Microwave Plasma Chemical Vapor Deposition of Homoepitaxial Diamond for M-i-P Diodes: A Study of Reactor Design, Growth Kinetics, and Surface Morphology

Open Access
Author:
Bresnehan, Michael Stephen
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 29, 2010
Committee Members:
  • Joan Marie Redwing, Thesis Advisor
  • David W Snyder, Thesis Advisor
Keywords:
  • MPCVD
  • Diamond
  • Surface Morphology
  • Growth Kinetics
  • M-i-P Diode
  • CVD Diamond
Abstract:
Due to its excellent electronic properties and ability to be used in high temperatures and harsh environments, there is significant interest in the use of diamond to replace silicon for high power, high frequency electronics. This interest is accelerating as large area single crystal substrates become commercially available. The diamond M-i-P (Metal – intrinsic diamond – P++ diamond) Schottky barrier diode takes advantage of intrinsic diamond’s extremely high hole mobility (3800cm2/Vs) that permits very high forward current densities (up to 100A/cm2 at 5V). However, the performance of this structure depends on the crystalline quality and purity of the intrinsic region. Defect and impurity centers act as traps, capturing charge carriers passing through the intrinsic region. This leads to power loss and degraded device efficiency. Therefore, a growth process is required that produces a thick, high quality, high purity intrinsic diamond layer. This thesis studies the effect of microwave plasma chemical vapor deposition (MPCVD) reactor design, process conditions, and pre-growth surface treatments on the quality, surface morphology, and growth rate of single crystal intrinsic diamond. A vertical bell-jar MPCVD reactor was purchased for homoepitaxial diamond growth. Several geometry modifications were explored to take full advantage of the growth kinetics of MPCVD diamond. By altering the reactor geometry, the plasma could be manipulated to provide a high plasma density, sufficient contact to the substrate, decreased plasma etching of the quartz chamber walls, and increased dissociation of gas species (as observed with optical emission spectroscopy). As part of the reactor study, the susceptor was adapted to enhance cooling of the substrate, allowing for high microwave input powers at low substrate temperatures. In this regime, growth rates approaching 20μm/hr were achieved in a high purity hydrogen-methane plasma. These films were verified with Raman spectroscopy and secondary ion mass spectrometry (SIMS) to be of high crystalline quality and purity. The effects of pre-growth surface etching and substrate misorientation on the surface morphology were also studied. Dislocations and surface defects can lead to macroscopic growth features (such as growth hillocks) that degrade device performance, encourage non-uniform p-type doping, and induce strain, defects, and impurities. Here, pre-growth reactive ion etching (RIE) is shown to remove surface damage and improve substrate surface roughness (Ra) from 0.795nm to 0.478nm. Substrate misorientation is shown to supply a high density of favorable nucleation sites in the form of crystallographic step edges. Both techniques are shown to suppress hillock formation and improve overall film quality. An initial study of nanocrystalline diamond growth is also reported. Grain size of polycrystalline diamond films was controlled in an attempt to deposit nanocrystalline diamond. This thesis studies the effects of argon concentration on the growth rate, grain size, and crystalline phase (as observed with Raman spectroscopy) of diamond films approaching nanocrystalline material. It was shown that argon concentrations from 90- 95% can yield diamond films with grain sizes below 1 μm and growth rates around 1μm/hr in the current reactor configuration.