Conventionally, as a method for forming quantum dots, the forming method based on Stranski-Krastanow mode (S-K mode) is known.
S-K mode is a mode in which a semiconductor crystal to be epitaxially grown grows two-dimensionally (film growth) at the start of the growth and grows three-dimensionally on the stage where the semiconductor crystal has exceeded the elastic limit of the film. A film having a larger lattice constant than the base material is epitaxially grown, whereby quantum dots of three-dimensionally grown islands are self-assembled.
S-K mode can easily make quantum dots self-assembled, and is widely used in the field of photosemiconductor devices, etc.
Recently, techniques in the fields of quantum information and quantum computation attract a great deal of attention. In these fields it is very important to form a quantum dot in a prescribed position in a prescribed size.
However, the conventional method for forming quantum dots described above makes positions and sizes of the formed quantum dots random.
As techniques of controlling positions for quantum dots to be formed in, the following techniques have been proposed.
For example, Japanese published unexamined patent application No. 2000-315654, the specification of U.S. Pat. No. 5,229,320 and Appl. Phys. Lett., Vol. 76, No. 2, p. 167-169, (2000) propose techniques of controlling positions for quantum dots to be formed in by forming in advance by means of electron beams concavities in the surface of a semiconductor substrate in positions for quantum dots to be formed in. When a semiconductor layer is grown on a semiconductor substrate with concavities formed in, the semiconductor layer tends to grow at a higher rate in the concavities, whereby quantum dots can be formed in the concavities.
Appl. Phys. Lett., Vol. 75, No. 22, p. 3488-3490, (1999) and Phys. Stat. Sol. (b) 224, No. 2, p. 521-525, (2001) propose a technique of forming in advance by means of an STM (Scanning Tunneling Microscope) deposits in positions for quantum dots to be formed in to thereby control the positions for the quantum dots to be formed in. When a semiconductor layer is grown on a substrate with deposits formed on, concavities are formed in the semiconductor layer surface above the deposits. When another semiconductor layer is grown on the semiconductor layer with the concavities formed in, quantum dots are formed in the concavities because said another semiconductor layer grows in the concavities at a faster growth rate.
Appl. Phys. Lett., Vol. 77, No. 16, p. 2607-2609, (2000) proposes a technique of forming trenches in the surface of a semiconductor substrate by means of an AFM (Atomic Force Microscope) to form quantum dots in the trenches.
However, in the above-described technique of forming the concavities by means of electron beams, the surface of the semiconductor substrate is contaminated with carbon when the concavities are formed by means of electron beams, which makes it difficult to form quantum dots of good quality.
In the above-described technique of controlling positions of quantum dots by using an STM, crystal defects take place in the semiconductor layer with the concavities formed in. It is difficult to form quantum dots of good quality on the semiconductor layer having the crystal defects. Furthermore, the diameter of the concavities are somewhat enlarged, and it is difficult to form fine quantum dots of a below 40 nm (including 40 nm)-diameter.
In the technique of controlling positions of quantum dots by using an AFM, the trenches are formed in the substrate surface by mechanically scraping the substrate surface with the probe of the AFM. Therefore, the configurations of the trenches are non-uniform. It is difficult to form quantum dots in a uniform size. The trenches are formed by mechanically scraping the substrate surface, which causes crystal defects in the trenches. It is difficult to form quantum dots of good quality without crystal defects in the trenches with such crystal defects.
Recently techniques in the fields of quantum information and quantum computation attract a great deal of attention as described above, and possibilities of applications of quantum dots attract attention. However, for further fundamental studies and application development of quantum dots, various technical barriers which have to be overcome are present.
For example, quantum dots have small sizes, and the quantum dots self-assembled by using S-K mode are distributed at random. No method that makes the self-assembled quantum dots distributed at random electrically accessible has been so far proposed.
On the other hand, conductor pads of an about 100 nm-size can be formed by using processes, such as electron beam lithography, reactive ion etching, etc., which can make submicron processing. Accordingly, in a case that the density of quantum dots is very low, it will be possible to form an electrode above a single quantum dot by using such process. That is, electrodes are formed suitably above the regions where quantum dots are expected to have been formed, by using a process which can make submicron processing. Then, whether or not the quantum dots are present below the electrodes is checked. The electrode formation and the following check are repeated many times, whereby the presence of a single quantum dot below the electrode could be often found. However, it is almost impossible to capture a single quantum dot in a case that the density of the quantum dots is high. Such method depending on the probability is inefficient to fabricate devices.
Japanese published unexamined patent application No. Hei 07-297381 (1995) proposes a method of electrically accessing quantum dots by means of electrodes in the form of a fine probe shape. However, in this case, in order to arrange electrodes above quantum dots, a special process for forming the quantum dots in an array or others is necessary. In Patent Reference 1, electric fields of the probe-shaped electrodes will be distributed far more widely than a size of the quantum dots. Accordingly, the electric fields of the electrode arranged above a certain quantum dot will influence quantum dots adjacent to said certain quantum dot.
As described above, the conventional techniques have established no method of making accurate electric access to each of quantum dots which have self-assembled at random.
It is very significant in various aspects of the fundamental studies, application developments, etc. of quantum dots to realize the accurate electric access to each of self-assembled quantum dots.
An object of the present invention is to provide a method for forming a quantum dot which can form a quantum dot of good-quality in a prescribed position and in a prescribed size.
Another object of the present invention is to provide a quantum semiconductor device which can make accurate electric access to a quantum dot and a method for fabricating the quantum semiconductor device.