@anturn Dots Feature

Quantum wires and dots show the way forward

Zh.1. Alferov, A.F. loffe Physico-Technical Institute

I

Concluding his three-part series on the pivotal role of compound semiconductor heterostructures, Professor Alferov of the loffe Institute in St. Petersburg reviews the development and recent exciting advances in quan- tum wires and, in more detail, quantum dot structures.The future trends and perspectives of these new types of heterostructures are discussed.

D uring the 198Os, progress in two-dimensional (2D) quan- tum well (QW) hetero

structures physics and its appli- cations attracted many scientists to the study of systems of lower dimensionality - quantum wires (QWr) and quantum dots (QDs). In contrast to quantum ‘wells’ where carriers are localized in the direc- tion perpendicular to the layers but move freely in the plane of the layer, in quantum ‘wires’ carriers are localized in two dimensions and move freely only along the ‘wire’ axis. If confined in all three directions, quantum ‘dots’, or ‘artifi- cial atoms’, with a totally discrete energy spectrum, are created. Figure 1 shows the density-of-state function in the cases of QWs, QWrs and QDs.

Figure 1. Schematic diagrams of the densi- ty-of-state functions for (a) quantum well, (b) quantum wife, and (c) quantum dot structures.

Experimental work on the fabri- esting applications of QWr lasers cation and investigation of QWr was being carried out by Y. structures began more than ten Arakawa and H. Sakaki. These years ago. At the same time, thee authors pointed to the possibility retical consideration of problems of weakening the threshold cur- concerning one of the most inter- rent dependence on temperature

- (d)

(cl

(b)

(a) (a) To = 104 “C

(b) To = 285 “C

(c) T, = 48 1 OC

(d) T,, = * “C

0.5 I I I I I I I I I I I I I

-60 -40 -20 0 20 40 60

Temperature (“C)

Figure 2. Temperature dependence of the normalized threshold current for dii%mnt double hetemstructure (OHS) lasers: (a) bulk; (b) quantum well; (c) quantum wire; and (d) quantum dot. T,, is the characteristic tempemtum of the device.

0981-1290/98/$19.000 1998 Elsevier Science Ltd. All rights reserved

Ill-Vs Review ??Vol.1 1 No. 2 1998 47

Quantum Dots Feature

\----+20nrn

cow I

InAs

GaAs

Figure 3. Vertical and lateral ordering of coupled InAslGaAs quantum dots. Cross-section high-resolution electron microscope image of QDs brmed by a three-cycle InAs-GaAs deposition (above), and the plan-view TEM image (below). (After N. N. Ledentsov Proceedings of 23rd International Conference on the Physics of Semiconductors, Berlin, Germany 1996 (M. Schemer, R. Zimmerman, eds) World Scientific, Singapore (1996), p. 19.)

for QWr lasers and of attaining full temperature stability for QD lasers (Figure 2). To date there are a sig- nificant number of both theoreti- cal and experimental papers in this field, exploring: transport and capacitance in wires; vertical and lateral tunnelling in QWr and QDs structures; and far-infrared (IR) photoluminescence (PL), Raman spectroscopy, optical gain and peculiarities in optical properties.

Nevertheless, the realization of the potential applications of QWr structures belongs to the future, and advances in this area are prov- ing rather slow.

The first semiconductor dots based on II-VI microcrystals in a glass matrix were proposed and demonstrated by A.I. Rkimov and A.A. Onushenko . This work sparked important theoretical stud- ies of QDs, initiated by A. Efros and

A.L. Rfros at the Ioffe Institute. However, because the semiconduc- tor QDs were introduced into an insulating glass matrix and the quality of the interface between the glass and semiconductor dot was rather poor, both fundamental studies and device applications were limited. More exciting possi- bilities have appeared since three- dimensional coherent QDs were fabricated in a semiconductor matrix by L. Goldstein and co workers in 1985.

Self-organization Several methods have been pro- posed for the fabrication of these structures. Indirect methods, such as the postgrowth lateral pattem- ing of 2D quantum wells, often suf- fer from insufficient lateral resolution and interface damage caused by the patterning proce- dure. A more promising route is fabrication by direct methods, that is, growth in V-grooves and on cor- rugated surfaces which may result in formation of QWrs and QDs. In the last few years, groups at the Ioffe Institute in Russia and Berlin Technical University in Germany have cooperated closely in this research and have contributed sig- nificantly to the field of QDs.

Eventually, it was concluded that the most exciting method for the formation of ordered arrays of QWrs and QDs is the phenomenon of self-organization on crystal sur- faces. Strain relaxation on step or facet edges may result in formation of ordered arrays of QWrs and QDs both for lattice-matched and lat- tice-mismatched growth. Spontan- eous formation of different ordered structures with a periodic- ity much larger than the lattice parameter on crystal surfaces was a subject of intense theoretical investigations. Outstanding contri- butions were made to this area in the early 1980s by AX Andreev, who studied faceting transitions in crystals, and VI. Marcher&o with his theory of the equilibrium

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Quantum Dots Feature

shape of crystalsThe ultimate limit is the fabrication of an ideal semi- conductor atom-like QD, having 6 function-like energy levels. In order to fully realize the advan- tages of this approach, one needs to create a dense and uniform army of wires and dots, otherwise inhomogeneous broadening can completely eliminate the advan- tages of reduced dimensional&y. The size of the nanostructures should not exceed a few nanome- tres in order that the energy sepa- ration between electron and hole sublevels is in the range of several kT at room temperature (R’I). In addition, they should be free from dislocations and defects.

One mechanism of ordered nanostructure formation is fac- eting, where a planar crystal sur- face rearranges into a periodic hill-and-valley structure in order to decrease the surface free energy. Subsequent heteroepitaxial growth on faceted surfaces under optimized growth conditions may result in formation of corrugated superlattices.

Another class of self-organized structures suitable for QWr and QD fabrication is related to the spontaneous formation of ordered arrays of monolayers: highly strained islands formed during sub monolayer deposition of material having large lattice mismatch with the substrate. Recently, very uni- form arrays of threedimensional nanoscale islands (QDs), which also exhibited lateral ordering, were realized by monolayer depo- sition in the InA&aAs system, both by molecular beam epitaxy (MBE) and metal organic chemical vapour deposition (MOCVD) growth methods [ 11.

Elastic strain relaxation on facet edges and island interaction via the strained substrate are the driving forces for the selt&ganization of ordered arrays of uniform, strained islands on crystal surfaces [2]. In most cases, experiments show the islands to have a rather narrow size distribution [l] and, further, that

under certain conditions coherent islands of InAs form a quasi-period- ic square lattice.The shape of QDs can be signiticantly modified dur- ing regrowth or post-growth arm- ealing, or by applying complex growth sequences. Short period alternating deposition of strained materials leads to a splitting of QDs and to formation of vertically coupled QD superlattice structures (Figure 3) [ 3,4]. Ground state QD emission, absorption and lasing energies are found to coincide [l]. The observation of ultra-narrow (X0.15 mev) luminescence lines from single QDs [l], which do not exhibit broadening with tempem- ture between 5-50K (Figure 4) is proof of the formation of an elec- tronic QD.

Superior properties QD lasers are expected to have superior properties compared with conventional QW lasers. High differential gain, ultmlow threshold current density and high tempem- ture stability of the threshold cur- rent density are expected to occur simultaneously Additionally, ord- ered arrays of scatterers formed in

an optical waveguide region may result in distributed feedback and/or in stabilization of single- mode lasing. Intrinsically buried QD structures spatially localize car- riers and prevent them from recombining nonradiatively at res- onator faces. Overheating of facets, which is one of the most impor- tant problems for high-power and high-efficiency operation of AlGaA&aAs and AlGaAs-InGaAs lasers, may thus be avoidable.

In 1994 we published results showing that InGa& QD lasers have ultrahigh temperature stabili- ty of the threshold current density, with a characteristic temperature (Td of about 350400 K over a wide temperature range (30150 K), and low threshold current den- sity (120 A.cme2) over a broad tem- perature range (70-150 K). In addition, single longitudinal mode lasing is observed both at low and high (300 K) temperatures. The characteristic temperature (Ta = 350 K) significantly exceeds the theoretical limit for a QW laser.

A relatively small energy separa- tion between the QD ground exci- ton state and the wetting layer (-100 mev), and between the QD

0 10 20 30 40 Lattice temperature (K)

50

res. limit

60

Figure 4. Temperature dependence of CL peak width at half maximum, compating sing/e dots with systems of higher dimensionalities. (After MGrundman et al., Phys. Rev. Lett. 74 (1995) 4043.)

Ill-Vs Review ??Vol.1 1 No. 2 1998 49

Quantum Dots Feature

& m InAs (‘)

Al content

42% f- SL AlGaAs(201$) (‘1 GaAs( 1

7x1017 cm-3 2x1019 cm-3 _ for GaAs ,. . I

Figure 5. VerticaNy coupled QD (VCQD) lasers: (a) schematic representation of the vertica/iy coupled three-sheet lnAslGaAs dots; (b) TEM cross-section of VCQDs; and (c) schematic of the laser structure. (After Zh.l.Abrov et al. 141.)

exciton and the GaAs barrier exci- ton states (-200 meV), results in effective evaporation of zero dimension excitons and carriers at high temperatures (> 170 K). As To increases, the lasing energy shifts closer to the wetting layer exciton energy In addition, the threshold current density increases to higher values of 950 A.cm-* at 300 K in order to compensate for the result- ing gain loss.

Growth of vertically coupled QDs (VCQDs) using short-period GaAs(InGa)As deposition allows the realization of lasing via the ground state at 300 K 141 (Figure 5). In spite of further increasing To Cr, = 430 K in the range 70-150 K) and decreasing threshold current u th = 40 A.cm-* at 80 K), Jr,., at RT

remained high (660 A.cm-*) and To again became equal to -60 K. InGaAs QDs in these lasers were grown inside a GaAs layer confined by short-period superlattices (SPSL), similar to the approach mentioned above [ 51.

QDs lasers grown by the MOCVD method demonstrated ultrahigh temperature stability (To = 530 K over the temperature range 70-220 K). Up to 220 K the threshold current density was about 50 A.cm-* and practically temperature insensitive. Optimiza- tion of growth parameters and structure geometry allowed the range of ultrahigh temperature sta- bility of the threshold current (To = 385 K) to be extended up to 50°C (323 K).

Recently at the Ioffe Institute we have studied the influence of the number of stacks (N) of InGaAs QDs on structumI and opti- cal properties and lasing in the InGaAsGaAs VCQD structures, grown by MBE on GaAs (100) sub sttates and inserted in the active region of AlGaAsGa& SPSL QW lasers. We have found that coupled dots are formed by a self-organized shape transformation effect, involv- ing InGaAs material transfer from the lower to the upper QD and its replacement with GaAs. Lateral size and volume of the upper QDs progressively increase with N. For large N, a QD-superlattice is formed in the vertical direction (Figure 6).

Increase in N leads to a dramat- ic decrease in Ju, at 300 K as a result of the increase in the optical confinement factor (from 900 A.cm-* for N = 1, to 260 A.cm-* for N = 6 and to 9OA.cm-* for N = 10) (Figure 6c). At the same time, the emission wavelength at RT increas- es with N, approaching the PL emission wavelength at low excita- tion densities (1.05 Pm, 300 K, N = 10) and the To in the vicinity of RT increases from 60 K (N = 1) to 150 K (N = 10).

Vertical cavity lasers based on QDs electronically coupled in the vertical direction were fabricated with Jth < 200 mA and CW RT operation.The performance of the first experimental surfaceemitting lasers (SEL) is comparable to that of the best QW SEL.

The most significant aspects of the development of the QD laser structure and the principal advan- tages of QDs and QWrs are summa- rized in Table 1.

Looking ahead Recently, impressive and welldoc- umented results have been achieved for short-wavelength light sources using II-VI selenides and III-V nitrides. The success in this research was mostly deter- mined by application of het-

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Quantum Dots Feature

erostructure concepts and meth- ods of growth which had been developed for III-V QWs and superlattices.The natural and most predictable trend is the appllca- tion of the heterostructure con- cepts, as well as their tech- nological methods and peculiarl- ties, to new materials. Different III- V, II-VI and IV-VI heterostructures, developed recently, are good examples of this.

But from a general and more fun- damental viewpoint, heterostruc- cures (the classical, QWs and SLs, QWrs and QDs) provide the way to create new types of materials: the hetero-semiconductors.

(4

The classical heterostructures, QWs and superlattices are now rel- atively mature, and we exploit many of their unique properties. QWr and QD structures are still in their infancy: exciting discoveries and new unexpected applications await us. Already, however, we can say that ordered equilibrium arrays of QDs may be used in many devices, such as lasers, light modula- tors, far-infrared (IR) detectors and emitters. Resonant turmelling via semiconductor atoms introduced in larger bandgap barrier layers may lead to signiticant improvement in device characteristics. More gener- ally, QD structures will be devel- oped both in ‘width’ and in ‘depth’. In ‘width’ means new material sys- tems to cover further regions of the energy spectrum.The most mature system, InGaAs/GaAs, has already found applications for the dramatic improvement in semiconductor Table 1. lmportent aspects of heterostructure quantum wires and dots

v InAs QDs (77K) V InAs QDs (3OOK) A InGaAs QDs (77K)

V A InGaAs QDs (3OOK)

I00 I I 1 2 I 3 1 4 I 5 I 6 I 7 I 8 I 9 I IO I 1 I Number of QD sheets

Figure 6. TEM plan-view (a) and cross-section (b) of VCQD structure with six sheets of QD. (c) Threshold currant density versus number of QD sheets in the active region of QD lasers.

laser characteristics it shows. QDs potentially attractive for mid- Recently, GaSb/GaAs type II QDs, IR laser applications. The lifetime which also have a welldefined rec- (degradation) problems of the tangular base, have been for-med on green and blue semiconductor a Gak (100) surface.The same con- lasers, and the more general prob cept of dot formation also works in lems of the creation of defect-free the InSb/GaSb system, making the structures based on wide-gap II-VI

r of the threshuld current - d&erete level

t in the electro-

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Quantum Dots Feature

and III-V (nitrides), could potentiai- ly be solved by using QD structures in these systems.

As to developments in ‘depth’: the degree of ordering depends on very complicated growth condi- tions, materials constants and spe- cific values of the surface free energy. The way to resonant tun- nelling and single electron devices is an indepth, detailed investiga- tion and evaluation of these para-

meters in order to achieve the maximal possible degree of order- ing. In geneml, it is necessary to discover stronger selforganization mechanisms for the creation of ordered arrays of QDs. Coupled arrays of se&organized QWrs and QDs are attractive for lateral Esaki- Tsu superlattices. Stacked dots - onedimensional superlattices - are still relative newcomers requiring considerable investigations.

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As has been discussed in this series of three articles, the physics and technology of semiconductor heterostructures have had an enor- mous impact not only on con- densed matter physics but also in our everyday lives. And this is sure to continue into the future.

Many scientists, past and pre- sent, have contributed to the progress in this area. The Ioffe Institute is proud to have been involved from the start and looks forward to further advances in this exciting field.

References [l] N.N.Ledentsov et al, ‘Lumines- cence and structural properties of (In,Ga)As/GaAs quantum dots’, Proceedings of 22nd International Confewnce on the Hysics of Senai-con- ductors. Vancouver Cam& 1334. (D.J. Lockwood, ed.) (World Scientific, Singapore (1995) 1985. r21 VA.Shchukin et a& ‘Spontaneous ordering of arrays of coherent strained islands’, %ys. Rev. Leti. 75 (1995) 296s. [3] VA.Shchukin et al., ‘Strain- induced formation and tuning of ordered nanostructures on crystal surfaces’, SU$ Sci. 352 (1996) 117- 122. [4] Zh.I.Alferov et aZ.,‘Injection het- erolaser based on vertically cou- pled quantum dots in GaAs matrix’, Fin Tech. Pohpr. 30 (1996) 351 (Semiconductors 30 (1996) 194). [5] Zh.I.Alferov et al., ‘Reducing the threshold current in GaAs-AlGaAs DHS SCH quantum well lasers a,=52 Acm-*; T=300 K) with quantum well restriction by short period superlattice of variable peri- od’, l%ma Zb. Zbn. Fiz. 14 (1988) 1803 (Sov. Pbys.: Tech. Pbys. Lett. 14 (1988) 782).

Gntactz 2b.l. Aljhw AX IO& PhysicoTechnical Institute, Russian Academy of Sciences, 26 Politechnicheskaya St., St. Petersburg, 19402 1, Russia. E-mail: alferov@dphsh.ioffe.issi.ru

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