Gallium Arsenide Doping for Solar Cells

Onyeka Dimkpah, Kaitlyn Estep, Erin Flynn, and Tyler Rice 12-9-2015

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Abstract

Solar energy has risen to prominence in the search for alternative energy. Research is working to develop solar cells capable of efficiently turning solar energy into harnessable electrical energy. In solar cell development, Gallium Arsenide has risen to prominence as a better semiconductor for the job. However, research is still ongoing to improve it further through processing, and one such process being explored is doping gallium arsenide with Zinc, Silicon, and Carbon. Zinc acts as a p-type dopant, while Silicon and Carbon both can act as amphoteric dopants, acting as donors or acceptors depending on what lattice position they fill. These dopants fill Gallium or Arsenic vacancies in the lattice structure, and their contribution to holes or electrons increases the concentration of charge carriers, thus shifting the fermi energy. The shift in the fermi energy can improve the Gallium Arsenide’s ability to convert solar energy to electrical energy, and this increased efficiency may make a future of solar power more attainable.

Introduction

In the search for cleaner energy sources, solar energy continues to rise in popularity as research progresses its viability as an alternative energy source. Advancements have led to more efficient light capture and energy conversion, with the current record of 46% efficiency being held by a four-junction solar cell developed by the Fraunhofer Institute for Solar Energy Systems [1]. However, most advanced solar cells with high efficiencies are currently confined to laboratories as their production requires expensive materials and small scale processes.

The record-breaking four-junction solar cell was one of the first of its kind, but with multiple junctions comes added complexities. However, simpler single-junction solar cells have also been improving, with the current single-junction record being held at 28.8% by a Gallium Arsenide thin film crystal cell developed by Alta Devices [1]. Gallium Arsenide, a group III-V compound semiconductor, has many technical advantages over the more famous Silicon semiconductors, but the use of Gallium Arsenide is currently crippled by the high cost of manufacturing, an issue currently trying to be tackled by researchers [2]. Despite its higher cost, gallium arsenide is still the preferred semiconductor for satellite solar panels due to higher efficiency making it worth its weight to launch into orbit.

Despite being the best known semiconductor for solar cells, setting a Gallium Arsenide crystal out in the sun does not a solar panel make. There are many steps involved in converting a gallium arsenide wafer to a solar cell, including processing the material itself to improve its solar energy conversion capabilities. One such process that has been shown to improve solar cell efficiency is doping the Gallium Arsenide followed by annealing. Through these processes, the crystal structure of the Gallium Arsenide is altered by the integration of the dopant atoms, which in turn alters the ability of electrons to move through the crystal as well as shifts the fermi energy depending on the dopant used.

Processing

Doping semiconductors is a well-known process, and there are multiple methods that can be used depending on the dopant and the substrate involved. With Gallium Arsenide, special considerations must be made for the fact that it is a compound material and high temperatures may result in dissociation of the atoms on the surface; this temperature sensitivity is part of what complicates attempts to grow bulk crystals [3]. However, several methods have been developed to successfully dope Gallium Arsenide with a variety of dopants, including Zinc, Silicon, and Carbon.

The method used to dope the Gallium Arsenide can vary depending on the dopant involved as well as other parameters, such as desired depth of dopant diffusion. One process used to grow doped

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Gallium Arsenide is metalorganic vapor phase epitaxy (MOVPE) [4]. In this process, vapor phases of a Gallium Alkyl and Arsenic Hydride are reacted in vapor phase inside a cold wall chamber to grow Gallium Arsenide [5]. Dopants can be added to the growth surface by being introduced into the chamber also in vapor phase. This processing method can be used to dope with Zinc, using a Zinc Alkyl as the precursor, or to dope with Silicon, using Silane as the precursor; both tend to fill in the Gallium’s position in the lattice. Carbon dopants, incorporated from the organic precursors, tend to fill the Arsenic’s position. The partial pressure ratio of Arsenic Hydride vapor to metal Alkyl vapors as well as the temperature in the reaction chamber impact the resultant dopant concentration. The higher the ratio of Arsenic Hydride partial pressure to both the other precursor partial pressures, the more Arsenic reacting to fall into place in the lattice; this leaves more Gallium vacancies for the Zinc or Silicon to react and fill. Thus the higher the ratio of Arsenic Hydride, the higher the concentration of dopant. The temperature dependence stems from the high elemental vapor pressure of Zinc and Silicon. Dopants already deposited onto the surface can evaporate and do so at a higher rate with increasing temperature, thus the higher the chamber temperature, the lower the dopant concentration.

Another common method used for doping Gallium Arsenide is molecular beam epitaxy (MBE). In this process, sources of the constituents are heated to high temperatures in separate chambers [6]. The high temperatures evaporate atoms off the sources and the vapor phase atoms. The vapor phase atoms in the high temperature chambers are released into the cold wall chamber for crystal growth in beams that are targeted towards a rotating, heated substrate. As the atoms hit the substrate, they condense onto the surface; the thermal energy provided by heating the substrate allows the atoms enough energy to move and arrange into a crystalline structure. Dopants can be incorporated during this growth process by being a third source, and the amount of dopant incorporated into the growing crystal depends on the amount of beam allowed to pass into the growth chamber and the partial pressure of dopant in the growth chamber [7]. The higher the partial pressure, the greater the amount of dopant incorporated.

Doping Gallium Arsenide influences its lattice structure and electrical properties, depending on the amount and identity of the dopant used. Zinc, Silicon, and Carbon doping are of interest at this time. Zinc dopants act as acceptors and have a high mobility within the lattice. Silicon and Carbon have been shown to work as both the acceptor and the donor depending on growth environments; this amphoteric behavior is of special interest in how it affects the structure and how it can be controlled.

Structure

Gallium Arsenide, in the most basic form, has a face centered cubic lattice of As atoms with Ga

atoms overlaid in the As lattice’s tetrahedral interstitials. Each Ga atom is bonded to four As atoms. This

structure is called cubic sphalerite, and is shown in Figure 1. Physically, GaAs is layered in sheets.

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Figure 1. Cubic sphalerite structure of GaAs.[8]

With MOVPE growth, Silicon and Zinc have a preferred lattice site of III, and Carbon has a

preferred lattice site of V. When doping, local changes in gas velocity and recirculation of gas that has

already been deposited result in non-uniform growth rate, materials properties, and alloy composition,

leaving unclear doping and compositional interfaces [5]. Zinc concentration increases with Arsenic

Hydride mole fraction in MOVPE during the growth of GaAs at a constant growth rate. This occurs

because of an increase in Ga vacancies, leading to more Zn inclusions, as well as decomposition in the

reaction of Zn and AsH3. Silane is the most common gas form of silicon used in the doping of GaAs. As

seen in Figure 2, the rate of incorporations of Si depends on the orientation of the GaAs crystal; the As

(111) has a higher electron concentration, more Si inclusions, than Ga (111). Also seen is that at higher

temperatures, there are higher rates of Si inclusions.

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Figure 2. Electron concentrations of multiple planes at multiple temperatures.[5]

Properties

Gallium Arsenide, as an undoped crystal, already has properties that enable it to perform as a better solar energy converter than Silicon. The band gap for Gallium Arsenide is 1.43eV, which corresponds to light of wavelength 0.873μm [1]. This wavelength is very near to the peak of the sun’s output energy distribution, meaning a high concentration of the photons coming to Gallium Arsenide from the sun match its band gap energy and can be absorbed [2]. Comparatively, Silicon has a band gap of 1.11eV, which corresponds to a larger wavelength that is not as prevalent in the sun’s output [1,2]. Gallium Arsenide also has a temperature coefficient of near zero, meaning its performance is not impacted by temperature, unlike silicon which has a negative coefficient indicating its performance degrades with an increase temperature [3]. Additionally, Gallium Arsenide can withstand moisture and UV radiation, making it more durable and longer lasting as a solar cell [4]. Despite Silicon’s significantly lower price, Gallium Arsenide’s properties are superior to make it more ideal for solar cells. However, these properties alone cannot make a high-efficiency solar cell and can be enhanced by the addition of dopants.

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Figure 3. Comparison of GaAs solar cell to Si solar cell; (top) Temperature rise of solar cell over

ambient temperature in a day of exposure; (bottom) Normalized performance of solar cells [3]

Doping Gallium Arsenide with Zinc using alters the band structure, carrier mobility, and luminescence properties of GaAs. Zinc acts as an acceptor, increasing the concentration of holes in the GaAs. It sits in the gallium sublattice, which contributes to it having a high rate of diffusion. Zinc diffusion, although it beneficial in creating a uniform p-type region, causes problems in multi-junction solar cells where the Zinc diffuses across to n-type doped layers. This creates a compensation effect which partially negates the effects of the n-type doping [10]. The doped GaAs, due to its increase in carrier concentration, experiences an increase in carrier mobility. With charge now able to move faster through its lattice, the GaAs functions as a more efficient photovoltaic material. Low mobility in photovoltaic devices has been shown to cause trap-assisted recombination, which means a generated electron-hole pair are more likely to annihilate one another before reaching the electrodes [11]. Zinc, despite the challenge presented by its high diffusivity, improves the capabilities of GaAs acting as a photovoltaic material in a solar cell.

GaAs can also be doped with carbon, a dopant of more recent interest. Unlike zinc, carbon dopant atoms tend to occupy the arsenic lattice sites of the GaAs crystal due to their smaller size making fit the smaller lattice spot more optimal [5]. However, carbon does not have as many electrons as arsenic and therefore tends to act as an acceptor, like zinc. This results in a p-type GaAs crystal, which again exhibits the increased charge mobility that allows for it to work better as a solar cell. However, since carbon tends to sit on the arsenic sublattice, it is more limited in diffusion [10]. Although this makes it harder to create a uniform doping, it means the carbon atoms do not create the compensation effects like zinc atoms do when near n-type regions. Carbon doping comes with its own set of challenges though. During growth, hydrogen can be introduced to the growing crystal and get trapped in the lattice [10]. The carbon atoms tend to complex with these hydrogen atoms, and these carbon-hydrogen complexes create impedances to electricity flow. This impedance negates some of the advantage of carbon doping. Luckily, it can be easily fixed as the hydrogen can be released from the lattice by simply annealing it under an inert atmosphere. Carbon doping can also cause problems during growth as it slows down growth rate and cause growth islands, dopant aggregation in a concentrated area that leads to unwanted

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complexing. This can be avoided by manipulation of the growth rate and doping rate. Carbon, typically a p-type dopant, has its own challenges and advantages in improving GaAs performance.

Silicon is used as a dopant for the growth of n-type GaAs in metalorganic vapor deposition for

many of the same reason Zinc and Carbon are used. It should be noted that Silicon and Carbon are amphoteric dopants, capable of acting as both donors and acceptors. Silicon, being able to act as both an acceptor and a donor depending on lattice position, can have a compensating effect; even though it tends to occupy Gallium sites and act as a donor, some may occupy Arsenic sites and thus turn into acceptors; this causes a compensation effect, where the concentration of conducting electrons is not as high as the silicon doping concentration due to some silicon atoms acting as acceptors. The low volatility and low diffusion coefficient of Silicon in GaAs are properties that make Silicon useful for the deposition of layers for device structures that require specific dopant profiles. Unlike other dopants Silicon concentrations in GaAs have a strong temperature dependence which can lead to problems obtaining uniform doping levels across large substrates. When GaAs is doped with Silicon, the material gains a larger whole concentration than electron concentration; the Fermi energy moves closer to the valence band than the conduction band as a result of the increased hole concentration. GaAs photoluminescence properties rely heavily on the type of dopant used. Because holes are the majority carriers in p-type GaAs there is less chance for electrons to be excited by any light source. The use of MOVPE allows explicit control of background doping which makes the process more precise.

Summary

In conclusion, GaAs is the prefered material that is used in the process of making solar cells. One way to prepare GaAs for becoming a solar cell is to dope it with Si, Zn, or C. This is then followed by an annealing process that allows for certain properties to be obtained. Two of the most popular ways to dope GaAs is to use MOVPE or MBE. Depending on how much dopant is used and how deep it penetrates into GaAs, there will be certain influences on the lattice structure and the electrical properties of GaAs, some of which are desired for the performance of the solar cell. On top of how GaAs is doped, the elements used to dope it will also change the properties of GaAs. Zinc will alter the bandgap, carrier mobility, and luminescence properties of GaAs. Carbon when used as a p-type dopant decreases the resistance of the base, affects the surface morphology, and roughens the surface of GaAs. Silicon does the same things as Zinc and Carbon, but because Silicon will donate to Gallium and be an acceptor to Arsenic, this causes a larger hole concentration which moves the Fermi energy closer to the valence band. This is just a snapshot of three potential dopants being explored to improve GaAs performance as a solar cell.

Author’s Contribution Statement

All authors have been given a reasonable opportunity to read the final version of this report and approve its submission. All the authors read 2-3 journal articles, took notes, and then uploaded those notes onto Google Drive so that everyone could use them to be able to write their parts of the paper. Onyeka wrote the properties portion, Kaitlyn wrote the structure, Erin wrote the process and edited, and Tyler wrote the intro, summary, and edited.

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