104
6 AlGaN-Based Multiple-Quantum-Well Materials and UV LEDs
pointed out that the hole concentration is much lower than the electron concentration
[53]. For AlGaN-based optoelectronic devices, the asymmetric electron and hole
concentration distribution would cause electron leakage. In other words, injected
electrons in the active region is likely to penetrate into the p-type region. In such
case, radiative recombination efficiency of the quantum wells would be affected
and reduced, accompanied by a long-wavelength parasitic luminescence in the
p-type region. Meanwhile, relatively low current spreading capacity of the n-type
and p-type regions would cause current crowding effect and heat accumulation. The
undesired effects mentioned above will cause device degradation, which will reduce
its luminous efficiency and reliability.
Low Light Extraction Efficiency: Another major factor limiting the EQE of
AlGaN-based UV LEDs is the lower light extraction efficiency. It can be mainly
attributed to three reasons. (1) The light emitted from the LED front side is nearly
completely absorbed due to the strong absorption of UV light by p-GaN. That’s why
UV LEDs generally adopt a flip-chip structure. There are also some reports of UV
LEDs with vertical structures [70, 71]. (2) The planar sapphire substrates cannot
effectively reduce the internal total reflection. There lacks a simple and effective
surface roughening method to process hard sapphire substrates. N. Lobo’s simulation showed that flip-chip UV LEDs with no mirrors and encapsulation had a light
extraction efficiency of only 7–9% [59]. (3) In AlGaN-based UV LEDs, the dominant
emission mode converts from the TE mode to the TM mode as the Al composition
increases and the wavelength decreases. For LEDs on the c-plane sapphire, the TE and
the TM polarized light propagates in the vertical and horizontal directions, respectively. Thus, TM polarized light is more difficult to be extracted from the vertical
direction than TE polarized light. Han-Youl Ryu’s simulation showed that the light
extraction efficiency of the TM mode in DUV LEDs is less than one tenth of that of
the TE mode [72].
Figure 6.6 shows the typical epitaxial structure of an AlGaN-based DUV LED
emitting at 280 nm [73]. It mainly includes 1 μm thick high-temperature AlN
template layer grown on the sapphire substrate, 10 to 20 pairs of AlGaN/AlN
superlattice structure for strain management and dislocation blocking, 2~3 μm thick
Si-doped Al 0.55 Ga 0.55 N n-type contact layer, 5 pairs of Al 0.5 Ga 0.5 N/Al 0.4 Ga 0.6 N
MQW active region, 20 nm thick Mg-doped Al 0.6 Ga 0.4 N electron barrier layer
(EBL), and a p
+ -GaN (100 nm)/p-AlGaN hole injection layer. Generally speaking,
DUV LEDs are fabricated in flip-chip structure to avoid optical absorption. Brief
fabrication process of such structure is shown in Fig. 6.7.
(1) Wafer cleaning: The cleaning process has an important influence on the process
and device performance, and is a critical step during the semiconductor process.
It starts with organic cleaning to remove various hydrocarbons such as stains and
oils. The wafer can be ultrasonically cleaned with hot acetone and isopropanol
or ethanol, then rinsed in deionized water and dried with N 2 . If there are residual
metal indium spots on the wafer, they can be dissolved in aqua regia.
(2) Meas etching: The mesa shape is determined by photolithography and ICP
etching. A mixed gas of BCl 3/ Cl 2 /Ar is usually used to form low-damage mesas.
6 AlGaN-Based Multiple-Quantum-Well Materials and UV LEDs
pointed out that the hole concentration is much lower than the electron concentration
[53]. For AlGaN-based optoelectronic devices, the asymmetric electron and hole
concentration distribution would cause electron leakage. In other words, injected
electrons in the active region is likely to penetrate into the p-type region. In such
case, radiative recombination efficiency of the quantum wells would be affected
and reduced, accompanied by a long-wavelength parasitic luminescence in the
p-type region. Meanwhile, relatively low current spreading capacity of the n-type
and p-type regions would cause current crowding effect and heat accumulation. The
undesired effects mentioned above will cause device degradation, which will reduce
its luminous efficiency and reliability.
Low Light Extraction Efficiency: Another major factor limiting the EQE of
AlGaN-based UV LEDs is the lower light extraction efficiency. It can be mainly
attributed to three reasons. (1) The light emitted from the LED front side is nearly
completely absorbed due to the strong absorption of UV light by p-GaN. That’s why
UV LEDs generally adopt a flip-chip structure. There are also some reports of UV
LEDs with vertical structures [70, 71]. (2) The planar sapphire substrates cannot
effectively reduce the internal total reflection. There lacks a simple and effective
surface roughening method to process hard sapphire substrates. N. Lobo’s simulation showed that flip-chip UV LEDs with no mirrors and encapsulation had a light
extraction efficiency of only 7–9% [59]. (3) In AlGaN-based UV LEDs, the dominant
emission mode converts from the TE mode to the TM mode as the Al composition
increases and the wavelength decreases. For LEDs on the c-plane sapphire, the TE and
the TM polarized light propagates in the vertical and horizontal directions, respectively. Thus, TM polarized light is more difficult to be extracted from the vertical
direction than TE polarized light. Han-Youl Ryu’s simulation showed that the light
extraction efficiency of the TM mode in DUV LEDs is less than one tenth of that of
the TE mode [72].
Figure 6.6 shows the typical epitaxial structure of an AlGaN-based DUV LED
emitting at 280 nm [73]. It mainly includes 1 μm thick high-temperature AlN
template layer grown on the sapphire substrate, 10 to 20 pairs of AlGaN/AlN
superlattice structure for strain management and dislocation blocking, 2~3 μm thick
Si-doped Al 0.55 Ga 0.55 N n-type contact layer, 5 pairs of Al 0.5 Ga 0.5 N/Al 0.4 Ga 0.6 N
MQW active region, 20 nm thick Mg-doped Al 0.6 Ga 0.4 N electron barrier layer
(EBL), and a p
+ -GaN (100 nm)/p-AlGaN hole injection layer. Generally speaking,
DUV LEDs are fabricated in flip-chip structure to avoid optical absorption. Brief
fabrication process of such structure is shown in Fig. 6.7.
(1) Wafer cleaning: The cleaning process has an important influence on the process
and device performance, and is a critical step during the semiconductor process.
It starts with organic cleaning to remove various hydrocarbons such as stains and
oils. The wafer can be ultrasonically cleaned with hot acetone and isopropanol
or ethanol, then rinsed in deionized water and dried with N 2 . If there are residual
metal indium spots on the wafer, they can be dissolved in aqua regia.
(2) Meas etching: The mesa shape is determined by photolithography and ICP
etching. A mixed gas of BCl 3/ Cl 2 /Ar is usually used to form low-damage mesas.