Doping of Wide Bandgap Semiconductors GaN

SiC

ZnO

Diamond

n-type dopant

Si on Ga site (~ 15 meV)

N on C site (~ 85 meV)

B on Zn (~30-60 meV)

N (~ 1.7 eV)

p-type dopant

Mg on Ga site

Al on Si site (~ 200 meV)

N on O site (~ 170-200 meV)

B (~ 370 meV)

(160 meV)

GaAs Si 6meV

Si P 45meV

C (Be) B ~28 meV 40meV

Zn ~ 340 meV n-conductivity*

~ 0.002 Ωcm

~ 0.01 Ωcm

~ 0.02 Ωcm

> 1000 Ωcm

p-conductivity*

0.2-2 Ωcm

0.5-2 Ωcm

0.5-40 Ωcm

10-100 Ωcm

* Experimental values

~ 0.001 Ωcm

Why difficult? • Dopant binding (or activation) energy ∝ m* • Maybe the ubiquitous H plays a big role?

Van Der Walle et al. Nature

Commonly Detected Deep Levels in High Quality uid n-GaN Grown by MOCVD and MBE (Steve Ringel, OSU) Defects: dislocation, stacking fault, VN, GaI, antisites, Impurities: C, O, H …, defect complexes: H-VGa, O-VN, H-MgGa … Deep Level Optical Spectroscopy (DLOS) and Deep Level Transient Spectroscopy (DLTS)

EC – ET

Ec •All concentrations reflect typical distributions seen for high quality UCSB material1,2

0.25 eV 0.60 eV 0.90 eV

•Individual concentrations can be significantly varied by growth conditions, impurities, etc.3

1.35 eV

•SI GaN has considerably different trap spectrum4

2.40-2.80 eV 3.04 eV 3.22 eV 3.28 eV

Ev

1013 1014 1015 Approx trap density (cm-3)

1. A. Hierro, et al., APL 76, 3064 (2000) 2. A. Hierro, et al., Phys. Stat.Sol. 228, 937 (2001) 3. A. Hierro, et al., APL 80, 805 (2002) 4. S.A. Ringel, et al., Proc. 2003 ISCS, in print.

Hydrogenic model for impurities Ionization of acceptor or donor impurities can be considered in the same fashion as the electron energy levels and radii are calculated for an isolated hydrogen atom. 4

2

∗

q Z m The impurity levels Edn are given by: Ec − Edn = 2 2 2n (4πε
) where n is a positive integer and Z is the number of unit charge of the ionized donor atom, i.e. Z = 2, for a doubly ionized donor.

 Z In terms of hydrogen ∆Edn = Ec − Edn = 13.6 atom ionization energies :  nε r 2  n The orbital radii of the r = 0.53 ε r  m  Å dn  Z  m∗  electrons are given by:  

  

2

 m∗     m  

Doping of semiconductors I For a semiconductor in thermal equilibrium, For flatband (charge neutral) condition,

np =

ni2

 − Eg = N c N v exp  kT

  

ρ (r ) ∆.E = = N d+ − N a− + p − n = 0 εε 0

For flatband (charge neutral) condition,

N a− 1 = N a 1 + g a exp Ea − E f

For flatband (charge neutral) condition,

N d+ 1 = N d 1 + g d exp E f − Ed kT

[(

[(

) kT ] ) ]

Here ga and gd are the acceptor and donor degeneracy factors. For GaAs: gd = 2, ga = 4 (heavy and light hole bands); For Si (six minima): gd = 12, ga = 4; For GaN: gd = 2, ga = 4

Doping of semiconductors II From the charge neutrality condition:

ρ (r ) ∆.E = = N d+ − N a− + p − n = 0 εε 0

We have,

Nd Na − 1 + g d exp E f − E d kT 1 + g a exp E a − E f

[(

 E f − Ec = N c exp   kT

) ]

[(

  Ev − E f  − N v exp    kT

) kT ]

  

The above equation is the generic equation governing the position of the Fermi level in a semiconductor at equilibrium. If the donor and acceptor concentrations and their activation energies are known, then the position of the Fermi level can be calculated. Note that if (Ef – Ed) and (Ef – Ea ) are >> 0, then impurities are fully ionized

p and n type doping of GaN • N-type doping for GaN is simple and similar to other common semiconductors – Si has Ea ~ 20 meV (mobility ~800 – 1000 cm2V-1s-1), Ge not so good for doping due to low incorporation

• P-type doping is complicated and quite different from other semiconductors – Mg has activation energy of ~0.2 eV (mobility few tens of cm2V-1s-1) – Mg forms complexes with hydrogen which has to be broken first, for Mg to act as acceptors – Mg-H complexes can be broken down by • Annealing in nitrogen atmosphere • Low energy electron beam irradiation (LEEBI); ~10KeV, 60 µA

Variation of resistivity with annealing temperature

Technological impact of doping problems • Fabrication of HBTs very difficult (poor base resistance) • Contact resistance high for LEDs and lasers (high series resistance, bad diode characteristics) • Other device structures involving p-type doping are not easy to fabricate • N-type doping of AlGaN with high Al composition is also difficult (activation energy increases with bandgap) • P-type doping for higher Al composition is even more difficult • InN samples are usually degenerately n-type doped. It is difficult to even make a mild p-type device

S.I. GaN… Growth GaN deposited on sapphire substrate typically shows n-type conductivity. SIMS of GaN film near sapphire interface: Oxygen thermally etched or diffused from the sapphire substrate

Residual impurities in precursors and gasses

• minimize O incorporation • compensation via C or dislocations

S.I. GaN… Fe doping Growth of GaN films grown on sapphire, first 0.3 µm Fe doped. Fe conc. (cm-3)

Sheet Resistance (W/sq.)

XRD FWHM (arcsec)

0

3×103

251/491

1.7×1018

2×105

-

1.3×1019

7×109

*

!

253/481

* XRD FWHM for (002) and (102) reflections

Highly insulating films of high structural quality. Sten Heikman et al., UCSB 2002

Crystal growth:n-type doping Ga(In)N: Si – shallow, [Si] < 1019 cm-3 because of SixNy formation and step

poisoning

AlxGa1-xN: Si forms DX center in at x > 0.5 + self-compensation through formation of VGa and VAl = triple acceptors

wide band gap!

C. Stampfl and C.G. Van de Walle, Appl. Phys. Lett. 72, 459 (1998)

1e+19

Example: on Al0.63Ga0.37N-on-sapphire base layers

n [cm-3]

0.3 µm thick AlxGa1-xN:Si films

1e+18

xAl= 0.2 xAl= 0.5

xAl= 0

xAl= 0.58 1e+17 1e+16

xAl= 0.6

0 1e-5

1e-4

Si/(Al+Ga)

1e-3

Crystal growth: p-type doping GaN:Mg ~ 160 - 230 meV deep, H-passivated

(p~ 1 x 1018 cm-3)

shallower in InxGa1-xN (~ 80 – 160 meV)!

GaN:Mg, Xing et al

InGaN:Mg K. Kamakura et al., J.Cryst. Growth 221 (2000) 267

problem: p-type doping of AlGaN with high Al mole fractions

Modulation Doping

Doping statistics

Kozodoy et al. 1999

Ion Implantation in GaN Manufacturability, reproducibility, flexibility and reliability But… •

Accurate dose control


Intrinsically low temperature process
Impurity Concentration profile and the structural changes can be tailored
Insensitivity to the lattice structure and defects, and the presence of impurities
Not constrained by the thermodynamics
Simplify the device processing


High resistivity to annealing.
Inability to epitaxially recrystallize the implantation induced amorphous layer by annealing
Complete recovery requires 1500oC annealing(2/3 of melting point), thus high N2 over 15Kbar N2 overpressure, and proper capping layers

Requirements for implantation doping by activation annealing
Remove the implantation-produced compensation defects and the lattice disorder
Electrically/optically activate implanted species by moving the interstitial dopants to substitutional sites via short-range diffusion.
No considerable redistribution of dopants after post-implantation annealing. Haijiang Yu, UCSB 2004

Thermodynamic analysis and the thermal processing Thermal Thermal Processing Processing Chamber Chamber 

Processing Temperature: ~1500oC



Pressure: 1500 psi (or ~100 bar) N2



2 inch wafer accommodation

Temperature 44% ~1500oC

RF Generator

Output power %

Graphite or Moly susceptor Thermocouple in alumina sheath

100 bar N2 1minute Sample surface temperature Thermocouple temperature

Phase Diagram

Expecting point 10 kbar needed

Our pressure

GaN stability at equilibrium

1500oC annealing with 100 bar N2:

- 100 bar N2  ~100oC

- Rapid processing

- 1500oC processing  >10 kbar N2

-Thermally stable capping layers (Davydov, et.al., phys.stat.sol.(a) 2001)

Rapid high pressure annealing of planar GaN •

GaN protected: – ~1500oC – ~Optimized – sputtered AlN – ~100 bar N2

~1300oC annealing no AlN cap ~100nm Sputtered AlN ~2um UID GaN Sapphire

Typical UCSB MOCVD GaN template (0002)~300 arcsec

(10 12) :~950 arcsec

Before annealing

~1500oC annealing 100 nm AlN cap

Ion implantation results 2

Rs (Ohm/square)

-3

Si concentration (cm )

10

15

100 keV, 5 X 10

-2

cm

1019 18

10

2

80 60

2

90 cm / V.s

70

40

60 2

80 cm / V.s

20

50

2

17

10

0

o

After 1500 C activation anneal As-implanted

16

10

90

95 cm / V.s

80

0

0.1

0.2

0.3

0.4

0.5

0

2

4

6

8

55 cm / V.s 10 12 14 16 15

Si activation (%)

Si implant in SI GaN at

20

100

105 cm / V.s

100

40

-2

Implanted Si dose (10 cm ) 0.6

Depth (µm) 7 After contact annealing

• Minimal dopant redistribution • Very low sheet resistance: ~ 20 ohm/square, normal doping: ~ 500 ohm/square • Contact resistance: 0.07 ohm mm for non-alloyed contacts 0.02 ohm mm for alloyed contacts normal doping: ~ 0.2-0.5 ohm mm Haijiang Yu et al. APL, 2004

Resistance ( Ω)

6

Before contact annealing

5 4 3 2 1 0

0

10

20

30

40

Distance (µ µm)

50

60

Polarization-induced doping

Debdeep Jena, 2001

Polarization doping Ga-face Al composition x increases

AlxGa1-xN

ρ = −∇ ⋅ P 3D

To maintain space charge neutrality, ns =ρ3D D. Jena et al. Phys Rev. B 67 153306 (2003)

• Interesting from theoretical standpoint to study such 3D charges which was found to have mobility limited by only alloy scattering at very low temperatures

Polarization-induced doping

• Wherever high conductivity bulk material is needed – - in MESFETs, JFETs, Regrown contacts, etc. Debdeep Jena, 2001

Alternative structure: Polarization-doped FET Graded AlXGa1-XN

Advantages over conventional MESFETs GaN

• Higher mobility: no impurity scattering • No carrier freeze-out • High Schottky barrier height • High breakdown field in AlGaN

Advantages over HEMTs • Ability to tailor gm curve for better linearity Charge = 1.6e18 cm-3 Mobility = 800 cm2/V.s

• Different grading schemes lead to different charge profiles

PolFET: Device Characteristics 100 Gain(dB), POUT(dBm)

Gain POUT PAE

24

80 60

20 16

40

12

20 0

4

8

12

16

20

Power-Added Efficiency (%)

28

PIN(dBm)

150µm x 0.7µm devices IDS = 850 mA/mm gm = 93 mS/mm fT = 19 GHz fMAX = 51 GHz

• Output Power = 4.3 W/mm @ 4GHz • PAE = 63% • Gain = 10 dB (VDS = 30V, IDS = 40 mA/mm)

• On sapphire Siddharth Rajan et al. 2004

Nitride MBE (Jim Speck, UCSB) rf plasma MBE growth diagrams • Basic considerations • Morphology control

Bulk transport and growth • n-GaN • p-GaN Polarization reversal

Alloys • AlGaN to AlN 2DEGs (AlGaN/GaN, AlN/GaN) • InGaN Wetting layer and purity issues Challenges: High In composition InGaN InN GaN, InN quantum dots Gen 930 at ND (Albert)

RF MBE GaN Growth Diagram Constant N* = 15.2 nm/min

Accumulation of Ga

Ga-flux (nm/min)

20

18

Ga-droplet Ga-stable

16

C

B

Intermediate Ga-stable

Steady-state Ga coverage

A 14

N-stable 12 550

600

650

Substrate Temperature (°C)

700

FGa < N*

Morphology: AFM Ga-droplet

Intermediate

5µm

Ga-flux (nm/min)

20

18

C

16

B

N-stable

A 14

12 550

600

650

Substrate Temperature (°C)

700

Structure: Cross-Section TEM

Ga-droplet

g(2110)

Intermediate

g(2110) {1013}

g(0110)

500nm

N-stable

Pits initiate at mixed and edge dislocations Decrease in pits Surface kinetics Ga-adlayer coverage

n-GaN: Transport Structure

Growth structure: Hall mobility measured on 30µm x 30µm vdP patterns

MBE-GaN film n-p-n+ isolation layers MOCVD-GaN template

1-2 µm thick NTD = 108 -1010cm-2

Sapphire substrate

Experiments: 1) Determine the effect of Dislocation Scattering by growing on templates with different dislocation densities 2) Determine the effect of growth regime by changing III/V ratio

Doped GaN: Transport Properties nH (cm-3)

nCV (cm-3)

784

2.0x1016

3.7x1016

858

2.9x1016

Doped with Si Ga-droplets Intermediate

15 0 550

1098

600 650 700 750 Substrate Temperature (°C)

800

Both decent n-type and p-type GaN achieved (GaN:Mg was grown under Ga crossover regime and Hall measurement results on the right) B. Heying et al. and I.P. Smorchkova et al.

2.9x1016

851 1.2x1016 semi-insulating

N-stable

2.3x1016

25

2

30

Hole mobility µ p (cm /Vs)

Ga-flux (nm/min)

45

Constant N-flux 3.3 nm/min

µH (cm2/Vs)

20 15 10 5

0

5 10 Hole concentration p (10

17

cm-3 )

15

Why MBE for Mg in GaN? Accurate p-n junction placement is possible

• Very sharp doping profiles

MOCVD

1020

1021

1018 10

Mg Conc [cm-3]

20

10

Concentration (atoms/cc)

1019

~ 9 nm/dec

~ 2.5 nm/dec

3

Mg concentration (atoms/cm )

MBE

1019 18

10

17

10

17

Si Conc [cm-3] 16

10

10

230 nm 400 nm

16

0

100

200 300 400 Depth (nm)

500

600

15

10

0

0.5

1 Depth (µm)

1.5

Growth Transition Series (constant Mg flux): Morphology

5 µm

Increasing Ga flux

Mg Growth Transition Increasing Ga flux

5 µm

‘Crossover’ p = 4.8x1017 cm-3 µ = 13 cm2/Vs

‘Droplet’ p = 5.7x1017 cm-3 µ = 11 cm2/Vs

‘Droplet’ p = 5.4x1017 cm-3 µ = 11 cm2/Vs

Intermediate or N-rich growth: insulating Optimal properties at the crossover!

Polarity Inversion: Sample Structures Single Mg-doped layers

Multiple Mg-doped layers

Ga-stable vs. N-stable

Ga-stable vs. N-stable

0.4 µm Mg-GaN Mg BEP = 1x10-8 torr (for Ga-rich: p = 8-9 x 1017)

120 nm Mg-GaN, Mg BEP = 1.8x10-8 torr 120 nm UID MBE GaN 120 nm Mg-GaN, Mg BEP = 1x10-8 torr 120 nm UID MBE GaN

30 nm UID MBE GaN

120 nm Mg-GaN, Mg BEP = 7.8x10-9 torr 120 nm UID MBE GaN

1 – 2 µm MOCVD GaN

120 nm Mg-GaN, Mg BEP = 3.5x10-9 torr 30 nm UID MBE GaN

Tgrowth = 650ºC

1 – 2 µm MOCVD GaN

N-Rich Single Mg-Doped Layers: Complete Polarity Inversion CBED

X-Section TEM

MBE

MBE MOCVD

MOCVD

0.2 µm

Ga-Rich Single Mg-Doped Layers: ‘Spike-Shaped’ Inversion Domains ID Formation TDs Isolated

Plan-View

Density ~109 cm-2 100 nm

MBE MOCVD

Cross-Section

400 nm

Multilayer Structures: N-rich 120 nm Mg-GaN, Mg BEP = 1.8x10-8 torr 120 nm UID MBE GaN 120 nm Mg-GaN, Mg BEP = 1x10-8 torr 120 nm UID MBE GaN 120 nm Mg-GaN, Mg BEP = 7.8x10-9 torr 120 nm UID MBE GaN 120 nm Mg-GaN, Mg BEP = 3.5x10-9 torr 30 nm UID MBE GaN

1 – 2 µm MOCVD GaN

MBE layers are fully inverted

Multilayer Structures: Ga-rich 120 nm Mg-GaN, Mg BEP = 1.8x10-8 torr 120 nm UID MBE GaN 120 nm Mg-GaN, Mg BEP = 1x10-8 torr 120 nm UID MBE GaN 120 nm Mg-GaN, Mg BEP = 7.8x10-9 torr 120 nm UID MBE GaN 120 nm Mg-GaN, Mg BEP = 3.5x10-9 torr 30 nm UID MBE GaN

1 – 2 µm MOCVD GaN

Inversion begins at first Mg layer

Model for Polarity Inversion: Growth on Dry Surfaces Ga wetting layer development 300 Å

GaN WL

ID nucleation on dry regions

630 Å c+ GaN

Growth c- Mg-GaN

1250 Å

2500 Å

Ga wetting layer develops gradually (Solution: pre-wet surface!)

c+ Mg-GaN

c+ GaN (no Mg)

Alloy Growth: AlGaN Idea: N-flux Ga-flux Al-flux

Excess Ga

• Full range of Al composition available

Al Incorporated Ga 0

0.1

0.2

0.3

0.4

• Composition is controlled with the group III component with the highest sticking coefficient (Al)

0.5

Beam Equiv. Press.(µtorr)

• III/V > 1 • Al comp. = 10 %

0.6

AlGaN/GaN III/V Study All Samples: Al0.12Ga0.88/GaN Ga Rich No Ga Droplets Spirals 77Κ µ= µ=11,200 cm2/Vs Ga Rich Ga Droplets Spirals 77Κ µ= µ=11,200 cm2/Vs

Ga-flux (nm/min)

45 No droplets Ga stable

30 15 0 550

Ga Rich No Ga Droplets AlGaN Spheres 4nm x 150nm 77Κ µ=8000 cm2/Vs

Ga droplets Ga stable

N stable 600 650 700 750 800 Substrate Temperature (°C)

N-flux 3.3 nm/min

Ga Rich No Ga Droplets AlGaN Spheres 5nm x 120nm 77Κ µ=3000 cm2/Vs

Transport Properties of Al.09Ga.91N/GaN Heterostructure

50000

30000 20000 Previous Record

10000

12

nsh 2.2 X 1012cm-2

0 10

1.25 1.20

Rxx/Rxx(B=0)

13

40000

10

4.2 K

60000

Magnetoresistance

10

Shubnikov-de Haas Oscillations

Al.09Ga.91N/GaN Tg = 750°C

Mobility (cm2/Vs)

Sheet Carriers (cm-2)

Temperature Dependent Hall Properties

1.15 1.10 1.05 1.00 0.95

100

Temperature (K)

0.90 0

MBE-AlGaN MBE-GaN Spacer

1

2

3

4

B(T)

MOCVD-Template

nSdH = 2e/(hD(1/B))

ref

5

6

AlN/GaN Growth of AlN under Ga wetting layer

AlN GaN

37 Å AlN

49 Å AlN

77K

300K 100 Å AlN

I.P. Smorchkova et al., JAP 90, 5196 (2001)

Alloy Growth: InGaN Growth Idea:

Example: InGaN Growth

N-flux Ga-flux In-flux

Excess In

• Composition is controlled with the group III component with the highest sticking coefficient (Al>Ga>In) • Full range of In composition available

Ga Incorporated In Beam Equiv. Pressure

C. Poblenz et al. 2003

• III/V > 1 for optimal morphology and properites

In Flux Series: Morphology and Transport

12.9% In, droplets µ = 110 cm2/Vs n = 1.5x 1018 cm-3

Excess In (nm/min)

600ºC 1.1x10-7 Ga BEP Variable In flux

60 40

In droplets In stable

20 0 400

N stable

• • • •

No droplets In stable

N-flux 2.3 nm/min

5.3% In, no droplets µ = 134 cm2/Vs n = 7.0x 1017 cm-3

500 600 Substrate Temperature (°C)

Structure: 0.5 µm InGaN on npn isolation structure 11% In, no droplets µ = 87 cm2/Vs n = 1.56x 1018 cm-3

Hall: 30 µm x 30 µm Greek Cross

0% In, no droplets

All AFM: 5 µm x 5 µm

Towards 2DEGs: AlGaN/InGaN 1.00E+14

Nsh

Al0.12Ga0.88N/In0.06Ga0.94N µ (77K) = 500 cm2/Vs InGaN/AlGaN AlGaN/GaN

1.00E+13

Al0.09Ga0.91N/GaN µ (77K) = 18,470 cm2/Vs 1.00E+12 0.001

0.01

0.1

1/T(K)

Low temperature freeze-out not observed in InGaN

Impurity Incorporation in MBE-In0.17Ga0.83N: SIMS Ga B.E.P.

N

N

In

In

5

5

10

150 nm

4

10

Oxygen 3

10

Indium 200

GaN 750°C

4

Oxygen 3

10

Indium

Boron

2

10

0

150 nm

10

Boron 2

InGaN 600°C

GaN 600°C

MOCVD GaN

Indium Desorption

Counts per second

Counts per second

10

GaN 750°C

InGaN 600°C

GaN 600°C

No Indium Desorption

MOCVD GaN

B.E.P.

Ga

400

600

Depth (a.u.)

800

1000

10

0

200

400

600

Depth (a.u.)

800

1000

Sensitivity of Ga Wetting Layer to Excess In Desorption of excess metal

Ga-rich GaN growth at 600ºC No desorption

Counts per second

1 .4 x 1 0

4

1 .2 x 1 0

4

1 .0 x 1 0

4

8 .0 x 1 0

3

6 .0 x 1 0

3

4 .0 x 1 0

3

2 .0 x 1 0

3

MOCVD GaN

GaN

GaN (In)

GaN

GaN (In)

GaN

150 nm

AlGaN markers

(In) indicates excess In present during growth

Oxygen Aluminum Boron

0 .0 0

500

1000

D e p th ( a .u .)

1500

2000

Structure of Ga and In Wetting Layers Ga-rich growth

In-rich growth

Laterally contracted Ga bilayer

In wetting layer

Why such high UID in InGaN? Difference in wetting layer (Ga bilayer vs In ‘monolayer’)! J.E. Northrup, J. Neugebauer, R.M. Feenstra, and A.R. Smith, Phys. Rev. B 61, 9932 (2000) H. Chen, R.M. Feenstra, J.E. Northrup, T. Zywietz, J. Neugebauer, and D.W. Greve, J. Vac. Sci. Tech. B 18, 2284 (2000)

Effect of Ga Flux on InGaN Composition and Growth Rate 0.18 0.16

%In by XRAY

0.14 0.12

0.14

0.1

0.08 0.06

0.07

0.04

Growth Rate (µ µm/hr)

In (BEP) = 4.05 x 10-7 Torr In Droplet Regime, 600°C

Maximum of 17% In incorporation at 600°C

0.02 0 0.00E+00

2.00E-08

4.00E-08

6.00E-08

8.00E-08

1.00E-07

1.20E-07

Ga Flux (B.E.P.)

FGa = 0

FGa > 0

Complete In wetting layer: ‘monolayer’ structure

Ga + In wetting layer: Complete Ga wetting layer: structure unclear fluid-like bilayer

Origin of Impurities Source

Impurity Possibility Reason ?

Outgassing of Ga, In, or N source

O, B

NO

SIMS

Background pressure in the system

O

NO

System base pressure is in mid 10-11 Torr range

O, B

NO

TEM

N source gas

O

YES

Low cracking efficiency of plasma source

N source

B

YES

Plasma could sputter crucible during growth

Reduced growth rate in the InGaN layer

~1% N source efficiency: 1018 UID = 1 ppm oxygen from total N source