Prospective n-type impurities and methods of diamond doping*

ELSEVIER Diamond and Related Materials 4 L 1995 I 1302- 1310
Abstract
Review
Prospective n-type impurities and methods of diamond doping*
G. Popovici a.b, M.A. Prelas
" Rockford Dianlond Technology, 501 S. Sixth Srreer. Ckattlpaign, IL 61820-5579, USA
Nirclenr Engineering Department, Universit!. of Missoilri-Collrn~birr, Colu~nbia. MO 6521 1. USA
Received 6 January 1995; accepted in final form 3 August 1995
A major seal of diamond thin film technology research has been the reproducible production of p-n junctions. which are the
basic units of many electronic devices. While p-type conductivity is relatively easily attained by boron doping, n-type conductivity
has proved much harder to achieve. The experimental and theoretical results on prospective donor impurities are reviewed. In
analogy with classical semiconductors, we will discuss the possibility of obtaining n-type diamond by using substitutional impurity
atoms (nitrogen and phosphorus) and interstitial atoms (Li and Na).
New methods of forced diffusion and ion assisted doping during growth are discussed. Methods of forced introduction of
impurities into the diamond lattice have an important advantage over traditional ion implantation methods. Ion implantation
introduces structural defects (vacancies, vacancy +interstitial, and their combinations) that are difficult to cure. Both methods,
forced diffusion and ion assisted doping during growth, introduce no additional structural defects, except that inherent to the
impurity itself.
Ker.n.ords. Diamond; n-type doping; Na, Li, P, N impurities in diamond
1. Introduction
Diamond films produced by chemical vapor depos-
itlon (CVD) have a wide array of potential applications.
With the advent of new technologies in this area, dia-
mond-based-electronics is becoming a reality. Diamond
(due to its wide band gap, as well as its resistance to
chemical and radiation damage) makes it a prime candi-
date for applications not possible with silicon technol-
ogy. This could include high temperature applications,
electronics for outer space, and instrumentation for
reactors.
However, the development of active electronic devices
technology requires solving the problem of reproducible
doping by donor and acceptor impurities to obtain a
p-n junction. It is well known that p-type conductivity
can be obtained by boron doping. The results reported
so far on n-type doping are controversial [I].
In this paper the experimental and theoretical results
on prospective donor impurities (N, P, Li, and Na) in
* Paper presented at the International Symposium on Diamond
Films. Minsk. Belarus, 4-6 May 1994.
Elseclcr Sclence S.A.
SSDl 0925-96?5( 95 )00319-.;
the diamond lattice are reviewed. New methods of forced
diffusion and ion assisted doping during growth are
discussed.
2. Hydrogen-like model of substitutional impurities in Si
and diamond
In classical semiconductors, n- and p-type doping is
usually attained by substituting a fraction from the host
lattice atoms with impurity atoms of neighboring groups
of the periodic table of elements. Elements of the group
111 are used as acceptors and those of group V as donors
to dope Si and Ge. The behavior of group I11 and V
atoms in the Si lattice is well described by the simple
theoretical model of hydrogen-like impurities.
According to the model, a donor atom in the lattice
is similar to a hydrogen atom; it is a positive ion,
keeping, by Coulomb force. its extra electron, which is
not used for covalent bonding by the host lattice. The
strength of the interaction between the electron and the
ion is determined by two factors: the charge carrier
effective mass and the screening ability of the host lattice.

This model takes into account only the properties of the
host lattice. There is a tacit assumption that the impurity
atom is not much different from the host lattice atom,
dimensionally and/or energetically. However, some
properties of the impurity itself, like the bonding energy
between the impurity nucleus and its electrons, are
sometimes quite important. The bonding energy of
electrons to the impurity atom is strong in atoms with
a small atomic number, in which the outer electrons are
weakly screened from their nuclei. This is the reason
why the hydrogen-like model does not work for the
nitrogen impurity in silicon. Nitrogen does not form a
shallow donor in the Si band gap. The energy of its
level. as observed experimentally, is l9OmeV [2-31,
whereas the hydrogen-like model gives AE=25 meV
[4]. Indium also does not form a shallow level in the
Si band gap (AE=0.16 eV [4]), which is, however, due
to other reasons. The In atom is big (covalent radius
I-= 1.62 A compared with I-= 1.18 A for Si) [5]. Thus,
the distortion of the Si host lattice becomes quite important
in the energy balance.
Diamond is unique in its electronic structure. Its four
outer electrons are bonded much more strongly to the
nucleus than in Si. The hydrogen-like model predicts an
acceptor activation energy AE, = 0.36 eV (E/E = 5.7 and
m,/rn,=0.75) for the group I11 substitutional impurities
[6]. and for group V elements a donor activation energy
AE,=0.19 eV (m,jm,=0.48) [6-81. The covalent radius
of a carbon atom in diamond is small. 0.77 A. Only
boron (I-=0.80 A) and nitrogen (r.= 0.6 A) have covalent
radii small enough to enter the diamond lattice without
distorting it. The boron acceptor level has an ionization
energy of about 0.37 eV. close to that predicted by the
hydrogen-like model. Aluminum. however. is too big an
atom (I-= 1.43 A) and is not an active center in diamond,
neither electric. nor optical. The behavior of nitrogen
and phosphorus in the diamond lattice is discussed in
the next section.
3. Substitutional impurities in diamond
Nitrogen can enter in substitutional positions at con-
centrations up to lOI9 cm-"93. The energy level of the
nitrogen substitutional impurity in the diamond lattice
was calculated by different methods with similar results
[ 10-121. Briddon et HI. [ 101 used an 86 atom cluster
with the central carbon atom replaced by nitrogen. They
found that the resulting electronic structure consists of
t\\-o levels: a level 1.3 eV above the valence band and a
level 0.5 eV below the conduction band. A trigonal
distortion of 78% was computed.
Bernholc and co-workers [ I I. 121 advanced another
model. In the substitutional position three nitsopen
electrons are bound to carbon atoms. Two remaining
electrons form a lone pair bound to the fourth carbon
atom. There is no loosely bound electron to form a
hydrogen-like shallow level. A distortion of the diamond
lattice of 25% was computed along the bond correspond-
ing to the lone pair, due to the charge repulsion of the
lone pair and the carbon electron. The energy level of
this state was found to be 1.5 eV. The experimental value
of the nitrogen energy level is 1.7-2.1 eV below the
bottom of the conduction band [ 13,141.
The single-substitutional nitrogen is a very effective
recombination center for electron-hole pairs. The intrin-
sic photocurrent generated by far-UV photons decreases
dramatically with increasing nitrogen concentration
[15,16]. The deep nitrogen donor level is an effective
compensator of acceptor states. reducing by six orders
of magnitude the conductivity of diamond films at room
temperature [17]. Therefore, nitrogen is a poisoning
impurity for p-type conductive diamond layers.
3.2. Phosphorus
Different methods used to calculate for the position
of the donor level of a substitutional phosphorus impu-
rity in the diamond band gap gave different results.
Studies by Bernholc and co-workers [11,12], using
pseudopotentials and a plane wave basis for impurities.
embedded in a periodically repeated supercell, found the
activation energy of phosphorus impurity centers in
substitutional sites to be 0.20 eV. However, the equilib-
rium solubility of phosphorus was predicted to be very
low, even at high temperatures. In contrast. a self consis-
tent local density approximation cluster calculation on
C:N and C:P impurities [18] determined the donor
levels in both systems to be deep levels. lying well below
the conduction-like states. Activation energies of 1.09 eV
for phosphorus and 0.9 eV for nitrogen impurities were
found.
The experimental results on phosphorus doping are
also contradictory. There have been unsuccessful
attempts to dope synthetic HTHP diamond with phos-
phorus [19]. In this case no effect on electrical conduc-
tivity was found and no phosphorus was detected in the
diamonds by secondary-ion mass spectroscopy (SIMS).
There exist in the literature claims of the successful
doping during CVD growth of diamond by phosphorus.
forming a shallow level [20]. However. a deep level
with an activation energy of 0.84-1.16eV was also
obtained by phosphorus doping during CVD growth
C21.223. Such an energy was predicted theoretically by
Jackson et al. [18]. Another possible explanation may
exist for obtaining a deep level due to phosphorus
doping. To explain the results of electron spin resonance
(ESR) on the phosphorus impurity in diamond. Toki?
et al. [33] calculated a 39 atom carbon cluster with the
central atom replaced by phosphorus. This rnodel did

not give a sa~isfactory explanation of the experimental
results. The iil~thors did. however. find a model which
explained the esperimental ESR results. In that model
the paramagnetic phosphorus is the second nearest
neighbor of a vacancy placed in the center of the cluster.
This result suggests that phosphorus does not enter in
singly dispersed form, but forms impurity-vacancy complexes.
which should have an activation energy different
from that of singly substitutional phosphorus. The formation
of a phosphorus-vacancy complex in order to
accommodate the large phosphorus atom may sometimes
be preferable comparing with the substitutional
position of phosphorus in the diamond lattice.
It seems that recently Prins [24] found a method to
put phosphorus in substitutional positions obtaining
n-type conductivity by implanting phosphorus into highpurity
natural. type IIa, diamond. Compared with the
doses traditionally implanted, very low doses of phosphorus
ions (P+ ~3.55 x 1010cm-2) were used for the
cold implantation rapid annealing process. This method
permitted a substantial reduction in the number of
defects created during in~plantation. The thermoelectric
(hot probe) measurements showed n-type conductivity.
An activation energy of electrical conductivity of
0.2-0.21 eV was measured.
4. Interstitial impurities
Impurity atoms can enter interstitla] sites of the dia-
mond lattice in the tetrahedral or hexagonal voids of
the lattice. It was shown theoretically [25] that for a
small ion in the diamond lattice, for which the repulsive
energy is small, the hexagonal site is an equilibrium site
and the tetrahedral site is a saddle point. For large ions,
the repulsive energy will dominate the picture, and the
tetrahedral site will be the equilibrium position.
The most promising interstitial dopants for diamond
are lithium and sodium. The ionic radius of Li is
0.060 nm and that of Na is 0.095 nm; their atomic radii,
however, are quite large: r(Li)=1.52 A and r(Na)=
1.66 A). If they diffuse in the neutral form through the
diamond lattice, the lattice distortions will be strong.
The energies of Li and Na impurities in diamond in
interstitial and substitutional positions have been calcu-
lated [I 13. As expected, the interstitial sites were found
to be energetically favored. The tetrahedral sites are
preferable for both Li and Na, because even the lithium
ion in an interstitial site is not small enough for the
diamond lattice. Activation energies of 0.1 eV and 0.3 eV
below the conduction band minimum have been com-
puted for Li and Na respectively. The solubility of these
dopants is expected to be low. The activation energy of
diffusion for Li and Na impurities has been predicted to
be 0.85 eV and 1.4 eV respectively [ 11,12 3. The mob~lity
of Li in the diamond lattice is predicted to be high [12].
Experiments on Li and Nil doping have not continned
the theoretical results. Lithium doping has been tried
during gro~vth [26]. using diffusion [?7 ] or ion impl~intation
[28]. &one of these methods have worked.
Lithium fluoride was used as an in situ doping source
[26] for homoepitaxial films grown by r.f. plasma discharge.
SIMS analysis showed that Li can enter up to
about lo2' cm-3 in the most heavily doped san~ples.
The films obtained had p-type conductivity and an
activation energy of 0.24eV. The conclusion of this
experiment was that Li is electrically inactive in diamond
and that the observed results were due to a boron
contamination (the growth system had also been used
for boron doping). However, there might be another
explanation. Elements of the seventh group of the periodical
table entering interstitial sites should form shallow
acceptor levels in diamond. So fluorine from the dopant
source ( LiF) might be responsible for p-type conductivity.
Another explanation of the failure might be that the
Li level is deeper than predicted and it diffuses as a
neutral atom. In the last case. the lattice distortions
would be strong and could introduce compensating
acceptor impurities. This effect is discussed later.
The diffusion of lithium into diamond from the vapor
phase has been studied at temperatures between 400 and
900 "C [27]. The presence of lithium was qualitatively
determined by SIMS. However, the change of electrical
conductivity due to the presence of lithium was small.
only two orders of magnitude, from 10-" to
R-' cm-'. The authors advanced the hypothesis of the
existence of acceptor-like states close to the valenceband
edge extending 1-2 eV into the band gap. A high
enough density of such states might be responsible for
the compensation of lithium donors. The nature of these
states is not known. The states might be formed due to
impurities, defects or localized valence bond anomalies
such as carbon-carbon double bonds. No comprehensive
research has been done so far to verify these hypotheses.
In summary, these experiments show that in many cases
the influence on electrical conductivity of defects introduced
in the diamond lattice during doping is stronger
than that of the impurity itself.
Na impurity also does not behave as predicted by
theory [ll]. Jamison et al. 1291 showed that Na is a
p-type dopant in diamond and forms a shallow level
(about 0.09 eV). These results are discussed later.
5. Forced doping methods
Methods of forcing impurities into the diamond lattice
have an important advantage over ion implantation.
which has been widely used for diamond doping [28].
The traditional ion implantation introduces structural
defects (vacancies, vacancy + interstitial, and their com bi-
nations) which are difficult to cure [30]. No additional

1308 G. Popovici, M. A. Prelas/Diamond and ' Relared Marerials 4 ( 1995) 1305-1310
structural defects are introduced by forced diffusion or
ion assisted doping during growth.
5.1. Elecrric field assisted difltision
The Debye temperature of diamond is about 2200 K,
the highest of all known solids, reflecting high structural
rigidity of the diamond lattice [31]. For comparison,
the Debye temperature of silicon is 635 K. At temperatures
lower than the Debye temperature, atoms oscillate
with amplitudes small compared with the interatomic
distance.
The diffusion of substitutional impurities is controlled
by the vacancy mechanism. The energy of formation for
vacancies in diamond is high, 7.2 eV (for Si it is 4.2 eV)
[32]. Hence, the temperature that would be necessary
for effective diffusion of substitutional impurities in
diamond is expected to be higher than the maximum
temperature which can be used for diffusion, which is
the temperature at which the graphitization of high
quality diamond begins (1900 K). Graphitization of
diamond films starts at lower temperatures. However,
promising results were obtained by using solid BN
sources for diffusion of boron and nitrogen into natural
diamond [33]. Rapid thermal processing ( 1400 "C for
30 s in argon) was used. SIMS profiles indicated a boron
concentration of about 1019 to a depth of 500 A.
A diffusion coefficient of lo-" cm2 s-' at 1400 "C was
determined.
It is known that fast diffusing of interstitial impurities,
such as Cu, Li, Na, can be introduced in silicon and
germanium lattices at relatively low temperatures
(200-400 "C for Si, which is much less than the diffusion
temperature for substitutional impurities of
800-1100 "C), if electric field assisted diffusion is used
[34,35]. However, field enhanced diffusion in diamond
has not, to our knowledge, been tried. We propose to
apply an electric field to enhance the diffusion of Li in
diamond thin films. If it is true that the predicted
ionization energy of the Li level is small (0.1 eV) [11,12],
Li atoms will be totally ionized at 1160 K, and the field
enhanced diffusion of Li should be effective.
We studied the diffusion of lithium, chlorine and
oxygen under electrical bias [36]. High quality free
standing diamond films, about 230 pm thick and polished
on both sides were used. The average crystallite
size was of the order of tens of micrometers. All samples
were grown under the same conditions. The Raman
spectra of the samples showed the diamond line
(1332cm-') only, and no graphite and amorphous
carbon lines. The cathodoluminescence spectra showed
a strong free exciton line which is a feature of good
crystalline quality [37]. The films were mounted on a
graphite base with an embedded tungsten heater. The
base temperature was monitored with a chromel-alumel
thermocouple. The dopant sources Li,CO, and LiCIO,
were of analytical purity (99%). The diffusion was
performed for 190 min at 1000 "C in an argon atmosphere.
An electric field of 200 V was applied to the
sample. The control sample had no electric field applied.
Large amounts of impurity (about 4 x 1019 ~ m - were ~ )
found to diffuse at relatively low temperatures. In con-
trast, the bias was found not to affect the concentration
of the diffused impurities. The last point can be explained
by the high diffusion coefficient through the grain bound-
aries and other defects inherent to polycrystalline films.
As the ionization energy of doping atoms in association
with other defects of the lattice must be different, usually
larger, to the energy level of the same impurity in the
singly dispersed form, the diffusion through the defects
on the grain boundaries was probably due to the neutral
atoms and was not influenced by electric field. The
diffusion experiments on Li doping in a type IIa natural
diamond, using nearly the same conditions, showed that
the Li content is two to three orders of magnitude
smaller in the single crystalline diamond than in the
polycrystalline films [38].
The diffusion in polycrystalline films under bias using
a Li salt, LiClO,, lead to a large change in conductivity
(about eight orders of magnitude). An activation energy
of about 0.25 eV was derived from the temperature
dependence of the conductivity [39]. The samples
showed p-type conductivity, as measured by conven-
tional hot probe. The hot probe measurements under
bias showed n- or p-type conductivity depending on the
applied bias. A hypothesis was advanced, that a p-type
surface inversion layer was formed on the diffused n-type
layer [40]. More research is needed to clarify this
problem.
The Hall effect measurements on the same polycrystal-
line films diffused under bias were carried out by two
independent groups. The measurements showed an
n-type conductivity with electron concentration about
10'' cm-3 and a sheet resistance about lo5 R!U r37.393.
Supposing the thickness of the diffused layer to be of
the order of 2 pm, the mobility of electrons could be
estimated: 50cm2 V-' s-'. We do not know which
impurity (or association of impurities) the n-type conduc-
tivity is due to in this case. More research is needed to
clarify this problem.
5.2. Ion assisted doping during growth
The basic difficulty with conventional doping during
growth is with the incorporation of dopant atoms into
the growing film. Many dopants are more likely to
re-evaporate rather than be incorporated into a growing
film surface. as their bonding energy is small compared
with the C-C bond energy (see Table I of Ref. 6). Even
if a dopant atom is incorporated into the sro\i.lng tilm.
there may also exist driving forces due to the strain
energy. related to peometncal factors. which ma cause

the dopanl atoms to segregate to the surface and eventually
re-evaporate.
Large improvements in the incorporation of In and
Sn in Si and GaAs grown by molecular bean1 epitaxy
have been demonstrated [41] using low energy
(60-300 eV) ionized dopant atoms, which were directed
onto the film during deposition. Ion assisted doping also
gave good results in obtaining p-type conductivity in
CdTe using As and P ions as dopants [42-441. The
ionized dopant atoms are implanted at a small depth
and have a much larger sticking coefficient than dopant
atoms in thermal equiIibrium with the growing surface;
hence. they are more likely to be incorporated into the
growing film.
The method of doping during growth was tried on
diamond by Jamison et a]. [29] with sodium, rubidium
and phosphorus. and proved to be efficient for embedding
the impurities into the growing diamond layer.
SIMS measurements showed that phosphorus entered
up to 5 x 10" C~I-~, while sodium and rubidium entered
up to 1020cm-3. Highly conductive p-type layers were
obtained by sodium doping. Energies of activation of 44
and 82 meV were measured for sodium.
The experiments on the influence of deformations on
diamond conductivity might be an explanation of p-type
conductivity in diamond caused by big atoms. The
investigation of electrical properties of deformed natural
diamond [45-471 has shown that the dislocations
change its electrical, optical and photoelectrical properties.
An insulating natural diamond of Ila type with
I initial electrical resistivity of 1016 i2 cm, after deformation
I resembled the semiconducting diamond IIb in terms of
I resistivity values. The resistivity varied between 1013 and
I lo2 R cm due to the plastic deformation. If the number
I
of dislocations is large (about 109-1010 cm-'), the electri-
I
I cal resistivity can become as low as 102 Rcrn. This
I change of resistivity was obtained without introducing
1 .
I any impurities in the diamond lattice. The activation
I
I
I
I
I
energy of electrically active centers produced by deformation
was 0.26-0.29eV. The conductivity was of
p-type. The results of this experiment suggest that the
introduction in the diamond lattice of prospective donor
I impurities with large atomic radii can produce not only
n-type centers, due to the impurity itself, but also com-
1
pensating p-type centers due to lattice deformations. As
I sodium is too large an atom (atomic radius r= 1.66 A,
I ionic radius r' =0.95 A) [5 1, the structural defects of
I the lattice due to the introduction of Na could be too
) large and might dominate the transport properties.
I 6. Conclusions
I
I
1
Shallow energy level n-type diamond doping by prospective
donor impurities N, P, and Na has been unsuccessful
(except for the recent work of Prins on
phosphorus doping using the cold in~plantat~on r~~pid
annealing process). There are vanous explanatlonb for
this behavior. The energy structure of the band gap of
diamond films, grown in different conditions, is not
known yet. The existence of considerable densities of
acceptor-like states close to the valence-band edge and
extending 1-2 eV into the band gap might be responsible
for the compensation of donors. The nature of these
states is not presently known. These states may be due
to impurities. and/or structural defects. Another hypothesis
is that high densities of structural defects introduced
in the lattice by large impurity atoms. might form a
relatively shallow acceptor level (about 0.3 eV) and
dominate the transport properties, giving p-type conductivity.
No exhaustive experiments have been done yet to
verify these hypotheses. However. the conclusion should
be drawn that the route to successful diamond doping
is in finding methods that permit a reduction of number
of defects created during the doping process.
Methods of forced doping are effective for introduction
of large amounts of dopants into the diamond lattice.
Forced diffusion of impurities from Li salts enabled
n-type conductivity to be obtained in high quality diamond
polycrystalline films. The measurements showed
an electron concentration of about 1015 ~ m - a ~ sheet ,
resistance of about lo5 Q/0 [36,37] with the estimated
electron mobility of 50 cm2 V-' s-'.
Acknowledgments
The authors would like to express many thanks to
Professor Peter J. Gjelisse of Florida State University
for the critical reading of the manuscript.
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