Novel materials and nanostructures for advanced optoelectronics

Q. Zhuang, P. Carrington, M. Hayne, A Krier Physics Department, Lancaster University, UK

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Outline u Brief introduction to

­  Lancaster University

­ Department of Physics

­  Research group - Semiconductor Physics and Nanostructures

u III-V compound materials and Nanostructures from MBE ­  QDs (self-assembled, V group exchange & droplets)

­  Nanowires

­  Dilute nitrides

u Summary

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Lancaster University Established 1964 Close to the Lake District 17,500 students 2,250 staff Ranked in top 10 universities in UK Ranked 131 in the World (THEWUR) Top ranked Physics Department in UK in RAE 2008

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

semiconductor physics and nanostructures

u Academic staff ­ Qiandong Zhuang (molecular beam epitaxy) ­ Manus Hayne (physics & applications of QDs) ­ Oleg Kolosov (scanning probe microscopy) ­ Tony Krier (Mid-infrared optoelectronics) ­ Three Fellowships

u Main research facilities ­ Epitaxial growth – 3 MBE ­ Photoluminescence (PL) ­ Magneto-PL up to 17 T ­ AFM & SEM ­ X-ray ­ Class 100 clean room for nano-fabrication

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Growth facilities u MBE-1&3 (VG V80H):

­  III: Ga, Al, In ­ V: As, Sb, N plasma ­ Dopant: GaTe, Be

u MBE-2: ­  Wide bandgap nitride nanowires ­ Graphene on BN

u Research focuses on III-V semiconductors ranging from MBE growth to fundamental studies, optoelectronic devices (telecom lasers, MID emitters & detectors, solar cells, thermophotovoltaic, next generation QDs flash memories)

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Self-assembled GaSb QDs

Type-II QDs (Leuven, Berlin, LNLS Brazil) [PRB, APL, PRB, PRB]

u Why GaSb/GaAs QDs? − Type II: flash memories

!

(b)

GaSb GaAs GaAs

!"

+"Ener

gy

z

(a)

Schematic band-gap diagram of GaSb/GaAs including band bending. z is the growth direction.

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Self-assembled GaSb QDs

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.60

10000

20000

30000

40000

50000

60000 4.2 K

Cou

nts

(arb

. uni

ts)

Energy (eV)

GaAs

WL

QDs

Height: 3 nm; Diameter: 36 nm Density: 4x1010 cm-2

10nm

Scanning tunnelling microscope image of a quantum ring cross-section; brighter regions indicate high Sb content.

Sb leaks out

Strong As-Sb exchange and Sb segregation during capping

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

InSb/InAs QDs by exchange

Temperature dependent PL was carried out in the range4–300 K. An Ar-ion laser operating at 488 nm was used forexcitation of the sample, which was contained in a continu-ous flow liquid helium cryostat. The midinfrared radiationwas analyzed using a Bentham 0.3 m monochromator anddetected using a 77 K InSb detector and conventional lock-intechniques. For EL measurements the samples were pro-cessed into the mesa etched homojunction p-i-n LEDs thatwere 750 !m in diameter using standard photolithographictechniques.

PL emission spectra !80 K" from samples grown at dif-ferent temperatures are shown in Fig. 1. In each case, twopeaks can be clearly identified, #3.0 !m from InAs and amuch stronger peak between 3.2–3.7 !m. The latter is as-sociated with the recombination of holes localized in theInSb QDs and electrons in the surrounding InAs layerscoupled to the holes via coulomb interaction !see inset ofFig. 1". The QD PL shifts toward longer wavelengths as thegrowth temperature decreases consistent with the formationof larger InSb QDs. The width of the QD peak also increaseswith the decreasing growth temperature, corresponding tothe recombination from a wider distribution of dot sizes. Forthe lower growth temperatures !345 and 320 °C" the peakfrom the InSb dots is much higher than that from InAs indi-cating stronger hole localization in the larger QDs, which isrequired for LEDs and lasers operating at high temperatures.The QD PL peak wavelength was also found to depend dif-ferently on growth temperature for the QD structures grownusing either !Sb2 ,As2" or !Sb4 ,As4". We found that fordimers the growth temperature should be reduced by50–100 °C in order to get the same PL wavelength whenusing tetramers. XRD measurements revealed that for thestructures grown using !Sb2 ,As2" the amount of InSb depos-ited is less than for !Sb4 ,As4" at the same growth tempera-ture. This causes the PL to shift toward shorter wavelengths,since the reversed As-to-Sb exchange interaction is easierunder As2 flux, which is more aggressive than As4. Thisindicates that there exists a dynamic equilibrium betweenSb-to-As and As-to-Sb exchange, which depends on thegrowth temperature and As, Sb fluxes. However, no differ-ence was found in the behavior of Sb2 and Sb4 molecules forthe growth temperatures used.

Figure 2 shows the temperature dependence of the PLfor QD structure grown at 450 °C. With the increasing tem-

perature, an anomalous blueshift of the InSb QD peak isobserved up to 60 K and is associated with state filling in theQD array, due to the electronic coupling between the dots atlow temperatures.12 At higher temperatures the redshift ofthe PL peak corresponding to the band gap narrowing withtemperature becomes the dominant effect. Despite bright RTPL observed from the InSb QDs grown by Sb-to-As ex-change, EL from LEDs containing such QDs was weak atRT. Low growth temperatures !"350 °C" are needed toform large dots with sufficient hole localization energy whenusing only Sb-to-As exchange. At such low growth tempera-tures, the material quality is poor and there is a strong sur-face segregation of Sb atoms in the surrounding InAs matrix,which forms rough interfaces.14 To produce InSb QDs forLEDs, larger dots at higher temperature were formed usingSb/As exchange at 430 °C, followed by additional MEEdeposition of 0.7 ML of InSb. LED structures containing tenInSb QD sheets separated by 19-nm-thick InAs barriers weregrown on n-type InAs !100" substrates. The use of an un-doped 30-nm-thick Al0.9Ga0.1As0.15Sb0.85 electron blockingbarrier further increased the RT EL intensity by a factor of 5.Figure 3 shows the RT EL spectra for one of the QD LEDswith a blocking barrier, where a single QD-related peak isobserved at 3.8 !m. Note that the InAs related emission at

FIG. 1. !Color online" PL Spectra !80 K" of the InSb QD structures grownat different temperatures. The numbers on the left correspond to the InSbnominal thickness calculated from XRD simulation.

FIG. 2. !Color online" Temperature-dependent PL spectra for the samplegrown at 450 °C !0.5 ML InSb thickness". The inset shows the temperaturedependence of the InSb QD peak.

FIG. 3. !Color online" RT emission spectra at various injection currentsfrom a QD LED structure with an AlGaAsSb barrier.

091101-2 Carrington et al. Appl. Phys. Lett. 93, 091101 !2008"

Downloaded 11 Sep 2008 to 194.80.32.9. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

u InAs-based QDs for mid-infrared light emitters

u Fabricate through an exchange technology

Currently developing InSb/InAs MIR Lasers on InP

Sb-As exchange

GaAs

Sb Flux

InAs

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

GaAs/AlGaAs QDs by droplets

5 ML Ga droplets Diameter: 71 nm; height: 25 nm

Density: 6.5±0.5x109 cm-2

200 nm Al0.32 Ga0.68 As

GaAs Sub

Expose to As4 to crystallize

u Why droplets for QDs? - No strain is required

Cap with GaAs or AlGaAs

More flexible in materials design More suitable for fundamental studies

Ga droplets

Sensitive to growth

conditions

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

GaAs/AlGaAs QDs by droplets

u Sharp PL emission u Persists up to room

temperature u Geometry of buried dots

to be studied by TEM u Aim to transfer to

antimonide QDs

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Nitride nanowires on Si(111)

Material and Energetic Criteria

� Band Gap (Eg) must be atleast 1.6-1.7 eV.

� Band Edges must straddleH2O redox potentials

� Rapid charge transfer

� Stable in aqueous solution

1.23 eV

1.6-1.7 eV

p-typeSemiconductor

Eg

CounterElectrode

H2O/H2

H2O/O2

All must besatisfied

simultaneously

u Bandgap must be 1.6-1.7 eV u straddle H2O redox potentials u Rapid charge transfer u Stable in aqueous solution

u Advantages of NWs? u Devices: Solar cells, lighting &

water splitting – H2 generation

Requirements:

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Nitride nanowires on Si(111)

InN grains InN Nanowires

1500 1550 1600 16500

500

1000

1500

2000

50 mW

100 mW

200 mW

Inte

nsity

(a.u

.)

Wavelength (nm)

400 mW

u  Monocrystalline u  Strain-free

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Dilute nitrides

u A few percent of nitrogen ­  reduces lattice constant ­  reduces band-gap ­ Reduced Auger

recombination

u Ga(In)AsN: 1.3 ~1.5 um emission on GaAs

u Possible for MIR optoelectronics?

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Physics of bandgap bowing

Shan W et al, Phys. Rev. Lett. 82 1221(1999)

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Dilute nitrides

u Develop GaAs-based dilute nitride lasers operating at 1.55 um up to a temperature of 85oC through optimising valence band offset ­ Adding Sb ­ Embedded AlGaAs barrier

u Oclaro studentship

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Dilute nitrides

InAsNSb LED

Sb as surfactant improves the dilutes

u InAsNSb on InAs or GaSb: covers MIR and far-infrared with suppressed Auger recombination

Developing nitride MIR lasers on InP in collaboration with Wisconsin University

Q Zhuang, Sino-UK Workshop, 6-7 December 2012, Beijing, China,

Summary

u Top 10 universities in the UK, top ranked Physics in RAE 2008

u MBE growth facilities - most of arsenic and antimonide materials and

u Active in MBE grown novel materials and nanostructures ­ GaSb/GaAs QDs/Quantum Rings ­  InSb/InAs QDs by exchange ­ GaAs/AlGaAs QDs by droplets ­ Novel dilute nitride materials ­ Wide bandgap nitride NWs on Si

u New materials & structures: graphene + BN