WO2006125272A1

WO2006125272A1 - Resonant defect enhancement of current transport in semiconducting superlattices

Info

Publication number: WO2006125272A1
Application number: PCT/AU2006/000704
Authority: WO
Inventors: Martin Andrew Green
Original assignee: Newsouth Innovations Pty Limited
Priority date: 2005-05-27
Filing date: 2006-05-26
Publication date: 2006-11-30

Abstract

An artificial amorphous semiconductor composite material is provided having a controlled bandgap and mobility. The material comprises a plurality of crystalline semiconductor material quantum dots substantially uniformly distributed and regularly spaced in three dimensions through a matrix of dielectric material, high bandgap semiconductor materi or thin layers of such materials. The bandgap and mobility of the composite material are determined by selecting the material parameters including the size of the quantum dots, and wherein the composition of the resulting matrix includes defects which enhance mobility based on resonance between defect sites in the matrix and the quantum stat of the quantum dot.

Description

Resonant defect enhancement of current transport in semiconducting superlattices

Cross-Reference to Related Applications

The present application claims priority from Australian Provisional Patent Application No 2005902712 filed on 27 May 2005, the content of which is incorporated herein by reference.

Introduction

The present invention relates generally to the field of semiconductor physics and in particular the invention provides improvements in a new class of materials known as artificial amorphous semiconductors with particular application to thin-film solar cells using those materials.

Background of the Invention

Semiconductor superlattices were first developed in the early 1970s as a way of extending the range of electronic properties and functions available to semiconductor materials. In the initial work on superlattices, thin layers of semiconductor 11, 12, 13, 14, 15, only a few nanometers thick, were grown on top of one another, over a substrate 16, as illustrated in Fig. 1, to form a one dimensional (ID) superlattice 17. By choosing suitable semiconductors for materials A & B, the energy states available to carriers can be manipulated by the period of the superlattice, as in Fig. 2. Confined electronic states 18 can be formed in regions low in electron or hole energy separated by barrier regions 19. If the spacing of such superlattices is sufficiently small, these states 18 can broaden out into bands. Carriers can be transported within such bands through the intervening barrier regions 19, by quantum mechanical tunnelling processes. For good transport, very tight specifications are required on the uniformity of the layer thicknesses of the layers 11, 12, 13, 14, 15 and the quality of the superlattice interfaces. These exacting requirements are met for a number of applications including lasers and light-emitting diodes based on Group III-V semiconductors

There is growing interest in extending these ideas to material where the size is controlled in all three spatial dimensions. This provides many challenges not only in finding ways to deposit material with regular structure in all three dimensions, but also in getting sufficient control over spatial uniformity for good transport properties. Summary of the Invention

According to a first aspect, the present invention consists in a an artificial amorphous semiconductor composite material having a controlled bandgap and mobility comprising a plurality of crystalline semiconductor material quantum dots substantially uniformly distributed and regularly spaced in three dimensions through a matrix of dielectric material, high bandgap semiconductor material or thin layers of such materials wherein the bandgap and mobility of the composite material are determined by selecting the material parameters including the size of the quantum dots, the composition of the matrix and the semiconductor material of the quantum dots, and wherein the composition of the resulting matrix includes defects which enhance mobility based on resonance between defect sites in the matrix and the quantum states in the quantum dot.

According to a second aspect of the invention a method of forming an artificial amorphous semiconductor material having a controlled bandgap and mobility comprises; forming a plurality of layers of a matrix of material comprising a dielectric or high bandgap semiconducting compound, wherein alternating layers are layers of stoichiometric matrix material and layers of semiconductor rich matrix material respectively, at least some of these layers including defects, or having a propensity to form defects during processing, and heating the layers of matrix material to cause quantum dots to form in the semiconductor rich layers of matrix material whereby they are uniformly distributed and regularly spaced in three dimensions through the matrix material, wherein the bandgap and mobility are determined by selecting the material parameters including the size of the quantum dots, the composition of the matrix and the semiconductor material of the quantum dots to achieve the desired parameters, and wherein the composition of the resulting matrix material includes defects which enhance mobility based on resonance between defect sites in the matrix and the quantum states in the quantum dot.

According to a third aspect, the present invention consists in a photovoltaic junction comprising an n-type region of artificial amorphous material adjacent a p-type region of artificial amorphous material forming a junction there between, the n-type and p-type artificial amorphous materials being integrally formed as a matrix of material containing defects and in which is substantially regularly disbursed a plurality of crystalline semiconductor material quantum dots and wherein the composition of the resulting matrix includes defects which enhance mobility based on resonance between defect sites in the matrix and the quantum states in the quantum dots, and the n-type and p-type regions are respectively doped with n-type and p-type dopant atoms.

According to a fourth aspect of the invention a method of forming an artificial amorphous semiconductor material photovoltaic cell comprises; forming a plurality of layers of a matrix material comprising a high bandgap semiconducting compound or a dielectric material comprising a compound of a semiconducting material, wherein alternating layers of the matrix material are layers of stoichiometric matrix material and layers of semiconductor rich matrix material respectively, and at least some of these layers include defects or have a propensity to form defects during processing, doping regions of the plurality of layers of matrix material with p-type and n- type dopants either simultaneously with their formation or subsequently, and heating the layers of matrix material to cause quantum dots to form in the semiconductor rich layers, wherein the bandgap and mobility are determined by selecting the material parameters including the size of the quantum dots, the composition of the matrix material and the semiconductor material of the quantum dots to achieve the desired parameters, and wherein the composition of the resulting matrix includes defects which enhance mobility based on resonance between defect sites in the matrix and the quantum states in the quantum dots.

A region in the vicinity of the junction between the n-type and p-type regions of the artificial amorphous material may be undoped or have a balance of n-type and p- type dopants whereby the region behaves as intrinsic material. The quantum dots are distributed in layers throughout the artificial amorphous material and, for photovoltaic applications each of the n-type and p-type regions will typically include 20-50 layers of quantum dots and preferably about 25 layers formed by providing that number of each of the alternating stoichiometric and semiconductor rich layers. The n-type and p-type regions are typically each in the range of 75 - 200 nm thick and preferably about lOOnm thick. This is achieved by creating each layer of matrix material with a thickness in the range of 1.5 to 2.5 nm and preferably about 1.9 to 2.1 nm and providing 25 of each of the stoichiometric and semiconductor rich layers (i.e. 50 layers in all) in each of the doped regions to give a cell having a thickness of 150 to 250 and preferably 200 nm thick. The matrix material may be selected from silicon oxide, silicon nitride or silicon carbide or a structure including layers of one or more of these materials possibly with layers of other materials included. The semiconductor material of the quantum dots is preferably silicon or a silicon alloy such as silicon alloyed with germanium.

The defects in the matrix material may comprise respectively oxygen, nitrogen or carbon vacancies in the silicon oxide, silicon nitride or silicon carbide matrix material. The term "oxygen vacancy" can refer to oxygen vacancies, defects known as

E' centres, dangling bonds and sub-stoichiometric oxygen nitrogen or carbon content, for example. Other matrix materials of interest include silicon oxy-nitride (i.e. a mixture of silicon oxide and silicon nitride) which will allow control of the barrier height in Fig. 7 and provides a possible mechanism for inclusion of defects, as well as influencing the barrier height, by inclusion of the non-Si element of one compound into the other.

Artificial amorphous material photovoltaic cells may be stacked in tandem with other artificial amorphous material photovoltaic cells and/or cells of more conventional material such as poly crystalline silicon cells. When a plurality of cells are stacked in tandem the bandgaps of the artificial amorphous material cells are preferably varied from cell to cell (and with respect to any base line silicon cell) whereby each cell is optimised for a different wavelength of incident light on the tandem structure. Conventional material may also be used adjacent to an artificial amorphous material layer to assist in connecting to the artificial amorphous material.

Brief description of the Drawings

Examples of the method will now described, with reference to the accompanying drawings in which:

Fig. 1 diagrammatically illustrates a prior art one dimensional (ID) superlattice; Fig. 2 diagrammatically illustrates confined electronic states created in regions low in electron or hole energy in the prior art superlattice of Fig. 1;

Fig. 3 diagrammatically illustrates a defect modelled as a potential well in a tunnelling barrier represented by a dielectric;

Fig. 4 diagrammatically illustrates a very simple arrangement of cubic quantum dots in a dielectric matrix used as a model for analysis;

Fig. 5 diagrammatically illustrates a modelling of the potentials represented by the band edges along the x, y and z directions of Fig. 4;

Fig. 6 diagrammatically illustrates the modelling of potentials of Fig. 4 with defects added; Figs. 7(a), 7(b) and 7(c) illustrate bulk band alignments between crystalline silicon and its carbide, nitride and oxide (estimated) respectively; Fig. 8 graphically illustrates the benefit calculated to occur from resonant defect enhancement of current for the modelled case.

Fig. 9 diagrammatically illustrates a superlattice structure formed by deposition of alternating stoichiometric and silicon-rich layers; Fig. 10 shows the layers of Fig. 2 after high temperature treatment showing crystalline silicon quantum dots;

Detailed description of an Example

Amorphous semiconductors are created by formation of silicon quantum dots in a dielectric or high band gap semiconducting matrix material, such as an oxide, nitride or carbide. The silicon is formed by phase separation from the matrix material layers, initially formed with excess silicon. The transport properties of such materials have been calculated to be poor but experimentally have proven to be surprisingly good.

A process for enhancing current flow in semiconductor superlattices based on resonance between defect sites in the matrix and the quantum states in the quantum dot is now described. This enhancement greatly relaxes requirements on quantum dot uniformity and spacing and allows the formation of practical quantum dot structures using simple approaches

It has been shown that resonant tunnelling can occur through defects formed by oxygen vacancies in silicon dioxide dielectrics thermally grown on silicon. We have also examined the effect of defect location on tunnelling current.

Referring to Fig. 3, the defect has been modelled as a potential well 21 in the tunnelling barrier 22 represented by the dielectric. Although limited to a ID simulation, comparison with the work of Stadele et alia allows conversions to 2D geometries by use of a capture cross-section. These capture cross-sections were found to be approximately the same size as that offered by a single atom in the dielectric.

An analysis of the benefits from this new concept is presented below using an adaptation of a simple technique recently published by O. Lazarenkova and

A. Balandin. The benefits identified by this analysis are much higher than might be expected due to the ability of defects to provide states that resonate with those in the quantum dot, regardless of the precise value of the latter. This resonance increases the widths of the quantum dot minibands greatly increasing the mobility of carriers between the dots. Thermal agitation of the defects is able to shift the energy of the defect state with time allowing it to periodically come into resonance with quantum dot states, even if this means that the defect state not always in the resonant energy position. If there is a distribution in energy of defect states arising from a multiplicity of defect types or from variation in their local environment, those of the correct energy for resonance will be automatically selected from the defect pool.

ESTIMATION OF ENHANCEMENT

For ease of calculation, a very simple arrangement of cubic quantum dots 23 in a dielectric matrix 24 is considered as illustrated in Fig. 4. In practice, dots 23 may better be approximated by spheres, although the thrust of the calculations to be described will remain unchanged. For these calculations, all dots 23 are assumed to be the same size and to be regularly spaced in the matrix 24 as shown. This represents the ideal conditions for conventional transport between the dots. The benefits of the defect resonances to be described will be greater in relative terms for geometries departing from this ideal

Along the x, y and z directions, the potentials represented by the band edges 22, 25 are modelled as shown in Fig. 5. Defects are represented in the approximate way shown in Fig. 6 but with an abrupt potential step approximating the potential well 21.

The figure of merit used in the comparison will be the carrier mobility μ, given by:

μ = qτ/m * (1)

where q is the electronic charge, m* is the effective mass of the carrier within the miniband and τ is the carrier scattering time. For bulk silicon and other commercial grade semiconductors, this scattering time is typically several hundred femoseconds in value. A lower value of 30 femoseconds will be assumed in the present calculations. Again, the specific details of these calculations do not affect the general conclusions from of the results. Since q and τ are constant, changes in mobility arise from changes in w* which are calculated using the approach of Lazarenkova and Balandin, previously mentioned. For these calculations, cubic silicon dots having edges 2 nm long are assumed, embedded in a silicon dioxide matrix 24 with a spacing varying between 1 nm and 4 nm between the dots 23. As shown in Fig. 7c, this gives a large tunnelling barrier at the band edges, restricting miniband transport. The situation will improve progressively in nitride and carbide (Fig. 7b), also shown, due to the smaller tunnelling barrier, although the advantages of the present approach will also apply to these. However, as shown in Fig. 8, when defects with the required properties in the barrier layer are assumed, the calculated mobility increases by about a hundredfold, if defects are present in sufficient quantities to ensure an electron transitioning between dots comes under the influence of at least one resonant defect, regardless of transit direction. This mobility increase is likely to cause conversion of quantum dot material that is a very poor conductor of electricity into quite a good conductor, with properties suited for many applications. Of particular interest is the use of such quantum dot arrangements as artificial, engineered semiconducting materials of controlled bandgap (artificial amorphous semiconductors), such as for application in photovoltaic solar energy conversion.

The invention requires the preparation of the quantum dot material by forming quantum dots in a matrix material in a way that encourages the presence of defects with states at the appropriate energy range for resonance. This is in contrast to the approach normally taken with dielectric materials where efforts normally are made to prepare material as free from defects as possible.

Referring to Fig. 9, to prepare the artificial amorphous material of interest, alternating layers of stoichiometric silicon oxide, nitride or carbide 124 are interspersed with layers of silicon-rich material 122 of the same type. These layers are formed on a substrate 125 which may be glass, ceramic or other suitable material depending on the particular application. On heating, crystallisation of the excess silicon occurs in the silicon-rich layers. As illustrated in Fig. 10, in order to minimise their free energy, the crystallised regions 123 are approximately, spherical of a radius determined by the width of the silicon-rich layer, and approximately uniformly dispersed within this layer. If the interspersed layers of stoichiometric material 124 are sufficiently thin, free energy minimisation encourages a symmetric arrangement of quantum dots 123 on neighbouring planes (either in a close-packed arrangement as shown or in related symmetrical configurations) of the dielectric material whereby they are uniformly distributed and regularly spaced in three dimensions through the dielectric material.

Suitable deposition approaches for .the layers 124, 122 include physical deposition such as sputtering or evaporation, including these in a reactive ambient, chemical vapour deposition including plasma enhanced processes, or any other suitable processes for depositing the materials involved. Suitable heating processes include heating in a suitable furnace, including belt or stepper furnaces, or heating by rapid thermal processes including lamp or laser illumination amongst others. For approaches resulting in hydrogen incorporation into the layers during deposition, several stages of heating may be required to allow the hydrogen to evolve prior to exposure to the higher crystallisation temperatures

Doping of the quantum dots 123 is achieved by incorporating standard silicon dopants during deposition of either type of layer 124, 122. Some of these are incorporated into nearby quantum dots 123, donating or accepting electrons from neighbouring atoms and imparting donor or acceptor properties. Alternatively, regarding dots 123 as artificial atoms, dots that differ chemically from neighbours, such as by the incorporation of Ge, also can give similar donor or acceptor properties.

Dopants can also be incorporated into the matrix or diffused into the dots through the matrix after the dots have been formed.

At least two different approaches for establishing resonant defect enhancement of current appear to have merit, these being: a) the preparation of dielectric material under conditions likely to lead to non- stoichiometric composition, leading to defects such as the oxygen-vacancy. b) the introduction of additional chemical impurities into the dielectric, such as transition metal ions, to create defects with states in the described energy range. A wide range of possible impurities and preparation approaches would be known to those skilled in the art.

The term "oxygen vacancy" can refer to oxygen vacancies, defects known as E'centres, dangling bonds and sub-stoichiometric oxygen content. In oxide materials ,oxygen vacancies are formed under conditions that produce an oxygen deficit in the oxide. In silicon oxide, this can occur if extra silicon is inserted in the layer (which is likely to happen in the present quantum dot devices as a consequence of the matrix material being a compound of the element included to form the quantum dots). Other methods of forming oxygen defects include: i) heating in a reducing atmosphere, such as one containing hydrogen, is another possible approach to producing an oxygen deficiency; ii) including contaminants, such as titanium or aluminium, which are likely to form compounds in the matrix with oxygen; iii) exposing matrix to UV light and heating to around 760K has been found to cause an oxygen deficit (C. Fiori and R. Devine, Phys. Lett., Vol. 52, p. 2081, 1984); iv) irradiation by electron beam or other high energy beam would be expected to produce similar results; v) adding germanium also has been found to lead to oxygen vacancies (G. Pacchioni and A. Basile, J. Non-Crystalline Solids, Vol. 254, p. 17, 1999) in silicon oxide. Embodiments may also employ defects other than oxygen vacancies. For example, implantation of ions, even silicon ions (CJ. Nicklaw et. al, IEEE Trans.

Nuclear Science, Vol. 47, p. 2269, 2000), can cause a host of defects in the matrix, including (but not restricted to) oxygen vacancies, and applying a high voltage stress can also cause defects in an oxide including oxygen vacancies.

Incorporation of transition metals into the oxide has been of interest in other microelectronic applications (e.g., Delima et al., Physica B, Vol. 192, p. 245, 1985). Transition metals have partly filled d or /electron cells and have atomic numbers of 21 to 28, 39 to 46, or 57 to 78 and can be incorporated during deposition or subsequently ion implanted. However some transition metals may have adverse effects if they find their way into the quantum dots and so selection of suitable candidates should be guided by observance of the relevant properties that may be available for bulk silicon/silicon-oxide interfaces.

Metals other than transition metals can also be considered as sources of defects in the matrix material. Some, such as Cu, Al, P, B, Ni, can be tolerated in quite high concentrations in bulk silicon while others such as Fe, Mn, Co, Cr can be tolerated in reasonable concentrations. These metals are of interest in the present application.

Silicon carbide has not been explored in this type of role to the same extent as oxide or nitride, however some similarities of behaviour can be expected. For example, the same considerations would apply to metallic defects in silicon nitride and silicon carbide as they would to the silicon oxide, as would the effect of UV light, high voltages, etc.

Defects in silicon nitride such as Si-Si bonds and Si and N dangling bonds produce defect states in such material. In Si-rich nitride, a specific Si dangling bond centre known as the K-centre can be identified (J. Robertson, Philos. Mag. B, Vol. 69, p. 307, 1994). Controlling the ratio of Si to N in the material can influence the concentration of the different types of defects.

In silicon oxy-nitrides or in silicon oxide and nitride containing carbide, the possibilities for defect inclusion are increased. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. An artificial amorphous semiconductor composite material having a controlled bandgap and mobility comprising a plurality of crystalline semiconductor material quantum dots substantially uniformly distributed and regularly spaced in three dimensions through a matrix of dielectric material, high bandgap semiconductor material or thin layers of such materials wherein the bandgap and mobility of the composite material are determined by selecting the material parameters including the size of the quantum dots, the composition of the matrix and the semiconductor material of the quantum dots, and wherein the composition of the resulting matrix includes defects which enhance mobility based on resonance between defect sites in the matrix and the quantum states in the quantum dot.

2 The material of claim 1 wherein quantum dots are distributed in layers throughout the artificial amorphous material.

3. The material of claim 2 wherein the matrix material is selected from silicon oxide, silicon nitride or silicon carbide, silicon oxy-nitride or a structure including layers of one or more of these materials.

4. The material of claim 3 wherein layers of other materials included between the layers of silicon oxide, silicon nitride or silicon carbide.

5. The material of claim 3 or 4 wherein the semiconductor material of the quantum dots is silicon or a silicon alloy.

6. The material of claim 5 wherein the semiconductor material of the quantum dots is silicon alloyed with germanium.

7. The material of claim 3, 4, 5 or 6 wherein the defects in the matrix material comprise respectively oxygen, nitrogen or carbon vacancies in the silicon oxide, silicon nitride or silicon carbide matrix material, E'centres, dangling bonds and sub- stoichiometric oxygen nitrogen or carbon content.

8. The material as claimed in any one of claims 2 to 7 wherein the artificial amorphous semiconductor composite material comprises an n-type region of artificial amorphous material adjacent a p-type region of artificial amorphous material forming a junction therebetween, the n-type and p-type artificial amorphous materials being integrally formed as a matrix substantially regularly disbursed crystalline semiconductor material quantum dots, and the n-type and p-type regions are respectively doped with n-type and p-type dopant atoms and form a photovoltaic junction.

9. The material of claim 8 wherein a region in the vicinity of the junction between the n-type and p-type regions of the artificial amorphous material is undoped or has a balance of n-type and p-type dopants whereby the region behaves as intrinsic material.

10. The material of claim 8 or 9 wherein each of the n-type and p-type regions includes 20-50 layers of quantum dots.

11. The material of claim 8 or 9 wherein each of the n-type and p-type regions includes 25 layers of quantum dots.

12. The material as claimed in any one of claims 8 to 11 wherein the n-type and p- type regions are each in the range of 75 - 200 nm thick.

13. The material of claim 12 wherein the n-type and p-type regions are each lOOnm thick and comprise layers of matrix material each with a thickness in the range of 1.5 to 2.5 nm.

14. The material of claim 13 wherein the n-type and p-type regions comprise layers of matrix material each with a thickness in the range of 1.9 to 2.1 nm.

15. The material as claimed in any one of claims 8 to 14 wherein the photovoltaic cell formed of artificial amorphous material is stacked in tandem with another artificial amorphous material photovoltaic cell.

16. The material as claimed in any one of claims 8 to 14 wherein the photovoltaic cell formed of artificial amorphous material is stacked in tandem with a crystalline or polycrystalline photovoltaic cell.

17. The material of claim 15 or 16 wherein the bandgaps of the artificial amorphous material cell or cells are varied from that of other cells in the stack whereby each cell is optimised for a different wavelength of incident light on the tandem structure.

18. The material as claimed in any one of claims 8 to 17 wherein a crystalline or polycrystalline material layer is provided adjacent to an artificial amorphous material layer and a contact is formed on the crystalline or polycrystalline material to provide connection to the artificial amorphous material layer.

19. A method of forming an artificial amorphous semiconductor material having a controlled bandgap and mobility comprises; forming a plurality of layers of a matrix of material comprising a dielectric or high bandgap semiconducting compound, wherein alternating layers are layers of stoichiometric matrix material and layers of semiconductor rich matrix material respectively, at least some of these layers including defects, or having a propensity to form defects during processing, and heating the layers of matrix material to cause quantum dots to form in the semiconductor rich layers of matrix material whereby they are uniformly distributed and regularly spaced in three dimensions through the matrix material, wherein the bandgap and mobility are determined by selecting the material parameters including the size of the quantum dots, the composition of the matrix and the semiconductor material of the quantum dots to achieve the desired parameters, and wherein the composition of the resulting matrix material includes defects which enhance mobility based on resonance between defect sites in the matrix and the quantum states in the quantum dot.

20. The method of claim 19 wherein the material of the layers of matrix material is selected from silicon oxide, silicon nitride or silicon carbide, silicon oxy-nitride or a structure including layers of one or more of these materials.

21. The method of claim 20 wherein layers of other materials are included between the layers of silicon oxide, silicon nitride or silicon carbide.

22. The method of claim 20 or 21 wherein the semiconductor material of the semiconductor rich layers and the subsequently formed quantum dots is silicon or a silicon alloy.

23. The method of claim 22 wherein the semiconductor material of the semiconductor rich layers and the subsequently formed quantum dots is silicon alloyed with germanium.

24. The method of claim 3, 4, 5 or 6 wherein the defects formed in the layers of matrix material comprise respectively oxygen, nitrogen or carbon vacancies in the silicon oxide, silicon nitride or silicon carbide matrix material, E' centres, dangling bonds and sub-stoichiometric oxygen nitrogen or carbon content.

25. The method as claimed in any one of claims 19 to 24 wherein the artificial amorphous semiconductor material forms a photovoltaic cell the method further comprising: doping regions of the plurality of layers of matrix material with p-type and n- type dopants either simultaneously with their formation or subsequently, and whereby the heating of the layers of matrix material to cause quantum dots to form in the semiconductor rich layers of matrix material, causes the quantum dots of each a particular dopant type in each doped region to be uniformly distributed and regularly spaced in three dimensions through the respective region of matrix material, adjacent p-type and n-type regions forming a photovoltaic junction of the photovoltaic cell.

26. The method of claim 25 wherein a region in the vicinity of the junction between the n-type and p-type regions of the artificial amorphous material is undoped or has a

^• balance of n-type and p-type dopants whereby the region behaves as intrinsic material.

27. The method of claim 25 or 26 wherein each of the n-type and p-type regions includes 20-50 layers of stoichiometric matrix material and 20-50 layers of semiconductor rich matrix material, whereby 20-50 layers of quantum dots are formed.

28. The method of claim 25 or 26 wherein each of the n-type and p-type regions each includes 25 layers of stoichiometric matrix material and 25 layers of semiconductor rich matrix material.

29. The method as claimed in any one of claims 25 to 28 wherein the n-type and p- type regions are each formed to be in the range of 75 - 200 nm thick.

30. The method of claim 29 wherein the n-type and p-type regions are each formed to a thickness of lOOnm and comprise layers of matrix material each with a thickness in the range of 1.5 to 2.5 nm.

31. The method of claim 30 wherein the n-type and p-type regions comprise layers of matrix material each with a thickness in the range of 1.9 to 2.1 nm.

32. The method as claimed in any one of claims 25 to 31 wherein the artificial amorphous material of the photovoltaic cell is formed stacked in tandem with another artificial amorphous material photovoltaic cell.

33. The method as claimed in any one of claims 25 to 31 wherein the artificial amorphous material of the photovoltaic cell is formed stacked in tandem with a crystalline or polycrystalline cell.

34. The method of claim 32 or 33 wherein each cell in the stack is optimised for a different wavelength of incident light on the tandem structure, by selecting different bandgaps for the artificial amorphous material in the artificial amorphous material cell or in each of such cells.

35. The method as claimed in any one of claims 25 to 34 wherein a crystalline or polycrystalline material layer is formed adjacent to an artificial amorphous material layer and a contact to the artificial amorphous material is formed on the crystalline or polycrystalline material layer.