EL ANÁLISIS DE CALIDAD REGULATORIA Y LOS GOBIERNOS LOCALES

Other IR absorption lines were observed to vary with heat treatment. The 7354 cm−1

absorption line, see Figure 6-14, increased in absorption from 1.8 to 7.0 cm−2 _with

heat treatments between 600 and 850 K. Other absorption lines 6857 and 6426 cm−1

in Sample J showed enhancement with heat treatment, see Figure 6-10(iii), but the variation of those line intensities observed in Sample H (Figure 6-14) was in-

sufficient to accurately distinguish. The enhancements of the 7354 cm−1 _{(1360 nm)}

line were also recorded using UV-vis absorption spectroscopy [36].

Time constraints did not permit the detailed study of the changes in the ab- sorption lines at 2807 and 2728 cm−1 _{observed during heat treatments. However,}

these should be studied further as potential candidates for the optical analogue of the NVH− _defect.

Chapter 6. Charge transfer and trapping in nitrogen doped CVD diamond 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 0 1 2 3 4 5 6 7 8 6 4 2 6 c m - 1 6 8 5 7 c m - 1 In te g ra te d a b s o rp ti o n ( c m -2 ) T r e a t m e n t t e m p e r a t u r e ( K ) ( 1 0 m i n t r e a t m e n t ) 7 3 5 4 c m - 1

Figure 6-14: Variation in the concentration of integrated intensity of 7354, 6857 and 6426 cm−1 _{absorption lines in Sample H, with isochronal heat treatment using}

the furnace method (see Section 6.2). The curves is provided as a guide to the eye only.

6.4

Discussion

In those nitrogen doped SC-CVD samples studied, the concentration of N0 S de-

creased with heat treatment (≤850 K). This affect is reversed by UV illumination at room temperature. Thus, this is not the result of a permanent change to the

defect, but a transfer of charge as described by Equation 6-1. FTIR absorption data is consistent with this interpretation, see Figure 6-10.

The activation energies for charge transfer observed here (see Table 6-3) are

insufficient to overcome the ionisation energy of N0

S (1.7 eV [1]), hence the transfer

of charge must occur through the valence band. This means that the activation

energies determined indicate the position of the traps with respect to the top of the valence band.

Two traps common to CVD diamond have been identified, X0

1 and NVH0, the

former through a discrepancy in charge balance and the latter through changes in the concentration of NVH−_{. In the CVD diamonds studied, X}0

1 must account

for 50–100% of the change in the concentration of N0

S. In Sample A, the X01

trap accounts for a large percentage of the change in the concentration of N0 S.

Chapter 6. Charge transfer and trapping in nitrogen doped CVD diamond

This suggests that the activation energy determined indicates the position of X0 1

(1.2(2) eV) with respect to the top of the valence band. Similarly, the activation energy determined from the increase in concentration of NVH− _{suggests that the}

trap, NVH0_{, lies 1.1(2) eV from the top of the valence band.}

Those samples with higher overall defect concentrations (samples I and J) have shown significantly lower activation energies (0.5(2) eV) for the change in

concentration of N0

S and NVH

−_{. In the case of N}0

S the defect concentrations are

approaching that where defect bands may be formed, ∼ 1017_cm−3 _{(1 ppm) [37,}

38] potentially affecting the activation energy. The concentration of NVH0 _is

significantly lower, but inhomogeneous defect distribution can lead to higher local

concentrations where defect bands could exist. Alternatively this could be the result of another trap (X0

2) which occurs in the higher concentration samples and

lies 0.5 eV from the valence band. It is also possible that additional contributing defects may have an affect upon the activation energies. Such affects have been

reported by Lagrangeet al. [38] in diamond and Lee et al. in silicon [39].

Four of the samples have been studied using TL spectroscopy. A TL peak

indicates the activation energy of carriers from their trapped states. The same process is suggested to be responsible for both the TL peaks and the charge transfer

observed by EPR and optical absorption spectroscopy because the temperatures observed are similar.

In samples A and C, the defect X1 accounts for ∼100% of the change in the

concentration of N0

S and both display a TL peak at ∼550 K. Whereas in samples

where NVH0 _{is present in significant concentrations the peak is severely lessened}

or not present at all. The defect labelled X1 may be responsible for the ∼550 K

feature and the presence of NVH− _{may act to broaden or reduce the intensity of}

it.

In the CVD diamonds studied, UV illumination at room temperature returned all observable defect concentrations to pre-heat treatment levels. It is suggested

that this occurs through charge transfer from the traps X−

1 or NVH

− _{to N}+ S via

the conduction band (see Figure 6-16). Figure 6-7 shows that the return in the

concentration of N0

S is spread over a range of energies. This implies that the

Chapter 6. Charge transfer and trapping in nitrogen doped CVD diamond CB VB NS0 X ~1.2eV W A R 5 W A R 7 W A R 8 0 +/0 +/0 N V H 0 ~1.0eV ~0.5eV 1 X2

Figure 6-15: Model suggested for the change in concentration of traps, X2, X1,

NVH−_{, WAR5}0_{, WAR7}0/− _{and WAR8}0/− _{as a result of thermal treatment. The}

defects are shown to lie 0.5 eV, 1.2 eV, 1.0 eV, 1.1 eV, 1.1 eV and 1.2 eV from the top of the valence band respectively.

CB VB N_S+ X ~4.5eV ~2.5eV ~1.0eV 1

Figure 6-16: Model for the photo-excitation of charge from the extended defect, X1, to N+S

at room temperature, restor- ing the pre-heat treatment concentrations.

the bottom of the conduction band. This is a large portion of the band-gap and if

a single defect type is responsible it might be expected to be extended in nature.

A potential candidate for X1 is the vacancy cluster. A vacancy cluster would

be expected to accept charge, and so might be expected to be EPR-active in one charge state. However, such a defect would occupy the already crowded central

g=2 region of the EPR spectrum. The size of the defect might also cause the lines to be broadened to a few mT, meaning that any changes would go undetected.

Another potential candidate for X1are dislocation boundaries which are commonly

but not uniquely found in CVD diamond [31].

The colour variation observed ‘by eye’ correlates with the changes in the ab-

sorption bands (see Table 6-4). In this work an absorption ramp as discussed by Jones [40] has been observed alongside bands at 360 and 520 nm, which decreases

in intensity with heat treatment. Any of these bands, or most probably a combina- tion of them, could be responsible for the variations in colour. These bands or the

aforementioned ramp, for which vacancy clusters have been suggested responsible [40], could also be optical analogues of the extended X0 _defect.

Chapter 6. Charge transfer and trapping in nitrogen doped CVD diamond

The changes in concentration of N0

S (Table 6-4) derived from measurements of

the 270 nm band [41] agree well with those determined by EPR.

6.4.1

Other contributing defects

Changes in a defect concentration can indicate its charge state. Here it is assumed that N0

S is the dominant donor and that the concentration of N 0

S is much larger

than the concentration of all other defects, because of this the position of defects in the band-gap were determined as follows; WAR5 (Chapter 9) decreased in

concentration with heat treatment, (Figure 6.6(b)), whilst WAR7 and WAR8 both increased in concentration. The activation energies are below 1.7 eV, suggesting

that the defect is a trap. The concentration of WAR5 reduces with heat treatment and at higher temperatures, when the energy available is greater than 1.7 eV and

electrons could be excited into the conduction band, the WAR5 signal does not recover. It is assumed that the decrease in concentration at 1.2 eV cannot be

the result of WAR5 donating electrons because it would be expected to re-trap electrons at higher temperatures, as is observed for WAR2 in Chapter 7, which

it does not. For that reason it is suggested that the WAR5 defect is accepting an electron, becoming more negative and so diamagnetic, thus the EPR-active

WAR5 defect is positive or neutrally charged. This is consistent with the defect model suggested in Chapter 9. The same argument can be made to explain the

increase in concentration of the WAR7 and WAR8 EPR defects and the defects responsible for the 7554 cm−1_{, 6857 cm}−1 _{and 6426 cm}−1 _{IR absorption lines. The}

defect concentration does not decrease at higher temperatures and so the defect must be more negative in its EPR-active state, see Figure 6-15.