Localized Excitons in InGaN
- PDF / 1,087,807 Bytes
- 12 Pages / 414.72 x 648 pts Page_size
- 105 Downloads / 246 Views
lasing mechanisms of cw In. 15Gao.85N MQW LD's having small potential undulations are well explained by the EHP picture with strong Coulomb enhancement [19]. However, those LD's having inhomogeneous MQWs exhibited characteristic gain peaks. EXPERIMENT Samples used in this study were grown on sapphire (0001) substrates by metalorganic vapor phase epitaxy (MOVPE) with low-temperature GaN buffer layers [1]. The AlGaN/InGaN 3D DH LED structures [1] were modified to have (i) a 50 nm-thick In0 .06Gao.94N active layer codoped with Si and Zn, (ii) the same structure as (i) except for an undoped In0 .06Ga0.94N active layer, (iii) the same structure as (i) except for a Si-doped (10'9 crn") In0 .06Gao.9N active layer. For PL and PL excitation (PLE) measurements, a 50 nm-thick undoped 3D Ino.o9Gao.91N layer was grown on a 50-nm-thick In0.01Ga0.99N:Si / 50-nm-thick Alo3 Gao.7N:Si / 2-p m-thick GaN:Si epilayer base. The blue/green SQW LED's have a 3 nm-thick undoped In.OGalxN QW (x=0.3 and 0.45), respectively [1]. For both spatially-resolved and integrated CL measurements, 3 nm-thick undoped InxGalxN (x=0.05, 0.2, and 0.5) SQW's were grown on a 3- pm-thick Si-doped (5x10' 8 cm") GaN layer. The SQW's were subsequently capped by a 6 nm-thick undoped GaN layer, or uncapped. Several MQW LD wafers [1] were prepared. One of them has ten periods of 2.5 nm-thick Ino2Ga0.gN wells and 7.5 nm-thick In 005Gao.95N barriers, whose pulsed lasing wavelength is 410 nm and the spontaneous EL peak is 399 nm (3.11 eV) at RT [1,13]. Its threshold current density (JQ)was 11.4 kA/cm2 .The other cw LD wafer group had three 3.5 nm-thick In0.1 Ga0 .85N wells. Two LD wafers were examined, namely CW1 and CW2. A remarkable difference between the two is that the potential fluctuation in the QW plane in CW1 is much weaker than that in CW2. The Jth for RT cw operation were 7.4 and 9.4 kA/cm2 , and the wavelength was 398 and 409 nm for CWl and CW2, respectively. The device lifetime was less than 1 hour for both LD's. As shown later, CW1 has small potential fluctuation, and showed a gain spectrum that can be fully explained by the EHP picture. EL and photovoltage (PV) spectra were measured using the LED or LD devices. PL, PLE, and modulated-electroabsorption (EA) spectra [13] were measured using the device wafers. For comparison, we also characterized single GaN epilayers by the photoreflectance (PR) measurements [20]. Both the PR and EA spectra were analyzed using the Lorentzian lineshape functional form. Static PL was mainly excited by the 325 nm line of a cw He-Cd laser. Time-resolved PL (TR-PL) measurements were also carried out using a N 2 pulsed laser or a frequency-doubled Ti:sapphire laser as an exciter. These measurements were carried out between 10 K and RT. Spatially-integrated or spatially-resolved CL [15] was excited by a cw electron beam (e-beam) with or without the e-beam scanning, and dispersed by a 25 cm focal-length grating monochromator coupled to a scanning electron microscope. The energy resolution of the CL spectrum was about 15 meV at 410
Data Loading...
