Dilute Nitrides

Introduction

Dilute nitrides are semiconductor alloys, which contain a small amount of nitrogen. Typically the nitrogen content is less than 5 % due to several material quality and fabrication related issues.

Introducing nitrogen into (In)GaAs causes a strong reduction of the band gap energy. In FIG. 1. the photoluminescence spectra of several GaAsN samples with different nitrogen contents are shown and the red shift of the PL wavelength is observed demonstrating the band gap reduction. The band gap reduction makes dilute nitrides potential materials for several applications, especially for long wavelength applications (1300 nm and 1550 nm).

One serious drawback related to dilute nitrides is apparent from FIG. 1. The luminescence intensity is decreasing radically with increasing nitrogen content. This is due to the defects introduced by the nitrogen incorporation. The fact that the material quality is strongly decreasing with nitrogen is very undesirable and efforts have been made to improve the crystal quality by, for example, thermal annealing treatments.

FIG. 1. Low-temperature photoluminescence spectra measured from GaAsN samples containing different amounts of nitrogen. The PL wavelength increases with increasing nitrogen content.

In-situ monitoring of dilute nitride QW structures

The need to monitor growth of structures increase as the structure of applications gets more complicated. Nowadays the active region of several components are often one or more quantum wells (QWs). However, typically only in-situ monitoring of thick layers is considered to be possible. QW monitoring is enabled when the wavelength of the light used for in-situ monitoring is absorbed into the material under monitoring.

When dilute nitride QW growth is monitored also answers to some material research and growth related issues are revealed. In addition, the complex refractive indices of several materials with several compositions are obtained in high temperature (the growth temperature is typically between 500 and 600 degrees C) at wavelength 635 nm.

FIG. 2. shows in-situ reflectance curve measured during growth of InGaAsN/GaAs multi-QW structure and below that a theoretical curve is shown. The theoretical curve has been obtained using matrix method. The real and imaginary parts of the material complex refractive index are obtained by simulations. The real and imaginary parts of the complex refractive index of InGaAs are shown in FIG. 3. Both exhibit a clear trend with increasing indium content. Because of the clear trend observed, the real time analysis of the QWs is made possible.

Typically the composition analysis of quarternary alloys is fairly difficult (for example by XRD measurements). However, complex refractive index measured during growth provides both real and imaginary parts for the composition analysis. Luckily it turns out that the real parts of complex refractive indices of nitrogen containing GaAs based materials are independent of the nitrogen content. Furthermore, the imaginary parts are strongly nitrogen dependent as shown in FIG. 4.

The knowledge of the complex refractive indices of different materials at their growth temperature enables the analysis of the compositions of the materials already during growth. Additionally, the material quality can be roughly assessed during growth.

FIG. 2. In-situ reflectance curve measured during growth of InGaAsN/GaAs multi-QW structure. Below the measured curve a theoretical one is shown. Vertical offset is added for clarity.

FIG. 3. Real and imaginary part of InGaAs complex refractive index determined from in-situ reflectance data as a function of the indium content.

FIG. 4. Imaginary parts of InGaAsN complex refractive index containing different amounts of indium and nitrogen. Both indium and nitrogen contents affect the imaginary part.

SESAMs

Dilute nitride quantum wells have been utilized to realize a semiconductor saturable absorber mirror (SESAM) structures for long wavelength applications (1300 nm and 1550 nm). Also, the neutralizing effect of the lattice strain that nitrogen has in the InGaAs lattice dilute nitrides have been used to fabricate thick QW stacks in order to maximize the SESAM modulation depth.

SESAMs operating at wavelengths 914 nm, 1064 nm, 1300 nm and 1500 nm have been fabricated in Micronova and are currently under testing.