Modification of the Electrical Properties in Pure and Doped GaSb by Doping with Er and Yb

J.L. Plaza' *, V. Corregidor2, J. Olvera1

, C. Algora2 and E. Dieguez1

1 Laboratorio de Crecimiento de Cristales, Departamento de Fisica de Mater/ales, Facultad de Ciencias, UniversidadAutonoma de Madrid, 28049 Cantoblanco, Madrid, Spain

*Corresponding autor: joseluis.plaza!@u(mJ.es, Tel. + 34914974784; Fax:+ 34914978579

2/nstituto de Energia Solar, E.T.S.J. Telecomunicacion, Universidad Politecnica de Madrid, Avda. CompluteJJSe 38; 28040 Madrid, Spain

Absh·act. 11-ty pe Te doped Gallium antimonide, Te-doped-Er codoped GasSb and and Te­doped-Yb-codoped GaSb single crystals have been grown by using Liquid Encapsulated Czochralski. Carrier mobility, density and resistivity along the crystal have been obtained. The axial dopant composition has also been studied by analysing different wafers by Induced Coppled Plasma (ICP). From these measurements it is concluded that codoping with Er and Yb greatly reduces the mobility and increases the resistivity in both cases compared to simply Te doped GaSb. Secondary Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) analysis have revealed Sb clustering in Er and Yb codoped Te-doped GaSb samples. These defects could be the responsible for the reduction of the carrier mobili ty and the increment in the resistiv ity. TPV cells, with size of 4 mm2have been developed in the Te-doped GaSb sample showing relatively high performance.

Keywords: GaSb, rare earth, liquid encapsulated Czochralski , thennophotovoltaic cells. PACS: 61.72.Vv; 61.72. Yx

INTRODUCTION

Among the III-V compound semiconductors, gallium ant imonide (GaSb) is of particular interest regarding to the fabrication of thennophotovoltaic cells which have shown high quantum efficiency [1]. However, further research is needed in order to increase their perfonnance up to acceptable and competitive levels.

It has been previously reported that the mobility of Te-doped GaSb polycrystals is enhanced when codoping with Er20 3 due to the gettering effect of the Rare earth elements [2]. However, to the best of our knowledge, this effect has not been considered in Liquid Encapsulated Czochralski (LEC) grown GaSb single crystals. However differences are expected compared tp polycristal line samples due to the existence of grain boundaries in the later ones.

In this work we analyse the effect of two codopants (Er20 3 and Yb20 3) on electrical properties (carrier type, mobility, density and resistivity of the samples) on LEC

CP890, Thermophotovoltaic Generation of Electricity, Seventh Conference edited by C. Algora and V. Corregidor

4:1 2007 American Institute of Physics 978-0-7354-0392-5/07/$23.00

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grown n-type Te doped GaSb. The results are correlated to the dopant concentration along the ingot and to clustering effects obseJVed in tl1e rare eartl1 codoped samples.

In order to check tl1e feasibility of these wafers as substrates for TPV converters, cells with 4 1mn2 have been manufactured and characterized by J-V cuJVes and spectral response. Zinc diffusion on n-type GaSb substrates has been used in order to create the necessary p-n junction for 'PP V cell. With the help of tl1e spectral response characterization, the properties of the. substrates (for example minorities carries or mobility) can be analyzed. To test the perfonnance of tl1ese substrates, the results will be compared witl1 results fonn similar converters produced on commercialn-GaSb:Te substrates.

EXPERIMENTAL

In order to study the effect of rare earth codoping Te- GaSb crystals, four pure and doped GaSb crystals were grown by using liquid encapsulated Czochralski (LEC). Tllis technique has proved to be successful in growing lligh quality GaSb single crystals [4,5]. The composition of the crystals was the following: Ingot (A): as grown GaSb; Ingot (B): GaSb:Te (1019cm·\ Ingot (C): GaSb:Te (10 19cm·\Er20 3 (5xl018

cm·\ Ingot (D): GaSb:Te (1019cm-\Yb20 3 (5xl018 cnf\ The compositions of Ingots (C) and (D) were chosen considerin~ previous studies [2] in wllich a concentration of rare earth (Er) in melt of 5xl01 cm·3 and Te: 1019 cm·3 resulted in a mobility enhancement in Bridgman grown polycrystalline samples.

Ga (6N), Sb( 6N), Te (6N), Er203 (3N) and Yb203 (3N) from Alfa-Aesar were used as raw materials. A 1nixture of B20 3 (3N) and Na3AlF6 (3N) 3 % mol was used as encapsulant. In order to analyse the different electrical and compositional properties of the materials along the crystal, three wafers were cut from each crystal perpendicular to the growth axis by using a diamond saw cutting maclline. After cutting, tl1e samples were che1nically etched and mechanically polished by using alumina powder (0.3 J.un).

The elemental composition on each wafer was studied by Induced Coupled Plasma (ICP) teclutique by using a ICP-MS Elan 6000 Perkin-Elmer Sciex with AS 91 autosampler system. The electrical properties at room temperature regarding to carrier mobility, type, density and resistivity were obtained by tl1e van der Pauw technique by using a home-made Hall effect measuring system with magnetic fields up to 0.7 Tesla. Secondary Electron Microscopy (SEM) and surface chentical studies by using Energy Dispersive X-Ray (EDX) analysis were also perfonned by using a scaruting electron microscope (SEM), Pllilips XL30 witl1 EDAX DX4i analyzer.

RESULTS AND DISCUSSION

In order to establish the actual dopant composition, ICP analysis was developed in three different samples along the crystals (B), (C), (D). Due to tl1e low concentration of Er and Yb only tl1e Te concentration could be ascertained from the ICP analysis. Tllis fact was already expected as it is known that the segregation of RE elements

~

100

during growth of GaSb crystals is high. Figure la shows the~Te concentration obtained from three different wafers cut along the growth axis from each crystal. The main feature of this plot is that Te concentration in Er and Yb codoped samples is much lower than in the case of barely Te doped GaSb even though the initial Te concentration in melt was the same in the three crystals. Tllis could indicate a phase separation due to the nlixing of rare earth and Te. It is well known that due to the electropositive character of rare earths, they tend to bind to electronegative elements like oxygen or, in tllis case, Te.

The results presented in figure 1 could demonstrate tl1at, during growth, the rare earth elements tend to fonn different phases of the kind RE-Te wllich would segregate and would not appear in the crystal. Probably these phases could be present in the remaining product (encapsulant + some GaSb) in the crucible and tl1e study of tllis remaining material would clearly justify the above statements.

Some evidences about phase separations have been obtained by means of a combined SEM-EDX analysis of tl1e etched surfaces of tl1e RE-codoped samples where Sb-rich clusters have been observed as shown in figure lb were a backscattered electron image, obtained witl1 the electron microscope, is presented. The brighter cluster indicates lligher atomic number than the surroundings. The composition of these clusters has been analysed by means of EDX resulting in Sb(93 at.%), Ga (7 at.%) . NoTe or RE (Er, Yb) elements have been detected in tl1ese clusters. However it must be considered tlmt tl1e detection limit of an EDX system is above 5 at.% and we expect Te and Er, Yb concentrations to be lower tl1an 0.01 at. %. However it has been reported in the literature [2], and references therein] the tendency of RE elements to fonn "pnictides" wllich are of the kind RE-Group V, in our case (Er, Yb)-Sb. Tllis behaviour is currently under investigation and will be published elsewhere.

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As the main goal of this work was to establish whether the rare earth ions (Er, Yb) had the same mobility enhancement effect in tl-type Te:doped GaSb single crystals as they have in Te:doped GaSb polycrystals, we have analysed the electrical properties of a number of samples taken from different regions along each crystal by using the van der Pauw technique.

Figure 2 shows the carrier mobility, detlsity and resistivity of the samples versus the solidified fraction. In all the cases negative values indicate electrons and positive, holes. These figures are also compared to those obtained from as-grown GaSb crystal. It is observed in the mobility vs solidified fraction plot (figure 2a), that tJtis parameter has been greatly reduced in the case of RE (Er, Yb) codoped samples compared to the values from the GaSb:Te crystal. Tltis behaviour is opposite to that observed in polycrystalline samples [2] and indicates the important role played by the grain boundaries.

It is known that in Te-doped polycrystals, the Te atotns preferentially incorporate into the grain boundaries [6]. Therefore, rare earth elements have better chance to induce t11eir gettering effect in tltis regions. However, in single crystals the absence of grain boundaries make more difficult for the RE elements to bind Te. It is also believed that the phase separation and the fonnat ion of pnictides discussed below could affect to these parameters due to. the fonnation of structural defects like that presented in figure lb which would be detrimental to the carrier mobility . The carrier concentration of the (Er, Yb) samples is slightly higher than in the case of GaSb:Te. It has to be pointed out t11at all the Te doped samples, even those codoped wit11 Er, Yb were n type in nature except the sample at the very top of the Er codoped crystal which resulted to be p type, probably due a bad mixing at the beginning of the growth process were an excess of Er over the Te concentration could have induced t11e p type character.

Finally it is observed from figure 2c that t11e resistivity of the RE codoped samples is much higher than in the case of the Te doped sample. Tlus could be due to the above mentioned defect generation induced by t11e fonnation of different phases (Pnictides (RE-Sb) and Re-Te compounds).

102

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PREPARATION OF TPV CELLS

In order to check the feasibility of these wafers as a substrates for TPV converters, we have manufactured cells with 4 nun2 in area (2 x 2 nun2

) . Due to the extremely low mobility of the Er, and Yb codoped samples only GaSb:Te crystals have been used as substrates. The p-n junction is formed from the Zn evaporation in a pseudo-closed graphite box at 475°C for 45 minutes. A first anodic oxidation is performed to remove the high Zn concentration of the surface and to achieve the desired Zn profile. Tllis anodic layer is eliminated using a diluted HCl solution. Finally a new anodic oxidation layer (up to 60 V) is done and maintained in order to act as an antireflection coating for the cells. The back contact is fonned from the thennal evaporation of AuGe/Ni/Au and then annealed at 250°C during 1 minute. Before tlte evaporation the back surface is polished with alumina powder in order to remove any trace of Zn from the evaporation. The front contact is defined by lithographic process and then successive metals (Cr/Au/Ni/Au) are thermally evaporated, ho alloy is necessary. Finally, the cells are electrically isolated. A photograph of one of the cells manufactured is shown in figure 3.

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FIGURE 3. Photograph of one of the 2 x 2 mm2 TPV cell manufactured on GaSb:Te doped wafers.

The cells were characterized by J-V characteristics curves and spectral response. With these teclmiques, specially spectral response we can analyse the material from the point of view of the minorities carriers. To test the perfonnance of these wafers as substrates, the results were compared with ones from similar TPV converters produced on commercials monocrystalline n-GaSb:Te substrates.

Figure 4 shows the illuminated J-V curves for different cells of the same wafer. The response for a TPV cell manufactured in a commercial substrate is shown for comparison. It can be observed a lower Voc value than the reference cell and almost the same photocurrent densities. It can be observed as well that even in the same wafer, the curves are different showing a dependence of the TPV cell response with the position in the wafer. The J-V curves are characterized by a low fill factor values (around 44%) in comparison with the reference one (63%). These results indicate the presence of shunt resistance leakages, possibly related with defects such as Te precipitation or inclusions.

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104

The internal quantum efficiency for the same TPV cells described above are shown in figure 5. In general, tlte TPV cells manufactured on GaSb:Te substrates grown by the CZ metl10d show a lower quantum efficiency values in comparison with the reference spectra. At high wavelengths, values below 85 % indicate low lifetime for the minority carriers, therefore low diffusion lengths and so higher dark currents (as it was discussed above). At low wavelengths (up to 1000 nm) tlte lower quantum efficiencies values show the low quality of tlte emitter, which could be related with the Zn profile (not deep as needed) or/and high recombination velocity due to poor surface preparation.

Longer Zn diffusion times (1 hour or more) and better surface treatments will be taken into account in furtlter studies. It has to be pointed out that the substrates are sawn from the ingot, so mechanical damages are expected in tlte substrates. Chemical polishing and surface preparation are needed in order to remove this damaged layers, and a better Zn diffusion profile is expected.

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CONCLUSIONS

In tltis work Er and Yb eo-doped GaSb:Te n-type single crystals have been prepared by LEC. The chemical analysis indicates Te impurity reduction in RE-doped samples. Hall measurements carried out by tlte van d~r Pauw teclutique also indicate the reduction in the mobility and increased resistivity in RE-doped GaSb single crystals, probably due to phase separation and Sb-rich cluster formation. Regarding tlte TPV cells manufactured on GaSb:Te substrates, good values of photogeneration densities but with low open circuit voltage are obtained from the illuminated J-V

105

characteristic cutves. From the external quantum efficiency spectra low lifetimes values are obtained. Furthennore, a better surface preparation should be studied in order to reduce the recombination velocity and a optimal Zn profile.

, ACKNOWLEDGEMENTS

This work has been supported by the Ministry of Education and Science of Spain under a Ramon y Cajal Contract and by the project MAT2003-09873. V. Corregidor acknowledges the MEC the Juan de la Cietva contract. Tllis work is partially supported by the European Commission through the funding of the project FULLSPECTRUM (Ref. N: SES6-CT-2003-502620).

REFERENCES

1. M.G. Drexhage, C.T. Moyniham, Sci. Am. 259 (5) 76 (1988). 2. J.L. Plaza, P. Hidalgo, B. Mendez, J. Piquems, E. Dieguez, Mat. Sci. Eng. B 91-92 529-533. (2002). 3. D. Martin, C.Algora, Sixth Conference on Thermophotovoltaic Generation of electricity, AlP

Conference Proceedings, 738, p. 311 (2004). 4. M. Kumagawa, N. V. Quang, Jpn. J. Apl. Phys. 19 (1980) 2509. 5. I. Hirai, T. Obokata, Jpn. J. Apl. Phys. 21 (1982)956. 6. P. S. Dutta, H. L. Bhat, V. Kumar, J. Appl. Phys. 81 5821 (1997).

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