Ž . Materials Science and Engineering C 15 2001 63–65 www.elsevier.comrlocatermsec Experimental determination of the Andreev reflection probability using ballistic point contact spectroscopy M. Jakob, J. Appenzeller ) , J. Knoch, H. Stahl, B. Lengeler II. Physikalisches Institut, RWTH Aachen, Templergraben 55, 52056 Aachen, Germany Abstract In this article, we present an experimental technique to determine the Andreev reflection probability—a quantity of central importance Ž . for superconductorrsemiconductor-hybride structures. This is achieved with a point contact PC formed in a two-dimensional electron Ž . Ž . gas 2DEG in front of a superconductorrsemiconductor SN interface. The PC in the InGaAsrInP heterostructure emits electrons towards the SN interface and detects the reflected carriers. Due to the specific sample setup, the vast majority of the detected carriers are Ž . retroreflected holes and the signal is a direct measurement of the Andreev reflection probability. AE is found to be up to 20% by measuring the point contact conductance. The experimental results are compared with the predictions of two theoretical models. It is found that the Andreev reflection probability significantly differs from theoretical expectations. The main advantage of the presented technique is the direct way of measuring the Andreev reflection probability, independent of any model describing the SN interface. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Andreev reflection; Superconductorrsemiconductor interface; Ballistic transport; Point contact spectroscopy 1. Introduction In recent years, superconductorrsemiconductorrsuper- Ž . conductor SNS structures have attracted much attention. The idea is to combine the low-power dissipation and low-noise properties of a superconductor and the control of the electron density in the semiconductor with the goal to generate and to control a supercurrent through the semi- w x conducting area 1,2,3 . Whether a supercurrent in the semiconductor exists or not depends on the so-called An- wx dreev reflection 4 , which mediates the phase information between the two superconductors. The Andreev reflection Ž . coefficient AE gives the probability for electrons being retroreflected as holes when impinging on an SN interface. Ž . Thus, the knowledge of AE is essential to determine the critical current in SNS structures. Usually, the Andreev reflection probability is obtained by fitting the normalized differential conductance G at an SN interface. Depend- SN Ž wx w x ing on the model e.g. BTK 5 , MBTK 6,7 , or models w x. including the proximity effect 8,9 , a more or less accu- rate fit can be generated, while the only justification is the quality of the fit. In this work, we present a direct and ) Corresponding author. Tel.: q 49-241-807079; fax: q 49-241- 8888306. E-mail address: j.appenzeller@physik.rwth-aachen.de Ž . J. Appenzeller . model independent determination of the Andreev reflection probability and show that a good fit of G does not SN Ž . automatically imply that AE was determined correctly. 2. Experimental determination of the Andreev reflec- tion probability Fig. 1 shows an electron micrograph of the geometry used which allows an almost exclusive detection of An- dreev reflected holes. Based on an In Ga AsrInP 0.77 0.23 heterostructure optimized for large elastic and inelastic w x mean free paths 10 , we defined a PC by means of reactive ion etching at a distance of L s 1 mm in front of a straight SN interface. Because the distance is roughly equal to the ballistic mean free path of the electrons, we are able to perform spectroscopic point contact measure- ments. Electrons emitted by the PC will impinge on the SN interface with suffering only a few scattering events. Be- low the transition temperature T of Nb, some of the C electrons are reflected at the SN interface as electrons and Ž . the others are retroreflected as holes Andreev reflection . By measuring the differential conductance G of the PC, Nb we are able to use the PC as detector for reflected carriers. We now discuss the contributions of the electrons and holes. 0928-4931r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0928-4931 01 00236-3