Electron distribution and capacitance–voltage characteristics of n-doped quantum wells

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The effect of multiple quantum well (MQW) parameters on the free carrier distribution (FCD) and the apparent carrier distribution (ACD) obtained from capacitance–voltage (C–V) profiling has been investigated using a self-consistent simulation technique and the C–V profiling technique. The FCD in MQW structures reveals large outer peaks and small inner peaks even when all the parameters of each quantum wells (QWs) are the same. Such a feature becomes more apparent as the barrier layer becomes thinner or the barrier doping level becomes lower. These characteristics are found to originate from the fact that the density of carriers confined in each well is mostly determined by the depletion region formed alongside the well via the charge neutrality condition. The ACD is found to vary drastically as the thickness or the doping level of barrier changes. When the Debye averaging process is prominent, the ACD peaks are broader and smaller than the FCD peaks and are displaced toward the bottom layer side. The ACD inner peaks even disappear completely when the Debye screening length is comparable to or larger than the QW period, while real free carriers are well confined in each well. The effect of temperature on the ACD through the Debye averaging process is also investigated. When the Debye length is much smaller than the QW period, the full width at half maximum of the ACD is determined by the change of the position expectation value of the two-dimensional differential carriers. This change of position expectation value is found to be much smaller than the well width for relatively narrow QWs. The accuracy of our simulation results is confirmed by the excellent agreements between the simulated ACDs and the measured ACDs of In0.2Ga0.8As/GaAs MQWs. As an example of extracting the qualitative informations from the measured C–V profile, the C–V profiles of partially strain relaxed InxGa1−xAs/GaAs MQWs with x=0.15 and x=0.25, grown by metal organic chemical vapor deposition, are discussed. These results show systematically how the QW parameters affect the FCD and the ACD.