Qifeng Shan

Carrier recombination mechanisms and efficiency droop in GaInN/GaN light-emitting diodes

We model the carrier recombination mechanisms in GaInN/GaN light-emitting diodes as R=An+Bn2+Cn3+f(n), where f(n) represents carrier leakage out of the active region. The term f(n) is expanded into a power series and shown to have higher-than-third-order contributions to the recombination. The total third-order nonradiative coefficient (which may include an f(n) leakage contribution and an Auger contribution) is found to be 8×10−29 cm6 s−1. Comparison of the theoretical ABC+f(n) model with experimental data shows that a good fit requires the inclusion of the f(n) term.

Transport-mechanism analysis of the reverse leakage current in GaInN light-emitting diodes

The reverse leakage current of a GaInN light-emitting diode (LED) is analyzed by temperature dependent current–voltage measurements. At low temperature, the leakage current is attributed to variable-range-hopping conduction. At high temperature, the leakage current is explained by a thermally assisted multi-step tunneling model. The thermal activation energies (95–162 meV), extracted from the Arrhenius plot in the high-temperature range, indicate a thermally activated tunneling process. Additional room temperature capacitance–voltage measurements are performed to obtain information on the depletion width and doping concentration of the LED.

Temperature dependent efficiency droop in GaInN light-emitting diodes with different current densities

The effect of chip area on the temperature-dependent light-output power (LOP) in GaInN-based light-emitting diodes (LEDs) is investigated. The larger the chip size, the faster the reduction in LOP with increasing temperature becomes, indicating that increasing the size of LED chips, a technology trend for reducing the efficiency droop at high currents, is detrimental for high temperature-tolerant LEDs. In addition, it is found that regardless of chip size, the temperature-dependent LOP is identical for the LEDs operating at the same current density.

Efficiency droop in AlGaInP and GaInN light-emitting diodes

At room temperature, AlGaInP pn-junction light-emitting diodes (LEDs) emitting at 630 nm do not exhibit an efficiency droop. However, upon cooling the AlGaInP LEDs to cryogenic temperatures, they show a pronounced efficiency droop. We attribute the efficiency droop in AlGaInP LEDs to electron-drift-induced reduction in injection efficiency (i.e., carrier leakage out of the active region) mediated by the asymmetry of the pn junction, specifically the disparity between electron and hole concentrations and mobilities, with the concentration disparity exacerbated at low temperatures.

On the temperature dependence of electron leakage from the active region of GaInN/GaN light-emitting diodes

Reduction in the light-output power in GaN-based light-emitting diodes (LEDs) with increasing temperature is a well-known phenomenon. In this work, temperature dependent external-quantum-efficiency versus current curves are measured, and the mechanisms of recombination are discussed. Shockley-Read-Hall recombination increases with temperature and is found to greatly reduce the light output at low current densities. However, this fails to explain the drop in light-output power at high current densities. At typical current density (35 A/cm2), as temperature increases, our results are consistent with increased Shockley-Read-Hall recombination and increased electron leakage from the active region. Both of these effects contribute to the reduction in light-output power in GaInN/GaN LEDs at high temperatures.

Analysis of thermal properties of GaInN light-emitting diodes and laser diodes

The thermal properties, including thermal time constants, of GaInN light-emitting diodes LEDs and laser diodes LDs are analyzed. The thermal properties of unpackaged LED chips are described by a single time constant, that is, the thermal time constant associated with the substrate. For unpackaged LD chips, we introduce a heat-spreading volume. The thermal properties of unpackaged LD chips are described by a single time constant, that is, the thermal time constant associated with the heat spreading volume. Furthermore, we develop a multistage RthCth thermal model for packaged LEDs. The model shows that the transient response of the junction temperature of LEDs can be described by a multiexponential function. Each time constant of this function is approximately the product of a thermal resistance, Rth, and a thermal capacitance, Cth. The transient response of the junction temperature is measured for a high-power flip-chip LED, emitting at 395 nm, by the forward-voltage method. A two stage RthCth model is used to analyze the thermal properties of the packaged LED. Two time constants, 2.72 ms and 18.8 ms are extracted from the junction temperature decay measurement and attributed to the thermal time constant of the LED GaInN/sapphire chip and LED Si submount, respectively.

Polarized light emission from GaInN light-emitting diodes embedded with subwavelength aluminum wire-grid polarizers

We demonstrate a back-emitting (sapphire-substrate emitting) linearly polarized GaInN light-emitting diode (LED) embedded with a subwavelength-sized aluminum wire-grid polarizer (WGP). Rigorous coupled wave analysis is implemented to study the polarization characteristics of such a WGP LED. The aluminum nanowire grating with a period of 150 nm is located on the sapphire backside of a GaInN LED structure and is fabricated by electron-beam lithography and inductively coupled plasma reactive-ion etching. A polarization ratio of 0.96 is demonstrated for a WGP GaInN LED in good agreement with simulation results.

U-turn feature in the efficiency-versus-current curve of GaInN/GaN light-emitting diodes

The onset of the efficiency droop in GaInN/GaN blue light-emitting diodes (LEDs), i.e., the maximum-efficiency point, typically occurs at current densities of 1–10 A/cm2 and the efficiency decreases monotonically beyond the onset. At typical operating current densities (10–100 A/cm2), LEDs are strongly affected by the droop. At cryogenic temperatures, an increase in the efficiency, i.e., a “U-turn” feature, is found in the droop regime of the efficiency-versus-current curve. The occurrence of the U-turn feature coincides with a distinct increase in device conductivity, which is attributed to an enhancement in p-type conductivity that in turn increases the injection efficiency.

Effect of Quantum Barrier Thickness in the Multiple-Quantum-Well Active Region of GaInN/GaN Light-Emitting Diodes

The dependence of the polarization-induced electric field in GaInN/GaN multiple-quantum-well light-emitting diodes (LEDs) on the GaN quantum barrier (QB) thickness is investigated. Electrostatic arguments and simulations predict that a thin QB thickness reduces the electric field in the quantum wells (QWs) and also improves the LED efficiency. We experimentally demonstrate that the QW electric field decreases with decreasing QB thickness. The lower electric field results in a better overlap of electron and hole wave functions and better carrier confinement in the QWs. A reduced efficiency droop and enhanced internal quantum efficiency is demonstrated for GaInN/GaN LEDs when the QB thickness is reduced from 24.5 to 9.1 nm.

Method for determining the radiative efficiency of GaInN quantum wells based on the width of efficiency-versus-carrier-concentration curve

We report a method to determine the radiative efficiency (RE) of a semiconductor by using room-temperature excitation-dependent photoluminescence measurements. Using the ABC model for describing the recombination of carriers, we show that the theoretical width of the RE-versus-carrier-concentration (n) curve is related to the peak RE. Since the normalized external quantum efficiency, EQEnormalized, is proportional to the RE, and the square root of the light-output power, LOP, is proportional to n, the experimentally determined width of the EQEnormalized-versus-n curve can be used to determine the RE. We demonstrate a peak RE of 91% for a Ga0.85In0.15N quantum well.