15Optical Processes and Excitons

OPTICAL REFLECTANCE 429Kramers-Kronig relations 430Mathematical note 432Example: conductivity of collisionless electron gas 433Electronic interband transitions 434

EXCITONS 435Frenkel excitons 437Alkali halides 440Molecular crystals 440Weakly bound (Mott-Wannier) excitons 441Exciton condensation into electron-hole drops (EHD) 441

RAMAN EFFECT IN CRYSTALS 444Electron spectroscopy with x-rays 447

ENERGY LOSS OF FAST PARTICLES IN A SOLID 448

SUMMARY 449

PROBLEMS 450

1. Causality and the response function 4502. Dissipation sum rule 4503. Reflection at normal incidence 4504. Conductivity sum rule and superconductivity 4505. Dielectric constant and the semiconductor energy gap 4516. Hagen-Rubens relation for infrared reflectivity of metals 4517. Davydov splitting of exciton lines 452

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428

Raman scattering (generic term): Brillouin scattering when acoustic phonon is involved; polariton scattering when optical phonon is involved.

Signs + for phonon emission (Stokes process)– for phonon absorption (anti-Stokes)

Two phonon creation.

Electron spectroscopy with x-rays (XPS): incident x-ray photon ejects valence or core electron from solid.

Photoelectron

X-ray

�, k

�, k

�′, k′

�, K

�, K

�′, K′

� = �′ ± �k = k′ ± K

Figure 1 There are many types of experiments in which light interacts with wavelike excitationsin a crystal. Several absorption processes are illustrated here.

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429

chapter 15: optical processes and excitons

The dielectric function �(�,K) was introduced in the preceding chapter todescribe the response of a crystal to an electromagnetic field (Fig. 1). The di-electric function depends sensitively on the electronic band structure of acrystal, and studies of the dielectric function by optical spectroscopy are veryuseful in the determination of the overall band structure of a crystal. Indeed,optical spectroscopy has developed into the most important experimental toolfor band structure determination.

In the infrared, visible, and ultraviolet spectral regions the wavevector ofthe radiation is very small compared with the shortest reciprocal lattice vector,and therefore it may usually be taken as zero. We are concerned then with thereal �� and imaginary �� parts of the dielectric function at zero wavevector;�(�) � ��(�) � i��(�), also written as �1(�) � i�2(�).

However, the dielectric function is not directly accessible experimentallyfrom optical measurements: the directly accessible functions are the reflect-ance R(�), the refractive index n(�), and the extinction coefficient K(�). Ourfirst objective is to relate the experimentally observable quantities to the realand imaginary parts of the dielectric function.

OPTICAL REFLECTANCE

The optical measurements that give the fullest information on the elec-tronic system are measurements of the reflectivity of light at normal incidenceon single crystals. The reflectivity coefficient r(�) is a complex function de-fined at the crystal surface as the ratio of the reflected electric field E(refl) tothe incident electric field E(inc):

(1)

where we have separated the amplitude �(�) and phase �(�) components ofthe reflectivity coefficient.

The refractive index n(�) and the extinction coefficient K(�) in thecrystal are related to the reflectivity at normal incidence by

(2)

as derived in Problem 3 by elementary consideration of the continuity of thecomponents of E and B parallel to the crystal surface. By definition, n(�) andK(�) are related to the dielectric function �(�) by

(3)��(�) � n(�) � iK(�) � N(�) ,

r(�) �

n � iK � 1n � iK � 1

,

E(refl)/E(inc) � r(�) � �(�) exp[i�(�)] ,

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where N(�) is the complex refractive index. Do not confuse K(�) as used herewith the symbol for a wavevector.

If the incident traveling wave has the wavevector k, then the y componentof a wave traveling in the x direction is

(4)

The transmitted wave in the medium is attenuated because, by the dispersionrelation for electromagnetic waves, the wavevector in the medium is related tothe incident k in vacuum by (n � iK)k:

(5)

One quantity measured in experiments is the reflectance R, defined asthe ratio of the reflected intensity to the incident intensity:

(6)

It is difficult to measure the phase �(�) of the reflected wave, but we showbelow that it can be calculated from the measured reflectance R(�) if this isknown at all frequencies.

Once we know both R(�) and �(�), we can proceed by (2) to obtain n(�)and K(�). We use these in (3) to obtain �(�) � ��(�) � i��(�), where ��(�) and��(�) are the real and imaginary parts of the dielectric function. The inversionof (3) gives

(7)

We now show how to find the phase �(�) as an integral over the re-flectance R(�); by a similar method we relate the real and imaginary parts ofthe dielectric function. In this way we can find everything from the experi-mental R(�).

Kramers-Kronig Relations

The Kramers-Kronig relations enable us to find the real part of the re-sponse of a linear passive system if we know the imaginary part of the responseat all frequencies, and vice versa. They are central to the analysis of opticalexperiments on solids.

The response of any linear passive system can be represented as the su-perposition of the responses of a collection of damped harmonic oscillatorswith masses Mj. Let the response function �(�) � ��(�) � i��(�) of the col-lection of oscillators be defined by

(8)

where the applied force field is the real part of F� exp(�i�t) and the total dis-placement is the real part of x� exp(�i�t). From the equation of motion,

Mj(d2/dt2 � �

j

d/dt � �2j )xj � F ,

x � �j

xj

x� � �(�)F� ,

��(�) � n2 � K2 ; � �(�) � 2nK .

R � E*(refl)E(refl)/E*(inc)E(inc) � r*r � �2 .

Ey(trans) � exp {[i[(n � iK)kx � wt]} � exp(�Kkx) exp[i(nkx � �t)] .

Ey(inc) � Ey0 exp[i(kx � �t)] .

430

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we have the complex response function of the oscillator system:

(9)

where the constants fj � 1/Mj and relaxation frequencies �j are all positive fora passive system.

If �(�) is the dielectric polarizability of atoms in concentration n, then fhas the form of an oscillator strength times ne2/m; such a dielectric responsefunction is said to be of the Kramers-Heisenberg form. The relations we de-velop also apply to the electrical conductivity �(�) in Ohm’s law, j� � �(�)E�.

We need not assume the specific form (9), but we make use of three prop-erties of the response function viewed as a function of the complex variable �.Any function with the following properties will satisfy the Kramers-Kronigrelations (11):

(a) The poles of �(�) are all below the real axis.(b) The integral of �(�)/� vanishes when taken around an infinite semi-

circle in the upper half of the complex �-plane. It suffices that �(�) → 0 uni-formly as |�| → �.

(c) The function ��(�) is even and ��(�) is odd with respect to real �.

Consider the Cauchy integral in the form

(10)

where P denotes the principal part of the integral, as discussed in the mathe-matical note that follows. The right-hand side is to be completed by an integralover the semicircle at infinity in the upper half-plane, but we have seen in (b)that this integral vanishes.

We equate the real parts of (10) to obtain

In the last integral we substitute s for �p and use property (c) that ���(�s) �

����(s); this integral then becomes

and we have, with

1s � �

�

1s � �

�

2 ss2

� �2 ,

��

0

��(s)s � �

ds ,

��(�) � 1� P ��

�� � �(s)s � � ds � 1� P ���

0 ��(s)s� � ds � � 0

�� ��(p)p � � dp� .

�(�) �

1�i P�

�

��

�(s)s � � ds ,

�(�) � �

j

fj

�2j � �2

� i��j

� � fj �2

j � �2 � i��j

(�2j � �2)2

� �2�2j

,

15 Optical Processes and Excitons 431

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the result

(11a)

This is one of the Kramers-Kronig relations. The other relation follows onequating the imaginary parts of Eq. (10):

whence

(11b)

These relations are applied below to the analysis of optical reflectance data;this is their most important application.

Let us apply the Kramers-Kronig relations to r(�) viewed as a response func-tion between the incident and reflected waves in (1) and (6). We apply (11) to

(12)

to obtain the phase in terms of the reflectance:

(13)

We integrate by parts to obtain a form that gives insight into the contribu-tions to the phase angle:

(14)

Spectral regions in which the reflectance is constant do not contribute to theintegral; further, spectral regions s � � and s � do not contribute much be-cause the function ln |(s � �)/(s � �)| is small in these regions.

Mathematical Note. To obtain the Cauchy integral (10) we take the inte-gral over the contour in Fig. 2. The function �(s) is analytic inthe upper half-plane, so that the value of the integral is zero. The contributionof segment (4) to the integral vanishes if the integrand �(s)/s → 0 is faster than|s|�1 as |s| → �. For the response function (9) the integrand → 0 as |s|�3; andfor the conductivity �(s) the integrand → 0 as |s|�2. The segment (2) con-tributes, in the limit as u → 0,

�(2)

�(s)s � � ds l �(�) � 0

�

iu ei� d�

u ei� � ��i�(�)

��(s)(s � �)�1 ds

�(�) � �1

2� ��

0 ln � s � �

s � �� d ln R(s)

ds ds .

�(�) � ��� P ��

0

ln R(s)s2

� �2 ds .

ln r(�) � ln R1/2(�) � i�(�)

� �(�) � �2�� P ��

0

��(s)s2

� �2 ds .

��(�) � �1� P ��

��

��(s)s � � ds � �1

� P ���

0 ��(s)s � � ds � ��

0 ��(s)s � � ds� ,

��(�) �

2� P ��

0

s��(s)s2

� �2 ds .

432

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to the integral, where s � � � u ei�. The segments (1) and (3) are by definitionthe principal part of the integral between �� and �. Because the integralover (1) � (2) � (3) � (4) must vanish,

(15)

as in (10).

EXAMPLE: Conductivity of Collisionless Electron Gas. Consider a gas of freeelectrons in the limit as the collision frequency goes to zero. From (9) the responsefunction is, with f � 1/m,

(16)

by the Dirac identity. We confirm that the delta function in (16) satisfies the Kramers-Kronig relation (11a), by which

(17)

in agreement with (16).We obtain the electrical conductivity �(�) from the dielectric function

(18)

where �(�) � x�/(�e)E� is the response function. We use the equivalence

(CGS) (19)

for the Maxwell equation can be written either as c curl H � 4��(�)E � i�E or as c curl H � �i��(�)E. We combine (16), (18), and (19) to find the conductivity of a collisionless electron gas:

(20)

For collisionless electrons the real part of the conductivity has a delta function at � � 0.

��(�) � i� �(�) �

ne2

m ��(�) �

i�� .

�(�) � (�i�/4�)[�(�) � 1] ,

�(�) � 1 � 4�P� /E� � �4�nex� /E� � 4�ne2�(�) ,

��(�) �

2m ��

0

(s)s2

� �2 ds � �

1m�2

,

�(�) � � 1m� lim

�l0

1� � i� � � 1

m� �1� � i�(�)� ,

�(1)

� �(3)

�

P ��

��

�(s)s � � ds � �i�(�) ,

15 Optical Processes and Excitons 433

1 32

4

�Figure 2 Contour for the Cauchy principal valueintegral.

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Electronic Interband Transitions

It came as a surprise that optical spectroscopy developed as an importantexperimental tool for the determination of band structure. First, the absorp-tion and reflection bands of crystals are broad and apparently featureless func-tions of the photon energy when this is greater than the band gap. Second,direct interband absorption of a photon �� will occur at all points in the Bril-louin zone for which energy is conserved:

(21)

where c is an empty band and v is a filled band. The total absorption at given �is an integral over all transitions in the zone that satisfy (21).

Three factors unraveled the spectra:

• The broad bands are not like a spectral line greatly broadened by damping,but the bands convey much intelligence which emerges when derivatives aretaken of the reflectance (Fig. 3); derivatives with respect to wavelength,electric field, temperature, pressure, or uniaxial stress, for example. Thespectroscopy of derivatives is called modulation spectroscopy.

�� � �c(k) � �v(k) ,

434

0

03.63.43.23.0

(a)

(b)

(c)

�, in eV

Figure 3 Comparison of (a) reflectance, (b) wavelength derivative reflectance (first derivative),and (c) electroreflectance (third derivative), of the spectral region in germanium between 3.0 and3.6 eV. (After data by D. D. Sell, E. O. Kane, and D. E. Aspnes.)

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• The relation (21) does not exclude spectral structure in a crystal, becausetransitions accumulate at frequencies for which the bands c, v are parallel—that is, at frequencies where

(22)

At these critical points in k space the joint density of states Dc(�v �

��)Dv(�v) is singular, according to the same argument we used in (5.37) toshow that the density of phonon modes D(�) is singular when k� is zero.

• The pseudopotential method for calculating energy bands helps identify thepositions in the Brillouin zone of the critical points found in modulationspectra. Band-band energy differences can be calculated with an accuracy asgood as 0.1 eV. The experimental results can then be fed back to give im-provements in the pseudopotential calculations.

EXCITONS

Reflectance and absorption spectra often show structure for photonenergies just below the energy gap, where we might expect the crystal to betransparent. This structure is caused by the absorption of a photon with thecreation of a bound electron-hole pair. An electron and a hole may be boundtogether by their attractive coulomb interaction, just as an electron is bound toa proton to form a neutral hydrogen atom.

The bound electron-hole pair is called an exciton, Fig. 4. An exciton canmove through the crystal and transport energy; it does not transport charge

�

�k[�c(k) � �v(k)] � 0 .

15 Optical Processes and Excitons 435

e

h

Figure 4a An exciton is a bound electron-hole pair,usually free to move together through the crystal. Insome respects it is similar to an atom of positronium,formed from a positron and an electron. The excitonshown is a Mott-Wannier exciton: it is weakly bound,with an average electron-hole distance large in com-parison with the lattice constant.

+– +–

+ – + +–

+ – + +–

–

+– +– –

+– +– –

Figure 4b A tightly-bound or Frenkel exciton shown local-ized on one atom in an alkali halide crystal. An ideal Frenkelexciton will travel as a wave throughout the crystal, but theelectron is always close to the hole.

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because it is electrically neutral. It is similar to positronium, which is formedfrom an electron and a positron.

Excitons can be formed in every insulating crystal. When the band gap isindirect, excitons near the direct gap may be unstable with respect to decayinto a free electron and free hole. All excitons are unstable with respect to theultimate recombination process in which the electron drops into the hole. Ex-citons can also form complexes, such as a biexciton from two excitons.

We have seen that a free electron and free hole are created whenever aphoton of energy greater than the energy gap is absorbed in a crystal. Thethreshold for this process is �� � Eg in a direct process. In the indirectphonon-assisted process of Chapter 8 the threshold is lower by the phonon en-ergy ��. But in the formation of excitons the energy is lowered with respect tothese thresholds by the binding energy of the exciton, which may be in therange 1 meV to 1 eV, as in Table 1.

Excitons can be formed by photon absorption at any critical point (22), forif k�v � k�c the group velocities of electron and hole are equal and the parti-cles may be bound by their coulomb attraction. Transitions leading to theformation of excitons below the energy gap are indicated in Figs. 5 and 6.

The binding energy of the exciton can be measured in three ways:

• In optical transitions from the valence band, by the difference between theenergy required to create an exciton and the energy to create a free electronand free hole, Fig. 7.

• In recombination luminescence, by comparison of the energy of the freeelectron-hole recombination line with the energy of the exciton recombina-tion line.

• By photo-ionization of excitons, to form free carriers. This experiment re-quires a high concentration of excitons.

We discuss excitons in two different limiting approximations, one by Frenkelin which the exciton is small and tightly bound, and the other by Mott andWannier in which the exciton is weakly bound, with an electron-hole separationlarge in comparison with a lattice constant. Intermediate examples are known.

��

436

Table 1 Binding energy of excitons, in meV

Si 14.7 BaO 56. RbCl 440.Ge 4.15 InP 4.0 LiF (1000)GaAs 4.2 InSb (0.4) AgBr 20.GaP 3.5 KI 480. AgCl 30.CdS 29. KCl 400. TlCl 11.CdSe 15. KBr 400. TlBr 6.

Data assembled by Frederick C. Brown and Arnold Schmidt.

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Frenkel Excitons

In a tightly bound exciton (Fig. 4b) the excitation is localized on or near asingle atom: the hole is usually on the same atom as the electron although thepair may be anywhere in the crystal. A Frenkel exciton is essentially an excitedstate of a single atom, but the excitation can hop from one atom to another byvirtue of the coupling between neighbors. The excitation wave travels throughthe crystal much as the reversed spin of a magnon travels through the crystal.

The crystalline inert gases have excitons which in their ground states cor-respond somewhat to the Frenkel model. Atomic krypton has its lowest strong

15 Optical Processes and Excitons 437

Exciton levels Energy gap, Eg

Conduction band(effective mass me)

Valence band(effective mass mh)

k

Figure 5 Exciton levels in relation to the conduction band edge, for a simple band structure withboth conduction and valence band edges at k � 0. An exciton can have translational kinetic en-ergy. Excitons are unstable with respect to radiative recombination in which the electron dropsinto the hole in the valence band, accompanied by the emission of a photon or phonons.

Energy gap

Exciton bindingenergy

Exciton levels

Conduction band continuum

Valence band continuum

0

Eg

Eg – Eex

Eex

Figure 6 Energy levels of an exciton created in a direct process. Optical transitions from the topof the valence band are shown by the arrows; the longest arrow corresponds to the energy gap.The binding energy of the exciton is Eex, referred to a free electron and free hole. The lowest fre-quency absorption line of the crystal at absolute zero is not Eex, but is Eg � Eex.

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atomic transition at 9.99 eV. The corresponding transition in the crystal isclosely equal and is at 10.17 eV, Fig. 8. The energy gap in the crystal is 11.7 eV,so the exciton ground state energy is 11.7 � 10.17 � 1.5 eV, referred to a freeelectron and free hole separated and at rest in the crystal.

The translational states of Frenkel excitons have the form of propagatingwaves, like all other excitations in a periodic structure. Consider a crystal of N atoms on a line or ring. If uj is the ground state of atom j, the ground state ofthe crystal is

(23)

if interactions between the atoms are neglected. If a single atom j is in anexcited state vj, the system is described by

(24)

This function has the same energy as the function l with any other atom l ex-cited. However, the functions that describe a single excited atom and N � 1 atoms in their ground state are not the stationary quantum states of theproblem. If there is any interaction between an excited atom and a nearbyatom in its ground state, the excitation energy will be passed from atom toatom. The eigenstates will have a wavelike form, as we now show.

j � u1u2

Á uj�1vjuj�1

Á uN .

�g � u1u2

Á uN � 1uN ,

438

0

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.50 1.51 1.52 1.53 1.54 1.55 1.56

Abs

orpt

ion

coef

ficie

nt �

in 1

04 cm

–1

Photon energy in eV

Eg

Absorption with formationof free electron-hole pairs

Exciton absorption

Figure 7 Effect of an exciton level on the optical absorption of a semiconductor for photons ofenergy near the band gap Eg in gallium arsenide at 21 K. The vertical scale is the intensity absorp-tion coefficient �, as in I(x) � I0 exp(��x). The energy gap and exciton binding energy are deduced from the shape of the absorption curve: the gap Eg is 1.521 eV and the exciton bindingenergy is 0.0034 eV. (After M. D. Sturge.)

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When the hamiltonian of the system operates on the function j with thejth atom excited, we obtain

(25)

where � is the free atom excitation energy; the interaction T measures the rateof transfer of the excitation from j to its nearest neighbors, j � 1 and j � 1. The solutions of (25) are waves of the Bloch form:

(26)

To see this we let operate on �k:

(27)

from (25). We rearrange the right-hand side to obtain

(28)

so that the energy eigenvalues of the problem are

(29)Ek � � � 2T cos ka ,

��k � �j

e

ijka[� � T(e

ika � e�ika)]j � (� � 2T cos ka)�k ,

��k � �j

e

ijka �j � �j

e

ijka [�j � T(j�1 � j�1)] ,

�

�k � �j

exp(ijka) j .

�j � �j � T(j�1 � j�1) ,

15 Optical Processes and Excitons 439

0

0.5

1.0

1.5

13.0 12.0 11.0 10.0�, eV

log(

I 0/I

)

Figure 8 Absorption spectrum of solid krypton at 20 K. (After G. Baldini.)

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as in Fig. 9. The application of periodic boundary conditions determines theallowed values of the wavevector k:

(30)

Alkali Halides. In alkali halide crystals the lowest-energy excitons are local-ized on the negative halogen ions, as in Fig. 4b. The negative ions have lowerelectronic excitation levels than do the positive ions. Pure alkali halide crystalsare transparent in the visible spectral region, which means that the exciton en-ergies do not lie in the visible, but the crystals show considerable excitonicabsorption structure in the vacuum ultraviolet.

A doublet structure is particularly evident in sodium bromide, a structuresimilar to that of the lowest excited state of the krypton atom—which is iso-electronic with the Br� ion of KBr. The splitting is caused by the spin-orbit in-teraction. These excitons are Frenkel excitons.

Molecular Crystals. In molecular crystals the covalent binding within amolecule is strong in comparison with the van der Waals binding between mol-ecules, so that the excitons are Frenkel excitons. Electronic excitation lines ofan individual molecule appear in the crystalline solid as an exciton, often withlittle shift in frequency. At low temperatures the lines in the solid are quitesharp, although there may be more structure to the lines in the solid than inthe molecule because of the Davydov splitting, as discussed in Problem 7.

k � 2�s/Na; s � �12N, �1

2N � 1, Á , 12N � 1 .

440

Wavevector k–�/a �/a0

Energy

�

� + 2T

� – 2T

Figure 9 Energy versus wavevector for a Frenkel exciton, calculated with positive nearest-neighbor transfer interaction T.

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Weakly Bound (Mott-Wannier) Excitons

Consider an electron in the conduction band and a hole in the valenceband. The electron and hole attract each other by the coulomb potential

(CGS) (31)

where r is the distance between the particles and � is the appropriate dielec-tric constant. (The lattice polarization contribution to the dielectric constantshould not be included if the frequency of motion of the exciton is higher thanthe optical phonon frequencies.) There will be bound states of the exciton sys-tem having total energies lower than the bottom of the conduction band.

The problem is the hydrogen atom problem if the energy surfaces for theelectron and hole are spherical and nondegenerate. The energy levels referredto the top of the valence band are given by a modified Rydberg equation

(CGS) (32)

Here n is the principal quantum number and � is the reduced mass:

(33)

formed from the effective masses me, mh of the electron and hole.The exciton ground state energy is obtained on setting n � 1 in (32); this

is the ionization energy of the exciton. Studies of the optical absorption linesin cuprous oxide, Cu2O, at low temperatures give results for the exciton levelspacing in good agreement with the Rydberg equation (32) except for transi-tions to the state n � 1. An empirical fit to the lines of Fig. 10 is obtained withthe relation (cm�1) � 17,508 � (800/n2). Taking � � 10, we find � 0.7 mfrom the coefficient of 1/n2. The constant term 17,508 cm�1 corresponds to anenergy gap Eg � 2.17 eV.

Exciton Condensation into Electron-Hole Drops (EHD)

A condensed phase of an electron-hole plasma forms in Ge and Si whenmaintained at a low temperature and irradiated by light. The following sequenceof events takes place when an electron-hole drop (EHD) is formed in Ge: Theabsorption of a photon of energy �� � Eg produces a free electron and free hole,with high efficiency. These combine rapidly, perhaps in 1 ns, to form an exciton.The exciton may decay with annihilation of the e-h pair with a lifetime of 8 �s.

If the exciton concentration is sufficiently high—over 1013 cm�3 at 2 K—most of the excitons will condense into a drop. The drop lifetime is 40 �s, butin strained Ge may be as long as 600 �s. Within the drop the excitons dissolveinto a degenerate Fermi gas of electrons and holes, with metallic properties:this state was predicted by L. V. Keldysh.

1� �

1me

�

1mh

,

En � Eg

�

�e4

2�2�2n2 .

U(r) � �e2/�r ,

15 Optical Processes and Excitons 441

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Figure 11 shows the recombination radiation in Ge from free excitons (714 meV) and from the EHD phase (709 meV). The width of the 714 meVline is accounted for by Doppler broadening, and the width of the 709 meVline is compatible with the kinetic energy distribution of electrons and holes ina Fermi gas of concentration 2 1017 cm�3. Figure 12 is a photograph of alarge EHD.

442

0

–1

–2

–3

17,100 17,200 17,300 17,400Photon energy in cm–1

ln (T

rans

mis

sion

)

2.12 2.13 2.14 2.15 2.16Photon energy in eV

n = 5n = 4n = 3

n = 2

Figure 10 Logarithm of the optical transmission versus photon energy in cuprous oxide at 77 K,showing a series of exciton lines. Note that on the vertical axis the logarithm is plotted decreasingupward; thus a peak corresponds to absorption. The band gap Eg is 2.17 eV. (After P. W. Baumeister.)

704 706 708 710 712 714 716

Inte

nsity

�, in meV

�F s

EHDFE

Figure 11 Recombination radiation of free electrons with holes and of electron-hole drops in Geat 3.04 K. The Fermi energy in the drop is �F and the cohesive energy of the drop with respect to afree exciton is �s. (After T. K. Lo.)

ch15.qxd 8/31/04 2:51 PM Page 442

15 Optical Processes and Excitons 443

Figure 12 Photograph of an electron-hole drop in a 4 mm disk of pure germanium. The drop isthe intense spot adjacent to the set screw on the left of the disk. The photograph is the image ofthe drop obtained by focusing its electron-hole recombination luminescence onto the surface ofan infrared-sensitive camera. (After J. P. Wolfe et al.)

0

10

20

30

40

50

60

1014 1015 1016 1017 1018 1019

Average concentration, all phases, pairs/cm3

Tem

pera

ture

, K

Free exciton gas

Mott criterion

Gas of unpaired electrons and holes

Gas-liquid coexistence

Liquid

Figure 13 Phase diagram for photoexcited electrons and holes in unstressed silicon. The diagramshows, for example, that with an average concentration near 1017 cm�3 at 15 K, a free-exciton gaswith saturated-gas concentration of 1016 cm�3 coexists with a (variable) volume of liquid droplets,each with a density of 3 1018 cm�3. The liquid critical temperature is about 23 K. Theoretical andexperimental values for the metal-insulator transition for excitons are also shown. (From J. P. Wolfe.)

ch15.qxd 8/31/04 2:51 PM Page 443

The exciton phase diagram for silicon is plotted in the temperature-concentration plane in Fig. 13. The exciton gas is insulating at low pressures.At high pressures (at the right of the diagram) the exciton gas breaks up into aconducting plasma of unpaired electrons and holes. The transition from exci-tons to the plasma is an example of the Mott transition, Chapter 14. Furtherdata are given in Table 2.

RAMAN EFFECT IN CRYSTALS

Raman scattering involves two photons—one in, one out—and is one stepmore complex than the one photon processes treated earlier in this chapter. Inthe Raman effect a photon is scattered inelastically by a crystal, with creationor annihilation of a phonon or magnon (Fig. 14). The process is identical to theinelastic scattering of x-rays and it is similar to the inelastic scattering ofneutrons by a crystal.

The selection rules for the first-order Raman effect are

(34)

where �, k refer to the incident photon; ��, k� refer to the scattered photon;and �, K refer to the phonon created or destroyed in the scattering event. Inthe second-order Raman effect, two phonons are involved in the inelasticscattering of the photon.

The Raman effect is made possible by the strain-dependence of the elec-tronic polarizability. To show this, we suppose that the polarizability � associ-ated with a phonon mode may be written as a power series in the phononamplitude u:

(35)

If u(t) � u0 cos �t and the incident electric field is E(t) � E0 cos �t, then theinduced electric dipole moment has a component

(36)�1E0u0 cos �t cos �t � 12�1E0u0[cos(� � �)t � cos(� � �)t] .

� � �0 � �1u � �2u2 � Á .

� � ��� � ; k � k� � K ,

444

Table 2 Electron-hole liquid parameters

Crystal Binding energy relative Concentration, Critical (unstressed) to free exciton, in meV n or p, in cm�3 temperature in K

Ge 1.8 2.57 1017 6.7Si 9.3 3.5 1018 23.GaP 17.5 8.6 1018 45.3C-SiC 17. 10. 1018 41.

Courtesy of D. Bimberg.

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Thus photons at frequencies � � � and � � � can be emitted, accompaniedby absorption or emission of a phonon of frequency �.

The photon at � � � is called the Stokes line and that at � � � is theanti-Stokes line. The intensity of the Stokes line involves the matrix elementfor phonon creation, which is just the matrix element for the harmonic oscilla-tor, as in Appendix C:

(37)

where nK is the initial population of phonon mode K.The anti-Stokes line involves phonon annihilation, with a photon intensity

proportional to

(38)

If the phonon population is initially in thermal equilibrium at temperatureT, the intensity ratio of the two lines is

(39)

with given by the Planck distribution function 1/[exp(��/kBT) � 1]. Wesee that the relative intensity of the anti-Stokes lines vanishes as T → 0, be-cause here there are no thermal phonons available to be annihilated.

Observations on the K � 0 optical phonon in silicon are shown in Figs. 15and 16. Silicon has two identical atoms in the primitive cell, and there is noelectric dipole moment associated with the primitive cell in the absence ofdeformation by phonons. But �1u does not vanish for silicon at K � 0, so thatwe can observe the mode by first-order Raman scattering of light.

The second-order Raman effect arises from the term �2u2 in the polariz-

ability. Inelastic scattering of light in this order is accompanied by the creationof two phonons, or the absorption of two phonons, or the creation of one andthe absorption of another phonon. The phonons may have different frequen-cies. The intensity distribution in the scattered photon spectrum may be quitecomplicated if there are several atoms in the primitive cell because of the

nK�

I(� � �)I(� � �)

� nK�

nK� � 1 � exp(���/kBT) ,

I(� � �) � �nK � 1 �u �nK� �2 � nK .

I(� � �) � �nK � 1 �u �nK� �2 � nK � 1 ,

15 Optical Processes and Excitons 445

�, k

�′, k′

�, K �, k

�′, k′

�, K

Stokes Anti-Stokes

Figure 14 Raman scattering of aphoton with emission or absorp-tion of a phonon. Similar processesoccur with magnons (spin waves).

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446

Siliconsample

0

1

2

3

4

–520 –500 +500 +520Raman shift, cm–1

Inte

nsity

20 K

20 K

Stokes460 K

460 K

770 K

770 K

Resolution

Anti-Stokes

(111)

LensLaserbeam

10–3

10–2

10–1

1

200 400 600 800Temperature, in K

Rel

ativ

e in

tens

ity, I

A/I

S

Figure 15 First-order Raman spectra of the K 0optical mode of a silicon crystal observed at threetemperatures. The incident photon has a wavelengthof 5145 Å. The optical phonon frequency is equal tothe frequency shift; it depends slightly on the temper-ature. (After T. R. Hart, R. L. Aggarwal, and B. Lax.)

Figure 16 Intensity ratio of anti-Stokes to Stokeslines as a function of temperature, for the obser-vations of Fig. 15 on the optical mode of silicon.The observed temperature dependence is in goodagreement with the prediction of Eq. (39): thesolid curve is a plot of the function exp(���/kBT).

–800 –700 –600 –500 –400 –300 –200 –100 0Raman shift, cm–1

Inte

nsity

Figure 17 Raman spectrum of GaP at 20 K. The two highest peaks are the first-order Ramanlines associated with the excitation of an LO phonon at 404 cm�1 and a TO phonon at 366 cm�1.All the other peaks involve two phonons. (After M. V. Hobden and J. P. Russell.)

ch15.qxd 8/31/04 2:51 PM Page 446

corresponding number of optical phonon modes. Second-order Raman spectrahave been observed and analyzed in numerous crystals. Measurements ongallium phosphide (GaP) are shown in Fig. 17.

Electron Spectroscopy with X-Rays

The next degree of complexity in optical processes involves a photon inand an electron out of the solid, as in Fig. 1. Two important techniques are x-ray photoemission from solids (XPS) and ultraviolet photoemission (UPS). Insolid state physics they are used in band structure studies and surface physics,including catalysis and adsorption.

XPS and UPS spectra can be compared directly with valence band densi-ties of states D(�). The specimen is irradiated with highly monochromatic x-rays or ultraviolet photons. The photon is absorbed, with the emission of aphotoelectron whose kinetic energy is equal to the energy of the incident pho-ton minus the binding energy of the electron in the solid. The electrons comefrom a thin layer near the surface, typically 50 Å in depth. The resolution ofthe best XPS spectrometer systems is less than 10 meV, which permits refinedstudies of band structure.

The valence band structure of silver is shown by Fig. 18, with the zero ofenergy set at the Fermi level. Electrons in the first 3 eV below the Fermi levelcome from the 5s conduction band. The strong peak with structure below 3 eVis from the 4d valence electrons.

Excitations are also seen from deeper levels, often accompanied by excita-tion of plasmons. For example, in silicon the 2p electron with a binding energyclose to 99.2 eV is observed in replica at 117 eV with single plasmon excitationand at 134.7 eV with two plasmon excitation. The plasmon energy is 18 eV.

15 Optical Processes and Excitons 447

0

10

20

30

40

50

9 5 0Binding energy, eV

Cou

nts

per

chan

nel,

thou

sand

s

Figure 18 Valence-band electron emission fromsilver, after Siegbahn and co-workers.

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ENERGY LOSS OF FAST PARTICLES IN A SOLID

So far we have used photons as probes of the electronic structure of solids.We can also use electron beams for the same purpose. The results also involvethe dielectric function, now through the imaginary part of 1/�(�). The dielec-tric function enters as Im{�(�)} into the energy loss by an electromagneticwave in a solid, but as �Im{1/�(�)} into the energy loss by a charged particlethat penetrates a solid.

Consider this difference. The general result from electromagnetic theoryfor the power dissipation density by dielectric losses is

(CGS) (40)

per unit volume. With a transverse electromagnetic wave Ee�i�t in the crystal,we have dD/dt � �i� �(�)Ee�i�t, whence the time-average power is

(41)

proportional to ���(�). The tangential component of E is continuous across theboundary of the solid.

If a particle of charge e and velocity v enters a crystal, the dielectric dis-placement is, by standard texts,

(CGS) (42)

because by the Poisson equation it is D, and not E, that is related to the freecharge. In an isotropic medium the Fourier component E(�,k) is related tothe Fourier component D(�,k) of D(r,t) by E(�,k) � D(�,k)/�(�,k).

The time-average power dissipation associated with this Fourier compo-nent is

whence

(43)�(�,k) � � 18�

� 1��� D2(�,k) �

18�

�

� �(�,k)��� 2 D

2(�,k) .

� 14�

� D2(�,k)�� 1���

cos �t � 1���

sin �t�[�sin �t]� ,

�(�,k) � 14�

Re{��1(�,k)D(�,k)e�i�t}Re{�i� D(�,k) e�i�t}�

D(r,t) � �grad e�r � vt �

,

� 14�

�E2(� � cos �t � ��sin �t)cos �t� � 18�

�� �(�)E2 ,

� � 14�

Re{Ee�i�t}Re{�i��(�)Ee�i�t}�

� � 14�

E � (�D/�t) ,

448

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The result is the motivation for the introduction of the energy loss function�Im{1/�(�,k)} and it is also a motivation for experiments on energy losses byfast electrons in thin films.

If the dielectric function is independent of k, the power loss is

(44)

where �k0 is the maximum possible momentum transfer from the primary par-ticle to an electron of the crystal. Figure 19 shows the excellent experimentalagreement between values of ���(�) deduced from optical reflectivity measure-ments with values deduced from electron energy loss measurements.

SUMMARY

• The Kramers-Kronig relations connect the real and imaginary parts of aresponse function:

• The complex refractive index N(�) � n(�) � iK(�), where n is the refrac-tive index and K is the extinction coefficient; further, �(�) � N2(�), whence��(�) � n2 � K2 and ���(�) � 2nK.

• The reflectance at normal incidence is

R � (n � 1)2 � K

2

(n � 1)2 � K

2 .

��(�) �

2� P ��

0

s��(s)s2

� �2 ds ; ��(�) � �2�� P ��

0

��(s)s2

� �2 ds .

�(�) � �2� e2

�v Im{1/�(�)}ln(k0v/�) ,

15 Optical Processes and Excitons 449

0

1

2

3

4

5

6

7

0 10 20 30eV

B

Cu

�"

0

1

2

3

4

5

6

7

0 10 20 30eV

B

C

Au

�"

Figure 19 ���(�) for Cu and Au; the bold lines are from energy loss measurements by J. Daniels, andthe other lines were calculated from optical measurements by D. Beaglehole, and L. R. Canfield et al.

ch15.qxd 8/31/04 2:51 PM Page 449

• The energy loss function �Im{1/�(�)} gives the energy loss by a chargedparticle moving in a solid.

Problems

1. Causality and the response function. The Kramers-Kronig relations are consis-tent with the principle that an effect not precede its cause. Consider a delta-function force applied at time t � 0:

whence F� � 1/2�. (a) Show by direct integration or by use of the KK relations thatthe oscillator response function

gives zero displacement, x(t) � 0, for t � 0 under the above force. For t � 0 the contour integral may be completed by a semicircle in the upper half-plane. (b) Evaluate x(t) for t � 0. Note that �(�) has poles at �(�2

0 � �2)1/2 � i�, both in the lower half-plane.

2. Dissipation sum rule. By comparison of ��(�) from (9) and from (11a) in the limit� → �, show that the following sum rule for the oscillator strengths must hold:

3. Reflection at normal incidence. Consider an electromagnetic wave in vacuum,with field components of the form

Let the wave be incident upon a medium of dielectric constant � and permeability� � 1 that fills the half-space x � 0. Show that the reflectivity coefficient r(�) asdefined by E(refl) � r(�)E(inc) is given by

where n � iK � �1/2, with n and K real. Show further that the reflectance is

*4. Conductivity sum rule and superconductivity. We write the electrical con-ductivity as �(�) � ��(�) � i���(�), where ��, ��� are real. (a) Show by a Kramers-Kronig relation that

(45)lim�

l�

� ��(�) � 2� ��

0 ��(s) ds .

R(�) � (n �1)2 � K

2

(n �1)2 � K

2 .

r(�) � n � iK � 1n � iK �1

,

Ey(inc) � Bz(inc) � Aei(kx��t) .

� fj � 2� ��

0 s��(s) ds

.

12

14

�(�) � (�20 � �2 � i��)�1

F(t) � (t) � 12�

��

��

e�i�t d� ,

450

*This problem is somewhat difficult.

ch15.qxd 8/31/04 2:51 PM Page 450

This result is used in the theory of superconductivity. If at very high frequencies(such as x-ray frequencies) ���(�) is identical for the superconducting and normalstates, then we must have

But at frequencies 0 � � � �g within the superconducting energy gap the real partof the conductivity of a superconductor vanishes, so that in this region the integralon the left-hand side is lower by ��n�g. There must be an additional contributionto ��s to balance this deficiency. (b) Show that if ��s(� � �g) � ��n(� � �g), as is ob-served experimentally, then ��s(�) can have a delta function contribution at � � 0,and from the delta function there is a contribution ���s (�) ��n �g/�. The deltafunction corresponds to infinite conductivity at zero frequency. (c) By elementaryconsideration of the classical motion of conduction electrons at very high frequen-cies, show that

(CGS)

a result found by Ferrell and Glover.

5. Dielectric constant and the semiconductor energy gap. The effect on ���(�) of an energy gap �g in a semiconductor may be approximated very roughly by substituting (� � �g) for (�) in the response function (16); that is, we take���(�) � (2�ne2/m�)�(� � �g). This is crude because it puts all the absorption atthe gap frequency. The factor 1/2 enters as soon as we move the delta functionaway from the origin, because the integral in the sum rule of Problem 2 starts at theorigin. Show that the real part of the dielectric constant on this model is

It follows that the static dielectric constant is ��(0) � 1 � �2p/�2

g, widely used as arule of thumb.

6. Hagen-Rubens relation for infrared reflectivity of metals. The complex refrac-tive index n � iK of a metal for �� 1 is given by

(CGS)

where �0 is the conductivity for static fields. We assume here that intrabandcurrents are dominant; interband transitions are neglected. Using the result ofProblem 3 for the reflection coefficient at normal incidence, show that

(CGS)

provided that �0 � �. This is the Hagen-Rubens relation. For sodium at room temperature, �0 2.1 1017 s�1 in CGS and � � 3.1 10�14 s, as deduced from

R 1 �(2�/��0)1/2 ,

�(�) � (n � iK)2 � 1 � 4�i�0

/� ,

��(�) � 1 � �2p

/(�2g � �2) , �2

p � 4�ne2/m .

12

��

0 ��(�) d� � �ne2/2m ,

�

�

��

0��s (�) dw � ��

0 ��n(�) d� .

15 Optical Processes and Excitons 451

ch15.qxd 8/31/04 2:51 PM Page 451

� � �0m/ne2. Radiation of 10 �m has � � 1.88 1014 s�1, so that the Hagen-Rubens result should apply: R � 0.976. The result calculated from experimental val-ues of n and K is 0.987. Hint: If �0 � �, then n2 K2. This simplifies the algebra.

*7. Davydov splitting of exciton lines. The Frenkel exciton band of Fig. 9 is doubledwhen there are two atoms A, B in a primitive cell. Extend the theory of Eqs. (25) to(29) to a linear crystal AB.AB.AB.AB. with transfer integrals T1 between AB and T2

between B.A. Find an equation for the two bands as functions of the wavevector.The splitting between the bands at k � 0 is called the Davydov splitting.

452

*This problem is somewhat difficult.

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