IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 5, SEPTEMBER/OCTOBER 2002 1045

Improved Luminescence From Quantum-DotNanostructures Embedded in StructurallyEngineered (In,Ga)As Confining Layers

L. Chen, Student Member, IEEE, V. G. Stoleru, and E. Towe

Abstract—Efficient luminescence of quantum-dot nanos-tructures embedded in active regions of lasers is importantfor low-threshold current density devices. This paper discussesan approach for structurally engineering confining (In,Ga)Aslayers into which InAs quantum dots are inserted to enhancetheir emission efficiency. It is shown that by inserting the dotsat the center of compositionally graded In Ga1 As layers, therelative emission efficiency can be increased by nearly an order ofmagnitude over the emission of dots inside a constant composition(In,Ga)As structure. This enhancement is thought to be a result ofthe high structural and optical quality of the confining layers.

Index Terms—Emission spectra, nanostructures, quantum dots,strain.

I. INTRODUCTION

I T HAS been established that the use of (In,Ga)As quantumdots in the active regions of certain optoelectronic devices,

such as lasers and infrared detectors, endows them with somevery desirable characteristics. Intersublevel detectors thatare sensitive to normal-incidence light, for example, can bedesigned and fabricated using (In,Ga)As quantum-dot nanos-tructures [1]; devices which operate under similar principles,but fabricated from quantum-well structures, do not share thisproperty [2]. Quantum-dot lasers with ultralow-threshold cur-rent densities and low sensitivity to temperature variations havealso been demonstrated [3], [4]. More recently, quantum-dotlasers emitting at the important telecommunications wavelengthof 1.3 m have been reported [4], [5]. The significance of thisdemonstration lies in the fact that the lasers can be fabricatedfrom a GaAs-based technology, with its mature and inexpen-sive processing base. For the lasers, however, the quantum-dotmedia do not often have sufficient gain for the devices tooperate at the ground-state wavelength [6]. This is due to thecombined consequence of the low density of states and the lowareal density of dots that have been used so far in the devices.Several techniques to avoid gain saturation before the thresholdof oscillation for the ground-state emission is reached havebeen used. One technique uses several dot layers to increase

Manuscript received June 11, 2002; revised July 29, 2002. This work wassupported by the the Department of Defense (DARPA/SPAWAR) under GrantN66001-02-1-8901.

The authors were with the University of Virginia, Charlottesville, VA 22904USA. They are now with the Laboratory for Photonics, Department of Electricaland Computer Engineering, Carnegie Mellon University, Pittsburgh, PA 15213USA.

Digital Object Identifier 10.1109/JSTQE.2002.804231

the modal gain; another increases the gain by coating thelaser facets to increase their reflectivities. Yet another methodincreases the gain by lengthening the laser cavity. The aim ofall three methods is to increase the modal gain (or reduce theloss) before the threshold of oscillation so that the device canemit at the ground state wavelength (of 1.3m).

There have been several efforts devoted to the study ofmethods for synthesizing dots that emit at 1.3m, and alsoto studying structures that improve the emission efficiency ofthe dots. For long-wavelength emission, the key parameter thatcontrols the wavelength is the size of the dot; large-size dotstend to emit at longer wavelengths than small-size ones. A wayto obtain large-size dots is to synthesize them by the alternatesupply of the group-III indium and group-V arsenic fluxes [7].The conventional approach is to supply the indium and arsenicfluxes continuously, but at such low rates that the InAs growthrate is extremely slow (0.01 monolayers per second) [8].

The efficiency of luminescence from quantum-dot structuresdepends on a number of factors. The most important of thesefactors include the capture of the carriers within the dots, theminimization of nonradiative recombination channels withinthe dots and in the surrounding matrix, and the eliminationof defects at the hetero-interfaces inherent in the structure.Experimental evidence suggests that burying the dots from thetop with a constant composition (In,Ga)As layer, rather than aGaAs layer improves the luminescence efficiency [9]. In an-other study, it has been shown that the laterally varying surfacestrain arising from the dots affects the growth of the (In,Ga)Asoverlayer. In particular, the strain is believed to induce an alloyphase separation of the (In,Ga)As overlayer; the indium atomsin the alloy are thought to preferentially migrate to the sitesof the InAs dots, thus increasing their sizes (volumes) [10].The approach of burying the dots with (In,Ga)As overlayershas recently been extended to its next logical step: that ofembedding the dots inside a constant composition (In,Ga)Asquantum-well structure. As expected, the emission efficiencyof the dots inside the well is dramatically improved over that ofdots between GaAs layers [11]. The enhancement of emissionefficiency is mainly due to the improved quality of the structuraland optical properties of the embedding layers, and the abilityof the potential barriers inherent in the embedding layers tocapture and confine carriers to the vicinity of the dots.

This paper discusses further structural modification of the(In,Ga)As layers surrounding the InAs quantum dots, and theresultant improvement in the luminescence of such structures.

1077-260X/02$17.00 © 2002 IEEE

1046 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 5, SEPTEMBER/OCTOBER 2002

Fig. 1. Quantum-dot active regions with (a) dots embedded between a topand a bottom GaAs layer, (b) dots on top of a GaAs layer, but covered withan (In,Ga)As layer on the top, and (c) dots sandwiched between symmetric(In,Ga)As layers (to constitute a “dots-in-a-quantum-well” structure).

II. BACKGROUND

There are three basic structures that are used in the ac-tive regions of quantum-dot lasers. In the first scheme, thequantum-dot array is embedded at the center of a GaAs wave-guide; this, in turn, is surrounded by symmetric cladding layers[13]. In the second scheme, a dot array on top of a GaAs layeris typically covered with an (In,Ga)As layer; this is usuallyfollowed by a layer of GaAs before the top cladding layer.There is, of course, usually an associated bottom cladding layer.The third scheme embeds the dots at the center of an (In,Ga)Aslayer. The basic period of each one of these structures can be,and is usually repeated several times to increase the modal gain.

Fig. 1 shows a schematic drawing of the three structures,along with the conduction band profile on the right-hand side.In the energy band profile, we have included a modificationdue to the wetting layer (which is usually omitted for clarity).In structure (a), there is a maximum lattice-mismatch betweenthe GaAs layer and the InAs (or InGaAs) dots. This mismatchcan lead to defects that can serve as nonradiative recombina-tion centers for injected electron–hole pairs. Another problemwith this structure is the segregation of indium from the dotsto the surrounding GaAs layer. Segregation modifies the sizeof the dots, and, hence, the emission wavelength. The struc-ture in scheme (b) is an improvement over structure (a). The(In,Ga)As layer that covers the dots serves to reduce the magni-tude of the lattice-mismatch, and hence the strain. The thicknessof this layer is on the order of the size of a quantum well whosepotential can capture and confine carriers to the vicinity of thedots. We want to comment further that as has been shown in sev-eral studies [9]–[11], most InAs dots covered with an (In,Ga)Asor an (In,Al,Ga)As alloy tend to emit at or near 1.3m. It ispointed out in [10] that the migration of the indium atoms to theInAs dot sites increases their volumes. This, in turn, lowers theground-state energy for both the conduction and valence bands,

leading to a narrower interband separation energy (which con-tributes to the long-wavelength emission). The migration of theindium atoms to the InAs dot sites has been called activatedalloy phase separation [10]. This is equivalent to spinodal de-composition [13]. All of these effects are beneficial to the emis-sion characteristics of the structure because they improve theradiative efficiency of the dots, as well as causing them to emitat the desirable wavelength of 1.3m. Because of the reducedstress in the structure, the contribution of the strain field to thepotential confining carriers to the dots is also reduced. As aconsequence, the combination of the large-size dot and the re-duced contribution of the strain to the confining potential leadto a ground-state energy that is lower, and, hence, the emissionat the desirable long wavelength as stated earlier. Structure (c)is the logical evolution of structure (b). Here, the quantum-dotarray is inserted at the center of an (In,Ga)As quantum well,which, in turn, is at the center of a GaAs waveguide. The ben-efits of this structure are the same as those of structure (b). Inthis case, however, the (In,Ga)As quantum well is symmetricabout the quantum-dot array. The stress induced by the latticemismatch is reduced below and above the dots. The likelihoodof defect generation is minimized in the neighborhood of thedots because the lattice constant of the dots is closer to that ofthe surrounding layers. There is also no issue of indium segre-gation here since there is indium in both the quantum-dot arrayand the adjacent confining layers. As in structure (b), the topand bottom (In,Ga)As layers serve to confine carriers close tothe vicinity of the dots. Overall, this structure is superior to theother two, and extremely low-threshold current lasers based onthis active region design have been demonstrated [14].

III. D OTSSANDWICHED BETWEENGRADED (In,Ga)As LAYERS

The basic idea of embedding dots inside (In,Ga)As layerswith graded compositions derives from the concept of a graded-index, separate confinement, heterostructure (GRINSCH) laser.Instead of merely embedding the dots at the center of a conven-tional constant composition (In,Ga)As quantum well, the dotsare inserted inside a graded-layer structure as shown in the insetof Fig. 3. This scheme has all the benefits of the previous struc-tures, including others that are absent from the previous struc-tures. Perhaps the most distinct advantage of this structure is thelower lattice mismatch between the GaAs layers and the InAsdots. The intervening (In,Ga)As layers can be graded in such away that the lattice mismatch is minimized. It is reasonable toexpect that because of the compositional abruptness inherent atthe InAs–GaAs quantum-dot interface, there is a higher likeli-hood of generating deleterious defects at this interface than ata strained (In,Ga)As–GaAs interface, for example. The gradualadjustment of the lattice constant (by compositional grading),therefore, helps lower the incidence of dislocations resultingfrom large accumulated strains.

The samples for our experimental study were grown in asolid-source molecular-beam epitaxy system on semi-insu-lating (001) GaAs substrates. The basic structure consistsof InAs quantum dots symmetrically sandwiched betweentwo (In,Ga)As layers; this structure, in turn, is inserted at thecenter of a 180-nm GaAs layer on top of a 250-nm GaAs

CHEN et al.: IMPROVED LUMINESCENCE FROM QUANTUM-DOT NANOSTRUCTURES 1047

(a) (b)

Fig. 2. Atomic force microscope surface images of quantum dots ofstructure A and B type samples on top of an (In,Ga)As layer. (a) Sample A:1.25�1.25�m. (b) Sample B: 1.25�1.25�m.

buffer layer on a GaAs substrate. The 250-nm GaAs bufferwas grown at a substrate temperature of 580C. The substratetemperature was then gradually lowered to 490C during thegrowth of the 90-nm GaAs layer which precedes the bottom(In,Ga)As layer on top of which the InAs dots are grown.The rest of the structure was grown at 490C. The growthrate for the InAs dots was 0.05 monolayers/s, and the beamequivalent As pressure was maintained at 4.010 torr.The substrate holder was continuously rotated to improveuniformity. Two samples, different in the essential details oftheir band structures, were grown. Sample A consists of 2.5monolayers of InAs dots sandwiched at the center of an 8-nmIn Ga As layer. Sample B consists of 2.5 monolayers ofInAs dots sandwiched at the center of an 8-nm InGa Aslayer; the bottom half of the InGa As layer is gradedfrom , at the GaAs interface to , at the InAsinterface; the top half is similarly graded from , atthe top InAs interface to at the GaAs interface. Thechange of indium composition in the graded InGa Asquantum well region is achieved by deposition of InAs–GaAssuperlattice layers that form a quasi-linear indium distributionalong the growth direction (see the inset of Fig. 3). The detailsof the growth method have been described elsewhere [15].We want to comment that during the growth of sample B, thereflection high-energy electron diffraction (RHEED) patternwas observed to gradually dim during the growth of the gradedIn Ga As layer adjacent to the GaAs buffer. This is an indi-cation of the gradual change in the lattice constant. For sampleA, however, the RHEED intensity was abruptly dimmed after afew seconds. This sample consisted of a constant compositionIn Ga As layer grown directly on top of the GaAs bufferlayer without any intermediate graded layer.

Another set of samples grown under identical conditions, butwithout the layers that cover the dots was grown for atomic forcemicroscope (AFM) surface analysis. The areal density of thedots for structure A was estimated to be about 2.510 cm ,and about 2.8 10 cm for structure B. The photomicro-graphs in Fig. 2 show the dot distributions for the two samples.These images are intended to convey qualitatively that the dotdensities are roughly the same for two samples. Because the em-bedded structures are not exactly the same as those used for theAFM surface studies, one should not read too much into theseimages.

Fig. 3. Room-temperature photoluminescence emission of InAs dotsembedded in a compositionally graded (In,Ga)As quantum-well structureand in a constant composition quantum well at an optical excitation level of�30 W/cm . The inset shows the schematic conduction band structure of thetwo samples studied.

Fig. 4. The ratio of the integrated photoluminescence intensity of sample B/Aas a function of excitation level at a sample temperature of 295 K.

IV. EMISSION CHARACTERISTICS

The emission characteristics of sample A and sample B wereanalyzed by photoluminescence spectroscopy from liquid ni-trogen to room temperature. Fig. 3 shows the room-tempera-ture emission spectra for the two samples. The emission for theground-state transition for both samples is well beyond 1.3m.Notice that there is a bump on the high energy side of bothspectra that suggests the presence of an emission from the firstexcited state. Both spectra are fairly broad, indicating inhomo-geneous broadening due to the variation in dot sizes. Fig. 4shows a plot of the ratio of the integrated intensity for sample Bover sample A, as a function of pump power at room tempera-ture. For excitation source fluxes of a little over 100 W/cm, theratio of the integrated photoluminescence intensity of sample Bto sample A is larger than 10. This enhancement in integratedintensity is a function of pump power; higher pumping fluxeslead to larger enhancements. Since the two samples were grownunder identical conditions, the only difference being the natureof the indium profile in the structures, one must attribute theemission enhancement for sample B to the graded indium pro-file. The density of the dots for the samples is comparable; it is

1048 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 5, SEPTEMBER/OCTOBER 2002

Fig. 5. The ratio of the integrated photoluminescence intensity of sample B/Aas a function of sample temperature at an optical excitation level of�10 W/cm .

therefore unlikely that the increase in emission intensity is dueto an increased dot density.

We have studied the temperature variation of the ratio of theintegrated emission intensities for sample A and sample B. Thetemperature of the samples was varied from 78 K to 295 Kduring the experiment. Fig. 5 shows a plot of the ratio of theintegrated intensity for sample B to sample A as a function oftemperature. At temperatures below 200 K, the enhancement inemission for sample B is not that significant; in fact, it is almostflat as a function of temperature. Beyond 200 K, however, thereis a dramatic rise in enhancement. The enhancement approachesan order of magnitude as room temperature is approached (forthe level of pump power used in this experiment). This trendholds for all excitation powers used.

V. DISCUSSION

Emission of long-wavelength (1.3m) radiation from GaAs-based quantum dots is highly desirable because it could lead tothe manufacture of low-cost lasers for telecommunications. Theprimary reason for the desirability is the existence of a matureand robust processing technology for GaAs-based devices. Thequantum-dot lasers would benefit from this infrastructure.

The growth and fabrication of 1.3-m quantum-dot laserson GaAs substrates is still in its infancy. In general, the growthconditions must be optimized for dots of the right physical andstructural parameters to be obtained. One important parameteris the dot size. This, together with the surrounding matrix,determines the emission wavelength, which is inextricablylinked to the strain distribution in and around the dots. Becauseof the nature of the self-assembled quantum dot, its size hastwo spatial components: lateral extent (diameter) and height.The dots in our experiments range in lateral size from about 15to 20 nm, with corresponding heights of between 3 and 5 nm.These estimates are based on our earlier work on InAs–GaAsand InGaAs–GaAs quantum dots [16], and on transmissionelectron microscope (TEM) and scanning electron microscope(SEM) observations. Based on these observations, we infer thatthe appropriate physical model for these dots is a truncatedpyramid. The dot size estimates are further corroborated byin situ RHEED observations where the thickness of an over-

layer needed to completely bury an array of dots (so that thelayer-by-layer growth mode is reestablished) can give a roughmeasure of the dot height. Using these data, we have carriedout electronic spectra calculations to determine the emissionwavelengths of dots of varying sizes. The energy calculationsrequire knowledge of the strain distribution in and aroundthe dots. The details of the calculations have been reportedelsewhere; here, we give only the salient points of the method.Beginning with the Lamé potential [17] in classical elasticitytheory, one can show that the stress components for the dot aregiven by [18]

(1)

where and are unit vectors in theth and th directions, suchthat

and . The parameter is the lattice mismatch,is the Young’s modulus, is Poisson’s ratio, and is a

Kronecker delta function. The point is on the surface of adot. The lattice mismatch is taken to be negative for a materialunder compression. The first integral in the equation above isperformed over the surface of the dot; the second integral is overthe volume of the dot. The volume of a square-based, truncatedpyramidal dot is defined by

(2)

where is the height of the untruncated pyramid,is its base,and , where represents a truncation factor. Theaxis is defined to be the [001] growth direction; the origin of thecoordinate system is at the center of the pyramid (on thebasal plane).

The strain components which enter the electronic structurecalculations are obtained from the stress as

(3)

The strain-dependent conduction band-edge is usually writtenas

(4)

where is the offset of the unstrained conduction band-edge;this is

(5)

is the strain-induced shift of the conduction band which isexpressed as

(6)

CHEN et al.: IMPROVED LUMINESCENCE FROM QUANTUM-DOT NANOSTRUCTURES 1049

Fig. 6. The dependence of the ground-state emission wavelength of InAsquantum dots embedded in an In Ga As quantum well on the sizeparameters of the dot (basewidth and height).

In (5), is the spin-orbit splitting, is the unstrainedbandgap, and is the unstrained average valenceband-edge. The parameterin (6) is the deformation potentialfor the conduction band.

The effect of strain on the valence band depends largely onthe symmetry of the strain. The heavy- and light-hole bands,

and , couple to the individual strain components viathe relations [19]

(7)

and

(8)

where , and . Here, and are thedeformation potentials, and and are the hydrostatic andthe biaxial strains, defined as

(9)

and

(10)

respectively.Our numerical calculations for the dependence of the peak

emission wavelength for dots of various sizes are summarized inFig. 6. These spectra are for dots expected to emit in the vicinityof the important telecommunications wavelength of 1.3m.Several ratios of the base to height dimension, , are plottedas parameterized variables, while the base size is changed. Notethat is defined as the truncated height of a pyramidaldot. Our analysis suggests that the emission wavelengths for thedots depend more acutely on their heights than on their lateralextent.

We have shown that InAs quantum dots embedded insidesymmetric, graded (In,Ga)As quantum wells, exhibit higheremission efficiencies than similar dots inside constant com-position wells. We argue that the enhancement in emission isprimarily due to the high structural and optical quality of thematrix surrounding the dots. The grading, we believe, has the

effect of minimizing the formation of defects that could actas nonrecombination centers. This hypothesis is supported byevidence from previous work on InAs–GaAs quantum dotsby Le Ru et al. [20], who observed similar enhancements inemission intensity. The enhancement in their case, however,was observed after subjecting the quantum-dot nanostructuresto a hydrogen-passivation treatment. The basic structure usedin the experiment by Le Ruet al. is similar to that of Fig. 1(a).The reason for the emission enhancement in their structures isthought to be related to the passivating effects of hydrogen. Itis suggested that the hydrogen neutralizes or passivates shallowand deep defects and impurities/dislocations at the InAs–GaAsinterfaces. The defects are attributed to the large lattice mis-match between the InAs and GaAs layers. If this is true, thegraded (In,Ga)As layers in our structure should minimize theformation of such defects in the first place; this is so becauseof the smaller lattice mismatch associated with the gradualgrading of the composition of the intervening (In,Ga)As layers.The grading results in improved structural and optical qualityand hence enhanced luminescence.

VI. SUMMARY

We have shown that by grading the potential of aquantum-well structure into which InAs dots are embedded,the luminescence efficiency can be enhanced. The increase inluminescence can be nearly an order of magnitude over thatobserved for dots inserted inside constant composition wells.The enhancement in luminescence is thought to be a resultof the improved structural and optical quality of the layerssurrounding the dots. Because of the compositional grading oneither side of the dots, the lattice mismatch between the dotmaterial and the surrounding matrix is reduced. A reduction inlattice mismatch leads to fewer stress-induced defects which inturn, results in an improved luminescence efficiency.

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L. Chen (S’99) received the B.E. degree from the University of Science andTechnology of China (USTC), Hefei, Anhui, and the M.E. degree from theChinese Academy of Sciences (CAS), Beijing. He was a Ph.D. student at theUniversity of Virginia, Charlottesville.

Currently, he is with the Department of Electrical and Computer Engineering,Carnegie Mellon University, Pittsburgh, PA. His research interests include semi-conductor quantum dots growth, characterization, and application on lasers anddetectors.

V. G. Stoleru received the B.S. and M.S. degrees from the University ofBucharest, Bucharest, Romania, and the Ph.D. degree from the University ofVirginia, Charlottesville.

She is currently a Postdoctoral Fellow at the Department of Electrical andComputer Engineering, Carnegie Mellon University, Pittsburgh. Her researchprojects include studies of ordering self-assembled III-V quantum-dot nano-structures grown by molecular beam epitaxy for optoelectronic devices appli-cations, by using prefabricated templates.

E. Towe received the S.B., S.M., and Ph.D. degrees from the MassachusettsInstitute of Technology, Cambridge, where he was also a Vinton Hayes Fellow.

He is currently on the faculty of Carnegie Mellon University, Pittsburgh, PA,where he is a Professor of electrical and computer engineering, and materialsscience and engineering. From 1997 to 2001, he divided his time between theDefense Advanced Research Projects Agency (DARPA), Arlington, VA, and theUniversity of Virginia, Charlottesville. At DARPA, he led the agency’s researchefforts in photonics.

Prof. Towe is the recipient of several awards, including the National ScienceFoundation Young Investigator Award, the Outstanding Technical AchievementAward from the Office of the U.S. Secretary of Defense, the Commonwealthof Virginia Scholar Award, and the Young Faculty Teaching Award from theUniversity of Virginia.