Mat. Res. Soc. Symp. Proc. Vol. 744 © 2003 Materials Research Society
M4.1.1
Ultraviolet Emitting SrS:Te Thin Films
J.M. Fitz-Gerald 1 , J. Hoekstra1 , J.D.Fowlkes2 , P.D. Rack2
1
University of Virginia, Dept of Materials Science and Engineering, 116 Engineer's Way
Charlottesville, VA 22904-4745
2
University of Tennessee, Department of Materials Science and Engineering, 603 Dougherty
Hall, Knoxville, TN 37996-2200
ABSTRACT
In the bulk SrS has an indirect bandgap of ~ 4.32 eV. When SrS is doped with tellurium,
ultraviolet emission occurs at 360 nm (for singlet) and 400nm (for Te-Te dimers) due to
recombination from bound exciton states. In this paper we discuss the ultraviolet emission of
pulsed laser deposited thin films of SrS:Te grown at room temperature on Si. Deposited film
thickness ranged from 0.1 – 1.5 µm, with optimized films grown at ~0.5 µm to avoid peeling and
cracking. Te doping was incorporated by both ion implantation and conventional diffusion of
deposited Te capping films. The characteristics of the ultraviolet emission will be discussed
along with results from microstructural, chemical and optical characterization techniques.
INTRODUCTION
The development of semiconductor based ultraviolet (UV) light sources is of critical
importance for miniaturized ultraviolet light sources, which have application in biological agent
detection, non- line-of-sight covert communications, water purification, equipment/personnel
decontamination, and white light generation. Currently, a significant amount of research is being
performed to extend the III-V nitride blue lasers and light emitting diodes into the ultraviolet
region. This research is focused on increasing the aluminum content of III-V alloys and
necessarily increasing the semiconductor bandgap. The main obstacle with this approach is the
limited concentration of p-type dopant that can be introduced in high aluminum content ternary
and quaternary III-V nitride alloys. In this paper we will discuss preliminary work (on SrS:Te)
investigating tellurium doped alkaline earth sulfide materials (SrS:Te, CaS:Te, and MgS:Te) as
an alternative material for UV and deep UV (DUV) solid-state light sources. Figure 1 shows the
alkaline earth sulfide bandgap versus the experimental (BaS:Te and SrS:Te) and calculated
(CaS:Te and MgS:Te) emission wavelengths for so-called Te singlet and Te-Te dimer bound
exciton emission. The calculations suggest that tunable solid-state emission at wavelengths from
430 to 265 nm is possible with this new class of materials.
The theory of bound exciton emission is not new, as the idea of an isoelectronic trap was
first introduced by Thomas et al. in 1965 when they used this theory to describe the
photoluminescence observed in GaP:N. 1,2 They suggested that even though nitrogen had the
same valence as phosphorus (isoelectronic), nitrogen acted as an electron trap in the GaP lattice.
It was reasoned that because nitrogen is significantly more electronegative than phosphorus,
electrons are preferentially trapped at nitrogen sites. Subsequently, the N-electron complex has a
net negative charge, which sets up a short range Coulombic attraction for holes. Finally, when
the hole is trapped, the electron and hole couple together and form an exciton. Because they are
bound spatially in the nitrogen vicinity they are called bound excitons. The localization of the
bound exciton causes the momentum to be diffuse (due to the uncertainty principle) and efficient
M4.1.2
Emission Wavelength
(nm)
radiation is realized in GaP, which has an indirect bandgap. Since this model was suggested, the
luminescent properties of other III-V, II-VI and I-VII materia ls have been described by it. The
focus of this discussion will be on bound excitonic II-VI semiconductor materials and in
particular MS:Te (where M = Zn, Cd, and Sr).
Te Singlet
Te-Te Dimer
450
SrS:Te
400
BaS:Te
350
CaS:Te
MgS:Te
300
250
3.5
4
4.5
Band Gap (eV)
5
5.5
Figure 1. Te singlet (blue) and Te- Te dimer (red) bound exciton emission wavelength as a
function of the alkaline earth sulfide bandgap. The filled shapes are experimentally
observed emission wavelengths and the un- filled points are calculated emission
wavelengths.
For MS:Te semiconductors, the bound exciton formation is similar to the GaP:N system
except that the sulfur host anions has a larger electronegativity than the tellurium dopant.
Because tellurium has a higher hole affinity than sulfur, holes are trapped at tellurium sites
forming a tellurium- hole complex. The tellurium- hole complex has a net positive charge, which
induces a Coulombic attraction for electrons. When the electron is trapped at this complex, a
bound exciton is formed.
Of the MS:Te systems, CdS:Te 3-7 and ZnS:Te 6,8-10 have been studied the most. Aten et
3,4
al. first reported luminescence at 77K in lightly doped CdS:Te (Te concentration ~ 1018 cm-3 ).
They observed an emission band at 600nm, which was largely quenched at room temperature.
Cuthbert and Thomas later investigated both lightly and heavily doped CdS:Te (Te
concentration 1018 cm-3 -1020 cm-3 ).5 They observed the 600nm emission band and discovered
another emission band at 730nm in heavily doped samples. They attributed the 600nm band to
single Te S sites (where Te S represents a Te atom occupying substitutionally a S lattice site) and
the 730nm band to Te S-Te S pairs or dimers. Electron beam excited fluorescence revealed that
while the 600nm band quenched significantly at 175K, the 730nm emission was stable up to
room temperature. Later Rossler extensively studied the CdS:Te system and from temperature
dependent photoluminescence excitation and emission spectroscopy he developed a
configuration coordinate model to describe the CdS:Te bound exciton behavior.7 The 600nm
emission zero-phonon line was estimated to be 2.36eV and the phonon energy of the ground state
and excited state was reported to be 21 and 14 meV, respectively. The thermal quenching and
luminescence was described and it was found that the addition of shallow donors helped raise the
temperature where significant luminescence quenching occurred. Isler and Strauss were the first
to report the optical properties of ZnS:Te.6 They showed two emission bands for the ZnS:Te
with peak energies at 3.2eV and 2.8 eV for the wurtzite phase and 3.1eV and 2.7eV for the
zincblende phase. They also attributed the high-energy peaks to Te S sites and the low energy
M4.1.3
peak to Te S-TeS pairs. Fukushima and Shionoya also studied the ZnS:Te system and determined
the phonon energies of the ground states and excited states.8 They developed a configuration
coordinate model to explain the bound exciton behavior in ZnS:Te and used a similar model to
describe the thermal quenching behavior that was observed.
Rack et al. have investigated the blue emission in BaS:Te 11 and thoroughly characterized
the ultraviolet emission in SrS:Te powder materials. 12,13
In both cases, efficient
photoluminescence (PL) from Te S and Te S-Te S was observed. SrS:Te powders were synthesized
and found to have two high-energy emission bands with peak wavelengths at 360 and 400nm.
The 360nm emission band dominated at lower tellurium concentrations and was attributed to Te S
bound excitons, whereas the 400nm band dominated at higher tellurium concentrations and was
ascribed to Te S-Te S bound excitons.
Figure 2 shows a schematic illustrating the Te S and Te S-Te S bound exciton and figure 3
shows the energy band diagram of the bound excitons. The zero-phonon lines of the Te S and
Te S-Te S were estimated to be 3.76eV and 3.44eV, respectively, and from these lines the bound
exciton binding energies were estimated to be 0.56eV (Te S) and 0.88eV (Te S-TeS). To
investigate the bound exciton process more extensively, PL emission spectra were taken as a
function of temperature. Using a configuration coordinate model, the phonon energies
associated with the peak broadening for each emission band was determined. The phonon
energy calculated for the Te S emission band was 37.8 meV, which was in excellent agreement
with the SrS longitudinal optical phonon energy (35meV). 14,15 This suggested that the Te S site is
coupled strongly to the vibrational energy of the SrS lattice. The calculated Te S-Te S phonon
energy was 18.4meV, which was close to the calculated SrTe longitudinal optical phonon energy
(22.8meV – calculated using the central force model for diatomic molecules). This further
suggests that the Te S-Te S pairs have a local vibrational mode, which is independent of the host
lattice.
S
Sr
S
Sr
S
S
Sr
S
Sr
S
Sr
S
Sr
S
Sr
Sr
S
Sr
S
Sr
S
Sr
Sr
S
S
Sr
Te h
Sr
S
Sr
S
Sr
S
Sr
Sr
S
Sr
Te
Sr
S
Sr
S
Sr
S
S
Sr
S
Sr
S
h
Te
e
a)
e
b)
Figure 2. Schematic illustrating the a) Te S and b) Te S-Te S bound excitons in SrS:Te.
M4.1.4
0.88 eV
SrS
BG=4.32eV
(Indirect)
BE TeS-TeS 0.56 eV
Bound
Exciton
hv = 3.44 eV
(360 nm)
Conduction
BE TeS
Band
Bound
Exciton
hv = 3.76 eV
( 330 nm)
Valence
Band
Figure 3. Energy band diagram of the SrS:Te bound excitons.
From PL intensity versus temperature data, the thermal quenching activation energies for the Te S
and Te S-Te S bound excitons were determined to be 25.5meV and 46.1meV, respectively.
Because the activation energies were significantly lower than the bound exciton binding energy,
the quenching activation energy was ascribed to the binding energy of the electron. By
comparing the non-radiative recombination rates of the Te S and Te S-Te S bound excitons, it was
predicted that the radiative recombination rates of the Te S bound excitons were ~ 1.8 times faster
than the Te S-Te S bound excitons. Luminescent decay measurements were performed to compare
the radiative rates of each bound exciton and the Te S bound excitons were found to be ~ 1.5
times faster than the Te S-Te S bound excitons, in excellent agreement with the predicted values.
In this paper we will extend from our previous powder materials work and discuss our initial
progress on the thin film synthesis and characterization of SrS:Te thin films.
EXPERIMENTAL DETAILS
All films for this research were deposited at room temperature using a pulsed excimer
laser (λ=248 nm) operating at 10 Hz with a 25 ns pulse (FWHM). A base pressure of 3x10-6
Torr was used for all experiments, while depositions were carried out in Ar and H2 S at pressures
ranging from 50 - 200 mTorr. Films were grown on (100) single crystal p-type Si substrates,
which were cut into 2 cm x 2cm pieces. A high purity pressed and sintered powder SrS
sputtering target 5 cm in diameter x 6 mm thick was used to deposit the bulk thin films while a
Te sputtering target 7.5 cm in diameter x 6 mm thick was used to deposit thin layers of Te.
Figure 4 illustrates the experimental setup and geometry used for all depositions. The excimer
laser was incident on the target surface at a 45° angle with a resulting spot area of ~0.186 cm2 for
the bulk SrS thin films and 0.15 cm2 for the Te thin film layers. The optimized deposition
fluence was found to be 1.6 J/cm2 for the SrS bulk thin films and 1.0 J/cm2 for the Te thin film
layers. A computer controlled scanning mirror was used to control laser beam movement over
the surface of the target during deposition. Deposition rates for the SrS thin films were 5.3
M4.1.5
Å/pulse while a deposition rate of 1.4 Å/pulse was used to deposit Te thin films onto the bulk
SrS thin films for doping. In the initial study, three ~ 0.5µm SrS films deposited on Si were
implanted with Te ++ at Los Alamos National Laboratories. Doses of 5x1012 , 5x1013 , 5x1014
atoms/cm2 were implanted at 150kV (300KeV for Te ++ implantation energy) which should yield
Te concentrations on the order of 1x1017 , 1x1018 , 1x1019 atoms/cm3 , respectively, subsequent to
the post implant drive–in and activation anneal. Another set of 0.5µm films were capped by
PLD with ~ 3, 6, 9, and 12 Å of Te (ie. 5, 10, 15 and 20 pulses), which should yield
concentrations of ~ 2x1019 , 4x1019 , 6x1019 , and 8x1019 atoms/cm3 , respectively. All samples
were annealed at 850°C in Ar for 3 hrs and were capped with another SrS thin film to minimize
sulfur outgassing during annealing.
(a)
scanning mirror
excimer laser
UV window
(b)
(c)
turbo
load
lock
substrate window
(d)
Figure 4 Experimental setup, (a) PLD system, (b) deposition system schematic, (c) digital
image taken during deposition of SrS at 200 mTorr Ar, 10 Hz, 1.6 J/cm2 , room temperature.
Digital photograph (d) illustrates a top view of the actual system.
Thin film characterization was performed with using a JEOL 6700F scanning electron
microscope equipped with a energy dispersive spectroscopy (EDS) and a fully integrated CL
system equipped with imaging and scanning monochrometer (0.2 nm resolution, 185 – 900nm)
and a LN 2 cold / hot stage operating from 90-375K. All CL spectra were taken with a 1 second
dwell time and a 2 nm scan step. X-ray diffraction was performed using a Phillips X-pert x-ray
diffractometer.
target
RESULTS AND DISCUSSION
Initial investigations into the SrS:Te system have been conducted to establish the efficacy
surrounding the use of pulsed laser deposition to grow thin films (0.1-1.5µm) of SrS with
controlled stoichiometry. For these experiments, thin films of SrS were grown in both Ar and
H2 S atmospheres at room temperature with laser energy densities ranging from 1.5 – 2.1J/cm2 .
The Ar pressure was varied from 50-200mTorr and it was clear from energy dispersive x-ray
analysis (EDS) and x-ray diffraction (XRD) that depositions performed at room temperature with
a backfill atmosphere of 200 mTorr Ar produced the most accurate stoichiometry, 1:1 ratio of Sr
to S as shown in Figure 5a,c. Thin film SrS depositions performed at room temperature in a H2 S
atmosphere produced thin films with a large amount of secondary SrS (with S/Sr ratio > 1) and S
complexes as shown in Figure 5b. This overpressure of S resulted in a Sr:S ratio of 1:2.4 as
measured by EDS. Scanning electron micrographs of SrS thin films produced in both Ar and
M4.1.6
H2 S are shown in Figure 6. It is clear from (a), (b) that the morphology of the thin films varied
significantly between the Ar and H2 S, due to the incorporation of extra S into the thin films. By
controlling the energy density and the growth time for the films deposited in Ar at 200 mTorr,
thin films were optimized for a thickness of 0.5 µm in Ar as show in Figure 7.
200mTorr Ar
a
150mTorr H2S
Sr:S = 1:2.4
b
Sr/S ratio
2.5
2
1.5
SrS
1
0.5
0
0
50
100
150
200
250
Argon Pressure (mTorr)
c
Figure 5. Results of SrS thin films grown by PLD in both Ar and H2 S atmospheres.
Representative X-ray diffraction of thin films grown in Ar (a), confirm that at room
temperature, semi-crystalline materials are produced with control over stoichiometry
(c) as measured by EDS. Thin films grown at room temperature in an H2 S atmosphere
tended to exhibit multiple SrS1+x and S complexes (b) with poor control over
stoichiometry.
M4.1.7
(b)
(a)
10 µm
10 µm
Figure 6. Representative SEM images of thin films of SrS grown in (a) Ar, and (b) H2 S.
2,000 pulses
200 mTorr Ar
a
10 µm b
100 nm
10 µm d
100 nm
4,000 pulses
200 mTorr Ar
c
Figure 7. Representative SEM images of optimized SrS thin films. Film thickness on the
order of 0.2µm – 0.5µm was achieved through deposition in 200 mTorr Ar at a fluence of
1.6 J/cm2 with laser pulse counts of (a,b) 2000 and (c,d) 4000 respectively.
To dope the SrS thin films with tellurium, sets of films were both ion implanted or capped with a
thin tellurium metallic layer. TRIM implantation simulations were performed assuming a
theoretical SrS density (3.7g/cm3 ), which resulted in a longitudinal range of 1062 Å and a
M4.1.8
longitudinal straggle of 384 Å. After the activation/drive-in anneal, cracking and delamination
of the SrS:Te films were observed. A previous investigation of the thin film stresses in sputter
deposited SrS revealed that significant tensile stresses develop in the SrS films deposited on Si.
These large stresses during annealing are due to the difference in the coefficient of thermal
expansion of Si (4x10-6 ) and SrS (25x10-6 ).16 While the original stress state of the as-deposited
films were not known, an estimate of the thermal stresses generated in the SrS:Te films during
the cool down to 850o C to room temperature is ~ 750 MPa in tension.
X-ray diffraction data taken on several annealed samples showed a reduction in the full width at
half maximum (FWHM) as compared to the as-deposited samples (sample sizes varied so direct
comparison of peak intensities could not be made). The reduction in the FWHM suggests that
the SrS grains grew during the post-deposition anneal which is consistent with previous work on
sputtered films.16
Analysis of the specific post-anneal dopant distribution has not been performed, however
normalized cathodoluminescence spectra (taken at 15keV) of the implanted (Figure 8 (a)) and
tellurium capped films (Figure 8 (b)) showed emission from the signature Te S and Te S-Te S bound
CL Intensity vs. Wavelength
1
CL Intensity vs. Wavelength
1
(a)
1x10^17
(b)
2x10^19
4x10^19
1x10^18
6x10^19
1x10^19
0.8
8x10^19
0.8
Te thin film
Normalized CL Intensity
Normalized CL Intensity
Te implanted
0.6
0.4
0.2
0.6
0.4
0.2
420 nm
410 nm
360 nm
360 nm
0
300
350
400
450
500
550
600
Wavelength
0
300
350
400
450
500
550
600
Wavelength
Figure 8. Cathodoluminescence measurements taken at 15 kV for both implanted (a), Te
capped films (b). Emission was expected to be dominated by the singlets for the lower doped
samples and by doublets for the higher doped samples but here the opposite trend is observed,
which may point to a diffusion gradient in the SrS films.
exciton peaks at ~ 360 and 410nm, respectively.
Both sets of films in Figure 8 show a
M4.1.9
counterintuitive trend in the peak ratio of the two peaks; namely that the TeS peak height
increases relative to the Te S-Te S peak height with increasing dopant concentration. It was
expected that with increasing the dopant concentration the Te S-Te S emission peak would
systematically increase because the probability of forming Te S-Te S increases with the Te
concentration. The observed trends indicate that the Te dopant is not fully driven into the sample
and a Te concentration gradient through the SrS thickness exists. Thus the Te S-Te S pairs are
spatially located in the near surface region and the Te S sites are located near the SrS-Si interface.
Samples with the highest Te concentration will have the largest concentration gradient thereby
enhancing diffusion and creating more singlets at the SrS / Si interface. The enhanced TeS
bound exciton emission at higher dopant concentration is observed in Figure 8 with the highest
TeS emission coming from the highest doped samples (~ 1x1019 and 8x1019 Te/cm3 ).
The optimization of the beam interaction volume will also have a significant effect on the
Te S and Te S-Te S emission. Figure 9 shows the Monte-Carlo simulations of the electron
penetration into the SrS/Si stack as a function of beam energy. The simulations show that the
optimum beam energy is between 7-10 kV, with higher energies (>10 kV) penetrating into the
underlying Si substrate and lower energies (<5-7 kV) failing to excite the complete volume of
the SrS film. In addition, the TeS bound exciton emission is anomalously high; however the
electron-hole pairs are preferentially generated at the tail of the electron beam distribution which
has better overlap with the TeS singlets distributed deeper into the film.
SrS
Si
5000 Å
SrS
5000 Å
Si
2 µm
15 kV
(a)
1 µm
10 kV
(b)
SrS
5000 Å
5 kV
(c)
SrS
2 kV
(d)
Si
2 µm
0.5 µm
5000 Å
Figure 9. Monte Carlo simulations of electron penetration depth as a function of beam energy
for (a, 15kV), (b, 10 kV), (c, 5 kV), and (d, 2 kV). It is clear from the lines representing the
SrS/Si interface that the optimum beam energy for (5000 Å) thick films is between 7-10kV.
M4.1.10
Figure 10 shows the CL intensity as a function of beam energy at 1, 5, 10, and 15 kV
respectively for the highest doped ion implanted sample (dose = 5x1014 Te/cm2 , Te concentration
= 1x1019 atom/cm3 ). The strongest CL emission is observed at 10 kV, confirming the
simulations as a function of beam penetratio n depth. Another observation that must be noted is
the shift of the singlet-doublet peak intensities at 15 kV between Figures 8, and 10 for the same
sample (1x1019 ). The CL spectra in Figure 10 was acquired with a 500x image resolution at a
faster beam scan rate (75x), while the CL spectra shown in Figure 8 were taken at 1000x with a
slow beam scan rate. It is speculated that along with the tellurium diffusion gradient, that several
processes such as charging, current saturation, radiative and non radiative energy transfer,
internal electric field gradients and carbon staining, are occurring which are affecting both the
TeS singlet and TeS-TeS doublet bound exciton emission. Detailed CL studies as well as
complimentary photoluminescence studies are being performed to elucidate the dominant
processes.
CL Intensity vs. Wavelength
140000
1x10^19, 1kV
1x10^19, 5kV
120000
1x10^19, 10kV
1x10^19, 15kV
100000
CL Intensity
Te implanted
80000
60000
40000
20000
360 nm
0
300
350
420 nm
400
450
500
550
600
Wavelength (nm)
Figure 10. CL intensity (not normalized) measured as a function of beam energy. Beam
energies were varied from 1-15 kV showing the strongest CL emission at 10 kV, which is
predicted by the MC simulations.
M4.1.11
CONCLUSIONS
SrS:Te phosphor thin films were synthesized and their luminescent properties were
investigated. Thin films (0.5 µm) were grown by pulsed laser deposition (PLD) in Ar. Te
doping was performed by both ion implantation and Te capping layers via PLD. Samples were
annealed in Ar at 850°C for 3 hours. Cathodoluminescence measurements taken at room
temperature show characteristic Te S and Te S-Te S bound exciton emission at 360 nm, and 410420 nm respectively, which is in excellent agreement with previous research efforts in SrS:Te
powders at room temperature. The strong emission of Te S-Te S (420 nm) at low doping
concentrations and a rise in the emission of Te S (360 nm) at high Te concentrations is
counterintuitive to prior research investigations suggesting that the there is a gradient in the Te
dopant concentration through the films. This trend is present in both sets of thin films and is
particularly noticeable for the highest doped films. Beam energies were varied from 1-15 kV on
a heavily doped sample (1x1019 ), confirming simulations of the beam interaction volume in
terms of CL emission.
M4.1.12
ACKNOWLEDGEMENTS
The authors would like to acknowledge Dr. Chris Wetteland for ion implantations performed at
Los Alamos Nation Laboratory. In addition, the authors would like to acknowledge financial
support from the AFOSR under DURINT equipment grant #F49620-01-1-0420.
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