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

A mesa structure PV type MCT is arranged at upper part and irradiated from the CdTe substrate side. The electrode on an active plane is designed as a lattice or a ring pattern to transmit a longer wavelength light than cutoff. A middle band (- 16 (mu)m) or a wide band (-24 (mu)m) MCT whose cutoff wavelength is longer than that of a PV type MCT is arranged at the lower part of the element. The incident light into a PC type MCT is a signal light with quite small background noise filtered coldly through a PV type MCT and contribute to the enhancement of a S/N ratio. Wavelength coverage of relative response curves for a narrow band PV type MCT and a wide band PC type MCT is shown in Fig. 1b.

DEVICE FABRICATION

Preparation and Characterization of PV and PC-Type MCT

We used high quality liquid phase epitaxy (LPE) wafers from Fermionics Corporation as a PV type and a PC type MCT detectors. A p-on-n type double layer heterojunction (DLHJ) epitaxial layer grown on a CdTe substrate was used as a PV-type MCT. An undoped n-type epitaxial layer grown on a CdZnTe substrate was used as a PC type MCT. The specification of these materials are shown on Table I. At first, the fabrication method of a PV type MCT is described below.

A wafer was divided into a certain size and several elements were formed on the epitaxial layer using both a photolithography and a liftoff methods. A round mesa structure was formed by etching over a width of about 100 (mu)m and a depth of 5 (mu)m using a spray etching with a etchant like Br/ethyleneglycol. Next, a passivation process which is a critical process affecting the diode properties largely, was performed. It is necessary to deposit a film of ZnS/CdTe layers quickly before contamination of a p/n interface exposed by the mesa etching occurs. CdTe and ZnS films were deposited to the thickness of about 100 and 300 nm, respectively, using an electron beam evaporation. Au/ Cr electrode films were formed by an electron beam method for Cr of 30 nm and by a combination method of resistance heating and sputtering for Au of 300 nm. A ZnS antireflection film was finally deposited to a thickness of about 1.3 (mu)m to act as an antireflection film for wavelength longer than cutoff. After these procedures, a single element chip was obtained by dividing the wafer using a precision wire cutter. The fabricated PV element chip was bonded to a cold aperture with FOV of 60. A PV type MCT unit was completed after bonding Au leads on the n and p layers. A result of characterization for the element with active area of 7.9 x 10 cm^sup 2^ and FOV of 600 is shown next.

Typical value of RoA of 50 Omega cm^sup 2^ was obtained at 80K. Quantum effficiency was calculated from a short circuit current assuming 300K background radiation and estimated as over 40%. A single beam spectrum is shown in Fig. 2a. A D* of about 5 x 10^sup 10^ cmHz^sup 1/2^ W^sup -1^ was obtained at the peak wavelength.

The fabrication process of a PC type MCT is described next. A standard photolithography technique was also used. An anodic oxidation film for passivation was formed on the whole surface at the thickness of 80 nm after etching of an epitaxial layer to a certain shape. We processed an anodic oxidation using 0.1N-KOH/ethileneglycol as an electrolyte. Anodic oxidation films on the electrode were removed by HF solving and an In film was deposited at about 500 nm for ohmic contact. Finally, ZnS film was deposited at about 1.3 gm for antireflection. A single element chip was glued to a metal substrate (Cu or Al) and lead wires were bonded to obtain a PC type MCT unit. For the characterization of the performance, a cold shield with FOV of 60 deg was attached and the unit was mounted to the cryostat. The single beam spectrum of a typical PC type MCT with cutoff of 18 (mu)m and an active area of 1 x 10^sup -2^ cm^sup 2^ is shown in Fig. 2b. A D* of about 2 x 10^sup 10^ cmHz^sup 1/2^ W^sup -1^ was obtained at a peak wavelength.