Miftahul Anwar

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.

Atom devices based on single dopants in silicon nanostructures.

Silicon field-effect transistors have now reached gate lengths of only a few tens of nanometers, containing a countable number of dopants in the channel. Such technological trend brought us to a research stage on devices working with one or a few dopant atoms. In this work, we review our most recent studies on key atom devices with fundamental structures of silicon-on-insulator MOSFETs, such as single-dopant transistors, preliminary memory devices, single-electron turnstile devices and photonic devices, in which electron tunneling mediated by single dopant atoms is the essential transport mechanism. Furthermore, observation of individual dopant potential in the channel by Kelvin probe force microscopy is also presented. These results may pave the way for the development of a new device technology, i.e., single-dopant atom electronics.

Effect of electron injection into phosphorus donors in silicon-on-insulator channel observed by Kelvin probe force microscopy

We have comparatively studied the effects of electron injection in individual phosphorus-donor potential wells at 13 K and 300 K by Kelvin probe force microscopy in silicon-on-insulator metal-oxide-semiconductor field-effect-transistors. As a result, at 13 K, localized single-electron filling into the phosphorus-donor potential well is found, reflecting single-electron tunneling transport through individual donors, whereas at 300 K, spatially extended and continuous electron filling over a number of phosphorus-donors is observed, reflecting drift-diffusion transport.

Single-Electron Charging in Phosphorus Donors in Silicon Observed by Low-Temperature Kelvin Probe Force Microscope

Kelvin probe force microscopy (KFM) working at low temperatures (13 K) is used to study local electronic potential fluctuations induced by individual phosphorus donors. Electronic potential maps were measured at the surface of thin phosphorus-doped channel of silicon-on-insulator field-effect transistors for different values of backgate voltage. We observed local changes of the potential profile with increasing backgate voltage, indicating electron injection in the channel. Single-step changes in the depth of the fine potential wells, observed by changing backgate voltage, are ascribed to single-electron charging in individual donors. For clusters of donors, with overlapped potential wells, electron charging occurs gradually, without single-step behavior, as the backgate voltage becomes more positive.

Si-Based Single-Dopant Atom Devices

We have recently proposed and demonstrated a new device concept, “Si-based single-dopant atom device”, consisting of only one or a few dopant atoms in the channel of Si field-effect transistors. The device characteristics are determined by a dopant, which is mediating electron or hole transport between source and drain electrodes. In this paper, our recent results on electronic and photonic applications are introduced. Furthermore, single-dopant images obtained by a scanning probe microscope are also presented.

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.

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.

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.