Yasuhiro Iguchi

1.3 µm Traveling-Wave GaInNAs Semiconductor Optical Amplifier

We fabricated a GaInNAs semiconductor optical amplifier (SOA) by applying a facet coating to a buried-ridge-stripe GaInNAs laser. Due to a low reflectivity (<0.1%) and a wide bandwidth (70 nm) coating, Fabry–Perot (FP) modes of the GaInNAs laser were suppressed sufficiently, and thus a 1.3 µm traveling-wave GaInNAs SOA was realized for the first time. Peak chip gains of more than 9.6 dB and a 3-dB-gain bandwidth above 49 nm (9 THz) were obtained simultaneously with a cavity length between 600 µm and 900 µm. In addition, on/off ratios between 20 and 30 dB were obtained by switching the current on and off, which seems sufficient for the SOA to work as a switching device. With the temperature characteristics, we found that the ASE intensity and the gain coefficient of the GaInNAs SOA were much less dependent on temperature than those of conventional InP-based SOAs. These results demonstrate the superior temperature characteristics of the GaInNAs SOA compared with conventional InP-based SOAs.

The growth of high quality GaAsSb and type-II InGaAs/GaAsSb superlattice structure

The effects of growth temperature and V/III flux ratio on the GaAsSb crystalline quality were studied. Phase separation was suppressed within a narrow window of growth temperature. 1-μm-thick GaAsSb layers which exhibited single peaks in X-ray diffraction (XRD) measurement were successfully obtained grown at the optimum temperature. XRD and photoluminescence (PL) measurements revealed that V/III flux ratio also has an optimum value. It was found that the type-II InGaAs/GaAsSb superlattice with GaAsSb grown at the optimized growth condition seems to have an excellent optical property, because the PL peak energy exhibited peculiar dependence on temperature suggesting transition between the conduction band of InGaAs and the valance band of GaAsSb.

MOVPE grown InGaAs/GaAsSb type II quantum well photodiode for SWIR focal plane array

Infrared sensors with type II quantum well structure have gained great attention and have shown advanced progress. InGaAs/GaAsSb type II quantum well structures are considered as an attractive material system for realizing low dark current PDs owing to lattice-matching to InP substrate. In this report, we describe successful operation of PIN-PDs with InGaAs/GaAsSb quantum wells grown by metal-organic vapor phase epitaxy (MOVPE). MOVPE method is well-known to have good uniformity which leads to mass-production of focal plane array. Planer type pin-PDs were adopted. The p-n junction was formed in the absorption layer by the selective diffusion of zinc. Electrical and optical characteristics of pin-PDs such as well number dependence of responsivity, were investigated. Dark current was 9.0 μA/cm2 at 233 K, which has better uniformity compared to those of MBE sample, and responsivity of 0.8 A/W in SWIR region were obtained. This result indicates that planer photodiode using MOVPE grown InGaAs/GaAsSb type II quantum wells is a promising candidate for consumer applications.

A coaxial-type 1.3/1.55 /spl mu/m WDM-PD module for optical access networks

A new 1.3/1.55 /spl mu/m WDM/photodiode(PD) module has been designed and fabricated for use in optical access networks. A 1.3 /spl mu/m-PD and a 1.55 /spl mu/m-PD were arranged in series along the optical axis in a coaxial package. The 1.3 /spl mu/m-PD with an In/sub 0.66/Ga/sub 0.34/As/sub 0.76/P/sub 0.24/ photosensitive layer (/spl lambda//sub g/=1.42 /spl mu/m) absorbs 1.3 /spl mu/m digital optical signals and it is transparent to 1.55 /spl mu/m wavelength. The 1.55 /spl mu/m-PD with an In/sub 0.53/Ga/sub 0.47/As photosensitive layer absorbs 1.55 /spl mu/m analog optical signals. Responsivity of the module is 0.82 A/W at 1.31 /spl mu/m wavelength and 0.75 A/W at 1.55 /spl mu/m wavelength, respectively. IMD2 of the 1.55 /spl mu/m-PD measured by two-tone method was -76.1 dBc at Pf(1.55 /spl mu/m) of -6 dBm. Bit error rate (BER) of the 1.3 /spl mu/m-PD was measured at 28.8 Mb/s by using an amplifier with bandwidth of 200 MHz and was found to be less than 10/sup -8/ at Pf(1.3 /spl mu/m) of -30 dBm.

Low dark current SWIR photodiode with InGaAs/GaAsSb Type II quantum wells grown on InP substrate

For sensing short wavelength infrared (SWIR) region (1.0–2.5 µm), photodiodes with In<inf>0.53</inf>Ga<inf>0.47</inf>As/GaAs<inf>0.5</inf>Sb<inf>0.5</inf> type II quantum well structure grown on InP substrate by solid source molecular beam epitaxy (MBE), were successfully fabricated. Low dark current was obtained by improving GaAsSb crystalline quality.

Raman Scattering Study of InGaAsP Quaternary Alloys Grown on InP in the Immiscible Region

We have studied the asymmetric broadening of the Raman spectra of InxGa1-xAsyP1-y grown on InP in the immiscible region using the spatial correlation model. The compositional dependence of Raman peaks are well explained by a simple model which does not account for the immiscibility. However, the broadening of the Raman line shape is greatly enhanced in the samples in the region of the immiscibility. In particular, the asymmetry of the spectral peak of the InAs-like longitudinal optical phonon is found to be enhanced in the immiscible region. This shows that the samples which include the immiscibility have been structurally changed to suppress the uniform distribution of the long wavelength phonons.

5×5 cm2 GaAs and GaInAs Solar Cells with High Conversion Efficiency

We fabricated GaAs and GaInAs solar cells with a large cell area of 5×5 cm2 as the first step toward their practical use. The 5×5 cm2 cells showed high conversion efficiencies equivalent to those of 1×1 cm2 cells owing to both a good uniformity of epitaxial film characteristics in a 3-inch wafer and the reduction of the series resistance of grid electrodes. Moreover, a higher conversion efficiency of 26.0% under 1-sun air-mass 1.5 global conditions was achieved for the 5×5 cm2 GaAs cell by optimizing the antireflection coating. This is the highest efficiency among GaAs cells ever reported. The 5×5 cm2 GaInAs cell showed an efficiency of 4.2% as the bottom cell under a GaAs top cell. Therefore, a conversion efficiency of more than 30% can be expected for the 5×5 cm2 GaAs/GaInAs mechanically stacked tandem cell.

The effect of a thin antimony layer addition on PdZn ohmic contacts for p-type InP

Abstract The effects of addition of thin Sb layers to PdZn ohmic contacts for p-type InP were investigated. The Sb layers reduced significantly the contact resistivity of the PdZn contacts, and provided excellent reproducibility and a wide annealing temperature range to produce low resistances. The minimum contact resistivity of 7×10−5 Ω cm2 was obtained for the Sb (3 nm)/Zn (20 nm)/Pd (20 nm) contacts, annealed at temperature ranging from 375°C to 400°C for 2 min, where a slash (/) sign indicates the deposition sequence. Since this annealing temperature is close to that used for preparation of typical AuGeNi ohmic contacts to n-InP, simultaneous annealing for p- and n-InP ohmic contacts can be achieved by using the Sb/Zn/Pd and AuGeNi ohmic contacts for p- and n-InP, respectively.

Raman Intensity of Phonon Modes in InGaAsP Quaternary Alloys Grown on (100) InP in the Region of Immiscibility

The Raman intensities of optical phonon modes in InxGa1-xAsyP1-y quaternary alloys lattice matched to (100) InP are studied in the compositional region of immiscibility. The peak intensity as well as the integral intensity of each phonon modes are investigated in terms of the relative density of corresponding bonds. It was found that the integral intensity is well interpreted in terms of the bond density. The replacement of a gallium atom with an indium atom produces a remarkable variation in the peak intensity.

Uncooled SWIR InGaAs/GaAsSb type-II quantum well focal plane array

Low dark current photodiodes (PDs) in the short wavelength infrared (SWIR) upto 2.5μm region, are expected for many applications. HgCdTe (MCT) is predominantly used for infrared imaging applications. However, because of high dark current, MCT device requires a refrigerator such as stirling cooler, which increases power consumption, size and cost of the sensing system. Recently, InGaAs/GaAsSb type II quantum well structures were considered as attractive material system for realizing low dark current PDs owing to lattice-matching to InP substrate. Planar type PIN-PDs were successfully fabricated. The absorption layer with 250 pair-InGaAs(5nm)/GaAsSb(5nm) quantum well structures was grown on S-doped (100) InP substrates by solid source molecular beam epitaxy method. InP and InGaAs were used for cap layer and buffer layer, respectively. The p-n junctions were formed in the absorption layer by the selective diffusion of zinc. Diameter of light-receiving region was 140μm. Low dark current was obtained by improving GaAsSb crystalline quality. Dark current density was 0.92mA/cm2 which was smaller than that of a conventional MCT. Based on the same process as the discrete device, a 320x256 planar type focal plane array was also fabricated. Each PD has 15μm diameter and 30μm pitch and it was bonded to read-out IC by using indium bump flip chip process. Finally, we have successfully demonstrated the 320 x256 SWIR image at room temperature. This result means that planer type PD array with the type II InGaAs/GaAsSb quantum well structure is a promising candidate for uncooled applications.