JP2830932B2 - Molecular beam epitaxial growth method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molecular beam epitaxial growth method, and more particularly, to a method for maintaining an evaporation source cell at an appropriate temperature after growth, reducing surface defects of the epitaxial layer due to oxides, and having a high purity. And a method for forming the same.

[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 epitaxial growth is a method of evaporating a material as a raw material under ultra-high vacuum and growing a target crystal on a heated substrate. As a device using the molecular beam epitaxial growth method, a high electron mobility transistor (HEMT) or the like is used. Has been put to practical use.

[0003] On the surface of the epitaxial layer grown by the molecular beam epitaxial growth method, minute projections and dimple-like defects are found, and the causes thereof have been studied. When classified by cause, it is divided into the following three. (1) Defects due to minute particles due to contamination in the growth chamber. (2) A defect called an oval defect caused by an oxide of a raw material such as Ga or a raw material due to spitting (a sudden phenomenon). (3) Classified as defects caused by dislocations in the substrate.

[0004] These surface defects cause deterioration of pinch-off characteristics in the characteristics after device fabrication, and lower the yield. It is known that the defect (1) can be reduced by cleaning the growth chamber, transporting the substrate, or keeping the substrate downward during growth. Regarding the defect (2), there is known a method of reducing the temperature distribution of the evaporation source cell at the time of growth by increasing the temperature at the tip of the crucible in the cell to prevent the generation of droplets at the tip of the crucible. Have been. However, even if the raw material is oxidized when it is not growing even in this method, formation of defects due to Ga oxide is observed.
This can be solved by raising the temperature of the evaporation source cell to a temperature equal to or higher than the temperature used for growth before the start of growth and performing a sufficient degassing operation to remove oxides in the raw material, but wastes the raw material. And increase costs.

Normally, the temperature of the evaporation source cell is raised to a temperature at which a molecular beam intensity required for growth is obtained during growth, and after completion of growth, the temperature is kept at a temperature at which no molecular beam is generated (see FIG. 1). This temperature is called the idling temperature). The temperature used for the growth is usually the temperature in the reduction region, and there is no problem that the raw material is oxidized. However, when the temperature is lowered, the raw material may be oxidized by the reaction with the residual gas in the growth chamber.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned drawbacks, and an object of the present invention is to always prevent the raw material from being oxidized without performing a degassing operation for each growth and to improve the surface defects. It is an object of the present invention to obtain a method for determining an idling temperature at which an epitaxial layer having a small number of layers can be obtained.

[0007]

That is, according to the present invention, the temperature of the evaporation source cell in the molecular beam epitaxy apparatus when the growth is not performed is controlled by the partial pressure ratio of hydrogen and water in the molecular beam epitaxy apparatus. (Ph ₂ / PH ₂ O), wherein the molecular source is maintained at a temperature not lower than the temperature at which the oxidation reaction of the evaporation source material does not proceed and at a temperature lower than the temperature at which the molecular beam intensity becomes 1/100 of that during the growth. A line epitaxial growth method is provided.

Another object of the present invention is to provide a molecular beam epitaxial growth method characterized in that the evaporation source material is Ga and / or In.

First, a case where GaAs of a group III-V compound semiconductor is epitaxially grown will be considered.

The redox reaction and the equilibrium constant of Ga are given by the following equations (1) and (2). _{2Ga + 3H 2 O = Ga 2} O 3 + 3H 2 (1) Kp = (PH 2 / PH 2 O) 3 (2)

Here, Kp is calculated using the thermodynamic constant of each molecule.
Can be calculated (here, Ga and Ga ₂ O ₃
Has an activity of 1). FIG. 1 shows the calculation results. The horizontal axis represents the reciprocal of the temperature of the figure, the vertical axis _{log (PH 2 / PH 2 O} )
Is taking. In this figure, in the region above the equilibrium constant curve (region indicated as region A in the figure), the reaction of equation (1) proceeds to the left, and the raw material is in the reduction region. On the other hand, in a region below the equilibrium constant curve (region indicated as region B in the figure), the reaction of equation (1) proceeds to the right, and the raw material is oxidized.

Usually, a quadrupole mass spectrometer is attached to the molecular beam epitaxial growth apparatus, and the residual gas in the growth chamber can be analyzed. Therefore, the residual gas in the growth chamber is analyzed to obtain the value of the partial pressure ratio of hydrogen and water (PH ₂ / PH ₂ O), and the temperature of the cell is selected so that the equilibrium constant of equation (2) becomes smaller than that value. If the temperature is set to the idling temperature, the oxidation of the raw material will not proceed. During growth, liquid nitrogen is introduced into the cryoshroud to lower the partial pressure of water in the growth chamber.However, after the growth is completed, the supply of liquid nitrogen is stopped. Pressure ratio (P
Since H ₂ / PH ₂ O) rapidly decreases, the partial pressure ratio when liquid nitrogen is not supplied to the cryoshroud may be used as a reference.

On the other hand, the upper limit of the idling temperature is determined by the molecular beam intensity. If the idling temperature is too high, the molecular beam will continue to evaporate even when it is not growing, which not only increases the consumption of raw materials but also accelerates the deterioration of the shutter. Therefore, the upper limit may be defined by the molecular beam intensity. It is appropriate that the normal value be equal to or lower than the evaporation source cell temperature at which the molecular beam intensity at the time of growth becomes 1/100.

[0014]

EXAMPLE Here, the result of the growth of a GaAs layer will be described. The growth chamber is 200
Baking the whole device at 3 ° C for 3 days, and the ultimate vacuum 5 ×
10 ^-9 Pa was obtained. In this state, the residual gas in the growth chamber was analyzed with and without liquid nitrogen in the cryoshroud. Hydrogen partial pressure and water pressure were determined by a quadrupole mass spectrometer. As a result, the partial pressure ratio of hydrogen and water was as follows in each state.

A state in which liquid nitrogen is put: (PH ₂ / PH ₂ O) = 1 × 1
0 ⁴ states not put liquid _{nitrogen: (PH 2 / PH 2 O} ) = 7 × 1
0 ²

Here, based on the equilibrium constants of the oxidation-reduction reaction of Ga shown in FIG. 1, the temperatures of Ga to be balanced at these partial pressure ratios are 611 ° C. and 790 ° C., respectively. On the other hand, Ga
Was measured using a beam monitor movable to the substrate position. 6 × 10 ⁻⁵ Pa was obtained as a molecular beam intensity of 1 μm / h, which is a normal growth condition. The Ga cell temperature at which the molecular beam intensity became 1/100 was 850 ° C.

From these preliminary experiments, it was found that the temperature range in which the molecular beam intensity is always in the reduction region in the oxidation-reduction reaction of Ga and the molecular beam intensity is 1/100 or less of the normal use intensity is 790-790
850 ° C. was obtained. Therefore, in this example, a growth experiment was performed 50 times at an idling temperature of 800 ° C.

FIG. 2 shows an oval defect (a defect caused by an oxide of Ga) of an epitaxial layer grown for each growth.
Defects caused by fine particles such as dust are excluded). In the present embodiment, during the growth, the degassing step of raising the Ga cell temperature to a temperature necessary for growth and raising it to a temperature higher than that is not performed. Although the surface defect density was high several times after the start of growth, it was able to maintain a low value stably at 10 defects / cm ² or less thereafter. The Ga raw material was not oxidized at the time of idling when liquid nitrogen was not contained in the cryoshroud, and the oval defect (defect caused by Ga oxide) density could be reduced without performing a costly degassing step.

In the above embodiment, G was used as the evaporation source material.
Although a has been described, it is also applicable to In. However, the influence of In on surface defects is smaller than that of Ga.

[0020]

As described above, by determining the idling temperature using the determination method according to the present invention,
Surface defects due to oxides and the like are reduced, and the number of defects is not increased even when the growth is repeated, so that an epitaxial layer having a low defect density can be stably obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a temperature change of an equilibrium constant of a redox reaction of Ga.

FIG. 2 is a diagram showing transition of surface defect density for each growth according to the present invention.

Claims (2)

(57) [Claims] 1. The temperature of an evaporation source cell in a molecular beam epitaxy apparatus when growth is not performed is determined according to the partial pressure ratio (PH ₂ / PH ₂ O) of hydrogen and water in the molecular beam epitaxy apparatus. Above the temperature at which the oxidation reaction of the evaporation source material does not proceed,
A molecular beam epitaxial growth method characterized in that the molecular beam intensity is maintained at a temperature at which the molecular beam intensity becomes 1/100 or less during the growth. 2. The method according to claim 1, wherein the evaporation source material is Ga and / or I.
2. The molecular beam epitaxial growth method according to claim 1, wherein n is n.