AlAs mirrors grown by gas-source molecular beam epitaxy

ELSEVIER

Materials

Chemistry

and Physics 51 (1997)

1-5

Strain-compensated InAsP/GaInP multiple-quantum-well waveguide modulators and in situ monitored AlGaAs/AlAs mirrors grown by gas-source molecular beam epitaxy C.W. Tu *, X.B. Mei Department

of Electrical

and Computer

Engineering,

Received

Uniivrsi@

18 December

of Cal$ornia,

San Diego, La Jolla,

1996; accepted 29 January

CA 92093-0407,

USA

1997

Abstract Gas-source molecular beam epitaxy (MBE) is used to grow high-quality strain-compensated InAs,P, -,/Ga,In, -YP multiple quantum wells on InP substrates for modulator applications at 1.3 p+rnwavelength. The composition of the InAs,P, --x well can be determined in situ by the As-to-In incorporation ratio from As- and In-induced intensity oscillations of reflection high-energy electron diffraction. Tensile Ga$n, -,P is used to compensate the compressive strain in the InAsP. A modified doping profile in a waveguide modulator is used to provide a better confinement of the depletion region in the p-i-n structure under reverse bias. An improved quantum-confined Stark effect is obtained. In situ monitoring and controlling reflectivity of the AlGaAs/AlAs quarter-wave mirrors is used for reflection-type modulators on GaAs. 0 1997 Published by Elsevier Science S.A. Keywords:

Gas-source

molecular

beam epitaxy;

Strain compensation;

InAsPiGaInP

1. Introduction Optoelectronics devices operating at 1.3 p,rn wavelength to reduce fiber loss and dispersion are of great interest for long-haul fiber communications. High-contrast, high-speed optical intensity modulators based on the quantum-confined Stark effect (QCSE), as well as multiple quantum well (MQW) lasers, have been made at this wavelength [ l-41. The strained InAsP quantum well has some advantages over the quatemary systems (InGaAsP or InGaAlAs) for these applications, such as the independent control of thickness and composition of InAsP and reduced alloy scattering. In order to obtain a high contrast ratio in modulators or better temperature characteristics in laser operation, a relatively large number of periods is desirable [ 5,6]. However, since InAs,P, --x quantum well layers grown on InP are under compression, the whole stack has a large net strain, which limits the number of MQW periods grown without strain relaxation. To solve this problem, tensile-strained Ga,.In, -,P has been employed as a substitute for InP as the barrier material [7,8] to compensate the compressive strain in InAsP. The net strain can be reduced and a stable structure with more periods achieved. Moreover, since the barrier strain intro* Correspondingauthor. 0254-0584/97/$17.00 PUSO254-0584(97)01918-4

8 1997 Published

by Elsevier

MQWs;

Waveguide

modulator;

In situ monitoring;

AlGaAsiAlAs

mirror

duces a new degree of freedom, the flexibility in structure design is increased. For waveguide modulators we also introduce pulse-doped layers on both sides of the MQW region to confine the electric field better [ 141. In this paper we report the successful growth of straincompensated InAsP/GaInP MQW structures for 1.3 pm wavelength waveguide modulators by gas-source molecular beam epitaxy (GSMBE) and also the in situ monitoring and control of AlGaAs/AlAs distributed Bragg reflector (DBR) mirrors for the purpose of fabricating reflection-type InAsP/ GaInP modulators. 2. Experimental The MQW samples were grown in a Varian GEN-II MBE system modified for GSMBE growth. Thermally decomposed ASH, and PH, were used as the group V sources, and elemental In and Ga as the group III sources. The growth temperature was set at 46O”C, and the InP growth rate was typically 1 pm h-‘. The growth rate was determined by intensity oscillations of reflection high-energy diffraction (RHEED). The layer thicknesses were determined by double-crystal X-ray (400) diffraction (DCXRD) . For surface-normal photocurrent measurement, mesas with a 500 mm diameter were defined by wet chemical etching on the sample surface. A ring-shaped AuZn/Cr contact was

Science S.A. All rights reserved

C.W. Tu, X.B. Mri / Materials Chemisrry and Physics51 (I 997) I-S

2

evaporatedonto thep + -In I).53Ga,,37A~ cap layer. In wasused as the n-type contact on the back sideof the substrate.The breakdown voltage of the ring diodesis typically 6 V at 2 p,A leakagecurrent. The surface-normalphotocurrent measurement wasperformed at room temperature.A beamof monochromatic light was focused onto the center window of the ring diode, and the photocurrent was amplified with a lockin amplifier. The reverse electric bias was applied via the ring-shapedelectrode and the back contact.

$

aE

0.8

I

T,=4SO”C

I

,

,,,

3. Results and discussion O-OcL--L--L--L---L--‘o

3. I. Gas-sourceMBE growth of InAsP We and others have shown previously that the ternary compound InAs,P, --1 is a viable alternative to the conventional quatemary compoundInGaAsP for 1.3 km modulator andlaserapplication [ I-41. The growth rate, or the thickness, is controlled by the In flux in MBE, and the wavelength, or the composition, is determinedindependently by the arsineto-phosphine flow-rate ratio [9]. Both parameterscan be determinedin situ by monitoring the group III- andgroup Vinduced RHEED intensity oscillations [ lo], so the material composition and thickness are easily more reproducible. Once the As and In incorporation rates, RAsand Rr,, respectively, at a desiredgrowth temperatureare calibrated by Asand In-induced RHEED oscillations, respectively, asshown in Fig. 1. Hou and Tu [ 91 show that when the ratio R,,/RI, is lessthan unity, all of As, dimersreact with In atomson the surfaceandthe remainingIn atomsreact with P, dimers.The excessP2 moleculesdesorb from the surface.Thus, the As compositionx in InAs,P, --xis simply the ratio R,,IR1,. Based on this idea of in situ controlling the As composition in InAs,P, -x, a seriesof InAs,P, -,/InP strained-layer superlattices (SLSs) were grown at 2 seemPH, flow rates and different R,,/R,, ratios. Shown in Fig. 2 is the ex situ determined As composition, by X-ray rocking curves of SLSs, versusthe R,,IRI, ratio. An excellent agreementcan be seen when x < OS. The discrepancyfors > 0.5 is becausethe large surface strain may affect the incorporation behavior of As

L

+In J t3

Fig. 1. The InAs RHEED Asduringr,
-AS

-In, +As I 10

oscillations

Time Is)

I 20

a

Ash

I 31

are induced by In during t,
Incorporatlon-Rate

Fig. 2. Ex situ determined As composition curves vs. in situ determined As composition [91.

Ratio

in InAsP from X-ray rocking from RHEED measurements

and P. Another possibility is that partial strain relaxation in InAs,TP,-,JInP SLS samplesmay lead to an underestimate of the As composition. Of course, there is an upper limit to the P, flux for this simple in situ method of composition detemlination. When the PH3flow rate is much higher than the ASH, flow rate (by about a factor of five), more P2 will be incorporated. As a result, the As composition in InAsP would start to decrease asthe P2flux increases.Therefore, to keepthis simplein situ method valid, it is important to useas small a V/III incorporation ratio as possible. Again, one should note that at higher substrate temperaturesdesorption of AS? and P2 is significant and the incorporation of one speciescan be influenced easily by the presenceof the other. Therefore, the in situ method is applicable only for substratetemperatures lower than about 500°C. 3.2. Stmin-compensated

InAsP/GnlnP MQWs

Since we are interestedin MQW light modulatorsbased on QCSE, a relatively large number of quantumwell periods ( > 20) is desirable,especially for a normal-incidencemodulator. Since the InAs,P, -.‘i quantum well layers (x= 0.4 for 1.3 pm and a strain of 1.3%)) grown on InP, are undercompression, the whole InAs,P,-,/InP MQW structure has a large net strain and the number of MQW period is limited by the onset of strain relaxation. Fig. 3 shows double-crystal rocking curves of InAs,,~P,,,/InP (90 A/130 A) MQWs with 11. 7 and 5 periods,correspondingto curves (a), (b), and (c), respectively [ 111. Curve (d) is a simulation, based on the dynamical theory, of curve (c) . It is clear that the lattice already starts to relax for the 7-period MQW as the superlatticesatellitepeakshave becomeslightly broader.For the 11-period MQW, thesuperlatticesatellitepeaksarehardly present,indicating almostcomplete lattice relaxation. If we use a GaJn, -,.P barrier layer, which has a tensile strain, a strain-compensatedMQW structure with a large number of periods can be obtained. The Ga compositiony

C.W. Tu. X.B. Mei /Mhlaterinls

(a): xl 1 (b): x7 (c): x5 (d): simulation

Cherrzi;rtq

rind

Physics

51 (1997)

of (c)

_

T A-f ,’

3

1-j

plnGaAsP

di I!-ln&PflnGaP

MQWs 1 , k /

n+ InP Substrate Fig. 5. Schematic

1 (g) -4500

,

,

,

-3000

-1500

0

Angle

(arcsec.)

Fig. 3. Double-crystal X-ray rocking curves of strain-uncompensated InAso4P0,&P (90 Ail30 A) MQWstructures with (a) 11, (b) 7and (c) 5 periods. (d) is a simulation of (c) based on the dynamical theory [ 1 I 1,

-

1

-2

Experimental Simulation

I

I

I

1

-1

0

1

2

Angle (deg.) Fig. 4. X-ray rocking curve (top) of a strain-compensated 21-period InAs0.1P06/G~.l&, 87P (93 A/ 135 A) MQW structure, and the bottom curve is a simulation based on the dynamical theory [ 111.

and the thicknessof the GaJn, -,P layer are designvariables. The critical numberof periodsincreasesrapidly with increasing Ga compositionuntil when the net strain is zero. We have P(86A112OA) MQWsupto grown InAso.~Po.a/Ga.171n0.83 50 periods without degradationof crystallinity. Fig. 4 shows an X-ray rocking curve and a simulation for a 21-period MQW, which showssharpsatellitepeakson both sidesof the substratepeak, indicating excellent crystallinity. The zerothorder peak is hidden in the substratepeak, indicating a nearly perfect strain compensation. An electroabsorption modulator is in principle a reverse biasedp-i-n structure with the active region in the undoped i-region. The i-region in our waveguide modulator structures, shown in Fig. 5, contains 6 periods of InAs0,4Po,,/ G~,,aIn,,,,P strain-compensatedMQWs with the well and barrier thicknessesN 100 A. The active MQW region issandwiched between a p-doped and an n-doped passive waveguiding layers with a bandgap ~~o.~~Gao.~~Aso.~9Po.71

diagram

of a waveguide

modulator,

wavelength of 1.15 mm. This structure provides a vertical optical mode size of approximately 2 mm, comparableto the mode size in a lensed fiber, so that the coupling loss is reduced.Note that the doping level in the p and n-type waveguiding layers is only 5 X 10” cme3, low enoughto reduce the propagation losscausedby free carrier absorption. Fig. 6 shows surface-normal electroabsorption measurements at room temperature for a 6-period InAs,,,P,,,/ G~.,Jn,,,,P (93 A/ 130 A) MQW, sandwichedbetweenn+ and p” contact layers, under reverse bias. A beamof monochromatized light wasfocusedonto the center window of the ring diode, and a photodetector was placed on the backside of the sample.The reverse electric bias was applied via the ring-shapedelectrode and the back contact. Excellent QCSE is obtained. At zero bias the half-width at half-maximum (HWHM) of the 11= 1 electron-heavy hole exciton peak is assmall as6 meV, which is excellent for this highly strained material systemand for materialsat this wavelength. The problemwith the abovementionedstructure,however, is that the depletion region extends from the i-region into the guiding layers underrevere biasbecauseof the relatively low doping level in these guiding layers and relatively thin iregion (thickness ci,= 0.13 pm). The wider depletion region causesa decreasein the electric field (E-field) in the MQWs at a given biasvoltage V,, resulting in a lower transmissionvoltage slope efficiency. Our solution to this problem is to insert a highly doped (2 X 10” cm-3) and thin (20 nm) p+

2 dV

,v 2v

----3v ~’ 4v

I

12X0

12400

I

rzml

wveiength Fig.

6. Electroabsorption

I

I

13200

1

133X

(A)

spectra of a strain-compensated

MQW.

C. W. Tu, X.B. Mei / Materials

Chemistv

and Physics 51 (1997)

1-5

29%. This result indicates that the pulse doped layers provide a better confinement of the depletion region and therefore increase the efficiency of the bias voltage. By comparing the experimental QCSE with the calculation values in Fig. 8, we obtained the E-field at each V,. The E-field in #922 is approximately 20% larger than that in #834. As a result, the transmission-voltage slope efficiency improves by 15% in the waveguide measurement [ 141, 3.3. AiGaAs/AlAs on GaAs

n+-bPsub

I

i Fig. 7. Modulator

*

layer structure

with pulse doping.

#834, &moLJtPD

-a0

I 50

I loo

I 150

I .x0

250

Fig. 8. QCSE energy shift in #834 and #922. The solid line is a calculation based on the effective well width model 191.

(n+ ) Ino.s;iGa.13As0.29P0.71layer (pulse-doped layer) in the p (n) guiding layer right on the side of the i-region as shown in Fig. 7. Since the two pulse-doped (PD) layers are very thin compared with the waveguide thickness ( = 1.7 mm), they do not cause serious free carrier absorption. Better confinement means that the depletion region is thinner under the same V,, so a larger E-field can be obtained in the MQWs. For comparison, two waveguide modulator samples, #834 and #922 were grown. #922 has the above mentioned pulsedoped layers, while #834 does not. In Fig. 8 we plot the QCSE energy shift of the n = 1 heavy hole exciton peak in these two samples against the reverse bias. The solid line is a theoretical calculation based on the effective well width model [ 121, assuming that the depletion region width equals the i-region width, di, and the E-field is uniform across the depletion region and is zero elsewhere. This situation is the ideal case where the confinement of the depletion region is perfect. From Fig. 8, one can see clearly that #922 (with PD) has a much larger QCSE than #834 (without PD) under a given bias. At 2 V reverse bias, the difference is as large as

DBR mirmrsfor

InAsP/GnInP

MQWs

It is desirable to integrate InP-based 1.3 pm MQWs with GaAs-based DBR to make reflection-type modulators. One reason is that GaAs-based DBR mirrors are more easily produced than InP-based mirrors. Besides heteroepitaxial growth, another method that is gaining popularity is wafer bonding. Here we use in situ monitoring of the reflectivity spectrum to realize an accurate control of the growth of AlGaAs/AlAs DBRs [ 131. The reflectivity-measurement system consists of a white light source,afiberopticcable, aspectrometerandacomputer with a spectrum simulation software. The white light is introduced to the sample surface via the fiber cable. The reflected signal is collected by the same cable and is sent to the spectrometer via a T connection. The reflection spectrum data are collected, displayed and stored by the computer. The measurementof one spectrumtakeslessthan a second. To grow a DBR mirror, we interrupt the growth aftergrowing for five periods,lower the substratetemperatureto lOO”C, measurethe reflection spectrumthrough the center viewport, and perform computer simulations with different structural parameters.By fitting the simulated curve to the measured one, we delermine the actual compositionsand thicknesses of the grown DBR. If theseparametersare different from the expected ones, we can introduce somecompensationin the rest of the DBR by varying the layer thicknesses. This processis shownin Figs. 9 and 10. The whole DBR structure consists of 14 periods of quarter wave AlAs/ Al,,G+~,As layers. The target wavelength of the DBR is 860 nm. Fig. 9 showsthe reflectivity spectrumof the first five and half periods. The reasonof choosingthe half period is to have an A1,,IGa,,,As insteadof an AlAs layer on the surface in order to reduce contaminationduring the growth interruption. The simulation showsthat the Alo, ,Ga,,As layers were 0.26 wavelength thick, a little thicker than the expected0.25 wavelength. We adjustedthe subsequentgrowth accordingly and obtained a DBR with the center wavelength close to 860 nm, asshownin Fig. IO.The integrationofInAsP/GaInP MQW modulators on these AlGaAslAlAs DBR mirrors is under development. 4. Conclusions We have shown that strain-compensatedInAsP/GaInP MQWs are an excellent alternative to InGaAsP/InP MQWs

C.W.

TLC, X.B. Mei

/ Materials

Chetniut~

und

Plzpics

51 (1997)

1-j

5

pulse-doped layers sandwiching the MQW confine the electric field in the MQW and result in better modulator characteristics. For surface-normal, reflection-mode InAsP/GaInP modulators, we have used in situ reflectivity measurement to produce the desired AlAs/Al,,,G~,,As DBR mirror.

Acknowledgements We wish to thank the DARPA OptoelectronicsTechnology Center for the support of the work and fruitful discussions with K.K. Loi, Professors W.S.C. Chang and H.H. Wieder.

0.0

Fig. 9. Reflectivity spectrum of a 5-period DBR. The dashed curve is a simulation using AlAs and Al,, ,Ga,,,As, 0.25 and 0.26 wavelength thick, respectively.

Fig. 10. Reflectivity

spectrum

of a 14-period

DBR.

for 1.3 pm modulators. The wavelength, or the mole fraction of InAs, is easily controlled by the As-to-In incorporationrate ratio through RHEED intensity oscillations in GSMBE. Strain compensation of using tensile-strained GaInP to balance the compressive-strained InAsP allows a large number of periods in MQWs. For waveguide modulators, special

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