Electron transport in a GaPSb film
© Lo et al.; licensee Springer. 2012
Received: 27 June 2012
Accepted: 12 October 2012
Published: 23 November 2012
We have performed transport measurements on a gallium phosphide antimonide (GaPSb) film grown on GaAs. At low temperatures (T), transport is governed by three-dimensional Mott variable range hopping (VRH) due to strong localization. Therefore, electron–electron interactions are not significant in GaPSb. With increasing T, the coexistence of VRH conduction and the activated behavior with a gap of 20 meV is found. The fact that the measured gap is comparable to the thermal broadening at room temperature (approximately 25 meV) demonstrates that electrons can be thermally activated in an intrinsic GaPSb film. Moreover, the observed carrier density dependence on temperature also supports the coexistence of VRH and the activated behavior. It is shown that the carriers are delocalized either with increasing temperature or magnetic field in GaPSb. Our new experimental results provide important information regarding GaPSb which may well lay the foundation for possible GaPSb-based device applications such as in high-electron-mobility transistor and heterojunction bipolar transistors.
III-V-based alloys and heterostructures have been attracting much interest because of their great device applications as well as their fundamental importance. A major issue of composing III-V-based systems is the miscibility gap in solids. It is known that the mixing enthalpy of the systems, such as GaAs-GaSb and GaP-GaSb, is proportional to the square of the difference in lattice constant of the two end binary components of the system. This reason prohibits the epitaxial growth of most alloys at ordinary growth temperatures. Therefore, the epitaxial growth of these systems was not achieved until the first growth of GaAs-GaSb was done in 1979 by carrying out the growth under a high-supersaturation condition, such as molecular beam epitaxy (MBE). The ternary alloy gallium phosphide antimonide (GaP1−xSb x ) was grown on GaAs for the first time using the organometallic vapor-phase epitaxy method in 1988. Since then, there has been a lack of work on gallium phosphide antimonide as well as on the transport behavior in such material.
with the exponent p = 1/3 for two-dimensional (2D) systems or p = 1/4 for three-dimensional (3D) ones. Here, R0 is a prefactor and T0 is the characteristic temperature related to the localization length. When considering interactions, the suppression of density of states near the Fermi energy would lead to p = 1/2 for both 2D and 3D systems in Equation 1, known as Efros-Shklovskii VRH. Another important phenomenon in the strong localization regime is referred to as negative magnetoresistance (NMR) which results from the suppression of quantum interference between forward scattering hopping paths as the magnetic field (B) is applied[8–11]. On the other hand, at high B where the hopping probability between different sites is significantly reduced due to the shrinkage of wave function, positive magnetoresistance (PMR) occurs. NMR and PMR refer to the case where the resistance decreases and increases, respectively, with increasing B.
with Boltzmann constant (kB), activation energy (Ea), and a prefactor (Ra). Such a crossover from VRH conduction to an activation one with increasing T has already been observed in Si delta-doped GaAs grown by MBE.
Since there is a dearth of work on the transport properties of GaPSb, important physical phenomena such as the type of carriers, the strength of carrier-carrier interactions, transport behavior, and so on require further investigations. In this work, we report extensive transport studies of a GaPSb film grown on a GaAs substrate. Such a device is fully compatible with the existing GaAs-based high-electron-mobility transistor (HEMT) technology. Moreover, the GaPSb-based material system may well be of great device applications in heterojunction bipolar transistor (HBT), high-power devices, and nanoelectronics[14–16]. We shall show that the carriers in GaPSb are electrons. Moreover, at low temperatures, electrons in GaPSb are strongly localized and can be described by 3D Mott VRH. Therefore, electron–electron interactions are negligible in GaPSb. Furthermore, we show that VRH and activation conduction can coexist, which is consistent with the observed peculiar T dependence of n. The measured gap from the observed activated behavior (approximately 20 meV) is comparable to thermal broadening at room temperature (approximately 25 meV). Such a result suggests that electrons are delocalized in nominally undoped GaPSb at room temperature. Our new experimental results provide important information for possible device applications as well as modeling using the GaPSb-based materials.
The undoped 720-nm-thick GaP0.71Sb0.29 was grown on a 4-in. (100) 2° off-axis toward (110) GaAs substrate by an Aixtron 2600G3 (Aixtron SE, Aachen, Germany) metal organic chemical vapor deposition, using trimethygallium and trimethyantimony as metal organic sources and phosphine (PH3) and arsine (AsH3) as hydride sources. The growth conditions for the GaP0.71Sb0.29 are similar to those published in. We used Vegard's law to determine the averaged Sb composition in this GaP0.71Sb0.29 bulk sample by X-ray diffraction. The reactor was heated up to 700°C to clean the substrate surface with AsH3 before the epitaxial growth and then cooled down to 530°C for the GaPSb growth. The V/III ratio and growth rate were approximately 1 and 1 μm/h, respectively. On top of the GaP0.71Sb0.29 film, a 20-nm GaAs cap was deposited. The Hall bar device was fabricated by standard photolithography and etched by a top-down process with a solution at the mixture ratio of H3PO4/H2O2/H2O = 1:1:10. Four-terminal measurements were performed by standard dc techniques in a top-loading He3 cryostat.
Results and discussion
For carriers in the conduction band, diffusive motion can dominate the transport, in which nonlinear V(I) does not occur. As inferred from the NMR shown in Figure2a, delocalization of carriers occurs with increasing B as well. However, as known from the nonlinear V(I) at various B for T = 4.2 K shown in Figure3, the carriers are still strongly localized. Therefore, the observed NMR should result from the quantum interference feature in the strong localization regime, which is different from the T-induced delocalization process.
We have performed extensive transport measurements on a GaPSb film grown by MOCVD. At a low T, variable range hopping dominates the transport, and then, a crossover from VRH to activation conduction occurs with increasing T. In the intermediate temperature range, coexistence of the VRH and activation conduction can be found, consistent with the observation of peculiar T dependence of carrier concentration. The observed nonlinear current–voltage relations further support that the carriers are strongly localized at low temperatures. The carriers can be delocalized either with increasing T or increasing B. The measured transport gap of approximately 20 meV is comparable to thermal broadening at room temperature (approximately 25 meV). Therefore, our results show that electrons can be thermally activated even in an intrinsic GaSbP film at room temperature. This interesting result may find applications in designing a transistor with a GaPSb base. Moreover, since our GaPSb film is grown on GaAs, such a device is fully compatible with existing GaAs-based HEMT technology. Our work may be useful for possible applications such as HEMT and HBT devices based on the GaPSb material system.
STL obtained his B.Sc. degree at National Taiwan University (NTU) in 2010 and is pursuing his Ph.D. degree at the Graduate Institute of Applied Physics, NTU. He won the Dr. An-Tai Chen Scholarship, Mr. Ming Kao Scholarship, and Creative Award for college students participating in special research project provided by the NSC in 2009. HEL obtained his master's degree at NTU in 2011. SWW obtained his master's degree at NTU in 2011. He won the prestigious Lam Research Award in 2012. HDL obtained his B.S. degree at Chinese Culture University, Taiwan, and his Ph.D. degree at Mississippi State University, USA, and currently works as a project engineer at the Electronics Testing Center, Tao-Yuan, Taiwan (ROC). YCC received his B.S. degree in physics from NTU in 1996 and his MSEE degree from National Chiao-Tung University (NCTU) in 2000. He currently is a Ph.D. student studying at the Graduate Institute of Electronics Engineering, NTU. HHL obtained his BSEE, MSEE, and Ph.D. degree at NTU and is currently a professor of the Department of Electrical Engineering, NTU. JCL obtained his master's degree at NTU in 2012. CTL obtained his B.Sc. degree at NTU in 1990 and his Ph.D. degree in physics at Cambridge University, UK, in 1996 and is currently a professor of physics at NTU. He is also a topical editor for Current Applied Physics and an associate editor for the Journal of Nanoscience Letters.
This work was funded by the NSC, Taiwan and by the National Taiwan University (grant number: 101R7552-2).
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