N-type GaAs Crystals Research Articles

Efficient electroluminescence in doped-GaAs via terahertz metamaterials

We investigate the highly efficient terahertz nonlinearity exhibited by n-type GaAs crystals under metallic metamaterials. An intense THz field applied to the metamaterials leads to impact ionization in the GaAs substrate, which emits electroluminescence in the near-infrared region. Even for a similar THz field strength, n-type GaAs emits near-infrared photons more efficiently than semi-insulating GaAs. We analyzed the luminescence lineshapes and intensity as a function of the excitation field strength, using Fermi–Dirac statistics and the density of states in the conduction band to quantify electron density and locate the Fermi level after the relaxation of excited hot electrons.

Noise-induced effects in nonlinear relaxation of condensed matter systems

Noise-induced phenomena characterise the nonlinear relaxation of nonequilibrium physical systems towards equilibrium states. Often, this relaxation process proceeds through metastable states and the noise can give rise to resonant phenomena with an enhancement of lifetime of these states or some coherent state of the condensed matter system considered. In this paper three noise induced phenomena, namely the noise enhanced stability, the stochastic resonant activation and the noise-induced coherence of electron spin, are reviewed in the nonlinear relaxation dynamics of three different systems of condensed matter: (i) a long-overlap Josephson junction (JJ) subject to thermal fluctuations and non-Gaussian, Lévy distributed, noise sources; (ii) a graphene-based Josephson junction subject to thermal fluctuations; (iii) electrons in a n-type GaAs crystal driven by a fluctuating electric field. In the first system, we focus on the switching events from the superconducting metastable state to the resistive state, by solving the perturbed stochastic sine-Gordon equation. Nonmonotonic behaviours of the mean switching time versus the noise intensity, frequency of the external driving, and length of the junction are obtained. Moreover, the influence of the noise induced solitons on the mean switching time behaviour is shown. In the second system, noise induced phenomena are observed, such as noise enhanced stability (NES) and stochastic resonant activation (SRA). In the third system, the spin polarised transport in GaAs is explored in two different scenarios, i.e. in the presence of Gaussian correlated fluctuations or symmetric dichotomous noise. Numerical results indicate an increase of the electron spin lifetime by rising the strength of the random fluctuating component. Furthermore, our findings for the electron spin depolarization time as a function of the noise correlation time point out (i) a non-monotonic behaviour with a maximum in the case of Gaussian correlated fluctuations, (ii) an increase up to a plateau in the case of dichotomous noise. The noise enhances the coherence of the spin relaxation process.

Enhancement of electron spin lifetime in GaAs crystals: The benefits of dichotomous noise

The electron spin relaxation process in n-type GaAs crystals driven by a fluctuating electric field is investigated. Two different sources of fluctuations are considered: (i) a symmetric dichotomous noise and (ii) a Gaussian correlated noise. Monte Carlo numerical simulations show, in both cases, an enhancement of the spin relaxation time by increasing the amplitude of the external noise. Moreover, we find that the electron spin lifetime vs. the noise correlation time: (i) increases up to a plateau in the case of dichotomous random fluctuations, and (ii) shows a nonmonotonic behaviour with a maximum in the case of bulks subjected to a Gaussian correlated noise.

Anisotropic electric-field-enhanced electron emission from deep-level defects in GaAs

Strongly anisotropic electric-field enhancements of the thermal emission rates of electrons from the EL3 and EL5 deep-level defects in n-type GaAs crystal have been revealed with the double-correlation deep-level transient spectroscopy. The results, analysed by taking into account both the Poole–Frenkel and phonon-assisted tunnel effects, evidence a strong coupling of the defects to the lattice vibronic modes. The defect potential anisotropy of EL3 is consistent with the defect identification as an off-centre substitutional oxygen on the arsenic site. The revealed surprising break-down of the emission-rate enhancement, for the electric field applied along the ⟨1 0 0⟩ crystallographic direction, is interpreted as resulting from a possible reorientation of the EL3 defect, owing to a jump of the oxygen ion into a neighbouring lattice site, driven by a strong electric field. On the other hand, a close pair divacancy complex is suggested to be responsible for the EL5 defect.

Electric-field-enhanced electron emission from the EL5 deep-level defect in GaAs

Double-correlation deep-level transient spectroscopy has been used to study the electric-field-induced thermal emission of electrons from the EL5 defect in n-type GaAs crystal. Strongly anisotropic electric-field enhancement of this emission, which was the largest for the electric field parallel to the 〈1 0 0〉 crystallographic direction and the smallest for the 〈1 1 1〉 configuration, points to the trigonal symmetry of the defect potential and a strong coupling of the defect to the lattice vibronic modes. In view of the revealed results it is argued that a close pair divacancy complex can be responsible for the EL5 defect.

Effect of Impurities and Structural Defects on the Transport Properties of n-Type GaAs Crystals

The carrier mobility and concentration in n-type GaAs single crystals, undoped, doped during growth (Te, Sn, and Ge), and neutron-transmutation doped (NTD) with Se and Ge, were measured as a function of temperature. Although the donor concentration was the same in all of the doped crystals, the carrier mobility and concentration in the GaAs crystals were found to be lower than those in the GaAs , GaA , and NTD GaAs crystals by about a factor of three and one order of magnitude, respectively. These results are interpreted in terms of the compensation ratio and predominant scattering processes in the crystals. The large compensation ratio in the GaAs crystals is due to the high acceptor concentration. The low carrier mobility in GaAs is associated with the strong scattering of charge carriers by ionized defects, because of their high density, and also, in contrast to the undoped, GaAs , and GaAs , and NTD GaAs crystals, with additional scattering by nonuniformly distributed structural defects. In calculations of the compensation ratio and analysis of the scattering mechanisms, the Brooks–Herring or Cornell–Weisskopf approximation was used, depending on the compensation ratio in the material.

Bistable behavior of a medium-deep center related to EL5 and EL6 in n-type bulk GaAs

We report characteristic behaviors of medium-deep electron traps (EL5, EL6, etc.) in n-type bulk GaAs crystals observed by isothermal constant-capacitance–voltage transient spectroscopy (CCVTS). Each CCVTS spectrum of EL5 and EL6 was broader than a theoretical one expected for a single level, and was found to consist of more than two trap components. Anomalous filling time dependence of CCVTS peak heights for these traps was observed when the filling pulse width was varied in a wide range, i.e., one component of EL6 decreased to about 50% of its maximum, while one component of EL5 increased and saturated. The decrement of the constituent of EL6 was nearly equal to the increment of that of EL5. These variations could be reversed by controlling electron occupation fractions of these traps by means of changing the interval time between two adjacent filling pulses in application of the isothermal “double-pulse CCVTS” method. Such interaction was commonly observed in n-type bulk GaAs irrespective of carrier concentration and growth method. These behaviors of medium-deep electron traps are interpreted well by a model of an electronically controlled bistable reaction.

Donor Neutralization by Fluorine Containing Plasmas in Si-Doped n-Type GaAs Crystals

CF4, SF6 and CF4/O2 plasma-induced damage in silicon-doped n-type GaAs crystals have been investigated by using Hall measurements, X-ray Photoelectron Spectroscopy (XPS) analysis and Secondary Ion Mass Spectrometry (SIMS) analysis. We observed that the carrier density degraded after plasma exposure and post annealing. The degree of deactivation of carrier density with post annealing was strongly dependent on the plasma chemistries and the self-bias Vdc of the plasmas. From SIMS analysis, fluorine contamination was observed in the surface region of GaAs crystals after CF4 plasma exposure. The internal diffusion of fluorine atoms and the localization of fluorine atoms in n-type GaAs layers were observed after post annealing at 400°C. From these results, we proposed a mechanism of plasma induced damage in which internally diffused fluorine atoms neutralize the donor silicon by forming a Si–F bond. We also evaluated the effective diffusion coefficient of fluorine in GaAs as 1.5×10-11 cm2/s (at 400°C).

Donor Neutralization by Fluorine Containing Plasmas in Si -Doped N -Type GaAs Crystals

AbstractCF4, SF6 and CF 4/O2 plasma-induced damage in silicon-doped n-type GaAs crystals have been investigated by using Hall measurements, and SIMS analysis. We observed that the carrier density degraded with post annealing after plasma exposure. The degree of deactivation of carrier density with post annealing was strongly dependent on the plasma chemistries and the self-bias Vdc of the plasmas. From SIMS analysis, fluorine contamination was observed in the surface region of GaAs crystals after CF4 plasma exposure. The internal diffusion of fluorine atoms and the localization of fluorine atoms in n-type GaAs layers were observed after post annealing at 400°C. From these results, we proposed a mechanism of plasma induced damage in which internally diffused fluorine atoms neutralize the donor silicon by forminq a Si-F bond. We determined the effective diffusion coefficient of fluorine in GaAs as 1.5 × 10−11 cm2/s (at 400°C).

Analysis of Large Impurity Atmospheres at Dislocations and Associated Point Defect Reactions in Differently n-Doped GaAs Crystals

The large impurity atmospheres at dislocations typical of n-type (Si- or Te-doped) GaAs crystals have been analysed by localized measurements of the free electron concentration, diffusion length and DSL etching velocity. The atmospheres always contain dopant atoms as well as point defects (complexes) whose formation and type depend on the type of dopant impurity and melt stoichiometry. The donor- or acceptor-like characteristics of such point defects (complexes) are responsible for the observed remarkable difference in the electrical and recombinative properties of the atmospheres between the differently doped crystals. The point defect reactions at the base of the formation of the slip traces are discussed. The possible mechanisms of the impurity-dislocation interaction leading to the formation of the atmospheres are also considered.

Influence of fast-neutron irradiation on the intensity of the copper-related luminescence band at hν m =1.01 eV in n-type GaAs

The influence of neutron irradiation (the energy E=2 MeV and the dose Φ=1013–1015cm−2) and subsequent anneals (the annealing temperature T a =400–700 °C and the annealing time is 30 min) of n-type GaAs(Te,Cu) crystals with an initial carrier concentration n0=2×1018 cm−3 on the intensity of the copper-related luminescence band with an emission maximum at hν m =1.01 eV is studied. A significant irradiation-induced increase in the intensity of the band is observed. It is attributed to a radiation-stimulated increase in the concentration of emitting centers (CuGaVAs pairs) as a result of the effective interaction of interstitial copper atoms with irradiation-induced gallium (VGa) and arsenic (VAs) vacancies, as well as VGaVAs divacancies.

CF 4 /O 2 plasma induced damage in Si-implanted n-type GaAs crystals

The CF4/O2 or CF4 plasma induced damage in Si-implanted n-type GaAs crystals has been investigated by using Hall measurement and secondary ion mass spectrometry (SIMS) analysis. We found that sheet resistance degraded after plasma exposure and postannealing. The degradation of the sheet resistance during postannealing at 200–400 °C, is mainly associated with the reduction of carrier density in the crystal. From SIMS analysis, fluorine contamination was observed in the surface region of GaAs crystals after CF4/O2 plasma exposure. The internal diffusion of fluorine atoms and the localization of fluorine in the Si-implanted n-type GaAs layer, were observed after postannealing at 400 °C. Based on these findings, a fluorine passivation related model is proposed, where Si–F bonds are formed in the Si-implanted n-type GaAs crystals.

Effect of fast-neutron irradiation on the photoluminescence of n-type GaAs(Te) crystals

Effect of fast-neutron irradiation on the photoluminescence of n-type GaAs(Te) crystals

Photocapacitance investigation of the two photoquenching levels in n-type GaAs crystals with excess arsenic atoms

The photocapacitance method is applied to the n-type GaAs crystal in which the excess arsenic atoms are introduced by the 67 h annealing at 900 °C under extremely high arsenic vapor pressure of 1950 Torr. The ion density photocapacitance spectra and the temperature dependencies of the photoinduced ion density measurements reveal two photoquenching levels with different level positions and thermal recovery temperatures, respectively.

Electrical characterization and detector performances of a LPE GaAs detector for X-ray digital radiography

The electrical characteristics of a LPE n-type GaAs crystal have been fully measured. The experimental results were used to build a model, including the parametrization of the electric field inside the crystal, and to study the features of such material as a detector in view of a possible application for digital radiography. The predictions of a Monte Carlo based on this model are in very good agreement with the measured performances of this crystal when used as a detector for X-rays in the diagnostic energy range.

Capacitance transient study of deformation-induced defects in n-type GaAs

Deep energy levels of deformation-induced defects in n-type GaAs crystals are studied by means of isothermal capacitance transient spectroscopy (ICTS). Analysis of the transient curves shows that three kinds of electron traps, termed ED1a, ED1b, and ED2, and one kind of hole traps, termed HD1, are induced by deformation. Additional deformation-induced acceptors, termed X, might also be introduced near the valence band. The energy levels and capture cross sections of ED1a and ED1b are determined as Ec − 0.55 eV, 2.9 × 10−17 cm2 and Ec − 0.60 eV, 1.3 × 10−15 cm2, respectively, and those of ED2 as Ec − 0.41 eV and 4.3 × 10−16 cm2. HD1 is observed with filling pulses longer than 1 ms. The energy level and hole capture cross section of HD1 are determined to be Ev + 0.66 eV and 8.4 × 10−13 cm2, respectively. HD1 is found to be less stable than ED1a and ED1b againstthermal annealing.

Dominant Midgap Levels in the Compensation Mechanism in GaAs

AbstractProperties of midgap levels in n-type GaAs crystals were studied by using trap density spectroscopy (TDS), capacitance-voltage and near infrared (NIR) absorption measurements. The TDS analysis for various n-type samples showed that the concentrations of the midgap level were from 2.0 × 1016 cm−3 to 3.4 × 1016 cm−3. However, those of EL2 in the same crystals were found to be from 1.1 × 1016 cm−3 to 1.3 × 1016 cm−3 by NIR absorption measurements at room temperature. On the other hand, in undoped semi-insulating GaAs crystals with carbon concentrations ranging from 6 × 1014 cm−3 to 2.4 × 1016 cm−3, the densities of the ionized EL2 determined by NIR absorption were only about 30 % of those of carbon acceptor determined by the localized vibrational mode absorption at room temperature. These differences suggest the presence of another midgap donor which does not give NIR absorption. The concentrations of this trap were estimated to be 0.8 to 1.6 times those of EL2 by decomposition of the TDS spectra into their components.

Defects in GaAs on Si studied with the positron-beam technique.

Positron-beam experiments have been carried out to characterize defects in metal-organic vapor-phase-epitaxially grown GaAs layers on (100)GaAs and (100)Si substrates. The Te-doped GaAs/GaAs layer (n=2×10 15 cm -3 ) appears to be free of vacancies which are present in similarly doped bulk n-type GaAs crystals. The heteroepitaxial GaAs/Si layer with the low (<10 7 cm 2 ) dislocation density is also free of positron trapping. At ahigher dislocation density of 10 7 -10 8 cm -2 positrons detect Si cluters.

Study of Medium-Deep Traps in Undoped GaAs Grown by Arsenic-Pressure Controlled Cz Method

ABSTRACTProperties of medium-deep traps in n-type undoped GaAs crystals grown by arsenic-pressure controlled Czochralski (PCZ) method were studied by using capacitance transient spectroscopy (CTS) and temperature-dependent Hall (TDH) measurement. EL3 and EL6-like (EM2) traps were dominant medium-deep traps in D LTS measurements. The dependence of EM2 peak height on the duration of filling pulse and the detailed analysis of CTS showed that the trap EM2 spectrum consisted of three electron traps which had thermal emission activation energies of approximately 0.35 eV and various capture cross sections with activation energies for electron capture. On the other hand, the trap EL6 spectrum in n-type LEC GaAs was described by two levels of traps with activation energies of 0.32 eV and 0.33 eV for electron emission, respectively. It was concluded that so-called EL6 formed a family and some of its components were classified as defects with large lattice relaxation.

Stoichiometry-dependent deep levels in n-type GaAs

The photocapacitance (PHCAP) method in the constant capacitance condition has been applied to determine the energy and density of deep levels within the limits of a certain depletion layer thickness as a function of the photon energy and the applied voltage. Samples used were various n-type GaAs crystals annealed for a long time under excess As vapor pressure and rapidly cooled. The present PHCAP investigations have revealed several stoichiometry-dependent deep levels including some photoquenching levels. These are Ec−0.66 and −0.72 eV deep donors, the photoquenching level at 1.03, 1.25, and 1.41 eV. The As-vapor-pressure dependence of the density of these deep levels in quasithermal equilibrium were also clarified. The 1.25-eV photoquenching level density increased monotonically with increasing applied As vapor pressure and finally saturated at extremely high As vapor pressures. The density of Ec −0.72 eV deep donors showed a similar dependence on the As vapor pressure, but showed less saturation under higher As vapor pressures. The Ec −0.72 eV deep donor is quite different from the so-called EL2 level. But the temperature dependence of the PHCAP responses (ΔVph -T) revealed strong interaction effects among these defect structures. From the photoluminescence results and conventional deep-level transient spectroscopy measurements,it was clarified that both Ec −0.66 and −0.72 eV deep donors had larger Frank–Condon shifts, 0.28 and 0.24 eV, respectively, compared with that of the so-called EL2 level (0.12 eV). The PHCAP measurements after photoexcitation were carried out to detect de-ionization of the photoionized deep levels. Additional de-ionization of the photoionized state of the 1.41-eV photoquenching level was detected adjacent to those of the EL2 level. These de-ionization could be detected only in the course of the photoquenching phenomenon in n-type GaAs. The As-vapor-pressure dependence of these deep levels shows that interstitial As atoms play a vital role in forming deep levels in annealed GaAs crystals.