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

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

3. Why II-VI?

4. 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

5. 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)

6. 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

7. 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

8. 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.

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

10. 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

11. 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

12. Gas Sensors Sensing material on top
Electrical resistance change via chemical reaction
Important features
Response
Gas selectivity
Very low concentrations

13. Nanostructures Increased surface area Decrease reflection
Increase efficiency
Growth
CVD
MOCVD
MBE
Hydrothermal

14. ZnO Structures Thin Film Nanowires

15. 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,

16. 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,

17. 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

18. References
Safa O. Kasap, Peter Capper (2006). Springer handbook of electronic and photonic materials. Springer. pp. 54,327. ISBN 
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: /adma
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,
Springer Handbook of Electronic and Photonic Materials, ISBN 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,
Fine, G.F.; Cavanagh, L.M.; Afonja, A.; Binions, R.; Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring. Sensors 2010, 10,

19. 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)