Chen Dunjun

Au/Pt/InGaN/GaN Heterostructure Schottky Prototype Solar Cell

A patterned Au/Pt/In0.2Ga0.8N/GaN heterostructure Schottky prototype solar cell is fabricated. The forward current-voltage characteristics indicate that thermionic emission is a dominant current transport mechanism at the Pt/InGaN interface in our fabricated cell. The Schottky solar cell has an open-circuit voltage of 0.91 V, short-circuit current density of 7mA/cm2, and fill factor of 0.45 when illuminated by a Xe lamp with a power density of 300 mW/cm2. It exhibits a higher short-circuit current density of 30 mA/cm2 and an external quantum efficiency of over 25% when illuminated by a 20-mW-power He-Cd laser.

Field plate engineering for GaN-based Schottky barrier diodes

The practical design of GaN-based Schottky barrier diodes (SBDs) incorporating a field plate (FP) structure necessitates an understanding of their working mechanism and optimization criteria. In this work, the influences of the parameters of FPs upon breakdown of the diode are investigated in detail and the design rules of FP structures for GaN-based SBDs are presented for a wide scale of material and device parameters. By comparing three representative dielectric materials (SiO2, Si3N4 and Al2O3) selected for fabricating FPs, it is found that the product of dielectric permittivity and critical field strength of a dielectric material could be used as an index to predict its potential performance for FP applications.

Photoresponse of the In0.3Ga0.7N metal–insulator–semiconductor photodetectors

In 0.3Ga 0.7N metal–insulator–semiconductor (MIS) and metal–semiconductor (MS) surface barrier photodetectors have been fabricated. The In0.3Ga0.7N epilayers were grown on sapphire by metalorganic chemical vapour deposition (MOCVD). The photoresponse and reverse current–voltage characteristics of the In0.3Ga0.7N MIS and MS photodetectors were measured. A best zero bias responsivity of 0.18 A/W at 450 nm is obtained for the In0.3Ga0.7N MIS photodetector with 10 nm Si3N4 insulator layer, which is more than ten times higher than the In0.3Ga0.7N MS pho-todetector. The reason is attributed to the decrease of the interface states and increase of surface barrier height by the inserted insulator. The influence of the thickness of the Si3N4 insulator layer on the photoresponsivity of the MIS photodetector is also discussed.

High-field-induced electron detrapping in an AlGaN/GaN high electron mobility transistor

A step stress test is carried out to study the reliability characteristics of an AlGaN/GaN high electron mobility transistor (HEMT). An anomalous critical drain-to-gate voltage with a negative temperature coefficient is observed in the stress sequence, beyond which the HEMT device starts to recover from degradation induced by early lower voltage stress. While the performance degradation featuring the drain current slump stems from electron trapping in the surface or bulk states during low-to-medium bias stress, the recovery is attributed to high field induced electron detrapping. The carrier detrapping mechanism could be helpful for lessening the trapping-related performance degradation of a GaN-based HEMT.

UV Light-Emitting Diodes at 340 nm Fabricated on a Bulk GaN Substrate

We demonstrate an ultra-violet light-emitting diode (UV-LED) fabricated on a bulk GaN substrate with electroluminescence (EL) emission centered at about 340nm. The UV-LED exhibits low reverse leakage current on the order of 10−9 A under −5 V at room temperature, which can be explained by the low defect density in the epi-structure. The evolution of EL spectra as a function of injection current levels reveals the improved heat dissipation of the LEDs with vertical geometry on the bulk GaN substrate. The unusual increase of EL intensity at elevated temperatures can be explained by thermally assisted p-dopant ionization.

Passive Quenching Electronics for Geiger Mode 4H-SiC Avalanche Photodiodes*

We design and fabricate 4H-SiC UV avalanche photodiodes (APDs) with positive beveled mesa, which exhibit low leakage current and high avalanche gain when working in the Geiger mode. The single photon counting performance of the SiC APDs is studied by using a passive-quenching circuit. A new method to determine the exact breakdown voltage of the APD is proposed based on the initial emergence of photon count pulses. The photon count rate and dark count rate of the APD are also evaluated as a function of quenching resistance.

Forward Current Transport Mechanisms of Ni/Au—InAlN/AlN/GaN Schottky Diodes

We fabricate two Ni/Au-In0.17Al0.83N/AlN/GaN Schottky diodes on substrates of sapphire and Si, respectively, and investigate their forward-bias current transport mechanisms by temperature-dependent current-voltage measurements. In the temperature range of 300–485 K, the Schottky barrier heights (SBHs) calculated by using the conventional thermionic-emission (TE) model are strongly positively dependent on temperature, which is in contrast to the negative-temperature-dependent characteristic of traditional semiconductor Schottky diodes. By fitting the forward-bias I–V characteristics using different current transport models, we find that the tunneling current model can describe generally the I–V behaviors in the entire measured range of temperature. Under the high forward bias, the traditional TE mechanism also gives a good fit to the measured I–V data, and the actual barrier heights calculated according to the fitting TE curve are 1.434 and 1.413 eV at 300K for InAlN/AlN/GaN Schottky diodes on Si and the sapphire substrate, respectively, and the barrier height shows a slightly negative temperature coefficient. In addition, a formula is given to estimate SBHs of Ni/Au—InAlN/AlN/GaN Schottky diodes taking the Fermi-level pinning effect into account.

GaN Schottky Barrier Diodes with High-Resistivity Edge Termination Formed by Boron Implantation

Edge termination is one of the key technologies for fabricating high voltage Schottky barrier diodes (SBDs), which could effectively reduce the peak electric field along the Schottky contact edge and enhance the breakdown voltage. We adopt a high-resistivity ring structure as the edge termination for planar GaN SBDs. The edge termination is formed by self-aligned boron implantation on the edge of devices to form a highly damaged layer. In the implant dose and energy ranges studied experimentally, the GaN SBDs show improved blocking characteristics in terms of reverse leakage current and breakdown voltage at higher implant dose or implant energy. Meanwhile, the forward turn-on characteristics of the GaN SBDs exhibit no apparent change.

Improved Programming Efficiency through Additional Boron Implantation at the Active Area Edge in 90 nm Localized Charge-Trapping Non-volatile Memory

As the scaling-down of non-volatile memory (NVM) cells continues, the impact of shallow trench isolation (STI) on NVM cells becomes more severe. It has been observed in the 90 nm localized charge-trapping non-volatile memory (NROM™) that the programming efficiency of edge cells adjacent to STI is remarkably lower than that of other cells when channel hot electron injection is applied. Boron segregation is found to be mainly responsible for the low programming efficiency of edge cells. Meanwhile, an additional boron implantation of 10° tilt at the active area edge as a new solution to solve this problem is developed.

High Sensitivity and Selectivity of AsP Sensor in Detecting SF6 Decomposition Gases

The sensing properties of monolayer arsenic phosphorus (AsP) for the adsorption of SF6, H2O, O2, and SF6 decomposition gases (SO2 and H2S) are theoretically investigated by the first-principle calculations. We calculate the adsorption energy, equilibrium distance, Mulliken charge transfer, and electron localization function (ELF) to explore whether AsP is suitable for detecting SF6 decomposition gases. By comparing the adsorption performance of SF6, H2O, O2, and H2S gases, we have revealed that the SO2 gas molecules could form stable chemisorption with AsP monolayer. The results demonstrate that AsP is highly sensitive and selective to SO2 gas molecules with robust adsorption energy and apparent charge transfer. Furthermore, the current-voltage (I–V) curves reveal that only the adsorption of SO2 can largely modify the resistance of AsP. Our results show that gas sensors based on AsP monolayer could be better than that of black phosphorene (BP) to diagnose the state of online gas-insulated switchgear (GIS).