M.Sc Thesis | |

M.Sc Student | Uri Ezra |
---|---|

Subject | Intersubband Electro and Photo-induced Absorption in InGaAs/InP Quantum Wells |

Department | Department of Physics |

Supervisors | Full Professors Gershoni David |

Professor Emeritus Ehrenfreund Eitan |

We report on investigations of In_{0.53}Ga_{0.47}As/InP
supperlattices using intersubband optical spectroscopy. We measured
intersubband absorption, photo- and electro-induced absorption, due to optical
transitions between the conduction minibands. The samples consist of 60Å
InGaAs well and InP barrier of various widths in each period. The barriers,
which separate between adjacent wells determine the overlap between the wave
functions of carriers within each quantum well, and thereby the energy width of
the superlattice's minibands. The narrower the barriers are, the wider the
minibands are. The energy width of a miniband (_{}) is related to the lifetime of a
carrier within this miniband (τ) by τ
= _{}/_{}. The tunneling
probability of an electron through the superlattice structure is calculated as
a function of its energy. This probability is, normally very small, but when
the incident electron energy coincides with a miniband energy, there is a
resonant amplification in the tunneling probability. The lifetime is then
evaluated from the full energy width at half the maximum of the resonant
tunneling peak. In our samples the first electronic level (e_{1}) is
quite deep, while the second level (e_{2}) is quite shallow.
Consequently, the width of the first superlattice miniband is rather narrow,
while that of the second one is quite wide. Since intersubband transitions are
vertical in momentum space, their energy is extremely sensitive to the momentum
component along the superlattice axis of the e_{1 }electron. When
external electric field is applied to a superlattice, its energy bands ‘bend’
and change slightly. In addition there is a flow of carriers through the
structure and their steady state population undergoes redistribution. Under
these conditions τ represents the dwell time that a
carrier spends in its miniband.

The density of electrons in the
first miniband (n_{1}) can be measured by the intersubband absorption
(e_{1}-e_{2}). Using electro-or photo-induced absorption the
induced changes in this density can be extracted. By correlating the measured
density with the measured current, we tried to get an estimate for the lifetime
τ. In the most straightforward case, the dwell time
is deduced from the current density (_{}) by: τ=n_{1}/_{}. We measured
electron density of ~ 10^{10 }cm^{-2 }per period due to
unintentional doping. However, two orders of magnitude smaller variations were
measured in the electro- induced absorption. This means that only a small
fraction of the resident electron population is influenced by the applied bias.

We found that in the differential electro induced intersubband absorption spectra there are both positive and negative spectral regions. This means that under bias, electrons of different momentum redistribute differently.

These differences are
qualitatively explained in terms of “effective mass filtering”. From the second
derivative of the calculated e_{1} dispersion we get the electron
effective mass and thereby its mobility. In the Brillouin zone center (k_{z}=0)
the electrons have positive relatively small in absolute value, effective mass.
In the zone edge (k_{z}= _{}/l_{z , }where_{ }l_{z}
is the superlattice period) they have negative relatively small in absolute
value effective mass. In between, the effective mass changes sign and its
absolute value is rather large. Therefore, zone edge and zone center electrons
are rather mobile, but they move in opposite directions when subjected to an
applied electric field. In contrary, mid zone electrons are very heavy and
immobile.

We outline a more sophisticated
model, which takes into account the Poisson and the continuity equations in
order to quantitatively account for our experimental* *observations.