A study was performed on the experimental determination of electron and hole mean drift distance in chemical vapor deposition polycrystalline diamond. Air was used as an absorbing layer in order to change the energy of the impinging α particles. The results showed that the pumping process was much more effective on hole conduction

Angelone, M

Marinelli, M

Milani, E

Pillon, M

Pucella, G

Tucciarone, A

Verona-Rinati, G

Archivio della Ricerca - Università di Roma Tor vergata

APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 26 30 JUNE 2003Experimental determination of electron and hole mean drift distance:
Application to chemical vapor deposition diamond
Marco Marinelli, E. Milani, G. Pucella, A. Tucciarone, and G. Verona-Rinatia)
INFM-Dipartimento di Ingegneria Meccanica, Universita` di Roma ‘‘Tor Vergata,’’ Via del Politecnico 1,
I-00133 Roma, Italy
M. Angelone and M. Pillon
Associazione EURATOM-ENEA sulla Fusione, Via E. Fermi 27, I-00044 Frascati-(Roma), Italy
~Received 19 February 2003; accepted 23 April 2003!
A simple technique for measuring the electron and hole mean drift distance in chemical vapor
deposition polycrystalline diamond in the as-grown and in the so-called pumped state obtained by
90Sr b-particle irradiation is presented. To this purpose, the efficiency h of a diamond-based particle
detector was measured using a 5.5-MeV 241Am a-particle source. In particular, two different
experimental setups were specifically designed and realized in order to perform a systematic study
of the device efficiency as a function of the a-particle penetration depth, both in the positive and
negative bias polarization. In the first setup, air is used as an absorbing layer in order to change the
energy of the impinging a particles, while in the second one, the measurements were performed in
vacuum and the incidence angle was varied in the 0°–80° range. The advantages of the latter setup
are evidenced. The theoretical formula for the mean drift distances of carriers is derived using a
properly modified Hecht model, and fitted to the data, allowing a separate evaluation of the charge
collection distances of each carrier type (le5meteE and lh5mhthE). The obtained results
unambiguously show that the pumping process is much more effective on hole conduction, lh being
much greater than le in the pumped state. © 2003 American Institute of Physics.
@DOI: 10.1063/1.1586475#Chemical vapor deposition ~CVD! diamond has been
shown to be a suitable material for a great number of tech-
nological applications, such as particle and UV radiation
detectors,1–4 and even more so if it is driven in the so-called
pumped state by ionizing irradiation, which is known to dras-
tically improve the transport properties of synthetic
diamond.5,6 However, despite the large diffusion of the pro-
cedure, few authors have carefully studied the physical
grounds of the pumping process in order to discriminate its
effect on electron and hole propagation in the detection
process,7–12 so that very little is known to date about the
contribution of each carrier type to the transport mechanism
in the as-grown and pumped states.
The aim of the present work is to carefully separate and
evaluate the charge collection distance of electrons and holes
in polycrystalline CVD diamond, both in the as-grown and
pumped states, thus giving a better insight on the transport
properties of such material. To this purpose, a 130-mm-thick
film was grown in our labs by microwave plasma chemical
vapor deposition and a particle detector was realized and
tested with a 241Am a-particle source, as described
elsewhere.13 The mean drift distance of each carrier type can
be separately obtained by systematically studying the effi-
ciency h of the device as a function of the incident particle
penetration depth G, and by analyzing the experimental
curves in the framework of a properly modified Hecht
model. Indeed, the contribution of one type of carrier ~de-
a!Electronic mail: gianluca.verona.rinati@uniroma2.it4720003-6951/2003/82(26)/4723/3/$20.00
Downloaded 22 Jul 2003 to 160.80.91.68. Redistribution subject topending on bias polarity! gradually decreases as the penetra-
tion depth is reduced.
A crucial point of this work was the design and realiza-
tion of a suitable experimental procedure in order to care-
fully control the a-particle penetration depth in the CVD
diamond. For this purpose, two different approaches are pro-
posed and discussed. In a first experiment, an absorbing layer
with increasing thickness is used in order to progressively
reduce the energy of the impinging a particle. This was done
by simply measuring the a spectra in air, at different source-
to-sample distances. The residual energy released in the
sample by the a particles, and hence the values of the pen-
etration depth G, were estimated using a standard nuclear
physics simulation program ~SRIM!. In the second experi-
mental setup, the sample is mounted inside a vacuum cham-
ber on a rotating holder in such a way that the incidence
angle u of the a-particle can be varied in a 0°–90° range
with respect to the sample surface. Of course, in this case, G
is reduced by a factor cos u with respect to the normal inci-
dence value G0513.54 mm.
The efficiency of the diamond-based device is then mea-
sured using these techniques both in the as-grown and
pumped states and under positive and negative bias polariza-
tion. All the measurements were done using a 1-V/mm ap-
plied electric field. Typical a spectra measured on our
samples are reported in Refs. 7 and 13. The obtained results
are summarized in Figs. 1 and 2 for air and tilt measure-
ments, respectively. The highest penetration depth value in
Fig. 1 is obtained by measuring the sample efficiency in
vacuum, both in the as-grown and pumped states.
It is evident that very similar behavior of the efficiency3 © 2003 American Institute of Physics
 AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
4724 Appl. Phys. Lett., Vol. 82, No. 26, 30 June 2003 Marinelli et al.curves was observed in both cases. Nevertheless, as far as
Fig. 1 is concerned, the higher the sample-to-a-particle
source distance, the lower the energy released in the sample.
As a consequence, the measured spectrum is shifted to the
lower-energy side and the signal-to-noise ratio decreases sig-
nificantly. This effect is evident in Fig. 1, where the error
bars become increasingly higher at low penetration depth,
making measurements eventually impossible at low values of
G, especially in the as-grown state. In the tilt measurements
~Fig. 2!, the total energy released by the impinging particles
stays constant for all the u values, so that both the shape and
the peak position of the a spectra are not greatly affected by
the tilting procedure. This is why, in this case, lower penetra-
tion depth regions can be reached with approximately con-
stant error bars on the efficiency measurements.
Let us discuss now the obtained results from the trans-
port mechanism point of view. For this purpose, a modified
Hecht model is derived for a homogeneous parallel plate
detector with blocking contacts. We assume that the detector
is positively biased when a positive voltage is applied to the
growth surface ~top electrode!. The mean drift distance le(x)
of an electron generated at a distance x from the top elec-
trode of a positively biased detector is
le~x !5
*0
xsP~s !ds1x*x
‘P~s !ds
*0
‘P~s !ds
, ~1!
where P(s)5e2s/le is the probability for an electron to drift
for a distance s @which cannot exceed the distance x from the
electrode, hence Eq. ~1!# in an infinite material. le5meteE
is the charge collection distance associated with the electron,
FIG. 1. Efficiency curves as a function of the penetration depth G for air
measurements in positive ~square! and negative ~circle! bias polarity; ~a! in
the pumped state and ~b! in the as-grown state. Solid lines are the fit curves.Downloaded 22 Jul 2003 to 160.80.91.68. Redistribution subject towhere me is the electron mobility, te is the electron lifetime,
and E the electric field. A similar formula holds for holes.
The calculated results for electrons and holes drifting in a
positively biased detector whose thickness is D are
le~x !5le~12e2x/le!; lh~x !5lh~12e ~D2x !/lh!. ~2!
The average drift distances over the whole detector are
calculated by averaging over all the generated electron–hole
pairs, using the energy density u(x) associated with the im-
pinging particles: Le5^le&u(x) and Lh5^lh&u(x) . Le and Lh
are not directly experimentally accessible, but are correlated
to the efficiency h of the detector by
h5
Le1Lh
D . ~3!
Since the Bragg energy distribution u(x) associated with an
a particle is approximated well by the exponential function
u(x)5A1Bex/C with a cutoff at the penetration depth G,
the efficiency h for positive polarization is as follows:
h5
le1lh
D 2
1
D@AG1BC~eG/C21 !#
H BCFle e ~1/C21/le!G2112C/le
1lhe
2D/lh
e ~1/C11/lh!G21
11C/lh
G1A@2le2~e2G/le21 !
1lh
2e2D/lh~eG/lh21 !#J . ~4!
FIG. 2. Efficiency curves as a function of the penetration depth G for tilt
measurements in positive ~square! and negative ~circle! bias polarity; ~a! in
the pumped state and ~b! in the as-grown state. Solid lines are the fit curves. AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
4725Appl. Phys. Lett., Vol. 82, No. 26, 30 June 2003 Marinelli et al.Obviously, in the case of a negatively polarized detector, the
roles of electrons and holes are interchanged. Equation ~4!
reduces to the usual Hecht formula when u(x) is constant
(B50 and G5D) and le5lh .
In order to analyze the efficiency curves reported in Figs.
1 and 2, a few additional considerations have to be made.
The energy distribution u(x) is strongly dependent on the
geometry of the adopted experimental setup. In the standard
case ~normal incidence in vacuum! the parameters A5A0 ,
B5B0 , C5C0 , and G5G0 were calculated by fitting the
SRIM simulated Bragg energy distribution with the function
u(x) ~see Table I!. On the other hand, when the a-particle
penetration depth is changed either by an air absorbing layer
or by a tilting procedure, the function u(x) must be properly
modified. In the former case, a cutoff is to be introduced in
the low-energy side of u(x) corresponding to an axis shift of
G02G , where G is the new penetration depth. In the latter
case, G5G0 cos u, so that the x axis is compressed by a
factor cos u. The corresponding transformations of the geo-
metrical parameters A, B, C, and G are also summarized in
Table I.
The experimental curves reported in Figs. 1 and 2 were
then fitted by Eq. ~4! using the proper transformations of A,
B, C, and G. Very good agreement is observed between the
theoretical curves and the experimental data. It is worth not-
ing that the curves relative to both positive and negative
polarization are simultaneously fitted in each case by the
very same values of le and lh , whose values are reported in
Table II. Comparable values of the two fitting parameters are
obtained from the two different setups, thus demonstrating
TABLE I. Transformations of the parameters A, B, C, and G, relative to
translations and compressions of u(x). A0 , B0 , C0 , and G0 values are
calculated for normal incident 5.5-MeV a particles.
Normal incidence
~vacuum!
Translation
~air!
Compression
~tilt!
A A05267.7 keV/mm A0 A0G0 /G
B B0511.17 keV/mm B0 e (G02G)/C0 B0G0 /G
C C053.45 mm C0 C0G/G0
G G0513.54 mm G G
TABLE II. Values of le and lh obtained by the fitting procedure for both
experimental setups.
Air Tilt
As-grown Pumped As-grown Pumped
le @mm# 8.661.0 8.361.3 8.660.6 8.461.0
lh @mm# 8.161.0 36.161.5 6.860.6 37.661.1Downloaded 22 Jul 2003 to 160.80.91.68. Redistribution subject totheir experimental reliability. These charge collection dis-
tances are intrinsic properties of the material used and de-
scribe well the contribution to the transport mechanism due
to each carrier type. From our data, it is evident that the
pumping process is confirmed to be much more effective on
hole conduction, lh being much greater than le in the
pumped state.7
In conclusion, electron and hole mean drift distance in
CVD polycrystalline diamond were carefully studied in the
as-grown and the so-called pumped state. Two different ex-
perimental setups were specifically designed and realized.
The tilt method was more accurate than the air measurement,
and allowed the device characterization in a wider range of
penetration depths. Electron and hole contributions to the
conduction mechanism were successfully separated and
evaluated by fitting the efficiency curves, obtained in both
device polarizations, in the as-grown and pumped states,
with a properly modified Hecht model. As a result, the
pumping process is confirmed to be much more effective on
hole conduction, lh being much greater than le in the
pumped state. Work is in progress to test such experimental
determinations of the physical transport properties of CVD
diamond with much more penetrating ionization sources,
such as high energy ions.
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Experimental determination of electron and hole mean drift distance: Application to chemical vapor deposition diamond

MARINELLI, MARCO

MILANI, ENRICO

VERONA RINATI, GIANLUCA

Pillon, M.

English

VERONA RINATI, G

Cineca Institutional Research Information System

Marco Marinelli

E. Milani

G. Pucella

A. Tucciarone

G. Verona-Rinati

M. Angelone

M. Pillon

Crossref

https://art.torvergata.it/retrieve/handle/2108/131483/270706/Apl%202003%20Experimental%20determination.pdf

Experimental determination of electron and hole mean drift distance: Application to chemical vapor deposition diamond

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