Instrument to identify defects and impurities in wide band gap semiconductors via excited states

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The researchers propose to develop a new instrument to measure the electrical properties of defects in wide band gap semiconductors. The most important wide gap materials at the present time are compounds made from metals from group III of the periodic table and nitrogen such as GaN and InGaN. These are referred to as III-N materials. They are used to make low energy lighting, LASERs and efficient RF and high power transistors. Today's generation of these devices does not function as well as would seem possible from the properties of the materials and at the present time functionality and performance is limited. LEDs and LASERs (other than blue) are less efficient at generating light than expected and in the case of the transistors several aspects of performance are less than desirable. This is due, at least in part, to the presence of defects in the component materials and devices. These defects are difficult to identify using existing techniques. They may be due to impurities or imperfections in the crystal lattice resulting from the crystal growth or introduced during the device manufacturing process. The research group at Manchester have over thirty years of experience in solving defect problems in other materials such as GaAs and Si. Devices made from these materials have revolutionized society through mass produced electronics and communication technologies. The ability to measure, understand and control defects, particularly electrically active defects, has played a major role in this immense technological achievement and instruments devised, developed and licensed from Manchester have played a role in this. In the case of the III-N materials detecting defects and quantifying their properties is much more difficult and no technique exists at the moment which can look at all the band gap and quantify the recombination paths and trapping centres which degrade III-N devices.The defining feature of the new instrument is that it uses sub-band gap light from tunable semiconductor LASERs to create excited states of the defects. Carriers are then thermally ionised to the semiconductor bands from the excited states. Because the optical excitation stimulates a bound to bound transition, a fine line spectrum can be obtained which is a fingerprint of the defect species and its location in the lattice. In the case of many defects being present, the emission rates will be separated using our existing Laplace DLTS processing. Recombination and trapping parameters can be obtained using the methodologies developed for variants of DLTS and LDLTS. One of our project partners (Santa Barbara University in California) will undertake theoretical studies aimed at associating the excited state spectra with chemical species and/or the structure of the defect with a view to generalized identification rather than using correlation with previously obtained spectra.The instrument development is complementary to the EPSRC contracts currently in progress at many UK universities for the development of III-N materials and devices. In the initial phase of the instrument development, collaborations with consortia led by Cambridge and Glasgow for testing materials and power devices have been negotiated. This will be broadened to embrace other groups as the project progresses. Industrial interest in the project has resulted in strong support from five companies in the field of manufacture of III-N materials, LEDs, GaN power devices and instrumentation. Four of these are UK based. The potential benefits to society of a successful completion of this contract are enormous in facilitating greater improvements in domestic lighting and enabling new applications of the III-N materials to be developed for example in efficient short wavelength UV GaN LEDs. These could be used in cheap low maintenance drinking water sterilization, a pressing concern in the developing world.