IES83668Y1 - A method and apparatus for analysis of semiconductor materials using photoacoustic spectroscopy techniques - Google Patents
A method and apparatus for analysis of semiconductor materials using photoacoustic spectroscopy techniquesInfo
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- IES83668Y1 IES83668Y1 IE2003/0396A IE20030396A IES83668Y1 IE S83668 Y1 IES83668 Y1 IE S83668Y1 IE 2003/0396 A IE2003/0396 A IE 2003/0396A IE 20030396 A IE20030396 A IE 20030396A IE S83668 Y1 IES83668 Y1 IE S83668Y1
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- sample
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- light source
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Abstract
ABSTRACT A photoacoustic spectrometer apparatus adapted to enable an observation and characterisation of non—radiative sub bandgap defects in narrow and large bandgap materials using photoacoustic spectroscopy techniques, the apparatus providing for an irradiation of a sample material provided within a photoacoustic cell and the subsequent detection and processing of an acoustic signal emitted by the sample, the apparatus comprising a light source having a polychromatic output substantially in the photonic energy range 0.5 eV to 6.2eV, focusing means adapted to couple the output from the light source onto the sample material, the focusing means providing for an alignment and focusing of the light emitted from the light source so as to provide a substantially parallel incident light onto the sample material, and means for detecting and acquiring the acoustic signal emitted by the sample in response to the irradiation. A method of providing an acoustic signal spectrum emitted by a sample material provided within a photoacoustic cell following irradiation of the sample by an incident light beam is also provided.
Description
Title
A method and apparatus for analysis of semiconductor materials using photoacoustic
spectroscopy techniques.
Field of the Invention
The invention relates to Photoacoustic Spectroscopy (PAS), and in particular to
techniques and apparatus using photoacoustic spectroscopy in the analysis of
semiconductor materials. Such materials may include direct or indirect bandgap
semiconductors having bandgaps in the range of about 05 eV < EG < 6.2 eV.
Examples of such semiconductors include infrared detector materials (e .g. InAs), and
wide bandgap semiconductors for blue/violet/UV light emissions and high
temperature electronics (e .g. GaN, SiC or diamond). Further examples include Si and
GaAs.
Photoacoustic spectroscopy techniques can be divided into direct and indirect
methodologies. In direct photoacoustic spectroscopy, a photoacoustically generated
wave is produced in a sample Where an excitation beam is absorbed. The
photoacoustically generated wave is typically measured by a piezoelectric transducer
bonded to the sample. The invention is, however, more particularly directed to indirect
techniques. In indirect photoacoustic spectroscopy, an acoustic wave is generated in a
coupling medium adjacent to a sample to be analysed, e.g. via a “thermal piston”
effect, resulting in a heating of an air column directly above the excited region of the
sample. The heating of the air column results in measurable pressure fluctuations.
The inherent advantage of indirect photoacoustic spectroscopy for the investigation of,
for example, stress and defect related phenomena in semiconductor materials is that it
is non—invasive compared with direct photoacoustic spectroscopy.
Background to the Invention
Advances in the microelectronics industry have been underpinned by improvements in
the quality of the constituent device materials. Consider for example silicon: in the
1970s the dislocation density was of the order of 103 cm2, whereas today defect free
wafers with a diameter of up to and greater than 400 mm are being used in the
production of microprocessors. The characterisation and understanding of defects
within semiconductor materials is necessary if device performance is to be enhanced.
The structural and opto-electronic properties of a material are interrelated and thus
neither can be examined in isolation of the other. Of particular importance is the
influence of structural defects on the opto-electronic properties of the material, as
these are known to affect carrier diffusion lengths, radiative and non-radiative
recombination processes.
Photoacoustic spectroscopy (PAS) is a non—invasive photo—calorimetric technique that
can probe the non-radiative thermal de-excitation channels of a sample and hence
compliments absorption and other spectroscopic analysis methods. Only light
absorbed within the sample can generate a photoacoustic response and thus, elastic
scattering or transmission of light through the sample does not influence this highly
sensitive technique. Photoacoustic spectroscopy can be used to measure amongst
others, the absorption spectrum, lifetime of photo—excited species and thermal
properties of a sample.
The photoacoustic effect was first reported in 1880 by Alexander Graham Bell in a re-
port to the American Association for the Advancement of Science. After the work of
Bell, the photoacoustic effect was largely ignored until the latter half of the 20”‘
century because the technical equipment, such as phase sensitive amplifiers and
microphones, necessary to obtain accurate results did not exist. The first theoretical
description of the photoacoustic effect in non-gaseous samples was made in the early
l97()’s and several classical extensions were made to this theory before the first semi-
classical description of the photoacoustic effect in semiconductors was published in
the early 1980’s. Essentially, these theories describe how light absorbed in a sample
following non—radiative de-excitation processes gives rise to a heat source in the
sample that may be distributed throughout the sample volume or confined to its
surface. This heat source generates both temperature and pressure fluctuations within
the sample, which in turn induce measurable pressure variations within the gas in
Contact with the sample.
The basic mechanism behind photoacoustic spectroscopy is as follows. Intensity
modulated monochromatic light is shone on a sample. Non-radiative de-excitation
processes following light absorption consequently heat the sample. By convective
processes, the sample in turn heats up a gas layer in the immediate vicinity of the point
of light absorption. The modulated nature of the light induces corresponding pressure
fluctuations in the gas due to repetitive heating and cooling of the sample. These
pressure fluctuations are detected in the case of indirect photoacoustic spectroscopy by
a microphone and are known as the photoacoustic signal. A photoacoustic spectrum
may be obtained by determining the photoacoustic signal of the sample as a function
of the wavelength and modulation frequency of the incident light.
The Extension of PAS to Semiconductors
It will be appreciated that electron excitations, having a finite lifetime, are generated
in the process of light absorption. This absorption of light is accompanied by the
generation of electron-hole pairs, which exist for a finite lifetime and move within the
sample, before transfening their energy back to the sample in the form of heat.
It is known that a photoacoustic spectrum can be used in an evaluation of the optical
absorption coefficient and the bandgap energy of a semiconductor sample. The first
theory of the photoacoustic effect in semiconductors was developed in the 1980s by
Bandeira et al.. Several groups tried to improve their theory, but all quintessentially
possessed the same foundations. In their study, Bandeira and co-workers were
interested in enhancing the photoacoustic effect from samples with low optical
absorption coefficients. To this end, they applied an electric field across the sample
perpendicular to the direction the incident photons made with the sample. The
subsequent Joule heating enhanced the contribution to the photoacoustic signal from
photoexcited carriers in the bulk. The application of this technique was limited to an
analysis, non—destructively, of the bandgap of semiconductors, direct or indirect.
This technique is also capable of analysing non-destructively:
l. The energy location of sub-bandgap defect levels, which ‘are the prime cause of
non—radiative recombination, and thus are detrimental to opto-electronic device
operation.
. The impact of dislocation generation in strained layer epitaxial systems for
modern electronic and opto-electronics materials and devices. In un-strained,
defect-free substrate material, one only observes an increase in the
photoacoustic signal during the bandgap transition. As an epitaxial layer is
grown on the substrate, any induced strain will modify the band-structure,
possibly providing alternative non-radiative recombination paths for
photoexcited carriers. The presence of such levels would be seen as peaks in
the spectrum below the bandgap energy. The energy levels of these defects can
be inferred directly from the PAS spectrum.
]. The optical absorption coefficient (B) of the semiconductor, for direct or
indirect bandgap materials. Through a knowledge of the normalised
photoacoustic spectrum and the thermal diffusion length of the sample, it is
possible to determine the optical absorption coefficient of the sample.
IV. Elastic and thermoelastic properties of the material under investigation.
Photoacoustic spectrometers for the analysis of gaseous substances are commercially
available. However, photoacoustic spectrometers for condensed matter analysis are
difficult to obtain and are often unsuitable in their construction to the varied needs of a
semiconductor experimentalist. This has been the impetus for the development of in-
house systems, which are typically designed for specific experimental conditions and a
narrow range of type of materials. The design process for many of these systems has
been quite arduous, expensive and very involved.
Photoacoustic spectrometry requires the use of an intensity modulated monochromatic
light source to induce the photoacoustic effect in the semiconductor. For this purpose,
pulsed and continuous lasers are popular. Due to the inherent wavelength properties of
such lasing devices, it will be appreciated that they are only useful over a narrow
photonic range, the range of operation of the laser. Zegadi et al. (Rev. Sci. Instrum.. 65
(7), July 1994) discuss the use of a non-laser device. They disclose the use of a short
are xenon lamp as a light source in the examination of spectra in‘ the near infrared
portion. Although this light source has specific application in the region of interest
described in Zegadi, it suffers in that the resolution of the incident light on the sample
is not as good as what is achievable using lasers. They nevertheless discuss how they
believe the resolution of their apparatus is a high resolution arrangement. It will be
appreciated from a review of their disclosure that this reference to high resolution is a
reference to for example “high energy resolution” as would be found in a typical
energy vs. PA Signal plot.
There is, therefore, a need to provide a photoacoustic system that has an extended
wavelength range such that it can be used in the analysis of a wide variety of
semiconductor sample types, yet maintains an incident light source of sufficient
spatial resolution so as to spatially distinguish the location of any defects detected on
the sample.
It is therefore an object of the present invention to provide a spectrometer having a
light source whose emission spectra is suitable to effect a radiation of samples of
differing semiconductor constituency yet maintains resolution so as to enable a spatial
discrimination of the location of detected defects in a sample.
Summary of the Invention
Accordingly the present invention provides a photoacoustic spectrometer for use in
the photonic energy range 05 eV to 6.2 eV for the observation and characterisation of
non-radiative sub bandgap defects in narrow and large bandgap materials.
In a preferred embodiment, a photoacoustic spectrometer is provided having a high
power short are lamp with a high resolution monochromator, wherein the
characteristics of the beam which is incident on the sample is alterable by the
provision of an optical system at the output of the monochromator to vary the spatial
resolution of the incident beam.
Desirably, the present invention provides a system which, at a reasonable cost,
acquires relative information about a sample of semiconductor material by
normalising the spectra, obtained using photoacoustic spectroscopy techniques, to that
of a known sample (e.g. carbon black powder).
In preferred embodiments, a high power short arc lamp is used and it will be
appreciated that such a lamp is more economical than a laser. However, it will be
further appreciated that in alternative embodiments, a photoacoustic spectrometer may
be provided having a laser system with an Optical Parametric Amplifier (OPA), or an
Optical Parametric Oscillator (OPO).
In a first embodiment, a photoacoustic spectrometer apparatus is provided which is
adapted to enable an observation and characterisation of non—radiative sub bandgap
defects in narrow and large bandgap materials using photoacoustic spectroscopy
techniques, the apparatus providing for an irradiation of a sample material provided
within a photoacoustic cell and the subsequent detection and processing of an acoustic
signal emitted by the sample, the apparatus comprising:
a light source having a polychromatic output substantially in the photonic
energy range 0.5 eV to 6.2 eV,
focusing means adapted to couple the output from the light source onto the
sample material, the focusing means providing for an alignment and focusing of the
light emitted from the light source so as to provide a substantially parallel incident
light onto the sample material, and
means for detecting and acquiring said acoustic signal emitted by the sample in
response to said irradiation.
The apparatus typically includes means for modulating the polychromatic light. The
means for modulating typically includes an optical chopper.
The apparatus desirably additionally includes a monochromator provided between the
light source and the focusing means, the monochromator adapted to convert the
modulated polychromatic light into modulated monochromatic light.
The monochromator may comprise means for altering the output slit width to optimise
one or more of incident power, spectral and spatial resolution, PA signal-to-noise
rations, etc. The altering means may be computer controlled.
Means may further be provided to vary the input slit to the monochromator for one or
more of the aforementioned reasons. The means for varying the slit may be manually
controlled or computer controlled. An advantage with computer control is that a
constant bandpass may be provided over the energy range of interest.
The means for detecting ‘said acoustic signal desirably includes a plurality of
microphones provided within the photoacoustic cell, the microphones being adapted
to detect an acoustic emission from the sample upon radiation by the light source and
couple that signal to the means for acquiring said signal. The microphones are
typically of the type known as electret microphones.
Preferably, the means for detecting and acquiring said acoustic signal emitted by the
sample detects and acquires a signal associated with a sub-range defined within the
photonic energy range output of the light source.
The means for detecting and acquiring said acoustic signal desirably includes data
acquisition and processing means and signal processing means. In one embodiment of
the invention, numerous spectra may be recorded. Data obtained by this means may be
numerically processed through for example, averaging or filtering processes or
algorithms. The means for detecting said signal desirably includes:
means for pre—amplifying an electrical signal, such as a low—noise pre-
amplifier, and
means for detecting signal, such as a lock-in amplifier.
In one embodiment of the invention, a signal from the microphone is passed through a
low—noise pre—amplifier to a lock-in amplifier. The amplified signal may then be
passed to a computer.
It will be appreciated that in some embodiments of the invention, a plurality of
microphones may be provided. The respective signals from the plurality of
microphones may be added and sent as a single signal to a lock-in amplifier. It will be
appreciated that additional circuitry may be provided to maximise the signal. This
would have the effect of preventing the generation of a weak lock-in signal due to
signals out of phase quadrature combining.
The photoacoustic cell may be provided with an inert atmosphere such as helium gas.
The use of such gases which have associated high thermal conductivity will absorb
more heat from the sample than for example air.
The apparatus may additionally include means for cooling the cell to below 273K.
The means for cooling may be adapted to maintain the cell at temperatures in a
controllable range from about 77K to about room temperature by means of apparatus
such as a cryostat (for temperatures down to 77K) or Peltier cooler (for 215K — 273K
range). It will be appreciated that alternative arrangements may be provided for
cooling the cell. It will further be appreciated that in accordance with the classical
photoacoustic theory of Rosencwaig and Gersho that this cooling will serve to
improve the photoacoustic signal-to—noise ratio.
The apparatus may include means for heating the cell.
In one embodiment, such heating means may comprise means for applying an
electric field across the sample material in a direction perpendicular to the direction of
the incident light, to encourage Joule heating of the sample.
Preferably, the apparatus comprises means for recording the acoustic signal over a
range of temperatures, so as to enable the relationship between temperature and the
acoustic signal to be investigated.
The focusing means are desirably adapted to provide an incident light of not greater
than about 15 mm in diameter, the diameter of the incident light being substantially
equivalent to the spatial resolution achievable.
The focusing means desirably includes a first and second optical focusing system: the
1st optical focusing system adapted to provide for a maximisation of the photonic
throughput thereby maximising the intensity of light incident on the sample (and
hence the photoacoustic signal as it is directly proportional to the intensity of the
incident light), and the 2nd optical focusing system being adapted to provide for an
increment in the spatial resolution of the light,
The first optical system is desirably adapted to provide for a magnification factor of
the image of less than 1 and preferably about 0.3. To effect such a magnification
factor, the first optical system typically utilises a two lens arrangement, the
magnification of the system being a combination of the combined magnification of the
individual lenses. The first lens is desirably configured to provide a virtual image as
the source image which then provides a real image as the incident light on the sample.
A variance of the distance between the two lens may be effected to vary the size of the
beam incident onto the sample. Typically, the first and second lenses are provided by a
biconcave and biconvex lens respectively.
The 2nd optical focusing signal is adapted to provide for spatial mapping of the
photoacoustic signal produced at the cell. Spatial resolution of the beam may be
provided by a tight focusing of the incident light (to ~1 mm spot size, yielding best
resolutions also of ~1 mm). Means may be provided to allow relative movement
between the beam and the sample to enable different portions of the sample to be
analysed. Desirably, the photoacoustic cell is mountable on a computer controlled X-
Y translational stage so as to enable a movement of the cell relative to the incident
light. Such relative movement enables the formation of a spatially resolved map (for
example ~ 1mm lateral spatial resolutions) of the PA signal to be computed.
The second optical system desirably comprises a plurality of optical components
which are adapted to re—configure the spatial dimensions of the light emitted from the
monochromator so as to form an incident light beam which, in a preferred
embodiment, is substantially circular in cross—section. It will be appreciated however
that alternative geometrically shaped beams may be provided.
Typically, the second optical system includes:
a cylindrical lens adapted to re-configure the dimensions of the light incident
thereon, the cylindrical lens providing a light beam substantially circular in cross
section as an output thereof,
a concave mirror adapted to re-direct and focus the substantially circular light
onto a lst plane mirror which is adapted to further re-direct the light beam so as to
provide a source image for the first optical system, and
a 2nd plane mirror adapted to receive the magnified output from the first
optical system and re-direct that light onto the sample within the photoacoustic cell.
The combination of the lens and mirror assembly provided by the first and second
systems desirably delivers a circular shaped beam incident on the sample with a
diameter variable between 1mm and 12 mm.
The combination of a cylindrical lens, which it will be appreciated effects a focusing
in one plane only, and a concave mirror, which focuses in both planes,, generates an
image on the 1st plane mirror substantially equal in size in both horizontal and vertical
directions,
The apparatus is desirably adapted to provide for a fully automated spatial resolved
photoacoustic scan of a sample material. This provides for an analysis of the results of
the scan during or shortly after the scan (i.e. in real time) and then a subsequent
repositioning of the beam on areas of interest to obtain further results in such regions.
Desirably, the apparatus comprises means for varying the chopping frequency of the
incident beam. Varying the chopping frequency enables the penetration depth of the
beam into the sample to be controlled. It will be appreciated that such depth
controlling enables depth profiling of the sample to be investigated. The means for
varying the chopping frequency may include the computer in conjunction with the
lock—in amplifier.
The present invention further provides a method of providing an acoustic signal
spectrum emitted by a sample material provided within a photoacoustic cell following
irradiation of the sample by an incident light beam, the method comprising the steps
of:
A. providing a light source having a polychromatic output substantially in
the photonic energy range 05 eV to 6.2 eV,
B. setting the wavelength of the light source to an initial first irradiating
Wavelength,
C. irradiating the sample with said light source and detecting the acoustic
signal emitted by the sample at said wavelength,
D. incrementing the wavelength by a sequence of increment values so as
to provide a plurality of irradiating wavelengths and detecting the
acoustic signal emitted by the sample at each of said irradiating
wavelengths, and
relating each of the detected acoustic signals to the incident Wavelength effecting
generation of said acoustic signal.
Desirably, the sample material is selected from groups within the periodic table
having one or more of the following:
A. infrared detector materials (e .g. InAs),
B. wide bandgap semiconductors for blue/violet/UV light emissions,
c) high temperature electronics (e .g. GaN, SiC or diamond), and
d) Si or GaAs based materials.
It will be appreciated that the apparatus of the present invention is not limited to use in
the methods described herein. It is therefore possible that the apparatus may be
adapted for use in alternative experimental procedures. For example, the temperature
of the gas in the cell may be variable. By examining a particular defect level as a
function of temperature it may be possible to obtain direct information on phonon
mediated or non-radiative processes. In one embodiment, liquid helium may be use to
cool the sample held in the cell, at which temperature the non-radiative or phonon
mediated effects should disappear or be significantly reduced.
These and other features of the present invention will be better understood with
reference to the following drawings.
Brief Description of the Drawings
Figure l is a schematic representation of a photoacoustic spectrometer of the present
invention,
Figure 2 is a schematic representation of the focusing system used in the
photoacoustic spectrometer of Figure 1,
Figure 3 is a plot of fractional variation in object distance verses magnification for a
single lens.
Figure 4 shows an optical system consisting of two lenses separated by a distance d,
used in Configuration (a) for maximal intensity throughput for the PAS system.
Figure 5 is a flow chart showing sequence steps that may be undertaken, in accordance
with the present invention, so as to provide a photoacoustic energy scan of a sample
material.
Detailed Description of the Drawings
Figure 1 is a schematic representation of a photoacoustic spectrometer according to
one embodiment of the present invention.
In use, an optical chopper 2 modulates polychromatic light from a 300 W Xenon arc
lamp 4 as it is focused, using a coupling lens 3, onto an entrance slit of a
monochromator 8. The amplitude—modulated light that enters the monochromator 8
undergoes diffraction in accordance with the grating equation:
gxl = asin .9 (1)
where g is the order of the reflection, /1 the monochromatic diffracted wavelength, at
the line spacing of the grating and Hthe diffraction angle. The order sorting filter
wheel at the output of the monochromator ensures that only light with wavelength /1 is
transmitted and the harmonic contribution from wavelengths /l/g where g _>. 2 are
rejected.
At this stage the monochromatic light enters a dual configuration focusing system 10,
from which it is reflected into a photoacoustic cell 22 which, in use, contains a sample
of material to be analysed. The modulated monochromatic light is shone on the
sample. Non-radiative de—excitation processes following light absorption consequently
heat the sample. By convective processes, the sample in turn heats up a gas layer in
the immediate vicinity of the point of light absorption. The modulated nature of the
light induces corresponding pressure fluctuations in the gas due to repetitive heating
and cooling of the sample. These pressure fluctuations are detected by a microphone
and are known as the photoacoustic signal.
The resulting electrical or photoaeoustic signal is first pre—amplified in a low—noise
pre-amplifier 26 before detection of the signal is performed with a lock—in amplifier
28. An internal frequency generator in the lock-in amplifier 28 provides the reference
frequency for the optical chopper 4.
A photoaeoustic spectrum may be obtained by determining the photoaeoustic signal of
the sample as a function of the wavelength and modulation frequency of the incident
light.
The photoacoustic cell is mounted on a computer controlled X-Y translational stage.
Thus, fully automated spatial resolved photoaeoustic scans are possible. Using
paraxial ray theory analysis, general component specifications are derived. Ray tracing
analysis is used to determine the size of the beam striking the mirror as a function of
the inter-lens distance.
The entire system is controlled by a personal computer 30 using software such as
LabView® software and the National Instruments IEEE 488.2 GPIB interfacing
protocol.
Focusing System
The dual configuration focusing system 10 in the Photoacoustic Spectrometer of
Figure 1 is shown in detail in Figure 2. It will be understood that the optics of the
present invention are adapted to provide for intensity, maximisation and spatially
resolved photoaeoustic studies. In this section, the operation of the focusing optics
will be explained. The only major design constraint is that the materials used in the
lenses have to be as optically transparent as possible over the wavelength range of
interest i.e. 200 nm to 2.4 pm and also that the entrance window to the cell must be
substantially transparent over these ranges.
The focusing system of Figure 2 comprises two subsystems. According to the present
invention the focusing system is adapted to include optics which provide for:
(a) intensity maximisation, and
(b) spatially resolved photoacoustic studies.
The first focusing subsystem is designed to maximize photonic throughput and
thereby the intensity of light incident on the sample 20. A shown in Figure 2, the first
subsystem comprises a biconcave lens 12 and a biconvex lens 14 separated by a
distance d.
In photoacoustic spectroscopy, the photoacoustic effect is directly proportional to the
intensity of the incident light source 10. This implies that as much of the power from
the output port of the monochromator 8 should be focused into as small an area as
possible. It will be appreciated that there is a lower limit to the area of the focused
beam. If this area is too small, insufficient heating of the gas in contact with the
sample within the cell will occur, thus failing to generate an appreciable pressure
variation that can be measured by the microphone in the gas cell. Conversely, if the
beam size is too big and hence the intensity of the incident light source 10 is too low,
then an insufficient signal may be generated. Using the example of the lens
arrangement in the first focusing subsystem of Figure 2 as providing a magnification
factor of approximately m = 0.3, it will be understood that for example, a 3 mm by 12
mm output beam from the monochromator 8 will be converted into a beam
approximately 1 mm by 4 mm in size.
Starting with the basic lens equation:
irzta ‘”
and refonnulating the magnification of a single lens yields:
where u is the object distance, v is the image distance and the subscript sl denotes
single lens. This equation implies that for a real image to be formed by a single lens,
two constraints must be satisfied:
If either of these constraints are ignored a virtual image will be generated. The second
constraint implies us; = afi; for some a > 1. Consequently, equation (3) may be re-
written as:
"ls! : 1-:
a -1
Figure 3 is a plot of fractional variation in object distance verses magnification for a
single lens. The plot demonstrates that a single lens only provides realistic real image
focusing in the magnification range 1s m _<_ 2. Outside this range the object distance
becomes physically impractical with respect to the numerical aperture of the lens.
Therefore, a two lens imaging system is used in the focusing system of the invention.
Figure 4 shows an optical system consisting of two lenses separated by a distance d.
Biconvex lenses have been drawn, however the mathematical derivation in the text
applies to lenses with both positive and negative focal lengths once the sign
convention is adhered to.
The object distances have been chosen to be greater than the focal lengths of the
lenses to ensure real images are created. The magnification of the system is the
combined magnification of the individual lenses:
v, v2
mr,2 ‘ mimz ‘ —’
ul uz
Since
and U2 2 d ~v1,
flvl
::>m =—————— (7)
L2 d(u1 ‘ f1)"”1f1
Referring to the first focusing sub-system of Figure 2 for use in the photoacoustic
spectrometer of Figure 1, it will be understood that in order to ensure a maximal
throughput of intensity from the short-arc Xe lamp in the PAS system, the biconcave
lens 12 has a focal length f1 = -30 mm and the biconvex lens 14 has a focal length f2 =
mm. In use, this configuration provides a combined magnification mm = -0.32 for
an inter—lens separation distance of 50 mm. The minus sign in the magnification
means the image of the source is inverted. This is inconsequential as we assume equal
irradiance in all parts of the beam, i.e. the beam is the same above and below the
principal axis.
This lens configuration, which is seen implemented in Figure 2, assures irradiance of
the sample under test over regions of up to 15 mm in diameter, thus allowing for large
signal-to-noise ratios in the photoacoustic signals, and consequently for rapid analysis
times over the uniquely large energy excitation range of 0.5 eV through to 6.2 eV.
By varying the distance between the biconcave lens 12 and the biconvex lens 14 one
can vary the size of the beam that strikes the second plane mirror. Therefore, in use, it
is possible to vary the size of the beam which is reflected onto the sample using the
second plane mirror 16.
The lens used in the system of Figure 2 are desirably as optically transparent as
possible over the wavelength range of interest, i.e. 200 nm to 2.4 mm.
The second focusing assembly of the photoacoustic spectrometer of Figure 1 is
designed for spatial mapping of the photoacoustic signal within the sample. As shown
in Figure 2, the second subsystem comprises a concave mirror 17, a cylindrical lens
, a first plane mirror 19 and a second plane mirror 16.
The combination of the concave mirror 17 and cylindrical lens 18 (which focuses in
one plane only) generates an image on the first plane mirror 19 that is the same size in
the horizontal and vertical directions.
The output optical configuration for spatially resolved photoacoustic studies is now
described. As shown in Figure 2, the second sub-system focuses a 3 mm by 12 mm
rectangular beam at the output port of the monochromator 8 into a circular shaped
beam incident on the sample with a diameter variable between 1 mm and 12 mm. This
design has been formulated in an iterative manner; we will therefore only describe the
optimal solution.
Photoacoustic gas cell
In order to efficiently detect any photoacoustic signal emitted from an excited sample
it is important that the photoacoustic cell within which the sample is placed meets a
number of requirements. These requirements will be well known to those skilled in
the art, and typically require a confined volume of gas whose volume is definable. An
upper transparent window is provided through which the incident light can pass and
impinge on the sample contained within the cell. One or more microphones are
desirably provided within the cell to detect the acoustic signal emitted by the sample
upon excitation.
Typically, the internal diameter of the active volume of such a cell is 30mm and the
gas column length is 5 mm, the cell having a fixed gas volume of 35crn3. It will be
appreciated however that these values are exemplary only and it is not intended to
limit the dimensions of the cell of the present invention to any one set of values Air at
atmospheric temperature and pressure is typically used as the gas in the cell. However,
other gases, e.g. Ar or He can be introduced instead of air. As the PA signal is
proportional to the thermal conductivity of the gas in Contact with the sample, it will
be appreciated that by replacing the air in the cell with a gas having a different thermal
conductivity to that of air, the photo acoustic signal generated within the cell may be
enhanced. For example, helium could increase the signal by a factor of 6 relative to
air.
The cell is made large not only to accommodate large samples, but also to minimise
the effect of reflected and scattered light from the sample on the cell walls. The cell
may be made from H—30 Aluminum. When mechanically polished, this metal
becomes highly reflective. Spectrosil WF (quartz) windows, with a thickness of 0.2
mm, have been used as they possess transmittance in excess of 99% for all incident
light in the photon range 0.42 eV to 6.2 eV. Sealing means such as rubber O-rings
and or neoprene cushions may be provided to ensure a good acoustic seal from the
ambient surroundings.
The microphone used to detect the photoacoustic signal may be a single FG3329
electret microphone manufactured by Knowles Electronics. This miniature cylindrical
device, of dimension 2.59 mm in diameter and 3.22 mm in length, is housed in an
antechamber under the sample stage. This ensures that no scattered light induces noise
from the microphone diaphragm. Only one microphone is used according to this
embodiment of the invention, although it will be appreciated that cell can be easily
modified accommodate a detector array. The microphone possesses a nominally high
flat sensitivity of 25 mV/Pa in the frequency range 100 Hz to 10 kHz making the
microphone ideal for studies at different chopping frequencies. In the case of a
detector array the signal to noise ratio scales with the square root of the number of
microphones.
Short Arc Xenon Lamp
According to the present invention a polychromatic light source is used to provide the
incident light. An example of such a light source is a 300 W xenon short are lamp as
manufactured by LOT Oriel, with an arc size of 0.7mm - 2.4 mm, primary condensing
optics, secondary coupling optics and with a high voltage power supply to provide the
radiation source. A parabolic reflector is situated behind the lamp to enhance device
efficiency. As the arc lamp is at the focal point of the primary condensing lens, this
lens provides a collimated beam for the secondary lens, which in turn performs the
number matching with the monochromator situated at its focal point, thus maximising
throughput. The lamp provides reasonable constant irradiance from 250 nm to 2400
nm. The lamp is ozone free and consequently suffers from strong attenuation below
nm (above 4.96 eV).
In comparison to other non—ozone free lamps of similar output, the constant irradiance
above the oxygen cut-on wavelength makes the lamp quite suitable for photoacoustic
spectroscopy.
Optical Chopper
A variable frequency enclosed optical chopper manufactured by LOT Oriel is inserted
in the path of the collimated beam between the primary and secondary condensing
lenses. Hence, the collimated light is intensity modulated before being focused on the
entrance slit of the monochromator. An ancillary benefit of the enclosure is the
minimisation of acoustic noise arising from the air being chopped. The enclosure
surrounding the wheel also aids in safeguarding the user against hazardous scattered
light from the are lamp. The chopping frequency may be varied from sub-Hz to 3 kHz
by selection of an appropriate chopping wheel. In an exemplary embodiment, the
system is configured to operate at a maximum modulation frequency of 350 Hz. The
maximum modulation frequency depends on the chopper used. It will be noted that as
chopping frequency increases the output signal decreases, and at very high chopping
frequency (about 1 kHz) thenno-viscous damping occurs, which is undesirable in the
application of the present invention. To avoid effects of thermally reflected waves and
therrno—viscous damping, the usable chopping frequency range is typically of the order
of about 1() Hz to about 1 kHz.
The device may operate in stand-alone mode or from a supplied external reference. In
the exemplary system configuration described herein, the reference signal for the
optical chopper is supplied by an internal signal generator in the lock—in amplifier. The
reason for this is twofold: firstly, it ensures that the detection frequency and excitation
frequency are identical with a zero phase difference between them, and secondly, as
the frequency generated by the lock-in amplifier is fully programmable this enables
control software to be written where the modulation frequency can be varied at the
users discretion, by setting a desired modulation frequency value.
High Resolution Monochromator
An example of a monochromator that is suitable for use in the arrangement of the
present invention is the Cornerstone 260 monochromator manufactured by LOT Oriel.
It has entrance and exit focal lengths of 260 mm, a potential spectral operating range
of 180 nm to 20pm depending on the diffraction gratings used. The afore-mentioned
limitations of the Xenon arc lamp that are imposed by ozone attenuation below 250
nm (above 4.96 eV) may be overcome by purging the instrument with nitrogen. Thus,
through nitrogen purging, it is possible to extend the use of the Xenon arc lamp in the
analysis of semiconductor materials to those having bandgaps greater than about
.6eV and as such all materials in the range 05eV to 6.2eV.
The device has a motorised triple grating turret, which facilitates rapid broad-spectrum
scans at a maximum scan rate of 175 nm/s.
If monochromatic light strikes a grating, then a fraction of the light is diffracted into
each order in accordance with the grating equation, as will be well known by those
skilled in the art. The fraction diffracted into any order can be termed the efficiency of
the grating in that order. Gratings are not equally efficient at all wavelengths for
numerous reasons as the efficiency can be tuned by changing the number of grooves
(or lines) in the grating, the groove facet angles and the shape or depth of the grating
lines. The optimisation of efficiency by appropriate groove shaping is known as
blazing. The blaze wavelength is the wavelength for which the grating is most
efficient. Generally two types of grating are used: holographic and ruled. Holographic
gratings provide good spectral resolution at the expense of reduced intensity, whilst
ruled gratings offer increased intensity over the spectral range of interest at the
expense of spectral resolution. The resolution of a grating increases and the
throughput decreases with the number of grating lines. With these technical points and
knowledge of the arc lamp spectrum in mind the following gratings were used in the
monochromator, although it will be appreciated that different configurations may
require alternative parameters.
Grating No. Type No. of Grooves (1/mm) 7» range (nm) Blaze
2» nm
Holographic 1200 180 — 650 250
Ruled 1200 450 4 1400 750
Ruled 600 900 — 2800 1600
Table 2.1: Gratings used in monochromator.
The performance of a monochromator may be evaluated in terms of its resolution,
accuracy, precision and dispersion. The bandpass is the spectral width of radiation
passed by a monochromator when illuminated by a light source with a continuous
spectrum. By reducing the width of the input and output slits of the monochromator,
the bandpass may also be reduced until a limiting bandpass is reached. The limiting
bandpass is termed the resolution of the device. In spectral analysis, the resolution is a
measure of the ability of the instrument to separate two spectral lines that are close
together. The resolution of the Cornerstone 260 is 0.15 nm for a 12001/mm grating
when used with entrance and exit slits with dimension 10pm X 2 mm. By judicious
variation of the input and output slit widths, a relatively constant bandpass can be
obtained for the entire wavelength range of a photoacoustic spectral scan. Attached to
the monochromator input and output ports are continuously variable micrometer
driven slits whose width may be varied from 4pm to 3 mm and their height from 2
mm to 15mm.
The monochromator has an accuracy of 0.35 nm and will reproduce wavelengths to a
precision of 0.08 nm. It has an efficiency above 80% for blaze wavelengths and
exhibits high dispersion, typically 0.31 mm/nm and 0.16 mm/nm for the wavelength
ranges 180 nm to 1400 nm and 900 to 2800 nm, respectively.
Coupled to the output slit of the monochromator is a six-position filter wheel. For the
spectral range of interest, three filters are necessary to remove the effect of higher
order harmonic contamination in the output spectrum. The filters and the associated
grating wavelength ranges that they operate for are presented in Table 2.2. These order
sorting filters will also minimise the effect of stray re-entrant light in the
monochromator. In a system with the potential to scan such a large range of
wavelengths, this component becomes an integral part of the system. The filter change
mechanism is controlled directly by the monochromator, which itself may be
controlled using a dedicated hand controller, the IEEE 488.2 GPIB or the RS-232
communication protocols.
Filter N o. Cut-on wavelength (nm) Grating No. 7» range (nm)
2 324 1 340 — 650
2 850 — 1200
6 1600 3 1750 — 2400
Table 2.2: Filters used with monochromator.
It will be appreciated that the degree of spectral resolution achievable using hand
driven slits is not as high as what may be provided by computer controlled slits. It will
therefore be appreciated that if good spectral resolution is desired that a utilisation of
such computer controlled motor driven slits enable one to vary the slit width
according to the spectral position of interest and thereby achieve a constant spectral
bandpass over the energy range of interest.
Electrical Hardware
It will be understood that by using a lock in amplifier in combination with a data
acquiring system that it is possible to acquire data from the lock-in amplifiers whilst
simultaneously controlling the monochromator.. It is possible to single out the
component of a signal at a specific reference frequency and phase. An important part
of this process is that noise signals at frequencies other than the reference frequency
are rejected. Hence, they do not affect the measurement.
An example of a lock-in amplifier which may be used in the photoacoustic
spectrometer of the present invention is the Stanford Research Systems SR83O digital
signal processing amplifier. The amplifier converts the amplified experimental signal
using a 16 bit ADC that has a sampling frequency of 256 kHz. An anti-aliasing filter
prevents higher frequency inputs from aliasing below 102 kHz. Using the digital
PSD’s linear multipliers, the digitised experimental signal is multiplied with the
digitally computed reference signal. The reference sine wave may be considered
“pure” as all hannonics are attenuated with a dynamic reserve of 100 dB. The lock-in
amplifier has time constants from 10us to 30 ks with 6 dB to 24 dB per octave filter
rolloff. The internally generated reference signal is accurate to within 25 ppm and
phase measurements can be made with a resolution of 0.01 °. The amplifier provides
dual inputs and outputs in conjunction with an output reference port. Inputs can be
supplied in differential or single ended mode. The two data displays can be used for
the display of X and Y or R and 9. The digital lock-in is better than it analogue
counterpart as it does not suffer from drift in the PSD’s.
Low-noise Preamplifier
To further enhance the electrical signal generated by the microphone, the use of a
Stan—ford Research Systems SR552 low-noise bipolar input voltage preamplifier may
be employed. The preamplifier is designed to supply gain to the experimental detector,
before the signal to noise ratio is permanently degraded by capable capacitance and
noise pick—up. The amplifier has an input impedance of 100 kOhm + 25 pF and a
common mode rejection ratio of 110 dB at 100 Hz. Signals can be supplied in
differential or single—ended mode. It has a full-scale sensitivity from 10 nV to 200
mV. Thus the preamplifier minimises noise and reduces measurement time in noise
limited experiments. The power and control signals for the device are supplied
directly by the SR830 lock-in amplifier.
When used in conjunction with the lock-in amplifier, the gain is set to 10. It is
therefore necessary to divide all output measurements by 10 to obtain the true
measurement value.
Electrical Connections
Due to the small size of the microphone and its positioning within the photoacoustic
cell, attachment of coaxial cables to the signal lines was not possible. Therefore, small
insulated wires from the microphone were connected inside a Gaussian shield to a
twisted coaxial network that was fed to the input preamplifier in a differential mode
configuration. The output from the preamplifier was also fed to the lock-in amplifier
through twisted pair coaxial cables in a differential mode configuration. The
microphone requires a supply voltage of 0.9 V to 1.6 V volts and this was supplied
from a standard laboratory power supply. All of the electronic circuitry i.e. the
microphone and its power supply, the lock-in amplifier, the optical chopper driver and
the monochromator were all powered from a mains voltage supply independent to that
supplying the arc lamp power supply. This ensured any potential voltage variations
due to the operation of the lamp did not couple into the rest of the system. A plurality
of switchable microphones may also be used to enhance the photoacoustic signa1-to-
noise ratio.
Control Hardware
The full potential of the optical and electrical equipment previously described can
only be harnessed by placing the entire system under the control of a personal
computer. All the equipment in the system may be controlled directly or indirectly via
the IEEE 488.2 GPIB or RS-232 communication standards. Conventional GPIB
provides a modular robust approach for interfacing up to fifteen devices on a single
data bus. Unlike RS-232, where parameters such as baud rate, parity and the number
of stop bits have to be known, any device adhering to the GPIB standard may be
connected to the bus with little or no knowledge of its communication requirements.
In conjunction, RS-232 does not readily permit simultaneous communication with
several devices without the use of sophisticated hardware or software routines.
GPIB devices communicate with each other by sending device-dependent messages
and interface messages through the interface system. Device—dependent messages,
commonly known as data messages, contain device specific information such as
programming instructions that control its operation. Interface messages are primarily
concerned with bus management. Interface messages perform functions such as
initialising the bus and addressing devices. GPIB devices may be categorised as
talkers, listeners and controllers. Listeners are devices that may receive data
transmitted by a talker. For example, in the spectrometer, the lock-in amplifier acts as
both a talker (transmitting data to the computer) and a listener (acquiring data from the
microphone via the preamplifier). The controller, a PCI card in this application,
manages the flow of information on the bus by sending commands to all the devices.
Devices are usually connected via a shielded 24-conductor cable with both a plug and
receptacle connector at each end. The bus uses negative logic with standard TIL
levels. In order to achieve the high data transmission rates, nominally 1.5 Mbytes/s
when using a PCI controller, the physical distance between devices is limited as
follows. The maximum separation between any two devices should be less than 4 m
and the average device separation must not be greater than 2 in over the entire bus.
The total cable length must not exceed 20 m.
Software
The software developed for the photoacoustic spectrometer essentially falls into two
functional categories, that of data acquisition and data processing. The data
acquisition software is responsible for the communication and control of all the
experimental apparatus. Due to the low signal to noise ratio inherent in photoacoustic
spectroscopy, once spectra have been recorded they normally have to undergo
processing of some sort before they can be interpreted correctly.
Data Acquisition Software
An ancillary benefit of the IEEE 488.2 GPIB standard is that several companies have
developed sophisticated high-level application development tools to enable engineers
to provide application specific user interfaces to their GPIB fostered systems. Lab-
View, a product of National Instruments Inc., is a graphical program development
environment. LabView programs are called virtual instruments, or VIs for short,
because their appearance and operation mimic the actual operation of the device they
communicate with. A VI consists of an interactive user interface, a data-flow diagram
that serves as the source code and icon connections that allow the VI to be called from
higher level Vls. When amalgamated with LabView GPIB compliant instrument
drivers, sophisticated control systems may be developed with little time overhead.
Several Vls or alternative software code may be used to control both the
monochromator and the lock—in amplifier.
Photoacoustic Energy Scan
The present invention provides a photoacoustic spectrometer for the characterisation
of sub-bandgap absorption defects. The method is typically effected using a plurality
of method steps such as:
A. providing a light source having a polychromatic output substantially in
the photonic energy range 0.5 eV to 6.2 eV,
B. setting the wavelength of the light source to an initial first irradiating
wavelength,
C. irradiating the sample with said light source and detecting the acoustic
signal emitted by the sample at said wavelength,
D. incrementing the wavelength by a sequence of increment values so as
to provide a plurality of irradiating wavelengths and detecting the
acoustic signal emitted by the sample at each of said irradiating
wavelengths, and
E. relating each of the detected acoustic signals to the incident wavelength
effecting generation of said acoustic signal.
It will be appreciated that such methodology may be effected using a plurality of
different techniques. One example of a typical VI that may be used for such studies
will be examined with reference to the flowchart for the photoacoustic energy scan VI
illustrated in Figure 5. It will be appreciated that the parameters utilised in this
exemplary embodiment are typical of values that may be used and are not intended to
limit the present invention to such values. Prior to starting the VI, the user enters the
start wavelength, end wavelength and wavelength increment for the scan. The user
specifies the number of scans that are to be performed and also provides the details of
where the data is to be stored. The V1 is then started and proceeds according to the
flowchart and to the following steps:
Step 1: Moving to the initial wavelength,
Step 2: Pausing for .25,
Step 3: Increasing wavelength by a user—se1ectable increment,
Step 4: Pausing for 51, where I is the time constant for a lock-in input filter,
Step 5: Reading and displaying the current wavelength,
Step 6: Reading the photoacoustic signal magnitude from the lock—in amplifier,
Step 7: Reading the photoacoustic signal phase from the lock—in amplifier,
Step 8: Repeating Steps 3 to 7 until the current wavelength equals the final
wavelength,
Step 9: Writing data to file,
Step 10: Repeating Steps 1 to 9 in accordance with the number of scans preset
by the user.
Step 11: Finish.
Data Processing Software
In a normal photoacoustic energy scan, the data is collected from a number of
different spectra. Since the noise is assumed to be Gaussian in nature simple
statistically averaging should improve the signal to noise ratio. The averaged spectrum
is next nonnalised to an averaged spectrum of a known sample such as carbon black
powder. As averaging cannot remove all of the spectral noise, some filtering is
necessary at this stage. All of these processes will now be examined.
Spectral Averaging
During a photoacoustic energy scan, the signal magnitude and phase are read from the
lock—in amplifier as the wavelength of the incident light is varied. These three
elements are recorded in an ASCII text file. For a spectrum from a carbon black
powder where the photoacoustic effect is quite strong, nominally five scans across the
wavelength range of interest are recorded. For a semiconductor sample it is necessary
to record twenty or more scans to minimise the effect of noise. Having recorded the
spectra, they are statistically averaged. A C program has been written that reads the
directory where the spectra are stored, loads them into memory and takes the simple
average. The average is then stored in a separate file for normalisation.
Spectral Normalisation
When a photoacoustic spectrum is recorded, superimposed on the signal from the
sample itself is a photoacoustic signal due to the spectral distribution of the optical
system, the cell and the microphone. Nonnalisation is the process where these errors
are corrected.
This is performed by normalising the photoacoustic response of the specimen with
that of a fine powder of carbon black. The latter acts as a true light trap with flat
response at all wavelengths. During the normalisation procedure the user is asked to
enter the name of the averaged spectrum to be normalised along with the name of the
averaged carbon black spectrum used in the process. Each sample spectral point is
divided by its corresponding carbon black point. The normalised data is then stored in
a file for filtering.
Spectral Filtering
Due to the fact that the constructed spectrometer is a single beam one, and therefore,
by the process of normalisation fluctuations in the normalised photoacoustic spectrum
are bound to happen. The previous averaging and normalisation processes in
themselves could enter some element of noise which could alter to some extent the
final result. It is therefore desirable to perform some form of spectral filtering or
smoothing to remove any noise. Two filters have been developed for use with the
data. The first is a simple n—point smoothing window where each spectral point is
replaced by a local average of the nL data points to the left and nR data points to right
of it. The user specifies the number of points to be averaged. In general, it is not
recommended to average more than five points at a time or the spectrum may be
corrupted. Mathematically the form of this filter may be expressed as follows:
. = nERCnfi+n (8)
n:—n,_
where g,~ is the filtered value for the spectral point fi_ The coefficient on = 1/( nL+
(nL+1))-
It will be appreciated that such a moving average filter Works quite well for carbon
black powders as their signal to noise ratio is quite high. However, for spectra from
semiconductor samples it is dangerous to use such a filter. Suppose the spectrum
could be approximated by a function that is sufficiently differentiable such that its
second order derivative exists. In such cases, the moving average filter has the
mathematical property of reducing the value of the function when a local maximum
occurs. In a spectroscopy application, this implies a narrow spectral line will have its
height reduced and its width increased. Since these parameters are themselves of
physical interest, such filtering is obviously erroneous. Note however, that a moving
average filter will preserve the area under the peak of interest i.e. the zeroth moment.
For spectroscopy applications a filter is required that preserves the zeroth and higher
order moments. One such filter that may be implemented is the Savitzky-Golay filter.
This type of filter has the advantage that it operates directly in the time—domain, and
therefore, data does not have to be transferred back and forth between the Fourier
domain. This avoids the risk of any loss of information, i.e. introduction of noise, due
to algorithms such as the fast Fourier transform that might be used for such a process.
The basic idea behind the Savitzky-Golay filter is to find filter coefficients on that
preserve higher order moments. Equivalently, the idea is to approximate the
underlying function with a moving polynomial window. At each point f; a polynomial
is least—squares fitted to all n points in the moving window, and then g, is set to be the
value of that polynomial at position i.
Photoacoustic Depth Profiling
The apparatus of the present invention may be adapted to provide for photoacoustic
depth profiling. It will be appreciated that as the probe depth, i.e. the depth to which
the incident beam is irradiated or penetrates into the sample, is adjusted by varying the
chopping frequency, it may be possible to carry out depth profiling on the sample.
The relationship between the thermal diffusion length of the heat source (the probe
depth) generated by the exciting light source and the radial frequency of the
chopping/modulation of the light source is shown in the following equation:
%.)“ <9>
wherein a is the thermal diffusion coefficient, co is the chopping frequency, and (1. is
the thermal diffusitivity, and where
“=7/ac
(10)
wherein k is the thermal conductivity, ,0 is density and C is the specific heat.
As shown in figure 1, the PC can control the optical chopper via the lock-in amplifier.
Thus, by varying the modulation frequency through a judicious selection of a suitable
optical chopper, it is possible to probe information from different thermal lengths
within a sample.
For example, with a sample of Si, at 50 Hz and 300 Hz the probe depths are 742
micrometers and 302 micrometers. This is in contrast to the penetration depth of the
incident optical photons - suppose they have energy approx. 1.3 eV then their
penetration depth is about 100 micrometers. Therefore PAS can obtain information
from regions (i .e. probe depths) not conventionally accessible through optical
techniques such as photoluminescence and Raman spectroscopy (both of which
incidentally are radiative techniques). Thus one could envisage probing dopant
distributions in wafers as a function of depth and position on the wafer and building
up a 3D tomographic image of same. This may be used, for example, in the study of
the thermoelastic properties of semiconductor materials in particular at dopant
interfaces.
lt will be appreciated that means may be provided for cooling or heating the cell to
allow the effect of temperature on the photoaccoustic signal to be monitored. The
means for cooling may be adapted to maintain the cell at temperatures in a
controllable range from below 273K. For temperatures down to about 77K, apparatus
such as a cryostat (for temperatures down to 77K) or Peltier cooler (for 215K - 273K
range) may be used. Alternatively, liquid helium may be used to ‘cool the sample
further, at which temperature the non-radiative or phonon mediated effects should
disappear or be significantly reduced. In one embodiment, the cell may be adapted so
as to allow liquid helium to pass through the walls of the cell to cool the sample
indirectly. It will be appreciated that in accordance with the classical photoacoustic
theory of Rosencwaig and Gersho that this cooling will serve to improve the
photoacoustic signal—to-noise ratio
Again, when heating the cell, the means for heating may heat the sample directly or
indirectly. For example, the cell may include a heating stage which may be used to
control the temperature of the cell over a specific range. It will be appreciated that the
sample may be heated directly by applying an electric field across the sample material
in a direction perpendicular to the direction of the incident light, thus encouraging
Joule heating of the sample.
It will be further appreciated that it may therefore be possible to record the acoustic
signal over a range of temperatures, so as to enable the relationship between
temperature and the acoustic signal to be investigated. Software may be provided to
enable a photoacoustic spectra to be recorded during heating or cooling of the sample.
The effect of change of temperature on the stress of the sample may then be
investigated.
It will be understood that herein has been described a method and apparatus for
investigating the photoacoustic spectrum of sample materials in an extended
wavelength range to that hereinbefore possible. In preferred embodiments, the range is
over the entire spectrum of about 05eV to about 6.2eV, although it will be
appreciated that for certain materials sub-ranges within this extended range may be
determined as being more suitable and as such the obtaining of data from a complete
spectrum will not be required. It will be further appreciated that the methodology and
system of the present invention enable an investigation of distinct areas within a
sample material such that specific defects may be associated with specific areas on the
sample material and that other areas may be defined as being substantially defect free.
Such spatial analysis of a sample material provides a more efficient analysis of a
sample material than hereinbefore possible.
The words “comprises/comprising” and the words “having/including” when used
herein with reference to the present invention are used to specify the presence of
stated features, integers, steps or components but does not preclude the presence or
addition of one or more other features, integers, steps, components or groups thereof.
Claims (4)
- A photoacoustic spectrometer apparatus adapted to enable an observation and characterisation of non-radiative sub bandgap defects in narrow and large bandgap materials using photoacoustic spectroscopy techniques, the apparatus providing for an irradiation of a sample material provided within a photoacoustic cell and the subsequent detection and processing of an acoustic signal emitted by the sample, the apparatus comprising: a) a light source having a polychromatic output substantially in the photonic energy range 0.5 eV to 6.2 eV, b) focusing means adapted to couple the output from the light source onto the sample material, the focusing means providing for an alignment and focusing of the light emitted from the light source so as to provide a substantially parallel incident light onto the sample material, and c) means for detecting and acquiring said acoustic signal emitted by the sample in response to said irradiation.
- The apparatus as claimed in claim 1 further including means for modulating the polychromatic light.
- A method of providing an acoustic signal spectrum emitted by a sample material provided within a photoacoustic cell following irradiation of the sample by an incident light beam, the method comprising the steps of: a) providing a light source having a polychromatic output substantially in the photonic energy range 0.5 eV to 6.2 eV, b) setting the wavelength of the light source to an initial first irradiating wavelength, C) irradiating the sample with said light source and detecting the acoustic signal emitted by the sample at said wavelength, d) incrementing the wavelength by a sequence of increment values so as to provide a plurality of irradiating wavelengths and detecting the acoustic signal emitted by the sample at each of said irradiating wavelengths, and relating each of the detected acoustic signals to the incident wavelength effecting Ln generation of said acoustic signal.
- 4. A method substantially as hereinbefore described with reference to and/or as illustrated in
Publications (2)
Publication Number | Publication Date |
---|---|
IE20030396U1 IE20030396U1 (en) | 2004-11-17 |
IES83668Y1 true IES83668Y1 (en) | 2004-11-17 |
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