pulsed laser annealing of p implanted diamond

16 Pages·2009·0.24 MB·English

PULSED LASER ANNEALING OF P
IMPLANTED DIAMOND
M. G. Allen, S. Prawer, D.N. Jamieson
School of Physics. University of Melbourne, Parkville, Victoria, 3052.
Australia.
R. Kalish
Solid State Institute and Physics Department Technion - Israel Institute
of Technology, Haifa, 32000, Israel.
Abstract
Diamond deeply implanted with 4MeV P to a dose of lxl015atoms/cm2 is an­
nealed by a focused pulsed laser that is selectively absorbed by the implanted dam­
aged layer. Laser treatment with multiple pulses at ever increasing power effects
excellent regrowth as measured by channeling RBS, surface profilometry, and by
optical transmission. The importance of the deep implantation and the potential of
this method for doping diamond is demonstrated.
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The superior mechanical, heat conducting and electrical properties of diamond
hold considerable promise for the use of diamond in high-temperature, radiation-hard,
very fast electronic devices and detectors. Effective n- and p-type doping of diamond
is required for the realization of such devices. Ion implantation is a practical means
to incorporate dopants into the diamond lattice in a controlled way. However, for the
successful doping of diamond by this method it is necessary to (i) repair the radiation
damage induced by the ion implantation and (ii) electrically activate the dopant. In the
present work radiation damage is annealed using a high power pulsed laser which enables
the investigation of the effects of high temperature (~ 3U00K), high pressures (5-10 GPa)
and very rapid annealing times (10-400ns) on the regrowth kinetics of diamond. These
conditions are not otherwise available in the laboratory.
The effects of single pulse laser irradiation on deeply buried C ion beam damaged
layers in diamond has recently been reported by us [1]. In that work graphitization.
annealing and severe ablation were observed to occur over a narrow range of laser power.
These effects were explained in terms of the phase diagram of carbon near the triple point.
For a certain range of laser powers, partial annealing of the damaged layer was possible.
Graphitization, which would usually have occurred for the doses employed, was avoided
by the use of a constraining cap of undamaged diamond formed by the deep MeV implants
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which maintains the damaged layer under high pressure. In the present work we report
on the use of a multiple-pulse laser annealing scheme to effect much superior regrowth
over a larger area in heavily damaged P ion-implanted diamond, thus demonstrating the
potential of this annealing scheme for P and other potential dopants in diamond.
In our experiments type Ha, optically polished, < 110 > oriented, diamond slabs
were used. Implantation of 4.0 MeV Phosphorus to dose 1 x 1015atoms/cm2 [R± AR =
p P
1.31 ± 0.07//m] was performed at 77K, leaving an almost undamaged surface cap of dia­
mond about 1/zm thick.
Laser irradiation was performed using a Q-switched, frequency doubled (532 nm)
Nd-YAG laser focused through a conventional x 10 microscope objective to a spot approx­
imately 15 /mi in diameter. Larger annealed spots could also be generated by defocusing
the laser beam. The 532 nm line was used because it is selectively absorbed by the buried
damaged layer. The pulse duration was 15-40 ns.
The diagnostics used to assess the radiation damage and degree of annealing were
optical spectroscopy (400-800nm) to measure changes in optical absorption, surface pro-
filometry to measure swelling and compaction, and channeling Rutherford Backscattering
Spectroscopy (c-RBS) to quantitatively assess lattice perfection as a function of depth.
The application of c-RBS to small laser annealed spots < 30//m in diameter required
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the use of Channeling Contrast Microscopy (CCM). This technique involves scanning a
highly focused (~ 1/im) probing beam (in the present case 1.4 MeV H+) over the sample
which has been aligned in channeling orientation with respect to the proton beam [2].
The backscattered yield is recorded as a function of position on the sample and images
are generated in which differences in crystal quality show up as contrast in the image. It
is then possible to extract from any region of the image the c-RBS spectrum.
Figure 1(a) is an optical micrograph taken with transmitted light of the unim-
planted/implanted boundary of the diamond, together with a surface profile over this
boundary. The ion implantation has caused the diamond to turn brown and to swell up
by about 30nm. An optical micrograph of a laser treated spot together with its surface
profile is shown in Figure 1(b).
A method which was found to optimize annealing of the damage involves multiple,
variable energy laser pulses. The laser power was ramped up with each successive pulse
according to the scheme shown in Figure 2, which produced the excellent regrowth shown
in Figure 1(b). The maximum amount at which the energy density can be increased
on every successive pulse without causing graphitization of the implanted layer and cap
was found to be approximately 15% per pulse up to 12 pulses. With this scheme the
energy density at which graphitization occurs for a single pulse can be exceeded by the
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third pulse. It is important to note that multiple pulses of the same power produce little
change in the annealing after the first pulse. The importance of the overlying constraining
cap is demonstrated by the fact that for the same laser power a shallower implant (640 ke\"
P to a dose of 1 x 1015atoms/cm2) graphitizes during a single laser pulse, while the deep
implant anneals, even though the deeper implant used here had higher optical absorption
than the shallow implant and consequently reached a higher temperature during the pulse.
In the region of the GR-1 band at 550nm the optical transmission of the as-implanted
diamond normalized to unimplanted diamond was 14%. Following the multiple pulse laser
irradiation this increases to 61%. However as is evident from Fig 1(b) the best annealing
occurs at the very centre of the laser spot over about a 2/mi radius, whereas the spatial
resolution of the microspectrophotmeter employed in the measurement was about 10/mi.
Hence the figure of 61% should be viewed as a lower limit, the transmission in the centre
of the spot being considerably higher than this.
In addition to the bleaching of the ion beam induced absorption clearly shown in
Fig 1(b). evidence for annealing is also provided by the surface profile. This shows that
the diamond has been compacted following the multiple laser pulses to its preimplantation
density, reversing the effects of defect induced swelling shown in Fig 1(a). Ablation can
be ruled out as the cause for the dip evident in Fig 1(b) for the following reasons. Firstly.
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tlie surface profile is smooth, and is unlike the rough surfaces obtained when diamond
is ablated/sputtered by pulsed laser irradiation. Secondly, when ablation does occur the
whole of the surface cap is removed [1] which is clearly not the case here and thirdly,
graphitizatiun is always observed to occur at lower laser powers than those required for
ablation. In the present case no graphitization occurs indicating that the laser power is
insufficient t; cause ablation.
Figure 3 shows the channeled RBS spectrum from the centre of the laser annealed
spot shown in Figure 1(b). Figure 4 shows the CCM map of this spot and the region from
which the c-RBS spectra have been extracted. As pointed out above the contrast in the
image is due to the differences in the backscattered yield from different regions of the
specimen.
Figure 3 shows that the implantation has created a damaged buried layer in
which the backscattered yield almost reaches 80% of the random level. Followirg laser
annealing Figure 3 shows that this peak completely disappears and the crystal quality
has been returned to near its uniniplanted level.
The combined results of compaction, increased optical transmission and high RBS
channeling quality, all indicate that excellent regrowth has occurred in the central 10/mi
diameter of the laser spot.
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Aii estimate of the maximum temperature in the implanted region of 2500K is
obtained for the first pulse by solving a ID heat equation usnig simulation software
SLIM [3]. Previous work [1] has shown that a single laser pulse can remove at most :$0-
")0(X of the point defect damage. So it is clear that the time during which the implanted
region is held at this elevated temperature (~ 10ns) is insufficient to achieve complete
annealing. In the present work further annealing is achieved by additional pulses, which
must be increased in energy density because of the reduction in optical absorption with
each successive pulse. If pulses of the same energy are used no additional annealing is
achieved. It seems a reasonable assumption that the increase in energy between pulses
ensures that the damaged layer can be raised to at least the same temperature as was
attained in the first puise.
Previously we have shown [1] that annealing takes place in the solid, rather than
the liquid state. Using the data presented above it is possible to estimate the solid
state diffusion coefficient of defects in the following manner. Firstly, whilst the defect
concentration following ion implantation is not known precisely an estimate of the number
of vacancy/interstitial pairs may be obtained with the aid of the computer code THIM
[1] using a displacement energy of 45 eV. For 1 x 1015P/cm2 an average separation of
O.^fim between defects was obtained. Our results show that during one laser pulse of
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duration. t~ 20ns. most, but not all of the defects diffuse and recombine. This gives an
order of magnitude estimate of the diffusion length. A, of defects as 0.3nm during a 20 ns
pulse at the elevated temperature, which leads to an estimate of the diffusion coefficient
I)= A2/4t ~ I x 10~8 cm2/s at T=2500K. Remarkably this is four orders of magnitude
greater than the coefficient for self diffusion in Si at ~1700K [5].
Notwithstanding the uncertainties in the above estimates, it is clear that the
central result reported here is the removal of point defects in extremely short annealing
times, thus ensuring that only short range effects may occur. These effects probably
include the recombination of interstitials (both C and P) with vacancies which are in close
proximity. However, given that in general interstitials are more mobile than vacancies,
the short anneal times may help to prevent agglomeration of vacancies into clusters which
may prove to limit the annealing and the electrical activity of dopants.
The excellent regrowth reported here establishes laser annealing as an effective
practical means for the removal of latt-ce damage following ion implantation. In particular
the laser may be directed onto the sample while in the implanter so that a wide variety of
in-situ annealing schemes can be devised, such as multiple implantation/arnealing cycles
which have been found to achieve a high level of electrical activity for B implants [6],
However, ultimately, the success of laser annealing for the activation of dopants
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will be measured by the electrical properties of the laser annealed regions and for these
measurements larger annealed areas electrically isolated from the surrounding material
are required. We have produced annealed areas of size 60 x 25/im2 by defocusing the laser
beam, and even larger areas by scanning a CW focused beam over the sample surface.
Klect rical measurements of these annealed samples are currently in progress.
RK wishes to thank the staff of The School of Physics. University of Melbourne
for their hospitality. The support of the Australian Research Council and the Department
of Industry. Technology and Commerce is greatly acknowledged. MGA acknowledges the
assistance of an APR A.
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References
['] S.Prawer. D.N.Jamieson and R.Kalish. Phys. Rev. Lett. 69. 2991-2994 (1992).
[2] D.N. Jamieson. R.A. Brown, C.G. Ryan and J.S. Williams. Nucl. Instruni. Metli.
Phys- Res.. B 54.213 (1991).
[•i] R. Singh and J. V'iatella, "Simulation of Laser Interaction with Materials (SLIM)."
I niversity of Florida. 1992 (unpublished).
[4] J. Ziegler, J.P. Biersack and J.Cuomo (unpublished) 1980.
[5] J.W. Mayer. S.S.Lau. Electronic Materials Science. Macmillan (1990).
[6] J.F Prins. Physica B (in print 1992).
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