M. Rudziński

Nonpolar GaN substrates grown by ammonothermal method

In this letter, the authors demonstrate large size m-plane GaN substrates grown by ammonothermal method. These substrates have excellent structural quality. The concentration of threading dislocation density is below 5×104 cm−2 and the full width at half maximum for the symmetrical and asymmetrical peaks equals 16 and 19 arc sec, respectively. Also good optical quality, the energy gap-related transition is clearly observed at room temperature in photoluminescence and contactless electroreflectance spectra. GaN epilayers deposited on these substrates exhibit intrinsic narrow exciton lines which are very sensitive to the optical selection rules typical for hexagonal symmetry, proving truly nonpolar character of the material.

Non-polar and semi-polar ammonothermal GaN substrates

In this paper we review the developments of producing non-polar (i.e. m-plane and a-plane) and semi-polar (i.e. (20.1)-plane) wafers by ammonothermal method. The growth method and polishing results are described. We succeeded in producing 26 mm × 26 mm non- and semi-polar wafers. These wafers possess outstanding structural and optical properties, with threading dislocation density of the order of 104 cm−3. Detailed studies of homoepitaxial layers, as well as AlGaN heterostructures are also presented, showing the potential of studied ammonothermal substrates in the fabrication of optoelectronic devices.

Investigations of GaN surface quantum well in AlGaN∕GaN transistor heterostructures by contactless electroreflectance spectroscopy

AlGaN∕GaN transistor heterostructures, caped by ∼2nm GaN layer, were investigated by contactless electroreflectance (CER) spectroscopy at room temperature. Below the AlGaN-related transition CER spectra have shown a clear resonance at the energy of ∼3.7eV, i.e., at much higher energy than the GaN band gap energy. The observed feature has been connected with the optical transition within the GaN cap layer. It was concluded that a surface GaN quantum well has been created by the deposition of nominally undoped (or Si-doped) GaN cap layer on AlGaN∕GaN transistor heterostructures.

Photoreflectance investigations of a donor-related transition in AlGaN∕GaN transistor structures

Below-bandgap photoreflectance (PR) features observed for GaN layers and undoped AlGaN∕GaN transistor structures have been analyzed in this letter. In addition to PR signal associated with the interference oscillations a strong PR feature at ∼3.37eV has been resolved for some AlGaN∕GaN structures. This feature has been attributed to an electron transition between the valence band and a donorlike state located ∼50meV below the conduction band. An absorptiontype experiment, such as PR spectroscopy, makes it possible to observe such a transition because this donorlike state is ionized by the strong internal electric field existing in the GaN layer at the AlGaN∕GaN interface. The existence of this electric field with a magnitude of ∼210kV∕cm has been confirmed by the observation of GaN-related Franz-Keldysh oscillations in the PR spectra. Obtained results show that donorlike states located ∼50meV below the conduction band are one of the sources of high concentration of the two dimensional electron gas in undo...

Contactless electroreflectance of GaN bulk crystals grown by ammonothermal method and GaN epilayers grown on these crystals

Room temperature contactless electroreflectance (CER) spectroscopy has been applied to study the energy gap, optical quality, and band bending for n-type and semi-insulating GaN crystals grown by ammonothermal method. Broad CER resonances typical of band-to-band absorption with opposite phases, which indicates opposite band bendings, have been clearly observed for the two types of GaN crystals. In addition, GaN epilayers have been grown by metalorganic chemical vapor deposition on these crystals and characterized by CER spectroscopy. Very narrow CER resonances (∼15 meV), typical of high quality material, have been observed for these epilayers. It confirms the excellent usefulness of ammonothermal GaN substrates for GaN homoepitaxy.