High-sensitivity, wide-dynamic-range avalanche photodiode pixel design for large-scale imaging arrays

Abstract

Abstract. We present a detailed design study for a novel solidstate focal plane array of silicon avalanche photodiodes (APDs), with each detector in the array IntroductionThere is a growing need in science and industry for largescale detector arrays capable of imaging with high sensitivity and high speed near room temperature, over a wide range of natural illumination conditions from direct sunlight to a cloudy moonless night. Existing solid-state camera technology does not enable imaging in the visible spectrum over such a wide dynamic range near room temperature. In this design study, we demonstrate through detailed calculation means that a novel back-illuminated, wide dynamic range avalanche photodiode ͑APD͒ device design will enable imaging with high sensitivity near room temperature at −30°C, over the full range of natural illumination conditions from the AM 0 in space orbit around Earth, to ultralow light levels when the illuminance at the camera lens E l = 0.0001 lux ͓lumens/ m 2 ͔, as on a cloudy moonless night when far from city lights. The high pixel sensitivity is achieved using a compact readout circuit, ca-

Design of a high sensitivity emitter-detector avalanche photodiode imager using very high transmittance, back-illuminated, silicon-on-sapphire

“…The quantum efficiency of the 27 μm emitter-detector pixel with etched laser diode cavity has a reduced quantum efficiency for the visible and near infrared wavelengths compared to a silicon mesa detector pixel without an etched cavity for the laser diode. 5,8 At the GaN-VCSEL diode wavelength of emission however, the sapphire microlens helps to focus light into the APD silicon and away from the crystallographically etched laser diode cavity, thereby preserving high detector quantum efficiency at the laser wavelength. The Si-ðAlN∕a-SiN 0.62 Þ-sapphire-(MgF 2 ) substrate enables nearly two times as much light to be transmitted into the APD detector at the laser wavelength λ 0 ¼ 370 nm compared to Si-(AlN)-sapphire-(MgF 2 ), thereby improving the unity SNR threshold of the 27 μm emitter-detector pixel to a maximum possible range of 27,500 m. The improvement in the detection range of the optical radar pixel photonic device can be regarded as significant.…”

“…5,8 It has been shown that the silicon mesa APD will operate with high quantum efficiency and the array will achieve 100% sensitive-area-fill-factor due to the antireflective layer between the sapphire and silicon and due to the spherical sapphire microlens. In the conventional silicon mesa APD, the microlens focuses optical rays under full height of the silicon mesa and away from the mesa sidewalls that have a reduced height as well as away from the optical dead space between pixels where the silicon has been etched, to enable low resistance anode contacts and direct pixel-topixel optical crosstalk isolation.…”

“…The APD is fabricated in the remaining silicon mesa using conventional silicon processing technology and is specially designed to operate over a wide dynamic range in a dual-mode, operating in a linear mode with or without internal gain for passive imaging in daylight conditions and in single-photon sensitive Geiger-mode for active or passive imaging under low light level conditions. 5,8 A monolithic sapphire microlens beneath each pixel is optimized to effectively shape and relay the laser beam to an object of interest while its secondary function is to concentrate reflected radiation from an object into the detector active area and away from the silicon mesa sidewalls, thereby compensating the optical dead space between pixels resulting from the mesa isolation etch. A low resistance aluminum (Al) or copper (Cu) metal anode grid fills the space between pixels and also inhibits direct optical crosstalk between adjacent pixels.…”

“…The primary purpose of having a short pulse laser light source in each pixel however, is not for brightening a scene under low light conditions for collecting 2D intensity images, since the APD detectors are already sufficiently sensitive for passive imaging at the lowest natural illumination conditions. 8 The purpose rather of having a short pulse laser light source in each pixel, is for providing an optical pulse to measure the return time-offlight of the photons reflected from an object in the scene conjugated to the pixel containing the light source, using a range gating technique. The range from the focal plane array to objects in a scene can be measured by delaying arming of the APD in Geiger-mode following an emitted optical pulse from the pixel, by a time interval corresponding to when the optical returns from a specific object distance are expected to arrive, and disarming the APD after a fixed gate-on time interval to set the range gate.…”

“…(7) and P ce is the hydrostatic energy shift of the conduction band defined in Eq. (8). The effective mass, block-diagonalized Hamiltonian matrix derived using the k · p method, for the valence band of a bulk, strained-layer, wurtzite semiconductor is given in Eq.…”