Norihide Yamada

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.

Internal quantum efficiency of highly-efficient InxGa1−xN-based near-ultraviolet light-emitting diodes

The internal quantum efficiency (IQE) of highly-efficient near-UV light-emitting diodes, which shows an external quantum efficiency of 43% at 406 nm, has been measured by excitation power and temperature-dependent photoluminescence (PL). Assuming peak PL quantum efficiency at 8 K is 100%, peak IQE at 300 K was measured to be as high as 63%. At the injected carrier density, which corresponds to 20 mA current injection, IQE and light extraction efficiency were estimated to be about 54% and 80%, respectively.

Pit formation in GaInN quantum wells

The formation of pits in GaInN quantum wells (QWs) has been studied by atomic force microscopy and transmission electron microscopy. It is found that the pits have a hexahedron cone morphology with six sidewalls on 〈1101〉 planes and dislocations connected to their vertexes. The dislocations may induce the formation of pits during the growth of GaInN QWs.

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.

Radiative and nonradiative lifetimes in GaInN/GaN multiquantum wells

This paper analyzes a simplified rate equation model of localized exciton emission in GaInN. Expressions for temperature dependent photoluminescence (PL) efficiency and decay time are derived and compared with time integrated (TIPL) and time resolved photoluminescence (TRPL) data for a series of multiquantum well light emitting diodes with varying In composition in the active region. Time resolved photoluminescence is measured up to relatively high temperature (540 K) and a decreasing efficiency coupled with a peak energy decay time that is weakly dependent on temperature is observed. The decay time at peak emission energy begins to decrease at a temperature that depends on the In content in the quantum wells. The analysis developed here demonstrates that application of the expressions τr=τpl/η and τnr=τpl/(1−η) is not sufficient to determine radiative and nonradiative lifetimes from TRPL and TIPL data in the GaInN system. (Here τr is the radiative decay time, τnr is the nonradiative decay time, τpl is th...

Second‐harmonic generation from GaAs/AlAs vertical cavity

We have demonstrated second‐harmonic generation in a GaAs/AlAs vertical cavity grown on a (311)B GaAs substrate. Second‐harmonic light of 492 nm was observed and its efficiency was measured to be 1.4×10−4 %/W. The cavity confines high intensity of fundamental field, resulting in efficient second‐harmonic generation. Quasiphase matching was realized with the stacked GaAs/AlAs layers inside the cavity, which was designed taking into account the strong absorption of second‐harmonic power in GaAs layers. We show that conversion efficiency of the GaAs/AlAs vertical cavity could be more than 10%/W if the optimization is completed.

Blue vertical-cavity surface-emitting lasers based on second-harmonic generation grown on (311)B and (411)A GaAs substrates

We have studied blue vertical-cavity surface-emitting lasers (VCSELs) based on second-harmonic generation (SHG) grown on (411)A and (311)B GaAs substrates in order to investigate suitable substrate orientations for SHG-VCSELs. The comparison among substrate orientations has been made on three parameters, SHG conversion efficiency, transparency current density and gain coefficient. The transparency current density and the gain coefficient are characterized by edge emitting lasers grown on the above substrates. We also discuss the transparency current density and the gain coefficient for (311)A reported previously by Takahashi et al. [M. Takahashi, M. Hirai, K. Fujita, N. Egami, and K. Iga, J. Appl. Phys. 82, 4551 (1997)]. SHG conversion efficiency is 38 and 30% W for SHG-VCSELs grown on (311)B and (411)A substrates, respectively, which is consistent with theory, assuming identical nonlinear coefficients for the A face and B face. Transparency current density for (311)A, (311)B and (411)A is 80, 105 and 60 ...

Second-harmonic generation in vertical-cavity surface-emitting laser

A concept for a short-wavelength compact laser is proposed, in which second-harmonic coherent light is produced by converting fundamental light lased in a vertical-cavity surface-emitting laser. A layer is incorporated inside the laser cavity particularly for efficient second-harmonic generation. This layer consists of second-order optical nonlinear crystals which are preferably III–V- or II–VI-system compound semiconductors epitaxially grown with crystal orientation tilted from . Simulation indicates that the device will produce several hundred µ W with AlAs/GaAs alternating layers for second-harmonic generation and an InGaAs active layer for lasing.

Improvement of far-field pattern in nitride laser diodes

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.

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.