Near band-edge optical properties of cubic GaN with and without carbon doping

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Abstract

We report the results of studying the optical properties of cubic GaN thin films with photoluminescence and photoluminescence excitation spectroscopies. The films are deposited by plasma-assisted molecular beam epitaxy on GaAs (001) substrates, with and without intentional doping with carbon atoms (p-type doping). The evolution of the optical spectra of the C-doped samples is consistent with a picture in which carbon enters into N-vacancies at low concentrations, producing a marked improvement in the crystalline properties of the material. At higher concentrations it begins to form complexes, possibly due to interstitial occupation. The temperature dependence on the absorption edge of the doped material is also measured and is analyzed with standard theoretical models.

Introduction

The fabrication of the first c-InGaN/GaN double-heterostructure LED grown by MOCVD has been recently reported [1]. Contrary to their hexagonal counterparts, the cubic structures can be grown free from modulation due to spontaneous polarization and strain-induced piezoelectric fields. Thus, the spatial separation of the carriers wave function, induced by the quantum-confined Stark effect in the hexagonal phase, is avoided in the cubic phase. For this reason the cubic polytype nitride-based quantum wells are expected to have increased optical recombination efficiency. Hence, the growth and characterization of high-quality c-GaN epitaxial layers are essential first step on the road to high-performance devices fabricated with this material. In addition, for the fabrication of devices with this material, it is essential to be able to introduce p- and n-type doping in a controlled manner. This involves introducing dopant impurities, which produce shallow acceptor or donor levels. Understanding how these impurities enter into the GaN lattice is of fundamental importance for achieving useful doping.

In the present work, we report the results of a study of the optical properties of cubic GaN thin films, deposited by plasma-assisted molecular beam epitaxy on GaAs (001) substrates, with and without intentional doping with carbon atoms (p-type doping). We used photoluminescence (PL) and photoluminescence excitation spectroscopy (PLE) to study the spectral region near the fundamental absorption edge of these samples. We observe a clear step-like absorption edge, resulting from the merging of the free exciton with the continuum. Quantitative values for the absorption edge energy and lifetime broadening are obtained. The temperature dependence on the absorption edge of the doped material is also measured and is analyzed with standard theoretical models. The evolution of the optical spectra of C-doped samples as the concentration of the dopant increases is consistent with a picture in which carbon enters into N-vacancies at low concentrations, producing a marked improvement in the crystalline properties of the material. At higher concentrations, it begins to form complexes, possibly due to interstitial occupation.

Section snippets

Experimental

Cubic GaN films were grown by rf-plasma assisted MBE on semi-insulating (001) GaAs substrates at a temperature of 720 °C. The growth rate was of 0.07 μm/h and the total thickness of each film was approximately 1 μm. Carbon doping was achieved, as explained in detail in Ref. [2], by electron beam evaporation of a graphite rod, with the e-beam power maintained constant between 0 and 400 W during the growth. The C-flux was calibrated by growing C-doped GaAs in the same manner and assuming the same

Results and discussion

PL and PLE spectra of the undoped sample (sample A in Table 1) are displayed in Fig. 1. The PL spectrum obtained here by low power density excitation with a filtered Xe-arc is entirely similar to the spectra obtained by excitation with laser sources reported by the authors of Ref. [3]. Two clear narrow structures characterize this spectrum, attributed to acceptor–donor transitions (3.15 eV) and bound exciton (3.26 eV), respectively. In the PLE experiment, best results are obtained when centering

Summary and conclusions

We have presented experimental optical data that give a strong indication of the lattice sites occupied by carbon during growth of carbon-doped GaN by rf-plasma assisted MBE on semi-insulating (001) GaAs. The resulting picture for the carbon incorporation into the c-GaN lattice is the C-atoms initially enter the sites of N-vacancies in the native material, thus improving its crystalline quality. The higher the C concentration, the better the crystallinity of the sample, up to a critical power

Acknowledgements

Financial support from the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), both in the form of research grants (01/01067-4 and 98/12779-0) and of postdoctoral fellowships for some of the authors (J.R.L.F. and J.A.N.T.S.) is gratefully acknowledged. Funding from the Deutsche Forschungsgemeinschaft (DFG) is also acknowledged.

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