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

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M E . The edge gain would be higher than the interior gain due to the higher electric field at the edge arising, for example, from junction curvature or from a narrower n-region at the surface. Minority photocarriers collected laterally over a length L OPT by diffusion from beyond the edge of the junction would experience the edge gain, whereas those collected within the interior of the junction area would experience the interior gain. This leads to a simple expression for the measured gain as a function of diode radius: M 2 2 2 M1DR J + M E[(R J + LOPT ) − R J ] = (2) 2 (R J + LOPT ) The solid curve in Fig. 7 is the best fit of this equation to the gain data. With L OPT set equal to 12 µm, the value determined from the analysis in Fig. 8, the best fit of this equation to the data of Fig. 7 yields values for the edge gain of 45 and the interior gain of 85. Spatial Dependence of Avalanche Gain Our previous papers 3,4 reported spot scan data for avalanche gain at V=-5 V in these large-area diodes at 80 K. Those data (Fig. 9 in Ref. 34) showed that the gain is uniform within the interior of the junction area, but that there is a hint of higher gain at the edges. This higher gain at the edges is seen more clearly in the spot scan data in Fig. 9 of another element in that same 4×4 array. The spatially-dependent avalanche gain, defined as the ratio of the profile at -5.0 V to the profile for V=0, is plotted in Fig. 9 on the right-hand axis. The avalanche gain is spatially uniform over the interior of the photodiode, with an average value of 10.8, which is in close agreement with the gain data at 80 K for this film taken in the full-illumination configuration (see Fig. 3). There is a small but consistently observed increase in the FWHM at -5.0 V compared to the V=0 case, with the value of 3.8 µm for the data in Fig. 9, which is responsible for the small increases in the gain profile at the edges of the junction seen in Fig. 9. This increase is most probably not due to the voltageinduced increase in the depletion width, because the higher doping of the p-region causes the depletion region width to increase inward, toward the n-contact. We interpret this increase as due to higher gain at the edges of the diode, which is consistent with the dependence of measured gain on junction area in Fig. 7. Excess Noise Factor F(M) -12-
Noise measurements were done at Voxtel, Inc. A Test Chip from film 163, packaged in a flatpack, was mounted inside a windowed vacuum cryochamber. Stable, switchable flood illumination is projected through the window with an OZ Optics OZ-2000 1550 nm stabilized fiber-coupled diode laser. Bias voltage is applied to the diode by an HP4145A semiconductor parameter analyzer (SPA). A bias-T couples the DC component of the diode current to the SPA, and sends the AC component to an HP8447D high-speed preamp. The preamp can feed either a spectrum analyzer or a noise figure meter. In this way, both the average gain (calculated in the usual way from the DC component of the photocurrent) and the noise power spectral intensity at a given frequency and bias voltage can be read simultaneously. An HP8566B spectrum analyzer is used to choose frequency bands with low environmental noise, and an HP8970B noise figure meter makes calibrated noise power measurements. Noise power measurements are taken both in the dark and under illumination. By subtracting the noise power of the dark current, the noise power of the photocurrent can be isolated. This allows one to measure the multiplication noise and reject environmental noise. The HP8970B noise figure meter outputs noise power spectral intensity (S P in W/Hz), and the impedance of the test circuit must be found to convert these measurements to noise current spectral intensity (S I in A²/Hz). We use the noise measurement at unity gain (low bias voltage) for this normalization, rather than a measurement of the diode impedance in isolation, because this gives the most direct measurement of the relevant impedance – including effects from the mounting of the detector, its interaction with the preamplifier and noise figure meter, and anything specific to the frequency band used for the noise power measurement. The normalization is accomplished using Schottky’s theorem, S I = 2 q I (A²/Hz), along with a direct measurement of the DC photocurrent I PH : R = S P / S I = S P / (2 q I PH ) (3) Between 20 and 30 independent measurements of the light and dark noise power at unity gain are used to find R. Subsequently, noise current spectral intensity S I is measured at a variety of different values of M, taking 10 independent measurements of light and dark noise power for each gain point. Finally, excess noise factor F(M) is calculated using the noise current spectral intensity theorem for avalanche multiplication: -13-
Page 1 and 2: Characterization of HgCdTe MWIR Bac
Page 3 and 4: INTRODUCTION The Hg 1-x Cd x Te ele
Page 5 and 6: advantages of the HgCdTe energy ban
Page 7 and 8: More recent results on HgCdTe e-APD
Page 9 and 10: current and dark current plus dc ph
Page 11: temperature than the dashed curves,
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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