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Example I: Type-II InGaN/ZnGeN2 Quantum Wells for High Efficiency Light-Emitters
Problem:
InGaN quantum wells (QWs) have been used as active regions for high efficiency blue light-emitting diodes (LEDs). However, the efficiency of the devices decease significant as the emission wavelength extends to green and longer wavelength regime, caused by 1) charge separation in InGaN QWs using higher In-content InGaN and thicker layer; and 2) In-segregation and defects in InGaN with high-In content. This has become a bottleneck to achieve the monolithic white light LEDs for solid state lighting.
Solution:
Utilizing the advantages of 1) closely-lattice matching between GaN and ZnGeN2; and 2) a large band offset between GaN and ZnGeN2, we designed a type-II InGaN-ZnGeN2 QW structure to address the issue mentioned above. The band structure engineering from the type-II QW leads to a significant enhancement of electron-hole wavefunction overlap resulting in enhanced radiative efficiency in InGaN QWs emitting in the green wavelength regime. Using a lower In-content InGaN layer, the type-II QW can lead to 4.6-4.9 times enhancement of radiative recombination rate for LED emitting at 530 nm [1, 2]. ZnGeN2 layer position and thickness in the InGaN QW have a strong impact on the band structure and electron-hole wave function overlap [3].
References:
[1]. L. Han, K. Kash, H. Zhao, Proc. of SPIE, Vol.9003, 90030W-1 (2014).
[2]. L. Han, K. Kash, H. Zhao, Journal of Applied Physics, 120, 103102 (2016).
[3]. J. Grgat, L. Han, H. Zhao, “Analysis of Position and Thickness Dependence of ZnGeN2 Layer in Type-II InGaN-ZnGeN2 Quantum Wells Light-Emitting Diodes”, the IEEE/OSA Conference on Lasers and Electro-Optics (CLEO) 2017, San Francisco, CA, May 2017.
Example II: GaN-ZnGeN2 Coupled Quantum Wells for Near-Infrared Quantum Cascade Lasers
Background and Problem:
Quantum cascade lasers (QCLs) using intersubband transitions have been used for compact, high power and room-temperature continue-wave (cw) operations. InP-and Ga(In)As-based QCLs have been demonstrated with superior performance in the mid-IR to far-IR for applications such as gas sensing, medical imaging, security screening and etc.. However, QCLs operating at shorter wavelengths (λ<3μm) for the applications in free-space communication and spectroscopy is limited by the materials with available conduction band offset and by material transparency. III-nitride semiconductors (GaN, AlN, InN and their alloys) are suitable candidates for near-IR intersubband devices due to their large band gap, large conduction band offset (>1ev) and electron-longitudinal (LO) optical phonon energy (~90meV). For example, intersubband transitions have been studied in the GaN/AlN or GaN/AlInN heterostructures. However, the material/device performance suffered from the incompatibility of the growth conditions and the large lattice mismatch (between GaN/AlN) induced strains and dislocations in the heterostructures. Up to date, electrically-injected intersubband transition in the near-IR has not been achieved.
Solution:
Utilizing the large band-offset between GaN and ZnGeN2 (∆Ec=1.4 eV; ∆Ev=1.5 eV), a lattice-matching coupled QW GaN-ZnGeN2 heterostructure is designed to target for near-IR QCL application. In the design [4-6], GaN serves as the well region and ZnGeN2 layer serves as the barrier layers. By tuning the thickness of the wells and barrier, near-IR intersubband transitions (λ~1.7-3 um) can be achieved between the conduction subband levels (E3-->E2), with the optimized electron-LO phonon scattering process between E2-->E1 (~ 92 meV).
References:
[5]. H. Zhao, L. Han, J. Grgat, “Novel Device Designs Enabled by Lattice-Matched GaN-ZnGeN2 Heterostructures”, SPIE Photonics West 2017, Gallium Nitride Materials and Devices XII, San Francisco, CA, Feb. 2017.
[6]. L. Han, C. Lieberman, H. Zhao, “Electron-Photon and Electron-LO Phonon Scattering Rates in Closely-Lattice-Matched GaN-ZnGeN2 Coupled Quantum Wells”, J. Appl. Phys., (in print)
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