Admittance characterization and analysis of trap states in AlGaN/GaN heterostructures

Abstract

We present a systematic study on the admittance characterization of trap states in AlGaN/GaN heterostructures. An equivalent circuit model was developed to analyze the spatial location, energy distribution density, and time constant of the AlGaN/GaN trap states. Results show that AlGaN/GaN interface trap states are responsible for the trapping behavior in the 10 kHz -1 MHz frequency range. An increase of AlGaN layer thickness or Al composition leads to greater trap state density at the AlGaN/GaN interface.1 Introduction The large conduction band discontinuity and high density two-dimensional electron gas (2DEG) in the AlGaN/GaN interface make AlGaN/GaN heterostructure field-effect transistors a promising candidate in high-power microwave amplifier applications [1,2]. However, the development of AlGaN/GaN heterostructure field-effect transistors (HFETs) is still largely hindered by the limiting effect of the trap states in AlGaN/GaN heterostructures [3]. These trap states may be located at the AlGaN surface, in the AlGaN barrier layer, at the AlGaN/GaN heterointerface, or in the GaN buffer layer. The presence of trap states in AlGaN/GaN HFETs can cause a voltage delay in device operation through trapping and de-trapping process, thereby degrading the power handling capability at high frequency. In this paper, we provide a qualitative and quantitative evaluation on the electronic properties of trap states. Bias voltage-and frequency-dependent admittance measurements were performed on AlGaN/GaN heterostructures. A frequency dispersion of the admittance was observed, and analyzed by deploying an equivalent circuit model.

Investigation of Trap States in AlInN/AlN/GaN Heterostructures by Frequency-Dependent Admittance Analysis

“…In general, for Schottky diode fabrication, the semiconductor surface is inevitably covered with a native thin insulating interfacial oxide layer if the semiconductor surface is prepared by the usual polishing and chemical etching process, in which the evaporation of metal is carried out in a conventional vacuum system. [19][20][21][22] The interfacial oxide layer is only a few monolayers thick. If this layer's thickness is smaller than 30 Å , most of the states are in equilibrium with the metal.…”

“…2, the frequency dispersion of the admittance strongly depends on the external bias, and becomes significant in the deep accumulation regime (at zero or near very small reverse voltage), in turn indicating that surface trap states are the dominant trapping mechanism in the 10 kHz to 1 MHz frequency range. 8,19 Because of these reasons, the component related to the interface trap states can be eliminated, and the effect of the surface trap states can be extracted by comparing the measured admittance values at the deep accumulation and weak depletion regime.…”

“…However, the bulk states have time constants as long as milliseconds, making their effects unobservable by admittance and currentvoltage measurement methods. 8,9,19,23 These traps states can be detected by using the deep-level transient spectroscopy (DLTS) method. 24 For these reasons, the bulk states were not considered in this analysis.…”

“…The method described by Schroder for the interface trap states in a metal-oxide-silicon system was used in these studies for the analysis of AlInN/AlN/GaN heterostructures with care given to the surface traps. 8,9,16,17,19,23,25 The parallel capacitance C p and conductance G p /x can be obtained from measured C m and G m /x curves by using the relation 8,17 In the equation, we take the barrier capacitance C b as the total of the C AlInN and C GaN cap capacitance values. C b was determined from the plateau in the C-V curve associated with the accumulation of electrons in the two-dimensional electron gas channel.…”

“…[15][16][17][18][19] The density of trap states of GaN metal-oxide-semiconductor (MOS) and AlGaN/GaN structures has been evaluated by using frequencydependent capacitance and conductance measurements. 16,17 Miller et al 8 and Chu et al 19 reported on investigations of trap states in AlGaN/GaN heterostructure field-effect transistors (HFETs), and Stoklas et al 9 reported on a trap density evaluation in AlGaN/GaN MOS heterostructure field-effect transistors (MOSHFETs) by using similar experimental methods.…”