New applications for gallium nitride
by S.J. Peartona, C.R. Abernathya, M.E. Overberga, G.T. Thalera, A.H. Onstinea, B.P. Gilaa, F. Renb, B. Loub, and J. Kimb
GaN-based visible light-emitting diodes and laser diodes are already commercialized for a variety of lighting and data storage applications. This materials system is also showing promise for microwave and high power electronics intended for radar, satellite, wireless base stations, and utility grid applications; for biological detection systems; and for a new class of spin-transport electronics (spintronics) in which the spin of charge carriers is exploited.
The explosive increase of interest in the AlGaInN family of materials in recent years has been fueled by the application of blue/green/UV light-emitting diodes (LEDs) in full-color displays, traffic lights, automotive lighting, and general room lighting (using so-called white LEDs)1. In addition, blue/green laser diodes can be used in high storage-capacity digital versatile disk (DVD) systems2. AlGaN-based photodetectors are also useful for solar-blind UV detection and have applications as flame sensors for control of gas turbines or detection of missiles.
aDepartment
of Materials Science and Engineering of Chemical Engineering University of Florida, Gainesville, FL 32611 USA E-mail:
[email protected] bDepartment
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There are currently major development programs in the US for three newer applications of GaN-based materials and devices, namely: • UV optical sources capable of operation down to 280 nm for use in airborne chemical and biological sensing systems, allowing direct multi-wavelength spectroscopic identification and monitoring of UV-induced reactions. • Power amplifiers and monolithic microwave integrated circuits (MMICs) for use in high performance radar units and wireless broadband communication links; and ultra high power (>1 MW) switches for distribution control on electricity grid networks. • Room temperature ferromagnetic semiconductors for use in electrically-controlled magnetic sensors and actuators; high density ultra-low power memory and logic; spin-polarized light emitters for optical encoding, advanced optical switches, and modulators; and devices with integrated magnetic, electronic, and optical functionality.
ISSN:1369 7021 © Elsevier Science Ltd 2002
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Table 1 Physical properties of selected semiconductors.
Si
GaAs
GaN
AlN
6H-SiC
Electron mobility (cm2/V.s) at RT
1.1 indirect 1400
1.4 direct 8500
6.2 direct 135
2.9 indirect 600
Hole mobility (cm2/V.s), RT Saturation velocity (107 cm/s) Breakdown field (106 V/cm) Thermal conductivity (W/cm) Melting temperature (K)
600 1 0.3 1.5 1690
400 2 0.4 0.5 1510
3.4 direct 1000 (bulk) 2000 (2D-gas) 30 2.5 >5 1.5 > 1700
14 1.4
40 2 4 5 > 2100
Bandgap (eV) at 300°C
2 3000
Properties of GaN semiconductors The three binary compounds in the AlGaInN system, namely GaN, InN, and AlN, span the range of bandgaps from 1.9-6.2 eV. In addition, they have significantly smaller lattice constants than Si, the more established III-V compound semiconductors (such as GaAs, InP, and GaP), and II-VI semiconductors with wide bandgaps (Fig. 1). The ternaries InGaN and AlGaN have been produced over the entire composition range between their component binaries, while InAlN is less well-explored. GaN and AlN are fairly well lattice-matched to SiC substrates, which have the advantages of dopability and high thermal conductivity relative to more commonly used Al2O3 substrates. Table 1 compares the physical properties of GaN and AlN with other well-known semiconductors. The nitrides are wellsuited to high temperature applications because of their wide bandgaps and low intrinsic carrier concentrations. The electron mobility in GaN is quite high considering the magnitude of the bandgap and is even higher in selectively doped AlGaN/GaN heterostructures where two-dimensional electron gases may form, producing high sheet carrier densities3. These densities are enhanced by the strong piezoelectric and polarization effects present in the AlGaN/GaN structures. These properties make the nitrides well-suited to high-frequency applications. Finally, GaN possesses a very high breakdown field, allowing devices to support large voltages for high power operation. SiC is also well-suited to these applications and has been developed for electronics over a much longer period than GaN. However, as shown in Fig. 2, GaN still has greater potential for these applications because of its larger bandgap and higher carrier velocity and mobility. The top part of Fig. 2 shows theoretical and experimental data for the critical breakdown field of various semiconductors as a function of their bandgaps. While diamond supports the largest field
Fig. 1 Bandgap versus lattice constant for the nitrides and other technologically important semiconductors.
strength, the difficulty of synthesizing large electronic-grade crystals and subsequently doping and contacting to form devices has limited progress for this material4. A combined figure-of-merit (CFOM) for high frequency, high power/high temperature applications can be defined as5 CFOM = χεµVdEb2 (1) where χ is the thermal conductivity, ε the dielectric constant, µ the electron mobility, Vd the drif velocity,and Eb the breakdown field. The bottom part of Fig. 2 shows the resulting values for GaN, 4H-SiC, GaAs, and Si, illustrating the potential of GaN for these applications.
GaN-based UV optical sources The current generation of visible GaN-based LEDs and laser diodes generally employs InGaN quantum well active regions, with AlGaN cladding layers. Replacing the InGaN with GaN quantum wells typically produces operating wavelengths in the 350-360 nm region6. The use of AlGaN quantum wells is capable of shifting the LED emission to below 340 nm7. However, these devices typically show large decreases in
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Fig. 2 Critical breakdown field versus bandgap for various semiconductors (top), and combined figure-of-merit for high frequency/high temperature/high power applications (bottom)5.
optical output intensity with increasing operating temperature. In some cases the use of quaternary AlGaInN can produce improved lattice matching to the cladding layer and better UV emission efficiency8-12. There are many inherent difficulties in pushing to even shorter wavelengths. Hirayama et al.13 reported photoluminescence measured at 77 K in the range 230-280 nm from AlxGa1-xN(AlyGa1-yN) multiquantum wells grown on SiC substrates. However, the intensity at room temperature from the AlGaN quantum wells was much lower than from InGaN-based structures. There is much work to be done on improving the material quality of AlGaN-based structures, such as the reduction of native point defects and impurities, control of strain and stress, and increasing the conductivity of p-AlGaN, which is essential for achieving acceptable p-ohmic contact resistance. In actual LED structures, there is a need to optimize current injection cavity design and light extraction through the contact layers. The large band-offsets in this system are also a challenge in terms of designing layer structures that minimize the turn-on voltage and enhance carrier transport.
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UV LEDs or laser diodes can be employed in systems for real-time measurement of fluorescence spectra from airborne biological and chemical particles. This data can then be employed as part of a compact biological or chemical agent warning sensor with fast response and high detection sensitivity. The UV wavelengths are necessary to produce fluorescence in many of the targeted chemicals and biological agents.
GaN-based power electronics There is increasing interest in the replacement of mechanical relays in power flow control circuits used in the electricity grid and other applications, such as electric automobiles and hybrid electric military vehicles. Presently, the design and limitation of Si curtails the use of power electronics in utility transmission and distribution systems14. Devices based on SiC, and especially GaN, offer a power handling capability a factor of ten higher15. Most of the Si devices of the last decade are based on gate turn-off thyristors (GTOs), with emerging devices including insulated gate bipolar transistors (IGBTs) and MOS-controlled
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thyristors (MCTs). There is also the beginnings of serious interest in the utility industry for static compensators (to provide voltage support on lines by generating or absorbing reactive power without the need for large external reactors or capacitor banks), verified power controllers (to comprehensively control power flow), and dynamic voltage restorers (to protect sensitive loads from line disturbances). These would all be components in a flexible ac transmission system. Inclusion of power electronics in such systems enables achievement of soft-switching to eliminate harmonics and dramatically improves power controllers and converters for ship propulsion and avionics. Currently the realization of such systems is limited by the inadequacy of Si-based semiconductor-controlled rectifiers (SCRS) and GTOs in terms of: • Maximum voltage ratings 60 V) and hence a large dynamic range30. Since one of the potential applications for HEMTs is in broad-band satellite transmission for communications, television, and weather forecasting systems, it is necessary that they be radiation-resistant. Fig. 4 shows the effect of 40 MeV proton irradiation at doses corresponding to ten or 100 years in low-earth orbit on the dc characteristics of AlGaN/GaN HEMTs31. Even for the highest proton dose, the transconductance only decreases by 30%. Similarly the RF performance was only degraded by 30% for this dose. The main degradation mechanism is the removal of carriers from the device channel by radiation-induced traps. The devices are also very resistant to γ-ray damage32. One frequently reported problem in these devices is that the RF power obtained is still much lower than expected from the dc characteristics33-35. This problem is manifested by a collapse in drain current or by frequency dispersions in the transconductance and output resistance, leading to severely reduced output power and power-added efficiency. Several mechanisms have been identified as the causes, including the presence of surface states between the gate and drain that deplete the channel in this region over a time constant long enough to disrupt modulation of the channel charge during large signal operation or of trap states in the buffer layer (Fig. 5, top). Several studies have shown that the use of SiNx passivation layers can be effective in reducing the effects of surface states34-36. One drawback of typical plasma enhanced chemical vapor deposited SiNx is the high hydrogen content, which could migrate into the GaN or the gate metallization. Two alternative candidates for HEMT passivation are MgO and Sc2O3, which are under development as gate dielectrics
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GaN for spintronic applications There is currently much interest in the science and potential technological applications of spin-transport electronics (or spintronics), where the spin of charge carriers (electrons or holes) is exploited to provide new functionality for microelectronic devices39-41. The phenomena of giant magnetoresistance and tunneling magnetoresistance have been exploited in all-metal or metal-insulator-metal
Fig. 5 Schematic of an AlGaN/GaN HEMT showing possible mechanisms for the currentcollapse phenomena (top), and power measurements from an HEMT before and after Sc2O3 passivation (bottom).
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magnetic systems for read/write heads in computer hard drives, magnetic sensors, and magnetic random access memories (MRAM)42. The development of magnetic semiconductors with practical ordering temperatures could lead to new classes of devices and circuits, including spin transistors, ultra-dense non-volatile semiconductor memory, and optical emitters with polarized output. In the past, most of the attention on ferromagnetic semiconductors has focused on the (Ga,Mn)As40 and (In,Mn)As43 systems. In samples grown in single-phase by MBE, the highest Curie temperatures reported40 are ~110 K for (Ga,Mn)As and ~35 K for (In,Mn)As. A tremendous amount of research on these materials systems has led to some surprising results, such as the very long spin lifetimes and coherence times in GaAs44 and the ability to achieve spin transfer through a heterointerface45, either semiconductorsemiconductor or metal-semiconductor. One of the most effective methods for investigating spin-polarized transport is by monitoring the polarized electroluminescence output from a quantum well LED into which the spin current is injected. Quantum selection rules relating the initial carrier spin polarization and subsequent polarized optical output can provide a quantitative measure of the injection efficiency46. There are a number of essential requirements for achieving practical spintronic devices in addition to the efficient electrical injection of spin-polarized carriers. These include the ability to transport the carriers with high transmission efficiency within the host semiconductor or conducting oxide, the ability to detect or collect the spin-polarized carriers, and the control of the transport through external means such as biasing of a gate contact on a transistor structure. Nitta et al.47 demonstrate that a spin-orbit interaction in a semiconductor quantum well could be controlled by applying a gate voltage. These key aspects of spin injection, spin-dependent transport, manipulation, and detection form the basis of current research and future technology. The use of read sensors based on metallic spin valves in disk drives for magnetic recording is already a $100 billion per year industry. It should also be pointed out that spintronic effects are inherently tied to nanotechnology, because of the short (~1 nm) characteristic length of some of the magnetic interactions. Combined with the expected low power capability of spintronic devices, this should lead to extremely high packing densities for memory elements. While the
progress in synthesizing and controlling the magnetic properties of III-arsenide semiconductors has been astounding, the reported Curie temperatures (TC) are too low to have significant practical impact. A key development that focused attention on wide bandgap semiconductors as being the most promising for achieving high TCs was the work of Dietl et al.48. They employed the original Zener model of ferromagnetism49 to predict TC values exceeding room temperature for materials such as GaN and ZnO containing 5% of Mn and a high hole concentration (3.5 x 1020 cm-3). In the period after the appearance of Dietl’s paper, remarkable progress has been made on the realization of materials with TC values at or above room temperature. There are a number of existing models for the observed magnetism in semiconductors and conducting oxides. The Dietl near-field model considers ferromagnetism to be mediated by delocalized or weakly localized holes in p-type materials50. The magnetic Mn ion provides a localized spin and acts as an acceptor in most III-V semiconductors, so that it can also provide holes. This treatment assumes that the Mn-doped III-V materials are charge transfer insulators and does not apply when d-shell electrons participate in charge transport. The spin-spin coupling is assumed to be a longrange interaction, allowing use of a mean-field approximation. The TC for a given material, Mn concentration, and hole density is then determined by a competition between the ferromagnetic and antiferromagnetic interactions. The model takes into account the anisotropy of the carrier-mediated exchange interaction related with the spin-orbit coupling in the host material. The TC is proportional to the density of Mn ions and holes. It is certainly fair to say that the origin of ferromagnetism in wide bandgap semiconductors is still not totally understood. Many aspects of the experimental data can be explained by the mean-field model (which is based on the Ruderman-Kittel-Kasuya-Yosida, or RKKY, interaction). However, ferromagnetism has been observed in samples that have very low hole concentrations, in insulating material, and, more recently, in n-type material. Recent models have taken account of these observations of ferromagnetism in non-degenerate samples and included consideration of indirect exchange interactions caused by virtual electron excitations from magnetic impurity acceptors to the valence band51 or the effects of positional disorder52, which lead to unusual spin and charge transport properties and the shape of
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Fig. 6 Schematic of GaN-based spin-LED (top), and blue light emission from this device (bottom).
the magnetization curve. In some cases, positional disorder of the magnetic impurities is found to enhance the ferromagnetic transition temperature53. An alternative approach using local density functional calculations indicates that the magnetic impurities may form small nano-size clusters (just a few atoms in dimension), which produce the observed ferromagnetism54. This would be difficult to detect by most characterization techniques. Additional studies have predicted which magnetic dopants should be most effective in GaN (e.g. V, Cr, Mn, Fe) without additional doping to produce carriers55 and have identified chemical trends. Numerous recent reports56-58 have found room temperature ferromagnetism in (Ga,Mn)N. Note that not all of the materials are degenerately-doped p-type. In the mean-field theories it is difficult to achieve ferromagnetism in n-type semiconductors because of their generally smaller s-d interaction. In these types of theory, the sp-d interactions are regarded as the effective magnetic field acting on the carriers, so that when spontaneous magnetization and holes are present, the resultant spin-splitting in the valence band lowers the system energy. The initial work on this material59,60 involved either microcrystals synthesized by nitridization of pure metallic Ga in supercritical ammonia or bulk crystals grown in reactions of Ga/Mn alloys on GaN/Mn mixtures with ammonia at
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~1200°C. These samples exhibit ferromagnetic properties over a broad range of Mn concentrations, as did some early MBE-grown films61. More recent reports on epi-growth of (Ga,Mn)N have detailed a range of growth conditions producing single-phase material and the resulting magnetic properties62. In general, no second phases are found for Mn levels below ~10% for growth temperatures of ~750°C. The (Ga,Mn)N retains n-type conductivity under these conditions and is singlephase as measured by e-ray diffraction. When the Mn concentration is increased significantly, peaks from tetragonal Mn0.6Ga0.4 become visible. In accordance with most theoretical predictions, magnetotransport data show the anomalous Hall effect, negative magnetoresistance, and magnetic resistance at temperatures that are dependent on the Mn concentration. For example, in films with very low (
