12.2 Quantum Dot LED
269
field in China and abroad mainly focuses on the study of tunable growth and optical
properties of quantum dots. Wu et al. systematically calculated the optical properties
of InGaN quantum dots. The results show that the wavelength can be adjusted in the
entire visible range. The electron-hole overlap integral in the quantum dots is larger
than the quantum well, which is conducive to radiative recombination [19]. Schulz
et al. calculated the polarization electric field in InGaN quantum dots, showing that
the quantum dot structure can reduce the polarization effect [20]. Park et al. found
that the quantum dots with small size have strong luminescence than that of quantum
dots with large size, which is attributed to the weak polarization effect in small-sized
quantum dots [21]. Zhang and Xu et al. prepared blue, green and red quantum dot
LEDs based on InGaN quantum dots [22]. The yellow-green InGaN quantum dot
multilayer stack structure was successfully prepared in Tsinghua University in China
[23]. The InGaN quantum dot with green emission was successfully prepared by the
Institute of Semiconductors of the Chinese Academy of Sciences, with anomalous
dependence between PL intensity and temperature [24]. There are few studies on
nitride based quantum dot LEDs. The Chua research group prepared InGaN quantum
dot single-chip white LEDs [25]. However, the luminous efficiency was quite low. In
general, although InGaN quantum dots initially show unique luminescence properties
and potentials in applications, the controllable growth of InGaN quantum dots is still
difficult. In addition, the luminescence efficiency is still low, which requires to further
improve the quality of material to achieve high-efficiency and high-performance
nitride based quantum dots LED.
In addition to nitride-based quantum dot LEDs, the coating of monolayer of
colloidal CdSe/ZnSe quantum dots on the nitride LED as a fluorescent conversion
layer as shown in Fig. 12.14 has attracted attention in recent years [26].
In addition to applications in light-emitting diodes, quantum dots are also widely
used in lasers, photodetectors, solar cells, and single-photon sources. Compared with
traditional quantum well lasers, quantum dot lasers can reduce threshold current
density, improve temperature stability, and easily achieve single mode lasing. As
the free dimension of the active region is reduced, from bulk materials, quantum
wells, quantum wires, to quantum dots, the threshold current density Jth of the laser
can be effectively reduced, and the temperature stability of Jth can be improved.
For photodetectors, compared with quantum well, carriers in quantum dots have
longer lifetime due to stronger quantum constraints. This is beneficial to improve the
detection rate, responsiveness, photoconductivity gain of photodetectors, and dark
current. For solar cells, in addition to the long carrier lifetime of quantum dots, this
can facilitate the extraction of photogenerated carriers. The quantum dot absorption
spectrum can be adjusted in a large range with the quantum dot size, which is also
beneficial to enhance the light absorption and improve the solar conversion efficiency.
Besides, single photon source based on single quantum dot luminescence plays an
important role in quantum information processing and is widely studied at present.
269
field in China and abroad mainly focuses on the study of tunable growth and optical
properties of quantum dots. Wu et al. systematically calculated the optical properties
of InGaN quantum dots. The results show that the wavelength can be adjusted in the
entire visible range. The electron-hole overlap integral in the quantum dots is larger
than the quantum well, which is conducive to radiative recombination [19]. Schulz
et al. calculated the polarization electric field in InGaN quantum dots, showing that
the quantum dot structure can reduce the polarization effect [20]. Park et al. found
that the quantum dots with small size have strong luminescence than that of quantum
dots with large size, which is attributed to the weak polarization effect in small-sized
quantum dots [21]. Zhang and Xu et al. prepared blue, green and red quantum dot
LEDs based on InGaN quantum dots [22]. The yellow-green InGaN quantum dot
multilayer stack structure was successfully prepared in Tsinghua University in China
[23]. The InGaN quantum dot with green emission was successfully prepared by the
Institute of Semiconductors of the Chinese Academy of Sciences, with anomalous
dependence between PL intensity and temperature [24]. There are few studies on
nitride based quantum dot LEDs. The Chua research group prepared InGaN quantum
dot single-chip white LEDs [25]. However, the luminous efficiency was quite low. In
general, although InGaN quantum dots initially show unique luminescence properties
and potentials in applications, the controllable growth of InGaN quantum dots is still
difficult. In addition, the luminescence efficiency is still low, which requires to further
improve the quality of material to achieve high-efficiency and high-performance
nitride based quantum dots LED.
In addition to nitride-based quantum dot LEDs, the coating of monolayer of
colloidal CdSe/ZnSe quantum dots on the nitride LED as a fluorescent conversion
layer as shown in Fig. 12.14 has attracted attention in recent years [26].
In addition to applications in light-emitting diodes, quantum dots are also widely
used in lasers, photodetectors, solar cells, and single-photon sources. Compared with
traditional quantum well lasers, quantum dot lasers can reduce threshold current
density, improve temperature stability, and easily achieve single mode lasing. As
the free dimension of the active region is reduced, from bulk materials, quantum
wells, quantum wires, to quantum dots, the threshold current density Jth of the laser
can be effectively reduced, and the temperature stability of Jth can be improved.
For photodetectors, compared with quantum well, carriers in quantum dots have
longer lifetime due to stronger quantum constraints. This is beneficial to improve the
detection rate, responsiveness, photoconductivity gain of photodetectors, and dark
current. For solar cells, in addition to the long carrier lifetime of quantum dots, this
can facilitate the extraction of photogenerated carriers. The quantum dot absorption
spectrum can be adjusted in a large range with the quantum dot size, which is also
beneficial to enhance the light absorption and improve the solar conversion efficiency.
Besides, single photon source based on single quantum dot luminescence plays an
important role in quantum information processing and is widely studied at present.