JPH0810671B2 - Compound semiconductor layer - Google Patents

Description

TECHNICAL FIELD The present invention relates to a compound semiconductor layer in which a compound semiconductor is epitaxially grown on a crystalline substrate and the epitaxial growth layer is used as an electronic element or an optical element,
In particular, the present invention relates to a high quality compound semiconductor layer having few crystal defects such as dislocations.

[Conventional technology]

It is a well known fact that the epitaxial growth technique has been put to practical use mainly in the manufacture of semiconductor devices.
Compound semiconductor crystals are being widely used as a material for devices such as ultra-high-frequency oscillators, high-speed logical operation devices, and semiconductor lasers by taking advantage of their characteristics. However, from the viewpoint of crystal perfection, the present situation is far below that of Si, which has achieved almost no dislocation. Taking GaAs crystal, which is the most representative compound semiconductor, as an example,
Even with the horizontal Bridgman method, which can obtain crystals with relatively few dislocations, a dislocation density of the order of 10 ³ / cm ² is common,
The pulling method gives a high density of 10 ⁴ to 10 ⁶ / cm ² .

By the way, in recent years, since the molecular beam epitaxial method and the vapor phase growth method using an organic metal enable the epitaxial growth of an ultrathin layer and the growth of a multi-component compound, they are essential means for realizing an element using the epitaxial growth layer. Has been done. When homoepitaxial growth is performed on a substrate crystal by these methods, most of the substrate dislocations penetrate into the epitaxial growth layer.

For example, reference (Shinohara et al., Applied Physics 58 (1985) 3449, M.
Shinohara, T.Ito, Y.Imamura: J.Appl.Phys.58 (1985) 34
As shown in 49), when homoepitaxial growth is carried out on a GaAs substrate by molecular beam epitaxy, 90% of substrate dislocations are propagated in the epitaxial growth layer. Therefore, high-density dislocations are present in the epitaxial growth layer as in the substrate crystal.

In the conventional method of suppressing the propagation of dislocations from the substrate to the epitaxial growth layer, as shown in FIG. 8, different materials 2 and 3 having greatly different lattice constants are formed on the substrate crystal 1.
After the superlattice spacer layer 4 in which the layers are laminated is grown, a desired crystal 5 is grown. This utilizes the fact that tensile stress acts on the crystal of the different material having the smaller lattice constant and the compressive stress acts on the crystal of the larger lattice constant due to the large difference in the lattice constant. This is based on the theory that Matthews et al. (Crystal Growth Journal, Crystal Growth Journal, 27 (1974) 118, JW Matthews and
AEBlakeslee, J. Crystal Growth, 27 (1974) 118). And in this case, the substrate dislocations are crystals 2 and 3
The condition of bending at the interface of was dependent on the film thickness of the crystals 2 and 3, the lattice constant difference, and the like.

[Problems to be solved by the invention]

According to the theory of Matthews et al., The condition that the substrate dislocation is bent at the interface is almost equal to the condition that the misfit dislocation 7 enters the interface as shown in FIG.
That is, under the condition that the penetration of substrate dislocations is prevented, the occurrence of high density misfit dislocations cannot be avoided. In this case, there is a problem that a misfit dislocation has a dislocation penetrating into the epitaxial growth layer 5 again,
Depending on the density of misfit dislocations, the dislocation density in the epitaxial growth layer 5 may increase more than the dislocation density of the substrate. This can be found, for example, in the literature (Shinohara et al., Japan Applied Physics, 24 (1985) L818, M. Shinohara, T. Ito, K. Yamada, Y. Im.
amura: Jpn.J.Appl.Phys.24 (1985) L818). Further, it is difficult to control the material composition and film thickness of the crystals 2 and 3, and the epitaxial growth layer 5 and the superlattice spacer layer 4 are
High-density misfit dislocations easily occur at the interface with and, which causes a problem that the quality of the epitaxial growth layer 5 deteriorates.

In addition, by using a material with a small lattice constant difference from the substrate crystal in the same structure as in FIG. 8 (specifically, substrate crystal 1 and crystal 3 are GaAs, 2 is AlAs), misfit dislocations do not occur. A method of controlling the propagation of dislocations from a substrate has been reported in. However, since the strain in the superlattice spacer layer 4 is small, the propagation of substrate dislocation cannot be completely blocked.

In FIG. 8, although it is assumed that the epitaxial growth layer 5 and the crystal 3 are joined,
The same result is obtained even when the crystal and the crystal 2 are joined.

The present invention has been made in view of such a point,
The purpose thereof is to obtain a compound semiconductor layer that can completely prevent dislocations contained in a substrate from propagating into an epitaxial growth layer without generating misfit dislocations.

[Means for solving problems]

In order to achieve such an object, the present invention provides a compound semiconductor layer having a superlattice spacer layer having a laminated structure in which a thin layer made of a first crystal and a thin layer made of a second crystal are sequentially laminated alternately. In the above, a thin crystal composed of a crystal in which at least one of the lower end of the conduction band energy and the upper end of the valence band energy in the relative relationship between the first crystal and the second crystal is discontinuous and the lower end of the conduction band energy is relatively high. The layer is doped with an impurity exhibiting n-type conductivity, or a thin layer made of a crystal having a relatively low valence band energy upper end is doped with an impurity exhibiting p-type conductivity. .

[Action]

In the compound semiconductor layer according to the present invention, propagation of substrate dislocations is suppressed without causing misfit dislocations.

〔Example〕

The compound semiconductor layer according to the present invention has the following features.

In the molecular beam epitaxial growth method or the vapor phase growth method using an organic metal, in order to prevent the dislocation of the substrate crystal from propagating to the epitaxial growth layer, a misfit dislocation is formed at any interface between the substrate crystal and the epitaxial growth layer. With the spacer layer inserted without generating.

The spacer layer is composed of a superlattice layer in which two kinds of crystals having conduction band energy or valence band energy discontinuous with each other are deposited.

Crystals on the high side of the conduction band energy lower end were doped with n-type impurities, or crystals on the lower side of the valence band energy upper end were doped with p-type impurities.

The conventional technique for suppressing the propagation of substrate dislocations utilizes bending of substrate dislocations in the direction perpendicular to the growth direction due to the stress stored in the heteroepitaxial layer, and is based on the mechanical action acting on the dislocations. Is. On the other hand, in the compound semiconductor layer according to the present invention, paying attention to the electrical properties of dislocations, modulation doping is performed in the heteroepitaxial layer to spatially separate positive and negative charges at the heteroepitaxial layer interface. , It is different from the prior art in that the dislocation is bent at the interface of the heteroepitaxial layer due to the electrical action caused by it.

FIG. 1 is a constitutional view showing one embodiment of the compound semiconductor layer according to the present invention, showing a case where GaAs which is a typical compound semiconductor is epitaxially grown. 1 is a substrate made of GaAs single crystal grown by the pulling method, 8 is an undoped GaAs layer of 100 Å thickness, 9 is 100 Å thickness of 5 × 10 ¹⁷ / cm ³ doped with Si as an impurity showing n-type conductivity AlAs
Layers, 4a is a superlattice spacer layer formed by repeated growth of layers 8 and 9, and 5 is a superlattice spacer layer grown on superlattice spacer layer 2
This is a GaAs epitaxial growth layer with a thickness of μm. The dislocation density of the substrate in this example is 10 ⁵ / cm ² or less.

Fig. 2 shows the energy band of the GaAs and Si-doped AlAs2 periods of the superlattice spacer layer 4 of Fig. 1, and shows the energy band. Substrate dislocations bend at the interface between GaAs and AlAs, It is a figure for estimating and explaining the mechanism which does not propagate theoretically. In FIG. 2, 1a and 1b are GaAs layers, 2a and 2b are AlAs layers, 10 is a conduction band energy lower end, and 11 is a valence band energy upper end.

In the above structure, AlAs-doped Si
Is activated and electrons accumulate on the GaAs side, which has a low conduction band energy. As a result, activated donor impurities having a positive charge remain on the AlAs side, and electrons having a negative charge accumulate on the GaAs side. Therefore, as shown in FIG. The structure is such that positive and negative charges exist separately.

Dislocations existing in GaAs crystals, which are III-V group compound semiconductors, are called α-dislocations in which group III Ga atoms are arranged in a line at the ends of extra atomic planes, and β dislocations in which group V As atoms are arranged. There are two types of 60-degree dislocations, called what is called. The α-dislocation acts electrically as a donor and the β-dislocation acts as an acceptor. The electronic state of the dislocation can be explained from the relationship with the dangling bond, and the donor has a neutral charge when it has an electron and a positive (+) charge when it has emitted an electron. Further, it is known that the acceptor has a neutral charge in the state of having holes and a negative (-) charge in the state of releasing holes (for example, Applied Physics Vol. 46, June issue, 1977, 631). Page, Nakata et al. "Electrical properties of dislocations in GaAs"). In addition, normal 650 ℃
In the growth temperature region of ~ 800 ℃, the Fermi level is close to the center of the conduction band and the valence band, so the dislocations are electrically activated.

Considering this, in FIG. 2 for explaining the mechanism of the embodiment of FIG. 1, dislocations move from the left side (this is the substrate side) of the drawing to the right side (this is the epitaxial growth layer side). Considering the case of propagation, α-dislocations are GaAs layers
It is derived that it receives attractive force from 1a and 1b and repulsive force from AlAs layers 2a and 2b. On the other hand, the electric field concentration occurs at the hetero interface from an electrical point of view, and it is possible to change the dislocation propagation direction to the direction along the interface like the stress concentration.

Therefore, when α dislocations propagate from the GaAs layer to the AlAs layer, for example, from the GaAs layer 1a to the AlAs layer 2b,
Bent in the direction perpendicular to the growth direction at the interface between la and AlAs layer 2b,
It does not propagate to the epitaxial growth layer. Similarly, β translocation
Since it receives attractive force from the AlAs layer and repulsive force from the GaAs layer,
When propagating from 2b to GaAs layer 1b, it is bent at the interface between AlAs layer 2b and GaAs layer 1b in the direction perpendicular to the growth direction, and does not propagate into the epitaxial growth layer.

As a result, dislocations existing in the GaAs crystal are bent at the GaAs-AlAs interface and do not propagate to the epitaxial growth layer.

3 and 4 are photographs of crystal defects showing the effect of the compound semiconductor layer according to the present invention. FIG. 3 shows dislocations in the epitaxial growth layer observed by electron beam induced current method using a scanning electron microscope after forming a titanium Schottky barrier on the surface of the epitaxial growth layer 5 having the structure shown in FIG. FIG. 4 shows the dislocation of the substrate at the same place as in FIG. 3 observed by the molten KOH etching method. The substrate dislocation density shown in Fig. 4 is approximately 1.5 × 10 ⁵ /
While it is cm ² , the dislocation density in the epitaxial growth layer shown in FIG. 3 is zero. That is, this example shows that dislocation propagating from the substrate to the epitaxial growth layer can be completely prevented, and a dislocation-free epitaxial layer can be grown.

The inventors of the present application directly observed the dislocation direction in the superlattice spacer layer with a transmission electron microscope, and actually measured that the dislocation was running parallel to the hetero interface due to the structure as described above. Specifically, as shown in FIG.
A superlattice spacer layer 4 of Å to 3000Å was formed so as to satisfy the energy band structure shown in FIG. 2, and then a GaAs film was epitaxially grown to a thickness of 2 μm. Next, in the thus obtained sample, the epitaxial GaAs film and the GaAs of the substrate were removed by etching, and the sample was observed from above the main surface obliquely by a transmission electron microscope. FIG. 7 is a photograph of crystal defects showing the observation result. The black line in FIG. 7 indicates a dislocation line, and it can be seen that the dislocation line runs in a plane parallel to the hetero interface. The arrow 12 in the figure indicates the direction of the <110> crystal direction.
On the other hand, as described above, it is clear by observation by electron beam induced current method that dislocations do not exist (penetrate) in the epitaxial GaAs film, so dislocation lines bend at the hetero interface of the superlattice spacer layer. As a result, it is predicted that the upper epitaxial growth layer has not been reached.

FIG. 5 and FIG. 6 are examples in which the state of propagation of substrate dislocations into the epitaxial growth layer was examined in the case where no impurities were doped in FIG. 1, that is, when there was no electrical effect due to electric charges. This corresponds to the technique performed conventionally. FIG. 5 is a photograph of crystal defects showing the results of observing dislocations in the epitaxial growth layer in the same manner as in FIG.
The figure is a photograph of crystal defects showing the results of observing the substrate dislocation in the same manner as in FIG. The black dot visible in FIG. 5 corresponds to one dislocation. In this case, about 40 of substrate dislocations
It can be seen that% is propagated into the epitaxial growth layer.

As described above, it has been proved that the compound semiconductor layer according to the present invention is far superior to the conventional compound semiconductor layer, that is, the effect is remarkable. Conventionally, it has been known that dislocations existing in the epitaxial growth layer cause deterioration of laser lifetime. In addition, it is reported that when a field effect transistor (FET) is made using a substrate with dislocations, the operating voltage (threshold voltage) of the FET near the dislocations is different from the characteristic at the location without dislocations. Therefore, the use of the compound semiconductor layer according to the present invention is effective in extending the life of the laser or making the FET device characteristics uniform. In particular, the uniformization of the FET element characteristics is an essential requirement for using the element as an integrated circuit, so that the characteristics of the present invention are utilized.

In this embodiment, an example in which the AlAs layer is doped with Si is shown in FIG. 1, but the same effect was experimentally obtained even when the AlAs layer was doped with p-type Be. However, in this case, ionized acceptors (negative charges) exist on the AlAs layers 2a, 2b side in FIG. 2, and positive charges accumulate on the GaAs layers 1a, 1b side. Therefore, the interface where the α-dislocation and β-potential are bent is opposite to the case where the n-type doping described above is used, but there is no difference in the effect of electrically bending the dislocation at the interface.

Further, in this embodiment, an example in which GaAs is used as a substrate and a GaAs-n-type AlAs superlattice is used as a superlattice spacer layer is shown, but discontinuous propagation also occurs in other material systems as shown in FIG. It goes without saying that the same effect can be expected by doping a n-type or p-type dopant using a combination having a lower band energy or a higher valence electron energy.

Furthermore, in this embodiment, the epitaxial growth layer 5 and GaAs are
Although the structure in which the layers 8 are joined is adopted, the epitaxial growth layer 5
Even if the AlAs layer 9 is bonded to the AlAs layer 9, the effect is the same.

〔The invention's effect〕

As described above, according to the present invention, the first crystal and the second crystal of the superlattice spacer layer having a laminated structure in which the thin layers made of the first crystal and the thin layers made of the second crystal are alternately laminated in sequence. At least one of a conduction band energy lower end and a valence band energy upper end in a crystal relative relationship is made discontinuous, and a thin layer made of a crystal having a relatively high conduction band energy lower end is doped with an impurity exhibiting n-type conduction. Or a thin layer made of a crystal having a relatively low valence band energy upper end doped with an impurity exhibiting p-type conduction, and epitaxial growth was performed using a substrate with a high dislocation density. Even when dislocations do not occur, the propagation of substrate dislocations can be suppressed, so that an epitaxial growth layer without dislocations can be obtained above the superlattice spacer layer. There is an effect that can be.

[Brief description of drawings]

FIG. 1 is a structural view showing an embodiment of a compound semiconductor layer according to the present invention, FIG. 2 is a structural view showing a mechanism of a superlattice spacer layer forming the compound semiconductor layer of FIG. 1, FIG. 3 and FIG. FIG. 4 is a photograph of crystal defects showing dislocation densities of the epitaxially grown layer and the substrate in the compound semiconductor layer of FIG. FIG. 7 is a photograph of a crystal defect shown in FIG. 7, FIG. 7 is a photograph of a crystal defect showing bending of a dislocation line in the compound semiconductor layer of FIG. 1, and FIG. 8 is a constitutional view showing a conventional compound semiconductor layer. 1 ... GaAs substrate, 4a ... Superlattice spacer layer, 5 ... Epitaxial growth layer, 6a, 6b ... Substrate dislocation, 8 ... GaAs
Layer, 9 ... Si-doped AlAs layer.

Claims (1)

[Claims] 1. A compound semiconductor substrate, an epitaxial growth layer, and a superlattice spacer layer, wherein the superlattice spacer layer is interposed between the compound semiconductor substrate and the epitaxial growth layer, and the superlattice spacer layer is a first crystal. A compound semiconductor layer having a laminated structure in which a thin layer made of s and a thin layer made of a second crystal are sequentially laminated alternately.
N-type in a thin layer made of a crystal in which at least one of the lower end of the conduction band energy and the upper end of the valence band energy is discontinuous and the lower end of the conduction band energy is relatively high. 2. A compound semiconductor layer characterized by being doped with an impurity exhibiting p-type conductivity or by doping a thin layer made of a crystal having a relatively low valence band energy upper end with an impurity exhibiting p-type conductivity.