Patent Application
20110042712
The present invention broadly relates to a semiconductor material and relates particularly to a gapless semiconductor material.
A field of technology that exploits both the spin state and charge of electrons is commonly referred to as ‘spintronics’. Materials that are currently being used for spintronic applications include diluted magnetic semiconductors, ferromagnetic materials and half metallic materials.
Diluted magnetic semiconductors do not achieve 100% electron spin polarisation in most cases and the speed of mobile electrons is reduced due to electron scattering. Diluted magnetic semiconductors are also currently confined to use at relatively low temperatures as they must be ferromagnetic in order to show some degree of spin polarizations.
Conductive ferromagnetic materials can also be used to create spin polarised currents for spintronic use but are not able to achieve %100 electron spin polarisation. Again this reduces electron mobility due to electron scattering. Further, ferromagnetic materials are not semiconducting and so their applications are limited to selected spintronic devices such as spin valves.
Half metallic materials can be used to achieve 100% spin polarization, but the charge carriers and their concentration cannot be adjusted or controlled. Consequently, the half metallic materials cannot be used for semiconductor based spintronic device applications.
There is a need for technological advancement.
The present invention provides in a first aspect a new type of gapless semiconductor material having electronic properties that can be characterized by an electronic band structure, the electronic band structure comprising valence and conduction band portions VB1 and CB1, respectively, for a first electron spin polarisation, and valence and conducting band portions VB2 and CB2, respectively, for a second electron spin polarisation;
Throughout this specification the term “gapless” is used for an energy gap that of approximately 0.1 eV or smaller than 0.1 eV.
The gapless semiconductor material typically is arranged so that the Fermi level is, without an external influence, positioned in the proximity of the maximum of VB1.
The first energy level typically is a maximum of VB1 and the second energy level typically is a minimum of the one of CB1 and CB2.
Throughout this specification, the term “external influence” is used for any force, field or the like that results in a shift of the Fermi level relative to the electronic bands of the gapless semiconductor material. For example, the external influence may be provided in the form of an electrical field associated with a voltage applied across the gapless semiconductor material.
Gapless electronic transitions, requiring only a very small excitation energy, are possible between VB1 and the one of CB1 and CB2. However, an energy gap is defined between VB2 and the other one of CB1 and CB2 and energy is required for electronic excitations from VB2 to CB1 or CB2. Consequently, the gapless semiconductor material has the significant advantage that gapless electronic excitations are possible and all excited electrons and/or hole charge carriers, up to a predetermined excitation energy, have the same spin polarization.
The bandgap may be a direct or an indirect bandgap. Further, gapless transitions may be a direct or indirect transitions.
Because gapless electronic transitions are possible, the electronic properties of the gapless semiconductor material typically are very sensitive to a change in external influences, such as a change in an external magnetic or electric fields, temperature or pressure, light and strain etc. The full spin polarisation reduces electron scattering probabilities and consequently the electron mobility typically is relatively large, such as 1 to 2 orders of magnitude larger than that of conventional semiconductor materials. The gapless semiconductor material according to an embodiment of the present invention combines the advantages of gapless electronic transitions in a semiconductor material with full spin polarisation and consequently opens avenues for new applications, such as new or improved “spintronic”, electronic, magnetic, optical, mechanical and chemical sensor devices applications.
The energy maximum of VB1 and the energy minimum of the one of CB1 and CB2 may for example have an energetic separation in the range of 0-0.01 eV, 0-0.02 eV, 0-0.04 eV, 0-0.05 eV, 0-0.06 eV, 0-0.08 eV, 0-0.1 eV and may also have a slight overlap.
The predetermined energy depends on the energetic position of the energetic band portions relative to each other. The predetermined energy typically is within the range of 0 eV to EG or 0 to 0.5 EG (EG: bandgap energy). The bandgap energy EG typically is in the range of 0.2 to 5 eV or 0.2 to 3 eV.
The gapless semiconductor material typically is arranged so that electronic properties are controllable by controlling the position of the Fermi level relative to the energy bands. For example, the gapless material may be arranged so that a shift of the Fermi level position relative to the energy bands by a predetermined energy results in generation of fully polarised free charge carriers. In one specific example the gapless semiconductor material is arranged so that a predetermined shift of the Fermi level relative to the energy bands results in a change in one type of fully polarised charge carriers to another type of fully polarised charge carriers with and without a change in polarisation.
The gapless semiconductor may be arranged so that electrons excited from VB1 or VB2 to CB1 or CB2 have full spin polarisation. Alternatively or additionally, the gapless semiconductor may be arranged so that hole charge carriers in VB1 or VB2 have full spin polarisation.
In a first embodiment of the present invention the maximum of VB1 and the minimum of CB1 are positioned in the proximity of each other and typically in the proximity of the Fermi level. In this embodiment the bandgap EG is defined between VB2 and CB2. For example, the maximum of VB2 may be positioned at the Fermi level and the minimum of CB2 may be positioned at an energy of EG above the Fermi level. In this case all electrons that were excited from VB1 to CB1 have the same spin polarisation for an excitation energy up to EG. Alternatively, the minimum of CB2 may be positioned at the Fermi level or the maximum of VB2 may be positioned below the Fermi level. In this case all hole charge carriers in VB1 have the same spin polarization for an excitation energy up to EG. In a further example, the material may be arranged so that the Fermi level is positioned substantially in the middle of the bandgap. In this case all electrons excited from VB1 to CB1 have the same spin polarisation for an excitation energy up to 0.5 EG and also all corresponding hole charge carriers in VB1 have the same spin polarisation.
In a second embodiment of the present invention the maximum of VB1 and the minimum of CB2 are positioned in the proximity of each other and typically in the proximity of the Fermi level. In this embodiment a first bandgap is defined between VB1 and CB1 and a second bandgap typically is defined between VB2 and CB2. A gapless electronic transition from VB1 to CB2 is associated with a change in spin polarisation. In this embodiment the gapless semiconductor material is arranged so that electrons excited from VB1 to CB2 have full spin polarisation up to an excitation energy that corresponds to an energy difference between the minimum of CB1 and the minimum of CB2 and corresponding hole charge carriers of VB1 have full opposite spin polarisation.
In the above-described second embodiment of the present invention the gapless semiconductor material typically is arranged so that, if the Fermi level is shifted by an external influence below the maximum of VB1 to a position at or above the maximum of VB2, fully polarised hole charge carriers are generated in VB1. Further, the gapless semiconductor material typically is arranged so that, if the Fermi level is shifted by the external influence above the maximum of VB1 to a position at or below the minimum of CB2, CB2 includes fully polarised electrons, which are polarised in a direction that is opposite to that of the polarised hole charge carriers in VB1 generated by lowering the Fermi level.
The gapless semiconductor material may have a dispersion relation that is at least in part a substantially quadratic function of momentum. Alternatively, the material may also have a dispersion relation that is at least in part a substantially linear function of momentum.
The gapless semiconductor material may be provided in any suitable form and typically comprises an indirect or direct gapless semiconductor material that is doped with magnetic ions.
The gapless semiconductor material may comprise a material that is associated with a transition from half metal to magnetic semiconductor. In one specific embodiment of the present invention the gapless semiconductor material is provided in the form of an oxide material, such as a material of the type AxByOz where A is a group 1, group 2 or rare earth element. B is a transition metal and the parameters x, y and z are typically within the range of 0-4. For example, the gapless material may comprise a lead-based oxide, and typically comprises PbPdO2. In this embodiment the gapless semiconductor material is doped with Cobalt ions and at least some, typically approximately 25%, of the Palladium ions of the PbPdO2 are replaced by the Cobalt ions. The inventor has observed that PbPdO2 doped with Cobalt is a material that has electronic properties in accordance with the above-described second specific embodiment of the present invention.
Alternatively, the gapless semiconductor material may comprise any suitable type of graphene (a single layer of graphite with or without doping and with or without modifications to surfaces and/or edges or any type of gapless semiconductor material or narrow band materials that is doped in a suitable manner.
The valence band and conduction bands of the gapless semiconductor material may have band bendings that are chosen so that excited polarised electrons and hole charge carriers have differing speeds whereby separation of the excited electrons and hole charge carriers from each other is facilitated.
The present invention provides in a second aspect a source of polarized light, the source comprising:
The other one of CB1 and CB2 typically is CB2. The excitation source may be a photon source. The source of polarised light typically is arranged so that electron transitions from VB2 to the either CB1 or CB2 are substantially avoided.
The above-defined source of polarized photons typically is arranged so that excited electrons and holes have a spin that is predetermined by possible electronic transitions and recombination of the excited electrons and the holes typically results in emission of polarized photons.
The present invention provides in a third aspect a source of polarized light, the source comprising:
VB1, VB2, CB1 and CB2 typically have energy levels that are arranged so that the first energy bandgap is defined between VB1 and CB1 and the second energy bandgap between VB2 and CB2. The excitation source typically is arranged for exciting electrons from VB1 to CB1 and arranged so that an excitation energy is insufficient for exciting electrons from VB2 to CB2.
The excitation source may be a photon source. The source of polarized electrons typically is arranged so that electronic excitations form VB1 to CB2 and/or from VB2 to CB1 are substantially avoided.
The present invention provides in a fourth aspect a gapless semiconductor material comprising an oxide material and having electronic properties that can be characterized by an electronic band structure, the electronic band structure comprising a valence band VB and a conduction band CB;
The oxide material typically is of the type AxByOz where A is a group 1, group 2 or rare earth element. B is a transition metal and the parameters x, y and z are typically within the range of 0-4. In one specific example the gapless semiconductor material is a lead-based oxide such as PbPdO2.
Alternatively, the gapless semiconductor material may be provided in the form of AxByCzDqOt where A and B are a group 1, group 2 or rare earth element, C and D are transition metal and elements in III, VI, and V family, O is oxygen, and the parameters x, y, z, q, t are within the range of 0-12.
The present invention provides in a third aspect an electronic device comprising the gapless semiconductor material in accordance with the first or second aspect of the present invention.
The electronic device typically comprises a component for generating an external influence and thereby shifting a Fermi level position of the gapless semiconductor material relative to energy bands. Further, the electronic device may comprise a separator for separating excited polarised electrons and hole charge carriers from each other. In one embodiment the separator is arranged to operate in accordance with the principles of the Hall effect.
The invention will be more fully understood from the following description of specific embodiments of the invention. The description is provided with reference to the accompanying drawings.
Embodiments of the present invention provide a gapless semiconductor material that is arranged for full spin polarization of excited electrons and/or hole charge carriers up to a predetermined excitation energy. The gapless semiconductor material combines the advantages of gapless semiconductor transitions with those of full spin polarization and consequently opens new avenues for new or improved electronic, magnetic, optical, mechanical and chemical sensor devices applications
With reference to
In this embodiment the maximum of the valance band portion VB2 is also positioned at the Fermi level, but the minimum of the conducting band portion CB2 is separated from the maximum of the valance band portion VB2 by a bandgap. Consequently, for electronic transitions from the valance band into the conducting band the only available empty electronic states are those of CB1 that are positioned at an energy between the Fermi level and the minimum CB2 if the excitation energy is below an energy that corresponds to the bandgap. In this case, all excited electrons are fully polarized.
The energetic position of the Fermi level relative to the energy bands of the gapless semiconductor material can be altered by an external influence such as an external voltage applied across the gapless semiconductor material. The charge carrier concentration may be controlled by choosing the position of the Fermi level relative to the energy bands. For example, if the Fermi level is lifted relative to the energy bands to a position below the minimum of CB2, the conducting band portion CB1 has occupied electronic states that are fully polarized.
For example, the Fermi level position may be lifted to a slightly higher energy, but below the minimum of CB1. In this case, CB2 would contain occupied electronic states that are fully polarized. If, on the other hand, the Fermi level is slightly shifted to a lower position but above the maximum of VB2, fully polarized hole charge carriers are generated in VB1. The generated hole charge carriers have a polarization that is opposite that of the occupied electronic states generated by lifting the Fermi level. Consequently, it is possible to change the type of charge carriers and their polarization by controlling the Fermi level position using an external influence.
The gapless semiconductor may for example be provided in the form of an AxByOz oxide material, where A is a group 1, group 2 or rare earth element. B is a transition metal or III, IV, V family elements and the parameters x, y and z are within the range of 0-4. In this example the gapless material comprises PbPdO2. In this embodiment the gapless semiconductor material is doped with Co ions and approximately 25% of the Pd ions of the PbPdO2 are replaced by the Co ions.
The PbPdO2 material may be formed by mixing powders of PdO, PdO and CoCO3. The mixture is then palletized and then sintered at a temperature of approximately 600-900° C. for approximately 3-10 hours. For the manufacture of thin film samples a bulk target of Pb—Pd—Co—O may initially be formed and then a pulsed laser deposition method may be used to deposit the thin film material on suitable substrates at a temperature of approximately 400-900° C. in an atmosphere of Argon with oxygen partial pressure.
It is to be appreciated by a person skilled in the art that the gapless semiconductor material may be provided in many different forms. Generally, the specific gapless semiconductor material having the described properties typically comprises a gapless semiconductor material that is doped with a suitable dopant, typically magnetic ions. Alternatively, the gapless semiconductor material may comprise any other suitable type of material doped with magnetic ions including graphine and Hg based IV-VI materials such as HgCdTe, HgCdSe or HgZnSe.
a) shows an electronic band structure for PbPdO2 calculated for high symmetry points in the Brillouin zone.
a) indicates that there is no forbidden band or bandgap present at the Γ point indicating that PbPdO2 is a typical direct gapless semiconductor (direct refers to transitions across the bandgap).
b) shows a spin resolved electron band structure of PbPdO2 with a 25% doping level of Co. The solid lines in 5(b) indicate the band structure of “spin up” electrons. The dotted lines in
b) shows that for Co-doped PbPdO2, the highest valence band of the spin up electrons is adjacent the Fermi level at the Γ points. The lowest conduction band is also adjacent the Fermi level at the U point and between the T and Y points. The valence band of the spin up electrons (VB1) and the conduction band of the spin down electrons (CB2) is therefore shown to be indirectly gapless.
The band structures shown in
The electronic device 100 comprises a separator 106 that is arranged to separate electrons from hole charge carriers. The separator 106 is arranged for generating a magnetic field. Electrons and hole charge carriers that move through the material 102 in a direction as indicated by arrows in
Although the invention has been described with reference to particular examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. For example, the gapless semiconductor material may not be an oxide material. Further, a person skilled in the art will appreciate that the band structure diagrams shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008901173 | Mar 2008 | AU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AU2009/000293 | 3/12/2009 | WO | 00 | 11/9/2010 |
