As-grown Defects Research Articles

Cz-Silicon Produced from Solar Grade and Recycled Materials. Part I: Bulk Properties and Formation of As-Grown Defects

The current paper is the first of two, investigating the properties of Czochralski-silicon (Cz-silicon) materials and solar cells produced with recycled and compensated silicon materials. In this work, impurity levels in two experimental Cz-silicon ingots were characterized by glow discharge mass spectroscopy and FTIR. The combined effects of these impurities on recombination properties were investigated by photoluminescence imaging and compared to bulk properties of a reference ingot. Ring pattern distribution of as-grown micro-defects in the ingots, related to elevated carbon and oxygen levels, were delineated based on the results of two steps of dry oxidation. Moreover, the mechanism of defect formation in the experimental Cz-Si during solidification and oxidation is elaborated in light of thermodynamic theories. Meanwhile, the positive effect of phosphorus in compensated feedstock is discussed in relation to the minority carrier lifetime based on the quasi-steady-state photoconductance results, as well as on the restricted formation of oxygen-related defects during solidification by analyzing samples after oxidation and copper decoration.

Investigating the effect of carbon on oxygen behavior in n-type Czochralski silicon for PV application

The objective of the current work was to understand the effect of carbon as an impurity in silicon in terms of the formation of as-grown oxygen defects and the subsequent behavior of these defects in n-type Czochralski (Cz) silicon during heat treatment.Three n-type Cz ingots with different carbon levels were used in the investigation. Copper decoration was used to quantify the number of as-grown defects, while a two-step oxidation process (4 h at 750°C and 16h at 1050°C) was used to study the evolution of as-grown defects, that is, the formation and morphology of oxygen precipitates (stacking faults and smaller precipitates) in the silicon during heat treatment. Carrier Density Imaging (CDI) revealed the defect distribution and distinguished their states.Results from the study show that substitutional carbon enhances the etch pit density on the copper decorated samples; indicating an enhanced concentration of defects when the carbon level in the material increases. The higher number density but smaller size as-grown oxygen defects is concluded to be induced by the presence of substitutional carbon, given the oxygen precipitate formation pattern and morphology. Vacancies introduced by carbon did not, however, affect the density of voids significantly and we hence conclude that vacancies were largely consumed by the formation of oxygen complexes, as illustrated by the presence of a higher number density of as-grown oxygen defects in samples with a high carbon concentration. The highest effective minority carrier lifetime of as-grown wafers after amorphous-Si (a-Si) passivation was found on the sample with the highest carbon concentration, and the lifetime in the all samples showed stronger dependence on the oxygen concentration than on the carbon content.

Radial oxygen precipitation of a 12” CZ silicon crystal studied in-situ with high energy X-ray diffraction

The radial oxygen nucleation and precipitation in Czochralski-grown silicon crystals is observed in real time via the local evolution of the X-ray Bragg intensity. Using a high X-ray energy and a divergent beam setup, it becomes possible to examine the impact of the local as-grown point defect configuration on the formation of bulk micro defects in a crystal of macroscopic dimensions. As a salient feature the transition from vacancy- to interstitial-dominated regions in a 12” crystal can be identified, and its influence on the precipitation kinetics is quantified through effective oxygen diffusion constant. It appears enhanced by up to a factor of six in vacancy-rich regions as compared to crystal volumes which are identified as point-defect free where it agrees well with the literature value.

Spectral broadening due to post-growth annealing of a long-wave InGaAs/GaAs quantum dot infrared photodetector with a quaternary barrier layer

We present the effect of post-growth rapid thermal annealing on multilayer, uncoupled In0.50Ga0.50As/GaAs quantum dot infrared photodetectors (QDIPs) with a combination of quaternary In0.21Al0.21Ga0.58As and GaAs capping. Long wave photoresponse (~0.11eV or 10.2μm) with narrow spectral width (14%) is obtained from as-grown QDIPs. The spectral width increases to 45% for QDIP annealed at 800°C. It is likely that a large inter-diffusion effect because of annealing changes the composition of the quantum dots and capping layer, thus increasing the QDIP spectral width. Dark current as well as noise current is decreased for 650°C annealed QDIP compared to as-grown detector. Passivation of as-grown defect by post growth annealing predicted the improvement of dark current. Such improvement and broadening of characteristics may render such QDIPs suitable for use in broadband infrared imaging applications.

Charge Transport and Degradation in HfO2 and HfOx Dielectrics

We combine experiments and simulations to investigate leakage current and breakdown (BD) in stoichiometric and sub-stoichiometric hafnium oxides. Using charge-transport simulations based on phonon-assisted carrier tunneling between trap sites, we demonstrate that higher currents generally observed in HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> are due to a higher density of the as-grown oxygen vacancy defects assisting the charge transport. Reduction of the dielectric BD field ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BD</sub> ) in HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> is explained by the lower zero-field activation energy ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A,G</sub> ) of the defect generation process, as extracted from time-dependent dielectric BD experiments.

As-grown deep-level defects in n-GaN grown by metal–organic chemical vapor deposition on freestanding GaN

Traps of energy levels Ec −0.26 and Ec −0.61 eV have been identified as as-grown traps in n-GaN grown by metal–organic chemical vapor deposition by using deep level transient spectroscopy of the Schottky contacts fabricated by resistive evaporation. The additional traps of Ec −0.13 and Ec −0.65 eV have been observed in samples whose contacts are deposited by electron-beam evaporation. An increase in concentration of the Ec −0.13 and Ec −0.65 eV traps when approaching the interface between the contact and the GaN film supports our argument that these traps are induced by electron-beam irradiation. Conversely, the depth profiles of as-grown traps show different profiles between several samples with increased or uniform distribution in the near surface below 50 nm. Similar profiles are observed in GaN grown on a sapphire substrate. We conclude that the growth process causes these large concentrations of as-grown traps in the near-surface region. It is speculated that the finishing step in the growth process should be an essential issue in the investigation of the surface state of GaN.

A detailed investigation on the impact of post-growth annealing on the materials and device characteristics of 35-layer In0.50Ga0.50As/GaAs quantum dot infrared photodetector with quaternary In0.21Al0.21Ga0.58As capping

The effect of post-growth rapid thermal annealing on 35-layer In0.50Ga0.50As/GaAs quantum dot infrared photodetector (QDIP) with quaternary In0.21Al0.21Ga0.58As capping has been investigated. Transmission electron microscopy showed some as-grown defects were removed by post growth annealing treatment. An increase in the compressive strain in the heterostructure due to annealing was identified from X-ray diffraction curve. A two-color photoresponse in the long-wave region (8.5 and 10.2μm) was observed in both as-grown device and those annealed at 650°C temperature. A three-fold enhancement in peak responsivity was observed in the QDIPs annealed at 650°C (1.19A/W) compared to that in the as-grown (0.34A/W). Detectivity also increased by two fold from as-grown to 650°C annealed device. The changes are attributed to the removal of as-grown defects and dislocations during epitaxial growth. These removals changed the confinement potential profile, which resulted in an improvement in the detectivity and responsivity of the annealed sample.

Depth dependent lattice disorder and strain in Mn-implanted and post-annealed InAs thin films

The lattice order degree and the strain in as-grown, Mn-implanted and post-implanted annealed InAs thin films were investigated with depth resolution by means of Rutherford backscattering spectrometry in channeling conditions (RBS/C). Three main crystallographic axes were analyzed for both In and As sublattices. The behaviour of the induced defects was evaluated in two regions with different native defects: the interface and the surface. The results show that Mn implantation and post-implantation annealing are anisotropic processes, affecting in a different way the In and As sublattices. The mechanisms influencing the enhancement and deterioration of the crystal quality during the implantation are discussed in relation to the as-grown defects and the segregation of the elements.

Micro Defect Size in Si Single Crystal Grown by Czochralski Method

Single crystals for 300 mm wafer are grown by horizontal magnetic Czochralski method. 300 mm wafers are made from the vertical samples cut from crystal along ingot axial direction. Micro defects in various defect regions are investigated with various measurement methods. In order to investigate the size of the defects, the locations of defects are identified, the wafers with the defects are cut in cross-sectional direction, and the sizes of the defects are measured by transmission electron microscopy (TEM). The voids more than 20 nm size exist in vacancy-rich region. Any as-grown defect is not observed by any available measurement tools in the region having nuclei of oxidation-induced stacking fault (P-band), pure silicon in a vacancy-dominant crystal region (Pv), and pure silicon in an interstitial-silicon-dominant crystal region (Pi). High sensitive laser scattering tomography system with the detection limit of 20 nm size is used to investigate as-grown defects in P-band, Pi, and Pv regions. It is concluded that there are no as-grown defects more than 20 nm size in P-band, Pi, and Pv regions.

GaN optical degradation during high energy Sn5+ ion irradiation

GaN(0001) epilayers grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD) have been irradiated with 75 MeV Sn5+ ions at the fluences of 1011, 1012, and 1013 ions/cm2. Structural and optical studies reveal that GaN epilayer withstands 75 MeV Sn5+ ion irradiations up to 1011 ions/cm2 ion fluences. High resolution X-ray diffraction results showed that the FWHM corresponding to (0002) plane increased from 227 to 279 arc-seconds after Sn-ions irradiation. Red shift was observed in the yellow luminescence (YL) emission by photoluminescence (PL), corresponds to the concentration of ion fluences. Donor-bound exciton (DBE) and free exciton (FEA, FEB and FEC) emissions were observed for as-grown and irradiated GaN samples up to 1012 ions/cm2 at 2K PL measurements. Free excitons are dominated by low-temperature PL measurements for as-grown and irradiated GaN samples at 1011 and 1012 ion fluences. Atomic force microscopy images show the RMS roughness increases with increasing Sn-ion fluences by removing as-grown GaN surface defects.

Asymmetric angular dependence of Jc in coated conductors prior to and after fast neutron irradiation

Angular dependent measurements of Jc were performed on a commercial coated conductor (SuperPower) consisting of a 1μm thick YBCO layer grown on a MgO IBAD buffer layer. An asymmetric behavior of the angular dependence of Jc (Jc(Ф)) was found with a changing distance between the two peaks at different temperatures and applied magnetic fields. One peak always occurs when the field is oriented parallel to the tape surface, the other smaller peak is located in the perpendicular orientation at high fields, but slightly shifted (by up to 10°) at 77K and low magnetic fields. This peak shift, the overall Jc(Ф) asymmetry and the influence of fast neutron irradiation on Jc(Ф) are discussed. The spherical defects, introduced by collisions of fast neutrons with the lattice atoms, are randomly distributed, add to the as-grown defect structure and change the critical current anisotropy by altering both peaks.

Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure

Effect of post-growth thermal annealing within the temperature range 200 to 430 oC for 15 min on the luminescent characteristics of CdSe/ZnSe quantum dot (QD) heterostructure was studied. Annealing at lower temperatures (Tann 270 oС) results in an increase by a factor of 2-3 of the intensity of two photoluminescence bands observed, the first being caused by excitonic transitions in QDs and the second one being connected with the defect complex including a column II vacancy. The effect is supposed to be caused by annealing of as-grown nonradiative defects. Annealing at higher temperatures (Tann > 270 oС) stimulates a decrease of the QD photoluminescence band intensity and up to 100 meV blue shift of its peak position. The former is explained by generation of extended defects and reduction of the QD density. The blue shift observed at 370-430 oС is ascribed to diffusion of cadmium from QDs that also results in reduction of the QD density. It is found that the energy of excitonic transitions in the wetting layer does not change upon annealing. Lower thermal stability of QDs as compared to that of the wetting layer has been explained by strain-enhanced lateral Cd/Zn interdiffusion via vacancies. The presence of column II vacancies in the wetting layer is proved by characteristics of defect-related PL band and its excitation spectra.

Wafer Mapping Using Deuterium Enhanced Defect Characterization

Deuterium (as well as other hydrogen isotopes) binds with a wide range of morphological defects in semiconductors and, as such, becomes distributed similarly to those defects. Thus, the deuterium profile within the sample serves as the basis of a technique for defect mapping known as amethyst wafer mapping (AWM). The efficiency of this technique has been demonstrated by evaluation of ion-induced damage in implanted Si, as well as as-grown defects in HgCdTe (MCT) epilayers. The defect tagging or decoration capability of deuterium is largely material independent and applicable to a wide range of defect morphologies. A number of analytical techniques including ion channeling and etch pit density measurements were used to evaluate the AWM results.

Effects of thermal treatment on optically active vacancy defects in CVD diamonds

In this paper we report on the preliminary results obtained in as-grown and annealed CVD diamond with positron annihilation spectroscopy. We observe clustering of as-grown vacancy defects in annealing above 1400 ∘ C . The optical activity of the vacancy defects is studied by shining monochromatic light in the visible range during the positron measurements.

Microstructure and defect investigations of the as-grown and annealed ZnO/Si thin films

The microstructure and defects of as-grown and annealed ZnO/Si thin films were investigated by high-resolution transmission electron microscopy (HRTEM). The cross-sectional bright-field TEM images showed that the ZnO thin films consisted of columnar grains. The selected-area electron diffraction pattern showed that the ZnO/Si thin film were c-axis oriented and the deviation angle along the ZnO (0001) direction with respect to the growth direction was no more than 5°. The grain boundaries can be classified into three types: low-angle boundaries, boundaries near a 30 deg angle with (101¯0) facet structures and large-angle boundaries. In addition to grain boundary, the as-grown and annealing processing defects were also investigated by HRTEM. The crystal quality of the ZnO/Si thin films can be improved by annealing at 600 °C. However, at too high an annealing temperature of 800 °C, the newly produced processing defects such as three layer stacking fault and double stacking fault were formed. The research was useful for us to find optimal growth conditions to improve ZnO/Si thin film quality.

Thermal stability of defects in p-type as-grown 6H-SiC

An isochronal annealing series was performed in order to investigate the thermalbehaviour of as-grown defects in Al-doped 6H-SiC. Five hole traps have been detected,by means of deep level transient spectroscopy (DLTS), above the valence band(EV) in the 0.2–1.3 eV energy range. One trap, labelled MZ2, anneals out after a1800 °C heat treatment, while the others display higher thermal stability. The nature of thesecentres is discussed on the basis of the previous experimental and theoretical data found inthe literature. Particular emphasis has been given to a trap labelled MZ3, which weidentified as the D-centre, and to the effect that the electric field has on its emission timeconstant.

Identifying Contributions to Electrical Instability in High-k Gate Stacks: As-Grown vs. Generated Defects

Stress-induced changes in the high-k gate stack electrical characteristics may originate in both the Hf-based dielectric film and the SiO2-like interfacial layer, in which instability could be induced by different mechanisms. In this work, we focus on the contributions from as-grown defects and defect precursors in each of these layers, as well as stress-generated defects, under low and high voltage stress regimes. Physical data, consistent with ab initio calculations, is presented in support of proposed theoretical explanations for the mechanisms that give rise to electrical performance.

Two-dimensional small-angle X-ray scattering from as-grown and heat-treated synthetic quartz

The small-angle X-ray scattering (SAXS) in as-grown and heat-treated quartz crystals was investigated as a function of the azimuth angle around the primary beam. For this, samples parallel to (10\bar 10) were extracted from Z- and −X-growth sectors of a synthetic quartz bar which had the OH content evaluated by infrared spectroscopy (IRS). In addition, SAXS and IRS were independently recorded as a function of heating temperature. As a result, the two-dimensional SAXS images revealed an anisotropic pattern randomly decorated by low-intensity Kossel lines. The intensities were projected along specific directions or were axially integrated around the primary beam. It was observed that the Porod invariant (Q) increased and the Kossel lines moved slightly to higher q values with increasing temperature. The effect of the sample orientation on the Q value and the lack of a clear relationship between Q and OH content suggested that the diffuse scattering due to the periodicity of the crystal lattice played an important role in the small-angle scattering of quartz. The net scattering intensities produced by heat-treatment at 873 K were attributed to molecular water aggregates created by the diffusion of as-grown OH defects.

Effect of ion implantation on quantum well infrared photodetectors

Ion implantation is a postgrowth processing technique which, when combined with annealing, can be used to tune the absorption wavelength of quantum well devices. We have implanted and annealed, three different quantum well infrared photodetector structures, and measured the absorption spectra of the samples by Fourier transform spectroscopy. The peak absorption wavelength shift of each structure has been calculated as a function of diffusion length by simulating the diffusion processes. We found different diffusion rates for the structures and attribute this to different numbers of as-grown defects. Our results indicate that agglomeration of single defects into defect clusters limits the ability of ion implantation to tune the wavelength of a structure with a higher number of as-grown defects. Thus, a structure with the lowest number of as-grown defects is most useful for fabricating a multi-color quantum well photodetector by ion implantation, because in this case ion implantation can enhance the diffusion rate considerably leading to large red- shift in peak absorption wavelength.

Electron Trap Generation in High-&lt;tex&gt;$kappa$&lt;/tex&gt;Gate Stacks by Constant Voltage Stress

Positive bias constant voltage stress combined with charge pumping (CP) measurements were applied to study trap generation phenomena in SiO 2/HfO2/TiN stacks. Using gate stacks with varying thicknesses of the interfacial SiO2 layer (IL) or high-kappa layer and analysis for frequency-dependent CP data developed to address trap depth profiling, the authors have determined that the defect generation in the stress voltage range of practical importance occurs primarily within the IL on as-grown precursor defects most likely caused by the overlaying HfO2 layer. The generated traps can be passivated by a forming gas or nitrogen (N2 ) anneal, whereas a postanneal stress reactivates these defects. The results obtained identify the IL as one of the major targets for reliability improvement of high-kappa stacks