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Measuring the hole chemical potential in ferromagnetic
Ga1xMnxAs/GaAsheterostructures by photoexcited resonant
tunneling
O. Thomas, O. Makarovsky, A. Patan,a L. Eaves, R. P. Campion, K.
W. Edmonds,C. T. Foxon, and B. L. GallagherSchool of Physics and
Astronomy, University of Nottingham, Nottingham NG7 2RD, United
Kingdom
Received 21 December 2006; accepted 22 January 2007; published
online 23 February 2007
The authors investigate the optical and electrical properties of
a p-i-n GaAs/ AlAs resonanttunneling diode in which the p-type
layer is the ferromagnetic alloy semiconductor Ga1xMnxAsx=3%. The
high density of Mn acceptors affects significantly the
electrostatic potential profile ofthe heterostructure and inhibits
hole tunneling from Ga1xMnxAs.The authors use photoconductivityto
probe this potential and measure the hole chemical potential in
Ga1xMnxAs relative tothe band edges of the adjacent undoped GaAs
layers. 2007 American Institute of
Physics.DOI:10.1063/1.2709624
The dilute ferromagnetic semiconductor alloyGa1xMnxAs has
emerged as acandidate material system forapplications in
spintronics.14 Substitutional Mn in GaAsMnGaisan acceptor impurity
with a hole binding energy of
110 meV.5 At high Mn concentrations, the acceptor impurityband
tends to merge with the valence band. Hence the holesbecome mobile,
even at low temperatures, and act as itiner-ant carriers of the
ferromagnetic interaction between theMnGa
2+ ions.1,6,7 A Curietemperature of up to 173 K has re-cently
been achieved.8 At present there is no firm consensusconcerning the
dependence on Mn content of the energy gapof Ga1xMnxAs and the
location of the chemical potential prelative to the conduction
andvalence band edges. Opticalspectroscopy measurements9 indicate
that for high xx= 5 % 7 % , p lies within the Mn-impurity band.
Roomtemperature scanning tunneling microscopy has measured aband
gap of 1.230.05 eV for x 3%.10 A knowledge of
these material parameters is essential for the design,
model-ing, and operation of Ga1xMnxAs heterostructure
devices,including p-i-n light-emitting diodes. In this type of
device,spin-polarized holes can be injected from Ga1xMnxAs intothe
intrinsic i region of the diode to give polarized
elec-troluminescenceELemission.3
In this letter we investigate the electronic and
opticalproperties of a p-i-ndouble barrier resonant tunneling
diodeRTD incorporating an AlAs/ GaAs/ AlAs quantum wellQW and a
p-doped Ga1xMnxAs layer with x=3%. Thehigh density of Mn acceptors
in the p-type layer affects sig-nificantly the electrostatic
potential profile of the diode andinhibits the injection of holes
from Ga1xMnxAs. Photocon-
ductivityPCmeasurements allow us to probe the potentialprofile
and to locate precisely the position of the chemicalpotential in
Ga1xMnxAs with respect to the band edges inthe adjacent GaAs
layers. This information is particularlyuseful as it determines the
bias required for hole injectioninto the QW subbands.
The sequence of layers, grown by molecular beam epi-taxy on
100semi-insulating GaAs substrates, is as followssee inset of Fig.
1: a 300 nm Si-doped GaAs layer n =21018 cm3; 100 nm of Si-doped
GaAs n =2
1017 cm3; an undoped intrinsic iregion consisting of a20 nm GaAs
spacer layer, a 6 nm GaAs QW formed betweentwo 5 nm AlAs barriers,
and a 10 nm GaAs spacer layer; thetop 50 nm Ga1xMnxAs layer with
x=3% was grown at
250 C. We have also studied a control sample of similardesign,
except that the Ga1xMnxAs layer was replaced by a1 m layer of
carbon-doped GaAs p =21018 cm3. Thelayers were processed into 200 m
diameter mesa diodeswith a ring-shaped electrode on the top of the
mesa for ELand for current-voltage, IV, measurements under
opticalexcitation with the 633 nm line of a HeNe laser. We
havestudied annealed and unannealed devices. Annealing hasbeen
shown to significantly reduce compensation inGa1xMnxAs through Mn
interstitial outdiffusion
11 and canlead to hole densities close to the Mn concentration
of 6.61020 cm3 for x=3%.12 In this work, we only considersamples
that were unannealed or else annealed only for short
aAuthor to whom correspondence should be addressed; electronic
mail:[email protected]
FIG. 1. Low-temperatureT=4.2 K IVcurve of the p-i-ndiode
contain-ing the p-Ga1xMnxAs layer upper curve and of the control
sample inwhich the p layer is doped with shallow carbon acceptors
bottom curve.The inset is a sketch of the layer composition of the
p-i-ndiode containingthe p-Ga1xMnxAs layer.
APPLIED PHYSICS LETTERS 90, 082106 2007
0003-6951/2007/908/082106/3/$23.00 2007 American Institute of
Physics90, 082106-1article is copyrighted as indicated in the
article. Reuse of AIP content is subject to the terms at:
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http://dx.doi.org/10.1063/1.2709624http://dx.doi.org/10.1063/1.2709624http://dx.doi.org/10.1063/1.2709624http://dx.doi.org/10.1063/1.2709624http://dx.doi.org/10.1063/1.2709624http://dx.doi.org/10.1063/1.2709624http://dx.doi.org/10.1063/1.2709624http://dx.doi.org/10.1063/1.2709624http://dx.doi.org/10.1063/1.2709624http://dx.doi.org/10.1063/1.2709624http://dx.doi.org/10.1063/1.2709624
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times 3 h at relatively low temperatures T=150 C.Under these
annealing conditions, only limited outdiffusionof compensating Mn
interstitials will occur, and a hole den-sity of around 4.11020 cm3
is expected.12,13
Figure1 compares the low-temperature T=4.2 K IVcurve of the
p-i-ndiode containing the p-Ga1xMnxAs layer
with that of the control sample in which the p layer is
dopedwith shallow carbon acceptors. For the control sample,
thestrongest peak in IVcorresponds to electron tunneling intothe
lowest conduction subband E1; the three weaker peaks,HH1, LH1, and
HH2, arise from resonant hole tunneling,14,15
where LHn and HHn refer to QW subbands with light Lorheavy Hhole
character at wave vector k=0 and n =1, 2 isthe subband quantum
number. In contrast, the diode incorpo-rating the Ga1xMnxAs layer
reveals only a single resonancein IVdue to electron tunneling into
E1.
As shown in Fig.2,the low-temperature T=12 K ELspectrum of the
diode containing the p-Ga1xMnxAs layerconsists of a strong band
around 1.50 eV arising from exci-
tonicXand carbon-acceptors-related e-C0
recombinationin the GaAs layers and a sharp line at 1.65 eV
involvingradiative transitions between the edges of the quantized
sub-bands E1HH1 of the QW. The weaker features around1.4 eV are due
to recombination of electrons with holesbound to residual Mn
acceptors in the GaAs layers e-Mn0.The EL from the QW has a weak
circular polarization1% , which will be discussed in a subsequent
article. Thisemission indicates that holes are injected into the QW
fromthe p-Ga1xMnxAs layer despite the absence of discerniblehole
resonances in IV. As shown in Fig.2b,the QW ELpeak intensity
increases sharply at 1.85 V corresponding tothe fall in current
just beyond the E1 peak in IV. In con-
trast, the bias dependence of the QW EL intensity of thecontrol
sample follows closely that of the tunnel current.15
The steplike increase of the QW EL emission Fig.2bjust beyond
the E1 peak demonstrates that hole injectionfrom the Ga1xMnxAs p
layer into the QW is controlled bythe electron current. In
addition, the absence of hole reso-nances inIV Fig.1indicates that
the current is dominatedby electron tunneling. To understand this
unusual behaviorwe consider the effect of the high density of Mn
acceptors onthe electrostatic potential profile of the diode.
Figure 3ashows a schematic diagram of the band edges of the
hetero-structure at a voltage VFBcorresponding to the flatband
con-dition at which the electric field in the intrinsic region
iszero. In this diagram, we assumed that the band gap ofGa1xMnxAs
is smaller than that of GaAs, as indicated byrecent experiments.10
The potential step at the interface be-
tween Ga1xMnxAs and the GaAs spacer layer is made up oftwo
contributions: athe band-edge discontinuity due to thesmaller band
gap of Ga1xMnxAs and bthe dipolar chargeat the edge of the
Ga1xMnxAs layer created by negativelyionized Mn acceptors and holes
which diffuse into GaAs.Since the chemical potential on the n-type
side of the barriersis very close within a few meV to the energy Ec
of theGaAs conduction band edge, at V=VFB the hole
chemicalpotential in Ga1xMnxAs lies at a well-defined energyp =eVFB
below Ec in the adjacent GaAs layers, seeFig.3a.
For VVFB, hole injection from Ga1xMnxAs into theQW is inhibited
by the potential step at the
Ga1xMnxAs-spacer layer interface. Hence the current isdominated
by resonant transmission of electrons through E1,
FIG. 2. a Low-temperature T=12 K EL spectrum for the p-i-n
diodecontaining thep-Ga1xMnxAs layerV=1.89 V.bBias dependence of
theEL intensity for the QW emission circlesand of the tunnel
current solidline.
FIG. 3. aSchematic diagram of the conduction and valence band
edges ofour p-i-n diode containing the p-Ga1xMnxAs layer biased at
V= VFB forwhich the electric field in the intrinsic region of the
device is zero. bSchematic of the dynamics of photoexcited
electrons and holes when thediode is biased at VVFB.
cLow-temperature T=4.2 K IV character-istics for different
intensities of laser excitation P=0, 4, 11, 18, and35 W cm2. The
inset shows the hole and electron resonances in the differ-ential
conductance, dI/dV, plot for Pmax=35 W cm
2. The right insets showthe intersection point, VFB, in
photocurrent for our p-i-n diode containingthe p-Ga1xMnxAs layer
top insetand for our control sample in which thep-type GaAs layer
is doped with shallow carbon acceptors bottom inset.
082106-2 Thomas et al. Appl. Phys. Lett. 90, 082106 2007
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as is evident from Fig. 1. These electrons recombine withholes
in the Ga1xMnxAs and GaAs layers on the p side ofthe diode, thus
reducing further the density of holes near thebarrier that are
available for tunneling into the QW. How-ever, when the electron
tunnel current falls beyond the E1peak, the number of available
holes increases, leading to thesharp rise of the QW EL intensity at
1.85 V shown inFig.2b.
A key parameter of our Ga1xMnxAs heterostructure isthe hole
chemical potential energy p, which we derive fromthe bias
dependence of the PC response at T=4.2 K. For VVFB, the
photocreated electrons and holes are swept by theelectric field in
opposite directions and form accumulationlayers adjacent to the
tunnel barriers, see Fig. 3b. Since thedark current is negligible
for VVFB, the current measuredunder illumination is due to the
photocreated carriers tunnel-ing through the barriers. The
photocurrent is zero at the flat-band condition, i.e., whenV=
VFB=p/e, from which we de-termine the position ofp.
The PC response of the Ga1xMnxAs diode exhibits anegative
photocurrent at biases below VFB=1.410 V and aresonant peak at
V=1.18 V, see Fig. 3c. The IV curvesmeasured under different
excitation powers all intersect atVFB at which the photocurrent is
zero. We observea similar PC effect in our control sample, but the
intersectionpoint occurs at a significantly higher bias, VFB=1.514
Vsee right insets of Fig. 3c. This corresponds top =1.514 0.005 eV,
which as expected, is close to the bandgap of pure GaAs at T=4.2 K
Eg =1.519 eV. The corre-sponding energy for the Ga1xMnxAs diode is
considerablysmaller, p =1.4100.005 eV. Hence, we deduce that
thereis an interface barrier of height Uh =Evp =0.11 eV be-tween
holes at p in the Ga1xMnxAs layer and the valenceband edge, E
v=1.519 eV, in the adjacent GaAs layers. This
barrier explains the absence of distinct hole resonant
tunnel-
ing peaks in the dark IV curve of our Ga1xMnxAs diodeand may be
also relevant to the observation of highly asym-metric resonant
tunneling peaks in the IV curves of p-i-pGa1xMnxAs RTDs.
16 A similar barrier height was derivedfrom thermionic emission
data in p-i-p Ga1xMnxAs diodeswith x=1.4% 5.1%.17 We note that our
value of p coin-cides with that of the ground state of the isolated
substitu-tional Mn acceptor impurity in GaAs,5 thus indicating
thatthe chemical potential remains pinned to this level as the
Mnconcentration changes from the dilute to the alloy regime.
Finally, we consider the resonant features in the biasregion
where the PC is negative. We attribute the peak at1.18 V to
resonant tunneling of the photocreated electrons
from the accumulation layer at the right hand barrier throughthe
E1 subband of the QW and into the n-doped contactlayer. The current
path is completed by the diffusion of thephotoexcited holes towards
the Ga1xMnxAs layer, see Fig.3b.A similar process involving
resonant tunneling of pho-tocreated holes from the accumulation
layer into the HH1and HH2 QW subbands gives rise to two weak
resonant fea-tures in the differential photoconductance plot,
dI/dV, see
inset of Fig.3c.The presence of these hole resonances
inphotoexcitation and their absence in the dark current forVVFBis
further evidence that hole injection into the QWfrom the Ga1xMnxAs
layer is inhibited by a potential barrierat the Ga1xMnxAs-spacer
layer interface.
In conclusion, we have studied a p-i-n light-emittingresonant
tunneling diode in which the p-type hole emitter isGa1xMnxAs x=3%.
The incorporation of Mn leads to a
significant modification of the electronic band
properties.Photoconductivity measurements as a function of bias
pro-vide a means of determining precisely the position of thehole
chemical potential in the Ga1xMnxAs layer with respectto the band
edges in adjacent GaAs layers. This informationshould be of use in
the design and study of spintronic devicesinvolving hole injection
from ferromagnetic Ga1xMnxAs.
The work is partly supported by EPSRC U.K.. Theauthors thank D.
Taylor and J. Chauhan for processing ourdevices.
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082106-3 Thomas et al. Appl. Phys. Lett. 90, 082106 2007
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