The present invention relates to a semiconductor device composed of a Group III nitride semiconductor layer typically represented by (InxAl1-x)yGa1-yN (0≦x≦1, 0≦y≦1), i.e., a so-called gallium-nitride-based compound semiconductor layer and to a method for fabricating the same.
Studies have been made on the application of a Group III nitride semiconductor, i.e., a semiconductor composed of GaN, AlN, InN, or a mixed crystal thereof not only to a short-wavelength light-emitting device but also to an electronic device. In particular, the development of a heterojunction field effect transistor (HFET) device using a high-density 2-D electron gas formed at the heterojunction interface between AlxGa1-xN (0≦x≦1) and GaN as a high-output RF device has been pursued because of its high dielectric breakdown voltage and high saturation speed and its high possibility of allowing full use of these advantageous physical properties inherent in a nitride semiconductor.
Referring to FIG. 13, a conventional semiconductor device having a Group III nitride semiconductor layer will be described herein below.
As shown in FIG. 13, a buffer layer 102 is formed on a substrate 101 and a GaN layer 103 is formed on the buffer layer 102. An AlxGa1-xN layer 104 is formed on the GaN layer 103. Ohmic contacts 105 are formed in the AlxGa1-xN layer 104 and a gate electrode 106 is formed on the AlxGa1-xN layer 104.
In a HFET, the flow rate of a 2-D electron gas flowing in the channel region is normally controlled by adjusting a voltage applied to the gate electrode 106 forming a Schottky junction with the surface barrier layer of the AlxGa1-xN layer 104. Thus, the gate electrode having the Schottky junction formed on the surface of the nitride semiconductor layer is typically formed by depositing a metal gate electrode material directly on the surface of the nitride semiconductor layer. In this case, the reactivity between the gate electrode material and the nitride semiconductor layer should be increased to provide enhanced adhesion therebetween.
However, the increased reactivity between the gate electrode material and the nitride semiconductor layer presents a serious problem in minimizing a leakage characteristic at the Schottky junction formed finally. To minimize a leakage current, a material low in the reactivity (e.g., Au, Pt, Pd, or the like) to the nitride semiconductor layer may be used normally appropriately for the gate electrode. However, the gate electrode using the material low in reactivity is easily delaminated from the surface of the nitride semiconductor layer because it is low in the reactivity to the nitride semiconductor layer and therefore low in the adhesion to the surface of the nitride semiconductor layer.
To solve the problem of easy delamination of the gate electrode using a material low in the reactivity to the nitride semiconductor layer such as Au, Pt, or Pd, a method which enhances the adhesion of the gate electrode to the nitride semiconductor layer by vapor depositing Ti, Ni, Si, or the like in conjunction with Au, Pt, Pd, or the like has been adopted normally.
However, since Ti, Ni, or Si is a metal high in the reactivity to the nitride semiconductor layer, it chemically reacts with the nitride semiconductor layer to induce a defect such as a nitrogen void in the nitride semiconductor layer. Consequently, a leakage current flowing through the formed Schottky junction is increased significantly.
It has been proved that the adhesion of the gate electrode to the nitride semiconductor layer is enhanced by using Pd containing an extremely small amount of Si to form the gate electrode on the surface of the nitride semiconductor layer for the purpose of forming a Schottky junction which allows a reduction in the occurrence of a leakage current. If the amount of Si added to the material of the gate electrode is minimized, however, the need to suppress an effect of mechanical disturbance in the process of fabricating a semiconductor device after the formation of the gate electrode (e.g., the avoidance of excessive ultrasonic cleaning) grows enormously. In addition, the minimized amount of Si added to the material of the gate electrode mostly causes a new problem of delamination of the gate electrode that has once adhered to the surface of the nitride semiconductor layer if the gate electrode is formed on an epitaxial layer having an extremely rough surface and insufficient crystallinity or if a high-temperature heat treatment process is performed.