Chapter 1 Introduction
1.1 Significance of the dissertation
The synthesis of semiconductor nanomaterials has aroused worldwide interest in the last few years. Given their large surface area to volume ratios, size effects and possible quantum confinement effects, nanomaterials are predicted to exhibit new and enhanced properties relative to those of the corresponding bulk materials and offer routes to fabricating novel nanodevices. The wide band gap (3.4 eV at 5K), large exciton binding energy (60 meV), and radiation hardness of ZnO make it an excellent candidate UV light-emitter [1,2] for use in lasers, light-emitting diodes (LEDs), and other UV light-emitting devices. Under the right preparation conditions, pure excitonic emission from ZnO can be achieved, and UV lasing in bulk and nanowire ZnO have been observed.
The size-tunable optical properties of quantum confined semiconductor nanocrystals have motivated further investigations into the luminescence of ZnO nanocrystals (quantum dots). The synthesis of other semiconductors as colloidal nanocrystals has opened up possibilities for their uses in many new applications. For example, CdSe quantum dots (QDs) have been prepared showing band-edge emission at a range of wavelengths in the visible with high quantum yields, given proper surface passivation. Strongly emitting CdSe QDs and related materials have been tested for use in biolabelings [50], in LEDs, or as quantum dot lasers [51]. ZnO is particularly attractive for similar applications because of the current interest in UV emitters, but the luminescence of colloidal ZnO QDs is usually dominated by visible emission from a trap state [52]. High UV emission quantum yields have not yet been observed in ZnO nanocrystals, limiting their potential
uses. In this dissertation, we fabricated high-quality ZnO QDs by a simple sol-gel method to solve above problem and discussed the optical features of crystal structures and lattice dynamics, Raman vibrational properties, and exciton-LO-phonon coupling in order to completely apply in photonic devices. Furthermore, we theoretically studied the influence of electronic behavior in ZnO finite structures using semi-empirical tight binding (SETB).
1.2 Basic properties of ZnO and general review of ZnO nanostructures
1.2.1 Basic properties of ZnOZinc oxide (ZnO) is a material with great potential for a variety of practical applications, such as piezoelectric transducers [53], optical waveguides [54], surface acoustic wave devices [55], varistors [56], phosphors [57], transparent conductive oxides [58], chemical and gas sensors [59], spin functional devices [60], and UV-light emitters [1,2]. The interest in ZnO is fueled and fanned by its prospects in optoelectronics
applications owing to its direct wide band gap (Eg ~ 3.3 eV at 300 K). Some optoelectronic applications of ZnO overlap with that of GaN, another wide-gap semiconductor, which is widely used for production of green, blue-ultraviolet, and white light-emitting devices. However, ZnO has some advantages over GaN among which are the availability of fairly high-quality ZnO bulk single crystals and a large exciton binding energy (60 meV). ZnO also has much simpler crystal-growth technology, resulting in a potentially lower cost for ZnO-based devices. The basic materials parameters of ZnO were also shown in Table 1 [3]. To realize any type of device technology, these parameters were important to have control over the concentration of intentionally
introduced impurities (dopants), which are responsible for the electrical properties of ZnO. The dopants determine whether the current (and, ultimately, the information processed by the device) is carried by electrons or holes.
Wurtzite zinc oxide has a hexagonal structure (space group C6mc) with lattice parameters a = 0.3296 nm and c = 0.52065 nm. The structure of ZnO can be simply described as a number of alternating planes composed of tetrahedrally coordinated O2− and Zn2+ ions, stacked alternately along the c-axis as shown in Fig. 1-1, in which a1, a2, and c are the unit vectors in a unit cell, the open and closed circles show the cation and
anion atoms, respectively. The tetrahedral coordination in ZnO results in non-central symmetric structure and consequently possesses piezoelectricity and pyroelectricity.
Fig. 1-1 The wurtzite structure model of ZnO.
Another important characteristic of ZnO is containing polar surfaces. The most common polar surface is the basal plane. The oppositely charged ions produce positively charged Zn-[0001] and negatively charged O-[000-1] surfaces, resulting in a normal dipole moment and spontaneous polarization along the c-axis as well as a divergence in surface energy. To maintain a stable structure, the polar surfaces generally have facets or exhibit massive surface reconstructions, but ZnO ±[0001] are exceptions: They are atomically flat, stable and without reconstruction [4, 5]. Efforts to understand the superior stability of the ZnO ±[0001] polar surfaces are at the forefront of research in today’s surface physics [6-9]. The other two most commonly observed facets for ZnO are [1120] and [1-100], which are non-polar surfaces and have lower energy than the [0001] facets.
1.2.2 General review of ZnO nanostructures
Nanostructured ZnO materials have received broad attention due to their distinguished performance in electronics, optics and photonics. From the 1960s, synthesis of ZnO thin films has been an active field because of their applications as sensors, transducers and catalysts [58]. In the last few decades, a variety of ZnO nanostructure morphologies, such as nanowires [10-12], nanorods [13-16], tetrapods [17-19], nanoribbons/belts [20-22], and nanoparticles [23, 24] have been reported. ZnO
nanostructures have been fabricated by various methods, such as thermal evaporation [16-21], metal–organic vapor phase epitaxy (MOVPE) [15], laser ablation [16],
hydrothermal synthesis [13, 14], sol-gel method [23, 24] and template-based synthesis [12]. Recently, novel morphologies such as hierarchical nanostructures [25],
bridge-/nail-like nanostructures [26], tubular nanostructures [27], nanosheets [28], nanopropeller arrays [29, 30], nanohelixes [29, 31], and nanorings [29, 31] have, amongst others, been demonstrated. Some of the possible ZnO nanostructure morphologies are shown in Fig. 1.2. Several recent review articles have summarized progress in the growth and applications of ZnO nanostructures [32–34].
Fig. 1.2 Representative scanning electron microscopy images of various ZnO nanostructure morphologies [32-34].
Additionally, as the dimensions of semiconductors are reduced to the nanometer scale, the optical properties of these are much different from their bulk materials [35-37].
There are two incompatible physical mechanisms in modifying the energy band structure of nanostructures, i.e., the quantum confinement effect (QCE) and surface states [38].
These two mechanisms compete with each other to influence PL spectra. For nanodots or nanostructures in ZnO system with diameters less than 10 nm, the QCE plays a dominant role as has been much reported [39, 40]. On the other hand, the surface-to-volume ratio also brings much influence on the system’s Hamiltonian when the material size is reduced to the nanometer scale [41, 42]. The predominance of surface states is responsible for many novel physical features of nanomaterials [43, 44].
Recently, Guo et al. [45] exhibited significantly enhanced UV luminescence, diminished visible luminescence and excellent third-order nonlinear optical response with poly (vinyl
pyrrolidone) (PVP) modified surface of ZnO nanoparticles. Norberg and Gamelin [46]
observed that changes in nanocrystal size, shape, and luminescence intensities have been measured for nanocrystals capped by dodecylamine (DDA) and trioctylphosphine oxide after different growth times. They explained the green trap emission intensities show a direct correlation with surface hydroxide concentrations. Contrary to expectations, there is no direct correlation between excitonic emission quenching and surface hydroxide concentrations. The nearly pure excitonic emission observed after heating in DDA is attributed to the removal of surface defects from the ZnO nanocrystal surfaces and to the relatively high packing density of DDA on the ZnO surfaces. Furthermore, Shaish et al.
[47] showed that intensity relations of below-band-gap and band-edge luminescence in
ZnO nanowires depend on the wire radius. The weight of this surface luminescence increases as the wire radius decreases at the expense of the band-edge emission. Pan et al. [48] also predicated a significant increase in the intensity ratio of the deep level to the
near band edge emission is observed with ever-increasing nanorod surface-aspect ratio.
Thus, in quantum-size nanostructures, surface-recombination may entirely quench band-to-band recombination, presenting an efficient sink for charge carriers that unless deactivated may be detrimental for electronic devices. Although there were many experiments to describe the influence of surface states and electronic behavior in ZnO nanostructure, it is still lack of experimental and theoretical studies [61, 62] of the influences of crystalline size on electronic structure and surface states in ZnO.
1.3 Motives
In this dissertation, we experimentally and theoretically study the influences of finite crystallize on optical properties and electronic behavior of ZnO quantum structures. In experiment, we first show how to grow high-quality ZnO QDs by a simple sol-gel method. The average size of nanoparticles can be tailored by the appropriate concentration of zinc precursor. For optical properties analysis, Raman scattering can yield important information about the nature of the solid on a scale of the order of a few lattice constants, it can be used to study the microscopic nature of structural and/or topological disorder. Raman scattering thus has been widely used to study the influence of phonon demeanor in finite size of ZnO crystalline. Furthermore, PL measurement is a suitable tool to determine the crystalline quality and the presence of impurities in the material as well as exciton fine structures. It is imperative to fully characterize the excitons in ZnO since not only are excitons a sensitive indicator of material quality but also they play an important role in the stimulated emission and gain processes in real device structures. Especially, the exciton-phonon interaction has significant influence on the optical properties of nanostructure semiconductors. The influence of crystal structure and morphology were studied in detail recently in ZnO nanostructure system, but the effects on the optical properties of fabricated nanostructures are still unknown.
Since, we will investigate the interesting optical features of crystal structures and lattice dynamics, Raman vibrational properties, and exciton-LO-phonon coupling in use of Raman and PL spectroscopy.
In theoretical, we present the electronic band structure and total density of states (DOS) of ZnO and ZMO compound crystallization using the nearest- and the
next-nearest-neighbor SETB approach sp3 model. We limit the number of nonzero tight-binding parameters to one-center on-site integrals and the nearest neighbor two-center integrals, as discussed by Slater and Koster [49]. We also used SETB method to investigate the electronic stricture and surface states of ZnO finite well structure considering non-relaxed and non-reconstructed surfaces with growth different directions.
1.4 Organization of the dissertation
This dissertation is organized as follows. Chapter 2 presents a general concept of crystal structures, lattice dynamics, fundamental optical transitions, and tight binding method. In Chapter 3, we demonstrated the synthesis of the ZnO QDs and show the brief illustrations of characterization techniques. In Chapter 4, the morphology, crystal structures, and lattice dynamics were discussed with difference crystallize size of ZnO QDs. Chapter 5 elucidates the increase of exciton binding energy may result from the decrease of exciton Bohr radius making the exciton less polar thereby reducing the coupling to LO phonons. Chapter 6 and Chapter 7 we calculated the electronic structure and surface states in the wurtzite structure of ZnO from bulk to nanostructures using sp3 semi-empirical tight-binding model. Finally, in Chapter 8, we conclude the studies on the ZnO finite structures and propose several topics of the future works.
References
1. D. EzgFr, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S.
J. Cho, H. MorkoA, J. Appl. Phys 98, 041301 (2005).
2. R. Triboulet, J. Perriere, Prog. Cryst. Growth Charact. Mater 47, 65 (2003).
3. D. P. Norton, Y. W. Heo, M. P. Ivill, K. Ip, S. J. Pearton, M. F. Chisholm, and T.
Steiner, Materials Today 7, 34 (2004).
4. O. Dulub, L. A. Boatner and U. Diebold, Surf. Sci 519, 201 (2002).
5. B. Meyer and D. Marx, Phys. Rev. B 67, 035403 (2003).
6. P. W. Tasker, J. Phys. C: Solid State Phys 12, 4977 (1979).
7. O. Dulub, U. Diebold and G. Kresse, Phys. Rev. Lett 90, 016102 (2003).
8. A. Wander, F. Schedin, P. Steadman, A. Norris, R. McGrath, T. S. Turner, G.
Thornton and N. M. Harrison, Phys. Rev. Lett 86, 3811 (2001).
9. V. Staemmler, K. Fink, B. Meyer, D. Marx, M. Kunat, G. S. Gil, U. Burghaus and C.
Woll, Phys. Rev. Lett 90, 106102 (2003).
10. M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P.
Yang, Science 292, 1897 (2001).
11. M. H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater 13, 113 (2001).
12. C. Liu, J. A. Zapien, Y. Yao, X. Meng, C. S. Lee, S. Fan, Y. Lifshitz, S. T. Lee, Adv.
Mater 15, 838 (2003).
13. B. Liu, H. C. Zeng, J. Am. Chem. Soc 125, 4430 (2003).
14. M. Guo, P. Diao, S. Cai, J. Solid State Chem 178, 1864 (2005).
15. W. I. Park, Y. H. Jun, S.W. Jung, G.-C. Yi, Appl. Phys. Lett 82, 964 (2003).
16. A. B. Hartanto, X. Ning, Y. Nakata, T. Okada, Appl. Phys. A 78, 299 (2003).
17. Y. Dai, Y. Zhang, Z. L. Wang, Solid State Commun 126, 629 (2003).
18. V. A. L. Roy, A. B. Djurisic, W. K. Chan, J. Gao, H. F. Lui, C. Surya, Appl. Phys.
Lett 83, 141 (2003).
19. H. Yan, R. He, J. Pham, P. Yang, Adv. Mater 15, 402 (2003).
20. Z.W. Pan, Z. R. Dai, Z. L. Wang, Science 291, 1947 (2001).
21. H. Yan, J. Johnson, M. Law, R. He, K. Knutsen, J. R. McKinney, J. Pham, R.
Saykally, P. Yang, Adv. Mater 15, 1907 (2003).
22. Y. B. Li, Y. Bando, T. Sato, K. Kurashima, Appl. Phys. Lett 81, 144 (2002).
23. K. F. Lin, H. M. Cheng, H. C. Hsu, L. J. Lin, and W. F. Hsieh, Chem. Phys. Lett 409, 208 (2005).
24. C. J. Pan, K. F. Lin, W. T. Hsu, and W. F. Hsieh, J. Appl. Phys 102, 123504 (2007).
25. J. Y. Lao, J. G. Wen, Z. F. Ren, Nano Lett 2, 1287 (2002).
26. J. Y. Lao, J. Y. Huang, D. Z. Wang, Z. F. Ren, Nano Lett 3, 235 (2003).
27. Y. J. Xing, Z. H. Xi, X. D. Zhang, J. H. Song, R. M. Wang, J. Xu, Z. Q. Xue, D. P.
Yu, Solid State Commun 129, 671 (2004).
28. J.-H. Park, H.-J. Choi, Y.-J. Choi, S.-H. Sohn, J.-G. Park, J. Mater. Chem 14, 35
(2004).
29. Z. L. Wang, X. Y. Kong, Y. Ding, P. Gao, W. L. Hughes, R. Yang, Y. Zhang, Adv.
Funct. Mater 14, 943 (2004).
30. P. X. Gao, Z. L. Wang, Appl. Phys. Lett 84, 2883 (2004).
31. X. Y. Kong, Z. L. Wang, Nano Lett 3, 1625 (2003).
32. Z. Y. Fan, J. G. Lu, J. Nanosci. Nanotechnol 5, 1561 (2005).
33. G. C. Yi, C. R. Wang, W. I. Park, Semicond. Sci. Technol 20, S22 (2005).
34. Y.W. Heo, D. P. Norton, L. C. Tien, Y. Kwon, B. S. Kang, F. Ren, S. J. Pearton, J. R.
LaRoche, Mater. Sci. Eng 47, 1 (2004).
35. X.Y. Kong, Y. Ding, R. Yang and Z. L. Wang, Science 303, 1348 (2004).
36. S. Nakamura, M. Senoh, N. Iwasa, T. Yamada, T. Matsushita, Y. Sugimoto, H.
Kiyoku, Appl. Phys. Lett 69, 1568 (1996).
37. L. Bergman, X.B. Chen, J.L. Morrison, J. Huso, A.P. Purdy, J. Appl. Phys 96, 675 (2004).
38. L.T. Canham, Appl. Phys. Lett 57, 1046 (1990).
39. Y. Kayanuma, Phys. Rev. B 38, 9797 (1988).
40. M.D. Mason, G.M. Credo, K.D. Weston, S.K. Buratto, Phys. Rev. Lett 80, 5405 (1998).
41. F. Koch, V. Petrova-Koch, T. Muschit, J. Lumin 57, 271 (1993).
42. J.B. Xia, K.W. Cheah, Phys. Rev. B 59, 14876 (2003).
43. J.C. Tsang, M.A. Tischler, R.T. Collins, Appl. Phys. Lett 60, 2279 (1992).
44. I. Shalish, H. Temhin, V. Narayanamurti, Phys. Rev. B 69, 245401 (2004).
45. L. Guo, S. Yang, C. Yang, P. Yu, J. Wang, W. Ge, and G. K. L. Wong, Appl. Phys.
Lett 76, 2901 (2000).
46. N. S. Norberg and D. R. Gamelin, J. Phys. Chem. B 109, 20810 (2005).
47. I. Shalish, H. Temkin, and V. Narayanamurti, Phys. Rev. B 69, 245401 (2004).
48. N. Pan, X. Wang, M. Li, F. Li, and J. G. Hou, J. Phys. Chem. C 111, 17265 (2007).
49. J. C. Slater and G. F. Koster, Phys. Rev 94, 1498 (1954).
50. K. Boldt, O. T. Bruns, N. Gaponik, and A. Eychmuller, J. Phys. Chem. B 110, 1959 (2007).
51. A. Nemchinov, M. Kirsanova, and N. N. Hewa-Kasakarage, J. Phys. Chem. C 112, 9301 (2008).
52. A. B. Djurisic and Y. H. Leung, small. 2, 944 (2006).
53. Z. L. Wang, Appl. Phys. A. 88, 7 (2007).
54. E. Hutter, and J. H. Fendler, Adv. Mater 16, 1685 (2004).
55. C. R. Gorla, N. W. Emanetoglu, and S. Liang, J. Appl. Phys 85, 2595 (1999).
56. G. G. Zhao, R. P. Joshi, H. P. Hjalmarson, Journal of the American Cermaic Society
91, 1188 (2008).
57. G. X. Hu, B. Kumar, H. Gong, E. F. Chor, and P. Wu, Appl. Phys. Lett. 88, 101901 (2006).
58. R. Martins, P. Barquinha, I. Ferreira, L. Pereira, G. Goncalves, and E. Fortunato, J.
Appl. Phys 101, 044505 (2007).
59. Y. Sun, G. M. Fuge, N. A. Fox, D. J. Riley, and M. N. R. Ashfold, Adv. Mater 17, 2477 (2005).
60. S. J. Pearton, D. P. Norton, K. Ip, Y. W. Heo, and T. Steiner, Pross. Mater. Sci. 50, 293 (2005).
61. J. W. Chiou, J. C. Jan, H. M. Tsai, C. W. Bao, W. F. Pong, M. H. Tsai, I. H. Hong, R.
Klauser, J. F. Lee, J. J. Wu, and S. C. Liu, Appl. Phys. Lett. 84, 3462 (2004).
62. J. W. Chiou, H. M. Tsai, C. W. Bao, F. Z. Chien, W. F. Pong, C. W. Chen, M. H.
Tsai, J. J. Wu, C. H. Ko, J. F. Lee, and J. H. Guo, J. Appl. Phys 104, 013709 (2008).
Chapter 2 Theoretical background
In this chapter, we will discuss in detail the crystal structures, lattice dynamics, excitons-related emissions, quantum size effect and tight binding method. Lattice
dynamics corresponding to the selection rules, lattice vibrational properties, and polar-optical phonon scattering mechanism are discussed. Optical transitions in ZnO
have been studied by a variety of experimental techniques such as optical absorption, transmission, reflection, photoluminescence (PL), and cathodoluminescence
spectroscopies, etc. In Section 2.2, we reviewed some fundamental issues related to the optical properties of ZnO single crystal by PL measurement. In Section 2.3, the
quantum effect was described in nanostructures using effective mass model. Finally, we
detail the SETB approach sp3 model of wurtzite semiconductors.
2.1 Crystal structures and Lattice dynamics
2.1.1 Crystal structures [1]
Most of the group II-VI binary compound semiconductors crystallize in either cubic
zinc-blende or hexagonal wurtzite structure in which each anion is surrounded by four cations at the corners of a tetrahedron, and vice versa. This tetrahedral coordination is
typical of sp3 covalent bonding, but these materials also have a substantial ionic character.
ZnO is a II-VI compound semiconductor whose ionicity resides at the borderline between
covalent and ionic semiconductor. The crystal structures shared by ZnO are wurtzite (B4), zinc blende (B3), and rocksalt (B1), as schematically shown in Fig. 2-1. At
ambient conditions, the thermodynamically stable phase is wurtzite. The zinc-blende ZnO structure can be stabilized only by growing on cubic substrates, and the rocksalt
structure may be obtained at relatively high pressures.
Fig. 2-1 Stick and ball representation of ZnO crystal structures: (a) cubic rocksalt (B1), (b) cubic zinc blende (B3), and (c) hexagonal wurtzite (B4). The shaded gray and black spheres denote Zn and O atoms,
respectively.
The wurtzite structure has a hexagonal unit cell with two lattice parameters, a and c,
in the ratio of c/a= 83 =1.633
and belongs to the space group of C or P66v4 3mc. A schematic representation of the wurtzitic ZnO structure is shown in Fig. 2-2. The structure is composed of two interpenetrating hexagonal-close-packed (hcp) sublattices,
each of which consists of one type of atom displaced with respect to each other along the threefold c-axis by the amount of u = 3/8 = 0.375 in an ideal wurtzite structure. The
fractional coordinate, the u parameter, is defined as the length of the bond parallel to the c axis in unit of c. Each sublattice includes four atoms per unit cell and every atom of one
kind (group-II atom) is surrounded by four atoms of the other kind (group VI), or vice versa, which are coordinated at the edges of a tetrahedron.
Fig. 2-2 Schematic representation of a wurtzitic ZnO structure having lattice constants a in the basal plane and c in the basal direction; u parameter is expressed as the bond length or the nearest-neighbor distance b
divided by c, and α and β are the bond angles.
In a real ZnO crystal, the wurtzite structure deviates from the ideal arrangement, by changing the c/a ratio or the u value. It should be pointed out that a strong correlation
exists between the c/a ratio and the u parameter when the c/a ratio decreases. The u parameter increases in such a way that those four tetrahedral distances remain nearly
constant through a distortion of tetrahedral angles due to long-range polar interactions.
The lattice parameters of a semiconductor usually depend on the following factors: (i)
free-electron concentration acting via deformation potential of a conduction-band minimum occupied by these electrons; (ii) concentration of foreign atoms and defects and
their difference of ionic radii with respect to the substituted matrix ion; (iii) external strains (for example, those induced by substrate); and (iv) temperature. The lattice
parameters of any crystalline material are commonly and most accurately measured by high resolution x-ray diffraction (HRXRD). For the wurtzite ZnO, the lattice constants
at room temperature determined by various experimental measurements and theoretical calculations are in good agreement. The lattice constants mostly range from 3.2475 to
3.2501 Å for the a parameter and from 5.2042 to 5.2075 Å for the c parameter. The c/a ratio and the u parameter vary in a slightly wider range, from 1.593 to 1.6035 and from
0.383 to 0.3856, respectively. The deviation from that of the ideal wurtzite crystal is probably due to lattice stability and ionicity. It has been reported that free charge is the
dominant factor responsible for expanding the lattice proportional to the deformation potential of the conduction-band minimum and inversely proportional to the carrier
density and bulk modulus. The point defects such as zinc antisites, oxygen vacancies, and extended defects, such as threading dislocations, also increase the lattice constant,
albeit to a lesser extent in the heteroepitaxial layers.
2.1.2 Symmetry properties and phonon modes of inelastic cross sections [2, 3]
The symmetry properties of the scattering cross sections are determined by the
symmetry properties of second-order susceptibility for the excitation concerned. The spatial symmetry properties of the scattering medium lead to further connections between
the cross section measured in different experiments on the same sample. The Stokes cross section for different polarization of the incident and scattered light are often related
by the spatial symmetry, and the cross section is sometimes required to vanish for certain polarizations that depend on the nature of the excitation.
The spatial symmetry of the scattering medium is formally specified by its symmetry group, the group of all spatial transformations that leave the medium invariant.
Individual atoms and molecules have spatial symmetries characterized by a point group consisting of rotations and reflections that leave the atom or molecule invariant. The
atomic arrangements in a regular crystal lattice are characterized by a space group that contains translations in addition to rotations and reflections. The effects of the
translational invariance of a crystal are largely accounted for in the momentum conservation conditions, and the residual effects of the spatial symmetry derive from the
crystal point group that remains on removal of translations from the space group. There are 32 different crystal point groups. The effects of spatial symmetry are particularly
crystal point group that remains on removal of translations from the space group. There are 32 different crystal point groups. The effects of spatial symmetry are particularly