2. Experimental procedure

Figure 1. a: Static output characteristics (IDS-VDS-VGS) at T=85K. The gate voltage is first increased from 0 to -10 V

Then trapping effects due to the substrate deep levels can occur. On the other hand, it is well known that SiC / insulator interface presents high density of surface states. As an example, when using a SiO2 passivation, a high interface state density (in the 1011 to 1012 cm-2 range) between SiO2 and SiC is still present. Consequently, the second hypothesis is consistent with the well known remaining passivation problem for SiC devices. Understanding the nature of the surface states in MESFETs could be a key to solve the problems. However, only a few works were reported on this topic. Conductance Deep Level Transient Spectroscopy cd\_z/z,%0\*!z\_z%/z\*z!""%%!\*0z0++(z0+z+0%\*z%\*"+.)0%+\*z+10z0.,/z%\*z/!)%¥ conductors, such as the activation energy, the capture cross section and the density of traps (in the case of Capacitance DLTS). The advantage of CDLTS over DLTS is the possibility to

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rent decrease is no present for the second set of curves

282 Physics and Technology of Silicon Carbide Devices

Figure 2. Conductance DLTS spectra for a 4H-SiC MESFET.

\$!zGw%z
/z/01 %! z%\*z0\$%/z3+.'z3!.!z.!(%6! z0zz!/!.\$z\* z!\$\*+(¥ ogy (TRT) in Orsay (France) using the same fabrication process for all the transistors. The epitaxial layer structures were prepared by CVD on semi-insulating substrates supplied by CREE. The layer stack consists of three layers: a P-type buffer layer with a thickness of 0.3 µm and a doping level of 1.1016 cm-3, an N-type active layer with 0.3-0.4 µm thickness and doping level Nd = 1-2.1017 cm-3, and an N+ contact layer with a thickness and doping of 0.2 µm and Nd = 1019 cm-3. The lift-off process for the fabrication of the device has already been described elsewhere [5]. The first step consists in reactive ion etching for the channel recess. An evaporation of Ti/Pt/Au stack is realized for the gate contact. The surface is passivated by a deposited oxide layer. The measurements presented in this work have been realized on short test transistors with a gate of 1µm and a gate width of 100 µm. For all the transistors the source gate distance is 0.5 µm and the gate drain distance is 2 µm.

The current (or conductance) deep level transient spectroscopy (CDLTS) measurements were performed by applying both a drain to source voltage of 8 V, in the linear regime, and a drain to source voltage of 18V in the saturation region. The steady-state reverse bias and "%((%\*#z,1(/!z2+(0#!/z,,(%! z0+z0\$!z#0!z3!.!zwDCz\* zC^z\$!z .%\*z/+1.!z1..!\*0z0.\*/%¥ ent were recorded using a numerical multimeter (HP 34401 A). The transient were treated numerically using the Lang method as in classical capacitance DLTS. The measurements were carried out between 80K and 600K in a nitrogen cooled cryostat.

be the same, but the trap location is different. Indeed, the degradation in current can be due to the presence of deep centers located in the vicinity of the channel surface. Trapping/ detrapping phenomena on these centers change the charge density near the surface. When we apply VDS for the first measurement we force these traps to be charged. In the case of electron traps, for instance, this charge is negative and a parasitic depletion at the channel surface reduces the drain current. At low temperature the emission kinetic of the traps is 2!.5z/(+3^z+\*/!-1!\*0(5\_z3\$!\*z0\$!z/!+\* z)!/1.!)!\*0z%/z,!."+.)! \_z0\$!z,./%0%z !,(!¥ tion region is still present and the current is reduced. When the temperature is increased, the traps are thermally activated and the phenomenon disappears. The same effect of gate lag has been observed recently and was undoubtedly attributed to surface trapping phenomena by an other group. Indeed, in their work, Cha et al [18fz \*+0%! z 0\$0z 0\$!z,\$!\*+)!\*+\*z !¥ pends strongly on the channel surface: current lag is lower for recessed gate and buried gate geometry than for unrecessed and channel recessed ones. Another parasitic effect observed +\*z/+)!z+"z+1.z0.\*/%/0+./z+\*/%/0/z%\*z\*z%),+.0\*0z(!'#!z1..!\*0z%\*.!/%\*#z3%0\$z0\$!z0!)¥ perature as can be seen for VGS = -10 V in Fig. 1 a and 1b. We will focus in the following on

Conductance Deep-Level Transient Spectroscopic Study of 4H-SiC MESFET and Traps

http://dx.doi.org/10.5772/51212

285

I-V-T measurements have been performed on the Schottky gate of defective MESFETs in the 0!),!.01.!z .\*#!z DCCwGIC^z\*z 0\$!/!z %+ !/z3!z \$2!z \*+0z +/!.2! z \*5z \*+)(+1/z !¥ havior like multi barrier height in direct characteristics. The ideality factor for all transistors ranges from 2 at 300K to 1.3 at 400K. These values show that the dominant mechanism of transport in our case is the generation recombination (G-R) mechanism. This result confirms

The current DLTS measurements were performed on the MESFET by applying a small drain to source voltage, Vds of 8V while keeping the source grounded. This low Vds was used to !\*/1.!z0\$0z0\$!z
z+,!.0!/z3%0\$%\*z0\$!z(%\*!.z.!#%+\*z+"z0\$!z1..!\*0w2+(0#!z\$.0!.%/¥

With a gate bias of 4V in the neighbourhood of the threshold voltage, the drain current

Figure 2 gives current DLTS spectrum under a gate pulse, showing five traps called (B1, B2, B3, B4 and B5). The apparent activation energies and capture cross-sections are deduced

ture and localization of traps B1 to B4 has been discussed in a previous publication [6]. On the other hand, with a gate bias of -10V the device is biased closer to the threshold voltage, 0\$!z .%\*w1..!\*0zz%/z/!\*/%0%2!z 0+z 0.,/z%\*z 0\$!z\$\*\*!(z\* z0z 0\$!z1""!.w\$\*\*!(z%\*0!.¥ face. A current DLTS spectrum under a gate pulse (Vgs switching from 0-4V and 0-10V) is depicted in Figure 4. The peak amplitude of the B1 (0.18eV), B2 (0.44eV) B3 (0.57eV) and B4 (0.79) traps is invariant with the change of the gate bias which improves their localisation at

/endz2!./1/zDCCCuzc%#1.!zFdz+"z((z+/!.2! z0.,/^z\$!z\*¥

the transistors showing these parasitic effects in output characteristics.

the presence of (G-R) centers in the structure.

DLTS is sensitive to traps in the channel.

from the Arrhenius plot of Ln (T2

the channel surface [13].

3.2. Part A

tics.

Figure 4. Conductance DLTS under gate pulse spectrum with Vgs = -4V and -10V.

#### 3. Results and discussion

#### 3.1. Output Characteristics

Drain-source current voltage (IDS-VDSdz)!/1.!)!\*0/z/zz"1\*0%+\*z+"z#0!z2+(0#!z\* z0!)¥ perature have been performed. Output characteristics registered at different temperatures show several parasitic effects. The first anomaly observed on IDS-VDS characteristics consist in decreasing of the current when we perform the VGS sweep consecutively increasing the negative gate voltage (i.e pinching the channel) and next decreasing VGS to zero (i.e opening the channel). This effect observed at 85K becomes more and more important for low gate voltages (Fig. 1a), i.e. when the current flows in the physical channel near the surface. This degradation in current vanishes progressively when the temperature is raised and totally disappears above 470K (Fig. 2b)^z2%+1/(5\_z0\$%/z!\$2%+.z%/z 1!z0+zz0\$!.)((5z0%20! z!"¥ fect. In a previous work [17fz3!z+/!.2! z (/+z \*z!""!0z+"z1..!\*0z+((,/!\_z10z)+.!z,.+¥ nounced for high gate voltage, when the current flows near the substrate. We attributed this to the presence of deep center located in the SI substrate. Basically, the explanation here can be the same, but the trap location is different. Indeed, the degradation in current can be due to the presence of deep centers located in the vicinity of the channel surface. Trapping/ detrapping phenomena on these centers change the charge density near the surface. When we apply VDS for the first measurement we force these traps to be charged. In the case of electron traps, for instance, this charge is negative and a parasitic depletion at the channel surface reduces the drain current. At low temperature the emission kinetic of the traps is 2!.5z/(+3^z+\*/!-1!\*0(5\_z3\$!\*z0\$!z/!+\* z)!/1.!)!\*0z%/z,!."+.)! \_z0\$!z,./%0%z !,(!¥ tion region is still present and the current is reduced. When the temperature is increased, the traps are thermally activated and the phenomenon disappears. The same effect of gate lag has been observed recently and was undoubtedly attributed to surface trapping phenomena by an other group. Indeed, in their work, Cha et al [18fz \*+0%! z 0\$0z 0\$!z,\$!\*+)!\*+\*z !¥ pends strongly on the channel surface: current lag is lower for recessed gate and buried gate geometry than for unrecessed and channel recessed ones. Another parasitic effect observed +\*z/+)!z+"z+1.z0.\*/%/0+./z+\*/%/0/z%\*z\*z%),+.0\*0z(!'#!z1..!\*0z%\*.!/%\*#z3%0\$z0\$!z0!)¥ perature as can be seen for VGS = -10 V in Fig. 1 a and 1b. We will focus in the following on the transistors showing these parasitic effects in output characteristics.

I-V-T measurements have been performed on the Schottky gate of defective MESFETs in the 0!),!.01.!z .\*#!z DCCwGIC^z\*z 0\$!/!z %+ !/z3!z \$2!z \*+0z +/!.2! z \*5z \*+)(+1/z !¥ havior like multi barrier height in direct characteristics. The ideality factor for all transistors ranges from 2 at 300K to 1.3 at 400K. These values show that the dominant mechanism of transport in our case is the generation recombination (G-R) mechanism. This result confirms the presence of (G-R) centers in the structure.

#### 3.2. Part A

The current (or conductance) deep level transient spectroscopy (CDLTS) measurements were performed by applying both a drain to source voltage of 8 V, in the linear regime, and a drain to source voltage of 18V in the saturation region. The steady-state reverse bias and "%((%\*#z,1(/!z2+(0#!/z,,(%! z0+z0\$!z#0!z3!.!zwDCz\* zC^z\$!z .%\*z/+1.!z1..!\*0z0.\*/%¥ ent were recorded using a numerical multimeter (HP 34401 A). The transient were treated numerically using the Lang method as in classical capacitance DLTS. The measurements

were carried out between 80K and 600K in a nitrogen cooled cryostat.

Figure 4. Conductance DLTS under gate pulse spectrum with Vgs = -4V and -10V.

Drain-source current voltage (IDS-VDSdz)!/1.!)!\*0/z/zz"1\*0%+\*z+"z#0!z2+(0#!z\* z0!)¥ perature have been performed. Output characteristics registered at different temperatures show several parasitic effects. The first anomaly observed on IDS-VDS characteristics consist in decreasing of the current when we perform the VGS sweep consecutively increasing the negative gate voltage (i.e pinching the channel) and next decreasing VGS to zero (i.e opening the channel). This effect observed at 85K becomes more and more important for low gate voltages (Fig. 1a), i.e. when the current flows in the physical channel near the surface. This degradation in current vanishes progressively when the temperature is raised and totally disappears above 470K (Fig. 2b)^z2%+1/(5\_z0\$%/z!\$2%+.z%/z 1!z0+zz0\$!.)((5z0%20! z!"¥ fect. In a previous work [17fz3!z+/!.2! z (/+z \*z!""!0z+"z1..!\*0z+((,/!\_z10z)+.!z,.+¥ nounced for high gate voltage, when the current flows near the substrate. We attributed this to the presence of deep center located in the SI substrate. Basically, the explanation here can

3. Results and discussion

284 Physics and Technology of Silicon Carbide Devices

3.1. Output Characteristics

The current DLTS measurements were performed on the MESFET by applying a small drain to source voltage, Vds of 8V while keeping the source grounded. This low Vds was used to !\*/1.!z0\$0z0\$!z
z+,!.0!/z3%0\$%\*z0\$!z(%\*!.z.!#%+\*z+"z0\$!z1..!\*0w2+(0#!z\$.0!.%/¥ tics.

With a gate bias of 4V in the neighbourhood of the threshold voltage, the drain current DLTS is sensitive to traps in the channel.

Figure 2 gives current DLTS spectrum under a gate pulse, showing five traps called (B1, B2, B3, B4 and B5). The apparent activation energies and capture cross-sections are deduced from the Arrhenius plot of Ln (T2 /endz2!./1/zDCCCuzc%#1.!zFdz+"z((z+/!.2! z0.,/^z\$!z\*¥ ture and localization of traps B1 to B4 has been discussed in a previous publication [6]. On the other hand, with a gate bias of -10V the device is biased closer to the threshold voltage, 0\$!z .%\*w1..!\*0zz%/z/!\*/%0%2!z 0+z 0.,/z%\*z 0\$!z\$\*\*!(z\* z0z 0\$!z1""!.w\$\*\*!(z%\*0!.¥ face. A current DLTS spectrum under a gate pulse (Vgs switching from 0-4V and 0-10V) is depicted in Figure 4. The peak amplitude of the B1 (0.18eV), B2 (0.44eV) B3 (0.57eV) and B4 (0.79) traps is invariant with the change of the gate bias which improves their localisation at the channel surface [13].

The anomalous hole trap like peaks observed in current DLTS spectra of GaAs MESFET have been previously studied and have been associated with surface states present in the ungated regions between the gate and the source-drain electrode, acting as trapping sites for !(!0.+\*/z !)%00! z ".+)z 0\$!z #0!z ! #!z 1.%\*#z 0\$!z .!2!./!z %/^z 1\$z \*z !4,(\*0%+\*z%/z /1,¥ ported by the fact that the temperature dependence of the thermal emission current from the gate edge to the ungated region observed in the DLTS measurements induces the change of the DLTS peak height of the hole-like trap when the value of t2 was 4t1 at the constant value of Vp and Vm (Figure 5).

Figure 6. Ids transient showing the progressive change from emission (375 K) to capture (420 K).

 \*z 0\$%/z,0z3!z\*z/01 5z 0\$!z/1."!z 0.,/z%\*z
zGw%z3!z\*z1/! z^z\$!.!¥ "+.!\_z%\*z+1.z/),(!/\_z3!z!4,!0z0+z+/!.2!z!)%//%+\*z".+)z0\$!z\$\*\*!(zc+.z1""!.u\$\*\*!(z%\*¥ terface) electron traps. In this case, the depletion width under the gate (or at the buffer/ channel interface) is reduced and then an increase in Ids is observed. The corresponding C-DLTS signal is positive. Nevertheless, a progressive change from emission to capture was observed in the Ids transient measurement between 375 K and 420 K (Fig. 6)^z \$!z +..!¥ sponding C-DLTS is shown on Fig. 6. A broad positive peak due to electron emission by at "+1.z %""!.!\*0z(!2!(/z%/z+/!.2! ^z\$!z/%#\*01.!/z+"z0\$!/!z0.,/z\$2!z!!\*z.!,+.0! z%\*zz,.!2%¥ ous work [15f^z \$!z %\*0!.!/0%\*#z ,+%\*0z +)!/z ".+)z 0\$!z \*!#0%2!z ,!'z 3\$%\$z %/z (!.(5z +¥

Conductance Deep-Level Transient Spectroscopic Study of 4H-SiC MESFET and Traps

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287

B In capacitance DLTS they can be due to a measurement artifact when a high frequency

B Even if this kind of behavior is frequently called "hole like", a hole emission process is not

B A mid-gap amphoteric level can exchange with both bands. The activation energy of 0.9

B The last explanation is the presence of a conductive layer at the channel/passivating layer interface in the ungated regions. This conductive layer constitutes a tank for electrons

C2 w2

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3.3. Part B: Surface Traps in 4H-SiC MESFET

served. We will focus here, on this level.

Different mechanism can explain the presence of a negative peak:

ously cannot be the case in C-DLTS measurement.

eV for level B5 rules out this explanation.

modulation is used for the differential capacitance measurement (R2

possible in our samples. Where do these holes could come from?

which can be captured by a trap located close to the interface.

Figure 5. Conductance DLTS spectra in the linear regime (VDS = 8 V) at VR = -4 V and for deferent emission rates

Thermal emission from the bulk electron traps, if present, in the active channel region below 0\$!z#0!z3+1( z.! 1!z 0\$!z !,(!0%+\*z3% 0\$z1\* !.z 0\$!z#0!\_z 0\$%/z3+1( z%\*z 01.\*z1/!z\*z%\*¥ crease in the measured source to drain current Ids. However, the emission of electrons from the interface electron traps into the surface conduction channel would increase Is^z/z0\$!z/1.¥ face channel forms an additional conduction path between the gate and the drain (source), the interface trap emission would thus decrease Ids. This is because the change in the surface current Is and the change in the bulk channel flow in opposite directions.

The appearance of the hole-like trap signal B4 (Ea=0.78eV) only for Vm lower than -10V near the pinch-off voltage leads us to conclude that the hole-like trap is not related to the surface states at the ungated regions, but is related to channel-buffer interface.

The independence of the CDLTS signal associated to all the traps with tp lead us to conclude the absence of all types of micropipes which can be responsible for the high power device deficiencies [13].

Figure 6. Ids transient showing the progressive change from emission (375 K) to capture (420 K).

#### 3.3. Part B: Surface Traps in 4H-SiC MESFET

The anomalous hole trap like peaks observed in current DLTS spectra of GaAs MESFET have been previously studied and have been associated with surface states present in the ungated regions between the gate and the source-drain electrode, acting as trapping sites for !(!0.+\*/z !)%00! z ".+)z 0\$!z #0!z ! #!z 1.%\*#z 0\$!z .!2!./!z %/^z 1\$z \*z !4,(\*0%+\*z%/z /1,¥ ported by the fact that the temperature dependence of the thermal emission current from the gate edge to the ungated region observed in the DLTS measurements induces the change of the DLTS peak height of the hole-like trap when the value of t2 was 4t1 at the constant value

Figure 5. Conductance DLTS spectra in the linear regime (VDS = 8 V) at VR = -4 V and for deferent emission rates

current Is and the change in the bulk channel flow in opposite directions.

states at the ungated regions, but is related to channel-buffer interface.

Thermal emission from the bulk electron traps, if present, in the active channel region below 0\$!z#0!z3+1( z.! 1!z 0\$!z !,(!0%+\*z3% 0\$z1\* !.z 0\$!z#0!\_z 0\$%/z3+1( z%\*z 01.\*z1/!z\*z%\*¥ crease in the measured source to drain current Ids. However, the emission of electrons from the interface electron traps into the surface conduction channel would increase Is^z/z0\$!z/1.¥ face channel forms an additional conduction path between the gate and the drain (source), the interface trap emission would thus decrease Ids. This is because the change in the surface

The appearance of the hole-like trap signal B4 (Ea=0.78eV) only for Vm lower than -10V near the pinch-off voltage leads us to conclude that the hole-like trap is not related to the surface

The independence of the CDLTS signal associated to all the traps with tp lead us to conclude the absence of all types of micropipes which can be responsible for the high power device

of Vp and Vm (Figure 5).

286 Physics and Technology of Silicon Carbide Devices

deficiencies [13].

 \*z 0\$%/z,0z3!z\*z/01 5z 0\$!z/1."!z 0.,/z%\*z
zGw%z3!z\*z1/! z^z\$!.!¥ "+.!\_z%\*z+1.z/),(!/\_z3!z!4,!0z0+z+/!.2!z!)%//%+\*z".+)z0\$!z\$\*\*!(zc+.z1""!.u\$\*\*!(z%\*¥ terface) electron traps. In this case, the depletion width under the gate (or at the buffer/ channel interface) is reduced and then an increase in Ids is observed. The corresponding C-DLTS signal is positive. Nevertheless, a progressive change from emission to capture was observed in the Ids transient measurement between 375 K and 420 K (Fig. 6)^z \$!z +..!¥ sponding C-DLTS is shown on Fig. 6. A broad positive peak due to electron emission by at "+1.z %""!.!\*0z(!2!(/z%/z+/!.2! ^z\$!z/%#\*01.!/z+"z0\$!/!z0.,/z\$2!z!!\*z.!,+.0! z%\*zz,.!2%¥ ous work [15f^z \$!z %\*0!.!/0%\*#z ,+%\*0z +)!/z ".+)z 0\$!z \*!#0%2!z ,!'z 3\$%\$z %/z (!.(5z +¥ served. We will focus here, on this level.

Different mechanism can explain the presence of a negative peak:


This phenomenon has been previously observed and is well-known in the case of GaAlAs/ SigN, interface for GaAs MESFETs [16]. The presence of a large amount of interfacial traps (in the 1012 cm2 range) at the SiC/SiO2 interface is consistent with this explanation. These defects acting as a conductive layer at the channel surface can also explain the leakage current observed on DC characteristics (surface current flowing from the gate to the drain). To strengthen the hypothesis of a capture process on surface states, an additional C-DLTS measurement in the saturation regime (Vd=18V) has been performed. From The result displayed on Fig. 7 we clearly observe the absence of the negative peak. Indeed, in these polarization conditions the channel between the gate and the drain is almost fully deserted even for Vg = 0 V and then, it is no more modulated by the gate pulses. As the gate-drain distance is 4 times higher than the source gate one (respectively 2 um and 0.5 um), the response in C-DLTS measurement is almost insensitive to the channel/SiO2 interface. This is why peak Bs is no more observed.

Figure 7. Current DLTS spectra in the saturation regime (Vog = 15V). Only positive peaks, corresponding to electron emission are observed in this case.

#### 4. Conclusion

In summary, deep levels in 4H-SiC MESFET were directly measured by means of the draincurrent DLTS technique. Three kinds of electron traps called B1, B2, and B3 with the activa tion energies of 0.18eV, 0.44eV and 0.57eV respectively. These traps are located in the channel surface.

In part B we have been used Conductance DLTS for 4H-SiC MESFETs characterization and revealed capture phenomenon of electrons present at the channel/passivating layer (SiC/ SiO2) interface or at the channel/buffer or buffer/substrate interface. One kind of hole-like trap signal with activation energy of 0.90 eV is observed in conductance DLTS measurements on 4H-SiC MESFETs when the device is biased in the linear regime with a gate reverse pulse in the neighbourhood of the threshold voltage. This result shows the interest of this technique for the analysis of trapping phenomena due to SiC/SiO2 interfacial defects. The conductance DLTS using a gate pulse closer to the pinch-off voltage shows one additional hole trap HL2 with activation energies of 0.56eV located at the channel/buffer or buffer/SI substrate interface.

The understanding of trapping phenomena due to surface and interface states and the surface passivation breakout is of main interest for the future industrial development of high power RF transistors on wide band gap materials (SiC MESFETs and AIGaN/GaN HEMTs).
