Application Notes:

Spectroscopic Ellipsometry
for Telecom Applications:
Semiconductor Devices

The telecommunications industry is growing larger than ever mainly due to the demand for faster information transfer. Fiber optic communication is rapidly becoming the backbone for voice, video, and internet data transfer. As this industry matures, the components for broadband fiber networking undergo continuous research and development. These components include VCSEL and edge-emitting lasers, thin-film DWDM filters and waveguides for multiplexing/demultiplexing, EDFA and Raman amplifiers, photodiode detectors, and more. Thin films are critical to the performance of these devices. Spectroscopic ellipsometry (SE) is uniquely suited to measure both film thickness and refractive index at telecom wavelengths in the infrared. This article surveys both ex situ and in situ SE applied to the single and multilayer film structures of telecom devices.

Transmitters and Receivers
Optical communication literally begins and ends with a transmitter and receiver. In the
transmitter, lasers produce light to travel down the fiber encoded with information. In the end, the receiver converts this light to an electronic signal via a photodiode detector. Thin alloy semiconductor films play an important role in both components.

A. Laser Sources
Semiconductor lasers are designed to operate in the near infrared at 1310 nm or 1550 nm for fiber-optic communication. Another important wavelength is 980nm used for pump lasers.

Semiconductor lasers create light via optical transitions between energy levels in a direct-bandgap semiconducting film. Ternary and quaternary alloy semiconductors offer the
flexibility to tune the desired emission wavelength by changing the composition of the materials, as the bandgap is dependent on the ratio of materials in the alloy. For instance, the quaternary material In_xGa_1-xAs_yP_1-y can provide laser emission in the 1.1µm < λ < 1.6µm range depending on alloy ratio (x and y). Similarly, the ternary material Al_xGa_1-xAs can be varied for use in pump lasers.

B. Photodiode Detectors
The same energy levels that produce laser light will absorb light to excite electrons to an
elevated state. Absorption frees electrons from their atoms to create a measurable electric current which is directly related to the intensity of light shining on the detector. The energy levels adjust with alloy concentration, resulting in varying amount of absorption at
different wavelengths. This variation in optical properties is described by the material optical constants, commonly known as n and k or ε₁ and ε₂. The optical constant shape corresponds to the material’s electronic transitions. Thus, the optical constants become a “fingerprint” for the semiconductor.

For example, in Al_xGa_1-xAs, the direct bandgap shifts toward shorter wavelengths with
increasing aluminum concentration, x. This is seen in Figure 1 as an absorption edge shift toward shorter wavelengths, along with similar shifts in each high-energy transition.

Spectroscopic Ellipsometry
Spectroscopic ellipsometry takes advantage of the changing optical constants in a semiconductor to determine alloy ratio. Accurate alloy ratio measurements require a database of the optical constants for different compositions. Fortunately, the energy bands shift systematically with changing composition. Thus, a series of five to ten samples that cover the full range of compositions will provide enough information to setup an alloy model. The alloy model creates the correct optical constants for any specified alloy ratio, x. If an alloy ratio is specified between two of the original database values, an appropriate interpolation is performed. This is enhanced by using the alloy-shifting model developed by Snyder. In practice, the optical constants can be properly described by simply selecting the correct alloy ratio. In Figure 2, we show the ellipsometric measurement from a bi-layer
semiconductor stack. Five fit parameters were used to match this data: three layer thicknesses (including the surface oxide) and two alloy compositions.