Name: fabrication of schottky diodes on zn-polar bemgzno/zno heterostructure grown by plasma-assisted molecular beam epitaxy
Uploaded: 2018-10-23T13:00:00.000+00:00
Duration: 14 min 16 s
Description: Watch this Scientific Journal Video about Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy at JoVE.com

This work was supported by Air Force Office of Scientific Research (AFOSR) under Grant FA9550-12-1-0094.

1. Growth and Preparation of GaN Template for MBE Growth
<ol>
	<li>Growth of semi-insulating GaN template by MOCVD
	<ol>
		<li>Ensure that the gate valve isolating the load-lock chamber from the reactor is closed. Vent the load-lock with N2 to atmospheric pressure. Open the load-lock to take out the substrate holder.</li>
		<li>Load the 2-inch sapphire substrate on the holder and then place the holder back into the load lock. Pump down the load-lock pressure down to 2.5&#215;10-2 Torr by a mechanical dry pump.</li>
		<li>Vent the load-lock with N2 to equalize its pressure with that in the reactor chamber (15 Torr).</li>
		<li>Open the load-lock isolation gate valve and load the holder into the sample holder assembly in the reactor. Turn on the rotation motor and set the substrate-holder rotation speed of 100 rpm. Then close the gate valve.</li>
		<li>Ensure that the cooling water is running, both H2 and NH3 supply is sufficient, mass flow controller (MFC) and pressure controller (PC) readings are the same as the setpoints, and the substrate holder is rotating.</li>
		<li>Switch the system gas from N2 to H2. Turn on the in situ optical reflectance monitor.</li>
		<li>Initiate the growth by starting the recipe file, which preinstalls all the growth parameters including the ramping of substrate temperatures, gas flow rates, and reactor pressure, and switching of valves.</li>
		<li>Ramp up the reactor pressure to 30 Torr in 3 min and the substrate temperature to 1055 &#176;C to desorb residual contaminations from the substrate surface for 3 min in H2 environment.</li>
		<li>Ramp down the substrate temperature to 941&#160;&#176;C for the nucleation and growth of low-temperature (LT) AlN buffer layer.</li>
		<li>Stabilize the trimethylaluminum (TMAl) flow as 12&#160;standard cubic centimeter per minute (sccm) and ammonia (NH3) flow as 7 sccm and the substrate temperature for 3 min.</li>
		<li>Initiate the growth of the LT-AlN layer by switching TMAl from vent to the run line.</li>
		<li>Control the thickness of the LT-AlN layer by using a custom-built reflectivity measurement system, which includes a 637-nm laser diode, a p-i-n Si photodetector, and a Labview-based software. Determine the growth rate from the period of reflectivity oscillations19,20. In selected samples, use cross-sectional scanning electron microscopy and transmission electron microscopy images to confirm the accuracy of in situ monitoring.</li>
		<li>Keep the growth for 6 min to reach a thickness of ~20 nm, then ramp up the substrate temperature to 1100 &#176;C in 3 min without a growth interruption and continue the growth of AlN layer to a thickness of 300 nm, as monitored by the evolution of the oscillation of the in situ optical reflectance.</li>
		<li>Switch TMAl from run to vent and then to idle line to stop the AlN growth. Stabilize the trimethylgallium (TMGa) flow at 15.5&#160;sccm, ramp up the NH3 flow to 7000 sccm and stabilize for 1 min. Ramp up the reactor pressure to 76 Torr in 1 min. Ramp up the substrate temperature to 1107 &#176;C in 1 min.</li>
		<li>Nucleate and grow a GaN recovery layer with a thickness of ~400 nm, while monitoring the reflectivity evolution. Initially the reflectivity exhibits a sharp decrease when GaN islands nucleate on AlN surface and then recovers the intensity to the original level corresponding to an atomically flat surface when the islands coalesce.</li>
		<li>Ramp up the substrate to 1124&#160;&#176;C in 2 min. Grow the high-temperature semi-insulating GaN layer with a thickness of ~2.5 &#956;m. Stop the growth by switching TMGa from run to vent and then to idle line.</li>
		<li>Cool down the substrate to room temperature over a 40 min period.</li>
		<li>Ramp down the reactor pressure to 15 Torr over a 1.5 min period.</li>
		<li>Unload the substrate from the reactor by following the reverse procedure of steps 1.1.1-1.1.4.</li>
	</ol>
	</li>
	<li>GaN template preparation and loading to MBE reactor
	<ol>
		<li>Cut the 2-inch GaN template into 6 equal pie-shaped pieces by using a diamond scriber.</li>
		<li>Prepare aqua regia acid solution inside the acid fume hood by adding nitric acid (HNO3, 68.0-70.0 w/w%, 50 mL) to hydrochloric acid (HCl, 36.5-38.0 w/w%, 150 mL) slowly into a quartz beaker.</li>
		<li>Put the aqua regia beaker on a hot plate with a temperature of 220 &#176;C.</li>
		<li>After the appearance of an orange/red color and gas bubbles, soak one pie-shaped GaN template in the solution and boil it for 10 min.</li>
		<li>Rinse the GaN template in running de-ionized (DI) water for 3 min.</li>
		<li>Soak the GaN template in HCl (36.5-38.0 w/w%):H2O solution (1:1) for 3 min to remove Ga oxide.</li>
		<li>Rinse the GaN template in running DI water for 5 min.</li>
		<li>Dry the template with N2 gas.</li>
		<li>Put the cleaned GaN template on a Mo holder and load it into MBE load-lock chamber immediately.</li>
		<li>Start pumping down the load-lock by a mechanical dry pump.</li>
	</ol>
	</li>
</ol>
2. MBE Growth of BeMgZnO/ZnO Heterostructures
<ol>
	<li>Preparation of the effusion cells
	<ol>
		<li>After pumping down the load-lock for 1 h, start the preparation of the Zn, Mg, Be effusion cells. Set the upper zone temperature of the double-zone Zn cell to 525 &#176;C with a ramping rate of 17 &#176;C/min, wait for 5 min then ramp down to 515 &#176;C with a ramping rate of 5 &#176;C/min. Set the Mg cell temperature to 570 &#176;C with a ramping rate of 15 &#176;C/min, after reaching the set point, wait for 10 min then ramp the Mg cell down to 300 &#176;C. Set the Be cell temperature to 900 &#176;C with a ramping rate of 10 &#176;C/min, after reaching the set point, wait for 3 min then ramp the cell down to 650 &#176;C.</li>
		<li>After 30 min, set the lower zone temperature of the double-zone Zn cell to 360 &#176;C with a ramping rate of 10 &#176;C/min.</li>
		<li>After pumping down the load-lock for 2 h to reach a pressure of ~5&#215;10-7 Torr, turn on the reflection high-energy electron diffraction (RHEED) system, and load the GaN template into the MBE chamber.</li>
		<li>Tune the angle of the GaN template by rotating the manipulator to monitor the RHEED pattern evolution along [1-100] azimuthal direction.</li>
		<li>Set the lower zone temperature of the double-zone Zn cell to 355 &#176;C with a ramping rate of 10 &#176;C/min.</li>
	</ol>
	</li>
	<li>Polarity control of ZnO on GaN and growth of LT-ZnO buffer
	<ol>
		<li>Ramp up the substrate temperature to 615 &#176;C with a ramping rate of 13.6 &#176;C/min to desorb the residual contaminations from the substrate surface for 15 min.</li>
		<li>Ramp down the substrate temperature from 615 to 280 &#176;C with a ramping rate of 13.6 &#176;C/min for the growth of LT-ZnO. When the temperature reaches 550 &#176;C, open the Zn cell shutter to expose the GaN template surface with Zn flux. Turn on O2 plasma power supply, set the power as 100 W, and check to verify that O2 gas line is closed.</li>
		<li>When the temperature reaches 280 &#176;C, set the O2 plasma power to 400 W, set the O2 flow rate to 0.3 sccm to ignite the plasma then decrease the O2 flow rate to 0.25 sccm.</li>
		<li>Wait for 1 min, then open the O2 shutter to start the growth of LT-ZnO buffer layer.</li>
		<li>Record a RHEED pattern every 5 min. After growing for approximately 15 min corresponding to a buffer thickness of ~20 nm, when the RHEED pattern changes from stripes (2D mode) to elliptical spots (3D mode), close both Zn and O2 shutters to stop the growth.</li>
		<li>Set the O2 flow rate to 0.4 sccm, and set the substrate temperature to 730 &#176;C with a ramping rate of 13.6 &#176;C/min to anneal the LT-ZnO buffer layer. Set the lower zone temperature of the double-zone Zn cell to 345 &#176;C with a ramping rate of 10 &#176;C/min for the growth of HT-ZnO layer.</li>
		<li>When the substrate temperature reaches the set-point of 730 &#176;C, wait for 5 min and check the ZnO surface by RHEED. When the RHEED pattern transits from 3D to 2D, stop annealing by ramping down the substrate temperature to 700&#160;&#176;C.</li>
	</ol>
	</li>
	<li>Growth of high-temperature ZnO layer
	<ol>
		<li>When the substrate temperature reaches 700&#160;&#176;C and stabilizes, increase the O2 flow rate to 3.2 sccm.</li>
		<li>Start the growth of HT-ZnO layer by opening both Zn and O2 shutters concurrently.</li>
		<li>Grow the HT-ZnO layer for ~140 min to reach a thickness of ~300 nm. Record the RHEED patterns several times during the growth to confirm the 2D growth mode.</li>
		<li>Stop the growth of HT-ZnO layer by closing both Zn and O2 shutters concurrently.</li>
	</ol>
	</li>
	<li>Growth of BeMgZnO barrier
	<ol>
		<li>Set the O2 flow rate to 0.3 sccm, set the Be cell temperature to 820 &#176;C with a ramping rate of 10 &#176;C/min, set the Mg cell temperature to 510 &#176;C with a ramping rate of 15 &#176;C/min, and set the substrate temperature to 325 &#176;C with a ramping rate of 13.6 &#176;C/min for the growth of the BeMgZnO barrier.</li>
		<li>When the substrate temperature stabilizes, increase the O2 flow rate to 1.25 sccm, and start the growth by concurrent opening the Zn, Mg, Be, and O2 shutters.</li>
		<li>Grow the BeMgZnO barrier layer for ~12 min to reach a thickness of ~30 nm. Record RHEED patterns several times during the growth to monitor the growth mode evolution.</li>
		<li>Stop the growth of BeMgZnO layer by closing the Mg and Be shutter, while keeping the Zn and O2 shutter open for 1 min to have a ~2 nm thick ZnO cap layer.</li>
		<li>Finish the growth by closing both Zn and O2 shutters.</li>
		<li>Ramp down the substrate temperature to its standby temperature 150 &#176;C. Decrease the O2 flow rate to 0.25 sccm.</li>
		<li>When the substrate temperature is below 250 &#176;C, decrease the O2 plasma power to 100 W, turn off O2 plasma power supply, decrease the O2 flow rate to 0, close the O2 gas line, and cool down the cell temperatures to standby conditions.</li>
		<li>Wait for the substrate temperature to reach the standby temperature 150 &#176;C, open the growth chamber gate valve and unload the wafer holder to the load-lock chamber.</li>
		<li>Vent the load-lock chamber with N2 gas and take out the sample.</li>
	</ol>
	</li>
</ol>
3. Characterizations
<ol>
	<li>Roughly measure the thickness of the sample by using a step-profiler, through the covered area on the edge of the sample.</li>
	<li>Evaluate the thickness, strain and structural quality of the heterostructure by using high resolution X-Ray diffraction (HRXRD) (2q-w scans of (0002) reflection).</li>
	<li>Cut the sample into 5&#215;5 mm2 square piece by using a diamond scriber. Investigate the electronic properties of the sample by using temperature dependent Hall Effect measurements in the van der Pauw geometry, with indium (In) dots as contact electrodes.</li>
	<li>Check the surface morphology by using atomic force microscopy (AFM).</li>
</ol>
4. Fabrication of Schottky Diodes
<ol>
	<li>Fabrication of Ohmic contacts on BeMgZnO/ZnO heterostructures
	<ol>
		<li>Degrease the sample (~20&#215;20 mm2 in size) with acetone in an ultrasonic cleaner for 5 min, followed by cleaning with methanol in the ultrasonic cleaner for 5 min, rinsing in DI water for 5 min, and blowing dry with N2.</li>
		<li>Spin coat photoresist with 1000 rpm for 3 s and then 3000 rpm for 30 s.</li>
		<li>Soft bake the photoresist at 100 &#176;C for 140 s.</li>
		<li>Expose with UV light through the Ohmic contact mask at 6.5 mW UV lamp power for 2.38 min on photolithography mask aligner.</li>
		<li>Post bake the photoresist at 110 &#176;C for 80 s.</li>
		<li>Develop in a developer for 60 s with a shaking frequency of 1/s.</li>
		<li>Rinse in DI water for 3 min, and blow dry with N2.</li>
		<li>Load the sample into the electron beam evaporator.</li>
		<li>Without heating the sample, deposit Ti/Au with a thickness of 30/50 nm, as measured by quartz crystal thickness monitor.</li>
		<li>Lift-off in acetone, followed by cleaning in methanol for 5 min, rinsing in DI water for 5 min, and blowing dry with N2.</li>
		<li>Anneal the contact by rapid thermal annealer (RTA) at 300 &#176;C for 30 s.</li>
		<li>Check contact resistance through transition line model (TLM) measurement21.</li>
	</ol>
	</li>
	<li>Fabrication of Schottky contacts on BeMgZnO/ZnO heterostructures
	<ol>
		<li>Follow steps 4.1.1-4.1.7 for photolithography of Schottky contact.</li>
		<li>Treat the sample surface with remote O2 plasma for 5 min with an O2 flow of 35 sccm and an RF power of 50 W.</li>
		<li>Follow steps 4.1.8-4.1.10 for the deposition of Ag with a thickness of 50 nm.</li>
		<li>Characterize the obtained structure by I-V measurements for the Schottky diodes17.</li>
	</ol>
	</li>
</ol>

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R. C., Kumar, K. J., Subrahmanyam, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Physical+properties+of+silver+oxide+thin+films+by+pulsed+laser+deposition:+effect+of+oxygen+pressure+during+growth.">Physical properties of silver oxide thin films by pulsed laser deposition: effect of oxygen pressure during growth.</a> Journal of Physics D: Applied Physics. 42 (13), 135411 (2009).</li></ol>

The authors have nothing to disclose.

Incorporation of BeO into MgZnO to form the quaternary BeMgZnO provides the feasibility to tune the extent and sign of strain in the quaternary and hence significantly increases the 2DEG density8. The representative results show that the Be0.02Mg0.26ZnO/ZnO heterostructure results in a 2DEG density close to the desired plasmon-LO phonon resonance electron density (~7&#215;1012 cm-2)24. Although the electron mobility of the heterostructure strongly depends on the MBE growth parameters such as the substrate temperature and VI/II ratio of both the HT-ZnO and the BeMgZnO barrier layer, the 2DEG density is weakly dependent on the growth conditions and mainly determined by the Be and Mg content in the barrier.
A GaN template is used for the growth of BeMgZnO/ZnO heterostructures with high crystalline quality owing to the moderate lattice mismatch of 1.8% between GaN and ZnO, compared with a large lattice mismatch of 18% between sapphire and ZnO. To avoid any conductive parallel channel, it is critical to have a high resistance in the M&#937;/square range for the GaN template. In our case, this is achieved by growing at a low chamber pressure of 76 Torr to enhance carbon compensation. To ensure the polarity control in the BeMgZnO/ZnO heterostructures (Zn-polarity), careful surface treatment of GaN template is indispensable. Any oxidation or contamination introduced during the preparation on the GaN surface would induce Zn- and O- mix-polarity in the heterostructures even the determinant VI/II ratio &#60;1.5 is fulfilled.
Any chemical reaction between the metal and the semiconductor, the presence of surface contaminants, states, defects in the vicinity of the surface, and the diffusion of metal into the semiconductor are common problems in the field of the fabrication of Schottky contacts. A variety of methods has been reported in the literature for preparing the surface of ZnO for Schottky contact fabrication. Among them are etching in HCl (or other acids), physical etching with Ar+, UV Ozone cleaning, treatment in H2O2, and O2 plasma (or mixture with He)25,26,27,28. The etching procedures aim for the removal of a surface layer with thickness a ranging from a few nanometers to microns and therefore cannot be applied for HFET devices. The UV-Ozone cleaning or O2 plasma procedure removes only the surface layer. Therefore, it is well suited for the surface preparation of our BeMgZnO/ZnO heterostructures.
Usually Schottky contacts are achieved by depositing a high work function metal such as Pd, Pt, Ir, etc. In contrast, Ag has a low work function of 4.26 eV. Despite that, devices utilizing Ag electrode can show rectifying behavior owing to the formation of an interface silver oxide layer caused by partial oxidation of Ag with oxygen from ZnO matrix. The so formed oxide layer is transparent for electrons and has higher work function compared to Ag. Raju et al. have reported work functions around 5.5 eV for AgO grown by pulsed laser deposition (PLD), which is 1.3 eV higher than that of Ag, and close to the characteristic of Pd, Pt, and Ir29. Our results indicate that that Ag electrode (with O2 plasma pretreatment on the surface of ZnO heterostructure) is a promising contact metal for the formation of Schottky diodes.
We have demonstrated a method for fabricating high quality Schottky contacts for ZnO-based HFETs. MOCVD grown GaN template with careful surface preparation just prior to MBE growth and a low VI/II ratio &#60;1.5 during ZnO nucleation ensure the Zn-polar orientation of the ZnO-based heterostructures with high quality. MOCVD is a widely-used mature technique for epitaxy of GaN for various applications. The MBE procedure described in this work indicates the combinability of MOCVD and MBE techniques, and GaN and oxide semiconductors for electronic devices. Incorporation of a small amount of Be into the BeMgZnO barrier layer results in HFETs with high 2DEG density, high electron mobility, and high thermal stability, for enhanced high speed performance.

Heterostructure field effect transistors (HFETs) based on two dimensional electron gas (2DEG) have a promising potential for the applications in high speed electronic devices1,2,3. Zinc oxide (ZnO) as a wide bandgap (3.4 eV) semiconductor with high electron saturation velocity has gained considerable attention as a platform for HFETs4,5. Conventionally used barrier material MgZnO ternary necessitate a very high Mg content (&#62;40%) grown at low substrate temperatures (300 &#176;C or lower)6,7, and as such these structures are apt to degrade under high power operations and during thermal treatments, even if the unwanted charge density in the barrier is low enough for gate modulation. To circumvent this obstacle, we have proposed and adopted BeMgZnO as the barrier, in which the strain sign in the barrier can be switched from compressive to tensile via the incorporation of beryllium (Be), making the spontaneous and piezoelectricpolarizations to be additive. As a result, high 2DEG concentration can be achieved with relatively moderate Mg content. Utilizing this approach, high 2DEG densities is observed near the plasmon-LO phonon resonance (~7&#215;1012 cm-2) in BeMgZnO/ZnO heterostructures while the Mg content below is 30% and the Be content is only at 2~3%8.
Due to its similar crystal symmetry, UV and visible light transparency, robust physical and chemical properties, and low cost, c-plane sapphire is widely employed for epitaxy of both GaN and ZnO. Thanks to the remarkable progress achieved in the growth technology of GaN-based electronic and optoelectronic devices on saphhire, high quality GaN templates can be easily produced on sapphire substrates by using AlN or low-temperature (LT) GaN buffer, despite its large lattice mismatch of 16% with sapphire9. Epitaxial growth of ZnO, which has an even larger in-plane lattice mismatch of 18% with sapphire, is relatively well understood for O-polar variety, while the growth of Zn-polar material in two-dimensional mode is not well established. Due to the moderate lattice mismatch of 1.8%, epitaxy of ZnO on GaN is an attractive alternative.
Both MOCVD and MBE are the most successful semiconductor deposition techniques for fabricating high-quality thin films and heterostructures with high reproducibility. The main reason that MBE is less popular than MOCVD for epitaxy of GaN is the cost and inadequacy for mass production. The growth rate in GaN by MOCVD can be several micrometers per hour, and tens of 2 inch (50 mm) diameter wafers or those as large as 6-8&#34; can be grown in one run9. Here, we also adopt MOCVD for the growth of GaN in our study. For the growth of ZnO-based heterostructures, however, more reports on the formation of 2DEG are realized by MBE at the present time prior to the commercialization of the potential applications10,11,12. Recently, we have developed MBE growth of high quality ZnO heterostructures with an accurate control of surface polarity on Ga-polar GaN templates13. It was found that with Zn pre-exposure treatment, ZnO layers so grown exhibited Zn-polarity when nucleated with low VI/II ratios (&#60;1.5), while those nucleated with VI/II ratios above 1.5 exhibited O-polarity. To avoid parallel conduction channel through GaN templates, we adopted carbon compensated semi-insulating GaN MOCVD grown under low-pressure conditions on AlN buffer for the subsequent growth of ZnO-based HFET structures.
Prior to our work14, there has been no reports on the investigation of Schottky diodes on BeMgZnO/ZnO heterostructures. Only several studies have reported on Schottky contacts to MgZnO15,16, e.g., with an ideality factor of 2.37, a barrier height of 0.73 eV, and a rectification ratio of only 103 15. Various Schottky metals have been used for ZnO17, and among them, silver (Ag) has been widely adopted, due to a relatively high Schottky barrier height of 1.11 eV on bulk ZnO with an ideality factor of 1.08 18.
In this work, we aim to fabricate high-quality Schottky diodes for the applications in ZnO-based high-speed HFET devices. The following protocol applies specifically to the fabrication of Ag/BeMgZnO/ZnO Schottky diodes by e-beam evaporation of Ag on the BeMgZnO/ZnO heterostructures grown by plasma-assisted MBE on MOCVD-deposited GaN templates.

<table><tbody><tr><td>MOCVD</td><td>Emcore</td><td></td><td>custom-built</td></tr><tr><td>MBE</td><td>SVT Associates</td><td></td><td></td></tr><tr><td>TMAl</td><td>SAFC</td><td>CAS: 75-24-1</td><td></td></tr><tr><td>TMGa</td><td>SAFC</td><td>CAS: 1445-79-0</td><td></td></tr><tr><td>NH3</td><td>The Linde group</td><td>CAS: 7664-41-7</td><td></td></tr><tr><td>H2</td><td>National Welders Supply Co.</td><td>supplier part no. 335-041</td><td>Grade 5.0</td></tr><tr><td>O2</td><td>National Welders Supply Co.</td><td>supplier part no. OX 300</td><td>Industrial Grade Oxygen, Size 300 Cylinder, CGA-540</td></tr><tr><td>Mg</td><td>Sigma-Aldrich</td><td>Product No.: 474754-25G</td><td>MAGNESIUM, DISTILLED, DENDRITIC PIECES, 99.998% METALS BASIS</td></tr><tr><td>Be</td><td>ESPI Metals</td><td>Stock No. K646b</td><td>Beryllium pieces, 3N</td></tr><tr><td>Zn</td><td>Alfa Aesar, Thermo Fisher Scientific Chemicals Inc.</td><td>Product No.: 10760-30</td><td>Zinc shot, 1-6mm (0.04-0.24in), Puratronic, 99.9999%</td></tr><tr><td>Au</td><td>Kurt J. Lesker</td><td>part no. EVMAUXX40G</td><td>Gold Pellets, 99.99%</td></tr><tr><td>Ag</td><td>Kurt J. Lesker</td><td>part no. EVMAG40QXQ</td><td>Silver Pellets, 99.99%</td></tr><tr><td>Ti</td><td>Kurt J. Lesker</td><td>part no. EVMTI45QXQ</td><td>Titanium Pellets, 99.995%</td></tr><tr><td>Developer</td><td>Rohm and Haas electronic Materials LLC</td><td>MF-CD-26</td><td>Material number 10018050</td></tr><tr><td>Photoresist</td><td>Rohm and Haas electronic Materials LLC</td><td>SPR 955</td><td>Material number 10018283</td></tr></tbody></table>

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.

The left column of Figure 1 shows the evolution of RHEED pattern recorded along the [1-100] azimuthal direction during MBE growth of a Be0.02Mg0.26ZnO/ZnO heterostructure with a 300 nm thick HT-ZnO layer and a 30 nm thick Be0.02Mg0.26ZnO barrier. The right column shows representative surface morphologies at different growth stages (not from the same sample). As evidenced from the appearance of a spotty RHEED pattern, the LT-ZnO buffer layer is of three-dimensional (3D) island growth mode nature. Its surface morphology was improved by thermal annealing treatment at a temperature above 700 &#176;C. It is clearly seen that the surface transformed from 3D to 2D morphology. The subsequent HT-ZnO layer continues to grow in a 2D mode, followed by the 2D growth of Be0.02Mg0.26ZnO layer without the formation of a second phase. AFM measurements have shown that the GaN template has a root mean square (RMS) roughness of 0.28 nm for 5&#215;5 &#956;m2 scan. A smooth surface with an RMS roughness of 0.35 nm is obtained for the HT-ZnO layer without a barrier by growing under O-rich condition and an RMS roughness of 0.45 nm is observed after the growth of BeMgZnO barrier.
HRXRD triple-axis 2&#952;-&#969; scan for a typical Zn-polar Be0.02Mg0.26ZnO/ZnO heterostructure with a 300 nm thick HT-ZnO layer, and a 50 nm thick Be0.02Mg0.26ZnO barrier layer is shown in Figure 2. The reflections at 34.46 o, 34.54 o, and 34.75 o are consistent with (0002) reflections of ZnO, GaN, and Be0.02Mg0.26ZnO, respectively. Note that the broadening of the reflection from Be0.02Mg0.26ZnO is due to its thinness. The tensile biaxial strain in the ZnO layer is an indication of the Zn-polar heterostructure, as investigated in our previous study13. Be and Mg contents in the BeMgZnO quaternary were calculated from the Bragg angle of its XRD (0002) reflection and emission photon energy in LT-photoluminescence (LT-PL) spectrum measured at 13 K (not shown).
Figure 3 shows the results of temperature-dependent Hall Effect measurements for a Be0.02Mg0.26ZnO/ZnO heterostructure. The sheet carrier concentration reduced from 8.8&#215;1012 cm-2 to 6.4&#215;1012 cm-2 when the sample was cooled down from room temperature (293 K) to approximately 100 K. With further cooling to 13 K, the sheet carrier concentration saturates at 6.2&#215;1012 cm-2. This finding manifests that the observed reduction in electron concentration is originated from the contributions from parallel conduction channels which include defective nucleation layer and HT-ZnO layer as well as the Be0.02Mg0.26ZnO barrier, if any. This trend has also been reported for MgZnO/ZnO heterostructures10,22. The electron mobility in the Be0.02Mg0.26ZnO/ZnO heterostructure monotonically increases with decreasing temperature; both the 293 K mobility of 206 cm2/Vs and the 13 K mobility of 1550 cm2/Vs are comparable to the values in the literature22,23. The evolution of the electronic properties as a function of temperature clearly indicates the presence of 2DEG at the Be0.02Mg0.26ZnO/ZnO heterointerface.
Figure 4 shows the current-voltage (I-V) curves measured at room temperature for four representative Ag/Be0.02Mg0.26ZnO/ZnO Schottky diodes with a Schottky area of 1.1&#215;10-4 cm2 within one wafer. The forward currents increase exponentially with applied voltage up to 0.25 V, beyond which the voltage drops across the series resistance become apparent. The highest Schottky barrier height of &#934;ap of 1.07 eV was attained with an ideality factor n of 1.22. Rectification ratios of about 1&#215;108 are achieved by using the current values measured at V=&#177;2 V.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58113/58113fig1.jpg" /> 
Figure 1. Surface characterization. Left column shows the RHEED patterns taken along the [1-100] azimuthal direction during MBE growth of a Be0.02Mg0.26ZnO/ZnO heterostructure, and right column presents the surface morphologies of the GaN template, HT-ZnO layer, and Be0.02Mg0.26ZnO layer measured by AFM. LT-ZnO buffer technology enables the 2D-mode growth of high-quality ZnO heterostructures on low lattice-mismatched GaN templates. <a href="https://www.jove.com/files/ftp_upload/58113/58113fig1large.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/58113/58113fig2.jpg" /> 
Figure 2. HRXRD of the heterostructure. HRXRD triple-axis 2&#952;-&#969; scan of a typical Zn-polar Be0.02Mg0.26ZnO/ZnO heterostructure with a 50 nm thick Be0.02Mg0.26ZnO barrier layer . The reflections at 34.46 o, 34.54 o, and 34.75 o are consistent with (0002) reflections of ZnO, GaN, and Be0.02Mg0.26ZnO, respectively. <a href="https://www.jove.com/files/ftp_upload/58113/58113fig2large.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/58113/58113fig3.jpg" /> 
Figure 3. Electronic Properties of the heterostructure. Temperature dependences of sheet carrier density and electron mobility of a Zn-polar Be0.02Mg0.26ZnO/ZnO heterostructure. <a href="https://www.jove.com/files/ftp_upload/58113/58113fig3large.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/58113/58113fig4.jpg" /> 
Figure 4. Schottky diodes. Typical I-V characteristics of four representative Ag/Be0.02Mg0.26ZnO/ZnO Schottky diodes measured at room-temperature. The similarity of the four I-V curves indicates the high in-wafer uniformity of the sample. <a href="https://www.jove.com/files/ftp_upload/58113/58113fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy at JoVE.com

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

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 ...

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7.6K Views.  Virginia Commonwealth University. 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.

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

Fabrication of Spatially Confined Complex Oxides

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from the inherent strong electron correlations that reside within them. These materials can also possess electronic phase separation in which regions of vastly different resistive and magnetic behavior can coexist within a single crystal alloy material. By reducing the scale of these materials to length scales at and below the inherent size of the electronic domains, novel behaviors can be exposed. Because of this and the fact that spin-charge-lattice-orbital order parameters each involve correlation lengths, spatially reducing these materials for transport measurements is a critical step in understanding the fundamental physics that drives complex behaviors. These materials also offer great potential to become the next generation of electronic devices 1-3. Thus, the fabrication of low dimensional nano- or micro-structures is extremely important to achieve new functionality. This involves multiple controllable processes from high quality thin film growth to accurate electronic property characterization. Here, we present fabrication protocols of high quality microstructures for complex oxide manganite devices. Detailed descriptions and required equipment of thin film growth, photo-lithography, and wire-bonding are presented.

We describe the use of pulsed laser deposition (PLD), photolithography and wire-bonding techniques to create micrometer scale complex oxides devices. The PLD is utilized to grow epitaxial thin films. Photolithography and wire-bonding techniques are introduced to create practical devices for measurement purposes.

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Synthesis and Characterization of High c-axis ZnO Thin Film by Plasma Enhanced Chemical Vapor Deposition System and its UV Photodetector Application

In this study, zinc oxide (ZnO) thin films with high c-axis (0002) preferential orientation have been successfully and effectively synthesized onto silicon (Si) substrates via different synthesized temperatures by using plasma enhanced chemical vapor deposition (PECVD) system. The effects of different synthesized temperatures on the crystal structure, surface morphologies and optical properties have been investigated. The X-ray diffraction (XRD) patterns indicated that the intensity of (0002) diffraction peak became stronger with increasing synthesized temperature until 400 oC. The diffraction intensity of (0002) peak gradually became weaker accompanying with appearance of (10-10) diffraction peak as the synthesized temperature up to excess of 400 oC. The RT photoluminescence (PL) spectra exhibited a strong near-band-edge (NBE) emission observed at around 375 nm and a negligible deep-level (DL) emission located at around 575 nm under high c-axis ZnO thin films. Field emission scanning electron microscopy (FE-SEM) images revealed the homogeneous surface and with small grain size distribution. The ZnO thin films have also been synthesized onto glass substrates under the same parameters for measuring the transmittance.
For the purpose of ultraviolet (UV) photodetector application, the interdigitated platinum (Pt) thin film (thickness ~100 nm) fabricated via conventional optical lithography process and radio frequency (RF) magnetron sputtering. In order to reach Ohmic contact, the device was annealed in argon circumstances at 450 oC by rapid thermal annealing (RTA) system for 10 min. After the systematic measurements, the current-voltage (I-V) curve of photo and dark current and time-dependent photocurrent response results exhibited a good responsivity and reliability, indicating that the high c-axis ZnO thin film is a suitable sensing layer for UV photodetector application.

We offered a method to directly synthesize high c-axis (0002) ZnO thin film by plasma enhanced chemical vapor deposition. The as-synthesized ZnO thin film combined with Pt interdigitated electrode was used as sensing layer for ultraviolet photodetector, showing a high performance through a combination of its good responsivity and reliability.

In this study, zinc oxide (ZnO) thin films with high c-axis (0002) preferential orientation have been successfully and effectively synthesized onto ...

Preparation of ZnO Nanorod/Graphene/ZnO Nanorod Epitaxial Double Heterostructure for Piezoelectrical Nanogenerator by Using Preheating Hydrothermal

Well-aligned ZnO nanostructures have been intensively studied over the last decade for remarkable physical properties and enormous applications. Here, we describe a one-step fabrication technique to synthesis freestanding ZnO nanorod/graphene/ZnO nanorod double heterostructure. The preparation of the double heterostructure is performed by using thermal chemical vapor deposition (CVD) and preheating hydrothermal technique. In addition, the morphological properties were characterized by using the scanning electron microscopy (SEM). The utility of freestanding double heterostructure is demonstrated by fabricating the piezoelectric nanogenerator. The electrical output is improved up to 200% compared to that of a single heterostructure owing to the coupling effect of the piezoelectricity between the arrays of ZnO nanorods on the top and bottom of graphene. This unique double heterostructure have a tremendous potential for applications of electrical and optoelectrical devices where the high number density and specific surface area of nanorod are needed, such as pressure sensor, immuno-biosensor and dye-sensitized solar cells.

One-step fabrication method for obtaining freestanding epitaxial double heterostructure is presented. This approach could achieve ZnO coverage with a higher number density than that of the epitaxial single heterostructure, leading to a piezoelectric nanogenerator with an increased output electrical performance.

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Epitaxial Growth of Perovskite Strontium Titanate on Germanium via Atomic Layer Deposition

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and conformality by taking advantage of self-limiting reactions between vapor-phase precursors and the growing film. Perovskite oxides present potential for next-generation electronic materials, but to-date have mostly been deposited by physical methods. This work outlines a method for depositing SrTiO3 (STO) on germanium using ALD. Germanium has higher carrier mobilities than silicon and therefore offers an alternative semiconductor material with faster device operation. This method takes advantage of the instability of germanium's native oxide by using thermal deoxidation to clean and reconstruct the Ge (001) surface to the 2&#215;1 structure. 2-nm thick, amorphous STO is then deposited by ALD. The STO film is annealed under ultra-high vacuum and crystallizes on the reconstructed Ge surface. Reflection high-energy electron diffraction (RHEED) is used during this annealing step to monitor the STO crystallization. The thin, crystalline layer of STO acts as a template for subsequent growth of STO that is crystalline as-grown, as confirmed by RHEED. In situ X-ray photoelectron spectroscopy is used to verify film stoichiometry before and after the annealing step, as well as after subsequent STO growth. This procedure provides framework for additional perovskite oxides to be deposited on semiconductors via chemical methods in addition to the integration of more sophisticated heterostructures already achievable by physical methods.

This work details the procedures for the growth and characterization of crystalline SrTiO3 directly on germanium substrates by atomic layer deposition. The procedure illustrates the ability of an all-chemical growth method to integrate oxides monolithically onto semiconductors for metal-oxide semiconductor devices.

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and ...

Chemistry

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

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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 ...

Aerosol-assisted Chemical Vapor Deposition of Metal Oxide Structures: Zinc Oxide Rods

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, their high cost production and/or incompatibility with microfabrication technologies, due to the use of pre-deposited catalyst-seeds and/or high processing temperatures exceeding 900 &#176;C, represent a drawback for a widespread use of these methods. Here, however, we report the synthesis of ZnO rods via a non-catalyzed vapor-solid mechanism enabled by using an aerosol-assisted chemical vapor deposition (CVD) method at 400 &#176;C with zinc chloride (ZnCl2) as the precursor and ethanol as the carrier solvent. This method provides both single-step formation of ZnO rods and the possibility of their direct integration with various substrate types, including silicon, silicon-based micromachined platforms, quartz, or high heat resistant polymers. This potentially facilitates the use of this method at a large-scale, due to its compatibility with state-of-the-art microfabrication processes for device manufacture. This report also describes the properties of these structures (e.g., morphology, crystalline phase, optical band gap, chemical composition, electrical resistance) and validates its gas sensing functionality towards carbon monoxide.

Columnar zinc oxide structures in the form of rods are synthesized via aerosol-assisted chemical vapor deposition without the use of pre-deposited catalyst-seed particles. This method is scalable and compatible with various substrates based on either silicon, quartz or polymers.

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, ...

A Fabrication and Measurement Method for a Flexible Ferroelectric Element Based on Van Der Waals Heteroepitaxy

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on high-density data storage and low-power consumption capabilities. However, the high-quality oxide based nonvolatile memory on flexible substrates is often constrained by the material characteristics and the inevitable high-temperature fabrication process. In this paper, a protocol is proposed to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica. The versatile deposition technique and measurement method enable the fabrication of flexible yet single-crystalline non-volatile memory elements necessary for the next generation of smart devices.

In this paper, we present a protocol to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica.

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on ...

A Standard and Reliable Method to Fabricate Two-Dimensional Nanoelectronics

Two-dimensional (2D) materials have attracted huge attention due to their unique properties and potential applications. Since wafer scale synthesis of 2D materials is still in nascent stages, scientists cannot fully rely on traditional semiconductor techniques for related research. Delicate processes from locating the materials to electrode definition need to be well controlled. In this article, a universal fabrication protocol required in manufacturing nanoscale electronics, such as 2D quasi-heterojunction bipolar transistors (Q-HBT), and 2D back-gated transistors are demonstrated. This protocol includes the determination of material position, electron beam lithography (EBL), metal electrode definition, et al. A step by step narrative of the fabrication procedures for these devices are also presented. Furthermore, results show that each of the fabricated devices has achieved high performance with high repeatability. This work reveals a comprehensive description of process flow for preparing 2D nano-electronics, enables the research groups to access this information, and pave the way toward future electronics.

The article aims to introduce a standard and reliable fabrication procedure for the development of future low dimensional nanoelectronics.

Two-dimensional (2D) materials have attracted huge attention due to their unique properties and potential applications. Since wafer scale synthesis of 2D ...

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.

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.

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 ...

Schottky Barrier Diode

Schottky barrier diodes are specialized semiconductor devices characterized by their unique construction. This construction involves combining a metal layer with a moderately doped n-type semiconductor material. This combination leads to the formation of a Schottky barrier, a pivotal element that defines the diode&#39;s operational characteristics. The core functionality of Schottky barrier diodes is their capacity to allow current to flow in only one direction due to their distinctive metal-semiconductor junction construction.
A charge transfer occurs when the metal comes into contact with a semiconductor. This process aligns the Fermi levels of the two materials, which, depending on the initial Fermi level of the semiconductor relative to the metal, results in the formation of a depletion region around the contact area. In n-type semiconductors, this depletion region is populated with positive charges from donor ions that are uncompensated, whereas, in p-type semiconductors, the depletion region contains negative charges, thus ensuring the diode&#39;s directionality in current flow.
The Schottky barrier diode distinguishes itself from conventional pn-junction diodes through its operational speed and efficiency. Since it operates primarily through majority carriers and lacks minority-carrier charge-storage effects, it is significantly faster, making it ideal for switching applications. Additionally, the forward voltage drop across a Schottky diode is lower than pn-junction diodes, which leads to reduced power dissipation during operation.
With these characteristics, Schottky diodes are extensively utilized in applications requiring fast switching and low power loss, such as voltage clamping and transient protection circuits. Their rapid response and low forward voltage make them an indispensable component in modern electronic circuit design, especially where efficiency and speed are critical parameters.

Schottky barrier diodes are semiconductor devices formed by joining a metal layer with a moderately doped n-type semiconductor material.
When a metal ...

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

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