two-step growth of metamorphic gaas/algaas mirror on an inp substrate by mocvd
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Thin Solid Films 542 (2013) 317–326
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Two-step growth of metamorphic GaAs/AlGaAs mirror on an InPsubstrate by MOCVD
Yoshitaka Ohiso ⁎, Ryuzo IgaNTT Photonics Laboratories, NTT Corporation, Atsugi, Kanagawa 243–0198, Japan
⁎ Corresponding author. Tel.: +81 462403282; fax: +E-mail address: [email protected] (Y. Ohis
0040-6090/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2013.07.006
a b s t r a c t
a r t i c l e i n f oArticle history:Received 12 December 2012Received in revised form 3 July 2013Accepted 4 July 2013Available online 12 July 2013
Keywords:Metamorphic III-V semiconductorDistributed Bragg reflectorMetal-organic chemical vapor depositionTwo-step growthSurface morphology
This paper describes a high-reflectivity metamorphic undoped GaAs/Al0.98Ga0.02As distributed Bragg reflectorgrown on an InP substrate by metalorganic chemical vapor deposition (MOCVD). Optimal two-step growthconditions at low and high growth temperatures can provide a smooth surface morphology, leading to ahigh reflectivity (N99.5%) with little optical scattering loss. We also show that a large mismatched interfacedoes not create dislocations in the active layer at a slow cooling rate after the growth sequence. These resultsindicate that metamorphic GaAs/Al0.98Ga0.02As directly grown on an InP substrate by MOCVD is promising forapplication to InP-based vertical cavity surface-emitting laser structures.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Vertical surface emitting lasers (VCSELs) are very attractive as lowcost, low electrical power consumption and high-speed modulationlight sources for optical communications. 850-nm VCSELs for localnetworks have already been demonstrated operating at up to 40Gbit/s [1], and 49 Gbit/s has been achieved at −14 °C for 980-nmVCSELs [2]. In contrast, long wavelength VCSELs (LW-VCSELs) operat-ing at 1.3 or 1.55 μm have the same potential, but they have yet tofind practical applications for extended optical fiber communications.850- and 980-nm devices consist of a GaAs/Al(Ga)As distributedBragg reflector (DBR) with high reflectivity and high thermal conduc-tivity. In terms of candidate DBR materials for the LW-VCSELs, thereare no alternative semiconductor materials that match an InP substratewith high thermal conductivity. Recently, high-speed modulation (40Gbit/s) has been demonstrated by using 1.55 μm LW-VCSELs with di-electric mirrors [3]. However, a dielectric mirror has no electrical con-ductivity, and the VCSEL fabrication process is complicated because anintra-cavity structure is inevitable to fabricate the electrical injectionpath. As a result, there is a possibility that the cavity length (without in-cluding the penetration depth in the DBR) is relatively long because ofthe electrical path layer and the long photon lifetime, which is a disad-vantage for very high-speedmodulation. There have been some reportsof electrical conductive GaAs/Al(Ga)As DBRs that were stacked on anInP-based layer by wafer fusion technology [4] or by molecular beam
81 462404301.o).
rights reserved.
epitaxy (MBE) growth [5]. These VCSELs have achieved room-temperature continuous-wave operation at 1.3 or 1.55 μm. However,the wafer fusion process requires two or three substrates and is alsovery complicated, which makes it difficult to reduce fabrication costs.Direct metamorphic growth by MBE is very attractive, but the MBEmethod is now rarely used for growing InP-based active layers, whichare produced in huge numbers bymetalorganic chemical vapor deposi-tion (MOCVD).
This paper describes the characteristics of a GaAs/Al0.98Ga0.02AsDBR grown on an InP substrate by MOCVD for InP-based VCSEL appli-cations. To achieve extremely high reflectivity (more than 99.5%) forhigh performance VCSELs, it is most important to produce a smoothmetamorphic GaAs/AlGaAs interface with little optical scatteringloss by MOCVD.
In this paper we first describe the optimal growth conditions at aconstant growth temperature. Surface roughness is more clearly re-vealed on an exactly oriented substrate than on an oriented substrate,so we investigated the growth conditions using an (001) InP (exactlyoriented) substrate. We then present the impact of the misorientationof the substrate. Second, we describe the optimal conditions fortwo-step growth involving an initial buffer layer to achieve a bettersurface morphology. Finally, we discuss the influence of a highlystrained DBR on the active layer.
2. Experiment
There is a lattice mismatch of about −3.7% between GaAs-basedmaterial and InP. A number of studies have reported that island
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formation in the early stages of heteroepitaxial growth can lead to anelastic relaxation of the lattice parameter and three-dimensional [3D]growth [6,7]. There have also been several reports on highly-strainedGaAs/InP growth under various conditions, such as a low growthtemperature, a high V/III ratio, a high growth rate, and using amisorientated substrate. These approaches have been recognized asproducing a low dislocation density [8–11], and have focused onhigh crystal quality at the topmost surface, regardless of that of thebuffer layer. In this study, our goal is to achieve very high reflectivity,so the first priority is to find a way of growing very flat interfacesacross all the DBR layers. To obtain optimal growth conditions, we es-timated the conditions using in-situ reflectance, as grown reflectancespectra and atomic force microscopy (AFM).
The test structure consisted of 29.5 pairs of GaAs/Al0.98Ga0.02AsDBRs. Each layer had a λ/4 optical thickness at a center wavelengthof 1300 nm. All the samples were grown in a Veeco D180 LDMrotating-disk reactor operating at 900 rpm. The reactor pressurewas 9.3 kPa and the total hydrogen flowwas about 50 slm. The reflec-tance was measured with an in situ monitoring system using a lightemitting diode operating at a wavelength of 930 nm during the entiregrowth period. The source materials were trimethylgallium (TMG),trimethylaluminum (TMA), and arsine (AH3).
Heteroepitaxial growth is inherently accommodated by an inter-face network of dislocations as soon as the lattice-mismatched layerexceeds the critical thickness. In this study, the critical thickness ofa GaAs layer is calculated to be about 14 Å from the Matthew andBlakselee model [12]. When the layer thickness is exceeded, 3Dgrowth is favored in highly strained systems [13]. However, 3Dgrowth can cause large surface undulations, which lead to a large op-tical scattering loss with DBRs. With very high reflectivity (N99.5%),even a small optical scattering loss can develop into a major problem.
Reducing the surface roughness of GaAs/AlGaAs layers lattice-mismatched to an InP substrate depends on whether or not the sur-face diffusion length of Ga adatoms can be restricted in the earlystage until the development of 3D growth. It appears that a GaAslayer is more sensitive to 3D growth than an AlGaAs layer, becauseGa adatoms have a greater migration length than Al adatoms [14].
3. Results
3.1. Optimal growth conditions
3.1.1. Growth rateFig. 1(a) shows the in-situ reflectance of the DBR wafers during the
entire growth period. TheDBR layers consisted of GaAs and Al0.98Ga0.02Aslayers with abrupt interfaces in this work. The DBR layer growth rateswere 1.8 and 3.0 μm/h. The GaAs and AlGaAs growth rates were almostthe same for the same growth sequence. The samples were grown at650 °C with a V/III ratio of 120. The in-situ reflectance for the DBR struc-ture exhibited a periodic curve in relation to growth time. The peak-valley points of the curves coincided with an optical thickness of λ/4 atthe growth temperature (not room temperature). The number of peakswas the same as the number of DBR pairs. Ideally, the peak and valley re-flectivity with a low optical scattering loss maintains the same value re-gardless of the number of DBR pairs. The initial reflectivities of the twowafers were not maintained as the number of pairs increased as shownin Fig. 1(a). These results suggest that the optical scattering loss generat-ed at the GaAs/InP interface becomes larger as the number of pairs in-creases. After the growth, the topmost layers of both samples exhibitedlarge undulations when observed with an optical microscope at roomtemperature. Comparing the two samples using an optical microscope,we found that the surface morphology was somewhat smoother whenthe growth rate was 3.0-μm/h. From these results, we can assume thatwhen the growth rate is high, a subsequentGaAs layermay cover the sur-face until the Ga adatoms of the previous GaAs layer diffuse and 3D
growth begins properly, resulting in the suppression of 3 D growth tosome degree.
3.1.2. V/III ratioWith MBE growth, a high As pressure can reduce surface mobility
and prevent the formation of an equilibrium 3D morphology [5]. Al-though a high V/III ratio can also reduce the surface mobility forMOCVD growth [14], a large surface roughness can be observed forthe Al(Ga)As layer with a low V/III ratio. It is well known that as theAl content increases, the surface roughness becomes larger with ahigh V/III ratio.
Fig. 1(b) shows the in-situ reflectance of the DBRs with V/III ratiosof 6 and 120 and a 650 °C growth temperature. Although both reflec-tance values exhibit damping curves as the number of pairs increases,the reflectivity decay with a V/III ratio of 120 is somewhat smaller.This may be because a lower V/III ratio can enhance the reconstruc-tion of an Al(Ga)As layer, which is more effective for producing aflat surface than the suppression of 3D growth by a high V/III ratio[15].
3.1.3. Growth temperatureWhen the reduction in the Ga diffusion length is taken into ac-
count, a low growth temperature is promising for the GaAs layer[10]. Fig. 2(a) shows the dependence of the in-situ reflectance of theDBRs at a 3.0-μm/h growth rate and V/III ratio of 6 on various growthtemperatures (550, 600, and 650 °C). Although the reflectance valuesof the three samples are very close when the DBRs are less than tenpairs, the reflectance at a high growth temperature shows a decaycurve as the pair number increases. When the thickness of the DBRlayers exceeds ten pairs (that is, a total thickness of about 2 μm),the residual strain in the GaAs or AlGaAs must be almost completelyrelaxed [9]. These results suggest that anisotropic kinetic energy is re-stricted within a thin layer at a low growth temperature, but once thetotal thickness exceeds the thickness with free strain, the surfacemorphology becomes noticeably rougher.
Fig. 2(b) shows the details of the in-situ reflectance of the threesamples for a few pairs. Note that the reflectivity of the initial GaAslayer clearly exhibited a positive curve for the growth temperatureof 550 °C and negative curves for 600 and 650 °C. If an initial GaAslayer with an ideal flat surface is grown, the reflectivity will exhibita positive curve. This is because the refractive index of GaAs is largerthan that of InP at 930 nm. These results indicate that the surfaceroughness of the initial GaAs layer at a low growth temperature issmaller than that of samples prepared at a higher growth tempera-ture. Fig. 3 shows cross-sectional scanning electron microscope(SEM) images of 29.5 pairs of DBRs at growth temperatures of (a)550 °C and (b) 650 °C. For the sample at 550 °C, the GaAs/AlGaAs in-terface of the initial layers was clearly observed and the upper layer(more than two pairs) had the appearance of a crumbling wall. Onthe other hand, the GaAs/AlGaAs interfaces of the sample at 650 °Cwere clearly observed up to the topmost layer, but some dips wereobserved within the initial layers.
These results agree well with those for the in-situ reflectanceshown in Fig. 2(b), suggesting that if the initial GaAs layer is grownat 550 °C and the upper layers are grown at 650 °C, we will be ableto obtain a smoother surface morphology across all the layers. Thisgrowth method is similar to the two-step growth technique, whichis widely used for the heteroepitaxial growth of GaAs/Si [7].
3.1.4. MisorientationThe above optimal growth conditions under a constant growth
temperature do not appear to provide the DBR properties requiredfor LW-VCSEL applications as we will discuss in Section 3.2, becauseVCSELs require ultimate reflectivity (N99.5%) [16,17]. Using amisorientated substrate may be an effective way to achieve small sur-face roughness, because the migration of Ga adatoms in the [110]
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Fig. 1. In situ reflectance spectrum of the DBR during the entire growth period. (a) Growth rates of 3 μm/h (dotted line) and 1.8 μm/h (solid line) (b) V/III ratio of 6 (dotted line) and120 (solid line).
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direction is suppressed. The surface migration of Ga atoms is general-ly anisotropic on a (100) GaAs substrate and depends on such growthconditions as the AH3, TMG, and TMA mole fractions [18]. The diffu-sion constant of the [110] direction must be larger than that of110h i
under the above optimal conditions [14]. A valid way to sup-press the diffusion constant in the [110] direction is to form terracesby employing a surface inclined in the [110] direction. Fig. 4 showsthe in-situ reflectance of metamorphic DBR samples grown on (a) a(100) InP substrate (exactly oriented substrate) and (b) an InP sub-strate angled 7˚off towards the [110] direction InP substrate (7˚offsubstrate: sample B). The growth temperature was constant at
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Fig. 2. (a) In situ reflectance spectrum of the DBR during the entire growth period at growthpairs of DBR.
650 °C (not two-step growth), the V/III ratio of 6 and growth rate of3 μm/h were the same. The in-situ reflectances of the exactly orientedand 7˚off substrates are clearly different. The reflectance of the 7˚offsubstrate maintained the same peak value of about 40% up to 29.5pairs and that of the exactly oriented substrate decreased as thepair number decreased due to the large optical scattering loss. The re-flectance of a 2˚ off substrate was also observed and the result wassimilar to that of the exactly oriented substrate. Fig. 5 shows imagesof the topmost layer of (a) an exactly oriented substrate and (b) a7˚off substrate obtained with a Nomarski interference microscope at1000 × magnification. The typical surface morphology of the exactly
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temperatures of 550, 600, and 650 °C. (b) Details of the reflectance spectrum for a few
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Fig. 3. SEM images of GaAs/AlGaAs metamorphic DBR. (a) Growth temperature constant at 550 °C. (b) Growth temperature constant at 650 °C Details of SEM images of the DBR fora few pairs are shown at the bottom of the figure.
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Fig. 4. In situ reflectance spectrum of the DBR on an exactly oriented and a 7˚off sub-strate at a growth temperature of 650 °C and a V/III ratio of 6.
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oriented substrate exhibits random roughness as shown in Fig. 5(a),and the roughness of the 7˚off substrate is smaller as shown inFig. 5(b). However, these results for the 7˚off substrate are not suffi-cient to confirm high reflectivity (N99.5%) as we discuss later whendescribing the AFM image result.
3.1.5. Initial GaAs LayerTo obtain a DBR with high reflectivity, we must investigate the op-
timal growth conditions for the initial GaAs layer in the two-stepgrowth. Fig. 6 shows representations of 5 × 5 μm2 area AFM mea-surements emphasizing the shapes at the surfaces of the bufferGaAs layers. The GaAs layer thickness was the same at 100 nm andthe growth temperature was 550 °C. For a V/III ratio of 30, pyramidaldislocation tangles (PDTs) [11] were observed regardless of themisorientation. These results indicate that the AsH3 pressure is insuf-ficient on the substrate at this growth temperature. The incorporationof As into InP in the initial stage comes from the excess As atoms andcan result in the formation of an As-stabilized GaAs surface prior togrowth [18]. Fig. 7 shows representations of AFM measurements em-phasizing the shapes at the surfaces of the GaAs layers at various V/IIIratios on the exactly oriented and 7˚off substrates. As the V/III ratiobecame higher, the PDTs disappeared and the surface roughness de-creased. The minimum root mean square (RMS) of the AFM is about2.5 nm within a 50 × 50 μm2 area. Note that the RMS value of the
(a) (b)
µ µ
Fig. 5. Surface morphology of 29.5 pairs of DBRs on (a) exactly oriented (b) 7˚off substrates.
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exactly oriented substrate is lower than that of the 7˚off substratewith a V/III ratio of 30, and it is almost the same (about 2.5 nm) ata higher V/III ratio for the 7˚off substrate.
Fig. 8 shows in-situ reflectance results for a DBR obtained by thetwo-step growth method (sample C). After the growth of an initial20-nm thick GaAs buffer layer on an exactly oriented InP substrateat 550 °C and a V/III ratio of 120, a GaAs layer with a thickness ofabout 74 nm and 29 pairs of GaAs/AlGaAs layers were sequentiallygrown at 650 °C and with a V/III ratio of 6. The GaAs and AlGaAsgrowth rates were both 3 μm/h. The peak of the periodic reflectivityremained at a constant 42% up to the topmost layer in Fig. 8(a), andthe reflectivity of the initial GaAs layer clearly shows a positivecurve in Fig. 8(b). The peak reflectance of the 7˚off substrate obtainedwith two-step growth was slightly less than that of the exactly orient-ed substrate.
From the above results, we can conclude that, although the impactof misorientation is valid for a constant growth temperature, the
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Fig. 6. AFM images of 5 × 5 μm2 area of 100-nm GaAs grown on (a) exactly oriented and (bwhite arrows in (a) and (b).
two-step growth method is more effective under the optimum condi-tions of a high V/III ratio at a low growth temperature and a low V/IIIratio at a high growth temperature.
3.2. Reflectivity spectra of as grown DBR
After the growth of the DBR structures, the reflectivity was mea-sured with a double monochromator under an incident light angleof 5˚ at room temperature. Fig. 9(a) shows the reflectance spectra ofthe 29.5 pairs of DBRs grown on an exactly oriented InP substrate(sample A) and GaAs substrates at a constant temperature of650 °C, a V/III ratio of 6, and a growth rate of 3.0 μm/h. A peak reflec-tivity of more than 99.5% can be achieved at 1300 nm for a GaAs sub-strate, while it is less than 90% for sample A due to a large opticalscatting loss. Microscope observation reveals that sample A has amilky surface. Fig. 9(b) shows the reflectance spectra of 29.5 pairsof DBRs grown on 7˚off InP substrates (sample B). The 650 °C growth
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Fig. 7. AFM images of 50 × 50 μm2 area of 100-nm GaAs grown at various V/III ratios. (a)–(c) Exactly oriented substrate (top row); (d)-(f) 7˚off substrate (bottom row). (a) and(d) ratio of 30 (left line); (b) and (e) ratio of 60 (middle line); (c) and (f) ratio of 120 (right line). The RMS values are (a) 4.1, (b) 2.6, (c) 2.6, (d) 8.2, (e) 2.8, and (f) 2.5 nm.
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Fig. 8. In situ reflectance spectrum on an exactly oriented substrate realized after using two-step growth. In the first step, the low growth temperature and V/III ratio are 550 °C and120, respectively. The GaAs layer is 70 nm thick. In the second step, the high growth temperature and V/III ratio are 650 °C and 6, respectively. (a) Reflectance during the entiregrowth. (b) Details of the reflectance for a few pairs of DBR.
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Fig. 9. Optical reflectivity spectra of GaAs/AlGaAs DBR under constant growth conditions. (a) On a GaAs and an exactly oriented InP substrate. (b) On a 7˚off substrate. The growthtemperature and V/III ratio are 650 °C and 6, respectively.
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temperature and V/III ratio of 6 were constant during the entiregrowth period. Although the surface morphology of the topmost ofthe 29.5 pairs of DBRs was relatively smooth, the reflectivity at1300 nm reached the 99% level, which is insufficient to provide highperformance VCSELs.
Fig. 10 shows cross-sectional SEM images of sample C. Fig. 10(a)shows the entire growth layer and (b) shows the details for a numberof pairs. These images are in good agreement with the results in Fig. 8.They show that the GaAs/AlGaAs interfaces up to the topmost layerremain very flat and there are no dips around the initial layers be-cause of the two-step growth effect. Fig. 11 shows the reflectancespectra of the 29.5 pairs of DBRs of sample C. A reflectivity of morethan 99.5% is achieved at the center wavelength of 1300 nm, whichagrees well with the theoretical matrix method model deducedfrom the refractive index. These results indicate that the two-stepgrowth technique is promising for the GaAs-based DBRs directly
(a)
Fig. 10. SEM photographs of a GaAs/AlGaAs metamorphic DBR obtained by
grown on an InP substrate. The x-ray spectrum corresponding to the(004) peak of the epitaxial structure exhibits a relatively narrowlinewidth of 180 arcsec. The lattice-mismatch ratio between the InPsubstrate and the AlGaAs/GaAs mirror is −3.68%, which almost coin-cides with the value between free strained GaAs and InP.
AFM images of these samples were obtained to investigate thesurface morphology in detail. Fig. 12 shows tapping-mode AFM im-ages of the topmost layer of (a) sample B (constant growth tempera-ture and 7˚off substrate) and (b) sample C (two-step growth andexactly oriented substrate). The respective RMS values across a50 × 50 μm2 area are 8.4 and 2.5 nm.
The fraction of the light incident normal to the surface that isscattered is given by the following equation [19]
IsIi¼ 16π2σ2R
λ2 ; ð1Þ
(b)
using two-step growth. (a) Entire growth. (b) Details for a few pairs.
Calc.Exp.
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Fig. 11. Experimental and calculated optical reflectivity spectra of GaAs/AlGaAs DBRunder the two growth conditions.
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where σ and R are the RMS and the Fresnel reflection coefficient at theinterface, respectively. From Eq. (1) the optical scattering loss per inter-face can be estimated as Is/Ii = 1.7 × 10−1%, which corresponds to anoptical absorption coefficient of 160 cm−1. It is also estimated as Is/Ii = 1.5 × 10−2%, which is coincides with a 14 cm−1 absorption coeffi-cient resulting from the RMS in sample C. If this scattering loss occursuniformly in GaAs/AlGaAs interfaces over all the DBR layers, we can ex-pect the reflectivities of samples B and C to be less than 99% and 99.5%,respectively, from the previous calculation. If the RMS value is assumedto be proportional to the distance from theGaAs/InP interface (based onthe RMS of 0 nm at the GaAs/InP interface), a total reflectivity of morethan 99.5% is estimated for sample C. These calculations suggest thatsurface roughness may increase gradually from the GaAs/InP interfaceto the topmost layer. Indeed, the light incident to the DBR comes fromthe substrate side (the reverse of this experiment) for the VCSEL appli-cation. This directly grown mirror is suitable for high-performanceLW-VCSELs, because the expected effective reflectivity of the VCSEL'sstructure is higher than this experimental result.
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Fig. 12. Representative AFM images of the surface of 29.5 DBR pairs. (a) Sample B on a 7˚off ssamples were grown at a V/III ratio of 6 and a growth temperature of 650 °C.
3.3. Photoluminescence intensity
A total thickness of more than 6 μm is required for a GaAs/AlGaAsDBR in order to obtain a reflectivity of 99.5%. Such thick strainedlayers are inherently accompanied by a network of dislocations andlarge residual strain. Once a GaAs/AlGaAs DBR structure has beengrown on the epitaxial layers including an active layer, there is thepossibility that the dislocations of the large mismatched interfacethat are generated will propagate to the active layer and changetheir optical property. Moreover, dislocations can also be created inthe active layer by the different thermal expansion coefficients ofGaAs/InP during the post-growth cooling stage.
The influence of the highly-strained DBR structure on the activelayer was investigated. The samples were grown on InP-based layerswith an underlying active layer. The spacer layer thickness betweenthe active layer and the GaAs/InP interface was investigated as a pa-rameter. The growth conditions were the same as those for sampleC except for the cooling rate after the metamorphic growth. The sam-ples were measured after the DBR layers had been removed by wetchemical etching. At a 60 K/min cooling rate, no PL intensity was ob-served for the sample at a thickness of 200 nm. The result was thesame even for a thickness of 600 nm. The recombination centers inthe active layer may have been generated after the growth of themetamorphic layer. On the other hand, at a cooling rate of 8 K/minthe PL intensity of the active layer at a distance of 200 nm exhibits en-hancement rather than degradation before and after growth as shownin Fig. 13(a). The peak wavelength of the PL was slightly blue-shifted.Fig. 13(b) shows x-ray rocking curves for the (400) reflections of theactive layer before and after DBR growth. They are almost the sameexcept for a slight reduction in the satellite intensity. These resultssuggest that the intermixing of the Al and Ga between the well andthe barrier layer in the active layer may be accelerated by high tem-perature growth.
To confirm the high quality of the active layer, we characterized theGaAs/InP interface by transmission electronic microscopy (TEM) ofcross sections. Fig. 14 shows TEM images of the cross-sectional viewsprojected along [110]. The metamorphic materials were regrown onthe InP, InAlAs spacer layer, and active layers. The thickness betweenthe interface and active section was 400 nm (InP and InAlAs) with
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ubstrate (b) Sample C on an exactly oriented substrate grown by two-step growth. Both
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Fig. 13. (a) Photoluminescence (PL) spectra for the active layer before and after DBR growth. The PL peak is slightly blue-shifted from 1276.5 to 1268.2 nm. (b) X-ray rocking curvesfor the (400) reflections of the active layer before and after DBR growth.
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this sample. Fig. 14(a) shows a TEM micrograph of metamorphic DBRson the InP-based layer. The lattice near the interface is highly dislocated.Although the blocking of some threading dislocation propagations inthemetamorphic GaAs/AlGaAs interface was observed, the dislocationsgenerated at the GaAs/InP interface propagated to a depth of 100 to200 nm into the underlying InP-based layers. However, no dislocationswere observed in the underlying active layer in Fig. 14(b). These resultsindicate that a slow cooling rate after themetamorphic growth can pre-vent the degradation of the underlying active layer if the spacer layerthickness exceeds 200 nm.
300 nm
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InP spacer
InAlAs space
active layer
/GaAs
AlGaAs
Fig. 14. (a) TEM micrograph of GaAs/AlGaAs DBR on the InP-based a
4. Conclusion
A GaAs/AlGaAs metamorphic mirror grown on an InP substrate byMOCVD was investigated. The optimal growth conditions, consistingof the growth rate, V/III ratio and growth temperature, and two-step growth can provide a smooth surface morphology leading to ahigh-reflectivity mirror (99.5%) with a low optical scattering loss. Interms of the cooling rate, we demonstrated that highly strainedDBRs can be stacked without any degradation in the quality of the ac-tive layer.
(b)
100 nm
r
GaAs
InP spacer
active layer
InAlAsspacer
AlGaAs
ctive layer. (b) Detailed TEM micrograph of the InP spacer layer.
326 Y. Ohiso, R. Iga / Thin Solid Films 542 (2013) 317–326
These results indicate that this metamorphic mirror with highthermal conductivity directly grown on an InP substrate by MOCVDis promising for LW-VCSEL applications.
Acknowledgements
We thank Drs. M. Mitsuhara, M. Arai, and M. Kotoku and Mr. J.Asaoka for fruitful technical discussions and support. We also thankDr. H. Oohashi for her encouragement.
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