We present experimental evidences of quantum confinement in borosilicate glasses with a new microcrystallite CdTexS1-x semiconductor. The microcrystallite sizes are controlled by the heat-treatment time and temperature. Transmission electron microscopy measurements show the microcrystallites average diameters near 55 angstrom for the sample treated for the longest time. We observe a red shift from 570 to 640 nm in the absorption and photoluminescence spectra as the size increases. These shifts agree with the expected quantum-confined energies, varying from 0.80 to 0.60 eV. The absorption spectra also show a second feature which can be assigned to the second quantum-confined transition.59212715271

ALVES, OL

BARBOSA, LC

CESAR, CL

GALEMBECK, F

NETO, JAM

Applied Physics Letters

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Quantum size effects on CdTe x S1−x semiconductordoped glass
J. A. Medeiros Neto, L. C. Barbosa, C. L. Cesar, O. L. Alves, and F. Galembeck 
 
Citation: Applied Physics Letters 59, 2715 (1991); doi: 10.1063/1.105894 
View online: http://dx.doi.org/10.1063/1.105894 
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/59/21?ver=pdfcov 
Published by the AIP Publishing 
 
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Quantum size effects on CdTe$, --x semiconductor-doped glass 
J. A. Medeiros Neto, L. C. Barbosa, and C. L. Cesar 
Institute de Rsica-Unicamp, P. 0. Box 6165, 13081~Campinas-SP, Bras2 
0. L. Alves &d F. Galembeck 
Institute de Quimica-Unicamp, P. 0. Box 61.54, I308I-Campinas-SP, Bras2 
(Received 14 December 1990; accepted for publication 19 September 1991) 
We present experimental evidences of quantum confinement in borosilicate glasses with a new 
microcrystallite CdTe2i _ X semiconductor. The microcrystallite sizes are controlled by 
the heat-treatment time and temperature. Transmission electron microscopy measurements 
show the microcrystallites average diameters near 55 A for the sample treated for the 
longest time. We observe a red shift from 570 to 640 nm in the absorption and 
photoluminescence spectra as the size increases. These shifts agree with the expected 
quantum-confined energies, varying from 0.80 to 0.60 eV. The absorption spectra also show a 
second feature which can be assigned to the second quantum-confined transition. . 
Quantum-confined effects in semiconductor systems of 
low dimension have attracted considerable attention. Be- 
side the well-known multiple quantum well structures’ 
which provide confinement in one dimension, semiconduc- 
tor microcrystallites with sizes comparable to the exciton 
Bohr radius exhibit three-dimension quantum confine- 
ment. For II-VI semiconductors, the Bohr radii are typi- 
cally in the range of 20-80 A. Most of the experimental 
investigations have been concentrated in materials like 
CdSe$i _ x and CuCl in the form of crystallites grown in 
borosilicate glass hosts or precipitated colloids in 
solution.*-“ 
The quantum confinement strongly modifies the opti- 
cal properties like absorption, photoluminescence, and the 
nonlinear refractive index and opens up interesting prob- 
lems in the physics of low-dimensional systems. Further- 
more, the nonlinear properties of these material&* may 
have important applications in optoelectronics, such as pi- 
cosecond optically bistable switches and fiber or other 
waveguides of highly nonlinear materials.‘-” 
Theoretical and experimental studies show an en- 
hancement in the nonlinear properties due to confinement 
effects in the semiconductor microcrystallites.1”‘4 The 
quantum confinement increases with the ratio between the 
Bohr radius and the crystallite size, and is strongly depen- 
dent on the bulk semiconductor properties. CdTe,S, _ X al- 
loys have larger Bohr radii (74 A) than CdSe,Si _ X (54 
A) alloys. Therefore, it is easier to reach the strong con- 
finement region in the CdTe,Si --x system which makes it 
more promising than CdSe,Si ,.... X for optoelectronics appli- 
cations, as has been pointed out before.14 
In this letter we give experimental evidences of quan- 
tum confinement in CdTe$Si _ ,-doped borosilicate glasses. 
We controlled the microcrystallite sizes with an adequate 
choice of heat-treatment time and temperature. From the 
absorption data we estimate the particle’s diameters of dif- 
ferent heat-treatment time samples. The shortest diameter 
is 44 A while the largest is 54 A. Transmission electron 
microscopy (TEM) performed on the smallest and the 
largest crystallite samples give an average particle’s diam- 
eters of 39 and 55 8, with a typical 18% dispersion. When 
the particle size increases, we observe a red shift in the 
photoluminescence and absorption peaks. The absorption 
spectra also show another feature, assigned to the next 
subband transition, which together with the red shift are 
strong evidences of quantum confinement effects. 
Experimental glasses were prepared by melting a glass 
host containing Si02, H,BO,, Na,C03, and ZnO mixed 
with CdO, Te, and S in the CdTe,&& stoichiometry. 
Other aspects of the growth are described elsewhere.15 Sub- 
sequent heat treatment produces the semiconductor micro- 
crystallites. The four samples used here were annealed at 
580 “C for 40, 60, 90, and 110 min. Optical absorption 
measurements were done with a Cary-Varian 2300 model 
spectrophotometer and the photoluminescence spectra 
were obtained with 10 mW of the 488.8 nm line of a Spec- 
tra Physics argon laser and a Spex monochromator. 
TEM micrographs were obtained in a Zeiss EM 902 
microscopy equipped with an electron-energy-loss spec- 
trometer (EELS). We used an agata mortar to fracture the 
doped glass. The resulting fine particles were dispersed in 
n-propanol and applied over parlodium-carbon-coated 
copper mesh grids. The observation was done on sharp 
edges or bubbles, in which there was sufficient contrast. 
Figure 1 shows a typical micrograph. The electron diffrac- 
tion rings shown in the inset reveal a typical crystalline 
structure. Figure 2 shows the size histograms for two sam- 
ples with 40 min [2(a)] and 110 min [2(b)] amrealing time 
at 580 “C. The average diameters are 39 *7 A and 55 f 10 
k for the 40 and 110 min samples, respectively. Although 
the two distributions overlap (39 + 7~55 - 10 A) their 
size separation (16 A) is much larger than the average’s 
dispersion z 0 z 3 A. Therefore there is a clear average 
size difference for the several annealing times. 
Figure 3 shows the absorption spectra measured at 
room temperature for samples of the same batch with dif- 
ferent heat treatment times. The inset shows the room- 
temperature photoluminescence spectra of the same sam- 
ples. In the absorption spectrum of the sample with the 
shortest heat-treatment time, there is a clearly resolved first 
feature followed by a plateau and a shoulder. The photo- 
luminescence peak is about 100 meV below the first ab- 
2715 Appl. Phys. Lett. 59 (21), 18 November 1991 0003-6951/9i /462715-03$02.00 @I 1991 American Institute of Physics 2715  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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FIG. 1. TEM micrograph and electron diffraction pattern of 
CdTe& _ x doped glass for 110 min annealed sample with an average 
diameter 55 .&. 
sorption peak. As the heat-treatment time increases, the 
whole absorption and photoluminescence spectra shift to 
longer wavelengths, but with less separation between the 
shoulder and the first feature. The clear subband structures 
and the shift in both the absorption and photoluminescence 
spectra to longer wavelengths with heat-treatment time are 
consistent with quantum confinement effects due to the 
0 -Jl ’ KY-.. 
20 30 40 50 60 70 80 
OIAMETER 6, 
FIG. 2. Histogram of microcxystallite sizes obtained from TEM measure- 
ments for the 40 and 110 min annealed samples. 
600 
WAVELENGTH (nun) 
FIG. 3. Room-temperature absorption spectra for the samples annealed al 
580°C for (a) 40, (b) 60, (c) 90, and (d) 110 min. 
microcrystallite size. The first absorption feature is consis- 
tent with the IS heavy-hole subband to the 1s conduction- 
subband transition, whereas the shoulder is consistent with 
the IP-IP heavy hole to conduction the quantum-confined 
transition. I4 
Our data can be explained accordingly to current 
models” which take into account the confinement energies 
under the effective mass approximation and neglect the 
Coulombic interaction. The energy Ej of the jth opticai 
transition is given by 
where a is the particle’s radii, p is the reduced electron- 
hole mass, Eg is the bulk band gap, and the two values 
xX = 3.1416 and ;uP = 4.493 are the first non-null roots of 
the zero and first-order spherical Bessel functions. 
In this model the separation between the first two tran- 
sitions (IS-1Sand lP-1P) AEsp = (% - d) (2i2/2pa2) de- 
pends only on the reduced mass and the particles radii. 
Figure 4 shows a plot of E, vs Ep - E, We measured both 
Es and Ep by taking the average between the maximum and 
the minimum on the first derivative of the spectrum. The 
straight line which fits the points has a slope of 0.93, very 
close to x+(x, - II,“, = 0.96 value obtained with the 
above model. The constant term is 1.45 eV, which is con- 
sistent with the reported values for the CdTe bulk band 
gap. We estimate the microcrystallite diameter using an 
effective mass p = 0.1 and Eq. ( 1) as 44, 48, 50, 54 A for 
the 40, 60, 90, and 110 min of heat treatment at 580 X!! 
2716 Appt. Phys. Lett., Vol. 59, No. 21, 18 November 1991 Neto ef al. 2716 
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143.106.108.115 On: Wed, 19 Nov 2014 16:41:09
2.30 I- ~ 1 
, ,g5 I t -...-J--..L - 
0.55 0.60 0.65 0.70 0.75 0.60 0.65 0.90 
Ep- ES (eW 
FIG. 4. Plot of the first transition energy Es vs Ep - Es obtained from the 
absorption spectra. 
respectively. These estimates are in good agreement with 
the TEM measurements 39 * 7 A and 55 f 10 A for the 40 
and 110 min treatment time samples, respectively. 
In conclusion, we show quantum confinement behavior 
in the new CdTe,S, _ X semiconductor microcrystallite- 
doped glass. This semiconductor can be more attractive 
than the presently used CdSe,Sr _ X quantum box due to its 
smaller Bohr radii. The red shift in the absorption spectra 
as the size increases agree with a particle in a spherical well 
model. The estimated sphere’s diameters are 44, 48, 50, 
and 55 A with the correspondingly confinement energies of 
0.80,0.64,0.60, and 0.58 eV for the 40, 60, 90, and 110 min 
annealing time at 580 “C, respectively. TEM measurements 
on the 40 and 110 min annealing time samples give an 
average diameter of 39 f 7 A and 55 f 10 A, respectively. 
The authors would like to acknowledge the 
Telecommuicacses Brasileiras S/A (Telebras ) and the 
Chemistry for Electronics Program (PQME-FINEP/IQ) 
for financial support. We also acknowledge Dr. Jose A. 
Sanjurjo for the luminescence measurements. 
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2717 Appl. Phys. Lett., Vol. 59, No. 21, 18 November 1991 Neto et al. 2717  This article is copyrighted a  indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditi ns. Downloaded to IP:
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QUANTUM SIZE EFFECTS ON CDTEXS1-X SEMICONDUCTOR-DOPED GLASS

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QUANTUM SIZE EFFECTS ON CDTEXS1-X SEMICONDUCTOR-DOPED GLASS

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