Growth and investigation of AlN/GaN and (Al,In)N/GaN based Bragg reflectors

In this work we investigate the synthesis of Bragg reflectors consisting either of AlN/GaN or of (Al,In)N/GaN by plasma-assisted molecular beam epitaxy (MBE). In addition, we study the impact of Si-doping on the surface morphology and the structural and electrical properties of AlN/GaN Bragg reflectors in order investigate the feasibility of obtaining vertically conducting Bragg mirrors. The structures are grown on conducting SiC(0001) substrates. A brief introduction to the fundamental concepts used for the growth of AlN and GaN is given. Metal-stable growth at high temperature yield excellent results in terms of surface morphology and crystal quality and have therefore been widely adopted. We investigate the growth of AlN in detail in order to obtain AlN layers with smooth surfaces and with a high crystal quality thus making these layers suitable for AlN/GaN Bragg reflectors. Si-doping of AlN is performed in order to study the impact of Si-doping on the surface morphology, the structural quality and the electrical properties of AlN. The obtained surface morphology and structural quality for both un-doped and Si-dopedAlN layers is comparable to those of GaN layers grown in the same system. In addition, semi-conducting AlN layers are demonstrated. A Si concentration of 6×1020 cm−3 yields an electron concentration of 7.4×1017 cm−3 with a mobility of 11 cm2/Vs and a resistivity of 1.5 Ωcm. InN growth experiments are performed as part of the optimization of the ternary compound (Al,In)N. The concept of metal-stable growth which is used for AlN and GaN, is not directly applicable for InN since InN decomposes at a temperature at which In does not yet desorb. The ternary (Al,In)N is subject to spinodal decomposition like in the case of (In,Ga)N but to an even higher degree. A growth temperature of 450◦C and above gives rise to phase separation. Growth at 300◦C induces crack-formation caused by tensile stresses due to crystallite coalescence and grain boundary formation. Homogeneous and crack-free films are obtained at a growth temperature lying around 400◦C under N-rich conditions. All films exhibit a low tilt comparable to GaN layers from the same system, but a large twist (≈1–2◦). The Bragg reflectors based on AlN/GaN exhibit smooth surfaces and interfaces of a very high integrity which is evidenced by reflectancemeasurements that yield reflectances of 99% andmore for these structures. Furthermore, the AlN/GaN Bragg reflectors are crack-free as revealed by differential interference contrast optical microscopy. Reciprocal space maps unambiguously show that the AlN/GaN Bragg reflectors are crack-free due to a relaxation process that results in strain-compensated structures. Key to this relaxation process is that the first layer, which always is GaN, is thin enough (45.5 nm) to relax only partially. We find that Si-doped AlN/GaN Bragg reflectors are vertically conducting. A doping level of 1.4×1020 cm−3 gives a specific series resistance of 2×10−3 Ωcm2. In contrast, the undoped structures are insulating. The surprisingly high vertical conductance is an effect of auto-ionization of the Si-donors caused by the internal electrostatic fields. Finally, we investigate the growth of lattice matched (Al,In)N/GaN Bragg reflectors. Under the appropriate growth conditions these structures are found to be homogeneous and crack-free and have abrupt interfaces and smooth surfaces. The reflectance is lower than expected despite the promising morphological properties. This might possibly be attributed to residual absorption or scattering of the incident light in the (Al,In)N layers.