Space-resolved photoluminescence of ZnS:Cu,Al nanocrystals fabricated by sequential ion implantation

Atsushi Ishizumi, C. W. White, Yoshihiko Kanemitsu
2004 Applied Physics Letters  
We report on photoluminescence ͑PL͒ properties of Cu-and Al-doped ZnS nanocrystals fabricated by sequential implantation of Zn ϩ , S ϩ , Cu ϩ , and Al ϩ ions into Al 2 O 3 matrices. The spatially resolved PL spectrum has been studied by a scanning near-field optical microscope ͑SNOM͒. In the SNOM image, bright spots are observed on the sample surface. The PL spectrum at each bright spot is broad and is not sensitive to the monitored positions. The broad SNOM-PL spectrum at each spot is very
more » ... lar to the macroscopic PL spectrum measured by conventional optics. The donor-acceptor pair luminescence process in nanocrystals is discussed. Much interest has been focused on the fabrication and optical properties of semiconductor nanocrystals doped with luminescence centers, such as transition-metal and rare-earth ions, 1 since it is reported that Mn 2ϩ doped ZnS nanocrystals show high photoluminescence ͑PL͒ efficiency and a short PL lifetime. 2 The doped nanocrystals have usually been fabricated by chemical synthesis methods. 2,3 However, in the case of the doped nanocrystals fabricated by the chemical methods, it is suggested that doped ions are at the surface of nanocrystals. 4 The luminescence properties depend on the surrounding chemical environment. 3 On the other hand, it has been demonstrated that ion implantation is one of the most versatile methods for the fabrication of compound semiconductor nanocrystals embedded in transparent matrices. 5,6 In particular, impurity-doped semiconductor nanocrystals can be simply fabricated by sequential ion implantation of the elements forming compound semiconductors and impurities. 7 In this work, we have fabricated ZnS nanocrystals doped with Cu and Al ͑ZnS:Cu,Al nanocrystals͒ by ion implantation followed by thermal annealing, and have studied their PL properties. Spatially resolved PL spectra of individual ZnS:Cu,Al nanocrystals were measured by means of a scanning near-field optical microscope ͑SNOM͒. The donoracceptor ͑DA͒ pair luminescence related to Cu and Al impurities is clearly observed in the nanocrystal samples. The spectral shape of the SNOM PL is very similar to that of the far-field PL measured by conventional optics. The origin of the broad PL spectrum of single ZnS:Cu,Al nanocrystals is discussed. The c-axis oriented single crystal ␣Al 2 O 3 was used as a host material. Cu-and Al-doped ZnS nanocrystal samples were fabricated by ion implantation of Zn ϩ (1.0 ϫ10 17 cm Ϫ2 at 420 keV͒, S ϩ (1.0ϫ10 17 cm Ϫ2 at 225 keV͒, Cu ϩ (2.0ϫ10 15 or 1.0ϫ10 16 cm Ϫ2 at 400 keV͒, and Al ϩ (2.0ϫ10 15 or 1.0ϫ10 16 cm Ϫ2 at 170 keV͒ into the Al 2 O 3 substrate, and then the samples were annealed at 1000°C for 60 min in a flowing 96% Arϩ4% H 2 atmosphere. Two samples with different impurity concentrations ͑but equal doses of Cu and Al ions͒ were prepared: the lowdoped sample (2.0ϫ10 15 cm Ϫ2 ) and the high-doped sample (1.0ϫ10 16 cm Ϫ2 ). X-ray diffraction examination indicated that the ZnS nanocrystals are prepared as a mixture of hexagonal and cubic ZnS crystals. 8 The far-field macro-PL spectra were measured under a 310 nm light excitation, using a cooled charge-coupled device ͑CCD͒ detector and a 32 cm monochromator. The spatially resolved PL spectra were measured by a SNOM system ͑JASCO, NFS-330͒. For the SNOM-PL measurements, the sample was illuminated with 325 nm He-Cd laser light through an aperture of fiber probe with a pure-SiO 2 core. The PL signals from the sample were collected by the same aperture ͑a so-called illumination and collection mode͒ and detected by a cooled CCD detector through a 25 cm monochromator. The spectral sensitivity of the measuring system was also calibrated using a tungsten standard lamp. Figure 1 shows macro-PL, PL excitation, and absorption spectra of ͑a͒ the low-doped and ͑b͒ the high-doped samples at 14 K. In the low-doped sample, four peaks are clearly observed in the absorption spectrum, and their peak energies are indicated by the arrows in Fig. 1͑a͒ . It is found that the energies of the four peaks almost coincide with the exciton energy ͑3.81 eV͒ in the cubic ZnS bulk crystal and the A, B, and C exciton energies ͑3.88, 3.90, and 3.98 eV͒ in the hexagonal ZnS bulk crystal, respectively. 9 This result shows that the ZnS nanocrystal sample is a mixture of cubic and hexagonal ZnS crystals, and this observation is consistent with x-ray diffraction examination. 8 The broad PL band is ob-a͒
doi:10.1063/1.1689738 fatcat:3ivyfirqpfanhhvsxrnavl2zbu