Effect of antimony on uniform incorporation of nitrogen atoms in GaInNAs films for solar cell application

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Abstract

In this work, we have investigated the effect of antimony (Sb) on the uniform incorporation of nitrogen atoms in GaInNAs alloy grown with different levels of Sb fluxes. The photoluminescence (PL) intensity and full-width at half maximum (FWHM) are both improved for a narrow range of Sb flux between 1×10−8 and 5×10−8 Torr, in which Sb plays the role of surfactant. On the other hand, higher level of Sb flux deteriorates the PL characteristics most likely due to Sb-related defects. Furthermore in temperature dependent PL measurements, drastic peak energy shifts were observed in all samples, which indicate a strong carrier localization. Although GaInNAs sample showed a large energy shift of 53 meV, supply of Sb decreased the localization energies to 13–22 meV. These results show that optimized amount of Sb, maintaining a high growth temperature of 520 °C, enhances the homogeneity of potential energy in conduction band of GaInNAs alloy, since the carrier localization is led by inhomogeneous N incorporation.

Highlights

► We introduce antimony as a surfactant in MBE-grown GaInNAs films. ► Optimum amount of antimony improves PL properties of GaInNAs films. ► Carrier localization in GaInNAs CB is observed due to potential fluctuation. ► Potential fluctuation is improved by introducing antimony during GaInNAs growth. ► Reduction of potential fluctuation leads to improved solar cell performances.

Introduction

The present record efficiency of 43.5% [1] in a triple-junction tandem cell has attracted much interest in multi-junction solar cell design, and there is a strong quest for a potential material which can add to a four-junction tandem cell. The most promising candidate is an InGaAs alloy due to its compatibility with the III–V based technologies [2], [3], [4]. However, high lattice mismatch is a strong concern in this system, and though a graded buffer layer approach can overcome this constraint, the buffer layer should be appropriately chosen so that the bandgap is above or equal to the overlying junction. Moreover, the thick graded layers involve the complexity of the precise growth procedures.

Dilute nitride semiconductor alloy, GaInNAs, is a promising candidate for applications to multi-junction tandem solar cells [1], [5], since its band-gap energy can be tuned to a required band-gap, for example 1.0 eV in a four-junction tandem solar cell, while maintaining its crystal structure lattice-matched to the GaAs and Ge substrates. However, this alloy requires a low temperature growth, and both its optical and electrical characteristics are degraded even if a few percent of nitrogen (N) atoms are added to the host Ga(In)As, due to compositional fluctuations and phase separations [6], [7].

Recently, use of antimony (Sb) in molecular beam epitaxy (MBE) is shown to be effective to obtain the long wavelength emission toward the telecommunication wavelength of 1.3 and 1.55 μm in heavily strained GaInxNAs(Sb) (x∼30%)/GaAs quantum well (QW) systems [7], [8], [9]. In these reports, Sb has been proposed to be a surfactant element to modify the surface free energy by segregating to the growth front. One of the plausible effects is to decrease the N-related localized energy level observed beneath the conduction band [10]. Such effects are usually interpreted from the energy shift in photoluminescence (PL) between the ground state of GaInNAs(Sb) QWs and N-related localized state by state filling with variable excitation power or thermal redistribution of carriers with increasing temperature. On the other hand, although the interaction between the Ga(In)NAs conduction band (CB), or mobility edge, and the N-related localized state has been reported in a “low-strained” Ga(In)NAs bulk sample [11], the validity of introduction of Sb on the growth of Ga(In)NAs films applicable to solar cell structure has not been fully addressed so far. Yuen et al. reported a degraded PL emission efficiency in the low strained GaInxNAsSb (x∼10%) layer compared to the Sb free GaInNAs sample [12]. Jackrel et al. also reported the solar cell performances of GaInNAs grown with and without Sb along with a thorough investigation of respective material properties [13]. In their study, the role of Sb in low strained GaInNAs alloys is not clear enough. On one hand, they found improvement in the current density, which was due to the wider depletion width resulting from decreased background doping density. On the other hand, capture cross section of deep trap centers increased with Sb addition, and the minority carrier lifetime decreased.

In this work, we have focused on the effect of Sb on nitrogen-induced localized states in the conduction band (CB) of Ga(In)NAs alloys. Because these localized states originate from the nitrogen compositional fluctuations, the surfactant effect of Sb is expected to influence the kinematics of N incorporation during molecular beam epitaxy (MBE) growth of these alloys. For this purpose, we have grown a series of thin films with a variation of controlled fluxes of Sb starting from very dilute amount, and investigated by using photoluminescence (PL) spectroscopy. We show that the conduction band fluctuations are significantly suppressed when optimized amount of Sb is introduced, and this has led to an improved carrier collection measured in a solar cell structure.

Section snippets

Experimental

GaInNAs(Sb) films and solar cells were grown by radio frequency-MBE (RF-MBE) with continuous supply of atomic hydrogen [14], [15]. For the films, the growth conditions, such as growth temperature of 520 °C [15], [16] and As2 flux of 1×10−5 Torr in beam equivalent pressure (BEP) were kept identical except for the Sb beam flux, which was varied in the range of 0–1×10−7 Torr. Sb and N concentrations were calibrated by secondary ion mass spectroscopy (SIMS) measurement analyzed in a separately grown

Results and discussion

Fig. 1 shows plots of N and Sb compositions determined by SIMS measurement for GaIn0.076NAs(Sb) layers grown with different Sb fluxes of 0, 1×10−8, 5×10−8, and 10×10−8 Torr. The secondary ion signals of In are also plotted in Fig. 1. We can observe that N and In incorporations in GaIn0.076NAsSb layers do not show any dependence on the Sb beam flux for the given GaInNAs layers studied here. Because the sticking coefficient of In is unity in typical GaInAs growth, all the supplied In atoms can

Conclusion

In conclusion, the effect of Sb introduction on N-induced localized states in our H-MBE grown GaInNAs(Sb) alloys was investigated. Supplying Sb flux in a range between 1×10−8 and 10×10−8 Torr can improve homogeneous incorporation of N into host GaInAs material, and reduces the localization energies. Meanwhile, larger amounts of Sb supply can induce degradation of PL characteristics, which suggests that Sb induced defects, such as SbGa antisites, are generated. Therefore, small amount of Sb

Acknowledgments

This work is performed under SOLAR QUEST Project supported by the Incorporated Administrative Agency New Energy, Industrial Technology Development Organization (NEDO) and Ministry of Economy, Trade and Industry (METI), Japan.

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