Excess Carrier Transport Research Articles

Effect of low-energy electron irradiation on voltage-capacity curves of Al/SiO2/Si structure

Charging of dielectric targets by electron irradiation is a well-known phenomenon which should be taken into account in characterization of dielectric materials and coatings with electron microscopy, in electron beam lithography, in development of dielectric coatings for spacecrafts and other fields of science and engineering. Charging kinetics is strongly affected by spatial distribution of electrons and holes formed by irradiation. At the experimental electron beam energy electron penetration depth is smaller than dielectric thickness and this allows identifying the contribution of excess carrier transport to trap formation at the SiO2/Si interface. Low-energy electron beams have been shown to substantially affect C–V curve slope, i.e., to form traps at the interface. We have studied the effect of bias applied to the test structure before and after electron beam irradiation. The experiment has shown that bias of either polarity applied to the test MOS structure before low-energy electron beam irradiation practically does not affect the C–V curves of the test structure. Positive bias applied to the metallization layer during low-energy electron beam irradiation has a strong effect on the C–V curve pattern while negative bias affects the C–V curves but slightly. Study of the stability of the changes caused by electron beam irradiation has shown that the C–V curves of the test structure restore slowly even at room temperature. Application of negative bias decelerated charge relaxation.

Low-energy electron beam irradiation effect on Al/SiO2/Si structure voltage-farad characteristics

The effect of electron irradiation with energy of 2.5 keV on the MOS structure Al/SiO2/Si capacitance-voltage (C-V) characteristics have been studied. At chosen beam energy the electron penetration depth is lower than the dielectric thickness that allows to reveal the contribution of excess carrier transport to the trap formation on the SiO2/Si interface. It was established that the electron beam irradiation leads to a significant change in the C-V characteristics slope, i.e. to to the trap formation at the interface. A study of effect of bias applied to the investigated structure before and during the electron beam irradiation was carried out. It was established that while the bias applied before irradiation practically did not affect the C-V characteristics of the investigated MOS structure, the positive voltage applied to metallization during irradiation produced a pronounced effect on the C-V curve changes. At the same time the C-V characteristics after irradiation with zero and negative voltage were very similar. The investigation of stability of changes produced by the electron beam irradiation showed that the C-V curves are slowly restored even at room temperature. An applied negative bias was found to slow down the charge relaxation process.

(Invited) Non-Destructive Three-Dimensional Imaging of Extended Defects in 4H-SiC

Grazing-incidence X-ray topography and photoluminescence (PL) imaging are utilized to understand the nature and behavior of extended defects in 4H-SiC. Although these methods are useful for non-destructive defect analysis, they provide two-dimensional (2D) images with low depth resolution. Advanced three-dimensional (3D) imaging methods are thus expected to be used for characterizing extended defects. In particular, very high-voltage SiC bipolar devices with a thick epilayer require defect imaging as deep as 100-200 μm. We established a 3D imaging technique using an X-ray microbeam and a novel fine slit, which was successfully applied to characterizing threading screw dislocations (TSDs) [1, 2], threading edge dislocations (TEDs), and basal-plane dislocations (BPDs) [3, 4] in 4H-SiC. However, this method requires a large-scale synchrotron-radiation facility. In this study, we show how second-harmonic generation (SHG) and two-photon-excited photoluminescence (2PPL) imaging techniques are powerful tools for 3D analysis of extended defects in 4H-SiC epilayers [5, 6]. The SHG and 2PPL methods can be performed by a multi-photon microscope in individual laboratories. The SHG method provides clear 3D images of 3C-inclusions because 3C-SiC is SHG-active, but not 4H-SiC host crystal in c-axis incidence (Fig. 1). The 2PPL method yields 3D images; not only of 3C-inclusions but also 8H stacking faults (Fig. 2), and single Shockley stacking faults in the epilayers. The 2PPL method also achieved 3D imaging of TSDs, TEDs, and BPDs using band-edge emission [7]. Since band-edge emission quenches near defects, these dislocations can be visualized as dark contrasts on a bright background. We obtained 3D images of TSDs and TEDs extending ~200 μm from the surface (Fig. 3). Unlike 2D PL imaging with uniform illumination, the dark-contrast imaging is governed by the diffusion and transport of excess carriers injected from a scanning focal point. A simulation study was conducted to reveal the mechanism of the dark-contrast imaging. Each of the dislocations provided 2D dark-contrast images stacked depthwise, from which the tilt angles of dislocations were determined. Figure 4 shows the plots of φ versus θ for 89 TSDs in the 4º off epilayer, where θ denotes the angle of the dislocation line from the c-axis, and φ the counter-clockwise angle of (0 0 0 1) projected dislocation lines from the step-flow [1 1 -2 0] direction. Their dislocation lines incline in the step-flow direction. It is also shown that the regions of left- and right-handed (LH and RH) dislocations clearly differ. The plots in Fig. 4 include those of 1c and c+a dislocations. We found that 1c dislocations had very similar tilt angles, whereas those of c+a dislocations were largely spread out. The tilt angles were also investigated for 105 TSDs in the 8º off epilayer, which exhibits larger θ angles and a smaller φ range than the 4º off-cut case. We examined the tilt angles of more than 300 TEDs and found that the TEDs not only inclined in the step-flow direction, but also the direction of the extra half planes [7]. The results of the tilt-angle analyses cannot be explained by the energy minimization model in bulk materials. We consider that the directions of TEDs and TSDs depend upon the interactions between dislocations and advancing steps on a growing surface. [1] R. Tanuma, T. Kubo, F. Togoh, T. Tawara, A. Saito, K. Fukuda, K. Hayashi, and Y. Tsusaka, Phys. Status Solidi A 204, 2706 (2007). [2] R. Tanuma, T. Tamori, Y. Yonezawa, H. Yamaguchi, H. Matsuhata, K. Fukuda, and K. Arai, Material Sci. Forum 615-617, 251 (2009). [3] R. Tanuma, D. Mori, I. Kamata, and H. Tsuchida, Appl. Phys. Express 5, 061301 (2012). [4] R. Tanuma, D. Mori, I. Kamata, and H. Tsuchida, J. Appl. Phys. 114, 023511 (2013). [5] R. Tanuma and H. Tsuchida, Appl. Phys. Express 7, 021304 (2014). [6] R. Tanuma and H. Tsuchida, Mat. Sci. Forum 778-780, 338 (2014). [7] R. Tanuma, M. Nagano, I. Kamata, and H. Tsuchida, Appl. Phys. Express 7, 121303 (2014). Figure 1

Modeling Inductive Switching Characteristics of High-Speed Buffer Layer IGBT

In this study, a physics-based compact model for high-speed buffer layer insulated gate bipolar transistor (IGBT) is proposed. The model utilizes the 1-D Fourier-based solution of ambipolar diffusion equation (ADE) implemented in MATLAB and Simulink. Based on the improved understanding on the inductive switching behavior of a high-speed buffer layer IGBT, the ADE is solved for all injection levels instead of high-level injection only as usually done. Assuming high-level injection condition in the buffer layer, the excess carrier transport, redistribution and recombination in the buffer layer are redescribed. Moreover, some physical characteristics such as the low conductivity of N-base at turn-on transient and free holes appeared in the depletion layer during turn-off process are also considered in the model. Finally, the double-pulse switching tests for a commercial field stop IGBT and a light punch-through carrier-stored trench bipolar transistor are used to validate the proposed model. The simulation results are compared with experiment results and good agreement is obtained.

A class of hierarchical nanostructures: ZnO surface-functionalized TiO2 with enhanced sensing properties

Combining the versatility of the electrospinning technique and the hydrothermal growth of nanostructures enabled the fabrication of TiO2/ZnO composite nanostructures. XRD, FESEM, TEM and EDX analysis confirmed the growth of ZnO nanosheets with a diameter of about 500 nm on the surface of TiO2 nanofibers. Gas sensing properties of the sensors fabricated from the as-prepared TiO2/ZnO, TiO2 and ZnO were systematically investigated. The TiO2/ZnO sensor exhibits a response of 15.7 to 100 ppm of ethanol at 280 °C, which is much better than pure TiO2 and ZnO. The enhanced sensing performance to ethanol is mainly attributed to the formation of heterojunctions between TiO2 and ZnO, which leads to the interfacial transport of excess carriers.

Role of extended defects in the transformation of InGaN/GaN multiple quantum well structure optical properties under low energy electron beam irradiation

AbstractIt is shown that threading extended defects can enhance the excess carrier transport through the InGaN/GaN multiple quantum well (MQW) region. The enhanced excess carrier transport through a GaN film is also revealed that was explained by a carrier transport along extended defects. A possible role of threading extended defects in the low energy electron beam irradiation induced modification of electrical and optical properties in InGaN/GaN MQW structures are discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancement of Gas Sensing Properties of CdS Nanowire/ZnO Nanosphere Composite Materials at Room Temperature by Visible-Light Activation

CdS nanowire/ZnO nanosphere materials (CdS/ZnO) with hierarchical structure were synthesized by a three-step solvothermal process. XRD, FESEM and TEM analysis confirmed the growth of ZnO nanospheres on the surface of CdS nanowires (NWs). The transient photovoltage (TPV) measurements revealed that the interface between CdS and ZnO can inhibit the recombination of photogenerated excess carriers and prolong the lifetime of excess carriers in CdS/ZnO materials. Moreover, the CdS/ZnO materials exhibit a dramatic improvement in optoelectronic performance and visible-light-irradiation gas sensing activity, which gave 1 order of magnitude larger than that of CdS NWs in response to formaldehyde. The enhancement of sensing properties is attributed to the interfacial transport of excess carriers.

Sub-Bandgap Laser Light-Induced Excess Carrier Transport Between Surface States and Two-Dimensional Electron Gas Channel in AlGaN/GaN Structure

Variations of the channel resistance of a AlGaN/GaN high electron mobilities transistor caused by subsequently incident photons with sub-bandgap energies after ultraviolet light-induced changes became stable were studied. Temperature-dependent measurements yielded a 0.342 eV thermal activation energy and wavelength-varying measurements yielded a 0.8 eV cut-off photon energy. The ratio between the two values is very close to the theoretical value of the ratio between the valence and conduction band discontinuities. A qualitative description about the transports of excess carriers through the band discontinuities, is also proposed and consistent with the experimental results.

DIFFUSION LENGTH AND EFFECTIVE CARRIER LIFETIME IN III-NITRIDES

The results of excess carrier diffusion length investigations in GaN and in the light emitting structures (LES) based on MQW InGaN / GaN with different ordering of mosaic structures are presented. It is confirmed that the diffusion length values in these structures indeed are essentially lower than 1 μm. The Electron Beam Induced Current investigations have shown that the effective diffusion length measured on GaN structures could be determined by the mosaic domain boundary effect on the excess carrier transport. Such nanosize domains could also determine the lateral localization in the MQW light emitting diodes. The mosaic structure ordering effect on the quantum efficiency of LES and high values of the effective carrier lifetime in well-ordered LES are discussed.

Ambipolar drift of spatially separated electrons and holes

We report on the ambipolar transport process of photogenerated, spatially separated charge carriers in the doping layers of a $p\ensuremath{-}i\ensuremath{-}n$ diode under the influence of lateral electric fields. By including externally applied electric fields into the theory of ambipolar diffusion of spatially separated electrons and holes, we show that the transport of excess carriers can be described as a combined drift and diffusion process. Compared to the well-known ambipolar transport in bulk material, the ambipolar diffusion process is enhanced by several orders of magnitude, whereas the ambipolar drift can be described by the same ambipolar mobility as in bulk material if the electric fields in both doping layers are identical. One major difference of the ambipolar drift of electrons and holes propagating in different layers in comparison to bulk material is the possibility to control the ambipolar mobility dynamically by changing the dark carrier densities by varying the reverse bias applied to the $p\ensuremath{-}i\ensuremath{-}n$ structure. Most interesting however, is the fact that the ambipolar drift can be controlled by different external fields for electrons and holes. In order to verify the predictions of our theoretical description of the ambipolar transport of spatially separated electrons and holes, we have developed a new pump-and-probe technique that allows for a direct temporally and spatially resolved investigation of the various transport scenarios. The results agree very well with the theoretical simulations.

The transport of excess current carriers in an inhomogeneous semiconductor with position-dependent band gap

The theory of excess carrier transport with zero magnetic field in a nondegenerate semiconductor with position-dependent energy gap is presented. It is assumed that the change of energy gap is moderate at the distance of the Debye length and in consequence the quasi-neutrality principle is fulfilled. As the basis the transport equations of the thermodynamics of irreversible processes are chosen. Formulae for the total current density J and for the effective field are derived. It is shown that in addition to the ohmic component the expression for contains the Dember effect, bulk photovoltage (Tauc's effect) and component, the last of these being connected with the position dependence of mobility which is rather unavoidable in the case of a variable energy gap. The continuity equation is formulated as an extension of Roosbroeck's continuity equation for a homogeneous semiconductor. Most typical applications of one-dimensional equations in an approximate form are examined, which is important from the practical point of view.

Influence of defects on electron–hole plasma recombination and transport in a nipi-doped InxGa1−xAs/GaAs multiple-quantum well structure

The nonlinear optical and transport properties of a nipi-doped InxGa1−xAs/GaAs multiple-quantum well sample (x=0.23) has been studied using a novel approach called electron-beam-induced absorption modulation (EBIA). The absorption in the sample is modulated as a result of screening of the built-in electric field in the nipi structure due to excess carrier generation. The change in field causes a Stark shift of the first quantized optical transitions in QWs which are situated in the intrinsic layers. In EBIA, a scanning electron probe is used to locally generate an electron–hole plasma that is used to study the spatial distribution of defects that impede excess carrier transport and reduce the lifetime of spatially separated carriers. The Stark shift in the MQW structure is imaged with micrometer-scale resolution and is compared with cathodoluminescence imaging results which show dark line defects resulting from strain-induced misfit dislocations. Theoretical calculations using Airy functions in the transfer-matrix method with a self-consistent field approximation were used to determine the energy states, wave functions, and carrier recombination lifetimes of the MQW as a function of the built-in field. A quantitative phenomenological analysis is employed to determine the built-in field, excess carrier lifetime, and ambipolar diffusion coefficient as a function of the excitation density. The defects are found to create potential barriers and recombination centers which impede transport and markedly reduce the excess carrier lifetime.

Lateral transport in GaAs/AlGaAs quantum wells

The lateral transport of excess carriers in metal organic chemical vapor deposition grown GaAs/AlxGa1−xAs(x=0.5) 20 Å thick single quantum well structures is studied by cathodoluminescence (CL) between 5 K≤T≤170 K. Impurity and native defect related transitions are found to take place around grown-in dislocations where well thickness variations are also found. The CL intensity of free excitons and/or transitions involving impurities and native defects is measured as a function of distance from the dislocation to the point of excitation. Using a simple diffusion model, we are able to determine the hole diffusion length, L=1.5 μm, in slightly n-doped SQW. This represents a novel method for the direct determination of the diffusion length in sufficiently defect-free material.

Analytic time domain characterization of p-i-n photodiodes: Effects of drift, diffusion, recombination, and absorption

An analytic impulse response function of high-speed p-i-n photodiodes is given in direct time domain, incorporating the effects of drift, diffusion, recombination, and absorption. In the limit of no carrier diffusion and large excess carrier lifetime, the result reduces to the well-known theory of Lucovsky et al. [J. Appl. Phys. 35, 622 (1964)]. In this analysis the output photocurrent is obtained in terms of photogenerated carrier concentrations from the energy balance consideration. The excess carrier transport is described by a Green’s function method. Additionally the internal electric field is explicitly examined. The present formulation is a generalization of Yariv’s one-carrier-current model which ascribes the externally detected current to the time-varying induced charge on the electrodes.

Transport of excess carriers in silicon wafers

Contactless transient photoconductivity measurements are used to study excess charge carrier transport in single-crystal silicon wafers. It is shown that the minority carrier diffusion constant and the minority carrier lifetime can be determined from the decay behaviour by varying the thickness of the wafer. The presence of non-uniform defect densities can be studied by their influence on the diffusion of excess carriers. This is demonstrated by investigation of a proton-irradiated wafer and of a wafer after an internal gettering treatmen.

Flux method for the analysis of excess carrier transport in multilayer devices in the presence of electric field

The flux method of treating the one-dimensional steady-state transport problem of excess charge carriers, created by the generating sources, is analysed for multilayer devices in the presence of electric field. General analytical expressions for the forward (parallel to the electric field) and backward (antiparallel to the electric field) fluxes of excess carriers at any arbitrary boundary of the multilayer device are derived in terms of the reflection and transmission coefficients of the individual layers and boundaries. As an application of this general analysis, a simple example of a multilayer device consisting of only two layers is considered. The results are shown to reduce those obtained by other calculations in appropriate limiting cases.

Some new results on the electronic properties of a-Si and a-SiN x

Fast time-resolved techniques have been used to investigate the transport of excess carriers in the tail states of a-Si as well as carrier-tail-state interactions. In the first part new results on low temperature tail state hopping transport of electrons and holes will be reported and discussed. Also the interpretation of electron transport in compensated a-Si will be considered. The second part of the paper is concerned with the electronic properties of a-SiN x and the possibility of controlling these by doping.

Transport Properties of Amorphous Silicon / Silicon Oxide Heterostructures

ABSTRACTThe transport of excess carriers in glow-discharge deposited a-Si:H/insulator heterostructures has been studied by time-of-flight and xerographic discharge techniques. Efficient injection of photocarriers from a-Si:H into, and transport through, relatively thick SiOx:N:H has been achieved. A mobility-lifetime product approaching 18−6 cm2V−1 is found for electrons in SiOx:N:H, and time resolved measurements indicate a room temperature mobility of 5×10−6 cm2v−1s−1 at a field of 2×104 Vcm−1, suggesting an electron lifetime of the order of 8.2 seconds. The results are contrasted with transport measurements on thermally grown SiO2 on Si and a transport model involving hopping through defect states within the gap of SiOx:N:H is discussed.

Transport in photo-conductors—I: Focal plane processing

Transport of excess carriers in a photo-conductor allows photo-carriers to be sampled remote from their point of generation. In paper I the separation of a photo-conductor into a generation and sampling volume is described in terms of equivalent signal processing associated with the sampling volume. Device geometry for time-delayed integration, multiplexing and edge enhancement are discussed. The Green's function for excess carriers in an infinite solid is used to give a physical description of signal and noise and their modification by transport. Diffusion is identified as a cause of image degradation; noise correlation is shown to limit the processing advantage described. In paper II a more detailed analysis is outlined to include the effects due to finite length and give a discussion of coherent and incoherent sampling.

Space-charge effect on excess carrier transport in semiconductors: A. Ostrowski. Electron Tech., Warsaw7, ( [formula omitted]), 39 (1974)

Space-charge effect on excess carrier transport in semiconductors: A. Ostrowski. Electron Tech., Warsaw7, ( [formula omitted]), 39 (1974)