1

Polarization doping: new opportunities for III-nitride

optoelectronics

Sergey Yu. Karpov

Workshop “Intensive Discussion on Crystal Growth of Nitride Semiconductors , Sendai, Japan, 22-23 October, 2012

STR Group – Soft-Impact, Ltd (St.Petersburg, Russia)

2 Polarization doping for High-Electron Mobility Transistors

AlGaN

GaN

polarization charge (PC) located at

the interface

electron density

distribution

2DEG formation due to

polarization doping

A. Bykhovsky et al., J. Appl. Phys. 74 (1993) 6734 – strong piezoeffect

F. Bernardini et al., Phys. Rev. B 56 (1997) R10024 – spontaneous polarization

at the moment, polarization doping is routinely exploiting in AlGaN/GaN

and AlInN/GaN HEMTs, providing the sheet electron concentration in the

range of ~0.8-2.6××××1013 cm-2

3 Polarization impact on LED and laser diode performance

504 510 516 522 528

-4

-3

-2

-1

0

1 I = 10 mA

2.661 eV

Ene

rgy

(eV

)

Distance (nm)

n-GaN / 5××××(InGaN/GaN) /

p-AlGaN / p-GaN LED

structure

Negative impact of polarization charges on

performance of [0001] LED and LD structures:

• interface polarization charges induce built-in

electric field in InGaN QW resulting in a large

separation of electron and hole wave functions;

this leads to reduction of the photon emission

rate and, eventually, in a lower IQE of the

[0001] LED structure

• electric field induced in the MQW active

region leads also to increased operation voltage

of the LEDs

• polarization charges at the LED structure

interfaces provide the conduction and valence

band alignment favoring the electron leakage in

the p-side of the heterostructure

4 Fundamentals of distributed polarization doping (DPD)

0.3

0.5

0.1

XAlN

0.3

0.5

0.1

XAlN

0.3

0.5

0.1

XAlN

[0001]

[0001]

[0001]

nominal: uniform

composition

graded-down: descending composition

graded-up: ascending

composition

positive PC

negative PC

zero PCgraded-

composition AlGaN

GaN

polarization charge (PC)

zdPd

ρ z−=

Distributed polarization doping (DPD) has been initially proposed

for III-nitride FETs

Experiment: D. Jena et al., Phys.Stat.Solidi (c) 0 (2003) 2339 – n-type DPD

PCs are formed at interfaces

only

p-type DPD

inducing holes

in the material

n-type DPD

inducing electrons

in the material

5 How to attain high hole concentration by DPD?

Simulation: K. A. Bulashevich et al., Proc. 3 rd APWS, Jeonju (2007) 192

102

103

104

1650 1700 1750 1800

-5

-4

-3

-2

-1

0

1

electroncurrent

hole current

Ene

rgy

(eV

)

Distance (nm)

T = 300 K

j = 5.3 kA/cm 2

Par

tial c

urre

nt d

ensi

ty (

A/c

m2 )

1014

1015

1016

1017

1018

1019

1020

1021

1022

1650 1700 1750 1800

-5

-4

-3

-2

-1

0

1

electrons

holes

Ene

rgy

(eV

)

Distance (nm)

300 K

j = 5.3 kA/cm 2

Con

cent

ratio

n (c

m-3)

!

such a high hole density can never

be obtained by

conventional doping with

Mg impurities

UV laser diode structure with graded-composition EBL

simulations of a UV laser diode operation have demonstrated a new way for

achieving high hole concentration in graded-composition III-nitride alloys,

which is quite promising for design of advanced optoelectronic devices

6 Experimental confirmation of the p-type DPD concept

Experiment: J. Simon et al., Science 327 (2010) 60

acceptable hole

mobility

[0001]

temperature independent hole density

7 Acceptor-free deep-UV LED structure

360 380 400 420 440 460 480

-7

-6

-5

-4

-3

-2

-1

0

1

2λλλλ = 280 nm

holes

Ene

rgy

(eV

)

Distance (nm)

electrons

j = 50 A/cm 2

1016

1017

1018

1019

1020

1021C

once

ntra

tion

(cm

-3)

0.4

0.6

0.8

1.0

injectorlayer

n-contactlayer

AlN

frac

tion

p-contactlayer

SQW

5.4 5.5 5.6 5.7 5.8 5.9 60

20

40

60

80

100

SQW width 2 nm 3 nm 4 nm

Cur

rent

den

sity

(A

/cm

2 )Forward bias (V)

1 10 100 1000

270

280

290

300

310

0.0

0.1

0.2

0.3

4 nm

3 nm

Pea

k w

avel

engt

h (n

m)

Current density (A/cm 2)

2 nm

SQW width 2 nm 3 nm 4 nm

IQE

LED characteristics depend strongly on the SQW width

p-type DPD

provides high hole

density in the

whole EBL/p-

contact layer

without using of

Mg acceptors !

Simulation by SiLENSe 5.1 (http://www.str-soft.com/products/SiLENSe)

[0001] →→→→

8 Feasibility of polarization-doped p-n junction

Experiment: S. Li et al., Phys. Stat. Solidi (c) 7/8 (2011) 2182

d = 100 nm d = 100 nm

diode characteristics

has been demonstrated by DPD p-n junction

carrier density depends on the

composition gradient in the AlGaN alloysDPD p-n junction structure

grown by PAMBE

9 Feasibility of polarization-doped UV LED structures

Experiment: S. D. Carnevale et al., Nanoletters 12 (2012) 915

deep-UV nanowire LED structures with 7 nm GaN SQW

and 110 nm DPD AlGaN claddings were grown be

PAMBE (with the nanowire diameter of ~25 nm)

both SQW and DPD-AlGaN cladding designs were

far from optimal ones

10 Low -resistance p-type Ohmic contact based on polarization doping

10-4 10-3 10-2 10-1 1001016

1017

1018

1019

1020

1021

modeling experiment :

n-type DPD p-type DPD

Car

rier

conc

entr

atio

n (c

m-3)

Composition gradient (nm -1)

graded AlGaN/GaN

desirableregion

10-4 10-3 10-2 10-1 10010-8

10-7

10-6

10-5

10-4

10-3

DPD AlGaN

Mg-doped GaN & 7%-AlGaN

Con

tact

res

ista

nce

(ΩΩ ΩΩ

·cm

2 )Composition gradient (nm -1)

desirableregion

annealed Ni/Au contacts to graded AlGaN alloys

with the Al content descending along the [0001]

direction from a certain value to zero

Experiment and model: S. Nikishin et al., Appl. Phys. Lett. 95 (2009) 164502100 150 200 250 300 350 400

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

Mg doped GaN DPD-AlGaN with

composition gradient of 5x10 -3 nm -1

Con

tact

res

ista

nce

(ΩΩ ΩΩ

·cm

2 )

Temperature (K)

11 Feasibility of Ohmic contacts formed to graded-composition AlGaN

Experiment: S. Li et al., Appl. Phys. Lett. 1018 (2012) 122103

n-contact

p-contact

100 nm graded-composition Al 0.1Ga0.9N

linear I-V characteristics is demonstrated for Ohmic contact to acceptor-free DPD AlGaN

diode characteristics of the DPD AlGaNp-n junction, impurity-doped GaN p-njunction, and GaN Schottky barrier

12Conclusions

simulations show that the use of distributed polarization doping (DPD) provides new opportunities for design of advance optoelectronic devices like LEDs, laser diodes, and solar cells

first experimental studies support the DPD concept in general but they do not utilize in full measure all its advantages, being carried out under conditions far from optimal ones

a prototype of deep-UV LED structure is proposed and examined theoretically; it is found to be quite promising for improving the p-type conductivity in the heterostructure and forming low-resistance p-contacts to the LED

implementation of the DPD concept in the device fabrication technology will require special approaches for precise control of the composition profile and strain relaxation in the device structures during their epitaxial growth