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>
	<li>Hughes, W. C., 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+growth+and+properties+of+GaN+films+on+GaN/SiC+substrates.">Molecular beam epitaxy growth and properties of GaN films on GaN/SiC substrates.</a> J. Vac. Sci. Technol., B. 13 (4), 1571-1577 (1995).</li><li>McSkimming, B. M., Wu, F., Huault, T., Chaix, C., 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=Plasma+assisted+molecular+beam+epitaxy+of+GaN+with+growth+rates+2.6+&#181;m/hr.">Plasma assisted molecular beam epitaxy of GaN with growth rates 2.6 &#181;m/hr.</a> J. Cryst. Growth. 386, 168-174 (2014).</li><li>Grandjean, N., Massies, J., Leroux, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nitridation+of+sapphire.+Effect+on+the+optical+properties+of+GaN+epitaxial+overlayers.">Nitridation of sapphire. Effect on the optical properties of GaN epitaxial overlayers.</a> Appl. Phys. Lett. 69 (14), 2071-2073 (1996).</li><li>Corrion, A. L., Wu, 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=Growth+regimes+during+homoepitaxial+growth+of+GaN+by+ammonia+molecular+beam+epitaxy.">Growth regimes during homoepitaxial growth of GaN by ammonia molecular beam epitaxy.</a> J. Appl. Phys. 112 (5), 054903 (2012).</li><li>Mazumder, B., 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=Atom+probe+analysis+of+AlN+interlayers+in+AlGaN/AlN/GaN+heterostructures.">Atom probe analysis of AlN interlayers in AlGaN/AlN/GaN heterostructures.</a> Appl. Phys. Lett. 102 (11), 111603 (2013).</li><li>Feezell, D. F., Speck, J. S., DenBaars, S. P., Nakamura, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Semipolar+(2021)+InGaN/GaN+Light-Emitting+Diodes+for+High-Efficiency+Solid+State+Lighting.">Semipolar (2021) InGaN/GaN Light-Emitting Diodes for High-Efficiency Solid State Lighting.</a> J. Disp. Technol. 9 (4), (2013).</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=True+Green+Semipolar+InGaN-Based+Laser+Diodes+Beyond+Critical+Thickness+Limits+Using+Limited+Area+Epitaxy.">True Green Semipolar InGaN-Based Laser Diodes Beyond Critical Thickness Limits Using Limited Area Epitaxy.</a> J. Appl. Phys. 114 (18), 183101 (2013).</li><li>Wong, M. H., 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=N-polar+GaN+epitaxy+and+high+electron+mobility+transistors.">N-polar GaN epitaxy and high electron mobility transistors.</a> Semicond. Sci. Technol. 28 (7), 074009 (2013).</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=Charge+control+in+N-polar+InAlN+high-electron-mobility+transistors+grown+by+plasma-assisted+molecular+beam+epitaxy.">Charge control in N-polar InAlN high-electron-mobility transistors grown by plasma-assisted molecular beam epitaxy.</a> J. Vac. Sci. Technol., B. 33 (6), 061207 (2015).</li><li>Piquette, E. C., Bridger, P. M., Beach, R. A., McGill, T. C. <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+Buffer+Layer+and+III/V+Ratio+on+the+Surface+Morphology+of+GaN+Grown+by+MBE.">Effect of Buffer Layer and III/V Ratio on the Surface Morphology of GaN Grown by MBE.</a> Symposium G '-' GaN and Related Alloys. , (1998).</li><li>Tarsa, E. J., 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=Homoepitaxial+growth+of+GaN+under+Ga-stable+and+N-stable+conditions+by+plasma-assisted+molecular+beam+epitaxy.">Homoepitaxial growth of GaN under Ga-stable and N-stable conditions by plasma-assisted molecular beam epitaxy.</a> J. Appl. Phys. 82 (11), 5472-5479 (1997).</li><li>Kuzmik, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Power+electronics+on+InAlN/(In)GaN:+Prospect+for+a+record+performance.">Power electronics on InAlN/(In)GaN: Prospect for a record performance.</a> IEEE Electron Device Lett. 22 (11), 510-512 (2001).</li><li>Fern&#225;ndez-Garrido, S., Ga&#269;evi&#263;, &. #. 3. 8. 1. ;., Calleja, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+comprehensive+diagram+to+grow+InAlN+alloys+by+plasma-assisted+molecular+beam+epitaxy.">A comprehensive diagram to grow InAlN alloys by plasma-assisted molecular beam epitaxy.</a> Appl. Phys. Lett. 93 (19), 191907 (2008).</li><li>Katzer, D. 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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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,...

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...

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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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8.4K Görüntüleme.  Naval Research Laboratory. 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 mate...