Hsueh-Hsing Liu

High-Performance AlGaN/GaN Schottky Diodes With an AlGaN/AlN Buffer Layer

High-performance AlGaN/GaN Schottky barrier diodes are fabricated on a composite AlGaN/AlN buffer layer with low screw-type and high edge-type dislocation densities. Without edge termination, the devices with 30-μm anode-to-cathode spacing exhibit a high breakdown voltage (<i>V</i>_B) of 3489 V, a low leakage current (<i>I</i>_R) of less than 0.2 μA at -2000 V, and a low specific on-resistance (<i>R</i>_on) of 7.9 mΩ·cm², resulting in a figure of merit (<i>V</i>_B²/<i>R</i>_on) as high as 1.54 GW/cm². Their switching characteristics as revealed by the reverse-recovery transient waveform exhibit a short reverse-recovery time of 17 ns.

Effects of Lens Shape on GaN Grown on Microlens Patterned Sapphire Substrates by Metallorganic Chemical Vapor Deposition

The epitaxial growth of GaN on patterned c-plane sapphire substrates having microlenses with a flat top, a dull tip, or a sharp tip is carried out. The growth mode, dislocation density, residual strain, and optical properties of GaN are investigated and correlated with the shape of the microlens. Because the growth of GaN does not take place on top of the microlens with a sharp tip, this type of patterned substrate leads to a wider low dislocation density lateral growth region, while it also gives rise to a higher compressive residual strain in GaN. For GaN grown on the microlens with a dull tip, many dislocations appear, resulting from the extra facets on the lens. It, however, has the lowest compressive strain among the samples studied. This work provides a guideline for preparing microlens patterned sapphire substrates for potential applications in high brightness InGaN light emitting diodes as both dislocation density and strain influence their internal quantum efficiency.

Low-resistance smooth-surface Ti/Al/Cr/Mo/Au n-type Ohmic contact to AlGaN/GaN heterostructures

A comparative study of the specific contact resistivity and surface morphology of Ti/Al/Ni/Au, Ti/Cr/Mo/Au, and Ti/Al/Cr/Mo/Au metal contact stacks on AlGaN/GaN heterostructures is reported. Compared to the conventional Ti/Al/Ni/Au contact, the Ti/Al/Cr/Mo/Au contact has much smoother surface and achieves minimum specific contact resistivity of 1.1×10−6 Ω cm2. This contact maintains its low contact resistivity after storage at 200 °C for 100 h in N2. The robustness of this contact is attributed to the Cr and Mo layers, which suppress the formation of Al–Au alloys in the contact stack.

Efficiency Enhancement of InGaN LEDs With an n-Type AlGaN/GaN/InGaN Current Spreading Layer

This letter reports an InGaN light-emitting diode (LED) structure that has an n-type Al0.1Ga0.9N/GaN/In0.06Ga0.94N current spreading layer under its multiple-quantum-well active region. As indicated by simulation, the Al0.1Ga0.9N/GaN/In0.06Ga0.94 N heterostructure induces a higher electron concentration than an n-AlGaN/GaN cladding layer and an n-GaN/InGaN current spreading layer that are used in conventional LEDs. As a result, the proposed n-type spreading layer is expected to alleviate current crowding and improve external quantum efficiency. Experimentally, the light output uniformity across the chips is greatly improved. The output power and wall-plug efficiency are enhanced by about 18.2% and 22.2% at an injection current of 350 mA for the LEDs employing the new double-heterostructure current spreading layer.

Improved Surface Morphology and Edge Definition for Ohmic Contacts to AlGaN/GaN Heterostructures

We demonstrate that Ti/Al/Cr/Mo/Au ohmic contact has an extremely smooth surface morphology of 29.0 nm and a low specific contact resistivity (ρc) of 1.1×10-6 ohm-cm2 on n-type AlGaN/GaN heterostructures. The use of Cr interlayer in Ti/Al/Cr/Mo/Au contacts leads to significantly improved contact morphology without any degradation on the contact resistance. This is attributed to the reduced inter-diffusion and reaction between the layers in the contact stack.

Growth of high quality AlN on sapphire by using a low-temperature AlN interlayer

Aluminum nitride is a material of great potential for high power electronic devices, UV photonic devices as well as acoustic devices. However, the lack of a good crystal growth technology for bulk material and substrate hinders the development of these AlN-based devices. While AlN has been successfully grown on sapphire substrate for some time, the presence of a large number of dislocations in the material is still a major barrier to overcome [1]. In this work, we demonstrate a low-dislocation-density AlN template on sapphire by inserting an AlN interlayer by metal-organic chemical vapor deposition. The main idea of our approach is to change the growth mode in the course of the epitaxial growth by decreasing growth temperature and changing V/III ratio. As the growth mode changes, dislocations tend to be redirected and/or form dipole half loops via annihilation processes [2]. The etch-pit-density of the AlN templates is reduced from 3.6×109 cm-2 to 1.7×109 cm-2. Accordingly, the full width at half maximum of the (0002) x-ray rocking curve is reduced from 37 arcsec to 12 arcsec. The result indicates that the AlN template has low screw and mixed type dislocations. AlGaN/GaN Schottky diodes fabricated on this high quality AlN template exhibit very high breakdown voltage (> 2000 V), which sets a record-high figure of merit of 1.15 GW/cm2.

Growth of crack-free semi-polar (1-101) GaN on a 7°-off (001) Si substrate by metal-organic chemical vapor deposition

This paper reports a novel selective growth method for growing crack-free semi-polar (1-101) GaN on 7°-off (001) Si substrates by adding SiO2 stripes in perpendicular to the V-grooves on Si. This method can effectively reduce the thermal stress between GaN and Si substrate so that crack-free (1-101) GaN films as thick as 1 μm is achieved after coalescence even without an AlN interlayer. Cathodoluminescence measurements show the presence of low dislocation density areas, which can be attributed to the bending of dislocations toward to the [1-100] and [11-20] directions during the facet growth.

Epitaxial lateral overgrowth of GaN on AlGaN/(111)Si micropillar array fabricated by polystyrene microsphere lithography

The authors report on the growth of GaN on AlGaN/(111)Si micropillar array by metal-organic chemical vapor deposition. Using the substrates with micropillar array, 2 μm-thick GaN films without cracks can be achieved. Transmission electron microscopy, atomic force microscopy, and micro-Raman studies indicate that the dislocation density and residual stress of the GaN grown on micropillar array are also reduced. The results reveal the potential of this type of substrates for growing GaN-based devices as well as preparing GaN freestanding substrates.

Efficiency improvement of GaN light emitting diodes on Si by double island growth method

This paper reports the double island growth method to reduce threading dislocation density in GaN epilayers grown on (111) silicon substrates by metal-organic vapor phase epitaxy. The overall dislocation density can be effectively reduced to 2.6×109 cm-2. The output power and wall-plug efficiency are enhanced by about 27 % and 34.5 % at an injection current of 20 mA for the light emitting diodes (LEDs) using the new double island structure.

Investigations of photo-assisted conductive atomic force microscopy on III-nitrides

We have employed photo-assisted conductive atomic force microscopy (PA-CAFM) to obtain high-resolution current images on III-nitride surfaces. From the statistical results of current distribution, it is revealed that the full-width at half-maximum (FWHM) value is very sensitive to the dislocation density of III-nitride films even if the dislocation density is very low (~10^8cm^-^2), suggesting that the FWHM value of current statistics can be an indicator of III-nitride quality. The results acquired by PA-CAFM are consistent with those obtained from etching-pit density and X-ray diffraction measurements. In addition, it is also revealed that PA-CAFM can observe the current distribution of InN dots without external bias influences, directly indicating the dependence of InN dots and dislocations. Our experimental results indicate that PA-CAFM is a promising method for investigating the electrical and structural properties of a nanometric area in III-nitride materials.