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3 Light-Emitting Diodes: There is Life Outside of Silicon!

3 Light-Emitting Diodes: There is Life Outside of Silicon!

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322



4 Semiconductors



Figure 4.75. Illustration of dip-pen nanolithography, used to write nanoscale features of CdS on mica and

SiOx substrates. Reproduced with permission from Ding, L.; Li, Y.; Chu, H.; Li, X.; Liu, J. J. Phys. Chem.

B 2005, 109, 22337. Copyright 2005 American Chemical Society.

Table 4.5. Summary of the Ink-Substrate Combinations Used to Date for DPN[82]

Molecular ink

Substrate

Alkylthiols (e.g., ODTa and MHAb)

Au

Ferrocenylthiols

Au

Silazanes

SiOx, GaAs

Proteins

Au, SiOx

Conjugated polymers

SiOx

DNA

Au, SiOx

Fluorescent dyes

SiOx

Sols

SiOx

Metal salts

Si, Ge

Colloidal particles

SiOx

Alkynes

Si

Alkoxysilanes

SiOx

ROMP materials

SiOx

Thioacetamide/cadmium acetatec

SiOx, mica

a

1-Octadecanethiol.

b

16-Mercaptohexadecanoic acid, or thiohexadecanoic acid.

c

Ding, L.; Li, Y.; Chu, H.; Li, X.; Liu, J. J. Phys. Chem. B 2005, 109, 22337.



layer analogous to SiO2 onto Si, CVD is used to grow films such as GaS for surface

passivation. Many other direct bandgap semiconductors such as II–VI (e.g., ZnS,

ZnSe) and III–V (e.g., GaN, GaP) are widely used for optoelectronic (light emission)

and photonic (light detection) applications. The most important applications for nonSi semiconductors are light-emitting diodes (LEDs) and solid-state lasers. Unlike Sibased devices, the bandgap of compound semiconductors may be significantly

altered by varying the stoichiometry of the composite elements.



4.3. Light-Emitting Diodes: There is Life Outside of Silicon!



323



Figure 4.76. Band diagram of a p–n junction with a) no bias voltage, and b) with forward bias, V,

resulting in photon emission. Reproduced with permission from Kasap, S. O. Principles of Electronic

Materials and Devices, 3rd ed., McGraw-Hill: New York, 2007. Copyright 2007 The McGraw-Hill

Companies.



A light-emitting diode is a p–n junction device that is comprised of a direct

bandgap semiconductor(s). The recombination of an electron-hole pair (EHP)

results in photon emission, with energy equivalent to the bandgap, Eg (Figure 4.76).

The LED may be structured as a simple multilayer (Figure 4.77a), or as a heterojunction that is comprised of two different bandgap semiconductors (Figure 4.77b).

The latter is preferred for high-intensity LED applications, since the emission is

confined to certain regions of the device.

At present, the applications for LEDs have been centered on commercial electronic displays such as clock radios, microwave ovens, watches, etc. However, the

availability of LEDs in a spectrum of colors has opened the floodgates for new

applications. The greatest breakthrough was realized in the 1990s with the discovery

of wide bandgap blue LEDs, making it possible to create any color of light

(Table 4.6). Approximately 10–15% of the traffic lights in the United States have

now been replaced with LED-based lamps. New automobiles also utilize this

lighting for ultrabright brake and turn-signal lighting. The higher initial cost of the

LEDs is quickly recovered due to their greater efficiency in converting electrical

current to light emission relative to incandescent lighting. That is, whereas LEDs

consume 20–1,000 mW, an incandescent bulb of similar brightness consumes ca.

50–150 W. Indeed, once an inexpensive white LED becomes commercially available, our society will forever change as we shift from our longstanding reliance on

inefficient and short-lived incandescent bulbs. The light emission from LEDs is

classified as a type of luminescence, which is different from incandescence – the

generation of light from a material as a result of its high temperature. Since LEDs

glow as a result of an electrical current, this emission is referred to as electroluminescence. Two other common types of luminescence include chemoluminescence



4 Semiconductors



324



Figure 4.77. Light-emitting diode (LED) structures. Shown are (a) a simple multilayer structure of

epitaxial p- and n-type layers, and (b) a double heterostructure with accompanying band diagram,

illustrating light emission from the p-type region due to confinement between wide-bandgap

surrounding layers.

Table 4.6. Comparison of the Observed Colors of LEDs

Observed color

Wavelength of

Semiconductor

emission (nm)

(Infrared)

Red

Red

Orange

Orange

Yellow

Green

Blue

Ultraviolet

White



880

660

633

612

605

585

555

470

395





GaAlAs/GaAs

GaAlAs/GaAlAs

AlGaInP

AlGaInP

GaAsP/GaP

GaAsP/GaP

GaP

GaN/SiC

InGaN/SiC

InGaN/SiC



(induced by a chemical reaction(s); e.g., “glow sticks”), and photoluminescence

(induced through photon excitation; e.g., vaseline glass discussed in Chapter 2).

If the emission is prolonged, lasting long after the stimulation source is removed,

it is known as phosphorescence; otherwise, the short-lived process is termed

fluorescence.



4.3. Light-Emitting Diodes: There is Life Outside of Silicon!



325



Table 4.7. Bandgaps of III–V Semiconductors

Semiconductor

Bandgap (eV)

AlN

AlP

GaN

GaP

GaAs

GaSb

InN

InP

InAs



6.02

2.45

3.50

2.35

1.35

0.67

1.95

1.27

0.36



As we saw earlier in this chapter, the wavelength (and color) of light emitted by a

direct bandgap material through electron-hole recombination is influenced by its

bandgap. In order to change the wavelength of emitted radiation, the bandgap

of the semiconducting material utilized to fabricate the LED must be changed.

For instance, gallium arsenide has a bandgap of 1.35 eV (Table 4.7), and emits in

the infrared (ca. 900 nm). In order to decrease the wavelength of emission into the

visible red region (ca. 700 nm), the bandgap must be increased to ca. 1.9 eV. This

may be achieved by mixing GaAs with a material with a larger bandgap, such as GaP

(Eg ¼ 2.35 eV). Hence, LEDs of the chemical composition GaAsxP1Àx may be used

to produce bandgaps from 1.4 to 2.3 eV (and varying colors), through adjustment of

the As:P ratio.

The bandgap and concomitant wavelength of light that is emitted from LEDs is

related to the bond strength between atoms in the lattice. For these compounds, as

the bond strength increases, there is more efficient overlap between molecular

orbitals that gives rise to a larger bandgap between bonding and antibonding MOs

(i.e., valence and conduction bands of the infinite lattice, respectively). For a

particular Group 13 metal, as one moves down the Group 15 Period, the bonding

interaction between III–V elements will become weaker through the interaction of

more diffuse atomic orbitals. For instance, the bond strengths of Ga–N and Ga–As

bonds are 98.8 and 50.1 kcal molÀ1, respectively. The larger bandgap for GaN

relative to GaAs translates to a short wavelength (blue color) of emitted light that is

observed.

Most white LEDs employ a semiconductor chip emitting at a short wavelength

(blue), and a wavelength converter that absorbs light from the diode and undergoes

secondary emission at a longer wavelength. Such diodes emit light of two or more

wavelengths, that when combined, appear as white. The most common wavelength

converter materials are termed phosphors (e.g., ZnS – Figure 4.55), which exhibit

luminescence when they absorb energy from another radiation source. Typical LED

phosphors are present as a coating on the outside of the bulb, and are composed of an

inorganic host substance (e.g., yttrium aluminum garnet, YAG) containing an

optically active dopant (e.g., Ce). Use of such a single-crystal phosphor produces

a yellow light, upon combination with blue light gives the appearance of white.

A similar result has recently been produced through use of CdSe nanoparticles (see



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4 Semiconductors



Chapter 6).[83] White LEDs may also be made by coating near ultraviolet (NUV)

emitting LEDs with a mixture of europium-based red and blue emitting phosphors,

plus green emitting copper- and aluminium-doped zinc sulfide (ZnS:Cu,Al). It is

also possible for LEDs to emit white light without the use of phosphors. For

instance, homoepitaxially grown ZnSe crystals simultaneously emit blue light

from the film, and yellow light from the ZnSe substrate.

Recently, there has been much interest in organic light-emitting diodes (OLEDs).

Flat-panel televisions, cellular phones, and digital cameras are already beginning to

employ this technology; it is only a matter of time before the “holy grail” of flexible

display screens and luminous fabrics are produced. OLED displays offer many

benefits relative to standard CRTs and LCDs such as enhanced brightness, lower

power consumption, and wider viewing angles.

The multilayered structure and electroluminescent mechanism of OLEDs is illustrated in Figure 4.78. Depending on whether small organic molecules or long



Figure 4.78. Multilayered structure of OLEDs/PLEDs. Also shown are the relative energy levels for

individual layers; light is emitted as a result of the radiative recombination of electron-hole pairs.



4.3. Light-Emitting Diodes: There is Life Outside of Silicon!



327



Figure 4.79. Molecular structures of commonly used OLED/PLED materials. Shown are: (a) Alq3 (tris

(quinoxalinato)Al (III)) used as an electron-transport material; (b) DIQA (diisoamylquinacridone) used as

an emissive dopant; (c) BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) used as an exciton/hole

blocking agent; (d) NPB (1,4-bis(1-napthylphenyl amino)biphenyl); (e) PFO (9,9-dioctylfluorene) used as

an emissive polymer in PLEDs; (f) PEDOT–PSS (poly-3,4-ethylenedioxythiophene–polystyrene

sulfonate) used as a hole transport material in PLEDs.



repeating-unit polymers are used (Figure 4.79), the diodes are referred to as OLEDs

or PLEDs, respectively. Under positive current, electrons and holes are injected into

the emissive layer from opposite directions – from the cathode and anode, respectively. The metal cathode is usually an alkaline earth or Al, which readily release a



328



4 Semiconductors



valence electron (i.e., possess a low work function). The holes that migrate from the

anode are blocked from further transport to the cathode by an organic layer of 2,9dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, Figure 4.79c), which has a relatively low-lying HOMO.

In contrast, the anode is usually tin-doped In2O3 (ITO, typically 1:9 Sn:In), since

this material is transparent and highly conductive (ca. 1 Â 104 WÀ1 cmÀ1).[84]

The substitutional replacement of In3+ ions with Sn4+ ions results in n-doping of

the lattice, injecting electrons into the conduction band. The doping of Sn into the

In2O3 lattice may also result in SnO sites; this introduces holes in the lattice that

reduces its conductivity. The conductivity of ITO is due to both Sn dopants and O

vacancies, as represented by the general formula In2ÀxSnxO3À2x.[85] Since the

organic hole-transport polymer is in direct contact with ITO in an OLED, the surface

properties of ITO is important for effective hole injection. It has been shown that

surface treatments such as UV ozone cleaning or Ar/O2 plasma treatments, result in

an increase in the work function of the ITO surface as the Sn:In ratio is decreased

(and O concentration is increased).

The recombination of a hole and electron creates a quasiparticle known as an

exciton, which releases a photon of energy. Organic molecules in the emissive layer

facilitate exciton formation – likely through hole-trapping, followed by Coulombic

attraction to the free electrons. A challenge in OLED design is to ensure that an

equal number of holes and electrons meet in the emissive layer. This is not trivial,

since holes migrate much slower than electrons in conductive organic molecules.

The organic molecules used in OLEDs/PLEDs are p-conjugated, meaning that pz

orbitals on adjacent (–C¼C–C¼)n units overlap, resulting in p valence and p*

conduction bands. The observed color will depend on the HOMO–LUMO gap of

the molecule, which may be fine-tuned by altering the length of the conjugation,

or nature of the molecular backbone (e.g., substitution of electron withdrawing/

donating groups, etc.).

A limitation of traditional OLEDs/PLEDs is their relatively poor quantum efficiency. An exciton may exist in either a single t (total spin, S ¼ 0) or triple t state

(S ¼ 1), with the triplet outweighing the singlet by a 3:1 ratio. The molecules in the

emissive layer are referred to as fluorophores, which yield fluorescent radiation only

when a singlet exciton is formed. Hence, the theoretical maximum efficiency that is

possible is only 25%. In an effort to improve the efficiency, a number of recent

studies have been devoted to using organometallic compounds (transition metals

with organic ligands), which allow for rapid intersystem crossing (ISC) from the

excited singlet to light-emitting triplet states (Figure 4.80). The process of collecting

the excitons in the lowest excited triplet state is referred to as triplet harvesting,

and results in a 100% quantum efficiency of the OLED.[86] This effect is most

pronounced for complexes of 4d and 5d transition metals with well-shielded valence

electrons, which exhibit large spin–orbit coupling[87] and concomitant mixing of

singlet and triplet states.[88]



4.3. Light-Emitting Diodes: There is Life Outside of Silicon!



329



Figure 4.80. Illustration of triplet harvesting. In the absence of a triplet emitter, the triplet excitation

energy is converted to heat, losing 75% of the quantum efficiency. Also shown are a variety of

organometallic complexes, and their relative spin–orbit coupling values – directly proportional to their

use in phosphorescent OLEDs. Reproduced with permission from Yersin, H. Top. Curr. Chem. 2004, 241,

1. Copyright 2004 Springer Science and Business Media.



330



4 Semiconductors



4.4. THERMOELECTRIC (TE) MATERIALS

As our society attempts to wean itself from a dependence on fossil fuels, many

alternative energy sources are being investigated. One interesting potential is from

the conversion of waste heat (e.g., vehicle engine heat) into electricity. This conversion is due to the thermoelectric effect, first discovered for junctions (or thermocouple) of two dissimilar metals. When metals are welded together, free electrons are

able to drift across the junction in a preferential direction, based on the different

atomic lattice structures of each metal. The migration of electrons effectively leaves

one metal electron-deficient (positively charged), and the other metal negatively

charged.

If the two metals are held at different temperatures, a voltage will result that is

proportional to the DT. The ratio (DV / DT) is referred to as the Seebeck coefficient,

a, related to the band structure of the materials involved. The a value for semiconductors is at least two orders of magnitude larger for semiconductors relative to

metals, giving rise to much greater voltages. Contrary to the Seebeck effect, when a

current is passed through a closed circuit of the two metals, heat is produced at one

of the junctions and is absorbed at the other. Since the latter junction is cooled, this

Peltier effect has been largely exploited for thermoelectric cooling applications

(e.g., auto seat coolers, computer component cooling).

A TE device consists of a heat source and sink, joined together via n-type and

p-type semiconductor materials (Figure 4.81).[89] The figure of merit, ZT (Eq. 22), of

the device is useful to determine its suitability for power generation or refrigeration –

a means to describe the transport properties of the TE material. An effective TE



Figure 4.81. Schematic of a thermoelectric device, which may be used for power generation or

refrigeration.



4.4. Thermoelectric (TE) Materials



331



material should have a high Seebeck coefficient (heat conversion efficiency) and

electrical conductivity, as well as low thermal conductivity to maintain thermal

isolation in the device. The thermal conductivity is related to the transfer of heat

through a material through either electron transport or quantized lattice vibrations

(phonons). Hence, the ideal TE material has been described as a phonon-glass/

electron-crystal, having the electrical properties of a crystalline lattice, and thermal

properties of an amorphous/glass-like solid.

ð22Þ



ZT =



a2 s

l



where a is the Seebeck coefficient (mVKÀ1; 1–10 for metals, 150–250 for

semiconductors); s is the electrical conductivity; and l is the thermal conductivity

(electronic + lattice terms).

Semiconductors are much more effective TE materials (greater ZT values) than

metals due to their significantly greater Seebeck coefficients and lower thermal

conductivities. A barrier toward higher ZT values for semiconductors is their

relatively small electrical conductivity, especially at low temperatures. In order

to overcome this limitation, the chemical composition of the semiconductor may

be fine-tuned to yield a small bandgap material (!6 kBT; kB ¼ Boltzmann cst;

i.e., !0.16 eV at 300 K), or one that is sufficiently doped with an intermediate

concentration (ca. 1019–1021 cm3) of electronic/thermal carriers exhibiting high

mobility through the lattice.

The most widely studied TE material is Bi2Te3, consisting of a hexagonal unit

cell with repeating. . .[Te–Bi–Te–Bi–Te. . . Te–Bi–Te–Bi–Te]. . . units (Figure 4.82).

Whereas Te–Bi layers are bound by strong covalent interactions, the bonding

between adjacent Te layers is through weak van der Waals interactions. This results

in bulk anisotropic electrical and thermal conductivity, being most pronounced

along planes that are perpendicular to the c-axis of the unit cell. To further improve

ZT values, Bi2Te3 crystals may be doped with n- or p-type dopants, with most

desired compositions of Bi2Te2.7Se0.3 (n-type) and Bi0.5Sb1.5Te3 (p-type), yielding

ZT ¼ 1 at room temperature. This improvement results from a decrease in thermal

conductivity of the lattice, brought about by phonon scattering by the dopant atoms

(i.e., perturbing the symmetry of the lattice, affecting the organized lattice vibration

modes).

The following list represents the primary materials classes that have been

designed in an effort to optimize ZT for thermoelectric applications.[90] The general

strategy is to dope the lattice with sufficient carriers (n- or p-dopants), while also

interrupting the phonon transport through the solid through the introduction of large

interstitial atoms.

(i) Complex Solid-State Inorganic Lattices (e.g., CsBi4Te6, Bi2ÀxSbxTe3ÀySey,

ZrTe5, AgnPbmMnTem+2n (M ¼ Sb, Bi), AgPb10SbTe12, “Half-Heusler alloys”

MNiSn (M ¼ Zr, Hf, Ti), Zr0.5Hf0.5Ni0.5Pd0.5Sn0.99Sb0.01). For half-Heusler

alloys, the unit cell is a combination of a NaCl lattice of two metals, with the

third metal occupying tetrahedral interstitial sites. For instance, for TiNiSn,



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4 Semiconductors



Figure 4.82. Unit cell of Bi2Te3 – a widely studied thermoelectric material. The blue atoms are Bi, and

the pink atoms are Te. Reproduced with permission from Tritt, T. M.; Subramanian, M. A. MRS Bull.

2006, 31, 188. Copyright 2006 Materials Research Society.



the Ti and Sn form the NaCl lattice, with Ni occupying 1/2 of the available

tetrahedral interstitial sites (i.e., Ti:Ni:Sn ¼ 4:4:4 atoms per unit cell). This

combination offers a great deal of control over electronic/thermal conductivity

of the solid. While the introduction of Sn (p-doping) increases electrical conductivity, heavy metal atoms such as Ti and Ni cause a decrease in thermal

conductivity through phonon scattering.

(ii) Crystal Structures with “Rattlers” (e.g., Rare-earth (e.g., La, Ce, Nd, Sm, Eu,

etc.) doped AB3-based (A ¼ Co, Fe, Ru, Os, Ir; B ¼ P, As, Sb), Figure 4.83),[91]

or complex antimonide skutterudites (e.g., b-Zn4Sb3, Yb14MnSb11,

AyMo3Sb7ÀxTex),[92] or clathrates (e.g., Ba8Ga16E30, E ¼ Ge, Si)(see endnote

89). In these structures, dopant atoms are weakly bound to the cage, and “rattle”

in response to increasing temperature. As the atom within the cages becomes

smaller/heavier, the amount of structural disorder will increase causing a larger

decrease in lattice thermal conductivity.

(iii) Oxides (e.g., NaCo2O4, Ca3Co4O9,[93] Al0.02Zn0.98O, b-SrRh2O4[94]). These

structures consist of CoOx layers, which serve as effective electronic transport



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