Photoconductivity and electrical transport properties in PbTe single quantum well samples

• Autor(a) principal: PENA, Fernando Silva
• Data de Publicação: 2019
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Repositório Institucional da UNIFEI (RIUNIFEI)
• Texto Completo: https://repositorio.unifei.edu.br/jspui/handle/123456789/2078

We investigated the photoconductivity effect in 10 and 14 nm well thickness n-type PbTe/Pb1xEuxTe quantum wells, with x values of 0.12 and 0.1, for a temperature range of 300–10K using infrared light. Also, magnetotransport properties measurements are performed in 8, 10, 15, 20 and 30 nm well width p-type PbTe/Pb0.9Eu0.1Te quantum wells, at magnetic fields up to 33 T and temperature varying from 0.35 to 300 K, under dark and illuminated condition. For the n-type PbTe quantum wells, the measurements revealed that at high temperatures, the photoresponse has small amplitude. As temperature decreases to T~75 K, the photoconductivity amplitude increases reaching a maximum value 10 times higher than the original value before illumination. From Hall measurements performed under dark and light conditions, we show that this effect is a result of carrier concentration increase under illumination. Unexpectedly, for further reduction of temperature, the amplitude starts to decrease again. The electrical resistance profiles indicate that the transport occurs through barriers and the well that behave as two parallel channels. For temperatures below 75K, transport is more effective in the quantum well, where the signal reduction can be associated with the electron-electron scattering due to the increase in the carrier concentration that occurs under illumination. We also used the random potential model to explain the origin of the persistent effect observed in the photoconductivity curves. We compare magnetotransport measurements in p-type PbTe/Pb0.9Eu0.1Te quantum wells with different widths: 10, 15 and 20 nm, revealing clearly QHE and SdH oscillations, which evidences the two-dimension electron gas formation and the high quality of the samples. The 10 nm well width presented odd non-integer filling factors sequence (2.3; 3.4; 5.6; 6.9 and 8.8). The non-integer values may be associated to edge states, since that parallel conduction in the interface, between the barrier and well, was not enhanced or destroyed by illumination, although the carrier concentration was enhanced. Further investigations are necessary to clarify this effect and we consider the approximated integers 2; 4; 6; 7 and 9 calculated from experimental B values. For the 15 and 20 nm QW thickness, the SdHO and QHE also appear, however, it reveals an even integer filling factors sequence. The 10 nm QW thickness odd sequence originated from the first sub-band Landau level spin splitting; according to the Fermi level compared to the longitudinal and transversal resistance together with the first longitudinal sub-band spin splitting simulation. This assumption VI agrees with the FFT analysis that reveals the main frequency peak and two other ones, which may be the second harmonic spin splitting, up and down. The same FFT profile appears for the other two samples. Nevertheless, the 8 and 30 nm well width sample presented an insulator profile behavior and neither SdHO nor QHE were present. It is probably because the indium contacts do not diffuse through the PbTe layer well.