1.4 Defects and impurities in semiconductors
10 min read•august 20, 2024
Defects and impurities in semiconductors are crucial factors that shape their electrical, optical, and mechanical properties. These imperfections in the crystal lattice can be intentional or unintentional, ranging from point defects to larger volume defects.
Understanding and controlling these defects is essential for optimizing semiconductor devices. Various techniques are used to characterize and manage defects, including electrical and optical measurements, structural analysis, and methods to reduce or passivate unwanted imperfections. This knowledge is fundamental for improving device performance and reliability.
Types of defects
- Defects in semiconductor materials play a crucial role in determining their electrical, optical, and mechanical properties
- Understanding the different types of defects is essential for controlling and optimizing semiconductor devices
- Defects can be classified based on their dimensionality and the way they disrupt the crystal lattice
Point defects
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- Zero-dimensional defects that involve single atoms or vacancies in the crystal lattice
- Examples include vacancies, interstitials, and substitutional impurities
- Point defects can significantly impact the electrical and optical properties of semiconductors
Line defects
- One-dimensional defects that extend along a line in the crystal lattice
- Dislocations are the most common type of line defect
- Line defects can affect the mechanical properties and introduce energy levels within the bandgap
Planar defects
- Two-dimensional defects that form at the interface between two regions of the crystal
- Examples include grain boundaries, stacking faults, and twin boundaries
- Planar defects can influence the electrical and mechanical properties of semiconductors
Volume defects
- Three-dimensional defects that extend over a significant volume of the crystal
- Voids, precipitates, and inclusions are examples of volume defects
- Volume defects can impact the mechanical, thermal, and electrical properties of semiconductors
Point defects
- Point defects are localized imperfections in the crystal lattice that involve single atoms or vacancies
- They can be intrinsic (vacancies and interstitials) or extrinsic (substitutional impurities)
- Point defects introduce energy levels within the bandgap and can alter the electrical and optical properties of semiconductors
Vacancies
- Missing atoms from their regular lattice sites
- Vacancies can be created during crystal growth or by radiation damage
- They act as acceptor-like defects and can trap electrons or holes
Interstitials
- Atoms occupying non-lattice sites between regular lattice positions
- Interstitials can be formed during crystal growth or by radiation damage
- They act as donor-like defects and can introduce additional energy levels within the bandgap
Substitutional impurities
- Foreign atoms replacing the host atoms in the crystal lattice
- Intentionally introduced to control the electrical properties (doping)
- Examples include boron (acceptor) and phosphorus (donor) in silicon
Frenkel defects
- Pairs of vacancies and interstitials created by the displacement of an atom from its regular lattice site to an position
- Commonly observed in ionic crystals and can be induced by radiation damage
Schottky defects
- Pairs of cation and anion vacancies in ionic crystals
- Maintain the overall charge neutrality of the crystal
- Can be created during crystal growth or by
Line defects
- Line defects, also known as dislocations, are one-dimensional imperfections in the crystal lattice
- They can be classified into edge, screw, and mixed dislocations based on their geometry
- Dislocations introduce strain fields and can affect the mechanical, electrical, and optical properties of semiconductors
Edge dislocations
- Formed by the insertion or removal of an extra half-plane of atoms in the crystal lattice
- Characterized by a line defect perpendicular to the extra half-plane
- Introduce dangling bonds and can act as recombination centers for charge carriers
Screw dislocations
- Formed by a shear displacement of the crystal lattice along a line defect
- The Burgers vector is parallel to the dislocation line
- Can introduce deep energy levels within the bandgap and affect the electrical properties
Mixed dislocations
- Combination of edge and screw components
- Most dislocations in real crystals are mixed dislocations
- Can have complex strain fields and interactions with other defects
Burgers vector
- A vector that represents the magnitude and direction of the lattice distortion associated with a dislocation
- Determined by performing a closed loop around the dislocation and comparing it with the same loop in a perfect crystal
- Characterizes the type and strength of the dislocation
Dislocation density
- The number of dislocations per unit volume or area of the crystal
- Expressed in units of cm^-2 or cm^-3
- Higher dislocation densities can lead to increased carrier scattering and reduced carrier
Planar defects
- Planar defects are two-dimensional imperfections that occur at the interface between two regions of the crystal
- They can be classified into grain boundaries, stacking faults, twin boundaries, and antiphase boundaries
- Planar defects can influence the electrical, mechanical, and optical properties of semiconductors
Grain boundaries
- Interfaces between two differently oriented crystalline regions (grains) in a polycrystalline material
- Act as scattering centers for charge carriers and can reduce carrier mobility
- Can segregate impurities and defects, leading to localized changes in electrical properties
Stacking faults
- Irregularities in the stacking sequence of atomic planes in a crystal
- Commonly observed in close-packed structures (FCC, HCP)
- Introduce localized strain and can affect the band structure and optical properties
Twin boundaries
- Special type of grain boundary where the orientation of the crystal on one side is a mirror image of the other side
- Can be formed during crystal growth or by mechanical deformation
- Have lower energy than general grain boundaries and can improve the mechanical properties
Antiphase boundaries
- Interfaces between two regions of the crystal that have the same structure but are shifted by a fraction of the lattice parameter
- Commonly observed in ordered alloys (GaInP, GaAsSb)
- Can introduce deep energy levels and affect the electrical and optical properties
Volume defects
- Volume defects are three-dimensional imperfections that extend over a significant volume of the crystal
- They can be classified into voids, precipitates, inclusions, and cracks
- Volume defects can impact the mechanical, thermal, and electrical properties of semiconductors
Voids
- Empty spaces or cavities within the crystal
- Can be formed during crystal growth or by the agglomeration of vacancies
- Reduce the mechanical strength and thermal conductivity of the material
Precipitates
- Secondary phases formed by the aggregation of impurities or excess constituents in the crystal
- Can be intentionally introduced to control the electrical properties (gettering)
- Can also form unintentionally during crystal growth or processing
Inclusions
- Foreign particles or phases embedded within the crystal
- Can be introduced during crystal growth or processing
- Act as scattering centers for charge carriers and can reduce carrier mobility
Cracks
- Macroscopic fractures in the crystal
- Can be caused by mechanical stress, thermal shock, or defect interactions
- Severely degrade the mechanical properties and can lead to device failure
Defect formation
- Defects in semiconductors can form during crystal growth, processing, or operation
- Understanding the thermodynamics and kinetics of defect formation is crucial for controlling and minimizing their impact on device performance
Thermodynamics of defects
- Defects are thermodynamically stable if their formation reduces the overall free energy of the system
- The equilibrium concentration of defects depends on temperature, pressure, and chemical potentials
- Higher temperatures generally lead to higher equilibrium defect concentrations
Defect equilibrium concentration
- The concentration of defects in a crystal at thermodynamic equilibrium
- Determined by the balance between defect formation and annihilation processes
- Can be calculated using statistical thermodynamics and the defect formation energy
Defect formation energy
- The energy required to create a defect in an otherwise perfect crystal
- Depends on the type of defect, the host material, and the environmental conditions
- Lower formation energies lead to higher equilibrium defect concentrations
Defect migration
- The movement of defects within the crystal lattice
- Driven by concentration gradients, stress fields, or electric fields
- Plays a crucial role in defect interactions, aggregation, and annihilation
Impurities in semiconductors
- Impurities are foreign atoms that are intentionally or unintentionally incorporated into the semiconductor crystal
- They can be classified based on their concentration, energy levels, and electronic behavior
- Impurities play a crucial role in controlling the electrical properties of semiconductors
Intentional vs unintentional impurities
- Intentional impurities are deliberately introduced to control the electrical properties (doping)
- Unintentional impurities are inadvertently incorporated during crystal growth or processing
- Unintentional impurities can have detrimental effects on device performance
Shallow vs deep impurities
- Shallow impurities introduce energy levels close to the band edges (within a few kT)
- Deep impurities introduce energy levels far from the band edges (mid-gap states)
- Shallow impurities are more effective in controlling the electrical properties
Donor impurities
- Impurities that can donate electrons to the conduction band
- Examples include group V elements (P, As, Sb) in silicon
- Donor impurities increase the electron concentration and create n-type semiconductors
Acceptor impurities
- Impurities that can accept electrons from the valence band, creating holes
- Examples include group III elements (B, Al, Ga) in silicon
- increase the hole concentration and create p-type semiconductors
Amphoteric impurities
- Impurities that can act as both donors and acceptors depending on the host material and the lattice site they occupy
- Examples include silicon in GaAs and germanium in silicon
- Amphoteric impurities can compensate each other and affect the net doping concentration
Effects of defects and impurities
- Defects and impurities can have significant effects on the electrical, optical, mechanical, and thermal properties of semiconductors
- Understanding these effects is crucial for designing and optimizing semiconductor devices
Electrical properties
- Defects and impurities introduce energy levels within the bandgap, which can act as traps, recombination centers, or scattering centers for charge carriers
- They can alter the , mobility, and lifetime, affecting the conductivity and device performance
- Examples include reduced carrier mobility due to ionized impurity scattering and increased leakage current due to deep level defects
Optical properties
- Defects and impurities can introduce new optical transitions, modify the band structure, and affect the absorption and emission of light
- They can be used to engineer the optical properties of semiconductors for specific applications (LEDs, lasers, detectors)
- Examples include the use of nitrogen- centers in diamond for quantum sensing and the use of rare-earth in optical fibers for amplification
Mechanical properties
- Defects, particularly dislocations and grain boundaries, can significantly influence the mechanical properties of semiconductors
- They can act as sources of stress concentration, reduce the yield strength, and promote crack propagation
- Examples include the reduced fracture toughness of polycrystalline silicon solar cells compared to single-crystal cells
Thermal properties
- Defects and impurities can scatter phonons and reduce the thermal conductivity of semiconductors
- They can also introduce localized heating and thermal stress, which can affect device reliability
- Examples include the reduced thermal conductivity of heavily doped silicon and the thermal runaway in power devices due to defect-induced leakage current
Defect and impurity characterization
- Characterizing defects and impurities in semiconductors is essential for understanding their impact on device performance and developing strategies for defect control
- Various characterization techniques are used to probe the electrical, optical, structural, and chemical properties of defects and impurities
Electrical characterization techniques
- Techniques that measure the electrical properties of semiconductors, such as conductivity, carrier concentration, and mobility
- Examples include Hall effect measurements, capacitance-voltage (C-V) profiling, and deep level transient spectroscopy (DLTS)
- Provide information about the energy levels, concentrations, and capture cross-sections of defects and impurities
Optical characterization techniques
- Techniques that probe the optical properties of semiconductors, such as absorption, emission, and reflectance
- Examples include photoluminescence (PL) spectroscopy, Raman spectroscopy, and Fourier-transform infrared (FTIR) spectroscopy
- Provide information about the band structure, , and vibrational modes of the material
Structural characterization techniques
- Techniques that investigate the atomic structure and morphology of semiconductors
- Examples include (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM)
- Provide information about the crystal structure, lattice parameters, defect types, and spatial distribution of defects
Chemical characterization techniques
- Techniques that analyze the chemical composition and bonding of semiconductors
- Examples include secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES)
- Provide information about the elemental composition, chemical states, and impurity concentrations in the material
Defect and impurity control
- Controlling defects and impurities is crucial for improving the performance, reliability, and yield of semiconductor devices
- Various strategies are employed to reduce the concentration of unwanted defects and impurities and to mitigate their effects
Growth techniques for defect reduction
- Optimizing the crystal growth conditions to minimize the formation of defects and impurities
- Examples include the use of high-purity source materials, controlled growth atmospheres, and in-situ monitoring techniques
- Techniques such as molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) enable precise control over the growth process
Gettering techniques
- Intentionally introducing impurities or defects to attract and immobilize unwanted impurities away from the active device regions
- Examples include the use of phosphorus diffusion gettering in silicon and the use of epitaxial gettering layers in III-V semiconductors
- Gettering can effectively reduce the concentration of detrimental impurities and improve device performance
Passivation techniques
- Treating the semiconductor surface to reduce the density of surface states and minimize their impact on device performance
- Examples include the use of hydrogen passivation in silicon and the use of sulfur passivation in III-V semiconductors
- Passivation can improve the carrier lifetime, reduce surface recombination, and enhance the stability of devices
Annealing techniques
- Applying thermal treatments to the semiconductor to promote the annihilation, redistribution, or transformation of defects and impurities
- Examples include rapid thermal annealing (RTA) for dopant activation and furnace annealing for defect reduction
- Annealing can help to restore the crystal quality, reduce the concentration of unwanted defects, and optimize the electrical properties of the material
Key Terms to Review (16)
Acceptor impurities: Acceptor impurities are specific types of dopants introduced into semiconductor materials to create holes in the crystal lattice, thereby enhancing the material's ability to conduct electricity. These impurities typically come from elements in group III of the periodic table, such as boron or aluminum, which have fewer valence electrons than the semiconductor host, like silicon. By accepting electrons from the semiconductor’s valence band, acceptor impurities facilitate the formation of p-type semiconductors, essential for various electronic devices.
Activation Energy: Activation energy is the minimum amount of energy required to initiate a chemical reaction or a change in the state of a system. This concept is crucial in understanding how defects and impurities in semiconductors can influence their electrical properties, as these factors can significantly alter the energy barriers that charge carriers must overcome for conduction to occur.
Carrier Concentration: Carrier concentration refers to the number of charge carriers (electrons and holes) in a semiconductor material, typically expressed in terms of carriers per cubic centimeter. This concept is crucial as it directly impacts the electrical properties of semiconductors, influencing conductivity, behavior under electric fields, and interactions with defects and impurities.
Defect Concentration: Defect concentration refers to the number of defects present in a semiconductor material per unit volume. These defects can include vacancies, interstitials, and impurities, and they significantly affect the electrical and optical properties of the semiconductor. A higher defect concentration can lead to increased scattering of charge carriers, affecting conductivity, mobility, and overall device performance.
Defect Density: Defect density refers to the number of defects per unit volume in a semiconductor material, typically expressed in units such as cm$^{-3}$. These defects can include vacancies, interstitials, and dislocations, and their presence can significantly impact the electronic and optical properties of semiconductors. Understanding defect density is crucial for evaluating the quality of semiconductor materials and their performance in devices, influencing recombination processes and fabrication techniques.
Defect energy levels: Defect energy levels are electronic states within the band gap of a semiconductor caused by the presence of defects or impurities. These levels can act as recombination centers for charge carriers, significantly impacting the electrical properties and behavior of the semiconductor material. Understanding these energy levels is crucial for improving device performance and designing better semiconductor materials.
Dopants: Dopants are impurities intentionally added to semiconductor materials to modify their electrical properties. By introducing specific atoms or molecules into the semiconductor lattice, dopants can either increase the number of free charge carriers, creating n-type or p-type semiconductors, which are essential for the functioning of various electronic devices.
Electron Paramagnetic Resonance: Electron paramagnetic resonance (EPR) is a spectroscopic technique used to study materials with unpaired electrons, providing insights into the electronic structure and dynamics of paramagnetic centers. This method is particularly useful for investigating defects and impurities in semiconductors, as it allows researchers to analyze the local environment and interactions surrounding unpaired electrons, which can significantly affect semiconductor properties.
Frenkel defect: A Frenkel defect is a type of point defect in a crystal lattice where an atom or ion is displaced from its normal position to an interstitial site, creating a vacancy and an interstitial defect. This defect occurs in ionic and covalent solids and plays a significant role in understanding the properties of semiconductors, as it influences electrical conductivity and material behavior.
Interstitial: An interstitial refers to a type of point defect in a crystal lattice where an atom or ion occupies a space in the lattice that is not normally occupied. These defects can significantly impact the properties of semiconductor materials, affecting their electrical, thermal, and mechanical behaviors. Understanding interstitials is crucial when studying how defects and impurities influence the overall functionality of semiconductors.
Mobility: Mobility refers to the ability of charge carriers, such as electrons and holes, to move through a semiconductor material when subjected to an electric field. This movement is influenced by factors like temperature, electric field strength, and the presence of defects or impurities in the material, which can either enhance or hinder the mobility of these charge carriers.
Point Defect Theory: Point defect theory refers to the study of localized imperfections in the crystalline structure of materials, particularly in semiconductors. These defects can occur as vacancies, interstitials, or substitutions and significantly influence the electronic properties and behavior of semiconductor devices. Understanding these point defects is crucial for optimizing performance and reliability in electronic applications.
Schottky Defect: A Schottky defect is a type of point defect in a crystalline solid, specifically in ionic crystals, where an equal number of cations and anions are missing from their lattice sites, creating vacancies. This vacancy creation disrupts the regular arrangement of ions in the crystal lattice, which can influence the material's electrical and thermal properties. The presence of Schottky defects plays a crucial role in understanding how defects and impurities affect the overall behavior of semiconductors.
Thermal Equilibrium: Thermal equilibrium occurs when two systems reach the same temperature and no net heat flows between them. This condition is essential in understanding various phenomena in semiconductors, where heat transfer can influence charge carrier dynamics and the behavior of defects and impurities.
Vacancy: A vacancy is a type of point defect in a crystal lattice that occurs when an atom is missing from its regular lattice position. This missing atom creates an empty site, which can significantly influence the physical properties of the material, such as electrical and thermal conductivity. The presence of vacancies is crucial for understanding how defects and impurities affect semiconductor behavior and carrier diffusion.
X-ray diffraction: X-ray diffraction is a powerful technique used to study the structure of crystalline materials by measuring the intensity and angles of X-rays scattered off the material. This technique provides essential information about the arrangement of atoms within a crystal, allowing for the determination of crystal structure, lattice parameters, and defects. The analysis of diffraction patterns also helps in understanding various properties of materials, including their electronic and mechanical characteristics.