Ion implantation is a key' process for the introduction ofdopants in semiconductor technology. It involves bombarding thesubstrate material with energetic ions. One of the principleside effects of particle irradiation into crsystallinesemiconductor materials, like Si, is the disruption of theoriginal lattice caused by collisions with incident ionscreating atomic displacements. Point defects mediate dopantdiffusion in semiconductors, at temperatures where ordinarythermal diffusion is negligible. They also introduce energylevels into the forbidden gap of semiconductors.
These energy levels can serve as recombination centres forelectrons and holes. While recombination centres are unwantedin, for example silicon photo detectors, where carrier lossshould be kept at a minimum, they are deliberately introducedin silicon power devices to control their switching propertiesas they are effective 'lifetime killers'. For this defectengineering in silicon, proton irradiation is widely usedbecause of the fact that localised damage regions are createdand the damage profile can be precisely controlled by justadjusting the incident ion energy.
Implantation induced damage is a function of variousparameters like the energy of the incident ion, mass, dose,dose rate, substrate temperature among others. In this work anattempt has been made to study the effects of variation of theabove mentioned factors on the resultant damage.Very low dosesof ions have been employed and hence, defects formed constitutea very dilute regime of concentration. Deep level transientspectroscopy (DLTS) has been used for sample characterisationafter implantation. The most common defects observed in allroom temperature implanted silicon samples, irrespective ofmass, are the divacancy (V2) and the vacancy-oxygen(VO) centres in n-type,the divacancy (V2) and the carbon-oxygen (CO centres in p-type. Theconcentration of defects increases linearly with increasingdose, provided doses are not very high where these defectsinteract with each other forming higher order complexes. Doserate studies have revealed that the concentration of pointdefects decreases with increasing dose rate and this isatrributed to the fact that the rapidly diffusing siliconself-interstitials (I's) annihilate vacancies created inadjacent ion tracks, due to overlap of cascades, therebyreducing vacancy concentration and thus preventing theformation of vacancy type defects. The dose rate effect is evenmore pronounced for heavy' ion implantation and occurs at lowerdose rates owing to the larger size of an individual collisioncascade and a high density of I's. Implantation at elevatedsubstrate temperatures have indicated a relaxation of thelattice strain created due to ion bombardment and favoured theproduction of unperturbed V2centres as well as VO centres.
Implantation induced defects like V2and VO centres which involve broken or danglingbonds are easily passivated at room temperature by hydrogen andcopper. If introduced in a controlled fashion, copper andhydrogen can form. electrically active complexes withdefects.
Annealing studies show that V2and VO centres are less stable in Czochralskigrown (CZ)ion implanted silicon compared to electron irradiatedfloat zone silicon (FZ). This is because of a largeconcentration of impurities in CZ silicon like oxygen andcarbon and highly disordered regions in ion implanted sampleswhich act as effective traps for migrating point defects.
Institutionen för elektronisk systemkonstruktion , 1997. , 50 p.