The Technion-Israel Institute of Technology announced the first completion of an aluminum gallium nitride (AlGaN) quantum cascade detector (QCD) grown on a 4 inch diameter silicon substrate. The paper was published on December 7, 2018. In the IEEE Electron Device Letters journal. The detection wavelength of the QCD is in the mid-infrared range (3~8 μm). In quantum cascade structures, long wavelengths are excited by intersubband transitions (ISBT).
The research team said: "Successfully preparing gallium nitride (GaN) ISBT optoelectronic devices on silicon substrates has a positive effect on integrated optoelectronic technology, demonstrating the enormous potential for ultra-high-speed operation in the ultra-wide spectral range."
Figure 1 (a) Schematic diagram of the QCD structure. (b) Calculate the conduction band distribution of the 1.5-cycle QCD active region and the shifted squared envelope function of the electron-bound state; the red vertical arrow indicates the optical transition, and the gray arrow indicates the direction of electron transport in the extractor region.
The QCD in Fig. 1 is a 30-cycle cascade structure based on the inter-subband transition energy of 267 meV, and the corresponding wavelength is 4.49 μm. Longitudinal optical (LO) phonon-assisted tunneling promotes electron flow between energy levels. The infrared excited structure produces a photovoltage.
An AlN/Si (111) 4 inch template for the QCD structure was prepared using a metal organic chemical vapor deposition (MOCVD) process. A device layer was grown using a plasma assisted molecular beam epitaxy (PAMBE) method at 720 °C.
The device was fabricated on a 7 mm x 7 mm chip diced on the epitaxial wafer. A 700 μm x 700 μm mesa was realized by inductively coupled plasma (ICP) etching. The metal contacts consist of titanium/aluminum (Ti/Al). The center of the top of the countertop is kept away from the contact metal and serves as a window for the front illumination into the absorbing layer.
The determination of the peak detection wavelength is hindered by the absorption of carbon dioxide at a nearby 4.3 μm (Fig. 2). The researchers extracted a 4.14 μm peak at 18 K and a red shift to 4.5 μm at 150 K. (Redshift refers to the phenomenon that the electromagnetic radiation of an object increases in wavelength for some reason. In the visible range, the spectrum of the spectrum shifts toward the red end. A distance, that is, the wavelength becomes longer and the frequency decreases.) The 18K detection line width is a full width at half maximum (FWHM) of 1.26 μm. The peak response also decreases with increasing temperature, and disappears significantly into the noise above 150K.
Figure 2. Spectral response of the device. The decline is due to the absorption of carbon dioxide. Illustration: Temperature dependence of the signal, peak signal at 18K.
Absolute responsivity is measured using a 1000K SiC spherical black source. At a temperature of 18K, a 162 pA photocurrent was measured corresponding to a response of 44 μA/W. The detection rate in combination with responsiveness, blackbody aperture area, and current noise spectral density (15.4fA/√Hz) is 2x108 Jones at 19K.
In the paper, the researchers proposed some suggestions for improvement: optimizing the absorption, extraction efficiency and resistance trade-off; reducing the redundant diagonal transition from the ground state to the extractor quantum well, increasing the resistance and reducing the dark current; The energy level of the stage is adjusted to the LO phonon step to increase the electron transfer rate; the electron density in the quantum well is increased by increasing the doping to obtain a larger absorption.
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