STUDENT PROJECTS

Introduction

In the semiconductor industry, GaN is important for blue Laser Diodes (LDs) and Light Emitting Diodes (LEDs). In order to maximize efficiency for optoelectronic devices that utilize GaN products, a low contact resistance and an ohmic contact are needed. Previously, the contact resistance has been found to be as low as 10^-4 Ω cm². The optoelectronic devices do not work efficiently if they do not behave ohmically.

It was the aim of this research project to investigate the influence of different annealing conditions for the contact resistance (ρ_c), analyze the microstructure of the electrodes, find the relationship between the microstructure, annealing conditions, and ρ_c, and then explain the mechanism. The influence of the different annealing conditions affecting the contact resistance will be explained, but the microstructure of the electrodes has not been found. This paper will then hypothesize the mechanism based on results from similar experiments done by others.

Experiment
The semiconductor wafer is as-grown with a sapphire base with the GaN layer on top of it. It was made in the MOCVD process. Because there is a great difference in lattice constants between the sapphire and the p-type GaN, a buffer layer of GaN about 40-50μ thick is placed between the two. It was then activated to become p-type GaN by doping the wafer with MgH. The hydrogen needs to be removed so that there will be a hole concentration. With the GaN doped with MgH, there is no hole concentration. To activate the Mg and get a p-type semiconductor, the wafer needs to be heated in a Hot Wall furnace made of quartz.

The sample that was used in the main experiment was activated at 800C for 20 minutes. Another small experiment was done in the activation where five different temperatures of the GaN samples were activated at: 700C, 750C, 800C, 850C, and 900C. These samples were then measured for hole concentration.

The wafer then could be prepared for deposition of the electrodes. The wafer was first cleaned with CCl₄:H₂O on a 1:1. Then acetone, next alcohol, and finagling deionizing water all were used to rinse the wafer. The wafer underwent a supersonic rinse. It was dried at 90C for 30 minutes. The photoresistor was put onto the wafer. It was dried again at 90C for 30 minutes.

Photolithography was the next procedure. Reactive Ion Etching (RIE) was used to create lines of mesas on which to put the electrodes. In the RIE technique, ions collide with the sample. They etch down the sample. The photoresistor protects the area that isn’t carved down by the ions. The sample is rinsed again. Following this, another model is put on the sample in the form of squares in a line. The model is made of material similar to glass. In between the squares is the photoresistor material. Ultraviolet light is shined on the sample. After the light hits the sample, the desired lines of mesed squares remain. This is the second part of the photolithography technique.

The sample is rinsed with HCl:H₂O with a 1:1 to remove the photoresistor material. The sample is then ready to be deposited with the metal for the electrodes. The base pressure is 4 x 10^–8 Mbarr. The electrodes are 400 Å high and 200μ in length and width. The height consists of 200 Å of Nickel and 200 Å of Gold. The sample is now ready to be annealed.

There were four different conditions in which the sample was annealed. First, the entire wafer was broken up into smaller pieces. The same original wafer is used throughout all the different annealing conditions, but different pieces of it are exposed to the different conditions. The first condition was varying the ambient. The samples were annealed at varying percentages of a O₂ and N₂ blend at 500C for 10 minutes. The second condition varies the time that the samples were annealed. The experiment tested values between 15 seconds and 10 minutes at 500C in O₂. The third condition varied the temperature at which the sample was annealed in N₂. The temperatures ranged from 350C to 700C. The final condition also varied the temperature, but this test was carried out in O₂ instead of N₂. Both the third and the fourth conditions were annealed for 10 minutes.

Following the annealing, the samples could then be measured for their contact resistivity, ρ_c, and an ohmic contact. To do this, the popular Transmission Line Method (TLM) was used. The samples had lines of square electrodes. A probe was placed at the beginning of the line for an input current and a probe at the last square for the output current. The current was kept constant throughout the measurement. Next, two other probes were used to measure the voltage between adjacent electrodes. The distance between the electrodes decreases, which lead to the difference in voltage.

The data was linearly fit. The following equations were used to find ρ_c:

Results and Discussion
One can see from the graphs that definite minimums were found for all four of the situations. There are speculated answers as to why the minimums of the graphs occur at certain values. Again, to understand these answers and to see if our samples have similar results, one needs to look at the cross-sectional area of the sample. This will be done in the near future.

After the final physical experiment is performed, a discussion section will be added to this entire project. On a broader scale, the methods described in this paper can all be performed together to yield the best contact resistivity for a semiconductor.

Research supported by the NSF International REU program through Illinois State University.