- Source: Lateral epitaxial overgrowth and pendeo-epitaxy
Epitaxy (prefix epi- means "on top of") refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epitaxial film or epitaxial layer. Epitaxial growth and semiconductor device fabrication are technologies used to develop stacked crystalline layers of different materials with specific semiconductor properties on a crystalline substrate, commonly silicon or silicon carbide materials, to achieve the desired performance of the microelectronic devices, such as transistors and diodes. The crystal structure of these layers is with high density of imperfections, such as dislocations and stacking faults. Therefore the microelectronic engineers and technologists have developed different techniques to eliminate or minimize the density of these structural defects in order to improve the microelectronic devices operation . One such approach is the Selective Area Growth technology.
Lateral epitaxial overgrowth (LEO) along with pendeo-epitaxy (PE) are selective area growth (SAG) techniques, developed in the late 1990s and early 2000s for epitaxial growth of wide bandgap semiconductor materials, such as gallium nitride: gallium nitride (GaN) on silicon carbide (SiC) substrate., GaN on sapphire (Al2O3) substrate, and GaN on silicon (Si) substrate. Epitaxial GaN is relevant to a semiconductor device technology important in microelectronics and chip manufacturing for development of high-power, high frequency, high temperature electronic devices. LEO and PE are technologies that are not limited to the wide bandgp GaN materials. Conventional epitaxial growth techniques of GaN on SiC, sapphire and Si substrate are known to produce high density of structural defects, mainly edge and screw dislocations and stacking faults, in the order of 109-1010 cm-2. PE and LEO, the latter also referred to epitaxial lateral overgrowth (ELO), are known to enable two to four orders of magnitude lower density of dislocations, compared to conventional growth,as revealed via transmission electron microscopy [3]. Having device layers of low defect density enables improved device characteristics and performance
= Lateral epitaxial overgrowth (LEO)
=LEO involves growing a seed GaN layer of the material on a substrate, then etching a patterned mask on the surface of the seed layer, commonly silicon dioxide or silicon nitride, leaving some GaN seed windows exposed that act as crystallographic template for the subsequent growth of the GaN layer (Figure 1). The new LEO film grows simultaneously from the GaN windows both vertically and at the same time extends laterally over the mask, forming wings of much lower density of structural defects (mostly treading dislocations). The wings can merge together to form a continuous GaN film, or remain separated by seams. Notably LEO process drastically reduces the defects in the crystal structure of the laterally grown areas by filtering them out at the mask interface. LEO can be performed from the vapor phase, depending on the material and the growth conditions via epitaxial growth techniques such as metalorganic vapour-phase epitaxy (MOCVD) or hydride vapour-phase epitaxy (HVPE).
= Pendeo-epitaxy
=Initially PE was developed as an alternative technology and complementary approach to lateral epitaxial overgrowth (LEO) of GaN on SiC substrate. Pendeo-epitaxy of GaN involves growing a continuous GaN film, commonly with high density of dislocations, as a seed layer on a substrate (SiC, sapphire or Si), then etching away portions from the GaN film (seed layer) thus leaving GaN seed stripes or columns, separated by trenches. The subsequent PE layer grows simultaneously from the tops and the side walls of the GaN stripes or columns (Figure 2). Thus, the top and the side walls of these columns act as homoepitaxial seed layers, which act as crytallographic templates for the subsequent vertical and lateral growth of continuous PE GaN layers. The regions of lateral growth are again with two to four orders of magnitude lower density of dislocations. Importantly, the film grows laterally from the side walls of the columns and extends horizontally over the trenches without touching the initial seed layer, forming wings of low crystallographic defect density. Hence pendeo-epitaxy, a term from Latin pendare, meaning to hang down, suspend. The wings can merge to form a continuous film or remain separated by seams. As with LEO, pendeo-epitaxy mechanism reduces the crystallographic defects in the film by avoiding the direct contact with the substrate, eliminating the lattice mismatch and the thermal mismatch stress/strain. Pendeo-epitaxy is mainly performed from the vapor phase via MOCVD and HVPE, and initially is used for growing gallium nitride (GaN) microelectronic device structures.
In the case of GaN material system, LEO and PE technology was initiated in the late nineties and early 2000s in Prof. R.F. Davis group at NCSU. The PE and LEO technologies are not limited to the development of low defect density wide bandgap GaN layers, important for the microelectronics industry, but also for many other epitaxial materials systems (Si, SiC, diamond etc.). Modeling of the LEO and PE growth processes reveals improved stress/strain characteristics and the concomitant improved characteristics in the microelectronic devices fabricated thereby. The strong microelectronics relevance of PE and LEO technologies to enable low density of dislocations in the semiconductor layers was documented in numerous patents.
