Kwon-Young Choi

High-power GaN-based blue-violet laser diodes with AlGaN∕GaN multiquantum barriers

AlGaN∕GaN multiquantum barriers (MQBs) were introduced into violet AlInGaN laser diodes with an InGaN multiquantum-well structure, resulting in drastic improvements in lasing performance. Comparing with conventional AlGaN single electron blocking layer (EBL), lower threshold current of 32mA and higher slope efficiency of 1.12W∕A at room temperature has been achieved by using the AlGaN∕GaN multiquantum barrier. This improvement implies that p-type AlGaN∕GaN MQBs are more effective in suppressing the overflow of electrons than p-type AlGaN single EBL. Effective barrier heights of the MQBs should be higher than the single EBL due to the quantum effect of MQBs and the enhancement of p-type doping efficiency. Additionally, the effect of strain on InGaN multiquantum wells from the single EBL can be reduced by using the AlGaN∕GaN MQBs structure.

Highly stable temperature characteristics of InGaN blue laser diodes

We report stable temperature characteristics of threshold current and output power in InGaN blue laser diodes emitting around 450nm. The threshold current is changed by <3mA in operation temperature range from 20to80°C, and even negative characteristic temperature is observed in a certain temperature range. This peculiar temperature characteristic is attributed to originate from unique carrier transport properties of InGaN quantum wells with high In composition, which is deduced from the simulation of carrier density and optical gain. In addition, slope efficiency is also maintained well and wall plug efficiency is even improved as temperature increases.

Single-mode blue-violet laser diodes with low beam divergence and high COD level

We demonstrate GaN-based high-power single transverse-mode laser diodes (LDs) emitting at 405 nm. LD structures are designed to exhibit a high level of catastrophic optical damage and small beam divergence angle. By the control of refractive index profiles, we achieved a vertical beam divergence angle of as low as 17.5/spl deg/ and maximum output power of as high as 470 mW under continuous-wave operation condition. In addition, nearly fundamental transverse-mode operation is demonstrated up to 500-mW pulsed output power by far-field investigation.

High power AlInGaN-based blue-violet laser diodes

High power and high efficiency AlInGaN-based laser diodes with 405 nm were fabricated for the post-DVD applications. Magnesium doped AlGaN/GaN multiple quantum barrier (MQB) layers were introduced into the laser diode structure, which resulted in considerable improvement in lasing performances such as threshold current and slope efficiency. Asymmetric waveguide structure was used in order to improve the characteristics of laser diodes. Aluminum content in the n-cladding layer was varied in connection with the vertical beam divergence angle and COD level. By decreasing Al content in the n-cladding layer, the vertical divergence angle was reduced to 17 degree and the COD level was enhanced to over 300mW. The maximum output power reached as high as 470 mW, the highest value ever reported for the narrow-stripe GaN LDs. In addition, the fundamental transverse-mode operation was clearly demonstrated up to 500 mW-pulsed output power.

A new bottom-gated poly-Si thin-film transistor

We have proposed and fabricated the new bottom-gated poly-Si TFT with a partial amorphous-Si (a-Si) region by employing the selective laser annealing. The channel layer of the proposed TFTs is composed of poly-Si region in the center and a-Si region in the edge. The TEM image shows that the local a-Si region is successfully fabricated by the effective cut out of the incident laser light in the upper a-Si layer. Our experimental results show that the reverse leakage currents are decreased significantly in the new poly-Si TFT compared with conventional one. This reduction is due to the suppression of field emission currents by local a-Si region like that of a-Si TFTs while the ON currents are kept almost the same due to the considerable inducement of electron carriers in the short a-Si channel by the positive gate bias.We have proposed and fabricated the new bottom-gated poly-Si TFT with a partial amorphous-Si (a-Si) region by employing the selective laser annealing. The channel layer of the proposed TFTs is composed of poly-Si region in the center and a-Si region in the edge. The TEM image shows that the local a-Si region is successfully fabricated by the effective cut out of the incident laser light in the upper a-Si layer. Our experimental results show that the reverse leakage currents are decreased significantly in the new poly-Si TFT compared with conventional one. This reduction is due to the suppression of field emission currents by local a-Si region like that of a-Si TFTs while the ON currents are kept almost the same due to the considerable inducement of electron carriers in the short a-Si channel by the positive gate bias.

The grain growth blocking effect of polycrystalline silicon film by thin native silicon oxide barrier during the excimer laser recrystallization

A method to control the recrystallization depth of amorphous silicon (a-Si) film during the excimer laser annealing is presented. By irradiating the XeCl excimer laser on the triple film structure of a-Si/thin native silicon oxide (∼20 A)/thick a-Si layer, only the upper a-Si film is recrystallized, whereas the lower thick a-Si film remains amorphous. The thin native silicon oxide layer blocks the grain growth and prevents the upper grains from growing into the lower a-Si. As a result, the thin oxide film sharply defines the boundary between polycrystalline silicon (poly-Si) and the a-Si layer. The poly-Si/a-Si double layer is useful for the fabrication of high-performance poly-Si thin film transistors.

Recent progress of high-power InGaN blue-violet laser diodes

We report on the development of GaN-based violet laser diodes (LDs) for the high-capacity optical storage application and blue LDs for the laser projection display application. InGaN LDs with emission wavelength of ~405 nm are already being adopted for next-generation optical-storage systems. We present results on >400 mW single-mode output power under pulsed operation which can be employed in 100 Gbyte multi-layer BD systems. We designed LD layer structures to exhibit high level of catastrophic optical damage (COD) and small beam divergence. In addition, GaN-based blue LDs with emission wavelength of ~450 nm have also been developed for the application to the blue light sources of laser display systems. We demonstrate single-mode blue InGaN LDs with >100 mW CW output power. Interestingly, we observed anomalous temperature characteristics from the blue InGaN LDs, which has shown highly-stable temperature dependence of output power or even negative characteristic temperature (T0) in a certain operation temperature range. This unusual temperature characteristic is attributed to originate from unique carrier transport properties of InGaN QWs with high In composition, which is deduced from the simulation of carrier density and optical gain.

High-power blue-violet laser diodes with improved beam divergence and high COD level

GaN-based laser diodes for 405 nm high-power applications are demonstrated. By decreasing the Al concentration of n-cladding layers, the vertical divergence angle was reduced to <18/spl deg/ and the average COD level was increase to >300 mW.

The Recrystallization Depth Control of the Excimer-Laser-Recrystallized Poly-Crystalline Silicon Film

A novel method to control the recrystallization depth of amorphous silicon (a-Si) film during the excimer laser annealing (ELA) is proposed in order to preserve a-Si that is useful for fabrication of poly-Si TFT with a-Si offset in the channel. A XeCl excimer laser beam is irradiated on a triple film structure of a-Si thin native silicon oxide (~20A)/thick a-Si layer. Only the upper a-Si film is recrystallized by the laser beam irradiation, whereas the lower thick a-Si film remains amorphous because the thin native silicon oxide layer stops the grain growth of the poly-crystalline silicon (poly-Si). So that the thin oxide film sharply divides the upper poly-Si from the lower a-Si.