Physica E 6 (2000) 297–300 www.elsevier.nl/locate/physe Universality at a quantum Hall – Hall insulator transition in a Si= Si 0:87 Ge 0:13 2D hole system R.B. Dunford a ; * , N. Grin a ; 1 , M. Pepper a , P.J. Phillips b , T.E. Whall b a Cavendish Laboratory, University of Cambridge, Madingley Rd, Cambridge CB3 OHE, UK b Department of Physics, University of Warwick, Coventry, CV4 7AL, UK Abstract The temperature dependence of the magnetoresistivity of a Si= Si0:87Ge0:13 2D hole system with n =2:2 × 10 11 cm -2 and  = 16 900 cm 2 = Vs (at 50 mK) has been studied. An insulating phase is observed for ¡ 1, with xy remaining nite and close to h=e 2 , indicating a possible quantised Hall insulator state. The data on either side of the  = 1 to Hall insulator transition can be scaled to a single pair of curves with a scaling exponent of  =0:68 ± 0:05. Temperature-independent conductivities close to  =0:69 and  =1:5 are found to appear at values of  xx =0:5e 2 =h (xy =0:5e 2 =h) and xx =0:5e 2 =h (xy =1:5e 2 =h), respectively, in agreement with theory. ? 2000 Elsevier Science B.V. All rights reserved. PACS: 73.20.Dx; 73.40.Hm; 71.27; 71.45 Keywords: Quantum Hall eect; Metal–insulator transition The 2D hole system (2DHS) in a compressive bi- axially stressed Si 0:87 Ge 0:13 channel is an unusual sys- tem because of the complex valence band structure, high eective mass and large, anisotropic g * -factor. This makes it an interesting system to study univer- salities in conduction at the B = 0 metal–insulator transition [1,2], the quantum Hall eect and the Hall insulator regimes. New phenomena have been ob- served in this system, such as an unanticipated insu- lating phase at  =1:5 which has been attributed to the unusual Landau level degeneracy [3–9]. * Corresponding author. Tel.: 44-1223-337469; fax: 44-1223- 337271. E-mail address: rd225@cam.ac.uk (R.B. Dunford) 1 Currently at National Microelectronics Research Centre, Lee Maltings, Prospect Row, Cork, Ireland. We report on the magnetic eld-driven integer quantum Hall eect (QHE) to insulator transitions in a high-mobility, low-density 2DHS Si= Si 0:87 Ge 0:13 heterostructure. At zero magnetic eld, the sample is on the metallic side close to the B = 0 metal–insulator transition [1,2]. The sample was grown by MBE on a bonded silicon-on-insulator substrate [10]. Firstly a 200 nm Si buer layer was grown, followed by a 25 nm compressive biaxially stressed Si 0:87 Ge 0:13 channel, a 20 nm Si spacer layer, a 50 nm doped Si supply layer (B : N A =2 × 10 18 cm -3 ) and a 30 nm Si cap. The sample mobility of  = 16 900 cm 2 = Vs at a hole density of n =2:2 × 10 11 cm -2 (at 50 mK) is one of the highest ever reported for 2D holes in a Si-based system. The ratio of the transport relaxation time to 1386-9477/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII:S1386-9477(99)00158-7