Characterization of HgCdTe MWIR Back-Illuminated II-VI-07 ... - Voxtel

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µm at 77 K, Elliott et al. 17 deduced that the reverse bias I(V) curves (for reverse bias beyond -1.4 V) are limited by avalanche (impact ionization) mainly due to one carrier (electron); the largest gain reported is 5.9 at -1.4 V. Reisinger et al. 18 measured a gain of 50 at -5 V in their front-illuminated Hg-diffused n-on-p photodiodes with a cutoff of 11.95 µm at 77 K, and concluded that α e >100×α h from analysis of the shape of spectral response curve as function of reverse bias voltage. It was, however, the paper by Beck et al. 1 in 2001 that first reported the clear and compelling advantages of the electron-initiated avalanche process, based on data on MWIR HgCdTe lateral-collection “paround-n” photodiodes with p-type absorber regions: avalanche gain increases exponentially with reverse bias voltage, is quite uniform from element to element, and is essentially noiseless (i.e., F(M)≈1.0). Soon thereafter, a theory by Kinch et al. 2,19,20 of the physics of electron and hole avalanche multiplication in HgCdTe related the large inequity between α e and α h to three key features of the HgCdTe energy band structure: the electron effective mass is much smaller than the heavy hole effective mass, which means that electrons have a much higher mobility, a much lower scattering rate by optical phonons, and a factor-of-two lower ionization threshold energy; there are no subsidiary minima in the conduction band to which energetic electrons can scatter; and the light holes are not important. The key features of this theory were substantiated by Monte Carlo simulations by the group at the University of Texas at Austin. 21 Since the seminal papers of Beck et al. 1 and Kinch et al., 2,19,20 there have been reports of HgCdTe e- APDs in other configurations and architectures, all of which corroborate the essential features of the electroninitiated avalanche process initially reported by Beck et al. 1 Baker et al. 22,23 report a 1.55 µm range-gated 320×256 FPA for 3D imaging with a lateral-collection “loophole” HgCdTe e-APD array having 24×24 µm² unit cells, cutoff wavelengths of 4.2-4.6 µm, and gains over 100 at -7 V, operating at 90 K. Back-illuminated implanted planar n + -n - -p e-APD 1×64 arrays grown by MBE on CdZnTe are reported by Vaidyanathan et al. 24 with a gain of 1000 at -10.5 V for a 4.2 µm cutoff at 78 K, and a gain >100 at -3.5 V for a 10.3 µm cutoff at 78 K. Hall et al. 25 report gain at 78 K of 100 at -12 V for a back-illuminated MWIR n-p-P structure grown in situ by MOVPE on GaAs with an x=0.36 gain layer. -6-
More recent results on HgCdTe e-APDs in the front-illuminated lateral-collection “p-around-n” architecture from the DRS group include characterization 26-28 of both SWIR (2.2 µm) and LWIR (9.7 and 10.8 µm), and a 128×128 gated IR imaging FPA 29,30 with 40×40 µm² unit cells, cutoff wavelengths of 4.2 to 5 µm, median gains of 946 at -11 V, and a sensitivity of less than one photon for a 1 µs gate width. A gain of 5300 at -12.5 V is reported in the paper by Perrais et al. 31,32 for a back-illuminated planar p- i-n 30×30 µm² photodiodes with cutoff wavelength of 5.0 µm at 78 K, formed by boron implantation in a HgCdTe film grown by MBE. This is the highest reported gain for a HgCdTe e-APD, and is consistent with the gains reported for all other e-APDs with shorter cutoff wavelengths, given that the e-APD gain increases exponentially with both cutoff wavelength and reverse bias voltage. Recently, this group reported 32,33 the first measurements of pulse response and bandwidth in HgCdTe e-APDs, with a bandwidth value of ~400 MHz for 1.55 µm radiation, in a device with a 5.3 µm cutoff wavelength at 80 K, and with a gain of ~2800 at -11 V, corresponding to a gain-bandwidth product of ~ 1.1 THz. The bandwidth exhibits only a weak decrease with increasing gain, dropping from 500 MHz at unity gain to 400 MHz at 2800, in agreement with the theoretical predictions for a k=0 APD. 26,27 This group also reported 34 gain of 4 at -1.3 V for a cutoff wavelength of 9.2 µm at 77 K, and a gain of 20 at -22.5 V for a cutoff wavelength of 2.4 µm at 150 K. HgCdTe e-APD DESIGN The device cross section for our back-illuminated planar n-on-p HgCdTe e-APD is shown in Figure 1. It is fabricated in a single-layer p-type HgCdTe film grown on an IR-transparent CdZnTe substrate. Radiation is incident on the CdZnTe surface, which may have a thin-film antireflection coating or interference filter. Radiation is absorbed within the p-type region, creating electron-hole pairs that diffuse and drift toward the depletion region. The p-side is much more heavily doped than the n-side, so nearly the entire width W of the depletion region lies within the n-region. When the photogenerated minority carrier (electron) reaches the edge of the depletion region, it is accelerated by the strong electric field. It gains energy rapidly and begins the avalanche multiplication process. As reverse bias voltage is applied, the depletion region expands further into the n-region, toward the surface, and the field increases, resulting in higher avalanche gain. -7-
Page 1 and 2: Characterization of HgCdTe MWIR Bac
Page 3 and 4: INTRODUCTION The Hg 1-x Cd x Te ele
Page 5: advantages of the HgCdTe energy ban
Page 9 and 10: current and dark current plus dc ph
Page 11 and 12: temperature than the dashed curves,
Page 13 and 14: Noise measurements were done at Vox
Page 15 and 16: The breakdown voltage increases wit
Page 17 and 18: NASA Contract CA28C from the Jet Pr
Page 19 and 20: 15. T.J. de Lyon, B. Baumgratz, G.
Page 21 and 22: 29. J. Beck, M. Woodall, R. Scritch
Page 23 and 24: Unit Cell CdTe Passivation Indium B
Page 25 and 26: 1000 100 Gain at -9.0 V 10 235-G, E
Page 27 and 28: 1E-7 10000 Relative Response 1E-8 1

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