Determination of energy barrier profiles for high-k dielectric materials utilizing bias-dependent internal photoemission

Abstract

We utilize bias-dependent internal photoemission spectroscopy to determine the metal/dielectric/ silicon energy barrier profiles for Au/ HfO 2 / Si and Au/ Al 2 O 3 / Si structures. The results indicate that the applied voltage plays a large role in determining the effective barrier height and we attribute much of the variation in this case to image potential barrier lowering in measurements of single layers. By measuring current at both positive and negative voltages, we are able to measure the band offsets from Si and also to determine the flatband voltage and the barrier asymmetry at 0 V. Our SiO 2 calibration sample yielded a conduction band offset value of 3.03± 0.1 eV. Measurements on HfO 2 give a conduction band offset value of 2.7± 0.2 eV (at 1.0 V) and Al 2 O 3 gives an offset of 3.3± 0.1 (at 1.0 V). We believe that interfacial SiO 2 layers may dominate the electron transport from silicon for these films. The Au/ HfO 2 barrier height was found to be 3. The study of high-k dielectrics as a replacement for SiO 2 in complementary metal-oxide-semiconductor devices has become a field of enormous interest. [1][2][3] In this letter, we investigate the band-offset characteristics of high-k dielectrics on silicon. We utilize internal photoemission spectroscopy, a simple optical method developed in the 1960s, 4,5 which has seen recent renewed interest in order to gain information about barrier heights, trap states and interface dipoles in high-k dielectrics. 6 In this technique, a bias is applied across a dielectric structure, while tunable monochromatic light shines on the sample. At a threshold photon energy, electrons from the substrate (or metal gate) are excited by internal photoemission over the dielectric barrier. 7 This threshold energy corresponds to the barrier height of the dielectric. Using bias-dependent internal photoemission spectroscopy, we have determined a barrier height profile as a function of voltage. By measuring the barrier height at both positive and negative voltages, band offsets with respect to silicon (and also the metal gate) can be determined in addition to the flatband voltage and barrier asymmetry at 0 V.In our experimental system, we utilize a 1000 W Hg-Xe lamp with a monochromator as our light source. We use a voltage source/femtoammeter to apply the bias across the sample and to measure the current at each bias. The system is computer controlled by LabView so that the light can be scanned between 1 and 6 eV at any bias and photon energy step size. A multifunction optical meter is used to determine the lamp output spectrum to normalize the photoemission yield. Fused silica lenses are used to focus the light onto the top gold contact of the sample, which is held vertically.The dielectric samples are grown on degenerately phosphorous doped n-type silicon to minimize the voltage drop across the depletion region in the silicon, and enhance the accuracy of our measurement. The dielectrics presented in this letter are HfO 2 and Al 2 O 3 grown by atomic layer deposition. 8,9 Before depo...

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