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In situ Fermi level control of point defects in wide bandgap semiconductors

In situ Fermi level control of point defects in wide bandgap semiconductors
Date:15.10.2014
Affiliation:North Carolina State University, Material Science and Engineering Department, Raleigh 27695, NC, USA
Inviting person:Tilo Baumbach, KIT-ANKA, IPS, LAS
Speaker:Marc Patrick Hoffmann
Time:11:00 a.m.

Abstract:

Charged point defects in compound semiconductor thin films strongly determine their electronic and optical properties. In case of high doping of wide band gap semiconductors, low conductivities can be observed due to (self-)compensation by native defects and impurities.To have control of incorporationof unwanteddefectsduring the growth of thin films is highly desirable.We developeda novel in situ scheme to control point defects in wide band gap semiconductorsduring the growth. It uses above band gap UV-illumination to change the position of the quasi Fermi levels in a semiconductor.Thus, the formation energy of compensating defects increases leading to a decrease in its incorporation.

Using GaN and AlGaN as a model system it will be demonstrated that a significant improvement of the electrical properties can be achieved in wide band gap materials.In n- and p-type doped AlGaN films grown by MOCVD, point defects such as hydrogen, carbon, nitrogen or gallium vacancies and their corresponding complexes lead to dopant compensation, resulting in a high resistivity and a low mobility of these films. Here, results from p-type GaN:Mg and n-type GaN:Si and AlGaN:Si are discussed in detail. Samples were characterized using SIMS, photoluminescence, Hall measurements, XRD and TEM. In p-type GaN:Mg the passivation and self-compensation of Mg acceptors by H and VN can be significantly reduced by above-bandgap illumination. The H concentration is reduced by an order of magnitude and no post growth annealing is needed to activate the samples when grown with UV-light. In addition, UV-growth leads to lower resistivity values and a decrease in defect photoluminescence intensity in heavily Mg-doped GaN. In Si doped (n-type) GaN and AlGaN, UV-illumination during the growth, reduces the Si donor compensation by acceptor defects such as impurities or native defects (e.g. C or VGa/Al). Here, UV-growth leads to an increase in mobility and free carrier concentrations. A significant reduction of PL intensity related to compensating defects is also observed in these n-type films grown under UV illumination.The developed point defect control scheme is of great interest for all semiconductors that suffer from compensation and low free carrier conductivity.