We are researching a whole family of opto-electronic devices based on the original concept of lateral p-n junctions. Lateral p-n junctions have particular properties and are promising for applications in opto-electronic integrated circuits. We fabricated light-emitting diodes (LEDs), vertical-cavity surface-emitting lasers and photodiodes using lateral p-n junctions, and demonstrated high-density integration in LED arrays.

1. Introduction
　A p-n junction (boundary of n-type region and p-type region) is a basic structure for opto-electronic devices. We have developed a new method to fabricate p-n junctions (Fig.1). In this method, layers of compound semiconductor material are deposited using an amphoteric dopant (one that can be n-type or p-type according to the deposition conditions) on a patterned substrate that has more than one surface on it. We can obtain n-type and p-type regions simultaneously after one deposition according to the orientation of the surfaces, and a lateral p-n junction is formed at the boundary of the two regions.

　　　

Fig. 1. (a) Lateral p-n junction and (b) conventional p-n junction.

　Lateral p-n junctions thus prepared are promising as next-generation devices because they have the following advantages.

1. They are easy to fabricate, since they are formed by only a one-time crystal growth with only one dopant.
2. The junction area is controlled by the thickness of the layer with silicon impurities that can be as thin as one atomic single layer. Therefore, the junction area can be made orders of magnitude smaller than in conventional p-n junctions.
3. The n-type and p-type regions are side by side on the substrate surface; therefore, all electric contacts can be made on the same side of the substrate using a coplanar geometry.
4. Electrically insulating substrates can be used, simplifying electrical isolation of multiple devices made on the same substrate.
5. The junction can be clad between non-doped epitaxial layers that reduce leakage current and produce carrier confinement.

　We proposed and demonstrated various devices that use these advantageous properties of lateral junctions and fabricated high-density linear arrays of light emitting diodes (LEDs). In this article, we introduce these devices.

2. Lateral junction devices
2-1 Light emitting diodes
　Conventional LEDs have a large portion of the light-emitting area covered by the top contact. This configuration obstructs light emission and reduces total efficiency. On the other hand, lateral-junction LEDs (LJ-LEDs) have contacts placed on the sides of the light-emitting area, and light emission is not obstructed. We fabricated LJ-LEDs with various active layer structures and compositions [2].

2-2 Lasers
　This p-n junction can also be used for making vertical-cavity laser diodes. We demonstrated a lateral-junction vertical-cavity surface-emitting laser (LJ-VCSEL) [3], which emits light perpendicular to the substrate. The structure of the LJ-VCSEL is similar to the LJ-LED, with the addition of multilayer dielectric mirrors (distributed Bragg reflectors) on the top and bottom of the active layer. Laser emission at room temperature was observed for a current higher than 2.3 mA (Fig. 2).

Fig. 2. Emission spectrum of a lateral-junction vertical-cavity surface-emitting laser.
Red: above the threshold current; Blue: below the threshold current.

2-3 Photodetectors
　For optical wireless communication systems, photodetectors, which detect light, are important devices. To make the system faster, we need photodetectors with a faster response, which can be accomplished by the reduction of the capacitance of the devices. Fast operation of lateral-junction photodetectors was demonstrated, taking advantage of the small junction area of the LJ devices, which led to small capacitance of the devices [4].

　　　
Fig. 3. Optical micrograph of the light emission from the array of lateral
junction LEDs with a 2400-dpi density.

4. Conclusion
　We will develop opto-electronic devices for optical wireless communication systems with higher performance using lateral p-n junctions, which have many advantages.

[1] H. Ohnishi, M. Hirai, K. Fujita and T. Watanabe, "Lateral tunneling devices on GaAs (111)A and (311)A patterned substrates grown by MBE using only silicon dopant", Jpn. J. Appl. Phys. 35, 1168 (1996).
[2] P. O. Vaccaro, H. Ohnishi and K. Fujita, "A light emitting device using a lateral junction grown by molecular beam epitaxy on GaAs (311)A-oriented substrates", Appl. Phys. Lett. 72, 818 (1998).
[3] P. O. Vaccaro, H. Ohnishi and K. Fujita, "Lateral-junction vertical-cavity surface-emitting laser grown by molecular beam epitaxy on a patterned GaAs (311)A substrate", Appl. Phys. Lett. 74, 3854 (1999).
[4] P. O. Vaccaro, M. Uwani, T. Ohachi, M. Tani and M. Kurosawa, "Lateral junction photodetector grown by molecular beam epitaxy on a GaAs (311)A-oriented substrate", 26th Intl. Symp. Compound Semic., Berlin, Germany, Aug. 22-26, 1999.