- Email: [email protected]
MBE growth of In(Ga)As quantum dots for entangled light emission
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 1811–1814
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
MBE growth of In(Ga)As quantum dots for entangled light emission C.A. Nicoll a,, C.L. Salter a,b, R.M. Stevenson b, A.J. Hudson a,b, P. Atkinson a, K. Cooper a, A.J. Shields b, D.A. Ritchie a a b
Cavendish Laboratory, J.J. Thomson Avenue, Cambridge CB3 OHE, UK Toshiba Research Europe Limited, 208 Cambridge Science Park, Milton Road, Cambridge CB4 0GZ, UK
a r t i c l e in fo
abstract
Available online 17 October 2008
Radiative biexciton decay in a single semiconductor quantum dot (QD) is a process by which entangled pairs of photons can be generated for quantum information applications. The observation of entangled light from a QD requires minimal splitting of exciton states and the ability to isolate the neutral biexciton and exciton photoluminescence (PL) emission of the individual dot. As a consequence, the growth of QDs for this purpose is subject to simultaneous constraints on areal dot density, dot emission energy, and wetting-layer (WL) emission energy. In this work we will describe modifications to the molecular beam epitaxial (MBE) growth of In(Ga)As QDs performed to address these requirements, for the realization of samples which generate entangled light of increasing quality. & 2008 Elsevier B.V. All rights reserved.
PACS: 03.67.Bg 73.21.La 78.55.Cr 78.67.Hc 81.07.Ta 81.15.Hi Keywords: A1. Photoluminescence A3. Molecular beam epitaxy A3. Self-assembled quantum dots B1. Gallium arsenide B1. Indium arsenide B3. Entangled photon emitters
1. Introduction 1.1. Entangled light from self-assembled QDs Entangled light is of great interest for quantum communication [1] and quantum logic [2] applications. As proposed by Benson et al. [3], and first demonstrated experimentally by Stevenson et al. [4], entangled photon pairs can be generated by biexciton decay in a single semiconductor quantum dot (QD). Two photons emitted by the radiative decay of a neutral biexciton state via one of two degenerate exciton states (illustrated schematically in Fig. 1 (a)) can be polarization-entangled. The two-photon state is ideally a maximally entangled state which, in a rectilinearly polarized basis, can be written pffiffiffi Cþ ¼ ðjHxx Hx i þ jV xx V x iÞ= 2. (1)
1.2. Limitations on measured entanglement The growth of In(Ga)As QDs ideally suited for use as entangledlight sources has been the focus of some effort [4–6]. One major Corresponding author.
E-mail address: [email protected] (C.A. Nicoll). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.10.006
challenge is that the two intermediate exciton states in a QD are typically separated by an exchange-induced fine-structure splitting S, which varies with shape, composition and strain [7–9]. This splitting leads to polarization-dependent photon energies and the destruction of entanglement. It has been observed experimentally [10,11] that for In(Ga)As dots in GaAs, S is minimized in dots emitting near 1.4 eV or 885 nm. This is attributed [10] to the strong modification of the electron–hole exchange energy in these short-wavelength dots in which one of the carriers is weakly confined. A simultaneous requirement of the molecular beam epitaxial (MBE) growth is that the areal density of QDs must be small enough that emission from a single dot can be isolated, either in an etched mesa structure or under an aperture in a metal mask. The simplest way to achieve high emission energy and low dot density in In(Ga)As/GaAs QD wafers is to stop deposition near the onset of dot formation, while the dots are very small, and cap entirely with GaAs before raising substrate temperature (T sub ). Nonuniformity in dot size, intrinsic to small, immature dots, is not detrimental for single-dot investigations, and the first demonstration [4] of entangled photon pairs from the biexciton–exciton cascade was made using a sample grown in this way. We will refer to this variety of dots as ‘‘Type A.’’ As is evident from Fig. 2, the WL peak is very close in energy to the dot lines near 885 nm, and its emission contributes
ARTICLE IN PRESS 1812
C.A. Nicoll et al. / Journal of Crystal Growth 311 (2009) 1811–1814
As4 . InAs was deposited over a GaAs buffer of at least 200 nm thickness, at rates from 0:01 to 0:03 mm=h, and capped with at least 150 nm of GaAs at a rate of 1 mm=h. The arsenic flux was selected such that the V/III beam equivalent pressure ratio was 11212 : 1 during GaAs growth, and was not explicitly altered for dot growth and interruptions. The quantity of InAs deposited was chosen iteratively, based on PL characterization, to generate a low density of dots under the growth conditions used. The capping procedure differs between types of sample. Fig. 1. (a) Schematic of radiative biexciton–exciton cascade in an ideal QD. Decay from the biexciton state (XX) to the ground state (GS) can take place via one of two degenerate optically active exciton states (X). The polarization, but not the energy, of each of the two emitted photons will depend on the decay path taken. (b) When the single-exciton states X are not degenerate, the photon energies reveal the decay path without knowledge of the polarizations.
Fig. 2. PL spectrum of In(Ga)As dots in a microcavity structure: small dots emitting near 885 nm can exhibit small exciton-state splitting, but suffer from strong background emission due to the spectral proximity of the broad wettinglayer peak.
significantly to the detected signal. As this dilutes the entangled light in the measurement, it is desirable to reduce the background in the region of the dot biexciton and exciton lines. The objective of this work was to accomplish this background reduction in lowdensity In(Ga)As QD wafers with single-dot emission near 885 nm by modifying the MBE growth, to yield increased entanglement purity in the collected light.
3.1. Interruption after dot deposition Introducing a growth interruption after InAs deposition can effect a blueshift in the WL emission as indium is redistributed from the WL to the dots [12]. This approach was used in what will be referred to as ‘‘Type B’’ samples. Dots were grown at T sub 490 C, as measured using a kSA BandiT band-gap thermometry system, at an InAs deposition rate of 0:01 mm=h. Group III deposition was halted for a period of up to 5 min under As4 flux, and a layer of GaAs 4–20 nm thick was deposited to cap the dots before T sub was raised to 625 C for deposition of the remainder of the GaAs capping layer. Fig. 3(a) shows the change in wetting-layer (WL) emission between interruption durations of 0, 2, and 4 min. A slight blueshift in the WL emission is seen with increasing interruption time, resulting in a significant reduction in background intensity near 885 nm. A useful (very low) density of small dots still emit at this wavelength. A similar trend in WL emission with interruption time can be discerned in Figure 4 of Ref. [13]. Small QDs are by far in the minority, as the growth interruption causes the main distribution to shift to larger, mature dots [13] (Fig. 3(b)). Excited states of these mature dots contribute a background which increases superlinearly with pump power. An example of this background is seen in Fig. 3(c) for a QD in a
2. Outline of experiment In this investigation, self-assembled In(Ga)As/GaAs QD wafers were grown by solid-source MBE and characterized by lowtemperature (t10 K) photoluminescence (PL) spectroscopy. PL was excited above the GaAs band gap by a pulsed laser operating at 80 MHz. Unless otherwise stated, the power used was that required to excite on average 1 exciton per cycle within the dot (P0 ). Ensemble PL was carried out on unpatterned chips to evaluate dot density and emission energies. For isolation of emission from individual dots, apertures were patterned in an Al film evaporated on the surface. For single-dot correlation measurements, QDs were incorporated into a weak 1l GaAs cavity, with GaAs/AlGaAs distributed Bragg reflectors (DBRs) above and below to enhance photon-collection efficiency. Twophoton cross-correlation measurements were made to determine fidelity with the maximally entangled state.
3. Growth of QD development wafers All QD development layers were grown on semi-insulating, (1 0 0)-oriented GaAs substrates in a VG V80H MBE reactor using
Fig. 3. PL from Type B samples. (a) Reduction in background from WL near 885 nm with increasing duration of growth interruption prior to capping of dots. (b) In the samples with 2- and 4-min interruptions, almost all dots emit at 41 mm. (c) Excited states of the low-energy dot ensemble at higher pump laser power (3P 0 ) in a sample with a 5-min interruption and a cavity designed to enhance emission at 885 nm.
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microcavity. While a reduction in the background signal from the WL has been achieved, this new source of background emission is problematic at high pump powers. In addition, it is well known that the exposed wafer surface incorporates more impurity atoms with increasing interruption time. Type B dot preparation leaves the sample vulnerable to incorporation of charged impurities directly into the exposed dots and WL.
3.2. Interruption and indium desorption after partial capping For ‘‘Type C’’ samples, the growth interruption immediately after InAs deposition was replaced with a growth interruption, accompanied by a T sub ramp, after deposition of a partial cap of GaAs over the dots [14–16]. InAs was deposited at 0:03 mm=h at T sub 505 C and covered with 2–4 nm of GaAs. Growth was then interrupted and T sub was ramped upward at 1:8 C=s. After 60 s, deposition of the GaAs cap was resumed and T sub equilibrated at 630 C. Decreasing the partial-cap thickness was found to shift the WL peak to increasingly short wavelengths. This trend too is observable in the data of Ref. [13] (see Figure 5 therein). During the interruption, desorption of indium from uncovered tops of dots limits the maximum QD height [15,16]. Indium alloyed into, or segregated onto the surface of, the thinly capped regions between dots [17] can also be desorbed. This appears to affect the WL composition in the finished structure enough to cause an observable change in its PL emission. In Fig. 4 we contrast PL spectra of a wafer of Type C (pre-interruption cap thickness 2 nm) and a Type A wafer. For the Type A sample of Figs. 2 and 5 optical pyrometry was used and not band-gap thermometry, so a specific Type A control sample was grown, using the same T sub and As4 overpressure parameters as for the Type C sample, but with 20 nm of GaAs over the dots before starting a T sub ramp during continued growth. Both have low densities of dots, emitting in the region of 885 nm. In the Type C sample, the contribution of the WL peak at 885 nm is smaller due to a 5 nm blueshift relative to the WL of the Type A sample. Neither contains the undesirable long-wavelength dots of Type B. Thus, the Type C capping process reduces background emission at 885 nm due to the WL, without the complication of the background due to excited states of larger dots seen in Type B, and without exposing uncapped dots to long interruptions. Fig. 5 compares signal-to-background ratios for neutral biexciton and exciton lines from single dots of Types A and C. A reduction in relative background strength of a factor of 5 is
Fig. 5. Neutral biexciton and exciton emission from dots of (a) Type A, and (b) Type C. PL was excited above band with power 1:5P 0. Relative background emission is lower by a factor of 5 in the Type C sample.
seen between these two dots. Such a reduction is expected to improve the measured degree of entanglement.
4. Entanglement measurements Single-dot two-photon correlation measurements were performed in rectilinear, diagonal, and circular polarization bases on a microcavity structure containing Type C dots. PL emission was filtered using spectrometers tuned to the exciton and biexciton lines, and detected with avalanche photodiodes (APDs). Correlation (anticorrelation) was observed as expected of linearly (circularly) polarized entangled photon pairs. The fidelity of þ emission with Cþ (Eq. (1)) was found to be f ¼ 0:68 0:03; significantly more than for the Type A dots in Ref. [4].
5. Conclusion We have investigated the effects of capping methods on background light emission from low-density In(Ga)As QD wafers for entangled photon pair generation. By capping dots partially with 2 nm of GaAs and desorbing excess In during a growth interruption, we successfully increased the photon pair intensity to background light ratio by more than a factor of 5, while retaining emission wavelengths near the optimum 885 nm required for minimal polarization splitting and high-fidelity entanglement. Improved fidelity with the maximally entangled state was measured using dots with reduced background emission. A reduction in background emission becomes more crucial in larger-area devices and may be key to implementing an entangled LED.
Acknowledgements The authors wish to thank Mr. Douglas Heftel for technical support in the Cavendish MBE facility and Dr. Robert Young for valuable discussions. This work was partially funded by EPSRC through QIPIRC as well as the EU through FP6 projects SANDiE, QAP and the NanoEPR project of NanoSci-ERA. References
Fig. 4. PL from a sample of Type C (upper pane) and a Type A control (lower pane). In both cases, dots are small due to the amount of indium deposited, but when the indium desorption is carried out after the partial (2 nm) cap, WL emission is blueshifted significantly.
[1] [2] [3] [4]
A.K. Ekert, Phys. Rev. Lett. 67 (1991) 661. E. Knill, R. Laflamme, G.J. Milburn, Nature 409 (2001) 46. O. Benson, C. Santori, M. Pelton, Y. Yamamoto, Phys. Rev. Lett. 84 (2000) 2513. R.M. Stevenson, R.J. Young, P. Atkinson, K. Cooper, D.A. Ritchie, A.J. Shields, Nature 439 (2006) 179. [5] R.J. Young, R.M. Stevenson, P. Atkinson, K. Cooper, D.A. Ritchie, A.J. Shields, New J. Phys. 8 (2006) 29.
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C.A. Nicoll et al. / Journal of Crystal Growth 311 (2009) 1811–1814
[6] L. Wang, A. Rastelli, O.G. Schmidt, J. Appl. Phys. 100 (2006) 064313. [7] E. Blackwood, M.J. Snelling, R.T. Harley, S.R. Andrews, C.T.B. Foxon, Phys. Rev. B 50 (1994) 14246. [8] D. Gammon, E.S. Snow, B.V. Shanabrook, D.S. Katzer, D. Park, Phys. Rev. Lett. 76 (1996) 3005. [9] O. Stier, M. Grundmann, D. Bimberg, Phys. Rev. B 59 (1999) 5688. [10] R.J. Young, R.M. Stevenson, A.J. Shields, P. Atkinson, K. Cooper, D.A. Ritchie, K.M. Groom, A.I. Tartakovskii, M.S. Skolnick, Phys. Rev. B 72 (2005) 113305. [11] D.J.P. Ellis, R.M. Stevenson, R.J. Young, A.J. Shields, P. Atkinson, D.A. Ritchie, Appl. Phys. Lett. 90 (2007) 011907.
[12] T.J. Krzyzewski, T.S. Jones, J. Appl. Phys. 96 (2004) 668. [13] S. Fafard, Z.R. Wasilewski, C. Nı` Allen, D. Picard, P.G. Piva, J.P. McCaffrey, Superlattices Microstructures 25 (1999) 87. [14] R.P. Mirin, J.P. Ibbetson, J.E. Bowers, A.C. Gossard, J. Crystal Growth 175–176 (1997) 696. [15] G.D. Lian, J. Yuan, L.M. Brown, G.H. Kim, D.A. Ritchie, Appl. Phys. Lett. 73 (1998) 49. [16] Z.R. Wasilewski, S. Fafard, J.P. McCaffrey, J. Crystal Growth 201–202 (1999) 1131. [17] P. Howe, E.C. Le Ru, R. Murray, T.S. Jones, J. Crystal Growth 278 (2005) 57.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
MBE growth of In(Ga)As quantum dots for entangled light emission C.A. Nicoll a,, C.L. Salter a,b, R.M. Stevenson b, A.J. Hudson a,b, P. Atkinson a, K. Cooper a, A.J. Shields b, D.A. Ritchie a a b
Cavendish Laboratory, J.J. Thomson Avenue, Cambridge CB3 OHE, UK Toshiba Research Europe Limited, 208 Cambridge Science Park, Milton Road, Cambridge CB4 0GZ, UK
a r t i c l e in fo
abstract
Available online 17 October 2008
Radiative biexciton decay in a single semiconductor quantum dot (QD) is a process by which entangled pairs of photons can be generated for quantum information applications. The observation of entangled light from a QD requires minimal splitting of exciton states and the ability to isolate the neutral biexciton and exciton photoluminescence (PL) emission of the individual dot. As a consequence, the growth of QDs for this purpose is subject to simultaneous constraints on areal dot density, dot emission energy, and wetting-layer (WL) emission energy. In this work we will describe modifications to the molecular beam epitaxial (MBE) growth of In(Ga)As QDs performed to address these requirements, for the realization of samples which generate entangled light of increasing quality. & 2008 Elsevier B.V. All rights reserved.
PACS: 03.67.Bg 73.21.La 78.55.Cr 78.67.Hc 81.07.Ta 81.15.Hi Keywords: A1. Photoluminescence A3. Molecular beam epitaxy A3. Self-assembled quantum dots B1. Gallium arsenide B1. Indium arsenide B3. Entangled photon emitters
1. Introduction 1.1. Entangled light from self-assembled QDs Entangled light is of great interest for quantum communication [1] and quantum logic [2] applications. As proposed by Benson et al. [3], and first demonstrated experimentally by Stevenson et al. [4], entangled photon pairs can be generated by biexciton decay in a single semiconductor quantum dot (QD). Two photons emitted by the radiative decay of a neutral biexciton state via one of two degenerate exciton states (illustrated schematically in Fig. 1 (a)) can be polarization-entangled. The two-photon state is ideally a maximally entangled state which, in a rectilinearly polarized basis, can be written pffiffiffi Cþ ¼ ðjHxx Hx i þ jV xx V x iÞ= 2. (1)
1.2. Limitations on measured entanglement The growth of In(Ga)As QDs ideally suited for use as entangledlight sources has been the focus of some effort [4–6]. One major Corresponding author.
E-mail address: [email protected] (C.A. Nicoll). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.10.006
challenge is that the two intermediate exciton states in a QD are typically separated by an exchange-induced fine-structure splitting S, which varies with shape, composition and strain [7–9]. This splitting leads to polarization-dependent photon energies and the destruction of entanglement. It has been observed experimentally [10,11] that for In(Ga)As dots in GaAs, S is minimized in dots emitting near 1.4 eV or 885 nm. This is attributed [10] to the strong modification of the electron–hole exchange energy in these short-wavelength dots in which one of the carriers is weakly confined. A simultaneous requirement of the molecular beam epitaxial (MBE) growth is that the areal density of QDs must be small enough that emission from a single dot can be isolated, either in an etched mesa structure or under an aperture in a metal mask. The simplest way to achieve high emission energy and low dot density in In(Ga)As/GaAs QD wafers is to stop deposition near the onset of dot formation, while the dots are very small, and cap entirely with GaAs before raising substrate temperature (T sub ). Nonuniformity in dot size, intrinsic to small, immature dots, is not detrimental for single-dot investigations, and the first demonstration [4] of entangled photon pairs from the biexciton–exciton cascade was made using a sample grown in this way. We will refer to this variety of dots as ‘‘Type A.’’ As is evident from Fig. 2, the WL peak is very close in energy to the dot lines near 885 nm, and its emission contributes
ARTICLE IN PRESS 1812
C.A. Nicoll et al. / Journal of Crystal Growth 311 (2009) 1811–1814
As4 . InAs was deposited over a GaAs buffer of at least 200 nm thickness, at rates from 0:01 to 0:03 mm=h, and capped with at least 150 nm of GaAs at a rate of 1 mm=h. The arsenic flux was selected such that the V/III beam equivalent pressure ratio was 11212 : 1 during GaAs growth, and was not explicitly altered for dot growth and interruptions. The quantity of InAs deposited was chosen iteratively, based on PL characterization, to generate a low density of dots under the growth conditions used. The capping procedure differs between types of sample. Fig. 1. (a) Schematic of radiative biexciton–exciton cascade in an ideal QD. Decay from the biexciton state (XX) to the ground state (GS) can take place via one of two degenerate optically active exciton states (X). The polarization, but not the energy, of each of the two emitted photons will depend on the decay path taken. (b) When the single-exciton states X are not degenerate, the photon energies reveal the decay path without knowledge of the polarizations.
Fig. 2. PL spectrum of In(Ga)As dots in a microcavity structure: small dots emitting near 885 nm can exhibit small exciton-state splitting, but suffer from strong background emission due to the spectral proximity of the broad wettinglayer peak.
significantly to the detected signal. As this dilutes the entangled light in the measurement, it is desirable to reduce the background in the region of the dot biexciton and exciton lines. The objective of this work was to accomplish this background reduction in lowdensity In(Ga)As QD wafers with single-dot emission near 885 nm by modifying the MBE growth, to yield increased entanglement purity in the collected light.
3.1. Interruption after dot deposition Introducing a growth interruption after InAs deposition can effect a blueshift in the WL emission as indium is redistributed from the WL to the dots [12]. This approach was used in what will be referred to as ‘‘Type B’’ samples. Dots were grown at T sub 490 C, as measured using a kSA BandiT band-gap thermometry system, at an InAs deposition rate of 0:01 mm=h. Group III deposition was halted for a period of up to 5 min under As4 flux, and a layer of GaAs 4–20 nm thick was deposited to cap the dots before T sub was raised to 625 C for deposition of the remainder of the GaAs capping layer. Fig. 3(a) shows the change in wetting-layer (WL) emission between interruption durations of 0, 2, and 4 min. A slight blueshift in the WL emission is seen with increasing interruption time, resulting in a significant reduction in background intensity near 885 nm. A useful (very low) density of small dots still emit at this wavelength. A similar trend in WL emission with interruption time can be discerned in Figure 4 of Ref. [13]. Small QDs are by far in the minority, as the growth interruption causes the main distribution to shift to larger, mature dots [13] (Fig. 3(b)). Excited states of these mature dots contribute a background which increases superlinearly with pump power. An example of this background is seen in Fig. 3(c) for a QD in a
2. Outline of experiment In this investigation, self-assembled In(Ga)As/GaAs QD wafers were grown by solid-source MBE and characterized by lowtemperature (t10 K) photoluminescence (PL) spectroscopy. PL was excited above the GaAs band gap by a pulsed laser operating at 80 MHz. Unless otherwise stated, the power used was that required to excite on average 1 exciton per cycle within the dot (P0 ). Ensemble PL was carried out on unpatterned chips to evaluate dot density and emission energies. For isolation of emission from individual dots, apertures were patterned in an Al film evaporated on the surface. For single-dot correlation measurements, QDs were incorporated into a weak 1l GaAs cavity, with GaAs/AlGaAs distributed Bragg reflectors (DBRs) above and below to enhance photon-collection efficiency. Twophoton cross-correlation measurements were made to determine fidelity with the maximally entangled state.
3. Growth of QD development wafers All QD development layers were grown on semi-insulating, (1 0 0)-oriented GaAs substrates in a VG V80H MBE reactor using
Fig. 3. PL from Type B samples. (a) Reduction in background from WL near 885 nm with increasing duration of growth interruption prior to capping of dots. (b) In the samples with 2- and 4-min interruptions, almost all dots emit at 41 mm. (c) Excited states of the low-energy dot ensemble at higher pump laser power (3P 0 ) in a sample with a 5-min interruption and a cavity designed to enhance emission at 885 nm.
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microcavity. While a reduction in the background signal from the WL has been achieved, this new source of background emission is problematic at high pump powers. In addition, it is well known that the exposed wafer surface incorporates more impurity atoms with increasing interruption time. Type B dot preparation leaves the sample vulnerable to incorporation of charged impurities directly into the exposed dots and WL.
3.2. Interruption and indium desorption after partial capping For ‘‘Type C’’ samples, the growth interruption immediately after InAs deposition was replaced with a growth interruption, accompanied by a T sub ramp, after deposition of a partial cap of GaAs over the dots [14–16]. InAs was deposited at 0:03 mm=h at T sub 505 C and covered with 2–4 nm of GaAs. Growth was then interrupted and T sub was ramped upward at 1:8 C=s. After 60 s, deposition of the GaAs cap was resumed and T sub equilibrated at 630 C. Decreasing the partial-cap thickness was found to shift the WL peak to increasingly short wavelengths. This trend too is observable in the data of Ref. [13] (see Figure 5 therein). During the interruption, desorption of indium from uncovered tops of dots limits the maximum QD height [15,16]. Indium alloyed into, or segregated onto the surface of, the thinly capped regions between dots [17] can also be desorbed. This appears to affect the WL composition in the finished structure enough to cause an observable change in its PL emission. In Fig. 4 we contrast PL spectra of a wafer of Type C (pre-interruption cap thickness 2 nm) and a Type A wafer. For the Type A sample of Figs. 2 and 5 optical pyrometry was used and not band-gap thermometry, so a specific Type A control sample was grown, using the same T sub and As4 overpressure parameters as for the Type C sample, but with 20 nm of GaAs over the dots before starting a T sub ramp during continued growth. Both have low densities of dots, emitting in the region of 885 nm. In the Type C sample, the contribution of the WL peak at 885 nm is smaller due to a 5 nm blueshift relative to the WL of the Type A sample. Neither contains the undesirable long-wavelength dots of Type B. Thus, the Type C capping process reduces background emission at 885 nm due to the WL, without the complication of the background due to excited states of larger dots seen in Type B, and without exposing uncapped dots to long interruptions. Fig. 5 compares signal-to-background ratios for neutral biexciton and exciton lines from single dots of Types A and C. A reduction in relative background strength of a factor of 5 is
Fig. 5. Neutral biexciton and exciton emission from dots of (a) Type A, and (b) Type C. PL was excited above band with power 1:5P 0. Relative background emission is lower by a factor of 5 in the Type C sample.
seen between these two dots. Such a reduction is expected to improve the measured degree of entanglement.
4. Entanglement measurements Single-dot two-photon correlation measurements were performed in rectilinear, diagonal, and circular polarization bases on a microcavity structure containing Type C dots. PL emission was filtered using spectrometers tuned to the exciton and biexciton lines, and detected with avalanche photodiodes (APDs). Correlation (anticorrelation) was observed as expected of linearly (circularly) polarized entangled photon pairs. The fidelity of þ emission with Cþ (Eq. (1)) was found to be f ¼ 0:68 0:03; significantly more than for the Type A dots in Ref. [4].
5. Conclusion We have investigated the effects of capping methods on background light emission from low-density In(Ga)As QD wafers for entangled photon pair generation. By capping dots partially with 2 nm of GaAs and desorbing excess In during a growth interruption, we successfully increased the photon pair intensity to background light ratio by more than a factor of 5, while retaining emission wavelengths near the optimum 885 nm required for minimal polarization splitting and high-fidelity entanglement. Improved fidelity with the maximally entangled state was measured using dots with reduced background emission. A reduction in background emission becomes more crucial in larger-area devices and may be key to implementing an entangled LED.
Acknowledgements The authors wish to thank Mr. Douglas Heftel for technical support in the Cavendish MBE facility and Dr. Robert Young for valuable discussions. This work was partially funded by EPSRC through QIPIRC as well as the EU through FP6 projects SANDiE, QAP and the NanoEPR project of NanoSci-ERA. References
Fig. 4. PL from a sample of Type C (upper pane) and a Type A control (lower pane). In both cases, dots are small due to the amount of indium deposited, but when the indium desorption is carried out after the partial (2 nm) cap, WL emission is blueshifted significantly.
[1] [2] [3] [4]
A.K. Ekert, Phys. Rev. Lett. 67 (1991) 661. E. Knill, R. Laflamme, G.J. Milburn, Nature 409 (2001) 46. O. Benson, C. Santori, M. Pelton, Y. Yamamoto, Phys. Rev. Lett. 84 (2000) 2513. R.M. Stevenson, R.J. Young, P. Atkinson, K. Cooper, D.A. Ritchie, A.J. Shields, Nature 439 (2006) 179. [5] R.J. Young, R.M. Stevenson, P. Atkinson, K. Cooper, D.A. Ritchie, A.J. Shields, New J. Phys. 8 (2006) 29.
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C.A. Nicoll et al. / Journal of Crystal Growth 311 (2009) 1811–1814
[6] L. Wang, A. Rastelli, O.G. Schmidt, J. Appl. Phys. 100 (2006) 064313. [7] E. Blackwood, M.J. Snelling, R.T. Harley, S.R. Andrews, C.T.B. Foxon, Phys. Rev. B 50 (1994) 14246. [8] D. Gammon, E.S. Snow, B.V. Shanabrook, D.S. Katzer, D. Park, Phys. Rev. Lett. 76 (1996) 3005. [9] O. Stier, M. Grundmann, D. Bimberg, Phys. Rev. B 59 (1999) 5688. [10] R.J. Young, R.M. Stevenson, A.J. Shields, P. Atkinson, K. Cooper, D.A. Ritchie, K.M. Groom, A.I. Tartakovskii, M.S. Skolnick, Phys. Rev. B 72 (2005) 113305. [11] D.J.P. Ellis, R.M. Stevenson, R.J. Young, A.J. Shields, P. Atkinson, D.A. Ritchie, Appl. Phys. Lett. 90 (2007) 011907.
[12] T.J. Krzyzewski, T.S. Jones, J. Appl. Phys. 96 (2004) 668. [13] S. Fafard, Z.R. Wasilewski, C. Nı` Allen, D. Picard, P.G. Piva, J.P. McCaffrey, Superlattices Microstructures 25 (1999) 87. [14] R.P. Mirin, J.P. Ibbetson, J.E. Bowers, A.C. Gossard, J. Crystal Growth 175–176 (1997) 696. [15] G.D. Lian, J. Yuan, L.M. Brown, G.H. Kim, D.A. Ritchie, Appl. Phys. Lett. 73 (1998) 49. [16] Z.R. Wasilewski, S. Fafard, J.P. McCaffrey, J. Crystal Growth 201–202 (1999) 1131. [17] P. Howe, E.C. Le Ru, R. Murray, T.S. Jones, J. Crystal Growth 278 (2005) 57.