Transport Properties of a Superconductor - Semiconductor Ohmic Contact

We report electrical measurements of ' a sandwich structure consisting of a niobium electrode in contact with a thin lightly doped n type InGaAs layer. The bottom of the sandwich is a degenerate layer of n-type InGaAs used to collect the current. The semiconductor layers are grown by molecular beam epitaxy (MBB). These three layers are the essence of the proposed superconducting-base, semiconductor-isolated transistor (SUBSIT), lacking only the emitter tunnel junction. It could also form the basis for a superconducting FET type device. We have observed a resistance rise, beginning just below the transition temperature of the niobium, and continuing to at least 2 K. Nonlinear IV curves are also measdred, and may be interpreted in some iustances as.space cliarge limited current flow. Introduction Contacts between supercondoctors and semiconductors have been proposed for use in 3 terminal amplifying and switching devices for at least the last ten years. Silver and his group'.' at Aerospace proposed an FET-like device in which the charge induced on the semiconductor surface by a gate potential would modulate the proximity effect on the semiconductor surface. T.D. Clark3 proposcd a similar device at around the same time. More recently, Frank4 and co-workers proposed a bipolar-like mechanism that would use the semiconductor to isolate the superconductive base from the collector. This device has been named SUBSIT (for Superconducting-Base Semiconductor-Isolated Transistor). Experimentally, Silver could demonstrate a reduction in resistance in his dcvices on InAs, but never demonstrated a supercurrent. Such supercurrents have now been observed in silicon and in both p type and n type Inlks by Nishino', Takayangi6 , Kawakamij and Kleinsasselg. Experimental work on the bipolar mechanism has been reported by KobayashiQ, and by Tamura". In no device, of either bipolar or field effect design, has useful gain been achieved. We have been studying the properties of a niobium contact to a ternary semiconductor, n type Ln,,Gal5As. Such a contact could eventually form the base-coilector structure of a bipolar SUBSIT, or a source or drain contact in an FET type device. o u r experiments show that the proximity effect is not necessarily as simple in a semiconductor contact as it is in contacts to norinal metal. Our work shows that the theory of the superconductive proximity effects needs to be extended to cover the situation when the "nomial metal" is actually a semiconductor. In particular, we observe a resistance increase below the critical temperature of the superconductor, rather than a decrease as the usual proximity effect theory and some other measurements would lead one to expect. Manuscript received September 30. 1986 727 Structure Layers Our samples are simiiar to those measured by others in that the contact of interest is between a deposited layer of niobium anti a semiconductor surface. Our samples differ from the FET work, however, in that we pass current through and measure voltage across a thin semiconductor layer that is in contact to a thick degenerate layer underneath., That is, our samples have a sandwich structure, with the semiconductor layer of interest between deposited niobium above and degenerate semiconductor below. As shown in the cross section in Fig. 1, we have used 100 nm thick niobium, 200 nm thick semiconductor and 1000 nm thick degenerate layer. n Layer Active p Layer Niobium n+ InGaAs 200 nm Contact 40 nm 100 nm 1000 nm