1 Gbps free-space deep-ultraviolet communications based on Ⅲ-nitride micro-LEDs emitting at 262 nm

The low modulation bandwidth of deep-ultraviolet(UV) light sources is considered as the main reason limiting the data transmission rate of deep-UV communications. Here, we present high-bandwidth Ⅲ-nitride microlight-emitting diodes(μLEDs) emitting in the UV-C region and their applications in deep-UV communication systems. The fabricated UV-C μLEDs with 566 μm2 emission area produce an optical power of 196 μW at the 3400 A∕cm2 current density. The measured 3 dB modulation bandwidth of these μLEDs initially increases linearly with the driving current density and then saturates as 438 MHz at a current density of 71 A∕cm2, which is limited by the cutoff frequency of the commercial avalanche photodiode used for the measurement. A deep-UV communication system is further demonstrated. By using the UV-C μLED, up to 800 Mbps and 1.1 Gbps data transmission rates at bit error ratio of 3.8 × 10-3 are achieved assuming on-off keying and orthogonal frequency-division multiplexing modulation schemes, respectively.

Lateral polarity control ofⅢ-nitride thin film and application in GaN Schottky barrier diode

N-polar and Ⅲ-polar GaN and AlN epitaxial thin films grown side by side on single sapphire substrate was reported.Surface morphology,wet etching susceptibility and bi-axial strain conditions were investigated and the polarity control scheme was utilized in the fabrication of Schottky barrier diode where ohmic contact and Schottky contact were deposited on N-polar domains and Ga-polar domains,respectively.The influence of N-polarity on on-state resistivity and Ⅰ–Ⅴ characteristic was discussed,demonstrating that lateral polarity structure of GaN and AlN can be widely used in new designs of optoelectronic and electronic devices.

A theoretical and experimental evaluation of Ⅲ–nitride solar-blind UV photocathode

We have developed a superior solar-blind ultraviolet （UV） photocathode with an AlxGa1_xrN photocathode （x ～ 0.45） in semi-transparent mode, and assessed spectra radiant sensitivity related to practical use. Betbre being grown over a basal plane sapphire substrate by low-pressure metal organic chemical vapor deposition （MOCVD）, a reasonable design was made to the photocathode epitaxy structure, focusing on the AlxGa1_xN： Mg active layer, then followed by a comprehen- sive analysis of the structural and optical characterization. The spectra radiant sensitivity is peaked of 41.395 mA/W at wavelength 257 nm and then decreases by about 3 to 4 decades at 400 nm demonstrating the ability of this photocathode for solar-blind application prospects.

Plasma-assisted MBE of GaN pn-junction Grown on Si（111） Substrate

We investigated growth of GaN pn-junction layers grown on silicon（111） by plasma-assisted molecular beam epitaxy system and its application for photo-devices. Si and Mg were used as n- and p-dopants, respectively. The reflection high energy electron diffraction images indicated a good surface morphology of GaN pn-junction layer. The thickness of GaN pn- junctions layers was about 0.705 nm. The absence of cubic phase GaN showed that thiS layer possessed hexagonal structure. According to XRD symmetric rocking curve ω/2θ scans of （0002） plane at room temperature, the full width at half-maximun of GaN pn-junction sample was calculated as 0.34°, indicating a high quality layer of GaN pn-junction. Surprisingly, there was no quenching of the A1 （LO） peak, with the presence of Si- and Mg-dopants in sample. The pn-junctions sample has a good optical quality which was measured by the photoluminescence system. For photo-devices applications, Ni and A1 were used as front and back contacts, respectively. The current-voltage characteristics of the devices showed the typical rectifying behavior of heterojunction. The photo-current measurement was performed using a visible-lamp under forward and reverse biases. From the temperature-dependent measurements, the current at low bias exhibited much stronger temperature dependence and weaker field dependence. The effect of thermal annealing on front contact Ni was also carried out. The front contact Ni was annealed at 400 and 600 ℃ for 10 min in the nitrogen ambient. The results showed that 600 ℃ treated sample had a higher gain at 1.00 V/e than 400 ℃ treated and untreated samples.

Comparison of resonant tunneling diodes grown on freestanding GaN substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy

AlN/GaN resonant tunneling diodes(RTDs)were grown separately on freestanding Ga N(FS-GaN)substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy(PA-MBE).Room temperature negative differential resistance(NDR)was obtained under forward bias for the RTDs grown on FS-GaN substrates,with the peak current densities(Jp)of 175-700 kA/cm^(2)and peak-to-valley current ratios(PVCRs)of 1.01-1.21.Two resonant peaks were also observed for some RTDs at room temperature.The effects of two types of substrates on epitaxy quality and device performance of GaN-based RTDs were firstly investigated systematically,showing that lower dislocation densities,flatter surface morphology,and steeper heterogeneous interfaces were the key factors to achieving NDR for RTDs.

Optical and magnetic properties of GaN epilayers implanted with ytterbium

We have studied the optical and magnetic properties of ytterbium implanted GaN epilayer grown on (0001) sapphire by metalorganic chemical vapor by deposition (MOCVD). Samples were implanted at room temperature with Yb ions at dose 4 1015 cm-2 and energy of 150 keV. The implanted samples were annealed at 1000 C in N2 at atmospheric pressure to recover implantation damages. The photoluminescence (PL), PL excitation (PLE), and PL kinetics have been studied with continuous and pulse photo-excitations in 360-1100 nm spectral range at different temperatures. The characteristic Yb3+ ion emission spectra were observed in the spectral range between 970-1050 nm. Theoretical fittings of the experimental PL temperature and PL kinetics data suggest that Yb3+ ions are involved in at least two major luminescence centers. The PLE spectra indicate that excitation of the Yb3+ ion occurs via electron-hole pair generation and complex processes. Magnetization versus magnetic field curves shows an enhancement of magnetic order for Yb-implanted samples in 5 K to 300 K temperature range. The Yb-implanted GaN sample showing weak ferromagnetic behavior was compared with the ferromagnetic in situ doped GaYbN material.