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Optical properties and defects in GaAsN and InGaAsN films and quantum well structures
Solid-State Electronics 46 (2002) 2147–2153 www.elsevier.com/locate/sse
Optical properties and defects in GaAsN and InGaAsN films and quantum well structures A.Y. Polyakov a,*, N.B. Smirnov a, A.V. Govorkov a, A.E. Botchkarev b, N.N. Nelson b, M.M.E. Fahmi b, J.A. Griffin b, A. Khan b, S. Noor Mohammad b, D.K. Johnstone c, V.T. Bublik d, K.D. Chsherbatchev d, M.I. Voronova d, V.S. Kasatochkin a a
Institute of Rare Metals, B. Tolmachevsky 5, Moscow 109017, Russia Department of Electrical Engineering, Howard University, 2300, Sixth St. NW, Washington, DC 20059, USA c AFOSR, 801 North Randolf Street, Room 732, Arlington, VA 22203-1977, USA Department of Materials Science of Semiconductors, Moscow Institute of Steel and Alloys, Leninsky Prospekt 4, Moscow 117936, Russia b
d
Received 17 January 2002; received in revised form 24 March 2002; accepted 1 April 2002
Abstract Photoluminescence and microcathodoluminescence spectra of thick-film GaAsN and InGaAsN structures and GaAs/ InGaAsN, AlGaAs/InGaAsN quantum wells (QWs) were studied for InGaAsN layers with low nitrogen concentration of 0.35–0.5%. It is shown that in thick-film structures the bandedge luminescence intensity is strongly decreased in the row homoepitaxial GaAs, GaAsN on GaAs buffer, GaAsN, GaAs on GaAsN buffer, InGaAsN which correlates with the increasing concentration of electron traps with activation energy 0.53–0.55 eV. The type of defect bands in the thickfilm structures was found to strongly depend on composition of the layers. For the GaAs/InGaAsN QW structures the intensity of luminescence was found to be more than an order of magnitude higher than in InGaAsN single films. Ó 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Dilute ternary and quaternary solid solutions of GaAs1x Nx , Iny Ga1y As1x Nx (x < 0:1) are of great interest to various electronic and optoelectronic applications because of the strong downward bowing of the GaAs or InGaAs bandgap due to incorporation of small amounts of nitrogen (see e.g. [1–4]). This allows to fabricate laser diodes for the 1.2–1.3 lm spectral range on cheap GaAs substrates instead of more expensive InP substrates needed for the InGaAsP/InP system (see e.g. [5]). It has also been demonstrated that, with InGaAsN (x ¼ 0:02, y ¼ 0:07) films, the spectral response of the
*
Corresponding author. Tel.: +7-095-239-9090; fax: +7-095953-3862. E-mail address: [email protected] (A.Y. Polyakov).
solar cells can be shifted to about 1 eV which allows to prepare tandem InGaAsN/GaAs solar cells with very high conversion efficiency [4]. In heterojunction bipolar transistors (HBTs) based on III–V materials there is a strong drive to decrease the turn-on voltage of the devices in order to increase the battery lifetime in portable devices (see e.g. a recent review in [6]). Decreasing the InGaAsN bandgap should be instrumental in achieving that goal. Again, very attractive results in that respect have been reported on even not fully optimized GaAs/ InGaAsN heterojunctions [6]. Of course, good lattice matching to GaAs is an advantage in such applications. Therefore, the quaternaries have an edge over ternary solid solutions when growing on GaAs substrates. However, if one considers the possibility of growth on Si substrates GaAsN would be of potentially great interest. All that makes studies of electrical and optical properties of GaAsN and InGaAsN films and quantum well (QW)
0038-1101/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 2 ) 0 0 1 7 8 - 8
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structures of great scientific and practical interest. Several groups have published rather extensive studies of these properties (see e.g. [4,6–10]) but it is felt that our knowledge in that respect is not still adequate. In the present paper we discuss the luminescent properties of very dilute (x < 0:04) thick-films of GaAsN and InGaAsN and of AlGaAs/GaAsN, AlGaAs/InGaAsN and GaAs/InGaAsN QW structures.
2. Experimental 2.1. Growth procedure All GaAsN samples studied in this work were grown in Varian GenII molecular beam epitaxy (MBE) machine equipped with valved cracker to precisely control the As2 flow. EPI-radio frequency (RF) plasma source of nitrogen was used to introduce nitrogen into the films and the effective nitrogen flow could be varied by varying the RF plasma source power. Growth was performed on (0 0 1) oriented GaAs substrates, typically at 500 °C growth temperature for GaAsN and InGaAsN, with typical growth rates of GaAsN and InGaAsN films of 0.75 lm/h. The RF plasma source power was typically set at 150–250 W. Some of the GaAsN and InGaAsN films and QWs were Si doped to electron concentration of about 1017 cm3 . In that case the films were grown on Si doped (to 1018 cm3 ) GaAs substrates. Other InGaAsN films were undoped and grown on semi-insulating SI-GaAs substrates. Two types of QW structures were prepared. In type I structures three GaAs, GaAsN or InGaAsN wells were grown between the Al0:25 Ga0:75 As barriers (the exact structures will be discussed below). In type II structures single QWs or double QWs of InGaAsN were inserted between the GaAs barriers. Type I QWs were uniformly Si doped and grown on Si doped GaAs substrates. Type II structures were undoped and grown on SI-GaAs substrates. 2.2. Characterization The composition of GaAsN films was determined from high-resolution X-ray diffraction (HRXRD) patterns as follows. Measurements were performed on double-crystal spectrometer with CuKa1 radiation and Ge(1 1 1) monochromator crystal. Rocking curves were taken for two asymmetric reflections (5 1 1) obtained by rotating the sample by 180° around the normal to the film’s surface (so called h / and h þ / scheme). From the distance between the film and the substrate peaks for these two (5 1 1) family planes the parallel epar and the normal enorm components of deformation could be calculated which allowed to obtain the real relative change
of the lattice parameter Da=aGaAs due to incorporation of nitrogen from the equation Da=aGaAs ¼ enorm C11 =ðC11 þ 2C12 Þ þ 2epar C12 =ðC11 þ 2C12 Þ; ð1Þ
where aGaAs is the lattice parameter of cubic GaAs, Da is the difference of the lattice parameters of the GaAsN film and the GaAs substrate, C11 and C12 are the elastic moduli of GaAsN taken in our case to be equal to those of GaAs (i.e. C11 ¼ 11:88 1011 dyn/cm2 , C12 ¼ 5:38 1011 dyn/cm2 [11]) because the N mole fraction in our films was very low. The nitrogen mole fraction x was then calculated from Vegard’s law as x ¼ ðDa=aGaAs Þ=½ðaGaAs aGaN Þ=aGaAs ;
ð2Þ
where aGaN is the lattice parameter for cubic GaN taken [12]. The approach taken above is similar to to be 4.52 A the one used in paper [10] to take into account tetragonal distortion of the GaAsN films when determining the films compositions from X-ray measurements. For InGaAsN, independent calculation of both the nitrogen and the indium mole fractions from X-ray rocking curves alone was not possible. Therefore it was assumed that the indium incorporation on the group III lattice site in our MBE growth was not affected by the presence of small amounts of nitrogen in the growth atmosphere (a reasonable enough assumption). Thus the indium mole fraction y was determined from calibration performed for InGaAs growth. After that the nitrogen mole fraction x was calculated from the unstrained value of the relative change of the lattice parameter of the InGaAsN film Da in respect to the GaAs substrate Da=aGaAs . The latter was deduced from the two (5 1 1) reflections from Eq. (1) as discussed above for GaAsN films. The unstrained lattice parameter of the quaternary Iny Ga1y As1x Nx film was expressed through the lattice parameters of constituent components as discussed by Adachi [11]: aInGaAsN ¼ yaInN þ ðx yÞaGaN þ ð1 xÞaGaAs ¼ aGaAs þ yðaInN aGaN Þ þ xðaGaN aGaAs Þ: ð3Þ The lattice parameter of cubic InN in (3) was taken as [12]. From (1) and (3) the mole fraction of ni4.98 A trogen could be easily obtained as x ¼ ½yðaInN aGaN Þ=aGaAs Da=aGaAs =ðaGaAs aGaN Þ=aGaAs :
ð4Þ
Superlattice satellites in double-crystal rocking curves measured on all studied QW structures were too weak to be detected and the zero-order superlattice peaks (see e.g. [13]) were only observed as shoulders so that HRXRD measurements were not particularly informa-
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tive in that case and the composition of the InGaAsN or GaAsN barriers in the AlGaAs/InGaAsN or GaAs/ InGaAsN QWs was assumed to be the same as for similarly grown thick layers. Luminescence spectra of the grown films were measured either at 77 K using a red He–Ne laser excitation and a photomultiplier tube for detector (photoluminescence (PL) spectra below) or at 90 K with excitation from a 25 kV probing beam of a scanning electron microscope (microcathodoluminescence (MCL) spectra below). Details of experimental setups can be found e.g. in [14,15].
2.3. Studied samples 2.3.1. Thick-films Two types of structures were grown. In type I structures growth was performed on Si doped GaAs substrates, all the films of the structure were uniformly doped to about 1017 cm3 , the first film, film 1 adjacent to the substrate was 0.5 lm thick, the second film on top of the first film was 0.25 lm thick. In type II structures a 1 lm thick undoped InGaAsN film was deposited on semi-insulating GaAs substrate. The samples were as follows: Type I structures: (1) sample M1874: 0.5 lm thick n-GaAs film on n-type GaAs substrate; (2) sample M1871: 0.5 lm thick n-GaAs buffer, 0.25 lm thick n-GaAs1x Nx film; (3) sample M1875: 0.5 lm thick n-GaAs1x Nx (same growth conditions and same N composition as for M1871), 0.25 lm thick n-GaAs film on top; (4) sample M1864: 0.5 lm thick n-GaAs buffer, 0.25 lm thick n-In0:01 Ga0:99 As1x Nx film. Type II structures: Sample M1899: SI-GaAs substrate, undoped 1 lm thick In0:07 Ga0:93 As1x Nx film.
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QW structures: Type I QW structures were uniformly Si doped to about 1017 cm3 , were grown on n-type GaAs substrates and consisted of n-Al0:25 Ga0:75 As buffer (0.5 lm thick), thick QWs of GaAs, GaAsN or InGaAsN three 50 A (thickness established from growth rates of thick films) thick barriers and the top Al0:25 separated by 100 A Ga0:75 As cap layer that was 0.17 lm thick. The samples were as follows: (1) sample M1865, GaAs QWs; (2) sample M1866, GaAs1x Nx QWs with GaAsN wells grown similarly to the thick-film sample M1871; (3) sample M1870, In0:01 Ga0:99 As1x Nx QWs with InGaAsN wells grown similarly to the thick-film sample M1864. Type II QW structures were undoped and grown on SI-GaAs substrates. The first sample, sample M1897, was a single QW sample consisting of 0.5 lm undoped thick undoped In0:07 Ga0:93 As1x Nx GaAs buffer, 100 A QW grown similarly to the thick-film sample M1899, 0.125 lm thick top GaAs cap. The second sample, sample M1898, differed from InGaAsN QWs sepaM1897 by that it had two 100 A thick GaAs barrier. The composition of rated by 250 A the QWs was the same as in M1897 and in the thick film sample M1899.
3. Results 3.1. X-ray measurements Da=aGaAs values calculated from the two (5 1 1) rocking curves from Eq. (1) and corresponding nitrogen mole fractions x calculated either from Eq. (2) for GaAs1x Nx or from Eq. (4) for Iny Ga1y As1x Nx for the thick-film structures M1871, M1864 and M1899 are presented in Table 1. The In compositions y used in
Table 1 Structure and composition of the used samples Sample no.
Structure
Da=aGaAs
N mole fraction x
In mole fraction y
M1874 M1871 M1864 M1875 M1865 M1866 M1870 M1899 M1897 M1898
GaAs (0.5 lm) GaAsN (0.25 lm)/GaAs (0.5 lm) InGaAsN (0.25 lm)/GaAs (0.5 lm) GaAs (0.25 lm)/GaAsN (0.5 lm) /50 A ) 3QW AlGaAs/GaAs (100 A /50 A ) 3QW AlGaAs/GaAsN (100 A /50 A ) 3QW AlGaAs/InGaAsN (100 A InGaAsN (1 lm) Single QW GaAs/InGaAsN Double QW GaAs/InGaAsN
– 7 104 1:4 104 – – – – 4:7 103 – –
– 0.0035 0.0035 0.0035 – 0.0035 0.0035 0.005 0.005 0.005
– – 0.01 – – – 0.01 0.07 0.07 0.07
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these calculations and obtained from prior calibration on InGaAs films are also presented in the table. It can be seen that the nitrogen composition in samples M1871 and M1864 was close to x ¼ 0:0035 (i.e. 0.35 at.% nitrogen) and in sample M1899 it was close to x ¼ 0:005 (i.e. 0.5 at.%). Thus the InGaAsN film M1864 is closely lattice matched to GaAs (the lattice matching condition for the Iny Ga1y As1x Nx is y ¼ 3x [4]) while the M1899 InGaAsN film is slightly mismatched. The films according to HRXRD measurements were fully strained and no relaxation of mismatch via misfit dislocations formation occurred. This was confirmed by the lack of dark lines due to misfit dislocations network in MCL and electron beam induced current (EBIC) images of the films. From above results it was concluded that the QW composition in QW samples M1866, M1870, M1897 and M1898 was respectively GaAs0:9965 N0:0035 , In0:01 Ga0:99 As0:9965 N0:0035 and In0:07 Ga0:93 As0:995 N0:005 . These results are summarized in Table 1.
3.2. PL and MCL spectra 3.2.1. Type I thick films and type I QW structures 77 K PL spectra measured on thick-film GaAs (M1874), GaAsN (M1871) and InGaAsN (M1864) samples are presented in Fig. 1. In Fig. 2 we compare the 77 K PL spectra taken on the single layer GaAs film M1874 and on sample M1875 in which 0.25 lm thick GaAs layer was grown on top of 0.5 lm thick GaAs0:9965 N0:0035 film. From Fig. 1 it can be seen that the PL spectrum of our MBE grown GaAs shows the 1.50 eV bandedge line and a broad weak defect band centered near 1.2 eV. Incorporation of 0.35% of As into the GaAsN film in sample M1871 decreased the energy of the bandgap line to 1.42 eV which is in good agreement with the bandgap reduction observed for such compo-
Fig. 1. 77 K PL spectra of GaAs sample M1874, GaAsN sample M1871 and InGaAsN sample M1864 (see text and Table 1).
Fig. 2. 77 K PL spectra of the GaAs sample M1874 and the GaAs/GaAsN sample M1875 (see text and Table 1).
sition in paper [7]. Thus, for this sample the compositions determined from X-ray and PL measurements coincide very reasonably. The position of the defect band in the GaAsN sample M1871 was slightly red shifted compared to the GaAs sample M1874 (to 1.165 instead of 1.2 eV) and the relative intensity of this defect band compared to the bandedge line increased by more than an order of magnitude compared to the GaAs sample. In the InGaAsN sample M1864 the bandedge line was observed near 1.42 eV, i.e. very close to the GaAsN with the same nitrogen mole fraction which is probably reasonable since the the bandgap decrease due to the incorporation of 1% In is very small [16]. Strangely, the intensity of the bandgap line in the InGaAsN sample M1864 was more than two times lower than in the GaAsN sample M1871, although the latter is measurably lattice mismatched while the former is very closely lattice matched to GaAs (see previous section). The intensity of the defect band in the InGaAsN sample was lower than in the GaAsN but considerably higher than in GaAs and the peak energy was shifted to 1.25 eV. Comparison of PL spectra (see Fig. 2) of the single layer GaAs sample M1874 and of sample M1875 (0.25 lm GaAs film grown on top of the 0.5 lm thick GaAs0:9965 N0:0035 layer) is very instructive. The intensity of the GaAs bandedge line in sample M1875 is almost an order of magnitude lower than in the GaAs sample, the intensity of the GaAsN peak at 1.42 eV (coming from the GaAsN underlayer) is practically the same as in the GaAsN film M1871, no defect bands at either 1.165 eV or 1.2 eV are observed. Instead an intense defect band at about 1 eV was detected in this sample. (All the spectra in Figs. 1 and 2 were taken with the same excitation and registration conditions and the intensities of various lines in the spectra do reflect real changes in the luminescence efficiencies for different bands in various samples.)
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3 it can be seen that the intensity ratio for QW structures is reversed, i.e. the intensity for the GaAsN QW is an order of magnitude higher. And that even despite the fact that the half-width of the QW peaks is considerably higher in GaAsN and InGaAsN QWs compared to GaAs QW (respectively about 25 meV for the latter and about 40 meV for the former).
Fig. 3. 90 K MCL spectra of the 3QW AlGaAs/GaAs sample M1865, 3QW AlGaAs/GaAsN sample M1866 and 3QW AlGaAs/InGaAsN sample M1870 (see text and Table 1).
The intensity of QW-related luminescence coming from the AlGaAs/GaAs (sample M1865), AlGaAs/GaAsN (sample 1866) and AlGaAs/InGaAsN (sample M1870) 3QW structures was low with our laser excitation. Hence for these samples we did 90 K MCL measurements instead because of the higher possible excitation intensity. The spectra obtained are presented in Fig. 3. For the AlGaAs/GaAs QW sample M1865 the spectrum consisted of the GaAs substrate peak near 1.5 eV, the AlGaAs peak at 1.84 eV and the GaAs QW peak at 1.675 eV corresponding to quantum confinement shift of 0.19 eV (the 90 K GaAs bandgap value was taken to . be 1.49 eV) and the QW width of approximately 54 A For the AlGaAs/GaAsN sample M1866 the QW peak position was at 1.648 eV. MCL spectra measurements on the thick GaAsN sample M1871 (not shown here) gave the bandgap position of 1.41 eV instead of 1.42 eV which is consistent with the measurement temperature change from 77 K in PL to 90 K in MCL measurements if the temperature coefficient for the bandgap width is close to that of GaAs [17]. Then the quantum confinement energy in the GaAsN QW sample would be 0.235 eV, i.e. slightly higher than for the GaAs sample, either because of the slightly lower GaAsN bandgap than in the reference thick-film sample M1871 or a slightly narrower well than in the AlGaAs/GaAs QW sample M1865. Similar situation is observed for the AlGaAs/InGaAsN QW structure M1870. Here the QW-related peak is observed at 1.637 eV which gives the quantum confinement energy of 0.224 eV if the bulk bandgap is taken to be 1.41 eV. That is, again either the bandgap is lower or the QW narrower. It should be pointed out that the intensity of the QW peak in the InGaAsN QW sample M1870 is the lowest which agrees with our observations on thick films (see Fig. 1). However, if one compares the intensities of thick film and QW GaAs and GaAsN structures in Figs. 1 and
3.2.2. Type II thick films and QW structures Because type II structures were undoped they were relatively highly resistive and showed very low PL intensity, particularly for the thick-film sample M1899. Therefore, MCL spectra measurements were preferred in that case. 90 K MCL spectrum of the single film In0:07 Ga0:93 As0:995 N0:005 sample M1899 is shown in Fig. 4. The GaAs bandedge peak near 1.5 eV and the InGaAsN bandedge peak near 1.385 eV can be clearly seen. The bandgap of unstrained In0:07 Ga0:93 As is expected to be 1.43 eV according to paper [16]. The lattice parameter of our InGaAsN film is slightly higher than that of GaAs. Thus the film should experience tensile stress. Effects of tensile stress on the bandgap of InGaN were analyzed in some detail in paper [18] and from the results presented there one expects the effective bandgap to be even slightly higher than in the unstrained case. Hence the quite measurable decrease of the bandgap in our InGaAsN film compared to InGaAs can be ascribed to the effect of nitrogen incorporation. The bandgaps of InGaAsN ternary solid solutions have not been well mapped as a function of N mole fraction, particularly for lattice mismatched compositions. For lattice matched In0:07 Ga0:93 As0:98 N0:02 the bandgap has been reported to be close to 1 eV at room temperature, i.e. about 1.1 eV at 90 K. If the films were unstrained and the bandgap dependence on nitrogen mole fraction were linear, for 0.5% N in solid solution we should have measured the bandgap value of 1.35 eV instead of 1.385 eV in our experiment. More detailed studies are
Fig. 4. 90 K MCL spectra of the thick-film InGaAsN sample M1899, single QW GaAs/InGaAsN sample M1897 and double QW GaAs/InGaAsN sample M1898 (see text and Table 1).
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necessary to understand whether the observed difference should be ascribed to strain effects as discussed above. 90 K MCL spectra for the single QW sample M1897 and double QW sample M1898 with the well compositions similar to that of the thick-film sample M1897 are also shown in Fig. 4. The MCL intensity for the QW structures is more than an order of magnitude higher than for the thick film, both for the InGaAsN and the GaAs peaks (the latter obviously due to absorption of the GaAs signal in the thick InGaAsN film). The signal intensity for the double QW sample M1898 in the InGaAsN peak region is about twice that for the single QW sample M1897 which supports the attribution of the signal to to the InGaAsN wells. The peak energy of the InGaAsN peak is shifted to 1.43 eV, i.e. by 45 meV compared to bulk InGaAsN film M1899, owing to the quantum confinement effect.
4. Discussion Several observations can be made in the above data. First, the intensity of bandedge luminescence in thickfilm samples decreases in the order M1874 (GaAs film), M1871 (GaAsN film), M1875 (GaAs film on GaAsN underlayer) and M1864 (InGaAsN film) (see Figs. 1 and 2). Deep levels transient spectroscopy (DLTS) measurements to be reported in detail elsewhere show that the main difference between the studied samples is in the density of deep electron traps with activation energy 0.53–0.55 eV. These traps are not detected in the GaAs film M1874, their density is very high (4:8 1015 cm3 ) in the InGaAsN sample M1864, about two times lower (2:4 1015 cm3 ) in the GaAs/GaAsN sample M1875 and still lower (1:2 1015 cm3 ) in the GaAsN sample M1871. This correlates perfectly with the observed bandedge signal changes at either 1.5 eV in the GaAs films or at 1.42 eV in the GaAsN and InGaAsN films. The origin of such traps is not clear but, for GaAs, it has been suggested that they could be related to off-center substitutional oxygen on As site [19]. The nature of additional oxygen contamination is not clear at the moment but it is somehow related to turning on the RF plasma source of nitrogen when growing GaAsN or InGaAsN. Obviously this contamination process is enhanced upon introduction of indium since the density of the 0.55 eV traps is the highest in the InGaAsN film. A certain memory of the growth ambience would be required to explain the enhanced 0.55 eV traps concentration in the GaAs film grown on top of the GaAsN underlayer (sample M1875). Secondly, the deep defects bands in thick-film samples are seriously affected by the composition of the layer. In the GaAsN sample M1871 and the InGaAsN sample M1864 having the same bandgap of 1.42 eV the defect
band positions are at 1.165 eV in the first case and at 1.25 eV in the second. Very interestingly, in the GaAs/ GaAsN film M1875 the dominant defect band is at 1 eV, i.e. very different from either the GaAs film M1874 (the dominant band at 1.2 eV) or the GaAsN film M1871 (1.165 eV band). From which of the films, the top GaAs or the bottom GaAsN, comes the 1 eV band in the GaAsN sample M1875 is not quite clear at present. Preliminary results do suggest that the band in question is due to the top GaAs film. In that case it would match perfectly in energy a transition involving electrons trapped by the 0.55 eV center and the free holes in the valence band. (In the GaAsN sample M1871 and the InGaAsN sample M1864 the transition energy would be expected to shift to 0.9 eV where the sensitivity of the photomultiplier tube used is quite low which might explain why such a band is not detected in these samples.) What is perfectly clear is that, whilst the bandedge peak energy and intensity due to the GaAsN underlayer in the GaAs/GaAsN film M1875 are the same as in the GaAsN/GaAs sample M1871 (the intensity, in fact, is even slightly higher because some weak absorption in the top GaAs layer of the structure should occur) the strong defect band at 1.165 eV is manifestly absent in the GaAsN film of the M1875 sample (see Figs. 1 and 2). Why would the crystalline quality of a thicker lattice mismatched GaAsN film grown directly on GaAs substrate (as in the GaAs/GaAsN sample M1875) be better than the quality of a thinner GaAsN film grown on thick GaAs buffer layer (as in the GaAsN/GaAs sample M1871) is not obvious. One explanation that comes to mind is that growth of the top GaAs layer could be in certain respects equivalent to post-growth annealing of the underlying GaAsN film proven to be very beneficial to suppressing deep centers in GaAsN and InGaAsN (see e.g. [4,20]). But it is apparent that more studies are necessary to clarify these issues. As for the QW samples, it is of interest and perhaps of practical importance that the order of intensities is reversed for QW-related signals of the AlGaAs/GaAs QWs and the AlGaAs/GaAsN QWs compared to that observed for thick films. As seen in Fig. 3 the signal from the GaAsN QWs in sample M1866 is stronger than the signal from GaAs QWs in sample M1865 while the signal from the InGaAsN QWs (sample M1870) still remains the lowest. Apparently the situation with deep centers in GaAsN QWs compared to GaAs QWs is not as bad as for thick films. For more In-rich QWs in type II QW structures M1897 and M1898 as compared to the thick-film structure M1899 the situation is similar to the above described situation for GaAsN, i.e. the intensity of luminescence from QW structures is more than an order of magnitude stronger than from the thick film sample M1899 (see Fig. 4). Since the latter is rather heavily
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lattice mismatched this observation seems to be perfectly reasonable. As mentioned in the previous section we observed a slight increase in the quantum confinement-related energy shifts for GaAsN and InGaAsN QWs compared to GaAs QWs in type I QW structures with AlGaAs barriers (0.22–0.24 eV in GaAsN and InGaAsN as opposed to 0.19 eV in GaAs, see Fig. 3). In principle this increase could be due to somewhat increased N incorporation efficiency in thin structures but the thickness variations in GaAsN or required to explain the difference (48–49 A InGaAsN instead of 54 A in GaAs) are quite small and more serious studies are necessary to prove or disprove such an assumption. 5. Conclusions We have shown that the bandedge luminescence intensity for thick-film structures of InGaAsN decreases in the following order: GaAs homoepitaxial films, GaAsN films with GaAs layer on top, GaAs films on GaAsN underlayer, InGaAsN films on GaAs buffer. Observed changes correlate with corresponding decrease in the density of 0.55 eV electron traps that, according to literature, could be due to off-center substitutional oxygen on arsenic sites. However, in QW structures the situation with luminescence efficiency of GaAsN versus GaAs QWs is greatly improved and, in fact, the latter show a higher intensity of the QW-related signal. The type and intensity of defect bands in InGaAsN films are a strong function of composition (e.g the dominant defect bands are 1.2 eV in homoepitaxial GaAs, 1.165 in GaAs0:9965 N0:0035 films on GaAs buffer, 1 eV in GaAs films on GaAs0:9965 N0:0035 underlayer, none in the GaAsN underlayers of the GaAs/GaAsN structures). Acknowledgements The research at Howard University was supported by the US Air Force Office of Scientific Research Grant no.
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F49620-02-1-0008. The authors would like to thank Mrs. E.F. Astakhova for assistance in samples preparation.
References [1] Sakai S, Ueta Y, Teauchi Y. Jpn J Appl Phys, Part 1 1993;32:4413. [2] Wei SH, Zunger A. Phys Rev Lett 1996;76:664. [3] Bellaiche L, Wei SH, Zunger A. Phys Rev B 1996;54:17568. [4] Kurtz SR, Allerman AA, Jones ED, Gee JM, Banas JJ, Hammons BE. Appl Phys Lett 1999;74:729. [5] Kondow M, Nakatuka S, Kitani T, Yazawa Y, Okai M. Electron Lett 1996;32:2244. [6] Pearton SJ, Ren F, Zhang AP, Lee KP. Mater Sci Eng R 2000;30:55. [7] Pozina G, Ivanov I, Monemar B, Thordson JV, Andersson TG. J Appl Phys 1998;84:3830. [8] Rao EVK, Ougazzaden A, LeBellego Y, Yuhel M. Appl Phys Lett 1998;72:1409. [9] Uesugi K, Morooka N, Suemune I. J Cryst Growth 1999;201/202:355. [10] Uesugi K, Morooka N, Suemune I. Appl Phys Lett 1999; 74:1254. [11] Adachi S. J Appl Phys 1982;53:8775. [12] Popovici G, Morkoc H. Growth and doping defects in IIInitrides. In: Pearton SJ, editor. GaN and related materials II. New York: Gordon and Breach Science Publishers; 1999. p. 93–172. [13] Herman MA. Semiconductor superlattices. Berlin: Akademie-Verlag; 1986. [14] Markov AV, Polyakov AY, Smirnov NB, Govorkov AV, Eremin VK, Verbitskaya EM, et al. Nucl Instr Meth A 2000;439:651. [15] Gorbylev VA, Chelniy AA, Polyakov AY, Smirnov NB, Wilson RG, Milnes AG, et al. J Appl Phys 1994;76:7390. [16] Goetz KH, Bimberg D, Jurgensen H, Solders J, Solomon AV, Glinskii F, et al. J Appl Phys 1983;54:4543. [17] Willardson RK, Beer AC. In: Semiconductors and Semimetals, vol. 3. New York: Academic Press; 1967. [18] Wang TY, Stringfellow GB. J Appl Phys 1990;67:344. [19] Skowronski M. Mat Sci Forum 1992;83–87:377. [20] Buyanova IA, Pozina G, Hai PN, Thinh NQ, Bergman JP, Chen WM, et al. Appl Phys Lett 2000;77:2325.
Optical properties and defects in GaAsN and InGaAsN films and quantum well structures A.Y. Polyakov a,*, N.B. Smirnov a, A.V. Govorkov a, A.E. Botchkarev b, N.N. Nelson b, M.M.E. Fahmi b, J.A. Griffin b, A. Khan b, S. Noor Mohammad b, D.K. Johnstone c, V.T. Bublik d, K.D. Chsherbatchev d, M.I. Voronova d, V.S. Kasatochkin a a
Institute of Rare Metals, B. Tolmachevsky 5, Moscow 109017, Russia Department of Electrical Engineering, Howard University, 2300, Sixth St. NW, Washington, DC 20059, USA c AFOSR, 801 North Randolf Street, Room 732, Arlington, VA 22203-1977, USA Department of Materials Science of Semiconductors, Moscow Institute of Steel and Alloys, Leninsky Prospekt 4, Moscow 117936, Russia b
d
Received 17 January 2002; received in revised form 24 March 2002; accepted 1 April 2002
Abstract Photoluminescence and microcathodoluminescence spectra of thick-film GaAsN and InGaAsN structures and GaAs/ InGaAsN, AlGaAs/InGaAsN quantum wells (QWs) were studied for InGaAsN layers with low nitrogen concentration of 0.35–0.5%. It is shown that in thick-film structures the bandedge luminescence intensity is strongly decreased in the row homoepitaxial GaAs, GaAsN on GaAs buffer, GaAsN, GaAs on GaAsN buffer, InGaAsN which correlates with the increasing concentration of electron traps with activation energy 0.53–0.55 eV. The type of defect bands in the thickfilm structures was found to strongly depend on composition of the layers. For the GaAs/InGaAsN QW structures the intensity of luminescence was found to be more than an order of magnitude higher than in InGaAsN single films. Ó 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Dilute ternary and quaternary solid solutions of GaAs1x Nx , Iny Ga1y As1x Nx (x < 0:1) are of great interest to various electronic and optoelectronic applications because of the strong downward bowing of the GaAs or InGaAs bandgap due to incorporation of small amounts of nitrogen (see e.g. [1–4]). This allows to fabricate laser diodes for the 1.2–1.3 lm spectral range on cheap GaAs substrates instead of more expensive InP substrates needed for the InGaAsP/InP system (see e.g. [5]). It has also been demonstrated that, with InGaAsN (x ¼ 0:02, y ¼ 0:07) films, the spectral response of the
*
Corresponding author. Tel.: +7-095-239-9090; fax: +7-095953-3862. E-mail address: [email protected] (A.Y. Polyakov).
solar cells can be shifted to about 1 eV which allows to prepare tandem InGaAsN/GaAs solar cells with very high conversion efficiency [4]. In heterojunction bipolar transistors (HBTs) based on III–V materials there is a strong drive to decrease the turn-on voltage of the devices in order to increase the battery lifetime in portable devices (see e.g. a recent review in [6]). Decreasing the InGaAsN bandgap should be instrumental in achieving that goal. Again, very attractive results in that respect have been reported on even not fully optimized GaAs/ InGaAsN heterojunctions [6]. Of course, good lattice matching to GaAs is an advantage in such applications. Therefore, the quaternaries have an edge over ternary solid solutions when growing on GaAs substrates. However, if one considers the possibility of growth on Si substrates GaAsN would be of potentially great interest. All that makes studies of electrical and optical properties of GaAsN and InGaAsN films and quantum well (QW)
0038-1101/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 2 ) 0 0 1 7 8 - 8
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structures of great scientific and practical interest. Several groups have published rather extensive studies of these properties (see e.g. [4,6–10]) but it is felt that our knowledge in that respect is not still adequate. In the present paper we discuss the luminescent properties of very dilute (x < 0:04) thick-films of GaAsN and InGaAsN and of AlGaAs/GaAsN, AlGaAs/InGaAsN and GaAs/InGaAsN QW structures.
2. Experimental 2.1. Growth procedure All GaAsN samples studied in this work were grown in Varian GenII molecular beam epitaxy (MBE) machine equipped with valved cracker to precisely control the As2 flow. EPI-radio frequency (RF) plasma source of nitrogen was used to introduce nitrogen into the films and the effective nitrogen flow could be varied by varying the RF plasma source power. Growth was performed on (0 0 1) oriented GaAs substrates, typically at 500 °C growth temperature for GaAsN and InGaAsN, with typical growth rates of GaAsN and InGaAsN films of 0.75 lm/h. The RF plasma source power was typically set at 150–250 W. Some of the GaAsN and InGaAsN films and QWs were Si doped to electron concentration of about 1017 cm3 . In that case the films were grown on Si doped (to 1018 cm3 ) GaAs substrates. Other InGaAsN films were undoped and grown on semi-insulating SI-GaAs substrates. Two types of QW structures were prepared. In type I structures three GaAs, GaAsN or InGaAsN wells were grown between the Al0:25 Ga0:75 As barriers (the exact structures will be discussed below). In type II structures single QWs or double QWs of InGaAsN were inserted between the GaAs barriers. Type I QWs were uniformly Si doped and grown on Si doped GaAs substrates. Type II structures were undoped and grown on SI-GaAs substrates. 2.2. Characterization The composition of GaAsN films was determined from high-resolution X-ray diffraction (HRXRD) patterns as follows. Measurements were performed on double-crystal spectrometer with CuKa1 radiation and Ge(1 1 1) monochromator crystal. Rocking curves were taken for two asymmetric reflections (5 1 1) obtained by rotating the sample by 180° around the normal to the film’s surface (so called h / and h þ / scheme). From the distance between the film and the substrate peaks for these two (5 1 1) family planes the parallel epar and the normal enorm components of deformation could be calculated which allowed to obtain the real relative change
of the lattice parameter Da=aGaAs due to incorporation of nitrogen from the equation Da=aGaAs ¼ enorm C11 =ðC11 þ 2C12 Þ þ 2epar C12 =ðC11 þ 2C12 Þ; ð1Þ
where aGaAs is the lattice parameter of cubic GaAs, Da is the difference of the lattice parameters of the GaAsN film and the GaAs substrate, C11 and C12 are the elastic moduli of GaAsN taken in our case to be equal to those of GaAs (i.e. C11 ¼ 11:88 1011 dyn/cm2 , C12 ¼ 5:38 1011 dyn/cm2 [11]) because the N mole fraction in our films was very low. The nitrogen mole fraction x was then calculated from Vegard’s law as x ¼ ðDa=aGaAs Þ=½ðaGaAs aGaN Þ=aGaAs ;
ð2Þ
where aGaN is the lattice parameter for cubic GaN taken [12]. The approach taken above is similar to to be 4.52 A the one used in paper [10] to take into account tetragonal distortion of the GaAsN films when determining the films compositions from X-ray measurements. For InGaAsN, independent calculation of both the nitrogen and the indium mole fractions from X-ray rocking curves alone was not possible. Therefore it was assumed that the indium incorporation on the group III lattice site in our MBE growth was not affected by the presence of small amounts of nitrogen in the growth atmosphere (a reasonable enough assumption). Thus the indium mole fraction y was determined from calibration performed for InGaAs growth. After that the nitrogen mole fraction x was calculated from the unstrained value of the relative change of the lattice parameter of the InGaAsN film Da in respect to the GaAs substrate Da=aGaAs . The latter was deduced from the two (5 1 1) reflections from Eq. (1) as discussed above for GaAsN films. The unstrained lattice parameter of the quaternary Iny Ga1y As1x Nx film was expressed through the lattice parameters of constituent components as discussed by Adachi [11]: aInGaAsN ¼ yaInN þ ðx yÞaGaN þ ð1 xÞaGaAs ¼ aGaAs þ yðaInN aGaN Þ þ xðaGaN aGaAs Þ: ð3Þ The lattice parameter of cubic InN in (3) was taken as [12]. From (1) and (3) the mole fraction of ni4.98 A trogen could be easily obtained as x ¼ ½yðaInN aGaN Þ=aGaAs Da=aGaAs =ðaGaAs aGaN Þ=aGaAs :
ð4Þ
Superlattice satellites in double-crystal rocking curves measured on all studied QW structures were too weak to be detected and the zero-order superlattice peaks (see e.g. [13]) were only observed as shoulders so that HRXRD measurements were not particularly informa-
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tive in that case and the composition of the InGaAsN or GaAsN barriers in the AlGaAs/InGaAsN or GaAs/ InGaAsN QWs was assumed to be the same as for similarly grown thick layers. Luminescence spectra of the grown films were measured either at 77 K using a red He–Ne laser excitation and a photomultiplier tube for detector (photoluminescence (PL) spectra below) or at 90 K with excitation from a 25 kV probing beam of a scanning electron microscope (microcathodoluminescence (MCL) spectra below). Details of experimental setups can be found e.g. in [14,15].
2.3. Studied samples 2.3.1. Thick-films Two types of structures were grown. In type I structures growth was performed on Si doped GaAs substrates, all the films of the structure were uniformly doped to about 1017 cm3 , the first film, film 1 adjacent to the substrate was 0.5 lm thick, the second film on top of the first film was 0.25 lm thick. In type II structures a 1 lm thick undoped InGaAsN film was deposited on semi-insulating GaAs substrate. The samples were as follows: Type I structures: (1) sample M1874: 0.5 lm thick n-GaAs film on n-type GaAs substrate; (2) sample M1871: 0.5 lm thick n-GaAs buffer, 0.25 lm thick n-GaAs1x Nx film; (3) sample M1875: 0.5 lm thick n-GaAs1x Nx (same growth conditions and same N composition as for M1871), 0.25 lm thick n-GaAs film on top; (4) sample M1864: 0.5 lm thick n-GaAs buffer, 0.25 lm thick n-In0:01 Ga0:99 As1x Nx film. Type II structures: Sample M1899: SI-GaAs substrate, undoped 1 lm thick In0:07 Ga0:93 As1x Nx film.
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QW structures: Type I QW structures were uniformly Si doped to about 1017 cm3 , were grown on n-type GaAs substrates and consisted of n-Al0:25 Ga0:75 As buffer (0.5 lm thick), thick QWs of GaAs, GaAsN or InGaAsN three 50 A (thickness established from growth rates of thick films) thick barriers and the top Al0:25 separated by 100 A Ga0:75 As cap layer that was 0.17 lm thick. The samples were as follows: (1) sample M1865, GaAs QWs; (2) sample M1866, GaAs1x Nx QWs with GaAsN wells grown similarly to the thick-film sample M1871; (3) sample M1870, In0:01 Ga0:99 As1x Nx QWs with InGaAsN wells grown similarly to the thick-film sample M1864. Type II QW structures were undoped and grown on SI-GaAs substrates. The first sample, sample M1897, was a single QW sample consisting of 0.5 lm undoped thick undoped In0:07 Ga0:93 As1x Nx GaAs buffer, 100 A QW grown similarly to the thick-film sample M1899, 0.125 lm thick top GaAs cap. The second sample, sample M1898, differed from InGaAsN QWs sepaM1897 by that it had two 100 A thick GaAs barrier. The composition of rated by 250 A the QWs was the same as in M1897 and in the thick film sample M1899.
3. Results 3.1. X-ray measurements Da=aGaAs values calculated from the two (5 1 1) rocking curves from Eq. (1) and corresponding nitrogen mole fractions x calculated either from Eq. (2) for GaAs1x Nx or from Eq. (4) for Iny Ga1y As1x Nx for the thick-film structures M1871, M1864 and M1899 are presented in Table 1. The In compositions y used in
Table 1 Structure and composition of the used samples Sample no.
Structure
Da=aGaAs
N mole fraction x
In mole fraction y
M1874 M1871 M1864 M1875 M1865 M1866 M1870 M1899 M1897 M1898
GaAs (0.5 lm) GaAsN (0.25 lm)/GaAs (0.5 lm) InGaAsN (0.25 lm)/GaAs (0.5 lm) GaAs (0.25 lm)/GaAsN (0.5 lm) /50 A ) 3QW AlGaAs/GaAs (100 A /50 A ) 3QW AlGaAs/GaAsN (100 A /50 A ) 3QW AlGaAs/InGaAsN (100 A InGaAsN (1 lm) Single QW GaAs/InGaAsN Double QW GaAs/InGaAsN
– 7 104 1:4 104 – – – – 4:7 103 – –
– 0.0035 0.0035 0.0035 – 0.0035 0.0035 0.005 0.005 0.005
– – 0.01 – – – 0.01 0.07 0.07 0.07
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these calculations and obtained from prior calibration on InGaAs films are also presented in the table. It can be seen that the nitrogen composition in samples M1871 and M1864 was close to x ¼ 0:0035 (i.e. 0.35 at.% nitrogen) and in sample M1899 it was close to x ¼ 0:005 (i.e. 0.5 at.%). Thus the InGaAsN film M1864 is closely lattice matched to GaAs (the lattice matching condition for the Iny Ga1y As1x Nx is y ¼ 3x [4]) while the M1899 InGaAsN film is slightly mismatched. The films according to HRXRD measurements were fully strained and no relaxation of mismatch via misfit dislocations formation occurred. This was confirmed by the lack of dark lines due to misfit dislocations network in MCL and electron beam induced current (EBIC) images of the films. From above results it was concluded that the QW composition in QW samples M1866, M1870, M1897 and M1898 was respectively GaAs0:9965 N0:0035 , In0:01 Ga0:99 As0:9965 N0:0035 and In0:07 Ga0:93 As0:995 N0:005 . These results are summarized in Table 1.
3.2. PL and MCL spectra 3.2.1. Type I thick films and type I QW structures 77 K PL spectra measured on thick-film GaAs (M1874), GaAsN (M1871) and InGaAsN (M1864) samples are presented in Fig. 1. In Fig. 2 we compare the 77 K PL spectra taken on the single layer GaAs film M1874 and on sample M1875 in which 0.25 lm thick GaAs layer was grown on top of 0.5 lm thick GaAs0:9965 N0:0035 film. From Fig. 1 it can be seen that the PL spectrum of our MBE grown GaAs shows the 1.50 eV bandedge line and a broad weak defect band centered near 1.2 eV. Incorporation of 0.35% of As into the GaAsN film in sample M1871 decreased the energy of the bandgap line to 1.42 eV which is in good agreement with the bandgap reduction observed for such compo-
Fig. 1. 77 K PL spectra of GaAs sample M1874, GaAsN sample M1871 and InGaAsN sample M1864 (see text and Table 1).
Fig. 2. 77 K PL spectra of the GaAs sample M1874 and the GaAs/GaAsN sample M1875 (see text and Table 1).
sition in paper [7]. Thus, for this sample the compositions determined from X-ray and PL measurements coincide very reasonably. The position of the defect band in the GaAsN sample M1871 was slightly red shifted compared to the GaAs sample M1874 (to 1.165 instead of 1.2 eV) and the relative intensity of this defect band compared to the bandedge line increased by more than an order of magnitude compared to the GaAs sample. In the InGaAsN sample M1864 the bandedge line was observed near 1.42 eV, i.e. very close to the GaAsN with the same nitrogen mole fraction which is probably reasonable since the the bandgap decrease due to the incorporation of 1% In is very small [16]. Strangely, the intensity of the bandgap line in the InGaAsN sample M1864 was more than two times lower than in the GaAsN sample M1871, although the latter is measurably lattice mismatched while the former is very closely lattice matched to GaAs (see previous section). The intensity of the defect band in the InGaAsN sample was lower than in the GaAsN but considerably higher than in GaAs and the peak energy was shifted to 1.25 eV. Comparison of PL spectra (see Fig. 2) of the single layer GaAs sample M1874 and of sample M1875 (0.25 lm GaAs film grown on top of the 0.5 lm thick GaAs0:9965 N0:0035 layer) is very instructive. The intensity of the GaAs bandedge line in sample M1875 is almost an order of magnitude lower than in the GaAs sample, the intensity of the GaAsN peak at 1.42 eV (coming from the GaAsN underlayer) is practically the same as in the GaAsN film M1871, no defect bands at either 1.165 eV or 1.2 eV are observed. Instead an intense defect band at about 1 eV was detected in this sample. (All the spectra in Figs. 1 and 2 were taken with the same excitation and registration conditions and the intensities of various lines in the spectra do reflect real changes in the luminescence efficiencies for different bands in various samples.)
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3 it can be seen that the intensity ratio for QW structures is reversed, i.e. the intensity for the GaAsN QW is an order of magnitude higher. And that even despite the fact that the half-width of the QW peaks is considerably higher in GaAsN and InGaAsN QWs compared to GaAs QW (respectively about 25 meV for the latter and about 40 meV for the former).
Fig. 3. 90 K MCL spectra of the 3QW AlGaAs/GaAs sample M1865, 3QW AlGaAs/GaAsN sample M1866 and 3QW AlGaAs/InGaAsN sample M1870 (see text and Table 1).
The intensity of QW-related luminescence coming from the AlGaAs/GaAs (sample M1865), AlGaAs/GaAsN (sample 1866) and AlGaAs/InGaAsN (sample M1870) 3QW structures was low with our laser excitation. Hence for these samples we did 90 K MCL measurements instead because of the higher possible excitation intensity. The spectra obtained are presented in Fig. 3. For the AlGaAs/GaAs QW sample M1865 the spectrum consisted of the GaAs substrate peak near 1.5 eV, the AlGaAs peak at 1.84 eV and the GaAs QW peak at 1.675 eV corresponding to quantum confinement shift of 0.19 eV (the 90 K GaAs bandgap value was taken to . be 1.49 eV) and the QW width of approximately 54 A For the AlGaAs/GaAsN sample M1866 the QW peak position was at 1.648 eV. MCL spectra measurements on the thick GaAsN sample M1871 (not shown here) gave the bandgap position of 1.41 eV instead of 1.42 eV which is consistent with the measurement temperature change from 77 K in PL to 90 K in MCL measurements if the temperature coefficient for the bandgap width is close to that of GaAs [17]. Then the quantum confinement energy in the GaAsN QW sample would be 0.235 eV, i.e. slightly higher than for the GaAs sample, either because of the slightly lower GaAsN bandgap than in the reference thick-film sample M1871 or a slightly narrower well than in the AlGaAs/GaAs QW sample M1865. Similar situation is observed for the AlGaAs/InGaAsN QW structure M1870. Here the QW-related peak is observed at 1.637 eV which gives the quantum confinement energy of 0.224 eV if the bulk bandgap is taken to be 1.41 eV. That is, again either the bandgap is lower or the QW narrower. It should be pointed out that the intensity of the QW peak in the InGaAsN QW sample M1870 is the lowest which agrees with our observations on thick films (see Fig. 1). However, if one compares the intensities of thick film and QW GaAs and GaAsN structures in Figs. 1 and
3.2.2. Type II thick films and QW structures Because type II structures were undoped they were relatively highly resistive and showed very low PL intensity, particularly for the thick-film sample M1899. Therefore, MCL spectra measurements were preferred in that case. 90 K MCL spectrum of the single film In0:07 Ga0:93 As0:995 N0:005 sample M1899 is shown in Fig. 4. The GaAs bandedge peak near 1.5 eV and the InGaAsN bandedge peak near 1.385 eV can be clearly seen. The bandgap of unstrained In0:07 Ga0:93 As is expected to be 1.43 eV according to paper [16]. The lattice parameter of our InGaAsN film is slightly higher than that of GaAs. Thus the film should experience tensile stress. Effects of tensile stress on the bandgap of InGaN were analyzed in some detail in paper [18] and from the results presented there one expects the effective bandgap to be even slightly higher than in the unstrained case. Hence the quite measurable decrease of the bandgap in our InGaAsN film compared to InGaAs can be ascribed to the effect of nitrogen incorporation. The bandgaps of InGaAsN ternary solid solutions have not been well mapped as a function of N mole fraction, particularly for lattice mismatched compositions. For lattice matched In0:07 Ga0:93 As0:98 N0:02 the bandgap has been reported to be close to 1 eV at room temperature, i.e. about 1.1 eV at 90 K. If the films were unstrained and the bandgap dependence on nitrogen mole fraction were linear, for 0.5% N in solid solution we should have measured the bandgap value of 1.35 eV instead of 1.385 eV in our experiment. More detailed studies are
Fig. 4. 90 K MCL spectra of the thick-film InGaAsN sample M1899, single QW GaAs/InGaAsN sample M1897 and double QW GaAs/InGaAsN sample M1898 (see text and Table 1).
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necessary to understand whether the observed difference should be ascribed to strain effects as discussed above. 90 K MCL spectra for the single QW sample M1897 and double QW sample M1898 with the well compositions similar to that of the thick-film sample M1897 are also shown in Fig. 4. The MCL intensity for the QW structures is more than an order of magnitude higher than for the thick film, both for the InGaAsN and the GaAs peaks (the latter obviously due to absorption of the GaAs signal in the thick InGaAsN film). The signal intensity for the double QW sample M1898 in the InGaAsN peak region is about twice that for the single QW sample M1897 which supports the attribution of the signal to to the InGaAsN wells. The peak energy of the InGaAsN peak is shifted to 1.43 eV, i.e. by 45 meV compared to bulk InGaAsN film M1899, owing to the quantum confinement effect.
4. Discussion Several observations can be made in the above data. First, the intensity of bandedge luminescence in thickfilm samples decreases in the order M1874 (GaAs film), M1871 (GaAsN film), M1875 (GaAs film on GaAsN underlayer) and M1864 (InGaAsN film) (see Figs. 1 and 2). Deep levels transient spectroscopy (DLTS) measurements to be reported in detail elsewhere show that the main difference between the studied samples is in the density of deep electron traps with activation energy 0.53–0.55 eV. These traps are not detected in the GaAs film M1874, their density is very high (4:8 1015 cm3 ) in the InGaAsN sample M1864, about two times lower (2:4 1015 cm3 ) in the GaAs/GaAsN sample M1875 and still lower (1:2 1015 cm3 ) in the GaAsN sample M1871. This correlates perfectly with the observed bandedge signal changes at either 1.5 eV in the GaAs films or at 1.42 eV in the GaAsN and InGaAsN films. The origin of such traps is not clear but, for GaAs, it has been suggested that they could be related to off-center substitutional oxygen on As site [19]. The nature of additional oxygen contamination is not clear at the moment but it is somehow related to turning on the RF plasma source of nitrogen when growing GaAsN or InGaAsN. Obviously this contamination process is enhanced upon introduction of indium since the density of the 0.55 eV traps is the highest in the InGaAsN film. A certain memory of the growth ambience would be required to explain the enhanced 0.55 eV traps concentration in the GaAs film grown on top of the GaAsN underlayer (sample M1875). Secondly, the deep defects bands in thick-film samples are seriously affected by the composition of the layer. In the GaAsN sample M1871 and the InGaAsN sample M1864 having the same bandgap of 1.42 eV the defect
band positions are at 1.165 eV in the first case and at 1.25 eV in the second. Very interestingly, in the GaAs/ GaAsN film M1875 the dominant defect band is at 1 eV, i.e. very different from either the GaAs film M1874 (the dominant band at 1.2 eV) or the GaAsN film M1871 (1.165 eV band). From which of the films, the top GaAs or the bottom GaAsN, comes the 1 eV band in the GaAsN sample M1875 is not quite clear at present. Preliminary results do suggest that the band in question is due to the top GaAs film. In that case it would match perfectly in energy a transition involving electrons trapped by the 0.55 eV center and the free holes in the valence band. (In the GaAsN sample M1871 and the InGaAsN sample M1864 the transition energy would be expected to shift to 0.9 eV where the sensitivity of the photomultiplier tube used is quite low which might explain why such a band is not detected in these samples.) What is perfectly clear is that, whilst the bandedge peak energy and intensity due to the GaAsN underlayer in the GaAs/GaAsN film M1875 are the same as in the GaAsN/GaAs sample M1871 (the intensity, in fact, is even slightly higher because some weak absorption in the top GaAs layer of the structure should occur) the strong defect band at 1.165 eV is manifestly absent in the GaAsN film of the M1875 sample (see Figs. 1 and 2). Why would the crystalline quality of a thicker lattice mismatched GaAsN film grown directly on GaAs substrate (as in the GaAs/GaAsN sample M1875) be better than the quality of a thinner GaAsN film grown on thick GaAs buffer layer (as in the GaAsN/GaAs sample M1871) is not obvious. One explanation that comes to mind is that growth of the top GaAs layer could be in certain respects equivalent to post-growth annealing of the underlying GaAsN film proven to be very beneficial to suppressing deep centers in GaAsN and InGaAsN (see e.g. [4,20]). But it is apparent that more studies are necessary to clarify these issues. As for the QW samples, it is of interest and perhaps of practical importance that the order of intensities is reversed for QW-related signals of the AlGaAs/GaAs QWs and the AlGaAs/GaAsN QWs compared to that observed for thick films. As seen in Fig. 3 the signal from the GaAsN QWs in sample M1866 is stronger than the signal from GaAs QWs in sample M1865 while the signal from the InGaAsN QWs (sample M1870) still remains the lowest. Apparently the situation with deep centers in GaAsN QWs compared to GaAs QWs is not as bad as for thick films. For more In-rich QWs in type II QW structures M1897 and M1898 as compared to the thick-film structure M1899 the situation is similar to the above described situation for GaAsN, i.e. the intensity of luminescence from QW structures is more than an order of magnitude stronger than from the thick film sample M1899 (see Fig. 4). Since the latter is rather heavily
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lattice mismatched this observation seems to be perfectly reasonable. As mentioned in the previous section we observed a slight increase in the quantum confinement-related energy shifts for GaAsN and InGaAsN QWs compared to GaAs QWs in type I QW structures with AlGaAs barriers (0.22–0.24 eV in GaAsN and InGaAsN as opposed to 0.19 eV in GaAs, see Fig. 3). In principle this increase could be due to somewhat increased N incorporation efficiency in thin structures but the thickness variations in GaAsN or required to explain the difference (48–49 A InGaAsN instead of 54 A in GaAs) are quite small and more serious studies are necessary to prove or disprove such an assumption. 5. Conclusions We have shown that the bandedge luminescence intensity for thick-film structures of InGaAsN decreases in the following order: GaAs homoepitaxial films, GaAsN films with GaAs layer on top, GaAs films on GaAsN underlayer, InGaAsN films on GaAs buffer. Observed changes correlate with corresponding decrease in the density of 0.55 eV electron traps that, according to literature, could be due to off-center substitutional oxygen on arsenic sites. However, in QW structures the situation with luminescence efficiency of GaAsN versus GaAs QWs is greatly improved and, in fact, the latter show a higher intensity of the QW-related signal. The type and intensity of defect bands in InGaAsN films are a strong function of composition (e.g the dominant defect bands are 1.2 eV in homoepitaxial GaAs, 1.165 in GaAs0:9965 N0:0035 films on GaAs buffer, 1 eV in GaAs films on GaAs0:9965 N0:0035 underlayer, none in the GaAsN underlayers of the GaAs/GaAsN structures). Acknowledgements The research at Howard University was supported by the US Air Force Office of Scientific Research Grant no.
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F49620-02-1-0008. The authors would like to thank Mrs. E.F. Astakhova for assistance in samples preparation.
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