Deep Level Centers Research Articles

Nature of low-energy optical emission in doped AlGaN∕GaN heterostructures

Photoluminescence (PL) in modulation-doped and nominally undoped AlxGa1−xN∕GaN heterostructures was studied and compared with PL spectra of GaN films grown on sapphire substrates. It is demonstrated that optical emission in the energy range of 3.3–3.46eV related to the two-dimensional electron gas radiative processes can be completely suppressed in modulation-doped AlxGa1−xN∕GaN heterostructures. Instead of this, an intense broad long-wavelength emission attributed to the recombination of donor-acceptor pairs in the lower energy range of 2.7–3.3eV is revealed. This spectral transformation is explained by the presence of deep-level defect-related acceptor centers in AlxGa1−xN∕GaN heterostructures introduced at the modulation doping of the AlxGa1−xN barrier layer.

Ab initiosupercell calculations on aluminum-related defects in SiC

Ab initio supercell calculations of the binding energies predict complex formation between aluminum and carbon interstitials in SiC. In high-energy implanted SiC aluminum acceptor can form very stable complexes with two carbon interstitials. We also show that carbon vacancy can be attached to shallow aluminum acceptor. All of these defects produce deep levels in the band gap. The possible relation of these defects to the recently found aluminum-related deep-level transient spectroscopy centers is discussed. © 2007 The American Physical Society.

Discovery of classes among deep level centers in gallium arsenide

By using thermal electron emission data from the literature in addition to own measured results for totally 40 data sets of deep level centers in GaAs, we demonstrate that they can be divided into two different classes. We find that all the centers of this population, except one, can be interpreted as following the Meyer–Neldel rule. Under this condition, the magnitude of the matrix element determining their capture cross section is the most probable partitioning mechanism.

About the Nature of Recombination Current in 4H-SiC pn Structures

The forward current was investigated in 4H-SiC p+n structures grown by sublimation epitaxy. The doping level, Nd-Na, of the n-layer was about (3-4)x1016 cm-3 and the diode area was in the range from 1x10-5 to 2x10-4 cm2. The observed current can be considered as current due to bulk recombination in the space charge region of the pn junction via deep level center or due to surface recombination. The criterion which was performed in this study to differentiate such currents was the investigation of recombination current versus perimeter/area ratio dependence. It was found that no pronounced difference in the recombination current parameters for diodes with different perimeter/area ratio was observed, i.e. current due to surface recombination was not observed for the 4H-SiC pn structures investigated.

Probing deep level centers in GaN epilayers with variable-frequency capacitance-voltage characteristics of Au∕GaN Schottky contacts

Under identical preparation conditions, Au∕GaN Schottky contacts were prepared on two kinds of GaN epilayers with significantly different background electron concentrations and mobility as well as yellow emission intensities. Current-voltage (I-V) and variable-frequency capacitance-voltage (C-V) characteristics show that the Schottky contacts on the GaN epilayer with a higher background carrier concentration and strong yellow emission exhibit anomalous reverse-bias I-V and C-V characteristics. This is attributed to the presence of deep level centers. Theoretical simulation of the low-frequency C-V curves leads to a determination of the density and energy level position of the deep centers.

Effect of the levels of intrinsic defects in the CdP2 band gap on electrical characteristics of corresponding structures with the Schottky barrier

The electrical characteristics of Schottky barriers formed on n-type cadmium diphosphide are studied. It is established that the space-charge region at the metal-semiconductor interface represents in fact a Schottky layer formed owing to a high concentration of deep-level centers. The charge transport in the conducting direction for these structures is related to the above-barrier emission of electrons and is consistent with the diffusion theory for one or two types of charge carriers. The high concentration of ionized centers in the space-charge region gives rise to the tunneling mechanism of breakdown in the blocking direction. The frequency dependences of the complex conductance are governed by the exchange of charge carriers between the conduction band and donors that specify the conductivity type of the material and also by the recharing of the centers with a large depth of levels. Good agreement between the reported results and the theory is obtained.

Deep level centers in InGaN/GaN heterostructure grown on sapphire and free-standing GaN

Deep level transient spectroscopy has been used to characterize the deep levels in InGaN/GaN grown on sapphire substrate as well as on free-standing GaN. The deep levels at E c − E t ∼ 0.17–0.23 eV and E c − E t ∼ 0.58–0.62 eV have been detected in our samples which are present in GaN samples reported by others. These two deep levels have been attributed by us to threading dislocations as they exhibit logarithmic capture kinetic behavior and are found to be substantially reduced in its trap concentration (∼ from 10 14 to 10 12 cm − 2 ) in GaN grown on free-standing GaN template. Other than the two deep levels, an additional level at E c − E t ∼ 0.40–0.42 eV has been identified in both samples, which is believed to be related to In segregation. AFM image shows region of pits formation in InGaN epilayer for sample grown on u-GaN using sapphire substrate while the latter gives a much smoother morphology. From the X-ray diffraction space mapping, the mosaicity of the sample structure for both samples were studied. Dislocations do not play a significant role in the structural properties of InGaN grown on free-standing GaN since the FWHM based on the Δ ω is relatively small (± 0.15°) in the case of InGaN/GaN on free-standing GaN substrate as compared to that on sapphire (± 0.35°). The wider spread in Δ ω–2 θ value for InGaN layer on free-standing GaN also suggested the effect of compositional pulling with increasing InGaN layer thickness.

Effect of stationary ionization of traps near the midgap on the spectrum of thermally stimulated capacitance of semiconductor devices

An algorithm for calculating the parameters of deep-level centers in the space-charge region of semiconductor devices on the basis of an analysis of the curves of thermally stimulated capacitance in the presence of recombination centers with the levels located near the midgap is suggested. The method is tested using the commercial AL-107 GaAs-based light-emitting diodes. The results of calculations of the deep-center parameters in the materials under study are consistent with the results of other methods of analysis.

Numerical simulation of radiation damage effects in p-type silicon detectors

Abstract In the framework of the CERN-RD50 Collaboration, the adoption of p-type substrates has been proposed as a suitable approach to optimize the long-term radiation hardness of silicon detectors. In this work, we present a numerical model for the simulation of radiation damage effects in p-type silicon, developed within the general-purpose device simulator DESSIS. The model includes radiation-induced deep-level recombination centers in the semiconductor band-gap and the Shockley–Read–Hall statistics. In particular, two deep-level defects have been introduced: one located at EC−0.42 eV, corresponding to a single charge state divacancy and a second one located at EC−0.46 eV, corresponding to a single charge state tri-vacancy. For simulation purposes we have considered a simple, two-dimensional test structure, consisting of a single diode of 40 μm width and 300 μm depth, surrounded by a 6 μm wide guard ring. The n+ implant region depth is 1 μm, with donor concentration of ND=1018 cm−3 implanted on a high-resistivity p-type substrate (NA=5×1012 cm−3). The results of simulations adopting the proposed radiation damage model for p-type substrate have been compared with experimental measurements carried out on similar test structures irradiated with neutrons at high fluence. A good agreement with the experimental data has been obtained for the depletion voltage and diode leakage current. The simulated current damage constant (α=3.75×10−17 A cm−1) is in satisfactory agreement with values reported in the literature. A preliminary study of charge collection efficiency as a function of the fluence is also reported.

Investigation of The Electrical and Chemical Properties of Plasma-Treated AlGaN

ABSTRACTThe surface properties of n-type AlxGa1−xN (x=0-0.5) exposed to inductively couple plasma were studied systematically using metal contact measurements and X-ray photoelectron spectroscopy (XPS). Cl2/BCl3 and Ar plasma treatment considerably increased the surface conductivity of AlxGa1−xN (x<0.3) and thus improved the characteristics of Ti/Al-based ohmic contacts, but degraded the surface and contact properties of AlxGa1−xN (x≥0.3). XPS measurements revealed a significant increase in oxygen incorporation and energy shifts of the core-level spectra in plasma-treated samples. In contrast to an upward shift of the surface Fermi level in plasma-treated GaN, the surface Fermi level moved away from the conduction band in Al0.5Ga0.5N upon plasma treatment. These findings suggest that the nature of plasma damage in AlGaN appears to be a function of the Al mole fraction. Plasma-induced defects in high-Al AlGaN may act as deep-level compensation centers, degrading the surface electrical properties and contact characteristics.

Field-enhanced ionization of deep-level centers as a triggering mechanism for superfast impact ionization fronts in Si structures

We investigate the origin of free carriers that initiate impact ionization in depleted high-voltage p-n junctions under dynamic breakdown conditions and deterministically trigger superfast ionization fronts that propagate several times faster than the saturated drift velocity. We argue that in Si structures triggering occurs due to the field-enhanced ionization of process-induced deep-level centers identified as sulfur impurities. This impurity is a double-level electron trap with low recombination activity. It is present in high-voltage Si structures due to the side effect of widely used fabrication technology. We calculate the field and temperature dependences of the ionization probability for the upper midgap level (0.28eV) and midgap level (0.54eV) in electric fields up to 5×105V∕cm as well as the occupation of these levels at different temperatures. The emission of free electrons is sufficient to trigger the ionization front from zero temperature to ∼400K, in agreement with experiments. At room temperature the front is triggered due to the phonon-assisted tunneling from the midgap level with an ionization energy of 0.54eV. For temperatures below 200K all double-level centers are in the ground state and the front is triggered due to the direct tunneling from the upper midgap level with an ionization energy of 0.28eV.

Deep-level defects introduced by 1 MeV electron radiation in AlInGaP for multijunction space solar cells

Presented in this paper are 1 MeV electron irradiation effects on wide-band-gap (1.97 eV) (Al0.08Ga0.92)0.52In0.48P diodes and solar cells. The carrier removal rate estimated in p-AlInGaP with electron fluence is about 1cm−1, which is lower than that in InP and GaAs. From high-temperature deep-level transient spectroscopy measurements, a deep-level defect center such as majority-carrier (hole) trap H2 (Eν+0.90±0.05eV) was observed. The changes in carrier concentrations (Δp) and trap densities as a function of electron fluence were compared, and as a result the total introduction rate, 0.39cm−1, of majority-carrier trap centers (H1 and H2) is different from the carrier removal rate, 1cm−1, in p-AlInGaP. From the minority-carrier injection annealing (100mA∕cm2), the annealing activation energy of H2 defect is ΔE=0.60eV, which is likely to be associated with a vacancy-phosphorus Frenkel pair (Vp-Pi). The recovery of defect concentration and carrier concentration in the irradiated p-AlInGaP by injection relates that a deep-level defect H2 acts as a recombination center as well as compensator center.

Deep levels induced by InAs/GaAs quantum dots

Self-organised InAs/GaAs quantum dots (QDs) were formed by molecular beam epitaxy using the Stranski–Krastanov growth mode. Deep-level transient spectroscopy as well as secondary ion mass spectrometry have been used to characterise structures containing the QDs. DLTS depth profiling procedures indicate that deep level-related defects are localised in GaAs in the vicinity of the QD plane. For the first time, we report the presence of a deep level-related trap with an extremely high thermal activation energy of E c − 1.03 eV. An electron trap at E c − 0.78 eV can be identified as the well-known level related to the EL2 family. We conclude that a third trap revealed at E c −0.57 eV is the familiar PL killer related to the intrinsic point defect-oxygen complex. The latter is confirmed by results of the SIMS study, which indicates that the amount of oxygen accumulated at the InAs/GaAs heterointerface is increased. This paper demonstrates that the EL2 and oxygen-related deep-level centers occur by the presence of InAs/GaAs QDs. We present the hypothesis that deep states could be a factor limiting the efficiency of QD-based devices.

Dynamic avalanche breakdown of a p-n junction: Deterministic triggering of a plane streamer front

We discuss the dynamic impact ionization breakdown of a high voltage p-n junction which occurs when the electric field is increased above the threshold of avalanche impact ionization on a time scale smaller than the inverse thermogeneration rate. The avalanche-to-streamer transition characterized by generation of dense electron-hole plasma capable of screening the applied external electric field occurs in such regimes. We argue that the experimentally observed deterministic triggering of the plane streamer front at the electric-field strength above the threshold of avalanche impact ionization, yet below the threshold of band-to-band tunneling, is generally caused by field-enhanced ionization of deep-level centers. We suggest that the process-induced sulfur centers and native defects such as EL2, HB2, and HB5 centers initiate the front in Si and GaAs structures, respectively. In deep-level-free structures the plane streamer front is triggered by Zener band-to-band tunneling.

Observation of Deep Level Centers in 4H and 6H Silicon Carbide Metal Oxide Semiconductor Field Effect Transistors

Utilizing an very sensitive electron spin resonance (ESR) technique, spin dependent recombination (SDR) we have identified interface and near interface trapping centers in 4H and 6H SiC/SiO2 metal oxide semiconductor field effect transistors (MOSFETs). We extend our group’s earlier observations on 6H devices to the more technologically important 4H system and find that several centers can play important roles in limiting the performance of SiC based MOSFETs.

Deep‐level transient conductance spectroscopy of high resistivity semiconductors

We describe a deep-level transient-conductance (DLTC) spectrometer for high resistivity semiconductors, which uses a radiofrequency (∼40 MHz) marginal oscillator as conductance detector. The DLTC spectra are generated by periodically filling the deep-level trapping centres by carriers stimulated by a pulsed GaAs laser. Then the trap-emptying conductance's signal process through an exponential Miller correlator as the sample temperature is slowly ramped. A simple capacitive coupling of the samples to the oscillator tank circuit eliminates problems such as unwanted defect annealing and other material changes often associated with the high-temperature techniques necessary for ohmic contact formation. Representative deep-level spectra for CdTe, CdZnTe, HgI and gamma-irradiated Si are given. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Selective detection of deep level centers by x-ray absorption fine structure questioned

A new scheme of capacitance-detection for the x-ray absorption fine structure (XAFS) was reported recently [M. Ishii, Phys. Rev. B 65, 085310 (2002)]. The absorption of x-rays leads to oscillations in the steady state capacitance of a Schottky barrier diode which were interpreted as a site-selective detection of deep level centers. We have performed XAFS at the K-edge of gallium and arsenic on GaAs and AlGaAs Schottky-diodes containing DX and EL2 defect centers. For the detection, different techniques were applied: the photocurrent under short circuit conditions, the steady state impedance, and x-ray induced capacitance transients were measured. The results are compared with conventionally detected XAFS spectra (absorption, fluorescence). No evidence for a site-selectivity of the capacitance EXAFS is found in our studies. Based on a detailed analysis of the different detection techniques we strongly question the proposed “site-selectivity”.

Structural defects at the semiconductor-ferroelectric interface

Structural defects at the interface between Pb0.95La0.05Ti0.8Zr0.2O3 and La1.85Sr0.15CuO4 were studied using the method of isothermal current relaxation. Two cases were considered: (a) the width of the defect-containing layer is much smaller than the width of the space-charge region and (b) the width of the defect-containing layer is greater than the width of the space-charge region. It is shown that, for the samples studied, the width of the defect-containing layer exceeds 50–100 A, and the density of states for deep-level centers in the energy interval Ev+(0.55–0.65) eV is about 3×1020 cm−3 eV−1; this value corresponds to a surface state density of about 2×1014 cm−2 eV−1. It is shown that the density of states for deep-level centers increases from the interface into the depth of the semiconductor.

Quantum dynamics of phonon assisted emission of carriers at deep level centers

A formalism is proposed to investigate quantum dynamics of localized states involving highly non-adiabatic time-evolution of electron–lattice systems. The effect of electron itinerancy is projected onto the dynamics of local variables through an integral kernel of Volterra's integral equation. The method is applied to the problem of thermal emission of carriers at deep level centers in semiconductors. It is shown that the real situation is in the adiabatic limit, and the probability of thermal emission of the trapped carriers is one per a single lattice oscillation, if the amplitude of the oscillation exceeds a critical value but zero if not.

Contribution of the disturbed dislocation slip planes to the electrical properties of plastically deformed silicon

The deep-level centers introduced by the plastic deformation at 680°C in p-type silicon are quantitatively studied by the electron beam induced current (EBIC) and DLTS techniques. It is shown that the DLTS signal is higher than that could be ascribed to the centers located at (or close to) dislocations. In opposite, the number of electrically active defects in the dislocation trails, which is estimated from the EBIC contrast, easily explains the DLTS signal. The possible nature of defects in the dislocation trails is discussed.