Abstract:We report on the mechanism of strain-influenced quantum well (QW) thickness reduction in GaN/AlN short-period superlattices grown by plasma-assisted molecular beam epitaxy. Density functional theory was used to support the idea of a thermally activated exchange mechanism between Al adatoms and Ga surface atoms that is influenced by the strain state of the GaN QWs. These ab initio calculations support our experimentally observed reduction in QW thickness for different intrinsic strains.
“…(2)-(4) we calculated the depth profiles of strain and composition, as well as the thickness of the AlGaN-graded layer. Despite the high substrate temperature and a thermally activated exchange mechanism between Al adatoms and Ga surface atoms, 20 we obviously observe fringes on the XRD curve, which indicates a quasi-flat interface. This allows for the precise determination of the thickness of the graded layers of $90 6 2 m, which has been confirmed by transmission electron microscopy.…”
Here, we demonstrate X-ray fitting through kinematical simulations of the intensity profiles of symmetric reflections for epitaxial compositionally graded layers of AlGaN grown by molecular beam epitaxy pseudomorphically on [0001]-oriented GaN substrates. These detailed simulations depict obvious differences between changes in thickness, maximum concentration, and concentration profile of the graded layers. Through comparison of these simulations with as-grown samples, we can reliably determine these parameters, most important of which are the profiles of the concentration and strain which determine much of the electrical properties of the film. In addition to learning about these parameters for the characterization of thin film properties, these fitting techniques create opportunities to calibrate growth rates and control composition profiles of AlGaN layers with a single growth rather than multiple growths as has been done traditionally.
“…(2)-(4) we calculated the depth profiles of strain and composition, as well as the thickness of the AlGaN-graded layer. Despite the high substrate temperature and a thermally activated exchange mechanism between Al adatoms and Ga surface atoms, 20 we obviously observe fringes on the XRD curve, which indicates a quasi-flat interface. This allows for the precise determination of the thickness of the graded layers of $90 6 2 m, which has been confirmed by transmission electron microscopy.…”
Here, we demonstrate X-ray fitting through kinematical simulations of the intensity profiles of symmetric reflections for epitaxial compositionally graded layers of AlGaN grown by molecular beam epitaxy pseudomorphically on [0001]-oriented GaN substrates. These detailed simulations depict obvious differences between changes in thickness, maximum concentration, and concentration profile of the graded layers. Through comparison of these simulations with as-grown samples, we can reliably determine these parameters, most important of which are the profiles of the concentration and strain which determine much of the electrical properties of the film. In addition to learning about these parameters for the characterization of thin film properties, these fitting techniques create opportunities to calibrate growth rates and control composition profiles of AlGaN layers with a single growth rather than multiple growths as has been done traditionally.
“…However, the trends in thickness change remain the same. The observed thickness reduction mainly for the GaN QW can be explained by the following [23, 24]: (i) the Al-N binding energy is much higher than the Ga-N binding energy, (ii) the exchange between the Ga atoms of the QW and the Al adatoms of the barrier is thermally activated, and (iii) the strain in the GaN QWs influences the Al-Ga exchange mechanism.Fig. 4The XRR profiles of S20 ( green ), S10 ( blue ), and S5 ( red ).…”
Superlattices (SLs) consisting of symmetric layers of GaN and AlN have been investigated. Detailed X-ray diffraction and reflectivity measurements demonstrate that the relaxation of built-up strain in the films generally increases with an increasing number of repetitions; however, an apparent relaxation for subcritical thickness SLs is explained through the accumulation of Nagai tilt at each interface of the SL. Additional atomic force microscopy measurements reveal surface pit densities which appear to correlate with the amount of residual strain in the films along with the appearance of cracks for SLs which have exceeded the critical thickness for plastic relaxation. These results indicate a total SL thickness beyond which growth may be limited for the formation of high-quality coherent crystal structures; however, they may indicate a growth window for the reduction of threading dislocations by controlled relaxation of the epilayers.
“…5. Here, we see that the surface is densely covered with depressions, which are evidently the emergence of mixed or pure screw TDs with a Burger's vector, |b m | 2 = (1/3 × a) 2 + c 2 , in the 11 23 ½ direction, or |b s | = c, in the [0001] direction [25]. The formation of pits at TD cores is possible due to the strain energy density associated with surface-terminated threading dislocations being equivalent to a line of tension directed into the material [26].…”
Section: Afm Characterizationmentioning
confidence: 89%
“…However, the trends in thickness change remain the same. The observed thickness reduction mainly for the GaN QW can be explained by the following [23,24]: (i) the Al-N binding energy is much higher than the Ga-N binding energy, (ii) the exchange between the Ga atoms of the QW and the Al adatoms of the barrier is thermally activated, and (iii) the strain in the GaN QWs influences the Al-Ga exchange mechanism.…”
Superlattices (SLs) consisting of symmetric layers of GaN and AlN have been investigated. Detailed X-ray diffraction and reflectivity measurements demonstrate that the relaxation of built-up strain in the films generally increases with an increasing number of repetitions; however, an apparent relaxation for subcritical thickness SLs is explained through the accumulation of Nagai tilt at each interface of the SL. Additional atomic force microscopy measurements reveal surface pit densities which appear to correlate with the amount of residual strain in the films along with the appearance of cracks for SLs which have exceeded the critical thickness for plastic relaxation. These results indicate a total SL thickness beyond which growth may be limited for the formation of high-quality coherent crystal structures; however, they may indicate a growth window for the reduction of threading dislocations by controlled relaxation of the epilayers.
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