X-ray diffraction (XRD) allows the study of the crystal quality and phase compo-sition without destruction or modification. The periodical arrangement of crystal lattice planes with a lattice spacing of some Ångström results in the diffraction of X-ray waves if there wavelength is less than the lattice spacing. The diffraction angle θ is a function of X-ray wavelength λ and the lattice spacing dhkl of lattice planes with the Miller indices hkl:
2dhklsinθ =nλ, (4.1)
whereas n is a positive integer [273; 274]. A typical XRD spectrum shows several diffractions which are identified with the database values from [275].
The experimental XRD line profile is a convolution of the instrumental and phys-ical curve which is furthermore a convolution of strain and crystallite size [274]. For the correction of the instrumental broadening a standard sample must be measured under instrumental conditions which are identical to the sample measurements. This standard sample must have the same chemical composition such as the sample ma-terial and the crystallite size must be large enough to eliminate all particle-size broadening. As we have no clear identification of the chemical composition of the secondary phases within ZnO, the crystallite size was estimated with the Scherrer relation [276]:
B = 0.94·λ
DBcosθ (4.2)
whereas B is the full width at half maximum (FWHM), λ is the X-ray wavelength and DB the crystallite size. For the calculation of the crystallite size the diffraction peak was fitted with the Gaussian function:
fGauss = 1 whereas σ >0 is the standard deviation andµ the expected value. FWHM is than given by:
F W HM = 2√
2 ln 2σ≈2.35·σ (4.4)
All XRD-spectra were measured with a D8 Discover from Bruker AXS [277] which works with a copper X-ray source at a wavelength of 1,540601 Å. All spectra were taken in the unlocked θ-2θ mode from 25◦ to 85◦. Due to minor variances between the surface normal and the c-axis of the single crystals, all implanted single crystal samples were aligned via a fast θ-2θ scan and a subsequent rocking curve
measure-4.3 Raman spectroscopy 55 ment. During the fastθ-2θscan 2θf astand θf astof the (0002) diffraction of ZnO was determined, whereas 2θf ast was used for the rocking curve measurement. Thereby the 2θf ast angle between the sample and the detector is fixed and the intensity is measured as function of the incident beam angleθ. The rocking curve gives the per-fect θrocking angle and the discrepancy between θf ast and θrocking is the adjustment of θ for the unlocked θ-2θ measurement.
4.3 Raman spectroscopy
Raman spectroscopy is a non-invasive research method for chemical analysis or solid state physics by inelastic scattering of monochromatic light within the studied sam-ple [278–280]. It provides access to the lattice dynamics with the information about the chemical composition, orientation, or crystalline quality [281]. However, elec-tronic and magnetic properties could be addressed, by Raman resonance effects and Raman scattering from magnons, respectively [281]. All this informations were obtained from frequency position and intensity, frequency width, and line shape.
The Raman shift is given by the frequency difference between the scattered light and the monochromatic excitation light due to the generation or annihilation of el-ementary excitations [278; 279; 281]. The excitation light transfers an electron onto a virtual electronic state or an excited electronic state, whereas the latter one is designated as resonant Raman scattering and shows an enhanced intensity if the number of such electronic states is large [278; 279]. During the recombination of this virtual electron state energy could be transfered via generation (Stokes Raman scattering) or annihilated (Anti-Stokes Raman scattering) of elementary excitation which results in positive or negative Raman shift (see figure 4.2) [279; 281], respec-tively. Below room temperature only few phonons are thermally excited, therefor the Stokes scattering process is dominant [281]. All presented Raman spectra show only the spectral part of Stokes Raman signals, whereas all spectra are recorded with a small offset to the Rayleight scattering peak at 0 cm−1 (see figure 4.2). During the inelastic scattering process energy from the incident photon ~ωi is transfered to the sample by generation or vice versa by annihilation of elementary excitation, which is assumed to be phonons [281]. The energy ~Ωs and wave vector ~qj of the participating phonon are given by [278; 279]:
Stokes process: ~ωi−~ωs =~Ωs and ~ki−~ks =~qj
Anti-Stokes process: ~ωs−~ωi =~Ωs and ~ks−~ki =~qj (4.5)
515 wavelength (nm) frequency (cm-1)
520 525
510 505
500 530
20037 19837 19637 19437 19237 19037 18837 Anti-Stokes
Raman scattering
Stokes
Raman scattering Rayleigh scattering
Scattering intensity
Raman shift (cm-1)
-600 -400 -200 0 200 400 600
Figure 4.2: Schematic drawing of an Raman spectrum excited with an green laser source (514.5 nm) representing the positive wavelength shift of Stokes Raman scattering, the negative shift of Anti-Stokes scattering, and Rayleight scattering (from [281]).
whereas~ωs is the energy of the scattered photon,~ki and~ks are the wave vectors of the incident and scattered light, respectively. Raman scattering involving elementary excitations are characterized by well-defined (Ω, q) pairs due to the conservation of energy and momentum [278; 279]. All Raman measurements within this thesis were applied in backscattering geometry, by what the momentum vectors (4.5) could be simplified into scalar functions [281]:
For Stokes process: qj = 1
c(n(ωi)ωi−n(ωs)ωs), (4.6) whereas c is the velocity of light and n(ω) is the index of refraction. Due to the much longer wavelength of incidentλi and scattered lightλscompared to the lattice constant a0 and much smaller wave vectors ki, ks and qj than the wave vector of the Brillouin zone boundary, the phonon wave vector is: qj ≈ 0 for first order Ra-man scattering. Therefore, one-phonon RaRa-man scattering is restricted to radiation-phonon interactions at the center of the Brillouin zone [278; 279], which is in contrast to multi-phonon scattering. In that case only the sum of the phonon wave vectors must be close to zero, for which reason also phonons from outside the Brillouin zone
4.3 Raman spectroscopy 57
spectrometer CCD
laser
microscope
objective
inhomogeneous sample
beam splitter
Figure 4.3: Schematic drawing of a micro-Raman setup: The excitation laser light is inserted through a beam splitter and focused on the sample by an objective.
The backscattered light is collected by the objective and deflected by the beam splitter into the spectrometer (from [281]).
center can be involved. A detail description of the Raman method and theory is given in [278–281].
All Raman measurements were performed as micro-Raman or macro-Raman ex-periments in collaboration with the work group of Prof. Dr. J. Geurts at the Univer-sity of Würzburg. An optical microscope was inserted into the setup for micro-Raman measurements (see figure 4.3) [281]. Micro-Raman scattering with an optical micro-scope gives the advantage for a lateral resolution with the opportunity to detect and record inhomogeneities on the sample surface [281]. The excitation laser light is coupled into the microscope (Leica DM LM on Renishaw Raman system RM 1000 and Olympus BHT on Dilor XY) via a beam splitter and focused onto the sample by an objective (Leica DM LM: 50x/Olympus BHT: 10x, 50x ULWD1, 80x ULWD, and 100x) [281]. As visible in figure 4.3, the objective lens collects the Raman scattered light from the sample which is then split from the excitation laser light by the beam
1UltraLong WorkingDistance objective
splitter and deflected into the spectrometer Renishaw RM 10002 or Dilor XY3 [281].
Thereby, the Ramishaw setup is equipped with a camera for optical photography.
For excitation several laser lines from an argon ion laser4 or a HeNe laser5 [281].
The Dilor XY sample holders for mirco-Raman and macro-Raman measurements are equipped with a liquid helium cryostat, whereas the macro-Raman setup could be utilized as magneto-cryostat with a maximum magnetic field of 6 Tesla [281].