Structural and elastic properties of InN and InAlN with different surface orientations and doping
Sammanfattning: Group–III nitrides, InN, GaN, AlN, and their alloys, have revolutionized solid state lighting and continue to attract substantial research interest due to their unique properties and importance for optoelectronics and electronics. Among the group–III nitrides, InN has the lowest effective electron mass and the highest electron mobility, which makes it suitable for high–frequency and high power devices. InxAl1–xN alloys cover the widest wavelength region among any semiconductor systems with band gaps ranging from 0.6 eV (InN) to 6.2 eV (AlN). Thus, InxAl1–xN is promising for light emitting diodes and laser diodes in a wide spectral range from infrared to deep ultraviolet, as well as for solar cell applications. InxAl1–xN thin films are also extensively studied in relation to their application for Bragg reflectors, microcavities, polariton emission and high electron mobility transistors. Despite the intense research, many of the fundamental properties of InN and InxAl1–xN remain controversial. For example, the material lattice parameters, stiffness constants, structural anisotropy and defects in nonpolar and semipolar films, effect of impurities and dopants are not established. Furthermore, to fabricate InN based devices, reliable n– and p–type doping should be achieved. At present, control and assessment of p–type conductivity using Mg doping of InN is one of the most outstanding issues in the field.This thesis focuses on: i) Establishing the structural and elastic properties of InxAl1−xN with arbitrary surface orientations (papers I to III); ii) Studying structural and free-charge carrier properties of non/semi-polar and zinc-blende InN (papers IV and V) and iii) Establishing the effects of doping (p and n) on lattice parameters, structural and free-charge carrier properties of InN (Papers VI and VII). The work includes ab initio calculations and experimental studies of InN and InxAl1−xN materials grown in world−class laboratories in Japan, Europe and the USA.The first part of the thesis includes general description of the basic material properties. Next, the structural and elastic properties and defects in InxAl1−xN and InN are discussed. The experimental techniques and relevant methods used to characterize the materials are described, as well as details on the ab initio calculations used in this work are provided. Part II consists ofseven papers.In Paper I we present the first theoretical analysis on the applicability of Vegard’s linear rule in InxAl1−xN alloys in relation to strain related elastic and piezoelectric properties. We derive the elastic stiffness constants and biaxial coefficients, as well as the respective deviations from linearity by using ab initio calculations. The stress−strain relationships to extract composition from the lattice parameters are derived in different coordinate systems for InxAl1−xN with an arbitrary surface orientation. The error made in the composition extracted from the lattice parameters if the deviations from linearity are not taken into account is discussed for different surface orientations, compositions and degrees of strain. The strain induced piezoelectric polarization is analyzed for InxAl1−xN alloys grown psudomorphically on GaN. We establish the importance of the deviation from linearity in the extracted strain values in respect to the piezoelectric polarization.Paper II reports the lattice parameters of InxAl1−xN in the whole compositional range using first-principle calculations. Deviations from Vegard’s rule are obtained via the bowing parameters, which largely differ from previously reported values. The paper discusses for the first time the implications of the observed deviations from Vegard’s rule on the In content extracted from x-ray diffraction.Paper III discusses the lattice parameters and strain evolution in Al−rich InxAl1−xN films with composition. Decoupling of compositional effects on the strain determination was accomplished by measuring the In contents in the films both by Rutherford backscattering spectrometry (RBS) and x−ray diffraction (XRD). It is suggested that strain plays an important role for the observed deviation from Vegard’s rule in the case of pseudomorphic films. It is found that Vegard’s rule in the narrow compositional range around the lattice matching to GaN may be applicable.Paper IV reports the first study of structural anisotropy of non-polar InN and semi−polar InN grown on sapphire and γ-LiAlO2 substrates. The on−axis rocking curve (RC) widths were found to exhibit anisotropic dependence on the azimuth angle. The finite size of the crystallites and extended defects are suggested to be the dominant factors determining the RC anisotropy in a-plane InN, while surface roughness and curvature could not play a major role. Furthermore, strategy to reduce the anisotropy and magnitude of the tilt and minimize defect densities in a−plane InN films is suggested. The semipolar InN was found to contain two domains nucleating on zinc−blende InN(111)A and InN(111)B faces. These two wurtzite domains develop with different growth rates, which was suggested to be a consequence of their different polarity. We found that a− and m−plane InN films have basal stacking fault densities similar or even lower compared to nonpolar InN grown on free−standing GaN substrates, indicating good prospects of heteroepitaxy on foreign substrates for the growth of InN−based devices.Paper V reports the development of appropriate methods based on X-ray diffraction and Infrared spectroscopic ellipsometry to identify wurtizte and zinc-blende InN and quantify their phase ratio. Detailed analysis on the formation of the cubic and wurtzite phases is presented and their evolution with film thickness is discussed in detail. The free-charge carrier and phonon properties of the two phases are discussed together with the determination of the surface electron accumulation.Paper VI studies the effect of Mg doping on the structural parameters and free−charge carrier properties of InN. We demonstrate the capability of infrared spectroscopic ellipsometry to identify p−type doping. The paper provides important information on the effect of Mg doping on extended defects and lattice parameters, and also discussed the relationship between doping, defects and carrier mobility.Paper VII presents the first study on the effect of impurities on the lattice parameters of InN using first principle calculations. We considered both the size and the deformation potential effect for Mg0, Mg−, Si+ and O+ and Hi+. The incorporation of H on interstitial site and substitutional O leads to expansion of the lattice. On the other hand, incorporation of Si or Mg leads to contraction of the lattice. The most pronounced effect is observed for Si. Our results indicate that the experimentally observed increase of the in−plane lattice parameter of Mg doped InN cannot be explained neither by the size nor by the deformation potential effect and suggest that the growth strain is changed in this case. The reported size and deformation potential coefficients can be used to elucidate the origin of strains in InN epitaxial layers and the degree of electrically active impurities.
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