Growth of Inrich InGaNGaN SQW

1
Growth of In-rich InGaN/GaN multi-quantum well
structures by metalorganic chemical vapor
deposition and their optical properties
Soon-Yong Kwon, Seong-Il Baik, Hee Jin Kim,
Young-Woon Kim, Jung-Won Yoon, Hyeonsik M.
Cheong, Yoon Soo Park, and Euijoon
Yoon School of Materials Science and
Engineering, Seoul National University Seoul
151-742, Korea Dept. of Physics, Sogang
Univeristy, Seoul 121-742, Korea School of
Physics, Seoul National University, Seoul
151-742, Korea
2
Motivation

• InN or In-rich InGaN on GaN
• Large lattice mismatch
• Highly defective epilayers
• Defect control
• Dislocation Reduction
• Growth interruption before GaN capping

3
Growth procedure of In-rich InGaN/GaN SQW

• Low-pressure MOCVD
• GaN growth on sapphire 1080 oC, 2 mm
• In-rich InGaN growth 730 oC, 90 sec
• TMI flow rate 305 sccm, NH3
  flow rate 4 slm
• Only TMIn and ammonia were
  supplied.
• Growth interruption (GI) 730 oC, 0 30 sec
• GaN capping layer 730 oC, 20 nm

4
Growth of In-rich InGaN/GaN SQW

• TMI flow rate 305 sccm, NH3 flow 4slm, InGaN
  growth time 90 sec, InGaN growth temp. 730oC

• Thickness fluctuations in the 2.5 nm QW layer
• Many structural defects in QW layer and LT-capped
  GaN layer

S. Y. Kwon et al., phys. stat. sol. (c) 0, 2830
(2003) H. J. Kim et al., phys. stat. sol. (c) 0,
2834 (2003)
ICNS-5, Nara, 2003
5
Introduction of GI in In-rich InGaN/GaN SQW

• The InGaN QW layer thickness is about 1 nm.
• The InGaN/GaN interface became very flat with 10
  sec GI.
• Low temperature grown GaN capping layer has much
  less defects.

S. Y. Kwon et al., phys. stat. sol. (c) 0, 2830
(2003)
6
Effect of GI on optical properties
12K PL

• As the growth interruption (GI) time increased,
  the PL emission efficiency from InGaN layer
  improved with peak position blue-shifted.

S. Y. Kwon et al., phys. stat. sol. (c) 0, 2830
(2003)
7
Medium Ion Energy Scattering (MEIS)
S. Y. Kwon et al., phys. stat. sol. (a) accepted
for publication

• There exists line broadening in indium peak.
• From the simulation using the SIMPLE program,
  the 0.43-nm-thick InGaN
• layer was In-rich and it has 6070 indium
  contents. There was about 10
• (0.12-nm-thick InGaN) and 30 (0.25-nm-thick
  InGaN) indium intermixing at
• top and bottom InGaN/GaN interfaces,
  respectively.

8
Surface of LT GaN-capped In-rich InGaN

• By introducing GI time, the dislocation density
  was lowered by one order of magnitude and LT-GaN
  surface shows a spiral growth mode.
• InGaN layer grown with GI would have a smooth
  surface, which is similar to that grown on the
  HT-grown GaN layer with flat surface.

S. Y. Kwon et al., phys. stat. sol. (c) 0, 2830
(2003)
9
Strain Relaxation during InN Growth on GaN
Y. F. Ng et al., Appl. Phys. Lett. 81, 3960 (2002)
PA-MBE grown InN layer

• 2D growth condition
• High substrate temp. (gt420oC)
• high In flux (In/N gt1)
• GaN growth temp. 600650oC

• According to Matthews and Blakeslees formula,
  the critical thickness for dislocation formation
  is less than 1 ML.
• The strain in the epitaxial InN is initially
  relieved by defects (dislocations) rather than by
  surface islanding.
• For 2D growth, about 80 of the total strain is
  relieved within the first 2MLs while the
  relaxation of the remaining strain is at a very
  slow rate.

10
Effect of InN Growth Time on Defect Density
2 mm x 2 mm
S. Y. Kwon et al., J. Appl. Phys., submitted
11
Stacking of InGaN/GaN MQW with different GI times

• InGaN QW layer already flattened and thinned to
  1 nm at 3 sec GI and
• its thickness was nearly unchanged to 10 sec GI.
• - Severe decomposition in In-rich InGaN layer
  due to relatively high growth
• temperature
• - More drastic decomposition at regions of
  swelling surface during GI
• - Better thermal stability of InGaN near
  interface due to stronger bond strength

12
Influence of interfacial roughness

• In MQW A B, we observed four strong PL peaks
• corresponding to four InGaN layers with
  different GI.
• In MQW B, the emission wavelengths originated
  from
• four InGaN layers were well-fitted with the
  results of
• SQWs, however, in MQW A, that of InGaN layer
  with
• 3 sec GI is quite different from the result of
  SQW.
• In MQW A, the first InGaN/GaN QW layer would be
• quite rough and defective so that the second
  InGaN
• layer with 3 sec GI would be influenced by first
  QW
• layer and its original character diminished.

13
Improvement of InGaN/GaN QW layer quality

• The crystalline quality of InGaN layer was
  greatly improved after 5 sec
• GI in MQW B, which is well-fitted with the
  results of SQWs.
• The introduction of 10 sec GI was very effective
  to improve the crystalline quality of In-rich
  InGaN/GaN QW layer.

14
Growth of In-rich InGaN/GaN MQWs using 10 sec GI

• We grew 10 periods of In-rich
• InGaN/GaN MQW structure using
• 10 sec GI.
• Atomically flat 1-nm-thick InGaN
• QW layers were well grown.

HRTEM images of MQW C
15
Near-UV emission from In-rich InGaN/GaN MQW
He-Cd laser (325 nm), excitation power 0.65 mW

• We obtained strong near-UV emission from MQW C
  at room temperature.
• Use of very thin In-rich InGaN/GaN MQWs can be a
  new candidate
• for near-UV source.
• However, optimization of number of QW layers is
  needed.

16
Time-resolved PL of MQW C at 10 K
Ti sapphire laser related
Tisapphire laser (367 nm), excitation power 2
mW
In collaboration with Prof. D. Lee K. J.
Lee, Chungnam Natl University

• From TR-PL measurement, the PL decay time was
  1.75 ns in 1 nm InGaN
• MQW.
• For comparison, we measured the PL decay time of
  thick InGaN MQW,
• however, it passed the limit indicating much
  larger PL decay time.

17
Excitation power dependent PL
TR-PL at 10 K
PL spectra at 10 K

• For MQW C, the PL peak energy and PL decay times
  are almost constant against excitation intensity.
• In the 1 nm QW, electrons and holes are strongly
  confined, leading to
• a large overlap between electron and hole wave
  functions. This results in
• a constant PL energy and a fast PL decay,
  independent of excitation intensity.

18
Temp. Dependent PL of In-rich InGaN/GaN MQWs
10 InGaN(1nm)/GaN(2nm) MQW
He-Cd laser (325 nm), excitation power 0.2 mW

• About 24 meV red-shift of InGaN QW peak from
  10K to 300K
• - No S shape dependence of PL peak position
• - No W shape dependence of FHWM value
• No QD-like features from optical properties

19
Introduction of two-step growth method in active
InGaN QW layer

• Two-step growth method in active InGaN QW layer
  was introduced
• to increase In-rich InGaN QW layer thickness
  and/or local In composition
• within In-rich InGaN QW layer.

20
Growth of In-rich InGaN/GaN MQW using two-step
growth method
GaN
MQW D
sapphire

• We grew 4 periods of In-rich InGaN
• /GaN MQW using two-step growth
• method.
• No increase in InGaN QW thickness
• No thickness fluctuations in InGaN
• QW

HRTEM images of MQW D
21
Strong near-UV and blue emissions at RT

• Strong near-UV (400 nm) and blue emissions (450
  nm) at RT were observed
• from In-rich InGaN/GaN MQW using two-step growth
  method.
• PL efficiency of 450 nm peak is much higher than
  that of 400 nm peak,
• indicating better carrier localization of 450 nm
  peak.

22
Summary

• In-rich InGaN/GaN MQWs were successfully grown by
  MOCVD by introducing growth interruption before
  GaN capping, resulting in strong room temperature
  PL.
• Time-resolved PL shows that it has 1.75 ns
  lifetime at 10K.
• Temperature and power-dependent PL measurement
  suggests that the thin, In-rich InGaN/GaN MQWs
  are a good candidate for the active layers of
  near-UV light sources.