The Microstructure And Electrical Properties Of Oxygen Ion Implanted Nanocrystaline Diamond Films

The preparation of n-type diamond films with high quality, highcarrier mobility and low resistivity have hampered the developments ofdiamond in the electronic application area for a long time. The intrinsicnanocrystalline diamond （NCD） films were prepared by usinghot-filament chemical vapor deposition method. These films areimplanted by oxygen ion with different doses. The impacts of annealingtemperature and the ion implantation dose on the microstructure andelectrical properties of NCD films were investigated systematically.Researches relating to this work provide meaningful scientificsignificances and high practical values to the preparation and applicationsof n-type diamond films with better electrical properties.Oxygen ion with the dose of1×10¹²cm-2was implanted into NCD films followed by different temperature annealing. Themicrostructure and electrical properties of NCD films have beeninvestigated. The results show that the nano-sized diamond grains havedifferent growth mechanism under different annealing temperatures. Thatis, the size of grains grows bigger through a phase transition fromamorphous carbon grain boundaries （GBs） to diamond phase duringannealing with temperature below1000℃, and the size of diamondgrains is3-5nm before and after annealing. However, the Ostwaldripening process occurs by aggregating the small grains to20-50nm insize after1000℃annealing, which is almost one order of magnitudelarger than that of the unannealed sample. In addition, the hydrogenbonded to the TPA chains can be out-diffused at such high annealingtemperature significantly, and results in the shortness of the length of thechains. Without hydrogen, part of short TPA chains with the main-C-C=C-chain characteristic feature can form a six-membered ring ofgraphite-like structure during such high temperature annealing, whichapparently increases both the size and the disordering of the graphiteclusters. Hall effect measurements show that both oxygen and carbon ionimplanted NCD films exhibit a p-, n-type transition phenomenon at theannealing temperature range of725-800℃. The NCD films exhibitp-type conductivity when they are annealed below this temperature range,while they exhibit n-type conductivity when they are annealed above this temperature range. It is found that900℃annealing can better restore thedamages caused by ion implantation and activate more electronic carriersin the NCD films, so that the films exhibit better n-type conductivity. Theoxygen dopants probably occupy the substitutional or interstitial positionsof the diamond crystal lattice so as to donate extra electrons for theconduction of the films, and promote the grains offering conduction.Oxygen ion implanted NCD films with different doses wereprepared and all annealed at900℃subsequently. The microstructure andelectrical properties of NCD films have been investigated. The resultsshow that the ion implantation can etch the crystal and amorphous phasesin the NCD films. The heterogeneous elements （oxygen ions） show arather more significant etching effect on the loosen-structured amorphouscomponents among the films, like sp²graphite clusters andtrans-polyacetylene （TPA） chains. Meanwhile, the ion implantation canalso increase the compressive stress in the films to a very high level. It isobserved that the relative high oxygen ion implantation dose (1×10¹⁴cm^-2) would significantly damage the quality of NCD films. Hall effectmeasurements show that, NCD films implanted with different low oxygenion doses and subsequently annealed at900℃exhibit better n-typeconductivity with high mobility. Specifically, the sample implanted withdose of1×10¹¹cm^-2has a mobility as high as303cm2·V^-1·s^-1, which isexpected to prepare the pn junctions of NCD films. With the means of Raman, XPS and Flourier transform IR（FTIR） spectroscopy, the microstructural changes of sp²and sp³carbon aswell as the hydrocarbons in the1×10¹⁴cm^-2oxygen ion dose implantedultrananocrystalline diamond （UNCD） films were studied. A phasetransition mechanism occurring between the diamond phase andamorphous GBs in the films at different annealing temperatures isobtained. The results show that the¹210cm^-1peak in the Ramansp^ectrum can be assigned to the hydrogen-rare amorphous sp³C clustersand/or extremely small sized diamond grains （marked as a-sp³C clusters）.As the films were annealed in the range of800-900℃, the hydrogenstarted to diffuse and reacted with the surface terminated hydrogens ofdiamond grains, likely to form H2. As a result, some active sitessimultaneously are created on the surfaces of diamond grains, which thenused to promote the a-sp³C clusters connecting to the diamond grains,and finally results in the a-sp³C clusters transforming to diamond phase.However, bulk hydrogen out-diffusion occurs at1000℃. As lack ofsurface terminated hydrogen, the stability of diamond phase is damaged.Therefore, diamond phase forced to collapse to be smaller and less andeven transform to the a-sp³C clusters, inducing a converse phasetransition from diamond phase to a-sp³C clusters. The hydrogen diffusionprocess in the UNCD films accounts for the phase transitions duringannealing.