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Contract No. DE-AC36-99-GO10337 

A.G. Norman, M.C. Hanna, P. Dippo, D.H. Levi, 

R.C. Reedy, J.S. Ward, and M.M. Al-Jassim 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005

February 2005 • NREL/CP-520-37405 

InGaAs/GaAs QD Superlattices: 

MOVPE Growth, Structural and 

Optical Characterization, and 

Application in Intermediate-Band 

Solar Cells 
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InGaAs/GaAs QD SUPERLATTICES: MOVPE GROWTH, STRUCTURAL AND OPTICAL 

CHARACTERIZATION, AND APPLICATION IN INTERMEDIATE-BAND SOLAR CELLS 

A. G. Norman, M. C. Hanna, P. Dippo, D. H. Levi, R. C. Reedy, J. S. Ward, and M. M. Al-Jassim 

National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, Golden, CO 80401 

ABSTRACT

We report on the growth and characterization of

InGaAs/GaAs quantum dot (QD) superlattices for 

application in intermediate-band solar cells (IBSCs). Good 

optical and structural quality QD superlattices with up to

50 periods were obtained by metal-organic vapor-phase

epitaxy (MOVPE) growth on {113}B GaAs substrates.

Solar cells containing Si ∂-doped and undoped QD

superlattice absorption regions have been fabricated and

their performance compared with control cells containing

undoped GaAs or undoped InGaAs/GaAs superlattice 

absorption regions. The QD superlattice cells exhibited

photoresponses extended to longer wavelengths than the

control cells. The introduction of QDs to the absorbing

region of the solar cells resulted in a decrease in the opencircuit voltages and, in some cases, a decrease in the

short-circuit currents of the cells. 

INTRODUCTION 

In the future, materials costs and availability are likely 

to drive the evolution of photovoltaic technology toward

cells performing at the highest possible efficiencies [1,2]. 

New concepts will be required to produce this ‘third

generation’ of low-cost, high-performance PV devices

[1,2]. One of the largest losses associated with

conventional single-junction solar cells is that subbandgap photons do not contribute to the device current. 

Luque and Marti [3,4] proposed increasing the efficiency of

ideal solar cells by using an intermediate band lying inside

the forbidden energy gap of the semiconductor host 

material to absorb photons with energies below the

semiconductor bandgap. The states within the

intermediate band need to be partially filled to provide

good absorption. This is so an electron from the valence

band can be excited to an unoccupied level in the 

intermediate band, from which it or another electron can

then be excited to the conduction band (see Fig. 1). By

this mechanism, the absorption of two photons of low

energy can result in an electron-hole pair of higher energy

than the semiconductor host-material bandgap. The

electrons in the conduction band generated by this

process add to those generated by the normal one-step

absorption process across the barrier semiconductor

material bandgap, thus adding extra current flow in the

device. If the cell is properly contacted, these extra

electrons can be extracted at a high voltage limited only by

the bandgap of the barrier material. Assuming optimal 

photon selectivity, they showed that the limiting efficiency 

was as high as 63.2% under maximum concentration

conditions at 300 K (sun assumed to be a black body at

6000 K), which is much greater than the 40.7% limiting

efficiency of a conventional single-gap solar cell under the 

same operating conditions. Under the above conditions, 

the optimum values of the gaps (see Fig. 1) were

predicted to be EL = 0.71 eV, EH = 1.24 eV, and EG = 1.95

eV . 

EG

VB

CB

IB

hv2

hv1 hv3 EH

EL

Fig. 1. Sketch of the energy gaps and photon

absorption processes involved in the operation of

an intermediate band solar cell. 

Marti et al. [5–8] proposed realizing the concept of an

IBSC by using QD technology. In a small-bandgap QD 

surrounded by a higher-bandgap barrier material, the

wave function of an electron can be confined to discrete

CB

VB

QD Barrier material

1.95 eV

Distance from QD center

0.87 eV

0.71 eV

1.24 eV

RD

Energy

E1 (level in dot)

Fig. 2. Simplified band structure of ideal spherical 

QD in QD intermediate-band solar cell [5]. 

1 
energy levels (see Fig. 2). The position of these levels 

can be tuned by controlling the size of the QD. In Fig. 2,

the top of the valence band (VB) has been assumed to be 

the same in the QD and barrier semiconductor material,

and band bending due to space-charging effects has been

ignored. The optimum barrier material bandgap of 1.95

eV is shown. In this example, electrons confined in the

QD have a single energy level E1 lying at 0.71 eV below 

the conduction band (CB) edge produced by a potential

well depth of 1.08 eV below the CB edge resulting from a 

QD material of bandgap 0.87 eV [5]. AlGaAs barriers and 

InGaAs QDs are potential materials to produce such a

band structure. Ideally, however, one would prefer a

combination of QD and barrier materials with a zero VB 

offset, because a non-zero VB offset will result in a

reduction in the open-circuit voltage of the final QD IBSC 

[9]. If the QDs form an ordered 3D array, packed close

enough together such that the wave functions of the

carriers in the individual QDs overlap with each other, then

the discrete energy levels of a single QD broaden to

become mini-bands. A mini-band at the appropriate

energy position can then act as the intermediate band

required for an IBSC. A close-packed 3D array of QDs is 

also required for a high absorption coefficient. A 

schematic of the proposed QD IBSC device structure is

shown in Fig. 3.

A possible way of obtaining the ordered 3D array of

QDs required for the QD IBSC is by the strain-induced

Stranski-Krastanov growth process [10,11]. Growth of a 

coherently strained epitaxial layer results in the formation

of strained islands after a certain critical thickness; this is 

driven by a reduction in the total energy. These strained

islands of a narrow-bandgap semiconductor form QDs 

when surrounded by a higher bandgap barrier

semiconductor material. A single layer of these strained

islands forms a 2D array of QDs that can show some

lateral ordering [11,12]. A 3D array of QDs is then formed

by vertical stacking of 2D arrays of QDs separated by thin

barrier layers. Strain coupling between adjacent QD

layers can result in an ordered vertical stacking of the QDs 

and the formation of a 3D QD array or QD superlattice

(SL) [11,13]. 

In this work, we have chosen to grow by low-pressure

MOVPE InGaAs/GaAs QD superlattices (SLs) as a model 

system to test the concept of the IBSC. GaAs was chosen

as the barrier material (rather than the more optimum 

bandgap AlGaAs) because of the large difference in

growth temperature required to grow good-quality AlGaAs

and InGaAs QDs by MOVPE.

Contact

p n

Contact

Barrier semiconductor

Quantum dots

Fig. 3. Schematic of the QD intermediate band 

solar cell. 

EXPERIMENTAL

InGaAs/GaAs QD SLs, containing up to 50 periods, 

have been grown by low-pressure MOVPE on both (001)

and {113}B GaAs substrates. Triethylgallium, 

trimethylindium, and arsine were used as sources for the 

MOVPE growth. The growth temperature was 550 ̊C and

the QD composition was aimed to be In0.47Ga0.53As. 

Annealing under H2 after the growth of each InGaAs QD

layer was found to be essential in order to grow goodquality QD SLs. The microstructure of the QD SLs was 

characterized by atomic force microscopy (AFM), 

transmission electron microscopy (TEM) and x-ray 

diffraction. The optical properties of the samples were

assessed by photoluminescence (PL) and absorption

measurements. Pin device structures incorporating Si ∂-

doped or undoped QD SL absorbing regions have been

grown and processed into mesa–type cells with 2-mmdiameter circular active regions. External quantum

efficiency (EQE) and current-voltage (IV) measurements

have been made from several cells on each wafer, and the

results were compared to measurements performed on

control cells with identical device structures (except for

containing undoped GaAs or undoped InGaAs/GaAs SL

absorbing regions). 

RESULTS 

Growth and characterization of QD SLs 

To obtain high-quality QD SLs, it was found that the

MOVPE growth conditions are critical, particularly the gasswitching procedures used at the end of growth of the

InGaAs QD layers. Pausing under AsH3 for 1 s after the 

growth of each InGaAs QD layer resulted in only a low

density of large InGaAs islands after growth of several

periods of the QD SLs, particularly on (001) substrates.

Incorporating a 30 s anneal under H2 after each QD layer 

Fig. 4. AFM image of a 50-period undoped 

InGaAs (6.1 ML)/GaAs (20 nm) QD SL grown on 

{113}B GaAs substrate. 

2 
was found to result in a much higher quality QD SL for

growth on {113}B substrates. Using these conditions, 

good lateral ordering of QDs occurred, as was determined

by AFM on a 50-period QD SL (see Fig. 4). Ordered

chains of QDs were present and aligned along [-110] on

the {113}B surface. The QDs had a number density of

1010 cm-2. The same growth conditions for QD SLs on

(001) substrates, however, resulted in clusters of InGaAs

QDs on top of surface mounds. QD SLs grown on {113}B

substrates using the H2 anneals showed very good vertical

stacking of the QDs along the [113] growth direction when

examined by cross-sectional TEM (see Fig. 5). In

comparison, the QD SLs grown on (001) substrates using

the same conditions exhibited little vertical stacking of 

QDs. 

Fig. 7. Room-temperature PL spectra of 50-

period InGaAs/GaAs QD SLs grown on {113}B

GaAs (solid line) and (001) GaAs (dashed line)

0.2 μm substrates. 

<113>B

<-110> pole BF

Fig. 5. <-110> pole bright-field TEM cross-section

image of a 50-period undoped InGaAs (6.1 

ML)/GaAs (20 nm) QD SL grown on {113}B GaAs

substrate. In QD SLs containing 25 and 50 periods, the misfit 

strain associated with the InGaAs QD layers resulted in

the introduction of misfit dislocations (see Fig. 6). Such

lattice defects are liable to result in a degradation of

optical properties and a reduction in minority-carrier

lifetimes and thus could reduce the performance of solar

cells fabricated from such QD stacks. 

The optical properties of the QD stacks were

assessed by PL. Room-temperature PL spectra of 50-

period undoped QD stacks grown using H2 pauses after

p+ GaAs substrate

100 nm p GaAs

100 nm n GaAs

30 nm n GaInP

200 nm n+ GaAs

50x(9 nm UD/1 nm Si:

GaAs/6.1ML InGaAs)

Au grid bar

Au contact

Fig. 8. Schematic diagram of QD IBSC

device structure containing a 50-period

Si ∂-doped InGaAs/GaAs QD SL 

absorbing region. 

0.2 μm

220 DF <-110> XS

InGaAs/GaAs QD SL

GaAs

<113>B

Fig. 6. 220 dark-field TEM cross-section image of 

a 50-period undoped InGaAs (6.1 ML)/GaAs (20 

nm) QD SL grown on {113}B GaAs substrate. 

Misfit dislocations are present at InGaAs/GaAs 

QD SL–GaAs interface. 

3 
growth of the InGaAs QD layers are shown in Fig. 7.

Despite the presence of misfit dislocations, the sample

grown on {113}B, which contains a good-quality wellordered QD SL (see Figs. 4 and 5), shows a single intense

peak at an energy of 1.12 eV. The sample grown on (001)

that contained a very poor quality QD SL exhibited

multiple PL peaks of lower intensity. 

Growth, fabrication, and characterization of QD solar

cells 

Test solar cell pin devices have been grown and

fabricated that incorporate Si ∂-doped and undoped

InGaAs/GaAs QD SL absorbing regions. A typical device

structure is shown in Fig. 8. The wafers were processed

into an array of small isolated mesa-type cells with a 2-

mm-diameter circular active region. The absorbing region 

thickness was kept constant at 0.5 μm by adjusting the

barrier layer thickness in QD SLs consisting of 12, 25, and 

50 periods. The performance of these QD solar cells was

compared with control cells having an identical device

structure, but without the QD layers. One control cell

contained an undoped GaAs absorbing region. The other

control cell contained an absorption region consisting of

an undoped SL of thin two-dimensional strained InGaAs 

quantum wells separated by thin GaAs barrier layers; to

determine the effect of thin two-dimensional InGaAs

wetting layers present in the QD SL samples on cell

performance. 

EQE measurements were performed on several cells

of each processed wafer using a calibrated Si cell as a 

reference cell. No antireflection coatings were used on

the devices. Some typical results obtained for different 

device structures grown on {113}B substrates are shown

in Fig. 9. The InGaAs/GaAs SL control cell exhibits a

photoresponse extended to slightly longer wavelengths

than the GaAs control cell. All the QD SL cells have

photoresponses extended to longer wavelengths than

both the control cells. Luque et al. [14] recently reported a

similar extended photoresponse for a 10-period Si ∂-

doped InAs/GaAs QD IBSC device grown by molecular

beam epitaxy. It is clear from our results that an increase

in the number of QD SL periods in the absorbing region of 

Si ∂-doped QD SL devices results in an increase in the

EQE at long wavelengths. A 50-period undoped QD SL

exhibited a higher EQE at longer wavelengths than the

equivalent Si ∂-doped structure (almost 1% still at a

wavelength of 1100 nm). 

Fig. 9. External quantum efficiencies of InGaAs/GaAs QD SL cells and InGaAs/GaAs 

SL and GaAs control cells grown on {113}B GaAs substrates. 

IV measurements have been performed on several

cells from each processed wafer. These measurements

were performed at 25 ̊C under an AM1.5G spectrum at an

irradiance of 1000 W/m2

 using a XT10 solar simulator. A 

Si reference cell was used to calibrate the illumination

level. Some typical light-IV results obtained for different 

device structures grown on {113}B substrates are shown

in Fig. 10. The QD SL solar cells show reduced opencircuit voltages (VOC) in comparison to the control cells.

For Si ∂-doped QD cells, the VOC decreases as the

number of periods is increased in the QD SL absorption 

region. Si ∂-doped QD cells containing 25 and 50 periods

also exhibited much reduced short-circuit current densities

(JSC) in comparison to the control cells and a 12-period Si

∂-doped QD cell. Interestingly, a 50-period undoped QD

cell had a JSC comparable to the control cells. 

DISCUSSION 

The IV measurements indicate that the addition of a

QD SL in the absorbing region reduces the VOC of the

cells. One of the causes of this is probably due to the VB

offset between the InGaAs QDs and the GaAs barrier

layers. If a large VB offset is present it leads to the

4 
introduction of a high number of closely spaced hole levels

into the QD VB with a separation < kT that can overlap

with the barrier layer VB continuum. This can enable

easy, rapid relaxation of holes, through phonon

interaction, from the VB continuum to the hole minibands

in the QDs. This will result in a reduction of the VOC of the 

cell of close to the value of the VB offset. The VB offsets

for InGaAs strained to GaAs were calculated by Kim et al. 

[15]. For In0.47Ga0.53As strained to GaAs, they predict a

VB offset of about ~0.23 eV. In the case of our 50-period

QD SLs, we see a reduction in VOC of 0.45–0.5 eV. Other 

sources for the reduction in VOC of these SLs with a high

number of periods could be segregation of In into the

barrier layers forming lower bandgap InGaAs alloy barriers

or increased recombination in the devices associated with

the quantum dots and the high density of misfit 

dislocations present in some samples. Alternatively, 

electrons from the CB of the barrier material may be

rapidly relaxing to the QD IB by the emission of phonons

causing the CB electron gas to collapse and no longer be

well separated from the IB electron gas. This means that

the CB electron gas will no longer have its own quasiFermi level well separated from the IB electron gas quasiFermi level resulting in a reduction in the VOC. To

overcome the reduction in VOC due to the VB offset, a new

QD/barrier materials system with as low a VB offset as

possible is probably required. Honsberg and Levy [9]

have recently identified InAsSb or InPSb QDs in AlAsSb

barriers as potential materials systems with very low VB

offsets. 

In the 25- and 50-period Si ∂-doped QD SLs, a large

reduction in JSC was measured in comparison to a 12-

period Si ∂-doped QD SL, a 50-period undoped QD SL, 

and the two control cells. TEM studies revealed that both

the 50-period Si ∂-doped and the 50-period undoped QD

SLs contained a high density of misfit dislocations. Both

samples showed reasonably good vertical stacking of 

QDs. These results suggest that the large reduction in JSC

in the 25- and 50-period Si ∂-doped samples may be 

related to the Si ∂-doping. These two samples also

exhibited reduced EQE for wavelengths below 900 nm in

comparison to the other cell structures. 

Fig. 10. Typical current-voltage characteristics measured for InGaAs/GaAs QD SL 

cells and InGaAs/GaAs SL and GaAs control cells grown on {113}B GaAs substrates. 

To achieve a high absorption coefficient for the QD 

SL cells, we require as high a QD density as possible. 

This means that one has to grow QD SLs with a large

number of periods. If the Stranki-Krastanov strain-induced

growth mechanism is used to form the QD SLs, then

eventually, as more QD SL periods are added the whole

SL structure will relax by the introduction of misfit

dislocations to relieve the misfit strain between the QD SL

and the substrate. To overcome this problem, strainbalanced QD SLs will be required. The compressive

strain in the QD layers can be compensated by growing

tensile-strained barrier layers. By the appropriate choice

of materials and layer thickness, a strain-balanced QD SL

can be grown that exhibits no net lattice mismatch to the 

substrate. If strain balance can be achieved, there should

be no limit to the number of periods that can be grown in

the QD SL. The reduction in dislocation density expected

in these strain-balanced structures should also lead to an

increase in the VOC of QD SL cells. Such strain-balanced

structures, e.g., InGaAs/GaAsP on GaAs substrates, have

been successfully grown previously for quantum-well solar

cells [16]. 

CONCLUSIONS 

We have grown by MOVPE on {113}B GaAs 

substrates up to 50-period InGaAs/GaAs QD SLs showing

both lateral and vertical ordering of QDs and good

structural and optical properties. Solar cells incorporating 

these QD SLs in the absorbing regions show

photoresponses extended to longer wavelengths than

GaAs and InGaAs/GaAs SL control cells. The QD solar

cells show reduced VOC and, in some cases, reduced JSC

in comparison to the control cells. Undoped QD SL

devices performed better than Si ∂-doped devices. To

avoid the introduction of harmful misfit dislocations in 

5 
[8] A. Martí, L. Cuadra, and A. Luque, “Design Constraints

of the Quantum-Dot Intermediate Band Solar Cell”,

Physica E 14, 2002, pp. 150-157. 

devices with a large number of periods, we may need to

grow strain-balanced structures, e.g., InGaAs/GaAsP QD

SLs on GaAs substrates. 

[9] C.B. Honsberg and M.Y. Levy, “Theory and 

Experimental Investigation of Approaches to >50% Solar

Cells”, DOE Solar Program Review Meeting Denver 

October 2004.

ACKNOWLEDGEMENTS 

This work was supported by the NREL Director’s

Discretionary Research and Development (DDRD)

Program.

[10] I.N. Stranski and L. Krastanov, Sitzungsber. Akad.

Wiss Wien. Math-Naturwiss. Klasse 146, 1937, pp. 797. 

[11] V.A. Shchukin, N.N. Ledentsov, and D. Bimberg,

REFERENCES Epitaxy of Nanostructures, Berlin: Springer-Verlag, 2004. 

[12] V.A. Shchukin, N.N. Ledentsov, P.S. Kopev, and D.

Bimberg, “Spontaneous Ordering of Arrays of Coherent 

Strained Islands”, Phys. Rev. Lett. 75, 1995, pp. 1968-

2971. 

[1] M.A. Green, “Third Generation Photovoltaics: Solar 

Cells for 2020 and Beyond”, Physica E 14, 2002, pp. 65-

70. 

[13] Q. Xie, A. Madhukar, P. Chen, and N. P. Kobayashi, 

“Vertically Self-Organized InAs Quantum Box Islands on

GaAs(100)”, Phys. Rev. Lett. 75, 1995, pp. 2542-2545. 

[2] M.A. Green. Third Generation Solar Cells: Advanced

Solar Energy Conversion, Berlin: Springer-Verlag, 2003. 

[3] A. Luque and A. Martí, “Increasing the Efficiency of

Ideal Solar Cells by Photon Induced Transitions at 

Intermediate Levels”, Phys. Rev. Lett. 78, 1997, pp. 5014-

5017. 

[14] A. Luque, A. Martí, C. Stanley, N. López, L. Cuadra,

D. Zhou, J.L. Pearson, and A. McKee, “General Equivalent

Circuit for Intermediate Band Devices: Potentials, Currents

and Electroluminescence”, J. Appl. Phys. 96, 2004, pp. 

903-909. 

[4] A. Luqe and A. Martí, “A Metallic Intermediate Band

High Efficiency Solar Cell”, Prog. Photovolt.: Res. Appl. 9, 

2001, pp. 73-86. [15] K. Kim, G.L.W. Hart, and A. Zunger, “Negative Band

Gap Bowing in Epitaxial InAs/GaAs Alloys and Predicted

Band Offsets of the Strained Binaries and Alloys on 

Various Substrates”, Appl. Phys. Lett. 80, 2002, pp. 3105-

3107. 

[5] A. Martí, L. Cuadra, and A. Luque, “Quantum Dot 

Intermediate Band Solar Cell”, Twenty Eighth IEEE PVSC, 

2000, pp. 940-943. 

[6] A. Martí, L. Cuadra, and A. Luque, “Partial Filling of a 

Quantum Dot Intermediate Band for Solar Cells”, IEEE 

Trans. Electron Devices 48, 2001, pp. 2394-2399. 

[16] N.J. Ekins-Daukes, K.W.J. Barnham, J.P. Connolly, 

J.S. Roberts, J.C. Clark, G. Hill, and M. Mazzer, “Strainbalanced GaAsP/InGaAs Quantum Well Solar Cells”, 

Appl. Phys. Lett. 75, 1999, pp. 4195-4197. 

[7] A. Martí, L. Cuadra, and A. Luque, “Quantum Dot 

Analysis of the Space Charge region of the Intermediate

Band Solar Cell”, Photovoltaics for the 21st Century,

Electrochemical Society Proceedings Volume 2001-10, 

2001, pp. 46-60. 

 

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4. TITLE AND SUBTITLE

InGaAs/GaAs QD Superlattices: MOVPE Growth, Structural and 

Optical Characterization, and Application in Intermediate-Band 

Solar Cells 

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A.G. Norman, M.C. Hanna, P. Dippo, D.H. Levi, R.C. Reedy, 

J.S. Ward, and M.M. Al-Jassim 

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14. ABSTRACT (Maximum 200 Words)

We report on the growth and characterization of InGaAs/GaAs quantum dot (QD) superlattices for application in 

intermediate-band solar cells (IBSCs). Good optical and structural quality QD superlattices with up to 50 periods were 

obtained by metal-organic vapor-phase epitaxy (MOVPE) growth on {113}B GaAs substrates. Solar cells containing Si 

∂-doped and undoped QD superlattice absorption regions have been fabricated and their performance compared with 

control cells containing undoped GaAs or undoped InGaAs/GaAs superlattice absorption regions. The QD superlattice 

cells exhibited photoresponses extended to longer wavelengths than the control cells. The introduction of QDs to the 

absorbing region of the solar cells resulted in a decrease in the open-circuit voltages and, in some cases, a decrease in 

the short-circuit currents of the cells. 

15. SUBJECT TERMS

PV; growth and characterization; quantum dot (QD); intermediate-band solar cells (IBSCs); metal-organic vaporphase epitaxy (MOVPE); photoresponses; open-circuit voltages; 

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