High Performance Optoelectronic Integrated Devices by Micromachining Technique

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Optoelectronic integrated devices in which light-receiving and light-emitting devices are vertically and directly integrated, produce various new optical functions important for optical signal processing and optical interconnections. By integrating vertically and directly conventional optoelectronic deices, not only the combined characteristics of the constituent devices, but also new functions due to the mutual interaction between them can be easily obtained. This leads to more general engineering concept that the higher the degree of integration, the higher the functionality of the device. Based on this concept, we have at first developed advanced optical logic devices with various flip-flop functions such as bistable and tristable flip-flops and an astable multivibrator by integrating multiple laser diodes and heterojunction phototransistors and utilizing internal optical and electrical couplings.In the application of the integrated devices to parallel signal processings, it is very … More important to possess a surface-emitting function. Recently, a vertical cavity surface-emitting laser (VCSEL) has been drawing much attention. When we use the VCSEL as the laser part of the integrated devices, however, the optical feedback from the VCSEL to the heterojunction phototransistors becomes too strong since the lasing output power directly enters the phototransistors. The integrated devices utilizes the spontaneous emission instead of the lasing emission as the optical feedback to obtain various functions. The strong optical feedback leads to the limited numbers of functions such as an optical awitching. To avoid this limitation, we have considered using a circular grating coupled surface-emitting device. The lasing oscillation occurs in the direction parallel to the substrate surface, and a part of the light is coupled out to the direction normal to the substrate surgace. Therefore, when we integrate the phototransistors on the position where there is no grating coupler, the feedback light becomes spontaneous light and various functions can be expected. In this work, we have for the first time achieved the lasing oscillation of the circular grating coupled surface-emitting laser with the embedded grating couplers which is suitable for integration. From the near-field pattern observation of the device, however, it was found that the lasing was observed mainly along the [110] direction and the pattern tended to spread into a wider area with increasing injection current. This behavior is due to a slight nonuniformity of the grating shapes in the azimuthal direction.To overcome the above problem, we have employed a new method to embed the gratings, that is, wafer-fusion technique. The basic idea for the process is as follows : two epitaxial wafers A and B were prepared (the wafer A has a separate confinement multiple quantum well structure, and the wafer B has second-order diffraction gratings), the surface of each wafer was pretreated by buffered HF solution, and the wafers were stacked under the pressure higher than 1.7N/cm^2 and heated in H_2 atmosphere for 30min at 620ﾟC.Then the wafers A and B were bonded as strong as to withstand various processes, and it was found from the SEM and TEM observation that the uniform air/semiconductor gratings were formed in the device having the depth and period about 0.2mum and 0.4mum, respectively. The rectangular-like grating shape obtained by the reactive ion etching was changed to somewhat circular shape due toe the mass-transport phenomenon. The fused interface bacame firmer and electronically more reliable with the aid of the mass-transport. It has been also found that the shape of the grating after wafer-fusion has almost no dependence on the crystalline orientation. Then the fused wafers were processed to a ridge waveguide structure, and the CW lasing oscillation has been successfully achieved at room temperature. These results are very encouraging to develop integrated devices with uniformly embedded circular grating coupled surface-emitting elements. Less