Session S19 - Focus Session: III-Nitrides Dislocations and Growth.
FOCUS session, Thursday morning, March 21
117, Indiana Convention Center

[S19.001] Dislocation Electrical Activity in III-nitride Films

The effect of dislocation type and MBE growth stoichiometry on the excess reverse bias gate leakage in GaN Schottky diodes will be discussed. By comparing high quality GaN films grown by MBE on either HVPE or MOCVD templates, we show definitive evidence that pure screw threading dislocations are the primary source of reverse bias gate leakage. This conclusion was reached by correlating the scanning current image taken at fixed reverse biases with simultaneously acquired topographic images, and by comparing the density of leakage spots to total dislocation density and distributions of different dislocation types obtained from cross sectional TEM measurements. While dislocation type is the primary factor in determining gate leakage, growth stoichiometry also has a large effect. The leakage current is 2 to 3 orders of magnitude higher for samples grown under Ga-rich conditions. TEM results indicate that excess Ga induced changes in dislocation core structure. This in terms causes the different electrical activity associated with screw/mixed dislocations in samples grown under different conditions. We will present arguments on how core structural differences between screw and edge dislocations in the template could lead to the drastically different electrical activity.

[S19.002] Stacking Fault Emissions from Hexagonal GaN films grown on SiC

The photoluminescence spectra of a series of thin undoped GaN films grown by MBE on 6H-SiC have been studied as a function of temperature using the 3.523 and 3.408 eV lines from the Ar ion laser as excitation source. Defect induced peaks are observed below the 3.46 eV band gap of hexagonal GaN at ~3.35, 3.33 3.26, 3.22 and 3.17 eV. The 3.26 eV peak is attributed to band gap emission from cubic GaN inclusions. The presence of such inclusions was confirmed by high resolution TEM. By comparing the spectra excited above and below the hexagonal GaN band gap, it is possible to identify the emission peaks arising from cubic GaN. We propose to explain the energies of the observed emission peaks based on a model in which cubic GaN form quantum wells inside the hexagonal host.

Research at Berkeley was supported by the US DOE under Contract No. DE-AC03-76SF00098. Research at HKU was supported by HK RGC grant No. HKU 7121/00P and by the NFSC/RGC joint research grant No. N-HKU028/00.

[S19.003] Evidence of Spatially Indirect Transition from Stacking Faults in Hexagonal GaN films grown on SiC. grown on SiC.

The photoluminescence spectra of a series of thin undoped hexagonal GaN films containing cubic GaN inclusions grown by MBE on 6H-SiC have been studied as a function of temperature and excitation power using the 3.523 and 3.408 eV lines of the Ar ion laser. The PL spectra of one sample, when excited below the hexagonal GaN band gap at 3.408 eV, was found to be dominated by a peak at \sim3.17 eV. The dependence of its lineshape and peak position on laser power suggests that it is a spatially indirect transition. We have been able to fit its power dependent lineshape assuming a Type II band alignment between cubic and hexagonal GaN. Such band alignment has been theoretically predicted[1] but so far not observed experimentally. The values of the conduction and valence band offsets extracted from our data are in good agreement with theoretical predictions. Research at Berkeley was supported by the US DOE under Contract No. DE-AC03-76SF00098. Research at HKU was supported by HK RGC grant No. HKU 7121/00P and by the NFSC/RGC joint research grant No. N-HKU028/00. [1]C. Stampfl and C. G. Van de Walle, Phys. Rev. B57, R15052 (1998).

[S19.004] Calculated potential profile near charged threading dislocations at metal/semiconductor interfaces

We have made finite element calculations of the expected potential profile around negatively charged threading dislocations (TDs) close to a metal-semiconductor interface, using a Pt contact on n-type GaN as a specific case. The potential was calculated as a function of the assumed linear density and energy level of TD-related traps. Our results showed good agreement with the classic model of Read [Philos. Mag. 45, 775 and 46, 111 (1954)] for an infinite dislocation away from any interface. Assuming 1 trap/c-axis lattice spacing (0.52 nm), we find that trap levels deeper than 1.2eV below the conduction band minimum (CBM) should produce a significant local increase in the Pt/GaN barrier potential, which should be observable by Ballistic Electron Emission Microscopy (BEEM). In fact, recent BEEM measurements by Im et al. on MBE-grown GaN films [PRL, 87, 106802, (2001)] showed no significant increased barrier at TDs, indicating a trap energy within 1.2 eV of the CBM or a smaller trap density. Work supported by ONR.

[S19.005] Investigation of Cracks in Hexagonal Nitride Semiconductors Grown on (111) Silicon by Micro-Raman Imaging

We present post-growth micro-Raman stress mapping of cracks in GaN, AlN, and AlGaN grown on (111) oriented silicon. Cracks with an average spacing of 100 microns are observed and are categorized into two types. The first type cracks both the epilayers (GaN, AlN, and AlGaN), and the substrate and the second type cracks only the epilayer (GaN). The micro-Raman stress mapping of the first type of crack shows that the epilayers are under biaxial tensile stress and the substrate is under compressive stress far away from the crack. At the crack position, the epilayers and the substrate are found to be relaxed from the equilibrium values of -0.5 GPa (AlN), -0.16 GPa (GaN), -0.6 GPa (AlGaN) and 0.36 GPa (Si). The stress mapping of the second type of crack reveals that the epilayer is under tensile stress, while the substrate is completely relaxed far away from the crack. However, at the crack position, the substrate is found be under tensile stress (-0.39 GPa) and the epilayer is partially relaxed from -0.2 GPa to -0.08 GPa. Polarization dependent measurements are also discussed. The stress mapping of both types of cracks is well described by the distributed-force model and good agreement is observed. Finite element calculations are performed to compare with the results.

[S19.006] RBS/Channeling study of epitaxially grown GaN thin films

The composition, thickness, growth mode and crystalline ordering of thin variable thickness GaN films grown in a wide temperature range (750-1050^oC) have been investigated using RBS/Channeling (He++ at 2.2MeV) and XPS. The films have been grown by MOCVD using NH_3 and TMGa as precursors (molar ration 1000) and H_2 and N_2 as buffer gasses. In-situ XPS analysis of samples indicates equal amounts of Ga and N, and different levels of C and O contamination. The RBS analysis shows that the films have variable thickness and the low temperature films completely cover the substrate while for temperatures 850^oC and higher the films grow island-like and cover 5%-60 % of the substrate. The channeling yield indicates an increase of crystalline order with higher deposition temperature and thickness of the film. The analysis shows that in the early stages of growth, the crystalline order and morphology of GaN film are strongly influenced by the substrate temperature. Epitaxial films with a good channeling yield of 2.94% have been grown at 950^oC having (0001) orientation.

[S19.007] Surfactants and Antisurfactants on GaN surfaces

Even small impurity/dopant concentrations on GaN surfaces may strongly affect growth rate, surface morphology, formation of extended defects, and electronic properties. However, our understanding about the basic mechanisms causing these effects are still in its infancy. We have therefore studied the atomic geometry, surface energy and adatom kinetics of a variety of clean and adsorbate covered surfaces employing first principles calculations. Based on these results we identify a number of mechanisms strongly affecting the surface morphology. For example, we explain why In at low concentrations may be used as a surfactant while at high concentrations it enhances the formation of nanometer-size surface defects (causing quantum-dot like In fluctuations). As a second example we will discuss Si and explain when and why it acts as an anti-surfactant.

[S19.008] Growth and Transport Properties of Al-rich Si-doped n-type AlGaN alloys

Al-rich AlGaN has potential applications in devices such as UV light emitters and heterojunction field effect transistors. High Al content AlGaN alloys are generally highly resistive. The realization of conductive Al_xGa_1-xN alloys with high Al concentrations (x > 0.5) is a challenge for material growth. We have grown successfully Si doped n-type Al-rich AlGaN alloys by MOCVD. These materials have been characterized by energy dispersive x-ray (EDX) spectroscopy, x-ray diffraction (XRD), and photoluminescence (PL) spectroscopy measurements. Dependence of film surface morphology on growth conditions was probed by AFM and SEM measurements. Variable temperature Hall-effect measurement was employed to study the transport properties of AlGaN epilayers. Our experimental results have confirmed that we have achieved n-type conduction in Al_xGa_1-xN alloy for x up to 0.7 by Si-doping. The resistivity of Al_0.7Ga_0.3N was found to be 0.46 Ømega-cm at room temperature with electron concentration and mobility of 6.2x10^17 cm^-3 and 21 cm^2/Vs, respectively. Implication of these high quality n-type Al-rich AlGaN alloys to the applications of UV emitters will be discussed.

[S19.009] Suppression of Spiral Growth in Molecular Beam Epitaxy of GaN on Vicinal 6H-SiC(0001)

The surface morphology of GaN thin films grown by molecular beam epitaxy on nominal flat and vicinal 6H-SiC(0001) substrates was characterized by in situ scanning tunneling microscopy. On flat substrates, screw dislocations emerge at the surface, creating straight steps along (1100) directions for 50 nm thick film. As growth proceeds, these steps wind around the dislocations and form spirals for film thickness larger than 100 nm. In contrast, the spiral growth was completely suppressed on vicinal substrates, and step-flow growth was observed. Based on the Burton, Cabrera, and Frank theory, a model is proposed to explain the suppression of the spiral growth on vicinal substrates.

[S19.010] Low-Pressure Growth of Bulk GaN from In-Ga Alloys

Polycrystalline GaN was grown at subatmospheric pressures with a plasma source of atomic N and Ga/In alloys ranging from pure Ga to 99 mole results, and measurements of the band edge luminescence spectra were consistent in all cases with growth of pure GaN rather than the alloy, conforming to the calculated phase diagram. The use here of Ga/In alloys enhances the solubility of N in the melt by as much as a factor of 100 compared to pure Ga, thus enhancing growth rates. These experiments are aimed toward the development of low-pressure growth of large bulk single crystal substrates of GaN.

[S19.011] Amorphous GaN prepared by ion beam assisted deposition at room temperature

Amorphous GaN thin films were deposited by ion beam assisted molecular beam evaporation at room temperature. The nitrogen ions were provided by a Kaufman type ion source with the beam energy and beam current in the range of 50-180eV and 0.5-7.0mA respectively. XRD, ED, and Raman spectroscopy show that the films are amorphous. RBS analysis was performed to study the elemental composition and the stoichiometry of the films. Depending on the deposition condition, films with Ga to N ratio of 1.0:0.5 to 1.0: 1.4 were obtained. The Ga rich films show dark appearance, while the N rich films are transparent. Some of these films lost some N and gained some O at room temperature after deposition, others have been stable against oxidation until annealed above 500^oC. The film with Ga to N ratio of 1.0:1.4 was studied further. The optical band gap is about 2.1eV from Tauc plot. XPS study shows that the binding energies for N and Ga to be consistent with only Ga-N bonding. The Auger parameter calculated from Ga 3d photoelectron peak and Auger Ga LMM peak is 1084.15eV.

[S19.012] Dislocation termination at thin Si3N4 layers in GaN epitaxy on silicon substrates

The effect of ultra-thin Si3N4 layers on the dislocation structure in GaN epitaxy has been studied by transmission electron microscopy and spatially resolved cathodoluminescence. LED structures were grown by metalorganic chemical vapor deposition. AlN buffer layers were used to reduce the dislocation densities. Transmission electron microscopy observation shows that screw dislocations are strongly affected by the Si3N4 layers while edge dislocations are only slightly affected. This reduction of dislocations can be directly visualized in cathodoluminescence images taken in cross section. Dislocations reactions leading to the termination of screw dislocations and their effect on luminescence efficiency will be presented.

Part S of program listing