GaAs Cr Research Articles

10.1007/s11453-008-2022-9

Diffusion of Cr into epitaxial GaAs in an open system in the temperature range of 750–850°C was studied. Temperature dependences of the diffusion coefficient and solubility of Cr in GaAs were determined. Temperature dependences of the diffusion coefficient and solubility of Cr are described by the Arrhenius equation with the parameters D 0 = 1.9 × 109 cm2/s and E = 4.1 ± 0.2 eV for the diffusion coefficient and N 0 = 2.3 × 1024 cm−3 and E 0 = 1.9 ± 0.4 eV for solubility. The effect of protective SiO2 filmon the Cr diffusion coefficient and morphology of the GaAs surface after diffusion was studied.

ϕ decay: A relevant source forK−production at energies available at the GSI Schwerionen-Synchrotron (SIS)?

We gratefully acknowledge the useful discussions with H. Oeschler and K. Redlich. In particular we thank K. Redlich for providing the calculations by the statistical model. The HADES Collaboration gratefully acknowledges support from the following: BMBF Grants 06TM970I, 06GI146I, 06F-140, and 06DR135 (Germany); GSI (TMFR1, GI/ME3, OF/STR); Excellence Cluster of Universe (Germany); Grants GA AS CR IAA100480803 and MSMT LC 07050 (Czech Republic); Grant KBN 5P03B 140 20 (Poland); INFN (Italy); CNRS/IN2P3 (France);GrantsMCYT FPA2000-2041-C02-02 and XUGA PGID T02PXIC20605PN (Spain); Grant UCY-10.3.11.12 (Cyprus); INTAS Grant 06- 1000012-8861; and EU Contract RII3-CT-2004-506078.

Diffusion of chromium into epitaxial gallium arsenide

Diffusion of Cr into epitaxial GaAs in an open system in the temperature range of 750–850°C was studied. Temperature dependences of the diffusion coefficient and solubility of Cr in GaAs were determined. Temperature dependences of the diffusion coefficient and solubility of Cr are described by the Arrhenius equation with the parameters D0 = 1.9 × 109 cm2/s and E = 4.1 ± 0.2 eV for the diffusion coefficient and N0 = 2.3 × 1024 cm−3 and E0 = 1.9 ± 0.4 eV for solubility. The effect of protective SiO2 filmon the Cr diffusion coefficient and morphology of the GaAs surface after diffusion was studied.

Properties of GaAsN/GaAs quantum wells studied by optical detection of cyclotron resonance

Effective masses and carrier recombination in GaNAs/GaAs quantum wells (QWs) with nitrogen composition up to 4.5%, have been studied by optical detection of cyclotron resonance (ODCR). When monitoring the PL emissions under the conditions of above-GaAs barrier excitation, the ODCR spectrum is dominated by the electron CR in GaAs with an effective mass value 0.066 m 0 . The ODCR mechanism is discussed in terms of hot carrier effects, resulting in a reduced carrier recombination in GaAs and an enhanced carrier trapping in the GaNAs QW. Under resonant excitation of the GaNAs QWs, only a broad ODCR signal can be observed, corresponding to an effective mass value of 0.12 m 0 and 0.19 m 0 when the N composition is about 1.2 and 2%, respectively. This is attributed to the electron CR in the GaNAs QW with a lower electron mobility. This sizeable increase in the electron effective mass is in agreement with earlier theoretical predictions.

Diffusion of chromium in gallium arsenide

The diffusion of Cr in GaAs was investigated under both in-diffusion and out-diffusion conditions. In-diffusion sources were CrAs(s)-GaAs(s)-Ga(l), three phase mixtures which, at a given temperature, provide an invariant chemical potential of the components. Unexpectedly deep penetration was observed and complex (two-branch) concentration profiles were often observed. The profiles were successfully modeled using an interstitial/substitutional/dissociative model. Different regimes of diffusion conditions gave rise to fast and slow effective diffusion coefficients. Subtle effects of doping and preannealing under arsenic overpressures confirmed the proposed model and served as a probe to study these subtle differences. Out-diffusion experiments led to slow effective diffusion coefficients, consistent with the diffusion model.

Redistribution of Cr in GaAs:Cr and of V in GaAs:V after implantation of Si, Be, or B and annealing in a controlled atmosphere

The redistribution of V in V-doped and Cr in Cr-doped semi-insulating GaAs substrates after implantation with Si, Be, B and subsequent controlled atmosphere annealing at 840 °C for 20 min is determined using secondary ion mass spectrometry. Both V and Cr show accumulation into zones of high defect densities at ∼Rp+ΔRp of the Si, Be, or B implantation profile, respectively, and a depletion which extends beyond that zone to ∼2–4×Rp. Some surface accumulation has also been observed. For all three cases studied, the V redistribution is much smaller than the Cr redistribution. It is therefore concluded that from thermal stability point of view semi-insulating GaAs:V should be a better substrate material for implantation type devices than is GaAs:Cr.

Ternary phase equilibria in the Ga-As-Cr system and its relevance to diffusion and solubility studies

The phase equilibria in the Ga-As-Cr system were determined for As ⩽ 50 a/o from room temperature to 1300°C by calorimetric and x-ray examination of appropriately prepared samples. Several corrections were made in the literature values for properties of Cr-As binary compounds. The ternary diagram shows that GaAs is not a dominant compound in the system and GaAs and Cr cannot coexist under equilibrium conditions. From one ternary tie triangle region and a solid plus liquid region, appropriate solubility and diffusion sources for Cr in GaAs for most temperatures can be obtained.

Lattice location of chromium in semi-insulating GaAs by ion channeling techniques

Proton-induced x-ray emission combined with ion channeling techniques has been used to investigate the lattice location of Cr in GaAs grown by the Czochralski method. The angular dependence of Cr x-ray yields about the 〈100〉 and 〈110〉 directions indicated that the Cr atoms occupied the substitutional sites with no trace of well-defined Cr interstitials at the concentrations studied. Fe and Ni impurities in Cr-doped GaAs were located at the substitutional sites whereas Cu impurities were more likely distributed among random sites with respect to the host atoms.

A model of Cr in GaAs

We have developed a model of Cr in GaAs which is consistent with a large body of experimental data. It relies or recent spectroscopic models and on our interpretation of redistribution and electrical data, all of which indicate the existence of Cr complexes. The existence of rapidly diffusing interstitial Cr donors is assumed and justified. The model offers a unified picture of the effects of implantation on the Cr profile. It contains mechanisms for compensation and redistribution, which offer an explanation of the semi-insulating properties of Cr doped GaAs and of the two apparently incompatible classes of diffusion and anneal data. The redistribution depends on how the Cr was incorporated and on the vacancy concentration profiles. A study of representatives of the two classes of redistribution data allows us to estimate a lower limit of interstitial Cr diffusion constant and of the vacancy diffusion lengths in GaAs.

The photoluminescence associated with trigonal Cr in GaAs: a discussion of two recently proposed models

Reports the results of Zeeman measurements up to 22 T on the 0.84 eV photoluminescence arising from trigonal Cr in GaAs. The measurements are interpreted in terms of effective Hamiltonians for the ground and excited states, both of which have trigonal symmetry. It is demonstrated that recent criticisms of the authors model (by Voillot and co-workers) (1980-81) are without substance. The authors model is also shown to give an accurate description of both the intensities and energies of the lines seen in high-resolution optical absorption measurements.

The material state of ion-implanted Cr in GaAs

Chromium-implanted GaAs has been studied using 120 keV implants to fluence levels from 1×1014 to 5×1016 cm−2. The postannealed materials state has been examined using Rutherford backscattering spectrometry, proton-induced x-ray excitation, channeling, and transmission electron microscopy. Annealing was performed in flowing H2 at 900 C for 15 min under 85-nm thick Si3N4 dielectric caps. Various stages of precipitation were observed having structure and composition that depended on total implanted fluence. For the highest fluence, coarse nonuniform second-phase precipitates were present with dimensions on the order of 50 to 100 nm. At lower fluences small defect cluster precipitates, with average diameters of 6 nm, were identified through black-white contrast analysis. The relative fraction of precipitated Cr was 40% at 1×1015 cm−2 and decreased to 9% at 5×1014 cm−2. The volume concentration of implanted Cr that is retained in solid solution after annealing is estimated at 2×1018 cm−3. Channeling analysis suggests that these atoms are retained in nonunique interstitial or other nonsubstitutional positions.

Redistribution of manganese after annealing of GaAs implanted with Si+ and Se+

The redistribution of Mn in Si+ and Se+ implanted Cr-doped semi-insulating GaAs substrates is studied by secondary-ion mass spectrometry as a function of implant dose, energy, and annealing temperature subsequent to annealing capless and with a SiO2 cap in a As4-H2 atmosphere. Mn accumulates within a depth of 0.5 μm from the GaAs surface with a peak at ∼500 Å from the surface for capless anneals. The Mn peak position is constant as the implant energy is varied. The role of the encapsulant in gettering Mn is described. The observed accumulation of Mn near the surface region of GaAs can cause p-type conversion of this region as reported by Klein. A higher solid solubility (≊ 3×1018 atoms/cm3) and a higher diffusion rate of Mn than of Cr in GaAs at 900 °C are indicated by the dose study. A preliminary model for the Mn diffusion mechanism under the above conditions is proposed.

Spectroscopic study of vanadium in GaP and GaAs

The near-infrared optical-absorption, luminescence, and luminescence-excitation spectra of liquid-encapsulation Czochralski-grown, vanadium- (V) doped GaP and GaAs have been investigated. The behavior of the characteristic V emission has been studied in magnetic fields up to 5.3 T. An analysis of the data within the framework of crystal-field theory indicates that the V-induced optical bands arise from crystal-field transitions within nonassociated ${V}^{2+}{3d}^{3}$ ions in the strong-field (low-spin) case. The electron-spinresonance spectrum of semi-insulating GaAs:V contains a characteristic signal which is identified as isolated ${V}^{3+}{3d}^{2}$. The characteristics of this signal show that V can act as a deep acceptor. The distribution coefficient of V in GaAs is close to that of Cr in GaAs.

Low-temperature gettering of Cr in GaAs

Gettering of Cr by back-surface damage has been investigated at temperatures in the range 300–400 °C, for anneal periods of 10–300 h. It has been shown that measurable concentrations of Cr can be detected and that the process can be characterized by an activation energy of ≃0.88 eV and a time-dependent term exp(ϑt1/2).

Chromium concentrations, depth distributions, and diffusion coefficient in bulk and epitaxial GaAs and in Si

Implantation of 200-keV chromium fluences from 4.5×1012 to 1.0×1015 cm−2 and SIMS profiling were used to measure the atom densities and depth distribution of Cr in bulk GaAs (Cr), in undoped liquid-phase epitaxial GaAs grown on a GaAs(Cr) substrate, and in Si. The Cr atom density in both GaAs(Cr) substrates was ∼1.6×1017 cm−3. The background-limited detection sensitivity of SIMS for Cr in GaAs was ⩽3×1015 cm−3. Chromium outdiffused into the epitaxial layer during growth at 800 °C. The diffusion coefficient for CR at 800 °C in GaAs was determined to be 6.7×10−12 cm2/sec. Channeling tails were observed for the Cr implanted in Si and in the undoped epitaxial GaAs, but not in the bulk GaAs(Cr). Ranges, range straggles, and other Pearson IV moments for the implanted Cr distributions are given.

Optically induced transient electron paramagnetic resonance phenomena in GaAs:Cr

The dynamics of EPR spectra of the charge states of Cr in GaAs during and following optical excitation are profoundly determined by the presence of other traps. Transients are slow, nonexponential, not thermally activated, and sample dependent. We show that the instability of Cr1+ and Cr2+ at low temperatures (< 10 K) is not thermal, but due to interimpurity transitions involving hole traps 0.45 eV from the valence band. The particular complexities of the GaAs:Cr system are described, but still allow the self-consistency of the EPR and earlier data to be demonstrated.

Diffusion of chromium in gallium arsenide

Both in-diffusion and out-diffusion experiments for Cr in GaAs have been performed, using radiotracer Cr so that profiles could be plotted. An analysis of the profiles shows that they can be explained on the basis of a model in which the Cr diffuses through the lattice interstitially. A simple interpretation of the solubility data implies that the Cr occurs on the group V site, in contradiction to previous work on the system. An attempt is made to resolve this paradox by considering the possible form of the Ga-As-Cr phase diagram at the temperatures used in these diffusions. Electrical measurements were also made on Cr-diffused samples. Interpretation of these measurements was difficult because conversion of the samples to p-type occurred during the experiment. It is demonstrated that this conversion was due to the heat treatment involved in the diffusion rather than to the introduction of Cr into the crystal. The electrical measurements are consistent with the idea that Cr introduces a deep acceptor level into GaAs.

Alloying of Au layers and redistribution of Cr in GaAs

Alloying of Au films on Cr-doped GaAs substrates and Sn-doped LPE layers grown on semi-insulating substrates has been investigated by TEM and SIMS profiling. Annealing at 350 °C for variable periods was found to produce rapid outdiffusion of Cr into regions of near-surface damage induced by strain effects at the interface and subsequent diffusion of Au into the GaAs.

Gettering of Cr in GaAs by back surface mechanical damage

Dislocation line structure introduced by mechanical damage at the back surface is shown to be effective in gettering background concentrations of Cr in doped GaAs wafers. Reverse annealing or “degettering” is shown to occur after extended annealing at 800 °C. In each case, the gettering behaviour can be correlated with the annealing of dislocation lines at the back surface. Es wird gezeigt, das in dotierten GaAs-Scheiben die durch mechanisches Damage an der Ruckseite verusachte Versetzungslinienstruktur als Getterer von Hintergrundkonzentrationen von Cr wirkt. Ausheilung oder „Entgettern” tritt nach langerem Gluhen bei 800 °C auf. In jedem Fall kann das Getterverhalten mit der Ausheilung der Versetzungslinien an der Ruckseite in Beziehung gesetzt werden.

Measurement of the chromium concentration in semi-insulating GaAs using optical absorption

Chromium has been observed to be responsible for a band of absorption in semi-insulating GaAs. The corresponding absorption coefficient has been carefully calibrated using three different chemical analysis methods. It is shown that the spatial distributions of Cr in semi-insulating GaAs ingots can be obtained very easily using optical absorption measurements and that the segregation coefficient of Cr in GaAs is equal to 8.9×10−4. The nature of the optical transitions due to the presence of Cr is discussed in detail and it is shown that they occur mainly on the Cr3+ in semi-insulating material.