The results showed a dependence of the hardness and the modulus of elasticity on the defect content. The influence of the sapphire nitridation pretreatment and growth conditions on the NW morphology was investigated.
NITRIDES
Historical review of the III-Nitrides
6. (aration of carriers) and this results in the existence of well-defined excitonic states. Due to QCSE, the radiative recombination of carriers is reduced, which reduces the efficiency of the devices.
Properties of III-Nitrides
- Crystal structure
- Elastic properties
- Defects observed in III-Nitrides
- Dislocations
- Stacking faults
In the new coordinate system (x' y', z') the stress components of equation (1.5) are transformed to the following:26. Therefore, most defects are expected to arise in the interfaces between the substrate and the epilayer.
GROWTH AND CHARACTERIZATION
Epitaxial growth of III-Nitrides
- Molecular Beam Epitaxy (MBE)
- The sapphire substrate
The growth techniques commonly used for the synthesis of the III- Nitride heterostructures and nanostructures are the Hydride Vapor Phase Epitaxy (HVPE), the Metalorganic Vapor Phase Epitaxy (MOVPE) and the Molecular Beam Epitaxy (MBE). Sapphire is the most common foreign substrate for the epitaxial growth of the III- Nitride materials.
Transmission Electron Microscopy (TEM)
- Basics of TEM
- TEM techniques for defect observation
- Phase contrast-High resolution TEM (HRTEM)
- Convergent beam electron diffraction (CBED)
- Z-contrast STEM
- Geometrical Phase Analysis for nanoscale strain calculations
- Specimen preparation for TEM observations
However, forward scattering (both coherent and incoherent) produces the majority of the signals used in the TEM. The stress field of the dislocations is also visible in the stress field (red dots in the interface area).
Complementary experimental and theoretical tech- niques
- Nanoindentation
- X-Ray Diffraction (XRD)
- Raman Spectroscopy
- Empirical potential calculations using Molecular Dynamics (MD)
To "smooth out" and reduce the amorphization towards the end of the milling, lower voltage is applied. The light interacts with excitations in the system (eg phonons), resulting in the energy shift of the laser photons, which provides information about the vibrational states of the system. It can also be used to observe other low-frequency excitations of the solid (plasmons).
Raman scattering of an anisotropic crystal provides information about the crystal orientation by studying the polarization of the scattered light relative to the crystal and the polarization of the laser light. This hypothesis does not take into account the role of electrons on the cohesive force of a solid. Despite this, this assumption can be justified if the Born-Oppenaheimer approximation is taken into account, which states that "the motion of the dislocation nuclei and electrons are distinguishable".
Tsiakatouras, “Molecular beam epitaxy of the GaN semiconductor on diamond and r-plane sapphire substrates,” Ph.D.
NONPOLAR INTERFACES AND EPILAYER PROPERTIES
Nonpolar a-plane GaN
- Crystallography of nonpolar a-plane GaN on r-plane sapphire and strain relaxation mechanisms
- Anisotropy of nonpolar GaN epilayers
- Growth details of the samples and experimental techniques
- Samples morphologies and diffraction contrast experiments
- HRXRD and Raman spectroscopy experiments
- Discussion and Conclusions
- Influence of the defect characteristics of a-plane nonpolar GaN on the mechanical properties of the epilayers
- Nanoindentation methodology
- Nanoidentation results and correlation with the defect structure of the nonpolar a-plane GaN epilayers
- Conclusions
Roder et al.10 observed a similar broadening, which they attributed to the defect content of the material. Its relationship with the anisotropy of the residual elastic strain (‘strain anisotropy’) will also be considered. The difference in the structural anisotropy values of samples D and F is notable and requires further investigation.
In both samples, Moiré fringes near the AlN/sapphire interface are due to a. Arrows indicate TD slip loops promoted by TD slope due to NL roughness. HRXRD strain measurements can be explained by referring to the defect content and grain structure of the samples.
Due to the fact that the a and m axes are loaded to different degrees, the epilayers exhibit orthorhombic deformation of the unit cell.
Nonpolar InN
- Growth details and experimental techniques
- Structural characterization of nonpolar a-plane InN
- Conclusions
As a result of these interactions, new TDs appear to originate from the top of the defect pyramids and continue to the surface of the film. The nanofaceting of this interface appears to promote the bending of TDs coming from the NL or originating from the thread arms of misaligned dislocations (MDs) at the GaN/InN interface. The structure of the GaN NL and the InN/GaN and GaN/Al2O3 interfaces are shown in detail in the HRTEM image in Figure 3.21 (a).
It is seen that the GaN/InN interface blocks some of the TDs coming from GaN, which become MDs, while others pass through to the InN. This is better illustrated in Figure 3.21(b), whereby part of the interface is analyzed by Bragg filtering and GPA51 to show the (0002) extra half planes and the strain fields of the regular MD array. A high density of interacting oblique defects is observed at the lower part of the epilayer.50.
Nanofaceting of NL promoted the formation of new TDs and TD interactions on inclined planes.
SEMIPOLAR INTERFACES AND EPILAYER PROPERTIES
Interfacial structure of semipolar (1122) AlN grown on m-plane sapphire
- Growth details and experimental techniques
- Structural characterization of the interfacial structure of ( 1122 ) AlN grown on m-plane sapphire without nitridation pre-treatment
- Structural characterization of the interfacial structure of (1122)AlN grown on m-plane sapphire with nitridation pre-
- Discussion on the nonpolar/semipolar nucleation
- Conclusions
Four samples were investigated to study the boundary structure of semipolar (1122) AlN with m-plane sapphire. The polycrystalline character of the interfacial zone can be seen in the cross-sectional HRTEM image in Figure 4.4(a) taken along the [1100]AlN|| [1120]Al O2 3 and shows moiré fringes at the interfacial zone. Boundary zone illumination indicates the existence of AlN crystals with basal planes aligned with the sapphire r-planes.
Along the [1123]AlN||[0001]Al O2 3 z.a. the parasitic non-polar m-plane AlN orientation within the interfacial zone is clearly identified, as can be seen in the HRTEM image of Figure 4.5. In the case of the nitration, the sapphire substrate is much rougher as shown in the HRTEM image of Figure 4.8. These wells appear to promote the disorder, which affects the introduction of the nonpolar phase and other misorientations.
In the perpendicular direction (i.e., [0001]Al2O3), the mismatch is accommodated by inclined crystal planes in the manner shown in Figure 4.10.
V-defects and interfacial structure of semipolar(1122) InGaN
- Structural characterization of the v-defects
- Conclusions
XTEM observations were performed to obtain a better insight into the 3D morphology of the pits and the related defects. One of the facets is projected as (1120), as they appear flat and perpendicular to the inclined (0002) basal planes. The second is that they must conform to the point symmetry of the (1122) surface, which includes the (1100) mirror plane.
On the other hand, viewed along [1 100]III N−, the dislocations were roughly aligned with the projection of the basal plane. The surface facets of the V-defect have an average orientation below an angle of 16o with (1122). Mixed thread dislocations with (a+c) BVs were identified at the bottom of the holes using TEM.
The clear white arrow shows a TD ending at the center of the v-well that is visible under both diffraction conditions.
Structural properties of PAMBE-grown semipolar InN epilayers
- Growth details of s-plane InN on r-plane sapphire and experi- mental techniques
- Structural characterization of semipolar s-plane InN on r-plane sapphire
- Interfacial structure and influence of RTA
- Structural characterization of semipolar ( 1122 ) InN on m-plane sapphire
- Conclusions
TEM and topological analysis were used to establish the coexistence of the two orientation variants. In addition to both variants, bumps were observed at the InN/sapphire interface, which were attributed to the growth of the InN interlayer. The semipolar orientation was found to exist in two variants due to the symmetry of the sapphire nucleation plane.
The HRTEM image of Figure 4.26 shows the coexistence of the two variants and the continuity of lattice planes is indicated. This GB orientation appears to include part of the interface between the two variants, shown in the HRTEM image of Figure 4.26. Furthermore, the holes observed at the InN/sapphire interface (Figure 4.29) are attributed to the dissolution of the sapphire protrusions and the out-diffusion of oxygen combining with metallic indium through the dissolution of the epilayer.
The SEM-EDS analysis showed indium oxide crystallites on the surface of the film (Figure 4.30(b)).
INTERFACES BETWEEN NONPOLAR AND SEMIPOLAR ORIENTATIONS OF
NITRIDE SEMICONDUCTORS
- Bicrystallography
- Modelling of interfacial structures and energies
- Conclusions
In Figure 5.4, the crystal on the right side of Figure 5.3 is shown at a higher magnification. The Burgers circuit around one of these dislocations shows that they are of the type shown in Figure 5.2(a) and have also been observed in GaN material. As shown in Figure 5.8(a), this boundary exhibits the largest energy of the three GBs studied for all three binary compounds.
In this way the interfacial structures on both sides of the SFs will be energetically degenerate and the GB energy decreases, as shown in Figure 5.8(b). This Burgers vector appears in the case of the 4 ML step observed in Figure 5.5 (c), since it can be decomposed as 3 + 1 ML, and the 3 ML step possesses almost zero Burgers vector, as shown in Figure 5.11 (c). This was consistent with experimental observations, i.e., the observed dissociations yielded a stable low-energy interface.
The nanomechanism of energy reduction by the emanation of SFs could further increase the SF density in nonpolar and semipolar materials, and also lead to cubic pockets.1 The energy reduction by introducing the defects was smaller in the case of InN.
QUANTUM DOT NANOSTRUCTURES
Semipolar InGaN/GaN quantum dots
- Growth details and experimental techniques
- Structural characterization of the InGaN QDs
- QD strain and indium content calculation
- Conclusions
The influence of the structure on the PL measurements has also been investigated, in correlation with the growth temperature. More details regarding the structural characterization of Polar Ingan QDs are presented by Koukoula et al.2. This bending must have been involved due to the interaction of the TDS with the elastically strained inganic layers.
The preferential nucleation of QDs on planes deviating from the exact (1122) orientation is attributed not only to the effect of local TD deformation, but also to the faceted surface roughness associated with the growth of (1122) oriented GaNs. Figure 6.3(b) shows QDs nucleated on inclined planes near the TD. Corresponding polar InGaN/GaN nanostructures showed surface and embedded QDs in the case of high T growth with similar QD height.
Strain measurements at GPA were performed and the indium content of the samples was estimated.
Semipolar GaN/AlN quantum dot nanostructures
- Growth details and experimental techniques
- Structural and morphological analysis of the GaN/AlN QDs
- Elucidation of the strain state of the GaN QDs and Finite Element analysis
- Conclusions
An increase in QD size is observed with increasing the amount of GaN deposited. Irregularities in the QD shapes were also found due to the truncation caused by nucleation on surface depressions. The images in Figure 6.10 also show that the vertical stacking of the QDs appears to follow the slope of the basal plane.
GPA12 was used to elucidate the strain state of the semipolar GaN QDs. The other is the electric shear field Di, which leads to the appearance of the built-in electric potential V. The 3D FE lattice describing the geometry of the rectangular-based (1122)-core QD is shown in Figure 6.13 (a).
The strain depression QD was found close to the top facet (1122) at the intersections with the side faces, which is consistent with the experimental observations as shown in Figure 6.12.
NANOWIRE NANOSTRUCTURES
- Growth details and experimental techniques
- Effect of nitridation pre-treatment and growth conditions
- Deposition of GaN on nitridated r-plane sapphire under metal- and nitrogen- rich conditions
- Structural characteristics of the a-plane GaN film
- Structural characterization of the NWs
- Orientation of the NWs and nonpolar/semipolar orientation relationship between the a-plane GaN and the
Then, the samples were nitrided under different plasma source conditions (RF power ranging from 80 to 400 watts) and for different times (from 16 min to 1 h). The NW density increased with increasing nitriding time and RF plasma source power (courtesy of Prof. Georgakilas at FORTH-IESL). When growing GaN under nitrogen-rich conditions, there are still steps on the r-plane of sapphire, which are promoted by pre-treatment of the substrate with nitriding.
The GaN islands were used as a template for the growth of the NWs under N-rich conditions. The arrows in the BF image indicate some of the thread displacements of the sample. Two semipolar variants of the NWs were observed (courtesy of Prof. Georgakilas at FORTH-IESL).
On the right side of the image is the FFT corresponding to the area recorded from the HRTEM image.