Cathodoluminescence studies of carbon-13 diamond

Journal of Luminescence 4O&41 (1988) 865—866 North-Holland, Amsterdam

865

CATH000LUMINESCENCE STUDIES OF CARBON-13 DIAMOND

Alan T COLLINS and Gordon DAVIES Wheatstone Physics Laboratory, King’s College London, Strand, London WC2R 2LS, UK Vibronic luminescence bands associated with seven point defects have been studied for synthetic diamonds grown using 99% C-13. Changes in the electronic and vibrational energies have been compared with those expected theoretically. This is the first study of point defects in a semiconductor material where the isotope of the host lattice has been almost completely changed. Changes in vibrational and electronic energies have been measured and compared with those predt~.4T~

~

0—13

~:,

pre-

x10

sent in many as-grown synthetic diamonds, the GRI centre, produced by radiation damage, and

—

c—ia

-

the 575 nm, 640 nm, 3.188 eV and 5RL centres produced by radiation damage and annealing. These centres have been investigated using

2. 80

2. 90

3. 00

PHOTON ENERGY

cathodoluminescence, supplemented by absorption measurements on the 640 nm and GR1 systems. The effects of the increased mass of the host lattice are clearly visible in the vibronic

--

xlO 3. 10

3. 20

(uV)

Cathodoluminescence spectra of the 3.188 eV centre in C-12 and C-13 diamonds at 77K.

sidebands associated with electronic transitions at point defects. A typical example is the 3.188 eV cathodoluminescence band shown in

a factor of around 0.962. For peak ‘a, however, the factor is 0.977, possibly suggesting that

figure 1. In the C-12 diamond distinct features include the phonon replica at 3.113 eV, the

this is associated with a nitrogen-carbon vibration. (Nitrogen is the dominant impurity in

sharp edge at 3.023 eV associated with the emis-

synthetic diamond.)

sion of an LO phonon (165 meV) and the 2L0 edge at 2.863eV. The energy spacings of the corres-

Another striking example of carbon isotope effects in a vibronic band is shown in figure 2.

ponding features from the zero-phonon line in the C-13 diamond are all reduced by a factor of

This is the 5RL cathodoluminescence band which is notable because of the sharp local-mode rep-

around 0.965, which is close to (12/13)~, as expected. The sharp lines around 3.0 eV in figure 1 appear to be local-mode replicas of the

licas of the zero—phonon line spaced at about 2 These allow the 237 meV (in C—12 diamond). anharmonicity of the vibration to be analysed

zero-phonon line. For all peaks except those labelled “a” the energy spacings from the zerophonon line in the C-13 diamond are reduced by

with some precision to yield phonon frequencies in C-12 and C-13 diamond of 237.6 and 228.7 meV respectively. The ratio of these energies is

0022—2313/88/$0350 10 Elsevier Science Publishers By. (North-Holland Physics Publishing Division)

866

A. T. Collins, G. Davies / Cathodoluminescence studies ofcarbon-13 diamond

TABLE I

Comparison of experimental and calculated isotope shifts. A negative sign means the line in C—13 diamond occurs at lower energy than in C—12 diamond. A ? denotes that data are not available, and an x indicates that this contribution is not applicable. Calculated shift Name of

Energy

Static

Dynamic

Local

Vibronic

Total

defect

(meV)

shift

shift

mode

coupling

shift

—1.6 to -2.0 5.0 1.25

x x x

—

GR1 640 nm 575 run H3 -

5RL

1400 1673 1945

1.26 to 2.5 0.25 1.25

2156 2463 3188 4582

0.4 3.96 small 1.7

? 0.6 1.75 3.0

____________________________________

x x x

x x x 2.0

x 0.15 x x

Experiment

-0.35 to +0.5 5.0 2.5

—0.5 2.9 2.1

>0.4 4.7 1.75 6.7

3.0 5.0 3.3 8.0

the zero—point motion of the defects, producing the “dynamic shift”.

This can be estimated from

the temperature-dependence of the energies of 1 For the H3 defect there the zero-phonon lines. is also a contribution arising from the vibronic

C13

—

coupling between two excited electronic states.1

z For most of the defects studied the vibrational modes have frequencies lying in the

-J 3. 6

lattice continuum between 0 and 165 meV. 3:8

4. 0

4. 2

PHOTON ENERGY

4:4

4. 6

(uV)

FIGURE 2

For

the 5RL defect (see figure 2 and table I) there

is a further contribution to the dynamic shift from

Cathodoluminescence spectra of the 5RL centre in C—12 and C—13 diamonds at 77K.

the localised modes of vibration.

Isotope shifts of eleven zero-phonon lines

0.963, showing that the vibrating atom must be

have been observed in C-13 diamond, and those for which uniaxial stress and temperature—

carbon. The frequency of the vibration is almost 50% higher than the LO phonon frequency,

dependence data are available are summarised in table I. The comparison between the measured

and this suggests that the vibration is associa—

and calculated isotope shifts is encouraging.

ted with a configuration which has an abnormally high “spring constant” - perhaps due to the carbon interstitial being wedged into a very tight space. The effects of the isotope change on the energies of the zero-phonon lines are listed in table I. Increasing the atomic mass reduces the lattice spacing.3 The resulting “static shift” of the zero-phonon lines can be calculated using their known responses to externally applied stresses. Changing the isotope also changes

REFERENCES 1.

G. Davies, Chem. and Phys. of Carbon 13 (1977) 1.

2.

A.T. Collins and P.M. Spear, J.Phys. C19 (1986) 6845.

3.

F.A. London, Zeit. fur Phys. Chem. 16 (1958) 302.