Optical and electrical properties of nitride semiconductors

• Main Authors: Da-Ren Hang, 杭大任
• Other Authors: Yang-Fang Chen
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/45667495407088859764

博士 === 國立臺灣大學 === 物理學研究所 === 90 === Abstract We present the studies of optical and electrical properties of low-dimensional III-V nitride semiconductor heterostructures. Photoluminescence (PL), modulation spectroscopy, and magnetotransport measurements are employed to characterize the physical properties of AlGaN/GaN heterostructures and an InAsN/InGaAs single quantum well. This thesis consists of five parts described as following. 1. AlGaN/GaN band offsets determined by deep-level emission We present studies of the compositional dependence of the optical properties of AlXGa1-XN (0 < x < 0.22) alloys by modulation spectroscopy and PL measurements. The yellow luminescence (YL), which is well known in GaN, has also been observed in AlGaN. The deep level responsible for the YL is used as a common reference level to determine the band alignment in AlGaN/GaN heterojunctions. Combining with the near-band-edge modulation spectra, the estimated ratio of conduction-to-valence band discontinuity is 65:35. Our results are close to the values obtained from PL measurements on Al0.14Ga0.86N/GaN quantum wells and those calculated by linear muffin-tin orbital method and linearized augmented plane wave method. 2. Effective mass of the two-dimensional electron gas in an Al0.2Ga0.8N/GaN heterojunction We have performed a magnetotransport study on an AlGaN/GaN heterostructure at low temperatures. The effective-mass values have been evaluated by analyzing the exact form of the temperature-dependent Shubnikov-de Haas (SdH) oscillation function. The values obtained increase with the magnetic field. This mass enhancement is attributed to conduction-band nonparabolicity. The effective mass variation with the magnetic field was extrapolated to zero-field, together with further correction due to band filling effect, yielding an effective mass of 0.185 ± 0.005 of the free-electron mass. Our result is in excellent agreement with the results obtained by first-principle calculations and tight-binding method and suggests the significance of magnetic-field-induced nonparabolicity in transport measurements. 3. Electrically detected and microwave-modulated Shubnikov-de Haas oscillations in an Al0.4Ga0.6N/GaN heterostructure We report the drastic enhancement pattern of SdH oscillations observed in an AlGaN/GaN heterostructure by microwave modulation. The dependence of the SdH pattern on microwave power and temperature is investigated. The underlying mechanism is attributed to the effect of carrier heating. This technique helps study the transport properties of two-dimensional electron gas (2DEG) in novel wide band-gap heterostructures, where moderate mobilities and heavier electron effective mass (rapidly damping SdH amplitudes) are frequently encountered. In addition, this method has the advantage of keeping the carrier concentration fixed and not requiring high-energy laser facilities compared with carrier-modulated SdH measurements. 4. Subband electron properties of modulation-doped AlGaN/GaN heterostructures We present studies on subband electron properties of two-subband-occupied AlGaN/GaN heterostructures by magnetotransport measurements. The second-subband population is manifested by the multi-frequency in the SdH oscillations. The modulated patterns of SdH oscillations due to multi-frequency can be drastically enhanced by employing the microwave modulation technique. We provide direct experimental evidence that the 2DEG in the second subband has a higher mobility than that in the first subband by means of microwave-modulated magnetotransport measurements. The carrier concentrations, 2DEG Fermi energy and quantum mobilities for each subband were determined. It was found that the second subband population ratio increases with spacer thickness up to 5 nm, while the subband separation decreases. Furthermore, studies on the quantum lifetimes of subband electrons suggest that the short-range scattering and interband resonant scattering have significant influence on the properties of the 2DEG in the modulation-doped Al0.22Ga0.78N/GaN heterostructures. 5. Shubnikov—de Haas oscillations of two-dimensional electron gas in an InAsN/InGaAs single quantum well We present the first investigation of SdH oscillations of 2DEG formed in an InAsN/InGaAs single quantum well (QW). The PL peak energy of the InAsN/InGaAs QW decreases in consequence of the bowing effect due to the incorporation of nitrogen atoms. The nitrogen content can be estimated by the PL peak to be 0.4 %. From the analysis of SdH oscillations, the effective mass is determined to be 0.1 ± 0.01 m0. The enhancement of the effective mass is mainly due to the incorporation of the nitrogen atoms in the InAs lattice. The large increase of the effective mass cannot be explained by the simple band anticrossing model. In addition, a temperature-independent magnetoresistivity at a critical magnetic field is observed. Our analysis supports the fact that the value of the critical exponent in quantum Hall effect is not universal.