High Performance Optoelectronic Integrated Devices by Micromachining Technique

光电集成器件将光接收和光发射器件垂直直接集成在一起，产生了各种新的光学功能，对光信号处理和光互连具有重要意义。通过垂直和直接集成传统光电器件，不仅可以获得组成器件的组合特性，而且可以容易地获得它们之间相互作用的新功能。这导致了更普遍的工程概念，即集成程度越高，设备的功能越高。基于这一概念，我们首先通过集成多个激光二极管和异质结光电晶体管并利用内部光和电耦合，开发了具有各种触发器功能的先进光学逻辑器件，如双稳态和三稳态触发器以及不稳定多振子。在将集成器件应用于并行信号处理中，具有表面发射功能是非常重要的。近年来，垂直腔面发射激光器（VCSEL）备受关注。然而，当我们使用VCSEL作为集成器件的激光部分时，由于激光输出功率直接进入光电晶体管，VCSEL对异质结光电晶体管的光反馈变得太强。该集成器件利用自发发射代替激光发射作为光反馈来获得各种功能。强的光反馈导致了有限的功能，如光开关。为了避免这种限制，我们考虑使用圆光栅耦合表面发射器件。激光振荡发生在平行于衬底表面的方向上，并且一部分光耦合到衬底表面法线方向上。因此，当我们将光电晶体管集成在没有光栅耦合器的位置时，反馈光变成自发光，可以实现各种功能。本文首次实现了适合集成的圆光栅耦合面发射激光器与嵌入式光栅耦合器的激光振荡。然而，从器件的近场模式观察中发现，激光主要沿着[110]方向被观察到，并且随着注入电流的增加，模式有向更宽的区域扩散的趋势。这种行为是由于光栅形状在方位角方向上有轻微的不均匀性。为了克服上述问题，我们采用了一种新的方法来嵌入光栅，即晶圆融合技术。该工艺的基本思路是：制备两个外延片A和B （A片具有单独的约束多量子阱结构，B片具有二阶衍射光栅），用缓冲HF溶液对每个外延片表面进行预处理，在高于1.7N/cm^2的压力下堆叠，并在H_2气氛中以620℃加热30min。然后将A晶片和B晶片粘合到足以承受各种工艺的强度，SEM和TEM观察发现，器件内形成了均匀的空气/半导体光栅，深度和周期分别约为0.2mum和0.4mum。由于质量输运现象，反应离子蚀刻得到的矩形光栅形状变成了圆形。在质量传输的帮助下，熔合界面变得更加牢固，在电子方面也更加可靠。还发现，晶圆融合后光栅的形状几乎不依赖于晶体取向。然后将熔接晶片加工成脊波导结构，在室温下成功地实现了连续波振荡。这些结果对开发均匀嵌入圆光栅耦合面发射元件的集成器件具有重要的启发意义。少

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