Study On The Novel Nitride Ingan Channle Heterostructures And HEMT Devices

GaN based heterostructures have attracted extensive attention due to the two dimensional electron gas?2DEG?in the channel,which is generated by the spontaneous and piezoelectric polarization.In the past two decades,significant progress has been achieved in the research of high electron mobility transistors?HEMT?based on AlGaN/GaN heterostructure.With the increasing demands for the device performance,the limitations of AlGaN/GaN heterostructures,such as inferior 2DEG confinement,restricted carrier mobility,deteriorated device reliability caused by lattice mismatch,have been revealed.In GaN is proposed as a novel material for channel layer,which is superior to the conventional GaN channel mainly in two aspects.Firstly,InGaN possesses higher carrier drift velocity due to its relatively lower electron effective mass,which is important to the promotion of devices'working frequency;Secondly,InGaN channel layer inserted between barrier and buffer layers is a suitable way to modulate the band structure and increase the band offset with the barrier and buffer layers,which is beneficial to enhance the 2DEG confinement,and then to increase the working efficiency and reliability of the devices.This dissertation mainly focuses on the growth and optimization of high-quality InGaN channel heterostructures as well as the fabrication and characterization of high-performance HEMT,and the major achievements are listed as follows:1.Based on time-division transport theory,pulsed metal organic chemical vapor deposition?MOCVD?is proposed for InGaN material growth,which is beneficial to the improvement in material quality.The properties of InGaN films grown between500°C⁸00°C is discussed,and the optimum temperature range is determined for the growth of In_0.05Ga_0.95N film.These results lay the foundation for the research on InGaN channel heterostructures.2.The optimization of two key issues in the research of InGaN channel heterostructures,growth temperature and thickness for channel layer,is performed based on the experiments,and the optimum growth temperature and thickness for In GaN channel is obtained as 740°C and 21 nm,respectively.High-quality Al_0.35Ga_0.65N/In_0.05Ga_0.95N heterostructure is successfully grown on sapphire substrate under the optimal condition.The heterostructure possesses smooth interface and surface with parallel atomic steps,and the root-mean-square?RMS?roughness are 0.19 nm and 0.15 nm,respectively.The transport properties of AlGaN/InGaN heterostructure are superior to that of conventional AlGaN/GaN heterostructure at elevated temperature?higher than 360 K?.Furthermore,the2DEG mobility of AlGaN/InGaN heterostructure at 300 K is 1681 cm²/Vs,with an increase of 30%compared to the best reported result.These results are reported by the international famous media“Semiconductor Today”.3.Nearly lattice-matched In_0.18Al_0.82N/In_0.05Ga_0.95N heterostructure is successfully grown on sapphire substrate,and Two-Step AlN interlayer is proposed in order to improve the transport properties.The employment of Two-Step AlN interlayer resolves the conflict between the channel material quality and interface morphology,and promotes the 2DEG mobility and density to 967 cm²/Vs and 1.85×10¹³ cm^-2,respectively.Two-Step AlN interlayer combines the advantages of constant high and low temperature interlayers,and provides an excellent interlayer growth mode for InAlN/InGaN heterostructures.4.Nearly lattice-matched In_0.18Al_0.82N/In_0.05Ga_0.95N/In_0.18Al_0.82N/In_0.05Ga_0.95N double channel heterostructure is proposed and grown successfully for the first time.It is performed by taking advantage of the high intensity of spontaneous polarization of InAlN barrier,which induces high density 2DEG in the channel.Double channel structure makes it possible to increase 2DEG density without adverse effects on the mobility,and then to decreases the sheet resistance.Benefiting from the fine control of pulsed MOCVD and the protection of Two-Step AlN interlayer,the interfaces of double channel heterostructure are flat and sharp.At 300 K,the 2DEG mobility and density of double heterostructure are 1119cm²/Vs and 2.53×10¹³ cm^-2,respectively,yielding a sheet resistance of 218.5?/?,which is the record of reported results of InGaN channel heterostructures.5.Al_0.35Ga_0.65N/In_0.05Ga_0.95N HEMT on sapphire substrate is successfully fabricated.The HEMT with gate length of 1?m and gate width of 2×50?m is fabricated on the AlGaN/InGaN heterostructure with 2DEG mobility of 1650 cm²/Vs and density of 1.18×10¹³ cm^-2.The HEMT exhibits maximum drain current of 1128.2 mA/mm and maximum transconductance of 188 mS/mm at 300 K,and the transconductance almost keeps the maximum at a gate voltage range of-3.2V^-0.2V,which is due to the superior 2DEG confinement of AlGaN/InGaN heterostructure.In addition,AlGaN/InGaN HEMT possesses excellent high-temperature performance.At 400 K and 500 K,the maximum drain current reaches 81 percent and 68 percent of that at 300 K,which is far superior to conventional AlGaN/GaN HEMT.Besides,the HEMT exhibits current gain cutoff frequency?fT?of 10.1GHz and maximum oscillation(f_MAX)of 14.0 GHz with the gate length of 1?m.In conclusion,this dissertation presents a study on the structure design,material growth,HEMT fabrication,and characteristic analysis for the novel InGaN channel heterostructures.The works significantly improve the depth of research on In GaN channel heterostructure,and the results in this dissertation will be the guidance and reference for further study of InGaN channel heterostructures based electronic devices.