Exciton quantum confinement in nanocones formed on a surface of CdZnTe solid solution by laser radiation
© Medvid' et al.; licensee Springer. 2012
Received: 15 August 2012
Accepted: 6 September 2012
Published: 20 September 2012
The investigation of surface morphology using atomic force microscope has shown self-organizing of the nanocones on the surface of CdZnTe crystal after irradiation by strongly absorbed Nd:YAG laser irradiation at an intensity of 12.0 MW/cm2. The formation of nanocones is explained by the presence of a thermogradient effect in the semiconductor. The appearance of a new exciton band has been observed after irradiation by the laser which is explained by the exciton quantum confinement effect in nanocones.
KeywordsNanocones Exciton quantum confinement effect Thermogradient effect CdZnTe Nd:YAG laser
Nowadays, nanostructures are one of the most investigated objects in semiconductor physics, especially the quantum confinement effect (QCE) in such quantum systems as quantum dots or 0D[1–5], quantum wires or 1D[6–8] and quantum wells or 2D[9–14]. In the case of nanostructures, the energy band diagram of the semiconductor is strongly changed. This leads to a crucial change of semiconductor properties, such as electrical properties (the change of free charge carriers concentration and their mobility), optical properties (absorption coefficient, reflectivity coefficient, and radiative recombination efficiency), and mechanical and thermal properties. Another possibility to change a property of a semiconductor is by using solid solution, such as Cd1−xZn x Te and Si1−xGe x , which change the component content. It was shown that the shapes and sizes of the mentioned quantum systems have more influence on the properties of a semiconductor than its component content. For example, the ‘blue shift’ of Si0.7Ge0.3 photoluminescence (PL) spectrum of nanocones is up to 1.2 eV, but the possible maximal shift of PL spectra due to change of x is only up to 0.33 eV. Moreover, the band of PL spectrum is broader and more intense due to QCE and graded band gap. Moreover, nanocones enhance radiation hardness of CdZnTe detector, as shown in.
In this paper, we report about the appearance of a new band in PL spectra of Cd1−xZn x Te solid solution irradiated by Nd:YAG laser, which is explained by exciton QCE in nanocones formed on the irradiated surface of the sample.
The laser processing was performed on the samples of Cd1−xZn x Te solid solution with x = 0.1 in ambient atmosphere at room temperature, pressure 1 atm, and 80% humidity. The surface of Cd1−xZn x Te sample was irradiated by pulses of Nd:YAG laser with the wavelength of λ = 532 nm, pulse duration of τ = 15 ns and power p = 1 MW. The spot of laser beam with 3 mm diameter was moved by 20 μm steps over the surface of the sample. Atomic force microscope (AFM) was used for the study of the irradiated surface morphology. The low-temperature PL at 5 K was carried out to investigate the optical properties of the nanostructures formed by laser radiation (LR) on the samples. He-Ne laser with λ = 632.8 nm was used as an excitation source.
Results and discussion
This process takes place in the following way: the irradiation of the Cd1−xZn x Te solid solution by the laser leads to the drift of Cd atoms toward the irradiated surface and of Zn atoms - in the opposite direction due to high gradient of temperature. This is so-called thermogradient effect (TGE). As a result, the formation of CdTe/Cd1−x 1Znx 1Te heterostructure, where x1 > x, takes place due to the replacement of Zn atoms by Cd atoms at the irradiated surface. At the same time, the opposite process takes place under the top layer. In the buried layer of the semiconductor, Zn atoms replace Cd atoms. At least three factors determine A0X and D0X exciton lines position in PL spectrum. They are as follows: the concentration of Zn atoms in the CdTe top layer and in CdZnTe buried layer, 2D EQC effect in the CdTe layer when its thickness is comparable with Bohr radius of the exciton, and the mechanical compressive stress of the CdTe top layer due to mismatch of CdTe and CdZnTe crystalline lattice constant.
It means that before the irradiation of the sample by the laser, the intensity of LO-ZnTe phonon band was three to four times higher than the intensity of TO and LO-CdTe phonon bands, but after irradiation the opposite situation in Raman spectra is observed.
The studies of the effect of highly absorbed laser radiation on the optical properties of the Cd1−xZn x Te (x = 0.1) compound have revealed the formation of nanocones on the surface of the semiconductor under irradiation by the Nd:YAG laser within the intensity range of 9.0-12.0 MW/cm2 and the simultaneous appearance of a new PL band at 1.87 eV, which is explained by the exciton quantum confinement effect in nanocones.
The TGE has the main role in redistribution of Zn atoms at the surface of Cd1−xZn x Te irradiated by the second harmonic of Nd:YAG laser. The graded band gap structure with open optical window is formed on the top of nanocones after the irradiation by Nd:YAG laser at the intensity 4.0-12.0 MW/cm2. Formation of a graded band gap with a close of optical window in Cd1−xZn x Te crystal is possible under irradiation by the second harmonic of Nd:YAG laser at the intensity 0.2-2.0 MW/cm2. A two-stage model of the nanocones formation on the surface of the Cd1−xZn x Te (x = 0.1) under the irradiation by Nd:YAG laser at the intensity 4.0-12.0 MW/cm2 was proposed.
Atomic force microscope
Exciton quantum confinement
- I Cd :
Cd interstitial atoms
Quantum confinement effect
- V Cd :
The authors gratefully acknowledge the financial support in part by the European Regional Development Fund within the projects ‘Sol–gel and laser technologies for the development of nanostructures and barrier structures’ and 2010/0221/2DP/184.108.40.206/10/APIA/VIAA/145, RTU PVS ID 1535.
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