Few electron double quantum dot in an isotopically purified 28Si quantum well

Abstract

We present a few electron double quantum dot device defined in an isotopically purified 28 Si quantum well (QW). An electron mobility of 5.5 · 10 4 cm 2 (Vs) −1 is observed in the QW which is the highest mobility ever reported for a two-dimensional electron system in 28 Si. The residual concentration of 29 Si nuclei in the 28 Si QW is lower than 10 3 ppm, at the verge where the hyperfine interaction is theoretically no longer expected to dominantly limit the T 2 spin dephasing time. We also demonstrate a complete suppression of hysteretic gate behavior and charge noise using a negatively biased global top gate.PACS numbers: 73.63. Kv, 73.23.Hk, 73.21.Fg, 28.60.+s, 72.70.+m, 73.50.Td Semiconductor quantum dots (QD) are among the candidates for a scalable implementation of electron spin based qubits in solid state systems. Silicon (Si) has been widely recognized as a well suited material system for decoupling electron spin qubits from their volatile solid state environment owing to the weak spin-orbit and weak hyperfine interaction. Very long spin relaxation times (T 1 ) on the order of seconds have been reported for Si on the basis of electrostatically defined QDs 1 , single phosphorous donors 2 or triplet-singlet relaxation times in double QDs 3 . Recently, also a spin dephasing time of T * 2 = 360 ns has been observed in a time ensemble measurement in a Si double QD 4 . These milestones highlight the great potential for quantum information processing in Si.The adverse impact from nuclear spins on electron spin coherence 5 can be further reduced in the Si material system by means of isotopic enrichment of the 28 Si isotope which has zero nuclear spin. Recent technological advances have enabled the fabrication of highly enriched 28 Si crystals 6 with isotopic fractions of the nuclear spin carrying 29 Si isotope smaller than 4 · 10 2 ppm. In such ultra-clean 28 Si bulk samples, the spin coherence time T 2 for donor-bound electrons 7 achieves unprecedentedly long values of T 2 = 10 s. This offers a promising perspective for qubit applications with electrostatically defined QDs in 28 Si heterostructures. However, the integration of isotopically purified material with low impurity concentrations into molecular beam epitaxy (MBE) or chemical vapor deposition growth processes is still a challenge. Hence, no QD devices have been demonstrated so far for two-dimensional electron systems (2DES) in 28 Si.In this letter, we report on the fabrication and characterization of an electrostatically defined few electron double QD within a high mobility 2DES in a MBEgrown 28 Si/SiGe heterostructure. We find a concentration of residual 29 Si nuclei in the quantum well (QW) smaller than 10 3 ppm and achieve a peak mobility of 5.5 · 10 4 cm 2 (Vs) −1 at a 2DES density of 3 · 10 11 cm −2 . We combine our double QD with a global top gate (TG) and demonstrate a strong suppression of hysteretic gate behavior and charge noise as a negative voltage is applied to the global TG.Our heterostructures are grown in a solid source MBE system equ...

Theory of spin relaxation in two-electron laterally coupled Si/SiGe quantum dots

“…30 Hereof only 29 Si has nonzero nuclear spin (I = 1/2), and purification can further reduce its abundance down to 0.05%. 31,32 For this reason, silicon-based quantum dots have become the new focus of interest, and recent progress emphasizes their perspectives. [33][34][35][36] Another advantage of silicon 8,29 over GaAs is a larger g factor, which allows spin manipulations in smaller magnetic fields.…”

Characterization and suppression of low-frequency noise in Si/SiGe quantum point contacts and quantum dots

“…It has been demonstrated for Si/SiGe QPCs defined by gate electrodes placed on a Al 2 O 3 insulated Si surface, i.e., metaloxide-semiconductor (MOS), that negatively biasing a global top gate gradually reduces the charge noise in the conductance. 17 The charge noise may be more significant in surface Schottky gate structures than in MOS gate structures because there is no direct leakage between the gate metal and the QPC channel in the latter, but no detailed studies on charge noise spectra in Si/SiGe QPCs have been reported to date.…”

Extremely high electron mobility in isotopically-enriched 28Si two-dimensional electron gases grown by chemical vapor deposition

“…This mobility may be the highest reported for modulation-doped Si 2DEGs grown by CVD regardless of 28 Si enrichment. In previous work of isotopically-enriched 28 Si 2DEGs grown by molecular beam epitaxy, the highest reported mobility was 55 000 cm 2 /V-s. 12 Enhancement-mode samples with enriched 28 Si only in the Si QW layer were made without n-type dopants with a SiGe setback layer of 60 or 150 nm on top of the 2DEG layer (Table I). With a metal gate of Cr/Au on top of 90-nm Al 2 O 3 , the Hall electron density increased with gate voltage and mobility increased rapidly with electron density (Fig.…”