Electrical Properties of CdTe Quantum Dots
A. Nowak1, E. Placzek-Popko1, T. Wojtowicz2, M. Wiater2, M. Guziewicz3,
Z. Gumienny1, and G. Karczewski2
1 Institute
of Physics, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27,
50-370 Wrocław, Poland
2 Institute of Physics, Polish Academy of Science, Al. Lotników 32/46, 02-668 Warsaw,
Poland
3 Institute of Electron Technology, Al. Lotników 32/46, 02-668 Warsaw, Poland
The electrical properties of CdTe quantum dots (QDs) embedded in the ZnTe matrix grown
by molecular beam epitaxy were studied by capacitance-voltage (C-V) measurements and
admittance spectroscopy. The layers of (0.7m) undoped ZnTe, (4.4m) p+-type ZnTe:N and
(0.6m) p-type ZnTe:N were deposited on the (001) GaAs substrate. The quantum dots were
grown by Stranski -Krastanow mode by depositing of 3 monolayers of CdTe on the (0.6m)
p-ZnTe:N film and were covered with the (0.5m) p-ZnTe:N cap. A sample with the same
configuration of layers but without quantum dots was grown for the reference purpose. The
Pd/Au ohmic contact was formed on the ZnTe:N after a part of the area was etched-off to the
open p-type ZnTe:N below the quantum dots layer. The 0.16mm2 squared In-Schottky
contacts were thermally evaporated on the remaining part of the sample. The quality of the
contacts was checked by current-voltage (I-V) measurements.
The capacitance-voltage characteristics were measured at room temperature. A characteristic
step within -1V and +0.5V bias voltage in the capacitance-voltage dependence for the sample
with quantum dots is observed. It is due to the accumulation of the carriers in the assembly of
quantum dots. The admittance measurements were taken at -1V voltage bias for temperatures
in a range of 77K and 320K. The conductivity versus frequency dependence exhibits a peak
shifting towards lower frequencies with decreasing temperature. The activation energy of the
conductance was obtained from the corresponding Arrhenius plot of the conductance peak.
The reference sample reveals the hole trap with the activation energy equal to 0.02 eV. For
the sample with quantum dots a nonlinear behavior of the Arrhenius plot is observed. The
nonlinearity has been attributed to two different traps with the activation energies
approximately equal to 0.2 eV and 0.02 eV. We suspect that the trap of the energy equal to
0.2 eV, observed only in the sample with quantum dots is related to the dots and the hole
emission from the quantum dot ground state to the valence band.