Semicond. Sci. Technol. 11 (1996) 1185–1188. Printed in the UK

Electroluminescence of GaInSb/GaSb strained single quantum well structures grown by molecular beam epitaxy A N Baranov, Y Cuminal, G Boissier, J C Nicolas, J L Lazzari†, C Alibert and A Joullie´ ´ Centre d’Electronique et de Microoptoelectronique de Montpellier (CEM2), Unite´ Mixte de Recherche CNRS No 5507, Universite´ de Montpellier II, Sciences et Techniques du Languedoc, 34095 Montpellier Cedex 05, France Received 25 March 1996, accepted for publication 16 May 1996 Abstract. GaSb-based heterostructures with strained-layer GaSb/Ga1−x Inx Sb(x = 0.26 or x = 0.35) single quantum wells and Al0.5 Ga0.5 Sb cladding layers were grown by molecular beam epitaxy. The thickness of the GaInSb well was varied from 3.0 to 14.5 nm. Intense room-temperature electroluminescence was observed from mesa diodes with peak emission wavelengths in the 1.9–2.2 µm spectral range. The dependence of emission energy on well thickness is consistent with the predictions obtained by an effective mass treatment assuming a rectangular type I quantum well and a conduction band offset 1Ec = 0.61Eg .

1. Introduction The GaInSb/GaSb strained system has potential applications in the 1.7–3 µm spectral region, for example in trace gas sensing, atmospheric pollution monitoring, absorption spectroscopy or long-haul telecommunications. Despite the expected improvement due to strain-induced modifications of the band structure, this system remains poorly explored compared with the GaInAsSb/GaSb lattice-matched one [1]. Several groups have reported the fabrication of devices which are mainly based on ‘bulk’ GaInSb layers. Ga0.6 In0.4 Sb photodiodes have been grown by metal organic vapour phase epitaxy (MOVPE) for detection at 2.5 µm [2]. Thick layers of Ga0.77 In0.23 Sb were also grown by molecular beam epitaxy (MBE) for the fabrication of planar transferred electron devices [3]. To our knowledge, light-emitting diodes (LEDs) grown by MOVPE are the only devices using compressively strained GaInSb/GaSb quantum wells (QWs) [4]. Ga1−x Inx Sb/GaSb QWs have been elaborated by MOVPE [4–8]. Their low-temperature photoluminescence has been studied [6, 7]. The interband magneto-optical properties of single and coupled wells have also been investigated [8]. However, photoluminescence at room temperature has not yet been reported [6–8]. † Present address: Centre de Recherche sur les M´ecanismes de la Croissance Cristalline (CRMC2), UPR CNRS 7261, Universit´e AixMarseille I, case 913, Campus de Luminy, 13 288 Marseille Cedex 09, France. c 1996 IOP Publishing Ltd 0268-1242/96/081185+04$19.50

In the present work, we report the electroluminescence (EL) of mesa diodes based on strained Ga1−x Inx Sb/GaSb single quantum wells grown for the first time by molecular beam epitaxy. The dependence of EL emission on well thickness, injection current and temperature was studied. 2. Experiment The investigated structures were prepared in a Varian Gen II MBE machine on (100)-oriented Te-doped (n = 5 × 1017 cm−3 ) GaSb substrates. The system was equipped with effusion cells containing elemental Ga, In, Al and Sb solid sources. Sb2 Te3 and Ge were used for n- and p-type doping respectively. The deposition temperature was 500 ◦ C, monitored both by an optical pyrometer and a thermocouple. The growth was in situ controlled by a reflection high-energy electron diffraction (RHEED) gun operating at 10 keV under grazing incidence. The growth rate and composition of the layers were determined by observing four to five first RHEED intensity oscillations recorded during the growth of the structure. They were verified by thickness and composition measurements realized on especially grown thick GaSb, GaInSb and AlGaSb layers. The GaSb substrates were first cleaned in isopropanol, etched at room temperature in HCl for 2 min, immediately rinsed in isopropanol, and dried with N2 . They were then soldered with In on a molybdenum mounting block. Prior 1185

A N Baranov et al

Figure 1. Schematic structure of the GaInSb/GaSb single QW based electroluminescent diode.

to crystal growth, the substrate temperature was raised to 520 ◦ C under a 1×10−6 Torr Sb4 flux for 10 min in order to desorb surface oxides. After this treatment a well defined (1 × 3) Sb-stabilized surface reconstruction was usually observed during subsequent GaSb deposition. A schematic diagram of the elaborated structures is given in figure 1. The strained QW structure consists of a GaInSb well embedded in two undoped 150 nm thick pGaSb layers. This active region is in turn enclosed between Te- and Ge-doped Al0.5 Ga0.5 Sb barrier layers used to obtain electron confinement of the injected carriers. The thickness of these barriers was kept below 50 nm to avoid the formation of misfit dislocations. The structure is terminated by a 1 µm thick Ge-doped GaSb contact layer. Doping levels are n ≈ (1–2) × 1018 cm−3 for the buffer layer and the first barrier, p ≈ 2 × 1017 cm−3 for the second barrier and p ≈ 1018 cm−3 for the top layer. They were estimated by Hall measurements on thick layers grown under the same conditions. From these structures we prepared 300 µm diameter mesa diodes using conventional photolithography. Electroluminescence measurements were performed at 80 K, 296 K and as a function of temperature within the 80–420 K range. Pulsed 50–400 mA injection currents with a 50% duty cycle and 3 ms pulse duration were used. The EL signal was detected through the 0.5 m path-length Jobin-Yvon model HRS2 spectrometer equipped with a lead sulphide photoconductive detector. 3. Results and discussion The nominal well widths and indium contents of the prepared samples are presented in table 1. The emission wavelengths corresponding to the maximum of the EL peak at 80 K and 296 K for low injection current are listed. The observed peak wavelengths vary from 1.68 µm to 1.96 µm at 80 K and from 1.90 µm to 2.17 µm at room temperature. Typical emission spectra recorded at room temperature are presented in figure 2. A bulk GaSb recombination band 1186

Table 1. Nominal characteristics and EL peak emission wavelengths (for a 50 mA forward current) of the investigated Ga1−x Inx Sb/GaSb QW structures. EL peak wavelength (µm) Sample number

x

Well thickness ˚ (A)

80 K

296 K

1 2 3

0.26 0.26 0.26

70 100 145

1.78 1.84 1.88

1.99 2.04 2.07

4 5 6 7

0.35 0.35 0.35 0.35

30 40 60 85

1.68 1.80 1.91 1.96

1.90 2.02 2.10 2.17

is observed at 1.73 µm but it remains weak for all samples. The EL peaks are narrow with a full width at half maximum (FWHM) of only 30–40 meV indicating good crystalline quality of the structures. The luminescence intensity is high and comparable with that of the best 2 µm latticematched GaInAsSb/GaSb LEDs prepared by liquid phase epitaxy [9]. No significant reduction of the luminescence efficiency was observed with increasing well width. This finding, and the narrow FWHM of the emission spectra, make us think that strain relaxation did not occur in the investigated structures. The EL spectra were found to be dependent on the forward current I across the diode. As an example the EL band of sample 5 at 80 K is centred at 1.80 µm for an injection current I = 50 mA. It shifts towards shorter wavelengths with increasing current: 1.77 µm for I = 100 mA and 1.75 µm for I = 400 mA. Simultaneously the band becomes broader, mainly on its high-energy side. At high forward current another band appears at 1.55 µm, corresponding to bulk recombination in the GaSb barriers. These findings are consistent with a filling of quantum levels by both electrons and holes and with a related expansion of the carrier wavefunctions into

Electroluminescence of GaInSb/GaSb MBE SQW Table 2. Material parameters used for the calculations of the fundamental electron–heavy-hole transitions at room temperature. Parameter

GaSb

InSb

Ga1−x Inx Sb

˚ Lattice constant (A)

6.096 [14]

6.479 [14]

6.096 + 0.383x

Elastic stiffness constant: c11 (×1012 dyn cm−2 ) c12 (×1012 dyn cm−2 )

0.88 [14] 0.40 [14]

0.66 [14] 0.36 [14]

0.88 − 0.22x 0.40 − 0.04x

0.79 [12] −6.85 [12] −2.0 [14]

0.36 [12] −6.17 [12] −2.1 [14]

0.79 − 0.43x −6.85 + 0.68x −2.0 − 0.1x

Hydrostatic deformation potential: for the valence band av (eV) for the conduction band ac (eV) Axial deformation potential b (eV) Direct bandgap energy Eg (eV) Spin–orbit splitting 10 (eV)

0.725 [16] 0.761 [16]

0.172 [15] 0.810 [14]

0.725 − 0.138x − 0.415x 2 [15] 0.761 + 0.049x

Kohn–Luttinger parameter: γ1 γ2 Electron effective mass me∗ /m0

13.3 [12, 14] 4.4 [12, 14] 0.042 [15]

36.3 [12, 14] 16.1 [12, 14] 0.0145 [15]

13.3 + 23x 4.4 + 11.7x 0.042 − 0.0445x + 0.017x 2 [15]

Figure 2. Room-temperature EL spectra of samples 3, 5 and 6 for a 50 mA forward current.

the GaSb barriers. Arrhenius representations of the integral intensity obtained for samples 5 and 6 are given in figure 3. Above 250 K a rapid thermal quenching of the EL intensity is observed. In the 250–420 K range the EL intensity was found to be proportional to exp(1E/kT ) with activation energies 1E = 115 meV for sample 5 and 1E = 132 meV for sample 6. These two values are in good agreement with the 111 meV and 135 meV confinement energies of the carriers in the well, given by the difference between the GaSb barrier bandgap and the EL emission peak energy. This result can be explained by the thermionic emission of carriers from the QW ground state [10, 11]. Between 120 K and about 250 K the EL intensity gradually decreases with a low activation energy, indicating the participation of a nonradiative process. We have noted that thermal quenching in this temperature range differed from diode to diode of the same wafer and we think this non-radiative channel can be

Figure 3. Arrhenius plot of the EL intensity of samples 5 and 6.

attributed to surface recombination. The dependence of the EL peak wavelength on the quantum well thickness was calculated by a classical effective mass treatment under the assumption of a rectangular and symmetric QW. In this approximation the well width L is expressed as a function of the fundamental level E0 according to [12] L = K(mw E0 )−1/2 arctan



mw (Q1Eg − E0 ) mb E0

1/2 (1)

√ where K = h ¯ 2, mw and mb are the effective masses in the well and the barrier respectively, and 1Eg is the difference between the (stressed) heavy-hole bandgaps (1Eg = Eg (GaSb) − Eghh (GaInSb)). Q defines the fractional conduction and valence band offsets 1Ec and 1Ev at the interface as follows: 1Ec = Q1Eg and 1Ev = (1 − Q)1Eg . The compressively stressed heavyhole bandgap Eghh was determined using the Van de Walle 1187

A N Baranov et al

at 18 K of Ga0.82 In0.18 Sb/GaSb single quantum wells with well thicknesses varying from 5.9 nm to 15.4 nm. Agreement between the predictions of their square well model and experimental data was obtained using a valence band offset 1Ev = 0.037 eV, i.e. a Q factor of 0.65. 4. Conclusion

Figure 4. Room-temperature dependence of the EL peak emission wavelength on well thickness. The full curves give the evolution of the fundamental e1 –hh1 transition wavelength calculated for 1Ec = 0.61Eg .

model [13]. The out-of-plane [001] electron effective mass me and [001] heavy-hole effective mass mhh in pseudomorphic GaInSb layers were calculated using the People and Sputz analytic expressions [14]. The material parameters involved were taken from [13, 15–17] and are listed in table 2. The calculated bandgap energies and out-of-plane effective masses of strained Ga1−x Inx Sb on (100) GaSb are fitted by the following equations: Eghh (300 K) = 0.725(1 − x) + 0.261x − 0.22x(1 − x) (2) [001]mhh = 0.222 + 0.0217x

(3)

[001]me = 0.042 + 0.0058x + 0.0187x − 0.0228x . (4) 2

4

One can note that the electron effective mass along the [001] direction remains close to the GaSb value for the whole range of compositions. The determination of the fundamental transition E(e1 –hh1 ) was performed by solving expression (1) simultaneously for both electrons and holes, the Q value being entered as a fitting parameter. As shown in figure 4, a good agreement between the theoretical curves and the experimental data was obtained with Q = 0.6 for both compositions, which implies 1Ev = 0.065 eV and 1Ev = 0.085 eV for Ga0.74 In0.26 Sb/GaSb and Ga0.65 In0.35 Sb/GaSb interfaces respectively. A comparable calculation was made by Qian and Wessels [7]. These authors studied the photoluminescence

1188

Light-emitting diodes with a Ga1−x Inx Sb/GaSb single quantum well (x = 0.26 or x = 0.35) and Al0.5 Ga0.5 Sb confining layers were grown successfully by MBE. Bright and narrow room-temperature electroluminescence with peak emission wavelength in the 1.9–2.2 µm range was obtained. The dependence of peak emission wavelength on well thickness was calculated using an effective mass treatment and assuming a rectangular and symmetric type I quantum well. A good agreement with experiment was obtained for a conduction band offset ratio Q = 0.6 for both compositions, which furnishes interface valence band offsets 1Ev = 0.065 eV and 1Ev = 0.085 eV for x = 0.26 and x = 0.35 respectively. References [1] Mikhailova M P and Titkov A N 1994 Semicond. Sci. Technol. 9 1279–95 [2] Pascal-Delannoy F, Bougnot J, Allogho G G, Giani A, Gouskov L and Bougnot G 1992 Electron. Lett. 28 531–2 [3] Kodama M 1993 Phys. Status Solidi a 140 481–90 [4] Krier A, Bissitt S A, Mason N J, Nicholas R J, Salesse A and Walker P J 1994 Semicond. Sci. Technol. 9 87–90 [5] Haywood S K, Chidley E T R, Mallard R E, Mason N J, Nicholas R J, Walker P J and Warburton R J 1989 Appl. Phys. Lett. 54 922–4 [6] Su Y K, Juang F S and Su C H 1992 J. Appl. Phys. 71 1368–72 [7] Qian L Q and Wessels B W 1993 J. Vac. Sci. Technol. B 11 1652–5 [8] Wong S L, Warburton R J, Nicholas R J, Mason N J and Walker P J 1993 Physica B 184 106–10 [9] Andaspaeva A, Baranov A N, Gusseinov A, Imenkov A N, Litvak L M, Filaterova G M and Yakovlev Yu P 1988 Sov. Tech. Phys. Lett. 14 377–8 [10] Devine R L S 1988 Semicond. Sci. Technol. 3 1171–6 [11] Storch D R, Schneider R P and Wessels B W 1992 J. Appl. Phys. 72 3041–5 [12] Fouillant C and Alibert C 1994 Am. J. Phys. 62 564–5 [13] Van de Walle C G 1989 Phys. Rev. B 39 1871–83 [14] People R and Sputz S K 1990 Phys. Rev. B 41 8431–9 [15] Auvergne D, Camassel J, Mathieu H and Joulli´e A 1974 J. Phys. Chem. Solids 35 133–40 [16] Madelung O (ed) 1982 Landolt-Borstein, Numerical Data and Functional Relationships in Science and Technology group III vol 17a (New York: Springer) [17] Alibert C, Joulli´e A, Joulli´e A M and Ance C 1983 Phys. Rev. B 27 4946–54