Study of SiC single-crystal sublimation growth conditions

MATERIALS SCIENCE & ENGINEERING ELSEVIER

B

Materials Science and Engineering B29 (1995) 90-93

Study of SiC single-crystal sublimation growth conditions I. G a r c o n ~, A. Rouault b, M. An ik in ~, C. Jaussaud ~, R. M a d a r b ~'LETI (CEA-Technologies A vanc~es) CEN/G, 17rue des Martyrs, 38054 Grenoble Cedex 9, France t'LMGP (CNRS & INPG) ENSPG, BP 46, 38402 St. Martin d'HOres, France

Abstract The development of silicon carbide microelectronics depends on the availability of high quality substrates. Nowadays, the most satisfactory technique for growing SiC single crystals is sublimation growth. In this paper, conditions for the sublimation growth of SiC are studied. Growth of crystals was carried out using a hot wall reactor. Acheson platelets were used as seeds and the source material was an abrasive silicon carbide powder. The temperature of source powder and substrate varied from 2150 °C to 2250 °C and 2000 °C to 2150 °C respectively. The growth chamber, a 90 mm diameter graphite crucible, was heated by r.f. induction. The thermal gradient was set by coil position. Crystal quality was investigated by X-ray diffractometry, optical microscopy, photoluminescence, and Raman spectroscopy. Growth rates from 0.1 to 2 mm h-J were obtained. Typical features of the grown crystals are 6H-SiC, rocking curve peaks with full width at half-maximum of 30 s, n-type crystals, n = 10 j7 cm -~ as measured by mercury probe. This work enabled us to establish limits for SiC growth conditions. Evolution of the single-crystal growth rate was investigated with temperature variation but also with variation in the silicon content of the source powder. The crystal quality was examined and related to growth conditions.

Keywords: Crystal growth: Characterization; Silicon carbide; Sublimation

1. Introduction

2. Crystal growth

Silicon carbide is a promising material for electronic applications; its physical and electronic properties are unmatched for high temperature, high power, high frequency and high radiation resistant device applications [1 ]. With a view to developing such applications, much work has been done to develop the technology to obtain good quality SiC single crystals. Until now, the best process for growing large SiC single crystals has been the method developed by Tairov and Tsvetkov in 1978 [2]. This process is based on sublimation source physical vapour transport. SiC single crystals were grown in the apparatus described below, following a given process. The crystals were then evaluated using different characterization techniques which are detailed. The results obtained combined with thermodynamic calculations allowed us to understand further the growth conditions. On the one hand, the limits for SiC growth were established. On the other, in addition to the effect of the temperature on the crystal quality and growth rate, the influence of the amount of silicon added to silicon carbide powder is discussed.

Growth experiments took place in an r.f. heated graphite crucible (Fig. 1); a 150 kHz, 40 kW HF generator provided the required power. Acheson platelets were used as seeds, and fixed on the top of the crucible,

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1. Garcon et al.

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Materials Science and Engineering B29 (1995) 90-93

while an abrasive SiC powder was placed in the bottom. Growth occurred by transport of SiC vapour species from the powder surface held at T~ to the seed surface at temperature T2 less than T I. Values of T 1 and T2 varied from 2150 °C to 2250°C and from 2000°C to 2150°C respectively. The thermal gradient between the source powder and the seed crystal was controlled by positioning the induction heating coil. T~ and T2 were measured by pyrometers on the top and bottom of the crucible. The distance between the powder and the seed was fixed at 10 mm. Graphite foam was used for thermal insulation. A turbomolecular pump was installed to obtain secondary vacuum. A photograph of the reactor is shown in Fig. 2. A typical growth sequence was as follows. (1) The coil is positioned at the etching level. A high temperature outgassing step under secondary vacuum is first needed. The system is purged several times to ensure that the environment is clean. (2) The coil is positioned at the etching level and the temperature is raised to ~ = 2100 °C and TI < T 2. Transport of the sublimed species now takes place from the seed surface to the SiC powder; this phenomenon results in in-situ etching of the substrate. The pressure is then stabilized at 1 Torr and the coil is maintained in this back transport position for 20 min. (3) The coil starts to move slowly to reach the growth position. The power is then adjusted to obtain the required temperatures.

91

(4) The duration of crystal growth itself is fixed at lh. The measured growth rates ranged from 0.2 to 2 mm h-I depending on the growth conditions. Fig. 3 illustrates a single crystal grown in our reactor.

3. Evaluation of grown crystals To identify SiC polytypes, three different characterization techniques were used. Firstly, X-ray diffraction gives information on the nature of the polytypes present in the same crystal. Fig. 4 shows the expanded X-ray diffraction pattern from the (0001) face of a crystal containing at least three polytypes: 6H, 4H, 15R. While taking the measurements the sample is rotated along the c axis. In addition to the main peaks at 2 0 = 35.6 ° and 75.4 ° which are characteristic of SiC (0001) planes, 11 sub-peaks appear. These sub-peaks result from the Renninger effect [3,4] and are typical of each polytype. Thus the 6H polytype is characterized

Fig. 3. Green 6H SiC crystal.

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Fig. 4. Expanded X-ray spectrum showing 6H, 4H, 15R polytypes in the same crystal.

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Materials &'ience and Engineering B29 (199+5)90-93

by five sub-peaks; their positions are given in Table 1. This technique is very useful for identifying all the polytypes present in the same crystal. However, it does not give any information about the 3C polytype. The second main technique used was photoluminescence imaging which provides a representation of geographic distribution of different polytypes in the same crystal. The luminescence of samples is obtained by illuminating the sample with a mercury lamp through a UV transmitting and visible absorbing filter. The excitation produced by the incidence of U V light causes electronic transitions inside the exposed sample which are characteristic of each polytype. Those electronic transitions then give rise to visible light emission specific to each polytype. The last technique, which has to be coupled with photoluminescence imaging, is microRaman spectroscopy. Each SiC polytype has a specific Raman spectrum [5-8]. Thus this technique allows the identification of SiC polytypes on small areas (500 jxm 2) and is very useful in identifying the polytype

Table 1 Sub-peak position and indexation for three polytypes ()~Cu) Polytype 6H 4H 15R

20

20

20

20

20

(hkl)

(hkl)

(hkl)

(hkl)

(hkl)

41.9 (007) 45.05 (005) 43.15 (0018)

48.2 (008) 54.7 (006) 50.8 (0021)

54.7 (009) 64.8 (007) 58.7 (0024)

61.4 (0010)

68.4 (ooll)

66.9 (0027)

nature of each zone revealed by the photoluminescence image. To estimate crystal quality, rocking curves were used. Fig. 5 shows a rocking curve of one of our grown crystals: the presence of several misoriented grains is shown up but full width at half-maximum (FWHM) values for each grain are in the range of 30 s. The electrical characteristics of our grown crystals were evaluated by mercury probe measurements: the crystals are n type and the carrier concentration was estimated at 1017_ 1 0 1 8 c m 3.

4. Study of growth conditions To gain a better understanding of the phenomena occurring during crystal growth, thermodynamic calculations were undertaken. These calculations were performed using a specific program (MELANGE) developed by Barbier at LTPCM in Grenoble [9]. This program, based on minimization of the Gibbs energy of the whole system, allows calculation of the equilibrium composition of complex systems consisting of gaseous or condensed species. According to these calculations and in agreement with previous works on the subject [10], SiC powder heated at 2300°C is decomposed into four main gaseous species: Si, Si2C , SiC2 and Si 2. Fig. 6 shows the evolution of partial pressure of each species as a function of sublimation temperature. Evolution of the total pressure as a function of sublimation temperature is illustrated in Fig. 7; when the sublimation temperature increases from 1900°C to 2600 °C, the total pressure varies from 10 2 Torr to 20 Torr. Limiting conditions of the formation of SiC can also be determined by thermodynamic calculations. If

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Materials Science and Engineering B29 (1995) 90-93

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Fig. 8 shows the growth rate as a function of percentage of silicon added and sublimation temperature. The dependence of the growth rate on temperature is confirmed. Concerning the experiments performed at 2200 °C the growth rate is found to decrease when the amount of added silicon increases. However, for small amounts of added silicon, the deposit becomes polycrystalline.

5. Conclusions '

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the gradient between souce and seed exceeds a critical value, the system equilibrium is perturbed and condensation of silicon occurs when the system is cooled down. As an example, if the sublimation temperature is fixed at 2300 °C, SiC will condense provided that the seed temperature is lower than 2127°C; a thermal gradient greater than 173°C will result in silicon formation. Within this thermal gradient range, condensation of SiC takes place easily, but many parameters contribute to the quality of grown crystals and their growth rates [11]; the influence of temperature, distance from source to seed and pressure have yet to be. Another interesting parameter which needs to be analysed in detail is the amount of silicon added to the SiC powder. Indeed, during the etching step, if no silicon is added to the source powder, the silicon pressure in the chamber is not sufficient and sublimation of the seed gives rise to surface carbonization. If growth takes place on such a surface, a polycrystalline deposit is formed.

SiC single crystals were grown and proved to be of good quality. Analysis of the amount of silicon added to the SiC powder shows that this parameter has a great influence on crystal quality and on the growth rate. Thermodynamic calculations determine limits to this sublimation process and give information about the equilibrium composition inside the crucible.

References [1] M.L. Locatelli and S. Gamal, J. Phys. 111 France, 3 (1993) 1101. [2] Yu.M. Tairov and V.F. Tsvetkov, J. Crvst. Growth, 43 (1978)

208. [3] M. Renninger, Z. Phys., 106 (1937) 141. [41 A.H. Gomes de Mesquita, Acta C~stallogr., 23 ( 1967) 610. [5] D.W. Feldman, J.H. Parker, Jr., W.J. Choyke and L. Patrick, Phys. Rev., 173 (1968) 787. [6] L. Patrick, Phys. Rev., 167 (1968) 809. [7] S. Nakashima, H. Katahama, Y. Nakakura and A. Mitsuishi, Phys. Rev. B, 33 (1986) 5721. [81 Z.C. Feng, A.J. Mascarenhas, W.J. Choyke and J.A. Powell, J. Appl. Phys., 64 (6) (1988) 3176. [9] J.N. Barbier and C. Bernard, in B.L. Kaufman (ed.), 15th Call)had Meet., Calphad ( 1986 ) 206. [10] Y.M. Tairov. V.F. Tsvetkov, lzv. Akad. Nauk. SSSR, Neorg. Mater., 13 (1977) 2382. Ill] D.L. Barrett, J., MacHugh, H.M. Hobgood, R.H. Hopkins, EG. MacMullin and R.C. Clarke. J. Cr),st. Growth, 128 (1993) 358.