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Wide Band Gap Electronic Materials

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Advanced Science Institutes Series

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3. High Technology - Vol. 1

Page 261


The logical subsequent development of this trend is to attempt the synthesis of amor-
phous superlattices based on carbon layers. Carbon is the origin of a whole class of ma-
terials, making possible the formation of amorphous superlattices based solely on carbon
compounds, and with these it is hoped to produce MSs with better thermodynamic sta-
bility and minimized diffusion processes between layers. Since the band gap (Eg) of
amorphous carbon films may be varied continuously from 0.4 to 3 eV, a-C:H deposition
conditions have an advantage over a-Si:H films which must be deposited with practically
constant Eg values [4-6]. This makes it possible to use a-C:H films with different band
gaps as layers of superlattices.

2. Experimental Details

Amorphous semiconductor multilayer structures were synthesized from alternating a-
C:H layers with optical band gaps ofEgl = 0.55 eV (a-C:H1) andEgz = 1.5 eV (a-C:Hz).

The carbon layers were obtained by DC magnetron sputtering of graphite target in argon
(a-C:H1) and by plasma-ion beam deposition in C6H1z (a-C:Hz). During deposition, pres-
sure and substrate temperatures were in the range (1-2)*10-3 torr and 330-350 K, respec-
tively. Carbon layers were deposited on fused quartz wafers and silicon substrates. The
deposition growth rates were about 50 A/min for a-C:HI and about 25 A/min for a-C:Hz.
The multilayer structures contained from 5 to 19 layers. The layer thicknesses were var-
ied from 10 A up to 200 A and from 30 A up to 50 A for a-C:H} and a-C:Hzlayers, re-

Properties of carbon layers depend on deposition conditions and thicknesses, and have
been described in previous papers [7].

3. Results and discussion

At the first stage x-ray properties of multilayer carbon structures were measured. Fig. 1
gives the results of reflecting property of the 31 layers structure with different optical
band gap and the thickness of an individual layer of 45 A. It is seen that the reflection
coefficient value for Bragg maximum of the ftrst order is 6% and for diffraction peak of
the third order-O.l%. The peak full with at half maximum of the first order is 0.065°. The
angular position of Bragg reflection peaks correspond exactly the calculated thickness of
MIS period, while absence of the second order diffraction peak shows thickness equality
of the neighboring layers in all structure periods.

Then the influence of layer thickness on the change of Eg a-C:HI layers in superlattices
was investigated.

For this purpose we performed the transmission and reflection spectra of the structures

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20 .-----------------,


R., %

fO t "

5 I .30

0.1 06 f.f

8, de!

Figure 1. X -ray diffraction pattern of multilayer carbon structure with equal thickness of layers in








~ J.SO




Figure 2. Dependence of (<x*E)I12 on E for multilayer structures with thickness of a-C:Hzlayers (5.0

nm) and a-C:H1layers: (1) d = 8.0 nm; (2) d = 3.5 om; (3) d = 2.0 nm; (4) d = 1.5 nm; (5) d =
1.0 om and (6) the separate a-C:Hz films.

Page 521



elastic constants, 335
electrical conductivity, 393
electrochemical reduction, 131
electroluminescence, 251
electron beam-pumped laser, 481
electronic disorder, 211
electronics states, 211
electronics states of defects and impurities, 91
electronic structure, 335
energy conversion, 111
energy conversion efficiency, 463
entropy, 362
epitaxial growth, 329
erbium (Er), 431
excimer, 463
excitation of luminescence, 69
exciton, 2m


field emission, 53
film structure pyrolysis, 305
f1uorescer, 463
fluorine containing plasma, 421
free energy,
surface, 63
boundary, 63

gallium nitride, 453
Gibbs-Curie's principle, 63
glow discharge, 285
growth rate, 321

heat capacity, 362
heat treatment, 305
heha-arnmoniacate boron hydride of magnesium, 313
heteroepitaxy, 305
heterostructures, 111
hexagonal cubic sp3-carbon, 211
high optical gap, 243, 285
high-power semiconductor devices, 453
high pressure. 321
high pressure synthesis. 401
high temperature, 321
high-temperture semiconductor devices,453
hydrogenation, 285
hyperfine interaction, 91
hypothetical compounds, 335


impurities, 69, 91
interfaces, 335
ionicity, 335
ion implantation, 69, 313
ion milling, 53, 225
ion sputtering, 291
isotopic impurity, 81



laser etching, 219
lattice thermal conductivity, 81
lithium. I. 15
lithium nitride, 321
Lorentzian line, 129
luminescence, 181
luminescence center. 391


Madelung potential theory, 481
magnetron sputtering, 235
mesa structure, 421
methane. 41
microstructura pararnagentic center. 91
microwave semiconductor devices, 453
molecular beam eptaxy (MBE), 329
MOMBE, 431

monocristal. 391
multi-quantum well (MQW) structure, 251


n-type doping, 31
n-type semiconductor, 91
negative electron affinity, 53
neutron irradiated laB-type dimond, 89
nitrate, 131
nitrite, 131, 329
nitrogen, I
nitrogen defects in diamond. 89
normal (N) process. 81
nuclear battery, 481
nucleation rate. 321

optical band gap. 251
optical centres, 69
optoelectronic applications, 69
optoelectronic device, 181

Page 522

optoelectronic semiconductor devices, 453
oxidation, 219
oxygen, 15, 115

p-type silicon, 291
paramagnatic defects, 129
patterning, 219
phase diagram, 362
phase formation, 265
phase transition, 313
phonon scattering, 81
phosphorus, 1,31,97
photocapacitance (PC), 161
photoehromic effects, 89
photodetector, 161
photoelectronics, 171
photoluminescence, 431
Photovoltaic Energy conversion of Nuclear energy
System (PENS), 463
photovoltaics, 171, 463, 487
physical properties, 401
plasma soure, 329
polymorphs, 362
polymorphism, 401
polytypes, 335, 401
polycondensation process, 63
positron annihilation, 123,447
prototype molecules, 31
pyrolysis at high pressue, 313


radiation defects, 69
radiation detector, 291
reactive ion etching, 427
rectifying juction, 475
resonant tunneling effect, 257
RF decomposition ofmethane, 235
room-temperature spectral holes, 89

sapphire substrate, 305
Schottky barriers, 171
semiconducting diamond, 69
shallow donor, 97
silicon, 427
silicon carbide, 427, 453
simulation, 97
sintering activation energy, 421
sodium, 1
spectral hole burning, 89
spectroellipsometry, 285


Auger electron spectroscopy (AES), 437
high resolution electron energy ion spectroscopy,

infrared, 105
mass, 105
secondary ion mass spectroscopy (SIMS), 431,

spindensity, 129
Stark energy level, 431
stressed diamond film, 487
structural relaxation process, 249
sulphur, 31
surface, 447
surface chemistry, 105
surface inversion layer, 161
optically activated, 207

synthesiz of cubic boron nitrid, 313

tantalum carbide, 265
Tauc's equation, 257
tetrahedraI carbon films, 271
thermal conductivity, 401
thermodynamic properties, 362
threshold condition, 487
tight-binding theory, 97
treatment pressure, 397
tunabl1ity, 187

ultraviolet phototransformers, 171
universal single-atom (USIA) mechanism, 249
Urbach tails, 271
UV-Vis and IR transmission, 171

wide band gap, 329
wide band gap nitrides, 401
wide band gap semiconductors, 453
wurtzite structure, 475

X-ray diffraction, 329

Young's modulus, 421

zero-phonon line, 397

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