Antonella Parisini

Electrical and photoluminescence properties of undoped GaSb prepared by molecular beam epitaxy and atomic layer molecular beam epitaxy

Abstract High-quality, unintentionally doped GaSb layers were grown by molecular beam epitaxy (MBE) and by atomic layer MBE (ALMBE) to study the influence of growth conditions on their transport and photoluminescence properties. While the hole mobility μ is only slightly dependent on the growth conditions, the 77 K hole concentration p shows minima at growth temperatures of ∼ 450°C and for Sb 4 /Ga beam equivalent pressure ratios (BEPRs) of ∼ 8; the concentrations of singly and of doubly ionizable acceptors and of a compensating donor have been obtained by simultaneously fitting the temperature dependence of μ and p in the 40–300 K range; from these results we show that the existence of minima of p can be related to an increased electrical compensation due to a reduced incorporation of acceptors. The 15 K photoluminescence spectra are generally dominated by the donor-acceptor pair transition (A, 779 meV) involving the neutral state of the native acceptor, except when the growth is carried out at relatively low temperatures with low BEPRs. Under the latter conditions: (i) a peak (788 meV) shows at an energy 25 meV below the band-gap energy and (ii) exciton related transitions have intensities comparable to that of the A recombination.

Electron mobility and physical magnetoresistance in n-type GaSb layers grown by molecular beam epitaxy

Electron mobility and low-field transverse physical magnetoresistance were measured in Te-doped GaSb layers grown by molecular beam epitaxy. The samples investigated had electron densities ranging from to ; measurements were taken in the 8 - 300 K temperature range. The high mobility values demonstrate that SnTe can be used as a source of Te doping with results comparable with GaTe. A detailed analysis of the magnetoresistance data demonstrates that in samples with high electron density the magnetoresistance is mainly due to mixed conduction of electrons in both and L conduction band minima: the analysis gives the temperature dependence of the and mobilities and of the energy separation between L and edges. is 82 meV at 300 K and 67 meV at 8 K and exhibits a non-monotonic behaviour within the temperature range explored. In samples with low electron density the magnetoresistance is mainly due to the energy distribution of carriers in the valley.

Preparation of GaSb by molecular beam epitaxy and electrical and photoluminescence characterization

Abstract The molecular beam epitaxy and the characterization of high-quality, non-intentionally doped and Te-doped GaSb are reported. The undoped layers have 77 K Hall hole concentrations p = 1−3 × 10 15 cm −3 and mobilities μ = 3000−5600 cm 2 V −1 s −1 , which compare favourably with the best results reported so far. Photoluminescence (PL) measurements at 70 K point to a relatively low concentration of intrinsic GaSb antisite defects, responsible for the p-type behaviour. A simultaneous fit of μ and p was performed in the 40–300 K temperature range by using a model with two acceptors, one of them having two charge states; the levels have energies consistent with those deduced by PL experiments. n-Doping was obtained approximately in the range 1 × 10 16 −1 × 10 18 cm −3 by using Te from an SnTe source. The mobility values are definitely higher than those reported in the literature and obtained with the same source, and they slightly exceed those achieved in GaSb doped using a GaTe Te source. A procedure for the analysis of electrical data has been set up and tested for n-GaSb with free carrier concentrations lower than a few 1017 cm−3.

Concentration dependence of optical absorption in tellurium-doped GaSb

The optical absorption of molecular-beam-epitaxy-grown and Te-doped GaSb layers is measured in the spectral region of the fundamental absorption over a temperature range extending from 10 K to 300 K. In accordance with the Burstein-Moss description, a filling of the conduction-band states, resulting in a change of the shape and a shift of the absorption edge to higher energies, is observed in the absorption spectra of Te-doped n-type GaSb layers, with electron density ranging from 1.1 × 10 16 to 7.6 × 10 17 cm −3 at room temperature. A quantitative description of the Burstein-Moss effect is performed and the Fermi-level energy and the electron density in thevalley are obtained as a function of the temperature in two different ways: (i) by comparing absorption spectra of heavily doped and unintentionally or lightly doped GaSb samples; (ii) through a direct fit of absorption data performed in the framework of Kanes band model. The values of the Fermi level and of electron density in thevalley which have been optically obtained resulted in satisfactory agreement with those obtained from electrical measurements. The bandgap narrowing and the perturbation of the conduction-band density of states due to heavy doping in small-effective-mass semiconductors, such as GaSb, is considered in the framework of some current theoretical models.

Photovoltage and photocurrent spectroscopy of p+-i-n+ GaAs/AlGaAs quantum well heterostructures

The photoelectric properties of p+‐i‐n+ heterostructures that contain a GaAs/AlGaAs superlattice, grown by molecular beam epitaxy, have been investigated. The nominally undoped multi‐quantum‐well region was excited by photons whose energies were varied within the spectral region of valence‐to‐conduction subband transitions. The dark‐ and light‐induced perpendicular transport was examined. The intense spectral features observed by photovoltage (PV) and photocurrent (PC) measurements were ascribed to excitonic transitions between the heavy and light hole electron states; their energetic location and line shape (half width at half peak and integrated area) were carefully studied in the temperature interval 10–300 K. The excitonic nature of the spectral peaks showing a conspicuous red shift under a reverse bias (Stark shift) were confirmed by electroreflectance measurements. The full analogy and the origin of the two (PV and PC) photoresponse signals are clearly proved. The spectral features can be explained ...

Al+ Implanted 4H-SiC: Improved Electrical Activation and Ohmic Contacts

The p-type doping of high purity semi-insulating 4H-SiC by Al+ ion implantation and a conventional thermal annealing of 1950 °C/5 min has been studied for implanted Al concentration in the range of 1 x1019 - 8 x 1020 cm-3 (0.36 μm implanted thickness). Sheet resistance in the range of 1.6 x 104 to 8.9 x102 Ω, corresponding to a resistivity in the range of 4.7 x 10-1 to 2.7 x 10-2 Ωcm for increasing implanted Al concentration have been obtained. Hall carrier density and mobility data in the temperature range of 140–600 K feature the transition from a valence band to an intra-band conduction for increasing implanted Al concentration. The specific contact resistance of Ti/Al contacts on the 5 x1019 cm-3 Al implanted specimen features a thermionic field effect conduction with a specific contact resistance in the 10-6 Ωcm2 decade.

A Shallow State Coexisting with the DX Center in Te‐Doped AlGaSb

The effects on the electrical properties of AlxGa1—xSb of a level of the Te impurity (D level) coexisting with the DX center were investigated through an extensive analysis of Hall data. The experiments were performed in MBE samples in a wide range of alloy compositions (0 ≤ x ≤ 0.50) and Te doping densities (1016 to 1018 cm−3). The possibility of observing carrier freeze-out to a D level, without competition with the DX center, is restricted to lightly doped samples and to a narrow range of alloy fractions close to the Γ–L crossing of conduction band edges. Within this range the D level is most likely coupled to the L minima. At higher alloy fractions, freeze-out to a D level can be observed only at a low temperature where the DX center occupancy is frozen. An example of this procedure is given for x = 0.40.

Control of the n-type doping in AlxGa1 − xSb: DX-center behavior of the Te impurity

Abstract Hall and photo-Hall data have been taken in Te-doped Al x Ga 1 − x Sb epitaxial layers of different AlSb molar fractions and doping densities in the 10 17 –10 18 cm −3 range. The evidence of the persistent photoconductivity effect at low temperatures reveals the presence of the DX center, whose occupancy level is as deep as the AlSb molar fraction increases. The temperature dependences of the Hall carrier density and mobility, n H ( T ) and μ H ( T ), have been carefully investigated by varying the density of the photoionized DX centers between the dark value and the saturation one. At low temperatures the electrical data are dominated by the occupancy of a Te-donor level in thermal equilibrium with the conduction band states, responsible of a semiconductor-to-metal transition when the density of the photoexcited electrons becomes sufficiently high. The isothermal μ H ( n ) curve is a single valued function, independent of the experimental procedure. Possible explanations of this result have been briefly discussed.

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

Microwave Annealing of Al+ Implanted 4H-SiC: Towards Device Fabrication

Carrier transport in Al+ implanted 4H-SiC for Al concentrations in the 5 × 1019 5 × 1020 cm-3 range and after 2000°C/30s microwave annealing are characterized. Each sample resistivity decreases with increasing temperature and attains values of about 102 Ωcm for temperatures > 600 K. At room temperature, resistivity decreases from 4 × 10-1 Ωcm to 3 × 102 Ωcm with the increase of implanted Al concentration. The onset of an impurity band conduction around room temperature takes place for implanted Al concentrations > 3 × 1020 cm-3. Al+ implanted and microwave annealed 4HSiC vertical p+-i-n diodes have shown promising forward characteristics.