Fabrication and characterization of AIGaN gan HEMTs

The state-of-the-art performance of MODFETs can be summarised by the following data collected from different devices: the average saturation drain-source current is about 1 A/mm in virtu

Chapter 1

Introduction

1.1 Review on III-V semiconductor transistors

The radio frequency (RF) semiconductor market is ever increasing due to the exploding development of cellular phones and satellite communications The bulk of this market has been occupied by Si and GaAs This however is likely to change in the near future as two other semiconductors emerge with greater capabilities They are SiC and GaN [1], [2]

The comparison of GaN with the other materials is shown in Figure 1.1 [1-5]: all semiconductors, which are candidates for the RF applications are schematically indicated

in a frequency-power diagram For some semiconductors (Si, GaAs), this represents the current situation while for others (SiC, GaN) it represents the expected one in the near future On the high frequency side, InP-based materials take advantage of small effective mass and high mobility to achieve record frequencies On the low frequency side, Si prevails for moderate powers Owing to the success of SiGe [6], the Si industry is now extending towards higher frequencies, about 2.5 GHz SiC is performing well for high power and low frequency, but is, however, limited to frequencies of a few giga-hertz (GHz) only [1]

GaN is in competition with SiC on its low frequency side and with GaAs on its low power/high frequency side (see Figure 1.1) Thanks to breakthroughs in technology that improve the thermal management (wafer fusion and flip chip, etc.) [7, 8], all technologies are moving towards higher power where heat dissipation is a prevailing barrier for optimum performance

Figure 1: Semiconductor materials for RF electronics RF power is plotted against Frequency [1-5]

The domain of RF applications is wide and in rapid growth As new applications emerge, there is a continuous shift towards higher frequencies On the low frequency side, Si and GaAs benefit from the exponential development of cellular phone (0.8 and 1.9 GHz) [9]

At high frequencies, satellite and terrestrial communications (1.6, 2.5, 5.2, 23, 28 GHz) [9] and military applications (X-band, 8 to 12 GHz) represent a strong potential for development of GaN [9] Applications at higher frequencies such as anticolliding radar (76 GHz) can also be implemented using GaN and InP [9]

The cost of RF systems depends on the power and frequency of use As the frequency increases, the difficulty to coherently add the individual powers coming from different transistors increases Dephasing has to be accounted for by a proper design and addition

of passive elements such as capacitors can take up to three-quarter of the area on the wafer As a result, high frequency high power modules are expensive In addition, a

RF Output Power 50W

10W

cooling system is usually required as high power applications generate substantial heat that deteriorates the device performance

The advantage of III-V semiconductors, which feature wide bandgaps, is tremendous They provide a larger RF power per unit area, which simplifies the design, and also, they work well at elevated temperatures, which allows reduced peripherals for cooling The power generated in each transistor under a large bias makes the impedance and power matching between different stages easier This is especially so for the AlGaN/GaN transistor which has a characteristic impedance three times larger than its counterpart, AlGaAs/GaAs [21] Thus, it is expected that using GaN rather than conventional III-V compounds such as GaAs will lead to the same performance at a lower price, or to a higher performance at a competitive price due to ease of design

The advantage of GaN over SiC is the possibility of making heterostructures Such heterostructure has been demonstrated to produce two-dimensional electron gas (2DEG)

at the heterojunction, and this makes possible several novel devices that can operate at frequencies beyond the capability of SiC This is due to the mobility of the 2DEG at about 3 times higher than the carriers in SiC metal-semiconductor FETs, (MESFETs) [10] Also, the 2DEG permits low resistance and low noise performance not possible in SiC The state-of-the-art performance of MODFETs can be summarised by the following data collected from different devices: the average saturation drain-source current is about

1 A/mm in virtue of a large 2DEG sheet carrier density (1013cm-2) that is possible by the very large conduction band offset at the AlGaN/GaN interface [2], [11] The transconductance can reach 270 mS/mm (for a 0.7µm gate length high electron mobility

transistor (HEMT)) [12] and the highest unity-gain bandwidth (f t) and unity-power gain

bandwidth (f max) could reach 67 and 140 GHz [13], respectively Extremely high breakdown voltage of 100V has been recorded, and power density of 10W/mm at 10 GHz has been attained due to good thermal management [14] Up to now, the best solution for good thermal management has been the usage of bulk SiC as a substrate as it has a very high thermal conductivity constant of about 3 W/Kcm (10 times that of sapphire) [31] It

is crucial to note that the highest temperature drop occurs at the first micron below the

channel region However, in order to obtain a decent quality GaN, it is necessary to grow more than one micron of GaN on the substrate (SiC or sapphire) [1] As a result, the good thermal conductivity of SiC is not fully exploited

In the arena of high temperature electronic applications, until recent times, electronics was kept far away from heat sources A temperature of up to 200°C was usually allowed

simply because silicon-on-insulator (SOI) can work up to this temperature [37] Semiconductors such as GaAs and InP, in general have low thermal conductivity, hence limiting operations at high power where much heat is dissipated due to large current flow, and this can drastically degrade performance III-nitride semiconductor transistors however, have found their way to excel in performance over these materials as they are several times more thermal conductive than their counterparts Domains of application include aerospace, automotive and terrestrial and high power transmitters in wireless communications In general, the substrate conductivity is not an issue and both SiC and sapphire are well adapted There have been many demonstrations of GaN transistors working in high temperature [15, 16] A peak electron drift velocity of 1 x 107 cm/s at 750K and 3 x 107 cm/s at room temperature was demonstrated [15] Presented also were the results of the DC and RF measurements showing that these devices can operate at least up to 300°C

1.2 Current Issues

Field-effect transistors fabricated using the AlGaN/GaN heterostructure offer the potential to produce a class of devices with excellent DC and RF performance Although the physics of the 2DEG that forms at the heterojunction is not completely understood at this time, it is clear that the sheet charge density is very high and of the order of 1013

cm-2, which is a factor of ten higher than that for the AlGaAs/GaAs 2DEG The sheet charge density is higher than would be expected from the standard 2DEG theory, which indicates that additional physical effects are probably involved Several explanations for this phenomenon using arguments such as spontaneous polarization and piezoelectric

polarization have been proposed [17] The 2DEG at the AlGaN/GaN heterojunction has excellent charge transport characteristics and the saturation velocity has a magnitude of about 3 x 107 cm/s [2] Room temperature mobility was found to be in the range of 1200

to 2000 cm2/Vs [18] The combination of high sheet charge density and good transport characteristics, has resulted in high current capability for the transistor This is in fact observed in AlGaN/GaN MODFETs where maximum channel current of over 1 A/mm is routinely obtained in experiments [19, 20] Table 1.1 summarises the key material parameters [21] for AlGaAs/GaAs, 4H SiC and AlGaN/GaN transistors It can be seen that with large energy bandgap, high breakdown voltage, high saturation velocity and good thermal conductivity, SiC and AlGaN/GaN devices have shown to be more promising candidates for high power and high frequency operations

Table 1.1: Summary of key material parameters for AlGaAs/GaAs, 4H SiC and AlGaN/GaN transistors [21]

Maximum sheet charge concentration (cm-2) 2-3 x 1012 N/A 1-5 x 1013

Saturation electron velocity (x107 cm/s) 1.0 2.0 3.0

and its relative materials such as AlGaN This breakdown field is about 3.3 MV/cm, much higher that that of GaAs of 0.4 MV/cm With such parameter, high drain bias operation is then possible due to high breakdown voltage Together with the high output current driving capabilities, GaN-based transistors with high power output will then become viable

In high frequency applications, GaN has a very high saturation/peak velocity, and this is critical for achieving high cut off frequencies, and therefore is competitive with GaAs Although the mobility of carriers in GaN is about 4 times lower than that of GaAs, it is sufficient for high frequency operations as the critical mobility for optimum RF power performance is approximately 500 cm2/Vs [23] Moreover, the sheet charge-mobility product is higher owing to the high sheet charge concentration in the AlGaN/GaN 2DEG This is vital for the development of low noise and high frequency transistors Another important property of GaN-based materials is the ability to utilise bandgap engineering in the design of device structures Modulation doped field effect transistor (MODFET) which utilises heterojunction design can result in higher sheet carrier concentrations, higher mobilities, better charge confinement, higher peak saturation drain currents, higher breakdown voltages, higher cutoff frequencies, etc This is in view of exploiting the advantages of spontaneous and piezoelectric polarization induced sheet charge and the growth of low defects AlGaN barrier layers with high aluminium mole fractions With this combination of high frequency operation, high breakdown voltage, and high drain current, GaN is an excellent candidate for high power microwave operations

Currently, numerous GaN-based MODFETs especially AlGAN/GaN, have been fabricated by researchers and the device performance was investigated These included small signal microwave performance, dc performance, power performance, etc

A comparison of the cutoff frequency versus gate length reported by several organisations was compiled by C Binari [24] It was found that the cutoff frequency has

an approximate negative proportion relationship with the gate length In the report of C Binari, the gate length ranges from 0.15 µm to 1 µm and the ft ranges from 15 GHz (for 1

µm) to 67 GHz (for sub-micron gate lengths) The highest reported values for ft and f max

are 67 and 140 GHz, respectively [13] Refinements and optimization in material structure and device design can actually be done to produce an overall improvement in the AlGaN/GaN MODFET small signal performance

Recent intensive research on AlGaN/GaN MODFETs grown on sapphire substrate has resulted in a steady increase in power density from 1.1 W/mm at 2 GHz [25] to 1.5-1.57 W/mm at 4 GHz [26, 27] and 1.7 W/mm at 10 GHz [28] The best power density ever reported is for HEMTs grown on a semi-insulating 4H-SiC substrate with 10 W/mm at 10 GHz and 4.1 W/mm at 16 GHz [29] Total power results have also been pushed up to 7.6 W/mm achieved at 4 GHz for HEMTs grown on sapphire and flip-chip mounted on AlN heat sinks [30] Such improvement has partially resulted from an increased understanding and application of the piezoelectric effect at the AlGaN/GaN interface that induces large sheet carrier concentrations, and also the of use of bandgap engineering to design and grow HEMT structures with larger Al mole fractions to allow a large sheet charge concentration to coexist with a higher carrier mobility [2] Gates with high Schottky barrier height have also been fabricated, yielding a high gate-drain reverse breakdown voltage of more than –80V [32] This is a result of using high Al mole fraction to effectively suppress thermionic gate leakage current at elevated temperature operations

GaN-based FET structures offer the potential of not only high power and high-speed operation, but also high temperature This is found to be beyond those of Si and GaAs, which have operated up to 400 °C [33] and 500 °C [34], respectively However, to date,

the operation of GaN-based FETs has only been reported up to 750 °C for doped-channel

(DC) AlGaN/GaN HFET [35], but none has been made for undoped channel HFETs up

to such high temperatures It has been proposed that AlGaN/GaN doped channel, HFETs performs better than undoped channel AlGaN/GaN HFETs at elevated temperatures because of a decrease in ionised impurity scattering [15] This is due to a larger carrier concentration at the 2DEG, which acts to screen off the ionized impurity scattering from the AlGaN barrier layer It was found by Binari et al [36] that for the

DC-doped-channel AlGaN/GaN HFET, an increase in temperature led to an increase in drain current However, this phenomenon might not be true for undoped channel HFETs

1.3 Motivation and Objectives of Current Project

It is clear at this juncture that the GaN-based devices such as the AlGaN/GaN HEMT has many properties that make them attractive for high power microwave applications, and their performance is greatly dependent on the fabrication process, device structure and material parameters In order to realize high performance AlGaN/GaN HEMTs for high power and high frequency applications, it is crucial to first develop and optimize a good fabrication process that is reproducible and cost effective Development and optimization

of fabrication processes such as the formation of ohmic contacts with very low specific contact resistance, and the laying of reliable gate metals with high Schottky barrier heights and low leakage currents are important This is because good ohmic and Schottky contacts allow devices to deliver high output current at low knee voltage with low gate leakage current, which are needed for high power applications High frequency measurements differ from those at low frequency, and it can only be realized by fabricating devices with dimensions specific to GHz probing Hence, designing a set of high frequency photomask is necessary

To our best of knowledge, there have not been many reports on the simulation of AlGaN/GaN HEMTs It is important to assess and quantify the performance that is realistically achievable in AlGaN/GaN HEMTs by studying optimal device geometry and material parameters Through simulations, we can investigate the feasibility of possible new device structure designs to improve device performance, before implementing these appropriate designs and material parameters to actual wafer growth and fabrication of HEMT devices In this way, we can save time and money by achieving as close to the required dc or rf performance without going through the process of trial and error on actual fabricated wafers Significant improvements in the quality and performance of the AlGaN/GaN system can then be realized in this way

It is therefore our objective to first, study the fabrication process of ohmic and Schottky contact formation on the AlGaN/GaN HEMT structure We aim to achieve a low specific contact resistance of the order of 10-7 Ωcm2 or lower, an improvement from 10-6 Ωcm2

achieved by other research groups [85-87] We shall be investigating the possibility of using surface treatment on the wafer and also etching procedures to achieve our objective We are also studying methods to obtain Schottky diodes on AlGaN/GaN devices with minimal reverse leakage current, high barrier heights and good thermal stability Till this date, little has been done in this area of research and information on thermal stability of Schottky contact on AlGaN/GaN HEMTs has not been published It is then our aim to fabricate AlGaN/GaN HEMT devices for dc measurements and characterization before going into the designing of a set of photomask for high frequency and high power measurements With the importance of simulation as mentioned earlier,

we shall extend our study into the potential of AlGaN/GaN HEMTs by running simulations of a possible new AlGaN/GaN HEMT device structure and compare it with the performance of HEMTs with conventional device structure reported in literature

1.4 Outline of Thesis

Chapter 1 has presented an introduction to the current status of the research and development of the AlGaN/GaN HEMT It has also spelt out the importance of the current project and the objective we hope to achieve at the end In Chapter 2, the fundamentals of the GaN related materials and the theory of the AlGaN/GaN HEMT are described The experimental procedures for the fabrication of ohmic contact, Schottky contact and AlGaN/GaN HEMTs are presented along with their characterizations in Chapter 3 In addition, the performance is discussed The design of photomasks suitable for the making of HEMTs for high frequency and/or high power applications is considered in Chapter 4 Chapter 5 shows the simulation results of a new AlGaN/GaN HEMT structure that may surpass the performance of existing conventional device structures reported in literature Last but not least, Chapter 6 presents the conclusions from the current work and some of the possible avenues for furthering the current research work

Chapter 2

Theoretical study of GaN related

semiconductors and devices

2.1 Introduction

In this chapter, the material study of some nitride-based semiconductors is presented It includes the crystal structure, the chemical, electrical and mechanical properties of materials such as GaN, AlN and AlGaN It also introduces the basic device structure of the AlGaN/GaN HEMT and its properties and characteristics such as the formation of the 2DEG, the piezoelectric effect and the carrier transport mechanism Finally, the measurement techniques for specific contact resistance and the Schottky barrier height are presented

2.2 Crystal Structures of Nitrides

Wurtzite (Wz), zincblende (ZB) and rocksalt structures are the three common crystal structures shared by group-III nitrides Under ambient conditions, the thermodynamically stable structure is wurtzite for bulk AlN, GaN and InN The zincblende structure for GaN and InN has been stabilized by epitaxial growth of thin films on {011} crystal planes of cubic substrates such as Si, MgO, and GaAs In these cases, the intrinsic tendency to form the wurtzite structure is overcome by topological compatibility The rocksalt, or

NaCl, structure can be induced in AlN, GaN and InN under very high pressures The wurtzite structure has a hexagonal unit cell and thus two lattice constants, c and a It contains six atoms of each type and consists of two interpenetrating Hexagonal Close Packed sublattices, each with one type of atom, offset along the c axis by 5/8 of the cell height

The zincblende structure has a unit cell containing four group III elements and four nitrogen elements The position of the atoms within the unit cell is identical to the diamond crystal structure Both structures consist of two interpenetrating face-centred cubic sublattices, offset by ¼ of the distance along a body diagonal Each atom in the structure may be viewed as positioned at the center of a tetrahedron, with its four nearest neighbours defining the four corners of the tetrahedron

There are some similarities between the wurtzite and zincblende structure In both cases, each group-III atom is coordinated by four nitrogen atoms, and conversely, each nitrogen atom coordinated by four group-III atoms The main difference between these two structures lies in the stacking sequence of the closest packed diatomic planes For the wurtzite structure, the stacking sequence of the (0001) plane is ABABAB in the <0001> direction, while the stacking sequence of the (111) plane in a zincblende structure is ABCABC in the <111> direction

A stick and ball representation of wurtzite structure is depicted in Figure 2.1 The wurtzite and zincblende structures differ only in the bond angle of the second-nearest neighbour, (see Figure 2.2)

Figure 2.1: A stick and ball diagram of a hexagonal structure

Figure 2.2: Stick and ball stacking model of crystals with wurtzite (a) an zincblende (b) orientations

As shown clearly, the stacking order of the wurtzite along the [0001] c direction is ABAB, meaning a mirror image but no in-plane rotation with the bond angles In the zincblende structure along the [111] direction there is a 60° rotation which causes a

stacking order of ABCABC, Figure 2.2b The wurtzite polytypes of GaN, AlN and InN form a continuous alloy system whose direct bandgaps range from 1.9 eV for InN, to 3.4

eV for GaN, to 6.2 eV for AlN Thus, the III-nitrides could potentially be fabricated into

optical devices, which are active at wavelengths ranging from the red well into the ultraviolet

2.3 Gallium Nitride

2.3.1 Chemical Properties of GaN

Since the first synthesized GaN in 1932, a large body of information has repeatedly indicated that GaN is an exceedingly stable compound with a large bandgap and exhibits significant hardness It is this chemical stability at elevated temperatures combined with its hardness that has made GaN an attractive material for high temperature and high power electronics While the thermal stability of GaN allows freedom of high-temperature processing, the chemical stability of GaN presents a technological challenge Conventional wet etching techniques used in the semiconductor processing have not been

very successful for GaN device fabrication For example, Maruska and Tietjen [38]

reported that GaN is insoluble in H2O, acids, or bases at room temperature, but does

dissolve in hot alkali solutions at very slow rate Pankove [39] noted that GaN reacts with

NaOH forming a GaOH layer on the surface, prohibiting wet etching of GaN To circumvent this difficulty, he developed n electrolytic etching technique for GaN Low-quality GaN has been etched at reasonably high rates in NaOH [40, 41], H2SO4 [42],

H3PO4 [43-45] Although these etches are useful for identifying defects and estimating their densities in GaN films, they are not very successful for the fabrication of devices Well-established chemical etching processes are required for the device-technology development Promising possibilities are the various dry-etching processes under

development, and reviewed by Mohammad et al [46]

2.3.2 Thermal and Mechanical Properties of GaN

In the hexagonal wurtzite structure, GaN has a molecular weight of 83.728 g/mol At 300K, the lattice parameters of this semiconductor are a0 = 3.1892 ± 0.0009 Å and c0 = 5.1859 ± 0.0005 Å However, for the zincblende polytype, the calculated lattice constant

based on the measured Ga-N bond distance in the wurtzite GaN is a = 4.503 Å The measured lattice constant of this polytype varies between 4.49 and 4.55 Å, indicating that the calculated value lies within acceptable limits [47] A high-pressure phase transition from the wurtzite to the rocksalt structure has been predicted and observed experimentally The transition point is 50 Gpa and the experimental lattice constant in the rocksalt phase is a0 = 4.22 Å Table 2.1 compiles the known properties of wurtzite GaN

Table 2.1 List of the known properties of Wurtzite and zincblende GaN

Wurtzite Polytype

Bandgap energy Eg (300K) = 3.42 eV Eg (4K) = 3.505 eV

Temperature coefficient dEg/dT = -6.0 x 10-4 eV/K

Pressure coefficient dEg/dP = 4.2 x 10-3 eV/kbar

Electron effective mass, me 0.22m0

Hole effective mass, mp >0.8m0

Zincblende polytype

Bandgap energy Eg(300K) = 3.2—3.3 eV

Lattice constant a = 4.52 Å

Index of refraction n(3eV) = 2.9

It is interesting to note that the lattice constants of GaN grown with higher growth rates was found to be larger When doped heavily with Zn [48], and Mg [49] a lattice

expansion occurs because at high concentrations, the group-II element begins to occupy the lattice sites of the much smaller nitrogen atoms

Measurements made over the temperature range of 300-900 K indicates the mean coefficient of thermal expansion of GaN in the c plane to be ∆a/a = 5.59 x 10-6 K-1 Similarly, measurements over the temperature ranges of 300-700 K and 700-900 K, indicates the mean coefficient of thermal expansion in the c direction to be ∆c/c = 3.17 x

10-6 K-1 and 7.75 x 10-6 K-1, respectively [38] Sheleg and Savastenko [50] reported a

thermal expansion coefficient near 600 K, perpendicular and parallel to the c-axis, of (4.52 ± 0.5) x 10-6 K-1 and (5.25 ± 0.05) x 10-6 K-1, respectively

Sichel and Pankove [51] measured the thermal conductivity of GaN for the temperature

range of 25-360 K The room temperature value of the thermal conductivity κ = 1.3

W/cmK is a little smaller than the predicted value of 1.7 W/cmK [52] Other thermal properties of Wz-GaN have been studied by a number of researchers The specific heat of Wz-GaN at constant pressure (Cp) is given by [53]

Cp (T) = 9.1 + (2.15 x 10-3 T) [cal/mol K] (2.1)

2.4 Aluminum Nitride

AlN exhibits many useful mechanical and electronic properties For example, hardness, high thermal conductivity, resistance to high temperature and caustic chemicals, make AlN an attractive material for electronic packaging applications The wide bandgap is also the reason for AlN to be touted as an insulating material in semiconductor device applications Piezoelectric properties make AlN suitable for surface-acoustic-wave device applications [54] However, the majority of this semiconductor stems from its ability to form alloys with GaN producing AlGaN and allowing the fabrication of AlGaN/GaN based electronic and optical devices, the latter of which could be active from the green wavelength into the ultraviolet

2.4.1 Thermal and Chemical Properties of AlN

When crystallized in the hexagonal wurzite structure, the AlN crystal has a molar mass of 20.495 g It is an extremely hard ceramic material with a melting point higher than

2000°C The thermal conductivity κ of AlN at room temperature has been predicted at ≈

3.2 W/cmK [55, 56], and values of κ measured at 300 K are 2.5 [57] and 2.85 W/cmK

[58] The measured thermal conductivity as a function of temperature is plotted in Figure 2.3

Figure 2.3: Thermal conductivity of single crystal AlN (Ref: 57)

The thermal expansion of AlN is isotropic with a room-temperature value of 2.56 x 10-6

K-1 The thermal expansion coefficients of AlN measured by Yim and Paff [59] have a

mean value of ∆a/a = 4.2 x 10-6 K-1 and ∆c/c = 5.3 x 10-6 K-1 The equilibrium N2-vapour pressure above AlN is relatively low compared to that above GaN, which makes it easier

to be synthesized Similar to GaN but even more so, AlN exhibits an inertness to many

chemical etches The surface chemistry of AlN was investigated by Slack and McNelly

[60] and it indicated that the AlN surface grows and oxide 50-100 Å thick when exposed

to ambient air for about a day However, this oxide layer was protective and resisted further decomposition of the AlN samples

Table 2.2 List of the known properties of Wurtzite and zincblende AlN

2.4.2 Electrical Properties of AlN

Electrical characterization on AlN has been limited to just resistivity measurements and not other measurements such as mobility because of some of its inherent properties These include low intrinsic carrier concentration, and deep level defects and impurity

energy levels Kawabi et al [61] conducted such a test and found the resistivity, ρ, of

transparent AlN single crystals to be 1011 – 1013 Ωcm However, impure AlN crystals

which, showed a bluish colour due to the presence of Al2OC, have much lower resistivities of 103 – 105Ωcm

The insulating nature of these AlN films has hindered meaningful studies on the electrical transport properties However, with the availability of refined growth techniques, AlN is presently grown with much improved crystal quality and shows both n- and p-type

conductions Edwards et al.[61] and Kawabe et al.[62] carried out some Hall

measurements on p-type AlN and found a rough estimate of the hole mobility to be, µp =

14 cm2 /Vs at 290 K

2.5 Aluminum Gallium Nitride (AlGaN) alloy

Good k nowledge of the compositional dependence of the barrier and well materials is a requirement in attempts to analyze heterosturctures in quantum wells and superlattice In the nitride system, a wide scope of possible options is available for the construction of such structures The barriers formed can be materials such as AlGaN or GaN; while depending on the barrier material, the wells can be constructed of GaN or InGaN layers The energy bandgap of AlxGa1-xN may be expressed by

Where Eg(GaN) = 3.4 eV, Eg(AlN) = 6.2 eV, x is the Al mole fraction, and b is the

bowing factor which until now has controversial values Yoshida et al [63] concluded in

their studies that as the Al mole fraction increases, the energy bandgap of AlxGa1-xN deviates upwards from a graph of Eg vsx when b = 0 This implied a negative value for

the bowing factor, b In contrast, Koide et al [64] observed that the bowing factor is positive as they concluded a downward deviation that is opposite to that of Yoshida

The resistivity of unintentionally doped AlGaN increases rather rapidly with increasing

Al mole fraction, so much so that AlGaN becomes almost insulating for Al fraction exceeding 20% As the Al mole fraction increases from 0 to 30%, the n-type carrier concentration drops from 1020 to 1017 cm-3, and the mobility increases from 10 to 30

cm2/Vs An increase in the native defect ionization energies with increasing Al mole fraction may be the explanation for this variation It is still not known how the dopant atoms such as Si and Mg respond to the variation of the AlN mole fraction in AlGaN AlGaN with Al mole fraction as high as 50-60% is dopable by both n-type and p-type impurity atoms Until now, a low Al mole fraction of about 15% is sufficient for good optical field confinement

2.6 Substrates for Nitride Epitaxy

Of the many challenges faced in the research of GaN, one of the major difficulties is the lack of a suitable material that is lattice matched and thermally compatible with GaN GaN, AlN and InN have been grown primarily on sapphire, most commonly the (0001) orientation In addition, III-nitrides have also been grown on Si, SiC, InP, ZnO, TiO2, and LiGaO2

2.7 The AlGaN/GaN High Electron Mobility Transistor

2.7.1 The structure of the conventional n + - AlGaN/GaN HEMT

The cross section of a conventional HEMT is shown in Figure 2.4 The source and drain contacts and the gate metallization are analogous to those in either Si-MOS or the compound semiconductors, such as GaAs MESFET devices The epitaxial layer structure

of the AlGaN/GaN HEMT grown and fabricated is illustrated in Figure 2.5 The device is grown on a AlN buffer layer to reduce the lattice mismatch of 49% between the GaN channel layer and the sapphire substrate The layers grown, from bottom to top are, a sapphire substrate, an AlN buffer layer, an undoped GaN “channel layer”, an undoped AlGaN “spacer layer”, a n-doped AlGaN “donor layer” and finally an undoped AlGaN

“cap layer” The role of each layer will become apparent in this section The thickness of individual layers and their doping have a direct influence on the device properties and the performance of the HEMT The gate lengths and the source-drain distance may vary according to speed, application, and yield requirements

Figure 2.4: Schematic of a conventional AlGaN/GaN HEMT

Figure 2.5: Epitaxial layer structure and conduction band diagram for a HEMT under positive gate bias

n + -AlGaN Donor Layer AlGaN Spacer

Layer

GaN Channel Layer AlSubstrate 2O3

Gate

Metal

AlN

2.7.2 Heterostructures in Semiconductors

In conventional semiconductor devices, only one type of semiconductor material is used

in the fabrication of the devices Control of current flow is achieved by creating a junction within the device structure Such device is called a homostructure, and one such example is Si-based metal-oxide-semiconductor (MOS), or the bipolar-junction transistor (BJT) If more than one semiconductor is used, causing a change in the energy bands within the structure, this type of devices is termed a heterostructure The ability to customize the energy-band structure adds flexibility to the design of new devices based

on doping and material variations in the various layers These changes in the energy band provide an additional means, independent of doping and applied external fields, to control the flow and distribution of the charge carriers throughout these devices

When two semiconductor materials with different bandgaps are joined together to form a heterojunction, discontinuities in both the conduction and valence band edges occur at the heterointerface For the HEMT, the wide-bandgap material, for example AlGaN, is n-doped with Si donors The added charges bend the band edges and create a triangular potential well in the conduction-band edge of the lower bandgap material, for example, GaN Electrons accumulate in this well and form a sheet charge analogous to the inversion channel in an SiO2/Si MOS structure The thickness of this channel is typically only 100 Å, which is much smaller than the de Broglie wavelength of the electrons in GaN which is given by λ = h/(2mn*kT)1/2 Hence the electrons are quantized in a two-dimensional system at the interface, and so the channel of the HEMT is called a two-dimensional electron gas (2DEG) An advantage to such a device structure is the physical separation between the donors and the electrons in the channel layer, thus reducing the impurity Coulombic scattering and, therefore, enhances the mobility as well as the effective velocity of the electrons under the influence of an electric field

2.7.3 Equations for the Two-dimensional Electrons as the current carriers

Within the framework of the effective mass approximation, the electronic subband energy levels, Ei, and the corresponding envelope wavefunction are the solutions of the

Schrödinger equation given by Stern et al [65]:

where T is the kinetic energy operator, and V(z) is the electrostatic potential which in turn is found from the solution of the Poisson equation:

ε

ρ( ))

(

d z

B d z

z dV dz

Since the electrostatic potential, V(z), given by equation (2.4), is only a function of the z coordinate, the envelope function ψ(x,y,z) can be written as (Stern et al):

where kx and ky are the x and y components of the wavevectors measured relative to the band edge, θ is the superlattice wavevector, and ξi(z) is the solution of Schrödinger equation which describes the one-dimensional bound motion:

) ( 2

2 2

)()(

)exp(

)exp(

2

2 2 2

2

2

y ik x ik E

y ik x ik y

m x

y x

+

=+

where mx and my are the principal effective masses for the motion parallel to the

interface, obtained from the bulk masses (Stern et al) Each eigenvalue Ei of (2.9) is the bottom of a continuum of energy levels called a ‘subband’ The subbands can be grouped into ladders with respect to the bulk conduction band minimum from which they originate Each subband energy level is found from the solution of (2.9) and is given by:

y x

x i

k m

k E

k

E

22

)

(

2 2 2

In order to progress further from the solution of equations (2.3) and (2.4) to determine the band bending, one has to specify the electrostatic potential Once V(z) is specified, one must solve the Schrödinger equation (2.3) and the Poisson equation (2.4) self-consistently

(Stern et al) One can, however, find a satisfactory physical picture for some limiting

cases The simplest cases are illustrated in Figure 2.6a, representing an infinite square

well (e.g (AlGaN/GaN/AlGaN heterostructure), and 2.6b, representing a triangular (asymmetric) well (e.g., (AlGaN/GaN) heterostructure)

(i) Infinite square well

Since wavefunction must vanish at z = ± Lz the Bohr-Sommerfeld quantization

conditions yields (Landau et al.) [66]:

i z

L

i dz

z

)1( +

2 2

)1(

L m

E

z z

i η π

(ii) Asymmetric triangular well

where Fs is the effective electric field at the interface

Then the Bohr-Sommerfeld quantization condition gives (Landau et al):

(

z s

i

m

i dz

z eF

and the solution of this equation gives the energy levels quantized in the z-direction as:

3 / 2 3

/ 2 3

/ 1 2

)4/3(2

πη

E1 E2 E3

Figure 2.7: Quantization in the z direction with dispersion in x and y directions leading to a stepped cumulative density of states with increasing energy

2.7.4 Piezoelectric effects on the 2DEG in AlGaN/GaN heterostructure

The group III nitrides possess a large spontaneous and piezoelectric polarization The presence of this strong polarization is supported by both theoretical calculations of its existence and the large electron concentrations which result at the 2DEG of the AlGaN/GaN heterojunctions in the transistor structures Simple models [67-69] have been used to calculate the electron concentration at a single heterointerface and support the hypothesis that the 2DEG found at the interface is induced by polarization effects, apart from the diffusion of electrons from the donor layer as mentioned in the previous section

The polarization present in the group III-nitrides such as GaN-based alloys, is due to the lack of inversion symmetry along the c-axis of the wurzite crystal structure In relaxed material there exist a built-in or spontaneous polarization [67], and this polarization is dependent on the Al mole fraction, x, with the following relation

Density of States

Energy

2D

This polarization points toward the substrate for Ga-face material and points toward the surface in N-face material The polarization in the material can be changed by placing it under strain This change is polarization is commonly called the piezoelectric polarization and is given by:

0

2

C

C e e a

a a

where a is the lattice constant under strain, and a 0 is the lattice constant of the relaxed

material The constants e 31 and e 33 are piezoelectric constants and C 13 and C 33 are elastic deformation constants The total polarization in a given layer is simply the sum of the

spontaneous and piezoelectric polarization, P = P SP + P PE The constants used are shown

in Table 2.3 [67], [70] At a heterojunction there is usually a change in the polarization on each side This abrupt change in polarization causes a bound sheet charge In general, the bound sheet charge is the polarization of the bottom layer minus the polarization of the top layer, σ = P(bottom) – P(top)

Table 2.3: The constants used to calculate the polarization in III-nitride layers P SP is the spontaneous

polarization e 33 and e 31 are piezoelectric constants C 13 and C 33 are elastic deformation constants and a 0 is the lattice constant

the N-face material, the positive bound charge is present at the upper interface and the 2DEG will be formed there

Figure 2.8: The direction of polarization and the location of the 2DEG in Ga-face and N-face AlGaN

HEMTs In both cases, the AlGaN layer is under tensile strain leading to both a spontaneous and piezoelectric component to the polarization For Ga-face material the direction of polarization causes the formation of a 2DEG at the lower interface In the N-face material the direction of polarization is reversed causing the 2DEG to form at the upper interface

The polarization induced sheet charge density, σ, is in fact dependent on the Al mole

fraction, x, an it can be calculated by the following:

|σ(x)| = |P PE (Al x Ga 1-x N) + P SP (Al x Ga 1-x N) – P SP (GaN)|

)0()()

(

)()()()

(

)()0(2)

(

33

13 33

x C

x C x e x e x

a

x a a

2DEG

2DEG Tensile

strain

With this high positive polarization induced sheet charge at the AlGaN/GaN interface for Ga-face material, the maximum possible sheet carrier concentration found at the 2DEG at the interface of the unintentionally doped structure is [71]:

)()

()

de

x e

x x

x m x E x

where the ground subband level of the 2DEG is given by:

3 / 2

* 0

2 0

)(

)()(88

9)

x

x n x m

e x

ε ε

πη

with effective electron mass, m * (x) ≈ 0.22me, band offset [74, 75]:

where the band gap of AlGaN is measured to be [76]:

Eg(x) = xEg(AlN) + (1-x)Eg(GaN) – x(1-x)1.0 eV (2.30)

= x6.13 + (1-x)3.42 – x(1-x)1.0 eV

2.8 Carrier Transport

Current conduction and thus the resistance of the semiconductor material and device, is determined by the ease with which the carriers can traverse through the structure As the carriers travel through a semiconductor, they undergo a variety of interactions with the host material In a perfect static crystal, carriers would accelerate indefinitely by the applied electric field However, semiconductor crystal contains defects, intentionally added impurities, and even at very low temperatures, the semiconductor is in constant motion and far from being static Therefore as carriers travel through the semiconductor, they experience various events referred to as scattering, the most effective of which are

by charged impurities and/or centers, and by lattice vibrations An additional scattering mechanism is that due to charged dislocations, which can be partially screened at high doping levels

There are many types of lattice vibration such as acoustic and optical In covalent polar semiconductors as GaAs and GaN, the Longitudinal Optical (LO) polar optical phonon scattering is the dominant scattering mechanism associated with lattice vibrations The ionic nature of the bonds in these semiconductors is such that, as the neighboring atoms move away from one another, an electric polarization results, which causes an electric

field to form This field interacts with a moving charge particle, which is termed the polar optical phonon scattering, and dominates the mobility at high temperatures Moreover the lack of center symmetry in compound semiconductors, particularly wide-bandgap nitrides, causes them to be piezoelectric in which phonons scatter electrons

The conductivity of a semiconductor is determined not only by the number of available free carriers but also the freedom with which those carriers can move about within the crystal This freedom of movement is known as carrier mobility It is function of lattice temperature, the electric field, the doping concentration, and the material quality of the semiconductor, which determines the amount of defects and dislocations

The effective electron concentration and ionized impurity concentration primarily affect the low temperature mobility values At higher temperatures, e.g room temperature, the mobility values are primarily determined by polar optical phonon scattering At low sheet carrier densities, these scattering processes are screened, which explains the increase in mobility to more than 2000 cm2/Vs for ns>1013 cm-2 At very high sheet carrier densities, the average distance of the 2DEG to the AlGaN/GaN interface becomes smaller due to occupation of higher energy subbands Depending on the surface quality, this can decrease the mobility significantly due to increase in interface roughness scattering [77]

2.9 Current Flow in Metal-Semiconductor Junctions

In cases when defects are not involved, there are three mechanisms [78, 79] that govern the current flow in a metal-semiconductor system

can surmount the top of the barrier, which should be small for contacts, by thermionic emission For low-doped or high-barrier semiconductors, on the other hand, the vast majority of electrons would be unable to cross in either direction into the semiconductor; and ohmic behavior is not observed

2.9.2 Thermionic-Field Emission (TFE)

For intermediately doped semiconductors, ≈ 1017 < ND < 1018 [cm-3], the depletion region

is not sufficiently thin to allow direct tunneling of carriers that are more or less in equilibrium But if the carriers gain a little energy, they may be able to tunnel Consequently, both thermionic emission and tunneling take place

2.9.3 Field Emission (FE)

For heavily doped semiconductors, ND > ≈ 1018 cm-3, the depletion region is narrow, and direct electron tunneling from the metal to the semiconductor is possible In the absence

of a good match between the metal and the semiconductor work functions, which is generally the case, this is the best approach to pursue for ohmic contacts Figure 2.9 illustrates the 3 ways of current flow at the metal-semiconductor interface

(b)

(c)

Figure 2.9: Schematic description of (a) the thermionic emission, (b) thermionic field emission, and (c)

direct tunneling field emission

2.10 Contact Resistance Measurement

The contact resistance measurement is used to measure the degree of ohmic behavior of the metal contact on the semiconductor, or simply how well current is flowing across the metal-semiconductor interface

The contact resistance of HEMTs are measured using the transmission line method

(TLM) developed independently by Berger [80] and Murrman and Widmann [81] and described in detail by Berger [82] A schematic diagram of the TLM is shown in Figure 2.10 where d is the contact length In using this model, it was assumed that (a) the

interfacial resistance, given by the shunt conductance, is independent of the sheet resistance Rs of the semiconductor layer beneath the contact, and (b) only the horizontal current flows in the semiconductor layer beneath the contact For the case in which the

width of the metal (w) is nearly equal to the width of the mesa (W) or for W the contact

resistance of the TLM is:

q Φ b

TFE

q Φ b

FE

Rc = Vi/Ii = Z coth (αd), (2.32)

where Z is the characteristics impedance, d is the length of the contact area and α is the

attenuation constant α = √(Rs/ρc), and Rs and ρc are the sheet resistance and specific contact resistivity, respectively

w

d Figure 2.10: Schematic diagram of the TLM pattern

The accuracy of the specific contact resistivity ρc measured by TLM depends on the accuracy of the electrical and dimensional measurements of the TLM device and on the validity of assumptions made in the analysis For example, the resolutions of the dimensional measurements of the width and length of the contacts and their separation are limited to 1µm if one uses the optical microscope

TLM patterns are fabricated with rectangular metal pads of 100 µm wide and 50 µm

long, and separated by a gap varying from 5 µm to 30 µm Once the mesas are defined,

the resistance between the two contacts can be measured using a four-point probe arrangement which results in current flow between two probes, while the measuring voltage with the other two to eliminate any error due to the contact resistance between the probe and the metal contact Using the method of least squares to fit the experimental data with a straight line, one can determine the contact resistance Rc from the plot of the total resistance between two metal pads Rt versus gap length (the distance between the contacts)

W

2.11 Schottky Barrier Height Measurement

The following thermionic emission equation can be used to analyze the I-V characteristics of a Schottky contact -

qVd I

d ln 0

Hence by plotting ln I versus Vd, I0 and n can be obtained and used to solve equation (2.34) to find the required barrier height, φb

by the reporting of the successful fabrication of big dimension AlGaN/GaN HEMTs and their dc characterisations Another issue involved in this study includes the effect of long term heat treatment on the dc characteristics of the AlGaN/GaN HEMT and the Schottky contact

3.2 Ohmic Contact

Many research groups have reported ohmic contact metallization on AlGaN/GaN HEMTs

by annealing as-deposited metal layers on the surface of the n-AlGaN [1-3] The formation of low resistance ohmic contact on structures with high Schottky barrier height surface layer such as n-GaN (and/or AlGaN) requires a heavily doped region near the metal contact to facilitate carrier tunneling However, the formation of these low resistance ohmic contacts appears to be a complex process as it involves solid-state interfacial reactions between metal and the adjoining semiconductor layer(s) These reactions are dependent on the thickness and composition of the n-doped-nitride epi-layer and the annealing conditions, and annealing is essential to these solid-state reactions

Owing to the complexity of these reactions, annealing at high temperatures of 800°C to

900°C [1-3] could possibly be insufficient to allow the metal layers to completely react

with the AlGaN barrier layer to reach the two-dimensional electron gas (2DEG) [4], which forms the channel of the HEMT device In such cases, the unreacted AlGaN layer would then serve as a barrier for tunneling effect and hence increase the contact resistance We report the electrical properties of Ti/Al/Pd/Au metal contacts on induction coupled plasma (ICP) etched AlGaN/GaN HEMT This Ti/Al based metal system is used

as it has been reported to form good metal-semiconductor interface on AlGaN/GaN surfaces for good carrier transport across the interface [5] Pd is used as the third layer as

it is capable of preventing Au from in-diffusing towards the Ti/Al-GaN interface which may lower the contact resistance at the metal-semiconductor interface [6] The effects of annealing temperature and the gas composition of ICP etching on the ohmic behaviour are also reported

3.2.1 Experimental Procedure

The AlGaN/GaN HEMT epi-layer structure used in this project was grown by SVT Associates, Inc and based on their design [7] It is shown in Figure 3.1 and grown on (0001) sapphire substrate using the molecular beam epitaxy (MBE) technique Itconsists

of, starting from the bottom, a 400nm AlN nucleation layer, a 1.5µm unintentionally

doped (UID) GaN channel layer followed by a 5nm unintentionally doped AlGaN spacer layer, a 5nm AlGaN donor layer doped with Si to a concentration of 1x1019 cm-3, an UID 15nm AlGaN layer and finally a 2nm UID GaN cap layer to protect the AlGaN cap layer from oxidation The aluminium mole of AlGaN is 0.15 We named this HEMT structure design “α” to differentiate from another AlGaN/GaN HEMT structure design to be

introduce later which is called the “conventional AlGaN/GaN HEMT structure”

The HEMT epi-wafer was first cleaned using organic solventsby placing the wafer in boiling Acetone for 10 minutes, followed by ultrasonic treatment in Methanol for another

10 minutes, and finally rinsed by DI water This was followed by mesa structure formation using Inductively Coupled Plasma (ICP) etching with Cl2/Ar gas chemistry

The contact pads of the Linear Transmission line method (LTLM) patterns were then laid

on electrically isolated mesas using typical photolithography procedure The wafer was scribed and cleaved into several samples using a diamond cutter and they were grouped into 3 main groups for ICP etching to 3 different target depths (details are described in Section 3.2.3)before ohmic metallization

Figure 3.1: The AlGaN/GaN HEMT device structure α used in our project

3.2.2 ICP Etching

The ICP etcher used in our study is a PlasmaTherm 790 reactor with a load-locked stainless-steel chamber system It has a 1kW ICP power source operating at 2MHz to control the ion flux, and a 500W RIE power source operating at 13.6MHz to control the ion energy This is connected to a liquid nitrogen-cooled wafer chuck where the substrate

is located The cooling liquid nitrogen is used to bring the temperature of the sample to a very low temperature region (down to sub-zero degrees) to prevent the baking effect and

5nm UID AlGaN spacer layer 5nm doped AlGaN 1x10 19 cm -3 donor layer 15nm UID AlGaN barrier layer 2nm UID GaN cap layer

2DEG

400nm AlN nucleation layer Sapphire Substrate