J.-Q. Xi

Junction and carrier temperature measurements in deep-ultraviolet light-emitting diodes using three different methods

The junction temperature of AlGaN ultraviolet light-emitting diodes emitting at 295nm is measured by using the temperature coefficients of the diode forward voltage and emission peak energy. The high-energy slope of the spectrum is explored to measure the carrier temperature. A linear relation between junction temperature and current is found. Analysis of the experimental methods reveals that the diode-forward voltage is the most accurate (±3°C). A theoretical model for the dependence of the diode forward voltage (Vf) on junction temperature (Tj) is developed that takes into account the temperature dependence of the energy gap. A thermal resistance of 87.6K∕W is obtained with the device mounted with thermal paste on a heat sink.

GaInN light-emitting diode with conductive omnidirectional reflector having a low-refractive-index indium-tin oxide layer

Enhancement of light extraction in a GaInN light-emitting diode (LED) employing a conductive omnidirectional reflector (ODR) consisting of GaN, an indium-tin oxide (ITO) nanorod low-refractive-index layer, and an Ag layer is presented. An array of ITO nanorods is deposited on p-type GaN by oblique-angle electron-beam deposition. The refractive index of the nanorod ITO layer is 1.34 at 461nm, significantly lower than that of dense ITO layer, which is n=2.06. The GaInN LEDs with GaN∕low-n ITO/Ag ODR show a lower forward voltage and a 31.6% higher light-extraction efficiency than LEDs with Ag reflector. This is attributed to enhanced reflectivity of the ODR that employs the low-n ITO layer.

Distributed Bragg reflector consisting of high- and low-refractive-index thin film layers made of the same material

A conductive distributed Bragg reflector (DBR) composed entirely of a single material—indium tin oxide (ITO)—is reported. The high- and low-refractive-index layers of the DBR are deposited by oblique-angle deposition and consist of ITO thin films with low and high porosities, which yield an index contrast of Δn=0.4. A single-material DBR with three periods achieves a reflectivity of 72.7%, in excellent agreement with theory.

Very low-refractive-index optical thin films consisting of an array of SiO 2 nanorods

The refractive-index contrast in dielectric multilayer structures, optical resonators, and photonic crystals is an important figure of merit that creates a strong demand for high-quality thin films with a low refractive index. A SiO2 nanorod layer with low refractive index of n = 1.08, to our knowledge the lowest ever reported in thin-film materials, is grown by oblique-angle electron-beam deposition of SiO2. A single-pair distributed Bragg reflector employing a SiO2 nanorod layer is demonstrated to have enhanced reflectivity, showing the great potential of low-refractive-index films for applications in photonic structures and devices.

Omnidirectional reflector using nanoporous SiO 2 as a low-refractive-index material

Triple-layer omnidirectional reflectors (ODRs) consisting of a semiconductor, a quarter-wavelength transparent dielectric layer, and a metal have high reflectivities for all angles of incidence. Internal ODRs (ambient materials refractive index n >> 1.0) are demonstrated that incorporate nanoporous SiO2, a low-refractive-index material (n = 1.23), as well as dense SiO2 (n = 1.46). GaP and Ag serve as the semiconductor and the metal layer, respectively. Reflectivity measurements, including angular dependence, are presented. Calculated angle-integrated TE and TM reflectivities for ODRs employing nanoporous SiO2 are R(int)/TE = 99.9% and R(int)/TM = 98.9%, respectively, indicating the high potential of the ODRs for low-loss waveguide structures.

Junction Temperature in Ultraviolet Light-Emitting Diodes

The junction temperature and thermal resistance of AlGaN and GaInN ultraviolet (UV) light-emitting diodes (LEDs) emitting at 295 and 375 nm, respectively, are measured using the temperature coefficient of diode-forward voltage. An analysis of the experimental method reveals that the diode-forward voltage has a high accuracy of ±3°C. A comprehensive theoretical model for the dependence of diode-forward voltage (Vf) on junction temperature (Tj) is developed taking into account the temperature dependence of the energy gap and the temperature coefficient of diode resistance. The difference between the junction voltage temperature coefficient (dVj/dT) and the forward voltage temperature coefficient (dVf/dT) is shown to be caused by diode series resistance. The data indicate that the n-type neutral regions are the dominant resistive element in deep-UV devices. A linear relationship between junction temperature and current is found. Junction temperature is also measured by the emission-peak-shift method. The high-energy slope of the spectrum is explored in the measurement of carrier temperature.

Enhanced Light Extraction in GaInN Light-Emitting Diode With Pyramid Reflector

GaInN light-emitting diodes (LEDs) that employ a reflector consisting of an array of three-dimensional (3-D) SiO2 pyramids and a Ag layer are demonstrated to have enhanced light extraction compared with GaInN LEDs with planar Ag reflector. Ray tracing simulations reveal that the pyramid reflector provides 14.1% enhancement in extraction efficiency. Consistent with the simulation, it is experimentally demonstrated that GaInN LEDs with the pyramid reflector show 13.9% higher light output than LEDs with a planar Ag reflector. The enhancement is attributed to the appearance of an additional escape cone for light extraction enabled by the 3-D pyramid reflector

Enhanced light-extraction in GaInN near-ultraviolet light-emitting diode with Al-based omnidirectional reflector having NiZn∕Ag microcontacts

Enhancement of light extraction in a GaInN near-ultraviolet light-emitting diode (LED) employing an Al-based omnidirectional reflector (ODR) consisting of GaN, a SiO2 low-refractive-index layer perforated by an array of NiZn∕Ag microcontacts, and an Al layer is presented. A theoretical calculation reveals that a SiO2∕Al ODR has much higher reflectivity than both a SiO2∕Ag ODR and a Ag reflector at a wavelength of 400nm. It is experimentally shown that GaInN near-ultraviolet LEDs with GaN∕SiO2∕Al ODR have 16% and 38% higher light output than LEDs with SiO2∕Ag ODR and Ag reflector, respectively. The higher light output is attributed to enhanced reflectivity of the Al-based ODR in the near-ultraviolet wavelength range.

Omni-directional reflectors for light-emitting diodes

This article discusses possible solutions to limitations in light extraction efficiency of light-emitting diodes (LEDs) using new types of triple-layer omni-directional reflectors (ODRs). The ODRs have lower mirror losses than metal reflectors and distributed Bragg reflectors (DBRs). High-reflectivity ODRs have been incorporated into AlGaInP LEDs and GaInN LEDs. It is shown that the ODR significantly increases light extraction from ODR-LEDs as compared to reference LEDs employing a DBR or metal reflector. Other examples of innovative concepts to be presented include novel materials with unprecedented low-refractive index, which further enhance the optical properties of ODRs.

Light-emitting diodes with integrated omnidirectionally reflective contacts

An electrically conductive omnidirectional reflector (ODR) is demonstrated as p-type ohmic contact for an AlGaInP light-emitting diode (LED). The ODR comprises the semiconductor, a metal layer and an intermediate low-refractive index dielectric layer. The SiO2 dielectric layer, located between a GaP and a silver layer, is perforated by an array of AuZn micro-contacts thus enabling electrical conductivity. It is shown that the ODR-LED has a significantly higher light-extraction efficiency as compared to LEDs employing distributed Bragg reflectors (DBRs). For devices emitting in the red wavelength range, external quantum efficiencies of 18 % and 11 % are obtained for ODR- and DBR-LEDs, respectively. The performance of the ODR-LED can be further increased by replacing the SiO2 dielectric with materials having a refractive index << 1.45. Performance characteristics of such powerful reflectors will be presented.