JP4268405B2 - ZnO crystal growth method, ZnO crystal structure, and semiconductor device using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a ZnO crystal, a growth method thereof, and a semiconductor device using the same.
[0002]
[Prior art]
Conventionally, ZnO has been grown directly on a sapphire substrate using an RS (radical source) -MBE method. As a source for Zn, a solid source for Zn in a K cell (Knudsen cell) is used. As a source for O, oxygen radicals generated by using RF or ECR or the like as an oxygen gas as a gas source are used.
[0003]
In particular, the RF-MBE method using RF uses the most common high frequency (13.56 MHz) on a commercial basis. O radicals are generated by introducing O ₂ gas, which is a raw material gas, into an electrodeless discharge tube in the MBE chamber. The O radical blows into the MBE chamber and becomes an O radical beam. A ZnO thin film is grown by simultaneously irradiating a sapphire substrate with an O radical beam and a Zn beam from a K cell.
[0004]
A semiconductor device such as an LED (Light Emitting Diode) or an LD (Laser Diode) including a pn junction can be manufactured using ZnO which is one of II-VI group semiconductors.
[0005]
The crystallinity of a semiconductor crystal material constituting a semiconductor device such as an LED or LD has a significant influence on the electrical characteristics, optical characteristics, and element reliability (element lifetime) of the semiconductor element. The better the crystallinity of the semiconductor crystal material constituting the semiconductor element, the better the optical and electrical characteristics and reliability of the semiconductor element.
[0006]
[Problems to be solved by the invention]
The ZnO crystal grown directly on the sapphire substrate using the conventional RS (radical source) -MBE method has the following problems. That is, the difference in lattice constant between the sapphire substrate and ZnO (the lattice constant mismatch is about 18%) is large. In addition, there is a large difference of 2.6 times in the coefficient of thermal expansion between the two.
[0007]
Many crystal defects are introduced into the grown ZnO crystal.
[0008]
FIG. 8 shows a measurement result of the ZnO crystal by the XRC (X-ray rocking curve) method when the ZnO (0002) crystal is directly grown on the sapphire (0001) substrate. ZnO crystal is grown by the RS-MBE method under the conditions of growth temperature: Tg = 650 ° C., Zn partial pressure: P _zn = 1 × 10 ⁻⁷ Torr, oxygen flow rate: O ₂ = 2 SCCM, RF output: 300 W did.
[0009]
As a result of XRC measurement, an approximately normal distribution curve having peak intensity around 17.5 degrees was observed for Omega. The full width at half maximum (FWHM) was as large as 0.5 ° (1800 arcsec). From the measurement result of FIG. 8, it was found that the ZnO crystal grown under the above conditions has poor crystallinity.
[0010]
Figure 9 shows the results of the grown ZnO crystals was measured by PL (P hoto L uminesence) method in the above conditions. The horizontal axis represents the energy of the emitted light from PL. A sharp peak having a high intensity and a narrow half-value width was observed near an energy of 3.35 eV. The peak energy value of PL substantially coincides with the forbidden band width (3.3 eV) of ZnO. It is understood that the emission peak is due to recombination of electrons and holes between the conduction band and the valence band.
[0011]
A very broad peak was also observed from 1.8 eV to around 2.7 eV as energy. This broad peak is understood to be caused by light emission between deep levels existing in the forbidden band. This suggests that many crystal defects exist in the ZnO crystal.
[0012]
An object of the present invention is to provide a crystal growth method for reducing a crystal defect of a ZnO crystal grown on a sapphire substrate and growing a high-quality ZnO crystal.
[0013]
Another object of the present invention is to provide a ZnO crystal grown on a sapphire substrate and a semiconductor device using the same.
[0014]
[Means for Solving the Problems]
According to one aspect of the present invention, a ZnO single crystal buffer layer is formed on a sapphire substrate by a RS-MBE method at a temperature between 200 ° C. and 600 ° C. under conditions of zinc (Zn) rich or oxygen (O) rich. A step of growing, a step of subjecting the ZnO single crystal buffer layer to a surface flattening treatment by heat treatment at a temperature from 600 ° C. to 800 ° C. for 2 minutes to 60 minutes, and a ZnO single crystal subjected to the surface flattening treatment. And a method of growing a single crystal ZnO layer on the crystal buffer layer.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0016]
Note that the “low temperature growth ZnO growth temperature” defined in the present specification is, for example, about 200 ° C. to 600 ° C., generally 100 ° C. higher than the temperature of crystal growth for growing a ZnO single crystal. The temperature is about 400 ° C lower. The “growth temperature of the high temperature grown single crystal ZnO layer” is generally a growth temperature suitable for growing a ZnO single crystal, is higher than the above “growth temperature of low temperature grown ZnO”, and is 800 ° C. The lower temperature, for example, 650 ° C.
[0017]
A method for growing a II-VI compound semiconductor crystal according to the first embodiment of the present invention will be described with reference to FIGS.
[0018]
FIG. 1 shows a crystal growth apparatus (hereinafter referred to as “RS-MBE apparatus”) using a radical source molecular beam epitaxy (RS-MBE) method as an example of an II-VI group compound semiconductor crystal growth apparatus.
[0019]
The RS-MBE apparatus A includes a chamber 1 in which crystal growth is performed, and a vacuum pump P that keeps the chamber 1 in an ultrahigh vacuum state.
[0020]
The chamber 1 includes a Zn port 11 for evaporating Zn, an O radical port 21 for irradiating O radicals, and an N radical port 31 for irradiating N radicals.
[0021]
The Zn port 11 includes a Knudsen cell (hereinafter referred to as a K cell) 17 that houses a Zn (purity 7N) raw material 15 and is heated and evaporated, and a shutter S ₁ .
[0022]
The O radical port 21 introduces an oxygen gas, which is a raw material gas, into the electrodeless discharge tube, and jets O radicals generated using a high frequency (13.56 MHz) into the MBE chamber 1. An orifice S ₂ is provided for the O radical beam.
[0023]
The N radical port 31 introduces nitrogen gas, which is a raw material gas, into the electrodeless discharge tube, and ejects N radicals generated using high frequency (13.56 MHz) into the MBE chamber 1. A shutter S ₃ is provided for the beam of N radicals.
[0024]
The structure of the radical ports 21 and 31 is a structure in which an induction coil is wound outside the discharge tube provided in the outer shield tube.
[0025]
In the chamber 1, there are provided a substrate holder 3 for holding a sapphire substrate S as a base for crystal growth, and a heater 3 a for heating the substrate holder 3. The temperature of the sapphire substrate S can be measured by the thermocouple 5. The position of the substrate holder 3 can be moved by a manipulator 7 using a bellows.
[0026]
The chamber 1 includes a gun 41 of a reflection electron beam diffractometer (RHEED device) and a screen 55 of the RHEED device provided for monitoring the grown crystal layer. Using the gun 41 of the RHEED apparatus and the screen 55 of the RHEED apparatus, the growth can be performed while monitoring the state of crystal growth (growth amount, quality of the grown crystal layer) in the MBE apparatus A.
[0027]
The temperature of crystal growth, the thickness of the crystal growth film, the degree of vacuum in the chamber, and the like are appropriately controlled by the control device C.
[0028]
Hereinafter, the step of growing ZnO on the sapphire substrate S will be described in detail.
[0029]
Crystal growth is carried out by opening and closing appropriate S ₃ from the shutter S ₁ by RS-MBE method.
[0030]
As a method for generating a radical source, an RF-MBE method using RF is used. O radicals are generated by introducing O ₂ as a raw material gas into the electrodeless discharge tube using a high frequency of 13.56 MHz. By blowing O radicals into the MBE chamber 1 in a high vacuum state, an O radical beam is obtained. A ZnO thin film is grown by simultaneously irradiating the sapphire substrate S with an O radical beam and a Zn beam from a K cell.
[0031]
FIG. 2 shows an outline of the semiconductor crystal structure according to the present embodiment.
[0032]
A ZnO buffer layer 101 is formed on the sapphire (0001) substrate S, and a ZnO single crystal layer 105 is formed on the buffer layer 101.
[0033]
The process for creating the structure shown in FIG. 2 will be briefly described below.
[0034]
First, ZnO is grown on the sapphire substrate S at a low growth temperature and under Zn-rich conditions. In the state irradiated with oxygen radicals, the temperature is raised to a normal growth temperature suitable for single crystal growth of ZnO. This state is maintained for about 20 minutes, for example. The surface of the low temperature growth ZnO layer is flattened by the high temperature heat treatment. Thereby, the buffer layer 101 is formed. A ZnO single crystal layer 105 is grown thereon.
[0035]
The buffer layer 101 alleviates lattice mismatch and a large difference in thermal expansion coefficient caused by a large shift in lattice constant between the sapphire substrate S and the ZnO single crystal layer 105, and the ZnO single crystal layer 105 caused by these. Prevent the introduction of distortion into the inside.
[0036]
FIG. 3 shows the surface state of the ZnO buffer layer 101 observed by the AFM method (Atomic Force Microscopy).
[0037]
FIG. 3A shows the surface of the buffer layer 101 when grown under a Zn-rich condition, and FIG. 3B shows the surface of the buffer layer 101 when grown under an O-rich condition.
[0038]
A Zn-rich ZnO crystal is a crystal in which x is contained at a ratio smaller than 1 in ZnO _1-x .
[0039]
It can be seen that when the ZnO buffer layer 101 is grown under a Zn-rich condition, the planarity of the surface of the ZnO buffer layer 101 is improved as compared with the case where it is grown under an O-rich condition. In addition, the grown low-temperature grown ZnO layer is considered to be a single crystal in which grain boundaries exist, both when grown under oxygen-rich growth conditions and when grown under Zn-rich growth conditions.
[0040]
Hereinafter, a more detailed crystal growth method will be described.
[0041]
The (0001) plane of the sapphire substrate S is wet-etched for 60 minutes in a solution of H ₃ PO ₄ : H ₂ SO ₄ = 1: 3 heated to 110 ° C.
[0042]
After performing the above surface treatment, the sapphire substrate S is mounted on the substrate holder 3 (FIG. 1).
[0043]
Surface treatment with oxygen plasma was performed in an MBE apparatus for 1 hour under conditions of a substrate temperature of 550 ° C., an oxygen flow rate of 2 SCCM, and an RF power of 150 W. By processing the surface of the sapphire substrate S in the MBE apparatus, the surface of the sapphire substrate S is cleaned.
[0044]
After the substrate surface treatment, first, the buffer layer 101 is grown. Unlike normal single crystal ZnO substrate growth conditions, growth is performed under low temperature and Zn rich conditions (low temperature growth ZnO layer). The beam amount of Zn is 2.7 × 10 ⁻⁷ Torr.
[0045]
An RF plasma source of O is used as the oxygen beam supply source. Pure oxygen (purity 6N) gas is introduced into the O radical port 21 and radicalized using a high frequency oscillation source.
[0046]
The flow rate of oxygen as a gas source is 5 × 10 ⁻⁵ Torr and the RF power is 300 W at a flow rate of 1.5 SCCM as the partial pressure of oxygen in the chamber. The growth temperature is in the range of 300 ° C to 600 ° C. The thickness of the buffer layer is in the range of 10 to 100 nm.
[0047]
Here, the value of the pressure indicates an instruction value of a nude ion gauge attached to the substrate holder position (growth position).
[0048]
FIG. 4 shows the relationship between the Zn partial pressure (Pzn) and the growth rate when the substrate temperature Tg is 650 ° C. The O ₂ partial pressure was ₂ SCCM, and the RF output was 300 W. The partial pressure of Zn was changed from 1.3 × 10 ⁻⁷ Torr to 2.7 × 10 ⁻⁷ Torr.
[0049]
When the partial pressure of Zn is increased from 1.3 × 10 ⁻⁷ Torr to 2.15 × 10 ⁻⁷ Torr, the growth rate of ZnO increases from 0.10 μm / hr to 0.26 μm / hr. When the partial pressure of Zn is in the range of 2.15 × 10 ⁻⁷ Torr to 2.7 × 10 ⁻⁷ Torr, the growth rate of ZnO is approximately constant from about 0.26 μm / hr to about 0.27 μm / hr. Indicates.
[0050]
After the ZnO buffer layer grown at a low temperature is grown, the surface of the buffer layer is planarized. As the planarization treatment, heat treatment is performed at a high temperature for growing a single crystal, for example, 650 ° C. for 2 minutes to 60 minutes. The low-temperature grown ZnO buffer layer 101 that has been grown at a low growth temperature is considered to be a single crystal having a grain boundary and epitaxially grown so that each grain exhibits the same anisotropy. It is considered that irregularities such as those observed in AFM observation were observed mainly due to the grain boundaries between the grains.
[0051]
By applying the above heat treatment to the low temperature growth ZnO buffer layer, it is considered that the single crystal of each grain is solid phase grown, the grain size is increased, and the surface is flattened.
[0052]
In particular, when grown under a Zn-rich condition, the initial surface irregularities are smaller than when grown under an oxygen-rich condition, so that an excellent flat surface can be easily obtained by the planarization treatment. When a ZnO layer is grown at a high temperature on a low-temperature grown ZnO buffer layer having an excellent planar surface, a single crystal ZnO layer having good crystallinity is easily obtained.
[0053]
The temperature for growing the low-temperature grown ZnO layer is preferably between 200 ° C. and 600 ° C.
[0054]
The low-temperature grown ZnO buffer layer has a small grain size in an as-grown state and a grain boundary is observed, so that it looks like a polycrystal in AFM observation. However, analysis by X-ray diffraction or RHEED method shows the characteristics of a single crystal.
[0055]
This phenomenon is observed in the growth with GaN or ZnO. By subjecting the low-temperature grown ZnO buffer layer to a high-temperature heat treatment, it is considered that irregularities caused by grain boundaries and the like grow as in the case of solid-phase growth, and the surface becomes flat. Even if an attempt is made to grow single crystal ZnO on a non-planar ZnO surface, the crystallinity is not improved by experience.
[0056]
Next, a ZnO single crystal (high temperature grown ZnO single crystal layer) is grown on the planarized low temperature grown ZnO buffer layer.
[0057]
The growth conditions are a substrate temperature of 650 ° C. and a Zn (7N) beam amount of 8.0 × 10 ⁻⁵ Torr. The oxygen partial pressure is 8 × 10 ⁻⁵ Torr at a flow rate of 2.0 SCCM, and the RF power is 300 W.
[0058]
The thickness of the grown high temperature grown ZnO single crystal layer is 1 μm.
[0059]
As the growth conditions for the high-temperature grown ZnO single crystal layer, it is preferable to perform the growth for 2 minutes to 60 minutes at a temperature between 600 ° C. and 800 ° C.
[0060]
The crystallinity of the high-temperature-grown ZnO single crystal layer when grown under the above conditions was evaluated.
[0061]
FIG. 5 shows the measurement result by XRC.
[0062]
The peak value is 17.35 °, which is almost the same as the value in FIG. The full width at half maximum is 0.06 ° (216 arcsec). Compared to the half width (0.5 °) of ZnO grown under the conventional conditions shown in FIG. 8, the half width was reduced to about 1/8. A significant reduction in the half-value width is achieved by providing a low temperature growth ZnO buffer layer and performing a planarization treatment on the surface, and then growing a high temperature growth ZnO single crystal layer on the surface at a high growth temperature. It is understood that this is due to the improvement in crystallinity of the single crystal layer.
[0063]
FIG. 6 shows a PL spectrum of the ZnO single crystal layer.
[0064]
A sharp peak was observed at an energy of 3.359 eV. In the case of a ZnO single crystal grown without a low-temperature grown ZnO buffer layer (FIG. 9), a very broad peak from 1.8 eV to around 2.7 eV is not observed. This is understood to be due to a decrease in light emission from deep levels existing in the forbidden band of the ZnO layer. It is considered that crystal defects in the ZnO crystal are reduced and crystallinity is greatly improved.
[0065]
When many crystal defects are introduced into the ZnO crystal, strong n-type conductivity is exhibited even when no impurity is introduced. The high temperature growth ZnO single crystal layer grown by using the above crystal growth method has very few crystal defects. It is also possible to realize ZnO exhibiting p-type conductivity, which was difficult with a ZnO single crystal grown by a conventional crystal growth method. It is considered that the light emission efficiency is very high because crystal defects that form a non-light emitting center are greatly reduced.
[0066]
Using ZnO doped with N as an impurity as the p-type semiconductor, it is possible to realize a LED (L ight E mitting D iode ) including a p-n junction Daodo using ZnO and Ga-doped as an n-type semiconductor.
[0067]
7, using the ZnO of N doped as p-type semiconductor shows a cross-sectional structure of LED (L ight E mitting D iode ) including a p-n junction Daodo using ZnO and Ga-doped as an n-type semiconductor.
[0068]
The LED shown in FIG. 7 includes a sapphire substrate 301, a non-doped low-temperature growth ZnO buffer layer 305 having a thickness of 100 nm grown thereon, and an n-type (Ga-doped: 1 μm thickness) grown thereon. × 10 ¹⁸ cm ⁻³ ) A high-temperature-grown ZnO single crystal layer 311 and an N-doped p-type high-temperature-grown ZnO single crystal layer 315 formed thereon and having a thickness of 100 nm.
[0069]
The n-type ZnO layer 311 is in contact with the first electrode 321.
[0070]
In order to form the n-type ZnO layer, other group 3 elements such as Al may be doped instead of Ga.
[0071]
The N-doped p-type ZnO layer 315 is processed into an island shape. The p-type ZnO layer 315 processed into an island shape is covered with an insulating film 318 made of, for example, Si ₃ N ₄ . On the upper surface of the p-type ZnO layer 315, for example, a substantially circular opening is formed through the insulating film 318.
[0072]
On the periphery of the upper surface of the p-type ZnO layer 315, a ring-shaped second electrode 325 is formed. At least a part of the lower surface of the ring-shaped second electrode is in contact with the peripheral portion of the upper surface of the p-type ZnO layer 315. The radially outer portion of the ring-shaped second electrode 235 has a structure that rides on the insulating film 318.
[0073]
In the above structure, when a positive voltage is applied to the second electrode with respect to the first electrode 321, a forward current flows through the pn junction. Minority carriers (electrons) injected into the p-type ZnO layer 315 and majority carriers (holes) in the p-type ZnO layer 315 recombine with light emission. Upon recombination of electrons and holes, light having an energy substantially equal to the energy gap of the forbidden band is emitted from the opening. That is, electrical energy is converted into light energy.
[0074]
By the above operation, light having a wavelength of about 370 nm, for example, is emitted from the opening of the ring-shaped second electrode 235 of the LED.
[0075]
In this embodiment, the LED has been described as an example of a semiconductor element using a pn junction between a p-type ZnO layer and an n-type ZnO layer. However, the p-type ZnO layer 315 and the n-type ZnO 311 are combined. It is also possible to form a laser element. In addition, it goes without saying that electronic devices such as FETs and bipolar transistors, other optical devices, and semiconductor devices combining these devices can be manufactured in combination with the p-type ZnO layer 315.
[0076]
Note that a semiconductor crystal or a semiconductor device can be formed using a ZnO-based ternary or quaternary mixed crystal including a ZnO single crystal with good crystallinity.
[0077]
As mentioned above, although this invention was demonstrated along embodiment, this invention is not restrict | limited to these. Various conditions for crystal growth and other process parameters can be selected. It will be apparent to those skilled in the art that other various modifications, improvements, combinations, and the like are possible.
[0078]
【The invention's effect】
Single crystal ZnO with good crystallinity can be grown on a sapphire substrate.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view schematically showing an MBE apparatus used in a crystal growth method according to a first embodiment of the present invention.
FIG. 2 shows a laminated structure of a sapphire substrate / low temperature growth ZnO buffer layer / high temperature growth ZnO single crystal layer grown by the crystal growth method according to the first embodiment of the present invention.
FIG. 3 shows a result of measuring a surface state of a ZnO buffer layer grown by a crystal growth method according to a second embodiment of the present invention by an AFM method. (A) is grown under Zn-rich conditions, and (b) is grown under O-rich conditions.
FIG. 4 shows the relationship between the growth rate of a high-temperature-grown ZnO single crystal layer at a growth temperature of 650 ° C. and the partial pressure of Zn.
FIG. 5 shows an XRC method for a high-temperature-grown ZnO single crystal layer in a laminated structure of a sapphire substrate / low-temperature-grown ZnO buffer layer / high-temperature-grown ZnO single crystal layer grown by the crystal growth method according to the first embodiment of the present invention. It is a measurement result by.
FIG. 6 shows a PL measurement method of a ZnO single crystal layer in a laminated structure of a sapphire substrate / low temperature growth ZnO buffer layer / high temperature growth ZnO single crystal layer grown by the crystal growth method according to the first embodiment of the present invention. It is a measurement result.
FIG. 7 is a schematic cross-sectional view showing a structure of a semiconductor light emitting device (LED) according to a second embodiment of the present invention.
FIG. 8 is a measurement result of a ZnO single crystal layer measured by an XRC method in a laminated structure of a sapphire substrate / low temperature growth ZnO buffer layer / high temperature growth ZnO single crystal layer grown by a conventional growth method.
FIG. 9 is a measurement result of a high temperature growth ZnO single crystal layer measured by a PL measurement method in a laminated structure of a sapphire substrate / low temperature growth ZnO buffer layer / high temperature growth ZnO single crystal layer grown by a conventional growth method.
[Explanation of symbols]
A RS-MBE device C Control device P Vacuum pump S Substrate 1 Chamber 3 Substrate holder 3a Heater 5 Thermocouple 7 Manipulator 11 Zn port 15 Zn raw material 17 Knudsen cell 21 O radical port 31 N radical port 100 ZnO substrate 101 ZnO buffer layer (Low temperature growth ZnO buffer layer)
105 ZnO single crystal layer (high temperature grown ZnO single crystal layer)
301 Sapphire substrate 305 ZnO layer buffer layer (low temperature growth ZnO buffer layer)
311 n-type ZnO layer (high temperature grown ZnO single crystal layer)
315 p-type ZnO layer (high temperature grown ZnO single crystal layer)
318 Insulating film 321 First electrode 325 Second electrode

Claims (8)

A step of growing a ZnO single crystal buffer layer on a sapphire substrate by a RS-MBE method at a temperature between 200 ° C. and 600 ° C. under zinc (Zn) rich or oxygen (O) rich conditions;
Performing a surface flattening process on the ZnO single crystal buffer layer at a temperature from 600 ° C. to 800 ° C. by heat treatment for 2 minutes to 60 minutes;
And a step of growing a single crystal ZnO layer on the ZnO single crystal buffer layer subjected to the surface flattening treatment.   The method for growing a ZnO crystal according to claim 1, wherein the step of growing the ZnO single crystal buffer layer is a step of vapor phase growth under Zn-rich conditions. A sapphire substrate,
Zinc (Zn) rich or oxygen (O) formed thereon at a temperature between 200 ° C. and 600 ° C. and subjected to surface planarization treatment by heat treatment at a temperature from 600 ° C. to 800 ° C. for 2 minutes to 60 minutes. ) A rich , undoped ZnO single crystal buffer layer;
A ZnO crystal structure including a ZnO single crystal layer formed above the ZnO single crystal buffer layer.   The ZnO single crystal buffer layer is a layer obtained by growing ZnO at a temperature lower than 600 ° C., and the ZnO single crystal layer formed on the ZnO single crystal buffer layer is higher than the ZnO single crystal buffer layer and The ZnO crystal structure according to claim 3, further comprising a high-temperature-grown ZnO single crystal layer grown at a temperature lower than 800 ° C.   The ZnO crystal structure according to claim 3 or 4, wherein a thickness of the ZnO single crystal buffer layer is 10 to 100 nm.   The ZnO crystal structure according to claim 4 or 5, wherein the high-temperature-grown ZnO single crystal layer is a ZnO layer having n-type conductivity doped with Ga or Al.   The ZnO crystal structure according to any one of claims 4 to 6, wherein the high-temperature-grown ZnO single crystal layer is a ZnO layer having p-type conductivity doped with N. A sapphire substrate,
Zinc (Zn) -rich, which is formed at a temperature between 200 ° C. and 600 ° C. so as to be in contact with the substrate, and has been subjected to a surface flattening treatment at a temperature from 600 ° C. to 800 ° C. for 2 minutes to 60 minutes An oxygen (O) rich ZnO single crystal buffer layer that is not doped with impurities ;
A ZnO single crystal layer formed above the ZnO single crystal buffer layer,
The ZnO single crystal layer is a semiconductor device having a pn junction between a ZnO layer having n-type conductivity doped with Ga or Al and a ZnO layer having p-type conductivity doped with N.

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