1. 1 Tyndall National Institute, Lee Maltings, Dyke Parade, Cork, Ireland
Matthew Smith
The effect of a varied NH3 flux in MOVPE growth of AlN interlayers for InAlN/GaN HEMTs on miscut sapphire substrates
MD Smith1,2,TC Sadler1, VZ Zubialevich1, HN Li1,2
PJ Parbrook1,2
1 Tyndall National Institute, Lee Maltings, Dyke Parade, Cork, Ireland
2 Electrical & Electronic Engineering Department, University College Cork, Ireland

2. Introduction
Scope of the talk:
InAlN HEMTs for space applications
AlN interlayers: Why? How effective?
MOVPE growth series: sapphire, V/III ratio, cool down phase
AFM analysis
Electrical measurement results
Conclusions

3. InAlN/GaN HEMTs for space
AlGaN/GaN HEMTs offer good power densities and high operating frequencies. High temperature/radiation dose failure mechanisms linked to relaxation of strain at the heterojunction...
Lattice matched InAlN/GaN can generate 2DEG densities of > 1013 cm-2 without any piezoelectric polarization:
No in-plane strain allows performance to be maintained > 800 oC in harsh radiation environments.
High AlN content provides good thermo-chemical stability at the surface.
Medjdoub et al, Open Electrical & Electronic Engineering Journal 2 (2008) 1-7

4. AlN interlayers
Provides an additional energetic barrier between the channel layer and the alloyed barrier layer.
Further confines the 2DEG wave function to the binary GaN layer, suppressing alloy disorder scattering
10 nm-InAlN ~ 1 μm-GaN
10 nm-InAlN nm-AlN ~ 1 μm-GaN
Channel electron mobility improves with AlN thickness to ~ 1 nm:
If the layer is too thin its effects aren’t as pronounced
Too thick and interfacial rougheness scattering dominates.
Medjdoub et al, Open Electrical & Electronic Engineering Journal 2 (2008) 1-7

5. Wafer preperation
Want to visualise AlN/InAlN interface by imaging 1 nm AlN layers, and compare roughening features with the GaN channel for different growth conditions.
Full structure:
Samples grown via MOVPE on c-plane sapphire substrates miscut by either 0.1o or 0.4o toward the m-plane, with a high temperture AlN stage beneath the GaN buffer.

6. MOVPE of 1 nm-AlN
AlN only 1 nm ~ 4 monolayers, so have low Al(CH3)3 flux (5.2 μmol/min) to control growth rate and use NH3 flux to vary V/III ratio.
Cool down stage critical: need to consider increasing decomposition rate of exposed GaN under low pressure & high temperature conditions when preparing for room temperature wafer extraction.
V/III ratio: molar flow rate of NH3 divided by combined metal-organic flow rates
‘Cool down A’ employs the NH3 flow rate that would be used for InAlN barrier layer growth, with an additional temperature stabilization stage.
‘Cool down B’ maintains the NH3 flow rate used in the 1 nm-AlN layer.

7. 1 nm-AlN growth series
Grow 1 nm-AlN on 1-µm GaN and compare AFM surfaces for different growth conditions:
Sample ID
Substrate miscut
1 nm-AlN NH3 flow (mmol/min)
1 nm-AlN V/III ratio
Cool down
NH3 flow (mmol/min)
Temperature
NB-1
0.4o
2.6
500 B
Direct
NB-2 13
2500 “
NA-1
0.52
100 A 56
Models HEMT growth
NA-2
“ “ “
NA-3
130
25000
NA-4
0.1o
NA-5
NA-6

8. GaN channel morphology
View surfaces using tapping-mode AFM, done immediately after growth to suppress environmental contamination
University of Cambridge DoITPoMS website
Fareed et al., J. Cryst. Growth pg 348
GaN exhibits step flow growth, i.e. the step width is generally conserved, characteristic of the moderate surface diffusion length of GaN adatoms.
Threading dislocation density of ~ 109 cm-2, manifesting as nm-scale pits at step edges (edge + mixed) and terminations (screw + mixed).
rms surface roughness ~ 0.8 ± 0.3 nm

9. 1 nm-AlN AFM analysis NB-2 NB-1
Cool down B: ~200 nm diameter pits extending > 30 nm down into the GaN channel.
Pits of such size catastrophic to performance
V/III = 500; PD ~ 12 μm-2 . V/III = 2500 PD = ~3 μm-2
More ammonia present in reactor  less pitting
Pits etched when ammonia level below critical level AND temperature above critical level (~800 °C)
Cool down A: simulate route to InAlN deposition (same high ammonia flow) but cools fast at point InAlN barrier layer growth would start.
NB-1
NB-2

10. 1 nm-AlN AFM analysis (2) 500 25000 V/III ratio AFM surface: Comment
0.1o miscut substrate
0.4o miscut substrate
500
Can see some departure from GaN morphology, but ~1 μm-2 pits remain.
25000
Much less pitting suggests underlying GaN well protected, but loss in terrace regularity and directionality
NA-2
NA-2
NA-5
5 nm
NA-2
5 nm
500nm
0 nm
rms : ~0.7 nm
rms : ~1.1 nm
0nm
NA-6
5 nm
NA-3
5 nm
0 nm
rms : ~0.9 nm
0 nm
rms : ~0.8 nm

11. Electrical performance
For the samples grown on 0.1o miscut sapphire, clear improvement in dc performance with 1 nm-AlN V/III ratio up to 25000, as underlying GaN is better protected from decomposition during cool down.
For the samples grown on 0.4o miscut sapphire there is a similar improvement until V/III ratio reaches some critical point, where the more pronounced stepping features lead to roughness scattering degrading conductivity.
Neither samples showed any significant improvement in 2DEG density.

12. Conclusions
Like thickness, AlN interlayer V/III ratio is an important growth parameter and can be optimised for device performance.
Accurate visualisation of 1 nm AlN layers requires consideration of GaN decomposition during cool down phase.
Need to consider the effect of the substrate miscut when selecting AlN interlayer growth conditions to balance GaN channel decomposition with interfacial rougheness scattering.

13. Acknowledgements