Final Report Summary - ALIGHT (AlGaInN materials on semi-polar templates for yellow emission in solid state lighting applications)
Executive Summary:
Gallium nitride (GaN) based crystals are the primary materials used for the light emitting diodes (LEDs) that ,due to their superb efficiency, compactness and long lifetime are causing a revolution in lighting. The present LEDs are based on a polar crystal orientation which is readily available but has known deficiencies when it comes to high output optical power at high input current. Semipolar crystal orientations should theoretically result in better LEDs but are only available in small sizes and at great expense. The ALIGHT project has sought to develop scalable processes to realise large area semipolar GaN templates and to demonstrate their use in LED devices. The semipolar orientation chosen by the project should ultimately lead to higher efficiency especially at green and yellow wavelengths and excellent polarization characteristics. The project ran from June 2012 to May 2015 with a consortium of 6 members consisting of the Tyndall National Institute, University College Cork (Ireland), the University of Cambridge (UK), the University of Ulm, Max Plank - MPIE, Ferdinand Braun - FBH and OSRAM (all in Germany). Through strong collaboration, the consortium successfully developed high quality, large diameter (100 mm) sapphire wafers with semipolar GaN layers that serve as templates for the subsequent growth of LED structures. Fundamental atomic scale theoretical work has revealed the correct properties of the materias as well as the influence of different conditions on the growth of the materials. The materials were characterised at the nanoscale to help understand the evolution of the growth process under different experimental situations allowing the optimsation of the template formation process. The growth of quantum wells and doping of semipolar structures were optimised. Technological processes to realise LEDs were addressed. LEDs emitting from the blue to the yellow region were demonstrated with good performance considering this early stage of their development. The first 100 mm diameter semipolar LED wafers were processed on an industrial manufacturing line which we believe is a world first. These templates, which in addition to LEDs, may also be used as a basis to develop new types of sensors and electronic devices are ready for the next stage of development. Many reserachers have been trained in the science and technology of these important materials. The results of the work have been published in leading international conferences and journals listed on our website http://www.alight-project.eu/(s’ouvre dans une nouvelle fenêtre) while considerable knowhow in these materials is held by the partners.
Project Context and Objectives:
Gallium nitride is a key material for our modern society. It is used in the highest performing light emitting diodes (LEDs) and in power electronics. A reduction in total electricity use, and thus CO2 emissions, of 10% -15% associated with lighting and a further 10% associated with power electronics is possible with the widespread introduction of GaN technologies. The factors that need to be overcome in order to accelerate the widespread uptake of LEDs are in providing high quality light with a colour balance from the luminaire which is attractive to the consumer while delivering the light at an acceptable cost, and by further improving the conversion efficiency. While great inroads have been made, the LEDs currently on the market suffer from a reduction in their efficiency at higher currents, called droop, thus limiting the total amount of power from a single chip. Furthermore, the colour can change as a function of current and age as the white light is obtained using a down-converting phosphor leading to an energy loss. Therefore, a combination of colours including direct yellow emission would be desirable for higher efficiency and colour control. A solution to these issues can be obtained by basing the LEDs on semi-polar planes where there is a reduction of the polarization fields. However, the necessary semi- polar substrates are difficult to engineer resulting in them being costly and having limited wafer size.
The ALIGHT project addressed this challenge by investigating new approaches to obtaining large area, low defect density, semi-polar (11-22) GaN wafers onto which LEDs can be grown. The approach was based on structured r-plane (10-12) sapphire and structured (113) silicon substrates.
Despite the fact that devices based on GaN materials are commercially available some of the fundamental properties of the alloys are not well understood with many difference values for the same parameter reported in the literature. An example is the evolution in the bandgap with alloy composition. Additional objectives of ALIGHT were to understand the source of the discrepancies and to clarify the values of the actual parameters.
The quality of the semipolar templates available at the start of ALIGHT was not adequate to impact commercially. The size of wafers was limited to 50mm diameter and the quality needed to be improved in terms of uniformity, flatness, reduction of additional features. We set a target to reduce stacking fault and threading dislocation density.
At the start of the project the performance of LEDs based on the available semipolar templates was un-quantified in the literature. However, high quality devices grown on bulk substrates were reported. The objective was to come close and even to surpass the performance of these bulk substrate LEDs.
To address these objectives required contribution from many scientific and technological disciplines: Modeling and simulation of the fundamental materials, surface chemistry and epitaxial growth dynamics, the development of technological and growth processes to initiate and coalesce GaN to obtain a high quality semipolar surface, to use the most advanced analytic techniques to understand the materials at the micron and nanoscale and to relate these back to the technological steps being undertaken; to develop epitaxial processes for the layers that are needed in highly efficient blue and yellow light emitters LED structures,
Special facilities and equipment was needed to address these objectives. For example Metal-Organic Vapour Phase Epitaxy (MOVPE) and Hydride VPE (HVPE) techniques were used for the growth of the materials. X-ray diffraction, photo-luminescence and atomic scale imaging using transmission electron microscopy and atom probe tomography were needed. Fabrication facilities were required to realise the devices which were finally evaluated in suitable packages.
Project Results:
The description of the main scientific and technological achievements are in the attached report
Potential Impact:
We have essentially achieved our stated goal of low defect density, large area (100 mm diameter), (11-22) GaN templates and the realisation of LEDs on these substrates using wafer scale processing. The LEDs show excellent output power for this stage of their development, they show enhanced polarized emission and other useful properties when compared with c-plane LEDs. Freestanding LEDs have also been demonstrated.
Our work is an essential step in the development of GaN materials which one should realise have only had detailed investigation in the last 20 years. As a result GaN and its alloys are relatively poorly understood when compared to silicon. GaN materials are one of the most important and are now beginning to have a major socio-economic impact and it is essential that we learn about these materials and the associated devices as we can expect new innovations to arise. Just consider how societies’ use of lighting has changed in the last decades from reliance on incandescent lamps through fluorescent lamps to now the more widespread use of LEDs. This is because of the incredible improvement in the light generating efficiencies to the ability to provide a complete range of colours, the small size, and long lifetime . These LED devices are also used in displays, medical diagnostics and therapy and even communications and used in multiple new innovations by all sorts of companies. These GaN materials and their alloys (AlInGaN) are the basis for a new generation of highly efficiency power devices. The improved efficiency leads to environmental benefits reducing the amount of power required translating into lower greenhouse emissions. The project looked at recycling materials and identified the opportunities for recovery of Ga which is now being implemented. The materials used are safe and do not have mercury as needed in compact fluorescent lights.
The ALIGHT website (http://www.alight-project.eu/(s’ouvre dans une nouvelle fenêtre)) has reported on the activities during the project and will be archived with a record describing the project, including the publication list and publishable summaries.
The knowledge gained has been disseminated in many ways: at conferences and through the leading journals in the field. For example, the ALIGHT concortium members have participated and had invited talks at the major conferences IWN. The consortium has also participated in national conferences.
The papers have been published in high ranking journals establishing a strong reputation for ALIGHT and for European research in nitride materials. Most of the papers have involved collaboration across the consortium. Additional publications are anticipated and the full list and links to the papers can be accessed from the ALIGHT website.
As a result of the work in the project many researchers – from students to professors - have detailed expertise in the physics and technology of semipolar GaN templates and the associated LEDs. More than 5 PhD students have been involved in the work and are completing their theses. More than 10 different postdoctoral researchers were involved in the work during various stages of the prohject and are developing the careers in academia or industry. Many technical staff worked on the project while several intern students obtained their first experience with research on semiconductor devices. The academic members have used their learning to teach both graduate and undergraduate courses. The members of the consortium have been actively involved in outreach to school children in an effort to promote future career choices in the scientific fields.
The ALIGHT project co-organised two workshops on “Measurement and analysis of X-Ray diffraction of semipolar GaN” at Ulm and a joint “Symposium on Solid State Lighting” held in Nov 2014 in Lund Sweden.
All partners hope to benefit from the knowledge gained in the project through future developments in the materials and the technology. The templates developed are immediately available to others in small volume through the FBH by engaging in discussion. The partners are willing to enter into discussions with companies wanting to develop products on the templates or LEDs. The partners are keen to strengthen existing collaborations and develop new partnerships based on the knowledge gained. These could be industrially relevant or with a view to deeper scientific understanding. The knowhow gained here is influencing approaches to LED optimisation.
List of Websites:
http://www.alight-project.eu/(s’ouvre dans une nouvelle fenêtre)
Gallium nitride (GaN) based crystals are the primary materials used for the light emitting diodes (LEDs) that ,due to their superb efficiency, compactness and long lifetime are causing a revolution in lighting. The present LEDs are based on a polar crystal orientation which is readily available but has known deficiencies when it comes to high output optical power at high input current. Semipolar crystal orientations should theoretically result in better LEDs but are only available in small sizes and at great expense. The ALIGHT project has sought to develop scalable processes to realise large area semipolar GaN templates and to demonstrate their use in LED devices. The semipolar orientation chosen by the project should ultimately lead to higher efficiency especially at green and yellow wavelengths and excellent polarization characteristics. The project ran from June 2012 to May 2015 with a consortium of 6 members consisting of the Tyndall National Institute, University College Cork (Ireland), the University of Cambridge (UK), the University of Ulm, Max Plank - MPIE, Ferdinand Braun - FBH and OSRAM (all in Germany). Through strong collaboration, the consortium successfully developed high quality, large diameter (100 mm) sapphire wafers with semipolar GaN layers that serve as templates for the subsequent growth of LED structures. Fundamental atomic scale theoretical work has revealed the correct properties of the materias as well as the influence of different conditions on the growth of the materials. The materials were characterised at the nanoscale to help understand the evolution of the growth process under different experimental situations allowing the optimsation of the template formation process. The growth of quantum wells and doping of semipolar structures were optimised. Technological processes to realise LEDs were addressed. LEDs emitting from the blue to the yellow region were demonstrated with good performance considering this early stage of their development. The first 100 mm diameter semipolar LED wafers were processed on an industrial manufacturing line which we believe is a world first. These templates, which in addition to LEDs, may also be used as a basis to develop new types of sensors and electronic devices are ready for the next stage of development. Many reserachers have been trained in the science and technology of these important materials. The results of the work have been published in leading international conferences and journals listed on our website http://www.alight-project.eu/(s’ouvre dans une nouvelle fenêtre) while considerable knowhow in these materials is held by the partners.
Project Context and Objectives:
Gallium nitride is a key material for our modern society. It is used in the highest performing light emitting diodes (LEDs) and in power electronics. A reduction in total electricity use, and thus CO2 emissions, of 10% -15% associated with lighting and a further 10% associated with power electronics is possible with the widespread introduction of GaN technologies. The factors that need to be overcome in order to accelerate the widespread uptake of LEDs are in providing high quality light with a colour balance from the luminaire which is attractive to the consumer while delivering the light at an acceptable cost, and by further improving the conversion efficiency. While great inroads have been made, the LEDs currently on the market suffer from a reduction in their efficiency at higher currents, called droop, thus limiting the total amount of power from a single chip. Furthermore, the colour can change as a function of current and age as the white light is obtained using a down-converting phosphor leading to an energy loss. Therefore, a combination of colours including direct yellow emission would be desirable for higher efficiency and colour control. A solution to these issues can be obtained by basing the LEDs on semi-polar planes where there is a reduction of the polarization fields. However, the necessary semi- polar substrates are difficult to engineer resulting in them being costly and having limited wafer size.
The ALIGHT project addressed this challenge by investigating new approaches to obtaining large area, low defect density, semi-polar (11-22) GaN wafers onto which LEDs can be grown. The approach was based on structured r-plane (10-12) sapphire and structured (113) silicon substrates.
Despite the fact that devices based on GaN materials are commercially available some of the fundamental properties of the alloys are not well understood with many difference values for the same parameter reported in the literature. An example is the evolution in the bandgap with alloy composition. Additional objectives of ALIGHT were to understand the source of the discrepancies and to clarify the values of the actual parameters.
The quality of the semipolar templates available at the start of ALIGHT was not adequate to impact commercially. The size of wafers was limited to 50mm diameter and the quality needed to be improved in terms of uniformity, flatness, reduction of additional features. We set a target to reduce stacking fault and threading dislocation density.
At the start of the project the performance of LEDs based on the available semipolar templates was un-quantified in the literature. However, high quality devices grown on bulk substrates were reported. The objective was to come close and even to surpass the performance of these bulk substrate LEDs.
To address these objectives required contribution from many scientific and technological disciplines: Modeling and simulation of the fundamental materials, surface chemistry and epitaxial growth dynamics, the development of technological and growth processes to initiate and coalesce GaN to obtain a high quality semipolar surface, to use the most advanced analytic techniques to understand the materials at the micron and nanoscale and to relate these back to the technological steps being undertaken; to develop epitaxial processes for the layers that are needed in highly efficient blue and yellow light emitters LED structures,
Special facilities and equipment was needed to address these objectives. For example Metal-Organic Vapour Phase Epitaxy (MOVPE) and Hydride VPE (HVPE) techniques were used for the growth of the materials. X-ray diffraction, photo-luminescence and atomic scale imaging using transmission electron microscopy and atom probe tomography were needed. Fabrication facilities were required to realise the devices which were finally evaluated in suitable packages.
Project Results:
The description of the main scientific and technological achievements are in the attached report
Potential Impact:
We have essentially achieved our stated goal of low defect density, large area (100 mm diameter), (11-22) GaN templates and the realisation of LEDs on these substrates using wafer scale processing. The LEDs show excellent output power for this stage of their development, they show enhanced polarized emission and other useful properties when compared with c-plane LEDs. Freestanding LEDs have also been demonstrated.
Our work is an essential step in the development of GaN materials which one should realise have only had detailed investigation in the last 20 years. As a result GaN and its alloys are relatively poorly understood when compared to silicon. GaN materials are one of the most important and are now beginning to have a major socio-economic impact and it is essential that we learn about these materials and the associated devices as we can expect new innovations to arise. Just consider how societies’ use of lighting has changed in the last decades from reliance on incandescent lamps through fluorescent lamps to now the more widespread use of LEDs. This is because of the incredible improvement in the light generating efficiencies to the ability to provide a complete range of colours, the small size, and long lifetime . These LED devices are also used in displays, medical diagnostics and therapy and even communications and used in multiple new innovations by all sorts of companies. These GaN materials and their alloys (AlInGaN) are the basis for a new generation of highly efficiency power devices. The improved efficiency leads to environmental benefits reducing the amount of power required translating into lower greenhouse emissions. The project looked at recycling materials and identified the opportunities for recovery of Ga which is now being implemented. The materials used are safe and do not have mercury as needed in compact fluorescent lights.
The ALIGHT website (http://www.alight-project.eu/(s’ouvre dans une nouvelle fenêtre)) has reported on the activities during the project and will be archived with a record describing the project, including the publication list and publishable summaries.
The knowledge gained has been disseminated in many ways: at conferences and through the leading journals in the field. For example, the ALIGHT concortium members have participated and had invited talks at the major conferences IWN. The consortium has also participated in national conferences.
The papers have been published in high ranking journals establishing a strong reputation for ALIGHT and for European research in nitride materials. Most of the papers have involved collaboration across the consortium. Additional publications are anticipated and the full list and links to the papers can be accessed from the ALIGHT website.
As a result of the work in the project many researchers – from students to professors - have detailed expertise in the physics and technology of semipolar GaN templates and the associated LEDs. More than 5 PhD students have been involved in the work and are completing their theses. More than 10 different postdoctoral researchers were involved in the work during various stages of the prohject and are developing the careers in academia or industry. Many technical staff worked on the project while several intern students obtained their first experience with research on semiconductor devices. The academic members have used their learning to teach both graduate and undergraduate courses. The members of the consortium have been actively involved in outreach to school children in an effort to promote future career choices in the scientific fields.
The ALIGHT project co-organised two workshops on “Measurement and analysis of X-Ray diffraction of semipolar GaN” at Ulm and a joint “Symposium on Solid State Lighting” held in Nov 2014 in Lund Sweden.
All partners hope to benefit from the knowledge gained in the project through future developments in the materials and the technology. The templates developed are immediately available to others in small volume through the FBH by engaging in discussion. The partners are willing to enter into discussions with companies wanting to develop products on the templates or LEDs. The partners are keen to strengthen existing collaborations and develop new partnerships based on the knowledge gained. These could be industrially relevant or with a view to deeper scientific understanding. The knowhow gained here is influencing approaches to LED optimisation.
List of Websites:
http://www.alight-project.eu/(s’ouvre dans une nouvelle fenêtre)
