Estruturas de barreira dupla de PbTe/PbEuTe crescidas por epitaxia de feixe molecular / PbTe/PbEuTe double barrier structures grown by molecular beam epitaxy

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

This work reports the growth of PbTe/Pb(1-x)Eu(x)Te double barrier (DB) structures by molecular beam epitaxial and the device processing aiming the resonant tunneling measurement. The samples were grown on (111) BaF(2) substrates at 300°C. Resistivity and Hall effect measurements were performed on reference films to determine the most suitable electrical properties of each layer of the DB structure. PbTe layers with electron concentration of n ~ 10(17) cm(3) for the spacers and the well were controlled by stoichiometric deviation through the additional Te flux. The highest Hall mobility at 77 K of 1.5x10(4) cm(2)/Vs was obtained just near the n to p transition range, which occurred abruptly at a beam flux ratio between Te and PbTe close to 0.02. To obtain layers with higher electron concentration, a study of Bi doping on PbTe was performed. PbTe films with n values between 1x10(17) e 4x10(19) cm(-3) were obtained by increasing the Bi(2)Te(3) effusion cell flux. Resistivity and Hall effect measurements at temperature from 10 to 320 K were done for these films. The results indicate that Bi atoms were effectively incorporated to PbTe as active donors. No thermal activation was observed for these doping levels, indicating that Bi has added resonant levels with the conduction band. Mobility curves show that the PbTe layers tend to a metallic behavior, when the electron concentration increases. Values of n around 10(19) cm(-3) are suggested as the most appropriate for the contact layers in the device. The beam flux ratio of 2.2 between Te and Eu was considered as the most suitable for the growth of the Pb(1-x)Eu(x)Te barrier reference layers. Using the Fourier transform infrared spectra, the europium concentration of x = 0.050 was obtained, close to the nominal concentration of x = 0.044 predicted from the beam flux ratio between Eu and PbTe. Considering a band offset of 50% between the conduction and valence bands, a barrier height of 150 meV is found for the PbTe/Pb(0.95)Eu(0.05)Te interface. A set of PbTe/Pb(0.95)Eu(0.05)Te DB samples was grown with nominal thicknesses, estimated by the growth rates, of 12 nm for the spacers, 5 nm for the well, from 2.5 to 100 nm for the barriers and 36 nm for the cap layers. To obtain the structural parameters, high-resolution x-ray diffraction spectra were measured around the (222) Bragg peak. From the best fit of the calculated spectra, using Takagi-Taupin equations, to the measured ones, the individual thickness of the layers that compose the DB structure was precisely determined. A good agreement between the nominal thicknesses and the values obtained from the x-ray analysis was achieved, except for layers thinner than 10 nm, where a discrepancy up to 40% was observed. The x-ray analysis also revealed that abrupt interfaces were obtained during the MBE growth, and that the Pb(0.95)Eu(0.05)Te barriers are completely strained to the PbTe layers inside the DB structures up to thicknesses of 100 nm. The IxV curves of the DB mesa devices exhibit an ohmic behavior, confirming the Bi doping efficiency in the contact layers. Leak currents through the mesa edges were considered to be the main reason which prevents the detection of resonant tunneling in the PbTe/PbEuTe double barrier structures.

ASSUNTO(S)

ciências e tecnologia de sensores e materiais

ACESSO AO ARTIGO

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