Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

This method can help answer key question about voltage control over current in zinc oxide-based heterojunction field effect transistors with two-dimensional electron gas using Schottky contacts. The main advantage of this technique is that the gate of the field effect transistor can be defined in one photo step. The implications of this technique extend across new generations of high-frequency, high-power field effect transistors by taking advantage of high electron saturation diversity in zinc oxide.

Though this method provides insight into the nature of Schottky contact in stability on single-site, it can also be applied to other single-site based devices such as solar plant detectors and the chemical or biosensors. To begin the procedure, load a sapphire substrate two-inch in diameter 380 micrometers thick C-plane into a metal organic CVD instrument, and prepare the system for deposition. Once the system is ready, ramp the reactor pressure to 30 torrs and the substrate temperature to 1, 055 degrees Celsius in a hydrogen atmosphere over the course of 35 minutes.

Hold it at that temperature for three minutes to desorb the residual contaminants. Then, ramp the substrate down to 941 degrees Celsius over the course of three minutes. After letting the temperature stabilize for two minutes, set the trimethylaluminum flow to 12.0 SCCM, and the ammonia flow to seven SCCM.

Let the flow rate stabilize for three minutes. Then, switch the trimethylaluminum flow to the run line to initiate the low temperature aluminum nitride layer growth. Over the course of six minutes, grow about 20 nanometers of aluminum nitride on the substrate as measured by reflectivity oscillations.

After that, without interrupting growth, ramp the substrate to 1, 100 degrees Celsius in three minutes. Continue the aluminum nitride growth until the layer is 300 nanometers thick. Then, direct the trimethylaluminum flow away from the reactor.

Start the trimethylgallium flow at 15.5 SCCM and let it stabilize for two minutes. Then, grow about 400 nanometers of gallium nitride on the substrate. An initial decrease in reflectivity will be observed during gallium nitride nucleation.

The reflectivity will recover to the original level when the gallium nitride islands coalesce. Once the gallium nitride is 400 nanometers thick, ramp the substrate temperature to 1, 124 degrees Celsius in two minutes without interrupting growth. Grow about 2.5 nanometers of a high temperature semi-insulating gallium nitride layer.

Then, direct the trimethylgallium flow away from the reactor to stop the growth. Cool the newly formed gallium nitride template to room temperature and unload it from the reactor. Next, slice the template into six equally-sized pieces.

In a fume hood, heat a hotplate to 220 degrees Celsius and prepare 200 milliliters of a one-to-one by volume mixture of concentrated hydrochloric acid in deionized water. Then, place 150 milliliters of concentrated hydrochloric acid in a 300-milliliter quartz beaker. Slowly add 50 milliliters of concentrated nitric acid to obtain an aqua regia solution.

Heat the aqua regia solution on the hotplate until the solution is orange-red and bubbly. Then place one gallium nitride template piece in a polytetrafluoroethylene basket and boil it in aqua regia for 10 minutes. Rinse the template in flowing deionized water for three minutes.

Then soak the template in the hydrochloric acid solution for three minutes. Rinse the template again in flowing deionized water for five minutes and then dry it with nitrogen gas. Within five minutes, load the clean template into a two-six molecular beam epitaxy instrument load lock and start pumping it down.

After pumping down the load lock with the clean gallium nitride template for an hour, prepare the zinc magnesium and beryllium effusion cells. Turn on the reflection high-energy electron diffraction system and load the template in to the MBE. Next, ramp the substrate to 615 degrees Celsius at 13.6 degrees Celsius per minute, and hold it at that temperature for 15 minutes to desorb residual contaminants.

Then, start ramping the substrate down to 280 degrees Celsius. When the substrate reaches 550 degrees Celsius, open the zinc cell shutter to expose the gallium nitride surface to the zinc flux. Turn on the oxygen plasma power supply, set the power to 100 watts, and confirm that the oxygen gas line is closed.

When the substrate reaches 280 degrees Celsius, set the oxygen plasma power to 400 watts. Set the oxygen flow to 0.3 SCCM to ignite the plasma, and then reduce it to 0.25 SCCM. Wait one minute and then open the oxygen shutter to start the low-temperature zinc oxide buffer layer growth.

Record a read pattern along the one negative one zero zero azimuthal direction every five minutes during growth. After about 15 minutes, the read pattern will change from 2D mode to 3D mode, indicating a buffer thickness of about 20 nanometers. Close the zinc and the oxygen shutters to stop the growth.

Then, increase the oxygen flow rate to 0.4 SCCM. Start ramping the substrate to 730 degrees Celsius at 13.6 degrees Celsius per minute. Ramp the lower zone temperature of the double-zone zinc cell to 345 degrees Celsius at 10 degrees Celsius per minute.

Wait for five minutes when the substrate reaches 730 degrees Celsius, and then start monitoring the zinc oxide surface by read. When it changes to 2D mode, the buffer layer has been annealed. Cool the substrate to 680 degrees Celsius.

Then, increase the oxygen flow rate to 3.2 SCCM, and open the zinc and oxygen shutters to grow a 300-nanometer thick, high-temperature zinc oxide layer. Set the oxygen flow rate to 0.3 SCCM afterwards. Ramp the beryllium cell to 820 degrees Celsius at 10 degrees Celsius per minute, and the magnesium cell to 510 degrees Celsius at 15 degrees Celsius per minute.

Cool the substrate to 325 degrees Celsius at 13.6 degrees Celsius per minute. Once the substrate temperature stabilizes, gradually increase the oxygen flow rate to 1.25 SCCM. Then, concurrently open the zinc, magnesium, beryllium, and oxygen shutters to start the beryllium magnesium zinc oxide barrier growth.

Grow an approximately 30 nanometer-thick layer of beryllium magnesium zinc oxide over the course of 12 minutes. Periodically acquire read patterns to monitor the growth mode evolution. Then, acquire a final read pattern and close the magnesium and beryllium shutters to end the beryllium magnesium zinc oxide growth.

Leave the zinc and oxygen shutters open for one more minute to grow a roughly two-nanometer thick zinc oxide cap layer. To begin the diode fabrication, sonicate the beryllium magnesium zinc oxide zinc oxide heterostructure sample in acetone and methanol for five minutes each in sequence. Rinse the sample in deionized water for five minutes, and dry it under a stream of nitrogen gas.

Then, spin-coat the sample with i-Line positive photoresist. Soft-bake the photoresist at 100 degrees Celsius for 140 seconds. Mask the sample, and expose it to a 6.5 watt UV lamp for 2.38 minutes.

Post-bake the photoresist at 110 degrees Celsius for 80 seconds. Then, shake the sample in photoresist developer for 60 seconds with a shaking frequency of one hertz. Rinse the developed sample in deionized water for three minutes, and dry it under nitrogen gas.

Next, treat the sample with remote oxygen plasma with an oxygen flow of 35 SCCM in an RF power of 50 watts for five minutes. Lastly, load the sample into an electron beam evaporator and deposit 50 nanometers of silver. Lift off with acetone to form the contacts, and clean and dry the sample with methanol, water, and nitrogen gas.

Read-patterns of the low temperature zinc oxide buffer layer initially showed elliptical spots, indicating a growth mode of 3D islands. Annealing at above 700 degrees Celsius produced a 2D surface morphology. The subsequent layers both grew in 2D mode.

Atomic force microscopy showed a small increase in root mean square roughness with each layer. X-Ray diffraction showed reflections consistent with 0002 reflections of zinc oxide, gallium nitride, and beryllium magnesium zinc oxide. The broadening of the beryllium magnesium zinc oxide reflection was attributed to the thinness of that layer.

All-effect measurements of the heterostructure showed a decrease in sheet carrier concentration with decreasing temperature, with saturation at about 13 kelvin. The electron mobility monotonically increased with decreasing temperature. The observed values at 293 kelvin and 13 kelvin were consistent with literature values.

These trends indicate the presence of two-dimensional electron gas at the beryllium magnesium zinc oxide zinc oxide interface. Current density voltage curves at room temperature for silver beryllium magnesium zinc oxide zinc oxide Schottky diodes showed gate currents increasing exponentially with applied forward voltage up to 0.25 volts, after which voltage-drops across the series resistance became apparent. The similarity between the curves indicated high in-wafer uniformity of the sample.

The highest apparent Schottky barrier height was observed with an ideality factor of 1.22. Since this method performed is critical for union to precisely control the surface polarity of zinc oxide on gallium polar gallium nitride templates, Failure in polarity control results in a heterostructure with no two-dimensional electron gas. Maintaining all-sector ratio below 1.5 during single-site nucleation ensures that the single-site based heterostructures have all zinc polar orientation.

While attempting this procedure, remember to carefully clean the sample surface both before growing the beryllium magnesium zinc oxide zinc oxide heterostructures on gallium nitride template and before fabricating the Schottky contacts on the heterostructures. Following this procedure, other methods like an RTM and XPS can be employed to get insight into the nature of single-site silver interface at the nanoscale level. We hypothesize that the formation of conductive silver oxide at the zinc oxide silver interface results in the stable Schottky contact.

Therefore, this approach paves the way towards high-quantity stable Schottky contact on single-site. This has implications for devices that rely on Schottky contact including H-phase photodetectors and chemical and biosensors. Don't forget the strong solvents and beryllium-containing compounds can be extremely hazardous.

Chemical protection gear, a mask, and gloves, should be always be worn during this procedure. Wear a dust mask when loading and unloading samples for MB growth. However, it should be mentioned that the total amount of beryllium evaporated in the MB system is a few tenths of micrograms, with most of it buried in the chamber walls in the form of beryllium-poor zinc beryllium oxide.