Towards Integrated AlGaN/GaN Based X-Band High-Power Amplifiers

Transcription

1 Towards Integrated AlGaN/GaN Based X-Band High-Power Amplifiers PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr. R.A. van Santen, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op maandag 5 juli 2004 om uur door Bart Jacobs geboren te Valkenswaard

2 Dit proefschrift is goedgekeurd door de promotoren: prof.dr.-ing. L.M.F. Kaufmann en prof.dr.-ing. E. Kohn copromotor: dr. F. Karouta Druk: Universiteitsdrukkerij Technische Universiteit Eindhoven Ontwerp omslag: Paul Verspaget CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Jacobs, Bart Towards integrated AlGaN/GaN based X-band high-power amplifiers / by Bart Jacobs. - Eindhoven : Technische Universiteit Eindhoven, Proefschrift. - ISBN NUR 959 Trefw.: galliumnitridehalfgeleiders / veldeffecttransistoren / passieve elektronische componenten / 3-5 verbindingen. Subject headings: wide band gap semiconductors / high electron mobility transistors / coplanar waveguides / III-V semiconductors.

3 Aan mijn ouders

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5 Contents 1 Introduction Advantages of the Gallium Nitride Material System Applications Class A Amplifier Example Objectives and Outline of this Thesis The Gallium Nitride Material System Crystal Structure and Material Properties Material Growth Substrates Polarization Effects Undoped AlGaN/GaN HEMT Structures Summary Reactive Ion Etching of GaN-Based Materials Introduction Reactive Ion Etching Setup and Working Principles Etching of GaN Description of Plasma Behavior Based on the DC Bias Etching of AlGaN Metal-Semiconductor Contacts on AlGaN/GaN Heterostructures Introduction Metal-Semiconductor Contacts Fermi Level Pinning Barrier Formation Metal-Induced Gap States Origin of the 2DEG in AlGaN/GaN Structures Experimental Approach Ohmic Contacts Metallization Scheme and Epitaxial Structure The Transfer Length Method Optimization of the Ohmic Contact Measurement Accuracy, Reproducibility, Line Definition and Morphology i

6 ii CONTENTS Concluding Remarks Schottky Contacts on AlGaN/GaN FET Structures Schottky Metallization Wafer Description and Schottky Layout Reference Tests Forward and Reverse Characteristics Optimization of Pre-Metallization Treatments Discussion AlGaN/GaN High Electron Mobility Transistors Introduction Principles of Operation of the HEMT Breakdown High-Frequency Operation Dispersion Phenomena in AlGaN/GaN HEMTs Drain Lag and Buffer-Related Current Collapse Surface-Related Gate Lag and Transient Response Optimization of AlGaN/GaN HEMT Processing Process Optimization Submicron HEMTs Summary and Discussion Passive Components on AlN Introduction Matching Networks Quarter-Wavelength Transformer L-type Matching Networks Microstrip and Coplanar Waveguide Technology Substrate Material and Integration Technique Constructing a CPW Library Coplanar Transmission Lines Introduction Geometrical Quasi-TEM Range Quasi-TEM Approximation Non-CPW Modes Quasi-TEM Behavior of CPW Lines Transmission Line Model Conformal Mapping Results Assessment of the Quasi-TEM Range De-embedding Algorithm for CPW Lines The Modified TRL Approach Summary of the Algorithm Comparison with Algorithm Based on Power Waves Quasi-TEM Coplanar Lines on AlN

7 CONTENTS iii Scalable Model for the Line Capacitance Scalable Model for the Propagation Constant Influence of the Measurement Setup Scalable Model for the Adaptor Design Considerations Summary Orthogonal Elements Introduction Coplanar Bend Calibration Development of a Scalable Model Properties of Coplanar Bends Coplanar T-junctions and Crosses Measurements on Multi-Ports Development of a Scalable Model Results Summary Capacitors and Resistors Introduction Tapers Capacitors Breakdown Mechanisms Resistors Series Resistor Resistor to Ground Model Accuracy and Discussion Summary Application Examples: Matching Networks Introduction Stub-Tuning Ohm Matching Network to-1 Combiner Conclusions Conclusions HEMTs on AlGaN/GaN Heterostructures Passive Components on AlN Concluding Remarks A Waveguide Circuit Theory 163 A.1 Introduction A.2 Waveguide Voltage, Current and Characteristic Impedance A.3 The Scattering Matrix

8 iv CONTENTS A.3.1 Travelling Waves A.3.2 Power Waves A.4 The Transmission Line Model B Processing of CPW Elements on AlN 171 B.1 Properties of Ceramic AlN B.2 Process Flow C Scalable Models 175 C.1 Functions for the Adaptor C.2 Functions for the Bend C.3 Functions for the Taper C.4 Functions for the Capacitor C.5 Functions for the Resistor C.6 Finding a Suitable Multi-Variable Function C.6.1 Functions for T-junctions and Crosses List of Symbols 181 List of Acronyms 187 Summary 191 Samenvatting 193 Dankwoord 195 Curriculum Vitae 196

9 Chapter 1 Introduction 1.1 Advantages of the Gallium Nitride Material System For a new semiconductor material system to become successful, it must have clear advantages over current solutions. Often the new material system must outperform other materials, enable new applications, or promise significant cost reductions. Table 1.1 shows some important properties for the most common semiconductors today (Si, gallium arsenide (GaAs) and indium phosphide (InP)) and the recently emerging wide bandgap semiconductors (WBGS) silicon carbide (SiC) and gallium nitride (GaN). For electronic applications, the added value of WBGS can be found in the combination of a high breakdown voltage with a high electron velocity. This promises the realization of high-frequency, high-power applications that cannot be realized in the other material systems. Another advantage is related to the bandgap itself. A wide bandgap implicates strong bindings in the material making it less susceptible to chemicals and temperature variations. Devices made using these materials can therefore be used in harsh environments. In addition, WBGS can be used to fabricate light emitting devices in the blue to ultraviolet range. Clearly WBGS have advantages, but applications must be found that commercially justify the development of these materials into mature technologies. Some of these applications will be discussed in the next section. 1.2 Applications Almost all of the early research done on WBGS was directed towards optoelectronic applications. This was due to the fact that blue was the only color missing on the commercial light emitting diode (LED) market. Before GaN became available, SiC was used but its indirect bandgap resulted in rather poor efficiencies. Using the indium gallium aluminum nitride (InGaAlN) alloy system, highly efficient LEDs with wavelengths ranging from ultraviolet to blue/green can be realized. GaN-based LEDs can be used in conjunction with yellow and orange LEDs, made using the aluminum gallium indium phosphide (AlGaInP) material system, to realize full- 1

10 2 Introduction Table 1.1: Material properties for various semiconductors at 300K taken from [1] unless otherwise stated. Property Si GaAs InP 4H SiC GaN AlGaAs/ InAlAs/ AlGaN/ InGaAs InGaAs GaN Bandgap (ev) Electron mobility (cm 2 /Vs) a b 2000 c Saturated (peak) electron velocity (1.0) (2.1) (2.3) (2.0) (2.7) (x 10 7 cm/s) 2DEG sheet electron N.A. < < N.A density (cm 2 ) Critical breakdown field (MV/cm) Therm. conductivity [2] (W/cmK) 3.3 s a,b,c parameters for the corresponding heterostructures (aluminum gallium arsenide (AlGaAs)/indium gallium arsenide (InGaAs), indium aluminum arsenide (In- AlAs)/InGaAs and aluminum gallium nitride (AlGaN)/GaN), respectively. s thermal conductivity of semi-insulating SiC. color LED television. Numerous other applications are possible including traffic lighting, automotive lighting, and possibly general lighting. White LEDs have been made by coating the inside of the ultraviolet LED with phosphorus. Like with tubular lighting (TL), this will convert the ultraviolet photon into visible light. Blue lasers were first demonstrated by Nakamura et al. from Nichia at the end of 1995 [3]. These devices could result in a four- to eightfold increase in optical storage capacity. In 1998, the same authors demonstrated room-temperature continuous wave (CW) operation with a lifetime of more than hours [4]. GaN optoelectronic devices represent a multi-billion dollar market but several issues still remain. The most important ones are related to the p-type doping of the material and obtaining high reflectivity mirror cavities. Low p-type doping leads to high contact resistances and problems with current spreading [5]. This process is difficult because the dopants, mostly Mg, have a high activation energy. In addition, during growth interstitial hydrogen forms a Mg:H complex that passivates the acceptor. To activate the Mg, a hightemperature annealing step is needed. Most of the development of WBGS for electronic applications is directed towards high-power, high-frequency devices. Impressive power data on the order of 4-6W/mm at 4GHz for SiC metal-semiconductor field-effect transistors (MESFETs) and 10-11W/mm at 10GHz for AlGaN/GaN high electron mobility transistors (HEMTs) has been demon-

11 1.2 Applications 3 strated 1. GaN-based HEMTs usually show a better high-frequency performance because of the large mobilities that can be achieved in these structures. Both SiC and GaN offer the advantage of a high power density and good thermal properties, which can result in smaller power modules with fewer cooling requirements. In addition, if higher supply voltages are used, the input and output impedances become much higher, which reduces the complexity of the impedance matching problem. Recently, Cree has announced SiC MESFETs capable of producing 10W at 1dB compression at 2.4GHz with 13dB gain 2, while RF Micro Devices is sampling customer evaluation samples of their GaN-based HEMTs delivering 20W CW designed for universal mobile telecommunications system (UMTS) applications 3. For now, both companies are focused on the basestation market where they have to compete with the much cheaper but less performing (1W/mm) Si laterally diffused metal-oxide-semiconductor (LDMOS) field-effect transistor (FET) technology. Realizing heterojunction bipolar transistors (HBTs) in GaN is difficult due to problems with achieving sufficiently high p-type doping in the base. Furthermore, the current in these devices flows in parallel with the dislocations in the material. This could cause difficulties with parallel conduction paths and reliability. A large portion of the research on WBGS for electronic applications is related to military industry. In the United States, the office of naval research (ONR) is actively involved in coordinating and financing research programs carried out at several universities. One of the target applications is a broadband amplifier for a multi-function radar [1]. Such a system would lower the total area of radar arrays to be exploited by adversaries, avoid possible electromagnetic conflict between different arrays and reduce overall costs. Another example is a low-noise amplifier (LNA), which is more robust due to the high breakdown field and good thermal conductivity. Noise figures of 0.6dB at 10GHz [7] and third-order-intermodulation intercept points 10dB higher than those of GaAs-based amplifiers [8] have been published. Another application area is high-temperature electronics. Usually, electronic systems that control and monitor high-temperature devices, like a jet engine, are located in cooler areas. This requires wiring between the electronic system and the sensors. If these systems could be placed in the high-temperature area, the total amount of wiring could be reduced. Degraded wiring has been the cause of major airplane tragedies like TWA flight 800 (near Long Island NY in 1996) and SwissAir flight 111 (near Nova Scotia in 1998) [9]. WBGS have to compete with low-power Si and Si on insulator (SoI) technologies that are used today for applications up to C. The added value for WBGS therefore lies in the C range. Issues for this market are related to reliable contact technology and the development of new packaging technologies that can withstand these temperatures and the associated oxidation. The total market for high-temperature electronics is expected to reach $ 900 million by the year It is difficult to predict the market share of WBGS compared to Si or SoI. Strategies Unlimited have estimated the total GaN electronic device market to reach $ 500 million by the end of this decade. However, economies of scale in the optoelectronic market, as well as improvements in material and substrate growth in this area, could accelerate industrialization of GaN-based electronic devices. 1 See [6] for a comparison between SiC and GaN devices. 2 Part number CRF-24010: Product sheet available on the internet. 3 Press release in compound semiconductor.net 3 march 2003.

12 C 4 4 Introduction I? D A I = N E A 4 I F A 8 E I = N 8 =? 8 E I 1 =? 8 C I 8 8 F I J E A J E A ) * Figure 1.1: Class A amplifier circuit (A) and current-voltage characteristics (B). 1.3 Class A Amplifier Example This thesis investigates the use of GaN for high-frequency, high-power amplifiers. To demonstrate the most important issues for these amplifiers we will use an example of a class A amplifier as illustrated in Figure 1.1. The gate of the FET, e.g. an AlGaN/GaN HEMT, is connected to an AC voltage source (V ac ) that has a DC offset of V in. In a first approximation, the FET acts as a voltage-controlled current source that can be described by the transconductance (g m ): g m = I ds V gs (1.1) where I ds is the drain-source current and V gs the gate-source voltage. Applying a sinusoidal voltage to the gate will result in a sinusoidal drain-source current (I ac ). The total drainsource current can then be written as: I ds (t) = I ac (t) + I dc (1.2) where I dc is the DC component corresponding to the operating bias point as indicated by the black dots in the figure. For the drain-source voltage (V ds ) we can write: V ds (t) = V dc I ac (t)r = V dc + I dc R I ds (t)r (1.3) where V dc is the drain-source DC operating bias point and R the load resistor. This relation describes the so-called loadline in the (I ds,v ds ) plane. Voltage and current move along this line in the time-domain 4. As illustrated in Figure 1.1, the maximum AC power that can be delivered to the load without waveform clipping equals: P out = (V ds,max V k )I ds,max 8 4 Any reactive elements in the FET itself are neglected. (1.4)

13 1.3 Class A Amplifier Example 5 where V k is the knee voltage and V ds,max and I ds,max are the maximum drain-source voltage and current, respectively. The power added efficiency (PAE) of the class A amplifier can be found using: P AE = P out P in = P out (1 1/G p ) (1.5) P dc P dc where G p is the power gain and P in and P dc are the AC and DC power supplied to FET, respectively. Assuming an infinite gain, we find: P AE = V ds,max V k 2(V ds,max + V k ) (1.6) Hence, for an ideal FET, which has V k = 0, the maximum efficiency equals 0.5 or 50%. Power added efficiency is a figure of merit that indicates how much of the supplied DC power is converted into AC power. Applying biasing strategies other than class A and/or using waveshaping techniques, efficiencies well above 50% can be obtained. As mentioned above, the gain of the amplifier also determines the PAE. Generally speaking, if the gain drops below 10dB, it has a deteriorating effect on efficiency. There are limits up to which frequency the amplifier can be used efficiently. Several figure of merits exist that describe the high-frequency limits of a device. Here we will use the cut-off frequency f t, which is the frequency at which the short-circuit current gain equals unity. For a HEMT, f t can be calculated using: f t g m (1.7) 2πC gs where C gs is the gate-source capacitance. Because it mainly depends on intrinsic material properties, this figure of merit is particularly useful to discriminate between different semiconductor technologies. This feature can be exposed by re-writing the drain-source current in saturation as: I ds = qσ 2 W g v sat (1.8) where qσ 2 is the charge density in the channel of the HEMT, v sat the saturated electron velocity, and W g the gate width. Applying Eq. (1.1) gives: C gs qσ 2 g m = W g v sat = W g v sat = v satc gs (1.9) V gs L g W g L g with L g the gate length. Inserting this in Eq. (1.7) yields: f t v sat 2πL g (1.10) Hence, f t is inversely proportional to L g /v sat, which is the transit time of electrons under the gate. The cut-off frequency and power gain are closely related. This will be demonstrated using the simple equivalent circuit illustrated in Figure 1.2. In this figure R in and R ds represent the input and output resistance of the device, respectively. The current (I) through the load resistor can be computed using: I = g m V gs R ds V ac R ds + R = g R ds m 1 + jωc gs R in R ds + R (1.11)

14 6 Introduction 8 =? 4 E 8 C I C 8 C I + C I I 4 Figure 1.2: Simple equivalent circuit for a FET. where ω is the angular frequency. For high frequencies (ωc gs R in 1), the power delivered to the load is given by: P out = I 2 R 2 = V 2 ac 2R 2 in ( ) 2 ( ) 2 ft Rds R (1.12) f R ds + R where f is the frequency. At high frequencies the power delivered to the FET simply equals Vac/2R 2 in. The gain is therefore given by: G p = P out P in = 1 R in ( ) 2 ( ) 2 ft Rds R (1.13) f R ds + R This relation clearly illustrates the need for high f t devices. Based on the equations derived above, we can now understand why GaN-based HEMTs are indeed excellent candidates for high-power, high-frequency power amplifiers (see table 1.1): A high saturated electron velocity enables high f t devices and increases maximum current and therefore power. High electron sheet densities in the channel allow large current swings, again increasing power output. High breakdown fields allow large voltage swings, which also increase output power. The high electron mobility in the HEMT structure reduces the knee voltage, thereby increasing efficiency and power output. A wide bandgap increases the maximum thermal dissipation in the FET. In the discussion above we tacitly assumed that the impedance of the load can be chosen to optimize output power. Most radio frequency (RF) systems however, have a 50Ω input and output. We therefore need matching networks to transform the 50Ω to the desired impedance levels. These networks are usually a combination of inductors and capacitors. Especially at high frequencies, losses in these networks tend to increase due to the skin effect and substrate losses. As an example, consider a HEMT that has a power gain G p and dissipates P dc, which is connected to both an input and output matching network. The losses in these networks are

15 1.4 Objectives and Outline of this Thesis 7 P loss,in and P loss,out, respectively. The efficiency of this combined circuit can be computed from: P AE = P in (P loss,in G p P loss,out 1) P dc = P AE HEMT P loss,out G p 1/P loss,in G p 1 (1.14) where P AE HEMT is the efficiency of the HEMT alone: P AE HEMT = P inp loss,in G p P loss,in P in P dc (1.15) Losses in the output matching network are more critical for efficiency than losses in the input matching network due to the multiplication with G p. As a numerical example we take a HEMT with G p = 15dB and 50% efficiency. Losses in the output network as low as 0.5dB already reduce the total efficiency by 5%. 1.4 Objectives and Outline of this Thesis This project was started in 1998 as a cooperation between the Eindhoven University of Technology (TUE) and TNO Physics and Electronics Laboratory (TNO-FEL) in The Hague. The main drive for this research was to evaluate the use of AlGaN/GaN based HEMTs for X-band (8-12GHz) high-power amplifiers (HPAs) and, by developing a GaN technology base, get a clear understanding of the drawbacks and advantages involved with this new material system both from a theoretical and practical perspective. The main goal of this thesis is to develop the technologies needed in a high-power amplifier based on AlGaN/GaN HEMTs. As mentioned in the previous section, this not only involves the active devices, but also includes passive devices needed for impedance matching and biasing. These two different aspects of the power amplifier are reflected in the outline of this thesis. The first part of this thesis describes the development of a GaN technology base for the fabrication of AlGaN/GaN HEMTs. It consists of the following chapters: Chapter 2 presents a general overview of the GaN material system. We will look at how the material is grown, discuss the crystal structure and how this affects the properties of the AlGaN/GaN epitaxial layer structure. Mesa etching is needed to electrically isolate adjacent devices. Any damage introduced to the etched surface could lead to surface conduction and should therefore be avoided. A suitable etch process is described in chapter 3 Ohmic and Schottky contacts will be described in chapter 4. The drain and source of a FET are examples of ohmic contacts. The resistance of these contacts should be as low as possible. Gate contacts are Schottky contacts. Key parameters of these contacts are the reverse current and the breakdown voltage. The latter is related to the maximum voltage swing that can be applied to the FET.

16 8 REFERENCES In chapter 5 we will combine all the process steps described above into a HEMT process flow. As we will see, merely combining process steps results in devices that are useless at both DC and RF frequencies. First of all, the leakage currents are too high. Secondly, the devices show a severe amount of frequency dispersion, which means that the available current swing at high frequencies is considerably lower than at DC. To solve these problems the entire HEMT process needs to be optimized. The second part of this thesis describes the construction of a library with passive components. These devices are of the coplanar type using ceramic aluminum nitride (AlN) as substrate material. The reasons for choosing this technology instead of e.g. microstrip are given in Chapter 6. At the start of this project, there was little experience with coplanar elements nor was there a process flow available. As a result, all of the elements needed for impedance matching and interconnect in the power amplifier had to be fabricated, measured and modelled. It is important that all these elements have a high current carrying and thermal capability. Furthermore, they should be low-loss and dispersion-free, i.e. their behavior should not depend on frequency. The work on passive components is spread out over several chapters: Chapter 7 deals with coplanar transmission lines. Characteristic impedance and propagation constant are measured and compared to an analytical model based on conformal mapping. To connect different pieces of transmission line or to make turns in circuits, one can use bends, T-junctions or crosses. These elements are presented in chapter 8. To complete the component library, nickel chromium (NiCr) resistors and silicon nitride (SiNx) metal-insulator-metal capacitors (MIMCAPs) will be discussed in chapter 9. It is important to check the breakdown voltage of the SiNx used in the capacitors. This should be well above the maximum voltage occurring in the circuit. In chapter 10, the accuracy of the models for the various passive components will be demonstrated using two examples of a matching network. In chapter 11, we will briefly review the most important conclusions that can be drawn from the work described in this thesis and how these findings can be translated in future prospects for the GaN material system in electronic applications. References [1] R.T. Kemerlay, H.B. Wallace, and M.N. Yoder, Impact of Wide Bandgap Microwave Devices on DoD systems, Proceedings of the IEEE, vol. 90, no. 6, p. 1059, [2] L. Liu and J.H. Edgar, Substrates for Gallium Nitride Epitaxy, Material Science and Engineering Reports, vol. 37, no. 3, p. 61, [3] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, and Y. Sugimoto, InGaN-Based Multi-Quantum-Well-Structure Laser Diodes, Japanese Journal of Applied Physics Letters, vol. 35 part2, no. 1B, p. L74, 1996.

17 REFERENCES 9 [4] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho, Continuous- Wave Operation of InGaN/GaN/AlGaN-Based Laser Diodes Grown on GaN Substrates, Applied Physics Letters, vol. 72, no. 16, p. 2014, [5] S.P. DenBaars, Gallium-Nitride-Based Materials for Blue to Ultraviolet Optoelectronic Devices, Proceedings of the IEEE, vol. 85, no. 11, p. 1740, [6] R.J. Trew, SiC and GaN Transistors- Is There One Winner for Microwave Power Applications?, Proceedings of the IEEE, vol. 90, no. 6, p. 1032, [7] N.X. Nguyen, M. Micovic, W.S. Wong, P. Hashimoto, P. Janke, D. Harvey, and C. Nguyen, Robust Low Microwave Noise GaN MODFET with 0.6dB Noise Figure at 10GHz, Electronics Letters, vol. 36, no. 5, p. 469, [8] T. Jenkins, L. Kehias, P. Parikh, Y.F. Wu, P. Chavarkar, M. Moore, and U. Mishra, Linearity of High Al-Content AlGaN/GaN HEMT s, Device Research Conference Digest, p. 201, [9] P.G. Neudeck, R.S. Okojie, and L.Y. Chen, High-Temperature Electronics- A Role for Wide Bandgap Semiconductors?, Proceedings of the IEEE, vol. 90, no. 6, p. 1065, 2002.

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19 Chapter 2 The Gallium Nitride Material System Several aspects make the GaN material system totally different from conventional semiconductors like Si or GaAs. First of all, there is the material growth itself, which is mostly heteroepitaxial. Because GaN substrates are not readily available, growth on different materials like sapphire or SiC is required. The lattice mismatch that exists between these materials and GaN results in dislocation densities on the order of /cm 2. Despite these large densities, e.g. GaAs has typical dislocation densities of /cm 2, excellent device performance has been achieved. A second difference is the large piezoelectric and spontaneous polarization fields that exist in this material system. At interfaces within the epitaxial structure, the polarization changes abruptly. According to Gauss law, this will give rise to electric fields. Electrons and holes present in the material will try to re-arrange themselves to cancel these fields. This property is used to achieve high sheet carrier densities in HEMT structures that are free of doping. This chapter presents a brief overview of the GaN material system. We will especially focuss on the two topics mentioned above for several reasons. First of all, the choice of substrate not only determines the properties of the HEMT, but it also influences integration possibilities. Secondly, a good description of the polarization effects is very important for understanding the operation of AlGaN/GaN HEMTs. 2.1 Crystal Structure and Material Properties Usually, GaN has a wurzite structure that consists of alternating biatomic closely-packed planes of Ga and N pairs stacked in an ABAB sequence, see Figure 2.1. Adjusting substrate and growth conditions can however result in metastable zincblende structures. Due to difficulties with the growth of these crystals, most device research is however done using wurzite crystals. Almost all GaN work is done on crystals grown along the c-axis ([0001] direction). Crystals with this orientation have the so-called Ga-face polarity, crystals grown in the [000 1] direction possess N-face polarity. The definition of polarity is related to the Ga-N 11

20 12 The Gallium Nitride Material System? = N E I / =? F = A? = Figure 2.1: GaN wurzite crystal structure (Ga-face polarity). bonds parallel to the c-axis. In Ga-face crystals, the Ga atom is below the N atom, for N-face crystals we have the opposite. This distinction is important because most GaN structures are grown on a foreign substrate. If we were to have pure GaN crystals, the polarity could be changed by flipping the crystal. In addition, crystals with different polarities also behave differently. For instance, N-face crystals have lower dislocation densities and can be etched chemically [1] whereas Ga-face crystals have smoother surfaces, have a lower background concentration and are chemically inert. Almost all HEMT structures are based on Ga-face crystals due to better electron transport properties [2]. For completeness, the most important material properties of GaN are given in table Material Growth The best substrate for GaN growth, is GaN itself. Conventional bulk material growth techniques from a melt, like Czochralski or Bridgman, are not possible due to the high melting point of 2573K [6]. Furthermore, to prevent decomposition, high N 2 pressures have to be applied (60kbar at the melting point) [6]. Still, two techniques have been developed that have produced crystals large enough for homoepitaxy: hydride vaporphase epitaxy (HVPE) and high-pressure growth. The best structural properties for GaN substrates have been achieved using the high nitrogen pressure solution growth (HNPSG) approach [7]. These substrates are made from a gallium melt saturated with 1 atomic percent nitrogen at temperatures up to C obtained by nitrogen pressures of 20kbar. Dislocation densities for these crystals can be as low as 10/cm 2 [8]. However, the high oxygen concentration in these crystals results in electron concentrations on the order of /cm 3. Mg doping can be used to obtain semi-insulating properties [8]. To date, crystals are about 1cm 2 but due to the extreme conditions necessary for growth, it is difficult to scale them up to larger sizes. HVPE can be used to produce thick (> 300µm) free-standing epitaxial GaN layers. The basic mechanism is the reaction between gallium monochloride (GaCl) and ammonia (NH 3 ) at C to form GaN. The GaCl is formed within the reactor vessel by

21 2.2 Material Growth 13 Table 2.1: GaN material properties at 300K or otherwise stated [3]. electron mobility 900 cm 2 /Vs [4] saturated electron velocity cm/s [5] hole mobility 13 cm 2 /Vs breakdown field V/cm [4] thermal conductivity 2.1 W/cmK heat capacity 35.3 J/molK dielectric constant 9.5 electron mass 0.22 m 0 bandgap 3.44 ev melting point > 2573 K (at 60 kbar) lattice constants a 0 = a (300K) nm c 0 = c (300K) nm a a 0 /a 0 (1400K) c c 0 /c 0 (1400K) flowing hydrochloric acid (HCl) gas over liquid Ga. The growth rates for this process are on the order of 100µm/hr, which is comparable to the high-pressure process. HVPE is typically done using foreign substrates like sapphire, Si, GaAs or SiC, which can be removed after growth. Typical dislocation densities that can be obtained using this technique are therefore higher than the homoepitaxial high-pressure process (10 6 /cm 2 [9]). However, it gives a lower unintentional n-type doping (typically on the order of /cm 3 ) and the quality of the film improves with thickness [10]. Furthermore, the grown GaN substrate can be used as seed material in subsequent growth runs. Semiinsulating behavior can be obtained by Zn doping. Although both techniques are promising, GaN substrates are not readily commercially available. Most of the work on GaN is therefore conducted on heteroepitaxial material. This approach poses two problems, notably the mismatch in crystal symmetry and the mismatch in lattice constants between GaN and the substrate. The general approach is to use a thin (10-100nm) AlN or GaN nucleation layer 1 grown at a low temperature ( C), which forms a transition between the substrate and the actual GaN film. By optimizing the growth parameters for this layer, e.g. growth temperature and thickness, high-quality films can be obtained. Treating the substrate prior to growth, e.g. nitridation of sapphire substrates, also influences the final quality of the GaN films. Metal-organic vapor-phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are the common methods used for GaN heteroepitaxy. Trimethylaluminum, trimethylgallium, and ammonia are the precursors for the MOVPE growth of Al x Ga 1 x N. Both hydrogen and nitrogen can be used as a carrier gas. The deposition temperature varies between 1 This layer is sometimes called the buffer layer. We will use the term nucleation layer to differentiate between this layer and the undoped GaN layer present in HEMT structures.

22 14 The Gallium Nitride Material System Table 2.2: Properties of various substrates used for GaN heteroepitaxy at room temperature taken from [3] unless otherwise indicated. Data for GaN is included for reference. Property GaN AlN Si 6H-SiC Sapphire Crystal symmetry wurz. wurz. cubic hex. hex. Lattice constants a 0 = a (300K) (nm) c 0 = c (300K) (nm) N.A Lattice constants a (1400K) (nm) c (1400K) (nm) N.A Thermal conductivity (W/cmK) [11] 1.5 [4] Lattice mismatch with GaN(%) and C, which is considerably larger than for MBE (600 to C). In MBE, a solid source is used for Ga, while RF or electron cyclotron resonance (ECR) plasma sources are used to create atomic nitrogen. MOVPE and MBE show comparable growth rates of about 1µm/hr Substrates Table 2.2 presents some properties for the substrates that are most frequently used for heteroepitaxy: sapphire, SiC, Si and AlN. Each of these substrates will be discussed next. Sapphire Sapphire, single-crystal aluminum oxide, is the most frequently used substrate for the growth of GaN. Like Si, it can be grown using the Czochralski method. Sapphire is electrically isolating but has a poor thermal conductivity, which limits the power handling capability of devices. Furthermore, it has a large lattice mismatch with GaN resulting in high dislocation densities (10 10 /cm 2 ) in the epitaxial film [12]. Usually, GaN is grown on the c-plane of sapphire. This results in c-plane oriented films but with the [0001] plane of GaN rotated by 30 0 with respect to the sapphire. This rotation reduces the lattice mismatch from 30% to 13.9%. Due to this rotation, the cleavage planes of both materials are not aligned. Obtaining smooth cleaved surfaces, e.g. needed for laser fabrication, is therefore very difficult. Due to the large lattice mismatch at growth temperature, GaN films cannot be grown strained to lattice match the sapphire. One would expect the film to deposit fully relaxed. Because the thermal expansion coefficients of sapphire are larger than those of GaN, compressive strain is induced upon cooling down. Usually, a compressive strain of 0.7GPa

23 2.2 Material Growth 15 remains at room temperature for films of 1-3µm thick [3]. Both doping and nucleation layer thickness are known to influence the stress. Sapphire is a non-polar substrate. Films deposited by MOVPE on c-plane sapphire are normally Ga-face regardless of the nucleation layer. With MBE the polarity can be chosen. An AlN nucleation layer will give Ga-face polarity whereas a GaN layer will result in N-face polarity. However, without a proper nucleation layer and/or substrate treatment, epitaxial films of mixed polarity may be deposited. For the work described in this thesis, MOVPE-grown Ga-face epitaxial structures have been used. SiC There are over 250 different polytypes of SiC. These polytypes reflect one-dimensional variations of the stacking sequence of closely-packed biatomic planes. For epitaxial growth of GaN, the 4H and 6H polytypes are commonly used. H stands for hexagonal crystal symmetry and the numbers refer to the number of layers of Si and C atoms before the atomic arrangement repeats. SiC is mostly grown using sublimation techniques like the modified Lely process [13]. SiC is a polar material and it is available in both polarities. Generally speaking, Siterminated SiC results in Ga-face polarity of the GaN film and C-terminated SiC gives N-face polarity [14]. Although the lattice mismatch is only 3%, it is still large enough to cause high dislocation densities on the order of /cm 2, similar to GaN films grown on sapphire. Reasons for this are the roughness of the SiC substrates (1nm root mean square (RMS) compared to 0.1nm for sapphire) and the damage introduced during the polishing process, e.g. etching remnants and scratches. Different pre-treatments like wet/dry etching or annealing can be used to alleviate these effects. Furthermore, GaN and AlN nucleation layers are used to improve the quality of the epitaxial film. Because the thermal expansion coefficients of SiC are smaller than those of GaN, most epitaxial films will be under tensile strain. However, the amount of strain and sometimes even its sign, can strongly be influenced by adjusting the nucleation layer. One advantage of SiC is its high thermal conductivity. This makes SiC an excellent choice for high-power applications. It does however come with a high price. Semiinsulating, n- and p-type substrates are available. Si The reasons for trying Si as a substrate are obvious. Si substrates are cheap, have a high degree in crystal perfection and are available in very large sizes. It also introduces the possibility of combining GaN and Si devices on the same wafer. However, the lattice and thermal mismatch is very large. To date, decent performance can be obtained with devices on Si(1 1 1) [15] but more work is needed to approach the results obtained on sapphire or SiC. If the growth problems can be overcome, Si is a very attractive candidate. Due to its moderate thermal conductivity of 1.5W/cmK, devices on Si will not likely achieve the same power densities as devices on SiC. Furthermore, obtaining low background doping in Si substrates is rather difficult. However, the low costs of using Si substrates may prevail.

24 16 The Gallium Nitride Material System AlN From all of the substrates mentioned sofar, AlN is the best match to GaN. Not only do the lattice constants match at room temperature, they are also matched at the growth temperature of GaN. Epitaxial layers of GaN grown on AlN substrates have shown low defect densities on the order of /cm 2, which is about four orders of magnitude smaller than layers grown on SiC [11]. In addition, the thermal conductivity of AlN approaches that of SiC making it an excellent substrate for power devices. Apart from being a prime candidate for GaN heteroepitaxy, AlN could be used for optoelectronic devices. Its bandgap of 6.2eV promises the development of deep-uv light sources. AlN is usually grown by sublimation techniques using 6H-SiC as a seed material or not using a seed at all (self-seeding). Due to the high temperatures involved (2300K) and the highly reactive Al species, designing a durable reactor is complicated. Cracking induced by the difference in thermal expansion coefficients between AlN and the SiC, or the crucible in case of self-seeding, can be significant [16]. A high-pressure approach, in which AlN is grown from a solution of atomic nitrogen in liquid aluminum, is also pursued [17]. Crystal-IS 2, an incubator of the Rensselaer Polytechnic Institute, is currently the only company industrializing the (sublimation) growth of bulk AlN. Substrates as large as 1cm 2 are available at the time of writing this thesis. Defect densities on the order of 500/cm 2 have been demonstrated. It needs no explanation that the availability of lowcost AlN substrates could accelerate the development of the III-nitride material system. However, obtaining a low background doping may prove to be difficult due to the high reactivity of aluminum with oxygen. 2.3 Polarization Effects Due to lack of crystal symmetry, the center points of the positive and negative charge in GaN films do not coincide. This gives rise to large polarization fields (P ) within the material. Polarization that exists in fully relaxed films (zero strain), is referred to as spontaneous polarization (P SP ), whereas polarization induced by strain is referred to as piezoelectric polarization (P P E ). Both polarization fields are parallel to the c-axis and are usually described as vectors pointing from the Ga towards the N atom. The piezoelectric polarization can be computed from [18]: ( ) ( ) c c 0 a a 0 P P E = e 33 ɛ z + e 31 (ɛ x + ɛ y ) = e e 31 (2.1) c 0 a 0 where a and c are the lattice constants of the strained layer, a 0 and c 0 the relaxed values for this layer (see Figure 2.1), e 33 and e 31 the piezoelectric constants of the material (see table 2.3), and ɛ i the strain in direction i with the z direction parallel to the c-axis. The strain within the c-plane is assumed isotropic (ɛ x = ɛ y ). The deformation in the c-plane is related to the change in lattice constant c 0 by the elasticity tensor. For GaN-based materials we have [18]: 2 See c c 0 c 0 = 2 C 13 C 33 a a 0 a 0 (2.2)

25 2.3 Polarization Effects 17 Wurzite AlN GaN P SP (C/m 2 ) e 31 (C/m 2 ) e 33 (C/m 2 ) C 13 (GPa) [18] C 33 (GPa) [18] ) / = / = 5 = F F D E H A , - / G I 2 G I 2 G I 2! Table 2.3: Polarization and elasticity constants from [19] unless otherwise stated. Figure 2.2: The various polarization fields for Ga-face material. where C 13 and C 33 are the elasticity constants of the layer. Combining Eq. (2.1) with Eq. (2.2) gives: P P E = 2 a ( ) a 0 C 13 e 31 e 33 (2.3) a 0 C Undoped AlGaN/GaN HEMT Structures Consider the undoped AlGaN/GaN heterostructure illustrated in Figure 2.2. The AlGaN layer is so thin that it adjusts its lattice to the underlying GaN layer. The polarization in the AlGaN layer will therefore contain a piezoelectric component 3. Both the piezoelectric and spontaneous polarization are negative, meaning that they point towards the substrate. The impact of these fields can be shown by Gauss law: ɛ s E = P + ρ (2.4) where ɛ s is the dielectric constant of the corresponding layer, E the electric field and ρ the net free charge, which is zero for an ideal undoped structure. This equation shows that a gradient in the polarization field is equivalent to free charge. At the different interfaces in the structure, the polarization fields change abruptly. For instance, if we look at the AlGaN/GaN interface, we can introduce an equivalent polarization charge carrier density (σ P 2 ) by using the integral form of Eq. (2.4): ɛ s E nda = P nda = A (Ptot,GaN P tot,algan ) = qaσ P 2 (2.5) where da is a surface element of the total integration surface (A), n the corresponding normal vector, q the elementary charge, and P tot = P SP + P P E the total amount of polarization. For charge neutrality, similar carrier densities must be introduced at the Air/AlGaN (σ P 1 ) and GaN/sapphire (σ P 3 ) interfaces, see Figure 2.2. To compute the values of these densities, Eq. (2.3) must be used in combination with the values for the spontaneous polarization. Figure 2.3 presents the results of these calculations as a 3 The reader might notice the absence of the piezoelectric component in the GaN layer, although it was stated in the previous section that residual stress is present after growth. This component is assumed to be included in the spontaneous polarization of the GaN layer.

26 18 The Gallium Nitride Material System Figure 2.3: Equivalent polarization charge carrier density at the three interfaces for a Ga-face AlGaN/GaN structure on sapphire as a function of Al content. Negative values correspond to negative charges. function of Al content. For these calculations, Vegard s law was used to calculate the lattice parameters and the polarization and elasticity constants needed in Eq. (2.3). As a result of the electric field setup by the equivalent polarization charges, electrons in the AlGaN will try to cancel this field by moving towards the AlGaN/GaN interface thereby forming a two-dimensional electron gas (2DEG). Even for undoped structures, high sheet carrier densities (10 13 /cm 2 ) have been achieved [18]. This is somewhat surprising as the entire structure is undoped. But as we will see in chapter 4, the electrons in the 2DEG may stem from interface states at the AlGaN surface [20]. Despite the advantages of using undoped structures, like increased breakdown voltage or reduced gate leakage, there are some drawbacks related to the trapping and de-trapping time constants of the surface states. These drawbacks will be discussed extensively in chapter 5. In chapter 4, it will be shown that the 2DEG concentration is an increasing function of AlGaN thickness. High sheet carrier densities can be obtained by choosing a high Al content (increase in conduction band discontinuity and piezoelectric polarization) and growing relatively thick AlGaN layers. However, one should be careful not to exceed the critical thickness. This will result in relaxation thereby lowering the piezoelectric field and sheet carrier concentration. Also, for high Al content, electron mobility will decrease due to increased surface roughness scattering [21]. For these reasons, the AlGaN thickness for most HEMT structures is between 20 and 30nm with an Al content ranging from 25-35%. 2.4 Summary GaN-based HEMTs offer the possibility to combine high voltages and currents at high frequencies. Despite this potential, early development in this material system was hampered by the unavailability of a suitable substrate. Currently, SiC, Si and sapphire are the most frequently used substrates for heteroepitaxy. Due to the lattice mismatch with these substrates, pure monocrystalline GaN films cannot be grown without the use of an intermediate layer. Although the defect densities obtained in these films are several or-

27 REFERENCES 19 ders of magnitude larger than those typically found in GaAs, excellent device results have been obtained. At the time of writing this thesis, the most impressive power densities are reported for AlGaN/GaN HEMTs on SiC. It is the high thermal conductivity of SiC that makes this material an excellent substrate for power devices. To avoid the high costs of SiC, much effort is being spent in developing Si as substrate material. The advantages of using Si include large wafer sizes and increased integration possibilities. Another substrate candidate is monocrystalline AlN. These crystals present a very low lattice mismatch to GaN and have a thermal conductivity similar to that of SiC. Furthermore, the very wide bandgap of this material presents new opportunities for optoelectronic devices. However, much like Si, it will be difficult to obtain semi-insulating material. Homoepitaxy on GaN substrates should offer the highest crystal quality possible. Commercial availability of these substrates is still limited, but this will most likely change in the future. Obtaining low background doping levels is crucial if these substrates are to be used in electronic devices. References [1] J.L. Weyher, P.D. Brown, A.R.A. Zauner, S. Müller, C.B. Boothroyd, D.T. Ford, P.R. Hageman, C.J. Humphreys, P.K. Larsen, I. Grzegory, and S. Porowski, Morphological and Structural Characteristics of Homoepitaxial GaN Grown by Metalorganic Chemical Vapour Deposition (MOCVD), Journal of Crystal Growth, vol. 204, no. 4, p. 419, [2] O. Ambacher, J. Smart, J.R. Shealy, N.G. Weimann, K. Chu, M. Murphy, W.J. Schaff, L.F. Eastman, R. Dimitrov, L. Wittmer, M. Stutzmann, W. Rieger, and J. Hilsenbeck, Two-Dimensional Electron Gases Induced by Spontaneous and Piezoelectric Polarization Charges in N- and Ga-face AlGaN/GaN Heterostructures, Journal of Applied Physics, vol. 85, no. 6, p. 3222, [3] L. Liu and J.H. Edgar, Substrates for Gallium Nitride Epitaxy, Material Science and Engineering Reports, vol. 37, no. 3, p. 61, [4] R.T. Kemerlay, H.B. Wallace, and M.N. Yoder, Impact of Wide Bandgap Microwave Devices on DoD systems, Proceedings of the IEEE, vol. 90, no. 6, p. 1059, [5] B. Gelmont, K. Kim, and M. Shur, Monte Carlo Simulation of Electron Transport in Gallium Nitride, Journal of Applied Physics, vol. 74, no. 3, p. 1818, [6] J. Karpinski, J. Jun, and S. Porowski, Equilibrium Pressure of N 2 over GaN and High Pressure Solution Growth of GaN, Journal of Crystal Growth, vol. 66, no. 1, p. 1, [7] S. Porowski, High Pressure Growth of GaN: New Prospects for Blue Lasers, Journal of Crystal Growth, vol. 166, no. 1, p. 583, [8] K. Saarinen, J. Nissilä, P. Hautojärvi, J. Likonen, T. Suski, I. Grzegory, B. Lucznik, and S. Porowski, The Influence of Mg Doping on the Formation of Ga Vacancies and Negative Ions in GaN Bulk Crystals, Applied Physics Letters, vol. 75, no. 16, p. 1276, 1999.