Biomedical Engineering Reference
In-Depth Information
The chapter is divided into three sections. First we will discuss the sample
structures used in this investigation along with the experimental details and an
explanation of the important spectroscopic features. This will be followed by a brief
explanation of the Stark effect in QDMs. We will then conclude with a specific
example where an optically generated field, arising from photovoltaic effects, is
characterized in detail using the Stark effect in QDMs.
11.2
Sample Structure
The samples described in this chapter consist of two sequential layers of InAs QDs
grown using molecular beam epitaxy [
17
,
18
]. All of the samples discussed in this
chapter were grown on n
-doped GaAs substrates to facilitate the fabrication of
Schottky diode structures. In a typical sample an 80 nm thick layer of undoped
GaAs is deposited on the doped substrate. The first layer of QDs is then formed due
to the strain induced by the lattice mismatch between the GaAs and the deposited
InAs material. This involves the standard growth process of the formation of a
two-dimensional wetting layer (WL) on which, after a critical thickness, the QDs
begin to nucleate. The QD layer is then partially capped with GaAs and an Indium
flush technique [
19
], where the temperature is increased to redistribute and partially
remove the exposed InAs, is used to control the height, and therefore the bound
state energies, of the QDs. After the first layer has been truncated another layer
of undoped GaAs is deposited. This layer forms the potential barrier between the
QDs and the thickness, and therefore the coupling, can be precisely controlled.
The strain-driven Stranski-Krastanow growth mode is extremely well suited for
the formation of QDMs as after the barrier growth, the QDs formed in the second
layer will preferentially nucleate above those in the first layer. The second layer
of QDs can then be truncated in a manner similar to the first. The remainder
of the device consists of 230 nm of undoped GaAs followed by a 40 nm thick
layer of Al
0.3
Ga
0.7
As and a 10 nm GaAs capping layer. To complete the device
structure a semi-transparent layer of titanium, 5 nm thick, and an aluminum shadow
mask, 100 nm thick, are deposited to form the Shottky contact. Using negative
tone photoresist and e-beam lithography an aluminum shadow mask is created with
micron sized apertures and alignment marks to facilitate the spectroscopic study of
individual QDMs (Fig.
11.1
).
+
11.3
Experimental Setup
The majority of the measurements were done with a standard photoluminescence
(PL) setup. The excitation was provided by a tunable Spectra-Physics Ti:Sapphire
laser. Typical excitation was non-resonant (800-900 nm), into, or higher in energy
than, the quasi-continuum of the InAs WL. After relaxation of the electron-hole
