Vol.3 No.4 2011

21/72

Research paper : Development of single-crystalline diamond wafers (A. Chayahara et al.)−262−Synthesiology - English edition Vol.3 No.4 (2011) nucleus occurs in insufficient growth conditions. In many cases, this abnormal nucleus forms a pyramidal structure with grain of {111} orientation growing on top of the {100} face. This is thought to originate from the dislocation within the seed crystal, polishing flaw, etch pit, or fluctuation during growth. The methods to effectively control the occurrence and spread of such abnormal nuclei include the parameter control, step flow growth using an off-substrate, and nitrogen addition. These are explained below.1) parameter controlIn diamond crystal growth, the facets (automorphic face of crystal) that manifest are mostly {111} and {100} faces. Setting the growth rate perpendicular to those faces as V100 and V111, the parameter can be defined as follows[27][28]: = 3 V100/V111Since V100 and V111 have different dependencies for the growth conditions such as pressure, methane concentration, and temperature, changes according to the growth conditions. In the case of polycrystal synthesis, the change of is used for controlling the orientation. That is, the growth continues even if the growth is started from a nucleus with specific orientation, and as the film grows, the crystal orientation near the growth surface becomes oriented in the direction determined by . Also, the form of the crystal grain is determined by . At about =3, the polycrystal growth assumes the <100> orientation, and is effective in controlling the occurrence of abnormal nuclei in the growth on the {100} face used often as single-crystal growth surface. In this case, if the origins of the abnormal nuclei are present and the {111} oriented grains grow on top, the growth of {100} around it is fast, the abnormalities become buried, and they will not be able to grow into large abnormal grains. However, in this condition, the defects that occur during the dislocation and growth on the substrate are carried over to the growth direction, and may remain as perforating dislocations. Also, since the differs by devices, the condition that yields a certain is explored by mapping the by changing the growth conditions.2) Step flow on the off-substrateIt is known that the step flow growth occurs when the growth is done on a polished off-substrate that is inclined within a few degrees from the {100} face. When the step passes this part faster than the growth of the abnormal nucleus on the terrace, the abnormal nucleus cannot grow and flat growth can be expected. It is an effective method for forming a semiconductor grade film[29].In the case of the growth of a large crystal, even if the growth is started using the off-substrate as the seed crystal, the step growth occurs initially over the whole surface of the substrate as in the film growth, but the off is disengaged at the edge of the substrate, and finally the entire growth surface becomes the {100} face[30]. Of course, the effect of controlling the abnormal nuclei cannot be expected after that.3) Nitrogen additionDuring the epitaxial growth on the {100} face, the occurrence of the abnormal nucleus can be controlled by adding a small amount of nitrogen to the raw material gas. The increases with ultralow volume addition of nitrogen. When the amount is increased, the {111} face can no longer grow normally and becomes polycrystalline. Either way, the growth of the {111} face is inhibited, and as a result, the abnormal nuclei are controlled. This method is used when synthesizing large single crystals by maintaining high-speed prolonged growth. However, when nitrogen is added, deep donor level and carrier trap are introduced and an insulating body is formed. Also, due to the defects accompanying nitrogen, absorption occurs in the visible light range. The control of the {111} face growth by nitrogen addition is thought to occur as the nitrogen atoms bond strongly to the {111} face at three coordinates, and the carbon atoms cannot bond to the {111} face covered with nitrogen[22][26].Figure 3 shows the differential interference microscope image of the growth surface at various nitrogen flow rate. When nitrogen is not added, a pyramidal projection (growth hill) can be observed, and there is a typical surface form of the diamond synthesized by the CVD method on the {100} surface. This growth hill remains in the crystal as a large structural defect, and is a factor that inhibits the formation of a thick film by prolonged epitaxial growth. The growth hill is not seen at all when nitrogen is added, but rough surface caused by macro step bunching is observed. When more nitrogen is added, the steps become nonlinear and disturbed. When nitrogen is added, the surface roughens, but since there will be no growth hills, thick film and bulk formation by prolonged growth becomes possible. Conventionally, the single-crystal growth by CVD method was difficult due to the occurrence of abnormal nuclei, but as the result of control of abnormal grain growth by the addition of ultralow volume of nitrogen, the bulk single-crystal growth by CVD method became possible.3.3 Size increaseThe photograph of the sample synthesized over a long time on a 1 cm square substrate is shown in Fig. 4. In the {100} growth by nitrogen addition, the crystal diameter does not expand as shown in the photograph. To obtain the large diameter, the lateral face ({100} face) of the grown crystal is polished, and the growth is repeated on top of that face as shown in Fig. 5. Figure 6 shows an example of the crystals obtained by this method.The similar lateral growth is seen in the method for reducing defects for the SiC single-crystal growth called the RAF method[31]. The sublimation method is used for the single-crystal growth of silicon carbide SiC, which is expected to be

※このページを正しく表示するにはFlashPlayer9以上が必要です