RU2816671C1 - Method of making diamond schottky diode - Google Patents

Abstract

FIELD: electronic equipment.

SUBSTANCE: invention relates to electronic engineering and is used in a method for manufacturing a diamond Schottky diode. Method includes preparation by polishing of substrate 2 from synthetic monocrystal 1 of diamond doped with boron, chemical deposition from gas phase on one side of polished substrate 2 of diamond film with low degree of doping with boron, forming an anode on the reverse side of polished substrate 2 in the form of an ohmic contact and forming a cathode with a Schottky contact on the diamond film. Substrate 2 used is a diamond plate cut from growth sectors {001} and/or {113} of diamond monocrystal with cuboctahedral habit, grown by temperature gradient method at high pressure and temperature in high-pressure apparatus of "toroid" type with average level of doping with boron in range of 10¹⁷ to 10¹⁹ atom/cm³ and degree of compensation of acceptor impurities of not more than 1 %.

EFFECT: reduction of voltage drop on boron-doped substrate 2 when direct current flows through the diode.

1 cl, 2 dwg

Description

Field of technology

The present invention relates to the field of electronic engineering, in particular to the design and manufacturing technology of semiconductor diodes with a Schottky barrier, and can be used in high-current high-voltage and solid-state high-frequency electronics.

Prior Art

Currently, despite significant advances in silicon electronics, existing silicon-based diodes are not able to meet the growing needs of industry. Based on the combination of the most important parameters for electronic devices, synthetic diamond is one of the most promising wide-gap materials for the following reasons:

- the value of the critical electric field strength for diamond (10 ⁷ V/cm) exceeds by almost an order of magnitude the corresponding indicators for silicon carbide, which makes it possible to obtain higher blocking voltages;

- large band gap leads to extremely low leakage currents over a wide range of operating temperatures;

- high thermal conductivity reduces the thermal resistance of the crystal and makes it possible to reduce the size of power devices;

- the chemical inertness and mechanical hardness of diamond makes it possible to create reliable devices for harsh operating conditions;

- the radiation resistance of diamond allows the use of devices based on it under conditions of natural cosmic and artificial radiation without the use of special protective housings.

There are known methods for manufacturing a semiconductor diode with a Schottky barrier based on an epitaxial layered diamond structure (US patent No. 6833027 B2, IPC C30B 29/04, published 12/21/2004; US patent No. 5352908, IPC H01L 29/48, published 10/04/1994) . When implementing such methods, the following operations are performed. A working substrate of p-type conductivity is prepared from a synthetic diamond single crystal heavily doped with boron. Next, using the chemical vapor deposition method, a layer (film) of lightly doped (or high-purity) single-crystal diamond is formed on one side of the diamond substrate. On the back side of the diamond substrate, an anode is formed in the form of an ohmic contact, for example, using a sublayer of carbide-forming transition metals (titanium, tantalum, molybdenum, vanadium, etc.). A cathode is formed on the grown film of lightly doped diamond in the form of contact with the Schottky barrier, for example, using noble metals (gold, platinum, palladium).

These methods make it possible to obtain diodes with a Schottky barrier based on synthetic diamond with breakdown voltages U _MAX of no more than 20-50 V and leakage currents I _LEAK of more than 10 mA. In addition, the voltage drop in the open state of U _ON for such diodes is tens of volts, which limits the range of operating direct currents I _ON to tenths of amperes.

The disadvantages associated with low breakdown voltages U _MAX and high leakage currents I _LEAK are eliminated by the known method of manufacturing a diode with a Schottky barrier, chosen as a prototype (patent RU 2488912 C2, IPC H01L 21/329, published 07.27.2013). In this known method, a prepared and polished substrate of a synthetic diamond single crystal with a high degree of boron doping is additionally subjected to an ion plasma etching operation to remove a surface layer with a thickness of at least 10 μm before deposition of a diamond film with a low degree of boron doping. After deposition of the diamond film, the resulting epitaxial layered diamond structure is annealed. In this case, the decrease in the diode voltage drop in the forward direction is due to a decrease in the thickness of the transition region near the substrate-film interface and the removal of impurity hydrogen from the structure. An increase in the breakdown voltage U _MAX and a decrease in leakage current I _LEAK is achieved by forming a protective structure in the form of an expanded electrode with a special profile. To do this, a protective dielectric layer is formed from several layers of dielectrics applied sequentially, differing in the etching rate during the formation of a window using lithography, which leads to the formation of an expanded electrode with an angle of inclination of the electrode wall to the plane of the diamond film of less than 20°.

This known technical solution makes it possible to create Schottky diodes with various combinations of differential on-state resistance values (R _ON ) and breakdown voltages (U _MAX ). The open resistance is the sum of the resistance of the doped substrate and the resistance of the drift (working) layer: R _ON =R _SUB +R _DRIFT . In this case, by selecting the parameters of the drift layer, it is possible to select the ratio of the thickness w_drift and the doping level of the drift layer B_drift such as to minimize the resistance R _DRIFT for a given (required) U _MAX . In addition, there are alternative diode designs that can further reduce the voltage drop across the forward drift layer. For example, in the utility model RU174126 U1 MPK: H01L 29/872, the claimed technical solution is to create a diamond diode with a Schottky barrier based on multilayer structures from a synthetic diamond single crystal, characterized by a voltage drop reduced by at least two times when current flows in the forward direction .

In all cases, the substrate resistance R _SUB remains unchanged and contributes to the total on-state resistance R _ON . For high-current diodes (current density I _ON > 20 A/cm ² at U _ON = 4 V or less), the substrate resistance becomes decisive. This leads to high resistive energy losses on the diode element when forward current flows.

The essence of the invention

The problem to be solved by the claimed technical solution is to reduce the voltage drop across a boron-doped substrate when direct current flows through the diode.

This problem is solved in the present invention by the fact that in the method of manufacturing a diamond Schottky diode, including preparing a conductive substrate from a synthetic single crystal of diamond doped with boron by polishing, chemical vapor deposition on one side of the polished substrate of a diamond film with a low degree of boron doping, forming on on the back side of the polished anode substrate in the form of an ohmic contact and the formation of a cathode on the diamond film with a contact in the form of a Schottky barrier, according to the present invention, a diamond plate cut from growth sectors {001} and/or {113} is used as a conducting boron-doped diamond substrate a single crystal of diamond with a cuboctahedral habit, grown by the temperature gradient method at high pressure and temperature in high-pressure apparatuses of the “toroid” type with an average level of boron doping in the range from 10 ¹⁷ to 10 ¹⁹ atoms/cm ³ and a degree of compensation of acceptor impurity of no more than 1%.

In addition, according to one embodiment of the present invention, before forming the ohmic contact, the diamond substrate is further thinned using plasma etching to a thickness of 50 to 5 μm.

To produce substrates for diamond Schottky diodes with vertical geometry, bulk boron-doped diamond single crystals are used. During crystal growth using the temperature gradient method at high pressure and temperature (TG-HPHT), doping occurs due to the capture of impurities through the growth faces of the crystal. In this case, the rates of impurity capture are determined by the surface energy and the kinetics of the addition of carbon atoms and impurities on a specific growth face. It is known that at an average level of doping of diamond with boron in the range of 10 ¹⁷ -10 ¹⁹ atom/cm ³ , the growth of crystals of cuboctahedral habit occurs (the appearance of the crystal during the entire synthesis contains faces or growth sectors {001}, {113}, {111}) . Studies carried out by the authors of the present invention have shown that the growth sectors {001} and {113} of crystals demonstrate 25 times lower resistivity at room temperature due to a reduced (no more than 1%) degree of compensation of the acceptor impurity (in other words, the degree of compensation of the main impurity by parasitic donor impurities). This is probably due to the fact that the boron/nitrogen capture rate ratio in the {001}, {113} sectors is higher than in the {111} sector. A low degree of compensation leads to a relatively high degree of ionization of the impurity (high concentration of free charge carriers) already at room temperature, which in turn ensures a low resistivity of the diamond plate, no more than 5 Ohm×cm.

During the growth of a homoepitaxial diamond layer, dislocations present in the single-crystal diamond substrate are inevitably inherited into the growing layer. Vertical through dislocations in the homo-epitaxial layer lead to an increase in diode leakage currents ( _ILEAK ). In turn, it was found that the growth sectors {001} and {113} of a boron-doped diamond single crystal with a cuboctahedral habit have better crystalline perfection and a lower dislocation concentration.

Thus, the present invention is based on the fact that in the manufacture of a Schottky diode, a substrate is used from a synthetic diamond single crystal with low resistivity and high structural perfection, which is achieved through the selection of growth sectors {001}, {113} (regions) of a bulk boron-doped single crystal diamond of cuboctahedral habit, from which the substrate is cut.

Brief description of drawings

The invention is illustrated by drawings, in which:

Fig. 1 is a diagram of cutting a diamond doped single crystal of cuboctahedral habit to prepare the substrate plate used in the method of the present invention;

Fig. 2 is a diagram of a diamond Schottky diode manufactured by the method of the present invention.

Carrying out the invention

The method of the present invention is carried out as follows.

A synthetic single crystal of diamond with a cuboctahedral habit is grown using the temperature gradient method at high pressure and temperature in high-pressure toroid-type apparatuses. The average level of boron doping of diamond is from 10 ¹⁷ to 10 ¹⁹ atom/cm ³ , and the degree of compensation for the acceptor impurity in diamond is no more than 1%. A diamond plate is cut out from the growth sectors {001} and/or {113} of the grown single crystal 1, which is then used as a conducting boron-doped diamond substrate 2. The cutting diagram for such a single crystal 1 is shown in Fig. 1. The substrate 2 is polished.

A diamond film 3 with a low degree of boron doping is applied to one side of the polished substrate 2 using chemical vapor deposition. On the other (reverse) side of the polished substrate 2, an anode is formed in the form of an ohmic contact 4. A cathode 5 with a contact in the form of a Schottky barrier 6 is applied to the diamond film 3.

Industrial applicability

Example.

Synthetic diamond single crystal 1 cuboctahedral habit type IIb is grown by the temperature gradient method at high pressure and temperature (t=1500°C, p=5.5 GPa) in a Fe-Al-CB growth medium with the necessary addition of boron in a concentration of 4×10 ¹⁷ atom /cm ³ and the degree of compensation for acceptor impurities is no more than 1%. From the growth sectors {001} and/or {113} of the grown single crystal 1, a plate of crystallographic orientation (001) with dimensions of 4×4 mm is cut out using laser cutting (an automated system with a nanosecond laser). The cut plate is polished on both sides and thinned to a thickness of 0.1-0.2 mm. Polishing is carried out using equipment from Dialit (Israel) for high-quality mechanical grinding and polishing of poly- and single crystals of diamonds. The resulting polished plate is used as a medium-doped single-crystal p+ diamond substrate 2 for the manufacture of a Schottky diode. Such a substrate has a resistivity at room temperature of ρ = 4.7 Ohm×cm. On the surface of the substrate 2, a homoepitaxial single-crystal layer of p-diamond film 3 with a thickness of 6 μm is grown in a device for gas-phase deposition using RF plasma. The boron concentration in the p- layer is 1×10 ¹⁶ atom/cm ³ . On the opposite surface of the medium-doped p+ substrate, an anode is placed in the form of an ohmic three-layer contact 4 measuring 3.3×3.3 mm (area 0.1 cm ² ), by sequential vacuum magnetron sputtering of a titanium layer (30 nm, 200 W, 10 min) with annealing at 800°C in vacuum, a platinum layer (50 nm, 100 W, 5 min) and a gold layer (150 nm, 100 W, 5 min). On the surface of a lightly doped layer of p-diamond film 3, by vacuum magnetron sputtering of platinum (50 nm, 150 W, 5 min), a cathode 5 is formed in the form of a metal contact 3.3 × 3.3 mm in size with a Schottky barrier.

The use of a medium-boron-doped substrate 2, cut from the growth sectors {001} and/or {113} of a bulk boron-doped diamond single crystal 1 of cuboctahedral habit, significantly reduces the series resistance of the diode in the on-state R _ON (by at least 2 times) and, therefore, reduces the forward voltage drop U _ON . In addition, the substrate, cut from the growth sectors {001} and/or {113} of a bulk boron-doped diamond single crystal of cuboctahedral habit, has high crystalline perfection (dislocation concentration 10 ² cm ^-2 or less). This makes it possible to grow on the specified substrate a homoepitaxial diamond film 3 with high crystalline perfection, which provides reduced leakage currents and increased breakdown voltage of the Schottky diode in the closed state. Taken together, this leads to reduced electrical (ohmic) losses when using a diode in power electrical circuits.

An additional reduction in the series resistance R _SUB introduced by the boron-doped substrate 1 can be achieved by thinning it. The limit of mechanical polishing of a diamond plate is considered to be 100 microns thick. Further thinning is achieved by plasma etching in a plasma, for example, in a fluorine-containing plasma) to a thickness of 50 to 5 μm.

A diamond Schottky barrier diode manufactured by the method of the present invention has a forward voltage drop of 4 V when a forward current I _ON = 10 A flows, which corresponds to a current density J _ON = 100 A/cm ² . High forward current values are achieved due to resistive self-heating of the device. Thus, a more than twofold increase in forward current was achieved at a voltage U _ON = 4 V compared to a diamond diode with a Schottky barrier manufactured using the method according to patent RU 2488912.

Claims (2)

1. A method for manufacturing a diamond Schottky diode, including preparing a substrate from a synthetic single crystal of diamond doped with boron by polishing, chemical vapor deposition of a diamond film with a low degree of boron doping on one side of the polished substrate, and forming an anode in the form of an ohmic contact on the reverse side of the polished substrate. and forming a cathode with a Schottky contact on the diamond film, characterized in that a diamond plate cut from the growth sectors {001} and/or {113} of a diamond single crystal with a cuboctahedral habit, grown by the temperature gradient method at high temperatures, is used as a conducting boron-doped diamond substrate. pressure and temperature in high-pressure apparatuses of the “toroid” type with an average level of boron doping in the range from 10 ¹⁷ to 10 ¹⁹ atom/cm ³ and the degree of compensation for acceptor impurities of no more than 1%. 2. The method according to claim 1, characterized in that before the formation of the ohmic contact, the diamond substrate is additionally thinned using plasma etching to a thickness of 50 to 5 μm.