1. Field of the Invention
This invention relates to a method of monitoring surface roughness of a crystal. More particularly, it is concerned with a method of in situ monitoring the form or morphology of the surface of a growing crystal when a crystal is grown on a semiconductor material by molecular beam epitaxy (MBE) or metal organic vapor phase epitaxy (MOVPE). According to this method, the surface roughness that occurs on the surface of a growing crystal can be detected in a high resolution of from a single atomic layer to several atomic layers. This invention also relates to a crystal growth equipment making use of such a method.
2. Description of the Prior Art
When a crystal of a semiconductor of the same kind or a different kind is grown on a semiconductor substrate, what is often questioned is the difference in lattice constant between the substrate and the growing crystal. If the difference in lattice constant is great, a lattice defect ascribable to the difference in lattice constant may occur even when a grown crystal is made to have a thickness of 10 nm or less, resulting in a serious lowering of the quality of the grown crystal to make it difficult to manufacture devices of practical use. If, however, the difference in lattice constant is relatively small, it is possible to make a crystal with a certain degree of layer thickness to grow without occurrence of the lattice defect. The maximum layer thickness at which a crystal can be grown without occurrence of the lattice defect is primarily determined by the combination of substrates and crystals to be grown. This layer thickness is usually called critical layer thickness.
For example, when an In.sub.0.15 Ga.sub.0.85 As crystal is grown on a GaAs substrate mainly composed of (001) planes, the critical layer thickness is presumed to be about 30 nm or so. It is important for manufacturing various electrical and optical devices to accurately catch hold of this critical layer thickness according to the combination of various substrates and various crystals to be grown thereon. For such purpose, in usual instances, laborsome methods have been conventionally used such that samples with different layer thicknesses are prepared, double crystal X-ray diffraction is carried out on the respective samples to examine changes in lattice constants and the critical layer thickness is determined from the results obtained. In order to determine the critical layer thickness without preparing a number of samples with different layer thicknesses, it is necessary to find the lattice constant of a growing crystal by in situ monitoring the crystal surface while the crystal is being grown. From this point of view, some of conventional methods have employed the reflected high energy electron diffraction (RHEED) process to find lattice constants from diffraction patterns of crystals in the course of crystal growth. This method, however, is disadvantageous in that it not only requires apparatus such as a television camera, a video tape recorder, a video digitizer and a personal computer so that the line spaces of a diffraction pattern can be measured in a good precision, but also results in a poorer measurement precision when a measuring area has a smaller difference in lattice constant. Moreover, although the method according to the RHEED process can be used for the crystal growth carried out in a high vacuum as in the MBE, there is the disadvantage that it can not be used for the crystal growth carried out under the atmospheric pressure or in a low vacuum as in the MOVPE.
As discussed above, the conventional techniques have been disadvantageous such that they require fairly much labor or expensive apparatus to determine the critical layer thickness. In addition, the values themselves of critical layer thicknesses determined by these conventional methods are actually different depending on the measuring methods. Critical layer thicknesses determined by X-ray diffraction are generally larger than the values measured by RHEED. In the case of X-ray diffraction, the lattice constant to be found is a lattice constant (a.perp.) in the direction perpendicular to the surface on which a crystal grows, and also an average value with respect to a certain degree of thickness. On the other hand, in the case of RHEED, what is found is a lattice constant (a.parallel.) in the direction parallel to a growth plane on the very outward surface of a grown crystal. This difference is presumed to bring about the differences in values of critical layer thicknesses. In order to determine the critical layer thickness in a higher precision, it becomes necessary to use a more laborsome method in which a number of samples with varied thicknesses of grown crystals are prepared, the defects present in the grown crystals are counted by any method to diagram the relationship between layer thickness and lattice defect density and the critical layer thickness is determined from the results obtained.