JPH0710931U - Radical cell and molecular beam epitaxy system - Google Patents

Abstract

(57) [Summary] (Modified) [Objective] In the molecular beam epitaxy method, in order to dope nitrogen to a II-VI group compound semiconductor such as ZnSe, nitrogen radicals are applied to the growing crystal. The exit of the radical cell is a small orifice. Since the vacuum chamber has a high vacuum and the radical cell has a low vacuum, the differential pressure is maintained by the orifice. In this case, the pressure of the radical cell changes due to the pressure variation of the vacuum chamber, and the radical production amount also varies. Then, the doping amount of nitrogen changes. The purpose is to prevent this. [Configuration] Valves 2 and 6 are provided on the inlet side and the outlet side of the radical cell 10. The valve 6 on the outlet side is a vacuum chamber 7
It also acts to maintain the differential pressure between the high vacuum inside and the low vacuum inside the cell. The pressure of the radical cell is measured and the valve opening on the outlet side is adjusted so that the pressure becomes constant.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a radical cell used in molecular beam epitaxy and using nitrogen gas as a radical. The molecular beam epitaxy apparatus is to grow a thin film crystal on a substrate by applying a molecular beam of a raw material to a heated substrate in a chamber evacuated to an ultrahigh vacuum. Since it is an ultra-high vacuum, the mean free path of the molecule is long and the molecular beam goes straight from the cell to the substrate.

When the raw material is a solid, it is placed in a crucible of a molecular beam cell and heated with a heater to be vaporized or sublimated to form a molecular beam. Molecular beam cells include crucibles, heaters, reflectors, shutters and the like. When the raw material is gas, a gas source cell is used. There is a gas cylinder outside the equipment, from which gas is introduced into the ultra-high vacuum chamber. The gas source cell also has a crucible, a heater, and a reflector, and the heater heats the gas to give kinetic energy. A small opening is provided above the crucible to maintain the difference between the pressure on the cell side and the pressure in the chamber.

[0003]

[Prior art]

 The molecular beam epitaxy method can be used for forming III-V compound semiconductor thin films, Si semiconductor thin films, and thin films of many kinds of substances such as dielectric films. The present invention relates to a molecular beam epitaxy apparatus capable of doping nitrogen into an epitaxial growth film of a II-VI group semiconductor, for example, ZnSe. ZnSe is important as a material for blue light emitting elements. The semiconductor light emitting device includes a light emitting diode and a semiconductor laser. Infrared, red, yellow, green, etc. light emitting elements already exist. There has been a long-standing demand for elements that generate blue color, but sufficient performance has not yet been obtained. The blue light emitting element must form a pn junction using a semiconductor crystal with a wide band gap.

A wide band gap is required as a material for a blue light emitting element. In addition, it is required to be a direct transition type, to facilitate pn junction, to easily obtain a large-sized substrate, and to facilitate the device process. There is currently no material that satisfies all of these conditions. GaN, SiC, ZnSe, ZnS, GaAlN, ZnSSe and the like satisfy any of the conditions.

The subject matter of the present invention is a II-VI group compound semiconductor. For example, ZnSe is a direct transition type semiconductor having a band gap of 2.7 eV. However, this has a problem that a large single crystal substrate cannot be formed and a p-type cannot be formed. Many methods such as a high-pressure Bridgman method, a sublimation method, an iodine transport method, a solution growth method, and a piper method have been tried for producing a ZnSe single crystal substrate. Since ZnSe sublimes at high temperature unless high pressure is applied, it is difficult to form a melt, and the formation of a single crystal substrate has not yet been successful.

Therefore, it is attempted to use GaAs as a substrate and heteroepitaxially grow a ZnSe thin film thereon. The board problem is now avoided. However, it is necessary to form a pn junction to form a light emitting device. For this purpose, n-type crystals and p-type crystals must be produced. However, it is difficult to control the conduction type.

It is difficult to produce a p-type or n-type conduction type crystal. In the case of non-dope, it becomes n-type. However, the carrier density is extremely low. Conventionally, Al, Ga, In, etc. have been known as n-type impurities of ZnSe. However, these impurities rather decrease the n-type carrier density when the doping amount is increased. The maximum n-type carrier density was 10 ¹⁷ cm ^-3 . When the concentration is low as described above, the resistance in the forward direction is large, which is insufficient as a pn junction of the light emitting element.

However, more difficult is to make a p-type ZnSe crystal. First of all, I wondered what would become a p-type dopant. There is a theory that Na, Li, etc. become p-type dopants, and there is an attempt to do so. There is also an attempt to grow ZnSe by MBE (Molecular Beam Epitaxy). Various p-type dopants have been tried, but no p-type dopant has been obtained.

Nishizawa et al. Announced that they manufactured a p-type ZnSe crystal doped with Li by the vapor pressure controlled temperature difference method. This is not a thin film, but a bulk crystal. 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 reproducible. In addition, the carrier density of p-type impurities is low. It does not exceed 8 × 10 ¹⁶ cm ^-3 . In addition, Li easily diffuses in ZnSe, and the characteristics of the pn junction are not constant.

MIGITA et al. Say that a p-type ZnSe thin film was grown on a GaAs substrate using nitrogen as a dopant by a molecular beam epitaxy method. M.MIGITA, A.TAIKE, M.SHIIKI &H.YAMAMOTO; "p-type conduction of ZnSe Hig hly Doped With Nitrogen by Metalorganic Molecular Beam Epitaxy", J. Cryst al Growth 101 (1990) 835-840

This is a molecular beam epitaxial growth method. Dimethyl Zn (DMZ) is used as a Zn raw material, and hydrogenated selenium H ₂ Se is used as a selenium raw material. These are not put into a usual molecular beam cell, but are introduced into the inside of a molecular beam epitaxial device from a gas source cell. Nitrogen is also introduced from the gas source cell in the form of NH ₃ gas. The substrate is GaAs. It is said that this was heated in a susceptor to 250 ° C. to 450 ° C. and vapor-phase grown at 1 to 4 × 10 ⁻⁵ Torr. This is p-type ZnSe, which is said to have a carrier density of 10 ¹⁸ cm ^-3 at a temperature of about 450 ° C. However, it is 10 ¹³ cm ⁻³ at 300 ° C. As a result, p-type ZnSe is obtained.

However, this method does not have good reproducibility and does not have such characteristics due to additional tests. Further, even though it is a p-type, there is a problem that the carrier density is too low. Even if nitrogen functions as a p-type dopant, it may not sufficiently penetrate into the ZnSe thin film. For this reason, the density of the p-type carrier may be too low.

Subsequently, it was found that chlorine is effective as an n-type impurity. After this discovery, chlorine was used as an n-type impurity. This uses zinc chloride (ZnCl ₂ ) as a raw material. Chlorine doped ZnSe can also be made by the molecular beam epitaxy method. The GaAs substrate is simultaneously irradiated with a molecular beam cell of Zn, Se and ZnCl ₂ . K. Ohkawa, T. Mitsuyu and O. Yamazaki: Ext. Abstr. 18th Conf. Solid State Dev ices and Materials, Tokyo, 1986, p635

When chlorine is used as an n-type impurity, a carrier density of about 10 ¹⁹ cm ⁻³ can be realized. The carrier density is improved by an order of magnitude as compared with the case of Ga-dope.

As described above, nitrogen has been studied as a p-type impurity. However, nitrogen is difficult to enter into the ZnSe crystal. Although various attempts have been made to dope nitrogen into ZnSe by liquid phase epitaxy, MOCVD method, MBE method, 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. This is a method in which nitrogen gas is turned into active nitrogen by electric discharge, and a molecular beam is applied to the substrate together with a Zn molecular beam and a Se molecular beam. K.Ohkawa, T.Karasawa and T.Mitsuyu: 6th Int.Conf.Molecular Beam Epitaxy, San Diego, 1990, PIII-21, J.Cryst.Growth 117,375 (1991) K.Ohkawa, A.Tsujimura, S.Hayashi , S. Yoshii and T. Mitsuyu: Ext.Abstr.1992 Int. Conf. Solid State Devices and Materials, Tsukuba, 1992 p.330 Thus, with chlorine as n-type and nitrogen as p-type, ZnSe of the molecular beam epitaxy method was used. A pn junction is now available.

[0017]

A radical cell was attached to a molecular beam epitaxy apparatus, and nitrogen was used as a radical beam to irradiate a GaAs substrate to grow a ZnSe thin film of nitrogen doping. The proposed radical cell still has a problem in stability, and the nitrogen doping amount fluctuates with time. That is, the fluctuation of the p-type carrier concentration in the thickness direction of the thin film is large.

FIG. 2 shows the structure of a radical cell attached to a molecular beam epitaxy apparatus. The cell body 1 is a cylindrical container, and is connected to a gas cylinder (not shown) via a pipe 11. A valve 2 for controlling the amount of nitrogen gas is provided in the middle of the pipe 11. An RF coil 3 is provided on the outer periphery of the cell body 1. A high frequency is applied to this to generate a high frequency electromagnetic field inside the cell body. As a result, the gas is excited and enters a plasma state. The plasma 4 is a collection of nitrogen ions, neutral active species, electrons and the like.

A small orifice 8 opens at the top of the cell body. Nitrogen radicals and ions enter the ultra-high vacuum chamber through the orifice 8. The radical is a radical ray that travels straight because the mean free process is long. With this structure, the cell body is fixed to the cell attachment portion inside the chamber, and the RF coil exists inside the chamber. The valve 2 is outside the chamber and can be operated from outside. The cell body in which plasma is generated is inside the chamber.

When a crystal is grown by a molecular beam epitaxy apparatus, a solid source is 10 ⁻⁸ to 10 ⁻⁹ Torr, and a gas source is also grown in a high vacuum close to this. In a radical cell, a high frequency glow discharge must be maintained in order to generate radicals. In order to maintain high frequency glow discharge, a low vacuum degree of about 10 ^-1 to 10 ^-2 Torr is preferable. The cell has a low vacuum, the chamber has a high vacuum, and the pressure difference is large. In order to maintain the differential pressure, the tip of the cell has a small orifice 8. Differential pressure is exhausted to maintain the pressure difference between the two.

However, the pressure in the chamber fluctuates during epitaxial growth. In addition, the state of plasma in the cell changes as the flow rate of nitrogen gas changes. The amount of nitrogen dopant that is doped into ZnSe varies depending on the change in plasma state. Therefore, the electrical characteristics of the crystal vary. There was also a problem in reproducibility of crystal growth.

This is due in part to the fact that the opening of the orifice at the tip of the radical cell cannot be adjusted by the pressure fluctuation in the chamber. Another reason is that it is difficult to maintain a constant pressure inside the radical cell.

It is desirable to stabilize the generation of nitrogen radicals in the radical cell and control the amount of dopant contained in the ZnSe crystal to a constant level so that a thin film crystal of stable quality can be manufactured. Is the purpose. Further, the second object of the present invention is to enable the nitrogen-doped thin film crystal growth with high reproducibility without changing the conditions by repeated growth.

[0024]

[Means for Solving the Problems]

 In the radical cell of the present invention, the portion of the pipe between the radical cell and the molecular beam epitaxy device is lengthened, a pressure adjusting valve is provided in the pipe portion, and a pressure gauge is provided in the cell body. The pressure gauge signal is fed back to automatically adjust the opening of the pressure control valve to keep the pressure inside the radical cell constant. That is, it differs from the conventional one in that the pressure of the radical cell is monitored and the cell pressure is controlled.

[0025]

[Action]

 A pressure regulating valve is provided instead of the orifice of the radical cell, which can maintain the pressure difference between the chamber and the cell. If the opening cannot be adjusted like an orifice, the effect of pressure fluctuations in the chamber affects the cell. However, in the present invention, since the valve is used, the cell pressure can be controlled freely even if the pressure in the chamber fluctuates. The amount of plasma generated is a function of gas supply amount, temperature, high frequency power, gas pressure, etc. However, since there are valves before and after the cell, not only the gas supply amount but also the gas pressure can be controlled.

In the present invention, the pressure in the cell is monitored, and when the pressure fluctuates, the opening of the pressure regulating valve is changed so as to restore this. In this way, the pressure in the cell can be kept constant even if the pressure in the chamber fluctuates. Then, the amount of radicals generated is constant over time, and the amount of dopant taken into the crystal thin film becomes stable. A uniform thin film crystal of p-type carrier can be grown. It is also possible to obtain high-quality epitaxially grown crystals with little variation in dopant concentration between products.

[0027]

【Example】

 An embodiment of the present invention will be described with reference to FIG. The radical cell is provided as part of the molecular beam epitaxy system. The cell body 1 is a cylindrical container, which is outside the molecular beam epitaxy apparatus. Of course, it can be housed inside, but the outside is more convenient. The cell body 1 is connected via a pipe 11 to a cylinder of nitrogen gas (not shown). The pipe 11 has a valve 2 for adjusting the amount of gas entering the cell. On the outer periphery of the cell body 1 is a radio frequency (RF) coil 3. A high frequency is applied to this to generate a high frequency electromagnetic field inside the cell to convert nitrogen gas into plasma 4.

In the present invention, a pressure gauge 5 is provided on the side of the cell to measure the pressure P inside the cell. Further, a pipe 12 is further formed in front of the cell body 1. There is a second valve in the middle of the pipe 12. Here, it is referred to as the pressure control valve 6.

In FIG. 2, since the cell is inside the chamber, the front of the cell becomes an orifice immediately. However, in the embodiment of the present invention, the cell body is exposed to the outside of the chamber. The pipe 12 is required for connection. The tip of the radical type pipe 12 is simply an opening 13. This may be narrowed down to an orifice. The actuator 9 of the pressure adjusting valve 6 can feed back the signal from the pressure gauge 5 to automatically adjust the opening.

The radical cell 10 has such a structure, but both the opening Q of the valve 2 on the inlet side and the opening R of the pressure adjusting valve 6 on the outlet side can be adjusted. The power W of the high frequency coil 3 is also an adjustable variable. There are three control variables. Plasma density depends on radio frequency (RF) power, gas supply, plasma output, etc. If these variables are controllable, the plasma density and the plasma output can be finely controlled.

Here, in order to keep the pressure P in the cell constant, the signal of the pressure gauge 5 is fed back so that the actuator 9 of the pressure adjusting valve 8 is automatically adjusted. Since it is convenient for maintenance and the like that the pressure gauge and the actuator 9 are outside the chamber, most of the radical cells are outside the chamber. Therefore, a long extension pipe 12 is required to connect the cell and the inside of the chamber. Of course, it is also possible to house the radial cell inside the chamber. Even in this case, the extension pipe is necessary because the pressure adjusting valve 6 is attached to the outlet side of the cell.

The other structure is the same as that of a normal molecular beam epitaxy apparatus. The ultra-high vacuum chamber 7 is a container made of metal such as stainless steel or aluminum. A liquid nitrogen shroud 14 is provided along the inner wall. This is for adsorbing gas and increasing the degree of vacuum. The transfer device 15 is provided so that the substrate can be transferred to another vacuum chamber (not shown). A substrate holder 16 is provided at the tip of the transfer device, and the substrate 17 is attached to the substrate holder 16. The substrate holder 16 includes a substrate rotating and heating mechanism. Above the substrate 17 is a quadrupole mass analyzer 18.

In addition to the radical cell, some molecular beam cells 19 for converting a solid raw material into a molecular beam are provided. Only one is shown here for simplicity. A crucible 20 for containing a solid raw material 21, a heater 22 for heating the raw material into a liquid or sublimation, a reflector 23 for not releasing heat, and the like are concentrically provided. A shutter 24 is provided so as to be openable and closable close to the upper opening of the crucible. The cell is installed on the ultra-high vacuum flange 25. Posts 26 support the crucible, heater, and reflector against the flange. In the case of a solid material, such an isolated molecular beam cell is used. The opening of the crucible 20 faces the substrate, and only when the shutter 24 is opened, the molecular lines fly to the substrate.

The radical cell 10 has a main body outside the chamber, and the extension pipe 12 can pass through the ultra-high vacuum flange 31. Although the front end of the pipe 12 is an opening 13, a shutter 32 is also provided above the opening 13 to block the radical rays from passing therethrough. Radical rays fly toward the substrate 17 only when the shutter is open.

[0035]

[Effect of device]

 In the conventional radical cell, since only the valve opening at the cell inlet can be controlled, it was not possible to effectively adjust the pressure fluctuation of the radical cell and the amount of radicals produced. The present invention increases the controllable variable by one by adding a pressure regulating valve to the outlet of the cell, and increases the measurable variable by one by measuring the cell pressure.

As a result, the pressure of the radical cell body can be kept constant. Stable plasma can be maintained even if the flow rate of gas introduced into the radical cell or the high frequency power is changed. This stabilizes the dopant concentration of the epitaxially grown thin film. It is also possible to reduce variations in dopant concentration between different thin films.

Further, since plasma can be generated in the radical cell without being affected by the pressure fluctuation in the high vacuum chamber, stable radical supply can be performed under optimum conditions regardless of film forming conditions.

[Brief description of drawings]

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

FIG. 2 is a cross-sectional view of a radical cell according to a conventional example.

[Explanation of symbols]

 1 Cell Main Body 2 Valve 3 RF (Radio Frequency) Coil 4 Plasma 5 Pressure Gauge 6 Pressure Control Valve 7 Ultra High Vacuum Chamber 8 Orifice 9 Actuator 10 Radical Cell

Claims (2)

[Scope of utility model registration request] 1. A molecular beam epitaxy apparatus for introducing nitrogen into a II-VI group semiconductor crystal as radicals, the cell body being a space for passing nitrogen gas into plasma. , A high-frequency coil installed around the cell body to excite nitrogen into plasma by a high-frequency electromagnetic field, a valve provided on the inlet side of the cell body, a pressure control valve provided on the outlet side of the cell body, and a pressure on the cell body And a pressure gauge for detecting the pressure, and the opening of the pressure adjusting valve on the cell outlet side is automatically adjusted by the pressure of the cell body. 2. A molecular beam epitaxy apparatus for flying a raw material as a molecular beam toward a substrate in an ultrahigh vacuum to grow a thin film crystal on the substrate, the chamber being capable of being pulled to the ultrahigh vacuum, An evacuation device, a transfer mechanism that transfers a substrate, a substrate holder, a molecular beam cell that uses a solid source as a molecular beam, and a radical cell that uses nitrogen as a radical. A cell body that is a space for making, a high-frequency coil that is provided around the cell body to excite nitrogen into plasma by a high-frequency electromagnetic field, and an extension pipe for carrying radicals generated in the cell body into the chamber, A valve provided on the inlet side of the cell body, a pressure adjusting valve provided on the outlet side of the cell body, and a pressure gauge for detecting the pressure of the cell body,
The opening of the pressure control valve on the cell outlet side is automatically adjusted by the pressure of the cell body, the cell body is outside the chamber of the molecular beam epitaxy apparatus, and the radicals are introduced into the chamber by an extension pipe. Molecular beam epitaxy device characterized in that