JP5216677B2 - Zinc oxide substrate surface treatment method and zinc oxide crystal production method - Google Patents

Description

  The present invention relates to a surface treatment method of a zinc oxide substrate for obtaining a zinc oxide substrate having excellent surface cleanliness and flatness, and production of a zinc oxide crystal for crystal growth on the surface of the zinc oxide substrate treated by the method. Regarding the method.

So far, a sapphire (Al ₂ O ₃ ) substrate has been used for the growth of zinc oxide-based semiconductor crystals. However, since the lattice constant of a sapphire substrate is significantly different from that of a zinc oxide-based semiconductor, many crystal defects are generated when a zinc oxide crystal is grown on the substrate, and the obtained crystal is difficult to use in a device. Met.

  On the other hand, in recent years, single crystal zinc oxide substrates have been realized, and application to crystal growth of zinc oxide based semiconductors is expected. By growing a zinc oxide crystal using a single crystal zinc oxide substrate, the occurrence of crystal defects, which is a problem when using a sapphire substrate, can be reduced. However, at present, the cleanliness and flatness of the surface of the single crystal zinc oxide substrate are insufficient, and it is difficult to use it for crystal growth of zinc oxide.

For example, if there is a foreign substance on the substrate surface, the semiconductor crystal to be grown not only causes crystal defects due to the foreign substance but also enters the semiconductor crystal as an impurity. Thus, the crystallinity and electrical characteristics of a semiconductor crystal in which crystal defects and impurities are mixed are deteriorated. Therefore, usually, the surface of the substrate used for semiconductor crystal growth is cleaned by various methods to remove foreign substances.
On the other hand, if the substrate surface is rough, the surface of the semiconductor crystal to be grown is also rough due to the rough shape of the substrate surface. For example, light-emitting diodes (LEDs) and laser diodes (LDs) have a laminated structure that sandwiches a very thin thin film of several atomic layers called a quantum well structure, so that the applied thin film is sufficiently flat in units of atomic layers. It is because it is necessary to have sex.
Therefore, in order to make it possible to apply a semiconductor crystal to an element, it is important to grow the semiconductor crystal using a substrate having excellent surface cleanliness and flatness.

  Conventionally, as a substrate surface cleaning method, for example, a method of removing impurities by etching the surface of a zinc oxide substrate with hydrochloric acid (see Non-Patent Document 1), a surface of the zinc oxide substrate is trifluoroacetic acid (TFA). (See Non-patent Document 2), a method of etching a metal oxide substrate in an oxygen atmosphere and then etching the surface with a nitric acid solution (see Patent Document 1), zinc oxide A method in which the substrate is heated in an oxygen atmosphere and then the surface is etched with an alkaline solution (see Patent Document 2), and a method in which the surface of the gallium oxide substrate is heat-treated in a vacuum chamber (see Patent Document 3). It is disclosed.

JP 2006-45354 A JP 2007-1787 A JP 2006-310850 A

J. et al. Appl. Phys. 83 (2), 15, January 1998 Superlattices and Microstructures 38 (2005) 349-363

However, the method described in Non-Patent Document 1 has a problem that the etching rate varies depending on the polarity of the surface of the zinc oxide substrate and is hardly etched on the Zn polar surface, whereas the surface is severely corroded on the O polar surface. was there. In addition, when holes (pits) due to crystal defects exist on the substrate surface, etching proceeds at an accelerated rate, and large pits are formed on the substrate surface.
The method described in Non-Patent Document 2 only discloses etching on the O-polar surface, and further, when the etching amount is 70 nm or more, the surface becomes cloudy and the substrate surface becomes rough. It was.

On the other hand, the method described in Patent Document 1 is applied to a single crystal substrate made of ReScO ₃ (Re represents at least one of La, Ce, Pr, Nd, Sm, Eu, Cd, Tb, Dy, Ho, and Y). It is not intended for a zinc oxide substrate. When a zinc oxide substrate is heat-treated in an oxygen atmosphere, heavy metals attached to the substrate surface combine with oxygen (O) on the substrate surface to form a stable oxide layer. The oxide layer cannot be removed by wet etching depending on the type.
In the method described in Patent Document 2, the etching rate increases when the pH of the alkaline solution increases, but the flatness of the substrate surface deteriorates. Furthermore, in wet etching, the etching state varies greatly depending on the solution state and pH, and it is difficult to obtain the cleanliness and flatness reproducibility of the substrate surface. As described above, the method described in Patent Document 2 has a problem that a substrate having excellent surface cleanliness and flatness cannot be easily obtained.
Further, in the method described in Patent Document 3, oxygen is desorbed from the substrate surface and the flatness of the substrate surface is deteriorated. Patent Document 3 also discloses that the substrate surface is irradiated with an N beam after the heat treatment, but this is not for cleaning the substrate surface but for the purpose of modification. The method described in Patent Document 3 is not intended for a zinc oxide substrate.

Further, in the cleaning by wet etching, it is inevitable that the substrate is exposed to the atmosphere after etching, and when the substrate is used after that, it is necessary to clean again in the apparatus to be used. It was.
On the other hand, a method of cleaning the substrate surface by a dry process has been studied, and a method of performing plasma processing in a vacuum is also disclosed, but because the plasma is high energy, the substrate surface is roughened. There was a problem.

  The present invention has been made in view of the above circumstances, and a surface treatment method of a zinc oxide substrate for obtaining a zinc oxide substrate excellent in surface cleanliness and flatness, and a zinc oxide substrate treated by the method It is an object of the present invention to provide a method for producing a zinc oxide crystal using

To solve the above problem,
The invention according to claim 1 is characterized in that the surface of the zinc oxide substrate is etched by irradiating the surface of the zinc oxide substrate heated under vacuum conditions with a gallium molecular beam. It is a processing method.
The invention according to claim 2 is characterized in that the pressure under the vacuum condition is 1.3 × 10 ^{−8 to} 5.3 × 10 ⁻³ Pa, and the surface of the zinc oxide substrate according to claim 1 It is a processing method.
The invention according to claim 3 is the surface treatment method for a zinc oxide substrate according to claim 1 or 2, wherein the heating temperature of the zinc oxide substrate is 600 to 1000 ° C.
The invention according to claim 4 is characterized in that the intensity of the gallium molecular beam is 1.3 × 10 ^{−8 to} 1.3 × 10 ⁻⁶ Pa. It is the surface treatment method of the described zinc oxide substrate.
The invention according to claim 5 is characterized in that the surface of the zinc oxide substrate is treated by the method according to any one of claims 1 to 4 and then zinc oxide crystals are grown on the surface. It is a manufacturing method of a crystal.

  According to the present invention, a zinc oxide substrate having excellent surface cleanliness and flatness can be obtained. Further, by using the substrate, a high-quality zinc oxide crystal suitable for use in an element can be obtained.

It is a schematic block diagram which illustrates the processing apparatus of the surface of a zinc oxide board | substrate used by this invention. It is a schematic block diagram which illustrates the manufacturing apparatus of the zinc oxide crystal | crystallization used by this invention. It is an AFM image of the substrate surface in Example 1, (a) is an AFM image before etching, (b) is an AFM image after etching about 40 nm. It is a RHEED image of the substrate surface in Example 1, (a) is a RHEED image before an etching, (b) is a RHEED image after etching about 40 nm. FIG. 2 is an AFM image of a substrate surface in Example 2, (a) is an AFM image before etching, and (b) is an AFM image after etching several nm. It is a RHEED image of the substrate surface in Example 2, (a) is a RHEED image before etching, (b) is a RHEED image after etching several nm.

<Surface treatment method of zinc oxide substrate>
The surface treatment method for a zinc oxide substrate of the present invention is characterized in that the surface of the zinc oxide substrate is etched by irradiating the surface of the zinc oxide substrate heated under vacuum conditions with a gallium (Ga) molecular beam. And
The structure of dry etching the surface of a substrate with a Ga molecular beam under vacuum conditions has not been known so far, and the present invention having such a structure can be applied to an element and has never been produced with high quality oxidation. Zinc crystals are provided.
Hereinafter, the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a schematic configuration diagram illustrating a processing apparatus used in the present invention. The processing apparatus 1 shown here includes a metal Ga evaporation source 11, a Ga evaporation source power supply 12, a vacuum chamber 13, an electron gun 14, and a fluorescent screen 15. The metal Ga evaporation source 11 further includes a heater 11a, and is electrically connected to a Ga evaporation power source 12. By energizing the heater 11a, the housed metal Ga raw material 2 can be heated. The electron gun 14 and the fluorescent screen 15 constitute a reflection type high-speed electron beam diffractometer (RHEED).

In the present invention, the surface of the zinc oxide substrate can be treated by using, for example, the molecular beam epitaxy (hereinafter abbreviated as MBE) method using the processing apparatus 1.
The zinc oxide substrate 3 is installed at a predetermined location in the vacuum chamber 13, and the metal Ga raw material 2 is heated by a heater under reduced pressure under reduced pressure. As a result, a Ga molecular beam is emitted from the metal Ga raw material 2 in accordance with the solid vapor pressure curve. By irradiating the surface of the zinc oxide substrate 3 with this Ga molecular beam, the surface is reduced and etched. At this time, the surface of the zinc oxide substrate 3 at the time of etching can be observed in real time by irradiating the surface of the zinc oxide substrate 3 from the electron gun 14 and detecting the diffraction image with the fluorescent screen 15.

  In the present invention, it is not always necessary to use the electron gun 14 and the fluorescent screen 15, but the surface treatment of the zinc oxide substrate 3 can be performed more easily by etching while observing the substrate surface using these. Can do. Further, the substrate surface may be observed using an apparatus other than RHEED.

The pressure under the vacuum condition is preferably 1.3 × 10 ^{−8 to} 5.3 × 10 ⁻³ Pa (1.0 × 10 ^{−10 to} 4.0 × 10 ⁻⁵ Torr), preferably 1 More preferably, it is 3 × 10 ^{−8 to} 1.3 × 10 ⁻⁶ Pa (1.0 × 10 ^{−10 to} 1.0 × 10 ⁻⁸ Torr). The higher the degree of vacuum, the better. However, the lower limit of the above range is the lowest pressure that can be achieved in a general vacuum chamber. Moreover, etching can be sufficiently performed by setting the pressure to be equal to or lower than the upper limit of the above range. Usually, when the pressure exceeds the upper limit of the above range, irradiation with a Ga molecular beam becomes difficult.

  The heating temperature of the zinc oxide substrate under vacuum conditions is preferably 600 to 1000 ° C. Etching can be sufficiently performed by setting the temperature to 600 ° C. or higher. Usually, when the heating temperature is less than 600 ° C., the desorption of Ga oxide from the substrate surface becomes insufficient. Moreover, by setting it as 1000 degrees C or less, the detachment | desorption of oxygen and zinc from the zinc oxide board | substrate surface by heating can fully be suppressed, and the flatness of a board | substrate surface can be made still higher.

The intensity of the irradiated Ga molecular beam is preferably 1.3 × 10 ^{−8 to} 1.3 × 10 ⁻⁶ Pa (1.0 × 10 ^{−10 to} 1.0 × 10 ⁻⁸ Torr). . By setting it to the lower limit value or more, the intensity of the Ga molecular beam can be clearly confirmed, irradiation can be easily controlled, and etching can be sufficiently performed. Moreover, the residual of Ga oxide on the substrate surface can be effectively suppressed by setting it to the upper limit value or less.

  The etching rate can be controlled by changing the intensity of the Ga molecular beam and the temperature of the substrate. In the present invention, the etching rate is not particularly limited, but it is preferably 5 to 40 nm / hour, more preferably 10 to 30 nm / hour, because the flatness of the substrate surface can be further enhanced.

  The etching depth of the substrate surface is not particularly limited as long as the effects of the present invention are not impaired. Usually, about 50 nm or less is sufficient, and 10 nm or less may be used.

  In order to perform etching with a Ga molecular beam with higher accuracy, it is preferable to remove in advance the foreign matter adhering to the surface of the zinc oxide substrate to be irradiated with the Ga molecular beam. For example, organic substances and the like may be removed by performing a degreasing process.

  In the present invention, both the O-polar surface and the Zn-polar surface of the zinc oxide substrate can be precisely etched. As a result, the flatness of the surface of the zinc oxide substrate can be made extremely high. For example, the RMS roughness of the surface can be made 1 nm or less.

  The apparatus for treating the surface of the zinc oxide substrate is not limited to the one shown in FIG. 1, and any apparatus that can heat the zinc oxide substrate under vacuum conditions and irradiate a Ga molecular beam. The zinc oxide substrate surface-treated according to the present invention is suitable for use in the production of zinc oxide crystals. Therefore, as will be described later, it is preferable that the processing apparatus is provided with a configuration for producing zinc oxide crystals. In this case, since the metal Ga is also a dopant for developing n-type conductivity in the zinc oxide crystal, the Ga-doped zinc oxide crystal can be grown immediately after treating the substrate surface, simplifying the manufacturing process. Is possible.

  Since the surface-treated zinc oxide substrate according to the present invention has a high surface cleanliness, the zinc oxide crystal grown using this surface has a high quality with suppressed crystal defects and impurities. . In addition, since the zinc oxide substrate is excellent in surface flatness as described above, a zinc oxide crystal having sufficient flatness in units of atomic layers can be obtained.

<Method for producing zinc oxide crystal>
The method for producing a zinc oxide crystal of the present invention is characterized in that the surface of a zinc oxide substrate is treated by the method of the present invention, and then zinc oxide crystals are grown on the surface.
In the present invention, the crystal growth may be performed by a known method except that the zinc oxide substrate surface-treated according to the present invention is used as the zinc oxide substrate on which the zinc oxide crystal is grown.

  FIG. 2 is a schematic configuration diagram illustrating a manufacturing apparatus used in the present invention. The manufacturing apparatus 5 shown here is obtained by adding a configuration for growing zinc oxide crystals to the processing apparatus 1 shown in FIG. Specifically, a metal zinc (Zn) evaporation source 51 and a Zn evaporation source power source 52 are provided. The metal Zn evaporation source 51 further includes a heater 51a, and is electrically connected to the Zn evaporation source power source 52. By energizing the heater 51a, the stored metal Zn raw material 6 can be heated. Further, an oxygen (O) plasma supply source 56 and an RF power source 57 are provided, and these are electrically connected. Further, the O plasma supply source 56 is connected to oxygen (O) by piping through an oxygen (O) valve 58. ) Is connected to the source 7, and O radicals are generated by the RF plasma excited by the RF power source 57. 2, the same components as those of the processing apparatus 1 shown in FIG. 1 are denoted by the same reference numerals, and detailed description thereof is omitted.

In the present invention, the manufacturing apparatus 5 can be used, for example, to manufacture zinc oxide crystals as shown below.
First, the surface treatment of the zinc oxide substrate 3 is performed by the above method. Next, heating is continued as it is without removing the treated zinc oxide substrate from the manufacturing apparatus 5. The heating temperature at this time is not particularly limited as long as it is a temperature necessary for crystal growth of zinc oxide, and may be the same as the temperature at the surface treatment, for example. Then, the metal Zn raw material 6 is heated and supplied into the vacuum chamber 13. At the same time, the O valve 58 is opened to supply oxygen (O ₂ ) gas from the O source 7 into the vacuum chamber 13. Then, the O plasma generated by exciting the RF plasma with a predetermined output by the high frequency generated by controlling the RF power source 57 by setting the O ₂ partial pressure in the vacuum chamber 13 to a predetermined range is obtained. The plasma is supplied from the plasma supply source 56 into the vacuum chamber 13. Thereby, Zn and O plasma are supplied to the surface of the treated zinc oxide substrate, and zinc oxide crystals can be grown.

The apparatus for producing zinc oxide crystals is not limited to that shown in FIG. 2, and any apparatus having a configuration for performing surface treatment of a zinc oxide substrate according to the present invention and growth of zinc oxide crystals may be used.
The conditions for generating O ₂ partial pressure and O plasma during crystal growth may be appropriately adjusted according to known methods.

  The zinc oxide crystal produced according to the present invention is excellent in crystallinity and electrical characteristics and particularly suitable for use in various devices because it suppresses the incorporation of crystal defects and impurities and is excellent in surface flatness. is there.

Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the present invention is not limited to the following examples.
[Example 1]
(Surface treatment of O-polar single crystal zinc oxide substrate)
Using the processing apparatus shown in FIG. 1, the O-polar surface of the single crystal zinc oxide substrate was surface-treated by the MBE method. First, the O-polar single crystal zinc oxide substrate was degreased and placed in a vacuum chamber to obtain an ultrahigh vacuum state of 1.3 × 10 ⁻⁸ Pa (1 × 10 ⁻¹⁰ Torr) and heated to 650 ° C. . Then, the power source for the evaporation source was controlled, the heater was energized, and the metal Ga raw material was heated to 785 ° C. The metal Ga raw material was irradiated on the surface of the heated zinc oxide substrate as a metal Ga molecular beam from a metal Ga evaporation source according to a solid vapor pressure curve. The intensity of the metal Ga molecular beam at this time was 1.2 × 10 ⁻⁶ Pa (9.0 × 10 ⁻⁹ Torr). The etching rate was 20 nm / hour. In this state, the surface treatment was performed while observing the state of the substrate surface with RHEED. FIG. 3 shows an AFM image of the substrate surface. FIG. 3A shows an AFM image before etching, and FIG. 3B shows an AFM image after etching by about 40 nm. As apparent from FIG. 3B, the substrate surface after the etching has a high cleanliness, and the RMS roughness is about 0.8 nm, confirming that the flatness is extremely high. FIG. 4 shows an RHEED image of the substrate surface. FIG. 4A shows an RHEED image before etching, and FIG. 4B shows an RHEED image after about 40 nm etching. From FIG. 4B, after etching, (2 × 2) surface reconstruction was observed, and it was confirmed that the activity of dangling bonds on the outermost surface that greatly affects the growth of zinc oxide crystals was high. .

[Example 2]
(Surface treatment of Zn-polar single crystal zinc oxide substrate)
Using a Zn polar single crystal zinc oxide substrate, the Zn polar face of the single crystal zinc oxide substrate was surface-treated in the same manner as in Example 1. FIG. 5 shows an AFM image of the substrate surface. FIG. 5A shows an AFM image before etching, and FIG. 5B shows an AFM image after etching several nm. As apparent from FIG. 5B, the substrate surface after the etching has a high cleanliness and the RMS roughness is about 0.2 nm, and it was confirmed that the flatness is extremely high. FIG. 6 shows an RHEED image of the substrate surface. FIG. 6A shows an RHEED image before etching, and FIG. 6B shows an RHEED image after etching several nm. FIG. 6 confirms the streak pattern observed when the halo pattern due to the non-uniform crystal state of the surface in the background is reduced and the flatness of the surface is high.

[Example 3]
(Growth of zinc oxide semiconductor crystal using surface-treated O-polar single crystal zinc oxide substrate)
In the same manner as in Example 1, the O-polar surface of the single crystal zinc oxide substrate was surface-treated, and after etching, (2 × 2) surface reconstruction was confirmed.
Next, a zinc oxide semiconductor crystal was grown while maintaining the substrate temperature at 650 ° C. Zinc (Zn) as a raw material was supplied to the vacuum chamber from the metal Zn evaporation source by controlling the power source for the metal Zn evaporation source and energizing the heater to heat the metal Zn source. On the other hand, oxygen (O) was supplied by operating a valve to introduce oxygen (O ₂ ) gas supplied from an oxygen source into the vacuum chamber. At this time, the O partial pressure in the vacuum chamber is set to 2.7 × 10 ⁻³ Pa (2.0 × 10 ⁻⁵ Torr), and RF plasma is 300 W at a high frequency of 13.56 MHz generated by the RF power source. Excitation was performed at the output, thereby generating O radicals. This O radical was supplied from the O plasma supply source to the treated surface of the single crystal zinc oxide substrate to grow a zinc oxide semiconductor crystal.
The manufactured zinc oxide crystal was free from crystal defects, mixed with impurities, and excellent in surface flatness.

  INDUSTRIAL APPLICABILITY The present invention can be used for manufacturing various devices that require excellent electrical characteristics.

  DESCRIPTION OF SYMBOLS 1 ... The processing apparatus of the surface of a zinc oxide substrate, 2 ... Metal Ga raw material, 3 ... Zinc oxide substrate, 6 ... Metal Zn raw material, 7 ... O source, 11 ... Metal Ga Evaporation source, 11a: heater, 12 ... power source for Ga evaporation source, 13 ... vacuum chamber, 51 ... metal Zn evaporation source, 51a ... heater, 52 ... power source for Zn evaporation source 56 ... O plasma supply source, 57 ... RF power supply, 58 ... O valve

Claims (5)

  A surface treatment method for a zinc oxide substrate, comprising: irradiating a surface of a zinc oxide substrate heated under vacuum conditions with a gallium molecular beam to etch the surface of the zinc oxide substrate. 2. The surface treatment method for a zinc oxide substrate according to claim 1, wherein a pressure under the vacuum condition is 1.3 × 10 ^{−8 to} 5.3 × 10 ⁻³ Pa. ³ .   3. The surface treatment method for a zinc oxide substrate according to claim 1, wherein a heating temperature of the zinc oxide substrate is 600 to 1000 ° C. 3. 4. The surface treatment of a zinc oxide substrate according to claim 1, wherein the intensity of the gallium molecular beam is 1.3 × 10 ^{−8 to} 1.3 × 10 ⁻⁶ Pa. 5. Method.   A method for producing a zinc oxide crystal, comprising treating a surface of a zinc oxide substrate by the method according to any one of claims 1 to 4 and then growing a zinc oxide crystal on the surface.

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