Electrical transport properties of a nanorod GaN p-n homojunction grown by molecular-beam epitaxy

Young S. Park, Chang M. Park, J. W. Lee, H. Y. Cho, T. W. Kang, Kyung Hwa Yoo, Min Soo Son, Myung Soo Han

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Abstract

We investigated the electrical properties of a GaN nanorod p-n junction diode patterned on a Si O2 substrate using e-beam lithography. The electron transport mechanisms were characterized by temperature-dependence and current-voltage measurements. At a low temperature, the current-voltage curves showed that the current slowly increased with a given voltage for the forward bias and hardly changed for the reverse bias, indicating the tunneling current dominates through the deep trap barrier. At a high temperature, however, the current-voltage curves exhibited strong temperature dependence suggesting that thermionic emission with an activation energy of 19.1 meV over a barrier dominated.

Original languageEnglish
Article number066107
JournalJournal of Applied Physics
Volume103
Issue number6
DOIs
Publication statusPublished - 2008

Bibliographical note

Funding Information:
p - n homojunction grown by molecular-beam epitaxy Park Young S. 1 a) Park Chang M. 1 Lee J. W. 1 Cho H. Y. 1 Kang T. W. 1 b) Yoo Kyung-Hwa 2 Son Min-Soo 2 Han Myung-Soo 3 1 Quantum Functional Semiconductor Research Center and Department of Physics, Dongguk University , Seoul 100-715, Republic of Korea 2 Department of Physics, Yeonse University , Seoul 120-749, Republic of Korea 3 Micro-Optics Team, Korea Photonics Technology Institute , Gwangju 500-460, Republic of Korea a) Electronic mail: yspark@dongguk.edu. b) Author to whom correspondence should be addressed. Electronic mail: twkang@dongguk.edu. 15 03 2008 103 6 066107 06 11 2007 21 01 2008 26 03 2008 2008-03-26T14:38:44 2008 American Institute of Physics 0021-8979/2008/103(6)/066107/3/ $23.00 We investigated the electrical properties of a GaN nanorod p - n junction diode patterned on a Si O 2 substrate using e-beam lithography. The electron transport mechanisms were characterized by temperature-dependence and current-voltage measurements. At a low temperature, the current-voltage curves showed that the current slowly increased with a given voltage for the forward bias and hardly changed for the reverse bias, indicating the tunneling current dominates through the deep trap barrier. At a high temperature, however, the current-voltage curves exhibited strong temperature dependence suggesting that thermionic emission with an activation energy of 19.1 meV over a barrier dominated. KRF-2006-312-C00162 Low-dimensional structures (nanowires or nanorods) are well known to have great prospects for providing further understanding of fundamental physical science and for new and unique technological applications. 1,2 Because of the large band gap and structural confinements of nanostructures, the fabrication of visible and UV optoelectronic devices, with relatively low power consumption, is potentially feasible. 3,4 Group III-nitride wide band gap semiconductors have attracted much attention because of possible uses in many important applications such as blue/UV light emitting diodes (LEDs), laser diodes, and high temperature/high-power electronic devices. 5,6 Bulk GaN has many defects, such as point and threading dislocations that induce deep levels. However, if we use a nanowire or a nanorod to study electrical and optical properties, we can disregard bulk defects because the nanowire or nanorod is a perfect single crystal. 7 This reduction of defects is expected to improve device performance and facilitate investigation of the nanowire or nanorod’s fundamental properties. The fabrication and characterization of a p - n junction diode for a one-dimensional structure have been very important in fabricating devices. There are a number of models describing the temperature dependent current-voltage ( I - V ) characteristics for the Schottky barrier or p - n junction diode. Studies on the transport mechanism of the GaN p - n junction diode have been reported by Das and Pal, 8 Shah et al. , 9 and Kozodoy et al. 10 A study of the electrical properties of the well-defined p - n junction GaN nanowire diodes, synthesized using vapor-liquid-solid synthesis, has been reported by Chung et al. 11 This study indicated that transport occurs by means of tunneling through a voltage-dependent barrier in the overall temperature range. Although we reported the formation and the deep level properties of the p - n junction in our previous paper, 12 the detailed electrical transport was not discussed. In this paper, we report on the electrical transport mechanism in a GaN nanorod p - n junction diode. In order to study this mechanism, temperature-dependent I - V measurements were performed. The samples used in this study were grown on Si (111) substrates by using rf-plasma assisted molecular-beam epitaxy. The Si substrates were degreased and etched with HF. A reconstructed ( 7 × 7 ) reflection high energy electron diffraction pattern was obtained for the Si substrate after thermal treatment for 30 min at 1000 ° C . Detailed growth conditions of the GaN nanorods have been reported elsewhere. 13 To achieve the n - and p - Ga N nanorods, solid sources of Si and Mg were used as dopants, respectively. The carrier concentrations were evaluated to be 3 × 10 18 cm − 3 for the n - Ga N nanorod and 1 × 10 17 cm − 3 for the p - Ga N nanorod. The carrier concentrations were calibrated by standard n - and p - Ga N epilayers grown under the same temperature as the Si and Mg effusion cell. In order to study the deep levels in the junction of the depletion region, we patterned linear electrodes of Ti ∕ Au between the p - and n - Ga N nanorod regions using e-beam lithography. The Ti ∕ Au electrode showed Ohmic contact. 14 A rapid thermal annealing at 700 ° C for 30 s was performed. To confirm the p - n junction diode structure, cathodoluminescence measurements were carried out by using a MonoCL2™ system installed on field emission scanning electron microscopy equipment with beam energy of 10 kV at 296 K . The detailed result has been reported elsewhere. 12 The I - V characteristic curves of the p - n junction nanorod diode were measured in a temperature range of 6 – 296 K . Figure 1 shows the I - V curve measured at several different temperatures. As shown in this figure, the temperature behavior is divided into two different trends; that is, the electrical transport mechanism is different at the low and high temperature regions. Figure 1(a) represents the I - V curves for the low temperature range from ∼ 6 to ∼ 100 K . The I - V characteristic curve shows nonlinear and clear rectifying behavior at 6 K with a turn on voltage of ∼ 7.2 V for the forward bias and a reverse bias breakdown of ∼ − 9 V . The I - V curves hardly change with temperatures for the reverse bias. However, the current changes of the forward bias slightly increase at a given bias with the increasing temperature. These behaviors are similar with previous results in the temperature range of 20 – 300 K , which means that an electron tunneling through a voltage-dependent barrier dominates. 8,11 Unlike the other research results, the turn on voltage for the forward bias is too high implying the existence of a barrier in the band gap. By increasing the temperature up to ∼ 298 K , the turn on voltage reduces to ∼ 0.4 and ∼ − 0.8 V for the forward and reverse biases, respectively. The I - V curve exhibits strong temperature dependence for the forward and reverse bias suggesting that a single tunneling can be neglected. One interesting feature is that the current rapidly increases with increasing temperature below 2 V for the forward bias. In our pervious paper, we reported on the deep level which was located in the band gap. The electrons, captured in the deep level, start to flow across the p - n junction. Significant reverse bias leakage can be observed in this nanorod diode indicating the presence of trap levels within the band gap due to the intrinsic defect. For many reasons, the I - V characteristics of Schottky or junction diodes deviate from the ideal factor. The current flowing through a p - n junction may be expressed as I = I s { exp [ e ( V − V th ) ∕ n k B T ] − 1 } , where n is the ideality factor, I is the forward current, I s is the reverse saturation current, V is the applied voltage, and V th is the voltage where the current begins to increase in the forward direction (forward bias threshold voltage). n is unity in the Shockley theory for p - n junctions in the absence of recombination, as well as for thermionic emission theory and diffusion theory for Schottky diodes. In order to extract more detailed information, we fitted the I - V curve for all of the temperature range. Figure 2 shows the ideality factor as a function of temperature. We observed that the ideality factor varied from ∼ 2460 for 6 K to ∼ 13 for 298 K , with a large deviation with ideal unity. These were much closer values to the ideal factor than those of Das and Pal 8 near a similar temperature range. Kim et al. 15 reported an ideality factor of 17.8 at room temperature for the Schottky diodes based on a single GaN nanowire. Chung et al. reported an ideality factor of 5.5–6.5 over all of the temperature range. 11 The large deviation of n comes mainly from an insulating interfacial layer between the Al electrode and GaN nanowire or the AlN layer with evaporated Al to form the electrode. In our case, however, we used a Ti ∕ Au electrode for the Ohmic contact which was, for the above reason, neglected. If the forward current in a p - n junction is dominated by recombination of minority carriers injected into the neutral regions of the junctions, the current gives an ideality factor of 1. On the other hand, recombination of carriers in the space charge region, mediated by recombination centers located near the intrinsic Fermi level, results in an ideality factor of 2; that is, only one distribution level at the mid-band-gap region with identical capture-cross section for electrons and holes contributes an ideality factor of 2. The high ideality factors ( n ⪢ 2 ) in GaN-based LEDs were attributed to deep level assisted tunneling due to temperature-independent I - V characteristics. We have already reported the deep level located near 0.4 eV below the conduction band. The inset shows the dependence of n as a function of the inverse of temperature. n can be expressed as n = A + B ∕ T for the p - n homojunction. 16 As expected earlier, the slope is divided into two regions; that is, one region is high temperature and the other is low temperature. We calculated the values of A and B for the high temperature region to be − 69.7 , and 2.26 × 10 4 K , respectively. These agreed well with the results of Das and Pal. 8 However, the values of those for the low temperature region were calculated to be 82.3 and 1.35 × 10 4 K , respectively. At low temperatures, the quantum tunneling of electrons through the energy barrier formed by a difference energy level at k space between the conduction band and deep defect level would become a main transport mechanism, indicating that the thermionic emission current can be neglected. At high temperature, however, the electrons pass over the barrier with thermal activation energy. The resistance linearly decreases from 0.05 to 0.0034 M Ω with the increasing temperature from ∼ 100 to 296 K . The temperature-dependent resistance near the zero bias voltage could be fitted to the thermal activation energy in the form of exp ( E a ∕ k B T ) . The temperature dependence of the linear resistance is well fitted to the thermal activation energy form at the high temperature region as shown in Fig. 3 . We calculated the value of the activation energy to be 19.1 meV . The activation energy can be related to the energy difference ( E C F ) between the conduction band and the Fermi level. This energy barrier plays a role in barring an electron’s flow through the p - n junction. In conclusion, we have investigated the electrical transport for a GaN nanorod p - n junction diode grown by molecular-beam epitaxy. At a low temperature, the current-voltage curves show that the current slowly increases with a given voltage for the forward bias and hardly changes for the reverse bias, indicating that the tunneling current is overwhelmed through the deep trap barrier. At a high temperature, on the other hand, the current-voltage curves exhibit strong temperature dependence, suggesting that thermionic emission, with activation energy of 19.1 meV over a barrier formed by the deep level, dominates. This work was supported by a Korea Research Foundation Grant funded by the Korean government (MOEHRD, Basic Research Promotion Fund) (KRF-2006-312-C00162), QSRC, and the Terabit Level Nano Device Project as part of the 21st Century Frontier Project at Dongguk University. FIG. 1. The temperature-dependent I - V characteristic curve of a p - n junction GaN nanorod diode. (a) The low temperature range from 6 to ∼ 100 K . (b) The high temperature range above 200 K . The inset shows the logarithmic current vs the forward bias voltage. FIG. 2. The variation of the ideality factor as a function of temperature. The inset shows the ideality factor as a function of the inverse temperature. FIG. 3. Resistance as a function of inverse temperature near the zero bias voltage for the p - n junction diode.

All Science Journal Classification (ASJC) codes

  • Physics and Astronomy(all)

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