2005 International Conference on Thermoelectrics (2005)

InGaAs with embedded ErAs nano-particles is a promising material for thermoelectric applications. The incorporation of erbium arsenide metallic nanoparticles into the semiconductor can provide both charge carriers and create scattering centers for phonons. Electron filtering by heterostructure barriers can also enhance Seebeck coefficient by selective emission of hot electrons. 2.1μm-thick ErAs/InGaAs superlattices with a period of 10 nm InAlGaAs and 20 nm InGaAs were grown using molecular beam epitaxy, and the effective doping is from 2x10 to 1x10 cm . Special device patterns were developed for the measurement of the cross-plane Seebeck coefficient of the superlattice layers.. Using these device patterns, the combined Seebeck coefficient of superlattice and the substrate were measured and the temperature drops through the superlattice and InP substrate were determined with 3D ANSYS simulations. The Seebeck coefficient of the superlattice layers is obtained based on the measurements and simulation results. Introduction The Seebeck coefficient of thermoelectrical material is of great importance for thermoelectrical devices working either as coolers or generators. For TE coolers, the cooling power is proportional to the Seebeck coefficient of the element material, and the output power of thermal-toelectricity power generator is proportional to the square of the Seebeck coefficient. Superlattice structure can enhance thermoelectrical device performance by the selective emission of hot carriers above the barrier layer thorough thermionic emissions. And the figure of merit ZT of a semiconductor can also be improved by incorporating of semi-metallic nano-particles into the material to form phonon scattering centers and increase carrier concentration. The metal-based superlattices with tall barriers can achieve a large effective thermoelectric figure of merit by electron filtering. In-plane and cross-plane Seebeck coefficient for superlattices are usually different, mainly because of filtering effects. The in-plane Seebeck coefficient measurement is straightforward: building up a temperature difference across the sample and measuring the output voltage. By comparison, the measurements of cross-plane Seebeck coefficient are more obscure, for the temperature difference across superlattice layer of a few microns can not be directly measured preciously. In our previous work, we used the same device pattern and measured SiGe/Si superlattice cross-plane Seebeck coefficients. In this paper, we report the measurements of cross-plane Seebeck coefficients of ErAs/InGaAs superlattices. Materials Growth and Structure Four ErAs/InGaAs superlattice samples were grown using a Varian Gen II molecular beam epitaxy system on (100) ntype InP substrate. The samples have the superlattice structure. Erbium particles are randomly distributed through InGaAs layers, which provide 2×10 cm doping. Silicon codoping for the four superlattice samples are 0, 2×10, 4×10 and 8×10 cm, thus the effective doping of erbium and silicon dopants for the four samples are 2×10, 4×10, 6×10 and 8×10 cm respectively. The structure of superlattice is illustrated in Figure 1 and the compositions of the four samples are shown in table 1. Table 1: Four ErAs/InGaAs superlattice samples have structure with different silicon co-doping. Superlattice structure Effective doping (cm) 1 70×(10nm (InGaAs)0.6(InAlAs)0.4/20nm (n-InGaAs)0.997Er0.003 randomly) 2×10 2 70×(10nm (InGaAs)0.6(InAlAs)0.4/20nm (n-InGaAs)0.997Er0.003 randomly, 2×10 Si) 4×10 3 70×(10nm (InGaAs)0.6(InAlAs)0.4/20nm (n-InGaAs)0.997Er0.003 randomly, 4×10 Si) 6×10 4 70×(10nm (InGaAs)0.6(InAlAs)0.4/20nm (n-InGaAs)0.997Er0.003 randomly, 8×1018 Si) 1×10 0-7803-9552-2/05/$20.00 ©2005 IEEE 485 2005 International Conference on Thermoelectrics Figure 1: Schematic of device structure Device fabrication We fabricated the test patterns both on superlattice and substrate. The device was fabricated with first forming a 3μm-mesa by etching the superlattice layer down to the InP substrate using reactive ion etching. Ni/GeAu/Ni/Au layers were used as contact metal on both n and p type ErAs/InGaAs superlattice on the top of the mesa and InP substrate at the bottom of the mesa. 1μm-thick Ti/Au heater wires were formed on top of contact area. The heater wire and contact metal layer were isolated using 300 nm SiO2/SiN double layers. As devices sit both on top of the superlattice mesa and on the substrate, cross-plane Seebeck of the superlattice and the Seebeck of bulk InP substrate can be measured using these devices. Figure 2 is the CCD picture of these Seebeck measurement devices. Figure 3 illustrates the measurement set-up. Figure 2: Seebeck measurement devices sitting on both top of the 3 μm superlattice mesa and on the InP substrate respectively. Figure 3: Schematic of Seebeck coefficient measurement setup Results and Discussions The measurement of bulk Seebeck for the InP is straight forward using the devices sitting on the InP substrate. When current was applied through the device heater wire, there’s a temperature rise beneath the wire area. By measuring the temperature difference ∆T and voltage output ∆V between this heater wire point and another metal contact, the Seebeck coefficient α = ∆V/∆T can be known. Figure 4 shows the measurement results. 0 200 400 600 80