SPIN-VCSEL HETEROSTRUCTURE
A schematic of the spin-polarized vertical-cavity surface-emitting laser (spin-VCSEL)
heterostructure as well as the n-contact variations for the control devices are shown in Fig. 1.
The spin-polarized VCSEL (spin-VCSEL) is grown by molecular-beam epitaxy (MBE) and
consists of a 29.5-pair, 81.2 nm Al0.8Ga0.2As/72.4 nm GaAs distributed
Bragg
reflector (DBR)
5-pair ZnSe/MgF
2 DBR
stack for the bottom mirror; a 280.6 nm full-wave GaAs cavity
with
an active region comprised
p+-Al
0.8Ga0.2As/GaAs DBR
of five, 5 nm In0.2Ga0.8As QWs separated by 10 nm GaAs barriers;
a hybrid top mirror
GaAsand
λ-cavity
formed from one quarter-wave pair of p-doped (NA = 5×1018 cm-35) In
81.2GanmAs
AlQWs
0.8Ga0.2As/72.4 nm
0.2
0.8
GaAs DBR and a 5-pair 197.8 nm MgF2/105.8 nm ZnSe dielectric DBR stack. For the bottom
DBR mirror, only the three Al0.8Ga0.2As/GaAs pairs nearest the cavity
are n-doped (ND = 7×1017
+-Al Ga As/GaAs DBR
3-pair
n
0.8
0.2
cm-3), and the spin injector is subsequently deposited by molecular-beam
epitaxy on the topmost
+
n-GaAs layer. We use a 10 nm Fe/15 nm n -Al0.1Ga0.9As
Schottky
tunnel
barrier DBR
as the spin
26.5-pair i-Al0.8Ga0.2As/GaAs
injector for the spin-VCSEL. Nonmagnetic metallization is subsequently deposited to form pads
for Au wire bonding.
semi-insulating GaAs(001) substrate
5-pair ZnSe/MgF2 DBR
(a)
p+-Al0.8Ga0.2As/GaAs DBR
nonmagnetic
AuGe-based metallization
5 nm Al
GaAs λ-cavity
10 nm Fe
5 In0.2Ga0.8As QWs
15 nm
3-pair n+-Al0.8Ga0.2As/GaAs DBR
26.5-pair i-Al0.8Ga0.2As/GaAs DBR
(b)
nonmagnetic
n-GaAs
(7×1017
cm-3)
(1×1019 cm-3)
5 nm Al
10 nm Fe
nonmagnetic
AuGe-based metallization
AuGe-based
Fig. 1. Schematic
of themetallization
laser heterostructure
showing the n-contact 5variations
for the (a) Fe
nm Al
spin-polarized, (b) Fe control,
and (c) nonmagnetic
10 nm Fe
VCSELs.
+
19
-3
15 nm n -Al0.1Ga0.9As (1×10
0.1Ga0.9As
n-GaAs (7×1017 cm-3)
semi-insulating GaAs(001) substrate
(a)
n+-Al
cm )
n-GaAs (7×1017 cm-3)
(c)
nonmagnetic
AuGe-based metallization
n-GaAs (7×1017 cm-3)
CONTROL
VCSELS5 nm Al
(b)
10 nm Fe
Two control devices are fabricated in order to provide unambiguous evidence of electrical
spin injection inAuGe-based
our nonmagnetic
spin-VCSEL.
All design parameters remain identical to the Fe spin-VCSEL
metallization
except for the n-contact, and the n-contact variations are depicted schematically in Figs. 1(b) and
-3)
(7×10
cmfollows:
1(c). The controln-GaAs
devices
are17as
(1) a nonmagnetic VCSEL in which nonmagnetic nohmic metallization replaces the regrown Fe/Al0.1Ga0.9As Schottky tunnel barrier and (2) a Fe
control
10 nm Fe and 5 nm Al layers are deposited via MBE regrowth atop
nonmagnetic
(c) VCSEL in which
nonmagnetic n-ohmic
metallization.
AuGe-based
metallizationThe in-plane properties of thin-film ferromagnets are often
influenced significantly
by the substrate material; however, the out-of-plane magnetization curve
n-GaAs (7×1017 cm-3)
is not expected to differ for a 10 nm Fe film whether it is grown epitaxially on (Al)GaAs or atop
1
an evaporated Au film. Confirmation that this expectation has been realized in our Fe layers is
crucial for comparison of the Fe spin-polarized and Fe control VCSELs. We have performed
superconducting quantum interference device (SQUID) magnetometry measurements on 10 nm
Fe films grown on GaAs(001) substrates by low-temperature epitaxy. Two cases are considered
here: (1) a 10 nm film is grown epitaxially atop a GaAs buffer layer and (2) a 10 nm Fe films is
deposited directly atop a 200 nm thick, electron-beam evaporated Au film. In both cases, the Fe
films are grown in an MBE chamber using identical growth conditions (i.e., substrate
temperature of 40 ºC and a Fe growth rate of 0.35 nm/min). The magnetization curves for the
two Fe films taken at 50 K, as shown in Fig. 2, are nearly identical.
Fig. 2. Normalized magnetization curves for 10 nm
thick Fe layers grown atop epitaxial GaAs and
evaporated Au layers taken at 50 K.
SPIN-VCSEL CHARACTERISTICS
Reflectivity (%)
80
60
16 nm
40
20
800
900
1000
1100
Wavelength (nm)
1200
10
(b)
0.15
8
0.12
6
0.09
4
0.06
2
0.03
0
50
100
150
200
0.00
Slope Efficiency (mW/mA)
(a)
T = 300 K
Threshold Current (mA)
100
PL Intensity (arb. units)
In order to optimize laser performance at cryogenic temperatures, we introduce a 16 nm gain
offset at room temperature as evidenced by the offset between the gain peak and resonance
cavity wavelengths shown in Fig. 3(a). The threshold current density exhibits the expected
parabolic dependence on temperature, which is caused by thermal detuning of the lasing mode
and gain peak. The 16 nm gain offset yields a minimum threshold current density at 150 K as
shown in Fig. 3(b).
Temperature (K)
Fig. 3. (a) Reflectivity and photoluminescence (PL) spectra of the as-grown Fe spin-polarized
VCSEL heterostructure showing a 16 nm red shift of the gain peak with respect to the laser cavity
resonance wavelength at room temperature, and (b) temperature dependence of the threshold
current and slope efficiency for a 20 µm mesa diameter Fe spin-polarized VCSEL.
2
The background polarization of our VCSELs is measured at 50 K using a rotating GlanThompson linear polarizer. The polarization axis of the linear polarizer is aligned along a <110>
crystallographic axis. We find that the emission of our VCSELs is highly linearly polarized
above threshold with maximum and null intensities observed along orthogonal <110> directions,
as shown in Fig. 4. Below threshold the VCSEL emission is essentially unpolarized. Similar
polarization behavior is observed for 17 µm and 20 µm mesa diameter VCSELs.
Normalized Intensity (a.u.)
1.0
o
o
o
o
60
30
0
90
0.8
0.6
0.4
0.2
0.0
936
942
948 936
942
948 936
942
948 936
942
948
Wavelength (nm)
Fig. 4. Electroluminescence spectra as a function of polarizer angle for a 15 µm diameter VCSEL
at 50 K. The polarizer is referenced to a <110> crystallographic direction.
CONTROL EXPERIMENTS
A gradual decrease of ΔIth/Ith,0 was observed for fields > 30 kOe and was attributed to the
gradual decrease in total light intensity with increasing magnetic field. This trend toward lower
overall emission intensity with increasing magnetic field is observed for both the spin-polarized
and nonmagnetic VCSELs, shown in Fig. 5. However, the emission enhancement due to a spininduced threshold current reduction more than offsets this effect and only becomes apparent in
the threshold reduction data for magnetic fields H > Hsat.
0.2
B = 0 kG
B = 5 kG
Light (mW)
Light (mW)
0.3
Fig. 5. Light versus current characteristics for a
nonmagnetic VCSEL taken at 50 K. Inset shows the
magnetic field dependence of the light intensity at a
continuous-wave bias of 5 mA.
I = 5 mA
0.11
0.10
0.1
-4
-2
0
2
4
Magnetic Field (kG)
(d)
0.0
0
T = 50 K
2
4
Current (mA)
6
3
Magneto-photoluminescence measurements were performed on the spin-VCSEL to ascertain
the severity of a potential parasitic contribution from magnetic circular dichroism (MCD). In
these experiments, the Fe spin-VCSEL sample was illuminated with linearly-polarized laser
excitation to generate unpolarized electron-hole pairs directly in the laser cavity. We measured
ΠC of the resulting photoluminescence using a photoelastic modulator technique utilizing lock-in
detection. A negligible degree of circular polarization was observed in this control experiment
as shown in Fig. 6, which indicates that a parasitic contribution from MCD is absent in the Fe
spin-VCSEL. Parasitic contributions from stray field or magneto-optical effects were assessed
by analyzing ΠC for the Fe control VCSEL. As shown in Fig. 6, we measure ΠC < 1.5% for
applied magnetic fields up to 40 kOe in the Fe control VCSEL, which is significantly less than
the saturation value of ΠC measured for the Fe spin-VCSEL. Thus, parasitic contributions from
stray field or magneto-optical effects cannot account for the observed electroluminescence
polarization signal observed in the Fe spin-VCSEL.
Fig. 6. Degree of circular polarization
for the electrically-pumped Fe control
VCSEL and optically-pumped Fe spinpolarized VCSEL measured at 50 K.
THRESHOLD REDUCTION
With an estimated multiple quantum well (MQW) spin relaxation time of 300 ps, the spindependent rate equations predict that a pump current spin polarization of 19.8% is required to
achieve a threshold current reduction of 11%. If the MQW spin relaxation time can be increased
to > 1 ns, Fig. 7 suggests that the maximal threshold reduction expected for a Fe spin-VCSEL is
only 31%.
Fig. 7. Theoretical threshold current
reduction versus pump current spin
polarization for active region spin
relaxation times of 300 and 1000 ps.
As the mesa diameter is reduced from 30 µm to 17 µm, we find that the threshold current decreases
appropriately such that the threshold current density remains fairly constant, at least to within reasonable
tolerances. However, the scaling toward lower threshold currents ceases to hold for 15 µm spin-VCSELs.
For the two 15 µm spin-VCSELs discussed in the letter, threshold currents of ~6 mA and ~7 mA were
measured at 50 K [refer to Figs. 2(a) and 3(e) appearing in the letter], suggesting a threshold current
4
density of 3400–3960 A/cm2 which is greater than that measured in a 20 µm devices [Jth ≈ 2260 A/cm2,
Fig. 3(b)]. Many groups have reported a dramatic increase in threshold current density for small-aperture
selectively-oxidized VCSELs and small mesa diameter etched-post VCSELs, which can exceed a factor
of five. The scalability of etch-posted VCSELs is ultimately limited by optical loss which results in
higher threshold gain for small diameter VCSELs. Scattering losses from the mode interaction with the
etched sidewalls and diffractive losses from the mode spread outside of the index guide cause an increase
in the threshold gain which requires higher carrier densities to support. Optical losses are greater for
small diameter and deeply-etched-post VCSELs for which etching proceeds through the active region.
Given our contact geometry we cannot avoid etching through the active region in our spin-VCSEL. The
threshold current density increases dramatically if the threshold gain rises to sufficiently high levels that
gain saturation effects begin to dominate. Small changes in gain will then require very large changes in
current density for lasing to occur.
5