**2. Fabrication of AlSb thin films**

AlSb film was prepared from dc magnetron sputtering of Al and Sb target (Kurt J. Lesker, Materials Group, PA) simultaneously in a sputtering chamber. Sputtering is a physical vapor deposition process whereby atoms are ejected from a solid target material due to bombardment of the target by plasma, a flow of positive ions and electrons in a quasineutral electrical state (Ohring, 2002). Sputtering process begins when the ion impact establishes a train of collision events in the target, leading to the ejection of a matrix atom (Ohring, 2002). The exact processes occurring at the target surface is depends on the energy of the incoming ions. Fig. 1 shows the schematic diagram of sputtering using DC and RF power. DC sputtering is achieved by applying large (~2000) DC voltages to the target (cathode) which establishes a plasma discharge as Ar+ ions will be attracted to and impact the target. The impact cause sputtering off target atoms to substrates.

Fig. 1. Schematic Diagrams of (a) DC sputtering and (b) RF sputtering (Ohring, 2002)

AlSb film was prepared from dc magnetron sputtering of Al and Sb target (Kurt J. Lesker, Materials Group, PA) simultaneously in a sputtering chamber. Sputtering is a physical vapor deposition process whereby atoms are ejected from a solid target material due to bombardment of the target by plasma, a flow of positive ions and electrons in a quasineutral electrical state (Ohring, 2002). Sputtering process begins when the ion impact establishes a train of collision events in the target, leading to the ejection of a matrix atom (Ohring, 2002). The exact processes occurring at the target surface is depends on the energy of the incoming ions. Fig. 1 shows the schematic diagram of sputtering using DC and RF power. DC sputtering is achieved by applying large (~2000) DC voltages to the target (cathode) which establishes a plasma discharge as Ar+ ions will be attracted to and impact

the target. The impact cause sputtering off target atoms to substrates.

Fig. 1. Schematic Diagrams of (a) DC sputtering and (b) RF sputtering (Ohring, 2002)

**2. Fabrication of AlSb thin films** 

In DC sputtering, the target must be electrically conductive otherwise the target surface will charge up with the collection of Ar+ ions and repel other argon ions, halting the process. RF Sputtering - Radio Frequency (RF) sputtering will allow the sputtering of targets that are electrical insulators (SiO2, etc). The target attracts Argon ions during one half of the cycle and electrons during the other half cycle. The electrons are more mobile and build up a negative charge called self bias that aids in attracting the Argon ions which does the sputtering. In magnetron sputtering, the plasma density is confined to the target area to increase sputtering yield by using an array of permanent magnets placed behind the sputtering source. The magnets are placed in such a way that one pole is positioned at the central axis of the target, and the second pole is placed in a ring around the outer edge of the target (Ohring, 2002). This configuration creates crossed *E* and *B* fields, where electrons drift perpendicular to both *E* and *B.* If the magnets are arranged in such a way that they create closed drift region, electrons are trapped, and relies on collisions to escape. By trapping the electrons, and thus the ions to keep quasi neutrality of plasma, the probability for ionization is increased by orders of magnitudes. This creates dense plasma, which in turn leads to an increased ion bombardment of the target, giving higher sputtering rates and, therefore, higher deposition rates at the substrate.

We employed dc magnetron sputtering to deposit AlSb thin films. Fig. 2 shows the schematic diagram of Meivac Inc sputtering system. Al and Sb targets were placed in gun 1 and 2 while the third gun was covered by shutter. Both Al and Sb used were purchase from Kurt J. Lesker and is 99.99% pure circular target with diameter of 2.0 inches and thickness of 0.250 inches.

Fig. 2. Schematic diagram of dc magtron sputtering of Al and Sb targets.

Firstly, a separate experiment was conducted to determine the deposition rate of aluminum and antimony and the associated sputtering powers. Al requires more sputtering power than Sb does for depositing the film at same rates. Next Al and Sb was co-sputtered to

AlSb Compound Semiconductor as Absorber Layer in Thin Film Solar Cells 347

(1)

1 11 21 31 *<sup>T</sup> <sup>d</sup> <sup>I</sup> T R R R Se*

Where, *α* is the absorption coefficient antd *d* is the thickness of the semiconductor film. *R1*, *R2* and *R3* are the Fresnel power reflection coefficient and the Fresnel reflection coefficient at semiconductor - substrate and substrate – air interface. *S* measures the scattering coefficient

UV Visible Spectrophotometer (Lambda 850) was used to measure the absorption and transmission data. This system covered the ultraviolet-visible range in 200 – 800 nm. The procedures in the Lambda 850 manual were followed. Figure 4 shows the transmittance spectra of the AlSb thin films. The films have an strong absorption in the visible spectral range up to 550 nm for film with Al:Sb ratio 2:5. Similarly for films with Al:Sb in the ratio of 1:3, 1:1 and 3:7 have strong absorption up to 700 nm. The films were transparent beyond

**3. Optical characterization of AlSb thin film** 

of the surface.

these levels.

0

*I*

The transmittance in the thin film can be expressed as (Baban et al. 2006):

Fig. 4. Transmittance Spectra of AlSb films with different Al:Sb growth ratios.

normalizing the Transmittance in the transparent region as (Baban et al. 2006):

metallic thus absorbing most of the light in visible spectrum.

reflectance spectra by famous using Tauc relation (Tauc, 1974)

The film with Al:Sb ratio of 7:3 didn't have a clear transmittance spectra and thus not shown in the figure. This was because the increasing the content of aluminum would make the film

Absorption coefficient of a film can be determined by solving equation 1 for absorption and

Optical band gap of the film was calculated with the help of transmission spectra and

 

<sup>1</sup> ln *Tnormalized <sup>d</sup>*

*n g*

(2)

*h dh E* (3)


produce 1 micron AlSb film in different deposition ratio for Al:Sb. The film was annealed at 200 C in vacuum for 2 hrs and cooled down naturally. Table 1 summarizes the deposition parameters of different AlSb films.

Table 1. Deposition Parameters of Different AlSb films.

The film was deposited on glass slides for electrical and optical characterization. The microscopic glass substrate (1 cm x 1 cm) was cleaned using standard substrate cleaning procedure as follows: soaked in a solution of 90% boiling DI and 10% dishwashing liquid for five minutes, followed by soaking in hot DI (nearly boiled) water for five minutes. The substrate was then ultra sonicated, first in Acetone (Fisher Scientific) and then isopropyl alcohol (Fisher Scientific) for 10 minutes each. The substrate was then blown dry with nitrogen.

The morphology of the AlSb film was checked by SEM and was used to validate the grain size and crystalline nature of AlSb particles and shown in Fig. 3.

The AlSb grains were found to have been developed after annealing of the film due to proper diffusion and bonding of Al and Sb. Only low magnified image could be produced before annealing the film and holes were seen on the surface. The AlSb microcrystal is formed with an average grain size of 200 nm. Also seen are holes in the film which are primarily the defect area, which could act as the recombination centers. Better quality AlSb film could be produced if proper heating of the substrate is employed during deposition process.
