FellowMSc, PhD Oleg Babchenko
Project NameDesign and Fabrication of Diamond-on-GaN Hybrid Structures for MEMS Devices
Host organisationInstitute of Electrical Engineering
Duration of the project01.05.2015 - 30.04.2018

Abstract
The project is based on combination of working experience and theoretical knowledge from the research fields related to technology of diamond thin films and manufacturing of electronic devices on the base of AlGaN/GaN heterostructures. As the one of project outcomes the design for hybrid electronic devices should be proposed. The stated goal is efficient thermal management of AlGaN/GaN based devices by diamond passivation and/or passivation of diamond-based devices by metal oxides. It is expected that the project resolving will enable fabrication of hybrid high power electronic devices and/or micro-electro-mechanical systems potentially operating at high frequencies and/or high temperatures.

Project Summary with Interim Results

The project implementation was realized in several directions simultaneously regarding the project objectives, settled targets and current requirements of the research team in the Institute of Electrical Engineering Slovak Academy of Sciences (IEE SAS). The project objectives, stated in the work plan according to the key factors of a successful diamond  growth  on  foreign  substrates  and  its  further  utilizing  for  electronic  devices fabrication, were as follows:

1).        The study  on the formation  of  thermodynamically favorable  and stable  sites  on foreign substrates for diamond growth.

2).        The finding correlations between the growth process parameters, the properties of formed diamond layers and influence on substrates.

3).        The design, fabrication and characterization of hybrid electronic devices.

4).        The study on the potential of diamond based electronic devices.

Thus the studies related to first mentioned objective were guided by necessity of diamond films growth on the backside of thin AlGaN/GaN membranes and 3D structured Si support. In this case, the stable sites for diamond growth in the form of diamond seeds (nuclei) were distributed over the foreign substrate by several methods. Such seeding (nucleation) strategies as ultrasound agitation and/or immersing of samples in the aqueous and alcohol diamond powder suspension as well as nucleation via using diamond particles embedded in the polymer matrix were tested. The comparison of mentioned seeding strategies reveals that despite effectiveness of ultrasound nucleation on the planar and 3D structured substrates it is was not applicable for thin AlGaN/GaN membranes. The last ones break apart during the ultrasound agitation. On the other hand, gentle and reasonably good for planar substrates immersing in the nucleation solution was unable to bring diamond seeds (nucleation sites) to the bottom part of etched in the Si holes (membrane back side and sidewalls). Finally, seeding using diamond particles embedded in the polymer matrix was so far the most efficient strategy for nucleation of thin AlGaN/GaN membranes and 3D structured Si support.

The comparison of two types of diamond deposition equipment (hot filament and microwave plasma systems) was realized within second project objective. The observed preliminary results do not clearly reveal significant difference in the both systems using for diamond chemical vapor deposition (CVD) on backside of AlGaN/GaN membranes and 3D structured Si support. The more detailed comparison is expected. Also within the second project objective were realized preliminary experiments on the diamond growth on the top surface of circular high electron mobility transistors (CHEMTs). The deposition process at standard conditions (e.g. which were efficient for diamond growth on the AlGaN/GaN membrane backside) reveal lack of stability for initially proposed high temperature stable Ir/Au and IrO2/Au gates. Due to not optimized process conditions or structures design (e.g. not sufficient Au adhesion) the deterioration of mechanical properties (in some cases even delamination) and thus electronic properties was observed. Therefore, it was proposed to perform research on the HEMT design modification.

The supportive study that give impact on second project objective was related to the investigation  of  overall  AlGaN/GaN  heterostructures  stability  against  hydrogen  plasma (widely used for diamond growth). In this case the CHEMT devices and structures for circular transmission line measurements (CTLM) were fabricated on solid supported (Sbase) AlGaN/GaN heterostructures. Next, the bare and protected by thin SiNx layer CHEMT and CTLM structures were exposed to the low temperature hydrogen plasma. The electronic properties of structures, evaluated before and after the hydrogen plasma treatment, as well as scanning electron microscopy (SEM) and secondary ion mass spectroscopy (SIMS) data give us better understanding of the processes related to diamond growth and their influence on the AlGaN/GaN heterostructures. For example, positively we found no degradation of AlGaN surface (hence underlying GaN) detectable by SEM even after 1 hour of hydrogen plasma treatment. On the other hand, SIMS reveals incorporation of hydrogen in AlGaN layer which seems to be the main reason of observed sheet resistance increasing and slight deterioration of electronic properties of the devices (e.g. increase of gate leakage current). As general feedback the observed results indicate that protection against hydrogen penetration into the heterostructure bulk is needed rather than protection against heterostructure physical or chemical damage.

The research work related to structures design (i.e. third project objective) was targeted on formation of high temperature stable gate contacts for CHEMT that are able to withstand diamond growth. As outcome of previous studies we observe that in some cases initially used high temperature stable Ir/Au and IrO2/Au Schottky contacts suffers during the diamond deposition process. The modification of gate electrodes dealt with the proposal to use Ir/Al based gates composed from several sequencing multilayers. The influence of different formation procedures (high temperature oxidation) on structural and electronic properties of Schottky contacts was analyzed in details using Auger electron spectroscopy, X-rays diffraction analysis, SEM, and electrical measurements. The best gate contacts show high performance and operational stability up to the temperatures as high as 60C.

The expected final results are in line with the described in the Annex1 and intend AlGaN/GaN based electronic structures with diamond passivation/support.