INDUSTRY MBE FOR NITRIDES Reactor portfolio


Our STE3N* series of MBE reactors that we released in 2003 combine an extremely high temperature on the substrate – it can be up to 1200°C with a high N/III ratio and an ammonia flow rate of up to 1000 sccm. The heating stage is highly reliable during a long growth run, aiding deposition of structures featuring thick AlN/AlGaN buffer layers and active layers with extremely low dislocation densities.


The STE3N* is available in either a two or three chamber format (the STE3N2 and STE3N3, respectively), with the latter equipped with a buffer preparation chamber.


The STE3N* systems can be configured to combine the ammonia injector with a nitrogen plasma source, which can be used in combination with ammonia MBE for growing active layers of InGaN, InAlN and magnesium-doped AlGaN. Both systems have specially designed indium, gallium and aluminium effusion cells for providing long-term life and growth rates of up to 2 µm/hr, when operating as ammonia MBE tools.


Other features of our STE3N* growth tools are the patented design of the substrate holder and the heating stage of the growth manipulator. This enables high heating uniformity, regardless of substrate material, and wafer diameters of up to 100 mm, allowing this tool to fulfil the requirements of engineers in R&D labs and those responsible for pilot production.


Our latest addition to the nitride MBE portfolio, the STE75, is also capable of the same growth conditions for high- temperature deposition of AlN buffer layers, but features more compact cryopanels for a limited number of samples per growth series.


Superior samples We have used our ammonia MBE tools to develop a complex, very special AlN/ AlGaN buffer layer that holds the key to the growth of extremely low dislocation density films on several types of mismatched substrates. Using c-plane sapphire, for example, we can form a buffer comprising 200-400 nm-thick AlN grown at a substrate temperature of 1100-1150 °C, followed by an AlGaN/AlN superlattice and Alx


Ga1-x N


(x=0.1-0.3) transition layers deposited at 900-920 °C.


According to scanning transmission electron microscope images, the dislocation density in these samples is 2-4 x 1010 4-6 x 109


cm-2 cm-2


cm-2 GaN 9 x 108


in the AlN buffer, falling to in the AlGaN layer grown


after the superlattice (see Figure 1). Meanwhile, dislocation density in the top GaN active layer is just 9-10 x 108


latter value is comparable to GaN grown on sapphire by MOCVD, and far lower than that associated with conventional MBE.


The improvements to material quality have produced a substantial increase in electron mobility, which reaches a maximum value of 600-650 cm2


V-1 s-1 . The AlN 2-4 x 1010 cm-2


Figure 1. An scanning tunnelling electron microscopy image of an AlN/SLS/AlGaN/GaN heterostructure reveals that the dislocation density in nitride films grown by MBE can rival those found in MOCVD-grown samples


in


a 1.5-µm-thick, lightly doped GaN top layer (silicon doping of 3-5 x 1016


cm-3 ).


This value is comparable to that of good quality, MOCVD-grown GaN, and in good agreement with calculations determining the relationship between mobility and dislocation density.


The GaN layer can be capped with an Alx


Ga1-x N barrier layer with variable


composition. By changing the value of x from 0.25 to 0.4, electron sheet density can be varied from 1.0-1.8 x 1013


cm-2


while mobility is adjusted from 1300-1700 cm2


V-1 s-1 , . This enables the


channel sheet resistance for the two- dimensional electron gas to be tuned between 230-400 Ohm/sq. Higher electron sheet densities are possible by replacing the AlGaN barrier with lattice- matched InAlN – this yields an electron sheet density of 2.3-2.5 x 1013 mobility of 1200-1300 cm2


V-1 s-1


cm-2 .


to the special cryopanels and pumping system design, the vacuum levels are almost unchanged when ammonia flow hits 1000 sccm! This attribute of our tool will make it very attractive for producing many device structures.


Several Russian research centres and companies have employed our MBE tools to grow DHFET epiwafers, which have been used to make prototypes of advanced high power microwave transistors. Testing of these devices reveals a high quality active region and absence of the current collapse effect, thanks to the adoption of a double heterostructure design that is significantly different from that of a conventional GaN/ AlGaN FET.


and a


Historically, only PAMBE could be used to grow the high-quality, lattice- matched InAlN layer, which requires an indium content of 18 percent. The lower temperature limit for ammonia MBE is determined by ammonia cracking efficiency, and this is negligible at temperatures lower than 500 °C.


Increase the temperature above this value and typical ammonia flow leads to very low indium content – it is insufficient for growing InAlN lattice-matched to GaN.


However, Speck’s group have shown recently that if the ammonia flow is increased to 1000 sccm, growth of InAlN that is lattice-matched to GaN is possible. Motivated by this result, we have tried to grow such structures by PAMBE and ammonia MBE, realising good results in both cases. It is worth noting that thanks


30 www.compoundsemiconductor.net October 2013


What is also very encouraging for us is that the high power transistors produced from wafers grown in our STE3N* MBE system are very robust, according to long-term ageing tests.


Our efforts have meant that within a relatively short space of time, we have come a long way from a small R&D company to being the sole Russian MBE manufacturer, competing successfully with world-leading companies. One of the biggest factors behind this success is our continuous work with partners and customers. We offer not only technical training, but enhanced process training, including development and implementation of client-oriented processes. Creating a strong relationship with our customers and the constant monitoring of their work allows us to continuously improve our equipment.


© 2013 Angel Business Communications. Permission required.


-1 x 109 AlGaN 4-6 x 109 cm-2 cm-2 SLS


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