Conference Session: B6
6096
Disclaimer—This paper partially fulfills a writing requirement for first year (freshman) engineering students at the University
of Pittsburgh Swanson School of Engineering. This paper is a student, not a professional, paper. This paper is based on publicly
available information and may not provide complete analyses of all relevant data. If this paper is used for any purpose other
than these authors’ partial fulfillment of a writing requirement for first year (freshman) engineering students at the University
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THE USE OF GERMANIUM-TIN SEMICONDUCTORS IN OPTICAL
COMPUTING
Connor Bomba, cjb115@pitt.edu, Bursic 2:00, Timothy Cropper, tsc27@pitt.edu, Sanchez 10:00
Abstract - Microprocessors have been under constant
modification since the first one was manufactured in 1971.
Engineers have been changing the architecture of the chip to
increase their speed and make them more powerful. They have
achieved this mostly by increasing the transistor count, adding
more cores, and designing the chip to dissipate heat efficiently.
Engineers had basically hit a wall when it came to
improving processing capability, but recent studies have
shown that it is possible to use photons to transmit data within
a microprocessor. Germanium is already in use in processors
as a semiconductor, but lately scientists have found benefits in
doping germanium with small amount of tin. When grown on
silicon under specific conditions they produce a chip with
integrated optics. Meaning that these germanium-tin
semiconductors are capable of producing light emissions
(lasers) that transmit photons as data.
In this paper, we discuss the recent evolution of processors
and where this technology could be heading in the near future.
Explicitly, we dive into the design and production of computer
processors to try and find where the faults are in current
processors. We then use this information to evaluate if the
eventual integration of germanium-tin semiconductor lasers
would be a step in the right direction for the future of
processors.
Key Words — Transistors, Semiconductors, Capacitance,
Utilization Wall, Silicon, Germanium-tin laser
INTRODUCTION
Starting with the first microprocessor in 1971, until 2002,
processor speed was increasing exponentially. After that
processing speed slowed down and has been slowly increasing
over the years. Engineers have been able to continually
increase the speed of these processors by increasing the
number of transistors inside, but this cannot continue forever,
as current processors contain over 1 billion transistors. Since
increasing the transistor count can only go so far, eventually
we will have to introduce a new way to continue to make
increase processing speeds [1].
One way to address this issue is to change how processors
work. By transitioning to using germanium-tin semiconductors
University of Pittsburgh Swanson School of Engineering 1
Submission Date 2016-03-04
that are capable of producing photons to transmit data, we
could potentially make processing speeds faster. Currently
germanium-tin semiconductors do not work at temperatures
that make them useful in a working environment, but engineers
are hopeful that with more research, germanium-tin
semiconductors could be used for optical computing [2].
COMPUTER COMPONENTS
Microprocessors
A microprocessor is the component in a computer that
handles the logic and acts as a director for the other parts of the
computer, such as the memory, storage, and peripherals: input
and output devices such as a keyboard or a monitor. These
processors need to be fast in order to deal with the mass
amount of information that is being delivered to it from other
parts of the computer and from peripherals. As we progress,
technology is becoming more and more complicated; as we
transition from using buttons to touch screens for input, and
we output displays at higher resolutions, these processors must
keep up, and so far they have. But the way to increase
efficiency of a microprocessor needs to shift to redesigning
how it works, as opposed to modifying the current model.
Semiconductors
A semiconductor is the most basic part of a microprocessor.
A semiconductor is simply a material that is more conductive
than an insulator but less conductive than a conductor, and the
most commonly used semiconductor in microprocessors is
silicon. This makes them useful in electronics because it is
possible to change their conductivity, which allows the
semiconductor to be used to create different parts of a
microprocessor, such as transistors and diodes. The
conductivity can be changed with different processes, such as
changing the temperature, establishing a magnetic or electric
field, adding in other metals (called doping), and introducing
light or heat. Since all of these actions alter the conductivity,
semiconductors can be used to detect when these changes
occur [3].
Connor Bomba
Timothy Cropper
is attributed to the increasing transistor density...” [1]. This
shows the importance of transistors in microprocessors.
However, Moore’s Law cannot reasonably continue
forever, considering there will be a point at which transistors
are unable to be made any smaller, or the mass amount of
transistors
would
cause
overheating
within the
microprocessor, this is known as the “Utilization Wall” [4].
An article by George Strawn published by the Institute of
Electrical and Electronics Engineers Computer Society states
that, as far as the number of transistors in processors goes,
Moore’s Law could be coming to an end [5], and with a 2012
Intel processor containing 1.4 billion transistors, that limit
could be getting closer and closer. This means that engineers
will have to look in a new direction in order to make computers
faster. The previous article even mentions that one of the next
possible steps to take in order to improve the speed of
microprocessors, is optical computing. This is where we
believe germanium-tin semiconductor lasers could be
introduced.
Transistors
One of the aforementioned uses for semiconductors in
microprocessors is a transistor. Transistors work like switches,
and they tell the computer if a signal is present or not: on or
off, and can also control the amount of current that is flowing
through a wire, thus amplifying the signal. This is necessary in
computers, since they operate on a binary system of 1s and 0s.
It is logical then, that having more options of switches that can
be on and off would give a processor more options for what it
is capable of, and allow for more data to be handled at once.
The measure of how fast a computer processes data is known
as clock speed. This leads to Moore’s Law, which states that
the number of transistors on a microprocessor doubles every
other year as processors become more advanced [1].
Optical Computing
In the past, the main source of data transfers and signals in
a processor have come through wires as electrons. But as we
push to make processors smaller and smaller so that they can
fit into thinner smartphones and laptops, we are running into
the problem of manufacturing wires that are small enough to
fit in microprocessors. As the wires get narrower, they have
diminishing returns on how much space electrons have to
move around; this is known as quantum confinement [6].
To battle the quantum confinement, engineers must move
on to new innovations. One of their options to explore is
known as optical computing. Optical computing is very similar
to fiber optic cables, which work by transmitting data as
photons, or tiny packets of light. Since the data is being
transmitted as photons, it is able to move at the speed of light
which is much faster than data is currently transferred by
electrons moving through nanowires.
In order to make the switch from traditional computing
using electrons and transistors, optical transistors will be put
in to use. An optical transistor is similar to a regular transistor
in that it takes in a signal and is able to amplify it or change it
in some way. The main difference comes from the fact that the
signal that goes into and comes out of an optical transistor will
be light, in the form of photons.
While the increased speed of optical computing sounds
ideal, there are still drawbacks that may keep it from being
commonplace. One concern is that optical computing loses
close to 30% of its energy when converting from electric
energy to light. So the cost of having faster processing speed
may come in the form of requiring more power in order to
compensate for the conversion loss. Most of this energy that is
lost is turned into heat that could pose a risk of overheating the
microprocessor. Another concern is that converting back and
forth between electric energy and light takes time, which could
FIGURE [1]
Rate of Processing Speed Increase
As seen in the previous charts, the number of transistors on
microprocessors has been steadily increasing and roughly
adhering to Moore’s Law with each new generation of
microprocessor from Intel. Roger Uy published an article
through the Computer Technology Department at De La Salle
University and stated that “The increasing trend in clock speed
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Timothy Cropper
end up slowing down the entire process of optical computing
[Nolte].
could be the future of processing speed. In studying the
chemical make-up of silicon, it was discovered that
germanium has an indirect band gap and a direct band gap that
is not too far above the indirect gap. The direct band gap is
fabricated by researchers through a process that adds
phosphorus to the germanium bonded with tin. This puts the
metal under a mechanical strain. Once strained to a certain
amount, the direct gap is within reach and thus effective for
use. This means that the electrons and positively charged
particles are able to move between different energy levels to a
level of ease [9]. Another discovery made during research of
the topic is that since germanium, tin, and silicon are all group
IV elements, they can be combined onto each other handily.
The ease of combination between silicon, germanium and
tin atoms is crucial to the entire process. These elements are
able to react together so nicely because of the valence electrons
they possess. Valence electrons are the outer most electrons of
an atom that are not in a full shell; because they are not secured
by a full shell, the atoms react easily. All of these elements
have four valence electrons and since eight is usually desired
for stability, the four valence electrons of germanium and the
four from tin are a perfect match.
Current trials of the germanium-tin laser are promising but
have significant flaws. One of these being the operating
temperature. One current study being done has the laser
functioning at -183 degrees Celsius. For obvious reasons, this
is not practical because it cannot be implemented into devices.
However, researchers are optimistic that they will be able to
increase the operation to a temperature in which the laser can
be useful in devices [2].
Naturally, silicon has a competent range of ultraviolet and
infrared light intake. Since silicon and germanium-tin are
similar in atomic structure, they both will have quality
absorption. Different concentrations of germanium to tin have
been tested in research in an attempt to find the ideal
combination. In doing so, it was discovered that the
concentration of tin effects what wavelengths of light can be
absorbed. For example, when the germanium quantity is over
ninety percent, the wavelengths absorbed are between 1.31 and
1.55 micrometers (10−6 𝑚). This is important because most
communication systems that transmit data use a signal within
this wavelength range.
Nanowires
Current microprocessors use a specific type of wire known
as a nanowire. A nanowire is a wire that is constructed out of
nanoparticles and has a diameter measured in nanometers
(1x10-9 meters). These wires are typically made of
semiconducting material such as silicon or germanium, and
they are used to create the transistors used in microprocessors.
As processors become smaller, there is less room even for
nanowires, which is why we must try to move away from them
[6].
GERMANIUM-TIN SEMICONDUCTORS
When anything becomes innovated to its maximum
potential, it is necessary to research new ways of
accomplishing the task. Without constant innovation, the
world will reach a standstill in technological capabilities. For
this reason, germanium-tin semiconductors are needed.
Before the technology implemented in the germanium-tin
laser can be truly appreciated and recognized as innovative,
one must understand the basis of it. The whole functionality of
this laser is to bypass the increasingly thin copper wiring that
is limiting the capabilities of computing speed. But with
copper being the best overall option for wiring, wiring will
need to be eliminated, in a sense, all together. The noted
benefits of germanium-tin are not only to increase processing
speed, but also to attempt decrease in power consumption. This
is ideal for any electronic device [2].
In a perfect world, communication in computing devices
would solely use silicon to do all the work. A significant
amount of data linking the Internet today is moved via lasers.
This is done by producing light that is sent down glass fibers.
These fibers lead to a different computing system where the
data is then sent to its necessary location. Silicon is capable of
producing light to a very small degree, but the output produced
by it is not nearly efficient enough. This is due to what is
known as an indirect band gap. An indirect band gap is the
result of poor coherence between the energy that electrons
have in the atomic structure compared to the energy of
positively charged particles [7]. The consequences of an
indirect band gap are tremors and oscillations; this is bad news
in high precision computing device. Additionally, this
movement can cause unwanted heat production. It is less than
ideal to have complex computer systems have shaking parts
[8].
For this reason, optoelectronics have a semiconductor
component to them in order to produce the light needed for
faster transmission speeds [8]. When scientists realized that
silicon could not produce the results needed for practicality,
they went to the drawing board. After some time, they made
an advancement, that with some refinement and improvement,
Molecular Beam Epitaxy
To get the germanium-tin combination to adhere correctly
a few different processes can be taken. One of the processes is
Molecular Beam Epitaxy, or MBE [10]. MBE is a process that
fabricates infinitesimally thin films made from a variety of
materials. It proves to be one of the more preferred methods
because of the repeatability that it offers.
The basics of the design of an MBE reactor is that it
contains three different chambers. The first chamber uses an
electron emitter to force excess electrons upon the germaniumtin to decrease reaction time. The material is then moved to the
second chamber, called the intermediate chamber, in which
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Connor Bomba
Timothy Cropper
ions of the entity are ejected towards the sample to further
increase the growth. The reason this process produces such a
precise product lies in the use of electron and ion emitters.
They release the materials with acute precision. This gives the
product very high tolerances of error. Finally, the germaniumtin is moved to the third chamber where it sits to harden.
If a company is going to potentially manufacture tens of
thousands of units, they will want to ensure that the structures
are consistent and perform to the desired level. Additionally,
surfaces made through MBE are incredibly smooth [11].
However, hot wall reactors do have advantages over cold.
Cold wall reactors have a bigger difference in temperature.
This temperature difference can affect the consistency of how
thick the layer is. Furthermore, another problem lies in the size
of the batch being made. If there is little material being coated
in the reaction chamber, the amount of thermal stress put on
the product can be too much for it to handle. As a result,
damage can be done to the item being coated. From a produce
stand point, this is less than ideal. [12].
Below is a basic example of a hot wall reactor. This reactor
is set up to coat multiple items; therefore, this model has
shelves in it. At the top is where the gases needed for the
reaction to take place are put in. As you can see, the heating
elements are on the left and right sides and force heat inward
to aid the reaction. Finally, there is an exhaust vent to deal with
all of the non-useful products.
Chemical Vapor Deposition
Much like MBE, chemical vapor deposition, or CVD, also
applies extremely thin layers of composites to materials.
Researchers see a lot of potential in this area of CVD for
specific use in semiconductor uprising. In the development of
silicon based semiconductors, CVD has been at the forefront
of their development. As a matter of fact, CVD is even used to
purify the silicon so that it can be used in components. A
consistent quality of material is crucial in the production of any
product.
In the chemical vapor deposition process, several reaction
types are used with two standing above the rest; hot wall and
cold wall. The hot wall system is a chamber in which the
materials you wish to be coated are loaded inside of. Next the
chamber is heated, usually by coils, to high temperatures. Once
threshold temperature is reached, the gases needed for the
particular reaction are introduced. After the reaction is
complete, the gases are pumped out. Hot chambers are often
designed and built to the size specifications needed for the
product being made. Additionally, the only major limitation of
the hot wall reactors are the materials they are made out of.
Due to the intense heat, design and materials used may vary
slightly for ideal efficiency.
The other major type of chemical vapor deposition reactor
is the cold wall reactor. These are built similarly to the hot wall
reactors. However, the reaction that takes place varies
significantly. In this case, the material being made is heated
and the chamber is chilled to very low temperatures.
Additionally, both chambers have increased pressure put upon
them to aid the reaction. Typically, when pressure is increased
for a reaction, atoms collide more frequently and the process
time is reduced.
Furthermore, cold wall reactors have a few noted
advantages over its hot wall counterpart. The biggest
advantage cold has over hot is that the chamber has less
material left on the walls after the reaction is done. This is
valued by producers because more often than not, the materials
undergoing chemical vapor deposition are rather expensive.
The more that is left on chamber walls is less profit they will
make. This also means less cleaning time which is crucial if
CVD is being used to produce items in high quantity. Finally,
cold wall reactors get their desired temperatures quicker than
hot walls do.
FIGURE [12]
Hot Wall Reactor for Chemical Vapor Deposition.
DRAWBACKS OF GERMANIUM-TIN
SEMICONDUCTORS
Although germanium-tin semiconductors have several
promising upsides, there are downfalls with it. Most notably of
these is power consumption. The semiconductor itself uses less
energy but in this process a new need for energy arises. This
need is for the converting of electrical energy to photons and
the reverse once the process is complete. In total, the energy
saved in this process is used in the conversion and thus no net
benefit [13] [2].
Additionally, this technology is still in the midst of
research. Currently, functioning prototypes have been made
but they must operate at extremely low temperatures. For this
reason, germanium-tin semiconductors are not near a point
where they could be implemented. This raises the question: Is
this technology worth pursuing?
OTHER TECHNOLOGY OPTIONS
Quantum Computing
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Timothy Cropper
something faster, after all, processors are already fast and
capable of doing our current tasks.
Faster and more efficient processors could make certain
jobs no longer a necessity. One example is that if an employee
is able to complete the same task, possibly data processing, in
less time, then an employer would not feel the need to pay 2 or
more people to process the data that another employee could
process in the same time. Another example would be that if
employees are being paid based on how much work they get
done, being able to complete more work would force an
employer to increase pay. If the employer was forced to
increase pay, and they are not able to, then the employer would
likely fire some employees.
In addition to possibly taking jobs away, it also might not
be the best use of resources to constantly strive for the next
best processor. Time and money could be placed into solving
other problems such as cancer research. We already have fast
processors that get the job done, so is it necessary to go faster?
When will we be satisfied?
Overall we think the best option is to continue making the
advancements with micro processing speed. While companies
may be required to pay more, they would also have the
possibility of benefitting from employees being able to work
faster. The speed of processors would also in aid cancer
researcher, as researches most likely rely on computers to
analyze their findings. Increasing the speed of a
microprocessor would most likely benefit those that use them
as opposed to harming them.
Quantum computing uses the theories of quantum physics
in order to process electronic data and logic. Just as current
computers use bits as a way of storing data, quantum
computers use qubits (quantum bits). In a traditional computer,
a bit is represented by being on or off: a 1 or a 0. On the other
hand, a qubit in a quantum computer could be a 1, 0, or a
superposition of either, which starts to get into the theories of
quantum mechanics and how particles behave at sub-atomic
levels. The best way to explain superposition is that it is similar
to Schrödinger’s cat and the Electron Cloud theory. With the
electron cloud theory, you can predict the general location of
an electron, but you cannot know the exact location; just as a
qubit is known to be between a 1 and a 0, but the exact value
is not known [14].
Since quantum computers operate based on quantum
theories, this is also where they would end up being the most
beneficial. Quantum computers would be better at working
with extremely large numbers and data sets. In a study by the
International Association for Cryptologic Research that used
80 processors and took almost two years, researchers were able
to factor a 232-digit number. The researchers stated that if only
one processor had been used, then it would have taken around
1500 years to do complete the same process [15]. With a
quantum computer designed to process these types of large
numbers, the process would not take as long. In addition to
processing large data, quantum computers would also be used
for solving quantum algorithms, and simulating chemistry and
physics processes that deal with quantum mechanics [16].
CONCLUSION
Optical vs. Quantum Computing
Germanium-tins’ ability to produce photons and be
combined with silicon allows it to be a useful as a
semiconductor. Since silicon alone does not produce enough
required light to be used in optoelectronics, adding in the light
emitting germanium-tin changes how the semiconductor could
be used within a processor. Instead of using silicon to construct
transistors and diodes that work with electronic pulses, the
photons emitted by germanium-tin would be used as the data
carriers, being intercepted by sensors and optical transistors
that can be used with photons as opposed to electrons.
There has not yet been established an ideal formula for
germanium-tin to be produced. Research is ongoing to
determine the ratio of germanium to tin for specific purposes.
Not all ratios are equal because the amount of tin effects the
wavelength of light produced from it. Also, one mean of
production has not placed itself above the rest. Both Molecular
Beam Epitaxy and Chemical Vapor Deposition are proven
processes to work. However until researchers determine a way
for germanium-tin to exist at more reasonable temperatures,
neither MBE nor CVD is clearly more effective.
Overall, germanium-tin has high potential to revolutionize
computing speeds and can have a significant impact on the
technology in this world. It is capable of getting current
technology over the utilization wall and into the next era of
When it comes to choosing the possible future of
computers, the two largest competitors are transitioning to
optical computing, or transitioning to quantum computing.
While both of these options have clear pros and cons, they also
have extremely different potentials for their uses. Optical
computing has the ability to make consumer computers and
computer devices faster and more efficient, and quantum
computing could be used by researches that want to study more
about the fundamentals of quantum physics. While quantum
computing could be an option for general consumers, the
current market price of $15 million [17] is sure to put buyers
off. Ultimately, it is probably best to explore both options, as
we do not know which one will be more useful in the future,
and we should not rule out an options that are still in the early
stages of development.
ETHICS
There were only a few ethical issues we ran into while
researching. The first being that faster and more efficient
processors have the possibility to put some people out of work.
The other reason is that we are wondering if it is really the best
use of time and money to constantly work on making
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Connor Bomba
Timothy Cropper
processing if proven effective, therefore it should be further
researched.
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ACKNOWLEDGEMENTS
We would like to thank our writing instruction Janine
Carlock for her continued recommendations on how to
improve the quality of our content. We would also like to thank
our Chairs Leigh Pardun, Ken Norris, and our Co-Chair Anita
Jain for their guidance throughout the process.
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