EL2 Defect Research Articles

EL2-related metastable defects in semi-insulating GaAs

Thermally stimulated current spectroscopy, photoquenching, and photorecovery have been used to reveal the EL2-related metastable defect levels in semi-insulating GaAs. It has been found that one set of metastable levels is directly related to EL2 and the other set is indirectly related to EL2 defect through charge transfer. The origin of these EL2-related metastable levels has been discussed in the context of recently proposed three-center-complex model of EL2 in semi-insulating GaAs.

The influence of impurities on radiative recombination via EL2 centers in gallium arsenide single crystals

The influence of Cd and Se atoms on the quantum efficiency of emission induced by EL2 defects in GaAs single crystals is studied in detail. The diffusion of impurities under conditions of vacuum is compared to that in an As atmosphere. It is ascertained that the character and degree of variation in the quantum efficiency are governed by the vacancy type and concentration in the crystals and are related to the probability of formation of complexes that consist of EL2 centers and impurities.

Anti-Stokes Bands of Intrinsic Radiation in Undoped GaAs Crystals

It has been established that undoped gallium crystals exhibit anti-Stokes radiation with energy corresponding to the interband recombination of these crystals. Its appearance is most probably determined by EL2 defects, and its intensity depends on the product of the cross sections for photoionization of electrons and the holes from these defects.

High-field ODMR investigation of the EL2 defect in semi-insulating GaAs

The microscopic structure of the EL2 defect in semi-insulating GaAs has remained controversial. It is generally agreed that EL2 is an As antisite-related defect, but whether it is an isolated AsGa defect or an AsGa-related pair defect, has not been unambiguously decided. In a previous investigation with electron nuclear double resonance (ENDOR) detected via the magnetic circular dichroism (MCD) of the near infrared absorption of EL2+ (MCDA-ENDOR) at 24GHz (K-band) the local symmetry could not be determined conclusively due to complicated multiple ENDOR line splittings caused by strong effects of pseudo-dipolar couplings. We report on an MCDA-ENDOR investigation at 95GHz (W-band) where the undesired 75As ENDOR line splittings due to pseudo-dipolar coupling were suppressed. The surprising result is that the angular dependence of the nearest 75As shell shows almost Td symmetry. A small non-equivalence of the nearest neighbours is hidden in an unresolved ENDOR line width for B0||[001] and can be a most 1.5% of the superhyperfine (shf) constants. The lines of a further 75As shell, interpreted as the fifth shell, are split into at least two subshells indicating a pair defect in spite of the almost equivalence of the nearest neighbours. It is discussed whether EL2 is an AsGa+–VGa3− pair defect following recent theoretical calculations showing that this pair defect has the observed properties of the shf interactions.

Defect identification for the AsGa family

The AsGa family consists of at least four distinctly different point defects including the technologically important EL2 defect. While the different members are easily distinguished from their MCDA spectra, the differences of the hf and shf interactions as derived from ODEPR and ODENDOR are rather small.We present ab initio calculations using the LMTO–ASA Green's function method for a variety of defect models that might be relevant for the identification of AsGa-related defects. We confirm the identification of the isolated AsGa and show that the {AsGa–X2} defect must be identified with the nearest-neighbor antistructure pair rather than with the {AsGa–VAs} pair. For the {AsGa–X1} defect a distant antistructure pair is a likely candidate. For the EL2, the most important member of the AsGa family, we have not found a conclusive defect model. The recent ODENDOR data are similar to those of the distant orthorhombic {AsGa–VGa} pair, which, however is a triple acceptor and not a donor.

Far‐infrared absorption due to shallow acceptors in semi‐insulating GaAs in dependence on EL2 bleaching

AbstractFar‐infrared absorption in semi‐insulating GaAs due to electronic transitions of shallow acceptors was measured in dependence on conditions of bleaching the GaAs intrinsic defect EL2. The far‐infrared spectra were compared with the results of near‐infrared spectroscopy of EL2 fundamental state. It was found that under conditions of low intensity illumination the neutral acceptors can coexist with the neutral EL2 defect. The model is proposed which explains the effects that were not observed previously, such as depopulation of neutral shallow acceptors at low temperature, and the charge redistribution between different shallow acceptor levels at increasing temperature for the samples co‐doped with zinc and carbon. It is shown that far‐infrared spectroscopy is suitable for quantitative evaluation of the shallow acceptor concentration in GaAs material. The corresponding calibration factors are given. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitation photocapacitance study of EL2 in n -GaAs and its relation to non-stoichiometry

The excitation photocapacitance measurements are performed on n-GaAs to obtain charge transition characteristics of EL2 defects in a full spectral region. It is shown that the threshold photon energy for EL2 ++ to EL2 + transition is changed as a function of the primary excitation photon energy. It is also shown that the Frank–Condon shifts ( d FC) changed. It is considered that the lattice relaxation around the EL2 defect is affected by the deviation from the stoichiometric composition.

A study of deep levels in vanadium-doped GaAs grown by OMVPE

Vanadium(V)-doped GaAs layers grown by metal organic vapour phase epitaxy using vanadium tetrachloride as a doping source were characterized by deep level transient spectroscopy. The only deep level detected in undoped GaAs is EL2. After V doping, the EL2 defect was not detected. This observation is explained by a reaction involving V and the EL2 arsenic antisite. V doping creates an electron trap (E1) at 0.14 eV below the conduction band, associated with the transition V3+/V2+, and a broad band (E2) of electron traps. The E1 concentration increases with increasing V doping to reach a maximum value of 1016 cm−3 for a total V concentration of about 1018 cm−3. The E2 band is characterized by a defect distribution similar to the so-called U band. We propose that it is associated with intrinsic defects introduced by the growth conditions.

Defect Engineering and Atomic Relocation Processes in Impurity-Free Disordered GaAs and AlGaAs

ABSTRACTImpurity-free disordering (IFD) of GaAs and AlxGa1-xAs epitaxial layers using SiOx capping in conjunction with annealing was studied by deep level transient spectroscopy (DLTS) and capacitance-voltage (C-V) measurements. Three dominant electron traps S1 (EC– 0.23 eV), S2* (EC– 0.53 eV), and S4 (EC– 0.74 eV) are created in disordered n-type GaAs. The electron emission rate of S1 is enhanced in the presence of an externally applied electric field. We propose that S1 is a defect that may involve As-clustering or a complex of arsenic interstitials, Asi, and the arsenic-antisite, AsGa. S2* is shown to be the superposition of two defects, which may be VGa-related. S4 is identified as the defect EL2. Our preliminary results indicate that the same set of defects is created in disordered n-type AlxGa1-xAs, with S1 pinned to the conduction band edge, while S2* and S4 are pinned relative to the Fermi level. In contrast to disordering in n-type GaAs, IFD of p-type GaAs results in the pronounced increase in the free carrier concentration in the near-surface region of the disordered layer. Two electrically active defects HA (EV+ 0.39 eV) and HB2 (EV+ 0.54 eV), which we have attributed to Cu- and Asi/AsGa-related levels, respectively, are also observed in the disordered p-GaAs layers. IFD causes segregation of Zn dopant atoms and Cu towards the surface of IFD samples. This atomic relocation process poses serious limitations regarding the application of IFD to the band gap engineering of doped GaAs-based heterostructures.

A study of the electric field transient at the Au/semi-insulating GaAs contactunder an alternating current square-pulse bias

Positron lifetime spectroscopy combined with a 50 V AC appliedsquare-wave bias has been used to probe the electric field in the 2 μmregion next to an Au/SI-GaAs contact. The electric fields spatially and timeaveraged over the reverse half-period at different temperatures (255–295 K) andpulsing periods (∼1 ms–10 ks) have been obtained using a numerical differentiationtechnique. It is found that within the temperature range studied, the electric fieldfirst increases as a function of time to a maximum of about 10 kV cm−1 and thendecreases, before finally saturating at the DC value of 7 kV cm−1. The increase ofthe electric field with time is attributed as previously to the electron emissionfrom the EL2 defect. The subsequent electric field decrease is associated with theonset of a thermally activated process with an energy barrier of 1.03 ± 0.15eV. This process, which causes the neutralization of the space charge region,may be tentatively attributed to the recombination centres EL5 or EL6.

Oxygen Plasma Damage in GaAs Directly Exposed to Surface-Wave Plasma

We have developed a down-flow ashing system for microprocessing of large substrates. The system is equipped with a surface-wave-plasma source of which the “slot-type” antenna was specially designed to generate a uniform and high-density plasma. In this work, the oxygen-plasma-induced damage to the GaAs surface was investigated as a function of the plasma-substrate distance D and exposure time T. These samples were evaluated by X-ray photoelectron spectroscopy (XPS), ellipsometry, photoluminescence (PL), deep level transient spectroscopy (DLTS) and current–voltage (I–V) measurements. XPS results showed that the part of the oxide layer near the surface consists mainly of Ga2O3 and As2O3 while deeper in the oxide layer, Ga2O3 predominantly exists. Ellipsometry results showed that the thickness of the oxide layers increases up to 100 Å with decreasing D and increasing T. PL intensity of free exciton (FE) and (e,A0) observed in GaAs decreased with decreasing D and increasing T. The origin of this decrease in PL intensity was confirmed by DLTS measurements to be due to an increase in EL2 (0.73 eV) defect level density. I–V measurement showed that the effective barrier height decreases with decreasing D. This is probably due to the creation of surface states and to an increase in EL2 density, which lead to an increase in the recombination current at the interface or depletion layers. These results demonstrate that a low amount of damage can be ensured at D≧157 mm. The developed ashing system and condition is currently being used for the mass production of actual semiconductor devices.

Electron emission from deep level defects EL2 and EL6 in semi-insulating GaAs observed by positron drift velocity transient measurements

A ±100 V square wave applied to a Au/semi-insulating SI–GaAs interface was used to bring about electron emission from and capture into deep level defects in the region adjacent to the interface. The electric field transient resulting from deep level emission was studied by monitoring the positron drift velocity in the region. A deep level transient spectrum was obtained by computing the trap emission rate as a function of temperature and two peaks corresponding to EL2 (Ea=0.81±0.15 eV) and EL6 (Ea=0.30±0.12 eV) have been identified.

Passivation of GaAs(110) with Ga2O3 thin films deposited by electron cyclotron resonance plasma reactive molecular beam epitaxy

Gallium oxide thin films deposited by electron cyclotron resonance plasma molecular beam epitaxy on GaAs(110) surfaces are reported. Room temperature photoluminescence spectra show an enhancement over as-is surfaces by greater than an order of magnitude for semi-insulating wafers. This enhancement is corroborated by low temperature photoluminescence spectra, showing a reduction in AsGa, OAs, and carbon-related emissions. The bonding configuration at the interface to GaAs was investigated by x-ray photoelectron spectroscopy depth profiling and secondary ion mass spectroscopy: Arsenic oxide related compounds were below the sensitivity limits of the former technique, while carbon (both in the film and in the vicinity of the interface) was below the sensitivity limit of the latter technique. Photoluminescence enhancement is also attributed to hydrogen passivation of EL2 defects, which is found to be stable following deposition at temperatures of 400 °C on semi-insulating and p-type wafers.

Defect formation in GaAs grown by organometallic vapor phase epitaxy and the structure of the native defect EL2

Abstract Existing data describing the dependence of the concentrations of the defects S As , Zn Ga , Si Ga , Sb Ga , and EL2 in GaAs grown by organometallic vapor phase epitaxy on the partial pressures of arsine ( p As ) and the dopant-containing gases in the input gas stream are analyzed using a simple model. It is found that (1) As 4 is the dominant species of As in the vapor in equilibrium with the solid at the growth interface, (2) the electron concentration governing the charge state of the defect being formed is proportional to p As 3/4 independent of the dopant concentration, (3) the Fermi level during the defect formation process lies above the ionization levels of deep donors but below that of shallow donors, and (4) the atomic structure of the mid-gap electron trap, EL2, is stoichiometrically equivalent to the arsenic antisite, As Ga .

Application of positron annihilation lifetime technique to the study of deep level transients in semiconductors

Unlike its conventional applications in lattice defect characterization, positron annihilation lifetime technique was applied to study temperature-dependent deep level transients in semiconductors. Defect levels in the band gap can be determined as they are determined by conventional deep level transient spectroscopy (DLTS) studies. The promising advantage of this application of positron annihilation over the conventional DLTS is that it could further extract extra microstructure information of deep-level defects, such as whether a deep level defect is vacancy related or not. A demonstration of EL2 defect level transient study in GaAs was shown and the EL2 level of 0.82±0.02 eV was obtained by a standard Arrhenius analysis, similar to that in conventional DLTS studies.

Effect of proton irradiation on the characteristics of GaAs Schottky barrier diodes

Proton irradiation (80 keV) effect on the electrical properties of Au/n-GaAs Schottky barrier diodes (SBDs), fabricated on an epitaxially grown undoped n-GaAs has been studied for a range of particle fluences from 1×10 13 to 1×10 15 particles/cm 2. Current–voltage ( I–V), capacitance–voltage ( C–V) and deep level transient spectroscopy (DLTS) measurements have been carried out to study the change in characteristics of the devices and the defects introduction due to implantations, respectively. Reverse leakage current ( I R) of the device is the most sensitive parameter to the incident proton irradiation. The I R was increased (1.0×10 −9 to 7.95×10 −5 A) upon irradiation and it strongly depends on the particle fluence. Annealing of irradiated diodes shows reduction in the I R and particularly, enhancement in barrier heights of the diodes. A change of effective free carrier concentration in the material has been observed from the C–V measurements for the irradiated diodes when compared to the unirradiated diode. From DLTS measurements, it was found that the low energy proton irradiation of the SBDs increases the concentration of EL2 defects and the concentration was estimated to be 6.42×10 14 cm −3.

Magneto-optical and ODEPR investigations of native defects in substrate-free LT-MBE grown GaAs

As-grown substrate-free LT-GaAs (200°C) and annealed samples (up to 660°C) have been investigated with magnetic circular dichroism of absorption (MCDA) and optically detected electron paramagnetic resonance (MCDA-EPR) in K- and W-bands. MCDA-EPR spectra of all samples reveal several As Ga-related defects with different excitation spectra, that indicate their different structures in as-grown and annealed material. A strong change occurs in samples annealed above 400°C. The ODEPR spectra show another yet unknown defect which we attribute to a Ga vacancy-related defect. Samples annealed at 500°C exhibit EL2-like MCDA spectra. From the above band gap MCDA spectra of the EL2-like defects evidence is found for their low symmetry. The bleaching properties and spin-lattice relaxation times of the different As Ga-related defects were investigated and compared with the properties of the EL2 defect in bulk semi-insulating GaAs.

On the problem of the EL2 structure in semi-insulating GaAs: high-frequency ODEPR/ODENDOR measurements in W-band

For almost two decades the structure model of the EL2 defect in semi-insulating (SI) GaAs has been controversially discussed. Neither the isolated As Ga with T d symmetry, nor the As Ga–As i pair model nor any other As Ga-related defect model could be unambiguously established. The reason was that the analysis of previous optically detected electron nuclear double resonance (ODENDOR) and optically detected electron paramagnetic resonance (ODEPR) spectra measured at 24 GHz was difficult and not unambiguous because of forbidden transitions, pseudo-dipolar couplings and higher order effects. However, the ODEPR spectra of the EL2 defect measured in W-band (93 GHz) can be fitted by first order perturbation calculation of the spin Hamiltonian thus allowing a much simpler analysis compared to X- or K-band. From ODEPR measurements in different SI GaAs samples both in K- and W-band we show that the ODEPR spectra of the EL2 defect together with the K-band ODENDOR data are not consistent with the As Ga–As i pair model, but the highly symmetric isolated As Ga model has to be excluded as well. First ODENDOR measurements in W-band are also reported.

Detection of oxygen-related defects in GaAs by exo-electron emission spectroscopy

The influence of intentional introduction of oxygen, at the surface of GaAs, on its native surface states was studied. Oxygen was made to interact with the surface of GaAs by three different means: (1) by growing native oxides, (2) exposing to oxygen plasma in an electron cyclotron resonance (ECR) plasma reactor and by (3) high energy oxygen ion irradiation. Thermally stimulated exo-electron emission (TSEE) spectroscopy was used to estimate the relative densities and energies of the surface states induced by the three different modes of introducing oxygen. Out of the two native defect levels found in GaAs by TSEE; at 325 K (0.7 eV below E c) and at 415 K (0.9 below E c); the former is seen to get broadened or split into multiple peaks in each of the methods. Multiple peaks in TSEE signify the presence of a closely spaced band of defect levels. Therefore the results exclusively point out that oxygen-related complexes contribute to the formation of a band of defects centered at 325 K in TSEE which is correlated to an energy level 0.7 eV below E c known as the EL2 defect level. The results reported in this paper thus confirm that the TSEE peak at 0.7 eV below E c is related to oxygen induced defects whereas the peak at 0.9 eV is not affected by the presence of oxygen-related species.

Slow domains in semi-insulating GaAs

Semi-insulating GaAs shows current oscillations if a high dc voltage is applied to a sample. These oscillations are caused by traveling high-electric-field domains that are formed as a result of electric-field-enhanced electron trapping. This article describes the various types of experiments that have been carried out with this system, including recent ones that use the electro-optic Pockels effect in order to measure the local electric fields in the sample in a highly accurate manner. An historical overview of the theoretical developments is given and shows that no satisfying theory is currently available. A list of all the required ingredients for a successful theory is provided and the experimental data are explained in a qualitative manner. Furthermore, the main electron trap in semi-insulating GaAs is the native defect EL2, the main properties of which are described.