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 beam flux ion gauge measurement and average the second two. Adjust the cell temperature to achieve the desired flux according to previous flux/temperature calibrations.</li>
</ol>
2. Substrate Preparation and Loading
<ol>
	<li>Ex situ substrate cleaning
	<ol>
		<li>Load epi-ready N-polar GaN substrates directly, without any ex situ cleaning. If the wafer has been exposed to air for more than a few hours, rinse under acetone (30 sec), isopropanol (30 sec) and de-ionized (DI) water (60 sec). Rinsing is preferred to immersion when possible as it tends to leave less particles on the wafer surface. 
		NOTE: For Ga-polar GaN substrates a more rigorous surface clean is recommended.23</li>
	</ol>
	</li>
	<li>Load wafer
	<ol>
		<li>Close the load-lock isolation gate valve, and vent with N2.</li>
		<li>Load the wafer on the holder and return the cassette to the load lock. Turn on the load lock roughing pump and open the roughing pump valve and manifold valve.</li>
		<li>When the manifold pressure has dropped below 0.1 Torr, close the manifold and roughing pump valves. Turn off the roughing pump and open the load-lock turbo-pump isolation valve.</li>
		<li>Allow the load lock to pump down for 30-60 min. Ideally, use a load lock pressure of 10-6-10-7 Torr before transferring to the preparation chamber.</li>
		<li>Open the load lock into the preparation chamber, and transfer the wafer to the trolley using wobble stick. Then use the trolley to transfer the wafer to the outgassing station in the preparation chamber. Move the trolley manually along a rail using a rotary feedthrough on the preparation chamber.</li>
	</ol>
	</li>
	<li>Outgas wafer
	<ol>
		<li>Ramp the outgassing station heater temperature to 700 &#176;C over 10 min.</li>
		<li>After 30 min, ramp the temperature back down to 100 &#176;C. When the temperature is &#8804; 250 &#176;C, transfer the wafer back to the trolley using the wobble stick at the outgassing station.</li>
	</ol>
	</li>
	<li>Load wafer into the growth chamber
	<ol>
		<li>Lower the substrate manipulator to the load position, open the preparation/growth chamber gate valve and transfer the wafer holder to the manipulator.</li>
		<li>Raise the manipulator to growth position, near the substrate heater.</li>
		<li>Remove the trolley and close the gate valve.</li>
		<li>Open the N2 bottle valve, regulator valve and isolation needle valve. Set the mass flow controller (MFC) to 1.5 sccm (or as needed to get a chamber pressure of 3-4 x 10-5 Torr). The optimal pressure for plasma ignition is strongly system dependent. Note that the N2 must be ultra-high purity (preferably better than 6N) and an additional filter is used in line to further reduce impurities.</li>
		<li>With the N* and main shutters closed, turn on the plasma RF power supply and auto-matching network controller. Increase the RF power until the plasma ignites.</li>
		<li>Set the RF power and N2 flow to the desired process conditions, in this case 350 W and 2.0 sccm. The N* flux resulting from given plasma conditions is system dependent, but here, these conditions yield a GaN growth rate of 5.0 nm/min, or a N* flux of 1.8 nm-2sec-1. Monitor the plasma stability with a spectrometer attached to the rear window of the plasma chamber.</li>
	</ol>
	</li>
	<li>In situ surface preparation: Ga deposition and desorption
	<ol>
		<li>Ramp up the substrate heater to 10 &#176;C above the desired GaN growth temperature with a ramp rate of &#8804;1 &#176;C/sec. In this case, use an estimated wafer temperature of 730 &#176;C.</li>
		<li>Turn on the RHEED system to watch for degradation of the wafer surface and monitor Ga coverage. Manually turn on the substrate rotation. Set up the triggered RHEED acquisition software to collect a RHEED pattern once per substrate rotation to provide a static image while the substrate is rotating during growth.</li>
		<li>Open the substrate shutter and Ga shutter for 1 min. Ensure the RHEED intensity decreases then reaches a plateau as Ga accumulates. Ga flux should be similar to that used for GaN growth. Here, the Ga flux is approximately, 3.7 nm-2sec-1.</li>
		<li>Close the shutter for 2 min and ensure the RHEED intensity increases and reaches a plateau before the end of the 2 min, indicating Ga desorption.</li>
		<li>Repeat Steps 2.5.2-2.5.4 three times,24 and then ramp the substrate to the GaN growth temperature.</li>
	</ol>
	</li>
</ol>
3. HEMT Growth
<ol>
	<li>Buffer growth
	<ol>
		<li>Initiate growth by opening the N* shutter for a 1 min nitridation.</li>
		<li>Grow a thin 1-3 nm N-rich AlN nucleation layer by opening the Al shutter. This AlN layer can help prevent threading dislocation generation,25 however, the AlN layer complicates X-ray diffraction (XRD) measurements and is not recommended for XRD-based calibration samples. Use the same Al flux as for the InAlN layer, or about 0.36 nm-2sec-1, giving a growth rate of approximately 1 nm/min.</li>
		<li>Close the Al and N* shutter and immediately open the Ga shutter for 10 sec to allow Ga to saturate the surface, the RHEED intensity should decrease rapidly. Open the N* shutter (with the Ga shutter still open) and grow 5 min of GaN. Use Be or C doping, especially for freestanding GaN substrates, to prevent electrical leakage currents in the buffer.</li>
		<li>Close the Ga and N* shutters for a 1 min growth interruption. Monitor the RHEED intensity. If the RHEED intensity immediately increases, then the Ga flux is not high enough. If the RHEED intensity increases after &#62;30 sec or does not reach a plateau within 1 min then the Ga flux is too high. See Figure 3.</li>
		<li>Increase the substrate temperature by a few degrees (or reduce the Ga effusion cell temperature) to compensate for high Ga flux seen in 3.1.4. If the Ga flux was too low, decrease the substrate temperature (or increase the Ga effusion cell temperature) to compensate.
		<ol>
			<li>Repeat Steps 3.1.3-3.1.5 until there is a 15-30 sec delay before the RHEED intensity increases and the RHEED intensity reaches a plateau before 1 min.</li>
		</ol>
		</li>
		<li>Continue repeating Steps 3.1.4-3.1.5 until the desired GaN thickness is reached. Determine thickness by multiplying the total growth time by the calibrated growth rate. For Ga-rich growth, determine the growth rate by the N* flux, which can in turn be calculated using XRD to measure the thickness for a known growth time in a separately grown calibration sample.</li>
	</ol>
	</li>
	<li>InAlN barrier growth
	<ol>
		<li>Wait an additional 1 min after the final GaN growth step to ensure all Ga has evaporated.</li>
		<li>Quickly ramp down to the InAlN growth temperature to approximately 630 &#176;C. Allow the substrate temperature to stabilize for about 2 min.</li>
		<li>Open the In, Al, and N* shutters. The RHEED intensity should decrease and reach a plateau within the first 3 min. Continued decrease of RHEED intensity may indicate accumulation of In, which is detrimental to InAlN growth. The RHEED pattern should remain streaky, indicating a smooth surface. The In and Al fluxes are about 0.31 and 0.36 nm-2sec-1, giving a group-III limited growth rate of 1.25 nm/min.</li>
		<li>Close the In, Al, and N* shutters after the desired barrier thickness has been reached. For N-rich growth, determine the growth rate by the total group-III flux. Measure the growth rate using XRD on a separately grown InAlN calibration sample. For a 15 nm InAlN barrier using the conditions given here, open the shutters for 12 min 30 sec.</li>
	</ol>
	</li>
	<li>Interlayer and channel growth
	<ol>
		<li>First open the Ga shutter for 5 sec, then open the N* shutter and grow the GaN interlayer. The substrate temperature should still be at the InAlN growth temperature.</li>
		<li>Open the Al shutter without closing the Ga or N* shutters to grow the AlN interlayer. The Al flux for the interlayer should be the same or slightly higher than the N* flux, ideally using a different Al effusion cell then that used for the InAlN barrier layer, to avoid having to change the cell temperature. See step 3.2.4.</li>
		<li>Close the N* and Al shutters but leave the Ga shutter open. Ramp the substrate temperature to the GaN channel temperature.</li>
		<li>After 30 sec close the Ga shutter. Wait 30 sec (or as soon as the RHEED intensity starts to increase) and re-open the Ga shutter. Continue to cycle the Ga shutter until the substrate has reached the GaN channel growth temperature. This will protect the surface while preventing excessive Ga accumulation.</li>
		<li>Open the Ga shutter for 5 sec, then open the N* shutter and grow the GaN channel.</li>
		<li>Close the Ga, N* and main shutters. Ramp the substrate temperature down to 200 &#176;C, turn off the N* plasma and shut off the N2 gas flow.</li>
		<li>Ramp the cells down to their standby temperatures if finished for the day.</li>
		<li>Wait for the substrate temperature to decrease below 250 &#176;C and the chamber pressure to drop below 8 x 10-7 Torr, then open the growth chamber gate valve and transfer the wafer holder back to the trolley.</li>
		<li>Follow the reverse of Step 2.2 to transfer the wafer back to the load lock, vent with N2 and remove the wafer. Make sure the growth chamber gate valve is closed before opening the load lock, and that the load lock turbo pump gate valve is closed before venting.</li>
		<li>Follow Steps 2.2.3-2.2.5 to return the cassette and load lock to high vacuum.</li>
	</ol>
	</li>
</ol>
4. Characterization
<ol>
	<li>Characterize the material quality using optical microscopy to check for pits, cracks or Ga-droplets that may have formed during the channel growth, XRD to check the interfacial and structure quality, and AFM to check the surface morphology.20,21</li>
	<li>If Ga-droplets are present, immerse the wafer in concentrated HF acid for 5-10 min to remove the droplets without damaging the chemically sensitive N-polar surface.</li>
	<li>Measure sheet resistance using Lehighton contactless resistance measurements.</li>
	<li>Process the sample to allow electrical characterization, including Hall and CTLM measurements and dc and RF transistor characterization.9,26</li>
</ol>

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The authors have nothing to disclose.

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,28 or allowing a continuous N*-flux to incorporate the excess Ga when the Ga shutter is periodically closed.24
A growth interruption (to allow Ga evaporation) is used in this case because it reduces wear on the shutters implicit in metal-modulated epitaxy, is tolerant to wafer temperature non-uniformity,29 and avoids ambiguity in the actual growth time (and layer thickness) as well as prevents potential impurity incorporation as a result of using N* flux to incorporate the Ga-wetting layer.24 This technique results in streaky RHEED and smooth surfaces/interfaces with rms roughness &#60;1 nm if the correct Ga-flux and growth/interrupt times are used. In addition to the RHEED transients described in Figure 3, insufficient Ga-flux can be indicated by spotty RHEED patterns and a rough surface (for N-rich conditions) or streaky RHEED with a plateau/trench rough surface morphology (for Ga-rich intermediate conditions).28
The maximum growth time between interrupts is somewhat flexible, but is limited by macroscopic Ga-droplet formation. Once droplets form and coalesce, they are very difficult to fully evaporate and RHEED brightening can still occur in the presence of Ga droplets, incorrectly suggesting a Ga-free surface. GaN growth times of up to 20 min between interrupts have been used to reduce shutter wear.29 Ga-droplets found on the wafer surface after growth can originate from droplets formed during the buffer layer growth (though they can also form without significant negative consequences during the GaN channel growth). Macroscopic Ga-droplets which formed during the buffer can cause Ga incorporation and/or displacement of In in the InAlN barrier layer, leading to reduced carrier concentrations and/or cracking of the barrier layer due to strain, which is more severe in lower temperature regions of the wafer.
Growth interrupts at or near the channel can cause severe device degradation for HEMTs. However, if Ga-flux is periodically supplied and allowed to desorb during the interrupt, the degradation in mobility can be avoided and the interrupt can be used to ramp the substrate to a higher growth temperature more favorable for GaN growth.15 Using this technique, the mobility has been increased by approximately 150 cm2/V&#8729;sec relative to samples with the GaN channel grown at the InAlN growth temperature. A GaN and/or AlN interlayer must be grown prior to increasing the growth temperature to avoid damaging the InAlN surface.
We have demonstrated a method to grow high quality N-polar InAlN-barrier HEMTs. In situ Ga deposition and desorption is utilized to prepare a clean surface for epitaxial growth. The HEMT epitaxial layer structure incorporates an InAlN-barrier layer with homogeneous composition, AlN and GaN interlayers to control channel charge density and mobility, and a GaN channel growth procedure with a temperature ramp which can further optimize the mobility. PAMBE and the procedures described here are widely applicable to III-nitride epitaxy, including RF and high-power electronics and visible-through-UV optoelectronics.

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="https://cloudflare-jove-com.remotexs.ntu.edu.sg/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="https://cloudflare-jove-com.remotexs.ntu.edu.sg/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="https://cloudflare-jove-com.remotexs.ntu.edu.sg/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 lattice constant as the GaN substrate, indicating fully coherent growth for a reasonably thick InAlN layer. The full width at half maximum (FWHM) in the q&#8214; direction is very close to the same for the InAlN layer and GaN substrate, suggesting that no additional dislocations or other structural defects have been introduced in the InAlN layer. The broadening of the InAlN layer in q&#8869; is caused by limited coherence length broadening in the relatively thin InAlN layer.
The AFM images shown in Figure 5 compare two In0.18Al0.82N layers with similar composition grown at 500 &#176;C and 630 &#176;C. To achieve the same composition, the In/group-III flux ratio was increased from 0.18 at 500 &#176;C to 0.47 at 630 &#176;C. At the lower growth temperature, the surface indicates a quasi-3D growth mode, while steps are seen on the 630 &#176;C sample, suggesting 2D step-flow growth. Transmission electron micrographs show elimination of the honeycomb morphology for In0.18Al0.82N samples grown at high temperature.19,20
XRD measurements of full N-polar InAlN-barrier HEMT structures grown on N-polar GaN substrates shown in Figure 6(a) agree well with XRD simulations. The simulations are sensitive to all of the various layers, and such agreement indicates both good interfacial quality and a high degree of control over the individual layer thicknesses. The AlN and GaN IL thicknesses were 1.4 and 1.5 nm, respectively. Hall effect measurements result in measured &#181; = 1,400 cm2/V&#8729;sec, ns = 2.2 x 1013 cm2, and Rs = 200 &#937;/&#9633;.
N-polarity for epitaxial layers grown on C-polar SiC and N-polar GaN substrates was confirmed by comparing the sheet resistance of test structures and full HEMT structures. Samples having only an InAlN layer grown on GaN or GaN/AlN buffer layer were highly resistive, indicating the absence of a two-dimensional electron gas. Samples with a GaN channel on top of the InAlN layer show a low sheet resistance from 170-300 &#937;/&#9633; and evidence of a two-dimensional electron gas in Hall measurements, confirming N-polar orientation.
Based on initial materials characterization, samples may be processed into HEMT devices. The first stage of processing involves depositing the Ti/Al/Ni/Au (20/100/10/50 nm) Ohmic contacts by e-beam evaporation and annealing at 750 &#176;C for 30 sec under N2 ambient, followed by device isolation using a Cl2/BCl3/Ar inductively-coupled plasma etch. The source-drain spacing for the devices discussed here is 5 &#181;m. At this point several process monitoring structures can be electrically tested, including linear and circular transfer length measurement patterns for sheet and contact resistance and Hall effect patterns for channel charge density and mobility. Selected samples are then processed for full HEMT structures by depositing 1 &#181;m long, 150 &#181;m wide Pt/Au gates. HEMTs with the epitaxial structure described above grown on an N-polar GaN substrate and fabricated with Pt/Au Schottky gates show excellent dc characteristics, with a maximum current of 1.5 A/mm (for a 5 &#181;m source-drain spacing and unoptimized contacts) and a threshold voltage of -1.6 V, as shown in Figure 6.
<img alt="Figure 4" src="/files/ftp_upload/54775/54775fig4.jpg" /> 
Figure 4: XRD of InAlN thin films. (a) 2&#952;/&#969; XRD diffraction scans of 50 nm and 200 nm thick InAlN thin films grown on freestanding N-polar substrates and (b) an XRD reciprocal space map about the reflection, showing the 200 nm thick InAlN layer has the same q&#8214; coordinate and is thus coherent to the GaN substrate. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54775/54775fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/54775/54775fig5.jpg" /> 
Figure 5: AFM of InAlN thin films. AFM micrographs of In0.18Al0.82N samples grown at (a) 500 &#176;C with an In/group-III ratio of 0.18 and (b) 630 &#176;C with a In/group-III ratio of 0.47. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54775/54775fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/54775/54775fig6.jpg" /> 
Figure. 6: XRD and device results of an InAlN HEMT. (a) XRD &#969;/2&#952; scan about the 0002 reflection of a full N-polar InAlN HEMT structure grown on an N-polar GaN substrate with inset layer thicknesses used to generate the XRD simulation. (b) HEMT transfer curve and (c) IV curves for the same device in (a) processed with a Pt/Au Schottky gate. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54775/54775fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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 ...

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8.5K Views.  Naval Research Laboratory. 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.

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

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 ...

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 ...

Demonstration of Equal-Intensity Beam Generation by Dielectric Metasurfaces

The fabrication and characterization protocol for a metasurface beam splitter, enabling equal-intensity beam generation, is demonstrated. Hydrogenated amorphous silicon (a-Si:H) is deposited on the fused silica substrate, using plasma-enhanced chemical vapor deposition (PECVD). Typical amorphous silicon deposited by evaporation causes severe optical loss, impinging the operation at visible frequencies. Hydrogen atoms inside the amorphous silicon thin film can reduce the structural defects, improving optical loss. Nanostructures of a few hundreds of nanometers are required for the operation of metasurfaces in the visible frequencies. Conventional photolithography or direct laser writing is not feasible when fabricating such small structures, due to the diffraction limit. Hence, electron beam lithography (EBL) is utilized to define a chromium (Cr) mask on the thin film. During this process, the exposed resist is developed at a cold temperature to slow down the chemical reaction and make the pattern edges sharper. Finally, a-Si:H is etched along the mask, using inductively coupled plasma&#8211;reactive ion etching (ICP-RIE). The demonstrated method is not feasible for large-scale fabrication due to the low throughput of EBL, but it can be improved upon by combining it with nanoimprint lithography. The fabricated device is characterized by a customized optical setup consisting of a laser, polarizer, lens, power meter, and charge-coupled device (CCD). By changing the laser wavelength and polarization, the diffraction properties are measured. The measured diffracted beam powers are always equal, regardless of the incident polarization, as well as wavelength.

A protocol for the fabrication and optical characterization of dielectric metasurfaces is presented. This method can be applied to the fabrication of not only beam splitters, but also of general dielectric metasurfaces, such as lenses, holograms, and optical cloaks.

The fabrication and characterization protocol for a metasurface beam splitter, enabling equal-intensity beam generation, is demonstrated. Hydrogenated ...