Nobuyuki Kajihara

Development of LPE-grown HgCdTe 64 x 64 FPA with a cutoff wavelength of 10.6 μm

We have developed a hybrid HgCdTe focal plane array (FPA) for wavelengths from 8 to 11 micrometers . We describe how we fabricated our back illuminated 64 X 64-element photodiode array on a liquid phase epitaxial (LPE) HgCdTe wafer, and a Si CCD multiplexer with line address readout. We optimized carrier concentration in the p-type HgCdTe layer to maximize charge injection efficiency to the Si CCD readout circuit to more than 99.3%. We achieved excellent uniformity of characteristics of the photodiode array, which is very important for an IRFPA, by using LPE HgCdTe grown with a tipping method, and passivating the photodiode array with an anodic sulfide of HgCdTe. We obtained an average product of zero-bias resistance and area (RoA) of 9.1 (Omega) cm2 with a cutoff wavelength of 10.6 micrometers at 77 K. We used line address readout to give a large charge storage capacity of 4 X 107 electrons. We estimated a noise equivalent temperature difference (NETD) of 0.08 K with F/2.5 optics, including fixed pattern noise. We tried some preliminary experiments to reduce the crosstalk from photogenerated carriers which spread laterally into the epitaxial layer. We improved the modulation transfer function (MTF) at Nyquist spatial frequency from the conventional 35% to 60% by using a crosswise drain structure around each photosensitive n+ on p diode.

Reduction of Hg1−xCdxTe native oxide during the SiNx deposition process

Electron cyclotron resonance plasma chemical vapor deposition enables low‐temperature SiNx deposition on Hg1−xCdxTe (MCT). The SiNx film has an excellent interface on MCT with a low surface state density of 1.0×1011 cm−2 eV−1 and a low fixed charge of −1.4×1011 cm−2. A detailed analysis by x‐ray photoelectron spectroscopy and Auger electron spectroscopy spectra of Te, Cd, and Si at the SiNx/MCT interface indicated that the SiNx deposition reduces the naturally grown MCT native oxide. The oxygen taken from Te oxidizes SiH4 and produces silicon oxides which remain in the SiNx film. Since a chemical shift caused by oxidized Cd at the fresh surface of MCT is very slight, intentionally oxidized samples were used to confirm the above reaction. The analysis of the shape of the Si(2p) peak at the interface indicated that the silicon oxides are composed of SiO and SiO2. Thermodynamic considerations support such a mechanism.

Study on Deep Level Traps in p-HgCdTe With Dltfs

The authors studied the characteristics of deep-level traps in p-type HgCdTe diodes using the Deep Level Transient Fourier Spectroscopy (DLTFS) method. For both holes and electrons, two types of traps were observed. The DLTFS signal intensity of one type of trap increased with the carrier density in the HgCdTe, while the other did not exhibit a monotonic increase. While measuring the stability of these traps during cooling cycles, the DLTFS signal intensity of the first group was almost constant while that of the latter fluctuated with every cooling cycle. Stable traps originated from Hg vacancies, unstable traps are attributed to vacancy-impurity complex defects.

Evaluation of HgCdTe using laser beam induced current

We have developed a nondestructive evaluation method for HgCdTe. We focused on laser beam induced current (LBIC) which features a high specific resolution and nondestructive evaluation. The LBIC technique shows the electrically active regions in HgCdTe wafer as an image. We have considered the measurement temperature versus the LBIC signal. The LBIC technique at room temperature (300 K) can be used to evaluate non-uniformities in carrier concentrations in HgCdTe more sensitively. Using etch pit studies and secondary ion mass spectroscopy (SIMS), we have identified that non-uniformities of carrier concentration in the HgCdTe wafer arise from metal impurities around dislocation clusters. This nondestructive technique is useful for screening HgCdTe wafers before fabricating devices.

Development of mid-wavelength QWIP FPA

A novel quantum-well infrared photodetector (QWIP) with peak responsivity in the mid-wavelength (MW) range was characterized, and the performance of a focal-plane array (FPA) based on the MW-QWIP was investigated. InGaAs/AlGaAs quantum wells were used for the QWIP, resulting in a peak absorption wavelength in the range of 4.5~5.0 μm. The doping concentration and In composition of the well layers were varied to improve the photocurrent of the QWIP. The relationship between the noise of the QWIP and the number of multi quantum well (MQW) layers was also investigated, and the optical gain g was estimated. The noise-equivalent temperature differences (NETDs) of QWIPs with various numbers of MQW layers were calculated, and the optimum number of MQW layers was evaluated. It was found that controlling the In composition of the wells was very effective for improving the photocurrent. As a result, a MW-QWIP FPA with a NETD of 21 mK at an operation temperature of 80 K, an integration time of 16 ms, and F2.0 optics was fabricated.

CdTe films deposited by helicon sputtering for p-type Hg0.77Cd0.23Te surface passivation

We investigated the passivation properties of CdTe films that were deposited on p-type Hg0.77Cd0.23Te using the helicon sputtering method. Crystallinity and morphological microstructure of the CdTe films were determined by transmission electron microcopy and atomic force microscopy, respectively. We found that the CdTe films are polycrystalline and provide good morphological properties: they have no granular-type defects and no pin-holes. Electrical measurements of metal-insulator semiconductor devices showed that the properties of the CdTe/HgCdTe interface mainly depend on deposition pressure and the conditions of post-deposition annealings. It was observed that CdTe deposition at higher pressure reduces the surface deposition damage that is responsible for increasing the positive fixed charges, the slow traps and other degradations of the HgCdTe surface. Moreover, post-deposition annealing at a temperature above 100 degrees Celsius improves the thermal stability of the electrical properties of the interface. A gate-controlled diode that was produced with post-deposition annealing showed that CdTe films deposited by helicon sputtering were essentially suitable for passivation of HgCdTe photodiode arrays.

256 x 256 element HgCdTe hybrid IRFPA for 8- to 10-um band

We developed a 256 by 256 element HgCdTe hybrid infrared focal plane array (IRFPA) for the 8 to 10 micrometer band. We used three technologies to develop this high-performance, long-wavelength, large-scale IRFPA. The first innovation was to glue a sapphire substrate to a thinned Si readout circuit to reduce the thermal expansion mismatch with a HgCdTe diode array fabricated on a CdZnTe substrate. The second was to fabricate an interlaced switched- FET readout circuit using a 3 micrometer CMOS process. This readout circuit has a storage capacity of more than 107 electrons and two video outputs capable of a 3.5 MHz data rate. The third was a HgCdTe diode array with an anodic sulfide passivation film and an optimized cutoff wavelength to reduce dark current and achieve high sensitivity. The noise equivalent temperature difference (NETD) was 0.06 K using f/2.5 optics. After 1000 thermal cycles (300 K - 80 K), there were no significant indium bump failures nor notable degradation in detector performance.

Feedback Direct Injection Current Readout For Infrared Charge-Coupled Devices

We are proposing current readout for infrared charge coupled devices (IRCCDs) which can operate at higher temperatures. Feedback direct injection (FDI) consists of a simple amplifier of gain, A<1, connected to a photodiode and a conventional direct injection (DI) circuit. The amplifier feeds the source voltage of an input field-effect transistor back to the photodiode, making the diodes resistance effectively 1/(1-A) times larger. This improves conventional direct injection current readout efficiency. FDI was used in a medium-wavelength IRCCD operating at a high temperature. We made a 64-element HgCdTe linear IRCCD using FDI. The device operates at 195 K with an NETD of 0.5 K.