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  • H10F71/1276—The active layers comprising only Group III-V materials, e.g. GaAs or InP comprising growth substrates not made of Group III-V materials
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  • H10F77/143—Shape of semiconductor bodies; Shapes, relative sizes or dispositions of semiconductor regions within semiconductor bodies comprising quantum structures
  • H10F77/1437—Quantum wires or nanorods
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  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy
  • Y02E10/544—Solar cells from Group III-V materials
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  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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  • Y02E10/50—Photovoltaic [PV] energy
  • Y02E10/548—Amorphous silicon PV cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

• the present invention relates to semiconductor tunnel diodes, and in particular to tunnel diodes made using nanowires.
• the tunnel diode is based on interband tunneling of charge carriers.
• the tunnel diode is made up by two layers of degenerately doped semiconductor material of different doping types in contact.
• n ++ will denote degenerate doping with donors and p ++ degenerate doping with acceptors.
• Fig. 1 schematically illustrates the current (A) through the tunnel diode when a voltage (V) is applied across the junction made up by these layers.
• the Fermi level will be in the conduction band for the n ++ side, and in the valence band for the p ++ side. This leads to charge carriers on each side of the junction having the same energy and opposite charge, thereby allowing tunneling and subsequent annihilation of the charge carriers.
• Fig. 2 in a)-d) schematically illustrates the band diagram for points A-D as indicated in Fig. 1 , respectively.
• Ec is the conduction band energy
• Ev is the valence band energy
• EF is the Fermi level energy
• EFH and EF p are the Fermi level energy for the n-type and p-type sides of the junction with an applied voltage, respectively.
• the tunneling rate scales inversely exponential both to the height of the barrier made by the band gap between the conduction band and the valence band, as well as the thickness of the barrier.
• the thickness is given by the depletion region width, which is given by the doping concentration in the material. Also, the electron and hole masses are important for the tunneling rate. In point A, at zero voltage, the junction function as an ohmic resistance, and in this analogue, the resistance is inversely proportional to the tunneling rate.
• Tunnel diodes have been used for oscillators, amplifiers, heterojunction bipolar transistors, as well as pressure gauges and light emitting diodes.
• Other applications for tunnel diodes are in low-power memory cells, so- called tunneling SRAMs, and in latches monolithically integrated with a standard CMOS process, as well as for interconnects in monolithically integrated multi- junction solar cells.
• tunnel diodes Although a great potential benefit in many applications the use of tunnel diodes is limited due to unsatisfactory performance primarily due to technical barriers in fabrication. Fabrication of prior art tunnel diodes is commonly based on epitaxial thin film growth and photolithography and etching, and hence, tunnel diodes are primarily made in Si, Ge and GaAs based materials and the scalability is limited.
• one object of the invention is to provide improved tunnel diodes.
• a tunnel diode according to the invention comprises a p-doped semiconductor region and an n-doped semiconductor region forming a pn-junction.
• the pn-junction is formed at least partly within a nanowire, either in an axial or a core-shell configuration.
• the p-doped semiconductor region comprises a degenerately doped p++ segment adjacent to a degenerately doped n++ segment of the n-doped semiconductor region.
• the semiconductor materials of the tunnel diode can be chosen so that they are the same on both sides of a junction, i.e. a homojunction device. It is also possible to have different semiconductor materials on different sides of the junction i.e. a heterojunction device. In this case, there are different types of material combinations, resulting in type-I (straddling gap) or type-II (staggered gap) combinations, wherein the a n ++ segment is grown on a p ++ segment or a p ++ segment is grown on a n ++ segment. Another possibility is to combine materials so that the conduction band energy for the material on one of the sides of the junction is lower than the valence band energy for the material on the other side of the junction. This is a type-Ill (broken gap) heterojunction, which does not require degenerate doping.
• the nanowire geometry allows strain relaxation via the surface, allowing for a much broader range of heterostructure combinations than for thin-film growth as the requirement for lattice matching is essentially removed. This opens up the potential of using type-II and type-Ill material combinations that cannot formed by prior art techniques. These material combinations have the promise of much better performance due to the reduced tunnel barrier height. Additionally, heterostructures forming quantum wells at one or both sides of the junction may be utilized, forming a so-called resonant interband tunnel diode. Reduced lattice mismatch requirements also opens up for growth of compound semiconductors on semiconductor substrates not readily made with prior art techniques, such as III-V semiconductors on Si.
• the present invention provides tunnel diodes made of compound semiconductor materials selected from the group of: Ga, P, In, As, thereby forming type-I (Straddling gap) heterojunction tunnel diodes or type-II (Staggered gap) heterojunction tunnel diodes.
• type-I Spaddling gap
• type-II Staddling gap
• type-II Staddling gap
• Sb-based compound semiconductors type-Ill (Broken gap) heterojunction tunnel diodes can be formed.
• These type of tunnel diodes improves the transmission properties of the tunnel diode.
• the Sb content can be increased to levels not possible in prior art technology, i.e. binary, ternary, quaternary and quinary Sb-base compounds can be formed and combined with other semiconductor compounds although the lattice mismatch may be significant.
• Such high Sb content would be detrimental in many prior art devices, in particular for optoelectronic devices since light would be absorbed in a region of high Sb content.
• a method of manufacturing a tunnel diode comprises the steps of providing a semiconductor substrate; growing a nanowire on the semiconductor substrate, whereby a pn-junction 6 comprising a p-doped semiconductor region 4 and an n-doped semiconductor region 5 at least partly within the nanowire 1 is formed.
• tunnel diodes are a required building block to render nanowire multi-junction solar cells possible.
• a multi -junction solar cell comprising the tunnel diode according to the invention is provided.
• a fundamental feature of nanowires is the narrow lateral size and the epitaxial, potentially defect-free, growth.
• the bottom-up approach of nanowire growth is easily scalable to smaller diameters and avoids the defects often induced in top- down processes based on etching.
• Embodiments of the invention are defined in the dependent claims. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims.
• Fig. 1 schematically illustrates the VI curve for a tunnel diode
• Figs. 2a-d schematically illustrate band diagrams for different points (A) -(D) representing different properties of the tunnel diode points, in (A) ohmic resistance, in (B) current maximum, in (C) negative differential resistance, and in (D) normal diode in forward bias;
• Fig. 3 schematically illustrates a nanowire tunnel diode in axial configuration according to the invention
• Fig. 4 schematically illustrates a nanowire tunnel diode in core-shell configuration according to the invention
• Fig. 5 schematically illustrates different tunnel diode configurations according to the invention
• Fig. 6 schematically illustrates a chart of different materials combinations for a tunnel diode according to the invention
• Fig. 7 schematically illustrates different III-V compound tunnel diode junctions according to the invention.
• Fig. 8 schematically illustrates different compound tunnel diode junctions comprising Sb according to the invention.
• Fig. 9 is a schematic diagram of the growth process of example 1 according to the invention.
• Fig. 10 shows a measurement of the current through a single nanowire with light (solid line) and without (dash-dot) in accordance with example 1 according to the invention
• Fig. 1 1 shows a SEM picture (left) of nanowire heterojunction tunnel diodes made up by n-type InP and (right) a VI curve for a single nanowire.
• nanowire is to be interpreted as a structure being essentially of nanometer dimensions in its width or diameter. Such structures are commonly also referred to as nanowhiskers, nanorods, etc.
• VLS vapour-liquid-solid
• the present invention is limited to neither such nanowires nor the VLS process.
• Other suitable methods for growing nanowires are known in the art and is for example shown in international application No. WO 2007/ 102781. From this it follows that nanowires may be grown without the use of a particle as a catalyst. Thus selectively grown nanowires and nanostructures, etched structures, other nanowires, and structures fabricated from nanowires are also included.
• the p-doped semiconductor region 4 comprises a degenerately doped p++ segment 4' adjacent to a degenerately doped n++ segment 5' of the n-doped semiconductor region 5, however not limited to this, as explained in the following.
• the functionality of the tunnel diode is as described in the background. During operation the tunnel diode has to be connected to terminals arranged at end portions of the tunnel diode in order to apply a voltage over the tunnel diode.
• the nanowire 1 is grown from an upper surface of a semiconductor substrate
• the semiconductor substrate 3 forms part of the tunnel diode or a semiconductor device comprising the tunnel diode the nanowire 1 protrudes from the semiconductor substrate 3 in a direction parallel with a normal direction, or in an pre-determined inclined relationship, with of the surface.
• the substrate 3 may be only a passive carrier for the nanowire 1 or part of an electrical circuit comprising the tunnel diode, for example being functional as one of the connecting terminals or forming part of the pn-junction.
• the semiconductor substrate 3 itself has to be doped or provided with a doped, or conductive, layer at the upper surface. Such layers are commonly referred to as buffer layers.
• a tunnel diode with an axial configuration comprises at least a degenerately doped p++ segment 4' epitaxially grown on a degenerately doped n++ segment 5' within a nanowire 1 that protrudes from an upper surface of a semiconductor substrate 3.
• a tunnel diode with a core-shell configuration comprises a degenerately doped p++ segment 4' epitaxially grown as a shell 8 enclosing at least a portion of a degenerately doped n++ segment 4' of a nanowire core 9.
• Fig. 3 and 4 illustrates one embodiment where the nanowire 1 is electrically connected to the substrate 3 and a dielectric layer is arranged on the upper surface, however not limited to this.
• the nanowires 1 of Figs. 3 and 4 comprises additional segments of different doping and / or composition arranged along the length of the nanowire and / or radially enclosing at least a portion of the nanowire in a core-shell configuration in order to form functional parts analogous to different semiconductor devices such as field-effect transistors, photodetectors, light emitting diodes, etc.
• the semiconductor materials of the tunnel diode can be chosen to be the same on both sides of a junction, i.e. forming a homojunction as schematically illustrated in Fig. 5a, or to have different semiconductor materials on different sides of the junction i.e. forming a heterojunction as schematically illustrated in Fig. 5b -f. In this case, there are different types of material
• type-I straddling gap
• type-II staggered gap
• a n ++ segment is grown on a p ++ segment or a p ++ segment is grown on a n ++ segment.
• Another possibility is to combine materials so that the conduction band energy for the material on one of the sides of the junction is lower than the valence band energy for the material on the other side of the junction, resulting in type-Ill (broken gap) heterojunction.
• the junction may comprise some intermediate layer of different composition, i.e. at least one of the segments 4', 5' comprises a sub -segment in an end portion adjacent to the other segment 4', 5', as long as this does not significantly affect the tunneling properties.
• the tunnel diode comprises at least a degenerately doped n++ segment 5' epitaxially connected to a degenerately doped p++ segment 4'.
• the tunnel diode comprises at least a degenerately doped n++ segment 5' epitaxially connected to a degenerately doped p++ segment 4'.
• Type-I heterojunctions shown in Fig. 7 are p++ GaP/n++ InAs, p++ GaP/n++ GaAs and p++ InP/n++ InAs and type-II heterojunctions shown in Fig. 7 are p++ GaP/ n++ InP, p++ GaAs/n++ InAs and p++ GaAs/n++ InP, whereof preferred combinations are type-I p++ InP/n++ InAs and type-II p++ GaP/ n++ InP, p++ GaAs/n++ InAs and p++ GaAs/n++ InP.
• suitable semiconductor materials for the tunnel diode include, but are not limited to, combinations of binary, ternary, quaternary and quinary compound semiconductors from the group of Ga, P, In, As, Sb.
• the compound semiconductors may also include Al.
• exemplified materials are GaP 2.78eV, GaAs 1.42eV, GaSb 0.73eV, InP 1.35eV, InAs 0.36eV, InSb 0. 17eV.
• the chart of Fig. 6 and the examples of Figs. 7-8 gives an overview of suitable heterostructure combinations for the tunnel diode.
• the heterostructure combinations comprising a Sb-based material, schematically illustrated in Fig. 8, are of particular interest.
• Preferred compound semiconductor combinations are the type-I combinations n++ InAs/p++ GaP and n++ InAs / p++ InP as well as the type-II combinations n++ InP/p++ GaP, n++ InP/p++ GaAs, n++ InP/p++ GaSb, n++ InAs/p++ GaAs and n++ InSb/p++ GaSb. Further preferred combinations are the type-Ill combinations n- or i-type InAs / p- or i-type GaSb and n- or i-type InAs /p- or i-type InSb. In the chart preferred combinations are indicated by a "+"-sign, and more preferred materials are indicated by a "++"-sign.
• the tunnel diode comprises at least a degenerately doped n++ segment 5' epitaxially connected to a degenerately doped p++ segment 4'.
• the tunnel diode comprises at least a degenerately doped n++ segment 5' epitaxially connected to a degenerately doped p++ segment 4'.
• Type-I heterojunctions shown in Fig. 8 are p++ GaP/n++ GaSb, p++GaP/n++ InSb, p++ GaAs/n++ GaSb, p++ InP/n++ InSb and p++ GaAs/n++ InSb.
• Type-II heterojunctions shown in Fig. 8 are p++ InP/n++ GaSb and p++ GaSb/n++ InSb.
• Type-Ill heterojunctions shown in Fig. 8 are p++ InAs/n++ GaSb and p++ InAs/n++ InSb. As mentioned above the requirements on doping for these type III heterojunction segments are moderate as compared to prior art technology.
• a tunnel diode comprising a heterojunction formed by adjacent degenerately doped segments in accordance with the invention may comprise one or more additional segments of different doping and/ or composition in connection to the degenerately doped segments.
• a n/p-doped segment having significantly lower doping level, optionally of different material composition is placed adjacent to a n++/p++ degenerately doped segment, or as shown in Fig. 5f, a n and p-doped segments having significantly lower doping level, optionally of different material composition, placed adjacent to a the n++ and p++ degenerately doped segment, respectively.
• suitable methods for growing nanowires are known in the art and is for example shown in PCT application No. WO 2007/ 102781 , incorporated by reference.
• a method for manufacturing a tunnel diode according to the invention comprises the steps of:
• a pn- junction 6 comprising a p-doped semiconductor region 4 and an n-doped semiconductor region 5 at least partly within the nanowire 1 is formed.
• Nanowire growth is initiated by supplying suitable precursor gases.
• Material composition may be varied by changing the concentration or the composition of these gases during growth.
• the step of growing preferably further comprises the step of degenerately doping at least a p++ segment 4' of the p-doped region 4 and a n++ segment 5' of the n-doped region 5.
• the doping may be accomplished by supplying the dopant in gas phase during growth.
• Suitable precursor gases for formation of the nanowires and nanowire segments comprising compound semiconductors made of InGaAsSbP-materials include, but are not limited to: AsH3, TBP, TBAs, TMln, TMGa, TEGa, TESb, and TMSb.
• Suitable gases for doping include, but are not limited to: DMZn, DEZn, TESn, H 2 S, and H 2 Se.
• a homojunction tunnel diode is demonstrated. Further the example demonstrates how two diodes, acting as photovoltaic cells, in an InP nanowire are monolithically contacted with the tunnel diode.
• the nanowire was nucleated on a Si substrate in accordance with techniques according to prior art, and the growth of the nanowire was then continued, comprising the following steps:
• TMln, PH3 and TESn were supplied to the growth reactor.
• TMln and PH3 are the precursors for the InP, while Sn is incorporated from the TESn precursor, resulting in n-doping of the InP.
• a small flow of HC1 was added to the gas mixture to remove any growth on the sidewall of the nanowire. This flow was maintained throughout the growth of the wire.
• a flow of DEZn was added to the gas mixture in the growth reactor to achieve an extrinsic p-doped region.
• the DEZn flow was turned up to increase the incorporation of Zn, resulting in a section with substantially higher doping level. This is the first section of the tunnel diode.
• the DEZn flow was chosen so that only a small increase would have resulted in a loss of epitaxial growth of the nanowire.
• the flow of DEZn was enough to reach degenerate doping in spite of the surface pinning of InP, which is beneficial for n-type doping rather than p-type doping.
• the DEZn flow was turned off completely, and a large flow of TESn was immediately turned on instead.
• Sn can be incorporated in InP nanowires to a very high level without losing epitaxial growth, it was possible to achieve an abrupt change in doping in spite of the Au seed particle acting as a buffer of the doping atoms. Due to the surface pinning of the InP and the high flow of Sn, only a fraction of the available Sn needs to be incorporated into the nanowire to achieve n-type degenerative doping and thereby avoiding the delay of the buffering effect in the Au.
• a flow of DEZn was added to the gas mixture in the growth reactor to achieve an extrinsic p-doped region.
• the growth temperature was kept at 420° C throughout the whole process.
• a schematic diagram of the growth process is shown in Fig. 9 with the corresponding doped sections of the InP nanowire.
• a single wire was broken off from the silicon substrate and metal contacts were made to each end of the wire. This device was investigated by measuring the current through the wire as a function of the applied voltage. The measurement data can be seen in Fig. 10.
• the applied voltage that is needed to keep the current through the wire at 0 A is known as the open-circuit voltage (Voc).
• Voc open-circuit voltage
• the relatively high Voc proves the functionality of the tunnel diode as this would not be possible if the two rectifying diodes were not contacted in series through the tunnel diode.
• This type of device is known as a tandem photovoltaic cell.
• Fig. 1 1 shows a SEM picture (left) of nanowire heterojunction tunnel diodes made up by n-type InP (lower part of wire) and p-type GaAs (top part of wire) . Fabrication of the structures of Fig. 1 1 comprised the steps of:
• TMIn and PH3 are the precursors for the InP, while Sn is incorporated from the TESn precursor, resulting in degenerate n-doping of the InP.
• the growth temperature was 420° C.
• this device was investigated by breaking of single wires and contacting each end.
• the current through a single wire as a function of the applied voltage can be seen in Fig. Z (right).
• the NDR region 18 the device shows the characteristics of negative differential resistance. Thereby, this device functions as a heterojunction tunnel diode in a III-V nanowire.
• suitable materials for the substrate include, but are not limited to: Si,
• Ge SiGe, GaAs, GaP, GaAs, InAs, InP, GaN, A1 2 0 3 , SiC, GaSb, ZnO, InSb, SOI (silicon-on-insulator), CdS, ZnSe, CdTe.
• Suitable materials for the nanowires and nanowire segments include, but are not limited to: GalnAsPSb, GaAsSb, InAsSb, GaPSb, InPSb, GaAsPSb, InAsPSb, InGaAsP, InGaAsSb, InGaPSb, InGaAsPSb AlGalnN, AllnP, BN, GalnP, GaSb, GaAs, GaAsP, GaAlInP, GaN, GaP, GalnAs, GalnN, GaAlInP, GaAlInAsP, GalnSb, Ge, InAs, InN, InP, InAsP, InSb, Si, ZnO.
• Possible donor dopants are Si, Sn, Te, Se, S, etc, and acceptor dopants are Zn, Fe, Mg, Be, Cd, etc.
• AB binary compound consisting of an element A and an element B
• a x Bi- x where 0 ⁇ x ⁇ l .
• ternary, quaternary and quinary compounds when mentioned in a general context, such as when referring to InGaAsSbP-materials, 0 ⁇ x ⁇ 1.

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