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Research progress of heteroepitaxy single crystal diamond and related electronic devices

October 29,2023

1. Heterogeneous epitaxial single crystal diamond

Microwave plasma chemical vapor deposition (MPCVD) is the mainstream method for the preparation of high-quality single crystal diamond.  According to the substrate selection, it can be divided into two types: homogeneous epitaxy and heteroepitaxy. Homogeneous epitaxy uses single crystal diamond as the substrate, and large-area single crystal is obtained through three-dimensional growth technology and mosaic splicing technology. Single-crystal diamond substrates with sizes of 40 mm × 60 mm can be obtained using mosaic splicing technology. Heteroepitaxy epitaxial materials are different from substrate materials, diamond single crystal heteroepitaxy technology has been developed over the years, from the initial epitaxial diamond grain to the complete heteroepitaxy single crystal diamond film, now can epitaxial growth of nearly 4 inches of single crystal diamond substrate (see Figure 1), crystal quality is also constantly improving.


Figure 1 Current largest size heterogeneous epitaxial single crystal diamond substrate

The coefficients of thermal expansion of MgO and SrTiO3 differ greatly from diamond, so when a temperature suitable for diamond epitaxy is reached (dotted line in Figure 2), high stress in the diamond film deposited on the oxide substrate can cause the diamond to easily fragment or fall off the substrate. Al2O3 and Si substrates have the advantages of low cost, large area substrate, high crystal quality, and relatively small thermal mismatch with diamond, so they have become the mainstream substrates for heteroepitaxy single crystal diamonds.


Fig. 2 Thermal stress-deposition temperature variation relationship between different substrates and deposited diamonds

As shown in Figure 3, at the beginning of the BEN process, an amorphous carbon layer is first formed on the surface of Ir (see Figure 3(a)), and under the acceleration of the electric field, the carbon ions excited by microwaves are continuously injected into the subsurface of Ir until saturation, and when the concentration of carbon continues to increase, the C atoms on the Ir subsurface will precipitate to form a primary diamond nucleus. After the formation of the primary diamond nucleus, the C atoms around it are normalized by the interaction force between the C atoms to form a regularly arranged diamond nucleus (see Figure 3(c)). After the bias is turned off, and within 5~10 s of the beginning of the rapid growth process of diamond, the amorphous carbon on the surface of Ir will be etched in a hydrogen-rich environment.


Figure 3 Schematic diagram of diamond BEN process

Single-crystal diamond substrates with sizes of 10 mm× 10 mm × 1 mm were successfully prepared, with a Raman half-peak width of 3.7 cm-1 and good crystal quality.


Figure 4 (a) Diamond microneedle preparation process; (b) diamond microneedles formed by chemical etching of NI; (c) 10 mm× 10 mm× 1 mm heterogeneous epitaxial single crystal diamond substrate; (d) Comparison of temperature changes during growth with and without micron needle substrates; (e) Raman spectra of a diamond substrate

This method has the following advantages: 1) the use of ELO to improve the quality of diamond crystals; 2) Diamond micro needles can effectively relieve the stress caused by lattice mismatch of diamond and oxide, and solve the problem of poor heat dissipation due to substrate warpage during rapid growth; 3) It can realize the automatic peeling of diamond and substrate. This method may allow multiple iterations on the resulting diamond substrate to continuously improve the quality of the diamond crystal.


Figure 5 1-inch heterogeneous epitaxial diamond substrate[25]

In 2022, Kasu's team performed heteroepitaxy growth of diamond on a α-Al2O3 substrate with a surface bias of direction of 7°, and its substrate structure is shown in Figure 6(a). The experiment found that the diamond showed a step growth mode during the rapid growth process, and the tensile stress inside the crystal was released, which improved the crystallization quality, and a single-crystal diamond substrate with a size of 2 inches was successfully prepared (see Figure 6(b))


Figure 6 (a) Off-axis growth schematic; (b) 2-inch heterogeneous epitaxial single crystal diamond substrate; (c) XRD rocking curve half-peak full-width full-spectrum of 2-inch single crystal diamond (004) surface

As shown in Figure 7, after adding the buffer layer containing metal W, the number of etching pits on the diamond surface is significantly reduced, and the dislocation density is greatly reduced.


Fig. 7 SEM photo of MPCVD heteroepitaxy diamond surface after H2/O2 plasma treatment

2. Power electronics based on heterogeneous epitaxial single crystal diamond substrate

The n-type doping technology of diamond faces the problem of high activation energy of the donor, and its technology is still being explored. Current diamond-based MOSFETs are mainly prepared using hydrogen terminals as conductive channels. When hydrogen terminal diamond is exposed to air, nitrogen dioxide, ozone, or contact with some transitional oxides such as V2O5, MoO3, etc., surface electrons will be transferred to the surface adsorbent, causing the surface energy band to bend, and then form a two-dimensional hole gas (2DHG) on the surface.


Fig.8 Schematic diagram of the energy band forming two-dimensional hole gas on the surface of hydrogen terminal diamond[59]

The device structure is shown in Figure 9. Its maximum source-leakage current is -288 mA/mm. Experiments show that the 100 nm Al2O3 passivation layer effectively suppresses the leakage of the device, achieves a breakdown voltage of -2608 V in the off state, and the breakdown electric field is 2 MV·cm-1, which is comparable to the current SiC and GaN-based MOSFETs.


Figure 9 Schematic diagram of MOSFET structure cross-section (a) of 100 nm Al2O3 overlay and ID-VDS(b) in off state

In 2022, Kasu et al. will use chemical-mechanical polishing (CMP) technology to polish heterogeneous epitaxial single crystal diamond substrates to improve surface flatness and reduce defects. After 200 h of CMP treatment, the surface roughness of the diamond was 0.04 nm, and the resistance size of the hydrogen terminal surface block was 3.55 kΩ/sq, and the result was shown in Figure 10(a).


Figure 10 (a) Model parameters of gold/hydrogen terminal diamond transmission line under different CMP treatments; (b) ID-VDS curve in open state of MOSFET; (c) effective mobility; (d) ID-VDS curve in off state

In the same year, the research team prepared "modulated doped" diamond MOSFETs. As shown in Figure 11, NO2 and hydrogen terminal channels were separated by nitrogen dioxide doping above the Al2O3 gate dielectric layer at 8 nm, the mobility was increased to 496 cm2/(V·s), the breakdown voltage reached -3326 V, the maximum drain current density was -0.42 A/mm, and the BFOM was 820.6 MW/cm2, which proved that heteroepitaxy single crystal diamond is expected to be used in RF power devices.


Figure 11 (a) Schematic diagram of the cross-sectional structure of the MOSFET; (b) Variation in effective mobility of modulated doped MOSFETs with carrier concentration

As shown in Figure 12, the current-voltage characteristics of the p-i-n diode exhibit good rectification characteristics. Increasing the forward current leads to a sublinear increase in the integral intensity of defect luminescence, while a superlinear increase in the integral intensity of free exciton luminescence. This significant trend is the same trend observed with p-i-n diodes prepared with homogeneous epitaxial growth films on conventional HTHP synthetic diamond substrates. It indicates the potential of heterogeneous epitaxial single crystal diamond substrates in future diamond-based electronic devices.


Figure 12 (a) P-I-n device structure and test schematic; (b) Diode forward conduction characteristics

Since the n-type doping technology of diamond is not yet mature, the current diamond-based Schottky diode is mainly realized by forming a Schottky junction between p-type diamond and metal. From the structure, it can be divided into vertical type, quasi-vertical type and horizontal type, and its structure is shown in Figure 13.


Figure 13 (a) Vertical Schottky diode; (b) quasi-vertical Schottky diodes; (c) Transverse Schottky diode

In 2021, Sittimart et al. suppressed defects by inserting a buffer layer containing tungsten metal. Quasi-vertical Schottky barrier diodes were prepared on heterogeneous epitaxial crystals with a side length of 5 mm. After inserting the buffer layer, in-plane uniformity is improved, all Schottky diodes exhibit excellent rectification, and leakage current is suppressed, as shown in Figure 14.


Figure 14 I-V characteristics of 20 Schottky diodes without (a) and (b) buffer layers at room temperature

Figure 15 compares the electrical properties of diamond Schottky diodes reported in recent years. The abscissa is the breakdown voltage and the ordinate is the specific on-resistance. It can be seen from Figure 15 that the performance of Schottky diodes based on heteroepitaxy single crystal diamond substrates is generally not as good as that of homoepitaxy diamond substrates, mainly because the crystal quality of current heteroepitaxy single crystal diamond substrates is difficult to reach the level of homogeneous epitaxy. Further improving the quality of heteroepitaxy single crystal diamond crystal is the key to improving device performance.


Figure 15 Diamond Schottky diode performance comparison chart