Cross section TEM image of sample nNC2 (see Table I). The inset shows one NC in high resolution transmission electron microscopy.
Measurement setup used for the characterization of charging and discharging processes in the direct tunneling regime.
Measurement principle for the characterization of (a) the electron charging process from an inversion layer in the substrate, (b) the hole charging process from an accumulation layer in the substrate, (c) the discharging process after charging the NCs with electrons, and (d) the discharging process after charging the NCs with holes. The measurement principle is illustrated for -channel devices, but can be applied analogously for -channel devices.
Principle of data analysis. (a) Transient drain current during the electron charging process at different gate voltages (sample nNC1). (b) Transfer characteristic of sample nNC1, assembled from the values of the drain current at the beginning of the charging process.
Measured small signal conductance (a) and measured capacitance (b) for sample pNC1. The curves represents the 10th cycle between −1, 1 V and −3, 3 V. The sweep rate was 0.1 V/s, no delay time was used. Source/drain contacts were floating.
(a) Absolute value of the CP charge as a function of frequency for the NC samples nNC2, pNC1, and the reference sample pRef1. The maximum/minimum values for the gate voltage square pulse were −2/0.5 V (sample pRef1) and −1.5/1.5 V (sample nNC2 and pNC1). The rise/fall times were fixed to . (b) Depth profile of the volume trap density in the tunnel oxide, calculated from the curves and Eq. (2). The error bars shows the influence of a charging/discharging of the NCs during the measurements.
(a) Normalized conductance vs frequency for several ratios of surface and Fermi potential in depletion. The curves refer to the forward sweep of the large (filled symbols) and the small hysteresis (Fig. 5). (b) Calculated density of trap states at the interface for sample pNC1 and pRef1, calculated after (Ref. 24). The reference curve was obtained by Berglund’s method (Ref. 25).
(a) Transient flatband voltage shift during the electron charging process from inversion for different charge voltages . The solid lines illustrate the characteristics of sample nNC1, while the dotted lines account for sample nNC3 (Ref. 27) (b) Transient flatband voltage shift during the hole charging process from accumulation for different charge voltages for sample nNC1 (solid symbols) and sample nNC3 (open symbols) (Ref. 27) For all measurements, was fixed at 0.1 V. Note that the equilibrium value of the flatband voltage differs from due to a small amount of electrons stored on the NCs at (compare Table I).
(a) Transient flatband voltage shift during the electron discharging process at after 1 s charging from inversion at different charge voltages . The solid symbols account for sample nNC1, while the open symbols account for sample nNC3 (Ref. 27) (b) Transient flatband voltage shift during the hole discharging process at after 1 s charging from accumulation at different charge voltages . The solid lines illustrate the characteristics of sample nNC1, while the dotted lines account for sample nNC3 (Ref. 27). For all measurements, was fixed at 0.1 V. Note that the equilibrium value of the flatband voltage differs from due to a small amount of electrons stored on the NCs at (Table I).
Calculated band diagram for sample nNC1 for one hole stored per NC at . Standard MOS equations based on the charge sheet approximation [Eq. (1)] have been used. The quantum confinement is taken into account according to Niquet et al. (Ref. 32). The voltage drop across the NC is neglected. For , the scale of the -axis is adapted (factor 1/50). Possible discharging mechanisms are illustrated in the enlarged section (see text).
Comparison of the tunnel current density through the tunnel oxide during the hole discharging process (solid symbols) and the electron charging process (open symbols) for sample nNC1. The current density and the voltage drop across the tunnel oxide are obtained from the transient flatband voltages (Sec. III C). The curves were assembled from the four different characteristics obtained for different values of [Figs. 8(a) and 9(b)], respectively.
Structural and electrical parameters of the samples investigated. The denotation refers to disorder averaged values taking into account the size distribution of the Ge NCs. The structural parameters were obtained by TEM and spectral ellipsometry measurements (maximum error ±0.15 nm). The substrate doping was determined from characteristics analyzing the maximum/minimum capacitance. The equilibrium values of and were determined from the transfer characteristics of the devices. Here, the maximum error is estimated to ±0.05 V.
Interfacial broadening parameter of sample pNC1, obtained for the different sweep directions for different maximum/minimum voltages [after Brews (Ref. 24)].
Article metrics loading...
Full text loading...