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A dual ion beam epitaxy system
H5H B
Nuclear Instruments and Methods in Physics Research B70 (1992) 579-582 North-Holland
Beam Interactions with Materials & Atoms
A dual ion beam epitaxy system Su Shijun and Jiang Weisheng
China Institute of Atomic Energy, Beijing, China
Qin Fuguang and Wang Xiangming
Institute of Semiconductors, Academia Sinica, Beijing, China
The design and characteristics of a dual ion beam epitaxy system (DIBE) are discussed. This system is composed of two beam lines, each providing a mass-separated ion beam converging finally with the other into the target chamber . The ions are decelerated and deposited on a substrate which can be heated to a temperature of 800°C. Currents of a few hundred microamp6res are available for both beams and the deposit energies are in the range from tens to 1000 eV . The pressure of the target chamber during processing is about 7 x 10-R Pa. Preliminary experiments have proved that compound semiconductor materials such as GaN can be synthesized using the DIBE system. 1 . Introduction A dual ion beam epitaxy system has been built at the Institute of Semiconductors, Academia Sinica, to study new semiconductor materials and multi-layer film structures . Mass-separated low energy ion beam deposition technology has the character of epitaxy at low temperatures and of material purification for the exploration * Supported partially by the National Natural Science Foundation of China .
and research of new materials. We have developed a dual ion beam epitaxy system (DIBE), of which the function is substantially expanded compared with conventional single-beam systems. In this paper the design and characteristics of the DIBE are presented .
2. Design and characteristics of the DIBE A higher epitaxial growth rate is necessary for thin film deposition using ion beams, and therefore higher Mass analyzer
Fig. 1 . Schematic diagram of the DIBE. 0168-583X/92/$05 .00 0 1992 - Elsevier Science Publishers B.V . All rights reserved
IX . RELATED TECHNIQUES
580
Su Shijun et al. /A dual ion beam epitaxy system
intensities of ion beam currents are required, for instance a few dozen microampères at least. The energy of the decelerated ions should be as low as possible to reduce self-sputtering and lattice damage of deposited films . On the other hand, the purity of the materials is an important characteristic of the system . According to the discussion above, a low energy double ion beam epitaxy system (DIBE) has been designed and manufactured. The layout of the system is shown in fag . 1. This system is composed of two beam lines . In each beam line the ion beam is focused twice and deflected by an electrostatic deflector, then deposited on the substrate with a certain energy after passing through the deceleration electrode system . A Freeman-type ion source is used, which can produce ions of most elements in the periodic table. The extracted beam current can reach more than 10 mA, while the accelerating voltage is 20-30 kV. In the DIBE, both beam lines have two stages . The first stage is a mass analyzer with a 90° symmetric focusing dipole magnet, which is the same for both beam lines . The second stage of one beam line is a magnetic quadrupole doublet (MO), and the other one is an electric quadrupole doublet (EQ) . Each of these takes the image of the first stage as the object, and forms a quasi-square beam spot on the substrate. Another reason for dividing the beam line into two stages is to provide the possibility of differential vacuum pumping, so as to obtain ultrahigh vacuum in the target chamber . The electrostatic deflectors are used to reduce the tingle and throw off the high energy neutral particles produced in the charge exchange process. The beam trajectories are calculated with the program TRANSPORT. The parameters of these two beam lines and of the beam envelopes are given in ref. [1] .
â T 'ïo
w
C N C
400
300
3. Experimental results As a performance test of this new equipment, preliminary research on preparing epitaxial films has been carried out.
cd 200 m
3.1. Single beam experiment
100 01
Compared with a single beam, deceleration of two assembled ion beams is more complex, as the incident angles in the transverse direction are larger . In such a case, the component velocities of the decelerated ions will cause a large number of ions to return. Besides, in order to reduce the effect of space charge, the distance within which the ion energy is reduced from 20-30 keV to about 100 eV is very short, and therefore high voltage sparking occurs easily. Ion trajectories in the deceleration area are calculated under the condition of considerable space charge [2]. To ensure that the film is deposited uniformly, the substrate holder can be oscillated with low speed in the transverse direction. The vacuum pumping system is an important part of DIBE. The differential pumping method is used, in which the vacuum is increased step by step from the ion source to the target chamber. When the two ion beams deposit on the substrate, the pressure in the chamber is about 7 x 10 -6 Pa. Before the analyzing slits, oil-diffusion pumps are used ; after the slits, oilfree pumps, such as turbomolecular pumps, sputter ion pumps and titanium sublimation pumps are used . On the earthed substrate, approximating a few hundred microampères of argon ion beam can be obtained, and the spot is 1-2 cm' in area. The transmission coefficient from the ion source to the substrate is about 50-70% . Deceleration experiments were carried out. The relationship between the deposit energy and argon ion beam intensity on the substrate is shown in fig. 2. It can be seen that the lower the energy of the decelerated beam, the lower the beam intensity on the substrate. The reason for this is that the transverse velocity of the ions causes the ions to return when the energy is lower. When the deposit energy is 100 eV, the beam intensity of argots ions on the substrate is about 130 WA for the EQ beam line, and 200 pA for the MQ beam line . For other elements, as there are many isotopes, the beam intensity can reach a few dozens to 100 wA .
100
300 400 200 Deposit energy (ev)
Fig. 2. The relationship between the deposit energy and the argonionbeam intensity on the substrate .
In this experiment polished p-SiQII) wafers were used as the substrates . After a routine cleaning treatment the substrate was transferred into the ultrahigh vacuum target chamber. Before film deposition the substrate was bombarded by a Cl' beam with an energy of 300 eV for 5 minwhile keeping the substrate
Su Shijun et al. /A dual ion beam epitaxy system
Table 1 The influence of different substrate temperatures on the growingfilm structure for the epitaxy of Ge on Sî(111) 74 G + 74 Ge + 74 Ge + ]on Energy [eV] 78 78 78 Current [WA] 60 60 85 Target chamber vacuum [Pal 1 .2x 10 -° 1 .2x 10 -4 1 .2x 10 -4 Substrate temperature 350 423 450 10C] Film structure monocrystal monocrystal monocrystal +twins temperature at about 600°C, and then film deposition with a 74Ge+ ion beam began. The 74Ge+ ion beam current density was about 90 H.A/cm2. The energy of 74Ge+ ions arriving at the substrate was about 80 eV, and the substrate temperature was kept constant . It was changed, however, for different experiments to observe the influence of substrate temperature on the crystallographic structures of the deposited films under examination. The experimental results are summarized in table 1. A RHEED pattern of such a film is shown in fig. 3. The epitaxial growth, even though there are some twins in the film, can be found at substrate temperatures as low as 300°C, and the film structure tends to become more perfect with substrate temperatures above 400°C. 3.2. Double beam experiment To synthesize the compound material GaN, Ga + and N+ ions were extracted at an energy of 25 keV from two independent ion sources with Ga 203 + CCI4 and N2 as feed materials, respectively. Both 69Ga + and 14N+ beams were decelerated and finally converged onto the substrate surface to form the compound film. During the film deposition the substrate was vibrated
Fig. 3. RHEED pattern of the epitaxial Ge film on Sî(111) grownat a substrate temperature of 300°C.
58 1
Fig. 4. RHEED pattern of the epitaxial GaN film on Sî(111). horizontally by the manipulator to ensure the homogeneity of the grown films . Polished Sî(111) and sapphire (1102) wafers have been used as substrates . Before transfer into the target chamber, the Si wafers were cleaned by routine procedures. The sapphire wafers were pre-treated, which included boiling in a solution of H3 PO4 +H 2 SO3 (1 :3) and cleaning by routine procedures. At the beginning of film deposition both beams were adjusted to their optimum conditions resulting in an ion beam current of about 100 p,A for the 69Ga + beam and 150 wA for the t4N+ beam. During film deposition the substrate temperature was kept at approximately 600°C. The substrate vibration rate was about 10 times/min at an amplitude of t10 mm, and the film deposition rate was about 2.5 .&/min . The deposition process lasted two hours, and finally a grown GaN film of about 300 A thick deposited on an area of 4 cm2 was obtained. It should be noted that when sapphire substrates were used, a special electron gun was placed adjacent to the substrate so that during film deposition, the positive charge accumulated on the insulating substrate surface could be neutralized by the electrons supplied by the electron gun. Figs. 4 and 5 show the RHEED patterns of the epitaxial GaN film deposited on Sî(111)
Fig. 5. RHEED pattern of the epitaxial GaN film on sapphire (1102). IX. RELATEDTECHNIQUES
582
Su Shijun et al. / A dual ion beam epitaxy system
and sapphire (1102), respectively . It has been shown that most of the films deposited on sapphire are monocrystalline but with some defects, while the films deposited on Si are much more perfect . Auger electron spectroscopy of the epitaxial films indicates that the obtained GaN films are in good stoichiometry .
for film deposition, especially, the dual-beam structure can be used to do research works better than a singlebeam system.
4. Conclusion
[11 Mao Naifeng et al ., Atomic Energy Science and Technology 20 (1986) 298, in Chinese. [21 Fang Jin-ging and Hua Da-ping, Commun. i n Theor. Phys. (Beijing) 2 (6) (1983) 1505 .
The preliminary experiments show that DIBE constructed in our laboratory has many good performances
References
Nuclear Instruments and Methods in Physics Research B70 (1992) 579-582 North-Holland
Beam Interactions with Materials & Atoms
A dual ion beam epitaxy system Su Shijun and Jiang Weisheng
China Institute of Atomic Energy, Beijing, China
Qin Fuguang and Wang Xiangming
Institute of Semiconductors, Academia Sinica, Beijing, China
The design and characteristics of a dual ion beam epitaxy system (DIBE) are discussed. This system is composed of two beam lines, each providing a mass-separated ion beam converging finally with the other into the target chamber . The ions are decelerated and deposited on a substrate which can be heated to a temperature of 800°C. Currents of a few hundred microamp6res are available for both beams and the deposit energies are in the range from tens to 1000 eV . The pressure of the target chamber during processing is about 7 x 10-R Pa. Preliminary experiments have proved that compound semiconductor materials such as GaN can be synthesized using the DIBE system. 1 . Introduction A dual ion beam epitaxy system has been built at the Institute of Semiconductors, Academia Sinica, to study new semiconductor materials and multi-layer film structures . Mass-separated low energy ion beam deposition technology has the character of epitaxy at low temperatures and of material purification for the exploration * Supported partially by the National Natural Science Foundation of China .
and research of new materials. We have developed a dual ion beam epitaxy system (DIBE), of which the function is substantially expanded compared with conventional single-beam systems. In this paper the design and characteristics of the DIBE are presented .
2. Design and characteristics of the DIBE A higher epitaxial growth rate is necessary for thin film deposition using ion beams, and therefore higher Mass analyzer
Fig. 1 . Schematic diagram of the DIBE. 0168-583X/92/$05 .00 0 1992 - Elsevier Science Publishers B.V . All rights reserved
IX . RELATED TECHNIQUES
580
Su Shijun et al. /A dual ion beam epitaxy system
intensities of ion beam currents are required, for instance a few dozen microampères at least. The energy of the decelerated ions should be as low as possible to reduce self-sputtering and lattice damage of deposited films . On the other hand, the purity of the materials is an important characteristic of the system . According to the discussion above, a low energy double ion beam epitaxy system (DIBE) has been designed and manufactured. The layout of the system is shown in fag . 1. This system is composed of two beam lines . In each beam line the ion beam is focused twice and deflected by an electrostatic deflector, then deposited on the substrate with a certain energy after passing through the deceleration electrode system . A Freeman-type ion source is used, which can produce ions of most elements in the periodic table. The extracted beam current can reach more than 10 mA, while the accelerating voltage is 20-30 kV. In the DIBE, both beam lines have two stages . The first stage is a mass analyzer with a 90° symmetric focusing dipole magnet, which is the same for both beam lines . The second stage of one beam line is a magnetic quadrupole doublet (MO), and the other one is an electric quadrupole doublet (EQ) . Each of these takes the image of the first stage as the object, and forms a quasi-square beam spot on the substrate. Another reason for dividing the beam line into two stages is to provide the possibility of differential vacuum pumping, so as to obtain ultrahigh vacuum in the target chamber . The electrostatic deflectors are used to reduce the tingle and throw off the high energy neutral particles produced in the charge exchange process. The beam trajectories are calculated with the program TRANSPORT. The parameters of these two beam lines and of the beam envelopes are given in ref. [1] .
â T 'ïo
w
C N C
400
300
3. Experimental results As a performance test of this new equipment, preliminary research on preparing epitaxial films has been carried out.
cd 200 m
3.1. Single beam experiment
100 01
Compared with a single beam, deceleration of two assembled ion beams is more complex, as the incident angles in the transverse direction are larger . In such a case, the component velocities of the decelerated ions will cause a large number of ions to return. Besides, in order to reduce the effect of space charge, the distance within which the ion energy is reduced from 20-30 keV to about 100 eV is very short, and therefore high voltage sparking occurs easily. Ion trajectories in the deceleration area are calculated under the condition of considerable space charge [2]. To ensure that the film is deposited uniformly, the substrate holder can be oscillated with low speed in the transverse direction. The vacuum pumping system is an important part of DIBE. The differential pumping method is used, in which the vacuum is increased step by step from the ion source to the target chamber. When the two ion beams deposit on the substrate, the pressure in the chamber is about 7 x 10 -6 Pa. Before the analyzing slits, oil-diffusion pumps are used ; after the slits, oilfree pumps, such as turbomolecular pumps, sputter ion pumps and titanium sublimation pumps are used . On the earthed substrate, approximating a few hundred microampères of argon ion beam can be obtained, and the spot is 1-2 cm' in area. The transmission coefficient from the ion source to the substrate is about 50-70% . Deceleration experiments were carried out. The relationship between the deposit energy and argon ion beam intensity on the substrate is shown in fig. 2. It can be seen that the lower the energy of the decelerated beam, the lower the beam intensity on the substrate. The reason for this is that the transverse velocity of the ions causes the ions to return when the energy is lower. When the deposit energy is 100 eV, the beam intensity of argots ions on the substrate is about 130 WA for the EQ beam line, and 200 pA for the MQ beam line . For other elements, as there are many isotopes, the beam intensity can reach a few dozens to 100 wA .
100
300 400 200 Deposit energy (ev)
Fig. 2. The relationship between the deposit energy and the argonionbeam intensity on the substrate .
In this experiment polished p-SiQII) wafers were used as the substrates . After a routine cleaning treatment the substrate was transferred into the ultrahigh vacuum target chamber. Before film deposition the substrate was bombarded by a Cl' beam with an energy of 300 eV for 5 minwhile keeping the substrate
Su Shijun et al. /A dual ion beam epitaxy system
Table 1 The influence of different substrate temperatures on the growingfilm structure for the epitaxy of Ge on Sî(111) 74 G + 74 Ge + 74 Ge + ]on Energy [eV] 78 78 78 Current [WA] 60 60 85 Target chamber vacuum [Pal 1 .2x 10 -° 1 .2x 10 -4 1 .2x 10 -4 Substrate temperature 350 423 450 10C] Film structure monocrystal monocrystal monocrystal +twins temperature at about 600°C, and then film deposition with a 74Ge+ ion beam began. The 74Ge+ ion beam current density was about 90 H.A/cm2. The energy of 74Ge+ ions arriving at the substrate was about 80 eV, and the substrate temperature was kept constant . It was changed, however, for different experiments to observe the influence of substrate temperature on the crystallographic structures of the deposited films under examination. The experimental results are summarized in table 1. A RHEED pattern of such a film is shown in fig. 3. The epitaxial growth, even though there are some twins in the film, can be found at substrate temperatures as low as 300°C, and the film structure tends to become more perfect with substrate temperatures above 400°C. 3.2. Double beam experiment To synthesize the compound material GaN, Ga + and N+ ions were extracted at an energy of 25 keV from two independent ion sources with Ga 203 + CCI4 and N2 as feed materials, respectively. Both 69Ga + and 14N+ beams were decelerated and finally converged onto the substrate surface to form the compound film. During the film deposition the substrate was vibrated
Fig. 3. RHEED pattern of the epitaxial Ge film on Sî(111) grownat a substrate temperature of 300°C.
58 1
Fig. 4. RHEED pattern of the epitaxial GaN film on Sî(111). horizontally by the manipulator to ensure the homogeneity of the grown films . Polished Sî(111) and sapphire (1102) wafers have been used as substrates . Before transfer into the target chamber, the Si wafers were cleaned by routine procedures. The sapphire wafers were pre-treated, which included boiling in a solution of H3 PO4 +H 2 SO3 (1 :3) and cleaning by routine procedures. At the beginning of film deposition both beams were adjusted to their optimum conditions resulting in an ion beam current of about 100 p,A for the 69Ga + beam and 150 wA for the t4N+ beam. During film deposition the substrate temperature was kept at approximately 600°C. The substrate vibration rate was about 10 times/min at an amplitude of t10 mm, and the film deposition rate was about 2.5 .&/min . The deposition process lasted two hours, and finally a grown GaN film of about 300 A thick deposited on an area of 4 cm2 was obtained. It should be noted that when sapphire substrates were used, a special electron gun was placed adjacent to the substrate so that during film deposition, the positive charge accumulated on the insulating substrate surface could be neutralized by the electrons supplied by the electron gun. Figs. 4 and 5 show the RHEED patterns of the epitaxial GaN film deposited on Sî(111)
Fig. 5. RHEED pattern of the epitaxial GaN film on sapphire (1102). IX. RELATEDTECHNIQUES
582
Su Shijun et al. / A dual ion beam epitaxy system
and sapphire (1102), respectively . It has been shown that most of the films deposited on sapphire are monocrystalline but with some defects, while the films deposited on Si are much more perfect . Auger electron spectroscopy of the epitaxial films indicates that the obtained GaN films are in good stoichiometry .
for film deposition, especially, the dual-beam structure can be used to do research works better than a singlebeam system.
4. Conclusion
[11 Mao Naifeng et al ., Atomic Energy Science and Technology 20 (1986) 298, in Chinese. [21 Fang Jin-ging and Hua Da-ping, Commun. i n Theor. Phys. (Beijing) 2 (6) (1983) 1505 .
The preliminary experiments show that DIBE constructed in our laboratory has many good performances
References