Yukihiro Tsuji

Application of Nanoimprint Lithography to Fabrication of Distributed Feedback Laser Diodes

We have succeeded in employing nanoimprint lithography (NIL) to form the diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. Uniform gratings and phase-shifted gratings with periods of 232 nm have been formed by using a reversal-tone NIL with a step-and-repeat imprint tool. Line edge roughness has been sufficiently low with the fabricated gratings. DFB LDs fabricated by NIL have indicated comparable characteristics with LDs fabricated by electron beam lithography. We have also demonstrated that phase-shifted DFB LDs show better uniformity in characteristics than uniform-grating DFB LDs. The results of this study indicate that NIL has high potential for the fabrication of DFB LDs.

Study of reactive ion etching for reverse tone nanoimprint process

We have used reverse nanoimprint for fabricating diffraction gratings of distributed feedback laser diodes. Generation of residues in the etching process of resin is a serious issue leading to poor line edge roughness of the grating patterns. We have found that the residues are composed of oxide products from Si-containing resin. We have successfully suppressed the generation of the residues by optimizing oxygen partial pressure of reactive ion etching (RIE). We have also succeeded in effectively removing the residues by utilizing sputtering effect of RIE.

1.3 µm GaInNAs Bandgap Difference Confinement Semiconductor Optical Amplifiers

A GaInNAs bandgap difference confinement (BDC) semiconductor optical amplifier (SOA) utilizing a bandgap difference between an active region and a cladding region for current confinement was developed for the first time. Due to strong current and optical confinements in the lateral direction, this SOA exhibited a 4.3 dB larger chip gain and 2.1 dB smaller fiber coupling loss than the conventional GaInNAs-buried-ridge-stripe (BRS) SOA. In addition, the gain dependence of the GaInNAs-BDC-SOA on temperature was found to be much smaller than that of the conventional InP-based-SOA, and comparable to that of the GaInNAs-BRS-SOA.

Evaluation of nanoimprint lithography as a fabrication process of phase-shifted diffraction gratings of distributed feedback laser diodes

The authors have succeeded in employing nanoimprint lithography (NIL) to form diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. Uniform gratings and phase-shifted gratings with periods of 232 nm have been formed by using a reversal NIL with a step-and-repeat imprint tool. Line edge roughness has been sufficiently low with the fabricated gratings. DFB LDs fabricated by NIL have indicated comparable characteristics with LDs fabricated by electron beam lithography. LDs show high long-term stability in threshold current. The authors have also demonstrated that phase-shifted DFB LDs show better uniformity in characteristics than uniform-grating DFB LDs. The results of this study indicate that NIL has high potential for fabricating DFB LDs.

Fabrication of mesa-type InGaAs pin PDs with InP passivation structure on 4-inch diameter InP substrate

We have successfully fabricated mesa-type InGaAs pin PDs with an InP passivation structure on a 4-inch diameter InP substrate. The dark current and capacitance of PDs with 80-/spl mu/m mesa diameter are 0.278 nA and 335 fF at a reverse bias voltage of 5 V, respectively. A responsivity is 0.91 A/W at an incident light wavelength of 1310 nm. A 3-dB bandwidth for PDs with 80-/spl mu/m mesa diameter is 7.98 GHz at a reverse bias voltage of 5 V and an incident light wavelength of 1550 nm. The results show that the pin PDs on a 4-inch diameter InP substrate have sufficient characteristics for practical applications.

Evaluation of nanoimprint lithography as a fabrication method of distributed feedback laser diodes

We have succeeded in employing nanoimprint lithography (NIL) to form the diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. Uniform gratings and phase-shifted gratings with periods of 232 nm have been formed by using a reverse NIL with a step-and-repeat imprint tool. Line edge roughness has been sufficiently low with the fabricated gratings. DFB LDs fabricated by NIL have indicated comparable characteristics with LDs fabricated by electron beam lithography. We have also demonstrated that phase-shifted DFB LDs show better uniformity in characteristics than uniform-grating DFB LDs. The results of this study indicate that NIL has high potential for the fabrication of DFB LDs.

Highly Uniform Fabrication of Diffraction Gratings for Distributed Feedback Laser Diodes by Nanoimprint Lithography

We have used a nanoimprint technique to fabricate diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. We have aimed to establish the fabrication process featuring the high reproducibility of the period and linewidth of grating corrugations, which leads to an increase in the production yield of DFB LDs. The combination of the reverse tone nanoimprint and optimized etching techniques has contributed to the improvement of the reproducibility. The variation in grating period has been less than 0.2 nm and the variation in linewidth has been less than 10 nm over the six wafers. The results of this study indicate that our fabrication process for the diffraction gratings utilizing the nanoimprint technique has a high potential for the fabrication of DFB LDs.

GaInNAs Distributed Feedback (DFB) Laser Diode

First successful operation of a buried-ridge-type GaInNAs-DFB laser was realized. Under CW condition, it oscillated with a threshold current of 18 mA at 25degC, and it could oscillate up to 110degC with good I-L linearity and with SMSR > 40 dB.

Pretreatment for surface leakage current reduction in type-II superlattice MWIR photodetectors

Focal plane array based on InAs/GaSb type-II superlattice (T2SL) is expected as an alternative to HgCdTe. To get more competitive performance of T2SL detector, we need building up more reliable fabrication process. Especially, mesa formation and passivation with understanding of surface leakage mechanism is critical issue. Generally, the existence of dangling bonds at crystal surface or damaged layer and native oxides on etched mesa sidewall leads to surface leakage currents, which mostly degrade the detector performance. Many researchers adopted SiO2 film as an effective passivation layer, which was deposited by plasma enhanced chemical vapor deposition at low temperature. However, good passivation requires not only stable film, but also an effective surface treatment before passivation. There are few reports, which discuss the relation between treatment before passivation and device characteristics in T2SL photodetectors. In this work, we present dry etching mesa formation and the effect of pretreatment of passivation on T2SL p-i-n photodetector fabrication. We investigate R0A-Perimeter/Area relation and R0A temperature dependence with in-situ plasma treatment prior to the passivation. From results of electrical characterization and interface analysis using STEM, it is recognized that in-situ N2 plasma treatment was effective to surface leakage reduction.

GaInNAs DFB Laser with Buried GaAs Grating

We for the first time fabricated a GaInNAs DFB laser having a buried GaAs grating in the center current injection region. A successful DFB laser oscillation with the threshold current of 34 mA was obtained