Low leakage current density (as low as 10 08 A/cm 2 at an applied voltage of 5 V) and high breakdown electrical field (larger than 4.5 MV/cm) of the liquid phase chemical-enhanced oxidized GaAs insulating layer enable the application to the GaAs MOSFET. The oxide layer is found to the composite of Ga 2 O 3 , As, and As 2 O 3 . The n-channel depletion mode GaAs MOSFET's are demonstrated and the I I I-V V V curves with complete pinch-off and saturation characteristics can be seen. A transconductance larger than 30 mS/mm can be achieved which is even better than those of the MESFET's fabricated on the same wafer structure.Index Terms-Compount semiconductor GaAs, depletion mode, liquid phase oxidation, MOSFET.
INTRODUCTIONNearly all of the planar semiconductor electronic devices are adversely affected by the electrical properties of the semiconductor surface and interface. Silicon, being the only exception, owes its unrivalled success to the ability of its oxide to passivate and impart excellent electrical properties to its surface. With niche areas in optoelectronics, and very high speed and low power data processing, III-V compound semiconductors are plagued with unacceptably large interface traps and a lack of a suitable passivation method to impart acceptable qualities to the interface. 1 Passivation of the GaAs surface has been shown to improve the electrical characteristics of GaAs devices. One example is improvement in the electrical characteristics of GaAs metal-insulatorsemiconductor diodes. 2 For this purpose, the GaAs surface is exposed to agents involving S (i.e., S passivation), such as Na 2 S and (NH 4 ) 2 S, and P 2 S 5 / (NH 4 ) 2 S has been used as the S passivation source. Elemental S, selenium, and tellurium have also been used to passivate the GaAs surface. 2,3 The purpose of this study is to investigate the effect of S passivation of GaAs on improving electrical characteristics of metal-oxide-semiconductor (MOS) structures with the oxide layers prepared by the liq-uid-phase chemical-enhanced oxidation (LPCEO) technique. 4,5 In this paper, the electrical properties of oxide layers on the (NH 4 ) 2 S x -passivated GaAs surface were realized by using current-voltage measurements. The oxide layer thickness and refractive index were measured by ellipsometry. In addition, the chemical composition of oxide layers was analyzed by secondary ion mass spectroscopy (SIMS). According to the results of SIMS, a model is proposed to illustrate the chemical composition of the oxide layer on the S-passivated GaAs surface and to express improvement of the device parameter. The effect of different surface treatments on the oxidation kinetics is characterized by analyzing their activation energies (E a ) using Arrhenius plots.
EXPERIMENTALSamples of GaAs wafers were first cleaned by organic solvents (2-propanol, acetone, and methanol) and etched in H 2 SO 4 :H 2 O 2 :H 2 O (4:1:1). After rinsing with deionized (DI) water, some samples were passivated with (NH 4 ) 2 S x for 0.5-1 h. To investigate the effects of pretreatments, we further employed other samples treated by NH 4 OH or HCl in 10-20% concentration, respectively. Then the as-grown specimens were rinsed with DI water and dried with nitrogen. After this step, the wafers were immediately transferred into an oxidation system under constant temperature. The experimental apparatus and processes of the LPCEO technique can be found in Ref. 4.The properties of GaAs oxide layers prepared by liquid-phase chemical-enhanced oxidation of the (NH 4 ) 2 S x -passivated GaAs surface are investigated. The initial oxidation rate is suppressed and the refractive indices of oxide layers are lower after S passivation. In accordance with the depth profiles measured by secondary ion...
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