Deep-ultraviolet (DUV) photodetectors based on wide bandgap (WB) semiconductor materials have attracted strong interest because of their broad applications in military surveillance, fire detection and ozone hole monitoring. Monoclinic β-Ga2O3 with ultra-wide bandgap of ~4.9 eV is a promising candidate for such application because of its high optical transparency in UV and visible wavelength region, and excellent thermal and chemical stability at elevated temperatures. Synthesis of high qualityβ-Ga2O3 thin films is still at its early stage and knowledge on the origins of defects in this material is lacking. The conventional epitaxy methods used to grow β-Ga2O3 thin films such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) still face great challenges such as limited growth rate and relatively high defects levels. In this work, we present the growth of β-Ga2O3 thin films on c-plane (0001) sapphire substrate by our recently developed low pressure chemical vapor deposition (LPCVD) method. The β-Ga2O3 thin films synthesized using high purity metallic gallium and oxygen as the source precursors and argon as carrier gas show controllable N-type doping and high carrier mobility. Metal-semiconductor-metal (MSM) photodetectors (PDs) were fabricated on the as-grown β-Ga2O3 thin films. Au/Ti thin films deposited by e-beam evaporation served as the contact metals. Optimization of the thin film growth conditions and the effects of thermal annealing on the performance of the PDs were investigated. The responsivity of devices under 250 nm UV light irradiation as well as dark light will be characterized and compared.
Group III-nitride (Al-, In-, Ga-, N) material system has been well studied and widely applied in optoelectronics such as light emitting diodes (LEDs) for solid state lighting. In contrast, the group II-IV-nitride is rarely studied, yet it can expand the material properties provided by III-nitrides. For example, ZnGeN2 has a similar bandgap and lattice constant as those of GaN. Recently, theoretical studies based on first principle calculation indicate a large band offset between GaN and ZnGeN2 (Delta_Ec=1.4 eV; Delta_Ev=1.5 eV). Utilizing the novel heterostructures of GaN (InGaN)/ZnGeN2, we studied the following two types of device structures: 1) Type-II InGaN-ZnGeN2 quantum wells (QWs) for high efficiency blue and green LEDs; 2) Lattice-matched GaN-ZnGeN2 coupled QWs for near-IR intersubband transitions. The design of type-II InGaN-ZnGeN2 QWs leads to a significant enhancement of the electron-hole wavefunction overlap due to the strong confinement of the holes in the ZnGeN2 layer as well as the engineered band bending. Simulation studies based on a self-consistent 6-band k∙p method indicate an enhancement of 5-7 times of spontaneous emission rate for an appropriately designed type-II InGaN-ZnGeN2 QWs for LED applications. For the coupled QW structure, it is comprised of two GaN QWs separated by a thin ZnGeN2 barrier layer, with thick ZnGeN2 layers as outer barriers surrounding the QWs. Our studies indicate that with optimized ZnGeN2 barrier thickness, the energy separation between E1 and E2 can be tuned to 92 meV for the resonance of the electron and LO-phonon scattering.
Strain-compensated type-II InGaN-ZnGeN2-AlGaN quantum wells (QWs) are studied as improved active regions for light-emitting diodes (LEDs). Both the band gap and the lattice parameters of ZnGeN2 are very close to those of GaN. The recently predicted large band offset between GaN and ZnGeN2 allows the formation of a type-II heterostructure. The deep confinement of holes in the ZnGeN2 layer allows the use of a low In-content InGaN QW to extend the emission wavelength into the green wavelength region. A thin layer of AlGaN surrounding the QW is used as a strain compensation layer. Simulation studies of the proposed type-II QW indicate an enhancement of 5.6-6.8 times the spontaneous emission rate compared to InGaN-GaN QWs emitting in the green wavelength region.
GaN microdomes are studied as a broadband omnidirectional anti-reflection structure for high efficiency multi-junction concentrated photovoltaics. Comprehensive studies of the effect of GaN microdome sizes and shapes on the light collection efficiency were studied. The three dimensional finite difference time domain (3-D FDTD) method was used to calculate the surface reflectance of GaN microdomes as compared to that of the flat surface. Studies indicate significant reduction of the surface reflectance is achievable by properly designing the microdome structures. Formation of the GaN microdomes with the flexibility to tune the size and shape has been demonstrated by using reactive ion etching (RIE) of both GaN and the self-assembled silica monolayer microspheres. Characterizations of the angle-dependence light surface reflectance for both micro-domes and flat surface show the similar trend as the simulation.
The enhancement of light extraction efficiency for thin-film-flip-chip (TFFC) InGaN QWs LEDs with GaN microdomes
on n-GaN layer was studied. The three dimensional FDTD method was used to calculate the light extraction
efficiency for the TFFC InGaN QWs LEDs emitting at visible spectral regime, as compared to that of the conventional
TFFC InGaN QWs LEDs. The calculation indicates significant dependence of the p-GaN layer thickness on the light
extraction efficiency. Significant enhancement of the light extraction efficiency (2.5-2.7 times for λpeak=460nm and 2.7-
2.8 times for λpeak=550nm) is achievable from LEDs with GaN micro-domes with optimized micro-dome diameter and