Implantation and Dry Etching of Group-III-Nitride Semiconductors
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expected to enhance device operation, further device advances will also require improved processing technology. In this article, we review developments in two critical processing technologies for photonic and electronic devices: ion implantation and plasma etching. Ion implantation is a technology whereby impurity atoms are introduced into the
semiconductor with precise control of concentration and profile. It is widely used in mature semiconductor materials systems such as silicon or gallium arsenide for selective area doping or isolation. Plasma etching is employed to define device features in the semiconductor material with controlled profiles and etch depths. Plasma etching is particularly necessary in the Ill-nitride materials systems due to the lack of suitable wet-etch chemistries, as will be discussed later. Figure 1 shows a laser-diode structure (after Nakamura3) where plasma etching is required to form the laser facets that ideally should be vertical with smooth surfaces. The first Ili-nitride-based laser diode was fabricated using reactive ion etching (RIE) to form the laser facets but suffered from rough mirror facet surfaces that contributed to scattering loss and a high lasing threshold. This is a prime example of how improved material quality alone will not yield optimum device performance. Improved processing—in this case, improved etching techniques—will also be needed. With this in mind, advances in plasma etching of the III-nitride materials are reviewed with a particular emphasis on the use of high-density plasmas based on electronNi/Au
Figure 1. Schematic of a Ill-nitride-based blue laser diode (after Nakamura in Reference 3) showing (a) the epitaxial layer structure and (b) the device after processing that includes a deep plasma etch requiring vertical sidewails and smooth facet morphology and the formation of n- andp-type ohmic contacts. Advances in plasma-etching technology are expected to improve the sidewall morphology and reduce scattering losses in the laser.
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MRS BULLETIN/FEBRUARY 1997
Implantation and Dry Etching of Group-Ill-Nitride Semiconductors
cyclotron-resonance (ECR) and inductively-coupled-plasma (ICP) sources. With these sources, considerable improvements in etch rate and sidewall (facet) morphology have been demonstrated. Figure 2 shows the process flow for a GaN junction field-effect transistor (JFET) that requires precise plasma etching as well as multiple implantation steps.7 In this case, the plasma etch must have a well-controlled etch rate to remove 50-100 nm of p-type GaN in the source and drain regions of the JFET. Implantation is used to form the junction gate, n-channel, and n+ source and drain regions. This implantation technology will be equally applicable to GaN metalsemiconductor field-effect transistors and heterostructure field-effect transistors to reduce the access resistance of the device. The etch process would also be applicable for the recess etch in a recessed gate transistor. As will be discussed later, to activate the implanted dopants, an an
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