An additional IR laser with a fixed energy, well below the QD ground state transition, generates excess free holes from deep levels in GaAs. The excess electron and hole will separately experience a diffusion, due to the time separation between the two events of their generation, to eventually become captured into the QD. Although the generation rates of excess carries are much lower than that of the electron-hole pairs generation in the WL, they influence considerably the QD emission at low temperatures. The integrated PL intensity increases by several times compared to single laser excitation and the QD exciton spectrum is redistributed in favor of a more neutral charge configuration. The dependence of the observed phenomenon on the powers of the two lasers and the temperature has been studied and is in consistence with the model proposed. The concept of dual excitation could be successfully applied to different low-dimensional semiconductor structures in order to manipulate their charge state and emission intensity.
This work was supported by grants from the Swedish Foundation for Strategic Research (SSR) and Swedish Research Council (VR). Financial support from the Wenner-Gren Foundation and the program „Low-Dimensional Quantum Structures“ of the Russian Academy of Sciences is achnowledged by E.S.M. V.D. is thankful for financial support from the Swedish Foundation for International Cooperation in Research and Higher Education (STINT) and from the Bulgarian National Science Fund.
PACS: 78.67.Hc, 71.55.Eq, 73.50.Gr 1. Introduction of generation of the electron and the hole is large compared to the time for carrier migration, capture and relaxation to In photoluminescence (PL) studies of low-dimensional the lowest energy state (typically some tens of ps ). Such semiconductor structures, the optical generation of free separate generation of electrons and holes can be realized carriers normally takes place in the barriers, while the e. g. by means of two excitation sources initiating different desired radiative recombination, at least at low temperatures, electronic transitions involving defect levels.
occurs after carrier diffusion into the lowest energy state of Semiconductor quantum dots (QDs) effectively confine the potential well. In the commonly employed single laser electrons and holes on the nanometer length scale in all excited PL, the electron (e) and the hole (h) in the e-h three directions and are a typical example of potential pair are generated simultaneously by the same photon and wells surrounded by barriers, where the optical generation experience a joint diffusion, which results in a considerable of free carriers normally takes place. Recent modeling emission in the barriers with rather short recombination of QD lasers  has emphasized the importance of the times ( ns [1,2]). However, if the electrons and the carrier capture mechanism on the threshold current, with a holes are generated not as a pair, but separately in time resulting larger current for exciton capture than for separate (and space), they will diffuse separately, to be captured electron and hole capture. Mazurenko et al.  reported and recombine in the potential minimum, resulting in an on an infrared (IR) laser induced increase (up to 40%) enhanced PL emission. The recombination in the barriers of the PL output from InAs/GaAs QD ensembles, excited can be avoided, if the time interval between the two events with photon energy above the GaAs band gap. This 1878 V. Donchev, E.S. Moskalenko, K.F. Karlsson, P.O. Holtz, B. Monemar, W.V. Schoenfeld...
observation was explained in terms of an optical release A conventional diffraction-limited µPL setup was used in of carriers previously captured from the QDs into deep these studies. The excitation of a single QD was performed traps . However, due to the high dot density used , by two cw Ti–Sp laser beams, focused on the sample the authors were neither able to observe any PL signal from surface down to a 2 µm spot by means of a microscope the wetting layer (WL), nor to distinguish between different objective. The primary laser, used to excite the PL, was charged exciton complexes in the QD PL. An enhancement tuned from hex = 1.493 to 1.515 eV, while the second of the radiative efficiency due to photoionization of deep laser was set to a fixed energy in IR, hIR = 1.233 eV.
levels (DLs) by an additional below-band-gap excitation was The power of the primary laser P0 was varied in the observed also in epitaxial GaAs . range 17 nW < P0 < 10 µW, while the IR laser power was The spectroscopy of a single QD has proven to be an varied in the range 0.55 < PIR < 100 µW. The PL signal extremely sensitive probe to excess charges in the QD was dispersed by a single grating 0.45 m monochromator and its surrounding [7–12], since it allows monitoring and detected by a liquid-nitrogen-cooled Si CCD camera.
of the spectral fine structure related to different charged The measurements were performed at temperatures T in the exciton complexes. In our previous studies [9–12] on range 5 < T < 65 K using a continuous flow cryostat. Eight InAs/GaAs QD structures with a dot density as low as single QDs were studied, revealing a similar behavior as the 106 cm-2 (allowing simultaneous monitoring of the PL QD presented here. A detailed description of the sample emission from a single QD and the surrounding WL), we and the experimental set-up is given elsewhere [9–11].
have demonstrated that a proper selection of the photon excitation energy hex, will allow single charge carriers, 3. Experimental results and discussion electrons and/or holes, to be generated in the sample and subsequently be captured by the QDs. This is reflected The threshold energy Eth described above determines in the charge distribution of the QD excitonic states, the primary laser excitation energy above which excess as monitored by low-temperature (5K) micro-PL (µPL) electrons are generated. This threshold energy is defined spectra of the individual QD.
GaAs GaAs by Eth = Eg - EA, where Eg = 1.519 eV is the GaAs In this paper, we report on dual laser excited µPL studies band gap and EA is the shallow acceptor ionization energy.
of self-assembled InAs/GaAs single QDs. The primary laser, The values of Eth have been found to be either 1.GaAs tuned in the range Eth < hex < EX, provides e-h pairs or 1.493 eV  (depending on the QD surrounding) in via photon absorption in the WL, as well as excess free accordance with the ionization energies of the main residual electrons from shallow acceptors in the GaAs barriers .
acceptors in MBE grown GaAs: Si (EA = 34 meV) and C An additional IR laser generates excess holes via electronic (EA = 26 meV), respectively . The results presented excitations involving DLs. The powers of the two lasers are in this paper are for hex 1.493 eV, which implies that chosen in such a way, that the average time interval between electrons from both types of acceptors are excited in the two individual events of an excess electron and an excess GaAs conduction band by the photons of the primary laser.
hole generation is much longer than the time required for The solid curves in Fig. 1 show µPL spectra of the QD carrier migration in the WL and collection into the QDs.
measured at (a) 5 and (b) 55 K under single laser excitation When the principal excitation is complemented with the with hex = 1.503 eV. The dotted curves represent µPL IR laser, it is found that the QD PL intensity increases spectra recorded under the same experimental conditions, several times, while the WL PL remains almost unchanged.
but with an additional IR laser excitation. Altogether three Another striking observation is the charge redistribution emission lines, marked as X, X- and X2-, can be seen on monitored in the QD µPL spectrum. With dual laser Fig. 1. They have been identified as the neutral (X), single excitation, the QD becomes more neutral. The dependence (X-) and double (X2-) negatively charged exciton , of these interesting effects on the laser powers and the corresponding to the 1e1h, 2e1h and 3e1h QD ground state temperature is studied.
charge configurations, respectively. The small peak labeled X3- is tentatively ascribed to the triple negatively charged exciton (4e1h).
2. Samples and experimental set-up The µPL spectrum, obtained at 5 K, excited with a single The sample studied was grown by molecular-beam- laser is dominated by the doubly negativety charged exciton epitaxy (MBE) employing a special growth procedure to X2-. This is observed to be dominant for all hex in GaAs obtain a very low QD density, corresponding to an average the range Eth < hex < EX. The predominance of X2inter-dot spacing of about 10 µm. The self-assembled InAs is explained in terms of excess electrons generated via QDs are lens-shaped with a typical lateral size (height) of optical electronic transitions from shallow acceptors to the 35 nm (4.5 nm). They are located on a thin ( 0.5nm) conduction band in the GaAs barriers  (represented by WL sandwiched between two 100 nm thick layers of GaAs. arrow 1 in Fig. 2). Under dual laser excitation, the µPL A short-period 40 2nm/2nm AlAs/GaAs superlattice is spectrum changes dramatically: the line X2- is quenched used as a buffer layer between the structure and the semi- and, instead, the neutral exciton X becomes predominant.
insulating GaAs substrate. Besides, an overall increase of the integrated QD PL Физика твердого тела, 2006, том 48, вып. Enhancement of the photoluminescence intensity of a single InAs/GaAs quantum dot... intensity IQD by more than 5 times relatively to the single PL laser excited PL intensity is observed. For all hex in the above-mentioned range, the addition of the IR laser i) redistributes the µPL spectrum towards a more neutral state, i. e. in favor of the X PL line and ii) increases IQD several times. The IR laser alone does not give any PL detectable PL, as expected from its energy, which is well below the QD emission energy.
No change which could be induced by the IR laser is detected for the dot PL intensity, when the QD is excited directly, i. e. for hex below the absorption edge of the WL. This allows us to exclude capture processes of carriers from the QD to DLs followed by an optical excitation to the barriers and recapture into the dot (see dashed arrows in Fig. 2), as has been considered earlier . Further, it is important to note that the considerable increase of IQD PL (> 5 times) is not accompanied by an analogous increase of the integrated µPL intensity of the WL, IWL, which PL increases only by 5–10%. Taking into account all the experimental observations described above, the IR laser induced changes in the PL spectra can be understood in terms of generation of excess holes. Since hIR is in the extrinsic region of the sample, we assume that the holes are generated by optical electronic transitions between the valence band and DLs in the GaAs (represented by Figure 1. Micro-PL spectra of a single InAs/GaAs QD under arrow 2 in Fig. 2). As the QD is already populated with single (solid lines) and dual (dotted lines) laser excitation meaexcess electrons by the primary laser (see above), excess sured at (a) 5 and (b) 55 K with hex = 1.503 eV, P0 = 40 nW, positive charges are effectively attracted. This increases and PIR = 100 muW.
the probability for a more neutral charge distribution, i. e.
for the 2e1h and 1e1h charge configurations (X- and X).
The observed increase of IQD can be explained by the PL combined generation of excess electrons by the primary laser and excess holes by the IR laser with generation rates ge and gh, respectively. A permanent generation of excess electrons and holes requires a continuous replenishment of the acceptors with electrons and the DLs with holes. Most likely this occurs by electron capture from the DLs to the acceptors (represented by undulated arrow in Fig. 2). It is worth noting that PL measurements of GaAs have suggested that the acceptors tend to cluster in the neighborhood of the DL centers .
The generation rate of additional e-h pairs in the QD gad is on top of the generation rate gQD, originating from the primary laser absorption in the WL (represented by arrow 3 in Fig. 2). gQD is in fact the observed IQD PL without the IR laser excitation. IWL could be considered PL to be equal to the carrier generation rate in the WL gWL. Introducing a collection efficiency wc of the photo Figure 2. A schematic reqresentation of the conduction band carriers from the WL to the QD as wc = IQD/IWL, gQD (Ec) and valence band (Ev) edges in the growth direction together PL PL can be expressed as gQD = wcgWL = wcWLdWLP0/hex, with the acceptor (A) and the deep level (DL) positions in the sample studied. The thin (thick) vertical arrows correspond to the where dWL = 0.5 nm is the WL thickness and in a first primary (IR) laser excitation, while the curved arrows represent approximation the absorpsion coefficient of bulk InAs at the capture processes into the QD and the deep levels discussed hex, WL = 104 cm-1  is used. From the expression in the text. The undulated arrow represents the electron capture wc = IQD/IWL, wc can be evaluated to be 2 · 10-3 for PL PL from the deep levels to the shallow acceptors.
hex used in this study.
Физика твердого тела, 2006, том 48, вып. 1880 V. Donchev, E.S. Moskalenko, K.F. Karlsson, P.O. Holtz, B. Monemar, W.V. Schoenfeld...
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