Photon Upconversion in Heavily Doped Semiconductors

Abstract: In this thesis the luminescence properties of highly doped semiconductors are studied with focus on degenerately n-doped InP. It is demonstrated how photoluminescence measurements on degenerately doped semiconductors allow an estimation of the doping concentration without need for electrical contacts. The degenerate doping can furthermore reveal the conduction band structure for energies higher than the bandgap, which is exploited to experimentally support the existence of a theoretically predicted second conduction
band minimum in wurtzite InP.

Excitation energy dependence measurements reveal band-to-band absorption for photon energies much lower than the Fermi energy. That absorption causes not only downconverted photoluminescence with photon energies lower than the excitation energy, but also upconverted photoluminescence with photon energies higher than the absorbed laser photon. From the results of the detailed study of this novel upconversion mechanism in degenerately n-doped InP nanowires and bulk InP we propose the following explanation:

An elevated electron gas temperature in degenerately doped semiconductors allows absorption of photon with energies much lower than the Fermi energy. Band-to-band absorption of photons with energies lower than the Fermi energy excites holes with k-values lower than kF and scattering of the photexcited holes to higher k-values allows k-conserving radiative recombinations with photon energies higher than the energy of the absorbed photon. Similar upconversion luminescence is observed for degenerately n-doped
bulk GaAs and degenerately p-doped GaAs nanowires, which suggest that
similar photon upconversion could be observed in many degenerately doped direct band semiconductors.

The three most important findings about degenerately doped direct band semiconductors are. There is significant photon upconversion for excitation energies between Eg and EF. The charge carrier recombination rate is higher than, or comparable to the scattering rate of the minority carriers. And, the radiative recombination is strongly dominated by k-conserving vertical transitions in contrast to the common assumption of relaxation of the k-selection rule in degenerately doped material.

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