Cathodoluminescence Studies of Bound Excitons and Near Band Gap Emission Lines in Boron- and Phosphorus-Doped CVD-Diamon
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Mat. Res. Soc. Symp. Proc. Vol. 423 * 1996 Materials Research Society
RESULTS AND DISCUSSION a) Boron-doped CVD-diamond films Fig. 1 shows a selection of CL spectra demonstrating the general trends upon increasing boron concentrations. The topmost spectrum exhibits dominantly free exciton radiation as TO-, TA-, and TO- plus Or-phonon assisted recombination lines (FEW: 5.27 eV, FETA: 5.32 eV, and FET°+r : 5.105 eV). Weakly, observed is radiation from excitons localized at isolated boron atoms ("bound exciton", BE: 5.215 eV). These line positions are in accord with the basic luminescence data in Ref. [2]. This sample was not intentionally boron doped but contaminated in the growth apparatus. All other samples in Fig. 1 were intentionally boron-doped during hot filament CVD-growth. The boron concentrations were determined by SIMS and increase from top to bottom. They are listed in Table I for all samples investigated. In Fig. 1, for sample TD 219A with [B] = 40 ppm, the FE lines have almost disappeared; the BET° line has largely increased and broadened yet maintaining its energy position. As a new feature, the BET°+°r transition is visible, and towards lower energies, there is a rise in intensity up to the boron-related emission band peaking at 4.5 eV [3]. For the next concentration, [B] = 825 ppm
(sample B 113), the bound exciton lines have shifted and broadened even more. Finally, for [B] = 3500
Wavelength (nm)
Z60
Z•o
z30
240
ppm (sample B1110), this trend is continued: The BET° and BET°+°r lines merge due to their largely enhanced widths, however, the two components remain distinguishable with energy spacing of around 165 meV = hfl (Or). At the same time, the lines are shifted down in energy significantly. A lineshape analysis demonstrates that the BE spectra
V
4 T
8KFIM
a)
BE,
3
\
B,
o.,ý 'Ir //
,further ,
b)
of all samples can be generated progressively from
T•219A
""the low-doping c)
71
case by simple Gaussian-broadening
Table 1: of the boron-doped diamond films indicating the List total boron and phosphorus concentrations as determined by SIMS measurements. The substrates were in all cases Si:B. Sensitivity limit of SIMS < 0.01 ppm. Sample
B-concentration (ppm) 20 Nr 3 S.2 40 TD219A 81 Nr 5 S.2 140 WD 13 225 Nr 7 S.2 340 W)D 12 500 Nr 8 S.2 630 Nr 9 S.2 715 Nr 10 S.2 825 B113 3500 Bll0
Photon Energy (eV)
Fig. 1:
Near-band edge boron-doped diamond CLspectra at 80 K. (a) Spectrum from a nominally undoped MW-CVD film grown on Si:B exhibiting TO-phonon and (TO+Or)phonon assisted free exciton radiation (FE', FETO+Or) along with boron bound exciton radiation (BET°). (b - d) Spectra from hot filament grown films with intentional boron doping of 40 ppm, 825 ppm, and 3500 ppm, respectively (see Table 1).
694
P-concentration (ppm) -
< 0.01 -
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