Low-power Pulsed-laser Annealing Research Articles

Hydrogen patterning of Ga 1−xMn xAs for planar spintronics

We demonstrate two patterning techniques based on hydrogen passivation of Ga 1− x Mn x As to produce isolated ferromagnetically active regions embedded uniformly in a paramagnetic, insulating host. The first method consists of selective hydrogenation of Ga 1− x Mn x As by lithographic masking. Magnetotransport measurements of Hall-bars made in this manner display the characteristic properties of the hole-mediated ferromagnetic phase, which result from good pattern isolation. Arrays of Ga 1− x Mn x As dots as small as 250 nm across have been realized by this process. The second process consists of blanket hydrogenation of Ga 1− x Mn x As followed by local reactivation using confined low-power pulsed-laser annealing. Conductance imaging reveals local electrical reactivation of micrometer-sized regions that accompanies the restoration of ferromagnetism. The spatial resolution achievable with this method can potentially reach <100 nm by employing near-field laser processing. The high spatial resolution attainable by hydrogenation patterning enables the development of systems with novel functionalities such as lateral spin-injection as well as the exploration of magnetization dynamics in individual and coupled structures made from this novel class of semiconductors.

Electrical and optical characterization of a Zn-implanted InP laser annealed in a nitrogen atmosphere

Semi-insulating InP samples have been implanted with Zn in order to obtain p-type films. The implanted samples have been annealed by low-power pulsed-laser annealing in a N2 controlled atmosphere and in a vacuum. The Hall resistance measurements show that remarkable conductivity is obtained only in the case where the annealing is performed in a N2 atmosphere. The measured carrier activation energy evidences that the conduction is indeed due to the ionization of the implanted Zn atoms. Moreover, the mobility temperature behaviour has been used to evidence the carrier scattering mechanisms in the implanted and annealed layer. Lastly, the optical absorption coefficient of these samples has been measured by means of the photo-thermal deflection absorption technique in order to evidence the modification on the optical properties induced by the ion beam and laser irradiation.

Temperature and ion flux dependence of damage structures in Zn+ implanted and laser annealed GaAs

High resolution transmission electron microscopy has been used to investigate the lattice damage dependence on the implantation temperature and ion flux in Zn+ implanted gallium arsenide (GaAs). The implantation parameters were chosen in the amorphization–crystallization transition regime in order to investigate the damage typology while the amorphization regime is approached. It is shown that the approach to the amorphization regime is related to the appearance of stacking faults related extended defects lying on (111) planes. Subsequently, the samples have been annealed by low-power pulsed-laser annealing (LPPLA). In the best annealing conditions the resulting structure is characterized by a low density of extended defects in the implanted layer showing that LPPLA induced solid phase epitaxial regrowth is able to anneal out even `crystalline' defects.

High resolution transmission electron microscopy of elevated temperature Zn+ implanted and low-power pulsed laser annealed GaAs

Cross-sectional high resolution transmission electron microscopy was applied to 140 keV Zn+ implanted GaAs at elevated temperature (110±10 °C). Gaussian-like in-depth distributions of damage clusters, retaining some features of the original crystal lattice, were observed. The distribution maximum was found located between about 55 and 80 nm below the implanted surface. Low-power pulsed-laser annealing of the implanted samples induced both migration and clustering of radiation defects in the region extending from the surface and 80 nm depth, combined with nearly complete recrystallization of the material below this layer.

In-depth characterisation of electrical carrier activation in Zn +-implanted and laser annealed InP

Low-power pulsed-laser annealing has been applied to Zn +-implanted InP samples in N 2 atmosphere, attaining structural reordering and high electrical activation. The in-depth hole carrier concentration distribution has been compared with the in-depth implanted Zn distribution: a [Zn] plateau appears where the activation is about 100%.

Structural reordering and electrical activation of ion-implanted GaAs and InP due to laser annealing in a controlled atmosphere

The effects of the ambient atmosphere in the annealing chamber on the electrical and structural characteristics of Zn-implanted III-V compound semiconductors, processed by low-power pulsed-laser annealing are presented. The samples were analyzed using several complementary experimental techniques: Reflection high-energy electron diffraction, Rutherford backscattering spectroscopy, Raman spectroscopy, secondary ion mass spectroscopy, and electrical measurements. During the laser beam irradiation in the presence of gas inlet into the annealing chamber the ambient gas atoms diffused well into the target changing the stoichiometry and the electrical parameters. Redistribution of the implanted impurity was also observed. By varying the type of gas used and its pressure, it was possible to achieve electrical activation of up to 80%. It seems all structure and electrical parameters achieve their best values at the same ambient atmosphere and density of the deposited laser power P, e.g., 1.5 atm of ${\mathrm{N}}_{2}$ and $P=6.5 {\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ for InP.

ELECTRICAL CARRIER ACTIVATION IN ZN+ IMPLANTED AND LOW-POWER PULSED-LASER ANNEALED INP IN NITROGEN ATMOSPHERE

Low-power pulsed-laser annealing was applied to Zn+-implanted InP samples. In order to avoid surface oxidation during the treatment, the laser irradiation was carried out in inert ambients of different gases (Ar or N2) at different pressures. Reflection high energy electron diffraction (RHEED), Rutherford backscattering spectroscopy (RBS) and electrical measurements were used to analyse the physical properties of the samples before and after the laser annealing process. In particular, it has been possible to demonstrate that the chemical properties of the gas used play a crucial role in the electrical carrier activation (about 80%) of the processed InP sample.

Effects of low-power pulsed-laser annealing on electrical properties of Zn-implanted InP

Abstract Low-Power Pulsed-Laser Annealing (LPPLA) has been applied to Zn-implanted InP samples kept in a controlled atmosphere (Ar or N2). The chemical properties of the used gas play a crucial role to give rise to the electrical carrier activation (about 80%) of the processed InP.

Activation of electrical carriers in Zn-implanted InP by low-power pulsed-laser annealing

Low-power pulsed-laser annealing was applied to Zn+-implanted InP samples. In order to avoid surface oxidation during the treatment, the laser irradiation was carried out in inert ambient of nitrogen at different pressures. The analytical techniques used include Rutherford backscattering spectroscopy, reflection high energy electron diffraction, and electrical measurements. The highest carrier activation, about 80%, was achieved at the same laser power density (6.5 MW/cm2) at which the best crystal recovery was obtained.

InP crystals-ion implantation and laser annealing: RHEED, XPS and computer simulation studies

Reflection high-energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS) were used to demonstrate the effects of Zn +-ion implantation and subsequent low-power pulsed laser annealing (LPPLA) on the crystallinity and the P/In ratio near the surface of InP. The influence of ion-beam effects during depth profiling by a 1 to 5 keV Ar beam on the composition of surface layers is discussed in terms of XPS data compared to computer simulation results using a dynamic model based on TRIM. Relative enrichment in P of the topmost surface layer of virgin samples along with the P deficiency below the surface is confirmed by both XPS and computer simulation data, the effect being more pronounced for higher energies of the depth-profiling Ar + beam. Severe C and O contamination is found on the ion-implanted InP surface exposed to air. The P/In ratios at and below the surface (to about 10 nm depth) differ from that for the virgin sample. Both RHEED and XPS demonstrate the efficiency of laser pulses at a power density of 6 MW/cm 2 in recovering the structure and stoichiometry of ion-implanted crystals of InP.

XPS depth profiling of laser-annealed Zn+-implanted GaAs

Data on the effects of 140 keV Zn+-implantation in (100) GaAs and the consequent low power pulsed laser annealing (LPPLA) on the As/Ga ratio and the chemical states of the elements at the surface and in the subsurface region are presented. The results include the depth distribution of the elements for virgin, as-implanted and implanted+LPPLA [30×(4.5/7.5) MW/cm2] samples. The X-ray photoelectron spectra of as-implanted samples show that a low-intensity Zn2p peak is observed after 20 min of Ar+ sputtering with an energy of 3 keV, corresponding to about 20 nm of etched material. The depth profiling XPS analysis confirms the `recovering' of the stoichiometry of Zn+-implanted specimens after LPPLA with laser pulses of a power density in the energy window of (5–7 MW/cm2). At laser pulse power densities outside of this energy window (4.5 and 7.5 MW/cm2) Zn appears again in the XP spectra after 20 min sputtering as in the case of as-implanted GaAs.

Low power pulsed laser annealing of Zn+-implanted InP: first endeavours

In this paper we report a first attempt to apply the low power pulsed-laser annealing (LPPLA) technique, already successfully used on implanted GaAs, to a low dose Zn-implanted InP. The aim of this work is to check the LPPLA efficiency and to define, in analogy with the GaAs case, the eventual existence for the InP of an energy window in which the crystallinity is recovered without stoichiometric changes of the material.Reflection high energy electron diffraction analysis and electrical measurements have revealed good crystalline reordering and a decrease in the sheet resistivity in the laser-annealed samples.

Solid-phase epitaxy induced by low-power pulsed-laser annealing of III-V compound semiconductors.

Low-power pulsed-laser annealing (LPPLA) was applied to III-V compound semiconductors GaAs and InP. The effects have been analyzed using several experimental techniques such as reflection high-energy electron diffraction, Rutherford backscattering spectroscopy, x-ray photoelectron spectroscopy, and electrical measurements. In addition, a calculation method was developed to study the heat propagation in the irradiated sample during the LPPLA process. The irradiation conditions, realizing a uniform surface laser-energy distribution, made possible a unidimensional approach. The results obtained experimentally and by numerical modeling agree well if one assumes that a solid-phase epitaxy takes place. The XPS measurements for GaAs and InP show, in particular, that a range of the irradiation power density exists where the LPPLA can effectively restore the lattice order without appreciable alteration of the surface stoichiometry. At higher power density of irradiation, the As and P vacancies introduced by the laser, in GaAs and InP, respectively, may no longer be neglected. \textcopyright{} 1996 The American Physical Society.

Lattice defects in ion implanted GaAs

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Lattice Electron Microscopy and Image Processing of Ion-Implanted and Laser-Annealed GaAs Structures

The efficiency of the Low-Power Pulsed-Laser Annealing (LPPLA) as a treatment to restore the ion implantation induced crystal disorder is well-known, but it is still open the question regarding the way in which is possible to model the LPPLA dynamic effects. At this purpose we believe that a deeper knowledge about the modification of the ion-induced extended defects (dislocations, stacking faults, clusters), as a consequence of the LPPLA treatment, is relevant. In our opinion, a technique suitable to gain this information could be the lattice image digital processing. In this paper we reported the experimental results concerning the use of this technique applied to some HRTEM images of low-dose implanted GaAs samples. In particular, the appearance of different types of extended defects in the implanted material and their annealing with LPPLA have been put in evidence.

Laser-Induced Reduction of Carrier Activation Energy in Zn-Implanted GaAs

It has been recently established that the crystallinity of ion-implanted GaAs samples can be restored by using a low-power pulsed-laser annealing (LPPLA) technique. In this paper we report an improvement of carrier activation achieved after low-temperature processing (added to LPPLA) at temperatures low enough to avoid substantial losses of As. By using this combination of techniques we have reduced the sheet resistivity to values comparable with those obtained by standard techniques without any specimen encapsulation (or similar) processing. The pretreatment by LPPLA seems to reduce the Zn activation threshold down to values which prevent the sublimation of As.

XPS study of laser‐annealed ion‐implanted GaAs

AbstractData on the effects of 140 keV Zn+ implanation of (100)GaAs and the consequent low‐power pulsed laser annealing (LPPLA) on the composition of the surface layers are presented. The implantation does was 1014 cm −2 and the sample temperature was kept at 110 ± 10°C. THE LPPLA was carried out by 10–30 pulses of a Q‐switched ruby laser (λ = 694.3 nm, τ = 25 ns and P0 = 4–6 MW cm−2) equipped with a spatial homogenizer. The crystal surface was studied by XPS analysis combined with depth profiling with the use of 1 keV Ar+.The results presented include the depth redistribution of oxygen and the As/Ga ratio as well as the oxide thickness for (a) virgin, (b) as‐implanted and (c) implanted and then annealed samples.

High Voltage Transmission Electron Microscopy of Low Power Pulsed Laser Annealing of Zn+ Implanted GaAs

Crystals of (100) GaAs implanted with Zn+-ions were treated by low-power pulsed-laser annealing (LPPLA) technique. The first investigations about the effect of this treatment on the damage structure distribution in the GaAs by means of high voltage transmission electron microscopy are shown. Typical rearrangement and accumulation trends of the initially uniformly dispersed damage clusters were established after the LPPLA. The morphology of gallium oxide films grown on the surface of GaAs during LPPLA treatment and their specific behavior under electron bombardment during the observation in the microscope are reported too.

Temperature behavior of implanted and pulsed laser irradiated GaAs

Recent studies have shown that the Low-Power Pulsed-Laser Annealing (LPPLA) of ion-implanted GaAs specimens can be realized in a power-density window in which a complete structural reordering is guaranted. As the experimentally employed conditions allow us to describe the theoretical problem in an unidimensional space domain, we describe here a method to investigate the in-depth temperature behavior during the low-power pulsed-laser irradiation of ion-implanted semiconductors. The application of this method to GaAs specimens shows that the upper limit of the energy density window is connected with the exceeding of the critical temperature Tc below which the As evaporation rate is negligible.

Experimental Conditions Required to Achieve Low-Power Pulsed-Laser Annealing of Implanted GaAs

The experimental conditions necessary to obtain low-power pulsed-laser annealing (LPPLA) of implanted samples of GaAs are described. Simple theoretical estimates have been used to show that the sample's temperature evolution is such that it does not produce appreciable GaAs decomposition. Particular attention is paid to the spatial distribution of the energy over the laser beam cross section and the optical system used in the experiments. It is also pointed out that below the melting threshold, but above the upper limit of the energy window, within which the LPPLA is obtained, a structural disorder of the material is induced by the laser itself.