Zn Diffusion In GaAs Research Articles

Identification of the dissociative and kick-out diffusion mechanisms of Zn diffusion in GaAs by photoluminescence analysis

Abstract Most investigations on Zn diffusion in GaAs were processed using the Zn–As alloy sources to prevent the As atoms from escaping GaAs wafers, while we found that the Zn diffusion would change fundamentally if Zn–Ga alloy sources were used. The Ga atoms from the diffusion sources suppressed the formation of the high-concentration surface region in Zn profiles, thus converting a kink-and-tail profile into a box profile. The photoluminescence (PL) analysis was used to identify the diffusion mechanisms. The Ga vacancy defects were found in the surface region of the kink-and-tail profile, indicating that the dissociative mechanism dominated; the PL spectrum in the tail region of kink-and-tail profile and the main region of box profile showed the same signals, no Ga vacancy defects were found, thus the kick-out mechanism dominated.

Zinc diffusion in gallium arsenide and the properties of gallium interstitials

We have performed zinc diffusion experiments in gallium arsenide at temperatures between $620\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and $870\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ with a dilute $\mathrm{Ga}\ensuremath{-}\mathrm{Zn}$ source. The low Zn partial pressure established during annealing realizes Zn surface concentrations of $\ensuremath{\leqslant}2\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, which lead to the formation of characteristic S-shaped diffusion profiles. Accurate modeling of the Zn profiles, which were measured by means of secondary ion mass spectroscopy, shows that Zn diffusion under the particular doping conditions is mainly mediated by neutral and singly positively charged Ga interstitials via the kick-out mechanism. We determined the temperature dependence of the individual contributions of neutral and positively charged Ga interstitials to Ga diffusion for electronically intrinsic conditions. The data are lower than the total Ga self-diffusion coefficient and hence consistent with the general interpretation that Ga diffusion under intrinsic conditions is mainly mediated by Ga vacancies. Our results disprove the general accepted interpretation of Zn diffusion in GaAs via doubly and triply positively charged Ga interstitials and solves the inconsistency related to the electrical compensation of the acceptor dopant Zn by the multiply charged Ga interstitials.

An experimental investigation of Zn diffusion into InP and InGaAs

In this report, we investigate the profiles of zinc (Zn) in n-type InP ⟨100⟩ single crystal wafers and InGaAs epitaxial film prepared by semi-closed ampoule Zn diffusion. The different annealing effect on junction depth and concentration for InP and InGaAs is studied, and the difference is tentatively ascribed to solubility for each material. The maximum net acceptor concentration in InP changes from 3.3 × 1017 cm−3 to 3.3 × 1018 cm−3 before and after annealing, and the later value is close to the solubility limit for closed-ampoule Zn diffusion InP. Annealing does not make the maximum net acceptor concentration and depth in InGaAs change much. Our experiments show that Zn diffusion InGaAs can be used as a good ohmic contact material for photoelectric devices due to its high acceptor concentration.

Be diffusion in molecular beam epitaxy-grown GaAs structures

Beryllium diffusion is considered in heavily doped p-type GaAs structures grown by molecular beam epitaxy (MBE). Secondary ion mass spectrometry (SIMS) measurements performed on samples which underwent rapid thermal processing (RTP) experiments at 850 °C for 30 s show that (i) Be diffusion is faster in p/p+/p structures than in p/p+ ones and (ii) an increase of the As4/Ga flux ratio during the MBE growth affects Be diffusion only in p/p+ structures. These results are discussed by modeling Be diffusion according to a substitutional–interstitial diffusion mechanism where Be transition from substitutional to interstitial takes place by a kick-out process. The modeling procedure, which has been previously used to simulate Be and Zn diffusion in GaAs and in other related compounds, has been modified in order to account for the lacking of equilibrium in the initial concentration of Ga interstitials. It is shown that when the lacking of equilibrium is accounted for at the beginning of the annealing experiment, a satisfactory description of the SIMS results has been achieved in both p/p+ and p/p+/p structures, also considering samples grown by different As4/Ga flux ratios. The modeling results allow us to conclude that in the p/p+/p structures the concentration of point defects (e.g., IGa) in the regions cladding the base layer affects, to a major extent, Be diffusion during RTP, so that an efficient reduction of Be diffusivity is expected only when the whole structure is grown by high V/III flux ratios.

Modeling of Be diffusion in GaAs layers grown by MBE

Beryllium diffusion is modeled in order to simulate the Be depth profiles obtained by SIMS measurements on p/p + and p/p +/p GaAs structures which underwent rapid thermal annealing (RTA) experiments at 850 °C for 30 s. The simulation procedure, which has been previously used to simulate Be and Zn diffusion in GaAs and in other related compounds, is critically discussed. It is shown that usual assumptions do not allow to simulate measured Be profiles in p/p +/p structures and that a qualitative agreement between measured and simulated Be profiles can be achieved in both p/p + and p/p +/p structures by considering proper initial conditions.

Defect formation during Zn diffusion in GaP and GaSb

The defect structures induced by zinc diffusion in intrinsic GaP single crystals at temperatures between 1182K and 1358K and in an intrinsic GaSb single crystal at 883K are characterized for diffusion times t ≤ 90 min by analytical transmission electron microscopy. The results are compared with Zn concentration depth profiles obtained by secondary ion mass spectroscopy. In regions near the Zn diffusion front dislocation loops of interstitial type and Ga-rich precipitates are found. Closer to the surface a loose network of dislocations, dislocation loops and faceted voids with Ga-rich precipitates are observed. In addition, a surface layer of polycrystalline Zn 3 P 2 is formed at the surface of GaP at higher diffusion temperatures. The formation of diffusion-induced defects can be understood in a similar way as the defect formation during Zn diffusion in GaAs and InP.

Short interval open tube diffusion of Zn in GaAs at low temperatures

Short interval open tube Zn diffusion in GaAs at low temperatures is studied for application to heterostructure lasers. The electrochemical capacitance voltage (ECV) profiling technique is used to obtain the carrier concentration versus depth profiles for Zn diffused samples. Diffusion rate is found to be somewhat different from the values obtained for longer durations employing similar techniques. Results are applied to improve the ohmic contact quality for Al-free semiconductor lasers grown in our laboratory. No deterioration is observed in the light versus current (L-I) characteristics of these devices fabricated after Zn diffusion.

Reactions of Gallium Vacancies during Annealing and Zn Diffusion in GaAs: Si

Reactions of Gallium Vacancies during Annealing and Zn Diffusion in GaAs: Si

Modeling of zinc-indiffusion-induced disordering of GaAs/AlAs superlattices

A model for the effect of Zn indiffusion on enhancing the GaAs/AlAs superlattice (SL) disordering process, which combines recently proposed models for Ga self-diffusion and Zn diffusion in GaAs, is presented. Four coupled partial differential equations describing the process were solved numerically. Satisfactory agreement between the simulated results and experimental data available in the literature is obtained. At a given temperature, the used values for the diffusion coefficient and the thermal equilibrium concentration of the responsible point defect species, the doubly positively charged Ga self-interstitials IGa2+, are a consistent splitting of the known Ga self-diffusion coefficient dominated by IGa2+. Quantitatively, the SL disordering enhancement is mainly due to the Fermi-level effect while an IGa2+ supersaturation also makes a small contribution. Because of p-doping by Zn acceptor atoms, the IGa2+ concentration is increased tremendously via the Fermi-level effect. An IGa2+ supersaturation also develops because the IGa2+ generation rate is higher than its removal rate. The enhanced SL disordering process mainly proceeds under the Ga-rich SL composition conditions. The Zn-indiffusion-enhanced Al-Ga interdiffusion coefficient shows an apparent dependence on the Zns− concentration differing slightly from a quadratic relationship.

Theory of Zn-enhanced disordering in GaAs/AlAs superlattices.

The microscopic mechanisms for Zn diffusion in GaAs and Zn-induced interdiffusion in GaAs/AlAs superlattices are investigated by ab initio molecular dynamics. Among the various proposed mechanisms for Zn diffusion, kick-out by Ga interstitials has the lowest activation energy. Zn in-diffusion generates nonequilibrium group-III interstitials, which are bound to Zn by Coulomb forces. The interstitials follow the Zn diffusion front and disorder the superlattice. The calculated activation energies for these processes are in good agreement with the experimental data.

Diffusion mechanism of chromium in GaAs

The diffusion of the substitutional Cr atoms (Crs) in GaAs results from the rapid migration of the interstitial atoms (Cri) and their subsequent changeover to occupy Ga sites (or vise versa), a typical substitutional-interstitial-diffusion (SID) process. There are two possible ways for the Cri-Crs changeover to occur: the kick-out mechanism in which Ga self-interstitials are involved, and the dissociative mechanism in which Ga vacancies are involved. The Crs in-diffusion profiles are of characteristic shapes indicating the dominance of the kick-out mechanism, while the Crs out-diffusion profiles are error-function shaped, indicating the dominance of the dissociative mechanism. In this study, an integrated SID mechanism, which takes into account both the kick-out and dissociative mechanisms, is used to analyze Cr diffusion results. Going beyond just qualitative consistency, the Cr in- and out-diffusion features in GaAs are explained on a quantitative basis. It is confirmed that the kick-out mechanism dominates Cr in-diffusion while the dissociative mechanism dominates Cr out-diffusion. Parameters used to fit existing experimental results provided quantitative information on the Ga self-interstitial contribution to the Ga self-diffusion coefficient. The values obtained are consistent with those obtained from a study of Zn diffusion in GaAs, and with available experimentally determined Al-Ga interdiffusion coefficients.

A model for the Zn diffusion in GaAs by a photoluminescence study

To study the mechanism of zinc diffusion in GaAs, we diffused zinc from a ZnAs2 source into Si-doped GaAs samples (n ≊ 1.3 × 1018 cm−3) at different temperatures (from 575 °C up to 700 °C) in sealed evacuated quartz tubes. The samples are characterized by the depth profile of the photoluminescence at different temperatures. The photoluminescence spectra show characteristic emission associated to deep levels of gallium and arsenic vacancies. A detailed analysis of the spectra demonstrates the role played by vacancies in the Zn diffusion process. The spatial correlation between the luminescence spectra and the Zn concentration obtained from secondary ion mass spectroscopy measurements has been demonstrated.

ALCHEMI studies on the location of zinc in GaAs

The diffusion of Zn in GaAs is anomalous in that the diffusion coefficient D is proportional to the Zn concentration squared. Further, the diffusion rate of the column III species in III-V layer structures (ie. Ga and Al in GaAs-GaAlAs) can be increased by 105 with the addition of doping levels of Zn3. The column V sites are not affected. As an aid to understanding the diffusion process we have located the position of the Zn in the GaAs lattice.There are four high symmetry positions in the zincblende structure that the Zn could occupy: Ga, As, T (Tetrahedral interstitial, located at 1/2 1/2 1/2 with four nearest neighbors at 0.433 a0) and H (Hexagonal interstitial, located at 5/8 5/8 5/8 with six nearest neighbors at 0.415 a0). Interstitial diffusion involves hopping between alternating T and H sites with the energy barrier to diffusion being equivalent to the difference in the potential energy of the two sites. Figure 1 indicates the possible low-index orientations for ALCHEMI studies which can differentiate between these sites.

Application of thin silicon films to closed-tube Si and Zn diffusion in GaAs and AlxGa1−xAs

The application of thin Si films for selective area Si and Zn diffusion into GaAs and AlGaAs is described in detail. The concentration-dependent pair diffusion coefficient and the activation energy for Si diffusion into AlxGa1−xAs under As overpressure for 0≤x≤0.8 are measured using both secondary ion mass spectrometry and junction depth studies. Furthermore, Si films are seen to act as an ideal mask for Zn diffusion at temperatures below 750 °C. Ideal lateral Zn diffusion profiles are routinely observed using these films, independent of stress at the interface which provides new insight into the nature of laterally enhanced diffusion.

Open-tube Zn diffusion in GaAs using diethylzinc and trimethylarsenic: Experiment and model

We have characterized the diffusion of Zn into GaAs from the organometallic sources diethylzinc and trimethylarsenic. This method produces surface hole concentrations in excess of 1020 cm−3 with good control of junction depths as shallow as 0.1 μm. Smooth surface morphology is retained. The profile shape is much more complex than the accepted interstitial-substitutional Zn-diffusion model would predict. To explain the observed profiles, a new model for Zn diffusion is proposed and implemented in a computer simulation.

Zinc-Diffusion in GaAs Using a Sintered Ternary Source

A ternary Zn3As2–ZnAs2–GaAs source for the diffusion of Zn in GaAs has been developed by a low temperature sintering technique that does not require special safety precautions. Wafers diffused using this source remained free of damage. Doping concentrations and diffusion depths agree with the results of the best high temperature Ga/As/Zn diffusion sources.

Zn diffusion in GaAs obtained by a simple open-tube technique

Zn diffusion into 2-in.-diam semi-insulating GaAs wafers has been carried out by a simple, open-tube diffusion method using a commercially available GaAsZn solid source and without surface coating or encapsulation. High-quality (μ=75–100 cm2/V s) layers with a flat carrier concentration and sharp cutoff Zn distribution have been obtained. The p-type layers obtained by this method are uniform across the wafer, and the technique can be easily scaled up. Selective diffusion of Zn was also demonstrated with this technique, using a plasma-deposited Si3N4 as a diffusion mask.

Use of In-Zn-As Ternary Sources for the Diffusion of Zn in GaAs by the Semiclosed-Box Method

Liquid sources consisting of the In-Zn-As ternary system have been successfully used for the diffusion of Zn in GaAs by the semiclosed-box method. The surface concentration and junction depth were easily controlled over a wide range of source composition. The experimentally observed effects of the zinc and arsenic fractions in the source on the diffusion agree well with a model derived from interstitial-substitutional interchange.

Thermal Pulse Diffusion of Zn in GaAs from an Elemental Source

We have studied Zn diffusion from an electroplated elemental source, using rapid thermal annealing techniques to investigate a range of diffusion times from 3 to 30s. Improved surface morphology is obtained by exploiting the high vapor pressure of Zn to remove excess source material, leaving only one to two monolayers of Zn tightly bound to the substrate. Resultant diffusions form shallow (0.1–0.2 μm) heavily doped regions. Junction properties are almost independent of diffusion time and linear in temperature over the range investigated.

Monolayer surface doping of GaAs from a plated zinc source

We have investigated surface doping and diffusion of Zn in GaAs using rapid thermal processing. The use of an aqueous RuCl3 pretreatment has enabled us to electroplate uniform ≊100-Å-thick Zn layers directly on the GaAs surface. The excess Zn is removed with a 500 °C vacuum ‘‘evaporation,’’ leaving a modified surface layer which may serve as a dopant source for rapid thermal diffusion. Evaporation of A1 onto such surfaces yields Schottky diodes with enhanced barrier heights which have been used to fabricate metal-semiconductor field-effect transistor structures.