丁玉成等综述Ordering, positioning and uniformity of quantum dot arrays

Nano T oday(2012)7,94—123REVIEWOrdering,positioning and uniformity of quantumdot arraysHongbo Lan a,b,∗,Yucheng Ding a,∗a Centre for Micro and Nano Manufacturing,State Key Laboratory for Manufacturing System Engineering,Xi’an Jiaotong University,Xi’an710049,Chinab Nanomanufacturing and Nano-Optoelectronics Lab,Qingdao T echnological University,Qingdao266033,ChinaReceived30October2011;received in revised form16February2012;accepted20February2012Available online16March2012KEYWORDSQuantum dot arrays;Uniformity andordering;Site control;Patterned substrateSummary Quantum dot(QD)arrays have now been attracting tremendous attention due tothe potential applications in various high performance devices(e.g.,QD lasers,3rd generationsolar cells,single photon emitters,QD memories,etc.),the fundamental investigation of quan-tum computing and quantum communication,and in the exploration or observation of novelphysical phenomena.Uniform and regular QD arrays with precisely controlled positions andsizes may serve as a template for the next generation of nanoelectronic and optoelectronicdevices.Currently,the major challenging issues in commercialized application of QD arraysinclude fabrication of large-area,defect-free,highly uniform and ordering QDs,accurate posi-tioning for individual QD nucleation site,and reproducibility in size and spatial distribution,which all crucially determines optoelectronic performance and consistency for these QDs-basedfunctional devices and the investigation of fundamental physical properties for QDs.Over thepast decade,enormous attempts have been made to improve the ordering,positioning,unifor-mity,and defect reduction for obtaining perfect QD arrays over a large area with long rangeordering.This article provides a review of some major attempts and progresses recently madefor enhancing the ordering,positioning and uniformity for QD arrays,with an emphasis on theproblems which has been well addressed to reach the current state of the arts.Furthermore,theprospects,challenges and trends for producing high quality QD arrays with high ordering,uni-formity,positioning and defect reduction,are addressed.Finally,some potential or promisingsolutions for achieving perfect QD arrays are discussed.©2012Elsevier Ltd.All rights reserved.∗Corresponding authors at:Centre for Micro and Nano Manufac-turing,State Key Laboratory for Manufacturing System Engineering,Xi’an Jiaotong University,Xi’an710049,China.T el.:+862983399518;fax:+862982660114.E-mail addresses:hblan99@(n),ycding@(Y.Ding).IntroductionQuantum dots(QDs)are nanoscale and three-dimensionallyconfined semiconductor structures,which exhibit quantumeffects such as sharp density of states.Semiconductor quan-tum dot structures have now been attracting great interestbecause of their peculiar physical properties as well as1748-0132/$—see front matter©2012Elsevier Ltd.All rights reserved.doi:10.1016/j.nantod.2012.02.006Ordering,positioning and uniformity of quantum dot arrays95their potential applications in high performance devices[1]. The unique advantages of quantum dot ensembles or indi-vidual QD can be fully harvested only if the dots are as uniform as possible in shape and size and the correspond-ing nucleation sites are precisely positioned.In particular, quantum dot arrays with high uniformity,positioning and ordering have demonstrated the great potential for a wide variety of applications,ranging from nanoelectron-ics,optoelectronics quantum functional devices operating at telecom wavelengths,to quantum information process-ing and quantum computing(e.g.,QD lasers,solar cells, single-electron transistors,single photon emitters,QD mem-ories,infrared photodetectors,etc.)[2—4].However,the industrialized application of QD arrays is faced with some major challenging issues,including generation of defect-free,highly uniform and ordering QDs over a large-area, accurate controlling of individual QD nucleation site,as well as reproducibility in size and spatial distribution,which can play a crucial role in the realizations and further per-formance improvement of these QDs-based devices and in the investigation of fundamental physical properties con-cerning the QDs.Since optical and electrical properties, and acute responses essentially depend upon the unifor-mity of QD size,shape and composition,and upon a precise positioning of individual QD nucleation site,as well as pre-dictable and reliable controlled applications,large-area QD arrays with uniformity and perfect long range ordering are demanded.The large sizefluctuation of QDs results in signif-icant inhomogeneous broadening,diminishing the potential advantages of their use in optoelectronic devices,such as those with low threshold current density,narrow gain band-width,and increased critical temperature.Numerous studies have been performed to explore the processes for producing QD arrays with accurate site con-trol(high ordering and positioning)and with identical size, shape,and composition(high uniformity).Such quantum dot arrays would be desirable for realizing high performance lasers or photonic crystal devices,single photon emitters, as well as for inducing efficient coupling between QDs and cavity modes[2—4].For instance,an ideal QD laser should consist of high density3D QD arrays with equal size and shape surrounded by a higher bandgap material which con-fines the injected carriers.The whole structure is embedded in an optical waveguide consisting of lower and upper cladding layers[5].One of the most promising and chal-lenging methods for achieving high power efficiency(>60%) solar cells is to utilize periodic3D arrays of the semiconduc-tor quantum dots[6,7].Highly ordered InAs/InP quantum dot arrays(1D,2D,3D)are the basis for the realization of novel quantum functional devices operating at telecom wave-lengths[8].For single photon emitters and device integration,the ability to place an individual QD precisely at a pre-defined location is of great ly,QD arrays with accurately positioned nucleation sites and a low density are critical.Moreover,the precise position control of individual quantum dots in an array is also necessary for the integration in photonic cavities or for further processing of the devices[8].Laterally ordered semiconductor QD arrays are highly demanded due to the shrinking feature sizes in microelec-tronics and especially in the emerging quantum functional devices employing semiconductor QDs[9,10].The control of lateral QD alignment has been considered as the key to next generation quantum functional and optoelectronic devices where quantum mechanical and electromagnetic interactions of single and multiple electrons and photons are well-controlled within the ordered QD networks[3,8]. Perfectly ordered QD arrays are also required for the obser-vation of novel physical phenomena such as interference effects[11]and enhancement of excitonic optical nonlin-earities[3,8].The realization of well-positioned QD arrays, thus,gives rise to interesting physical phenomena which modify the fundamental material properties,providing the basis for nanoelectronics/optics in future functional devices utilized for quantum information processing and quantum computing.In addition,coupled QD array system with dot-to-dot interaction is a very promising candidate for the logical unit in a quantum computer,as called‘‘qubit’’or quantum bit,which in principle can be realized by a two-level quantum system,e.g.,the horizontal or vertical polarization of a photon or the up and down states of an electronic spinning[12,13].Thus,not only the size,shape, and emission wavelength control are needed,but also is a spatially well-positioned QDs nucleation essential[9,14,15].QDs can be formed by epitaxial growth from the vapor phase molecular beam epitaxy(MBE),metallo-organic chemical vapor deposition(MOCVD),chemical synthesis (colloidal chemistry or electrochemistry)and semiconductor processing methods.Conventional semiconductor process-ing techniques based on lithography and etching are faced inherently with problems such as limited resolution,high cost,and possible introduction of surface defects,which tend to degrade the optical and electrical properties of QD-based devices[16].One of the most promising approaches for fabricating low-dimensional semiconductor QDs is a self-assembly process under the Stranski—Krastanov(S—K) growth mode.The self-assembled quantum dots nucleate spontaneously under certain conditions during MBE and MOCVD,when a material is grown on a substrate to which it is not lattice-matched.The growth initially progresses with a pseudomorphic growth mode forming a strained2Dfilm, the wetting layer.When this strain accumulates in thefilm and reaches a certain amount at the critical thickness,the growing material matrix may experience a phase transition from a2D layer-by-layer growth to a3D island growth.This growth mode is known as Stranski—Krastanov(S—K)growth. The important term to determine the S—K growth mode is the lattice mismatch between the two epitaxial layers.For instance,the common combination of InAs and GaAs has a lattice mismatch of7%,Ge/Si system has a lattice mismatch of4%.The islands can be subsequently buried to form the QDs.The semiconductor QDs grown by S—K growth mode has been well established as a reliable growth technique, and also considered as one of the most promising candidates for applications in QD-based devices due to the ease in fab-rication and their high-quality,defect-free properties.The QDs grown by S—K growth mode are defect-free giving rise to structural and optical properties of a high quality.How-ever,due to the inherent limitation of the self-assembling process(this approach is not ideal and is complicated by the randomness of the QD size distribution and inherent presence of the wetting layer.Furthermore,wetting layers are inevitably produced as a step in the formation process of self-assembled QDs),the QDs sizefluctuation,materialn,Y.Dingintermixing,and randomly distributed nucleation sites are inevitable.But,the large size variation of these QDs typically results in significant inhomogeneous broadening, diminishing the potential advantages of their use in opto-electronic devices.For an ideal case,the energy levels of all QDs should be the same.This means that the size,shape,and composition of the QDs need to be identical to the greatest extent.Then,the inhomogeneous broadening of QD lumines-cence due to sizefluctuations can be eliminated,resulting in a real concentration of the electron energy states[8].In addition,controlling the nucleation site of quantum dots is also one of the key issues in the nano-scale design of optoelectronic functional QD devices.Furthermore,the site-control plays an essential role for these QDs used for quantum computing and quantum communications,since a regular array of quantum bits and a single photon emit-ter can be formed by this technique.However,this random nucleation makes it difficult to address each individual self-assembled QD.It is a big challenge to reproduce and exactly control the nucleation site of each individual QD.Moreover, another disadvantage associated with self-assembly is its inability to create long-range-ordered structures[17].The QDs were formed using the S—K growth mode.The area densities range between2×1010and l×1011cm−2with a typical size distribution of∼10%[5].Some functional QD devices such as QD laser,QD memory require a large num-ber of QDs in the desired region with a high density.On the other hand,considering the application of QDs in sin-gle photon-emitting devices,such as a microcavity quantum electro-dynamics device,in order to isolate single QD emis-sion in practical device structures a QD density approaching 108cm−2is required(the density of QDs should be rather low).In other words,it is preferred that one QD is formed at the center of the cavity where the electricfield is maxi-mum,in order to achieve high-performance operation.It is very difficult for QDs grown by S—K growth mode to obtain low density QDs.It is almost impossible to place QDs in an organized way and at predesigned locations.Therefore,in a summary,applicability of QD arrays may at least be faced with the following challenging issues:1)Homogeneity(uniformity)of QD arrays in size,shape andcomposition;2)Accurate positioning of individual QD nucleation site inQD arrays;3)Proper control(tuning)of QD arrays density from lowdensity to high density;4)Generation of large area,high density,long-range order-ing QD arrays with high optical property;5)Mass production of perfect QD arrays over large area.In order to achieve large-area,defect-free QD arrays with high uniformity and ordering(also called perfect QD arrays),a number of investigations and efforts have been performed over the past decade.The objective of this paper is to present a comprehensive review of recent advances in creating highly ordering and uniform,well-positioning QD arrays.Furthermore,the prospects,challenges and future development directions of making high quality QD arrays are also addressed and discussed.As a result,some possible or promising solutions for achieving perfect QD arrays are discussed.The paper may provide a reference and direction for the further explorations and studies of perfect QD arrays.The rest of this paper is organized as follows.Sec-tion‘Basic characteristics and a classification of QD arrays’introduces principal characteristics and classification of QD arrays.Section‘Formation of QD arrays on non-patterned substrates’presents the formation of QD arrays on non-patterned substrates.Generation of QD arrays on patterned substrates are described in detail in section‘Generation of QD arrays on patterned substrates’.Furthermore,some other methods for fabricating QD arrays are also addressed in section‘Other approaches for formation of QD arrays’.In addition,three key issues are discussed in section‘Discus-sions for three key issues’.Prospects,challenges and trends in producing perfect QD arrays are elaborated in section ‘Discussions for three key issues’.Finally,section‘Conclu-sions’summarizes this paper.Basic characteristics and a classification of QD arraysPrincipal characteristicsSome key concepts or characteristics arefirstly addressed which can play an important role to accurately describe the properties of QD arrays and better understand the nature of QD arrays.These features of QD arrays are described as follows.Density(high density and low density)The density of QD arrays(i.e.,the number of QDs over a unit area)can be roughly divided into high density(>1010cm−2) and low density(<108cm−2).Some QD devices require a large number of QDs in the desired region with a high density. For instance,the requirement of QD density for the inter-mediate band solar cells(IBSC is about5—10×1010cm−2); the sheet density is estimated to be about1011cm−2for high efficient QD lasers.On the other hand,some QD-based devices and applications(e.g.,single photon emitters and microcavity quantum electro-dynamics device),low density (dilute)QD arrays and single QD are required and desirable. In other words,it is preferred that one QD is formed at the center of the cavity where the electricfield is maximum,in order to achieve high-performance operation.The density ranging from2×1010to l×1011cm−2with a size distribu-tion of∼10%is the typical range for S—K QD ensembles. S—K growth beyond this range is certainly difficult but may be achieved and was reported.However,it is very difficult to achieve low density QD arrays by the self-assembly pro-cess.The QDs growth on patterned substrates has the ability to achieve both high dense QDs and low dense QDs.Growth on the patterned substrates has been shown to result in a low QD density,by preferential QD nucleation in nanoholes or growth of QDs on pyramidal structures.Alternatively, it is also possible to significantly control reducing the QD density on conventional substrates through accurate con-trol of growth conditions,for example by reduction of the InAs quantum dot growth rate together with further sam-ple treatment such as mesa etching to isolate the single QD emission has been demonstrated[9,18,19].Ordering,positioning and uniformity of quantum dot arrays 97Ordering (lateral ordering and vertical ordering;short-range ordering and long-range ordering)The ordering of QD arrays involves generally two aspects:lateral ordering and vertical ordering;short-range ordering and long-range ordering [3,17,20].The lateral ordering of QDs (lateral alignment of QDs)refers to that the QDs can accurately control the nucleation site in a planar .The lateral ordering of self-assembled QDs and QD arrays has been under investigation because many emerging QD-based devices such as single photon emitters or single turnstile devices will eventually require the strict positioning of individual QD.A variety of approaches have been employed to obtain lat-eral ordering.The most promising approach to force QDs into lateral order seems to be the growth on patterned sub-strates.It is important to form electrically and optically active QD layers with perfect crystal quality far away from the contaminated interface.Therefore,the vertical align-ment of QDs is a useful technique to transfer the initial surface modulation to the growth front through laterally strain-modulated buffer layer .This buffer layer can consist of a laterally strain-modulated superlattice or stacked self-assembled QDs.The self-assembled QDs stacked would form a three-dimensionally ordered QD superlattice.Vertical ordering can be improved by the multilayer stacking pared to the vertical ordering,the lateral ordering of QDs and QDs arrays may play a much more important role for QD-based devices and applications.Various methods includ-ing strained multilayer stacking,pre-patterned substrates,multi-atomic steps,cleaved edge overgrowth,and multi-layer stacking on high-index substrates have been proposed to enhance the lateral and vertical ordering of QDs and QD arrays [3].Three-dimensional stacking of self-assembled QDs in multilayers or superlattice structures has turned out to be one of the most effective techniques for control-ling the vertical and lateral arrangement of the QDs.The semiconductor QDs grown by S—K growth mode can obtain QD arrays with short-range ordering.The primary disadvan-tage associated with self-assembly is its inability to create long-range-ordered structures.However ,long-range order characterizes physical systems in which remote portions of the same sample exhibit correlated behavior .Long-range ordering quantum dot arrays might be useful for a high inte-gration of single quantum dot devices,or to realize one,two,and three-dimensional quantum dot crystals.It is this lack of long-range order that prevents the widespread use ofself-assembly for many technological applications.The QDs growth on the patterned substrates has the ability to create the QD arrays with long-range ordering.Unfortunately ,the technique is hard to make such defect-free QD arrays hav-ing long-range ordering [21].Many efforts to obtain perfect long-range order over large are via the self-assembly process have been frustrated in practice as ensuing patterns contain defects [17].Dimensionality (1D,2D,3D QD arrays)According to the difference of the spatial dimension,there are three kinds of types for QD arrays which involve one-dimensional (1D),two-dimensional (2D),and three-dimensional (3D)QD arrays,as shown in Fig.1.High uniform and ordering 1D,2D QD arrays can be achieved by epitaxial growth on the patterned substrates [3,8,22].3D QD arrays have been formed by successive deposition of epitaxial QD layers;after the first layer of epitaxial QDs is formed,suc-cessive layers tend to form with the QDs in each layer aligned on top of each other .One can add a new dimension to the study of quantum-dot lasers by stacking the individual lay-ers.Because of the interacting strain fields,islands in one layer tend to form above those in the layer below,if the sep-aration is not too great.Stacking layers of dots in the laser structure increase the amount of active materials in devices.Significant reduction in photoluminescence linewidth seen from the stacked layers is attributed to the more uniform dot sizes caused by the correlated nucleation of the dots.Better uniformity of dots is achieved at the top surface of a stack than an iso1ated monolayer .Vertical correlation and elec-tronic coupling/decoupling between the dots open up the possibility of some novel micro-and optoelectronic devices.Fig.2schematically shows the formation of vertical stacks due to stress distribution [24].Material (group III—V;group II—VI,group IV—IV)Semiconductor QDs and QD arrays can be fabricated using different material systems which can be categorized mainly into three types:III—V (e.g.,GaAs,InAs,InP ,etc.),II—VI (e.g.,CdSe,CdT e,ZnS)and IV—IV (Si/Ge).III—V and II—VI compound semiconductors have shown high luminescence efficiencies and have been widely used in room temperature LEDs and lasers.However ,such compound semiconductors are currently difficult to integrate with conventional electronic devices,due to the processincompatibilityFigure 1Type of ordered QD arrays (a)1D QD arrays,(b)2D QD arrays,(c)3D QD arrays.Reprinted with permission from Reference [23].Copyright 2008Springer .n,Y.DingFigure2Schematic of spontaneous synthesis of nanostructures via growth of highly strained materials[24].with the current CMOS technique.Silicon is a well-known indirect-bandgap semiconductor.Bulk silicon does not emit visible light and cannot be applied in photodevices.Many efforts have been performed to resolve the low quantum efficiency of silicon associated with its indirect bandgap. One important approach is the combination of Si with other semiconductors,such as Ge for heterostructures.Due to the simplicity and the sophisticated integration technique on Si,Ge/Si system has been an interesting subject of self-assembled islands.A number of investigations have been conducted to produce the Si/Ge QDs and QD arrays [25—27].Area(large area and small area)For QD-based devices,non-uniformity of size,shape and composition is clearly undesirable,as the resulting struc-tures will exhibit a broad range of electronic and optical properties,effectively smearing out the sought-after zero-dimensional behavior of the dot ensemble[28].Currently, producing the uniform and ordering QD arrays is mainly confined in the small areas.It is considerably difficult to gen-erate large area(∼cm2)QD arrays with high uniformity and ordering by using existing techniques.In addition,it is hard even impossible to obtain large-scale ordered arrays with-out some forms of external guidance.The most promising approach to grow large-scale arrays of quantum dots is self-assembly.Unfortunately,large-scale self-assembled arrays are imperfect,consisting of multiple domains,domain walls, and defects.It is easy to understand why large-scale self-assembled arrays are seldom perfect.Considerfirst the case where the deposition is isotropic.When theflatfilm destabilizes to a self-assembled array,it loses orientational and translational symmetries.Hence there is a‘‘Goldstone mode’’of equivalent solutions,which in this case are related to each other by translations and/or rotations.In other words,an array that emerges spontaneously can have any orientation,and the specific direction along which the arrays form depends on uncontrollable factors like local stochas-tic effects.In a large scale array,sites far from each other emerge with independent orientations.As these arrays grow beyond a characteristic size,they fail to merge to a sin-gle domain,thus leaving a pattern with multiply oriented domains and associated defects[21].Epitaxial processes(MBE,MOCVD,CBE)Fabrication methods of self-assembled QD arrays mainly include:MBE,MOCVD,and CBE.The basic principle for MOCVD,MBE,and CBE is demonstrated in Fig.3.Molecu-lar beam epitaxy is one of several methods of depositing single crystals.MBE takes place in high vacuum or ultra high vacuum(10−8Pa).The most important aspect of MBE is the slow deposition rate(typically less than1000nm/h),which allows thefilms to grow epitaxially.The slow deposition rates require proportionally better vacuum to achieve the same impurity levels as other deposition techniques.During the operation,reflection high energy electron diffraction (RHEED)is often used for monitoring the growth of the crystal layers.In systems where the substrate needs to be cooled,the ultra-high vacuum environment within the growth chamber is maintained by a system of cryopumps, and cryopanels,chilled using liquid nitrogen or cold nitro-gen gas to a temperature close to77Kelvin(−196◦C).MBE has several advantages over metal—organic chemical vapor deposition.For example,films with lower point defect con-centration have been achieved.Problems in MBE are due to the difficult temperature control of the effusion cells since the change in effusion cells temperature is dramat-ically altered the beamflux and growth rate.Moreover,the temperature profile the group III cells change with consump-tion,thus the regular calibration is required.Metal—organic chemical vapor deposition,also known as organometallic vapor phase epitaxy(OMVPE)or metal—organic vapor phase epitaxy(MOVPE),is a chemical vapor deposition method of epitaxial growth of materials,especially compound semi-conductors from the surface reaction of organic compounds or metalorganics and metal hydrides containing the required chemical elements.In contrast to MBE the growth of crys-tals is by chemical reaction and not physical deposition. This takes place not in a vacuum,but from the gas phase at moderate pressures(2—100kPa).As such,this technique is preferred for the formation of devices incorporating ther-modynamically metastable alloys,and it has become a major process in the manufacture of optoelectronics.The advan-tage of MOCVD growth is theflexibility of the vapor sources and the ability to mass production which makes the tech-nique very suitable for the industry.However,the safety and responsible environmental care have become major fac-tors of paramount importance in the MOCVD-based crystal growth of compound semiconductors.Chemical beam epi-taxy(CBE)is a newer development in epitaxial technology which combines the beam nature of MBE and the use of all vapor sources as in MOCVD.CBE is a process in which metal alkyls(e.g.,triethylgallium),as group III sources,and hydrides(e.g.,mine).As group V sources,are used to formOrdering,positioning and uniformity of quantum dot arrays99Figure3Fundamental epitaxial processes(a)MOCVD,(b)MBE,(c)CBE[8,29].the corresponding intermetallic crystalline compound in an ultra-high vacuum(UHV)chamber.The beam nature in CBE allows it to prepare semiconductor heterostructures with monolayer abruptness and thickness control as achieved by MBE,where the use of vapor sources provides pre-cise reproducibleflux control with instantaneous response [8,29—31].A classification of QD arraysBased on the different criteria,QD arrays can be categorized into six forms,as shown in Fig.4.T able1presented a summary for previous studies and literatures regarding the enhancement of ordering,posi-tioning and uniformity for QD arrays and various fabrication methods for QD arrays.According to the summarized results, most research works focus on the following several aspects: epitaxial growth using MBE;materials based on III—V;high density;and lateral ordering as well as accurate position-ing for individual QD nucleation site.Producing QD arrays using the MOCVD process should be widely and deeply inves-tigated.In addition,absolutely accurate controlling and positioning individual QD nucleation site need also be further conducted in the future works.Formation of QD arrays on non-patterned substratesThe self-assembled QD arrays grown by S—K mode on a non-patterned substrate are promising candidates for producing quantum functional devices due to their ease in fabrication and their high-quality,defect-free properties.Substantial knowledge has been acquired for the growth on planar sub-strates.However,QDs grown using S—K mode are usually randomly distributed on the growth surface and suffer from fluctuations in size and strain in a random manner.It is almost impossible to place QDs in an organized way and at predesigned locations.The sizefluctuation also results in large inhomogeneous broadening in their energy spec-trum.This seriously limits potential-device applications of the QDs.For the QDs growth on the patterned substrates, the structural and,in particular,optical properties of the dots are significantly degraded due to defects which usually arise from lithographic and etching imperfections.Recently,the technique of‘‘self-organized anisotropic strain engineering’’has been developed and led to a novel method for QD ordering maintaining high structural prop-erty and optical quality.The lateral ordering of epitaxial QDs by self-organized anisotropic strain engineering relies on the creation of well-ordered multilayered superlattice。