Jun Ohsawa

A GaAs Micro Solar Cell with Output Voltage over 20 V

Twenty-four micro-solar cells connected in series have been fabricated on semi-insulating (SI) GaAs substrates for application in the fields of micro-electromechanical systems. The array was formed in an area of 0.8×1.0 mm2, and exhibited an open-circuit voltage of 22.5 V under illumination of 5 mW at the wavelength of 815 nm. Calculation and experiment has demonstrated that, unlike conventional solar cells, the shunt resistance deteriorates the output characteristics far more seriously than the series resistance in the case of micro-solar cells. Leakage currents in both the unit diode and the substrate were evaluated separately. A quantitative estimate based on the measurements revealed that photocurrents generated in the surface of the SI substrate could function as a vital shunt resistance. Light-blocking metal films were successfully employed to obtain high output voltage.

Comparison of Spectral Responses between Front- and Back-Incidence Configurations in GaN Metal–Semiconductor–Metal Photodetector on Sapphire

Front and back illumination of a metal–semiconductor–metal structure on a 2-µm-thick GaN layer showed obvious differences in the spectral responsivity in the wavelength range of 300–500 nm. Pt/Au interdigitated electrodes on an unintentionally doped n-GaN were confirmed to be of extremely low leakage Schottky type, and simulations of the electrostatic potential distribution have revealed that the depletion regions do not prevail throughout the thick GaN layer even at a bias of 10 V. The difference observed in the wavelength region shorter than the fundamental absorption edge is due to incomplete depletion of the GaN layer off the Schottky contacts in conjunction with short optical penetration depths, while the back-incidence responsivity in the longer wavelength region reflects extrinsic optical absorptions characteristic to the epitaxial crystal.

Different Bias-Voltage Dependences of Photocurrent in Pt/InGaN/GaN and Pt/GaN Schottky Photodetectors on Sapphire

Two structures under back illumination showed opposite bias polarity dependence in the photocurrent of Schottky barrier contacts, where a combination of Pt/Au metal films was formed on unintentionally doped n-type layers. The contact with a 2-µm-thick GaN layer exhibited a higher photocurrent for reverse biasing as expected, whereas the same contact with an additional 20-nm-thick InGaN layer on GaN exhibited a much higher current for forward biasing. This current was maintained down to a small reverse bias voltage, which indicates that the thin InGaN layer with an In content of 15% behaves like a p-type semiconductor. The result can be understood by the internal electric field in the InGaN layer as well as the fact that 400 nm light illuminated from the back side is absorbed in the thin layer just under the contact metal.

Selective Detection of Blue and Ultraviolet Light by An InGaN/GaN Schottky Barrier Photodiode

Light in wavelength ranges of 400 and 350 nm was selectively detected by changing the bias voltage of the diode. Piezoelectric field in a 20-nm-thick InGaN layer on an n-GaN/sapphire structure in combination with a back-incidence configuration was successfully utilized. The principle is based on a fact that a strained InGaN layer has an inward electric field, whereas the n-type underlayer has an outward field. Using a circular Schottky electrode of 1 mm diameter surrounded by an ohmic electrode, a responsivity of 0.06 A/W was obtained for both wavelength ranges by choosing a bias of either 0 or -5 V.

Contactless Measurement of Young's Modulus Using Laser Beam Excitation and Detection of Vibration of Thin-Film Microresonators

Youngs modulus of a single-crystal thin silicon film was estimated by a noncontact method. The resonance frequencies of a microresonator fabricated using the film were measured by excitation and detection using multiple laser beams. A special configuration was adopted for the resonator to suppress the heating effect of the excite beam. Clear resonances were observed in the frequency characteristics of vibration. Youngs modulus was determined from an elastic modification analysis of a spring-mass system using the resonance frequency together with the geometrical dimensions of the resonator. The deduced value of Youngs modulus was 152±17 MPa, which agrees with reported values within experimental errors. The cantilever beams of the resonator had trapezoidal cross sections caused by the reactive ion etching (RIE) used in the fabrication, and their widths showed nonuniformity. The error mainly comes from the nonuniformity of the beam width along the cantilever.

Low-Dark-Current Large-Area Narrow-Band Photodetector Using InGaN/GaN Layers on Sapphire

A metal–semiconductor–metal structure was fabricated on a 20-nm-thick InGaN layer, and showed a responsivity of over 0.1 A/W to the wavelength of 400 nm at a bias of 1 V. The photocurrent was almost linear to the power of the incident light in three decades. The Schottky interdigital electrodes kept the dark current to less than 100 pA at 10 V despite its large detecting area of 1 mm2. The device showed fast responses on the order of 10 ns to optical impulses from a 407 nm laser diode. Biasing at 1 V or lower is effective in suppressing the sensitivity in the wavelength range shorter than 350 nm when illuminated from the substrate side through a 2-µm-thick GaN layer, resulting in a narrow-band detector for the 400 nm band.

Contactless excitation and measurement method for inspection of microstructures and thin films

A specially designed resonator to avoid the change of the resonance characteristics by thermal influences of the laser beam was fabricated on a silicon on insulator wafer. This resonator design enables one to apply a lateral driving force to generate lateral vibration. The resonator was excited using a laser diode which was driven with sinusoidal current, and the vibration was detected by measuring intensity fluctuation of the He–Ne laser beam reflected on the resonator mass. Four evident resonance frequencies were successfully observed in the range from 100 Hz to 100 kHz.

Self-Aligned Formation of Porous Silicon Membranes Using Si Diaphragm Structures.

We report a new method of forming porous silicon membranes that is useful for micromachining applications. Using a p+ layer on an n-type silicon substrate, silicon diaphragms were formed by anisotropic etching with p+ etch stop characteristics. By choosing an appropriate polarity of current, local porosification of the membranes was achieved by anodic oxidation in hydrofluoric acid. Optical transmission was measured in the wavelength range from 400 nm to 900 nm. Transmittance of the membrane was calculated from reflectance and absorption coefficients using data from the literature. Quite reasonable agreement was obtained between the measured and calculated values, which confirms that the membrane had been converted uniformly to porous silicon.

Higher Picosecond Photoresponsivity Realized by Introducing Hole-Capturing Levels of Iron in GaAs

Control of transient carrier populations has been achieved by the use of carrier-capturing properties of relevant individual deep levels. Slow contribution from holes to transient photocurrents has been successfully suppressed so that the fast component of electron current becomes dominant. This has been realized by introducing deep acceptor levels of iron into undoped semi-insulating GaAs bulk material. Photoresponses to picosecond light pulses with and without iron doping were compared. A sharp peak at the leading edge, whose width is less than 100 ps, is increased for the iron-doped one. An additional cause of this could be reduced electron capture due to a concomitant decrease of deep donors such as electron levels No. 2 (EL2) and No. 6 (EL6). Iron-diffused epitaxial material, which has much lower concentrations of grown-in deep levels, showed the same electron-dominated characteristics, confirming the effectiveness of the hole-capturing nature of the iron level.

Higher Resistivities Obtained by Iron-Diffusion into Undoped Semi-Insulating GaAs

A simple diffusion process at low temperatures of 500–550°C can increase the resistivity of GaAs by one order of magnitude; deep acceptors of iron on the order of 1014 cm-3 were introduced into liquid encapsulated Czochralski(LEC)-grown GaAs wafers from a spin-on source to obtain higher resistivity in the range of 108 Ωcm. Control of the Fermi level onto the middle of the energy gap was accomplished by compensating deep donors of electron level No. 2 (EL2) by the acceptors located at Et-Ev=0.46 eV. A calculation based on carrier-compensation concurs with the trend in the resistivity variation experimentally obtained by iron-diffusion in the temperature range of 450–600°C.