Investigation of Mg doping profile in the pcladding layer for highbrightness AlGaInPbased light emitting diodes.

14, 6124–6127, 2014 www.aspbs.com/jnn Investigation of Mg Doping Proﬁle in the p-Cladding Layer for High-Brightness AlGaInP-Based Light Emitting Diodes Hwa Sub Oh1, Ho Soung Ryu1 3, Joo

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Article Journal of Nanoscience and Nanotechnology

Vol 14, 6124–6127, 2014 www.aspbs.com/jnn

Investigation of Mg Doping Proﬁle in the p-Cladding Layer for High-Brightness AlGaInP-Based Light Emitting Diodes

Hwa Sub Oh1, Ho Soung Ryu1 3, Joon Mo Park1, Hyung Joo Lee2, Young Jin Kim2,

In Kyu Jang2, Ji Hoon Park2, Joon Seop Kwak3, and Jong Hyeob Baek1 ∗

1LED Device Research Center, Korea Photonics Technology Institute,

971-35 Wolchul-dong, Buk-gu, Gwangju, Korea

2Process Engineering Department, Kodenshi Auk Incorporation, 513-37 Eoyang-doing, Iksan city, Chonbuk, Korea

3Department of Printed Electronics Engineering, Sunchon National University,

Chonnam 540-742, Korea

We investigated 590 nm light-emitting diodes appropriate for full-color display applications in terms

of their electrical and optical behaviors during operation according to their Mg doping proﬁle in the

p-cladding layer As the hole concentration in the “b” zone of the p-cladding layer is increased

from 34 × 1017 to 67 × 1017, the light output power increases by 41% due to the enhancement

of the hole injection into the active region and also due to the minimization of the carrier overﬂow

problem However, at an oversaturation of Mg doping with excess [Cp2Mg]/[III] in the “b” zone, the

internal quantum efﬁciency degrades because of the decrease in hole concentration because of the

oversaturated material problem

Keywords: Mg Doping Proﬁle,p-Cladding Layer, AlGaInP-Based LED

1 INTRODUCTION

Recently, AlGaInP-based light-emitting diodes (LEDs)

have experienced an impressive evolution in both device

performance and market volume In particular,

high-brightness LEDs are gaining interest for use in commercial

applications such as automotive lighting, full-color

dis-plays, and general illumination To increase their

util-ity in these applications, improved performance such

as shorter wavelengths and high-powered devices have

been pursued.1–5However, (AlxGa1−x 05In05P

heterostruc-tures have a small conduction band offset that limits

their electron conﬁning potential.6 This weaker electron

conﬁnement subsequently leads to electron heterobarrier

leakage in AlGaInP heterostructure LEDs, especially in

short-wavelength devices, where a fraction of the electrons

injected into the active region have a sufﬁcient thermal

energy to escape into the p-cladding layer To overcome

this problem, AlGaInP-based LED structures require an

∗ Author to whom correspondence should be addressed.

optimized doping proﬁle in thep-cladding layer in order to

prevent carrier overﬂow and to gain a higher light output power (Pout In the ﬁeld of AlGaInP-based LEDs that

emit a short peak wavelength at around 590 nm, how-ever, the effects of the doping proﬁle in the p-cladding

layer on LED performance has yet to be systematically studied

In this study, we investigate the behaviors of electri-cal and optielectri-cal characteristics according to their Mg dop-ing proﬁle in the p-cladding layer by analyzing device

performances

2 EXPERIMENTAL DETAILS

We conducted metal-organic vapor phase epitaxy (MOVPE) to grow LED structures on a 2-in (100) GaAs substrate that was tilted 10 toward 011 to sup-press spontaneous ordering in the GaInP and AlGaInP epilayers.6 Here, trimethylaluminum (TMAl), trimethyl-gallium (TMGa), and trimethylindium (TMIn) were used

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Figure 1 Schematic of LED structures having different Mg doping

pro-ﬁles in thep-cladding layer.

as the group-III sources; phosphine (PH3 and arsine

(AsH3 were the group-sources, and disilane (Si2H6 and

biscyclopentadienylmagnesium (Cp2Mg) were the n- and

p-type doping sources, respectively The growth

temper-ature was set at 700 C, for a growth rate of about

1.5m/h The multi-quantum well (MQW) structures for

emitting a 590 nm peak wavelength consisted of 4.5 nm

(Al02Ga08 05In05P quantum wells separated by 30 nm

(Al05Ga05 05In05P barrier layers, which were sandwiched

between two Al05In05P cladding layers To improve the

LED performance relative to the Mg doping

concentra-tion, the p-cladding layer was divided into three zones:

“a”, “b”, and “c” zones In addition, a 10-m-thick p-GaP

window layer capped thep-cladding layer in order to

eval-uate the actual device performance Schematics of the LED

structures are depicted in Figure 1

Table I presents the detailed Mg doping proﬁles for

the three zones of thep-cladding layer in the LED

struc-tures Note that the “a” zone in the p-cladding layer is

intentionally left undoped and that the “c” zone is

rela-tively lightly doped with 10×10−4[Cp

2Mg]/[III] in order

to prevent Mg diffusion into the active region To

inves-tigate and optimize the hole injection into the active

region and to prevent electron overﬂow, the “b” zone in

the p-cladding layer is doped with [Cp2Mg]/[III] values

of 10 × 10−4, 20 × 10−4, 30 × 10−4, and 50 × 10−4 in

the Test 1, Test 2, Test 3, and Test 4 LED structures,

respectively To determine the hole concentration in the

p-cladding layer according to the [Cp2Mg]/[III] value,

electrochemical capacitance–voltage (ECV) measurements

at 300 K were performed To evaluate the devices, the

Table I Detailed Mg doping proﬁles for “a,” “b,” and “c” zones in the p-cladding layer of LED structures.

Mg Doping proﬁle atp-cladding layer

No [Cp2Mg]/[III] Hole conc [cm−3] [Cp2Mg]/[III] Hole conc [cm−3] [Cp2Mg]/[III] Hole conc [cm−3]

wafers were sectioned into 300 m × 300 m chips,

with 80-m-diameter metal contacts located on the top

p+-contact layer After dicing, the chips were mounted

onto TO-18 headers with no epoxy encapsulation before being measured using a large-area Si photodiode that was placed on top of the device

3 RESULTS AND DISCUSSION

Figure 2 presents the typical current–voltage (I–V )

char-acteristics and light output power (Pout of LEDs having

different Mg doping proﬁles in the p-cladding layer The I–V characteristics in the “b” zone in the p-cladding layer

show similar behaviors regardless of thep-doping

concen-tration These data indicate that the Mg doping level in the “b” zone is not signiﬁcantly affected by the operating

voltage, in terms of device performance

Figure 2(b) then shows the light output-current charac-teristics of LEDs having different Mg doping levels in the

“b” zone In addition, the relative increase of Pout (RIP) of the same LEDs at a 200 mA operating current is shown

in the ﬁgure inset As the [Cp2Mg]/[III] value in the “b”

zone is increased from 10 × 10−4 to 30 × 10−4, the RIP

increases from 1.0 to 1.4, though at a further increase of [Cp2Mg]/[III] to 50 × 10−4, the RIP degrades to 1.3.

The improvement of internal quantum efﬁciency is due

to the fact that the highlyp-doped “b” zone increases the

potential barrier in thep-cladding layer, which minimizes

the electron overﬂow problem, and the improved hole con-ductivity helps to enhance the hole injection into the active region.7The RIP decrease in the Test 4 LED structure indi-cates that the excess [Cp2Mg]/[III] deteriorates the internal quantum efﬁciency As such, it is important to optimize the [Cp2Mg]/[III] value in thep-cladding layer in order to

improve the light output power

One possible explanation for the decreased light out-put of the LEDs under an excess [Cp2Mg]/[III] value

is the degradation of the active region by Mg diffu-sion Meneghesso et al.8 reported that the degradation of optical power is accompanied by both an increase of a generation-recombination current at a low forward bias and an increase of the device ideality factor To exam-ine the inﬂuence of Mg diffusion into the active region, theI–V characteristics plotted on a log–log scale and the

ideality factor of LEDs with different Mg doping proﬁles

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Investigation of Mg Doping Proﬁle in thep-Cladding Layer for High-Brightness AlGaInP-Based Light Emitting Diodes Oh et al.

Voltage [V]

Test 1

Test 2

Test 3

Test 4

0

200

400

600

800

1000

1200

1400

0.8

1.0

1.2

1.4

1.6

Pout

Current [mA]

Test 1 Test 2 Test 3 Test 4

0

20

40

60

80

100

(a)

(b)

Figure 2 (a) ForwardI–V characteristics and (b) light output power of

LEDs having different Mg doping proﬁles in thep-cladding layer The

inset in (b) shows the relative increase of brightness for the same LEDs.

at p-cladding layer are shown in Figure 3 However, it

should be noted that no leakage current increase at a

low bias was observed In addition, the LEDs with

dif-ferent Mg doping levels in the “b” zone did not induce

1.6

1.7

1.8

1.9

2.0

Voltage [V]

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10

100

Figure 3 Forward I–V characteristics plotted on a log–log scale of

LEDs having different Mg doping proﬁles in thep-cladding layer The

inset shows the ideality factors of the same LEDs.

signiﬁcant changes in ideality factors Hence, these results imply that the decrease in the light output of LEDs with

an excess [Cp2Mg]/[III] value cannot be explained sim-ply based on degradation caused by Mg diffusion into the active region Indeed, these results indicate that the intentionally undoped “a” zone and relatively low-doped

“c” zone efﬁciently prevent Mg diffusion into the active

region

Another possible explanation for the decreased light out-put of the LEDs with excess [Cp2Mg]/[III] in the “b”

zone is directly related to the decreased hole concentra-tion The hole concentration of thep-type Al05In05P layer

measured using the ECV shows that as the [Cp2Mg]/[III] value is increased from 10 × 10−4 to 30 × 10−4, the hole

concentration increases from 34 × 1017to 67 × 1017, and that at a further increase to 50 × 10−4, the hole

concen-tration decreases to 53 × 1017 due to the oversaturated material problem.9 These results indicate that the hole concentration at the “b” zone in the p-cladding layer is

directly inﬂuenced by improvement in the spontaneous light-emitting efﬁciency and that carefully optimizing the [Cp2Mg]/[III] value is important for attaining the high-est hole concentration—and thereby improving the device performance

4 CONCLUSION

To make 590 nm high-brightness LEDs that are appro-priate for full-color display applications, we studied the electrical and optical behaviors of device performances according to their Mg doping proﬁle in the p-cladding

layer As the [Cp2Mg]/[III] value in the “b” zone was

increased from 10 × 10−4 to 30 × 10−4, the light output

power increased by 41% due to the enhanced hole injec-tion into the active region and also by minimizainjec-tion of the carrier overﬂow problem In addition, the oversaturation

of Mg doping in the “b” zone with excess [Cp2Mg]/[III] deteriorated the internal quantum efﬁciency by decreas-ing the hole concentration As a result, by evaluatdecreas-ing the

I–V characteristics and ideality factors, we found that the

intentionally undoped “a” zone and relatively low-doped

“c” zone efﬁciently acted to prevent Mg diffusion into the

active region

Acknowledgment: This work was supported by a Korea Research Foundation Grant from the ATC project (No 10035863) provided by the Ministry of Knowledge Economy, Korea

References and Notes

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4 M Eichfelder, W M Schulz, M Reischle, M Wiesner, R Robach,

M Jetter, and P Michler, Appl, Phys Lett 95, 131107

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5 H S Oh, S M Kim, J H Joo, J H Baek, and J S Kwak, J Nanosci.

Nanotechnol 11, 1503 (2011).

6 J Johansson, W Seifert, T Junno, and L Samuelson, J Cryst Growth

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7 P Blood, Mat Sci Eng B66, 174 (1999).

8 G Meneghesso, S Levada, R Pierobon, F rampazzo, E Zanoni,

A Cavallini, A Castaldini, G Scamarcio, S Du, and

I Eliashevich, IEDM Technical Digest, IEEE, San Francisco (2002),

p 103.

9 G J Bauhuis, P R Hageman, and P K Larsen, J Crystal Growth

191, 313 (1998).

Received: 17 March 2013 Accepted: 17 April 2013

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