Fases de desarrollo del proyecto de Investigación

C h a p te r 7: A tm o sp h eric E ffects on Surface C on du ctivity

annealed [Looi 1998], as at lower temperatures some but not all o f the hydrogen termination is removed and thus there is less transfer doping through dipoles.

In vacuum the hydrogen termination is not exposed to significant amounts of oxygen and hence it is far more stable. Obviously, as the sample is heated, adsorbates will be dislodged and the conductivity will decrease as seen in figures 7.2 and 7.3. when the diamond is cooled after annealing, it stabalises at a much lower value of conductivity as the adsorbates are no longer present. The trend in figure 7.4 is ju st thermally stimulated current, as there are far fewer adsorbates the dominant process on cooling is the freezing out of thermally generated bulk charge carriers. Figure 7.5 shows how the conductivity is replaced by venting the sample to air, this is due to the redeposition of adsorbates at the surface of the diamond, a phenomenon also seen by Sung Gi and Ristein [Sung Gi 1995, Ristein 2001]. Figures 7.7 and 7.8 show the reproducibility of this effect and hence the stability of the hydrogen dipole under this vacuum. Looi et al [Looi 1998] suggested that the annealing process was identical in air and in vacuum, however, they did not measure the sample after the chamber had been vented to air and hence did not see the conductivity return. Szameitat et al [Szameitat 2000] reported similar observation to those above but found that the rate of return of conductivity was very low. It is not clear why their results conflict with those above or reported in the literature [Ristein 2001]. They suggest that the adsorbate required to induce conductivity is not very common in atmospheric air, in contradiction with the model [Sung Gi 1995]. Another possibility is that the air in their lab had a very low humidity or there was very poor circulation in their vacuum chamber and hence the sample took a long time to saturate.

Figures 7.9 and 7.10 show persistent photoconductivity in hydrogenated diamond films. Figure 7.9 (A) shows that the film is in fact sensitive to even visible light, but this is not entirely surprising as the film is polycrystalline and hence contains many defects. Figure 7.9 (B) shows the increased sensitivity expected to UV light and a clear persistent photoconductivity effect, this effect has been observed in other wide bandgap materials including diamond [Gonon 1997, Horng 1999]. The decrease in current after pumping is due to small heating effect from the illumination. There is also the effect of the negative electron affinity (NEA) of the hydrogen terminated

C h a p ter 7: A tm o sp h eric E ffects on Surface C on du ctivity

diamond surface, if an electron can reach the eonduction band, i.e. through pumping then it can leave the sample and enter the vaeuum. However, it has been shown that the NEA of hydrogen terminated diamond is not m aintained unless under UHV conditions [Ristein 1998]. It is therefore more likely that the UV effect is related to persistent photoconduetivity, as defect states in the films can be used to “ladder” electrons into the adsorbate layer.

This model so far requires the presence of the hydrogen dipole, but this alone is not sufficient. Figure 7.11 shows the effeet of removing the hydrogen dipole under vacuum. The conductivity is totally annihilated after 1 second of argon bombardment and does not return upon venting the sample to air. This means that the atom beam is not just removing the adsorbates from the diamond surface as they would come back as air was let into the chamber. The increase in conduetivity after 1 second is slight and due to damage on the surface from the atom bombardment. After bombardment it is impossible to rehydrogenate the sample without prior surface treatments to remove surface damage. This damage can be removed by oxygen plasm a treatment or just annealing at high temperatures in air to “burn off” the near surface. A similar result is recorded in the literature where one side of a sample was electron bombarded and another not; upon contact with air, the un-irradiated side became conductive while the electron iiTadiated area did not [Ristein 2001]. As the energy of the electrons was very low (IK eV ), any argument about the bombardment effecting the bulk is void.

Hall measurements have also been used to quantify the effects of gases on the surface conductivity layer [Sung Gi 1999]., these measurements showed an increase in carrier concentration in oxidising gas (NOg) and a decrease in reducing gas (NH3) atmospheres.

Figures 7.12 and 7.13 show direct evidence of the reduetion in carrier eoneentration due to applied vacuum and the relative stability of the m obility under these conditions. Figure 7.15 shows the increase in hall voltage during annealing and hence the decrease in carrier concentration, the offset also increases marking the increase in the sheet resisitivity. The value at 440K in this figure shows the signal before the film

C h a p ter 7; A tm o sp h eric E ffects on Surface C o n d u ctivity

sloped. Figure 7.16 shows the analysis of this data, the mobility is seen to increase slightly as the carrier concentration decreases. The carrier concentration obviously decreases because of the desorbtion of adsorbates, and at higher temperatures there are also thermally stimulated carriers. W hen the sample is brought back to room temperature without venting, the carrier concetration drops further still due to the loss of the thermally stimulated carriers. Annealing has a rather more subtle effect on the mobility values. As the sample is heated the carriers have more thermal energy and hence are less confined by the surface dipole. This means that they will experience far less coulomb scattering and surface scattering and hence the mobility increases. This increase is ultimately limited by phonon scattering. When the sample is brought back down to room temperature the mobility value drops because the thermal energy is decreased leading to enhanced confinement and the associated increased scattering.

C h a p te r 7: A tm o sp h eric E ffects on Su rface C on du ctivity