Characterization of GaN and Al 0.35 Ga 0.65 N/GaN Heterostructures by Scanning Kelvin Probe Microscopy
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Characterization of GaN and Al0.35Ga0.65N/GaN Heterostructures by Scanning Kelvin Probe Microscopy G. Koley and M. G. Spencer Department of Electrical and Computer Engineering Cornell University, Ithaca, New York 14853 ABSTRACT Scanning Kelvin probe microscopy (SKPM) technique operated in feedback mode has been used to characterize GaN (unintentionally n-type doped, n+ doped and semi-insulating), and Al0.35Ga0.65N/GaN heterostructures (with varying Al0.35Ga0.65N thickness) grown by metalorganic chemical vapor deposition and molecular beam epitaxy. SKPM was used to measure the surface potential on these materials. The measurement technique was calibrated using metal calibration samples of Pt, Au, Ni and Al. The BSBH for n-doped GaN was measured to be 0.7 eV, which is in good agreement with values reported in the literature. Growth features such as dislocations present on the surfaces of III-nitrides were also investigated for their electrical properties using SKPM and non-contact mode atomic force microscopy, simultaneously. The dislocations have been found to be negatively charged for GaN as well as Al0.35Ga0.65N/GaN heterostructure samples.
INTRODUCTION There have been a lot of research initiatives in the field of III-nitride semiconductor devices in recent years [1,2]. Due to large piezo-electric coefficients of the nitrides, these materials are suitable for fabrication of AlxGa1-xN/GaN heterostructure field effect transistors (HFETs) without any intentional doping [3]. However, the origin of the two-dimensional electron gas (2DEG) present at the AlxGa1-xN/GaN interface is not well understood. Various models have been proposed to explain the observation of 2DEG, assuming barrier potential at the AlGaN bare surface [3] or GaN/substrate interface [4]. On the other hand, due to non-availability of proper lattice and thermally matched substrates, nitride epitaxial layers can have as much as 1010 cm-2 dislocations. These dislocations can get charged up and lead to many detrimental effects, such as, scattering electrons [5], and reducing the 2DEG at the AlxGa1-xN/GaN interface [6]. In the present study, we have used scanning Kelvin probe microscopy (SKPM) technique to measure the surface potential on GaN and AlGaN/GaN heterostructures grown by metalorganic chemical vapor deposition (MOCVD). The threading dislocations, as well as, other extended defect structures on the III-nitride epi-layers have also been characterized.
EXPERIMENTS The SKPM technique has been described in detail elsewhere [7,8], so we will discuss the salient features of the measurements only briefly. For SKPM measurements, sinusoidal ac signal (at frequency ω) is applied to the probe tip (made of p+ doped Si) along with a dc feedback voltage. The dc feedback voltage, Vdc, (referenced to the sample, which is biased at ground E4.4.1
potential by establishing an Ohmic contact to it) is applied to the tip to nullify the ω-component of the force between the tip and the sample. Neglecting the influence of any charges present on the surface, Vdc is given
