**4.6.3 Some properties of the device structures supplied with zirconium dioxide films**

Photoelectric characteristics and noise of germanium photodiodes supplied with ZrO2 and SiO2 films described above have been investigated. Monochromatic sensitivity of these photodiodes is typical for germanium devices and equals to 0.5-0.6 A/W (at 1.06 and 1.55 µm wavelengths). The change SiO2 over ZrO2 resulted in somewhat decrease of a dark current (on average for 10%). Main improvement of the photodiodes quality achieved due to the application of ZrO2 films revealed at the noise studies. Under the voltage exceeding operational one (that corresponds to accelerated reliability testing conditions) the check samples with SiO2 films have shown pulse noise of telegraphic type in the oscillogram, which can be associated with processes of energizing- deenergizing of the surface

Fianite in Photonics 165

As it is apparent from Fig. 23, the reflection minimum was approached at min =0.97 μm. Thus, the experimental results are in conformity with the theory practically complete.

Therefore, the ZrO2 film ensures high antireflection quality: at min the reflection loss does not exceed 2-3 %. The data obtained confirm that ZrO2 is an excellent material for

In theory, it is possible to eliminate the reflection completely (at the corresponding thickness of a film *d*) at *nd* = √ *n*f, where *n*f – optical refraction constant of a semiconductor. Since for Si and Ge the constants equal to 3.7 and 4, respectively, the reflection is completely eliminated at *nd* = √ *n*<sup>п</sup> 2. Therefore, a dielectric having its optical refraction constant *nd* = √ *n*~2 (at *n* = 3.7÷4) can be considered as an optimal material for the antireflecting film for solar cells and the other photosensitive devices. Theoretically, it is the case at the film thickness, which is equal to a quarter of optical wavelength *W* = *λ*/4*nd* , such dielectric allows a complete

The refraction constant of SiO2 (*n* = 1.47) is considerably lower than that value. At this *n* value it is impossible to maintain the reflection loss lower than 10%. Refraction constants of fianite and ZrO2 are within (2.15÷2.18) and (2,13÷2.2), respectively, - that is close to the above optimum value. Thus providing an evidence that fianite and ZrO2 are very promising as antireflecting coatings for solar cells and the other photosensitive devices based on Ge, Si

Experimental dependencies of antireflection (as dependencies of the reflection on

Experimental study of antireflective properties of fianite oxide applied to Ge was performed. By the reason that, germanium photodetectors are designed for detecting radiation generated by lasers with wavelengths λ= 1.06; 1.3; 1.54 µm, the thickness of the antireflective fianite film was chosen as W=1300 Å; such thickness provides for minimal reflection losses in the said wavelength range λ= 1.06-1.54 µm. Fig. 23 a shows the comparison of experimental (thin line) and theoretical (bold line) R(λ) curves. The

. ( 1) ( )( 1)sin (2 / )

*П ок*

4 .

*ок П П ок ок ок*

According to the above formula, reflection may fall practically to zero at the optimal value of nok (note, that in case of SiO2 anti-reflective film, for which nok=1.47, it is impossible to obtain reflection lower than 10%). The minimal reflection is achieved at the following

min *Wnок*

*n W* 2

The plots apparently demonstrate that the reflection drops to 0 – 1.5 % in the minima.

<sup>4</sup> <sup>1</sup> <sup>2</sup> <sup>2</sup> <sup>2</sup> <sup>2</sup> <sup>2</sup> <sup>2</sup>

*n n n n n*

wavelength) of fianite films on Si and Ge have been plotted (Fig. 24).

theoretical R(λ) curve was calculating using the following formula:

*<sup>n</sup> <sup>n</sup> <sup>R</sup>*

**5. Silicon and III-V solar cells with fianite antireflecting layer** 

**5.1 Anti-reflection properties of fianite film on Ge and Si** 

antireflecting films, as well as fianite.

elimination of the reflection loss (R=0).

and AIIIBV compounds.

wavelength λmin:

conducting channels [59]. The defects occurring because of the presence of pores in SiO2 films are a probable cause of arising of the channels. In the batch with ZrO2 protective films only shot noise, which is in principle unavoidable, was observed. More detailed results of the device studies are presented in [60].

Thus, the studies performed on fianite and zirconium dioxide films, as well as on the device structures developed using these films have demonstrated the advantages of zirconia-based solid solutions in application to photosensitive apparatus technology.

#### **4.7 Studies of optical properties of ZrO2 films**

Optical refraction of ZrO2 equals to 1.982.1, that is close to fianite one, therefore this material is also promising for antireflection coatings. Determination of the refraction constant *n* and monitoring of the film thickness *d* were carried out using ellipsometry technique. The experimentally determined values of d depended on duration of the films growth and varied within 600Å - 1100Å range.

The films obtained have shown rather high refraction: ~22.1. These values were significantly higher than that of SiO2 (1.45).

In theory, considering an incident beam from air (vacuum), it is possible to decrease the reflection to zero when the refraction constant of an antireflecting film corresponds the following equation:

$$n = \sqrt{n\_{\\_}} \,\, \_{\\_}$$

where *пп* – refraction constant of a semiconductor. In case of Si and GaAs *пп* ~ 3.54, thus *<sup>п</sup> п* ~ 1.92. Therefore, the ZrO2 films obtained actually satisfy perfect antireflection of Si and GaAs – based devices from the viewpoint of n. Moreover, the difference in n-values of SiO2 and ZrO2 films provides an opportunity for the antireflection over a broad spectral range due to application of binary SiO2+ ZrO2 antireflecting coatings. The dependency of the reflection constant on wavelength of silicon sample coated with ZrO2 film of 1200Å thickness is presented in Fig. 22. Theoretical absorption minimum corresponds to = 4 nd=42.10.12 1 μm.

Fig. 22. The dependency of the reflection constant on wavelength of silicon sample coated with ZrO2 film of 1200Å thickness.

conducting channels [59]. The defects occurring because of the presence of pores in SiO2 films are a probable cause of arising of the channels. In the batch with ZrO2 protective films only shot noise, which is in principle unavoidable, was observed. More detailed results of

Thus, the studies performed on fianite and zirconium dioxide films, as well as on the device structures developed using these films have demonstrated the advantages of zirconia-based

Optical refraction of ZrO2 equals to 1.982.1, that is close to fianite one, therefore this material is also promising for antireflection coatings. Determination of the refraction constant *n* and monitoring of the film thickness *d* were carried out using ellipsometry technique. The experimentally determined values of d depended on duration of the films

The films obtained have shown rather high refraction: ~22.1. These values were

In theory, considering an incident beam from air (vacuum), it is possible to decrease the reflection to zero when the refraction constant of an antireflecting film corresponds the

*<sup>п</sup>*<sup>=</sup> <sup>n</sup> *<sup>n</sup>* ,

where *пп* – refraction constant of a semiconductor. In case of Si and GaAs *пп* ~ 3.54, thus *<sup>п</sup> п* ~ 1.92. Therefore, the ZrO2 films obtained actually satisfy perfect antireflection of Si and GaAs – based devices from the viewpoint of n. Moreover, the difference in n-values of SiO2 and ZrO2 films provides an opportunity for the antireflection over a broad spectral range due to application of binary SiO2+ ZrO2 antireflecting coatings. The dependency of the reflection constant on wavelength of silicon sample coated with ZrO2 film of 1200Å thickness is presented in Fig. 22. Theoretical absorption minimum corresponds to = 4

Fig. 22. The dependency of the reflection constant on wavelength of silicon sample coated

solid solutions in application to photosensitive apparatus technology.

the device studies are presented in [60].

**4.7 Studies of optical properties of ZrO2 films** 

growth and varied within 600Å - 1100Å range.

significantly higher than that of SiO2 (1.45).

following equation:

nd=42.10.12 1 μm.

with ZrO2 film of 1200Å thickness.

As it is apparent from Fig. 23, the reflection minimum was approached at min =0.97 μm. Thus, the experimental results are in conformity with the theory practically complete.

Therefore, the ZrO2 film ensures high antireflection quality: at min the reflection loss does not exceed 2-3 %. The data obtained confirm that ZrO2 is an excellent material for antireflecting films, as well as fianite.
