In order to simplify the management of the traditional network, software-defined networking (SDN) has been proposed as a promising paradigm shift that decouples control plane and data plane, providing programmability to configure the network. With the deployment and the applications of SDN, researchers have found that the controller placement directly affects network performance in SDN. In this paper, the state of the art of controller placement problem is surveyed from the perspective of optimization objective. First, we introduce the overview of SDN and controller placement problem. Then, we classify this paper of controller placement problem into four aspects (latency, reliability, and cost and multi-objective) depending on their objective and analyze specific algorithms in different application scenarios. Finally, we identify some relevant open issues and research challenge to deal with in the future and conclude the controller placement problem.

Magnetointersubband scattering (MIS) oscillations of two-dimensional electron gas (2DEG) in Al0.22Ga0.78N/GaN heterostructures have been investigated by means of magnetotransport measurements at low temperatures and high magnetic fields. Double periodic Shubnikov–de Haas oscillations modulated by MIS oscillations have been observed due to the intersubband scattering of the 2DEG at the two lowest subbands in the triangular quantum well at the heterointerface. By using the fast Fourier transform analysis, it is found that the MIS oscillations become slightly weaker with an increase in temperature. From the MIS frequency, the energy separation between the first and the second subbands is determined to be 80 meV. The observation of the MIS effect indicates that the effective masses of the electrons in the first and second subbands are the same in Al0.22Ga0.78N/GaN heterostructures.

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A two dimensional electron gas (2DEG) was observed in a C-face 3C∕4H SiC heteropolytype junction. Sheet carrier concentrations of ∼3×1013cm−2 and Hall mobility of ∼314cm2∕Vs were measured at 77K. The temperature dependences of mobility and carrier concentration clearly demonstrate the presence of the 2DEG. Comparison with theory indicates that the carriers originate from both spontaneous polarization and unintentional degenerate nitrogen doping of 3C-SiC, suggesting a 77K 2DEG mobility ∼700cm2∕Vs in parallel with bulk hopping conduction. Mobility at high temperatures was phonon limited, indicating a Debye temperature of 1600K. Transmission line measurements yielded similar mobilities, with saturation currents of ∼3A∕mm, suggesting the utility of SiC heteropolytypes in microwave devices.

The authors report on the electronic properties of a rectifying Si face 3C∕4H SiC heteropolytype junction on n+ 4H SiC. Capacitance-voltage profiling of the junction at temperatures from 4–300K showed high apparent carrier concentration. A semiclassical model was used to explain the behavior. The model predicted a spontaneous polarization-induced valence band quantum well in the 3C, indicating a polarization charge of 9.7×1012cm−2 for 4H SiC, in good agreement with theory. The formation of a two-dimensional hole gas was predicted. Using a Poisson-Schrödinger solver to analyze the measurements, it was found that large (∼3.5×1012cm−2) mobile hole charge was induced in the n-doped 3C SiC.