Evaluation of polarization field in InGaN/GaN multiple quantum well structures by using electroluminescence spectra shift

In order to investigate the inherent polarization intensity in InGaN/GaN multiple quantum well(MQW) structures,the electroluminescence(EL) spectra of three samples with different GaN barrier thicknesses of 21.3 nm, 11.4 nm, and 6.5 nm are experimentally studied. All of the EL spectra present a similar blue-shift under the low-level current injection,and then turns to a red-shift tendency when the current increases to a specific value, which is defined as the turning point.The value of this turning point differs from one another for the three InGaN/GaN MQW samples. Sample A, which has the GaN barrier thickness of 21.3 nm, shows the highest current injection level at the turning point as well as the largest value of blue-shift. It indicates that sample A has the maximum intensity of the polarization field. The red-shift of the EL spectra results from the vertical electron leakage in InGaN/GaN MQWs and the corresponding self-heating effect under the high-level current injection. As a result, it is an effective approach to evaluate the polarization field in the InGaN/GaN MQW structures by using the injection current level at the turning point and the blue-shift of the EL spectra profiles.     

Influence of Growth Temperature and Trimethylindium Flow of InGaN Wells on Optical Properties of InGaN Multiple Quantum-Well Violet Light-Emitting Diodes

An InGaN multiple-quantum-well (MQW) violet-light-emitting diode (LED) is grown by low-pressure metalorganic chemical vapour deposition. It is found that photoluminescence wavelength of the InGaN MQW violet LED is lengthened with increasing growth temperature and with the increasing trimethylindium flow of the InGaN wells. The electroluminescence peak wavelength of the violet LED are about 401 nm with full width at half maximum of 14nm, and the output power in injection current of 2OmA at room temperature is 4.1mW.     

High power and high reliability GaN/InGaN flip-chip light-emitting diodes

High-power and high-reliability GaN/InGaN flip-chip light-emitting diodes （FCLEDs） have been demonstrated by employing a flip-chip design, and its fabrication process is developed. FCLED is composed of a LED die and a submount which is integrated with circuits to protect the LED from electrostatic discharge （ESD） damage. The LED die is flip-chip soldered to the submount, and light is extracted through the transparent sapphire substrate instead of an absorbing Ni/Au contact layer as in conventional GaN/InGaN LED epitaxial designs. The optical and electrical characteristics of the FCLED are presented. According to ESD IEC61000-4-2 standard （human body model）, the FCLEDs tolerated at least 10 kV ESD shock have ten times more capacity than conventional GaN/InGaN LEDs. It is shown that the light output from the FCLEDs at forward current 350mA with a forward voltage of 3.3 V is 144.68 mW, and 236.59 mW at 1.0A of forward current. With employing an optimized contact scheme the FCLEDs can easily operate up to 1.0A without significant power degradation or failure. The li.fe test of FCLEDs is performed at forward current of 200 mA at room temperature. The degradation of the light output power is no more than 9% after 1010.75 h of life test, indicating the excellent reliability. FCLEDs can be used in practice where high power and high reliability are necessary, and allow designs with a reduced number of LEDs.     

Study on electroluminescence from porous silicon light-emitting diode

Porous silicon (PS) light-emitting diode (LED) with an ITO/PS/p-Si/Al structure was fabricated by anodic oxidation method. Photoluminescence (PL) of the PS LED was measured with a peak at 593 nm, and electroluminescence (EL) was measured with a peak at 556 nm under the conditions of 7.5-V forward bias and 210-mA current intensity. The spectral width of EL was measured to be about 160 nm.     

Study on electroluminescence from porous silicon light-emitting diode

Porous silicon (PS) light-emitting diode (LED) with an ITO/PS/p-Si/Al structure was fabricated by anodic oxidation method. Photoluminescence (PL) of the PS LED was measured with a peak at 593 nm, and electroluminescence (EL) was measured with a peak at 556 nm under the conditions of 7.5-V forward bias and 210-mA current intensity. The spectral width of EL was measured to be about 160 nm.     

InGaN/GaN multiple quantum well solar cells with an enhanced open-circuit voltage

In this paper,InGaN/GaN multiple quantum well solar cells (MQWSCs) with an In content of 0.15 are fabricated and studied.The short-circuit density,fill factor and open-circuit voltage (V oc) of the device are 0.7 mA/cm 2,0.40 and 2.22 V,respectively.The results exhibit a significant enhancement of V oc compared with those of InGaN-based hetero and homojunction cells.This enhancement indicates that the InGaN/GaN MQWSC offers an effective way for increasing V oc of an In-rich In x Ga 1 x N solar cell.The device exhibits an external quantum efficiency (EQE) of 36% (7%) at 388 nm (430 nm).The photovoltaic performance of the device can be improved by optimizing the structure of the InGaN/GaN multiple quantum well.     

Indium-Induced Effect on Polarized Electroluminescence from InGaN/GaN MQWs Light Emitting Diodes

Polarization-resolved edge-emitting electroluminescence （EL） studies of In GaN/GaN MQWs of wavelengths from near-UV （390nm） to blue （468nm） light-emitting diodes （LEDs） are performed. Although the TE mode is dominant in all the samples of InGaN/GaN MQW LEDs, an obvious difference of light polarization properties is found in the InGaN/GaN MQW LEDs with different wavelengths. The polarization degree decreases from 52.4% to 26.9% when light wavelength increases. Analyses of band structures of InGaN/GaN quantum wells and luminescence properties of quantum dots imply that quantum-dot-like behavior is the dominant reason for the low luminescence polarization degree of blue LEDs, and the high luminescence polarization degree of UV LEDs mainly comes from QW confinement and the strain effect. Therefore, indium induced carrier confinement （quantum-dot-like behavior） might play a major role in the polarization degree change of InGaN/GaN MQW LEDs from near violet to blue.     

Effects of polarization and p-type GaN resistivity on the spectral response of InGaN/GaN multiple quantum well solar cells

Effects of polarization and p-type GaN resistivity on the spectral response of InGaN/GaN multiple quantum well(MQW) solar cells are investigated. It is found that due to the reduction of piezoelectric polarization and the enhancement of tunneling transport of photo-generated carriers in MQWs, the external quantum efficiency(EQE) of the solar cells increases in a low energy spectral range(λ 370 nm) when the barrier thickness value decreases from 15 nm to 7.5 nm. But the EQE decreases abruptly when the barrier thickness value decreases down to 3.75 nm. The reasons for these experimental results are analyzed. We are aware that the reduction of depletion width in MQW region, caused by the high resistivity of the p-type GaN layer may be the main reason for the abnormally low EQE value at long wavelengths(λ 370 nm).     

Au/Pt/InGaN/GaN Heterostructure Schottky Prototype Solar Cell

A patterned Au/Pt/In0.2Ga0.8N/GaN heterostructure Sehottky prototype solar cell is fabricated. The forward current-voltage characteristics indicate that thermionie emission is a dominant current transport mechanism at the Pt/InGaN interface in our fabricated cell. The Sehottky solar cell has an open-circuit voltage of 0.91 V, short-circuit current density of 7mA/cm^2, and fill factor of 0.45 when illuminated by a Xe lamp with a power density of 300 mW/cm^2. It exhibits a higher short-circuit current density of 30 mA/cm^2 and an external quantum efficiency of over 25% when illuminated by a 20-roW-power He-Cd laser.     

Photocurrent properties of high—sensitivity GaN ultraviolet photodetectors

GaN epilayers were grown on sapphire substrates by metal-organic chemical vapour deposition. Metal－semiconductor－metal photoconductive detectors were fabricated using this material. The photocurrent properties of the detectors were measured and analysed. The spectrum response shows a high sensitivity in the wavelength region from 330 to 360nm, with a peak at 358nm and a sharp cutoff near 360nm. The maximum responsivities at 358nm were 700A/W (2V) and 7000A/W (30V). The relationship between responsivity and bias indicates that the responsivity increases linearly with bias until 30V. The influence of the spacing between two electrodes on the detector responsivity was also studied.