Presence of voids in GaN/AlN multilayer system on Si affects the mechanical, crystallographic, and optical properties of the material structure. Specifically, the stress control, crystal quality, and optical performance of GaN on Si are influenced by voids.
Fig. B.5 Void rate under various testing conditions.
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To investigate the influence from voids on stress control efficiency in GaN growth on Si, two samples had been grown. The only difference between them was voids existed in one and not in the other. The growth conditions of AlN buffer layer, GaN layers and AlN interlayers were controlled to be the same with each other. In GaN on Si, in order to balance the tensile stress occurs during cooling, it is well known that the compressive stress in GaN layers should be well maintained.
It was clear that compressive stress was introduced much more efficiently in the sample without voids than the one with voids. The compressive stress was released rapidly in the sample with voids. Under the influence from voids, there were two sources of stress releasing effect. The first came from GaN itself. As shown in Fig. B.1, due to the presence of voids, GaN layers became loose in lateral direction. GaN layer was separated into domains by the spacing of voids. The contact area of between GaN and underlying AlN interlayer was also reduced by voids and then GaN would be less stressed. Consequently, the shear stress in lateral direction of GaN layer dropped. On the other hand, as shown in Fig. B.4a, many openings in AlN interlayers were generated by the formation of voids in underlying GaN.
These openings broke AlN IL and the capability of inducing shear stress in overlying GaN was degraded. Openings also acted as the sources of defects generated in overlying GaN.
Introduction of defects made GaN relax more rapidly.
Fig. B.7 (a) FWHM of XRD rocking curve of samples with and without voids; (b) types of the interaction between AlN interlayer and dislocations.
Fig. B.6 Curvature transition curves during the growth of the samples with and without voids.
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Voids also deteriorate the crystal quality of GaN. The crystal quality of GaN without voids was better than that with voids. This was assigned to the more dislocation introduction by the openings in AlN interlayer due to the formation of voids. As represented in Fig. B.7b, there are three types of behavior of the interaction between the dislocations and AlN interlayer. Type a is the propagation of dislocation through the interlayer. This doesn’t affect the dislocation density. Type b is new dislocation introduction generated on the upper interface of AlN interlayer. In interlayer, the dislocation, grain boundaries and the openings caused by the voids in underlying GaN can be the sources of new dislocations. This behavior increases the dislocation density in overlying GaN. Type c is blocking. Some propagating dislocations are blocked by AlN interlayer and this is favorable to the reduction of dislocation density. However, voids are barely capable of blocking dislocations, because if there are dislocations in the area of voids, they have already propagated into AlN interlayer. After the formation of voids, the dislocation which had propagated into the interlayer could not be
Fig. B.8 Effects of voids on the optical properties of GaN on Si, (a) cross section of voids in GaN/AlN structure; (b) plane view of etched voids in GaN after removing overlying AlN interlayer; (c) illustration of scattering by the facets of voids; (d) reflectance pattern at 950 nm,
(e) reflectivity and (f) photoluminescence characterization of samples with and without voids respectively.
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GaN GaN Optical head
Scattered light cannot be detected.
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reflectivity
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intensity (a.u.)
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Peak of gratings Light
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removed while the etching of GaN. Therefore, compared with the sample without voids, openings above the voids in AlN interlayer may introduce additional dislocations into overlying GaN which leads to quality degradation.
Moreover, voids brought more significant effect to the optical properties of GaN/AlN multilayer structure on Si than to crystal quality. The cross-section view and plane view of voids in GaN layers were shown in Fig. B.8a and b. From the plane view it was found that etching of GaN occurred along the regular facets of GaN. Such voids with crystal facets are good light scattering structure. When light comes to the facets, it would be reflected or scattered to various directions which it not normal to the plane of (0001) of GaN anymore.
Only a part of the light goes back to the direction which it comes along, as represented in Fig.
B.8c. As a result, as plotted in Fig. B.8d, as the GaN thickness increased and the density of voids accumulated, the intensity of reflected incident light with wavelength of 950 nm from the curvature and temperature monitor head was decreasing. The same thing happened with the reflectivity measurement of these two samples. Depending on the wavelength, the reflectivity of GaN with voids was about 0.05 lower than that without voids. More significantly, in the photoluminescence (PL) measurement, no PL light could be collected from the sample with voids. The normal peak around 550 nm from GaN was obtained. These experimental results proved that the assumption of scattering by the voids was reasonable.
There are some potential applications of the light scattering effect of void structure in devices. Such light scattering structure is favorable in solar cells to trap lights and increase the optical path length in the device to enhance its light-electricity conversion efficiency.
Another application is in LEDs. Due to scattering of the voids, the intensity of emitted light from LED would be less angle-dependent. Especially, it may be helpful to improve the external quantum efficiency of LESs on GaN-on-Si wafer if the influence of voids on the stress control and GaN quality is ignorable. Because on GaN-on-Si wafer, a portion of the light emitted from the LED active area may be absorbed by Si substrate. The presence of void structure reduces the intensity of the light goes to the Si substrate side.