CIGSe/ZnO interface formation

In CIGSe/ZnO stacks the question arises whether the formation of InO_x or GaO_x domi-nates, or whether an (In_y,Ga_1−y)O_xalloy forms. Fig. 5.5 shows an SEM image of a sample fabricated with a modified three stage process at T = 525 without sodium addition, as described in Sec. 2.1. The CIGSe layer grows poly-crystalline with no morphologic correlation to the ZnO layer. No inter-facial layer can be directly identified. To study the interface formation between CIGSe and ZnO TEM images were taken and XPS spectra of cleaved samples were recorded.

TEM analysis Fig. 5.6a shows a HR-TEM image of the hetero-junction region. In the region between the CIGSe and the ZnO an approximately 6 nm thick layer can be iden-tified. Fig. 5.6b shows an interface image taken at a different, thinner sample area. The interfacial layer is marked in red within this image. The layer thickness varies between 4 and 6 nm, thus around +/-20 %. To identify the crystalline structure of the interfa-cial layer a fourier transform (FFT) of this image was calculated at the interface, the ZnO and the CIGSe area. The FFT images of the ZnO and the CIGSe areas show dis-tinct peaks originating from the periodicity of the lattice planes, with dZnO=0.27 nm and dCIGSe=0.32 nm. The FFT of the interface layer does not show any peaks characteristic for a crystalline structure, which leads to the conclusion that the layer has an amorphous structure. The chemical composition of the interface layer is obtained by a TEM-EDX line scan over the interface at a thicker part of the TEM sample. The EDX profile is shown in Fig. 5.7a. Clearly visible from this measurement is that the amorphous layer consists of a segregation of Ga. No segregation of In or Cu is visible. The O profile is shifted 4 nm from the Zn profile towards the CIGSe layer, indicating that the Ga is bound to oxygen within a few nanometer thick layer. The FWHM of the Ga EDX signal peak is 22 nm, which is broader than the width of the actual interfacial layer, since it is the projection of a rough interface.

(a) (b)

(c)

Figure 5.6: a)HR-TEM image of the CIGSe/ZnO interface. The CIGSe is grown at 525‰ without additional NaF supply. b)HR-TEM image of the same CIGSe/ZnO interface recorded at a thinner sample area used for EDX analysis. The interface layer between CIGSe and ZnO is coloured in red. c) From left to right: FFT of the ZnO area, peak corresponding to 2.7 ˚A(spacing of the (002) lattice planes). FFT of the interface area, no peaks visible. FFT of the CIGSe area, peak corresponding to 3.5 ˚A(spacing of the (112) lattice planes). The same amount of pixels were used for all three FFTs.

(a) (b)

Figure 5.7: a)TEM-EDX line profile showing the elemental depth profiles at the CIGSe/ZnO interface b) TEM-EDX map of the same interface region. The GaO_x layer is clearly visible due to the increased GaK line signal at the GaO_xlayer.

A two-dimensional EDX map of the Ga Signal from the same interface region is also shown in Fig. 5.7b. The Ga-rich layer observed in the one-dimensional plot can be observed along the hetero-interface. It covers the whole interface relatively homogeneous, with a variation of the FWHM of the Ga EDX signal peak of around +/-20 % along the interface, similar to the variation of the GaO_x thickness observed in Fig. 5.6b.

XPS analysis Further insight into the elemental composition of the interfacial layer was obtained by quantitative XPS measurements. To analyse the interface, the sample was cleaved by thermal shock at the interface between CIGSe and ZnO, as described in Sec. 2.5.2. The layer remaining on the glass substrate was the ZnO layer and the layer attached to the silver epoxy the CIGSe layer. As seen in Tab. 5.1 the surface of the ZnO layer contains all elements of ZnO and CIGSe. The quantitative analysis (details in Sec. 2.5.2) shows that it mainly consists of Ga, O and Se with impurities of Cu, In and Zn in the range of a few atomic percent. No sodium was detected at the interface.

The information depth of the Ga and Zn XPS signal is around 2 nm. Assuming that the GaO_x layer is free of holes, the Zn signal does not originate from the ZnO underneath the interfacial layer. The concentration of Zn within the interfacial layer may increase closer to the ZnO layer though. The information depth varies for the different elements between 1.6 nm and 6.1 nm and the error of the quantitative results are around 60 %, thus the chemical composition can only be approximated to (In_y,Ga1−y)(O_z,Se1−z)_x:Zn,Cu , with y and z close to 1. In the following the interfacial layer will be named GaO_x whenever the In concentration is below 5 at.%, else it will be named (In_y,Ga_1−y)O_x or InO_x for y=1.

The surface of the CIGSe layer consists mainly of Cu poor CIGSe with around 1 at.%

Zn and O contamination. The low Cu-content cannot be confirmed by the TEM-EDX depth profile in Fig. 5.7a.

Table 5.1: Quantitative results from XPS measurements shown in atomic % of all elements within a sample. The sample was cleaved at the CIGSe/ZnO interface. Care is to be taken since the relative error of the atomic concentrations is between 50% and 60%.

inf. depth in ZnO ZnO surface inf. depth in CIGSe CIGSe surface

nm at.% nm at.%

Cu 2.4 3 3.1 13

In 4.6 5 5.8 25

Ga 1.6 44 2.1 6

Se 6.1 17 7.8 54

O 4.2 32 4.1 1

Zn 2.0 1 2.6 1

Na 4.2 0 2.3 0

Temperature dependence

In this section, the temperature dependence of the interfacial oxide formation is studied with the help of GDOES depth profiling. All samples were grown similar to the sample used for the TEM/XPS analysis and without external sodium supply. Figs. 5.8a-c show the ratios of the cations to selenium for various CIGSe deposition temperatures, each together with the Zn signal as a reference.

Interestingly, for the sample deposited at 420‰, all cations, Cu, In and Ga, show a slight increase of their ratio to Se at the interface. Following the arguments given in the previous section, this originates from an anion substitution from selenium to oxygen within a thin layer at the interface. In this case the interfacial layer consists of all elements in similar concentration, forming a Cu(In_y,Ga1−y)(O_z,Se1−z)_x layer.

At the deposition temperature of 520‰it is known from the TEM and XPS measure-ments in the previous section, that a 6 nm thick GaO_x layer forms with impurities from Cu, In, Zn and Se. The GDOES signal ratios in Fig. 5.8b show indeed that mainly the Ga/Se ratio increases at the interface to ZnO, whereas The Cu/Se and the In/Se ratios show only minor deviations.

If the temperature is further increased to 550‰ (Fig. 5.8c), the Ga/Se ratio at the hetero-interface also increases further compared to the deposition at 520‰. Indicating the growth of a thicker GaO_x layer at higher deposition temperatures. Interestingly though, at this temperature, the Cu/Se ratio at the hetero-interface increases noticeably as well, indicating detectable Cu diffusion into the GaO_x layer at such elevated temperatures.

The Ga/Se ratios for the three different profiles are shown together in Fig. 5.8d. The signal peak intensities of the ratios were calculated by fitting the signal peaks with Voigt functions using the horizontal dashed line in Fig. 5.8d as the baseline. The peak area of the Ga/Se ratio of the 420‰sample is 25 %, and the intensity of the 550‰sample 150 % of the peak intensity of the 520‰ sample. In the previous section, the GaO_x thickness for a 520‰ sample was shown to be approximately 6 nm, leading to an estimated GaO_x thickness of 9 nm for the 550‰sample. The interfacial oxide layer of the 420‰ sample, consists of all elements and is therefore estimated to be around 3 nm thick.