The invention relates to a semiconductor substrate and a method for forming the same, and more particularly to a semiconductor substrate formed with an epitaxial layer made of compound semiconductors thereon and a method for forming the same.
Compound semiconductors such as GaAs and InP have been received a great deal of attention rather than silicon semiconductor as possessing excellent properties such as high electron mobility being suitable for high frequency and high speed devices as well as showing a radiative recombination due to a direction transition being suitable for optical devices such as light emitting diodes.
Such compound semiconductors are, however, engaged with the following disadvantages. In view of increasing the productivity of compound semiconductor devices, it is preferable to use a semiconductor having a possible large diameter. It would, however, be difficult to obtain a mono-crystal ingot of compound semiconductors with a large diameter as the compound semiconductors comprises a plurality elements contrary to the silicon ingot, although such large diameter mono-crystal ingot is suitable for epitaxial growth thereof. Further, the compound semiconductor substrate possesses a relatively low thermal conductivity rather than the silicon substrate. The low thermal conductivity causes a thermal accumulation in the substrate. This prevents the compound semiconductor device to exhibit any excellent performance and also results in a reduction of efficiency of power consumption of the device. Moreover, such the compound semiconductors as GaAs and InP are costly materials.
On the other hand, silicon seems to have different properties from the properties of the compound semiconductors such as GaAs and InP. For example, it seems relatively easy to obtain a silicon substrate with a large diameter. Silicon has a relatively high thermal conductivity. Silicon is also inexpensive material.
The compound semiconductor transistors or the light emitting diodes using the compound semiconductors requires the compound semiconductors to form an active layer on a substrate in which the compound semiconductor active layer has such a small thickness as several micrometers, while the substrate has such a large thickness as several hundreds micrometers. In the above viewpoints, in order to bring out the respective advantageous properties of the compound semiconductors and silicon, it was proposed to epitaxially grow an active layer made of compound semiconductors on a silicon substrate with a large diameter. Such silicon substrate having a large diameter formed thereon with the thin active layer made of the compound semiconductors are engaged with problems concerned with the lattice mismatching between silicon and the compound semiconductors such as GaAs and InP. For example, silicon has a lattice constant of 0.543 nanometers, while GaAs and AlAs have lattice constants of 0.565-0.566 nanometers respectively. Such a difference in the lattice constant or the lattice mismatching between silicon and the compound semiconductors to be grown on silicon makes it difficult to achieve a direct epitaxial growth of the compound semiconductors on silicon.
To settle the above problems as to the lattice mismatching between silicon and the compound semiconductors, it was proposed to use direct or indirect epitaxial growth of compound semiconductors on the silicon substrate. It is disclosed in the Japanese laid-open patent application No. 61-203630 to use a molecular beam epitaxy method in which an epitaxial growth of amorphous GaAs layer on a silicon substrate at a low temperature of 200.degree. C. is carried out for local crystallization thereof by irradiation of laser bean such as Ar laser beam to form such grains of crystals as to allow for subsequent recrystal-growth of GaAs by use of the molecular beam epitaxy at 550.degree. C. as a high temperature of the semiconductor. This prior art has such a problem as described below. It is required to conduct precise and strict scanning operation of Ar laser bean in which a laser beam is irradiated on the amorphous GaAs layer in stripe or in matrix thereby the laser bean irradiation in stripe or in matrix makes a mono-crystal GaAs in stripe or in matrix involved in the amorphous GaAs in which a pitch of the stripe or the matrix is required to be in the range of from 15 to 100 micrometers to obtain a good surface homology of the resultant single crystal GaAs layer. This results in the necessarily of complicated and strict processes.
An another prior art is also disclosed in the Japanese laid-open patent application No. 2-94431. A spontaneous oxide film on a surface of a silicon substrate is removed to avoid an influence to an epitaxial growth of compound semiconductor on a silicon substrate. A silicon substrate is subjected to a heat treatment at a temperature of 1000.degree. C. in hydrogen within a heat reactor to remove a spontaneous oxide film. At a temperature in the range from 600.degree. C. to 700.degree. C., a surface of the silicon substrate from which the oxide film was removed is subjected to a flow of AsH.sub.3 to be covered with As. The silicon substrate covered with As is introduced into a metal organic chemical vapor deposition apparatus for subsequent GaAs growth on the silicon substrate. The silicon substrate with the GaAs is again introduced into the heat reactor for being subjected to a heat treatment at a temperature of 900.degree. C. in AsH.sub.3. In this prior art, although an object of the latter heat treatment is to reduce a dislocation due to lattice mismatching, dislocations tend to be generated due to differences in both the lattice constant and the coefficient of thermal expansion. This makes it difficult to obtain a semiconductor substrate of a good crystal structure.
Other prior art is also disclosed in the Japanese laid-open patent application No. 3-55437 in which a silicon substrate is subjected to an organic solvent cleaning using organic solvents such as trichlene, acetone and isopropyl alcohol. The silicon substrate is then immersed for five minutes into an etchant where H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 =4:1 thereby a thin oxide film is formed on a surface of the silicon substrate. A 5% HF liquid is used to remove the oxide film which will be repeated a few times. The silicon substrate is then immersed into the etchant where H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 =4:1 for five minutes to form a thin oxide film on the surface of the silicon substrate. This oxide film is to protect the silicon substrate from suffering any pollution during introduction of the silicon substrate into a vacuum. The substrate is then introduced into a preparation room in an apparatus for molecular beam epitaxy for a subsequent heat treatment for thirty minutes at a temperature in the range of from 1000.degree. C. to 1050.degree. C. in an ultra high vacuum to not only remove the oxide film from the surface of the silicon substrate but also form a crystal step of biatomic layer on the surface of the silicon substrate. The temperature is dropped down to 300.degree. C. for subsequent introduction of the silicon substrate into the growth chamber. The temperature of the substrate is subjected to an irradiation of As molecular beam during increase of the temperature of the silicon substrate up to 600.degree. C. for a subsequent irradiation of As and Ga molecular beams to form a GaAs layer on the silicon substrate. In this prior art, the temperature of the substrate is increased up to 1000.degree. C. or more within the growth chamber. It seems difficult to enlarge the size of the apparatus for crystal growth on account of thermal resistivity of materials for the apparatus. This conventional method is unsuitable for mass production.