JPH09115834A - Molecular beam evaporation source cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the micro particles of molecular beam evaporation source material from causing defect on the surface of epitaxial layer even when the residual quantity of material is small by providing a mesh at the molecular beam ejection port of molecular beam evaporation source cell in a molecular beam epitaxial growth system employing a solid state material. SOLUTION: The molecular beam evaporation source cell comprises a crucible 2 having an opening, a heater 3, a heat shield 4, a cell cap 1 and a thermocouple 6 wherein a material metal is placed in the crucible 2. A mesh 7 made of Ta is inserted between the crucible 2 and cell cap 1. The mesh 7 is a disc having same diameter as the crucible 2 and the size of mesh is 500μm. Ta is selected as a mesh material because of it high melting point, low degassing and easy machining. In order to attain a molecular beam intensity of conventional cell using the inventive mesh 7, cell temperature must be set higher by about 60 deg.C and thereby the mesh 7 is set to be heated independently.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a molecular beam evaporation source cell of a molecular beam epitaxial growth apparatus,
The present invention relates to an improvement for reducing defects on the surface of an epitaxial layer due to fine particles of a molecular beam evaporation source material. In particular, it relates to a molecular beam evaporation source cell in which the solid material of the molecular beam evaporation source is As.

[0002]

2. Description of the Related Art As a crystal growth method for forming an ultra-thin film on a semiconductor substrate, a molecular beam epitaxial growth method has recently attracted attention and has been studied. Molecular beam epitaxy is a method of evaporating a raw material under ultra-high vacuum to grow a target crystal on a heated substrate.As a device using the molecular beam epitaxy method, a high electron mobility transistor (HEMT), etc. Has been put to practical use.

Minute protrusions and dimple-like surface defects are found on the surface of the epitaxial layer grown by the molecular beam epitaxial growth method, and the cause thereof has been studied. These surface defects are classified according to their causes: (1) defects due to solid fine particles of the molecular beam evaporation source raw material, (2) defects called oval defects due to oxides in the raw material of the molecular beam evaporation source and raw materials due to spitting , (3) Three types of defects caused by dislocations in the substrate.

These surface defects cause deterioration of the pinch-off characteristics in the characteristics after device fabrication, resulting in a decrease in yield. It is known that the defect (2) can be reduced by devising the temperature distribution of the molecular beam evaporation source cell. It is known that (3) can be reduced by the off-angle of the surface orientation of the substrate.

The defect caused by the raw material of the molecular beam evaporation source of (1) is caused by using the conventional flake-shaped raw material by making the metal of the raw material of the molecular beam evaporation source into an ingot having a shape matched to the crucible. It is known that it can be reduced more than. However, when the epitaxial growth is repeated continuously and the remaining amount of the molecular beam evaporation source raw material decreases, the ingot of the molecular beam evaporation source raw material collapses, and it is the same as when using the flaky molecular beam evaporation source raw material. The defect (1) is likely to occur. Normally, when the raw material of the molecular beam evaporation source is in such a state, the raw material of the molecular beam evaporation source is recharged. However, a considerable amount of the molecular beam evaporation source raw material remains in the molecular beam evaporation source cell, which causes a high cost in terms of use efficiency of the molecular beam evaporation source raw material.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned drawbacks. Even when the remaining amount of the raw material becomes low, the surface defects of the epitaxial layer due to the fine particles of the molecular beam evaporation source raw material are eliminated. Another object of the present invention is to provide a molecular beam evaporation source cell which can prevent and effectively use the molecular beam evaporation source raw material to the end.

[0007]

In the molecular beam epitaxial growth, when the molecular beam evaporation source raw material is a solid metal raw material, the molecular beam is formed by sublimation of the solid metal raw material. When the flaky metal raw material is heated in a vacuum, the metal raw material is gradually decomposed into fine flakes by heating.
The fine particles of the molecular beam evaporation source raw material are scattered during this decomposition, and a part of the fine particles reaches the substrate where the crystal is about to grow and becomes a surface defect. For example, the surface of a GaAs epitaxial layer grown by molecular beam epitaxial growth has a size of several tens.
It is observed in a state in which As microparticles of μm to several 100 μm are embedded.

The present inventor diligently studied the structure of the molecular beam evaporation source cell in consideration of the generation of fine particles in the course of such fine solid metal raw material, and as a result, the molecular beam was prevented from reaching the substrate. It has been found that the surface defect of the epitaxial layer due to the fine particles of the molecular beam evaporation source raw material can be reduced by providing the shielding plate for the fine particles between the evaporation source raw material and the substrate. That is, the present invention is a molecular beam evaporation source cell in a molecular beam epitaxial growth apparatus using a solid material, in the molecular beam evaporation source cell, characterized in that a mesh is provided at the ejection port of the molecular beam of the molecular beam evaporation source cell. In addition, the molecular beam evaporation source cell is characterized by being provided with a mechanism capable of independently heating the mesh portion.
Furthermore, the molecular beam evaporation source cell is characterized in that the solid raw material is As or Sb.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The position of the mesh needs to be provided between the substrate and the raw material of the molecular beam evaporation source, but in order not to shield other molecular beams, the molecular beam of the molecular beam evaporation source cell Attach to the spout of the wire. Furthermore, since the mesh is heated at the same time as the molecular beam evaporation source material during film formation, clogging of the mesh due to precipitation can be prevented to some extent. The mesh size is preferably 0.01 mm or more and 1 mm or less. If the mesh size is larger than 1 mm, the effect of preventing scattering of fine particles is lost, and if it is less than 0.01 mm, there is an effect of preventing scattering of fine particles, but it is difficult to obtain sufficient molecular beam strength. As the material of the mesh, a material that is less outgassed and does not react with the raw material is used. For example, a material such as pBN (pyrolytic boron nitride), Ta or Mo can be used.

Further, in order to effectively prevent clogging of the mesh, a heating mechanism capable of heating only the mesh portion may be provided. For example, coating the surface with pG (pyrolytic graphite) and providing a conductive layer
A mesh made of pBN coated with BN, Ta, or Mo is used, and the mesh portion can be independently heated by energizing the mesh.

This molecular beam evaporation source cell is basically effective for a molecular beam evaporation source raw material that can be sublimated as a solid to obtain a molecular beam. For example, in addition to As, it can be used as a molecular beam evaporation source material such as Sb.

[0012]

【Example】

(Embodiment) An embodiment according to the present invention is shown in FIG. The molecular beam evaporation source cell is composed of a crucible 2 having an opening, a heater 3, a heat shield 4, a cell cap 1 and a thermocouple 6, and a metal as a raw material is put in the crucible 2. A mesh 7 made of Ta was inserted between the crucible 2 and the cell cap 1. As shown in FIG. 2, the mesh 7 is a disk having the same diameter as the brim of the crucible 3, and the mesh size is 50.
It was set to 0 μm. Here, Ta is selected as the material because it is a refractory metal and degassing is small and processing is easy. In addition, in order to obtain the same molecular beam intensity as that obtained in the conventional cell by using the mesh 7, it was necessary to set the cell temperature high by about 60 ° C. The mesh 7 has a mechanism through which an electric current can flow so that the mesh 7 can be independently heated as needed (not shown in FIG. 1).

Using this molecular beam evaporation source cell, molecular beam epitaxial growth of a GaAs epitaxial film was continuously repeated under the following film forming conditions. (Film forming conditions) Substrate: GaAs substrate Substrate temperature: 600 ° C. Growth rate: 1.0 μm / h Molecular beam intensity: 1.0 × 10 ^-3 Pa (vapor pressure corresponding to molecular beam by nude ion gauge)

As a result, no defect due to As fine particles was observed on the GaAs epitaxial film regardless of the change in the shape of the As raw material due to the increase in the remaining amount of As and the number of film formations. Similar results were obtained when the GaAs epitaxial film was similarly subjected to molecular beam epitaxial growth while a constant current was applied to the mesh 7 to heat the mesh 7.

(Comparative Example) FIG. 3 shows an example of a conventional molecular beam evaporation source cell. The cell is composed of a crucible 2 having an opening, a heater 3, a heat shield 4, and a cell cap 1. A metal as a raw material is put in the crucible 2. Using this cell, molecular beam epitaxial growth of a GaAs epitaxial film was repeated under the same conditions as in Example 1.

As a result, as the epitaxial growth was repeated, the number of GaAs epitaxial films in which defects due to As fine particles were generated on the GaAs epitaxial film increased. Therefore, in the conventional cell, since the minute As particles in the crucible directly hit the substrate simultaneously with the molecular beam, it is impossible to prevent the generation of surface defects on the GaAs epitaxial film.

[0017]

As described above, by using the molecular beam evaporation source cell according to the present invention, surface defects on the epitaxial film due to the fine particles of the molecular beam evaporation source raw material can be reduced. Further, the epitaxial layer having a low defect density can be stably obtained irrespective of the amount and shape of the molecular beam evaporation source raw material, so that the molecular beam evaporation source raw material can be effectively used until the end.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a molecular beam evaporation source cell according to the present invention.

FIG. 2 is a diagram showing an example of a mesh plate of a molecular beam evaporation source cell according to the present invention.

FIG. 3 is a diagram showing an example of a conventional molecular beam evaporation source cell.

[Explanation of symbols]

 1: Cell cap 2: Crucible 3: Heater 4: Heat shield 5: Cell base plate 6: Thermocouple 7: Mesh

Claims (3)

[Claims] 1. A molecular beam evaporation source cell in a molecular beam epitaxial growth apparatus using a solid material, wherein a molecular beam evaporation source cell is provided with a mesh at a molecular beam ejection port. 2. The molecular beam evaporation source cell according to claim 1, wherein a mechanism for independently heating the mesh portion is provided. 3. The molecular beam evaporation source cell according to claim 1 or 2, wherein the solid raw material is As or Sb.