The cubic zincblende polymorph of InGaN is a very promising candidate for optical applications. Its absence of spontaneous polarization fields in the (001) orientation and lower fundamental band gap compared to its wurtzite counterpart makes it especially interesting for efficient light-emitting diodes (LEDs) and micro-LEDs. Therefore, understanding the optical properties and band structure of this material plays a key role in the development of possible future applications. For this, the current study presents the optical properties in the form of the dielectric function of thin film ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ for entire composition range $(0\le x\le 1)$. The investigated spectral regions range from the near-infrared (0.5 eV) to the vacuum ultraviolet (8.8 eV). We focus on the spectral ranges above 4 eV to study the redshift of the higher energy transition (${\mathrm{E}}_1$ at the $L$-point of the band structure and ${\mathrm{E}}_2$ at the $X$-point) as the In composition changes from GaN to InN. The characteristic energy of these transitions at critical points in the band structure are determined by a critical point model based on the third derivative of the dielectric function. Bowing fits of the ${\mathrm{E}}_1$ and ${\mathrm{E}}_2$ transitions yield bowing parameters of $b_1=1.01\mathrm{eV}$ and $b_2=0.50\mathrm{eV}$. Furthermore, we determined the dielectric limit as ${\varepsilon }_{\infty }={\varepsilon }_1(E\to 0)$, yielding a bowing parameter of $b_{\infty }=1.58$.

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• Electronic structure
• III-V semiconductors
• Narrow band gap systems
• Ellipsometry