DX centers in selectively Si-doped GaAs---AlAs superlattices

DX center has been characterized in four GaAs---AlAs superlattices grown by MBE at 580°C. The structures are uniformly Si-doped or selectively Si-doped in the AlAs layers or in the middle of the GaAs layers or on both sides of the interfaces. Deep level transient spectroscopy measurements (DLTS) put in evidence one dominant electron trap, with an activation energy for thermal emission of about 0.42eV for all the superlattices. This defect shows a thermally activated capture cross section and a large concentration except for the case where the only GaAs layers are doped. For the first time, a study of the capture reveals a capture activation energy of 0.36-0.37 eV, which allows us to locate the DX level nearly 60 meV below the conduction miniband. From these results, we show that the observed DX center is related to silicon in the AlAs layers. For the case when the AlAs barriers are not doped, the DX level is due to the Si diffusion from the middle of the wells towards the barriers, the Si atoms having diffused during the growth.     

Electrical properties of DX centers in GaAs and AlGaAs

Abstract

The DX center, the lowest energy state of the donor in AIGaAs with x < 0.22, is responsible for the reduced conductivity as well as the persistent photoconductivity observed in this material at low temperature. Extensive studies of the properties of this deep level in Si-doped AIGaAs are reviewed here. Data are presented showing that the characteristics of the DX center remain essentially unchanged when it is resonant with the conduction band (x < 0.22) and that, independent of other compensation mechanisms, the DX center therefore limits the free carrier concentration in Si-doped GaAs to a maximum of about 2 × 10¹⁹ cm^?3. Recent measurements suggesting that the lattice relaxation involves the motion of the Si atom from the substitutional site toward an interstitial site are also presented. Evidence for the negative U model, that the DX level is the two electron state of the substitutional donor, is discussed.     

High pressure and deep donors (DX centers) in gaas and algaas

Abstract

The presented paper is a review of some interesting phenomena characteristic for the DX center — a metastable donor in GaAs and AlGaAs. We concentrate on a presentation of microscopic models of the DX centers and their consequences. We point out some controversies concerning the nature of this donor impurity and the profits resulting from the wide use of pressure methods in resolving these controversies.     

Charge trapping in n-AlxGa1-xAs “insulators” and related device instabilities

In some devices based on GaAs/Al_xGa₁-_xAs heterostructures, the Al_xGa₁-_xAs plays the role of a wide band gap “insulator”. These devices are therefore excellent systems for studying charge trapping in Al_xGa₁-_xAs. It is a poorly understood property of Al_xGa₁-_xAs that incorporation of any n-type dopant results in the formation of a deep electron trap, the DX center. Recent experiments on heterostructure devices have probed both thermal and athermal (hot electron) capture and emission by the DX center. By observing the trapping behavior as the composition (Al mole fraction) of the alloy is varied, the relationship between the trap level and the band structure of the host material has been clarified. A remarkable result is the observation of electron trapping at alloy compositions where the trap state is resonant with the conduction band.     

AlGaAs∶Sn中DX中心电子俘获势垒的精细结构

采用定电容电压法,测量了n型Al026Ga074As∶Sn中DX中心电子热俘获瞬态,以及不同俘获时间后的电子热发射瞬态;并对瞬态数据进行数值Laplace变换,得到其Laplace缺陷谱(LDS).通过分析LDS谱,确定了电子热俘获和热发射LDS谱之间的对应关系,从而得到热俘获系数对温度依赖关系,以及与Sn相关的DX中心部分电子热俘获势垒的精细结构;通过第一原理赝势法计算表明,Sn附近的AlGa原子的不同配置是电子热俘获势垒精细结构产生的主要原因     

Pressure dependence of electron scattering by DX centres in GaAs and AlGaAs

Abstract

The electron mobility of heavily n-dopped GaAs and AlGaAs increases rapidly with applied hydrostatic pressure as carriers are trapped at DX centres. This has been taken as evidence against the Chadi and Chang (CC) negative charge state model of the DX centre. We argue that DX^? centres are formed close to d⁺ centres in highly doped samples, and that the mobility data is in fact fully consistent with the CC model, when such dipole-like correlations are included.     

Pressure studies of deep levels in semiconductors

Abstract

The effects of pressure on the energetics and kinetics of electron emission and capture processes by several important deep levels in Si are discussed. The results yield the first quantitative measures of the breathing mode lattice relaxations accompanying these processes.     

The DX center and other tin-related defects in AlGaAs semiconductors

Al _x Ga_1-xAs semiconductors doped with both natural and enriched¹¹⁹Sn have been studied by Mössbauer spectroscopy (MS) to help to determine the atomic-scale nature of a deleterious, deep-level defect known as the DX center. Spectra have been acquired in the dark at 76 K and under sub-bandgap illumination at 4 or 10 K to distinguish the DX center from the substitutional shallow donor defect. Although electrical differences are clearly detected in these two states, no difference in the Mössbauer spectra are observed. Unexpected high Sn contents, determined by quantitative MS, demonstrate a large non-electrically active Sn fraction in some samples and this may be obscuring the observation of the DX center by both X-ray absorption spectroscopy and MS. Grinding the single-crystal layers into fine powders leads to an Sn defect that is attributed to a surface-oxidized site.     

Individual,attractive defect centers in the SiO2-Si interface of μm-sized MOSFETs

In a micron-sized p-channel MOSFET, the alternate capture and emission of holes into a Coulomb-attractive defect center is analyzed by the random telegraph signal in the source-drain current. Anomalous switching is observed with a high channel conductance when the defect center is occupied, and a low channel conductance after re-emission. The rate constants show an inverse symmetry for capture and emission. The measured results are interpreted by a tunneling transfer of a hole bound in the channel at the attractive center to the defect center 2.4 nm deep in the oxide and vice versa. The energy offset of the two stable configurations can be linearly varied by the gate voltage. An excited state 40 meV above the ground state is observed for the defect level. The anomalous switching is caused by a mobility change rather than by a change in mobile charge carrier density. The tunneling transfer for Coulomb-attractive centers differs from the transfer observed for Coulomb-repulsive centers where activated emission is reported.     

Line emission and alignment effects in state selective electron transfer in He2+-Li(2s, 2pΣ, 2pΠ) collisions

Classical Trajectory Monte-Carlo (CTMC) method has been used to investigate state selective electron capture by He²⁺ ions colliding with Li(2s) and Li(2p) in as well as alignments in the energy range 1-15 keV/amu. He⁺(4l) electron capture, line emission [He II(n = 4 3)] cross-sections and alignment parameters have been calculated and analyzed in the light of the available results. The undulatory structure of the capture and emission cross-sections have been explained qualitatively in terms of a quasi-molecular ion formation. Projectile impact energy and spatial overlap play crucial role in determining the alignment effects. Received 3 July 1998 and Received in final form 3 June 1999     

On the possible mixing of electron capture and positron emission channels in the decay of single-electron ions and neutral atoms

The mixing (interaction) of electron capture and positron emission channels when both channels are allowed in terms of energy is used in an attempt to explain oscillations of K capture rate that may have been observed in recent experiments.     

Far-IR radiation of hot holes in germanium for mutually perpendicular directions of uniaxial pressure and electric field

The advantages of crossed directions of an electric field and uniaxial pressure over a parallel configuration for obtaining lasing (λ≅100 μm) in p-type germanium are substantiated. The results of the first experiments investigating far-IR emission for mutually perpendicular directions of the field and pressure are reported. The pressure dependences of the radiation intensity in various electric fields are explained by the different hole occupation of the states of an impurity center which are split by uniaxial pressure. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 257–261 (25 August 1999)     

Observation of the triplet metastable state of shallow donor pairs in AlN crystals with a negative-U behavior: a high-frequency EPR and ENDOR Study

Theoretical predictions about the n-type conductivity in nitride semiconductors are discussed in the light of results of a high-frequency EPR an ENDOR study. It is shown that two types of effective-mass-like, shallow donors with a delocalized wave function exist in unintentionally doped AlN. The experiments demonstrate how the transformation from a shallow donor to a deep (DX) center takes place and how the deep DX center can be reconverted into a shallow donor forming a spin triplet and singlet states.     

Si DX centers in GaAs at large hydrostatic pressures

A number of experimental and theoretical studies indicate that DX centers in GaAs, its alloys and other III–V semiconductors have negative U properties. Using far infrared localized vibrational mode (LVM) spectroscopy of Si donors in GaAs under large hydrostatic pressure in a diamond anvil cell we have discovered an LVM of the Si DX center. From the ratio of the LVM absorption lines of Si_Ga and Si_DX and the compensation in our GaAs samples, we show unambiguously that two electrons are trapped when the ionized shallow Si donors transform into negatively charged DX centers, in full agreement with the negative U model.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Assessment of persistent-photoconductivity centers in MBE grown Al x Ga1-x as using capacitance spectroscopy measurements

Deep-level transient spectroscopy and thermally stimulated capacitance measurements were used to investigate the properties of deep traps in Si-dopedn-Al _x Ga_1–xAl layers grown by molecular beam epitaxy. Two electron traps at electron emission activation energies of 0.44 and 0.57 eV have been detected. Both traps were studied in detail and found to be the origin of the persistent-photo-conductivity phenomenon in this material. The nature of both traps is the same as of the DX center in liquid phase epitaxial material reported by Lang et al. The electron capture cross-sections are _n 1 = _n 2=8.3×10^–22cm² atT=205K. Activation energies ofE 1= 0.33eV andE 2=0.37eV at temperatures higher than 125 K were determined by DLTS measurements and by direct measurements of the capture transient. In order to allow for the variation of the free-electron concentration during the capture process, a new method for the evaluation of the electron capture crosssection was developed.     

Erbium electroluminescence excitation in amorphous hydrogenated silicon under thermally stimulated deep-center tunneling ionization

A study of the electroluminescence of erbium-doped, amorphous hydrogenated silicon, a-Si:H 〈Er〉, is reported. It has been found that the electroluminescence intensity at the wavelength λ=1.54 μm corresponding to the ⁴ I _13/2→⁴ I _15/2 intra-4f shell transition in Er passes through a maximum near room temperature. The unusual temperature and field dependences of the electroluminescence indicate electric-field induced multi-phonon tunneling emission of electrons from deep centers. The electroluminescence of Er³⁺ ions is due to their becoming excited as conduction-band electrons are captured by neutral dangling bonds (D ⁰ centers), which form when erbium is incorporated into the amorphous matrix. This Auger process transforms the center from its neutral state, D ⁰, to a negatively charged state, D ⁻, and the energy released in the capture is transferred by Coulomb interaction into the erbium-ion 4f shell. The steady-state current through the electroluminescent structure is supported by the reverse process of multi-phonon tunneling-electron emission from the D ⁻ center to the conduction band. The proposed theoretical model is in a good agreement with experimental data. Fiz. Tverd. Tela (St. Petersburg) 41, 210–217 (February 1999)     

Possible explanation of oscillations of the electron capture rate in decays of one-electron ions

On the basis of the idea of mixing (interaction) between the electron capture and the positron emission channels in the β ⁺ decay in the cases when both channels are energetically allowed, we attempt to explain oscillations of the K-capture rates that were possibly seen in recent experiments.     

The role of cation vacancies in excitation mechanism of re-ions in alkaline-earth sulphides

Abstract

The VUV-excitation spectra and thermostimulated emission for intrinsic and impurity luminescence of alkaline-earth sulphides doped by cerium have been investigated. The intensity of activator emission is higher in samples with artificially created cation vacancies. The role of cation vacancies is displayed as a result of the common analysis of the temperature dependence of impurity luminescence with the curves of the thermostimulation of the hole traps created due to the presence of the cation vacancies. For room temperatures the hole traps on cation vacancies become unstable. Therefore the trap level of cation vacancy serves as a bridge for the capture of holes from the valence band to 4f RE level.     

Calculation of positronium emission fraction from metallic surfaces

Abstract

Theoretical calculations of positronium emission fraction are compared with experiments. The results show that positronium emission fraction is quite sensitive to screening effects. Thus, positronium formation will be a very good example for testing dynamical screening effects. An experimental angle-resolved energy spectrum of positronium is required for more detailed discussion.