AlGaInN Materials and Heterostructures

Molecular Beam Epitaxy Growth of GaN and Related Alloys :

Task leader: Chris Boney cboney@uh.edu

This research group focuses on thin film deposition of III-N materials by molecular beam epitaxy (MBE). The system consists of a cylindrical growth chamber with a gravity held 3" diameter sample holder. It uses effusion cells for Al, Ga, In, Mg, and Si material delivery and a rf-plasma source for generation of atomic nitrogen. The manipulator allows for x-y-z motion and rotation of the sample during growth. Reflection high-energy electron diffraction (RHEED) characterization is available for in-situ growth monitoring.

Based on our experience, control of GaN MBE growth is significantly more complicated than that of other "traditional" III-V semiconductors. This is due to higher growth temperatures (700-850°C), low surface mobility of group III elements, necessity of keeping a near-unity III/N flux ratio during the growth, and the unavoidable use of substrates with large lattice and thermal mismatches.

Our current studies are focused on the growth of device quality materials on sapphire and Si wafers. Projects include development of integrated UV-Vis chemical sensors, advanced particle detectors employing AlN and AlGaN, and field emission devices made from GaN, AlN, and AlGaN.

In situ control of the GaN -based materials growth by time of flight mass spectroscopy of recoiled ions :

Task Leader: Esther Kim EstherK@space.svec.uh.edu

Secondary ion mass spectroscopy (SIMS) is the most widely used and sensitive ex situ ion-based technique for material composition analysis. Unfortunately, in its present state SIMS does not allow real time thin film characterization under process conditions. On the other hand, time of flight mass spectroscopy of recoiled ions (TOF MSRI), a technique developed by Ionwerks, overcomes SIMS imitations through detection of high energy recoils in a glancing geometry. The advantages of TOF MSRI are: operation at pressures up to 1 mTorr, isotopic sensitivity to all elements including hydrogen, high sensitivity to nitrogen, and virtually non-destructive analysis nature.

The TOF MSRI technique has been successfully integrated in our laboratory with a GaN growth reactor. The reactor permits GaN deposition by both gas source molecular beam epitaxy (GSMBE) using ammonia as a nitrogen precursor and chemical beam epitaxy (CBE) using organometallics for group III precursors. The combination of time-of-flight low energy ion scattering (TOF ISS) and RHEED makes for a powerful and unique in-situ surface characterization tool. They have allowed us to study the influence of growth conditions on the thin film quality (crystallinity, surface roughness, surface structure, and true surface termination), impurity or elemental incorporation mechanisms and interlayer diffusion studies, and direct measurement of the surface III to V ratio.

Currently we are investigating selective area growth of III-N compounds on various buffer layers. Such studies are directed toward fundamental understanding of important issues such as: (I) epitaxial lateral overgrowth dependence on crystal structure, termination, and vicinal surfaces; (II) selective epitaxy for field emission tip arrays; and (III) new type of buffer layer formation.