JPS61274312A - Compound semiconductor device - Google Patents

Abstract

PURPOSE:To make it possible to form the buffer layer of high resistivity without doping the impurity, by applying the high resistive layer to the buffer layer the high resistive layer is formed by the alternate crystal growths of the island type with every 0.1-30 atomic layers of the different kind of compound semiconductors. CONSTITUTION:For the semiinsulative or conductive substrate 1, InAs and InSb, for the conductive substrate as well as InP and GaAs to make the semiinsulative substrate are available. On the substrate 1, the buffer layer 2 is formed by the alternate crystal growth of the island type of the two different kind of compound semiconductors. The structure of this buffer layer is not an orderly layer but an piled island form with a complicated cross section. The thickness of growth is equivalent to 0.1-30 atomic layers. On the buffer layer 2, the active layer 3 is formed by epitaxial growth, which is doped with suitable impurity to obtain the N or P-type. The ohmic junction electrodes 4, 4 and the Schottky junction electrode 5 are installed on the active layer 3.

Description

DETAILED DESCRIPTION OF THE INVENTION (g) Industrial Application Field This invention relates to a compound semiconductor device that can be used as a high-speed microwave device. A high-resistance thin film crystal layer is formed on a compound semiconductor substrate, and a high-mobility active layer is formed on the thin film crystal layer to form a semiconductor device.

Among these, a method of forming a thin film crystal layer with high resistance is provided by the present invention.

(a) Prior Art Semiconductor devices are generally manufactured by epitaxially growing a high-mobility active layer on a semi-insulating substrate. A buffer layer may be provided between the active layer and the substrate. This is to prevent lattice defects contained in the substrate from being introduced into the active layer.

When forming a buffer layer and an active layer by epitaxial growth, lattice matching between these thin films and the substrate is important. In other words, the lattice constants of both crystals must be equal.

It is desired that the active layer has high electron or hole mobility and high density of these carriers.

It is better for the substrate to have high insulating properties. This is because conductivity makes it impossible to insulate between elements.

At present, the material that exists as a good insulating substrate material is S
i 1GaAs 5InP only. Other than this -
Group V compounds cannot be used as semi-insulating substrates.

Although InAs and InSb are compound semiconductors having high mobility, they cannot be made semi-insulating.

Therefore, using InAs or InSb itself as a substrate,
It is not possible to create an active layer on top of this.

Ino, 53Gao, uAs are mixed crystals with high mobility. Moreover, it is highly valuable as a light emitting element and a light receiving element. A high resistance substrate cannot be made with this composition itself.

Therefore, a field effect transistor (FET) using this
When fabricating this, semi-insulating InP is used as a substrate. This is because InP has the same lattice constant as Ino, 53Gao, and 4yAs, and can be made to have high resistance.

When forming an active layer on a substrate, a buffer layer is often interposed between the active layer and the substrate in order to prevent crystal defects and contamination on the substrate surface from being brought into the active layer. The buffer layer is a semiconductor crystal layer having the same composition as the substrate, or a semiconductor crystal layer different from the substrate but having the same lattice constant.

It is desirable that the buffer layer has high resistance.

FIG. 4 shows a cross-sectional view of a cell F'lli:T in which Ino, 53 Gao, and 47 As are used as active layers. Semi-insulating In
A buffer layer 22 is formed on the P substrate 21, and an active layer 23 of In6.5aGa (1,47A!1) is formed thereon.

In order to form an FET, an ohmic junction type Wi 24. which is to be used as a source electrode or a drain electrode is placed on the active layer 23.
24, and a Schottky junction electrode 25 used as a gate electrode.

The buffer layer 22 has a relatively high resistance, and the operating layer 23
Allows current to flow selectively only to

For example, as the buffer layer 22, a P-type InP layer, a P-type I
n00s3Gao, 47As layer, A[o-4aIno,
52As layer is used. All of these are lattice matched to the substrate and the active layer.

The electron mobility of the operating layer 23 at room temperature is 7000 to 180
00Q12/Vsec. The electron mobility of the buffer layer 22 at room temperature is 1000α27Vsec. It can be seen that the buffer layer has a sufficiently high resistance.

The InP layer used for the buffer layer is usually an n-type semiconductor with high electron mobility (2000 to 4000 cm2/Vsec
). Ino, 53cao, 47 As layers also,
It is usually n-type and has high electron mobility.

In order to use these n-type semiconductors as a buffer layer, it is necessary to increase the resistance. For this reason, Zn, Be.

These layers are doped with p-type impurities such as Mg. A deep acceptor level is formed by the p-type impurity,
Electron mobility decreases and the number of electrons as carriers also decreases. Therefore, it becomes a p-type semiconductor with high resistance.

However, these p-type impurities expand into the active layer in the solid phase when the active layer is further formed on the buffer layer. Since the p-type impurity diffuses into the active layer, it reduces the electron mobility in this layer and greatly impairs the electrical characteristics of the FET.

Furthermore, if p-type impurities are used in the crystal growth apparatus, the inside of the crystal growth apparatus will be contaminated with the p-type impurities.

The conventional method of doping InP or InGaAs with p-type impurities to form a buffer layer cannot necessarily be said to be superior because of the influence it has on the active layer and the inside of the crystal growth apparatus.

As Pa'/77 layer, Al o, 4s InO, s+
If As is used, a high-resistance buffer layer can be formed without doping with p-type impurities. This has the same lattice constant as Ino, 53Gao, and 47As, and can be lattice matched.

However, this buffer layer has some technical difficulties in manufacturing.

First, Al is required in addition to In1Ga and As, which are the raw materials for the active layer. Therefore, the structure of the crystal growth apparatus becomes complicated.

Next, since both the buffer layer and the active layer are ternary mixed crystals,
It is necessary to mutually control lattice matching conditions. This complicates the growth technology.

Thirdly, since Ad is an element that is extremely easily oxidized,
Oxygen accumulates on the surface of the buffer layer. When oxygen diffuses into the active layer, it forms electron traps within the active layer.

The presence of electron traps reduces electron mobility within the active layer and degrades FET characteristics.

For this reason, Alo, 4s is used as a buffer layer.
The conventional method of using Ino, 52As layers is also not necessarily optimal.

The ones described above have operating layers of Ino, 53Gao, 4
This is about the case of yAs.

The active layer is In-Ga having a higher mobility than this, -
, As (x > 0.53), InAs, etc., there is no semi-insulating semiconductor substrate having the same lattice constant as these.

Therefore, in order to fabricate an FET using In, GaAs (x > 0.53), or InAs as a dynamic -3m layer, there are even more difficult problems than in the case described above. Two solutions are currently proposed.

(1) One uses semi-insulating InP or GaAs for the substrate, and the buffer layer has a composition of In, P, As or In%
This means that the composition of Ga and As is continuously changed little by little, and the lattice constant is changed from one that is equal to that of the substrate to one that is equal to that of the active layer.

The composition ratio can be changed using molecular beam epitaxy.

(2) Since a semi-insulating substrate having the same lattice constant as the active layer cannot be obtained, a conductive substrate having the same lattice constant is used.

Then, on top of this, a high resistance buffer layer is made to have a high resistance. If the active layer is made of InAs, then the substrate is also made of conductive InAs, for example. An insulating buffer layer having a matching lattice constant is epitaxially grown on this layer.

However, these proposals have not yet been successful. (1) Gradually changing the composition of the buffer layer,
This method complicates the growth technology and has not been successful to date. Furthermore, in order to increase the resistance of the buffer layer, it is necessary to dope p-type impurities, which diffuse into the active layer and work to lower electron mobility.

In method (2) above, p is used to make the buffer layer high in resistance.
Type impurities must be doped.

Then, there is a problem that these diffuse into the active layer and reduce electron mobility.

What has been described above is a conventional technique for making a buffer layer.

We will also briefly explain the conventional techniques for creating the operating layer.

For example, G with a different lattice constant is placed on an InP substrate.
It has been proposed to epitaxially grow aAs and InAs alternately in layers. This is because it is not a buffer layer but an active layer with low resistance and high mobility. By molecular beam epitaxial
AAs and InAs are grown alternately. What is important is that it is layered. This means that when the x1Y axis is taken in the direction of the substrate surface and the Z axis is taken in the direction perpendicular to the substrate surface, the composition depends only on the Z coordinate and does not depend on the x1Y coordinate.

GaAs grows uniformly in a direction parallel to the surface, forming layers of the same height. On top of this, a layer of InAs is uniformly grown.

In the molecular beam epitaxial apparatus, it is necessary to rotate a manipulator to heat the substrate to an appropriate temperature, and to strictly control the temperature of the molecular beam source.

What is grown epitaxially in layers in this way is
A superlattice structure of aAs and AlAs has already been grown on a GaAs substrate to fabricate a high-speed FET. In this case, the purpose is to dope only the GaAs layer with Si to increase the electron density of the active layer.

In any case, these had to be layered.

However, in reality, it is difficult to epitaxially grow a large number of thin films in layers.

Gauze 1 Problems to be Solved by the Present Invention An object of the present invention is to solve the above-mentioned problems in forming a buffer layer in a compound semiconductor FET. In other words, even if the active layer and the substrate have different lattice constants, the active layer must be a layer with few defects and high electron mobility, and the buffer layer must be a layer with high resistance and p
There is no need to dope type impurities.

B) Means for Solving the Problems The present invention uses, as a buffer layer, a high-resistance layer in which crystals of different compound semiconductors are grown alternately in island shapes of 0.1 to 30 atomic layers.

As mentioned above, the method of forming a superlattice by growing different types of compound semiconductors in layers is well known.

In the present invention, the growth is performed not in a layered manner but in an "island shape". This is the important point. Until now, it has not been done to form the Ephitaxia mu layer in the form of islands.

For lack of a better word, we call it island-like.

However, this is not a concept defined as a technical term, but a name given by the inventor.

The X1 and Y axes are taken along the two sides of the board, and the Z axis is taken at right angles to these. As mentioned above, growing in a layered manner means that a layer of a certain composition is formed uniformly in the x and y directions.

Let A and B be two types of compound semiconductors. This alternates in the Z-axis direction with Zl, Z2, Z3, . . . as boundaries. That is, the layered structure alternates as follows: z=z NZA I Z = ZNZ B z =: Z2 to Z3A regardless of the X and Y coordinates.

The term "island-like" refers to the fact that the compound A is locally deposited in mountains on the substrate. There may be places where the A compound is not present and the substrate is exposed.

The height of compound A is not uniform and varies depending on the X and Y coordinates.

Compound B grows on top of this. Since the B compound is also island-shaped, some of it is attached to the substrate, and some of it is attached to the A compound. Furthermore, the height of the B compound deposited differs.

FIG. 1 is a perspective view illustrating the formation of such island-like compounds A and B. ial shows an example in which the A compound was grown in an island shape on the substrate, and (b) shows an example in which the B compound was grown in an island shape on the substrate. (C) shows an example in which compound A was further grown. (d) schematically shows the buffer layer grown in this way.

Although it is said to be island-like, if only a small number of atoms are grown, only one small island will be formed. As the number of atoms increases, the islands become larger and the number increases to two or three. The height also increases. As the number of atoms increases further, adjacent islands coalesce,
The height of the islands increases, but the number decreases.

Since A, B, A, B are grown alternately, it is assumed that they are numbered 1st layer, 2nd layer, etc. in order from the bottom. Tsumashi, A1, B2, A3, B4, A5, etc.

The difference between AlB and B is abstracted, and both are written as C for simplicity.

The structure of the Patsubua layer can be written as C1, C2, C3, . . . However, since it is not layered, C1+1 is C
It is not necessarily above h.

The area where Ck and Ch are in contact divided by the area of the entire semiconductor substrate is written as 5 (kh).

In case of layered +1) Only when k w h ± 1 5 (kh):1, (II) In the case of k f−h±1 5 (kh) = Q.

In the case of an island, this is not the case; even in the case of
There is a probability that it is 0.

Also, regarding the thickness, the thickness of Ck in the Y direction is T(k,
If it is written as X, Y), then T(k, X, Y) if it is layered.
: T(k) (11 could be written. However, this is not the case in the case of an island.

There is a distribution in the x1y direction. On the substrate, T(k,
By integrating Y) and dividing it by the area of the substrate, you will find the average height. This is written as T(k).

It is. So is the area of the substrate.

Let the thickness of one atomic layer be σ. T(k) divided by σ is a value expressing the average thickness of Ck in terms of the number of atomic layers.

Let's call this N(k).

1~ N(k) = −T(k) +a+σ It is.

If it is layered, the dispersion of T(kSXlY) in the XY plane is 0.
It is.

JJ(T(k,X,Y) −T(k))dXdY=0
(41 However, since it is an island-like distribution, this dispersion is
Not 0. Letting the standard deviation be Qk, it can be defined as follows. Since it is island-like, Qk is not 0.

However, since the formation of islands is stochastic, it is difficult to control Qk to a constant value.

Therefore, an island can be defined by saying that the average standard deviation is greater than 0.1, for example. An island shape can be defined by n k ==z where n is the number of layers.

Of course, layered and island-like are not complementary. A border area exists. For a good layered single crystal, the left side of equation (6) should be 0.01 or less.

The concept of "island" is explained below.

It is easier to grow islands on a substrate using a molecular beam epitaxial growth apparatus than to grow layers.

Possible methods include making the temperature of the substrate non-uniform, not opening the shutter of the molecular beam cell fully, and providing a directional guide at the exit of the molecular beam cell. Since the substrate is rotated, island-like growth can be achieved.

In this way, islands of A, B, A, B, etc. are grown alternately. The growth thickness is 0.1 to 30 atomic layers.

This is the average thickness N(k) mentioned earlier.

When finally grown in an island shape, the surface of the buffer layer should be smooth. In other words, if there are any depressions left, growth will be concentrated there. The result is a beautiful mirror surface. This seems strange.

However, the inventor confirmed this experimentally.

It can be said that the present inventor is the first to notice such a phenomenon.

The substrate is InP, and its lattice constant is ao.

Suppose that a ternary mixed crystal is grown on top of this. Let this lattice constant be a. The lattice matching is evaluated by the ratio of the lattice constant deviation to ao. The difference in lattice constants (alao) is written as Δa. Lattice matching is often evaluated by 1Δa/aoj.

In the case of a ternary mixed crystal, these atoms will always be ejected in the same proportion from the molecular beam cell toward the substrate. A lattice constant a can be defined depending on the composition ratio. In many cases, it is a linear function of the composition ratio.

When growing a ternary system of In, Ga, and As as a mixed crystal on an InP substrate, the lattice matching coefficient is 1Δa/a
If of is larger than about 0.4 to 0.5%, the surface of the grown layer will become hot and the flatness will deteriorate.

When growing ASB compounds alternately instead of mixed crystals,
There is no clear lattice constant a1. In this case, the lattice constant a of the grown layer is tentatively defined by the lattice constant a1 of the mixed crystals having the same composition.

This definition is reasonable considering that a is a linear function of the composition ratio X and that both the islands of compound A and the island of compound B are minute islands of 0.1 to 30 atomic layers be.

When compound A and compound B are grown in multiple layers alternately in the form of islands as in the present invention, the lattice matching condition is relaxed. When grown in multiple layers in the form of an island, even if the lattice misalignment is about 2%,
I was able to obtain a flat mirror surface.

The island-like multilayered structure has the excellent feature that the lattice matching conditions are significantly relaxed and a flat surface can be easily obtained.

Another advantage of island growth is that it has high resistance.

Consider a case where InAs and GaAs are grown alternately in island shapes on an InP substrate.

The lattice constant of the substrate is a. The lattice constant of the epitaxial growth layer thereon is set to al. Al varies depending on the composition ratio of InAs and GaAs.

If InAs is 100%, Δa/a, = +3.1
It's %. Δa is the value of (al-ao). GaAs
If is 100%, Δa/ao = -8.7%.

Regarding the mixed crystal of InxGa1-xAs', instead of using X as a parameter, Δa/aO can be used as a parameter.

Figure 2 shows In3cGa, , As using rnP as a substrate.
3 is a graph showing the specific resistance of a grown layer when epitaxially growing a mixed crystal of . The right end is Δa/
a(1 is +8.1%, which means 1 nAs is 100%
corresponds to the limit of

At the left end, Δa/a 0 is -3.7% and GaAs is 1
00%” limit.The resistivity is about 10Ω1 on the left side (GaAs side) and about 10Ω on the right side (InAs side).
J, and changes linearly with (Δa/ao) during this time. In any case, the specific resistance is extremely small.

However, in the case of island-like growth, Δa /
When eL6 was defined and the specific resistance was measured, it was found that the specific resistance value became significantly large. The specific resistance is one to two orders of magnitude higher than that of a mixed crystal with the same composition ratio. In FIG. 2, the resistivity of the island-like growth layer is shown by white circles.

Point P indicates the specific resistance of three atomic layers of GaAs and one atomic layer of InAs grown alternately in an island shape. here·
...The atomic layer is the value obtained by dividing the average thickness by the thickness of the atomic layer, and corresponds to N(k) in equation (3).

Although it is called three atomic layers, the three atomic layers are not formed uniformly over the entire surface of the substrate.

In this case, the composition ratio X is 0.25. Since the corresponding lattice constant a1 of the mixed crystal is known, Δa/
a 6 is calculated and the value is plotted on the horizontal axis in Figure 2.

Point 9 indicates the specific resistance of two atomic layers of GaAs and two atomic layers of InAs grown alternately in an island shape.

The R point is one atomic layer of GaAs, three atomic layers of InAs,
It shows the resistivity of islands grown alternately.

At points Q and R, the specific resistance is more than 100 times that of a mixed crystal with the same composition ratio.

The layers grown alternately in the form of islands in this manner satisfy the following two requirements for a buffer layer.

(1) Surface flatness (2) High resistance These are features that cannot be obtained with mixed crystals or ternary systems grown in alternating layers. This is first achieved through island-like growth.

An active layer of InAa or InxGa, -, As is epitaxially grown on the buffer layer thus formed.

Furthermore, an ohmic junction electrode used as a source or drain electrode and a Schottky junction electrode used as a gate electrode are provided on the active layer.

FIG. 3 is a cross-sectional view of such a compound semiconductor device. 1 is a semi-insulating or conductive compound semiconductor substrate. Not only a semi-insulating substrate such as InP or GaAs, but also a conductive substrate may be used. rnAs,
InSb etc. can also be used.

On the substrate 1, two different compound semiconductors are alternately placed.
The buffer layer 2 is formed by growing it in an island shape. This is not a neat layered structure, but a stack of islands, so it has a complex cross-section.

An operating layer 3 is epitaxially grown on the buffer 7 layer 2. Dope with appropriate impurities to make it n-type or p-type.

On the active layer 3, an ohmic contact electrode 4.4 and a Schottky contact electrode 5 are provided.

Although this figure shows one FET, in reality, a large number of FETs are fabricated on a wafer in all directions.

EXAMPLE On a conductive n-type InP substrate, three atomic layers of InAs and one atomic layer of GaAs (corresponding to point R in FIG. 2) are grown alternately in island shapes to form a buffer with a thickness of 10 μm. formed a layer.

This specific resistance was 8Ω.

On the Pa'/77 layer, In, 75 Ga0. ．． A ternary mixed crystal of 5As was epitaxially grown. The thickness of this active layer was 0.1 μm, and the carrier density was IXlocM.

On top of this, a Schottky junction electrode (gate) and ohmic junction electrodes (source, drain) were provided to form an FET. The ohmic junction electrode is made of AuGeNi. The Schottky junction electrode is Ad.

The FET manufactured in this manner had no leakage current to the substrate side and exhibited good FET characteristics.

The lattice mismatch between the substrate (InP) and the active layer was 2.2%, but even with such a large mismatch, the electron mobility of the active layer was sufficiently high.

In addition to the above-mentioned examples, experiments were also conducted in which the average thickness of each layer of GaAs and InAs was varied between 0.1 and 30 layers and island-like growth was performed. The buffer layer thus produced was flat and had high resistance.

In the above example, since there was a large amount of InAs, there were 10 irregularities with respect to InP. However, the lattice mismatch with the substrate may be - or zero.

In the above embodiment, the reason why the substrate was made of conductive InP was to confirm the insulation properties of the buffer layer.

Of course, a semi-insulating substrate may also be used.

The combination of different types of compound semiconductors constituting the buffer layer is not limited to InAs and GaAs. It is sufficient that the thin film crystal layer has a mirror-like surface and has a high resistance when grown in an island-like manner and is grown alternately. In addition to InP, GaAs can also be used for the substrate.

(b) Effects (1) A high-resistance buffer layer can be created without doping with impurities.

(2) Since there is no p-type impurity in the buffer layer (when the active layer is n-type), it will not diffuse into the active layer and reduce electron mobility.

(3) Since the buffer layer has sufficient insulation, a conductive substrate can be used as the substrate. The range of substrate selection is expanded.

(4) No strict lattice matching conditions are imposed between the substrate and the active layer. There may be a difference in lattice constant.

(5) The buffer layer becomes a highly flat mirror surface, and the active layer grown thereon has fewer crystal defects.

When a mixed crystal with the same composition is grown on a substrate, even if the surface of the mixed crystal has a lattice misalignment that is uneven, it becomes flat in the case of island-like growth.

(1) Idea The buffer layer of the present invention has high resistance and good surface flatness. This may seem strange, but it can be thought of as follows.

Because the crystals are grown in the form of islands, there are layered minute parts on the islands, but there are stacking faults between the islands.
ng f'aults) occurs. This is a defect caused by a slippage between the planes of a distance equal to a fraction of the crystal repeat.

Since stacking faults are planar defects, dislocations (d
islocation ) has higher energy.

■ In the case of -v compounds, the stacking fault energy is high, so
This usually does not occur easily. Dislocations often occur during the growth of many single crystals. When creating a mixed crystal with a large lattice mismatch with the substrate by epitaxy, stacking faults do not occur very often, and the problem is caused by edge dislocations running parallel to the interface between the substrate and the epitaxial layer, called lattice misalignment dislocations. be.

However, in the present invention, the particles are grown in the form of islands, and stacking faults are actively created at the boundaries between the islands.

From a three-dimensional perspective, InAs surrounded by stacking faults
It is considered to be a collection of crystals with a layered structure of GaAs and GaAs.

Even if there is a large lattice mismatch between the buffer layer and the substrate, most of the energy based on this lattice mismatch is relaxed only by stacking faults, and no lattice misalignment dislocations occur. Since the energy of stacking faults is much larger than the energy of dislocations, the formation of stacking faults eliminates the energy available for generating lattice misalignment dislocations.

In this way, lattice misalignment dislocations due to lattice misalignment between the substrate and the buffer layer do not occur, and only stacking faults occur.

Stacking faults act as local trap levels for electrons. Therefore, the electron mobility becomes extremely low and the buffer layer becomes highly resistive.

Furthermore, since stacking faults suppress the occurrence of lattice misalignment dislocations, the surface of the buffer layer formed by stacking in the form of islands has a high degree of flatness and a mirror surface.

For the amount of epitaxial growth, 0.1 to 3
The reason for 0 atomic layer is as follows.

If it is less than 0.1 atomic layer, the islands become too fine and approach the limit of mixed crystal. This is no different from growing a mixed crystal.

If the thickness is 30 atomic layers or more, the height of the islands becomes too large, and irregularities begin to appear on the surface of the buffer layer, and at this point, the electrical properties of the active layer begin to be adversely affected.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the steps of growing crystals in an island shape on a substrate. (a) shows one layer grown on a substrate. Φ) is the second layer grown. (C) shows the third layer grown. (d) shows a buffer layer formed by growing multiple layers. FIG. 2 is a graph showing measured values of resistivity in the case of crystal (black circles) and in the case of island-like growth (open circles) of the present invention, using the lattice mismatch Δa/ao as a parameter. FIG. 3 is a cross-sectional view of an FET made by the method of the present invention. FIG. 4 is a cross-sectional view of a conventional FET. 1...Substrate 2...Buffer layer 3...Operation layer

Claims (3)

[Claims] (1) A semi-insulating or conductive semiconductor substrate and different types of compound semiconductors are alternately placed on the semiconductor substrate at 0.1
1. A compound semiconductor device comprising: a buffer layer formed by island-like crystal growth of ~30 atomic layers; and a high mobility compound semiconductor operating layer formed on the buffer layer. (2) The compound semiconductor device according to claim (1), wherein the buffer layer is lattice matched to the substrate. (3) The compound semiconductor device according to claim (1), wherein the buffer layer has a lattice mismatch with the substrate.