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Short-pulse chemical beam epitaxy
Journal of Crystal Growth 136 (1994) 200—203 North-Holland
J ~
CRYSTAL GROW T H
o~
Short-pulse chemical beam epitaxy Suian Zhang
a
Jie Cui
a
Akihiko Tanaka
b
and Yoshinobu Aoyagi
a
RIKEN, The Institute of Physical and Chemical Research, Hirosawa 2-1, Wako-shi, Saitama 351-0], Japan ~ Bentec Co., Sakae-cho 6-1, Tachikawa-shi, Tokyo 190, Japan
Short-pulse chemical beam epitaxy has been proposed and studied. The short pulses with supersonic characteristics and a width of milliseconds were generated by high speed valves and the related pumping lines on a purpose-built CBE system. Using a time-of-fight technique, we verified the dependence of pulse properties on the source pressures and the valve on-time. The results indicate that modulation of molecular kinetic energy and accurate control of molecule supply were obtained. GaAs epitaxial growth with use of trimethylgallium pulses was carried out and investigated by means of RHEED (reflection high-energy electron diffraction) observation. It was demonstrated that the newly developed short-pulse chemical beam epitaxy has the advantage of high controllability.
1. Introduction As is well known, in most of the conventional CBE (chemical beam epitaxy) and MBE (molecular beam epitaxy) systems, beam switching is made by using mechanical shutters and mechanical valves, whose response times are typically around 0.1 s or longer. Needless to say, further improvement on the speed of beam switching is required for advanced applications to fabrications of nanostructures such as vertical superlattices and quantum wires. In the present research, we propose to use short-pulse chemical beams to improve the controllability of molecule supply as well as surface migration and chemical reactions, based on the views described below. Firstly, by using short pulses with response times shorter than 1 ms, it is expected that the controllability on the molecule number can be significantly improved. Secondly, according to gas dynamics [1], the kinetic energy of molecules in a beam extracted from a supersonic jet can be modulated in a range up to. several electron volts when a mixture of source gas and light molecular gas is used [21. Thirdly, use of short pulses of chemical beam allows us to manipulate surface reactions at different instants while keeping the vacuum of growth chamber at a high level. In this paper we
report experimental investigations on the shortpulse properties and the application to epitaxial growth of GaAs.
2. Experimental procedure A purpose-oriented CBE system as shown in fig. la was built for the present study. For the sake of clarity, only one line of the gas handling system is sketched in the figure. To grow GaAs, AsH3 (100%) and trimethylgallium (TMG) were respectively used as arsenic and gallium sources. The AsH3 gas was cracked to As2 at 1100°C.To generate a pulsed TMG beam, a high-speed valve with a 0.8 mm diameter orifice was installed 350 mm away from the substrate. The source pressure at the inlet of the high speed valve was controlled by a mass-flow controller and a pressure control valve linked to the corresponding absolute pressure gauge. To ensure an ideal shape of the TMG pulse, a high-vacuum differential pump was set at the outlet of the high-speed valve. The high vacuum of the growth chamber was maintained by using a high-capacity turbomolecular pump and a standard liquid-nitrogen shroud. The base pressure of the growth chamber was in the 10_In Torr range when the gas introduction valves were
0022-0248/94/$07.00 © 1994 — Elsevier Science B.V. All rights reserved SSDI 0022-0248(93)E0292-F
S. Zhang ci al. (a)
Pump
/ Short-pulse chemical beam epitaxy
GaAs epitaxial layers were grown on (001)
UHV pump
pcv MFC
HSV
Chamber
UHV
~pump
Trigger
_rT*1
II
growing surface.
~orRHEED
UHV pump Chamber
~cv
~
~
uHv
FIG head
driver
Trigge
I
Computer
L~.i
FIG
I
control unit
_______
Trigger
Transient
The short pulses of TMG generated with
source pressures of 25, 50 and 100 Torr are
pump
Hsv
vapor
semi-insulating substrates treatedThe bygrowth a standard cleaning and etching procedure. ternperature was measured using an infrared pyrometer. Throughout the growth, a RHEED (reflection high-energy electron diffraction) optical system withangle incident a primary of 2°energy was used of 20 to keV monitor and the an
3. Results and discussion
Pump
(b)
201
Signal
1c0~~er
MFC:
Mass Flow Controller PressureGauge HSv: High Speed valve PCv: Pressure Control valve FIG: Fast Ionization Gauge
PG:
Fig. 1. Schematic illustrations of the short-pulse chemical beam epitaxy system (a) and the time-of-flight measurement setup (b).
closed. At the time of growth the background pressure was in the i0~ Torr range, depending on the AsH3 flow rate and TMG beam intensity. To measure the temporal distribution of the generated pulses, a time-of-flight (TOF) technique was used. On the TOF setup (fig. ib), the high speed valve itself was employed as the pulse gate, while a fast ionization gauge with a response time of 3 ~s placed at the position of the substrate was used as the detector. The gauge head has an open fly-through structure of Ba yard—Alpert type, and permits good linearity up to high pressures in the 10_2 Torr range. The output signal, which is proportional to the molecular density, and the logic signal of valve operation were recorded by a transient converter,
shown in fig. 2, where the beam intensity was measured in an equivalent pressure as scaled by a vertical bar. As the time reference, the logic signal (rectangular line) of the valve operation is also given in the same figure. The separation between the TMG pulse and the signal of valve
action on the horizontal axis corresponds to the time of molecular travel from the valve to the head of the ionization gauge, where the substrate was placed at the time of growth. The decrease of the flight time with increase of source pressure shows that the molecular kinetic energy was increased. Since the pressure at the outlet of the
Source Pres. (Torr)
a) -~
QTorr~
0..
0
100 >‘
-~
E -_____
-10
0
10
20
30
40
Time (ms) Fig. 2. Time-of-flight traces of TMG pulses. The rectangular line shows the logic signal of the valve operation.
202
S. Zhang et al.
/ Short-pulse chemical beam epitaxy
Source Pres
Source Pres.=5OTorr
a)
p
~
~2x10~
Torr
0 I-
k~orr ___
-a a)
.8 C E (‘a a)
E
‘i
a)
0 -10
0
10
20
30
50
100
Time (ms)
high-speed valve was kept very low, the Mach number of the supersonic jet increased as the source pressure was raised from 25 to 100 Torr, based on the principle of gas dynamics. This caused the modulation of molecular kinetic energy obtained in fig. 2. At the same time, the higher Mach number, which determines a narrower velocity distribution, led to the formation of a shorter pulse at a higher source pressure. By a simple calculation of intensity integral for the TMG pulses, it can be revealed that the total number of molecules in each pulse is exactly proportional to the source pressure. Fig. 3 shows TMG pulses obtained with on-time of 2, 5 and 10 ms, respectively. As predicted, the intensity integral of the TMG pulse was found to be proportional to the valve on-time. Furthermore, generation of a series of well shaped short-pulses was confirmed, as shown in fig. 4, where TMG was injected in a frequency as high as 10 Hz. This indicates that exact control of the molecular supply can be made by simply counting the number of pulse injections. Using the short pulses of chemical beams, we have grown GaAs at different conditions. A quartz plate with several holes was inserted on the line determined by the high speed valve and the substrate. As shown in fig. 5, the RHEED intensity
200
250
300
Time (ms)
40
Fig. 3. TMG pulse (dotted line) generated with valve on-time of 2, 5 and 10 ms, respectively, as marked by the corresponding logic signal (rectangular lines) of the valve operation.
150
Fig. 4. TMG pulse train (dotted line) generated at 10 Hz. The rectangular line shows the corresponding logic signal of the valve operation.
changes with the increase of the TMG pulse width. This indicates that the precise supply of TMG molecules and the precise control of growth
Valve Action
2OkeV 5Oms /
/
~
4Oms
.~
3Oms /
0 W w
2Oms AsH3=1 sccm
cc
.
1 Oms
T
5ms
-
500
=580°C sub
-.
___________________
LJT
0
I
I
I
500
1000
1500
,
Time (ms) Fig. 5. RHEED specular beam intensity observed after feeding a single pulse of TMG in width of 5 to 100 ms, respectively.
S. Zhang et al.
/ Short-pulse chemical beam epitaxy
rate were realized. Concerning the physical explanations, the details are under further investigation and will be published elsewhere [31.At the present stage, it is believed that the modulated TMG adsorption and desorption, the related chemical reactions, the TMG decomposition, the surface migration, the nucleation, and the phase transition between the surface reconstructions are the major factors involved there. Considering the importance of adsorption and desorption controls as well as surface migration and chemical reaction controls in many epitaxial growth techniques, the use of short-pulse chemical beams is quite promising.
4. Summary and conclusion Experimental study on the short-pulse chemical beams for epitaxial growth has been made by using a time-of-flight technique. The results mdicate that control of the pulse width and the molecular kinetic energy was obtained. Moreover lt was found that the molecular supply can be accurately controlled over a wide range by changing the source pressure, the valve on-time and the
203
number of pulse injections. Using TMG shortpulses, epitaxial growth of GaAs was carried out. The obtained results suggest that the use of short-pulse chemical beams has a great potential for improving the controllability of epitaxy. It is also believed that the short pulse technique can serve as a new tool to deepen the understanding of the growth mechanism.
Acknowledgments This work was partially supported by a Special Grant for Promotion of Research from The Institute of Physical and Chemical Research (RIKEN). The authors would like to thank Dr. S. Iwai, Dr. K. Ozasa and Dr. T. Meguro for useful discussions.
References [1] A.
Kantrowitz and J. Grey, Rev. Sci. Instr. 22 (1951) 328. [2] N. Abuaf, J.B. Anderson, R.P. Andres, J.B. Fenn and D.G.H. Marsden Science 155 (1967) 997. [31S. Zhang, J. Cui, A. Tanaka and Y. Aoyagi, to be published.
J ~
CRYSTAL GROW T H
o~
Short-pulse chemical beam epitaxy Suian Zhang
a
Jie Cui
a
Akihiko Tanaka
b
and Yoshinobu Aoyagi
a
RIKEN, The Institute of Physical and Chemical Research, Hirosawa 2-1, Wako-shi, Saitama 351-0], Japan ~ Bentec Co., Sakae-cho 6-1, Tachikawa-shi, Tokyo 190, Japan
Short-pulse chemical beam epitaxy has been proposed and studied. The short pulses with supersonic characteristics and a width of milliseconds were generated by high speed valves and the related pumping lines on a purpose-built CBE system. Using a time-of-fight technique, we verified the dependence of pulse properties on the source pressures and the valve on-time. The results indicate that modulation of molecular kinetic energy and accurate control of molecule supply were obtained. GaAs epitaxial growth with use of trimethylgallium pulses was carried out and investigated by means of RHEED (reflection high-energy electron diffraction) observation. It was demonstrated that the newly developed short-pulse chemical beam epitaxy has the advantage of high controllability.
1. Introduction As is well known, in most of the conventional CBE (chemical beam epitaxy) and MBE (molecular beam epitaxy) systems, beam switching is made by using mechanical shutters and mechanical valves, whose response times are typically around 0.1 s or longer. Needless to say, further improvement on the speed of beam switching is required for advanced applications to fabrications of nanostructures such as vertical superlattices and quantum wires. In the present research, we propose to use short-pulse chemical beams to improve the controllability of molecule supply as well as surface migration and chemical reactions, based on the views described below. Firstly, by using short pulses with response times shorter than 1 ms, it is expected that the controllability on the molecule number can be significantly improved. Secondly, according to gas dynamics [1], the kinetic energy of molecules in a beam extracted from a supersonic jet can be modulated in a range up to. several electron volts when a mixture of source gas and light molecular gas is used [21. Thirdly, use of short pulses of chemical beam allows us to manipulate surface reactions at different instants while keeping the vacuum of growth chamber at a high level. In this paper we
report experimental investigations on the shortpulse properties and the application to epitaxial growth of GaAs.
2. Experimental procedure A purpose-oriented CBE system as shown in fig. la was built for the present study. For the sake of clarity, only one line of the gas handling system is sketched in the figure. To grow GaAs, AsH3 (100%) and trimethylgallium (TMG) were respectively used as arsenic and gallium sources. The AsH3 gas was cracked to As2 at 1100°C.To generate a pulsed TMG beam, a high-speed valve with a 0.8 mm diameter orifice was installed 350 mm away from the substrate. The source pressure at the inlet of the high speed valve was controlled by a mass-flow controller and a pressure control valve linked to the corresponding absolute pressure gauge. To ensure an ideal shape of the TMG pulse, a high-vacuum differential pump was set at the outlet of the high-speed valve. The high vacuum of the growth chamber was maintained by using a high-capacity turbomolecular pump and a standard liquid-nitrogen shroud. The base pressure of the growth chamber was in the 10_In Torr range when the gas introduction valves were
0022-0248/94/$07.00 © 1994 — Elsevier Science B.V. All rights reserved SSDI 0022-0248(93)E0292-F
S. Zhang ci al. (a)
Pump
/ Short-pulse chemical beam epitaxy
GaAs epitaxial layers were grown on (001)
UHV pump
pcv MFC
HSV
Chamber
UHV
~pump
Trigger
_rT*1
II
growing surface.
~orRHEED
UHV pump Chamber
~cv
~
~
uHv
FIG head
driver
Trigge
I
Computer
L~.i
FIG
I
control unit
_______
Trigger
Transient
The short pulses of TMG generated with
source pressures of 25, 50 and 100 Torr are
pump
Hsv
vapor
semi-insulating substrates treatedThe bygrowth a standard cleaning and etching procedure. ternperature was measured using an infrared pyrometer. Throughout the growth, a RHEED (reflection high-energy electron diffraction) optical system withangle incident a primary of 2°energy was used of 20 to keV monitor and the an
3. Results and discussion
Pump
(b)
201
Signal
1c0~~er
MFC:
Mass Flow Controller PressureGauge HSv: High Speed valve PCv: Pressure Control valve FIG: Fast Ionization Gauge
PG:
Fig. 1. Schematic illustrations of the short-pulse chemical beam epitaxy system (a) and the time-of-flight measurement setup (b).
closed. At the time of growth the background pressure was in the i0~ Torr range, depending on the AsH3 flow rate and TMG beam intensity. To measure the temporal distribution of the generated pulses, a time-of-flight (TOF) technique was used. On the TOF setup (fig. ib), the high speed valve itself was employed as the pulse gate, while a fast ionization gauge with a response time of 3 ~s placed at the position of the substrate was used as the detector. The gauge head has an open fly-through structure of Ba yard—Alpert type, and permits good linearity up to high pressures in the 10_2 Torr range. The output signal, which is proportional to the molecular density, and the logic signal of valve operation were recorded by a transient converter,
shown in fig. 2, where the beam intensity was measured in an equivalent pressure as scaled by a vertical bar. As the time reference, the logic signal (rectangular line) of the valve operation is also given in the same figure. The separation between the TMG pulse and the signal of valve
action on the horizontal axis corresponds to the time of molecular travel from the valve to the head of the ionization gauge, where the substrate was placed at the time of growth. The decrease of the flight time with increase of source pressure shows that the molecular kinetic energy was increased. Since the pressure at the outlet of the
Source Pres. (Torr)
a) -~
QTorr~
0..
0
100 >‘
-~
E -_____
-10
0
10
20
30
40
Time (ms) Fig. 2. Time-of-flight traces of TMG pulses. The rectangular line shows the logic signal of the valve operation.
202
S. Zhang et al.
/ Short-pulse chemical beam epitaxy
Source Pres
Source Pres.=5OTorr
a)
p
~
~2x10~
Torr
0 I-
k~orr ___
-a a)
.8 C E (‘a a)
E
‘i
a)
0 -10
0
10
20
30
50
100
Time (ms)
high-speed valve was kept very low, the Mach number of the supersonic jet increased as the source pressure was raised from 25 to 100 Torr, based on the principle of gas dynamics. This caused the modulation of molecular kinetic energy obtained in fig. 2. At the same time, the higher Mach number, which determines a narrower velocity distribution, led to the formation of a shorter pulse at a higher source pressure. By a simple calculation of intensity integral for the TMG pulses, it can be revealed that the total number of molecules in each pulse is exactly proportional to the source pressure. Fig. 3 shows TMG pulses obtained with on-time of 2, 5 and 10 ms, respectively. As predicted, the intensity integral of the TMG pulse was found to be proportional to the valve on-time. Furthermore, generation of a series of well shaped short-pulses was confirmed, as shown in fig. 4, where TMG was injected in a frequency as high as 10 Hz. This indicates that exact control of the molecular supply can be made by simply counting the number of pulse injections. Using the short pulses of chemical beams, we have grown GaAs at different conditions. A quartz plate with several holes was inserted on the line determined by the high speed valve and the substrate. As shown in fig. 5, the RHEED intensity
200
250
300
Time (ms)
40
Fig. 3. TMG pulse (dotted line) generated with valve on-time of 2, 5 and 10 ms, respectively, as marked by the corresponding logic signal (rectangular lines) of the valve operation.
150
Fig. 4. TMG pulse train (dotted line) generated at 10 Hz. The rectangular line shows the corresponding logic signal of the valve operation.
changes with the increase of the TMG pulse width. This indicates that the precise supply of TMG molecules and the precise control of growth
Valve Action
2OkeV 5Oms /
/
~
4Oms
.~
3Oms /
0 W w
2Oms AsH3=1 sccm
cc
.
1 Oms
T
5ms
-
500
=580°C sub
-.
___________________
LJT
0
I
I
I
500
1000
1500
,
Time (ms) Fig. 5. RHEED specular beam intensity observed after feeding a single pulse of TMG in width of 5 to 100 ms, respectively.
S. Zhang et al.
/ Short-pulse chemical beam epitaxy
rate were realized. Concerning the physical explanations, the details are under further investigation and will be published elsewhere [31.At the present stage, it is believed that the modulated TMG adsorption and desorption, the related chemical reactions, the TMG decomposition, the surface migration, the nucleation, and the phase transition between the surface reconstructions are the major factors involved there. Considering the importance of adsorption and desorption controls as well as surface migration and chemical reaction controls in many epitaxial growth techniques, the use of short-pulse chemical beams is quite promising.
4. Summary and conclusion Experimental study on the short-pulse chemical beams for epitaxial growth has been made by using a time-of-flight technique. The results mdicate that control of the pulse width and the molecular kinetic energy was obtained. Moreover lt was found that the molecular supply can be accurately controlled over a wide range by changing the source pressure, the valve on-time and the
203
number of pulse injections. Using TMG shortpulses, epitaxial growth of GaAs was carried out. The obtained results suggest that the use of short-pulse chemical beams has a great potential for improving the controllability of epitaxy. It is also believed that the short pulse technique can serve as a new tool to deepen the understanding of the growth mechanism.
Acknowledgments This work was partially supported by a Special Grant for Promotion of Research from The Institute of Physical and Chemical Research (RIKEN). The authors would like to thank Dr. S. Iwai, Dr. K. Ozasa and Dr. T. Meguro for useful discussions.
References [1] A.
Kantrowitz and J. Grey, Rev. Sci. Instr. 22 (1951) 328. [2] N. Abuaf, J.B. Anderson, R.P. Andres, J.B. Fenn and D.G.H. Marsden Science 155 (1967) 997. [31S. Zhang, J. Cui, A. Tanaka and Y. Aoyagi, to be published.