Investigation on the Electronic Structure and Optical Properties of ZnO Nanofilms

The electronic structure and optical properties of wurtzite ZnO nanofilms with different thickness are investigated systematically by using the first-principles approach. The results indicate that the valence band properties of the ZnO nanofilms are mainly determined by the Zn: 3d state and O: 2p state. And its conduction band properties are determined by Zn: 4s state and Zn: 4p state. The band gap decreases with the thickness of nanofilms increasing in [0001] direction. It is also found that the interband transition absorption edge of ZnO nanofilms decreasing from 5.5 eV to 2.7 eV with the thickness of nanofilms increasing from single layer to five layers. The interband transition of reflection spectrum occurs mainly in the range of 10 eV to 18 eV, which is in line with the ionic bonding characteristic of wurtzite ZnO.

Intersubband Transitions in In0.3 Ga0.7As/GaAs Multiple Quantum Dots of Varying Dot-Sizes

ABSTRACTThe optical absorption spectra of intersubband transitions in In0.3Ga0.7As/GaAs multiple quantum dots (MQDs) grown by molecular beam epitaxy were investigated. By varying the number of In0.3Ga0.7As monolayers deposited, a series of samples with varying dot sizes ranging from 10 – 50 monolayers were obtained. The quantum dots grown with size less than 15 monolayers or more than 50 monolayers did not yield any observable measurements of intersubband transition. This suggests that there exist a critical upper and lower limit of In0.3Ga0.7As quantum dots for infrared detectors. A wavelength range of 8.60 – 13.70 μm is achieved for structures grown with the above monolayers range. The theoretical line-shape of the intersubband transition absorption was compared to the experimental measurements. From the lineshape, it was deduced that bound-to-continuum transtition is present in thick quantum dots and bound-to-bound transition is present in thinly grown quantum dots.