Method for forming quantum dots

Disclosed is a method which enabled the precise formation of a group of quantum dots. A device which functions on the principle of a transmission type electron microscope is used to produce a beam of electrons which are passed through a thin crystal membrane in order to produce an electron beam diffraction image. The energy distribution of the diffracted electron beam is used to produce masks, enable epitaxial growth and dry etching involved with the microscopic fabrication operations. For example, a thin GaAs membrane is used to form a diffracted electron beam image on a GaAs layer formed on a substrate. Carbon is then supplied and used to form carbon layers on the the locations where the beam energy is strongest. These carbon layers are used as a mask which allow selective etching of the GaAs layer. An AlGaAs insulating layer is then epitaxially grown on the exposed surface portions of the AlGaAs substrate to fill the spaces between each of the quantum dot defining GaAs portions which project up from the AlGaAs substrate surface.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of forming quantum dots for 
quantum effect device. 
Quantum dots are regions which are sized to shut in electrons of electron 
quantum mechanical type waves having a wave length in the order of 10 nm. 
Currently, there is widespread research into devices which include 
individual quantum dots which are fabricated in semiconductive materials 
using laser beams and photolithographic technology. 
The next generation of such devices however, will not limited to the use of 
individual quantum dots and will include a plurality of closely grouped 
dots. Among these new devices which are under investigation are those 
which use the so called tunnel phenomenon which occurs between this kind 
of closely grouped quantum dots. 
In order to fabricate devices which include lines of closely arranged 
quantum dots it has been thought to use electron beams and 
photolithography. In this connection, a work piece has been coated with an 
organic photoresist which is subject to reaction by an electron bean. This 
photo resist is patterned via irradiation with a finely focussed electron 
beam, and the patterning used as a mask for etching via which microscopic 
fabrication operations are carried out. 
However, with this method, the electrons from the irradiated electron beam, 
scatter within the resist layer which is coated on the work piece, and 
react therewith. This phenomenon is referred to as a proximity effect. 
When using the above type of method, due to the proximity effect, the 
proximity with which a group of quantum dots can be formed is limited to 
about 50 nm, and this prevents the desired tunnel effect from being 
obtained. 
An alternate method of forming quantum dots which also uses an electron 
beam but which is not limited to the previously mentioned resist coating, 
is such that the resist can be applied by supplying the materials from 
which the resist is formed, in gaseous form and irradiating the surface of 
the work piece with the finely focussed electron beam. An example of such 
a method is disclosed in U.S. Pat. No. 5,171,718 filed in the name of 
Ishibashi et al. 
With the just mentioned type of technology it is possible to achieve a 10 
nm degree precision, and as the above mentioned proximity effect is 
absent, it is possible to form quantum dots in the required proximity of 
one another. 
However, scanning near the peripheral sections of the areas is very 
difficult and the production of quantum dots on a large scale cannot be 
improved. Further, as only one electron beam is available for scanning, 
the formation efficiency of a plurality of quantum dots on a large scale, 
is poor and an extremely long scanning time is required for the 
production. 
OBJECT AND SUMMARY OF THE INVENTION 
In view of the technical drawbacks mentioned above, it is an object of the 
present invention to provide a method by which groups of closely spaced 
quantum dots can be simultaneously produced with the required precision. 
The quantum dot fabricating method according to the present invention 
features the use of a crystal layer through which a de Broglie wave can 
pass and subsequently form a diffraction image. A plurality of quantum 
dots cam be simultaneously formed by using the diffraction distribution 
strength of the diffraction image on the surface of the work substrate. 
The above mentioned de Broglie wave can take the form of an electron beam, 
X rays, .differential. rays, a neutron beam or the like type of energy 
wave. The crystal material can be metallic or semiconductive crystalline 
material. Macro lattices can be formed and include portions of 
non-crystalline material and/or the like type synthetic materials. 
Further, the invention is not limited to the use of a simple single crystal 
membrane and the diffraction pattern can be obtained using a plurality of 
membranes or a compound membrane as the situation demands. 
Upon the de Broglie wave entering the crystal material, due to the crystal 
arrangement, the electrons of the wave undergo scattering. With the wave 
scattered in this manner the relationship between the relative positions 
of the electrons exhibits a fixed phase difference. A diffraction image is 
produced via mutual interference. That is to say, the diffraction strength 
distribution of the de Broglie wave of the electron wave etc., manifests 
itself and, due to the crystal construction and in accordance with the 
interference, the diffraction image includes zones of high energy and 
zones of low energy. 
The present invention is such that the specific absorption, dissolution and 
excitation qualities related with this energy distribution, enable 
microscopic fabrication operations including mask formation, epitaxial 
growth and dry etching. 
As the dimensions of the quantum dots are essentially the same as the wave 
lengths of the electrons which pass through the semiconductor crystal 
(Viz., about 10 nm is good) and as this is much larger than the 
semiconductor crystal lattice spacing, it is adequate to use a diffraction 
image with a comparatively low magnification. 
Accordingly, with the present invention, it is possible to simultaneously 
produce batches of microscopically patterned high quality quantum dots. 
This of course renders it possible to manufacture quantum dot devices with 
extremely high efficiency. 
Further, the crystal material is such that material which is epitaxially 
grown on a substrate, can be selectively etched. In this case, after a 
membrane of a crystal material is epitaxially grown on a substrate, it is 
possible to selectively etch the substrate so that a portion of the 
membrane through which the de Broglie wave passes is completely exposed 
and so that it is surrounded by a portion or portions of the substrate 
which acts as a support. With this arrangement, it is possible to directly 
mount the membrane on the surface in which the quantum dots are to be 
formed by way of the supporting portions. This disposition is such that 
the effect of external vibration on the diffraction pattern image 
produced, is prevented and it is possible to irradiate for prolonged 
periods due to the increased stability. The reproducibility of the process 
is also increased. Further, it is possible to produce crystal diffraction 
lattices which have large surface areas.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention will now be disclosed with reference to the drawings 
in which the preferred embodiments of the same, are shown. 
First, the electron beam diffraction device which is used in connection 
with the first-third embodiments of the present invention is shown in FIG. 
1. 
As schematically shown in FIG. 1, this arrangement is an electron beam 
diffraction device which functions on the same principle as a transmission 
type electron microscope (TEM). That is to say, an electron beam (e.sup.-) 
which is emitted from an electron source 1, transmitted by way of a 
collimating lens 2 to a thin crystalline GaAs membrane 3. This thin 
crystalline GaAs membrane 3 is that, in order for the electron beam to 
pass therethrough, the electrons, which are scattered, give rise to mutual 
interference. The electron beam which is grouped by a diffraction 
figure(s) formed on the membrane 3, is then passed through an object lens 
4 and then an incidence lens 5 which is disposed in a specimen chamber 6 
and onto a wafer 7. 
The magnification of the diffraction image(s) is determined by the 
objective and incidence lenses 4, 5. In this connection, the instant 
electron beam diffraction device can use a magnification which is small 
when compared with that of normal electron microscopes. By way of example, 
if the lattice spacing of the thin GaAs membrane is "a", and the period of 
the quantum dots is na (wherein n is natural number), then it is 
acceptable to set the magnification at n times. 
With this type electron diffraction device, it is possible using the 
diffraction figure(s) on the thin GaAs membrane 3, to form microscopic 
pattern masks, grow epitaxial layers and etch the same. The actual 
production steps involved in the fabrication of the various embodiments 
will now be set forth in detail with reference to FIGS. 2-5. 
First, a AlGaAs layer 11 is formed on either a AlGaAs or a GaAs substrate. 
Following this, a GaAs layer 12 is epitaxially grown on the thus prepared 
wafer. The thickness of this GaAs layer matches the height of the quantum 
dots. 
This wafer is set in a specimen chamber 6 of an electron diffraction device 
of the nature illustrated in FIG. 1. Under these conditions, an electron 
beam e.sup.- is generated, caused to undergo diffraction as it passes 
through the thin GaAs membrane 3, and used to irradiate the wafer set in 
the chamber 6. This results in the diffraction pattern d being formed on 
the upper surface of the GaAs layer in accordance with the diffraction 
figure formed in the GaAs layer 3. This diffraction pattern d is such as 
to result in the diffracted ray strength distribution, which is induced by 
the crystal diffraction of the GaAs layer 3, being distributed to form a 
periodically arranged pattern of the nature shown in FIG. 2. 
After the diffraction pattern D image is formed on the upper surface of the 
GaAs layer 12, a small amount of carbon is introduced into the specimen 
chamber 6. This carbon adheres to the zones on the upper surface of the 
GaAs layer 12 where the diffraction forming electron energy has been 
absorbed. This results in the carbon masks patterns M shown in FIG. 3. The 
spacing L.sub.1 with which the diffraction pattern d influenced mask 
pattern M is arranged, can be selected within the range of 50-200 .ANG. 
depending on the manner in which the magnification n is selected. 
Following the formation of the carbon mask pattern, etching is carried out 
either in the above mentioned specimen chamber 6 or in a separate etching 
device. This etching can be carried out using RIE (reaction type ion 
etching) by way of example, and is such as to remove the portions of the 
GaAs layer which are not covered with the carbon mask pattern M. This 
results in the arrangement depicted in FIG. 4. 
When the complete etching is finished, the carbon mask pattern is removed 
and a AlGaAs layer 13 is formed over the exposed portions of the AlGaAs 
layer 11 via epitaxial growth. The result of this is that, as shown in 
FIG. 5, a plurality of quantum dots Qd are formed in the positions which 
were marked out with the carbon mask pattern. Each of the dots are 
mutually insulated from one another by the AlGaAs layer 13. 
Second Embodiment 
This embodiment is described with reference to FIGS. 6a and 6b and utilizes 
a quantum dot formation process wherein the quantum dots are formed via an 
electron beam diffraction pattern influenced epitaxial growth. 
First, the ion beam diffraction device of FIG. 1 is used in the manner 
schematically shown in FIG. 6a, to form an image on the upper surface of a 
AlGaAs substrate 21 using the electron beam which passed through the thin 
GaAs membrane 3. Under these conditions, material in the form of trimethyl 
gallium and trimethyl arsenic gas, is introduced into the specimen chamber 
6. Subsequently, the electron beam energy decomposes the gases and GaAs 
layer 22 are selectively developed via epitaxial growth in the path of the 
electron beams in the manner illustrated in FIG. 6b. 
Although not shown in the figures, after the epitaxial growth of the GaAs 
layers 22 in the zones defined by the diffraction pattern, an insulating 
AlGaAs layer is formed on the upper surface of the AlGaAs substrate 21 in 
the areas which are not covered with the GaAs layers 22, via epitaxial 
growth. This defines the quantum dots. 
Third Embodiment 
The third embodiment of the invention is such that the diffraction pattern 
is used in combination with dry etching. This process will now be 
explained with reference to FIGS. 7a and 7b. 
First, a GaAs substrate 31 on which the quantum dots are to be formed, is 
set in the specimen chamber 6 of the electron beam diffraction device 
shown FIG. 1. Etching gas is then introduced into the specimen chamber 6 
and is absorbed by the upper surface of the GaAs substrate 31. 
Next, similar to the first and second embodiments, the electron diffraction 
device is used as schematically shown in FIG. 7a, so that the diffracted 
electron beams which pass through the GaAs layer 3, produce an image on 
the upper surface of the GaAs substrate 31. 
Subsequently, as shown in FIG. 7b, the zones wherein the diffraction 
strength is strong, selectively absorb the electron beam energy and become 
excited. This result in the GaAs substrate being etched and the formation 
of the microscopic bores 32 in accordance with the diffraction pattern. 
After this, the interiors of the microscopic bores 32 can have GaAs of a 
different band gap size material, epitaxially grown therein to define the 
quantum dots. 
Fourth Embodiment 
In accordance with this embodiment, an adnation type crystal diffraction 
lattice unit is formed using electron beam irradiation and attached to a 
wafer. This unit is used to form the quantum dots. The process via which 
this is carried will be disclosed with reference to FIGS. 8-12. 
First, as shown in FIG. 8, a AlGaAs membrane 42 is formed on a GaAs 
substrate 41 using a MOCVD type epitaxial growth technique. The size of 
the GaAs substrate 41 is selected to be the same as that of the crystal 
lattice. Further, in this embodiment the thickness of the GaAs substrate 
is selected in accordance with the distance from the surface in which the 
quantum dots are to be formed that the crystal diffraction lattice should 
be supported. The thickness t of the AlGaAs membrane 42 is selected to be 
in the order of several tens of .ANG. and such that the electron beam 
which passes therethrough will be diffracted. 
Following this, a resist layer 43 is formed on a side of the GaAs substrate 
opposite to that on which the AlGaAs membrane is attached. This resist 
layer 43 has an essentially inverted U-shape and is formed along three 
edges of the GaAs substrate in the manner shown in FIG. 9. 
Next, etching is carried out using the resist layer 43 as a mask until the 
GaAs membrane 41 is removed. This etching can be carried out using Cl or F 
class etchant using a RIE (reaction ion etching) technique. At this 
etching stage the surface of the AlGaAs membrane 42 which is below the 
GaAs layer 41 includes projections. This surface which includes 
projections is coated with a low vapor pressure AlF.sub.3 which exhibits 
highly selective protective characteristics with respect to the AlGaAs 
membrane 42. 
After this, the resist layer 43 is removed thus completing the crystal 
diffraction lattice as depicted in FIG. 10. The inverted U-shape pattern 
into which the GaAs substrate is formed, serves as a support for the 
crystal diffraction lattice. 
Next, the inverted U-shaped GaAs substrate 41 is placed in contact with a 
surface 44a of a processing material substrate 42 in which quantum dots 
are to be formed, in the manner depicted in FIG. 11. Under these 
conditions, the spacing with which the AlGaAs membrane 42 which forms the 
diffraction image(s), is maintained from the surface 44a, is determined by 
the thickness of the GaAs substrate 41. 
With this arrangement even if vibration should be applied to the processing 
material substrate 44, as the crystal diffraction lattice is supported by 
the GaAs substrate 41, the relative positions of the crystal diffraction 
lattice and the processing material substrate 44 do not change. The effect 
of this is to stabilize the diffraction pattern and to improve the 
reproducibility of the process. It also renders it possible to irradiate 
using the electron beam for long periods of time. 
Under these conditions, as shown in FIG. 12, the AlGaAs membrane 42 which 
is exposed to the electron beam e- irradiation, induces the formation of 
an electron diffraction pattern image 45 on the surface 44a of the 
processing material substrate 44. At this time, as an opening is formed in 
the GaAs substrate 41, carbon, etching gas etc., can be introduced in the 
manner indicated by the bold arrow K, and a mask can be formed in 
accordance with the diffraction image pattern 45, epitaxial growth, dry 
etching and the like can be carried out. In other word, it is not 
necessary to remove the GaAS substrate and it can be left in contact with 
the processing material substrate 44 enabling stable microscopic 
fabrication steps to be carried out. 
Fifth Embodiment 
This embodiment is a variation of the forth embodiment and features the 
formation of GaAs layers on both sides of the AlGaAs membrane. This 
increases the rigidity with which the diffraction lattice is supported 
when it is used for the formation of quantum dots. 
As shown in FIG. 13, firstly, a AlGaAs membrane 52 is expitaxially grown on 
the surface of GaAs substrate 51. The thickness of this membrane is in the 
order of 10 nm--a given number of nm thick so as to provide the required 
crystal characteristics. 
Further, in this embodiment a GaAS layer 53 is formed on the upper surface 
of the AlGaAs membrane 52. As will become apparent later, during 
fabrication this GaAs layer 53 provides a framework which increases the 
mechanical strength of the arrangement. 
Next, a resist layer 54 is formed on the GaAs layer 53. As shown in FIG. 
14, this resist layer 54 is patterned so as to extend about the four edges 
of the GaAs layer 53. 
After this, the resist layer 54 is used as a mask and RIE is carried out. 
This etching uses Cl or F type etchant. As a result of this etching, the 
surface of AlGaAs membrane 52 becomes rough and AlF.sub.3 is used as an 
etching stopper. 
When the etching is completed, the resist layer 54 is removed and the 
arrangement illustrated in FIG. 15 is obtained. 
After this, the same resist/etching techniques are used on the GaAs layer 
51 to form a support member which extends around three sides of the AlGaAs 
layer 52 and has an opening into which carbon and etchant gas can be 
introduced and supplied to the surface of the processing material 
substrate on which the diffraction pattern image is projected. 
Other than this the process is the same as disclosed in connection with the 
fourth embodiment and the GaAs substrate is placed on the surface of a 
processing material substrate and exposed to irradiation by an electron 
beam. The electron beam diffraction pattern image which is formed is used 
to form mask patterns and to control etching and the like as disclosed in 
connection with earlier embodiments. 
With the crystal diffraction lattice of the instant embodiment, as the 
AlGaAs layer is supported on both sides by the GaAs substrate 51 and the 
GaAs layer 53, the mechanical rigidity of the arrangement is increased. As 
a result, as the lattice supported on the substrate on which fabrication 
operations are being carried out, any problems which tends to stem from 
vibration on the formation of the diffraction pattern is even further 
reduced. 
It will of course be appreciated that the present invention is not limited 
to the above described five embodiments and it is possible within the 
scope of the present invention that X rays or the like type of energy 
waves be used in place of the above described electron beam diffraction. 
Further, in connection with the energy irradiation of the substrate, the 
gases which can be used can be varied in order to provide selective 
effects and/or from various types of material. For example, in connection 
with the first embodiment, the mask material is not limited to carbon and 
other organic or inorganic materials can be used. In the case of inorganic 
materials it is possible to use silane type gas with oxygen in order to 
form a silicon oxide layer membrane. Alternatively, a tungsten metallic 
mask can be formed by introducing a fluro-tungsten gas. 
In the case the work piece takes the form of a semiconductor, for example a 
chemical compound type semiconductor and the substrate is made of a 
material the etching of which can be controlled to a desired ratio, it is 
possible to use a different compound type semiconductor as a mask. 
In the same manner the second embodiment is such that if a given type of 
layer is formed on a fabrication substrate and the quantum dots can also 
be formed in this particular layer, it is feasible to prepare a different 
compound semiconductor or use different semiconductive or metallic 
material layers. 
Further, the size of the crystal lattice described in connection with the 
fourth and fifth embodiments, it is possible to select the sizes to vary 
in the order of a number of mm to a number of cm.