Exciton quantum confinement in nanocones formed on a surface of CdZnTe solid solution by laser radiation
 Artur Medvid'^{1}Email author,
 Natalia Litovchenko^^{2},
 Aleksandr Mychko^{1} and
 Yuriy Naseka^{2}
DOI: 10.1186/1556276X7514
© Medvid' et al.; licensee Springer. 2012
Received: 15 August 2012
Accepted: 6 September 2012
Published: 20 September 2012
Abstract
The investigation of surface morphology using atomic force microscope has shown selforganizing 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/cm^{2}. 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.
Keywords
Nanocones Exciton quantum confinement effect Thermogradient effect CdZnTe Nd:YAG laserBackground
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[15]. Another possibility to change a property of a semiconductor is by using solid solution, such as Cd_{1−x}Zn_{ x }Te[16] and Si_{1−x}Ge_{ x }[17], which change the component content. It was shown[18] 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 Si_{0.7}Ge_{0.3} 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[19].
In this paper, we report about the appearance of a new band in PL spectra of Cd_{1−x}Zn_{ 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.
Methods
The laser processing was performed on the samples of Cd_{1−x}Zn_{ x }Te solid solution with x = 0.1 in ambient atmosphere at room temperature, pressure 1 atm, and 80% humidity. The surface of Cd_{1−x}Zn_{ 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 lowtemperature PL at 5 K was carried out to investigate the optical properties of the nanostructures formed by laser radiation (LR) on the samples. HeNe 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 Cd_{1−x}Zn_{ 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 socalled thermogradient effect (TGE)[20]. As a result, the formation of CdTe/Cd_{1−x 1}Zn_{x 1}Te heterostructure, where x_{1} > 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 A^{0}X and D^{0}X 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 LOZnTe phonon band was three to four times higher than the intensity of TO and LOCdTe phonon bands, but after irradiation the opposite situation in Raman spectra is observed.
Conclusions
The studies of the effect of highly absorbed laser radiation on the optical properties of the Cd_{1−x}Zn_{ 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 9.012.0 MW/cm^{2} 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 Cd_{1−x}Zn_{ 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 4.012.0 MW/cm^{2}. Formation of a graded band gap with a close of optical window in Cd_{1−x}Zn_{ x }Te crystal is possible under irradiation by the second harmonic of Nd:YAG laser at the intensity 0.22.0 MW/cm^{2}. A twostage model of the nanocones formation on the surface of the Cd_{1−x}Zn_{ x }Te (x = 0.1) under the irradiation by Nd:YAG laser at the intensity 4.012.0 MW/cm^{2} was proposed.
Abbreviations
 AFM:

Atomic force microscope
 0D:

Quantum dots
 1D:

Quantum wires
 2D:

Quantum wells
 EQC:

Exciton quantum confinement
 I _{Cd} :

Cd interstitial atoms
 LR:

Laser radiation
 PL:

Photoluminescence
 QCE:

Quantum confinement effect
 TGE:

Thermogradient effect
 V _{Cd} :

Cd vacancies.
Declarations
Acknowledgment
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/2.1.1.0/10/APIA/VIAA/145, RTU PVS ID 1535.
Authors’ Affiliations
References
 Alivisatos A: Semiconductor clusters, nanocrystals, and quantum dots. Science 1996, 271: 933–937. 10.1126/science.271.5251.933View ArticleGoogle Scholar
 Norris D, Bawendi M: Measurement and assignment of the sizedependent optical spectrum in CdSe quantum dots. Phys Rev B 1996, 53: 16338–16346. 10.1103/PhysRevB.53.16338View ArticleGoogle Scholar
 Kunz A, Weidman R, Collins T: Pressureinduced modifications of the energy band structure of crystalline CdS. J Phys C: Solid State Phys 1981, 14: L581L584. 10.1088/00223719/14/20/004View ArticleGoogle Scholar
 Lee C, Mizel A, Banin U, Cohen M, Alivisatos A: Observation of pressureinduced directtoindirect band gap transition in InP nanocrystals. J Chem Phys 2000, 113: 2016–2020. 10.1063/1.482008View ArticleGoogle Scholar
 Yoffe A: Lowdimensional systems  quantumsize effects and electronic properties of semiconductor micro crystallites (zerodimensional systems) and some quasi2dimensional systems. Adv Phys 1993, 42: 173–266. 10.1080/00018739300101484View ArticleGoogle Scholar
 Xia Y, Yang P, Sun Y, Wu Y, Mayers B, Gates B, Yin Y, Kim F, Yan H: Onedimensional nanostructures: synthesis, characterization, and applications. Adv Mater 2003, 15: 353–389. 10.1002/adma.200390087View ArticleGoogle Scholar
 Krutarth T, Hyungsang Y, Herman C, Moon J, Walter H: Quantum confinement induced performance enhancement in sub5nm lithographic Si nanowire transistors. Nano Lett 2011, 11: 1412–1417. 10.1021/nl103278aView ArticleGoogle Scholar
 Reed M, Randall J, Aggarwal R, Matyi R, Moore T, Wetsel A: Observation of discrete electronic states in a zerodimensional semiconductor nanostructure. Phys Rev Lett 1988, 60: 535–537. 10.1103/PhysRevLett.60.535View ArticleGoogle Scholar
 Fowler A, Fang F, Howard W, Stiles P: Magnetooscillatory conductance in silicon surfaces. Phys Rev Lett 1966, 16: 901–903. 10.1103/PhysRevLett.16.901View ArticleGoogle Scholar
 Tulkki J, Heinamaki A: Confinement effect in quantum well dot induced by InP stressor. Phys Rev B 1995, 52: 8239. 10.1103/PhysRevB.52.8239View ArticleGoogle Scholar
 Xiao X, Liu C, Sturm J, Lenchyshyn L, Thewalt M, Gregory R, Fejes P: Quantum confinement effects in strained silicongermanium alloy quantum wells. Appl Phys Lett 1992, 60: 2135–2137. 10.1063/1.107061View ArticleGoogle Scholar
 Kuo Y, Lee Y, Ge Y, Ren S, Roth J, Kamins T, Miller D, Harris J: Strong quantumconfined Stark effect in germanium quantumwell structures on silicon. Nature 2005, 437: 1334–1336. 10.1038/nature04204View ArticleGoogle Scholar
 Dingle R, Wiegmann W, Henry C: Quantum states of confined carriers in very thin AlxGa1–xAsGaAsAlxGa1−x as heterostructures. Phys Rev Lett 1974, 33: 827–830. 10.1103/PhysRevLett.33.827View ArticleGoogle Scholar
 Parsons C, Thacker B, Szmyd D, Peterson M, McMahon W, Nozik A: Characterization and photocurrent spectroscopy of single quantum wells. J Chem Phys 1990, 93: 7706–7715. 10.1063/1.459350View ArticleGoogle Scholar
 Li J, Hong X, Liu Y, Li D, Wang Y, Li J, Bai Y, Li T: Highly photoluminescent CdTe/poly(Nisopropylacrylamide) temperaturesensitive gels. Adv Mater 2005, 17: 163–166. 10.1002/adma.200400448View ArticleGoogle Scholar
 Reno J, Jones E: Determination of the dependence of the bandgap energy on composition for Cd1−xZnxTe. Phys Rev B 1992, 45: 1440–1442. 10.1103/PhysRevB.45.1440View ArticleGoogle Scholar
 Sun K, Sue S, Liu C: Lowdimensional systems and nanostructures. Physica E 2005, 28: 525–530. 10.1016/j.physe.2005.05.063View ArticleGoogle Scholar
 Medvid' A: Nanocones formed on a surface of semiconductors by laser radiation: technology, model and properties. In Nanowires Science and Technology. Edited by: Viena LN. Rijeka: INTECH; 2010:61–82.Google Scholar
 Medvid' A, Mychko A, Dauksta E, Naseka Y, Crocco J, Dieguez E: The effect of laser radiation on CdZnTe radiation hardness. JINST 2011, 6: C11010. 10.1088/17480221/6/11/C11010View ArticleGoogle Scholar
 Medvid' A: Redistribution of point defects in the crystalline lattice of a semiconductor in an inhomogeneous temperature field. Defect Diffus Forum 2002, 89–102: 210–212.Google Scholar
 Medvid' A, Fedorenko L, Korbutjak B, Kryluk S, Yusupov M, Mychko A: Formation of graded bandgap in CdZnTe byYAG:Nd laser radiation. Radiat Meas 2007, 42: 701–703. 10.1016/j.radmeas.2007.01.070View ArticleGoogle Scholar
 Medvid' A, Onufrijevs P, Chiradze G, Muktapavela F: Impact of laser radiation on microhardness of a semiconductor. AIP Conf Proc 2011, 1399: 181–182.View ArticleGoogle Scholar
 Thomas D, Hopfield J: Excitons and band splitting produced by uniaxial stress in CdTe. J Appl Phys 1961, 32: 2298–2304. 10.1063/1.1777063View ArticleGoogle Scholar
 Yonenaga I: Hardness, yield strength, and dislocation velocity in elemental and compound semiconductors. Mater Trans 2005, 46: 1979–1985. 10.2320/matertrans.46.1979View ArticleGoogle Scholar
 Kayanuma Y: Quantumsize effects of interacting electrons and holes in semiconductor microcrystals with spherical shape. Phys Rev B 1988, 38: 9797–9805. 10.1103/PhysRevB.38.9797View ArticleGoogle Scholar
 Lee H, Park H, Lee I, Kim T: Formation and optical properties of CdTe/ZnTe nanostructures with different CdTe thicknesses grown on Si (100) substrates. Appl Phys Lett 2007, 102(103507):1–5.Google Scholar
 Campbell H, Fauchet P: The effect of microcrystal size and shape on the one phonon Ramanspectra of crystalline semiconductors. Solid State Commun 1986, 58: 739–741. 10.1016/00381098(86)905132View ArticleGoogle Scholar
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