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Optical Properties of Nanostructure Formed on a Surface of CdZnTe Crystal by Laser Radiation A.Medvid’ 1,2 , A.Mychko 1 , N.Litovchenko 2 , O.Strilchuk 2 , Yu.Naseka 2 , P.Onufrijevs 1 , A.Pludons 1 1 Riga Technical University, 14 Azenes Str., LV-1048, Riga, Latvia mychko@latnet.lv 2 Institute of Semiconductor Physics, 45, prospect Nauki, Kyiv, 252028, Ukraine SUMMARY Studies of the effect of high absorbed laser radiation on optical properties of the Cd 1-x Zn x Te (x=0,1) compound have revealed formation of nano-structures on the semiconductor surface at irradiation by Nd:YAG laser within the intensity range of 4 – 12 MW/cm 2 . The effect of exciton quantum confinement is present in structures diameter of which at the top of nano-hills is 10 – 15 nm and less. A graded band gap structure with optical window is formed on top of the nano-hills. Abstract Self-organizing structures of nanometer size are observed on the surface of crystalline irradiated by strongly absorbed Nd:YAG laser radiation (LR) at intensities within 4 - 12 MW/cm 2 . The effect of exciton quantum confinement manifested by a shift to higher energies of the A 0 ,X exciton line of the photoluminescent spectrum is present in structures of 10 – 15 nm in diameter at the top of nano-hills. A graded band gap structure with optical window develops at the top of nano-hills. EXPERIMENTAL DETAILS Single crystals of Cd 1-x Zn x Te (x=0,1), growing from the melt by the modified Bridgman method under an overpressure of Cd, were used in our experiments. The samples have been irradiated by the second harmonic (λ = 532 nm, τ =10ns) of the Nd:YAG laser in Q-modulation regime with the intensity from 4 MW/cm 2 to 12 MW/cm 2 . The irradiated surface of crystal was covered with a thin layer of SiO 2 in order to avoid material evaporation at laser heating. The thickness of SiO 2 layer was 0,3μ m and it was transparent for laser radiation. The sizes of the samples were 10x10x2 mm 3 . Photoluminescence (PL) was employed as the main investigation tool. The PL method allows to investigate the energy structure and concentration of luminescence centers. PL spectra were measured at 14,5 K using 532 nm line of laser with excitation powers of less than 200mW. Irradiation was carried out at room temperature and atmospheric pressure. DISCUSSION The PL spectrum is rather complex which consist of an intense line (A 0 , X) at 1,6248 eV ascribed to bond excitons to shallow acceptors (Cd vacancies - V Cd ). The PL band around 1,4794 eV is caused by recombination of donor-acceptor pairs (DAP). The shift of the A 0 ,X line is not observed at intensities below 4 MW/cm 2 . It is threshold intensity of LR causes of A 0 ,X line shift. The shift increases with the radiation intensity reaching 7,5 meV at 12 MW/cm 2 . At higher intensities the SiO 2 film is destroyed, due to ablation of material. The A 0 ,X exciton line is seen to shift toward higher energies with the increase of radiation intensity. After irradiation at intensity of 12 MW/cm 2 the surface morphology changed: nano-size structures of about 10 nm high had grown on the microstructures as seen in Fig. 4. The nanostructures begin forming at intensities I ≥ 4 MW/cm 2 . The shift of the exciton line is explained by the effect of quantum confinement in the nano-structures emerging on the semiconductor surface [8, 9]. At such size of nano- structures the effect of exciton quantum confinement is observed as the increase of exciton energy and decrease of exciton radius. The effect, on its turn, is related to the increase of the energy gap of the semiconductor. The increase of exciton energy is proportional to the decrease of cross section area toward the top of the nano-hill. The exciton activation energy determined from thermal dependence of the position of the A 0 ,X exciton line in Cd 1-x Zn x Te (x=0,1) crystals irradiated at intensity of 4 MW/cm 2 in the 14,5 – 300 K range was estimated to be 40 meV, respectively. The increase of exciton energy on 7,5 meV proves the presence of the exciton quantum confinement effect at the top of nano- hills. The energy gap of the Cd 1-x Zn x Te crystal increases along the axis of the nano-hill perpendicular to the sample surface. Thus, a graded band gap structure with optical window is formed in the nano-hill. Fig. 3. Three-dimensional AFM image of before laser irradiation Fig. 4. Three-dimensional AFM image of self-organized nanohills grown at intensity of YAG:Nd laser 12 MW/cm 2 . Fig. 1. PL spectra of Cd 1-x Zn x Te samples before and after irradiation by the laser 4 6 8 10 12 1,622 1,624 1,626 1,628 1,630 E,eV I, MW/cm 2 Fig.2. Photo-luminescence line of the A 0 ,X exciton in Cd 1-x Zn x Te (x=0,1) before and after irradiation measured at 14,5 K Fig.5. Position of the A 0 ,X exciton line of Cd 1-x Zn x Te (x=0,1) samples at different irradiation intensities. 1,4 1,5 1,6 1,7 0 5 10 15 20 25 30 35 40 45 no irradiated 4 MW/cm 2 8 MW/cm 2 12 MW/cm 2 FL intensity (arb.units) Photon energy, eV 532nm excitation energy T=14,5K 0 10 20 30 40 50 60 70 80 7,0 7,5 8,0 8,5 9,0 9,5 10,0 10,5 11,0 ln(lum(I)) 1000/T lum at 761nm 4MW/cm 2 E a =~0,04 eV Fig. 6. Temperature dependent of PL intensity of exciton line. 1,60 1,61 1,62 1,63 1,64 1,65 1,66 0,3 0,6 0,9 normalized lum. int. Photon energy, eV nonirradiated 4MW/cm 2 8MW/cm 2 12MW/cm 2 532nm excitation energy T=14,5K