Electronic Theses and Dissertations (PhDs)
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Item The Low–Temperature Properties of Boron–Implanted Diamond Materials(University of the Witwatersrand, Johannesburg, 2024-08) Mahonisi, Nyiku Clement; Naidoo, MervinThe physio-chemical properties of semiconducting diamond materials under extremely low temperatures have fundamental implications in Condensed Matter Physics. Highly doped boron diamonds have been shown to reach a superconductive state at critical temperatures (Tc) ranging from 4 K − 10 K, albeit such properties are ”at the moment” only attributed to heavily boron-doped synthesized samples via HPHT and CVD growth methods. Theoretical predictions have shown that by exceeding the current solubility limit of boron in diamond, an increase in Tc beyond the 4 K − 10 K is possible, even close to room temperatures. However, in order to gain such a feat, an increase in active boron concentration beyond the metal-to-insulator transition (MIT) is an absolute necessity, and hence, non-equilibrium doping fabrication processes such as CVD growth and ion implantation are required. In this study, we explore carefully the properties of degenerate diamond layers with p-type impurity bands via low energy and low fluence ion implantation. The study involves the utilization of the non-equilibrium ion implantation technique to fabricate boron layers that stretches from the diamond surface to some depth that is a few nm deep into the diamond matrix. A total of seven samples were processed uniquely with respect to ion energies, ion concentrations and annealing temperatures. Three samples (A, B and D) with varying parameters were implanted with both carbon and boron ions in regions where the carbon distribution overlaps with the boron distribution at concentrations close to the MIT level ∼ 4 × 1020 ions/cm3. The carbon implants were used to induce vacancy defects very close to the surface such that the boron ions that are subsequently implanted would simply diffuse into the carbon vacant sites. Furthermore, four samples (BE,ME and SE) were implanted with only boron ions also with varying processing parameters to establish a correlation between the two set of implanted samples. Lastly, one sample (C) was implanted similarly to the first set of samples , albeit, with only carbon ions in order to ascertain the boron-related defects. Initially, all the samples were annealed at 650◦C in order to recover the crystalline state. Spectral analysis clearly confirms the formation of boron-related defects for the carbon and boron implants with dynamic annealing at 200◦C and at 400◦C, respectively. That is, the Lorentzian-like component ∼ 1200 cm−1 and the asymmetric line shape ∼ 1300 cm−1. However, vacancy and interstitial peaks at ∼ 1500 cm−1 and ∼ 1620 cm−1, respectively, are also prevalent. Albeit, dynamic annealing limits the graphitization of the diamond structure with no detection of graphitic features. A concentration and thermal annealing dependency was also established. The second batch of samples were multi implanted with boron (i.e., within the same buried region for samples ME and SE and overlapping implanted regions for sample BE to form a box profile). Boron-related features were not wholly detected for all these samples. After multiple implantation processes it was evident that the diamond structure of samples BE and SE were adversely affected with prominent graphitic features forming. Very faint boron-related features appear for sample ME after nine multiple boron implantation processes with low energy and low fluence at room temperature (RT). However, the spectra show the onset of broadening effects associated structural damage. Expectantly, boron-related features are absent for the carbon implanted sample C with very minimum damage to the diamond spectra. Low resistance tri-layer metal contacts of Ti/Cr/Au were deposited onto the surfaces of the samples in a 4-point probe Van der Pauw configuration. Ohmic behavior was confirmed from electronic transport measurements. Thus, the conduction properties of the samples are also reported in this thesis. A very clear dependence on the fabrication method used to create the boron buried layers is demonstrated by the electronic response of the samples. Sample A experiences Space Charge Limited Currents (SCLCs) related to high localization effects that compensate the movement of free charge carriers through the diamond matrix such that the reported data is limited to a temperature range that is between 300 K → 100 K. The carrier concentration of sample B determined from Hall effect measurements indicate a contribution of both electrons and holes, likely due to the amphoteric vacancy states induced by carbon implantation. However, a careful increase in boron fluence and thermal annealing averts such effects. Samples BE and SE showcase a high level of conductivity with variable range hopping (VRH) mechanisms at low temperatures ∼ 10 K that suggests an increase of the localization length ξloc with low TES values. The overpopulation of boron ions within the nm−sized channels of these samples results in amorphous regions that contribute to the conduction properties of the materials. Hence, a very clear difference of the conduction properties of the samples is demonstrated.