PACS: 73.40.Gk; 73.61.Ey Numerical simulations of AlN/GaN-based resonant tunnelling diode structures are presented, employing self-consistent real-time Green's functions. The simulated current-voltage characteristics show strong asymmetry effects due to polarization charges at the heterointerfaces in the doublebarrier region.Introduction During the last decade there has been tremendous progress in semiconducting III-nitride material science and technology. Early, very successful device applications in the field of optoelectronics (blue LEDs and laser diodes) and power microelectronics (HEMTs) have further driven the improvement of the materials and in turn of the devices. Even though the nitrides are far from the level of perfection seen in the technology of GaAs, the next natural step is towards quantum devices. In this sense, the resonant tunnelling diode (RTD) is an attractive candidate and offers the possibility of fundamental investigations of quantum phenomena as well as for modern device applications. Up to now, little attention has been paid to these types of structures and their functionality, when realized on the basis of nitride semiconductors.Kikuchi et al. [1] were the first to report the successful fabrication of RTDs based on AlN/GaN double-and multi-quantum well structures. The typical negative differential resistance (NDR) region in the current-voltage (I-V) characteristic was clearly observed, and, interestingly, a strong asymmetry was revealed: the NDR was only measured for one bias polarity and not for the opposite one (at least in the voltage range investigated). Due to the polarization charges built up at the wurtzite nitride heterojunctions, opposite electric fields are induced in the barrier and well regions of the device, which obviously are related to the observed asymmetry. Previous model calculations of GaNbased RTD structures showed that the effect of polarization fields is to shift the resonances in the transmission probability in comparison to the same hypothetical structure without polarization fields [2]. The model presented here goes a step further and deals with the self-consistent calculation of the current across the device structure under nonequilibrium conditions. This provides a direct comparison with the experiment and we are able to reproduce the measured asymmetry of the I-V characteristics.
PACS : 73.20.At; 73.40.Kp; 81.15.Hi The growth of III-nitrides on Si(111) substrate by plasma assisted molecular beam epitaxy is addressed. Growth optimization of GaN layer is reported. Different III-N flux ratio, substrate temperature and buffer layer structures were chosen as optimization parameters. Significant improvement of the GaN properties is achieved using AlGaN/AlN buffers. Temperature-dependent Hall effect experiments show a complex behavior, whereby p-and n-type conductivity is observed.Introduction Silicon is an attractive substrate for GaN-based devices because of its physical properties, crystal quality, doping capability, thermal stability, low cost and well known Si technology. The crystal perfection is better than any other substrate material used for GaN epitaxy and its surface could be prepared with extremely smooth finishes. To date, the quality of GaN epitaxial layers on silicon has been much poorer than that on sapphire or silicon carbide, due to large lattice and thermal expansion coefficient mismatch, and the tendency of silicon to form an amorphous silicon nitride layer when exposed to reactive nitrogen sources. Nevertheless, GaN devices have been already demonstrated on Si substrates including LEDs and HEMTs [1]. In this work we first concentrate on the III-nitride MBE growth on Si(111) substrates and finally on the Hall effect characterization of 2DEG AlGaN/GaN heterostructures.
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