JPH028450B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a method for manufacturing a high-speed semiconductor device, particularly a semiconductor device in which a two-dimensional electron layer formed at a heterojunction interface is used as a channel.

(Description of Prior Art) High-speed integrated circuits mainly made of silicon have been known. However, recently, instead of this conventional high-speed integrated circuit, ultra-high-speed integrated circuits using semiconductor elements that effectively utilize GaAs/GaAlAs heterojunctions formed by molecular beam epitaxy (MBE) or metal organic pyrolysis (MOCVD) have been developed. is proposed.

FIGS. 1 and 2 are cross-sectional views for explaining the structure and manufacturing method of an example of a conventional semiconductor element used in such an ultra-high-speed integrated circuit.

FIG. 1 shows a conventional field effect transistor (hereinafter simply referred to as FET) in which a channel layer is a highly mobile two-dimensional electron gas at a GaAs/AlGaAs heterojunction interface (hereinafter simply referred to as heterointerface). This FET element is mounted on a GaAs substrate 1 with high purity.
After a GaAs layer 2 and an AlGaAs layer 3 are successively grown by molecular beam epitaxy (hereinafter simply referred to as MBE), a source electrode 4, a drain electrode 5, and a gate electrode 6 are formed on the AlGaAs layer 3. . Note that the regions surrounded by dotted lines 8 and 9 are regions that act as a source region and a drain region, respectively. The FET element configured in this way has high speed
Operates as a FET. This uses a high-purity GaAs semiconductor layer with a small bandgap (forbidden band width),
In a superlattice structure in which AlGaAs semiconductor layers with a large bandgap are grown alternately, or in a single heterojunction structure of both semiconductor layers, modulation doping of impurities, that is, n-type doping selectively only in the AlGaAs layer, is performed. Due to the difference in electron affinity with AlGaAs, electrons in the AlGaAs layer transfer to the high-purity GaAs layer, forming a layer of electron gas that spreads two-dimensionally at the hetero interface as shown by the broken line 7 in Figure 1. It is explained that since the electron beam is spatially separated from the ionized donor impurity, there is less Coulomb scattering, and therefore the electron mobility is high especially at low temperatures, resulting in high-speed operation.

However, since the surface of the FET element shown in Fig. 1 is an AlGaAs layer, not only the characteristics of the shot junction of the gate 6 become unstable, but also the AlGaAs
Since the number of carriers in layer 3 is large, in order to modulate the concentration of secondary electron gas by gate voltage,
There was a drawback that the thickness of the AlGaAs layer 3 had to be precisely controlled. Furthermore, since an ohmic electrode is formed on the two-dimensional electron gas via the AlGaAs layer, there is also the drawback of increased contact resistance.

On the other hand, another conventional FET device as shown in FIG. 2 has been proposed in which the semiconductor layer forming the heterojunction of the FET device in FIG. After successively growing the GaAs buffer layer 12, AlGaAs buffer layer 13, n-type AlGaAs layer 14, and high-purity GaAs layer 15 by the method, the high-purity GaAs layer 1
It has a structure in which a source electrode 16, a drain electrode 17, and a gate electrode 18 are formed on the 5, respectively.
Here, the regions surrounded by dotted lines 20 and 21 are regions that act as a source and a drain, which are formed using normal semiconductor manufacturing techniques. In this FET element, high-purity GaAs15 and its lower n
A channel layer, that is, a two-dimensional electron layer (gas) as shown by a broken line 19 is formed at the hetero interface with the type AlGaAs layer 14.

The FET element with the structure shown in FIG. 2 eliminates the drawbacks of the FET with the structure shown in FIG.
Literature publication "Journal of Applied Physics"
H. Morkoc, TJ Drummond and R. Vol. 53, No. 2, February 1982, pp. 1030-1033.
As is clear from the description in the paper by Fischer et al. (particularly FIG. 3), the mobility of the two-dimensional electron gas 19 is the same as the mobility of the two-dimensional electron gas 7 of the FET device shown in FIG. 1 grown under the same conditions. For example, the average mobility of the latter 78〓 obtained at a substrate temperature in the range of 600-680℃ during growth is 90000 cm ² /Vs, whereas the average mobility of the former 78〓 obtained up to a substrate temperature of 680℃ during growth is much lower. The mobility of 78〓 obtained in the range is about 2000cm ² /Vs, and it is 8500cm ² /Vs at the maximum filter substrate temperature of 700℃. In particular, the mobility of the former, that is, the FET element with the structure shown in Figure 2, is It was found that the growth rate significantly depends on the substrate temperature during growth using the MBE method.

Conventionally, when growing a GaAs or AlGaAs film on a GaAs substrate by the MBE method, the film is grown after confirming that the surface oxide film on the GaAs substrate evaporates. The evaporation temperature of this surface oxide film is approximately
The temperature of the substrate during this growth is approximately 580℃.
The temperature is set to 580℃ or higher. On the other hand, the literature publication "Applied Physics Letter" Vol.38, No.9, 1981
WI published on pages 708 to 710 on May 1, 2018
Wang, S. Judaprawira, CECWood and LF.
The paper by Eastman et al. and “Applied Physcs
Letter” Vol. 38, No. 6, No. 427, March 15, 1981
PDKirchner, JM, published on pages ~429
As is clear from papers by Woodall, JL Freeout, GDPettit, etc., it has been reported that the higher the substrate temperature, the better the growth of AlGaAs films. However, in the case of the FET element with the structure shown in Fig. 2, even if the substrate temperature during growth is set as high as 700°C, the mobility at 78〓 is about 8500 cm ² /Vs, and this element The drawback is that the mobility is too low to be applied to high-speed semiconductor devices.

(Purpose of the Invention) The present invention was developed in view of the above-mentioned drawbacks of conventional semiconductor devices.The purpose of the present invention is to eliminate the need for precise control of the thickness of the high-purity GaAs layer, provide low contact resistance, and provide two-dimensional electronic An object of the present invention is to provide a method for manufacturing a high-speed semiconductor device with high gas mobility.

(Structure of the Invention) In order to achieve this object, the method according to the present invention is characterized in that the substrate temperature during the formation of the heterojunction is set to a temperature lower than the evaporation temperature of the surface oxide film of the substrate.

This substrate temperature is approximately 20℃~ based on the evaporation temperature.
It is preferable to set the temperature range to 180°C lower.

Further, this substrate can be a GaAs substrate, the first semiconductor layer can be an AlGaAs layer, and the second semiconductor layer can be a GaAs layer.

(Description of Examples) Examples of the present invention will be described below with reference to the drawings.

The present invention relates to the manufacture of a semiconductor element having a structure corresponding to the FET element having the structure shown in FIG. I will explain about it.

As shown in Figure 2, using the MBE method,
Approximately 1000 cells are placed on the GaAs substrate 11 sequentially from this substrate side.
GaAs buffer layer 12 with a thickness of about 1000 Å
AlGaAs buffer layer 13, a silicon-doped AlGaAs layer 14 with a thickness of about 500 Å as a first semiconductor layer, and a non-doped AlGaAs layer 14 with a thickness of about 5000 Å as a second semiconductor layer.
A heterojunction 2 between a silicon-doped AlGaAs layer 14 and a non-doped GaAs layer 15 by making the GaAs layer 15 continuous
form 2. Then, using normal semiconductor device manufacturing technology, the source and drain regions (areas surrounded by dotted lines 20 and 21) and non-doped
A source electrode 16, a drain electrode 17, and a gate electrode 18 are formed on the GaAs layer 15. In the semiconductor device formed in this way, a channel layer is formed by the two-dimensional electron gas 19 existing at the interface of the heterojunction 22, and this channel layer is controlled by the voltage applied to the gate electrode 18 to form a channel layer from the source electrode 16 to the drain. The current flowing through the electrode 17 can be modulated.

By the way, as already explained, in order to make this semiconductor device a high-speed device, it is necessary to increase the mobility of the secondary electron gas forming the channel layer 19, and this mobility depends on the substrate temperature during growth. It was related to. Therefore, the inventor of this application conducted an experiment to investigate the relationship between this mobility and the substrate temperature during growth, and found that there is a relationship as shown in FIG. 3. In this Figure 3, the horizontal axis is the substrate temperature (°C) at the time of the formation of the hetero interface where a two-dimensional electron gas is formed, and the vertical axis is the number of sheet carriers 5×10″cm ^-2
This is a diagram plotting the hole mobility of the two-dimensional electron gas at the liquid nitrogen temperature (77〓). In this case, the substrate temperature is expressed based on the temperature (T ₀ ) at which the GaAs surface oxide film evaporates. From the results of this experiment, it was found that the hole mobility of 77〓 becomes 40000 cm ² /V·sec or more when the substrate temperature is in the range of about T ₀ -20°C to _{T 0} -180°C.

The relationship shown in Figure 3 is obtained because
The surface accumulation effect of silicon dopant (Si) during growth by the MBE method decreases as the substrate temperature decreases, and lowering the substrate temperature increases the mobility accordingly, and further lowering the substrate temperature increases the hetero-interface. This is thought to be due to the fact that the flatness of the surface becomes worse and the mobility decreases.

Therefore, in the present invention, the silicon-doped AlGaAs layer 14 and the non-doped AlGaAs layer 14 are formed using the above-mentioned MBE method.
The substrate temperature when growing the heterointerface with the GaAs layer 15 is set at a temperature lower than the evaporation temperature, especially about 20°C to 180°C below the evaporation temperature, based on the temperature at which the surface oxide film of GaAs evaporates. It is preferable to set the temperature within a range.

(Effects of the Invention) According to the present invention, the MBE is placed on the upper side of the substrate.
As described above, the substrate temperature is set when growing the heterointerface between the n-type AlGaAs layer 14, which is the first semiconductor layer with a large band gap, and the high-purity GaAs layer 15, which is the second semiconductor layer with a small band gap, from the substrate side using the method. The temperature is set at a temperature range of approximately 20℃ to 180℃ lower than the evaporation temperature of the surface oxide film.
The mobility of the two-dimensional electron gas at the hetero-interface of the semiconductor device thus formed can be increased, and therefore a high-speed semiconductor device using this two-dimensional electron layer, that is, the electron gas 19, as a channel layer can be realized. It can be realized.

Furthermore, in the structure of a semiconductor device, such as an FET device, manufactured according to the method according to the present invention, since the gate electrode 18 is formed on the high-purity GaAs layer 15, n.
Unlike when the gate electrode is formed on the surface of the AlGaAs layer 14 of the mold, the voltage applied to the gate electrode is not reduced by the ionized donor impurities, and this applied voltage is directly transmitted to the two-dimensional electron gas layer.
Therefore, there is no need to precisely control the thickness of the high-purity GaAs layer 15, and high mutual conductance can be obtained. In addition, since the surface of the FET element is the GaAs layer 15, the characteristics of the shot junction of the gate electrode 18 are stabilized, and the characteristics of the shot junction of the gate electrode 18 are stabilized.
It is easy to form an ohmic electrode, and its resistance value can be easily reduced. Furthermore, GaAs is more stable than AlGaAs, making it possible to realize highly reliable devices.

(Description of Modifications) It is clear that the invention is not limited only to the embodiments described above, but can be modified and modified in many ways. For example, although the present invention has been described with respect to field effect transistors, it can also be applied to other high mobility devices and very high frequency devices.

Further, unlike the above-described embodiments, the first semiconductor layer may be formed directly on the substrate, or the heterojunction may have a superlattice structure.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing a conventional field effect transistor in which a high-mobility two-dimensional electron gas at a GaAs-AlGaAs heterointerface is used as a channel layer, and FIG. 2 is used to explain methods for manufacturing semiconductor devices according to the conventional method and the present invention. A schematic enlarged sectional view showing an embodiment of a field effect transistor similar to that shown in FIG. 1 but having a different structure from that shown in FIG. 1; FIG. 3 is shown in FIG. FIG. 3 is a characteristic curve diagram showing the relationship between substrate temperature and hole mobility during manufacturing of a field effect transistor. 11...Substrate (e.g. GaAs substrate), 12...
Buffer layer (e.g. GaAs), 13... Buffer layer (e.g. AlGaAs), 14... First semiconductor layer (e.g. n-type AlGaAs layer), 15... Second semiconductor layer (e.g. high purity GaAs layer), 16... Source electrode, 17...Drain electrode, 18...Gate electrode, 19...Two-dimensional electron gas (or channel layer),
20... A line indicating the boundary of the source region, 21... A line indicating the boundary of the gate region.

Claims (1)

[Claims] 1 Using molecular beam epitaxy on a GaAs substrate,
A step of growing a first semiconductor layer made of AlGaAs and a second semiconductor layer made of GaAs in this order to form a heterojunction between the two semiconductor layers, and a step of selectively performing modulation doping on the first semiconductor layer. In manufacturing a semiconductor device using a two-dimensional electron layer at a heterojunction interface as a channel, the substrate temperature at the time of forming the heterojunction is set to a temperature 20 to 180 degrees Celsius lower than the evaporation temperature of the surface oxide film of the substrate. A method for manufacturing a semiconductor device, characterized in that: