11

B. M. Arora received his B Tech degree in Electronics & Communication Engineering

from the Indian Institute of Technology, Kharagpur in 1965 and PhD degree in

Electrical Engineering from the University of Illinois, Urbana, in 1972. His early work

was in the area of plasma display panel. Subsequently, he studied photoluminescence

of CdSe and CdS. In 1972, he joined the Tata Institute of Fundamental Research (TIFR),

Mumbai where he worked on the growth of III-V compounds by liquid phase epitaxy,

anodic oxidation and passivation of GaAs, Schottky barrier junction properties and

defects in III-V compounds. Later at TIFR Professor Arora worked extensively on the

growth of III-V strained layer heterostructures using metal-organic vapor phase

epitaxy and their application to optoelectronic devices such as lasers and quantum well infrared

photodetectors. Professor Arora is currently with the Electrical Engineering Department of Indian Institute

of Technology, Bombay. (email: bmarora@ee.iitb.ac.in)

of size, the basic premise is very different. Reduction of size in MOS structures

is based on scaling [1, 2], while quantum structures are based on behaviour of

free electron confined to dimension smaller than the DeBroglie wavelength

(typical value for electrons in GaAs is ~50 nm), which is visualized as “particle

in a box” and referred to as quantum size effect or quantum confinement [3].

The three quantum structures: quantum wells (QWs), quantum wires (QWIs),

and quantum dots (QDs) are shown schematically in Fig.1, depending on

confinement of electrons in 1, 2 or 3 dimensions respectively. The dimension in

the confinement direction is typically 10 nm or smaller .Apart from these, two

other quantum structures in common use are i) multi-quantum wells (MQWs)

and ii) superlattices (SLs) as shown in Fig.1. As seen in the figure, the quantum

structures are composed of multiple semiconductors of different band gaps

in contact. Together they comprise a family called heterostructures.

Introduction

Quantum structures have been at the

forefront of major developments in

the science and technology of

semiconductors for nearly four

decades. Together with metal-oxide-

semiconductor (MOS) structure,

they form one of the two branches of

semiconductor science which led the

way to modern nano-science and

nano-technology. Although both

MOS and quantum structures have a

common feature, namely reduction

QUANTUM STRUCTURES OF III-V COMPOUND

SEMICONDUCTORSB. M. Arora

Sandip Ghosh

Department of Electrical Engineering,

Indian Institute of Technology-Bombay, Powai, Mumbai 400076

Department of Condensed Matter Physics and Material Science

Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005

Sandip Ghosh received his B.Sc. (Hons.) degree in physics from the University of Delhi

in 1990 and the M.Sc. degree in physics from the Indian Institute of Technology, Delhi,

in 1992 and Ph.D. from the Tata Institute of Fundamental Research (TIFR), Mumbai in

1998. After holding post doctoral positions at the University of Surrey, Guildford and

the Paul Drude Institute, Berlin, he joined the faculty at TIFR in 2002. His research

interests involve the study of electronic band structure related physics of

semiconductors and optoelectroinc devices, using optical spectroscopic techniques.

(email: sangho10@tifr.res.in)

Bulletin of Indian Physics Association Vol. 41 Issue 1 January 2011

Quantum structures generally

confine electrons in the smaller band

gap semiconductor with the larger

band gap semiconductor acting as

barrier.

Historically, advances in quantum

s t r u c t u r e s h a ve h a d s t r o n g

relationship with III-V compound

semiconductors since near ideal

heterostructures can be synthesized

from them. III-V semiconductors,

with the most well known member

GaAs, have been known since the

pioneering work of Welker [4]. They

are formed by combining elements

from column III (B,Al,Ga,In) and

column V (N,P, As ,Sb) of the periodic

table. They may be binary (GaAs, InP

etc), ternary (AlGaAs, GaAsP etc), or

quaternary (AlGaInAs, InGaAsP etc)

materials [5]. Quantum structures

are formed generally from single

crystalline materials. Most of the III-

V compounds grow in cubic zinc-

blende crystal structure (Fig.2). Over

the last decade III-nitrides (AlN,

GaN and InN ) have been extensively

investigated [6]. Nitrides have

predominantly hexagonal close

packed wurzite crystal structure

(Fig.2).

An impetus for quantum structures

came after the invention of GaAs

diode laser in 1962 in order to make it

a practical device operating at low

current density. The first crucial step

was the introduction of hetero-

structure GaAs-AlGaAs which forms

the basis of many subsequent

discoveries and inventions including

quantum well lasers [7], quantum

well light modulators [8], quantum

cascade lasers [9], quantum well

infra-red detectors [10], resonant

tunnel structures [11], high electron

mobility transistors [12], and integral

12

and fractional quantum Hall effects [13]. Kroemer's early theoretical work

[14] had paved the way for subsequent developments of hetero-structures.

The next advance came from the work of i) Esaki and colleagues on electron

transport in superlattices [15], and of ii) Dingle and coworkers on optical

properties of quantum wells [16]. Apart from these composition modulated

structures, doping modulated structures nipi were also investigated [17].

Fig.1 : Semiconductor quantum structures. Electrons can tunnel across the layers in a

superlattice. In a MQW structure, they are confined to the smaller band gap. Also

shown are the density-of-states (DOS) for the electrons.

It was realized early that experiments on quantum wells and superlattices

required very high degree of crystal perfection of multiple thin crystalline

layers (known as epitaxial layers) as well as perfection of interfaces between

the epitaxial layers. All early work on heterostructures was done by growing

layers using liquid phase epitaxy (LPE) [18]. New techniques i) molecular

beam epitaxy (MBE) [19, 20] and ii) metal-organic vapour phase epitaxy

(MOVPE) [21] were invented for synthesis of very thin layers with near perfect

/ abrupt hetero-interfaces. Most of the above advances have come by using

arsenides and phosphides of Al, Ga and In. For example, communication

diode lasers use InGaAs for the light emitting QW layer and InGaAsP barrier

layers to form the waveguide, all grown on InP substrates.

More recently, great advances have been made in the use of nitrides of Al, Ga

and In . Apart from giving us sources and detectors in the UV, blue and green

regions, nitrides have provided us a vision of solid state lighting [22], very

high frequency compact electronics [23], and high temperature electronics

[24]. In this brief review, we discuss salient properties of heterostructures and

quantum structures, methods of synthesis, electronic structure and optical

properties, and conclude with a few applications.

A combination of two semiconductors of different band gaps forms a hetero-

junction. The band diagram of a hetero-junction has discontinuities at the

junction as shown in Fig.3. The discontinuity between the conduction band

Hetero-structures and Quantum structures of III-V compounds

13

edges is called the conduction band offset and corresponding discontinuity

between the valence band edges is the valence band offset. Three different

types of discontinuities are observable in different combinations of

semiconductors as shown in Fig.3 [25]. Although there are examples of all the

three types of heterostructures, in this review we dwell mainly on the type-I

heterostructures, which are the ones most widely used. Fig.4 shows band

diagram of a type I QW. As seen, transition from the small band gap to the

larger band gap is abrupt, making it a square potential QW. The synthesis of

hetero-structures has been refined such that the transition region can be

designed to create potential well of a desired shape such as parabolic or

triangular well.

E (k) = k /2m * + E (y,z) ....(2)

E = E (x,y,z) ....(3)

e x e em,n

e el,m,n

2 2

In a QD, the electron energy is

discrete in all three dimensions and

given as

In these equations, the localization

energy of the electrons is E (z) in a

QW, E (y,z) in a QWI, and E

(x,y,z) in a QD; l,m,n have integral

values. Similar relations hold for

localization energy of holes in the

valence band of quantum structures.

Localization energy is obtained by

solving the Schrodinger equation of

motion of electron/hole in the finite

potential well. [26].

Growth of high quality single crystal

multiple layers of different materials,

with different chemical compositions

and doping, different thicknesses of

layers with sub-nanometer control

and near perfect and abrupt

interfaces, imposes very stringent

requirement on the choice of

materials and method of synthesis

[27]. Molecular beam epitaxy (MBE)

and organometallic vapour phase

epitaxy (MOVPE) are widely used for

the synthesis of quantum structures.

Schematic diagrams of these

apparatus are shown in the Fig.5.

The MBE apparatus [20] is an ultra

high vacuum (UHV 10 Torr)

chamber with ovens containing

elemental Al, Ga, In, As (for growing

arsenides), Si (n type dopant) and Be (

p type dopant). Atomic/molecular

beams evaporate from the ovens kept

at elevated temperatures. The beams

reach the heated substrate where they

react and form the compounds which

grow epitaxially. The ovens are fitted

with shutters to prevent unwanted

beam from reaching the substrate.

en

em,n el,m,n

��10

�

Choice of Materials and

S y n t h e s i s o f Q u a n t u m

Structures

Fig.2 Zinc Blende and Wurzite crystal structures

Fig.3 The three types of heterostructure band lineups, Type-I, Type-II and Type-III.

In a QW, electrons are confined in one

dimension (along ), while they are

free in the other 2 dimensions ( and

). Such a system of electrons is called

2 dimensional electron gas (2DEG).

The energy of electrons in a QW of

width L is given by

E (k) = k /2m * + k /2m * + E (z)

.....(1a)

where

E (z) = n /2L m * .....

In a QWI electrons are free to move in

1 direction (say ) and confined in the

other two dimensions ( and ). For a

QWI, the electron energy is given as

z

x

y

x

y z

w

e x e y e en

en w e

2 2 2 2

22 2 2� (1b)

� �

�

Fig.4 A type-I GaAs/Al Ga As Quantum

Well (well width = L ) potential profile.

x 1-x

w

15

the area of optoelectronic devices such

as quantum dot lasers and quantum

dot infrared photodetectors [37].

materials) because of disorder in the random placement of components in the

sublattices. Ternary and quaternary materials are alloys and even though

crystalline, they have alloy disorder. For example, in Al Ga As, the number of

Al/Ga atoms will fluctuate from one cubic cell to another, even though globally

the mean composition is fixed. The alloy disorder effect is particularly

prominent in determining carrier transport properties at low temperatures in

quantum well structures. For example, electron mobility in GaAs QW can rise

to 10 cm /(V-s) or even higher at liquid He temperature [28]. On the other

hand, the corresponding electron mobility in an InGaAs QW does not exceed

10 cm /V-s because of alloy disorder in the In/Ga sublattice [30].

0.5 0.5

7 2

5 2

Fig.6 Effect of mismatch between lattice parameters of the substrate and the epi-

layer

The lattice matching requirement for near perfect growth is relaxed in the case

of very thin layers such as QWs. The difference in lattice mismatch is

accommodated by the biaxial compression (or tension) in the plane of the QW,

with concomitant extension (or shrinkage) of lattice parameter normal to the

plane of the QW [31] as shown in Fig.6. If the thickness of the epi layer exceeds

a critical limit, the layer relaxes with production of dislocations. The ability to

grow mismatched defect free layers has greatly enhanced the choice of

materials that can be used in devices. As is to be expected, built-in strain in

these layers modifies the electronic band structure in the layer. This turns out

to be highly beneficial in many cases and plays very important role in modern

devices which employ strain engineering [32]. Strained QW laser diodes have

lower lasing threshold current [33]. Strained Si-Ge layers are used to enhance

the electron mobility [34]. The role of strain in the synthesis of quantum dots is

even more extraordinary. Large lattice mismatch (~5%) between the substrate

and the layer (a > a ), reduces the critical thickness of defect free planar layer

to one or two atomic monolayers ( also called wetting layer), such that further

growth results in self assembled growth of nanometer dimension islands ,

which are essentially defect free QDs [35, 36]. It is even possible to build

multiple layers of QDs in this manner. Substantial advance has been made in

L S

Fig.7 Schematic of the electronic band

structure of bulk GaAs in terms of

e l e c t ro n e n e rg y ve rs u s c r ysta l

momentum ( ) around the band gap Eg.k

Spectroscopy of Quantum

Structures

As discussed above, the reduction in

the physical size of a semiconductor

crystal and the formation of

heterostructures, lead to significant

changes in the electronic band

structure (EBS). In order to tailor the

EBS to ones' needs for designing

better electronic devices, it is

important to be able to measure the

changes in the EBS. Optical

spectroscopy is one of the most

powerful tools for studying the EBS

and in this section we will describe a

few of the optical techniques used in

the study of semiconductor quantum

structures. First we note that the

o u t e r m o s t e l e c t r o n i c s h e l l

configuration of group III and group

V elements consist of and type

atomic orbitals (eg. in GaAs, Ga:4s ,

4p and As: 4s , 4p ). When the atoms

come close together to form a

compound III-V semiconductor

crystal, these orbitals undergo

s p2

1 2 3

16

MOVPE on a GaAs substrate, which

has several GaAs/Al Ga As QWs

of different thicknesses. We see that

although the material of the well

(GaAs) is the same throughout, the

change in the thickness has led to

different effective band gaps for the

wells, as identified by the different

peak PL emission energies. The GaAs

related emission is from the thick

GaAs cap layer on top of the sample.

Light emission through the edges of a

QW sample (perpendicular to )

shows addit ional interest ing

features.

Fig.8 b) shows that such electro-

luminescence emission from a

GaInP/AlGaInP QW is strongly

polarized with the transverse electric

(TE || ) component peaking at a

longer wavelength when compared

to the transverse magnetic (TM || )

component, the latter being weaker in

strength [38]. This can be understood

as follows. The confinement energies

are different for the hh and the lh

bands due to the different effective

masses of the holes in these bands.

This lifts the hh-lh degeneracy and

the lowest energy hh band now has

more holes resulting in a stronger e1-

hh1 emission at longer wavelengths

(lower energy). Also transition from

an like CB state to a like VB state

involves polarized light and so on.

The hh state around k , k = 0 is purely

composed of and orbitals and

therefore this emission is polarized.

The lh state has a small component

and therefore the e1-lh1 transition is

seen in the TM polarized emission at

shorter wavelengths (higher energy).

While PL spectroscopy is useful for

estimating the effective band gap

change, it does not reveal any

information about confined states of

the quantum structure lying at

0.27 0.73

z

x

z

x

x

s p

p p

p

x

x y

z

x y

hybridization and result in the formation of a single conduction (CB) and

three closely spaced valence bands (VB). Fig.7 shows a schematic of the EBS of

bulk GaAs in terms of electron energy versus crystal momentum ( ). Here one

is concerned with only the CB and VBs, that too near =0 (Brillouin zone

center) while neglecting filled bands at lower energies and empty ones at

higher energies because they typically do not directly affect device

performance. The curvature of the bands in space defines the effective mass

and accordingly the top two valence bands, which are degenerate (in cubic

crystals) at =0, are labeled as the heavy hole (hh), light hole (lh) and the third

as the spin orbit split off (so) hole band. The character of the cell periodic parts

of the wavefunction in these bands, which are like for CB and , or like

for the VBs, decide how these materials respond to polarized light. This topic

will be discussed later. Fig.4 showed the potential energy diagram of a GaAs

quantum well where one had plotted the energy of the band extrema at =0 as

a function of distance along , the film growth direction. We see that the lowest

energy state that the electron can occupy in the QW is not the CB bottom but an

energy higher by E (z). Similarly the hh occupies an energy higher by E (z)

(hole energies increase downwards). Thus the effective band gap of this

structure becomes Eg = Eg + E (z) + E (z). Referring to equation (1b) we

see that effective band gap increases as the width of well L reduces.

k

k

k

k

k

s p p px y z

z

e1 hh1

eff GaAs e1 hh1

w

Fig.8 a) Photo-luminescence(PL) spectrum of a sample which has several

GaAs/Al Ga As QWs of different well widths Lw. The QW emissions are all e1hh1

transitions. b) Polarization resolved edge emission spectrum from a GaInP/AlGaInP QW.

0.27 0.73

Photo-luminescence (PL) spectroscopy is a technique commonly used to

estimate such effective band gap change. Here one excites the sample with an

intense pump laser beam with photon energy much higher than the band gap

to be measured. The absorbed pump beam excites electrons to the CB leaving

behind a hole in the VB. Within a short time (typically pico-second) these

excited electrons (holes) loose energy and trickle down to the CB bottom (VB

top) and thereafter recombine radiatively. The photon energy of the resultant

emission will then correspond to the effective band gap. Thus by measuring

the emission spectrum, typically using a reflection-grating based

monchromator to disperse the emitted light and then detect it with an

appropriate photo-detector, one can estimate the effective band gap change.

Fig.8 a) shows the PL spectrum of a sample grown in our laboratory using

17

higher energies. One way to obtain such information is through photo-

luminescence excitation (PLE) spectroscopy, where in one detects the PL

emission at a particular wavelength while varying the wavelength of the

pump excitation. The working principle is that more the absorption, more

electron-hole pairs would be generated and hence more will be the

luminiscence output. Fig.9 shows the PLE spectrum of a 4.6nm thick

GaAs/Al Ga As single QW, which essentially mimics its absorption

spectrum. It consists of some sharp peaks riding on a staircase like

background. The latter is the signature of the step-function 2D density of

states previously shown in Fig.1. Each step indicates the onset of transition

between a CB and VB confined energy state in the QW. The sharp peaks just at

the edge of the steps are due to resonant absorption of photons that lead to

exciton formation. Excitons can be visualized as electron-hole pairs bound

together by coulomb attraction, just like the electron-proton in a hydrogen

atom. In 3D, the exciton binding energy for GaAs is about 4 meV and is

therefore easily ionized at room temperature (k T=26meV at 300K). However

an interesting aspect of 2D systems is that when the electron-hole pair is

constrained to move in 2D then the solution of the Schrodinger equation

shows that the binding energy becomes four times that in case of 3D motion.

In practice one gets a value close to 10 meV for exciton binding energy in a 5

nm GaAs QW. This allows the excitons to exist up to much higher

temperatures and they show up prominently in the absorption spectrum of a

QW. We also notice two closely placed transitions close to the band edge.

These arise because the hh-lh degeneracy is lifted in a QW as explained before.

0.36 0.64

B

second intense pump laser beam

incident on the sample. The pump

beam has photon energy higher than

the band gap and it generates

electron-hole pairs which modify the

built-in electric fields at the interfaces

of the QW. This leads to an electric

field modulation of the dielectric

function and consequently the

refractive indices change. In ER the

electric field is applied directly using

appropriate electrode arrangements

[39]. The dielectric function depends

on the electronic joint-density-of-

states. Since the joint-density-of-state

changes abruptly at a critical point

(van-Hove singularity) in the EBS,

such as at the onset of a new inter-

band transition, a perturbation like

the electric field affects the optical

response more around a critical

point. Thus the change in reflectance

shows sharp spectral features at

energies where a new inter-band

transition arises such as between the

higher confined states in a QW. The

difference nature of modulation

spectroscopy helps it pick up small

changes in reflectance over a large

background and from the resulting

spectral features one can determine

the critical point energies. Fig.10

shows the PR spectrum of another

GaAs/Al Ga As QW where one

finds sharp spectral features which

one can identify as arising due to the

GaAs (cap), Al Ga As barrier and

four transitions in the QW. The e1-

hh3 feature is weak because it is a

normally forbidden transition and

the e2-hh2 transition is weak because

the e2 level is close to the conduction

band edge and is therefore weakly

confined.

An absorption spectrum directly

gives information on higher energy

states. However, direct measurement

0.36 0.64

0.36 0.64

Energy (eV)1.6 1.7 1.8 1.9 2.0 2.1

Sig

na

l (a

rb.

un

it)

0

100

200

300

400

GaAs/Al 0.36Ga0.64As SQW

Lw~46A, 8K

Al0.36Ga0.64Ase1hh1

e1lh1

e1hh3

e2hh2

�D

PLE

PL

Fig.9 Photo-luminescence excitation (PLE) spectrum of a single GaAs/Al Ga As

QW of width Lw ~4.6 nm measured at 8K. The PL spectrum is also shown. The PLE

detection was at 1.62eV ( =765nm).

0.36 0.64

D�

PLE has a drawback in that it typically requires sample cooling to low

temperatures for the PL signal to be detected. However it is also possible to

identify higher energy transitions in quantum structures like single QWs

using the techniques of modulation spectroscopy such as Photo-reflectance

(PR) and Electro-reflectance (ER). In PR one measures the change in the

reflectance of a probe beam of variable wavelength due to influence of a

18

plane of the QW. The strain mixes the

VBs and modifies the relative

contributions of and to the hh

and lh bands giving rise to this

anisotropy. The anisotropic strain in

this case was specially engineered,

such structures are of interest for

polarization sensitive detection

applications.

As the expertise of synthesizing

good quality III-nitrides is

growing, there is continuous

refinement of the measurements of

their basic properties. Among the

heterostructure properties, the most

significant are the conduction band

discontinuity and the valence band

discontinuity at the heterojunction. It

turns out that determination of these

band discontinuities has presented

difficulties of reliability even for

the most well known heterojunction

AlGaAs-GaAs [41]. We chose to

measure the band discontinuities

b e t w e e n I n N a n d G a N

heterojunction by internal photo-

emission technique [43] using

junction of heavily doped n type InN

and lightly doped n type GaN. We

measure short circuit photo current

of this heterostructure, while shining

monochromatic light (through GaN

side) over a broad range of photon

energies 1.5 to 3.5 eV. Results of these

measurements are shown in

p px y

of the absorption coefficient through optical transmission experiments is

often difficult in case of quantum structures because the small dimensions

(for example the width of a single QW) result in barely measurable light

attenuation. To estimate the absorption spectrum, indirect methods are

adopted such as PLE discussed above as also Photo-Voltage (PV) and Photo-

Current (PC) spectroscopy. In PV measurements the quantum structure

typically has a built-in field across it, such as by virtue of being placed in the

i-region of a p-i-n diode structure. In this case, light absorbed by the

Fig.10 Photo-reflectance (PR) spectrum of a GaAs/Al Ga As single QW with well

width Lw ~4.6 nm at room temperature.

0.36 0.64

structure creates electron-hole pairs which get separated by the field and in

the process modifies the built in potential which is detected as a voltage

signal, much like in the case of a solar-cell. Fig.11 shows our measured PV

spectrum of MBE grown InAs QDs. These pancake shaped QDs with average

height ~9 nm and diameter ~40 nm are in a GaAs matrix. The PV spectrum

mimics the absorption spectrum of the QDs. Instead of very narrow atom like

transitions expected for a 0D system, we find that the spectrum can be

resolved into broadened Gaussians which actually represent inhomogenous

broadening of the spectrum due to a distribution in the size of the QDs (about

4% of the mean here) [40]. However we do see the dramatic effect of 3D

confinement in terms of the shift of the band gap from 0.35 eV for bulk InAs to

about 1.03 eV for these InAs QDs. It is also possible to estimate the absorption

spectrum by applying a bias to a quantum structure and then shining light on

it to measure the photo-current as a function of incident photon-energy. Fig.12

shows the lateral PC spectra of GaAs/Al Ga As QWs where one has ensured

that the conduction happens along the plane of the QW. The two spectra are

for two orthogonal polarizations of the light incident normally on the QW

plane (x-y plane). We again see spectral features similar to those seen in the

PLE spectrum, however one also notices that the hh and lh related exciton

transitions have different strength for the two polarizations. This is usually

not expected in a QW since both hh and lh bands have identical and

contributions. In this case our analysis showed [41] that the in-plane

polarization-anisotropy arises due to the presence of anisotropic strain in the

x 1-x

p px y

Energy (eV)1.0 1.1 1.2 1.3 1.4

PV

sig

nal (a

rb.

un

i ts)

0

1

2

3

4

5

6

p=1p=2

p=3

p=4

WL

x1/20

x1/300

GaAs

QD

Fig.11 Photo-voltage spectrum of InAs

quantum dots embedded in GaAs,

measured at room temperature.

19

phase epitaxy [44]. Fig.14 b) shows

crossectional micrograph of a typical

device fabricated by LPE [45].

Devices incorporating QWs and

other quantum structures are

fabricated by using MBE or MOVPE.

Diode lasers used for optical

communication are single mode

devices, integrated and tunable [46,

47]. On the other hand, diode lasers

have also been combined to make

arrays for delivering kilowatts of

power for industrial applications

[48]. Fig.15 shows characteristics of a

high power 0.98 micron laser device

fabricated at TIFR.

Inter-subband transitions provide a

mechanism for generating and

d e t e c t i n g i n f r a r e d r a d i a t i o n

i n vo l v i n g o n l y e l e c t r o n s i n

conduction band or holes in the

valence band. The emission or

absorption wavelength of radiation

i s d e t e r m i n e d b y q u a n t u m

confinement and is independent of

the fundamental band gap. The light

emitting device called quantum

cascade laser may have 20 to 100

periods of coupled quantum wells

and is perhaps the most intricate

quantum device [9]. Detectors based

on quantum wells are relatively

s i m p l e r a n d a r e e s s e n t i a l l y

photoconductors as shown in the

Fig.16. Inter subband transitions in

quantum wells require, apart from

e n e r g y a n d m o m e n t u m

conservation, that the polarization of

the radiation should be in direction

normal to the plane of the QWs.

Coupling of light to electrons in the

QW is achieved in a single detector

device by using the angle lapped

configuration as shown in Fig.16 [49].

In imaging devices, it is desirable that

2. Inter-subband transition

devices

Fig.13 Internal-photo emission spectrum of an InN/GaN hetero-junction. The

adjacent schematic shows the inferred band alignment at the heterojunction.

Fig.13.Three prominent features can be seen in this figure marked Region I,

Region II, and Region III. Region III is sharp peak arising from excitations of

electrons across the GaN band gap. Region II is caused by excitation of

electrons from the valence band maximum of InN to conduction band edge of

GaN. Region I is caused by excitation of electrons from the conduction band

(level marked E ) of InN to the bottom of conduction band of GaN. From these

threshold energies, we obtain the valence band discontinuity of InN/GaN

heterojunction to be ~0.85 eV and the corresponding conduction band

discontinuity to be ~1.82 eV as shown in the adjacent schematic band diagram.

Quantum structures form the basis of a very large number of optoelectronic

and electronic devices. Perhaps the most widely used device is the

semiconductor laser diode. They are small in size, have high gain and are very

reliable. Fig.14 shows the progressive reduction in the threshold current

density for lasing by using quantum structures for the active layer, because of

the higher gain. Higher gain of diode lasers is a result of modification of the

density of states.All laser structures till 1970 were synthesized by using liquid

F

Selected Applications

1. Diode Lasers

Fig.12 Polarized lateral photo-current (PC) spectrum of a GaAs/Al Ga As QW based

device structure (inset) where the QW is under anisotropic strain in the x-y plane.

x 1-x

Fig.17 Quantum Hall Effect measurement results. The figure shows the sample structure

with the InGaAs/InP QW, the potential profile, a photograph of the sample and results of

Hall effect measurements showing the QHE plateaus at various temperatures [52].

20

light is imaged on the plane of the

QWs. To achieve coupling to

electrons, the surface is formed into a

grating. Infrared detectors are also

made utilizing inter subband

transitions in quantum dots (QDIPs).

Coupling to electrons in QDs does

n o t h a v e t h e p o l a r i z a t i o n

dependence required by the QWs.

Infrared imaging cameras based on

QWIPs are used in astronomy (extra

terrestrial bodies emitting in

i n f r a r e d ) , m e d i c i n e ( c a n c e r

detection), defence (night vision),

and for industrial applications where

different parts of objects may be at

different temperatures.

A large concentration of high

mobility electrons can be generated

i n a q u a n t u m we l l t h r o u g h

modulation doping, where the

doped layer in the larger band gap

semiconductor is separated from the

QW and donates electrons to the QW.

A 2DEG structure of this type using

undoped InGaAs QW and n doped

InP as the donor layer is shown in

Fig .17 . The most prominent

electronic device based on 2DEG is

the high electron mobility transistor

(HEMT). Transistors fabricated from

structures similar to that in Fig.17

show excellent characteristics with

high gain and low noise upto

100GHz in frequency[50]. Further

improvements have been achieved

by using metamorphic HEMT, where

the substrate is GaAs, the QW is

InGaAs and the donor layer is

I n A l A s [ 5 1 ] . M o r e r e c e n t

developments of HEMTs based on

III- Nitrides [23] give advantages of

reduced size and higher power at

high frequencies because of electron

densities approaching 10 /cm .13 2

3. Two Dimensional Electron

Gas (2DEG) Devices

Fig.14 a) Progressive reduction in the threshold current density for lasing over the

years. b) Crossectional micrograph of an LPE grown QW laser

Fig.15 Structure and characteristics of a high power QW laser device fabricated at TIFR.

Fig.16 Structure and characteristics of a QW infrared photodetector fabricated at TIFR [49]

21

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Application of a suitable potential to

the gate electrodes essentially isolates

the region contained by them,

mimicking a QD structure. This has

been widely used in single electron

transistors to investigate coulomb

blockade and tunnel current transport

[55]. With the advent of the subject

of quantum information processing

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semiconductor quantum structures

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electron spin system in QD as a qubit.

Electrical and optical techniques have

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further progress may be expected.

The authors are grateful to many

colleagues who have contributed to

the development of the work at TIFR.

We would like to acknowledge K. L.

Narasimhan, A. Bhattacharya, S. S.

Chandvankar, T. K. Sharma (RRCAT

Indore), S. Ganguly (RRCAT Indore), S

Sen (Calcutta Univ.), G. Dohler

(Erlangen Univ.), S. Malzer (Erlangen

Univ.), K. S. Chandrasekharan, M. K.

Sanyal (SINP, Kolkata), S. Datta, B.

Karmakar, A. Arora, J. Bhattacharya,

M. R. Gokhale, A. P. Shah, S.

Subramanian, A. K. Srivastava, A.

Majumdar and Z. H. Mahmood. BMA

was with TIFR till 2008.

Acknowledgements

2DEG electrons are also the source of

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