Takashi Nakabayashi

Epitaxial graphene formation on 3C-SiC/Si thin films

By forming a thin 3C-SiC film on Si substrates and by annealing it at ~1500 K in vacuo, few-layer graphene is formed epitaxially on Si substrates. In this graphene-on-silicon (GOS) technology, graphene grows at least on three major low-index Si surfaces: (1 1 1), (1 0 0) and (1 1 0), which allows tuning of structural and electronic properties of epitaxial graphene by simply controlling the crystallographic orientation of the surface. A typical example can be found in the two types of graphene formed on 3C-SiC(1 1 1) surfaces; the one on 3C-SiC(1 1 1)/Si(1 1 1) shows a Bernal stacking with an interfacial buffer layer, while the one on 3C-SiC(1 1 1)/Si(1 1 0) shows a non-Bernal stacking without an interfacial buffer layer. Inserting an AlN interlayer between Si and 3C-SiC significantly contributes to improvement of the GOS quality. Moreover, thanks to the sealing effect of the AlN layer against Si out-diffusion, we can apply chemomechanical polishing of SiC surface to reduce the surface roughness, which can further accentuate the effect of H2 annealing of the surface. As a result, a D to G band intensity ratio as low as 0.4 is obtained.

Highly reliable AlGaInAs buried heterostructure lasers for uncooled 10 Gb/s direct modulation

High reliability (estimated median lifetime of 240,000 hours) of 1.3 /spl mu/m AlGaInAs buried heterostructure lasers has been demonstrated by more than 10,000 hours accelerated aging tests. Distributed-feedback lasers have successfully operated at 10 Gb/s at 95/spl deg/C.

Facet Passivation of GaInAsP/InP Edge-Emitting Laser Diode by Aluminum Ultrathin Layer Insertion

We investigated the suppression of facet degradation in GaInAsP/InP edge-emitting laser diodes (LDs). And we demonstrated that facet degradation is successfully suppressed using facet passivation by inserting an aluminum thin layer to the interface between the semiconductor facet and the facet coating film. The ultrathin aluminum layer was approximately 20 A thick. From photoelectron spectroscopy analysis of the passivated surface, it was found that this passivation is effective in reducing surface recombination. This reduction effect is caused by the suppression of both phosphorus shortage and oxidation. To investigate the passivation effect, we evaluated the tolerance to facet degradation by high-current injection test up to 600 mA and electrostatic discharge test up to 1.2 kV. Although about half of the tested LDs without passivation were degraded, all the LDs with passivation survived. This facet passivation method can suppress facet degradation markedly and it can be applied to not only GaInAsP/InP LD but also other InP-based LDs.

High Quality Graphene Formation on 3C-SiC/4H-AlN/Si Heterostructure

The growth of graphene on 3C-SiC/Si heterostructure is a promising approach, which provides low production cost, high scalability and easiness of nanoelectromechanical system fabrication. However, the quality of graphene is still insufficient for device applications due to mediocre morphological and structural quality of the 3C-SiC epilayers compared to bulk SiC crystals and to excessive Si out-diffusion from the Si substrate. Here, we propose a solution of inserting a 4H-AlN layer between 3C-SiC and Si, which allows us to polish the 3C-SiC film without worrying about enhancement of the Si out-diffusion despite the thinning after the polishing. With this insertion, a considerable quality improvement is achieved in our graphene on silicon.

Analysis of Reverse-Biased Electrostatic-Discharge-Induced Degradation of GaInAsP/InP Buried Heterostructure Laser Diode

We clarified the mechanism of the degradation of reverse-biased electrostatic discharge (ESD). Although the degradation mechanism of forward-biased ESD has been clarified, that of reverse-biased ESD remains unclear. We prepared a 1.31 µm GaInAsP/InP distributed feedback laser diode with a conventional pn-InP buried heterostructure for the analysis of the degradation mechanism of reverse-biased ESD. We used two approaches, namely, the failure analysis and numerical analysis of electric field. We found a defect inside an active layer in the early stage of degradation. Interestingly, such a defect was generated at the periphery of an active layer, but not at the center. On the other hand, we found that a high electric field occurs at an active layer under a reverse-biased condition, particularly at the periphery of an active layer. We also demonstrated the improvement in the tolerance to ESD with a decrease in electric field. From these results, we successfully confirmed that reverse-biased-ESD-induced degradation is caused by the concentration of an electric field.

Electrostatic-discharge-induced degradation of 1.3μm AlGaInAs∕InP buried heterostructure laser diodes

Degradation of 1.3μm AlGaInAs buried heterostructure laser diodes due to electrostatic discharge (ESD) is studied. The degradation mechanism of this material has not previously been clear and so the ESD tolerance was evaluated. Degradation occurred at 0.5 and 2.5kV for forward and reverse polarities, respectively. Because that ESD tolerance for forward polarity is insufficient for practical applications, we focused on it in analyzing the degradation mechanism. Elliptically shaped melted regions are observed in the active layer of the facet. Such regions developed inside a cavity under the application of ESD pulses. These results indicate that degradation is caused by melting due to optical absorption.

Electrostatic-Discharge-Induced Degradation Caused by Argon Ion Bombardment in Facet-Coating Process of GaInAsP/InP Laser Diode

Electrostatic discharge (ESD)-induced degradation is one of the most important reliability issues of GaInAsP/InP laser diodes. We investigated the relation between ion irradiation in the facet-coating process and ESD-induced degradation. We used electron-beam evaporation with two types of argon ion irradiation for facet-coating. One type of irradiation is used for facet cleaning, in which argon ions bombard the facet directly. Although the ion energy was as low as 40 eV, a lack of phosphorus and enhanced oxidation were found on the ion-irradiated surface. Furthermore, an increase in the surface recombination current and the enhancement of ESD-induced degradation were observed by extending the ion irradiation time. The other type of irradiation is used to promote evaporation. Here, argon ions do not bombard the facet directly. Thus, it had little effect on ESD-induced degradation. From these results, we successfully confirmed that direct ion irradiation increases surface recombination and accelerates ESD-induced degradation even if the ion energy is as low as 40 eV.

High reliability 1.3-μm buried heterostructure AlGaInAs-MQW DFB laser operated at 28-Gbit/s direct modulation

Clear eye opening at 28-Gbit/s direct modulation up to 75°C is achieved with 1.3-μm buried heterostructure AlGaInAs-MQW DFB laser. No failure had occurred at high optical output power aging test over 7000 hours.

Failure Analysis of InP-Based Edge-Emitting Buried Heterostructure Laser Diodes Degraded by Forward-Biased Electrostatic Discharge Tests

We investigated the forward-biased electrostatic discharge (ESD)-induced degradation that is one of the important reliability issues for InP-based edge-emitting buried heterostructure laser diodes. Although it has been suggested that the degradation mechanism is related to optical damage, the detailed mechanism has not been established. Thus, we carried out failure analysis. Two elliptically shaped degraded regions, which were introduced by the first and second ESD pulses, were observed in an active layer. A peculiar chain-like degradation was created along the longitudinal axis. This degradation was caused by light absorption at an active layer. We also investigated the relationship between forward-biased ESD tolerance and an aging test. We observed a decrease in tolerance as a result of the aging test for laser diodes with cleaved facets. This decrease was sufficiently suppressed by facet coating. We identified two reasons why facet coating is important. One is to obtain sufficient ESD tolerance under initial conditions, and the other is to suppress the decrease in ESD tolerance during aging.

Electrostatic discharge induced degradation of GaInAsP/InP laser diodes caused by argon ion irradiation in facet coating

The ESD induced degradation is one of the most important reliability characteristics of GaInAsP/InP LDs. We clarified the ESD induced degradation mechanism caused by the argon ion irradiation in the facet coating. By the angle resolved XPS analysis of ion-irradiated InP (100) surface, the lack of phosphors and oxidation were found. It means the generation of the dangling bonds and increase of the surface recombination. Then we investigated the ion irradiation effects before and during the facet coating on 1.31 /spl mu/m GaInAsP/InP DFB-LD. The increase of the surface recombination current and enhancement of the ESD induced degradation was observed by extending the ion irradiation time before the coating. And the ion irradiation during the coating did not affect the ESD degradation. These results indicate the facet condition before the coating is most important and the ion irradiation increases the surface recombination and accelerates the ESD induced degradation.