Fabrication of a-C Semiconductor Nanoparticles for Quantum Dots Surface Emitting Laser Using High-Density Plasma in Localized Area
Authors: Kobayashi Ryutaro , Nagata Yoshiya , Okafuji Keigo , Ohtomo Shinpei , Naragino Hiroshi , Honda Kensuke 
Published: February 2020

1. Introduction
Everyone uses and always carries mobile terminals in these modern days, when the information level in a society has been highly developed. Since the devices are used on　various scenes in a life, low power consumption and high-speed data communication at low cost are required. The semiconductor laser used in a terminal device has two problems: a high production cost and low velocity of light emitted from laser.Surface emitting laser has been developed as a low-cost semiconductor laser, but sufficient velocity of light has not been obtained.Lower velocity of light is caused by the process of energy calibration of light emitted from the semiconductor device.The size of conventional laser device is several micro meters at minimum, and multiple bonding orbitals are formed in electronic state of the semiconductors. In transition of highly-excited electrons to lower state, there is more than one state, which electrons can transit to. It results in a considerable variation in wavelength of light and energy of light must be calibrated. To homogenize emitted light, quantum size effect are tried to be employed. The reduction of device size to quantum size not having multiple energy levels can homogenize the emitting energy. With this method, calibration of energy is not necessary and the velocity of light can be improved. To realize low power consumption, high-speed communication, and low production cost, stack layer of semiconductor particles with a diameter of 10 nm in porous alumina was tried to be fabricated. We, however, found it hard to control the diameter of semiconductor particle under 10 nm and stack the particles .Moreover, rare metals such as Ga and In are commonly used as materials for semiconductor laser and increase costs. Thus, non-rare material semiconductor particle with a diameter of 10 nm should be developed. Our research group has reported that the method to fabricate nanoparticle of a-C semiconductor was established and particle diameter could freely be controlled in the range from 200 to 10 nm by using high-density plasma localize area. The objective of this study is to realize quantum dots surface emitting laser by stacking the a-C semiconductor particle in porous alumina template. Synthesis method which enhances the semiconductor properties of the nanoparticles up to the level applicable to a semiconductor laser was tried to be established. The a-C semiconductor nanoparticles of which optical gap are regulated by addition of Si atoms were synthesized by the plasma CVD method using hollow cathode electrodes.
2. Experiment Section
Localized high-density plasma regions were formed by inserting aluminum porous plate between cathode and anode electrodes. Performance of a-C semiconductor mobilities and back ground current tends to be degraded by sp2 carbon impurities. In order to evaluate the content of sp2carbon in a-C semiconductor nanoparticle, carrier mobilities in Hall effect measurement, current in photo electrochemical measurement without photo irradiation, and onset potential in photo current under the exposure to UV lamp were employed.
3. Result and Discussion
The average diameter of the synthesized semiconductor particles was approximately 15 nm. Semiconductor properties of nanoparticles could be controlled by RF power and flow rate of hydrogen gas during CVD synthesis. The amount of sp2 impurities can be reduced by lowering the plasma power from 32 to 20 W. As a result, carrier mobility was elevated from 3.4 to 5.4 cm2V-1S-1 because electron scattering was suppressed. Further, by increasing the flow rate of hydrogen gas from 50 to 200 sccm, sp2 defects on the surface were reduced by H2 plasma etching. It results in the decrease of leakage current from 1.3 to 0.5 μA/cm2, the positive shift of onset potential of photo current from -0.4 to 2.6 V, and clear rectification behavior. By reducing sp2 impurities, semiconductor properties of the nanoparticles can be improved. It is concluded that synthesize method of n-type semiconductor nanoparticles with a diameter of 15 nm and controllable semiconductor properties was successfully established.

Figure 1