Temperature-543

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23. Temperature-Insensitive Band-Gap III-VSemiconductors: Tl-III-V and III-V-Bi

Hajime Asahi

Thallium-containing III-V (Tl-III-V) and bismuth-containing III-V (III-V-Bi) alloy semiconductorswere first proposed as novel functional semi-conductors. They are alloys consisting of semi-conductors (III-V) and semimetals (Tl-V, III-Bi),and are important materials for the fabrication oftemperature-insensitive lasing wavelength laserdiodes as well as long wavelength infrared (LWIR)optical devices. However, the growth conditions foralloys containing Tl and Bi are very strict and thegrowth windows are narrow. In this chapter, theexpected properties of these semiconductors andthe experimental results for the growth, charac-terization, and device applications are described.

23.1 Tl-III-V Alloy Semiconductors ............ 54423.1.1 Expected Properties

of Tl-Based III-V Alloys....................... 54423.1.2 Growth of Tl-III-V Alloys..................... 54623.1.3 Temperature Dependence

of Physical Properties ......................... 54723.1.4 Tl-III-V LD Application........................ 549

23.2 III-V-Bi Alloy Semiconductors ............ 55023.2.1 Expected Properties

of Bi-Based III-V Alloys ...................... 55023.2.2 Growth of III-V-Bi Alloys .................... 55123.2.3 Optical Properties of III-V-Bi Alloys ..... 55223.2.4 III-V-Bi LD Application ....................... 553

23.3 Summary .......................................... 554

References ................................................... 554

Wavelength division multiplexing (WDM) technologyis very important for an optical fiber communicationsystem for increasing transport capacity and obtain-ing flexible network management. However, one of theproblems encountered when using InGaAsP/InP laserdiodes (LDs) in the WDM system is that the lasingwavelength fluctuates with ambient temperature vari-ation mainly due to the temperature dependence ofthe band-gap energy. Therefore, LDs in WDM systemsmust be equipped with Peltier elements to stabilize theLD temperature.

To solve this problem, Oe et al. proposed the useof temperature-insensitive band-gap semiconductors asan active layer of LDs [23.1]. Such a temperature-insensitive band-gap property was observed in the II-VIsemiconductor, HgCdTe, as shown in Fig. 23.1 [23.2].HgCdTe is usually used for the long wavelength in-frared (LWIR) detectors. CdTe is a semiconductor andHgTe is a semimetal. With the increase of HgTe com-position, their band-gap energy decreases. At the CdTe-rich side of HgCdTe, the band-gap energy decreaseswith the increase of temperature, while at the HgTe-richside of HgCdTe, the band-gap energy increases with theincrease of temperature. Therefore, at a certain HgTecomposition (0.48) region, the temperature-insensitive

behavior is expected [23.3]. Similar behavior was alsoobserved for HgCdSe, where the very small temper-ature dependence of refractive index was also con-firmed [23.4].

The origin of this temperature-insensitive band-gapenergy is considered as follows. Figure 23.2 shows thealloy composition dependence of the band structure forHgCdTe alloys [23.5]. At the CdTe side of HgCdTe,the �6 band behaves as a conduction band and the �8

band behaves as a valence band. Also in this com-position region, the band-gap energy decreases withincreasing temperature similar to usual semiconduc-tors. With increasing HgTe composition, the �6 bandgoes down and the band-gap energy decreases. By fur-ther increasing HgTe composition, the �6 and �8 bandscross and the band inversion occurs. After this band in-version, the �6 band becomes a valence band and the�8 light hole band forms a conduction band. The �8

heavy hole band still behaves as a valence band andHgCdTe becomes a semimetal. Therefore, the energydifference between the two bands increases with in-creasing temperature so that the band-gap energy forHgCdTe semiconductors at the HgTe side composi-tion increases with increasing temperature. At someintermediate alloy composition, we can observe the

© Springer International Publishing AG 2017S. Kasap, P. Capper (Eds.), Springer Handbook of Electronic and Photonic Materials, DOI 10.1007/978-3-319-48933-9_23

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544 Part C Materials for Electronics

1.0

–0.4

0

1.00.80.60.40.20CdTeHgTe x

Band-gap energy (eV)

~OK300 K

InterbandMagnetorefelection AT 77 K

InterbandMagnetorefelection AT 4 K

Optical absorption AT 300 K

Photovol studies AT 77 Kand 300 K

Photoluminescence AT 12 K

Hg1–xCdxTe

Fig. 23.1 Band-gap energy versus alloy composition forHgCdTe as a function of temperature. (After [23.2])

temperature-insensitive band-gap energy as shown inFig. 23.1 [23.2].

From the above-mentioned discussion, the alloysemiconductor consisting of a semiconductor anda semimetal is expected to show the temperature-insensitive band-gap behavior. Oe et al. suggestedGaAsBi and InGaAsBi alloys as new semiconductorsshowing temperature-insensitive band-gap energy forpossible application to the temperature-stable lasing

1.2

1.6

0.8

0.4

–0.3

0

1.00.80.60.40.20CdTeHgTe x

Band-gap energy (eV)

Hg1–xCdxTe

Semiconductor

T = 4.2 K

Semimetal

Г8

Г8Г6

Г6

Fig. 23.2 Alloy composition dependence of the bandstructure for HgCdTe alloys. (After [23.5])

wavelength LDs [23.1]. Asahi et al. also proposed TlIn-GaAs and TlInGaP as new semiconductors showingtemperature-insensitive band-gap energy [23.6, 7]. Sim-ilar to HgCdTe, it is also expected that these alloysemiconductors can be used for the LWIR detectors.

This chapter describes the expected properties ofthe Tl-III-V and III-V-Bi alloy semiconductors and thepresent status of their growth, characterization, and de-vice applications.

23.1 Tl-III-V Alloy Semiconductors

Alloy semiconductor HgCdTe is widely used forLWIR focal-plane arrays (FPA). However, HgCdTe isa weakly bonded II-VI compound with material growthand processing problems dominated by native defectsthat limit FRA performance and yield [23.8]. VanSchilfgaarde et al. proposed thallium (TI)-based III-V ternary alloy semiconductors, TlInSb, TlInAs, andTlInP as the possible candidates for future LWIR detec-tor materials [23.8–11]. Their theoretical calculationsof TlSb, TlAs, and TlP showed that they have negativeband gaps similar to HgTe (semimetal). Therefore, it isexpected that TlInSb, TlInAs, and TlInP should exhibitLWIR band gaps. Furthermore, they are III-V mate-rials and are compatible with the conventional III-Vdevice fabrication technologies. Therefore, it was ex-pected that these Tl-based alloys could overcome thedifficulties encountered in the widely used LWIR semi-conductor HgCdTe.

Interesting extensions of TlInSb, TlInAs, andTlInP alloys are quaternary alloys TlInGaP and

TlInGaAs [23.6, 7, 12]. These quaternary alloys canbe lattice matched to GaAs, InP, and InAs sub-strates. Lattice matching is very important to ob-tain good quality crystals. Furthermore, they are ex-pected to have temperature-insensitive band-gap en-ergies at some alloy compositions and they can beapplied to the fabrication of temperature-insensitivelasing wavelength LDs [23.6]. Further extension isa quinary alloy semiconductor TlInGaAsN, which con-sists of a semiconductor GaInNAs and a semimetalTlAs and is also expected to be applicable to fab-ricate the temperature-stable lasing wavelength andtemperature-stable threshold LDs [23.13] as discussedbelow.

23.1.1 Expected Properties of Tl-Based III-VAlloys

Figure 23.3 shows the expected relationships betweenthe band-gap energy (Eg) and the lattice constant for

Temperature-Insensitive Band-Gap III-V Semiconductors: Tl-III-V and III-V-Bi 23.1 Tl-III-V Alloy Semiconductors 545Part

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0.0

0.5

1.0

1.5

2.0

2.5

–0.5

–1.0

–1.5

0.5

1

2510

0.5 0.55 0.6 0.65

Band-gap energy (eV)

Lattice constant (nm)

Wavelength (μm)

Direct gapIndirect gapLattice-match

AIPAIAs

GaP

GaAs

GaSb InSbInAs

TIP

TIAs

TISb

AISbInP

GaNP

GaNAs

TIInGaAs

AIGaAsTIInGaP

TIInGaAsN

Fig. 23.3 Relationship for the band-gap energy versus al-loy composition for Tl-III-V alloys as well as usual III-Vsemiconductors. (After [23.6, 12, 13])

Tl-based III-V alloys [23.6, 12, 13] based on the the-oretical results for TlInSb, TlInAs, and TlInP by vanSchilfgaarde et al. [23.8, 11]. The lines for TlGaAs andTlGaP were predicted by Asahi et al. [23.6]. The datafor other III-V compound materials are also plotted inthe figure for reference.

Van Schilfgaarde et al. [23.8–11] have theoreticallycalculated the lattice constant, cohesive energy, elasticconstant, band-gap energy, effective mass, and electronmobility by the local density approximation first prin-ciple theory. They obtained the following results: (1)TlAs and TlP are formed in the zincblende structure,while TlSb is formed in the CsCl structure; (2) the lat-tice constants of TlSb (0:659 nm), TlAs (0:618 nm), andTlP (0:596 nm) are only 1:5�2% larger than those ofInSb (0:648 nm), InAs (0:606 nm), and InP (0:587 nm),respectively; (3) the cohesive energies are about twicethat of HgTe; therefore, bond strengths are higher; (4)TlSb, TlAs, and TlP are semimetals with negative bandgaps of �1:60, �1:34, and �0:27 eV, respectively, sim-ilar to HgTe (�0:3 eV); (5) TlInSb, TlInAs, and TlInPexhibit a direct band gap of 0:1 eV (12:4�m) at theTl compositions of 0.09, 0.15, and 0.67, respectively.These properties are promising for application to LWIRdetectors.

Thermodynamical properties of TlInP and TlInAswere theoretically estimated [23.14]. It was predictedthat only low concentrations of Tl can be achieved inTlInP (< 5% at 350 ıC) using gas-source molecularbeam epitaxy (GSMBE). Although much less than 1%Tl was expected to be soluble in TlInAs, the error esti-mates in calculation indicated that obtaining 15% Tl ispossible.

Quaternary alloys of TlInGaP and TlInGaAs pro-posed by Asahi et al. [23.6, 7] can be easily latticematched to InP, GaAs, and InAs substrates as shownin Fig. 23.3. TlInGaP lattice matched to InP can coverthe band-gap energy from 1:24 eV (0:92�m at 0K,InP) to less than 0:1 eV (12:4�m, TlGaP), and overthe whole lattice-matched composition the direct band-gap is expected to form. The addition of Al and Ascan further extend the energy (wavelength) range. TlIn-GaAs lattice matched to InP can cover from 0:75 eV(1:65�m, InGaAs) to about 0 eV (TlGaAs). These qua-ternary alloys can be applied to LWIR detectors andemitters. They are expected to exhibit the temperature-insensitive band-gap energy behavior at some alloycompositions similar to HgCdTe [23.6]. This is becausethese semiconductors are alloys consisting of semicon-ductors (InGaP, InGaAs) and semimetals (TlP, TlAs).These temperature-insensitive band-gap energy charac-teristics are very promising for the fabrication of LDswhose wavelength does not change with or is insensi-tive to ambient temperature variation. The band lineupof lattice-matched TlInGaP/InP heterostructures wassuggested to be type-I and the conduction band discon-tinuities are larger than the valence band offsets [23.6],also proved by the theoretical calculation [23.15].

Quinary alloy semiconductor TlInGaAsN was pro-posed by Asahi et al. to fabricate the temperature-stable lasing wavelength and temperature-stable thresh-old current TlInGaAsN/AlGaAs LDs [23.13]. TlIn-GaAsN is an alloy of a semiconductor InGaNAsand a semimetal TlAs as shown in Fig. 23.3. In-GaNAs/AlGaAs heterostructures are widely studiedto fabricate temperature-stable threshold current LDsin the wavelength range of 1�m because of theirlarge conduction band discontinuity [23.16]. However,there exists a long wavelength limit of 1:3�m inthis material system. This is because of the degrada-tion of the crystal quality as well as the reductionof valence band offset with increasing N composi-tion [23.17]. To overcome this problem, new alloy semi-conductor TlInGaAsN/AlGaAs heterostructures wereproposed [23.13].

As can be clearly seen in Fig. 23.3, much lowerband-gap energy, therefore, longer wavelength, can beeasily obtained for this new semiconductor comparedwith GaInNAs. TlInGaAsN/AlGaAs heterostructuresalso have a large conduction band discontinuity similarto GaInNAs/AlGaAs ones. Furthermore, TlInGaAsNis expected to have temperature-insensitive band-gapenergy because it is an alloy of a semiconductor anda semimetal, similar to TlInGaAs. Therefore, the fab-rication of temperature-stable lasing wavelength andtemperature-stable threshold current LDs is expected byusing TlInGaAsN/AlGaAs heterostructures.

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546 Part C Materials for Electronics

23.1.2 Growth of Tl-III-V Alloys

The growth of Tl-based III-V alloys has been attemptedby metal-organic vapor phase epitaxy (MOVPE)[23.18–20], gas source MBE (GSMBE) [23.6, 21–26],and solid source MBE (SSMBE) [23.27, 28].

In the MOVPE growth of TlInSb and TlInP, cy-clopentadiethylthallium (CPTl) and trimethylindium(TMIn) were used as Tl and In precursors, and trimethy-lantimony (TMSb) or tertiarybutyldemethylantimony(TBDMSb) and phosphine (PH3) as Sb and P precur-sors, respectively [23.18–20, 29]. The vapor pressure ofCPTl is as low as about 3 Torr even at 120 ıC. There-fore, high bubbler temperature and heated valves, andstainless delivery lines are needed.

In GSMBE or SSMBE growth, elemental Tl, In,and Ga were used as group III sources [23.6, 21–28].Thermally cracked arsine (AsH3), phosphine (PH3), orelemental As and Sb were used as group V sources. Tlvaporizes from the melt state, whichmeans that the con-trollability of the Tl flux is much better than that of Sb.The problem in using Tl metal is its ease of oxidation.The toxicity of Tl is known to be higher than that of As,so extreme care must be taken during treatment [23.6].Tl-condensed surfaces should not be touched with barehands and Tl flakes or vapors should not be inhaled:legal intake is documented to be at 600mg. From thistoxicity point of view, growth by MBE is much saferthan that by MOPVE. In MBE, the solid Tl is used andis stored in the growth chamber, whereas in MOVPE,the Tl is supplied and vented as a form of metal organic(MO) gas. It is considered that there is no essential dif-ference between GSMBE and SSMBE both in safetyand growth mechanism, though growth conditions maybe slightly different.

Growth of Tl-III-Sb Ternary AlloysThe growth of TlInSb was first conducted by Choiet al. [23.18]. They grew TlInSb layers with goodsurface morphology on GaAs and InSb substrates at455 ıC by low-pressure MOVPE. X-ray diffractionmeasurements showed resolved peaks of InSb andTlInSb. However, in contrast to the theoretical predic-tion, an increase in Tl incorporation led to a decreasein the lattice constant (higher diffraction angle). Thiswas also observed by Karam et al. [23.20]. The lat-ter obtained TlInSb with Tl composition of 0.1 (10%).Incorporation of Tl was also confirmed with Augerelectron spectroscopy and Rutherford backscattering(RBS) measurements. Infrared transmission measure-ments showed a shift of the absorption edge towardlonger wavelengths with increasing Tl composition forTlInSb [23.18, 20, 29].

Infrared photoconductors were fabricated fromTlInSb/InSb grown on (100) semi-insulating GaAs sub-strates [23.20, 30, 31]. A clear shift of response tolonger wavelength was observed. The cutoff wave-length extended to 11�m at 300K for the TlInSb (Tlcomposition of 6%) photoconductor [23.30]. The max-imum detectivity was 7:6�108 cmHz1=2 W�1 at 77K.

Growth of Tl-III-As Ternary AlloysThe growth experiments of TlInAs were conducted byGSMBE [23.25, 26, 32] and SSMBE [23.27, 28]. Take-naka et al. [23.25] confirmed the successful growth ofTlInAs on InAs substrate at 430 ıC by double crys-tal x-ray diffraction measurement (40 arcsec shift) aswell as by reflection high-energy electron diffraction(RHEED) intensity oscillation measurement, where theoscillation period was reduced by the addition of Tlflux during the growth of InAs (1:3% reduction). Luby-shev et al. [23.28] also reported the growth of TlI-nAs with Tl composition of 0:6%. However, Antonellet al. [23.26, 32] and Lange et al. [23.27] reported thatthe incorporation of Tl into InAs was difficult. Thegrowth of high Tl composition TlInAs has not beenaccomplished. The window for the appropriate growthconditions, especially substrate temperature, was nar-row.

The growth of TlGaAs was conducted on GaAssubstrates by SSMBE by Lubyshev et al. [23.28]. Theyreported the successful growth of TlGaAs with a Tlcomposition of 0.05 (diffraction peak separation of1260 arcsec between TlGaAs and GaAs in the (400)double crystal x-ray diffraction (DCXRD)) and a sec-ond metal phase (TlCGaCAs) on the surface. Theyobserved that the growth of TlGaAs is easier than thatof TlInAs.

Growth of Tl-III-P Ternary AlloysTlInP growth was conducted on InP substrates by bothMOVPE and GSMBE. Razeghi et al. [23.29] reportedthe successful growth of .TlP3/x.InP/1�x layers at520 ıC by MOVPE and observed clear photoresponsewith onset wavelengths of 5:5 and 8:0�m in infraredphotoconductivity measurements for two samples (es-timated compositions: x D 0:6 and 0.64), respectively.Yamamoto et al. confirmed the GSMBE growth ofTlInP by the DCXRD measurement [23.21] and bythe RHEED intensity oscillation measurement [23.22].The growth temperature was about 450 ıC and (2�4) RHEED patterns and specular surface morphol-ogy with SEM were observed [23.21]. With increasingTl=(TlCIn) flux ratio, the XRD peak was shifted tolower diffraction angles, indicating the increase in Tlincorporation. On the other hand, Antonell et al. [23.26]

Temperature-Insensitive Band-Gap III-V Semiconductors: Tl-III-V and III-V-Bi 23.1 Tl-III-V Alloy Semiconductors 547Part

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reported the difficulty of the growth of TlInP similar toTlInAs. Narrow growth window for the growth of thesematerials is probably the reason for this discrepancy.

TlGaP layers closely lattice matched to GaAs weresuccessfully grown on GaAs substrates at about 450 ıCby GSMBE [23.23]. However, TlGaP on GaAs showedthe phase separation into three stable compositions:TlGaP nearly lattice matched to GaAs, GaP-like Tl-GaP and TlP-like TlGaP. However, by adjusting Tland Ga fluxes, TlGaP layers without phase separa-tion and nearly lattice matched to GaAs were grown.Photoconductance measurement on TlGaP showed theabsorption in the 1:3�m wavelength region, agreeingwith the expected band-gap energies for the TlGaPlattice matched to GaAs. Furthermore, a small tem-perature variation of band-gap energy was observed asexpected.

Growth of Tl-III-V Quaternary AlloysQuaternary TlInGaP layers were successfully grownon InP substrates by GSMBE [23.22, 33, 34]. Duringgrowth, the RHEED pattern showed (2� 4) recon-structions [23.22]. The DCXRD measurement showeda lower angle shift for the TlInGaP peak with increas-ing Tl flux [23.33, 34], agreeing with the increase of Tlcomposition. No phase separation was observed. TheTl composition obtained was less than 0.1. The in-crease of photoconductivity (PC) was observed in thelonger wavelength region compared with the PC ofInP [23.24], clearly indicating the formation of narrowband-gap semiconductors.

Quaternary TlInGaA layers were also successfullygrown on InP substrates by GSMBE [23.25, 33, 34].The growth temperature was 450 ıC. The growth con-dition for alloys containing Tl was very strict. Forexample, the growth below 400 ıC resulted in segre-gation of Tl, and the growth above 460 ıC failed toincorporate Tl due to desorption of Tl atoms fromthe growing surface. During the growth at 450 ıC, theRHEED pattern showed (2� 2) reconstructions. Thegrown samples showed mirror surface and incorpora-tion of Tl. With increasing Tl flux, the x-ray diffractionpeak shifted toward lower angles and the Tl composi-tion was increased (Fig. 23.4) [23.33]. It is consideredthat during the growth of TlInGaAs, the large part ofincident Tl atoms is re-evaporated from the surface andonly a small part of them is incorporated into films, verysimilar to the case of MBE growth of GaN under usualgrowth conditions [23.35].

Growth of TlInGaAsN Quinary AlloysThe growth of TlInGaAsN was conducted by GSMBEwith thermally cracked AsH3 and plasma-enhanced N2

0.02

0

0.04

0.06

0.08

86420TI Flux (×10–7 Torr)

TI composition

TIInGaAs

Fig. 23.4 Dependence of Tl composition in TlInGaAs onTl flux during MBE growth. (After [23.33])

as As and N sources [23.36]. TlInGaAsN/GaAs quan-tum well (QW) structures were grown on GaAs (100)substrates. High Tl flux was needed for the incorpora-tion of Tl into the films.

The TlInGaAsN/TlGaAsN QW samples havinghigher N concentration in the QWs showed higher Tlincorporation [23.37]. The addition of N species dur-ing growth induced the increase of Tl incorporation intoTlInGaAs. This is considered due to the strong bonds ofTl–N.

23.1.3 Temperature Dependenceof Physical Properties

TlInGaAs: PhotoluminescencePhotoluminescence (PL) emission was observed fromthe grown TlInGaAs/InP samples in the temperaturerange of 10�300K. With increasing Tl composition,the PL peak energy was shifted toward lower en-ergy [23.38]. As the Tl composition increased, thetemperature variation of the PL peak energy decreasedand was much smaller than that of InAs, as shown inFig. 23.5. It is noteworthy that the band-gap energy(� 0:8 eV) of TlInGaAs is larger than that (0:356 eV)of InAs. In general, the wider the band gap a materialhas, the larger is the variation of band-gap energy withtemperature.

The temperature variation of as small as�0:03meV=K was observed in Fig. 23.5, whichcorresponds to the wavelength variation of 0:04 nm=K.This value is much smaller than those of lasing wave-lengths for the InGaAsP/InP Fabry–Pérot (FP) LDs(0:4 nm=K) as well as the InGaAsP/InP-distributedfeedback (DFB) LDs (0:1 nm=K). The InGaAsP/InP

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548 Part C Materials for Electronics

0.76

0.78

0.80

0.82

0.84

0.86

0.88

0.30

0.32

0.34

0.36

0.38

0.40

0.42

0 50 100 150 200 250 300Temperature (K)

PL Peak energy (eV) Band-gap energy (eV)

TIInGaAs (TI: 4.2 %)

TIInGaAs (TI: 13 %)

TIInGaAs (TI: 6.1 %)

InAs – 0.25 meV/K

– 0.12 meV/K

– 0.08 meV/K

– 0.03 meV/K

Fig. 23.5 Temperature variation of the PL peak energy forthe TlInGaAs/InP DH samples as a function of Tl compo-sition. (After [23.38])

3.74

3.72

3.7

3.68

3.66

360340320300280Temperature (K)

Refractive index (n)

x = 0.039x = 0.016x = 0

TIInGaAs λ = 1.0 μm

Fig. 23.6 Temperature variation of refractive indices forTlInGaAs (with different Tl composition x) and InGaAsat a wavelength of 1:0�m. (After [23.39])

DFB LDs are presently used in the optical fibercommunication systems. With further increase of Tlcomposition, real temperature-insensitive band-gapenergy temperature dependence is expected.

TlInGaAs: Refractive Indexand Band-Gap Energy

A spectroscopic ellipsometer (SE) was used to ob-tain the refractive indices of TlInGaAs in the photonenergy range of 1:2�2:0 eV (above band-gap energy re-gion) [23.39]. Optical reflectance and absorption mea-surements were carried out with a double-beam spec-trometer in the wavelength range of 1700�2500 nm(below band-gap energy region) to determine the re-

3.55

3.5

3.45

0.5 0.55 0.6 0.65Photon energy (eV)

Refractive index (n)

TIInGaAs

InGaAs

340 K300 K

340 K300 K

Fig. 23.7 Refractive index dispersions at T D 300 and340K. Gray-colored circles and triangles represent ex-perimental refractive indices at 300K and brown-coloredcircles and triangles represent those at 340K for TlInGaAs(Tl D 0:043) and InGaAs, respectively. Solid and dashedlines represent the calculated results with the first-orderSellmeier equation. (After [23.40])

Table 23.1 Temperature coefficients of the direct-bandedge E0 and the refractive index [n.� ! 1/] of InGaAsand TlInGaAs. (After [23.40])

Material dE0=dT(10�4 eV=K)

n�1.dn=dT)(10�5 K�1)

Ref.

InGaAs �3:48 [23.41]�3:5 9.8 [23.40]

TlInGaAs �2:1 5.0 [23.40]

fractive indices n and the band-gap energies E0 (Eg) ofTlInGaAs, respectively [23.40].

At the energies above band gap, the refractive indexn increases with photon energy [23.39]. However, thetemperature variation of n for TlInGaAs (Tl D 0:039)greatly decreases as compared to InGaAs, as can beseen in Fig. 23.6 at the wavelength of 1:0�m.

The photon energy dependencies of n at the ener-gies below band gap for TlInGaAs (Tl D 0:043) andInGaAs at 300 and 340K together with the calculatedresults by the first-order Sellmeier equation are shownin Fig. 23.7 [23.40]. The value of n of TlInGaAs isgreater than that of InGaAs. The n value increases withincreasing temperature. However, the temperature vari-ation of n of TlInGaAs is remarkably smaller than thatof InGaAs. Therefore, the temperature variation of ndecreases with the increase of Tl composition at thephoton energy both above and below Eg.

Table 23.1 [23.40] summarizes the temperature co-efficients of the E0 edge, dE0=dT , and those of the

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refractive index, n�1.dn=dT/, for TlInGaAs (Tl D 0and 0.043), together with those from [23.41]. Ta-ble 23.1 shows that the TlInGaAs alloy, although theTl composition is limited to small value, exhibits tem-perature stability not only in the E0 edge but alsoin n.

TlInGaAsN: Photoluminescenceand Electroluminescence

TlInGaAsN/GaAs and InGaAsN/GaAs double quan-tum well (DQW) LEDs with GaAs layers as barrier lay-ers were fabricated [23.36]. Reduction in the tempera-ture variation of electroluminescence (EL) peak energywas observed in the temperature range of 288�328Kby the addition of Tl into InGaAsN/GaAs DQW lay-ers; �0:62meV=K for the InGaAsN/GaAs DQW and�0:53meV=K for the TlInGaAsN/GaAs DQW. By re-placing GaAs barrier layers with TlGaAs barrier lay-ers, further reduction was obtained; �0:35meV=K forTlInGaAsN/TlGaAs DQW LEDs [23.36]. SIMS mea-surements indicated that this improvement is caused bythe increased incorporation of Tl into the DQW layersby using TlGaAs barrier layers.

TlInGaAsN/TlGaAsN triple QWs (TQWs) withTlGaAsN layers as barrier layers were investi-gated [23.37]. The TQW samples having higher Nconcentration in the TQWs have the highest Tl incor-poration without deterioration of the crystalline quality.The temperature dependence of the PL peak energy wasfound to be the least for the highest Tl containing TQWsample.

23.1.4 Tl-III-V LD Application

TlInGaAs/InP double heterostructure (DH) LD struc-tures were grown by GSMBE on (100) InP sub-strates [23.42]. Temperature variation of the EL peakwavelength as small as 0:06 nm=Kwas confirmed. Thisis much smaller than the 0:4 nm=K observed for In-GaAsP/InP FP LDs as well as the 0:1 nm=K observedfor InGaAsP/InP DFB LDs.

Laser operation was achieved under the pulsedcondition from 77 to 310K [23.42]. The thresholdcurrent density at room temperature was approxi-mately 7 kA=cm2, and the lasing wavelength was about1660nm. The characteristic temperature T0 value forthe temperature dependence of the threshold current inthe temperature range of 77�300K was approximately94K [23.42], which is similar to that of InGaAsP/InPLDs.

Several longitudinal-mode peaks were observed inthe lasing spectra. Each longitudinal mode peak wave-length is determined by the effective refractive in-dex of the TlInGaAs/InP waveguide and the cavity

1602

1604

1606

1608

1610

1600330320310300290280

Temperature (K)

EL peak wavelength (nm)

dλ/dT = 0.01 nm/K

Be-doped InGaAs (0.2 μm)Be-doped InP cladding layer (1.0 μm)Be-doped TIInP (0.2 μm)TIInGaAs (0.2 μm)Si-doped TIInP (0.2 μm)Si-doped InP cladding layer (0.5 μm)Si-doped InP sub.

Fig. 23.8 Temperature dependence of EL peak wave-length for the TlInGaAs/TlInP/InP SCH LED. (Af-ter [23.43])

length of the LD. The temperature variation of eachlongitudinal-mode peak wavelength was as small as0:06 nm=K [23.42]. However, in the wide temperaturerange, the main peak of the lasing spectrum movedto another peak of longer wavelength with increasingtemperature. The average temperature variation of themain peak wavelength was 0:3 nm=K [23.42]. The tem-perature variation of the main peak for the FP LDs ismainly determined by the temperature variation of thegain peak. The observed average temperature variationof 0:3 nm=K is due to the temperature variation of thegain peak.

Laser light propagates along both TlInGaAs ac-tive layer and InP cladding layer; hence the temper-ature variation of lasing wavelength is influenced bytheir temperature variations. The addition of Tl intoInP cladding layer results in the smaller temperature-dependent refractive index of cladding layer. The re-fractive index for TlInP was measured with the SE andit was confirmed that the refractive index of TlInP islarger than that of InP [23.43].

The InP/TlInP/TlInGaAs/TlInP/InP separate con-finement heterostructure (SCH) LDs were fabricatedand the improved temperature dependence of EL peakwavelength was obtained as shown in Fig. 23.8 [23.43].Temperature dependence of the EL peak wavelengthwas reduced to 0:01 nm=K. The insertion of TlInP layeralso increased the incorporated Tl composition in theTlInGaAs active layer.

The lasing spectra as a function of tempera-ture and the temperature dependence of main peakwavelength for the SCH LDs under pulsed opera-tion are shown in Fig. 23.9 [23.43]. The temperature

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16601640 1645 1650 1655

1649.4

1649.8

1650.2

1650.6

1651

1649302301300299298297

Wavelength (nm)

Wavelength (nm)Light output (a.u)a) b)

Temperature (K)

302 K

301 K

300 K

299 K

298 K

297 K

dλ/dT = 0.06 nm/K

Fig. 23.9 (a) Lasing spectra as a function of temperature and (b) temperature dependence of lasing peak wavelength forthe TlInGaAs/ TlInP/InP SCH LD under pulsed operation. (After [23.43])

variation of main peak wavelength is as small as0:06 nm=K within the experimental temperature range(297�302K). This value is much smaller than thoseof InGaAsP/InP FP LDs (0:4 nm=K) and InGaAsP/InPDFB LDs (0:1 nm=K). This reduced temperature vari-ation is considered to be due to the reduced tem-perature dependence of refractive indices for TlIn-GaAs and TlInP compared with those for InGaAs andInP [23.43].

The observed temperature variation of the mainpeak wavelength for the TlInGaAs/TlInP/InP SCH

LD is larger than that of the EL peak wavelength,which corresponds to that of gain peak or that ofband-gap energy [23.43]. The main peak in the FPLD is one of the longitudinal-mode peaks, which aredetermined by the effective waveguide refractive in-dex, that is, refractive indices of TlInGaAs activelayers, TlInP cladding layers, and InP cladding lay-ers, as well as the cavity (waveguide) length. There-fore, the temperature variation of the main peak can-not be smaller than that of the longitudinal-modepeak [23.43].

23.2 III-V-Bi Alloy Semiconductors

Bismuth-containing III-V (III-V-Bi) semiconductorswere first studied for the applications to mid-infraredand far-infrared optical devices [23.44]. InSbBi [23.45–48], InAsBi [23.49, 50], and InAsSbBi [23.49] weregrown by various growth methods, such as theCzochralski bulk growth method, RF sputtering, MBE,and MOVPE [23.44]. The solid solubility of Bi inInSb was reported to be as low as 2:6% in the bulkgrowth [23.45]. Low temperature MBE growth in-creased the Bi incorporation up to 5% [23.47].

Oe et al. [23.1] suggested the unique possible prop-erties (temperature-independent band-gap energy char-acteristics) of III-V-Bi semiconductors and proposedthe application to the temperature-insensitive wave-length LDs.

23.2.1 Expected Propertiesof Bi-Based III-V Alloys

Figure 23.10 shows the expected relationships be-tween the band-gap energy and the lattice constantfor the III-V-Bi alloy semiconductors [23.44]. As al-ready described, the alloy semiconductors consistingof a semiconductor and a semimetal are expected toexhibit the temperature-insensitive band-gap energysimilar to HgCdTe. The temperature-insensitive bandgap is caused by the reverse temperature dependenceof band gap (overlap) energy between the semicon-ductor and semimetal [23.1]. InGaAsBi was first sug-gested as a candidate for the temperature-insensitiveband-gap energy semiconductor. GaAs and InAs are

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0.05.4 5.6 5.8 6.0 6.2 6.4

0.5

1.0

1.5

Lattice constant (Å)

Band-gap energy (eV)

1.3 μm

1.55 μm

InSbBiInAsBi

InSb

GaAs

GaAsN

InP

GaSb

GaAsBi

GaNAsBi

InGaAsBiInAs

Fig. 23.10 Expected relationships between the band-gapenergy and lattice constant for the III-V-Bi alloy semicon-ductors. (After [23.44])

semiconductors and GaBi and InBi are semimet-als.

To adjust the lasing wavelength of GaAsBi andrelated alloys to the waveband of optical fiber com-munication, a quaternary alloy of GaNAsBi was pro-posed which can be lattice matched to GaAs [23.44].On the other hand, InGaAsBi is lattice matched toInP, but an additional element such as Al has to beadded to InGaAsBi to adjust its band gap to the wave-band of optical fiber communication [23.44]. GaAsBiand GaNAsBi have a characteristic of large spin–orbitinteraction, which helps in reducing the Auger recom-bination in LDs [23.51].

23.2.2 Growth of III-V-Bi Alloys

Growth of III-V-Bi Ternary AlloysIn MBE growth of InSbBi, it is imperative to control theV=III ratio (Sb=In flux ratio) in the range of near unityto achieve Bi incorporation into InSb [23.44, 47]. Bi in-corporation was suppressed at Sb=In ratio greater thanunity despite the existence of Bi atoms on the growingsurface. The strength of the In–Sb bond is considered tobe stronger than that of In–Bi. Therefore, the Sb atomsare preferentially incorporated and the Bi atoms are dif-ficult to be incorporated when the Sb=In flux ratio isgreater than unity. Furthermore, the In–Bi bond is easilyreplaced by the In–Sb bond. Another important condi-tion is that the growth temperature has to be set lowerthan 400 ıC to avoid re-evaporation of Bi atoms fromthe growing surface [23.44, 47]. The InBi molar frac-tion increases by decreasing the substrate temperature(Tsub) [23.47].

After the proposal of GaAsBi alloys as a tem-perature-insensitive band-gap semiconductor [23.1],

0.0

2.0

4.0

420400380360

GaBi molar fraction (%)a)

Substrate temperature (°C)

0.0

2.0

4.0

1050

GaBi molar fraction (%)b)

Bi flux (×10–8Torr)

GaAsBi

Bi flux2.8∙10–8Torr

GaAsBi

Tsub = 380 °C

Fig. 23.11 (a) GaBi molar fraction in GaAsBi against sub-strate temperature for the constant Bi flux. (b) GaBi molarfraction in GaAsBi against Bi flux for the same substratetemperature. (After [23.52])

GaAsBi was grown on GaAs substrates by MOVPE[23.53]. To obtain the GaAsBi alloys, growth at rel-atively low temperatures was needed because Bi hashigh vapor pressure. However, in the MOVPE growth,the insufficient decomposition of MOs at low tempera-tures induced the incorporation of carbon (C) impurityspecies into GaAsBi films. On the other hand, in theMBE growth, elemental Ga, As, and Bi are used assources [23.52, 54], and the incorporation of C impu-rity species is expected to be avoided.

GaAsBi was grown on GaAs (001) substrates atTsub less than 400 ıC by MBE [23.52, 54]. During thegrowth of GaAsBi, the RHEED pattern revealed the sur-face reconstruction of (2� 1), suggesting the existenceof Bi–Bi or Bi–As dimers [23.44, 55]. The unusualmetallic nature of Bi-induced (2� 1) was proposed asthe origin of the surfactant nature of Bi [23.44, 56].RHEED intensity oscillations observed during growthsuggest a layer-by-layer growth [23.57].

For the growth of GaAsBi, the GaBi molar frac-tion decreased with increasing Tsub under constant Gaand As fluxes as shown in Fig. 23.11a [23.52]. AtTsub higher than 400 ıC, Bi atoms are not incorpo-rated into GaAs. Therefore, the GaAsBi alloys haveto be grown at low temperature. However, the GaBimolar fraction increases with Bi flux, followed by thesaturation for the constant Ga and As fluxes at theconstant Tsub as shown in Fig. 23.11b. The unincorpo-rated Bi atoms are desorbed from the surface withoutthe formation of Bi droplets below critical Bi sup-ply, although excess Bi supply forms Bi-containingdroplets on the growth surface [23.52]. The droplet

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0 50 100 150 200 250 300 350

850 900 950 1000

1.35

1.40

1.45

1.50

1.55

1.25

1.30

1.35

1.40

1.45

Wavelength (nm)

Intensity (arb.units)a) b)

Temperature (K)

PL peak energy (eV) Band-gap energy (eV)

GaAs

GaAs0.976Bi0.024

300 K

240 K

200 K

160 K

120 K

80 K

60 K

40 K

10 K

× 2

× 2

× 2

× 2

× 4

GaAsBi

Fig. 23.12 (a) PL spectra as a function of temperature and (b) temperature variation of PL peak energy for the GaAsBi(GaBi D 0:024). For comparison, the band gap of GaAs is also shown. (After [23.53])

formation causes severe surface roughness and inhomo-geneity.

The range of the As flux is also very narrow to ob-tain GaAsBi films [23.52]. For the growth at Tsub D380 ıC with Ga flux D 3�10�7 Torr and Bi flux D 2�10�8 Torr, only GaAs without Bi incorporation wasgrown above As flux of 10�5 Torr, while below 10�6

Torr the surface of the film becomes rough and onlyheavily degraded film was grown. Therefore, to growthe device quality GaAsBi, careful adjustment of thesource supply and Tsub is needed.

Growth of III-V-Bi Quaternary AlloysGaNAsBi quaternary alloys can be lattice matched toGaAs substrate with the band gap of optical fiber com-munication wavelengths. They were grown on GaAs(100) substrate by MBE using solid sources of Ga,As, and Bi and RF-plasma-enhanced N2 gas at Tsub of350�400 ıC [23.58]. Precise control of As flux withina limited range and low temperature growth was crucialas in the case of GaAsBi. During growth, the (2� 1)RHEED pattern was observed also similar to GaAsBi,implying Bi-stabilized reconstruction. The GaBi molarfraction decreased with the increase of Tsub for the con-stant Ga, As, and Bi fluxes, which is similar to the caseof GaAsBi growth. Lattice matching between GaNAsBifilm and GaAs substrate was achieved by adjustingGaNmolar fraction [23.59].

The InGaAsBi quaternary alloy is another extensionof GaAsBi and can be lattice matched to InP [23.60].

Considering the band gaps of InAs (0:36 eV) andGaAs (1:42 eV), the band gap of InGaAsBi is ex-pected to cover the whole wavelength range usedin the optical fiber communication systems, and In-GaAsBi is a promising semiconductor to fabricate thetemperature-insensitive wavelength LDs [23.60].

InGaAsBi was grown on InP (100) substrate byMBE using solid sources of In, Ga, As, and Bi [23.60].Tsub was ranging from 270 to 350 ıC, which is lower by50 ıC than that for GaAsBi growth, corresponding tothe lower desorption temperature of Bi atoms from theInGaAs surface than that from the GaAs surface. Dur-ing the growth of InGaAsBi, the (1�3) streaky RHEEDpattern was observed. The XRD pattern showed a well-defined diffraction peak with distinct Pendellösungfringes, indicating the homogeneous alloy compositionand high epitaxial quality with smooth surface. RBSchanneling angular scans showed that Bi atoms are lo-cated exactly on the substitutional sites in InGaAs.

23.2.3 Optical Properties of III-V-Bi Alloys

Although the GaAsBi films were grown at low tem-peratures below 400 ıC by both MOVPE [23.53] andMBE [23.58, 61, 62], intense PL emission was ob-served at room temperature. This is considered tobe due to the surfactant effect of Bi atoms duringgrowth. Figure 23.12 shows the PL spectra as a func-tion of temperature and the temperature variation ofthe PL peak energy for the MOVPE-grown GaAsBi

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1.151.101.051.000.950.90Photon energy (eV)

Intensity (arb. units)

GaNAsBi Annealed

As-grown

Fig. 23.13 PL spectra of as-grown and annealed GaNAsBi(GaBi D 0:02). (After [23.58])

with GaBi molar fraction of 0.024 [23.53]. The tem-perature variation of the PL peak energy is as smallas 0:1meV=K, which is much smaller than that ofGaAs as can be seen in Fig. 23.12b. The results suggestthat the band gap of GaAsBi is less sensitive to tem-perature than the usual III-V semiconductors [23.53].The PL peak energy decreased with the increase ofGaBi molar fraction [23.58]. Reduction in the tem-perature variation of PL peak energy was also ob-served for the MBE-grown GaAsBi samples [23.61,62]. The temperature coefficient of PL peak energywas 0:23meV=K [23.61] and 0:15meV=K [23.62] forthe GaBi molar fraction of 0.013 and 0.025, respec-tively. The temperature coefficients of GaAsBi bandgaps obtained by photoreflectance spectroscopy were0.24, 0.23, and 0:15meV=K between 150 and 300Kfor samples with GaBi D 0:005, 0.013, and 0.026, re-spectively [23.63]; that is, with the increase of GaBimolar fraction the temperature coefficient of band gapdecreased.

To obtain intense PL emission for the grownGaNAsBi, post-growth thermal annealing was needed,similar to GaInNAs. PL intensity of the GaNAsBi(GaBi D 0:02) film was increased by approxi-mately fivefold after thermal annealing as shownin Fig. 23.13 [23.58]. PL peak energy of GaNAsBishowed no notable variation even after thermal an-nealing. The PL peak energy was decreased with theincrease of GaBi molar fraction [23.58]. The temper-ature coefficient of the PL peak energy for GaNAsBiwas very close to that for GaAsBi with the same GaBimolar fraction. Figure 23.14 shows the temperature

0.95

1.00

1.05

0.90

0.85

320240160Temperature (K)

Photon energy (eV)

2.0 % GaBi

3.7 % GaBi

4.7 % GaBi

In0.68Ga0.32As0.69P0.31

Fig. 23.14 Temperature variation of PL peak energy forGaNAsBi (GaBi D 0:02, 0.037, 0.047) and InGaAsP. (Af-ter [23.58])

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

26242220181614Temperature (°C)

Oscillation wavelength shift (nm)

0.37 nm/K

0.17 nm/K

GaAsGaAsBi: 3 %

Fig. 23.15 Lasing wavelength shift for the GaAsBi andGaAs FP-LDs from the lasing wavelength at 15 ıC in thetemperature range of 15�25 ıC. (After [23.64])

variation of PL peak energy for three samples [23.58].The temperature variation of PL peak energy forGaNAsBi is smaller than that of InGaAsP with similarband-gap energy (near 1300nm). It can be said thatthe temperature coefficients of GaAsBi and GaNAsBiare governed by GaBi molar fraction and not by GaNmolar fraction [23.58].

23.2.4 III-V-Bi LD Application

Photo-pumped lasing oscillation at 983 nm wasachieved in the MBE-grown GaAs/GaAsBi/GaAs

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(GaBi D 0:025) DH structure sample [23.62]. Thetemperature coefficient of the lasing wavelength de-creased to 40% of that of the band gap of GaAs inthe temperature range of 150�240K. By using theGaAs/GaAsBi/AlGaAs DH structure sample to im-prove the carrier confinement, the photo-pumped lasingoscillation at the longer wavelength of up to 1200 nmwas achieved [23.65]. The temperature coefficient ofthe lasing wavelength was about 40% to that of the1300 nm InGaAsP LDs in the temperature range of20�80 ıC.

Electrically pumped lasing oscillation at 947 nmwas demonstrated in the GaAsBi (GaBi D 0:022) FP-LD grown by MOVPE [23.66], though the temperature

coefficient of the lasing wavelength is unclear. GaAsBi(GaBi D 0:03 or 0.04)/AlGaAs SCH FP-LD was grownby MBE [23.64]. Lasing oscillation was observed inthe temperature range of 15�40 ıC. The characteristictemperature T0 for the GaAsBi (GaBi D 0:03)/AlGaAsSCH FP-LDwas calculated to be 125K, which is higherthan that of 1300 nm InGaAsP FP-LD (T0 D 60K) andis similar to that for the 980 nm InGaAs/GaAs LDs.High T0 is because of the stronger carrier confine-ment of the GaAsBi/AlGaAs system than that of theInGaAsP/InP system [23.64]. The temperature coeffi-cient of lasing wavelength was reduced to 0:17 nm=K,which is 45% of that (0:37 nm=K) for the GaAs FP-LDas shown in Fig. 23.15 [23.64].

23.3 SummaryTl-III-V and III-V-Bi alloy semiconductors are im-portant materials for the fabrication of temperature-insensitive lasing wavelength LDs as well as LWIRoptical devices. This chapter described the expectedinteresting properties of these semiconductors and the

present status of their growth, characterization, anddevice applications. It was shown that these alloysemiconductors have a potential to have temperature-insensitive band gap and to fabricate temperature-insensitive lasing wavelength LDs.

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