The nature of ion-implanted contacts to polycrystalline diamond films
Introduction
Polycrystalline highly resistive diamond layers may find application as detectors for ionizing radiation, as needed, for example, in medical radiation dosimetry [1]. The advantages that diamond based detectors for medical applications have over the more commonly used Si and other semiconductor detectors are, among others, the similarity of the atomic number of diamond (carbon) to that of the human body tissue, the radiation hardness of diamond and its high breakdown field.
The commonly used structure of a solid-state semiconductor detector is the vertical contact-insulator-contact configuration, in which the detector material is sandwiched between two contacts. The application of a strong electric field across the detector enables the collection of the charges produced by the ionizing radiation while stopping in or traversing through the detector. For the case of diamond-based detectors the material used is usually a fairly thick layer (thick enough to stop the radiation to be detected) of undoped polycrystalline CVD diamond.
Much work has been devoted to the improvement of the detector material as far as charge transport and collection efficiency are concerned. Less attention was given to the electrical contact to the diamond and its role to the performance of the detector. The contacts usually used are thin evaporated metal (i.e. Ti/Au) layers, layers of carbide forming metals or baked-on silver paint dots [2]. The use of ‘tissue equivalent’ materials also for the contacts is desirable when medical dosimetry applications are required. Such can easily be realized in diamond by just converting the contact area from the highly insulating diamond to graphite with its high metallic conductivity. The conversion of diamond to graphite has been studied extensively for the case of locally modified diamond to graphite by heavily damaging it by ion-implantation [3], [4]. Indeed, ion-implantation is the method of choice to obtain ohmic contacts to semiconducting (p- or n-type) doped diamond in which case heavy dose implantation (to a dose which exceeds the graphitization limit) of the dopant atom is performed into heated diamond or into diamond held at RT followed by thermal annealing [5]. Most of the implanted dopants reside in the so-obtained graphitic layer but a small tail of the implant distribution extend beyond it into the semiconducting diamond. Chemical etching removes the graphite, leaving a shallow very heavily doped near surface region, which forms very good ohmic contact to the less heavily doped diamond. Implantation into diamond held at room temperature without applying further annealing results in partial graphitization of the damaged region [3], with the advantage that this form of heavily damaged diamond is highly conductive, yet, at the same time, still maintains some of the diamond-like physical and chemical properties, i.e. it resists the acid etch, often applied to clean and oxidize the surface. The same method used to contact semiconducting doped diamond can be used also to form ohmic contacts to diamond layers, which are conductive due to implantation related damage. In this case, implantations of non-dopant atoms (C, Ar, Xe) are usually used.
These methods are useful for contacting fairly conductive diamond (due to previous doping or damaging), it is, however, unclear if it is applicable also to contact intrinsic, highly resistive, polycrystalline CVD diamond.
Previous studies, in which planar contact geometries were used, have shown that the increase in conductivity of polycrystalline undoped CVD diamond with increasing damaging ion implantation (by C and Xe ions) much resembles that of type IIa single crystal diamond [6]. It was thus concluded in Ref. [6] that polycrystalline and single crystal diamond respond similarly to radiation damage, and that the grain boundaries play only a minor role in the electric properties of damaged diamond. However, the resistivities addressed in that work were not as high as those required for diamond detectors and they were measured on the surface of the samples. The role that the granular nature of polycrystalline CVD diamond and the role that the grain boundaries play in the leakage through the layer was not searched for, and hence was not noticed in that work. More recent work [7] has shown that there exist conduction pathways through thick layers of undoped polycrystalline diamond layers, which are attributed to percolative current flow in channels along the grain boundaries. In these measurements material which was not particularly resistive was used, and evaporated Ti/Au contacts were applied to thick slabs of free-standing diamond films.
In the present work, we address the question of the suitability of using implantation-graphitized contacts to highly resistive CVD diamond layers, as required for detector applications. Interestingly, we find a substantial decrease in the measured resistivity for implanted contacts in comparison with metallic contacts. This is a peculiar observation considering the fact that the sample thicknesses always greatly exceeded the range of the ions used (and hence the thickness of the graphitized contact layer).
Section snippets
Experimental
Undoped, highly resistive, polycrystalline diamond films were grown on silicon substrates in a computer controlled (Astex model AX5010) microwave reactor that was never exposed to any possible dopants (boron). To achieve high nucleation densities, the substrates were ultrasonically pretreated, in an alcohol solution containing diamond particles of micro-meter size. This was followed by ultrasonic cleaning in de-ionized water and acetone. Nucleation and growth conditions are summarized in Table 1
Results and discussion
The average results of two-point resistance measurements for samples of different thicknesses and implanted or metallic contacts (Ag paint or evaporated Mo) are given in Table 2. Table 3 summarizes the effect of variation from the above procedure namely: B implantation rather than C implantation, lower implantation dose, and post treatment effects (annealing and etching) which were all done on the same sample.. The resistances (R) through sample number 6 for both implanted and non-implanted (Ag
Conclusions
A dramatic decrease in diamond film resistivity using ion-implanted (graphitized) or B doped CVD diamond contacts has been observed. The present data indicate that evaporated or baked on metallic contacts to undoped, poly-crystalline diamond films seem to suffer from potential barrier, which limits current flow through the layer. This barrier is substantially reduced by using graphitized or highly conductive B doped contacts such as obtained by high dose ion-implantation or CVD over-growth.
Acknowledgements
This work was supported in part by the EU Project G5RD-CT-2001-00603.
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