Acceptor activation of Mg-doped GaN by microwave treatment
Shoou-Jinn Changa)and Yan-Kuin Su
Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, Republic of China
Tzong-Liang Tsai, Chung-Ying Chang, Chih-Lih Chiang, Chih-Sung Chang, Tzer-Peng Chen, and Kuo-Hsin Huang
Department of Research and Development, United Epitaxy Company, Hsinchu 300, Taiwan, Republic of China
共Received 1 May 2000; accepted for publication 22 November 2000兲
A microwave treatment method different from thermal annealing and low-energy electron beam irradiation was proposed to activate Mg dopants in p-type GaN epitaxial layer. From photoluminescence spectra and Hall effect measurements, it was shown that microwave treatment is a very effective way to activate the acceptors in Mg-doped p-type GaN layer. The activation of Mg dopant in p-type GaN layer may be explained as the breaking of magnesium–hydrogen bonding due to the microwave energy absorption. © 2001 American Institute of Physics.
关DOI: 10.1063/1.1340864兴
Gallium nitride共GaN兲 and related group III nitrides are promising materials for manufacturing green, blue, violet, and ultraviolet light emitting devices such as light emitting diodes共LEDs兲 and laser diodes 共LDs兲. To fabricate such blue LEDs and LDs, it is necessary to achieve high conductive p-type GaN. However, it is difficult to achieve p-GaN with a high hole concentration. Sometimes, it is difficult to even achieve a p-type conductivity. It has been pointed out that hydrogen passivation1,2 is the reason to prevent acceptors from activation in p-type GaN. In order to achieve a highly conductive p-type GaN, low-energy electron beam irradia-tion 共LEEBI兲3,4 and thermal annealing5,6 are two popular methods that were used to activate the p-type impurity in GaN.
Akasaki et al.3have used LEEBI to convert the compen-sated Mg-doped GaN into conductive p-type material. How-ever, with acceleration voltage of 5–15 kV, an electron beam can only reach a depth of about 0.5m. Therefore, LEEBI is not an effective way to convert a thick high-resistive Mg-doped GaN into a p-type conducting material. Nakamura et al.5proposed a method to reduce the resistivity of p-type GaN by thermally annealing the GaN samples in a nitrogen atmosphere at elevated temperatures. In order to effectively activate Mg dopants in GaN, the annealing process should be carried out in the temperature range of 600–1200 °C. At such a high temperature, thermal dissociation of GaN may occur because the dissociation pressure gradually increases when the temperature is higher than 700 °C. In this letter, we report the acceptor activation of Mg-doped GaN layer by micro-wave treatment, which can be accomplished at a lower tem-perature and with a shorter cycle time.
The p-type GaN epitaxial layers used in this study were grown on c-face 共0001兲 sapphire substrates by low-pressure metalorganic chemical vapor deposition. Trimethylgallium 共TMGa兲 and ammonia (NH3) were used as the source mate-rial of Ga and N, respectively.
Biscyclopentadienylmagne-sium (Cp2Mg) was used as the p-type dopant source. Before epitaxial growth, sapphire substrate was first heated at 1150 °C in a stream of hydrogen to clean the substrate sur-face. Then, the substrate temperature was cool down to 510 °C so as to deposit a 25 nm GaN buffer layer onto the substrate. During GaN buffer layer growth, the flow rate of TMGa and NH3 were kept at 1.19⫻10⫺5 and 7.14 ⫻10⫺2mol/min, respectively. The temperature was then raised to 1130 °C to grow the 4 m p-type Mg-doped GaN layer. The acceptor activation of Mg-doped GaN layer was performed by a 2.45 GHz, 560 W microwave treatment with different process time. For comparison, a 730 °C, 20 min furnace annealed Mg-doped GaN sample was also prepared. The activation of acceptors was assessed by photolumines-cence and Hall measurement. For Hall effect measurement, Ni/Au layers were deposited onto the surface of the Mg-doped GaN layer as the p-type contact. The resistivity and carrier concentration of Mg-doped GaN samples were then measured by a Polaron Hall effect measurement system. Fig-ure 1 shows the photoluminescence spectra of the p-type Mg-doped GaN 共a兲 with a 5 min microwave treatment, 共b兲
a兲Electronic mail: [email protected]
FIG. 1. The photoluminescence spectra of Mg-doped GaN共a兲 with micro-wave treatment共b兲 with thermal annealing 共c兲 as grown without any treat-ment.
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with a 730 °C, 20 min thermal treatment, and 共c兲 as-grown layer without any treatment. It can be seen that Mg-doped GaN layers with microwave treatment and with furnace an-nealing both showed a strong 437.5 nm blue peak compared with the as-grown sample without any treatment. Figures 2 and 3 show the resistivity and hole concentration of Mg-doped GaN layer with different amount of microwave treat-ment time. These data were measured by Hall measuretreat-ment. From these two figures, we can see that the measure resis-tivity 共i.e., 1.1–1.65 ⍀ cm兲 and hole concentration 共i.e., 9.75⫻1017cm⫺3–2.15⫻1018cm⫺3) were almost indepen-dent of the microwave treatment time. On the other hand, the measured resistivity and hole concentration, of the 730 °C, 20 min furnace annealed Mg-doped GaN layer, were about 3.2 ⍀ cm and 6.58⫻1017cm⫺3, respectively. Such a result suggests that p-type dopants in Mg-doped GaN can also be effectively activated by microwave treatment. The activation of Mg-doped p-type GaN layer may be explained as the
breaking of magnesium–hydrogen bonding due to the ab-sorption of microwave energy. The fact that the measured resistivity and hole concentration were almost independent of microwave treatment time suggests that this microwave energy absorption is a very fast process. We can see from Fig. 2 that the resistivity decreased to about 1.45⍀ cm in just 5 s and a longer microwave treatment time can not signifi-cantly further decrease the resistivity of the Mg-doped GaN sample.
Microwave treatment is also a very effective way to con-vert the whole thick as-grown high resistive p-type GaN layer into low resistive layer. To prove this, we have re-moved a 1-m-thick layer from the surface of a total 4- m-thick Mg-doped GaN, by both reactive ion etching共RIE兲 and chemical wet etching, after microwave treatment. We found that the measured hole concentration is about 9⫻1016cm⫺3 after removing the 1-m-thick layer by RIE. On the other hand, the measured hole concentration is about 6.8 ⫻1017cm⫺3after removing the 1-m-thick layer by chemi-cal wet etching. For RIE etched sample, although the carrier concentration at a depth of 1 m from the surface is lower than that of the surface, it is still much higher than that of the as-grown p-type GaN sample. For the wet chemical etched sample, the carrier concentration at a depth of 1m from the surface is almost the same as that at the surface. For the RIE etched sample, we believe the reason why the carrier con-centration at a depth of 1m from the surface is lower may be due to the damage caused by RIE.
In conclusion, we have shown that microwave treatment is an effective way to activate the acceptors in p-type Mg-doped GaN layer. Using microwave treatment, we can easily convert a high resistivity Mg-doped GaN layer into a con-ductive p-type GaN layer.
The authors would like to thank Professor S. F. Homg for his assistance in photoluminescence measurement. This work is partially supported by the Science Park Administra-tion. This work is also partially supported by the National Science Council under Contract No. NSC-89-2215-E-006-005.
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FIG. 2. Resistivity of Mg-doped GaN with different microwave treatment time.
FIG. 3. The carrier concentration of Mg-doped GaN by Hall measurement with different microwave treatment time.
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