Study On GaN-Based Double Heterostructures

Due to unique application potentials in electronic devices with high frequency,high power, high temperature, high voltage, and anti-irradiation, GaN-basedsemiconductor (including InN, GaN, AlN and their alloys) have achieved rapiddevelopment in the last 10 years. Although great improvements has been achieved inAlGaN/GaN HEMTs with high power and high frequency, but a low carrierconfinement of conventional AlGaN/GaN single heterojunction HEMT device result insome severe questions, such as low power gain, bad pinchoff under high drain voltage,and layge buffer leakage current. To solve these problems, the GaN doubleheterostructure (DH) HEMT devices have be presented to improve the 2DEGconfinement. By introducing the back barrier obtain better confinement of channel, GaNdouble heterostructure devices can effectively prevent short channel effects, andimprove the breakdown characteristics in high-power applications, which are verysuitable for high temperature environments. The main research of this paper focused onGaN-based double heterostructure materials and device characteristics.The main results in the paper are as follows:1. The energy band diagram and carrier distribution of AlGaN / GaN, Al_xGa_1-xN/GaN/Al_yGa_1-yN/ GaN, Al_xGa_1-xN/GaN/Al_yGa_1-yN and AlGaN / GaN / InGaN / GaN structurewere simulated and optimized by solving 1D Poisson equation.For Al_xGa_1-xN/GaN/Al_yGa_1-yN/ GaN structure, a high back-barrier can be achievedby using AlGaN back barrier layer with high Al composition and large thickness.However, a severe parasitic channel can be formed and deteriorate device performance.Two methods were presented to solve this queations. One method is using a lower Alcomposition and thinner back barrier layer, and another better method is using a backbarrier with Al composition gradient from 0 to 0.1.There is no parasitic channel in Al_xGa_1-xN/GaN/Al_yGa_1-yN structures due to usingAlGaN buffer layer. By increasing Al composition, the height of back barrier wasincreased. The GaN channel layer grown on AlGaN buffer has a compressive strain. Sothe lattice mismatch and strain of top AlGaN layer will be decreased, which ispropitious to device reliability. But the piezoelectric polarization have weakened,resulting in smaller 2DEG density.Although the band gap energy of InGaN is less than that of GaN, the larger polarization field which is opposite to the AlGaN layer can be formed between theInGaN and the GaN, which will raise the potential energy. The mismatch between GaNand InGaN will generate parasitic channel. However, the carrier in parasitic channel canspill over into the main channel, thereby increasing the 2DEG density in the potentialwell.2. Different kinds of AlGaN/GaN double heterostructure materials with the top barrierlayer with Al composition of 30% and high Al composition were grown by MOCVD .The characteristics of these materials were anAl_yzed by AFM, XRD, CV, Hall and PLmeasurement.Seen from the AFM test results, it was found that the mismatch between backbarrier layer and channel layer will affect the surface morphology of the material.Al_xGa_1-xN/GaN/Al_yGa_1-yN/ GaN structure (back barrier with Al compositon of 10%)and AlGaN / GaN / InGaN / GaN structure have a poor surface.Only the CV curve of Al_0.3Ga_0.7N / GaN/Al_0.1Ga_0.9N / GaN structural had shownthe doubel channel phenomenon. The parasitic channel can effectively suppress byusing Al_xGa_1-xN/GaN/Al_yGa_1-yN (y = 0 0.1) / GaN structure, Al_xGa_1-xN/GaN/Al_yGa_1-yN structures and AlGaN / GaN / InGaN / GaN structure.It was found from the resultes of Hall measurement that while the 2DEG densitywas small, the mobility of DH was lower than of SH. Due to double-heterostructureconfinement improvement, 2DEG was shifted to the interface, the interface roughnessand alloy disorder scattering became important in the DH. When the 2DEG density ishigh, the 2DEG is very close to the interface, no matter what material is. They weresubject to interface roughness scattering and alloy disorder scattering. However, the SHhave poor confinement, then the electron can spill over and become 3D electronic. Thus,the mobility of DH was higher than of SH.3. The AlGaN/GaN single and double heterojunction HEMTs were fabricated. Thedevice characteristics in room temperature and high temperature and the breakdownvoltage of the HEMTs were studied.Because the double heterostructure HEMT have better channel confinement, unlikethe conventional single heterojunction HEMT, the drain induced barrier lowing (DIBL)effect of the double heterostructure HEMT was prevented effectively, and the off-statecurrent and the subthreshold current was very small.The CV results under high temperature showed that the buffer layer leakage ofdouble heterostructure HEMTs was very small, and the DH HEMTs can work at highertemperature than the SH HEMTs. Under high temperature, the mobility of DH 2DEG was not affected by the background electrons with low mobility. For singleheterojunction HEMTs, the 2DEG mobility under high temperature will decreasedrapidly and degraded device performance at high temperature.Moreover, it was shown that a good carrier confiment of DH will increase thebreakdown voltage. As double-heterostructure materials have good confinement andcan effectively inhibit the breakdown caused by buffer leakage, which greatly improvedthe breakdown characteristics of HEMT devices.