D. Stiévenard

Native defects in gallium arsenide

We describe information which has been obtained on point defects detected in various types of GaAs materials using electron paramagnetic resonance as well as electrical and optical techniques. From a comparison of their characteristics and those of simple intrinsic defects (As and Ga interstitials, vacancies and antisites) it is concluded that native defects are not simple intrinsic defects, with the exception of the antisites, but complexes formed by the interaction of such defects between themselves or with impurities. Particular emphasis is given to the As antisite complexed with an As interstitial, the so‐called EL2 defect which plays a major role in the electrical properties of bulk materials. Differential thermal analysis, positron annihilation, and x‐ray diffraction demonstrate that bulk materials contain a large concentration of vacancy‐related defects and As precipitates located along dislocations which play the role of gettering centers. Presumably, bulk materials also contain other As clusters ...

Nanooxidation using a scanning probe microscope: An analytical model based on field induced oxidation

The formation of a nanometer-size oxide pattern on silicon using a scanning probe microscope (SPM) has been widely reported in the literature. No analytical model has been proposed, however, to explain the variation of the oxide height with both polarization and speed of the SPM tip. In this letter, we explain quantitatively the variation of the oxide height with the polarization and the speed of the tip with a model based on field induced oxidation. Data analysis also allows us to estimate the thermal activation energy of the oxidation process, (∼0.15 eV). This low value is compared with activation energies measured for thermal and plasma oxidation of silicon.

Characterization of scanning tunneling microscopy and atomic force microscopy-based techniques for nanolithography on hydrogen-passivated silicon

A comparison between scanning tunneling microscope (STM) and atomic force microscope (AFM) nanolithography techniques based on local oxidation of silicon is proposed. This work deals with the three different near-field microscopy techniques, namely, STM, AFM in contact mode (CM-AFM), and tapping mode (TM-AFM), all of them operated in air. The thickness and width of oxide stripes are studied as a function of the applied probe–sample voltage, the speed of the probe and the setpoint (current, applied force, and vibration amplitude for STM, AFM contact, and tapping, respectively). The advantages and drawbacks of each technique are analyzed, establishing TM-AFM as the best candidate for scanning probe microscope nanolithography.

ARE ELECTRICAL PROPERTIES OF AN ALUMINUM-POROUS SILICON JUNCTION GOVERNED BY DANGLING BONDS ?

Using an aluminum–porous p+ silicon junction, we have realized a sensor which dc current increases up to two orders of magnitude in the presence of ammonia, as for a series of various gases. To interpret quantitatively this phenomenon, we assume that the conductivity is governed by the width of a channel resulting from the partial depletion of silicon located between two pores. This depleted region is due to the charges trapped on surface states associated with the Si–SiO2 interface where SiO2 is the native silicon oxide. When some gas is adsorbed, mainly on Si–H bonds, we propose there is an electrical screening of the interface states (mainly dangling bonds located in the neighborhood of the Si–H bonds), leading to a decrease of the depleted region, i.e., an increase of the width of the channel and thus an increase of the current.

Nanooxidation of silicon with an atomic force microscope: A pulsed voltage technique

The use of an atomic force microscope (AFM) as an active tool to realize silicon nanolithography is now well known, using a continuous voltage applied between the AFM tip and the surface. The main drawback of this technique is the poor reliability of the tip due to the strong tip-surface interaction. An original way which both increases the reliability and improves the nanolithography resolution is the use of pulsed voltages instead of continuous polarization. In such a case, the interaction time of the tip with the surface under electric field decreases. The frequency oscillation (in noncontact mode) of the cantilever is taken as a reference, and pulsed voltages with variable phase and duty cycle are used. We show that the variation of the phase allows a 100% modulation of the oxide width. Finally, combining this lithography technique with wet etching, a 17.5 nm wide and 5.5 nm height nanowire has been obtained starting from a silicon-on-insulator substrate.

Scanning tunneling microscopy and scanning tunneling spectroscopy of self-assembled InAs quantum dots

We present cross-sectional scanning tunneling microscopy images and scanning tunneling spectroscopy results of InAs quantum dots grown on GaAs. The samples contain 12 arrays of quantum dots. The analysis of the scanning tunneling microscope images reveals the self-alignment of the dots as well as the different dot interfaces with the under- and overgrown GaAs layers. We measure the strain distribution along the [001] direction in the (110) plane. The roughness of the dot interfaces along the [110] direction is also estimated and local spectroscopy of the dots evidences the electronic confinement (measured gap of 1.25 eV compared with 0.4 eV for bulk InAs).

Atomic-scale study of GaMnAs/GaAs layers

Cross-sectional scanning tunneling microscopy was used to study GaMnAs diluted magnetic semiconductors grown by low temperature molecular beam epitaxy. The Ga1−xMnxAs layer, containing a concentration of x=0.005, shows that the dominant defect in the material is the arsenic antisite. Mn ions can also be resolved and show a signature distinct from the arsenic antisites. Spectroscopic measurements are perfomed to study the variation of the Fermi level between the Ga0.995Mn0.005As and GaAs layers. The Mn ions act as acceptor dopants. However, for x=0.005, the Mn concentration in comparison with the As antisite concentration is too small to induce a significant change of the Fermi level from the midgap position, preventing the layer from being ferromagnetic.

Electrical behaviour of aluminium-porous silicon junctions

In order to analyse the electrical behaviour of aluminium-porous silicon junctions, we studied the current-voltage and temperature I(V, T) characteristics of a series of junctions, having a typical porosity of 45% and a layer thickness ranging from 2 to 30 jam. Under forward bias, the current can be fitted by the law I=I s (exp(q(V-R s I)/nkT)), with R s a serial resistance ranging from a few kiloohms to a few tens of megaohms as the thickness increases. Moreover, analysis of I s vs. the temperature alows us to determine a built-in potential φ b0 of the order of 0.40 eV, showing pinning of the Fermi level on a density of interface states associated with dangling bonds. Under reverse bias, the logarithm of the current follows a quite linear law, showing that the conduction is limited by a surface mechanism associated with hopping of the carriers from site to site, each site corresponding to a dangling bond, in agreement with theoretical results (in a porous layer, the carriers are not on the dopants but are localized on dangling bonds). Each site can be modelled as a square well potential and the electrons, to escape this well, have typically to overcome a barrier of the order of 0.10-0.30 eV, giving an extension of the associated wave function of the order of a few reciprocal a˙ngstroms. Finally, the possibility of variable range hopping is also considered and ruled out

Charge injection in individual silicon nanoparticles deposited on a conductive substrate

We report on charge injection in individual silicon nanoparticles deposited on conductive substrates. Charges are injected using a metal-plated atomic force microscope tip, and detected by electric force microscopy (EFM). Due to the screening efficiency of the conductive substrate, up to ∼200 positive or negative charges can be stored at moderate (<10 V) tip–substrate injection voltage in ∼40 nm high nanoparticles, with discharging time constants of a few minutes. We propose an analytical model in the plane-capacitor approximation to estimate the nanoparticle charge from EFM data. It falls in quantitative agreement with numerical calculations using realistic tip/nanoparticle/substrate geometries.

Formation of silicon islands on a silicon on insulator substrate upon thermal annealing

Starting from silicon on insulator substrates, we show that a thermal treatment (in the 600–900 °C range) induces the creation of silicon islands. To characterize the island formation as well as the initial silicon layer thickness, we use in situ Auger electron spectroscopy analysis in an ultrahigh vacuum chamber. The island size and density are studied with an ex situ atomic force microscope. We show that the formation temperature of the islands increases from 575 to 875 °C as the initial silicon layer thickness increases from 1 to 19 nm. For the 1 nm thickness, the minimum island size is reached (semispherical shape with a 16 nm diameter). The phenomena involved in the island formation are discussed and the study of the variations of the calculated stress tensor (IMPACT software) as a function of the thermal treatment explain the behavior of the top silicon layer.