Doping Density Profile Research Articles

Accessing the band alignment in high efficiency Cu(In,Ga)(Se,S)2 (CIGSSe) solar cells with an InxSy:Na buffer based on temperature dependent measurements and simulations

We present a one-dimensional simulation model for high efficiency Cu(In,Ga)(Se,S)2 solar cells with a novel band alignment at the hetero-junction. The simulation study is based on new findings about the doping concentration of the InxSy:Na buffer and i-ZnO layers as well as comprehensive solar cell characterization by means of capacitance, current voltage, and external quantum efficiency measurements. The simulation results show good agreement with the experimental data over a broad temperature range, suggesting the simulation model with an interface-near region (INR) of approximately 100 nm around the buffer/absorber interface that is of great importance for the solar cell performance. The INR exhibits an inhomogeneous doping and defect density profile as well as interface traps at the i-layer/buffer and buffer/absorber interfaces. These crucial parameters could be accessed via their opposing behavior on the simulative reconstruction of different measurement characteristics. In this work, we emphasize the necessity to reconstruct the results of a set of experimental methods by means of simulation to find the most appropriate model for the solar cell. Lowly doped buffer and intrinsic window layers in combination with a high space charge at the front of the absorber lead to a novel band alignment in the simulated band structure of the solar cell. The presented insights may guide the strategy of further solar cell optimization including (alkali-) post deposition treatments.

Design of Band Engineered HgCdTe nBn Detectors for MWIR and LWIR Applications

In this paper, we present a theoretical study of mercury cadmium telluride (HgCdTe)-based unipolar n-type/barrier/n-type (nBn) infrared (IR) detector structures for midwave IR and longwave IR spectral bands. To achieve the ultimate performance of nBn detectors, a bandgap engineering method is proposed to remove the undesirable valence band discontinuity that is currently limiting the performance of conventional HgCdTe nBn detectors. Our proposed band engineering method relies on simultaneous grading of the barrier composition and doping density profiles, leading to efficient elimination of the valence band discontinuity. This allows the detector to operate at |V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bias</sub> | <;50 mV, rendering all tunneling-related dark current components insignificant and allowing the detector to achieve the maximum possible diffusion current limited performance.

Secondary Electron Microscopy Dopant Contrast Image (SEMDCI) for Laser Doping

Laser doping has been the subject of intense research over the past decade, due to its potential to enable high-efficiency, low-cost silicon solar cell fabrication. Information about the doping profile that is created by the process is critical for process optimization but is generally difficult to obtain. We apply the technique of secondary electron image (SEI) contrast to the characterization of the cross sections of laser-doped lines. We demonstrate that this technique can be used for a large range of different dopant sources and different laser doping methods and that good dopant contrast can be obtained under a relatively wide range of microscope parameters. Comparison of dopant contrast and doping density profiles shows that the substrate doping is an important parameter that can significantly influence the dopant contrast, particularly at low (~1018 cm-3) and high (~10 20 cm-3 ) dopant densities. When suitable calibration samples are used, the technique can be employed to obtain quantitative dopant density images for p-type laser-doped regions, albeit currently over a limited range of dopant densities and with relatively large error. Furthermore, the technique can be used to evaluate the risk of metallization shunts near the edges of dielectric film windows that are opened by the laser.

Modeling metastabilities in chalcopyrite-based thin film solar cells

Cu(In,Ga)Se2-based thin film solar cell devices exhibit metastable electrical behavior. This behavior is often ascribed to intrinsic defects that can change configuration accompanied by large lattice relaxations. We extended the thin film solar cell simulation software scaps to enable the simulation of the metastable behavior of this kind of defects. The statistics that are needed to describe metastable defects are discussed. The procedure that has been implemented is introduced, and special attention is paid to the convergence of the method for high defect densities. The model is demonstrated by simulating the effect of voltage induced metastabilities on the capacitance-voltage characteristics. Some of the features present in the measured apparent doping density profiles can be directly related to presence of metastable defects.

Speckle Defect by Dark Leakage Current in Nitride Stringer at the Edge of Shallow Trench Isolation for CMOS Image Sensors

The leakage current in a CMOS image sensor (CIS) can have various origins. Leakage current investigations have focused on such things as cobalt-salicide, source and drain scheme, and shallow trench isolation (STI) profile. However, there have been few papers examining the effects on leakage current of nitride stringers that are formed by gate sidewall etching. So this study reports the results of a series of experiments on the effects of a nitride stringer on real display images. Different step heights were fabricated during a STI chemical mechanical polishing process to form different nitride stringer sizes, arsenic and boron were implanted in each fabricated photodiode, and the doping density profiles were analyzed. Electrons that moved onto the silicon surface caused the dark leakage current, which in turn brought up the speckle defect on the display image in the CIS.

The growth of ultra-uniform B-doped Si/SiGe multiple quantum wells by RTCVD for mid-IR applications

A process for the growth of uniform boron-doped Si/SiGe multiple quantum wells by rapid thermal chemical vapour deposition for studying intersubband transitions for quantum cascade laser applications has been developed. The doping density profiles are extremely sharp (∼2 nm/decade and ∼3 nm/decade for the leading and trailing boron edges, respectively) measured by high resolution secondary ion mass spectroscopy, and the well-to-well uniformity is excellent. A higher temperature is used for the silicon layers (625 °C) compared to the SiGe layers (525 °C) to keep the growth rate of both layers in the range of 0.1 nm s−1.

Parameter extraction using novel phenomena in nano-MOSFETs with ultra-thin (EOT = 0.46–1.93 nm) high-K gate dielectrics

Novel features of and a new technique for parameter extraction are outlined here for MOS devices with ultra-thin high-K MOS devices. These parameters include the channel doping density, the doping density profile, and the flat-band voltage—all very important for the high-K technology, besides the surface potential, the gate dielectric capacitance, and the accumulation surface potential quotient. The reliability of the new technique was confirmed by comparison with the results from other techniques. The experimental results indicate diffusion of metal impurities into the channel region during the device processing.

Spin-polarized electron transport at ferromagnet/semiconductor Schottky contacts

We theoretically investigate electron spin injection and spin-polarization sensitive current detection at Schottky contacts between a ferromagnetic metal and an n-type or p-type semiconductor. We use spin-dependent continuity equations and transport equations at the drift-diffusion level of approximation. Spin-polarized electron current and density in the semiconductor are described for four scenarios corresponding to the injection or the collection of spin polarized electrons at Schottky contacts to n-type or p-type semiconductors. The transport properties of the interface are described by a spin-dependent interface resistance, resulting from an interfacial tunneling region. The spin-dependent interface resistance is crucial for achieving spin injection or spin polarization sensitivity in these configurations. We find that the depletion region resulting from Schottky barrier formation at a metal/semiconductor interface is detrimental to both spin injection and spin detection. However, the depletion region can be tailored using a doping density profile to minimize these deleterious effects. For example, a heavily doped region near the interface, such as a delta-doped layer, can be used to form a sharp potential profile through which electrons tunnel to reduce the effective Schottky energy barrier that determines the magnitude of the depletion region. The model results indicate that efficient spin-injection and spin-polarization detection can be achieved in properly designed structures and can serve as a guide for the structure design.

The Influence of Stressing at Different Biases on the Electrical and Optical Properties of CdS/CdTe Solar Cells

ABSTRACTCadmium Sulfide/Cadmium Telluride (CdS/CdTe) devices are subject to stress under various biases. Striking differences are observed with the Current-Voltage, and Capacitance- Voltage measurements for cells degraded at 100°C in dark under forward (FB), open circuit (OC), and reverse (RB) biases. RB stress provides the greatest degradation, and the apparent doping density profile shows anomalous behavior at the zero bias depletion width. Thin films of CdS, both doped and undoped, with Cu are characterized with photoluminescence (PL). The PL spectra from the CdS films are correlated with the CdS spectra from stressed devices, revealing that Cu signatures in the CdS layer of stressed devices are a function of stress biasing. Device modeling using AMPS-1D produces IV curves similar to that in RB degraded devices, by only varying the trap level concentration in the CdS layer.

A new charge-control model for single- and double-heterojunction bipolar transistors

A new charge-control relation is derived for heterojunction bipolar transistors. The relation is valid for arbitrary doping density profiles and for all levels of injection in the base. It is applicable to both single- and double-heterojunction transistors. The model is an improvement over another recently proposed charge-control model that was valid only for constant doping density and low injection in the base. Large- and small-signal equivalent circuit models are also presented for heterojunction bipolar transistors. Comparisons with numerical and experimental data show excellent agreement. >

Redistribution of delta-doped Sb in Si

Redistribution of delta-doped Sb in Si with various areal concentrations upon post-growth annealing was investigated by means of secondary ion mass spectrometry (SIMS). The diffusion profiles are dependent on the initial doping density and non-Gaussian profiles were obtained except for the lowest-doped sample. The doped layers are stable up to 550°C, regardless of the doping density. For treatment at temperatures higher than 700°C, the upper limit of the doping density was found to be set at 0.01 monolayer (ML). It was also found that there is no crystalline quality difference affecting the diffusion of Sb between the recrystallized and the MBE-grown layers.

Electrical and structural properties of pulse laser-annealed polycrystalline silicon films

Doped epitaxial films of Si on single-crystal high-resistivity Si substrates have been prepared using ion implantation and Q-switched ruby laser annealing of LPCVD polycrystalline Si layers. Films, doped with B or As in the range 1017to 5 × 1020cm-3were studied by the measurement of their resistivities, Hall mobilities, and doping density profiles. The good film quality achieved permitted the fabrication of p-channel MOS transistors which, through measurements of threshold voltage and transconductance, yielded additional data on the surface mobility and the integrity of the Si-SiO 2 interface. The electrical properties of the films compared favorably with those of similarly doped single-crystal material, and transmission electron microscopy was used to confirm the good structural quality of the epitaxial growth.

An analytic model for minority-carrier transport in heavily doped regions of silicon devices

A simple analytic description of the minority-carrier current injected into typical diffused (or ion-implanted) heavily doped regions of silicon bipolar devices is derived. The effects of energy-bandgap narrowing, majority-carrier degeneracy, Auger recombination, a doping-density gradient, and surface recombination are accounted for tractably by using key approximations. Numerical solutions of the minority-carrier continuity equation support the model and facilitate the evaluation of the model parameters, which follow directly from the doping-density profile. The accuracy of the model is within the bounds of uncertainty emanating from present equivocal characterizations of bandgap narrowing and of Auger carrier lifetimes.

Extension of Existing Models to Ion-Implanted MESFET's

This work extends uniform MESFET's models to MESFET's with arbitrary doping density profiles. Numerical computations have carried out for ion-implanted devices with a Gaussian doping density profile. Good agreement is found between theory and experiment.