Chirped InGaAs quantum dot molecules for broadband applications
 Nirat Patanasemakul^{1},
 Somsak Panyakeow^{1} and
 Songphol Kanjanachuchai^{1}Email author
DOI: 10.1186/1556276X7207
© Patanasemakul et al; licensee Springer. 2012
Received: 28 December 2011
Accepted: 6 April 2012
Published: 6 April 2012
Abstract
Lateral InGaAs quantum dot molecules (QDMs) formed by partialcap and regrowth technique exhibit two groundstate (GS) peaks controllable via the thicknesses of InAs seed quantum dots (x), GaAs cap (y), and InAs regrowth (z). By adjusting x/y/z in a stacked QDM bilayer, the GS peaks from the two layers can be offset to straddle, stagger, or join up with each other, resulting in multiGS or broadband spectra. A nonoptimized QDM bilayer with a 170meV fullwidth at halfmaximum is demonstrated. The temperature dependencies of the emission peak energies and intensities from the chirped QDM bilayers are well explained by Varshni's equation and thermal activation of carriers out of constituent quantum dots.
Keywords
Quantum dot molecules InAs InGaAs Chirp Broadband PhotoluminescenceBackground
Narrow and broadband optoelectronic devices in the 0.9 to 1.55μm range benefit greatly from selfassembled InGaAs quantum dots (QDs). The conflicting narrow and broad bandwidth requirements lead to different procedures during the growth of QD active layers. While processes for narrowband devices such as lasers seek to minimize the inhomogeneities intrinsic to StranskiKrastanow (SK) QDs [1], those for broadband devices such as superluminescent diodes (SLDs) seek to maximize [2–4] and enhance them by adopting a chirped structure [5] where several QD layers are stacked in a widergap matrix and by varying the properties of the individual QD layers, the matrix, or both [6–9]. The number of stacks in strained heteroepitaxy, however, should be kept low to minimize accumulated strain [10]. To reduce the stack number in chirped QD structures without compromising bandwidth, the QDs can be replaced by certain types of lateral quantum dot molecules (QDMs). QDMs can be broadly described as a system of coupled QDs where QDs are spaced closely vertically, in the growth direction, or laterally, in the growth plane [11]. Vertical QDMs have been a subject of intense interest since the demonstrations of QD coupling [12] and entanglement of quantum states [13] which form the foundation of quantum computation [14]. Vertical coupling, however, cannot be tuned postgrowth in contrast to lateral QDMs whose tunnel barriers can be tuned electrically using surface or side gates and thus extend the scope of quantum interactions that can be studied. In contrast to their potentials as coupled quantum systems, lateral QDMs are often overlooked as device active layers mainly because selfassembly of lateral QDMs is more involved and partly because optical properties of QD and QDM ensembles do not differ significantly. We recently developed a capping and regrowth procedure [15] to form lateral QDMs where each molecule comprises a large, central QD (cQD) and several small, satellite QDs (sQDs). Our QDMs exhibit structural and optical bimodalities [16] and have been incorporated in solar cells [17]. Despite many reports of other lateral QDM growth strategies [18–21], a device structure designed specifically to take advantage of intrinsic QDM characteristics has been lacking. In the simplest case of two QDs per molecule, however, devices such as photodetectors have been reported [22].
In this letter, we propose and demonstrate an alternative chirped structure whose active layer is a lateral InGaAs QDM bilayer. The proposed structure departs from conventional chirped QDs and quantum wells but offers superior broadband luminescence or response in the nearinfrared (NIR) region with only a few QDM layers grown under standard conditions. The temperaturedependent photoluminescence of the chirped QDM structures can be well explained using multiGaussian distributions, each with characteristic activation energy and fitting parameters related to escape channels for carriers in cQDs and sQDs.
Methods
All InGaAs QDM samples are grown on (001)GaAs substrates by solid source molecular beam epitaxy (MBE). After oxide desorption at 610°C, 300nm GaAs is grown at 580°C, followed by a QDM layer and 100nm GaAs. Growth stops here for single QDM layer structures. For chirped QDM bilayer structures, growth continues with an additional QDM layer and a final 100 nm GaAs. The decoupled lower and upper QDM layers are different. A typical QDM layer is formed via the partialcap and regrowth technique [15] where x monolayers (MLs) of InAs seed QDs are grown at 500°C, then capped by yML GaAs at 470°C after which nanoholes are formed and used as a template for regrowth of zML InAs QDs at 470°C. After regrowth, GaAs capping proceeds at 470°C for the first 10 nm and at 500°C for the remaining 90 nm. Each QDM layer can be controlled by varying x, y, and z; the constituent nanostructure is thus referred to hereafter as x/y/z QDMs. The formation of QDs, nanoholes, and QDMs can be observed in situ via reflection highenergy electron diffraction patterns, while the morphologies of layers of interest are probed ex situ by atomic force microscopy (AFM). Series of samples where either a single QDM layer or chirped QDM bilayer constitutes the active layer are grown and characterized by photoluminescence (PL). In all samples, x = 1.9 or 2 ML, y = 6 to 25 ML, and z = 1.4 to 2 ML. Samples are mounted in a closedcycle He cryostat and excited by a 476.5nm Ar^{+} laser. The PL signals are dispersed in a 1m spectrometer and collected by a cooled InGaAs detector.
Results and discussion
The morphology and optical properties of single QDM layers will first be discussed, followed by the optical properties of the chirped QDM bilayers, their temperature dependencies, and the physical mechanisms that govern them.
Single QDMs
The 20K PL spectra of two sample series with a single QDM layer in each sample are shown in Figure 2a for 2/25/z QDMs where z = 1, 2, or 2.5 ML and in Figure 2b for 2/y/1.4 QDMs where y = 6, 10, or 25 ML. Figure 2a shows that as the regrowth thickness for 2/25/z QDMs increases from 1 to 2 ML, the number of groundstate (GS) peaks increases from 1 to 2 which correlates with surface morphology before and after the nucleation of sQDs, respectively. cQDs are present in both cases. The low (high) energy peak is thus assigned to emissions from cQDs (sQDs). As z increases to 2.5 ML, the cQD peak remains almost unaltered while the sQD peak redshifts and broadens slightly. The cQD peak remains almost unaltered because the nanoholes are being filled and saturating, while the sQD peak redshifts and broadens because the resulting greater material availability makes sQDs grow proportionately. Similar double GS peaks in lateral QDMs have been reported [23], and their temperature dependencies have been discussed in detail elsewhere [24].
Lateral QDMs spectra thus exhibit three basic PL characteristics: single peak, nonoverlapping double peak, and overlapping double peak  controllable via z. In chirped QDM bilayer structures, two of the three characteristics can be combined to yield broadband or multiGS emissions which can serve as active materials for SLDs [26] or dualwavelength QD lasers [27], respectively.
Chirped QDM bilayers
TypeII chirps require the cQD_{2} peak to be between the cQD_{1} and sQD_{1} peaks, and the sQD_{1} peak to be between the cQD_{2} and sQD_{2} peaks. Figure 4b shows an example of this design with spectra from the single 1.9/6/1.7 QDM_{2} layer (bottom curve), single 2/25/2 QDM_{1} layer (middle), and QDM_{1}/QDM_{2} bilayer (top) samples grown under otherwise identical conditions. The spectrum of the bilayer sample is almost a linear combination of the spectra of the two single QDM layers, indicating that the 100nm GaAs spacer is sufficiently thick to decouple the bilayer [29]. The strong cQD_{2} peak removes the cQD_{1}sQD_{1} dip, giving a smooth spectrum.
TypeIII chirps can be formed if the two QDM layers have sufficiently different capping thicknesses, to ensure that the cQD_{1} and cQD_{2} peaks are well separated, and thick regrowth, so that the sQD_{1} peak is at a lower energy than the cQD_{2} peak. This is demonstrated in a 2/15/1.7 QDM_{1} and 2/6/1.4 QDM_{2} bilayer. The 20K PL spectra of the bilayer sample is shown (upper curve) in Figure 4c, together with those of separately grown single QDM_{2} layer reference (lower). The latter  with a misleading single peak in the linear plot in Figure 2b  can be well described by a double Gaussian function (dashed lines) corresponding to the cQD_{2} and sQD_{2} peaks which appear on the right half of the upper curves whose left half can also be described by another double Gaussian function corresponding to QDM_{1} (not shown). The four Gaussian peaks can be resolved almost down to the noise floor as seen when comparing the PL spectra under nominal (I_{0} = 0.45 W/cm^{2}, upper curve) and reduced (I_{0}/100, middle) excitations, confirming that all peaks are ground states. Ignoring the small dip in the middle, the fullwidth at halfmaximum (FWHM) of this twostack chirped QDM structure is 170 meV, broader than the 125 meV achieved in a fourstack chirped QD structure [6], yet slightly narrower than the 200 meV in a sixtystack straincompensated structure [9], illustrating the potentials of chirped QDM structures as a broadband material.
Temperature dependencies
A broadband spectrum at low temperatures is the most important characteristic of a material destined for broadband applications, but does not necessarily translate to useful roomtemperature devices. The latter require proper barrier design to minimize carrier loss by nonradiative recombination (NRR) mechanisms which can be deduced from temperaturedependent PL measurements.
The PL spectra of the three chirped QDM bilayers from the 20K base temperature to 225 K are shown in Figure 4d for typeI, Figure 4e for typeII, and Figure 4f for typeIII designs. The overall luminescence of the three types can be seen to gradually decrease as temperature increases. The spectra are discernable from the noise floors up to approximately 230 K for all structures. This does not rule out the material for roomtemperature operation since cladding layers such as Al(Ga)As/GaAs superlattices can be employed to increase luminescence efficiency of the structures. The lowenergy peaks slowly redshift as the temperature increases and can be accurately modeled using Varshni's equation with bulk InAs parameters (see later). The highenergy end of the spectra is quenched at temperatures lower than the lowenergy end, which is in good agreement with temperaturedependent characteristics of single QDM layers which have been explained by thermal activation of carriers out of QDMs into the wetting layer (WL) [16, 24].
In order to model the temperature dependencies of the chirped QDM bilayers, a few assumptions are made. One, the bilayers are completely decoupled; the spectra of the bilayers are thus a linear combination of the spectra of individual QDM layers: QDM_{1} and QDM_{2}. This is a realistic assumption since the GaAs spacer layer between the bilayer is 100nm thick, almost twice the 200 ML determined by Xie et al. to be the thickness necessary to completely decouple stacked InAs/GaAs layers [29]. In addition, the three chirped structures are designed such that one or both of the lower QDM emission peaks are lower in energy than the upper QDM layer, minimizing reabsorption in surfaceemitting applications.
Two, the temperature variation of cQDs's peak energies follow Varshni's equation with bulk InAs parameters. The cQDs nucleate inside the nanohole template dug out of seed InAs QDs and are thus expected to exhibit the same temperature dependencies as typical InAs QDs. The seed InAs QDs are grown at a low rate of 0.01 ML/s, resulting in uniform QDs with typical FWHM in the range of 28 to 40 meV. For uniform InAs/GaAs QD ensembles, the temperature dependencies of GS peaks typically follow Varshni's equation with bulk InAs parameters, while those of FWHM remain virtually constant from 20 to 300 K [30, 31]. As a first approximation, the cQDs's FWHM are assumed constant.
Three, the temperature variation of sQDs's peak energies follow Varshni's equation up to about 80 K, above which they decrease at a faster rate of 1 meV/K. This assumption is based on a previous observation of single QDM layers [16] and usually referred to as a sigmoidal behavior resulting from carrier redistribution among nonuniform QD ensembles [32–35]. The FWHM of such ensembles exhibits a temperature anomaly where it decreases at an intermediate temperature and increases again to recover, or even exceed, the lowtemperature value as the temperature increases towards room temperature. Only nonuniform sQDs (thin regrowth) follow the sigmoidal and anomalous behaviors; uniform sQDs (thick regrowth) are treated the same way cQDs are.
where I_{ i } is the normalized intensity of a PL peak at the base temperature, E_{ i } is the GS peak energy, Γ is the broadening parameter, A = 3 × 10^{8} is the preexponential factor, E_{ WL } = 1.42 eV is the WL's energy appropriate for our structures [16], k is the Boltzmann constant, and η_{ i } is an ideality factor indicating the effective potential barrier felt by carriers in cQDs or sQDs of the layer of interest. The summation over i from 1 to 4 represents the four GS peaks of the QDM bilayer originating from the two constituent ensembles  cQDs and sQDs  of each layer. The numerator inside the summation is simply the Gaussian function describing inhomogeneity associated with SK QDs [36], while the denominator signifies the PL intensity reduction term similar in form and cause to those of quantum wells [37] which has been applied to QD ensembles with varying degrees of sophistication [34, 38–41]. The only difference here is the introduction of the ideality factor η which varies between 1 and 2. The lower limit η = 1 is the ideal case which implies that electrons and holes in the QDs are lost in pairs through thermal barriers [37]. The upper limit η = 2 indicates that the effective barrier is half the ideal case which is possible if (1) the loss of either carrier electrons or holes is the ratelimiting factor in which case band offsets would dictate the effective barrier [37] or (2) multiple loss mechanisms/channels exist and are acting in parallel. Besides thermionic emissions of carriers from QDMs to NRR centers in the WL and bulk GaAs, other possible NRR centers and transport channels present in our lateral QDMs are the nanohole template, interfacial defects, and straininduced localized states [42], and, considering the close proximity between cQD and sQDs in a QDM, may possibly involve tunneling [41]. If two independent, thermallyactivated NRR channels dominate, the intensity reduction term can be accurately modeled using a double activation energy term as reported by Heitz et al. [30]. However, if there are more than two channels with different temperature dependencies, with no clear dominant mechanisms, and with possible interaction, the introduction of the semiempirical ideality factor η is a simple approach which can significantly reduce the number of unknowns yet still yield satisfactory results.
Simulation parameters for the PL maps and spectra of chirped QDM bilayers
Type I  straddled  Type II  staggered  Type III  brokengap  

I _{ i }  E _{ i }  FWHM  η _{ i }  I _{ i }  E _{ i }  FWHM  η _{ i }  I _{ i }  E _{ i }  FWHM  η _{ i }  
(eV)  (meV)  (eV)  (meV)  (eV)  (meV)  
QDM_{2}  
sQD_{2}  0.071  1.114*  51.8*  1.11.9  0.044  1.213*  49.5*  11.8  0.7  1.220*  65.9*  1.02 
cQD_{2}  0.358  1.085  28.3  1.01.6  0.591  1.120  40.0  11.4  0.7  1.170  33.0  1.12 
QDM_{1}  
sQD_{1}  0.094  1.214*  77.7*  1.11.9  0.315  1.160*  53.0*  11.8  1.0  1.121  42.4  1.12 
cQD_{1}  1.000  1.048  30.6  1.01.6  1.000  1.077  40.0  11.4  1.0  1.078  33.0  1.12 
In order for chirped InGaAs QDM structures to achieve broadband NIR performance at room temperature, a few design rules should be followed. One, the lowenergy emission boundary should be redshifted without causing intensity drops at intermediate wavelength. This can be achieved by introducing an additional QDM layer with nominally the same x/y/z but using a strainreducing capping layer instead of GaAs. Nishi et al., for example, shows that replacing GaAs capping with 7nm In_{0.2}Ga_{0.8}As redshifts the spectrum by as much as 140 meV [43]. This third QDM layer should be the bottom layer to minimize reabsorption.
Two, the highenergy portion of the spectra should require a greater thermal energy to quench. This can be achieved by changing the barrier material and/or removing the WL. Sanguinetti et al. compared the effect of thermal quenching of luminescence between QDs grown by standard SK epitaxy, with WL, and droplet epitaxy (DE), without WL, and reported a much smaller intensity reduction of the latter due to the absence of WL [44]. If such strategy is employed in our design, and provided no other NRR mechanisms are introduced by the lowtemperature growth typical of DE, E_{ A } should increase by approximately 100 meV (the difference between bulk GaAs bandgap and E_{ WL } ). Alternatively, or additionally, E_{ A } can be increased by sandwiching the active QDM structures between AlGaAs/GaAs superlattices, but doing so may affect peak energy positions [45]. The highenergy portion of the spectra should be narrow to fully benefit from this design rule; z should thus be appropriately thick in proportion to the thicknesses of x and y. If cQDs and sQDs of all layers are narrow (< 45 meV), the temperature dependencies of all energy peaks will follow Varshni's equation; the lowtemperature FWHM and the PL profile will thus be maintained.
Not all lateral QDMs reported to date are compatible with the designs proposed in Figure 3 since at least two different QD ensembles are required. This rules out the essentially oneQDM ensembles grown on superlattice [19] or on nanoholes created by in situ etching [18] or by GaAs capping of InAs QDs under As_{2}[23]. For QDMs regrown on GaAs nanoring template [46] formed by nanodrilling [47], however, the designs are applicable since the dots inside and outside the rings are of different sizes. It is interesting to note that though the quad QDMs grown by DE on GaAs nanomounds [20] do have two different QD ensembles, the central QDs are GaAs without barriers. They thus emit at bulk values and are incompatible with the designs. If grown on AlGaAs, however, the emissions from DE GaAs cQDs and SK InGaAs sQDs can be combined to yield a broader bandwidth reported here.
Conclusions
Chirped bilayer structures employing lateral InGaAs QDMs as active layers are proposed. Ground state emissions from cQDs and sQDs in the bilayer are combined in three basic configurations  straddled, staggered, and brokengap  to yield multiGS and broadband spectra. A nonoptimized, 170meV FWHM chirped QDM bilayer is demonstrated, establishing lateral QDMs as a promising broadband NIR material as the bandwidth can be further broadened simply by increasing the stack number and chirping each additional layer with different x, y, or z or replacing the GaAs cap layer with a strainreducing InGaAs layer. By keeping x constant and varying y and z, we demonstrated the three basic chirping configurations. The temperaturedependent behaviors of the PL spectra of the chirped QDM bilayers are well described in the context of inhomogeneous broadening of constituent QDs and carrier loss to NRR centers via thermallyactivated channels. The ideality factor η introduced in the intensity reduction term implies a temperaturedependent effective potential barrier, indicating the presence of loss mechanisms/channels in addition to thermionic emission from QDMs to the WL.
Abbreviations
 AFM:

Atomic force microscopy
 cQD:

Central quantum dots
 DE:

Droplet epitaxy
 FWHM:

Fullwidth at halfmaximum
 GS:

Ground state
 MBE:

Molecular beam epitaxy
 ML:

Monolayer
 NIR:

Near infrared
 NRR:

Nonradiative recombination
 PL:

Photoluminescence
 QD:

Quantum dot
 QDM:

Quantum dot molecule
 SK:

StranskiKrastanow
 SLD:

Superluminescent diodes
 sQD:

Satellite quantum dots
 WL:

Wetting layer
Declarations
Acknowledgements
S. Thainoi and P. Changmoang are acknowledged for maintaining the MBE and PL systems. This work is partially funded by Chulalongkorn University, Nanotec, Thailand Research Fund (DPG5380002) and by the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission (EN1180A55).
Authors’ Affiliations
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