Origin of brown coloration in diamond

Measurements of the absorption spectra of brown natural type IIa diamond as well as brown nitrogen-doped CVD diamond are reported. These are largely featureless and increase almost monotonically from about 1–$5.5\mathrm{eV}$. It is argued that the brown coloration is due to an extended defect and not to a point defect. First principles modeling studies demonstrate that the spectra could be attributed to vacancy disks lying on $\left\{111\right\}$ planes. Such disks are unstable above about 200 vacancies and should relax to dislocation loops in natural diamond. Hydrogen is shown to passivate the optical activity of the disks.

Figure 1

Experimentally measured absorption spectra of five natural type IIa crystals and two samples of single crystal CVD grown brown diamond. The natural stones exhibit a featureless continuous absorption with no evidence of any threshold. The CVD samples show in addition some broad peaks which may be related to point defects. The absorption coefficient of CVD-4 has been multiplied by 0.01 for ease of comparison.

Figure 2

Band structure of bulk diamond, calculated in a 2 atom cell. The energies of states above $E_v$ are scaled by 1.3 to reproduce the direct and indirect bandgaps at 5.5 and $7.3\mathrm{eV}$, respectively. The valance band top is set to $0\mathrm{eV}$.

Figure 3

Illustration of the removal of a $\left\{111\right\}$ double plane (boxed) in bulk diamond and the subsequent surface rebonding leading to a vacancy disk with chains of $\pi$-bonded atoms along $⟨1\overline{1}0⟩$, shown in white for clarity.

Figure 4

View of the {111} plane with a $(2\times 1)$ Pandey chain reconstruction. The $\pi$-bonded atoms are smaller and colored white for clarity. The length of the $\pi$-bond is $1.426\mathrm{\AA }$.

Figure 5

Band structure of the vacancy disk in a 40 atom cell. The valance band top at $\Gamma$ is set to $0\mathrm{eV}$ and the higher energy levels are scaled by 1.3 as described in the text. Note the absence of any pronounced gap in the spectrum suggesting a featureless absorption spectrum. Dashed lines are unoccupied levels, while solid lines show filled levels. The center of the Brillouin zone is marked $\Gamma$, and other $\mathbf{k}$-points denoted with their positions relative to the primitive unit vectors of the Brillouin zone of the 80 atom cell.

Figure 6

Absorption of natural and CVD grown brown diamond compared to absorption of $\left\{111\right\}$ vacancy disk in an 80 atom cell. The absorption of the vacancy disk has been multiplied by a factor of $0.00025$ for clarity.

Figure 7

Log-log plot of absorption coefficient of natural type IIa brown diamond, and two samples of nitrogen doped CVD diamond along with lines $E^2$ and $E^3$ which indicate the energy dependence of the absorption. The calculated absorption coefficient for a $\left\{111\right\}$ vacancy disk in an 80 atom cell, multiplied by a factor of $0.00025$, is also shown. Note that it follows closely the absorption of the more transparent CVD-1 sample. The theoretical spectra below $2\mathrm{eV}$ and above 5eV are not reliable (see text). The broad band at $4.6\mathrm{eV}$ in CVD-4 is attributed to substitutional nitrogen but the bands at 2.48 and $3.39\mathrm{eV}$ are unaccounted for and may be related to other defects not detected in natural IIa brown and which are less prominent in CVD-1.

Figure 8

Hydrogenation of the [111] surfaces, passivating the $\pi$-bonds. Hydrogen atoms shown in white.

Figure 9

Band structure of the hydrogen terminated $\left\{111\right\}$ vacancy disk. It can be clearly seen that the band gap is cleared of states, allowing no optical absorption in the visible range. Valance band top set to $0\mathrm{eV}$. Dashed lines are unoccupied levels, while solid lines show filled levels. Brillouin zone points denoted as before. Energy levels above the top of the valance band are scaled by a factor of 1.3.

Figure 10

Illustration of the removal of a $\left\{110\right\}$ double plane (boxed) in bulk diamond and the subsequent surface rebonding leading to a vacancy disk with chains of $\pi$-bonded atoms along [001], shown in white for clarity.

Figure 11

Band structure of an infinite vacancy disk on the {110} plane. The break in levels in the region of the top of the bulk band gap, leads to a nonfeatureless absorption spectrum. Dashed lines are unoccupied levels, while solid lines show filled levels. Brillouin zone points denoted relative to the primitive unit cell. Energy levels above $E_v$ are scaled by a factor of 1.3.

Figure 12

The calculated optical absorption of a vacancy disk on the {110} plane scaled by a factor 0.0025 for comparison, compared to the disk on the {111} plane (scaled by a factor $0.00025$) and one natural and one CVD sample. Absorption from the {110} disk clearly shows several features such as a broad band at $\sim 2.8\mathrm{eV}$ and a dip in absorption around $4\mathrm{eV}$ where there are fewer band gap levels.