Stimulated Emission from Excitons in a Quantum Wire Laser Fabricated by Cleaved Edge Overgrowth
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Mat. Res. Soc. Symp. Proc. Vol. 326. ©1994 Materials Research Society
7nm
2 n
U) 0
z AIGaAs x =0.35
Z I co0 3
(a)
I I I 5 7 9 QW WIDTH (nm) (b)
Figure 1: Contours of constant probability (I-I-, 0.1, 0.2,..-.,0.9) for electrons confined at the T-intersection of two QWs (a) and binding energy with respect to the lowest QW state of electrons to the QWR as a function of QW width for equally thick wells (b). EXPERIMENTAL A schematic cross-section of the QWR laser structure is shown in Fig. 2. The multiple quantum well (MQW) layer consists of 22 GaAs QWs separated by A10.35 Gao.6 5 As barriers as illustrated in the magnified area. The first MBE growth formed the layer structure to the right of the arrow marked "cleave". After the first growth the multilayer was removed from the MBE machine, then polished from the back-side to a total thickness of 100 ptm and reinserted into the growth chamber. The MBE machine was brought to running condition and a portion of the sample was cleaved off presenting a fresh surface for overgrowth. The layers grown on the cleaved edge are shown to the left of the arrow marked "cleave". The 22 QWRs which form at the T-intersections of the 7 nm wide QWs are, in turn, embedded in a T-shaped dielectric waveguide. The purpose of the A10. 5Ga0 .5As and A10. 1Ga0. 9As is to confine an optical mode to the vicinity of the QWR array (see Fig. 2) as confirmed by waveguide calculations within effective refractive index approximation. In order to achieve lasing in the 11D structures, mirrors were cleaved perpendicular to the axis of the QWRs resulting in 600 pim long optical cavities. The cleave mirrors were left uncoated for all experiments reported here. Low-temperature (1.7K) optical excitation of the samples with uncoated mirrors was performed by focusing the output of a dye laser tuned to A = 775 nmn to a stripe of about 700 yrm in length and 5 prm in width oriented parallel to the QWRs. RESULTS AND DISCUSSION Figure 3 compares emission spectra of our QWR laser below and above threshold for stimulated emission. At low excitation power (0.25 mW) exciton recombination in the QWRs and QWs is observed at about 1.564 and 1.58 eV, respectively [16], indicating that 1D state is about 16 meV deep with respect to the well. The doublet structure in the QW emission is believed to originate from slightly different confinement energies for the QWs grown along the [001] direction and the [110] oriented QW formed during overgrowth. With increasing pump power the contrast in the Fabry-Perot oscillations, which develop on the low energy side of the QWR peak, increases and at about 10 mW stimulated emission occurs. Further increase of the pump power leads to a
402
direction of
c
[001
overgrowth [110] .4 ........cleave ,.die 7 nm
direction to
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