Variation of electroluminescence line halfwidth in GaAs diodes

NOTES Solid-Srorc

Eklronics

was also measured in the same ways and is included simply for comparative purposes. Physical properties of the first two diodes are given in Table 1. The injection current was supplied in 100 nsec pulses at 60 Hz, and the output radiation was detected by an Amperex CVP-I50 type S-l photomultiplier tube after passing through a Spex 1400 double grating spectrometer. In Fig. 1 the spectral output of a typical GaAs diode for different currents is shown. It is to be noted that increased current causes an upward shift in output light intensity and spectral position and also a narrowing of the halfwidth. The variation of the first two parameters with current has been widely published (e.g. Pankove[l] and Wittmann[2]), and thus this topic will not be pursued here. Figure 2 shows the decrease in halfwidth which occurs with increasing current. All three diodes show nearly the same characteristics, the current variation of Ahu being slightly greater for the commercial diode than for the others. The crosses terminating the curve mark the halfwidth AhVth at which the first mode of stimulated emission

Pergamon Press 197 I. Vol. 14, pp. 175-178. Printed in Great Britain

Variation

(Received

of electroluminescence diodes

line halfwidth

8 April 1970; in revisedfbrm29

June

in GaAs

1970)

NOTATION

h

Plank’s constant (eV-set) frequency ( I /set) Ah: EL halfwidth (eV) RerL(hu) external radiation (photons/set-eV) efficiency (dimension711, power conversion less) A area of p-n junction (cm2) e electronic charge (Coulombs) J current density (A/cm2) plus diffraction coefficient _ %fl absorption (l/cm) A NEGLECTED part of the great amount of experimental data on GaAs diode properties is a simultaneous display of the current and temperature variations of the electroluminescence (EL) line halfwidth. Although EL line halfwidth narrowing with increasing current is already known, data is fragmentary. One purpose of this paper is to demonstrate the decrease of EL halfwidth with increasing current at various temperatures and to suggest a plausible reason for it. Another purpose is to show that the EL halfwidth at the threshold for lasing does not increase sufficiently with temperature to account for more than a small part of the temperature dependence of the threshold current. The above two properties were noted in preliminary measurements on both diffused and epitaxially grown, degenerately doped diodes. One from each group was chosen for more detailed measurements, and the results are given here. A commercial diode* (denoted by C in Figs. 2 and 3)

I.““‘-“‘.“1

hv, ev Fig. I. Spectral distribution at I l.YK of EL emission from GaAs diode 28-E for three different current values. The spectral appearance is similar for the other diodes tested.

*A developmental type RCA-TA2628 laser diode in modified TO-46 package with low-absorbing glass window. Detailed knowledge of physical properties not available. 175

176

NOTES

Tuble I. Drrtcr for GaAs laser diodes. Fabry-Perot swfuces ure cleuved rend IIIIcarted on 5-D and 28-E. Currier wncentrmtion and mobility ut room tempercituw 5-I) (diffused)

n-region

p-region

dopant

Si

Zn

carrier concentration

I O’“/cm:’

carrier mobility

Junction thickness

==8pm

area

4.5 X 10 ~.’cm’

diffusion time at 900°C

3 hr

thickness

+Ipm

I WS/cm:’

2850 cm’/\/-set

parameters

28-E (epitaxial) dopant

Si

carrier concentration carrier mobility

Zn

I O’x/cm”

7.8 x 10’H/cm:’

2794 cm’/V-set

86 cm’/V-set

area

diffusion time at 900°C

-

becomes the

evident.

multimode

appears

about

the

and

In

kept narrow mode

slitwidths

of

Fermi

halfwidth

will

A\hu,,. To avoid this Spex

could

be

easily

level

in

in the donor The

with increased

as

not greatly

resultant

into

of finite

width.

quasi-Fermi

level p-side

The region states).

current level

when stimulated

an upward

position

temperatures,

the

by additional

does

picture

absorption

of the EL emission the quasi-Fermi Thermal

20 min

smearing

u1[.5] that

increases

EL is In

and the

the

EL

Fabry-Perot halfwidth we

current the

before

by

offer

diffraction.

the

over

faces

(the

this gain is greatest emission.

sharper

and the halfwidth

emission

did

that

current

the

is due

more

following

narrowing

ex-

of Al~u

in Fig. 2: At than loss by

current

increases

loss by absorp-

a significant

range of

it is equal to this loss plus that from

taneous

here

As

significantly

inducing

and it may exceed

Fabry-Perot

Since

Reasoning of Danilova

mirrors

Akv

with increasing current as depicted low currents. gain is much less and

halfwidth.

with current.

for the subthreshold

absorption

about

effect on any

and the observation

emission,

planation

smearing

to have a significant

this model

stimulated

higher

complicated

at the higher energy edge

varicltion of the halfwidth

EL

At

is slightly

levels of both holes and electrons.

but is unlikely with

shift in amplitude occur).

due to thermal

tion and diffraction[7]

(due to degen-

of pertinent

of injection

change the El. linewidth

the

(although

spectral

so does gain[6].

diodes[4].

the trcticc

by absorption

occupancy

rise of the

GaAs

distance along

this model an increase

devel-

electrolumines-

of the quasi-Fermi

a hole-band

affected

the energy

donor tail and takes into

with

of emission

electron

of the quasi-

have recently

doped

the drop

the junction

and

et

band tail since this level

authors

degenerately

consideration

lineshape

The

is omitted

reduce

31 and increases

It asserts an exponential

ah well

were

for the EL measurements

in the EL halfwidth

current[2,

involved.

cence

1400

identified.

oped a model for low temperature

erate

been

can not be due to a lowering

rises with

from

has

0.2 and 0.S A.

The decrease

range

of

of the

near threshold

was between current

lasing

so that the separate lines of the multi-

emission

resolution

in

such a case it is not

threshold

a measurement

the

is poor, emission

of the EL. spectrum

be less than the EL halfwidth problem.

resolution

of stimulated

fashion.

that

reached,

optical

the center

a smeared-out known

When structure

8.2 x 10m4cm’

not

the

El.

mainly

emission.

condition).

line

observed

will

be

less than if stimulated

occur.

narrowing

threshold

near the peak of spon-

Hence

it is concluded

of A/tv

with

to the gradual

does not

stimulated

This

emission

patible with that of Danilova

interpretation

increasing growth i\

of

com-

rt ct/.[51who obtained

177

NOTES

that nU does depend on absorption, diffraction, and the internal quantum efficiences; and that the temperature dependence of these factors affects that of Q,. RpS’(hv,) is proportional to the internal Aux density at frequency vp. At threshold the internal flux density at, this frequency must be sufficient to insure that gain via stimulated emission equals loss by absorption plus diffraction (oI,,,) as well as transmission through the Fabry-Perot faces. Hence the temperature dependence of cypff determines that of the external radiation threshold intensity R;,ff(hu,,). Thus at threshold we have from equation ( I):

lr

D

/-

Y

4 )O

YL

OM

where the temperature behavior of Jr,, established by the factor in brackets arises from that of (Y,,~ and the internal quantum efficiences for stimulated and spontaneous emission. The temperature dependence of Ahr+,, also affects JLh. The quantity Ahv reflects the energy range for the electrons and holes at low currents. However, at higher currents where stimulated emission occurs, Ahv narrows even more and begins to represent only the energy range for which radiative transitions occur at the highest rate. Nevertheless, since thermal smearing

A.&

CURRENT, A -

of EL line as a function of current at Fig. 2. Halfwidth different temperatures for three CiaAs diodes. I he letters E, D and C denote epitaxial, diffused and commercial, respectively.

for only one temperature current-emission linewidth data which was somewhat incomplete over the subthreshold range. The total external radiation can be written in terms of current density J, the area of the p-n junction A. and power conversion efficiency[8] r), as I

R’“‘(hv) dhv = R’“‘(hv,) Ahv = nPA$,

(1)

where hv, is the energy at which the EL emission peaks, Rest(hv) is the number of photons per unit time per unit energy interval emitted externally, and the linewidth is written Ahv since it is almost identical to the halfwidth. The quantity nl, can be expressed in terms of internal quantum efficiences for stimulated and spontaneous emission and factors which involve integration over all directions while taking account of absorption and reflections [9]. The important point to note here is simply

T,“K-Fig. 3. Comparison of temperature depen,‘_itces of EL line halfwidth evaluated at threshold and the threshold current for three GaAs diodes. The areas of the p-n junctions are listed in ‘Table 1.

178

NOTES

does increase trons

and

stimulated Hence

the energy

holes

exist,

emission

band

a broadening

is mainly

and

does not fully

holes

the

rise

electron-hole Other

effects[lO,

(Yangand

1I]

halfwidth It

more

is

perature

two

whereas

of two or three. is not

Hence

the

perature through

that

just be

an

orders

of

efficiences.

variation

the

one

of Ahv,,,.

increase

quantum

I? June,

(Receiwd

I Y70: in reui,~edfbrm

24 July

1970)

of both

Rather

tions

on crystal

semiconductor proposed

in fundamental

growth

is

and in the fabrication

devices. Various

to obtain

wafer

invchtigaof

methods have been

the doping

profile[l-IO].

Of

capacitance technique [ I-31 is used. It involves measurement

these the differential the most commonly

as a function

bias for a Schottky

diode or an abrupt p+n junction.

current

magnitude

from

ofJ,,, with tem-

due to a wider

energy

from

increase

tem-

relates

and resultant

smearing

in (Y,~, and the internal

quantum

The

technique

before

of interpolating

accuracy

between

two

computation

and is limited

in

by the necessity

point5 to obtain

the

slope of the capacitance

vs. voltage curve. The use

of an automated

data handling

digital

system

has

been suggested to speed up the computationl41. This note describes the

differential system

which

the doping profile. used

a simple method of automacapacitance

technique

using

;I direct

plot of

described

can be

provides

The technique

in conjunction

with

abrupt p+n junction J. LYNN

considerable

and numerical

an analog Ackno,cled~~f~Pnt-The authors wish to thank R. A. Shatas for helpful comments pertaining to the preparation of this paper.

requires

of reverse applied

the final result is obtained

resolution

ing

efficiences.

Solid Stcrte Physic,.7 Brunch Physical Sciencrs Lrrhorutory Redstone Arsenal, Alahamrt.

in a semiconductor

parameter

of the capacitance

of

its

profile

DOPING

an important

threshold

the increase

to temperature

THE

and the threshold

and holes resulting

smearing. changes

discussed.

to

Ahvlh rises by only a factor

primarily

of electrons

indirectly

itself

is due

be an increase

would of

seen

A technique for directly plotting the doping profile of semiconductor wafers

range

A~v,*

temperature

at threshold

than

11 to 300°K

range

though

As

would

decreases

current.

in width.

temperature

of the energy

3 shows the temperature

the EL rises

increase with

elec-

and also

suggested that the main reason

smearing.

of smearing

which

the energy range occupied

of Jth with

effect

Figure

will

even

represent

by them. Pilkuhn[8]

over

of Ahvlh

due to an increase

of electrons

for

range

the spontaneous

a Schottky

diode or an MIS

diode.

an

capacitor.

SMITH

H. K. WITTMANN

C-V

rrlmtiotts

For a Schottky

U.S.A.

capacitance

C,

diode, the relation the

doping

profile

between

the

n(.u) and

the

applied bias V is given by: REFERENCES I. J. I. Pankove, Phys. Reo. Lett. 9,283 ( 1Y62). 2. H. R. Wittmann, Phgs. stotw solidi. 35. 865 3. D. F. Nelson, M. Gershenson, A. Ashkin,

4. 5.

6. 7. 8. 9. IO.

I I.

( 1969).

L. A. D’Asaro and J. C. Sarace, Appl. Phys. Lett. 2, 182 (1963). J. I_. Smith and I-1. R. Wittmann, Phys. status .w/idi. (a)2.371 (1970). T. N. Danilova, L. M. Kogan, S. S. Meskin, D. N. Nasledov and B. V. Tsarenkov. Soviet Phys-Solid Sttrtr 8, I963 ( 1967). F. Stern, Phys. Rev. 148, I 86 ( 1966). W. Susaki. /nncm .I. cr~ool. Phvs. 5. 845 (1966). M. H. Pilkuhn. Plrgs. .&m skidi. 2.5. 9 ( 1968). G. Cheroff, F. Stern and S. Triebwasser, Appl. PI2ys. Lett. 2, I73 (1963). M. Pilkuhn, H. Rupprecht and S. Blum, So/id-St. Electron. 7.90s ( 1964). M. Pilkuhn and H. Rupprecht. J. opp/. Phys. 38, 5 (1967).

and xx-

0

c

where A is the area of the diode, 4 is the electronic charge

and t,? is the permittivity

ductor.

For an abrupt ~+n junction

ing in the high resistivity

of the semiconn(r) is the dop-

side of the junction

and

x is the distance from the edge of the junction. For an MIS

diode, in the deep depletion

the space charge

extends

region,

into the semiconductor

*Strictly speaking n(x) i\ the majority rather than the doping profile [ 11. 121.

carrieI-

profile