Optical spectroscopic study of n-type diamond
M. Nesladek, K. Meykens, K. Haenen, L. Stals, and T. Teraji*, S. Koizumi*
Materials Physics Division, Institute for Materials Research, Limburgs Universitair Centrum,
Universitaire Campus, Wetenschapspark 1,B-3590 Diepenbeek, Belgium.
*Research
Center for the Advanced Materials, National Institute for Research in Inorganic Materials (NIRIM) 1-1 Namiki,
Tsukuba 305, Japan.
Preparation of n-type diamond has been one of the challenges in the diamond field, not successful in
the past. Recently the group at NIRIM has succeeded in deposition of thin n-type P-doped CVD diamond films
[1,2]. Hall effect measurements confirmed n-type conductivity with the activation energy of the carrier
concentration of 0.40-0.60 eV [1,2]. Here, we present the first spectroscopic study of the phosphorus optical
activity in diamond, using the constant photocurrent method [3] (CPM) and an optical absorption technique photothermal deflection spectroscopy (PDS) [4]. Two new optically active defects are detected in the gap of Pdoped CVD diamond: the first at about 0.56 eV (denoted as X P1) and the second one at about 0.81 eV (denoted
as XP2) of the optical excitation energy. Additionally, at liquid-He temperature, a phonon-induced oscillatory
photoconductivity is observed, allowing to estimate the position of P-excited states.
The samples investigated were PH3-doped, 0.5-3 m thick CVD diamond films, deposited epitaxially
on polished {111} Ia and Ib diamond substrates. The sample A contained 2 part per million (ppm) phosphorus
and samples B and C about 50 ppm phosphorus (measured by SIMS). The electrical conductivity of the
sample A was below the detection limit of the van der Pauw - Hall effect measurement, but in the coplanar
contact configuration we could measure the activation energy of the dark conductivity which was 0.55-0.60
eV. The sample B showed mobility of 50 cm2V-1s-1 and the activation energy of the carrier concentration of
about 0.57 eV [2]. The conductivity of the sample C was below the detection limit of the van der Pauw
method. The samples were characterised by the PDS and CPM. CPM [3] is a common optical method allowing
to study the optical cross section  of defects (where N is the concentration of the impurity atoms
and  the optical absorption coefficient). Also, the optical excitation threshold EI (e.g. the energy position of a
defect within the forbidden optical gap) can be determined. The PDS and the quasi steady-state photocurrent
(7-13~Hz modulation) CPM set-ups are described in Refs. [4,5]. The CPM measurements are carried out at
liquid nitrogen (LNT), liquid helium (LHeT) and at room temperatures (RT). The PDS is measured only at RT.
3
optical cross section  (arb.units)
1
10
LNT
RT
2
10

XP2
0
1
10
10
experiment
theory
experiment
XP1 + XP2
(theory)
0
10

-1
XP1
10
0
-1
10
XP1
XP2
1
2
energy (eV)
3
0
1
2
energy (eV)
optical cross section  (arb.units)
CPM measurement
10
3
Fig. 1 Room temperature (RT) and liquid nitrogen temperature (LNT) optical cross section spectrum for the P-doped
epitaxial diamond (sample A). Dotted line shows a theoretical fit of the photoionization cross section.
The RT and LNT CPM spectra (Fig.1) reveal two new optically active defects. The first at about 0.6
eV (denoted as XP1) and the other one at about 0.8 eV (denoted as XP2). The LNT spectrum is sharper, less
broadened by acoustic phonons. Numerical fitting of the data by the Inkson formula for the spectral
dependence of the optical cross section [5,6], convoluted with a gaussian phonon-broadening term [6], yields
EXP1= 0.56 eV±0.03 and EXP2 = 0.81± 0.03 eV. The EXP1 value is in a very good agreement with the Hall effect
data. The CPM data show that in the sample B only the X P1 level is present. The samples A and C show both
the XP1 and the XP2 levels. Thus, the XP2 defect could act as a compensation level. The exact nature of the XP2
level (compensation, neutralisation complex-like defect, etc.) is currently studied.
-11
-11
1.0x10
 (arbitrary units)
photocurrent (arb. units)
1.5x10
-12
5.0x10
0.0
0.5
0.6
8
0.5
0.6
0.7 energy (eV)
6
4
0.7
0.8
0.9
energy (eV)
Fig. 2 Liquid helium temperature (LHeT) photocurrent spectra for the B sample. The arrows indicate minima in
photoconductivity, separated by a distance of about 165 meV. The inset shows the PDS optical absorption spectrum.
In Fig. 2, the LHeT photocurrent spectrum is plotted on the linear scale for the sample B. We can
clearly observe distinct minima in the photocurrent. In literature, photocurrent minima have been observed for
boron-doped diamond (IIb) and attributed to an oscillatory photoconductivity, due to a thermalisation of photoexcited holes by emissions of LO phonons [7]. A fine inspection of the spectra suggests at least two sets of
equidistant minima. The energy difference between the minima agrees well with the LO phonon energy in
diamond (165 meV). The positions of the collection levels (usually excited defect states) can be calculated by
subtracting the LO phonon energy from the lowest observed minimum of the set. This suggests for the
position of the first and the second excited state about 0.51-0.52 eV and 0.55-56 eV. The inset of Fig. 2 shows
the RT-PDS optical absorption spectrum. Two sharp absorption features in the spectrum are approximately at
the positions, calculated from the lowest photoconductivity minima.
To conclude, the spectroscopic study of the n-type diamond films revealed two new optically active
defect levels. The position of the first level at about 0.56 eV is in a good agreement with the Hall measurement.
The origin of the second level at 0.81 eV, which might act as a compensation level is unknown. Additionally,
observed LT oscillatory photoconductivity suggests the positions of the P-excited states.
Acknowledgement
This work was supported by the Belgian FWO research Program No G.0014.96. The authors would like to thank Dr.
Etienne Gheeraert from LAPES-CNRS in Grenoble (currently at NIRIM) for fruitful discussions.
References
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