Comparative Magneto-Photoluminescence Study of Ensembles and of

NANO LETTERS

Comparative Magneto-Photoluminescence Study of Ensembles and of Individual InAs Quantum Dots

2009 Vol. 9, No. 1 353-359

Evgenii S. Moskalenko,†,‡ L. Arvid Larsson,†,* Mats Larsson,† Per Olof Holtz,† Winston V. Schoenfeld,§ and Pierre M. Petroff§

Downloaded by YEUNGNAM UNIV on September 10, 2015 | http://pubs.acs.org Publication Date (Web): December 10, 2008 | doi: 10.1021/nl803148q

Department of Physics, Chemistry and Biology (IFM), Linko¨ping UniVersity, S-581 83 Linko¨ping, Sweden, A. F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021, Polytechnicheskaya 26, St. Petersburg, Russia, and Materials Department, UniVersity of California, Santa Barbara, California 93106 Received October 17, 2008

ABSTRACT We report on magneto-photoluminescence studies of InAs/GaAs quantum dots (QDs) of considerably different densities, from dense ensembles down to individual dots. It is found that a magnetic field applied in Faraday geometry decreases the photoluminescence (PL) intensity of QD ensembles, which is not accompanied by the corresponding increase of PL signal of the wetting layer on which QDs are grown. The model suggested to explain these data assumes considerably different strengths of suppression of electron and hole fluxes by a magnetic field. This idea has been successfully checked in experiments on individual QDs, where the PL spectra allow to directly monitor the charge state of a QD and, hence, to conclude about relative magnitudes of electron and hole fluxes toward the QD. Comparative studies of different individual QDs have revealed that the internal electric field in the sample (which was altered in the experiments in a controllable way) together with an external magnetic field will determine the charge state and emission intensity of the QDs.

Transport of carriers prior to capture into the QDs inevitably plays a major role in the understanding of the carrier dynamics and related physics in samples with QDs. This is because most of the experiments with QDs employ carrier injection into the sample by electrical or optical means, that is, not directly into the QDs, but in the barriers or into the wetting layer (WL) on which the QDs normally are grown. Intensive studies of the carrier capture mechanisms in the past decade have revealed optical phonon assisted,1,2 Augerlike,3 shakeup4 processes and carrier relaxation through the band tail states of the WL, with a subsequent emission of localized phonons.5 The lateral carrier transport (in the plane of the WL) could be affected by carrier hopping between QDs,6 by trapping of migrating particles into localized states of the WL,7 or into nonradiative centers8 in the surrounding media. It has also been suggested that the carrier drift could be considerably influenced by a long-range attractive potential caused by the strain field surrounding the QD.1 On the other hand, strain-induced potential barriers in the barrier/QD9 and * Corresponding author. E-mail: [email protected]. † Linko ¨ ping University. ‡ Russian Academy of Sciences. § University of California. 10.1021/nl803148q CCC: $40.75 Published on Web 12/10/2008

 2009 American Chemical Society

in the WL/QD interface10 were considered to limit the carrier capture into the QD. The important role of the electric field directed in the growth direction of the sample, on carrier capture into and escape out of the QD, was demonstrated by studies of an electric current passing through the QDs.11 In our previous contribution,12 we demonstrated that the internal built-in electric field in the sample considerably affects the lateral carrier transport. A magnetic field directed perpendicular to the plane of the structure was observed to limit the lateral transport of carriers.13 In the present paper, we experimentally demonstrate that when an external magnetic field is applied, lateral transport of carriers should be considered to take place in crossed electric and magnetic fields rather than subjected to only a magnetic field. In our studies, we apply an external magnetic field perpendicular to the plane of the sample (Faraday geometry) to study ensembles of QDs with different densities down to single quantum dots (SQD) by means of photoluminescence (PL). Surprisingly, we find that the total (spectrally integrated) decrease of the PL intensity from the QD ensembles is not accompanied by the corresponding increase of the PL intensity from the WL. To explain these experimental results, we suggest a model according to which an


external magnetic field applied in Faraday geometry suppresses the lateral transport of electrons toward the QDs considerably more than for holes. To check this model, we employ micro-photoluminescence (µ-PL) spectroscopy of individual QDs, where the µ-PL spectra directly reflect the charge state of the SQD.14 Analysis of the experimental data obtained for different SQDs demonstrate the crucial role played by an internal electric field for the carrier transport. Sample and Experimental Setup. The sample studied was grown by Molecular Beam Epitaxy on a semi-insulating GaAs (100) substrate. The buffer layer was prepared with a short-period superlattice 40 × 2 nm/2 nm AlAs/GaAs at a growth temperature of 630 °C. On top of a 100 nm GaAs layer, the QDs were formed from a 1.7 monolayer thick InAs layer deposited at 530 °C. A first growth interruption of 30 s was used to improve the size distribution. The dots were covered with a thin GaAs cap layer with a thickness of tcap ) 3 nm before a crucial second growth interruption of 30 s. Finally, a 100 nm thick GaAs layer was deposited to protect the QDs. As a result, lens-shaped InAs QDs were developed with an averaged height and diameter of 4.5 and 35 nm, respectively. The sample was grown without rotation of the substrate, so that a gradual variation of In flux is achieved across the wafer, resulting in a gradient in the density of the dots across the epitaxial layer.15 Accordingly, different parts of the same sample with the desired density of QDs could be selected. µ-PL of a part of the sample with extremely low QD density allows studies of a single QD in a diffraction-limited µ-PL setup. A Ti:sapphire laser beam was focused down to a spot diameter of 2 µm on the sample surface. The excitation energy of the laser (hνex) could be tuned in the range from 1.23 to 1.77 eV with a maximum excitation power (P0) of 5 mW. For dual laser excitation, a semiconductor laser operating at a fixed energy of 1.589 eV (with a maximum power output of 200 nW) was used as the principal excitation source, while a Ti:sapphire laser was operating in the infrared (IR) at a fixed excitation energy of hνIR ) 1.23 eV (with a maximum excitation power of PIR ) 100 µW). The sample was positioned inside a continuous-flow cryostat, which allowed the temperature (T) to change from 3.8 K up to room temperature. The PL signal passed through a single-grating 0.55 m monochromator combined with a nitrogen-cooled CCD camera, allowing a spectral resolution of 0.1 meV. A magnetic field (B) up to 5 T could be applied in Faraday geometry with respect to the plane of the sample, which was inserted in the center of a superconducting magnet of solenoid type. To study QD ensembles, another cryostat with a superconducting magnet was used, allowing a magnetic field up to 14 T. Both the optical excitation and collection of the PL from the sample, was provided by the same optical fiber. The PL signal was dispersed through a 0.85-m double-grating monochromator coupled to a LN2-cooled Ge detector. It is important to note that the QD ensembles were also checked in the µ-PL setup allowing us to record only a restricted range of the QDs PL energies (due to the limited sensitivity of the 354

Figure 1. PL spectra of (a) MQDs_1, (b) MQDs_2, and (c) MQDs_3 measured at hvex ) 1.687 eV, P0 ) 1 mW, and T ) 4.2 K for B ) 0 and B ) 13 T applied in Faraday geometry shown by thin and thick solid lines, respectively. (d) PLE spectra of MQDs_3 measured at T ) 4.2 K for B ) 0 (thin solid line) and for B ) 12 T (thick solid line) applied in Faraday geometry. The detection is at the energy that corresponds to the maximum intensity of QDs PL band in c. The arrow in (d) indicates the value of hvex used to obtain data in (a-c). Insets in (a-c) show the plot of IQD(B)/IQD(B ) 0) as a function of B.

CCD camera). The experimental results achieved were similar to those obtained when using the setup with the Ge detector. Experimental Results and Discussion. Low-temperature PL spectra of three sample parts with different QD densities measured at zero magnetic field are shown in Figure 1a-c. Each of the PL spectra consists of two PL bands that correspond to the emission from the QDs and the WL. The decreasing QD density as going from Figure 1a down to Figure 1c is evidenced by the decreasing spectrally integrated PL intensity of the QDs (IQD) versus the WL (IWL). Indeed, upon excitation with hνex ) 1.687 eV (used to obtain data in Figure 1), that is, considerably exceeding the GaAs barrier energy (EGaAs ) 1.518 eV), photoexcited electrons and holes will migrate in the GaAs barriers and in the WL, releasing their kinetic energy prior to capture into the QDs. Those carriers that have not reached spots close to the QDs will remain away from the QDs and recombine there, contributing to the PL of the WL. Consequently, assuming the same Nano Lett., Vol. 9, No. 1, 2009


efficiency of the carrier’s transport processes, one can predict a higher probability for QDs to be populated with carriers (and, hence, stronger IQD compared to IWL) for more dense QD ensembles, as is indeed inferred from Figure 1. When an external magnetic field (B) is applied perpendicularly to the sample plane, that is, in Faraday geometry, PL spectra will definitely change (Figure 1a-c): While the QDs emission intensity, IQD, is considerably decreased, IWL remains almost the same. The decrease of IQD with increasing B-field is explained in terms of suppression of carrier’s transport in the GaAs and the WL toward the QDs due to the bending of the carrier’s trajectories under the action of Lorentz force from the external magnetic field. At a sufficiently high field, the orbits of a carrier become a completed circle, with a spatial extension characterized by the cyclotron radii. It is predicted that with an increasing QD density, the effect of the magnetic field on the transport of carriers to the QDs is decreased, because the probability for the carrier to approach and become captured into a QD is proportional to the total number of QDs within the cyclotron orbit.13 In other words, one has to compare the cyclotron radii with the averaged interdot separation distance. This prediction agrees well with the experimental results, as demonstrated by the plots of IQD measured as a function of B (IQD(B); insets in Figure 1). Indeed, for the high density QDs ensemble, a decrease of IQD by 1.3 times is determined upon an increase of B from 0 up to 13 T (inset in Figure 1a), while a corresponding decrease of IQD(B ) 0)/IQD(B ) 13 T) ≈ 4.2 is monitored for the low density QDs ensemble (inset in Figure 1c). It is important to note that at excitation below the WL, that is, directly into the QDs, without carrier transport prior to capture into the QDs, IQD for all the dot densities was essentially unaffected by the field, consistent with the conclusion that a decreasing IQD with increasing field is entirely due to suppression of carrier’s motion by an external magnetic field. The most surprising result deduced from the PL spectra (Figure 1) is that the decrease of IQD with increasing field (∆IQD ) IQD(B ) 0) - IQD(B ) 13 T)), is not equal to the increase of the WL PL intensity (∆IWL ) IWL(B ) 13 T) IWL(B ) 0)). This fact can only be explained if we assume that the applied magnetic field suppresses the fluxes of electrons (Φe) and holes (Φh) toward the QDs with different efficiencies. Otherwise, an equal number of electrons and holes (which have not reached the QDs with increasing B) should remain in the WL and, accordingly, result in an increase of IWL by the same amount as the decrease of IQD, that is, ∆IWL ) ∆IQD. The usual criterion to evaluate the strength of the magnetic field applied is to compare ωc × τsc with unity,16 where ωc ) e*B/m is the cyclotron frequency, where e* is elementary charge, m and τsc are the effective mass and scattering time of a carrier, respectively. For a magnetic field to be strong, ωc × τsc should exceed 1, because to form a cyclotron orbit, it is required that a carrier should be able to accomplish at least one turn along the orbit before it scatters. The value of pωc achieved at B ) 13 T for an electron (pωce) and for a Nano Lett., Vol. 9, No. 1, 2009

hole (pωch) is calculated to be 22.6 and 3.4 meV, respectively.17 Typical scattering times τsce (τsch) for the carriers have earlier been evaluated to be τsce ) 3.40 ps and τsch ) 0.74 ps.19 Thus, we find ωce × τsce ) 23.1 and ωch × τsch ) 0.75 at B ) 13 T. Obviously a magnetic field of 13 T is still small to affect the motion of holes, while already at B ) 1 T, the kinetic energy of electrons should become quantized in units of pωce (i.e., Landau levels are formed). Consequently, our experimental results will be explained in terms of a model assuming Φe to be considerably affected by the field applied in Faraday geometry, while Φh is left unaffected. The effects of an external magnetic field on PL (Figure 1) remain almost the same at any hνex > EGaAs as can be deduced from a comparison of PL excitation (PLE) spectra recorded at different fields (see, e.g., Figure 1d). It is important to note that the decrease of IQD as a function of B was measured also at excitation with EWL < hνex < EGaAs (where EWL corresponds to the WL PL peak) but was found to be smaller compared to the case of hνex > EGaAs. This difference stems from the fact that at excitation below the band gap of GaAs, carriers can propagate in the plane of the thin WL while in case of hνex > EGaAs carriers are allowed to migrate in the rather thick (100 nm) layers of GaAs. The reduction of thickness of the layer in which carriers propagate leads to a considerable decrease of the carrier’s mobility and, hence, its scattering time due to the increased role of carrier scattering at interface roughnesses (see ref. 18 and references therein). Consequently, the value of τsce for electrons propagating in the WL could be considerably reduced (with respect to the case of GaAs) and could in fact break down the criterion ωce × τsce > 1 even for B ) 13 T. As a result we restrict ourselves to the case of hνex > EGaAs, where the effects shown in Figure 1 are more clear. To experimentally verify the idea that an external magnetic field applied perpendicularly to the plane of the sample suppresses Φe considerably more than Φh, spectroscopy of a SQD will be employed. Indeed, as shown in our previous work,14 each SQD could be in a neutral or a charged state depending on the ratio between Φe and Φh. Accordingly, if Φe and Φh are affected differently by a magnetic field, this should result in a change of the SQD’s charge state and, hence, in the PL spectra of the SQD with increasing field. Figure 2a shows µ-PL spectra of an individual QD measured in Faraday geometry for various fields with above GaAs band gap excitation (hνex ) 1.687 eV). The essentially different excitation power (P0 ) 1 mW and 20 nW) used to obtain the spectra shown in Figures 1 and 2 correspond to approximately the same power density (≈0.5-0.6 Wcm-2) if one takes into account the size of the laser spot on the sample surface (400 vs 2 µm). At B ) 0, the µ-PL spectrum is entirely dominated by the PL line marked X0, which splits into several components with increasing field (Figure 2a). IQD is decreased by 5 times, when B is increased from 0 up to 5 T (inset in Figure 2a), that is, the decrease of IQD for the SQD is considerably stronger than that for the case of any high density QDs ensembles (inset in Figure 1a-c). This observation is consistent with our conclusion that the sparser 355


Figure 2. µ-PL spectra of SQD_1 measured at T ) 4.2 K, P0 ) 20 nW for (a) hvex ) 1.687 eV and (b) hvex ) 1.540 eV at a number of B-fields as indicated in the figure. The magnetic field is applied in Faraday geometry. The insets show a plot of IQD(B)/IQD(B ) 0) as a function of B.

the QD ensemble is, the higher is the quenching of IQD with increasing magnetic field. In striking contrast with the µ-PL spectrum recorded at B ) 0 with hνex ) 1.687 eV (Figure 2a), the µ-PL spectrum of the same SQD obtained under excitation with hνex ) 1.540 eV at B ) 0 consists of two PL lines marked as X1- and X2- with no contribution of X0 recorded (Figure 2b). All these lines were identified in our previous work14 as the neutral (X0), singly (X1-), and doubly (X2-) negatively charged exciton complexes, that is, these lines are detected in the µ-PL spectra when the charge configuration of a QD at the emission event consists of 1e1h, 2e1h, and 3e1h, respectively. Such charge transformation, as evidenced in µ-PL spectra, measured at B ) 0 as a function of hνex, was explained in terms of different residual kinetic energies for the electrons (Eer),14 calculated with respect to the bottom of the conduction band of GaAs20 by release of a cascade of optical phonons. It was also concluded that at B ) 0 the collection area for holes, which of course could change depending on hνex, does not exceed that for electrons achieved at the same value of hνex, as is justified by the experimental fact that no positively charged exciton (X+) could be monitored in the µ-PL spectra at B ) 0 for any hνex > EGaAs, and the essentially (∼ 30 times) lower hole mobility (µh) compared to electron mobility (µe) in the GaAs barriers.22 A spectacular effect is seen in Figure 2b: Upon an increasing magnetic field, the amplitude of the X2- line decreases and instead the X1- and X0 lines gain intensities. At further increased field, even the X1- line decreases, while the X0 line is progressively enhanced to dominate at B ) 5 356

T (Figure 2b). Based on the above given interpretation of the X0, X1-, and X2- lines, it can be concluded that the magnetic field applied in Faraday geometry makes the charge state of the SQD more “positive”.19 These experimental data provide a direct demonstration that a magnetic field indeed suppresses Φe considerably more than Φh. Following this conclusion, one can predict that the positively charged exciton should develop in the µ-PL spectrum for a sufficiently high B-field at excitation conditions such that Φe ) Φh at B ) 0. Indeed, these conditions are fulfilled at excitation with hνex ) 1.687 eV, because at B ) 0, the µ-PL spectrum entirely consists of the neutral exciton X0 line (Figure 2a). It is clear that at B ) 5 T, the PL spectrum consists of more than two PL lines (Figure 2a), while in the case of the “pure” neutral exciton, X0, the magnetic field applied in Faraday geometry should split the PL line into only two components due to the Zeeman effect,21 as is observed in Figure 2b for B ) 5 T. Combining these data (compare lower PL spectrum in Figure 2a with upper PL spectrum in Figure 2b), one could interpret the multiline structure (in Figure 2a) as a manifestation of the positively charged exciton appearance. It should be noted that in contrast to the strong decrease of IQD with increasing B-field measured at hνex ) 1.687 eV (inset in Figure 2a), IQD(B) recorded at hνex ) 1.540 eV remains almost unchanged in the region of 0 < B < 5 T (inset in Figure 2b). We remind here that IQD means the spectrally integrated intensity of the total PL spectrum (including all PL lines). As stated above, our data are consistent with a B-field induced suppression of Φe, which is stronger than that of Φh. Indeed, each photon is due to the recombination of 1e and 1h. Accordingly, if there is a deficit of carriers of one sign, for example, holes (as in Figure 2b for B ) 0), IQD will be determined by the minority carriers, that is, holes in this example. As the magnetic field does almost not influence Φh, the probability for the SQD to be populated with holes and, hence, IQD, should be almost B-field independent. The only effect of the B-field should be a decreasing number of majority carriers (electrons), which results in a redistribution effect in favor of the X0 line (Figure 2b). In Figure 2a, the SQD at B ) 0 is populated with electrons and holes with equal probabilities and, as an increase of B results in a decrease of Φe, electrons become the minority carriers with increasing B. This explains the progressive decrease of IQD with increasing B-field (inset in Figure 2a). It should be pointed out that the correlation between the dependence of IQD on the B-field applied in Faraday geometry and the accumulation of extra negative charge in the QDs at B ) 0 (as revealed in single dot spectroscopy in Figure 2a and b) could be used as an effective tool (criterion) to distinguish whether QDs are negatively charged or not. (We stress here that this idea could only be used if extra negative charge accumulated in the QDs is entirely due to optical pumping, and not due to residual impurities which are inevitably present in the sample because of nonintentional doping during the growth procedure of each sample.) In this connection it would be interesting to mention that in the case Nano Lett., Vol. 9, No. 1, 2009


Figure 3. µ-PL spectra of SQD_2 measured at T ) 4.2 K, P0 ) 20 nW, hvex ) 1.540 eV for various B-fields applied in Faraday geometry, as indicated in the figure. The inset shows a plot of the SQD’s neutrality degree, expressed by the parameter R ) IX°/IQD as a function of B for SQD_1 and SQD_2 by filled and open symbols, respectively.

of high dot density, IQD progressively decreases as a function of B, as was stated above, by approximately the same amount for any excitation energy hνex > EGaAs, and also for hνex ) 1.540 eV (see Figure 1d), in deep contrast with the experimental findings shown in the inset in Figure 2b for the low dot density. Accordingly, we argue that in the case of high dot density, most of the individual QDs of the ensemble (Figure 1a-c) are neutral at B ) 0, even at excitation with hνex ) 1.540 eV. This conclusion is in contradiction to the case of SQD (Figure 2b) at a first glance. However, this contradiction could be avoided if one takes the collection areas for electrons (σe) and holes (σh) into account. Indeed, for the case of only one QD located within the laser spot area, σe could be as large as the laser spot, while in the case of high dot density, the upper limit of σe for each individual QD of the ensemble can not exceed the average interdot distance. Consequently, in the case of dense ensembles of QDs, σe may not exceed σh, even for excitation energies most “favorable” for optical charging. Figure 3 shows µ-PL spectra of another individual QD recorded at the same experimental conditions as used in Figure 2b. While the µ-PL spectrum measured at B ) 0 in Figure 3 is almost the same as the corresponding µ-PL spectrum in Figure 2 b, the spectra are considerably different at B ) 5 T. This difference becomes more evident from a plot of the parameter representing the QD’s degree of neutrality, defined as R ) IX0/IQD, where IX0 is the spectrally integrated PL intensity of the X0 line (inset in Figure 3). For similar experimental conditions, the ability of an external magnetic field to suppress Φe changes from one SQD to another, that is, depends on the particular sample spot studied. To explain this difference, we need to consider a model of the carrier migration in the sample prior to capture into the SQD. The suppression of Φe with increasing B-field has been explained in terms of a slowing down of the electron motion, which, in turn, considerably increases the probability for electrons to be captured into localizing potentials on their Nano Lett., Vol. 9, No. 1, 2009

way toward the QD.19 These localized potential fluctuations (with lateral sizes from 5 up to several 100s of nm25) are due to the growth-induced variations of the alloy composition and the strain along the plane of the WL.7,25,26 In addition, there is an internal, built-in electric field (F) inside the structure with a component directed in the plane of the sample. The origin of F is suggested to be due to ionized impurities spatially separated from the QD.27-29 Our earlier experiments performed on SQD and QD ensembles allowed us to estimate a characteristic size of the localizing potentials of L ≈ 47 nm and a time and space-averaged magnitude of the internal field of F ≈ 500 V cm-1.30,12 We have also demonstrated that a complementary IR laser effectively compensates the internal electric field.12 The velocity that a carrier could achieve in an electric field is given by VFe(h) ) µe(h) × F, which for T ) 4.2 K is calculated to be VFe ) 45 × 106 cm/s and VFh ) 1.5 × 106 cm/s, respectively.22 The carrier drift velocity in crossed electric and magnetic fields is given for both electrons and holes by Vdr ) F/B. It is seen that at B ) 5 T, Vdr ) 1 × 106 cm/s, which is evidently less than VFe, and hence, an electron is essentially slowed down compared to the case of B ) 0 (by ∼ 45 times). Within the model for the probability of carrier capture into localized potentials, introduced in ref 19, we can write Φe as Φe(B ) 0) ) Φe(0) )

Φe(B) )

C 1+

C LB 1+ e τacF

L τeacµeF

(1a)

(1b)

where C is a constant and τace ≈ τsce ) 3.4 ps is the characteristic time for an electron to emit an acoustic phonon. The B-field in eq 1b should exceed 0.56 T to provide ωce × τsce g 1. Thus, with F ) 500 V cm-1 and L ) 47 nm, one can estimate Φe(0)/ Φe(B ) 5 T) ≈ 2.3. This estimate is in reasonable agreement with the data obtained for SQD_1 (Figure 2b and the inset in Figure 3). Indeed, the dominance of the X2- line in the µ-PL spectrum in Figure 2b at B ) 0 means Φe(0)/ Φh(0) ≈ 3, while to observe a predominantly “neutral” charge state of the SQD (i.e., R ≈ 1), the condition Φe(B)/Φh(B) ≈ 1 should be fulfilled. It is also interesting to note that Vdr at B ) 5 T turns out to be approximately the same as VFh, a circumstance that gives additional support to the conclusion that Φh remains almost unaffected by B. It follows from eqs 1a and 1b that the ratio Φe(0)/Φe(B ) 5 T) decreases (increases) with an increasing (decreasing) internal field, F. Consequently, the experimental fact that the parameter R as a function of B measured for SQD_2 is (for any 0 < B < 5 T) less than that for SQD_1 (inset in Figure 3) is interpreted in terms of a larger internal field in the vicinity of SQD_2 compared to the vicinity of SQD_1. To experimentally check this idea, it is desirable to modify the internal field, F in a controllable way. As stated above and elsewhere,12 an additional IR laser can (depending on its excitation power, PIR) effectively compensate this field. Figure 4 shows three pairs of µ-PL spectra recorded with single (thin solid lines) and dual (thick solid lines) laser 357

effective experimental tool to manipulate the charge state and emission intensity of an individual QD. Acknowledgment. This work was supported by grants from the Swedish Foundation for Strategic Research (SSF) and the Swedish Research Council (VR).


References

Figure 4. µ-PL spectra of SQD_1 measured at T ) 4.2 K, P0 ) 20 nW, hvex ) 1.589 eV, PIR ) 50 µW, hvIR ) 1.23 eV and at a number of B-fields (as indicated in the figure) applied in Faraday geometry for single (thin solid lines) and dual (thick solid lines) excitation conditions. The inset shows a plot of the SQD’s neutrality degree, expressed by the parameter R ) IX°/IQD as a function of B for single and dual laser excitation conditions by open and filled symbols, respectively.

excitation in the presence of a B-field. The SQD’s degree of neutrality, R, is shown as a function of B for the case of single and dual laser excitations (inset of Figure 4). For the principal excitation, we used here a semiconductor laser with hνex ) 1.589, which results in a predominant X2- line in the µ-PL spectrum. For the case of dual laser excitation, the “neutralization” of the SDQ’s charge state becomes more complete compared to the case of single laser excitation. Indeed, the R-parameter measured for dual laser excitation exceeds that for single laser excitation at any B g 1 T (see inset in Figure 4). This, in turn, means that Φe is suppressed more effectively when an IR-laser (which partially compensates F) illuminates the sample in addition to the principal laser, as expected from our model. It follows directly from eq 1a that Φe(B) is a function of the ratio B/F and, hence, if the electric field F has been reduced, the same flux Φe (and accordingly, R) can be achieved for a lower magnetic field, B. We finally point out that the great similarities between the insets of Figures 3 and 4 is a strong support for our interpretation that the different redistribution strengths observed for the two SQDs (SQD_1 and SQD_2) can be attributed to different magnitudes of the built-in electric field around the two dots. Conclusions. The PL intensity of QD ensembles was found to be considerably reduced by an external magnetic field applied in Faraday geometry, while, surprisingly, the PL emission from the WL was unchanged. This phenomenon is explained in terms of different efficiencies of suppression of the electron and hole fluxes toward the QDs by the magnetic field. The suggested explanation has been tested on individual QDs and as a result it has been demonstrated that an external magnetic field applied in Faraday geometry as well as a complementary IR laser could be used as an 358

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Reithmaier, J. P.; Klopf, F.; Scha¨fer, F. Phys. ReV. B: Condens. Matter Mater. Phys. 2002, 65, 195315. (22) It should be noted that for the case of electrons, experimental data on mobility are available for the range of T’s down to 4.2 K,23 while for the case of holes the lowest value of T is 20 K.24 To extract the value of µh at T ) 4.2 K, we used the following procedure: First, on the base of formulas given in ref 26, the value of µh was calculated for T ) 20 K. It was found to differ by only 30% compared with µh measured in ref 24 for T ) 20 K. Second, we calculated µh ) 2970 cm2/V s for T ) 4.2 K and compared this value with µe ) 90000 cm2/V s, as was measured for T ) 4.2 K23 (i.e., µe is larger than µh by a factor of 30.3). It is important to note that the ratio µe/µh obtained from experiment23,24 at T ) 20 K is ≈ 28. As a result, we consider µh ≈ 3000 cm2/V s could be chosen as a good approximation for T ) 4.2 K. (23) Wolfe, C. M.; Stillman, G. E.; Lindley, W. T. J. Appl. Phys. 1970, 41, 3088. (24) Hill, D. E. J. Appl. Phys. 1979, 41, 1815.

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Comparative Magneto-Photoluminescence Study of Ensembles and of

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