Semiconductor nanowires for novel one dimensional devices

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Physica E 21 (2004) 560–567

www.elsevier.com/locate/physe

Semiconductor nanowires for novel one-dimensional devices

L Samuelsona;∗, M.T Bj*orka, K Depperta, M Larssonb, B.J Ohlssonc, N Paneva,

A.I Perssona, N Sk*olda, C Thelandera, L.R Wallenbergb

a Solid State Physics, The Nanometer Structure Consortium, Lund University, Box 118, Lund S-221 00, Sweden

b Materials Chemistry/the Nanometer Consortium, Lund University, Box 124, Lund S-221 00, Sweden

c QuMat Technologies AB, Lund, Sweden

Abstract

Low-dimensional semiconductors o8er interesting physical phenomena but also the possibility to realize novel types of devices based on, for instance, 1D structures By using traditional top-down fabrication methods the performance of devices is often limited by the quality of the processed device structures In many cases damage makes ultra-small devices unusable In this work we present a recently developedmethodfor bottom-up fabrication of epitaxially nucleatedsemiconductor nanowires basedon metallic nanoparticle-inducedformation of self-assemblednanowires Further development of the vapor–liquid– solid growth method have made it possible to control not only the dimension and position of nanowires but also to control heterostructures formedinside the nanowires Basedon these techniques we have realizeda series of transport devices such

as resonant tunneling andsingle-electron transistors but also optically active single quantum dots positionedinside nanowires displaying sharp emission characteristics due to excitons

? 2003 Elsevier B.V All rights reserved

PACS: 81.07.Vb; 73.40.Kp; 78.67.Hc; 73.40.Gk; 73.23.Hk

Keywords: Nanowire; Heterostructure; Quantum dot; Resonant tunneling; Coulomb blockade

1 Top-down vs bottom-up fabrication methods

Quantum device structures are traditionally created

via lithographic techniques, i.e by advanced

pattern-ing accompaniedby some type of structurpattern-ing, like

etching This approach has a number of problems,

such as damage induced by the processing, often

resulting in deadlayers andlimitedperformance of

the resulting devices An example is quantum wells

(QWs) formedin a planar growth mode, later followed

by patterning of the surface by wire or dot features,

∗Corresponding author Tel.: +46-46-222-7679; fax:

+46-46-222-3637.

E-mail address: lars.samuelson@ftf.lth.se (L Samuelson).

features which are then transferredto the QW via an anisotropic etch Another well-known example is the corresponding fabrication of 1D resonant tunneling devices, again via the initial epitaxial formation of a double-barrier structure surrounding a QW, later fol-lowedby etching a narrow mesa structure containing the 1D–0D–1D tunneling device [1] These top-down fabrication techniques have hadonly limitedsuccess due to process induced damage

Di8erent approaches to self-organization of quan-tum structures via bottom-up methods have been developed allowing ultra-small dimensions without the use of the most extreme lithography methods andoften with almost perfect anddefect-free device properties The formation of quantum dots (QDs) by 1386-9477/$ - see front matter ? 2003 Elsevier B.V All rights reserved.

doi:10.1016/j.physe.2003.11.072

strain-induced nucleation of ultra-small islands via

the Stranski–Krastanow growth mode [2] has ledto

high-quality optical QD structures Out of the many

approaches aiming at nanowire formation we

men-tion the use of preformedstructures where selective

growth on certain crystalline facets leads to

forma-tion of sharp V-grooves [3] The intersection of such

grooves o8ers a nucleation site for nanowires This

andmost other methods are however not able to form

heterostructures inside the nanowires

2 Vapor–liquid–solid growth mode of nanowires

Already during the 1960s Wagner and others [4]

studiedthe formation of micrometer-sizedwhiskers

that couldbe formedfrom a catalytically active metal

particle positionedat a crystalline surface Strong

ef-fort was put on the fabrication of silicon whiskers

formedvia the interface between a goldparticle anda

silicon substrate The mechanism of this vapor–liquid–

solid (VLS) growth mode is traditionally described

as a formation of a eutectic alloy between Au andSi

anda controlledestablishment of a supersaturation of

the melt, which induces a transformation from the

liq-uidalloy phase to a phase where the melt co-exists

with the solidSi This process can be comparedwith

that of liquid-phase epitaxy used for growth of e.g

GaP from a Ga metal that is kept supersaturatedby

the addition of GaP powder The supersaturation and

the continuedgrowth of the epitaxial crystal is

main-tained via a gradual decrease of the temperature of the

melt

In the beginning of the 1990s Hiruma and

co-workers [5] grew semiconductor whiskers at

nanometer dimensions and studied them for their

pos-sible use in electronics andphotonics The nanowires

(primarily GaAs andInAs) were nucleatedfrom

goldnanoparticles createdby heating an evaporated,

sub-monolayer thick, goldKlm Among the

achieve-ments can be mentionedthe successful formation

of pn-junctions [6] andthe demonstration of

injec-tion luminescence from such nanowire light-emitting

diodes [7] Starting in the late 1990s, the group of

Lieber at Harvardhas demonstratedthe potential for

growth of doped nanowires for electronics, photonics

andbiosensor applications [8 10]

3 Formation of heterostructures in nanowires Around1996, Hiruma et al., reportedthe possibility

to switch the composition of III–V nanowires from InAs to GaAs [11], and also made some rudimentary analysis of the composition variations The next step

in this Keldcame 2 years ago when high-resolution electron microscopy imaging revealedthe composi-tion andstructural quality of the InAs/GaAs interface [12] Furthermore, we also reportedthe mapping of the lattice constants of the two binary materials These studies proved the interfaces to be abrupt on an atomic scale andfree of defects Later also the InAs/InP sys-tem was shown to possess highly abrupt andperfect interfaces [13–15] In Fig 1, the color-coded lattice map of an InAs/InP superlattice wire is shown Here

Fig 1 Left-handKgure shows a color-codedrepresentation of the origin of di8raction spots from the two lattices of InAs (green) and InP (red) The right-hand Kgure shows a high-resolution electron microscope image of the same structure from which one can deduce that the multiple layer structure of alternating InAs and InP segments are perfect from a crystalline point of view, are free

of strain within less than 10 nm from the interface, andhave an interface abruptness on the atomic level.

the thinnest InP layer (red) is merely 1:5 nm thick.

The formation of such thin layers is due to the very

slow growth rates andlow pressures of the chemical

beam epitaxy approach to nanowire growth This

sep-arate study also included the demonstration of how the

nanowire geometry is able to absorb the lattice

mis-match andhow the system relaxes within about 10 nm

from the heterointerface, which is a result of generic

value since it opens the possibility to combine di8erent

materials without having to maintain lattice matching

as in regular epitaxy At the same time reports came

on composition modulation in the SiGe system [16]

along a nanowire andtransitions within a nanowire

be-tween GaAs andGaP [17], including the observation

of luminescence from segments of GaAs surrounded

by GaP material, however, without any detailed

analysis of the abruptness of the transition regions

4 Properties of single barriers in nanowires

From knowledge of the bandstructure andband

alignment in bulk III–V materials one couldpredict

that the conduction bando8set between InAs andInP

shouldbe at least 0:5 eV, a number which is not known

since it is not possible to form interfaces between

the two materials on a macroscopic scale In order to

Fig 2 (a) Banddiagram of a single barrier nanowire (b) A comparison between the I–V characteristics of a homogeneous n-type InAs nanowire andone with an 80 nm thick InP barrier inserted Thermal activation of the current, (c), gives an activation energy of about 0:6 eV for the conduction band o8set.

investigate the bandalignment andthe electronic prop-erties of the InAs/InP interfaces we studied the trans-port through InAs nanowires containing thick InP re-gions (80–100 nm), which function as barriers and prevent transport From analysis of the thermionic emission of electrons over the InP barriers (Fig.2), we deduced a conduction band o8set of about 600 meV [14], a number indicating that this material system couldbe highly interesting for heterostructure elec-tronics andphotonics By varying the thickness of the InP barrier the e8ective resistance can be variedfrom the few kL level for a barrier of zero thickness, via tunnel controlledresistance in the range of 100s of kL

up to ML andfor very thick barriers up to the GL to

TL level A couple of examples are shown in Fig.3

5 1D resonant tunneling devices One of the most demanding devices to fabricate and one that is very strongly basedon quantum phenom-ena, is the resonant tunneling diode These heterostruc-ture devices are, in the conventional form, made by surrounding a thin QW by two tunnel barriers, and on either side of these barriers a low-band gap material acting as emitter andcollector The functionality is basedon the resonant alignment of occupiedstates

Fig 3 Illustration of the dynamical range that can be covered by

inserting InP segments of di8erent thicknesses (80, 7, and 0 nm

blue, red, and green, respectively) into an InAs nanowire.

in the emitter relative to the quantizedstates in the

QW Hence, transmission through the device is

pos-sible only for certain ranges of appliedbias, giving

rise to sharp peaks in the current–voltage

characteris-tics andto regions of appliedvoltages having negative

di8erential conductance This phenomenon is also of

great value for the use of resonant tunneling devices

in oscillator circuits [18]

One of the holy grails of low-dimensional devices

has been to try to realize resonant tunneling devices

for lower dimensions, i.e with the emitter and

collec-tor being 1D nanowires andthe central region a QD,

so that the Kltering element is fully discrete One of the

Krst serious attempts to realize this, was the 1D–0D–

1D resonant tunneling device made by researchers at

Texas Instruments during the late 1980s Fig.4shows

illustrations taken from their publications andshows

their top-down method for fabricating such devices

[1,19] First, the GaAs/AlGaAs multi-layer structures

are grown, with the GaAs QW surrounded by two

Al-GaAs tunnel-barriers, in turn surrounded by Al-GaAs as

3D emitter andcollector regions Then electron beam

lithography anddry etching is usedto form

freestand-ing rods (wires) of di8erent diameters In the bottom

part of Fig 4 the resulting I–V characteristic of the

thinnest nanowires that were foundto be electrically

active after the processing (200 nm), can be seen It

is quite clear that top-down fabrication methods are

not quite able to produce the device dimensions for

which quantization e8ects are very signiKcant

Simi-lar conclusions can be made from other more recent

attempts to fabricate RTDs via QDs [20]

Fig 4 Illustrations taken from publications from Texas Instru-ments from the late 1980s, where top-down methods were used

to fabricate ultra-narrow mesas containing double-barrier resonant tunneling structures The smallest dimensions for which the pro-cessed devices were still operating were just on the boarder line of dimensions for which any quantization e8ects could be resolved.

Obviously, there is a needfor bottom-up and low-damage methods to produce such advanced heterostructure devices In Fig 5 is shown a low-resolution transmission electron microscopy (TEM) image of such double-barrier structures made

by chemical beam epitaxy (CBE) [21] We have cho-sen to work with InAs-basednanowires, meaning that the emitter andcollector regions are made of InAs,

as is the central QD, while the barriers are made of

Fig 5 Low-resolution TEM image of nanowires transferredfrom

the wafer and deposited on a TEM-grid The dimensions of the

wires are 40–50 nm in diameter, with the InP tunnel barriers about

5 nm thick andthe height of the central QD about 15 nm.

InP (compare Figs.1 3) We have primarily worked

with wire diameters between 40–60 nm and tunnel

barrier thicknesses of about 5 nm andthe width of

the central QD of about 15 nm

These types of nanowires are mechanically broken

o8 from the wafer where they were grown

andtrans-ferredto a SiO2-terminatedsilicon wafer Ohmic

con-tacts are formedto the nanowire via electron-beam

lithography, metal evaporation andlift-o8 techniques,

in a similar way as how the single-barrier devices were

made In order to verify that the barriers are actually

in the proper positions relative to the contacts usedin

the measurements, a selective etch was usedthat

re-moves InAs andleaves InP virtually intact This leaves

a topographic image of the double barrier, which will

reveal if the contacts have been improperly placed

relative to the tunneling structure

The current–voltage characteristics of the fabricated

devices showed an e8ective blocking of the current

for biases up to more than 50 mV This blocking is

followed, in a symmetric manner, by a sharp peak

(in Fig 6(c)) at about 80 mV with a full-width at

half-maximum of about 5 mV, which can be converted

into an energy sharpness of the transition of about

2 meV The inset shows that these devices are robust

to charging andhysteresis e8ects, since the voltage

traces for upwards and downwards sweeps are

virtu-ally identical At higher biases features are seen which

can be interpretedas tunneling into excitedstates

In some samples, this characteristic feature in the

I–V curves is replacedby a doublet feature which,

Fig 6 (a) Shows a TEM image of the DBRT-structure, linedup with the energy banddiagram for the emitter-barrier-dot-barrier-collector structure (b) The energy band structure in the emitter andcollector regions are those of a series of 1D density-of-states curves, for each of the laterally quantized states while the energy structure of the central QD is fully quantizedas expectedfor a 0D QD structure (c) shows the current–voltage characteristics of the devices, with a sharp peak for biases for which the bandof occupiedstates are linedup with the lowest

QD state This peak is followedby a dip until the Krst excited states is linedup with the emitter states.

although not proven, may indicate a situation with the

position of the Fermi level such that more than one

injecting emitter state is being populated No charging

e8ects are seen in these devices probably due to the

fact that tunneling out of the QD is so fast that no

electron accumulation in the QD takes place under

these experimental conditions

6 1D single-electron transistor devices

With the technologies described above for

forma-tion of single tunnel barriers anddouble barrier

struc-tures, the methods are available for the building of

ideal single-electron transistor (SET) devices, i.e

de-vices in which predeKned tunnel barriers surround

a central islandwith a capacitance suMciently small

so that the energy requiredto addanother electron

to the dot is large compared to the thermal energy,

kBT Such cases have been extensively studied

us-ing for instance Al=Al2O3technology [22] or with the

use of nanoparticles with controlledtunnel barriers

[23] More recently, carbon nanotubes (CNTs) have

been employedto make SET-type of devices, either

by utilizing defect-related tunnel barriers inside the

CNTs [24] or by using conducting CNTs as leads to

a nanoparticle acting as the central island[25] In one

case, an InP nanowire was usedas the SET island[26],

connectedto the source anddrain via high-resistance

contacts, in a way similar to how CNTs have been

usedfor SET devices However, it is clear that the

ten-ability of the barriers of InP inside an InAs nanowire

is very promising to builddesignedSETs having

pre-dictable properties

In Fig.7, a qualitative comparison between a DBRT

anda DB-SET device is shown, comparing cases with

the same materials anddimensions but with the change

in the length of the central islandfrom a size (for the

DBRT device) where quantization e8ects dominate

the energies of the island, to a large size of the island

for which the quantization is only very small but the

charging energies are large (for the DB-SET device)

In the SET-device, the charging energy, which can be

written as EC=e2=C, is dominated by the capacitances

of the barriers and is only weakly dependent on the

length of the island Hence, for a suMciently large

islandthe charging energy EC will totally dominate

over the quantization energies

Fig 7 Principles of how the resonant tunneling device can be convertedinto a single-electron transistor using identical technol-ogy but with the size of the islandextendedfrom about 15 nm up

to about 100 nm, such that the quantization energies are reduced

to almost zero while the charging energies are still large.

Fig 8 Principle of the blockade and single-electron transfer through the islandin a single-electron transistor as function of source–drain and gate biases.

The principles of how the transport of electrons through the SET occurs as function of the applied source–drain bias and the gate voltage is sketched

in Fig.8, from which is seen the expectedCoulomb blockade situation for small values of the applied source–drain bias This Kgure also illustrates how a gate applied to the system can push the ladder of states in the islandfor N; N + 1, etc electrons down (for positive gate voltages) such that more electrons can be added to the island or current be transported through these levels, one by one Experimental data [27] for the designed nanowire SET devices are pre-sentedin Fig.9, which shows the current vs source– drain voltage characteristics for two settings of the gate potential, one chosen such that the blockade is maximum andone (nearest) minimum, in which the blockade is perfectly lifted This behavior, in itself,

Fig 9 (a) Current vs source–drain voltage for the SET shown

in Fig 7 , displaying ideal Coulomb blockade as well as complete

lifting of the blockade controlled by the gate potential (b) Periodic

gate oscillations.

is indicative of that an ideal single island has been

formedin the nanowire between two tunnel barriers

In Fig.9(b) it is seen how the single-electron

tunnel-ing is perfectly periodic in gate voltage, a phenomenon

that can be followedandcontrolledfor more than 50

electrons being added to or removed from the central

island

7 Optically active QDs inside nanowires

One of the great challenges in the use of

nanos-tructures is the possibility to use a single QD as an

ideal photonic quantum emitter If a single QD can be

addressed in such a fashion that it can be controllably

excitedby a single exciton, it may be employedas a

single-photon-on-demand source [28], which would

be highly interesting for quantum optics in general

andfor quantum cryptography in particular High

op-tical quality andwell deKnedandspectroscopically

Fig 10 Photoluminescence spectra from a single GaInAs QD positionedinside a GaAs nanowire shown as function of the excitation intensity For the lowest excitation intensities, a single sharp line is seen, believedto be due to the single exciton For higher excitation intensities new excitonic features appear.

sharp exciton emission is a necessary pre-requisite for these applications Furthermore, the controlled injection of one electron–hole pair into the single QD must be achieved, preferably via a combination of resonant tunneling via tunnel barriers surrounding the active QD andCoulomb blockade preventing more than one carrier to be injected So far, strain-induced QDs, or etchedout pillars, have been proposedfor these applications We have recently foundthat

it is possible to grow single QDs in nanowires with high luminescence quality andwith sharp lu-minescence lines [29] An example is shown in Fig.10, with luminescence emission spectra for vary-ing excitation intensities The appearance of the svary-ingle exciton emission line, with line width of about 100 –200 eV andthe appearance of new exciton emis-sion lines for higher excitation intensities (probably due to the recombination of the bi-exciton) is clear Together with our previously publishedresonant neling results from similar QDs, surrounded by tun-nel barriers, hope is given that an electrically driven single-photon-on-demandsystem may be o8eredby self-assembledsemiconductor nanowires containing single QDs

[1] J.N Randall, M.A Reed, T.M Moore, R.J Matyi, J.W Lee,

J Vac Sci Technol B (1988) 302.

[2] W.Seifert, N Carlsson, M Miller, M.-E Pistol, L Samuelson,

L.R Wallenberg, Prog Cryst Growth Charact 33 (1996) 423.

[3] R Bhat, E Kapon, D.M Hwang, M.A Koza, C.P Yun,

J Cryst Growth 93 (1988) 850.

[4] R.S Wagner, in: A.P Levitt (Ed.), Whisker Technology,

Wiley, New York, 1970, pp 47–119.

[5] K Hiruma, et al., J Appl Phys 77 (1995) 447.

[6] K Haraguchi, T Katsuyama, K Hiruma, K Ogawa, Appl.

Phys Lett 60 (1992) 745.

[7] K Haraguchi, T Katsuyama, K Hiruma, J Appl Phys 75

(1994) 4220.

[8] Y Cui, C.M Lieber, Science 291 (2001) 851.

[9] J Wang, M.S Gudiksen, X Duan, Y Cui, C.M Lieber,

Science 293 (2001) 1455.

[10] Y Cui, Q Wei, H Park, C.M Lieber, Science 293 (2001)

1289.

[11] K Hiruma, H Murakoshi, M Yazawa, T Katsuyama,

J Cryst Growth 163 (1996) 226.

[12] Presentedat MSS10, Linz, Austria, July 2001;

B.J Ohlsson, et al., Physica E 13 (2002) 1126.

[13] L Samuelson, invitedtalk “1D stacking of strainedquantum dots via selforganisation and during whisker growth”, MRS Boston, 2001.

[14] M.T Bj*ork, et al., Nano Lett 2 (2002) 87.

[15] M.T Bj*ork, et al., Appl Phys Lett 80 (2002) 1058 [16] Y Wu, R Fan, P Yang, Nano Lett 2 (2002) 83 [17] M Gudiksen, L.J Lauhon, J Wang, D Smith, C.M Lieber, Nature 415 (2002) 617.

[18] S Luryi, Appl Phys Lett 47 (1985) 490.

[19] M.A Reed, et al., Phys Rev Lett 60 (1988) 535 [20] J Wang, et al., Appl Phys Lett 65 (1994) 1124 [21] M.T Bj*ork, et al., Appl Phys Lett 81 (2002) 4458 [22] T.A Fulton, G.J Dolan, Phys Rev Lett 59 (1987) 109.

[23] S.-B Carlsson, T Junno, L Montelius, L Samuelson, Appl Phys Lett 75 (1999) 1461.

[24] H.W.Ch Postma, T Teepen, Z Yao, M Gri8oni, C Dekker, Science 292 (2001) 76.

[25] C Thelander, et al., Appl Phys Lett 79 (2001) 2106 [26] S De Franceschi, et al., Appl Phys Lett 83 (2003) 344 [27] C Thelander, et al., Appl Phys Lett 83 (2003) 2052 [28] A Imamoglu, Y Yamamoto, Phys Rev Lett 72 (1994) 210.

[29] N Panev, et al., Appl Phys Lett 83 (2003) 2238.