L. D. Bell

Localized charge injection in SiO2 films containing silicon nanocrystals

An atomic-force microscope (AFM) is used to locally inject, detect, and quantify the amount and location of charge in SiO2 films containing Si nanocrystals (size ~2–6 nm). By comparison with control samples, charge trapping is shown to be due to nanocrystals and not ion-implantation-induced defects in samples containing ion-beam-synthesized Si nanocrystals. Using an electrostatic model and AFM images of charge we have estimated the amount of charge injected in a typical experiment to be a few hundred electrons and the discharge rate to be ~35±15 e/min.

Charging of single Si nanocrystals by atomic force microscopy

Conducting-tip atomic force microscopy (AFM) has been used to electronically probe silicon nanocrystals on an insulating substrate. The nanocrystal samples were produced by aerosol techniques and size classified; nanocrystal size can be controlled in the size range of 2-50 nm with a size variation of less than 10%. Using a conducting tip, the charge was injected directly into the nanocrystals, and the subsequent dissipation of the charge was monitored. Estimates of the injected charge can be made by comparison of the data with an intermittent contact mode model of the AFM response to the electrostatic force produced by the stored charge.

Direct control and characterization of a Schottky barrier by scanning tunneling microscopy

Scanning tunneling microscopy (STM) methods are used to directly control the barrier height of a metal tunnel tip‐semiconductor tunnel junction. Barrier behavior is measured by tunnel current‐voltage spectroscopy and compared to theory. A unique surface preparation method is used to prepare a low surface state density Si surface. Control of band bending with this method enables STM investigation of semiconductor subsurface properties.

Ballistic‐electron‐emission microscopy investigation of Schottky barrier interface formation

Ballistic‐electron emission microscopy (BEEM) has been used to investigate the origin of defects at the Au/GaAs(100) Schottky barrier interface. In addition, molecular beam epitaxy (MBE) and in situ fabrication methods have been employed to control Schottky barrier interface properties. BEEM characterization combined with MBE methods has enabled the development of a near‐ideal Schottky barrier interface with drastically reduced defect density.

Ballistic electron emission microscopy and spectroscopy of Au/GaAs interfaces

The Au/GaAs interface is currently the subject of much investigation. However, a complete understanding of this interface, and the ability to precisely control Au/GaAs interface properties, is still lacking. Previous work has focused primarily on interfaces prepared on the GaAs(110) cleaved surface and on the chemically or thermally prepared GaAs(100) surface. This paper presents the first Schottky barrier results for the Au/GaAs(100) interface prepared completely in situ on GaAs grown by molecular‐beam epitaxy. The resulting interface displays unexpected properties which can be interpreted in terms of enhanced electrode interdiffusion. In addition, the capability of molecular‐beam epitaxy for in situ processing enables the stabilization of this interface against diffusion, and allows the formation of a Au/GaAs system with nearly ideal properties. Newly developed ballistic electron spectroscopy and imaging techniques demonstrate that the heterogeneity present at the interface of Au/GaAs(100) fabricated on...

High total dose tolerance of prototype silicon nanocrystal non-volatile memory cells

We report the first results pertinent to the high total dose tolerance of Si nanocrystal nonvolatile memory cells. The studied prototype nc-Si field effect transistors made by ion implantation retained virtually unchanged write/erase characteristics, typical for the two-state devices, to cumulative doses exceeding 15Mrad(Si).

Metal/GaN Schottky Barriers Characterized by Ballistic-Electron-Emission Microscopy and Spectroscopy

Ballistic-electron-emission microscopy (BEEM) and spectroscopy have been used to characterize the Pd/GaN and Au/GaN interfaces. BEEM spectra yield a Schottky barrier height for Au/GaN of ∼1.05 eV that agrees well with the highest values measured by conventional methods. For both Pd and Au, a second threshold is observed in the spectra at about 0.2–0.3 V above the first threshold. Imaging of these metal/GaN interfaces reveals transmission in nearly all areas, although the magnitude is small and spatially varies. Attempts to perform BEEM measurements on other GaN material have resulted in no detectable transmission in any areas, even at voltages as high as 3.5 V.

New electron and hole spectroscopies based on ballistic electron emission microscopy

Ballistic electron emission microscopy (BEEM) is shown to be one of a class of carrier transport nanospectroscopies which measure both elastic and inelastic transport of electrons and holes in thin film structures. In BEEM, electrons are injected into a metal film from a tunnel tip and are collected in the substrate after ballistic transport through the film. An analogous spectroscopy can be performed using hole transport into a p‐type semiconductor substrate. In the present work, it is shown that inelastic scattering in metal films results in electron‐hole pair creation which can be detected by similar techniques. A theory for this spectroscopy successfully explains the experimental results.

Effect of n+GaN cap polarization field on Cs-free GaN photocathode characteristics

We report on a Cs-free GaN photocathode structure in which band engineering at the photocathode surface caused by Si delta doping eliminates the need for use of cesium for photocathode activation. The structure is capped with a highly doped n+GaN layer. We have identified that n+GaN cap thickness plays an important role in limiting the effect of polarization induced charges at the GaN surface on the photocathode emission threshold. Physics based device simulations is used for further analysis of the experimental results. Our findings clearly illustrate the impact of polarization induced surface charges on the device properties including its emission threshold.

Models for quantitative charge imaging by atomic force microscopy

Two models are presented for quantitative charge imaging with an atomic-force microscope. The first is appropriate for noncontact mode and the second for intermittent contact (tapping) mode imaging. Different forms for the contact force are used to demonstrate that quantitative charge imaging is possible without precise knowledge of the contact interaction. From the models, estimates of the best charge sensitivity of an unbiased standard atomic-force microscope cantilever are found to be on the order of a few electrons.