Measurement of the Activation Energy in Phosphorous Doped Polycrystalline Diamond Thin Films Grown on Silicon Substrates

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ABSTRACT Although many researchers have studied boron-doped diamond thin films in the past several years, there have been few reports on the effects of doping CVD-grown diamond films with phosphorous. For this work, polycrystalline diamond thin films were grown by hot filament chemical vapor deposition (HFCVD) on p-type silicon substrates. Phosphorous was introduced into the reaction chamber as an in situ dopant during the growth. The quality and orientation of the diamond thin films were monitored by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Current-voltage (I-V) data as a function of temperature for golddiamond film-silicon-aluminum structures were measured. The activation energy of the phosphorous dopants was calculated to be approximately 0.29 eV. INTRODUCTION In order for polycrystalline diamond film to be considered for electronic applications, an effective and controllable method for both p- and n-type doping must be found. There are many reports on the formation of prototype devices on diamond doped with boron. 14 The difficulty in doping of diamond arises from the fact that C atoms have a small tetrahedral radius of 0.77 A, and almost all impurity atoms (except B) are larger and deform the lattice. The solubility of a dopant atom in diamond is related to its formation energy (Ef) which is calculated based on the bulk cohesive energy of diamond and bulk cohesive energy of the impurity. The B atom has a negative Ef which explains its ease of incorporation into the diamond lattice. Phosphorous, on the other hand, has a low equilibrium solubility in diamond as a result of its high formation energy (Ef = 10.7 eV), which explains the difficulty of n-type doping of diamond with phosphorous. Okano et. al. 5' 6 used phosphorous pentoxide (P2 0 5 ) as a source of n-type doping. They have reported the formation of a diamond p-n junction diode, but since the electron mobility is extremely low (only 50 cm 2 /v.s), n-type conductivity may be due to lattice damage and not P incorporation into the diamond lattice. The main problem with doping diamond film n-type seems to be the low degree of ionization of the P atoms at room temperature. Other dopant sources such as Li and Na as well as the effect of N impurity (as a shallow donor) have been investigated. 7 Ramesham et. al. 8 found an increase in the resistivity of their diamond films as a result of P incorporation. As a general rule, it is often difficult to make ohmic contacts to large bandgap semiconducting materials such as diamond. Most of the methods utilized to form ohmic contacts to bulk semiconducting diamond have involved roughening, graphitizing the diamond surface, or carbide formation under the contact.! Ohmic contact to p-type homoepitaxially grown diamond has been created by intense argon-fluoride excimer laser radiation. 9 In order to measure the activation energy of the P atoms incorporated in a diamond film,

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Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society

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