Random Telegraph Noise (RTN) – an often encountered problem

use QPC device as charge noise sensor control RTN with Vtop

power spectral density

two-level RTN: Lorentzian, 1/f2 above corner freq.

ensemble of fluctuators with homogeneous distribution of eff : 1/f

Equivalent Gate Voltage Noise VEG

noise is reduced exponentially with less negative VG

RTN time scales?

device with single fluctuator

independent of temperature (to 4.2K), i.e. tunneling, not thermally activated process

typical for INchanges in OUT less pronounced, with both increasing and decreasing trends

Origin of RTN?

simulations:- 1D not enough (full screening)- 2D self-consistent “nextnano”

comparison: VTOP = 0 and VTOP = -1.7 V

steeper slope for VTOP = 0trap energy lifter for VTOP = -1.7 V“smaller” barrier

leakage eliminated for ETRAP > m

partial depletion using VTOP reduces oreliminates tunneling from Schottky gate

deep trapping state

comparison: VTOP = 0 and VTOP = -1.7 Videntical carrier depletion width

more radial fields for VTOP = 0tunneling in more sideways directions(access traps at surface)

OUT depends on E-field at trap locationwide range of behaviors as seen

bias cooling vs. surface gate

two techniques consistent

bias cooling:- apply (positive) bias to Schottky gates during cool-down- carriers (negative) are frozen in deep traps (DX-centers) at low T- when bias removed, non-equilibrium trap occupation maintained

presence of additional negative charges lowers VG

distributed doping delta doping

Role of DX centers?

x = 0.1 : no DX centers available (above conduction band edge)i.e. here, DX cannot be the intermediate charge trap for noise

x= 0.2, 0.3 : DX levels well below C band

noise for bias-cooled distributed doping

noise for bias-cooled delta doping

noise for bias-cooled delta doping

observations:• x = 0.1 most, x = 0.3 least noise(low band gap x=0.1, more tunneling)

• lower noise with less neg. VGabove VG ~ -0.4 V constant noise

• very negative VG, ~ 2(few active traps)

• VG > -0.4 V : ~< 1(other ensemble charge noise sample)( < 1 : nonuniform distribution corner freq.

• DX centers excluded

conclusions

• charge noise in GaAs/AlGaAs devices dominated by electron trapping leakingfrom Schottky gates

• noise reduced if tunneling is eliminated, baseline charge noise remains (of different origin)

• bias cooling or top gates to reduce tunneling / noise(top gate more versatile, in-situ, works in some cases where bias fails)

• introducing additional high bandgap AlAs layer under cap, or thin insulator under gates for noise reduction