Nobel Lecture: Fascinated journeys into blue light^*

Figure 1

Band structures of GaN (direct transition type) and Si (indirect transition type).

Figure 2

A schematic structure of a $p\text{−}n$ junction LED.

Figure 3

Number of publications (INSPEC) and activities related to nitrides from 1969 to 2002. All events are marked in the years when they were first achieved. Most of the important results were achieved by MOVPE using an LT-buffer layer after 1986. It is clear that the start of the steep increase in the number of publications and accomplishments is due to the key inventions (high-quality GaN, $p$- and $n$-type conductivity control, and $p\text{−}n$ junction blue/UV LED) in the late 1980s and 1990. Achievements with underlines are works done by Akasaki’s group. Green: crystal growth; blue: devices; red: conductivity control and physics.

Figure 4

A schematic and a photograph of a MIS-type GaN blue LED developed in 1978.

Figure 5

Schematic drawings of the reactor part of the MOVPE system before and after the reactor design was changed.

Figure 6

Schematics of homoepitaxy (e.g., GaN on the GaN case) and heteroepitaxy (e.g., GaN on the sapphire case).

Figure 7

A procedure with newly developed low-temperature buffer layer technology for high-quality GaN.

Figure 8

Scanning electron micrographs of GaN on sapphire (a) without the LT-buffer layer and (b) with the LT-buffer layer. (c) A photograph of specular and transparent GaN film grown on sapphire with the LT-buffer layer.

Figure 9

4.2 K photoluminescence spectra of undoped GaN films on sapphire substrates (a) without (dashed line in the inset) and (b) with (solid line) LT-buffer layers.

Figure 10

(a) Scanning electron micrographs and (b) reflection high-energy electron diffraction patterns during the GaN growth on sapphire with a LT buffer.

Figure 11

A bright-field image of GaN on sapphire with a LT-AlN buffer by cross-sectional TEM.

Figure 12

A dark-field image of GaN on sapphire with a LT-AlN buffer by cross-sectional TEM.

Figure 13

A growth model of GaN on sapphire with a LT-buffer layer. “Geometric selection” of the GaN fine crystals was suggested.

Figure 14

(a) GaN $p\text{−}n$ junction blue/UV LEDs (black dots). An electric current is being passed only through the centered LED that is emitting blue light. (b) $I\text{−}V$ characteristics of $p\text{−}n$ junction LED (left) and MIS LED (right). (c) Electroluminescence spectra of GaN $p\text{−}n$ junction blue/UV LED.

Figure 15

Electron concentrations in GaN (●) and AlGaN (○) as a function of ${\mathrm{SiH}}_4$ flow rate.

Figure 16

Chronological developments in external quantum efficiency ${\eta }_{\mathrm{ext}}$ of nitride-based blue LEDs (1971–2000).

Figure 17

Chronological developments in threshold power for stimulated emission by optical pumping (before 1995) and current injection (after late 1995) from nitrides (1971–2000).

Figure 18

77 K PL intensities of GaInN/GaN QWs as a function of the GaInN well width.

Figure 19

A stimulated emission spectrum from a GaInN/GaN QW device under pulsed current injection at room temperature.

Figure 20

Calculated longitudinal piezoelectric field in strained ${\mathrm{Ga}}_{0.9}{\mathrm{In}}_{0.1}\mathrm{N}$ on GaN as a function of the polar angle from (0001).