Schematic of the PL/TRPL measurement setup. Steady-state PL measurements are performed with a continuous-wave HeCd laser source (325 nm) and phase-sensitive detection by use of a lock-in amplifier. TRPL measurements are performed with a FTMLTS source (266 nm) and time-correlated single-photon counting (TCSPC) electronics. A frequency- quadrupled, Q-switched Nd:YVO4 laser serves as a pump source for the nanowire laser studies. PMT, photomultiplier tube; CCD, charge-coupled device camera for imaging; PC, personal computer.
Photoluminescence spectra of as-grown GaN nanowires (solid line) and of a single GaN nanowire dispersed onto a fused silica substrate (dashed line). T = 3 K. The vertical dashed lines indicate the expected strain-free positions of the free A exciton (XA ) and the donor-bound exciton (D 0 XA ) lines and their first two phonon replicas (FE-LO, DBE-LO, FE-2LO, DBE-2LO). Inset: the PL spectrum of the dispersed nanowire over an extended range of photon energies (down to 1.8 eV).
FESEM images of a typical n-type GaN nanowire from the B982 growth run after dispersal onto a transparent substrate. (a) Image of entire nanowire (15 μm in length). (b) Image of top end of same nanowire (average diameter of 284 nm). The arrows indicate the positive polar c-axis and growth directions. The dispersal substrate here is indium-tin-oxide-coated fused silica.
(Color online) Temporal evolution of the PL spectrum for a dispersed n-type GaN nanowire with a diameter of ∼460 nm and a length of 15.1 μm at T ≈ 3 K. The pump source for this measurement is a frequency-tripled mode-locked Ti:Sapphire laser. The scale bar indicates the normalized PL intensity. The upper inset shows an example TRPL trace taken at a PL emission wavelength of 359 nm (3.45 eV); the solid line represents the best fit for a single exponential decay. The lower inset is a FESEM image of the dispersed nanowire (oriented vertically in the image).
PL lifetimes vs excitation fluence for the same dispersed GaN nanowire pictured in Fig. 2 at a temperature of T ≈ 100 K. The dotted line serves as a guide for the eye. The error bar represents the uncertainty in the measurements of the lifetimes.
Temperature-dependent PL lifetimes from single-exponential fits for two nanowire excitation fluences—190 microJoules/cm2 (squares) and 25 microJoules/cm2 (triangles)—and PL lifetimes (excluding subnanosecond transients) for a free-standing HVPE-grown bulk GaN sample (crosses) at an excitation fluence of ∼20 μJ/cm2. The solid line represents the expected radiative lifetimes of free excitons in bulk GaN as per Ref. 9 . The size of the symbols represents the uncertainty associated with the measurements of the PL lifetimes. The vertical dashed lines bound the region where radiative recombination is dominant at the higher nanowire excitation fluence.
(Color online) Room-temperature luminescence and lasing spectra from a dispersed n-type GaN nanowire pumped at 7 different peak pump intensities from a frequency-quadrupled, Q-switched Nd:YVO4 laser source. The n-type GaN nanowire was 12.4 μm long and 245 nm in diameter with flat ends and no measurable taper. The inset shows the peak nanowire emission intensity vs the pump intensity. The lasing threshold for this optically pumped nanowire is 225 kW/cm2.
Temperature-dependent lasing threshold intensities for a dispersed n-type GaN nanowire with an average diameter of 350 nm, length of 13.6 mm, and taper of 75 nm. The pump source here is a frequency-quadrupled Q-switched Nd:YVO4 laser. Lasing threshold intensities are lower where excitonic radiative recombination was shown to be strong despite the higher fluence and quasisteady-state nature of the pump source. The vertical dashed lines bound the same temperature range highlighted in Fig. 4 .
Article metrics loading...
Full text loading...