Periodic Report Summary - MORGAN (Materials for Robust Gallium Nitride)
Concept and objectives
The MORGaN project addresses the need for new materials for electronic devices and sensors that operate in extreme conditions, especially high temperatures, highly corrosive solutions and high electric fields. It will take advantage of the excellent physical properties of diamond and gallium nitride (GaN) based heterostructures. The combination of the two substances will provide the best materials and devices for ultimate performance in extreme environments.
The packaging and metallisation of an electronic device or sensor are equally important considerations in extreme conditions and environments. In addition to chemical stability, metal contacts must be stable and the package must be thermally compatible with the device requirements. Advanced 3D ceramic packaging and new metallisation techniques based on the emerging technology of MN+1AXN alloys are also explored.
Devices and sensors designed to operate in harsh environments need new semiconductor materials which are stable, especially at high temperature and have substrate and package combinations that enable rapid heat extraction or capability to withstand high temperature. Chemical inertness is also an advantage, especially if there is a need to monitor highly corrosive chemical agents.
Diamond has been known for centuries for its excellent optical and mechanical properties. Another property of diamond, which is less well known to the man in the street, is its tremendous thermal conductivity, reaching 2000 Wm-1°C-1 for mono-crystal, which is the highest of any solid material. As such, diamond is potentially the ultimate substrate for many high temperature or extreme power applications.
Gallium nitride (GaN) alloys have been studied intensively for over thirty years but breakthroughs were demonstrated only about fifteen years ago in optical applications (e.g. blue LED, blue laser, lighting, etc.) and more recently have demonstrated impressive power handling from DC to microwave operation with a breakdown field reaching more than 5 MV cm-1. These wide band gap materials are suitable for high voltage (>10 kV) switching applications. This is crucial for the next generation of efficient long distance power distribution systems required for geographically dispersed renewable energy sources.
Diamond-based substrates
Element six (E6) is a major supplier of diamond-based materials. During the first period of the MORGaN project, they delivered to the consortium single crystal diamond of various crystalline orientations ((100), (110), and (111)), polycrystalline diamond substrates and they are currently optimising the manufacturing of Si/polycrystalline 2-inch to 4-inch substrates. The latter task is extremely difficult for these substrates due to the significant thermal mismatch between diamond and silicon. To confirm the necessity for diamond-based substrates, E6 and ATL did implement temporary solutions to minimise delays and confirm the interests of diamond-based substrate solutions.
Compliant substrate technology
A specific MORGaN activity is focused on the mechanical strain handling of GaN. University of Bath and FORTH are involved in the development of strategies for growing high quality GaN-based epitaxial layers on heterogeneous substrates. One solution is using nanopillars which act to absorb the thermal strain. One of the potential applications for this approach is the fabrication of cantilevers with low residual stress and high quality.
InAlN/GaN heterostructure
MORGaN is exploring the new InxAl1-xN/GaN material, which has the advantage of being crystal lattice-matched, for x = 0.17, allowing a large flexibility of Inx Al1-x N epitaxial film thicknesses on top of GaN. This allows a lower intrinsic mechanical stress, thus minimising material degradation mechanisms. This advantage does not come at the expense of electrical performance thanks to a large spontaneous polarisation leading to a factor of three improvement in current density.
An important activity during the first period was the growth optimisation and delivery of InAlN/GaN heterostructures grown on sapphire and silicon carbide to the consortium by ATL and AIX teams. This activity was important to the MORGaN project the teams involved in device activity and to demonstrate state of the art power results. EPFL also delivered a large amount of InAlN/GaN grown on low cost substrates for electrochemical sensors.
For the first time in the world EPFL and FORTH demonstrated the growth of a operational HEMT on single crystal diamond using molecular beam epitaxy. The limited size of those substrates (4x4 mm2) is however an issue for manufacturing. On a different avenue of research, AIX optimised the growth of GaN on 6-inch Si (111), which is compatible with manufacturing, but has a much lower thermal efficiency (the thermal conductivity of silicon is more than ten times worse than that of single crystal diamond).
High performance materials for extreme environments
This work package is related to the packaging and metal interconnections. Developments were undertaken to make housings compatible with operation above 500 °C. Various test vehicles were developed and tested at high temperatures. Various packages were examined and a strong collaboration with teams involved in sensor development took place to define the best solutions. IVF, MG, FCUB and G&H have been major contributors to this work. FCubic is developing 3-D layer manufacturing machining. They are studying various powder materials such as silver and copper, for efficient cooling systems.
For interconnections, MN+1AXN phases are a new class of electrically conducting solids that are thermodynamically very stable. They have been investigated now for about fifteen years and around fifty MN+1AXN alloys are known to exist. 'M' represents an early transition metal such as Sc, Ti, V, Cr, Zr, Nb, Mo, Hf and Ta, but Ti is most commonly used. The 'A' group is mostly IIIA or IVA, usually Al or Si. The 'X' element is Carbon or Nitrogen. In the first period, TiAlC, CrAlC and Ti3SiC2 were tested; the latter seems the most stable. Leading partners on these materials are IC, ITE, TUU and ATL.
Sensors
The III-N system has other desirable properties for sensor applications in extreme environments. It is the only highly polar semiconductor matrix that has ceramic-like stability and can form heterostructures. It has the highest spontaneous polarisation (for AlN), with a Curie temperature above 1000 °C (for AlN). The high surface stability of InAlN/GaN heterostructures enables gas sensing at 800 °C, rivalling current SiC gas sensors. Various sensors have been manufactured during the first period, e.g. single- and double-clamped cantilever structures on Si substrates with Wheatstone bridge integration suppressing the temperature dependence from room temperature to 300 °C. Moreover, diamond cantilevers were tested, in line with MORGaN 'spirit'. Finally diamond-GaN pH sensors were processed and tested successfully from pH =1 to pH = 13.
Passivation and top heat spreaders
Power electronics requires low electrical defect densities to ensure high electrical efficiencies. This can be generally achieved by optimising HEMT and using specific surface treatment. Various dielectrics were used from controlled oxidation procedures to high dielectrics such as Al2O3 or ZrO2 layers. Diamond coating of HEMT devices can be an efficient solution to improve the thermal behaviour of microwave devices. This can also be an interesting approach for protecting devices from harsh environment such as corrosive solutions.
Common semiconductors cannot preserve their electronic transport properties during diamond (NCD) deposition due to the huge thermal budget, which is in the range of 800 °C for hours. However, InAlN associated to GaN appears to be a good candidate and the first encouraging HEMT devices were demonstrated.
Power electronics
One of the main objectives of the MORGaN project for power electronics will be to develop technology to graft GaN-based material on diamond, but also to coat diamond on GaN-based devices. MORGaN will give rise to amplifiers combining the excellent thermal properties of polycrystalline diamond with the electrical efficiency of GaN compounds. MG is in charge of rapid material and electrical characterization using a specifically designed mask set. Within three weeks they are able to give feedback to the epitaxy teams, including full DC mapping. This is an extremely important asset to the project. Significant effort is also dedicated to thermal modelling and many methods of amplifier cooling were explored, for one of the MORGaN objectives is to develop the technology to 1 kW L-Band amplifiers.
A large survey and modelling (ATL, IVF) of the different package cooling strategies was carried out including heat pipe, water cooling, classical air cooling and De Laval nozzles. This work included the impact of device technology (substrate and localized heat spreaders). Solutions based on water cooling and micro-channels were designed and the first heat exchangers were manufactured by FCUB at the end of the period.
During the course of the project, MOSHEMT using localised InAlN oxidation below the gate (EPFL+TUU) were made and a record 11.6 W/mm was measured by IEMN (admittedly with a moderate 38 % power added efficiency (PAE)). ATL materials and components demonstrated excellent microwave power performance using InAlN/GaN HEMT (Schottky gate), with a record value at 3.5 GHz of power densities reaching more than 10 W/mm and 70 % PAE.
High efficiencies are critical to reduce power consumption and 'simplify' the thermal management. The measured PAEs in MORGaN are the state of the art for InAlN/GaN HEMT, and among the best ever reported measurements for devices with such large gate width (up to 2 mm). EPFL epitaxies together with TUU processes were able to demonstrate ft of 21 GHz together with fmax of 42 GHz using single crystal diamond substrate as measured by IEMN. This is a 'first in the world' result.
Transversal activities
The MORGaN project benefits from the support of different teams capable of high-class materials characterisation (MFA, STU, FORTH, ITE, IEE, UoB, ATL, AIX, EPFL), electromechanical modelling and characterisation (UJF, UoB, MG, STU), and electrical transport (IEMN, ATL, STU), and thermal modelling (ATL, IVF, TUW). Electrical characterisation and ageing tests are also carried out in many places such as IEMN, TUW, STU, ATL, while thermal characterisation is mostly done by GLG and TUW. Having so many teams collaborating on these advanced materials and devices, together with the integration teams carrying out the testing, is important to improve both materials and devices.
Training
There have been three rounds of WP9.3 research applications which have approved fourteen visits between MORGaN partners or attendance of a key conference (WP9.3). These visits were felt to be very useful for advancing the spread of knowledge and expertise through the different teams. A major MORGaN workshop was held at STU (24-26th May 2010) for training of twenty young MORGaN researchers (WP9.2). Very good feedback was received from the attendees and both UoB and STU must be thanked for their contributions. Moreover WP9 support has been approved for the 19th Hetech conference (October 18 - 20, 2010) which is to be organised by FORTH (WP9.3).
Dissemination and monitoring of innovation
Dissemination has been very active during the first reporting period and the results are in line with the planned activity. Key highlights include: promotional leaflets and posters, project newsletters, project presence at a wide range of public events, a distribution list of interested parties, articles in trade magazines and scientific press, and a project website, which can be found at http://www.morganproject.eu/. The monitoring of innovation was active only at the end of the first period. A questionnaire was prepared to be used by the consortium to evaluate the innovation levels of the project.
Conclusion
The MORGaN consortium has demonstrated state of the art results (a new GaN material approach, packaging for harsh environments, sensor technology for ambient temperatures above 500 °C, HEMT on single crystal diamond, HEMT with heat spreader, high efficiency HEMT, etc). Mastering of Si on polycrystalline composite substrate is an issue but various solutions are under evaluation. This had an impact on the project but alternative sapphire and SiC substrates bypassed the associated delay. Globally, MORGaN appears a clear source of innovations.
The MORGaN project addresses the need for new materials for electronic devices and sensors that operate in extreme conditions, especially high temperatures, highly corrosive solutions and high electric fields. It will take advantage of the excellent physical properties of diamond and gallium nitride (GaN) based heterostructures. The combination of the two substances will provide the best materials and devices for ultimate performance in extreme environments.
The packaging and metallisation of an electronic device or sensor are equally important considerations in extreme conditions and environments. In addition to chemical stability, metal contacts must be stable and the package must be thermally compatible with the device requirements. Advanced 3D ceramic packaging and new metallisation techniques based on the emerging technology of MN+1AXN alloys are also explored.
Devices and sensors designed to operate in harsh environments need new semiconductor materials which are stable, especially at high temperature and have substrate and package combinations that enable rapid heat extraction or capability to withstand high temperature. Chemical inertness is also an advantage, especially if there is a need to monitor highly corrosive chemical agents.
Diamond has been known for centuries for its excellent optical and mechanical properties. Another property of diamond, which is less well known to the man in the street, is its tremendous thermal conductivity, reaching 2000 Wm-1°C-1 for mono-crystal, which is the highest of any solid material. As such, diamond is potentially the ultimate substrate for many high temperature or extreme power applications.
Gallium nitride (GaN) alloys have been studied intensively for over thirty years but breakthroughs were demonstrated only about fifteen years ago in optical applications (e.g. blue LED, blue laser, lighting, etc.) and more recently have demonstrated impressive power handling from DC to microwave operation with a breakdown field reaching more than 5 MV cm-1. These wide band gap materials are suitable for high voltage (>10 kV) switching applications. This is crucial for the next generation of efficient long distance power distribution systems required for geographically dispersed renewable energy sources.
Diamond-based substrates
Element six (E6) is a major supplier of diamond-based materials. During the first period of the MORGaN project, they delivered to the consortium single crystal diamond of various crystalline orientations ((100), (110), and (111)), polycrystalline diamond substrates and they are currently optimising the manufacturing of Si/polycrystalline 2-inch to 4-inch substrates. The latter task is extremely difficult for these substrates due to the significant thermal mismatch between diamond and silicon. To confirm the necessity for diamond-based substrates, E6 and ATL did implement temporary solutions to minimise delays and confirm the interests of diamond-based substrate solutions.
Compliant substrate technology
A specific MORGaN activity is focused on the mechanical strain handling of GaN. University of Bath and FORTH are involved in the development of strategies for growing high quality GaN-based epitaxial layers on heterogeneous substrates. One solution is using nanopillars which act to absorb the thermal strain. One of the potential applications for this approach is the fabrication of cantilevers with low residual stress and high quality.
InAlN/GaN heterostructure
MORGaN is exploring the new InxAl1-xN/GaN material, which has the advantage of being crystal lattice-matched, for x = 0.17, allowing a large flexibility of Inx Al1-x N epitaxial film thicknesses on top of GaN. This allows a lower intrinsic mechanical stress, thus minimising material degradation mechanisms. This advantage does not come at the expense of electrical performance thanks to a large spontaneous polarisation leading to a factor of three improvement in current density.
An important activity during the first period was the growth optimisation and delivery of InAlN/GaN heterostructures grown on sapphire and silicon carbide to the consortium by ATL and AIX teams. This activity was important to the MORGaN project the teams involved in device activity and to demonstrate state of the art power results. EPFL also delivered a large amount of InAlN/GaN grown on low cost substrates for electrochemical sensors.
For the first time in the world EPFL and FORTH demonstrated the growth of a operational HEMT on single crystal diamond using molecular beam epitaxy. The limited size of those substrates (4x4 mm2) is however an issue for manufacturing. On a different avenue of research, AIX optimised the growth of GaN on 6-inch Si (111), which is compatible with manufacturing, but has a much lower thermal efficiency (the thermal conductivity of silicon is more than ten times worse than that of single crystal diamond).
High performance materials for extreme environments
This work package is related to the packaging and metal interconnections. Developments were undertaken to make housings compatible with operation above 500 °C. Various test vehicles were developed and tested at high temperatures. Various packages were examined and a strong collaboration with teams involved in sensor development took place to define the best solutions. IVF, MG, FCUB and G&H have been major contributors to this work. FCubic is developing 3-D layer manufacturing machining. They are studying various powder materials such as silver and copper, for efficient cooling systems.
For interconnections, MN+1AXN phases are a new class of electrically conducting solids that are thermodynamically very stable. They have been investigated now for about fifteen years and around fifty MN+1AXN alloys are known to exist. 'M' represents an early transition metal such as Sc, Ti, V, Cr, Zr, Nb, Mo, Hf and Ta, but Ti is most commonly used. The 'A' group is mostly IIIA or IVA, usually Al or Si. The 'X' element is Carbon or Nitrogen. In the first period, TiAlC, CrAlC and Ti3SiC2 were tested; the latter seems the most stable. Leading partners on these materials are IC, ITE, TUU and ATL.
Sensors
The III-N system has other desirable properties for sensor applications in extreme environments. It is the only highly polar semiconductor matrix that has ceramic-like stability and can form heterostructures. It has the highest spontaneous polarisation (for AlN), with a Curie temperature above 1000 °C (for AlN). The high surface stability of InAlN/GaN heterostructures enables gas sensing at 800 °C, rivalling current SiC gas sensors. Various sensors have been manufactured during the first period, e.g. single- and double-clamped cantilever structures on Si substrates with Wheatstone bridge integration suppressing the temperature dependence from room temperature to 300 °C. Moreover, diamond cantilevers were tested, in line with MORGaN 'spirit'. Finally diamond-GaN pH sensors were processed and tested successfully from pH =1 to pH = 13.
Passivation and top heat spreaders
Power electronics requires low electrical defect densities to ensure high electrical efficiencies. This can be generally achieved by optimising HEMT and using specific surface treatment. Various dielectrics were used from controlled oxidation procedures to high dielectrics such as Al2O3 or ZrO2 layers. Diamond coating of HEMT devices can be an efficient solution to improve the thermal behaviour of microwave devices. This can also be an interesting approach for protecting devices from harsh environment such as corrosive solutions.
Common semiconductors cannot preserve their electronic transport properties during diamond (NCD) deposition due to the huge thermal budget, which is in the range of 800 °C for hours. However, InAlN associated to GaN appears to be a good candidate and the first encouraging HEMT devices were demonstrated.
Power electronics
One of the main objectives of the MORGaN project for power electronics will be to develop technology to graft GaN-based material on diamond, but also to coat diamond on GaN-based devices. MORGaN will give rise to amplifiers combining the excellent thermal properties of polycrystalline diamond with the electrical efficiency of GaN compounds. MG is in charge of rapid material and electrical characterization using a specifically designed mask set. Within three weeks they are able to give feedback to the epitaxy teams, including full DC mapping. This is an extremely important asset to the project. Significant effort is also dedicated to thermal modelling and many methods of amplifier cooling were explored, for one of the MORGaN objectives is to develop the technology to 1 kW L-Band amplifiers.
A large survey and modelling (ATL, IVF) of the different package cooling strategies was carried out including heat pipe, water cooling, classical air cooling and De Laval nozzles. This work included the impact of device technology (substrate and localized heat spreaders). Solutions based on water cooling and micro-channels were designed and the first heat exchangers were manufactured by FCUB at the end of the period.
During the course of the project, MOSHEMT using localised InAlN oxidation below the gate (EPFL+TUU) were made and a record 11.6 W/mm was measured by IEMN (admittedly with a moderate 38 % power added efficiency (PAE)). ATL materials and components demonstrated excellent microwave power performance using InAlN/GaN HEMT (Schottky gate), with a record value at 3.5 GHz of power densities reaching more than 10 W/mm and 70 % PAE.
High efficiencies are critical to reduce power consumption and 'simplify' the thermal management. The measured PAEs in MORGaN are the state of the art for InAlN/GaN HEMT, and among the best ever reported measurements for devices with such large gate width (up to 2 mm). EPFL epitaxies together with TUU processes were able to demonstrate ft of 21 GHz together with fmax of 42 GHz using single crystal diamond substrate as measured by IEMN. This is a 'first in the world' result.
Transversal activities
The MORGaN project benefits from the support of different teams capable of high-class materials characterisation (MFA, STU, FORTH, ITE, IEE, UoB, ATL, AIX, EPFL), electromechanical modelling and characterisation (UJF, UoB, MG, STU), and electrical transport (IEMN, ATL, STU), and thermal modelling (ATL, IVF, TUW). Electrical characterisation and ageing tests are also carried out in many places such as IEMN, TUW, STU, ATL, while thermal characterisation is mostly done by GLG and TUW. Having so many teams collaborating on these advanced materials and devices, together with the integration teams carrying out the testing, is important to improve both materials and devices.
Training
There have been three rounds of WP9.3 research applications which have approved fourteen visits between MORGaN partners or attendance of a key conference (WP9.3). These visits were felt to be very useful for advancing the spread of knowledge and expertise through the different teams. A major MORGaN workshop was held at STU (24-26th May 2010) for training of twenty young MORGaN researchers (WP9.2). Very good feedback was received from the attendees and both UoB and STU must be thanked for their contributions. Moreover WP9 support has been approved for the 19th Hetech conference (October 18 - 20, 2010) which is to be organised by FORTH (WP9.3).
Dissemination and monitoring of innovation
Dissemination has been very active during the first reporting period and the results are in line with the planned activity. Key highlights include: promotional leaflets and posters, project newsletters, project presence at a wide range of public events, a distribution list of interested parties, articles in trade magazines and scientific press, and a project website, which can be found at http://www.morganproject.eu/. The monitoring of innovation was active only at the end of the first period. A questionnaire was prepared to be used by the consortium to evaluate the innovation levels of the project.
Conclusion
The MORGaN consortium has demonstrated state of the art results (a new GaN material approach, packaging for harsh environments, sensor technology for ambient temperatures above 500 °C, HEMT on single crystal diamond, HEMT with heat spreader, high efficiency HEMT, etc). Mastering of Si on polycrystalline composite substrate is an issue but various solutions are under evaluation. This had an impact on the project but alternative sapphire and SiC substrates bypassed the associated delay. Globally, MORGaN appears a clear source of innovations.
Contact
Record Number: 46400 /
Last updated on: 2011-01-17
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