MOCVD Growth Of Antimonide Semiconductors For Thermophotovoltaic Cells And The Device Simulation

Recently, narrow band-gap thermphotovoltaic (TPV) cells applied for low-temperature (<1000℃) radiation conversion have received extensive attention because they have high stability and safety. Antimonide-based GaxIn1-xAs1-ySby quaternary alloys have a large range of band-gaps from 0.296 eV (4.2μm) to 0.726 eV (1.7μm) when lattice-matched to GaSb substrates, which covers the typical wavelength range of low-radiator temperature TPV cells. Quantum dots (QDs) applied in TPV cells can realize the intermediate band structures and the multiple carrier excitation process, and then improve the conversion efficiency significantly. InAs1-xSbx QDs have the responsive wavelength of 2-3μm, which also covers the typical wavelength range of low-radiator temperature TPV cells. Most of the recently reported antimonide-based TPV cells are fabricated by metalorganic chemical vapor deposition (MOCVD) technique. However, compared with the traditional matured semiconductors, the epitaxial growth quality of the antimonides is relatively poor. Moreover, it is difficult to establish the complete physical model for the GaxIn1-xAs1-ySby TPV cell since the material parameters of the quaternary alloys are difficult to be obtained. It is stringent to build the theoretical foundation in order to carry out practical research.Aiming at the above-mentioned problems, our work has been focused on studying the properties of antimonide semiconductor epilayers and InAs1-xSbx QDs grown by low pressure metalorganic chemical vapor deposition (LP-MOCVD) technigue and simulating the GaxIn1-xAs1-ySby TPV cell structures.Ternary InAs1-xSbx and quaternary GaxIn1-xAs1-ySby alloys were grown by LP-MOCVD. The properties and growth characteristics of epitaxial layers were investigated. Growth temperature is a key growth parameter because it influences greatly the surface morphology, the crystalline quality and the alloy compositions of the epitaxial layers. The low growth temperature leads to the accumulation of Sb on the surface because of the strong metallic and low volatile character of antimony atoms. High growth temperature leads to the high density surface defects with tetrahedral and strip pyramids in epitaxial layers, and the profiles of the pyramids are (111) crystal plane with the lowest surface energy and elongate to the [110] direction. The surfactant characters of Sb and the (2×4) surface reconstruction may account 吉林大学博士学位论文for forming the pyramids. Vapor III/V ratio is also an important growth parameter. The excess group III metals and Sb appear on the surface as a separate phase for the high and low III/V ratios, respectively. The optimized growth temperature range is 480-500℃and theⅢ/Ⅴratio range is 0.85～1 for InAs1-xSbx ternary alloys. The optimized growth temperature is 520℃and the III/V ratio range is 0.8-1 for GaxIn1-xAs1-ySby quaternary alloys. High quality InAs1-xSbx and GaxIn1-xAs1-ySby epitaxial layers have been achieved. The InAs1-xSbx epitaxial layer with the highest Sb composition of 0.215 was achieved by choosing the growth temperature of 480℃and III/V ratio of 0.9. Unintentionally GaSb-rich and InAs-rich GaxIn1-xAs1-ySby shows the p-type and n-type electric conduction, respectively. The lowest hole concentration is 5.91×1015cm-3 and the highest hole mobility is 415cm2/V.s for GaSb-rich GaxIn1-xAs1-ySby, which is comparable with the results reported in the literatures.Self-assembled InAs1-xSbx QDs have been grown, and the effects of the growth parameters on the surface morphologies such as the shape, size, density and uniformity of the QDs were investigated in detail. The ultra low and high growth temperature both lead to big volume and low density, which were relevant to the surfactant character of Sb and the high mobility of In adatoms, respectively. The optimum growth temperature range is 460～470℃for high density and ordered array QDs. The growth time is also a key growth parameter. It shows that the forming of local nonequilibrium process by importing precursors alternately to separate group III and V species can reduce the mobility of In adatoms on growth surface and improve the density of QDs. The high density and ordered array QDs were obtained by controlling the growth time and periods of the valve on and off sequences of TMIn and AsH3. The experimental results show that the uniformity InAs1-xSbx nanotubes were formed without on and off sequences of precursors. The interruption time has some effect on the QDs but not so strong. Even the interruption time was zero the process still provided interruption time as the temperature did not decrease fast enough. The "antimonidation" process was introduced, e.g. the TMSb was imported before the growth, and Sb adatoms pre-adsorbed on the epitaxial layer can reduce the mobility of In adatoms. But the "antimonidation" has not improved the density significantly. Since the TMSb was imported in the growth process, and the surfactant effects of Sb adatoms also shown in growth process. The growth pressure also influences the morphology characters, so the experimental results demonstrate that the high density QDs were obtained at the low growth pressure of 2000Pa. The high density of 5×1010cm-2 and ordered array of InAs1-xSbx QDs were achieved with the optimized growth parameters, which can be applied for low-temperature radiator TPV cells.The complete GaxIn1-xAs1-ySby material parameters related to the performances of TPV cells were simulated, and the material and the device structure physical model were established. The maximum output power density (Pmax) and the energy conversion efficiency (ηcel) are the figures of merit of TPV cells, which depends on the photovoltaic device-related properties (e.g.short-circuit current density (Jsc), open-circuit voltage (Voc) and fill factor (FF)). These properties are primarily determined by the material parameters and the device structures. The basic material parameters of GaxIn1-xAs1-ySby are calculated by interpolating the related binary and ternary alloy data. The intrinsic carrier concentration is calculated using a semi-empirical formula. The minority carrier mobility is calculated using an extended Caughey-Thomas model, which accounts for both temperature and carrier concentration. The recombination rates of the main minority carrier recombination mechanisms are calculated by semi-empirical formulas. The absorption coefficients are calculated using Adachi's model, and modified to include temperature effects.The P-GaSb window/P-GaxIn1-xAs1-ySby active region/N-GaxIn1-xAs1-ySby active region/N-GaSb structure TPV cells have been simulated systematically, and the effects of the material and device structure parameters on device performances were analyzed. The simulations are carried out for a TPV system with a fixed spectral control filter at a radiator temperature of 950℃and cell temperature of 27℃.It shows that adopting the thick P-GaxIn1-xAs1-ySby active region with longer minority carrier diffusion length as the main optical absorption region can improve the carrier collection efficiency, and the thin N-GaxIn1-xAs1-ySby active region contributes little to the Jsc. But N-region parameters still influence Voc through the recombination current density (Jo). Decreasing N-region thickness can suppress the effects of the bulk recombination mechanisms on Voc, but increase the effect of the back surface recombination (SNh) which needs to be suppressed for improving Voc.The carrier concentration and minority carrier recombination mechanisms of P-GaxIn1-xAs1-ySby active region have the strong effects on device performance. The recombination mechanisms of P-region have the different effects on Voc for different injection levels. The Pmax andηcel are limited by the different recombination mechanisms for the different carrier concentration ranges, and it shows that the recombination rate needs to be suppressed to improve the device performances. The SRH and surface recombination can be suppressed by improving the material growth and the surface passivation process, respectively. The Auger and radiative recombination can be suppressed by reducing the carrier concentration. In addition, the P-region thickness also has an important effect on device performances, and the optimum thickness is achieved for the maximal Pmax andηcel.Adopting the P-GaSb window as the passivation layer for the P-GaxIn1-xAs1-ySby active region can suppress the front surface recombination and improve device performances. Increasing P-GaSb window carrier concentration leads to the upwards energy-band bending and the minority carrier recombination rate decreasing, for the front interface of the P-GaxIn1-xAs1-ySby active region, resulting in increase in device performances. However, the heavily doping leads to decrease minority carrier diffusion length, and the front surface of the window layer has the high surface recombination velocity, therefore, it is appropriate to reduce the window thickness to increase the device performances.The cell temperature has the strong effect on the cell properties. The cut-off wavelength of the internal quantum efficiency increases with temperature, leading to increase in Jsc. However, both Voc and FF decrease with temperature, which leads to the decrease in Pout andηcell.The device performances variations with temperature are linear, and the calculated temperature coefficient of Jsc, Voc, FF, Pout andηcell for Gao.84In0.16As0.14Sb0.86 TPV cells are 1.8mA cm-2.℃-1,-1.03mV.℃-1,-0.0011℃-1,-3.38mW.cm-2.℃-1 and-0.155℃-1, respectively.The optimized material and device structures have been achieved for Ga0.8In0.2As0.18Sb0.82 TPV cells. For the optimized cells operated in a TPV system with the fixed spectral control filter and the radiator temperature of 950℃, the Pmax andηcel at 30℃are 0.76W/cm2 and 31%, respectively. The back surface reflector (BSR) can increase the back surface reflectance, and therefore increase the photon absorption. Increasing the photon absorption by BSR can also increase the photo recycling factor ((?)), resulting in decrease in radiative recombination rate. For the back surface reflectance of 100%, the Pmax andηcel are 0.82W/cm2 and 33%, respectively. The actual optical path is increased by BSR, resulting in the optimum P-region thickness decrease from 4-5μm to 2.5-3μm, which can reduce the cost of the TPV cells.