Photon Upconversion in Heavily Doped Semiconductors

Sammanfattning: Popular Abstract in English Imagine a solar powered flash light. Each photon absorbed by the flash light’s solar cell creates electric energy and the flash light’s light source converts the electrical energy again into photons. The purpose of such a device may be questionable, because you would be surprised if it would create more light than what it absorbs. However, under certain conditions it is possible that an absorbed photon creates two or more photons or that an absorbed photon creates a photon with a higher energy. That one absorbed photon can cause the emission of two (or more) photons is not very surprising, as long as the total energy of the emitted photons is less or equal to the energy of the absorbed photon. It is more surprising if the emitted photon has more energy than the absorbed photon and that is what is studied in this work. Instead of a solar powered flash light I studied the light emitted by a sample after absorption of photons, a method, which is called photoluminescence. If the emitted photons have lower energy than the absorbed photons (which is typically the case) the process is called photon downconversion. If the emitted photons have higher energy than the absorbed photons the process is called photon upconversion. One possible mechanism for photon upconversion is the simultaneous absorption of two photons, called two-photon absorption, followed by emission of one photon with the combined energy of both photons. The probability of simultaneous absorption of two or more photons depends strongly on the light intensity hitting the sample. For low light intensities the probability is very low. I was quite surprised when I observed photon upconversion for the first time, because I used relatively low laser light intensities and could exclude two-photon absorption as main upconversion mechanism. I was studying the photoluminescence of doped InP nanowires when I detected photons with energies higher than the laser photon energy. The experiment is rather simple, but apparently nobody before has studied photon upconversion in highly doped semiconductors. InP is a direct semiconductor, which means it absorbs light much stronger than the indirect silicon. Each semiconductor has a certain photon energy range where it most efficiently converts absorbed photons into electric energy. Thus, to make a very efficient solar cell, different materials need to be combined. Nanowires are very small structures, only up to one tenth of a micrometer in diameter and a couple of micrometers long. At such small dimensions it is possible to combine the very different semiconducting materials necessary for highly efficient solar cells. Pure semiconductors have quite high electric resistances, but incorporation of specific atoms into the crystal may change the local conductance dramatically. Such incorporation is called doping. Most electronic devices would not work without doping. In the studied sample the InP was doped with sulfur, which means some of the phosphorus atoms in InP were replaced by sulfur atoms. The sulfur atoms have one electron more than the phosphorus atoms and every additional electron increases the electronic conductance. If the concentration of electrons is sufficiently high they can be treated as an ensemble of particles with average kinetic energy and temperature. The thermal energy of such an electron gas can increase the energy of an emitted photon. However, in my experiments the emitted photon energy was more than the thermal energy higher than the absorbed photon energy. To explain the observed upconversion we have to consider that electrons are Fermions and thus follow the Pauli exclusion principle, which means if two electrons have otherwise identical quantum states they cannot have the same energy. If more electrons are added to the system, the additional electrons will occupy higher energy states. The electrons always try to minimize their energy, but if the electrons are heated from the surrounding crystal or external sources they can gain energy if the final state is not already occupied by an electron. Such additional heating of the electrons together with the Pauli exclusion principle can explain the observed upconversion. The upconversion mechanism I discovered, will not enable perpetual motion or a solar powered flash light with a higher light output power than light input power, however, it may be relevant for future optoelectronic devices. In the present experiments it has proven to allow new ways to study the processes preceding the emission of a photon and how the electrons interact with the surrounding material.