Search

A Si infrared photodetector that operates without requiring to be cooled was fabricated, and its properties were evaluated. The function of this device is to detect, as an electrical signal, carrier density changes due to stimulated emission utilizing a phonon-assisted process via dressed photons. The photosensitivities of this device were 0.21 A W−1, 0.03 A W−1, and 0.01 A W−1 for wavelengths of 1.3 μm, 1.6 μm, and 2.0 μm, respectively, when the forward current density was 50 A cm−2, and the device exhibited a higher sensitivity for wavelengths greater than the cutoff wavelength.

The three-dimensional imaging of threading dislocations in GaN films was demonstrated using two-photon excitation photoluminescence. The threading dislocations were shown as dark lines. The spatial resolutions near the surface were about 0.32 and 3.2 µm for the in-plane and depth directions, respectively. The threading dislocations with a density less than 108 cm−2 were resolved, although the aberration induced by the refractive index mismatch was observed. The decrease in threading dislocation density was clearly observed by increasing the GaN film thickness. This can be considered a novel method for characterizing threading dislocations in GaN films without any destructive preparations.

Halide vapor phase epitaxy of thick GaN films was demonstrated on ScAlMgO4 (SCAM) substrates, and their self-separation was achieved. The 320-µm-thick GaN film was self-separated from the SCAM substrate during the cooling process after the growth. This separation phenomenon occurred because of both the c-plane cleavability of SCAM and the difference in the thermal-expansion coefficients between GaN and SCAM. The dark-spot densities for the GaN films on the SCAM substrates were approximately 30% lower than those on sapphire substrates. These results indicate that SCAM substrates are promising for fabricating a high-quality freestanding GaN wafer at a low cost.

We fabricated high-output-power 255 and 280 nm light-emitting diodes (LEDs) using direct bonding. The LED chips were bonded to sapphire lenses at room temperature using either atomic diffusion bonding or surface-activated bonding. The LEDs with lenses had a higher light extraction efficiency than conventionally structured LEDs. As a result, at a forward current of 350 mA, the output power of the 255 nm LED increased by a factor of 2.8, reaching 73.6 mW, while that of the 280 nm LED increased by a factor of 2.3, reaching 153 mW.

The polarization-related electric field in semipolar {1122} InGaN/GaN quantum wells (QWs) is investigated. Experimentally, the direction and magnitude of the electric field are determined by bias-dependent photoluminescence. Using both (1122) and the reverse (1122) QWs enables the electric field direction, which indicates that the polarization direction flips between the polar (0001) plane and semipolar (1122) plane, to be reliably determined. Lattice distortion due to lattice mismatch, which is the origin of the polarization effect, is theoretically assessed by the valence force field model, and the analysis supports the experimental result.

We realized room-temperature lasing of blue and green GaN-based vertical-cavity surface-emitting lasers (VCSELs), for the first time, by current injection. The blue GaN-based VCSEL had a threshold current of 1.5 mA and a threshold voltage of 3.3 V under continuous-wave operation. Its maximum output power was 0.70 mW and its laser emission wavelength was 451 nm. The green GaN-based VCSEL had a threshold current of 22 mA and a threshold voltage of 6.3 V under pulsed current operation. Its maximum output power was estimated to be over 0.80 mW and the laser emission wavelength was 503 nm.

We fabricated high-output-power 280-nm light-emitting diodes (LEDs) by employing high-crystal-quality AlN templates and optimized epitaxial structures. The emission wavelength, output power, forward voltage, spectral linewidth, and external quantum efficiency of the fabricated device measured at 20 mA were 281.0 nm, 2.45 mW, 7.53 V, 10.6 nm, and 2.78%, respectively. In the case of DC operation, the output power increased with time probably resulting from enhanced p-type activation by junction heating. We also fabricated a multi-chip device which consisted of 26 small-chip LEDs. It produced 223 mW at a pulse injection current of 1850 mA.

We succeeded in developing InGaN-based green laser diodes (LDs) with a wavelength of 515 nm under continuous-wave (cw) operation by improving the growth condition of epitaxial layers and structures of LDs. The LD structures were grown on conventional c-plane free-standing GaN substrates by metal organic chemical vapor deposition (MOCVD). The threshold current and threshold voltage at 515 nm were 53 mA and 5.2 V, respectively. The lifetime of 510–513 nm LDs was estimated to be over 5000 h under cw operation with an optical output power of 5 mW at 25 °C.

We compared the lasing characteristics of GaN-based vertical-cavity surface-emitting lasers (VCSELs) fabricated using a GaN substrate with those fabricated using a sapphire substrate. The original substrates are removed from the devices after the devices have been bonded to Si substrates. Consequently, with the exception of the cavity length, the two kinds of fabricated VCSELs have almost the same structures. The VCSELs fabricated using a GaN substrate have a higher maximum output power (0.62 mW) and longer lifetimes than those fabricated using a sapphire substrate. Even for the VCSELs fabricated with a GaN substrate, 10-min operation causes their threshold current to increase.

We report the demonstration of CW lasing at room temperature in a GaN-based vertical-cavity surface-emitting laser (VCSEL) by current injection. The active region of the VCSEL consisted of a two-pair InGaN/GaN quantum well active layer. The optical cavity consisted of a 7-λ-thick GaN semiconductor layer and an indium tin oxide layer for p-contact embedded between two SiO2/Nb2O5 dielectric distributed Bragg reflectors. The VCSEL was mounted on a Si substrate by wafer bonding and the sapphire substrate was removed by laser lift-off. Under CW operation for an 8-µm aperture device, the threshold current was 7.0 mA and the emission wavelength was approximately 414 nm.

Monolithic polychromatic light-emitting diodes (LEDs) based on micro-structured InGaN/GaN quantum wells are demonstrated. The microstructure is created through regrowth on SiO2 mask stripes along the [1100] direction and consists of (0001) and {1122} facets. The LEDs exhibit polychromatic emission, including white, due to the additive color mixture of facet-dependent emission colors. Altering the growth conditions and mask geometry easily controls the apparent emission color. Furthermore, simulations predict high light extraction efficiencies due to their three-dimensional structures. Those observations suggest that the proposed phosphor-free LEDs may lead to highly efficient solid-state lighting in which the color spectra of light sources are synthesized to satisfy specific requirements for illuminations.