Magneto-Optic Studies of GaN Films and GaN/AlGaN Heterostructures
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ABSTRACT Magneto-studies have been carried out for several MOCVD grown GaN thin films and GaN/AlGaN heterostructures at magnetic fields up to 30 T and at temperatures between 4.2 K to 100 K. Electron cyclotron resonance was observed in two heterostructures with high mobilities (p > 2000 cm 2/V.s), the effective mass obtained from the cyclotron resonance measurement is 0.23±0.01 m0, where mo is the mass of free electron. For Si-doped thin film GaN there was no sign of electron cyclotron resonance even when samples were heated up to 100 K. However, a Is to 2p+ absorption line was observed for Si-doped GaN samples. A binding energy of 29 meV and low frequency dielectric constant of 10.4 is obtained. INTRODUCTION GaN belongs to group-III nitride semiconductor where the large energy gap is expected to play a significant technological role such as the blue light emitting diode, and in electric power generation [1]. Until recently, sample quality has prevented direct measurement of the effective mass via cyclotron resonance since this technique requires cor>1 where - is the scattering time. We present here cyclotron resonance (CR) measurements on free electrons on the 2 dimensional electron gas (2DEG) on GaN/Al.Ga,.,N heterojunction, and on impurity transition on Si-doped GaN films using high magnetic fields. Recently, Meyer [2] et. al. and Alt [3] et. al. have reported effective mass of m" 0.236±0.005 mo where in. is the free electron mass using transmission experiment on unknown residual donors on GaN. Our measurements employ Fourier transform spectroscopy and high magnetic field to probe the cyclotron resonance absorption in GaN/Al.Ga1 .0 N heterostructures, and the Is to 2p+ absorption of Si donor in GaN films. EXPERIMENT AND RESULTS The magneto-optical experiments were performed at the National High Magnetic Field Laboratory in Tallahassee, Florida. A Bruker 113v interferometer with focusing parabolic mirrors was used to direct the infrared radiation down a light pipe to the sample [4]. Static magnetic fields up to 30 T were obtained and the temperature range is from 2 K to 100 K. After passing through the sample the infrared radiation is picked up using a composite Si bolometer. Both the Si-doped GaN films and the GaN/AIGaN heterostructures were grown by metalorganic vapor phase epitaxy on a 0.5 mm thick sapphire substrate with a AIN buffer layer that is about 350 A thick. The Si donor films are about 2 pm thick. In the case of the heterostructure samples, the 697 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
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Figure 1.Field dependence of the cyclotron resonance. Inset shows representative resonance absorption for sample 1.
GaN is 3 pm thick that is capped off with a 500 A thick AIGaN layer. Figure 1 shows the cyclotron resonance position versus field for spectra at 4.2 K for 2 hete
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