Exciton diamond

Huffman, D. R., L. A. Schwalbe, and D. Schiferl, 1982. Use of smoke samples in diamond-anvil cells to measure pressure dependence of optical spectra application to the ZnO exciton, Solid State Commun. (in press). 

In order to investigate the spatial distribution of B atoms in a B-doped HOD film, Graham et al. [415] deposited a 4-pm thick B-doped diamond layer on an undoped HOD film of 30-pm thickness. The B-doped layer was deposited using CH4, H2, and B2H2, where B/C = 44 ppm in the source gas. The specimen was thinned from the HOD film side so that TEM and CL measurements could be done for the same position of the specimen. In the CL spectrum related with dislocations, there were two bands at 2.87 eV (431 nm) and 2.32 eV (535 nm) due to bound excitons. A comparison between the TEM image and the monochromatic CL images for 2.87 and 2.32 eV indicated that the B dopants were distributed uniformly within the film on the submicron level. Furthermore, the incorporation of B dopants created dislocations in the film. 

The FEs can bind to neutral shallow impurities and become bound excitons (BEs), with a value of Eex slightly larger than the one of the FE. The difference is called the localization energy E oc of the BE. For the P donor, it is 4 meV in silicon, but 75 meV in diamond. E oc is given approximately by Haynes empirical rule [20] as 0.1 A, where A is the ionization energy of the impurity. BEs are created by laser illumination of a semiconductor sample at an energy larger than Eg and the study of their radiative recombination by PL... 

Fig. 3.10. Calculated direct excitonic gap of wurtzite-type (upper line) and sphalerite-type (lower line) CdS spherical clusters as a function of the cluster radii, compared with the experimental results. Full diamonds and circles are for sphalerite-and wurtzite-type clusters, respectively. The exciton binding energy in bulk CdS is 0.03eV (after [94]). Copyright 1996, American Institute of Physics...

Defects and impurities, in general, play a comparably important role for the luminescence properties of nanodiamond like they do for the bulk material. Owing to their existence, there are electronic states situated within the bandgap, which allow for inducing luminescence in nanodiamond samples also with longer wave radiation. Upon excitation with wavelengths between 300 and 365 nm, fluorescence bands are observed at more than 400 nm. They arise from various nitrogen defects. In comparison to bulk diamond, the Ufetime of the excited states is rather short, which possibly is due to the effect of surface states and to the increased density of excitons on the surface. 

There has been some interesting progress on the excited-state calculations in insulating solid systems. Mitas and Martin [59] calculated an excited state for a solid by evaluating an exciton in a compressed nitrogen insulating solid. A similar calculation was also carried out for diamond... 

Fig. 11.10. The DMRG calculated transition energies in pam-phenylene oligomers of a number of and states as a function of 1/AI, where N is the number of repeat units. Calculated from the Pariser-Parr-Pople model with unscreened parameters U = 10.06 eV, tp = 2.539 eV, td = 2.684 eV, ts = 2.22 eV, and e = 1. The low-lying states are branches of the n = 1 family of Mott-Wannier excitons and the low-lying states are branches of the n = 2 family of Mott-Wannier excitons. (See also Fig. 6.5.) l Rf (large, open circles), (open, down triangles), (up triangles), 4 R(" (diamonds) 2 21 (small, solid circles), 3 21 ...

The spectrum of Fig. 5a is compared in Fig. 6 with the edge emission from a natural type Ilb diamond. Here, in addition to the free exciton peaks, we see features D, and D(, which are associated with the recombination of excitons bound to the boron acceptor. The very weak zero-phonon lines Do and Dj (not visible in Fig. 6b) occur at energies (Do) = Eg - E and (DJ) = E g - 4x where Eg and are the energies of the excitons associated with the upper and lower valence bands, and 4 and E 4 are the binding energies of the upper and lower valence band excitons to the neutral acceptors. The peaks Dj and D( are TO phonon replicas of Dq and D, and a further replica D2 is clearly visible. 

The intensity of the bound-exciton peaks, relative to that of the free-exciton features, gives an indication of the uncompensated boron concentration in the region of the diamond examined. For the diamond shown in Fig. 6b this is about 5 X 10 cm as determined from Hall effect measurements (31). A very weak peak D, due to the accidental presence of a small concentration of boron (estimated as 3 X 10 cm ), is also evident in the low-temperature spectrum in Fig. 5. 

Edge emission can be detected only from diamonds that are relatively free from defects. The first observation of free-exciton luminescence from CVD diamond was made by Collins et al. (49), who examined individual particles grown by the microwave process. Kawarada et al. (50,51) subsequently observed intrinsic edge emission from single particles of their CVD material and also bound-exciton recombination from boron-doped polycrystalline diamond films. Weak intrinsic free-exciton luminescence has also been observed from polycrystalline... 

The free-exciton emission is strongest in diamonds with low concentrations of defects (46), but even in the best natural diamonds this luminescence is weak compared with the luminescence observed in the visible spectral region (discussed briefly in Sec. III.D). By contrast, measurements in the author s laboratory in 1995 have shown that in very high purity synthetic diamonds the free-exciton emission is strong, compared with the visible luminescence. Some polycrystalline CVD specimens examined also exhibit relatively strong edge emission, and in a few homo-epitaxial layers of CVD diamond the free-exciton luminescence is dominant. This indicates that diamond can now be manufactured with a considerably lower defect density than that found in the best natural diamonds. 

Figure 10.9 Near-edge X-ray absorption fine structure (NEXAFS) for clean (a) and hydrogen-terminated (b) diamond (100). The resonances Spi, Srj,4, and Sh imply unoccupied surface states in the band gap for the respective surfaces. For the clean surface, the symmetry of the surface core excitons could even be determined from...

Experimental data on unoccupied surface states are again only indirectly obtained via NEXAFS [68]. As for the clean diamond (100) surface, a surface core exciton with an excitation energy 4.8 eV lower than that of the bulk core exciton is observed that can be taken as a qualitative confirmation for the existence of unoccupied surface states within the band gap (S in Figure 10.15a). For C(lll)l x 1 H only the... 

When considering surface states in the band gap one should distinguish occupied (donorlike) and unoccupied (acceptorlike) states. Those of the latter type were not directly accessible experimentally so far, but in fact found in band structure calculations of all the surfaces discussed above. Quahtative confirmation of their existence within the band gap was for the (100) and the (111) surfaces obtained from NEXAFS in form of clear surface core exciton resonances. The unoccupied surface states are not electronically active for p-type material, but are expected to become important for n-type diamond. Occupied surface states in the band gap are found only for the clean diamond (111) surface, but can be removed by hydrogen or oxygen termination. All diamond surfaces are semiconducting. In the case of the clean C(lll)2xl and C(110)lxl surfaces, which show symmetric and unbuckled 7T-bonded rows of surface atoms, many-body effects are responsible for the opening of a surface band gap, which caimot be modeled theoretically on the DFT level. Table 10.2 summarizes the reconstructions and surface state distributions of the diamond surfaces discussed above. 

An entirely new situation arises for a semiconductor with NEA. Here the threshold for photoemission of electrons is the band gap energy, that is, a bulk rather than a surface property, and novel phenomena are to be expected. Tbis is indeed the case, and the most spectacular of these phenomena is certainly the contribution of bulk excitons to the photoelectron yield of diamond surfaces with NEA as first reported by Bandis and Pate [73, 107]. In addition, the depth from which electrons contribute to the yield is no longer limited by the inelastic mean free path of some tens of Angstroms but by the diffusion length of electrons and excitons of the order of micrometers, a fact that is responsible for the near 100% quantum efficiency of NEA diamond surfaces alluded to earlier (Section 10.3). 

With this in mind, let us now return to Figure 10.27 and have a closer look at the spectral features at threshold. It has been shown by different groups that the shape of the yield spectrum in this region is indeed determined by the bulk absorption coefficient ( >) of diamond [73, 114], and Figure 10.29 gives a recent example of the Yield near 5.5 eV. The three pronounced thresholds at 5.26, 5.48, and 5.54 eV and a fourth one just discernible at 5.32 eV are identical to the ones seen in the optical absorption spectrum [115]. It is disconcerting, however, that these thresholds correspond to the excitation of excitons, that is, bound electron-hole pairs that are a priori not expected to contribute to free electron emission into vacuum. 

Figure 1030 Energy diagram for the indirect optical excitation of excitons in diamond with the involvement of phonons to overcome the (c-vector difference between valence band maximum at P and conduction band minimum near X.

There are two rather spectacular consequences of the excitonic nature of the absorption processes that lead to electron emission near threshold in diamond. One is the oscillatory yield (Figure 10.31a) the other is the strong influence of surface band bending on the Yield that has considerable diagnostic potential. 

Figure 10.32 (a) Yield spectra of two hydrogenated diamond (100) surfaces, one with (0.18 iS) and one without (25 pS) surface conductivity. The differences in the spectra are accounted for by the influence of surface band bending on the electrons and not on the excitons as sketched in the inset. 

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