Electric-field-induced charge noise in doped silicon: Ionization of phosphorus donors

Abstract

We report low frequency charge noise measurement on silicon substrates with different phosphorus doping densities. The measurements are performed with aluminum single electron transistors (SETs) at millikelvin temperatures where the substrates are in the insulating regime. By measuring the SET Coulomb oscillations, we find a gate voltage dependent charge noise on the more heavily doped substrate. This charge noise, which is seen to have a 1/f spectrum, is attributed to the electric field induced tunneling of electrons from their phosphorus donor potentials.With their high charge sensitivity single electron transistors (SETs) provide a valuable tool for investigating charge transport at the single electron level. These sensitive electrometers have been employed to directly observe electron traps in Al 2 O 3 [1], and electron tunneling through GaAs quantum dots [2] amongst other systems. With particular relevance to this work, SETs have been used to study both time and frequency domain charge noise [3,4,5,6] in semiconductor substrates. Charge noise in semiconductors has long been of interest [7], and recently has become an important issue for solid-state quantum computation where it is crucial that such noise is minimized to extend quantum coherence times.We study the effect of electric fields on the charge noise in Si substrates with different doping levels. At millkelvin temperature the substrates are in the insulating regime (n < n MIT = 3.45 × 10 18 cm −3 ). For the intentionally doped substrate (n = 8 × 1016 cm −3 ) we find an additional 1/f charge noise on application of an electric field. By contrast, this charge noise was not measurable in the nominally intrinsic sample (n = 1×10 14 cm −3 ). We therefore infer that its origin is electrons tunneling from their phosphorus donors [8,9]. While this ionization effect can occur for both substrates, in the intentionally doped material a larger and more easily measurable charge redistribution is present due to the higher doping density. Dopant ionization has previously been measured by transient currents due to the build-up of depletion regions in p-n junctions [10,11], the slow formation of accumulation layers in MOS capacitors [12] and hysteretic effects in MOSFETs [13]. These experiments were mostly performed at an intermediate temperature range (T = 4K − 20K) where ionization can be thermally activated by the Poole-Frenkel effect [14]. At our experimental temperatures (T ∼ 100mK), electric field assisted ionization is no longer dominated by thermal activation but is expected to be purely field assisted tunneling.We fabricate the SETs using a standard bilayer resist and a shadow evaporation process. The electrostatic gate structure is evaporated Ti/Au and patterned in a separate electron beam lithography step (see fig. 1b inset). Both silicon substrates are terminated by 5nm of SiO 2 grown by thermal oxidization.The SET conductance was measured using standard lock-in techniques at an excitation frequency of 1kHz and an amplitude of 20µV . For the noise measure...

External field control of donor electron exchange at theSi∕SiO2interface

“…This limits the operating temperature to a few Kelvin (in current experiments, temperatures as low as 0.1 K are being used). 41,42 The inverse of the variational parameters, 1/α and 1/β, are proportional to the confinement lengths in the z-direction and the xy-plane respectively. Both depend on the distance d of the donor from the interface as shown in Fig.…”