High-resolution Laplace Deep Level Transient Spectroscopy Research Articles

Alloy splitting of the FeGa acceptor level in dilute AlxGa1−xN

The results of conventional deep-level transient spectroscopy (DLTS) and high-resolution Laplace DLTS measurements of the FeGa(0/−) acceptor level in dilute AlxGa1−xN layers (x ≤ 0.05) grown by MOVPE technique (metal–organic vapor phase epitaxy) on native ammono-GaN substrates are analyzed and discussed. It is shown that the electron emission signal related to the FeGa acceptor level in AlxGa1−xN splits into individual components due to aluminum fluctuations in the second-nearest neighbor (2NN) shell around the FeGa impurity atoms. The calculations of the probability of finding a given number of aluminum atoms in the 2NN shell of the FeGa defect agree well with the experimental concentrations determined from Laplace DLTS peak intensities. This finding shows that in dilute AlxGa1−xN layers grown by MOVPE, aluminum and iron atoms are randomly distributed in the material. Finally, we demonstrate that the energy level of the FeGa acceptor with no Al atoms in the 2NN shell in the AlxGa1−xN samples shifts linearly with the aluminum content and the shifts are 28 and 55 meV relative to that in GaN for the samples with x = 0.025 and 0.05, respectively.

M-Center in Neutron-Irradiated 4H-SiC

We report on the metastable defects introduced in the n-type 4H-SiC material by epithermal and fast neutron irradiation. The epithermal and fast neutron irradiation defects in 4H-SiC are much less explored compared to electron or proton irradiation-induced defects. In addition to the carbon vacancy (Vc), silicon vacancy (Vsi) and carbon antisite-carbon vacancy (CAV) complex, the neutron irradiation has introduced four deep-level defects, all arising from the metastable defect, the M-center. The metastable deep-level defects were investigated by deep level transient spectroscopy (DLTS), high-resolution Laplace DLTS (L-DLTS) and isothermal DLTS. The existence of the fourth deep-level defect, M4, recently observed in ion-implanted 4H-SiC, has been additionally confirmed in neutron-irradiated samples. The isothermal DLTS technique has been proven as a useful tool for studying the metastable defects.

Origin and anomalous behavior of dominant defects in 4H-SiC studied by conventional and Laplace deep level transient spectroscopy

Several deep level defects were observed by conventional deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS (LDLTS) in n-type 4H-SiC junction barrier Schottky diodes. We have shown that the broad DLTS peak labeled Z1/2 has, in fact, two components, Z1 and Z2, with activation enthalpies for electron emission of 0.63 eV and 0.68 eV, respectively. The reorientation process between these two components was observed. A combination of double-correlated DLTS and LDLTS demonstrated an anomalous reduction of the emission rate and an increase of the activation enthalpy of Z2 with an increase of the reverse bias applied to the diode. The possible explanation of this phenomenon could be correlated with a tensile stress in epitaxial SiC layers. The results observed are discussed in the frame of the model that correlates Z1 and Z2 with carbon vacancies (VC), located at hexagonal (h) and cubic (k) lattice sites, respectively. We also discussed the origin of other traps E0–E5 with particular emphasis on a N-related shallow donor level located at 0.04 eV below the conduction band, which has never been previously reported by DLTS studies.

Laplace DLTS study of the fine structure and metastability of the radiation-induced E3 defect level in GaAs

In this paper we used high-resolution Laplace deep-level transient spectroscopy (DLTS) to study the electrical properties of the E3 defect family introduced in GaAs by MeV electron irradiation. We found that the peak conventionally referred to as the E3 contained 3 components which we labeled E3a, E3b and E3c, with E3a being the most prominent component. The activation energy of E3a for different carrier densities varied between 0.36 and 0.375 eV. From dopant dependent introduction rate measurements, we found that the introduction rates of the E3a and E3b defect did not depend on doping density. However, the E3c concentration increased with increasing carrier density. Furthermore, the E3c was found to be metastable: it was reversibly removed during minority carrier injection at low temperatures and re-introduced by annealing above 160 K under zero bias. Interestingly, annealing measurements revealed that, of the three components, the E3b annealed first at around 500 K while both the other components annealed together at a higher temperature of 525 K. By comparing our results with previous studies, we concluded that the origin of E3a is VAs, E3b is Asi and E3c is VGa–SiGa.

(Invited) Deep-Level Analysis of Passivation of Transition Metal Impurities in Silicon

Experimental results on the interactions of transition metal (TM) impurities with hydrogen and on passivation by the displacement of the metals from interstitial to substitutional sites in crystalline silicon will be presented in this talk. Iron, titanium and vanadium from the 3d series, and Zr and Mo from the 4d series have been studied. The results have been obtained by means of deep level transient spectroscopy (DLTS), high resolution Laplace DLTS, minority carrier transient spectroscopy and secondary ion mass spectroscopy. The 3d metals are the most important TM series in relation to recombination in silicon. The most recombination active site is with the 3d metal in an interstitial position. It is found that in the presence of excess vacancies 3d TMs tend to move to substitutional sites and are quite stable in this position with no or much reduced recombination activity and much reduced diffusion. The experimental results on passivation of TMs with hydrogen show that in general the reactions of H atoms with TM impurities are much more effective in n-type material than in p-type Si. These findings can be easily explained considering charge states of the interacting species. Essentially the initial H-TM reaction is driven coulombically at equilibrium conditions. We have managed to promote reactions of hydrogen atoms with Fe, Mo and V impurities in p-type silicon by changing charge states of the interacting defects either by shifting the Fermi level upon the application of reverse bias to Schottky barrier diodes or by illumination of the contaminated and hydrogenated Si samples with the use of light emitting diodes with photon energies above the Si band gap energy. It has been found that the binding of hydrogen to the metal atoms, in all cases we have examined, is rather weak … of the order of 2 eV. This implies that the passivated fraction of the TMs resulting from the reaction of hydrogen with metals in bulk silicon is diminishingly small at device processing temperatures. Based on the results obtained we try to understand some of the previously published data on lifetime improvement in Si crystals by hydrogenation. In some of the commercially important cases, particularly those reporting on multi-crystalline slices, this is far from simple as other important effects such as enhanced diffusion, precipitation and gettering can take place during hydrogenation simultaneously with hydrogen-metal reactions. It appears that in some cases hydrogen-metal reactions does not seem to be the most important processes in enhancing the minority carrier lifetime of the silicon.

Tutorial: Junction spectroscopy techniques and deep-level defects in semiconductors

The term junction spectroscopy embraces a wide range of techniques used to explore the properties of semiconductor materials and semiconductor devices. In this tutorial review, we describe the most widely used junction spectroscopy approaches for characterizing deep-level defects in semiconductors and present some of the early work on which the principles of today's methodology are based. We outline ab-initio calculations of defect properties and give examples of how density functional theory in conjunction with formation energy and marker methods can be used to guide the interpretation of experimental results. We review recombination, generation, and trapping of charge carriers associated with defects. We consider thermally driven emission and capture and describe the techniques of Deep Level Transient Spectroscopy (DLTS), high resolution Laplace DLTS, admittance spectroscopy, and scanning DLTS. For the study of minority carrier related processes and wide gap materials, we consider Minority Carrier Transient Spectroscopy (MCTS), Optical DLTS, and deep level optical transient spectroscopy together with some of their many variants. Capacitance, current, and conductance measurements enable carrier exchange processes associated with the defects to be detected. We explain how these methods are used in order to understand the behaviour of point defects and the determination of charge states and negative-U (Hubbard correlation energy) behaviour. We provide, or reference, examples from a wide range of materials including Si, SiGe, GaAs, GaP, GaN, InGaN, InAlN, and ZnO.

Properties of a previously unobserved donor-related electrically active defect in Ge induced by alpha particle irradiation

Alpha particle irradiation was used to study the radiation-induced defects in n-type germanium (Ge). Investigation of the well-known antimony (Sb)-vacancy complex (commonly known as the E-center) in Ge, with an activation energy of 0.37eV (E0.37), has led to the discovery of another defect with a DLTS signature virtually indistinguishable from the E-center, but with different annealing characteristics. We shall refer to this new defect as the E-prime. Although the two defects are easily distinguishable by annealing, the DLTS signal produced by the E-center and E-prime were not distinguishable through conventional deep level transient spectroscopy (DLTS). Separation of the two peaks was only possible through the use of low noise equipment in conjunction with high resolution Laplace-DLTS. The activation energy of the Sb-vacancy and the E-prime was determined to be 0.370±0.005eV and 0.375±0.005eV. Depth profiles showed uniform distributions of both defects below the Schottky junction.

Electrical characterization of electron irradiated and annealed lowly-doped 4H-SiC

The effect of high energy electron (HEE) irradiation on nickel Schottky contacts fabricated on lowly-doped n-type 4H-SiC was investigated by deep level transient spectroscopy (DLTS) and high resolution Laplace-DLTS. The Schottky contacts were deposited by resistive evaporation of nickel and were observed to be of good rectification quality from current-voltage measurements. DLTS was performed up to 350K to investigate the presence of defects before and after HEE irradiation. HEE irradiation was observed to induce three deep level defects below 350K at 0.42eV, 0.62eV and 0.76eV below the conduction band minimum. These deep level defects are labelled E0.42, E0.62 and E0.76. Defects E0.42 and E0.76 were observed after the same electron fluence and were annealed out at the same temperature, suggesting that the defects could be strongly related. The effect of HEE irradiation and annealing on as-grown defects was also investigated and is reported.

Characterization of deep-level defects in GaNAs/GaAs heterostructures grown by APMOVPE

AbstractConventional deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS techniques were used to study electrical properties of deep-level defects in dilute GaNAs epitaxial layers grown by atmospheric-pressure metalorganic vapourphase epitaxy (APMOVPE) on the GaAs substrate. Three samples with nitrogen concentrations of 1.2 %, 1.6 % and 2.7 % were investigated. In DLTS and LDLTS spectra of the samples, four predominant electron traps were observed. On the basis of the obtained electrical parameters and previously published results, one of the traps was associated with N-related complex defects, while the other traps with common GaAs-like native defects and impurities, called EL6, EL3 and EL2.

Electrical properties of p-ZnTe/n+-GaAs junctions grown at different Te/Zn beam equivalent pressure ratios by molecular beam epitaxy

Electrical properties of undoped and N-doped p-ZnTe/n+-GaAs diodes grown by molecular beam epitaxy at different Te/Zn beam equivalent pressure (BEP) ratios were investigated. For undoped p-ZnTe layers, the diodes exhibited pronounced rectifying features at room temperature, with a rectification ratio of more than 104 and leakage currents of about 10−6 A/cm2. The leakage current increased in samples grown with a higher Te/Zn BEP ratio, and a higher net free carrier concentration, which can be correlated with the presence of zinc vacancies, was observed in such samples. Intentional doping of p-type ZnTe by nitrogen further increased the net free carrier concentration and leakage currents in the structures. This effect can be ascribed to the formation of NTe defects acting as shallow acceptors in ZnTe. Deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS studies of undoped p-ZnTe/n+-GaAs diodes showed two dominant peaks H330 and H330′ in samples grown with the Te/Zn BEP ratio below 2. We tentatively assigned H330 to an oxygen-related defect, which also resulted in a broad photoluminescence band at about 1.78 eV.

Optical and Electrical Study of Defects in GaN In Situ Doped with Eu3+ Ion Grown by OMVPE

A silicon/europium-codoped GaN epilayer (GaN:Si,Eu3+) and a control sample (GaN:Si) have been investigated using optical spectroscopy and various deep-level transient spectroscopy (DLTS) techniques. Three electron traps in GaN:Si,Eu3+, as well as one electron trap and one hole trap in GaN:Si, were observed in the temperature range from 35 K to 400 K by DLTS. High-resolution Laplace DLTS revealed multiple, closely spaced defect energy levels associated with trap B of GaN:Si,Eu3+ and trap D of GaN:Si, respectively. Minority-carrier transient spectroscopy (MCTS) measurements revealed a shallow hole trap in GaN:Si,Eu3+, whose presence was validated using the rare-earth-structured isovalent (RESI) hole trap model. The identified DLTS energy traps were compared with other energy traps reported in literature. Also, the energy transfer mechanism between the GaN host and Eu3+ ion was considered and discussed using the combination of electrical and optical studies reported here and in literature.

Study of Defects in GaN In Situ Doped with Eu3+ Ion Grown by OMVPE

In this work, GaN epilayer in situ doped with Eu3+ ions was deposited on the top of an undoped n-GaN/LT-GaN/sapphire structure by organometallic vapor-phase epitaxy. A set of different ohmic and Schottky contacts on GaN:Eu3+ surface was fabricated by electron-beam evaporation for performing deep level transient spectroscopy (DLTS) measurement. The deep defect energy levels in GaN:Eu3+ epilayer were assessed by standard DLTS and high resolution Laplace DLTS (L-DLTS). Three dominant DLTS peaks were observed in the temperature range from 35 K to 400 K. The calculated activation energies of these defect energy levels were 0.108 ± 0.03 eV (Trap A), 0.287 ± 0.04 eV (Trap B) and 0.485 ± 0.06 eV (Trap C) below conduction band edge, respectively. High resolution L-DLTS conducted for the GaN:Eu3+ epilayer revealed at least four closely spaced defect energy levels associated with Trap B. It is proposed that these defect energy levels correspond to the selected optically active Eu3+ ion centers in GaN host previously identified by optical studies in this material (Fujiwara and Dierolf in Jpn J Appl Phys 53:05FA13, 2014).

Electrical characterization of deep levels created by bombarding nitrogen-doped 4H-SiC with alpha-particle irradiation

Deep-level transient spectroscopy (DLTS) and Laplace-DLTS were used to investigate the effect of alpha-particle irradiation on the electrical properties of nitrogen-doped 4H-SiC. The samples were bombarded with alpha-particles at room temperature (300K) using an americium-241 (241Am) radionuclide source. DLTS revealed the presence of four deep levels in the as-grown samples, E0.09, E0.11, E0.16 and E0.65. After irradiation with a fluence of 4.1×1010 alpha-particles-cm−2, DLTS measurements indicated the presence of two new deep levels, E0.39 and E0.62 with energy levels, EC – 0.39eV and EC – 0.62eV, with an apparent capture cross sections of 2×10−16 and 2×10−14cm2, respectively. Furthermore, irradiation with fluence of 8.9×1010 alpha-particles-cm−2 resulted in the disappearance of shallow defects due to a lowering of the Fermi level. These defects re-appeared after annealing at 300°C for 20min. Defects, E0.39 and E0.42 with close emission rates were attributed to silicon or carbon vacancy and could only be separated by using high resolution Laplace-DLTS. The DLTS peaks at EC – (0.55–0.70)eV (known as Z1/Z2) were attributed to an isolated carbon vacancy (VC).

Carbon-Hydrogen Complexes in <i>n</i>- and <i>p</i>-Type SiGe-Alloys Studied by Laplace Deep Level Transient Spectroscopy

A deep level transient spectroscopy (DLTS) study on n- and p-type diluted Si1-xGex alloys (x=0, 0.011, 0.026, 0.046, and 0.070) is presented. Defect levels of several carbon-hydrogen (CH) complexes are observed. The high-resolution Laplace-DLTS technique allows us to detect configurations of defects which contain different numbers of Ge atoms in the first and second-nearest neighbourhood of the CH complexes. The electrical properties of the defects will be analysed and their origin will be discussed.

Electrical Characterisation of electron beam exposure induced Defects in silicon

The defects introduced in epitaxially grown p-type silicon (Si) during electron beam exposure were electrically characterised using deep level transient spectroscopy (DLTS) and high resolution Laplace-DLTS. In this process, Si samples were first exposed to the conditions of electron beam deposition (EBD) without metal deposition. This is called electron beam exposure (EBE) herein. After 50minutes of EBE, nickel (Ni) Schottky contacts were fabricated using the resistive deposition method. The defect level observed using the Ni contacts had an activation energy of H(0.55). This defect has an activation energy similar to that of the I-defect. The defect level is similar to that of the HB4, a boron related defect. DLTS depth profiling revealed that H(0.55) could be detected up to a depth of 0.8μm below the junction. We found that exposing the samples to EBD conditions without metal deposition introduced a defect which was not introduced by the EBD method. We also observed that the damage caused by EBE extended deeper into the material compared to that caused by EBD.

Isolated Ti in Si: Deep level transient spectroscopy, minority carrier transient spectroscopy, and high-resolution Laplace deep level transient spectroscopy studies

Combining deep level transient spectroscopy (DLTS), high-resolution Laplace DLTS, and minority carrier transient spectroscopy studies, we question the identification of the dominant Ti-related defects introducing deep levels (E40, E150, and H180) in n- and p-type Si. The observed results cannot unambiguously support the models previously reported for these defects. The presence of the Poole-Frenkel effect describing the enhancement of the emission rates of E40 as a function of electric field is not consistent with the previous assignment of this defect to the single acceptor, whereas the absence of the enhancement of the emission rate of E150 under different reverse bias applied to the diode does not confirm the previous attribution of this defect to the single donor in n-type Si. The attribution of H180 to the double donor is in good agreement with our results. In contrast, the identical depth profiles obtained for E40 and E150 in bulk of as-grown, hydrogenated and annealed samples cannot be explained by the assignment of these levels to different defects.

Correlation between barrier inhomogeneities of 4H-SiC 1 A/600 V Schottky rectifiers and deep-level defects revealed by DLTS and Laplace DLTS

Electrical properties of commercial silicon carbide (SiC) Schottky rectifiers are investigated through the measurement and analysis of the forward current–voltage (I–V) and reverse capacitance–voltage (C–V) characteristics in a large temperature range. Some of devices show distinct discrepancies in specific ranges of their electrical characteristics, especially the excess current dominates at voltage<1V and temperature<300K. Standard deep level transient spectroscopy (DLTS) revealed the presence of a single deep-level defect with activation energy of about 0.3eV, exhibiting the features characteristic for extended defects (e.g. dislocations), such as logarithmic capture kinetics. Furthermore, high-resolution Laplace DLTS showed that this deep level consists actually of three closely spaced levels with activation energies ranging from about 0.26eV to 0.29eV. A strong correlation between these two techniques implies that the revealed trap level is due to extended defects surrounded by point traps or clusters of defects. On the basis of obtained specific features of the deep-level defect, it was proposed that this defect is arguably responsible for the observed Schottky barrier inhomogeneities.

Zinc oxide thin films on silicon carbide substrates (ZnO/SiC): electro-optical properties and electrically active defects

The electrical and optical properties of heterojunctions formed by thermally deposited ZnO thin films on n-type 4H-SiC substrates have been investigated. Current–voltage characteristics of the fabricated light emitting devices revealed excellent rectifying behaviors with a typical leakage current lower than 1 nA at a reverse bias of −3 V, and with a forward current at 3 V in the range of 2 mA. A study of the electroluminescent characteristics of ZnO/SiC heterojunctions over the temperature range of 50–450 K showed an emission peak around 410 nm and a broad defect-related electroluminescence at room temperature in the visible range for a forward current of 300 mA. Electrically active deep level centers in ZnO and n-type 4H-SiC epilayers have been investigated by deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS (LDLTS). Additionally, LDLTS has successfully been employed to resolve the closely spaced hole trap energy levels.

Donor levels of the divacancy-oxygen defect in silicon

The elimination of divacancies (V2) upon isochronal and isothermal annealing has been studied in oxygen-rich p-type silicon by means of deep level transient spectroscopy (DLTS) and high resolution Laplace DLTS. Divacancies were introduced into the crystals by irradiation with 4 or 6 MeV electrons. The temperature range of the divacancy disappearance was found to be 225-300 °C upon 30 min isochronal annealing in the samples studied. A clear anti-correlation between the disappearance of V2 and the appearance of two hole traps with activation energies for hole emission of 0.23 eV and 0.08 eV was observed. It is argued that these traps are related to the first and second donor levels of the divacancy-oxygen (V2O) complex, respectively. Significant electric field enhancement of the hole emission from the second donor level of the V2O center occurred in the diodes studied. It is shown that in the range of electric field from 4 × 103 to 1.2 × 104 V/cm the emission enhancement is associated with phonon-assisted tunnelling.

Transition metals (Ti and Co) in silicon and their complexes with hydrogen: A Laplace DLTS study

The electrical properties of transition metals (TM) in Si were often determined by means of the conventional deep level transient spectroscopy (DLTS) technique. In the present study we demonstrate that the poor resolution of the conventional DLTS technique could lead to the wrong interpretation of the experimental data. On the example of well-known lifetime killers such as titanium and cobalt we show that the dominant defects appeared in samples doped with these impurities were previously wrongly attributed to different charged states of interstitial Ti and substitutional Co. Applying the high-resolution Laplace DLTS technique we re-examine the origin of these defects. Some of these defects will be shown to react with H and shallow acceptors leading to the appearance of new complex defects in the band gap of Si. Their nature will also be discussed in the present paper.