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One-Dimensional Nanostructures of Ternary AlGaN and Metal Doped- Zinc Oxide with Tunable Bandgaps: Growth, Characterization and Optical Properties (2011 - 2013)
Last Update
22.07.2013

Principal Investigators:

  • In Taiwan

Dr. Li-Chyong Chen

Center for Condensed Matter Sciences,

National Taiwan University, Taipei-106, Taiwan

Tel: +886 2 33665249/ Fax: +886 2 23655404; E-mail: chenlc@ntu.edu.tw

  • In Latvia

Dr. Habil. Phys. Baiba Berzina

Institute of Solid State Physics (ISSP), University of Latvia

8 Kengaraga Str., LV-1063 Riga, Latvia

Phone: +371-67253592; Fax: +371-67132778; E-mail: baiber@latnet.lv

  • In Lithuania

Prof. Habil. Dr. Kestutis Jarasiunas

Institute of Applied Research, Department of Semiconductor Optoelectronics

Vilnius University, Saulėtekio al. 9, building III

LT 10222Vilnius, Lithuania

Phone:+ 370 5 2366036 Fax +370 5 2366070

E-mail: kestutis.jarasiunas@ff.vu.l

Motivation

Prompt development of modern optoelectronic devices induces requirement for new prospective materials with definite features and maximally reduced dimensions. For this purpose ternary AlGaN and ZnO structures are prospective materials. As it is known material properties and especially their optical features are highly dependent on defect content in the material. Therefore, one and the same material synthesized under different conditions can possess various properties.

Ternary AlGaN consists of two nitrides AlN and GaN with hexagonal crystalline structure but different energy band gaps (6,2 eV and 3,4 eV, respectively) forming homogenous solid solution with variable band gap dependent on a ratio of Al and Ga content in the material. The exciton emission also depends on the band gap energy and, therefore, it could be modified between the energies above covering a large spectral range of ultraviolet light. During the last decade the AlxGax-1N material with an Al content x varying between the interval 0,2<x<1 is thoroughtly studied by N.Nepal et al. (APL 87 2005; APL 88 2006 etc.) and the features of exciton luminescence were established. In the present study the research is concentrated on thin films of AlxGax-1N with low content of Al, x not exceeding 0,12.

ZnO is one of prospective wide band gap material (Eg = 3,3 eV). The present research is concentrated on studies of ZnO thin films. ZnO films will be grown in three main crystallographic orientations, including two nonpolar a-plane (11-20) and m-plane (10-10) and one polar c-plane (0001) which optical and electrical properties will be studied.

Aim

Synthesis of ternary AlGaN semiconductor materials with alterable band gap energy Eg as well as varied ZnO structures and research of their optical and electrical properties in order to elaborate materials with predictable features available for application in optoelectronics.

The project realization.

Taiwanese side is working on sample synthesis using chemical vapour deposition (CVD) and molecular beam epitaxy (MBE) methods as well as structural characterization using different up-to-date microscopy methods.

Lithuanian side is working on material electrical and optical properties using methods of modulation spectroscopy, spectroscopic ellipsometry and time-resolved spectroscopy.

Latvian side is working on material spectral characterization using study of photoluminescence together with its excitation spectra and absorption spectra within a wide temperature range between 8 K and 300 K.

The results achieved. (The Latvian side)

AlxGax-1N

The features of an exciton luminescence, which is sensitive to the material band gap energy Eg were studied for the thin layers of AlxGax-1N with different x values not exceeding 0,1 within a temperature range from 8 K up to 300 K. The samples were excited with the 263 nm laser light. The following results are obtained.

  • For AlxGa1-xN samples with small Al content not exceeding 0,1 an increase of x results in a blue shift of the exciton luminescence band’s position caused by the band gap enlargement. This relevance is observable for different x at all fixed temperature points in 8 K - 300 K range.
  • For AlxGa1-xN an increase of temperature between 8 to 300 K results in a red shift of the exciton band maximum position caused by the narrowing of the band gap with the rise of the temperature due to expansion of the crystalline lattice and sensitivity of the electron-phonon interaction to the temperature.
  • Increase of the temperature of AlxGa1-xN results in decrease of the exciton luminescence intensity caused by the thermal delocalisation of the localized excitons responsible for the luminescence observed.
  • For AlGaN synthesis the substrate temperature is important factor to ensure a desirable Al content in the material.

ZnO

The photoluminescence spectra and absorption of ZnO films grown in three main crystallographic orientations accordingly to the α-, c- and m-plains were studied. It was found that the spectral position of the exciton luminescence band as well as an edge of the optical absorption depends on material crystallographic orientation.

Mutual workshops

2011, 18-25 July. Workshop in Riga. Participants – physicists from Taiwan (2), Lithuania (2) and Latvia.

2013, 22-28 May. Workshop in Taipei. Participants – physicists from Taiwan and Latvia (3).

Student work

2013. Jana Grigorjeva “ Exciton luminescence of ternary AlGaN” (magister’s degree)