Basic Research on Optoelectronic Devices for Neural Networks

本文从光电集成电路的角度对大规模神经网络的神经元器件和光互连进行了基础研究。研究了基本可行性和技术障碍。来自其他神经元的光输出乘以突触权重，并在人工神经元中求和。然后，光输出与sigmoid函数，这是很容易产生的LED连接到一个差分放大器发出。突触连接的功能在神经元中是最重要的。首先，光电电路的开发，它作为一个突触连接与可变的突触权重。阐明了光互连的基本功能和最佳性能。然后，开发了一种具有自组织功能的新设备，其突触权重适应于教师的信号。这些光电基础器件和电路均采用硅集成电路工艺制造。阐明了大规模集成的可行性，如nA量级的极低电流操作和简单的结构，并研究了单芯片OEIC所需的GaAs在Si上外延生长的初始生长机制。为了在Si上生长高质量的GaAs，必须基本上避免初始的三维生长。通过引入具有较强化学键的AlAs初始层或通过固相外延生长的薄GaAs层，实现了准二维生长。此外，通过增强Ga迁移，在GaAs（111）B和（110）的同质外延中获得了镜面。

A basic study on neuro-devices and optical interconnection has been conducted for large-scale neural networks in the viewpoint of an optoelectronic integrated circuit (OEIC). The fundamental feasibiity and technological barriers were investigated.Optical outputs from other neurons were multiplied by synaptic weights and summed in the artificial neuron. Then the optical output was emitted with the sigmoid function, which is easily generated by an LED connected to a differential amplifier. The function of synaptic connection is most important in the neuron. Firstly, an optoelectronic circuit was developed, which acted as a synaptic connection with variable synaptic weights. As a result, the basic function as well as the preferable performances of the optical interconnection were clarified. Then a new device with a self-organizing function was developed, whose synaptic weight is adapted to a teacher's signal. These optoelectronic fundamental device and circuits were fabricated using Si IC technology. Preferable characteristics for large-scale integration were clarified such as a very low current operation of the order of nA and a simple structure.Initial growth mechanisms were also studied in epitaxial growth of GaAs on Si, which could be needed for one-chip OEIC. The initial three dimensional growth must be avoided essentially for a growth of high quality GaAs on Si. A quasi-two dimensional growth was realized by introducing an AlAs initial layer having a stronger chemical bond or a thin GaAs layer grown by solid phase epitaxy. In addition, a mirror surface was obtained in homoepitaxy of GaAs(111)B and (110) by enhancing a Ga-migration.