Monitoring the formation of oxide apertures in micropillar cavities

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by  Morten P. Bakker

Monitoring the formation of oxide apertures in micropillar cavities
Morten P. Bakker,1 Henk Snijders,1 Donald J. Suntrup III,2 Tuan-Ahn
Truong,2 Pierre M. Petroff,2 Martin P. van Exter,1 and Dirk Bouwmeester1,2
1Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands
2University of California Santa Barbara, Santa Barbara, California 93106, USA
(Dated: January 30, 2013)
Animagingtechniqueispresentedthatenablesmonitoringofthewetthermaloxidationofathin
AlAslayerembeddedbetweentwodistributedBraggreflector(DBR)mirrorsinamicropillar. After
oxidationweconfirmbywhitelightreflectionspectroscopythathighqualityopticalmodesconfined
to a small volume have been formed. The combination of these two optical techniques provides a
reliableandefficientwayofproducingoxideaperturedmicropillarcavitiesforwhichthewetthermal
oxidation is a critical fabrication step.
3
1
0
Wet thermal oxidation is a widely used technique that monitor a sample while it is being oxidized and stop the
2
allows for optical (electrical) confinement by making use oxidation when only a small unoxidized area is left at
n
of the large difference in effective refractive index (resis- thecenterofa micropillarstructure. Afteroxidation, we
a
J tivity) between oxidized and unoxidized regions[1]. It is perform an in situ characterization at room temperature
9 an important fabrication step in the production of verti- of the optical modes defined by the optically confining
2 calcavitysurfaceemittinglasers(VCSELs)[2]andofcer- oxide aperture.
tain optical microcavities [3]. Embedding self-assembled The sample is grown by molecular beam epitaxy
]
s quantum dots (QDs) in such microcavities enabled the on a GaAs [100] substrate and is constructed as fol-
c study of cavity quantum electrodynamics in solid state
i
t systems[4]. Suchdevicesareofinterestforsingle-photon
p
sources [5] and for hybrid quantum-information devices
o
[6].
.
s
c A real challenge, however, is to obtain a high fabri-
i
s cation reliability in the wet oxidation process, since the
y oxidationratedependsstronglyonparameterswhichare
h
difficulttoreproduce,suchastheroughnessoftheetched
p
sidewalls or the Al composition to within an ∼1% accu-
[
racy. As repetitive calibrations can be costly, techniques

2
tomonitortheoxideformationinsituareimportant. All
v
three techniques reported so far make use of the change
8
4 inthereflectivityspectrumasoneormoreAlcontaining
2 layers are being oxidized. The average change in reflec-
6 tivityofalargepatternedareacanbemonitored[7]. The
.
1 reflectivity contrast between a fully oxidized DBR mir-
0 ror and unoxidized areas can be directly viewed through
3 a charge coupled device (CCD) camera using broadband
1
light [8]. Finally a buried, thin oxide aperture layer can
:
v be imaged [9], but this technique is delicate as the ap-
Xi plied wavelength interval has to be tuned carefully such
that the reflectivity contrast allows for sufficient spatial
r
a resolution.
In this paper, we present a new imaging technique
based on measuring the reflectivity, while scanning with
FIG.1. (a)SEMimageofthethreeetchedtrenchesthatform
amonochromaticbeamoveranetchedmicropillarstruc-
a micropillar mesa connected to the bulk material. (b) SEM
ture. Weuseawavelengthontheredsideofthephotonic
cross-sectional image of the center of a micropillar. Al O
x y
stopband,suchthatabsorptioninthesemiconductorma-
is darker than the Al Ga As layers. (c) Reflection scans
x 1−x
terial is low for temperatures up to 450◦C. For an oxi- (light: high reflectivity) taken with a 1064 nm laser for dif-
dizedsampleweareabletoclearlyidentifythreeregions: ferent temperatures at the corner of an etched trench (white
unoxidized regions, regions where only a thin aperture box in Fig. 3 shows this corner for a different micropillar).
layerhasbeenoxidizedandregionswhereboththeaper- (d) Reflectivity as a function of the distance along the white
lines in (c).
ture region and the DBR mirror are oxidized. We then
2
FIG. 2. (a) Reflectivity scan (light: high reflectivity) of a 1064 nm laser on an etched micropillar before the oxidation was
started. (b-e) Reflectivity scans for different times after the oxidation was started. Every scan takes about 1.5 minutes. (f)
Reflectivity scan taken after the oxidation was stopped after 29.5 minutes.
lows: twoDBRmirrors,comprisingalternatingλ/4-thick
layers of GaAs and Al Ga As, embed an aperture
0.9 0.1
layer, consisting of a thin 10 nm AlAs layer in between
Al Ga As and Al Ga As, and an active layer
0.75 0.25 0.83 0.17
containing InGaAs/GaAs self-assembled quantum dots
inside a PIN diode structure. Trenches are etched down
to the bottom DBR using reactive ion etching, leaving
a 30 µm circular micropillar connected by three narrow
unetched channels to the bulk material (see Fig. 1a).
Global electrical contacts to the doped layers allow for
control of the electrical field inside every micropillar in
an array of 42 micropillars.
FIG.3. Reflectivityscan(light: highreflectivity)ofthesame
The conversion of Al Ga As into Al O takes place
x 1−x x y micropillar as in Fig. 2, taken over a larger region at room
by applying H O vapor to the sample at a typical pro-
2 temperature.
cess temperature of 400-450◦C. Special care is taken to
prevent dry oxidation with oxygen, as it has been re-
ported that this can prevent further wet oxidation [1].
edge of an etched trench (see Fig 1(c)). The reflectivity
First native oxide is removed from the Al Ga As by
x 1−x change with refractive index is temperature dependent.
dipping the sample for 20s in NH OH, then it is rinsed
4 In Fig. 1(d) the reflection intensities along the white
with demi-water and placed in isopropanol. The sample
lines in Fig 1(c) are presented. Starting from the left
is finally clamped to a holder, equipped with a heater
side of the graph where the etched trench is, the reflec-
and a thermocouple, inside an oxidation chamber fitted
tivityrapidlychangesfromlowtohighwhentheoxidized
withaviewport. Isopropanolismaintainedonthesample
DBR is reached. In the middle a constant reflectivity is
surface until the chamber has been filled with nitrogen
visible, corresponding to an oxide layer with a constant
gas, in order to prevent exposure to oxygen. Steam is
thickness. On the right side the gradient in the oxide
applied by flowing nitrogen carrier gas through a water-
layer, at the end of the oxide aperture, causes a varying
filled bubbler heated to 95◦C.
reflectivity after which the reflectivity of the unoxidized
InFig1(b)aSEMcrosssectionalimageshowsthatthe region is more or less constant.
oxide penetration depth varies for different layers. Our Whenperformingamonitoredoxidationwiththisspe-
goal is to view and monitor the growth of the oxidized cificwafermaterial,atemperatureof420◦Cischosen. At
layers in situ with a non-invasive optical technique. thistemperaturethecontrastbetweentheoxideaperture
A NA=0.40 and 20 mm working distance objective is and the unoxidized regions is larger than at higher tem-
used to image the sample surface on a CCD camera and perature, and the oxidation rate is fast enough to com-
to focus incident light to a diffraction limited spot size. pletetheoxidationinabout30minutes. Inordertomon-
In order to differentiate between regions with a different itortheoxidationatadifferenttemperaturewithsimilar
extent of oxidation, we monitor the reflectivity of a fo- contrast, alightsourcewithadifferentwavelengthcould
cused Nd:YAG 1064 nm laser beam that we scan across be used.
the sample. In order to find the operating temperature Firstwetakeanunoxidizedsampleandperformaspa-
that yields the best reflectivity contrast, we take spatial tial reflectivity scan of the micropillar center. Fig. 2 (a)
reflectivity scans of an already oxidized sample at the shows a homogeneous reflectivity for the unetched re-
3
2(f), which was recorded at 420◦C.
In order to characterize the confined optical modes of
the device, the center of the micropillar is illuminated
usingasuperluminescencediode(Superlum,920-980nm),
withawavelengthrangecoveringtheopticalmodes, and
thereflectionspectraarerecordedwithaspectrometeras
a function of position [10]. Fig. 4(a) shows spatial scans
of the reflected intensity at the resonance wavelengths of
the first six Hermite-Gaussian modes. Fig. 4(b) shows
thereflectionspectraatsixpositions,markedinFig. 4(a)
withblackdotsandnumbers. Thesepositionscorrespond
to the minima in the reflection spectra and demonstrate
aclearspatialseparationofthedifferentmodes. Inorder
to determine the Q-factor, we fit a Lorentzian reflection
dip to the Ψ mode and find a Q-factor 10,000-12,000.
00
For the average wavelength splitting ∆λ, between the
fundamental Ψ and first order Ψ and Ψ modes, we
00 10 01
find ∆λ≈0.62 nm.
An important figure of merit of microcavities is the
Purcell factor P = 4π32(λn0)3VQ, where λ0 is the cavity
wavelength, n is the refractive index, Q is the quality
factor and V is the mode volume. Assuming there is
a quadratic confining potential present, giving rise to
Hermite-Gaussian optical modes, this can be rewritten
as: P = 12F(∆λ) [11]. Here F = λ0Q is the cavity
π λ0 2nLeff
finesse and L is the effective cavity length. By com-
eff
bining the measured Q-factor 10,000-12,000, ∆λ ≈ 0.62
nm with n ≈ 3 and L ≈ 1.4 µm estimated from
eff
the device structure, we find an estimated Purcell fac-
tor P =2.8−3.4.
Higher Purcell factors can be obtained by perform-
FIG.4. (a)Reflectivityscans(light: lowerreflectivity)forthe
indicatedwavelengths,between955.5-957.5nm,asafunction ing the oxidation longer and thereby reducing the cav-
ofpositioninthecenterofamicropillar. (b)Reflectionspec- ity volume. Also the Q−factor, limited by absorption
traatthesixpositionsthataremarkedwithablackdotand in the doped layers, can be further increased by cooling
labelled with numbers 1-6. An offset is added between each the device to a temperature of 4K, where such micropil-
trace. Theredlineincurve1isafittodeterminetheQ-factor
larscontainingquantumdotsaretypicallyoperated. The
of the Ψ mode.
00 difference in the wavelength splitting, between the fun-
damental Ψ and either one of the first order Ψ and
00 10
Ψ modes, is a result of elipticity arising from prefer-
01
gions and a lower reflectivity for the etched parts. Once
ential oxide formation rates and can be compensated for
the oxidation process is started, continuously scans are
by etching elliptical micropillars. Finally we would like
recorded, which take about 1.5 minutes each. Several of
tonotethatthefundamentalcavitymodeinmicropillars
these scans are shown in Fig 2 (b-e).
is typically polarization non-degenerate due to birefrin-
Thefrontoftheoxideaperture, inthiscasecharacter- gence, although we were limited by the resolution of the
ized by a lower reflectance, starts to penetrate towards spectrometerof0.019nmandcouldnotcharacterizethis.
the micropillar center. After 29.5 minutes the oxide ta- Permanent tuning to compensate for this birefringence
per has advanced to within 3 µm of the pillar center and has been demonstrated [12] and could be used as a final
we stop the oxidation by shutting off the water vapor step in the fabrication process.
flow and purging the oxidation chamber with nitrogen. In conclusion, we have developed a procedure that al-
Several minutes after the oxidation was stopped a final lows for the fabrication and characterization with high
scan is taken (Fig. 2 (f)). accuracy of high quality micropillar cavities that, oper-
Once the sample has cooled down to room tempera- atingatcryogenictemperatures,areofinterestforquan-
ture, a large spatial reflectivity scan is taken, see Fig. tum information applications.
3. Theunoxidizedcenter, havingthesamereflectivityas This work was supported by NSF under Grant No.
regionsfarawayfromtheetchedtrenches,isseentohave 0901886, the Marie-Curie Award No. EXT-CT-2006-
the same size and shape as the center of the scan in Fig. 042580andFOM-NWOGrantNo. 08QIP6-2. Wethank
4
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Review Letters 104, 160503+ (2010).
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