BACKGROUND

As is known in the art, semiconducting diamond can provide an order of magnitude increase in power handling over other semiconductor materials, such as silicon (Si), gallium arsenide (GaAs), and gallium nitride (GaN). This is made possible by diamond's higher thermal conductivity and a higher breakdown voltage than any device-quality semiconductor.

A diamond field-effect transistor (FET) can be made by forming a p-type surface conductive layer on the diamond surface. One approach to forming the p-type surface conductive layer is to terminate the diamond surface with hydrogen (H) atoms and then over-coat the H-terminated surface with a material that contains negative charge. Impressive current, voltage and operational frequencies have been demonstrated with such surface conductive FETs. However, with existing diamond FETs, device operation may be limited by the resistance of the surface layer and the inherent resistance between the surface conductive layer and metal contacts to the semiconductor. Such ohmic contacts may be likely to degrade, exhibit increasing resistance and burn out at high currents, e.g., currents >0.5 A mm−1. In addition, contacts exhibiting lower contact resistances (e.g., contacts <3 Ω-mm using Au or Pt) generally do not adhere well to the diamond surface and may not be suitably robust for device processing and fabrication.

SUMMARY

Disclosed herein are embodiments of a diamond semiconductor structure that has low resistance (e.g., ˜0.50 Ω-mm) important for high power diamond transistors. The current flowing in and out of the ohmics does not suffer from current crowding common to other metal ohmics on diamond. This makes the ohmics more robust and results in higher reliability for devices subjected to high temperatures and high operational current. Also disclosed are methods for fabricating robust low-resistance contacts suitable for high-frequency operation of diamond FETs. Embodiments of the present disclosure have experimentally yielded DC power density of 200 W/mm without the use of heat-sinks. Embodiments of the present disclosure can be used in high-frequency, high-power applications, such as power radio frequency (RF) amplifiers.

According to one aspect of the present disclosure, a semiconductor structure can include: a diamond substrate having a surface conductive layer; a heavily doped region formed in the diamond substrate; and a metal contact positioned over the conductive surface layer such that a first portion of the heavily doped region is covered by the metal contact and a second portion of the heavily doped region is not covered by the metal contact.

In some embodiments, the surface conductive layer may include: an H-terminated surface of the diamond substrate; and a material that contains negative charge. In some embodiments, the heavily doped region may include diamond doped with Boron in a concentration greater than 1019cm−3. In some embodiments, the heavily doped region can have a width in the range of 0.1 to 10 μm and a depth in the range 0.2 to 0.4 μm. In some embodiments, the heavily doped region may have a width greater than 10 μm. In some embodiments, the metal contact can include at least one of Molybdenum (Mo), Tungsten (W), Titanium (Ti), or Tantalum (Ta), or other carbide forming material. In some embodiments, the metal contact may include a plurality of metal layers, wherein the plurality of metal layers includes an interface layer in contact with the heavily doped region and a bonding layer for contact with wire. In some embodiments, the interface layer can include Molybdenum (Mo) and the bonding layer comprises Gold (Au).

According to another aspect of the present disclosure, a method can include: applying a mask over a surface of a diamond substrate; forming one or more highly doped regions in the diamond substrate using the mask; removing the mask from the surface of the diamond substrate; depositing metal over the surface of the diamond substrate such that, for each of the one or more highly doped regions, a first portion of the heavily doped region is covered by the metal and a second portion of the heavily doped region is not covered by the metal; and forming a conductive layer at the surface of the diamond substrate.

In some embodiments, applying the mask over the surface of a diamond substrate can include etching the surface of the diamond substrate. In some embodiments, forming the one or more highly doped regions can include growing diamond doped with Boron. In some embodiments, depositing the metal can include depositing at least one of Molybdenum (Mo), Tungsten (W), Titanium (Ti), or Tantalum (Ta), or other carbide forming material. In some embodiments, forming the conductive layer at the surface of the diamond substrate can include: terminating the surface of the diamond substrate with Hydrogen (H) atoms; and overcoating the H-terminated surface with a protective material. In some embodiments, forming the one or more highly doped regions can include forming a first highly doped region corresponding to a drain of a field-effect transistor (FET) and forming a second highly doped region corresponding to a source of the FET. The method can further include depositing a gate on the conductive layer.

