Investigation of AlN/GaN superlattices and their application to group III nitride devices

The deposition of group III nitrides still faces unsolved challenges concerning stress and strain of the deposited layers. Stress does not only lead to relaxation of the material via defect incorporation but also destabilizes the growth of ternary alloys as e. g. AlGaN which leads to high defect densities and changing compositions. One possible solution to this problem are compositional superlattices (SL) with periodicities on the nm scale. SL consisting of the more stable binary materials AlN and GaN form virtual alloys which have highly interesting electronic, optic and stress-controlling properties. In this thesis, the deposition of ultra-this AlN/GaN-SL (1.3 nm/3 nm) via metal-organic vapor phase epitaxy (MOVPE) was optimized and the superior structural quality was demonstrated. In analogy with a statistically mixed AlGaN alloy, tensile stress is building up in the layer stack with increasing SL layer thickness. It was shown that the critical layer thickness of the SL is higher compared to a classic AlGaN alloy with comparable Al content originating from the specific distribution of stress in the SL.Subsequently, SL heterostructure field-effect transistors (SL-HFET) were developed in which several two-dimensional electron gases (2DEG) generated at the AlN/GaN interfaces in the SL served as parallel channels. With the distribution of carriers into more than one channel, the carrier mobility was enhanced in the low (backbarrier effect) and high carrier concentration regime (reduced interface and phonon scattering). The carrier mobility was increased from 1690 cm2/Vs for a standard single-channel HFET to 1900 cm2/Vs for a triple-channel transistor. Besides the improved mobility, this approach also led to a linearization of DC- and AC-gain profiles and large-signal power amplification.AlN/GaN-SL were also used in distributed Bragg reflectors (DBR) where they replace the classical AlGaN alloy. Layer cracking was successfully controlled in 10-pair SL/GaN-DBR with an AlN content of 26 % in the low-index SL layers. Reflectivities of up to 80 % with 10 DBR pairs were shown. The accommodation of tensile strain occurred via a staircase pattern of TD. Here the TD bend into the direction at every GaN/SL interface of the DBR and kink back into the original propagation direction after a characteristic length. The concepts of SL-DBR and SL-HFET were successfully merged to create a novel electro-optic modulator (patent pending RWTH Aachen University). The modulation principle relies on the depletion or accumulation of carriers in one low-index SL layer in a SL-DBR via an electric field. This affects the dielectric properties including the refractive index of the SL due to electro-optic effects. The accumulated carriers are located in 2DEG at the AlN/GaN interfaces in the SL layer similar to the parallel transistor channels of the SL-HFET. In this way, the reflectivity of a single SL layer showed an increase of absolute +0.5 % when biased with +8 V. Simulations showed that with this approach the reflectivity of a 51-pair SL-DBR can be modulated from 94 % to 35 % with only one electrically addressed SL layer. Besides the application in optical modulators, this innovative modulation concept can also be used in laser diodes, projection systems and optical amplifiers