ABNORMAL LASING BEHAVIORS IN THIN P-CLAD

INGAAS QUANTUM WELL LASERS

C. H. WU1, P. S. ZORY2 and M. A. EMANUEL3

1Institute of Nuclear Energy Research, P.O.Box 3-11, Lung-Tan, Tao-Yuan, Taiwan 3252University of Florida, Department of Electrical Engineering, Gainesville, Florida, 32611, U.S.A.

3Lawrence Livermore National Lab, Livermore, California, 94550, U.S.A.

(Received 14 June 1997; in revised form 21 July 1997)

AbstractÐThin p-clad (250 nm) InGaAs quantum well (QW) lasers with 200 nm p+-GaAs cap layerthickness and various device con®gurations are studied. For wide stripe (w = 50 mm) lasers, abnormallasing characteristics of very high thresholds, current on-time dependent lasing and micro-second longdelays are observed. With 300 nm ridge-height, micro-second long lasing delays are found in w = 6 mmlasers, and Q-switching in w = 2.5 mm and 3.5 mm lasers, respectively. The unusual lasing behaviors areattributed to the decrease of transverse optical con®nement caused by the increase of p+-GaAs caplayer thickness. To improve device performance and eliminate lasing delay, p+-GaAs cap layer thick-ness of wide stripe lasers and stripe width of narrow stripe devices have to be not greater than 170 nmand 1.5 mm, respectively, in thin p-clad (250 nm) laser structure. # 1998 Published by Elsevier ScienceLtd. All rights reserved

1. INTRODUCTION

Recently, thin p-clad laser structure was studied in

index guided lasers[1] and distributed bragg re¯ec-

tion (DBR) lasers[2]. In fabricating such devices

with thin p-clad con®gurations, the contact re¯ectiv-

ity has to be high to reduce the modal loss[3]. Since

the p-cladding layer thickness is thin (<500 nm)

and the associated refractive index of p+-cap layer

at the lasing wavelength is close to that of active

region, the variations of p+-cap layer thickness

could be important in determining laser perform-

ance[4].

In our continuous work on thin p-clad (250 nm)

laser structure, di�erent types of lasing delay are

observed for lasers fabricated with 200 nm p+-

GaAs cap layer thickness and various device con-

®gurations. For wide-stripe (stripe-width

w = 50 mm), gain-guided lasers, micro-second long

lasing delay similar to those reported on homo-

structure, single heterostructure (SH) lasers and

gain-guided narrow stripe (wR10 mm) QW

lasers[5±8] are obtained. In addition, lasing

threshold of this thin p-clad laser is abnormally

high and current on-time dependent. For the

narrow stripe (wR6 mm), 300 nm ridge-height

lasers, stripe-width shows great in¯uence on device

lasing delay performance. Micro-second long lasing

delays are observed for w = 6 mm lasers and Q-

switching obtained for both w = 3.5 mm and 2.5 mmlasers. When w is reduced to 1.5 mm, no lasing

delay is found.

The work reported here is organized as follows:

Section 2Ðdescription of thin p-clad laser material

used and the basic parameters responsible for the

long time delays; Section 3Ðpresentations of wide-stripe lasers fabricated and the e�ects of QW refrac-tive index change on threshold gain of diode lasers;

Section 4Ðdetails of narrow-stripe, 300 nm ridge-height lasers fabricated and the measured results;Section 5Ðconclusions.

2. LASER STRUCTURE AND THEORETICAL

CALCULATIONS

The thin p-clad laser structure used in this studyis shown in Fig. 1, where p-cladding layer thicknessis only 250 nm and p+-cap layer thickness tcap=200 nm. As mentioned before, the variations oftcap could substantially a�ect diode laser perform-ance, to check this point optical con®nement factor

G and modal loss ai of laser structure stated aboveare calculated as a function of tcap. In the compu-tations, a lasing wavelength of 950 nm and shinyAu metal with a refractive index of 0.17-i5.7[9] used

as p-contact are assumed. The calculated results areshown in Fig. 2. As can be seen from this plot thatG and ai remain relatively constant when

tcapR100 nm and change slowly when increasedfrom 100 nm to 140 nm. As tcap is larger than140 nm, both G and ai change quickly. At

tcap=200 nm, G decreases smaller than 0.4% and aiincreases as high as 66 cmÿ1. The calculated resultsindicate that much more material gain is required

for thin p-clad lasers to reach the threshold con-dition if p+-cap layer thickness is increased from100 nm to 200 nm. For example, the thresholdmaterial gth of the thin p-clad laser with cavity

Solid-State Electronics Vol. 42, No. 3, pp. 405±410, 1998# 1998 Published by Elsevier Science Ltd. All rights reserved

Printed in Great Britain0038-1101/98 $19.00+0.00PII: S0038-1101(97)00205-0

405

length L = 500 mm and 100 nm p+-cap layer is

01200 cmÿ1, while gth of laser with 200 nm p+-cap

layer thickness is increased to as much as approx

22 000 cmÿ1. Consequently, thin p-clad (250 nm)

diode lasers fabricated with 200 nm p+-cap layer

thickness could have a much higher lasing threshold

than those lasers with tcap =100 nm.

3. WIDE-STRIPE LASERS

To check the predictions of laser performance

stated before, oxide-de®ned wide stripe lasers with

w = 50 mm are formed by using pulsed-anodic oxi-

dation technique to remove part of the p+-cap

layer on the outside stripe region and replaced by a

native oxide[10]. Alloyed metallogy of Ge/Au/Ni

and non-alloyed Au are deposited as the n-contact

and p-contact metal, respectively. It is noted that p-

contact metal can not be annealed to induce extra

modal loss. Diode lasers with various cavity lengths

are cleaved, soldered on the In-coated copper

blocks and characterized.

The average threshold current for the samples

with 500 mm cavity length is Ith01000 mA when

measured at 2 m s current pulse-width and 1000 Hz

repetition rate. The measured results are about 20

times higher than that of devices fabricated with

similar structure and 100 nm p+-cap layer thick-

ness[3]. In addition to the abnormally high

threshold performance, there is an unusual lasing

turn-on delay between the application of the

injected current and the onset of stimulated emis-

sion. Figure 3(a) shows the schematic diagram of

lasing delay observed, where tth is de®ned as the

threshold current on-time (under constant repetition

rate) at which the stimulated emission begins. At

certain input current I and constant current on-time

Fig. 1. Thin p-clad InGaAs SQW diode laser structure used in this study.

Fig. 2. The calculated dependence of modal loss ai andoptical con®nement factor G on the p+-GaAs cap layerthickness of thin p-clad InGaAs SQW laser structure

described in Fig. 1.

C. H. Wu et al.406

t, if t is smaller than tth diode laser only operate in

the below threshold condition and no stimulated

emission obtained. In the case of trtth, stimulated

emission starts and laser operates in the previous

threshold condition. Besides the lasing delay,

threshold current Ith depends strongly on the injec-

tion current on-time t. Figure 3(b) shows the

measured variations of Ith as a function of t. It is

noticed that the average Ith decreases signi®cantly

from 1000 mA to 720 mA when t is increased from

2 m s to 10 m s and then becomes relatively

unchanged with further increase of t.The calculated results of Fig. 2 have shown that

thin p-clad laser structure with 200 nm p+-cap layer

has very small optical con®nement and large mode

loss. In order to understand the dominant factor

for the abnormal behaviors, 50 mm stripe-width,

thin p-clad (250 nm) lasers with p+-cap layer

=100 nm and the same G factor but di�erent mode

loss are fabricated and lasing characterizations com-

pared. In the fabrications, either Au-metal or Ni-

metal is used as the p-contact[3]. The associated aiand G of these laser types are ai(Au)= 3 cmÿ1,G(Au)= 2.3%, and ai (Ni)= 70 cmÿ1, G(Ni)=

2.3%, respectively. It is very interesting to ®nd that

both laser types do not show lasing delay and

apparently points out that decrease of G is the main

factor responsible for the abnormal lasing delay

observed in thin p-clad QW lasers. This is also con-

sistent with the experimental results reported in

double heterostructure (DH) lasers[11], where G is

big and no lasing delay is observed.

For QW lasers, lasing delay times on the order of

micro-seconds were reported in narrow stripe, gain-

guided devices[6]. The long delay is attributed to

the time it takes for the active region under the

stripe to heat to the point where net mode gain

exceeds mirror loss. Net mode gain increases as

active region temperature rises because the associ-

ated rise in the refractive index increases the overlap

of the mode pro®le along the QW plane (lateral

mode overlap) with the peaked material gain region

under the stripe. For our wide-stripe QW lasers,

there is no strong index guiding along the junction

plane. The cause of lasing delay observed in wide

stripe lasers presented here should be di�erent from

that of narrow-stripe, gain-guided lasers and has to

do with the time it takes for the active region heat-

ing to increase the overlap of the mode pro®le per-

pendicular to the QW plane (vertical mode overlap)

with the material gain in the QW. This mode over-

lap with QW material gain increases with the rise of

QW refractive index and is closely related to the

injected current density as well as the injected cur-

rent on time[12]. For InGaAs QW lasers, carrier-

induced refractive index depression larger than 0.5

was also reported[13]. To understand the e�ects of

refractive index change Dn of QW on thin p-clad

laser performance, we have calculated the variations

of threshold material gain gth as a function of Dnwhen two di�erent p+-cap layer thickness

tcap=100 nm and tcap=200 nm are considered. The

calculated results are shown in Fig. 4, where a facet

re¯ectivity of 0.3 and a cavity length of 500 mmare assumed. It is noticed from this plot that

gth changes tremendously for the tcap =200 nm

laser from gth(Dn = 0) = 2.2 � 104 cmÿ1 to

gth(Dn=ÿ 0.5)= 4.2�104 cmÿ1 and gth(Dn=+0.5)

=0.98�104 cmÿ1 when Dn is changed from 0 to

Fig. 4. The calculated dependence of threshold materialgain gth on the refractive index change of quantum well Dnfor thin p-clad InGaAs SQW laser with di�erent p+-GaAscap layer thickness: 100 nm and 200 nm, where a facetre¯ectivity of 0.3 and a cavity length L= 500 mm are

assumed in the computations.

Fig. 3. (a) Schematic diagram of lasing delay behaviorsobserved in wide-stripe, thin p-clad InGaAs SQW lasers.(b) The variations of input current I as a function ofthreshold current pulse time tth for diode lasers with500 mm cavity length, where the input current repetition

rate is 1000 Hz.

Abnormal lasing behaviors 407

ÿ0.5 and +0.5, respectively. In contrast, under the

same variations of Dn, only a small change of gthfrom gth(Dn=0)=1.2�103 cmÿ1 to gth(Dn=ÿ 0.5)

= 1.4� 103 cmÿ1 and gth(Dn= + 0.5) = 1.0�103 cmÿ1 is found for the tcap=100 nm laser. The

variations of gth induced by Dn of tcap=100 nm

laser is so small that Ith could remain relatively

unchanged at any current on time point

measured and no lasing delay observed. However,

for lasers with tcap=200 nm, the variations of gthinduced by Dn is so big that Ith could change signi®-

cantly at di�erent current on time and cause lasing

delay.

The calculated results in Fig. 2 indicate that thin

p-clad laser behaviors could also be further

improved by reducing the p+-cap layer thickness.

To check this point, thin p-clad lasers with 50 mmstripe-width and three di�erent p+-cap layer thick-

ness: 170 nm, 130 nm and 70 nm are fabricated and

characterized. The di�erent p+-cap layer thickness

are obtained by etching away part of the 200 nm

p+-cap layer by the same method stated above and

followed by oxide stripper to remove the remaining

native oxide. Figure 5 shows the measured depen-

dence of Ith and ai on the p+-cap layer thickness. It

is interesting to note that Ith decreases abruptly

from 1000 mA to 150 mA when tcap is decreased

from 200 nm to 0170 nm and then decreases slowly

with further reduction of tcap. At tcap070 nm, Ith is

reduced to 050 mA. In addition, signi®cant

improvements of ai are achieved from 64 cmÿ1 to

34 cmÿ1 when tcap is reduced from 200 nm to

170 nm. At tcap0130 nm, the measured ai is08 cmÿ1

and changed slowly to03 cmÿ1 while tcap is reduced

to 070 nm. The most important feature of the

measurements is that no lasing delay occurs when

tcap is smaller than 170 nm. These measured results

of Ith and ai are in good consistence with the theor-etical predictions.

4. NARROW-STRIPE, 300 NM RIDGE-HEIGHT LASERS

In a previous study we showed that strong lateralwaveguiding e�ects could be obtained from thin p-

clad, narrow-stripe (w = 5 mm) lasers with lasingperformance comparable to those of conventionalthick p-clad lasers[1]. For similar thin p-clad laser

structure in this study we are interested in under-standing whether the strong waveguiding e�ectscould be obtained in narrow-stripe lasers. To checkthis point, lasers with various stripe-width

w = 6 mm, 3.5 mm, 2.5 mm and 1.5 mm on 500 mmcenters are de®ned by using standard photolithogra-phy technique. Ridge laser structures are formed by

etching away the outside stripe p+-cap materialthrough the use of chemical solutionNH4OH+ H2O2+50H2O and followed by anodic

oxidation to grow native oxide on the outside striperegion. The resultant ridge height is 0300 nm afterthe Dektak measurements. Wafer is then thinned to0100 mm and deposited with n-type and p-type con-

tact metals as described in the previous section.Lasers with various cavity lengths are cleaved, sol-dered on the In-coated copper blocks and charac-

terized.Table 1 shows the summarized results of

narrow-stripe diode lasers with 500 cavity lengths

and various stripe widths when measured at 2 m scurrent pulse width and 1000 Hz repetition rate. Itis noted that all sample types except the

w = 1.5 mm one show di�erent types of lasingdelays. Additionally, lasing threshold for thesamples with lasing delay are quite high comparedwith the results of a previous study[1] and greatly

decrease when w is decreased to 1.5 mm. Thew = 6 mm sample shows very high Ith, approx 250mA, and micro-second long lasing delay similar to

those observed in the wide-stripe lasers. For thesamples with w = 2.5 mm or w = 3.5 mm, a lasingdelay transition from Q-switching to long time

delay with the increase of injection currents isobserved. The lasing delay transition with the inputcurrent of w = 2.5 mm sample is shown in Fig. 6(a)±(d). At I = 80 mA, it is clear to see that stimulated

emission starts at the time position correspondingto the ending edge of the input current pulse i.e. Q-switching. As input current I is increased to 100

mA, both Q-switching and long time lasing delayare observed. When I is increased further to 120mA, Q-switching disappeared and long time delay

dominates laser performance. For w = 1.5 mmlasers, no lasing delay is observed.The totally di�erent lasing behaviors observed in

various stripe-width samples could be attributed tothe changes of lateral waveguiding e�ects in di�er-ent laser con®gurations. Since it is believed thatwaveguiding e�ect is a two-dimension function. In

Fig. 5. The variations of threshold current Ith and modalloss ai of thin p-clad InGaAs SQW diode lasers as a func-tion of p+-GaAs cap layer thickness when measured at 2

m s current pulse width and 1000 Hz repetition rate.

C. H. Wu et al.408

order to check this point and simplify the calcu-lations, the e�ective refractive index step Dn(lateral)between stripe region and outside stripe region iscalculated as a function of ridge-height. In the com-

putations, it is assumed that p+-cap layer, p-AlxGa1 ÿ xAs graded-layer and part of p-clad layer

of the outside-stripe region are removed and

replaced by a native oxide with a refractive index of1.8, the whole structure being covered with Au.

From the calculated results, at 300 nm ridge-heightthe corresponding Dn(lateral) is 3.5�10ÿ2. This

Dn(lateral) value is 020 times larger than that ofsimilar laser structure with 100 nm p+-cap layer

and the same ridge-height[1]. For w = 6 mm, 3.5 mmand 2.5 mm samples, Dn(lateral) of 3.5� 10ÿ2 may

not be su�cient to provide strong waveguiding toovercome the lack of transverse optical con®ne-

ment. However, for both w= 3.5 mm andw = 2.5 mm samples, the transverse optical con®ne-

ment could be somehow improved by Dn(lateral)such that Q-switching behavior is observed. This is

because that total loss of diode laser could decreasefaster than the gain and con®nement such that

stimulated emission can occur at the end of currentpulse[12]. The decrease of stripe-width w make the

improvements become more pronounced. As w is

reduced to 1.5 mm, Ith decreases signi®cantly and nolasing delay is observed. The measured results lead

us to believe that Dn(lateral) of 3.5� 10ÿ2 is su�-cient for the w = 1.5 mm samples to have strong

waveguiding to overcome the weakness of trans-verse optical con®nement. To further check the

waveguiding e�ects, lateral far-®eld intensity distri-

butions of diode lasers are measured and shown inFig. 7. As can be seen from this plot that multi-

lobe patterns are obtained for lasers with

w = 6 mm, 3.5 mm, 2.5 mm and their lobe numberdecreases with the decrease of stripe-width. For

w = 1.5 mm laser, single-lobe pattern is achieved.The diminishment of high order mode patterns in

lateral far-®eld intensity with the reduction of stripe

width w indicates clearly the improvements of wave-guiding e�ects in these narrow-stripe, thin p-clad

laser structures. In the measured CW P±I character-istics of w = 1.5 mm laser at room temperature, a

lasing threshold of Ith=29 mA and a total di�eren-

tial quantum e�ciency of 50% are obtained. Themeasurements of narrow stripe, thin p-clad lasers

described before not only show that stripe-width

plays an important role in determining laser per-formance but also point out that w is more crucial

for this laser structure than similar device structurewith 100 nm p+-cap layer to operate in low

threshold and single spatial mode regime.

Table 1. Summary of narrow stripe, 300 nm ridge-height thin p-clad QW laser lasing behaviors when measured at 2 m s current pulsewidth and 1000 Hz repetition rate, where the diode laser cavity length is 500 mm

Narrow stripe thin p-clad InGaAs SQW diode lasers with 300 nm ridge height

Stripe width: w

Laser per formance 1.5 mm 2.5 mm 3.5 mm 6 mm

Average threshold current:Ith 25 mA 80 mA 170 mA 280 mALasing delay behavior NO lasing delay Q-switching Q-switching Long time delay

Fig. 6. The transition of lasing behavior from Q-switchingto long time delay of a w= 2.5 mm, 300 nm ridge-height,thin p-clad InGaAs QW laser when measured at 2 msec.current pulse width, 1000 Hz repetition rate and variousinput current I: (a) I= 60 mA, (b) I= 80 mA, (c)

I= 100 mA, (d) I = 120 mA.

Fig. 7. Normalized lateral far-®eld intensity distributionsof thin p-clad InGaAs QW lasers with 300 nm ridge-heightand various stripe width when measured at I= 1.3 Ith, 4m s current pulse width, 1000 Hz repetition rate and room

temperature.

Abnormal lasing behaviors 409

5. CONCLUSIONS

In summary, lasing delay obtained in wide-

stripe, thin p-clad QW lasers are reported for the®rst time. This lasing delay has to do with the timeit takes for the active region heating to raise the as-

sociated refractive index and increase the overlap ofthe mode pro®le perpendicular to the QW plane(vertical mode overlap) with the material gain inQW. In addition, thin p-clad diode laser perform-

ance can be greatly improved and no lasing delayobserved when p+-cap layer thickness is reducedbelow 170 nm. Moreover, for 300 nm ridge-height,

narrow stripe lasers, stripe width has to be reducednot greater than 1.5 mm in order to overcome thede®cit of transverse waveguiding and make lasers

operate in low threshold regime.

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C. H. Wu et al.410