By doping an organic compound functioning as an electron donor (hereinafter referred to as donor molecules) into an organic compound layer contacting a cathode, donor levels can be formed between respective LUMO (lowest unoccupied molecular orbital) levels between the cathode and the organic compound layer, and therefore electrons can be injected from the cathode, and transmission of the injected electrons can be performed with good efficiency. Further, there are no problems such as excessive energy loss, deterioration of the organic compound layer itself, and the like accompanying electron movement, and therefore an increase in the electron injecting characteristics and a decrease in the driver voltage can both be achieved without depending on the work function of the cathode material.

Light Emitting Device

We report on the optical properties of the InGaN-based red LED grown on a c-plane sapphire substrate. Blue emission due to phase separation was successfully reduced in the red LED with an active layer consisting of 4-period InGaN multiple quantum wells embedding an AlGaN interlayer with the Al content of 90% on each quantum well. The light output power and external quantum efficiency at a dc current of 20 mA were 1.1 mW and 2.9% with the wavelength of 629 nm, respectively. This is the first demonstration of a nitride-based red LED with the light output power exceeding 1 mW at 20 mA.

https://iopscience.iop.org/article/10.7567/APEX.7.071003/pdf

Development of InGaN-based red LED grown on (0001) polar surface

The present disclosure relates to a semiconductor light-emitting element, which comprises: a first light-emitting part, a second light-emitting part, and a third light-emitting part, wherein each of the light-emitting parts includes multiple semiconductor layers including a first semiconductor layer, an active layer, and a second semiconductor layer, laminated in sequence, the first semiconductor layer having a first conductivity, the active layer generating light through the recombination of an electron and a hole, and the second semiconductor layer having a second conductivity different from the first conductivity; a non-conductive reflecting film which is formed to cover the multiple semiconductor layers and reflects light generated from the active layer; a first electrode which is provided to electrically communicate with the first semiconductor layer of the first light-emitting part and which supplies one of an electron and a hole; a second electrode which is provided to electrically communicate with the second semiconductor layer of the second light-emitting part and which supplies the other of the electron and the hole; and an auxiliary pad formed on the non-conductive reflecting film which covers the third light-emitting part.

Semiconductor light emitting element

In this paper, we review the recent developments (in years 2010–2011) of energy-saving solid-state lighting The industry of white light-emitting diodes (LEDs) has made significant progress, and today, white LED market is increasing (mostly with increasing LED screen and LED TV sales) The so-called “lighting revolution” has not yet really happened on a wide scale because of the lighting efficiency at a given ownership cost Nevertheless, the rapid development of the white LEDs is expected to soon trigger and expand the revolution

Advances in the LED Materials and Architectures for Energy-Saving Solid-State Lighting Toward “Lighting Revolution”

We fabricated two types of high-efficiency white light-emitting diodes (LEDs) composed of an InGaN multi-quantum well (MQW), which emit light of two or three different colors without phosphors. The Type-1 white LED emits light of two colors (blue and yellow) from the MQW active layer, while the Type-2 LED emits light of three colors (blue, green and red). When the Type-1 white LED was operated at a forward current of 20 mA at room temperature, the color temperature (Tcp), average color rendering (Ra) and luminous efficiency were 7600 K, 42.7 and 11.04 lm/W, respectively. When the Type-2 white LED was operated at a forward current of 20 mA at room temperature, Tcp, Ra and luminous efficiency were 5060 K, 80.2 and 7.94 lm/W, respectively.

Phosphor Free High-Luminous-Efficiency White Light-Emitting Diodes Composed of InGaN Multi-Quantum Well : Optics and Quantum Electronics

In order to enhance light output power of green light-emitting diodes (LEDs) on a sapphire (0001) substrate, we employ 1.5 nm-thick AlGaN interlayer between an InGaN well layer and an upper GaN barrier layer. AlGaN interlayer controls asymmetric band profile due to piezoelectric field, and thus it controls the quantum-confined Stark effect. Although both photoluminescence intensity and electroluminescence intensity of a conventional MQW decreases drastically as the emission wavelength becomes longer, the intensity drop at spectral range of 530–580 nm is suppressed by utilizing an AlGaN interlayer. The maximum light output power of 12 mW at 532 nm and external quantum efficiency of 25.4% have been achieved for the LED employing Al0.30Ga0.70N interlayer at a driving current of 20 mA. This result indicates that the band-engineering approach is effective for an IQE enhancement at pure green or longer wavelength, even if a polar (0001) face is utilized.

Enhanced light output power of green LEDs employing AlGaN interlayer in InGaN/GaN MQW structure on sapphire (0001) substrate

According to one embodiment, a nitride semiconductor device includes a foundation layer and a functional layer. The foundation layer is formed on an Al-containing nitride semiconductor layer formed on a silicon substrate. The foundation layer has a thickness not less than 1 micrometer and including GaN. The functional layer is provided on the foundation layer. The functional layer includes a first semiconductor layer. The first semiconductor layer has an impurity concentration higher than an impurity concentration in the foundation layer and includes GaN of a first conductivity type.

Nitride semiconductor device, nitride semiconductor wafer, and method for manufacturing nitride semiconductor layer

We have demonstrated low-threading-dislocation-density (low-TDD) GaN grown on Si substrate using the technique of multiple modulation of dislocation behavior that combines GaN island formation on SiN interlayer with recoating GaN island surface with SiN cap layer. TEM analysis showed the dislocation bending and/or blocking is brought about at the surface of GaN islands by recoating GaN island surface with SiN cap layer. TDD was found to be as low as 2.6×108/cm2 with FWHMs of 289 and 256 arcsec for (0002) and (10–12) X-ray rocking curves (XRCs), respectively, which is a value comparable to that for GaN grown on a sapphire substrate. By reducing TDD, the light output power at 20 mA (12.5 A/cm2) was enhanced by a factor of 1.5 and the maximum EQE increased up to 59% for the blue LED. This result indicates that the reduction of TDD is an effective approach for obtaining the high-efficiency GaN-on-Si-based LED. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reduction of threading dislocation by recoating GaN island surface with SiN for high-efficiency GaN-on-Si-based LED

According to one embodiment, a nitride semiconductor wafer includes a silicon substrate, a lower strain relaxation layer provided on the silicon substrate, an intermediate layer provided on the lower strain relaxation layer, an upper strain relaxation layer provided on the intermediate layer, and a functional layer provided on the upper strain relaxation layer. The intermediate layer includes a first lower layer, a first doped layer provided on the first lower layer, and a first upper layer provided on the first doped layer. The first doped layer has a lattice constant larger than or equal to that of the first lower layer and contains an impurity of 1×1018 cm−3 or more and less than 1×1021 cm−3. The first upper layer has a lattice constant larger than or equal to that of the first doped layer and larger than that of the first lower layer.

Nitride semiconductor wafer, nitride semiconductor device, and method for growing nitride semiconductor crystal

The influence of InGaN growth conditions on the structural instability of green light-emitting InGaN quantum wells (QWs) is investigated. The multiple quantum wells (MQWs) were grown by metal-organic chemical vapor deposition (MOCVD), varying the growth rate of InGaN well layers. It is found that the structural deterioration of MQWs attributable to high-temperature post-growth of GaN on MQWs is suppressed by decreasing the growth rate. Atomic force microscope measurement reveals the reduction of In-rich clusters that are fragile under high-temperature process on the surface of MQWs grown at low growth rates. It is demonstrated that the suppression of the formation of these In-rich clusters at the lower growth rates lead to the improvement of the thermal stability and the enhancement of optical properties of green light-emitting diodes. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Naoharu Sugiyama

Papers

Enhanced light output power of green LEDs employing AlGaN interlayer in InGaN/GaN MQW structure on sapphire (0001) substrate

Nitride semiconductor device, nitride semiconductor wafer, and method for manufacturing nitride semiconductor layer

Reduction of threading dislocation by recoating GaN island surface with SiN for high-efficiency GaN-on-Si-based LED

Nitride semiconductor wafer, nitride semiconductor device, and method for growing nitride semiconductor crystal

Impact of InGaN growth conditions on structural stability under high temperature process in InGaN/GaN multiple quantum wells