Two-dimensional metal-insulator transition and in-plane magnetoresistance in a high-mobility strained Si quantum well

Abstract

The apparent metal-insulator transition is observed in a high quality two-dimensional electron system (2DES) in the strained Si quantum well of a Si/Si1−xGex heterostructure with mobility µ = 1.9 × 10 5 cm 2 /Vs at density n = 1.45 × 10 11 cm −2 . The critical density, at which the thermal coefficient of low T resistivity changes sign, is ∼ 0.32 × 10 11 cm −2 , so far the lowest observed in the Si 2D systems. In-plane magnetoresistance study was carried out in the higher density range where the 2DES shows the metallic-like behavior. It is observed that the in-plane magnetoresistance first increases as ∼ B 2 ip and then saturates to a finite value ρ(Bc) for Bip > Bc. The full spin-polarization field Bc decreases monotonically with n but appears to saturate to a finite value as n approaches zero. Furthermore, ρ(Bc)/ρ(0) ∼ 1.8 for all the densities ranging from 0.35 × 10 11 to 1.45 × 10 11 cm −2 and, when plotted versus Bip/Bc, collapses onto a single curve.The two-dimensional (2D) metal-to-insulator transition (MIT) has been of great current interests. 1,2 According to the well-established scaling theory, 3 any amount of disorder in a non-interacting 2D electron system (2DES) will localize the carriers at zero temperature (T ) and zero magnetic (B) field, and the ground state of the 2DES is an insulator, whose conductivity logarithmically goes to zero as T → 0. Recent experimental studies in high quality dilute 2D systems where the electron-electron (e − e) interaction is large, however, have shown the existence of a metallic-like state and an apparent metal-insulator transition. There, the magnitude of the 2DES resistivity (ρ) undergoes a change from decreasing with decreasing T , (dρ/dT > 0), a metallic behavior, to increasing with decreasing T , (dρ/dT < 0), an insulating behavior, at a critical density n c .One of the fundamental questions in this apparent MIT problem is the nature of the metallic state 4 and its response to a pure in-plane magnetic field (B ip ). 5,6,7,8,9,10,11,12,13 It is observed that in conventional clean Si-MOSFET's the in-plane magnetoresistance (MR) of the 2DES, ρ(B ip ), first increases as B 2 ip at low B ip . After a critical B field B c , which has been identified as the full spin polarization B field for the 2DES, 7,8,9 the in-plane MR saturates to a constant value ρ(B c ).14 The enhancement of ρ(B ip ) under high in-plane B field in this 2DES is attributed to the reduction of screening of charged impurities in a Fermi liquid, caused by the loss of spin degeneracy, 15,16,17 and a ratio of ρ(B c )/ρ(0) = 4 or ∼ 1.2 is expected when the background impurity scattering 15 or the remote ionized impurity scattering 17 dominates. Thus, it is surprising that close to the critical density n c , the enhancement can be as large as several orders of magnitude.1,2,5,7,8,10,11 Furthermore, it has been shown that under B ip the metallic state is suppressed, and completely destroyed for n < 1.5n c . A recent study has demonstrated that the disappearance of the positive dρ/dT in large B ip is due t...

Conductivity of a spin-polarized two-dimensional electron liquid in the ballistic regime

“…3͒. Concerning the data for Si/ SiGe quantum wells, 4 one can evaluate the slope ratio for n s = 0.515ϫ 10 11 cm −2 at about 0.45, which is consistent with our results. …”

“…21 It is worth comparing the behavior of 2D electron system in Si MOSFETs to that in another two-valley system, Si/ SiGe quantum wells. Transport properties of the latter system have been found to be very similar to those of silicon MOSFETs, 4,14,22 although the disordered potential in both cases is different resulting, particularly, from the presence/ absence of a spacer. However, the peculiarities near the onset of complete spin polarization are less pronounced in Si/ SiGe quantum wells than in MOSFETs: only a weakening, but not absence, of the temperature dependence of the resistance has been observed in the metallic regime in a partially spinpolarized state.…”

“…However, the peculiarities near the onset of complete spin polarization are less pronounced in Si/ SiGe quantum wells than in MOSFETs: only a weakening, but not absence, of the temperature dependence of the resistance has been observed in the metallic regime in a partially spinpolarized state. 4 Theoretically, the effect of the weakening of the ͑T͒ dependence near the onset of complete spin polarization has been found for the 2D electrons in Si/ SiGe quantum wells in the frames of screening approach. 23 We now compare the experimental ratio of the slopes A * ͑B sat ͒ / A * ͑0͒ with theoretical predictions.…”

“…3͒ and Si/ SiGe heterostructures. 4 Interactions lead, in particular, to critical behavior of the Pauli spin susceptibility 5,6 and sharply increasing effective mass at low electron densities. [7][8][9] These phenomena ͑at least in Si MOSFETs͒ are not dominated by spin exchange effects, since the Landé g factor is found to be close to its value in a bulk semiconductor and the effective mass is insensitive to the degree of spin polarization.…”

Fractional quantum Hall effect in a dilute magnetic semiconductor

“…Illumination turned out to be crucial to observe fractional states as it increases the quantum scattering time τ q by a factor of 5 to a value of ∼3 ps. It is thus comparable to the values observed in GaAs and Si based 2DESs [see [18], for GaAs from Refs. [19,20], and for CdTe from Ref.…”