sic_usp_1994_5,441,011_sublimation growth of single crystal sic

7/31/2019 Sic_usp_1994_5,441,011_sublimation Growth of Single Crystal Sic

1/6

111111111111111111111111111111111111111111111111111111111111111111111111111United States Patent [19]Takahaski et ale

[11][45]

US005441011APatent Number:Date of Patent:

5,441,011Aug. 15, 1995

[54] SUBLIMATION GROWTH OF SINGLECRYSTAL SIC[75] Inventors: Jun Takahaski; Masatoshi Kanaya,both of Sagamihara, Japan[73] Assignee: Nippon Steel Corporation, Tokyo,Japan[21] Appl. No.: 213,055[22] Filed: Mar. 15, 1994[30] Foreign Application Prior ity DataMar. 16, 1993 [JP] Japan 5-055687

[51] Int. 0 .6 C30B 23/00; H01L 21/20[52] U.S. O 117/84; 117/105;117/951; 437/100; 148/DIG. 148[58] Field of Search 117/84, 88, 89,951,117/105; 437/100; 148/DIG. 148[56] References Cited

U.S. PATENT DOCUMENTS2,854,364 9/1958 Lely.3,147,159 9/1964 Lowe 437/1003,236,780 2/1966 Ozarow 117/9513,343,920 9/1967 Lowe 117/951

6

3,615,930 10/1971 Knippenber et aI 17/9514,147,572 4/1979 Vodakov et aI. 117/9514,866,005 9/1989 Davis et aI. .5,211,801 5/1993 Stein 117/951FOREIGN PATENT DOCUMENTS

63-57400 1111988 Japan.W089/04055 5/1989 WIPO 437/100PrimaryExaminer-Mary WilczewskiAttorney;Agent, orFirm-Pollock, Vande Sande &Priddy[57] ABSTRACTA method of growing a first SiC single crystal on a seedcrystal including a second SiC single crystal, comprisesthe steps of setting a SiC source material at an initialtemperature, growing the first SiC single crystal on theseed crystal including the second SiC single crystal at atemperature lower than the initial temperature of thesource material and gradually decreasing the sourcematerial temperature from the initial temperature during at least a predetermined period during the growingstep.

7 Claims, 2 Drawing Sheets


2/6

u.s. Patent Aug. 15, 1995 Sheet 1 of 2 5,441,011

FIG. I6

FIG. 2

10


3/6

u.s. Patent Aug. 15, 1995 Sheet 2 of 2 5,441,011

FIG. 3

- 2400'- ' 2380.J ~ow 2300el>t-

ol I i I0 2 4 6 8GROWTH TIME ( hr )


4/6

SUBLIMATION GROWTH OF SINGLE CRYSTALSIC

5,441,0111

SUMMARY O F TH E INVENTION

2Generally, in the crystal growth, the quality of agrown crystal is considered to be better at a lowergrowth rate. When the source material temperature inthe sublimation recrystallization method is decreased asBACKGROUND O F T HE INVENTION 5 time passes, the amount of vaporized SiC source mate-1. Field of the Invention rial produced decreases. In addition, the amount ofThis invention relates to a method of growing single vaporized source material (which means a flux includ-crystals of high quality silicon carbide (SIC) for use as ing SiCz, SizC, and Si) reaching the grown crystal de-a substrate wafer for producing blue light emitting di- creases, and the growth rate decreases, too. In this way,odes, electronic devices or the like, and more particu- 10 a ~ g h - q u a l i t y single crystal is conside:ed to be grown

lady to a method of growing single crystals of SiC by a w ~ t h l ~ levels of crystal defects WIthout polytypessublimation recrystallization process on a seed crystal of mixed In .a SiC single crystal. In the crystal growth of SiC according to the subli-2. Description of the Related Ar t mation method, however, since the lid of a crucible inA SiC single crystal, being a material chemically 15 which a seed crystal is mounted is constantly heated tostable and resistant to high temperatures and radiant a temperature higher than 22000 c., crystal defectsrays, holds high hopes for application in environment- included in the seed crystal progressively extend asresistant semiconductor devices. Having a wide band- black linear defects into the already grown region of thegap, SiC single crystals are currently being used as crystal. This defect elongates in the growing directionmaterial for short-wavelength light emitting diodes. In 20 of the crystal with passage of heating time. Therefore,fact, 6H-SiC has a bandgap of about 3.0 eV at room in the early stage of growth, the silicon carbide sourcetemperature, and is used as the material for blue light material is raised to a relatively high temperature toemitting diodes. Fo r this reason, there has been a keenly promote the generation of the source material gas infelt need for better quality SiC single crystal ingots for 25 large quantities at a far greater growth rate than thethe manufacture of high quality wafers to be used for speed of enlargement of the linear defect mentionedapplications mentioned above. above. Thereafter, a good-quality crystal portion fromAs a production process of SiC single crystals by which a desired wafer is made isgrown at lower speed.sublimation recrystallization, the process disclosed in In this crystal growth technique, the portion of theJP-B-63-57400 (by Siemens Actiengesellshaft, Federal 30 crystal which was grown at high speed in the first halfRepublic of Germany) has been known, in which pro- period of the crystal g rowth plays a role of a buffercess the crystal growth is carried out at a fixed tempera- layer which confines the defect where it is, so that theture. According to this production method, however, crystal grown in the latter half period has a better qual-there are high possibilities of an occurrence of black ity than the seed crystal used. Furthermore, accordinglinear defects extending from the seed crystal and an 35 to this method, impurities and nitrogen which becomesinterminglement of polytypes other than the desired 6H a doner, or the like contained in the source material andpolytype, and a decrease of crystal quality was wit- the crucible, vaporize in the early stage of growth bynessed in the later stage of the crystal growth when the high temperature, and they are trapped in the early-source material is running short. stage-growing portion of the crystal or discharged from

It is disclosed in U.S. Pat. No. 4,866,005 and WO-A- 40 the system. Consequently, in the middle and the latter89/04055 (Davis et al.) that the source material temper- stages when silicon carbide is grown at lower tempera-ature is fixed or is increased as the source material de- ture, the crystal has better quality because the crystalcreases. This is aimed to obtain a large SiC crystal by has already been deprived of the impurities in thehaving the low-temperature portion of the source mate- source material and the residual nitrogen.rial sublimate and thereby allowing the crystal to grow 45 In this crystal growth technique, i f the source mate-for a long time after the high-temperature portion of the rial temperature is decreased very quickly, this hassource material runs short. adverse effects on the grown crystal. Therefore, it isadequate to gradually decrease the source material temperature as the growth time passes. As for a pattern ofTh e object of the present invention is to grow good- 50 source material temperature decrease, the temperaturequality SiC single crystal ingots with low levels of crys- decrease rate need not be fixed unlesssharp changes aretal defects and with a fewer or without polytypesmixed not involved.in. Th e crystal grown by the crystal growth technique asIn order to achieve the above object, according to the mentioned above is obviously superior in transparencypresent invention, there is provided a method of grow- 55 particularly in the latter stage ofgrowth than the crystaling SiC single crystals including the steps of maintain- grown at a constant source material temperature. Th eing a SiC source powder at a first temperature, and cause is considered to be related to the ratio between Cgrowing a SiC single crystal on a seed crystal including and Si in the source material gas. According to a tem-a SiC single crystal at a temperature lower than the perature-pressure diagram of S iC+C system shown ontemperature of the SiC source powder which is gradu- 60 page 279 o f IEEE Trsns. Electron Devices ED-30ally decreased from the first temperature with passage (1983) 277, it is considered that in the early stage ofof time. growth the source material gas has an excessive content

In the process of growing a SiC single crystal, the of Si, but the content of C as the material of a graphitetemperature of the SiC source powder may be de- crucible becomes excessive with passage of time. If thecreased at a rate of 10 C. to 150 C. per hour. 65 material temperature is decreased as mentioned above,Furthermore, in the process of growing a SiC single the C gas pressure is prevented from becoming exces-crystal, the temperature of the SiC source powder may sive, so that pressures of C and Si gases move in thebe decreased at a rate of 50 C. to 10C. per hour. direction of coming closer to each other. This isproba-


5/6

5,441,011

EMBODIMENT 1An undoped SiC single crystal was grown using the(0001)c face of a {000I} wafer of 6H polytype as a seedcrystal, and setting the source material temperature at2380C. at the start of growth, the seed crystal temperature at 2320 C., and the atmospheric pressure at 20Torr. FIG. 3 shows the profile of the source materialtemperature throughout the growth process. As shownin this figure, from the time two hours after the start ofthe crystal growth process, the source material temperature was decreased at the rate of about 10C. per hour,and the crystal growth was continued for about eighthours. On the other hand, from the time two hours afterthe start of the crystal growth process, the seed crystaltemperature was lowered at the rate of about 6 C. perhour.

Th e thus grown crystal had a large (0001) facet at thetop and was excellent in transparency.Out of {oool}wafers cut from this ingot, wafers fromthe vicinity ofthe seed crystal were found to have manyblack crystal defects. However, the wafers producedfrom the central and the upper portion end of the ingotdid not have black defects as mentioned above. Th ewafers from the portion near the seed crystal were alittle greenish, even though the ingot was formed un-

4cause the sublimation of the source material stops in ashort time, but if this decrease rate is too low, it cannotbe expected that the effect of the present invention isachieved. If the decrease rate is higher than 15C. per5 hour, a sufficient growth time cannot be secured in adesired range of growth rate, a large single crystal cannot be formed. If the decrease rate is lower than 1C.per hour, a deterioration of crystal quality is witnessed.In addition, when productivity is taken into consider-

10 ation, the most desirable decrease rate is 5 C. to 10C.per hour.On the other hand, it is desirable to set the seed crys-tal temperature 30 C. to 120C., more preferably, 50C. to 80C. lower than the sourcematerial temperature,and set the thermal gradient at 5 to 25 C./cm, morepreferably, 10to 20C./cm. Furthermore, it is prefera-ble to set a fixed seed crystal temperature, or decreasethe seed crystal temperature at a rate lower than that ofthe source material temperature. Therefore, preferably,the thermal gradient is decreased gradually.With regard to the relation between temperature andpressure, the single crystal growth rate is desired to bein the range of 0.2 mm to 2.5mm per hour, more preferably, 0.4 mm to 1.6mm per hour throughout the growthprocess. High growth rates exceeding 2.5 mm per hourare not adequate because the crystallinity decreases atsuch a high growth rate, while at lo w growth rates lessthan 0.2 mm per hour, productivity is low. In the present invention, it is important to gradually decrease thesource material temperature and, as a result, reduce thegrowth rate gradually.FIG. 2 shows a single crystal ingot grown by themethod mentioned above. In FIG. 2, the seed crystal islocated at the lower position, so that the direction ofcrystal growth is on the side opposite the seed crystal.Th e portion of the grown single crystal ingot closer tothe seed crystal contains lots of crystal defects andimpurities and, therefore, has a poor crystal quality. Fo rblue light emitting diodes and electronic devices, waferscut from the center region and the top region of thecrystal are most suitable because the wafers from thoseregions are higher in crystal quality.

3bly th e reason why th e decrease in the crystal quality inthe latter stage of growth is suppressed.

Preferred embodiments of the present invention will 15be explained in detail with reference to the accompanying drawings.FIG. 1 shows an example of a single crystal growthapparatus used in a method of growing a SiC singlecrystal according to the present invention. As shown in 20FIG. 1, a graphite crucible used in the single crystalgrowth apparatus comprises a closed-end crucible 1,and a crucible lid 3 made of graphite for covering theopening of the crucible 1 having a mounting portion forholding a SiC substrate seed crystal 5. Th e crucible 1 25and the crucible lid 3 are covered on their side faces andtop and bottom faces with a heat insulating material 6made of a graphite felt, and are housed in a vessel capable of evacuation to a vacuum by an evacuation systemand capable of pressure control of the internal atmo- 30sphere by using an inert gas, such as argon. Heating isdone, for example, by a high frequency induction coilwound around the vessel. The crucible temperature ismeasured, for example, by a two-color pyrometerwhich receives light from the bottom portion of the 35crucible through an optical path 7 with a diameter of 2to 4 mm provided at the center of the graphite feltcovering the bottom portion of the crucible. Th e temperature thus measured is regarded as the source material temperature. Th e temperature of the crucible lid is 40measured through a similar optical path provided in thesame felt as that of the top portion, and the measuredtemperature is regarded as the seed crystal temperature.To give an example, crystal growth as describedbelow is performed. 45Th e vessel isevacuated, and the source material temperature is raised to about 2000 C. Thereafter, whileintroducing an inert gas into the vessel, the interiorpressure iskept at about 600Torr, and the source material temperature is raised to a target temperature. Th e 50SiC crystal growth should desirably be started by reducing the pressure by spending 10 to 90 min, and setting the atmospheric pressure at 2 to 50 Torr, morepreferably, 10 to 20 Torr and the source material temperature at 2200 to 2500C., more preferably, 2300to 552400 C. At low temperatures below 2200 C., it isdifficult for the source material to sublime, and at hightemperatures over 2500 C., it becomes difficult for abetter quality single crystal to be formed owing to thermal etching at the crystal surface, for example. Th e 60source material temperature should preferably be reduced at a rate of 1C. to 15 C. per hour or morepreferably at a rate of 5C. to 10C. per hour. Note thatin the early stage of growth, in order to form a bufferlayer at a high growth rate, it is preferable to grow a 65crystal for some time at a lower temperature decreaserate or at a fixed temperature. To o fast a decrease rateof the source material temperature is not adequate be-

BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a sectional view schematically showing theconstruction of an example of a SiC single crystal

growth apparatus;FIG. 2 isa singlecrystal ingot grown by this method;andFIG. 3 shows an example of the relation betweencrystal growth time and source material temperature.DESCRIPTION OF TH E PREFERREDEMBODIMENTS


6/6

ting diodes and environment-resistant electronic devices.We claim:1. A method of growing a first SiC single crystal ona seed crystal including a second SiC single crystal, saidmethod comprising the steps of:setting a SiC sourcematerial at an initial temperature;growing said first SiC single crystal on said seedcrystal including said second SiC single crystal at atemperature lower than said initial temperature ofsaid source material; andgradually decreasing said source material temperature from said initial temperature during at least apredetermined period during said growing step.2. A method according to claim 1, wherein saidsource material temperature is decreased at a rate of 1C. to 15 C. per hour.3. A method according to claim 1, wherein saidsource material temperature is decreased at a rate of 5C. to 10 C. per hour.4. A method according to claim 1, wherein said seedcrystal temperature is set 30 C. to 120 C. lower thansaid initial temperature of said source material.5. A method according to claim 1, wherein said seedcrystal tempera ture is set 50 C. to 80 C. lower thansaid initial temperature of said source material.6. A method according to claim 1, wherein said seedcrystal temperature remains fixed.7. A method according to claim 1,further comprisingthe step of:decreasing said seed crystal temperature at a ratelower than that of said source material temperature.

* *

6

* * *

5,441,011

EMBODIMENT 2

5doped. This isconsidered owing to doping with residualnitrogen left in the system. In addition, yellowish partswere observed on the wafers. The difference in colorwould be due to the kinds of polytypes. The investiga- 5tion of those portions by Raman scattering clarified thatthe greenish portion is formed by 6H polytypic crystals,and the yellowish portion by 4H polytypic crystals. Onthe other hand, the wafers taken from the central portion and the upper portion of the ingot were found to be 10remarkably transparent and to have only 6H polytype.

The crystal growth was performed under the sameconditions as in Embodiment 1. I t ought to be noted 15that to investigate the growth rate, marking was doneby having nitrogen gas flow at one-hour intervals. Wafers were taken from a grown crystal by vertical slicingalong the face including the c axis. The growth rate was 20estimated according to the occurrence of a green portion formed by doping with nitrogen. In one hour afterthe start of the crystal growth, the crystal grew about1.6 mm in the c-axis direction. In contrast, in one hourprior to the end of the crystal growth, the crystal grew 25no more than about 0.5 rom.From this, it was confirmedthat the growth rate decreased gradually.By using the present invention, it is possible to growSiC single crystal ingots with a few or without poly- 30types mixed in and with low levels of crystal defects,along with low levels of impurities, and to supply highquality single crystal wafers useful in various applications using SiC single crystals, such as blue light emit-

35

40

45

50

55

60

65

sic_usp_1994_5,441,011_sublimation growth of single crystal sic

Documents