High Quality AlN Templates Grown by HVPE for High Performance Wireless Applications

By AZoNano

Table of Contents

Introduction

III-V nitride semiconductors are known to be excellent candidates for high-frequency, high-power, RF power amplification. The key advantages of III-V nitrides over other semiconductor materials are listed below:

• III-V nitrides have large bandgaps hence they have corresponding large breakdown electric fields
• Superior thermal conductivity
• Good electron transport properties
• The ability to form heterostructures.

Advantages of Nitride Heterostructures

When compared to other III-V semiconductors and even SiC, these nitride heterostructures have very high 2DEG densities that are essential for high power, high electronic mobility transistors (HEMTs), intended to be used in high-power compact energy-efficient transmission amplifiers for 4G wireless mobile stations. A conventional AlGaN/GaN heterostructure is typically formed by epitaxially depositing a layer of AlGaN on a thick GaN layer on insulating or semi-insulating substrates such as SiC or sapphire. Strain induced and spontaneous polarizations lead to a high positive polarization in the AlGaN, resulting in a two-dimensional electron gas (2DEG) at the AlGaN/GaN boundary.

Optimised AlN Templates

There is significant improvement in the performance of HEMT devices when conventional AlGaN/GaN heterostructures were grown directly on AlN layer using SiC substrates. While inserting these AlN templates, the dislocation scattering mechanism and the electron spillover into the bulk are reduced and the 2DEG confinement is enhanced. Such application has increased the demand for higher quality AlN templates on SiC in order to enhance the device performance of the new HEMTs. At Oxford Instruments - TDI, the group led by V. Ivantsov V. Soukhoveev, and A. Volkova, have recently optimized the growth procedure to enhance structural properties and surface morphology of thick AlN layers deposited through hydride vapor-phase epitaxy (HVPE) on off-axis 6H-SiC substrates.

Improved Quality of AlN Templates

By using optimal nucleation and growth conditions, the group can produce AlN layers with FWHM of approximately 40 arcsec of rocking curve for reflex measured by high resolution X-ray diffraction (HRXRD), which is a great improvement over the previously reported results of approximately 150 arcsec. The line width is very close to that of the SIC substrate, showing that the AlN epitaxial layer has a remarkably low screw dislocation density (£10⁶ cm^-2) and small tilting around the normal to the basal plane as shown in Figure. 1.

Figure 1. The XRD rocking curves taken from the SiC substrate and HVPE deposited AlN layers (symmetrical 00.6 and 00.2 reflexes, respectively). Note the remarkably low difference between the FWHMs of the substrate and the epitaxial layer that suggests high structural perfection of the AlN layer. The present method also showed a drastic improvement as compared to the previous reported data.

Surface Morphology of Optimised AlN Templates

The reciprocal space mapping of asymmetric reflexes and measured lattice parameters also suggest a fully relaxed state of the epitaxial layer. The surface morphology of the AlN layer is further characterized by atomic force microscopy (AFM). The mirror-like surface of the layer exhibits less than 2.5 nm root mean square (RMS) roughness over 10x10 um² area as shown in Figure 2.

Figure 2. Atomic force microscopy measurements over 10x10 um² scan area of AlN layer shows ~2 nm RMS in surface roughness.

Summary

Using the sophisticated technique, the group is able to produce high quality AlN templates with up to 20 μm in thickness with low bowing of 80 μm, making these templates perfect for high volume production of HEMTs.

Bernard Scanlan, General Manager of the Oxford Instruments - TDI division, stated that the Oxford Instruments - TDI team has continuously strived to improve its HVPE template products. The company is elated to see a considerable increase in demand of these AlN template products in the near future, he added.

About Oxford Instruments Plasma Technology

Oxford Instruments Plasma Technology provides a range of high performance, flexible tools to semiconductor processing customers involved in research and development, and production. They specialise in three main areas:

• Etch
  • RIE, ICP, DRIE, RIE/PE, Ion Beam
• Deposition
  • PECVD, ICP CVD, Nanofab, ALD, PVD, IBD
• Growth
  • HVPE, Nanofab

This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma technology.

For more information on this source, please visit Oxford Instruments Plasma technology.