For Review Only - University of Toronto T-Space · For Review Only Experimental study on the mechanism governing spectral shifts in low power 670 nm AlGaInP multiple quantum well

For Review O

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Experimental study on the mechanism governing spectral

shifts in low power 670 nm AlGaInP multiple quantum well (MQW) laser diodes at temperature range (5oC-45oC).

Journal: Canadian Journal of Physics

Manuscript ID cjp-2015-0588.R2

Manuscript Type: Article

Date Submitted by the Author: 19-Feb-2016

Complete List of Authors: chackrabarti, santosh; jamia millia islamia,

sharma, dhrub; tribhuvan university Joseph, Shereena; Jamia Millia Islamia Faculty of Natural Sciences Zaker, Tho-alfiqar ; Jamia Millia Islamia Faculty of Natural Sciences hafiz, aurangzeb; Jamia Millia Islamia kafle, ram; ttribhuvan university

Keyword: AlGaInP, red shift, photoluminescence, wavelength, band gap narrowing

https://mc06.manuscriptcentral.com/cjp-pubs

Canadian Journal of Physics

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Experimental study on the mechanism governing spectral shifts in low power 670 nm

AlGaInP multiple quantum well (MQW) laser diodes at temperature range (50C-45

0C).

Santosh Chackrabarti1, Dhrub Sharma

2,a, Shereena Joseph

1, Tho-alfiqar A Zaker

1, A.K.Hafiz

1

and Ram Kafle2

1. Dept. of Physics, Jamia Millia Islamia, New Delhi- 110025,India

2. Dept. of Physics, Tribhuvan University, Kathmandu, Nepal, P.O.Box8212

a. Corresponding author: Tel. +9779841722106, Fax. +9771126934553

E-mail address: [email protected](Dhrub Sharma)

Abstract

We report on the temperature dependent spectral shifts in low power 670 nm AlGaInP

multiple quantum well (MQW) red laser diodes due to band gap narrowing at room

temperatures (50C-45

0C). The spectral shift mechanism is explored with a threshold current

density of 11.41 kA/cm2 and a good characteristic temperature of 114K. The PL peak

intensity shifts towards the higher wavelength and the full width at half maximum (FWHM)

increases with the increase in temperature from (50C-45

0C). We use Hamiltonian system

considering the effective mass approximation to formulate the carrier concentrations. The

band gap narrowing value determined by a simple formula amounts to 59.15 meV and

displays N1/3

dependence at higher densities. The carrier density dependence conveys that the

red shift of the spectral emission is due to band gap narrowing.

Keywords: AlGaInP; Red shift; Photoluminescence; Wavelength; Band gap narrowing

PACS: 98.62.Py, 78.55.Cr

1 Introduction

AlGaInP is the material of choice for the long wavelength part of the visible spectrum

ranging from red to yellow-green. AlGaInP alloys do not rely on deep level-mediated

transitions that tend to saturate. As AlGaInP alloys grown on GaAs substrates do not suffer

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lattice mismatch so by properly controlling the alloy composition of the AlGaInP material

system, one can grow a high quality AlGaInP epilayer on a GaAs substrates without

generating many misfit dislocations. Compared to AlGaAs multiple quantum well (MQW)

laser diode (LD)s, the direct-indirect transition in AlGaInP MQW LDs occurs at a higher

energy so they can be used for all colours of the long-wavelength region of the visible

spectrum down to yellow-green wavelengths. AlGaInP materials technology has steadily

advanced over the past three decades leading to the high performance edge-emitting lasers

and vertical cavity surface emitting lasers. These laser diode (LD)s have been intensively

developed for 630-700 nm wavelength range applications such as bar code readers, digital

versatile disk players, lasers pointers[1-4].

The presence of large concentration of free carriers in a semiconductor lattice can cause an

apparent reduction in the band gap structure. The band gap of a crystal has been a central

character in the calculation of spontaneous and emission spectra [5-7]. In the past, several

studies have been made on the temperature dependent mechanism governing the

characteristics of the lasers diodes. Accordingly the wavelength shift emission with its

temperature variation has been studied by several authors [8-10].

Free carriers may be introduced in a semiconductor in a variety of different ways most often

by impurity doping, but also by optical injection by an applied voltage. Changes in the band

gap arise from changes in the electron-electron interaction due to the free carriers and if

present, the effects of the impurity centre themselves, which may lead to strain effects and

electron scattering. The overall effect is the band gap narrowing (also called band gap

shrinkage) of the semiconductor. Taking this fact into consideration, researchers have been

interested in the study of spectral characteristics in AlGaInP MQW LDs due to band gap

narrowing. Yen et al.[11] theoretically analysed 630nm band GaInP-AlGaInP tensile strained

quantum well (QW) lasers with doping in the active region at 300K. The threshold current

density was strongly sensitive as the n-type dopant (Al) concentration was increased. On

comparing the data showing calculation results of emission wavelengths for the triple lasers

(with Al content 0.4, 0.5 and 0.6 respectively), the wavelengths were longer instead being

shorter. They attributed the result to the large amount of band gap narrowing due to high

carrier density in the active region at threshold. C.Y.Liu et al.[12]demonstrated the

temperature dependent photoluminescence(PL) measurements in the temperature range from

10 to 230K. The PL energy increased with temperature from 10 to 70K and decreased above

70K. The latter was attributed to temperature induced band gap narrowing in which the

calculated interband transition energy decreases monotonously as per the famous Varshni

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energy gap relation[ 13].At temperatures above 70 K, most of the trapped carriers attained

more thermal energy and were thermalized out from the local potential traps. These free

carriers recombine in the quantum well through interband transition recombination process.

Similarly, the line width of the PL peak followed an increased linear dependence above

~150K. Kaneko et.al.[14] synthesized 607-640nm wavelength quasi-quaternary (QQ) lasers

with 20nm QQ active layers using GaInP/AlInP QQ compounds at 5000C. The band gap

energy of the active layers in the lasers, which was evacuated by photoluminescence (PL)

measurement, shifted linearly from 649 to 578nm as a function of the equivalent Al content.

They also evaluated the dependency of the PL integrated intensities from the QQ active

layers of the lasers on the equivalent Al content at 4.2K, 77K and room temperature

respectively. The room temperature PL intensities decreased with the increased equivalent Al

content while the intensities at 4.2K and 77K were almost constant, reason being suppression

of carrier overflow at low temperatures due to the shrinking of carrier distribution in the

energy bands. Such results indicate that the systematic investigation of the spectral properties

of this particular MQW laser is necessary for understanding its light emission mechanisms.

In this paper, we make experimental study on the temperature dependent spectral changes in

670nm AlGaInP MQW laser due to band gap narrowing at room temperatures (50C-45

0C).

The temperature range is important to consider since most of the diode laser based operations

are executed in this range. We have reported results of the wavelength red shift, the

mechanism, and the feasibility of controlling the laser oscillation by the wavelength shift of

670nm AlGaInP MQW lasers at (50C-45

0C). To our best knowledge no work regarding the

spectral shift has been explained at this temperature range for the 670nm AlGaInP MQW

lasers. The paper has been organised as follows: in Section 2 we briefly describe the physical

background of band gap narrowing while in Section 3 we describe the experimental set up. In

Section 4 the details of the results obtained is discussed. Finally, in Section 5 we conclude the

paper

2 Theory

Mostly III-V semiconductors are polar and may also be doped sufficiently with impurities to

form a degenerate electron gas. As an electron gas, they may have effective density which is

very high. Semiconductors possess a suitable environment to study the high –density electron

gas.

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The many body system is a collection of ions and electrons. It is charge neutral so the ions

and electrons have the same average charge density ieZnne =0 where0n and

in are the

particle densities of electrons and ions. The ions of the valence Z are considered as rigid

objects. The excitations of the inner core electrons can be neglected since these energetic

events are not induced by the motion of the ions. The core excitations will contribute only a

small amount of dielectric screening which can be included as a phenomenological high

frequency dielectric constant iε . The Hamiltonian for the system may be expressed as [15]

2 2 2 1 1( ) ( )......(1)

2 2 2 2

ie i i i i

i i j ii i j

P P eH V r R V R R

m M r r

κκ κ β

κ κ κβκ ≠

= + + + − + −−

∑ ∑ ∑ ∑ ∑

where the first two terms are the kinetic energies of electrons at ir and ions at Rκ

respectively. The potential ( )ei iV r Rκ− is between an electron at ir and Rκ .Similarly the

potential ( )iiV R Rκ β− is between the two ions. Both of these potentials are unscreened so

they behave at large distances as [15]

2

l im ( ) . . . . . . . . . . . . . . . . . . .( 2 )e ir

i

Z eV r

rε→ ∞

−=

2 2

lim ( ) . . . . . . . . . . . . . . . . .(3 )i iR

i

Z eV r

Rε→ ∞

−=

The coulomb form of 2 is probably valid for all ion pairs. However the electron-ion

interaction eiV must always be treated realistically at small distances. The electron- electron

interactions are also included in the Hamiltonian. They cause the potentials to be screened.

Using the equations (1) and (3), the many electron Hamiltonian form can be written as [16]

2 2 2 20

, ,

1 1.........(4)

2 2 2

ie c

i i j i ii i j i i

P e Ze eH N E

m r r r R r Rκ βκ βε ε∗≠

′= + − − + − − −

∑ ∑ ∑ ∑r rr r

where, im ∗ is the conduction band effective mass, N ′ is the total no of electrons and 0

cE is the

energy of the k=0 state in the pure crystal. The electron states are Bloch waves in the periodic

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potential and must be calculated with the full effects of correlation and exchange which has

only been done approximately.

The system of all electrons in the conduction band (CB) can be approximated by single

electrons moving in the field of screened impurity potential, provided the whole CB is shifted

by an amount equal to the exchange energy e

cE [17]

Ignoring the small changes in effective mass, from eq (4) the one electron Hamiltonian can be

written as

2

2 0

2( ) ....................................(5 )

8

c e

a c c p c

c

H E E V x Emπ ∗

∇= − + + + −h

c

cp ExVxV −= )()( ............................................... (6)

where, ( )pV x is the potential fluctuation aboutc

cE i.e. the superposition of all screened

potentials creating the shift in the CB due to the individual impurities and 0

cE is the energy

of the K=0 CB state. V(x) is the screened potential of the randomly distributed impurities.

When ( )pV x =0, the solution of Ha are plane waves and the density of states (DOS) is

parabolic for the energy E > 0

cE -e

cE +c

cE . When the fluctuation ( )pV x is attractive,

localised band states below CB is expected. The Fermi energy relative to the bottom of the

band can be calculated with a good accuracy in the formalism of parabolic DOS

The fundamental band gap is the lowest energy needed to promote one electron from the

valence band to the conduction band. Electrons in both the conduction band and the valence

bands are affected by the additional interactions. The onset of the band gap shrinkage is due

to the Coulomb interactions and the mutual exchange between the added free electrons in the

conduction band and electron impurity scattering. This leads to an increase in the energy of

the valence band maximum and decrease in the energy of the conduction band minimum.

Simultaneously there is the change in the shape (non parabolic nature) of the conduction band

that has the strongest effect in the optical transitions. However there is rigid shift of the band

with no change in the electron effective mass [18] .Shrinkage effects are determined by free

carrier density and are nearly independent of the impurity concentration [19]. The expression

for the band gap shrinkage originally derived by Wolff [17] is

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3/13/1)

3)(

2( N

eE

so

g πεπε−=∆ .......... (7)

oε is the permittivity of the free space, sε is the static dielectric constant and N is the

concentration of free electrons or holes. The predicted shrinkage is proportional to the cube

root of the carrier concentration. Correlation effects become vital when the inter electron

(hole) spacing becomes comparable in size to the effective Bohr radius of the electrons

(holes).

3 Experimental details

In order to study the emission spectrum and wavelength shift of MQW we used Hitachi

670nm HL6722G index guided AlGaInP red laser diode for our experiment. Light from laser

diode was fed to the input slit of the monochromator via objective microscope (25X) and

collimating lens. Light from the exit slit of the monochromator was focused on to the high

speed silicon detector (DET10A) via microscope objective (25X). The diffraction grating in

the monochromator was precisely rotated step-by-step by a stepper motor .The spectral

resolution of this setup was about (0.1nm), allowing only longitudinal modes.

Laser mount, detector, lens and microscope objective were fixed on high precision XYZ

micro positioning mounts on the optical table. The monochromator was mounted on top of

two high precision optical jacks (from Sandvic Company). The key component of our

experiment- a lock-in amplifier was used to perform signal to noise ratio (S/N) determination

and phase sensitive measurements. This required the use of mechanical light chopper which

was mounted between the lens and microscope objective in front of entrance slit of

monochromator. The signal output from the lock-in amplifier was measured using high

sensitive programmable digital multimeter (CADOO62).

The wavelength from monochromator, the injection current, and the output from the lock-in

amplifier were recorded by digital video camera (Model HD-DV Camcorder) and the time

resolution of this measurement was determined by frame of the video camera system which

was maximum of 30fps, analyzed by a personal computer.

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4 Results and discussions

The experimental setup described in above section was used to study the effect of the

temperature on the spectral emission of the laser diode at fixed injection current . For fixed

injection current, the lasers output increases rapidly as the temperature is lowered. As the

temperature was raised from 50C to 45

0C in step of 10

0C, the threshold current changed from

(23.94mA) to (48.06 mA) giving an average change of (0.603 mA/0C). The ln(Ith) vs case

temperature was plotted on a semi logarithmic scale as shown in fig (1). Since threshold

current follows an exponential low [20], we have determined the value of the characteristics

temperature (To) which is commonly used to characterize the temperature sensitivity of the

laser diode from the following expression [21].

−=

0

12

1

2 exp)(

)(

T

TT

TI

TI

th

th .............................. (8)

where Ith (T) denotes the value of the threshold current at temperature T. (To) of (114 K) was

found

0 10 20 30 40 50

3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

3.9

Fig.1. Threshold current as a function of temperature for AlGaInP mqw laser

T0= 114 K

ln (

Ith )

Case Temperature(0C)

Since the energy gap (Eg)and the refractive index(n) are two fundamental properties bearing

significant impact on the band structure of semiconductors, we used following Moss’

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relation[22] who proposed a basic relationship between these two properties using the general

theory of photoconductivity based on the study of[22-27]:

m

n

e µλ774

= ..................................... (9)

where, eλ is the wavelength corresponding to the absorption edge

For eλ =670nm, n= 2.65

In terms of energy gap, this is [28]:

4.16 0.85 gn E= −

....................................... (10)

Using this relation the band gap energy for our laser diode was found to be 1.77 eV. The

value of static dielectric constant for our laser diode is εs=7.022. The carrier concentration N

is determined by the relation

2NqdBJ eff= ................................... (11)

where, J is the threshold current density, d is the total active layer thickness, Beff is the

effective recombination coefficient (~1.5×10-10

cm3/s) and q is the electron charge. In these

calculations, we assumed no leakage current. We took threshold current as 35.85 mA at 298K

for which the Jth was found to be 11.41 kA/cm2. For our sample, d=0.5µm; using equation

(11), the value of N is 3.08×1024

m-3

. Implying these values in the expression of band gap

shrinkage in equation (7) we have,

gE∆ = 59.15 meV

This calculated band gap shrinkage value for our 670 nm AlGaInP MQW laser indicates the

N1/3

dependence due to the exchange and correlation effects at higher carrier densities [29,

30]. To our knowledge no band gap shrinkage value has been determined for the AlGaInP

MQW laser diode at this particular temperature range.

The peak intensity decreased and the lasing wavelength jumps toward longer wavelength as

temperature is raised. Fig.(2) shows the shift in wavelength at the gain peak due to a heat sink

temperature change. The peak shift from (669.4nm) at (100C) to (673.6nm) at (37

0C).The measured

laser spectrum had a temperature coefficient of (0.155nm/0C). Furthermore, at the end point

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temperature where two cavity modes appear exited equally, implying that the gain peak was the

midpoint between the two cavity modes. The spectral line width or the full width at half maximum

(FWHM) gets wider with increasing temperature. This line has a FWHM of (~ 0.3nm) at (200C).

Fig.2. Wavelength vs output intensity of AlGaInP MQW laser as a function of temperature

10 15 20 25 30 35 40

669

670

671

672

673

674

wav

elen

gth

(nm

)

case temperature(0C)

GaInP/AlGaInP

N~(3.083X1024

m-3 )

Fig.3 Wavelength as a function of temperature for AlGaInP MQW laser

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The temperature dependent line width (FWHM) of the laser structure is shown in fig.4. At

temperature range up to 400C, the FWHM presents the regular behavior of III-V

semiconductor materials i.e. a monotonic increase with increasing temperature. Since for the

interband transition, the FWHM pattern follows a liner dependency [31] and this was also

observed in our case. The values of FWHM were 0.28nm, 0.32nm, 0.37nm and 0.43nm for

100C, 19

0C, 28

0C and 37

0C respectively. As the temperature is increased, most of the carriers

attain more thermal energy to overcome the small potential barriers in the local potential and

these carriers recombine in the quantum well through the interband transition recombination

process. The interband transition energy decreased monotonously as the band gap decreases

with the temperature [13] and consequently the peak showed red shift.

10 15 20 25 30 35 40

0.26

0.28

0.30

0.32

0.34

0.36

0.38

0.40

0.42

0.44

FWHM (nm)

T(0C)

GaInP/AlGaInP

N~3.083X1024m

-3

Fig.4 : FWHM of the laser emission as a function of temperature

Photoluminescence (PL) measurements were carried out on the multi structure layers at (283K-313K).

A typical curve of PL intensity vs temperature is shown in Fig. 5. The dots are the experimental

data. As the temperature is increased, the PL intensity decreases. The value of the emission intensity

decreased to 8.7, 7.6 and 6.7 at 190C, 280C and 370C respectively from 9.7 at 100C. This is considered

to be caused by two reasons: One is the dependence of the PL intensity on impurity doping.i.e. defects

which works as non radiative centres can be induced in the GaInP active layers by impurity doping.

the photogenerated carriers were likely lost to the barriers where they were possibly captured by the

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nonradiative centres which we suppose may have originated from defects and impurities in the

AlGaInP barrier layers. The other is the incomplete disordering of the ordered structure. This may be

probably due to the insufficient carrier concentration in our experiment because a concentration

higher than 1018

cm-3

is necessary for complete disordering of the ordered structure in GaInP layer

[32,33].

10 15 20 25 30 35 40

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

Int.

(a.

u)

T (0C)

GaInP/AlGaInP

N~3.083X1024

m-3

Fig.5: intensity as a function of temperature

A semiconductor laser's output spectrum depends strongly on case temperature, the gain peak

decrease and the wavelength shift is due to the change in the bandgap with temperature. The

phenomena leading to carrier and stimulated photon losses in semiconductor laser are rather

well known. For carrier losses it is recombination processes, spontaneous emission, and

electron leakage through spreading current or escape from the active region into the p-

cladding region. Modifying the temperature also changes the laser cavity length; refractive

index and gain curve

5 Conclusion

In conclusion, we studied the spectral characteristics of 670 nm MQW AlGaInP laser at room

temperatures (50C-45

0C). With the band gap shrinkage value of 59.15 meV, the lasing

wavelength jumps toward longer wavelength (red shift) as temperature is raised. The peak

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shifted from 669.4nm at (100C) to 673.6nm at (37

0C). The value of the emission linewidth

decreased with increasing operating temperature. It is actually confirmed in laser devices that

the band gap energy difference due to the atomic arrangement brought about the different

oscillation wavelengths. Room temperature based spectral shift mechanism was studied with

a threshold current density of 11.41 kA/cm2 . Further reduction in the threshold current density

and laser operation at shorter wavelength can be achieved by optimizing the MQW laser

structures especially the carrier concentrations.

Acknowledgements: The authors wish to thank Dr. Rayees A. Zargar, Dr. Rizwan Hussain and Jyoti

Bansal of Physics department at Jamia Millia Islamia for their helpful discussions.

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For Review Only - University of Toronto T-Space · For Review Only Experimental study on the mechanism governing spectral shifts in low power 670 nm AlGaInP multiple quantum well