Short-wavelength light beam in situ monitoring growth of InGaN/GaN green LEDs by MOCVD
Nanoscale Research Letters
Short-wavelength light beam in situ monitoring growth of InGaN/GaN green LEDs by MOCVD
Xiaojuan Sun 0 1
Dabing Li 0
Hang Song 0
Yiren Chen 0 1
Hong Jiang 0
Guoqing Miao 0
Zhiming Li 0
0 State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences , 3888 Dongnanhu Road, Changchun, 130033 , Peoples' Republic of China
1 Graduate University of the Chinese Academy of Sciences , Beijing, 100039 , Peoples' Republic of China
In this paper, five-period InGaN/GaN multiple quantum well green light-emitting diodes (LEDs) were grown by metal organic chemical vapor deposition with 405-nm light beam in situ monitoring system. Based on the signal of 405-nm in situ monitoring system, the related information of growth rate, indium composition and interfacial quality of each InGaN/GaN QW were obtained, and thus, the growth conditions and structural parameters were optimized to grow high-quality InGaN/GaN green LED structure. Finally, a green LED with a wavelength of 509 nm was fabricated under the optimal parameters, which was also proved by ex situ characterization such as highresolution X-ray diffraction, photoluminescence, and electroluminescence. The results demonstrated that shortwavelength in situ monitoring system was a quick and non-destroyed tool to provide the growth information on InGaN/GaN, which would accelerate the research and development of GaN-based green LEDs.
InGaN/GaN; Green LED; MOCVD; in situ monitoring
The anticipated high commercial demand for solid-state
light and high quality outdoor display applications has
significantly accelerated the development of green
lightemitting diodes (LEDs). However, the efficiency of green
LEDs is still far away from the expectation due to the
challenges of high-quality growth of InGaN/GaN
multiple quantum wells (MQWs) [1-8]. Compared to the
blue LEDs, growing InGaN/GaN MQWs is more
complex and difficult since more indium content is required
in the active layer for green emissions and relatively
lower growth temperature. Moreover, due to the low
miscibility of InN in GaN, high volatility of InN and the
low thermal decompositional efficiency of ammonia
(NH3) at low temperature, indium separation, and
roughness interface usually exist in high In-content
InGaN/GaN MQWs [9,10]. Furthermore, the defects
and compressive strain in the InGaN well further
decrease the optical transformation of InGaN/GaN MQW
LEDs, especially for green LED [11-15]. Besides the
growth challenge, the lack of direct in situ monitoring
system for InGaN/GaN MQWs' growth is another
important factor to hamper obtaining high-quality,
highIn-content InGaN/GaN MQWs. Generally, the InGaN/
GaN MQW LEDs were characterized by ex situ tools, e.
g., high-resolution X-ray diffraction (HR-XRD),
transmission electron microscopy, and scanning electron
microscopy, to evaluate their structural and interfacial
qualities, which are not powerful tools for production
because they are time-consuming and are destroyed.
Therefore, the in situ monitoring system is required to
monitor the whole growth process and provide the
related information on growth rate and interfacial
The traditional reflectometers with 950 and/or 633 nm
usually are not very helpful to monitor InGaN/GaN
MQWs' growth due to no obvious intensity modulation
of the reflected light. According to Fabry-Perot
oscillation, the phase and the amplitude of oscillations depend
on the wavelength of the incident light as well as the
thickness of the growing layer and optical constants of
the materials. Also, the maximum reflectance occurs if
the multiplication of the refractive index and layer
thickness is equal to the even number of the half incident
light wavelength. Thus, for InGaN/GaN MQWs' growth,
phase difference and constructive or destructive
interference hardly occur when the incoming light is 950 or
633 nm due to both the low transparent InGaN and thin
InGaN/GaN MQWs' layer. Instead, the in situ
monitoring system with short wavelength can show an intensity
modulation, related to interference effects.
In this paper, five-period InGaN/GaN MQW green
LEDs were grown by metal organic chemical vapor
deposition (MOCVD) with 405-nm light beam in situ
monitoring system. With the direct and precise
monitoring system, the optimized growth condition was
obtained easily and high-quality green LEDs with
509 nm were fabricated.
The InGaN/GaN MQW green LEDs were grown on
cplane sapphire substrates by MOCVD with 405-nm light
beam in situ monitoring system. Trimethylgallium/
triethylgallium (TMGa/TEGa), trimethylindium (TMIn),
and NH3 were the precursors of gallium, indium, and
nitrogen, respectively. Silane and biscyclopentadienyl
magnesium were used as the n- and p-type dopants,
respectively. The growth process was as follows: firstly,
the substrate was thermally cleaned at 1,050°C for
10 min before a 25-nm-thick low-temperature (LT)-GaN
buffer layer was deposited at 500°C. After the growth of
LT-GaN buffer layer, the growth temperature was
ramped to 1,010°C, and a 2-μm-thick undoped GaN and
1-μm-thick Si-doped GaN epilayers were grown in
sequence. Then, the growth temperature was decreased to
grow InGaN/GaN MQWs. For the InGaN/GaN MQWs'
growth, a modulated temperature growth mode was
employed, that is, the growth temperature for GaN
barrier and InGaN well layers was different. Here, the
growth temperature for barrier and well layers were 780°
C and 650°C, respectively. Finally, a p-GaN layer was
deposited. The schematic InGaN/GaN green LED
structure was shown in Figure 1.
In order to obtain high-quality InGaN/GaN MQWs,
the parameters for InGaN/GaN growth were optimized
based on the in situ monitoring curve. For in situ
monitoring system, the incident light is partly reflected from
the epilayer surface; another partly penetrates into the
epilayer and is reflected at the interface between the
epilayer and substrate and then travels back to the epilayer
surface leading to the intensity modulation of the reflected
light. The maximum reflectance occurs when the path
difference is equal to the even number of half wavelength:
where n is the refractive index; d, thickness of epilayer; m,
even number; and λ, incident light wavelength.
Considering the relationship between the thickness of epilayer and
the growth rate:
where r is the growth rate of the epilayer, and t is the
growth time. Then, the growth rate can be estimated from
the reflectance of in situ monitoring system:
Furthermore, the amplitude of the reflectance
increases with the increase of the In content, and the
intensity of the reflectance damps more and more with the
interface of GaN and InGaN becoming rougher and
rougher. Thus, the information of In composition and
interface morphology of InGaN/GaN MQWs can be
obtained from the reflectance of in situ monitoring
To activate the Mg-doped GaN, the samples were
annealed by rapid thermal annealing at 750°C for
10 min. HR-XRD and photoluminescence (PL) were
employed to characterize structural and optical
properties of InGaN/GaN MQWs.
Figure 1 Schematic structure of InGaN/GaN MQW LEDs.
Figure 2 Reflectance traces of 950 and 405 nm and true
temperature transients for InGaN/GaN MQW LEDs.
Results and discussion
Figure 2 shows the reflectance traces for 950 and
405 nm and true temperature transients for typical
InGaN/GaN MQW LEDs' growth. As can be seen, the
950-nm light beam in situ monitoring system can still be
used for GaN growth analysis, while it becomes
unpowerful for InGaN/GaN MQWs evolution. However, the
405-nm signal is sensitive to InGaN/GaN quantum wells
and independent on the GaN underlayer due to the GaN
layers absorbing the light at 405 nm and causing no
further oscillations. The relationship between the bandgap
and temperature is as follows :
Since the growth temperature for the GaN underlayer is
1,010°C, the bandgap of GaN is 2.797 eV. Then, the
absorbing wavelength is correspondingly 443 nm. Thus,
the bandgap of GaN at 1,010°C is narrow enough to
absorb the light with a wavelength of 405 nm. With
increasing the layer thickness, the intensity maxima and minima
approach the constant value of reflectance characteristic
for the layer surface, as shown in Figure 2. Then, the
growth of InGaN quantum wells on top of the GaN buffer
can be studied in detail. Obviously, differently from the
950-nm reflectance, the 405-nm data taken during buffer
layer growth and MQWs growth are no longer correlated,
which indicate that small deviations in the GaN growth
rate do not limit the quantitative analysis of the 405-nm
data measured during the MQWs growth.
It can also be seen that for the 405-nm data, the
quantum wells and barriers are distinguished due to the
refractive index less sensitive to temperature variations
at 405 nm than at 633 nm. Thus, the information of
wells and barriers' growth rate, thickness, and interface
roughness can be obtained by fitting or comparing the
405-light beam in situ monitoring curves, which is the
direct evidence for optimizing InGaN/GaN MQWs'
growing conditions and investigating the complex
evolution of InGaN/GaN MQWs. Furthermore, the amplitude
of the 405-nm reflectance oscillation enhances with the
increased of indium content, resulting from the changed
of refractive index n. Besides, it is known that the strain
effect between GaN barrier layer and InGaN well layer
can also cause the change of refractive index. Therefore,
the InGaN composition as well as the strain effect can
be derived from the amplitude of Febry-Perot oscillation
in the measured 405-nm reflectance transient.
Figure 3 displays two typical reflectance traces of
InGaN/GaN MQW green LEDs. As can be seen, all the
wells and barriers are clearly distinguished since the
405-nm in situ reflectance signal is not sensitive to
Figure 3 Reflectance traces of InGaN/GaN MQW green LEDs'
growth monitored by 405-nm in situ system.
temperature changes between wells and barriers' growth.
The blue curve shows the data for a nonideal InGaN/
GaN MQWs' growth (sample A). It displays the
reflectance oscillation damping, indicating the interface
between wells and barriers becoming rougher and rougher.
As known, the growth of InGaN/GaN is very complex
and sensitive to the growth condition, especially the
growth temperature. Due to the high volatility of InN,
the high In composition of InGaN/GaN MQWs should
be grown at low temperature. However, the lower
growth temperature results in a poor crystalline quality
of GaN barrier attributed to the low surface mobility of
adatoms in the low growth temperature as well as the
increase of nitrogen vacancy due to the low cracking
efficiency of ammonia.
As mentioned above, variations of the indium content
as well as of the morphology of the GaN and InGaN
layers influence the 405-nm reflectance transients during
MQWs' growth. In addition, the InGaN growth rate can
also modulate the oscillation character of 405-nm
reflectance signal due to the change of interference beam
path difference. Thus, the growth information can be
directly derived from the 405-nm light beam in situ
reflectance traces. For high In-content InGaN/GaN
MQWs, an extremely high V/III ratio is needed to
conquer the nitrogen deficiency on the growing surface
. However, an optimized flow rate of TMGa and
TMIn is also required to obtain high In composition.
Using 405-nm short-wavelength light beam in situ
monitoring system, the optimized source flow rate can be
easily obtained. Furthermore, the 405-nm signal gives the
direct evidence that rather than the In flow rate, the
growth temperature influences the In content of InGaN
well layer deeply. The red reflectance trace in Figure 3
shows the optimized growth data for sample B.
Figure 4 PL spectra for the samples A and B.
To further confirm the quality of the samples growth
monitored by 405-nm short-wavelength light beam in
situ system, the room temperature PL and HR-XRD have
also been studied. Figure 4 exhibits the PL spectra of the
two InGaN/GaN MQW LEDs before and after
optimization according to the 405-nm signal. It has been
shown that for the nonideal growth sample A, the PL
peak wavelength displayed poor optical property with a
strong yellow luminescence, caused by impurities or
defects, and nearly no obvious green luminescence can
be seen. Otherwise, the optimized sample B shows
strong green luminescence about 509 nm, indicating the
InGaN/GaN MQW green LEDs had a high quality.
These results agree with the information gained by the
405-nm signal, meaning the trustworthy of the 405-nm
in situ monitoring system.
Figure 5a shows the HR-XRD curves for samples A and
B. For sample B, five numbers of satellite peaks are clearly
observed, but no satellite peak is observed for sample A,
which suggests that the interface of sample B is flatter than
that of sample A due to the satellite peaks reduced by the
roughness interface for MQWs' structure. Meanwhile, for
the sample B, the fringe peaks (secondary satellite peaks)
between satellite peaks can be also observed, which means
the MQWs' quality is good. According to the numbers of
fringe peaks, the total period number could be deduced,
that is 3 (fringe numbers) + 2 = 5 (total periods), which is in
good agreement with the designed period numbers, further
indicating excellent layer periodicity and interface quality
for sample B. In order to get more information of the
MQWs structure, a simulation was carried out to fit the
experimental HR-XRD pattern (Figure 5b). According to the
simulation, the thicknesses of well and barrier are 3.0 and
17.0 nm with an error ±0.1, respectively. Furthermore, the
In content in the InGaN well layer is 0.21.
Figure 5 Measured HR-XRD curves and simulation. (a) Measured
HR-XRD curves for sample A, and (b) measured HRXRD curves and
its simulation for sample B.
Figure 6 displays the electroluminescence character of
the sample B with five-period InGaN/GaN MQWs
optimized according to the 405-nm in situ monitoring
system. Obviously, the high brightness green light further
confirmed the high quality of InGaN/GaN MQW green
LEDs optimized by 405-nm light beam in situ
In summary, five-period InGaN/GaN MQW green LEDs
were grown by MOCVD with 405-nm light beam in situ
monitoring system. The results showed that 405-nm
reflectance trace could provide the growth information,
such as the interface morphology, the In composition,
the growth rate, and so on. Thus, according to the in
situ 405-nm light monitoring signals, the parameters for
growth of high-quality, high In-content InGaN/GaN
MQW green LEDs can be optimized easily. The PL
spectra and HR-XRD curve and electroluminescence
character confirmed the high quality of InGaN/GaN
MQW green LEDs optimized by 405-nm in situ
monitoring data. The results show that the short-wavelength
Figure 6 Electroluminescence photo of the optimized InGaN/GaN MQW green LEDs.
in situ monitoring system is a powerful, noninvasive
real-time tool for the growth of InGaN/GaN MQWs.
XS and YC are assistant professors, DL, HS, HJ, and GM are professors, and ZL
is an associate professor at the State Key Laboratory of Luminescence and
Applications, Changchun Institute of Optics, Fine Mechanics and Physics,
Chinese Academy of Sciences, 3888 Dongnanhu Road, Changchun, 130033,
Peoples' Republic of China. XS and YC are also affiliated to the Graduate
University of the Chinese Academy of Sciences, Beijing, 100039, Peoples'
Republic of China.
This work was partly supported by the National Key Basic Research Program
of China (grant no. 2011CB301901) and the National Natural Science
Foundation of China (grant nos. 51072195 and 51072196).
1. Romano LT , McCluskey MD , Van de Walle CG , Northrup JE , Bour DP , Kneiss M , Suski T , Jun J : Phase separation in InGaN multiple quantum wells annealed at high nitrogen pressures . Appl Phys Lett 1999 , 75 : 3950 - 3952 .
2. Zhao DG , Jiang DS , Zhua JJ , Wang H , Liu ZS , Zhang SM , Wang YT , Jia QJ , Yang H : An experimental study about the influence of well thickness on the electroluminescence of InGaN/GaN multiple quantum wells . J Alloys and Compounds 2010 , 489 : 461 - 464 .
3. Niu NH , Wang HB , Liu JP , Liu NX , Xing YH , Han J , Deng J , Shen GD : Effects of growth interruption on the properties of InGaNGaN MQWs grown by MOCVD . Opto Lett 2007 , 3 : 0001 - 0003 .
4. Kim DJ , Moon YT , Song KM , Choi CJ , Ok YW , Seong TY , Park SJ : Structural and optical properties of InGaN/GaN multiple quantum wells: the effect of the number of InGaN/GaN pairs . J Crys Growth 2000 , 221 : 368 - 372 .
5. Li HD , Wang T , Jiang N , Liu YH , Bai J , Sakai S : Interactions between inversion domains and InGaN/GaN multiple quantum wells investigated by transmission electron microscopy . J Crys Growth 2003 , 247 : 28 - 34 .
6. Niu NH , Wang HB , Liu JP , Liu NX , Xing YH , Han J , Deng J , Shen GD : Enhanced luminescence of InGaN/GaN multiple quantum wells by strain reduction . Solid-State Elec 2007 , 51 : 860 - 864 .
7. Niu NH , Wang HB , Liu JP , Liu NX , Xing YH , Han J , Deng J , Shen GD : Improved quality of InGaN/GaN multiple quantum wells by a strain relief layer . J Crys Growth 2006 , 286 : 209 - 212 .
8. Kim S , Lee K , Park K , Kim CS : Effects of barrier growth temperature on the properties of InGaN/GaN multi-quantum wells . J Crys Growth 2003 , 247 : 62 - 68 .
9. Lin YS , Ma KJ , Hsu C , Feng SW , Cheng YC , Liao CC , Yang CC , Chou CC , Lee CM , Chyi JI : Dependence of composition fluctuation on indium content in InGaNÕGaN multiple quantum wells . Appl Phys Lett 2000 , 77 : 2988 - 2990 .
10. Cho HK , Lee JY , Sharma N , Humphreys CJ , Yang GM , Kim CS , Song JH , Yu PW : Effect of growth interruptions on the light emission and indium clustering of InGaN/GaN multiple quantum wells . Appl Phys Lett 2001 , 79 : 2594 - 2596 .
11. Wu XH , Elsass CR , Abare A , Mack M , Keller S , Petroff PM , DenBaars SP , Speck JS , Rosner SJ : Structural origin of V-defects and correlation with localized excitonic centers in InGaN/GaN multiple quantum wells . Appl Phys Lett 1998 , 72 : 692 - 694 .
12. Sharma N , Thomas P , Tricker D , Humphreys C : Chemical mapping and formation of V-defects in InGaN multiple quantum wells . Appl Phys Lett 2000 , 77 : 1274 - 1276 .
13. ChoH K , Lee JY , Kim CS , Yang GM : Formation mechanism of V defects in the InGaN/GaN multiple quantum wells grown on GaN layers with low threading dislocation density . Appl Phys Lett 2001 , 79 : 215 - 217 .
14. Takeuchi T , Wetzel C , Yamaguchi S , Sakai H , Amano H , Akasaki I , Kaneko Y , Nakagawa S , Yamaoka Y , Yamada N : Determination of piezoelectric fields in strained GaInN quantum wells using the quantum-confined Stark effect . Appl Phys Lett 1998 , 73 : 1691 - 1693 .
15. Kim IH , Park HS , Park YJ , Kim T : Formation of V-shaped pits in InGaN/GaN multiquantum wells and bulk InGaN films . Appl Phys Lett 1998 , 73 : 1634 - 1636 .
16. Temperature Dependences. [http://www.ioffe.ru/SVA/NSM/Semicond/GaN/ bandstr.html#Dependence]
17. Harris JC , Brisset H , Someya T , Arakawa Y : Growth condition dependence of the photoluminescence properties of InxGa1-xN/InyGa1-yN multiple quantum wells grown by MOCVD . Jpn J Appl Phys 1999 , 38 : 2613 - 2616 .