JPS6381919A - Superlattice structure and manufacture of multilayer film - Google Patents

Abstract

PURPOSE:To inexpensively, economically end safely manufacture a superlative structure, in which an ideal quantum size effect appears, and a multilayer film, by changing temperatures of an atomic crystal substrate, raw crystal or alloy, and solution, periodically up and down within a temperature range. CONSTITUTION:Growth solution 3 is interposed in a sandwitch shape between a seed crystal 1 substrate and raw crystal or alloy 2, which is of superlattice structure aiming at growth and which has components of a multilayer film. Then, temperatures in this growth system are changed uniformly in this space and changed periodically up and down within a temperature range. Concentration of the solute becomes high in the gravitational direction on the side of the seed crystal, and it becomes low on the side of the raw crystal or raw alloy. When an solid-liquid interface between the seed crystal substrate and the solution is held flatly and horizontally, the solute concentration becomes uniform. During a down period in the periodical temperature cycles, the solute is precipitated on the side of the seed crystal coming from this solution 3, and growth layers 4 and 5 can be formed in uniform composition and thickness. Besides, a distance between the seed crystal 1 substrate and the raw crystal or alloy 2 is made sufficiently small.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for manufacturing superstructures and multilayer films, and in particular, to semiconductor crystals and metals, which are currently being rapidly researched and developed for application to new electronic devices, etc. The present invention relates to a superlattice structure using crystals and a method for manufacturing multilayer films.

[Conventional technology]

Currently, superlattices and multilayer films made of thin films of semiconductor crystals or metal crystals are attracting attention as they exhibit new properties different from those of the original materials depending on their combination. 19
In 1970, Ezaki et al. devised a superlattice structure crystal in which two types of ultra-thin semiconductor crystals are alternately and regularly arranged. He pointed out that constraints may affect the wave function of electrons or holes, giving rise to new physical properties never seen before. Several years later, this proposal was substantiated by superlattice crystals grown by molecular beam epitaxy (MBE).

At the same time, research on superlattices and multilayer films of metal crystals has been actively conducted recently. Interesting phenomena such as changes in critical temperature and critical magnetic field have been observed.

In particular, in semiconductor crystals, in addition to the artificial periodic I-tension, it is possible to create electrons with a two-dimensional character that is different from the conventional one by using the Doji-like potential at the interface. Devices that utilize these quantum theoretical effects include semiconductor quantum well radar and HEMT (High Electron Mobili) that utilizes the behavior of two-dimensional electrons.
ty Transistor), etc., is currently being actively researched and developed. The method for growing these superlattice structures or multilayer films is the vapor phase growth method using organic metals (
MOCVD method), molecular beam epitaxy (MBE method), and liquid phase epitaxy (LPE method) are used, but MBE method or MOCVD method is mainly used. MOCV
Taking an IV compound semiconductor as an example, method D involves thermally decomposing or reacting an organic metal, which is a raw material for group Ⅰ elements, and a hydride, which is a raw material for group Ⅰ elements, in a gas phase to form a 1- It is said that it is possible to control the growth of monomolecular molecules in a reduced pressure atmosphere using a method of depositing group V compound semiconductors. However, since organic metals are used as raw materials, carbon is mixed into the grown layer, making it very difficult to control not only the conductivity type but also the impurity concentration. In addition, hydrides, which are raw materials for group V elements,
For example, arsine (AsH3), phosphine (PH3), etc. are highly toxic substances, so this is an industrially dangerous growth method. Furthermore, due to the reaction in the gas phase, most of the precipitation occurs in areas other than the substrate, resulting in low yields and making it impossible to manufacture devices at low cost. Since the MBE method supplies the constituent molecules to be grown to the substrate in the form of molecular beams, it is easy to adjust the minute amount of supply, and is said to be most suitable for growing ultra-thin films. Naturally, in order to increase the rate at which molecules reach the substrate, the inside of the growth chamber is drawn to an ultra-high vacuum. In addition, the degree of vacuum in this background is an important issue in controlling the conductivity type and impurity concentration of the growth layer, but the current vacuum technology has a limit of 10"-" Torr, so it is possible to control the impurity concentration with high precision. Not only is it difficult to control the concentration, but also a large number of defects called oval defects occur on the surface of the thin film grown layer by the MBE method. Although improvements have been made by using organic metals as raw materials, they have not yet been put into practical use. Furthermore, since ultra-high vacuum is used, it is impossible to manufacture devices at low cost due to maintenance of the vacuum system, etc. The LPE method has a higher growth rate than the MOCVD and MBE methods, and when growing mixed crystals in the normal gradual method, the composition of the grown layer changes with cooling. Although few attempts have been made to create films, it is well known that high-quality crystal layers can be obtained.

[Problem that the invention seeks to solve]

As mentioned above, the MOCVD method or the MBE method is generally used to fabricate semiconductor superlattice structures and multilayer films. These crystal growth methods are suitable for growing thin films, but because growth is performed in a state that deviates from a thermodynamic equilibrium state, it is not possible to obtain a high-quality grown layer. Furthermore, since it is essentially difficult to control the impurity concentration in these crystal growth methods, it is difficult to obtain an ideal quantum size effect when actually fabricating a device with a superlattice structure.

Furthermore, considering the danger of the raw materials used, the maintenance of the growth equipment, the maintenance of the vacuum system, and the yield of the growth layer relative to the amount of raw materials used, this method cannot be considered as an industrial method in the future. In the LPE method, growth is performed in a thermodynamically close to equilibrium state, so very high quality growth can be obtained. Furthermore, by considering the segregation coefficient, appropriate donor impurities or accessor impurities can be added to achieve the desired growth. It is possible to perform growth with a desired impurity concentration of conductivity type (p or n).

However, in the conventional LPE method, the growth rate is high and it is not suitable for growing thin films, in the gradual method, the amount of precipitation is determined by the cooling temperature range, and in the case of mixed crystals, composition changes due to cooling are inevitable. there were.

The purpose of the present invention is to solve the above-mentioned problems and to provide a method of growth that is cheaper, more economical and safer than the currently used growth methods.
The present invention provides a method for manufacturing a superlattice structure and multilayer film that exhibits an ideal quantum size effect.

[Means for solving problems]

The method of the present invention utilizes the fact that the gravity field and the specific gravity of a solution depend on the concentration of solute contained in the solution.

Specifically, a growth solution is inserted in a sandwich-like manner between a seed crystal substrate and a raw material crystal or raw material alloy having components of a superstructure and multilayer film to be grown. At this time, if the solute concentration increases and the specific gravity of the seed solution increases, the seed crystal substrate is placed below the solution, and conversely, if the specific gravity decreases, the seed crystal substrate is placed above the solution to allow crystal growth. Configure the system. The temperature of this growth system is then changed spatially uniformly and temporally up and down periodically over a certain temperature range. During the temperature increase in such periodic temperature cycles, solute is supplied from the raw material crystal or raw material alloy to the solution. Due to the difference in specific gravity, a concentration distribution is formed in which the concentration of solute is high on the side of the seed crystal substrate in the direction of gravity of the solution, and the concentration is low on the side of the raw material crystal or raw material alloy. here,
Due to the action of gravity, the horizontal concentration distribution of the solute becomes uniform.

Therefore, by keeping the solid-liquid interface between the seed crystal substrate and the solution flat and horizontal, the solute concentration near the solid-liquid interface becomes uniform. When the temperature decreases during periodic temperature cycles, solutes are precipitated from this uniform solution onto the seed crystal side, forming a growth layer with a uniform composition and uniform thickness. Furthermore, what is important is to make the thickness of the solution, that is, the distance between the seed crystal substrate and the raw material crystal or raw material alloy, sufficiently small.

By reducing the amount of solution as much as possible, the absolute amount of solute dissolved in the solution is reduced, and as a result, the amount grown during one temperature cycle is reduced, allowing thin film growth. Furthermore, for the same reason, it is desirable to grow at as low a temperature as possible in order to lower the solubility in the solution.

Transport of solute from the raw material crystal or raw material alloy to the seed crystal side is carried out at high speed due to diffusion action accelerated by gravity or natural convection caused by the difference in specific gravity. However, when the thickness of the solution is thin, that is, the typical length of the solution phase is small, and the length is within the diffusion length, natural convection does not occur and solutes are supplied only by diffusion accelerated by gravity. It is carried out. Further, even if natural convection occurs, convection occurs only in the solution layer on the low concentration side. Since the solution layer with a high solute concentration formed near the seed crystal by this solute transport acts as a stratosphere, this high concentration solution layer is not affected by convection.

From the above, it is possible to grow a multilayer film with an arbitrary number of layers by using the growth method of the present invention, which uses liquid phase growth without changing the composition of the grown layer, and without changing the composition of the growth layer due to temperature cycles. It's obvious. Furthermore, by considering the thickness of the solution, the growth temperature, and the amplitude of temperature fluctuations, it is possible to fabricate superlattice structures and multilayer films made of ultrathin films that exhibit quantum size effects. Of course, since the method of the present invention uses liquid phase growth, it goes without saying that the conductivity type and impurity concentration of the grown layer can be easily controlled.

From the above, it has become clear that all of the above objects can be achieved by the method of the present invention.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the superstructure and multilayer film manufacturing method of the present invention will be described below with reference to drawings.

Example 1 In the case of a 1-V group compound or a l-V group compound mixed crystal, a superlattice structure in which n and p are periodically doped in one growth by using amphoteric impurities such as St, Ge, Sn, etc. can be realized. Interesting optical data have been obtained on this selective Dogg type superlattice structure made of 1-V group compound semiconductors, and it is currently being manufactured by vapor phase epitaxy or molecular beam epitaxy using organic metals.

However, at present, high-quality growth is not possible using these growth methods. An example in which a selectively doped periodic structure was fabricated by using an amphoteric impurity will be shown below. For GaAs, GaSb, AtGaAs, etc., St
By considering the growth temperature, cooling rate, and impurity concentration in growth doped with amphoteric impurities such as Sn, Ge, etc., the position where the impurity enters can be changed from the group II side to the group V side, and a pn junction can be formed in one growth. It is a well-known technique to make . By repeating the temperature process using the growth method of the present invention, it is possible to create a desired number of periodic structures. FIG. 1 is a diagram useful in explaining the growth of a periodic structure of conduction type using the change in conduction type depending on the growth temperature.

Here, a case is shown in which the specific gravity of the solution decreases as the concentration of solute in the solution increases. Then, the seed crystal 1 is placed above the solution 3, and the raw material crystal 2 is placed below the solution 3.

Needless to say, an appropriate amount of amphoteric impurity atoms should be mixed into the solution and raw material crystal. Figure 1(d)
, Th and Tt indicate the upper and lower limits of the growth temperature cycle, and Tc indicates the temperature at which type reversal of conductivity occurs.

First, the seed crystal 1, the solution that will become the solution 3, and the raw material crystal 2 are heated to a temperature several degrees higher than the temperature Th while being kept at a constant temperature, held for a certain period of time, and then cooled at a constant rate. At time t81, the solution is put between the seed crystal 1 and the raw material crystal 2. FIG. 1(,) shows this state. The conductivity type of the grown layer 4 until the temperature reaches Tc is the conductivity type when grown above the temperature Tc, that is, either p-type or n-type. This state is shown in the first
It is figure (b).

When the temperature drops below Tc, type reversal of the grown layer occurs, and the conductivity type of the grown layer 5 becomes different from the conductivity type of the grown layer above the growth temperature Te. The next cycle begins with the temperature rising in this state. By repeating this process, it is possible to achieve growth having a conductive periodic structure. Here, growth of GaAa will be described as an example. When growing GaAs from Ga solvent, Si in the solution
At a concentration of 0.15 atm %, type reversal of the grown layer occurs at 800°C.

That is, at temperatures above 800°C, an n-type growth layer is obtained;
At temperatures below 00°C, it becomes a p-type growth layer. Therefore, Te and Tt are 800℃, ←, Tt are 810℃ and 790℃, respectively.
It was set to 0.15 atm% of St was dissolved in a Ga solution that saturates GaAs at 810°C, held for 10 minutes, and then inserted between a GaAs seed crystal and a raw material crystal.
The temperature was lowered at a rate of 0°/min. The GaAs crystal used is Z
n-doped p-type (111)B1.5XL5cTL2-
? The thickness of the l' solution is approximately 3 mm. After lowering the temperature to 790℃, hold for 10 minutes and reduce to 8 at 2'0/rnin.
The temperature was raised to 10°C and held for 10 minutes. After repeating this temperature cycle 5 times, the temperature was lowered, and the removed substrate was placed between the bones and then subjected to stain etching.
A p-type 7 μm five-period structure was observed. By considering the growth temperature and cooling rate, it is possible to further realize a periodic structure of the thin film. Further, for other IV group compounds such as GaSb and IV group compound mixed crystals such as AtGaAs, the growth temperature and cooling rate, specifically, the cooling rate is changed during slow cooling. Or Si.

Needless to say, it is possible to fabricate a selectively doped superlattice structure by selecting the concentration of amphoteric impurities such as Ge and Sn. Furthermore, the impurities to be used are not limited to group Ⅰ elements such as Si, Ge, Sn, etc., and it is possible to fabricate this structure by utilizing the compensation effect of double doping of Zn-Te. Also, II
-For materials whose crystal structures change depending on the growth temperature, such as those found in Vl group compound semiconductors, it is possible to rheotaxially grow different crystal structures by increasing and decreasing the temperature around the critical temperature.

Example 2 Group III element semiconductor that does not form a solid solution such as Ge-GaSb and ■-V
Group compounds also have a region where phase separation occurs, so-called m
In the m-■ group compound mixed crystal and the ■-■ group compound mixed crystal in which there is a safety gap of 1, a periodic Peter junction structure is realized by growth layers made of the respective constituent elements. Here, Ge-GaSb will be described as an example. Since Ga is used as a solvent, the seed crystal is placed below the solution and the raw material alloy is placed above the solution, taking into account the specific gravity of each solution.

Since it is impossible to make a Ge-GaSb alloy, Ge
:Garb=1:3 raw material is melted at 1000''C, thoroughly stirred for 2.1 hours, and quenched to be used as the raw material alloy.Ge, G in this alloy
The particle size of each aSb particle was about 100 μm.

A Ga solution containing saturated Ge and GaSb was heated at 510°C.
After inserting between the n-type Ge (1,], ],)-plane seed crystal and the raw material alloy and holding for 10 minutes,
It was cooled at 490° C. with a cooling rate of 100° C. After holding at that temperature for 10 minutes, increase the temperature by 5°C at a heating rate of 1°O/min.
The temperature was raised to 10°C and held for 10 minutes. After repeating this temperature cycle five times, the temperature was lowered, and the seed crystals removed were cut, polished, and etched.
A 5-periodic structure consisting of a Ge layer of 20 μm followed by a 20 μm GaSb layer was observed. That is, during cooling, Ge with a large segregation coefficient grows first, but at that time, GaSb does not form a solid solution in Ge, so no growth of GaSb occurs, and G grows near the substrate.

Growth of GaSb occurs after the degree of supersaturation of GaSb decreases, and by repeating the temperature process, it is possible to continuously form a periodic structure as shown in FIG.

Here, 15 is a Ge seed crystal substrate, 16 is Ge-GaSb
The raw material alloy, 17 is a Ga-Ge-8b solution, 18 is a Ge growth layer, and 19 is a GaSb growth layer. This is Cd
If-VI group compounds that do not dissolve in solid solution such as 5-CdTe or QaA with m1scability gap
-V group ternary mixed crystals such as sSb, AtxGa, -xA
sySb,-,,GaxInl-xAsySbl-.

It is applied to m-v group quaternary mixed crystal systems such as In(PxA3.-x)ySb and -A9. That is, m1scibili
Since the elements of the mixed crystal do not form a solid solution within the ty gap, heteroepitaxial growth of each component becomes possible.

Example 3 An example is shown in which two types of ultra-thin semiconductor crystals are alternately stacked at regular intervals using the method of the present invention.

An example of the slide stage used in FIG. 3 is shown. In FIG. 3, 8 and 8' are solutions 7 and 7 that are adjusted to precipitate different semiconductor crystals at respective growth temperatures.
' is a raw material crystal or raw material alloy whose component is the semiconductor crystal to be grown.

The seed crystal substrate 6 can be placed on the raw material on both sides by sliding, and at this time, the solution remains in the minute gap on the raw material crystal or raw material alloy.
The seed crystal 6 pushed downward by the lid 11 comes into contact with the solution, and a sandwich structure is realized in which the solution is sandwiched between the seed crystal substrate and the raw material crystal or raw material alloy. After the substrate is brought into contact with one of the raw material sides to perform growth for one cycle, it is slid and brought into contact with the other raw material side to perform growth for one cycle. By repeating this, 2
It is clear that different types of semiconductor crystals or mixed crystals can be grown alternately. Here, the thickness of the solution is adjusted by the weight of the lid, etc., but it can be made as thin as several tens of micrometers. Although FIG. 3 shows an example where the specific gravity of the solute is smaller than that of the solution, it goes without saying that a superlattice structure can be realized by the same method even in the opposite case.

Here, an example will be shown in which a superlattice structure made of InAs and GaAs was manufactured. G ILCL47 I n Q, s 5 A
8 has a lattice match with InP and a good epitaxial growth layer can be obtained, and is attracting attention as a material for long wavelength lasers, while a large value of mobility of 10,000 cm2/v, s or more at room temperature has been obtained. Therefore, it is expected to be used as a material for ultra-high-speed, high-frequency devices. High-purity carbon bose as shown in Figure 3 is used, and the raw material is GaAs.
and wrapped around the (111) B-face of InAs. The upper limit of the temperature cycle is 600°C and the lower limit is 597°C, that is, the temperature amplitude is 3°C. The solution is Ga at 600℃
A Ga solution in which As is saturatedly dissolved and an In solution in which InAs is saturatedly dissolved are prepared. N-type InP substrate (111)
The entire coat is heated in an electric furnace without contacting the B side, lX1crrt2, with either solution and held at a temperature several degrees higher than 600°C. After that, 0.2°Q/min
At a temperature of 600°C, that is, at time ts in FIG. 4(a), the InP substrate was slid and the I
It is brought into contact with an In solution in which nAs is saturatedly dissolved. Figure 4 (
a) shows this state. 20 is InP substrate, 21 is In
As raw material crystal, 22 is an In solution. 597°C'!
After cooling at , the state is maintained for 10 minutes from time ts2 to tll. FIG. 4(b) shows this state. 23 is a growth layer of InAs. At time t11, the InP substrate is slid and moved again to a position where it does not come into contact with either solution, and at time t1. from 0.2°C/min
Start heating up. Then time t2. to t3°6
The temperature is maintained at 00° C., and cooling is started at time t51, and at the same time, GaAs is grown by contacting the solution on the Ga side.

FIG. 4(c) shows this state, 24 is a GaAs growth layer, 25 is a QaAs raw material crystal, and 26 is a Ga solution. Here, both the Ga solution and the In solution are about 100%
It is rumored that it is about μm. This operation was repeated, and each growth was performed 30 times.

After the growth, this sample was folded and etched appropriately, and then observed using an electron microscope.
A periodic structure in which 30 X GaAs thin film layers and approximately 700 XInAs thin film layers were stacked alternately was observed.

This superlattice structure of (InAs)n(GaAs)rr is composed of GaO,43In with a bulk thickness of about 5,111 m.
o, 57As. Furthermore, by reducing the thickness of the solution, lowering the growth temperature, reducing the temperature width, or changing the temperature amplitude during each growth, the thickness of each thin film, that is, the number of layers of each molecule, can be increased. It is clear that it is possible to realize a superlattice structure that changes arbitrarily and exhibits remarkable quantum size effects. Furthermore, by introducing a doping gas during growth to change the conductivity type of the grown layer, it is possible to realize a structure that has a pn junction band structure as a whole while having a superlattice structure. Of course, when the positions of the substrate crystal and the source crystal are reversed in the growth of two types of semiconductor crystals, for example, in the case of GaAs growth using a Ga solution and AtAs growth using an At solution, yl? It is clear that a superlattice structure can be realized by creating a structure that can be inverted.

Also, is a superlattice structure created by combining three or more types of semiconductor crystals a problem? This can be easily achieved by increasing the amount of solution in the substrate, and a superlattice structure with a semiconductor mixed crystal can also be realized by using a raw material alloy having a composition intended for growth. This example has revealed that a superlattice structure of two or more types of ultrathin films, such as semiconductor crystals, mixed crystals, or metal crystals, can be manufactured easily and at low cost.

Especially in semiconductor crystals, GaAs/AtAs system, G
aAs/AtGaAs-based InAs/GaAs-based InGa
All I- such as As/InP system, GaAs/Garb system, etc.
Group V compound semiconductor or PbSnTe/P
It is applied to all II-Vl group compound semiconductors such as bTe type, ZnSe/ZnS type, etc.

〔Effect of the invention〕

As explained above, by the method of the present invention, it is possible to fabricate superlattice structures and multilayer films exhibiting ideal quantum size effects inexpensively, economically, and safely by repeating temperature cycles. When a lid well radar, a HEMT, etc. are manufactured using this method, the manufacturing cost can be reduced.

[Brief explanation of the drawing]

Figure 1 is a diagram illustrating an example of fabricating a superstructure using selective dougs according to the method of the present invention, and Figure 2 is a diagram illustrating an example of fabricating a superstructure using a selective doug method according to the method of the present invention.
Figure 3 shows an example of a Ge/Garb superstructure using the ty gap. Figure 3 is an example of a slide goze for producing a superlattice structure using two different types of semiconductor crystal thin films. Figure 4 The diagram is 1. nAs/
This figure shows an example of fabricating a GaAs superlattice structure. DESCRIPTION OF SYMBOLS 1... Seed crystal, 2... Raw material crystal or raw material alloy, 3... Growth solution, 4... Growth layer, 5... Growth layer, 6... Seed crystal, 7, 7' ... Raw material alloy or raw material crystal, 8.8'... Growth solution, 9... Slide boat for growth, 10... Slider, 11... Lid, 12, 12'... Operation rod , 13...quartz tube,
14... Electric furnace, 15... Ge iM child crystal substrate,
16-Ge-GaSb alloy, 17-Ga-Ge-8
b solution, 18... Ga growth layer, 19... Garb growth layer, 20... InP substrate, 21... InAs raw material crystal, 22... In-As solution, 23- InAs thin film growth layer, 24- GaAs thin film growth, 25...G
aAs raw material crystal, 26...Ga-As solution, g...
gravitational acceleration.

Claims (1)

[Claims] At least one of a seed crystal substrate and a raw material crystal or raw material alloy having components of a superstructure and multilayer film to be grown is arranged on the upper side and the other one on the lower side, and when the crystal growth starts, the seed crystal is The space between the substrate and the raw material crystal is filled with a solution that precipitates a solid phase containing superstructures and multilayer films for the purpose of growth, and the temperatures of the seed crystal substrate, raw material crystal or raw material alloy, and solution are periodically adjusted in a certain temperature range. A method for producing superstructures and multilayer films, characterized by transporting dissolved solute from the raw material crystal or alloy side to the seed crystal side by vertically moving the solute up and down, and depositing the solute on the seed crystal substrate.