Surface photovoltage spectroscopy of minority carrier diffusion lengths in undoped and Si‐doped GaN epitaxial films

Abstract

Undoped-and Si-doped GaN epitaxial films were grown on sapphire substrates by metal-organic chemical vapor deposition. Variable angle spectroscopic ellipsometry (VASE) and surface photovoltage (SPV) spectroscopy were used to determine the minority carrier diffusion lengths in the films. Both the reflectivity (R) and absorption coefficient (α) could be calculated from the VASE measurements. In the SPV spectra at room temperature, a strong transition with a threshold at 3.42 eV was observed in both films, while an exciton-related absorption was observed only in the undoped GaN. The minority carrier diffusion lengths were measured to be about 200 nm for the undoped GaN and about 20 nm for the Si-doped GaN. Moreover, a large divergence in the range of 1/α in the undoped GaN was found and attributed to a high surface barrier energy.Gallium nitride (GaN) and its alloys are promising materials for optoelectronics, high-power electronics, and high-temperature electronics due to their physical and electrical properties [1]. Despite high background electron concentrations in GaN films, GaN-based devices have shown good efficiencies [2]. Since the minority carrier transport properties play a significant role in the performance of bipolar devices, measurement of properties such as diffusion length and lifetime are essential to understanding and improving device performance. The diffusion length of minority carriers, L, has been mainly been determined with surface photovoltage (SPV) and electron beam induced current (EBIC) measurements [3,4]. Most previous studies on the SPV of GaN have extracted the diffusion lengths of minority carriers, ignoring the effect of reflectivity, R, and incident light flux, Φ, on the photon energy [5,6]. In this study, L in n-type GaN films was investigated using both variable angle spectroscopic ellipsometry (VASE) and SPV measurements. Moreover, the reflectivity, R(E), and the incident light flux, Φ(E), as a function of the photon energy were considered in the analysis, in contrast with the previous analyses of GaN.Two GaN films were grown on sapphire substrates by metallorganic chemical vapor deposition. One was unintentionally doped (undoped) with a resulting background electron concentration of 1.5 × 10 17 cm -3 , and the other was Si-doped with an electron concentration of 5.8×10 18 cm -3 . The thickness of both GaN layers was about 1.1 µm. In order to obtain refractive index and extinction coefficient, VASE measurements were performed in the range of approximately 2 eV to 6 eV. The incident light was polarized and the incident angle was fixed at 75 degrees. For the SPV measurements, monochromatic light from a Xe lamp was chopped at a frequency of 320 Hz. The sample stage was grounded and the surface photovoltage was measured at room temperature.

Surface photovoltage in undoped n-type GaN

“…27,28,32 However, this is not the case for GaN layers grown by metalorganic chemical vapor deposition ͑MOCVD͒ and molecular beam epitaxy ͑MBE͒ on sapphire substrates. A high density of threading dislocations ͑10 9 -10 10 cm −2 ͒ results in a very small minority carrier diffusion length of 0.02-0.3 m, [33][34][35][36] smaller than the typical depletion region width in undoped GaN samples. Here, we develop a phenomenological model with analytical expressions for steady-state SPV and its transients that can be used for undoped GaN and other semiconductors with short diffusion lengths.…”

Determination of carrier diffusion length in p- and n-type GaN

“…Various experimental techniques such as electron beam induced current (EBIC), 1,2,3 junction based photocurrent, 4 surface photo-voltage spectroscopy 5 have been devised for its determination and a wide range of values ranging from 50 nm to 3.4 µm have been reported depending on the material quality, doping levels, and the techniques used. 1,2,3,4,5,6,7,8 In this study, we measured the diffusion length of the photogenerated carriers in n-and ptype GaN, along the c-direction using photoluminescence (PL) spectroscopy and compared the results with cathodoluminescence (CL) line-scan measurements.…”