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Quantum Wens

CU-Hsiang Lin, Ruj Q. Yang, D, Zhang, S. J. Murry and S. S. Pei Space Vacuwn Epitaxy Center, University of Howton: TX 77204-5507

A. A. All- and S, R. Kurtz Saxidia National Laboratories, Albqurque, Nm 87 123-0603

I

We have dernonstrateci rm-temperature CW operatian of rype-ll quantum cascade (QC> light emitting diodes at 4.2 )un using InAs/LndjaSb,lsAISb type-II quaatum wells. The t y p e 4 QC configuration utilizes sequential multiple photon emissions in a staircase of coupfed type-n quantum wells. The device was grown by molecular Seam epitaxy on a p-type GaSb substrate and was composed of 20 periods of active regiaos separated by digitally graded quantum well kjection regions. The maximuan average output power is about 250 p W at SO K, and 140 pW at 300 K a1 a repetition rate of 1 kHz with a duty cycle of 50%.

Keywords: -E, Md-R h.&and, qwtum well, typed qww well, quantum cascade, laser

1. INTRQDUCTION

Mid-inhed (W) lasers emittLng from 3 to 5 pm have hqmtant applications in infrared (IR) countmeasures, IR lidar, remote sensiag, and enviromental monitoring such as detection of trace gases, hydrocarbon fuel emission, md other common pollutants. However, both milibry sllhd comemid M B q s r e m s are limited due 10 a lack of adequare sources. Ciurently, MIR solid state lasers tend to be bulky with little wavelength agility, while optical parmeaic 0x;illators (which use ti nonlinear cx-ytal to down convert or up convert the output wavelength of pump sources) we cmnpiex and expensive. Semiconductor diodes lasers have significant advantages in terms of cost, volume, weight, relaibility, power dissipation, and manufacture. The availability of compact high-ptwer MlR dccmducror diode hsers operating at ambient or themo-electrically cooled temperahnes would significantly enhance the capability of c m t MZR technology.

W f i l Lead-salt lasers emitting at wawiengths longer thao 3 pin have been available for a number of years. These lasers with emission wavelengths of 3 pm have exhibited puisd operation at tmperntures up to 290 K [l], and cw apmtioax up to 223 K 121. However, they are not expected to prodwe high

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output pavier (31 mw) because of the low thermal conductivity of lead-salt alloys, and their susceptibility to damage, For narrowp bandgap lINl semicmducms such as HgCdTe alloys, the first repofled laser diodes operated & pulse injection up to 90 K at the wavelength of 2.86 pm. It is thus very desirable to develop semicanductor Lasers wing m-V alloys, because these materials offer good metallurgical and hemal properties and because better-quality substrates are readily available.

Elecentiy, MIR lasers utilizing Sb-based type4 otructuras are showing a stead progress 12-93. ROQII~- temperam operation of Lase: diodes at waveleq$hs shorter than 2.8 pm has been r e h d [3-5]. However, due to the inDinsio nature of me-I iaterband traasitions, the lasing wavelmgtb is limited to the marerial bandgap, and the device performance suffers &om non-radiative Augz recoinbinarioa. For wavelength longer than 4 pm, M3R lasers based on we-l interbaud structure has proven to be quite chdleghg.

MIR lasets bared on lnAsllnGaSb type4 superlattices { SLs) [ 191 have achieved similar performance as those based on Sb-based type-I structures. A rnaxhm operating temperature = 255 K was achieved at 3.2 pin [lo], By simply changhg the M s and LnGaSb layer thichesses, the laing wavelength of t y p e 4 l.uMhGaSb lasers could vary h r n MIR to long TR. This is an important advanrap of InAs/InGaSb t y p e 4 lasers compared to the Sb-based -1 devices in term of epitaxy growth, since the composition contrd, composition uafoamity, and growth reproducibility of InGaAsSb, hAlAsSb, axi .4lGaAsSb are very dificult. Additionally, due to the Unique feateue of type-ll structures, the Auger recombinatian could bt deamatically suppnssed +&ougb careful bandgap enginering. For the gain spectrum, InAshGaSb rype-U sl-mtures can display strong osciUato1 strengrh as Long as the the layers are thin enough to afhw e~~Xkph hterpenekation of the electron and hole wavehetiom, and hence large gain should be attainable from typc-n structures. Recently, we have demonstrated above room-temperature operations of oprticdy-pumped MIR lasers based 03 InAslInraSblAISb tyye-U quanrum wells (QWs) with record rnax,imw opmMg te~~peiaturs T, and ch~acteristic temperature To [11-15]. Laser ezllissim at 4.2 to 4 5 pn has been observed at temperatures up to 310 K, and a peak output power exceeding 2 Wifacet was observed at 200 K [t5].

Type-I quantum cascade (QC) lasers [ 163 which utilize sequential multiple photon emksions between subbands in a staircase of coupled type4 EnGrtllsh4LAs QW structure promises significantly improved perf5m.ce of MLR lasers. The recent demonstran'ons of a high power rmm-ternperanue QC l a w at 5 pm [ 163 and a Img w~selength IR (- 1 1 p) QC laser aperathg up to 200 K [I71 indicate a great potential for the QC configuration. The QC lasers are espr=cMly suitable for obtaining high output powers. However, the type4 QC lasers still have a relatively low radiative efEcicncy (4x10") due to a fast non-mdiative phonon relaxation between subbands, which leads to a high threshold c m t l t densiry and subscnntial heating.

in contrary, the type-ll QC lasers, as originally proposed in Ref. 18, are based on interband transitions where &e phonon relaxation is inerinSjcdly suppressed due to tbe opposite curvatures of conduction and valence band strucntres. Figure I shows &e schematic Ctrawing of a n-Pype QC laser based on rype-TI QWs [ 19,201. Under a forward bias, electrons are injected from an injection region into the InAs well. The hGaSb, MSb, md GaSb layers are thick en~ugh to eflicimtly block che direct maneling of electrons out of the ph4s well, resulting in a reduced leakage current.

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AlSb \

i I 1’1 h AlSb

91Sb ?

AIS$ L

A

&

active region - injection region 34 Akb

Figure 1. Schematic drawing of a n-type-QC structure based on type4I QWs.

2. EWERIMOEM The type-a QC light emitting diode (LED) was composed of 20 periods of active regions separated

by n-type injection regions which serve both as the collectors for the preceding active regions and emitters for the follow% ones. As shown in Figure 1, the active region was composed of several couples QWs, which were sequentially made of 23 A AISb, 25.5 A InA5, 34 A InGaSb, and 15 A AlSb layers. The injection region consists of digitidly graded W A l S b (WUSb) QWs in which b InAs layexs are Si doped at 6% and the AlSb and M S b layers are wdoped, The whole device s t r u c ~ m is strain balanced and lattice matched to tbe GaSb substrate. To improve the power efficiency, we have been careful in the design of active regions and injection regions in order to minimize the losses due to inrerband and intersubband absorption, rtnd Auger recombination. Under a fornard bias, electrons are injected from an injection region into the level Ee which is in the bandgap region of the adjacent InGaSb layer. Since the electrons are effectively confined in the lnAs well with the InGaSb, AlSb, and GaSb barrier layers, they tend to relax to rhe hole state Eh id the adjaceaK valence band QW. The resulting photon emissions are shown in Figure 1. Therefore, the leakage current is reduced. Electrons at state E” WiU then CTOSS the thin AlSb baniex and GaSb layer by nu~reling and scattering into the conduction band of the next injection regon because of a strong spatial interband coupling (an unjque feanue existed in Sb-based type-l[I heterastructure), and are ready for the next interband transition. Since the relaxation time fiom E, to Eh is much longer than the carrier t~aasportation h e at Eh, a papulatian inversion is easily established.

ca-’

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1 -80 -60 -40 -20 0 20

Theta (minutes of arc) 40 60

Figure 2.004 DCXRD spectnm; of a type-ll QC device.

The rype-ll QC LEDs we= grnwn in a Riber 32 marecul~beam epitaxy W E ) system on a p typc GaSb substrate using an EPI As vdved nacker cell and an EPI Sb c&er cell, A 3OCIO-a CaSb bufkr layer adjacent to the subsvate was p w n at 510 O C . ‘The W A S b cladding layers and the active ~gim was gmwn at 440 O C , Izuring the growttl the GaSb layers ctisplayed gad l x 3 reQection Mgh- awgy elzEtron dZhction (WEED) pattern and the JnAs layers extdbited good 2x1 lZlW3D panem. After grrpwth, the sample was anaew at 490 C for 10 minuter, Growth ratts were calibrated to within E? Sb using FU-IEED and were czmfhwd by doutde-crystal x-ray diffraction (7.X-j measuremenrs. ‘ik background doping density was Imv los an‘ n-ape. for lnAs and was about 2~10’~ cm‘ p m for GaSb. Vshg the satnc: growoh rate for InAs and GaSb, the background Sb in InAs layers was less #an 1% and rhe backgrownd As in GaSb was less !lm 3%, with a VmI BEP ratio af 3 for InAs and 2 for GaSb. The BEP ON/OFF ratio with shutters opened or c l d was abut 8 far &a, and was about 160 for Sbl, When h SWGa V/m BEP ratio was increased, the backgrcrwd As in GaSb ckreasd However, the backgraund Sb ia InAs layers was not sensitive to the As flux, This implies W the sticking coefficient of b a c k g m d Sb in lnAs at 440 O C Is almast 100%. Fmm (PL) specua, the material quality of GaSWAlSb QWs is mt sensitive to the SblGa VmI BJ2 ratio as lory: as the ratio is larger than 1.8. In order to h p m e the layer thicbess and ccxn@tim conmi, we have also investigated the flux t r d m s of La, Ga, and Al elfusion cells due to shurter cpmtim [21], Using the =I SUMO cells fQr In

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Phonon fiergy (mew

Fig. 3a. EL spectra at 80 K and 300 K.

A u m p b e n t (mA)

F i p 3b. Average ourput powi VE average atrrenr.

5

-1 0 .'I .s - 1 .o -0 .s 0 .a 0.5

V o l t a g e (V) ( L i g h t E m i s e i o n - . F o r w a r d E i r a " )

1.0 L .5

Rg. 3c. Chmt VS. applied voltage of the type.I? QC LED at 80 K and 3oQ K,

3, CONCLUSlQNS

With impraved design ofthe acme region and injetion region, we fi;a.it improved tlie output power mare than three orders of mapmde compared to ow first reported type-u QC LED However, the perEorm3nce of &is device is still strongly limited by b e p a r material quality (striations m the murfrace), possibly due the poor crystal quality of the GaSb substrate. Further improvemts kr the substrate; quality and MBE growtk wil l significaarrly improve the device performance. To acichiewe rwm-cemperature MX lasers based on me-11 QC configuration, furrhzr mvestigations in the device design, especially the carrier trmsporcatiot! and Auger recombmatian, s\d! be the key issues.

4, ACKNOWLEDGMENTS The authors at UR thank J. R. Meyer and H. Q. Le for helpful discussion, H. C. Lh for the detailed

characterizations of earlier cleviccs, and C.-X. Thang for DCXEUl measurement. The wurk at UH is partially supported by NASA contract No. NAW-977 and TcSCH. The work at Sandia National Laboratories is supported by Dept. of Energy under Contract No. DE-AC34-94AL8500.

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