MOCVD Growth Of Antimonide Semiconductors And Simulation Of Antimonide-based Thermophotovoltaic Devices

The ternary antimonide alloy InAs_1-xSb_x could cover the wavelength ranges of 3～5μm and 8～14μm,where the atmospheric absorption is minimum,by adjusting its solid composition x.And the quaternary antimonide alloy Ga_xIn_1-xAs_1-ySb_y could cover a large range of wavelength from 1.7μm to 4.2μm,when lattice-matched to GaSb substrates.The narrow band-gap antimonide alloys have received considerable interests because of their potential applications of thermophotovoltaic devices. Thermophotovoltaic cell(TPV) is a sort of photovoltaic devices which could convert infrared radiation to electricity.As a prospective renewable energy technology, thermo- photovoltaic technology has become a new research hotspot.Recently,the experimental results reported that GaInAsSb/GaSb thermophotovoltaic cells could be obtained by MBE,LPE and MOCVD techniques.And GaInAsSb/GaSb thermophotovoltaic cells with a cutoff wavelength of 2μm(bandgap of 0.55eV) have internal quantum efficiency of more than 90%have been demonstrated in some relevant reports.Metal-Organic Chemical Vapor Deposition(MOCVD),a non-equilibrium technology,has proved itself as a unique and important epitaxial crystal growth technology.Many studies about growth characters of InAsSb and GaInAsSb thin films deposited by MOCVD technique have been done recently.In our work,the growth characters of InAsSb materials by LP-MOCVD system have been studied. Firstly,the InAsSb layers were obtained by LP-MOCVD.In the meantime,the manipulation of MOCVD equipment employed to grow InAsSb materials was mastered;Secondly,the Characterization of InAsSb epitaxial layer samples have been performed with a range of analysis tools,High Resolution X-ray Diffraction(HRXRD), XPS and Van der Pauw Hall measurements.Combining the conclusions of every testing above and the growth parameters,we have made a detail discussion of the effect of growth parameters to the properities of InAsSb layers,and got the conclusions are as follows:(1).There are several significant MOCVD growth differences between the arsenides,or phosphides and the high quality antimonides.For As or P nased materials,the high chemical bonding strength and high vapor pressure of As and P lead to a wide growth windows for tempture.On the other hand,because of the unique characteristic of Sb,there is a very narrow growth window for tempture.In addition, the lack of stable group V hydride for antimony source and insulating antimonide substrates introduce many difficulties to MOCVD growth of antimonides.(2).We have made the detail discussion about the influence of growth parameters such as growth temperature,vaporⅤ/Ⅲratio in reactor on the crystal quality of InAsSb epitaxial layer prepared by MOCVD.Due to the unique characteristic of antimonide,under the typical growth conditions the InAsSb alloy MOCVD process is the "kinetically controlled regime".Both growth temperature and vaporⅤ/Ⅲratio are crucial growth parameters because they drastically influence the crystal quality of the epitaxial layers.By the discussion,we find the optimal growth temperature range is 450-520℃and vaporⅤ/Ⅲratio range is 1.0-2.0, respectively.(3).Hall measurement was used to study the electrical properties.We have obtained carrier concentration and mobility of InAs_1-xSb_x epitaxial layer.Firstly,the undoped InAs_1-xSb_x epitaxial layers with higher Sb content with higher background carrier concentration indicated more impurities.This could be explained as more alkyl antimony compound source lead to more carbon incorporation.Secondly, because we have not done proper action to reduce the effect of lattice mismatch,the mobility of we samples is lower than the ratio reported from other articles.Meanwhile,in order to design and fabricate high-proformance GaInAsSb TPV cells that could be applied to the wavelength around 2.2μm,we introduced the theoretical analysis of Ga_0.87In_0.13As_0.12Sb_0.88 -based thermophotovoltaic cells into our work.First,we utilized a semi-empirical model to analysise dielectric function of Ga_xIn_1-xAs_1-ySb_y materials.The dependence of absorption coefficient a of Ga_xIn_1-xAs_1-ySb_y alloy on temperature and alloy composition are presented in the energy range of 0～5eV.In this model,we have taken into account contributions to dielectric function from corresponding inter-band transitions of various critical points in the Ga_xIn_1-xAs_1-ySb_y energy band structure.As well as introduced the damped harmonic oscillator model into calculation.The related basic material parameters of Ga_xIn_1-xAs_1-ySb_y alloy used in the calculation are obtained by interpolating the known binary and ternary compound data.An appropriate interpolation method was adopted in this work.Then the Ga_0.87In_0.13As_0.12Sb_0.88 alloy based TPV devices were analyzed.We discussed the influence of the relevant device and material parameters on dark current and quantum efficiency.The simulation results of dark current show that a variety of recommbination mechanisms in p-GaInAsSb affecting the dark current have something to do with the carrier concentration in p-type layer.In the range of low carrier concentration,dark current is mainly restricted by the surface recommbination mechanism.While in the range of high carrier concentration,the dark current is limited mainly by Auger recommbination mechanism.The radiation and SRH recommbination mechanisms have less effect on dark current density of devices. Combining with a variety of recombination mechanisms in the compound,the parameters of p-GaInAsSb influence the dark current of TPV cells.Lowering surface recombination velocity can reduce surface recombination current density,lowering carrier concentration can inhibit the Auger recommbination current density,and attenuation of the GaInAsSb material would be helpful to decrease the dark current of TPV cells.Thus,a reasonable choice of material thickness,carrier concentration and the surface passivation techniques would benefit the proformance of devices.Besides, radioactive recombination current depends on light recycling factor,and SRH has much to do with the lifetime of SRH recommbination.According to the discussion, we can see that the radiation recombination contributes little to the total dark current, when we set the parameters as the optimal values in other relevant articles,combined with zero effect of light recycling factor.And if we take light recycling factor into accout,radioactive recombination current will be further reduced.Though SRH recombination current is relatively high in the case of low carrier concentration,its contribution to total dark current is not as large as that of surface recombination.For the purpose of further reduction of SRH recombination current,the improvement of the material quality and the SRH minority carrier lifetime are needed.Finally,we analyzed the internal quantum efficiency(IQE) of Ga_0.87In_0.13As_0.12Sb_0.88 based homo-juction diode with p-on-n structure.We focused on effects of device parameters on IQE,including the layer thickness,doping concentration of p-type and n-type Ga_0.87In_0.13As_0.12Sb_0.88,the recombination velocity of top and back surface.And a relationship between IQE and these parameters has been established.Through the simulation,it is found that,in the p-on-n structure,we should choose a relatively thick p- GaInAsSb material as light-absorbing layer,in order to obtain a high IQE.The preferable doping concentration of n and p areas should be in the range of 10¹⁷～10¹⁸cm^-3.Then according to the device sizes,set values of doping concentration.For n +-p junction,combined with dark current of the simulation results,if the concentration of p-doped area is around 10¹⁷ cm^-3,doping concentration of n-GaInAsSb should be between 3x10¹⁷ and 10¹⁸cm^-3,to obtain a small dark current.At the same time,reduction of the top surface recombination velocity is an effective method to improve the IQE of TPV cells.However,the back surface recombination velocity plays an insignificant role in affecting the IQE. Finally,we studied the IQE of TPV devices with optimal width of active region.In this work,the manipulation of MOCVD equipment employed to grow InAsSb materials and some material testing methods were mastered.Meanwhile,in order to design and fabricate high-proformance GaInAsSb TPV cells that could be applied to the wavelength longer than 2.2μm,we performed the theoretical analysis on dark current and internal quantum efficiency of Ga_0.87In_0.13As_0.12Sb_0.88 -based thermophotovoltaic cells into our work.The results of this work can be a foundation for producing thermophotovoltaic cells based on GaInAsSb materials in the future.