II-VI Semiconductor Materials, Devices, and Applications EE 4611 Presenter: Lera Beletsky April 11, 2016

Outline Materials Bandgap Engineering Applications Devices Structure Properties Bandgap Engineering Applications Devices Current Research

Why II-VI?

Compounds Any combination of: Most commonly used: Cd, Zn, Hg O, S, Se, Te Most commonly used: CdS, CdSe, CdTe ZnO, ZnS, ZnSe, ZnTe CdO and mercury compounds of lesser interest

Structure Zinc Blende (Sphalerite) Wurtzite Zinc blende and diamond structure seen with silicon very similar – difference in ionic sizes (vary depending on how many other atoms it’s next to) http://som.web.cmu.edu/structures/S011-ZnS.html http://som.web.cmu.edu/structures/S014-ZnO.html

Properties Carrier concentration, etc Band gaps Hard to measure Dependent on growth technique Band gaps Direct Wide band gap (>2eV) devices operate at higher: Voltages Frequencies Temperatures Compound Band Gap (eV) CdS 2.42 CdSe 1.74 CdTe 1.49 ZnO 3.37 ZnS 3.54(zb) / 3.91(wz) ZnSe 2.7 ZnTe 2.25

Properties of ZnO pH range: 6.95-7.37 Non-toxic n-type Easily wet etched Transparent when doped with Al Band gap: 3.37eV Absorbance ~380nm Exciton binding energy: ZnO: 60meV GaN: 25 meV Efficient operation at room temperature https://en.wikipedia.org/wiki/Zinc_oxide

Bandgap Engineering Changing bandgap with doping Wurtzite phase MgZnO ZnO: 3.37eV MgO: 7.8eV Wurtzite phase MgZnO Highest Mg content: 55% Bandgap: 4.55eV Benefits Absorbance ~270nm Retains attractive ZnO properties Big expansion in semiconductor industry using band gap engineering ZnO bandgap: 3.37 eV MgO bandgap: 7.8 Alloying results in something in between Diagram: Band gap on y-axis Function of Mg content Pure ZnO bandgap starts at 3.37 eV Addition of Mg increases band gap Opposite, adding Zn to pure MgO decreases band gap ZnO has better electrical properties than MgO - ZnO is being doped with Mg Up until a certain point, crystal structure remains the same Problem: ZnO wurtzite and MgO cubic Mixed phase begins to occur, undesirable, film not uniform Groups trying to incorporate more Mg into MgZnO alloy Highest content reported so far: 55% corresponding to bandgap of 4.55 eV Why important? Remember electromagnetic spectrum Material begins absorbing shorter wavelengths or deep UV light and Retains attractive ZnO properties Create detector for missiles or flame detection - they give off UV light A. Ohtomo et al. MgZnO as a II-VI widegap semiconductor alloy. Appl. Phys. Lett. 72, 2466, 1988.

Applications Solar cells Photoresistors Lasers Blue LEDs (ZnSe replaced by GaN) Tetrahertz imaging (ZnTe) LCD displays Gas sensors ZnO nanowire FETs

Solar Cells http://www.auo.com/?sn=192&lang=en-US A-Si is pretty low efficiency GaAs is very expensive CIS are emerging – still high cost http://www.auo.com/?sn=192&lang=en-US

CdTe/CdS Solar Cells Pros Drawbacks Thin film Easy to make Less expensive than Si Better solar absorption profile Drawbacks Efficiency ~12-14% (c-Si ~13-20%) Toxicity http://www.nrel.gov/pv/thinfilm.html

Gas Sensors Sensing material on top Electrical resistance change via chemical reaction Important features Response Gas selectivity Very low concentrations http://www.nature.com/am/journal/v2/n2/full/am201040a.html

Nanostructures Increased surface area Decrease reflection Increase efficiency Growth CVD MOCVD MBE Hydrothermal http://spie.org/newsroom/technical-articles-archive/3261-antireflective-nanostructures-for-high-efficiency-optical-devices

ZnO Structures Thin Film Nanowires http://www.hindawi.com/journals/apec/2012/834961/fig8/ http://www.nature.com/am/journal/2010/201008/full/am2010154a.html

ZnO-Based Gas Sensors Regular thick film Nanostructured film Deore, M.K. and Jain, G.H. Synthesis, characterisation and gas sensing application of nano ZnO material. Int. J. Nanoparticles. 2014, 7, 57-72.

ZnO-Based Gas Sensors % Selectivity = max response other gases: max response target gas Deore, M.K. and Jain, G.H. Synthesis, characterisation and gas sensing application of nano ZnO material. Int. J. Nanoparticles. 2014, 7, 57-72.

Summary Elements from the IIB (Cd, Zn, Hg) and VI (O, S, Se, Te) groups Electrical properties are highly dependent on growth method Wide and direct band gaps LEDs, lasers, photoresistors, THz imaging, and LCD displays Current research Nanostructures Solar cells Gas sensors

References Safa O. Kasap, Peter Capper (2006). Springer handbook of electronic and photonic materials. Springer. pp. 54,327. ISBN 0-387-26059-5. http://link.springer.com/chapter/10.1007%2F978-1-4615-5247-5_36 A. Ohtomo et al. MgZnO as a II-VI widegap semiconductor alloy. Appl. Phys. Lett. 72, 2466, 1988. Manekkathodi, A., Lu, M-Y., Wang, C.W. & Chen, L.-J. Direct growth of aligned zinc oxide nanorods on paper substrates for low-cost flexible electronics. Adv. Mater. Published online: 28 May 2010; doi:10.1002/adma.201001289. http://solarcellcentral.com/solar_page.html Afzaal, M. and O’Brien, P. Recent developments in II-VI and III-VI semiconductors and their applications in solar cells. J. Mater. Chem., 2006, 16, 1597-1602. Springer Handbook of Electronic and Photonic Materials, ISBN 978-0-387-26059-4. Springer-Verlag US, 2007, p. 325 Deore, M.K. and Jain, G.H. Synthesis, characterisation and gas sensing application of nano ZnO material. Int. J. Nanoparticles. 2014, 7, 57-72. Fine, G.F.; Cavanagh, L.M.; Afonja, A.; Binions, R.; Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring. Sensors 2010, 10, 5469-5502. http://spie.org/newsroom/technical-articles-archive/3261-antireflective-nanostructures-for-high-efficiency-optical-devices

Key Points Wide and direct band gaps very useful for optoelectronics Bandgap engineering improves properties of ZnO while retaining its benefits CdTe/CdS solar cells are cheaper than Si (just need to increase efficiency) CdTe/CdS solar cells cover more of the solar spectrum than c-Si Growth of nanostructured materials like ZnO nanowires improves performance (i.e. gas sensing, solar reflection)