon acceptor at 831 nm. At higher detector gain we can also see a phonon replica of the acceptor peak at 852 nm. The quanturn well thickness as calculated from the PL peak energy, is consistently within one monolayer of the expected value, which confirms the RHEED result that the heterojunctions are indeed monosayer aurupt. In this sample the full width at half maximum of the PL peak is 7.9 meV for the 14 A quantum well and 5 meV for the 47 A well.
5. Conclusions It has been shown that this design of gas handling system conveniently provides a wide range of reagent flux with excellent stability and reproducibility. This approach allows the mass flow controller to be placed upstream from the bubbler
thereby avoiding potential problems of clogging. In addition it has proved very useful in obtaining reasonable flux rates (and therefore reasonable growth rates) from reagents with a very low vapor pressure, such as triethylalurninum. The novel compensated vent/run gas switching system allows the growth of heterojunctions with monolayer abruptness. These properties are essential if MOMBE is to seriously challenge MOVPE and MBE as a source of quality epitaxy for advanced Ill—V devices and circuits.
Acknowledgements The authors wish to thank Glenn Tessmer for the photoluminescence measurements and Baker Drake for expert technical assistance. The support of Alan W. Swanson and Walter J. Butler is also greatly appreciated.
References [1] S.D. Hersee and J.M. Ballingall. J. Vacuum Sci. Technol. A8 (1990) 800. [2] E. Veuhoff, W. Pletschen. P. Balk and H. LUih. J. Crystal Growth 55 (1981) 30. [3] W.T. Tsang, J. Electron. Mater. 15 (1986) 235. [4] W.T. Tsang, E.F. Schubert. T.H. Chiu. J. Cunningham. E.G. Burkhardt. J.A. Ditzenburger and F. Agyekum, Appl. Phys. Letters 51(1987) 761. [5] CF. Schaus, W.J. Schaff and JR. Shealy, J. Crystal Growth 77 (1986) 360. [6] S.D. Hersee, M. Baldy and P. Assenat. J. Physique 43 [7] [8] [9]
(1982) C5193. J.S. Roberts, N.J. Mason and M. Robinson, J. Crystal Growth 68 (1984) 422. T.A Heppell. Vacuum 37 (1987) 593. W.T. Tsang, T.H. Chiu. J. Cunningham and A. Robertson. AppI. Phys. Letters 50 (1987) 1376.