光電半導體元件與 高功率雷射系統之研究

1. 光電半導體元件與高功率雷射系統之研究 陳秀芬 楊勝州 屠嫚琳 吳佩璇 謝尚衛

2. Outline • Experimental and numerical investigation of 590-nm AlGaInP light emitting diodes and vertical-cavity surface-emitting lasers (陳秀芬) • 紫光氮化銦鎵面射型半導體雷射之設計與分析 (楊勝州) • Numerical study on the optical properties of a 655-nm vertical-cavity surface-emitting laser for PMMA-d8 optical fiber communication (屠嫚琳) • Investigation of the characteristics of electronic overflow in 415-nm violet InGaN semiconductor laser (吳佩璇) • Introduction of passive Q-switching theory and tunable Cr:YSO Q-switched Cr:LiSAF laser (謝尚衛) 光電半導體專題研究(91學年度)

3. Experimental and numerical investigation of 590-nm AlGaInP light emitting diodes and vertical-cavity surface-emitting lasers 指導教授：郭艷光 博士 報告者：陳秀芬 班級:物理系四年乙班

4. Abstract • We experimentally investigated the optical properties of several 590-nm AlGaInP light emitting diodes of different structures with a photoluminescence measurement system. • The optical properties of the 590-nm AlGaInP multiple quantum well structures are also numerically investigated with the PICS3D simulation program. • In the mean time, the optical characteristics of the 590-nm AlGaInP vertical-cavity surface-emitting lasers are numerically evaluated with the PICS3D simulation program. 光電半導體專題研究(91學年度)

5. PL spectrum of 590-nm LEDs at room temperature The intensity of Sample A is much lower than that of Sample B. Both of their wavelength is about 590 nm. The efficiency of Sample C is worse, and its wavelength is about 605 nm. 光電半導體專題研究(91學年度)

6. PL spectrum of Sample B at different temperatures It’s obvious that the central wavelength has “blue shift” when the temperature decreases. In the mean time, the intensity becomes higher, and FWHM (full width at half maximum) becomes smaller. 光電半導體專題研究(91學年度)

7. Wavelength and intensity of Sample B as a function of temperature When temperature decreases, the central wavelength becomes shorter and the intensity becomes higher. 光電半導體專題研究(91學年度)

8. Wavelength and intensity of Sample B as a function of input power If we increase the input power at room temperature, the central wavelength does not change obviously but the intensity increases accordingly. 光電半導體專題研究(91學年度)

9. Comparison between experiment and simulation We used the PICS3D program to simulate the 590-nm active region. The results obtained from simulation are in good agreement with those obtained experimentally. 光電半導體專題研究(91學年度)

10. Initial structure of 590-nm VCSEL The material of active region is Al0.095Ga0.46In0.445P (5 nm) / Al0.25Ga0.25In0.5P (10 nm) , which has a strain of about 0.27％. 光電半導體專題研究(91學年度)

11. L-I curves of different well materials The Al0.13Ga0.37In0.5P (5 nm) / Al0.25Ga0.25In0.5P (10 nm) is used as the material of the unstrained active region of the 590-nm VCSEL. The VCSEL with strained active region has lower threshold current and better laser efficiency. 光電半導體專題研究(91學年度)

12. 紫光氮化銦鎵面射型半導體雷射之設計與分析 指導教授：郭艷光 博士 報告者：楊勝州 班級:物理系四年甲班

13. Introduction • 用一套由加拿大Crosslight 公司所提供的商用套裝模擬軟體叫 PICS3D模擬軟體來做模擬。 • 由於氮化物剛在發展階段，其特性不是很明確，所以我就設計一個由電激發出紫光的氮化銦鎵面射型半導體雷射來探討它的發光特性及雷射輸出效能。 • 參考文章把發光波長設計在408 nm。 光電半導體專題研究(91學年度)

14. 初始元件結構圖 活性層量子井材料在井(well)部分為3 nm的In0.087Ga0.913N，井障(barrier)為4 nm的In0.001Ga0.999N，spacer厚度為0.15 mm的GaN ，DBR的材料成分設計為GaN與Al0.27Ga0.73N，n-type的DBR對數為60對，其反射率為99.6 ％ ，p-type的DBR對數為50對，摻雜濃度同樣為1x1018 cm-3，每隔20對DBR增加一層氮化鎵薄膜為緩衝層以緩衝應力，最後把整個元件半徑設計為10 mm。 光電半導體專題研究(91學年度)

15. 發光功率與電流關係圖 當元件結構設計為兩個量子井且共振腔長度為一個波長時其雷射光輸出的臨界電流約為15.5 mA左右。 光電半導體專題研究(91學年度)

16. 不同n-DBR對數的反射率頻譜圖 由上圖中可看出當我們把n-DBR對數設計為50對時，其雷射波長(408 nm)的最大反射率為98.8％，若增加到60對時其反射率增加到99.6％，且隨著DBR對數的增加，反射率所含蓋的頻譜範圍小。 光電半導體專題研究(91學年度)

17. 溫度與發光波長關係圖 由圖中可得知：當溫度為20 K時，其發光波長約在397.8 nm左右， 也就是說隨著溫度的降低，整個元件的發光波長會往短波長移動，會有藍位移的現象發生。 光電半導體專題研究(91學年度)

18. 不同元件半徑下的雷射光輸出功率圖 當元件半徑設計為6 mm時，有最低的臨界電流約4 mA，隨著元件半徑的增加，臨界電流也跟著增加，且發光效率變差。 光電半導體專題研究(91學年度)

19. Numerical study on the optical properties of a 655-nm vertical-cavity surface-emitting laser for PMMA-d8 optical fiber communication 指導教授：郭艷光 博士 報告者:屠嫚琳 班級:工教系四年乙班

20. Introduction • In this research, I used commercial simulation software PICS3D of Canada Crosslight incorporation. • It can be used to design device structure and material content of 655-nm vertical-cavity surface-emitting laser (VCSEL), so we can obtain all kinds of optic and electronic characteristics. 光電半導體專題研究(91學年度)

21. The absorption spectrum of PMMA-d8 (polymethylmethacrylate-d8) plastic optical fiber Between 655 and 700 nm, there is a relatively low-loss window, which is about 18 dB/km. 光電半導體專題研究(91學年度)

22. One column and current spreading structures One column structure of 655-nm red VCSEL Current spreading structure of 655-nm red VCSEL 光電半導體專題研究(91學年度)

23. L-I curves of one-column structure The threshold current is about 13.9 mA when radius = 10 mm, and 3.5 mA when radius = 5 mm. The simulation results indicate that the VCSEL of a smaller radius has a lower threshold current. 光電半導體專題研究(91學年度)

24. Comparison between one column and current spreading structure The threshold current is about 3.5 mA for one column structure, and 1.6 mA for current spreading structure. The simulation results indicate that the current spreading structure has a better laser performance than that of the one column structure. 光電半導體專題研究(91學年度)

25. Main-side mode suppression ratio (dB) When the operation current is 0.8 mA (below the threshold current), the main-side mode suppression ratio is about 11 dB. When the operation current is increased to 2 mA (above the threshold current), the main-side mode suppression ratio is increased to about 39 dB. 光電半導體專題研究(91學年度)

26. Laser performance as a function of temperature Simulation results show that when the temperature is increased, the laser performance deteriorates accordingly. 光電半導體專題研究(91學年度)

27. Characteristic temperature of the VCSEL Generally, for a semiconductor laser the relation between threshold current and temperature can be characterized by I = I0e(T/To) mA. The semiconductor laser is less sensitive to temperature when the characteristic temperature, T0, is high. If we best-fit the data shown in the above figure with this equation, a characteristic temperature of about 137K is obtained. 光電半導體專題研究(91學年度)

28. Investigation of the characteristics of electronic overflow in 415-nm violet InGaN semiconductor laser Adviser: Yen-Kuang Kuo Reporter: Pei-hsuan Wu Class: Physics 4B

29. Introduction • The InGaN semiconductor materials have played an important role in the short wavelength laser diodes (LD) and light emitting diodes (LED) owing to the relatively wide bandgap (1.95 ~ 6.2 eV) and high emission efficiency. • The reasons why nitride-based semiconductor lasers have high threshold currents include: (1) the high density of crystal defects caused by the lack of a lattice-matched substrate, (2) the high resistance of p-type layers. 光電半導體專題研究(91學年度)

30. Cap：GaN (0.1 m, p doping=51017 cm-3) Cladding：GaN (0.02 m, p doping=51017 cm-3) Barrier：In0.04Ga0.96N Well：In0.105Ga0.895N Barrier：In0.04Ga0.96N Well：In0.105Ga0.895N Barrier：In0.04Ga0.96N Cladding ：GaN (0.25 m, n doping=11018 cm-3) Buffer：GaN (0.4 m, n doping=11018 cm-3) Initial numerical structure of InGaN DQW laser p-contact The active region is assumed to be 20 mm in width and 600 mm in length. The reflectivities of the two end mirrors are assumed to be 80% and 90% respectively. n-contact 光電半導體專題研究(91學年度)

31. Issues of investigation • Improve the laser output efficiency (1) Change the number of QW • Improve the electronic overflow (1) Use the blocking layer (AlGaN) (2) Change the aluminum composition (3) Increase the p-doping level 光電半導體專題研究(91學年度)

32. L-I curves of the InGaN laser structures ofdifferent quantum well numbers The threshold current decreases and slope efficiency increases when the number of quantum wells increases. Laser Power (mW) 光電半導體專題研究(91學年度)

33. L-I curves with different p-AlxGa1-xN blocking layers in the InGaN laser structure When the aluminum composition in the AlxGa1-xN blocking layer, x, increases the threshold currents decreases and the laser performance increases accordingly. 光電半導體專題研究(91學年度)

34. Using the blocking layer to improve the electronic overflow Overflow current density and active-layer current densityas a function of total current density for (a) initial structure, and (b) improved structure with a blocking layer. (a) (b) 光電半導體專題研究(91學年度)

35. Increasing the p-doping level to improve the overflow current P-doping:5× 1017cm-3 P-doping:1× 1018cm-3 P-doping:5× 1018cm-3 P-doping:1× 1019cm-3 The overflow current is reduced with increased p-doping level. We also notice that the laser performance is improved (i.e., the threshold current is lower and the slope efficiency is higher) with increased p-doping level. 光電半導體專題研究(91學年度)

36. Conclusion • We use the Lastip numerical simulation to investigate the current overflow of InGaN DQW structure. The simulation results indicate that it is possible to improve the electronic current overflow by increasing the doping level of the p-type epi-layers and adding an AlGaN blocking layer in the p-type region. • Simulation results suggest that the improved laser structure has a lower threshold current and a higher slope efficiency. 光電半導體專題研究(91學年度)

37. Introduction of passive Q-switching theory and tunable Cr:YSO Q-switchedCr:LiSAF laser 指導教授：郭艷光 博士 報告者:謝尚衛 班級:物理系四年甲班

38. Introduction of Cr:LiSAF laser • After the invention of Cr:LiCAF laser in 1988 (tunable from725nm to840nm and peaked near780nm), Steven Payne invented the tunable Cr:LiSAF solid-state laser in 1989. • Cr:LiSAF laser has a long tuning range between780and 920nm and is peaked at830nm. • The lifetime of Cr:LiSAF is about67ms that is much longer than that of Ti:sapphire laser (3.2 ms). Hence, Cr:LiSAF laser can be pumped efficiently by flashlamps. 光電半導體專題研究(91學年度)

39. The Cr:YSO Saturable Absorber Deka et al. made the first spectral analysis of Cr:YSO saturable absorber in 1992. From many other papers, it has been demonstrated that Cr:YSO is an efficient saturable absorber for ruby, Cr:LiSAF, and Ti:sapphire lasers. Therefore, we expect that we can get high-power laser output from the Cr:LiSAF laser with Cr:YSO saturable absorber. 光電半導體專題研究(91學年度)

40. Definition of quality factor, Q • In an optical resonatorthe quality factor Q is defined as the ratio of the energy stored in the laser cavity to the energy loss per cycle. Therefore, the quality factor of a laser resonator can be altered by varying the cavity loss. 光電半導體專題研究(91學年度)

41. The three coupled rate equations for Cr:LiSAFpassive Q switching simulation In these three equations,nis the photon number in the laser cavity;Ngis the population inversion andNais the ground state electron numbers of the saturable absorber. 光電半導體專題研究(91學年度)

42. Level 2 Level 3 (Upper Laser Level) Emit Light Level 4 (Lower Laser Level) Four- and three-level lasers Fast • The dynamics of most lasers, including the Cr:LiSAF laser, can be described by the four energy levels shown above. These lasers are called "4-level lasers" that usually have good laser efficiency. • For some lasers (e.g., ruby) the lower laser level is in fact the ground state. These lasers are called "3-level lasers" that usually have relatively poor laser efficiency. Pumping Fast Level 1 (Ground State) 光電半導體專題研究(91學年度)

43. Pumping System Saturable Absorber Laser Output Active Medium Mirror (Output Coupler) Mirror (Total Reflector) The structure diagram ofpassive Q switching 光電半導體專題研究(91學年度)

44. Output energy and pulsewidth as a function of pumping rate (the wavelength is at 880 nm) This figure shows that higher output energy and narrower pulsewidth may be obtained at higher pumping level. 光電半導體專題研究(91學年度)

45. Output energy and pulsewidth as a function of reflectivity of output coupler Output Energy (mJ) Pulsewidth (ns) Reflectivity of output coupler Simulation results indicate that high output energy and narrow pulsewidth may be obtained when the reflectivity of output coupler is low. 光電半導體專題研究(91學年度)

46. Output energy as a function of ground-state electron number of saturable absorber Na0(1015) From the figure we note that the output energy increases when the ground-state electron number of saturable absorber increases, and when the wavelength of the laser decreases. 光電半導體專題研究(91學年度)

47. Conclusion • In conclusion, we have experimentally and numerically investigated the optical properties of 590-nm AlGaInP LEDs and vertical-cavity surface-emitting lasers (VCSELs). • We have designed a VCSEL that has good laser performance in violet spectral region. • We have designed a 655-nm VCSEL for PMMA-d8 optical fiber communication. • We have designed a violet InGaN edge-emitting laser diode for application in optical data storage system. • We have numerically investigated the passive Q-switching performance of a tunable Cr:YSO Q-switched Cr:LiSAF laser. • All of the above research results have been published in the Taiwan optoelectronic and physical annual meetings, and the special issue in 2003 optical fiber communication of the Journal of Optical Engineering (Taipei). 光電半導體專題研究(91學年度)

48. Thank you for your attention!