The Blue Laser Diode (chapter 11~15)

1. The Blue Laser Diode (chapter 11~15) Speaker:Meng-Lun Tsai National Changhua University of Education National Changhua University of Education

2. Outline • Room-Temperature Pulsed Operation of Laser Diodes • InGaN-Based Multi-Quantum-Well Laser Diodes InGaN Multi-Quantum-Well Laser Diodes with Cleaved Mirror Cavity Facets InGaN Multi-Quantum-Well Laser Diodes Grown on MgAl2O4 Substrates Optical Gain and Carrier Lifetime of InGaN Multi-Quantum-Well Laser Diodes Ridge-Geometry InGaN Multi-Quantum-Well Laser Diodes National Changhua University of Education

3. Outline • Room Temperature CW Operation of InGaN MQW LDs First Room-Temperature Continuous-Wave Operation of InGaN Multi-Quantum-Well Laser Diodes InGaN Multi-Quantum-Well Laser Diodes with Modulation-Doped Strained-Layer Superlattices cladding layers grown on the ELOG substrate National Changhua University of Education

4. InGaN-Based Multi-Quantum-Well Laser Diodes National Changhua University of Education

5. InGaN-Based Multi-Quantum-Well Laser Diodes National Changhua University of Education

6. InGaN-Based Multi-Quantum-Well Laser Diodes • GaN buffer : A 300-Å-thick GaN buffer layer grown at a low temperature of 550 °C, this layer served as a buffer layer of the thick AlGaN film growth to prevent cracking of the film. • p-type Al0.2Ga0.8N:Mg : This layer was used to prevent dissociation • of InGaN layers during the growth of the p-type layers. • The 0.1-μm-thick n-type GaN:Si and p-type GaN:Mg were light guiding layers. • The n-type Al0.15Ga0.85N:Si and p-type Al0.15Ga0.85N:Mg acted as cladding layers for confinement of the carriers and the light emitted from the active region of the InGaN MQW structure. National Changhua University of Education

7. InGaN-Based Multi-Quantum-Well Laser Diodes • It is difficult to cleave the GaN crystal grown on the c-face sapphire substrate.Therefore, reactive ion etching (RIE) was employed to form mirror cavity facets. • The surface of the p-type GaN layer was partially etched with Cl2 plasma until the n-type GaN layer was exposed in order to make a stripe LD. • The roughness of the facet surface was approximately 500 Å. • High refection facet coatings (60-70 %) were used to reduce the threshold current. • A Ni/Au contact was evaporated onto the entire area of the p-type GaN layer, and a Ti/Al contact onto the n-type GaN layer. National Changhua University of Education

8. Results and Discussion • The electrical characteristics of LDs were measured under pulsed current condition (pulse width is 2 μs, pulse period is 2 ms) at room temperature . • The output power from one facet was measured by a Si photodetector. National Changhua University of Education

9. Results and Discussion • Stimulated emission was not observed up to a current of 1.7 A. • The threshold current was about 1.7 A,which corresponded to a threshold current density of 4 KA/cm2. • A differential quantum efficiency of 13 % per facet and pulsed output power of 215 mW per facet were obtained at a current of 2.3 A. National Changhua University of Education

10. Results and Discussion • At injection currents below the threshold, spontaneous emission appeared with a FWHM of 20 nm and a peak wavelength of 410 nm. • Above the threshold current , a strong stimulated emission at 417 nm with a FWHM of 1.6 nm was the dominant emission. National Changhua University of Education

11. InGaN Multi-Quantum-Well Laser Diodes with Cleaved Mirror Cavity Facets • Sapphire substrate with (1120) orientation (a-face) has been conventionally used for GaN growth. • In the present section, LDs fabricated using wide-band-gap Ш-V nitride materials grown on a-face sapphire cleaved along (1102) (r-face) are presented. • It is difficult to cleave GaN crystal grown on c-face sapphire substrate. • Therefore they used a-face sapphire as a substrate in order to cleave the sapphire substrate along r-face. • The cleaved facets of the epitaxial layers and substrate were mirror-like. National Changhua University of Education

12. Results and Discussion • The threshold current was about 1.15 A, which corresponded to a threshold current density of 9.6 KA/cm2. • A differential quantum efficiency of 4.2 % per facet and pulsed output power of 76 mW per facet were obtained at a current of 1.5 A. • At a current of about 1.25 A, a link was observed in the light output curve. National Changhua University of Education

13. InGaN Multi-Quantum-Well Laser Diodes Grown on MgAl2O4 Substrates • In the present section, they discuss LDs fabricated onto spinel (MgAl2O4) substrate to investigate the cleaving behaviour, with the aim to improve the roughness of the laser mirror facets. • GaN growth has been reported onto spinel substrate , which has a smaller lattice mismatch (9.5 %) with respect to GaN than sapphire (13 %). • Kuramata et al. found that smooth cleaved facets of GaN grown on spinel substrates could be obtained.However, they used polished facets as a mirror cavity for LDs. • The roughness of the facet surface was approximately 50 Å after polishing. National Changhua University of Education

14. InGaN Multi-Quantum-Well Laser Diodes Grown on MgAl2O4 Substrates • The electrode size on the p-type GaN Layer was 5 μm× 500 μm ,and the electrode size on the n-type GaN layer was 80 μm× 500 μm. National Changhua University of Education

15. Results and Discussion • The threshold current was about 320 mA, which corresponded to a threshold current density of 13 KA /cm2. National Changhua University of Education

16. Results and Discussion • This demonstrates that the emission is strongly TE-polarized, and indicates the laser operation at a current above 320 mA. National Changhua University of Education

17. Results and Discussion • Typical far-field radiation patterns of the InGaN MQW laser structure in the planes parallel and perpendicular to the junction. • The beam FWHP level for the parallel • and perpendicular far-field patterns are 5° and 17°, respectively. National Changhua University of Education

18. Optical Gain and Carrier Lifetime of InGaN Multi-Quantum-Well Laser Diodes • In this section, the optical gain and carrier lifetime of InGaN MQW LDs is investigated experimentally. • The external differential quantum efficiency decreases with increasing cavity length. • The external differential quantum efficiency is given by • αi : intrinsic loss ηi :internal quantum efficiency R=30 % • αi and ηi : are calculated as 54 cm-1 and 86 %, respectively. National Changhua University of Education

19. Optical Gain and Carrier Lifetime of InGaN Multi-Quantum-Well Laser Diodes • The threshold gain Gthis given by where Γ is the confinement factor. • Γis found to be 0.7 assuming that the light wave propagates only into the 0.2 μm-thick GaN guiding layers in the structure. • Gthis calculated as 110 cm-1. National Changhua University of Education

20. Optical Gain and Carrier Lifetime of InGaN Multi-Quantum-Well Laser Diodes • The delay time of the laser emission was measured by pulsed current modulation of the LDs. • The delay time td is given by td=τsln(I/I-Ith) where τs is the minority carrier time, I is the pumping current, and Ith is the threshold current. • From the figure, τs is calculated as 2.5 ns. Also, the carrier density is given by nth=Jthτs/(ed) where nth is the carrier density at the laser threshold, Jth is the threshold current density, d is the thickness of the active layer and e is the elementary charge. • nthis calculated as 1.3 × 1019 cm3. National Changhua University of Education

21. Optical Gain and Carrier Lifetime of InGaN Multi-Quantum-Well Laser Diodes • The radiative and nonradiative recombination lifetimes are given by ηi = τs/τr , τs-1=τr-1+τnr-1 where τr is the radiative recombination lifetime of minority carriers, τnr is the nonradiative recombination lifetime of minority carries. • Using ηi = 86 % and τs= 2.5 ns, and τnr = 18 ns were obtained. National Changhua University of Education

22. Ridge-Geometry InGaN Multi-Quantum-Well Laser Diodes • For CW operation, the efficiency of the LDs must be greatly improve, and the threshold currents and voltages must be decreased. • In the present section, ridge-geometry LDs grown on a-face sapphire substrates are described which were fabricated with the aim to improve the characteristics of InGaN/GaN/AlGaN SCH LDs. • In order to make a comparison between the stripe-geometry and ridge- geometry LDs, The stripe-geometry LD was also fabricated. • The area of the stripe- and ridge-geometry LDs was 10 μm× 600 μm. National Changhua University of Education

23. Results and Discussion National Changhua University of Education

24. Results and Discussion • The threshold current of the ridge-geometry LDs was 187 mA which corresponded to a threshold current density of 3 KA/cm2. • On the other hand, the threshold current of the stripe-geometry LDs was 370 mA, which corresponded to a threshold current density of 6 KA/cm2 and the differential quantum efficiency was 5 %. • For the ridge-geometry LD, The differential quantum efficiency of 30 % per facet and pulsed output power of 3.5 mW per facet were obtained at a current of 194 mA. • For the stripe-geometry LD, The differential quantum efficiency of 5 % per facet and pulsed output power of 3.5 mW per facet were obtained at a current of 430 mA. National Changhua University of Education

25. Results and Discussion • These differences are probably due to the high lateral confinement of the light propagation and to the absence of etching damage in the gain region of the active layer due to the ridge geometry. • On the other hand, the stripe-geometry LDs were easily broken, probably because of the large heat generation due to the high operating currents and voltages. • Using ridge-geometry Ш-V nitride based LDs, the threshold current and differential quantum efficiency were greatly improved in comparison to those of strip-geometry LDs. National Changhua University of Education

26. Results and Discussion • The characteristic temperature T0, which is used to express the temperature dependence of the threshold current in the form of Ith(T)=I0exp(T/T0) • The characteristic temperature of the threshold current of the LDs was 185 K, which is higher than that (150 K) of II-VI based LDs. National Changhua University of Education

27. First Room-Temperature Continuous-Well Operation of InGaN Multi-Quantum-Well Laser Diodes National Changhua University of Education

28. First Room-Temperature Continuous-Well Operation of InGaN Multi-Quantum-Well Laser Diodes • This figure shows the result of a lifetime of RT CW-operated LDs where the operating current is shown as a function of time under a constant output power of 3 mW per facet controlled by an auto-power controller. • The operating current gradually increases due to an increase of the threshold current from the initial stage and sharply increases above 1 s. • This short lifetime is probably due to large heat generation caused by the high operating currents and voltages. National Changhua University of Education

29. InGaN Multi-Quantum-Well Laser Diodes with Modulation-Doped Strained-Layer Superlattices cladding layers grown on the ELOG substrate • It is difficult to grow the thick AlGaN cladding layer reguired for optical confinement, due to the formation of cracks in the layers during growth. • The crack are caused by the stress introduced in the AlGaN cladding layers due to lattice mismatch, and difference in the thermal expansion coefficients of the AlGaN and GaN layers. • Nakamura et al. demonstrated the InGaN MQW-structure LDs which have AlGaN/GaN modulation-doped straind-layer superlattice (MD-SLSs) within the range of critical thickness as cladding layers instead of thick AlGaN layers. • Modulation doping of the SLSs was performed to reduce the operating voltage of the LDs. • The epitaxially laterally overgrown GaN (ELOG) on sapphire was used as a substrate to reduce the number of threading dislocations of the GaN epilayer. National Changhua University of Education

30. InGaN Multi-Quantum-Well Laser Diodes with Modulation-Doped Strained-Layer Superlattices cladding layers grown on the ELOG substrate • A Al0.14Ga0.86N/GaN MD-SLS cladding layer consisting of 120 25-Å-thick Si-doped GaN separated by 25-Å-thick undoped Al0.14Ga0.86N layers. National Changhua University of Education

31. InGaN Multi-Quantum-Well Laser Diodes with Modulation-Doped Strained-Layer Superlattices cladding layers grown on the ELOG substrate • The selective growth of GaN was performed on a 2-μm-thick GaN layer grown on a (0001) C-face sapphire substrate. • The 1-μm-thick silicon dioxide (SiO2) mask was patterned to form 4-μm-wide stripe windows with a periodicity of 11 μm in the GaN (1100) direction . • After 20-μm-thick GaN layer growth on the mask pattern, the coalescence of the selectively grown GaN made it possible to achieve a flat GaN surface over the entire substrate. National Changhua University of Education

32. InGaN Multi-Quantum-Well Laser Diodes with Modulation-Doped Strained-Layer Superlattices cladding layers grown on the ELOG substrate • They were able to reduce the operating voltage using the ELOG substrate and MD-SLS, in comparison with the value for previous LDs. • InGaN Multi-Quantum-Well Laser Diodes with Modulation-Doped Strained-Layer Superlattices cladding layers grown on the ELOG substrate were demonstrated to have an estimated of more than 10000 h under CW operation at 20 °C. National Changhua University of Education

33. InGaN Multi-Quantum-Well Laser Diodes with Modulation-Doped Strained-Layer Superlattices cladding layers grown on the ELOG substrate • The InGaN MWQ-structure LD was modulated using a pulsed current (pulse width: 15.5 ns, pulse duty ratio: 1 %) • Form the measurement of the delay time of the laser emission as a function of the current intensity, the carrier lifetime (τn ) was estimated to be 1.8 ns using the formula td=τnln(I/I-Ith). • The self-pulsation is due to a saturable absorber in the waveguide region of the LDs. • The n-type In0.14Ga0.86N layer (compliance layer) just beneath the n-type Al0.14Ga0.86N/GaN MD-SLS layer probably function as a saturable absorber. National Changhua University of Education

34. InGaN Multi-Quantum-Well Laser Diodes with Modulation-Doped Strained-Layer Superlattices cladding layers grown on the ELOG substrate National Changhua University of Education

35. Result Violet-laser recorder comes to market National Changhua University of Education

36. Result • Sony has announced plans to start sales of the world's first DVD recorder that uses a violet laser light source, according to Reuters. The recorder will use the format developed by the Blu-Ray consortium, of which Sony is a founder member. • The new machine will be available in Japan from mid-April, priced at 450 000 yen (about $3800). Conventional DVD recorders using red lasers are priced at 50 000-70 000 yen ($420-590). National Changhua University of Education