WO2002033758A1

WO2002033758A1 - An isotope structure formed in an indirect band gap semiconductor material

Info

Publication number: WO2002033758A1
Application number: PCT/AU2001/001305
Authority: WO
Inventors: Kohei Itoh
Original assignee: Silex Systems Limited
Priority date: 2000-10-17
Filing date: 2001-10-17
Publication date: 2002-04-25
Also published as: AUPR083300A0

Abstract

A semiconductor structure has a density variation in a predetermined direction, and an external stress is applied perpendicular to the predetermined direction. The structure can be a stressed isotope superlattice (10), with density variation parallel to the crystallographic direction of the dominant intervalley scattering (for example, [100] for silicon, and [111] for germanium), and with in-plane stress arising from lattice mismatch with a single crystal overlayer or substrate (11). The external stress may also be applied mechanically, and serves to suppress or enhance phonon scattering in intervalley directions perpendicular to the predetermined direction. Applications include optical devices, transistors, quantum wires and quantum dots.

Description

TITLE: AN ISOTOPE STRUCTURE FORMED IN AN INDIRECT BAND GAP SEMICONDUCTOR MATERIAL

FIELD OF THE INVENTION

The present invention relates to an isotope structure and in particular to an isotope structure formed in an indirect band gap semiconductor material.

The invention has been developed primarily for advanced integrated electronic and optical devices and will be described hereinafter with reference to those applications. However, the invention is not limited to those particular fields of use and is also applicable to many other areas. DISCUSSION OF THE PRIOR ART

A wide variety of today's semiconductor devices such as LSI circuits, VLSI circuits, ULSI circuits, high-frequency devices, light-emitting and detecting devices, are fabricated using semiconductor elements such as Si, Ge, Ga and As, amongst others. Si is preferred in many LSI circuit applications due to its low market price, ease of fabrication, and chemical purity. However, the application of Si to high frequency devices is limited due to both the relatively low mobility of the carriers and the indirect band-gap structure of Si. Accordingly, the fabrication of many of today's high frequency devices, as well as light emitting and detecting devices, are not based on Si.

Where high frequency operation is the critical design factor, it is preferable to select semiconductor materials from those having higher carrier mobility, such as the compound semiconductors. This facilitates the fabrication of devices with broader operational frequency bands than can be covered by the single materials such as Si.

The carrier mobility in a number of compound semiconductors, for example GaAs, is much higher than that in Si and, as a result, a large number of today's high frequency devices are made of compound semiconductors instead of Si. Two theoretical approaches have been proposed to increase the carrier mobility in Si; these being:

1. The application of external stress; and

2. Decreasing the dimension of Si active regions to the quantum limits. In the first approach mentioned above, the application of the external stress has been realised by epitaxial growth of Si thin films on top of substrate single crystals. For example, the growth of a thin film of SiGe alloy which has a slightly different lattice constant from that of the Si. In the second approach mentioned above, there has been some work done to achieve small active regions approaching the quantum limit using selective growth and etching of Si-based structures.

Notwithstanding these efforts, neither approach has provided Si based high frequency devices and light emitting and detecting devices of superior performance to those made of compound semiconductors. Therefore, compound semiconductors still dominate today's market of high frequency and optical semiconductor devices. However, these compound semiconductors are expensive, difficult to process and handle, and not entirely compatible with the Si-based materials that included in present- day LSI circuits.

It has been conventional wisdom that it is not possible to change the intrinsic properties of bulk semiconductor materials without applying external perturbations such as stresses, electric fields, magnetic fields, or the like. However, it has been realised very recently that the control of isotopic composition can lead to changes in the intrinsic properties, including carrier mobility and the efficiency to emit and detect light. More particularly, in Japanese patent application 7-83029 it is disclosed that thin layers of different isotopic composition GaAs are stacked in sequence. The rationale is that this should increase the electron mobility in the GaAs by forming GaAs isotope superlattices. It is suggested that the scattering of moving electrons by longitudinal optical (LO) phonons is suppressed since the phonons, which act as scatterers, are confined in certain isotopic layers due to the artificially created periodicity of the atomic mass. On the other hand, a PCT application having the publication number

WO96/25767, describes a method to increase carrier scattering by certain phonons by making isotope superlattices. Such a phonon resonator selectively increases the density of phonons whose momentum corresponds to a change in the wavevectors for indirect band to band electron-hole recombination and for intervalley electron scattering. It is postulated that by carefully adjusting the thickness of layers composing the isotope superlattice, the phonon resonator allows for more efficient scattering of carriers by phonons. In turn, more carrier scattering by specific phonons leads to more efficient band to band electron-hole recombination for light emission and detection, and to more efficient intervalley electron scattering for formation of electron-electron pairs that may eventually increase the carrier mobility.

Neither of the proposals disclosed in these two applications has been confirmed theoretically nor experimentally.

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. DISCLOSURE OF THE INVENTION

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. According to a first aspect of the invention there is provided an isotope structure formed in an indirect band gap semiconductor material, the structure including: a first region in the material having a first density; and a second region in the material having a second density and being adjacent to the first region whereby the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material, the material having stress applied externally in a direction substantially perpendicular to the predetermined direction.

Preferably, the material includes a plurality of first and second regions that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

According to a second aspect of the invention there is provided a semiconductor isotope superlattice composed of indirect band gap semiconductor elements such as, for example, Si and Ge of periodically varying density comprising: at least one first region of a first density; and at least one second region of a second density, said first and second regions being adjacent to one another and alternating in said structure, being substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the given indirect band-gap semiconductor material, with stress applied externally in the direction substantially perpendicular to the direction of the density periodicity of said structure.

Preferably, said first region comprises a first layer of one to sixty atomic layers thickness and said second region comprises a second layer of one to sixty atomic layers thickness. Preferably also, semiconductor isotope superlattice is grown epitaxially on a single crystal substrate whose lattice constant is different from that of said superlattice in order to apply in-plane stress to said semiconductor isotope superlattice.

In a preferred form, the semiconductor isotope superlattice is one in which a single crystal over-layer whose lattice constant is different from that of said superlattice is grown in order to apply in-plane stress to said semiconductor isotope superlattice.

Preferably, the semiconductor isotope superlattice is stressed externally by means of mechanical methods.

According to a third aspect of the invention there is provided a semiconductor quantum wire composed of indirect band gap semiconductor elements such as, for example, Si and Ge of periodically varying density comprising: at least one first region of a first density; and at least one second region of a second density, said first and second regions being adjacent to one another and alternating in said structure, being substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the given indirect band-gap semiconductor material, with stress applied externally in a direction substantially perpendicular to the direction of the density periodicity of said structure.

Preferably, said first region comprises a first layer of one to sixty atomic layers thickness and said second region comprises a second layer of one to sixty atomic layers thickness.

Preferably also, the semiconductor isotope quantum wire is grown epitaxially on a single crystal substrate whose lattice constant is different from that of said quantum wire in order to apply in-plane stress to said semiconductor isotope quantum wire. In a preferred form the semiconductor isotope quantum wire is one in which a single crystal over-layer whose lattice constant is different from that of said quantum wire is grown in order to apply in-plane stress to said semiconductor isotope quantum wire.

Preferably, the semiconductor isotope quantum wire has been stressed externally by means of mechanical methods.

According to a fourth aspect of the invention there is provided a semiconductor quantum dot composed of indirect band gap semiconductor elements such as, for example, Si and Ge of periodically varying density comprising: at least one first region of a first density; and at least one second region of a second density, said first and second regions being adjacent one another and alternating in said structure, being substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the given indirect band-gap semiconductor material, with stress applied externally in a direction substantially perpendicular to the direction of the density periodicity of said structure.

Preferably also, the semiconductor isotope quantum dot is grown epitaxially on a single crystal substrate whose lattice constant is different from that of said quantum dot in order to apply in-plane stress to said semiconductor isotope quantum dot.

In a prefened form the semiconductor isotope quantum dot is one in which a single crystal over-layer whose lattice constant is different from that of said quantum dot is grown in order to apply in-plane stress to said semiconductor isotope quantum dot. ,

According to a fifth aspect of the invention there is provided a semiconductor isotope superlattice formed in an indirect band gap semiconductor material, the superlattice including: a first region in the material having a first density; and a second region in the material having a second density and being adjacent to the first region whereby the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material, the material having stress applied externally in a direction substantially perpendicular to the predetermined direction.

Preferably, the material includes a plurality of first and second regions that are alternated with each other such that the density of the material varies periodically in the predetermined direction. Preferably also, the first region includes a first layer of one to sixty atomic layers thickness and the second region includes a second layer of one to sixty atomic layers thickness. More preferably, the material is grown epitaxially on a single crystal substrate. Even more preferably, the substrate has a lattice constant that differs from that of the material such that in-plane stress is applied to the superlattice. In a preferred form, the superlattice includes a single crystal over-layer. More preferably, the over-layer has a lattice constant that differs from the lattice constant of the material such that in-plane stress is applied to the material.

Preferably, the material is stressed externally by mechanical means. According to a sixth aspect of the invention there is provided a semiconductor quantum wire formed in an indirect band gap semiconductor material, the wire including: a first region in the material having a first density; and a second region in the material having a second density and being adjacent to the first region whereby the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material, the material having stress applied externally in a direction substantially perpendicular to the predetermined direction.

Preferably also, the first region includes a first layer of one to sixty atomic layers thickness and the second region includes a second layer of one to sixty atomic layers thickness. More preferably, the material is grown epitaxially on a single crystal substrate. Even more preferably, the substrate has a lattice constant that differs from that of the material such that in-plane stress is applied to the material.

In a prefened form, the material includes a single crystal over-layer. More preferably, the over-layer has a lattice constant that differs from the lattice constant of the material such that in-plane stress is applied to the material. Preferably, the material is stressed externally by mechanical means.

According to a seventh aspect of the invention there is provided a semiconductor quantum dot formed in an indirect band gap semiconductor material, the dot including: a first region in the material having a first density; and a second region in the material having a second density and being adjacent to the first region whereby the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material, the dot having stress applied externally in a direction substantially perpendicular to the predetermined direction.

In a preferred form, the dot includes a single crystal over-layer. More preferably, the over-layer has a lattice constant that differs from the lattice constant of the material such that in-plane stress is applied to the material.

Preferably, the material is stressed externally by mechanical means.

According to an eighth aspect of the invention there is provided a method of forming a semiconductor isotope superlattice in an indirect band gap semiconductor material, the method including: providing in the material a first region having a first density; providing in the material a second region having a second density such that the first and second regions are adjacent to each other and the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material; and applying stress externally to the material in a direction substantially perpendicular to the predetermined direction. Preferably, the method also includes forming a plurality of first and second regions in the material that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

According to a ninth aspect of the invention there is provided a method of forming a semiconductor quantum wire in an indirect band gap semiconductor material, the method including: providing in the material a first region having a first density; providing in the material a second region having a second density such that the first and second regions are adjacent to each other and the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material; and applying stress externally to the material in a direction substantially perpendicular to the predetermined direction.

Preferably, the method includes the step of forming a plurality of first and second regions in the material that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

According to a tenth aspect of the invention there is provided a method of forming a semiconductor quantum dot in an indirect band gap semiconductor material, the method including:

providing in the material a first region having a first density; providing in the material a second region having a second density such that the first and second regions are adjacent to each other and the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material; and applying stress externally to the material in a direction substantially perpendicular to the predetermined direction.

Preferably, the method includes the step of forming a plurality of first and second regions in the material that are alternated with each other such that the density of the material varies periodically in the predetermined direction. According to an eleventh aspect of the invention there is provided a semiconductor structure including: a many-valley semiconductor material; a first region formed in the material and having a first density; and a second region formed in the material, the second region having a second density and being adjacent to the first region whereby the density of the material varies in a predetermined direction for suppressing the electron-phonon interactions corresponding to intervalley electron scattering of electrons by phonons.

According to a twelfth aspect of the invention there is provided a semiconductor structure including: a many-valley semiconductor material; a first region formed in the material and having a first density; and a second region formed in the material, the second region having a second density and being adjacent to the first region whereby the density of the material varies in a predetermined direction for enhancing the electron-phonon interactions corresponding to intervalley electron scattering of electrons by phonons. Preferably, the material includes a plurality of first and second regions that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

According to another aspect of the invention there is provided a semiconductor integrated circuit including a plurality of active semiconductor devices, at least one of the devices being selected from the group comprising: a superlattice as described above; a wire as described above; and a dot as described above.

In some preferred embodiments of the invention the isotope structures provide improved performance of electronic devices. In other preferred embodiments of the invention the isotope structures provide improved performance of opto-electronic devices. In still further prefeπed embodiments of the invention the isotope structures provide improved performance of a semiconductor integrated circuit.

The preferred embodiments of the present invention provide an improved many- valley semiconductor structure in which optical and electronic properties are modified due to partial or complete suppression or alternatively enhancement of electron-phonon interactions coπesponding to intervalley electron scattering of electrons by phonons. The phonon resonator proposed in the past (and as described in PCT application no. WO96/25767) requires the formation of density modulation in the direction parallel to the direction of the major intervalley electron scattering, that is, parallel to crystallographic direction <100> for Si and to <111> for Ge. However, the present invention has, in part, arisen from the appreciation that it is often the case that there are a multiple number of equivalent intervalley directions in a given many-valley semiconductor. For example, for Si and Ge there are three and four respectively. This means the phonon resonator proposed in PCT application no. WO96/25767 utilises only one of many directions for the resonance so that the effect of such implementation remains questionable.

The preferred embodiments of the present invention also provide an indirect bandgap material that functions as a non-resonator or alternatively resonator for phonons having the magnitude and direction of wavenumbers corresponding to the electronic transition of major intervalley electron scatterings. Specifically, the preferred embodiments provide a stressed isotope superlattice which is composed of many- valley (equivalent to the definition of indirect band-gap) semiconductor elements such as, for example, Si, Ge, and any combination of Si and Ge, of periodically varying isotope mass modulation by spatial control of the Si and Ge isotope distribution, with the direction of the density periodicity being parallel to the direction of major intervalley electron scattering, that is, parallel to the <100> direction for Si and parallel to the <111> direction for Ge. The superlattice is stressed in the direction perpendicular to the direction of the density periodicity by means of the epitaxial formation of said superlattice on a single crystal substrate having a lattice constant different from that of the superlattice and/or epitaxial formation of an over-layer on the top of said superlattice, and/or mechanical methods. In some embodiments, the structure of the present invention displays increased photon emission or absorption capability relative to known indirect bandgap materials owing to modification of the electronic band-structure and phonon properties associated with intervalley electron scattering. In other embodiments, the structure provides improved electrical properties such as carrier mobility and electrical conductivity, thanks to the partial or complete suppression of the intervalley electron scattering. The stressed- isotope superlattices of the present invention can be incorporated in active regions of a variety of optical and electronic devices. Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to". BRIEF DESCRIPTION OF THE DRAWINGS Preferced embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of a stressed Si isotope superlattice composed of alternating layers of ²⁸Si and ³⁰Si isotopes grown on a Si_xGeι_-x substrate in the <100> direction, that is, parallel to the direction <100> of the major intervalley electron scattering for Si;

Figure 2 is a schematic diagram of a stressed Si isotope superlattice composed of alternating layers of ²⁸Si and ³⁰Si isotopes having a Si_xGe_1-x over-layer. The growth direction is <100>, that is, parallel to the direction <100> of the major intervalley electron scattering for Si; Figure 3 is a schematic diagram of a stressed Si isotope superlattice composed of alternating layers of ²⁸Si and ³⁰Si isotopes. The isotope superlattice is stressed by means of a mechanical method using a solid ring and a point force from the bottom; Figure 4 is a phonon dispersion (phonon energy E vs. wavevector k) in bulk Si in the <100> direction; Figure 5 is a phonon dispersion (phonon energy E vs. wavevector k) in a ²⁸Si₆/³⁰Si₆ isotope superlattice in the <100> direction, demonstrating an example of the zone folding of the longitudinal acoustic (LA) phonon dispersion;

Figure 6 illustrates the Brillouin zone (14-hedron) of Si in reciprocal (wavevector) space;

Figure 7 illustrates the effect of in-plane stress on the Brillouin zone (14-hedron) of Si in reciprocal (wavevector) space;

Figure 8 is a portion of a high-electron-mobility-transistor (HEMT) device utilising the stressed-isotope superlattice of the present invention. Figure 9 is a portion of an integrated circuit utilising the stressed-isotope superlattice according to the present invention;

Figure 10 is an embodiment of the invention having a stressed-Si isotope quantum wire fabricated on a substrate; and

Figure 11 is another embodiment of the invention having a stressed-Si quantum dot fabricated on a substrate.

DESCRIPTION OF PREFERRED EMBODIMENTS

The stressed-isotope superlattice structure of the prefeπed embodiments lead to improved optical and/or electrical properties of indirect band-gap semiconductors due to the partial or complete suppression or alternatively enhancement of intervalley electron scattering of electrons. Specifically, a periodic layered structure comprising an isotope superlattice in which each layer is separately enriched for different particular isotopes of semiconductor elements, with the direction of the periodicity parallel to the direction of the dominant intervalley electron scattering, is utilised. In some embodiments, the thickness of each layer in the isotope superlattice is selected such that the maximum wavenumber of phonons in the direction of the dominant intervalley electron scattering becomes smaller than the wavenumber needed for the intervalley scattering of electrons. In other embodiments the thickness of each layer in the isotope superlattice is selected such that the wavenumber of phonons in the direction of the dominant intervalley electron scattering becomes integer multiples of the magnitude of the wavevector coπesponding to the dominant intervalley electron scattering.

One example of such a structure is shown in Figure 1. More particularly, Figure 1 shows a ²⁸Si_n/³⁰Si_m superlattice 10 grown on a Si_xGeι _-x substrate 11 with the thicknesses of the ²⁸Si and ³⁰Si layers n and m, respectively, in the units of an atomic mono-layer. Naturally occurring Si is composed of three isotopes in the fixed compositions, 92.2% ²⁸Si, 4.7% ²⁹Si, and 3.1% ³⁰Si. An isotopically enriched ²⁸Si layer in isotope superlattices is defined as a layer that contains more than 92.2% of Si isotopes. Similarly, isotopically enriched ²⁹Si and ³⁰Si layers contain isotopic compositions of² Si and ³⁰Si isotopes that exceed 4.7% and 3.1%, respectively. Preferably, the composition of an enriched isotope in any particular layer of an isotope superlattice approaches 100%. The ²⁸Si_n/³⁰Si_n superlattice 10 shown in Figure 1 is composed of alternating layers of isotopically enriched ²⁸Si and ³⁰Si layers in the crystallographic direction <100>. The number of periods, that is, the number of ²⁸Si and ³⁰Si layer pairs, is two in the example shown in Figure 1. In other embodiments, the number of periods is larger and, indeed, in some embodiments, approaches the limit of infinity.

The notation <100> includes all <100> directions that are equivalent with respect to the crystal symmetry, that is, directions [1,0,0], [0,1,0], [0,0,1], [-1,0,0] etc. The substrate 11, in this embodiment, is a single crystal type that has a lattice constant close to, but different from, the Si superlattice. In other embodiments, different crystal substrates are used.

The Si_xGei_-x substrate 11, whose lattice constant is larger than that of superlattice 10, applies uniform in-plane tensile stress to superlattice 10 in the direction perpendicular to the growth direction <100>. The subscript x < 1 denotes the content of Si in the Si-Ge alloy semiconductor.

Figure 2 shows a second prefeπed embodiment of the present invention. A ²⁸Si_n/³⁰Si_m isotope superlattice 10, as described above for Figure 1, is grown on a substrate 11 which, in this embodiment, is Si. However, in other embodiments, other single crystal substrates, for example Si_xGeι_-x, are used. In any event, the lattice constant of the substrate is different from that of Si.

Superlattice 10 includes an over-layer 30 whose lattice constant is different from that of the superlattice and which is grown epitaxially on top of the superlattice. Layer 30 applies a uniform in-plane stress to the superlattice in the direction perpendicular to the growth direction <100>.

Figure 3 shows a third prefeπed embodiment of the present invention. Again, a Sι_n/ Sι_m isotope superlattice 10, as described above for Figure 1 and Figure 2, is grown on a substrate 11. In this embodiment the substrate is Si, but in other embodiments use is made of other single crystal substrates. Some other examples include Si_xGe_1-x. The intention is to provide a substrate with a lattice constant that is different from that of Si.

As best shown in Figure 3, superlattice 10 includes a solid ring 31 for supporting the top layer of the superlattice. This applies an upward point force from the back of the substrate. The superlattice 10 is consequently bent spherically due to the upward point force from below and the solid-ring support at the top. This bending leads to creation of in-plane stress within superlattice 10.

To demonstrate the mechanism of partial or complete elimination of intervalley electron scattering in stressed-isotope superlattices, consideration is had to the structure shown in Figure 1 with the thickness of the Si layer n = 6 and that of Si layer m = 6. That is, ²⁸Si_ό ³⁰Si₆ superlattices. Figure 4 shows the schematic of phonon dispersion curves (energy E of phonons vs. wavenumber k) for the <100> direction in bulk Si. The lattice constant a_<ιoo_> in the <100> direction coπesponds to the 2 atomic layer thickness of Si in the <100> direction. The maximum value of the phonon wavevector in Figure 4

is π/a_<1oo_> as defined by the Brillouin zone edge.

Figure 5 shows the effect of the ²⁸Sie/³⁰Si6 isotope superlattice formation on the phonon dispersion curves. Due to the new periodicity a'<₁₀o>= 6a<ιoo> defined by the isotope superlattice, phonon dispersion curves are folded into the region 1/6 of the bulk

Brillouin zone. That is, within the region 0 < k < π/6a_<1Qo_> when only positive

wavenumbers are considered. Such zone- folding is usually refened to as a mini- Brillouin zone as shown in Figure 5. For clarity, only the zone folding of the longitudinal acoustic (LA) phonon is shown in Figure 5. In this case the maximum

wavenumber of phonons neglecting umklapp scattering becomes π/6a<ιoo> as defined by

the edge of the mini-Brillouin zone edge.

Figure 6 shows the Brillouin zone (14-hedron) of unstressed-Si in the reciprocal (wavevector) space. Six ellipsoidal surfaces A, B, B', C, C, C", and C" are constant energy surfaces of the six equivalent conduction bands (valleys) for carrier electrons. The dominant intervalley electron scattering in Si is of the type in the <100> direction between the valleys A and B, C and C", and C and C". Such intervalley transitions are

mediated by phonons having wavenumbers between 0.3π/a<ιoo> and 0.4π/a_<1oo>

conesponding to the distance between A and B, C and C", and C and C" where a_<ιoo> is the lattice constant of Si in the <100> direction. The distance between A and B, C and C", and C and C" across the zone-boundaries known in the art varies between

0.3π/a<ιoo> and 0.4π/a<ιoo>.

It is possible to eliminate the scattering between A and B - for example by growing the isotope superlattice in the direction [100] as defined in Figure 6 and by choosing the thickness of each layer, n and m, appropriately - so that the maximum wavenumber of phonons in the [100] direction becomes smaller than the distance between A and B. For example, the mini -Brillouin zone edge in the [100] direction

becomes less than 0.3π/a_<ιoo> when n>4 and m>4 in a Si isotope superlattice grown in

the [100] direction. In this case the intervalley electron scattering between A and B in Figure 6 is partially or completely eliminated since the phonons in the [100] direction no longer possess momentum large enough to induce such scattering. However, the scatterings between C-C" and C'-C" still exist in the example just discussed since the phonon dispersion in [010] and [001] directions as defined in Figure 6 are not modified and remain unchanged from the ones in unstressed bulk Si.

Thus, an unstressed Si isotope superlattice grown in [100] direction allows for the elimination of intervalley electron scattering in [100] direction only, and not those in the

[010] and [001] directions.

The prefeπed embodiments demonstrate the method to eliminate intervalley scatterings completely. In some embodiments this is achieved for the three directions in Si all at once. The isotope superlattices of the present embodiments are grown in the direction substantially parallel to the direction of the dominant intervalley electron scatterings, and are stressed externally in the direction substantially peφendicular to the direction of the mass modulation. The most prefeπed direction of the isotope superlattice for the case of Si is <100> since it is parallel to the directions of the dominant intervalley scatterings between valleys A and B, C and C", and C and C" in Figure 6.

For example, Figure 6 illustrates a superlattice that has been grown in the direction [100]. A uniform stress, either compression or tensile, is applied in the plane containing the [010] and [001] axes. Figure 7 shows the effect of the uniform tensile stress (as for the example of the structure shown in Figure 1) on the shapes of Brillouin zone (14- hedron) and constant energy surfaces of the conduction band. Due to the enlargement of the lattice constant in the [010]-[001] plane, the Brillouin zone is compressed in the [010]- [001] plane of reciprocal space. More importantly, the constant energy surfaces for free electrons in the conduction bands shown in Figure 7 change - due to the stress - significantly from those of unstressed ones shown in Figure 6.

The constant energy surfaces C, C, C", and C" shown in Figure 6 are much smaller than the surfaces A, A', B, and B' indicating that most of conducting electrons concentrate in the valleys A, A, B, and B' rather than in C, C, C", and C". Thus, the effect of intervalley scattering between valleys C and C", and C and C" are no longer important since there exists only a small number of electrons in valleys C, C, C", and C" with respect to the number of electrons in valleys A, A', B, and B'. The scatterings of the types between valleys A and B are the only important passes for the intervalley scattering. The application of compression in-plane stress also leads to a similar effect. Therefore, the application of the stress by, for example, methods shown in Figures 1, 2 and 3, are efficient methods to confine electrons in the conduction-band valleys whose longitudinal axes are parallel to the direction of the mass modulation of the isotope superlattice.

As will be now apparent to one skilled in the art - from the teaching herein - the elimination or alternatively enhancement of the intervalley scattering between A and B for the case shown in Figure 6 are the most important objectives for the preferred embodiments described above since the scattering between C and C", and C and C" are no longer significant.

In some embodiments, the electron mobility, that is, the device operational speed, is increased significantly by the complete elimination of the intervalley scattering in Si electronic devices. To eliminate the intervalley scattering between A and B for the example shown in Figure 7, it is necessary to provide the conditions such that the mini- Brillouin zone edge shown in Figure 7 takes a value smaller than the distance between

A and B, that is, 0.3π/a<ιoo> or 0.4π/a_<10o_>. Such conditions are realised by choosing n >

4 and m > 4 for the stressed ²⁸Si_n/³⁰Si_m isotope superlattice grown in the <100>

direction, for the case of the distance between A and B equal to 03π/a< oo>. The

condition becomes n > 3 and m > 3 if the distance between A and B is 0.4π/a<₁Qo>.

In some embodiments the optical properties of indirect gap semiconductors are improved using the stressed-isotope superlattices of the present invention. It has been described in PCT WO96/25767 that it is possible to promote radiative electronic transitions for enhanced light emission and detection, for example, in silicon by increasing the number of phonons that have wavenumbers conesponding exactly to the distance between A and A', and B and B' through the zone center ZC in Figure 7. The distance between A-B' and B-A' through the zone center ZC known in the art lies

between 0.8π/a<ιoo> and 0.85π/a<ιoo>. It is possible to maximise the number of phonons in the [100] directions by appropriately choosing the values of n and m in ²⁸Si_n/³⁰Si_m isotope superlattices grown in the [100] direction. If the distance between

A-B' and B-A through the zone centre ZC in Figure 7 is taken to be 0.8π/a<ιoo>,

n = m = 5/ should be chosen where / is the integer 1, 2, 3, etc. for the same reason discussed in PCT WO96/25767.

The above mentioned approaches are employed to find out the appropriate thickness of layers in isotope superlattices composed of semiconductor elements other than Si. The eight equivalent conduction bands of Ge lie in the direction <111>, that is, it is most desirable to grow stressed isotope superlattices in the direction <111> for the case of Ge.

The intervalley scattering of electrons are known to degrade or to enhance a variety of electrical and optical properties of many- valley semiconductors. It is therefore of great advantage to implement the stressed isotope superlattice of the present invention in active regions of various electrical and optical devices widely used in today's technology. The active regions refer to the regions through which electric currents flow in electric devices and to the regions in which light emission and/or detection accompanied by electric c rents occur in optical devices. In case of Si, for example, the electron mobility increases by a factor of about 3 if intervalley electron scattering is completely eliminated. This leads to a mobility compatible or even larger than some of the compound semiconductors, for example, GaAs, that are widely used for fabrication of high speed and high frequency devices.

Figure 8 shows one of many possible examples of the application of the present invention. More specifically, Figure 8 is a cross-section of a SiGe based high-electron- mobility-transistor (HEMT) device that utilises the stressed-isotope superlattice according to the present invention. The structure is superficially very similar to the one discussed by Takagi et al. in Journal of Applied Physics, Vol. 80, 1567 (1996). However, there are a number of significant differences. For example the structure of Figure 8 contains a ²⁸Si_n/³⁰Si_m isotope superlattice layer instead of standard Si as discussed by Takagi et al.

The Figure 8 embodiment includes a two dimensional electron gas layer that is formed at the top of the ²⁸Si_n/³⁰Si_m layer due to the appropriate band-offset induced by the in-plane stress arising from the lattice mismatch between the n-SiGe layer and the ²⁸Si_n/³⁰Si_m layer. The thickness n and m of the isotope superlattice is controlled in such a way that it reduces intervalley scattering completely, as discussed above. As a result, the electron mobility in the HEMT structure increases significantly - due to the presence of the isotope superlattice layer - leading to much higher device operational speed and much improved capability for handling high-frequency signals.

Reference is now made to Figure 9, where there is illustrated another application of stressed-isotope superlattice. More specifically, a portion of an integrated circuit is fabricated in an n-type epitaxial layer of stressed-isotope superlattice 10 that is made in accordance with the present invention. The isotope superlattice 10 is stressed by having a SiGe buffer layer underneath or a SiGe buffer over-layer on the top. In other embodiments different means of stressing are used. The circuit is fabricated on p-type silicon substrate 11. The illustrated portion in

Figure 9 includes a metal-oxide-semiconductor field effect transistor (MOSFET) 15 and an NPN bipolar transistor 16, each of which is isolated from the other and from other devices on the chip by p+ regions 19 that are diffused through isotope superlattice 10 into substrate 11. MOSFET 15 includes a drain metallisation 18 in contact with a p+ drain region 22, a source metallisation 21 in contact with a p+ source region 23 and a gate metallisation 20 isolated from electrical contact with the superlattice layer 10 by a SiO₂ layer 17. NPN transistor 16 includes a collector metallisation 28 in contact with an n+ region 27, a base metallisation 24 in contact with a p-type base region 26 and an emitter metallisation 25 in contact with an n+ emitter region 27.

In other embodiments the stressed semiconductor isotope superlattice is implemented in the active regions of a wide variety of devices other than integrated circuits. Examples of these variety of devices including light emitting devices, light emitting diodes, semiconductor solid state lasers, optical detectors, optical modulators, electrical conductors, planar transformers, diodes, bipolar transistors, field effect transistors, integrated circuits, and the like.

In a further embodiment the stressed-isotope superlattices of the invention is implemented as a naπow wire, as best shown in Figure 10. In still further embodiments the stressed-isotope superlattices of the invention is implemented as a small dot, as best shown in Figure 11.

The isotope quantum wire and quantum dot are preferably stressed by having an underlying layer that has a lattice constant different from that of the quantum wire or dot. Alternatively, or in addition, the stressing is affected by having an over-layer that has a lattice constant that is different from that of the quantum wire or dot. It will be appreciated by those skilled in the art that the present advanced semiconductor technology allows the fabrication of naπow wires and small dots whose dimensions are compatible with that of electron and hole wavefunctions that are confined in the structures. These wires and dots are very often referred to as quantum wires and quantum dots, respectively. The optical and electrical properties of semiconductors change significantly when they are formed into quantum wires and dots. For example, an indirect band-gap semiconductor like Si can become a direct band-gap semiconductor when it is made into quantum wires and dots. That is, it can emit and detect light more efficiently. Moreover, the emission and detection of light by quantum wires and dots can be made even more efficient by implementing the isotope superlattice of the present invention in the active regions of optical devices based on quantum wires and dots. For the same reasons, the electrical properties of electronic devices utilising quantum wires and dots, for example single electron transistors and diodes, can be improved by implementation of the stressed isotope superlattice of the present invention.

Although the invention has been described with reference to specific examples it will be appreciated by those skilled in the art that it may be embodied in many other forms.

Claims

1. An isotope structure formed in an indirect band gap semiconductor material, the structure including: a first region in the material having a first density; and a second region in the material having a second density and being adjacent to the first region whereby the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material, the material having stress applied externally in a direction substantially perpendicular to the predetermined direction.

2. A structure according to claim 1 wherein the material includes a plurality of first and second regions that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

3. A semiconductor isotope superlattice composed of indirect band gap semiconductor elements such as, for example, Si and Ge of periodically varying density comprising: at least one first region of a first density; and at least one second region of a second density, said first and second regions being adjacent to one another and alternating in said structure, being parallel to the crystallographic direction of the dominant intervalley electron scattering of the given indirect band-gap semiconductor material, with stress applied externally in the direction substantially perpendicular to the direction of the density periodicity of said structure.

4. A superlattice according to claim 3 wherein the first region comprises a first layer of one to sixty atomic layers thickness and said second region comprises a second layer of one to sixty atomic layers thickness.

5. A superlattice according to claim 3 that is grown epitaxially on a single crystal substrate whose lattice constant is different from that of said superlattice in order to apply in-plane stress to said semiconductor isotope superlattice.

6. A superlattice according to claim 3 in which a single crystal over-layer whose lattice constant is different from that of said superlattice is grown in order to apply in- plane stress to said semiconductor isotope superlattice.

7. A superlattice according to claim 3 that is stressed externally by means of mechanical methods.

8. A semiconductor quantum wire composed of indirect band gap semiconductor elements such as, for example, Si and Ge of periodically varying density comprising: at least one first region of a first density; and at least one second region of a second density, said first and second regions being adjacent to one another and alternating in said structure, being substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the given indirect band-gap semiconductor material, with stress applied externally in a direction substantially peφendicular to the direction of the density periodicity of said structure.

9. A wire according to claim 8 wherein the first region comprises a first layer of one to sixty atomic layers thickness and said second region comprises a second layer of one to sixty atomic layers thickness.

10. A wire according to claim 8 that is grown epitaxially on a single crystal substrate whose lattice constant is different from that of the wire in order to apply in- plane stress to said semiconductor isotope quantum wire.

11. A wire according to claim 8 in which a single crystal over-layer whose lattice constant is different from that of said wire is grown in order to apply in-plane stress to said semiconductor isotope quantum wire.

12. A wire according to claim 8 that has been stressed externally by means of mechanical methods.

13. A semiconductor quantum dot composed of indirect band gap semiconductor elements such as, for example, Si and Ge of periodically varying density comprising: at least one first region of a first density; and at least one second region of a second density, said first and second regions being adjacent one another and alternating in said structure, being substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the given indirect band-gap semiconductor material, with stress applied externally in the direction substantially peφendicular to the direction of the density periodicity of said structure.

14. A dot according to claim 13 wherein said first region comprises a first layer of one to sixty atomic layers thickness and said second region comprises a second layer of one to sixty atomic layers thickness.

15. A dot according to claim 14 that is grown epitaxially on a single crystal substrate whose lattice constant is different from that of said quantum dot in order to apply in-plane stress to said semiconductor isotope quantum dot.

16. A dot according to claim 14 in which a single crystal over-layer whose lattice constant is different from that of said quantum dot is grown in order to apply in-plane stress to said semiconductor isotope quantum dot.

17. A semiconductor isotope superlattice formed in an indirect band gap semiconductor material, the superlattice including: a first region in the material having a first density; and a second region in the material having a second density and being adjacent to the first region whereby the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material, the material having stress applied externally in a direction substantially peφendicular to the predetermined direction.

18. A superlattice according to claim 17 wherein the material includes a plurality of first and second regions that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

19. A superlattice according to claim 17 wherein the first region includes a first layer of one to sixty atomic layers thickness and the second region includes a second layer of one to sixty atomic layers thickness.

20. A superlattice according to claim 17 wherein the material is grown epitaxially on a single crystal substrate.

21. A superlattice according to claim 20 wherein the substrate has a lattice constant that differs from that of the material such that in-plane stress is applied to the superlattice.

22. A superlattice according to claim 21 including a single crystal over-layer.

23. A superlattice according to claim 22 wherein the over-layer has a lattice constant that differs from the lattice constant of the material such that in-plane stress is applied to the material.

24. A superlattice according to claim 17 wherein the material is stressed externally by mechanical means.

25. A semiconductor quantum wire formed in an indirect band gap semiconductor material, the wire including: a first region in the material having a first density; and a second region in the material having a second density and being adjacent to the first region whereby the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material, the material having stress applied externally in a direction substantially peφendicular to the predetermined direction.

26. A wire according to claim 25 wherein the material includes a plurality of first and second regions that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

27. A wire according to claim 25 wherein the first region includes a first layer of one to sixty atomic layers thickness and the second region includes a second layer of one to sixty atomic layers thickness.

28. A wire according to claim 25 wherein the material is grown epitaxially on a single crystal substrate.

29. A wire according to claim 28 wherein the substrate has a lattice constant that differs from that of the material such that in-plane stress is applied to the material.

30. A wire according to claim 25 wherein the material includes a single crystal over- layer.

31. A wire according to claim 30 wherein the over-layer has a lattice constant that differs from the lattice constant of the material such that in-plane stress is applied to the material.

32. A wire according to claim 25 wherein the material is stressed externally by mechanical means.

33. A semiconductor quantum dot formed in an indirect band gap semiconductor material, the dot including: a first region in the material having a first density; and a second region in the material having a second density and being adjacent to the first region whereby the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material, the dot having stress applied externally in a direction substantially peφendicular to the predetermined direction.

34. A dot according to claim 33 wherein the material includes a plurality of first and second regions that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

35. A dot according to claim 33 wherein the first region includes a first layer of one to sixty atomic layers thickness and the second region includes a second layer of one to sixty atomic layers thickness.

36. A dot according to claim 33 wherein the material is grown epitaxially on a single crystal substrate.

37. A dot according to claim 36 wherein the substrate has a lattice constant that differs from that of the material such that in-plane stress is applied to the material.

38. A dot according to claim 33 including a single crystal over-layer.

39. A dot according to claim 38 wherein the over-layer has a lattice constant that differs from the lattice constant of the material such that in-plane stress is applied to the material.

40. A dot according to claim 33 wherein the material is stressed externally by mechanical means.

41. A method of forming a semiconductor isotope superlattice in an indirect band gap semiconductor material, the method including: providing in the material a first region having a first density; providing in the material a second region having a second density such that the first and second regions are adjacent to each other and the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material; and applying stress externally to the material in a direction substantially peφendicular to the predetermined direction.

42. A method according to claim 41 including forming a plurality of first and second regions in the material that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

43. A method of forming a semiconductor quantum wire in an indirect band gap semiconductor material, the method including: providing in the material a first region having a first density; providing in the material a second region having a second density such that the first and second regions are adjacent to each other and the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material; and applying stress externally to the material in a direction substantially peφendicular to the predetermined direction.

44. A method according to claim 43 including forming a plurality of first and second regions in the material that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

45. A method of forming a semiconductor quantum dot in an indirect band gap semiconductor material, the method including: providing in the material a first region having a first density; providing in the material a second region having a second density such that the first and second regions are adjacent to each other and the density of the material varies in a predetermined direction that is substantially parallel to the crystallographic direction of the dominant intervalley electron scattering of the material; and applying stress externally to the material in a direction substantially peφendicular to the predetermined direction.

46. A method according to claim 45 including forming a plurality of first and second regions in the material that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

47. A semiconductor structure including: a many-valley semiconductor material; a first region formed in the material and having a first density; and a second region formed in the material, the second region having a second density and being adjacent to the first region whereby the density of the material varies in a predetermined direction for suppressing the electron-phonon interactions corresponding to intervalley electron scattering of electrons by phonons.

48. A structure according to claim 47 wherein the material includes a plurality of first and second regions that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

49. A semiconductor structure including: a many-valley semiconductor material; a first region formed in the material and having a first density; and a second region formed in the material, the second region having a second density and being adjacent to the first region whereby the density of the material varies in a predetermined direction for enhancing the electron-phonon interactions coπesponding to intervalley electron scattering of electrons by phonons.

50. A structure according to claim 49 wherein the material includes a plurality of first and second regions that are alternated with each other such that the density of the material varies periodically in the predetermined direction.

51. A semiconductor integrated circuit including one or more active semiconductor devices, at least one of the devices being selected from the group comprising: a superlattice as defined in claim 1 or claim 17; a wire as defined in claim 8 or claim 25; and a dot as defined in claim 13 or claim 33.