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Muons create better understanding of organic semiconductors
RESEARCH NEWS
Muons create better understanding of organic semiconductors ELECTRONIC MATERIALS Electronic devices based on organic semiconductors, such as Alq3 (tris[8-hydroxy-quinoline] aluminum) are revolutionising electroluminescent displays and large-area electronics. Organic semiconductors are economical, can be easily processed in large areas, have tunable electronic properties and easily grow into high quality thin films. Charge transport in organic conductors is fundamental to their operation, yet many aspects of organic charge transport are poorly understood. Techniques such as time-of-flight (TOF) and transportelectroluminescence (TEL) can be useful for comparing properties of thin film device materials prepared under varying conditions. However, it is very difficult to extract intrinsic transport properties from such bulk transport measurements without using large single crystals. Muon spectroscopy can be used for mobility measurements of conductors with very low concentrations of carriers. This constrasts with more familier methods which are more suited to highly doped materials. The muon technique is highly sensitive to intrinsic transport properties, even in polycrystalline material. In contrast, macroscopic transport probe measurements
The crystal structure of Alq3 are usually dominated by extrinsic properties, such as inter-grain hopping. A recent ‘proof-of-principle’ study using muon spectroscopy has confirmed the one-dimensional nature of the charge carrier transport in the organic semiconductor Alq3 and given an upper limit to the charge carrier mobility [Drew et al., Physical Review Letters (2008) 100, 116601]. Drew et al. found that the carrier mobility at large electric fields in the most recent TOF and TEL measurements for Alq3 was significantly smaller than
the mobility measured in the muon measurements in zero electric field. But by correcting the macroscopic TOF/TEL measurements for bottlenecks in the transport chain, the macroscopic mobility was found to be very similar to that in the muon measurements. With the relationships between measurement techniques understood, muon spectroscopy techniques can now be applied to other organic materials.
Alan Drew
Cosmic screening ELECTRONIC MATERIALS A new instrument is to be built at the ISIS Second Target Station dedicated to microchip irradiation testing. Neutrons in the atmosphere can collide with microchips and upset microelectronic devices. These episodes also affect computers on the ground, but the problem is 300 times greater at high altitude. A silicon microchip in an aircraft may be struck by a neutron every few seconds. When a neutron hits silicon, the resulting reaction causes an electrical charge shower that can interfere with electronic equipment. Problems that occur at altitude in the aerospace and avionic industries are also emerging in ground based electronic systems. Smaller electronic circuitry is more vulnerable to neutron buffeting, so the problem is compounded by the drive for smaller and more powerful systems and devices. Access to neutron facilities replicating the cosmic neutron spectrum, particularly in Europe, to test
the quality and susceptibility of components under accelerated conditions has become increasingly important to the wider electronics industry. Recognising this need, the Science and Technology Facilities Council (UK) and the Consiglio Nazionale delle Ricerche (Italy) have joined together to design and build a new neutron testing instrument at ISIS called Chipir [Frost et al., IRPS, April 2009 Montreal]. ISIS is one of the only facilities in the world capable of producing enough very high energy neutrons to perform such accelerated testing. Chipir will result in the creation of the world’s best screening facility. The new instrument will enable manufacturers to test their electronic components and manage the effects of cosmic radiation and is expected to be operational by 2012.
Jim Sutton
Airplane Cockpit. © iStock
NEUTRON SCATTERING SPECIAL ISSUE
69
Muons create better understanding of organic semiconductors ELECTRONIC MATERIALS Electronic devices based on organic semiconductors, such as Alq3 (tris[8-hydroxy-quinoline] aluminum) are revolutionising electroluminescent displays and large-area electronics. Organic semiconductors are economical, can be easily processed in large areas, have tunable electronic properties and easily grow into high quality thin films. Charge transport in organic conductors is fundamental to their operation, yet many aspects of organic charge transport are poorly understood. Techniques such as time-of-flight (TOF) and transportelectroluminescence (TEL) can be useful for comparing properties of thin film device materials prepared under varying conditions. However, it is very difficult to extract intrinsic transport properties from such bulk transport measurements without using large single crystals. Muon spectroscopy can be used for mobility measurements of conductors with very low concentrations of carriers. This constrasts with more familier methods which are more suited to highly doped materials. The muon technique is highly sensitive to intrinsic transport properties, even in polycrystalline material. In contrast, macroscopic transport probe measurements
The crystal structure of Alq3 are usually dominated by extrinsic properties, such as inter-grain hopping. A recent ‘proof-of-principle’ study using muon spectroscopy has confirmed the one-dimensional nature of the charge carrier transport in the organic semiconductor Alq3 and given an upper limit to the charge carrier mobility [Drew et al., Physical Review Letters (2008) 100, 116601]. Drew et al. found that the carrier mobility at large electric fields in the most recent TOF and TEL measurements for Alq3 was significantly smaller than
the mobility measured in the muon measurements in zero electric field. But by correcting the macroscopic TOF/TEL measurements for bottlenecks in the transport chain, the macroscopic mobility was found to be very similar to that in the muon measurements. With the relationships between measurement techniques understood, muon spectroscopy techniques can now be applied to other organic materials.
Alan Drew
Cosmic screening ELECTRONIC MATERIALS A new instrument is to be built at the ISIS Second Target Station dedicated to microchip irradiation testing. Neutrons in the atmosphere can collide with microchips and upset microelectronic devices. These episodes also affect computers on the ground, but the problem is 300 times greater at high altitude. A silicon microchip in an aircraft may be struck by a neutron every few seconds. When a neutron hits silicon, the resulting reaction causes an electrical charge shower that can interfere with electronic equipment. Problems that occur at altitude in the aerospace and avionic industries are also emerging in ground based electronic systems. Smaller electronic circuitry is more vulnerable to neutron buffeting, so the problem is compounded by the drive for smaller and more powerful systems and devices. Access to neutron facilities replicating the cosmic neutron spectrum, particularly in Europe, to test
the quality and susceptibility of components under accelerated conditions has become increasingly important to the wider electronics industry. Recognising this need, the Science and Technology Facilities Council (UK) and the Consiglio Nazionale delle Ricerche (Italy) have joined together to design and build a new neutron testing instrument at ISIS called Chipir [Frost et al., IRPS, April 2009 Montreal]. ISIS is one of the only facilities in the world capable of producing enough very high energy neutrons to perform such accelerated testing. Chipir will result in the creation of the world’s best screening facility. The new instrument will enable manufacturers to test their electronic components and manage the effects of cosmic radiation and is expected to be operational by 2012.
Jim Sutton
Airplane Cockpit. © iStock
NEUTRON SCATTERING SPECIAL ISSUE
69