WO2002015279A1 - A semiconductor isotope superlattice - Google Patents

A semiconductor isotope superlattice Download PDF

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Publication number
WO2002015279A1
WO2002015279A1 PCT/AU2000/000976 AU0000976W WO0215279A1 WO 2002015279 A1 WO2002015279 A1 WO 2002015279A1 AU 0000976 W AU0000976 W AU 0000976W WO 0215279 A1 WO0215279 A1 WO 0215279A1
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Prior art keywords
density
region
semiconductor
isotope
parallel
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PCT/AU2000/000976
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French (fr)
Inventor
Kohei Itoh
Original Assignee
Silex Systems Limited
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Publication date
Application filed by Silex Systems Limited filed Critical Silex Systems Limited
Priority to AU2000264174A priority Critical patent/AU2000264174A1/en
Priority to PCT/AU2000/000976 priority patent/WO2002015279A1/en
Publication of WO2002015279A1 publication Critical patent/WO2002015279A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/04—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a quantum effect structure or superlattice, e.g. tunnel junction
    • H01L33/06—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a quantum effect structure or superlattice, e.g. tunnel junction within the light emitting region, e.g. quantum confinement structure or tunnel barrier
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B25/00—Single-crystal growth by chemical reaction of reactive gases, e.g. chemical vapour-deposition growth
    • C30B25/02—Epitaxial-layer growth
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/60—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape characterised by shape
    • C30B29/68—Crystals with laminate structure, e.g. "superlattices"
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    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/04—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their crystalline structure, e.g. polycrystalline, cubic or particular orientation of crystalline planes
    • H01L29/045—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their crystalline structure, e.g. polycrystalline, cubic or particular orientation of crystalline planes by their particular orientation of crystalline planes
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    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/06—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions
    • H01L29/10—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions with semiconductor regions connected to an electrode not carrying current to be rectified, amplified or switched and such electrode being part of a semiconductor device which comprises three or more electrodes
    • H01L29/1025—Channel region of field-effect devices
    • H01L29/1029—Channel region of field-effect devices of field-effect transistors
    • H01L29/1033—Channel region of field-effect devices of field-effect transistors with insulated gate, e.g. characterised by the length, the width, the geometric contour or the doping structure
    • H01L29/1054—Channel region of field-effect devices of field-effect transistors with insulated gate, e.g. characterised by the length, the width, the geometric contour or the doping structure with a variation of the composition, e.g. channel with strained layer for increasing the mobility
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    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/15—Structures with periodic or quasi periodic potential variation, e.g. multiple quantum wells, superlattices
    • H01L29/151—Compositional structures
    • H01L29/152—Compositional structures with quantum effects only in vertical direction, i.e. layered structures with quantum effects solely resulting from vertical potential variation
    • H01L29/155—Comprising only semiconductor materials
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    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/16—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only elements of Group IV of the Periodic System
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    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
    • H01L29/76—Unipolar devices, e.g. field effect transistors
    • H01L29/772—Field effect transistors
    • H01L29/78—Field effect transistors with field effect produced by an insulated gate
    • H—ELECTRICITY
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    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/028—Inorganic materials including, apart from doping material or other impurities, only elements of Group IV of the Periodic System
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0352—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
    • H01L31/035236—Superlattices; Multiple quantum well structures
    • H01L31/035254—Superlattices; Multiple quantum well structures including, apart from doping materials or other impurities, only elements of Group IV of the Periodic System, e.g. Si-SiGe superlattices
    • H—ELECTRICITY
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    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
    • H01S5/3425—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers comprising couples wells or superlattices
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
    • H01S5/3427—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in IV compounds

Definitions

  • the invention relates to an isotope superlattice and in particular to a
  • the invention has been developed primarily for use in optical integrated circuits
  • the carrier mobility in a number of compound semiconductors for example,
  • GaAs is much higher than that in Si so that a large number of today's high frequency
  • thin films on top of substrate single crystals for example, SiGe alloys, which have a
  • control of isotopic composition can lead to change in the intrinsic properties
  • the carrier mobility and the efficiency to emit and detect light including the carrier mobility and the efficiency to emit and detect light.
  • the efficiency to emit and detect light including the carrier mobility and the efficiency to emit and detect light.
  • Japanese patent 7-83029 suggests that it is possible to increase the electron mobility in
  • GaAs by stacking thin layers of different isotopic composition in sequence, that is, by
  • Such a phonon resonator increases the density of phonons selectively whose momentum corresponds to
  • the phonon resonator allows for more efficient scattering of
  • isotope superlattice composed of semiconductor elements such as, for example, Si and
  • Ge of periodically varying density comprising:
  • said first and second regions being adjacent one another and alternating in said
  • the first region comprises a first layer of one to sixty atomic layers
  • said second region comprises a second layer of one to sixty atomic layers
  • the superlattice is composed of constituents of indirect
  • band-gap semiconductors including, for example, Si, Ge, and SiGe alloys.
  • quantum wire composed of indirect band-gap semiconductor elements including, for
  • Si and Ge of periodically varying density comprising:
  • said first and second regions being adjacent one another and alternating in said
  • the first region comprises a first layer of one to sixty atomic layers
  • said second region comprises a second layer of one to sixty atomic layers
  • the longitudinal direction of the wire is non-parallel to the
  • quantum dot composed of many- valley semiconductor elements including, for example,
  • Si and Ge of periodically varying density comprising:
  • At least one second region of a second density said first and second regions being adjacent one another and alternating in said
  • the isotope superlattices serve to provide improved performance of
  • the isotope superlattices serve to provide improved
  • the device including:
  • first and second regions are adjacent to and alternate with one another to
  • the first region includes a first layer of one to sixty atomic layers
  • said second region includes a second layer of one to sixty atomic layers
  • the indirect band-gap semiconductor material is selected
  • semiconductor quantum wire being formed in an indirect band-gap semiconductor material having a predetermined crystallographic direction in which the dominant
  • the device including:
  • first and second regions are adjacent to and alternate with one another to
  • the first region includes a first layer of one to sixty atomic layers
  • the second region includes a second layer of one to sixty atomic layers
  • the wire extends longitudinally and non-parallel to the
  • quantum dot being formed in many-valley semiconductor material having a
  • the device including:
  • first and second regions are adjacent to and alternate with one another to
  • WO96/25767 utilises only one of many directions for the resonance so that the effect of
  • invention aim to decrease the intervalley electron scattering in many- valley
  • bandgap material that functions as a non-resonator or alternatively resonator for
  • an isotope superlattice which is composed of many-
  • Si and Ge with periodically varying isotope mass modulation with the
  • structures of the present invention displays increased photon emission or absorption capability relative to known indirect bandgap materials due to modification of phonon
  • the structure in the present invention provides improved
  • the isotope superlattices of the preferred embodiments of the invention are preferred.
  • optical and/or electronic devices are incorporated in active regions of one or more optical and/or electronic devices.
  • Figure 1 is a schematic diagram of a Si isotope superlattice which is composed
  • Figure 2 illustrates phonon dispersion (phonon energy E vs. wavevector k) in
  • Figure 3 illustrates phonon dispersion (phonon energy E vs. wavevector k) in
  • LA longitudinal acoustic
  • Figure 4 illustrates the Brillouin zone (14-hedron) of Si in the reciprocal
  • Figure 5 illustrates a portion of an integrated circuit that utilises an isotope
  • Figure 6 is a schematic representation of a Si isotope superlattice quantum wire
  • Figure 7 is a schematic representation of a Si isotope superlattice quantum dot
  • embodiments an enhancement - of intervalley electron scattering of electrons.
  • a periodic layered structure comprising an isotope superlattice in which
  • each layer is enriched for a particular isotope of a semiconductor element
  • isotope superlattice is selected such that the maximum wavenumber of phonons in the
  • Figure 1 shows the 28 Si n / 30 Si m
  • superlattices 10 grown on a substrate 11 with the thicknesses of the 28 Si and 30 Si layers n
  • Naturally occurring Si is composed of three isotopes in the fixed compositions
  • superlattice is defined as a layer that contains more than 92.2% of 28 Si isotopes.
  • isotopically enriched 29 Si and 30 Si layers contain isotopic compositions of 29 Si
  • the composition includes 0.15% and 30 Si isotopes that exceed 4.7% and 3.1%, respectively.
  • the composition includes 0.15% and 30 Si isotopes that exceed 4.7% and 3.1%, respectively.
  • the 28 Si n / 30 Si n superlattice 10 shown in Figurel is composed of alternating layers
  • the substrate 11 in some embodiments is formed from Si of the natural isotopic
  • Figure 2 shows the schematic of phonon dispersion curves
  • a ⁇ > in the ⁇ 111> direction corresponds to two atomic layers thickness of Si in - li ⁇
  • Figure 3 shows the effect of the 28 Si 6 / 30 Si 6 isotope superlattice formation on the
  • Such zone-folding is usually referred to as a mini-Brillouin zone as
  • LA longitudinal acoustic
  • Figure 4 shows the Brillouin zone (14-hedron) of Si in reciprocal (wavevector)
  • V3 ⁇ k Br ⁇ lll> - ⁇ I a ⁇ n ⁇ > • C 1
  • the mini-Brillouin zone edge k ⁇ - ⁇ , ⁇ in the ⁇ 111> direction is given by:
  • n and m should be even numbers to promote phonon
  • the eight equivalent conduction bands of Ge lie in the direction
  • the active regions refer to the regions through which electric currents flow in
  • GaAs which is widely used for fabrication of high speed and high
  • Figure 5 shows a portion of an integrated circuit fabricated in an n-type epitaxial
  • the circuit is fabricated on p-
  • the illustrated portion in Figure 5 includes a metal-oxide-
  • MOSFET semiconductor field effect transistor
  • NPN bipolar transistor NPN bipolar transistor
  • Both transistors 15 and 16 are isolated from the other and from other devices on the chip
  • Transistor 15 includes a drain metallization 18 in contact with a p + drain region
  • Transistor 16 includes a collector metallization 28 in contact with an n + region
  • integrated circuits For example, alternative embodiments include light emitting
  • modulators electrical conductors, planar transformers, diodes, bipolar transistors, field
  • quantum wires and quantum dots are quantum dots
  • an indirect band-gap semiconductor like Si can become a direct band-gap
  • quantum wires and dots for example single electron transistors and diodes, are also possible.

Abstract

A 28Sin/30Sim supelattices (10) is grown on a substrate (11) where the thicknesses of the ?28Si and 30¿Si layers n and m, respectively, is in the unit of atomic layers. The 28Sin/30Sin superlattice (10) is composed of alternating layers of isotopically enriched ?28Si and 30¿Si layers in the crystallographic direction <111>. The number of periods, that is, the number of ?28Si and 30¿Si layer pairs in this embodiment, is two. The superlattice (10) is grown in a direction that is not parallel to the direction of the dominant intervalley electron scattering. The most preferred direction of the isotope superlattice for the case of Si is <111> since it has the same angles to the directions (A-B, C-C'', C'-C''').

Description

Title: A SEMICONDUCTOR ISOTOPE SUPERLATTICE
TECHNICAL FIELD
The invention relates to an isotope superlattice and in particular to a
semiconductor isotope superlattice.
The invention has been developed primarily for use in optical integrated circuits
and will be described hereinafter with reference to that application. However, the
invention is not limited to this particular field of use and is also suitable for other
integrated circuits.
BACKGROUND 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, As, etc. Si is preferred in
many LSI circuit applications due to its low market price, ease of fabrication, and
chemical purity. However, due to the relatively low mobility of carriers and to the
indirect band-gap structure of Si, many of today's high frequency devices, as well as
light emitting and detecting devices, are not fabricated with Si. Selection of
semiconductor materials having high carrier mobility is important for the fabrication of
a high frequency device to broaden the frequency band that can be covered by the single
device. The carrier mobility in a number of compound semiconductors, for example,
GaAs is much higher than that in Si so that a large number of today's high frequency
devices are made of compound semiconductors instead of Si. Two methods have been
proposed to increase the carrier mobility in Si; (1) application of external stress and (2)
decreasing the dimension of Si active regions to the quantum limits. In the former case application of the external stress has been realised very often by epitaxial growth of Si
thin films on top of substrate single crystals, for example, SiGe alloys, which have a
slightly different lattice constant from that of Si. In the later case, small active regions
of quantum limit have been realised using selective growth and etching of Si-based
structures. However, these methods are yet to provide Si-based high frequency devices
and light emitting and detecting devices of equal or superior performance compared to
ones made of compound semiconductors. Therefore, compound semiconductors still
dominate today's market of high frequency and optical semiconductor devices, though
the industry strongly prefers Si-based materials that are compatible with present-day
LSI circuits.
It has been believed by many that it is not possible to change the intrinsic
properties of bulk semiconductor materials without applying external perturbations such
as stress, electric field, magnetic field, etc. However, it has been realised very recently
that the control of isotopic composition can lead to change in the intrinsic properties
including the carrier mobility and the efficiency to emit and detect light. For example,
Japanese patent 7-83029 suggests that it is possible to increase the electron mobility in
GaAs by stacking thin layers of different isotopic composition in sequence, that is, by
forming GaAs isotope superlattices. In this prior patent, scattering of moving electrons
by longitudinal optical (LO) phonons can be 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, in the international patent application designated
by the publication number WO96/25767, there is described a method to increase the
carrier scattering by certain phonons by making isotope superlattices. Such a phonon resonator increases the density of phonons selectively whose momentum corresponds to
change in the wavevectors for indirect band to band electron-hole recombination and for
intervalley electron scattering. By carefully adjusting the thickness of layers composing
isotope superlattices, the phonon resonator allows for more efficient scattering of
carriers by phonons. More carrier scattering by specific phonons leads to more efficient
band to band electron-hole recombination for light emission and detection and also to
more efficient intervalley electron scattering for formation of electron-electron pairs that
may eventually increase the carrier mobility.
Neither of the above-mentioned proposals has been confirmed theoretically or
experimentally.
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 a semiconductor
isotope superlattice composed of 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 with the direction of the density periodicity of said structure being non-parallel
to the crystallographic direction of the dominant intervalley electron scattering of the
given indirect band-gap semiconductor material. Preferably, 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. More preferably, the superlattice is composed of constituents of indirect
band-gap semiconductors including, for example, Si, Ge, and SiGe alloys.
According to a second aspect of the invention there is provided a semiconductor
quantum wire composed of indirect band-gap semiconductor elements including, 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 with the direction of the density periodicity of said structure being non-parallel
to the crystallographic direction of the dominant intervalley electron scattering of the
given indirect band-gap semiconductor material.
Preferably, 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. More preferably, the longitudinal direction of the wire is non-parallel to the
to the crystallographic direction of the dominant intervalley electron scattering of the
given many- valley semiconductor material.
According to a third aspect of the invention there is provided a semiconductor
quantum dot composed of many- valley semiconductor elements including, 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 with the direction of the density periodicity of said structure being non-parallel
to the crystallographic direction of the dominant intervalley electron scattering of the
given many-valley semiconductor material.
Preferably, the isotope superlattices serve to provide improved performance of
electronic devices. More preferably, the isotope superlattices serve to provide improved
perforaiance of opto-electronic devices. Even more preferably, the isotope superlattices
serve to provide improved performance of semiconductor integrated circuits.
According to a fourth aspect of the invention there is provided a semiconductor
device being formed in an indirect band-gap semiconductor material having a
predetermined crystallographic direction in which the dominant intervalley electron
scattering occurs, the device including:
at least one first region of a first density; and
at least one second region of a second density different from the first density,
wherein the first and second regions are adjacent to and alternate with one another to
provide a density periodicity for the device which is non-parallel to the predetermined
crystallographic direction.
Preferably, the first region includes a first layer of one to sixty atomic layers
thickness and said second region includes a second layer of one to sixty atomic layers
thickness. More preferably, the indirect band-gap semiconductor material is selected
from the group: Si; Ge; and SiGe alloys.
According to a fifth embodiment of the invention there is provided a
semiconductor quantum wire being formed in an indirect band-gap semiconductor material having a predetermined crystallographic direction in which the dominant
intervalley electron scattering occurs, the device including:
at least one first region of a first density; and
at least one second region of a second density different from the first density,
wherein the first and second regions are adjacent to and alternate with one another to
provide a density periodicity for the device which is non-parallel to the predetermined
crystallographic direction.
Preferably, 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 wire extends longitudinally and non-parallel to the
predetermined crystallographic direction.
According to another aspect of the invention there is provided a semiconductor
quantum dot being formed in many-valley semiconductor material having a
predetermined crystallographic direction in which the dominant intervalley electron
scattering occurs, the device including:
at least one first region of a first density; and
at least one second region of a second density different from the first density,
wherein the first and second regions are adjacent to and alternate with one another to
provide a density periodicity for the device which is non-parallel to the predetermined
crystallographic direction.
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 corresponding to intervalley electron scattering of electrons by phonons.
The phonon resonator proposed in the past (PCT 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, it is often the case that there are multiple number of equivalent
intervalley directions in a given many- alley semiconductor. For example there are
three for Si and four for Ge. This means the phonon resonator proposed in PCT
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 aim to decrease the intervalley electron scattering in many- valley
semiconductor structures instead of increasing such scattering as proposed for the
phonon resonator in PCT WO96/25767.
The preferred embodiments of the present invention make use of 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, in one
embodiment, there is provided an isotope superlattice which is composed of many-
valley - otherwise known as "indirect bandgap" - semiconductor elements such as, for
example, Si and Ge, with periodically varying isotope mass modulation with the
direction of the density periodicity being non-parallel to the direction of major
intervalley electron scattering directions. That is, non-parallel to the <100> direction
for Si and non-parallel to the <111> direction for Ge. hi some embodiments, the
structures of the present invention displays increased photon emission or absorption capability relative to known indirect bandgap materials due to modification of phonon
properties associated with intervalley electron scattering.
In other embodiments, the structure in the present invention provides improved
electrical properties such as carrier mobility and electrical conductivity, again, due to
the partial or complete suppression of the intervalley electron scattering.
Preferably, the isotope superlattices of the preferred embodiments of the invention
are incorporated in active regions of one or more optical and/or electronic devices.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred 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 Si isotope superlattice which is composed
of alternating layers of 28Si and 30Si isotopes grown in the <111> direction, and that are
not parallel to the direction <100> of the major intervalley electron scattering direction
for Si;
Figure 2 illustrates phonon dispersion (phonon energy E vs. wavevector k) in
bulk Si in the <111> direction;
Figure 3 illustrates phonon dispersion (phonon energy E vs. wavevector k) in
28Si6/30Si6 isotope superlattices in the <111> direction that demonstrates an example of
the zone folding of the longitudinal acoustic (LA) phonon dispersion;
Figure 4 illustrates the Brillouin zone (14-hedron) of Si in the reciprocal
(wavevector) space;
Figure 5 illustrates a portion of an integrated circuit that utilises an isotope
superlattice of the present invention; Figure 6 is a schematic representation of a Si isotope superlattice quantum wire
fabricated on a substrate in accordance with the present invention; and
Figure 7 is a schematic representation of a Si isotope superlattice quantum dot
according to another preferred embodiment of the invention and which is fabricated on a
substrate.
MODES FOR CARRYING OUT THE INVENTION
The isotope superlattice structure of the preferred embodiments of the present
invention allows improved optical and or electrical properties to be gained from indirect
band-gap semiconductors due to the partial or complete suppression - or in some
embodiments an enhancement - of intervalley electron scattering of electrons.
Specifically, a periodic layered structure comprising an isotope superlattice in which
each layer is enriched for a particular isotope of a semiconductor element, with the
direction of the periodicity non-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 electron scattering of electrons. One example of
such a structure is shown in Figure 1. More particularly, Figure 1 shows the 28Sin/30Sim
superlattices 10 grown on a substrate 11 with the thicknesses of the 28Si and 30Si layers n
and m, respectively, in the unit of atomic layers.
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 an isotope
superlattice is defined as a layer that contains more than 92.2% of 28Si isotopes. Similarly, isotopically enriched 29Si and 30Si layers contain isotopic compositions of 29Si
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 28Sin/30Sin superlattice 10 shown in Figurel is composed of alternating layers
of isotopically enriched 28Si and 30Si layers in the crystallographic direction <111>. 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 much larger than
two and, where performance requirements outweigh space considerations, the number of
periods is extremely large.
It will be appreciated by those skilled in the art that the notation <111> includes
all <111> directions that are equivalent with respect to the crystal symmetry. That is,
directions [1,1,1], [-1,1,1], [ -1,-1,1], etc.
The substrate 11 in some embodiments is formed from Si of the natural isotopic
composition. However, in other embodiments use is made of Si of the tailor-made
isotopic composition, SiGe alloys, or other semiconductor materials.
To demonstrate the mechanism of partial or complete elimination of intervalley
electron scattering in isotope superlattices, reference is once again made to the structure
shown in Figurel with the thickness of the 28Si layer n=6 and that of 30Si m=6, that is,
28Si6/30Si6 superlattices. Figure 2 shows the schematic of phonon dispersion curves
(energy E of phonons vs. wavenumber k) for the <111> direction in bulk Si. The lattice
constant a<πι> in the <111> direction corresponds to two atomic layers thickness of Si in - li ¬
the <111> direction. The maximum value of the phonon wavevector in Figure 2 is
π/a<m> as defined by the Brillouin zone edge.
Figure 3 shows the effect of the 28Si6/30Si6 isotope superlattice formation on the
phonon dispersion curves. Due to the new periodicity a'<UI>=6a<ιn> 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<m> when only positive wavenumbers
are considered. Such zone-folding is usually referred to as a mini-Brillouin zone as
shown in Figure 3. For clarity, only zone folding of the longitudinal acoustic (LA)
phonon is shown in Figure 3. In this case, the maximum wavenumber of the phonon,
neglecting umklapp scattering, becomes π/6a<m> as defined by the edge of the mini-
Brillouin zone.
Figure 4 shows the Brillouin zone (14-hedron) of Si in 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<H)0> and 0.4π/a<100>
corresponding to the distance between A and B, C and C", and C and C" where a<100>
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<100> and 0.4π/a<100> 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
4, 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<100>, when n>4 and m>4 in a Si isotope superlattice
grown in [100] direction. In this case, the intervalley electron scattering between A and
B in Figure 4 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 the [010] and [001] directions as defined in
Figure 4 are not modified and remain unchanged from the ones in bulk Si. Thus, a Si
isotope superlattice grown in the [100] direction allows for the elimination of intervalley
electron scattering in the [100] direction only and not the ones in the [010] and [001]
directions.
The preferred embodiments of the present invention demonstrate a structure to
eliminate intervalley scatterings of all three directions, for example in Si, all at once.
The isotope superlattices of the preferred embodiments are grown in a direction that is
not parallel to the direction of the dominant intervalley electron scattering. The most
prefened direction of the isotope superlattice for the case of Si is <111> since it has the
same angles to the directions A-B, C-C", and C'-C" in Figure 4.
It is possible to eliminate all of the three scatterings by making the maximum
of the wavenumber in <111> direction short enough, that is, the [100], [010], [001]
components of the maximum vector in <111> direction become shorter than the
distance between A-B, C-C", and C'-C" in Figure 4, respectively. Such a condition can be found, for example, as follows. The Brillouin zone edge kBr<111> in the <111>
direction in terms of a<)00> is given by:
, V3 π kBr<lll> = -^ I a<n ι> • C1
The mini-Brillouin zone edge k^-^,^ in the <111> direction is given by:
kMBr<111> = 0.3 x — (2)
5 a<ι n>
when the distance between the valleys in <100> direction is chosen to be 0.3π/a<100>. In
this case the ratio kBr<m>: k^^m;, is given by 0.099:0.500 » 1:5. That is, five zone
foldings in the <111> direction correspond exactly to the <111> components of the
vectors between valleys A-B, C-C", and C'-C" in Figure 4. Therefore, intervalley
scattering can be suppressed significantly, for example, when n>6 and m>6 are chosen
for 28Sin/30Sim isotope superlattices grown in the <111> direction. If the same calculation
is repeated for the valley distance 0.4π/a<100>, the condition in the above example
changes to n>4 and m>4.
In some embodiments the optical properties of indirect-gap semiconductors are
improved using the isotope superlattices of the present invention which are grown in the
direction not parallel to the direction of the dominant intervalley electron scatterings. 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 corresponding exactly to
the distance between A and B' through the zone centre ZC in Figure 4. Likewise, it is
preferred to increase the number of phonons having wavenumbers corresponding to the
distances C-C" and C'-C" through the zone centre ZC in Figure 4. The distance A- B', C-C", and C'-C" through the zone centre ZC known in the art lies between
0.8π/a<100> and 0.85π/a<ι00>_
To increase the number of phonons in all three directions simultaneously, the
most preferced direction of the isotope superlattice for the case of Si is <111> since it
has the same angles to the directions A-B', C-C", and C-C"9 through the zone centre
ZC in Figure 4. It is possible to maximise the number of phonons in all of the three
directions by appropriately choosing the values of n and m in 2SSin/30Sim isotope
superlattices grown in the <111> direction. If the distance between A-B', C-C", and
C'-C" through the zone centre ZC in Figure 4 is taken to be 0.85π/a<100>, the [100],
[010], and [001] components of the wavevector in <111> direction become the same as
0.85π/a<100> when n=m=1.8 « 2.
If the distance between A-B', C-C", and C'-C" through the zone centre ZC
in Fig. 4 is taken to be 0.8π/a<100>, the [100], [010], and [001] components of the
wavevector in the <111> direction become the same as 0.8π/a<100> when n=m=l .9 * 2.
In fact any integer multiple of ∞ 2 for the values of n and m should satisfy the phonon
resonance condition. That is, n and m should be even numbers to promote phonon
resonance between two conduction band valleys for improved optical performance.
The above mentioned approaches are also employed to find out the appropriate
thickness of layers in isotope superlattices composed of semiconductor elements other
than Si. For example, the eight equivalent conduction bands of Ge lie in the direction
<111>. That is, it is most desirable to grow isotope superlattices in the direction <100>
for the case of Ge. The intervalley electron 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 isotope superlattices of the preferred
embodiments of the present invention in active regions of various electrical and optical
devices widely used in semiconductor 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 currents occur in optical devices. In case of Si, for example, the electron
mobility increases by a factor of about three if intervalley electron scattering is
completely eliminated. This leads to electron mobility that is comparable with or even
larger than some compound semiconductors. For example, such as that obtainable
through the use of GaAs, which is widely used for fabrication of high speed and high
frequency devices.
Figure 5 shows a portion of an integrated circuit fabricated in an n-type epitaxial
layer of isotope superlattice 10 of the present invention. The circuit is fabricated on p-
type silicon substrate 11. The illustrated portion in Figure 5 includes a metal-oxide-
semiconductor field effect transistor (MOSFET) 15 and anNPN bipolar transistor 16.
Both transistors 15 and 16 are isolated from the other and from other devices on the chip
by p+ regions 19 that are diffused through the isotope superlattice 10 into the substrate
11.
Transistor 15 includes a drain metallization 18 in contact with a p+ drain region
22, a source metallization 21 in contact with a p+ source region 23 and a gate metallization 20 isolated from electrical contact with the superlattice layer 10 by a SiO2
layer 17.
Transistor 16 includes a collector metallization 28 in contact with an n+ region
27, a base metallization 24 in contact with a p-type base region 26 and an emitter
metallization 25 in contact with an n+ emitter region 27.
In other embodiments the semiconductor isotope superlattice of the present
invention is implemented in the active regions of a wide variety of devices other than
integrated circuits. For example, alternative embodiments include light emitting
devices, light emitting diodes, semiconductor solid state lasers, optical detectors, optical
modulators, electrical conductors, planar transformers, diodes, bipolar transistors, field
effect transistors, and integrated circuits, amongst others.
In further embodiments the isotope superlattices of the invention is implemented
as a narrow wire - as best shown in Figure 6 - and as a small dot - as best shown in
Figure 7. It is also possible with advanced semiconductor technology to fabricate
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 and are quantum
structures that are well suited for use with optical and electrical devices.
It is has been shown that 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. The emission and detection of light by quantum wires and dots is made more
efficient by implementing the isotope superlattices of the preferred embodiments 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 utilizing
quantum wires and dots, for example single electron transistors and diodes, are also
improved by implementation of the isotope superlattices according to 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

CLAIMS:
1. A semiconductor isotope superlattice composed of 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 with the direction of the density periodicity of said structure being non-parallel
to the crystallographic direction of the dominant intervalley electron scattering of the
given indirect band-gap semiconductor material.
2. The semiconductor isotope superlattice of claim!, 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.
3. The semiconductor isotope superlattice of claim 1, which is composed of
constituents of indirect band-gap semiconductors including, for example, Si, Ge, and
SiGe alloys.
4. A semiconductor quantum wire composed of indirect band-gap semiconductor
elements including, 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 with the direction of the density periodicity of said structure being non-parallel
to the crystallographic direction of the dominant intervalley electron scattering of the
given indirect band-gap semiconductor material.
5. The semiconductor quantum wire of claim 4, 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.
6. The semiconductor quantum wire of claim 4 whose longitudinal direction is non-
parallel to the to the crystallographic direction of the dominant intervalley electron
scattering of the given many- valley semiconductor material.
7. A semiconductor quantum dot composed of many- valley semiconductor elements
including, 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 with the direction of the density periodicity of said structure being non-parallel
to the crystallographic direction of the dominant intervalley electron scattering of the
given many- valley semiconductor material.
8. The semiconductor structure of claim 1, 4, or 7 wherein said isotope superlattices
serve to provide improved performance of electronic devices.
9. The semiconductor structure of claim 1, 4, or 7 wherein said isotope superlattices
serve to provide improved performance of opto-electronic devices.
10. The semiconductor structure of claim 1, 4, or 7 wherein said isotope superlattices
serve to provide improved performance of semiconductor integrated circuits.
11. A semiconductor device being formed in an indirect band-gap semiconductor
material having a predetermined crystallographic direction in which the dominant
intervalley electron scattering occurs, the device including: at least one first region of a first density; and
at least one second region of a second density different from the first density,
wherein the first and second regions are adjacent to and alternate with one another to
provide a density periodicity for the device which is non-parallel to the predetermined
crystallographic direction.
12. A device according to claim 11 wherein said first region includes a first layer of
one to sixty atomic layers thickness and said second region includes a second layer of
one to sixty atomic layers thickness.
13. A device according to claim 11 wherein the indirect band-gap semiconductor
material is selected from the group: Si; Ge; and SiGe alloys.
14. A semiconductor quantum wire being formed in an indirect band-gap
semiconductor material having a predetermined crystallographic direction in which the
dominant intervalley electron scattering occurs, the device including:
at least one first region of a first density; and
at least one second region of a second density different from the first density,
wherein the first and second regions are adjacent to and alternate with one another to
provide a density periodicity for the device which is non-parallel to the predetermined
crystallographic direction.
15. A wire according to claim 14 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.
16. A wire according to claim 14 or claim 15 which extends longitudinally and non-
parallel to the predetermined crystallographic direction.
17. A semiconductor quantum dot being formed in many- valley semiconductor
material having a predetermined crystallographic direction in which the dominant
intervalley electron scattering occurs, the device including:
at least one first region of a first density; and
at least one second region of a second density different from the first density,
wherein the first and second regions are adjacent to and alternate with one another to
provide a density periodicity for the device which is non-parallel to the predetermined
crystallographic direction.
PCT/AU2000/000976 2000-08-15 2000-08-15 A semiconductor isotope superlattice WO2002015279A1 (en)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH09106946A (en) * 1995-10-11 1997-04-22 Mitsubishi Electric Corp Semiconductor device, semiconductor laser and high-electron mobility transistor device
US5917195A (en) * 1995-02-17 1999-06-29 B.A. Painter, Iii Phonon resonator and method for its production
JPH11297624A (en) * 1998-04-08 1999-10-29 Japan Science & Technology Corp Thin-film element and its manufacture

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5917195A (en) * 1995-02-17 1999-06-29 B.A. Painter, Iii Phonon resonator and method for its production
JPH09106946A (en) * 1995-10-11 1997-04-22 Mitsubishi Electric Corp Semiconductor device, semiconductor laser and high-electron mobility transistor device
JPH11297624A (en) * 1998-04-08 1999-10-29 Japan Science & Technology Corp Thin-film element and its manufacture

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Title
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