A spectacular effect was observed when, in addition to Figure 2. µPL spectra of SQD (a) and MQDs 1 (b) measured at the excitation of L0 at hex = 1.471 eV, the sample was hex = 1.471 eV, hIR = 1.240 eV, for a number of temperatures as illuminated with an infra-red laser, LIR. Indeed, the µPL indicated in the figure and at excitation with a single (dotted lines) spectrum of the sample (solid lines in Fig. 1) undergoes and double (solid lines) laser, respectively at an excitation power a dramatic change in this case: While IWL increases by a of P0 = 200 nW and PIR = 100 µW. The curved dash-dotted lines factor of 2.5, the SQD emission almost completely vanishes. in (a) are guides for the eye.
Физика твердого тела, 2005, том 47, вып. Effect of the electric field on the carrier collection efficiency of InAs quantum dots This experimental finding is unclear and further studies are needed to reveal its nature. However, some insight which can elucidate the situation can be achieved from spectral diffusion — a phenomenon previously observed in single self–assembled quantum dots [28,29]. In particular, it was found that the electric field induced by the charges, which are located in the close vicinity of a QD, initiated an energy shift of the QD spectral lines, which were quasi-periodic in time [28,29]. Consequently, the widths of the spectral lines registered in the time-integrated regime (as in the present manuscript) is expected to broaden in case of the existence of quasi-periodic (in time) local electic fields around the SQD under study . That is why the experimentally observed broadening of the QD PL line and its energy shift detected in the temperature range around T = 25 K (Fig. 2, a) could be regarded as an evidence for the presence of a local electric field arond the QD.
The effect of change of IQD induced by the IR laser could quantitatively be described in terms of a quenching ” rate“, RQD, which is defined as a ratio of IQD measured with dual laser excitation (Idual) to be compared with the QD corresponding single laser excitation PL intensity (Isignle).
QD RQD values measured for a SQD are shown by solid squares in Fig. 3, a. It is seen that RQD being as low as 0.1 at T = 5 K, progressively increases to reach the value of 1 at T 60 K, i. e. the quenching effect gradually disappears with increasing T.
A qualitatively similar behaviour could be recorded for another sample spot with a higher dot density (MQDs 1).
The µPL spectra in this case consist of a number of sharp lines superimposed on a broad PL band in the spectral range of 1.270-1.310 eV (Fig. 2, b). It is clearly seen that the IR laser also induces a quenching of the PL signal at low Figure 3. RQD (a) for hIR = 1.240 eV and PIR = 100 µW temperatures. The quenching rate is found to increase from and normalized values of Isingle (b) for SQD, MQDs 1 and QD RQD 0.5 at T = 5 K with increasing temperature to reach MQDs 2 measured for a number of T ’s at hex = 1.471 eV RQD 1 already at T 30 K (solid circles in Fig. 3, a).
and P0 = 200 nW shown by solid squares, circles and triangles, This T is essentially lower than the temperature needed respectively.
to completely cancel the quenching for the SQD (60 K) (Fig. 3, a).
In sharp contrast to these observations, similar quenching measured with dual laser excitation (solid lines in Fig. 2, a) effects were practically not registered at a sample position reveal both a redistribution of the emission lines in favour with even higer dot density (MQDs 2). In fact, RQD 0.of the X line and a progressively vanishing quenching effect already at T = 5K (solid triangles in Fig. 3, a) and stays with increasing T.
at a value around 1 for T > 5 K. In the following we will The redistribution effect in favour of X at elevated present a qualitative model which explains the quenching temperatures is clearly illustrated in Fig. 2, a. This fact phenomenon as well as its disappearance at increased clearly demonstrates that LIR supplies the sample with extra temperature and dot density.
h’s which can be captured by the QD, and accordingly To explain the quenching effect we note first that at an effectively neutralizing“ its charge configuration. The excitation energy of hex = 1.410 eV of the principal laser ” generation of extra h’s is assumed to take place in the GaAs L0 (the vertical solid arrow hex in the inset in Fig. 1) barriers as a result of an IR laser induced electron excitation IR laser had no effect on IQD (results not shown here).
from the GaAs valence band into DLs always present in It means that LIR can not influence the number of e-h the GaAs bandgap  (arrow IR2 in the inset in Fig. 1). paris which already have been captured into the QD (at It is interesting to note that the PL spectrum measured these experimental conditions e’s and h’s are not subjected under dual–laser excitation at T = 25 K is very broad and to transport along the plane of the WL prior to capture a peak appears between the X and X- lines (Fig. 2, a). into the QD). Consequently, the reason for the observed Физика твердого тела, 2005, том 47, вып. 2070 E.S. Moskalenko, K.F. Karlsson, V. Donchev, P.O. Holtz, W.V. Schoenfeld, P.M. Petroff decrease of IQD, registered for an excitation energy of The experimentally measured RQD can be expressed in hex = 1.471 eV, is the influence of the LIR on the transport terms of collection efficiencies in the following way:
properties of the carriers in the WL plane. Secondly, dual RQD = · RWL, (1a) IQD is proportional to the collection efficieny as was single explained above. Accordingly the observed decrease of IQD where means a considerabel reduction of measured under dual Idual WL RWL =, (1b) laser excitation (dual) with respect to the corresponding Isingle WL single laser excitation conditions (single). Thirdly, as was stated above, LIR supplies the sample with excess h’s. Isignle(Idual) is the WL PL intensity with single (dual) laser WL WL Consequently, the model suggested should explain how the excitation. To explain the observed increase of RQD with increasing T (Fig. 3, a) we need, according to Eq. 1a, apperance of the surplus h’s can affect the carrier transport information on temperature evolution of both RWL and in the WL plane.
We propose the following model to explain the influence Fig. 4, a shows RWL for a SQD measured as a function of the extra charge on the collection efficiency of the photoof temperature. It is seen that its value is progressively created carriers from the WL into the QD. The existence reduced down to RWL 1 at elevated T ’s. Consequently, of an electric field F in the plane of the WL is assumed.
according to Eq. 1a, the only reason for the observed Photo-excited e’s and h’s, generated at arbitrary spots within increase of RQD (Fig. 3, a) is the increase of dual/signle as the laser illumination, move along the plane of the WL for T becomes higher. Obviously there could be two basically some time ( ) decreasing their kinetic energy until they different reasons for the increase of dual/signle : single bind together and recombine as excitons, contributing to reduces down to dual as T increases or, on the contrary, IWL. The QD can capture carriers / excitons only for the dual could increase significantly to reach the value of single case, when the capture time from the WL into the QD is at elevated temperatures. The first possibility would imply a less than. Accordingly, a rather high velocity is needed to temperature-induced screening of the built-in electric field, have a non-vanishing probability for carrier transport and which facilitates carrier capture into the QD. The second capture into QD. The carriers are assumed to achieve a possibility would correspond to the experimental situation rather high velocity with respect to the thermal velosity that the carrier capture into the QD at high T ’s becomes so when their transport is influenced by the field F. effective that the role of the field F for the carrier transport progressively diminishes with temperature.
The origin of the built-in field is at present not known and To distinguish between these two possibilities we have further studies are needed to reveal its nature. The origin studied the temperature evolution of Isignle and Isingle is believed to be due to donors and acceptors which are QD WL positioned in the vicinity of the QD [15,16]. An e from (Fig. 4, b). It is seen that with increasing T Isingle WL is progressively reduced (by more than two orders of the donor atom can be captured by an acceptor, giving rise to a built-in field F with a component along the WL plane magnitude for the total T range studied), while Isingle QD as a result of the charge separation. When the surplus h’s, reveals an essential increase (> 5 times) when T changes photoexcited by the LIR, appear in the WL, the carriers from 5 up to 40 K to dominate the µPL spectrum of a will move along the direction of the built-in field. If these sample for T > 40 K. The considerable decrease of Isingle WL extra h’s are localized at the interface potential fluctuations, (Fig. 4, b) could be explained in the following way. At low T’s photoexcited carries are captured into localized states they could stay there for a rather long time (until an e of the WL (processes shown by arrow r in the inset to recombine with is approaching), providing an effective in Fig. 1) and stay there until they recombine radiatively screening of the field F. (A more detailed discussion of contributing to the PL signal of the WL (arrow IWL in the screening mechanism is given in ). Consequently, the inset in Fig. 1). The existence of such localized states the transport of e’s and h’s, excited in the WL by L0 and in the WL is evidenced by the observation of a number LIR will be determined by the thermal velocity which is of sharp peaks (separated from each other by < 1meV) assumed to be essentially lower than the drift velocity at superimposed on the low-energy tail of the WL emission the unscreened field conditions, and accordingly a decrease band (Fig. 1). As T increases, carriers become delocalized of IQD (RQD < 1) is expected.
and their thermal velocity increases. These two effects It is important to note that the absolute value of RQD result in a more efficient transport of carriers along the measured at given experimental conditions (with respect to plane of the WL. Consequently, the probability for carrier to P0, PIR and T ) for other individual SQDs was found to approach a QD (proportional to c) or alternatively a CNR exhibit variations depending on the particular SQD under (proportional to nr) is considerably increased at elevated study. This is reasonable since the concentration and space T’s. This eventually results in both processes. The increased distribution of impurity atoms in the close vicinity of a probability for capture at a CNR will contribute to the given SQD, which determines the value of F, should have quenching of the WL PL, while increased probability to variations depending on their exact location in the sample. become trapped in a QD will contribute to the enhancement Физика твердого тела, 2005, том 47, вып. Effect of the electric field on the carrier collection efficiency of InAs quantum dots of the SQD PL signal (Fig. 4, b). The crucial role of the IWL (RWL > 1). At elevated T ’s, the capture to the CNRs carrier transport prior to the trapping into the CNR has becomes more efficient, as explained above. Consequently, the LIR-induced effect of carrier release from the CNRs been described elsewhere .
The observed temperature dependence of the RWL (arrow IR1 in the inset in Fig. 1), which initiates an essential increase of the IWL at low T ’s, starts to play a minor role (Figl. 4, a) can be qualitatively explained in terms of an at elevated T ’s. This explains the gradual decrease of RWL essentially increased value of nr at elevated T ’s. Indeed, with increasing T (Fig. 4, a).
at low T’s, i. e. low values of nr, carriers which are We thus conclude that the observed increase of the IQD is released from the CNRs (as a result of influence of LIR) entirely determined by T -induced changes in the transport were captured into localized states of the WL prior to of carriers. This conclusion is justified by the following capture back to the CNRs. This resulted in the increase of experimental observation. At a principal laser excitation energy of hex = 1.410 eV (shown by the vertical solid arrow hex in the inset in Fig. 1), i. e. at excitation directly into the dot, when no transport of carriers along the plane of the WL is needed prior to capture into the dot, the SQD PL signal does not change in the temperature range 5 < T < 60 K (see inset in Fig. 4, b).
Fig. 4, c shows the experimentally derived values of dual and single measured for a SQD as a function of temperature. It is seen that both dual and single progressively increase with increasing T, with dual single at elevated temperatures, which in turn results in the disappearance of the IQD quenching effect. Consequently, we explain the experimentally observed fact of the progressive increase of the RQD up to 1 at elevated T ’s (Fig. 3, a) in terms of a considerable increase of the QD collection efficiency, rather than by a decrease of single down to dual. In other words, at increased temperatures, the SQD collects carriers so effectively that the role of the internal field which improves the carrier transport at low T ’s becomes negligible.
It is interesting to note, that the increase of T results in a qualitatively similar behaviour of IQD and IWL measured on high dot density spots. Typical examples of the temperature evolution of Isingle measured for the MQDs 1 and MQDs QD and compared with the SQD are shown in Fig. 3, b. An increase of the Isingle by approximately 2.7 times was QD recorded for MQDs 1 which is essentially less than the corresponding value of 5.5 measured for the SQD (Fig. 3, b).
In contrast to these observations, only a small increase (up to 50%) of the Isingle was revealed for MQDs 2. The QD observed behaviour (Fig. 3, b) is consistent with the idea of a temperature-improved transport of carriers in the plane of the WL. In fact, the different sample positions reveal different values of single at T = 5 K, namely 0.01, 0.1 and 2.3 for the SQD, MQDs 1 and MQDs 2, respectively.
We note here that in the upper limit of the value of collection efficiency (signle ) all the carriers generated are able to capture into the quantum dots even at low T.
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