106
6 AlGaN-Based Multiple-Quantum-Well Materials and UV LEDs
(3) N and p electrode deposition: In order to obtain good ohmic contact performance on the n-AlGaN and p-GaN layers, it is necessary to separately fabricate
n-type and p-type electrodes. The n-electrode usually adopts Ti/Al metal system
and is annealed at 850 °C for 30 s in N 2 atmosphere in a rapid annealing furnace.
The p-electrode usually adopts Ni/Au metal system, and is annealed at 550 °C
for 1 min in air atmosphere. Before evaporating the p and n electrodes, it is necessary to remove the surface oxidation of n-AlGaN and p-GaN by immersing in
acid solutions.
(4) Passivation isolation: In order to reduce the leakage current, avoid short circuit
and suppress the possible surface recombination of exposed quantum wells after
mesa etching, we use a SiO 2 or SiN layer deposited by plasma enhanced chemical vapor deposition (PECVD) as a passivation layer to protect the quantum
wells and isolate the n and p electrodes.
(5) Metal-pad deposition: via-holes in the passivation layer are formed by
photolithography and etching. Thick metal electrodes such as Cr/Pt/Au or
Al/Ti/Au or Ti/Au are then filled in the via-holes with designed shapes.
(6) Flip-chip bonding: The DUV LED chips are flip-chip mounted on high
thermal conductivity substrates or sub-mounts with metal bump electrodes. The
substrate has an important influence on the electrical and thermal properties of
DUV LEDs. Silicon-based or ceramic (AlN, etc.)-based substrates are usually
used.
(7) Package: The package has an important influence on the light extraction,
light field distribution, optical output power and reliability of UV LEDs. The
traditional epoxy resin is suitable for UV LEDs with wavelength longer than
380 nm due to its disadvantages on its high temperature heat-resistance and UV
transmittance.
Figure 6.8a–e show micrographs of the DUV LED chip after dicing, the silicon
sub-mount, the flip-chip bonded DUV LED chip on the silicon sub-mount, the encapsulated DUV LED chip and the spectrometer-integrating sphere system for output
power testing, respectively.
The LOP-I-EQE curves (light output power-current-external quantum efficiency
curves) of a bare 293-nm DUV LED and the EL spectrum at 20 mA are shown
in Fig. 6.9a, b, respectively. At an injection current of 20 mA, the DUV LED has
an output power of 1.97mW, a corresponding EQE of 2.32%, and an EL spectrum
with a full width at half maximum of 9.7 nm. At 30 mA, the LOP reaches 2.47mW
and begins to saturate. When the current increases to 40 mA, the LOP saturates
at a value of 2.59mW and a corresponding EQE of 1.53%. The saturation of LOP
comes from the aggravation of thermal effect with the increasing current density and
measurement time.
Figure 6.10a, b show the I-V curves of the DUV LED. The forward voltage is
reduced to 5.1 V at 20 mA after optimization. The leakage current of −10 V is as
low as 10
−4 mA.
6 AlGaN-Based Multiple-Quantum-Well Materials and UV LEDs
(3) N and p electrode deposition: In order to obtain good ohmic contact performance on the n-AlGaN and p-GaN layers, it is necessary to separately fabricate
n-type and p-type electrodes. The n-electrode usually adopts Ti/Al metal system
and is annealed at 850 °C for 30 s in N 2 atmosphere in a rapid annealing furnace.
The p-electrode usually adopts Ni/Au metal system, and is annealed at 550 °C
for 1 min in air atmosphere. Before evaporating the p and n electrodes, it is necessary to remove the surface oxidation of n-AlGaN and p-GaN by immersing in
acid solutions.
(4) Passivation isolation: In order to reduce the leakage current, avoid short circuit
and suppress the possible surface recombination of exposed quantum wells after
mesa etching, we use a SiO 2 or SiN layer deposited by plasma enhanced chemical vapor deposition (PECVD) as a passivation layer to protect the quantum
wells and isolate the n and p electrodes.
(5) Metal-pad deposition: via-holes in the passivation layer are formed by
photolithography and etching. Thick metal electrodes such as Cr/Pt/Au or
Al/Ti/Au or Ti/Au are then filled in the via-holes with designed shapes.
(6) Flip-chip bonding: The DUV LED chips are flip-chip mounted on high
thermal conductivity substrates or sub-mounts with metal bump electrodes. The
substrate has an important influence on the electrical and thermal properties of
DUV LEDs. Silicon-based or ceramic (AlN, etc.)-based substrates are usually
used.
(7) Package: The package has an important influence on the light extraction,
light field distribution, optical output power and reliability of UV LEDs. The
traditional epoxy resin is suitable for UV LEDs with wavelength longer than
380 nm due to its disadvantages on its high temperature heat-resistance and UV
transmittance.
Figure 6.8a–e show micrographs of the DUV LED chip after dicing, the silicon
sub-mount, the flip-chip bonded DUV LED chip on the silicon sub-mount, the encapsulated DUV LED chip and the spectrometer-integrating sphere system for output
power testing, respectively.
The LOP-I-EQE curves (light output power-current-external quantum efficiency
curves) of a bare 293-nm DUV LED and the EL spectrum at 20 mA are shown
in Fig. 6.9a, b, respectively. At an injection current of 20 mA, the DUV LED has
an output power of 1.97mW, a corresponding EQE of 2.32%, and an EL spectrum
with a full width at half maximum of 9.7 nm. At 30 mA, the LOP reaches 2.47mW
and begins to saturate. When the current increases to 40 mA, the LOP saturates
at a value of 2.59mW and a corresponding EQE of 1.53%. The saturation of LOP
comes from the aggravation of thermal effect with the increasing current density and
measurement time.
Figure 6.10a, b show the I-V curves of the DUV LED. The forward voltage is
reduced to 5.1 V at 20 mA after optimization. The leakage current of −10 V is as
low as 10
−4 mA.