JPH0714765A - Radical cell and molecular beam epitaxy system - Google Patents

Abstract

PURPOSE:To grow a high quality crystalline thin film of ZnSe by providing a radical cell for producing radical beam of nitrogen with a mesh electrode and applying a bias voltage thereto when a ZnSe crystal thin film is grown by molecular beam epitaxy using nitrogen as a p-type dopant. CONSTITUTION:A radical cell 8 comprises a crucible 18, a high frequency coil 19, and a shield 20 wherein high frequency current is fed to the high frequency coil 19 in order to generate plasma of nitrogen gas. The material of radical, i.e., nitrogen gas, is introduced from a gas cylinder to the crucible 18 through an introduction pipe 21 coupled with the crucible 18 at the bottom thereof. Furthermore, a mesh electrode 28 fixed with a metal mesh at the tip of a metal rod is disposed at an opening of the crucible 18 and a voltage is applied from a bias power supply 29 through the metal rod. Consequently, positive ions of nitrogen are braked and the nitrogen, otherwise the colliding against a substrate at high speed, is decelerated. This constitution suppresses the possibility of disturbing the crystal structure of ZnSe growing on the substrate.

Description

Detailed Description of the Invention

[0001]

This invention relates to molecular beam epitaxy.
The present invention relates to a radical cell capable of converting a raw material into plasma and introducing it into a chamber. The molecular beam epitaxy method is a method in which a substrate is provided in an ultrahigh vacuum chamber, several kinds of raw materials are heated to form molecular beams, and the substrate is irradiated with the raw material compound to form a thin film of the raw material compound. is there. Several molecular beam cells are provided along the peripheral wall of the chamber. This consists of a crucible, a heater, a reflector, a thermocouple, and a support. One molecular beam cell is provided for each raw material. The ultra-high vacuum lengthens the mean free path of gas atoms, and goes straight from the molecular beam cell to reach the substrate. A compound semiconductor thin film such as GaAs can be formed on a suitable substrate. When doping impurities, a molecular beam cell containing the element is provided. The molecular beam epitaxy method has an advantage that a device having a fine structure can be manufactured with good reproducibility because the supply of raw materials can be strictly controlled.

[0002]

2. Description of the Related Art The materials described above are solid materials. When the raw material is gas, it is not a closed cell like a molecular beam cell, but a gas source cell, which is a cell connected to an external gas cylinder.
A heater, a reflector, or the like is provided in the opening of the cell. A baffle plate may also be provided to limit the flow of gas. In any case, the raw material becomes a molecular beam because it is evaporated by heating. Although it is a molecular beam, it is in the ground state as an atom or molecule. Only energy has kinetic energy. Chemical activity is low.

Various blue light emitting elements have been considered. The material is a semiconductor having a wide band gap and a direct transition type. Moreover, a pn junction must be formed. However, there is no material that satisfies such conditions.

ZnSe has been studied for a long time as a material for a blue light emitting device. This has a wide band gap and is suitable for the condition of direct transition type. However, there were many difficulties, and a blue laser of ZnSe could not be realized. First, it is difficult to make a bulk ZnSe crystal. Since this sublimes at high temperature, high pressure must be applied to grow crystals.
Bulk crystals cannot be manufactured by the conventional Czochralski method or Bridgman method. Since ZnSe substrate cannot be used, G
Heteroepitaxy is often performed using aAs as a substrate. A ZnSe thin film is epitaxially grown on the substrate.

However, it is difficult to control the conductivity type. To make a pn junction, a p-type thin film and an n-type thin film are required. Both types are difficult, but p-type ZnSe is particularly difficult to make.
N-type crystals are obtained by doping n-type impurities. Conventional ZnSe n-type impurities include Al and G
a, In, etc. were known. However, when the amount of doping is increased, the n-type carrier density is rather lowered, and 10 ¹⁷ c
m ^-3 was the maximum n-type carrier density.

P-type doping is more difficult. Nishizawa et al.
It was announced that a p-type ZnSe crystal doped with i was manufactured by a vapor pressure controlled temperature difference method. J.Nishizawa, K.Itoh, Y.Okuno
and F. Sakurai: J. Appl. Phys. 57, 2210 (1985) However, the production of Li-doped ZnSe is difficult and poor in reproducibility. Further, the carrier density of p-type impurities is low. 8 x 10
^It does not exceed ¹⁶ cm ^-3 . In addition, Li easily diffuses in ZnSe, and the characteristics of the pn junction are not constant.

Therefore, chlorine has come to be used as an n-type impurity. This is made from zinc chloride (ZnCl ₂ ). Chlorine doped ZnSe is a molecular beam epitaxy
It can also be made by law. Zn, Se,
The ZnCl ₂ molecular beam cell is irradiated at the same time. K.
Ohkawa, T.Mitsuyu and O.Yamazaki: Ext.Abstr.18th Con
f. Solid State Devices and Materials, Tokyo, 1986, p6
When 35 chlorine is used as an n-type impurity, a carrier density of about 10 ¹⁹ cm ^-3 can be realized. It is improved by almost an order of magnitude over the Ga-dope case.

Nitrogen has been investigated as a p-type impurity. However, it is difficult for nitrogen to enter the ZnSe crystal.
Although various attempts have been made to dope nitrogen into ZnSe by liquid phase epitaxy, MOCVD, MBE, etc., none of them could introduce nitrogen into ZnSe. Nitrogen did not enter ZnSe and did not become a p-type impurity.

However, Okawa et al. Proposed a radical doping method in which nitrogen is used as a radical to dope ZnSe. The inventor has invented a method in which nitrogen gas is turned into active nitrogen by electric discharge and is applied to a substrate together with a molecular beam of Zn and a molecular beam of Se as a molecular beam. K. Ohkawa, T. Karasawa and T. Mi
tsuyu: 6th Int.Conf.Molecular Beam Epitaxy, San Dieg
o, 1990, PIII-21, J.Cryst.Growth 117,375 (1991) K.Ohkawa, A.Tsujimura, S.Hayashi, S.Yoshii and T.Mit
suyu: Ext.Abstr.1992 Int.Conf. Solid State Devices
and Materials, Tsukuba, 1992 p. 330 Since these authors do not show the structure of the molecular beam epitaxy device, it is not clear what kind of device was used. However, it is presumed that it may be something like FIG.

FIG. 3 is a schematic view showing a molecular beam epitaxy apparatus based on the inventor's imagination and provided with a radical cell. A liquid nitrogen shroud 2 is provided on the inner wall surface of the ultra-high vacuum chamber 1. It is connected to another vacuum chamber by a connecting pipe 3. The other vacuum chamber and the chamber 1 are separated by a gate valve 4. The transfer device 5 automatically transfers the sample substrate. The sample substrate 7 is attached to the sample holder 6 downward. Molecular Beam Epitaxy Several molecular beam cells are provided along the wall of the device. One of these is nitrogen
Radical cell 8 for The other molecular beam cell 9 uses a solid raw material as a molecular beam. Many are provided, but only one is shown here.

The molecular beam cell 9 comprises a crucible 10 and a heater 1.
1, reflector 12, column 13, ultra-high vacuum flange 1
4, thermocouple 15, shutter 16 and the like. This heats and evaporates solids such as zinc and selenium into molecular beams. The molecular beam goes straight toward the substrate.

The radical cell 8 includes a crucible 18, a high frequency coil 19, a shield plate 20, an introduction pipe 21, a valve 22,
The shutter 23 and the like are included. A shutter 17 is also provided just below the substrate.
There is. From the radical cell, nitrogen radicals become radical rays and fly to the substrate. The reaction occurs on the substrate,
ZnSe containing nitrogen grows.

[0013]

When a II-VI group compound semiconductor crystal such as ZnSe is manufactured by a molecular beam epitaxy apparatus, nitrogen gas is converted into plasma in order to obtain a crystal having a p-type conductivity type. The substrate was irradiated with the molecular beams of the group II and VI elements. Even if nitrogen gas is used as a molecular beam, it does not enter the inside of the ZnSe crystal, but it is taken into the inside of the growing ZnSe when activated by plasma. In order to generate plasma, high-frequency waves or microwaves are applied to the nitrogen molecular beam cell so that nitrogen is brought into a plasma state. Plasma nitrogen has high activity and various kinetic energy. Some are ions and some are neutral radicals.

Neutral nitrogen travels straight from the molecular beam to the substrate due to the ultrahigh vacuum of the molecular beam epitaxy system. The source of transporting neutral nitrogen molecules is kinetic energy given by high frequency waves or microwaves. In the case of ions, not only that but also acceleration due to the potential difference between the molecular beam cell and the substrate. Those with the appropriate kinetic energy enter the growing thin film and form part of the crystal structure. However, nitrogen ions with higher energy also exist in the nitrogen plasma.

In the case of ions, they are further accelerated by the bias of the molecular beam cell and the substrate and collide with the substrate. The impact of fast nitrogen ions on the substrate disturbs the crystallinity of the growing thin film. The zinc blende type crystal structure is disordered, and Zn and S
There is also a portion where the ratio of e is not locally 1: 1. If the crystallinity is poor, even if p-type ZnSe is formed by nitrogen doping, it cannot be used for a light emitting device or the like. This is because when it is used as a semiconductor laser, the electric-light conversion efficiency is poor and the electric resistance increases.

If nitrogen is not turned into plasma, nitrogen is added to Zn.
It is not possible to do so on Se. However, high-energy nitrogen ions are present in the plasma, which collide with the thin film and reduce the crystallinity of the thin film. It is an object of the present invention to solve such a problem in growing nitrogen doped ZnSe by molecular beam epitaxy. In other words, energy
It is an object of the present invention to provide a radical cell in which high-concentration nitrogen ions do not collide with the substrate.

[0017]

In the radical cell of the molecular beam epitaxy apparatus of the present invention, a high frequency or a microwave is applied to nitrogen to form a plasma, which is blown toward a substrate. A net consisting of The mesh is used as a mesh electrode to apply a desired bias. When a positive bias is applied, the positive ions of nitrogen lose kinetic energy. Some lose most of their energy and cannot pass through the mesh electrode. Even positive ions that have been able to pass through the mesh electrode have lost energy due to the bias of the mesh electrode, so that the impact force applied to the substrate is relaxed.

[0018]

In the nitrogen molecular beam cell of the present invention, a mesh electrode is provided between the cell opening and the substrate, and a positive voltage is applied to the mesh electrode. Nitrogen radicals contain high-energy nitrogen ions, which are decelerated near the mesh electrode. The degree of deceleration depends on the bias of the mesh electrode. Nitrogen ions are slowed down and some are returned from the mesh electrode towards the cell. Some pass through the mesh electrode and reach the substrate even when decelerated. Even in the latter case, since the energy when colliding with the substrate is low, the impact on the substrate is small. It does not adversely affect the crystallinity of the growing thin film. Nitrogen doped ZnSe having excellent crystallinity can be grown.

It is the neutral radicals that have a large effect on the nitrogen doping. Neutral radicals do not feel an electric field at the mesh electrode, so they can pass through the mesh electrode as they are. It does not reduce the amount of neutral radicals that reach the substrate and contribute to the reaction. Since the mesh electrode removes only harmful fast positive ions and leaves neutral radicals as they are, it does not hinder the growth of ZnSe.

[0020]

Embodiment An embodiment of the present invention will be described with reference to FIG. Along the inner wall of the ultra-high vacuum chamber 1, a liquid nitrogen shroud 2
Is provided. The chamber 1 communicates with another vacuum chamber (not shown), and the connecting pipe 3 is provided with a gate valve 4. The transfer device 5 is inside the vacuum chamber so that the sample substrate can be transferred. At the tip of the transfer device 5, there is a sample holder 3, which holds the substrate 7 downward. Several molecular beam cells are provided on the wall surface of the vacuum chamber 1 obliquely below. One of them is a radical cell 8 for introducing nitrogen. The solid-state molecular beam cell 9 is a molecular beam made of a solid raw material such as zinc or selenium. There are several, but only one is shown here for simplicity. The molecular beam cell 9 is composed of a crucible 10, a heater 11, a reflector 12, a column 13, an ultrahigh vacuum flange 14 and the like.

A solid raw material is placed in the crucible 10. This is for example a PBN crucible. The heater heats and vaporizes the raw material. The reflector 12 traps the heat of the heater in a reflective crucible. The column 13 is on the ultra-high vacuum flange 14,
It is for supporting the reflector 12, the heater 11, the crucible 10 and the like. A thermocouple 15 is in contact with the bottom of the crucible to monitor the temperature of the raw material. A shutter 16 is provided above the opening of the crucible 10. This can block the flow of molecular beams. The flow of the molecular beam can be freely controlled by opening and closing the shutter 16. The molecular beam cell for solid raw materials is thus a closed system.

The radical cell 8 is not closed because the raw material is gas. It is connected to an external high-purity gas cylinder (not shown) by a raw material introduction pipe. It includes a crucible 18, a high frequency coil 19, a shield plate 20, and the like.
A high frequency is applied to the high frequency coil, and the nitrogen gas is turned into plasma. A microwave can be used instead of the high frequency coil. A raw material introduction pipe 21 is continuous with the lower bottom of the crucible 18. Nitrogen gas, which is a raw material of radicals, is introduced from the gas cylinder into the crucible 18 through the introduction pipe 21. The valve 22 controls the flow rate of nitrogen.

The top of the crucible 18 is closed by a lid,
There is a small hole in the center. Nitrogen ions, radicals, etc. will fly out from this. The crucible 18 may be provided with a baffle plate for decelerating the gas flow. The shutter 23 is provided above the opening of the crucible 18 so as to be openable and closable. The chamber 1 needs to be pulled to an ultra high vacuum. Titanium sublimation pump 2 through a valve 24 in a part of the chamber
5, an ion pump 26 is provided. By combining these pumps, the inside of the chamber is 10 ^-10 to 10 ^-11.
The ultra high vacuum of Torr can be maintained. A quadrupole mass spectrometer 27 is provided above the chamber. It monitors the thickness of thin films grown on a given substrate.

In addition to the above structure, in the present invention, the mesh electrode 28 is provided in the opening of the crucible of the radical cell 18. In this, a metal net is attached to the tip of a metal rod, and a voltage is applied by the bias power supply 13 via the metal rod. Thus, when a positive voltage is applied to the mesh electrode 28, the positive ions of nitrogen are damped by this.
Therefore, the nitrogen that should collide with the substrate at a high speed is decelerated. The possibility of disturbing the crystal structure of ZnSe growing on the substrate is reduced. The bias voltage can be set freely, so select the optimum value.

FIG. 2 is an enlarged perspective view of the vicinity of the radical cell. There is a small opening 30 at the top of the crucible, on which a shutter 23 and a mesh electrode 28 are provided. The mesh electrode 28 is also rotatable so that it can be removed from above the crucible opening 30 if it is not needed. Neutral radicals can pass through the mesh electrode without any resistance. The positive ions are thereby slowed down and some return to the crucible that cannot pass through them. The impact of positive ions does not impair the crystallinity of the thin film grown on the substrate.

[0026]

When a ZnSe crystal thin film is grown by molecular beam epitaxy using nitrogen as a p-type dopant, a mesh cell is provided in a radical cell for obtaining a radical line of nitrogen, and a bias voltage is applied to the mesh electrode. Therefore, high-speed positive ion components can be removed. Since the fast positive ions do not collide with the substrate with a strong impact, the crystallinity of the growing thin film is not impaired. High quality crystalline ZnSe thin films can be grown. ZnSe in which n-type impurities are Cl and p-type impurities are nitrogen
The method of the present invention is particularly effective when manufacturing a semiconductor laser by molecular beam epitaxy.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a molecular beam epitaxy apparatus including a radical cell according to an embodiment of the present invention.

FIG. 2 is an enlarged perspective view of the vicinity of the opening of the radical cell of the present invention.

FIG. 3 is a schematic sectional view of a molecular beam epitaxy apparatus including a radical cell according to a conventional example.

[Explanation of symbols]

 1 Vacuum Chamber 2 Liquid Nitrogen Shroud 4 Gate Valve 5 Conveyor 6 Sample Holder 7 Substrate 8 Radical Cell 9 Molecular Beam Cell 10 Crucible 11 Heater 14 Ultra High Vacuum Flange 16 Shutter 17 Shutter 18 Crucible 19 High Frequency Coil 21 Inlet Pipe 22 Valve 28 Mesh electrode 29 Bias power supply

Claims (4)

[Claims] 1. A gas inside the crucible is turned into plasma by introducing a nitrogen gas into the crucible, an introduction pipe connected to the gas cylinder for introducing the nitrogen gas, a crucible continuously provided to the introduction pipe, and a high frequency current provided around the crucible. The high-frequency coil, a shutter provided above the opening of the crucible, a mesh electrode provided above the opening of the crucible, and a power source for biasing the mesh electrode, are used for the molecular beam epitaxy.
A radical cell, which is used in a device to generate radical rays of nitrogen. 2. A crucible, an introduction pipe connected to a gas cylinder for introducing nitrogen gas, a crucible continuously provided to the introduction pipe, a microwave oscillator for supplying microwaves into the crucible to convert nitrogen gas into plasma, and a crucible. Which is used in the molecular beam epitaxy apparatus to generate a radical line of nitrogen, which is composed of a shutter provided above the opening of the mesh, a mesh electrode provided above the opening of the crucible, and a power source for biasing the mesh electrode A radical cell characterized by the above. 3. An ultra-high vacuum chamber, a vacuum evacuation device for drawing the ultra-high vacuum chamber to a vacuum, a plurality of molecular beam cells provided along the outer wall of the chamber, and a radical cell for converting this into radicals through a source gas. , A transport device for automatically transporting the sample substrate in a vacuum, and a sample holder for holding the sample substrate, the molecular beam cell is used to turn the solid raw material into a molecular beam, and the radical cell converts nitrogen gas into plasma. Used to be a radical line, the radical cell is connected to a gas cylinder for introducing nitrogen gas, a crucible continuously provided to the introduction pipe, and by supplying a high-frequency current around the crucible, A high frequency coil that turns the gas inside the crucible into plasma, a shutter that is provided above the opening of the crucible, and a mesh electrode that is provided above the opening of the crucible. , A molecular beam epitaxy apparatus comprising a power supply for biasing a mesh electrode. 4. An ultra-high vacuum chamber, a vacuum evacuation device for drawing the ultra-high vacuum chamber to a vacuum, a plurality of molecular beam cells provided along the outer wall of the chamber, and a radical cell that turns the radicals through a source gas. , A transport device for automatically transporting the sample substrate in a vacuum, and a sample holder for holding the sample substrate, the molecular beam cell is used to turn the solid raw material into a molecular beam, and the radical cell converts nitrogen gas into plasma. Used to generate radical rays, a radical cell is connected to a gas cylinder to introduce nitrogen gas, a crucible continuously provided in the introduction tube, and a microwave inside the crucible to supply nitrogen gas to the plasma. A microwave oscillator, a shutter provided above the opening of the crucible, and a mesh electrode provided above the opening of the crucible,
A molecular beam epitaxy device comprising a power supply for biasing a mesh electrode.