Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P5: Nitride-Based Microelectronics |
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Sponsoring Units: FIAP Chair: Joanna Mirecki-Millunchick, University of Michigan Room: LACC 502B |
Wednesday, March 23, 2005 11:15AM - 11:51AM |
P5.00001: Toward a Self-Consistent Model of Carbon States, Semi-Insulating Conductivity and Yellow Luminescence in GaN:C Grown by Molecular Beam Epitaxy Invited Speaker: Carbon doping of GaN is of great interest to generate semi-insulating (SI) buffer layers used in AlGaN/GaN heterojunction field effect transistors grown by molecular beam epitaxy (MBE). However, the specific mechanism(s) responsible for SI behavior involving C-related bandgap states in GaN until recently had been un-verified experimentally due to difficulties in determining deep level properties within SI wide bandgap materials. Moreover, C-related states in GaN are under increasing scrutiny due to the observation of yellow luminescence (YL) in \textit{both} n-type and SI GaN:C since the prevalent model for YL in GaN requires requires gallium vacancy (V$_{Ga})$ defects, which are not expected to form in significant concentrations for SI GaN. However, the fact that YL is observed for SI GaN doped with C while not being observed for SI GaN doped with Fe, suggests additional roles for C-related states in the GaN bandgap that may also have implications for potential parasitic effects in GaN electronics. Hence a full understanding of C-related deep states in SI GaN:C is necessary. This presentation will focus on each of the aspects noted above. First, our recent development of a lighted capacitance-voltage defect profiling measurement that allows full quantification of deep level concentrations and energy levels within SI GaN throughout its bandgap will be described. By applying this method with deep level optical spectroscopy (DLOS) to a systematic MBE-grown GaN sample set with well-controlled carbon doping, carbon states responsible for the SI behavior are identified. From this knowledge and by comparing to photoluminescence studies of these same samples to monitor the YL dependence on both conductivity and carbon concentration, the controversy over the existence of YL in SI GaN:C is addressed. A model based on a coordinate configuration diagram will be presented, showing the first self-consistent picture of C-related bandgap states and how they influence both SI behavior and deep level YL. [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:27PM |
P5.00002: Invited Speaker: |
Wednesday, March 23, 2005 12:27PM - 1:03PM |
P5.00003: Dispersion in AlGaN/GaN HEMTs Invited Speaker: Robert Coffie Gallium nitride high electron mobility transistors (HEMTs) are living up to their potential as the high power high frequency transistor of the future. The record power density at 4 GHz now exceeds 30 W/mm [1], which is more than an order of magnitude greater than GaAs-based transistors. Achieving this record performance required controlling the phenomena known as ``dispersion'' in AlGaN/GaN HEMTs. Dispersion is the term used to indicate that the dynamic characteristics of a device are different from the static characteristics. The two sources of dispersion are trapped charge and self-heating [2]. Although electron trapping can occur in many parts of the device, the surface has been identified as one of the dominant sources of dispersion in AlGaN/GaN HEMTs. At frequencies below Ka-band, a combination of surface passivation (typically SiN) and a field plate structure can be used to control dispersion. As the frequency of operation is pushed above 30GHz, the penalty paid in capacitance introduced by field plates prevents field plate structures from being used. In addition, as the gate length is decreased in order to increase the operating frequency, the peak electric field in the channel increases causing dispersion to increase and reliability to decrease. Understanding and controlling dispersion is the key for obtaining good power performance and reliability in AlGaN/GaN HEMTs. This talk will explain dispersion from the virtual gate model of the surface. Two primary methods for measuring dispersion (pulsed I(V) and RF I(V)) will be shown. Finally, data showing how dispersion affects reliability of AlGaN/GaN HEMTs will be given. [1] Y.-F. Wu, et al., IEEE Electron Device Lett., vol. 25, no. 3, pp. 117-119, March 2004. [2] P. H. Ladbrooke et al., Electron. Lett., vol. 31, no. 21, pp. 1875-1876, Oct. 1995. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:39PM |
P5.00004: III-V Nitride Materials and Electron Devices Invited Speaker: The electronic properties of III-V nitride materials make them applicable to high power microwave transistors. They are grown by molecular beam epitaxy or metal organic vapor phase epitaxy, and are direct band gap materials, with alloy gaps ranging from $<$.7V to $>$6.2V. With no native substrates, SiC and sapphire are used. The dislocation properties are acceptor-like for larger band gaps, and donor-like for smaller band gaps. These Wurtzite Crystal materials, grown on the Ga face along the C-axis, have strong spontaneous and piezoelectric polarization. AlGaN/GaN heterojunctions have net polarization sheet charge, inducing an electron sheet charge. Growth without intentional impurity doping, yields $\sim $1X10$^{13}$/cm$^{2}$ electrons in a sheet of 25{\AA} FWHM thickness. Field-effect transistors are fabricated for microwave power amplification.. Their properties include CW normalized power levels $>$10W/mm at 10 GHz. Breakdown electric field strength in the GaN is 3 megavolts/cm, and the sheet current density is 1A/mm. The semiconductor surface is passivated, with silicon nitride, for charge stability. The experimental electron transit velocity of 1X10$^{7}$ cm/s is one third of that originally predicted by Monte Carlo simulations, since the build-up of longitudinal optical phonons was ignored in the simulations. Processing and properties of these transistors will be covered. [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 2:15PM |
P5.00005: Nitride semiconductor material growth by rf-MBE for electronic device applications Invited Speaker: Gallium nitride and related materials are now beginning to realize their potential for electronic device applications, including high electron mobility transistors (HEMTs). Rf-plasma-assisted MBE is an attractive method of growing these materials due to its low background impurity incorporation, and recently there have been impressive results on MBE-grown electronic devices. However, significant growth issues remain, including the elimination or reduction of buffer conduction, threading dislocation densities, and trapping in or near the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. Recent work at the U.S. Naval Research Laboratory has addressed these issues, including the use of Be-doped GaN to reduce buffer conduction, the effect of the AlN nucleation layer on buffer conductivity and dislocation density, homoepitaxial growth of GaN on free-standing GaN substrates to reduce the threading dislocation density, and the investigation of trap states in AlGaN/GaN HEMT structures. For example, we have observed that buffer conduction can be reduced by several orders of magnitude by using Be-doped GaN layers and that the detailed growth conditions of the AlN nucleation layer on SiC substrates can significantly affect buffer conductivity and Hall mobilities in the 2DEG. Further, we have achieved room-temperature Hall mobilities of 1920 cm$^{2}$/V-s in AlGaN/GaN HEMT structures grown on free-standing GaN substrates. [Preview Abstract] |
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