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Electrical Signature of Excitonic Electroluminescence and Mott Transition at Room Temperature

Mostly optical spectroscopies are used to investigate existence of excitons, Mott transitions and other exquisite excitonic condensed phases of matter. On the other hand, electrical signatures of excitons are hardly explored. Here we examine steady state, small signal, electrical response of AlGaInP based multi-quantum well laser diodes to identify and investigate the presence of excitons in electroluminescence. This frequency dependent response shows bias activated capacitance following a phenomenological rate equation. Dynamic dependence of this response on frequency reverses after light emission. This results in negative activation energy which we explain with the formation of a stable, intermediate transition state whose average energy matches well with excitonic binding energy of these III-V semiconductors. Hence we identify the presence of excitons which are also responsible for observed electroluminescence at low charge injections by electrical measurements alone. Further increase in charge injection suppresses the electrical signature of excitons representing their Mott transition into electron-hole plasma which is supported by standard optical measurements. This kind of correlation between electrical and optical properties of excitons was not systematically investigated in the past. Therefore, this study demonstrates a fresh experimental approach to electrically probe the rich and complex physics of excitons.