Carrier Concentration Profiles Research Articles

Quantifying the local Seebeck coefficient with scanning thermoelectric microscopy

We quantify the local Seebeck coefficient with scanning thermoelectric microscopy, using a direct approach to convert temperature gradient-induced voltages (V) to Seebeck coefficients (S). We use a quasi-3D conversion matrix that considers both the sample geometry and the temperature profile. For a GaAs p-n junction, the resulting S-profile is consistent with that computed using the free carrier concentration profile. This combined computational-experimental approach is expected to enable nanoscale measurements of S across a wide variety of heterostructure interfaces.

Depth profiling of strain and carrier concentration by cleaved surface scanning of GaN Gunn-diode: confocal Raman microscopy

Confocal micro-Raman spectroscopy was applied to study the cleaved surface of vertical GaN Gunn-diode structure grown by molecular-beam epitaxy. The analysis of lateral scanning along the cleaved edge reveals the depth profile of elastic strain, quality of the crystal structure, and the concentration of charge carriers. Results are compared with that of axial confocal Raman depth profiling normal to the structure's surface. Decrease of compressive strain near the cleaved edge in the direction from the substrate to the structure's surface and in the growth plane towards the cleaved edge is shown. The decrease in charge carrier concentration in the undoped n0-GaN channel region in comparison with the n+-GaN contact region is identified. Peculiarities of the resulting spatial profiles of free charge carriers and their correlation with the initial doping profile are discussed.

A look underneath the SiO2/4H-SiC interface after N2O thermal treatments

The electrical compensation effect of the nitrogen incorporation at the SiO2/4H-SiC (p-type) interface after thermal treatments in ambient N2O is investigated employing both scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM). SSRM measurements on p-type 4H-SiC areas selectively exposed to N2O at 1150 °C showed an increased resistance compared to the unexposed ones; this indicates the incorporation of electrically active nitrogen-related donors, which compensate the p-type doping in the SiC surface region. Cross-sectional SCM measurements on SiO2/4H-SiC metal/oxide/semiconductor (MOS) devices highlighted different active carrier concentration profiles in the first 10 nm underneath the insulator–substrate interface depending on the SiO2/4H-SiC roughness.The electrically active incorporated nitrogen produces both a compensation of the acceptors in the substrate and a reduction of the interface state density (Dit). This result can be correlated with the 4H-SiC surface configuration. In particular, lower Dit values were obtained for a SiO2/SiC interface on faceted SiC than on planar SiC. These effects were explained in terms of the different surface configuration in faceted SiC that enables the simultaneous exposition at the interface of atomic planes with different orientations.

Study of carrier concentration profiles in Al-implanted Ge by micro-Raman spectroscopy under different excitation wavelengths

The distribution profile of Al implanted in crystalline Ge has been investigated by micro-Raman spectroscopy. Using different excitation laser lines, corresponding to different optical penetration depths, the Al concentration at different depths beneath the sample surface has been studied. We have found a strong correlation between the intensity of the Al–Ge Raman peak at ~370 cm−1, which is due to the local vibrational mode of substitutional Al atoms, and the carrier concentration profile, obtained by the spreading resistance profiling analysis. A similar connection has been also observed for both shape and position of the Ge–Ge Raman peak at ~300 cm−1. According to these experimental findings, we propose here a fast and nondestructive method, based on micro-Raman spectroscopy under different excitation wavelengths, to estimate the carrier concentration profiles in Al-implanted Ge. Copyright © 2013 John Wiley & Sons, Ltd.

Defect spectroscopy of Cu(In,Ga)Se2-based thin film solar cells on polyimide substrate

Cells made on a polyimide substrate, with sodium supplied at different stages of the absorber deposition, have been studied by defect spectroscopy methods. In all of the cells, admittance spectroscopy measurements revealed the presence of two defect levels. Discussion on the origin of the observed levels, based on the correlation with free carrier concentration profiles and photocurrent results, is presented. We attribute one of the levels as belonging to interface defects, and the second one to a bulk deep defect. A distinctive feature of defects' sensitivity to a hole concentration, which leads to unusual behaviour of defect parameters is discussed.

Capacitance‐voltage and admittance investigations of InGaN/GaN MQW LEDs: frequency dependence

AbstractCapacitance‐frequency dependences for InGaN/GaN multiple‐quantum‐well light‐emitting structures grown by MOCVD are investigated at various temperatures and bias voltages. The charge relaxation in a quantum well is found to be satisfactorily described by two emission processes with different dependences of emission rate on temperature. It is shown that in typical InGaN/GaN lightemitting structures, one or a few quantum wells can be filled with electrons even for the relatively high reverse biases applied to the structure and contribute to the measured capacitance. This allows to explain the observed temperature and frequency dependences of apparent carrier concentration profiles obtained under capacitance–voltage profiling in these structures. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Validity of Calibrated Photoluminescence Lifetime Measurements of Crystalline Silicon Wafers for Arbitrary Lifetime and Injection Ranges

We investigate the validity of calibrated photoluminescence lifetime measurements of crystalline silicon wafers for arbitrary lifetime and injection ranges. Absolute lifetime images are obtained from steady-state photoluminescence measurements by relating the photoluminescence signal to the excess carrier density. Since the luminescence signal is expected to be related to the integral of the depth distribution of the excess carrier density, an adequate calibration of the luminescence signal requires a secondary method which yields the integral of the depth distribution of the excess carrier density in absolute units. In this paper, we investigate the applicability of steady-state photoconductance measurements for the calibration of the photoluminescence signal. We derive a generalized relation linking the photoluminescence signal with the excess carrier density, considering the impact of an inhomogeneous carrier concentration profile. We experimentally verify the impact of the carrier distribution on the photoluminescence calibration by investigating two silicon wafers with different electronic bulk properties. Finally, we propose an iterative correction procedure reducing the deviations due to an inhomogeneous carrier density profile of calibrated photoluminescence-based lifetime measurements significantly.

Analysis of the temperature dependence of the capacitance-voltage characteristics of InGaN/GaN multiple quantum well light-emitting structures

The capacitance-voltage characteristics and frequency dependences of the capacitance and conductance of InGaN/GaN multiple quantum well light-emitting structures are studied in the frequency range of 60 Hz-5 MHz and the temperature range of 77–300 K. It is shown that carrier relaxation in quantum wells can be described by two emission processes, i.e., the thermally activated one and with the power-law temperature dependence of the emission rate. It is also shown that one or several quantum wells in typical InGaN/GaN-based light-emitting structures can remain filled with electrons even at comparatively high reverse biases. This makes it possible to explain the depth shift of the apparent carrier concentration profiles, obtained from the capacitance-voltage characteristics, with decreasing temperature.

Electrical and optical simulations of a polymer-based phosphorescent organic light-emitting diode with high efficiency

A comprehensive numerical device simulation of the electrical and optical characteristics accompanied with experimental measurements of a new highly efficient system for polymer-based light-emitting diodes doped with phosphorescent dyes is presented. The system under investigation comprises an electron transporter attached to a polymer backbone blended with an electronically inert small molecule and an iridium-based green phosphorescent dye which serves as both emitter and hole transporter. The device simulation combines an electrical and an optical model. Based on the known highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of all components as well as the measured electrical and optical characteristics of the devices, we model the emissive layer as an effective medium using the dye's HOMO as hole transport level and the polymer LUMO as electron transport level. By fine-tuning the injection barriers at the electron and hole-injecting contact, respectively, in simulated devices, unipolar device characteristics were fitted to the experimental data. Simulations using the so-obtained set of parameters yielded very good agreement to the measured current–voltage, luminance–voltage characteristics, and the emission profile of entire bipolar light-emitting diodes, without additional fitting parameters. The simulation was used to gain insight into the physical processes and the mechanisms governing the efficiency of the organic light-emitting diode, including the position and extent of the recombination zone, carrier concentration profiles, and field distribution inside the device. The simulations show that the device is severely limited by hole injection, and that a reduction of the hole-injection barrier would improve the device efficiency by almost 50%. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Contrast distortion induced by modulation voltage in scanning capacitance microscopy

With a dark-mode scanning capacitance microscopy (SCM), we directly observed the influence of SCM modulation voltage (MV) on image contrasts. For electrical junctions, an extensive modulated area induced by MV may lead to noticeable changes in the SCM signal phase and intensity, resulting in a narrowed junction image and a broadened carrier concentration profile. This contrast distortion in SCM images may occur even if the peak-to-peak MV is down to 0.3 V. In addition, MV may shift the measured electrical junction depth. The balance of SCM signals components explain these MV-induced contrast distortions.

Calculation of charge carrier concentration profile and magnetoresistance in modulation doped structures with a wide potential well

We investigated Fermi charge carriers' concentration profile in modulation doped structures with a wide quantum well. We have formulated hydrodynamic equations both in the absence of magnetic field and in parallel magnetic field. We have obtained analytical solutions for the carrier concentration as a function of the tangential coordinate in the potential well. In a quantum area near the interface, we carried out quantum mechanical calculations taking into account the effect of electric and magnetic fields. The carrier concentration profile and magnetoresistance in parallel magnetic field are presented for modulation doped Si/SiGe/Si structures.

Simple power-loss analysis method for high-efficiency Interdigitated Back Contact (IBC) silicon solar cells

A simple analytical method is presented to perform a power-loss analysis of high-efficiency silicon Interdigitated Back Contact (IBC) solar cells. The method assumes one-dimensional current flows for both minority and majority carriers, as well as a constant gradient (or linear profile) of minority carrier concentration from the front to the back of the solar cell. The power-loss analysis method is applied to a real IBC silicon solar cell with a conversion efficiency of 23.3%.

Doping characterization for germanium-based microelectronics and photovoltaics using the differential Hall technique

In this coming decade, complementary metal-oxide-semiconductor microelectronic devices may undergo a major change with the implementation of germanium channels. Likewise, the performance of photovoltaic cells based on elemental semiconductors will continue to be optimized. Both technologies will rely on a detailed and thorough understanding of electrical properties, and here, precise doping characterization will play a key role. The differential Hall technique combines resistivity and Hall-effect measurements with successive surface layer removal, allowing one to measure independent carrier concentration and mobility depth profiles. In this Letter, we apply the technique for both microelectronic- and photovoltaic-relevant doping structures in germanium. Controllable and uniform layer removal is achieved with tailored depth resolution (<1–20 nm) for a range of doping structures (30–600 nm).

A near-field microwave technique for the deep profiling of free carrier concentrations in semiconductors

A technique for the profiling of free carrier concentration N(z) in semiconductors, based on the near-field measurements in microwave frequency range, is proposed. A high accuracy in retrieving the N(z) function with characteristic spatial scales of 10–100 nm using 2–3 probes with apertures of 3–15 μm is demonstrated.

Industrial standardization and quantification of the carrier concentration in semiconductor devices using electric SPM

Scanning capacitance microscopy (SCM) is one of techniques imaging two-dimensional carrier distribution in semiconductor devices. We have demonstrated that quantitative analysis of carrier concentration can be performed using calibration curves, which are statistically prepared for respective materials like silicon, silicon carbide, etc. The procedure was circulated to research organizations in domestic round robin test, and this method was found to be available from the viewpoint of comparability among users for the industrial standardization. It was also applied to other microscopes, and their profiles of carrier concentration coincide with each other, suggesting compatible to next-generation microscopes which would come up in the future.

Modelling of the Behaviour of Solar Cell Under High Injection Effect Conditions

An I-V characteristic measurements of silicon solar cell were carried out, the result showed a saturation of open circuit voltage Voc when sunlight concentration reaches 7 suns in our case, which imposed a limitation on the solar cell's efficiency. For understanding the carrier's transport mechanisms in the cell, a simulation of solar cell behaviour under sunlight concentrating –up to 25 suns-has been carried out. The minority carrier concentration profiles in the silicon solar cell's base were represented, by solving the continuity equation via suggesting a mathematical sample. The results enable us to disputes high injection effect at which tacks place when a high concentration of minority carrier -comparable to majority carriers-is appeared in thin area beside the depletion region.

Evaluation and Modeling of a Photo-Electrochemical Reactor for Hydrogen Production Operating under High Photon Flux

A photo-electrochemical reactor for water splitting with a Si(IV)-doped Fe2O3 photoanode was designed to investigate the effects on photocurrent densities of electrode potential, photon flux and 'front' and 'back' illumination of the photoanode. As a design tool to optimize the reactor performance, a 1-D kinetic model incorporating photogeneration, diffusion, migration, recombination and surface reaction terms, was developed to predict minority carrier concentration profiles and hence photocurrent densities. Model predictions were in good agreement with experimental photocurrent densities, which increased linearly with increased light intensity. Photocurrent densities decreased by 90 % when the direction of illumination was switched from hν -> electrolyte | semiconductor to hν -> glass | fluorinated tin oxide | semiconductor | electrolyte, due to the resulting decrease in hole concentrations at the semiconductor | liquid junction.

High-Vacuum Evaporation of n-CuIn3Se5 Photoabsorber Films for Hybrid PV Structures

Thin films of Cu-In-Se (CISe) photoabsorber with an overall composition of CuIn3Se5 were deposited onto glass/indium tin oxide (ITO) substrates from a polycrystalline bulk CuIn3Se5 source using the high-vacuum evaporation technique. Thermal conditions for the substrates during the evaporation process and the subsequent annealing in vacuum were selected to prepare polycrystalline n-CuIn3Se5 photoabsorber layers for use in hybrid photovoltaic structures based on an inorganic photoabsorber and conductive polymer functional layers. The CISe layers were deposited at a substrate temperature of 200°C and were annealed at temperatures from 300°C to 500°C in vacuum. Part of the as-deposited CISe was annealed twice, in argon and in vacuum at 500°C. These layers exhibited high photosensitivity and photoconductivity when illuminated with white light at an intensity of 100 mW/cm2. The results showed that the chalcopyrite structure of the prepared CISe photoabsorber films adhered well to the glass/ITO substrate. The average value of charge carrier concentration and the profile of charge carrier concentration in the annealed CISe photoabsorber layer were calculated using impedance spectroscopy.

Calculation of Charge Carrier Concentration Profile in a Wide Potential Well with Electric and Magnetic Fields

We investigated an equilibrium state of Fermi electrons in modulation doped structures with a wide quantum well in a strong parallel magnetic field. We studied the charge carrier system with a sufficiently high density, such that the de Broglie wavelength of electrons is smaller than the potential well width. We have formulated hydrodynamic equations for this carrier system both in the absence of magnetic field and in a parallel magnetic field. We have obtained analytical solutions for the charge carrier concentration as a function of coordinates in the potential well. In a quantum area near the interface, we carried out quantum mechanical calculations taking into account the effect of electric and magnetic fields. The concentration profile is presented for modulation doped Si/SiGe/Si heterostructures. We discuss large positive magnetoresistance in a strong parallel magnetic field in these structures.

Confocal Raman depth‐profile analysis of the electrical and structural properties in III‐nitride structures

AbstractScanning confocal Raman spectroscopy was used to non‐destructively evaluate the structural and electronic properties of n+/n0/n+‐GaN Gunn‐diode structures. The depth profiles of the free carrier concentration and mobility were obtained from a line shape analysis of the ω–, ω+ coupled phonon‐plasmon modes. It was found that the intensity profiles of the Raman polar A1(LO) phonon mode depend on the thickness of the undoped n0 layer, while the intensity profiles of the nonpolar E2high mode were unaffected by the doping. The change in frequency and linewidth of E2high mode throughout the thickness of the GaN layers, however reveal structural inhomogeneity in the diode structures. The carrier concentration and mobility exhibit only slight variations with thickness with the largest variation being at the free surface of the device. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)