A Thermodynamic Approach to Ohmic Contact Formation to p-GaN
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UCTION GaN alloys have received great interest in the past decade due to applications in photonic and electronic devices. However, because of the low free hole concentrations of p-GaN (1017 cm-3) and lack of a metal with a work function φ equal to or greater than the bandgap plus electron affinity (Eg + χs = 7.5 eV), attempts to make low resistance ohmic contacts to p-GaN have been unsuccessful[1-12]. The purpose of this paper is to propose a general scheme by which metallization schemes for ohmic contacts can be systematically selected. The scheme is called “NOG” (Nitride-forming metal Over Gallide-forming metal) and is based on the thermodynamic stabilities of these phases during interfacial reactions between the metallization layers and the GaN semiconductor. PRINCIPLES OF “NOG” SCHEME Since as-grown GaN are intrinsically n-type, there may be a high concentration of native N vacancies, VN, which is equivalent to a Ga-rich condition in the film. An opposite situation could be postulated: if a N-rich condition could be created in as-grown GaN films, the extra N atoms could create Ga vacancies and intrinsically p-GaN films However, this postulated condition might still be achieved by interfacial reactions in the contact region. If extra N atoms could be kept between the contact metal layer and the bulk p-GaN film, a N-rich condition could be formed at the metal/GaN interface. The 1
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extra N atoms could fill the VN positions and create Ga vacancies which would act as acceptors. If the Ga vacancy acceptors were sufficiently shallow and reached a high concentration, the interfacial region could become p+-GaN and current transportation could be dominated by field emission or thermionic field emission. The principle of the “NOG” scheme is illustrated in Figure 1. A gallide-forming metal is followed by a nitride-forming metal covered with a layer of protective metal (such as Au). Under a suitable annealing condition, the gallide-forming metal would react with GaN to form stable gallides and release N atoms. This first metal layer must both dissociate the GaN lattice and prevent or slow down the process of nitrogen out-diffusion. The second nitride-forming metal would help keep the released N atoms at the contact interfacial region and create a N-rich condition.
Protection metal (Au) Nitride-forming metal Gallide-forming metal p-GaN
Figure 1. Principle of the “NOG” scheme. all transition metals may be classified into three groups: the late, early and middle transition metals based on the enthalpy of the metallurgical reactions, which was called gallide-forming, nitride-forming and neutral metals in this paper. The metallurgical reaction of these metals to GaN was reported in reference [13] and [14]. EXPLANATION OF LITERATURE Many studies have been reported along with postulated mechanisms to explain reduced contact resistance, such as GaN re-growth [12], H extraction [1] and Ni oxidation [2]. A few representative contact schemes will be discussed based on the principle o
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