JPH0783029B2 - Semiconductor device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to a novel semiconductor device having a carrier traveling region of a superlattice structure formed by superposition of isotopes.

[Outline of Invention]

The present invention is based on the periodic stacking of a plurality of kinds of compound semiconductor thin films using the same constituent element but the isotopes having the same atomic number and different mass numbers as at least one corresponding element of the constituent elements. In a sense, it is chemically homojunction, but in terms of mass number, it forms a carrier traveling region of superlattice structure by superposition of heterojunctions.
Heterogeneous compounds, such as LO phonons (longitudinal optical phonons) can be confined and carrier lattice interactions can be modulated, and at the same time the crystal purity can be further improved by homogenizing the chemical properties of each compound semiconductor thin film in the superlattice structure. To solve various problems in superlattice structure by stacking semiconductor thin films.

[Conventional technology]

Recently, various semiconductor devices using a superlattice structure by repeatedly stacking a compound semiconductor thin film, for example, a GaAs compound semiconductor thin film and a different AlAs compound semiconductor thin film have been attempted.

A semiconductor layer having this type of superlattice structure can bring out various physical properties such as confining LO phonons and modulating carrier (for example, electron) -lattice interaction to improve carrier mobility. It is expected to be applied to various semiconductor devices. However, in the superlattice structure of the compound semiconductor thin film formed by this kind of different substances, the crystal purity problem due to a large amount of impurities being taken into one of the compound semiconductor thin films due to the difference in the chemical activity of each compound semiconductor thin film or the compound semiconductor thin film. There is a problem in that only the modulation of carrier-lattice interaction cannot be practically taken out due to the occurrence of a side effect such as a change in chemical and physical properties such as a mismatch of a heterojunction surface between thin films.

[Problems to be solved by the invention]

The present invention solves various problems in a superlattice structure due to periodic stacking of compound semiconductor thin films made of different materials in the semiconductor device having the above-mentioned superlattice structure, and enables high-speed operation semiconductor devices such as various field effect transistors (FETs) and hot transistors. A semiconductor device suitable for application to an electron transistor (HET), a heterojunction bipolar transistor (HBT), etc. is provided.

[Means for solving problems]

The present invention provides a carrier traveling region having a superlattice structure by periodically laminating a plurality of types of compound semiconductor thin films. Each compound semiconductor thin film forming this superlattice structure is made of compound semiconductors that are isotopes. Then, for the element having a small atomic number that constitutes the compound semiconductor, the thickness of the compound semiconductor thin film containing this isotope having a smaller mass number is d _1, and the thickness of the compound semiconductor thin film containing the other isotope having a larger mass number is Let d be ₂ (Vs is the saturation velocity of the electron, m is the effective mass of the electron, Is selected as h / 2π and h is Planck's constant), and the thickness of the other compound semiconductor thin film is selected so that its thickness d ₂ is several atomic layers or less.

[Action]

The superlattice structure having the above-described structure has a periodic laminated structure of a plurality of kinds of compound semiconductor thin films, but chemically shows the same characteristics due to the use of an isotope as a constituent element of each compound semiconductor thin film, and has physical properties. Specifically, it is possible to confine LO phonons, which is a characteristic peculiar to the superlattice structure.

Explaining this, there are isotopes having the same atomic number but different mass numbers, and the heavier the element, the broader the mass number. Examples of this isotope include Ga ⁶⁹ ,
Mention may be made of ^{Ga 71, Zn 64 ~Zn 70,} Se 74 ~Se 82 or the like. In this case, the dispersion relation of GaAs is Ga ⁶⁹ As ⁷⁵ and Ga ⁷¹ As ⁷⁵ as shown in Fig. 3 even if Ga has a small mass difference.
There is a slight misalignment in the branch. In FIG. 3, the horizontal axis represents qa / π (q is wave number, a is a value of 1/2 of lattice constant).
Now, when the isotope superlattice structure semiconductor layer is GaAs, that is, the first compound semiconductor thin film is Ga ⁶⁹ As ⁷⁵ , and the second compound semiconductor thin film is
Considering the case where the compound semiconductor thin film of is Ga ⁷¹ As ⁷⁵ and these are alternately repeated and laminated, in this case, the difference between the LO phonon frequencies of Ga ⁶⁹ As ⁷⁵ and Ga ⁷¹ As ⁷⁵ is
As shown in Fig. 3, each LO branch has q =
Due to the property of becoming completely flat at 0, the Ga ⁶⁹ As ^75- like LO phonon decays to 1 / e in Ga ⁷¹ As ^75. Damping length = 1 / K = a / 0.2π4Å (K is an imaginary number) The wave number is K = 0.2π / a). In other words, if the thickness d ₂ of the semiconductor thin film Ga ⁷¹ As ⁷⁵ of the second compound containing the isotope with a large mass number is selected to be a few atomic layers or less, the Ga ⁶⁹ As ^75- like phonon will be rapidly attenuated here. I understand that it will be done. By the way, this attenuation is due to GaAs in the superlattice structure of different materials of GaAs and AlAs.
The steepness is almost the same as the attenuation of the typical LO phonon in AlAs. On the other hand, the Ga ⁷¹ As ^75- like LO phonon is q0 in the Ga ⁷¹ As ⁷⁵ semiconductor thin film, but when it enters Ga ⁶⁹ As ⁷⁵ , the wave number q = 0.25π / a (wavelength λ = 8a, which is 4 atoms). (Compared to the confinement of a layer to a semiconductor thin film)).

So, for example, Ga ⁶⁹ As ^75- like LO phonons and Ga ⁷¹ As ⁷⁵
Manner respectively to that shown in Figure 4 A and B the wave function of the LO phonons, Ga ⁶⁹ As ⁷⁵ basis LO phonon is hardly present in the Ga ⁷¹ during As trapped in Ga ⁶⁹ As, also Ga ⁷¹ As
^{The 75} -type LO phonon can be q0 in Ga ⁷¹ As ⁷⁵ , but has a large wave number of q = 0.25π / a in Ga ⁶⁹ As ⁷⁵ .

By localizing the LO phonons by the superlattice structure in this way, it is possible to realize a high-speed semiconductor device or the like by modulating the momentum and hence the probability of scattering with electrons (carriers).

That is, the LO phonon-electron scattering probability W is LO phonon-electron scattering matrix elements such as;
It can be written using M _LO-e and final density of states ρ (f),
This M _LO-e is proportional to the reciprocal of phonon momentum, so now
Assuming that the thickness of the first compound semiconductor thin film made of Ga ⁶⁹ As ^{75 with} a small mass number is d ₁ , the local wave vector (= π / d ₁ ) due to confinement is the momentum (= mV
s) conditions to make it larger, That is, If so, M _LO-e becomes sufficiently smaller than that in bulk.
For example, for Ga ⁶⁹ As ⁷⁵ LO phonons, Is achieved, and this LO phonon wavefunction is forbidden at q = 0 because the possible phonon momentum q is remarkably limited as shown in Fig. 4A. M _LO-e is a bulk in the above superlattice. It becomes extremely small compared to the time. Therefore, Ga
The scattering probability of electrons due to ⁶⁹ As ^75- like LO phonons becomes very small. In addition, Ga ⁷¹ As ^75- like LO phonon scattering is q >> 0 in Ga ⁶⁹ As ⁷⁵ , so the scattering probability is small. Therefore, the only scattering is Ga ⁷¹ As from ⁷⁵ to ⁷⁵ Ga ⁷¹ As due to LO phonons.
Electron scattering in ⁷⁵ . By the way, while d ₁ can take values in the range of several atomic layers to several tens of atomic layers, Ga ⁷¹ As ⁷⁵ is almost 2
In the atomic layer, the above Ga ⁶⁹ As ^75- like LO phonon confinement can be accomplished, so the ratio becomes 1 / 3d ₂ / d ₁ 1/40, and the spatial probability is calculated for the scattering probability of the entire superlattice structure. This space can be reduced to 1/10 or less of the usual value by setting d ₂ to a few atomic layers or less, and the LO phonon scattering probability is Ga ⁶⁹ As ⁷⁵ −Ga ⁷¹ As.
^{In a 75} superlattice structure, it can be about an order of magnitude smaller than GaAs.
Therefore, scattering of electrons due to LO phonons is suppressed, and high electron mobility is expected.

In the above description, the Ga ⁶⁹ As ⁷⁵ -Ga ⁷¹ As ⁷⁵ superlattice structure has been described, but other combinations such as Zn ⁶⁴ Se ⁷⁴ -Zn ⁷⁰ Se.
⁸² or Zn ⁶⁴ Se ⁸² −Zn ⁷⁰ Se ⁷⁴
As shown in FIGS. 5 and 6, the LO phonon branch exhibits similar characteristics to the Ga ⁶⁹ As ⁷⁵ −Ga ⁷¹ As ⁷⁵ example described in FIG. Even when is used, electron (carrier) scattering can be suppressed, and thus high electron mobility can be obtained.

In the case of the above superlattice structure, the constituent semiconductor thin films are chemically stable and homogeneous because the constituent semiconductor thin films are of the same composition and the same element, and therefore, the deviation of impurities and the crystallinity are less likely to occur, and each semiconductor thin film Since no hetero-barrier occurs in the superlattice structure due to the different kinds of thin films, the carrier, for example, the electron transport direction is the stacking direction of the superlattice structure, that is, the direction along the junction surface, but in the direction orthogonal to this. Regardless of whether it is present or not, it can be treated as if it were a single semiconductor bulk.

[Embodiment] An example of the FET according to the present invention will be described with reference to FIG.
In this case, a superlattice structure semiconductor layer (3) made of the above-mentioned isotope, which becomes a carrier transit region, is formed on a substrate (11), for example, a semi-insulating GaAs substrate. The superlattice structure semiconductor layer (3) is, for example, a compound semiconductor thin film (1) with Ga ⁶⁹ As ⁷⁵ and a thickness d ₁ 200 Å and a compound semiconductor thin film (2) with a thickness d ₂ of Ga ⁷¹ As ⁷⁵ of 2 atomic layers. And are sequentially and alternately grown epitaxially. The formation of the superlattice structure semiconductor layer (3), that is, the formation of the compound semiconductor thin films (1) and (2) is performed by, for example, a metal organic chemical vapor deposition method, that is, a MOCVD method (Metal Organic).
Chemical Vapor Deposition) or molecular beam epitaxy, that is, MBE (Molecular Beam Epitaxy). Then, a Schottky gate electrode G is formed on the superlattice structure semiconductor layer (3), and a source electrode S and a drain electrode D are ohmicly deposited on both sides thereof to form a Schottky gate type FET.
In this case, a channel, that is, a carrier, for example, an electron transit region is formed in the superlattice structure semiconductor layer (3) under the gate electrode G. Then, in this case, in the superlattice structure semiconductor layer (3), an n-type or p-type impurity is doped at a required concentration similarly to a normal single-layer compound semiconductor layer, so that the An FET that uses electrons as carriers or holes as carriers is configured.

Further, FIG. 2 shows a case where the present invention is applied to a double junction bipolar transistor HBT. In this example, n
In the case of configuring a pn type HBT, in this case, for example, an n type low resistivity GaAs emitter electrode take-out layer (14) is epitaxially grown on a semi-insulating GaAs semiconductor substrate (11), and it will be described later. N with a large band cap that forms a hetero-type emitter pn junction Jε with the base layer (16)
Epitaxially grow an emitter layer (15) of a type, for example an AlGaAs semiconductor layer. And this emitter layer (15)
A base layer (16) consisting of the semiconductor layer (3) having the above-mentioned isotope superlattice structure is epitaxially grown on the n-type Al having a large band cap.
Epitaxial layer made of GaAs, that is, collector layer (1
7) and a similar n-type GaAs semiconductor layer (1
8) is epitaxially grown to form a collector junction Jc by a heterojunction pn junction between the collector layer (17) and the base layer (16), and an n-type low specific resistance GaAs semiconductor, for example, is formed on the collector junction Jc. Collector electrode extraction layer (1
9) Epitaxially grow. Each of these layers (14) (1
5) Further, each compound semiconductor thin film of the superlattice structure semiconductor layer (3) constituting the base layer (16), and further layers (17) (18)
(19) can be formed by continuous epitaxial growth. Then, a part of the semiconductor layers (18) and (17) is removed by etching, a part of the base layer (16) is exposed to the outside, and the base electrode B is ohmically adhered to the exposed part. Part of the layers (19) (18) (17) and (16) is removed by etching to expose a part of the emitter electrode extraction layer (14) to the outside, and the emitter electrode E is ohmicly deposited there. A collector electrode C is ohmicly deposited on the collector electrode take-out layer (19).

In the example shown in FIG. 2, the present invention is applied to an npn type HBT, but it may be a pnp type.

Furthermore, the present invention is not limited to the Schottky type FET described above, but can be applied to various FETs, for example, a secondary electron gas type FET, or other various semiconductor devices.

〔The invention's effect〕

As described above, according to the present invention, the mobility of carriers can be reduced by confining LO phonons by making the carrier traveling region a superlattice structure with respect to the mass number and reducing the scattering probability due to the carrier-LO phonon interaction. Modulation, that is, for example, carrier mobility can be increased, and a semiconductor device capable of high-speed operation can be obtained, and, despite having a superlattice structure, each of the constituent semiconductor thin films has an isotope composition. As a result, since the chemical characteristics can be made uniform, inconveniences such as inconsistency between the semiconductor thin films and heavy effective mass can be effectively avoided, and a semiconductor device having stable characteristics can be realized.

[Brief description of drawings]

1 and 2 are enlarged schematic sectional views of respective examples of the semiconductor device according to the present invention, FIGS. 3, 5, and 6 are LO phonon dispersion diagrams of respective compound semiconductors, and FIG. 4 is It is a wave function curve figure. (11) is a semiconductor substrate, and (3) is a superlattice structure semiconductor layer made of an isotope.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location H01L 29/68 29/73 29/778 29/812 H01L 29/203 29/72

Claims (1)

[Claims] 1. A carrier transit region having a superlattice structure formed by periodically laminating a plurality of types of compound semiconductor thin films, each of the compound semiconductor thin films being made of compound semiconductors that are mutually isotopes, and constituting the compound semiconductor. Regarding the compound semiconductor thin film of the above 1 which contains an element whose atomic number is young and whose mass number is small, the thickness d ₁ is (Vs is the saturation velocity of the electron, m is the effective mass of the electron, Is h / 2π and h is Planck's constant), and the thickness of the compound semiconductor thin film containing the isotope with a large mass number is
A semiconductor device characterized in that d ₂ is selected to have a thickness corresponding to several atomic layers or less.