Name: plasma-assisted molecular beam epitaxy of n-polar inaln-barrier high-electron-mobility transistors
Uploaded: 2016-11-24T13:00:00
Description: Watch this Scientific Journal Video about Plasma-assisted Molecular Beam Epitaxy of N-polar InAlN-barrier High-electron-mobility Transistors at JoVE.com

The authors thank Mr. Neil Green for assistance with sample preparation. This work was supported by the Office of Naval Research under funding from Dr. P. Maki. MTH was supported by a National Research Council Postdoctoral Fellowship.

1. Effusion Cell Ramp and Flux Calibration
<ol>
	<li>Confirm liquid N2 is flowing to the cryo-panels and that the growth chamber has reached base pressure.</li>
	<li>Ramp up the effusion cells to their beam flux measurement (BFM) temperature at a ramp rate of 1 &#176;C/sec for Ga and In cells, and 10 &#176;C/min for Al. Wait 1 hr for cells to thermally stabilize.</li>
	<li>Open the shutter of each cell for 30-60 sec, and then close the shutter for 1-2 min. Repeat three times for each cell. Discard the first ...

<ol>
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S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+beam+epitaxy+of+InAlN&#8725;GaN+heterostructures+for+high+electron+mobility+transistors.">Molecular beam epitaxy of InAlN&#8725;GaN heterostructures for high electron mobility transistors.</a> J. Vac. Sci. Technol., B. 23 (3), 1204-1208 (2005).</li><li>Kaun, S. W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=GaN-based+high-electron-mobility+transistor+structures+with+homogeneous+lattice-matched+InAlN+barriers+grown+by+plasma-assisted+molecular+beam+epitaxy.">GaN-based high-electron-mobility transistor structures with homogeneous lattice-matched InAlN barriers grown by plasma-assisted molecular beam epitaxy.</a> Semicond. Sci. Technol. 29 (4), 045011 (2014).</li><li>Hoke, W. E., Torabi, A., Mosca, J. J., Kennedy, T. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermodynamic+analysis+of+cation+incorporation+during+molecular+beam+epitaxy+of+nitride+films+using+metal-rich+growth+conditions.">Thermodynamic analysis of cation incorporation during molecular beam epitaxy of nitride films using metal-rich growth conditions.</a> J. Vac. Sci. Technol., B. 25 (3), 978-982 (2007).</li><li>Koblm&#252;ller, G., Reurings, F., Tuomisto, F., Speck, J. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Influence+of+Ga/N+ratio+on+morphology,+vacancies,+and+electrical+transport+in+GaN+grown+by+molecular+beam+epitaxy+at+high+temperature.">Influence of Ga/N ratio on morphology, vacancies, and electrical transport in GaN grown by molecular beam epitaxy at high temperature.</a> Appl. Phys. Lett. 97 (19), 191915 (2010).</li><li>Zhou, L., Smith, D. J., McCartney, M. R., Katzer, D. S., Storm, D. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Observation+of+vertical+honeycomb+structure+in+InAlN&#8725;GaN+heterostructures+due+to+lateral+phase+separation.">Observation of vertical honeycomb structure in InAlN&#8725;GaN heterostructures due to lateral phase separation.</a> Appl. Phys. Lett. 90 (8), 081917 (2007).</li><li>Ahmadi, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Elimination+of+columnar+microstructure+in+N-face+InAlN,+lattice-matched+to+GaN,+grown+by+plasma-assisted+molecular+beam+epitaxy+in+the+N-rich+regime.">Elimination of columnar microstructure in N-face InAlN, lattice-matched to GaN, grown by plasma-assisted molecular beam epitaxy in the N-rich regime.</a> Appl. Phys. Lett. 104 (7), 072107 (2014).</li><li>Hardy, M. T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Morphological+and+microstructural+stability+of+N-polar+InAlN+thin+films+grown+on+free-standing+GaN+substrates+by+molecular+beam+epitaxy.">Morphological and microstructural stability of N-polar InAlN thin films grown on free-standing GaN substrates by molecular beam epitaxy.</a> J. Vac. Sci. Technol., A. 34 (2), 021512 (2016).</li><li>Hardy, M. T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Indium+incorporation+dynamics+in+N-polar+InAlN+thin+films+grown+by+plasma-assisted+molecular+beam+epitaxy+on+freestanding+GaN+substrates.">Indium incorporation dynamics in N-polar InAlN thin films grown by plasma-assisted molecular beam epitaxy on freestanding GaN substrates.</a> J. Cryst. Growth. 245, (2015).</li><li>Leszczynski, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Indium+incorporation+into+InGaN+and+InAlN+layers+grown+by+metalorganic+vapor+phase+epitaxy.">Indium incorporation into InGaN and InAlN layers grown by metalorganic vapor phase epitaxy.</a> J. Cryst. Growth. 318 (1), 496-499 (2011).</li><li>Storm, D. F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultrathin-barrier+AlN/GaN+heterostructures+grown+by+rf+plasma-assisted+molecular+beam+epitaxy+on+freestanding+GaN+substrates.">Ultrathin-barrier AlN/GaN heterostructures grown by rf plasma-assisted molecular beam epitaxy on freestanding GaN substrates.</a> J. Cryst. Growth. 380, 14-17 (2013).</li><li>Storm, D. F., Katzer, D. S., Meyer, D. J., Binari, S. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Oxygen+incorporation+in+homoepitaxial+N-polar+GaN+grown+by+radio+frequency-plasma+assisted+molecular+beam+epitaxy:+Mitigation+and+modeling.">Oxygen incorporation in homoepitaxial N-polar GaN grown by radio frequency-plasma assisted molecular beam epitaxy: Mitigation and modeling.</a> J. Appl. Phys. 112 (1), 013507 (2012).</li><li>Storm, D. F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+interfacial+oxygen+on+the+microstructure+of+MBE-grown+homoepitaxial+N-polar.">Effect of interfacial oxygen on the microstructure of MBE-grown homoepitaxial N-polar.</a> J. Cryst. Growth. 409, 14 (2014).</li><li>Meyer, D. 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Growth of a high quality GaN buffer layer is critical to achieving high electron mobility in any III-nitride HEMT. In the case of an N-polar InAlN HEMT, the buffer layer growth is complicated by the requirement that all Ga be removed from the surface prior to InAlN growth. There are a variety of techniques to accomplish this in addition to the procedure described here, such as metal-modulated epitaxy,27 using growth conditions at the edge of the intermediate Ga coverage and Ga droplet accumulation regime,...

Molecular beam epitaxy (MBE) is a versatile epitaxial thin film growth technique that employs an ultra-high vacuum environment with base pressures as low as 10-11 Torr to ensure low impurity incorporation in the grown film. The composition and growth rate of the epitaxially grown layers are determined by controlling the temperature of each effusion cell, and thus the evaporated flux of the various source materials. In the case of III-nitride epitaxy, the group III-elements (In, Al, Ga) are typically provided by effusion cells while the active nitrogen (N*) flux is provided by either an N2 plasma1,2 (RF plasma-assisted MBE: PAMBE or RFMBE) or ammonia (NH3-MBE).3,4 MBE growth is characterized by lower growth temperatures and sharper interfacial abruptness than other epitaxial growth techniques, such as metalorganic chemical vapor deposition.5 A schematic is shown in Figure 1.
<img alt="Figure 1" src="/files/ftp_upload/54775/54775fig1.jpg" /> 
Figure 1: MBE system schematic. Schematic showing the load lock, transfer system, outgassing station and growth chamber. <a href="http://cloudflare.jove.com/files/ftp_upload/54775/54775fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
III-nitrides can be grown on substrates having a variety of crystal orientations. The most commonly used orientation is the Ga-polar c-plane, which allows the formation of a two-dimensional electron gas without doping by utilizing the difference in polarization between the barrier layer, typically AlGaN, and GaN channel. Various non-polar and semi-polar orientations of GaN have received significant attention for optoelectronics due to reduced polarization effects in the quantum wells,6,7 which also makes these orientations less desirable for HEMT applications. N-polar oriented devices are attractive for next-generation high-frequency HEMT operation due to several intrinsic advantages over conventional Ga-polar devices.8 The barrier layer in N-polar devices is grown beneath the GaN channel as shown in Figure 2, resulting in a natural back barrier that aids electrostatic control of the channel and reduces short channel effects, while allowing easier current access to the GaN channel and reducing contact resistance. The barrier can also be controlled separately from the channel, so that as the channel thickness is scaled down for high-frequency devices the barrier design can be modified to compensate for channel charge lost to Fermi level pinning effects.
<img alt="Figure 2" src="/files/ftp_upload/54775/54775fig2.jpg" /> 
Figure 2: Epitaxial layer schematic. Layer structure of (a) an N-polar HEMT and (b) a Ga-polar HEMT for comparison. <a href="http://cloudflare.jove.com/files/ftp_upload/54775/54775fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
HEMTs used in high-speed, high-power amplifiers are normally grown on SiC substrates to take advantage of the high thermal conductivity of SiC. Low threading dislocation density freestanding GaN substrates can be employed to improve the electron mobility,9 thus improving the high-frequency performance. Following the growth of an AlN nucleation layer, a thick GaN buffer is grown to spatially separate the impurities at the regrowth interface from the HEMT channel and improve electrical isolation. Unlike other III-V materials, GaN grown by PAMBE typically needs growth conditions with a group-III/V ratio greater than 1, i.e., metal-rich conditions,10,11 in order to achieve a smooth surface morphology. InxAl1-xN is an alternative barrier material for III-nitride HEMTs, and has received significant attention recently because it can be grown lattice matched to GaN for x &#8776; 0.18 and can generate over twice the channel charge relative to AlGaN barriers due its high spontaneous polarization.12-15 Unlike AlGaN barriers, Ga will incorporate preferentially to In in InAlN layers,16 thus care must be taken to ensure the surface is free of excess Ga after the Ga-rich GaN buffer layer growth and prior to InAlN growth.
Control of Ga on the surface can be accomplished by suppling a Ga flux slightly less than the flux required for Ga-droplet formation. However, this growth window is small, and insufficient Ga surface coverage will cause the surface morphology to degrade into plateau/trench morphology while excess Ga flux will result in Ga accumulation and macroscopic droplet formation.17 Reflection high-energy electron diffraction (RHEED) intensity can be used to monitor Ga accumulation and desorption. Ga surface coverage is indicated by a reduction in RHEED intensity, and any lag between closing the Ga (and N*) shutters and the initial increase in RHEED intensity indicates accumulation of Ga, as shown in Figure 3.
<img alt="Figure 3" src="/files/ftp_upload/54775/54775fig3.jpg" /> 
Figure 3: Monitoring Ga coverage with RHEED intensity. RHEED intensity signal measured from RHEED pattern acquired under rotation using triggered acquisition. Insufficient Ga flux is indicated by an immediate increase in intensity after closing the shutters (not shown). Saturated/ideal Ga coverage is indicated by a delay between shutter closure and abrupt RHEED brightening and excess Ga coverage in seen as both a delay in initial RHEED brightening as well as a more gradual intensity increase resulting in full intensity recovery taking longer than 60 s. <a href="http://cloudflare.jove.com/files/ftp_upload/54775/54775fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Achieving high quality InAlN by PAMBE is complicated by the presence of lateral composition fluctuations, resulting in a &#34;honeycomb&#34; microstructure consisting of Al-rich domains surrounded by In-rich boundaries.18 Elimination of this microstructure is achieved by using a substrate temperature about 50 &#176;C above the onset of In desorption,15,19,20 or approximately 630 &#176;C for N-polar InAlN. In this high temperature growth regime, the InxAl1-xN composition is a strong function of substrate temperature, with higher temperatures resulting in lower In incorporation. The In flux can be increased to compensate for In lost to evaporation, although in practice the maximum In flux is limited by a reduction in incorporation efficiency with increasing In flux.21 In addition to reducing the substrate temperature or increasing the In flux, increasing the growth rate can also increase the In composition due to the &#34;In burying effect&#34;, where incoming Al atoms trap In and prevent it from evaporating.21,22 Higher growth rates can be achieved by increasing the In and Al flux proportionally. To keep the growth conditions N-rich, the N* would need to be increased as well, which can be achieved by increasing the RF plasma power, increasing the N2 flow rate, improving the plasma chamber design, or increasing the aperture plate hole density.
Additional epitaxial layers in InAlN-based HEMTs include GaN and AlN interlayers (ILs) and a GaN channel. An AlN IL inserted between the barrier and channel can increase mobility &#181; as well as channel sheet charge density ns. The increase in mobility is attributed to reducing electron wave function overlap with the InAlN barrier and subsequent alloy scattering.9 To ensure high-quality growth of the AlN IL, an excess of Ga flux is supplied during growth to act as a surfactant. A GaN IL can be used between the AlN IL and barrier to further improve the mobility while reducing channel charge. The GaN channel can be grown at the same temperature as the InAlN barrier, allowing continuous growth from the barrier though the ILs and channel. Improved mobility has been obtained by interrupting growth after the AlN IL and increasing the growth temperature before growing the GaN channel. In this case a protective Ga surface coverage has to be maintained during the interrupt to prevent mobility degradation.
The following protocol applies specifically to InAlN-barrier HEMTs grown on N-polar GaN substrates. It can be directly extended to growth on C-polar 4H- or 6H-SiC substrates by including a 50 nm thick N-rich AlN layer.

<table border="0" cellpadding="0" cellspacing="0" width="749">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:309px;">Freestanding N-polar GaN wafer</td>
			<td style="width:145px;">Kyma</td>
			<td style="width:128px;"></td>
			<td style="width:167px;">10 mm x 10 mm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:309px;">C-polar SiC wafer</td>
			<td style="width:145px;">Cree</td>
			<td style="width:128px;">W4TRE0R-L600</td>
			<td>3 inch diameter</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:309px;">Microelectronics grade acetone</td>
			<td style="width:145px;">Fischer Scientific</td>
			<td style="width:128px;">A18-4</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:309px;">Microelectronics grade isoproponal</td>
			<td style="width:145px;">J.T. Baker</td>
			<td style="width:128px;">9079-05/JT9079-5</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:309px;">Al source material (6N5 pure)</td>
			<td style="width:145px;">UMC</td>
			<td>ALR62060I</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:309px;">Ga source material (7N pure)</td>
			<td style="width:145px;">UMC</td>
			<td>GA701</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:309px;">In source material (7N pure)</td>
			<td style="width:145px;">UMC</td>
			<td>IN750</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:309px;">ULSI N2 source gas (6N pure)</td>
			<td style="width:145px;">Matheson Tri-gas</td>
			<td>G2659906D</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:309px;">PRO-75 MBE system</td>
			<td style="width:145px;">OmicronScientia</td>
			<td style="width:128px;"></td>
			<td></td>
		</tr>
	</tbody>
</table>

plasma-assisted molecular beam epitaxy of n-polar inaln-barrier high-electron-mobility transistors

Plasma-assisted molecular beam epitaxy is well suited for the epitaxial growth of III-nitride thin films and heterostructures with smooth, abrupt interfaces required for high-quality high-electron-mobility transistors (HEMTs). A procedure is presented for the growth of N-polar InAlN HEMTs, including wafer preparation and growth of buffer layers, the InAlN barrier layer, AlN and GaN interlayers and the GaN channel. Critical issues at each step of the process are identified, such as avoiding Ga accumulation in the GaN buffer, the role of temperature on InAlN compositional homogeneity, and the use of Ga flux during the AlN interlayer and the interrupt prior to GaN channel growth. Compositionally homogeneous N-polar InAlN thin films are demonstrated with surface root-mean-squared roughness as low as 0.19 nm and InAlN-based HEMT structures are reported having mobility as high as 1,750 cm2/V&#8729;sec for devices with a sheet charge density of 1.7 x 1013 cm-2.

X-ray diffraction (XRD) scans of InAlN thin films shown grown on N-polar GaN substrates in Figure 4(a) are single peaked both for 50 and 200-nm-thick films. The XRD scan of the 50 nm thick InAlN film exhibits Pendell&#246;sung fringes up to 15th order, indicating very high interfacial quality. The asymmetric reciprocal space map in Figure 4(b) shows that the 200 nm thick InAlN layer has the same q&#8214;, and thus the same in-plane l...

Watch this Scientific Journal Video about Plasma-assisted Molecular Beam Epitaxy of N-polar InAlN-barrier High-electron-mobility Transistors at JoVE.com

Plasma-assisted Molecular Beam Epitaxy of N-polar InAlN-barrier High-electron-mobility Transistors

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&#8729;sec.

Plasma-assisted molecular beam epitaxy is well suited for the epitaxial growth of III-nitride thin films and heterostructures with smooth, abrupt ...

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

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Molecular Beam Mass Spectrometry With Tunable Vacuum Ultraviolet (VUV) Synchrotron Radiation

Tunable soft ionization coupled to mass spectroscopy is a powerful method to investigate isolated molecules, complexes and clusters and their spectroscopy and dynamics1-4. Fundamental studies of photoionization processes of biomolecules provide information about the electronic structure of these systems. Furthermore determinations of ionization energies and other properties of biomolecules in the gas phase are not trivial, and these experiments provide a platform to generate these data. We have developed a thermal vaporization technique coupled with supersonic molecular beams that provides a gentle way to transport these species into the gas phase. Judicious combination of source gas and temperature allows for formation of dimers and higher clusters of the DNA bases. The focus of this particular work is on the effects of non-covalent interactions, i.e., hydrogen bonding, stacking, and electrostatic interactions, on the ionization energies and proton transfer of individual biomolecules, their complexes and upon micro-hydration by water1, 5-9.
We have performed experimental and theoretical characterization of the photoionization dynamics of gas-phase uracil and 1,3-dimethyluracil dimers using molecular beams coupled with synchrotron radiation at the Chemical Dynamics Beamline10 located at the Advanced Light Source and the experimental details are visualized here. This allowed us to observe the proton transfer in 1,3-dimethyluracil dimers, a system with pi stacking geometry and with no hydrogen bonds1. Molecular beams provide a very convenient and efficient way to isolate the sample of interest from environmental perturbations which in return allows accurate comparison with electronic structure calculations11, 12. By tuning the photon energy from the synchrotron, a photoionization efficiency (PIE) curve can be plotted which informs us about the cationic electronic states. These values can then be compared to theoretical models and calculations and in turn, explain in detail the electronic structure and dynamics of the investigated species 1, 3.

Molecular Beam Mass Spectrometry With Tunable Vacuum Ultraviolet VUV Synchrotron Radiation

A molecular beam coupled to tunable vacuum ultraviolet photoionization mass spectrometer at a synchrotron provides a convenient tool to explore the electronic structure of isolated gas phase molecules and clusters. Proton transfer mechanisms in DNA base dimers were elucidated with this technique.

Tunable soft ionization coupled to mass spectroscopy is a powerful method to investigate isolated molecules, complexes and clusters and their spectroscopy ...

Spatial Separation of Molecular Conformers and Clusters

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold molecular beams. However, these beams often contain multiple conformers and clusters, even at low rotational temperatures. We present an experimental methodology that allows the spatial separation of these constituent parts of a molecular beam expansion. Using an electric deflector the beam is separated by its mass-to-dipole moment ratio, analogous to a bender or an electric sector mass spectrometer spatially dispersing charged molecules on the basis of their mass-to-charge ratio. This deflector exploits the Stark effect in an inhomogeneous electric field and allows the separation of individual species of polar neutral molecules and clusters. It furthermore allows the selection of the coldest part of a molecular beam, as low-energy rotational quantum states generally experience the largest deflection. Different structural isomers (conformers) of a species can be separated due to the different arrangement of functional groups, which leads to distinct dipole moments. These are exploited by the electrostatic deflector for the production of a conformationally pure sample from a molecular beam. Similarly, specific cluster stoichiometries can be selected, as the mass and dipole moment of a given cluster depends on the degree of solvation around the parent molecule. This allows experiments on specific cluster sizes and structures, enabling the systematic study of solvation of neutral molecules.

We present a technique that allows the spatial separation of different conformers or clusters present in a molecular beam. An electrostatic deflector is used to separate species by their mass-to-dipole moment ratio, leading to the production of gas-phase ensembles of a single conformer or cluster stoichiometry.

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold ...

Measurement of X-ray Beam Coherence along Multiple Directions Using 2-D Checkerboard Phase Grating

A procedure for a technique to measure the transverse coherence of synchrotron radiation X-ray sources using a single phase grating interferometer is reported. The measurements were demonstrated at the 1-BM bending magnet beamline of the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). By using a 2-D checkerboard &#960;/2 phase-shift grating, transverse coherence lengths were obtained along the vertical and horizontal directions as well as along the 45&#176; and 135&#176; directions to the horizontal direction. Following the technical details specified in this paper, interferograms were measured at different positions downstream of the phase grating along the beam propagation direction. Visibility values of each interferogram were extracted from analyzing harmonic peaks in its Fourier Transformed image. Consequently, the coherence length along each direction can be extracted from the evolution of visibility as a function of the grating-to-detector distance. The simultaneous measurement of coherence lengths in four directions helped identify the elliptical shape of the coherence area of the Gaussian-shaped X-ray source. The reported technique for multiple-direction coherence characterization is important for selecting the appropriate sample size and orientation as well as for correcting the partial coherence effects in coherence scattering experiments. This technique can also be applied for assessing coherence preserving capabilities of X-ray optics.

The measurement protocol and data analysis procedure are given for obtaining transverse coherence of a synchrotron radiation X-ray source along four directions simultaneously using a single 2-D checkerboard phase grating. This simple technique can be applied for complete transverse coherence characterization of X-ray sources and X-ray optics.

A procedure for a technique to measure the transverse coherence of synchrotron radiation X-ray sources using a single phase grating interferometer is ...

Speciation and Bioavailability Measurements of Environmental Plutonium Using Diffusion in Thin Films

The biological uptake of plutonium (Pu) in aquatic ecosystems is of particular concern since it is an alpha-particle emitter with long half-life which can potentially contribute to the exposure of biota and humans. The diffusive gradients in thin films technique is introduced here for in-situ measurements of Pu bioavailability and speciation. A diffusion cell constructed for laboratory experiments with Pu and the newly developed protocol make it possible to simulate the environmental behavior of Pu in model solutions of various chemical compositions. Adjustment of the oxidation states to Pu(IV) and Pu(V) described in this protocol is essential in order to investigate the complex redox chemistry of plutonium in the environment. The calibration of this technique and the results obtained in the laboratory experiments enable to develop a specific DGT device for in-situ Pu measurements in freshwaters. Accelerator-based mass-spectrometry measurements of Pu accumulated by DGTs in a karst spring allowed determining the bioavailability of Pu in a mineral freshwater environment. Application of this protocol for Pu measurements using DGT devices has a large potential to improve our understanding of the speciation and the biological transfer of Pu in aquatic ecosystems.

The technique of diffusive gradients in thin films (DGT) is proposed for speciation studies of plutonium. This protocol describes diffusion experiments probing the behavior of Pu(IV) and Pu(V) in presence of organic matter. DGTs deployed in a karstic spring allow assessment of the bioavailability of Pu.

The biological uptake of plutonium (Pu) in aquatic ecosystems is of particular concern since it is an alpha-particle emitter with long half-life which can ...

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal&#8211;semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 &#937;&#8722;1 cm&#8722;1, with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.

We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe2) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe2-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described.

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor ...

Effect of Bending on the Electrical Characteristics of Flexible Organic Single Crystal-based Field-effect Transistors

The charge transport in an organic semiconductor depends highly on the molecular packing in the crystal, which influences the electronic coupling immensely. However, in soft electronics, in which organic semiconductors play a critical role, the devices will be bent or folded repeatedly. The effect of bending on the crystal packing and thus the charge transport is crucial to the performance of the device. In this manuscript, we describe the protocol to bend a single crystal of 5,7,12,16-tetrachloro-6,13-diazapentacene (TCDAP) in the field-effect transistor configuration and to obtain reproducible I-V characteristics upon bending the crystal. The results show that bending a field-effect transistor prepared on a flexible substrate results in nearly reversible yet opposite trends in charge mobility, depending on the bending direction. The mobility increases when the device is bent toward the top gate/dielectric layer (upward, compressive state) and decreases when bent toward the crystal/substrate side (downward, tensile state). The effect of bending curvature was also observed, with greater mobility change resulting from higher bending curvature. It is suggested that the intermolecular &#960;-&#960; distance changes upon bending, thereby influencing the electronic coupling and the subsequent carrier transport ability.

This manuscript describes the bending process of an organic single crystal-based field-effect transistor to maintain a functioning device for electronic property measurement. The results suggest that bending causes changes in the molecular spacing in the crystal and thus in the charge hopping rate, which is important in flexible electronics.

The charge transport in an organic semiconductor depends highly on the molecular packing in the crystal, which influences the electronic coupling ...

Focused Ion Beam Fabrication of LiPON-based Solid-state Lithium-ion Nanobatteries for In Situ Testing

Solid-state electrolytes are a promising replacement for current organic liquid electrolytes, enabling higher energy densities and improved safety of lithium-ion (Li-ion) batteries. However, a number of setbacks prevent their integration into commercial devices. The main limiting factor is due to nanoscale phenomena occurring at the electrode/electrolyte interfaces, ultimately leading to degradation of battery operation. These key problems are highly challenging to observe and characterize as these batteries contain multiple buried interfaces. One approach for direct observation of interfacial phenomena in thin film batteries is through the fabrication of electrochemically active nanobatteries by a focused ion beam (FIB). As such, a reliable technique to fabricate nanobatteries was developed and demonstrated in recent work. Herein, a detailed protocol with a step-by-step process is presented to enable the reproduction of this nanobattery fabrication process. In particular, this technique was applied to a thin film battery consisting of LiCoO2/LiPON/a-Si, and has further been previously demonstrated by in situ cycling within a transmission electron microscope.

Focused Ion Beam Fabrication of LiPON-based Solid-state Lithium-ion Nanobatteries for In Situ Testing

A protocol for the fabrication of electrochemically active LiPON-based solid-state lithium-ion nanobatteries using a focused ion beam is presented.

Solid-state electrolytes are a promising replacement for current organic liquid electrolytes, enabling higher energy densities and improved safety of ...

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

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.

Attainment of high-quality Schottky contacts is imperative for achieving efficient gate modulation in heterostructure field effect transistors (HFETs). We present the fabrication methodology and characteristics of Schottky diodes on Zn-polar BeMgZnO/ZnO heterostructures with high-density two dimensional electron gas (2DEG), grown by plasma-assisted molecular beam epitaxy on GaN templates.

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device ...

Single-Digit Nanometer Electron-Beam Lithography with an Aberration-Corrected Scanning Transmission Electron Microscope

We demonstrate extension of electron-beam lithography using conventional resists and pattern transfer processes to single-digit nanometer dimensions by employing an aberration-corrected scanning transmission electron microscope as the exposure tool. Here, we present results of single-digit nanometer patterning of two widely used electron-beam resists: poly (methyl methacrylate) and hydrogen silsesquioxane. The method achieves sub-5 nanometer features in poly (methyl methacrylate) and sub-10 nanometer resolution in hydrogen silsesquioxane. High-fidelity transfer of these patterns into target materials of choice can be performed using metal lift-off, plasma etch, and resist infiltration with organometallics.

We use an aberration-corrected scanning transmission electron microscope to define single-digit nanometer patterns in two widely-used electron-beam resists: poly (methyl methacrylate) and hydrogen silsesquioxane. Resist patterns can be replicated in target materials of choice with single-digit nanometer fidelity using liftoff, plasma etching, and resist infiltration by organometallics.

We demonstrate extension of electron-beam lithography using conventional resists and pattern transfer processes to single-digit nanometer dimensions by ...

Preparing an Isotopically Pure 229Th Ion Beam for Studies of 229mTh

A methodology is described to generate an isotopically pure 229Th ion beam in the 2+ and 3+ charge states. This ion beam enables one to investigate the low-lying isomeric first excited state of 229Th at an excitation energy of about 7.8(5) eV and a radiative lifetime of up to 104 seconds. The presented method allowed for a first direct identification of the decay of the thorium isomer, laying the foundations to study its decay properties as prerequisite for an optical control of this nuclear transition. High energy 229Th ions are produced in the &#945;&#160;decay of a radioactive 233U source. The ions are thermalized in a buffer-gas stopping cell, extracted and subsequently an ion beam is formed. This ion beam is mass purified by a quadrupole-mass separator to generate a pure ion beam. In order to detect the isomeric decay, the ions are collected on the surface of a micro-channel plate detector, where electrons, as emitted in the internal conversion decay of the isomeric state, are observed.

Preparing an Isotopically Pure 229Th Ion Beam for Studies of 229mTh

We present a protocol for the generation of an isotopically purified low-energy 229Th ion beam from a 233U source. This ion beam is used for the direct detection of the 229mTh ground-state decay via the internal-conversion decay channel. We also measure the internal conversion lifetime of 229mTh as well.

A methodology is described to generate an isotopically pure 229Th ion beam in the 2+ and 3+ charge states. This ion beam enables one to investigate the ...