JPS6354231B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor device, and more particularly to a field effect that uses an electron storage layer with high electron mobility, which occurs at a heterojunction between a non-doped GaAs layer and an n-type AlGaAs layer, as a conductive channel. Concerning improvements in high electron mobility transistors (HEMTs).

HEMT is a heterojunction between two types of semiconductors with different electron affinities, where electrons in a semiconductor layer with a lower electron affinity (AlGaAs) move to the semiconductor layer with a higher electron affinity (GaAs) based on the difference in electron affinity. FIG. 1 shows an example of a HEMT structure, which is a semiconductor device that utilizes the phenomenon of forming a two-dimensional electron storage layer and has been proposed by the present inventors. 1st
In the figure, 1 is a semi-insulating GaAs substrate doped with chromium or the like, 2 is a non-doped GaAs layer, 3 is an n-type AlGaAs layer, 4 is a gate electrode in rectifying contact with the AlGaAs layer 3, 5 , 6 are source/drain ohmic electrodes made of AuGe alloyed up to the heterojunction interface, and 7 is an electron storage layer with high electron mobility formed in the GaAs layer 2 adjacent to the heterojunction. A major feature of this HEMT is that the electron mobility in the electron storage layer 7 becomes extremely large at low temperatures, for example 77°, where the effect of impurity scattering becomes the main cause of suppressing the electron mobility. Although the electron storage layer 7 is in the non-doped GaAs layer 2, it is extremely thin near the heterojunction and contains an n-type impurity that is generated within a range of about 100 Å and supplies electrons to the electron storage layer 7. Because it is spatially separated from the AlGaAs layer 3,
The electron mobility there will not be affected by impurity scattering. Extremely high electron mobilities are then achieved, especially at low temperatures where this impurity scattering effect prevents increases in electron mobility. It has been experimentally confirmed that this improvement in electron mobility reaches more than 10 times.

The HEMT shown in Fig. 1 uses a single electron storage layer 7, compared to a device that uses a large number of electron storage layers with a superlattice structure as a conductive channel, as disclosed in Japanese Patent Publication No. 55-500196. Since it is used as a conductive channel, it has the advantage of providing good FET characteristics. In particular, if parameters such as the n-type impurity concentration and thickness of the AlGaAs layer 3, the barrier height of the shot gate 4, and the difference in electron affinity between the two semiconductor layers are appropriately set, the AlGaAs layer 3 can be placed adjacent to the heterojunction. The depletion layer formed in the inner layer and the depletion layer generated by the shottock junction are connected in a thermal equilibrium state, and the n-type AlGaAs layer 3 is formed under the gate 4.
can be in a depleted state over the entire thickness. In this state, the electron concentration in the electron storage layer 7 is modulated with high sensitivity with respect to the voltage applied to the gate electrode 4, and the underlying non-doped GaAs layer 2 exhibits high resistance, so the impedance between the source and drain is only Therefore, the impedance between the source and drain is effectively modulated with high sensitivity to the gate input, and a high Gm is achieved. Furthermore, if the extension of the depletion layer is made large enough to reach a heterojunction, a HEMT with not only normally-on characteristics but also normally-off characteristics can be realized. In this way, the structure shown in Figure 1 has various excellent features among semiconductor devices that utilize a high-mobility electron storage layer at the heterojunction interface, and it is through this that a practical HEMT was realized for the first time. It is something that

The inventor further conducted various studies on the HEMT shown in Figure 1, and found that after heat treatment, the electron mobility in the electron storage layer tends to decrease, and this is due to the diffusion of impurities into the electron storage layer. , it is thought that the electron mobility decreases due to the impurity scattering effect. It is easy to infer that this impurity is mainly an n-type impurity, such as silicon, in the AlGaAs layer 3, which is essential for supplying electrons to the electron storage layer. It has been proposed in the past to leave the adjacent AlGaAs layer 3 undoped over a thickness of about 50 Å. However, this non-doped AlGaAs layer must be limited to an extremely thin thickness in order to generate the required electron storage layer, and it is difficult to completely suppress n-type impurity diffusion. The tendency for electron mobility to decrease is generally considered to be unavoidable.

On the other hand, according to experiments conducted by the present inventor, not only n-type impurities but also trace amounts of impurities that form deep levels to make the substrate 1 semi-insulating, such as chromium (Cr), have been detected in the electron storage layer. We obtained the knowledge that this further reduces the electron mobility. This Cr
Even though the non-doped GaAs layer 2 has a thickness of approximately 1 μm, it is distributed almost uniformly up to the heterojunction interface, and it is considered difficult to suppress the diffusion of impurities from the substrate 1 with the structure shown in Figure 1. It will be done.

Therefore, the present invention aims to provide a novel HEMT structure that can prevent impurities that create deep levels in a semi-insulating GaAs substrate from entering the electron storage layer and ensure high electron mobility. This is the purpose.

A semiconductor device according to the present invention is a semiconductor device having a GaAs layer with a low impurity concentration on a semi-insulating GaAs substrate, and an n-type AlGaAs layer provided in a heterojunction on the GaAs layer, board and said
The feature is that a non-doped high resistance AlGaAs layer is provided between the GaAs layer.

That is, according to the present invention, for example, the structure of FIG.
In HEMT, non-doped GaAs layer 2 and substrate 1
By interposing a non-doped high-resistance AlGaAs layer between the GaAs substrate 1 and the GaAs substrate 1, impurities such as Cr from the GaAs substrate 1 are prevented from diffusing and reaching the electron storage layer 7 during heat treatment, thereby preventing electrons in the electron storage layer This is intended to prevent a decrease in mobility. A thin non-doped AlGaAs layer like this
If a HEMT similar to the structure shown in Fig. 1 is created by first forming a GaAs layer on the substrate 1 and then growing it, and then growing a GaAs layer 2 on it,
After heat treatment, this non-doped AlGaAs layer contains Cr from the substrate at a higher concentration than the upper and lower GaAs layers.
It has been observed that the non-doped AlGaAs layer tends to contain impurities such as Cr, and it is therefore inferred that this non-doped AlGaAs layer has a gettering effect rather than blocking the diffusion of impurities such as Cr. In any case,
As will be described later, it has been confirmed that the presence of such an AlGaAs layer significantly alleviates the tendency for electron mobility to decrease in the electron storage layer due to heat treatment.

Embodiments of the present invention will be described in detail below. Second
The figure is a structural sectional view showing a field effect semiconductor device according to an embodiment of the present invention. In the figure, 11 is a Cr-doped semi-insulating GaAs substrate, 12 is a non-doped high resistance GaAs layer, 13 is also a non-doped high resistance AlGaAs layer, 14 is a non-doped GaAs layer, 15
is an n-type AlGaAs layer, 16 is an n-type GaAs layer, 1
7 is a shot gate electrode consisting of a three-layer structure of Ti-Pt-Au, 18 and 19 are AuGe ohmic electrodes for source and drain, 20 is a silicon dioxide (SiO ₂ ) film, and 21 is a heterojunction interface.
This is an electron storage layer formed on the GaAs layer 14 side.

Next, the manufacturing method and device parameters of the embodiment device shown in FIG. 2 will be explained in detail.

First, a Cr-doped semi-insulating GaAs substrate 1 is prepared and subjected to well-known molecular beam epitaxial growth (MBE).
The semiconductor layers 12 to 16 are successively grown on the substrate 1 by the technique. Parameters such as each layer thickness and impurity concentration are as follows. Non-doped GaAs layer 12
and the thickness of the non-doped AlGaAs layer 13 is 1000 mm.
Å, and the AlAs mixed crystal ratio of the AlGaAs layer 13 is 0.3.
That is, the composition was Al _0.3 Ga _0.7 As. non-dope
The thickness of the GaAs layer 14 is 6000 Å. n-type
The AlGaAs layer 15 uses 2x silicon as a donor.
Doped by 10 ¹⁸ cm ^-3 with a thickness of 600 Å
It is. This layer 15 also has a composition of Al _0.3 Ga _0.7 As. In particular, when these AlGaAs layers 13 and 15 have the same mixed crystal ratio,
This is convenient when performing multilayer growth using the MBE method. That is, while the molecular beam intensity of Al tends to fluctuate over time, the Al molecular beam intensity is monitored during the growth of the high-resistance layer 13 and adjusted to achieve the mixed crystal ratio required for the layer 15. If this is done, the Al molecular beams will continue to be generated in the same state during the subsequent growth of the GaAs layer 14, but they can be shielded by means such as a shutter.
When growing the AlGaAs layer 15, if the shielding of the Al molecular beam is removed, growth can be performed at the required mixed crystal ratio immediately after the start of growth. In HEMT, the characteristics of this heterojunction have a decisive influence on the device characteristics, so AlGaAs
It is particularly important to tightly control the composition of layer 15 immediately after the growth begins. Although not clearly shown in Figure 2, the AlGaAs layer 15 has a thickness of 50~
It is desirable to leave the layer undoped over 60 Å for the reasons mentioned above. It is well known that such precise impurity doping can be easily achieved in the MBE method by opening and closing a shutter that shields the impurity silicon molecular beam. The subsequently grown n-type GaAs layer 16 has a thickness of 500 Å and is doped with silicon as an n-type impurity at a concentration of 2×10 ¹⁸ cm ^-3 . It has the function of improving the ohmic contact properties of 19.

After the above-described continuous growth of the multilayer semiconductor layers 12 to 16 is completed, a SiO ₂ film 20 is deposited on the surface by well-known means such as sputtering. This _SiO2
After selectively removing the film 20 from above the source/drain contact regions and depositing the AuGe alloy,
This alloy film is patterned into the shape of the source/drain electrodes 18, 19, and then heat treated to form an alloy in order to ensure ohmic contact.
Instead of a single layer of AuGe alloy, a multilayer electrode structure of AuGe-Au may be used. Next, in the region where the gate electrode is to be formed, the SiO ₂ film 20 is selectively etched to open a window, and then GaAs is etched, and the etching is stopped when the AlGaAs layer 15 is exposed. Here, Ti, Pt, and Au are continuously deposited, and the AlGaAs in the etching groove is removed as shown in Figure 2.
The multilayer metal film 17 is patterned so as to remain on the exposed portion of the layer 15 to form a gate electrode 17, and the process is completed. In order to pattern the gate electrode 17, the photoresist film used for the selective etching of the SiO ₂ film 20 and the n-type GaAs layer 16 is left as is, and after continuous vapor deposition of a multilayer metal for the gate, A so-called lift-off method may be applied to remove the photoresist layer together with the metal film thereon.

The thus manufactured semiconductor device shown in FIG. 2 has a lower electron mobility in the electron storage layer 21 compared to a device having a structure without the non-doped AlGaAs layer 13, especially during various heat treatments essential during the manufacturing process. It was confirmed that there was little characteristic deterioration due to In addition, in FIG. 2, the AuGe alloy electrode 18,
Although the alloyed region between layer 19 and the underlying semiconductor is not shown, the layer 1
Alloying is performed by heat treatment until the heterojunction interface between 4 and 15 is reached.

Experimental results for clarifying the effect of the non-doped AlGaAs layer 13 on preventing a decrease in electron mobility according to the present invention will be described below. This experiment is the second
After heat-treating a substrate with a multilayer semiconductor structure similar to the HEMT shown in the figure at various temperatures, the electron mobility in the electron storage layer was measured to evaluate the tendency for mobility to decrease. . In order to make the effects of the present invention even more remarkable, we used a non-doped GaAs layer with a thickness of 1000 Å and a 1000 Å thick layer as samples.
Cr-doped semi-insulating non-doped AlGaAs layer
A total of four layers are laminated alternately on a GaAs substrate, and a non-doped layer corresponding to the GaAs layer 14 in Fig. 2 is placed on top of the GaAs substrate.
A substrate with a multilayer semiconductor layer structure having the same thickness, impurity doping concentration, and AlGaAs mixed crystal ratio as in FIG. 2 was prepared, except that the GaAs layer was provided with a thickness of 4000 Å.
Note that the layer corresponding to the AlGaAs layer 15 was undoped by a thickness of 60 Å from the interface with the underlying GaAs layer.

The comparative sample had exactly the same structure as the sample described above, except that a non-doped GaAs layer corresponding to layer 14 in FIG. 2 was grown directly on a semi-insulating GaAs substrate to a thickness of about 8000 Å.

After creating each of the above-mentioned samples by continuous multilayer growth using the MBE growth method, a SiO ₂ film with a thickness of 4000 to 5000 Å was formed on the surface by sputtering.
A 15 minute anneal in H ₂ was applied to each sample. For each sample after annealing, the electron mobility μ in the electron storage layer generated on the non-doped GaAs layer side at the interface with the n-type AlGaAs layer and the electron surface concentration Ns therein were measured. The measurement results are shown in FIG. In the figure, the horizontal axis is the annealing temperature, as-gr shows the state as grown, that is, without annealing, and the vertical axis shows the mobility μ and the electron surface concentration Ns. The measurements were conducted with each sample cooled to 77°C.

In FIG. 3, the measurement results are shown by white dots for the sample to which the present invention is applied, and by black dots for the comparative example. As is clear from this graph, as the temperature during heat treatment increases, the electron surface concentration Ns in the electron storage layer increases, and at the same time, the electron mobility μ decreases. From this, it is inferred that silicon diffuses from the n-type AlGaAs layer to the electron storage layer portion as a result of heat treatment. On the other hand, when comparing the inventive sample and the comparative example, it is found that there is almost no difference in electron mobility μ between the two samples without heat treatment, but after heat treatment at 700°C, the comparative example μ is 54000 (cm ² /V・
sec), whereas in the sample of the present invention it was 62000
(cm ² /V・sec), and after heat treatment at 730°C, the former had a value of 19000 (cm ² /V・sec), while the latter had a
27000 (cm ² /V·sec), which is a remarkable difference.
From this, it can be seen that the non-doped
The AlGaAs layer has the effect of suppressing impurities that create deep levels in the semi-insulating GaAs substrate from creeping up into the electron storage layer, and thereby has an excellent effect of mitigating the decrease in mobility caused by heat treatment. It is thought that it is played.

From the above experiments, it was found that the semiconductor device (HEMT) according to the present invention suppresses the tendency for the electron mobility in the electron storage layer, which should originally have high electron mobility, to decrease, especially due to heat treatment during the manufacturing process, and allows high speed operation. It can be seen that the operating characteristics can be secured. Although not mentioned in the explanation of Fig. 2, if the practical application of HEMT is to be further advanced, electronic The process of shallowly implanting impurity ions without affecting the storage layer is extremely useful, and the application of such ion implantation technology facilitates the integration of circuits, so it is necessary to make full use of ion implantation technology. The present invention is increasingly useful. In other words, if ion implantation is performed, annealing is essential to activate the implanted impurities, and the lower limit temperature is
In the case of AlGaAs, the temperature is approximately 700°C, so no matter how much effort is made to lower the temperature in other manufacturing processes, a significant decrease in electron mobility cannot be avoided. It is clear from FIG. 3 that the significant decrease in electron mobility that occurs when heat treatment at 700° C. or higher is included in the manufacturing process can be significantly alleviated by applying the present invention.

As described above, the present invention is based on GaAs with low impurity concentration.
In a semiconductor device (HEMT) that uses a high-mobility electron storage layer generated at a heterojunction between an n-type AlGaAs layer and an n-type AlGaAs layer as a conductive channel, impurities that form a deep level from a semi-insulating substrate creep up. In order to prevent this, at least one non-doped AlGaAs layer is interposed, thereby mitigating the decrease in electron mobility caused by the heating process and ensuring good high-speed operation characteristics. .

In addition, in a shot gate type FET that uses a normal n-type GaAs layer as a conductive channel, it has a high resistance.
The interposition of an AlGaAs layer as a buffer layer between a semi-insulating substrate is disclosed in, for example, Japanese Patent Laid-Open No. 54-12261.
As disclosed in the above-mentioned publication, such a known buffer layer is the non-doped buffer layer according to the present invention described above.
It is clear from the above explanation that the AlGaAs layer has completely different functions and effects. In addition, in the above example, the GaAs layer that forms the electron storage layer is all non-doped, but considering the operating principle of HEMT, the impurity concentration of this layer is n-type for electron supply.
If the impurity concentration is low, which is one order of magnitude lower than the donor concentration of the AlGaAs layer, the effect of improving electron mobility in the electron storage layer will occur, and the present invention has such an impurity concentration relationship. HEMT
It also extends to

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of the structure of a conventional high electron mobility transistor (HEMT), FIG. 2 is a cross-sectional view of the structure of a semiconductor device according to an embodiment of the present invention, and FIG. 2 is a graph showing the relationship between the heat treatment temperature and the electron mobility and electron surface concentration in the electron storage layer after the heat treatment. 11... Semi-insulating GaAs substrate, 12, 14... Non-doped GaAs layer, 13... Non-doped AlGaAs layer,
15...n-type AlGaAs layer, 16...n-type GaAs layer, 1
7... Multilayer electrode for gate, 18, 19... Source/drain electrode, 21... Electron storage layer.

Claims (1)

[Claims] 1 On a semi-insulating GaAs substrate, a substantially non-doped GaAs layer and an AlGaAs layer containing an n-type impurity are provided by forming a heterojunction on the GaAs layer.
a layer along the heterojunction interface;
A semiconductor device in which an electron storage layer with high electron mobility generated in a GaAs layer serves as a conductive channel, the GaAs layer being connected between the substrate and the GaAs layer and from the electron storage layer at the heterojunction interface. 1. A semiconductor device characterized in that a substantially non-doped AlGaAs layer is interposed at a position spaced apart from each other.