Prospective n-type impurities and methods of diamond doping*
Prospective n-type impurities and methods of diamond doping*
Prospective n-type impurities and methods of diamond doping*
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ELSEVIER Diamond <strong>and</strong> Related Materials 4 L 1995 I 1302- 1310<br />
Abstract<br />
Review<br />
<strong>Prospective</strong> n-<strong>type</strong> <strong>impurities</strong> <strong>and</strong> <strong>methods</strong> <strong>of</strong> <strong>diamond</strong> <strong>doping*</strong><br />
G. Popovici a.b, M.A. Prelas<br />
" Rockford Dianlond Technology, 501 S. Sixth Srreer. Ckattlpaign, IL 61820-5579, USA<br />
Nirclenr Engineering Department, Universit!. <strong>of</strong> Missoilri-Collrn~birr, Colu~nbia. MO 6521 1. USA<br />
Received 6 January 1995; accepted in final form 3 August 1995<br />
A major seal <strong>of</strong> <strong>diamond</strong> thin film technology research has been the reproducible production <strong>of</strong> p-n junctions. which are the<br />
basic units <strong>of</strong> many electronic devices. While p-<strong>type</strong> conductivity is relatively easily attained by boron doping, n-<strong>type</strong> conductivity<br />
has proved much harder to achieve. The experimental <strong>and</strong> theoretical results on prospective donor <strong>impurities</strong> are reviewed. In<br />
analogy with classical semiconductors, we will discuss the possibility <strong>of</strong> obtaining n-<strong>type</strong> <strong>diamond</strong> by using substitutional impurity<br />
atoms (nitrogen <strong>and</strong> phosphorus) <strong>and</strong> interstitial atoms (Li <strong>and</strong> Na).<br />
New <strong>methods</strong> <strong>of</strong> forced diffusion <strong>and</strong> ion assisted doping during growth are discussed. Methods <strong>of</strong> forced introduction <strong>of</strong><br />
<strong>impurities</strong> into the <strong>diamond</strong> lattice have an important advantage over traditional ion implantation <strong>methods</strong>. Ion implantation<br />
introduces structural defects (vacancies, vacancy +interstitial, <strong>and</strong> their combinations) that are difficult to cure. Both <strong>methods</strong>,<br />
forced diffusion <strong>and</strong> ion assisted doping during growth, introduce no additional structural defects, except that inherent to the<br />
impurity itself.<br />
Ker.n.ords. Diamond; n-<strong>type</strong> doping; Na, Li, P, N <strong>impurities</strong> in <strong>diamond</strong><br />
1. Introduction<br />
Diamond films produced by chemical vapor depos-<br />
itlon (CVD) have a wide array <strong>of</strong> potential applications.<br />
With the advent <strong>of</strong> new technologies in this area, dia-<br />
mond-based-electronics is becoming a reality. Diamond<br />
(due to its wide b<strong>and</strong> gap, as well as its resistance to<br />
chemical <strong>and</strong> radiation damage) makes it a prime c<strong>and</strong>i-<br />
date for applications not possible with silicon technol-<br />
ogy. This could include high temperature applications,<br />
electronics for outer space, <strong>and</strong> instrumentation for<br />
reactors.<br />
However, the development <strong>of</strong> active electronic devices<br />
technology requires solving the problem <strong>of</strong> reproducible<br />
doping by donor <strong>and</strong> acceptor <strong>impurities</strong> to obtain a<br />
p-n junction. It is well known that p-<strong>type</strong> conductivity<br />
can be obtained by boron doping. The results reported<br />
so far on n-<strong>type</strong> doping are controversial [I].<br />
In this paper the experimental <strong>and</strong> theoretical results<br />
on prospective donor <strong>impurities</strong> (N, P, Li, <strong>and</strong> Na) in<br />
* Paper presented at the International Symposium on Diamond<br />
Films. Minsk. Belarus, 4-6 May 1994.<br />
Elseclcr Sclence S.A.<br />
SSDl 0925-96?5( 95 )00319-.;<br />
the <strong>diamond</strong> lattice are reviewed. New <strong>methods</strong> <strong>of</strong> forced<br />
diffusion <strong>and</strong> ion assisted doping during growth are<br />
discussed.<br />
2. Hydrogen-like model <strong>of</strong> substitutional <strong>impurities</strong> in Si<br />
<strong>and</strong> <strong>diamond</strong><br />
In classical semiconductors, n- <strong>and</strong> p-<strong>type</strong> doping is<br />
usually attained by substituting a fraction from the host<br />
lattice atoms with impurity atoms <strong>of</strong> neighboring groups<br />
<strong>of</strong> the periodic table <strong>of</strong> elements. Elements <strong>of</strong> the group<br />
111 are used as acceptors <strong>and</strong> those <strong>of</strong> group V as donors<br />
to dope Si <strong>and</strong> Ge. The behavior <strong>of</strong> group I11 <strong>and</strong> V<br />
atoms in the Si lattice is well described by the simple<br />
theoretical model <strong>of</strong> hydrogen-like <strong>impurities</strong>.<br />
According to the model, a donor atom in the lattice<br />
is similar to a hydrogen atom; it is a positive ion,<br />
keeping, by Coulomb force. its extra electron, which is<br />
not used for covalent bonding by the host lattice. The<br />
strength <strong>of</strong> the interaction between the electron <strong>and</strong> the<br />
ion is determined by two factors: the charge carrier<br />
effective mass <strong>and</strong> the screening ability <strong>of</strong> the host lattice.
This model takes into account only the properties <strong>of</strong> the<br />
host lattice. There is a tacit assumption that the impurity<br />
atom is not much different from the host lattice atom,<br />
dimensionally <strong>and</strong>/or energetically. However, some<br />
properties <strong>of</strong> the impurity itself, like the bonding energy<br />
between the impurity nucleus <strong>and</strong> its electrons, are<br />
sometimes quite important. The bonding energy <strong>of</strong><br />
electrons to the impurity atom is strong in atoms with<br />
a small atomic number, in which the outer electrons are<br />
weakly screened from their nuclei. This is the reason<br />
why the hydrogen-like model does not work for the<br />
nitrogen impurity in silicon. Nitrogen does not form a<br />
shallow donor in the Si b<strong>and</strong> gap. The energy <strong>of</strong> its<br />
level. as observed experimentally, is l9OmeV [2-31,<br />
whereas the hydrogen-like model gives AE=25 meV<br />
[4]. Indium also does not form a shallow level in the<br />
Si b<strong>and</strong> gap (AE=0.16 eV [4]), which is, however, due<br />
to other reasons. The In atom is big (covalent radius<br />
I-= 1.62 A compared with I-= 1.18 A for Si) [5]. Thus,<br />
the distortion <strong>of</strong> the Si host lattice becomes quite important<br />
in the energy balance.<br />
Diamond is unique in its electronic structure. Its four<br />
outer electrons are bonded much more strongly to the<br />
nucleus than in Si. The hydrogen-like model predicts an<br />
acceptor activation energy AE, = 0.36 eV (E/E = 5.7 <strong>and</strong><br />
m,/rn,=0.75) for the group I11 substitutional <strong>impurities</strong><br />
[6]. <strong>and</strong> for group V elements a donor activation energy<br />
AE,=0.19 eV (m,jm,=0.48) [6-81. The covalent radius<br />
<strong>of</strong> a carbon atom in <strong>diamond</strong> is small. 0.77 A. Only<br />
boron (I-=0.80 A) <strong>and</strong> nitrogen (r.= 0.6 A) have covalent<br />
radii small enough to enter the <strong>diamond</strong> lattice without<br />
distorting it. The boron acceptor level has an ionization<br />
energy <strong>of</strong> about 0.37 eV. close to that predicted by the<br />
hydrogen-like model. Aluminum. however. is too big an<br />
atom (I-= 1.43 A) <strong>and</strong> is not an active center in <strong>diamond</strong>,<br />
neither electric. nor optical. The behavior <strong>of</strong> nitrogen<br />
<strong>and</strong> phosphorus in the <strong>diamond</strong> lattice is discussed in<br />
the next section.<br />
3. Substitutional <strong>impurities</strong> in <strong>diamond</strong><br />
Nitrogen can enter in substitutional positions at con-<br />
centrations up to lOI9 cm-"93. The energy level <strong>of</strong> the<br />
nitrogen substitutional impurity in the <strong>diamond</strong> lattice<br />
was calculated by different <strong>methods</strong> with similar results<br />
[ 10-121. Briddon et HI. [ 101 used an 86 atom cluster<br />
with the central carbon atom replaced by nitrogen. They<br />
found that the resulting electronic structure consists <strong>of</strong><br />
t\\-o levels: a level 1.3 eV above the valence b<strong>and</strong> <strong>and</strong> a<br />
level 0.5 eV below the conduction b<strong>and</strong>. A trigonal<br />
distortion <strong>of</strong> 78% was computed.<br />
Bernholc <strong>and</strong> co-workers [ I I. 121 advanced another<br />
model. In the substitutional position three nitsopen<br />
electrons are bound to carbon atoms. Two remaining<br />
electrons form a lone pair bound to the fourth carbon<br />
atom. There is no loosely bound electron to form a<br />
hydrogen-like shallow level. A distortion <strong>of</strong> the <strong>diamond</strong><br />
lattice <strong>of</strong> 25% was computed along the bond correspond-<br />
ing to the lone pair, due to the charge repulsion <strong>of</strong> the<br />
lone pair <strong>and</strong> the carbon electron. The energy level <strong>of</strong><br />
this state was found to be 1.5 eV. The experimental value<br />
<strong>of</strong> the nitrogen energy level is 1.7-2.1 eV below the<br />
bottom <strong>of</strong> the conduction b<strong>and</strong> [ 13,141.<br />
The single-substitutional nitrogen is a very effective<br />
recombination center for electron-hole pairs. The intrin-<br />
sic photocurrent generated by far-UV photons decreases<br />
dramatically with increasing nitrogen concentration<br />
[15,16]. The deep nitrogen donor level is an effective<br />
compensator <strong>of</strong> acceptor states. reducing by six orders<br />
<strong>of</strong> magnitude the conductivity <strong>of</strong> <strong>diamond</strong> films at room<br />
temperature [17]. Therefore, nitrogen is a poisoning<br />
impurity for p-<strong>type</strong> conductive <strong>diamond</strong> layers.<br />
3.2. Phosphorus<br />
Different <strong>methods</strong> used to calculate for the position<br />
<strong>of</strong> the donor level <strong>of</strong> a substitutional phosphorus impu-<br />
rity in the <strong>diamond</strong> b<strong>and</strong> gap gave different results.<br />
Studies by Bernholc <strong>and</strong> co-workers [11,12], using<br />
pseudopotentials <strong>and</strong> a plane wave basis for <strong>impurities</strong>.<br />
embedded in a periodically repeated supercell, found the<br />
activation energy <strong>of</strong> phosphorus impurity centers in<br />
substitutional sites to be 0.20 eV. However, the equilib-<br />
rium solubility <strong>of</strong> phosphorus was predicted to be very<br />
low, even at high temperatures. In contrast. a self consis-<br />
tent local density approximation cluster calculation on<br />
C:N <strong>and</strong> C:P <strong>impurities</strong> [18] determined the donor<br />
levels in both systems to be deep levels. lying well below<br />
the conduction-like states. Activation energies <strong>of</strong> 1.09 eV<br />
for phosphorus <strong>and</strong> 0.9 eV for nitrogen <strong>impurities</strong> were<br />
found.<br />
The experimental results on phosphorus doping are<br />
also contradictory. There have been unsuccessful<br />
attempts to dope synthetic HTHP <strong>diamond</strong> with phos-<br />
phorus [19]. In this case no effect on electrical conduc-<br />
tivity was found <strong>and</strong> no phosphorus was detected in the<br />
<strong>diamond</strong>s by secondary-ion mass spectroscopy (SIMS).<br />
There exist in the literature claims <strong>of</strong> the successful<br />
doping during CVD growth <strong>of</strong> <strong>diamond</strong> by phosphorus.<br />
forming a shallow level [20]. However. a deep level<br />
with an activation energy <strong>of</strong> 0.84-1.16eV was also<br />
obtained by phosphorus doping during CVD growth<br />
C21.223. Such an energy was predicted theoretically by<br />
Jackson et al. [18]. Another possible explanation may<br />
exist for obtaining a deep level due to phosphorus<br />
doping. To explain the results <strong>of</strong> electron spin resonance<br />
(ESR) on the phosphorus impurity in <strong>diamond</strong>. Toki?<br />
et al. [33] calculated a 39 atom carbon cluster with the<br />
central atom replaced by phosphorus. This rnodel did
not give a sa~isfactory explanation <strong>of</strong> the experimental<br />
results. The iil~thors did. however. find a model which<br />
explained the esperimental ESR results. In that model<br />
the paramagnetic phosphorus is the second nearest<br />
neighbor <strong>of</strong> a vacancy placed in the center <strong>of</strong> the cluster.<br />
This result suggests that phosphorus does not enter in<br />
singly dispersed form, but forms impurity-vacancy complexes.<br />
which should have an activation energy different<br />
from that <strong>of</strong> singly substitutional phosphorus. The formation<br />
<strong>of</strong> a phosphorus-vacancy complex in order to<br />
accommodate the large phosphorus atom may sometimes<br />
be preferable comparing with the substitutional<br />
position <strong>of</strong> phosphorus in the <strong>diamond</strong> lattice.<br />
It seems that recently Prins [24] found a method to<br />
put phosphorus in substitutional positions obtaining<br />
n-<strong>type</strong> conductivity by implanting phosphorus into highpurity<br />
natural. <strong>type</strong> IIa, <strong>diamond</strong>. Compared with the<br />
doses traditionally implanted, very low doses <strong>of</strong> phosphorus<br />
ions (P+ ~3.55 x 1010cm-2) were used for the<br />
cold implantation rapid annealing process. This method<br />
permitted a substantial reduction in the number <strong>of</strong><br />
defects created during in~plantation. The thermoelectric<br />
(hot probe) measurements showed n-<strong>type</strong> conductivity.<br />
An activation energy <strong>of</strong> electrical conductivity <strong>of</strong><br />
0.2-0.21 eV was measured.<br />
4. Interstitial <strong>impurities</strong><br />
Impurity atoms can enter interstitla] sites <strong>of</strong> the dia-<br />
mond lattice in the tetrahedral or hexagonal voids <strong>of</strong><br />
the lattice. It was shown theoretically [25] that for a<br />
small ion in the <strong>diamond</strong> lattice, for which the repulsive<br />
energy is small, the hexagonal site is an equilibrium site<br />
<strong>and</strong> the tetrahedral site is a saddle point. For large ions,<br />
the repulsive energy will dominate the picture, <strong>and</strong> the<br />
tetrahedral site will be the equilibrium position.<br />
The most promising interstitial dopants for <strong>diamond</strong><br />
are lithium <strong>and</strong> sodium. The ionic radius <strong>of</strong> Li is<br />
0.060 nm <strong>and</strong> that <strong>of</strong> Na is 0.095 nm; their atomic radii,<br />
however, are quite large: r(Li)=1.52 A <strong>and</strong> r(Na)=<br />
1.66 A). If they diffuse in the neutral form through the<br />
<strong>diamond</strong> lattice, the lattice distortions will be strong.<br />
The energies <strong>of</strong> Li <strong>and</strong> Na <strong>impurities</strong> in <strong>diamond</strong> in<br />
interstitial <strong>and</strong> substitutional positions have been calcu-<br />
lated [I 13. As expected, the interstitial sites were found<br />
to be energetically favored. The tetrahedral sites are<br />
preferable for both Li <strong>and</strong> Na, because even the lithium<br />
ion in an interstitial site is not small enough for the<br />
<strong>diamond</strong> lattice. Activation energies <strong>of</strong> 0.1 eV <strong>and</strong> 0.3 eV<br />
below the conduction b<strong>and</strong> minimum have been com-<br />
puted for Li <strong>and</strong> Na respectively. The solubility <strong>of</strong> these<br />
dopants is expected to be low. The activation energy <strong>of</strong><br />
diffusion for Li <strong>and</strong> Na <strong>impurities</strong> has been predicted to<br />
be 0.85 eV <strong>and</strong> 1.4 eV respectively [ 11,12 3. The mob~lity<br />
<strong>of</strong> Li in the <strong>diamond</strong> lattice is predicted to be high [12].<br />
Experiments on Li <strong>and</strong> Nil doping have not continned<br />
the theoretical results. Lithium doping has been tried<br />
during gro~vth [26]. using diffusion [?7 ] or ion impl~intation<br />
[28]. &one <strong>of</strong> these <strong>methods</strong> have worked.<br />
Lithium fluoride was used as an in situ doping source<br />
[26] for homoepitaxial films grown by r.f. plasma discharge.<br />
SIMS analysis showed that Li can enter up to<br />
about lo2' cm-3 in the most heavily doped san~ples.<br />
The films obtained had p-<strong>type</strong> conductivity <strong>and</strong> an<br />
activation energy <strong>of</strong> 0.24eV. The conclusion <strong>of</strong> this<br />
experiment was that Li is electrically inactive in <strong>diamond</strong><br />
<strong>and</strong> that the observed results were due to a boron<br />
contamination (the growth system had also been used<br />
for boron doping). However, there might be another<br />
explanation. Elements <strong>of</strong> the seventh group <strong>of</strong> the periodical<br />
table entering interstitial sites should form shallow<br />
acceptor levels in <strong>diamond</strong>. So fluorine from the dopant<br />
source ( LiF) might be responsible for p-<strong>type</strong> conductivity.<br />
Another explanation <strong>of</strong> the failure might be that the<br />
Li level is deeper than predicted <strong>and</strong> it diffuses as a<br />
neutral atom. In the last case. the lattice distortions<br />
would be strong <strong>and</strong> could introduce compensating<br />
acceptor <strong>impurities</strong>. This effect is discussed later.<br />
The diffusion <strong>of</strong> lithium into <strong>diamond</strong> from the vapor<br />
phase has been studied at temperatures between 400 <strong>and</strong><br />
900 "C [27]. The presence <strong>of</strong> lithium was qualitatively<br />
determined by SIMS. However, the change <strong>of</strong> electrical<br />
conductivity due to the presence <strong>of</strong> lithium was small.<br />
only two orders <strong>of</strong> magnitude, from 10-" to<br />
R-' cm-'. The authors advanced the hypothesis <strong>of</strong> the<br />
existence <strong>of</strong> acceptor-like states close to the valenceb<strong>and</strong><br />
edge extending 1-2 eV into the b<strong>and</strong> gap. A high<br />
enough density <strong>of</strong> such states might be responsible for<br />
the compensation <strong>of</strong> lithium donors. The nature <strong>of</strong> these<br />
states is not known. The states might be formed due to<br />
<strong>impurities</strong>, defects or localized valence bond anomalies<br />
such as carbon-carbon double bonds. No comprehensive<br />
research has been done so far to verify these hypotheses.<br />
In summary, these experiments show that in many cases<br />
the influence on electrical conductivity <strong>of</strong> defects introduced<br />
in the <strong>diamond</strong> lattice during doping is stronger<br />
than that <strong>of</strong> the impurity itself.<br />
Na impurity also does not behave as predicted by<br />
theory [ll]. Jamison et al. 1291 showed that Na is a<br />
p-<strong>type</strong> dopant in <strong>diamond</strong> <strong>and</strong> forms a shallow level<br />
(about 0.09 eV). These results are discussed later.<br />
5. Forced doping <strong>methods</strong><br />
Methods <strong>of</strong> forcing <strong>impurities</strong> into the <strong>diamond</strong> lattice<br />
have an important advantage over ion implantation.<br />
which has been widely used for <strong>diamond</strong> doping [28].<br />
The traditional ion implantation introduces structural<br />
defects (vacancies, vacancy + interstitial, <strong>and</strong> their com bi-<br />
nations) which are difficult to cure [30]. No additional
1308 G. Popovici, M. A. Prelas/Diamond <strong>and</strong> ' Relared Marerials 4 ( 1995) 1305-1310<br />
structural defects are introduced by forced diffusion or<br />
ion assisted doping during growth.<br />
5.1. Elecrric field assisted difltision<br />
The Debye temperature <strong>of</strong> <strong>diamond</strong> is about 2200 K,<br />
the highest <strong>of</strong> all known solids, reflecting high structural<br />
rigidity <strong>of</strong> the <strong>diamond</strong> lattice [31]. For comparison,<br />
the Debye temperature <strong>of</strong> silicon is 635 K. At temperatures<br />
lower than the Debye temperature, atoms oscillate<br />
with amplitudes small compared with the interatomic<br />
distance.<br />
The diffusion <strong>of</strong> substitutional <strong>impurities</strong> is controlled<br />
by the vacancy mechanism. The energy <strong>of</strong> formation for<br />
vacancies in <strong>diamond</strong> is high, 7.2 eV (for Si it is 4.2 eV)<br />
[32]. Hence, the temperature that would be necessary<br />
for effective diffusion <strong>of</strong> substitutional <strong>impurities</strong> in<br />
<strong>diamond</strong> is expected to be higher than the maximum<br />
temperature which can be used for diffusion, which is<br />
the temperature at which the graphitization <strong>of</strong> high<br />
quality <strong>diamond</strong> begins (1900 K). Graphitization <strong>of</strong><br />
<strong>diamond</strong> films starts at lower temperatures. However,<br />
promising results were obtained by using solid BN<br />
sources for diffusion <strong>of</strong> boron <strong>and</strong> nitrogen into natural<br />
<strong>diamond</strong> [33]. Rapid thermal processing ( 1400 "C for<br />
30 s in argon) was used. SIMS pr<strong>of</strong>iles indicated a boron<br />
concentration <strong>of</strong> about 1019 to a depth <strong>of</strong> 500 A.<br />
A diffusion coefficient <strong>of</strong> lo-" cm2 s-' at 1400 "C was<br />
determined.<br />
It is known that fast diffusing <strong>of</strong> interstitial <strong>impurities</strong>,<br />
such as Cu, Li, Na, can be introduced in silicon <strong>and</strong><br />
germanium lattices at relatively low temperatures<br />
(200-400 "C for Si, which is much less than the diffusion<br />
temperature for substitutional <strong>impurities</strong> <strong>of</strong><br />
800-1100 "C), if electric field assisted diffusion is used<br />
[34,35]. However, field enhanced diffusion in <strong>diamond</strong><br />
has not, to our knowledge, been tried. We propose to<br />
apply an electric field to enhance the diffusion <strong>of</strong> Li in<br />
<strong>diamond</strong> thin films. If it is true that the predicted<br />
ionization energy <strong>of</strong> the Li level is small (0.1 eV) [11,12],<br />
Li atoms will be totally ionized at 1160 K, <strong>and</strong> the field<br />
enhanced diffusion <strong>of</strong> Li should be effective.<br />
We studied the diffusion <strong>of</strong> lithium, chlorine <strong>and</strong><br />
oxygen under electrical bias [36]. High quality free<br />
st<strong>and</strong>ing <strong>diamond</strong> films, about 230 pm thick <strong>and</strong> polished<br />
on both sides were used. The average crystallite<br />
size was <strong>of</strong> the order <strong>of</strong> tens <strong>of</strong> micrometers. All samples<br />
were grown under the same conditions. The Raman<br />
spectra <strong>of</strong> the samples showed the <strong>diamond</strong> line<br />
(1332cm-') only, <strong>and</strong> no graphite <strong>and</strong> amorphous<br />
carbon lines. The cathodoluminescence spectra showed<br />
a strong free exciton line which is a feature <strong>of</strong> good<br />
crystalline quality [37]. The films were mounted on a<br />
graphite base with an embedded tungsten heater. The<br />
base temperature was monitored with a chromel-alumel<br />
thermocouple. The dopant sources Li,CO, <strong>and</strong> LiCIO,<br />
were <strong>of</strong> analytical purity (99%). The diffusion was<br />
performed for 190 min at 1000 "C in an argon atmosphere.<br />
An electric field <strong>of</strong> 200 V was applied to the<br />
sample. The control sample had no electric field applied.<br />
Large amounts <strong>of</strong> impurity (about 4 x 1019 ~ m - were ~ )<br />
found to diffuse at relatively low temperatures. In con-<br />
trast, the bias was found not to affect the concentration<br />
<strong>of</strong> the diffused <strong>impurities</strong>. The last point can be explained<br />
by the high diffusion coefficient through the grain bound-<br />
aries <strong>and</strong> other defects inherent to polycrystalline films.<br />
As the ionization energy <strong>of</strong> doping atoms in association<br />
with other defects <strong>of</strong> the lattice must be different, usually<br />
larger, to the energy level <strong>of</strong> the same impurity in the<br />
singly dispersed form, the diffusion through the defects<br />
on the grain boundaries was probably due to the neutral<br />
atoms <strong>and</strong> was not influenced by electric field. The<br />
diffusion experiments on Li doping in a <strong>type</strong> IIa natural<br />
<strong>diamond</strong>, using nearly the same conditions, showed that<br />
the Li content is two to three orders <strong>of</strong> magnitude<br />
smaller in the single crystalline <strong>diamond</strong> than in the<br />
polycrystalline films [38].<br />
The diffusion in polycrystalline films under bias using<br />
a Li salt, LiClO,, lead to a large change in conductivity<br />
(about eight orders <strong>of</strong> magnitude). An activation energy<br />
<strong>of</strong> about 0.25 eV was derived from the temperature<br />
dependence <strong>of</strong> the conductivity [39]. The samples<br />
showed p-<strong>type</strong> conductivity, as measured by conven-<br />
tional hot probe. The hot probe measurements under<br />
bias showed n- or p-<strong>type</strong> conductivity depending on the<br />
applied bias. A hypothesis was advanced, that a p-<strong>type</strong><br />
surface inversion layer was formed on the diffused n-<strong>type</strong><br />
layer [40]. More research is needed to clarify this<br />
problem.<br />
The Hall effect measurements on the same polycrystal-<br />
line films diffused under bias were carried out by two<br />
independent groups. The measurements showed an<br />
n-<strong>type</strong> conductivity with electron concentration about<br />
10'' cm-3 <strong>and</strong> a sheet resistance about lo5 R!U r37.393.<br />
Supposing the thickness <strong>of</strong> the diffused layer to be <strong>of</strong><br />
the order <strong>of</strong> 2 pm, the mobility <strong>of</strong> electrons could be<br />
estimated: 50cm2 V-' s-'. We do not know which<br />
impurity (or association <strong>of</strong> <strong>impurities</strong>) the n-<strong>type</strong> conduc-<br />
tivity is due to in this case. More research is needed to<br />
clarify this problem.<br />
5.2. Ion assisted doping during growth<br />
The basic difficulty with conventional doping during<br />
growth is with the incorporation <strong>of</strong> dopant atoms into<br />
the growing film. Many dopants are more likely to<br />
re-evaporate rather than be incorporated into a growing<br />
film surface. as their bonding energy is small compared<br />
with the C-C bond energy (see Table I <strong>of</strong> Ref. 6). Even<br />
if a dopant atom is incorporated into the sro\i.lng tilm.<br />
there may also exist driving forces due to the strain<br />
energy. related to peometncal factors. which ma cause
the dopanl atoms to segregate to the surface <strong>and</strong> eventually<br />
re-evaporate.<br />
Large improvements in the incorporation <strong>of</strong> In <strong>and</strong><br />
Sn in Si <strong>and</strong> GaAs grown by molecular bean1 epitaxy<br />
have been demonstrated [41] using low energy<br />
(60-300 eV) ionized dopant atoms, which were directed<br />
onto the film during deposition. Ion assisted doping also<br />
gave good results in obtaining p-<strong>type</strong> conductivity in<br />
CdTe using As <strong>and</strong> P ions as dopants [42-441. The<br />
ionized dopant atoms are implanted at a small depth<br />
<strong>and</strong> have a much larger sticking coefficient than dopant<br />
atoms in thermal equiIibrium with the growing surface;<br />
hence. they are more likely to be incorporated into the<br />
growing film.<br />
The method <strong>of</strong> doping during growth was tried on<br />
<strong>diamond</strong> by Jamison et a]. [29] with sodium, rubidium<br />
<strong>and</strong> phosphorus. <strong>and</strong> proved to be efficient for embedding<br />
the <strong>impurities</strong> into the growing <strong>diamond</strong> layer.<br />
SIMS measurements showed that phosphorus entered<br />
up to 5 x 10" C~I-~, while sodium <strong>and</strong> rubidium entered<br />
up to 1020cm-3. Highly conductive p-<strong>type</strong> layers were<br />
obtained by sodium doping. Energies <strong>of</strong> activation <strong>of</strong> 44<br />
<strong>and</strong> 82 meV were measured for sodium.<br />
The experiments on the influence <strong>of</strong> deformations on<br />
<strong>diamond</strong> conductivity might be an explanation <strong>of</strong> p-<strong>type</strong><br />
conductivity in <strong>diamond</strong> caused by big atoms. The<br />
investigation <strong>of</strong> electrical properties <strong>of</strong> deformed natural<br />
<strong>diamond</strong> [45-471 has shown that the dislocations<br />
change its electrical, optical <strong>and</strong> photoelectrical properties.<br />
An insulating natural <strong>diamond</strong> <strong>of</strong> Ila <strong>type</strong> with<br />
I initial electrical resistivity <strong>of</strong> 1016 i2 cm, after deformation<br />
I resembled the semiconducting <strong>diamond</strong> IIb in terms <strong>of</strong><br />
I resistivity values. The resistivity varied between 1013 <strong>and</strong><br />
I lo2 R cm due to the plastic deformation. If the number<br />
I<br />
<strong>of</strong> dislocations is large (about 109-1010 cm-'), the electri-<br />
I<br />
I cal resistivity can become as low as 102 Rcrn. This<br />
I change <strong>of</strong> resistivity was obtained without introducing<br />
1 .<br />
I any <strong>impurities</strong> in the <strong>diamond</strong> lattice. The activation<br />
I<br />
I<br />
I<br />
I<br />
I<br />
energy <strong>of</strong> electrically active centers produced by deformation<br />
was 0.26-0.29eV. The conductivity was <strong>of</strong><br />
p-<strong>type</strong>. The results <strong>of</strong> this experiment suggest that the<br />
introduction in the <strong>diamond</strong> lattice <strong>of</strong> prospective donor<br />
I <strong>impurities</strong> with large atomic radii can produce not only<br />
n-<strong>type</strong> centers, due to the impurity itself, but also com-<br />
1<br />
pensating p-<strong>type</strong> centers due to lattice deformations. As<br />
I sodium is too large an atom (atomic radius r= 1.66 A,<br />
I ionic radius r' =0.95 A) [5 1, the structural defects <strong>of</strong><br />
I the lattice due to the introduction <strong>of</strong> Na could be too<br />
) large <strong>and</strong> might dominate the transport properties.<br />
I 6. Conclusions<br />
I<br />
I<br />
1<br />
Shallow energy level n-<strong>type</strong> <strong>diamond</strong> doping by prospective<br />
donor <strong>impurities</strong> N, P, <strong>and</strong> Na has been unsuccessful<br />
(except for the recent work <strong>of</strong> Prins on<br />
phosphorus doping using the cold in~plantat~on r~~pid<br />
annealing process). There are vanous explanatlonb for<br />
this behavior. The energy structure <strong>of</strong> the b<strong>and</strong> gap <strong>of</strong><br />
<strong>diamond</strong> films, grown in different conditions, is not<br />
known yet. The existence <strong>of</strong> considerable densities <strong>of</strong><br />
acceptor-like states close to the valence-b<strong>and</strong> edge <strong>and</strong><br />
extending 1-2 eV into the b<strong>and</strong> gap might be responsible<br />
for the compensation <strong>of</strong> donors. The nature <strong>of</strong> these<br />
states is not presently known. These states may be due<br />
to <strong>impurities</strong>. <strong>and</strong>/or structural defects. Another hypothesis<br />
is that high densities <strong>of</strong> structural defects introduced<br />
in the lattice by large impurity atoms. might form a<br />
relatively shallow acceptor level (about 0.3 eV) <strong>and</strong><br />
dominate the transport properties, giving p-<strong>type</strong> conductivity.<br />
No exhaustive experiments have been done yet to<br />
verify these hypotheses. However. the conclusion should<br />
be drawn that the route to successful <strong>diamond</strong> doping<br />
is in finding <strong>methods</strong> that permit a reduction <strong>of</strong> number<br />
<strong>of</strong> defects created during the doping process.<br />
Methods <strong>of</strong> forced doping are effective for introduction<br />
<strong>of</strong> large amounts <strong>of</strong> dopants into the <strong>diamond</strong> lattice.<br />
Forced diffusion <strong>of</strong> <strong>impurities</strong> from Li salts enabled<br />
n-<strong>type</strong> conductivity to be obtained in high quality <strong>diamond</strong><br />
polycrystalline films. The measurements showed<br />
an electron concentration <strong>of</strong> about 1015 ~ m - a ~ sheet ,<br />
resistance <strong>of</strong> about lo5 Q/0 [36,37] with the estimated<br />
electron mobility <strong>of</strong> 50 cm2 V-' s-'.<br />
Acknowledgments<br />
The authors would like to express many thanks to<br />
Pr<strong>of</strong>essor Peter J. Gjelisse <strong>of</strong> Florida State University<br />
for the critical reading <strong>of</strong> the manuscript.<br />
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