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High-power hybrid GaN-based green laser diodes

with ITO cladding layer

LEI HU,

1,2

XIAOYU REN,

JIANPING LIU,

1,2,

*AIQIN TIAN,

LINGRONG JIANG,

1,2

SIYI HUANG,

WEI ZHOU,

LIQUN ZHANG,

AND HUI YANG

1,2

Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (CAS),

Suzhou 215123, China

School of Nano-Tech and Nano-Bionics, University of Science and Technology of China, Hefei 230026, China

*Corresponding author: [email protected]

Received 23 October 2019; revised 18 December 2019; accepted 22 December 2019; posted 24 December 2019 (Doc. ID 381262);

published 12 February 2020

Green laser diodes (LDs) still perform worst among the visible and near-infrared spectrum range, which is called

the “green gap.” Poor performanc e of green LDs is mainly related to the p-type AlGaN cladding layer, which on

one hand imposes large thermal budget on InGaN quantum wells (QWs) during epitaxial growth, and on the

other hand has poor electrical property especially when low growth temperature has to be used. We demonstrate

in this work that a hybrid LD structure with an indium tin oxide (ITO) p-cladding layer can achieve threshold

current density as low as 1.6kA∕cm

, which is only one third of that of the conventional LD structure. The

improvement is attributed to two benefits that are enabled by the ITO cladding layer. One is the reduced thermal

budget imposed on QWs by reducing p-AlGaN layer thickness, and the other is the increasing hole concentration

since a low Al content p-AlGaN cladding layer can be used in hybrid LD structures. Moreover, the slope efficiency

is increased by 25% and the operation voltage is reduced by 0.6 V for hybrid green LDs. As a result, a 400 mW

high-power green LD has been obtained. These results indicate that a hybrid LD structure can pave the way

toward high-performance green LDs.

https://doi.org/10.1364/PRJ.381262

1. INTRODUCTION

Laser diodes (LDs) based on III-nitride materials (Al, Ga, In)N

extend the wavelength of semiconductor lasers into the visible

and ultraviolet spectrum range, and therefore have been attract-

ing great attention in the past years due to their huge applica-

tions, such as information storage, laser lighting, laser display,

and recently emerging applications for atomic cooling and

metal processing [1–15]. However, semiconductor lasers with

wavelengths in the green spectrum range from 500 nm to

550 nm still perform worst among the visible and near-infrared

spectrum range, which is called the “green gap” [16–20]. Green

LDs are very important especially for display application to ob-

tain a wide color gamut. However, the performance of GaN-

based green LDs cannot meet the demand of laser display yet

and is a bottleneck. This is due to large lattice mismatch and

the difference in epitaxial growth temperatures between high-

indium-content green InGaN quantum wells (QWs) and the p-

AlGaN cladding layer. The growth temperature for green

InGaN QWs is around 700°C, while it is 950 –1000°C for

the p-AlGaN cladding layer, which means a large thermal

budget imposed on InGaN QWs during epitaxial growth of

the p-AlGaN cladding layer. A large thermal budget leads to

serious thermal degradation of green InGaN QWs [21–25].

Moreover, the p-AlGaN cladding layer has poor electrical prop-

erty, especially when low growth temperature has to be used in

the green LD structure. Therefore, these issues related to the

p-type AlGaN cladding layer have to be solved to improve the

performance of green GaN-based LDs.

In order to avoid these disadvantages associated with the

p-AlGaN cladding layer, silver (Ag) metal had been explored

as a cladding layer and metal p-electrode in GaN-based violet

LDs [26]. However, since the absorption coefficient (α)ofAgis

as high as 610,000 cm

−1

at 410 nm, it will induce a large in-

ternal loss of 30 cm

−1

, and thus the LD performance was not

improved although lasing had been achieved under electrical

injection. Indium tin oxide (ITO), which is conductive and

transparent in the visible spectrum range, has been widely used

as an electrode in GaN-based light-emit ting diodes. The ab-

sorption coefficient of ITO is 2 orders of magnitude lower

than that of metal. Meanwhile, the refractive index of ITO

is around 2, much lower than that of the p-AlGaN cladding

layer, and therefore provides sufficient optical confinement

for the laser cavity. ITO can be deposited at around 300°C

or lower, and therefore replacing the p-AlGaN cladding layer

Research Article

Vol. 8, No. 3 / March 2020 / Photo nics Research 279

with ITO can reduce the high-temperature growth time of the

p-AlGaN cladding layer, and thus reduce the thermal budget

imposed on green InGaN QWs. Moreover, using ITO as the

cladding layer also allows a thinner and lower Al content

p-AlGaN cladding layer to be used in hybrid LD structures,

meaning the electrical property of the cladding layer, in-

cluding hole concentration and conductivity, can be enhanced.

Therefore, hybrid GaN-based LDs using ITO as the p-cladding

layer are promising to improve the performance of GaN-based

LDs. There are a few reports about the design and fabrication

of hybrid GaN-based LDs using ITO as the cladding layer.

However, the performance is only comparable to LDs with

conventional structures [27–30].

In this work, we demonstrated that the performance of

green LDs can be improved greatly by using the ITO cladding

layer and reducing the thickness of the p-AlGaN cladding layer.

We first designed the LD structures by simulation in order to

reduce the internal loss, which was found to be dependent on

the remaining p-AlGaN cladding layer thickness and the ab-

sorption coefficient of the ITO layer. GaN-based green LDs

with an ITO cladding layer were then fabricated and charac-

terized. It was found that the threshold current density was re-

duced from 5kA∕cm

to 2.5kA∕cm

and the slope efficiency

was increased from 0.16 W/A to 0.2 W/A by using the ITO

cladding layer and reducing the thickness of the p-AlGaN clad-

ding layer. The cause for the improvement was attributed to

improved quality of green InGaN QWs due to reduced thermal

degradation imposed by the p-AlGaN cladding layer growth.

The threshold current density was further reduced to

1.6kA∕cm

when the average Al content of the p-AlGaN clad-

ding layer was reduced from 8% to 3.5%. Meanwhile, the op-

eration voltage was also reduced by 0.6 V. By employing the

ITO cladding layer, a high-power gre en LD with an output

power of 400 mW at the typical operation current density

of 9kA∕cm

for GaN-based green LDs has been achieved

in this work.

2. LASER DESIGN AND FABRICATION

The transfer matrix method was used to calculate the light field

distribution, the internal loss, and the confinement factor (Γ)of

our green LD structures [31]. The LD structure is shown in

Fig. 1(a). The epitaxial structure consisted of a silicon (Si)-

doped n-Al

0.07

0.93

N cladding layer; a Si-doped n-GaN layer;

a Si-doped n-In

0.04

0.96

N waveguide layer; two-period

unintentional-doped multiple quantum wells; an unintentional-

doped In

0.03

0.97

N waveguide layer; a magnesium (Mg)-

doped p-Al

0.15

0.85

N electron blocking layer; a Mg-doped

p-AlGaN cladding layer with average Al content of 8% and

3.5% and various thicknesses from 150 nm to 500 nm (p-

AlGaN/p-GaN superlattice structures with 2.5 nm p-AlGaN

and 2.5 nm p-GaN in a period were used in order to reduce

its resistivity); a p-InGaN contact layer; and an ITO p-electrode

or a palladium (Pd)/platinum (Pt)/gold (Au) metal p-electrode.

The refractive indices of (Al, In)GaN were taken from

Ref. [32], and the absorption coefficient was interpolated from

Ref. [33]. The absorption coefficients of p-AlGaN and Pd used

Fig. 1. (a) Structure of the hybrid LD with ITO cladding layer or conventional LD with Pd/Pt/Au electrode. (b) Calculated internal loss of the

hybrid LD and conventional LD as a function of p-AlGaN cladding layer thickness. (c) Calculated internal loss of hybrid LDs with various

absorption coefficients of ITO layers as a function of p-AlGaN cladding layer thickness.

280 Vol. 8, No. 3 / March 2020 / Photonics Research

Research Article

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