AlGaN based III-nitride tunnel barrier hyperspectral detector for infrared detection

ABSTRACT A III-nitride based tunable hyperspectral detector pixel is desc ribed with the potential for real time detection in a wide range of wavelengths including near infrared (NIR). This detector has intrinsic hyperspectral pixels, each pixel being tunable in real time through a range of wavelengths determined by pixel design. This will eliminate the need for external gratings and filters, substantially decreasing weight, size, and complexity and increasing robustness. The single pixel detector discussed here offers the potential of wavelength spectroscopy from UV to NIR. To allow for dynamic tunability a multilayered Al x Ga 1-x N (0<x<1) forms the triangular potential barrier. Inclusion of In x Ga 1-x N as the absorbing layer offers detection in the IR region. The results of our experiments are complimented and explained using extensive device simulation. Keywords: AlGaN, detector, hyperspectral 1. INTRODUCTION A hyperspectral image is a map of the spectral information of the emitted/reflected radiation in terms of the spatial coordinates of the surface of the subject. Such information is crucial to a range of applications including military, health, resource management and astronomy. Two materials can be differentiated on the basis of the emission profile over a wide range of wavelengths, instead of absolute intensity at any given wavelength, and hence hyperspectral imager needs to store intensity of incoming radiation as a function of wavelength for each pixel. Conventional hyperspectral imaging instruments [1, 2] use broadband detectors in conjunction with external gratings and filters to split the light into its constituent wavelengths. A line of pixels in the detector arra y collects the spectrally dispersed light from a spatial pixel. For the case of a 2D imaging array, the other dimension of the array is devoted to one of the dimensions of the spatial image. Vehicle motion or mechanical scan ning is used to build up the complete spatial image by sequentially recording these spectrally resolved image lines, to form a data cube of (x,y, ). (see Fig. 1 for one example). For the case of a 1D imaging array, mechanical scanning is also used for the other spatial dimension. One challenge in fabrication of conventional hyperspectral detectors is array uniformity. Fo r pushbroom instruments using a 2D array, each array row (in the spectral dimension) should have equivalent spectral response. Whiskbroom instruments reduce this requirement by using the same row for each spatial image pixel, but at the cost of an additional mechanical scan. In a pushbroom sensor, a line of sensors arranged cross-track is used. Different areas of the surface are imaged as the spacecraft flies forward. In a whisk broom sensor, a mirror scans across the sa tellites path (ground track), reflecting light into a single detector which collects data one pixel at a time. Thus, for particular applications, it may be desirable to use the tunability of the imager pixels to decrease the dimension of the array by one. This is possible because it would no longer be necessary to devote one dimension of the array to spectral information. The uniformity of the imager based on the proposed technology is expected to be very high (over 80%). This is because the array uniformity will be dependant in large part on the uniformity of the growth stoichiometry of the epitaxially grown absorbing layer and the barrier layer thicknesses, both of which are controlled by growth conditions. Given the current state of III-Nitride electronic and optoelectronic device technology high degree of alloy composition and thickness uniformity across a large area of the wafer (~2) is expected. [3] A further advantage of this technology is the flexibility to adapt the spectral resolution to suit the situation and measurement. Since wavelength resolution is no longer fixed by the grating and detector array geometry, real-time modification can be accomplished to accommodate low-resolution or high-resolution applications. A large body of