Growth of GaN Epitaxial Films on Polycrystalline Diamond by Metal-organic Vapor Phase Epitaxy.
University of Bath.
If electronic circuits and devices like transistors get hot their performance becomes much less efficient, often to the extent they do not work properly, if at all. Unfortunately, transistors almost always generate heat, creating the problem for engineers of keeping either individual devices or circuits cool. People are familiar with the use of fans in electrical appliances to achieve the necessary cooling, but these fans also use electricity and get hot, contributing to unwanted energy usage. A far simpler but technically challenging solution is to mount the heat-generating electronics on an excellent heat conductor and conduct heat away from where it is being generated. Unfortunately, any materials like the plastic or ceramic encapsulation used to protect the heat-generating electronic device impede the flow of heat away to cooler parts of the appliance or system. Overcoming this heat "resistance" problem requires growing the semiconducting material directly on an excellent conductor of heat. Diamond is the best thermal conductor known to mankind but growing good quality semiconductor crystals with different chemical composition and physical properties has not been possible until now. The paper describes processes for growing gallium nitride (GaN) crystals on polycrystalline diamond substrates which have nearly as high thermal conductivity as crystalline diamond but are much cheaper to manufacture. These processes exploit the existence of an ultra-thin silicon carbide (SiC) layer (typically about 1-3 nm in thickness) that forms during the growth of polycrystalline diamond on silicon (Si) wafers. Gallium nitride is a relatvely new semiconductor that has superior properties to silicon and is proving to be an ideal material for making the type of transistors that will be used in electric cars. The ability to grow gallium nitride on heat-extracting polycrystalline diamond wafers has the potential to greatly advance the efficiency of electronic systems that are prone to generating heat, like mobile phone masts and power control electronics in cars, thereby reducing energy consumption and the emission of greenhouse gases.
Heat extraction is often essential to ensuring efficient performance of semiconductor devices and requires minimizing the thermal resistance between the functional semiconductor layers and any heat sink. This paper reports epitaxial growth of N-polar GaN films on polycrystalline diamond substrates of high thermal conductivity with metal-organic vapor phase epitaxy by using a SixC layer formed during deposition of polycrystalline diamond on a silicon substrate. The SixC layer acts to provide the necessary structure ordering information for the formation of a single crystal GaN film at the wafer scale. It is shown that a three-dimensional island (3D) growth process removes hexagonal defects that are induced by the non-single crystal nature of the SixC layer. It is also shown that intensive 3D growth and the introduction of convex curvature of the substrate can be deployed to reduce tensile stress in the GaN epitaxy to enable the growth of crack-free layer up to a thickness of 1.1m. The twist and tilt can be as low as 0.65 and 0.39 respectively, values broadly comparable with GaN grown on Si substrates with a similar structure.
||Growth of GaN Epitaxial Films on Polycrystalline Diamond by Metal-organic Vapor Phase Epitaxy
||Metal-organic Vapor Phase Epitaxy, GaN-on-diamond, tensile stress reduction
||Electrical engineering > Power Electronics
Energy > Energy Efficiency
Materials sciences > Materials Synthesis and Growth
Optics, photonics and lasers > Optical Devices and Subsystems
||Faculty of Engineering & Design > Electronic & Electrical Engineering
Faculty of Engineering & Design > Mechanical Engineering
|Data collection method:
||Description of data
All the data used in this article takes the form measurable quantity (dependent variable) versus parameter varied under operator control (independent variable) and is recorded in tabulated form in spreadsheets. Apart from one data set referred to in a “non-quantitative way”, all the data used was presented in the above article in graphical form indicated as figures 1b, 2, 3 etc. Figure 1a is a sketch and therefore does not contain measurable data. The data-containing graphs are:
Figure 1b in file Fig_1b_curvature:
The bow of both kinds of PD substrates measured by moving a digitally-controlled stylus across the sample surface over a distance of 5 mm and measuring the vertical deflection of the stylus every few microns. The PDA substrate was found to have a convex bow (~1.5 μm over 5 mm) whilst PDB had a concave bow (~6 μm over 5 mm), where convex and concave are defined in the article.
Figure 2 in file Jiang_Fig_2_xps_data:
X-ray photo-electron spectroscopy (XPS): intensity of emitted x-ray photon versus binding energy. These data reveal the nature of the chemical bonds between the elements found on the surface or immediate sub-surface layer of the samples under consideration. This in turn provides information about the chemical nature of the surface on which the III-Nitride layers were grown.
In describing the data in this graph, reference is made to energy dispersive x-ray (EDX) measurements that confirm the presence of both silicon and carbon on the surface of the nominally polycrystalline diamond growth substrates, both in large quantities. Taken together with the chemical bonding information obtained from the XPS measurements (presented in figure 2) it was inferred that almost all the silicon present was bonded chemically to carbon, with some silicon bonded to oxygen.
Figure 3 in file Jiang_Fig_3_reflectivity:
These data were collected via in-situ measurements of the intensity of a visible laser beam (typically red light) from the surface of the III-Nitride layers as they were grown using a standard commercially available tool (specified in the article). The independent variable is time.
Figure 4 in file Jiang_Fig_4_Surface_MOR:
The data are images formed by the secondary electron emission, taken by digital camera during inspection of the surface of the grown III-Nitride layer by scanning electron microscopy. The data sets in the form of the intensity reaching each pixel in a two dimensional array of such pixels in the digital camera.
Figure 5 in file Jiang_Fig_5_comparison_of_XRD_3D_GaN(0002)_and_GaN(103):
These data are the intensity of a diffracted x-ray beam measured by rocking samples 3 and 4 (described in the article, especially in Table 1) when illuminated by a beam of x-rays under conditions that reveal the twist in the III-Nitride layers [beam alignment along crystallographic direction GaN(10-13)] and the tilt [ beam alignment along GaN(0002)].
Figure 6 in file Jiang_Fig_6_E2H Raman_comparison_3D_growth_time:
Raman spectra of Sample 3 and Sample 4 in the vicinity of the E2H phonon mode for two samples to reveal the impact of the duration of the 3-diminsional growth mode on the strain in the GaN epitaxial films.
Figure 7 in file Jiang_Fig_7_Raman_comparison of growth on bowed substrates:
Raman spectra in the vicinity of the E2H phonon mode for (a) Sample 5 grown on a concave PDB substrate and (b) Sample 6 grown on a convex PDA substrate to reveal the impact on the strain in the GaN epitaxial films of the growing the III-Nitride layers on bowed substrate.
|Data processing and preparation activities:
||Apart from the photographs the data processing involved only plotting measured dependent variable values against the corresponding measured independent variable values. Where applied, curve fitting performed by using the polynomial curve fitting tool available in Excel. Otherwise the data were not processed in any way.
|Technical details and requirements:
||Two kinds of PD substrate (hereafter called PDA and PDB) from different suppliers [Element Six Ltd, Diamond Materials GmbH] were used in the work, both supplied with their initial Si(111) growth substrate intact. In each case the Si(111) was removed by immersion in hot KOH or/and isotropic Si etching solution (a mixture of HNO3 and HF). A distinguishing feature of the two types of substrate after removal of the Si(111) substrate is their curvature, as shown in Figure 1a. The curvature measured by a stylus technique (Dektak Stylus profiler), shown in Figure 1b, occurs as a result of residual stress in the PD layers . PDA substrates exhibited a convex bow (~1.5 micron over 5 mm) while PDB substrates had concave bow (~6 micron over 5 mm).
X-ray photoelectron spectroscopy (XPS) measurements were performed under sub-contract at the University of Cardiff.
Energy dispersive x-ray spectroscopy (EDX) was performed a JEOL SEM6480LV electron microscope at the University of Bath.
Surface reflections were recorded during epitaxial growth of all samples using a LAYTEC EpiSense system.
X-ray diffraction (XRD) measurements were performed using a BEDE D1 system.
Raman spectroscopy was performed using a Renishaw inVia systemusing a laser source of wavelength = 530 nm).
The curvature of the substrates after epitaxial growth was measured with a Dektak stylus profiler.
All data collection was performed using the software supplied by the equipment manufacturers available at the time of purchase of the equipment.
All equipment was operated under the standard conditions recommended by the equipment manufacturers and all measurements were performed at room temperature, except the surface reflection measurements performed during epitaxial growth, which were performed at the temperatures specified in the manuscript.
||Sample preparation apart from the growth of the polycrystalline diamond substrates (which were supplied commercial manufacturers named in the manuscript) and epitaxial growth, was carried out in David Bullett Nanofabrication Facility at the Univesity of Bath.
Methodology and documentation files
|Allsopp, Duncanemail@example.com||0000-0003-4197-9852||University of Bath||Yes|
|Jiang, Quanzhong||University of Bath||No|
|Bowen, Christopherfirstname.lastname@example.org||0000-0002-5880-9131||University of Bath||No|
|Rights Holder||University of Bath|
|Funder name||Fundref ID||Grant number||Research project title|
|Engineering and Physical Sciences Research Council (EPSRC)||https://doi.org/10.13039/501100000266||EP/K024337/1||Novel High Thermal Conductivity Substrates for GaN Electronics: Thermal Innovation|
||University of Bath
||6 March 2017
||No restrictions nor limits on access to the dataset
|1 May 2013||31 August 2016|
|1 May 2013||31 October 2016|
||08 Mar 2017 10:32
||08 Mar 2017 11:07
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