This work was supported by the Natural Sciences and Engineering Research Council of Canada.

1. MgO substrate preparation
NOTE: Commercial one-side epi-polished (100) oriented single crystal MgO square substrates (1 cm x 1 cm) were employed for the X3N2 (X = Zn and Mg) thin film growth.
<ol>
	<li>High temperature annealing
	<ol>
		<li>Place the MgO on a clean sapphire wafer sample carrier with the polished side facing upwards in a furnace and anneal for 9 h at 1,000 &#176;C. Raise the temperature to 1000 &#176;C over a 10 min period. 
		NOTE: High temperature annealing removes carbon from the surface and reconstructs the surface crystal structure of the MgO single crystal substrates.</li>
		<li>Cool the MgO substrates to the room temperature (RT).</li>
	</ol>
	</li>
	<li>Substrate cleaning
	<ol>
		<li>Collect the annealed MgO substrates and rinse in deionized water in a clean borosilicate glass beaker.</li>
		<li>Boil the MgO substrates in 100 mL of acetone in a 250 mL borosilicate glass beaker for 30 min to remove inorganic carbon contamination from handling. 
		NOTE: Cover the beaker and do not allow the acetone to boil dry.</li>
		<li>Drain the acetone and rinse the MgO substrates in 50 mL of methanol.</li>
		<li>Blow-dry the substrates with nitrogen gas, then store the dry, clean substrates in the clean chip tray.</li>
	</ol>
	</li>
</ol>
2. Operation of VG V80 MBE
<ol>
	<li>Open the cooling water for the preparation chamber, cryoshroud on the growth chamber (see Figure 1), effusion cells, and quartz crystal microbalance sensor.</li>
	<li>Turn on the Ar-ion laser with a wavelength of 488 nm. The laser light is brought to the MBE chamber with an optical fiber from the laser, which is located in another room.</li>
	<li>Turn on the reflection high energy electron diffraction gun (RHEED), 13.56 Mhz radio frequency (rf) plasma generator, and quartz crystal microbalance (QCM) system.</li>
</ol>
3. Substrate loading
<ol>
	<li>Fast entry lock
	<ol>
		<li>Mount a clean MgO substrate on the molybdenum sample holder (Figure 2A) using tungsten spring clips.</li>
		<li>Turn off the turbo pump on the fast entry lock (FEL) and vent the FEL chamber with nitrogen. Open the FEL when the chamber pressure reaches atmospheric pressure.</li>
		<li>Remove the sampler holder cassette out of the FEL and load the sample holder with the substrate into the cassette.</li>
		<li>Load the cassette back into the FEL and turn the turbo pump back on.</li>
		<li>Wait for the pressure in the FEL to drop to 10-6 Torr.</li>
		<li>Increase the temperature of the fast entry lock to 100 &#176;C over a period of 5 min and degas the substrates with the holders for 30 min in the fast entry lock.</li>
	</ol>
	</li>
	<li>Make sure the pressure in the fast entry lock is below 10-7 Torr before opening the vacuum valve to the preparation chamber. Transfer the holder using the wobble stick transfer mechanism to the preparation chamber, then ramp up the degassing station to 400 &#176;C and allow it to degas for 5 h.</li>
	<li>Transfer the degassed holder by the trolley transfer mechanism to the sample manipulator in the growth chamber. Increase the substrate temperature up to 750 &#176;C over a period of 30 min and allow the sample to outgas in the manipulator for another 30 min. Make sure the cooling water is turned on in the cryoshroud to avoid overheating the cryoshroud.</li>
	<li>Drop the temperature of the substrate to 150 &#176;C for Zn3N2 film growth and 330&#176;C for Mg3N2 film growth using the thermocouple in the sample manipulator to measure the sample temperature.</li>
	<li>In-situ RHEED
	<ol>
		<li>Set the voltage on the electron gun to 15 kV and filament current to 1.5 A once the growth chamber pressure is below 1 x 10-7 Torr.</li>
		<li>Rotate the substrate holder until 1) the electron gun is aligned along a principle crystallographic axis of the substrate and 2) a clear single crystal electron diffraction pattern is visible.</li>
		<li>Take a picture of the RHEED pattern and save the picture.</li>
	</ol>
	</li>
	<li>Close the shutter on the effusion cell and stop the flow of nitrogen. Measure the RHEED pattern for the deposited film when the chamber pressure is below 10-7 Torr.</li>
</ol>
4. Metal flux measurements
<ol>
	<li>Use standard group III type effusion cells or low temperature effusion cells for Mg and Zn.</li>
	<li>Load the crucibles with 15 g and 25 g of high purity Mg and Zn shot, respectively.</li>
	<li>When the growth chamber has achieved a vacuum of 10-8 Torr or better, and before loading the substrate holder, outgas the Zn or Mg source effusion cells up to 250 &#176;C at a ramp rate of ~20 &#176;C/min and allow it to outgas for 1 h with the shutters closed.</li>
	<li>After the substrate has been loaded into the sample manipulator, heat the Zn and/or Mg effusion cells to 350 &#176;C or 390 &#176;C respectively, at a ramp rate of ~10 &#176;C/min, and wait 10 min for them to stabilize with the shutters closed.</li>
	<li>Use the retractable quartz crystal monitor to measure the metal flux. Position the quartz crystal sensor in front of the substrate inside the chamber. Make sure the substrate is fully covered by the detector so that no metal is deposited on the substrate.</li>
	<li>Input the density of the metal of interest (&#961;Zn = 7.14 g/cm3, &#961;Mg = 1.74 g/cm3) into the quartz crystal monitor (QCM) controller.</li>
	<li>To calibrate the flux, open the shutter for one of the metal sources and allow the effusion cell to deposit on the sensor. The QCM system converts its internal measurement of mass to thickness.</li>
	<li>Calculate the elemental flux from the slope of the increasing thickness as a function of time shown on the QCM. The rate of increase of the thickness over a few minutes is proportional to the elemental flux. In two example cases, a Zn flux of 0.45 nm/s and a Mg flux of 1.0 nm/s are obtained.</li>
	<li>Change the temperature of the effusion cells and repeat step 4.8 if the temperature dependence of the flux is required. The measured temperature dependence of the Mg and Zn flux is shown in Figure 3 for this specific growth system.</li>
	<li>When the flux measurements are complete, close the shutters on the effusion cells and retract the quartz crystal sensor.</li>
</ol>
5. Nitrogen plasma
<ol>
	<li>Turn off the filament current and high voltage on the RHEED gun to prevent damage in the presence of a high N2 gas pressure in the growth chamber.</li>
	<li>Open the gas valve on the high pressure N2 cylinder.</li>
	<li>Slowly open the leak valve slowly until the nitrogen pressure in the growth chamber reaches 3 x 10-5-4 x 10-5 Torr.</li>
	<li>Set the power of the plasma generator to 300 W.</li>
	<li>Ignite the plasma with the ignitor on the plasma source. A bright purple glow will be visible from the viewport when the plasma ignites, as shown in Figure 2B.</li>
	<li>Adjust the control on the rf matching box to minimize the reflected power as much as possible. A reflected power of less than 15 W is good; in this case, the reflected power is reduced to 12 W.</li>
</ol>
6. In situ laser light scattering
<ol>
	<li>Focus the chopped 488 nm Argon laser light reflected from the substrate in the growth chamber onto the Si photodiode so that an electrical signal can be detected by the lock-in amplifier. This is accomplished by adjusting the angle of the substrate by rotating the substrate holder around two axes and adjusting the position of the Si detector, then focusing the lens that collects the reflected light as shown in Figure 4.</li>
	<li>Open the shutter of one of the metal sources.</li>
	<li>Record the time-dependent reflectivity with a computer-controlled data logger. The growth of an epitaxial film will produce an oscillatory reflected signal with time associated with thin film optical interference between the front and back surfaces of the film.</li>
	<li>To protect the film from oxidation in air, deposit an encapsulation layer to protect the film from oxidation in air. This is especially important for Mg3N2 which oxidizes rapidly in air.</li>
	<li>In order to deposit a MgO encapsulation layer, close the nitrogen gas, switch to oxygen gas, repeat step 5.3, and increase the oxygen pressure to 1 x 10-5 Torr.</li>
	<li>Set the power of the plasma generator to 250 W and repeat step 5.5. The plasma starts at lower rf power with oxygen gas than with nitrogen gas.</li>
	<li>Open the shutter on the Mg source, and repeat step 6.4 for 5-10 min. 
	NOTE: This will produce a MgO film that is about 10 nm thick. The uncapped Mg3N2 films are yellow but fade quickly to a whitish color within 20 s upon exposure to air. Consequently, an encapsulation layer is required to allow time for measurements on the films before they oxidize after removal from the vacuum chamber.</li>
	<li>Close the gas valves, turn off the laser, and ramp down the substrate and cell temperatures to ~25 &#176;C in 30 min. Turn off the cooling water and the rf power to the plasma source.</li>
	<li>After several growth runs, the optical windows become covered with metal. Remove the metal by wrapping the window in aluminum foil and heating it with heating tape to 400 &#176;C and a temperature ramp rate of ~15 &#176;C/min or slower over the course of a weekend.</li>
</ol>
7. Growth rate determination
<ol>
	<li>Use Equation 1 below to describe the optical reflectivity of the sample11,19. 
	<img alt="Equation 1" src="/files/ftp_upload/59415/59415eq1.jpg" style="opacity: 0.9;" />&#160;(1) 
	Where: 
	<img alt="Equation 2" src="/files/ftp_upload/59415/59415eq2.jpg" style="opacity: 0.9;" />&#160;(1 - a) 
	<img alt="Equation 4" src="/files/ftp_upload/59415/59415eq4.jpg" style="opacity: 0.9;" />&#160;(1 - b) 
	<img alt="Equation 5" src="/files/ftp_upload/59415/59415eq5.jpg" style="opacity: 0.9;" />&#160;(1 - c) 
	<img alt="Equation 6" src="/files/ftp_upload/59415/59415eq6.jpg" style="opacity: 0.9;" />&#160;(1 - d)</li>
	<li>And where: n2 = 1.747 is the refractive index of the MgO substrate at a wavelength of 488 nm; &#952;0 is the angle of the incident beam measured with respect to the substrate surface normal; and t is time. The optical constants of the film (n1 and k1) and growth rate are obtained by fitting the reflectivity as a function of time in Equation 1.</li>
</ol>

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The authors have nothing to disclose.

A variety of considerations is involved in the choice of substrates and establishing the growth conditions that optimize the structural and electronic properties of the films. The MgO substrates are heated at high temperature in air (1000 &#176;C) to remove carbon contamination from the surface and improve the crystalline order in the substrate surface. Ultrasonic cleaning in acetone is a good alternative method to clean the MgO substrates.
The (400) X-ray diffraction peak for the Zn3N2 films was found to be narrower when the film was grown on high-temperature, annealed MgO substrates compared to when grown on unannealed substrates. The lattice constant of MgO (0.421 nm) is significantly smaller than (half of) the lattice constant of Zn3N2 (0.976 nm) or Mg3N2 (0.995 nm) and not well-matched to the semiconductor films. The lattice constants of the commercially available groups IV, III-V, and II-VI substrates are all larger than the lattice constants of Mg3N2 and Zn3N2. A more well-matched substrate is desirable. Silicon, which has a lattice constant of 0.543 nm, is somewhat more well-matched to Mg3N2 than MgO and is worth to be explored. Zn3N2 films were also grown on A-plane sapphire substrates. The structural quality of these films was not as good as that seen in the MgO substrates, as indicated by the RHEED spots and width of the (400) Zn3N2 X-ray diffraction peak. In the case of the sapphire substrates, the backside was coated with Cr (50 nm) and Mo (200 nm) to improve thermal coupling between the substrate holder and substrate.
The substrate temperature is measured during growth with a thermocouple located in the enclosed vacuum space between the substrate holder and heater, and it is not in physical contact with the substrate holder or the substrate. As a result, it was expected that the actual substrate temperature would be lower than the sensor temperature. Successful Mg3N2 and Zn3N2 growths were obtained with thermocouple temperatures in the 300-350 &#176;C and 140-180 &#176;C ranges, respectively.
High growth temperature increases the mobility of the ad-atoms on the growing surface and may be expected to produce material with fewer structural defects. However, at high substrate temperatures, the growth rate is lower, which is interpreted as being due to re-evaporation of metal atoms from the surface. At high metal flux, film growth rate is limited by the supply of active nitrogen. The active nitrogen is maximized at high rf power applied to the plasma source (300 W max) and at a high nitrogen flow rate. The N2 flow rate is limited by the maximum pressure in the growth chamber, which in this case was in the mid 10-5 Torr range. Ammonia is a possible alternative nitrogen source. Mg and Zn will react with NH3 at a high temperature without plasma activation; however, it is unclear whether the residence time of Mg and Zn atoms on the surface will be long enough to support film growth at temperatures for which NH3 will react with the metals.
In these experiments, effusion cells were used with pyrolytic boron nitride (PBN) crucibles with capacities of 40 cc for Mg and 25 cc for Zn. Figure 3 shows the temperature dependence of the Mg and Zn fluxes from the effusion cells. The straight lines in Figure 3 are fit to the measured temperature dependence of the fluxes. The fits have the form Flux = A exp(-B/kT), and the fitting parameters (A,B) are (8.5 x 1017 nm/s, 2.3 eV) and (1.3 x 1015 nm/s, 1.9 eV) for the Mg and Zn sources, respectively. The flux approximately doubles with each 10 &#176;C and 12 &#176;C increase in the effusion cell temperature for Mg and Zn, respectively. For the growth illustrated in Figure 6, the metal fluxes were near the maxima in Figure 3 (~1 nm/s with Mg flux higher than Zn flux) but the film growth rates were only 0.03 nm/s. This shows that the metal utilization efficiency is low, with Mg being lower than Zn and most of the metal re-evaporating.
The high N2 background pressure during growth precludes the continuous monitoring of the film growth with RHEED. Differential pumping of the RHEED gun can solve this problem. In situ optical reflectivity measurements serve as a complementary monitoring tool that is not affected by gas pressure and provides an accurate and reliable technique for determining the growth rate. The non-specular or diffuse reflectivity of the substrate can also be measured in situ and will provide information on surface roughness during growth.
The base pressure in the MBE growth chamber is 10-8 Torr with the N2 gas turned off. The cryoshroud in the growth chamber is cooled with water during film growth. Under these conditions, some residual oxygen contamination can be expected in the films. The residual water vapour pressure in the growth chamber can be reduced with a lower temperature coolant in the shroud, such as silicone oil at -80 &#176;C21.
In conclusion, this protocol describes how to grow single crystal films of Mg3N2 and Zn3N2 by plasma-assisted molecular beam epitaxy and provides suggestions for how the growth process can be changed to further improve film quality. These materials did not show photoluminescence at either room temperature or a low temperature. There is a need to further reduce the defect density in the films. Mg3N2-Zn3N2 alloys can also be grown by plasma-assisted molecular beam epitaxy.

The II3-V2 materials are a class of semiconductors that have received relatively little attention from the semiconductor research community compared to III-V and II-VI semiconductors1. The Mg and Zn nitrides, Mg3N2 and Zn3N2, are attractive for consumer applications because they are composed of abundant and non-toxic elements, making them inexpensive and easy to recycle unlike most III-V and II-VI compound semiconductors. They display an anti-bixbyite crystal structure similar to the CaF2 structure, with one of the interpenetrating fcc F-sublattices being half-occupied2,3,4,5. They are both direct band gap materials6, making them suitable for optical applications7,8,9. The band gap of Mg3N2 is in the visible spectrum (2.5 eV)10, and the band gap of Zn3N2 is in the near-infrared (1.25 eV)11. To explore the physical properties of these materials and their potential for electronic and optical device applications, it is critical to obtain high quality, single crystal films. Most work on these materials to date has been carried out on powders or polycrystalline films made by reactive sputtering12,13,14,15,16,17.
Molecular beam epitaxy (MBE) is a well-developed and versatile method for growing single-crystal compound semiconductor films18 that has the potential to yield high quality materials using a clean environment and high-purity elemental sources. Meanwhile, MBE rapid shutter action enables changes to a film at the atomic layer scale and allows for precise thickness control. This paper reports on the growth of Mg3N2 and Zn3N2 epitaxial films on MgO substrates by plasma-assisted MBE, using high purity Zn and Mg as vapor sources and N2 gas as the nitrogen source.

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			<td height="21" style="height:21px;width:222px;">(100) MgO</td>
			<td style="width:123px;">University Wafer</td>
			<td style="width:123px;">214018</td>
			<td style="width:276px;">one side epi-polished</td>
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			<td height="21" style="height:21px;width:222px;">Acetone</td>
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			<td>488 nm visible wavelength, 350 mW output power</td>
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			<td height="42" style="height:42px;width:222px;">Chopper&#160;</td>
			<td style="width:123px;">Stanford Research system&#160;</td>
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			<td style="width:276px;">&#160;Max. Frequency: 3.7 kHz&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:222px;">Lock-in amplifier&#160;</td>
			<td style="width:123px;">Stanford Research system&#160;</td>
			<td style="width:123px;">37909</td>
			<td style="width:276px;">DSP SR810, Max. Frequency: 100 kHz&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:222px;">Magnesium&#160;</td>
			<td style="width:123px;">UMC</td>
			<td style="width:123px;">MG6P5</td>
			<td style="width:276px;">99.9999%</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:222px;">MBE system</td>
			<td style="width:123px;">VG Semicon</td>
			<td style="width:123px;">V80H0016-2 SHT 1</td>
			<td style="width:276px;">V80H-10</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:222px;">Methanol&#160;</td>
			<td style="width:123px;">Alfa Aesar</td>
			<td style="width:123px;">L30U027</td>
			<td style="width:276px;">Semi-grade 99.9%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:222px;">Nitrogen</td>
			<td style="width:123px;">Praxair</td>
			<td style="width:123px;">402219501</td>
			<td style="width:276px;">99.998%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:222px;">Oxygen&#160;</td>
			<td style="width:123px;">Linde Gas</td>
			<td style="width:123px;">200-14-00067</td>
			<td style="width:276px;">&#62; 99.9999%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:222px;">Plasma source</td>
			<td style="width:123px;">SVT Associates</td>
			<td style="width:123px;">SVTA-RF-4.5PBN</td>
			<td>PBN, 0.11&#34; Aperture, Specify Length: 12&#34; &#8211; 20&#34;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:222px;">Si photodiode&#160;</td>
			<td style="width:123px;">Newport</td>
			<td style="width:123px;">2718</td>
			<td style="width:276px;">818-UV Enhanced, 200 - 1100 nm</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:222px;">Zinc&#160;</td>
			<td style="width:123px;">Alfa Aesar</td>
			<td style="width:123px;">7440-66-6</td>
			<td style="width:276px;">99.9999%</td>
		</tr>
	</tbody>
</table>

plasma-assisted molecular beam epitaxy growth of mg3n2 and zn3n2 thin films

This article describes a procedure for growing Mg3N2 and Zn3N2 films by plasma-assisted molecular beam epitaxy (MBE). The films are grown on 100 oriented MgO substrates with N2 gas as the nitrogen source. The method for preparing the substrates and the MBE growth process are described. The orientation and crystalline order of the substrate and film surface are monitored by the reflection high energy electron diffraction (RHEED) before and during growth. The specular reflectivity of the sample surface is measured during growth with an Ar-ion laser with a wavelength of 488 nm. By fitting the time dependence of the reflectivity to a mathematical model, the refractive index, optical extinction coefficient, and growth rate of the film are determined. The metal fluxes are measured independently as a function of the effusion cell temperatures using a quartz crystal monitor. Typical growth rates are 0.028 nm/s at growth temperatures of 150 &#176;C and 330 &#176;C for Mg3N2 and Zn3N2 films, respectively.

The black object in the inset in Figure 5B is a photograph of an as-grown 200 nm Zn3N2 thin film. Similarly, the yellow object in the inset in Figure 5C is an as-grown 220 nm Mg3N2 thin film. The yellow film is transparent to the extent that it is easy-to-read text placed behind the film10.
The surface of the substrate and the films were monitored in situ by RHEED. Figure 5A shows the RHEED pattern of a bare substrate with the electron beam incident along the [110] direction of the substrate. RHEED patterns for the deposited films in Figure 5B,C show that the crystal lattices of the Zn3N2 and Mg3N2 thin films are oriented in the plane of the substrate surface, as expected in the case of epitaxial growth. The disadvantage of RHEED for growth monitoring under the conditions used here is that the growth process must be stopped to allow the pressure to drop to 10-7 Torr and turn on the electron gun.
In contrast with RHEED, in situ optical reflectivity measurements are not affected by pressure in the chamber. To obtain the growth rate, in situ optical reflectivity was fit as a function of time shown in Figure 6 using Equation 1. In this equation, growth time t is the independent variable, and the optical constants of the film (n1, k1) and growth rate are fitting parameters. In Figure 6, the MgO substrate refractive index, angle of incidence, and wavelength are n2 = 1.747, &#952;0 = 36.5&#176;, and &#955; = 488 nm, respectively. The fitted refractive index of the film is n1 = 2.65, extinction coefficient is k1 = 0.54, and growth rate is = 0.031 nm/s for the Zn3N2 thin film as shown in Figure 6A. Similarly, the best fit refractive index for the Mg3N2 film is n1 = 2.4, extinction coefficient is k1 = 0.09, and growth rate is = 0.033 nm/s as shown in Figure 6B. The overall decrease in the specular reflection with time in Figure 6B is believed to be caused by an increase in surface roughness scattering as the Mg3N2 thin film becomes thicker. The effect of the roughness scattering was simulated by multiplying the calculated reflectivity with a decaying exponential, e-&#946;t, where &#946; = 810-5 s-1 and the growth time t ismeasured in seconds. 
Upon exposure to air, the uncapped yellow Mg3N2 films faded within minutes to a translucent white color. On the other hand, the Mg3N2 films that were capped with MgO were relatively stable. To further protect the capped Mg3N2 films from oxidation, the Mg3N2/MgO heterostructures were coated with a CaF2 layer deposited by electron beam evaporation. Uncapped Zn3N2 is more stable; however, the initially black Zn3N2 films also oxidized over time and turned grey within a few months. The oxidation reaction is believed to involve the formation of magnesium hydroxide and release ammonia according to the following chemical reaction20. A ZnO layer deposited using a similar method to the MgO can also be used as a protective layer to prevent oxidation.
Mg3N2 + 6H2O&#160;&#8594; 3Mg(OH)2 + 2NH3
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59415/59415fig1.jpg" /> 
Figure 1: View of the VG V80H molecular beam epitaxy growth chamber. This picture shows the MBE growth chamber with (clockwise) the RHEED screen and camera housing, quadrupole residual gas analyzer, optical hardware on laser light scattering port, Mg effusion cell, N-plasma source, rf matching box, and the Zn effusion cell. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59415/59415fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59415/59415fig2.jpg" /> 
Figure 2:&#160;Substrate holder and glow from plasma source. (A) Molybdenum sample holder plate with two tungsten wire clips holding the square MgO substrate in place. (B) Purple glow originating from the back window of the plasma source when operating with N2 gas. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59415/59415fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59415/59415fig3.jpg" /> 
Figure 3: Metal flux as a function of effusion cell temperature. The lines are fit to the temperature dependence of the metal fluxes using an Arrhenius relation as discussed in the text. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59415/59415fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/59415/59415fig4.jpg" /> 
Figure 4: Schematic of the in-situ laser light scattering setup. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59415/59415fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/59415/59415fig5.jpg" /> 
Figure 5: RHEED patterns. (A) RHEED diffraction pattern for MgO substrate. (B) RHEED pattern of as-grown Zn3N2 film with photograph of black Zn3N2 film. (C) RHEED pattern of as-grown Mg3N2 substrate with photograph of yellow Mg3N2 film. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59415/59415fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/59415/59415fig6.jpg" /> 
Figure 6: In situ specular reflectivity. In situ specular reflectivity at 488 nm of (A) Zn3N2 and (B) Mg3N2 films during growth. The calculated reflectivity (red line) is best-fit to the experimental data (blue circles) as discussed in the text. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59415/59415fig6large.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 Growth of Mg3N2 and Zn3N2 Thin Films at JoVE.com

Plasma-Assisted Molecular Beam Epitaxy Growth of Mg3N2 and Zn3N2 Thin Films

This article describes the growth of epitaxial films of Mg3N2 and Zn3N2 on MgO substrates by plasma-assisted molecular beam epitaxy with N2 gas as the nitrogen source and optical growth monitoring.

Plasma-Assisted Molecular Beam Epitaxy Growth of Mg3N2 and Zn3N2 Thin Films

This article describes a procedure for growing Mg3N2 and Zn3N2 films by plasma-assisted molecular beam epitaxy (MBE). The films are grown on 100 oriented ...

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7.5K Views.  University of Victoria. This article describes the growth of epitaxial films of Mg3N2 and Zn3N2 on MgO substrates by plasma-assisted molecular beam epitaxy with N2 gas as the nitrogen source and optical growth monitoring. 

Video: Plasma-Assisted Molecular Beam Epitaxy Growth of Mg3N2 and Zn3N2 Thin Films

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 ...

Nanomoulding of Functional Materials, a Versatile Complementary Pattern Replication Method to Nanoimprinting

We describe a nanomoulding technique which allows low-cost nanoscale patterning of functional materials, materials stacks and full devices. Nanomoulding combined with layer transfer enables the replication of arbitrary surface patterns from a master structure onto the functional material. Nanomoulding can be performed on any nanoimprinting setup and can be applied to a wide range of materials and deposition processes. In particular we demonstrate the fabrication of patterned transparent zinc oxide electrodes for light trapping applications in solar cells.

We describe a nanomoulding technique which allows low-cost nanoscale patterning of functional materials, materials stacks and full devices. Nanomoulding can be performed on any nanoimprinting setup and can be applied to a wide range of materials and deposition processes.

We describe a nanomoulding technique which  allows low-cost nanoscale patterning of functional materials, materials stacks  and full devices. Nanomoulding ...

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 ...

Electrospray Deposition of Uniform Thickness Ge23Sb7S70 and As40S60 Chalcogenide Glass Films

Solution-based electrospray film deposition, which is compatible with continuous, roll-to-roll processing, is applied to chalcogenide glasses. Two chalcogenide compositions are demonstrated: Ge23Sb7S70 and As40S60, which have both been studied extensively for planar mid-infrared (mid-IR) microphotonic devices. In this approach, uniform thickness films are fabricated through the use of computer numerical controlled (CNC) motion. Chalcogenide glass (ChG) is written over the substrate by a single nozzle along a serpentine path. Films were subjected to a series of heat treatments between 100 &#176;C and 200 &#176;C under vacuum to drive off residual solvent and densify the films. Based on transmission Fourier transform infrared (FTIR) spectroscopy and surface roughness measurements, both compositions were found to be suitable for the fabrication of planar devices operating in the mid-IR region. Residual solvent removal was found to be much quicker for the As40S60 film as compared to Ge23Sb7S70. Based on the advantages of electrospray, direct printing of a gradient refractive index (GRIN) mid-IR transparent coating is envisioned, given the difference in refractive index of the two compositions in this study.

Electrospray Deposition of Uniform Thickness Ge23Sb7S70 and As40S60 Chalcogenide Glass Films

A method of uniform thickness solution-derived chalcogenide glass film deposition is demonstrated using computer numerical controlled motion of a single-nozzle electrospray.

Solution-based electrospray film deposition, which is compatible with continuous, roll-to-roll processing, is applied to chalcogenide glasses. Two ...

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.

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 ...

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for Cu(In,Ga)Se2 Solar Cells

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction of degradation mechanisms and prevention of module field failure can consequently decrease investment risks as well as increase the electricity yield. An improved knowledge level can for these reasons significantly decrease the total costs of PV electricity. 
In order to better understand and minimize the degradation of PV modules, the occurring degradation mechanisms and conditions should be identified. This should preferably happen under combined stresses, since modules in the field are also simultaneously exposed to multiple stress factors. Therefore, two 'Combined Stress test with in situ measurement' setups have been designed and constructed. These setups allow the simultaneous use of humidity, temperature, illumination, and electrical biases as independently controlled stress factors on solar cells and minimodules. The setups also allow real-time monitoring of the electrical properties of these samples. This protocol presents these setups and describes the experimental possibilities. Moreover, results obtained with these setups are also presented: various examples about the influence of both deposition and degradation conditions on the stability of thin film Cu(In,Ga)Se2 (CIGS) as well as Cu2ZnSnSe4 (CZTS) solar cells are described. Results on the temperature dependency of CIGS solar cells are also presented.

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for CuIn,GaSe2 Solar Cells

Two &#39;Combined Stress test with in situ measurement&#39; setups, which allow real-time monitoring of accelerated degradation of solar cells and modules, were designed and constructed. These setups allow the simultaneous use of humidity, temperature, electrical biases, and illumination as independently controlled stress factors. The setups and various experiments executed are presented.

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction ...

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 Ultra-thin Color Films with Highly Absorbing Media Using Oblique Angle Deposition

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.&#160; We present an advanced method for fabricating ultra-thin color films with improved characteristics. The proposed process addresses several fabrication issues, including large area processing. Specifically, the protocol describes a process for fabricating ultra-thin color films using an electron beam evaporator for oblique angle deposition of germanium (Ge) and gold (Au) on silicon (Si) substrates.&#160; Film porosity produced by the oblique angle deposition induces color changes in the ultra-thin film. The degree of color change depends on factors such as deposition angle and film thickness. Fabricated samples of the ultra-thin color films showed improved color tunability and color purity. In addition, the measured reflectance of the fabricated samples was converted into chromatic values and analyzed in terms of color. Our ultra-thin film fabricating method is expected to be used for various ultra-thin film applications such as flexible color electrodes, thin film solar cells, and optical filters. Also, the process developed here for analyzing the color of the fabricated samples is broadly useful for studying various color structures.

We present a detailed method for fabricating ultra-thin color films with improved characteristics for optical coatings. The oblique angle deposition technique using an electron beam evaporator allows improved color tunability and purity. Fabricated films of Ge and Au on Si substrates were analyzed by reflectance measurements and color information conversion.

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.&#160; We present ...

Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films

Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous studies on flowing liquid films have neglected the contributions of spatial structures of waves to the impact outcome, while this has recently been shown to have a significant influence on the drop impact dynamics. In this report, we outline a step-by-step procedure to investigate the effect of periodic inlet forcing of a flowing liquid film leading to the production of spatiotemporally regular wave structures on drop impact dynamics. A function generator in connection with a solenoid valve is used to excite these spatiotemporally regular wave structures on the film surface while the impact dynamics of uniform-sized droplets are captured using a high-speed camera. Three distinct regions are then studied; viz. the capillary wave region preceding the large wave peak, the flat film region, and the wave hump region. The effects of important dimensionless quantities such as film Reynolds, drop Weber and Ohnesorge numbers parameterized by the film flow rate, drop speed, and drop size are also examined. Our results show interesting, hitherto undiscovered dynamics brought about by this application of film inlet forcing of the flowing film for both low and high inertia drops.

A protocol to study the contributions of waves to droplet impact dynamics on flowing liquid films is presented.

Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous ...

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 ...

Plasma-Assisted Molecular Beam Epitaxy Growth of Mg₃N₂ and Zn₃N₂ Thin Films

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