IL Investigations of Sapphire (Al₂O₃)

The production processes of intrinsic defects and the alteration of the luminescence properties by the irradiation with charged particles can be investigated on sapphire. Furthermore the application of IL allows the detection of Cr³⁺-impurities with a much higher sensitivity than those, which can be achieved with conventional IBA methods.

Contents:

Description of the sample
The IL spectrum of sapphire
The luminescence centers in sapphire
The alteration of the IL with irradiation time
IL images of sapphire
Literature

Description of the sample

The investigated sample was a sapphire single crystal (polished at both sides) with a thickness of 420 microns. Sapphire has a rhombohedral crystal structure. The optical properties can be modified for instance by doping the crystal  with Cr³⁺ impurities (possibility of stimulated emission: LASER). Then the Cr³⁺ ions replace Al³⁺ ions in the crystal structure (substitution).

The IL spectrum of sapphire

The sample was continuously irradiated with protons (energy E = 1430 keV, current I = 0.5 nA, fluenceF = 6.4x10¹²Â cm^-2 s^-1). During the irradiation IL spectra and IL images were collected. Fig.1 shows two IL spectra collected after 45 sec and 900 sec of irradiation respectively. One can see a luminescence band at a wavelength of 415 nm (2.98 eV), which intensity increases with irradiation time. This band is caused by an intrinsic defect, the F-center [1]. The peak at 694 nm originates from the extrinsic luminescence of Cr³⁺ impurities and consists of two emission lines (Cr³⁺-dublett) as can be seen in Fig.2 [2]. The intensity of the two lines decreases with irradiation time.

Fig.1: IL spectra of sapphire at different irradiation times.

Fig.2: Wavelength resolved Cr³⁺-dublett.

The luminescence centers in sapphire

F-center
The F-center consists of a oxygen vacancy, which is occupied by two electrons. After one electron has been excited, a de-excitation process in several single transitions takes place. The final transition of the electron to the ground state of the F-center leads to the emission of a photon with an energy of 3.0 eV [3]. Fig.3 shows the IL intensity vs. photon energy. The F-center luminescence band has a gaussian shape, a FWHM of 0.43 eV and is centered at 2.98 eV. This is in good agreement with previous investigations [3].

Cr³⁺-dublett
The Cr³⁺-dublett consists of the lines R1 (694.3 nm) and R2 (692.9 nm). They are related to the transition from the first excited state (²E) of a Cr³⁺ ion to its ground state (⁴A₂). Due to spin-orbit-coupling and the influence of the crystal field the first excited state is split into two energy levels, leading to the emission of two lines. Both lines arezero-phonon lines, which means that the related transitions are not affected by phonon interactions. Additionally transitions including the interaction with phonons take place. The creation (destruction) of phonons in a transition results in a photon energy reduced (enhanced) by the energy of the phonons. These transitions are responsible for the vibrational side bands in the IL spectrum (see Fig.3) [2].

Fig.3: Contribution of intrinsic and extrinsic luminescence centers to the IL spectrum of sapphire.

The alteration of the IL with irradiation time

The IL intensity was derived from the peak area of the Cr³⁺-dublett and the area under the 2.98 eV-band (see Fig.3). Fig.4 shows the relative IL intensity of F-center and Cr³⁺ emission vs. irradiation time.

Fig.4: Relative IL intensity of F-center and Cr³⁺ emission vs. irradiation time.

2.98 eV-band
At the beginning of the ion irradiation the IL intensity increases rapidly, stabilizes and remains constant after approximately 5 minutes. The increase of the IL intensity is caused by the increasing number of intrinsic defects produced by the ion beam. The fact, that the increase of the IL intensity stops, despite of a continuous ion irradiation can be explained by a transformation of optically active F-centers to inactive defect centers at a certain F-center concentration. According to Jeffries [4] the interactions between F-centers lead to aconcentration quenching of the luminescence at a concentration of 10¹⁹ cm^-3.In order to estimate the number of produced F-centers a detailed computer simulation was performed using the SRIM96 code [5]. At a depth of 15 microns from the surface of the sample each ion produces two oxygen vacancies/micron or 20000 oxygen vacancies/cm. Taking into consideration the ion fluence ofF = 6.4x10¹² cm^-2 s^-1, the density of O-vacancies increases by 6x10¹⁸ cm^-3 min^-1. Therefore a concentration quenching of the F-center luminescence after a few minutes of ion irradiation is very likely, despite the fact, that not all O-vacancies will be transformed into F-centers.

Cr³⁺-dublett

The IL intensity decreases with prolonged ion irradiation (see Fig.4). Possibly interactions between the increasing number of F-centers and the Cr³⁺ impurities are responsible for the suppression of the extrinsic luminescence of the Cr³⁺ ions.

 IL images of sapphire

The alteration of the luminescence properties of sapphire during ion irradiation can be further demonstrated with two IL images (see Fig.5). At the beginning of the ion irradiation the Cr³⁺ emission (red) dominates the F-center emission (blue) (Fig.5(a)), while later on the F-center emission clearly dominates the Cr³⁺ emission (Fig.5(b)) [5].

Fig.5: IL image of sapphire: (a) after 45 sec; (b) after 900 sec of ion irradiation.

Literature

[1]  K.H.Lee, J.H.Crawford,Jr. Phys. Rev. B19 (6), 3217 (1979).
[2]  D.F.Nelson, M.D.Sturge. Phys. Rev. 137 (4A), 1117 (1965).
[3] J.D.Brewer, B.T.Jeffries, G.P.Summers. Phys. Rev. B22 (10), 4900 (1980).
[4]  B.T.Jeffries, G.P.Summers, J.H.Crawford,Jr. J. Appl. Phys.51, 3984 (1980).
[5]  D.Spemann. Diploma Thesis, Universität Leipzig (1998).(siehe Zusammenfassung)