Time-resolved Free Carrier Absorption Research Articles

4H-SiC Auger recombination coefficient under the high injection condition

The on-resistance of bipolar devices depends on the carrier lifetime, which is determined by Shockley–Read–Hall, surface, radiation, and Auger recombination processes. Values for the Auger recombination coefficient have been previously reported, but the values were constant in each report. However, the Auger recombination coefficient should depend on the concentration of excited carriers and the presence of traps. In this study, we observed excited carrier recombination in 4H-SiC under the high injection condition using time-resolved free carrier absorption measurements. Consequently, we discovered that the Auger recombination coefficient is dependent on the excited carrier concentration and that the traps have negligible effects on the coefficient.

Nondestructive measurements of depth distribution of carrier lifetimes in 4H-SiC thick epitaxial layers using time-resolved free carrier absorption with intersectional lights.

To achieve low on-state and switching losses simultaneously in SiC bipolar devices, the depth distribution of the carrier lifetime within the voltage blocking layer and the techniques used for observing the carrier lifetime distribution are important considerations. We developed a measurement system of the time-resolved free carrier absorption with intersectional lights (IL-TRFCA) for the nondestructive measurements of the depth distribution of the carrier lifetime in 4H-SiC thick epilayers. To confirm the reliability of the measurement results, we also performed TRFCA measurements to the cross section of the samples. As a result, although the lifetimes are underestimated owing to an inevitable diffusion of the carriers from the measurement region, the system was able to observe a carrier lifetime distribution up to a depth of 250 μm. Our IL-TRFCA system demonstrated a depth resolution of ∼10 μm, which is the best resolution among previously reported nondestructive measurement techniques. We consider the proposed system to be useful for the development of SiC bipolar devices.

Observation of carrier lifetime distribution in 4H-SiC thick epilayers using microscopic time-resolved free carrier absorption system

The carrier lifetime is an important parameter for high voltage SiC bipolar devices because its distribution in drift layers affects the device performance. Observation techniques for carrier lifetime, along with the development of carrier lifetime control processes, are important to control carrier lifetime distribution. In this study, we developed a microscopic time-resolved free carrier absorption system that has a variable spot size of excitation light and two different probe light wavelengths (405 and 637 nm). By selecting a relatively small spot size of excitation light and the probe light of shorter wavelength (405 nm), the distribution of carrier lifetime was observed with a high spatial resolution of ∼3 μm. Additionally, by using a relatively large spot size of excitation light and the probe light that leads to stronger free carrier absorption (637 nm), an accurate measurement of carrier lifetime was obtained. The developed system enables the design and development of bipolar SiC devices with carrier lifetime distribution control.

Carrier recombination processes in Fe-doped GaN studied by optical pump–probe techniques

We applied time-resolved free carrier absorption and light-induced transient grating techniques for monitoring carrier dynamics in lightly Fe-doped GaN bulk crystals. Comparison of measured decay times at single- and two-photon carrier photoexcitation with those reported for higher Fe doping revealed two different branches of lifetime dependence on iron density. A deeper insight was reached by using different wavelengths for probing carrier dynamics, which enabled discrimination of electron and hole capture processes by Fe ions. The capture cross section for holes, σh = (4 ± 1) × 10−15 cm2 for the Fe2+* state (5E → 4T2 transition) was determined to be ten times larger than that for electrons, σe = (4 ± 1) × 10−16 cm2 for the Fe3+* state (4T1 → 3T1 transition). Thermal activation of the [Fe2+(5T2),hVB] complex, where hVB is a free hole, required an energy of 315 meV. Light-induced transient grating decay revealed decreasing diffusivity of electrons with Fe doping due to initial carrier capture to Fe3+ states. Simultaneous measurements of carrier diffusion coefficient and lifetime at low excitation conditions (3 × 1017 cm−3) revealed a decrease of bipolar diffusion length from 2 μm in the undoped GaN down to 0.08 μm in the Fe-doped samples at 300 K.

Influence of defects and indium distribution on emission properties of thick In-rich InGaN layers grown by the DERI technique

We report on the spatial variation of optical properties in thick, In-rich InGaN layers, grown by a novel droplet elimination by radical beam irradiation (DERI) technique. The increase of layer thickness causes layer relaxation and results in double-peaked photoluminescence spectra. Spatially resolved measurements show that the defects in the strained sub-layer are distributed inhomogeneously. An increase in the layer thickness results in faster nonradiative recombination due to increasing density of nonradiative recombination centers, as evidenced by time-resolved free carrier absorption, and facilitates larger indium incorporation in the upper part of the layer.

Free carrier absorption in self-activated PbWO4 and Ce-doped Y3(Al0.25Ga0.75)3O12 and Gd3Al2Ga3O12 garnet scintillators

Nonequilibrium carrier dynamics in the scintillators prospective for fast timing in high energy physics and medical imaging applications was studied. The time-resolved free carrier absorption investigation was carried out to study the dynamics of nonequilibrium carriers in wide-band-gap scintillation materials: self-activated led tungstate (PbWO4, PWO) ant two garnet crystals, GAGG:Ce and YAGG:Ce. It was shown that free electrons appear in the conduction band of PWO and YAGG:Ce crystals within a sub-picosecond time scale, while the free holes in GAGG:Ce appear due to delocalization from Gd3+ ground states to the valence band within a few picoseconds after short-pulse excitation. The influence of Gd ions on the nonequilibrium carrier dynamics is discussed on the base of comparison the results of the free carrier absorption in GAGG:Ce containing gadolinium and in YAGG without Gd in the host lattice.

Diffusion-limited nonradiative recombination at extended defects in hydride vapor phase epitaxy GaN layers

Time-resolved free-carrier absorption and transient grating techniques were applied to determine carrier lifetimes and diffusion coefficients in a set of hydride vapor phase epitaxy GaN layers of various thickness (from 10 to 145 μm). A linear increase in nonradiative carrier lifetime in 80–800 K range found to be in a correlation with decrease of the bipolar carrier diffusion coefficient. This correlation confirmed that recombination rate is governed by carrier diffusive flow to the grain boundaries of columnar defects. A model of diffusion-governed nonradiative lifetime was proposed for fitting the measured lifetime values in the layers of different thickness as well as lifetime dependence on temperature or threading dislocation density.

Fast and slow carrier recombination transients in highly excited 4H– and 3C–SiC crystals at room temperature

Nonequilibrium carrier recombination in highly excited epitaxial layers of 4H–SiC and free standing 3C–SiC was analyzed numerically and studied experimentally by the time-resolved free carrier absorption (FCA) technique. The measurement setup combined interband carrier excitation by a picosecond laser pulse and probing of carrier dynamics at excess carrier densities in the ΔN=1017–1020 cm−3 range by optically or electronically delayed probe pulses, thus providing temporal resolution of 10 ps and 10 ns, respectively. FCA decay kinetics at different excitation levels and subsequent numerical modeling were used to determine the bulk lifetime, surface recombination velocity, and bimolecular (B) and Auger recombination (C) coefficients at 300 K. Bulk lifetimes of ∼800 ns and ∼65 ns were determined in 4H and 3C epitaxial layers, respectively. The numerical fitting of FCA kinetics in the 4H layer provided values of B=(1.2±0.4)×10−12 cm3/s and C=(7±4)×10−31 cm6/s at lower excitations while the Auger coefficient decreased to C=(0.8±0.2)×10−31 cm6/s at ΔN∼1020 cm−3 due to screening of the Coulomb-enhanced Auger recombination. In 3C crystals, these values were measured to be B=(2.0±0.5)×10−12 cm3/s and C=(2.0±0.5)×10−32 cm6/s. The tendency for a strongly increased surface recombination rate in 3C at high excitation conditions was observed experimentally and associated with the screening of the surface potential by the high density carrier plasma.

Nonequilibrium Carrier Recombination in Highly Excited Bulk SiC Crystals

We applied time-resolved free carrier absorption (FCA) to monitor non-equilibrium carrier dynamics in 4H epilayers and 3C SiC bulk crystals at excess carrier densities in the N = 1017 - 1019 cm-3 range. The numerical fitting of FCA decay kinetics provided the linear and nonlinear carrier recombination rates in the 40-390 K range and the absorption cross-sections eh at 1064 nm. In 4H, the decrease of the bulk lifetime (800 ns) with excitation provided the bimolecular and Auger coefficients B=(1.2±0.4)×10-12 cm3/s and C=(7±4)×10-31cm6/s, respectively, at room temperature. These values for 3C were 55-150 ns, (2.0±0.4)×10-12 cm3/s, and (2±1)×10-32 cm6/s, respectively. The rate of linear and nonlinear recombination increased at lower temperatures. A value of eh =4.4×10-18 cm2 for 3C SiC at 1.064 m was found 2.3 times smaller than that for 4H SiC.

Fundamental band edge absorption in nominally undoped and doped 4H-SiC

Fundamental band edge absorption is investigated in nominally undoped (n<1014cm−3) and heavily doped (n∼8×1018cm−3) 4H-SiC by a spectroscopy technique based on spatially and time-resolved free-carrier absorption. The spectra are extracted over a wide absorption range (0.02–500cm−1) at temperatures from 75to450K. The experimental results are supported by an indirect transition theory with a unique set of dominating momentum-conserving phonons, showing good correlation with earlier findings of differential absorption measurements at 2K. Exciton binding energy of 30±10meV is derived from fitting the data at 75K. The detected polarization anisotropy of absorption with respect to c axis is shown to be consistent with the selection rules for the corresponding phonon branches. An analytical model related to constant degree of involved phonons describes well the obtained energy gap variation with temperature. Finally, doping induced band gap narrowing is characterized above the impurity-Mott transition and compared with theoretical calculations in the random phase approximation. The shape of the fundamental absorption edge at high carrier concentrations is discussed in terms of excitonic enhancement above the Mott transition, as recently detected in Si.

Spatially and time-resolved infrared absorption for optical and electrical characterization of indirect band gap semiconductors

The current status of the spatially and time-resolved free-carrier absorption (FCA) method is provided. The FCA technique allows monitoring carrier dynamics in a time scale from nanoseconds to miliseconds by employing either collinear or orthogonal geometry between pump and probe beams. A high spatial resolution is achieved allowing in-depth carrier profiles to be extracted. The method is particularly suited for investigation of injection-dependent optical and recombination phenomena: band gap optical absorption, Shockley–Read–Hall (SRH) lifetime, Auger recombination coefficient, and the injection-dependent surface (interface) recombination velocity. We summarize important aspects of the technique demonstrating numerous measurements that have been implemented in studies of bulk Si, epilaxial 4H-SiC and porous silicon.

Investigation of surface recombination and carrier lifetime in 4H/6H-SiC

A spatially and time-resolved free carrier absorption method is applied to quantify surface recombination losses as compared to the bulk in 4H- and 6H-SiC structures. The observed carrier lifetime variation is discussed in terms of crystalline quality, top-surface properties and junction effects at the epilayer-substrate interface. Surface recombination parameters in epilayers with differently processed surfaces are extracted from fitting experimental data with numerical simulations. The as-grown bare epilayer is characterized by 10 4 cm s −1 surface recombination velocity. Mechanical polishing increases this parameter to 5×10 5 cm s −1. No noticeable passivation of 6H-SiC surface by an oxide film is observed, whereas an increase of the surface recombination velocity up to 10 5 cm s −1 has been detected after dry oxidation of 4H-SiC.

Injection-level dependent surface recombination velocities at the Si–SiO2 interface

For determination of the effective surface recombination velocity (SRV) at the Si–SiO2 interface, a new method based on depth- and time-resolved free-carrier absorption measurements is put forward. The proposed technique is more advantageous than conventionally used methods and makes it possible to obtain the SRV and its injection-level dependence on a particular sample surface without additional knowledge of other silicon parameters. The experimental measurements have been performed on n–Si with polished facets and a cleaved edge. All surfaces were covered by thermally grown SiO2. The result is that the SRV increases with the injection level followed by saturation. The computer simulation of the excess carrier decay has shown that, for polished surfaces, the value of the SRV should be reduced in the high injection region and not only does depend on the injected level, but also on the initial excitation. It is proposed that temporal charging of the Si–SiO2 interface can be responsible for such behavior.

Depth- and Time-Resolved Free Carrier Absorption in 4<i>H</i> SiC Epilayers: A Study of Carrier Recombination and Transport Parameters

Depth- and Time-Resolved Free Carrier Absorption in 4<i>H</i> SiC Epilayers: A Study of Carrier Recombination and Transport Parameters

Evaluation of Auger Recombination Rate in 4H-SiC

The carrier lifetime in 4H-SiC material is studied using a time-resolved free carrier absorption technique. The Auger recombination coefficient {gamma}{sub 3} = (7{+-}1) x 10{sup -31} cm{sup 6}s{sup -1} is derived from the kinetics of a highly excited electron-hole plasma in low doped 4H-SiC epitaxial layers. In addition, the recombination rate of e-e-h Auger process has been evaluated separately in heavily doped n-type 4H-SiC, yielding {gamma}{sub 3}{sup eeh} = (5{+-}1) x 10{sup -31} cm{sup 6}s{sup -1} at room temperature. The e-e-h type band-to-band Auger recombination is shown to be the dominant mechanism in 4H-SiC at high carrier concentrations. (orig.) 6 refs.

The effect of emitter shortings on turn-off limitations and device failure in GTO thyristors under snubberless operation

The influence of the anode shorting on the turn-off failure of GTO thyristors is investigated. Two types of shorting patterns, viz., fingerand ring-type anode designs, are compared with a non-shorted design. The inductively loaded GTO unit cells are measured under snubberless operation close to the safe-operating area (SOA) limit. The critical anode blocking voltage was obtained for a constant anode current density by varying the turn-off gain, G. The investigation has shown that the turn-off voltage capability of both types of anode-shorted GTO's is deteriorated for high G values. The ring-type anode design, however, shows better voltage blocking capability than the finger type. This behavior is attributed to the formation of a cylindrical high drift-current density filament at the end of the storage and beginning of the fall-time periods. The electric field in this region, which is referred to as a quasi space-charge region (QSC), is strongly G dependent. Measurements based on the time-resolved free-carrier absorption (FCA) technique are used to map the carrier-density redistribution inside the unit cell during the turn-off switching cycle. For the first time, 3-D simulations of the GTO cell are also presented in order to support the model of dynamic punch-through failure at high G values, which was proposed in earlier papers. >

A time-dependent two-dimensional analysis of the turn-off process in a gate turn-off thyristor (GTO)

A time-resolved free-carrier absorption technique is used to describe transient gate turn-off thyristor (GTO) phenomena two-dimensionally. The measurements are spatially resolved in the anode-to-cathode direction, as well as in the direction determined by the cathode length. Effects associated with charge removal during the GTO turn-off cycle with respect to external circuit conditions are discussed. The behavior of resistively and inductively loaded GTOs is explained on the basis of experimental results. Modeled results published by others are used for comparison. >

A study of turn-off limitations and failure mechanisms of GTO thyristors by means of 2-D time-resolved optical measurements

The failure mechanisms of Gate-Turn-Off (GTO) thyristors are investigated. Measurements based on a time-resolved free-carrier absorption (FCA) technique are used to support the presented models. The measurements serve to map the local carrier densities two-dimensionally, at any time of the switching cycle. Inductively loaded GTOs under snubberless operation are studied close to the safe-operating area (SOA) limit. Several important features of the destructive process are established. First, the existence of a quasi space-charge region, QSC, in the n base and charge focusing at the anode side of the device is noticed during the fall-time period. Secondly, a piling up of holes in the p base is followed by current filamentation during the tail period. The sequential behaviour of different phenomena and their possible causal explanations are also established. The expansion of the QSC towards the anode emitter causes the enhancement and focusing of the charge distribution in the vicinity of this junction. Two possible failure mechanisms, viz. local dynamic punch-through in the n base and local dynamic avalanche injection in the blocking junction are discussed in the light of the experimental results. It is also suggested that both the above mentioned failure mechanisms can be part of the chain of events leading to device failure. Regardless of which mechanism dominates, a high peak of excess holes appears in the p base in the beginning of the tail period. This charge debiases the cathode junction locally, i.e. compensates the negative bias of the cathode junction, and current filaments connecting the cathode and anode sides of the device are formed. The excessive power dissipation in the dominant filament causes failure and permanent damage.