Although many benchmarks of nitride-based quantum confined light emitting structures and devices have been done, our long-term research in nitride-based microcavity structure included electrically driven continuous wave (CW) nitride-based VCSELs and single photon emitter structure has yet to be fulfilled. All previous steps will provide precious information and experience in the future. To achieve this goal, the
following actions require to be done:
(1) Improvement of nitride-based microcavity quality:
Improvement of microcavity quality can improve the collection efficiency of spontaneous emission, reduce the threshold current density of VCSELs structure, and raise the efficiency of light-matter interaction at a fundamental level. The improvement of microcavity quality included the reduction of the optical mirror loss and the internal material absorption loss in microcavity. Figure 6-1 shows the relationship of threshold current density of nitride-based VCSLE and both mirrors reflectance. Improvement of the material quality and the reflectance of DBR structure will be the initial steps in the further.
(2) Development of conductive AlN/GaN distributed Bragg reflectors:
DBR structure would be highly desirable with vertically conductive. The development of conductive DBR structures can reduce the thickness of n- or p-spacers and lets the semiconductor microcavity with shorter cavity length is possible. Although AlN is commonly believed to be invariably insulating by nature, and the huge conduction band offset between GaN and AlN (2.2 eV) seems to completely hinder a carrier flow across the heterointerfaces, A GaN/AlN DBR structure with vertically conducting is possible [1]. The fabrication of bottom AlN/GaN DBR structure with n-type doping will
be tested since we have obtained a high reflectance and crack-free AlN/GaN DBR structure.
(3)Development of a uniform current spreading p-side junction with low optical loss:
The need for lateral injection of holes from the p-side of the junction to the optically active volume poses a major challenge for nitride-based VCSELs. The commercial used of transparent contact layer is not suitable for fabrication of intracavity nitride-based VCSEL structures because of its large optical loss per single pass. Two approaches will be tested in this direction. First is using an indium-tin oxide (ITO) to replace TLC structure as the electrical contact to p-GaN. ITO will have a low optical loss if it was designed and placed at the standpoint of the light standing wave. Second is developing a nitride-based (Easki) tunnel junction structure which allows the use of low-resistivity n-type layers instead of high resistivity p-type layers as an ohmic contact layer. Both of these two structures may be very useful for nitride-based VCSEL structures.
(4) Development of multi-layer InGaN QDs structure grown by MOCVD:
To further increase the internal quantum efficiency and the filling factor of the InGaN QDs structure, not only the dot density should be increased, but the multi-layer QDs structure is essential for fabrication of high efficiency light emitting devices. The
fabrication issues of multi-layer QDs structure are included the critical growth condition for forming self-assembled QDs, the broad size distribution of QDs obtained by Stranski-Krastanov growth mode, and the new nonradiative centers not introduced during the stacking process. Althought the fabrication of high quality multi-layer QDs structure is more difficult than the MQW structure, the special epitaxial technique for InGaN QDs structure has to be established in the further.
(5) Combination of QDs structure and nitride-based microcavity Structure
Microcavity light emitting devices can increase the efficiency and speed of semiconductor spontaneous light emitters. However, small sized MCLEDs suffer from edge effects due to carrier loss and nonradiative recombination that can limit their efficiency. Using QDs structure as the active layer can resolve these effects for the strong carrier localizing capability. Furthermore, semiconductor microcavity structure combined with QDs structure is the best candidates for observing the research field of Cavity quantum electrodynamics (CQED), which is one of the core issues of modern optics research field. Despite the tough work of fabricating high quality nitride-based microcavity with QDs structure, nitride-based materials are the best candidates for executing the experiments of CQED at room temperature.
(6) Nitride-based single photon emitter:
Semiconductor QDs structure embedded in microcavity structure is the candidate of single photon emitter which is needed for quantum cryptography and quantum computer.
Because GaN-based materials in comparison to GaAs and InAs materials have wider direct bandgap, large conduction band offset, larger exciton binding energy, higher electrons saturation speed, high piezoelectric effect, and large electron effective mass, the GaN quantum confined microcavity structure is a much desirable device for the single photon emission allowing for higher-temperature operation. Improvement of the control on the density of nitride-based QDs structure and the resonant energy of the nitride-based microcavtiy will be the necessary step in the further.
Reference
[1] Tommy Ive, Oliver Brandt, Helmar Kostial, Thorsten Hesjedal, Manfred Ramsteiner, and Klaus H. Ploog, Appl. Phys. Lett., 85, 1970, 2004
Figure 6-1 The simulation results of the threshold current density under various mirror loss and scattering loss.
0.02 0.002 2E-4
0.1 1 10 100 1000 10000 100000 1000000
bulk dA=200nm αscatt=5cm-1 αscatt=30cm-1
αscatt=300cm-1 5 MQW dA=3nm
αscatt=5cm-1 αscatt=30cm-1 αscatt=300cm-1 SQW dA=3nm
αscatt=5cm-1 αscatt=30cm-1
jth[kA/cm2 ]
1-RRRF