Investigation of Ohmic mechanism for chlorine-treated p-type
GaN using x-ray photoelectron spectroscopy
Po-Sung Chen and Ching-Ting Leea兲
Institute of Microelectronics, National Cheng Kung University, Tainan, 701 Taiwan, Republic of China; Department of Electrical Engineering, National Cheng Kung University, Tainan, 701 Taiwan,
Republic of China; and Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, 701 Taiwan, Republic of China
共Received 19 January 2006; accepted 17 July 2006; published online 29 August 2006兲
To investigate the function and mechanism of oxidation, the surface of the chlorine-treated p-type GaN semiconductor was analyzed using x-ray photoelectron spectroscopy. The chlorinated surface treatment was performed by electrolyzing HCl chemical solution to generate HClO, which in turn could be used to oxidize the p-type GaN. The chlorinated surface treatment enhances the formation of GaOx on the GaN surface and removing GaOx layer from the surface thereafter leads to the
creation of additional Ga vacancies. Consequently, more holes are generated as a result of the generated Ga vacancies. Therefore, a relatively higher Ohmic performance with a specific contact resistance of 6.1⫻10−6⍀ cm2can be obtained for Ni/ Au metal contact subsequently patterned on the chlorine-treated p-type GaN via the enhanced formation of GaOx. © 2006 American Institute of
Physics.关DOI:10.1063/1.2336300兴
I. INTRODUCTION
Recently, wide-band-gap GaN-based compound semi-conductors have been widely and successfully used in elec-tronic devices,1 optoelectronic devices,2 and near white emission light sources.3 In those devices, sufficiently low Ohmic contact resistance between metal and semiconductor is needed in order to improve device performance and reli-ability. For n-type GaN-based compound semiconductors, useful Ohmic contact metals and surface treatment methods have already been proposed to obtain excellent Ohmic performance.4–6 However, comparable Ohmic performance for p-type GaN could not be easily achieved due to the in-herent difficulty of activating p-type dopants. In principle, the existence of Ga vacancies in GaN-based compound semi-conductors potentially can function as holes.7 To obtain a lower Ohmic resistance for application of high power de-vices, usually the Ga vacancies can be generated by com-pletely removing the native oxide on the surface of p-type GaN-based materials using 共NH4兲2Sxsurface treatment and
preoxidation process.8,9In this work, we present a method to increase Ga vacancies on p-type GaN surface through chlo-rinated surface treatment by electrolyzing HCl共aq兲 solution. The Ga vacancies induced on the surface of chlorine-treated p-type GaN were subsequently analyzed and investigated us-ing an x-ray photoelectron spectroscopy共XPS兲.
II. EXPERIMENTAL PROCEDURE
The epitaxial layers used in this work were grown on c-plane sapphire substrates using a metal organic chemical vapor deposition 共MOCVD兲 system. Trimethylgallium, am-monia 共NH3兲, and bis共cyclopentadienyl兲magnesium 共CP2– Mg兲 were used as the Ga, N, and Mg sources,
respec-tively. Following the growth of a highly resistive GaN buffer layer with a thickness of 650 nm at 520 ° C, an 800-nm-thick Mg-doped GaN layer was grown on the sapphire substrate at 1100 ° C. The grown samples were then annealed to activate the Mg dopants at 750 ° C for 30 min in a N2ambient. Using Hall measurement carried out at room temperature, the hole concentration and mobility of the Mg-doped GaN layer were determined as 5⫻1017cm−3and 10.2 cm2/ V s, respectively. Figure 1 illustrates the chlorination treatment system. A chemical aqueous solution of 1HCl+ 30H2O was used as the electrolytic solution. The grown p-type GaN sample was placed underneath the Pt anodic electrode and a voltage of 20 V was applied to the Pt electrode for 60 min. The chlo-rine was produced as a result of electrolyzing dilute HCl共aq兲 at the Pt anodic electrode. The produced chlorine was ad-hered to the p-type GaN sample and reacted with the sample. The Ga dangling bonds of the Ga-face p-type GaN surface grown by MOCVD system reacted with chlorine to form GaClx. The GaClx can easily be dissolved in the chemical
solvent10 in order to induce Ga vacancies on the surface of the p-type GaN sample. To investigate the function of
chlo-a兲Author to whom correspondence should be addressed; FAX:
886-6-2362303; electronic mail: [email protected] FIG. 1. 共Color online兲 Chlorination treatment system.
JOURNAL OF APPLIED PHYSICS 100, 044510共2006兲
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rinated surface treatment, both samples with and without the chlorinated surface treatment were first cleaned in chemical solutions of trichloroethylene, acetone, and methanol, and then rinsed with de-ionized water. The cleaned samples were then loaded into the vacuum chamber of a Perkin-Elmer PHI-5400 ESCA XPS system for analysis. The XPS mea-surements were performed using a monochromatic Mg K␣ x-ray source. To enhance the interface sensitivity, each XPS measurement was performed at a takeoff angle of 45°. A Au 4f7/2 peak at 83.86 eV and a Cu 2p3/2 peak at 932.65 eV were taken as the energy reference. The XPS core-level spec-tra were deconvolved into their related components using an interactive least-squares computer program, and the curves were gathered as the mixture of 80% Gaussian and 20% Lorentzian functions.
III. EXPERIMENTAL RESULTS AND DISCUSSION To investigate the function and mechanism of the chlorine-treated p-type GaN samples, the samples were ana-lyzed using XPS. Figures 2 and 3 show the depth profiles of Ga 3d and N 1s core-level spectra of the p-type GaN with and without chlorinated treatment, respectively. From the ex-perimental results of the XPS spectra, we can deduce the influence of chlorinated treatment into the surface only
within a depth of 1 nm. To remove the native oxide on the surface of the p-type GaN samples with and without the chlorinated treatment, the function and mechanism of chlo-rinated treatment can also be studied based on the Ga/ N ratio measured using XPS. According to the XPS measurement, Table I shows the ratio of Ga/ N for the chlorine-treated p-type GaN sample relative to the sample without chlori-nated treatment as a function of depth. Because the relative ratio of Ga/ N is approximately equal to 1 at a depth of 1 nm underneath the surface, the depth of the chlorinated treatment can thus be deduced to about 1 nm. The relative ratio of 0.5 was found on the surface of p-type GaN samples with chlo-rinated treatment. This phenomenon indicates the removal of Ga atoms from the surface due to the formation of GaClx
using chlorinated surface treatment. Therefore, more holes are induced as a result of additional Ga vacancies generated at the surface.
To better understand the chlorination treatment process, the composition of the resultant Pt cathode electrode was analyzed using energy dispersive spectrum system共EDS兲, as indicated in Table II. It can be seen that a small amount of gallium, chlorine, and oxygen adhered to the Pt cathode elec-trode after subjecting samples to the chlorination treatment in HCl solution. The oxygen thus detected believably comes from the de-ionized water. The chlorine is produced as a result of electrolyzing HCl solution, and as for the gallium, the dissolution of GaClxand the subsequent adherence of Ga
to the Pt electrode can be a contributing cause. The evidence of Ga adhered to the Pt cathode electrode indicates the cre-ation of Ga vacancies on the surface of p-type GaN.
To study the mechanism and the binding energy shift of Ga 3d and N 1s core-level peaks for the chlorine-treated p-type GaN samples, the surfaces of the samples with and without chlorination treatment were analyzed using XPS. Figures 4 and 5 show the Ga 3d and N 1s XPS core-level spectra for the chlorine-treated p-type GaN samples, respec-tively. For the Ga 3d XPS spectra shown in Fig. 4, the bind-ing energies of Ga–Ga, Ga–N, and Ga–O bonds are 18.6,11 19.54,12and 20.5 eV,13respectively. The relative intensity of Ga 3d spectrum for the chlorine-treated sample is smaller than that of the sample without chlorination treatment. Fur-thermore, the metallic gallium共Ga–Ga bonds at 18.6 eV due to Ga rich of p-type GaN samples兲14can hardly be observed
FIG. 2. 共Color online兲 Depth profile of Ga 3d XPS spectra for p-type GaN with and without chlorination treatment.
FIG. 3. 共Color online兲 Depth profile of N 1s XPS spectra for p-type GaN with and without chlorination treatment.
TABLE I. The共Ga/N兲 ratio of chlorine-treated p-type GaN relative to that of p-type GaN without chlorination treatment.
共Ga/N兲 0 nm共surface兲 1 nm 3 nm 5 nm
Ratio 0.5 0.97 1 1
TABLE II. Composition of Pt cathode electrode analyzed by energy disper-sive spectrum. Element at. % O 32.18 Ga 0.93 Pt 61.91 Cl 4.98
044510-2 P.-S. Chen and C.-T. Lee J. Appl. Phys. 100, 044510共2006兲
from the surface of chlorine-treated samples. This observa-tion can be ascribed to a reducobserva-tion of Ga concentraobserva-tion as a result of the removal of GaClxfrom the surface. The binding
energy of Ga–N bonds of the chlorine-treated samples is shifted by 0.4 eV toward the high-binding energy side com-pared to samples without chlorination treatment. The spectral intensity of Ga–O bonds of the chlorine-treated samples is larger than that of samples without chlorination treatment. The GaOx composition on the chlorine-treated p-type GaN
samples is attributed to the production of HClO in the solu-tion, which can be expressed as
Cl2+ H2O HCl + HOCl, 共1兲
where Cl2 is produced by electrolyzing HCl solution. The generated HClO in turn can oxidize p-type GaN to form GaOx; its mechanism can be depicted as
xHOCl + Ga xHCl + GaOx. 共2兲
The Ga 3d core-level peak is closely associated with the surface Fermi level position.15The shift of Ga 3d core-level peak to a higher energy side indicates that the surface Fermi level is being moved closer toward the conduction band edge.16 This phenomenon is responsible for an increase of surface barrier height due to the formation of GaOx. The
mechanism of forming GaOxin the chlorination treatment is
similar to the preoxidation process.9Subsequent removal of the GaOxlayer is expected to generate more Ga vacancies,
resulting in an increase in the hole concentration.
Figure 5 shows the N 1s XPS core-level spectra of the p-type GaN samples with and without chlorination treatment. The binding energies of N–Ga and N–H bonds are 397.5 eV 共Ref. 17兲 and 398.8 eV,18
respectively. The hydrogen source for the N–H bonds comes from the metal organic reactants.19 Notice that the binding energy of N–Ga bonds of the chlorine-treated p-type GaN samples exhibits a shift of 0.4 eV toward the high-binding side, compared with the samples without chlorination treatment. This result is similar to that of the Ga 3d core-level spectral analysis depicted in Fig. 4.
With Ni/ Au共50/600 nm兲 as the metal mask, the reac-tive ion etching 共RIE兲 was employed to pattern the mesa region 共100⫻1000 m2兲 for the transmission line method 共TLM兲 measurements using BCl3 gas to etch through the p-type GaN layer down to the undoped GaN buffer layer. Following the removal of metal mask, the p-type GaN with mesa pattern was directly put underneath the Pt anodic elec-trode and applied a voltage of 20 V for 60 min. The chlori-nation process is similar to that shown in Fig. 1, except that the mesa structure is shown instead of the planar structure of p-type GaN. A linear metallization pattern of contact pads with gap spacings of 5, 10, 15, 35, 50, and 60m, respec-tively, was lithographically defined over the mesa region. Prior to the deposition of the Ni/ Au 共20/100 nm兲 using an electron-beam evaporator, the GaOx layer and native oxide
layer on the p-GaN samples with and without chlorination treatment were removed using a chemical solution of HNO3: HCl 共1:3兲. The Ohmic alloying process was per-formed using a thermal furnace conducted at 500 ° C for 10 min in air ambient. The current-voltage共I-V兲 characteris-tics were measured based on the TLM technique using an HP 4145B semiconductor parameter analyzer and shown in Fig. 6. According to the TLM model,20the measured specific con-tact resistances for the p-type GaN samples with and without chlorination treatment are 6.1⫻10−6 and 7.2⫻10−4 ⍀ cm2, respectively.
FIG. 4.共Color online兲 Ga 3d XPS spectra for p-type GaN with and without chlorination treatment.
FIG. 5. 共Color online兲 N 1s XPS spectra for p-type GaN with and without chlorination treatment.
FIG. 6. Current-voltage characteristics of Ni/ Au contact to p-type GaN with and without chlorination treatment.
044510-3 P.-S. Chen and C.-T. Lee J. Appl. Phys. 100, 044510共2006兲
IV. CONCLUSIONS
In summary, the mechanisms involved for the chlorine-treated p-type GaN samples using the electrolysis process have been investigated. The HClO is produced in the elec-trolyte of dilute HCl solution via the electrolysis. The forma-tion of GaOxis achieved and further strengthened by relying
HClO to oxidize p-type GaN. Since GaOx is mainly
com-posed of Ga and O, additional Ga vacancies are expected to create after the removal of GaOx layer. Since Ga vacancies
knowingly behave as acceptors in GaN,21 additional holes can thus be induced in the chlorine-treated p-type GaN samples. Consequently, a much improved specific contact re-sistance of 6.1⫻10−6⍀ cm2 can thus be achieved via the enhanced formation of GaOxon the p-type GaN surface
us-ing electrolysis process. Accordus-ing to the XPS measurement indicated in Table I, the Ga/ N ratio of chlorine-treated p-type GaN is about half of that of p-type GaN without chlorination treatment. Although the Ga vacancies can be induced from the GaClxand GaOx formed on the
chlorine-treated p-type GaN surface, it can be deduced that the GaClx
is a major factor due to the direct formation of GaClx and
second order formation of GaOx.
ACKNOWLEDGMENT
This work was supported by the National Science Coun-cil of Taiwan, Republic of China under Contract No. NSC-94-2215-E006-013.
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