Yawara Kaneko

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

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.

Low-temperature-deposited AlGaN interlayer for improvement of AlGaN/GaN heterostructure

Abstract We have studied the effect of a low-temperature-deposited (LT-) AlGaN interlayer on AlGaN/GaN heterostructure. High-crystalline-quality and crack-free AlGaN layers covering the entire compositional range were realized using the LT-AlN interlayer. We also showed that AlGaN is applicable as the LT-interlayer under the condition that the AlN molar fraction of the LT-AlGaN interlayer is equal to or larger than that of the overgrown AlGaN layer. Furthermore, electrically conductive LT-Al 0.1 Ga 0.9 N has been realized with an intermediate deposition temperature and very high SiH 4 flow rate, in addition to the high-quality overgrown AlGaN layer. It is thought that the LT-interlayer technology can overcome the challenges of growing a high-quality AlGaN layer with high AlN mole fraction on GaN layers.

600-nm-Range GaInP/AlInP Strained Quantum Well Lasers Grown by Gas Source Molecular Beam Epitaxy

GaInP strained single quantum well (SSQW) lasers in the 630–710 nm range grown by gas source molecular beam epitaxy (GSMBE) using a novel shutter control method were systematically investigated. A good controllability in strain amount as well as good in-plane compositional homogeneity over 4-cm-long distance of the laser wafers were confirmed. For +1.1% compressive SSQW lasers (708 nm in wavelength), a very low threshold current density (J th) of 175 A/cm2 was obtained. These facts suggest that the novel shutter control method is very effective for fabricating GaInP SSQW lasers. Furthermore, the post-annealing effect on lasing characteristics of GaInP SSQW lasers was investigated. The increased p carrier density of Be-doped AlInP layers was obtained with the remarkable reduction in J th value of 633 nm tensile SSQW lasers from 850 A/cm2 to 555 A/cm2.

GaN Based Laser Diode with Focused Ion Beam Etched Mirrors

GaN-based MQWs-SCH laser diodes (LDs) with Fabry‐Perot resonator mirrors fabricated by focused ion beam (FIB) etching were demonstrated for the first time. The diodes show lasing by pulsed current injection at room temperature with a lasing wavelength near 410 nm. FIB etching of the mirrors significantly reduced the threshold current from 1.25 to 0.75 A. In addition we studied the dependence of I-L characteristics on the successive rotation of the etched mirror of a single device and found a strong angular dependence. A similar study of the tilting angle revealed a very weak variation.

Quantum-confined Stark effect in strained GaInN quantum wells on sapphire (0 0 0 1)

We have studied the quantum-confined Stark effect in strained GaInN/GaN quantum wells p-i-n structure on sapphire (0001) by using room temperature photoluminescence measurements with applied voltages. The photoluminescence peak in GaInN/GaN quantum wells was blue-shifted by increasing the applied reverse voltage. This is due to the cancellation of the piezoelectric field by the reverse bias. This piezoelectric field pointed from the growth surface to the substrate, and its magnitude was determined to be 1.3 MV/cm in strained Ga 0.84 In 0.16 N layers.

600 nm-Range GaInP/AlInP Multi-Quantum-Well (MQW) Lasers Grown on Misorientation Substrates by Gas Source Molecular Beam Epitaxy (GS-MBE)

The 600 nm-range GaInP/AlInP multi-quantum-well (MQW) lasers grown by gas-source molecular beam epitaxy were investigated. Nonradiative recombination at heterointerfaces was drastically reduced by use of superlattice cladding (SLC) layers and 15° misoriented substrates (from (100) toward the [011] direction), resulting in increments of PL peak intensities from active layers by factors of 20 and 2, respectively. In addition to these effects, we changed the well numbers optimizing at three. As a result, a very low threshold current density (Jth) of 2.0 kA/cm2 was obtained at a lasing wavelength of 629nm, without strain effects. Finally, experimental rapid deterioration of Jth and characteristic temperatures by shortening of the wavelengths was explained by leakage current components over the heterobarrier.

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.

Electrical Conductivity of Low-Temperature-Deposited Al0.1Ga0.9N Interlayer

We have realized a conductive AlGaN interlayer which was deposited at low temperatures ranging from 500 to 600°C. We studied the surface morphology and electrical property of an AlGaN/low-temperature-deposited (LT) AlGaN interlayer/GaN structure, dependent on the growth condition. It was found that the conductive LT-AlGaN interlayer and crack-free over-grown Al0.1Ga0.9N layer with a thickness of 1 µm were obtained with intermediate deposition temperature and SiH4 supply for the interlayer. Using the conductive LT-AlGaN interlayer technology, we demonstrated the low-voltage operation of a light-emitting diode. The conductive LT-AlGaN interlayer is promising technology for the realization of shorter wavelength emitters with high AlN molar fraction AlGaN cladding layers.

Growth of vertical cavity surface emitting laser material on (3 1 1)B GaAs by MBE

We report on the growth of VCSEL structures on (3 1 1)B GaAs substrates. For the growth of AlGaAs and AlAs, the best morphology and material quality were obtained for growth temperatures > 650°C. Surface morphology and mirror reflectivity degraded significantly at low growth temperatures ( < 600°C). From low-temperature photoluminescence (LTPL), we found a forbidden temperature range for the growth of InGaAs quantum well-active regions on (3 1 1)B substrates between 540 and 560°C. Active region growth temperatures in this range showed low intensity, broad LTPL, and poor laser characteristics. Cross-section TEM measurements show poor homogeneity for material grown in this temperature range. At higher temperatures (580°C), In desorption is greatly increased, so < 520°C was selected as the optimal growth temperature. Even with a non-optimized structure, the first reported VCSELs on (3 1 1)B were fabricated with a pulsed J th = 9 kA/cm 2 at - 40°C and 28 kA/cm 2 at room temperature. At - 40°C, 10 nW of SHG blue light at 485 nm was detected under pulsed conditions, and 2 nW was detected under CW conditions and was visible to the naked eye. By improving the structure we obtained CW lasing at room temperature with 300 A/cm 2 as a broad area laser and 1.4 kA/cm 2 as a VCSEL. A maximum power of 0.55 nW at 490 nm was detected CW at room temperature.

Raman scattering from interface phonons in GaInP/AlInP superlattice

Measurements of Raman scattering were performed on ternary alloy superlattices GaInP/AlInP grown by gas-source molecular beam epitaxy. We observed interface phonons whose frequencies are explained in terms of an electrostatic continuum model using the optical phonons of AlInP and GaInP bulk alloys. The Raman intensities of the interface phonons are enhanced when the incident photon energy is close to the exciton energy in the GaInP quantum wells and to the band gap energy in the AlInP. We measured Raman spectra of AlGaInP quaternary alloy, which have the same composition on a standpoint of average crystal structure, and compare it with the spectra of the superlattice.