Si Diffusion In GaAs Research Articles

Resolving the mystery of the concentration-dependence of amphoteric dopant diffusion in III-V semiconductors

Amphoteric dopants, such as silicon, exhibit a concentration-dependent diffusivity in III-V compounds characterized by a low to negligible diffusion at low dopant concentrations, which increases by orders of magnitude at high concentrations. The reason for this anomalous and non-intuitive diffusivity has largely remained a mystery. Leading models from literature, built to explain existing experimental results, make contradictory assumptions on the nature of the diffusing species, based only on thermodynamic considerations. In this work, we perform ab initio calculations to characterize the kinetics of dopant diffusion in III-V materials revealing how the nature of the primary diffusing species changes with concentration. We use these results to derive a new analytical model which fits historical, as well as more recent, experimental data on Si diffusion in GaAs and In0.57Ga0.43As. This work provides a fundamentals-based understanding of Si diffusion in III-V alloys as well as a new predictive analysis tool while showcasing a mutiscale approach of solving diffusion problems. We hope this work will help guide the design of the next generation of III-V based electronic and photonic devices.

Si diffusion in GaAs

Theoretical studies are carried out to ascertain the dominant mechanism of Si diffusion in GaAs. Lattice dynamical model calculations have shown that the most probable diffusion mechanism is through a single vacancy even though several experiments cannot fix the mechanism as substitutional, substitutional-interstitial pair or neutral defect pair.

Simulation of Under- and Supersaturation of Gallium Vacancies in Gallium Arsenide During Silicon in- and Outdiffusion

ABSTRACTThe diffusivity of Si in GaAs shows a dependence on the cubic power of its concentration or the concentration of electrons n under both in- and outdiffusion conditions. Hence, the diffusion of Si in GaAs is consistent with the Fermi-level effect model invoking the triply-negatively-charged Ga vacancies, , as the point defect species responsible for diffusion to occur on the Ga sublattice under n-doping condition. However, the Si diffusivity values of the indiffusion cases is several orders of magnitude smaller than those of the outdiffusion cases at the same Si concentrations. This means that the two apparent Si diffusivity values under intrinsic conditions will contain also the same discrepancy, which has been previously assessed to be due to a undersaturation in indiffusion cases and a supersaturation in outdiffusion cases. In this study we have calculated the under- and supersaturation values using the known Si diffusivities. We found that the GaAs surface states play a key role in the development of the under- and supersaturations.

Fermi-Level Effect on Group III Atom Interdiffusion in III-V Compounds: Bandgap Heterogeneity and Low Silicon-Doping

ABSTRACTHeavy n-doping enhanced disordering of GaAs based III-V semiconductor superlattice or quantum well layers, as well as the diffusion of Si in GaAs have been previously explained by the Fermi-level effect model with the triply-negatively-charged group III lattice vacancies identified to be the responsible point defect species. These vacancies have a thermal equilibrium concentration proportional to the cubic power of the electron concentration n, leading to the same dependence of the layer disordering rate. In this paper, in addition, we take into account also the electric field effect produced by the material bandgap heterogeneity and/or hetero-junctions. In heavily n-doped or long time annealing cases, this effect is negligible. At low n-doping levels and for short annealing times, the layer disordering rate can be enhanced or reduced by this effect. Available experimental results of low Si-doped and very short-time annealed samples have been satisfactorily fitted using the Fermi-level effect model.

Oxidation enhanced diffusion of Si in GaAs: The effect of excess As on diffusion depth and carrier concentration

An investigation of the fast diffusion of Si in GaAs from deposited surface layers oxidized in an Ar/H2O ambient indicates excess As, formed by oxidation of Ga originating from the substrate, is responsible for the enhanced diffusion of Si into the substrate. The formation of the SiO2 on the surface during oxidation prevents loss of the excess As, which accumulates in the remaining Si film. Higher H2O partial pressures during the oxidation produce higher As/Si ratios, resulting in an increase in the Si diffusion depth and concentration. However, the n-type carrier concentration decreases with higher As/Si ratios in the remaining Si layer. A second, nonoxidizing anneal on samples with the SiO2 and Si layer removed produced different effects on the carrier concentration, depending on whether As was free to escape from the substrate. The results indicate that excess As related defects such as gallium vacancies are probably responsible for the n-type compensation in the fast diffused samples.

Doping profile control and two-dimensional electron gas formation by Si diffusion into III-V compounds

The doping characteristics of Si-diffused III-V compounds have been investigated using fully dielectric diffusion sources consisting of undoped SiOx/SiN double-layered films prepared by plasma-enhanced chemical-vapor deposition. It is demonstrated that the rectangular-shaped carrier concentration profiles resulting from the Si diffusion in GaAs can be controlled by varying the deposition parameters of the SiOx/SiN films, i.e., the N2O flow rate in the SiOx deposition, the SiOx film thickness, and the NH3 flow rate in the SiN deposition. It is explained that the profile variations are determined by the quantity of an excess of Si in the SiOx bottom film and the outdiffusion rate of Ga and/or As from GaAs. Furthermore, the Si diffusion experiments are carried out in AlxGa1−xAs, and the formation of a two-dimensional electron gas with a Hall mobility of 5060 cm2/V s at 300 K and 97 500 cm2/V s at 77 K is achieved by diffusing Si into an undoped GaAs/Al0.22Ga0.78As heterostructure.

Si diffusion in GaAs and Si-induced interdiffusion in GaAs/AlAs superlattices.

Si diffusion in GaAs and Si-induced interdiffusion in GaAs/AlAs superlattices.

Diffusion kinetics of Si in GaAs and related defect chemistry

The diffusion of Si in GaAs was investigated using diffusion sources from different tie triangle regions on the Si-Ga-As ternary phase diagram which provide constant chemical potentials for the three components under isothermal conditions. The Si profiles are measured with secondary ion mass spectrometry and differ significantly for the different sources. The Ga-Si-GaAs tie triangle source produces p-type Si doping with concentration independent diffusion coefficients, and a neutral As or Ga vacancy is proposed as the dominant mobile defect for these conditions. Arsenic-rich sources from two tie triangle regions and also a Si-GaAs tie line source produce Si donor diffusion with concentration-dependent diffusion behavior. The concentration-dependent diffusion coefficients of donor Si for As-rich source diffusion are related to the net ionized donor concentration showing three different regimes: intrinsic regime, intermediate regime, and saturation regime. Ga vacancies are proposed to be responsible for donor Si diffusion in GaAs: a V0Ga and/or V−Ga mechanism for the intrinsic regime; and a V−Ga related mechanism for the extrinsic and saturation regimes. The Si-GaAs tie line source resulted in two branch type profiles, intermediate between the As-rich and the Ga-rich source diffusion cases.

Influence of background doping and implant damage on the diffusion of implanted silicon in GaAs

Diffusion of Si in GaAs is studied using implanted Si in undoped GaAs, implanted Si in Se-, Si-, and Zn-doped GaAs, and grown-in Si in epitaxial layer structures. No diffusion is observed in the undoped and Zn-doped GaAs cases, a moderate level is observed in the Si-doped case, and a significant amount is found for the Se-doped and nonimplanted Si-doped epitaxy cases. These results indicate that the diffusion is controlled by a Fermi level mechanism (probably via ionized gallium vacancies) and that implant damage inhibits diffusion by keeping the electron concentration and/or the ionized gallium vacancy concentration low.

Ga vacancy-assisted diffusion of Si in GaAs

A model for Si diffusion into GaAs based on the formation of Si+Ga-V−Ga pairs is developed. The model follows the formalism developed by M. E. Greiner and J. F. Gibbons [Appl. Phys. Lett. 44, 750 (1984)]. Using recently derived values for the Ga vacancy diffusivity, it is shown that the pair diffusion coefficient is approximately equal to three-quarters its value. The results of the model are demonstrated to be in good agreement with the Si diffusion data.

A model of Si diffusion in GaAs based on the effect of the Fermi level

We propose a quantitative model for Si diffusion in GaAs. In this model we incorporate the experimental result that Si is an amphoteric impurity which constitutes a shallow donor when occupying the Ga site, Si+Ga, and a shallow acceptor when occupying the As site, Si−As. At high concentrations Si+Ga and Si−As coexist. The amphoteric behavior of Si may be viewed as an effect of the Fermi level. We assume that Si+Ga and Si−As diffuse respectively on the Ga and As sublattices. That is, the diffusion of Si+Ga and Si−As is governed by the variously charged Ga vacancies and As vacancies (or self-interstitials), respectively. The experimentally observed Si diffusivity is concentration dependent which results from the Si amphoteric nature as well as from another effect of the Fermi level, which is its influence on the concentrations of the charged point-defect species. Satisfactory quantitative descriptions of available experimental results are obtained. An analysis of the data on Si diffusion into a Sn-doped GaAs substrate has led us to believe that the previously proposed Si-pair diffusion model may no longer be favored.

An Examination of the Mechanisms of Si Diffusion in GaAs

AbstractAn examination of the three available quantitative models of Si diffusion in GaAs has led to the conclusion that the Fermi-level effect mechanism plays the most essential role. In some experimental results a point defect concentration transient is involved which should be incoorporated in future models.

The Role of Charged Point Defects on the Diffusion Behavior of Silicon in GaAs

AbstractA multilayered Si doped MBE structure was used to study the effective diffusivity of Si and the results are modeled with an (n/ni)2 dependence over the temperature range of 750°C – 950°C. An activation energy, Ea′, of 4.0 eV is obtained which is higher than normally reported in the literature. This higher Ea′ value results from appropriate accounting of the temperature dependence of ni, which is often neglected in the expression for Deff. Si diffusion at a buried n+/n++junction of a MOCVD grown structure also follows an (n/ni)2 dependence. These results support a Fermi-level model of Si diffusion in GaAs and suggest that the local point defect chemistry of the GaAs, through Si doping, is responsible for this diffusion behavior, regardless of the proximity to the surface.

Defect-related Si diffusion in GaAs on Si

Preferential diffusion channels of silicon are evidenced in GaAs grown by metalorganic vapor phase epitaxy on Si(100). The density of these diffusion channels is found to be consistent with the measured dislocation density. In addition, combining scanning electron microscopy and x-ray fluorescence it is shown that a large amount of Si emerges at the surface inside small 〈011〉 overgrowth oriented defects (≊1 μm) present at the GaAs/Si surface.

Spatial localization and diffusion of atomic silicon in delta-doped GaAs

Spatial localization and diffusion of atomic Si is studied inσ-doped GaAs by employing capacitance-voltage (CV) measurements and rapid thermal annealing. It is found that diffusion and segregation are irrelevant inσ-doped GaAs grown at temperatures below 550° C. Combination of rapid thermal annealing and capacitance-voltage profiling is a novel method which is most sensitive to diffusion and it is shown that this method is able to detect diffusion on a length scale of 10A. CV-profile widths broaden from⪯40A to 137A upon rapid thermal annealing at 1000° C for 5 sec. The diffusion coefficient and the activation energy of atomic Si-diffusion in GaAs are determined to beDo = 4 × 10−4 cm2/s andEa = 2.45 eV, respectively. The basic theory of CV-measurements on a quantum-mechanical electron system is developed.

Diffusion of atomic silicon in gallium arsenide

Silicon impurities with an initial Dirac-delta-function-like distribution profile are diffused into GaAs using rapid thermal annealing. The diffusion of atomic Si is determined by a novel method of comparing experimental capacitance-voltage profiles with a corresponding self-consistent profile calculation. Capacitance-voltage profiles broaden from 30 to 137 Å upon rapid thermal annealing at 1000 °C for 5 s. The diffusion coefficient and the activation energy of atomic Si diffusion in GaAs are determined to be D0=4×10−4 cm2/s and Ea=2.45 eV, respectively. The diffusion coefficient is two orders of magnitude smaller as compared to Si-pair diffusion in GaAs.

Sensitivity of Si diffusion in GaAs to column IV and VI donor species

Secondary ion mass spectroscopy and carrier concentration measurements are used to characterize Si diffusion into GaAs wafers containing two fundamentally different forms of donors, the column IV donors Si or Sn and the column VI donors Se or Te. A decrease in the Si diffusion rate is found in GaAs containing the column VI donors compared to the column IV donors. This trend is consistent with the model in which the Si diffuses as donor-gallium-vacancy complexes. The decrease in the Si diffusion coefficient is attributed to the greater binding energy of column VI donor-gallium-vacancy nearest-neighbor complexes, thus reducing the concentration of free-gallium vacancies available to complex with the Si.

Effect of surface encapsulation and As4 overpressure on Si diffusion and impurity-induced layer disordering in GaAs, AlxGa1-xAs, and AlxGa1-xAs-GaAs quantum well heterostructures

Data are presented demonstrating that the surface encapsulant and the As4 overpressure strongly affect Si diffusion in GaAs and AlxGa1-xAs, and thus are important parameters in impurity-induced layer disordering. Increasing As4 overpressure results in anincrease in diffusion depth in the case of GaAs, and adecrease in diffusion depth for AlxGa1-xAs. In addition, the band-edge exciton is observed in absorption on an AlxGa1-xAs-GaAs superlattice that is diffused with Si and is converted to bulk crystal AlyGa1-yAs via impurity-induced layer disordering. In contrast, the exciton is not observed in absorption on GaAs diffused with Si in spite of the high degree of compensation. These data indicate that the Si diffusion process, and the properties of the diffused material, are different for GaAs and for AlxGa1-xAs-GaAs superlattices converted into uniform AlyGa1-yAs (0 ≤y ≤x ≤ 1) via impurity-induced layer disordering with the amphoteric dopant Si.

Thermal Diffusion of Buried Beryllium and Silicon Layer in GaAs Doped by Focused Ion Beam Implantation

The diffusion profiles of buried Be and Si doped by focused ion beam (FIB) implantation in GaAs after annealing were investigated. The diffusion coefficients of Be and Si were determined by fitting from the results of computer calculations. The diffusion coefficient of FIB-doped Be was enhanced by excess interstitial Be. Beryllium diffusion profiles expand by annealing with a high dose at 850°C. The concentration-dependent diffusion of Si in GaAs doped using a molecular beam was observed. The diffusion coefficient of Si heavily doped by FIB, however, was undetectably small in contrast with that of Be at 850°C.

Diffusion and electrical properties of silicon-doped gallium arsenide

The amphoteric nature of silicon in gallium arsenide is used to develop diffusion and electrical compensation mechanisms. The diffusion mechanism is based on the formation and diffusion of nearest-neighbor donor-acceptor pairs. General solutions are presented that predict abrupt diffusion fronts for a wide range of pairing conditions. Experiments support the application of this mechanism to Si diffusion in GaAs at high concentrations. A compensation mechanism for amphoteric dopants is developed as well. The compensation process is driven primarily by the free-electron concentration. Nearly complete compensation is predicted for large dopant concentrations.