Improvement of nonlinearity and extension of wavelength region using tandem (PV+PC) type HgCdTe detector (dual-MCT) in FTIR spectrometer

Journal of Electronic Materials, Jun 1999 by Abe, O, Kawasaki, K, Wakaki, M

RESULTS AND DISCUSSIONS

Improvement of Nonlinearity

The calibration curve measured for a versatile MCT is shown in Fig. 5. A pyroelectric detector TGS, which does not show nonlinearity was used for comparison. A Dual-MCT shows highly improved linearity comparable to TGS compared with a conventional PC type MCT at any wavelength.

It is required to reduce the incident light intensity smaller by 1/30 to the present value to operate conventional type MCT in a linear region, which imply the superiority ofthe Dual-MCT. But the improvement is available to a certain amount of light intensity. In the Dual-MCT, the linearity of PC type is corrected completely. On the contrary, the performance of the PV type gives the upper limit. In our experiments, nonlinearity of PV type becomes eminent over the limit and the increase of the absorbance gets slower. The improvement of nonlinearity by applying a further reverse bias is not preferable. Because the upper limit of the reverse bias is restricted to only about -0.5 mV for the safety of the device leading to little improvement, which cause a drawback to give clear increase of 1/f noise due to the soft breakdown of the PV type MCT and remarkable ingredient of SIN. The breakthrough may be come from the reduction of an active area to decrease the junction capacitance, which leads to the speed-up of a response time. The result to support the idea was obtained in the microscopic measurements mentioned below.

Single beam spectra of the Dual-MCT with an active area of 3.1 x 10^sup -4^ cm^sup 2^, cutoff of 16 ^(mu)m and FOV of 45 deg are shown in Fig. 6 for various incident intensities which are normalized to 1 at the maximum intensities. For microscopic measurements, the active area is small, but the density of light is high, then nonlinearity comparable to versatile MCT appears for a conventional microscopic MCT. However, a Dual-- MCT dose not show nonphysical energy at all and linearity is kept for the whole light intensity.

Rapid carrier relaxation is required for the improvement of linearity. In other word, a high speed response is required. The speed of a PV type MCT is proportional to the junction capacitance which also depends on the active area. The reduction of an active area leads to a speed-up of response time and as a result, improvement of linearity. In general, the response time of a PC type is determined only by a life time of a carrier and do not depend on an active area.

As a result, it is suggested to develop 2 D arrays of a PV type with a smaller active area to improve linearity of a versatile type further.

Extension of Wavelength Region

The second main feature of the Dual-MCT is to have flexibility of extension of a sensitive wavelength region. In a point of view of linearity, a wide band type MCT has better linearity even in a single PC type MCT compared with a middle band type MCT owing to the shortening of carrier lifetime leading to the speed up of response. But the response time is considerably longer compared with that of a PV type. On the other hand, it is very difficult to make a wide band PV type MCT. The problem is how to prevent the increase of a tunneling current. Pultz et al. have reported a 19 (mu)m cutoff PV detector,s but it is not available to commercial base. On the contrary, a Dual-MCT with high linearity is obtained easily by changing a PC type MCT to a wide band type. One more merit of the Dual-MCT is to give a large D* which cannot realize in a single PC type wide band MCT, because a PV type covers a 10 (mu)m region.