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Optical properties of CdTebelow and above the band gap: 

Optical properties of CdTe below and above the band gap P. Horodyský and P.Hlídek Institute of Physics, Charles University, Prague Direct measurement of interband absorption on very thin samples

Content: 

Content Preparation of thin samples Absorption coefficient close to the energy band gap Quality of ultra-thin samples Temperature dependence of free exciton (FX) peak in absorption

Preparation of thin samples : 

Preparation of thin samples Starting material - high quality CdTe monocrystals Mechanical polishing samples glued on quartz or saphire substrate (thickness 1-5 m) freestanding samples (thickness 10-20 m) Chemical etching (by brom-methanol), removal of several m glued samples (only one side) freestanding samples (both sides) Minimal thickness glued samples 0.8 m freestanding samples 4.8 m (3x3 mm2) CdTe Substrate Micron thick sample glued on substrate

Absorption spectra of thin and thick samplesroom temperature: 

Absorption spectra of thin and thick samples room temperature Absorption tail on both-side etched sample is much weaker Energy band gap of CdTe at RT is 1.50-1.51 eV Absorption edge on ‘thick’ sample is far from Eg

Occupation of edge states during photoluminescence excitationroom temperature: 

Occupation of edge states during photoluminescence excitation room temperature RT Photoluminescence arises from both localised and free states bellow and above Eg

Photoluminescence from thin free-standing samplelow temperature: 

Photoluminescence from thin free-standing sample low temperature Photoluminescence from 3 m below the long-time polished surface ehibits only slightly more broadened bound exciton lines compared to standard high-quality PL spectrum

Absorption of thin free-standing samplelow temperature: 

Absorption of thin free-standing sample low temperature Model for fitting the transmittance spectrum modulated by interference bellow Eg: losses by reflectance R(E) + absorption (two exponential functions for absorption tail) refractive index n(E) taken from Hlídek et al., Refractive index of CdTe: Spectral and temperature dependence, J. Appl. Phys. 90 (4), 2001

Absorption and Photoluminescence from ultra-thin sample low temperature: 

Absorption and Photoluminescence from ultra-thin sample low temperature More details: Grill et al., Defect-induced optical transitions in CdTe and Cd0.96Zn0.04Te , Semi. Sci. Tech. 17, 2002 More details on FX+LO absorption : Dillinger et al., Phonon-Assisted Exciton Transitions in AIIBVI semiconductors, Phys. Stat. Sol. 29, 1968

Absorption and Photoluminescence from ultra-thin sample low temperature: 

Absorption and Photoluminescence from ultra-thin sample low temperature FX linewidth of free-standing sample is 3-4 times narrover Above Eg absorption on a free-standing sample not accessible up to now Absorption on 10-100 m thick samples on BE should give concentrations FX absorption in literature: Marple., Optical Absorption Edge in CdTe: Experimental, Phys. Rev. 150, 1966

Absorption spectra on ultra-thin sample4-130 K: 

Absorption spectra on ultra-thin sample 4-130 K FX absorption peak will give reliable temperature dependence of Eg(T) A theoretical model for absorption could be established for comparison with experiment

Conclusion: 

Conclusion A method for preparation of ultra-thin samples of semiconductor CdTe was demonstrated It was shown that very thin free-standing samples can be produced by combining polishing and etching Temperature dependence of free exciton (FX) peak in absorption can be measured to high temperatures - a reliable temperature dependence of Eg will be derived soon