Biomedical Engineering Reference
In-Depth Information
distribution and resistivity of BDD using a finite element analysis. They found the domi-
nating conduction mechanism shifts from the valence-band conduction at intermediate
temperature to the hopping conduction at low temperatures. Chen et al. [43] correlated
the electron field emission properties of BDD with secondary ion mass spectroscopy and
infrared absorption and concluded that the solubility limit of boron in diamond is (B
+3
)
s
=
5
×
10
21
cm
−3
, and the largest boron concentration that can be incorporated as substitutional
dopants is only one-tenth of the solubility limit, (B
+3
)
d
= 5
×
10
20
cm
−3
. The diamond lattices
are strongly strained when heavily doped. Krutko et al. [44] produced p-type polycrys-
talline diamond layers by rapid diffusion of boron. Gevrey et al. [45] used a contactless
method to measure the conductivity of BDD films at microwave frequencies, which has
an advantage to avoid parasitic resistance or grain boundaries, although the skin effect
has to be considered. Lyman spectra in the infrared, electronic Raman scattering, and
cathodoluminescence in the UV have revealed much of the complex electronic structure of
this shallow acceptor. Nebel et al. [46] detected the long living excited states in BDD in the
energy regime 3.2-3.5 eV.
To date, only boron has clearly demonstrated and established itself conclusively to be
an active electrically active dopant, and a wide range of techniques have been applied to
BDD to investigate their electrical activation energy and defect levels. However, less work
has been found to characterize BDD films using AC impedance spectroscopy to the best
of the author's knowledge. Narducci and Cuomo [47] investigated the boron diffusivity in
diamond single crystals by impedance spectroscopy at a very early stage. They reported
that diffusivity of boron in diamond at up to 800°C will not make the thermal drift of
impurities a major path to device failure. However, their impedance measurement was
only up to 10
5
Hz without temperature dependence; the results presented still did not give
full understanding of responsible mechanism behind.
In this section, initial attention has been paid on boron-doped single-crystalline dia-
mond films. Room-temperature DC current-voltage measurements between two symmet-
ric electrodes across the surface of the diamond films were carried out. Voltage is set from
-10 to 10 V. Direct current current-temperature measurements were investigated using
Keithley 487 Picoammeter under rough vacuum from -185°C to 256°C. Voltage is set to 50 V
as power supply. Impedance measurement of the films was determined using a Solartron
1260A electrochemical impedance spectroscopy (EIS) in the frequency range from 0.1 Hz
to 10 MHz, and in the temperature range from -100°C to 300°C. The setup parameters are
0.05 V of AC amplitude, 1 s integration time, and no delay time.
Figure 4.9 shows room temperature DC current-voltage measurement on the surface
of the diamond films in two electrode directions: (1) from up to down and (2) from left to
right. Comparing the curves in both directions, Figure 4.9b measured from left electrode
to right shows nearly a perfect ohmic contact, which has a symmetric and linear character-
istic from the negative to positive voltage. It is easily estimated that the room temperature
resistance is about 1.1 M
Ω
. To investigate the DC temperature effect in this electrode direc-
tion, the current-temperature (-185°C to 256°C) curves are shown in Figure 4.10. It is found
that the current increases exponentially with the temperature increasing, which indicates
thermally activated conduction process. Linear curve fitting shows the sample has activa-
tion energy of 0.36 eV. The result is consistent with most of the other people reported on
BDD [29].
Simple DC electrical experiments have now been repeated to achieve the activation
energy of BDD, 0.37 eV, which is already well known within diamond community for
a number of years. For the first time, we applied impedance spectroscopy to the boron-
doped single-crystalline diamond sample to compare the AC results with DC and to
