Quasi van der Waals Epitaxy of Single Crystalline GaN on Amorphous SiO2/Si(100) for Monolithic Optoelectronic Integration.

The realization of high quality (0001) GaN on Si(100) is paramount importance for the monolithic integration of Si-based integrated circuits and GaN-enabled optoelectronic devices. Nevertheless, thorny issues including large thermal mismatch and distinct crystal symmetries typically bring about uncontrollable polycrystalline GaN formation with considerable surface roughness on standard Si(100). Here a breakthrough of high-quality single-crystalline GaN film on polycrystalline SiO2/Si(100) is presented by quasi van der Waals epitaxy and fabricate the monolithically integrated photonic chips. The in-plane orientation of epilayer is aligned throughout a slip and rotation of high density AlN nuclei due to weak interfacial forces, while the out-of-plane orientation of GaN can be guided by multi-step growth on transfer-free graphene. For the first time, the monolithic integration of light-emitting diode (LED) and photodetector (PD) devices are accomplished on CMOS-compatible SiO2/Si(100). Remarkably, the self-powered PD affords a rapid response below 250 µs under adjacent LED radiation, demonstrating the responsivity and detectivity of 2.01 × 105 A/W and 4.64 × 1013 Jones, respectively. This work breaks a bottleneck of synthesizing large area single-crystal GaN on Si(100), which is anticipated to motivate the disruptive developments in Si-integrated optoelectronic devices.

Three dimensional truncated-hexagonal-pyramid vertical InGaN-based white light emitting diodes based on ß-Ga2O3.

In this Letter, we describe the fabrication of three dimensional (3D) truncated-hexagonal-pyramid (THP) vertical light emitting diodes (VLEDs) with white emission grown on ß-Ga2O3 substrate. In the 3D n-GaN layer, it is noted that the longitudinal growth rate of the 3D n-GaN layer increases as the flow rate of N2 decreases and H2 increases. Moreover, the 3D THP VLED can effectively suppress the quantum-confined Stark effect (QCSE) compared with planar VLEDs due to the semipolar facets and strain relaxation. Thus, the internal quantum efficiency (IQE) of the 3D THP VLED has been doubled and the V-shaped pits have been greatly reduced. In particular, the 3D THP VLED enables multi-wavelength emission (448.0 nm and 498.5 nm) and also shows better light extraction efficiency (LEE), which presents an effective way for the realization of phosphor-free white LED devices.

Strain modulated luminescence in green InGaN/GaN multiple quantum wells with microwire array by piezo-phototronic effect.

We have demonstrated piezo-phototronic enhanced modulation in green InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with a microwire array (MWA) structure. It is found that an a-axis oriented MWA structure induces more c-axis compressive strain than a flat structure when a convex bending strain is applied. Moreover, the photoluminescence (PL) intensity exhibits a tendency to increase first and then decrease under the enhanced compressive strain. Specifically, light intensity reaches a maximum of about 123% accompanied by 1.1-nm blueshift, and the carrier lifetime comes to the minimum simultaneously. The enhanced luminescence characteristics are attributed to strain-induced interface polarized charges, which modulate the built-in field in InGaN/GaN MQWs and could promote the radiative recombination of carriers. This work opens a pathway to drastically improve InGaN-based long-wavelength micro-LEDs with highly efficient piezo-phototronic modulation.

Deep-ultraviolet aperiodic-oscillation emission of AlGaN films.

In this Letter, we describe an aperiodic-oscillation emission phenomenon originating from the Fabry-Perot effect in the deep-ultraviolet backscattering fluorescence spectrum of the $c$c-plane AlGaN film, which is related to the dispersion of its ordinary refractive index near the band edge. Based on this fluorescence spectrum, the ordinary refractive index of the AlGaN film near the band edge could be directly obtained. Certainly, by means of variable angle spectroscopic ellipsometry, the ordinary refractive index of the AlGaN film could be also achieved. Comparing the results obtained by both methods, we discovered that the refractive indices are quite similar, which suggests that the aperiodic-oscillation fluorescence spectrum is also a reliable approach to capture the refractive index of anisotropic optical films.

Near vacuum-ultraviolet aperiodic oscillation emission of AlN films.

An accurate measurement of the refractive index is necessary for the optical design of both deep ultraviolet laser diodes and light-emitting diodes. Generally, the refractive indices along different crystallographic axes of anisotropic thin films are measured using variable angle spectroscopic ellipsometry. However, there are still some limitations concerning this method. Here we proposed a potential method to measure the band edge refractive index of wide bandgap semiconductor. An aperiodic oscillation emission phenomenon due to the Fabry-Perot effect was observed in the fluorescence spectrum of an AlN film with a thickness of 1500 nm. Based on the characteristics of the fluorescence spectrum and the definition of Fabry-Perot effect, we obtained the ordinary refractive index of the AlN thin film near the band edge directly. This refractive index measurement method is a supplement to the variable angle ellipsometry, and it is a more direct and effective method for transferred film and thinner samples to measure the fluorescence spectrum.

Vacuum-Ultraviolet Photovoltaic Detector.

Over the past two decades, solar- and astrophysicists and material scientists have been researching and developing new-generation semiconductor-based vacuum ultraviolet (VUV) detectors with low power consumption and small size for replacing traditional heavy and high-energy-consuming microchannel-detection systems, to study the formation and evolution of stars. However, the most desirable semiconductor-based VUV photovoltaic detector capable of achieving zero power consumption has not yet been achieved. With high-crystallinity multistep epitaxial grown AlN as a VUV-absorbing layer for photogenerated carriers and p-type graphene (with unexpected VUV transmittance >96%) as a transparent electrode to collect excited holes, we constructed a heterojunction device with photovoltaic detection for VUV light. The device exhibits an encouraging VUV photoresponse, high external quantum efficiency (EQE) and extremely fast tempera response (80 ns, 104-106 times faster than that of the currently reported VUV photoconductive devices). This work has provided an idea for developing zero power consumption and integrated VUV photovoltaic detectors with ultrafast and high-sensitivity VUV detection capability, which not only allows future spacecraft to operate with longer service time and lower launching cost but also ensures an ultrafast evolution of interstellar objects.