Shallow Ohmic Contacts to p-InGaAs Based on Pd/Ge with Implanted Zn or Cd
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P. RESSEL', H. STRUSNY', D. FRITZSCHE**, H. KRAUTLE*" AND K. MAUSE** *Ferdinand-Braun-Institut fir H6chstfrequenztechnik, Rudower Chaussee 5, D-12489 Berlin, Germany **Telekom FTZ, Am Kavalleriesand 3, D-64295 Darmstadt, Germany ABSTRACT The development of shallow ohmic contacts for the thin p-lnGaAs layer (p< 1*1019 cm-3 ) of InPbased heterojunction bipolar transistors is a severe challenge to contact technology. While standard metallizations reveal several drawbacks, Pd/Ge-based systems emerged as promising candidates. In this work, Zn or Cd was implanted into the inner Pd contact layer for lowering contact resistivity. We present a study of the electrical and metallurgical properties of Pd/Ge contacts to p-InGaAs, and the influence of Zn and Cd implanted into the metallization. The resistivity of implanted and annealed (450575 "C) contacts is reduced up to one order of magnitude compared to the unimplanted contact. Backside SIMS measurements show that annealing leads to a very limited interdiffusion at the interface. Cd and Zn diffuse into the InGaAs layer to a depth of approx. 30 and 40 nm, respectively. Due to these features, this implanted Pd/Ge contact scheme is a promising candidate for shallow contacts to pInGaAs. INTRODUCTION Shallow low-ohmic contacts are required for the thin (50-100 nm) p-InGaAs base layer in high-speed InP/InGaAs heterojunction bipolar transistors (HBTs). To obtain a sufficiently high current gain needed for analog applications, the base layer doping is limited to 5-10*101" cm-3 . It is desirable to realize contact resistivities < 1*10.6 gcm2., 2 The small base thickness requires the use of nonalloyed or sintered contacts with only a very limited solid-state reaction between contact metal and InGaAs. Previous work shows that nonalloyed metallizations do not yield sufficiently low contact resistivity in the relevant
doping range.3'4 Therefore, an additional doping of the .base layer under the contact is necessary. This can be achieved employing alloyed contact systems with p-dopants like Zn or Be. Nevertheless, conventional Au-based metallizations like AuZn or AuBe suffer from inherent problems with the nonplanar metal - semiconductor interface and the comparably deep indiffusion of contact components. A potential solution to this problem is a nonalloyed contact system combined with controlled dopant diffusion from the contact during an annealing step. Pd/Ge-based metallizations are most promising as concluded from work on GaAs and InGaAs.5' 6 Here, the Pd germanide formation significantly reduces the extension of the reaction zone resulting in a shallow contact. Low contact resistivity is reported for the n-type semiconductors while p-doping > 10'9 cm-3 is necessary to achieve contact resistivities below 1*10`5 0cm2 .6' 7 To our knowledge, no attempt has been reported to introduce a p-dopant into the Pd/Ge metallization for improved contacts on p-InGaAs. This may be due to the wide acceptance of Pd/Ge as an ohmic contact for n-type semiconductors. However, the contact resistivities f
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