Cyril Pernot

Improved Efficiency of 255–280 nm AlGaN-Based Light-Emitting Diodes

We report on the fabrication and characterization of AlGaN-based deep ultraviolet light-emitting diodes (LEDs) with the emission wavelength ranging from 255 to 280 nm depending on the Al composition of the active region. The LEDs were flip-chip bonded and achieved external quantum efficiencies of over 3% for all investigated wavelengths. Under cw operation, an output power of more than 1 mW at 10 mA was demonstrated. A moth-eye structure was fabricated on the back side of the sapphire substrate, and on-wafer output power measurement indicated a 1.5-fold improvement of light extraction.

Solar-Blind UV Photodetectors Based on GaN/AlGaN p-i-n Photodiodes

We report on the fabrication and characterization of n-Al0.44Ga0.56N/i-Al0.44Ga0.56N/p-GaN ultraviolet solar-blind photodetectors. The diodes were fabricated by organometallic vapor phase epitaxy on low-defect-density AlGaN layers using a low-temperature interlayer technique. They present a long cutoff wavelength at 270 nm with high rejection of solar light and a responsivity of 12 mA/W. Low dark currents between 4 and 35 pA/mm2 at -5 V have been measured. A photocurrent decay time of 14 µs has been estimated in unbiased diodes. Due to the large dynamic resistance of the diode at 0 V bias, the detector itself shows a detectivity of 1.2×1013 cmHz1/2W-1.

Injection dependence of the electroluminescence spectra of phosphor free GaN-based white light emitting diodes

Multicolor, multi-quantum well light emitting diodes have been fabricated by molecular beam epitaxy by inserting quantum wells of various widths in the active region. The In content of the wells is 15%-20% and the color is governed by carrier confinement and the Stark effect. Combining a proper number of blue and yellow quantum wells allows to obtain monolithic white LEDs. The electroluminescence spectra of the diodes have been studied. At low injection, the luminescence intensity varies quadratically with the injection current, showing that the electroluminescence originates from the depleted region of the diode, and that non-radiative recombination paths exist. However, for higher injection currents, the luminescence efficiency of the wells situated near the n-side of the junction starts to vary linearly with the current, and this is accompanied by the appearance of GaN electroluminescence. We show that this is due to the entering of these wells into the neutral region of the diode, explaining the injection dependence of the color of these multicolor LEDs.

Demonstration of Flame Detection in Room Light Background by Solar‐Blind AlGaN PIN Photodiode

We have fabricated n-Al 0.44 Ga 0.56 N/i-Al 0.44 Ga 0.56 N/p-GaN heterojunction photodiodes with a cut-off wavelength of 275 nm. The multilayer device structure was grown by metal-organic vapor phase epitaxy using a low-temperature interlayer technique. Thanks to the high quality AIGaN, the responsivity dropped steeply by three, five and six orders of magnitude at the AIGaN bandedge of 275 nm, 600 nm, and 1 μm, respectively. The steep bandedge enables a minimum leakage of effective weak-flame luminescence between 250 and 280 nm. For flame detection, a visible cut-off filter to assist the selectivity to the flame luminescence was attached. In addition, the photodiode was operated at zero-bias so that the darkcurrent does not exceed the weak photocurrent induced by flame luminescence. This solar-blind photosensor thus successfully responded selectively to the flame luminescence regardless of whether the room was lighted or not.

AlGaN-Based Deep Ultraviolet Light-Emitting Diodes Fabricated on Patterned Sapphire Substrates

Deep ultraviolet (DUV) light-emitting diodes (LEDs) on patterned sapphire substrates (PSSs) have been clearly demonstrated. AlN templates grown on PSSs had average threading dislocation densities (TDDs) of as low as 5×107 cm-2. Flip-chip DUV LEDs fabricated on PSSs demonstrated a significantly high performance. The 266 nm LED exhibited an output power of 5.3 mW and an external quantum efficiency (EQE) of 1.9% at 60 mA DC, and the 278 nm LED had 8.4 mW output and an EQE of 3.4%. Moreover, the 70% lifetime was more than 700 h at 20 mA.

Low-Intensity Ultraviolet Photodetectors Based on AlGaN

We report on the fabrication and the characterization of low-intensity ultraviolet (UV) photodetectors based on AlxGa1-xN grown by organometallic vapor phase epitaxy, with threading dislocation density in the range of 5–8×109 to 2×1011 cm-2 and 6–7×107 to 1×109 cm-2. The detectors present dark current values below 100 fA at 10 V bias allowing a photocurrent to dark current ratio greater than one even at 40 nW/cm2. The photodetectors fabricated on low-density dislocation layers present a greatly reduced persistent photoconductivity. Also, they achieved a rejection ratio of 3 orders of magnitude between UV and visible light with cutoffs at 365 and 270 nm for x=0 and x=0.43, respectively.

Investigation of the Leakage Current in GaN P-N Junctions

We report on the fabrication and evaluation of leakage current based on GaN p-n junctions grown by metalorganic vapor phase epitaxy over sapphire. Si and Mg were used as n- and p-type dopants, respectively. Reactive ion etching was used to define mesas of different sizes and metal ohmic contacts were fabricated. Reverse dark current of 15 pA/mm2 was measured under -8 V bias at room temperature, and that of 900 pA/mm2 was measured under -5 V bias at 480 K. Surface leakage along the mesa sidewalls is suggested to be the main source of leakage under a low bias voltage, and tunneling effects appear to be the main reason for junction breakdown.

Improvement of Light Extraction Efficiency for AlGaN-Based Deep Ultraviolet Light-Emitting Diodes

AlGaN-based deep ultraviolet light-emitting diodes (LEDs) with aluminum reflective electrodes deposited to cover both p- and n-mesh contact electrodes have been fabricated. A 1.55-fold increase in light extraction efficiency has been demonstrated. Despite their reduced contact area, the LEDs exhibited only a slight increase in forward voltage of 0.45 V at 20 mA. Also, their 50% lifetime was estimated to be about 10,000 h at 20 mA DC at room temperature by extrapolation. Owing to the reflective electrodes, a 288 nm LED with external quantum efficiency as high as 5.4% was achieved. The light output power was 4.6 mW at 20 mA.

Uneven AlGaN multiple quantum well for deep-ultraviolet LEDs grown on macrosteps and impact on electroluminescence spectral output

AlGaN-based LEDs (λ < 300 nm) fabricated on n-AlGaN templates with a threading dislocation density larger than 5 × 108/cm2, which were grown on (0001) sapphire with a 1.0° miscut relative to the m-plane, showed external quantum efficiencies (EQEs) of 3.5, 3.9, 6.1, and 6.0% at 266, 271, 283, and 298 nm, respectively. These EQE values reveal significantly high internal quantum efficiencies (IQEs). This performance was obtained using an uneven multiple quantum well (MQW) grown on the AlGaN template with macrosteps having height larger than the well thickness. The electroluminescence spectra of the fabricated LEDs using this MQW structure shifts to a longer wavelength compared with those on sapphire with a miscut angle of 0.3° relative to the m-plane. Furthermore, the LEDs with this MQW show no deleterious effect on the lifetime, broader electroluminescence spectral widths, and higher output powers when using sapphire with a miscut of 1.0°.

Detailed feasibility study on a flame detector using AlGaN photosensors

Among the current flame detectors, UV selective photosensor has the highest performance thanks to its high ability to follow the turn down of burners. However, its utilization is limited to industrial-use because of its lifetime of 1-2 years, operation temperature below 120 degrees C, and its high system cost including power supply of 300V and cooling. Accordingly, solid sate flame detector is strongly desired. The required elements for the UV flame detection is as follows: a) photo current given by low intensity light should be greater than dark current; b) high rejection to the background is indispensable. AlGaN was chosen through material selection and feasibility study had been conducted. For a), using pn-GaN grown on single buffer layer, the leakage current comes from both mesa sidewall leakage and tunneling current through PN junction. Both leakages were approximately 2 orders larger to detect 1nW/cm2, but thought to be within outreach using multi-buffer crystal growth technique and sensor design. For b), AlGaN photocurrent fabricated on the multibuffer layer has shown responsivity difference of 3 orders at the bandage of 4.4 eV, which satisfied the high rejection in UV region of 300- 400nm.