On the Dynamical Behaviour of Excitons in CdS Single Crystals. II. Correlation between Intrinsic Excitonic Absorption and Photoconductivity

Abstract

An investigation is made of the excitation spectra of photoconductivity of CdS single crystals in the region of the fundamental absorption edge from 77 O K to room temperature. The results are compared with those of excitonic absorption measurements. On the basis of conclusions about the dynamical behaviour of excitons, equally drawn from absorption, emission, and photoconductivity measurements, a model is proposed of the photoconductivity in the region of exciton transitions, which explains the correlation between the exciton absorption and photoconductivity. This model involves that the diffusion length of the excitons don't exceed that of the ambipolar electron-hole pair diffusion.Anregungsspektren der Photoleitung von CdS-Einkristallen werden im Bereich der Grundgitterabsorptionskante zwischen 77 O K und Zimmertemperatur nntersucht. Die Ergebnisse werden mit MeBergebnissen der Exzitonenabsorption verglichen. Basierend auf den SchluBfolgerungen iiber das dynamische Verhalten von Exzitonen, die iibereinstimmend aus Absorptions-, Emissions-und Photoleitfahigkeitsmessungen gezogen werden, wird ein Modell der Photoleitfahigkeit im Bereich der Exzitonenubergange vorgeschlagen, welches den Zusammenhang zwischen der Exzitonenabsorption und der Photoleitfahigkeit erklart. Das Modell schlieBt ein, daB die Diffusionslange der Exzitonen nicht groBer ist als die fur die ambipolare Diffusion von Elektron-Loch-Paaren.

Electric-Field-Dependent Photoconductivity in CdS Nanowires and Nanobelts: Exciton Ionization, Franz–Keldysh, and Stark Effects

“…The exciton ionization has been observed in CdS crystal, where the correlation measurements between exciton absorption and PC have been conducted. Those two spectra gave the same spectral configuration, which manifests that excitons are ionized and contribute to the photocurrent . Therefore, the presence of the exciton peaks in our PC spectra demonstrates the exciton dissociation in our experiments.…”

“…Those two spectra gave the same spectral configuration, which manifests that excitons are ionized and contribute to the photocurrent. 33 Therefore, the presence of the exciton peaks in our PC spectra demonstrates the exciton dissociation in our experiments. An electric field applied to a semiconductor would cause the wave functions of electrons in conduction band and holes in valence band to change from Bloch functions to Airy functions.…”

“…For the incident photon energy above the bandgap, free carriers are generated which can be immediately swept out of the devices. However, absorption at the exciton peaks will initially create excitons bound together via Coulombic interactions which will not contribute to the photocurrent unless they are ionized through interactions either with optical phonons 33 or by field ionization 34 (including both the internal the electric field due to the surface depletion and the external electric field exerted through electrodes) in a time scale smaller than their lifetime. 11 In our CdS samples, the electron−phonon coupling is significant, which has been confirmed by the resonant Raman scattering experiments in similar II−VI nanostructures.…”

Эффекты ионной бомбардировки в спектрах краевой фотопроводимости и в вольт-амперных характеристиках кристаллов CdS

“…Поэтому их можно использовать для диагностики состояния приповерхностной области полупроводника. Согласно данным работы [10], в полупроводниках с прямыми разрешенными переходами за счет высокого коэффициента поглощения в спектральной области экситонных и межзонных переходов, достигающего значений порядка 10 5 см −1 , генерация экситонов происходит в тонком приповерхностном слое (в кристаллах CdS в слое толщиной ∼ 10 −5 см). Соответственно, это приводит к зависимости ТС спектров ФП от внешних и внутренних факторов, возмущающих состояние поверхности и (или) приповерхностной области полупроводника.…”