Experiment 5 (Making Ohmic contact)
1-To produce an Ohmic contact between Al and Si
Metal and semiconductor make two types of contact: either Ohmic or Schottky. The later one is a rectifying contact and the current-voltage (I-V) curve is non-linear. This contact is not useful if we want to investigate a device because the Schottky contact would leave its footprint on the measurement. In the Schottky contact an electronic barrier exists between the metal and semiconductor. This condition is related to the work-function of the two materials. Low-resistance, stable contacts are critical for the performance and reliability of integrated circuits and their preparation and characterization are major efforts in circuit fabrication. As a rule, Ohmic contacts on semiconductors form when the semiconductor is highly doped. A high doping would narrow the depletion region at the interface and allow electrons to flow in both directions easily at any potential by tunneling through the barrier.
When we deposit Al on Silicon, it is a bit like a Schottky contact. The purpose of this experiment is to turn the Al-Si contact into an Ohmic contact. Since this process involves heating the sample, we need to ensure that Al doesn’t get oxidized due to elevated temperature. An oxidized Al is an insulator and therefore we can never have an Ohmic contact.
Safety Considerations –
- Since we are using hydrogen gas in this experiment, keep an eye on the hydrogen detector. If its level is high, an alarm will sound. At that point, close the valve at the cylinder and notify security at 3717.
- When Bunsen burner is lighted to burn hydrogen, stay away from it. Any material placed near the flame can cause fire.
- After alloying is done don’t touch the samples as they are still hot. Use tweezers.
Experimental Procedure –
To make an intimate contact between the metal contacts and the Si surface, we will use the Alloying furnace.
1-Dry the samples with the N2 gun. Then load them into the alloying furnace.
2-Let N2 gas flow through the sealed reactor tube for 2 min. While waiting, light the Bunsen burner at the reactor tube exhaust.
3-Turn ON the furnace (Variac set to 80) and begin monitoring reactor temperature. When temperature reaches 350 0C ( about 20 min. after), change ambient to nitrogen/hydrogen ... **make sure that the gas begins burning at the exhaust within 2 min. …if not, shut OFF H2 and determine source ofproblem**
4-When a temperature of 450 0C is attained, begin alloy timing and reduce variac setting to 69. Perform the alloying for a total of 30 minutes.
5-When the alloying is complete, shut OFF the furnace and allow the system to cool in an H2 atmosphere until the temperature decreases to 3500C. At that point, the ambient may be changed to N2 and the Bunsen burner may be extinguished.
6-The samples may be removed from the reactor when the temperature decreases to below 80 0C. **don't forget to shut OFF the N2**
- Why do we need H2 ambiance for alloying?
- Why do we need to burn the gas that exits the furnace?