Laser diode structures were grown on (0001) Ga-polarity, conductive GaN substrates with low dislocation density 103-104 cm~3 (HPNS-GaN, Ammono-GaN), and with a dislocation density of 106-107 cm~2 (HVPE-GaN). For the laser diodes (LDs) operating at 405-420 nm, we used the structure shown in Fig. 2.25 (Skierbiszewski et al., 2005).
The 40-nm GaN:Si buffer layer and 450-nm Al0.08Ga0.g2N:Si cladding were grown under Ga-rich conditions at 720°C. The bottom waveguide, MQWs, EBL, top waveguide, top cladding, and contact layer were grown in In-rich conditions at 650°C. The active region consisted of three 3-nm In0.1Ga0.gN wells with 7-nm In0 02Ga0.g8N barriers. The devices were processed as ridge-waveguide, oxide- isolated lasers. The mesa structures were etched to a depth of 0.3 pm. The 20-pm-wide and 500-pm-long stripes were used as laser resonators. The oxidized Ni/Au ohmic contacts were deposited on the top surface of the devices, and Ti/Au contacts were deposited on the backside of the highly conducting n-GaN substrate crystal. The cleaved laser mirror facets were coated with symmetrically reflecting mirrors. Figure 2.26(a) shows the light current voltage (LIV) characteristics of the CW LDs with lasing threshold current density and voltage
Fig. 2.25. Transmission Electron Microscope image of the active region of a PAMBE laser diode (a), and structure details (b).
of 5.5 kA/cm2 and 5.7 V, respectively. Lasing was observed up to 60 mW of optical output power (30 mW per facet) at a wavelength of 411 nm, as indicated in Fig. 2.26(b) (Skierbiszewski et al., 2006). This confirms that growth of high- quality layers by PAMBE has been achieved; the smooth interfaces required for LDs can be obtained as shown in Fig. 2.25(a).
The optically pumped stimulated emission at 501 nm was a good starting point for true blue and green laser diodes. It is worth stressing here that not
Fig. 2.26. L-I-V characteristics of a PAMBE-grown cw laser diode (a) and lasing spectrum (b).
Fig. 2.27. Structural details of an SQW 455-nm laser diode.
only efficient QWs are essential for LDs, but also such design of the LDs ensuring confinement of the optical modes inside the laser waveguide. For the LDs operating at wavelengths longer than 430 nm, we found that the crucial factor for achieving lasing is optimization of the claddings and the waveguide. The details of a single quantum well (SQW) LDs structure lasing at 455 nm are presented in Fig. 2.27 (Skierbiszewski et al., 20126). In order to reduce penetration of the optical modes into the GaN substrate we have grown special claddings comprised of 2-pm heavily doped GaN:Si at the level of 7 x 1019 cm-3 and 4 pm GaN/Al0 05Ga0 95N superlattice. For further localization of the optical modes inside the waveguide, and to reduce losses in the gold p-type contact, we also used an “optical confinement layer” (OCL), 80 nm of In0 07Ga0.93N, and changed the upper cladding from InGaN/InAlGaN to GaN/AlGaN. As arises from theoretical calculations, the new claddings and waveguide with OCL helped significantly (i) to decrease absorption of the optical modes in the upper gold contact, and (ii) to reduce penetration of the optical modes to the GaN HVPE substrate. The other interesting feature of SQW LDs grown by PAMBE is a relatively small blue shift from spontaneous (at 465 nm) to lasing emission (at 455 nm); Fig. 2.28(b). This is achieved by employing OCL and staggered SQWs which reduce piezoelectric fields inside SQW.
We also introduced a new design of AlGaN-claddings-free separate confinement heterostructure (SCH) LDs grown by PAMBE, and demonstrated LDs operation from 435 nm to 460 nm (Skierbiszewski et al., 2012a). The LD structure details are shown in Fig. 2.29. The 0.5-pm GaN:Si buffer layer is followed by undoped 0.3-pm GaN. Next, the asymmetric high-indium-content undoped In0.08Ga0 92N waveguide (80 nm/40 nm) is grown. The active region consist of three 3-nm In0.13Ga0.87N wells with 7-nm In0.08Ga0.92N barriers. The p-type 20 nm In0.01 Al0.14Ga0.85N electron-blocking layer (EBL) doped with Mg is located at the top of the upper waveguide. The EBL doping level was 5 x 1019 cm-3. Then 0.5-pm GaN:Mg upper cladding is followed by a 60-nm In0 01Ga0 99N:Mg and 5-nm In01Ga0 9N:Mg contact layer. The devices were processed as ridge- waveguide, oxide-isolated lasers. The mesa structures were etched to a depth of 0.35 pm. The 3-pm-wide and 700-pm-long stripes were used as laser resonators. The oxidized Ni/Au ohmic contacts were deposited on the top surface of the devices, and Ti/Au contacts were deposited on the backside of the conductive
Fig. 2.28. L-I-V characteristics of PAMBE-grown 455 nm SQW laser diode operating in pulse mode (a), and spectrum below and above lasing threshold (b).
Fig. 2.29. The structure of the AlGaN-cladding free laser diode grown by PAMBE.
Fig. 2.30. The light-current-voltage characteristic of PAMBE laser diodes operating in continuous wave regime at 432 nm. The mesa for this LD was 3 x 700 pm.
GaN substrate crystal. The LDs have cleaved laser mirror facets covered by reflection (90%) and antireflection dielectric coatings.
The L-I-V characteristics for room-temperature continuous-wave (cw) operation for LDs at 432 nm are shown in Fig. 2.30. The threshold current density is
7.6 kA/cm2 and threshold voltage is about 8.2 V. The laser diodes are mounted p-down in copper clamps. The maximum optical power of 130 mW is achieved in cw mode with a differential gain of 0.5 W/A.