A. Parisini

Analysis of the hole transport through valence band states in heavy Al doped 4H-SiC by ion implantation

The temperature dependence of the Hall hole density and the Hall mobility data of heavy doped p-type 4H-SiC(Al) materials obtained by Al+ ion implantation have been analysed in the frame of the charge neutrality condition and the relaxation time approximation. Samples with implanted Al concentrations in the range 1019–1020 cm−3 and 1950 °C/5 min conventional annealing have been taken into account. The reliability of the calculation has been critically discussed by focusing the attention on both the validity limits of the models for the impurity scattering mechanisms and the adopted Hall factor. By introducing empirical mass anisotropy factors, reasons were given in favour of a generalized use of the unique experimental evaluation of the Hall factor reported by the literature for p-type 4H-SiC, assessed for an Al acceptor density in the range of 1.8 × 1015 cm−3–2 × 1018 cm−3. The simultaneous fits of the Hall hole density and mobility data indicate an electrical activation of the Al impurities of the order...

Oxidation kinetics of ion-amorphized (0001) 6H–SiC: Competition between oxidation and recrystallization processes

The wet oxidation of (0001) Si face 6H–SiC preamorphized by Ar+ implantation has been investigated in the temperature range between 750 and 950°C. Electron microscopy analysis has been performed to obtain information on the evolution of the amorphous layer during the oxidation process. The oxidation rate of the amorphous substrate is linear in time and assumes the value VOx(α)=3.8×107exp(−1.6∕kT)nm∕min. Due to the concomitant oxidation and recrystallization processes occurring at the two opposite faces of the amorphous layer, this expression is valid up to about 910°C for amorphous layer thickness of a few hundreds nanometer and processing time of a few tens of minutes. At higher temperatures, our oxidation data support the existence of a sudden increasing of the recrystallization that strongly reduces the time of surviving of the amorphous region and, consequently, the oxide thickness. The procedure to determine, for any fixed amorphous thickness, the most suitable experimental conditions giving rise to ...

Structural Characterization of Alloyed Al/Ti and Ti Contacts on SiC

Al-rich Al/Ti contacts on p-type SiC show good ohmic behaviour for a wide range of doping density after high temperature (> 950°C) post deposition annealing independently of the 4Hand 6H-SiC polytype [1, 2] employed. This ohmic contact behaviour appears to be closely related to the high Al content of the deposited metal layers. Several hypotheses have been proposed to explain the role of Al in such Al/Ti alloyed contacts mainly based on the doping effect of this element in SiC. However, Al/Ti-SiC alloyed contacts constitute a complex quaternary system where the formation and spatial distribution of different phases as well as loss of volatile Al could strongly affect the contact electrical characteristics. From recent structural investigations, two main experimental results deserve attention. On one hand, some of the phases expected from quaternary phase diagram based predictions have been identified in Al-rich Al/Ti-SiC alloyed contacts [3]. On the other hand, metallization spikes have been detected in this system. A high density of these metallic intrusions in the semiconductor is apparently bound to the ohmic behaviour and to the good contact-to-contact reproducibility observed [4]. Despite these experimental evidences several questions concerning the composition and the electrical behaviour of these contact regions are still open. Among these questions are the Al behaviour during the reaction that is not understood and the composition of the observed metallic spikes, suspected to enhance field emission, that is unknown. Additional structural investigations are thus needed to shed light upon these points. In this way, we report in this work a structural and chemical investigation of Al-rich Al/Ti-SiC alloyed contacts based on the Rutherford Back Scattering technique in the channeling mode (RBS-C) and Cross-Sectional Transmission Electron Microscopy (XTEM) in various analysis modes. These results are compared with those obtained from a preliminary structural investigation of Ti-SiC contacts.

Photoionization cross-section of the DX center in Te-doped AlxGa1−xSb

The photoionization of DX centers in Te-doped AlxGa1−xSb layers grown by molecular beam epitaxy is investigated by measuring the increase of the Hall free electron density after illumination by monochromatic light in the temperature range typical of the persistent photoconductivity (PPC) effect. The investigated samples have AlSb molar fractions in the 0.3⩽x⩽0.5 range and n-type doping in the 1017−1018 cm−3 range. An accurate investigation of the isothermal photoionization transients is performed to evidence features in the curve not directly related to the phenomenology of the DX center, the free electron density being influenced by the possible occupancy of other impurity levels. The transients show, in particular, an initial nonexponential behavior which is demonstrated as due to localization of a fraction of the photoexcited electrons into a nonmetastable impurity state which is responsible for the semiconductor-to-metal transition observed under the PPC regime. When this effect is accounted for, the ...

Microwave Annealing of High Dose Al+-implanted 4H-SiC: Towards Device Fabrication

High-purity semi-insulating 8° off-axis 〈0001〉 4H-SiC was implanted with Al+ at different doses and energies to obtain a dopant concentration in the range of 5 × 1019–5 × 1020 cm−3. A custom-made microwave heating system was employed for post-implantation annealing at 2,000 °C for 30 s. Sheet resistance and Hall-effect measurements were performed in the temperature range of 150–700 K. At room temperature, for the highest Al concentration, a minimum resistivity of 3 × 10−2 Ω cm was obtained, whereas for the lowest Al concentration, the measured resistivity value was 4 × 10−1 Ω cm. The onset of impurity band conduction was observed at around room temperature for the samples implanted with Al concentrations ≥3 × 1020 cm−3. Vertical p+-i-n diodes whose anodes were made by 1.5 × 1020 cm−3 Al+ implantation and 2,000 °C/30 s microwave annealing showed exponential forward current–voltage characteristics with two different ideality factors under low current injection. A crossover point of the temperature coefficient of the diode resistance, from negative to positive values, was observed when the forward current entered the ohmic regime.

Electrical and interfacial properties of GaAs/GaSb metal-organic vapour phase epitaxy heterostructures

The electrical properties of GaAs/GaSb heterojunctions grown by metal-organic vapour phase epitaxy were carefully investigated. The structures were formed by heavily p(Zn)-doped GaAs layers deposited on n(Te)-doped GaSb bulk crystal used as substrates. The current-voltage characteristics showed the formation of a GaSb p-n homojunction, which was expected to be induced by Zn diffusion into GaSb. Nevertheless, secondary ion mass spectrometry pointed out a small penetration depth of Zn atoms in the GaSb substrate, resulting unaffected by post-growth annealing processes. Electron beam induced current analysis demonstrated that the p-n junction interface was located more deeply into the substrate (∼1 μm). This result was confirmed by capacitance-voltage (C-V) and electrochemical C-V characterizations. Admittance spectroscopy led to attribute the change of conduction type from n to p in GaSb to the formation of additional shallow acceptor levels, activated by GaAs growth and post-growth thermal annealing proces...

Occupancy level of the DX center in Te-doped AlxGa1−xSb

Hall effect measurements were performed in Te-doped AlxGa1−xSb layers grown by molecular beam epitaxy to investigate the composition dependence of the DX center occupancy level. The investigated samples have AlSb molar fractions in the 0.25⩽x⩽0.50 range and n-type doping of about 1018 cm−3. A family of x≈0.40 samples of different doping (5×1015–1018 cm−3) were also studied. The Hall electron density data versus temperature were analyzed at high temperatures (T⩾150 K) where the DX center is at equilibrium, by assuming the negative-U model for the DX level and by taking into account the multivalley conduction effects. The DX level, degenerate in energy with the conduction band at low x values, enters the forbidden gap at x≈0.25 and then it becomes deeper with increasing x. In lightly doped samples, the introduction of a second level of the same Te impurity is required to fit the data; such level can be identified with the nonmetastable level which controls the low temperature electrical properties of the ma...

High Temperature Variable Range Hopping in Heavy Al Implanted 4H-SiC

In this work, we confirm and extend the results of a previous study where a variable range hopping transport through localized impurity states has been found to dominate the electrical transport properties of 3×1020 cm-3 and 5×1020 cm-3 Al+ implanted 4H-SiC layers after 1950-2000 °C post implantation annealing. In this study, samples with longer annealing times have been taken into account. The temperature dependence of these sample conductivity follows a variable range hopping law, consistent with a nearly two-dimensional hopping transport of non-interacting carriers that in the highest doped samples, persists up to around room temperature. This result indicates that the hole transport becomes strongly anisotropic on increasing the doping level. At the origin of this unusual electrical behavior, may be the presence of basal plane stacking faults, actually observed by transmission electron microscopy in one of the 5×1020 cm-3 samples

Effects of a resonant shallow state on the electrical properties of Te-doped AlxGa1−xSb

In AlxGa1−xSb, the Te impurity originates a simple donor state (D level), whose occupancy equilibrates rapidly with the conduction band; it coexists with a DX state which is responsible for the persistent photoconductivity effect. The D level is linked to the L conduction band minima and becomes resonant with the conduction band for x<0.20. Under these conditions, occupancy variations in the D level give rise to a nonomonotonic temperature dependence of the nH Hall density which is not attributable to mixed conduction effects. The role of the D level was investigated in samples of different alloy compositions (x<0.20) and doping levels (1016–1018 cm−3) and analyzed using a simplified model. The analysis confirmed the role of the occupancy variations in the D level in determining the temperature dependence of nH and the linkage of the level to the L conduction band edge.

Size effect on high temperature variable range hopping in Al+ implanted 4H-SiC

The hole transport properties of heavily doped 4H-SiC (Al) layers with Al implanted concentrations of 3  ×  1020 and 5  ×  1020 cm-3 and annealed in the temperature range 1950-2100 °C, have been analyzed to determine the main transport mechanisms. This study shows that the temperature dependence of the resistivity (conductivity) may be accounted for by a variable range hopping (VRH) transport into an impurity band. Depending on the concentration of the implanted impurities and the post-implantation annealing treatment, this VRH mechanism persists over different temperature ranges that may extend up to room temperature. In this framework, two different transport regimes are identified, having the characteristic of an isotropic 3D VRH and an anisotropic nearly 2D VRH. The latter conduction mechanism appears to take place in a rather thick layer (about 400 nm) that is too large to induce a confinement effect of the carrier hops. The possibility that an anisotropic transport may be induced by a structural modification of the implanted layer because of a high density of basal plane stacking faults (SF) in the implanted layers is considered. The interpretation of the conduction in the heaviest doped samples in terms of nearly 2D VRH is supported by the results of the transmission electron microscopy (TEM) investigation on one of the 5  ×  1020 cm-3 Al implanted samples of this study. In this context, the average separation between basal plane SFs, measured along the c-axis, which is orthogonal to the carrier transport during electrical characterization, appears to be in keeping with the estimated value of the optimal hopping length of the VRH theory. Conversely, no SFs are detected by TEM in a sample with an Al concentration of 1  ×  1019 cm-3 where a 3D nearest neighbor hopping (NNH) transport is observed.