Schematic illustration of GaN wurtzite crystal structure exhibiting the polarity along the c-axis. The small and large spheres indicate Ga and N, respectively. GaN with Ga-face (+c) polarity on left side and GaN with N-face (-c) polarity on right side. When the direction of the three bonds of the III-element is towards the substrate, the polar structure is defended as +c polarity. On the other hand, when that of the bonds is upward against substrate, it is defined as having -c polarity. The termination element on the surface is not specified unless it is explicitly mentioned in this article.
Optical microscope images of (a) +c GaN and (b) -c GaN films on sapphire substrates. The surface morphology of typical +c and -c GaN films are smooth and hexagonal facetted, respectively. The polarity was determined by the CAICISS method described in Sec. 2.4.
Schematic illustration of CAICISS apparatus with a chamber and block diagram. The incident and azimuth angles of the ion beam can be altered by moving the sample holder, which is equipped with a heater.
Schematic illustration explaining the focusing and shadowing effect. According to the definition given in Ref. [5], which was made by Katayama et al., the incident angle 
was changed from 90° (normal to the sample surface) towards the lower angle. The angular dependence of the CAICISS signal can be obtained because both effects correspond to the atomic arrangement on surface, as observed in the middle. Variations in the TOF spectra are induced by changing the incident angle, as shown on the right. The dependence of the integrated peak (colored area) of the TOF spectra on the angle corresponds to the CAICISS result shown on the bottom of the left-hand side, which can be used to determine the polarity and the surface structure.
Incident angular dependence of the Zn signal intensity when the specimen was tilted along the <110> azimuth. (a) single crystal (0001) Zn face (+c), (b) simulated curve of (a), (c) single crystal (0001)O-face (-c), and (d) simulated curve of (c). Simulation was based on a three-dimensional two-atom model for a virtual surface cut from an ideal bulk structure without any reconstruction. (Ohnishi Dr. Thesis p.70 Ref. [49]) The polarity of the GaN was determined from the angular dependence revealed by CAICISS by comparing these results with these results of ZnO, because ZnO has the same crystal structure and lattice constants that are very close to those of GaN.
TOF spectrum of the backscattered He+ ions used in the CAICISS analysis of an In0.5Ga0.5 SQW when the ion beam was irradiated at normal incidence to the sample. The inset depicts the TOF spectrum for In0.2Ga0.8N capped with 6-nm-thick GaN as a reference [after Ref. 51]. The In and Ga signals can be detected separately for each time-of-flight. The inset indicates how CAICISS analysis detects a region several nm deep below the surface.
Incident angle dependence of Ga and In scattered intensity at the [11
0] azimuth for In0.5Ga0.5N SQW. Variation of both the Ga and In signals indicates +c polarity judging from the CAICISS results on ZnO bulk given in Fig. 5. Indium atoms incorporated into an InxGa1-xN SQW were found to occupy substitutional Ga sites. [52]]
Equilibrium N2 pressure over III-V nitrides (solid) + III-metal (liquid). Lines for each nitride material are plotted together from Ref. 81
Models for the AlN thin films on c-plane sapphire substrates given in Ref. [58]. The color regions are added to explain the theoretical predictions. The structures in the brackets correspond to the polar structures of AlN described in Ref. [84]. (a) and (b) correspond to a +c polar surface. (c) and (d) to -c polarity.
Nitridation-temperature dependence of the atomic concentration on a sapphire surface determined by XPS analysis. When HT-GaN films were deposited on sapphire substrates nitrided at temperatures of more than 700°C, the films represented -c polarity and hexagonal facetted surfaces.
Angular dependence of Ga signal intensity for the buffer layers in CAICISS analysis: (a) as-deposited 20-nm buffer layer on non-nitrided sapphire, (b) as-deposited 20-nm buffer layer on nitirided sapphire and (C) annealed buffer layer of (b). The polarity of a buffer layers on nitrided sapphire substrates changed from +c to -c polarity, while there was no change for buffer layers on non-nitrided sapphire substrates. [after Ref. 35]. This is the first determination of the polarity for LT-GaN buffer layers, which has led to an understanding of the correlation between the MOCVD process and the polarity.
Incident angular dependence of Ga signal intensity at [110] azimuth of He+ beam in CAICISS analysis for a 210nm GaN buffer layer on nitrided sapphire: (a) as-deposited, (b)10min, (c) 20min, and (d) 30min annealing time. The lines in (c) and (d) are calculated according to the weight ratios of the +c:-c polarity material as being 5:5 and 2:8, respectively, assuming that they are of the same crystal quality i.e., the same intensity of CAICISS signal for +c and -c domains. The sharpening peak in (b) indicates crystallization, and the peak splitting at 72° in (c) suggests the existence of -c domains that are becoming exposed due to the evaporation of the buffer layer. [after Ref. 118]
Cross-sectional TEM images for the buffer layers annealed for (a) 20min and (b) 30min. The samples correspond to (c) and (d) in Figure 12, respectively. [after Ref. 118 ]
Dependence of (a) N1s and (b) Ga 3d peak positions for a 20-nm-thickness GaN buffer layer on H2 cleaned (open circles) and nitrided (closed circles) sapphire substrates on the annealing time under the N2 and H2 mixed ambient. The positions were detected by XPS analysis. [after Ref. 117 ] The sample on the nitrided substrate evaporated completely, and AlGaN was formed at the interface between the sample and the non-nitrided substrate.
(a) Dependence of absorbance at 3.6eV on annealing time for a 160nm GaN buffer layer on a nitrided sapphire substrate. (b) Variation of hexagonal facet size of GaN films deposited on annealed buffer layers in (a). The polarities of the HT-GaN layers are attributed to those of the annealed buffer layers. [after Ref. 35 ]
AFM images of annealed buffer layers on nitrided sapphire substrates before (on the left) and after (on the right) dipping in KOH solution for each time stated. (a) The sample deposited under a V/III ration of 20 000 and annealed in the H2 and N2 mixed ambient for 20min, and (b) the sample deposited under a V/III ratio of 5000 and annealed in an N2 ambient for 20min. [after Ref. 117]
Variation of (0002) peak intensity of LT-GaN (open circles) and AlN (closed circles) buffer layers of 20nm thickness as a function of annealing time. The intensity was enhanced due to the crystallization of the samples induced by the annealing, and it subsequently decreased due to the layer being thinned by sublimation. This indicates that the effects of mass transportation and sublimation are smaller for AlN buffer layers.
FWHM of 
(0002) for HT-GaN films deposited on GaN (open circles) and AlN (closed circles) buffer layers with the same thickness, when the buffer layers were annealed for the times shown in the figure. The conditions for obtaining better quality HT-GaN material are wider with GaN buffer layers [127]].
The FWHM value of (0002) for 1 µm HT-GaN films deposited on AlN (closed circles) and GaN (open circles) buffer layers with the thicknesses in the figure. The buffer layers were annealed for 10min under the optimum ambient. Their polarity changed as indicated by the surface morphology, and the polarity conversion occurred drastically in the case of the AlN buffer layer.
Influence on surface morphology of HT-GaN films of the V/III ratio in AlN buffer layers deposited at 1040 °C; V/III ratio= (a) 600, (b) 1800, (c) 6800, and (d) 13700. [127] ] A lower V/III ratio was required for the deposition of AlN buffer layer in order to obtain +c GaN film.
Summary of the key points for controlling the polarity of GaN films on sapphire substrates (as detailed in the Articles and the Recipes) in the time chart of the two-step MOCVD process. Our typical growth conditions are represented. The polarity at the end of each process is remarked with blue and red indicating +c and -c polarity, respectively, which can be used as the road map to control the polarity of GaN in MOCVD. [after Ref. 128]
Depth profiles of C-, O-, Al-, Si-, Ga- and GaN- for (a) +c GaN and (b) -c GaN films of 1µm in thickness. The intensities of impurities normalized to the GaN- count are listed at the bottom of each figure. The values normalized to the Ga+Ga+ ion in the brackets are evaluated from depth profiles using and O2+ primary beam [after Ref. 135]. The Al, C and O impurities were more readily incorporated into -c GaN films.
Comparison of PL and OA spectra at 8K and 300K for +c and -c GaN films. [after Ref. 142 ]
The line shape parameter S of +c and -c GaN as a function of the position of the acceleration energy E. The value of S depends on the size and density of vacancy-type defects. The diffusion length of positrons decreases with increasing defect density. [after Ref. 142]
Surface barrier heights measured in Ref. [147] for n-GaN shown as a function of the work function of the metal used for the contact formation.
Surface barrier heights measured in Ref. [147] for p-GaN shown as a function of the work function of the metal used for the contact formation.
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