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October 31, 2006 12:35 WSPC/145-JNOPM

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Journal of Nonlinear Optical Physics & Materials Vol. 15, No. 3 (2006) 323–329 c World Scientific Publishing Company

GaSb/AlGaSb COMPOUND SEMICONDUCTORS GROWN BY MOCVD FOR OPTOELECTRONIC APPLICATIONS

A. H. RAMELAN∗ and I. YAHYA Jurusan Fisika FMIPA, Universitas Sebelas Maret (UNS), Jl. Ir. Sutami No. 36 A Surakarta 57126, Indonesia ∗[email protected] PRASODJO Fakultas Kedokteran, Universitas Sebelas Maret (UNS), Jl. Ir. Sutami No. 36 A Surakarta 57126, Indonesia E. M. GOLDYS Division of Information and Communication Sciences, Macquarie University, Sydney 2109 NSW, Australia Received 30 April 2006 Alx Ga1−x Sb films in the regime 0 ≤ x ≤ 0.30 have been grown by metalorganic chemical vapor deposition on GaAs and GaSb substrates using TMAl, TMGa and TMSb precursors. We report the effects of growth conditions on the optical properties. Samples grown at temperatures of 540◦ C, 580◦ C and 600◦ C and a V/III ratio of 1 have been investigated. The Alx Ga1−x Sb layers grown at 580◦ C and 600◦ C with a V/III ratio of 1 and Al content in the range of 0.5% to 25% were found to exhibit excellent optical quality with a very high optical transmission at energies below the bandgap. The principle photoluminescence features observed are attributed to bound exciton and donor-acceptor transitions with FWHM comparable to the best values reported elsewhere. Keywords: Photoluminescence; bound exciton; donor-acceptor transitions; III/V compound semiconductors; MOCVD.

1. Introduction Currently, antimonide based compound semiconductors lattice matched to GaSb substrates have generated considerable interest. The energy band gaps not only cover a wide range of energy, from 1.58 eV (AlGaSb) to 0.3 eV (InGaAsSb), but are also nearly equal to spin orbit splitting of the valence band.1,2 GaSb is a basic binary compound of the AlGaSb, InGaSb, and InGaAsSb compound semiconductors. GaSb and InGaSb are suitable materials for microwave application due to their band structures in the particular range.3,4 The narrow energy gap of InGaAsSb is also useful for long wavelength photodevices.5,6 With regard to AlGaSb, it is a promising material for an avalanche photodiode (APD) due to its high value of 323


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the β/α ratio, where β and α are the hole and electron ionization coefficients,7,8 respectively. AlGaSb has proven to be a much more difficult material to grow by metalorganic chemical vapor deposition (MOCVD) than GaSb. The reason is the reactivity of Al, which forms strong bonds with carbon.5–7 It is not surprising then that frequently the MOCVD-grown AlGaSb and AlSb have been reported to exhibit poor electronic and optical properties. Chidley et al.8 found that both the crystallinity and electrical quality of MOCVD-grown Alx Ga1−x Sb were limited by carbon contamination from the TMAl material. Despite some reports on the growth of GaSb and AlGaSb,4–9 little data is available on the variations of optical properties with growth conditions of Alx Ga1−x Sb grown by MOCVD. With that in mind we carried out atmospheric pressure MOCVD growth of GaSb and Alx Ga1−x Sb and characterized the optical properties. In this report, we first describe briefly the growth conditions used for Alx Ga1−x Sb. The effect of growth temperature on optical properties are then reported. 2. Experimental Procedure 2.1. Growth conditions The GaSb and Alx Ga1−x Sb epilayers with V/III ratio of 1 were grown on GaSb (100) and SI-GaAs (100) substrates in a Thomas Swan atmospheric pressure horizontal MOCVD reactor. High purity TMAl, TMGa and TMSb from Morton International were used as precursors. These were held at 18◦ C, 9◦ C and 0◦ C, corresponding to molar flows of 0.45, 2.53 and 1.82 µmol/min, respectively, for 1 sccm hydrogen flow through the metalorganics. The GaSb and GaAs substrates were prepared by degreasing in trichloroethylene (TCE), rinsing in acetone and methanol, then chemically etched using HNO3 : HCl : CH3 COOH = 1 : 10 : 50 and H2 SO4 : H2 O2 : H2 O = 1 : 1 : 8 for GaSb and GaAs, respectively. The substrates were further rinsed in DI water and rapidly dried under a nitrogen jet and loaded into the reactor. 2.2. Characterization Low-temperature measurements of photoluminescence are necessary to obtain the fullest spectroscopic information by minimizing thermally-activated non-radiative recombination processes and thermal line broadening. The thermal distribution of carriers excited into a band contributes a line width of approximately kT /2 to the emission originating from that band. This makes it necessary to cool the sample to reduce the spectral width. The thermal energy kT /2 is only 0.85 meV at T = 20 K. For many measurements this is sufficiently low, but occasionally it is necessary to reduce this broadening further by reducing the sample temperature below 20 K. The sample was placed in a cryostat (Model 22C/350C), to enable PL measurements in the temperature range of 8.5–300 K. A diode laser with a wavelength of


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980 nm was used for excitation. The laser power could be varied from 0 to 10 mW. The optics in a PL apparatus was designed to ensure maximum light collection. To improve sensitivity, the excitation source was modulated by a chopper for synchronized detection. The PL emission from the sample was analyzed by a SPEX-270M Monochromator with a 1200 grooves/mm grating and detected by an InGaAs detector. A longpass filter with cutoff wavelength at 1 µm was used to block the scattered laser light and to pass the desired radiation. The detector output signal was fed to a lock-in amplifier and synchronized with the chopped laser signal for an enhanced signal-to-noise ratio. 3. Results and Discussion The low temperature PL spectra of undoped GaSb exhibit about 20 transitions in the energy range of 680–810 meV as listed in Table 1.10 Very few of these transitions have been associated with specific impurities or defects, although the residual acceptor, which is found in almost all GaSb epilayers, has been identified by Allegre and Averous11 as the structural defect complex VGa GaSb . PL measurements with varying temperature and excitation power were conducted on GaSb based samples grown at 540, 580 and 600◦C respectively and with a V/III ratio of 1. Figure 1 shows the PL spectra of GaSb grown at 540◦ C with V/III = 1. There are two peaks, i.e. a relatively sharp feature at 764 ± 5 meV and a poorly resolved peak at 791 ± 5 meV. As shown in Fig. 1 the peak position shifts to lower energy with increasing temperature. This may be related to the bandgap shrinkage with the increased temperature. In addition, the intensities of the high and low peaks decrease at different rates. This is still unexplained so far. The identification of luminescence emission remains a difficult task, although a careful comparison of the observed peak energies with those reported helps. Therefore Table 1 can be used for PL identification. Table 1 contains the reported peaks

Table 1. Energy (meV) 810 808.2 805.4 802.9 800.1 796.0 777.8 776 770 775 758 748.5 743.3

PL energies of undoped p-GaSb layer. Transition

Free exciton Excitons bound to donors Excitons bound to neutral Excitons bound to neutral Excitons bound to neutral Excitons bound to neutral Residual acceptor Residual acceptor Residual acceptor Bound exciton to acceptor LO phonon replica of BE4 LO phonon replica of A Unidentified

acceptor acceptor acceptor acceptor

Notation

Reference

FE D BE1 BE2 BE3 BE4 A A A BA BE4 -LO A-LO UI

10 10 10 10 10 10 10 12 11 10 12 10 10



A (764 meV)

25 K

Intensity (a.u.)

40 K

50 K

Integrated intensity (a.u.)

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BE4 10 (791 meV)

A peak

20

30

40

-1

1000/T (K )

77 K

100 K

0.70

0.75

0.80

0.85

0.90

Photon Energy (eV) Fig. 1.

PL spectra of GaSb grown at 540◦ C with a ratio V/III of 1.

energies for the two main commonly-observed transitions in GaSb, namely the transition BE4 at 796 ± 5 meV, attributed to an acceptor-bound exciton transition11 and the transition denoted as A at 770 ± 5 meV,10 related to a transition from the conduction band to the native acceptor. The two energies in GaSb grown at 540◦C with V/III ratio of 1 correlate well with these A and BE4 transitions. The PL intensity generally decreases as the temperature increases and a plot of PL intensity as a function of temperature is shown in the inset of Fig. 1. The PL quenching at elevated temperature is associated with the escape of carriers to a nonradiative recombination path. The thermal activation energy Ea for electronhole pair emission can be calculated by the Arrhenius relationship as given by the following equation: Ea1 Ea2 Io = 1 + C1 exp + C2 exp (1) I(T ) kT kT where k is Boltzmann’s constant, T is the absolute temperature, Ea1 and Ea2 are activation energies, and Io , C1 and C2 are constants. Using Eq. (1), the activation energies for the quenching are about 4.5 and 7.3 meV. The PL spectra of undoped GaSb grown by MOCVD at 580◦ C with a V/III ratio of 1 are shown in Fig. 2. At the lowest measurement temperature of 30 K three bands are visible, i.e. two relatively sharp features at 788 ± 5 meV and 772 ± 5 meV respectively, and in between there is a poorly resolved peak at an energy of 777 ± 5 meV. Again the assignment of these PL peaks is very difficult. However based on the GaSb transitions (Table 1), we tentatively associate the peak at 777 ± 5 meV with the


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Intensity (a.u.)

BE2

30 K

PL intensity (a.u)


BA

327

BE2 Peak

0 BE1

10

20

30

40

50

1000/T (K-1)

50 K

100 K

0.70

0.75

0.80

0.85

Photon Energy (eV) Fig. 2.

PL spectra of undoped GaSb sample grown at 580◦ C with a V/III ratio of 1.

bound exciton to acceptor transition (BA) involving native lattice defects formed by gallium vacancy-gallium antisite complexes (VGa GaSb ) in GaSb, as previously reported in Ref. 10. The strong emission band arising from two partially resolved excitonic emission lines at energies of 788 ± 5 meV and 772 ± 5 meV are assigned as BE1 and BE2 , respectively. An increase in the temperature gives rise to a rapid decrease of BE2 -line intensity but the BE1 -band remains relatively constant. The position of BE1 and BE2 shifted slightly with varying temperature. The PL intensity of BE2 -line as a function of temperature is shown in the inset of Fig. 2. The PL quenching at elevated temperature is again supposed to be associated with the escape of carriers to a nonradiative recombination path. The thermal activation energy Ea for electron-hole pair emission was found to be 7 meV. The BA-line disappears above 50 K. This behavior is typical for the thermal decay of a bound exciton and the emission is assigned to transition of bound excitons to neutral acceptors, as reported previously.13 Figure 3 shows the PL spectra of undoped GaSb grown at temperature of 600◦ C with a V/III ratio of 1. The spectra shows only 1 peak attributed to conduction band-native acceptor transition (A) at 768 ± 5 meV, similar to what was reported by Wu et al.13 Similar to the PL spectra of GaSb grown at 540◦ C, the peak position shifts to lower energy with rising temperature. The PL intensity decreases as the temperature increases and the plot of PL intensity as a function of temperature is shown in the inset of Fig. 3. The activation energy for the quenching of PL lines is about 9 meV.



768 meV

Intensity (a.u.)

25 K

PL Intensity (a.u.)

328

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A peak

0

10

20

30

40

1000/T (K-1)

100 K

0.65

0.70

0.75

0.80

0.85

0.90

Photon energy (eV) Fig. 3.

PL spectra of GaSb grown at 600◦ C with a V/III ratio of 1.

4. Conclusions The photoluminescence spectra of the GaSb epilayers have been observed and interpreted. The PL emissions of the GaSb layer grown at 540◦ C with a V/III ratio of 1 have been tentatively attributed to acceptor-bound exciton transition (BE) at a energy of 791 ± 5 meV and conduction band-native acceptor transition (A) at energy of 764 ± 5 meV. The PL spectra of the GaSb grown by MOCVD at 580◦C with V/III ratio of 1 shows two relatively sharp features at 788 ± 5 meV (BE1 ) and 772 ± 5 meV (BE2 ) respectively, and in between one poorly resolved peak (BA) at an energy of 777 ± 5 meV. Acknowledgments The authors would like to acknowledge KANTOR KEMENTERIAN RISET DAN TEKNOLOGI REPUBLIK INDONESIA for financial support in this project through RUT (RISET UNGGULAN TERPADU) XII.

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