Evidence for two energy levels associated with EL2 trap in GaAs

By photocapacitance technique, applied to n-type LEC-grown GaAs, two energy levels:E _v +0.45 eV andE _c –0.75 eV are identified, for the first time, as being associated with the EL2 trap. As follows from the analysis of photo-EPR results on highly resistive GaAs crystals, the same energy levels can be attributed to the arsenic antisite defect, As_GA. In view of these findings, it is argued that the occupied EL2 level corresponds to the neutral charge state of As_Ga defect.     

Isothermal capacitance transient spectroscopy (ICTS) study for midgap levels in Hb-GaAs by rapid thermal annealing

We studied the midgap levels by using isothermal capacitance transient spectroscopy (ICTS) in Hb-GaAs which had been processed by rapid thermal annealing (RTA). As the annealing time at 850 °C increased, the EL2 trap (E _c–0.81 eV) was transformed to the EX2 trap (E _c–0.73 eV) and eventually to the EX1 trap (E _c–0.87 eV). The diffusivity of the EL2 trap obtained from the experimental result of the heat treatment was about 1.02·10^–8cm²/s at 850 °C. This result indicate that the EL2 trap contains an interstitial arsenic atom. The result of the transformation to the EX1 and EX2 traps suggests that, when the EL2 trap is V_AsAS_iV_GaAs_Ga, the EX2 trap may be V_AsV_GaAs_Ga, which As_i is diffused out during a thermal annealing.     

电子顺磁共振“AsGa”的光淬灭行为与EL2亚稳态机理

根据Goltzene等人最近关于原生LEC,HB,压缩变形和中子辐照GaAs中电子顺磁共振(EPR)“As_Ga”谱低温光淬灭实验结果,结合Baraff和Schlüter最近关于EL2亚稳态的理论计算,本文认为可进一步证实作者之一在1981年提出的EL2(As_GaV_AsV_Ga)缺陷模型,同时还对Wager和Van Vechten最近提出的EL2(As_GaV_AsV_Ga)的亚稳态机理加以评述和修正。 关键词：     

Double anion antisite in GaAs - the simplest member of EL2 family?

Properties of an aggregate composed of two nearest-neighbour As_Ga defects are examined. Such an aggregate behaves like a dimer in which the two constituents are weakly coupled giving rise to a double near-degeneracy of electronic states of the system. The system is unstable against the Jahn-Teller (J-T) distortion that couples pairs of nearly degenerate states. Usually the distortion is stabilized by localization of a single electron, or a pair of electrons, at one of the two As_Ga defects. If one of the two electrons is excited into theT ₂ resonant state of individual As_Ga then the vibronic coupling can be strong, and the resulting J-T effect can give rise to a metastable behaviour of the system. It is argued that the (As_Ga)₂ aggregate is the best candidate for EL2 centre in GaAs crystals.     

Study of n type porous GaAs by photoluminescence spectroscopy: Effect of the etching time on the deep levels

Photoluminescence (PL) analysis is used to study porous layers elaborated by electrochemical etching of n⁺ Si-doped GaAs substrate with different etching times. Temperature and power dependence photoluminescence (PL) studies were achieved to characterize the effect of the etching time on the deep levels of the n⁺ Si-doped GaAs. The energy emission at about 1.46 eV is attributed to the band-to-band (B-B) (e-h) recombination of a hole gas with electrons in the conduction band. The emission band is composed of four deep levels due to the complex of (V_AsSi_GaV_Ga), a complex of a (Ga vacancy - donor - As vacancy), a (Si_Ga-V_Ga³⁻) defect or Si clustering, and a (gallium antisite double acceptor-effective mass donor pair complex) and which peaked, respectively, at about (0.94, 1, 1.14, and 1.32 eV). The PL intensity behavior as function of the temperature is investigated, and the experimental results are fitted with a rate equation model with double thermal activation energies.     

Characterization of GaAs/AlxGa1-xAs heterointerface defects by means of capacitive measurememts

x Ga_1-xAs heterojunctions grown by liquid-phase epitaxy. Interface states with hiqh concentration, N_t=3×10¹¹ cm^-2 and energy level E_c-E_t=0.14 eV distributed in a box 150 Å wide at the heterointerface and acting as electron traps are observed. The possible origin could be the isolated arsenic vacancy V_As in n-GaAs. Received: 25 April 1996/Accepted: 22 January 1997     

Using positron 2D-ACAR as a probe of point defects in GaAs: The As vacancy as a case study

Two-Dimensional Angular Correlation of positron Annihilation Radiation (2D-ACAR) experiments have been performed on n-type GaAs. By combining these results with those from positron lifetime experiments, the momentum distribution of the arsenic vacancy in its neutral (V _aAs ⁰ ) and negative (V _As ^– ) charge states have been extracted. These distributions were all normalized to the respective positron lifetime that characterizes them. The first thing to be noticed is that the momentum distributions of the vacancies, as seen by the positron, are fairly isotropic and structureless. The distribution forV _As ⁰ is more peaked than that ofV _As ^– while the latter is more intense in the large momentum regions of the spectra. From this, it can be inferred that VA. has a smaller open volume thanV _As ⁰ A closer look at the momentum distribution of the vacancies reveals that they are not entirely isotropic, but, in fact, have a bulk-like component. Finally, the experimental results for bulk GaAs andV _As ^– compare well in a qualitative manner with the momentum distributions that result from an ab-initio molecular dynamics calculation.     

On the recombination behaviour of iron in moderately boron-doped p-type silicon

The recombination lifetime and diffusion length of intentionally iron-contaminated samples were measured by the Surface Photo Voltage (SPV) and the Elymat technique. The lifetime results from these techniques for intentionally iron-contaminated samples were analysed, in particular for the aspect of the injection-level dependency of recombination lifetime. Based on theoretical considerations, a method for the analysis of deep-level parameters combining constant photon flux SPV and Elymat measurements has been developed. This method is based on a detailed numerical analysis of the Elymat technique, including the Dember electric field, the characteristics of the laser beam, the transport parameters of the semiconductor and multilevel Shockley-Read-Hall (SRH) recombination kinetics. The results of the numerical simulation are applied to the analysis of recombination lifetime measurements on intentionally iron-contaminated samples. We compared numerical simulations and experimental results from SPV and Elymat for p-type samples using the classical acceptor level atE _v +0.1 eV and the donor level of FeB pairs atE _c -0.3 eV as recombination centre. Better consistency in the interpretation of the results has been found in the doping range 10¹⁴–10¹⁶ cm^–3 supposing theE _c -0.3 eV level as predominant recombination centre. An attempt to extract the electron and hole capture cross-sections for this defect is made.     

A study of deformation-produced deep levels inn-GaAs using deep level transient capacitance spectroscopy

Deformation-produced deep levels, both of electron and hole traps, have been studied using deep level transient capacitance spectroscopy (DLTS) for an undopedn-type GaAs (HB grown) compressed at 440°C. Concentrations of two grown-in electron trap levels (E _c −0.65eV andE _c −0.74eV) and one grown-in hole trap level (E _v +∼0.4eV) increase with plastic deformation, while that of a grown-in electron trap level (E _c −∼0.3eV) decreases in an early stage of deformation. While no new peak appeared in the electron trap DLTS spectrum after plastic deformation, in the hole trap DLTS spectrum a broad spectrum, seemingly composed of many peaks, newly appeared in a middle temperature range, which may be attributed to electronic energy levels of dislocations with various characters.     

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.     

Dopant diffusion and segregation in semiconductor heterostructures: Part III, diffusion of Si into GaAs

We have mentioned previously that in the third part of the present series of papers, a variety of n-doping associated phenomena will be treated. Instead, we have decided that this paper, in which the subject treated is diffusion of Si into GaAs, shall be the third paper of the series. This choice is arrived at because this subject is a most relevent heterostructure problem, and also because of space and timing considerations. The main n-type dopant Si in GaAs is amphoteric which may be incorporated as shallow donor species Si_Ga ⁺ and as shallow acceptor species Si_As ^-. The solubility of Si_As ^- is much lower than that of Si_Ga ⁺ except at very high Si concentration levels. Hence, a severe electrical self-compensation occurs at very high Si concentrations. In this study we have modeled the Si distribution process in GaAs by assuming that the diffusing species is Si_Ga ⁺ which will convert into Si_As ^- in accordance with their solubilities and that the point defect species governing the diffusion of Si_Ga ⁺ are triply-negatively-charged Ga vacancies V_Ga ^3-. The outstanding features of the Si indiffusion profiles near the Si/GaAs interface have been quantitatively explained for the first time. Deposited on the GaAs crystal surface, the Si source material is a polycrystalline Si layer which may be undoped or n⁺-doped using As or P. Without the use of an As vapor phase in the ambient, the As- and P-doped source materials effectively render the GaAs crystals into an As-rich composition, which leads to a much more efficient Si indiffusion process than for the case of using undoped source materials which maintains the GaAs crystals in a relatively As-poor condition. The source material and the GaAs crystal together form a heterostructure with its junction influencing the electron distribution in the region, which, in turn, affects the Si indiffusion process prominently. Received: 19 April 1999 / Accepted: 3 May 1999 / Published online: 4 August 1999     

Mechanism for the creation of antisite defects,during combined climb-glide motion of dislocations in sphalerite-structure crystals

Processes are considered which lead to a formation of irregular jogs in a dislocation core, e.g., when in GaAs two As atoms are absorbed at an already existing jog. Then, after glide, the dislocation leaves an arsenic antisite defect behind. This mechanism can give an account of a correlation between the spatial distributions of EL2 donors (that are most likely related to the As_Ga defects) and dislocations in melt-grown GaAs crystals.     

Detection of surface defects under low-energy scattering

Computer simulation of Ga⁺ ion (E ₀ = 40–100 eV) scattering on a GaAs film with defects in the form of vacancies and K⁺ ions (E ₀ = 40–300 eV) on a V-target containing cerium has been carried out in the framework of the multiparticle interaction model. The simulation results show that low-energy scattering can be used as a tool for detection of surface defects.     

Atomistic mechanisms of dopant-induced multiple quantum well mixing and related phenomena

We discuss mechanisms of Ga self-diffusion and impurity diffusion in GaAs, and of layer mixing enhancement in GaAs/AlGaAs multiple quantum wells (MQWs). Ga self-diffusion and Ga-Al interdiffusion are governed by the triply negatively charged Ga vacancies, V _Ga ^3– , under intrinsic and n-doping conditions, and by doubly positively charged Ga self-interstitials, I _Ga ²⁺ , under heavy p-doping conditions. The mechanisms responsible for enhancing MQW mixing are the Fermi-level effect for the n-dopants Si and Te, and the combined effects of the Fermi-level and the dopant diffusion-induced nonequilibrium point defects for the p-dopants Zn and Be. For n-type GaAs the donor Si atoms diffuse primarily also via V _Ga ^3– . For p-type GaAs the interstitial-substitutional impurities Zn and Be diffuse via the kickout mechanism and induce a supersaturation and an undersaturation in the concentrations of I _Ga ²⁺ , respectively, under indiffusion and outdiffusion conditions.     

Formation of antisite defects by gliding dislocations in sphalerite-structure crystals

Conditions are discussed under which an interstitial atom absorbed in the core of a gliding 60° dislocation belonging to the glide set can be transformed into an antisite defect. The mechanism considered may be responsible for an increase in the concentration of As_Ga defects observed in GaAs single crystals after their plastic deformation.     

Solubility and point defect-dopant interactions in GaAs—II: Tin-doping

The form of the solubility curves for tin in both LPE and VPE grown GaAs are rationalised by a thermodynamic model which presumes that the dominant acceptor state due to tin is the donor-gallium vacancy complex Sn_GaV^?_Ga rather than the commonly postulated Sn^?_As.In LPE growth from tin-rich solutions where the arsenic concentration in the melt exceeds the gallium concentration, very low electron mobilities have been reported and this is shown to arise from auto-compensation of the Sn⁺_Ga donors by the Sn_GaV^?_Ga acceptors.     

Interstitial iron and iron-acceptor pairs in silicon

Deep levels in iron-dopedp-type silicon are investigated by means of Deep Level Transient Spectroscopy (DLTS) and the Hall effect. Pairs of Fe with the acceptors B, Al, and Ga are observed at 0.1, 0.19, and 0.24 eV above the valence band edgeE _v. For interstitial iron, (Fe _i ), a level energy ofE _v+0.39+-0.02 eV is obtained with DLTS after correction with a measured temperature dependence of the capture cross section. The Hall effect yieldsE _v+0.37 eV for Fe_i. The annealing behavior of levels related to Fe is investigated up to 160 °C. Iron participates in at least four different types of impurity states: Fe _i , Fe-acceptor pairs, precipitations (formed above 120 °C) and an additional electrically inactive state, which is formed at room temperature.     

A study of sputter-induced defects in magnetron-sputtered CoSi2 and TiSi2 Schottky barriers on n- and p-type GaP

Magnetron-sputtered CoSi₂ and TiSi₂ Schottky barriers on n- and p-type GaP were investigated. Their hitherto unknown barrier heights were determined to be 0.98 eV (for CoSi₂/n-GaP and CoSi₂/p-GaP), 0.91 eV (for TiSi₂/n-GaP), and 0.90 eV (for TiSi₂/p-GaP). It was found that magnetron-sputtering induced a compensated layer near the surface, both for n- and p-type GaP, with a thickness of about 0.05 m. As the dependence of the shift of the Mott-Schottky intercept with the V-axis on the substrate dopant concentration obeyed some specific law, we proposed that the defects are neutral complexes of dopant ions and sputter-induced native defects. These native defects were assumed to depend on the Fermi level position, namely the P_Ga antisite and the V_P vacancy for p-GaP and the V_Ga vacancy for n-GaP. The conversion between these defects occurs by nearest neighbour hopping of a phosphorus atom. The Schottky barrier heights obtained on p-GaP could be explained by Fermi level pinning at the surface due to the P_Ga defects. This could not be confirmed by n-GaP as the energy level position of the V_Ga was not available. The defects could be annealed out between 200° C and 300° C and the associated change of the Schottky barrier height corroborated the proposed model.On leave from the Université de Burundi, Faculté des Sciences, Bujumbura, Burundi     

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.     

Transition metal impurities in silicon: New defect reactions

Recent investigations on transition-metal impurities in silicon emphasizing the effect of the combined diffusion of two transition metals are presented and briefly discussed. The electronic properties and basic thermal kinetics are analysed by DLTS. The conversion of a Pd-related multivalent defect atE _c-0.35 eV andE _c-0.57 eV to the Pd-related defect atE _c –0.22 eV is observed, and a Pd-Fe complex level atE _c –0.32 eV is identified. The annealing characteristics of the multivalent Rh levels atE _c-0.33 eV andE _c-0.57 eV are observed, and used to analyse the influence of prior Rh doping on the Au diffusion. A complex formed by the codiffusion of Au and Cu is observed atE _v+0.32eV andE _v+0.42 eV, and shown to exhibit bistable behavior as does a similarly produced Au-Ni complex observed atE _v + 0.35 eV andE _v+0.48 eV.