Novel resistance reduction and phase changes of contacts to n-type InP by rapid thermal annealing
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Novel resistance reduction and phase changes of contacts to n-type InP by rapid thermal annealing J.S. Huang, T. Nguyen and N. Bar-Chaim Agere Systems, Optical Access Division, 2015 W. Chestnut Street, Alhambra, CA 91803 C.B. Vartuli and S. Anderson Agere Systems, 9333 S. John Young Parkway, Orlando, FL 32819 J. Shearer and C. Fisher Agere Systems, 2525 N. 12th Street, Reading, PA 19612 ABSTRACT We studied the influence of n-metal alloy on the long wavelength InP device performance. Various alloy schemes of rapid thermal annealing (RTA) were experimented to obtain the optimized contact resistance for the n-InP/AuGe/Ni/Au/Cr/Au metallization systems. Significant resistance reduction was achieved at 390°C for 45sec with wafer flattening step at 310°C. Using scanning transmission electron microscopy (STEM) and Auger electron spectroscopy (AES) analyses, we showed that resistance was correlated with interfacial reaction at the n-InP/metal. For the high resistance devices, little interfacial reaction between n-InP and Au occurred. For the low resistance devices, significant out-diffusion of P in the bottom Au and Ni layers occurred, forming Au-P and Ni-P metallic compounds. In addition, accumulation of Ge in the Ni layer was also detected. We suggest that Ni-P is very critical in obtaining low contact resistance for n-InP. INTRODUCTION Reduction of resistance is important for semiconductor lasers and photodiodes. Several issues may arise as a result of high resistance. First, high resistance can cause excessive device heating during operation and in turn affect device performance as well as reliability. Second, device speed will be reduced due to RC delay. One of the key contributions to high resistance is from contact resistance. Achieving low specific contact resistance to n-InP and p-InP is particularly important due to its wide use in the long wavelength application. Various combinations have been employed to make ohmic contacts to n-InP and p-InP [1-3]. In this paper, we experimented with various alloy schemes to investigate their effect on device resistance. Very low resistance was achieved at the optimized alloy condition. To correlate the resistance behavior with morphology, scanning transmission electron microcopy (STEM) and Auger electron spectroscopy (AES) were used to examine the samples of various resistances. STEM cross-section images revealed that the high device resistance was related to the insufficient interfacial reaction between InP and n-metal. For the low resistance samples, significant amount of interdiffusion was found. The entire bottom Au layer was consumed to form islands of Au-P compounds. A layer of Ni-P compound was present, which may be very critical in obtaining low contact resistance.
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EXPERIMENTAL Epitaxial p-type InP layers were grown on n-type Sn-doped InP substrate by metal organic chemical vapor deposition (MOCVD) technique. Prior to p-metal deposition, p-InGaAs was grown to form contact layer. Au/Zn/Au/Cr/Au metallization was deposited on the p-InGaAs layer and al
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