Beyond ZnOmaterials for Piezotronics and Nanogeneration
Dr. Max Migliorato
School of Electrical and ElectronicEngineering, University of Manchester, United Kingdom
Piezotronics is a term coined in by ProfZhong Lin Wang (Georgia Institute of Technology, Atlanta, USA) and describesthe exploitation of strain and deformation internal polarization fields inpolar semiconductors. Such fields already find applications in transducers andmicropositioner devices but are also known to be present in GaN based lightemitting diodes and lasers. Being a property of polar semiconductorspiezoelectricity is present in both III-V and II-VI compounds, such as thetechnologically important ZnO. For many years piezoelectricity was included inthe design of devices only to first order. In recent years a great deal ofevidence, both model and experimental data, has been generated that sucheffects need to be included to at least second order.  The inclusion of suchnonlinear effects produces surprising and non-intuitive results, notably thegeneration of fields of opposite polarity compared to the prediction of linearpiezoelectricity and the possibility of enhancing the piezoelectricpolarization by a factor of 5-10 under particular deformations. In thispresentation we will show the evidence for nonlinear effects and discuss thepossible applications to light emitting diodes, quantum dot emitters and energyharvesting devices. Our theory of Non Linearities, based on accurate quantummechanical calculations and a tight binding formulation of the elastic anddielectric properties of zincblende and wurtzite crystals, is capable of highlightingand correctly predicting the polarization properties of several polarsemiconductors under strain.
We observed strong non linearities in the strain dependence of both zincblendeand wurtzite III-V’s III-N and its alloys and even II-VI wurtzitesemiconductors. While revealing much smaller Spontaneous Polarization effectsin nitrides than previously reported, much larger values of the total(spontaneous + strain induced) polarization are observed compared to linearmodels, therefore highlighting the large effect of Non Linear Piezoelectricityin wurtzite semiconductors. Furthermore we will discuss new approach to designing higher outputefficiency nitride based LEDs through the introduction of a metamorphic layer.
Our recent work on III-V wurtzitecore-shell nanowires reveal much increased voltage generation are predicted (to be 3 orders ofmagnitude larger) than the typical values of ±3 V in homogeneous nanowires.Also considering properties such as voltage generation, bandgap discontinuityand mobility, such core-shell nanowires are candidates for high performancecomponents in piezotronics and nanogeneration . 
Our continued efforts towards developing anew breed of empirical interatomic potentials, namely MMP potentials, forsemiconductors and very recently, we observed strain induced semiconductinggraphene and submitted a patent application on “Graphene-basedElectronic Structure”. We will discuss the potential for Graphene as a piezoelectricmaterial.
Thesignificance of our work is that by incorporating our unique and well testedability of evaluating strain induced polarization in the design of compositesemiconductor structures we will in the near future be able to propose layoutswhere the piezoelectric fields, i.e. the ‘engine’ inside Energy Harvestingdevices, can be suitably enhanced, hugely increasing the ability of suchdevices to convert mechanical energy into an electrical potential difference.Energy harvesting technology is highly sought after by industry and policymakers because of the potential to reduce the need for 24h charging of portableelectronic devices.
Max A. Migliorato(Laurea in Physics, 1999, University of Rome “La Sapienza”, Italy, and PhD,2003, University of Sheffield, UK). In 2004 he was awarded a Royal Academy ofEngineering ? EPSRC Research Fellowship at the University of Sheffield. In 2007he was also awarded an RCUK Fellowship at the University of Manchester. Hiswork has always concentrated on atomistic modelling of semiconductornanostructures and he has pioneered the study of non linear polarizationeffects in III-V and II-VI semiconductors.
He is a researcher with 15 years ofexperience in modelling, simulations and characterization of semiconductormaterials and devices. His interests span from IIII-As, III-N, SiGe to Graphene.Over the years he has been awarded 3 EPSRC grants and 2 Research Fellowships.He has authored around 40 journal articles and received around 600 citations. Hisrecent work on Graphene, based on a novel technique of predicting crystaldeformation under external strain, has produced one patent application. He iscurrently the co-chairman of the international conference on Theory, Modellingand Computational methods for Semiconductors (TMCS).
Since March 2011 he is a Lecturer in thesame School, with research in modelling of electronic materials. For moreinformation see his web page: http://www.tmcsuk.org/migliorato/
 M. A. Migliorato et al, Phys. Rev. B74, 245332 (2006); R. Garg et al, Appl. Phys. Lett. 95, 041912(2009); J.Pal et al, Phys. Rev. B 84, 085211 (2011); H.Y.S. Al-Zahrani etal, Nano Energy 2 (6),1214 (2013); G Tse et al, J. Appl. Phys. 114 (7),073515 (2013).
 J.Pal et al, J. Appl. Phys. 114 (7),073104 (2013)
 H.Y.S. Al-Zahrani et al, Nano Energy(2014) (Accepted).
光学相干层析术（OpticalCoherence Tomography, OCT）是一种高速高分辨率无损伤的三维生物组织光学成像技术。具有1-10微米的轴向分辨率，比传统的超声波探测高1到2个数量级，在生物组织中成像深度可达数毫米量级，高速OCT系统可以实现高达百帧每秒的二维成像速度，在科学研究和医学临床具有着广泛的应用前景，目前的主要应用是活体诊断、医学治疗监测及工业监测等方面。此外，OCT成像技术结合多普勒效应、偏振效应等，可以实现生物组织偏振敏感的OCT成像和多普勒流体流速OCT成像。