High k Dielectrics for GaAs compound semiconductor passivation

• Main Authors: Pen Chang, 張翔筆
• Other Authors: M. Hong
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/93049863965507699962

碩士 === 國立清華大學 === 材料科學工程學系 === 93 === The compound semiconductor essentially offer the advantages of high electron mobility and semi-insulating substrate, thus to be anticipated outperform Si in MIS applications. High-frequency wireless communications, high-speed computations, and microwave high power applications could be realized by devices based on MOS type related structures. Using High�n �� Dielectrics is a direction solving scaling quantum limit. To obtain electrically and thermodynamically stable insulator for surface passivation that exhibits a low density of state (Dit) and low leakage current is one important critical challenge in the compound semiconductor device processing. Remarkably, our earlier work of in-situ deposition of Ga2O3(Gd2O3) for GaAs passivation producing low Dit and low electrical leakage MOS (metal oxide semiconductor) diode. Subsequent put in use Ga2O3(Gd2O3) as gate dielectric with ion implantation demonstrated a successful depletion-mode and enhancement-mode GaAs MOSFETs. We also discovered that using pure Gd2O3 (��=14) epitaxially growing on GaAs (100) substrate for passivation, showing an excellent insulting barrier. In first work, Rapid thermal processing (RTP) with different ramping rates of 20°C/sec and 10°C/sec to 600°C (holding at temperature in 30 seconds) and under various gases of N2, H2 and forming gas was used to anneal the Ga2O3(Gd2O3)/GaAs heterostructures. Another anneal to ~780°C under ultra high vacuum (UHV) was also performed. The interface was rough in the order of 0.5-0.8 nm after the RTP at different gases. We found that a slower ramping rate gives a smoother interface. Moreover, Studies using x-ray reflectivity (XRR) and high resolution transmission electron microscope (HRTEM) have shown that samples properly annealed under UHV have maintained smooth and abrupt interfaces with the interfacial roughness being less than 0.2 nm. This indicates the thermodynamic stability between Ga2O3(Gd2O3) and GaAs. The interfacial smoothness was maintained in the UHV anneal to ~780°C. After high temperature annealing, the oxide remains as amorphous the oxide remains as amorphous, an important aspect for device consideration. Current-voltage and capacitance-voltage measurements have shown low leakage currents (10-8 to 10-9 A/cm2), high dielectric constants (�� = 15), and accumulation and inversion, indicative of a low interfacial density of states (Dit) between gate dielectrics and GaAs. The attainment of a smooth interface between the gate dielectric and GaAs, even after high temperature annealing for activating implanted dopant, is a must to ensure the low Dit and to maintain high carrier mobility in the channel of the MOSFET. In another work I have extended the investigation to new high �� dielectrics of HfO2 (�� = 20) and Sc2O3 (�� = 12) to passivate the GaAs(100) surface with different conditions by the MBE growth method. HfO2 films are recently used to replace SiO2 on Si industry for 45 nm CMOS as alternative gate dielectrics. We grow HfO2 films at elevated series temperature by using our unique in-situ multi-chamber system, using many kinds of instruments to research their different properties. I-V (current-voltage) and C-V (capacitance-voltage) measurements showed excellent electrical properties. The low leakage current density JE from growing amorphous 69 Å HfO2 film on GaAs at 1 MV/cm is 10-5 A/cm2, a 15nm thick Sc2O3 film showed a low leakage of 10-9A/cm2 at 1 MV/cm, and a breakdown field of 2.5-3.0 MV/cm. A very abrupt interface about one atomic layer thickness was observed by HRTEM. Epitaxial growth of (100) cubic HfO2 on GaAs (100) was also achieved by depositions at elevated temperature over 210℃. The property of epitaxial film is observed by X-ray diffraction. Dielectric films in amorphous form are usually preferred over the crystalline form due to the absence of grain boundaries as easy pathways of leakage. To raise the recrystallization temperature by mixing high �� HfO2 dielectrics with other chemical additions like Al and Si is underway. We demonstrate that in-situ annealing to obtain the recrystalized films and subsequent regrowth at higher temperature is a good way to achieve high quality epitaxial HfO2 films. The oxide surface and interfacial roughness were measured by XRR and HRTEM. The oxide film surface morphology is routinely measured by atomic force microscopy (AFM). Extensive analyses using XPS is now in progress to examine the interfacial structure.