Bulk Single Crystal Ternary Substrates for a Thermophotovoltaic Energy Conversion System

1. Field of Invention

The present invention relates to the field of thermophotovoltaic (TPV) direct energy conversion. In particular, the present invention relates to bulk single crystal III-V semiconductor substrates as materials for use as both energy conversion devices and starting substrates for the epitaxial growth of TPV active layers.

2. Description of the Related Art

Thermophotovoltaic (TPV) energy systems convert thermal energy to electric power using, the same principle of operation as solar cells. In particular, a heat source radiatively emits photons that are incident on a semiconductor TPV cell. Photons with an energy greater than the bandgap (E.sub.g) of the semiconductor cell excite electrons from the valence band to the conduction band (interband transition). The resultant electron hole pairs (ehp) are then collected by metal contacts that can power an electrical load.

In order to maximize efficiency and/or power density of a TPV system, the bandgap of the semiconductor is matched to the radiator temperature, as described by L. D. Woolf in Optimum Efficiency of Single and Multiple Bandgap Cells in ThermophotovoltaIc Energy Conversion, Solar Cells, Vol. 19, pp. 19-30, 1987. For a given radiator temperature, increasing the bandgap will generally increase device efficiency, while decreasing the bandgap will generally increase power density, as described in KAPL Report 4778, Feasibility Assessment of Low Temperature Voltaic Energy Conversion, 1993.

Currently, bandgap variation is achieved by growing ternary (three-component) and quaternary (four-component) epitaxial III-V semiconductors layers on binary (two-component) substrates. III-V semiconductors are formed from elements from groups III and V of the Periodic Table. Ternary semiconductors vary both the bandgap and lattice constant as the composition changes between the binary endpoints. For example, FIG. 1 shows that the room temperature InGaAs bandgap (Energy Gap) and lattice constant (a.sub.o) can be varied between InAs (E.sub.g =0.36 eV, a.sub.o =6.0584.ANG.) and GaAs (E.sub.g =1.41 eV and a.sub.o =5.6533.ANG.). Only a limited number of ternary semiconductors can be grown lattice matched to available binary substrates, and these particular ternary semiconductors have only discrete energy bandgaps. For example, In.sub.0.53 Ga.sub.0.47 As is lattice matched to InP with a bandgap of 0.73 eV. A lattice-matched constraint, which is desired for the highest performance devices, severely limits the number of material options available for ternary epitaxial layers grown on binary substrates.

Quaternary semiconductors have the advantage of independent variation of both the bandgap and lattice constant. Thus, lattice matched growth of a number of different bandgaps of the same material may be accomplished. For example, InGaAsSb may be grown lattice matched to GaSb substrates with bandgaps that range from 0.7-0.3 eV. However, a number of these compositions lie within a region of thermodynamic instability (miscibility gap), that preclude fabrication of high quality TPV devices.

The present invention provides bulk single crystal ternary III-V semiconductor substrates that offer the advantage of synthesis of a single crystal material of a desired bandgap directly from a liquid melt. Ternary substrates may be used as a single crystal seed for the epitaxial growth of lattice-matched ternary or quaternary TPV active layers (a base layer and an emitter layer). This provides another degree of freedom when choosing and designing TPV devices from a number of available material systems.

In this regard, one embodiment of the present invention provides a thermophotovoltaic energy conversion device which includes a substrate formed from a bulk single crystal material having a bandgap (E.sub.g) of 0.4 eV<E.sub.g <0.7 eV and selected from the group of AlInAs, AlInSb, GaAsSb, GaInAs, GaInSb and InPAs. In this case, the ternary substrate forms the base layer of the TPV device and an emitter is formed by ion-implantation or diffusion of the appropriate dopant (n-type or p-type). Possible n-type dopants would be selected from group VI of the Periodic Table, and p-type would be selected from group II of the Periodic Table.

Another embodiment of the present invention provides a thermophotovoltaic energy conversion device which includes a host substrate formed from a bulk single crystal material selected from the group of AlAsSb, AlInAs, GaAsSb, GaInAs and InPAs, and lattice-matched ternary or quaternary III-V semiconductor active layers (an emitter and a base) are formed from one of the group of AlInAs, GaAsSb, GaInAs, InPAs, AlGaAsSb, AlGaInAs, AllnAsSb, AlInPAs, AlInPSb, GaInAsSb, GaInPAs, GaInPSb and InPAsSb. The emitter layer and the base layer are formed from the same material but are doped differently. When the host substrate is AlAsSb, the group forming the semiconductor active layers further includes GaInSb.

Yet another embodiment of the present invention provides a thermophotovoltaic energy conversion device that includes a host substrate formed from a bulk single crystal material selected from the group of AlGaSb, AlInSb and GaInSb, and lattice-matched ternary or quaternary III-V semiconductor active layers formed from one of the group of GaInSb, AlGaInSb, AlInAsSb, AlInPSb, GaInAsSb, GaInPSb and InPAsSb.

The present invention also provides a method of forming a thermophotovoltaic energy conversion device using the steps of forming a substrate from a bulk single crystal material having a bandgap (E.sub.g) of 0.4 eV<E.sub.g <0.7 eV, and selecting the substrate from the group of AlInAs, AlInSb, GaAsSb, GaInAs, GaInSb and InPAs. The ternary substrate will form the base layer of the TPV device and may be either n-type or p-type material. An emitter is formed on the substrate by ion implantation or by diffusion of the appropriate dopant. The emitter being formed may be either p-type or n-type material.

In one embodiment, the method of forming a thermophotovoltaic device includes the steps of forming a host substrate from a bulk single crystal material selected from the group of AlAsSb, AlInAs, GaAsSb, GaInAs and InPAs, and forming lattice-matched ternary or quaternary III-V semiconductor active layers formed from one of the group of AlInAs, GaAsSb, GaInAs, InPAs, AlGaAsSb, AlGaInAs, AlIAsSb, AlInPAs, AlInPSb, GaInAsSb, GaInPAs, GaInPSb and InPAsSb. When the host substrate is AlAsSb, the group forming the semiconductor active layers further includes GaInSb.

In another embodiment, the method of the invention includes the steps of forming a host substrate from a bulk single crystal material selected from the group of AlGaSb, AlInSb and GaInSb, and forming lattice-matched ternary or quaternary III-V semiconductor active layers formed from one of the group of GaInSb, AlGaInSb, AlInAsSb, AlInPSb, GaInAsSb, GaInPSb and InPAsSb.

The present invention is illustrated by way of example and not limitation in the accompanying figures in which like reference numerals indicate similar elements and in which:

FIG. 1 shows a graph of energy gap versus lattice constant for III-V semiconductors,

FIG. 2 is a cross-sectional diagram of a ternary substrate TPV device having a diff-used or ion-implanted junction according to the present invention;

FIG. 3 is a cross-sectional diagram of a ternary substrate TPV device according to the present invention having epitaxially grown lattice-matched ternary or quaternary active layers;

FIG. 4 shows a graph of energy gap and lattice constant versus composition of AlGaInAs;

FIG. 5 shows a graph of energy gap and lattice constant versus composition of AlGaInSb;

FIG. 6 shows a graph of energy gap and lattice constant versus composition of AlInAsSb;

FIG. 7 shows a graph of energy gap and lattice constant versus composition of AlInPAs;

FIG. 8 shows a graph of energy gap and lattice constant versus composition of AlInPSb;

FIG. 9 shows a graph of energy gap and lattice constant versus composition of GaInAsSb;

FIG. 10 shows a graph of energy gap and lattice constant versus composition of GaInPAs;

FIG. 11 shows a graph of energy gap and lattice constant versus composition of GaInPSb; and

FIG. 12 shows a graph of energy gap and lattice constant versus composition of InPAsSb.

For TPV systems, active layers (emitter and base) having bandgaps that range from 0.4 to 0.7 eV match well to radiator temperatures that vary between 800.degree.-2000.degree. C. FIG. 1 shows that there are six different ternary materials that fall into this bandgap range: AlInAs, AlInSb, GaAsSb, GaInAs, GaInSb, and InPAs. Bulk growth of these materials enables fabrication of simple diffused junction or ion-implanted junction TPV devices that are produced with a reduced cost in comparison to an epitaxially grown TPV device. FIG. 2 shows a cross-sectional diagram of a bulk grown ternary substrate TPV device having a junction that can be either a diff-used or an ion-implanted junction. An n- or p-type bulk single crystal ternary substrate 10 having a bandgap between 0.4-0.7 eV forms the base layer. A junction is formed on substrate 10 by ion-implantation or diffusion of the appropriate dopant to form a p- or n-type emitter 12. Front side metal grids 13 and back contacts 11 are formed on emitter 12 and substrate 10, respectively, using conventional techniques.

Ternary substrates also enable lattice-matched growth of ternary or quaternary semiconductor materials for TPV active layers. FIG. 3 shows a cross-sectional diagram of a ternary substrate TPV device having epitaxially grown lattice-matched ternary or quaternary active layers. An n- or p-type bulk single crystal ternary substrate 20 provides an epitaxial seed crystal. A first lattice-matched ternary or quaternary layer 21 is epitaxially grown on substrate 20. A second lattice-matched ternary or quaternary layer 22 is epitaxially grown on layer 21. Front side metal grids 13 and back contacts 11 are formed on layer 22 and substrate 20, respectively, using conventional techniques. Ternary substrates for the device of FIG. 3 may have any bandgap and only require lattice-matched active layers.

Substrates available for the device of FIG. 3 include AlAsSb, AlGaSb, AlInAs, AlInSb, GaAsSb, GaInAs, GaInSb, and InPAs. Potential lattice-matched active layers are determined by examining the energy gap and lattice constants of the various potential ternary and quaternary alloys, as shown in FIGS. 4-12. In particular, the lattice constant of a ternary substrate candidate must be matched to ternary or quaternary active layers with the appropriate bandgap of 0.4-0.7 eV and lattice constant. FIGS. 4-12 show that there are numerous potential high quality material systems that may be utilized for TPV applications. Table I tabulates all candidate ternary and quaternary active layers (for the embodiment as shown in FIG. 3) for each of the ternary substrates listed above.

While the present invention has been described in connection with the illustrated embodiments, it will be appreciated and understood that modifications may be made without departing, from the true spirit and scope of the invention.