JPH0815212B2 - Semiconductor device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor device, in particular, various compound semiconductor devices for epitaxially growing a compound semiconductor layer on a GaAs substrate to form an operating region, for example, a FET using a two-dimensional electron gas channel. , Related to compound semiconductor devices such as so-called HEMT.

SUMMARY OF THE INVENTION The present invention forms a high-quality compound by forming a buffer of (AlAs) _m (GaAs) _n superlattice structure on a GaAs substrate and epitaxially growing a compound semiconductor, for example, an AlGaAs-based compound semiconductor on the buffer. A semiconductor layer is formed.

[Conventional technology]

In a compound semiconductor device such as HEMT, for example, each AlGaAs semiconductor layer on a semi-insulating GaAs substrate doped with Cr, for example, an undoped GaAs semiconductor layer, an n-type AlGaAs semiconductor layer, and a GaAs semiconductor layer further thereon. Are, for example, MOCV
By the D (Metalorganic Chemical Vapor Deposition) method or the MBE (Molecular Beam Epitaxy) method, epitaxial growth is sequentially performed to form its operation region, that is, in the HEMT, each region including a region for forming a two-dimensional electron gas channel is formed. In this case, in order to obtain a high-quality epitaxial layer, the undoped GaAs layer in the portion forming the operating region, such as the HEMT described above, is several tens of μm.
It was necessary to grow it up to a certain thickness. This is because a thick growth film is formed in order to provide a function as a buffer layer so that diffusion of impurities such as Cr from the GaAs substrate and propagation of crystal defects do not occur from the interface side of the GaAs substrate. The buffer layer is intervened.

In recent years, attempts have been made to perform epitaxial growth through a buffer layer having a superlattice structure as a measure to obtain a high-quality epitaxial layer even if the epitaxial growth layer is thinned. For example, it has been reported that the optical properties of quantum well layers continuously grown on a superlattice buffer layer are improved (Fujii et al .; Spring 1984, Applied Physics Society, Proceedings 29P).
N-1, Arnold and others;
E-Electron Device Letters (IEEE Electron
Device Letters) Vol. EDL-5, No. 3, 82 (1984)).
However, in these reports as well, no report has been made on the investigation and knowledge of the mechanism of the superlattice buffer layer acting as a buffer layer and the optimum structure.

[Problems to be solved by the invention]

The present invention provides an effective structure based on the investigation of the function of the compound semiconductor layer as a superlattice buffer layer particularly when the compound semiconductor layer is epitaxially grown on a GaAs substrate. It is possible to form a high-purity and high-quality epitaxial layer in various semiconductor devices, for example, compound semiconductors such as HEMT, and improve the characteristics in various semiconductor devices, for example, HEMT.

[Means for solving problems]

In the present invention, as shown in FIG. 1, a desired operating region is formed on a semi-insulating GaAs substrate (1) doped with Cr, for example, through a superlattice buffer layer (2) having a specified structure. Semiconductor layer such as AlGaAs or AlGaInP
A semiconductor layer (5) of a system or the like is continuously formed by a series of epitaxial processes such as MOCVD or MBE.

The superlattice buffer layer (2) is (AlAs) _m (GaAs) _n ,
A superlattice structure having one period or more, with 6 ≦ m ≦ 12 and 6 ≦ n ≦ 12. That is, it was found that impurities are selectively gettered in the AlAs layer, and the thickness of the AlAs layer must be 6 atomic layers or more (m ≧ 6) for gettering. This improves the purity of the sandwiched GaAs and therefore the
The purity of GaAs will also be improved. AlAs m atomic layer (3)
And a n-atom layer (4) of GaAs are repeatedly laminated. The AlAs layer and GaAs can be selected so that n = m.

[Action]

As described above, the superlattice buffer layer (2) is formed by stacking atomic layers of (AlAs) _m and (GaAs) _n for one cycle or more on the semi-insulating GaAs substrate, and the semiconductor layer ( In the case of epitaxially growing 5), the semiconductor layer (5) is of high quality and high purity, that is, the invasion of Cr impurities from the substrate is suppressed. Therefore, a semiconductor layer having high electron mobility can be formed.

This is because impurities (eg, Cr) mixed during epitaxial growth from the GaAs substrate are get ringed by the AlAs layer by the superlattice buffer layer (2) of (AlAs) _m (GaAs) _n . This gettering of AlAs is suggested by the results of Raman scattering of mixed crystal and superlattice as shown in FIG. That is, even with an AlGaAs ternary mixed crystal, (AlAs) _m (G
Although two peaks of GaAs-like LO phonon and AlAs-like LO phonon can be seen by Raman scattering even in the aAs) _n superlattice, the intensity ratio of the latter, I _Al / I _Ga , to the former is not abundant in the mixed crystal. You can see that That is, in the figure
The mark and the circle mark are the Al _x Ga _1-x As mixed crystal and (AlAs) _m (GaA
s) I _Al / when changing each x / (1-x) (where x is the Al fraction and (x-1) is the Ga fraction) with the _n superlattice
I _Ga is shown, but in the case of a mixed crystal, x / (1-
This ratio is much smaller than x / (1-x) when it is made into a superlattice, in contrast to the straight line of x). Since the phonon peak intensity is generally proportional to the quality of the crystal, this seems to indicate that the quality of the AlAs layer deteriorates while the quality of the GaAs layer improves. In other words, it seems that gettering by AlAs occurs.

4, 5, 6 and 7 are (AlAs) _{m, respectively.}
(GaAs) _n superlattice structure m = n, and n = 6,8,12,2, respectively
The Raman spectrum is shown in Fig. 8. Fig. 8 shows Al _0.5 G
A similar Raman spectrum of a mixed crystal of a _0.5 As is shown. Curves (31) and (32) in FIG. 9 are (AlAs) _m (GaAs), respectively.
_This figure shows the dependence of each peak intensity of AlAs-like LO phonons and GaAs-like LO phonons on the n value in the Raman spectrum of n superlattice structure where m = n. Are plots of the measured values of the respective peak intensities when the respective n values of the AlAs-like phonon and the GaAs-like phonon are changed. According to this, AlAs
The LO intensity of the LO phonon is almost constant when n = 6 or more, and the intensity of the LO phonon of GaAs increases with the increase of n, and the peak intensity of the LO phonon of AlAs is m = n = 6 to 12.
It can be seen that it is sufficiently smaller than the GaAs-like LO phonon peak intensity. That is, since the peak intensity of phonons is proportional to the lifetime of LO phonons, it can be seen that the lifetime of phonons increases as n increases, that is, the purity of GaAs improves. Further, FIG. 10 shows the dependence of the half width of the LO phonon peak on the similar n value. In FIG. 10, the circles indicate the half width of the GaAs LO phonon.
The triangle marks show that of AlAs-like LO phonons. According to this, it can be seen that the half-widths decrease with increasing n and become almost constant when n = 6 or more. That is, since the inverse width of the phonon peak is proportional to the reciprocal of the LO phonon lifetime, it can be seen that the damping of the LO phonons decreases and the crystallinity of the epitaxial layer improves as n (= m) increases. That is, in the (AlAs) _m (GaAs) _n superlattice structure, impurities mixed during the epitaxial growth are gettered by the AlAs layer, and the n = m = 6 to 12 GaAs layer and the AlAs layer are improved in purity. It is shown that this structure is effective as a buffer layer.

〔Example〕

FIG. 2 shows an example in which the present invention is applied to a HEMT. In this example, a super-insulating (AlAs) ₆ (GaAs) _{6 having} a period of 2 to 3 is formed on a semi-insulating GaAs substrate (1) doped with Cr. A lattice buffer layer (2) is formed on which a first semiconductor layer (13) made of a 2 μm thick GaAs layer which is not doped with impurities, and a second semiconductor made of a non-doped AlGaAs layer thereon. A layer (14), on top of which, for example, n-type Al _0.3 Ga
_A third semiconductor layer (15) consisting of a _0.7 As layer was formed. These layers (2), (13) to (15) can be formed by a series of epitaxial processes by continuously switching the source gas supplied by the MOCVD method or the MBE method. And
A Schottky gate electrode (16) was deposited on the layer (15), and source and drain electrodes (17) and (18) were ohmicly deposited on both sides of the Schottky gate electrode (16). In this case, GaAs
The first semiconductor layer (13) of the AlGaAs second semiconductor layer (1
A two-dimensional electron gas channel is formed at the interface with 4), but since the interface portions of these layers (13) and (14) have excellent purity and crystallinity, higher electron mobility can be obtained. Therefore, a HEMT with high speed can be configured.

Further, the present invention is not limited to the above-mentioned examples, and in various compound semiconductor devices, when the operation layer or the active layer is epitaxially grown on the GaAs substrate, the buffer layer (2) is formed on the substrate, and the object is formed thereon. It is possible to form an operating layer or an active layer.

〔The invention's effect〕

As described above, in the present invention, (AlAs) _m (GaAs)
very thin buffer layer of _n of m = n = 6 to 12 with 2 to 3 periods of the superlattice structure (2) has the advantages of gettering impurities such as Cr from the substrate (1) side only provided Since it is possible to improve the purity of the epitaxial semiconductor layer formed thereon and to form a high-quality semiconductor layer with high carrier mobility, it is possible to improve the characteristics by using it when epitaxially operating regions of various compound semiconductor devices. be able to.

[Brief description of drawings]

FIG. 1 is an enlarged schematic cross-sectional view of a main part of an example of the device of the present invention,
FIG. 2 is a schematic enlarged cross-sectional view of an example in which the present invention is applied to HEMT, and FIGS. 3 to 7 are (AlAs) _m (GaAs) _{n, respectively.}
8 Raman spectrum of AlGaAs mixed crystal, and Figures 9 and 10 show (AlAs) _m (GaA
s) It is a figure which shows the dependence with respect to the value of n of the phonon peak intensity and half width of the Raman spectrum of _n (m = n). (1) is a GaAs substrate, and (2) is a superlattice buffer layer.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H01L 29/812

Claims (1)

[Claims] 1. On a GaAs substrate, (AlAs) _m (GaAs) _n with 6 ≦
It is formed by epitaxially growing a semiconductor layer to be an operating region through a buffer layer having a superlattice structure of m ≦ 12, 6 ≦ n ≦ 12 for one period or more.
A semiconductor device characterized in that the intensity of the As-like LO phonon peak is low and impurities from the GaAs substrate are gettered into AlAs.