Stimulated emission and ultrafast carrier relaxation in InGaN multiple quantum wells

Abstract

Stimulated emission (SE) was measured from two InGaN multiple quantum well (MQW) laser structures with different In compositions. SE threshold power densities (I th ) increased with increasing QW depth (x). Time-resolved differential transmission measurements mapped the carrier relaxation mechanisms and explained the dependence of I th on x. Carriers are captured from the barriers to the QWs in < 1 ps, while carrier recombination rates increased with increasing x. For excitation above I th an additional, fast relaxation mechanism appears due to the loss of carriers in the barriers through a cascaded refilling of the QW state undergoing SE. The increased material inhomogeneity with increasing x provides additional relaxation channels outside the cascaded refilling process, removing carriers from the SE process and increasing I th . 78.47.+p,78.66.Fd,78.45.+h,78.67.De Recent advances in the growth of group-III nitride heterostructures have made it possible to manufacture efficient green, blue, and ultraviolet emitters and detectors. [1][2][3] Carrier dynamics in such device structures are beginning to be understood through the use of high power, short pulse, regenerative and optical parametric amplifiers. [4][5][6][7] In this paper, stimulated emission (SE) in InGaN multiple quantum well (MQW) laser structures with differing QW depths is explored. SE threshold power densities increase with increasing QW depth, and ultrafast measurements of carrier dynamics reveal why.Two different InGaN MQW samples, differing primarily by QW In composition (x), were grown on c-plane sapphire in a modified two flow metalorganic chemical vapor deposition (MOCVD) reactor. 8 Both samples have ∼2 µm thick GaN:Si base layers. The 12% sample consists of 10 periods of 8.2 nm/3.4 nm In 0.07 Ga 0.93 N:Si/In 0.12 Ga 0.88 N QWs and is capped with a 100 nm thick GaN:Si layer. The 23% sample consists of 12 periods of 10 nm/3.5 nm In 0.03 Ga 0.97 N:Si/In 0.23 Ga 0.77 N QWs and is capped with 16 nm thick Al 0.10 Ga 0.90 N and 40 nm thick GaN:Si layers. The In compositions and the QW/barrier thicknesses are determined by high resolution X-Ray measurements. Material inhomogeneities, which arise from QW thickness fluctuations, compositional fluctuations, and In phase separation, have previously been observed to increase with increasing x and will be shown to play an important role in carrier relaxation and emission processes. 9,10 Fig. 1 shows the cw-PL for 3.81 eV low intensity (300W Xe lamp) and moderate intensity (25mW HeCd laser at 325 nm) excitations, and time-integrated PL (TI-PL) with a high intensity (∼15 µJ/cm 2 ) pulsed 3.31 eV excitation for both MQW samples. PL for the 12% sample shows a single peak at 3.01 eV for both Xe lamp (not shown) and HeCd excitations. The 23% sample shows at least two well-defined PL peaks for Xe-Lamp and HeCd laser excitation, the higher energy of which (2.52 eV) is assigned as the main PL from the QWs. The lower energy PL peak is associated with impurity (yellow) emission. For both samples, TI-PL from spont...

Stimulated emission and ultrafast carrier relaxation in AlGaN/GaN multiple quantum wells

“…The subpicosecond experimental technique, involving a wavelength tunable, ϳ100-fs pump, and a pulsed broadband continuum probe, has been described elsewhere. 4 To examine the relaxation of the total population of carriers, TRDT signals are spectrally integrated ͑SI͒ over all energies where photoexcited carriers were observed ͑Fig. 3͒.…”

Ultrafast carrier dynamics in nano-clustered InGaN

“…Hence, the transmission ratio essentially decreases with temperature. To obtain the carrier decay times, we used a model of double-exponential variation, convolved with a Gaussian function (the pump pulse), to fit the measured data [23], as illustrated by the dashed curves in Fig. 8.…”

Femtosecond studies of electron capture times in InGaN/GaN multiple quantum wells

“…[3][4][5] However, previous investigations of carrier capture by ultrafast laser spectroscopy have been restricted so far to a limited range of experimental conditions. 6,7 In this letter, we present capture time measurements on undoped blue-emitting InGaN/GaN multiple quantum well ͑MQW͒ structures by subpicosecond time-resolved differential transmission spectroscopy. We have investigated the dependence of the capture time on carrier density, temperature, and indium concentration.…”