Thomas Gessmann

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.

GaInN light-emitting diodes with RuO2/SiO2/Ag omni-directional reflector

A GaInN light-emitting diode (LED) employing an omni-directional reflector (ODR) is presented. The ODR consists of a RuO2 ohmic contact to p-type GaN, a quarter-wave thick SiO2 low-index layer perforated by an array of micro-contacts, and an Ag layer. Calculations predict a 98% angle-averaged reflectivity at λ=450 nm for an GaN/SiO2/Ag ODR, much higher than that for a 20 period Al0.25Ga0.75N/GaN distributed Bragg reflector (49%) and an Ag reflector (94%). It is shown that the RuO2/SiO2/Ag ODR has higher reflectivity than Ni/Au and even Ag reflectors, leading to a higher light extraction efficiency of GaInN LEDs with ODR. The electrical properties of the ODR-LED are comparable to those LEDs with a conventional Ni/Au contact.

Junction temperature in light-emitting diodes assessed by different methods

The junction temperature of red (AlGaInP), green (GaInN), blue (GaInN), and ultraviolet (GaInN) light-emitting diodes (LEDs) is measured using the temperature coefficients of the diode forward voltage and of the emission-peak energy. The junction temperature increases linearly with DC current as the current is increased from 10 mA to 100 mA. For comparison, the emission-peak-shift method is also used to measure the junction temperature. The emission-peak-shift method is in good agreement with the forward-voltage method. The carrier temperature is measured by the high-energy-slope method, which is found to be much higher than the lattice temperature at the junction. Analysis of the experimental methods reveals that the forward-voltage method is the most sensitive and its accuracy is estimated to be ± 3°C. The peak position of the spectra is influenced by alloy broadening, polarization, and quantum confined Stark effect thereby limiting the accuracy of the emission-peak-shift method to ±15°C. A detailed analysis of the temperature dependence of a tri-chromatic white LED source (consisting of three types of LEDs) is performed. The analysis reveals that the chromaticity point shifts towards the blue, the color-rendering index (CRI) decreases, the color temperature increases, and the luminous efficacy decreases as the junction temperature increases. A high CRI > 80 can be maintained, by adjusting the LED power so that the chromaticity point is conserved.

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.

Omnidirectional reflective contacts for light-emitting diodes

An electrically conductive omnidirectional reflector (ODR) is demonstrated in an AlGaInP light-emitting diode (LED). The ODR serves as p-type contact and comprises the semiconductor, a metal layer and an intermediate low-refractive index dielectric layer. The dielectric layer is perforated by an array of AuZn microcontacts thus enabling electrical conductivity. It is shown that the ODR significantly increases light extraction from an AlGaInP LED as compared to a reference LED employing a distributed Bragg reflector (DBR). External quantum efficiencies of 18% and 11% are obtained for the ODR- and the DBR-LED, respectively.

GaInN light-emitting diodes with omni directional reflectors

A high-reflectivity omni directional reflector (ODR) has been incorporated into a GaInN light-emitting diode (LED) structure. The ODR comprises a transparent, electrically conductive quarter-wave layer of indium tin oxide clad by silver and serves as an ohmic contact to p-type GaN. It is shown that ODR-LEDs have low optical losses and high extraction efficiency. Mesa-structure GaInN/GaN ODR-LEDs emitting in the blue wavelength range are demonstrated and compared to GaInN/GaN LEDs with semitransparent Ni/Au top contacts. The extraction efficiency of ODR-LEDs is higher as compared to conventional LEDs with Ni/Au contacts.

Performance characteristics of white light sources consisting of multiple light-emitting diodes

The performance characteristics of white light sources based on a multiple-LED approach, in particular dichromatic and trichromatic sources are analyzed in detail. Figures of merit such as the luminous efficacy, color temperature, and color rendering capabilities are provided for a wide range of primary emission wavelengths. Spectral power density functions of LEDs are assumed to be thermally and inhomogeneously broadened to a full width at half maximum of several kT, in agreement with experimental results. A gaussian line shape is assumed for each of the emission bands. It is shown that multi-LED white light sources have the potential for luminous efficacies greater than 400 lm/W (dichromatic source) and color rendering indices of greater than 90 (trichromatic source). Contour maps for the color rendering indices and luminous efficacies versus three wavelengths are given.

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.

AlGaInP light-emitting diodes with omnidirectionally reflecting submount

A novel AlGaInP light-emitting diode (LED) is presented that employs high-reflectivity omni-directional reflector (ODR) submounts. It is shown that the reflective-submount (RS) LED has a higher light-extraction efficiency than conventional LEDs. Red AlGaInP RS-LEDs bonded to Si-substrates are demonstrated using a silver-based ODR. The ODR is perforated by an array of small-area low-resistance ohmic contacts. The optical and electrical characteristics of the RS-LEDs are presented and compared to conventional AlGaInP absorbing substrates (AS) LEDs with distributed Bragg reflectors (DBR). It is shown that the light output from the RS-LED exceeds that of AS-LEDs by about a factor of two.

Reduction of Base Access Resistance in AlGaN/GaN Heterojunction Bipolar Transistors using GaInN Base Cap Layer and Selective Epitaxial Growth

One of the major challenges affecting the performance of Npn AlGaN/GaN heterojunction bipolar transistors (HBTs) is the high base access resistance, which is comprised of the base contact resistance and the base bulk resistance. A novel concept is proposed to reduce the base access resistance in Npn AlGaN/GaN HBTs by employing polarization-enhanced contacts and selective epitaxial growth of the base and emitter. In addition, this technique reduces the exposed base surface area, which results in a lower surface recombination current. Such a structure would enable better performance of AlGaN/GaN HBTs in terms of higher current gain and a lower offset voltage. Theoretical calculations on polarization-enhanced contacts predict p-type specific contact resistance lower than 10 –5 Ωcm 2 . Experimental results using transmission line measurement (TLM) technique yield specific contact resistances of 5.6×10 –4 Ωcm 2 for polarization-enhanced p-type contacts and 7.8×10 –2 Ωcm 2 for conventional p-type contacts.