This report looks at the present and future of research on opto-electronic devices aiming to create next-generation devices based on ATR's research activities in the field of compound semiconductor technology. The results introduced in previous articles, the 2-dimensional microcavity laser, Micro-origami, and lateral junctions, are all made of compound semiconductors, and are expected to be developed into small-sized high-performance opto-electronic devices by integration.

1. Introduction
　We are conducting research on the devices that were introduced in previous articles aiming to create next-generation opto-electronic devices. In order to realize optical wireless communication, we have to develop light emitting devices that generate optical beams, photodetectors that detect the beams, and beam-steering devices that properly control the beams in the direction of the target of the communication. In this special issue, we are introducing one article on light emitting/detecting devices, entitled "High-density integration of opto-electronic devices using lateral p-n junctions", as well as two articles on beam-steering devices, entitled "2-dimensional microcavity lasers" and "Optical semiconductor devices using 'Micro-origami' technique". These devices all consist of compound semiconductors, which is a combination of the elements in column III and those in column V in the periodic table (Table I). We have been working on the single-crystalline thin films of compound semiconductors regarding their growth, manufacturing, and applications into devices, and have developed them to the stage that we can propose devices for optical wireless communications.
　In this article, we describe some advantages of compound semiconductors and introduce our recent activities aiming at the realization of next-generation opto-electronic devices through the integration of various devices that are introduced in this special issue.

elements in I
column II

elements in
column IV

elements in
column V

boron (B)

carbon (c)

nitrogen (N)

aluminum (Al)

silicon (Si)

phosphorus (P)

gallium (Ga)

germanium (Ge)

arsenic (As)

indium (In)

tin (Sn)

antimony (Sb)

2. Advantages of compound semiconductors
　The most well-known semiconductor material is silicon (Si). We can obtain high-quality crystals of Si, and most LSIs are made from it. The crystals of III-V compound semiconductors can be composed by alternatively substituting Si atoms in a crystal consisting of only one kind of element in column IV with elements in column III and those in column V. One of the most famous III-V compound semiconductors is gallium arsenide (GaAs). Although it is difficult to make light emitting devices from Si, we can make high-performance ones with compound semiconductors, because they show efficient emission of light and the wavelengths can be continuously changed according to the composition of the materials. For instance, if we add Al to GaAs, which emits light with a wavelength of about 880 nm, to make AlGaAs, the wavelength becomes shorter. By using other III-V materials, we can obtain light emitting devices that emit light in a wide range of wavelengths, from ultra-violet, visible, to infrared.
　Moreover, it is well known that electrons can travel faster in compound semiconductors than in Si, which leads to another advantage in that we can obtain compound-semiconductor electronic devices operating faster than Si devices.

3. Next-generation opto-electronic devices by integration of beam-steering devices and light emitting/detecting devices
　As is mentioned in the previous articles, we can obtain small, fast, robust, and low-voltage-drivable beam steering devices making use of the growth and manufacturing technology of compound semiconductors. Furthermore, if they are combined with light emitting/detecting devices, it becomes possible to obtain next-generation opto-electronic devices. Figure 1 shows the schematic structure of this type of device.

　Although this kind of device has already been proposed, they are mainly made of Si. Manufacturing technology for Si is highly sophisticated, and a lot of high-performance devices and systems have been demonstrated [1]. However, most of these devices required the assembling of many parts, mainly due to the difficulty in fabricating light emitting devices using Si. On the other hand, if we use the compound semiconductor technology developed here at ATR, it becomes possible to fabricate integrated opto-electronic devices, which is the integration of light emitting/detecting devices and beam-steering devices. Since we might be able to achieve compact opto-electronic devices, which can control many beams with a high speed, by making use of our compound semiconductor technology, we expect that they will lead to next-generation devices.

Fig. 1. Schematic structure of next-generation opto-electronic devices,which
will be realized by compound semiconductor technology.

4. Aiming at eye-safe light sources
　In optical wireless communication, the light that is used must be completely safe for human eyes, so that no harm is done even if the light gets into the eyes. Lasers that meet this requirement are called eye-safe lasers, key devices in optical wireless communication. We can obtain lasers that shine in the infrared wavelength region by using a compound semiconductor, as is mentioned in this article. This is another advantage of the compound semiconductor.
　We are conducting research to create eye-safe lasers making use of the phenomenon that the emission wavelength of InAs, a kind of III-V compound semiconductor, changes according to the size when it is made as small as several tens of nm in diameter (quantum dots). We are trying to solve the problem that the light intensity becomes small as the wavelength becomes long by improving the film structure. Our recent results are shown in Figure 2 [2] .
　In the future, we are planning to combine eye-safe optical devices with the next-generation opto-electronic devices discussed in Section 3. Such a fruitful combination of technologies is made possible by the fact that all of these devices are based on the same III-V compound semiconductor.

　　　
Fig. 2. Photoluminescence spectrum of the materials for eye-safe lasers.

5. Conclusion
　We introduced our research aiming towards integrated devices of light emitting devices, which operate in wavelength regions that are safe to the human eye, photodetectors, and beam-steering devices. We must also make individual devices smaller to improve the degree of integration. To meet this requirement, we have begun research on a fine manufacturing technique using an electron beam lithography system and atomic microscope, both of which are capable of manufacturing sub-micron devices.
　By further developing our technology, we intend to make high-performance devices for the ultra-fast and large-capacity optical communication systems of the future.

[1] For example, see 2003 IEEE/LEOS Int. Conf. on Optical MEMS, Waikoloa, Hawaii, Aug. 18-21, 2003.
[2] S. Saravanan, P. O. Vaccaro, J. M. Zanardi, K. Kubota, and T. Aida, "InAs quantum dots on GaAs substrates with InGaAs strain reducing layer for long wavelength emission", phys. stat. sol. (c) 0, 1193 (2003).