| Summary: | 博士 === 國立交通大學 === 光電工程系所 === 96 === In this dissertation, the improvement in operation performance of III-V optoelectronic semiconductor light emitting devices, which include ultraviolet (UV) light-emitting diode (LED), 660-nm red resonant-cavity LED, 850-nm vertical-cavity surface-emitting laser (VCSEL), and 1.3-um edge-emitting laser (EEL), were studied. The key technologies for semiconductor light emitting devices to possess better output performance and high operation stabilities are the epitaxial crystal quality and the design of epitaxial structure. Noteworthily, the structure design is more important if we were to have a stable output performance in high temperature and high injection current operation. By using the epitaxial technology – metalorganic chemical vapor deposition (MOCVD) to grow the structures and advanced simulation programs to give theoretical analysis, the operation performances of semiconductor light emitting devices are investigated and improved. Mainly, we focus on confining the electrons effectively in the quantum well (QW) active region to reduce the electronic leakage current so as to improve the output performances.
In the research of UV LED, to emit an emission wavelength of 370 nm, quaternary AlGaInN is utilized as QW material during the epitaxy of UV LED. The device after standard process as 300×300 um2 size chip can provide a maximal output power of 4 mW and an external quantum efficiency of 1.2%. With an aim to enhance the output power of UV LED, we then theoretically investigate the effect of the number of QWs and the aluminum content in AlGaN electron-blocking layer on the UV LED output performance. After fitting in with the experimentally demonstrated output performance of UV LED, we find that the UV LED can provide a better output performance when the aluminum content in AlGaN electron-blocking layer is in a range of 19–21% and the AlGaInN QW number is in a range of 5–7.
In the research of 650-nm RCLED, it is known that the conduction band offset value in AlGaInP material QW active region is approximately 300 meV. When the device is under high temperature operation, the electron leakage problem may become more serious and consequently leading to the degradation of output performance. By means of widening the resonant cavity to a thickness of three wavelength (3 lambda), the degree of power variation between 25 and 95 ºC for the device biased at 20 mA is apparently reduced from -2.1 dB for the standard structure design (1-lambda�ncavity) to -0.6 dB. The current dependent far field patterns also show that the emission always takes place perfectly in the normal direction, which is suitable for plastic fiber data transmission. To optimize the RCLED structure, we continue numerically studying the structure dependent output performance by using an advanced simulation program. After fitting in with the experimentally demonstrated output performances of RCLEDs, we analyze the percentage of electron leakage current of the two structures, and we find that the stable temperature dependent output performance of 3-lambda-cavity RCLED is attributed to the reduction of electron leakage current.
In the research of 850-nm VCSEL, we first theoretically investigate the gain-carrier characteristics of In0.02Ga0.98As and InAlGaAs QWs of variant In and Al compositions. More compressive strain, caused by higher In and Al compositions in InAlGaAs QW, is found to provide higher material gain, lower transparency carrier concentration and transparency radiative current density over a temperature range of 25−95 ºC. Then we choose Al0.08Ga0.77In0.15As as QW material in the epitaxy of 850-nm VCSEL structure. After standard oxidation confinement process, this device can provide a threshold current of 1.47 mA with a slope efficiency of 0.37 mW/mA at 25 ºC, and the threshold current increases to 2.17 mA with a slope efficiency reduction of 32% when the device temperature is raised to 95 °C. To improve the operation performance of 850-nm VCSEL, a 10-nm-thick Al0.75Ga0.25As electron-blocking layer is employed in the QW active region for the first time, and the threshold current at 25 ºC is found reducing to 1.33 mA with an increment of slope efficiency to 0.53 mW/mA. When the device temperature raises to 95 °C, the threshold current increases by only 0.27 mA and the slope efficiency drops by only 24.5%. Numerical simulation is also done to analyze the effect of the electron-blocking layer on the output performance of 850-nm VCSEL, and the results show that the output performance is improved by the reduction of electron leakage current.
In the research of 1.3-um InGaAsN/GaAsN EEL, there has been several works investigating using strain GaAsN as direct barrier. Using GaAsN in the epitaxial growth can balance the highly compressive strain in InGaAsN QW and reduce the phenomenon of nitrogen out-diffusion from the well. However, it is a small bandgap material system, which indicates that the electron leakage may become more serious if adding more nitrogen into GaAsN barrier. Therefore, in the first instance, the temperature effects on the optical gain properties of single In0.4Ga0.6As0.986N0.014 QW with GaAsN barrier of different nitrogen compositions are studied for optimization. Theoretically, we suggest using GaAs0.995N0.005 as direct barrier can be a better choice with the considerations of epitaxial growth and electron confinement. Then we choose In0.4Ga0.6As0.986N0.014/GaAs0.995N0.005 as active region in the epitaxial growth of 1.3-um EEL structure. After standard process as 4×100 um2 size chip, this device can provide a threshold current of 84 mA with a slope efficiency of 0.09 mW/mA. When the device temperature increases to 105 °C, the threshold current becomes 188 mA and a characteristic temperature value (T0) of 118 K is obtained. To improve the operation performance of 1.3-um In0.4Ga0.6As0.986N0.014/GaAs0.995N0.005 EEL, we also try to insert a GaAs0.9P0.1 layer as electron blocking layer in the epitaxial growth of 1.3-um EEL. The threshold current of the device at 25 °C becomes 99 mA and a slope efficiency of 0.11 mW/mA is obtained. The threshold current at 105 °C only increases to 172 mA with a T0 value of 155 K and the reduction of slope efficiency becomes less. Numerical simulation is done to analyze the effect of the electron blocking layer on the output performance of 1.3-um EEL. The results show that the electron leakage current is reduced with the use of a high-bandgap GaAs0.9P0.1 layer. Further theoretical simulation work of investigating the effect of increasing the phosphide composition in GaAsP electron-blocking layer on the T0 value is also done. And, we find that increasing the phosphide composition in GaAsP to 15–20% can provide a better T0 value.
In a summary, the III-V optoelectronic semiconductor light emitting devices, which include 370-nm UV LED, 660-nm RCLED, 850-nm VCSEL, and 1.3-um EEL, are experimentally demonstrated and theoretically analyzed for a purpose of reducing the electron leakage current and thus improving the operation performance. We hope those all will turn into useful information in the design and epitaxy of optoelectronic semiconductor light emitting devices.
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