quantum dots laser

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Arraysof quantum dots promise improved

performancefor semiconductor lasersandhavethe

added advantage that theycanassem ble themselves

Quantum-dot lasers

KARLEBERL

bulk semiconductor material

SEMICONDUCTOR lasers

are key

components

in

many

of

the appliances that wetakefor granted in everyday life.

Fibre-optic communications and compact-disc players

are

perhaps the two best known examples. There is alsoa

world-wide research

and

development effort

to

improve

the performance of these devices by making them smaller,

brighter, more efficient

or

capable

of

lasing

at

new wave-

lengths. A new class

of

semicond uctor laser

- a

quantum-

dot laser that self assembles

-

is showing great promise

in

many

of

these areas.

Semicon ductor lasers emit light when

an

electron

in the

conduction band recombines with an empty electron

state

or

hole

in

the valence b and. The wavelength

of

the

lightisusually determined by thebandgap ofthe semi-

conductor

- the

energy difference between

the top of

the

valence band

and the

bottom

of the

conduction band.

Gallium indium phosphide,

for

example,

has a

band

gap

of about

1.9

eV, which leads

to the

emission

of

red light.

The band gap, whichisresponsibleformostofthe use-

ful properties

of

sem iconductors,

is due to the

quantum

nature of the electron waves as they travel throughthe

crystal. Quantum mechanics

forbids the electrons from

having energies inside

the

band gap.

In

a

bulk semiconductor,

such as silicon or gallium

arsenide,

an

electron

in the

conduction band behaves like

a free electron and can occupy

states with continuous values

of momentum and energy.

This picture changes dramat-

ically, however, when the

motion

of the

electron

is

restricted in one or more

dimensions (figure la

and

b .

Iftheelectrons are confinedin

a small enough region, their

energy levels will be quantized,

similar

to the

energy levels

in

an atom.

One way to

make

such

an

artificial atom

or

quantum dot

is to

surround

a small regionofsemiconduc-

tor with another semiconduc-

tor thathas alarger bandgap

(figure lc).

The

electrons

are

effectively confined

in the

region with

the

smaller band

gap as they seek to reduce

their energy.

Ina simplified modeltheconfinement potentialcan be

considered

as a

square well (figure Id .

In

this model

the

separationof the discrete energy levels depends inversely

on the size ofthequantumdot.Th e dot must be 10

nm or

smaller

for the

energy levels

of

the quan tum well

to be

separated

by

more than

the

thermal energy

at

room

tem-

perature. Such quantum dots contain only a few thousand

atoms

and

should

be

able

to

emit light

at

wavelengths

determine d by the energy levels

in

die do t, rather tha n

the

band-gap energy.

However, various processes conspiretostopasemicon-

ductor emitting light. Impurity atoms

can, for

example,

trap the charge carriers and prevent recombination.

Electrons

and

holes

can

also emit phonons

(i.e.

lattice

vibrations) rather than photon s when diey recombine. To

overcome these problems, the surface

of

the qu antum

dot

mustbe passivated :inother words,thesmall-band-gap

material must

be

completely embedded

in the

barrier

material without any crystal defects

and

impu rities.

Quantum dots are widely used for experiments that

semiconductor quantum dot

10

nm

band gap

light

emission

1 a) The periodic crystal potential in a piece of bulk

semiconductor material allows almost free motion for

electrons, which results in energetic bands separated by a

forbidden band gap. Within the conduction band, electrons

have continuous kinetic energy and momentum, o) The

situation is totally different for an electron confined in a very

small ~ 10 nm in diameter) semiconductor cluster and the energy spectrum becomes discrete like that for atoms.

c) A schematic illustration of a passivated quantum dot with the charge carriers purple) confined in the small-

band-gap material red), which is completely embedded in a large-band-gap material blue) without any crystal

defects and impurities present. The charge carriers are confined in all directions. The location of the electron is

indicated schematically and is similar to the orbitals in atoms, d) A model square-well confinement potential

showing the quantized energy states red lines).


3/7

4 8 Physics World September 1997

elastic strain relaxation

wetting layer

2(a) Cluster formation during epitaxial growth of a semiconductor

material (red) on top of another se miconductor with a smaller (by a few

per cent) lattice constant (blue). For example, indium phosphide has a

4%

larger lattice constant than gallium indium phosphide, so InP forms

clusters as shown in figure 3. The cluster formation is energetically

favourable, because the lattice can elastically relax compressive strain

and thus reduce strain energy, (>) In an alternative growth mode the

epilayer is laterally com pressed to match the substrate lattice. This so

called "pseudomorphic growth" is observed for heterostructures that

have only a small lattice mismatch.

probe the fundamental quantum behaviour of semicon-

ductors. They also have many potential device appli-

cations. Th e most promising of these are single-electron

transistors (see Harmans in further reading) and quan-

tum-dot lasers. The latter offer the possibility of improved

device performance and increased flexibility to adjust the

laser wavelength.

Early days

Semiconductor lasers have developed rapidly since their

invention in

1962.

Th e first semicondu ctor laser consisted

ofagallium arsenide p -n junction that was forward biased

such that electron-hole recombination took place in the

depletion region between the positively doped (p) and

negatively doped (n) regions. Polished surfaces (called

facets) perpendicular to the junction were used to form

the reso nant cavity needed for laser action. The se facets

fill the role played by mirrors in conventional lasers.

The performance of the early semiconductor lasers was

greatly improved by sandwiching the gallium arsenide

(GaAs) active layer between aluminium gallium arsenide

(AlGaAs) layers in a hetero structure device. This

improved laser performance for two reasons. First, the

AlGaAs layers have a larger

band gap than the GaAs

layers, which helps to confine

the charge carriers in the

active layer. Second, AlGaAs

has a lower refractive index

than GaAs, which confines the

light within the active layer

and thus acts as a waveguide.

In 1970 this concept lead to

the continuous room-tem-

perature operation of GaAs/

AlGaAs heterostructure lasers.

A key figure-of-merit for a

semiconductor laser is the

current density needed for lasing to start. This figure

should be as low as possible and by 1975 threshold cur-

rent densities of about 50 0Acm

2

had been achieved with

active layers abou t 0.1 |0.m thick. The practical use of

semiconductor lasers in real applications such as fibre-

optic data transmission began around this time.

In 1974 Raymond Dingle ofBellLaboratories in the US

demonstrated quantum confinement of charge carriers for

the first time. And in 1979 Won -Tien Tsan g, also of Bell

Labs, built the first semiconductor laser based on quan-

tum confinement. The active region in Tsang's laser was

only about 0.01 um (10 nm) thick, so the charge carriers

(i.e. electrons and holes) were only free to move in just

two dimensions. The se qua ntum-w ell lasers had three

advantages over previous devices: considerably higher

light amplification or optical gain ; m uch lower thresho ld

current densities (less than 50A cm

2

) and significantly

improved tem perature stability. The se advantages are all

linked to the redu ced energy spread of occupied states for

electrons and holes in quantum-well devices compared

with bulk semiconductors.

The next step - from quantum-well lasers to quantum-

wire lasers - came in the late 1980s. In quantum-wire

lasers the charge carriers are free to move in only one

direction, which further increases optical gain, reduces

threshold current densities (to less than 10A

cm

2

),

and

improves temperature stability.

In 1986 Masahiro Asada of the Tokyo Institute of

Technology and co-workers, building on earlier work by

Yasuhito Arakawa from Tokyo University, predicted that

quantum-dot lasers - lasers in which the active region

contains a large number of equally sized quantum dots -

would have even better characteristics. A quantum dot

confines the motion of the electrons in all three of the

spatial directions.

The discrete energy spectrum of the quantum dots

prom ises highly efficient lasing at a distinct energy. Two

key requirements in any laser are a population inversion

(i.e. there should be more electrons in the excited state

than in the ground state) and that the optical gain is

greater than the losses. Lasers also rely on stimulated

emission: in this process a photon of a given frequency

causes another photon of the same frequency to be emit-

ted. In a laser diode, only those electrons in the conduc-

tion-band minimum that recombine with holes in the

valence-band maximum contribute to stimulated emis-

sion. Since the electronic states in a quantum dot are

squeezed into discrete transition energies, fewer charge

carriers will be needed to create a population inversion,

which should lead to lower threshold currents than found

in existing laser diodes.

Temperature stability should also be improved in quan-

tum-dot lasers. In a conventional semiconductor laser the

3 A cross-sectional transmission electron m icrograph of an InP cluster on GalnP. The cluster was formed

during epitaxial growth of nominally three atomic layers of InP. Since the islands are extremely small and

appear spontaneously during growth , they are called self-assembling quantum dots.


4/7

PhytkaWo rid Septembar 1997 4 9

GaAs substrate

laser output

GalnP waveguide

^^

with one layer of InP

4 An optically pumped InP quantum-dot laser. The electron-hole pairs are excited by a ^ - i . * ^ ^ quantum d ots

focused stripe of green light incident from the left. The stimulated red -light emission (laser

output) originating from the dots is observed perpendicular to the excitation light. The experiment was performed at low temperature (8 K) under

continuous excitation , with the sample mounted in a cryostat. The layer of InP dots is in the middle of a GalnP film, wh ich is surrounded by AllnP

layers that have a smaller refractive index and thus keep the light close to the active layer.

spread of energy states usually causes the threshold cur-

rent to chang e significantly w ith tem perature. F or a q uan-

tum-dot laser, however, the discrete energy spectrum and

large separation between the energy levels almost elim-

inates any temperature d ependence.

Making quantum dots

Th e exp erimental challenge is to make 10 nm semicon-

ductor structures in an efficient and reproducible way. A

large number of dots are needed and they must be equal

in size and optically active. However, conventional semi-

conductor-processing techniques that are based on litho-

graphy and etching face inherent problems such as

limited resolution and the introduction of surface defects

during production.

Over the past few years several research groups have

started working on the direct synthesis of various semi-

conductor nanostrucrures, including quantum dots, by

combining epitaxial growth tech-

niques, such as molecular beam

epitaxy or chemical vapour de-

position, with lithography. Con-

ventional lithographic techniques

are used to create a pattern on

the surface before the quantum-

dot material is deposited. Epi-

taxial growth on patterned

substrates leads to the formation

of sharp edges or V-grooves on

the surface. Quantum dots and

wires can then be formed in a

self-adjusting way.

A much simpler idea is based

on the islands that form when a

semiconductor is deposited on a

substrate that is made from a

material with a significantly

smaller lattice constant. This

process is known as the Stranski-

Krastanov growth mode, and

quantum dots that have dimen-

sions of only a few atomic dis-

tances across have been grown

this way (figure 2a . An import-

ant point is that the size of the

S S pectrum of the laser output (a) and photoluminescence

spectrum (b) from the InP quantum-dot laser shown In figure

4. The laser-output intensity19about a factor of 200 times

more intensive than the lower spectrum.

quantum structure is controlled within the epitaxial de-

position in a self-adjusting process. For the first few

atomic layers, the atoms arrange themselves in a planar

layer called the wetting layer. As epitaxial growth pro-

ceeds, the atoms tend to bunch up and form clusters.

Cluster formation is energetically favourable, because the

lattice can elastically relax the compressive strain and thus

reduce the strain energy within the islands.

In an alternative growth mode known as pseu dom or-

phic grow th , the epilayer is laterally compressed to ma tch

the substrate lattice (figure

2b .

The lateral strain is auto-

matically introduced as the growth proceeds.

To see how these techniques work, consider the growth

of indium phosphide (InP) on gallium indium phosphide

(GalnP). GagjInojP has the same lattice constant as

gallium arsenide, so it is possible to grow good quality

G aln P layers on gallium arsenide sub strates. Th e lattice

constant of InP is about 4% larger. However, with opti-

mized molecular-beam-epitaxy growth conditions, defect-

free clusters or dots form after

1.5 or more monolayers of InP

have been deposited. For exam-

ple,

after three nominal layers

have been deposited, clusters can

be seen on top of the wetting

layer. There are about 5 x l0

1 0

dots per square centimetre and

they nucleate preferentially at the

atomic step s that are always

present on a surface. The dots

have a truncated pyramid shape,

and are about 15 nm in diameter

and 3 nm high (figure 3). Sur-

prisingly, the dot size varies by

only about 10%, which is essen-

tial for making a quantum-dot

laser. After the InP dots have

been grown, a thick layer of

GalnP is deposited to passivate

the dots.

The dot size strongly depends

on the amount of deposited dot

material. And since the energy

levels in the dot depend on its

size, so does the wavelength of

light emitted by the dot. Bulk


5/7

5 0 Ptiyttct Worid Septwntw S97

indium phosphide has a band gap

of 1.35 eV, which corresponds to a

wavelength of

1

[im.The growth of

two monolayers of InP on GalnP

produces dots with diameters of

abou t 12 nm , whereas for seven

monolayers the diameter is about

50 nm. The energy shift for 12 nm

dots is 1.85 eV, which corresponds

to red-light emission. The larger

50 nm dots have lower energy shifts

(1.65 eV) and emit in the infrared.

The pronounced increases in

energy are due to carrier confine-

ment and strain in the dots.

However, once more than seven

monolayers of InP have been

grown, various dislocations are

introduced and these reduce the

light output.

The size of the dots can also be

adjusted within a certain range by

changing the growth conditions.

For example, an extended interrup-

tion between the deposition of the

dot material and the growth of the capping layer leads to

larger dots with a lower dot density - smaller dots disap-

pear and larger dots grow higher during the interruption.

The same happens for very slow deposition rates. Larger

clusters are energetically favourable because there is less

strain at the top of large dots. However, short-wavelength

lasers (e.g. those working in the visible) require small dots,

which can be made by keeping the growth interruption

short. This also reduces the range of dot sizes.

The ability of self assembly to make regular arrays of

extremely small quantum dots without the need for litho-

graphy and etching makes it a highly promising technique

for the fabrication of quantum-dot lasers.

Let there be light

To make a quantum-dot laser we need a dense array of

equal-sized dots within the active region, and a mirror at

each end ofthe region to form the laser

cavity.

The energy

of the radiation depe nds on the ba nd gap of the dot m ater-

ial and the strain, and inversely on the size of the dots.

Electrons an d holes can be created in the dot by optical or

electrical excitation (figure 4).

In 1995 my group at the Max Planck Institute in

Stuttgart in collaboration with Andreas H angleiter's group

at the University of Stuttgart observed room-temperature

lasing in the visible range from an optically excited indium

phosphide q uantum -dot laser. Th e electron-hole pairs

were formed by focusing green light onto the active layer,

and red laser emission was observed emerging from the

mirrors of the laser structure (figure 5). Th e threshold

current for lasing was comparable with that obtained for

GalnP/AlGalnP quantum wells in a similar set-up.

Also in 1995 Dieter Bimberg's group at the Technical

University in Berlin, in collaboration with researchers

from the A F Ioffe Physical Technical Institute in St

Petersburg, demonstrated room-temperature operation

of an electrically pumped quantum-dot laser. This

device contained a closely packed stack of indium

arsenide (InAs) quantum-dot layers and emitted in the

infrared. Bimberg and co-workers also achieved thresh-

old current densities comparable with quantum-well

lasers (below 20

A cm

2

)

at liquid-nitrogen temperatures

6 A cross-sectional transmission electron micrograph

from a sample with three stacked layers of InP dots.

Stacking individual do t layers helps to Increase the total

amount of active material within the waveguide of a

laser structure.

and with better temperature sta-

bility. However, they have not yet

been able to match the perform-

ance of quantum-well laser diodes

at room temperature.

Stacking layers of dots in the laser

structure increases the amount of

active material in devices (figure 6).

In 1995 James Harris and co-

workers at Stanford University in

California reported a significant

reduction in photoluminescence

linewidth from stacked InAs dot

layers. This reduction is thought to

be due to the more uniform distri-

bution of dot sizes caused by the

correlated nucleation of the dots:

atoms that arrive during the growth

of the upper layer tend to accumu-

late and form new dots just above

those in the underlying layer. This

correlation - which is caused by the

strain field around the embedded

dots - is observed if the separation

between the layers is not too large.

Recently Bimberg and co-workers and Hideaki Saito's

group at NEC in Tsukuba have built laser diodes with a

stack of ten layers of dots.

Self-assembled futures

Although the research is still at an early stage, experiments

have already shown that self-assembled quantum dots do

make good lasers. Th e next challenge is to ma tch and sur-

pass the performance of quantum-well lasers. This would

give quantum-dot lasers a chance to move into appli-

cations such as ultrafast optical data transfer. A key aspect

of this challenge will be to improve our control over the

size distribution of the dots produced in the self-assembly

process. But given the rapid progress m ade so far, experts

in the field are confident that there will be significant per-

formance improvements in the near future.

Further reading

1986 Gain and the

box lasers

IEEE J.

M Asada, Y Miyamoto and Y Suematsu

threshold of three-dimensional quantum

Quantum

Electron.

QE-22 1915

D Bimberget

al.

1997 InGaAs-GaAs quantum dot lasers IEEE

J.QuantumElectron.

3 1

K Eberl, P M Petroff and P Demeester 1995 Low-dimensional

structures prepared by epitaxial growth or regrowth on pat-

terned substrates

NATO ASI

Series

E:

Applied Science VbL

29 8

(Kluwer, Dordrecht)

K Harmans 1992 Next electron, please...Physics Wbrld March

pp5O-53

A M oritzet al 1996 Optical gain and lasing in self-assembling

InP/GalnP quantum dotsA pp Phys.Lett.69 212

P M Petroff and S P DenBaars 1994 MBE and MO CVD growth

and properties of self-assembling quantum dot arrays in HI-V

semiconductor structuresSuperlatticesan dMicrostructures 15 15

H Saito et al. 1996 Room-temperature lasing operation of a

quantum-dot vertical-cavity surface-emitting laser

App. Phys.

Lea.

69 3140

Karl Eberl is in the Max-Planck-lnstitut fur Festkorperfof

schung,

Heisenbergstr. 1, 70569 Stuttgart, Germany (email:

[email protected])


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