Estruturas de barreira dupla de PbTe/PbEuTe ... - mtc-m17:80 - Inpe

PbTe/PbEuTe DOUBLE BARRIER STRUCTURES GROWN BY MOLECULARBEAM EPITAXYABSTRACTThis work reports the growth of PbTe/Pb 1-x Eu x Te double barrier (DB) structures bymolecular beam epitaxial and the device processing aiming the resonant tunnelingmeasurement. The samples were grown on (111) BaF 2 substrates at 300°C. Resistivityand Hall effect measurements were performed on reference films to determine the mostsuitable electrical properties of each layer of the DB structure. PbTe layers with electronconcentration of n ~ 10 17 cm -3 for the spacers and the well were controlled bystoichiometric deviation through the additional Te flux. The highest Hall mobility at77 K of 1.5x10 4 cm 2 /Vs was obtained just near the n to p transition range, whichoccurred abruptly at a beam flux ratio between Te and PbTe close to 0.02. To obtainlayers 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 theBi 2 Te 3 effusion cell flux. Resistivity and Hall effect measurements at temperature from10 to 320 K were done for these films. The results indicate that Bi atoms wereeffectively incorporated to PbTe as active donors. No thermal activation was observedfor these doping levels, indicating that Bi has added resonant levels with the conductionband. Mobility curves show that the PbTe layers tend to a metallic behavior, when theelectron concentration increases. Values of n around 10 19 cm -3 are suggested as the mostappropriate for the contact layers in the device. The beam flux ratio of 2.2 between Teand Eu was considered as the most suitable for the growth of the Pb 1-x Eu x Te barrierreference layers. Using the Fourier transform infrared spectra, the europiumconcentration of x = 0.050 was obtained, close to the nominal concentration ofx = 0.044 predicted from the beam flux ratio between Eu and PbTe. Considering a bandoffset of 50% between the conduction and valence bands, a barrier height of 150 meV isfound for the PbTe/Pb 0.95 Eu 0.05 Te interface. A set of PbTe/Pb 0.95 Eu 0.05 Te DB sampleswas grown with nominal thicknesses, estimated by the growth rates, of 12 nm for thespacers, 5 nm for the well, from 2.5 to 100 nm for the barriers and 36 nm for the caplayers. To obtain the structural parameters, high-resolution x-ray diffraction spectrawere 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 thelayers that compose the DB structure was precisely determined. A good agreementbetween the nominal thicknesses and the values obtained from the x-ray analysis wasachieved, except for layers thinner than 10 nm, where a discrepancy up to 40% wasobserved. The x-ray analysis also revealed that abrupt interfaces were obtained duringthe MBE growth, and that the Pb 0,95 Eu 0,05 Te barriers are completely strained to the PbTelayers inside the DB structures up to thicknesses of 100 nm. The IxV curves of the DBmesa devices exhibit an ohmic behavior, confirming the Bi doping efficiency in thecontact layers. Leak currents through the mesa edges were considered to be the mainreason which prevents the detection of resonant tunneling in the PbTe/PbEuTe doublebarrier structures.