A Si-integrated oxide-nitride deep-ultraviolet photodetector with remarkable photosensitivity is demonstrated. The proposed device topology is realized through the disordered nucleation of β-Ga2O3 crystals on monocrystalline TiN interlayers forming an oxide-nitride vertical heterostructure stack housed on a Si substrate. Spectral responsivity levels of about 240 A/W at illuminating power density levels of around 7.40 μW/cm2 were achieved.
Heavy reliance on extensively studied AlGaN based light emitting diodes (LEDs) to replace environmentally hazardous mercury based ultraviolet (UV) lamps is inevitable. However, external quantum efficiency (EQE) for AlGaN based deep UV emitters remains poor. Dislocation induced nonradiative recombination centers and poor electron-hole wavefunction overlap due to the large polarization field induced quantum confined stark effect (QCSE) in “Al” rich AlGaN are some of the key factors responsible for poor EQE. In addition, the transverse electric polarized light is extremely suppressed in “Al”-rich AlGaN quantum wells (QWs) because of the undesired crossing over among the light hole (LH), heavy hole (HH) and crystal-field split-off (SH) states. Here, optical and structural integrities of dislocation-free ultrathin GaN quantum disk (QDisk) (~ 1.2 nm) embedded in AlN barrier (~ 3 nm) grown employing plasma-assisted molecular beam epitaxy (PAMBE) are investigated considering it as a novel nanostructure to realize highly efficient TE polarized deep UV emitters. The structural and chemical integrities of thus grown QDisks are investigated by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). We, particularly, emphasize the polarization dependent photoluminescence (PL) study of the GaN Disks to accomplish almost purely TE polarized UV (~ 260 nm) light. In addition, we observed significantly high internal quantum efficiency (IQE) of ~ 80 %, which is attributed to the enhanced overlap of the electron-hole wavefunction in extremely quantum confined ultrathin GaN QDisks, thereby presenting GaN QDisks embedded in AlN nanowires as a practical pathway towards the efficient deep UV emitters.
A novel method to fabricate β-(AlGa)2O3 solar-blind photodetector has been demonstrated. The β-Ga2O3 thin film was first deposited on c-plane sapphire substrate by pulsed laser deposition (PLD) and following by high-temperature annealing (1000°C and above). With a proper annealing condition, the PLD deposited β-Ga2O3 film could be transformed from a binary to become a ternary β-(AlGa)2O3 film, which is resulted from the Al atoms of sapphire substrate diffused into the PLD deposited β-Ga2O3 layer by high-temperature driven, and the Ga atoms from β-Ga2O3 thin film to the substrate diffusion as well. By high-temperature driven interdiffusion method, β-(AlGa)2O3 thin film with designed Al composition and film thickness could be achieved, which could cover higher bandgap larger than 4.9 eV. With such a method, one can achieve β-(AlGa)2O3 Solar-blind photodetector with good crystal quality and surface morphology, which also could push the response wavelength even further to the DUV range.
Thin polymorphic gallium oxide films were grown on c-plane sapphire using pulsed laser deposition. The stacked thin films (ε-Ga2O3 and β-Ga2O3) were sequentially grown under the same conditions but in a different ambience. Our X-ray diffraction measurements and transmission electron microscopy images confirmed a β-Ga2O3/ε-Ga2O3 polymorphic heterostructure with rocking-curve widths of 1.4° (β-Ga2O3 (¯603)) and 0.6° (ε-Ga2O3 (006)). The crystallographic orientation relationships between c-plane sapphire and the heterogeneously nucleated ε-Ga2O3 buffer layer, as well as between the ε-Ga2O3 and β-Ga2O3 heterogeneous layers, were determined. Our study will aid in developing novel deep-ultraviolet optoelectronic devices, such as solar-blind and metal-insulator-semiconductor deep-ultraviolet photodiodes.
Polarization-matched quantum wells (QWs) can lead to maximized electron-hole wave functions overlap and low efficiency droop at high current density. By using the modern theory of polarization with hexagonal reference, c-plane InAlN/InGaN QWs were explored and designed for polarization matching. The simulation results show that, even on c-plane, polarization-matched structures can be achieved by adjusting strain and material composition. The In composition of larger than 35% of InAlN was required to match the total polarization of InGaN at any given composition. Considering the bandgap’s bowing factors of III-nitride ternary alloys, In0~0.1Ga1.0-0.9N as quantum barrier (QB) provided enough potential barriers for In0.35~0.45Al0.65-0.55N to form a multiple QW (MQW) structure. The results indicated that improper resistance of MQW and the existing fixed charge between the interfaces of p-type region/MQW and n-type region/MQW could result in nonuniform carrier distributions and current leakage, respectively. Furthermore, we found that In0.41Al0.59N/In0.1Ga0.9N polarization-matched MQW had proper resistance; however, such structure produced a huge polarization fixed-charge between the junction interface. By studying the strain level of InAlN QW and GaN QB, which can be grown on AlN/GaN superlattice templates, the In0.33Al0.67N/GaN polarization-matched MQW structure has been specifically designed with small resistance and without inducing improper polarization fixed charge. By optimizing the number and thickness of QWs, the 425nm LED has relative IQE of 56% and efficiency droop of only 7% at high current density of 333 A/cm2. This study provides guidance for development of In-rich InAlN materials.
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