TEM Structure Characterization of Ti/Al and Ti/Al/Ni/Au Ohmic Contacts for n-GaN
- PDF / 3,309,178 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 16 Downloads / 233 Views
ABSTRACT Transmission electron microscopy has been applied to characterize the structure of Ti/Al and Ti/Al/Ni/Au ohmic contacts on n-type GaN (-1017 cm-3) epitaxial layers. A thin polycrystalline cubic TiN layer epitaxially matched to the (0001) GaN surface was detected at the interface with the GaN substrate. This layer was studied in detail by electron diffraction and high resolution electron microscopy. The orientation relationship between the cubic TiN and the GaN was found to be: 11 (111 }Tr.N/{ 0 0 .1 } [1Ia,10 r.//[ .0]G, [l 1 2 ]TN//[ 1 0. 0 1G&. The formation of this cubic TiN layer results in an excess of N vacancies in the GaN close to the interface which is considered to be the reason for the low resistance of the contact. INTRODUCTION Nitride-based electronics and optoelectronics requires high reliability of metal contacts for on contact resistance. Recent success in fabrication of such devices as visible light-emitting diodes (LEDs)"-3 and metalsemiconductor field-effect transistors (MESFETs) 4 evidences that GaN and related compounds are very promissing for optoelactronics applications and, in particular, for high temperature applications. Nitrides are known to have an excellent thermal conductivity, high saturation velocity and large breakdown field. 5 Thus, a development of a low resistance ohmic contact for GaN is of great practical importance. A few attempts to achieve good ohmic contacts on GaN epilayers have been reported recently. 5 -8 Most of these metallization schemes include Ti as a first metal layer and reactive ion etching (RIE) of the GaN surface prior to the metal deposition. Low resistance contacts2 to GaN were formed as a result of these processes, even without annealing (e.g. 6.5x10-5 -2cm in the case of a Ti/Ag contact 6 ). Similar results were reported for W and WSix contacts. 8 The reason for the low resistivity of these contacts is still under discussion. It is resonable to assume that the low resistivity might be related to the interaction of Ti or W with the GaN. As a result, the formation of TiN or WN layers on the GaN surface and, associated with them, an excess of N vacancies near the interface is a possible cause for the low contact resistivity. However, until the present study no direct evidence of the formation of such a layer or its structure has yet been reported. n- and p-GaN because the performance of devices strongly depends
The structure and morphology of the Ti/Al and Ti/Al/Ni/Au thin composite layers which provide ohmic contacts on n-type GaN (_1017 cm- 3 ) epitaxial layers are the focus for the present study. Here, we report results on the metallurgy of GaN/Ti/Al and GaN/Ti/A1/Ni/Au contacts which form a thin cubic TiN layer on the GaN surface. 201 Mat. Res. Soc. Symp. Proc. Vol. 423 © 1996 Materials Research Society
EXPERIMENTAL 3 gm-thick n-GaN layers were grown by molecular beam epitaxy (MBE) on c-plane A120 3. 7 The GaN layers contained two sublayers, a 2 gm thick Si-doped layer with a carrier concentration of about 5x10 18 cm- 3 and a lgm thick undoped
Data Loading...
