We demonstrate photon antibunching from a single lithographically defined quantum dot fabricated by electron beam lithography, wet chemical etching, and overgrowth of the barrier layers by metalorganic chemical vapor deposition. Measurement of the second-order autocorrelation function indicates g(2)(0) = 0.395 ± 0.030, below the 0.5 limit necessary for classification as a single photon source.
© 2011 OSA
Single photon emitters are important for quantum key distribution (QKD) [1,2], quantum metrology, and quantum information processing applications [3,4]. Sources of single photons include epitaxial self-assembled quantum dots (SAQDs) [5–7], colloidal quantum dots , single molecules , and color centers in diamond [10,11]. Of these sources, epitaxial SAQDs are attractive due to their ease of fabrication and incorporation within other photonic structures such as optical cavities. While it has been demonstrated that an optical cavity can be positioned and fabricated around a single SAQD with a high degree of accuracy [12–14], the random nature of the SAQD nucleation process limits the scalability of this approach.
As a result, numerous techniques have been investigated for deterministically positioning a QD such as growth in etched nanoholes [15,16], atomic force microscopy , and growth in inverted pyramidal recesses . While these approaches have been shown to produce high optical quality site-controlled QDs, the bottom-up approach to fabrication implies that the QD properties depend critically on growth parameters and the properties of the prepatterned substrate such as the separation between QDs in the array and the individual pattern dimensions.
In recent work, we demonstrated a top-down approach to QD fabrication that utilizes a combination of electron beam lithography and wet etching of a preexisting quantum well [19,20]. Electron beam lithography controls the size and position of the quantum dot, while the emission wavelength can be tailored by adjusting the quantum well thickness and indium composition prior to the etching process. The interpretation of our previous results presumed that QDs fabricated using this technique act as single quantum emitters. However, to date no measurement has been performed to establish this definitively. In this report we provide the first demonstration that a single lithographically defined QD behaves as a single quantum emitter and is capable of producing single photons.
The fabrication process begins with a base structure grown by molecular beam epitaxy (MBE) on an undoped (100) GaAs substrate and consists of a 20-period GaAs/AlAs distributed Bragg reflector (DBR) stack to increase the extraction efficiency of light produced by the quantum dots, a 130 nm GaAs lower core, an 8 nm In0.2Ga0.8As quantum well, and a 10 nm GaAs cap. The DBR was designed to provide a peak reflectivity of ~99.6% at 910 nm. Including more DBR periods would in principle increase the reflectivity to be closer to the ideal value of 100%. However, we felt that the value of 99.6% was sufficiently close to the ideal value for the purposes of this experiment. In addition, the QD emission wavelength depends on the size of the QD as well as strain effects, which makes the exact alignment of the QD emission with the reflectivity peak of the DBR difficult in practice.
Electron beam lithography was performed with a scanning electron microscope with an acceleration voltage of 30 kV and beam current of 20 pA. Polymethyl methacrylate (PMMA) was used as the electron beam resist. Regular arrays of dots were patterned in square lattices with various pitches ranging from 500 nm to 5 µm. The dot diameters were also varied between approximately 60 nm and 130 nm_by modifying the electron beam dose. After development, 20 nm of titanium metal was evaporated on the sample, followed by liftoff in acetone. The metal dot patterns were then transferred into the underlying quantum well layer by use of a phosphoric acid-based etchant. The etch was timed to provide an etch depth of 25 nm, resulting in QDs with diameters ranging between approximately 10 nm and 80 nm. Note that the diameters of the QDs are smaller than the diameters of the titanium etch masks due to undercutting during the wet etching process. After etching, the titanium was stripped in buffered hydrofluoric acid. Figure 1 shows an array of 30 nm diameter QDs on a 1 µm pitch after stripping the etch mask. Following the etching step, the barrier layers were regrown in a low pressure MOCVD reactor and consist of a 130 nm GaAs upper core, an 80 nm AlAs confinement layer, and a 10 nm GaAs cap.
Optical measurements were performed at a temperature of 4 K in a liquid helium cryostat. A modelocked Ti:sapphire laser (795 nm center wavelength, 82 MHz repetition rate) was used as the excitation source. The pump beam was focused down to a spot size of approximately 4 µm with a 0.6 numerical aperture microscope objective. Light emitted from the QD was collected by the same objective and focused onto the input slit of a 0.75 m monochromator. An internal mirror in the monochromator was used to switch between a liquid-nitrogen-cooled CCD camera for recording emission spectra and a Hanbury-Brown-Twiss interferometer (HBTI) for measurement of the second-order correlation function. The HBTI consists of a 50/50 beamsplitter, two single-photon avalanche diodes (SPADs), and timing electronics for recording the time interval between detection events on the two SPADs. The start-stop time intervals are recorded and binned with a resolution of 256 ps to form a histogram that is proportional to the second order intensity correlation function. Figure 2 shows the spectra of three QD arrays with a pitch of 500 nm and different QD diameters of 80, 65, and 40 nm. The blueshift observed with decreasing QD diameter is the result of quantum confinement and strain effects, and is consistent with our previous work19, 20. This figure demonstrates the control over the emission wavelength which is made possible by the lithographic approach to QD fabrication.
Figure 3 shows the emission spectrum of a low density QD array consisting of 35 nm diameter QDs on a 2.5 µm pitch at a time-average pump power of 100 nW. Since the diameter of the pump spot is approximately 4 µm, the pump laser primarily excites a single QD in this array. The primary peak at 888.6 nm has a full width at half maximum of 0.16 nm or 260 µeV, significantly larger than the linewidths of typical self-assembled QDs . The resolution of the spectrometer used for this measurement was ~0.02 nm. While the origin of the larger linewidth requires further investigation, we suggest that it may be due to spectral diffusion caused by trapped charges near the etched interfaces of the QD and within the regrown barrier layer . As shown in the inset, the linear dependence of the integrated intensity on pump power below 200 nW suggests that this spectral line is due to emission from a single exciton state. The smaller peaks on the long-wavelength side of the primary peak are likely due to emission from adjacent QDs in the array that are at the edge of the pump beam. This was verified by moving the sample stage and observing the simultaneous decay of the primary peak and increase in the intensity of the adjacent peaks. In addition, the intensity of the adjacent peaks has a much weaker correlation with pump power than the single exciton line, making it unlikely that these peaks correspond to emission from biexciton or charged exciton states from the same QD. Although we fabricated lower density arrays of QDs with 5µm pitch, the emission linewidths of the QDs were found to vary across the sample between approximately 250 µeV and 800 µeV. The QD in this particular 2.5 µm pitch array was selected for measurement of the second order correlation function because it demonstrated one of the narrowest linewidths measured.
Figure 4 shows the second-order correlation function measured on the single-exciton line at 888.6 nm in Fig. 3. The measurement was performed at a pump power of 175 nW. The monochromator was set to pass 888.6 nm with a spectral bandwidth of ~0.1 nm to the HBTI. The theoretical DBR reflectivity at the wavelength of the exciton emission is ~98.4%, slightly lower than the reflectivity at the DBR design wavelength of 910 nm. The count rate on each SPAD was approximately 700 Hz for the duration of the measurement. Although the timing electronics allow us to collect data for time separations τ up to 16 µs, we show only the first few peaks around τ = 0 for clarity. The value of g(2)(0) was computed by taking the area of the peak at zero time delay and dividing by the average area of all other peaks. Performing this calculation without subtracting the background due to dark counts on the SPADs yields g(2)(0) = 0.395 ± 0.030. This value is below the 0.5 limit necessary for classification as a single photon source . The background in the g(2)(0) measurement was estimated by averaging the coincidence counts between pump pulses over a 1.5 ns time window. Measurement of the decay time of the QD luminescence at a pump power of 200 nW indicated a biexponential decay with a fast component of 470 ps and a slow component of 2.1 ns as shown in Fig. 5 . Thus we can safely assume that the majority of coincidence counts between pump pulses are due to dark counts and not long-lived QD emission. After subtracting the contribution due to dark counts we obtain g(2)(0) = 0.314 ± 0.029. The remaining counts at zero time delay may be due to the weak background emission around the single exciton line, as shown in Fig. 3, and imperfect spectral filtering of the two longer wavelength peaks caused by emission from adjacent QDs in the array. The slow component of the QD time resolved decay also suggests the presence of carrier traps in the barrier layer which may supply carriers to the QD over long time scales and result in the emission of additional photons after the production of the first photon within the same pump pulse.
In summary, we have demonstrated the first observation of single photon emission from a novel type of lithographically defined quantum dot. The fabrication process allows for control of the position and emission wavelength of the QD. Our previous work demonstrated that these QDs can be incorporated into a semiconductor laser diode , suggesting that it should in principle be possible to incorporate a single QD into other cavity types such as micropillars or photonic crystals. In addition, with further process development it may be possible to fabricate multiple QDs having identical properties, which may prove useful for generating quantum states of light that are not possible with self-assembled QDs due to statistical variations such as an on-demand source of two or more entangled photons by use of two or more identical QDs.
The authors thank Todd Harvey for assistance with the MBE growth. The work at Illinois was supported by the U.S. Department of Energy, Office of Basic Energy Sciences as part of an Energy Frontier Research Center and the National Science Foundation (ECCS 08-21979).
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