Type II diode lasers based on interface recombination at ... - CiteSeerX

Type II diode lasers based on interface recombination at 3.3pim Anthony Krier, Derek A. Wright, Victoria J. Ellarby Physics Department, Lancaster University, Lancaster, LA] 4YB, UK Victor V. Sherstnev, Konstantin D. Moiseev & Yuri P. Yakovlev Joffe Physico-Technical Institute, St Petersburg, ]95279, Russia ABSTRACT There is considerable interest in the realisation of room temperature mid-infrared diode lasers for a variety of applications, including remote gas sensing, infrared countermeasures and molecular spectroscopy. However the maximum temperature of operation in narrow gap Ill-V component alloys is limited by strong non-radiative Auger recombination and various band structure engineering techniques are being investigated to provide Auger suppression. In

our work we are investigating the possibility of obtaining a practical 3 .3 jim laser by making use of radiative recombination across single type II hetero-interfaces. Because transitions occur between confined electron and hole states

localised on either side of the heterojunction where the potential wells are triangular, there exists the possibility of tailoring the wave-function overlap to give good Auger suppression while still maintaining high radiative output. At the

same time growth from the liquid phase offers potentially lower SRH recombination. We compared two such

heterojunctions (1nAs094Sb006/InAs and Ga96In004As011Sb0 89/InAs) grown by rapid slider LPE and report on the photoluminescence and electroluminescence from the interfaces. The dependence of these interthce transitions on temperature, excitation intensity, band offset and polarisation is reported, with a view towards incorporating these in the active region of a practical laser.

Keywords: Hetero-interfaces, mid-infrared lasers, photoluminescence

1. INTRODUCTION

There is a growing interest in semiconductor lasers emitting in the mid-infrared spectral region (3 -5tm) since these lasers can be used in the trace gas detection of a variety of species which have characteristic absorption fingerprints in this range, leading to applications such as remote sensing, molecular spectroscopy and ecological monitoring. Other potential applications include infrared countermeasures and free space optical communications within the (3-5 jim) atmospheric transmission window. However, the room temperature operation ofnarrow-gap Ill-V semiconductor lasers is inhibited by Auger recombination which has a strong effect on the temperature dependence of the threshold current. As an alternative, intersubband quantum cascade lasers have been developed which may offer substantial output power at room temperature but are likely to be expensive to manufacture and are more appropriate for wavelengths >4jim '.During the last few years, the first Ill-V mid-IR VCSEL (2.3 jim) has been demonstrated 2 and strained Sb-based lasers, type II superlattices and "W"— structure interband lasers are also currently receiving much attention. These type H MQW and superlattice structures have already achieved some success and it is possible to engineer the Auger/radiative rate, but at

Novel In-Plane Semiconductor Lasers, Jerry R. Meyer, Claire G. Gmachl, Editors, Proceedings of SPIE Vol. 4651 (2002) © 2002 SPIE · 0277-786X/02/$15.00

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superlattices and "W"— structure interband lasers are also currently receiving much attention. These type II MQW and superlattice structures have already achieved some success and it is possible to engineer the Auger/radiative rate, but at the expense of increased complexity making them complex to grow and leading to uncertainty about cost-effective manufacturability.

In our laboratory, we have worked extensively on the liquid phase epitaxy (LPE) growth of InAs, InAsSb and InAsSbP and their heterojunctions, for the fabrication of LEDs4'5 and detectors 6 Using LPE technology we have previously fabricated diode lasers for the spectral range 2—3.5im in collaboration with the loffe Institute which operate

up to 160K. Together we aim to develop mid-JR interface lasers based on type II broken-gap GaSbfInAs and GaInSbIInAs/GaAlSb and related heterostructures. These are considered as promising alternative laser materials because of the spatial separation of electrons and holes and their localization in self-consistent quantum wells on opposite sides of the interfitce. In this way we aim to realise a room temperature 3.3im laser in a much simpler structure using interface quantum wells (IQW) rather than conventional type II QW or superlattices or cascaded structures employing thin multi-

layers. The advantages of this idea are its inherent simplicity and better e-h wavefunction overlap because of the triangular shape of the wells due to band bending at the interihce, resulting in higher radiative recombination probability. Furthermore, epitaxial growth from the liquid phase has the advantage that the crystallinity of the material is high since epitaxy occurs near thermodynamic equilibrium leading to lower point defect concentration and reduced Shockley-Read-

Hall (SRH) recombination. It is against this background that the present work reports on our investigation and comparison of the type II InAs/InAsSb and InAs/InGaAsSb hetero-interfaces where we identi1y the origin of the associated radiative transitions, with a view towards producing mid-infrared lasers fbr use in practical applications.

2. EXPERIMENTAL PROCEDURES 2.1 LPE growth of GaSb-rich and InAs-rich Ga1InAsSb1 To begin with, the bulk Ga1InAsSb1 quaternary solid solutions were grown lattice matched on (100) oriented InAs substrates by liquid phase epitaxy (LPE) at both ends of the composition range. The wide-gap GaSb-rich quaternary solid solutions were obtained in the range O.03 5pm) layers gives broad bands characteristic of graded bulk material.

of

Because

the

independence of the spectrum on composition we associated some of the peaks in fig. 4. with transitions

involving the InAs substrate as

6.5

shown in the schematic energy band diagram of figure 6. This figure is based on the well known radiative transitions normally observed in InAs and identifies the 416 meV

recombination transitions

with

6.0

band-band

and the 400 meV peak

with electron (or shallow donor) — acceptor recombination involving the

0.30 0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46 Energy / eV

acceptor states at 15 meV which are thought

to originate from native

lattice defects in InAs. The remaining features however are due

to

radiative

interface

transitions

at the

in much the same manner

.

. .

Fig. 4. PL emission spectra obtained from InAs1Sb / InAs single heterojunctions with Sb contents of x = 00 and 011

as in the Ga0841n016As0225b078 / InAs heterojunction described above. The 323, 350, 370 and 380 meV peaks are most likely due to transitions between electron and hole quantum well sub-bands localised at the interface. The 370 meV peak

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Wavelength / im 10

0.1

001 031

0.32 0.33 0.34 035 036 037 038 039 OAO 0.41 0A2 0A3 0A4 0A5 0A6

Energy / eV Fig. 5 The PL spectra (log plot) obtained from an InAs094Sb006 I IriAs single hetemjunction, using different laser excitation powers

4Ec =58meV

Ed =1-3meV

1ñASo8oSb020

416meV

AEv=I88meV

Fig. 6 The proposed energy band diagram in the InAsi, Sb ,/ InAs single heterojunction, showing the observed radiative transitions.(x - 0.20 at the interface

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which appears as a shoulder on the 380 meV band and the 323 meV bands are too weak to allocate reliably. However, the 350 and 380 meV peaks behaved similarly. Both these bands were polarised and exhibited the same blue shift with increasing laser power. The temperature quenching behaviour of the 350 and 380 meV emission bands was also similar, being less than that of the InAs—related transitions. This is strong evidence that both these bands are a result of interface transitions between quantum well sub-bands. This makes the 380 meV transition encouraging for use as the basis of a near room-temperature mid-infrared laser.

3.4 Investigation of electroluminescence in Ga084In016As022Sb078 I In083Ga017As082Sb018 single heterostructures interface Previously, strong electroluminescence was observed from the P-Ga84In016As022Sb078 I p-InAs

hetero-interface of 3.2 above in the

B77

B

spectral range 0.3-0.4 eV and over the temperature range 4.2-77K 8 The EL spectra at 77K contained two narrow

emission bands (hvA=0.310 eV and

hvB=0.378 eV). This was again consistent with assuming that the 2Delectrons from the quantum well in the narrow-gap layer recombine with holes localised near the heterointerface of the GaInAsSb quaternary solid solution9.

0.4

-

0.3 0.2

This led to the development of the

0.1

single hetero-interface P-GaInAsSb/ pInGaAsSb type II laser in which only a weak temperature dependence of

0.0

77 K

300 K 0.35

threshold current in the range 77-110 . . . . K, with a high characteristic temperature (T060 K) was obtained, indicative

of Auger

suppression.

0.45

0.40

0.50

Photon energy (eV) Fig 7a. EL spectra from the P-n heterojunction at 77 and 300 K

However, the threshold current density was high . 2 kA/cm2 which was mainly due to electron leakage across the P-p interface of the broken-gap heterostructure". This occurs by Auger ejection of hot electrons over the barrier into the wide gap GaInAsSb. To ftirther improve on this result it

E

was necessary to control electron leakage by increasing the electron confinement.

To improve the electron confinement we Pn studied the Ga 841n0 16As0 22Sb0 78/ In083Ga17Aso82Sbo8 heterojunction which has a

E 05 qU

higher barrier height (compared with P-p) and

EFP

investigated its electroluminescence properties. The

resulting spectra are shown in fig 7a and the corresponding energy diagram for the structure is shown in fig 4b. The EL spectra from our n-InAs / P-

Ga 841110 16As022Sb0 78/n-In083Ga17As0 82Sb018 heterostructure

at 300 K contained two intense

emission bands in the spectral range 0.25-0.45 eV as shown in fig.4a. The band A with peak photon energy

0

'Ci.

'i

Fig 7b. The energy band diagram ofthe P-n heterojunction interface

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199

at hvA=O.320 eV and half-width of 26 meV was more intense than the band B, hvB=O.355 eV, with a half-width of 68

meV. Band A was strongly polarised and exhibited a blue shift towards the higher energy emission band B with increasing drive current, before reaching a constant energy of '-M.345eV above l5OmA. The emission band B appeared at higher current after the emission band A, and its peak photon energy (hvB=O.383 eV) was the same as that observed in our PL measurements(hv2). The emission band B remained at constant spectral position over the whole range of drive current.

Consequently, it was established that the experimentally observed emission bands could be identified with interfaceinduced radiative transitions as shown in fig 7b. The emission band A was associated with tunnelling-assisted radiative transitions across the interface while the emission band B occurred in the narrow-gap solid solution.

A comparison of the EL spectra measured at 77 K and 300 K revealed a relative redistribution of the

EL peak intensity In contrast to the low temperature EL spectra where the emission band B was dominant the emission band A became narrower and exhibited higher intensity than the band B at room temperature Furthermore with increasmg temperature from 77 K to

'

I'

jfl(..573 3000

9 " '-

T0=25K ..

°°

300 K the decrease of the EL mtensity was observed to

be different for each emission band The intensity of

B decreased by almost one order while the

band

intensity of the emission band A decreased only by a factor of two ( see inset of 4a) The very low thermal quenching of EL intensity for the emission band A suggested the possibility for this transition of the single

,' 3

7yJ

a

600

0 'ii'" •

heterojunction to be used as the basis of the active region in a mid-infrared laser or a high-efficiency lightemitting diode operating at room temperature

I

60

.

I

I

I

I

80 100 120 140 16O1)2O0

Diode lasers The Temperature (K) InAs0 94Sb0 06/InAs heterojunction investigated using PL was enclosed within the active region of a DH laser with a lattice-matched InAsSbP Fig 8. Threshold current vs temperature for the P-n GaInAsSb waveguide. The structure was prepared in the form of a interface laser. double channel etched laser with a mesa stripe width of 55prn. A lasing wavelength of 3 .6 im was obtained at 77K and the laser operated up to 200K with a constant characteristic temperature, T0 45 K. This laser is limited by Joule heating and work is in progress to demonstrate pulsed operation at room temperature As mentioned above in 3 .3, a laser structure based on a single type H broken gap P-GaInAsSb/ p-InGaAsSb 3.5

heterointerface was fabricated previously. This laser exhibited a high characteristic temperature (T060 K) with a threshold current density of 2 kA/cm2, mainly due to electron leakage across the p-P interface of the broken-gap heterostructure. Following our characterisation of the P-n heterointerfàce electroluminescence a tunnel injection laser based on the type II P-GaInAsSb/n-InGaAsSb heterojunction was lubricated instead, which enabled us to fIjrther increase the electron confinement. A significant decrease in the threshold current and an improved temperature dependence was achieved. Mesa-stripe laser structures operating in the single mode regime were fabricated using standard photolithography and wet chemical etching with a stripe width of 1 10 im and cavity lengths L350-750 tm. We succeeded in raising the diode laser operating temperature up to T15O K with a characteristic temperature of T053 K. An improved maximum operating temperature of 192 K under pulsed mode conditions was also achieved as shown in fig 8. The threshold current density in the P-n tunnel injection laser was about th4O° A/cm2, which is five times less than in the previous P-p structure.

200

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3.6 New laser design Although encouraging, the results for this laser show that T0 decreases to 25 K at a temperature of 150 K. This is most likely related to the loss of holes into the n-

type narrow gap material by thermally assisted tunnelling as indicated in fig 4b. In order to prevent this

AE

it is necessary to re-design the structure. Our new design based on the transition A discussed above is

P-AIGaAsSb

shown in fig 9.and forms the basis of further current investigation. In this structure the p-n junction has been

reversed which allows the growth of wide gap A1GaAsSb to give asymmetric band offiet confinement of 0.7 eV for electrons and 0.4 eV for holes respectively

within a good waveguide. The structure also contains fewer layers leading to a reduced series resistance, and so an improved maximum operating temperature is

n—InAs

EF8

expected.

fl4 n-GainAsSb Fig 9. Proposed design for room temperature laser

4. CONCLUSIONS The details of radiative recombination at the P-p and P-n hetero-interfaces in PL and EL have been investigated and the transitions originating from recombination between localised electron-hole states on opposite sides of the interface have been clearly identified. Both GaInAsSb/InGaAsSb and InAsSbfInAs heterointerlhces were found to have interface related transitions and it was possible to realise mid-ir diode lasers in both systems which operated up to '200 K. Diode laser performance improvements have been realised for the GaInAsSb/InGaAsSb system by improving electron confinement as a result of using the P-n heterojunction with higher band offsets rather than the isotype P-p heterojunction. Our results enabled the design of a new laser structure which we believe is a good prospect for realisation of a room temperature midinfrared laser. Such a laser has considerable potential for use in a number of applications including for example, trace gas detection, infrared countermeasures, remote sensing and industrial process control.

ACKNOWLEDGEMENTS We would like to thank the EPSRC for providing a research grant to support this work and for providing a visiting fellowship for Dr K.D.Moiseev. This work was also supported in part by a grant from the Russian Basic Research foundation # 00-02-17047.

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REFERENCES

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