Yoshifumi Yamaoka

Bio: Yoshifumi Yamaoka is an academic researcher from Hewlett-Packard. The author has contributed to research in topics: Laser diode & Semiconductor. The author has an hindex of 4, co-authored 9 publications receiving 611 citations. Previous affiliations of Yoshifumi Yamaoka include Agilent Technologies.

Determination of piezoelectric fields in strained GaInN quantum wells using the quantum-confined Stark effect

Abstract: We have identified piezoelectric fields in strained GaInN/GaN quantum well p-i-n structures using the quantum-confined Stark effect. The photoluminescence peak of the quantum wells showed a blueshift with increasing applied reverse voltages. This blueshift is due to the cancellation of the piezoelectric field by the reverse bias field. We determined that the piezoelectric field points from the growth surface to the substrate and its magnitude is 1.2 MV/cm for Ga0.84In0.16N/GaN quantum wells on sapphire substrate. In addition, from the direction of the field, the growth orientation of our nitride epilayers can be determined to be (0001), corresponding to the Ga face.

Improvement of far-field pattern in nitride laser diodes

Abstract: We have clearly demonstrated a single spot in a far-field pattern of nitride-based laser diodes with a thick n-AlGaN layer/low-temperature-deposited buffer layer/sapphire. This AlGaN-based structure has realized a crack-free 1-μm-thick n-type Al0.06Ga0.94N cladding layer, leading to suppression of optical leakage from the waveguide region to the underlying layer and improvement of optical confinement. The threshold current of the laser diode is about 230 mA, which is comparable to or better than that of our laser diodes with the conventional GaN-based structure.

Melt-back etching of GaN

Abstract: Melt-back etching of GaN is proposed as a novel etching technique and is found to be prospective. The highest etching rate was 1.2 μm/h at a temperature of 1050°C. The etched surface was as good as the surface of an as-grown GaN crystal when the etching temperature is equal to or lower than 900°C, providing a practical etching rate of 0.1 μm/h.

Method for fabricating low-resistance contacts on nitride semiconductor devices

Abstract: A method for fabricating an electrical contact (12) on a surface (11) of a semiconductor comprising a group III element and nitrogen. The contact (12) is formed by depositing a metallic layer on the semiconductor surface and then annealing the layer at a tempeature greater than 400 °C for at least 4 hours. The method can be used to construct a Au/Ni contact on GaN semiconductor surface with substantially less resistivity than that obtained by conventional methods.

Method for fabricating semiconductor laser facets using combined cleave and polish technique

Abstract: A method for fabricating opto-electronic devices having a surface that includes a first mirror facet in a first semiconductor layer deposited on a wafer. The mirror facet is located in a first facet plane. In the method of the present invention, the wafer is divided along a first line to create a segment having the mirror facet located therein. The segment is fixed to a fixture that moves in relation to a dicing disk that has a first planar surface in which polishing grit is embedded. The segment is fixed to a mounting surface such that the first plane is aligned parallel to the first planar surface of the dicing disk. The dicing disk is caused to rotate while the first planar surface is in contact with the segment, but not in contact with the mounting surface, thereby polishing a surface of the segment. For opto-electronic devices that include a second mirror facet in a second facet plane, the second facet plane being parallel to the first facet plane and displaced therefrom, the wafer is also divided along the second line such that the segment also includes the second facet plane. The second mirror surface is polished by causing the dicing disk to rotate while the second planar surface is in contact with the segment along the second line. The present invention can be practiced with a conventional dicing saw.

Status and Future of High-Power Light-Emitting Diodes for Solid-State Lighting

Abstract: Status and future outlook of III-V compound semiconductor visible-spectrum light-emitting diodes (LEDs) are presented. Light extraction techniques are reviewed and extraction efficiencies are quantified in the 60%+ (AlGaInP) and ~80% (InGaN) regimes for state-of-the-art devices. The phosphor-based white LED concept is reviewed and recent performance discussed, showing that high-power white LEDs now approach the 100-lm/W regime. Devices employing multiple phosphors for "warm" white color temperatures (~3000-4000 K) and high color rendering (CRI>80), which provide properties critical for many illumination applications, are discussed. Recent developments in chip design, packaging, and high current performance lead to very high luminance devices (~50 Mcd/m2 white at 1 A forward current in 1times1 mm2 chip) that are suitable for application to automotive forward lighting. A prognosis for future LED performance levels is considered given further improvements in internal quantum efficiency, which to date lag achievements in light extraction efficiency for InGaN LEDs

Strain-induced polarization in wurtzite III-nitride semipolar layers

Abstract: This paper presents growth orientation dependence of the piezoelectric polarization of InxGa1−xN and AlyGa1−yN layers lattice matched to GaN. This topic has become relevant with the advent of growing nitride based devices on semipolar planes [A. Chakraborty et al., Jpn. J. Appl. Phys., Part 2 44, L945 (2005)]. The calculations demonstrate that for strained InxGa1−xN and AlyGa1−yN layers lattice matched to GaN, the piezoelectric polarization becomes zero for nonpolar orientations and also at another point ≈45° tilted from the c plane. The zero crossover has only a very small dependence on the In or Al content of the ternary alloy layer. With the addition of spontaneous polarization, the angle at which the total polarization equals zero increases slightly for InxGa1−xN, but the exact value depends on the In content. For AlyGa1−yN mismatched layers the effect of spontaneous polarization becomes important by increasing the crossover point to ∼70° from c-axis oriented films. These calculations were performed u...

Nitride semiconductor device

Abstract: A nitride semiconductor device including a light emitting device comprises a n-type region of one or more nitride semiconductor layers having n-type conductivity, a p-type region of one or more nitride semiconductor layers having p-type conductivity and an active layer between the n-type region and the p-type region. In such devices, there is provided with a super lattice layer comprising first layers and second layers which are nitride semiconductors having a different composition respectively. The super lattice structure makes working current and voltage of the device lowered, resulting in realization of more efficient devices.

Structural characterization of nonpolar (112̄0) a-plane GaN thin films grown on (11̄02) r-plane sapphire

Abstract: In this letter we describe the structural characteristics of nonpolar (1120) a-plane GaN thin films grown on (1102) r-plane sapphire substrates via metalorganic chemical vapor deposition. Planar growth surfaces have been achieved and the potential for device-quality layers realized by depositing a low temperature nucleation layer prior to high temperature epitaxial growth. The in-plane orientation of the GaN with respect to the r-plane sapphire substrate was confirmed to be [0001]GaN‖[1101]sapphire and [1100]GaN‖[1120]sapphire. This relationship is explicitly defined since the polarity of the a-GaN films was determined using convergent beam electron diffraction. Threading dislocations and stacking faults, observed in plan-view and cross-sectional transmission electron microscope images, dominated the a-GaN microstructure with densities of 2.6×1010 cm−2 and 3.8×105 cm−1, respectively. Submicron pits and crystallographic terraces were observed on the optically specular a-GaN surface with atomic force m...

Surface plasmon enhanced spontaneous emission rate of InGaN∕GaN quantum wells probed by time-resolved photoluminescence spectroscopy

Abstract: We observed a 32-fold increase in the spontaneous emission rate of InGaN/GaN quantum well (QW) at 440 nm by employing surface plasmons (SPs) probed by time-resolved photoluminescence spectroscopy. We explore this remarkable enhancement of the emission rates and intensities resulting from the efficient energy transfer from electron-hole pair recombination in the QW to electron vibrations of SPs at the metal-coated surface of the semiconductor heterostructure. This QW-SP coupling is expected to lead to a new class of super bright and high-speed light-emitting diodes (LEDs) that offer realistic alternatives to conventional fluorescent tubes.