Utilisation of terahertz scanning microscope for investigation of ultrafast photoconductivity of semiconductor nanocrystals

Abstract

Cı ́lem te ́to pra ́ce bylo studovat fotoexcitovane ́ povrchy polovodic ̌u ̊pomocı ́ pr ̌ı 'stroje THz SNOM (skenovacı ́ opticky ́ mikroskop v blı ́zke ́m poli) a popsat interakci mezi volny ́mi nosic ̌i na ́boje a THz za ́r ̌enı ́m, ktera ́ je zprostr ̌edkova ́na pomocı ́ hrotu AFM. Za tı ́mto u ́c ̌elem jsme vybrali nedopovane ́ polovodic ̌ove ́ destic ̌ky GaAs a InP a provedli experiment opticke ́ho c ̌erpa ́nı ́ - THz son- dova ́nı ́ v blı ́zke ́m poli s prome ̌nny ́m zpoz ̌de ̌nı ́m mezi c ̌erpacı ́m a sondovacı ́m pulzem. Pozorovali jsme rezonanci zpu ̊sobenou interakcı ́ hrotu s plazmono- vou excitacı ́, jejı ́z ̌ charakteristicka ́ frekvence se me ̌nı ́ v za ́vislosti na dobe ̌ po excitaci. Vzali jsme v u ́vahu dostupne ́ modely popisujı ́cı ́ rozptyl za ́r ̌enı ́ v ra ́mci dane ́ interakce: model bodove ́ho dipo ́lu a model konec ̌ne ́ho dipo ́lu (te ́z ̌ ne ̌kdy nazy ́vany ́ monopo ́lovy ́ model). Monopo ́lovy ́ model poskytuje po- drobne ̌js ̌ı ́ popis interakce, proto jsme ho pouz ̌ili pro popis nas ̌ich experi- menta ́lnı ́ch vy 'sledku ̊ za dodatec ̌ne ́ho pr ̌edpokladu, z ̌e fotoexcitace v polovo- dic ̌i vytva ́r ̌ı ́ jednu excitovanou vrstvu. V pr ̌ı ́pade ̌ GaAs zvoleny ́ model dobr ̌e popisuje experimenta ́lnı ́ vy 'sledky a poskytuje dobu z ̌ivota fotoexcitovany ́ch nosic ̌u ̊ a jejich...

The aim of this work was to study photoexcited surfaces of semiconduc- tors in a THz SNOM (scanning near-field optical microscope) device and describe the interaction between the free carriers and THz radiation me- diated by an AFM tip. To this aim, we selected undoped GaAs and InP semiconductor wafers and performed optical pump - near-field THz probe experiments as a function of the pump-probe delay with these compounds. A tip-plasmon resonance with a characteristic frequency depending on the pump-probe delay was observed. We reviewed available models of the ra- diation scattering in a SNOM: point dipole and finite dipole (or monopole) model. The finite dipole model provides a better insight into the interaction and we applied it to our experimental results under an assumption of a single photoexcited layer. In the case of GaAs the experimental results are well de- scribed by the model and provide the decay of the photocarrier concentration and scattering time. In the case of InP, long-lived carriers diffuse deeper into the bulk and an approximation of singe photoexcited layer is no longer valid.