Akira Ajisawa

Improvement in HgCdTe diode characteristics by low temperature post-implantation annealing

Hg1−xCdxTe diodes (x∼0.22) with different carrier concentrations in p type materials have been fabricated by employing an ion-implantation technique. The performances of the diodes, prior to and after low temperature postimplantation annealing, have been investigated in detail by model fitting, taking into account dark current mechanisms. Prior to the annealing process, dark currents for diodes with relatively low carrier concentrations are found to be limited by generation-recombination current and trap-assisted tunneling current, while dark currents for diodes with higher carrier concentrations are limited by band-to-band tunneling current. These dark currents in both diodes have been dramatically decreased by the low temperature annealing at 120∼150°C. From the model fitting analyses, it turned out that trap density and the density of the surface recombination center in the vicinity of the pn junction were reduced by one order of magnitude for a diode with lower carrier concentration and that the carrier concentration profile in a pn junction changed for a diode with higher carrier concentration. The improvements are explained by changes in both carrier concentration profile and pn junction position determined by interaction of interstitial Hg with Hg vacancy in the vicinity of the junction during the annealing process.

4*4 GaAs/AlGaAs optical matrix switches with uniform device characteristics using alternating Delta beta electrooptic guided-wave directional couplers

Integrated 4*4 GaAs/AlGaAs optical matrix switches constructed from 12 electrooptic directional couplers have been realized. In order to achieve uniform device characteristics, molecular beam epitaxy and reactive ion beam etching were chosen as the crystal growth technique and waveguide fabrication technique, respectively, in addition to the simplified tree structure as a matrix switch architecture. As a result, matrix switches with quite uniform device characteristics, such as small switching voltage deviation and little path dependence in +or-0.5-dB propagation loss, have been realized. >

5:1 on‐off contrast InGaAs/InP multiple quantum well Fabry–Perot étalon modulator

We report the first demonstration of an InGaAs/InP multiple quantum well Fabry–Perot etalon modulator. The obtained on‐off contrast is 5:1 at −16 V applied voltage for 1540 nm wavelength light. The absorption coefficient of the multiple quantum well around 1540 nm increases from 1000 to 6300 cm−1 as the applied voltage increases from 0 to −16 V, and the relative refractive index change is up to −0.9%.

HgCdTe and CdTe(1̄ 1̄ 3̄)B growth on Si(112)5° off by molecular beam epitaxy

CdTe(1 1 3)B epilayers were grown on Si substrates oriented (112)5° off toward the [1 1 1] direction by molecular beam epitaxy (MBE). A Zn irradiation process was developed in order to obtain a (1 1 3)B face. HgCdTe(1 1 3)B epilayers were grown on 20 μm‐thick CdTe/Si(112) 5° off, and characterized. These layers have double‐crystal x‐ray rocking curves with full width at half‐maximum as low as 64 arc s, and etch pit densities of 4.4×106 cm−2 and 2.6×105 cm−2 for as‐grown and thermal‐cycle annealed films, respectively. Photodiodes were also fabricated to demonstrate the capability of large‐area MBE‐HgCdTe/Si focal‐plane arrays.CdTe(1 1 3)B epilayers were grown on Si substrates oriented (112)5° off toward the [1 1 1] direction by molecular beam epitaxy (MBE). A Zn irradiation process was developed in order to obtain a (1 1 3)B face. HgCdTe(1 1 3)B epilayers were grown on 20 μm‐thick CdTe/Si(112) 5° off, and characterized. These layers have double‐crystal x‐ray rocking curves with full width at half‐maximum as low as 64 arc s, and etch pit densities of 4.4×106 cm−2 and 2.6×105 cm−2 for as‐grown and thermal‐cycle annealed films, respectively. Photodiodes were also fabricated to demonstrate the capability of large‐area MBE‐HgCdTe/Si focal‐plane arrays.

New thermally isolated pixel structure for high-resolution (640×480) uncooled infrared focal plane arrays

A new pixel structure with twice-bent beams and eaves structure, suitable for high-resolution uncooled infrared (IR) focal plane arrays (FPAs), is proposed. In comparison with previous results (FPA of 37-µm pixel pitch), the thermal conductance of the test device with the proposed pixel structure of 23.5-µm pitch is reduced about 2.5 times. The eaves structure, which is adopted to increase the fill factor of pixels, improves the responsivity by a factor of 1.3. A 640×480 bolometer-type uncooled IRFPA is demonstrated by utilizing the new pixel structure, with supplementary modification to improve thermal conductance and thermal time constant. It shows a noise equivalent temperature difference (NETD) of 50 mK for F/1.0 optics at 30 frames/sec, a thermal conductance of 0.03 µW/K, and a thermal time constant of 16 msec.

Performance of 320x240 uncooled bolometer-type infrared focal plane arrays

The performance of a 320 x 240 bolometer-type uncooled infrared (IR) Focal Plane Array (FPA) is described. Vanadium oxide thin film is adopted for the bolometer material, having a sheet resistance of approximately 10 kohms/square. It is patterned such that the bolometer resistance is by a factor of 10 larger than the sheet resistance. On-chip readout integrated circuit (ROIC) is designed to reduce signal drift, extend dynamic range for object temperature and extend ambient temperature range in which operates non-uniformity correction is carried out with about 1/10 fewer frequency than the former ROIC.The 320 x 240 FPA consists of pixels sensitive to IR radiation and optical black (OB) pixels covered with plate which shuts out IR radiation. Drift is reduced by current mirror circuit, using the OB pixels and digital compensation circuit based on voltage change of OB pixels resulting from change in operation temperature. Both the dynamic range and the ambient temperature range are extended by decreasing integration gain and developing low-noise, low-power and large swing operational amplifier(OP-AMP). Since decrease in integration gain degrades noise equivalent temperature difference (NETD), bias voltage for bolometer is increased by factor of 2 and bandwidth is reduced by route half. Finally, IR image was obtained with prototype camera and NETD value was found to be smaller than 0.1K for F/1 optics at 60Hz frame rate and thermal time constant was measured to be 12 msec.

Measurements of the polarization dependence of field‐induced refractive index changes in GaAs/AlAs multiple quantum well structures

The measurements of the polarization dependence of field‐induced refractive index changes in GaAs/AlAs multiple quantum wells, which are directly measured by using the modulation spectroscopy method for the TE and TM modes, are described. Clear polarization dependences, such as differences in the spectra shape and the peak wavelength, were observed. The polarization dependences originate from the different excitonic transitions: the n=1 heavy‐hole and light‐hole exciton for the TE mode and the n=1 light‐hole exciton for the TM mode. The experimental results show qualitatively good agreement with the theoretical calculation.

New thermally isolated pixel structure for high-resolution uncooled infrared FPAs

This paper proposes a new thermally isolated pixel structure, having a twice-bent beam structure and eaves structure, suitable for high-resolution uncooled infrared (IR) focal-plane arrays (FPAs). It also describes the properties of test devices, fabricated to verify the effect of the new pixel structure. Although the pixel size of the test devices is 23.5 μm × 23.5 μm, which represents a smaller area by a factor of about 2.5 than the 37 μm × 37 μm pixel size for the 320 × 240 bolometer-type uncooled IRFPA, previously developed by the authors, the test devices have beams with almost the same length as in the previous IRFPA by utilizing the new beam structure. In addition, the cross-sectional area of the beam is reduced. Accordingly, the thermal conductance of the test devices can be reduced by a factor of about 2.5. The eaves structure, which is adopted to increase the fill factor of pixels, improves the responsivity by a factor of 1.3, which is consistent with our calculations. By utilizing the new thermally isolated pixel structure, the test devices with 23.5 μm pixels enable us to achieve thermal sensitivity equivalent to the previous 37 μm pixels.

Development of MBE-grown HgCdTe 64 x 64 FPA for long-wavelength IR detection

The HgCdTe (MCT) 64 X 64 focal plane array (FPA) for long wavelength infrared (LWIR) detection was developed, using MCT epilayers grown by molecular beam epitaxy (MBE). The n-on-p photodiode array has a cutoff wavelength of 10.7 micrometers . The readout circuit, with off focal plane integration capacitance, was designed for 77 K operation. These components were fabricated independently and were hybridized. The 97.8% operability was obtained. Photodiode characteristics for each pixel were measured directly. Mean R

Hybrid 256 x 256 LWIR FPA using MBE-grown HgCdTe on GaAs

o)A value of 1.9 (Omega) (DOT) cm2 and quantum efficiency of 0.3 were obtained. Using an infrared camera system with nonuniformity correction function, the infrared image was successfully demonstrated. An NETD (noise equivalent temperature difference) value of 0.117 K was attained with an F/2.5 optical lens under the 300 K background condition.