Unipolar light emitting devices based on III-nitride semiconductor superlattices

1. Field of the Invention

The present invention relates to light emitting devices. More particularly, the invention relates to unipolar light emitting devices based on III-nitride semiconductors.

2. Description of the Prior Art

Recent developments in the field of III-nitride semiconductors give rise to a new generation of light emitting diodes and lasers for the visible spectral range. The main advantage of nitride semiconductors in comparison with other wide-band-gap semiconductors is their low degradation in optical devices. However, there is a problem in getting a good p-type conductivity for these materials, which blocks further development of high power lasers and light emitting diodes for the visible spectral range.

Various aspects of the present invention are set out below.

A light emitting device with a plurality of superlattices which are made exclusively of intrinsic or n-type III-nitride semiconductors or alloys.

A light emitting device with a plurality of superlattices which are made exclusively of intrinsic or n-type III-nitride semiconductors or alloys with active layers between the superlattices made of optically active impurities, impurity complexes or quantum dots.

A white light emitting device with at least three pairs of superlattics which are made exclusively of intrinsic or n-type III-nitride semiconductors or alloys with or without active layers between the superlattices made of optically active impurities, impurity complexes or quantum dots.

A white light emitting device with at least four graded superlattices which are made exclusively of intrinsic or n-type III-nitride semiconductors or alloys with or without active layers between the superlattices made of optically active impurities, impurity complexes or quantum dots.

A light emitting device with a plurality of superlattices which are made exclusively of intrinsic or n-type III-nitride semiconductors or alloys with active layers between the superlattices made of III-V or II-VI semiconductors with different conduction band discontinuities.

A unipolar light emitting device structure based on III-nitride semiconductor superlattices.

A Unipolar light emitting device generating light with a spectrum inside the spectral region from 400 nm to 4000 nm comprising:

a sapphire substrate;

a buffer layer;

a first n-cladding and contact layer made of n-type doped GaN, AlN, InN or their alloys;

a plurality of undoped or n-type doped superlattices with two or more 3-30 Å thick barriers made of GaN, AlN, InN or their alloys and two or more 3-30 Å thick wells made of GaN, AlN, IN or their alloys;

a second n-cladding and contact layer made of n-type doped GaN, AlN, InN or their alloys;

a transparent metallic alloy contact deposited on said second n-cladding and contact layer; and

a metallic contact to said first n-cladding and contact layer.

A unipolar light emitting device structure based on III-nitride semiconductor superlattices generating light with a spectrum inside the spectral region from 400 nm to 4000 nm comprising:

a sapphire substrate;

a buffer layer;

a first n-cladding and contact layer made of n-type doped GaN, AlN, InN or their alloys;

a plurality undoped or n-type doped superlattices with two or more 3-30 Å thick carriers made of GaN, AlN, InN or their alloys and a plurality of 3-30 Å thick wells made of GaN, AlN, InN or their alloys;

an active layer doped or δ-doped with rare earth metals, transition metals and their complexes with shallow donors or other impurities;

a second n-cladding and contact layer made of n-type doped GaN, AlN, InN or their alloys;

a transparent metallic alloy contact deposited on said second n-cladding and contact layer; and

a metallic contact to said first n-cladding and contact layer.

A unipolar white light emitting device structure based on III-nitride semiconductor superlattices comprising:

a sapphire substrate;

a buffer layer,

a first n-cladding and contact layer made of n-type doped GaN, AlN, InN or their alloys;

at least four undoped or n-type doped graded superlattices with a plurality of 3-30 Å thick barriers made of GaN, AlN, InN or their alloys and a plurality of 3-30 Å thick wells made of GaN, AlN, InN or their alloys;

at least three active layers for red, green and blue light generation made of semiconductors doped or δ-doped with rare earth metals, transition metals or their complexes with shallow donors or other impurities;

a second n-cladding and contact layer made of n-type doped GaN, AlN, InN or their alloys;

a transparent metallic alloy contact deposited on said second n-cladding and contact layer, and

a metallic contact to said first n-cladding and contact layer.

A unipolar white light emitting device according to the preceding paragraph but without some of the active layers.

The principal structure of a unipolar light emitting device (ULED) according to an example of the invention is shown in FIG. 1 .

The physical mechanism of the ULED's operation is illustrated in FIGS. 2 a,b . In the ULED, the radiation arises not due to recombination of electrons and holes as takes place in usual light emitting diodes, but due to electron transitions from a shallow sub-band superlattice into a deep sub-band superlattice accompanied by electron energy relaxation via emission of photons. The quantum efficiency of ULEDs based on n-type III-nitride superlattices increases with the increase of the light frequency. Our calculations show that the efficiency of such a ULED is limited by the non-radiative energy relaxation processes and is given by the equation: η = [ 1 + ω LO α 2  ɛ ∞  ɛ _  ω photon ]

where ω LO is the longitudinal phonon frequency, α=1/137 and is the fine structure constant, {overscore (ε)} −1 =ε ∞ −1 −ε υ −1 , ε ∞ and ε 0 are the optical and static dielectric constants respectively and hω photon is the energy of the emitting photon quanta.

For the visible spectral range a ULED efficiency of more than 10% can be achieved.

In the accompanying drawings:

FIG. 1 is a diagram exhibiting the ULED structure of Example 1 which is based on two III-nitride superlattices;

FIG. 2 a shows a schematic energy band diagram of energy (E) against co-ordinate position along the structure for an unbiased structure for explanation of the operation of the ULED of FIG. 1;

FIG. 2 b shows a schematic energy band diagram for a biased structure for explanation of the operation of the ULED of FIG. 1;

FIG. 3 is a diagram exhibiting the ULED structure of Example 2, which is based on two III-nitride superlattices with an active layer containing optically active impurities or quantum dots; and

FIG. 4 is a diagram exhibiting the ULED structure of Example 3 with four graded III-nitride superlattices for white light generation.