Study On Growth And Characteristics Of InAlN/InGaN/(Al)GaN Double-Heterostructure Materials

Due to the excellent electrical properties of III-V nitride-semiconductor heterostructures, GaN-based HEMTs are very promising for high-voltage, high-temperature and microwave-power devices. AlGaN/GaN heterojunction is the most commonly used structure among III-V nitride heterosturctures around the world, and quite a lot of remarkable achievements have been made based on AlGaN/GaN heterojunction-materials. However, as people further explore the potential of III-V nitride heterostructures for microwave-power applications, traditional AlGaN/GaN heterostructure can not meet increasingly strict requirements for performance and reliability, and several problems have hampered its wide applications. For example, firstly, to promote the high-frequence performance of a HEMT, the device size should be scaled down and the thickness of barrier layer in the heterojunction must be reduced proportionally to suppress the short-channel-effect and avoid the rapid reduction in the 2DEG density, which will deteriorate the heterojunction transport performance. Secondly, the 2DEG in traditional AlGaN/GaN HEMT devices can easily be excited by large voltage or high temperature and spill over from the channel, then degeneration of both carrier mobility and device reliability takes place. The former issue can be resolved by gate-recess process or replacing AlGaN barrier by thin-barrier materials, such as high-Al AlGaN, AlN and InAIN etc., among which the lattice-matched InAIN is very promising to improve the device reliability by eliminating anti-piezoelectric effect; and the second drawback can be overcomed by adoping back barrier, namely double heterostructure, which enhancing the 2DEG confinement in a heterojunction. For a common GaN-channel double-heterostructure, both AlGaN and InGaN are usually be employed as back barrier, and they can increase the conduction-band height of GaN buffer layer underneath the 2DEG channel so as to enhance the 2DEG confinement; another way to realize double-heterostructure is adopting InGaN channel. Owing to the narrower bandgap of InGaN mterials, a natural back barrier will form at the interface between InGaN channel and GaN buffer layer. Furthermore, the smaller electron effective mass in InGaN alloy is beneficial to further improve the performance of GaN-based heterosturcture materials and devices.Base on above-mentioned background information, in this dissertation, MOCVD growth of nearly lattice-matched InAlN/InGaN/(Al)GaN double-heterostructure materials and the fabrication of InAlN/InGaN/AlGaN double-heterostructure MIS-HEMTs are investigated. The major work and research results are listed as follows:1. The influences of temperature, chamber pressure, TMIn flow rate, NH3 flow rate, growth method and epitaxial template on InGaN layer during MOCVD growth are investigated. It is found that a higher temperature or chamber pressure will improve the crystalline quality and lower the InN mole fraction in InGaN. It should be noted that a too high or too low growth temperature might cause indium droplet on the surface, furthermore, a too low chamber pressure can not suppress the decomposition of InGaN effectively and will give rise to indium composition fluctuation. With increasing TMIn flow rate, both InN mole fraction and threading-dislocation density in InGaN increase and show a tendency of becoming saturated, simultaneously, the problem of indium droplets on the surface gets more obvious. Higher NH3 flow rate is helpful to increase the InN mole fraction in InGaN. For InGaN grown by MOCVD, pulsed growth manner can improve the crystalline quality but descend the InN mole fraction; in addition, the layer growth rate with pulsed growth manner is only 10%of that with continuous growth manner. During growth, the larger compressive strain provided by the growth template, the lower InN mole fraction it is in the InGaN film. Furthermore, larger compressive strain can suppress the problem of indium composition fluctuation better.2. For InGaN channel layer growth, to ensure the high quality of InGaN, proper growth parameters must be employed, thus too high indium composition and too thick channel thickness should be avoided. By combining AlGaN back barrier structure with InGaN channel,2DEG confinement can be further enhanced, at the same time, the more intensive compressive strain provided by AlGaN back barrier can suppress the problems of InN mole fraction fluctuation, phase separation and indium segregation in the channel.3. Both GaN bulk and InAlN/GaN heterojunction materials were grown on patterned/unpatterned sapphire substrates. By comparing the samples with the same structures on different substrates, it is demonstrated that the heterostructure materials grown on patterned sapphire substrates possess better crystalline quality, smoother heterojunction interface and improved surface morphology, as a result, higher heterostructure 2DEG mobility is achieved. Consequently, we conclude that, using patterned sapphire substrates for heterostructure epitaxy can further improve the performance and presents an important new method for III-V nitride heterostructure growth. In addition, by compraratively studying the InAlN/GaN heterostructure samples grown on different substrates, we find that the compressive strain will reduce the incorporation rate of indium in InAIN material, the same as the situation of InGaN.4. InAlN/InGaN/GaN double-heterostructure was grown and various methods were employed for characterization. Comparing to the common InAlN/GaN heterostructure, the 2DEG confinement is further enhanced in double-heterostructure; consequently, the heterojunction transport performance is superior at high temperature. At 593 K, for the InAlN/InGaN/GaN double-heterostructure, Hall measurement shows that the 2DEG mobility is still as high as 549 cm2/Vs and the sheet resistance is only 564 ohm/sq. On the basis of InAlN/InGaN/GaN double-heterostructure growth, by optimizing the growth method of AlGaN transition layer with gradually changed aluminum composition, InAlN/InGaN/AlGaN double-heterostructure was grown by adoping variable-temperature growth manner. The room-temperature 2DEG mobility in InAlN/InGaN/AlGaN double-heterostructure is as high as 1293 cm2/Vs, which is the highest mobility achieved in an InGaN channel.5. InAlN/InGaN/AlGaN double-heterostructure MIS-HEMTs were fabricated based on the InAlN/InGaN/AlGaN double-heterojunction wafer. The gate isolation dielectric is 10-nm-thick Si3N4, and the gate length and width are 0.5μm and 2x50 μm, respectively. A maximum drain current density of 1131.3 mA/mm and a maximum extrinsic transconductance of 155 mS/mm are demonstrated, moreover, the DIBL coefficient is as small as 6.6 mV/V, which is attributed to the enhanced 2DEG confinement in the double-heterostructure device. By alternating-current small-signal analysis, the device exhibits a current gain cutoff frequency and a maximum power gain cutoff frequency of 7 GHz and 14.5 GHz, respectively.