JP2012129459A - Method for growing zinc oxide-based semiconductor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for growing a p-type ZnO-based crystal of high quality, which is excellent in controllability of a p-type dopant concentration.SOLUTION: The method for growing a p-type ZnO-based crystal of high quality comprises a single-crystal growing process of growing a ZnO-based crystal layer using an organic metal compound not containing an oxygen atom in the molecular structure and a polar oxygen material by a MOCVD method. The single-crystal growing process includes a step of supplying TBP (Tertiary Butyl Phosphine).

Description

  The present invention relates to a method for growing a zinc oxide (ZnO) based semiconductor, and more particularly to a method for growing a p-type zinc oxide based semiconductor layer on a ZnO substrate by MOCVD.

  Zinc oxide (ZnO) is a direct transition type semiconductor having a band gap energy of 3.37 eV at room temperature, and is expected as a material for optical elements in the blue or ultraviolet region. In particular, the exciton binding energy is 60 meV and the refractive index n = 2.0, which is very suitable for a semiconductor light emitting device. Further, the present invention can be widely applied not only to light emitting elements and light receiving elements but also to 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 manufacturing a semiconductor element using zinc oxide, it is important to control the growth of high quality zinc oxide crystals and the conductivity type. Zinc oxide essentially exhibits n-type characteristics, and it is difficult to obtain high-quality p-type zinc oxide (for example, Patent Document 1). Therefore, a crystal growth method capable of obtaining high-quality p-type zinc oxide is desired.

  For example, as a crystal growth method for zinc oxide (ZnO), a method is known in which a ZnO crystal or amorphous material is grown at a low temperature, and then the quality is improved by high quality or rearrangement at a high temperature (for example, patents). Literature 2, Patent Literature 3). However, the zinc oxide grown at such a low temperature and improved in quality (or recrystallized) at a high temperature is likely to contain crystal defects, and has sufficient crystallinity for p-doped applications. I can't say that. For example, when such a ZnO crystal is doped with p-type dopant P (phosphorus), there is a possibility that P cannot be replaced with an O (oxygen) site. In addition, even if P (phosphorus) is substituted into the O site and can be doped in the ZnO crystal, doping with P (phosphorus) in high-quality treatment at high temperatures hinders quality improvement. There is a problem to become.

JP 2008-252075 A JP 2008-244011 A JP 2006-73726 A

  The present invention has been made in view of the above points, and the object thereof is excellent in growth of high quality zinc oxide (ZnO) crystals and control of conductivity type, particularly controllability of p-type dopant concentration. Another object is to provide a method for growing a high-quality p-type ZnO-based crystal.

  The method of the present invention is a method of growing a ZnO-based crystal layer on a ZnO single crystal substrate by MOCVD, and uses a metal oxide compound that does not contain oxygen atoms in the molecular structure and a polar oxygen material. A single crystal growth step for growing a crystal layer, wherein the single crystal growth step includes a step of supplying a TBP (tertiary butylphosphine) gas.

It is a figure which shows typically the structure of the MOCVD apparatus used for crystal growth. It is a figure which shows a crystal growth sequence. It is sectional drawing which shows the structure of the growth layer of the Example which grew the undoped ZnO crystal layer and P (phosphorus) doped ZnO crystal layer on the board | substrate. It is a result of the SIMS analysis of the growth layer by Example 1, and is a figure which shows P density | concentration of the P (phosphorus) dope ZnO layer with respect to the supply amount of TBP. 3 is a diagram showing an example of an SEM image of a P (phosphorus) -doped ZnO crystal layer according to Example 1. FIG. 6 is a diagram showing a (002) plane rocking curve of a growth layer according to Example 1. FIG. 6 is a diagram showing a (100) plane rocking curve of a growth layer according to Example 1. FIG. 6 is a diagram showing a (002) plane rocking curve of a growth layer of Comparative Example 1. FIG. 6 is a diagram showing a (100) plane rocking curve of a growth layer of Comparative Example 1. FIG. The results of Example 2, a diagram illustrating a P (phosphorus) concentration dependence on the flow rate of the water vapor (H ₂ O). It is a figure which shows the result of Example 3 and shows the dependence of P (phosphorus) density | concentration with respect to the flow volume of DMZn. It is the figure which plotted P (phosphorus) density | concentration in a ZnO crystal | crystallization with respect to Pgo (= F _TBP / F _H2O ). It is a figure which shows the SIMS result of the growth layer grown by Example 4 (TBP graded growth). 6 is a diagram showing an example of an SEM image of a growth layer of Comparative Example 2. FIG. 6 is a diagram showing a (002) plane rocking curve of a growth layer of Comparative Example 2. FIG. 10 is a diagram showing a (100) plane rocking curve of a growth layer of Comparative Example 2. FIG.

  In the following, a method for growing a high-quality P (phosphorus) -doped ZnO-based semiconductor crystal layer with few crystal defects on a ZnO single crystal substrate by MOCVD and control of P (phosphorus) concentration will be described in detail with reference to the drawings. Explained. Further, a comparative example for explaining the characteristics, configuration, and effects of the growth method and the growth layer of the example according to the present embodiment will be described in detail. In the drawings described below, substantially the same or equivalent parts will be described with the same reference numerals.

  FIG. 1 schematically shows the configuration of the MOCVD apparatus 5 used for crystal growth. Details of the apparatus configuration of the MOCVD apparatus 5 will be described below. The crystal growth material will be described in detail later.

[Device configuration]
The MOCVD apparatus 5 includes a gas supply unit 5A, a reaction vessel unit 5B, and an exhaust unit 5C. 5 A of gas supply parts are comprised from the part which vaporizes and supplies organometallic compound material, the part which supplies gaseous material gas, and the transport part provided with the function to convey these gas.

  An organometallic compound material that is liquid (or solid) at room temperature is vaporized and supplied as vapor. In this example, DMZn (dimethylzinc) was used as the zinc (Zn) source. An organometallic material supply system that supplies Cp2Mg (biscyclopentadienyl magnesium) as a magnesium (Mg) source and TEGa (triethylgallium) as a gallium (Ga) source is provided.

  First, the supply of DMZn will be described. As shown in FIG. 1, the nitrogen gas is set to a predetermined flow rate by a flow rate adjusting device (mass flow controller) 21S and sent to the DMZn storage container 21C through the gas supply valve 21M to saturate the DMZn vapor in the nitrogen gas. Then, the DMZn saturated nitrogen gas is taken out through the take-off valve 21E and the pressure adjusting device 21P to the first vent pipe (hereinafter referred to as the first VENT line (VENT1)) 28V during the growth standby, and to the first run pipe (hereinafter referred to as the first RUN) during the growth. Line (RUN1)) 28R. At this time, the internal pressure of the storage container is adjusted to be constant by the pressure adjusting device 21P. Further, the DMZn storage container is kept at a constant temperature in the thermostatic chamber 21T.

  The same applies to the case where other organometallic compound materials Cp2Mg and TEGa are supplied. That is, nitrogen gas having a predetermined flow rate is supplied to the storage containers 22C (Cp2Mg) and 23C (TEGa) for storing these materials, respectively, through the flow rate adjusting devices 22S and 23S, and the take-out valves 22E and 23E and the pressure adjusting device 22P are supplied. , 23P, these gases are supplied to the first VENT line (VENT1) 28V during growth standby and to the first RUN line (RUN1) 28R during growth.

In addition, H ₂ O (water vapor), which is a liquid material as an oxygen source, is supplied with a predetermined flow rate of nitrogen gas through the flow rate adjusting device 24S to the storage container 24C, and waits for growth through the take-off valve 24E and the pressure adjusting device 24P. Sometimes it is supplied to the second vent pipe (hereinafter referred to as the second VENT line (VENT2)) 29V, and at the time of growth to the second run pipe (hereinafter referred to as the second RUN line (RUN2)) 29R.

  As the p-type impurity source, TBP (tertiary butyl phosphine) gas was used. The TBP gas is supplied at a predetermined flow rate by the flow rate adjusting device 25S. It is supplied to the second VENT line (VENT2) 29V during standby and to the second RUN line (RUN2) 29R during growth. Note that the gas may be diluted with an inert gas such as nitrogen or Ar (argon).

  The liquid or solid material vapor and the gas material (hereinafter referred to as material gas) are supplied to the shower head 30 of the reaction vessel section 5B through the first RUN line (RUN1) 28R and the second RUN line (RUN2) 29R. The first RUN line (RUN1) 28R and the second RUN line (RUN2) 29R are also provided with flow rate adjusting devices 20C and 20B, respectively, and the material gas is supplied to the reaction vessel (chamber) 39 by a carrier gas (nitrogen gas). It is sent to the shower head 30 attached to the upper part.

  The shower head 30 has an ejection surface that faces the main surface (growth surface) of the substrate 10, and a large number of material gas ejection holes in the row and row directions (for example, several 10 to 10). 100) formed. The effective ejection diameter of the ejection surface is larger than the outer diameter of the substrate.

The ejection holes include a first ejection hole from which the organometallic compound material gas (group II gas) supplied from the first RUN line 28R is ejected, and H ₂ O (water vapor) (VI) supplied from the second RUN line 29R. A second ejection hole from which a group gas) is ejected. The gas from the first RUN line 28R and the gas from the second RUN line 29R are configured to be ejected from the first ejection hole and the second ejection hole, respectively, without being mixed. The first ejection holes and the second ejection holes are provided in substantially the same number and at intervals of several mm from each other, and are alternately arranged in each column and each row so that the organometallic compound gas and H ₂ O are uniformly mixed. ing.

  In the reaction vessel 39, a shower head 30 that blows a material gas onto the substrate 10, a substrate 10, a susceptor 19 that holds the substrate 10, and a heater 49 that heats the susceptor 19 are installed. The heater 49 can heat the substrate from room temperature to about 1100 ° C.

  Note that the substrate temperature in this embodiment refers to the temperature of the surface of the susceptor 19 on which the substrate is placed. That is, in the case of the MOCVD method, heat transfer from the susceptor 19 to the substrate 10 is performed by direct contact and a gas existing between the susceptor 19 and the substrate 10. Between the growth pressures of 1 kPa to 120 kPa (Pa: Pascal) used in this example, the surface temperature of the substrate 10 is about 0 ° C. to 10 ° C. lower than the surface temperature of the susceptor 19.

  The reaction vessel 39 is provided with a rotation mechanism for rotating the susceptor 19. More specifically, the susceptor 19 is supported by a susceptor support cylinder 48, and the susceptor support cylinder 48 is rotatably supported on the stage 41. Then, the rotation motor 43 rotates the susceptor support cylinder 48 to rotate the susceptor 19 (that is, the substrate 10). The heater 49 described above is installed in the susceptor support tube 48.

  The exhaust part 5C includes a container internal pressure adjusting device 51 and an exhaust pump 52. The internal pressure adjusting device 51 can adjust the pressure in the reaction container 39 to about 0.1 kPa to 120 kPa. .

[Crystal growth material]
In this example, as the organometallic compound material (or organometallic material), a material containing no oxygen atoms in the constituent molecules was used. 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. An example of the organic metal material containing oxygen is Zn (DPM) ₂ which is a DPM (Dipivaloylmethane) complex of Zn (zinc). An organometallic material containing oxygen is stable to oxygen (O ₂ ), water vapor (H ₂ O), alcohol, and the like, and needs to be heated to 300 ° C. or higher for reaction. In this way, the reaction system is completely different, and the apparatus requirements are also different.

  In this example and the comparative example, DMZn was used, but DEZn (diethyl zinc), Cp2Mg, MeCp2Mg (bismethylpentadienylmagnesium), EtCp2Mg (bisethylpentadienylmagnesium), etc. are used as the group II material. be able to. In addition, TMGa (trimethylgallium), TEGa, 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) was used.

  As the p-type impurity material, a compound that can easily be substituted for the O (oxygen) site of the zinc blende structure or the wurtzite structure in the crystal growth process is suitable. As described above, TBP (tertiary butyl phosphine) gas was used as the p-type impurity source.

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 such as H ₂ O (water vapor) or TBP onto the substrate surface is preferable. Specifically, a so-called 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. In this example and comparative example, high purity N ₂ (nitrogen) gas having a residual O ₂ concentration of less than 1 ppm was used.

  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 the density | concentration of the residual Li which is an impurity derived from a board | substrate by process, 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, a substrate ((0001) 0.5 ° off to [10-10]) in which the (0001) plane is inclined by 0.5 ° in the [10-10] direction, A so-called 0.5 ° off substrate (or 0.5 ° off substrate with the c-plane inclined 0.5 ° in the m-axis direction) was used.

  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.

[Growth method of ZnO single crystal]
In this example, first, a thin ZnO single crystal substrate with a damaged layer having an XRD (100) ω rocking curve FWHM of less than 40 arcsec was selected. The selected substrate was etched to remove the damaged layer. Good surface flatness was obtained by the etching. As an etching solution, a mixed solution in which a 0.2 mol / L solution of EDTA · 2Na (disodium ethylenediaminetetraacetic acid) and a 99% solution of EDA (ethylenediamine) were mixed at a ratio of 20: 1 was used. The etching rate of this etching solution (EDTA · 2Na: EDA = 20: 1) is 0.7 μm / h. Note that the etching solution is disclosed in Japanese Patent Application Laid-Open No. 2007-1787. Etching can be satisfactorily performed at a mixture ratio of the etching solution of about 5: 1 to 30: 1.

  The surface layer was etched by immersing in the etching solution at room temperature for 60 minutes (min). Thereafter, the etching solution was removed by washing with water and dehydrated by washing with an organic solvent (acetone or alcohol). Finally, the organic solvent was heated and dried in a steam atmosphere. Etching conditions such as temperature and time vary depending on the substrate surface treatment and storage conditions.

[Crystal growth sequence and growth layer]
FIG. 2 shows the crystal growth sequence of Example 1, and the growth method in this example will be described in detail below with reference to the figure.

  First, a ZnO single crystal substrate 10 (hereinafter also referred to as a ZnO substrate or simply a substrate) whose surface layer was etched was set on a susceptor 19 in a reaction vessel 39, and after evacuating to a vacuum, the reaction vessel pressure was adjusted to 10 kPa ( Time T = T1). Further, the ZnO substrate 10 was rotated at a rotation speed of 10 rpm by a rotation mechanism.

Next, nitrogen (N ₂ ) gas is supplied from the first RUN line (RUN1) 28R and the second RUN line (RUN2) 29R to the shower head 30 at a flow rate of 2000 cc / min (total 4000 cc / min), and the ZnO substrate 10 Sprayed on.

  The gas flow rate supplied to the shower head 30 from the first RUN line (RUN1) 28R and the second RUN line (RUN2) 29R was always kept constant. That is, when the organometallic material gas and the gaseous material are supplied at the time of growth standby and during the growth, the first RUN line (RUN1) 28R and the second RUN line (RUN2) 29R corresponding to the flow rates of the organometallic material gas and the gaseous material. The flow rate of the flow rate adjusting devices 20C and 20B provided in is increased or decreased to keep the gas flow rate supplied to the shower head 30 constant.

Next, the substrate temperature is raised from room temperature (RT) and the flow rate (F _H2O ) of H ₂ O (water vapor) is adjusted to 640 μmol / min and supplied to the ZnO substrate 10 from the second RUN line (RUN 2) 29R. (T = T2, in the figure: ON). Then, heat treatment (thermal cleaning) was performed at a substrate temperature of 800 ° C. for 7 min (T = T3 to T4).

Next, the pressure is increased to 80 kPa, and after waiting for 1 minute after the substrate temperature reaches a predetermined growth temperature Tg (Tg = 775 ° C. in this embodiment), the flow rate of _DMZn (F _DMZn ) is set to 10 μmol / min. And supplied to the ZnO substrate 10 to start crystal growth (T = T5, ON in the figure). When the growth time of 30 minutes (layer thickness 0.15 μm) had elapsed (T = T6), the supply of DMZn was stopped (in the figure: OFF). Thus, the growth was performed for 30 minutes, and as shown in the cross-sectional view of FIG. 3, an undoped ZnO single crystal layer 11 having a layer thickness of 0.15 μm (micrometer) was formed. Next, after heat treatment with water vapor (H ₂ O) for 30 seconds (sec) (T = T6 to T7), the supply of DMZn (flow rate: 10 μmol / min) was restarted (in the figure: ON). TBP was supplied after 2 seconds (T = T8), and a P (phosphorus) -doped ZnO single crystal layer 12 having a layer thickness of about 0.45 μm was grown in a growth time of 90 minutes (T = T8 to T9). More specifically, after the supply of TBP was completed (T = T9), the supply of DMZn was stopped 5 seconds later (T = T10).

  The growth temperature during the growth of P (phosphorus) -doped ZnO is preferably 600 ° C. or higher, more preferably 700 ° C. or higher. This is because if the growth is carried out at such a temperature, the migration of Zn, O, P, etc. and the precursor for supplying them becomes sufficiently large, and it becomes easier to bond due to the stable bonding position of each element on the growth surface. Furthermore, the growth temperature during the growth of P (phosphorus) -doped ZnO is preferably 850 ° C. or lower, more preferably 830 ° C. or lower. This is because the ZnO crystal may grow in an island shape rather than a layer shape even if it is attempted to grow beyond this temperature.

The TBP supply amount (F _TBP ) was set to 0.1 nmol / min, 1.0 nmol / min, 10.2 nmol / min, 49.9 nmol / min, and 102 nmol / min. Analysis and evaluation were performed on two substrates 15 with growth layers.

  In addition, an undoped ZnO crystal thin film is grown (layer thickness, 0.83 nm) for 10 seconds before the start of supply of TBP (T = T8). Will be described as a P (phosphorus) -doped ZnO single crystal layer 12.

After the completion of crystal growth (T = T10), while maintaining the pressure at 80 kPa, cooling was performed while flowing water vapor (H ₂ O) until the substrate temperature decreased to 200 ° C. (T = T10 to T11). Thereafter, the pressure was reduced to a pump vacuum (about 10 ⁻¹ Pa), and the supply of H ₂ O was stopped at the same time. The growth was completed until the substrate temperature reached room temperature. Note that the pressure reduction during cooling and the H ₂ O supply stop may be switched after waiting to room temperature (RT).

Using the MOCVD apparatus described above, a ZnO single crystal was grown on a ZnO single crystal substrate by a growth sequence similar to the crystal growth sequence of Example 1 (FIG. 2). In Example 2, the TBP flow rate is kept constant, and the water vapor (H ₂ O) flow rate (F _H2O ) is 120, 320, 640 μmol / min. Analysis and evaluation were conducted.

More specifically, while setting the growth temperature Tg to 775 ° C. and supplying water vapor (H ₂ O) (flow rate F _H2O = 120 or 320 or 640 μmol / min), the flow rate of _DMZn (F _DMZn ) is set to 10 μmol / min. An undoped ZnO single crystal layer 11 (layer thickness: 0.15 μm) was grown with a growth time of 30 minutes. Next, while the supply of DMZn was stopped for 30 seconds (FIG. 2, OFF), after heat treatment in a steam atmosphere, the supply of DMZn (flow rate 10 μmol / min) was restarted, and 10 seconds later, TBP was supplied. A P (phosphorus) -doped ZnO single crystal layer 12 (layer thickness: 0.45 μm) was grown at a growth rate of 90 minutes (flow rate: 49.9 nmol / min).

Using the MOCVD apparatus described above, a ZnO single crystal was grown on a ZnO single crystal substrate by a growth sequence similar to the crystal growth sequence of Example 1 (FIG. 2). In Example 3, the growth was performed three times with the flow rate of DMZn (F _DMZn ) being 2.5, _5.0, and 10 μmol / min, and analysis and evaluation of these three growth layer-attached substrates 15 were performed.

More specifically, the growth temperature Tg is set to 775 ° C., water vapor (H ₂ O) is supplied (flow rate 640 μmol / min), DMZn is supplied (flow rate: 10 μmol / min), and undoped with a growth time of 30 minutes. A ZnO single crystal layer 11 (layer thickness: 0.15 μm) was grown. Next, after stopping the supply of DMZn for 30 seconds and performing heat treatment in a water vapor atmosphere, the supply of DMZn (flow rate: 2.5 or 5.0 or 10 μmol / min) was restarted, and TBP was turned on 10 seconds later. Supply (flow rate 49.9 nmol / min) and grow P (phosphorus) doped ZnO single crystal layer 12 (layer thickness 0.45 μm) with growth time (360, 180 or 90 minutes) inversely proportional to DMZn supply flow rate did.

  Using the MOCVD apparatus described above, a ZnO single crystal was grown on a ZnO single crystal substrate by a growth sequence similar to the crystal growth sequence of Example 1 (FIG. 2). In Example 4, in the growth of the P (phosphorus) doped ZnO single crystal layer 12, the crystal growth was performed while changing the flow rate of TBP.

More specifically, the growth temperature Tg is set to 775 ° C., while supplying water vapor (H ₂ O) (flow rate 640 μmol / min) and DMZn (flow rate 10 μmol / min), undoped ZnO with a growth time of 30 minutes. A single crystal layer 11 (layer thickness: 0.15 μm) was grown. Next, after heat treatment was performed in a water vapor atmosphere while stopping the supply of DMZn for 30 seconds, the supply of DMZn (flow rate 10 μmol / min) was resumed, and TBP was supplied 10 seconds later, with a growth time of 90 minutes. A P (phosphorus) -doped ZnO single crystal layer 12 (layer thickness: 0.45 μm) was grown. Here, in the growth of the P (phosphorus) -doped ZnO single crystal layer 12, the TBP flow rate is linearly increased from 0.1 nmol / min to 50 nmol / min over the time from the start to the end of the crystal growth (hereinafter referred to as the following). Also called TBP graded growth).

[Comparative Examples 1 and 2]
In order to evaluate the ZnO-based single crystal layers grown in Examples 1 to 4 described above, crystal growth was performed as a comparative example using the following growth method and growth conditions.

[Comparative Example 1]
Comparative Example 1 was the same as Example 1 except that an undoped ZnO single crystal layer was grown instead of the P (phosphorus) doped ZnO single crystal layer 12 in Example 1. That is, except that TBP was not supplied in the growth process of the P (phosphorus) -doped ZnO single crystal layer 12 in Example 1, the ZnO substrate, the substrate etching process, the growth material, the growth sequence (FIG. 2) and Growth methods such as growth conditions were the same as those in Example 1.

[Comparative Example 2]
Comparative Example 2 was the same as Example 1 except that crystal growth was performed when the ratio of the flow rate of TBP to the flow rate of water vapor (molar flow rate ratio) Pgo (= F _TBP / F _H2O ) was four. It was. More specifically, the growth was performed with a TBP flow rate of 1020 nmol / min, in which case the water vapor flow rate was 120 μmol / min and 320 μmol / min (Pgo = 8.5 × 10 ⁻³ and Pgo = 3.2 × 10 ^{−3, respectively} ). And TBP flow rate of 6020 nmol / min, and in this case, water vapor flow rates of 120 μmol / min and 640 μmol / min (Pgo = 5.0 × 10 ⁻² and Pgo = 9.4 × 10 ⁻³ ), respectively. The four cases were analyzed and evaluated.

[Analysis and Evaluation Method for Crystal Growth Layer]
The surface morphology was evaluated by a differential polarization microscope, SEM (Scanning Electron Microscope) and 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).

  In the XRD analysis, since the ZnO-based crystal layer was grown on the c-plane ZnO single crystal substrate in the above examples and comparative examples, XRD 2θ measurement and ω (rocking curve) measurement were performed. (002) 2θ, and the orientation (degree of tilting and twisting) was evaluated by the full width at half maximum (FWHM) of (002) ω and (100) ω.

[Results of analysis and evaluation of crystal growth layer]
Table 1 below shows the growth conditions and evaluation results of the P (phosphorus) -doped ZnO crystal layer (E1-1, E1-2, E1-3, E1-4, E1-5) according to Example 1.

FIG. 4 shows the result of SIMS analysis of the growth layer according to Example 1, and shows the P (phosphorus) concentration (atoms / cm ³ ) of the P (phosphorus) doped ZnO single crystal layer 12 with respect to the _TBP supply amount (F _TBP ). FIG. In the figure, 1En is an index notation, and for example, 1E + 17 represents 1 × 10 ¹⁷ . When the supply amount of _TBP (F _TBP ) is increased from 0.1 nmol / min to 102 nmol / min, the P concentration becomes 2.0 × 10 ¹⁶ (atoms / cm ³ ) depending on the supply amount of _TBP (F _TBP ). ) To 1.4 × 10 ¹⁸ (atoms / cm ³ ).

FIG. 5 shows an example of an SEM image of the P (phosphorus) -doped ZnO single crystal layer 12 according to Example 1, and a P (phosphorus) -doped ZnO single crystal grown at a TBP supply rate F _TBP = 49.9 nmol / min. 2 is an SEM image of the surface of a layer 12. No nanocolumns or whiskers were observed, and it was confirmed that the two-dimensional growth had a very flat layered shape and a good surface morphology. Although pits are observed, the pits originate from the substrate (that is, defects and the like) and are not caused by film formation. That is, it decreases or disappears when the substrate quality is improved.

  6 and 7 are XRD measurement results of the growth layer according to Example 1, respectively, showing rocking curves of the (002) plane and (100) plane, and FIGS. 8 and 9 are (002) of Comparative Example 1 respectively. ) Surface and (100) surface rocking curves. The full width at half maximum (FWHM) of the rocking curve of the (002) plane of the P (phosphorus) doped ZnO crystal according to Example 1 (denoted as “ZnO: P” in the figure) is 29.3 arcsec, When compared with 29.3 arcsec (FIG. 8) of the undoped ZnO crystal of Comparative Example 1, it was confirmed that there was no disorder in crystallinity and orientation even when P (phosphorus) was doped. Similarly, with respect to the half width of the (100) plane rocking curve, the half width of Example 1 is 29.7 arcsec and the half width of Comparative Example 1 is 31.8 arcsec. Not confirmed.

  That is, when the crystallinity is lowered, the full width at half maximum is increased, but in Example 1, no deterioration is observed at all. Normally, when the crystallinity of ZnO crystal deteriorates, the n-type residual carrier concentration increases, but in this embodiment, there is no deterioration, that is, generation of n-type carriers that inhibit p-type formation by doping P (phosphorus). It is considered that doping is possible while suppressing this.

  Table 2 below shows the growth conditions and evaluation results of the P (phosphorus) -doped ZnO crystal layer (E2-1, E2-2, E2-3) according to Example 2.

FIG. 10 shows the results of SIMS analysis in Example 2, and shows the dependence of P (phosphorus) concentration (atoms / cm ³ ) on the flow rate (F _H2O ) of water vapor (H ₂ O). Even if the supply amount of TBP (flow rate 49.9 nmol / min) was kept constant and the supply amount of water vapor (H ₂ O) was changed, P could be doped into the ZnO crystal. The P concentration is 2.0 × 10 ¹⁸ atoms / cm ³ (F _H2O = 120 μmol / min), 1.2 × 10 ¹⁸ atoms / cm ³ (F _H2O = 320 μmol / min), 1.0 × 10 ¹⁸ atoms / cm ³ (F _H2O = 640 μmol / min). It was also found that the P concentration in ZnO changes in inverse proportion to the amount of water vapor (H ₂ O) supplied. In Example 2, as in Example 1, a flat single crystal film could be grown by two-dimensional growth with good crystallinity in terms of surface morphology and XRD.

  Table 3 below shows the growth conditions and evaluation results of the P (phosphorus) -doped ZnO crystal layer (E3-1, E3-2, E3-3) according to Example 3.

FIG. 11 shows the results of SIMS analysis in Example 3, and shows the dependence of the P (phosphorus) concentration (atoms / cm ³ ) on the flow rate of _DMZn (F _DMZn ). Even if the supply amount of TBP (flow rate 49.9 nmol / min) was constant and the supply amount of DMZn was changed, P could be doped into the ZnO crystal. The P concentration is 2.0 × 10 ¹⁷ atoms / cm ³ (F _DMZn = 10 μmol / min), 1.8 × 10 ¹⁷ atoms / cm ³ (F _DMZn = 5.0 μmol / min), 2.1 × 10 ¹⁷ atoms / cm ³ ( F _DMZn = 2.5 μmol / min). Thus, the P concentration in ZnO was almost constant without depending on the supply amount of DMZn. In Example 3, as in Example 1, a flat single crystal film having good crystallinity and two-dimensional growth could be grown from the viewpoint of surface morphology and XRD.

As described above, the following was found from Examples 1 to 3.
(1) From Example 1, the concentration of P in the crystal changes in proportion to the supply amount of _TBP (F _TBP ).
(2) From Example 2, the P concentration in the crystal changes in inverse proportion to the supply amount of steam (H ₂ O) (F _H2O ).
(3) From Example 3, the P concentration in the crystal does not depend on the supply amount of _DMZn (F _DMZn ) and is constant.

Thus, the P (phosphorus) concentration in the crystal changes according to the supply amount of TBP. However, it can be seen from the relationship between Example 2 and Example 3 that the P concentration in the crystal is determined only by the supply amount of TBP relative to the supply amount of water vapor (H ₂ O). FIG. 12 is a diagram in which the P concentration in the ZnO crystal is plotted against Pgo (= F _TBP / F _H2O ). Thus, it was found that the P concentration in the ZnO crystal can be controlled by Pgo (= F _TBP / F _H2O ), which is the ratio of the flow rate of TBP to the flow rate of water vapor (molar flow rate ratio).

For example, even if the supply amount of DMZn is increased or decreased as in Example 3, the P concentration in the crystal does not change if Pgo is constant. In a ZnO crystal, when oxygen is insufficient, the n-type residual carrier (electron) density tends to increase due to oxygen deficiency. That is, even when F _H2O is increased (that is, the VI / II ratio is increased) so that oxygen deficiency is difficult to occur, Tgo also increases when Tgo is increased (for example, when H ₂ O is increased). The concentration of P in the crystal can be made constant.

Phosphorus (P) combines with oxygen (O) to form a phosphoric acid compound. When a phosphoric acid compound is formed in the crystal, P is likely to enter the Zn site. In this case, since P works as an n-type impurity, it is not preferable. However, from the relationship between Examples 2 and 3, that is, the relationship of Pgo (= F _TBP / F _H2O ), it can be said that P is replaced with an O site. Therefore, in the MOCVD method for growing a ZnO crystal from Zn compound (DMZn) and water vapor (H ₂ O) containing no oxygen, an impurity material that does not contain an oxygen atom in the molecular structure is suitable as a p-type dopant. It is done. In particular, from the results of experiments related to the present invention, it was found that oxygen-free TBP is a suitable material.

FIG. 13 is a SIMS result of the growth layer grown according to Example 4 (TBP graded growth), and shows a P concentration profile in the depth direction from the growth layer surface. As described above, the water vapor (H ₂ O) flow rate (F _H2O ) is constant at 640 μmol / min, and the TBP flow rate is linear from 0.1 nmol / min (at the start of growth) to 50 nmol / min (at the end of growth). And crystal growth (TBP graded growth) was performed. That is, crystal growth was performed by increasing Pgo (= F _TBP / F _H2O ) from 1.6 × 10 ⁻⁷ (at the start of growth) to 1.6 × 10 ⁻⁴ (at the end of growth). As shown in FIG. 13, it was found that the P concentration increases from 2.0 × 10 ⁻⁶ atoms / cm ³ to 1.4 × 10 ¹⁸ atoms / cm ³ as Pgo increases. Similar to the above examples, a flat single crystal film with good crystallinity could be grown from the viewpoint of surface morphology and XRD. Pgo is preferably 1.6 × 10 ⁻⁷ or more from the viewpoint that the P concentration is practical.

  In the above example, the growth is performed at a high temperature of 775 ° C., and P diffusion during the growth of the ZnO crystal is expected. As can be seen from this example, the diffusion of P can be suppressed and the P concentration can be controlled by the supply amount of TBP (or Pgo). For example, for an undoped active layer, it is possible to suppress P diffusion by reducing Pgo in the vicinity of the active layer, or to increase the concentration in the vicinity of the electrode of the contact layer.

  Table 4 below summarizes the growth conditions and surface morphology evaluation results of the P (phosphorus) -doped ZnO crystal layer (C2-1, C2-2, C2-3, C2-4) of Comparative Example 2. .

FIG. 14 shows an SEM image of the growth layer C2-1 of Comparative Example 2. 15 and 16 show the (002) ω and (100) ω rocking curves of the growth layer C2-1 of Comparative Example 2, respectively. The half width of the (002) ω rocking curve is 54.2 arcsec, the half width of the (100) ω rocking curve is 403.5 arcsec, and the crystallinity is degraded as compared with the above example. Thus, when Pgo (= F _TBP / F _H2O ) was set to 3.2 × 10 ⁻³ or more, the crystal was polycrystallized, and the surface morphology was an island shape or a droplet shape. Therefore, in order to grow a P-doped ZnO crystal having good flatness and crystallinity, Pgo needs to be less than 3.2 × 10 ⁻³ .

As described above in detail, according to the present invention, an oxygen-free phosphorus compound (TBP) is formed by MOCVD using an organometallic compound that does not contain an oxygen atom in its molecular structure and water vapor (H ₂ O). ) Can be used as a dopant to grow a P (phosphorus) -doped ZnO-based crystal having good crystallinity. Further, according to the present invention, the P concentration in the ZnO crystal can be controlled by the ratio of the supply amount of TBP to the supply amount of water vapor (H ₂ O). Furthermore, a P (phosphorus) -doped ZnO crystal having a good crystal with Pgo of less than 3.2 × 10 ⁻³ can be obtained. Even at a high temperature growth of 775 ° C., P-doped ZnO crystal can be grown with good controllability from a low concentration to a high concentration by controlling the diffusion of P.

5 MOCVD apparatus 10 substrate 11 ZnO growth layer 12 P (phosphorus) doped ZnO single crystal layer

Claims (4)

A method of growing a ZnO-based crystal layer on a ZnO single crystal substrate by MOCVD,
A single crystal growth step of growing a ZnO-based crystal layer using an organometallic compound that does not contain an oxygen atom in the molecular structure and a polar oxygen material;
The single crystal growth step includes a step of supplying TBP (tertiary butylphosphine) gas. The polar oxygen material is water vapor (H ₂ O), and a molar flow rate ratio of the TBP to the water vapor is 1.6 × 10 ⁻⁷ or more and less than 3.2 × 10 ^−3. The method according to 1. The polar oxygen material is water vapor (H ₂ O), and a molar flow rate ratio of the TBP to the water vapor is 1.6 × 10 ⁻⁷ or more and 1.6 × 10 ⁻⁴ or less. The method according to 1.   The method according to claim 1, wherein the single crystal growth step is a step of growing a ZnO-based single crystal at 600 ° C. or higher and 850 ° C. or lower.