Materials Science in Semiconductor Processingzyang/Teaching/20172018... · Recent advances in diamond power semiconductor devices Hitoshi Umezawa a,b,c a Advanced Power Electronics

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Materials Science in Semiconductor Processing

journal homepage: www.elsevier.com/locate/mssp

Recent advances in diamond power semiconductor devices

Hitoshi Umezawaa,b,c

a Advanced Power Electronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 563-8577, Osaka, JapanbUniversité Grenoble Alpes, Institut NEEL, F-38000 Grenoble, Francec CNRS, Institut NEEL, F-38000 Grenoble, France

A B S T R A C T

Diamond is known as an ultimate material because of its superior properties and it is expected to be employed innext-generation power electronic devices. Progress in epitaxial growth and fabrication techniques such as p- andn-type doping control with low compensation and surface treatment have improved the performance of powerdevices. High forward-current density and long-term stability have been achieved for Schottky barrier diodesoperating at 400 °C. Fast turn-off operation with low loss and a high blocking capability of> 10 kV have alsobeen realized. In addition, high blocking voltages of more than 2 kV have been achieved for switching devicessuch as metal-semiconductor field-effect transistors (MESFETs) and metal-oxide semiconductor FETs. To max-imize device performance up to the material limit requires the development of fabrication techniques such asselective area doping, lithography, etching, formation of diamond/oxide interfaces and also defect reduction.Here, the current status of semiconductor diamond technology is reviewed.

1. Introduction

Diamond is a promising material for next-generation power elec-tronics that offer low power consumption and high-frequency opera-tion. High-temperature operation with a low leakage current is alsoexpected because of its extremely low intrinsic carrier concentrationand high built-in potential. Table 1 shows a comparison of the materialproperties of Si, 4H-SiC, GaN, Ga2O3 and diamond. Diamond has a highcarrier mobility (4500 and 3800 cm2/Vs for electron and holes, re-spectively) [1], a high breakdown field (> 10 MV/cm) [2], a low di-electric constant (5.7) and a very high thermal conductivity (2200 W/mK). Therefore, diamond based power devices are expected to sig-nificantly reduce both conduction and switching losses. The figure ofmerit (FOM) is one indicator of how much a device can be improved byusing a particular material. Baliga introduced two important FOMs forhigh power devices: Baliga's FOM (BFOM) and Baliga's high-frequencyFOM (BHFOM) [3]. The BFOM estimates the trade-off between con-duction loss and breakdown voltage for a unipolar device, and theBHFOM includes the switching loss associated with the gate capaci-tance for a field-effect transistor (FET). Diamond exhibits values forboth FOMs, as shown in Table 1. Huang also introduced the high-temperature FOM (HTFOM) and the chip-area FOM (CAFOM), whichtake into account the actual switching behavior and estimated high-temperature operation capability, and the reduction in chip area [4].For both the HTFOM and the CAFOM, diamond has the best value forwide-gap materials.

In the present decade, diamond growth techniques have been im-proved and doping control methods for p, p+, n-type and intrinsicdiamond have become available. The electrical properties of thesematerials can thus be characterized not only theoretically but also ex-perimentally using device structures. The material properties of dia-mond that have be determined experimentally are indicated in italicswith reference citations in Table 1. The carrier velocities of electronsand holes are estimated by the transient current technique [5] and alsoby the relationship between the gate length of FETs and the cut-offfrequency (only holes) [6]. The carrier mobility is obtained by time-of-flight [1] and Hall effect measurements [7]. A maximum breakdownfield of 9.5 MV/cm was reported by analysis of the doping profile andbreakdown voltage for a planar Schottky barrier diode (SBD) [8].Power device capabilities such as a high blocking voltage ofVmax>10 kV [9,10], high current operation at> 20 A [11], and fastoperation with low-loss switching [12] have been recently reported fordiamond SBDs. In this paper, recent progress in diamond semi-conductor devices is reviewed.

2. Diamond devices

2.1. Diodes

Taking into account carrier activation and carrier scattering at roomand elevated temperatures, the trade-off relationship between the on-resistance Ron and Vmax for a unipolar diamond device can be obtained.

https://doi.org/10.1016/j.mssp.2018.01.007Received 18 September 2017; Received in revised form 2 January 2018; Accepted 9 January 2018

E-mail address: [email protected].

Materials Science in Semiconductor Processing 78 (2018) 147–156

Available online 03 February 20181369-8001/ © 2018 Published by Elsevier Ltd.

T

The total Ron of an SBD consists of the drift layer resistance Rp-, theparasitic resistance of the p+ layer Rp+, and the ohmic contact ROhm, asfollows:

= + +− +R R R Ron p p Ohm (1)

For a one-dimensionally structured SBD, the total resistance Ron andspecific on-resistance RonS, can be expressed as:

= + +− + +R

dpμqS

ρ dS

ρS

,onp p p c

(2)

= + +−+ +R S

dpμq

ρ d ρ ,onp

p p c (3)

where − + +d d p μ S ρ ρ, , , , , , andp p p p c are the thicknesses of the p- and p+layers, the carrier concentration in the p- layer, the mobility, contactarea, resistivity of the p+ layer; and the specific contact resistance ofthe ohmic contact, respectively. The maximum blocking voltage Vmax

determined by avalanche breakdown for a unipolar diode can be ex-pressed as:

= ∙ −−−V E d

qN dε2

,max max pA p

dia

2

(4)

where E N ε E N ε, , , , , andmax A dia max A dia are the avalanche breakdownfield, acceptor concentration, and dielectric constant, respectively. Thecarrier concentration and mobility [13] are estimated using the fol-lowing equations which take into account the temperature effect.

∙ +− −

= ⎛⎝

⎞⎠

⎛⎝

− ⎞⎠

p N pN N p

πm kTh

exp EkT

( ) 2 * ,D

A D

A2

3/2

(5)

=+

∙⎛⎝

⎞⎠

−

( )μ

μ T

1 300,max

NN

γ

β

Aμ (6)

=+( )

ββ

1,max

NN

δAβ (7)

Here, N m k E andμ, *, ,D A max are the donor concentration in the drift layer,the effective mass of holes, the Boltzmann constant, the activation en-ergy for boron, and the carrier mobility at low doping concentration,respectively. N N β γandδ, , ,μ β are the fitting parameters for the empiricalmobility model. EA is also a parameter dependent on NA [14]. In thismodel, the doping dependence of Emax is not considered and is set to10 MV/cm.

Fig. 1 shows the unipolar limit of RonS for Si, SiC [15,16] and dia-mond as a function of Vmax. The unipolar limits for SiC and diamond at250 °C are also indicated. The obtained limit is the so called Baliga limitwithout taking into account the switching losses. RonS for diamond isalmost comparable to that for SiC at room temperature around Vmax =

2 kV; however, it becomes almost one order of magnitude lower fordevices with Vmax>10 kV. On the other hand, RonS lower by one totwo orders of magnitude for all Vmax values can be achieved at elevatedtemperatures. The optimum thickness and doping concentration for thedrift layer at 1 and 10 kV are estimated through this analysis to be1.7 µm with 1.1 × 1017/cm3, and 16 µm with 1.5 × 1016/cm3, re-spectively. The experimentally obtained RonS of diamond devices (openand solid circles in Fig. 1) are lower than the unipolar limit for Si andclose to the SiC limit at present; however, these are still much higherthan the expected characteristics for diamond. The reasons for thelimitation will be discussed later but this is mainly due to (1) a loweffective breakdown field strength because of field enhancement at theedge of the electrode, (2) an increase in leakage current through de-fects, and (3) non-optimized doping and thickness of the drift layers[17].

Both unipolar and bipolar diodes such as p-type-intrinsic-n-typediodes (PiNDs), SBDs, junction barrier Schottky diodes (JBSDs), metal-intrinsic-p type diodes (MiPDs), and Schottky pn diodes (SPNDs) havebeen experimentally reported. The cross-sectional structures and asummary of the device performance are listed in Table 2. The highestVmax, which is> 11.5 kV, has been obtained for a PiND without a mesastructure [10]. The breakdown voltage decreases because the leakagecurrent increases when a mesa structure is utilized. The increase of theleakage current is considered to be due to defects formed during themesa etching process. The forward current density for bipolar diamonddevices is low because of the short minority carrier lifetime. Thepseudo-vertical SBD (pVSBD) structure is well suited to diamond diodes

Table 1Material properties and FOMs for various semiconductors.

Si 4H-SiC GaN Ga2O3 Diamond

Bandgap EG [eV] 1.10 3.20 3.45 4.9 5.47Saturation drift velocity vS [x 107 cm/s] Electron 1.1 1.9 2.5 2 2.5, 1.9 [5]

Hole 0.8 1.2 – – 1.0, 1.4 [5], 0.6 [6]Carrier mobility μ [cm2/Vs] Electron 1500 1000 1500 300 4500 [1],

Hole 450 120 200 – 3800 [1], 2000 [7]Breakdown field Emax [MV/cm] 0.3 2.8 5 8 10–22, 9.5 [8]Dielectric constant εr 11.9 9.66 8.9 9.93 5.7Thermal conductivity λ [W/mK] 150 490 130 23 2200

Baliga's figure of merit (BFOM) ε μEr max 3 1.0 440 2950 3516 473078 (3380)

Baliga's high frequency FOM (BHFOM) μEmax 2 1.0 58 237 158 12510 (1486)

Huang's temperature FOM (HTFOM) λ ε E/ r max 2 1.0 0.43 0.07 0.01 0.46 (0.97)

Huang's chip area FOM (HCAFOM) ε μ Er 1 2 max 2/ 1.0 58 192 280 3887 (226)

Fig. 1. Trade-off relationship between RonS and Vmax for unipolar devices on Si, SiC anddiamond. Trade-off relationship on SiC and diamond at 250 °C are also indicated.

H. Umezawa Materials Science in Semiconductor Processing 78 (2018) 147–156

148

Table2

Summaryof

diam

onddiod

es.P

erform

ance

variab

leslistedareno

tob

tained

from

thesamede

vice,b

utarethebe

stva

lues

repo

rted

toda

te.

Type

Unipo

lar

Bipo

lar

Dev

ice

pVSB

DVSB

DSP

ND

MiPD

PiN

Structure

Vmax

2.5kV

1.8kV

[72]

>10

0V

2.5kV

[35]

>10

kV[10]

E max

>7MV/cm

[9,23]

2.7MV/cm

[80]

3.4MV/cm

[44]

4.2MV/cm

[35]

3.4MV/cm

[10]

I max

0.5A

20A

[11]

<10

0mA

<10

0mA

<10

0mA

J max

>4.5kA

/cm

2[29]

>1kA

/cm

2>

60kA

/cm

2[44]

7.5kA

/cm

2[36]

Rem

arks

Stab

leinterface@

400°C

&X-ray

Fast

turn-off

Notrad

e-off

betw

een

Smalltemp.

coeffi

cien

tof

Ron

Highe

stVmax

@25

0°C

t rr<

20ns

RonSvs

Vbr

Cha

lleng

eEd

ge-termination

Edge

-termination

Low

n-do

ping

withlow

compe

nsation

Highcu

rren

tcapa

bility

Long

lifetim

eof

electron

Current

capa

bility

Defects

Highn+

doping

H. Umezawa Materials Science in Semiconductor Processing 78 (2018) 147–156

149

because high crystal quality and low-cost semi-insulating substrates canbe utilized [18]. A heavily boron doped p+ type layer with a thicknessof 1–3 µm is first grown on the semi-insulating substrate before de-position of a lightly boron doped p- drift layer. Ohmic contacts aredirectly formed on the p+ layer after selective etching of the p- driftlayer. In this structure, the depletion layer extends vertically throughthe drift layer, however, the forward current flows laterally in the p+layer. The mean run length for carriers in the p+ layer increases withthe size of the main contact, accordingly, Rp+ is not decreased withincrease in S. The current capability of pVSBD is limited up to 5 A dueto this effect [19]. The large controllability of the Schottky barrierheight (SBH) for diamond is one of its important advantages. SBHs from1.2 to 3.4 eV have been reported for oxygen-terminated p-type diamond[20], which is comparable to the bandgap range from Si to SiC. Espe-cially, the ozone-treated surface exhibits a high SBH and a high elec-trical field strength of more than 2.5 MV/cm without the edge-termi-nation technique [21,22]. Utilizing this treatment, high reverseblocking voltages of> 1 kV and high current densities of> 1 kA/cm2

at 6 V have been realized for Zr SBD [23]. The power FOM (V R S/max on2 )

for this diamond SBD was 244 MW/cm2, which is the maximum valuefor diamond devices at present [24]. No breakdown behavior has beenfound for this device, because of the measurement limitation of the testequipment. However, the maximum breakdown field was esti-mated> 7.7 MV/cm from the doping concentration in the p- layer.

A few investigations of the Schottky interface have also been con-ducted. Carrier transport between metal and oxygen-terminated dia-mond under forward bias conditions can be explained using a ther-mionic emission model [21,25]; however, an inverse relationshipbetween the SBH and the electronegativity difference was also found[26,27]. Muret et al. and Teraji et al. have reported barrier in-homogeneity, which affects both the forward and reverse character-istics [27,28] and suggests the presence of interfacial defects or inter-facial charges that depend on the oxidation method [26].

Fig. 2 shows typical forward and reverse current-voltage char-acteristics for a pVSBD with a 50 µm sized Schottky contact. The pVSBDwas fabricated on a 1/2 in. wafer [29] as shown in the inset. Mo wasused as the Schottky metal with an SBH of 2.2 eV. At −7 V of the ap-plied voltage on Schottky contact, the forward current density was1800 A/cm2 at room temperature (RT), and increased to 4500 A/cm2 at250 °C. The reverse electrical field was more than 3.5 MV/cm, eventhough an edge termination structure was not utilized. No avalanchebreakdown was observed for these devices; however, the blockingvoltage was limited by the increase in leakage current. On this sub-strate, 75% of the fabricated devices had Emax greater than 3 MV/cm.The leakage current for diamond SBDs can be explained in terms of

thermionic field emission taking into account the barrier lowering ef-fect. This model agrees well with the measured leakage current, even atelevated temperatures [18,22,24,30].

Fig. 3 shows typical turn-off characteristics for a VSBD measured bythe double pulse method [12]. The VSBD was mounted on a metal/ceramic package designed for high-temperature and high-power de-vices, as shown in the inset of Fig. 3. The turn-off time (trr) was in-dependent of temperature and forward current density [31]. The re-verse recovery charge Qrr, which corresponds to the charge in thedepletion layer, was smaller than that for SiC SBDs because of the lowdielectric constant of diamond. The fast switching capability of dia-mond SBDs is one of their advantages for high-frequency low-loss cir-cuits.

The long-term stability of the Schottky interface was investigated athigh temperature, and the results are shown in Fig. 4. An initial shiftoccurs at about 1 h at 250 °C as the Schottky interface stabilizes;however, no further degradation of the SBH, RonS or the ideality factorat the Ru/diamond interface was found even after 1500 h at 400 °C and250 h at 500 °C [32]. Pt/diamond or WC/diamond interfaces also ex-hibit good thermal stability because of the extremely high activationenergy for platinum group metals or metal carbides diffusing into dia-mond [33]. The radiation hardness of diamond SBDs has also beenconfirmed after irradiation with X-rays at 10 MGy [34].

MiPDs are a potential structure for unipolar diamond devices.Brezeanu et al. realized blocking voltages greater than 2.5 kV using thisFig. 2. Typical current-voltage characteristics for a diamond SBD at RT and 300 °C.

Fig. 3. Diamond SBD mounted on metal/ceramic package and turn-off characteristicsunder high-temperature conditions.

Fig. 4. Current-voltage characteristics for a Ru/diamond SBD annealed at 400 °C.

H. Umezawa Materials Science in Semiconductor Processing 78 (2018) 147–156

150

diode structure [35]. Under forward biased conditions, the holes in-jected from the p+ layer flow with a high mobility in the intrinsic layer(i-layer), and the i-layer blocks high voltages under reverse biasedconditions. However, the forward current is governed by the space-charge-limited current and the current density is limited especially athigh temperatures [36,37]. The JBSD is a widely accepted structure,especially for SiC power devices [38]. A few trials to fabricate a similarstructure on diamond have been conducted. Kubovic et al. used a 10 nmthick n-type layer doped with nitrogen to make an np junction for acathode [39]. They observed current rectification at 1000 °C; however,there was no improvement in the breakdown field due to the presenceof defects.

Even though a high boron doping concentration is possible in dia-mond, lattice expansion cannot be ignored when the concentration ishigher than 1020/cm3 because of the larger covalent radius of boroncompared to carbon [40]. Kitagoh et al. and Kageura et al. confirmedrelaxation of the lattice which occurs after generation of large numberof dislocations using X-ray diffraction and transmission electron mi-croscopy [41]. They determined the critical thickness as 200 nm on 8×1021 /cm3 of boron doped film. Alegre et al. have estimated the criticalboron concentration as 3.2 × 1021 /cm3 on (001) growth and con-cluded that the generation of the dislocations is due to the proximityeffect [42]. Therefore, a drift layer grown on a stacked p+/substratesuch as pVSBD has a high density of dislocations originating from thedefective p+ layer. To avoid this effect, Nagase et al. first grew a p-drift layer on a high-quality semi-insulating substrate, and then grew ap+ contact layer [43]. The 50 µm thick substrate was then etched fromthe backside to prepare a Schottky contact on the p- layer. An RonS of5.89 mΩ-cm2 with a Vmax of 700 V was obtained using this structure.

The highest current density of more than 60 kA/cm2, which corre-sponds to an RonS of 0.03 mΩ-cm2, was reported by Makino et al. for aSchottky pn junction diode (SPND) [44,45]. They pointed out that thisnew concept device is not governed by the trade-off relationship be-tween RonS and Vmax because both characteristics can be designed in-dependently. They used a lightly doped n-type drift layer on a p+contact layer with a Schottky contact on top. The n-type layer was stilldepleted under forward biased conditions, so that holes injected fromthe p+ layer flowed through the n-type layer by the saturation velocity.Therefore, the forward current density was almost independent thethickness of the n-type layer. On the other hand, the blocking voltage isdetermined by the thickness of the n-type layer. Fast turn-off with trrless than 30 ns was also reported, which is not possible with conven-tional pn diodes [46]. The decrease in the n-type dopant concentrationwith an increase in the thickness of the drift layer to increase Vmax andmaintain the fully depleted condition of the drift layer in the forwardbiased regime.

2.2. Switching devices

Research aimed at utilization of diamond for switching devicesbegan in the 1980s using natural diamond crystals. Prins firstly realizeda bipolar junction transistor (BJT) using a natural p-type diamondcrystal as a base electrode with an n-type emitter and collector regionsformed by carbon ion implantation [47]. However, the current gain wasless than unity because of a geometrical problem attributed to thefabrication process. After this discovery, metal-semiconductor FETs(MESFETs) [48] and metal-insulator-semiconductor FETs (MISFETs)[49] were also realized on natural diamond crystals using diffusiondoping or ion implantation. The situation was dramatically changed inthe 2000s after the establishment of epitaxial growth by chemical vapordeposition (CVD). The typical device structures and a summary of theirperformance reported to date are listed in Table 3.

Schottky contacts with wide controllability of the SBH can be simplyfabricated on oxygen-terminated diamond surfaces. Therefore, deep-depletion mode MESFETs operate with high blocking voltages and lowgate leakage currents under high-temperature conditions. Fig. 5 show

typical current-voltage characteristics for a MESFET operated at roomtemperature and 300 °C [50]. The increase in the gate-drain distance(LGD) allows wide expansion of the depletion layer from the gate to thedrain, so that Vmax can be increased from 700 to 1500 V when LGD isincreased from 5 to 30 µm. Without considering field spikes at the drainedge of the gate electrode, Emax was estimated to be 2.15 MV/cm,which is almost comparable to that for GaN FETs [51]. The MESFEToperates with a low gate leakage current, even at 500 °C, due to its highSBH. Inserting oxide layer on the channel of MESFET enables accu-mulation mode operation. Pham et al. have demonstrated deep-deple-tion mode metal-oxide-semiconductor FETs (MOSFETs) [52]. They re-ported annealing at 500 °C dramatically decreases gate leakage currentunder accumulation condition.

Junction gate FETs (JFETs) are also the typical normally-on devices,in which the depletion layer in the channel beneath the junction gate iscontrolled by the gate bias voltage [53]. The wide built-in potentialof> 5 eV for a diamond pn junction makes normally-off type operationpossible [54]. The JFET is capable of blocking voltages of> 600 V atroom temperature and 200 °C with a high breakdown field of> 6 MV/cm. The high electric field strength is due to the quality of the lateral pnjunction interface [55,56]. Bipolar operation with a higher currentdensity of 458 A/cm2 has recently been achieved with a JFET [57].

Hydrogen-termination of a diamond surface plays an important rolewith respect to the electrical characteristics. When the diamond surfaceis terminated by hydrogen, which is the typical surface structure afterCVD growth, two-dimensional (2D) p-type surface conduction appears.This surface conductive layer is ideal for the channel of FETs because ithas a high sheet carrier concentration of> 1012 /cm2, a shallow depthof< 10 nm, and a low activation energy [58]. The SBH can be con-trolled based on the electronegativity of the metal; therefore, low-re-sistance ohmic contacts can be produced by deposition of Au or Pt onthe surface, and Schottky contacts are obtained with Al or Cu [59]. BothMES and MOSFETs with high current capability have thus been realized[60,61]. The current density reaches 1.3 A/mm, which is comparable toGaN high-electron-mobility transistors (HEMTs) [62]. Current andpower gains at frequencies of> 1 GHz have been observed with suchdevices [63]. The narrow channel means that reduction of the gatelength down to 0.1 µm does not cause a short channel effect [64], andthe cut-off frequency (fT) and maximum frequency of operation (fmax)can thus be improved to 50 and 100 GHz by refining the gate [65]. Ahigh current capability and a large value of fT and fmax have been alsofound for hydrogen-terminated polycrystalline diamond [66,67]. Re-cently, the breakdown voltage was characterized by Kitabayashi et al.[68] Similar to MESFET, Vmax for hydrogen-terminated surface channelFETs can be increased to 2 kV by increasing LGD to 24 µm, which is thehighest value for diamond FETs. The low activation energy for thechannel has meant that both a maximum drain current of 110 mA/mmand a Vmax of 1.7 kV have been achieved [69].

Matsumoto et al. realized inversion MOSFETs by utilizing OH ter-mination on (111) diamond [70]. They used a phosphorous doped n-type layer for the body and Al2O3 by atomic layer deposition was usedas the gate insulator. A p-type inversion channel was formed at a gatebias of 6.3 V. They determined the inversion mobility to be 8 cm2/Vsand pointed out that the presence of a high interface density of states (6× 1012 /cm2 eV) at the bunching steps degraded the mobility.

3. Edge-termination and defects in diamond

Even though diamond has superior material properties, edge-ter-mination techniques are required because breakdown occurs at theedges of electrodes where the potential distribution becomes steep.Hard breakdown initiated at the edge of electrodes has been frequentlyobserved for diamond devices. Typical edge-termination techniquesproposed for diamond devices are field-plate (FP) and junction termi-nation extension (JTE) for unipolar devices and mesa structures forbipolar devices.

H. Umezawa Materials Science in Semiconductor Processing 78 (2018) 147–156

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Table3

Summaryof

diam

ondsw

itch

ingde

vices.

Performan

ceva

riab

leslistedareno

tob

tained

from

thesamede

vice,b

utarethebe

stva

lues

repo

rted

toda

te.

Type

Unipo

lar

Bipo

lar

Dev

ice

MES

FET

JFET

H-FET

MOSF

ETBJ

T

Structure

Vmax

2.2kV

[tobe

publishe

d]>

600V[55]

2kV

[68]

<50

V<

50V

E max

2.1MV/cm

[50]

>6MV/cm

[55]

3.6MV/cm

[69]

I max

30mA

450A/cm

2[57]

1.3A/m

m[62]

<1mA

<1mA

J max

<3mA/m

m@25

0C

(bipolar

mod

e)Rem

arks

Stab

leop

eration

Normally

offSh

allow

chan

nel

Inve

rsionch

anne

l(normally-off)

HighT

f max>

100GHz

@50

0°C

&after10

MGyX-ray

sBipo

larmod

eNormally-on/

offVerticalstructure

Cha

lleng

eVerticalstructure

Verticalstructure

Reliability

HighVr

Long

lifetim

eof

electron

Verticalstructure

Highn+

doping

Dop

ingco

ntrol

Mob

ility

Low

n+laye

rresistivity

H. Umezawa Materials Science in Semiconductor Processing 78 (2018) 147–156

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Relaxation of the electric field at the edge of the electrode is possibleby inserting an insulator film between the Schottky electrode and thediamond surface. Ikeda et al. reported an optimized FP structure for adiamond SBD using Al2O3 and SiO2 as insulators. They noted that Vmax

was improved by more than 2 times with implementation of FP and theoptimal thickness of Al2O3 was around 1.5 µm, which is 1.7 timesthicker than that for SiO2 [71]. Reduction of the leakage current andimprovement of Vmax were confirmed experimentally [72]. However,the ideal breakdown was not obtained because field-enhancement atthe edge of the electrode was still present.

Effective improvement of the breakdown voltage has been expectedthrough the use of ramp oxide structures for FP, where the thickness ofthe insulator is increased at the outer side [73]. Brezeanu et al. opti-mized the structure of a ramp oxide FP for an MiPD by 2D simulationand reported that an ideal breakdown of 92% is expected for a rampoxide angle of 5.7°, and with a dielectric thickness and FP length of 3and 25 µm, respectively [74].

Due to the difficulty in producing low-resistance n-type layers byion implantation or high-quality n-type selective-area growth on p-typediamond surfaces, very few groups have realized JTE structures ondiamond. Huang et al. implanted H+ ions at the edge of VSBDs toobtain the same effect as JTE, and reported a Vmax of 3.7 kV [75]. Incontrast to this technique, an increase in surface ohmic leakage by asemi-insulating passivation layer on the surface will provide a uniformsurface potential distribution and improve the breakdown voltage. Thisis known as the semi-insulating polycrystalline-silicon (SIPOS) tech-nique, and is mainly used for high-voltage silicon devices [76]. A si-milar effect has been confirmed for diamond SBDs. Irradiation of dia-mond SBDs with X-rays at 10 MGy using the FP technique was found toslightly increase the leakage current to 10 µA/cm2. This leakage currentmay be due to charge transport through the irradiation defects in Al2O3,which improves the breakdown voltage of SBDs by 20% more [34]. Theleakage current can be explained by ohmic conduction in the weaklyreverse biased region and thermionic emission in the strongly biasedregion; therefore, breakdown still occurs at the edge of the electrode.

Field enhancement at the edge of the electrode can be experimen-tally visualized using electron-beam induced current (EBIC) imaging.Irradiation of diamond with an accelerated electron beam generateselectron-hole pairs. When a reverse bias is applied to the SBD, theminority carriers generated, i.e., electrons in diamond, are acceleratedby the electric field and multiplied in the drift layer. The carriers arecollected at the Schottky contact as a current, so that a current mapsynchronized with the scanning of an electron beam corresponds to theelectric field distribution. Fig. 6 shows topographic scanning electronmicroscopy (SEM) and EBIC images of a diamond SBD with an appliedreverse bias of 100 V, which corresponds to an average field of 1.3 MV/cm at the central area of the Schottky contact. As shown in Fig. 6(b), theSBD is surrounded with a laterally extended depletion layer that ap-pears as a high EBIC intensity area. However, the EBIC intensity is notuniform in the depletion layer and regions of very strong signal, hotspots, are present. One possible source of the hot spots is structuraldefects associated with device fabrication, particularly with regard tothe lithography and lift-off processes [77].

Defects also lead to degradation of device performance. Non-epi-taxial crystallites in the drift layer, which are polycrystalline particlesthat grow from contaminants on the substrate surface, are killer defectsin VSBDs [21] and MESFETs [78]. These crystallites can be almost re-moved by control of the growth conditions and through the use of off-cut substrates [79]. However, the effects of crystallographic defectssuch as threading dislocations have not yet been clarified. Fig. 7 showsthe cumulative relative frequency of Emax for pVSBDs as a function ofthe Schottky electrode radius [29,80]. No visible killer defects werefound in the active area of the chip in this case. All pVSBDs less than50 µm and more than 50% of those with a 300 µm radius have an Emax

of more than 2.5 MV/cm, even without edge-termination structures.However, degradation of the blocking capability becomes significantwhen the Schottky radius is larger than 450 µm. The density of devicedegradation defects can be estimated based on the assumption that thenumber of defects increases with the active area. The device yield isdetermined using Murphy's formula as follows [81]:

Fig. 5. Static characteristics for diamondMESFET and breakdown characteristics.

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= − − ∙ ∙Yield exp D S D S{[1 ( )]/ } ,0 02 (8)

where D0 and S are the defect density and the electrode area, respec-tively. Fitting using a yield threshold of 2–2.5 MV/cm gives D0 as600–1700 /cm2, which is much lower than the density of threadingdislocations in the film estimated by X-ray topography. This analysisincludes not only defects but also peripheral problems; however, thereis no significant difference in the amount of degradation trend for dif-ferent shaped devices, such as circular and rectangular devices. Furtherexperiments and analysis are required to independently evaluate theperipheral and areal effects for tracking killer defects.

X-ray topography is a powerful tool for characterizing crystal-lographic defects in diamond [82]. The dislocation density in substratesproduced at high pressure high temperature substrates is estimated tobe 104–105 /cm2 [83], and these dislocations propagate through theCVD film, together with additional dislocations originating from surfacepolishing defects [84]. Kato et al. attempted to reveal the correlationbetween the leakage current and the type of dislocations using X-raytopography, based on the assumption that each type of dislocation hasits own contribution to the increased leakage current. They concludedthat edge and threading mixed dislocations have a similar contributionto the leakage current [85]. Ohmagari et al. characterized the re-lationship between the leakage current for SBDs and band-A emission incathodoluminescence spectra, and concluded that only defects withfour-fold symmetrical luminescence patterns contribute to the leakagecurrent [86].

In conclusion, the current status of diamond semiconductor devices

has been reviewed in the present paper. The performance of diamonddevices has been improved since the establishment of homoepitaxialgrowth techniques and doping control; however, the lack of devicefabrication techniques still limits device performance. New processingtechniques to form edge-termination structures, especially on selec-tively doped substrates using ion implantation or selective area growth,together with MOS structures, are required for effective use of the at-tractive properties of diamond. A deeper understanding of surface, in-terface and defect structures is necessary in order to improve both de-vice fabrication and performance.

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