8.3 Quantum dot lasers and light-emitting devices

Quantum dot lasers and LEDs are game-changers in optoelectronics. They use tiny semiconductor crystals to control light emission, offering better performance than traditional devices. These nanostructures provide unique optical properties that can be fine-tuned for various applications.

The quantum confinement effect in these devices leads to discrete energy levels, enhancing stimulated emission and optical gain. This results in lower threshold currents, improved temperature stability, and narrower spectral linewidths for lasers and LEDs, making them ideal for displays and lighting.

Quantum Dot Laser Fundamentals

Stimulated Emission and Optical Gain

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• Stimulated emission occurs when an excited electron transitions to a lower energy state, emitting a photon
• Photon emission triggered by interaction with another photon of the same energy and phase
• Process amplifies light by creating multiple identical photons
• Optical gain results from stimulated emission exceeding absorption in the active medium
• Gain coefficient measures the amplification of light intensity per unit length
• Population inversion required to achieve positive optical gain
• Quantum dots provide discrete energy levels, enhancing stimulated emission efficiency

Quantum Confinement Effects in Lasers

• Quantum dots confine charge carriers in three dimensions
• Confinement leads to discrete energy levels, resembling atomic-like states
• Energy level spacing depends on quantum dot size and composition
• Increased density of states at specific energies enhances laser performance
• Reduced threshold current density compared to bulk or quantum well lasers
• Temperature stability improved due to reduced carrier thermal escape
• Spectral linewidth narrowed, resulting in higher-quality laser output

Quantum Dot LED Devices

QD-LED Structure and Operation

• Quantum dot LEDs incorporate a layer of semiconductor nanocrystals
• Active layer sandwiched between electron and hole transport layers
• Charge carriers injected from electrodes into transport layers
• Electrons and holes migrate to quantum dot layer
• Carriers captured by quantum dots, forming excitons
• Exciton recombination results in light emission (electroluminescence)
• Emission wavelength tunable by adjusting quantum dot size and composition
• Multi-color displays achievable using different-sized quantum dots in separate pixels

Charge Injection and Transport Mechanisms

• Charge injection occurs at electrode-transport layer interfaces
• Energy level alignment crucial for efficient injection
• Electron transport layer (ETL) facilitates electron movement (ZnO, TiO2)
• Hole transport layer (HTL) conducts holes to active region (PEDOT:PSS, NiO)
• Charge balance important for optimal device performance
• Carrier blocking layers prevent leakage and improve efficiency
• Charge trapping at quantum dot surface states can impact performance
• Surface passivation techniques reduce non-radiative recombination

Quantum Dot LED Performance

Color Purity and Spectral Characteristics

• Quantum dots exhibit narrow emission spectra due to discrete energy levels
• Full-width at half-maximum (FWHM) of emission peak typically 20-40 nm
• Narrow linewidth results in high color purity and saturation
• Color gamut exceeds that of conventional phosphor-based LEDs
• Emission wavelength precisely controlled by quantum dot size
• Enables accurate color reproduction in displays
• Reduced color crosstalk between adjacent pixels in QD-LED displays
• Quantum dot size distribution affects overall spectral width

Efficiency and Performance Limitations

• External quantum efficiency (EQE) measures overall device performance
• Internal quantum efficiency (IQE) relates to radiative recombination probability
• Light extraction efficiency affects how much generated light escapes the device
• Efficiency droop occurs at high current densities
• Auger recombination contributes to efficiency loss at high carrier densities
• Thermal quenching reduces efficiency at elevated temperatures
• Charge imbalance can lead to reduced efficiency and increased degradation
• Device lifetime affected by quantum dot stability and degradation mechanisms