Mid-wavelength infrared 1024x1024 pixel QWIP focal plane array

Abstract

A mid-wavelength 1024x1024 pixel quantum well infrared photodetector (QWIP) focal plane array has been demonstrated with excellent imagery. Noise equivalent differential temperature (NETD) of 19 mK was achieved at 95K operating temperature with f/2.5 optics at 300K background. This focal plane array has shown background limited performance (BLIP) at 90K operating temperature with the same optics and background conditions. In this paper, we will discuss its performance in quantum efficiency, NETD, uniformity, and operability.A quantum well designed to detect infrared (IR) light is called a quantum well infrared photodetector (QWIP). An elegant candidate for QWIP is the square quantum well of basic quantum mechanics [1]. Each period of the multi-quantum well (MQW) structure consists of coupled quantum wells of 40 Å containing 10 Å GaAs, 20 Å In 0.3 Ga 0.7 As, and 10 Å GaAs layers (doped n = 5x10 17 cm -3 ) and a 40 Å barrier of Al 0.3 Ga 0.7 As between coupled quantum wells, and a 400 Å barrier of Al 0.3 Ga 0.7 As. Stacking many identical periods (typically 50) together increases photon absorption. Ground state electrons are provided in the detector by doping the GaAs well layers with Si (see Fig. 1). This photosensitive MQW structure is sandwiched between 0.5 µm GaAs top and bottom contact layers doped n = 5x10 17 cm -3 , grown on a semiinsulating GaAs substrate by molecular beam epitaxy (MBE). Then a 0.7 µm thick GaAs cap layer on top of a 300 Å Al 0.3 Ga 0.7 As stop-etch layer was grown in situ on top of the device structure to fabricate the light coupling optical cavity [2][3][4][5].The MBE grown material was tested for absorption efficiency using a Fourier Transform Infrared (FTIR) spectrometer. The experimentally measured absorption quantum efficiency of this material at room temperature was 19%. The epitaxially grown material was processed into 200 µm diameter mesa test structures (area = 3.14x10 -4 cm 2 ) using wet chemical etching, and Au/Ge ohmic contacts were evaporated onto the top and bottom contact layers.The detectors were back illuminated through a 45° polished facet [5-7] and a responsivity spectrum is shown in Fig. 2. The responsivity of the detector peaks at 4.6 µm and the peak responsivity (Rp) of the detector is 170 mA/W at bias V B = -2 V. The spectral width and the cutoff wavelength are ∆λ/λ = 15% and λ c = 5.1 µm respectively. The peak quantum efficiency was 19% at bias V B = -2 V for a 45° double pass. The lower quantum efficiency is due to the lower Fig 1. Schematic diagram of the conduction band in a bound-toquasibound QWIP. A couple quantum well structure has been used to broaden the responsivity spectrum.