Minority Carrier Transient Spectroscopy Research Articles

Electrically active defects induced by thermal oxidation and post-oxidation annealing of n-type 4H-SiC

In this work, we have performed a detailed study of the defects created in the bulk of 4H-SiC after thermal oxidation and post oxidation annealing using deep level transient spectroscopy and minority carrier transient spectroscopy (MCTS). The study reveals the formation of several shallow and deep level majority carrier traps in the bandgap. The ON1 (EC−0.85 eV), ON2a (EC−1.05 eV), and ON2b (Ec−1.17 eV) levels are the most dominant and are observed across all the samples (EC denotes the conduction band edge). Three shallow levels Ti(k) (EC−0.17 eV), E0.23 (EC−0.23 eV), and C1/2 (EC−0.36/0.39 eV) are observed in the samples. For most of the majority carrier defects, the highest concentration is observed after an NO anneal at 1300°C. This behavior is sustained in the depth profile measurements where the defect concentration after the NO anneal at 1300°C is significantly higher than for the rest of the samples. The origin of most of the majority carrier defects has been attributed to C interstitial injection from the interface during thermal oxidation and annealing. MCTS measurements reveal two prominent minority carrier traps, labeled O0.17 (EV+0.17 eV) and B (EV+0.28 eV), where the concentration of O0.17 is independent of annealing parameters while the concentration of the B level increases after the NO anneal (EV denotes the valence band edge). Furthermore, the depth profiles of the defects are used to evaluate their diffusion parameters by solving the diffusion equation to fit the experimental profiles. The defect concentrations decay exponentially with depth, which evidences that the defects were created at or near the SiO2–SiC interface and migrate toward the bulk during oxidation and post-oxidation annealing.

Dual configuration of shallow acceptor levels in 4H-SiC

Acceptor dopants in 4H-SiC exhibit energy levels that are located deeper in the band gap than the thermal energy at room temperature (RT), resulting in incomplete ionization at RT. Therefore, a comprehensive understanding of the defect energetics and how the impurities are introduced into the material is imperative. Herein, we study impurity related defect levels in 4H-SiC epitaxial layers (epi-layers) grown by chemical vapor deposition (CVD) under various conditions using minority carrier transient spectroscopy (MCTS). We find two trap levels assigned to boron impurities, B and D, which are introduced to varying degrees depending on the growth conditions. A second acceptor level that was labeled X in the literature and attributed to impurity related defects is also observed. Importantly, both the B and X levels exhibit fine structure revealed by MCTS measurements. We attribute the fine structure to acceptor impurities at hexagonal and pseudo-cubic lattice sites in 4H-SiC, and tentatively assign the X peak to Al based on experimental findings and density functional theory calculations.

Investigation of carbon-related complexes in highly C-doped GaN grown by metalorganic vapor phase epitaxy

We investigated the C-related complexes in highly C-doped GaN by electron spin resonance (ESR) spectroscopy, Fourier transform IR spectroscopy (FTIR), and minority carrier transient spectroscopy (MCTS) measurements. In the ESR spectra, two resonances with g values of 2.02 and 2.04 were found to be assigned by (0/−) deep acceptor and (+/0) charge transition levels of carbon substituting for nitrogen site (CN). In the FTIR spectra, two local vibrational modes positioned at 1679 and 1718 cm−1 were confirmed to be associated with tri-carbon complexes of CN–CGa–CN (basal) and CN–CGa–CN (axial), respectively. In the MCTS spectra, we observed the hole trap level of E v + 0.25 ± 0.1 eV associated with the tri-carbon complexes, which are the dominant C-related defects, suggesting that these complexes affect the electronic properties in the highly C-doped GaN.

Impact of carbon injection in 4H-SiC on defect formation and minority carrier lifetime

Point defects in silicon carbide (SiC) can act as charge carrier traps and have a pronounced impact on material properties such as the mobility and carrier lifetime. Prominent among these traps is the carbon vacancy (VC) with a demonstrated detrimental effect on the minority carrier lifetime in 4H-SiC epitaxial layers. Hence, a variety of methods for VC removal have been proposed. Common to all is that they involve some sort of C-injection into the epi-layer that leads to annihilation of VC with C interstitials (Ci) via the reaction Ci＋ VC→0̸. However, many studies on injection of Ci for removal of VC do not take into account the potential effect of any additional defects that are formed as a result of excess C in the material, including electrically active defects that introduce energy levels in the SiC band gap. Herein, we study the formation and impact of carbon related defects that are introduced in 4H-SiC epi-layers by injection of excess carbon. C-injection is achieved by annealing 4H-SiC epi-layers covered by a graphitized photoresist known as a C-cap at 1250°C for different durations. The resulting appearance of defects in the samples is monitored using deep level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS) measurements, which reveal the formation of both minority and majority carrier traps as a result of the C-injection. Intriguingly, the injected carbon is also found to interact with a perennially present impurity in 4H-SiC epi-layers and a potentially lifetime limiting defect, namely boron. Furthermore, we monitor the minority carrier lifetime as a function of C-injection time and depth from the surface using cross-sectional time-resolved photoluminescence (TRPL) measurements. Both the defect distributions and the minority carrier lifetime are found to depend strongly on the duration of the C-cap anneal, with a marked depth dependence being present for all samples studied. The moderate temperature C-cap annealing treatment is proposed as a method for enhancing the carrier lifetime in n-type 4H-SiC epi-layers for power device applications.

Investigation of Electrically Active Defects in SiC Power Diodes Caused by Heavy Ion Irradiation

Deep level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS) are used to investigate electrically active defects in commercial SiC Schottky power diodes after heavy-ion microbeam irradiation at different voltages. The DLTS and MCTS spectra of pristine samples are analysed and compared to devices showing or not signatures of Single Event Leakage Current (SELC) degradation. An additional peak labelled ‘C’ with an activation energy of 0.17 eV below the conduction band edge is observed in the DLTS spectra of a sample degraded with SELC.

Minority Carrier Traps Induced by Neutron Reactions with 4H-SiC

This work presents the characterization of minority carrier traps in epitaxial n-type 4H-SiC after high fluence neutron irradiation using minority carrier transient spectroscopy (MCTS) in a temperature range of 20 K to 660 K. Three minority carrier trap levels are reported, labelled as X, B and Y, whose activation energies were estimated by Arrhenius analysis and where the B level is assigned to substitutional boron (BSi). The dynamic behaviour of the trap levels was studied by consecutive temperature scans.

Boron-Related Defects in N-Type 4H-SiC Schottky Barrier Diodes.

We report on boron-related defects in the low-doped n-type (nitrogen-doped) 4H-SiC semitransparent Schottky barrier diodes (SBDs) studied by minority carrier transient spectroscopy (MCTS). An unknown concentration of boron was introduced during chemical vapor deposition (CVD) crystal growth. Boron incorporation was found to lead to the appearance of at least two boron-related deep-level defects, namely, shallow (B) and deep boron (D-center), with concentrations as high as 1 × 1015 cm-3. Even though the boron concentration exceeded the nitrogen doping concentration by almost an order of magnitude, the steady-state electrical characteristics of the n-type 4H-SiC SBDs did not deteriorate.

The role of boron related defects in limiting charge carrier lifetime in 4H–SiC epitaxial layers

One of the main challenges in realizing 4H–SiC (silicon carbide)-based bipolar devices is the improvement of minority carrier lifetime in as-grown epitaxial layers. Although Z1/2 has been identified as the dominant carrier lifetime limiting defect, we report on B-related centers being another dominant source of recombination and acting as lifetime limiting defects in 4H–SiC epitaxial layers. Combining time-resolved photoluminescence (TRPL) measurement in near band edge emission and 530 nm, deep level transient spectroscopy, and minority carrier transient spectroscopy (MCTS), it was found that B related deep levels in the lower half of the bandgap are responsible for killing the minority carriers in n-type, 4H–SiC epitaxial layers when the concentration of Z1/2 is already low. The impact of these centers on the charge carrier dynamics is investigated by correlating the MCTS results with temperature-dependent TRPL decay measurements. It is shown that the influence of shallow B acceptors on the minority carrier lifetime becomes neutralized at temperatures above ∼422 K. Instead, the deep B related acceptor level, known as the D-center, remains active until temperatures above ∼570 K. Moreover, a correlation between the deep level concentrations, minority carrier lifetimes, and growth parameters indicates that intentional nitrogen doping hinders the formation of deep B acceptor levels. Furthermore, tuning growth parameters, including growth temperature and C/Si ratio, is shown to be crucial for improving the minority carrier lifetime in as-grown 4H–SiC epitaxial layers.

Comparison of the Properties of Defect States in Nitrogen‐Containing n‐ and p‐Type Float‐Zone Silicon: A Combined Deep‐Level Transient Spectroscopy and Minority‐Carrier Transient Spectroscopy Study

Defect states in nitrogen‐containing float‐zone silicon are investigated in both n‐ and p‐type materials using both deep‐level transient spectroscopy (DLTS) and minority‐carrier transient spectroscopy (MCTS). This enables a mapping of the defect landscape in the entire electronic bandgap and an investigation of whether the properties of the defects depend on the semiconductor type. Two defects, the E1/E2 pair and the E4/E6 pair, are investigated, and no evidence is found for the defect properties to depend on the semiconductor type.

Majority and Minority Charge Carrier Traps in n-Type 4H-SiC Studied by Junction Spectroscopy Techniques

In this review, we provide an overview of the most common majority and minority charge carrier traps in n-type 4H-SiC materials. We focus on the results obtained by different applications of junction spectroscopy techniques. The basic principles behind the most common junction spectroscopy techniques are given. These techniques, namely, deep-level transient spectroscopy (DLTS), Laplace DLTS (L-DLTS), and minority carrier transient spectroscopy (MCTS), have led to recent progress in identifying and better understanding the charge carrier traps in n-type 4H-SiC materials.

Minority Carrier Trap in n-Type 4H–SiC Schottky Barrier Diodes

We present preliminary results on minority carrier traps in as-grown n-type 4H–SiC Schottky barrier diodes. The minority carrier traps are crucial for charge trapping and recombination processes. In this study, minority carrier traps were investigated by means of minority carrier transient spectroscopy (MCTS) and high-resolution Laplace-MCTS measurements. A single minority carrier trap with its energy level position at Ev + 0.28 eV was detected and assigned to boron-related defects.

Combined DLTS/MCTS investigations of deep electrical levels of regular dislocation networks in silicon

Local electronic states of regular dislocation networks produced by n- and p-type silicon wafer bonding with different screw dislocation density were studied with deep-level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS). A drastic sadden changes of the electric level spectrum with increasing of dislocation density from two shallow bands located near the edges of valence and conduction bands towards two deep bands with energy positions about Ec - (0.22-0.26) eV and Ev + (0.4-0.53) eV were found. The origin of the electric level spectrum changes is ascribed to the changes of dislocation core structure from dissociated to perfect ones that occur when interdislocation distances became comparable with the dislocation equilibrium dissociation width. The obtained results correlate well with the results of recent studies of recombination activity of grain boundaries in mc-Si.

Deep defect levels in CuInSe2 single crystals using DLTS, MCTS and photoacoustic spectroscopy

In this paper, we investigate the presence of deep defect levels in as-grown p-conducting CuInSe2 single crystals using deep level transient spectroscopy, minority carrier transient spectroscopy and photoacoustic spectroscopy. The samples were cut from the middle of ingots grown by employing a vertical displaced Bridgman furnace. This material is attracting increasing interest in solar cells fabrication as the absorber layer. Aluminum Schottky contacts were deposited on the as-grown and etched material in an evaporation system pumped down to less than 10−6 mbar. Three majority carrier traps were observed using deep level transient spectroscopy, and one minority carrier trap was detected by minority carrier transient spectroscopy. The concentrations of the traps have been estimated. The results hence obtained are compared to deep defect levels observed in the measured spectral dependence of the samples prior to junction’s fabrication by using the technique of photoacoustic spectroscopy in the photon energy range 0.6 to 1.3 eV. The study shows a good agreement between the results of DLTS/MCTS to those of photoacoustic and to published data.

(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.

Thermally activated defects in float zone silicon: Effect of nitrogen on the introduction of deep level states

Float zone silicon (FZ-Si) is typically assumed to be an extremely high quality material, with high minority carrier lifetimes and low concentrations of recombination active defects. However, minority carrier lifetime in FZ-Si has previously been shown to be unstable following thermal treatments between 450 and 700 °C, with a range of unidentified deep level states being linked to reduced carrier lifetime. There are suspicions that nitrogen doping, which occurs from the growth atmosphere, and intrinsic point defects play a role in the degradation. This study aims to address this by using deep level transient spectroscopy (DLTS), minority carrier transient spectroscopy, Laplace DLTS, and photoluminescence lifetime measurements to study recombination active defects in nitrogen-doped and nitrogen-lean n-type FZ-Si samples. We find that nitrogen-doped samples experience increased degradation due to higher concentrations of deep level defects during thermal treatments compared to nitrogen-lean samples. In an attempt to explain this difference, in-diffusion of nickel has been used as a marker to demonstrate the existence of higher vacancy concentrations in the nitrogen-doped samples. The origin of the recombination active defects responsible for the thermally induced lifetime degradation in FZ-Si crystals is discussed.

Accurate method for estimating hole trap concentration in n-type GaN via minority carrier transient spectroscopy

We propose an analysis method for the accurate estimation of the hole trap (H1, EV + 0.85 eV) concentration in n-type GaN via minority carrier transient spectroscopy (MCTS). The proposed method considers both the hole occupation during a filling (current injection) period and the quick carrier recombination via the hole traps near the depletion layer edge immediately after a reverse bias is applied. The reverse bias voltage dependence of the MCTS spectrum indicates that an accurate trap concentration, as well as the hole diffusion length and electron capture cross section of the hole trap, can be determined.

Deep-level traps in lightly Si-doped n-GaN on free-standing m-oriented GaN substrates

In this study, we investigated the deep-level traps in Si-doped GaN epitaxial layers by metal-organic chemical vapor deposition on c-oriented and m-oriented free-standing GaN substrates. The c-oriented and m-oriented epitaxial layers, grown at a temperature of 1000 °C and V/III ratio of 1000, contained carbon atomic concentrations of 1.7×1016 and 4.0×1015 cm–3, respectively. A hole trap was observed at about 0.89 eV above the valence band maximum by minority carrier transient spectroscopy. The trap concentrations in the c-oriented and m-oriented GaN epitaxial layers were consistent with the carbon atomic concentrations from secondary ion mass spectroscopy and the yellow luminescence intensity at 2.21 eV from photoluminescence. The trap concentrations in the m-oriented GaN epitaxial layers were lower than those in the c-oriented GaN. Two electron traps, 0.24 and 0.61 eV below the conduction band (EC) minimum, were observed in the c-oriented GaN epitaxial layer. In contrast, the m-oriented GaN epitaxial layer was free from the electron trap at EC – 0.24 eV, and the trap concentration at EC – 0.61 eV in the m-oriented GaN epitaxial layer was lower than that in the c-oriented GaN epitaxial layer. The m-oriented GaN epitaxial layer exhibited fewer hole and electron traps compared to the c-oriented GaN epitaxial layers.

Lifetime degradation of n-type Czochralski silicon after hydrogenation

Hydrogen plays an important role in the passivation of interface states in silicon-based metal-oxide semiconductor technologies and passivation of surface and interface states in solar silicon. We have shown recently [Vaqueiro-Contreras et al., Phys. Status Solidi RRL 11, 1700133 (2017)] that hydrogenation of n-type silicon slices containing relatively large concentrations of carbon and oxygen impurity atoms {[Cs] ≥ 1 × 1016 cm−3 and [Oi] ≥ 1017 cm−3} can produce a family of C-O-H defects, which act as powerful recombination centres reducing the minority carrier lifetime. In this work, evidence of the silicon's lifetime deterioration after hydrogen injection from SiNx coating, which is widely used in solar cell manufacturing, has been obtained from microwave photoconductance decay measurements. We have characterised the hydrogenation induced deep level defects in n-type Czochralski-grown Si samples through a series of deep level transient spectroscopy (DLTS), minority carrier transient spectroscopy (MCTS), and high-resolution Laplace DLTS/MCTS measurements. It has been found that along with the hydrogen-related hole traps, H1 and H2, in the lower half of the gap reported by us previously, hydrogenation gives rise to two electron traps, E1 and E2, in the upper half of the gap. The activation energies for electron emission from the E1 and E2 trap levels have been determined as 0.12, and 0.14 eV, respectively. We argue that the E1/H1 and E2/H2 pairs of electron/hole traps are related to two energy levels of two complexes, each incorporating carbon, oxygen, and hydrogen atoms. Our results show that the detrimental effect of the C-O-H defects on the minority carrier lifetime in n-type Si:O + C materials can be very significant, and the carbon concentration in Czochralski-grown silicon is a key parameter in the formation of the recombination centers.

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.

Point defects in Ga-implanted SiC: Experiment and theory

We present an experimental and theoretical study of the electronic properties of Ga implanted silicon carbide (SiC). The dose of implanted Ga was selected to simulate the implant-tail region, typical of high-dose box-profile p-type doping implantation employed for device manufacture. Samples were electrically characterized by capacitance-voltage (C-V), deep level transient spectroscopy, and minority carrier transient spectroscopy. The thermal stability of the detected levels (seven majority carrier traps, five minority carrier traps) was investigated by performing an isochronal annealing prior to each characterization step. Density functional theory was employed to study both isolated (substitutional and interstitial Ga) and complex Ga-related defects (N- and vacancy-related) in order to gain more insight in the nature of the detected levels. Finally, based on the experimental and theoretical results, the possible role of Ga in the nature of the detected levels is discussed.