2.2 Schottky Junction
2.2.4 C-V Characteristic
The Schottky barrier height can be determined by analyze the capacitance-voltage measurement. [12, 16-18] The capacitance is measured with biasing DC and small AC voltage signal. AC signal induces the variation of charge and DC signal changes the state of semiconductor. In order to characterize the Schottky contact, reverse DC bias should be applied to metal in order that make depletion condition on semiconductor. Then, the depletion region WD can be expressed as Equation (9),
2 S
D bi
D
W V V
qN
(9)Accordingly,capacitance can be expressed as following equation.
1 2 2
2
D A S S A D
A
R bi A D
A q N N
C C A dQ A A
dV W V V N N
(10)
When NA ND, Equation (10) can be simplified to following equation,
1 2
2
S D bi
C A q N
V V
(11) In terms of voltage, Equation (11)
2
( 2) 2
S D
bi
q N A
V V
C
(12)The slope of
2 2
A
C curve is 2
S D
q N
, and the x-intercept is -Vbi, as shown in Figure 12. Finally, the Schottky barrier
B can be obtained by following equation.B qVbi qVn
(13)17 Figure 12 Typical C-V (
2 2
A
C vs V) characteristic of Schottky contact.
2.2.5 IPE Characteristic
The internal photoelectron emission (IPE) measurement method measures photocurrent by directly transferring electrons excited by an incident photon across two different interfaces. [19, 20] Interfaces such as electron energy barrier and charge transport characteristics are not affected by the electrical operation characteristics of the entire device. It is a very useful metric that allows you to find out local physical properties. According to Powell’s Model, the quantum yield is calculated as follows, and the electron energy barrier can be obtained from the threshold of the quantum yield.
IPE is a powerful technique for determining interfacial electron energy barriers using optical excitation of carriers in the metal film of a Schottky (metal/semiconductor) junction. The basic concept is to find the minimum photon energy required to excite an electron to surmount the energy barrier at interface.
We measure the photocurrent through the junction, which changes as the incident photon energy changes. By probing the threshold of photocurrent, we can determine the electron energy barrier at interface in precise quantitative manners.
In Schottky junction, ‘excitation over barrier’ and ‘band-to-band excitation’ are two major excitations to contribute to photocurrent as shown in Figure 13. To specify the Schottky barrier, the photocurrent which is induced by the excitation over barrier should be used, where the photon energy hv is in range of q
B hv E g.18
The Schottky barrier can be extracted by using following Fowler’s Formula. [21]
( B)2
Y hv q
(14)Where
Y
is photo yield, which is the photocurrent per absorbed photon. Therefore, we can simply specify the Schottky barrier by plotting Y1/2 vs hv Graph and find the x-intercept. Figure 13 shows an example of the Y1/2 vs hv Graph for Ni/SiC Schottky diode.Figure 13 Mechanism of IPE measurement in n-type Schottky contact with two major excitations of electrons.
19
Figure 14 Example of IPE measurement data for Ni/SiC Schottky junction. The former intercept is considered as excitation over barrier (
B) and the latter is crystal field splitting.20 2.2.6 Measurement Setup
We mainly used three types of measurement system in one probe station: current-voltage, capacitance- voltage, and internal photoemission (IPE). For the current-voltage measurement, Keithley 2636A source measure unit (SMU). Agilent E4980 LCR meter is used for capacitance-voltage measurement. For the IPE measurement system, Newport 6259 lamp Xenon arc lamp, Newport 69911 lamp power supply, Newport CS260 monochromator, and Newport 77634 optical fiber. All measurement software was programmed by Labview. Figure 16 shows an example, current-voltage measure software for Keithley 2636A SMU.
Figure 15 Photograph of measurement system.
21
Figure 16 Programmed equipment control software based on Labview (Keithley 2636A)
22
III Aluminum Nitride
III-nitrides materials are candidates for dielectric material of high-power electronic devices.
Aluminum nitride (AlN) is emerging material due to its superior property in wide band gap (Eg=6.19 eV), high thermal conductivity, high decomposition temperature, and high dielectric constant. (8.5) [22- 24] AlN is used for optoelectronic applications including UV detectors, short wavelength emitters, short wavelength detector, and lasers. The lattice mismatch between AlN and SiC as small as 1%, which is advantageous to grow single crystal AlN gate insulator on the SiC surface. [25] There have been few reports on SiC/AlN MISFET, but they were far from the practical usage because of their high gate leakage and high on resistance. [25-27] Direct deposition of AlN film make the high density of interface trap. [28]
3.1 Atomic Structure
AlN and SiC have very similar hexagonal crystal structure. [25, 29] Those properties make the AlN suitable for the silicon carbide (SiC) substrate. AlN has stable wurtzite phase, and is stable at high temperatures. The lattice constant for AlN is a=3.11 Å, c=4.98 Å at room temperature. This wurtzite structure is the origin of spontaneous polarization of AlN. The spontaneous polarization of AlN is known as ~8.1×10-2 C/m2, and the direction is inverse of c axis. (aluminum to nitride) [11, 30] This value is much larger than 4H-SiC.
3.2 Electrical Properties
AlN has high thermal conductivity reaching 285 W cm-1 K-1, [31-33] very high electrical resistance (1011~1013 Ω), wide bandgap (6.19 eV), and high dielectric constant (8.5). Those properties make it suitable to be utilized for the high-power semiconductors. AlN stabilizes in the inert gas atmosphere and dissolves around 2800°C and around 1800°C during a vacuum. In the atmosphere, the surface oxide layer is formed above 700°C, so it has stable properties up to 1,370°C. [34]
23
Figure 17 Atomic structure of wurtzite phase aluminum nitride.
24 3.3 Fabrication of Aluminum Nitride Thin Film
AlN film on 4H-SIC substrate (Si face) is prepared by UHV chamber and RF sputtering system (YSR- 06MF, Youngsin-RF co., Korea). Before the sputtering, SiC sample was cleaned with HF and thermal annealing was carried out to remove thin native oxide. [35] In the same chamber, the sputtering was done with the pure aluminum target with argon and nitrogen gases at 300 ℃. The deposition parameter is shown in Table 3. After the sputtering, AlN film was annealed with 800 ℃ for 30 min in the nitride ambient. [22]
Before the sputtering, the substrate was prepared with following processes. 4H-SiC wafer is cleaned with 49 % of HF solution for 1 minute and 100nm of SiO2 is deposited onto the Si face of SiC by using plasma enhanced chemical vapor deposition (PECVD) to protect the surface of Si-face during the cleaning and dicing processes. Dicing performed with 7 mm × 7 mm of size and cleaned using ultrasonication with acetone and methanol and native oxide on C-face is removed with diluted HF solution (1:50 for 49% HF:DI water) with 5 seconds. 100 nm of nickel is deposited onto the cleaned C- face by using e-beam evaporator to make Ohmic contact. The sample is annealed by using rapid thermal annealing (RTA) system at 1000 ℃ for 90 seconds. After the annealing, the Ohmic back contact is formed with lower than 5 Ω of resistance. Then, HF cleaning with 49% HF for 1 minute again to remove protective layer of Si-face.
TABLE 3. Sputtering parameters for AlN films
Base Pressure 5 × 10-9 Torr
Target Al
Substrate SiC
Sputtering gas Ar
Reactive gas N2
Gas flow ratio Ar:N2=1:1 Sputtering pressure 1.5 × 10-2 Torr Substrate temperature 300 ℃ Substrate-target distance 5 cm
RF power 85 W
25 3.3.1 Thin Oxide Remove
In order to deposit the AlN thin film neatly without other factors, the natural oxide layer up to 2 nm must be removed. Native oxide can raise the interface trap density which significantly degrades the channel mobility of SiC. [36, 37]
We used two step oxide removal processes including chemical and in-situ flashing treatment. HF is well-known wet etchant while almost not contaminating any metal. We dipped SiC substrate into 49%
of HF for 1 min and stored the substrate in the pure methanol solvent to protect additional oxidation.
Then sample is immediately loaded into the main chamber through the load lock chamber. UHV Chamber is consisted with load-lock chamber and main chamber as shown in Figure 18. The pressure of load-lock chamber is ~106 Torr and of main chamber is ~109 Torr. Before the sputtering process, in- situ annealing process with 620 ℃ for 5 minutes is done to remove native oxide.
Figure 18 Photograph of UV chamber and RF sputtering system.
26
Figure 19 Photograph of UHV chamber and RF sputtering system, inside the chamber.
3.3.2 RF Sputtering
Sputtering is well-establish process to deposit ionized target to substrate in vacuum. Ions (Ar+) in the plasma have high energy and accelerate to the cathode, collide with the cathode, and transfer energy to the atoms around the collision. When the transferred energy is greater than the binding force of the atoms, some of the atoms break the bond and bounce out. This is a mechanism of depositing a thin film as the separated atoms fly to the substrate located opposite the cathode. [38, 39]
In sputtering, since each particle has thermal energy and large kinetic energy, the energy is much higher than that of vacuum deposition, so the thin film is dense and has good adhesion compare to other deposition process. Sputtering is advantageous in uniformity, adhesion, various target including insulator, and availability of in-situ pre/post-cleaning in UHV chamber. However, the speed of deposited film formation is relatively low, and the condition of deposition is very sensitive to pressure, bias voltage, and substrate temperature. Sputtering is largely classified into two types, DC sputtering and RF sputtering.
27 Figure 20 Schematic diagram for DC sputter system.
Figure 21 Schematic diagram for RF sputter system.
28
Figure 22 and 23 show the schematic diagram of DC and RF sputtering. RF and DC sputtering. In DC sputtering, sputtering is not possible if the target is an oxide or insulator. These drawbacks can be solved by RF sputtering, especially at low pressures. While the sputtered material reaches the substrate, the scattering is ideally reduced, so that it can have a high sputtering yield value. In DC discharge, secondary electrons disappear to the anode before supplying sufficient energy to the ionization process, but in RF, both electrodes have a negative potential compared to plasma. Since it has, it can be sufficiently used for ionization by reflecting electrons. RF sputtering is capable of sputtering non-metals, insulators, oxides, dielectrics, etc. in addition to metals. While the sputtered material reaches the substrate, the scattering is ideally reduced, so that it can have a high sputtering yield value. [40-42]
3.4.3 Thermal Annealing
The property of aluminum nitride is also affected by the post annealing. In order to reduce the interface trap density (Dit), we heated sputtered sample at the N2 ambient. [22, 43] The annealing temperature was 800 ℃ for 30 min and the rise time was 50 ℃/min. J.P. Kar found that the bandgap of sputtered AlN film is increased by 0.2 and the significant reduction of interface trap density happened by these post annealing. [22] This is caused by the increase of aluminum-nitride bond density, and the incorporation with native oxide. According to J.P. Kar, the Dit is reduced from 4.6×1012 cm-2 eV-1 to 9.7×1011 cm-2 eV-1.
29
Figure 22 Schematic diagram for describing the principles of DC sputtering. If target is an insulator, sputter does not happen when it is saturated.
30
Figure 23 Schematic diagram for describing the principles of RF sputtering. Even if target is an insulator, sputtering is possible.
31
IV Metal/Semiconductor Junction with Thin Interfacial Layers and its Application
4.1 SiC Schottky Diode
SiC Schottky barrier diode is the first SiC device. [4, 44] SiC Schottky diode takes a crucial role in high-power, high-temperature devices because of its superior electrical, physical properties as described in previous chapter. [45-47] The breakdown voltage limit for conventional silicon Schottky diode is less than 200V but in case of SiC Schottky diode, is up to 10 times compare to silicon. Silicon carbide diodes were widely used for solar power conversion before active research on high voltage devices, including automobiles. High voltage diodes play a key role in solar energy converters, boost circuits, UPSs, charging systems, and automotive applications that are currently being studied most actively.
A crucial merit for SiC compare to other materials including GaN is compatibility for conventional processes of silicon. [48] SiC is preferred over GaN in the market, so SiC power semiconductors are partially commercialized and sold while GaN-based power semiconductor technology is not yet commercialized in its early stages. The physical properties of these materials are similar, but one of the reasons SiC is drawing attention is the type of bandgap. GaN is direct bandgap semiconductor which is more suitable features for use in devices through light than power semiconductor. SiC has the same direct bandgap property as silicon. SiC also has a very high thermal conductivity that is three times as high as thermal conductivity of Si or GaN. High voltage devices inevitably generate a lot of heat, so they must operate reliably at high temperatures, which is also very beneficial in terms of cooling efficiency. The breakdown voltage in commercialized Si Schottky barrier diode is limit around 200 V, while SiC is 1200~1700V.
32
TABLE 4. Electrical properties of silicon, 4H-SiC, and GaN which are mostly used in high-power devices.
Properties Silicon 4H-SiC GaN
Crystal structure Diamond Hexagonal Hexagonal
Band gap type Indirect Indirect Direct
Band gap energy (eV) 1.12 3.26 3.5
Electron mobility (cm2/V s) 1400 950 1250
Hole mobility (cm2/V s) 600 100 200
Thermal conductivity (W/cm ℃) 1.5 4.9 1.3
Relative dielectric constant 11.8 9.7 9.5
Breakdown Field (V/cm) 0.3×106 3.2×106 3.0×106
Saturation drift velocity (cm/s) 1×107 2.7×107 2.7×107
Thermal oxide O O X
33
4.2 Ni/4H-SiC Schottky Barrier modulation by Thin AlN Layer
There have been various tries to modulating Schottky barrier (
B) by inserting passivation layers. [6, 9, 49-52] The Schottky barrier height is very crucial because the electrical characteristic is strongly depend on the Schottky barrier height. [53] Though the Schottky barrier height is proportional to the metal work function, due to the Fermi-level Finning effect, there is a significant limitation in adjusting the Schottky barrier height. Considering Fermi-level pinning effect, the Schottky barrier can be expressed as( ) ( )
B S M CNL CNL S
(15)Where S is the pinning factor,
M is metal work function,
CNL is charge neutrality level of semiconductor, and
S is the electron affinity of semiconductor. [54] The pinning factor of n-type 4H-SiC is known to be between 0.6 and 0.7. [6] Additionally, the spontaneous polarization, as well as the metal work function or the pinning factor, also has a significant effect on the barrier. By inserting additional layer, it is possible to adjust the spontaneous polarization with various mechanisms.Lower Schottky barrier height is advantageous to avoid unnecessary power consumption. [55] The sample fabrication processes are described in Chapter 3. After the sputtering and post-annealing, 50 nm thickness of nickel is deposited by e-beam evaporator with 500 um dot patterned shadow mask. Figure 24 shows the current-voltage graph of substrate contact on the C-face. The resistance is measured below 10 Ω. We used n-type 4H SiC wafer with 0.5 µm of buffer layer with n-type 1×1018 cm-3 and 17.8 µm of epi-layer with n-type 2×1015 cm-3. The epi layers were grown on 8.06° miscut Si-face.
Figure 24 I-V graph of substrate Ohmic contact on C-face of 4H-SiC
34 4.2.1 I-V Measurement
Figure 25 and 26 show the I-V characteristic of Ni/SiC Schottky diode with ultra-thin interfacial aluminum nitride film respect to the sputtering time. Significant Schottky barrier reduction occurred.
The remarkable is the negligible change in on-current and leakage-current regardless the AlN film. The bare Ni/SiC sample has Schottky barrier with 1.721 eV and the barrier is decreases as the time of sputtering increased. After 120 sec, the barrier is rather increased and the ideality factor become higher, and the current density significantly decreased. There is a report that PbS thin layer between Au/6H- SiC Schottky diode decreases the Schottky barrier by 0.09 eV [52], and TiO2 thin film between Ni, Ti, Al/4H-SiC Schottky diode also decreases the barrier by 0.2~0.3eV but significantly increases the reverse leakage current. [6, 56]
After more than 150 sec of deposition the Schottky barrier even more increased and show very weird curve of plot. All samples except the sample of 150 sec sample show almost match to 1, which imply overall carrier transport process is obey on the thermionic emission. As the voltage is increased, Ni/SiC samples with interfacial layer show somewhat rounded shape of curve, which can be estimated as a tunneling and recombination. The barrier is significantly decreased reaching 0.66 eV while remaining high on current. As the thickness is increases, the current density tends to decreases but almost same until 90 sec. The reverse leakage current is below the limit for the SMU 2636A which has 10-13 of minimum current resolution and the current density was calculated with 500 um diameters of circle.
The current is measured with hysteresis with 530 ms of delay and 0.01 mV of step to reduce the noise.
Each sample has measured with at least 3 different dots and 3 times for same dots.
TABLE 5. Extracted Schottky barrier and ideality factor by I-V measurement with respect to the aluminum nitride deposition time.
Sample I-V Barrier (eV) Ideality factor Current density in 2.2V bias (A)
0 sec 1.710±0.013 1.035±0.010 37.98±15.17
30 sec 1.220±0.010 1.035±0.005 46.73±7.34
60 sec 1.113±0.014 1.034±0.004 41.04±4.03
90 sec 1.045±0.003 1.020±0.006 47.92±1.34
120 sec 1.048±0.014 1.022±0.009 27.44±1.68
150 sec 1.399±0.064 1.257±0.134 0.36±0.11
35
Figure 25 I-V graph for Ni/SiC Schottky barrier diode with ultra-thin AlN interlayer for (a) without AlN, (b) 30 sec, (c) 60 sec, (d) 90 sec, (e) 120 sec, (f) 150 sec of deposition (sputtering).
36
Figure 26 I-V graph for Ni/SiC Schottky barrier diode with ultra-thin AlN interlayer in (a) logarithm and (b) linear scale.
37 4.2.2 C-V Measurement
Figure 27 and 28 show the C-V characteristic of the sample with different time of sputtering. The capacitance of C = dQ/dV is measured by applying reverse bias and AC voltage with 50 mV of amplitude and 1 MHz of frequency with E4980a LCR meter. The reverse bias is sweep from 0 V to 3 V. The capacitance data is converted to 1/C2 in the graph to find the Schottky barrier. By using the equation (12),
2
( 2) 2
S D
bi
q N A
V V
C
, then the build-in potential is the x-intercept of the A/C2 graph.As a result, very similar parameter is extracted compare to the I-V measurement. The Schottky barrier significantly decreased reaches 0.72 eV. The targeted doping concentration is 2×1015 cm-3, and the measured results indicate reasonable doping concentrations, 2.73×1015 cm-3~3.3×1015 cm-3. The measured doping concentration tends to bigger as the deposition time increased. As in the case of I-V measurement, the Schottky barrier become higher after the 150 sec of deposition. 1/C2 versus bias voltage graph shows very linear shape but when the deposition time exceeds 150 seconds, the bending near the 0 V of bias is observed. C-V measurement is successively performed following I-V measurement. The capacitance measured 5 times with 3 different dots for 1 sample.
TABLE 6. Extracted Schottky barrier and Nd by C-V measurement with respect to the aluminum nitride deposition time.
Sample C-V Barrier (eV) Nd (×1015 cm-3)
Reference 1.746±0.006 2.835±0.017
30 sec 1.217±0.037 2.901±0.031
60 sec 1.089±0.014 2.775±0.030
90 sec 1.025±0.002 2.734±0.030
120 sec 1.026±0.021 3.279±0.029
150 sec 1.521±0.007 3.299±0.086
38
Figure 27 C-V graph for Ni/SiC Schottky barrier diode with ultra-thin AlN interlayer for (a) without AlN, (b) 30 sec, (c) 60 sec, (d) 90 sec, (e) 120 sec, (f) 150 sec of deposition (sputtering).
39
Figure 28 C-V graph for Ni/SiC Schottky barrier diode with ultra thin AlN interlayer
4.2.3 IPE Measurement
Internal photoemission measurements with range of 1.1 eV ~ 1.8 eV and 1.55 eV~ 2.07 eV have performed. Yield1/2-photon energy graph is shown in Figure 29. Typical silicon carbide Schottky diode shows two barriers. [19] The dash line indicates the linear interpolation procedure of yield1/2. Because
1/2
Y hv q
B, the x-intercept can be extracted as a barrier. The first region is believed as the Schottky barrier. [57] There can exist additional barriers originated in the crystal field splitting of the SiC. [58]The total photocurrent yield can be express Ytotal1/2