Effects of Surface Treatment Using Aqua Regia Solution p-type GaN
Journal on theof Change ELECTRONIC of Surface MATERIALS, Band Vol. 30, Bending No. 3, 2001 of
Special Issue Paper 129
Effects of Surface Treatment Using Aqua Regia Solution on the Change of Surface Band Bending of p-type GaN JONG KYU KIM,1 KI-JEONG KIM,2 BONGSOO KIM,2 JAE NAM KIM, 3 JOON SEOP KWAK,4 YONG JO PARK,4 and JONG-LAM LEE1,5 1.—Pohang University of Science and Technology (POSTECH), Department of Materials Science and Engineering, Pohang, Kyungbuk 790-784, Korea. 2.—Pohang University of Science and Technology (POSTECH), Beamline Research Division, Pohang Accelerator Laboratory, Pohang, Kyungbuk 790-784, Korea. 3.—Research Institute of Industrial Science & Technology (RIST), Pohang, Kyungbuk 790-784, Korea. 4.—Samsung Advanced Institute of Technology, Photonics Laboratory, Suwon 440-600, Korea. 5.—e-mail:
[email protected]
Effects of surface treatment on the change of band bending at the surface of ptype GaN were studied using synchrotron radiation photoemission spectroscopy, and the results were used to interpret the reduction of contact resistivity by the surface treatment. The contact resistivity on p-type GaN decreased from (5.1±1.2)×10–1 to (9.3±3.5) × 10 –5 Ω cm2 by the surface treatment using aqua regia prior to Pt deposition. Surface band bending was reduced by 0.58 eV and 0.87 eV after the surface treatments by HCl and aqua regia solutions, respectively. The atomic ratio of Ga/N decreased as the photoelectron detection angle was decreased, indicating that the surface oxide was mainly composed of Ga and O, GaOx , formed during high-temperature annealing for the generation of holes, and Ga vacancies, VGa, were produced below the GaO x layer. Consequently, the aqua regia treatment plays a role in removing GaOx formed on p-type GaN, leading to the shift of the Fermi level toward the energy levels of VGa located near the valence band edge. This causes the decrease of barrier height for the transport of holes, resulting in the good ohmic contacts to p-type GaN. Key words: Fermi level, band bending, ohmic contact, p-type GaN, photo-emission spectroscopy
INTRODUCTION Low resistance ohmic contacts to both n-type and ptype GaN are essential for the realization of GaNbased optoelectronic devices1–3 such as blue light emitting diodes (LEDs) and laser diodes (LDs). For ntype GaN, low resistance ohmic contacts (1000°C) and annealing for generation of holes (>700°C). During such high-temperature processes, oxides could be formed at the surface of p-type GaN leaving point defects below the oxide. In this case, the surface Fermi level might be pinned at an energy level of the surface states in the oxides. When the surface oxides are removed by certain treatment, the Fermi level position would shift to the energy level of point defects formed below the oxides. Therefore, the barrier height at the metal/GaN interface, thus the contact resistivity could be changed with the kind of oxide and point defects below the oxide that determine the Fermi level pinning position. To date, no result about this was experimentally provided. In the present work, we studied the effect of pretreatment of the surface of p-type GaN on the change of surface band bending in order to interpret the change of contact resistivity with the type of surface treatment. The surface band bending was directly measured using synchrotron radiation photoemission spectroscopy (SRPES). From these results, the ohmic contact formation mechanism on p-type GaN are discussed.
Fig. 1. A comparison of I-V curves with the type of surface treatment. The interspacing between the contact pads was 5 µm.
EXPERIMENTAL PROCEDURE GaN films used were grown by metal organic chemical vapor deposition (MOCVD) on the c-plane sapphire substrate. An undoped GaN layer with a thickness of 1 µm was grown, followed by the growth of 1-µm-thick p-type GaN doped with Mg. The grown samples were annealed for the generation of holes at 800°C, 4 min by rapid thermal annealing (RTA) under N2 atmosphere. The net concentration of holes in the film was determined to be 1.9 × 1017 cm–3 by the Hall measurements. First, the active region of p-type GaN for the evaluation of contact resistivity was defined using chemically assisted ion beam etching using Cl2. Three kinds of surface treatments were applied to the active region of the substrates. The first set was the as-grown p-type GaN to compare with the surface-treated samples. The second set was prepared by surface treatment using dilute HCl (HCl:deionized water = 1:1) for 30 sec, which is widely used as a pre-treatment to remove possibly native oxides [HCl-treated sample]. The third set was the surface treatment using boiling aqua regia (HCl:HNO3 = 3:1) for 10 min [aqua regiatreated sample]. After the surface treatments, all samples were rinsed for 1 min with deionized water, and then dried with N2 gas. The transmission line method (TLM) test structure with a length of 100 µm and a width of 200 µm was patterned on the surface-treated substrates. A 200-Åthick Pt layer was deposited using electron beam evaporator. The vacuum condition of the evaporator was maintained lower than 3 × 10–7 torr during the deposition of metal layers. After the metal deposition, the photoresist was lifted off. For the SRPES measurement, three samples were transferred to a loading chamber immediately after
Fig. 2. O 1s core level spectra with the depth below surface on p-type GaN, measured by x-ray photoelectron spectroscopy.
the surface treatment. The incident photon energy of 600 eV was used for the measurement of core level spectra of N 1s and O 1s and that of 130 eV was used for Ga 3d. The energy resolution of binding energy in both measurements was 0.1 eV. The valence band spectrum was calibrated with Au and Ta Fermi level. For the core level spectra, Au 4f was used as a reference. RESULTS AND DISCUSSION Figure 1 shows current-voltage (I-V) characteristics of Pt contacts on the p-type GaN, measured between the TLM pads with a distance of 5 µm. The Pt contact formed on as-grown sample showed Schottky behavior. The I-V curve was improved by the HCltreatment, but still non-linear. When the substrate was treated with aqua regia, the I-V curve became
Effects of Surface Treatment Using Aqua Regia Solution on the Change of Surface Band Bending of p-type GaN
Fig. 3. SRPES spectra of Ga 3d and N 1s core levels for the as-grown p-type GaN.
Fig. 4. SRPES spectra of Ga 3d and O 1s core levels as a function of on p-type GaN.
linear over the whole range of applied voltages. The four-point probe method was used in the measurement of contact resistivity. The ohmic contact resistance (Rc) in the unit of Ω-mm and the sheet resistance (Rs ) in the unit of Ω/o were determined from the intercept of y-axis and the slope of resistances at 0 V with interspacings of TLM pads. The specific contact resistivity (ρC ) was calculated by ρC = Rc 2/ Rs. The ρC was evaluated to be (5.1±1.2) × 10–1 Ω cm2 for the HCltreated sample and (9.3±3.5) × 10–5 Ω cm2 for the aqua regia-treated sample. The contact resistivity decreases by nearly four orders of magnitude only by the surface treatment using aqua regia, which is consistent with the previous results on Pd contact on p-type GaN.7 The lower contact resistivity in the Pt contact can be explained with the higher work function of Pt than Pd. The drastic decrease in the ohmic contact by the aqua regia treatment is attributed to the removal of surface oxide formed during the fabrication processes, such as the MOCVD growth and/or
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the activation process for the generation of holes in ptype GaN. In order to investigate the depth distribution of surface oxides on the as-grown sample, the depth distribution of O 1s spectrum was monitored through the repetition of XPS measurements. The depth was calibrated with the controlled erosion rate of the standard target by the Ar ion sputtering, and the resultant spectra of O 1s signals were displayed in Fig. 2. In the as-grown sample, O 1s signal was detected. When the sample was etched by 6.4 Å from the surface of p-type GaN, the O 1s peak disappeared. No O 1s peak was observed after further etching deeper to 12.8 Å below the surface. This indicates that the surface oxides only exist on the surface of GaN and they were formed during both the activation process and the growth. Figure 3 displays the SRPES spectra of Ga 3d and N 1s core levels for the as-grown sample. The N 1s spectrum was symmetric and nearly coincided with the Gaussian fit line, plotted with dotted line in Fig. 3. The Gaussian width did not change with the type of surface treatment. This suggests that there is only N-Ga bond in the N 1s spectra. However, the Ga 3d spectrum showed asymmetry. In addition, the Gaussian width of Ga 3d was measured to be 1.30 eV for the as-grown sample and 1.12 eV for the aqua regia treated one. This means that additional bonding is superimposed in the Ga 3d spectrum. Since O 1s spectra with high intensity were found in the survey scan, the superimposed peak could be attribute to the spectrum from Ga-O bonds. Therefore, the deconvolution of Ga 3d spectra was carried out. In the deconvolution of the Ga 3d spectra, the spin orbit splitting and hence the branching ratio of Ga 3d spectra were not considered, but the Gaussian width of 1.04 eV deduced from the Ga-N bonds in the aqua regia-treated sample was used. Consequently, the Ga-O bond could be separated, and shifted by 0.8 eV from the Ga-N bond toward higher binding energy, plotted with a dotted line in Fig. 3. This suggests that surface oxides were mainly composed of Ga and O, namely GaOx. Figure 4 displays Ga 3d core level spectra at the surface of p-type GaN with the surface treatments. The peak intensities of Ga 3d significantly increased after the aqua regia treatment. This indicates that GaOx forming on the surface of p-type GaN was reduced by the surface treatment. All Ga 3d spectra were deconvoluted into two components with the energy separation of 0.8 eV, namely Ga-N bond and Ga-O bond.15 The intensity of Ga-O bonds and O 1s spectra, not shown here, were reduced simultaneously in sequence with the HCl and the aqua regia treatment. The results indicate that the aqua regia treatment was effective in removing the GaOx layer on the surface of p-type GaN. The shift of the Fermi level position at the surface p-type GaN is accurately determined from the shift of core level spectrum in the photoemission spectra.11 In Fig. 4, the binding energies observed in Ga 3d shifted
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Kim, Kim, Kim, Kim, Kwak, Park, and Lee
Table I. The Change of Binding Energy of Ga 3d Spectra with the Type of Surface Treatment Measured by Synchrotron Radiation Photoemission Spectroscopy As-grown Binding 21.00 ± 0.03 energy (Eb) ∆Eb* —
HCl
Aqua Regia
20.42 ± 0.02
20.13 ± 0.02
0.58
0.87
* In the determination of binding energy change (∆Eb ), the binding energy in the as-grown sample was used as a reference.
Fig. 5. Change of the Ga/N ratio at the surface of p-type GaN with both the surface treatments and detection angle, evaluated from the integrated intensities of the Ga 3d and the N 1s spectra.
toward lower binding energy, which summarized in Table I. The binding energy shifted from 21.00 ± 0.03 to 20.42 ± 0.02 eV after the surface of GaN was treated by HCl, and to 20.13 ± 0.02 eV after the aqua regia treatment. This clearly showed that the Fermi level position at the surface of p-type GaN shifted by 0.58 eV and 0.87 eV toward the valence band edge through the HCl and the aqua regia treatments, respectively.
Thus, the surface band bending was reduced in sequence with the removal of GaOx by the HCl and the aqua regia treatment, respectively. In order to obtain the depth information of chemical compositions below the surface, angle-resolved scan of SRPES were performed. Smaller angle enhances the signal of photoelectrons emitting from the surface rather than from the bulk due to the inelastic meanfree path of photoelectrons. The relative Ga/N ratio with the detection angle were determined from the integration of intensities in the Ga 3d and the N 1s spectra. Figure 5 shows the relative Ga/N ratio at the surface of each sample with the detection angle. Data for the as-grown sample were used as a reference in the calculation of the Ga/N ratio. The Ga/N ratio decreased in sequence with the HCl and the aqua regia treatments independent of the detection angle. Note that the magnitude of decrease in the aqua regia-treated sample was pronounced at the smaller detection angle. The Ga/N ratio decreased to 0.91 at the detection angle of 90°, but decreased to 0.55 at 50°. This means that the surface oxide was mainly composed of Ga and O, GaOx, formed during the activation process, and resultantly Ga vacancies, VGa, were produced below the GaOx. There are two energy levels within the band gap of the compound semiconductor; donor and acceptor, which energy levels are located near the conduction band and the valence band edge, respectively. In GaN, N vacancies, VN, act as donors and VGa are acceptors.16 In the as-grown sample, the surface Fermi level might be located at the middle of the bandgap due to the surface states in the surface oxides. This causes a large surface band bending, resulting in the rectifying behavior in the I-V curve, as shown in Fig. 1. When surface oxides were removed by the aqua regia treatment, a thin layer of GaN containing a large amount of VGa was exposed on the surface. Thus, the Fermi level position shifts to the energy level of VGa, because their energy levels are located in the vicinity of the
Fig. 6. Energy band diagrams at the surface of p-type GaN with the surface treatments.
Effects of Surface Treatment Using Aqua Regia Solution on the Change of Surface Band Bending of p-type GaN
valence band edge. Therefore, band bending reduces by 0.87 eV, as shown in Fig. 6, resulting in a drastic decrease of contact resistivity. Meanwhile, the HCl treatment is a little effective in removing the surface oxide. Thus, the Fermi level at the surface decreased by 0.58 eV, leading to the slight improvement in ohmic contact behavior. CONCLUSION The effect of surface treatment on both the contact resistivity and the surface band bending of p-type GaN was studied. The contact resistivity on p-type GaN was decreased by four orders of magnitude by the surface treatment using aqua regia prior to Pt deposition. The shift of the Ga 3d core level spectra confirmed that the surface band bending was decreased by 0.58 eV by the HCl treatment and 0.87 eV by the aqua regia treatment. The Ga/N ratio with the photoelectron detection angle demonstrates that the surface oxide was mainly composed of Ga and O, GaOx , and a number of VGa were produced below the GaOx . Consequently, the aqua regia treatment plays a role in removing GaOx formed on p-type GaN, leading to the shift of the Fermi level toward the energy levels of VGa located near the valence band edge. This causes the decrease of barrier height for the transport of holes, resulting in good ohmic contacts to p-type GaN. ACKNOWLEDGEMENTS This work was supported in part by the Korea Institute of Science and Technology Evaluation and Planning (KISTEP) through the NRL projects.
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