A. Herms

Field influence on deep level transients at large deep to shallow trap ratios

The differential capacitance method is used to determine the enhancement of thermal emission rates by electric field. The customary analysis is modified for large ratios of deep and shallow trap concentrations. Experimental results are given for the DX center in AlxGa1−xAs at x=0.55.

Majority‐carrier capture cross‐section determination in the large deep‐trap concentration cases

A method to determine the thermal cross section of a deep level from capacitance measurements is reported. The results enable us to explain the nonexponential behavior of the capacitance versus capture time when the trap concentration is not negligible with respect to that of the shallow one, and the Debye tail effects are taken into account. A figure of merit for the nonexponential behavior of the capture process is shown and discussed for different situations of doping and applied bias. We have also considered the influence of the position of the trap level’s energy on the nonexponentiality of the capture transient. The experimental results are given for the gold acceptor level in silicon and for the DX center in Al0.55 Ga0.45As, which are in good agreement with the developed theory.

Fine speckle contrast in InGaAs/InP systems: Influence of layer thickness, mismatch, and growth temperature

This work is focused on the study of the fine speckle contrast present in planar view observations of matched and mismatched InGaAs layers grown by molecular beam epitaxy on InP substrates. Our results provide experimental evidence of the evolution of this fine structure with the mismatch, layer thickness, and growth temperature. The correlation of the influence of all these parameters on the appearance of the contrast modulation points to the development of the fine structure during the growth. Moreover, as growth proceeds, this structure shows a dynamic behavior which depends on the intrinsic layer substrate stress.

The influence of cobalt on the electrical characteristics of ZnO ceramics

Abstract The influence of cobalt on the electrical characteristics and the electroluminescence spectra of ZnO varistors has been investigated. By comparing the results obtained for samples with different percentages of cobalt we have found that the increase of the cobalt percentage produces higher non-linearity coefficients α and modifies favourably the I–V characteristics. On the other hand the electroluminescence spectra show that the cobalt concentration does not have the same influence on the radiative transition centred at 700 nm and at 950 nm. Our data rather indicate that the cobalt increase enhances the 950 nm peak more than the one at 700 nm. A model is reported to explain the results.

Optical and thermal cross-section of interface states from photocapacitance measurements

Abstract The photodepopulation transient of the interface states has been analyzed in order to obtain the capacitance variation under illumination in terms of the interface state distribution throughout the band gap, the temperature and the photon energy. From these results, two methods are reported to measure directly the majority carrier photoionization cross-section and the interface state density in MOS capacitors. Both methods are based on the analysis of the initial slope of the capacitance variation. The first method likens values obtained at different illumination delay times after bringing the MOS into depletion. The second uses the values measured at two near temperatures. In both cases, excellent agreement is obtained between optical and non-optical measurements of the interface state density. However, the optical cross-section of the interface states can be deduced without it being necessary to know their density previously. Likewise, from analysis of the dark capacitance transient during the delay time, the thermal cross section, σpt, can also be deduced. Experimental values have been measured using AlSiO2Si(p) structures. In agreement with other works, we observe that the profile of interface states does not depend on the temperature, and σpt increases exponentially as the energy value approaches that of the midgap and approaches a constant value of 1.6 × 10−12 cm2 for energies greater than about 0.45 eV. The optical cross-section shows behaviour similar to those reported for n-type Si, GaAs and InP. For energy values smaller than about 1.0 eV, σp0 can be fitted according to σp0 ∝ (hv - hv0)n with n about 3. For photon energy greater than 1.0 eV, the σp0 values deduced show a strong rise.

Dependence of the Defects Present in InAlAs/InP on the Substrate Temperature

The crystalline quality of InAlAs layers, grown by Molecular Beam Epitaxy on (100) InP substrates, has been investigated by Transmission Electron Microscopy in order to study the influence of InAlAs growth temperature (T g ) on the density of structural defects present in the layers. T g was varied from 300°C up to 530°C. The density of stacking faults and threading dislocations drops dramatically as T g increas

Time-dependent response of interface states determined by using differential isothermal transient spectroscopy

A method based on the differential analysis of the isothermal transients is proposed to study the dynamical properties of the charging and discharging of interface states, and several possibilities of using this method are shown. The results obtained in SiO2/Si and Si3N4/SiO2/Si samples are in agreement with the existence of a spatial and energy distribution of interface states within the insulator. From the experimental data, the concentration of traps within the insulator at 35 A is estimated to be 5×109 cm-2 eV-1, with a tunneling cross section ∼10-19 cm2, at Ec-E ≈ 0.2 eV.

Optical Assessment of the Interaction Between EL2 and EL6 Levels in Boron Implanted GaAs

The majority optical cross section and the concentration of levels with optical response induced by boron implantation at 1010 ions/cm2 in GaAs, have been obtained using the technique labelled Optical Isothermal Transient Spectroscopy, OITS, in an energy range between 0.8 eV and 1.4 eV. The experimental optical data measured at different annealing temperatures, show only the presence of the EL2 and EL6 electron trap levels. The optical behaviour found is in complete agreement with the existence of an interaction between these two deep levels, EL2 and EL6. Besides, this interaction has been found to be the origin of the U-band in the DLTS spectra and of the anomalous dependence of the level concentration on the annealing temperature deduced from the OITS and DLTS spectra.

Analysis of the edge region photocapacitance at constant bias in Te-doped Al0.55Ga0.45As

Abstract An analysis of the edge region photocapacitance at constant bias was used to determine the optical cross section of the DX center in Te-doped Al0.55Ga0.45As. The values obtained, at temperatures higher than those used by the conventional photocapacitance method, agreed with the large lattice relaxation model. The configurational diagram was prepared considering the effects of a high ratio between trap and shallow concentrations, as well as the electric field influence on emission rates. The data obtained allowed us to reaffirm that the DX center is lied to L conduction band minima.

Boron implantation influence on the backgating effect in GaAs MESFETs

Abstract An analysis of the electrical isolation between electrodes belonging to neighbour GaAs MESFET devices is reported. The isolation has been obtained by using single or double boron implantation. The results indicate that the leakage or backgate current is determined by the state of the semi-insulating GaAs surface layer and by the concentration of deep levels located in this layer, which depends on the implantation dose used. The dependence of this current on the implantation dose, contacts distance, annealing temperature and measurement temperature is discussed. Moreover, the threshold voltage for the increase of this current has been found to be directly related to the MESFET backgating threshold voltage.