We demonstrate an optically pumped AlGaN-based laser at 272 nm with two-step etched facets. Compared with a laser with cleaved facets, the laser with etched facets had a lower threshold and higher differential quantum efficiency.
Smooth and vertical facets for InGaN/GaN double heterostructure lasers grown on sapphire substrate are formed via a two-step method of dry etching and wet chemical etching. This two-step process consists of an inductively coupled plasma (ICP) etching with Ni metal as an etching mask to define the cavity length of the laser bars, followed by crystallographic wet etching in AZ400K developer to reduce the roughness and improve verticality of the sidewall. Optimization of ICP etching processes included the gas flow rate of Cl 2 /Ar and the radiofrequency (RF) power to obtain a heavily striated facet angle of 3° from vertical. Combined with the wet etching processes, the facets along 〈112̅̅0〉 direction became smooth and vertical due to the preferentially etching feature of AZ400K in exposing the (101̅0) and other (101̅n)-type planes because of their slow etch rates compared with the other planes. To confirm the feasibility of the two-step etching facets, optically pumping experiment was carried out on InGaN/GaN double heterostructure lasers. Lasing behaviors of longitudinal mode peaked at 375 nm with a narrow line width of 1.3 nm was demonstrated on the laser using this two-step etching process, and proved it an effective way for laser facets fabrication method.
We have studied the optical matching layers (OMLs) and external quantum efficiency in the evanescent coupling photodiodes (ECPDs) integrating a diluted waveguide as a fibre-to-waveguide coupler, by using the semi-vectorial beam propagation method (BPM). The physical basis of OML has been identified, thereby a general designing rule of OML is developed in such a kind of photodiode. In addition, the external quantum efficiency and the polarization sensitivity versus the absorption and coupling length are analysed. With an optical matching layer, the absorption medium with a length of 30 μm could absorb 90% of the incident light at 1.55 μm wavelength, thus the total absorption increases more than 7 times over that of the photodiode without any optical matching layer.
Owing to outstanding properties of two-dimensional electron gas (2DEG) and GaN-base material, such as high electron mobility and high critical electric field, AlGaN/GaN high electron mobility transistors (HEMTs) have been considered as the next-generation power semiconductor devices with high power conversion efficiency, high switching frequency and high-temperature operation capability. For power switching applications, enhancement mode (e-mode, or say normally-off) transistors are desired for fail-safe operation and silicon-compatible gate drive circuit. However, e-mode operation is difficult for AlGaN/GaN-based HEMTs because of the natural existence of 2DEG. To achieve e-mode operation, several approaches have been reported, including gate recess structure, fluorine plasma ion implantation, p-type gate structure and selective channel regrowth, etc. This paper reports normally-off AlGaN/GaN HEMTs with a recessed MOS-gate. After mesa isolation and Source and drain metal, the gate recess process used inductively coupled plasma (ICP). In order to avoid severe etching damage and obtain high drain current density, etching power was carefully optimized. The etching rate is slow for accurate etching depth control to leverage the current density and threshold voltage. After that, an O2 plasma treatment was applied using a plasma asher to oxidize the damaged semiconductor surface. The oxide layer was then removed in HCl: DI-water (1: 3). The next step is an atomic layer deposition (ALD) of Al 2 O 3 as the gate dielectric to increase the breakdown voltage. Follow after that, gate metal and pad metal. By the process, our team made three kinds of HEMTs with different recessed gate depths. The first one, which has been report before, exhibits a high threshold voltage of +4.6V, a specific on-resistance of 4mΩ-cm^2 and a drain current density of 108 mA/mm. This paper will show the others fabricated in the later researches. The device transfer curves show that this normally-off recessed MOS-gate AlGaN/GaN HEMTs exhibit threshold voltage of +0.9V and +2.1V, respectively. Additionally, the specific on-resistance and saturation drain current density of the device with 0.9V threshold voltage are 2.26mΩ-cm^2 and 326mA/mm, while 2.1V-Threshold Voltage-device are 3.03mΩcm^2 and 173mA/mm. Both of the two HEMTs have a breakdown voltage over 400V.
Spectral broadening is an important nonlinear effect during the interaction of intense femtosecond laser pulses with air. In this paper, we experimentally study the dependence of spectral broadening on pressure and pulse energy. It is found that increasing both pressure and pulse energy can enhance the spectral intensity, while only increasing pressure leads to obvious blue shift of spectra, and the pulse energy has little effect on it. To get a better insight of the mechanism of spectral broadening, the numerical simulation relating the instantaneous frequency to the time dependent pulse intensity and time dependent plasma density in air is carried out. The results indicate that the plasma generation induced self-phase modulation which is related to the pressure plays an important role in blue shift of spectrum. Besides the transmission of the laser pulse is measured and decreases with pressure. It proves that the energy transfer of the laser pulse is promoted by pressure. This study will be helpful to understand a deeper physical mechanism of femtosecond filament induced supercontinnum generation in air.
We investigate the effect of AlN/AlGaN superlattices (SLs) on crystal and optical properties of AlGaN epitaxial layers. The result indicates that the crystal quality of AlGaN layers is consistent within a wide range of SLs thicknesses, while the optical properties are opposite. With SLs thickness decreasing from 20/44 to 17/36 and 15/29 nm, the full-width at half maximum of X-ray rocking curves for (0002)- and ( )-plane of n-AlGaN layers grown on SLs are consistent of around 250 arcsec and 700 arcsec, respectively. Meanwhile, the center of the low optical transmittance band decreases from 326 to 279 nm and less than 266 nm as the SLs thickness decreases. 280 nm deep ultraviolet light-emitting diodes (DUV-LEDs) structures are further regrown on the n-AlGaN layers. The electroluminescent intensities of samples are 30% higher than that of the sample whose low optical transmittance band appears around 279 nm. Optical simulations reveal that the SLs acts as distributed Bragg reflectors, thus less photons of the corresponding wavelength escape from the sapphire backside.
GaN-based high electron mobility transistors (HEMTs) are promising candidates for millimeter wave amplifiers above 100 GHz for 6G communications. To satisfy those requirements, a maximum oscillation frequency $(\mathbf{f}_{\max})$ surpassing 400 GHz is essential, necessitating the deployment of advanced down-scaling and patterning technologies. Through the utilization of electron beam lithography, ultra-scaled InAlN/GaN HEMTs on sapphire with T-gates with gate length of~60 nm and source-drain distance of~300 nm were fabricated, reaching $\mathrm{f}_{\max}$ over 420 GHz with a corresponding f T exceeding 150 GHz. To the best of the author's knowledge, these scaled HEMTs exhibit the highest fmax among reported HEMTs on sapphire.
We demonstrate that low-fluence neutron irradiation can be a promising way to reduce the reverse leakage current of AlGaN/GaN heterostructures grown by MOCVD on sapphire substrates while maintaining other electronic properties almost unchanged.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
