Next-generation fiber-optic communication networks are supposed to work with data rates of over 100 Gb/s, which requires the development of fast optical devices such as (de)multiplexers, modulators and wavelength converters. For realistic commercial use, these devices must be compatible with current CMOS technology for high integration and cheap to produce, which requires the use of Si-based devices. A promising approach is the use of Si on insulator based waveguide structures, which are combined with a nonlinear material. Using this idea, a record-breaking ultra-fast demultiplexer achieving very high conversion rates from 170 Gbit/s to 42.5 Gbit/s was implemented previously by the groups of J. Leuthold and W. Freude.[1] Similar approaches might be used for electro-optical modulators and switches, which are investigated in subproject A4.4. For these devices, a highly-nonlinear electro-optic active material is required. In very close collaboration with A4.4, we explore nitrides, in particular AlN and GaN/AlN quantum wells, as possible non-linear active media.

AlN in its stable wurtzite structure configuration has a bandgap of 6.2 eV and is therefore transparent at wavelength of interest for signal transmission through optical fibers. In addition, it’s second order optical properties with a nonlinear coefficient d₃₃ = 11 pm/V[2] make it interesting for use in electro-optical modulators based on slot waveguides filled with non-linear material. While nitrides can be easily grown on highly lattice-matched substrates, such as 6H-SiC or LiAlO₂,[3] the growth on Si substrates is difficult due to large differences in the crystal structure and the thermal constants. Several attempts have been presented for nitrides growth on Si(111).[4] Thereby AlN was used as wetting and buffer layer for the subsequent growth of GaN and InN layers. And Little attention was given to the optical properties of AlN itself. In this project, we first study growth of vetical AlN films. This is the base step for later to place AlN into a slot waveguide structure and growt GaN/AlN quantum well structures. We have therefore started to perform a careful analysis of the growth parameters and their influence on the properties of AlN deposited on Si(111) planar substrates.

Device Structure and Epitaxial Relationship

A schematic drawing of the intended device structure is shown in Fig. 1. AlN will be placed inside a slot-waveguide made of Si (110) on SiO₂ substrates. The AlN film will be deposited on the Si (111) side facet. For this purpose, growth of AlN on Si (111) substrates will have to be optimized. While both the Si (111) and the AlN (0001) surface have a hexagonal lattice, the lattice parameter difference is large, 0.3823 nm for Si compared to 3.112 nm for AlN, as shown in Fig. 2. This results in large strain and a large formation of defects at the interface.

Nitride Growth on Si(111)

We used commercial Si (111) substrates. Before loading into the MBE chamber, they were cleaned by aceton followed by isoproponol and methnol for 5 min in an ultrasonic bath, respectively. The native oxide layer was removed by dipping them into a solution of 5% HF. Immediately after etching, the Si (111) substrates were into the loadlock chamber, where they were degassed at 130 ºC for 1 hour.

Final oxide removal was performed in the growth chamber by heating the substrates up to 950 ºC for 30 min or longer till a clear RHEED 7×7 reconstruction could be observed clearly at the temperatures lower than 830 ºC.

Growth of AlN Thin Films

All samples were grown by plasma-assisted molecular beam epitaxy on Si(111) substrates. Activated nitrogen N₂^* was supplied by an Oxford plasma cell, operated at 400 W with a N₂ flow of 0.5 sccm. The samples were grown at temperatures ranging from 575 °C to 900 °C with the same metal to nitrogen ratio. Characterization of AlN films was performed by high resolution X-ray diffraction (HR-XRD), scanning electrical microscopy (SEM) and atomic force microscopy (AFM).

SEM images, as shown in Fig. 4., reveal that there are no metal droplets on top of the AlN films grown at 575 °C and 900 °C, respectively, while there are metal droplets present on the films grown at 675 °C and   795 °C, respectively.

AFM images (Fig. 5) show that at 575 °C, the AlN film is composited by small grains. With the increase of the growth temperature, the grain size increases. At 900 °C, grains are no longer distinguishable and small holes are present.

Figure 6 shows HR-XRD measurements. With the increase of the growth temperature, the full-width at half-maximum (FWHM) of the AlN (0002) reflection peak in the HR-XRD rocking curve decreases from  5898.5 arcsec for films grown at 575 °C to 1784.9 arcsec for films grown at 900 °C, respectively, indicating better film quality for higher growth temperatures.[7]

The additional Al peaks in the HR-XRD spectra for films grown at 675 °C and 795 °C confirm the SEM observations that Al metal droplets exist on these samples.

References

List of Publications 2006-2011 as PDF

Subproject Report 2006-2010 as PDF