ICMCTF2005 Session D1-2: Carbon Nitride, Boron Nitride and Group-III (Al, Ga, In) Nitride MaterialsCarbon Nitride, Boron Nitride and Group-III (Al, Ga, In) Nitride Materials

Wednesday, May 4, 2005 2:10 PM in Room Royal Palm 1-3

Wednesday Afternoon

Start	Invited?	Item
2:10 PM		D1-2-3 Clear-Cut Evidence for Nitrogen-Excess in InN Thin Films H. Timmers, S.K. Shrestha (University of New South Wales at the Australian Defence Force Academy, Australia); K.S.A. Butcher, M Wintrebert-Fouquet, P.P.T. Chen (Macquarie University, Australia) The apparent native n-type characteristic of InN thin film materials from all common growth techniques has long been attributed to the prevalence of nitrogen vacancies, thought to arise from the low dissociation energy of InN and the thus inevitable difficulty of incorporating nitrogen during growth. The lack of appropriate nitrogen standards prevents the reliable and precise quantification of nitrogen concentrations in InN thin films with standard x-ray or sputtering techniques. Ion scattering with RBS is complicated by the high yield from the substrate, typically sapphire, which obscures the weak nitrogen signal and also prevents the identification of small amounts of oxygen in the film. The stoichiometry of a large number of state-of-the art InN films from all relevant growth technologies including MBE, MOCVD, Remote-Plasma-Enhanced-CVD and RF-sputtering has now been measured with Elastic Recoil Detection (ERD) analysis, which is particularly suited to the quantification of low-Z elements in combination with elements of high atomic number Z. It has been shown that the stoichiometry change during analysis accurately follows a model based on the concept of bulk molecular depletion. While it is clear from the data that, with very few exceptions, all measured InN thin films are contrary to expectation nitrogen-rich, the application of the model has allowed the extraction of N/In ratios with unprecedented precision. In addition, the oxygen and carbon content has been determined for all films studied. Interestingly, for RF-sputtering growth a correlation is observed between nitrogen-excess and apparent band-gap, while there appears to be no dependence on the relative large oxygen content (10-15%) in this type of InN material. The results are an important contribution to the present debate about the correct value of the electronic band-gap of the InN semiconductor, which will determine what type of future applications may be envisaged for this material.
2:30 PM		D1-2-4 Carrier Concentration Dependent Optical Properties of Wurzite InN Epitaxial Films on Si(111) Studied by Spectroscopic Ellipsometry C.-H. Shen, C.-L. Wu, S. Gwo (National Tsing-Hua University, Taiwan); H. Ahn (Industrial Technology Research Institute, Taiwan) Indium nitride (InN) is an interesting and potentially important semiconductor material with superior electronic transport properties over other group-III nitrides, which is suitable for high speed and high frequency electronic device applications. Recently, InN has received great attention due to the discovery of its narrow direct bandgap around 0.7 eV, in sharp contrast to the widely cited value of ~1.9 eV in the literature. It seems this controversy strongly depends on the crystalline quality of the InN samples. Epitaxially grown InN films typically have the narrow bandgap and show high crystalline quality compared to the sputtering-grown InN. The dispersion of optical properties is one of the most important material properties and the material properties of InN, especially near the bandgap, are still a conflicting issue. There have been several studies attempting to model the dielectric functions of InN in a wide spectral range. Since the optical properties of InN strongly depend on the growth process, constructing a unique model to describe the dispersion of dielectric function of InN is not a trivial matter. In this paper we will report the results of ellipsometric studies of the refractive index and the optical absorption of wurtzite InN epilayers grown on Si substrates with β-Si₃N₄/AlN(0001) double-buffer layer. Since for InN grown on Si substrates, Si atoms may work as donors to InN layer, the thickness or the type of buffer layer can play an important role for the electrical and optical properties of InN. We have determined AlN buffer layer dependent optical properties of InN by using the Adachi's model for the dielectric function. We found that the onset of optical absorption in the InN film is in the near-infrared region and the Burstein-Moss effect on optical absorption edge is observed for the InN epilayers grown with two different free-carrier concentrations.
2:50 PM	Invited	D1-2-5 A Comparative Study on InN Films Grown by Conventional and ArF Excimer Laser-Assisted MOVPE Techniques A. Yamamoto, H. Miwa, A. Hashimoto (University of Fukui, Japan) This paper reports the present status and issues to be solved for MOVPE InN. For the conventional MOVPE of InN using the pyrolysis of trimethyl-indium and NH₃, reactor design, growth temperature, gas pressure and V/III ratio are found to be critical growth parameters with respect to electrical and optical properties of MOVPE InN. Employment of a low-temperature GaN buffer has significant effects on film properties. Although the use of a GaN buffer improves morphological uniformity of MOVPE InN, it suppresses the grain growth of InN. The enhanced grain growth of InN is realized when no buffer is used. The highest mobility of 870 cm²/Vs is obtained for a film grown with a GaN buffer, while the lowest carrier concentration 5 x 10¹⁸ cm^-3 and the lowest photoluminescence (PL) peak energy 0.675 eV are obtained for InN films grown without buffers. A PL spectrum measured from the back side of the film through the sapphire substrate has a peak with an energy higher than that measured from the top surface of the film, indicating that a higher-carrier concentration region exists near the substrate. A discrepancy between PL peak energy and absorption edge is found to be a good measure of electrical and optical non-uniformity in InN. One of the essential problems in the conventional MOVPE of InN is that NH₃ has an extremely low thermal dissociation rate at the growth temperature of InN (600°C). To overcome this problem, we have developed the laser-assisted MOVPE where an ArF excimer laser (193 nm wavelength) dissociates NH₃ photolytically. An InN film can be grown in a wide range of growth temperature, from RT up to 700°C. Issues for the ArF excimer laser assisted MOVPE of InN will be also addressed.
3:30 PM		D1-2-7 Commensurately Matched InN/AlN Heterojunction Grown by Plasma Assisted Molecular-Beam Epitaxy C.-L. Wu, C.-H. Shen, H.-W. Lin, H.-M. Lee, S. Gwo (National Tsing-Hua University, Taiwan) Because of their wide bandgaps, chemical stability, mechanical strength, and superior transport properties (high electron mobility, large electron drift velocity, and high breakdown voltage), group-III nitride heterostructures are promising materials for high-frequency, high-speed, and high-power electronic devices. To realize such applications, it is important to be able to grow group-III nitride heterostructures with high crystalline quality, atomically abrupt interfaces, and large band offsets. The heterojunction formed between InN and AlN is very interesting because of a huge bandgap difference (0.7 eV vs. 6.2 eV) and superior electron transport properties in InN. However, the large differences in lattice parameters (~12% for InN grown on AlN) and the lack of III-nitride substrates post daunting challenges for growing high-quality InN/AlN heterostructures. We report here the growth and properties of commensurately matched InN/AlN heterojunction deposited on the Si(111) substrate by plasma-assisted molecular-beam epitaxy (PAMBE). By using the technique of in situ reflection high-energy electron diffraction (RHEED), we found that the pseudomorphic to commensurate lattice transition occurs within the first monolayer growth of InN on AlN. As a result, InN/AlN heterojunction can be made perfectly commensurate at the atomic level. Furthermore, very large heterojunction band offset and type-I band alignment were confirmed by X-ray photoelectron spectroscopy. Due to the excellent structural and electronic properties, we propose that the InN/AlN heterojunction technology can provide a platform for future device applications and for new functionality.
3:50 PM	Invited	D1-2-8 Nanoscale Imaging of Electronic Structure in In_xGa_1-xN/GaN Quantum Wells E.T. Yu (UCSB); X. Zhou (University of California, San Diego) In_xGa_1-xN/GaN quantum-well structures are of outstanding current interest for nitride semiconductor-based visible light emitters. Because of the high densities of point and extended defects in epitaxially grown nitride semiconductor material, characterization and understanding of local, nanoscale structure and electronic properties in such devices is essential to achieve effective control over and optimization of device characteristics and performance. We will discuss the use of scanning capacitance microscopy (SCM) and atomic force microscopy (AFM) to characterize structural and electronic properties of In_xGa_1-xN/GaN quantum-well structures at the nanoscale. SCM and AFM imaging under reverse-bias conditions reveals variations in local carrier density within the In_xGa_1-xN quantum-well layer that reflect the atomic step structure visible on the top GaN surface. A detailed analysis of SCM and AFM image data as well as spatially resolved scanning capacitance spectra indicates that these variations arise from monolayer fluctuations in In_xGa_1-xN quantum-well thickness. SCM images obtained under forward-bias conditions reveal features corresponding to localized In-rich clusters, within which increased electron accumulation occurs, in In_xGa_1-xN quantum wells grown under the appropriate conditions. The presence of these In-rich clusters is correlated with an increase in luminescence efficiency compared to that observed in structures within which such clusters are absent. Thus, using a combination of scanning probe imaging techniques, local nanoscale spectroscopy, and numerical simulation we are able to image and analyze nanoscale structure and electronic properties in subsurface In_xGa_1-xN/GaN quantum-well structures, and correlate the observed characteristics with various aspects of device behavior.
4:30 PM		D1-2-10 Self-Assembled InN Quantum Dots Grown on AlN and GaN Surfaces by Plasma-Assisted Molecular-Beam Epitaxy under Stranski-Krastanow Mode+ C.-H. Shen, H.-W. Lin, H.-M. Li, C.-L. Wu, S. Gwo (National Tsing-Hua University, Taiwan) Recently, III-Nitride Quantum Dot (QD) materials have attracted considerable interest due to their tremendous potential for optoelectronic device applications. For the case of InN QD, because of the narrow bandgap energy of bulk InN at ~0.7 eV, the quantum size effects in InN QDs might be applied to extend their emission wavelengths into the near-infrared region (especially at the important communication wavelength range of 1.3-1.55 µm) and possibly the visible red region, the unreachable wavelength ranges using other III-nitride materials. However, due to the difficulty of growing InN (low dissociation temperature of InN and high equilibrium vapor pressure of nitrogen), the preparation of InN QDs require a low growth temperature method and plasma-assisted molecular-beam epitaxy (PAMBE) is quite suitable for InN QD growth. In our experiments, the InN QDs were formed with different growth mechanisms. Here, we report the growth of InN QDs on AlN and GaN layers by using the Stranski-Krastanow (SK) growth mode and a thermal annealing method. By monitoring the lattice parameters using in situ reflection high-energy electron diffraction (RHEED) during MBE growth, the formation of InN QDs can be precisely controlled and analyzed systematically. Moreover, from the ex situ atomic force microscopy (AFM) measurements, the size and shape of InN QDs grown by different methods can be determined. According to the height/lateral size analysis of QDs, the InN QDs grown by the SK mode have better size/shape uniformity than the thermal annealing approach.
4:50 PM		D1-2-11 Synthesis, Structural and Optical Studies of InN Nanobelts and InN GaN Nanocables C.W. Hsu (Academia Sinica, Taiwan); M.S. Hu, W.C. Lai, K.H. Chen, L.S. Hong, C.W. Chen, L.C. Chen (National Taiwan University, Taiwan) InN nanobelts were synthesized successfully via a microwave enhanced metalorganic chemical vapor deposition system. Scanning electron microscope and transmission electron microscope investigations showed that the InN nanobelts were single crystals with a dimension of 30~50 nm in width, several nm in thickness, and 5~10 nm in length, and the growth direction was indexed to be [110]. Raman spectra clearly showed the A1(TO), E2 and A1(LO) phonon modes of wurtzite InN structure, at 445 cm-1, 489 cm-1 and 579 cm-1, respectively. However, a new peak at 550 cm-1 was also detected from our InN nanobelts. The origin of this signal was not clear yet and merits further investigations. Amorphous-GaN-shelled InN nanobelts were synthesized in a tube furnace by using gallium acetylacetonate and NH₃ as precursors. After thermal annealing process, an ultra sharp photoluminescence (PL) peak was observed at ~ 0.765-0.770 eV with a full-width-half-maximum of less than 10 meV. Furthermore, a small blue shift of this IR-PL emission from the core-shell structured InN GaN nanocables was observed compared to that obtained from bare InN nanobelts. This core-shell heterostructure can be a good candidate for passivation and enhanced optical properties of the 1-dimensional InN.
5:10 PM		D1-2-12 Electrical Properties of Single Gan Nanowire Field-Effect Transistor Patterned by Shadow Mask C.Y. Chang, B.J. Pong, Y.M. Laio, G.C. Chi (National Central University, Taiwan); J.T.H. Tsai (Tatung University, Taiwan); W.M. Wang, L.C. Chen (National Taiwan University, Taiwan); K.H. Chen (Academic Sinica, Taiwan) Fabrication and electrical properties of single GaN nanowire (NW) field-effect transistor (FET) are reported. A simple shadow mask process was used for patterning of the metal contact layers. Whereas GaN NWs were synthesized by chemical vapor deposition on Si substrate coated with a 5 nm thin Au metal film as the catalyst prior to GaN NWs growth. High-resolution TEM image showed that our nanowire samples exhibited single-crystalline wurtzite GaN structured and the NWs growth direction was in parallel to the [1-10] direction. The pristine GaN NWS were dispersed onto Si substrate (back gate) and pre-capped with a 320 nm-SiN_x layer as the insulator. To complete the device structure, a final E-gun deposition of a Ti/Au (30/120 nm) layer was carried out to form metal contacts to the two ends of single GaN nanowire using a copper gird as the shadow mask. After alloying such sample at 550 for 5 minute, formation of ohmic contact was confirmed by the linearity in the I-V characteristics of the conductance measured for the single GaN nanowire. FET characteristics of the individual GaN nanowire were further investigated. The GaN NW was found to be n-type and a carrier mobility of about 23 cm²/VZs was obtained. Temperature dependence of the conductance revealed that thermal emission dominated the transport mechanism.