JP5654910B2 - Zinc oxide substrate processing method - Google Patents

Description

  The present invention relates to a method for processing a zinc oxide (ZnO) substrate, and more particularly to a substrate processing method performed before crystal growth.

  Zinc oxide (ZnO) has physical properties that are extremely suitable for semiconductor light emitting devices, and is expected as a material for optical devices in the blue or ultraviolet region. Further, the present invention can be widely applied not only to light emitting elements but also to light receiving elements, surface acoustic wave (SAW) devices, piezoelectric elements, and the like. In addition, the raw material is inexpensive and harmless to the environment and the human body.

  In general, when a crystal growth of a ZnO-based semiconductor is performed on a ZnO substrate, substrate pretreatment such as substrate etching is performed to remove a damaged layer on the substrate surface, planarize, and remove a residue. For example, Patent Document 1 discloses etching using a mixed solution of EDTA · 2Na (ethylenediaminetetraacetic acid disodium) and EDA (ethylenediamine) as an etchant for a ZnO substrate formed by a hydrothermal synthesis method. Patent Document 2 discloses a first step of etching a zinc oxide-based material with a solution containing oxalic acid, and a second step of removing an etching residue in the first step using an alkaline solution. An etching method is disclosed. Patent Document 3 discloses that the final treatment of the substrate surface is performed by acidic wet etching at pH 3 or lower to prevent the generation of Zn hydroxide.

JP 2007-1787 A JP 2010-67824 A International Publication WO2009 / 001919

Colloid and Interface Chemistry 2nd Edition, Fumio Kitahara, Koichiro Aoki Co-translation / Yodogawa Shoten

  Although it is considered that the ZnO single crystal substrate has achieved high quality as a substrate for manufacturing semiconductor elements due to recent progress in substrate manufacturing technology, even when such a high quality ZnO substrate is used, The present inventors have obtained the knowledge that the following problems are present in substrate pretreatment such as etching when performing crystal growth, and have been made to solve such specific problems.

  In other words, in the conventional method, Si (silicon) remains on the substrate surface in some state (zinc silicide, attached silica, buried silica, etc.), and Si on the substrate surface diffuses into a ZnO-based semiconductor growth layer such as MgZnO.・ It has been made as a solution to such a problem by obtaining the knowledge that it remains, and the surface flatness and morphology of the growth layer are deteriorated, thereby inhibiting the crystal growth.

  An object of the present invention is to provide a growth pretreatment method for a ZnO substrate in which zinc silicide, silica, and the like do not remain. By such pretreatment, it is possible to obtain high-quality crystals having excellent surface flatness and no Si diffusion in crystal growth such as MOCVD (Metal Organic Chemical Vapor Deposition).

The present invention, the cleaning liquid substrates ZnO single crystal with principal Zn polar surface, and an etching step of etching with a solution containing EDTA chelate compound as an etchant, etching then an electrolyte solution having a ligand And a cleaning process for cleaning the substrate.

Is a sectional view showing a wafer subjected to crystal growth of the ZnO layer and Mg _x Zn _1-x O layer on a ZnO substrate for evaluation. The evaluation results of the growth layer using the substrate according to Example 1, specifically, XRD2θ scan (a), (100) plane rocking curve (b), AFM image (c), SIMS depth direction profile (d) FIG. It is a figure which shows the evaluation result of the growth layer using the board | substrate by Example 2, specifically, XRD2 (theta) scan (a), (100) plane rocking curve (b), and AFM image (c). It is a figure which shows the evaluation result of the growth layer using the board | substrate by Example 3, specifically, XRD2 (theta) scan (a), (100) plane rocking curve (b), and AFM image (c). It is a figure which shows the evaluation result of the growth layer using the board | substrate by Example 4, specifically, XRD2 (theta) scan (a), (100) plane rocking curve (b), and AFM image (c). It is a figure which shows the evaluation result of the growth layer using the board | substrate by Example 5, specifically, XRD2 (theta) scan (a), (100) plane rocking curve (b), and AFM image (c). The evaluation result of the growth layer using the substrate according to Comparative Example 1, specifically, XRD2θ scan (a), (100) plane rocking curve (b), AFM image (c), SIMS depth direction profile (d) FIG. It is a figure which shows the evaluation result of the growth layer using the board | substrate by the comparative example 2, specifically, XRD2 (theta) scan (a), (100) plane rocking curve (b), and AFM image (c). It is a figure which shows collectively the evaluation result of the growth layer using the board | substrate which performed the process by Examples 1-5 and Comparative Examples 1 and 2. FIG. It is a figure which shows the relationship between an etching mixed solution and cleaning liquid.

  Hereinafter, a pre-growth processing method for a ZnO substrate in which impurities including Si such as zinc silicide and silica do not remain will be described in detail with reference to the drawings. Further, a comparative example for explaining the features, configuration, and effects of the substrate processing method according to the present embodiment will be described in detail. In addition, a comparative example for explaining the characteristics of the crystal layer grown using the substrate that has been processed by the processing method will be described in detail.

[MOCVD equipment]
Crystal growth was performed using the substrate processed according to this embodiment and the comparative example, and the growth layer was evaluated and analyzed. First, the structure of a MOCVD (Metal Organic Chemical Vapor Deposition) apparatus used for crystal growth and the crystal growth material will be described below.

  The MOCVD apparatus used is composed of a gas supply unit, a reaction vessel unit, and an exhaust unit. The gas supply unit includes a portion for vaporizing and supplying the organometallic compound material, a portion for supplying a gaseous material gas, and a transport unit having a pipe for transporting these gases.

  The organometallic compound material gas and the gaseous material gas are supplied as carrier gas (nitrogen) to the reaction vessel through the first run pipe and the second run pipe, respectively, at the time of growth. A flow rate adjusting device (mass flow controller) is installed in each of the first run pipe and the second run pipe, and the air is sent to the shower head attached to the reaction vessel by the carrier gas adjusted to a predetermined flow rate. When waiting for growth, the organometallic material gas and the gaseous material gas are exhausted through the first vent pipe and the second vent pipe, respectively.

  The reaction vessel portion is provided with a showerhead that blows the organometallic material gas and the material gas onto the substrate, a susceptor that holds the substrate, and a heater that heats the susceptor. The substrate has a structure that can be heated from room temperature to about 1100 ° C. by a heater.

  Note that the substrate temperature in this embodiment refers to the temperature of the surface of the susceptor on which the substrate is placed. That is, in the case of the MOCVD method, heat transfer from the susceptor to the substrate is performed by direct contact and a gas existing between the susceptor and the substrate. Between the growth pressures of 1 kPa to 120 kPa (Pa: Pascal) used in this example, the surface temperature of the substrate is about 10 ° C. lower than the surface temperature of the susceptor.

  A rotation mechanism for rotating the susceptor is provided in the reaction vessel. More specifically, the susceptor is rotatably supported on the stage and is configured to be rotated by a rotary motor.

  The exhaust part is composed of an in-container pressure adjusting device and an exhaust pump, and has a structure in which the pressure in the reaction container can be adjusted to about 0.01 kPa to 120 kPa by the in-container pressure adjusting device.

[Crystal growth material]
In this example, a material that does not contain oxygen in the constituent molecules was used as the organometallic compound material. An organometallic material that does not contain oxygen is highly reactive with water vapor (oxygen material or oxygen source), and has a low growth pressure, or a flow rate ratio of water vapor to organometallic (MO) (F _H2O / _FMO ratio) or VI / ZnO-based crystals can be grown even in a region where the II ratio is low.

  In this example, DMZn (dimethylzinc) and Cp2Mg (biscyclopentadienylmagnesium) were used as Group II materials, but DEZn (diethylzinc), MeCp2Mg (bismethylpentadienylmagnesium), EtCp2Mg (bismuth). Ethyl pentadienyl magnesium). Further, TMGa (trimethylgallium), TEGa (triethylgallium), TMAl (trimethylaluminum), TEAl (triethylaluminum), TIBA (triisobutylaluminum), or the like can be used as the group III material.

As the oxygen material (hereinafter referred to as oxygen source), a polar oxygen material (polar oxygen source) is suitable. In particular, H ₂ O (water vapor) is greatly polarized to δ ⁺ and δ ⁻ on the side where hydrogen atoms are bonded in the molecule and on the side of the lone pair, and has an excellent adsorption ability to the oxide crystal surface.

The H ₂ O molecule has a tetrahedral structure with a hydrogen atom bond and a lone pair, and an oxide of sp ³ -type hybrid orbital (Zincblende / Cubic) or wurtzite (Wurtzeite / Hexagonal). Crystal growth is an excellent oxygen source that is preferentially oriented and adsorbed to oxygen sites. Similarly, other oxygen sources may be lower alcohols having a large dipole moment and O atoms taking sp ³ type hybrid orbitals. Specifically, as an oxygen source, in addition to H ₂ O (water vapor), lower alcohols, in particular, lower alcohols having 1 to 5 carbon atoms such as methanol, ethanol, propanol, butanol and pentanol can be used. . In this example, H ₂ O (ultra pure water: manufactured by Kanto Chemical Co., Inc., standard: Ultrapur) was used.

As the carrier gas (atmosphere gas), an inert gas that does not react with the crystal growth material described above is suitable. A gas that does not hinder the adsorption of the crystal growth material to the substrate surface, such as H ₂ O (water vapor) or TBP (tertiary butyl phosphine), is preferable. Specifically, an inert gas such as He (helium), Ne (neon), Ar (argon), Kr (krypton), Xe (xenon), or N ₂ (nitrogen) can be used as the carrier gas and the atmospheric gas. . In this example, N ₂ (nitrogen) gas having a residual O ₂ concentration of less than 1 ppm was used as a carrier gas.

  A ZnO (zinc oxide) substrate is a crystal having a wurtzite structure, and typical substrate cut-out surfaces include a c-plane that is a {0001} plane, an a-plane that is a {11-20} plane, {10- There are m-planes that are 10} planes and r-planes that are {10-12} planes. The c-plane includes a Zn polar plane (+ c plane) and an O polar plane (-c plane).

  In the examples and comparative examples described below, a ZnO single crystal substrate cut out from an ingot manufactured by a hydrothermal method was used. In addition, the board | substrate which reduced residual Li density | concentration by processes, such as high temperature heat processing (1000 degreeC or more), was used.

  The ZnO single crystal substrate 10 is preferably a substrate whose main surface (crystal growth surface) is a Zn polar surface (+ c plane) (hereinafter also referred to as a c-plane ZnO single crystal substrate). In the following examples and comparative examples, a substrate whose crystal growth surface is a Zn polar surface was used. Moreover, it is preferable that the substrate main surface (crystal growth surface) be inclined to either the a-axis or the m-axis. In the following examples and comparative examples, specifically, the so-called 0.5 ° off-substrate (or c-plane in the m-axis direction) in which the (0001) plane is inclined by 0.5 ° in the [10-10] direction. 0.5 ° off substrate tilted by 0.5 °) was used. Note that {} indicates an equivalent surface representative value, and () indicates a unique surface. For example, {111} = (111), (−111), (1-11), (11-1). Similarly, <> indicates a representative value of equivalent axial orientations, and [] indicates a unique axial orientation.

  The thickness of the substrate surface damage layer can be selected by a FWHM (full width at half maximum) value measured by (100) Ω. In Examples and Comparative Examples described below, a ZnO single crystal substrate having an XRD (100) ω rocking curve FWHM of less than 40 arcsec was used.

[Evaluation crystal growth]
FIG. 1 shows a cross-sectional view of a wafer on which crystal growth has been performed for evaluation. Specifically, a ZnO layer 11 and a Mg _x Zn _1-x O layer 12 were grown on the ZnO substrate 10. In Examples and Comparative Examples shown below, Mg _0.2 Zn _0.8 O (x = 0.2) was grown as the Mg _x Zn _1-x O layer 12, but the Mg composition x was 0 <x It may be ≦ 0.68.

  More specifically, the ZnO substrate 10 was set on the susceptor in the reaction vessel and rotated at 10 rpm. The reaction vessel was closed, nitrogen gas was supplied from the first run pipe and the second run pipe at a flow rate of 2000 cc / min (total 4000 cc / min), and sprayed onto the substrate 10 from the shower head. The pressure was 10 kPa.

  The gas flow rate supplied from the first run pipe and the second run pipe was always kept constant. For example, when DMZ vaporized gas is fed to the first run pipe at a flow rate of 100 cc / min, the flow rate of the flow controller provided at the base of the first run pipe is reduced to 1900 cc / min, and a total of 2000 cc / min. Was fed to the shower head.

The ZnO layer 11 was grown at a layer thickness of 100 nm by supplying a growth pressure of 80 kPa and a substrate temperature of 775 ° C., supplying H ₂ O (water vapor) at a flow rate of 800 μmol / min, and DMZn at a flow rate of 10 μmol / min. The Mg _0.2 Zn _0.8 O layer (hereinafter also simply referred to as MgZnO layer) 12 has a growth pressure of 5 kPa, a substrate temperature of 875 ° C., H ₂ O (water vapor) of 800 μmol / min, DMZn of 30 μmol / min, and Cp 2 Mg of 1. It was supplied at a flow rate of 0 μmol / min and grown with a layer thickness of 400 to 800 nm.

[Analysis and Evaluation Method]
The surface morphology was evaluated by AFM (Atomic Force Microscope). The crystal orientation and the defect / dislocation density were evaluated by X-ray diffraction (XRD). The impurity concentration in the crystal was evaluated by secondary ion mass spectrometry (SIMS).

  XRD analysis is a method of measuring X-rays diffracted from a measurement object using Cu Kα1 ray (X-ray) as a probe, and can evaluate the crystal arrangement state. The measurement includes 2θ measurement and rocking curve measurement. When a ZnO-based crystal layer was grown on a c-plane ZnO single crystal substrate, the c-axis length was evaluated by (002) 2θ measurement. The crystal orientation (degree of tilting and twisting) of the growth layer relative to the substrate was evaluated by the full width at half maximum (FWHM) of rocking curve measurement on the (002) plane and (100) plane. Specifically, tilting evaluation was performed by measuring the (002) plane rocking curve, and twisting evaluation was performed by measuring the (100) plane rocking curve. However, when the growth film is a thin film, the (002) plane rocking curve measurement is difficult to evaluate because the diffraction intensity of the substrate is strong. On the other hand, in the measurement of the rocking curve on the (100) plane, the orientation can be evaluated with high sensitivity even for a thin film (about 20 nm) by making it incident and diffracted at 89 ° with respect to the c-axis. When tilting is tilted toward the measurement surface, the (100) rocking curve cannot be used for evaluation, but when other surfaces are tilted, twist components are generated, so tilting can be evaluated at a rate of 2/3. . Therefore, in the present invention, the evaluation was performed mainly with the rocking curve of the (100) plane. As a matter of course, if the degree of tilting and twisting is large, the defect / dislocation density is high. Conversely, if it is small, the defect / dislocation density is low.

  In this example, the substrate 10 was subjected to pre-growth treatment using a mixed solution of EDTA · 2Na (ethylenediaminetetraacetic acid disodium salt) and EDA (ethylenediamine) as an etching solution.

And more particularly, EDTA-2Na (disodium ethylenediaminetetraacetate dihydrate, the chemical _{formula: C 10 H 14 N 2 Na} 2 O 8 · 2H 2 O) and 0.2 mol / l solution of, EDA (ethylenediamine ( Anhydrous), a mixed solution with chemical formula: C ₂ H ₈ N ₂ ) was used. The processing procedure was as follows.
(i) A protective sheet is attached to the back surface (−c surface, oxygen polar surface) of the ZnO substrate 10.
(ii) EDTA · 2Na: EDA is mixed at a volume ratio of 10: 1 and stirred for 15 minutes.
(iii) The ZnO substrate 10 is placed in the mixed solution of (ii) and etched for 2 hours. At this time, the etching rate is 0.7 μm / h. The etching time is determined by the thickness of the damaged layer.
(iv) A cleaning solution is prepared by diluting the mixed solution of (ii): pure water = 1: 10000.
(v) After the completion of the etching in (iii), the ZnO substrate 10 is placed in the solution in (iv) and washed. At the same time, remove the protective sheet (i).
(vi) Put the ZnO substrate 10 in acetone and put it in an ultrasonic cleaner for 5 minutes. Acetone is used for the purpose of dehydration. The ultrasonic wave was strong enough to generate bubbles.
(vii) Replace acetone and repeat step (vi) again.
(viii) The ZnO substrate 10 is put in IPA (isopropyl alcohol) heated in advance, and the ZnO substrate is pulled up in IPA vapor.

In the above procedure (ii), in the case of EDTA · 2Na: EDA = 10: 1 (volume ratio), for example, when EDTA · 2Na (concentration 0.2 mol / l) is 200 ml, EDA (anhydrous, density: 0.00). 9 g / cm ³ , molecular weight: 60.10) means that 20 ml is used. And the molar ratio at this time is calculated as follows.
・ Mole number of EDTA · 2Na solution (200 ml) = 0.2 × (200/1000) = 0.04 mol
-Number of moles of EDA (20 ml) = 20 (ml) x 0.9 / 60.10 = 0.2995 mol ≒ 0.30 mol
* EDTA * 2Na: EDA (molar ratio) = 0.04: 0.30 = 1: 7.5

The difference between the processing method of the present embodiment and the first embodiment will be described. The difference from Example 1 is the procedure (iv), in which a pre-growth treatment of the substrate 10 was performed using a diluted solution of EDA as a cleaning solution. That is,
(iv) A cleaning solution is prepared by diluting at a volume ratio of EDA: pure water = 1: 200.

  Then, similarly to Example 1, after the completion of the etching of (iii), the ZnO substrate 10 is put into the solution of (iv) and washed. At the same time, remove the protective sheet (i). Other steps are the same as those in the first embodiment.

The processing method of this example is different from Example 1 described above in procedure (iv), in which the substrate 10 was subjected to pre-growth processing using a diluted solution of EDTA · 2Na as a cleaning solution. That is,
(iv) EDTA · 2Na: pure water = 1: 100 is diluted to make a washing solution.

  Then, similarly to Example 1, after the completion of the etching of (iii), the ZnO substrate 10 is put into the solution of (iv) and washed. At the same time, remove the protective sheet (i).

In this example, the growth pretreatment of the substrate 10 was performed using a diluted solution of EDA as a cleaning solution. The processing procedure was as follows.
(i) A protective sheet is attached to the back surface (−c surface, oxygen polar surface) of the ZnO substrate 10.
(ii) Prepare 0.2 mol / l EDTA · 2Na.
(iii) The ZnO substrate 10 is placed in the solution (ii) and etched for 2 hours.
(iv) Prepare a cleaning solution so that EDA: pure water = 1: 200.
(v) After the completion of the etching in (iii), the ZnO substrate 10 is placed in the solution in (iv) and washed. At the same time, remove the protective sheet (i).
(vi) Put the ZnO substrate 10 in acetone and put it in an ultrasonic cleaner for 5 minutes. Acetone is used for the purpose of dehydration. The ultrasonic wave was strong enough to generate bubbles.
(vii) Replace acetone and repeat step (vi) again.
(viii) The ZnO substrate 10 is put in IPA heated in advance, and the ZnO substrate is pulled up in IPA vapor.

The processing method of this example differs from Example 4 described above in the procedure (iv), in which the substrate 10 was subjected to pre-growth processing using a diluted solution of EDTA · 2Na as a cleaning solution. That is,
(iv) EDTA · 2Na: pure water = 1: 100 is diluted to make a washing solution.

  The processing methods of the above-described examples can be summarized as follows. In Examples 1 to 3, etching is performed with a mixed solution (neutral or weakly alkaline) of EDTA · 2Na and EDA, and then in Example 1 is neutral to weakly alkaline. EDTA · 2Na + EDA (diluted solution), weakly acidic EDTA · 2Na (diluted solution) in Example 2, and alkaline EDA (diluted solution) in Example 3 were used as cleaning solutions. That is, treatment (cleaning) was performed with “electrolyte solution having a ligand” after etching.

  Further, in Examples 4 and 5, etching was performed with EDTA · 2Na (weakly acidic), and thereafter, alkaline EDA (diluted solution) was used in Example 4, and weakly acidic EDTA · 2Na (diluted solution) was used in Example 5. ) Was used as the cleaning solution. That is, treatment (cleaning) was performed with “electrolyte solution having a ligand” after etching.

[Comparative Examples 1-3]
In order to evaluate the substrate processing according to Examples 1 to 5 described above, the substrate was processed by the following method as a comparative example.

(Comparative Example 1)
In this comparative example, the growth pretreatment of the substrate 10 was performed using pure water as a cleaning solution. The processing procedure was as follows.
(i) A protective sheet is attached to the back surface (−c surface, oxygen polar surface) of the ZnO substrate 10.
(ii) EDTA · 2Na: EDA is mixed at a volume ratio of 10: 1 and stirred for 15 minutes.
(iii) The ZnO substrate 10 is placed in the mixed solution of (ii) and etched for 2 hours. At this time, the etching rate is 0.7 μm / h. The etching time is determined by the thickness of the damaged layer.
(iv) After completion of the etching in (iii), the ZnO substrate is washed with running pure water. At the same time, remove the protective sheet (i).
(v) Put the ZnO substrate 10 in acetone and put it in an ultrasonic cleaner for 5 minutes. Acetone is used for the purpose of dehydration. The ultrasonic wave was strong enough to generate bubbles.
(vi) Replace acetone and repeat step (vi) again.
(vii) The ZnO substrate 10 is put in preheated IPA, and the ZnO substrate is pulled up in IPA vapor.

(Comparative Example 2)
The processing method of this comparative example is different from the above-described comparative example 1 in the procedures (ii) and (iii). After etching using a diluted solution of EDTA · 2Na, the substrate 10 is prepared using pure water as a cleaning solution. The growth pretreatment was performed. That is, more specifically,
(ii) Prepare a 0.2 mol / l solution of EDTA · 2Na.
(iii) The ZnO substrate 10 is placed in the solution (ii) and etched for 2 hours.

[Evaluation results]
Below, crystal growth was performed using the substrate processed according to the above-described Examples 1 to 5 and Comparative Examples 1 and 2, and the crystal growth layer was evaluated. In the following, for ease of understanding, the substrates or growth layers of Examples 1 to 5 and Comparative Examples 1 and 2 may be abbreviated as EMB1 to 5, CMP1, and 2 in some cases.

  2, 3, 4, 5, and 6 show the evaluation results of Examples 1, 2, 3, 4, and 5, respectively. FIGS. 7 and 8 show the evaluation results of Comparative Examples 1 and 2, respectively. Specifically, FIGS. 2 (a), 3 (a), 4 (a), 5 (a), and 6 (a) are substrates subjected to the processing of Examples 1, 2, 3, 4, and 5, respectively. 7A and 8A show the XRD2θ measurement results of the growth layer using the substrates subjected to the processing of Comparative Examples 1 and 2, respectively. 2 (b), 3 (b), 4 (b), 5 (b), 6 (b), 7 (b), and 8 (b) are shown in Examples 1, 2, 3, 4, respectively. 5, the measurement result of the (100) plane rocking curve of Comparative Examples 1 and 2 is shown. 2 (c), 3 (c), 4 (c), 5 (c), 6 (c), 7 (c), and 8 (c) are shown in Examples 1, 2, 3, 4, respectively. 5, AFM evaluation results of Comparative Examples 1 and 2 are shown. The AFM evaluation results are for a 15 μm × 15 μm region on the growth layer surface. Further, FIG. 2D and FIG. 7D show the SIMS measurement results (depth profile) of Example 1 and Comparative Example 1, respectively. FIG. 9 shows the evaluation results of Examples 1 to 5 and Comparative Examples 1 and 2 in a table.

  As described above, in Comparative Example 1, crystal growth is performed on a substrate treated with pure water as a cleaning liquid after etching with EDTA · 2Na + EDA etchant. In Comparative Example 2, as a cleaning liquid after etching with EDTA · 2Na etchant. Crystal growth was performed on a substrate treated with pure water.

  First, the XRD measurement results of Comparative Examples 1 and 2 will be described. In Examples 1 to 5 and Comparative Examples 1 and 2, a structure in which a ZnO layer 11 (layer thickness, about 100 nm) is grown on a ZnO substrate 10 and then an MgZnO layer 12 (layer thickness, 400 to 800 nm) is stacked. It is said. The a-axis length and the c-axis length of the MgZnO crystal increase as the Mg composition increases, and the c-axis length decreases. In the XRD 2θ scan of Comparative Example 1 and Comparative Example 2 (FIGS. 7A and 8A), the (0002) diffraction peak (34.42 °) of the ZnO crystal and the MgZnO crystal on the higher angle side (0002) diffraction peaks were observed. Further, in the (100) plane rocking curve (FIGS. 7B and 8B) which is sensitive to the deterioration of the crystallinity of the growth layer, the half-value width tends to be widened.

  Further, referring to the AFM images of Comparative Example 1 and Comparative Example 2 (FIGS. 7 (c) and 8 (c)), although the MgZnO crystal is step flow grown, the growth surface has no directionality and is curved. Or it was an uneven | corrugated morphology (three-dimensional (3D) growth mode). From this morphology, it was found that the crystallinity was degraded due to the formation of domains in the growth layer.

Further, referring to the SIMS analysis result of Comparative Example 1 (FIG. 7 (d)), Al (aluminum) is below the lower detection limit (LM (Al)) in the ZnO growth layer 11 and the MgZnO growth layer 12, Diffusion from the ZnO substrate 10 to these growth layers was not observed. However, although Si (silicon) is below the lower detection limit value (LM (Si)) in the ZnO growth layer 11, diffusion into the MgZnO growth layer 12 was confirmed. The concentration was 3 × 10 ¹⁸ cm ³ , which was higher than the Si concentration contained in the ZnO substrate 11.

  Next, in Examples 1 to 5, since the a-axis length of the MgZnO crystal grows coherently to the a-axis length of the ZnO crystal, the c-axis length becomes slightly longer. From the XRD 2θ scan measurement results, a (0002) diffraction peak (34.42 °) of the ZnO crystal and a (0002) diffraction peak of the MgZnO crystal on the higher angle side were observed. In addition, the half width (FWHM) of the (100) plane rocking curve of the MgZnO crystal is equivalent to the half width of the ZnO substrate, there is no twist component, the defect and dislocation density are very low, and the crystallinity is good. I understood.

The description will be made with reference to AFM images of Examples 1 to 5 (FIGS. 2C to 6C).
The main surface (crystal growth surface) of the ZnO substrate 10 used in this example is a c-plane with Zn polarity, the vertical direction of the AFM image is the a-axis direction, and the horizontal direction is the m-axis direction. The c-axis is off by 0.5 °. In Example 1 to Example 5, the MgZnO crystal is step-flow grown in the m-axis direction, and as a result, has a stepped surface shape in the m-axis direction. Therefore, the AFM image has a vertical stripe shape in the a-axis direction (two-dimensional (2D) growth mode).

  Further, referring to the SIMS analysis result of Example 1 (FIG. 2D), Si and Al contained in the ZnO substrate 11 are within the lower detection limit (LM (Si), in the ZnO layer 11 and the MgZnO layer 12). LM (Al)) or less, and no diffusion into the growth layer was observed.

[Concentration of cleaning solution]
In Examples and Comparative Examples described below, as described above, EDTA · 2Na (concentration 0.2 mol / l) and EDA (anhydrous) were used as stock solutions, and these were mixed and diluted. When using this solution as a stock solution, it may be converted and applied. For example, when EDTA · 2Na having a concentration of 0.5 mol / l is used, it may be used in accordance with the molar concentration when a 0.2 mol / l solution is used. The same applies to EDA.

1. About Cleaning Solution of Example 1 1.1 Properties of Cleaning Solution The mixed dilution solution of EDTA · 2Na and EDA used in the cleaning solution can remove silica or zinc silicide on the surface of the ZnO substrate if it is an electrolyte solution or a solution capable of chelate coordination.

1.2 Concentration of cleaning solution It is preferable that etching is not performed (or the etching rate is very slow) for the purpose of cleaning the etching solution. In the diluted solution of pure water of the mixed solution, pure water is 100 or more (volume Ratio) is good.

  As another viewpoint, since the protective sheet on the back surface (O (oxygen) polar surface) of the ZnO substrate is peeled off during the cleaning process, the back surface side is not etched at that time (the O polar surface is easily formed with a weak acid or a weak base). It is preferable that pure water is 100 or more (volume ratio) with respect to the mixed solution 1 in the concentration of etching), that is, in a diluted solution of the mixed solution with pure water. A protective film (SiC, GaN, etc.) that does not peel off in chemical resistance and a growth atmosphere (temperature, gas, etc.) can also be used in place of the back protective sheet of the ZnO substrate.

  If the cleaning solution is too thin, the electrolyte and chelate concentrations will be thin, and silica will easily adhere to the substrate surface. Therefore, the pure water may be less than 10,000 (volume ratio) with respect to the mixed solution 1 in the mixed solution: pure water. preferable.

1.3 Concentration Range of Etching Mixed Solution and Cleaning Solution FIG. 10 shows the relationship between the etching mixed solution and the cleaning solution. The etching mixed solution (etchant) can have a practical etching rate such as EDTA · 2Na: EDA = 5: 1, 10: 1, 20: 1 (volume ratio). If it is 10: 1, the etching rate is 0.7 μm / h, and if it is 5: 1, it becomes faster. Conversely, if it is 20: 1, it will be slow.

  If the cleaning solution is slightly thinner than the etching mixed solution (etching mixed solution: pure water = 20: 1), the cleaning solution is easily etched during cleaning, and is more difficult to etch when diluted. All of the cleaning solutions shown in FIG. 10 exhibit good cleaning characteristics, but from the viewpoint of the cleaning solution, the range of EDTA · 2Na + EDA: pure water = 1: 100 to 1: 100000 is preferable.

In addition, when it is assumed that the total value of the volume before mixing coincides with the volume after mixing when the EDTA · 2Na aqueous solution is mixed with EDA (actually, the volume after mixing is slightly smaller), EDTA · 2Na: EDA = 5 The molar concentration in the case of mixing at 1: is calculated as follows. Here, since EDA uses an anhydride, the concept of molar concentration is not used, but the molar concentration is calculated for convenience. That is, when 1000 ml of EDA (anhydrous, density: 0.9 g / cm ³ , molecular weight: 60.10) is used,
[Mole number of EDA (mol)] = 1000 ml × 0.9 g / cm ³ ÷ 60.10 = about 15.0 mol
Therefore, the molar concentration when mixed with EDTA · 2Na: EDA = 5: 1 is
[EDTA · 2Na] = 0.2 mol / l x 5 ÷ 6 = 0.166 = about 0.17 mol / l
[EDA] = 15 mol / l x 1 ÷ 6 = 2.5 mol / l
It is. Similarly, the molar concentration when mixed with EDTA · 2Na: EDA = 20: 1 is
[EDTA · 2Na] = 0.2 mol / l x 20 ÷ 21 = 0.190 = about 0.19 mol / l
[EDA] = 15 mol / l x 1 ÷ 21 = 0.714 = about 0.71 mol / l
It is.

As described above, the mixing ratio (volume ratio) of the etchant which is a mixed solution of EDTA · 2Na and EDA is preferably EDTA · 2Na: EDA = 5: 1 to 20: 1. In this case, the molar concentrations of EDTA · 2Na and EDA in the etchant are in the range of 0.17 to 0.19 mol / l and 2.5 to 0.71 mol / l, respectively.
1.4 About EDTA Chelate Material EDTA chelate materials (compounds) include EDTA to which Na is not added, EDTA · 1Na in which Na is added, EDTA · 2Na in which Na is added, as well as EDTA · 3Na and EDTA · 4Na. Of these, acidic EDTA · 1Na and EDTA · 2Na are preferably used in combination with alkaline EDA.

2. About the cleaning liquid of Example 2 2.1 EDA dilution ratio In this example, anhydrous EDA was used. As in Example 1, as a result of testing the concentration suitable for the cleaning solution, it is sufficient that the concentration is lower than the EDA concentration of the etching mixed solution, but practically, the dilution ratio of EDA with pure water is EDA: pure water = 1: 100 to 1: 100,000 (volume ratio) is good.

When EDA: pure water = 1: 100, the concentration of the diluent is
[EDA] = 15.0 mol / l x 1/101 = 0.148 = about 0.15 mol / l
When EDA: pure water = 1: 100000, the concentration of the diluent is
[EDA] = 15.0 mol / l × 1 ÷ 100001 = 0.000148 = approx. 0.00015 mol / l
It is.

3. Regarding the cleaning liquid of Example 3, 3.1 Dilution ratio of EDTA · 2Na In this example, 0.2 mol / l EDTA · 2Na was diluted with pure water. As a result of testing the concentration suitable for the cleaning liquid, it is sufficient that the concentration is lower than the concentration of EDTA · 2Na in the etching mixed solution, but practically, the dilution ratio of EDTA · 2Na with pure water is EDTA · 2Na: pure water = 1: The range of 100 to 1: 10000 (volume ratio) is good.

When EDTA · 2Na: pure water = 1: 100, the concentration of the diluent is
[EDTA · 2Na] = 0.2 mol / l x 1 ÷ 101 = 0.00198 = about 0.0020 mol / l
Moreover, when EDTA · 2Na: pure water = 1: 10000, the concentration of the diluent is
[EDTA · 2Na] = 0.2 mol / l x 1 ÷ 10001 = 0.0000199 = about 0.000020 mol / l
It is.

4). About Cleaning Solution of Example 4.1 Etching Rate of EDTA · 2Na EDTA · 2Na hardly etches the ZnO crystal plane of the Zn polar plane. Specifically, the etching rate is 0.005 μm / h or less. However, the attached silica or zinc silicide layer can be sufficiently removed. EDTA · 2Na is suitable for etching in a state where there is no altered layer (process damage layer) on the ZnO substrate surface and no embedded silica.

  The molar concentration of the EDTA · 2Na etchant in the etching step is preferably in the range of 0.1 to 0.5 mol / l.

  4.2 The appropriate range of the EDA dilution ratio is the same as in Example 2.

5. About the cleaning liquid of Example 5, the appropriate range of the dilution ratio of 5.1 EDTA · 2Na is the same as that of Example 3.

[Causes and problems of Si residue / diffusion]
1. Problems of Comparative Example In Comparative Examples 1 and 2, after etching with EDTA · 2Na + EDA etchant or EDTA · 2Na etchant, cleaning using pure water was performed as a standard chemical removal method. When the ZnO crystal layer and the MgZnO crystal layer were grown on the ZnO substrate subjected to such treatment by the MOCVD method, the following problems were revealed.

  1.1 The first problem is the problem that Si (silicon) diffuses and remains in the MgZnO crystal layer as shown in the SIMS analysis result (FIG. 7D). This problem is considered to be a problem peculiar to the growth of the MgZnO crystal layer since the Si concentration in the ZnO crystal layer is below the detection limit.

When Si diffuses and remains in the MgZnO crystal layer, for example, the following problems occur. For example, in the case of an LED element having a DH (double hetero) structure, a three-layer structure of at least an n-type MgZnO layer, a ZnO layer as a light emitting layer, and a p-type MgZnO layer is formed. For n-type or p-type MgZnO, in order to efficiently inject electrons and holes (carriers) into the light emitting layer, the carrier density of each layer is about 1 × 10 ¹⁷ to 1 × 10 ¹⁹ cm ^−3. Impurity doping is performed. However, if residual impurities such as Si are mixed in the crystal layer at a high concentration, it becomes difficult to control the conductivity of the n-type or p-type crystal layer, which is an obstacle in manufacturing an LED with high luminous efficiency. Become.

  More specifically, Si is a substance that can be an n-type impurity, and when mixed into the p-type MgZnO crystal layer, the n-type residual carrier density is increased, making it difficult to form the p-type MgZnO layer. In addition, Si is problematic in the n-type MgZnO layer, the MgZnO light-emitting layer, and the like because it makes it difficult to control the conductivity or forms an unintended light-emitting center, thereby reducing the light-emitting efficiency of the LED. Furthermore, a high concentration of Si is a problem because it causes defects and dislocations in the growth layer and lowers the luminous efficiency of the LED.

  1.2 The second problem is a problem that the morphology of the MgZnO crystal layer becomes curved or uneven. That is, it is a problem that the growth of MgZnO crystals is hindered.

  As described above, the LED is formed by sandwiching the light emitting layer between layers having a larger band gap than the light emitting layer. At this time, an MgZnO layer may be used for band gap control, and it is assumed that another layer is grown on the MgZnO layer. When the growth layer becomes curved or uneven in crystal growth, there is a problem in that defects occur at the interface where the domains contact each other, and the luminous efficiency of the LED is lowered.

2. 2.1 Causes of Si Diffusion and Remaining in the MgZnO Crystal Layer 2.1 The inventor of the present application has found that Si is mixed into the MgZnO growth layer due to Si remaining in some state on the ZnO substrate surface. Got. More specifically, the inclusion of Si is included in the ZnO substrate because the concentration of Si in the MgZnO growth layer can be reduced to below the detection lower limit by devising the etching method of the ZnO substrate, particularly the cleaning method. It has been found that the main cause is not Si that is present but residual Si (zinc silicide, attached silica, buried silica, etc.) on the surface of the ZnO substrate.

2.2 Cause 1 of silica remaining on the ZnO substrate surface 1
A substrate for manufacturing a semiconductor element for epitaxial crystal growth is cut out from a single crystal ingot in a specific plane orientation (for example, (100) plane, (110) plane). The cut single crystal substrate is processed to a level with less disturbance in the atomic arrangement of the cut surface that becomes the growth surface by the steps of polishing (lapping) and mirror polishing (polishing). In particular, mirror polishing is polished by mechanical chemical polishing (CMP). At this time, colloidal silica (hydrated silicon oxide) is used as an abrasive. In CMP polishing with colloidal silica, a ZnO substrate is mirror-polished by mechanical action (abrasive grains) and chemical action. Therefore, zinc silicide (Zn—Si—O) is (partially) formed on the surface of the ZnO substrate, and Si remains. Also, colloidal silica tends to adhere to the substrate surface and cannot be easily removed. Further, since the ZnO substrate has a small Mohs hardness of 4, Si remains as a result of the colloidal silica being partially buried in the polished surface. For these reasons, a method of removing zinc silicide or colloidal silica by etching or cleaning after CMP polishing has been proposed, but this is not a sufficient treatment for growing MgZnO crystals with good crystallinity.

2.3 Cause 2 of silica remaining on the ZnO substrate surface 2
ZnO crystals can be wet etched with various reagents. By etching, Si residues (surface zinc silicide, attached silica, buried silica, etc.) on the substrate surface are released from the substrate surface and dispersed as hydrated silica in the etching solution. However, as a general cleaning step after etching, for example, when cleaning is performed using pure water, zinc silicide dispersed or dissolved in the etching solution is reattached or re-deposited on the surface of the ZnO substrate. Alternatively, zinc silicide is formed on the surface of the ZnO substrate. Furthermore, the colloidal silica hydrate also easily aggregates and adheres to the ZnO crystal surface.

2.4 Reason why problems are likely to occur in the MOCVD method In the case of the MOCVD method in which crystal growth is performed at a high temperature, the diffusion rate of Si in the ZnO crystal is high. Therefore, even if the Si concentration in the deposited ZnO crystal is low (for example, below the lower limit of SIMS detection), if Si remains on the substrate surface, it diffuses in the crystal and reaches the growth surface. Si existing on the surface of the growth layer inhibits two-dimensional growth in MgZnO crystal growth and promotes growth in a three-dimensional growth mode. In the three-dimensional growth mode, defects and dislocations occur at the coalescence interface of each domain, and the probability that Mg, Zn, and O elements move to and bond to stable sites on the crystal surface is reduced, and defects and transitions are likely to occur. Deteriorating the crystallinity of the growth layer. As a result, there arises a problem that the n-type residual carrier density derived from defect dislocations is increased and the Si residual density acting as an n-type impurity is increased. In particular, this problem is caused by zinc silicide scattered on the surface of the ZnO substrate.

2.5 Reason why problems easily occur in MgZnO growth The a-axis length and c-axis length of the MgZnO crystal (mixed crystal) lattice are longer than the a-axis length of the ZnO crystal and shorter than the c-axis length (for example, A. Ohtomo et al , Appl. Phys. Lett. 72, 19, p.2466, (1988)). Specifically, as the Mg composition of the MgZnO crystal increases, the a-axis length increases and the c-axis length decreases. However, when an MgZnO epitaxial crystal is grown on a ZnO single crystal substrate, the MgZnO crystal growth is hindered by stress based on lattice mismatch and Si diffusion from the lower ZnO layer, resulting in a bent morphology.

2.6 Growth Inhibition by Adhering Silica and Buried Silica Colloidal silica residue and buried colloidal silica residue adhering to the substrate are aggregates and cause growth inhibition locally on the growth surface. Specifically, pit formation and hill rock formation on the growth surface. As a matter of course, pits and hilllocks cause adverse effects such as current leakage when an LED element is formed. Therefore, it is better to remove colloidal silica residues and buried colloidal silica residues.

[Operation and effect of the present invention]
1.1 In the present invention, instead of pure water, which has been conventionally used, as a cleaning liquid in the cleaning step, an electrolyte solution having a ligand or a bidentate or more ligand (polydentate ligand) is included. A diluted solution of the electrolyte solution is used. Thereby, since a ligand coordinates (adsorbs) on a ZnO substrate or colloidal silica surface, formation of zinc silicide on the ZnO substrate surface can be suppressed and prevented. At the same time, adhesion (adsorption) of colloidal silica can be prevented. The buried colloidal silica is removed by an etching process.

  From another point of view, it is considered that the formation of zinc silicide can be suppressed and prevented by the slight progress of etching. The adhesion (adsorption) and prevention of colloidal silica can be considered similarly.

  From another viewpoint, it is considered that colloidal silica cannot be adhered due to steric hindrance by EDTA · 2Na or EDA coordinated on the ZnO crystal surface (Zn).

1.2 Advantages of using a chelating agent Generally, in order to prevent colloidal silica from adhering to a substrate (for example, a ZnO substrate), electrostatic repulsion is increased by making the zeta potential the same polarity and increasing the potential difference. , Silica adhesion can be prevented (dispersed). However, when a surfactant or chelating agent is used, the zeta potential changes (apparently) because it adsorbs (or coordinates) on colloidal silica or the substrate surface. For example, the zeta potential is larger than the potential of colloidal silica or the substrate, or the potential is reversed (for example, Non-Patent Document 1: “Colloidal and Interface Chemistry 2nd Edition”, Fumio Kitahara Koichiro Aoki / Published by Yodogawa Shoten Co., Ltd.). Thus, when a surfactant or a chelating agent is used, the physical properties of colloidal silica and the substrate are not a factor that determines the zeta potential. That is, it is not necessary to control the zeta potential with the pH of the solution. From another viewpoint, it is considered that the activator or chelating agent can be adsorbed (or coordinated) on the surface of the ZnO substrate to suppress the production of Zn (OH) ₂ .

1.3 Solution of the Present Invention EDTA · 2Na used in the present invention is a chelate compound and an electrolyte solution having a ligand. In addition, Non-Patent Document 1 describes an electric double layer, taking a surfactant and a chelating agent as examples, but it is understood that a ligand also has a similar effect on the molecular structure.

  In the treatment of the present invention, after etching, cleaning is performed with a diluted solution of an etching solution component without cleaning with pure water. However, since the reagent component of this etching solution is not desorbed in the steps of acetone dehydration and IPA drying, the reagent remains (adsorbs or coordinates) on the substrate surface. However, since the substrate is decomposed and scattered when the substrate is heated to a high temperature (especially 500 ° C. or more) in the growth of ZnO and MgZnO, the crystal growth is not adversely affected.

1.4 Effects of the Invention In Examples 1 to 3, after etching with EDTA · 2Na + EDA, which is a neutral or weak alkaline etchant, neutral or weak alkaline “EDTA · 2Na + EDA” or weak acid “EDTA” is used as a cleaning solution. -By using a diluted solution of an electrolyte solution having a ligand such as 2Na "or diluted alkaline" EDA ", formation of zinc silicide on the surface of the ZnO substrate can be suppressed. Furthermore, the colloidal silica which adheres can be suppressed.

  In Examples 4 to 5, after etching with EDTA · 2Na, which is a weakly acidic etchant, the electrolyte solution having a ligand such as “EDTA · 2Na” of weak acid or “EDA” of diluted alkali is used as a cleaning solution. By using the solution, formation of zinc silicide on the surface of the ZnO substrate can be suppressed. Furthermore, the colloidal silica which adheres can be suppressed.

  By applying such a pretreatment method, a substrate free from zinc silicide, colloidal silica, and buried colloidal silica can be provided, so that Si is not mixed into the MgZnO crystal layer even in the high temperature MOCVD method (that is, under detection). The MgZnO layer with less residual impurities can be grown.

  Further, it is possible to grow a MgZnO crystal layer having a vertical stripe morphology based on the substrate orientation and selective growth. That is, a crystal having a low defect density can be grown.

The grown MgZnO layer was observed for pit density with a differential interference microscope. In the method of the comparative example, the pit density varied greatly as 1 × 10 ⁴ to 1 × 10 ⁵ pieces / cm ² , whereas in the method of the present invention, 7 × 10 ^{3 to} 3 × 10 ⁴ pieces / cm ^2. It was confirmed that the density was low.

  Thus, it becomes possible to grow a MgZnO crystal layer with good crystallinity in the MOCVD method, to suppress Si residue in the MgZnO crystal layer, and to grow an MgZnO crystal layer with excellent flatness. .

  The mixed dilution solution of EDTA · 2Na and EDA, EDTA · 2Na diluted solution, and EDA diluted solution are the same components as the etching solution. Therefore, damage to the etched surface due to the interaction that is a concern when using another reagent solution. In addition, there is an advantage that unintended chemical solution does not occur on the etched surface.

  As described above in detail, according to the present invention, it is possible to provide a growth pretreatment method for a ZnO substrate in which zinc silicide, silica, or the like does not remain. More specifically, according to the present invention, the ZnO substrate is etched using a mixed solution of EDTA · 2Na and EDA or a solution containing an EDTA chelate compound such as EDTA · 2Na as an etchant, and a mixed diluted solution of EDTA · 2Na and EDA, The substrate after etching is cleaned using an electrolyte solution having a ligand such as EDTA · 2Na diluted solution or EDA diluted solution as a cleaning solution. By such substrate treatment, zinc silicide, silica, and the like on the substrate surface can be removed. Then, by using the substrate that has been subjected to such pretreatment, in crystal growth such as MOCVD, Si caused by zinc silicide, silica, or the like diffuses and remains in the ZnO-based semiconductor growth layer such as MgZnO. It is possible to obtain a high-quality crystal having excellent surface flatness and no Si diffusion without deteriorating surface flatness and morphology.

  Therefore, when applied to semiconductor elements, etc., excellent impurity concentration control, impurity concentration profile control, and steep interface control are possible, and a high-quality crystal stacked structure can be formed, thereby providing a semiconductor element with excellent characteristics. be able to. Furthermore, it is possible to manufacture a ZnO-based crystal semiconductor light emitting device having excellent electrical characteristics and light emission efficiency.

10 ZnO substrate 11 ZnO growth layer 12 MgZnO growth layer

Claims (11)

A method for processing a ZnO single crystal substrate having a Zn polar face as a main surface,
An etching step of etching the substrate using a solution containing an EDTA chelate compound as an etchant;
And a cleaning step of cleaning the substrate using an electrolyte solution having a ligand as a cleaning liquid after the etching. The etchant is a mixed solution of EDTA · 2Na (ethylenediaminetetraacetic acid disodium) and EDA (ethylenediamine),
The processing method according to claim 1, wherein the cleaning liquid is a mixed solution of EDTA · 2Na and EDA, or a diluted solution of pure water of any one of EDTA · 2Na and EDA.   The molar concentration of EDTA · 2Na and EDA in the mixed solution of the etchant is in the range of 0.17 to 0.19 mol / l and 2.5 to 0.71 mol / l, respectively. The processing method described.   The processing method according to claim 2 or 3, wherein the dilution ratio (volume ratio) of the diluent is in the range of 1:20 to 1: 10000.   The treatment according to claim 2 or 3, wherein the cleaning liquid is a diluted solution of EDTA · 2Na with pure water, and the molar concentration of the EDTA · 2Na is in the range of 0.0020 to 0.000020 mol / l. Method.   The processing method according to claim 2 or 3, wherein the cleaning liquid is a diluted liquid of EDA with pure water, and the molar concentration of the EDA is in the range of 0.15 to 0.00015 mol / l. The etchant is an EDTA · 2Na solution,
The processing method according to claim 1, wherein the cleaning liquid is a diluted liquid of any one of EDTA · 2Na and EDA.   The processing method according to claim 7, wherein a molar concentration of EDTA · 2Na in the etchant is in a range of 0.1 to 0.5 mol / l.   The processing method according to claim 8, wherein the cleaning liquid is a diluted liquid of EDA with pure water, and the molar concentration of the EDA is in the range of 0.15 to 0.00015 mol / l.   The processing method according to claim 9, wherein the cleaning liquid is a diluted solution of EDTA · 2Na with pure water, and the molar concentration of the EDTA · 2Na is in the range of 0.0020 to 0.000020 mol / l.   The processing method according to claim 1, wherein in the electrolyte solution having the ligand, the ligand is a multidentate ligand.