I am a senior studying electrical engineering. For my senior design project, me and my teammates are creating solar cells from scratch using spin-on doping techniques. We are in need of at least 10 p-type wafers for our project.
Subtrates for Spin-on-Doping of Semiconductors
What Wafers Can Be Used for Spin-on-Doping of Semiconductors?
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Research Using Silicon Subtrates Spin-On Doping of Semiconductors
What is Spin on Doping?
The electrical properties of a semiconductor can be changed by doping. When a pure silicon wafer is doped with phosphorus, it gains extra valence electrons that allow the crystal to conduct electricity.
Doping can be achieved by either diffusion or ion implantation. This article describes the spin on doping process which uses a non-hygroscopic solution rather than the toxic gases POCl3 and BBr3. This simplifies fabrication by eliminating the need for storing and using expensive ion implant equipment.
Substrate
The silicon wafer substrate is the foundation on which semiconductor devices are built. These devices include transistors, diodes, and ICs. It is a thin slice of material, typically made of:
- Pure Silicon
- Silicon Dioxide
- Aluminum Oxide
- Sapphire (Al2O3)
- Germanium (Ge)
- Gallium Arsenide (GaAs)
- Indium phosphide (InP)
The thickness of the wafer determines its characteristics. A thinner wafer has a higher resistance to the passage of electricity.
The spin on doping process is used to reduce the time required for the thermal diffusion of boron into the silicon substrate. The process combines the two furnace processes into one and allows for precise control of boron concentrations in the silicon substrate. The spin on doping process also provides a cost-effective alternative to the traditional ion implant method.
This method of doping uses a phosphorus-containing spin on dopant solution and a simple baking process. The P5O9 solution is commercially available from Filmtronics and contains a dopant compound and dissolved SiO2. It can be thinned with ethanol or isopropanol to modify concentrations. This solution does not damage the surface of the substrate and is suitable for use in a wide range of applications. The result is a uniform distribution of dopant across the substrate. The resulting boron doping is suitable for use in a variety of semiconductor devices. A typical application would be for a p-type silicon substrate.
Dopant
Doping a semiconductor with an impurity causes the energy bands of that material to shift. This allows electrons to move more freely in the valence band of the semiconductor and less freely in the conduction band, where the hole energy is. Depending on the type of dopant used, it can have an impact on both the conductivity and the energy levels in the energy band. For example, boron is a common dopant for silicon and when doped creates states near the conduction band. This is known as the donor energy band (EB).
Increasing the doping level of a semiconductor increases the number of free charge carriers. This can increase efficiency and allow for higher operation temperatures.
However, increasing the doping level also affects the mobility of these charge carriers. This is caused by impurity scattering, phonon scattering and electron-electron scattering. In some cases, the mobility can be improved by using a lower temperature or doping a smaller area.
There are many different dopants that can be used to dope a semiconductor. The most commonly used is phosphorus which is often added by diffusion of a phosphine gas. The phosphorus diffuses rapidly and forms wells in the semiconductor. Other dopants include arsenic, tin and gallium. Arsenic is especially useful because it has a low dissociation energy. This reduces the likelihood of ionized donors or acceptors being separated from each other and keeps them closer to the silicon lattice. This is important because ionized donors and acceptors attract each other and prevents them from moving freely in the semiconductor.
Deposition
Adding impurities to an intrinsic semiconductor changes the electrical and structural characteristics of the material. The doping of a semiconductor typically results in the creation of allowed energy states near the conduction and valence band edges that differ depending on the type of dopant atom added. These new energy states are close enough to the intrinsic Fermi level that the semiconductor acts like a conductor. The effect is illustrated in a band diagram, showing variations in the valence and conduction bands versus some spatial dimension, denoted as x.
Normally, the process of doping is achieved by two separate processes: a deposition and a high temperature drive-in diffusion. The deposition step uses a P5O9 solution that is commercially available from Filmtronics Inc., PA USA. The concentration of phosphorous in this solution can be modified with the addition or removal of ethyl alcohol to control surface doping. This solution is used because it does not require low temperature baking and therefore allows for rapid thermal activation.
The drive-in step is carried out by rapid thermal activation (RTD) at a temperature in the range of 900deg C to 1150deg C. During this process, the boron concentration in the sample is measured by a four-probe. The measured values indicate a good control of the doping level for this method. Moreover, the pn junction sheet resistivity is not greatly affected by variation of the annealing temperature or time.
Drive-in
The spin on dopant method is an alternative to traditional two furnace type methods that require careful handling to achieve p-type doping in semiconductor substrates. This approach allows the use of a single process step involving deposition of the dopant and a subsequent high temperature drive-in process for diffusion. The resulting process is less time consuming, more efficient and more economical compared to the two furnace method.
The curing temperature of the spin-on dopant film is important to determine because it determines the level of dopant activation at the silicon surface. In the present work, the curing temperature varied from 150 to 200 oC to explore its effect on dopant activation. The results show that a curing temperature of 175 oC for 60 minutes gives the best conversion efficiency (up to 4-5%).
The spin on dopant technology can be used as an alternative to conventional ion implantation or diffused doping processes in order to produce high quality silicon for use in advanced microelectronics and nanotechnology devices. This technology is particularly useful in the fabrication of n-type AlGaN/GaN high electron mobility transistors and other semiconductor devices that require p-type doping.