Ge diffusion into GaAs by pulsed laser irradiation

Ge diffusion into GaAs from thin evaporated layers as sources is reported. Irradiation with aQ-switched ruby laser gives rise ton-type diffused layers of a thickness from 240 to 710 Å. A strong compensation of the diffused layers, that cannot be removed by thermal annealing, was observed. From the present experimental results it can be inferred that the diffusion coefficient increases at the melting point by 5 to 6 orders of magnitude.     

CW CO2-laser annealing of arsenic implanted silicon

CW CO₂-laser annealing of arsenic implanted silicon was investigated in comparison with thermal annealing. Ion channeling, ellipsometry, and Hall effect measurements were performed to characterize the annealed layers and a correlation among the different methods was made. The laser annealing was done with power densities of 100 to 640 W cm⁻² for 1 to 20 s. It was found that the lattice disorder produced during implantation can be completely annealed out by laser annealing with a power density of 500 W cm⁻² and the arsenic atoms are brought on lattice sites up to 96±2%. The maximum sheet carrier concentration of 6×10¹⁵ cm⁻² was obtained for 1×10¹⁶ cm⁻² implantation after laser annealing, which was up to 33% higher than that after thermal annealing at 600 to 900°C for 30 min.     

Enhancement of minority carrier lifetime in GaAs1? x P x ( x =0.4; 0.65) by nitrogen implantation

Minority carrier lifetimes in nitrogen implanted GaAs_1-x P _x (x=0.4; 0.65) were measured at 77K by an optical phase shift method as a function of nitrogen dose and annealing temperature in order to investigate the dependence of the lifetime on the concentration of nitrogen isoelectronic traps. A large increase in the lifetime was observed after nitrogen implantation followed by annealing at and above 800°C. The maximum lifetimes were 22ns for GaAs_0.35P_0.65 and 6.7 ns for GaAs_0.6P_0.4. They were obtained by implantation to a dose of 5×10¹³ cm⁻² in GaAs_0.35P_0.65 and 10¹³ cm⁻² in GaAs_0.6P_0.4. The lifetime after nitrogen implantation followed by annealing was longer by a factor of 6–7 than that of the unimplanted sample.     

Nd: YAG laser annealing of gallium-implanted silicon

A Q-switched Nd: YAG laser with a pulse duration of 20 ns was used to investigate effects of laser annealing in gallium implanted silicon. Rutherford backscattering and Hall-effect measurements were performed to evaluate the annealed layer. Differential Hall-effect measurements were carried out to obtain carrier concentration profiles after annealing. It was found that a maximum sheet carrier concentration of 8×10¹⁵ cm⁻² can be obtained for a gallium implantation of 10¹⁶ cm⁻² by laser annealing with an energy density of more than 1.0 J cm⁻². Although the peak carrier concentration was found to be 8.0×10²⁰ cm⁻³, the annealed layer showed polycrystalline structures even after annealing with an energy density up to 4J cm⁻². The annealing took place in the solid phase in this energy density range.     

Passivation of laser induced defects in silicon by low energy hydrogen ion implantation

It is well established that the pulsed-laser annealing of as-implanted Si introduces electrically active defects. The aim of this paper is to show that these defects can be neutralized by low-energy hydrogen ion implantation. The techniques used for the sample characterization are current-voltage measurements and capacitance transient spectroscopy.     

Compensation in GaAs crystals due to anti-structure disorder

Complete equilibrium of native point defects in a GaAs crystal being in contact with the liquid phase (melt) is considered. Equilibrium relations for the concentrations of antisite defects, Ga_As and As_Ga, in dependence on the melt composition are derived. It is argued that the As_Ga defect is a double donor, the lower energy level of which corresponds to EL2, while the Ga_As is a double acceptor that can be identified with residual acceptor having the levels at 78 meV and 200 meV above the valence band, which was discovered by Elliot et al. It is shown that the presence of antisite defects together with an excess of background shallow acceptors leads, under reasonable assumptions, to a nearly intrinsic behaviour of LEC-grown GaAs, if the As atom fraction in the melt ranges from 0.471 to 0.535, in accordance with observations of Holmes et al.     

Front and back surface cw CO2-laser annealing of arsenic ion-implanted silicon

The annealing behavior of arsenic-implanted silicon under scanned cw CO₂-laser irradiation from front and back surfaces is investigated. Ellipsometry, Hall effect, Rutherford backscattering measurements and neutron activation analysis indicate an enhancement of annealing efficiency by laser irradiation from the back surface, which provides complete recovery of crystal damage, high substitutionality and electrical activation of implanted arsenic atoms without redistribution of concentration profile. The enhancement of annealing efficiency under back-surface irradiation is explained by the difference in laser reflection from the front and back surface of silicon wafers. No differences in the results are found for scanned and static annealing.     

Optical properties and origin of infrared light scattering centers in undoped semi-insulating GaAs crystals

The wavelength dependence and polarization characteristics of the infrared light scattered from an undoped GaAs crystal were investigated in the 90° angle infrared light scattering configuration. The scattering is Rayleigh scattering from scatterers which are always associated with the dislocations, and they are classified into three types,S, L _A , andL _G scatterers, according to their polarization characteristics. TheS, L _A , andL _G -scatterers are thought to be small As clusters, large As precipitates and large Ga precipitates, respectively.     

Nitrogen implantation in GaAs1−xPx (x=0.4; 0.65)

Nitrogen ions were implanted in GaAs_1−xP_x (x=0.4; 0.65) at room temperature at various doses from 5×10¹² cm⁻² to 5×10¹⁵ cm⁻² and annealed at temperatures from 600°C up to 950°C using a sputtered SiO₂ encapsulation to investigate the possibility of creating isoelectronic traps by ion implantation. Photoluminescence and channeling measurements were performed to characterize implanted layers. The effects of damage induced by optically inactive neon ion implantation on photoluminescence spectrum were also investigated. By channeling measurements it was found that damage induced by nitrogen implantation is removed by annealing at 800°C. A nitrogen induced emission intensity comparable to the intensity of band gap emission for unimplanted material was observed for implanted GaAs_0.6P_0.4 after annealing at 850°C, while an enhancement of the emission intensity by a factor of 180 as compared with an unimplanted material was observed for implanted GaAs_0.35P_0.65 after annealing at 950°C. An anomalous diffusion of nitrogen atoms was found for implanted GaAs_0.6P_0.4 after annealing at and above 900°C.     

Laser ablation and desorption from calcite from ultraviolet to mid-infrared wavelengths

3 ) at wavelengths ranging from the ultraviolet to the mid-infrared, including a Nd:YAG laser operated at the fundamental, second and third harmonics, and a tunable infrared free-electron laser (wavelength range 2.5–8 μm). The threshold for ablation and the topography of the irradiated spot were characterized by scanning electron microscopy. A clear indication of two distinct excitation mechanisms was observed, namely, cracks and fractures followed by exfoliation at ultraviolet to near-infrared wavelengths, in contrast to evaporative holes and scattered droplets in the mid-infrared. Plume emission/absorption spectroscopy, plume transmission and photoacoustic beam deflection were used to characterize the ablation plasma. The composition of atoms, molecules, or particles in the ablation plumes also has a distinctive variation as a function of the wavelength. The excitation mechanisms leading to ablation appear to be defect activation at ultraviolet to near-infrared wavelengths, molecular impurity absorption and resonant vibrational absorption of the calcite at mid-infrared. Received: 5 December 1996/Accepted: 6 January 1997     

Characterization of active layers in GaAs by microwave absorption

Free carrier electric microwave absorption is applied to the analysis of ion implanted and epitaxial active layers in GaAs. An improved version of a previously reported waveguide system is described. It allows a quick and nondestructive determination of the sheet resistance, carrier concentration and carrier mobility of active layers. The usefulness of the method for routine electric material characterization supporting a microelectronic device fabrication is demonstrated. Finally, some explorative microwave measurements of heterostructures and photo-induced effects are reported.     

Absence of dislocation-induced luminescence in GaAs

Photoluminescence measurements on highly deformed GaAs samples in the near band edge region as well as in the region far below the bandgap are reported. The radiative recombination gives no evidence for dislocation-induced states in the energy gap.     

Thermal equilibrium concentrations and effects of negatively charged Ga vacancies in n-type GaAs

We have calculated the thermal equilibrium concentrations of the various negatively charged Ga vacancy species in GaAs. The triply-negatively-charged Ga vacancy, V _Ga ^3– , has been emphasized, since it dominates Ga self-diffusion and Ga-Al interdiffusion under intrinsic and n-doping conditions, as well as the diffusion of Si donor atoms occupying Ga sites. Under strong n-doping conditions, the thermal equilibrium V _Ga ^3– concentration, , has been found to exhibit a temperature independence or a negative temperature dependence, i.e., the value is either unchanged or increases as the temperature is lowered. This is quite contrary to the normal point defect behavior for which the point defect thermal equilibrium concentration decreases as the temperature is lowered. This property provides explanations to a number of outstanding experimental results, either requiring the interpretation that V _Ga ^3– has attained its thermal equilibrium concentration at the onset of each experiment, or requiring mechanisms involving point defect non-equilibrium phenomena.     

Electrical activity and precipitation behavior of copper in gallium arsenide

The electrical properties and preferred lattice site of Cu in GaAs were investigated combining electrical and optical measurements with ion beam and structural analysis. From this comprehensive study it was determined that Cu introduces two levels in the band gap, that the concentration of electrically active centers introduced by Cu diffusion is considerably smaller than the total Cu concentration, that this ratio of electrically active to total Cu concentration depends strongly on the cooling speed after diffusion, and that the portion of Cu that remains electrically inactive forms Cu-Ga precipitates.     

Defect study on electron irradiated GaAs by means of positron annihilation

Measurements of the positron lifetime and Doppler-broadened annihilation-radiation have been performed in electron-irradiated GaAs. The positron lifetime at the irradiation induced defects was 0.250 ns at 300 K. The defect clustering stage was found to occur at around 520–620 K, and the coarsening and annealing stage is believed to be above 620 K. Similar annealing stages were also observed in GaAs lightly doped with Si (0.2×10¹⁸ cm^–3). Both the lifetime and the S-parameter in the irradiated GaAs were found to decrease with temperature from 300 K to 100 K, suggesting the coexistence of shallow traps in electron irradiated GaAs.     

Raman and point contact current-voltage characterization of laser-induced diffusion in GaAs

Raman scattering and point contact current-voltage (PCIV) measurements were used as characterization tools of tin-diffused GaAs layers. Diffusion was induced by irradiating GaAs substrates covered with thin tin layers single pulses of a ruby laser. Samples processed with the lowest energies show strong damage and incomplete electrical activation as deduced from Raman and PCIV measurements, respectively. Raman microprobe in depth analysis and PCIV profiles also suggest the presence of a damaged region with incomplete electrical activation at the boundary between the molten layer and the solid substrate.     

Defect study of proton-irradiated liquid-encapsulated Czochralski GaAs using the positron-annihilation technique

The positron lifetime of undoped Liquid-Encapsulated Czochralski (LEC)-GaAs and Si-doped (1.3×10¹⁸ cm^–3) LEC-GaAs was measured before and after irradiation with protons (dose 1×10¹⁵/cm², 15 MeV). In Si-doped GaAs, the decrease of positron lifetime at temperatures between 10 and 300 K are due to the decrease of the positron-diffusion length and the increase of the effective shallow traps such as antisite Ga_As. The annealing stage of the proton-irradiation-induced defects which show the different behavior from that of electron-irradiation-induced defects suggests that proton irradiation creates more complicated defect complexes, containing vacancies rather than isolated vacancy-type defects or simple complexes which have been observed during electron-irradiation processes. Above 700 K, proton-irradiation-induced defects such as vacancy-type defects and simple vacancy complexes are almost annealed out, while Si-induced defects such as Si_Ga-V_Ga complexes cannot be annealed out above 973 K.     

Laser ablation with short and ultrashort laser pulses: Basic mechanisms from molecular-dynamics simulations

Laser ablation is a technology widely used in many applications. Understanding in detail the mechanisms that lead to ablation remains a formidable challenge because of the complexity of the processes taking place, the variety of species involved, and the range of length and time scales covered. Atomic-level experimental information is difficult to obtain and must be augmented by theory. In this article, we briefly review the progresses that we have accomplished using a simple two-dimensional molecular-dynamics model, insisting on the importance of considering the thermodynamics of the evolution of the systems in order to understand ablation. Through the identification of the thermodynamic pathways followed by the material after irradiation, our model has provided significant insights on the physical mechanisms leading to ablation. It has been demonstrated in particular that these depend strongly on the fluence, and are actually determined by the effective amount of energy received within different regions of the target. Further, internal or external factors, such as inertial confinement, play a key role in determining the route to ablation - and thus the types and sizes of particles ejected - by constraining the thermodynamical evolution of the system. We have established that, for ultrashort pulses in strongly absorbing materials, ablation proceeds by either spallation, phase explosion or fragmentation; the latter, we demonstrate, is the most important mechanism. For longer pulses, ablation may also proceed by trivial fragmentation.     

Investigation of transient transport and recombination phenomena in semiinsulating GaAs

Photoconductivity and Hall voltage kinetics were measured simultaneously in SI GaAs monocrystals, using the pulsed neodimium laser excitation. The scattering and recombination centres were found to have a different influence at different time intervals of the transients (from 10 ns to some seconds). It is shown that in GaAs the photoconductivity relaxation in some time intervals can be interpreted correctly only by taking into account the mobility changes. The obtained resuls are explained in terms of recharging of the scattering centres and variations of the capture cross-section of charge carries on the local centres.     

Non-equilibrium point defect phenomena influencing beryllium and zinc diffusion in GaAs and related compounds

Beryllium and zinc are the main p-type dopants used for the fabrication of devices based on GaAs or related III-V materials. Both elements are substitutionally dissolved on the group III sublattice and diffuse via the kick-out mechanism which involves group III self-interstitials. Non-equilibrium concentrations of these self-interstitials have a strong influence on the diffusivities of Be and Zn with often drastic consequences on device behavior especially if Be or Zn is used to realize narrow base regions in heterojunction bipolar transistors (HBTs). Various situations in which non-equilibrium point defects play a role for Be and Zn diffusion are discussed such as: in-diffusion of these dopants from an outside source, diffusion of grown-in dopants, self-interstitial generation by Fermi level surface pinning of highly n ⁺-doped emitter cap or subcollector layers in HBTs, or recom bination-enhanced beryllium diffusion during device operation. Finally, we will comment on the diffusion behavior of carbon, which is dissolved on the group V sublattice in GaAs, is much less sensitive to non-equilibrium point defect, and, therefore, is increasingly used to replace Be and Zn as p-type dopants.On sabbatical leave from Duke University, School of Engineering, Durham, NC 27706, USA