JP3616745B2 - Manufacturing method of semiconductor device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lattice-mismatched stacked crystal structure and a method for manufacturing a semiconductor device such as an electronic element or an optical element using the same.
[0002]
[Prior art]
Conventionally, a problem in using a lattice mismatched stacked crystal structure in a semiconductor device, that is, a thin film crystal of a semiconductor having a lattice constant different from that on a substrate crystal (strictly, a lattice constant different in a direction parallel to the substrate surface). Various problems of the electrical properties of the grown thin film crystals during the growth are discussed.
[0003]
For example, in Journal of Applied Physics Vol. 67, No. 7 (1990), pages 3323 to 3327 (Journal of Applied Physics, Vol. 67, No. 7, pp 3323-3327), InAlAs carrier supply layer / InGaAs channel formation layer structure Improvements in electron mobility in the channel of several HEMT devices are discussed. In this paper, a buffer layer having a structure in which InGaAs crystal layers having different lattice constants are stacked stepwise between a GaAs substrate crystal and a thin film crystal constituting an active layer of a HEMT device, dislocation defects due to lattice mismatch are introduced. To improve the electron mobility in the channel. In this method, when the thickness of the entire buffer layer is 2.5 μm, the dislocation defect density of the active layer is about 10 ⁶ cm ⁻² and the electron mobility at room temperature is 8150 cm ² / Vs.
[0004]
In addition, in Applied Physics Letters Vol. 61, No. 8 (1992), p. 922 (Applied Physics Letters, Vol. 61, No. 8, p. 922), an InAlAs thin film crystal constituting an active layer of a GaAs substrate crystal and a HEMT device. Lattice mismatch by interposing only one InGaAs graded buffer layer with continuously changing lattice constant, or a buffer layer with a constant lattice constant between InGaAs graded layers. A method for reducing the dislocation defects due to GaN and improving the electron mobility of the HEMT device is discussed. In this method, when the In composition ratio is 0.3, the degree of lattice mismatch is small, and the thickness of the buffer layer is 1 μm, the electron mobility at room temperature is 8500 cm ² / Vs.
[0005]
[Problems to be solved by the invention]
The electron mobility at room temperature of the above-described prior art has remained at 8500 cm ² / Vs when the thickness of the buffer layer is 1 μm. That is, only the HEMT crystal lattice-matched on the GaAs substrate was obtained, and the effect of improving the electron mobility was insufficient.
[0006]
An object of the present invention is to provide a semiconductor thin film having a lattice constant parallel to the substrate crystal plane (hereinafter simply referred to as a lattice constant) different from that of the substrate crystal via a buffer layer made of a semiconductor having a thickness of 1 μm or less on the substrate crystal. An object of the present invention is to provide a lattice-mismatched stacked crystal structure in which crystals are stacked and the electron mobility at room temperature is larger than 8500 cm ² / Vs, and a semiconductor device using the same.
[0007]
[Means for Solving the Problems]
The purpose is to configure the buffer layer by laminating a plurality of first regions and a plurality of second regions in the stacking direction, increasing the lattice constant of the first region toward the semiconductor thin film crystal in the stacking direction, The thickness of the first region is set to a thickness at which lattice strain due to lattice mismatch with the substrate crystal is relaxed, and the second region is formed on and in contact with the surface of the first region on the semiconductor thin film crystal side. This can be achieved by making the lattice constant of the second region constant in the stacking direction and making the lattice constant of the buffer layer continuous in the stacking direction.
[0008]
Here, the lattice constants of the first region and the second region are controlled by the composition ratio of the constituent elements common to the first region and the second region. The at least one first region can be larger in the vicinity of the interface with the second region than in the second region in contact with the first region.
[0009]
In the present invention, as shown in FIGS. 4 and 5, even when the thickness of the buffer layer is 1 μm or less, a buffer layer structure having two or more steps is used (here, one step is one first step). And a second region in contact therewith)), and by finding that a lattice-mismatched stacked crystal structure with an electron mobility at room temperature greater than 8500 cm ² / Vs can be obtained. It was done.
[0010]
The reason why the electron mobility at room temperature is increased by the present invention is presumed as follows. First, in the first stage, the first region is grown to a thickness that relaxes the lattice strain accumulated therein, so that dislocation defects are generated in the first region during the formation of the first region. Next, the dislocation defects generated in the first region extend into the second region during the formation of the second region, but disappear by so-called dislocation bonding at the point where the dislocation defects meet each other. That is, dislocation defects generated in the first region are absorbed by the second region. Further, since the second region is lattice-matched with the first region, no new dislocation defect is generated in the second region. As a result, the second region acts as a substantially defect-free substrate crystal with respect to the first region in the second stage. This indicates that the second region is in the same state as the substrate crystal in the first stage. Therefore, the same action is repeated after the second stage.
[0011]
In short, the feature of the present invention is that it absorbs dislocation defects while there are few, so that the absorption efficiency of dislocation defects is good and the electron mobility can be increased.
[0012]
On the other hand, for example, in the case of one stage, since the lattice constant is changed at one time in one first region, the amount of change in the lattice constant is large, and a large number of dislocation defects are generated. Therefore, dislocation defects cannot be sufficiently absorbed by the second region.
[0013]
Further, the composition ratio of the constituent elements common to the first region and the second region, which controls the lattice constant of the first region and the second region, is set to the second region of at least one first region. In the vicinity of the interface, the buffer layer made larger than that in the second region in contact with the first region is particularly thick in a semiconductor thin film crystal such as a semiconductor laser or a bipolar transistor (about 200 nm or more), and becomes the center of operation. This is effective for an element in which carriers are minority carriers.
[0014]
The following can be inferred as an effect of this method. The relaxation of the lattice strain accompanying the increase of the lattice constant in the first region occurs little by little with the occurrence of new dislocation defects after it has occurred once, but the lattice strain remains without being completely relaxed. Therefore, when the composition ratio in the first region of the element that controls the lattice constant monotonously increases and approaches the composition ratio in the second region (see FIG. 2), The lattice constant at the interface with the second region is smaller than the lattice constant that it should originally have, and does not match the lattice constant of the second region. As a result, the possibility of the occurrence of dislocations in the second region remains. On the other hand, in the present method, the composition ratio of the element that controls the lattice constant is increased near the interface between the first region and the second region in anticipation of being smaller than the inherent lattice constant. Therefore, the lattice constants can be matched at the interface between the first region and the second region (see FIG. 8). As a result, the possibility of occurrence of dislocations in the second region can be reduced. Note that, after the composition ratio of the first region becomes larger than that of the second region, the composition ratio is continuously decreased to match that of the second region. As described above, the absorption efficiency of dislocation defects can be improved and the electron mobility can be increased, which is effective in a minority carrier device.
[0015]
In the present invention, it goes without saying that the continuity of the lattice constant between the regions constituting the buffer layer includes a deviation within 0.5%. Although the case where the carrier is an electron has been described, it goes without saying that the carrier is not limited to an electron but may be a hole. Further, by using such a lattice-mismatched stacked crystal structure and forming an active region of a semiconductor device in a semiconductor thin film crystal, it is possible to realize a semiconductor device with good characteristics.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Example 1
Hereinafter, a HEMT crystal and a HEMT device according to Example 1 of the present invention will be described with reference to FIGS.
[0017]
As shown in FIG. 1, an undoped InAlAs buffer layer 2 is 500 nm, an undoped InGaAs channel forming layer 3 is 40 nm, an undoped InAlAs spacer layer 5 is 2 nm, and n nm on a semi-insulating GaAs substrate 1 in order by molecular beam epitaxy. Type InAlAs carrier supply layer (Si doping amount: 3 × 10 ¹⁸ cm ⁻³ ) 5 is 15 nm, undoped InAlAs layer 6 is 10 nm, n-type InGaAs cap layer (Si doping amount: 3 × 10 ¹⁹ cm ⁻³ ) 7 is 30 nm To a HEMT crystal.
[0018]
Here, the In composition ratio of the InAlAs carrier supply layer and the InGaAs channel formation layer is 0.5. Further, the In composition ratio of the InAlAs buffer layer 2 was changed in five steps as shown in FIG. In addition, the composition ratio of In is changed by changing the temperature of the In molecular beam source without any interruption time for crystal growth.
[0019]
Further, for the InAlAs buffer layer 2 that is the main point of the present invention, the ratio of the thickness at each stage of the In composition ratio of the region where the In composition ratio continuously increases (first region), the thickness of the buffer layer, and The number of stages of the In composition ratio of the buffer layer was set variously and variously formed. First, FIG. 3 shows the relationship between the ratio of the thickness of the first region and the electron mobility at room temperature of the two-dimensional electron gas. The electron mobility exceeds the conventional 8500 cm ² / Vs when the ratio of the continuous region is in the range of about 0.1 to 0.45. Next, FIG. 4 shows the relationship of the electron mobility at room temperature of the two-dimensional electron gas to the thickness of the buffer layer when the ratio of the thickness of the first region is 0.2. In the prior art, high electron mobility is maintained even at a thickness of 1000 nm or less, in which the decrease in electron mobility is significant. In particular, the value of the electron mobility of about 10,000 cm ² / Vs at the thickness of the buffer layer of 500 to 1000 nm is equivalent to the lattice-matched HEMT crystal of the InP substrate. Since the buffer layer can be made thin, there is an effect that the crystal growth time can be shortened to half or less as compared with the prior art. Next, FIG. 5 shows the relationship between the number of steps in the In composition ratio of the buffer layer and the electron mobility at room temperature of the two-dimensional electron gas at a constant buffer layer thickness of 600 nm. The electron mobility exceeds 8500 cm ² / Vs of the prior art in two or more stages. The value of the electron mobility begins to decrease at 10 levels, indicating that the higher the number of stages, the better. This decrease in electron mobility is thought to be because the thickness of the region (second region) having a constant composition ratio that absorbs dislocation defects is reduced. Note that the thicknesses of the first region and the second region may not be the same in all stages. The starting point of the rise of the In composition is set to 0 in FIG. 2, but if it is difficult to set it to 0, it may be 0.15 or less. As a material for the buffer layer, an InAlGaAs quaternary material can be used if the above In composition is used.
[0020]
Next, a HEMT device having a gate length of 1500 nm as shown in FIG. 6 was fabricated using the HEMT crystal of FIG. As the HEMT crystal, a first region having a thickness ratio of 0.2 and a buffer layer thickness of 500 nm was used. The n-type InGaAs cap layer 7 is processed from the HEMT crystal shown in FIG. 1 by ordinary photolithography and electron beam exposure, and the source and drain electrodes 8 and 9 and the Schottky gate electrode 10 are formed to complete the HEMT device.
[0021]
This device had an external transconductance of 1.3 S / mm and a cutoff frequency of 250 GHz. These values are both about twice as large as those of the prior art, and are comparable to HEMT elements formed by lattice matching on an InP substrate.
[0022]
Example 2
A semiconductor laser according to Example 2 of the present invention will be described below with reference to FIGS. As shown in FIG. 7, an n-type InGaAs buffer layer 12 (thickness: 500 nm) is sequentially formed on an n-type GaAs substrate 11, and the In composition ratio is 0.5 and the Al composition ratio is changed from 0 to 0.5. At the same time, the n-type InAlGaAs graded layer 13 (thickness 200 nm) in which the Ga composition ratio changes from 0.5 to 0, the n-type InAlAs cladding layer 14 (thickness 1500 nm) in which the In composition ratio is 0.5, the In composition ratio Is an undoped InAlGaAs GRIN layer 15 (thickness 120 nm) in which the Ga composition ratio is changed from 0 to 0.5 and the Al composition ratio is changed from 0.5 to 0, and the In composition ratio is 0.5. Undoped InGaAs active layer 16 (thickness 15 nm), undoped InAl whose In composition ratio is 0.5, Al composition ratio changes from 0 to 0.5, and Ga composition ratio changes from 0.5 to 0 aAsGRIN layer 17 (thickness 120 nm), p-type InAlAs cladding layer 18 (thickness 1500 nm) with an In composition ratio of 0.5, In composition ratio 0.5, and Ga composition ratio changed from 0 to 0.5 At the same time, a p-type InAlGaAs graded layer 19 (thickness 200 nm) in which the Al composition ratio changes from 0.5 to 0, and a p-type InGaAs contact layer 20 (thickness 100 nm) in which the In composition ratio is 0.5 It was formed by the line epitaxy method. Si was used as the conductivity determining impurity of the n-type layer, and was doped 2 × 10 ¹⁸ cm ⁻³ , and Be was used as the conductivity determining impurity of the p-type layer, and was doped 1 × 10 ¹⁸ cm ⁻³ . The thickness from the n-type InAlGaAs graded layer 13 to the p-type InGaAs contact layer 20 is 3755 nm.
[0023]
Next, after forming the ohmic electrode 21 on the crystal surface and the back surface of the substrate, a semiconductor laser having a broad area contact structure was completed by cutting out to a resonator length of 300 μm and a width of 200 μm.
[0024]
Here, for the n-type InGaAs buffer layer 12 which is the main point of the present invention, as shown in FIG. 8, the number of steps of the In composition ratio of the buffer layer is set to 5, and the In composition ratio in each step is continuously increased. A region in which the In composition ratio is 5% larger than that of the layer (second region) formed thereon is provided in the changing region (first region). Further, the ratio of the thickness of the first region was set to 0.1 (the thickness of the first region was 10 nm, the thickness of the second region was 90 nm). In addition, when a region having an In composition ratio of 5% is provided in a part of the stage, a certain effect can be obtained. The thicknesses of the first region and the second region may not be the same at all stages. Although the rising start point of the In composition is set to 0 in FIG. 8, if it is difficult to set it to 0, it may be 0.15 or less. As a material for the buffer layer, an InAlGaAs quaternary material can be used if the above In composition is used.
[0025]
The threshold current density of this element was 500 A / cm ² , which was the same as that of a lattice-matched semiconductor laser using an InP substrate. As described above, according to this embodiment, a semiconductor laser having characteristics equivalent to those of a lattice matching semiconductor laser using an InP substrate can be obtained by using a GaAs substrate that is cheaper than an InP substrate. Further, if an electronic device is further formed on the GaAs substrate of this embodiment and the optical device and the electronic device are integrated (OEIC), the advantages of this embodiment can be further utilized.
[0026]
【The invention's effect】
According to the present invention, even when the thickness of the buffer layer is as thin as 1 μm or less, it is possible to realize a lattice mismatched stacked crystal structure having an electron mobility higher than 8500 cm ² / Vs at room temperature and a semiconductor device using the same.
[Brief description of the drawings]
1 is a longitudinal sectional view of a HEMT crystal HEMT crystal of Example 1 of the present invention.
FIG. 2 is a distribution diagram of the In composition ratio of the InAlAs buffer layer of Example 1 of the present invention.
FIG. 3 is a graph showing the relationship between the ratio of a continuously changing region of the In composition ratio of the InAlAs buffer layer of Example 1 of the present invention and the electron mobility of the InGaAs channel layer.
FIG. 4 is a diagram showing the relationship between the thickness of the buffer layer and the electron mobility of the InGaAs channel layer in Example 1 of the present invention.
5 is a graph showing the relationship between the number of In composition ratio steps of the buffer layer and the electron mobility of the InGaAs channel layer in Example 1 of the present invention. FIG.
FIG. 6 is a cross-sectional view of a HEMT device according to Example 1 of the present invention.
FIG. 7 is a cross-sectional view of a semiconductor laser according to the present invention.
FIG. 8 is a distribution diagram of the In composition ratio of the InGaAs buffer layer of Example 2 of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Semi-insulating GaAs substrate, 2 ... Undoped InAlAs buffer layer, 3 ... Undoped InGaAs channel formation layer, 4 ... Undoped InAlAs spacer layer, 5 ... n-type InAlAs layer, 6 ... Undoped InAlAs layer, 7 ... n-type InGaAs cap layer , 8 ... Source electrode, 9 ... Drain electrode, 10 ... Gate electrode, 11 ... n-type GaAs substrate, 12 ... n-type InGaAs buffer layer, 13 ... n-type InAlGaAs graded layer, 14 ... n-type InAlAs cladding layer, 15 ... Undoped InAlGaAsGRIN layer, 16 ... undoped InGaAs active layer, 17 ... undoped InAlGaAsGRIN layer, 18 ... p-type InAlAs cladding layer, 19 ... p-type InAlGaAs graded layer, 20 ... p-type InGaAs contact layer, 21 ... ohmic Electrode.

Claims (1)

A semiconductor device having a lattice mismatched laminated crystal structure in which a semiconductor thin film crystal having a lattice constant different from the substrate crystal in a direction parallel to the substrate crystal plane is laminated on the substrate crystal through a buffer layer made of a semiconductor In the manufacturing method, a buffer layer made of the semiconductor and a semiconductor thin film crystal are stacked by molecular beam epitaxy, and the buffer layer is alternately stacked with a plurality of first regions and a plurality of second regions in the stacking direction. The lattice constant of the first region is increased toward the semiconductor thin film crystal in the stacking direction, and the thickness of the first region is set to a thickness at which lattice strain due to lattice mismatch with the substrate crystal is relaxed, The second region is formed on and in contact with the surface of the first region on the semiconductor thin film crystal side, the lattice constant of the second region is constant in the stacking direction, and the lattice constant of the buffer layer is continuous in the stacking direction. Let The method of manufacturing a semiconductor device according to claim and.

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