Radiative Inter-Sublevel Transitionsin InGaAs/AlGaAs Quantum Dots

A. Weber1) (a), K. Goede (a), M. Grundmann*Þ (a), F. Heinrichsdorff (a),

D. Bimberg (a), V. M. Ustinov (b), A. E. Zhukov (b), N. N. Ledentsov (a, b),P. S. Kopev (b), and Zh. I. Alferov (b)

(a) Institut fur Festkorperphysik, Technische Universitat Berlin, Hardenbergstr. 36,D-10623 Berlin, Germany

(b) A. F. Ioffe Physical-Technical Institute, Politekhnicheskaya 26, 194021, St. Petersburg,Russia

(Received July 31, 2000, accepted October 2, 2000)

Subject classification: 73.21.La; 78.45.+h; 78.67.Hc; S7.12

We report on the emission of mid-infrared (MIR) radiation from InGaAs/AlGaAs quantum dotsupon photoexcitation and electrical injection. Mid-infrared radiation is achieved from bipolarquantum-dot lasers during near-infrared lasing. The MIR spectrum exhibits a peak at 16 mm and isdominantly TM polarized. The MIR intensity exhibits a superlinear dependence on the injection.These results are compared to optically pumped emission from InGaAs/GaAs quantum dots. Theunpolarized spectrum shows peaks at 15 and 10 mm. The MIR intensity increases sublinearly withexcitation power.

Mid-infrared (MIR) sources are of special interest for applications in optical IR spectro-scopy and gas detection. At the present time, the only coherent GaAs based MIRsources are unipolar or cascade lasers using inter-subband transitions of charge carriersin quantum wells [1–3].Quantum dots (QDs) are a promising system for improved mid- and far-infrared

optoelectronic devices [4]. This is due to the high oscillator strength, favorable polariza-tion selection rules of inter-sublevel transitions and reduced phonon scattering betweenzero-dimensional states.The feasibility of bipolar MIR lasers based on electron inter-sublevel transitions be-

tween confined QD levels during near-infrared interband QD laser operation has beentheoretically predicted [5, 6]. In this case, the MIR transition and population inversion areenabled by the depopulation of the ground state by stimulated interband emission. InRef. [6], the observation of MIR radiation in the 10–20 mm spectral range was reported.In this paper, we present MIR emission spectra from near-infrared (NIR) QD lasers

and optically excited MIR photoluminescence (PL) spectra of QD samples. Their re-spective MIR output power versus excitation power characteristics are compared.The lasers were grown by molecular beam epitaxy. The active region consists of four-

fold stacks of InGaAs self-assembled QDs in an AlGaAs matrix [7]. The QD surfacedensity per sheet is 4 � 1010 cm––2. The samples have been processed into edge emittersand lase at 940 nm at room temperature. They have been operated in a liquid nitrogen

1) Corresponding author; Tel.: +49-30-314 22073; Fax: +49-30-314 22569;e-mail: aweber@physik.tu-berlin.de

*) Present address: Universitat Leipzig, Institut fur Experimentelle Physik II, Linnestr. 5,D-04103 Leipzig, Germany.

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55 physica (b) 224/3

cryostat at 77 K under quasi-cw injection. The current was modulated with 20 kHz anda duty cycle of 1 : 1.The samples used for photopumping consist of a single layer of metal-organic chemi-

cal-vapor-deposition grown InGaAs QDs embedded in GaAs matrix. A seeding techniquewas used to enhance the surface density (2.1 � 1010 cm––2) and the size of the QDswhich emit at 1.3 mm [8]. The structure is embedded in AlGaAs barriers for strongercarrier confinement.The MIR emission has been spectrally resolved using a BRUKER 66v/S Fourier-

transform infrared spectrometer. The MIR emission was fed into the external inputport of the spectrometer (Fig. 1), which was equipped with a KBr beamsplitter. It wasdetected with a mercury-cadmium-telluride (MCT) detector (long wavelength cut-off at23.2 mm). For the measurements of MIR emission, the NIR radiation was blocked byan InAs (l < 3.5 mm) filter. The spectra were recorded using the step-scan mode of thespectrometer. Lock-in technique was used in order to suppress the ambient blackbodybackground radiation. Polarized spectra were recorded by putting a KRS-5 polarizerdirectly in front of the cryostat.In Fig. 2 the light output versus current (L–I) characteristics for the NIR QD laser

and its MIR emission are shown as well as the electrical input power vs. injection cur-rent. The NIR intensity, which was recorded using a large-area, calibrated Si photo-diode, exhibits a linear increase beyond the threshold current of 91 mA, correspondingto a threshold current density of jthr = 91 A/cm2. A maximum MIR output power of100 nW (per one facet) was observed.The L–I characteristic of the MIR radiation is superlinear and obeys a power law

with the exponent 2.3. Such superlinear L–I characteristic is typical for the below-threshold intensity of the MIR lasing mode, as a theoretical treatment of MIR emissionfrom NIR QD lasers shows [9]. Spontaneously emitted MIR radiation is expected toexhibit a linear increase. We suggest that the undirected spontaneous emission isstrongly absorbed in most directions by the doped cladding layers of the QD laser,which was not equipped with a MIR waveguide. Thus mainly the directed MIR radia-tion in the laser mode is detected, although no lasing is observed.

834 A. Weber et al.: Radiative Inter-Sublevel Transitions in InGaAs/AlGaAs QDs

Fig. 1. Experimental set-up for the detection of MIR emission

The polarized MIR emission spectra (Fig. 3) exhibit a dominantly TM polarized peakaround 16 mm (�80 meV) with a full width at half maximum of about 20 meV. Theobserved spectrum and its polarization prove that the observed MIR radiation is notdue to black-body radiation potentially arising from heat generated by the injectioncurrent, for which a L–I dependence proportional to the electrical input power (� j1.2)would be expected (Fig. 2). Also no increase of lattice temperature during laser opera-tion is observed, since the NIR lasing wavelength exhibits only a negligible shift be-tween near-threshold and high power operation. Instead we attribute the MIR radiationto a transition involving QD states. The energy position of the MIR peak indicates thatthe transition involves confined electron states since hole states have much smallerlevel separations in the 10 meV range. Assuming the QD electron ground state beingthe final state of the inter-sublevel transition, the polarization behavior indicates thatthe MIR transition involves an excited state with a node in the z-direction (growthdirection), which corresponds to the elongation direction of the electron wavefunctionsin electronically coupled stacked QDs.These experiments can be compared to photoluminescence (PL) spectra in NIR and

MIR from a non-lasing optically excited sample. In this case the ground state is notefficiently emptied by the NIR lasing pro-cess and filling of the energy levels in theQDs can occur.The sample was excited for NIR and

MIR photoluminescence spectra by achopped argon-ion laser emitting at514 nm. The laser beam was focussedonto the (001) sample surface by a lens.

phys. stat. sol. (b) 224, No. 3 (2001) 835

Fig. 2. Right scale: Intensity vs. injectioncurrent density j for the NIR QD laserand the MIR emission (note that theMIR values have been multiplied by2 � 106). The solid line for NIR is aguide to the eye, the dash-dotted linefor MIR represents a curve proportionalto j 2.3. Left scale: Electrical input powerPin vs. injection current density of theQD laser. The dotted line represents acurve proportional to j 1.2

Fig. 3. Polarized MIR spectra for an injectioncurrent of j = 1.2 kA/cm2

55*

The emitted light from the cleaved {110} edge of the sample was then analyzed anddetected in the same way as for the laser devices.NIR photoluminescence spectra for different excitation densities are shown on the

right-hand side of Fig. 4. The dashed line represents a high excitation spectrum, whererecombination also from higher QD states as well as from the wetting layer (WL) andGaAs substrate is visible. The energetic distance between the electron–hole recombina-tion of QD ground state and the first excited state (QD*) is D0 = 78 meV. The secondexcited state (QD**) is visible at D1 = 156 meV.The solid line in Fig. 4 shows the MIR and NIR spectra for a lower excitation den-

sity. The MIR spectrum, which is not significantly polarized, clearly shows a large peakat 84 meV with a full width at half maximum of 35 meV. A second peak with less inten-sity can clearly be distinguished at 120 meV.We attribute these peaks to radiative transitions of electrons from QD* and QD** to

the ground state, respectively. Because of their much smaller electron masses we expecttransitions of electrons being responsible for the major part of the excitonic recombina-tion energies D0 and D1. The main peak at 84 meV is in our opinion blue-shifted be-cause of the strongly descending detector responsivity near the detector cut-off. Thusthe apparent transition energy is higher than D0 = 78 meV.The L–P dependence in the inset of Fig. 4 shows in contrast to the laser experiments

a sublinear increase, possibly indicating saturation due to filling of the ground state or a

836 A. Weber et al.: Radiative Inter-Sublevel Transitions in InGaAs/AlGaAs QDs

Fig. 4. MIR (left) and NIR (right) photoluminescence spectra of the QD sample. The spectra withsolid lines were recorded at the same low excitation densities (the excitation power was 500 mW).A high excitation density NIR spectrum is plotted with a dashed line. The inset shows the inte-grated MIR emission vs. optical excitation power Pexc. The dashed line in the inset represents acurve proportional to P0:7

exc

decrease of MIR quantum efficiency with increasing carrier number. In contrast to Ref.[10] no decrease of the MIR emission is observed for high pump densities. under thechosen experimental conditions.

Acknowledgements This work has been supported by Deutsche Forschungsge-meinschaft (Grant No. Gr 1011/7-1) and by INTAS (Grant No. 97-751).

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