Maciej Ligowski

Observation of individual dopants in a thin silicon layer by low temperature Kelvin Probe Force Microscope

Detection of individual dopants in the thin silicon layer using Kelvin Probe Force Microscopy is presented. The analysis of the surface potential images taken at low temperatures (13K) on n-type and p-type samples reveals local potential fluctuations that can be attributed to single phosphorus and boron atoms, respectively. Results are confirmed by simulation of surface potential induced by dopants and by the back gate voltage dependence of the measured potential.

Single-Gated Single-Electron Transfer in Nonuniform Arrays of Quantum Dots

Single-electron transfer in single-gated one-dimensional quantum dot arrays is investigated statistically from the viewpoint of robustness against parameter fluctuations. We have found numerically that inhomogeneous quantum dot arrays formed in doped nanowires exhibit single-electron transfer over a wide range of parameters. This confirms our frequent experimental observation of single-electron transfer in doped-nanowire field-effect transistors. The most important result in this work is that three-dot arrays with small-large-small dot size distribution always allow single-electron transfer even if dot sizes fluctuate. This structure is, we believe, most promising for fabricating devices with high immunity against structural fluctuations on the nanometer scale. On the basis of these findings, we propose methods to fabricate high-yield single-electron transfer devices.

Single-Electron Transfer by Inter-Dopant Coupling Tuning in Doped Nanowire Silicon-on-Insulator Field-Effect Transistors

We demonstrate tunable single-electron turnstile operation in doped-nanowire silicon-on-insulator field-effect transistors. In these structures, electron transport occurs through dopant-induced quantum dots. We show that the substrate silicon can be used as a back gate to modulate the inter-dot coupling, which dictates the overlap between Coulomb domains in the charge stability diagrams of these devices. Since this overlap is a necessary requirement for single-electron turnstile, this procedure allows the optimization of the conditions for single-electron turnstile in doped-nanowire field-effect transistors.

Procedure for Calibrating Kelvin Probe Force Microscope

The paper presents novel method for calibrating Kelvin Probe Force Microscope. The method is based on measuring the surface potential of a reference sample and comparing it with the results obtained by KFM. Proposed method offers a calibration possibility in the whole measuring range of the microscope both in terms of absolute values and dynamic behavior. In this novel method the surface potential of a reference sample used for calibration can be adjusted to desired value instead of being defined by physical parameters of the sample. Presented results obtained with proposed method prove its validity.

KFM Observation of Electron Charging and Discharging in Phosphorus-Doped SOI Channel

Low temperature Kelvin Probe Force Microscopy (LT-KFM) can be used to monitor the electronic potential of individual dopants under an electric field. This capability is demonstrated for silicon-on-insulator field-effect-transistors (SOI-FETs) with a phosphorus-doped channel. We show results of the detection of individual dopants in Si by LT-KFM. Furthermore, we also observe single-electron charging in individual dopants located in the Si channel region.

Kelvin Probe Force Microscope Measurement Uncertainty

Kelvin Probe Force Microscopy is an attractive technique for characterizing the surface potential of various samples. The main advantage of this technique is its high spatial resolution together with high sensitivity. However as in any nanoscale measurements also in case of KFM it is extremly difficult to describe the uncertainty of the measurement. Moreover, a wide variety of measuring conditions, together with the complicated operation principle cause situation, where no standard calibration methods are available. In the paper we propose the model of the KFM microscope and analyze the uncertainty of the KFM measurement.

Dopant Freeze-out and Potential Fluctuations Observed by Low Temperature Kelvin Probe Force Microscope

Motivation Single Electron Devices (SEDs) are very promising for fabrication of future Ultra-Large Scale Integrated (ULSI) circuits, sensors, memories or metrological tools due to their ultimate properties of manipulating elementary charge. The possibility of significant reduction of parameters such as device size or power consumption makes SEDs being widely investigated at present. One of the approaches to achieve single electron transfer is by creating quantum dot (QD) arrays in the Si nanowire utilizing natural potential fluctuations caused by ionized dopant atoms 1 . Thus it is crucial to monitor the potential distribution inside doped nanowires. So far, none of the proposed methods 2,3 are capable of “looking” beyond several topmost layers . Low Temperature Kelvin Probe Force Microscope (LT-KFM) seems to be an appropriate tool for this purpose due to its high sensitivity to charges placed deeper in the device structure. Therefore we believe that KFM may be utilized to sensitively detect dopant induced potential fluctuations and for that goal we have investigated the surface potential of MOSFETs in the wide range of temperatures. We found direct evidence of the dopant freeze-out in nanodevice channel. Moreover we present the observation of potential fluctuations which may appear due to discrete distribution of dopants in the channel.

Single-Electron Transfer by Controlling the Dopant-Induced Quantum Dot Landscape

1. Single-electron transfer devices Single-electron devices (SEDs) have the capability of operating with individual charges, which makes them promising candidates for future electronics. 1 Among SEDs, an important category is formed by single-electron transfer devices, able to transfer elementary charges synchronized with external voltage pulses. Some design schemes have been already proposed based on precisely defined quantum dot (QD) arrays. The original single-electron turnstiles and pumps transfer electrons one-by-one during every cycle of one or, respectively, several ac gate voltages applied simultaneously to a 1D array of metallic QDs. Achieving such capabilities in semiconductor devices is essential for practical applications. In this direction, single-dot SEDs have been demonstrated to work as turnstiles and pumps by an appropriate control of the dot-lead coupling by ac-gates. 2-4

Si Single-Electron SOI-MOSFETs: Interplay with Individual Dopants and Photons

We have demonstrated that Si single-electron or single-hole SOI-MOSFETs with the multi-dots channel have attractive new functions such as photon detection and single-electron transfer. Multi-dots formed by selective-oxidation-induced patterning of the thin SOI layer have been used in the experiments of photon detection, while, most recently, we have utilized smaller dots consisting of individual dopant potentials in single electron transfer devices. Furthermore, in order to directly observe spatial landscape of single charges in the channel region, we have developed Low Temperature-Kelvin Probe Force Microscopy and succeeded in detecting single-dopant potential in the channel region. In this paper, photon detection by these devices will be primarily described.

KFM measurements of an ultrathin SOI-FET channel surface

In this work, we investigate surface potential of the thin silicon-on-insulator field-effect-transistor (SOI-FET) by Kelvin Probe Force Microscope (KFM) at different temperatures. It will be shown that the surface potential changes in the range of temperatures from 15 K to 100 K, indicating increasing number of ionized dopants.