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6-1-2008
Scanning tunneling optical resonance microscopy applied to indium arsenide quantum dot structures
Daniel P. Byrnes
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Recommended Citation Byrnes, Daniel P., "Scanning tunneling optical resonance microscopy applied to indium arsenide quantum dot structures" (2008). Thesis. Rochester Institute of Technology. Accessed from
This Thesis is brought to you for free and open access by RIT Scholar Works. It has been accepted for inclusion in Theses by an authorized administrator of RIT Scholar Works. For more information, please contact [email protected]. SCANNING TUNNELING OPTICAL RESONANCE MICROSCOPY APPLIED TO INDIUM ARSENIDE QUANTUM DOT STRUCTURES
By
Daniel P. Byrnes
B.A. State University of New York at Geneseo
(2004)
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in the Chester F. Carlson Center for Imaging Science of the College of Science
June 2008
Signature of the Author______
Accepted by______Coordinator, M.S. Degree Program Date CHESTER F. CARLSON CENTER FOR IMAGING SCIENCE COLLEGE OF SCIENCE ROCHESTER INSTITUTE OF TECHNOLOGY ROCHESTER, NEW YORK
CERTIFICATE OF APPROVAL
______
M.S. DEGREE THESIS
______
The M.S. Degree Thesis of Daniel P. Byrnes has been examined and approved by the thesis committee as satisfactory for the thesis requirement for the Master of Science degree
______Dr. Ryne Raffaelle
______Professor Richard Hailstone
______Dr. Michael Kotlarchyk
______Date
ii THESIS RELEASE PERMISSION ROCHESTER INSTITUTE OF TECHNOLOGY COLLEGE OF SCIENCE CHESTER F. CARLSON CENTER FOR IMAGING SCIENCE
Title of Thesis: SCANNING TUNNELING OPTICAL RESONANCE MICROSCOPY APPLIED TO INDIUM ARSENIDE QUANTUM DOT STRUCTURES
I, Daniel P. Byrnes, hereby grant permission to the Wallace Memorial Library of R.I.T. to reproduce my thesis in whole or in part. Any reproduction will not be for commercial use or profit.
Signature:______
Date:______
iii SCANNING TUNNELING OPTICAL RESONANCE MICROSCOPY APPLIED TO INDIUM ARSENIDE QUANTUM DOT STRUCTURES
By
Daniel P. Byrnes
B.A. State University of New York at Geneseo
(2004)
Submitted to the Chester F. Carlson Center for Imaging Science College of Science in partial fulfillment of the requirements for the Master of Science Degree at the Rochester Institute of Technology
ABSTRACT
The technique of Scanning Tunneling Optical Resonance Microscopy (STORM) has been investigated for use on nanostructures. It has been demonstrated as a viable technique to characterize both bulk and nanostructured materials by optically pumping the tip-sample junction with variable energy photons thereby changing the electronic signature in the scanning tunneling microscope allowing for the determination of the local absorption spectrum. STORM offers an alternative technique to characterize very small structures that lie beyond the limits of more conventional approaches.
iv ACKNOWLEDGEMENTS
I would like to thank NASA Glenn Research Center in Cleveland, Ohio for allowing me use of all of the equipment available. I am especially grateful for the support of Dr. Padetha Tin and Dr. Sheila Bailey during my residence at NASA Glenn.
Dr. Seth Hubbard was a source of inspiration, knowledge and encouragement at NASA as well as RIT. He grew all of the samples presented in this work and played a pivotal role during the course of my research.
I would like to thank my colleagues at Veeco Turbodisc for allowing me to use their time to finish the work presented in this manuscript. Thank you to my employer Dr.
Dong Lee for his support and encouragement to complete the research. I would also like to thank Dr. Ryne Raffaelle for offering encouragement. Finally, I would like to thank all of my friends and family. Thank you.
v TABLE OF CONTENTS
ABSTRACT...... iv
ACKNOWLEDGEMENTS ...... v
TABLE OF CONTENTS ...... vi
TABLE OF FIGURES...... vii
LIST OF TABLES ...... ix
I INTRODUCTION...... 1 1.1 MOTIVATION...... 1 1.2 SCANNING TUNNELING MICROSCOPY...... 7 1.3 ATOMIC FORCE MICROSCOPY...... 10 1.4 TI:SAPPHIRE RING LASER ...... 13 1.5 METAL ORGANIC CHEMICAL VAPOR DEPOSITION (MOCVD)...... 14
II EXPERIMENT ...... 15 2.1 STORM SETUP...... 15 2.2 STM TIP PREPARATION...... 17
III RESULTS...... 20 3.1 GALLIUM ARSENIDE STUDY...... 20 3.2 QUANTUM DOT MEASURMENT...... 25
IV CONCLUSIONS & RECOMMENDATIONS ...... 48 4.1 FUTURE RESEARCH AND DEVELOPMENT...... 48 4.2 CONCLUSION...... 52
V APPENDIX A ...... 53
VI REFERENCES ...... 57
vi TABLE OF FIGURES
Figure 1. Schematic of STM operation showing atomic interaction between the tip and sample...... 9
Figure 2. (a) Schematic of first AFM setup with STM as delflection detector. (b) Modern AFM using optical lever to sense the vertical position of the cantilever. ... 11
Figure 3. Schematic representation of STORM experimental setup...... 16
Figure 4. STM tip etching setup...... 19
Figure 5. Spectra of GaAs samples (a) STORM (b) PL (c) STORM and PL overlay for comparison...... 21
Figure 6. STORM spectra of GaAs samples with different doping levels...... 23
Figure 7. Low temperature PL of quanutm dot samples...... 24
Figure 8. Quantum dot low temperature PL spectra...... 26
Figure 9. AFM of Sample A (a) Height image (b) 3-D image...... 29
Figure 10. AFM of Sample B (a) Height image (b) 3-D image...... 30
Figure 11. AFM of Sample C (a) Height image (b) 3-D image...... 31
Figure 12. AFM of Sample D (a) Height image (b) 3-D image...... 32
Figure 13. AFM of Sample E (a) Height image (b) 3-D image...... 33
Figure 14. PL Spectra of quantum dot samples...... 35
Figure 15. Overlay of PL spectra for comparison...... 37
Figure 16. Sample B STORM and PL signal overlay...... 38
Figure 17. Sample C STORM and PL signal overlay...... 39
Figure 18. Sample D STORM and PL signal overlay...... 40
vii
Figure 19. Sample E STORM and PL signal overlay...... 41
Figure 20. STORM spectra ovelaid to show lamp/monochromator system efficiency effects...... 44
Figure 21. STORM spectra of all samples...... 45
Figure 22. STORM spectrum of sample A...... 47
Figure 23. Illustration showing increased output of second harmonic using a periodic poling technique...... 51
viii LIST OF TABLES
Table 1. Bandgap of doped GaAs as determined by STORM and PL...... 24
Table 2. Table showing the STORM and PL measurements on quantum dots...... 42
ix I INTRODUCTION
1.1 MOTIVATION
In recent years since the Kyoto Protocol, it has been recognized by the majority of
the planet that global warming has been accelerated by human activities. 1 The burning of fossil fuels for electricity generation produces most of the pollutants considered to be greenhouse gases which has lead to the active development of clean energy production with a major focus in solar technology. Solar cells have been around since the late
1800’s2 and there is currently a lot of research interest in photovoltaics.
There are several types of solar cell technology employed throughout the world, such as single junction, multijunction, concentrators, polymeric and solid state. The limiting factor in these technologies is the conversion efficiency. There are a variety of device issues related to current extraction, but the fundamental limit of efficiency is determined by the opto-electronic properties and quality of the active material. The maximum conversion efficiency for a single junction solar cell has been calculated to be about 30 %. 3 The reason for this number is photons of different energies have different
penetration depths. Thus, the further away from the junction the photon is absorbed via the creation of an electron-hole pair, the less likely the electron and hole will be separated
by the electric field in the junction. Electron-hole pairs that are created but do not contribute to current, but instead are absorbed via some non-radiative mechanism such as a crystal defect, lead to increased heat generation in the device, which decreases the
1 efficiency of the system. The higher energy photons contribute to surface heating of the material and the less energetic photons pass through the material with low probability of absorption. Both contribute to a decrease in efficiency. Due to the absorption physics,
placement of the junction and thickness of the solar cell layers is critical in device design.
For multijunction cells, the maximum conversion efficiency has been calculated
by taking the limit of an infinite stack of materials with bandgaps directly matched to the
solar spectrum. The theoretical limit for a multijunction cell has been calculated to be
approximately 66 %. 4 The current state-of-the-art terrestrial solar cells are triple-junction using an InGaP/GaAs/Ge stack. The materials in the InGaP/GaAs/Ge stack have
bandgaps of 1.9 eV, 1.43 eV, and 0.66 eV respectively, or roughly 650 nm, 865 nm, and
1880 nm respectively. The InGaP is used for absorption from about 300 nm to 700 nm, the GaAs is for absorption from 600 nm to about 900 nm, and the Ge is for increased infrared absorption from about 900 nm to over 1600 nm. Spectrolab reports efficiencies for extra-terrestrial applications of around 27 % at air-mass zero (AM0). 5 For terrestrial applications at AM1.5, these cells need to be used in conjunction with concentration devices to achieve high efficiency. For terrestrial applications under concentration, the current world record for efficiency is held by Spectrolab at 37.3 %. 6 Triple-junction cells are among the most efficient but also among the most expensive to produce. The complexity of the design requires that these structures are grown in a highly controlled environment, such as a metal organic chemical vapor deposition (MOCVD) system, unlike bulk silicon which can be manufactured very cheaply in large deposition systems.
2 Triple-junction cells also require tunnel junctions for current extraction that are deposited in between the active layers. Tunnel junctions are often a different composition than the active material. Thus, one has to take into account lattice mismatch and adjust growth
parameters to achieve a smooth interface. Furthermore, the tunnel junction is generally not perfect in charge transport contributing to power loss. Depending on the material, the tunnel junction may absorb incoming light leading to further inefficiency by taking away
potential power generation from the active material below. Since the solar cell junctions
are in a series circuit, the current is limited by the worst tunnel junction so great care
must be taken in selecting appropriate materials and growth conditions to insure a quality
junction.
For extra-terrestrial applications where the cost of space transportation is based
primarily on the payload mass, the triple-junction solar cells are preferred due to the their
power to mass ratio being higher than other types of cells (i.e. silicon). For terrestrial systems, however, triple-junction cells add to costs, because not only is production complicated, but they also need to be used in expensive solar tracking, and concentration systems for high efficiency operation.
An alternative approach to high efficiency solar cells is to use a single-junction
GaAs, p-i-n cell with InAs quantum dot stacks in the i layer, thereby reducing manufacturing complexity and thus expense, while maintaining high efficiency. Adding a stack of quantum dots can theoretically enhance the efficiency of a single junction cell
3 to over 60 %. 7 Furthermore, the bandgap of quantum dots is directly related to their size
(assuming a sphere), given by:
h 2 1 1 ∆E = + g 2 * * 8dQD me mh Equation 1
* * Where h is Planck’s constant, d is the quantum dot diameter, and me , mh are the effective masses of electrons and holes respectively. Thus, the absorption of the quantum dots can be tuned for optimum efficiency at specific wavelengths by controlling their diameter. 8
In order to optimize the fabrication and incorporation of nanostructured materials into solar cell structures, it is necessary to be able to understand and properly characterize these structures. The opto-electronic properties of nanostructured materials must be known in order to make predictions about and correlate with device performance.
Standard characterization techniques, such as photoluminescence and electroluminescence, tend to excite many nanostructures simultaneously. If the structures are not identical, then the individual spectra overlap and are indistinguishable from each other. Most nanoscale structures are formed through a process of self-assembly yielding a large range of sizes further complicating bulk spectroscopic analyses. Nanostructures can also be formed by arranging atoms or molecules in a specific manner with a scanning tunneling microscope (STM) 9 or optical tweezers,10 but these techniques are at present too time consuming for the large scale production required for solar cell arrays.
4 Since the opto-electronic properties of nanostructures are size-dependent, it is necessary to be able to characterize individual structures of different dimensions to determine which structure may be the most suitable for a particular device. One method of nanostructure characterization is near field scanning optical microscopy (NSOM).
NSOM is a form of scanning probe microscopy (SPM) and was first demonstrated in the visible wavelength regime in 1984 by Pohl et al. 11 NSOM uses a very small diameter, metallized optical fiber as a probe. It operates much the same way as an STM by
bringing the probe within angstroms of the sample and taking images in the near field.
Near field imaging is able to obtain resolutions far greater than conventional, lens-based microscopes because the proximity of sample and the NSOM system allows the Abbe diffraction limit to be avoided when taking optical images.12 With NSOM, one can
perform small scale spectroscopy since the probe is an optical fiber. A sample may be illuminated with various wavelengths to obtain spectroscopic data and optical images simultaneously. Resolutions as high as 25 nm have been demonstrated with NSOM.11
Micro-photoluminescence (µ-PL) is a photoluminescence (PL) scheme using a very small diameter pump beam with a spot size typically around 1 µm2. This system
will work well if the structures are sufficiently dispersed so that very few are illuminated
at the same time. Most PL systems have fixed wavelength pump lasers. This creates a
problem if the substrate and quantum dots have similar bandgap energy. This may cause
the substrate to absorb the pump laser as well as the quantum dots and since there will be
5 more signal coming from the substrate than the quantum dots, the quantum dot signal will
be indistinguishable from that of the substrate. 13
There are a variety of characterization techniques that fall in the category of
photo-assisted STM. The first photo-assisted STM measurement was demonstrated on a
semiconductor sample in 1987. 14 Van de Walle et al. used a He-Ne laser to increase
carrier density in semi-insulating GaAs, thereby reducing the resistivity of the sample.
The photoconduction allowed for imaging with both a positive and negative tip bias. It
was demonstrated by Van de Walle et al. that it is feasible to envision a scenario where insulating samples could be measured by STM under illumination.
The characterization technique discussed in this report is a photo-assisted STM technique called scanning tunneling optical resonance microscopy (STORM). STORM has been developed in collaboration with NASA Glenn Research Center and the
Rochester Institute of Technology and is a novel characterization technique that will allow opto-electronic measurements of individual nanostructures. 15
The STM operation is based on the quantum mechanical tunneling properties of electrons. To enhance the signal, a bias is applied to a sample thereby reducing the energy of the vacuum barrier and creating a larger probability of tunneling. By optically
pumping the tip-sample junction, a photoenhanced tunneling current may be detected,
provided the photon energy is greater than the work function of the sample. The
photoenhanced current can then be correlated to the electronic transitions of the sample yielding spectroscopic information about the energy level transitions. By using a tunable
6 radiation source to illuminate a sample, a unique current signature can be detected by
STM. As the illumination energy approaches the bandgap energy of a material, the material will exhibit a photo-enhanced tunneling current that can be detected by the STM using a lock-in technique. 16 The STM also allows imaging of the surface of the sample
so the relationship between the size of a nanostructure and its opto-electronic properties
can be seen.
1.2 SCANNING TUNNELING MICROSCOPY
The scanning tunneling microscope was first demonstrated at IBM in 1982 by
Gerd Binnig and Heinrich Rohrer. Binnig and Rohrer showed that their invention was
capable of imaging the atomic structure of silicon.17 The capability of the STM allowed
Binnig and Rohrer to share in a Nobel Prize four years after the first demonstration.
The STM operates by the quantum mechanical principle of electron tunneling.
The tip is brought within angstroms of the surface, which allows the tip and the surface
atoms’ wavefunctions to overlap. Similar to a particle in a box, the small region of space
between the tip and sample creates a vacuum barrier. Quantum mechanics allows for a
finite probability that a surface electron will be found at the STM tip. To assist the
tunneling, a bias is applied to the tip and sample. The bias effectively shifts the Fermi
level in the tip and sample to lower the barrier energy. The lower barrier energy allows
for an increased tunneling probability and higher signal. The STM is scanned across a
small area of the sample by piezo-electric drivers which are capable of angstrom
7 positioning resolution (Figure 1). The tunneling current is then processed by a feedback loop while the piezos respond to maintain a constant current or height.
The tunneling current can be approximated by the following relation:
− A ϕs J T ∝ e Equation 2 where J T is the tunneling current, φ is the barrier height, s is the tip-sample distance and
2m A = where h is Planck’s constant and m is the free electron mass.18,19 The h
exponential dependence of tunneling current on tip-sample separation means that the
tunneling current drops to virtually nothing within just a few nanometers. This feature
makes the STM extremely sensitive to height variations across a sample.
The STM does not measure the topography of a sample directly. It measures the
local density of states (LDOS) on the surface. For defect-free samples with a constant
work function across the surface, the LDOS closely matches the topographic profile. 20
The sensitivity to the LDOS makes the STM a useful tool in investigating the electronic
properties of a material; however, in the case of semiconductors, this can cause serious
problems since the LDOS of semiconductors tends to be strongly dependent on the applied bias. For example, the method to image the atomic arrangement of gallium and arsenic in GaAs is to take one image with a positive bias on the sample to image the As atoms and then reverse the bias to image the Ga atoms.
8 Y piezo Z piezo
X piezo
STM Tip Computer Controller
Sample
Figure 1. Schematic of STM operation showing atomic interaction between the tip and sample.
9 Then, one must overlay the two images to see the actual structure. This is due to the valence and conduction states being preferentially located on the As and Ga atoms respectively. 21 The dependence on sample bias can yield artifacts resulting in images that may have little or no resemblance of the actual surface topography for semiconducting materials.
1.3 ATOMIC FORCE MICRSCOPY
The atomic force microscope (AFM) was first introduced by Binnig et al. in
1986. 22 The AFM operates like a small record player. A tip is mounted on an extremely
flexible cantilever and allowed to be dragged across the surface of a material by the same
piezoelectrics that are used in the STM to keep a constant tip force on the sample. The
deflection of the tip is measured as the tip is raster scanned across the surface and relates
directly to the surface topography. The first design by Binnig et al. employed a STM
above the AFM cantilever to detect the deflection (Figure 2a). Many commercially
available systems use an optical lever as the feedback mechanism. The optical lever is a
way to measure the deflection of the AFM cantilever by reflecting a reference laser off it.
A position sensitive detector is then used to integrate the laser reflected signal to
determine the magnitude of the deflection (Figure 2b). The optical lever can magnify the
deflection of the tip over 1000 times providing the optical path length is far greater than
the cantilever length. This is how the AFM is able to achieve angstrom height sensitivity
and measure pico-Newtons of force. 23 It is possible to achieve atomic resolution with an
10 Position Sensitive Computer Controller Detector Computer Controller
Laser
xyz AFM Cantilever AFM Cantilever STM piezo
xyz xyz piezo piezo
Sample Sample
(a) (b)
Figure 2. (a) Schematic of first AFM setup with STM as deflection detector. (b) Modern AFM using optical lever to sense the vertical position of the cantilever.
11
AFM, but it tends to be more difficult than the STM due to the AFM tips having much larger tip radii. 24 However, one major advantage the AFM has over the STM is the capability to measure samples that are not conductive. The STM is incapable of measuring insulating samples and requires very careful operation when measuring semiconducting materials.
Tips for the AFM are generally made from SiN or some variation. Due to the tip composition, existing photolithography methods can be used to manufacture tips with minimal variation in shape from tip to tip. 25 If a manufacture error occurs, the resulting tip may not have the desired shape leading to artifacts in the image. Furthermore,
because the AFM tip is actually in contact with the sample, it may collect contaminants from the surface also causing image artifacts. Artifacts in AFM images are usually easily recognized by eye with experience. STM artifacts are much less obvious because of the complicated tip shape and electronic properties of the sample. This can lead to misinterpretation of image data. The AFM may be operated in a contact mode or tapping mode. Contact mode is when the AFM tip maintains contact with the sample during the entire imaging process. Tapping mode oscillates the cantilever close to its natural resonance frequency by a piezo on the tip holder, hence the tip “taps” the sample during imaging. Tapping mode significantly reduces lateral forces that may impart a torque on the cantilever which may cause image artifacts.
12 Recent advances in AFM tip technology have allowed for a wide range of sample
properties that can be quantified by AFM. Depending on the tip material or coating, one can measure magnetic, tensile, thermal, and electronic properties as well as chemical interactions while imaging the surface. The AFM can be operated in fluids and also in an ultra-low force mode for imaging living cells, even capturing movies of different cellular events. The versatility of the AFM is arguably greater than that of the STM, but both tools are powerful in materials science and give access to a large amount of information of material properties.
1.4 TI:SAPPHIRE RING LASER
The STORM technique requires a continuous, widely tunable radiation source. A source of tunable radiation that is commercially available is the Ti:Al 2O3 ring laser. The
Ti:Al 2O3 crystal has a fluorescence spectrum ranging from approximately 650 nm to
1050 nm and an absorption peak around 500 nm. This means that the 514 nm line of an argon ion laser or 532 nm diode pumped solid state (DPSS) laser may be used as a pump source and the realistic usable lasing range is from 700 nm to 1000 nm. The Ti:Sapphire has a broad lasing range due to the titanium impurity in the sapphire. The titanium undergoes Zeeman splitting of its energy levels. The energy levels are then broadened by
phonon vibrations and perturbations in the electric field of the crystal to cause overlapping energy states. 26 The Ti:Sapphire ring laser has a wide spectral range requiring three different sets of optics to access the entire lasing spectrum. Tuning of the
13 laser is accomplished by rotating a birefringent crystal filter to match the phase of the desired lasing wavelength.
1.5 METAL ORGANIC CHEMICAL VAPOR DEPOSITION (MOCVD)
The quantum dots for incorporation into the GaAs solar cell are grown epitaxially by metal organic chemical vapor deposition (MOCVD). MOCVD is a complex deposition system that uses low vapor pressure gases to flow material over a heated susceptor. A substrate material is placed in the susceptor and heated to allow the gases to
“crack” just above the susceptor, depositing the desired material uniformly across the substrate while the carrier gases are vented out of the system. The deposited material arranges itself on the host material into a crystalline structure. Quantum dots can be formed in an MOCVD system by the Stranski-Krastanow method. The dot material and substrate material must have sufficient lattice mismatch so that there is strain, but not enough to cause 3-D, or Volmer-Weber growth. The induced strain between the two different layers cause the quantum dots to form spontaneously leaving a thin wetting layer on the substrate. The formation of the quantum dots is a random process that is influenced by the substrate geometry, material defects, and reactor conditions. Hence, it is very difficult to control the size distribution and location of dots with MOCVD. 27
14 II EXPERIMENT
2.1 STORM SETUP
The STORM system uses a Coherent brand Verdi-V10 laser model which is a diode pumped, frequency-doubled neodymium yttrium vanadate in a ring configuration for a stable 532 nm output. The Verdi-V10 pumps the Coherent brand 899-01
Ti:Sapphire ring laser. The birefringent filter in the Ti:Sapphire laser has been retrofitted with a stepper motor for computer controlled wavelength tuning. The Ti:Sapphire output
passes through an intensity stabilizer to maintain constant illumination power throughout the duration of the experiment. A 90:10 beam splitter splits the beam with the 10 %
power beam passing into a Burleigh WA-1000 model wavemeter. The wavemeter measures the wavelength with an accuracy of .1 nm. The 90 % beam passes through a mechanical chopper before entering a multimode fiber for delivery to the tip-sample
junction. The output of the delivery fiber is collimated and has a spot size on the sample of about 1 cm2. The chopper is used as the reference signal in a Stanford Research
Systems lock-in amplifier. The scanning probe microscope (SPM) system is a Digital
Instruments/Veeco D3100. The SPM is enclosed in an acoustic hood on a mechanical vibration isolation table (Figure 3). The D3100 has the capability to be used as a STM or
AFM. Thus, the D3100 was used for all of the STM work as well as all of the AFM images.
15 Wavemeter Optical Power Meter
STM and Vibration Intensity Beam Isolation Hood Ti:Sapphire Stabilizer Splitter Verdi V-10
930-750 nm 532 nm
Chopper Fiber Coupler
Signal Access Module
Lock-in Amp STM Controller
STM Scanning Head
Sample Fiberoptic Stage Illumination
Figure 3. Schematic representation of STORM experimental setup.
16 The STM signal was extracted with a signal access module available from Veeco
Instruments, which runs in parallel with the STM, STM controller, and computer. The
STM signal travels to the lock-in amplifier from the signal access module and is locked- in with the chopper frequency. All of the wavelength, STM signal, and illumination
power were collected via GPIB and RS232 connections by a LabView acquisition
program.
2.2 STM TIP PREPARATION
The resolution of the STM is limited only by the tip radius.19 As the tip becomes
sharper, the resolving power is increased. Thus, an atomically sharp tip will be able to
image structures with atomic resolution.
Typically, STM tips are made from platinum/iridium alloy or tungsten. It is
possible to produce an atomically sharp tip by cutting a thin metal wire at an angle with a
pair of scissors. When the tip is cut from a wire, several jagged whiskers are generally
formed. If one whisker is longer than the others, tunneling will then take place from the
end of that single whisker. 28 Cutting wire with scissors tends not to yield a large amount
of high quality tips. There is a lot of human error introduced during the cutting
procedure, which can lead to numerous tips that are unsuitable for imaging.29 A more
reproducible method of fabricating STM tips is to use a chemical etch. Tungsten is easier
to etch than Pt/Ir and most commercially available etched tips are made from tungsten
wire. 30
17 Tips for the STORM experiment were fabricated from both cutting Pt/Ir wire and etching Pt/Ir wire. The etch used a 0.2 M solution of CaCl 2 and a graphite ring as an electrode. The graphite ring was lowered in the etch solution to a depth of 5 mm from the surface. The tip was then lowered in the solution to a depth of 3 mm below the surface. A variable DC power supply was used to generate an etch voltage of 20 V
(Figure 4). The etch was allowed to proceed until the current ceased to flow between the tip and graphite electrode. 31 Subsequently, the tip was then lowered back into the solution so that it broke the surface tension. A 5 V bias was then applied to micro-etch the tip to produce a sharper tip.32
Some of the tips were mechanically cut and then etched for a very short period of
time (no longer than 5 seconds). This fast etch is to remove any small whiskers, which
increases the probability of leaving one large whisker to detect current. This technique
can also be used to remove oxide layers that form on tips exposed to the ambient air. The
thin oxide layer that forms impedes the tunneling current, reducing the sensitivity of the
STM. Since all of the samples in question are semiconducting and a large bias voltage is
needed to detect tunneling current, any extra resistance due to tip contamination creates
decreased STM signal. The fast etch technique has also been shown to work for
commercially available, mechanically cut STM tips that have formed an oxide layer.33,34
18 Micrometer
Pt/Ir Wire
DC Power
Graphite Etch Solution Electrode
Figure 4. STM tip etching setup.
19 III RESULTS
3.1 GALLIUM ARSENIDE STUDY
The first STORM experiment was conducted to verify the bandgap of three bulk
GaAs samples grown with different doping levels. All of the samples were grown on a
Veeco D125 MOCVD system. Each sample was a single layer of n-type GaAs, 2 µm thick, grown on semi-insulating GaAs. Sample A51, A52, and A53 had doping levels of
6.34 x 10 16 cm -3, 1.47 x 10 17 cm -3, and 1.24 x 10 18 cm -3 respectively, which were
determined by Hall and chemical C-V measurements. PL measurements were taken on
an Accent RPM2000 with a 532 nm DPSS laser with a resolution of 10 Å full width half
maximum (FWHM). For the PL spectra, sample A51 has a peak at 1.42 eV with a
FWHM of 0.04 eV, sample A52 has a peak at 1.43 eV with a FWHM of 0.05 eV, and
sample A53 has a peak at 1.44 eV with a FWHM of 0.08 eV ( Figure 5). The STORM
measurements were performed in the center of each sample with the lateral translation
piezos disabled. The Ti:sapphire laser was tuned from 1.34 eV to 1.46 eV in one
nanometer increments. To extract the bandgaps from the absorption profiles, the curves
were first smoothed using a 5 point adjacent averaging technique to remove spurious data
points. Next, the derivative of the STORM signal was calculated and fitted with a
Gaussian curve. The energy value at the peak of the Gaussian curve was used to define
the bandgap as measured by STORM. This method of analysis determines the bandgap
of the material to be at the maximum slope, or inflection point of the absorption curve.
20 PL Spectra GaAs
A51 PL A52 PL A53 PL
PL Intensity (a.u.) Intensity PL
1.30 1.35 1.40 1.45 1.50 1.55 1.60 Energy (eV)
Figure 5. PL spectra of GaAs samples with different doping levels.
21 Analysis of the STORM curves show, for sample A51, a bandgap of 1.40 ± 0.02eV; for sample A52, a bandgap of 1.41 ± 0.02eV, and for sample A53, a bandgap of 1.41 ±
0.03eV (Figure 6). Sample A53 had the highest doping level and the largest FWHM for the PL signal compared to the other two samples. The A53 STORM curve also shows the weakest tunneling signal of the three samples. This experiment demonstrates the
STORM optically enhanced tunneling signal dependence on doping level. As a material
becomes more metallic through higher doping, the optical enhancement of tunneling current decreases due to a higher background tunneling current caused by the extra electrons in the dopant. This also demonstrates a limit to the usefulness of STORM. In the metallic regime, it may be more advantageous to use a different form of STM spectroscopy based solely on tunneling currents and bias voltages such as scanning tunneling spectroscopy (STS) to extract material properties. 35
However, for this experiment, the PL and STORM results show agreement within uncertainty. As such, STORM demonstrates viability as a local surface absorption spectroscopy technique for semiconducting materials (Figure 7, and Table 1).
22 STORM Spectra GaAs
A51 A52 A53
STORM Intensity (a.u.) Intensity STORM
1.32 1.34 1.36 1.38 1.40 1.42 1.44 1.46 Energy (eV)
Figure 6. Storm spectra of GaAs samples of different doping levels.
23 STORM PL Spectra Comparison
A51 PL A52 PL A53 PL A51 STORM A52 STORM A53 STORM
PL Intensity (a.u.) Intensity PL STORM Intensity (a.u.) Intensity STORM
1.35 1.40 1.45
Energy (eV)
Figure 7. STORM and PL spectra overlaid to show qualitative agreement.
Sample Dopant Eg (STORM) Eg (PL) Concentration A51 6.34 x 10 16 cm -3 1.40 ± 0.02eV 1.42 eV
A52 1.47 x 10 17 cm -3 1.41 ± 0.02eV 1.43 eV
A53 1.24 x 10 18 cm -3 1.41 ± 0.03eV 1.44 eV
Table 1. Bandgap measured by STORM and PL of GaAs with different dopant concentrations.
24 3.2 QUANTUM DOT MEASURMENT
All of the quantum dots grown for the STORM experiment were composed of indium arsenide and grown epitaxially via MOCVD on GaAs substrates. The first trial runs for quantum dot growth had extremely weak PL signal which subsequently required low temperature PL measurements to determine the bandgap range of the structures. The quantum dot samples were placed in a cryogenic chamber and cooled with liquid helium to a temperature of under 10 K. Samples were excited with the 514 nm line of a 5 W
Argon ion laser. The quantum dot peaks show a peak around 1.0 eV to 1.1 eV (Figure 8).
The low temperature PL measurements of the quantum dot samples show that the
bandgaps of the quantum dots fall into the most desirable energy range for incorporation
into solar cells, which is near 1.0 eV (see Appendix A). The Ti:Sapphire laser has a
minimum photon energy of about 1.27 eV and is not suitable to be used for STORM
measurements with these quantum dots. As a result of the photon energy of the
Ti:Sapphire laser being to high for a STORM measurement, a Tungsten Halogen lamp
and monochromator were substituted in place of the Ti:Sapphire laser, wavemeter,
intensity stabilizer and beam splitter. The lamp/monochromator system has a useful
output of 400 nm to 2000 nm.
The lamp/monochromator system produces substantially less irradiance as
opposed to a laser source. Thus, a 600 µm diameter fiber optic was used to deliver light
25
PL Intensity (a.u.)
InAs QD’s GaAs Substrate
Energy (eV)
Figure 8. Low temperature PL of quantum dot samples.
26 exiting the monochromator. The fiber output was collimated and angled to illuminate the tip-sample junction in the same method as the laser source. To avoid large output power fluctuations, the Tungsten Halogen lamp was operated at the maximum operation current.
Five different quantum dot structures were grown for the STORM experiment under different growth conditions. Sample A was grown on a 2 degree offcut, semi-
insulating (SI) GaAs substrate. The 300nm GaAs layer was grown with n-type doping of
approximately 1 x 10 18 cm -3 and a 6 Å InAs quantum dot layer on top. Sample B was
grown on n-type GaAs substrate with a 2 degree miscut. The 200 nm undoped GaAs
layer was grown with an 8 Å quantum dot layer deposited on top. Sample C and sample
D were identical structures grown during the same run, but on different offcut substrates.
Sample C was grown n-type 6 degree offcut GaAs, while sample D was grown a SI 2
degree offcut GaAs substrate. The structure for samples C and D was composed of a 200
nm GaAs layer followed by a stack consisting of 6 Å quantum dots with a 2nm GaAs
cap. This was followed by a 4 nm GaAs spacer with a 26 Å layer of GaP and finished
with a 4 nm GaAs spacer. This stack was repeated four times consecutively during the
growth. The final layer was a 6 Å q-dot layer for imaging by AFM. Sample E was
grown on 2 degree SI GaAs with a similar structure to samples C and D. The only
difference in the structure of sample E from that of C and D, was a tellurium doping of 5
x 10 17 cm -3 in the GaAs spacer layer. The addition of tellurium was to indirectly
incorporate the dopant into the quantum dot structures through diffusion.
27 All of the quantum dot samples were characterized by AFM and by photoluminescence on the Accent RPM2000. The size of the quantum dots were determined by AFM measurement. The dimensions of the quantum dots are in the range of 2 to5 nm vertically and 25 to 40 nm in diameter (Figure 9 through Figure 13). The larger dots present in the AFM images are over 10 nm tall and over 60 nm in diameter.
The larger dots do not contribute significantly to the PL signal. 36 Quantum dots with a height of greater than 10 nm have been shown to have weaker PL signal. As the quantum dots become taller, they begin to acquire the properties of the bulk material and lose the
beneficial effects of quantum confinement. Thus, large quantum dots are undesirable for
incorporation into solar cells (see Appendix A). Noticeable in the AFM image is the
different surface morphology of sample C as compared to the other samples. The
difference in morphology can be attributed to the substrate miscut upon which sample C
was grown. Sample C was grown on a 6 degree miscut substrate, indicating that the
terrace steps are narrower than the standard 2 degree miscut wafers. Since the quantum
dots preferentially nucleate on close to the step edges, a 6 degree miscut substrate does
not have step widths that are not sufficiently large to accommodate the quantum dots.
Hence, the quantum dots tend grow across multiple step edges. This phenomenon can
lead to defect propagation through the center of the dot causing it to contribute less to the
PL signal or current generation in a working device. 37 Sample C also has the lowest PL intensity quantum dot peak when compared to the other samples.
28 Sample A
(a)
(b)
Figure 9. AFM micrograph of sample A (a) Height image. (b) 3-D image.
29 Sample B
(a)
(b)
Figure 10. AFM micrograph of sample B (a) Height image (b) 3-D image.
30 Sample C
(a)
(b)
Figure 11. AFM micrograph of sample C (a) Height image (b) 3-D image.
31 Sample D
(a)
(b)
Figure 12. AFM micrograph of sample D (a) Height image (b) 3-D image.
32 Sample E
(a)
(b)
Figure 13. AFM micrograph of sample E (a) Height image (b) 3-D image.
33 All of the PL spectra for the quantum dot samples exhibit three distinct peaks.
These peaks correspond to the wetting layer (a thin quantum well that forms just before quantum dot nucleation) close to 1.34 eV and the GaAs substrate peak close to 1.43 eV.
The third peak can be seen at 1.20 eV (Figure 14). This corresponds to the quantum dots, however, the spectra have been truncated due to the limitations of the silicon detector.
Silicon responsivity drops rapidly close to 1.20 eV. Thus, the PL analysis displays only the leading edge of the emission of the quantum dots. An InGaAs detector array was also used for PL measurements; however the signal was too weak to be detected by the instruments. Some qualitative insight into the material quality can still be extracted from the PL data.
Sample A shows a more dominant GaAs peak in the PL profile than the other samples. This can be explained by increased PL from the doped GaAs layer due to the extra electrons available for emission. For sample E, the leading edge of the quantum dot signal is dominant. This also can be explained by the Te dopant in the quantum dots enhancing the PL signal. Sample B has an almost undetectable wetting layer emission
(Figure 14). Samples A and B are rather similar structures, however, the AFM shows that sample B has a lower density of large quantum dots compared to sample A (Figure 9 and Figure 10). Small differences in growth conditions lead to a weaker wetting layer signal in sample B than sample A. The major contribution to the PL signal in sample D is from the wetting layer and the leading edge of the quantum dot emission.
34 A B
C D
E
Figure 14. PL Spectra of quantum dot samples.
35 This is in contrast to sample C which has a noticeable GaAs signal, indicating that sample D has a higher quality InAs deposition. Sample D also has a stronger quantum dot signal than sample C and the differences in surface morphology indicate why this is so. Sample C displays tall, elongated nanostructures that have formed in the low lying areas on the surface. Many of these nanostructures may not be contributing to the PL due to the close step spacing of the 6 degree miscut substrate. The quantum dots have grown over multiple terrace steps, creating non-radiative defects, and resulting in the weaker emission spectrum. Sample D has both large and small quantum dots; however it has a higher density of quantum dots in the ideal size regime than sample C (Figure 12). Thus sample D exhibits more quantum dot emission in the PL spectrum. Figure 15 shows the relative PL intensities for all of the quantum dot samples measured with STORM.
Samples A through E were all measured with STORM and all of the samples exhibited photo-enhanced tunneling current in the energy region of the quantum dots, wetting layer, and GaAs substrate. A feature of the STORM measurement is that the spectral features are shifted to lower energies than shown in the PL data. This feature is consistent across all of the samples and is due to the applied bias during measurement.
All of the samples were negatively biased in the 300 mV to 600 mV range, decreasing the work function of the material relative to the vacuum barrier and lowering the effective
bandgap, resulting in red-shifted STORM absorption peaks (Figure 16 through Figure
19). Table 2 shows the difference in wetting layer energy as measured by STORM and
by PL.
36 PL Comparison
Sample A Sample B Sample C Sample D Sample E
PL Intensity (a.u.) Intensity PL
1.0 1.1 1.2 1.3 1.4 1.5 1.6 Energy (eV)
Figure 15. Overlay of PL spectra for comparison.
37 Sample B
STORM PL PL Intensity (a.u.) Intensity PL STORM Intensity (a.u.) Intensity STORM
0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Energy (eV)
Figure 16. Sample B STORM and PL signal overlay.
38 Sample C
STORM PL PL Intensity (a.u.) Intensity PL STORM Intensity (a.u.) Intensity STORM
0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Energy (eV)
Figure 17. Sample C STORM and PL signal overlay.
39 Sample D
STORM PL PL Intensity (a.u.) Intensity PL STORM Intensity (a.u.) Intensity STORM
0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Energy (eV)
Figure 18. Sample D STORM and PL signal overlay.
40 Sample E
STORM PL PL Intensity (a.u.) Intensity PL STORM Intensity (a.u.) Intensity STORM
0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Energy (eV) Figure 19. Sample E STORM and PL signal overlay.Table 2. Table showing the STORM and PL measurements. STORM shows a lower energy quantum dot peak and a higher energy wetting layer peak due to the applied bias.
41
STORM Peak Signal Sample B Sample C Sample D Sample E
Quantum Dots 1.14 eV 1.15 eV 1.18 eV 1.11 eV
Wetting Layer N/A 1.35 eV 1.36 eV 1.36 eV
PL Peak Signal Sample B Sample C Sample D Sample E
Wetting Layer 1.36 eV 1.33 eV 1.32 eV 1.33 eV
Table 3. Table showing the STORM and PL measurements. STORM shows a lower energy quantum dot peak and a higher energy wetting layer peak due to the applied bias.
42
Another noticeable feature of the STORM curves is a significant drop of intensity in signal as the illumination energy approaches 1.70 eV. This is caused by the lamp/monochromator system. The Tungsten Halogen lamp has a non-linear irradiance across the entire spectral region. Furthermore, the optics inside the monochromator contribute a different non-linearity in reflection efficiency. The result is a significant decrease in output power of the illumination system at 1.70 eV (Figure 20). These features are obvious in all spectra and are simply due to the illumination system efficiency. A similar situation was seen in the GaAs STORM experiment with hole
burning in the Ti:sapphire laser at 1.55 eV. Since 1.70 eV is out of the region of interest for quantum dot measurements, it does not affect the experimental results.
Sample E had a relatively weak STORM signal that is again due to the tellurium dopant contributing a higher background tunneling signal. As was seen in the GaAs study, increased n-type doping leads to a significant decrease in photo-enhanced tunneling current (Figure 5). This can be explained by the contribution of the electrons from the donor impurity. Extra electrons from the dopant increases the background tunneling current, thus significantly more electrons must be made available for tunneling through photo-excitation for the STORM system to detect the absorption. The intensity of the illumination lamp too low to obtain a good signal to noise ration from sample E.
Sample A exhibits a different STORM spectrum compared to the other samples in that the absorption features are more detailed (Figure 21).
43
STORM Spectra Comparison
Sample A Sample B Sample C Sample D
STORM Intensity (a.u.) Intensity STORM
0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 Energy (eV)
Figure 20. STORM spectra overlaid to show lamp/monochromator system efficiency effects.
44 STORM Spectra Comparison
Sample A Sample B Sample C Sample D Sample E
STORM Intensity (a.u.) Intensity STORM
0.6 0.8 1.0 1.2 1.4 Energy (eV)
Figure 21. STORM spectra of all samples.
45 Mini-band absorption occurs at 0.85 eV and 0.97 eV. A broader quantum dot absorption
peak occurs at 1.29 eV and there is a small wetting layer feature at about 1.36 eV. A
prominent spike in the STORM signal occurs at 1.40 eV and then the signal vanishes
(Figure 22). As with the previous STORM measurements on different samples, once the
illumination photons reach sufficient energy, a continuous absorption occurs as would be
expected for the system. Sample A does not continually absorb according to the STORM
data. All that is observed in the data is a rapid increase in absorption with a steep
decrease in signal. The explanation for this is that the photo-enhanced tunneling current
increased faster than the STM feedback loop could respond. This caused a rapid
oscillation in the tip-sample distance allowing the tip to contact the surface of the sample.
The STM tip was then retracted and damage to the tip and sample was verified through
an optical microscope. All of the other samples were measured with a different tip. The
reason that sample A exhibited mini-band absorption while the others did not is due to
the difference in tip radius. As was previously stated, the resolution of the STM is
dependent on the tip radius, thus a smaller tip will be able to detect smaller features. The
tip that was used to measure sample A had a small tip radius allowing tunneling from a
small sample of quantum dots. The tip used to measure samples B through E had a larger
radius thus sampling tunneling current from more quantum dots. Just as the PL
measurements do not show the features of quantum confinement due to the large sample
size, the STM is not able to detect mini-band states also due to a large sample size.
46 Sample A
STORM Signal PL Signal
PL Intensity (a.u.) Intensity PL STORM Intensity (a.u.) Intensity STORM
0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Energy (eV)
Figure 22. STORM spectrum of sample A.
47 IV CONCLUSIONS
4.1 CONCLUSION
In this work, STORM has been demonstrated to be a viable method of performing spectroscopy on nanostructured materials. Furthermore, it has been demonstrated that
STORM has overcome some of the problems associated with bulk characterization techniques including spectral sensitivity. The spectral range of STORM is limited by the tuning range of the illumination system, not the detector element. With the appropriate tip geometry, the sample size of STORM can be reduced such that spectroscopy on a single quantum dot may be achieved. In this way, STORM offers an alternative method to characterize nanostructures for the improvement of material design and for novel device design such as intermediate band solar cells.
4.2 FUTURE RESEARCH AND DEVELOPMENT
The quantum dots for incorporation into solar cells have bandgaps in an energy region where there are no commercially available tunable laser sources. Recently there has been a lot of effort put forth in researching the potential applications of optical
parametric oscillators (OPO) using periodically poled crystals as the non-linear medium to provide a widely tunable, solid-state, continuous wave radiation in the visible, near- infrared and mid-infrared.38
48 An OPO is very similar to a laser in design, but is a fundamentally different system. OPOs make use of non-linearities in the structure of the medium and require optical pumping to start the amplification process. OPOs also tend to be more tunable than lasers since the optical interaction does not involve discreet molecular states. In certain non-linear crystals, interaction with the pump signal (ω p) produces two
frequencies called the signal (ω s) and idler (ω i) which are related by the equation ω i+ ω s=
39 ωp. Many non-linear crystals are used for second harmonic generation (SHG) which is
a special case when ω i = ω s resulting in ½ the pump wavelength being conserved.
Typically, an OPO uses mirrors with reflectivities such that either the signal or idler frequencies are resonated to generate gain. OPOs may be designed to oscillate one, two or all three of the available frequencies. By allowing multiple frequencies to resonate in the optical cavity, the oscillation threshold drops dramatically and as such the pump
power can be decreased significantly. However, multiple wavelength oscillation places
severe restrictions on the optical cavity causing it to be extremely sensitive to external
influences. Thus, the environment has to be controlled very carefully.40
Lithium Niobate (LiNbO 3) has a transparency region from 350 nm to 5000 nm
making it very useful for near and mid-nfrared applications. Due to its large non-linear
coefficient of 23.2 pm/V, high damage threshold, and high quality manufacturing and
poling processes, periodically poled lithium niobate (PPLN) is a suitable medium for generation of continuous wave, near-infrared radiation in an OPO configuration.
49 The non-linear crystal, LiNbO 3, is a standard production, high purity crystal used
for many years in SHG applications. Recently, in an effort to increase the efficiency of
41 SHG from a LiNbO 3 crystal, the technique of periodic poling was developed. Periodic
poling is based on theory of Quasi Phase Matching (QPM).42 Due to dispersion effects, the fundamental and harmonic waves in a crystal have a phase velocity mismatch over a certain distance, the coherence length, where they are out of phase by 180 degrees causing destructive interference. As the two waves propagate energy is exchanged back and forth periodically resulting in a zero net gain. 40 QPM involves reversing the phase shift periodically so that the harmonic wave experiences a net gain. Periodic poling involves creating a photolithographic mask to create metal electrodes in a periodic fashion across the surface of the crystal. Then, a huge electric field is applied to
permanently reverse the ferroelectric domains of the crystal just between the top and
bottom electrodes, leaving the space between adjacent electrodes unaffected. When the two waves propagate through a poled crystal, instead of experiencing total destructive interference, they experience a net constructive interference for energy exchange resulting in higher output of SHG signal ().43 For a given pump wavelength and a desired
output wavelength, the spacing required to give the optimum conversion efficiency can
be calculated. Thus, different poling periods will correspond to different conversion
efficiencies for a given wavelength produce. With quasi phase matching, the phase
mismatch is never zero, but there is a large range of frequencies that are able to be
amplified. Birefringent phase matching, on the other hand, is much more sensitive to
50
Standard Lithium Niobate Periodically Poled Lithium Niobate
Figure 23. Illustration showing increased output of second harmonic using a periodic poling technique.
51 frequency differences since the crystal must be rotated such that both waves have the same phase velocity along a specific axis of propagation. This generally results in higher gain for birefrigent tuning than QPM systems. Furthermore, QPM allows for non-critical
phase matching throughout the entire transparency region by tuning the temperature of the crystal which facilitates power coupling for the largest element of the nonlinear tensor.44
Batchko et al. have reported a tuning range from 917 nm to 1266 nm using PPLN in a SRO, four mirror ring cavity using a 532 nm pump laser.45 Quasi-phase matched devices based on PPLN are able to provide wide tuning where the Ti:sapphire laser does not. Mainly in the energy range of the quantum dot bandgaps investigated in the STORM experiment.
52 V APPENDIX A
©2006 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.
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56 VI REFERENCES
1 “Kyoto Protocol” United Nations Framework Convention on Climate Change, http://unfccc.int/resource/docs/convkp/kpeng.html (1999). 2 "Charles Fritts." Encyclopædia Britannica < http://www.britannica.com/EBchecked/topic/220537/Charles- Fritts > (2009). 3 W. Shockley and H.J. “Detailed Balance Limit of p-n Junction Solar Cells” Queisser, J. Appl. Phys. 32 , 510 (1961). 4 A.J. Nozik, “Quantum Dot Solar Cells” Physica E 14 , 115 (2002). 5 J.E. Granata, C. Fetzer, J.H. Ermer, R.R. King, K. Edmondson, A. Stavrides, P. Hebert, P. Colter, G.S. Kinsey, M.S. Hojun Yoon Gillanders, N. Beze, K. Bui, J. Hanley, N.H. Karam, and B.T. Cavicchi, “Ultra triple-junction GaInP 2/GaAs/Ge solar cells: cell design and qualification status” Proc. of the IEEE World Conference on Photovoltaic Energy Conversion 1, 654 (2003). 6 R. R. King, R. A. Sherif, G. S. Kinsey, S. Kurtz1, C. M. Fetzer, K. M. Edmondson, D. C. Law, H. L. Cotal, D. D. Krut, J. H. Ermer, and N. H. Karam, “Bandgap Engineering in High-Efficiency Multi-junction Concentrator Cells” International Conference on Solar Concentrators for the Generation of Electricity or Hydrogen , NREL/CD-520-38172 (2005). 7 A. Luque and A. Martin, “Increasing the Efficiency of Ideal Solar Cells by Photon Induced Transitions at Intermediate Levels” Phys. Rev. Lett. 78 , 5014 (1997). 8 E. Kucur, J. Riegler, G.A. Urban, and T. Nann, “Determination of Quantum Confinement in CdSe Nanocrystals by Cyclic Voltammetry” J. Chem. Phys. 119 , 2333 (2003). 9 D.M. Eigler and E.K. Schweizer, “Positioning Single Atoms With a Scanning Tunneling Microscope” Nature 344 , 524 (1990). 10 Ashkin, A., Dziedzic, J. M. & Yamane, T., “Optical Trapping and Manipulation of Single Cells Using Infrared Laser Beams” Nature 330 , 769 (1987). 11 D.W. Phol, W. Denk and M. Lanz, “Optical Stethoscopy: Image Recording With Resolution λ/20” Appl. Phys. Letters 44 , 651 (1984). 12 E.H. Synge, "A Suggested Method for Extending the Microscopic Resolution Into the Ultramicroscopic Region" Phil. Mag. 6, 356 (1928). 13 C. Watatani, K. Edamatsu, T. Itoh, H. Hayashi, S. Shimomura, and S. Hiyamizu, “Micro- photoluminescence of Single GaAs/AlGaAs Quantum Dots Grown on a (411)A GaAs Surface” Quantum Electronics and Laser Science Conference 7-12 May , 57 (2000). 14 G. F. A. van de Walle, H. van Kempen, P. Wyder, and P. Davidsson, “Scanning Tunneling Spectroscopy on Photoconductive Semi-Insulating GaAs” Appl. Phys. Letters 50 , 22 (1987). 15 R.P. Raffaelle, T. Gennett, J. E. Lau, P. Jenkins, S.L. Castro, P. Tin, D.M. Wilt, A.M. Pal, and S.G. Bailey, “Scanning Tunneling Optical Resonance Microscopy (STORM)” Mat. Res. Soc. Symp. Proc. Vol. 738 , G3.3.1 (2003). 16 J.D. Patterson, J. Blatt, J. Burns, J.G. Mantovan, R.P. Raffaelle, “A Model of Enhanced STM Current due to Semiconductor Optical Absorption” J.of Phys. and Chem of Solids 63 , 257 (2001). 17 G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, “7x7 Reconstruction on Si(111) Resolved in Real Space” Phys. Rev. Letters 50 , 120 (1983). 18 G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, “Surface Studies by Scanning Tunneling Microscopy” Phys. Rev . 49 , 57 (1982). 19 J. Tersoff and D.R. Hamann, “Theory and Application for the Scanning Tunneling Microscope” Phys. Rev. Letters 50 , 1998 (1983).
57
20 J. Tersoff, “Anomalous Corrugations in Scanning Tunneling Microscopy: Imaging of Individual States” Phys. Rev. Letters 57 , 440 (1986). 21 R.M. Feenstra, J.A. Stroscio, J. Tersoff, and A.P. Fein, “Atom Selective Imaging of the GaAs (110) Surface” Phys. Rev. Letters 58 , 1192 (1987). 22 G. Binnig, C.F. Quate, and Ch. Gerber, “Atomic Force Microscope” Phys. Rev. 56 , 930 (1986). 23 S. Alexander, L. Hellemans, O. Marti, J. Schneir, V. Elings, and P.K. Hansma, “An Atomic-resolution Atomic-force Microscope Implemented Using an Optical Lever” J. Appl. Phys. 65 , 164 (1989). 24 Y. Sugimoto, P. Pou, M. Abe, P. Jelinek, R. Pérez, S. Morita and Ó. Custance, “Chemical Identification of Individual Surface Atoms by Atomic Force Micrsocopy” Nature 446 , 64 (2007). 25 J. Brugger, R.A. Buser and N F de Rooij, “Micromachined Atomic Force Microprobe With Integrated Capacitive Read-out” J. Micromech. Microeng. 2, 218 (1992). 26 P.F. Moulton, “Spectroscopic and Laser Characteristics of Ti:Al 2O3” J. Opt. Soc. Am. B 3, 125 (1986). 27 M. Kitamura, M. Nishioka, J. Oshinowo, and Y. Arakawa, “In Situ Fabrication of Self-aligned InGaAs Quantum Dots on GaAs Multiatomic Steps by Metalorganic Chemical Vapor Deposition” Appl. Phys. Letters 66 , 3663 (1995). 28 J. Garnaes, F. Kragh, K.A. Mørch, and A.R. Thölén, “Transmision Electron Microscopy of Scanning Tunneling Tips” J. Vac. Sci. Technol. A 8, 441 (1989). 29 A.H. Sørensen, U. Hvid, M.W. Mortensen, and K.A. Mørch, “Preparation of Platinum/Iridium Scanning Probe Microscopy Tips” Rev. Sci. Inst. 70 , 3059 (1999). 30 S. Kerfriden, A.H. Nahle, S.A. Campbell, F.C. Walsh and J.R. Smith, “Short Communication The Electrochemical Etching of Tungsten STM Tips” Electrochimica Acta 43 , 1939 (1998). 31 A.J. Nam, A. Teren, T.A. Lusby, and A.J. Melmed, “Benign Making of Sharp Tips for STM and FIM: Pt, Ir, Au, Pd, and Rh” J. Vac. Sci. Technol. B 13(4) , 1556 (1995). 32 A.J. Melmed, “The Art and Science and Other Aspects of Making Sharp Tips” J. Vac. Sci. Technol. B 9, 601 (1991). 33 B.L. Rogers, J.G. Shapter, W.M. Skinner, and K. Gascoigne, “A Method for Production of Cheap, Reliable Pt-Ir Tips” Rev. Sci. Inst. 71 , 1702 (2000). 34 Doo-In Kim and Hyo-Sok Ahn, “Etching Voltage Control Technique for Electrochemical Fabrication of Scanning Probe Microscope Tips” Rev. Sci. Inst 73 , 1337 (2002) . 35 R. Feenstra, J. Stroscio, and A. Fein, “Tunneling Spectroscopy of the Si(111)2 × 1 Surface” Surf. Sci. 181 , 295 (1987). 36 S.M. Hubbard, D. Wilt, S.Bailey, D. Byrnes, and R. Raffaelle, “OMVPE Grown InAs Quantum Dots for Application in Nanostructured Photovoltaics” Proc. of the IEEE World Conference on Photovoltaic Energy Conversion 1, 118 (2006). 37 S. Liang, H.L. Zhu, J.Q. Pan, X.L. Ye, W. Wang, “Growth of InAs Quantum Dots on Vicinal GaAs (100) Substrates by Metalorganic Chemical Vapor Deposition and Their Optical Properties”, J. Crystal Growth 289 , 477 (2006). 38 U. Strößner, J.P. Meyn, R. Wallenstein, P. Urenski, A. Arie, G. Rosenman, J. Mlynek, S. Schiller, and A. Peters, “Single-frequency Continuous-wave Optical Parametric Oscillator System With an Ultrawide Tuning Range of 550 to 2830nm” J. Opt. Soc. Am. B 19 , 1419 (2002). 39 S.Schiller, “Optical Parametric Devices” Elsevier Article # OPTC01251 , 2004. 40 Yariv, Amnon, Quantum Electronics (John Wiley & Sons, Inc., 1989). 41 M. Yamada, N. Nada, M. Saitoh, and K. Watanabe, “First-order Quasi-phase Matched LiNbO 3 Waveguide Periodically Poled by Applying an External Field for Efficient Blue Second-harmonic Generation” Applied Physics Letters 62 , 435 (1993). 42 J. A. Armstrong, N. Bloembergen, J. Ducuing, and P. S. Pershan, “Interactions Between Light Waves in a Nonlinear Dielectric” Phys. Rev. 127 , 1918 (1962).
58
43 S. Grilli; P. Ferraro, P. De Natale, B. Tiribilli, and M. Vassalli (2005). "Surface nanoscale periodic structures in congruent lithium niobate by domain reversal patterning and differential etching". Applied Physics Letters 87 , 233106 (2005). 44 L.E. Meyers, R.C. Eckardt, M.M. Fejer, R.L. Byer, W.R. Bosenberg, J.W. Pierce, “Quasi-phase Matched Optical Parametric Oscillators in Bulk Periodically Poled LiNbO 3” J. Opt. Soc. Am. B 12 , 2102 (1995). 45 R.G. Batchko, D.R. Weise, T. Plettner, G.D. Miller, M.M. Fejer, and R.L. Byer, “Continuous-wave 532- nm-pumped Singly Resonant Optical Parametric Oscillator Based on Periodically Poled Lithium Niobate” Optics Letters 23 , 168 (1998).
59