High temperature measurements of diamond thin films

g!AMOND RELATED MATERIALS Diamond and Related Materials 4 (1995) 673-677

ELSEVIER

High temperature measurements of diamond thin films F. Fontaine, Laboratoire

d’Etude

des Prop&t&

Electroniques 38042

A. Deneuville

des Solides, CNRS Grenoble

Cedex

associk d l’Universit& Joseph

Fourier,

BP 166,

9, France

Abstract Annealing at 1573 K of boron-implanted polycrystalline diamond films on silicon substrates introduces spurious effects originating from the shunt of the implanted layer by the substrate. The shunt resistance is dominated by the resistance to reach the substrate from the implanted region. It is ascribed to the grain boundaries of polycrystalline diamond. After the removal of the substrate, the electrical resistance of the implanted layer exhibits the expected behaviour for donor compensation. However, the higher activation energy for the high temperature resistance of the implanted layer requires the introduction of the partial ionization of the donor in this range of temperature and a high degeneracy factor g, = 1000 for the donor level. Keywords:

Ion implantation;

Boron doping; Electrical conductivity;

1. Introduction Because of its large forbidden gap and high thermal conductivity, diamond is remarkably well suited to high temperature electronics. Boron, the acceptor dopant atom, has a large ionization energy (0.37 eV Cl]), and is therefore completely ionized (as in normal electronic devices) only above a high temperature, which increases with the dopant level (typically above 880 K for Nboron = 1Ol6 cme3). Natural and synthetic diamond crystals, as well as thin films deposited by chemical vapour deposition (CVD), contain a lot of defects which create donor levels in the gap. The practical achievement of devices requires doping by ion implantation, and implantation defects also generate donor levels. Thus diamond always behaves as a compensated semiconductor, which increases the temperature needed for the complete ionization of the dopants. Therefore a knowledge of the electrical properties of boron-doped diamond in the high temperature region is important for both basic and device-related purposes. Until now, most of the electrical measurements on implanted material have been performed below 880 K [2-41, while other studies have indicated that some devices can be used at temperatures up to 973 K [S]. Previously, we have investigated (up to 600 K) the effectiveness of doping by ion implantation when the films are annealed at 1073 K [6]. However, this annealing temperature does not allow for full recovery of the implantation defects. In this study, we measure the 0925-9635/95/$09.500 1995Elsevier Science S.A. All rights reserved SSDZ 0925-9635(94)05296-4

Diamond

resistance in the range 300-1000 K of polycrystalline CVD diamond films doped by boron implantation ( 10’3-10’5 B cm-‘) and annealed at 1573 K. We show that, in coplanar geometry, the implanted layer is shunted by conduction through the silicon substrate. After measurements of the resistance on diamond membranes, a model of compensation is proposed to account for the experimental curve, in particular at high temperature.

2. Experimental

details

The diamond films Nere deposited at 880 “C on high resistivity silicon by the standard MPCVD method from a 0.5% CH,/99.50/; H2 gas mixture at a total gas pressure of 30 Torr during 8 h. These conditions lead to the deposition of 3 pm thick polycrystalline diamond. The films were doped by implantation at 77 K of 10’3-1015 B cm p2 (90 keV ions). The annealing process has been described elsewhere [7]. The deposition of a silicon nitride encapsulating layer allows for annealing at ! 573 K during 1 h without oxidation. After the removal of the silicon nitride layer, the resistance of the films was measured by a Keithley 236 Source-Measure Unit, in coplanar geometry, between two tungsten probe tips (1 mm apart) in contact with the diamond film. The temperature of the film was measured by a thermocouple placed on the diamond surface and electrically isolated from the electrical con-

614

I? Fontaine, A. Deneuville,lDiamond

and Related Materials 4 (1995) 673-677

tacts. The films were heated from their reverse sides through the silicon substrate or via a sapphire holder on silicon which absorbed the radiation of an IR lamp. A temperature controller/programmer displayed the temperature and regulated the power supply of the IR lamp. The resistance was measured in the temperature range 300-1000 K. The occurrence of oxidation at about 990 K at the experimental pressure (approximately lo-” Torr) set the upper limit for the temperature range [S]. -2

3. Results and discussion 3.1. Diamond,film

-3

on the silicon substrate

Voltage

Fig. 1 displays the resistance vs. the reciprocal temperature of polycrystalline CVD diamond films implanted by 10r3, lOi and lOi B cmP2 and annealed at 1573 K. Although the mean boron content in the implanted zone varies from 2 x 10” to 2 x lOi B cmP3, the relative resistance of the samples does not follow the dopant levels. At high temperature (above 500 K), the absolute resistances are similar, and the activation energies for the resistance, deduced from the logarithmic derivative, range from 0.63 to 0.8 eV. 3.2. Diamondjilm

membrane

The voltage used for the measurements measured I(I) characteristics between + were found to be symmetric (Fig. 2). characteristics were obtained at 600-700 on the implantation dose. No breakdown at f20 V, was observed.

was 2 V. The 20 and -20 V Nearly linear K, depending effect, at least

Fig. 2. I(V) characteristics 5 x 1Ol4 B cm-’ implanted

(V)

at room temperature, membrane.

500 and 700 K of a

Fig. 3 shows the resistance vs. the reciprocal temperature of the same diamond film, implanted with 1015 B cm m2 and annealed at 1573 K, before and after the etching of the silicon substrate. Over the whole range of temperature, the resistance of the membrane is much higher and the activation energy significantly lower than that of the same film on the silicon substrate. For these measurements, the contacts were placed at almost the same position on the sample. The previous effect is not due to any change in the “bulk’ resistance of the film itself during the annealing process. For comparison, the resistance of an unimplanted membrane, annealed at 1573 K in the same conditions, is shown in Fig. 3. The R(T) characteristics are well reproducible, as demonstrated in Fig. 4 where three measurements, made at three different positions on the 5 x 1014 B cme2 implanted membrane, are compared. 1o’i 10” IO’O lop 10” 10’ lo6 10S lo4

1

I,5

2 1000/T

2,5

3

3,s

(K“)

Fig. 1. Resistance vs. reciprocal temperature of 10’3, 1Ol4 and 1OL5 B cm-* implanted after annealing at 1573 K.

in the range 300-1000 K diamond films on silicon

1

1,s

2 1000/T

2,s

3

3,5

(K’)

Fig. 3. Resistance vs. reciprocal temperature m the range 300-~1000 K of diamond films, unimplanted and implanted by lOI B cm-’ and annealed at 1573 K, before and after etching of the silicon substrate.

F. Fontaine, A. DmeuviNe/Diamond and Related Materials 4 (1995) 673-677

615

substrate, the curves will exhibit the same well-defined activation energy close to 0.6 eV (half the silicon band gap). However, we measure some dispersion (0.6330.8 eV) in the activation energies centred around 0.66 eV. Therefore the conduction seems to be limited by the resistance RGB to reach the silicon substrate. The significant dispersion of the activation energies in the curves of Fig. 1 suggests a control of the resistance by the grain boundary network of the polycrystalline diamond film. In addition to the differences between the planar (1 mm) and the transverse (3 urn) distances, anisotropy in the grain boundary network may explain the shunt through the silicon substrate rather than simply by the underlying diamond film. Fig. 4. Comparison of the R(T) characteristics different positions on a 5 x 10 I4 B cm-’ implanted

3.3. Conduction

recorded at membrane.

three

path

From secondary ion mass spectroscopy (SIMS) measurements, the mean projected range of the boron ions (maximum concentration) is about 220 nm and the thickness of the implanted zone is about 500 nm, while the thickness of the film is much greater (3 urn). After annealing, the resistivity of the implanted layer is expected to be significantly lower than that of the underlying undoped diamond film. Nevertheless, the large increase in the resistance of the film when the silicon substrate is removed indicates that the main part of the current flows through the silicon substrate when it is present (Fig. 1). In that case, the system is roughly equivalent to the simple electrical circuit shown in Fig. 5: a resistor Rdia (the resistance of the doped diamond layer) in parallel with three resistors in series, 2R,, (twice the resistance to reach the silicon substrate) and Rsi (the resistance of the silicon substrate). For the samples of Fig. 1, 2 x R,, + R,i <
,

1

Fig. 5. Equivalent electrical circuit of the implanted diamond, bulk diamond and silicon substrate system with electrical contacts on the surface of the implanted diamond. Rdla is the resistance of the implanted layer, R,, is the resistance of the grain boundary network and Rsi is the resistance of the silicon substrate.

3.4. Diamond membrane:

donor compensation

mechanism

The classical model of the compensation effect for boron in diamond predicts that R/T3” roughly varies as exp C(O.37 eV)/kT] (neglecting mobility variations with temperature) and then saturates at high temperature [9,10]. It assumes that the compensating donor level is always completely ionized, i.e. the Fermi level Ef is far away. The variation of R/T3” vs. the reciprocal temperature is presented in Fig. 6 for the membrane of Fig. 3 and another membrane implanted with 5 x 1014 B cm-’ and annealed for 1 h at 1573 K. The activation energies are 0.31 eV below 500 K and 0.65 eV above 700 K for 1015 B cme2 implantation and 0.36 eV below 670 K and 1.33 eV above 850 K for 5 x 1Or4 B cmm2 implantation. The previous classical picture cannot describe the increase in the activation energy at high temperature. If we consider that E, can come sufficiently close to E, at our high temperatures of measurement, so that the donor level is partially occupied, the number of holes p

lOi”,“‘,‘,“““““‘,,“’ 1 1,5

2

2,5

1000/T

(K’)

3

3s

Fig. 6. R/T3” vs. the reciprocal temperature in the range 300-1000 K for diamond membranes implanted by 5 x 1014 and 10’s B cm-’ and annealed at 1573 K.

676

F Fontaine. A. DeneuvilleJDiamond

1o7

dLLy~I 0 0,5

I,5

2

2s

in the valence band is derived new charge neutrality equation

of a

N, exp [(E, - E,)/kT]

Nd 1 + gd exp& - &)/kTl whereNcqa)gd(a)and Edcalare the concentration,

15

1000/T

(K-‘)

from the resolution

i

1

3

Fig. 7. p/T”” vs. the reciprocal temperature for E,=0.37 eV, g, = 2, E,=1.2eV, g,=O.5, N,=10’9cmm3 and N,/N,= 10%. 50%. 90% and 99%.

degeneracy factor and energy respectively of the compensating (acceptor) level with regard to the valence band. Fig. 7 presents the original analytical solution of this equation for E,=0.37eV, g,=2, N,=1019cmp3, g,=O.5, Ed= 1.2 eV and for various compensation ratios N,/N,. The value of 1.2 eV for Ed was suggested by Prins [ 111, in agreement with photo-Hall effect measurements [ 121. At low temperature, the model predicts an activation energy of 0.37 eV and, at temperatures higher than 1000 K, an increase in the activation energy for high compensation. Because p is not very sensitive to the value of Ed, in order to account for the lower temperature observed experimentally for the increase in the activation energy, it has been suggested that the degeneracy factor g, is different from the classical value of 0.5 for a donor [ 131. Prins [ 133 proposed a value of g, as high as 1.9 x lo4 and introduces the concept of vacloids as the donor level. Fig. 8 presents the result of a new calculation of p for g,= 1000. The activation energy increases more rapidly and at a lower temperature, close to that observed experimentally. Because both the temperature of transition and the high temperature activation energy of p depend on Ed, Nd and g,, it is difficult at present to ascribe a precise value to each of these parameters. 3

4 (1995) 673-677

--.-.-II

1

1000/T

’ = 1 + g,

and Related Materials

2

2

>

2,5

3

(K-‘)

temperature for E, =0.37 eV, g,= 2, Fig. 8. p,lT3’* vs. the reciprocal E,=1,2eV, g4=1000, N,=10”cmm3 and N,!N,=lO%. 50% 90% and 99%.

4. Conclusions We have shown that, after annealing, at 1573 K, of polycrystalline diamond films implanted with boron at doses up to lOi B cm-‘, the implanted layer is shunted by the resistance of the silicon substrate Rsi and the resistances to reach the substrate 2R,,. The shunt resistance is limited by R,,, which is ascribed to the transverse resistance of the grain boundary network and presents an activation energy between 0.6 and 0.8 eV. This spurious effect is removed by etching of the silicon substrate. In the medium range of measurement temperatures, RIT312 activation energies close to 0.37 eV are obtained for 5 x lOi and 1015 B cme2 implanted samples as predicted by the compensation models. In order to reproduce the higher activation energies found experimentally at high temperatures, partial ionization of the donor and a high degeneracy factor gd = 1000 are introduced.

Acknowledgement We wish to acknowledge

DRET

for financial

support.

References [l]

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