Proc. of SPIE Vol O-1

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1 Photoluminescence Study of Self-Assembly of Heterojunction Quantum Dots(HeQuaDs) Kurt G. Eyink 1 ; David H. Tomich 1 ; S. Munshi 1 ; Bruno Ulrich 2 ; Wally Rice 3, Lawrence Grazulis 4, ; J. M. Shank 5,Krishnamurthy Mahalingam 6 1 Air Force Research Laboratory, Wright Patterson AFB OH 2 Bowling Green University, Bowling Green, OH 3 Wright State University, Dayton, OH 4 University of Dayton Research Institute, Dayton, OH 5 Southwestern Ohio Council for Higher Education, Dayton, OH 6 Universal Technology Corporation, Dayton, OH Recently quantum dots (QDs) have been the topic of extensive research. Unique properties arise in QDs due to a combination of the localized nature of their wavefunctions and a singularity in the associated density of states. Many strained III-V semiconductor film-substrate systems form QDs via a self-assembly process by means of a Stranski-Krastanov process. The strain relief responsible for the 3D nucleation causes a variation in the in-plane lattice constant which allows subsequent QD layers separated by thin spacer layers to be vertically stacked. Recently this concept has been extended to allow the formation of a heterojunction quantum dot (HeQuaD). In this structure an initial self-assembled QD (SAQD) is formed and then a different similarly strained material is nucleated on the initial SAQDs forming a crown on the underlying QD. This crown is also of a size appropriate to cause quantum confinement. In particular a stack of 4 layers of a HeQuaD structure of a GaSb crowned InAs SAQD on GaAs with GaAs spacer layers has been formed. The top HeQuaDs have been left uncapped to allow AFM analysis of the morphology. Photoluminescence of the HeQuaD has 3 peaks at ~0.95eV, 1.15eV, and 1.35eV. We have measured the intensity and temperature dependence of these PL peaks. Introduction Quantum dots are typically formed during molecular beam epitaxy of semiconductor systems via a self-assembly process. QDs have been observed to form in most III-V systems when a sufficiently large lattice mismatch exists between the substrate and the film. InAs on GaAs has been the most extensively studied binary system although QDs have also been formed with, InP on GaAs 1, GaSb on GaAs 2, InSb on InAs 3, and InSb on InP 4 as well as many ternary alloys. This self-assembly process is a nucleation and growth process in which the reduction in strain energy is balanced by the increase in surface tension. The randomness in the nucleation and growth process causes a variation in the size and location of the QDs. If several layers of QDs are stacked, it has been found that for thin spacer layers, the position of the QDs in an upper layer are linked to position of the QDs in the layer immediately below. Under most conditions, the QDs vertically align however under certain conditions additional QDs may nucleate between existing QDs. The rationale for vertical alignment has been related to the variation in the in-plane lattice constant. This variation is caused by the Quantum Dots, Particles, and Nanoclusters IV, edited by Kurt G. Eyink, Diana L. Huffaker, Frank Szmulowicz, Proc. of SPIE Vol. 6481, 64810O, (2007) X/07/$18 doi: / Proc. of SPIE Vol O-1

2 spacer layer lattice expanding to accommodate the underlying QDs. Overtop of the QDs, the spacer layer lattice expands to be closer to the lattice constant of the unstrained QDs. Therefore QDs which nucleate out in subsequent growth will be less strained over top of the buried QDs. Therefore the additional QDs will preferentially nucleate over the lower QD and cause stacking. In addition to causing stacking, the uniformity in QDs size is enhanced. Better alignment is found for thinner spacer layers. Recently this effect has been extended to from a heterojunction quantum dot (HeQuaD) 5,6. In this structure, an initial SAQD is formed. A subsequent growth of a similarly strained material is carried out without a spacer layer. It has been shown that this allows the nucleation of the second material on the initial QD. If the bandstructure is correct to allow for confinement of electrons or holes a new quantum structure is formed with an internal heterojunction. In this work we look at the preliminary results from the analysis of the photoluminescence from these structures. Experimental All samples were grown in a modular Varian GenII MBE machine equipped with valved As and Sb crackers. All growths were done on substrates of N+ GaAs(001). The substrate was heated until the oxide thermally decomposed and 3D diffraction spots were visible in the RHEED pattern. A buffer layer was grown at rate approximately a 0.5 µm/hr with an As 2 beam equivalent pressure of 6x10-6 Torr for a time of 2 hours. The substrate temperature after oxide desorption and buffer growth was monitored by an IRCON Modline PLUS V Series infrared pyrometer which detects radiation in a wavelength band of µm. InAs self-assembled quantum dots (SAQDs) were grown at temperatures between 450 C and 525 C with an As 2 overpressure 2-8 x10-6 Torr. Subsequently GaSb was grown at similar temperatures and pressures in the range of x10-7 Torr for various time periods. This structure was repeated 3 times with a GaAs spacer layer thick enough to prevent vertical stacking. The last structure was left exposed to allow atomic force microscopy analysis of the structures. Samples were analyzed using a Park Scientific Autoprobe CP atomic force microscope using a combination of tapping and contact modes to eliminate artifacts. Photoluminescence (PL) measurements were excited with the nm line of an argon laser at laser power which ranged from mw having a beam diameter of 2.25 mm. The PL spectra was acquired with a Fourier Transform Infrared (FTIR) Bomem DA3 spectrometer equipped with a liquid nitrogen cooled InAs detector. The experimental data was recorded over the temperature range of 5-80 K by employing a temperature controlled helium-cooled cryostat. Results and Discussion All HeQuaD growths contained 3D structures which could be generally classified into two basic types. One type was a smaller elongated feature which covered the majority of the surface and the other type was a larger hemispherical feature which Proc. of SPIE Vol O-2

3 occurred at a much lower density. These large 3D features were surrounded by regions devoid of other 3D structures. The smaller structures are typically elongated along the <110>. is a 5 µm x 5 µm image of a 15 second 1.75 ML GaSb growth onto a 2.1 ML InAs SAQD structure. Energy Dispersive Spectroscopy (EDS) analysis has previously been performed on these elongated structures. However the condition used produced features much larger than shown in. This analysis showed that these structures are composed of an InAs rich core with a GaSb rich crown region. Under the condition used here the smaller dots were near 70 nm along the 110 direction and nm in the perpendicular direction. The large features are similar to structures found with the growth of GaSb 7 and InAs 8 dots. In the case of InAs and GaSb dots these could be eliminated by growing with a higher group-v pressure. We have not observed the removal of these large 3D features with a change in the Sb over pressure from 1.7 x 10-7 Torr to 9.4 x 10-7 Torr. Photoluminescence was performed on a series of samples in which the time of GaSb deposition was changed between 15 and 22 seconds. Figure 2 shows the PL response in the photon energy region of 0.8 to 1.6 ev for the 15 and 22 second depositions. In each spectrum, three peaks are observed at approximately 0.9, 1.2, and 1.35 ev with corresponding approximate widths of 100, 100 and 25 mev respectively. The intensity and position of these peaks changed with GaSb deposition. The peak at 1.35 ev is significantly narrower than the other two. Peaks similar in energy to these are observed in InAs and GaSb QD growths. InAs SAQDs typically have a QD PL peak in the range of ev although extremely large QDs show a PL emission near 0.83eV. 9 GaSb QDs have an emission in a similar range 10. We currently believe the PL peak at 1.2eV is from the HeQuaD quantum structure or a QD type peak. Both InAs and GaSb have a wetting layer peak near the HeQuaD peak at 1.35 ev. The wetting layer peak from GaSb SAQDs on GaAs 10 occurs at ~1.33eV and from InAs SAQDs on GaAs 11 occurs at ~1.43 ev. In both cases this peak is much narrower than the PL emission from the QD distribution and similar to the HeQuaD peak at 1.35 ev. Comparing the intensity of the WL peak to QD peak show a major difference in the GaSb and InAs spectrum. In the case of GaSb QDs, the WL has a height comparable to the height of the QD peak as opposed to the InAs QDs the WL peak is much smaller than the QD peak. In the 15 seconds GaSb HeQuaD case, the 1.35 ev peak had an intensity comparable to the HeQuaD peak at 1.2 ev. With additional GaSb deposition of 22 seconds, the 1.35 ev peak became much less intense as compared to the 1.2 ev peak. Therefore if the 1.2 ev peak is a QD peak for the HeQuaD structure and the 1.35 ev peak is a WL peak, a change in the ratio of the WL peak to the QD peak was observed with GaSb deposition. EDS analysis of the wetting layer was performed and showed it to be composed of predominately InAs. This result rules out the formation of a GaSb like wetting layer when the InAs SAQDs were capped with GaSb as a cause for the high intensity of the 1.35 ev HeQuaD peak. It has been argued the increased intensity of the WL in GaSb is not due to composition but rather due to a much slower recombination in the GaSb QD due to Type-II alignment 10. The GaSb/InAs/GaAs HeQuaD is proposed to have separate confinement for the electron and holes, a similar behavior should be expected here as observed. However it should be noted the assignment of the 1.35 ev peak to the wetting Proc. of SPIE Vol O-3

4 layer has not been established yet. The peak at 0.9eV is an additional peak which we believe to be a ground state transition for the HeQuaD structure. Intensity dependent PL and temperature dependent PL was performed on a structure similar to the 15 second GaSb deposited structure. Figure 3 shows the PL response as the laser power was changed from 250mW to 2000mW corresponding to a power density of 6.3 W/cm 2 to 50 W/cm 2. The intensity of each peak changed linearly with input power. For type II aligned QDs such as GaSb, shifts in the peak position are expected to occur as the laser power is increased. Figure 4 is a plot of the variation in the peak position of all three peaks as a function of power density, P. As can be seen little or no variation was observed. This is in stark contrast to the P 1/3 dependence observed 12 for type II QDs such as GaSb on GaAs. Suzuki et al. 10 believe the cause of this effect arises from the electron states being localized in a triangular well adjacent to the GaSb. This energy level is expected to have an energy level whose position depends on cube root of the excitation power. In our case the electron and hole states are expected to be spatially confined in the InAs and GaSb respectfully so this effect may not be expected. More studies need to be performed to determine why our structure does not have the P 1/3 dependence. Figure 5 shows the PL response for analysis temperatures of 5K, 20K, 30K, 40K, and 80K. As the temperature was increased, the WL intensity dropped strongly for temperatures above 40K. The 1.2 ev peak showed little variation and the 0.95 peak started to show a slight temperature dependence around 80K. Although we do not have sufficient temperature dependent data to accurately analyze the emission characteristics of the HeQuaD structure, the data is plotted in an Arrhenius fashion in Figure 6. In the case of GaSb SAQDs, Suzuki et al found that the wetting layer had two activation energies 10. A 67 mev activation energy was observed for temperatures higher than 150K which are outside of the range studied here. In the K range they observed a thermal barrier of 18 mev. In our case, we see an activation energy of 6meV calculated from our 40K and 80K PL data. This value would indicate a much shallower level for this structure than for the GaSb data. Additional data must be obtained to confirm this point. It should be pointed out that the above results are consistent with our proposed structure of the HeQuaD 5. Figure 7 is a diagram for the strained InAs, and GaSb band line-up in quantum dots formed on GaAs 13. As can be seen from this plot in the GaSb/InAs/GaAs HeQuaD structure, the electrons would be localized in the InAs and the holes would be localized in the GaSb. This arrangement would result in reduced overlap between the electron and hole states. Therefore the sharp peak in our structure is likely our wetting layer. The intensity is high due to an increased lifetime in the HeQuaD state due to spatially separated electron and hole states. We do not observe the dependence on cube root of the laser power density since the electrons are not held in a region formed by s charge but rather are confined in the InAs layer. As such we should not expect to see the power dependence observed in the GaSb case. Proc. of SPIE Vol O-4

5 Conclusions The photoluminescence spectra was studied from a HeQuaD structure composed of a SAQD of InAs on GaAs crowned with GaSb. Three peaks have been observed in the PL spectra at approximately 0.95, 1.2, and 1.35eV. The line at 1.35 ev has been tentatively associated with the wetting layer. This peak occurs at a similar energy as the WL peaks for InAs and GaSb. The 1.35 ev peak quenches out with temperature in a similar manner to the GaSb wetting layer. An Arrhenius plot suggested an activation energy for thermal emission from the WL of 6meV. EDS analysis of the wetting layer showed the HeQuaD WL was composed mainly of InAs. The two other peaks at 0.95 and 1.2 ev are probably due to HeQuad levels and showed little or no quenching up to 80K. We are in the process of determining the cause of these emission peaks. The 1.35 ev peak has a comparable intensity to the 1.2 ev peak. The high intensity of the WL peak argues that the HeQuaD is a type-ii structure. The photoluminescence associated with these peaks was found to shift very little with laser power in contrast to GaSb-like type II QDs. We speculate this lack energy shift with laser power results from the confinement of both the electron and holes wavefunctions in wells. Proc. of SPIE Vol O-5

6 Figure 1. An AFM image of a 5 µµ x 5 µµ area of a HeQuaD structure is shown. The amount of GaSb deposition for this structure was 1.75ML. The inset in the image is of a 1 µµ x 1m area which shows the shape of the smaller dots. Proc. of SPIE Vol O-6

7 sec 22 sec Noramilzed Intensity X photon energy (ev) Figure 2. 5 K photoluminescence spectra of a two HeQuaD samples with 15 and 22 seconds of GaSb on top of 2.1 ML of InAs SAQDs on GaAs(001) is displayed. Three peaks are observed in the photoluminescence spectra at approximately 0.95eV, 1.2eV, and 1.35eV. The overall intensity has been normalized to the peak near 1.2 ev. Proc. of SPIE Vol O-7

8 Intensity mW, 5K 250mW, 5K 500mW, 5K 750mW, 5K 800mW, 5K 1000mW, 5K 1200mW, 5K 1500mW, 5K 1800mW, 5K 2000mW, 5K photon energy (ev) Figure 3. The photoluminescence response for various input laser power. Proc. of SPIE Vol O-8

9 10 Deviation in Peak Energy (mev) eV peak 1.25eV peak 0.95eV peak Excitation Power Density Figure 4. The variation in energy of the nominally 0.95 ev, 1.2 ev, and 1.35 ev peaks as a function of excitation power density. All curves show less than 5meV variation Proc. of SPIE Vol O-9

10 K 40K 30K 20K 5K Figure 5. The photoluminescence response for a HeQuaD plotted for various analysis temperatures. Proc. of SPIE Vol O-10

11 eV peak 1.2eV peak 0.95eV peak Peak Area 1000 E ~ 6meV /T (K -1 ) Figure 6. Logarithm of the integrated PL intensity plotted as a function of the reciprocal temperature. The curve indicates an energy of ~6meV for the thermal activation of the WL. The QD levels (0.95 and 1.2 ev) show essentially no temperature dependence up to 80K. Proc. of SPIE Vol O-11

12 2 E c 1 e - 0 E v h + -1 GaAs InAs GaSb Figure 7. Band line up for GaAs, InAs, and GaSb from Pryor ref 13 Proc. of SPIE Vol O-12

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