JP3081914B2 - Method of growing group III nitride semiconductor film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for growing a group III nitride semiconductor film by a molecular beam epitaxy method, and more particularly to an improvement for obtaining a flat growth surface.

[0002]

2. Description of the Related Art In order to grow a group III nitride semiconductor film such as GaN, AlN, and InGaN, a chemical vapor deposition method has been used so far.
Either the (CVD) method or the molecular beam epitaxy (MBE) method is used. However, the latter method is desirable in the future when considered as a construction material of a short wavelength optical element represented by a blue light emitting diode or the like, or a high output or high frequency element. In the former, ammonia is mainly used as a nitrogen source, but the growth temperature is 10
This is because the temperature is as high as 00 ° C. or more, and is not suitable for mixed crystal growth due to the difference in the incorporation efficiency between the group III elements. Further, it is difficult to control the growth on the order of atomic layers, and there is a problem that the p-type dopant is inactivated by hydrogen gas used as a carrier gas. Furthermore, there is no suitable method for real-time observation of the growth surface during growth, and there is a disadvantage that the elucidation of the crystal growth mechanism cannot be determined.

[0003]

On the other hand, the latter molecular beam epitaxy method does not have at least such disadvantages, and various "in-situ" observation methods for examining a growing surface during growth are used. Therefore, it is relatively easy to clarify the growth mode and growth mechanism. That said, there are still issues to be solved.

[0004] In the first place, it is generally difficult to obtain a bulk crystal of a group III nitride semiconductor, so that heteroepitaxial growth on a heterogeneous substrate is necessary. Even in the case of (1), about 13% of the lattice mismatch occurs with the grown film. Such a large lattice mismatch causes defects such as dislocation and distortion.

[0005] On the other hand, in a case where the molecular beam epitaxy method is essentially adopted in such a situation, crystal growth is performed under a considerably high vacuum environment. Failure to do so results in inconveniences, such as the escape of nitrogen atoms from the grown crystal and a markedly reduced growth rate. Therefore, a highly reactive nitrogen radical is required as a nitrogen source, and this must be obtained from an appropriate plasma source.

However, when nitrogen radicals are supplied from a plasma source, they react immediately upon encountering a group III element because of their high reactivity, and before the group III element reaches a thermally stable position in the crystal lattice. There is a problem that crystals are formed. For example, even if the same molecular beam epitaxy method is used for ordinary crystal growth of a group III-V compound semiconductor without using a radical, the group III element and the group V element are sufficiently two-dimensionally grown on the growth surface. It diffuses uniformly and bonds with each other with a reaction constant determined by the substrate temperature. However, when radicals are used, the group III element diffuses sufficiently on the growth surface, and the probability of reacting with nitrogen radicals before forming a flat surface as a whole increases. In addition, the plasma contains high-energy particles that can strike the growth surface and physically damage it.

[0007] In combination with the above, the growth surface of the group III nitride semiconductor film, particularly the growth surface in the early stage of growth, is unlikely to become a two-dimensionally flat layered surface, and tends to be uneven. Such a rough surface state is not recovered by simply continuing the growth thereafter, but may be rather severe. Of course, the lack of flatness significantly lowers the quality of the finally formed group III nitride semiconductor film, and is a major factor in lowering the performance of various electronic and optical devices manufactured using the same.

The present invention has been made in order to solve such a problem. When a group III nitride semiconductor film is formed by a molecular beam epitaxy method using nitrogen radicals, the flatness of the growth surface is restored. It is not intended to provide a method for obtaining it.

[0009]

According to the present invention, in order to achieve the above object, a group III element is sufficiently diffused two-dimensionally on a growth surface which is roughened or seems to be roughened. Is proposed to perform time width modulation on the irradiation of the radical beam toward the growth surface in order to increase the time required. In short, the radical beam is supplied intermittently. In this way, while the supply of the radical beam is stopped, the group III element can sufficiently diffuse two-dimensionally on the growth surface, and even in the concave portion of the roughened surface,
The probability of reacting with the subsequently supplied nitrogen radicals is increased. Therefore, the growth surface is flattened by repeating this radical beam intermittently as needed.

However, according to the knowledge of the present inventor, when the flatness is restored by adopting the surface flattening step of interrupting the radical beam as described above, the III method is performed similarly to the conventional method. It was found that the recovered flatness was not significantly impaired even when the radical beam was continuously irradiated together with the group element. Thus, in other words, such a surface planarization step according to the present invention can be used to reduce the
It may be inserted at any appropriate time between the start of the growth of the group II nitride semiconductor film and the completion of the growth to obtain a desired film thickness.

In order to confirm whether or not the flatness has been restored by the surface flattening step, as described above, the molecular beam epitaxy method employs various in-situ observation methods suitable for observing the growth surface. Therefore, it is preferable to use them. Among them, the reflection high-energy electron diffraction method, which is abbreviated as “RHEED”, is particularly preferable because the apparatus configuration is relatively simple and the observation operation is easy.

Conversely, by such an in-situ observation method, it is appropriate to set the time width modulation duty (finally the stop time and the supply time) of the radical beam necessary to restore the flatness of the surface. And the number of repetitions thereof, that is, if one stop and one supply are regarded as one cycle, it is possible to obtain information on how many times the cycle should be.

The surface flattening step according to the present invention may be inserted at least once during the growth process as described above. However, if necessary, for any reason, the surface flattening step is performed on the substrate surface. The surface planarization step may be continued from the start of the growth of the first group III nitride semiconductor film to the completion of the growth of the final layer having a desired film thickness.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1A shows an example of the structure of an apparatus that can be used in the present invention. However, the device configuration itself may be any one that can be applied to a known and existing molecular beam epitaxy method of this kind, in order to selectively stop the supply of the nitrogen radical beam containing the nitrogen excited species from the nitrogen radical source 12. An arbitrary configuration may be adopted except that necessary elements such as the shutter 15 and the valve 16 are essential. Briefly, just in case, in the growth chamber 10 maintained in a high vacuum environment, an appropriate sample substrate 20 such as sapphire on which a group III nitride semiconductor film is to be grown is inserted and set on the surface thereof. ,
The substrate 20 can be heated to a desired substrate temperature by the heater 13. As mentioned earlier, usually I
The group II nitride semiconductor film is formed by heteroepitaxial growth on a heterogeneous substrate.

A nitrogen species from a nitrogen source such as a nitrogen cylinder is supplied to a nitrogen radical source 12 via a valve 16 so as to be selectively opened and closed.
A radical beam containing nitrogen excited species is supplied therein. The nitrogen radical source 12 is generally configured as a suitably configured plasma beam generator. However, in the case shown in the figure, a shutter 15 is also provided at the exit of the radical beam as desired. When the shutter 15 is closed, the radical beam is shut off and is not supplied directly to the substrate surface 21.

On the other hand, a group III element source 11, which is generally used as a group III element evaporation source,
The group III element is supplied and reacts with the nitrogen radical, whereby the group III nitride semiconductor film is epitaxially grown on the growth surface 21 of the sample substrate 20.

The surface state of the group III nitride semiconductor film growing on the growth surface at that time is constituted by an appropriate in-situ observation device, preferably an electron beam irradiation source 31 and a wave receiving surface portion 32. Reflection high-energy electron diffraction (RHEED) system (31,
32). Growth surface 21 from electron beam irradiation source 31
The electron beam emitted toward the surface is reflected and diffracted by the growth surface 21 and reaches the wave receiving surface portion 32. The wave receiving surface portion 32 is generally configured as a display coated with a fluorescent substance. As is well known, if the growth surface 21 is a rough surface, it becomes a spot-like pattern in which bright spots appear, and a flat surface. , A streak pattern in which a plurality of linear bright lines are parallel.

However, if the growth is continued while the nitrogen radical beam from the nitrogen radical source 12 and the group III element from the group III element source 11 are continuously supplied, as shown in FIG. For this reason, irregularities are likely to occur on the growing surface 21 during the growth. Therefore, in order to recover the flatness, according to the present invention, as shown in FIG. Supply intermittently.

Although the method of shutting off the nitrogen radical beam may be arbitrary, for example, the shutter 15 shown in FIG.
Close it temporarily as shown in (A), or Figure 1 (A)
Can be performed by temporarily closing the valve 16 shown in FIG.

In any case, when the nitrogen radical beam is cut off, the group III element supplied to the growth surface 21 on the sample substrate 20 during that time is, as schematically shown in FIG. It can diffuse two-dimensionally uniformly on the growth surface which has been rough surface up to that time, and can enter into the concave portion of the rough surface.

Here, the time Toff during which the radical beam is stopped is desirably set to such a time that the group III element of one or two atomic layers is present on the growth surface. This can be defined as the time until the surface of the underlying group III nitride semiconductor film is no longer observed by an appropriate in-situ observation method. As shown in the drawing, when the RHEED device is used, by visually recognizing the wave receiving surface portion 32, when diffraction from the underlying group III nitride semiconductor film is no longer recognized,
It can be recognized that one or two atomic layers of Group III elements are present.

When this occurs, the shutter 15 or the valve 16 is opened, and the supply of the nitrogen radical beam is started again. Then, the supply time Ton (FIG. 1 (B)) until the nitrogen radical beam is cut off again is again set by the RHEED device.
The time is set to a point at which a pattern recognized as diffraction from the surface of the group III nitride semiconductor film is generated.

As described above, the on / off duty of the nitrogen radical beam, that is, the shutoff time Toff and the supply time Ton of the nitrogen radical beam can be desirably determined by an in-situ observation method. If one cycle of the supply is required, it is also possible to find out how many cycles are appropriate by repeating this, preferably by an in-situ observation method. In other words, when the diffraction pattern from the surface of the group III nitride semiconductor film is a spot-like pattern,
It can be recognized that the flatness of the growth surface has not yet been recovered, and if the pattern changes to a streak-like pattern after several intermittent nitrogen radical beams, the growth surface has been flattened to a satisfactory degree. I can judge.

Once the flatness of the growth surface is restored, the flatness is generally good even if a nitrogen radical beam is continuously supplied in addition to the group III element, as shown in FIG. 2 (B). Is maintained. However, if the appearance of the rough surface state is recognized again on the way, the intermittent supply step of the nitrogen radical beam according to the present invention may be restarted.

Conversely, if necessary, the interruption of the nitrogen radical beam may be continued from the start to the end of the growth, if necessary. Also,
Generally speaking, even when the method of the present invention is used temporarily, it is desirable that it is an initial growth stage.
The surface of the sample substrate 20 is also affected by the unevenness of the surface itself, and the degree of the rough surface of the growth surface is large in the initial growth process. This is because even if the simple growth process is continued, the possibility of finally obtaining a group III nitride semiconductor film having high flatness increases.

[0026]

EXAMPLES The method of the present invention can be applied to any group III element. Here, an example in which a Ga element is used to grow a GaN thin film will be described to confirm the effects of the present invention. Keep it.

On a sapphire substrate (0001) surface heated to 700 ° C., solid Ga and N ₂ high frequency plasma source (N ₂ flow rate = 1.5
(cm power, RF power = 400W), molecular beam epitaxy growth of hexagonal GaN through a low-temperature buffer layer of GaN by nitrogen radicals. A spot-like RHEED pattern was generated.

Therefore, according to the present invention, a surface flattening step is performed, and the nitrogen radical beam is interrupted for 4 to 5 cycles.
After repeating for 2-3 minutes, the RHEED pattern changed to a streak pattern, and the growth surface was flattened.
Thereafter, the growth was continued by switching to the continuous irradiation of the nitrogen radical beam, but the flatness of the growth surface was maintained.

FIG. 3 shows the X-ray diffraction width (X-ray FMVH) characteristics. When the modulation operation (intermittent) of the nitrogen radical beam according to the present invention is performed, the characteristics are clearly improved. Furthermore, the effect of applying the present invention to the case of using the AlN low-temperature buffer layer and the nitridation of the substrate itself as an initial process of heteroepitaxial growth was also observed.
As shown in FIG.

[0030]

According to the present invention, when a group III nitride semiconductor film is formed by a molecular beam epitaxy method using nitrogen radicals, the flatness of a growth surface which has conventionally tended to be uneven can be recovered. Therefore, the characteristics of various electronic and optical elements using the group III nitride semiconductor film produced by the method of the present invention as a functional part structure film can also be improved.
Regarding the fabrication of a nitride semiconductor heterostructure, it can contribute to improvement of the flatness and steepness of the interface, and various applications can be spread with high reliability.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of an apparatus configuration capable of implementing the method of the present invention and an explanation of the method of the present invention.

FIG. 2 is an explanatory view showing how a growth surface of a group III nitride semiconductor film is flattened according to the present invention.

FIG. 3 is an explanatory diagram of an effect on an X-ray diffraction width characteristic in an example of the method of the present invention.

[Explanation of symbols]

 10 Growth chamber 11 Group III element source 12 Nitrogen radical source 15 Shutter 16 Valve 20 Sample substrate 21 Growth surface 31 Electron beam irradiation source of RHEED device 32 Receiving surface of RHEED device

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 21 / 203,21 / 363 C30B 23/08

Claims (6)

(57) [Claims] 1. A group III element from a group III element source and a radical beam from a radical source containing a nitrogen-excited species are irradiated on a temperature-controlled substrate surface in a vacuum environment. A method for growing a group III nitride semiconductor film by molecular beam epitaxial growth of a nitride semiconductor film, comprising: a method for growing a group III nitride semiconductor film on the substrate surface; , III
A method of intermittently irradiating the radical beam with the supply of the group element to recover the flatness of the growth surface. 2. The method according to claim 1, wherein after the surface flattening step, the radical beam is continuously irradiated together with the group III element. 3. The method according to claim 1, wherein the recovery of the flatness is confirmed by an in-situ observation method capable of observing a growth surface state during growth. 4. The method according to claim 3, wherein the stop time and the irradiation time, and the number of repetitions of the stop and the irradiation when the radical beam is intermittently irradiated are determined by the in-situ observation method. Determining based on the results. 5. The method according to claim 3, wherein the in-situ observation method is a reflection high-energy electron beam diffraction method. 6. The method according to claim 1, wherein the step of planarizing the surface of the group III nitride semiconductor film on the surface of the substrate is continued from the start to the completion of the growth. A method for growing a group III nitride semiconductor film.