Summary
Molecular beam epitaxy is used to grow N-polar InAlN-barrier high-electron-mobility transistors (HEMTs). Control of the wafer preparation, layer growth conditions and epitaxial structure results in smooth, compositionally homogeneous InAlN layers and HEMTs with mobility as high as 1,750 cm2/V∙sec.
Transcript
The overall goal of this procedure is to observe the effect of various growth parameters on the structural and electrical quality of nitride semiconductors and their eventual impact on the performance of electronic devices.This method can help answer key questions in the fields of material science and electrical engineering, such as the impact of materials'defects on the speed and power of wide band gap transistors.The main advantage of this technique is that it can produce semiconductor materials with very abrupt interfaces and low impurity levels, which are very important for high performance devices such as transistors.Visual demonstration of this method is critical, as distinguishing between too much and not enough gallium coverage with weed is qualitative and difficult to describe.First, ensure that the cryo panels are cooled, and that the growth chamber is at base pressure.Ramp up the effusion cells to their beam flex measurement temperatures at the proper rates, and wait one hour for the cells to stabilize.For each cell, open the shutter for 30 seconds to collect a beam flex ion gage measurement, then close the shutter for one minute.Perform this process three times.Average the last two measurements for each cell.Use previous calibrations to set the cell temperatures to achieve the desired flexes.Close the load lock isolation gate valve and vent the load lock chamber with nitrogen gas.Load the epitaxy-ready and polar gallium nitride substrate on the holder, and place the sample cassette in the load lock.Close the load lock and shut off the nitrogen gas.Turn on the load lock roughing pump.Open the roughing pump valve and the manifold valve.Once the manifold pressure drops below 0.1 Torr, close the manifold and roughing pump valves and turn off the pump.Open the load lock turbo pump isolation valve, and pump down the load lock for 30 to 60 minutes.Open the load lock into the preparation chamber and use the wobble stick to transfer the substrate in its holder to the trolley.Using the trolley, move the substrate to the outgassing station.Ramp the outgassing station heater temperature to 700 degrees celsius over 10 minutes.Outgas the substrate for 30 minutes and then begin cooling.Once the temperature is below 250 degrees celsius, transfer the substrate to the trolley.Lower the substrate manipulator to the load position.Open the gate valve and transfer the substrate in its holder to the manipulator.Raise the substrate manipulator to the growth position and remove the trolley.Close the gate valve, and then open the nitrogen bottle valve, the regulator valve, and the isolation needle valve.Use the mass flow controller to bring the chamber to the optimal pressure for plasma ignition.Ensure that the active nitrogen flux and diffusion cell shutters are closed.Turn on the plasma RF power supply and the auto matching network controller.Increase the power until the plasma ignites.Set the power and nitrogen gas flow to appropriate process conditions for the system.Ramp the substrate heater to 10 degrees celsius above the desired gallium nitride growth temperature at a rate no higher than one degrees celsius per second.Turn on the read system to monitor the wafer surface.Turn on substrate rotation and prepare the read acquisition software.Set the gallium flux to ensure gallium deposition at the substrate temperature.Open the substrate and gallium shutters for one minute.The read intensity should initially decrease and then plateau as gallium accumulates.Close the gallium shutter for two minutes, during which time the read intensity should increase and the plateau as gallium desorbs.Repeat gallium depositon and absorption twice more, and then ramp the substrate heater up to the gallium nitride growth temperature.First, open the active nitrogen shutter for one minute to initiate buffer growth by nitridation.Then, open the aluminum shutter to grow a one to three nanometer nitrogen-rich aluminum nitride layer.Close the aluminum nitride and active nitrogen shutters, and immediately open the gallium shutter for ten seconds, during which time the read intensity should rapidly decrease.Then, open the active nitrogen shutter and grow gallium nitride for five minutes.Close the gallium and active nitrogen shutters for one minute, and monitor the read intensity during the growth interruption to assess the gallium flux.If the intensity immediately increases, the gallium flux is too low.Decrease the substrate temperature or increase the gallium effusion cell temperature.If the intensity increases after at least thirty seconds or does not plateau during the growth interruption, increase the substrate temperature or decrease the gallium effusion cell temperature.If the gallium flux was adjusted, grow another layer of gallium nitride and perform another one minute growth interruption.Once the read intensity increases between 15 to 30 seconds from the start time and plateaus within a minute, continue with the procedure.Continue growing gallium nitride in five minute increments with one minute interrupts until it reaches the desired thickness.Wait one minute after gallium nitride growth finishes to ensure all gallium has evaporated and then quickly ramp the substrate heater down to the indium aluminum nitride growth temperature.Allow the temperature to stabilize for two minutes.Open the indium, aluminum and active nitrogen shutters.The read intensity should decrease and plateau within three minutes, and the pattern should remain streaky.Once the barrier has grown to the desired thickness, close the indium, aluminum and active nitrogen shutters.To grow the gallium nitride interlayer, open the gallium shutter for five seconds and then open the active nitrogen shutter.Once the gallium nitride interlayer reaches the desired thickness, close the active nitrogen and gallium shutters.Start ramping the substrate heater to the gallium nitride channel growth temperature.After 30 seconds, close the gallium shutter.Wait thirty seconds or for the read intensity to increase, and then open the shutter.Continue cycling the gallium shutter until the substrate heater reaches the gallium nitride growth temperature, then open the gallium shutter.After five seconds, open the active nitrogen and aluminum shutters, and grow the aluminum nitride interlayer.After the aluminum nitride interlayer growth time has finished, close the aluminum shutter and grow the gallium nitride channel.Close the gallium, active nitrogen and main shutters.Ramp the substrate temperature down to 200 degrees celsius, turn off the active nitrogen plasma and shut off the nitrogen gas flow.Ramp the cells down to standby temperature.Once the substrate temperature is below 250 degrees celsius and the chamber pressure drops below 8x10
Tags
Plasma-assisted Molecular Beam Epitaxy
N-polar InAlN-barrier
High-electron-mobility Transistors
Growth Parameters
Structural Quality
Electrical Quality
Nitride Semiconductors
Performance Of Electronic Devices
Materials Defects
Wide Band Gap Transistors
Semiconductor Materials
Abrupt Interfaces
Low Impurity Levels
Transistors
Gallium Coverage
Cryo Panels
Growth Chamber
Effusion Cells
Beam Flex Measurement Temperatures
Shutter
Beam Flex Ion Gage Measurement
Cell Temperatures
Load Lock Isolation Gate Valve
