Dynamics of Photogenerated Charge Carriers in III-V Bulk and Nanowire Semiconductors

Sammanfattning: As a solution to solving energy consumption and environment problems, photovoltaics has become one type of the promising devices to convert solar energy into electricity directly. In some special areas like in space, a kind of photovoltaics with lightweight and reliable properties is needed to supply power. Therefore, photovoltaics based on the group III-V semiconductor nanowires has been emerged and developed nowadays. However, a large surface-to-volume ratio in nanowires leads to high-density surface traps and therefore could degrade the performance of photovoltaics. In general, the properties of applications are dominated by the behaviours of charge carriers in semiconductors. Therefore, it is important to understand all processes which are related to charge carriers in semiconductors. In this thesis, a series of surface passivation methods are applied to lower the density of trap states and consequently improve the lifetime of photogenerated charge carriers in group III-V bulk and nanowire materials. We show that the GaNAs and AlGaAs passivation layers help to lower the trap density at the GaAs surface. Similarly, we have investigated the AlyIn(1-y)P passivated GaxIn(1-x)P nanowires with a great potential for multi-junction photovoltaic applications. Concerning InP nanowires, we investigated why optimal HCl etching provides less surface defects. Although the density of surface defect in InP is lower than in GaAs, an insulating layer is still needed to isolate the active InP nanowires and the electrodes. In this respect, we demonstrated that an appropriate POx/Al2O3 capped layer works as a passivation and insulating layer. By means of several steady-state and time-resolved spectroscopies, such as time-resolved photoluminescence, transient absorption, and time-resolved terahertz spectroscopy, prospective passivation layers or conditions are screened for GaAs bulk, GaAs NW, InP NW, and GaInP NW materials. On the fundamental side, we find that charge trapping by several types of trap states dominates the primary steps of charge carrier dynamics and results in predominantly non-radiative recombination of photogenerated charges. Some trapping channels can be saturated via high charge generation rate under irradiation of the semiconductors by high intensity short optical pulses. Meanwhile, the atomic composition in ternary semiconductors, like Ga in GaxIn1−xP NWs plays a crucial role in the unexpected formation of deep traps. With the increase of Ga fraction, the fast electron trapping, hole trapping, and non-radiative recombination become more efficient.These spectrum studies in this thesis not only help us to select a potential passivation method for group III-V bulk and nanowires materials, but also reveal the carrier behaviour in these materials. Based on these understanding, several methods of characterization of the optoelectronic materials performance are derived.