Magnetotransport study on as-grown and annealed n- and p-type modulation-doped GaInNAs/GaAs strained quantum well structures
© Dönmez et al.; licensee Springer. 2014
Received: 26 November 2013
Accepted: 6 March 2014
Published: 24 March 2014
We report the observation of thermal annealing- and nitrogen-induced effects on electronic transport properties of as-grown and annealed n- and p-type modulation-doped Ga1 - xIn x N y As1 - y (x = 0.32, y = 0, 0.009, and 0.012) strained quantum well (QW) structures using magnetotransport measurements. Strong and well-resolved Shubnikov de Haas (SdH) oscillations are observed at magnetic fields as low as 3 T and persist to temperatures as high as 20 K, which are used to determine effective mass, 2D carrier density, and Fermi energy. The analysis of temperature dependence of SdH oscillations revealed that the electron mass enhances with increasing nitrogen content. Furthermore, even the current theory of dilute nitrides does not predict a change in hole effective mass; nitrogen dependency of hole effective mass is found and attributed to both strain- and confinement-induced effects on the valence band. Both electron and hole effective masses are changed after thermal annealing process. Although all samples were doped with the same density, the presence of nitrogen in n-type material gives rise to an enhancement in the 2D electron density compared to the 2D hole density as a result of enhanced effective mass due to the effect of nitrogen on conduction band. Our results reveal that effective mass and 2D carrier density can be tailored by nitrogen composition and thermal annealing-induced effects.
72.00.00; 72.15.Gd; 72.80.Ey
KeywordsGaInNAs Magnetotransport Shubnikov de Haas Transport Nitrogen-dependent effective mass
Dilute nitrides are technologically important materials due to their promising physical properties and potential application in optoelectronic technology. The strong nitrogen dependence of the bandgap energy makes dilute nitrides promising candidate for device applications, operating in near infrared region [1–3]. Therefore, in order to fully determine fundamental physical properties of this unconventional alloy system, an intense research has been devoted since its discovery. Much effort has been spent developing theoretical models and understanding peculiar nitrogen-induced effects on optical properties of dilute nitrides [1, 4–6]. Although the strong composition dependence of the bandgap energy compared to the conventional III-V alloys is attractive, it has been soon realized that the presence of nitrogen severely degrades the optical quality. Therefore, thermal annealing is commonly used a standard procedure to improve the optical quality of dilute nitrides, but at the expense of the blueshift of the bandgap [1, 7].
From the electronic properties' point of view, it has been demonstrated that incorporation of nitrogen gives rise to drastic decrease in electron mobility due to the N-induced scattering centers and enhanced electron effective mass [8–13]. On the contrary, in the presence of the nitrogen, it has been theoretically demonstrated that hole effective mass and hole mobility remain unaffected [14–16]. So far, much effort has been focused on nitrogen dependence of electron effective mass and electron mobility, ignoring the composition dependence of hole effective mass and hole mobility. Moreover, even it has been accepted as a standard procedure to improve optical quality, the effects of thermal annealing on electronic properties has not been considered.
The aim of the study presented here is to investigate the effect of nitrogen composition and thermal annealing on electronic transport properties of n- and p-type modulation-doped Ga0.68In0.32N y As1 - y/GaAs (y = 0, 0.009, and 0.012) strained quantum well (QW) structures.
The samples were grown on semi-insulating GaAs (100) substrates using solid source molecular beam epitaxy, equipped with a radio frequency plasma source for nitrogen incorporation. XRD measurements were used to determine nitrogen and indium compositions. The sample structures are comprised of 7.5-nm-thick QW with indium concentration of 32% and various nitrogen concentration (N% = 0, 0.9, and 1.2) and 20 nm doped (Be for p-type and Si for n-type) GaAs barriers. A 5-nm GaAs was used between GaInNAs and GaAs layer to separate charge and doping regions. The growth temperatures of GaInNAs, GaInAs, and GaAs were 420°C, 540°C, and 580°C, respectively. Post growth rapid thermal annealing was applied at 700°C for 60 and 600 s. The doping density was the same for both n- and p-type samples as 1 × 1018 cm-3. The samples were fabricated in Hall bar shapes, and ohmic contacts were formed by alloying Au/Ge/Ni and Au/Zn for n- and p-type samples, respectively.
Magnetotransport measurements were carried out using a 4He cryostat equipped with a 7 T superconducting magnet. In-plane effective mass, 2D carrier density, and Fermi energy were determined by analyzing the Shubnikov de Haas (SdH) oscillations as a function of temperature between 6.1 and 20 K. In order to evaluate the obtained results from SdH analysis, influence of nitrogen and thermal annealing on the bandgap was probed using photoluminescence (PL) measurements. PL was excited with an argon ion laser (514 nm), dispersed with a 0.5-m monochromator and detected with a thermo-cooled GaInAs photodetector.
Results and discussion
where Δρ xx , ρ0, EF, E1, ω c , m *, τ q , and μ q are the oscillatory magnetoresistivity, zero-field resistivity, Fermi energy, first subband energy, cyclotron frequency, effective mass, quantum lifetime of 2D carriers, and carrier mobility, respectively. The i represents the subbands. In Equation 1, the temperature dependence of the amplitude of the oscillations is included in the function D(χ). The exponential function in Equation 1 represents the damping of the oscillations due to the collision-induced broadening of Landau levels. The contribution of the higher subbands appears in SdH oscillations with different periodicity. We observed that the SdH oscillations has only one period, indicating that only the lowest subband is occupied. The observation of diminishing minima is an indication of absence of background magnetoresistance and presence of 2D carrier gas.
As seen in Figure 1a, the SdH oscillations are suppressed by either a positive (for N-free sample) or a negative (especially for n-type N-containing sample) background magnetoresistance. The minima of SdH oscillations decrease as the magnetic field increases for p-type N-containing samples due to negligible negative magnetoresistance than that of n-type sample. As for N-free samples, a pronounced positive magnetoresistance causes minima to increase with the magnetic field. The origin of the positive magnetoresistance is parallel conduction due to undepleted carriers in barrier layer, herein GaAs. On the other hand, the weak localization effect leads to negative magnetoresistance [19, 20]. The background magnetotransport makes the analysis of SdH oscillations difficult especially at low magnetic fields and high temperatures. In order to exclude the effect of the background magnetoresistance and to extract the SdH oscillations, we used the negative second derivative with respect to the magnetic field of raw magnetoresistance data (-∂2R xx /∂B2) (see Figure 1b). As can be easily seen from Equation 1, this method does not change the position of the peak or period of the oscillations and enables to subtract the slowly changing background magnetoresistance and amplifies the short-period oscillations [18, 19] as depicted in Figure 1b.
PL peak energies and observed blueshift amounts at 30 K
PL peak energy (eV)
Annealed (60 s)
Annealed (600 s)
Annealed (60 s)
Annealed (600 s)
Annealed (60 s)
Annealed (600 s)
Effective mass, 2D carrier density, and Fermi energy values found from analysis of SdH oscillations
Annealed (60 s)
Annealed (600 s)
Annealed (60 s)
Annealed (600 s)
Annealed (60 s)
Annealed (600 s)
Although all samples were doped with the same doping concentration, among n-type samples, among n-type samples, N-free ones have the lowest electron density. Moreover, the hole density is less than the electron density for the samples with the same nitrogen content. An enhancement of electron concentration in N-containing samples compared to the N-free ones was also observed in previous studies [8, 14–16] and explained in accordance with the BAC model, since N-induced flattening of conduction band leads to an increased density of states of electrons therefore a significant increase in 2D electron density. Upon thermal annealing, 2D electron density tends to increase in N-containing samples as a result of enhanced electron effective mass. As a result of almost thermal annealing insensitive effective hole mass, 2D hole density remains unaffected for the sample with 0.9% nitrogen. As nitrogen composition increases to 1.2%, the observed decrease in effective hole mass causes to reduce 2D hole density. The calculated Fermi energies change depending on both 2D carrier and effective mass, which are influenced by nitrogen composition and thermal-annealing-induced effects.
We have investigated the effect of nitrogen and thermal annealing on electronic transport properties of n- and p-type N-free and N-containing alloys using magnetotransport measurements. With an analysis of SdH oscillations at different temperatures, we have calculated in-plane effective carrier mass, 2D carrier density, and Fermi energy of the samples. Nitrogen-dependent enhancement of the both electron and hole masses has been observed in as-grown samples. Upon thermal annealing, the electron effective mass increased, whereas hole mass tends to decrease. The observed nitrogen dependence of electron mass has been explained in terms of strengthened interaction between localized nitrogen level and conduction band states. A tendency to decrease in hole mass upon annealing can be attributed to the reduction of well width and/or decrease in hole density. Even all samples have the same dopant density, the observation of higher 2D electron density than that of p-type samples with the same nitrogen composition and N-free samples has been explained with a stronger interaction of N level and conduction band states, which gives rise to enhancement of the density of states. The results revealed that effective mass in dilute nitride alloys can be tailored by nitrogen composition and also thermal-annealing-induced effects.
Shubnikov de Haas.
This work is supported by the TUBITAK project (project number 110 T874) and Istanbul University Scientific Research Projects Unit (project number IRP 9571) and The Ministry of Development, Turkey (project number 2010 K121050). We also acknowledge to the COST Action MP085 for enabling collaboration possibilities.
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