at the ordered GaInNP to GaAs heterointerface
H. P. Hsu1, Y. N. Huang1, Y. S. Huang*, 1, P. Sitarek2, K. K. Tiong3, and C. W. Tu4
1 Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
2 Institute of Physics, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, Wrocław 50-370, Poland
3 Department of Electrical Engineering, National Taiwan Ocean University, Keelung 202, Taiwan
4 Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, CA 92093-0407, USA Received 3 July 2008, revised 29 November 2008, accepted 7 December 2008
Published online 25 March 2009
PACS 78.20.Ci, 78.20.Hp, 78.55.Cr, 78.66.Fd
* Corresponding author: e-mail [email protected], Phone: + 886 2 27376385, Fax: + 886 2 27376424
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Lattice-matched Ga0.52In0.48P grown on a GaAs substrate has received considerable attention due to its potential applications in optoelectronic and electronic devices, such as semiconductor lasers [1], heterojunction bipolar transistors (HBTs) [2], and high-efficiency tandem solar cells [3]. Spontaneous ordering in Ga0.52In0.48P has been widely investigated in recent years [4 – 6]. Depending on the growth conditions, Ga0.52In0.48P could be grown on GaAs substrates in an orderly or a dis-orderly fashion which results in the discrepancy of band gap energy [7]. The ordering phenomenon in such com-pounds is a very important property as it allows changes in the electronic and optical properties.
A small amount of nitrogen incorporation is known to reduce the band gap energy in GaxIn1–xAs dramatically; the reduction results mostly from the lowering of the conduc-tion band [8]. A similar effect in nitrogen incorporated GaxIn1–xP has been reported [9, 10]. These properties thus, make GaxIn1–xNyP1–y a suitable candidate for the emitter and the collector of HBTs, especially for the blocked hole HBTs with a reduced offset and knee voltages [11].
De-spite its possible applications, to date there have been few reports on GaxIn1–xNyP1–y [12, 13].
In this study, we employ polarized piezoreflectance (PzR) and photoreflectance (PR) measurements to study the effects of nitrogen incorporation on Ga0.46In0.54NyP1–y al-loys. The Ga0.46In0.54NyP1–y epilayers were grown on GaAs(100) semi-insulating substrates by gas-source mo-lecular-beam epitaxy (GSMBE). The anisotropic optical properties of Ga0.46In0.54NyP1–y epitaxial layers along the [110] and [110] directions are characterized by polarized PzR spectra. The additional features below GaAs band edge region are observed in PR spectra. The origin of these in-ter-subband transition features are discussed and are attrib-uted to the type-II band alignment in Ga0.46In0.54NyP1–y/GaAs heterostructures.
2 Experimental detail The Ga0.46In0.54NyP1–y epilay-ers studied in this work were grown on GaAs(100) semi-insulating substrates by GSMBE using elemental gallium (Ga) and indium (In), thermally cracked arsin (AH3) and phosphine (PH3), and a RF plasma nitrogen radical beam Polarized piezoreflectance (PzR) and photoreflectance (PR) are
employed to study band alignment in Ga0.46In0.54NyP1–y/GaAs heterostructures grown by gas-source molecular-beam epi-taxy. The features near the band edge of Ga0.46In0.54NyP1–y show strong polarization dependence, indicating the existence of some degree of ordering of these samples. The PR spectra exhibit Franz – Keldysh Oscillations (FKOs) above the band edge of GaAs. The electric fields in the GaAs region are
eva-luated by analyzing the FKOs and found to decrease with in-creasing nitrogen content. The type-II band alignment at the Ga0.46In0.54NyP1–y/GaAs interface is concluded for the alloys with nitrogen content y larger than 0.5% based on the ap-pearance of additional features below band edge of GaAs. These features are attributed to the spatially indirect type-II transitions in the vicinity of interface region between Ga0.46In0.54NyP1–y and GaAs.
804 H. P. Hsu et al.: Type-II band alignment at the ordered GaInNP to GaAs heterointerface
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statussource. After removing the surface oxide from the GaAs substrate at 620 °C under As2 flux, a 1000 Å thick buffer GaAs layer was grown. The substrate temperature was then lowered to the growth temperature which was controlled between 380 °C and 480 °C and the nitrogen plasma was ignited. Growth was monitored by reflection high energy electron diffraction (RHEED). Four undoped 2350 Å thick Ga0.46In0.54NyP1–y layers with different nitrogen concentra-tions (denoted as samples A, B, C and D) were grown for this investigation. Sample A was the reference sample with no nitrogen content. Assuming that nitrogen incorporation does not affect indium composition, the nitrogen contents for samples B, C and D were then determined to be 0.5%, 1% and 2%, respectively, by X-ray diffraction measure-ments.
The PzR measurements were achieved by gluing the thin samples on a 0.15 cm thick lead zirconate titanate (PZT) piezoelectric transducer driven by a 200 V rms sinu-soidal source was at 200 Hz. The alternating expansion and contraction of the transducer subjected the sample to an al-ternating strain with a typical rms Δl/l value of ~10–5. In PR measurement, a mechanically chopped 325 nm line (~ 1 mW) of the He – Cd laser or a 670 nm laser diode served as the pumping beam. The modulating frequency was set at 200 Hz. The CASIX Rochon prisms were em-ployed for polarization dependent measurements. The ra-diation from a 150 W tungsten-halogen lamp filtered by a 0.25 m monochromator provided the monochromatic light.
The reflected light was detected by an UV-enhanced sili-con photodiode. The dc output of this photodiode was maintained constant by a servomechanism of a variable neutral density filter. A dual-phase lock-in amplifier was used to measure the detected signals. The entire data ac-quisition procedure was performed under computer control.
Multiple scans over a given photon energy range was pro-grammed until a desired signal-to-noise level has been at-tained.
3 Results and discussion Figure 1 shows room tem-perature PzR and PR spectra in the vicinity of band-edge transitions for four Ga0.46In0.54NyP1–y samples with different nitrogen contents y. The dashed and dotted curves repre-sent the experimental PzR and PR spectra, respectively.
The solid curves are least-squares fits to the first derivative Lorentzian lineshape function of the form [14, 15]
1 of the transitions, and the value of n depends on the origin of transitions. The energies of the band gaps obtained from the fits are indicated by arrows. As shown in Fig. 1, with nitrogen incorporation the linewidth increases due to alloy scattering and the red shifts of the PzR and PR features in-dicates band gap reduction. It is well know that the incor-
Figure 1 Room temperature PzR and PR spectra for Ga0.46In0.54NyP1–y samples with different nitrogen contents. The fit-ted band gap energy positions are indicafit-ted by arrows.
poration of a small amount of nitrogen in III – V semicon-ductors such as GaAs [16], GaP [17] and InP [18] results in a strong reduction of the band gap in these materials.
The reduction of band gap energy in Ga0.46In0.54NyP1–y can be described by a quadratic correlation ΔEg(y) = by(y – 1) [16, 17], where ΔEg(y) is the value of band gap reduction and b is the bowing coefficient. The obtained bowing coef-ficient b = 10.5 eV is found to be of the same order as the corresponding values associated with the incorporation of nitrogen in GaAs (10 – 20 eV) [16], GaP (14 eV) [17] and InP (16 eV) [18].
Figure 2 illustrates the room temperature polarized-PzR spectra of four Ga0.46In0.54NyP1–y samples with different N contents for E || [110] and E || [110] polarization. The features near band edge are strongly polarization depend-ent, indicating the existence of some degree of ordering for these samples. As seen in Fig. 2, the PzR spectra at 300 K show three distinct critical points transitions, denoted as E0, E0 + Δs and E0 + Δ0, as indicated by the vertical arrows in Fig. 2. Feature E0 is the fundamental band gap, E0 + Δs is
Figure 2 Room temperature polarized PzR spectra for Ga0.46In0.54NyP1–y samples with different nitrogen contents. The en-ergies of the near band edge transitions E0, E0 + Δs, and E0 + Δ0
obtained from fits are indicated by arrows.
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Table 1 Near band edge critical point transition energies E0, E0 + Δs, and E0 + Δ0for Ga0.46In0.54NyP1–y epilayers and energy level at GaInNP/GaAs interface with different nitrogen contents at room temperature obtained from PzR an PR measurements.
energy level at interface (eV)
samples E0
electric field at interface (kV/cm) Table 1. It should be noted that both the strain and atomic ordering induced the valence band splitting, while strain, ordering and clustering can all lead to a band gap reduction [19]. The existence of spontaneous ordering in GaInNP promoted by nitrogen incorporation was also supported by the high-resolution transmission electron microscopy [20]
as well as by the Raman studies [21].
Figure 3 illustrates room-temperature PR spectra in the range from 1.25 eV to 1.55 eV. The PR spectra exhibit Franz – Keldysh oscillations (FKOs) above the band edge of GaAs. The period of the FKOs is a direct measurement of the built-in electric field at the interface between GaAs buffer layer and Ga0.46In0.54NyP1–y. The position of the n-th extremum in the FKOs is given by [12]
0 3/2
π (4/3) [( n )/ ] ,
n = E -E Θ +χ (2)
where En is the photon energy of the n-th extremum, E0 is the band gap of GaAs, and χ is an arbitrary phase factor.
The electro-optic energy, is given by Θ,
3 2 2 2
(Θ) =q F /2 ,μII (3)
Figure 3 Room temperature PR spectra for Ga0.46In0.54NyP1–y sam-ples in the range of 1.25 eV to 1.55 eV. The transitions from Ga0.46In0.54NyP1–y/GaAs heterointerface are indicated by arrows.
where F is the electric field and μII is given by electron and heavy-hole along (001), respectively, in units of the free electron mass. The relevant electron ( )m and e* heavy-hole [mhh*(001)] are 0.067 and 0.34, respectively, for GaAs [22]. It has been demonstrated that the FKOs in the region of the direct gap are determined by m because of hh* the greater density of states in relation to the light-hole band [23]. From the FKOs, the electric field in the GaAs region for sample A, B, C and D is evaluated to be 21, 14, 12 and 11 kV/cm, respectively, showing a monotonous de-crease of the electric field as the nitrogen content inde-creases.
Figure 4 depicts the DC optical bias dependent PR spectra of sample B in the range between 1.32 eV and 1.52 eV. The optical bias was provided by an additional steady-state illumination of a 780 nm laser diode. FKOs above the GaAs band edge and inter-subband transition features below GaAs band edge were observed in the PR spectra. The built-in electric field determined from the pe-riod of the FKOs decreases slightly and the inter-subband transition energies shift toward lower energy region with the increase of the intensity of the DC optical bias. The decrease of the built-in electric field and redshift of inter-subband transition energies with the increase of the inten-sity of the DC optical bias can be understood by the existence of the interface states. As noted in Fig. 4, the decrease of the electric field reduces the intensity of the transition at ~ 1.38 eV. This indicates that it is an electric field tunnelling assisted transition and will be further dis-cussed in the following section.
As shown in Fig. 3, additional features (indicated by arrows) appear in the PR spectra at energies lower than the band gap energy E0 of GaAs for nitrogen incorporated samples. These features can be attributed to the transitions between the confined levels in the triangle potential well at the interface between GaAs and Ga0.46In0.54NyP1–y. As is well known, the band alignment for Ga0.46In0.54P/GaAs is type I (see Fig. 5(a)). The incorporation of nitrogen re-duces the band gap energy in Ga0.46In0.54NyP1–y with most of the reduction results from lowering the conduction band.
806 H. P. Hsu et al.: Type-II band alignment at the ordered GaInNP to GaAs heterointerface
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statusFigure 4 PR spectra of sample with nitrogen = 0.5% under dif-ferent optical bias in the range of 1.25 – 1.52 eV. The inset shows plots of (4/3π) (En – E0)3/2 as a function of the index n which la-bels the extrema.
The band alignment changes from type-I to type-II when the nitrogen contents become larger than 0.5%. As shown in Fig. 5(b) the energy minima for electrons and holes lie in different layers. This means that spatially separated elec-trons and holes are easily realized in such a system, in which electrons are confined in the Ga0.46In0.54NyP1–y and holes are localized in the GaAs layer. It is expected that the separation of opposite charges produces an electric field across the interface. A certain overlap of carrier wave functions allows indirect inter-subband transitions. Due to the electric field the wave functions can penetrate to for-bidden energy levels and the tunnelling–assisted transitions can take place even if there is a thin interfacial layer.
As can be seen in Fig. 5(b) triangular potential wells are formed in the conduction band and valence band at dif-ferent sides of interface of Ga0.46In0.54NyP1–y/GaAs het-erostructures. Larger nitrogen concentration implies a deeper triangular well and confined interface states inside the well. This line of reasoning is well illustrated by the experimental observation where only one inter-subband transition is observed in sample B in contrast to the obser-vation of two and three transitions in samples C and D, re-spectively. All inter-subband transition features have been well fitted to Eq. (1) using the first derivative of a Lor-entzian functional form with n = 2, which corresponds to an excitonic transition. The energies corresponding to each inter-subband transition are indicated by the arrows in Fig. 3 and also included in Table 1. From the nature of excitonic transitions and the fact of slightly redshift of inter-subband transition energies we attribute the below band edge features to the spatially separated electron – hole
Figure 5 Schematic diagram of the energy band alignment of Ga0.46In0.54NyP1–y/GaAs heterojunctions: (a) Type-I alignment for Ga0.46In0.54P/GaAs, (b) type-II alignment for Ga0.46In0.54NyP1–y/GaAs.
The dashed line represents the influence of the band-bending ef-fect under optical bias.
transitions in the interfaces, namely the inter-subband tran-sitions between holes on GaAs side and electrons on Ga0.46In0.54NyP1–y side, and therefore we conclude the type-II band alignment in the Ga0.46In0.54NyP1–y/GaAs interface. The suggested type-II alignment between Ga0.46In0.54NyP1–y/ GaAs agrees well with the recent report in the literature [24]. Izadifard et al. reported the observation of highly ef-ficient PL upconversion and appearance of a near-infrared emission in the nitrogen containing GaInNP/GaAs samples.
These two effects have been considered to be the charac-teristic signature of the spatially indirect type-II band alignment in the GaInNP/GaAs interface.
4 Summary Polarized piezoreflectance and photore-flectance are used to study the effects of nitrogen incorpo-ration on GSMBE grown Ga0.46In0.54NyP1–y epilayers on GaAs substrates. The features near the band edge of Ga0.46In0.54NyP1–y show strong polarization dependence, in-dicating the existence of some degree of ordering of the samples. The PR spectra exhibit FKOs above the band edge of GaAs. The electric fields in the GaAs region are evaluated by analyzing the FKOs and found to decrease with increasing nitrogen content. Additional features below band edge of GaAs are observed in nitrogen containing samples and are attributed to the spatially indirect type-II transitions in the vicinity of interface region between Ga0.46In0.54NyP1–y and GaAs.
Acknowledgements The authors acknowledge the sup-ports of National Science Council of Taiwan under Project No. NSC 96-2221-E-011-030 and the project-based personnel exchange programme between the NSC and PAS Project No. NSC96-2911-I-011-003.
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