Surface Characterization and Infrared Emission from AlN:Tm Film
A RESEARCH PROJECT
SUBMITTED TO THE GRADUATE SCHOOL
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE
MASTER OF ARTS
BY
Amani Alruwaili
DR. Muhammad Maqbool - ADVISOR
BALL STATE UNIVERSITY
MUNCIE, INDIANA
JULY 2015
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1.Introduction:
Rare-earth(RE) doped aluminum nitride (AlN)[1] and gallium nitride (GaN) thin films[210]growing on semiconductor substrates[11, 12] (e.g. silicon carbide, silicon, etc.) are studied
for their photoluminescence and electroluminescence properties[3]. The RE dopant gives rise of
sharp emission lines in UV, visible and infrared spectra, because the unfilled 4f-shell of RE
elements provides long lifetime metastable states, suitable for laser operations.
Another
characteristic property of the trivalent RE ions (e.g. Eu3+, Nd3+) is that their electronic transitions
usually occur within the 4f-shell, which is somewhat shielded from the host lattice by the
optically passive outer electronic shells. This reduces the influence of the host lattice on the
wavelengths, bandwidths and strengths of the relevant optical transitions. The RE dopant
properties are exploited to produce optical communication devices such as fiber lasers[13] and
amplifiers. However, conventional semiconductor hosts such as silicon and gallium arsenide
limits the performance of RE dopant because of lower solubility of rare earth and thermal
quenching[11], which reduces the photoluminescence efficiency under room temperature. A
wide band gap semiconductor host will increase the photoluminescence efficiency by reducing
the thermal quenching effect. Due to the wide band gap of AlN (around 6eV), the thulium(Tm) doped AlN will emit sharp infrared light if stimulated, making the AlN:Tm system a possible
candidate for lasers and optical amplifiers in the communication field. AlN also have other
valuable properties such as higher bond strength and melting point, better insulation strength,
better heat conductivity, better chemical and physical stability and higher optical damage
threshold comparing to conventional semiconductors, and it is also much cheaper than diamonds.
Moreover, both AlN and GaN have high radiation hardness, high avalanche breakdown electric
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field strength and large room temperature electron mobility. These properties make them suitable
for use in high-frequency and high-power optical and electrical device applications. Group IIInitrides are regarded as one of the most promising materials for applications as laser diodes
(LDs), photodiodes (PDs), light emission diodes(LED)[14, 15], and high-power and hightemperature electronic devices.
In this paper, we will investigate surface and optical properties of AlN film growing on Si
substrates and doped with Tm. The dopant species determines the transition spectra lines in the
stimulated luminescence, and the quality of luminescence is determined by the fabrication
process. In order to grow high quality AlN films, we use RF magnetron sputtering to deposit the
film on silicon substrates, by ion bombardment on aluminum targets in nitrogen atmosphere. The
dopant Tm is introduced in the sputtering process as well, implanting into the film. After
production of films on the substrates, we put the samples in the X-ray diffraction apparatus to
inspect its crystalline properties such as diffraction angle, grain size and strains on the film. We
also observe its optical emission via photoluminescence via laser stimulation and spectrum
analyzer.
2. Fabrication
We use single crystal Si wafers as substrates, which are cut either along (100) plane or (111)
plane. Different cleavage surfaces have impact on the film growth. The method of growing films
on substrate is magnetron RF sputtering, which is shown below:
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Fig.1 Scheme of Sputtering. In our case, the target is aluminum disk and the substrate is Si wafer.
The sputtering is done in nitrogen atmosphere, where sputtered aluminum atoms interact with
nitrogen to form aluminum nitride, which has chemical affinity with the substrate and deposit
there. Thulium target (not shown) is sputtered as well, and it will deposit in the substrate in the
form Tm3+, replacing some of the Al3+ ions in the film. The dopant density is controlled by the
size of thulium targets comparing to the aluminum targets.
The result AlN film has similar structure as the AlN crystal. The crystal has a structure
shown in the following figure:
Fig.2 The crystal structure of AlN. The gray sphere represents aluminum and the purple sphere
represents nitrogen. Thulium dopant replaces aluminum in the lattice. The left graph is the 3D
illustration, and the right one is the top-down view. Since the Tm ions has larger diameter than
the Al ions, it is expected that the crystal structure will be deformed near those impurity sites.
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3. Surface Characteristics
After depositing the film on the substrate, we want to examine its surface qualities via X-ray
diffraction. The principle of X-ray diffraction is shown as follows:
Fig.3 Illustration of Bragg diffraction. The X-ray is shining on the crystal structure at angle θ in
respect of the lattice surface, and the enhanced diffraction feature (bright rings) only occurs at
angles where Bragg’s Law is satisfied. Because the correct combination of wavelength, lattice
constant and incident angle must be met to produce a Bragg diffraction pattern, the common
method is to fix two out of three parameters: either scan a single crystal with continuous X-ray
frequency, or using crystal powder of random orientations with a single frequency X-ray source.
Here, we rotate the single crystal to scan the incident angles, and look for a peak in the power at
the receiving side.
The Bragg’s Law states that
,
(1)
If the crystal is perfect, the Bragg law is only satisfied at a special angle, such that the angular
spectrum will be a Dirac delta function. In real application, however, the crystal has dislocations
and grain boundaries, such that the spectrum will have finite linewidth. Assuming no other
irregularity arises besides grain boundaries, we have Scherrer’s equation:
,
(2)
where τ is grain size, K is dimensionless factor, called shape factor, which is close to 1, and we
choose 0.9, β is the angular spectrum peak’s full width half maximum (FWHM), in radians, and
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θ is the diffraction angle. Because other crystalline defects also broaden the spectrum line, this
estimate only gives a lower bound of the grain size.
As the baseline of our measurements, we use the following X-ray diffraction spectrum produced
by low temperature deposition of AlN film. Our single frequency X-ray source uses Kα- line of
copper, which has a wavelength of 0.15418 nm. The source emits X-ray due to high voltage
electron bombardment on copper anode, exciting the K-shell electrons of copper atoms.
Fig. 4 AlN thin film deposited at 77K on Si substrate shows only one peak at 28.4° for Si (100)
substrate. No peak for AlN appears, showing that AlN films are amorphous. Low temperature
facilitates glass transition, which freezes the material in the metastable phase with no crystal
symmetries. Similar techniques are used to manufacture amorphous metals and semiconductors
by thermal quenching.
We grow AlN film at higher temperatures to facilitate its crystallization, and the X-ray
diffraction shows the following:
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Fig. 5 AlN thin film deposited at 250°C on Si (100) substrate shows both peaks at 28° for Si (100)
substrate and 37.8° for AlN. The AlN’s feature is obscure, showing the crystal growth is
hindered in the sputtering and annealing process.
Fig. 6 AlN thin film deposited at 450°C on Si (111) substrate shows both peaks at 69.1° for Si
(111) substrate and 38.1° for AlN. The AlN’s feature is sharper, and the diffraction angle also
changes from Fig. 5, showing the crystal constant has been slightly altered in different growth
environment.
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Fig. 7 AlN thin film deposited at 700°C on Si (100) substrate shows both peaks at 28° for Si (100)
substrate and 35.92° for AlN. The AlN’s feature is much sharper, almost comparable with the
substrate strength, showing a well-homogenized AlN crystal growth along the substrate surface.
Higher temperature help forming tiny crystalline, and annealing makes them aggregate to larger
crystals with fewer defects.
Following the empirical formula of crystal strain:
,
(3)
where is the dimensionless strain of the crystal. Qualitatively, it is understood as the crystal
strain deforms the crystal, breaking the strict lattice symmetry, and broadens the X-ray
diffraction angle by allowing multiple crystal spacing.
Using Eq.1-3, we get the table for lattice constant, grain size and residue strain as function of
temperature, and plot them in Fig.8:
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Temperature(°C)
Strain
250
450
700
Grain Size(nm)
0.00293
0.0028
0.00249
36.54
37.91
45.18
Lattice Constant(nm)
0.238
0.2362
0.25
Table 1. The structural parameters for different temperatures.
Fig. 8 The surface characteristics of AlN films. We list the grain size, residue strain and lattice
constant of the AlN films. We see that higher temperature in the fabrication help us prepare
larger grain of single crystal AlN, and reduces strain in the crystal, improving its surface quality.
The interatomic spacing at 0.25 nm for the 700°C-grown film is also closer to the true single
crystal’s lattice constant (3.11A).
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4. Photoluminescence Characteristics
In order to study photoluminescence, we strike the samples with laser, and measure its
emission spectra. The incoming laser would excite transitions of Tm3+ ions (the AlN band gap is
too large to excite), by pumping them from ground state to several excited states. The cascading
decays from excited states give rise of distinctive spectra lines.
Here’s the energy diagram of Tm3+ ions[13]:
Fig. 9 Thulium energy diagram. Pumping laser with wavelength shorter than the transition line
from 0->1 to 0->5 will excite respective transitions to excited states. When atoms at excited
states decay, they may go through a cascading process and emit lines such as A54, A43, A32, etc.
when decaying from level 5.
Since we are going to study its infrared luminescence, we would like to excite lower energy
levels only, so we use 783nm laser and 532nm frequency-double Nd:YAG laser for the purpose.
We concentrate on near infrared spectra.
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Fig. 10 783nm photoluminescence measurement. We obtain two peaks, 802.67nm and
812.655nm. These two lines correspond to 3H6 to 3H4 transitions.
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Fig. 11 532nm photoluminescence measurement. The small peak at 611.534 nm is not shown on
the Tm3+ energy diagram, but the single peak at 810.768nm is corresponding to 3H6 to 3H4
transitions.
We note that the excitation frequency does not need to be exact transition frequency in the
energy diagram. The reason is that electrons from valence band can be excited into the
conduction band at any position, rather than the bottom of the band (see Fig.12).
The sharpness in the emission spectrum shows that our AlN:Tm film has high efficiency and
monochromaticity for communication or illumination purposes.
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Fig.12 The illustration of photoluminescence. Although the pure AlN band gap is too large to be
excited via our lasers in the visible and near IR range, the dopant levels, which are much lower,
can be excited. Any excitation larger than the dopant band gap would stimulate photon emission.
5. Conclusions
In this paper, we demonstrate the processes of fabrication of AlN:Tm film and tests of its
performances. We have shown that growth on Si wafer, cut at surface (100), on high temperature
as 700°C, resulting in high quality, low strain crystalline film. The optical properties of dopants
are examined through photoluminescence by laser stimulation. We found that 532nm laser
excitation result in sharp, bright illumination at near infrared transition at 810nm. The techniques
involved in these experiments may pave ways for future optical devices that relays, amplifies and
transmits near IR light signals.
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