REFERENTES HISTÓRICOS DE LA EDUCACIÓN FÍSICA Y EL DEPORTE

6.2.1 Erbium ion doped Ge-Ga-Se film deposition

Despite erbium ion doped Ge-Ga-Se glasses being intensively studied, only a few reports on erbium ion doped Ga-containing chalcogenide films were published so far. Takahiko et al. reported the properties of Er3+ doped Ga-Ge-Se films on fused silica substrates deposited using RF sputtering, and lifetimes of 1.8-2.6 ms for the Er3+ 4I13/2

state were observed when excited at 973 nm [219]. In Nazabal et al.’s reports, Er3+ doped Ga-Ge-Sb-S(Se) films were fabricated using pulsed laser deposition (PLD) and RF sputtering, and both physical and optical properties of the films were investigated [318]. The lifetime of the 4I13/2 state of Er3+ decreased from 1.6 ms to 1.1 ms in the

sputtered films as the erbium ion concentration increased from 0.3 to 1.5 at% (3.4x1019 to 1.65x1020 ions/cm3) without mentioning the pumping wavelength. A lifetime of 1.8 ms (equal to the values calculated using Judd-Ofelt theory) was achieved on a piece of Ga-Ge-Sb-S(Se) bulk glass with 0.05 at% Er3+ _{concentration after 60 minutes annealing}

[318].

A large part of the difficulty in film/waveguide amplifier fabrication, particularly in gallium containing chalcogenide glasses, relates to the difficulties in fabricating high quality erbium ion doped films. Thermal evaporation is perhaps the simplest, and therefore, most widely used method of preparing chalcogenide glasses films. However, many ternary and quaternary chalcogenide glasses display the undesirable property of forming phase separated molecular liquids on melting, with the different phases boiling off at different temperatures and rates. This can result in films with often quite different composition to the starting materials e.g. [213, 225]. Barely un-doped films obtained from rare-earth ion doped bulk targets using thermal evaporation were mentioned in Fick and Lyubin’s papers [128, 130]. The weight percentage of gallium was found to be significantly reduced from the value in the starting material in films deposited using standard thermal evaporation [319]. Among all the chalcogenide glass film deposition methods, elemental co-thermal evaporation provides a solution to this issue as it offers the possibility of controlling the evaporation rate of each element in the film independently. Thus, the final composition of the film can be controlled precisely.

For this reason, co-thermal evaporation was employed in this work, where each element had its own source and evaporation rate monitor, and thus the evaporation rate

of each element could be controlled accurately and independently to select any desired film composition. High purity Ge, Ga, Se and Er elements (5N) were used as starting materials, and evaporation was performed at a vacuum level ~1x10-5 Pa using the previously described chamber manufactured by Angstrom Sciences (in Section 3.2). Procedure of the Er3+:Ge-Ga-Se film evaporation was similar to the procedure of Er3+:As2S3 film thermal evaporation, which was described in Section 3.2.3. The film

thickness and linear refractive index (n) were measured using a dual angle spectroscopic reflectometer (SCI FilmTek 4000) using a Tauc-Lorentz model. The final film composition was determined by energy dispersive X-ray spectroscopy (EDS).

6.2.2 Erbium ion doped Ge-Ga-Se film characterisation

A film with a thickness of 1062±5 nm and refractive index of 2.433±0.002 at 1550 nm was deposited. Its composition was measured to be 24.60 at% Ge, 10.94 at% Ga, 63.74 at% Se and 0.71 at% (~2×1020 ion/cm3) Er which is in the optimal region based on the PL and lifetime measurements of the bulk glasses as noted in Section 6.1.6. An issue encountered during film evaporations was “spitting” of gallium particles out of the evaporation crucible. As will be discussed in this section, this led to a significant density of small particles in the films.

The Raman spectrum of the obtained films was measured with a Horiba Jobin Yvon 64000 spectrometer utilizing a 632 nm laser as the excitation source and a 50x near infrared objective with NA of 0.75. The obtained spectrum of an Er3+:Ge-Ga-Se film and a reference Raman spectrum of a 0.5 at% Er3+:Ge25Ga10Se65 bulk glass are shown in

Figure 6.9. Although the spectrum from the film is much noisier than that from the bulk glass due to the low film thickness, the main features at 202 cm-1 with a shoulder near 217 cm-1 and the lower intensity broad band from 270-330 cm-1 remain unchanged, implying the microstructure in film is similar to its bulk counterpart. The small extended band at ~450 cm-1 of the film arises from the thermally oxidized silicon (TOX) substrate upon which the film was deposited.

Figure 6.9 Raman spectra of co-thermal evaporated Er3+_{:Ge-Ga-Se film and 0.5 at%}

Er3+_:Ge

25Ga10Se65 bulk glass with excitation at 632nm.

The lifetime of the 4I13/2 state of Er3+ of this Er3+:Ge-Ga-Se film was measured using

the all fibre confocal set up mentioned in Section 3.3.2 with a 1490 nm pump. The intrinsic lifetime of this film was 0.87 ms, which is somewhat shorter than the ~1.35 ms observed in the corresponding bulk glass. Several factors could account for this. Firstly, the film was fabricated under non-equilibrium conditions in which a population of homopolar and ‘wrong’ bonds could be created, and these ‘wrong’ bonds may change the local environment for the erbium ions thereby degrading the performance. Secondly, local erbium ion clusters may also be formed during the evaporation as pure erbium metal was used as a source (i.e. polyatomic evaporation could occur as discussed with Er3+:As2S3 in Section 3.5), leading to quenching of the emission [131]. Thirdly, the

nanoscale homogeneity of the films was uncertain compared to the bulk glasses. The bulk glasses which were quenched from a metastable equilibrium melt were expected to be located in a region of the phase diagram where the glass was essentially single phase, rather than nanoscale phase separated. Given the clear issues with the gallium source and the non-equilibrium nature of the film growth, the film homogeneity cannot be guaranteed, and the “granular” nature often observed in evaporated thin films may also be relevant here [320]. Lastly, the erbium metal oxidisation and impurities introduced in the film during film evaporation may also cause a shorter lifetime.

The dependence of PL intensity and 1/e lifetime of Er3+ on pump power was also investigated in the obtained Er3+:Ge-Ga-Se films, which is shown in Figure 6.10(a). The emission has approximately quadratic pump power dependence in the measured range, which is similar to the bulk glasses. The 1/e lifetime of the 4I13/2 state of Er3+ in Ge-Ga-

compare the 1/e lifetime of film and bulk glass, the observed lifetime empirical formula (equation 6.1) was employed again to estimate the 1/e lifetime of a bulk sample with 0.7 at% Er3+ at a pump intensity around 300 kW/cm2. This produced an expected 1/e lifetime in the 0.7 at% Er3+ doped glass of 0.74 ms compared to the 0.52 ms measured in the film. The lifetime reduction in the film under pumping is smaller than that seen in the radiative lifetime (27% vs. 35%), and the shape of the decay curve in Figure 6.10(b) indicates that further reductions at the highest pump intensities expected in a typical waveguide device (up to ~1MW/cm2) will be modest leaving a workable lifetime around 0.5 ms.

Figure 6.10 PL intensity (a), and 1/e lifetime of the 4_I

13/2 state of Er3+ in the Er3+:Ge-Ga-Se

film (b) versus pump intensity in co-thermal evaporated films.

Measurements of film loss and erbium ion absorption in this Er3+:Ge-Ga-Se film were performed using a Metricon 2010 prism coupler, and the detail of this method can be found in Section 3.3.2. The resulting optical loss of the film, as well as a fitted absorption curve based on the absorption curve of the Er3+:Ge25Ga10Se65 (0.1 at%) bulk

Figure 6.11 Absorption spectrum of Er3+ _{doped Ge-Ga-Se films measured using prism}

coupler, with a fitted curve based on the absorption curve of the Er3+_:Ge

25Ga10Se65 (0.1

at%) bulk glass.

A minimum value of 0.8 dB/cm at 1650 nm is found outside the erbium ion absorption band. Given the observed particles in the as deposited film, it is likely that the loss is dominated by scattering off particulates. To investigate the film quality and particle induced scattering loss, the deposited film was inspected using dark field microscopy with x20 and x100 magnifications to estimate the particle density and size. Optical micrographs of these particles taken in dark field is shown in Fig 6.12. Custom software was used to count the particles in numerous images to ascertain an average particle density and also to estimate the size distribution. The density of particles was ~0.01 particles/μm2_{, with a mean particle diameter from 1.0 to 1.1 μm. The size}

distribution was always tightly bounded though with varying shape but the upper and lower bounds were 0.9 and 1.15 µm diameter based on multiple measurements of different parts of the wafer. Mie scattering would be expected to dominate in this size range, and here the r/ (r is the radius of particle, and  is the wavelength) ratio ranges from 0.3 in the 1550 nm band up to around 0.5 at 1000 nm. Mie scattering is a complex phenomenon requiring careful modelling for accurate predictions, but using the example of water droplets in air [321], this range of size parameter would result in a scattering cross-section that reduced by about a factor of two going from 1000 nm to 1550 nm. Additionally, the density of particles (i.e. a 1 mm wide beam typical of the set-up used would encounter 10,000 particles per mm of propagation length) and their likely metallic nature of the gallium droplets would be expected to induce non-negligible loss. It is expected that the particles could be eliminated with further refinement of the

evaporation set-up, most likely by using a baffled evaporation source that has no line of sight between the evaporant and the wafer.

Figure 6.12 Particles on the Er3+ _{doped Ge-Ga-Se film, which was taken with microscopy}

in dark field mode. The micro-sized particles has diameters ranging from 0.9 μm to 1.15 μm.

As it was established that the particles from gallium spitting cause a significant part of the propagation loss of the film, a modification of the film evaporation facility was implemented to resolve this issue and will be detailed in the following section.

6.3 Erbium ion doped Ge-Ga-Se film modification and