Naokatsu Yamamoto

Visible luminescence from photo-chemically etched silicon

Abstract A new photo-chemical etching method using H 2 O 2 as an oxidant was proposed to form a luminescence layer on a Si wafer. A single crystalline n-type Si wafer (100) having resisitivity of 35–45 Ω cm was photo-chemically etched in a mixture of HF and H 2 O 2 (HF:H 2 O 2 =6:1), and He–Ne laser (633 nm) was irradiated onto the wafer surface through the solution for photo-chemical reaction. Luminescence from photo-chemically etched layers was measured by photoluminescence (PL) using a He-Cd laser for excitation. It was found that the layer etched for 30 min shows strong yellow luminescence with a peak at 637 nm, while the layer etched for 5 min shows red luminescence with a peak at 695 nm. It is also confirmed that the etched layer exhibiting various color can be patterned. Blue luminescence (420 nm) can be obtained by dipping the layer etched for 30 min in a solution of ethanol and H 2 O (ethanol:H 2 O=1:1 for volume) for 148 h. The absorption peak of Si-O-Si is observed in the blue-luminescence layer by Fourier transform infrared (FTIR) absorption spectroscopy, indicating that the blue-luminescence is due to the oxidation of the photo-chemically etched layer.

Electroluminescence (EL) from photo-chemically etched silicon

Abstract Visible luminescence from Si-based materials has been investigated to develop new opto-electronic devices on a Si wafer. In this report, we propose a photo-chemical etching method in order to form a luminescent layer on a Si wafer. A comparison between electroluminescence (EL) and photoluminescence (PL) from the photo-chemically etched silicon is discussed. In the photo-chemical etching method, a Si wafer (100) with resisitivity of 35–45 or 0.22–0.38 ohm-cm is set at the bottom of a vessel filled with an etchant (HF+H 2 O 2 ), and a He–Ne laser (633 nm, 18.4mW/cm 2 ) is irradiated onto the surface for 30 min. An Au thin film (thickness 5 nm) is deposited onto the the etched layer, and a Au–Sb (1 wt%) film is deposited on the reverse side of the Si wafer to form ohmic contacts. The EL from the etched layer is observed by applying a voltage to the electrodes(−30∼+30V). PL from the etched layer is measured by He–Cd laser excitation (325 nm). As a result, it is clear that the peak wavelength of EL at forward bias coincides with a peak wavelength of PL. EL spectra at backward bias can be fitted by two gaussian functions, and one of them coincides with a peak wavelength of PL. In addition, the EL from the photo-chemically etched silicon can be explained schematically by an electrical circuit model.

Formation mechanism of silicon based luminescence material using a photo chemical etching method

Abstract A photo chemical etching method to form a silicon based luminescence material on crystalline silicon has been studied. An n-type Si wafer (100) having resisitivity of 0.22–0.38 or 35–45 Ohm/cm was photo chemically etched in a mixture of HF and H 2 O 2 (HF:H 2 O 2 =100:17∼250), and He–Ne laser (633 nm) was irradiated onto the wafer surface through the solution. A photoluminescence spectrum from the photo chemically etched silicon under a He–Cd laser excitation (325 nm) was measured. Rutherford back scattering spectroscopy measurement was carried out to calculate the thickness of the photo chemically etched silicon. Additionally, an electroluminescence device using etched silicon was fabricated. In this paper, we have proposed a schematic chemical reaction model and a chemical formula of the etching process to discuss the photochemical etching mechanism. As a result, it was found that the etching time presenting a maximum intensity of photoluminescence can be explained and calculated by the etching velocity of the photochemical etching process. Additionally, we demonstrated a yellow electroluminescence from photochemically etched silicon in order to develop the future opto-electronic device on a crystalline silicon substrate. We have exhibited here the usefulness of a photochemical etching method.

5G transport and broadband access networks: The need for new technologies and standards

In addition to new radio technologies, end-to-end transport networks will play a vital role in future 5G (and beyond) networks. In particular, access transport networks connecting radio access with core networks are of critical importance. They should be able to support massive connectivity, super high data rates, and real time services in a ubiquitous environment. To attain these targets, transport networks should be constructed on the basis of a variety of technologies and methods, depending on application scenarios, geographical areas, and deployment models. In this paper, we present several technologies, including analog radioover-fiber transmission, intermediate-frequency-over-fiber technology, radio-on-radio transmission, and the convergence of fiber and millimeter-wave systems, that can facilitate building such effective transport networks in many use cases. For each technology, we present the system concept, possible application cases, and some demonstration results. We also discuss potential standardization and development directions so that the proposed technologies can be widely used.

Radio-over-fiber network technology for millimeter-wave distributed radar systems

Millimeter-wave radar technology in a frequency band of 96 GHz is promising to detect small objects with a high range resolution because of its short wavelength and broadness of available bandwidths of 8 GHz. However, transmission distance in the millimeter-wave bands is limited by high atmospheric attenuation. Radio over fiber (RoF) technology should be implemented to transport these millimeter-wave signals to remote radar heads via an optical fiber network. In the study, we discuss the RoF network for distributed radar systems in the millimeter-wave and submillimeter-wave bands.

Radio over fiber-based radio relay link for indoor and in-car applications towards 5G/IoT era

Radio over single-mode, multimode, and plastic optical fiber links are demonstrated as a radio relay link for improvement of coverage areas. These techniques are capable for different deployment scenarios in several indoor conditions.