Patent Description:
Conventional light emitting diodes (LEDs) are built such that the diode's undoped active region, which contains one or more quantum wells, is located between p- and n-type doped layers. At present, commercially available light emitting diodes based on gallium nitride (GaN) and on alloys of gallium with aluminium (AlGaN) and indium (InGaN) emit light ranging from ultraviolet to green, depending on the composition of the alloy forming the quantum wells in the active region. Information about the structure of standard LEDs based on III group metal nitrides can be found, among others, in "<NPL>]. Light emission from a conventional LED is a single peak with the half-width of about a dozen nanometres. In the case of GaN-based diodes, the position of this peak as a function of current changes slightly, that is, by a few nanometres in the direction of the shorter wavelength and this effect is related to the screening of electric fields by the quantum-confined Stark effect (QCSE). Nevertheless, this effect is not strong enough to change the emission colour from red to blue in LEDs with a standard structure.

Devices wherein the colour of the emitted light can be changed within a broad range of the visible light spectrum are most often sets of independently controlled LEDs with strictly defined emission wavelengths. The most basic combination is made by blue (~<NUM>), green (~<NUM>) and red (~<NUM>) colour diodes. By independent control of the light emission power of each of the diodes it is possible to obtain light with any colour, including white light. The level of LED integration can occur at various stages of the engineering process. These can be large LEDs present in the RGB LED strips available on the market or micro-LED arrays. Methods of integration of single micro-LEDs for the purpose of production of multi-colour displays are disclosed in the review publication titled "<NPL>]. Such methods are described there as transfer and bonding of micro-LED chips made of group III metal nitrides and arsenides for dedicated drive substrates in various epitaxial processes, conversion of light from blue colour micro-LEDs to red and green colour with the use of luminophores or quantum dots or monolithic growth of multi-colour LEDs on the same substrate.

Monolithic displays containing diodes with all three primary colours can be made only within the III nitrides, because the band gap of the indium gallium nitride (InGaN) alloy is the only one to cover the whole visible spectrum. The emission wavelength increases in line with the increase in the indium content in the InGaN alloy and in order to achieve the red light emission the indium content in the quantum well alloy should be circa <NUM>%. Such a high indium content in planar structures is difficult to obtain due to high stresses of the InGaN layers and to the demanding conditions for epitaxy. Integration of indium in the micro-LED structure can be controlled at the nanostructure level, which is disclosed in the publication by <NPL>]. The epitaxy of the InGaN/GaN heterostructure by the PAMBE method, described in said publication, was carried out on a GaN substrate covered with a layer made of titanium, wherein openings with various diameters were left. The epitaxial structure growth occurred only in the openings left in the titanium mask and this way InGaN quantum dots were grown inside nanowires (dot-in-nanowire). Moreover, during the epitaxy of InGaN quantum dots the indium content increased strongly in line with the decrease in the diameter of the nanowires, which resulted in a shift of the emission length in the direction of red colour. This way, by selecting appropriate diameters of the openings in the titanium mask, a multi-colour micro-LED array with four emission colours was made: blue, green, orange and red. But even in this solution a single subpixel structure was still responsible for emitting light with a single colour. It is a disadvantage of the solution wherein three subpixels have different colours that the colour of the light from a single pixel becomes homogeneous for the eye only when observed from an appropriately long distance. This is because when observed from a short distance the component colours of subpixels can be distinguished. In addition, a single maximum for the wavelength that is different from those corresponding to the emissions from single subpixels cannot be achieved in the electroluminescence spectrum. In order to eliminate said disadvantages, such light sources were sought the emission wavelength of which could be changed continuously by changing the power supply voltage or the density of the current flowing through the structure.

Electrically tunable wavelength of emission from a single electroluminescent structure is disclosed in the publication by<NPL>]. These were LEDs made by the MOVPE method, built of a range of nanorods containing multiple InGaN/GaN quantum wells covered with a GaN:Mg layer. InGaN/GaN quantum wells can be distinguished in the structure of a single nanorod, which are located on the side walls, on the slants and on the top of the nanorods with various indium contents, being from <NUM>% on the side walls to <NUM>% on the top of the nanorods. By changing the power supply voltage, the emission wavelength was changed continuously from <NUM> (for <NUM> V) to <NUM> (for <NUM> V). This effect was explained by an appropriate model of electric field distribution in the nanorod structure. For the lowest power supply voltage, current with a low density flows preferably through InGaN wells with a high indium content on the top of the nanorods. In line with the increase in the power supply voltage, current with an increasingly higher density flows successively through the quantum wells on the slants and the side walls of the nanorods.

The solution of a LED with an electrically tunable emission wavelength, disclosed in <CIT>, is closest to the object of the present invention. Said publication relates to planar structures of light emitting diodes, wherein the active region is composed of three different sets of quantum wells with an increasing indium content in the direction from the n-type area to the p-type area, and these sets of quantum wells are additionally separated by intermediate carrier blocking layers. With an appropriate selection of the composition, width and doping of the carrier blocking layers for low current densities, carriers are injected effectively only to the well with the highest indium content, being <NUM>%, as a result of which red electroluminescence occurs. For higher current densities, the carrier injection effectiveness reaches the maximum successively for multi-quantum wells with the <NUM>% indium content and green electroluminescence and then with the <NUM>% indium content and blue electroluminescence. This way, by controlling the injection of carriers, electrically tunable electroluminescence ranging from <NUM> to <NUM> was obtained. However, red, green and blue light emissions are still achieved from InGaN wells with varied indium contents.

In the aforementioned publications, electroluminescence from diodes based on III nitrides is approached in a traditional manner, wherein the emission wavelength is determined by the indium content in the InGaN alloy forming the quantum well, while there are no publications which would suggest the possibility to obtain an electroluminescent structure wherein the emission wavelength could be changed within a broad range of the visible spectrum, that is, within the wavelength range from <NUM> to <NUM>, from a single and the same quantum well with a relatively low indium content, being for example circa <NUM>%.

It is an object of the invention to obtain a light emitting diode with a flat structure, enabling light emission within the visible spectrum, that is, with the light wavelength ranging from <NUM> to <NUM>, wherein the emission colour is changed by changing the intensity of the current powering such a diode.

This object is achieved by a diode according to the invention, which has an epitaxial layer structure built of nitrides of group III metals applied to a crystalline substrate. This structure is composed, counted from the crystalline substrate, of at least an n-type conductivity layer, a light emitting active region with one quantum well, and a p-type conductivity layer. The present invention consists in the active region that is composed, counted from the crystalline substrate, of a lower n-type doped barrier layer, a lower n-type doped part of the quantum well, an upper p-type doped part of the quantum well, and an upper p-type doped barrier layer. The width of each part of the quantum well ranges from <NUM> to <NUM>. The doping level of each part of the quantum well ranges from <NUM> × <NUM><NUM> cm-<NUM> to <NUM> × <NUM><NUM> cm-<NUM>, while the doping level of the barrier layer is up to <NUM>% of the doping level of the part of the quantum well that is adjacent to said barrier layer.

In one of the variants of the diode according to the invention, each part of the quantum well is made of the InxGa<NUM>-xN indium gallium nitride alloy, each barrier layer is made of the InyGa<NUM>-yN indium gallium nitride alloy, wherein the indium content in said alloys is determined by the following correlation: <NUM> ≤ y < x ≤ <NUM>.

In another variant of the diode according to the invention, the width of the upper part of the quantum well is between <NUM>% and <NUM>% of the sum of widths of both parts of the quantum well.

In still another variant of the diode according to the invention, between the upper barrier layer and the p-type conductivity layer is additionally an electron blocking layer, made of the AlxGa<NUM>-xN aluminium gallium nitride alloy wherein the aluminium content ranges from <NUM> to <NUM>, wherein the doping level of said layer ranges from 5x <NUM><NUM> cm-<NUM> to <NUM> × <NUM><NUM> cm-<NUM>.

The method according to the invention consists in fabricating an epitaxial layer structure on a crystalline substrate, with the structure's containing, counted from the crystalline substrate, at least an n-type conductivity layer, a light emitting active region with one quantum well and a p-type conductivity layer, of nitrides of group III metals in the process of nitrogen plasma assisted molecular beam epitaxy (PAMBE) growth. The invention is characterised in that an n-type conductivity layer, a four-layer active region and a p-type conductivity layer are successively fabricated on a crystalline substrate. While fabricating the active region, first its lower n-type doped barrier layer is fabricated. Subsequently, a lower n-type doped part of the quantum well is fabricated, after which an upper p-type doped part of the quantum well is fabricated. The fabrication of the active region is finished by applying an upper p-type doped barrier layer. While fabricating both parts of the quantum well, their width ranging from <NUM> to <NUM> and the doping level ranging from <NUM> × <NUM><NUM> cm-<NUM> to <NUM> × <NUM><NUM> cm-<NUM> are applied. While fabricating each barrier layer of the active region, the doping level of up to <NUM>% of the doping level of the part of the quantum well that is adjacent to the barrier layer being fabricated is applied.

In one of the variants of the method according to the invention, each part of the quantum well is made of the InxGa<NUM>-xN indium gallium nitride alloy, and each barrier layer is made of the InyGa<NUM>-yN indium gallium nitride alloy, by applying the indium content in said alloys that is determined by the following correlation: <NUM> ≤ y ≤ x ≤ <NUM>.

In another variant of the method according to the invention, an upper part of the quantum well is fabricated with the width being between <NUM>% and <NUM>% of the sum of widths of both parts of the quantum well.

In another variant of the method according to the invention, an electron blocking layer is applied additionally between the upper barrier layer and the p-type conductivity layer. For fabrication of said layer, the AlxGa<NUM>-xN aluminium gallium nitride alloy is used, wherein the aluminium content ranges from <NUM> to <NUM> and the doping level ranges from <NUM> × <NUM><NUM> cm-<NUM> to <NUM> × <NUM><NUM> cm-<NUM>.

In still another variant of the method according to the invention, the epitaxial growth of the layer structure is performed in metal-rich conditions. Epitaxial layers of said structure are applied at the temperature ranging from <NUM> to <NUM>, while nitrogen for epitaxial growth of the layer structure is supplied in the form of plasma excited by a power supply unit generating a power between <NUM> and <NUM> W, with the nitrogen flow between <NUM> and <NUM><NUM> per minute.

The invention breaks the conviction prevailing commonly among specialists that in order to obtain red electroluminescence of a LED fabricated using the technology of nitrides of group III metals, the indium content in the quantum well must be increased to circa <NUM>%. It has turned out unexpectedly that it is possible to obtain lighting from a single quantum well placed in the layer structure of a LED with a much higher wavelength than could be expected from the energy of the band gap of the alloy forming the quantum well, and that the wavelength of emission from such a quantum well can be changed continuously by changing the density of the current flowing through the LED structure. This is possible through an appropriate engineering of the doping of the active region containing one quantum well with the width between <NUM> and <NUM>.

The present invention is reflected in the drawing, with <FIG> presenting a schematic axonometric view of the diode according to the invention. <FIG> presents the light-current-voltage (LIV) characteristics of an embodiment of a diode according to the invention, while <FIG> presents the correlation between the emission wavelength and the density of the current powering said diode. <FIG> presents standardised spectra of electroluminescence of the same diode for various current densities ranging from <NUM> A/cm<NUM> to <NUM> A/cm<NUM>, while <FIG> presents points corresponding to said spectra in a chromaticity diagram according to the CIE <NUM> standard. <FIG> presents an example time diagram of control of the power supply of the diode according to the invention. <FIG> presents spectra of white light obtained by mixing blue and yellow light by the method according to the diagram in <FIG>, while <FIG> presents points corresponding to said spectra in a chromaticity diagram according to the CIE <NUM> standard. <FIG> presents diagrams of the band structures of the active region, which reflect the scheme of operation of a diode according to the invention for four different voltages powering said diode.

The present invention is presented in more detail in the embodiment described below. <FIG> shows that the embodiment of a diode according to the invention contains a set of epitaxial layers, which is composed successively, counted from the crystalline substrate <NUM>, of an n-type conductivity layer <NUM>, an active region (layers from <NUM> through <NUM>), an electron blocking layer <NUM> and a p-type conductivity layer <NUM>. The aforementioned active region is composed of four successive layers: a lower n-type doped barrier layer <NUM>, a lower n-type doped part <NUM> of the quantum well, an upper p-type doped part <NUM> of the quantum well, and an upper p-type doped barrier layer <NUM>. Such a diode contains a first current terminal <NUM>, connected electrically with the lower n-type conductivity layer <NUM>, and a second current terminal <NUM>, connected electrically with the p-type conductivity layer <NUM>.

This diode was fabricated in the process of nitrogen plasma assisted molecular beam epitaxy (PAMBE) growth in a Veeco GEN-<NUM> reactor. The substrate <NUM> with the thickness of <NUM>, made of gallium nitride, was heated in a prechamber of said reactor at the temperature of <NUM> for <NUM> minutes. The heating of the substrate <NUM> was continued for <NUM> minutes in a preparatory chamber at the temperature of <NUM>. The heated substrate <NUM> was transferred to a growth chamber. The layer epitaxy was performed in metal-rich conditions. Nitrogen in the form of plasma excited by radio frequencies by a power supply unit was used for the growth, wherein the plasma parameters varied depending on the currently grown layer. For the GaN layers, nitrogen flow was set at <NUM><NUM>/min and the power supply unit's power was set at <NUM> W. For the InGaN layers, said parameters were <NUM><NUM>/min and <NUM> W, respectively. The growth of the GaN layers was performed at the temperature of <NUM>, while of the InGaN layers - at the temperature of <NUM>. Metals forming the alloys, such as gallium, indium and aluminium, and n-type dopant materials, that is, silicon and germanium, were evaporated from standard Knudsen cells, while as a source of p-type magnesium dopant a special structure of the Knudsen cell was used, which had an additional valve inside the cell, which valve allows a faster adjustment of the magnesium dopant atom flux size during the growth. The purity of the elements used for the epitaxy exceeded <NUM>%. Firstly, on the substrate <NUM> a layer <NUM> (n-type) with the thickness of <NUM> was fabricated of GaN, with the silicon doping level of <NUM> × <NUM><NUM> cm-<NUM>. Subsequently, an active region (<NUM>, <NUM>, <NUM>, <NUM>) was fabricated, which at the bottom contained a lower barrier layer <NUM> with the thickness of <NUM>, made of the silicon-doped In<NUM>Ga<NUM>N alloy with the doping level of <NUM> × <NUM><NUM> cm-<NUM>. Another layer of said region was a lower part of the quantum well <NUM> with the thickness of <NUM>, made of the germanium-doped In<NUM>Ga<NUM>N alloy with the doping level of <NUM> × <NUM><NUM> cm-<NUM>. The next layer of the active region was an upper part of the quantum well <NUM> with the thickness of <NUM>, made of the magnesium-doped In<NUM>Ga<NUM>N alloy with the doping level of <NUM> × <NUM><NUM> cm-<NUM>. The last layer of the active region was an upper barrier layer <NUM> with the thickness of <NUM>, made of the magnesium-doped In<NUM>Ga<NUM>N alloy with the doping level of <NUM> × <NUM><NUM> cm-<NUM>. On the finished active region, an electron blocking layer <NUM> with the thickness of <NUM>, made of magnesium-doped GaN with the doping level of <NUM> × <NUM><NUM> cm-<NUM>, was fabricated. As the last one, a p-type conductivity layer <NUM> was fabricated, composed of a first sublayer with the thickness of <NUM>, made of magnesium-doped gallium nitride with the doping level of <NUM> × <NUM><NUM> cm-<NUM>, of a second sublayer with the thickness of <NUM>, made of the magnesium-doped In<NUM>Ga<NUM>N alloy with the doping level of <NUM> × <NUM><NUM> cm-<NUM>, and of a third sublayer with the thickness of <NUM>, made of the magnesium-doped In<NUM>Ga<NUM>N alloy with the doping level of <NUM> × <NUM><NUM> cm-<NUM>. On the top of the epitaxial structure fabricated by the aforementioned method, metallisation composed of a sequence of Ni/Au with the thickness of <NUM>/<NUM> was applied and subjected to the heating process in the nitrogen and oxygen atmosphere for one minute at the temperature of <NUM>. Subsequently, said structure was subjected to photolithography and reactive ion etching to form mesas with the dimensions of <NUM> × <NUM> and <NUM> × <NUM>, which were etched <NUM> deep, that is, to the level of the n-type conductivity layer <NUM>. On the top of the fabricated mesas, next Ni/Au/Pt metallisation with the thickness of <NUM>/<NUM>/<NUM> was applied in the form of grids with the dimensions of <NUM> × <NUM> on the mesas with the dimensions of <NUM> × <NUM> and in the form of solid squares with the dimensions of <NUM> × <NUM> on the mesas with the dimensions of <NUM> × <NUM>, composing the second (upper) current terminal <NUM>. Subsequently, the metallisations were heated again in the nitrogen and oxygen atmosphere for ten minutes at the temperature of <NUM>. At the last stage, metallisation of Ti/Al/Ni/ Au with the thickness of <NUM>/<NUM>/<NUM>/<NUM>, composing the first (lower) current terminal <NUM>, was applied on the side of the mesas.

The electrooptical properties of the fabricated structure were measured by the needle method, wherein a power supply unit and a voltage meter were interfaced with a spectrometer and an optical power meter. The optical power meter was placed at a constant height above the sample surface but it did not collect the whole light emitted from the sample to the outside, hence the optical powers presented in the chart (<FIG>) are purely qualitative. As reflected in <FIG>, the embodiment of a diode with the mesa size of <NUM> × <NUM> has current-voltage (I-V) characteristics in the forward direction that is typical of a LED, but the diode opening voltage is relatively low: circa <NUM> V. Optical power emitted from said diode increases approximately in a linear manner as a function of current density. <FIG> presents a semi-log plot with the change in the wavelength of the light emitted from the embodiment of a diode as a function of current density. The spectrum maximum shifts towards the shorter waves within the range from <NUM> to <NUM> in line with the increase in the density of the current flowing through the diode within the range from <NUM> A/cm<NUM> to <NUM> A/cm<NUM>. <FIG> presents standardised electroluminescence spectra for nine selected current densities, designated with letters from (a) through (i) in the drawing. In <FIG>, the same spectra (for the same current densities as in <FIG>) are presented in the chromaticity diagram of the CIE1931 colour space. As reflected in <FIG>, the colour of the light emitted from the diode according to the invention can be continuously changed within the whole visible spectrum, that is, from the red, to orange, yellow, green, cyan to blue, by changing the density of the current flowing through the diode, which is equivalent to a change in the voltage powering the diode.

Through appropriate power supply of the diode according to the invention, it is possible to control its power and mix the colours emitted by said diode in order to form white light and other colours in the CIE <NUM> space.

A scheme of such a power supply is presented in <FIG>. The current (I) or voltage (V) powering the diode changes stepwise. By selecting a power supply level, the wavelength of one of the peaks in the final spectrum is changed, and its intensity is adjusted by the duration. This way it is possible to set, for example, four power supply levels, with three responsible for emission of light of an appropriate primary colour: blue I<NUM>(V<NUM>), green I<NUM>(V<NUM>) and red I<NUM>(V<NUM>), and the zero level I<NUM>(V<NUM>), for which no emission occurs. The final lighting power can be reduced by prolonging the duration of the zero level.

<FIG> presents two spectra (A) and (B) of white light, obtained with the use of the power supply scheme described above, which uses two different power supply levels (V<NUM> and V<NUM>) for the embodiment of a diode according to the invention with the dimensions of <NUM> × <NUM>. In <FIG>, the same spectra as in <FIG> are presented in the chromaticity diagram of the CIE <NUM> colour space. Such a waveform of the power supply was achieved with the use of a programmable arbitrary power supply unit, Siglent SDG2042X. In the case of the spectrum designated with (A), cold white light was achieved when the diode was powered with voltages V<NUM> = <NUM> V and V<NUM> = <NUM> V, while the time of powering with voltage V<NUM> (for example, from t<NUM> to t<NUM> in <FIG>) was <NUM> times longer than the time of its powering with voltage V<NUM> (for example, from to to t<NUM> in <FIG>). In the case of the spectrum designated with (B), in turn, warm white light was achieved when the diode was powered with voltages V<NUM> = <NUM> V and V<NUM> = <NUM> V, while the time of powering with voltage V<NUM> was <NUM> times longer than the time of its powering with voltage V<NUM>. In both cases, voltage levels changed cyclically with the frequency of <NUM>, which means that the aggregate duration of both power supply voltages in one cycle was <NUM>.

Claim 1:
A light emitting diode with a variable light emission colour, having an epitaxial layer structure built of nitrides of group III metals applied to a crystalline substrate, which structure is composed, counted from the crystalline substrate, of at least an n-type conductivity layer, a light emitting active region with one quantum well, and a p-type conductivity layer, characterised in that the active region (<NUM>, <NUM>, <NUM>, <NUM>) is composed, counted from the crystalline substrate (<NUM>), of a lower n-type doped barrier layer (<NUM>), a lower n-type doped part (<NUM>) of the quantum well, an upper p-type doped part (<NUM>) of the quantum well, and an upper p-type doped barrier layer (<NUM>), wherein the width of each part (<NUM>, <NUM>) of the quantum well ranges from <NUM> to <NUM>, the doping level of each part (<NUM>, <NUM>) of the quantum well ranges from <NUM> × <NUM><NUM> cm-<NUM> to <NUM> × <NUM><NUM> cm-<NUM>, while the doping level of the barrier layer (<NUM>, <NUM>) is up to <NUM>% of the doping level of the part of the quantum well (<NUM>, <NUM>) that is adjacent to said barrier layer (<NUM>, <NUM>).